China stone activators

The prospect of utilizing non-metal materials for the adsorption and catalytic conversion of toxic environmental gases, as an alternative for the present-day precious metal catalyst is gaining interest, owing to its lower price as well as a better durablility^1,2,3,4,5,6. Among metal-free adsorbents, carbon based nanostructures, such as C₆₀, carbon nanotube (CNT) and graphene have received much attention^7,8,9. Similar interest is directed to, BN analogue: it was discussed that with modified electronic structures it can also lead to promising materials for gas capturing and catalytic convertors^10,11,12,13. The BN based monolayer and nanotube structures have been quite widely studied experimentally as well as theoretically^14,15. It is noteworthy that, a recent experimental study has demonstrated the possibility of systematically converting a graphene sheet to a hexagonal BN sheet via a chemical route¹⁶. Combining the chemical route with the lithography technique it is possible to produce uniform boron nitride structures without disrupting the structural integrity. Also the carbon based template can be used to synthesize the BN structures¹⁷. Inspired by these experiments, in the present work, we explore properties of fullerene-like BN cages, hereafter named as BN-60, which may be obtained as a result of atom by atom substitution of C₆₀ or by direct synthesis. The important point is that the network of pentagonal rings in BN-60 will lead to homonuclear bonds¹⁸. The BN cages, free of the homonuclear bonds, are made up of square and hexagon rings as discussed in previous literature^19,20,21. However, pentagon–octagon–pentagon line defects are found in the BN sheets, nanoribbons and single-walled BN nanotubes and are consequence of the existence of homonuclear bonds²². Under boron rich environment the large possibility of formation of frustrated B-B homonuclear bond has been reported²³. Also the pentagons with homonuclear bond form at the tip of the h-BN nanotube²⁴. In the present work, we found that the homonuclear bonds have decent reactivity, which is distinctly different from the conventional BN structures. We are particularly interested in the catalytic performance of homonuclear bonds for CO oxidation and CO₂ conversion.

The oxidation of CO is an important prerequisite for mitigating toxic CO gas. On a catalyst surface, CO oxidation follows Langmuir–Hinshelwood (LH) mechanism and the Eley–Rideal (ER) mechanism. LH mechanism involves the coadsorption of reactants onto the catalytic surface, followed by a surface reaction to form the products. ER mechanism, on the other hand, involves the direct reaction of a gaseous reactant with a chemisorbed one. Nitrogen-doped carbon nanotubes possess the ability to effectively catalyze the CO oxidation with activation energies ranging from 0.477 to 0.619 eV. A less negative charge on the dopant N atom is correlated with a higher activity for CO oxidation²⁵. Iron embedded graphene also proved to be a potential material for CO oxidation with activation energy of 0.58 eV²⁶. Graphene doped with Cu results in electronic resonance among the electronic states of the reactants and the Cu atom, leading to higher reactivity for oxidizing CO. The process proceeds first via an LH mechanism with barriers of 0.25 eV and 0.54 eV followed by ER reaction without energy barrier²⁷. Zhao et al. have investigated theoretically the possibility of CO oxidation on a Si embedded graphene surface and attributed to the charge transfer from the embedded Si atom to the 2π* orbital of O₂. The process proceeds first via LH mechanism with a barrier of 0.48 eV followed by ER mechanism²⁸. Fe encapsulated boron nitride cage has good CO to CO₂ conversion capabilities with an activation energy of 0.5 eV²⁹. The choice of dopants significantly alters the CO oxidation mechanism and hence the activity of boron nitride monolayer. This was demonstrated in our previous work employing carbon and oxygen as dopants, wherein the O dopant enabled chemisorption of CO, while C doped h-BN monolayer has lesser tendency to adsorb CO³⁰. O doping results in a larger bond length of a neighboring B atom, it’s out of plane displacement and less positive charge, synergistically contributing to stronger CO adsorption.

Metal-organic frameworks, Carbon and BN nanostructures, such as CNT and BN nanotube (BNNT)^31,32,33, have also been tested for CO₂ capture and storage. As the weak binding of CO₂ on such inert surfaces is a demerit, various methods to activate the surface have been tested. Suchitra et al. reported that Boron doped C₆₀ (BC₅₉) fullerene does not adsorb CO₂ molecule effectively but 1e⁻ charged BC₅₉ can strongly adsorb CO₂ with binding energy of −0.66 eV³¹. Huang et al. proposed about the remarkable CO₂ capturing ability of armchair graphene nanoribbons with dangling bond defect, the adsorption energy is about −0.31 eV³⁴. Sun et al. provided a route to increase the activity of a pure BN sheet to adsorb CO₂ by applying the electric field. By applying 1.36 eV of electric field the adsorption energy can be increased to −0.84 eV³⁵. Gao et al. demonstrated that single Ca atom anchored on C₆₀ can adsorb CO₂ with higher binding energy compared to pristine C₆₀³⁶. Shao et al. proposed the increase of chemical activity of BNNT by the substitutional doping of Al atom in-place of B site. The CO₂ binding energy varies with tube diameter and is in the range of about −0.03 to −5.08 eV³⁷.

In the present work, the BN analogues of C₆₀, which possess the frustrated homonuclear bonds because of pentagons, are investigated in the perspective of aforementioned CO oxidation and CO₂ conversion catalyst. The stability of the cages has been analyzed and discussed in detail. It has been discoursed how the pentagonal rings in the structure will generate the homonuclear B-B, B-B-B, N-N and N-N-N bonds. Throughout this work, we designate B1 notation for a single B atom surrounded entirely by nitrogen, B2 for two B atoms bonded together, making a B-B bond and B3 for two adjacent B-B bonds merged to form a B-B-B bond for simplicity. Similarly for N sites we consider the similar notation. The binding affinity of the different stable BN-60 cages considering B1, B2 and B3 sites to capture CO/CO₂/O₂ molecules has been estimated. The role of the sites (B1, B2 and B3) on the CO oxidation and CO₂ conversion are analyzed in detail using first principles approach.

Construction of BN-60 cages

Here we first outline the scheme adopted for the replacement of carbon atoms in C₆₀ cage, containing 12 pentagonal and 20 hexagonal rings with B and N atoms to construct the BN-60 cages. In general, there are many ways to construct the BN-60 cages containing different distribution of B and N atoms, B/N ratio and homonuclear B and N bonds. In this work we made three different classes of BN-60 cages, considering 1. Boron rich, 2. Nitrogen rich and 3. stoichiometric B:N environment. To make the BN-60 cages with these environments, our approach is to make homonuclear B and N bonds first, taking into account pentagonal rings and then following some rules. Initially, replacement of carbon atoms is done on pentagonal rings such that each pentagonal ring has one homonuclear bond of either type (B2 or N2). In fact, it is evident from recent experiments that the presence of homonuclear bonds on pentagonal rings of boron nitride structures are inevitable²⁰. Another important consideration is that we restrict the homonuclear bonds to a maximum of three B (B3) and three N (N3) atoms. So to make B rich cages, all pentagonal rings are filled by B2 bonds and each B2 is surrounded by three or four homonuclear B2 bonds with one carbon atom separating them. This constraint prevents the formation of homonuclear N2, B3 or N3 configurations. The remaining carbon atoms in both hexagonal and pentagonal rings are replaced by B and N atoms alternatively, to avoid the N2 bonds. To incorporate B3 bonds in the boron rich condition, two or three B2 configurations should be surrounded by two or three B2 bonds.

The N rich BN-60 cages are constructed in a similar fashion incorporating N2 bonds instead of B2 bonds. In order to make stoichiometric B:N type BN-60 cages, the 12 pentagonal rings should be equally shared by both B2 and N2 bonds (6 B2 and 6 N2). Arrange these B2 and N2 bonds by alternative or continuous way first so that the number of B2 and N2 bonds is same and now depending on the surrounding homonuclear bonds, rest of the carbon atoms in both hexagonal and pentagonal rings are replaced by both B and N atoms. The steps for constructing stoichiometric 1-B30N30 type cage with 5 B2, 2 B3 & 5 N2, 2 B3 bonds is demonstrated in the Fig. S1 of the supplementary information (SI). This approach would allow us to consider the effects of homonuclear bonds that are always likely to occur on pentagonal rings of BN nanostructures.

Stability of BN-60 cages

In order to analyze the stability of different BN-60 structures, the formation enthalpy (F.E) per atom has been estimated using the following equation,

where, E_tot is the total energy of the different type of homonuclear bonded BN-60 structure. n_B is the number of boron atoms replaced the carbon atoms; μ_B is the chemical potential of the boron atom (reference structure: α-rhombohedral phase of bulk boron); n_N is the number of nitrogen atoms replaced the carbon atoms; μ_N is the chemical potential of the nitrogen atom (reference structure: N₂ gas molecule). Higher negative values of formation enthalpy for BN-60 cages, as obtained from equation (1), indicate better stability of the system.

In Table 1, the formation enthalpy values for different type of homonuclear bonded BN-60 cages are summarized. In every case the E_F.E is negative which clearly indicates the stability of the systems. The formation enthalpy value is plotted against the configuration of the system and shown in Fig. 1b and it helps to obtain a clear idea about the relationship between the formation enthalpy and type of BN-60 cage. Among all, N rich systems show a higher negative value than others, hinting that N- rich cages are more favorable to be synthesized in normal pressure and temperature. It is also evident that higher number of B3 bonds compared to the B2 bonds leads to lower stability of the systems, which can be easily understood comparing the formation enthalpy of three different types of B30N30 cages (see Table 1). Based on the formation enthalpy and considering homonuclear B bonds, four BN-60 cages (one B rich, two N rich and one stoichiometric B:N cases) are selected for further calculation and are shaded in grey in Table 1. Geometry optimization for the different type of BN-60 cages was performed and the optimized structure is shown in Fig. 1a and Fig. S2. The B rich or more B bonds in the system leads structural distortion and it is visible in Fig. S2 and Fig. 1a, so it may act as a good medium for gas adsorption. We estimated the density of phonon states (DOPS) for all the BN-60 cages. No negative frequency states because of structural instability have been found. Here only DOPS of four structures are shown in Fig. S3 (B25N35, 1-B27N33, 1-B30N30, B34N26). This indicates that all the structures are highly stable. The DOPS for all the other structures are not shown here.

Density of states (DOS)

Next, we gain an understanding on the electronic properties of BN-60 cages. The densities of states (DOS) as a function of energy (eV) for four specific systems (B25N35, 1-B27N33, 1-B30N30, B34N26) are shown in Fig. 2. These results indicate that both B and N atoms in every structure contribute to the valence band maximum and conduction band minima. The contribution vary based on the B:N ratio in the BN-60 cages. For example in case of B25N35, N atoms mainly contribute valence bands whereas the B atoms contributed to the conduction band mostly. The B atoms‘ contribution to the valence bands increases in case of B34N26 cages. Also the B:N ratio and homonuclear B2, B3, N2 and N3 configuration play a major role for generating defect sates. In particular, the DOS for a 1-B30N30 has defect states very near to the Fermi level because of the higher number of B3 and N3 configuration in the system. More understanding about the role of B1 B2 and B3 configuration is discussed in the next section.

Adsorption of molecules

In order to test the catalytic capabilities of the BN-60 cages, we first study the adsorption of molecules, O₂, CO and CO₂. The high negative charge of nitrogen prevents the adsorption of any of the reactants on N1, N2 and N3 sites^30,38. Table 2 shows the calculated adsorption energies of the molecules on the three different boron sites. It can be seen that the reactivity of boron atom strongly depends on its environment. As expected, B1 sites show no evidence of adsorbing any reactant molecules, rendering them unsuitable for catalytic applications. On B1 sites, the O₂ adsorption energy (E_ad(O₂)) varies from 0.001/−0.057 eV (considering PBE/PBE-D functional) in 1-B27N33 to −0.094/−0.198 eV in B25N35, CO adsorption energy (E_ad(CO)) varies from 0.003 eV/−0.070 eV in B25N35 to 0.022 eV/−0.337 eV in B34N26 and CO₂ adsorption energy (E_ad(CO₂)) is rather very small in all the cases. Interestingly, B2 atoms are more reactive, with E_ad(O₂) ranging from −2.492/−2.622 in 1-B30N30 to −2.854/−2.988 in B25N35, E_ad(CO) varying from 0.028 eV/−0.344 eV in B34N26 to −0.539/−0.652 eV in 1-B27N33 and E_ad(CO₂) varying from 0.052/−0.079 in 1-B27N33 to −0.195/−0.375 eV in B25N35. The optimized geometries of B27N33 after the adsorption of molecules are shown in Fig. S4. The high affinity toward O₂ and CO combined with weak CO₂ adsorption suggest that B2 sites of BN-60 cages can be efficient in catalyzing CO oxidation, which is discussed in detail later. The B3 sites strongly adsorb the O₂, CO and CO₂ molecules, with (E_ad(O₂)) ranging from −2.903/−3.043 eV in 1-B27N33 to −3.470/−3.599 eV in 1-B30N30, E_ad(CO) ranging from −1.185/−1.294 in 1-B27N33 to −1.678/−1.784 eV in B34N26 and E_ad(CO₂) varying from −0.196/−0.382 eV in 1-B27N33 to −1.214/−1.361 eV in B34N26. The ability of B3 sites to anchor CO₂ molecules is of particular interest as these systems can be employed as metal-free CO₂ trapping agents to solve many environmental problems. On adsorption, the CO₂ molecule is strongly activated as is evident from the bent geometry of the molecule (Fig. S4).

Bader charge analysis also confirms that the B3 sites are the highest reactive sites. The net charges are given in units of e, with a positive charge indicating a deficit of charge and a negative charge indicating a surplus of charge. The Bader charge analysis is performed, taking 1-B27N33 as a representative case. Before adsorption, B1 has a net charge of +2.129. After the adsorption of CO, O₂ and CO₂ on 1-B27N33, the net charge on B1 is +2.146, +2.145 and +2.135 respectively (given in Table 3), indicating that there is very little charge transfer to the incoming molecules. The net charges on the two atoms constituting B2 changes from +1.344, +1.441 before adsorption to +1.423, +1.577 on CO adsorption, +2.19, +2.16 after O₂ adsorption and +1.319, +1.469 on CO₂ adsorption and are given in Table 3. These values indicate that the B2 sites are able to donate electrons to the O₂ and CO molecules whose net charges post adsorption are −1.545 and −0.268 respectively while the CO₂ molecule is unaffected and retains its neutral charge. The three atoms constituting the B3 sites have charges of +1.25, +0.880 and +1.26 before the adsorption of any molecules which changes to +1.519, +0.949 and +1.411 on CO adsorption; +1.414, +1.378 and +2.16 on O₂ adsorption and +1.405, +1.376 and +1.976 on CO₂ adsorption. The charges on O₂, CO and CO₂ in this case are −1.54, −0.387 and −1.418 respectively. A point to note is that the charge transfer to the adsorbates is the largest when the boron sites before adsorption have less positive charge and hence more electron density. The central atom of the B3 site hence has the strongest ability to adsorb the incoming molecules.

In order to understand the origin of the observed trend in reactivity, viz., B3>B2>B1, we plot the partial density of states (PDOS) of B1, B2 and B3, taking 1-B27N33 as an example as shown in Fig. 3. Inspection of the PDOS reveals that the occupied defect states of B3 sites lie near to the Fermi level (here normalized to lie at 0 eV), indicating their ability to easily transfer electrons to the reactant molecules. Also, the largest contribution to the B3 states comes from the central atom of B3.The defect valence band states of B2 sites are a little further away relative to the Fermi level, while the B1 states are far away from the Fermi level. Thus the position of the defect states because of the homonuclear configuration relative to the Fermi level governs the reactivity.

To gain more insight on the reactivity of B2 site and B3 sites, we calculated the charge density difference (CDD) upon the CO and O₂ adsorption on 1-B27N33, as depicted in Fig. 4. The isovalue is set at 0.005 e/Bohr³. The yellow and blue lobes represent the charge accumulation and the charge depletion, respectively. The CDD plots well explain that the B3 sites are more active than the B2 sites in interacting with an incoming molecule. All the plots demonstrate that O₂, CO and the surface undergo considerable charge redistribution on adsorption: the molecules acquire electrons from B27N33. The depletion of charge in the O-O and C-O bond regions of O₂ and CO imply that the molecules are strongly bound to the surface, resulting in the elongation of the intramolecular bond. The charge depletion from both the atoms of the B2 sites upon the O₂ adsorption can be clearly observed in the plot in Fig. 4(a). In addition, in the case of B3 sites (see Fig. 4(b)), the bond connecting the third B atom, which is indicated by an arrow and is not directly bonded with the incoming molecule, suffers from some amount of charge depletion, indicating that this site also plays a role in donating electrons to the incoming molecule. The observed trend of the higher reactivity of B3 site can be attributed to the overall charge donation feature of B3 atoms. The same observation is also found during CO adsorption. Even though CO molecule is attached to a single boron atom of either B2 or B3 (see Fig. 4(c,d)), the other boron atoms (indicated by arrows) constituting the homonuclear bonds also take part in electron donation to the incoming molecule, promoting the overall binding ability and hence B3 sites adsorb the strongest. This feature is also evident from Bader charge analysis, tabulated in Table 3, wherein all the boron atoms constituting the homonuclear bonds lose electronic charge on interacting with either CO or O₂.

Reactivity of homonuclear bond

The PDOS of the boron sites (B1, B2, B3) after the adsorption of molecules is shown in Fig. 5. It can be seen from Fig. 5a,d and g that there is no charge transfer from the B1 sites to any of the incoming adsorbate molecules. The molecular orbitals of O₂, CO and CO₂ retain their isolated characteristics, depicted in Fig. 5. Figure 5b,c show the PDOS of B2 and B3 sites and the O₂ molecule upon adsorption. The spin-up π* (O₂) states lie just above the Fermi level and can act as acceptor levels³⁹. In fact, upon adsorption on B2 and B3 sites, these states become occupied, shift downward and hybridize with the p states of B2 and B3 atoms, explaining the higher interaction. Figure 5e,f demonstrate the DOS of B2, B3 and CO molecule after adsorption. The antibonding (π*) states of CO lie at around 2 eV above the Fermi level. As the valence states of B2 and B3 are closer to the LUMO of CO, the interaction of the CO molecule with B2 and B3 sites results in charge transfer to π* orbital, resulting in stronger adsorption.

The CO2 molecule does not interact with the B2 sites,as can be seen from Fig. 5h. The chemisorption of CO₂ onto the B3 sites takes place in two steps: the bending of the CO₂ molecule followed by its adsorption^40,41. In order to understand this mechanism, we first performed a density of states analysis of an isolated bent CO₂ molecule, keeping the bond lengths and bond angles same as the adsorbed configuration (see Fig. S5). The density of states clearly illustrates the splitting of the HOMO (1π) and LUMO (2π*) orbitals of the CO₂ molecule into two states⁴². The split LUMO orbitals are named 2a and 2b, of which 2b can now readily accept electrons as it lies closer to the Fermi level. This can be understood by inspecting the DOS of B3 and CO₂ after their interaction (Fig. 5i), wherein the 2b states become occupied, resulting in the weakening of C-O bond of CO₂ and hence stronger adsorption. Based on the estimated results and analysis, we can conclude that for complete CO oxidation B2 sites are more suitable and for CO₂ capturing and conversion B3 sites are superior.

CO oxidation and free energy profile

Now we investigate the mechanism by which CO oxidation occurs on B2 site of 1-B27N33 and B25N35. This site is able to anchor both CO and O₂ implying that the CO oxidation may follow the LH mechanism. The free energy profile is constructed by taking ΔG = ΔE – TΔS + ΔZPE, where ΔE is the total energy change obtained from DFT calculations, ΔS denotes the entropy change and ΔZPE is the change in the zero point energies. TS of free molecules are obtained from ref. 43, while TS of the adsorbates and ZPE of the free molecules and adsorbates are estimated from the DFT calculations considering vibrational frequencies of the molecules in the harmonic approximation, freezing the BN cage⁴⁴. The ZPE correction is calculated as ZPE = ½∑_iћω_i, where ћ is the reduced Planck’s constant and ω_i is the frequency of the i^th vibrational mode of the adsorbate molecule. The entropic term of the free energy is calculated from:

where denotes the Boltzmann constant.

The images demonstrating the reaction steps of CO oxidation via LH mechanism is shown in Fig. S6. The initial step of LH mechanism is taken to be the one in which 2CO molecules and an O₂ molecule are far from the surface and do not interact. The co-adsorption of CO and O₂ on the B2 site is taken to be the next step and the optimized structure is shown in the inset of Fig. 6. The O₂ molecule which was initially in the triplet state loses its magnetic moment upon adsorption. Detailed information on the changes in the spin state of molecular oxygen upon adsorption is explained in the supporting information and tabulated in Table S1. The desorption of first CO₂ from the surface requires an activation energy (E_a) of around 1.14 eV in 1-B27N33 and 1.35 eV in B25N35. This step is hence the rate limiting step with the highest activation barrier. The overall reaction is exothermic (ΔG = −5.18 eV in 1-B27N33 and −5.16 eV in B25N35) and the remaining O atom migrates toward the epoxy site. The O atom then readily reacts with another incoming CO molecule to generate the next CO₂ molecule. This reaction requires that a thermodynamic barrier of 1.06 eV in 1-B27N33 and 1.08 eV in B25N35 be surmounted. The calculated values of E_a and adsorption energies of reactants are used to estimate the Sabatier activities of CO oxidation over B27N33 and B25N35. The SA can be used as a measure of the ability of the catalyst to catalyze the process of CO oxidation. The first reaction step (R1 of SI) is taken as the one in which CO is adsorbed and in the next step the O2 molecules adsorb (R2 of SI) on neighboring active sites. This results in the formation of a (O₂···CO)^* intermediate. The activation barrier for desorption of first CO₂ from this intermediate plays a decisive role in the overall activity. Also we found that the very high binding strength of molecules on the surface influences the activity. The Sabatier activities of 1-B27N33 and B25N35 are found to be −1.3 and −1.8 respectively. We have also calculated the SA of B30N30 and found it to be −0.61. The reaction rate is influenced by both the temperature and activation energies for CO oxidation. For instance, in the LH mechanism, after CO adsorption, O₂ adsorbs in a neighboring site, forming a (O₂···CO)^* intermediate. We have considered the removal of first CO₂ from this intermediate to be the rate determining step (R3 in the SI), because of the high activation barrier. The Arrhenius equation is:

where is the rate constant. Thus, the higher the temperature, the easier it is for the reactants to surmount the activation barrier⁴⁵. In this work, the calculations of the rate constants, rate and the Sabatier activity are performed at a temperature of 273 K. At higher temperatures, the activation processes are expected to be faster. The detailed reaction steps and calculation procedure are outlined in the supplementary information. We have also compared the calculated Sabatier activities of the BN nanocages with a few other conventional catalysts available in the literature and found that B30N30 cage performs good, showing the excellent activity (see Table S2 of supporting information).

CO oxidation on boron nitride nanotube with defects

The presence of similar kind of homonuclear B-B bonds have been observed in defective BN nanosheets and nanotubes. In particular the Stone-Wales defect (SW) in the BN based system has been studied theoretically⁴⁶. Also, spectroscopic studies suggest the existence of such defects is more feasible in the boron nitride nanotubes⁴⁷. A recent atomic resolution imaging study has also confirmed the presence of Stone-Wales like defects in boron nitride sheets⁴⁸. To investigate the activation processes on B-B sites, we take an example of a SW defect on a BNNT (henceforth named as SW-BNNT). To model this system, we have chosen a (7, 0) supercell consisting of 42 BN formula units. The SW defect is formed by rotating a BN bond by 90⁰. The O₂ molecule adsorbs at a B-B bond in the side-on fashion with adsorption energy of −2.97 eV, while CO adsorbs at the boron site of a B-B bond with adsorption energy of −0.08 eV, at a distance of 1.62 Ǻ from the surface. The adsorption energies of O₂ and CO on SW-BNNT are stronger than those on a pristine BNNT. This is in agreement with previous results⁴⁹. In order to compare the activity with the nanocages, we consider only the LH mechanism, wherein the CO molecule is adsorbed first followed by the adsorption of O₂, to form a CO···O₂ intermediate (see Fig. 7 for the entire energy profile). The removal of first CO₂ requires an activation barrier of 1.23 eV, leaving behind an oxygen atom in the epoxy position. The high value of E_a can be justified based on the weak adsorption of CO molecule unto the surface, implying that O₂ bond breaking is difficult. The reaction is exothermic by −5.23 eV. The reactive O atom then interacts with another CO molecule to form the second CO₂ molecule. We note in passing that CO oxidation via ER mechanism is also probable and may take place requiring a smaller activation barrier as the O₂ molecule adsorbs quite strongly in the side on fashion.

CO₂ conversion

As mentioned earlier, the B3 sites are able to capture CO₂ effectively in the BN cages. We examine here the possibility of effectively hydrogenating this activated CO₂ into formic acid, which is widely used as a chemical fuel. We have taken two examples of BN cages, namely 1-B27N33 and B30N30 to test the photocatalytic CO₂ conversion capabilities via a COOH mediated mechanism⁵⁰. The free energy profile for this process is shown in Fig. 8. We use similar convention to find the free energy change (ΔG) as discussed in previous section. Here it has been assumed that ‘’ is in equilibrium with , at pH = 0 and 0 V vs standard hydrogen electrode (SHE)^6,51 Initially the CO₂ molecule is considered to be far from the surface. The next step involves the adsorption of CO₂ onto the B3 sites, which is endothermic in 1-B27N33 by 0.18 eV (Fig. 8a). This is expected as the CO₂ molecule does not bind very strongly to 1-B27N33. But in the case of B30N30 (Fig. 8b), the adsorption of CO₂ is stronger and hence the first step is exothermic (ΔG = −0.46 eV). The next step, which is the hydrogenation of the activated CO₂ at its oxygen atom to form carboxyl (COOH), is uphill by 0.3 eV in 1-B27N33 and 0.73 eV in B30N30. The third step wherein the carbon atom of COOH is attacked by a hydrogen atom to form adsorbed formic acid is mildly endothermic by 0.14 eV in 1-B27N33 and endothermic by 0.10 eV in B30N30. Finally, the adsorbed product, HCOOH desorbs from the surface, with ΔG values being 0.34 eV and 0.60 eV in the case of 1-B27N33 and B30N30 respectively. The low endothermicity of the reaction steps occurring on the B3 sites of 1-B27N33 suggests an exciting possibility of hydrogenating CO₂ at near-room temperatures.

We design three different classes of BN-60 cages analogue to C₆₀ cage considering 1. B rich, 2. N rich and 3. stoichiometric B:N environments. The pentagonal rings developed the homonuclear B and N bonds in the BN-60 structures. The formation enthalpy per atom for different configurations has been estimated and found to be in the range of −0.322 to −0.619 eV. N rich BN-60 cages are more favorable compared to other environments and the stability of BN-60 cage with more B2 configuration is higher compared to cages with B3 configuration. The DOPS confirms the good stability of the BN-60 cages. We found that the ability to anchor gas molecules follows B3 > B2 > B1. This trend has been explained considering position of the defect state relative to the Fermi level (B2 > B3). The stronger adsorption of the O₂ and CO molecule on B2 and B3 sites is primarily because of charge transfer to π* orbital from the surface states. Only B3 sites can adsorb CO₂ molecule, through bending of CO₂ molecule, which results in the splitting of LUMO orbitals named 2a and 2b, of which 2b can readily accept electrons. The CO oxidation follows the Langmuir–Hinshelwood (LH) mechanism with Sabatier activity of −1.3 and −1.8 considering 1-B27N33 and B25N35 cage. We found that B3 sites can efficiently convert the CO₂ molecule to formic acid. Here we emphasize that the proposed science is a general understanding and will help us to proceed further for an efficient metal free catalyst.

We performed the calculations using spin polarized density functional theory as implemented in the Vienna ab-initio simulation package (VASP)⁵². The generalized gradient approximation (GGA) was employed for the exchange and correlation effects at Perdew–Burke–Ernzerhof (PBE)⁵³ and the potentials of the atoms were described by the projected augmented wave (PAW)⁵⁴ method. For long-range van der Waals attraction the Grimme’s method (DFT-D2) was used with PBE functional (denoted as PBE-D)⁵⁵. It was found that plane wave cut-off energy of 450 eV was sufficient to get well-converged results. Each BN60 cage was placed in a cubic supercell of size 15 Å, to avoid interactions between periodically repeating images which are at about 9 Å distances. Brillouin zone integration was performed at the Γ point only. All the structures were optimized until the total energy converged to less than 10⁻⁵ eV per atom and the maximum force converged to lower than 0.001 eVÅ⁻¹. Density functional perturbation theory (DFPT) has been used to calculate the density of phonon states (DOPS)⁵⁶. The nudged elastic band (NEB) method was employed to estimate the barrier energy⁵⁷.

How to cite this article: Sinthika, S. et al. Activation of CO and CO₂ on homonuclear boron bonds of fullerene-like BN cages: first principles study. Sci. Rep.5, 17460; doi: 10.1038/srep17460 (2015).

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1. Introduction

Analytical science to develop the methodology for the investigation of properties and structure of matter at level of single nucleus, atom and molecule, and scientific analysis to determine either chemical composition or elemental contents in a sample are indispensable in basic research and development, as well as in industrial applications.

Following the discovery of neutron by J. Chadwick in 1932 (Nobel prize, 1935) and the results of F. Joliot and I. Curie in 1934, neutron activation analysis was first developed by G. Hevesy and H. Levi in 1936. They used a neutron source (226Ra + Be) and a radiation detector (ionization chamber) and promptly recognized that the element Dy (dysprosium) in the sample became highly radioactive after exposure to the neutron source. They showed that the nuclear reaction may be used to determine the elements present in unknown samples by measuring the induced radioactivity.

Thereafter, the development of the nuclear reactors in the 1940s, the application of radiochemical techniques using low resolution scintillation detectors like NaI (Tl) in the 1950s, the development of semiconductor detectors (Ge, Si, etc.) and multichannel analyzer in the 1960s, and the advent of computers and relevant software in the 1970s, the nuclear technique has advanced to become an important analytical tool for determination of many elements at trace level. In spite of the developments in other chemical techniques, the simplicity and selectivity, the speed of operation, the sensitivity and accuracy of NAA have become and maintained its role as a powerful analytical technique. In 2011, Peter Bode describes in his paper “Neutron activation analysis: A primary method of measurements”, the history of the development of NAA overall the world [1].

Nowadays, there are many elemental analysis methods that use chemical, physical and nuclear characteristics. However, a particular method may be favoured for a specific task, depending on the purpose. Neutron activation analysis (NAA) is very useful as sensitive analytical technique for performing both qualitative and quantitative multielemental analysis of major, minor and traces components in variety of terrestrial samples and extra-terrestrial materials. In addition, because of its accuracy and reliability, NAA is generally recognized as the "referee method" of choice when new procedures are being developed or when other methods yield results that do not agree. It is usually used as an important reference for other analysis methods. Worldwide application of NAA is so widespread it is estimated that approximately 100,000 samples undergo analysis each year.

The method is based on conversion of stable atomic nuclei into radioactive nuclei by irradiation with neutrons and subsequent detection of the radiation emitted by the radioactive nuclei and its identification. The basic essentials required to carry out an analysis of samples by NAA are a source of neutrons, instrumentation suitable for detecting gamma rays, and a detailed knowledge of the reactions that occur when neutrons interact with target nuclei. Brief descriptions of the NAA method, reactor neutron sources, and gamma-ray detection are given below.

This chapter describes in the first part the basic essentials of the neutron activation analysis such as the principles of the NAA method with reference to neutron induced reactions, neutron capture cross-sections, production and decay of radioactive isotopes, and nuclear decay and the detection of radiation. In the second part we illustrated the equipment requirements neutron sources followed by a brief description of Es-Salam research reactor, gamma-ray detectors, and multi-channel analysers. In addition, the preparation of samples for neutron irradiation, the instrumental neutron activation analysis techniques, calculations, and systematic errors are given below. Some schemes of irradiation facilities, equipment and materials are given as examples in this section.

Finally, a great attention will be directed towards the most recent applications of the INAA and k0-NAA techniques applied in our laboratory. Examples of such samples, within a selected group of disciplines are milk, milk formulae and salt (nutrition), human hair and medicinal seeds (biomedicine), cigarette tobacco (environmental and health related fields) and iron ores (exploration and mining).

All steps of work were performed using NAA facilities while starting with the preparation of samples in the laboratory. The activation of samples depends of neutron fluence rate in irradiation channels of the Algerian Es-Salam research reactor. The radioactivity induced is measured by gamma spectrometers consist of germanium based semiconductor detectors connected to a computer used as a multichannel analyser for spectra evaluation and calculation. Sustainable developments of advanced equipment, facilities and manpower have been implemented to establish a state of the art measurement capability, to implement several applications, etc.

2. Neutron activation analysis

Neutron activation analysis (NAA) is a nuclear process used for determining the concentrations of elements in a vast amount of materials. NAA relies on excitation by neutrons so that the treated sample emits gamma-rays. It allows the precise identification and quantification of the elements, above all of the trace elements in the sample. NAA has applications in chemistry but also in other research fields, such as geology, archaeology, medicine, environmental monitoring and even in the forensic science.

3. Applications

It is hardly possible to provide a complete survey of current NAA applications; however, some trends can be identified [27]. At specialized institutions, NAA is widely used for analysis of samples within environmental specimen banking programmes [28]. The extensive use of NAA in environmental control and monitoring can be demonstrated by the large number of papers presented at two symposia organized by the IAEA in these fields: "Applications of Isotopes and Radiation in Conservation of the Environment" in 1992 [29] and "Harmonization of Health-Related Environmental Measurements Using Nuclear and Isotopic Techniques" in 1996 [30]. Similar trends can also be identified from the topics discussed at the regular conference on “Modern Trends in Activation Analysis (MTAA)” and at the symposia on "Nuclear Analytical Methods in the Life Sciences" [31-33]. Additional sources of recent information on utilizing NAA in selected fields, such as air pollution and environmental analysis, food, forensic science, geological and inorganic materials as well as water analysis can be found in the bi-annual reviews in Analytical Chemistry, for instance cf. Refs [34-42]. It follows from these reviews that NAA has been applied for determining many elements, usually trace elements, in the following fields and sample types:

1. Archaeology: samples and objects such as amber, bone, ceramics, coins, glasses, jewellery, metal artefacts and sculptures, mortars, paintings, pigments, pottery, raw materials, soils and clays, stone artefacts and sculptures can be easily analyzed by NAA.

2. Biomedicine: the samples and objects that can be analysed include: animal and human tissues activable tracers, bile, blood and blood components, bone, brain cell components and other tissues, breast tissue, cancerous tissues, colon, dialysis fluids, drugs and medicines, eye, faeces, foetus, gallstones, hair, implant corrosion, kidney and kidney stones, liver, lung, medical plants and herbs, milk, mineral availability, muscle, nails, placenta, snake venom, rat tissues (normal and diseased), teeth, dental enamel and dental fillings, thyroid, urine and urinary stones.

In this work, we have used the INAA technique to analyse the traditional medicinal seeds prescribed for specific treatment purposes, were purchased from local markets [43]. The samples were irradiated at Es-Salam research reactor, at a power of 5 MW for 6 h. The accuracy of the method was established by analyzing reference materials. Twenty elements were measured, with good accuracy and reproducibility (Table 6). The concentration of elements determined, was found to vary depending on the seeds (Fig.11). The daily intake of essential and toxic elements was determined, and compared with the recommended values. The probable cumulative intake of toxic elements is well below the tolerance limits.

1. Environmental: in this domain, related fields concerned by NAA are: aerosols, atmospheric particulates (size fractionated), dust, fossil fuels and their ashes, flue gas, animals, birds, insects, fish, aquatic and marine biota, seaweed, algae, lichens, mosses, plants, trees (leaves, needles, tree bark), household and municipal waste, rain and horizontal precipitations (fog, icing, hoarfrost), soils, sediments and their leachates, sewage sludges, tobacco and tobacco smoke, surface and ground waters, volcanic gases.

Recently, our laboratory is strongly involved in various areas of application of k₀-NAA. The present work focuses on the application of the k₀-NAA method in Nutritional and Health-Related Environmental field [44]. Tobacco holds a leading position among different commodities of human consumption. The adverse health effects of toxic and trace elements in tobacco smoke on smokers and non-smokers are a special concern. In the present study, the concentration of 24 trace elements in cigarette tobacco of five different brands of Algerian and American cigarettes have been determined by k₀-based INAA method. The results were compared with those obtained for samples from Iranian, Turkish, Brazilian and Mexican cigarettes tobacco. To evaluate the accurate of the results the SRM IAEA-140/TM was executed.

A multi-element analysis procedure based on the k₀-NAA method was developed at Es-Salam research reactor allowing to simultaneously determine concentrations for 24 elements (As, Ba, Br, Ca, Ce, Co, Cr, Cs, Eu, Fe, Hf, K, La, Na, Rb, Sb, Sc, Se, Sm, Sr, Ta, Tb, Th, Zn). The determination of toxic and trace elements in cigarette tobacco is important both from the point of view of health studies connected with smoking and more general aspects of the uptake of trace elements by plants (table 7). Because of its great sensitivity, k₀-NAA method is very suitable for determination of heavy metals such as As, Sb, Se and Zn. The accuracy of the results was checked by the analysis of standard reference material and good agreement was obtained with certified or literature values. The results of Algerian tobacco (table 8) were compared with analyses of Turkey [45], Iran [46], Mexican [47] and Brazilian tobacco [48].

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Bartolomeo Coppola,¹Jean-Marc Tulliani,¹Paola Antonaci,^2,3,* and Paola Palmero¹

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This review aims to provide a comprehensive assessment concerning alkali activation of natural stone wastes and minerals. In particular, the structure of the review is divided into two main sections in which the works dealing with alumino-silicate and carbonatic stones are discussed, respectively. Alumino-silicate stones are generally composed of quartz and feldspars, while carbonatic stones are mainly made of calcite and dolomite. The role of these minerals in the alkali activation process is discussed, attesting their influence in the development of the final product properties. In most of the works, authors use mineral additions only as fillers or aggregates and, in some cases, as a partial substitution of more traditional raw powders, such as metakaolin, fly ash, and granulated blast furnace slag. However, a few works in which alumino-silicate and carbonatic stone wastes are used as the main active components are discussed as well. Not only the raw materials, but also the entire alkali activation process and the curing conditions adopted in the literature studies here reviewed are systematically analyzed to improve the understanding of their effect on the physical, mechanical, and durability properties of the final products and to eventually foster the reuse of natural stone wastes for the purposes of sustainability in different applications.

Table 1

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Table 2

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Table 3

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2.1. Role of Raw Materials, Activators, and Curing Conditions

Different natural alumino-silicate minerals have been investigated in the scientific literature in combination with traditional alkali-activated materials. In particular, granite [46,47,48], albite [49], Pietra serena [50], Pisha sandstone [51,52], cordierite [53], and diatomite [54] have been used in combination with metakaolin (MK) [48,50,53] and fly ash (FA) [47,51]. As previously mentioned, only a few works investigated the use of alumino-silicate stones alone [46,49,51,52,54], i.e., without any other ‘active’ component. Alumino-silicate stones are mainly composed of quartz (SiO₂) and feldspars, such as microcline (KAlSi₃O₈) and albite (NaAlSi₃O₈). In some cases, some phyllosilicates belonging to the mica group, like biotite and muscovite, can also be present. As regards the oxides, these materials are mainly composed of silica (with values ranging between 50% and 70%) and alumina (between 15% and 35%), as reported in Table 2. A certain difference is presented by diatomite, which is a naturally occurring siliceous sedimentary rock that is characterized by a very high silica amount (80.3%) and a lower alumina content (6.1%) [54].

Clausi et al. [50] investigated the possibility of using an ornamental stone, i.e., Pietra Serena (an Italian sandstone) in combination with MK for the production of mortars. MK/Pietra Serena alkali-activated mortars can be used in restoration of cultural heritage because they mimic the natural stone, thus allowing aesthetic compatibility [17]. Crushed Pietra Serena was used for aggregate preparation, keeping the clayey fraction as well, which provided the necessary color shades for aesthetic reasons.

Tchadjié et al. [48] used granite waste from Cameroon in the preparation of MK-based geopolymers. In particular, the authors fused granite powder (at 550 °C for 2 h) with Na₂O at different weight percentages (from 10 to 60 wt.%, 10 wt.% of increments). Then, fused granite powder (d < 90 μm) was mixed with MK and activated with Na₂SiO₃ solution [48].

Hemra and Aungkavattana [53] investigated the influence of cordierite (Mg₂Al₄Si₂O₁₈) addition, from 0 to 50 wt.%, into MK-based geopolymers.

Hassan et al. [54] investigated the possibility of preparing geopolymers by mixing wood biomass ash (WBA) and diatomite. Even if diatomite is a sedimentary siliceous rock, it is mainly composed of alumino-silicates rich in amorphous iron.

Choi et al. [47] replaced fly ash (FA) and granulated blast furnace slag (GBFS) with a stone powder sludge derived from granite quarries. The authors investigated several replacement ratios (10, 20, and 30 wt.%, respectively) using the stone sludge as received, i.e., with a high water content (20.7%) [47].

An original contribution is provided by Palmero et al. [46], who exploited a granite quarry mud as a unique feedstock material for alkali-activated products, and this technology was the object of two recent patent applications [78,79]. Due to the almost fully crystalline nature of the raw powder, the developed material represents a clear innovation in the field of alkali-activation processes [46]. Similarly, Li et al. [51,52] investigated the alkali activation of Pisha sandstone (a Chinese sandstone) both alone [51,52] and mixed with FA [51]. Pisha sandstone is mainly composed of crystalline minerals, i.e., quartz and albite, with SiO₂ and Al₂O₃, accounting for approximatively 80 wt.% of the whole composition (Table 2). Feng et al. [49] investigated the possibility of activating pure albite, but it was previously thermally treated with sodium hydroxide or sodium carbonate. In particular, albite was mixed with different amounts (10%, 30%, and 50% with respect to albite mass) of sodium hydroxide or sodium silicate and then heated at four different temperatures (850, 900, 1000, and 1150 °C, respectively) for 30 min [49].

Details related to curing conditions and alkaline solutions of the different reviewed papers are reported in Table 2. It can be observed that, in some researches, the materials were cured at 80 °C [46,47,51,52], even if in most of the works, curing at room temperature was preferred [48,49,50,52,53,54].

In addition, the heat-curing was always performed for a short time (a few hours or a few days), followed by a longer room temperature curing. For example, in the studies reported in [46,47], specimens were cured at 80 °C for 24 h and then kept at 23 °C until testing.

The curing atmosphere plays a role, too. For instance, Li et al. [51] investigated different curing conditions for geopolymers prepared using a Chinese sandstone (i.e., Pisha sandstone) by exposing the samples at 80 °C for 24 h, followed by room temperature curing performed under air or water. Authors reported lower mechanical properties in the case of water-cured samples, probably due to excessive water, which hindered the polycondensation reactions [51].

A further important parameter that affects the performance of the fresh and hardened materials is the activating solution. Li et al. investigated several formulations: In [51], the authors kept Na₂SiO₃ and water content constant while varying NaOH concentration, thus obtaining solutions at three different SiO₂/Na₂O molar ratios (M_s), equal to 1.5, 2.0, and 3.0. They showed that the strength decreased by increasing M_s. On the contrary, in [52], the same authors tested different activators (Na₂SiO₃, Na₂CO₃, Na₂SO₄, and NaOH, respectively) in order to investigate the role of pH on the development of alkali-activated materials. They showed that the higher the pH, the stronger the materials.

A recent and interesting approach deals with the design and development of one-part geopolymers, which are mixtures of solid alumino-silicate precursors and solid alkaline chemicals; the addition of ‘just water’ is responsible for the activation, similarly to cement technology [80,81,82]. The term is in opposition to the traditional two-part geopolymers, in which the solid precursors are mixed with a liquid alkaline solution: Issues related to the high viscosity and pH of the solution make the scalability of geopolymers difficult, restricting them to relatively small-scale applications and pre-cast components. Two previous researches investigated the possibility of preparing one-part geopolymers with natural minerals [49,54]. In particular, Feng et al. [49] prepared alkali-activated materials starting from a thermally treated albite and adding only water. The authors fixed the water/solid ratio at 0.30 and cured samples at room temperature in sealed containers [49]. On the other side, Hassan et al. [54] prepared one-part geopolymers mixing a dry activator with diatomite. In particular, the dry activator was prepared mixing a CaCO₃-rich ash (WBA) with a dried solution of sodium hydroxide. Then, diatomite was mixed with different amounts of the dry activator and activated with water at a water/powder ratio of 0.27 and 3 wt.% of NaOH. Samples were finally cured at 23 °C and 99% relative humidity (RH).

2.2. Properties of Fresh and Hardened Materials

2.3. Role of Alumino-Silicate Minerals in the Alkali Activation Process

The role of alumino-silicate mineral waste on alkali-activated materials is still not completely clear. Some authors, in fact, attribute to these particles just a ‘filler’ effect. As an example, in Ref. [53], cordierite powder was added to MK (from 0 to 50 wt.%), showing a remarkable effect in improving thermal stability and thermal shock resistance, which was imputed to a filler effect of the particles, which reduced shrinkage and cracking.

However, most of the authors agreed on considering alumino-silicate minerals as not being inert in the alkali activation process and used several characterization techniques to demonstrate a possible role exerted by these mineral fines. As an example, Choi et al. [47] investigated the X-ray diffraction (XRD) phase composition of alkali-activated GBFS pastes containing granite sludge. The authors observed the formation of hydrotalcite (Mg₆Al₂CO₃(OH)₁₆·4(H₂O)), as already observed in alkali-activated slag [85]. However, the formation of this phase was favored in the granite-containing samples, as compared to ‘pure’-GBFS materials, suggesting a contribution of granite powder in the alkali activation process. Clausi et al. [50,76] suggested a role of Pietra Serena fines used in the preparation of MK-based mortars. In fact, by microstructural Scanning Electron Microscopy (SEM) observations, larger quartz and feldspar grains (contained in Pietra Serena) evidenced an incipient dissolution, and a limited formation of an alumino-silicate gel from Pietra Serena sludge was demonstrated [76].

In order to improve the reactivity of alumino-silicate mineral fines under alkaline condition, thermal activation was tested. Feng et al. [49] applied a thermal activation to waste albite particles and heated albite/sodium hydroxide or sodium carbonate dry mixtures between 850 and 1150 °C. For temperatures higher than 1000 °C, an almost completely amorphous structure was identified by XRD analysis, providing in this way a highly reactive geopolymer precursor. Similarly, Tchadjié et al. [48] applied an alkali fusion process to waste granite/sodium hydroxide pellets at 550 °C. After this treatment, the mineral composition of granite was modified, as ascertained by XRD analysis. In fact, while the raw powder was composed of well-crystallized quartz, biotite, almandine, and albite phases, in the melt sample, a decrease of these peaks was observed in addition to the appearance of the sodium silicate phase. In addition, the XRD patterns of the fused materials showed the presence of a halo, attesting the formation of an amorphous phase and a consequent high reactivity under alkaline condition.

In search of more environmentally friendly processes, some researchers tried to exploit alumino-silicate mineral fines as the only precursors of alkali-activated materials, without any thermal activation [46,51,52]. For instance, Palmero et al. [46] investigated the possibility of using granite mud as raw powder. The phase composition was investigated by XRD and phase quantification was carried out by Rietveld analysis for both raw powder and alkali-activated material. Compared to raw powder, a general decrease of the crystalline phases was observed; the biotite phase showed the highest weight decrease (44%), followed by albite (38%) and clinochlore (29.4%), while a lower reactivity was determined for microcline and quartz (around 22% for both phases). At the same time, an increase of the amorphous phase was determined. These results demonstrated a certain dissolution of crystalline alumino-silicate particles in a strong alkaline solution, and thus an active role in the alkali activation process. Further proof of the dissolution phenomena of alumina-silicate particles was obtained by field emission scanning electron microscopy equipped with energy dispersive X-ray spectroscopy (FESEM/EDX). An example is provided in Figure 3, showing an undissolved biotite grain surrounded by the matrix. Systematic EDX elemental profiles of the main elements were determined from the outer to the inner part of the grain, as shown by the white and yellow lines in Figure 3A,B). The Al Kα1 EDX profile is provided in Figure 3C, showing that the Al concentration was almost constant inside the grain, while it clearly decreased in correspondence with the biotite–matrix interface. However, though in lower concentration, all elements (Al, Fe, K, and Mg; see Figure 3D) were clearly detected in the matrix around the grain (a few microns from the interface), thus confirming the surface dissolution and the elemental diffusion in the surrounding binder. Through this mechanism, it was possible to produce materials characterized by a highly compact matrix (Figure 4A), with a good adhesion between the finer matrix and the undissolved particles (Figure 4B).

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Figure 3

(A,B): Field emission scanning electron microscopy (FESEM) micrographs at different magnifications of a biotite grain in a one-year-aged sample; (C): Energy dispersive X-ray spectroscopy (EDX) profile related to Al Kα1 along the line shown in (A,B). Reprinted from [46] under the license n. 4792980067658. In (D), the Al, Mg, K, and Fe profile evolution at the grain–matrix interface is highlighted (unpublished image by the authors).

Li et al. [51,52] confirmed the possibility of alkali-activating a Chinese sandstone mainly composed of quartz and albite with minor amounts of calcite (CaO content was 8.02% in [51] and 5.10% in [52], respectively). However, in this case, the authors ascribed to calcite, and not to alumino-silicates, the main role in the geopolymer process; in fact, by means of XRD, Fourier-transform infrared spectroscopy (FT-IR), and thermo-gravimetric analysis coupled with derivative thermo-gravimetric analysis (TGA/DTG), the authors attested the formation of calcium silicate hydrates (C-S-H) in alkali-activated Pisha sandstone samples [51,52].

3.1. Role of Raw Materials, Activators, and Curing Conditions

The carbonate materials considered here are mainly composed of two minerals: Calcium carbonate (CaCO₃) and calcium magnesium carbonate (CaMg(CO₃)₂). The first category includes marble (metamorphic rock), limestone, and travertine (sedimentary rocks), and is characterized by a CaO content between 35 and 55 wt.% (Table 3). The second group includes dolomite (sedimentary rock), in which MgO content varies approximately between 15 and 35 wt.%.

As stated in the introduction, most of the works considered the use of carbonate powders only as filler added to the raw powders traditionally used in the alkali activation process [50,55,56,57,58,59,60,61,62,63,64,65,66,67,68,69,70,71,72]; only two works investigated the use of carbonates alone [40,45]. In particular, carbonate fines have been used in combination with MK [50,55,57,67], FA [58,59,62,65,66,68], GBFS [56,59,61,65,68,69,70,72], and clays (bentonite [60], smectite [64], halloysite [71], undefined [65]).

The main difference among all the studied carbonate stone powders is the calcite and/or dolomite content, resulting in a different chemical composition that can be mainly expressed in terms of CaO and MgO content (Table 3). Indeed, articles can be summarized in three different groups according to the chemical nature of the starting minerals: mainly composed of calcite [45,55,56,57,58,60,63,64,65,66,67,68,69,70,71,72], with prevailing dolomite content [50,55,59,60,61,62] and a mixture of calcite and dolomite [72].

Concerning the calcite-rich group, Tekin [63] studied the possibility of using marble and travertine in combination with a waste volcanic tuff, a natural pozzolan containing zeolite. Marble’s and travertine’s chemical compositions were similar (Table 3), but travertine showed a higher porosity. Several formulations were investigated, in which marble or travertine ranged from ~20 to 80 wt.%, tuff being the remaining fraction [63]. Thakur et al. [66] prepared FA geopolymer bricks via extrusion, in which different marble fractions (i.e., from 10 to 80 wt.%) were added. Gao et al. [68] investigated ternary mixtures containing GBFS/FA/limestone with three different limestone contents: 10, 20, and 30 wt.%, respectively. Bayiha et al. [71] studied the alkali activation of thermally activated halloysite (a two-layered clay) with several limestone replacements (from 0% to 60%). Finally, Orteza-Zavala et al. [40] and Coppola et al. [45] used only calcite (limestone and marble, respectively) as raw powders in the alkali activation process.

In the case of dolomite-rich compositions, Cohen et al. [62] investigated the use of a quarry dust mainly composed of dolomite with calcite traces in combination with both cement and low-calcium FA. Several dolomite fractions (10, 20, 30, and 40 wt.%) as cement or FA replacements were investigated. Clausi et al. [50] investigated the possibility of using ornamental stone aggregates in the preparation of MK-based geopolymers for cultural heritage applications. The authors studied both a siliceous sandstone (Pietra Serena) and a dolostone (Pietra di Angera) with particles smaller than 0.5 mm, including fines.

Concerning mixed calcite–dolomite powders, Rakhimova et al. [72] investigated the role of three different limestone powders on the alkali activation of GBFS powders. In particular, the three powders were characterized by different calcite contents: 33% (with 66% dolomite and 1% quartz), 90% (with 9 wt.% quartz and 1 wt.% albite), and 100 wt.% [72]. Kürklü and Görhan [58] investigated the role of a quarry dust composed of calcite with dolomite traces as aggregate in geopolymers prepared from low-calcium FA (class F).

As regards the activating solution, Table 1 displays the general composition of the alkali activators used in the papers here reviewed, while Table 3 summarizes the main activating solution parameters (concentration, modulus, etc.) used in the case of carbonate-containing materials.

It can be easily observed that alkaline solutions containing sodium hydroxide (NaOH) and sodium silicate (Na₂SiO₃) are the most used [40,45,50,55,57,58,62,64,65,66,68,71]. Incidentally, it is worth noticing that, due to its reactivity properties as an alkaline activator, the use of sodium silicate is also well established in the cement and concrete industry, where it may act as a sealant, a densifying additive, or even a healing agent for self-healing applications [86,87,88,89]. Generally speaking, two parameters are mainly considered for the preparation of the activating solution: (i) The modulus M_s (i.e., the SiO₂/Na₂O molar ratio) for solutions containing both NaOH and Na₂SiO₃; (ii) the NaOH molar concentration, when only NaOH water solution is used [63].

Two studies [40,65] demonstrated the important effect of sodium silicate contained in the activating solution in increasing the materials performance. Salihoglu and Salihoglu [65] compared the behavior of marble sludge/fly ash geopolymers, activated using either a pure NaOH solution or a mixed NaOH/Na₂SiO₃ solution, and showed higher mechanical properties in the latter case. Similarly, Ortega-Zavala et al. [40] prepared alkaline solutions at three different SiO₂/Na₂O molar ratios: 0 (i.e., only NaOH, no sodium silicate), 1.0, and 1.5. Once again, the highest mechanical properties of alkali-activated limestone were achieved in the presence of sodium silicate.

Oppositely, in a previous work by Palmero and co-workers (unpublished results), the importance of NaOH in the activating solution was demonstrated. In fact, either a pure sodium silicate solution (SS, pH of 10.8) or a mixed NaOH/sodium silicate solution (SH/SS, pH of 12.7) was used for the activation of a carbonate mud. In this study, mixtures with different liquid-to-solid ratios (L/S) were investigated—precisely 45/55, 40/60, and 35/65—and cured for 14 or 28 days. The (unpublished) results achieved are displayed in Figure 5. It is possible to observe the key role played by NaOH in increasing the mechanical properties, since both flexural and compressive strengths of the NaOH-containing mixtures were higher than those of the specimens activated with only pure sodium silicate solution, suggesting a more effective species dissolution in a stronger alkaline medium. Furthermore, for NaOH-containing samples, the strengths increased by increasing the solid loading, while no meaningful differences were observed for the other mixtures. Finally, considering the influence of curing time, it is evident that the development of the mechanical properties for the mixtures without NaOH is delayed compared to the sodium-hydroxide-containing samples.

Similarly, Thakur et al. [66] prepared alkaline solutions based on NaOH and sodium metasilicate at two different NaOH molarities (2 and 4 M) for the activation of marble waste FA samples. In agreement with the results discussed above, materials prepared at the highest molar concentration (and therefore at the highest pH) showed higher mechanical properties and decreased water absorption.

Kürklü and Görhan [58] discussed the influence of both curing time (from 1 to 5 h, at 80 °C) and NaOH concentration (3, 6, and 12 M) on FA geopolymer mortars containing carbonate dust. In particular, the highest molar concentration was the most favorable condition in terms of apparent porosity and water absorption coefficient. At the same time, a longer curing time was more effective for the geopolymerization reactions to occur.

In [57], the combined effect of sodium and potassium hydroxides was investigated, as this study was based on the different roles played by these two alkaline metals in the geopolymerization process of calcined clays. Sodium, in fact, is known to increase the dissolution of amorphous phases, while potassium is known to promote a higher degree of polymerization reaction [90].

Interestingly, in some previous researches, sodium carbonate (Na₂CO₃) [59,69,70,72] was used to prepare the activating solution. Among activators, Na₂CO₃ is globally available and environmentally friendly. Moreover, Na₂CO₃ is easier to handle, characterized by lower pH, and cheaper than sodium silicate [91]. Yuan et al. [69,70] used sodium carbonate as an activator in GBFS/limestone geopolymers. Several GBFS replacements were investigated (5, 10, 15, and 30 wt.% in [69] and 10, 20, 30, 40, and 50 vol.% in [70]), and the obtained pastes were cured at room temperature and high RH (20 °C and >95%, respectively) [69,70]. Rakhimova et al. [72] exploited an alkaline solid waste as the activating ingredient; it was derived from the incineration of sewage of a petrochemical company and consisted mainly of Na₂CO₃ (91.4 wt.%) and NaOH (2.65 wt.%).

In [64], sodium citrate (Na₃C₆H₅O₇) was added to a sodium hydroxide–sodium metasilicate solution used to activate waste carbonate/calcined clay mixtures. The authors observed an improvement in the workability of the pastes due to citrate addition, leading to a less porous microstructure and higher mechanical properties of the hardened materials.

Some studies relate to the already mentioned one-part alkali-activated materials, in which the dry alkaline component is mixed with the raw powder so that only water is used to activate the mixture [56,60,61]. Abdel-Gawwad and Abo-El-Enein [56] mixed calcium carbonate and a sodium hydroxide solution (at different NaOH concentrations: 2%, 4%, and 6%, respectively) for the preparation of a dry activator to be used in combination with GBFS. Then, activation occurred by adding only water to the GBFS/dry activator mixtures. Peng et al. [60] investigated the possibility of preparing one-part alkali-activated materials using a mixture of bentonite, dolomite, and sodium carbonate. In particular, the authors prepared several mixtures of these three raw powders, which were then calcined at 1100 or 1200 °C. The calcination process resulted in the preparation of clinkers with several compounds (such as calcium aluminum oxide—Ca₃Al₂O₆, belite—Ca₂SiO₄, and sodium iron oxide—NaFeO₂ and MgO) that were hydrated by adding only water, as the alkali additive was already contained in the clinker. Yang et al. [30] prepared GBFS/calcined dolomite blends (2, 6, and 10 wt.% of the GBFS) for the preparation of one-part sodium carbonate activated pastes at different Na₂CO₃ amounts (10 and 15 wt.%).

A further key parameter in property development is curing, as it affects alkali activation reactions and material microstructure. A summary of the curing conditions used in carbonate-containing alkali-activated materials is given in Table 3.

As a common approach followed by several authors, fresh pastes are submitted to a short curing period (typically between 1 and 24 h) in an oven at temperatures between approximately 40 and 80 °C [40,45,55,56,57,58,60,62,63,66,67]. In some other works, samples were exposed to room temperature during the whole curing period [50,59,61,63,64,65,68,69,70,72]. The curing atmosphere was a key parameter, too, and several authors cured samples under wet conditions (90%–100% relative humidity) [40,45,50,56,59,61,62,63,64,68,69,70,72] or directly immersed under water [45]. When samples are cured in high-humidity conditions, or even water-cured, two opposite behaviors have been described in literature: on one side, this curing condition promotes the formation of C-S-H species, thus enhancing the mechanical properties and durability [45,63,64]; on the other side, it slows down the geopolymerization process and induces the formation of cracks in the cured samples [45,63,64]. As an example, Valentini et al. [64] cured clay/marble samples for 24 h at 95% RH, followed by 20 days of dry-curing or water immersion. The authors observed that if the same samples were cured under water, several macrocracks appeared due to volume expansion. Tekin [63] investigated several curing conditions: laboratory (22 °C and 40% RH) and heat-curing (45 or 75 °C for 24 h). Then, in all three cases, half of the specimens were cured in wet conditions (>95% RH) and the other half kept in laboratory conditions (35% RH) up to the specimens’ testing. Heat-curing at 75 °C increased the early strength of the pastes, but resulted in crack formation. Therefore, the optimal curing temperature suggested was 45 °C. In addition, the author observed that wet curing was not favorable for the geopolymerization reactions, and therefore for the development of the mechanical properties. Coppola et al. [45] investigated the role of curing conditions on alkali-activated marble sludge. In particular, after a short curing time at 60 °C under wet atmosphere, different curing environments were tested: air-curing (RH = 18% ± 2%), humid-curing (RH = 95% ± 2%), and water-immersion, all of them at 20 °C. The most favorable condition for C-S-H development and mechanical property increase was air-curing, suggesting an important role played by both short-time wet-curing (probably for hydrated species formation) and longer-time air-curing (for gel formation and polymerization).

3.2. Properties of Fresh and Hardened Materials

3.2.3. Durability Properties of Hardened Materials

The durability of alkali-activated materials containing carbonate fines was investigated by water immersion and absorption tests [45,56,64,66,71] by observing efflorescence occurrence [45,50,63,71], shrinkage [45,55,70,72], and microstructure [45,70].

The behavior in the presence of water is extremely important for construction materials. Considering water absorption, different authors claimed a positive effect played by sodium and calcium carbonate [56], marble [66], or limestone [71] due to their filler effect. For instance, Bayiha et al. [71] reported a decrease of absorbed water by increasing limestone content (up to 45 wt.%) in MK geopolymers, while a further increase (60 wt.%) was detrimental. In the first case, limestone contributed to paste densification by reducing capillary pores. On the contrary, at higher limestone contents, MK was not sufficient to provide the polycondensation reactions, and unreacted water evaporation resulted in formation of pores [71]. In FA/marble blends, Thakur et al. [66] observed that the higher the marble content, the lower the water absorption, thanks to the lower porosity of the samples. Similarly, Tekin [63] observed a lower apparent porosity and a consequent higher resistance to water absorption in waste marble and waste travertine-added pozzolan compared to the pure material. The author found the optimal conditions by also properly setting the NaOH molar concentration in the alkaline solution; as depicted in Figure 6a, only a strong alkaline medium (NaOH 10 M) allowed the full contrast of the water-induced degradation and failure.

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Figure 6

(a) Tuff/marble–travertine samples produced at different NaOH molar concentrations and submitted to water absorption tests. Reprinted from [63] under the license n. 4792980356522; (b) FESEM micrograph of hydrated sodium carbonate efflorescence (same materials used in [45] and humid-curing).

A well-known issue of geopolymers and alkali-activated materials is the efflorescence occurrence under wet conditions, which—in some cases—can lead to samples’ disintegration. The presence of carbonates seems to play a positive effect, limiting efflorescence appearance [50,71]. Clausi et al. [50] observed the positive effects of both Pietra di Angera and Pietra Serena in inhibiting the efflorescence occurrence in MK-based geopolymers. The authors affirmed that the presence of aggregates rich in aluminum (Pietra Serena) and calcium (Pietra di Angera) increased the crosslinking degree in the geopolymer gel, reducing Na⁺ cations’ mobility and, thus, sodium carbonate formation (i.e., efflorescence) [50]. Anyway, curing conditions play an important role in efflorescence occurrence; for instance, Coppola et al. [45] observed efflorescence formation in wet-cured carbonate geopolymers (Figure 6b), while in air-cured and water immersed samples, it did not appear.

As regards dimensional stability, a different effect played by the carbonate addition was reported. In fact, a reduction of shrinkage and cracking phenomena due to the presence of limestone in GFBS activated materials was observed by Rakhimova et al. [72].

On the other hand, most of the works reviewed here reported an increase of drying shrinkage with increasing mineral addition [55,70,71], which was mainly imputed to the presence of unreacted water. Yip et al. [55] evaluated the relative shrinkage of alkali-activated MK/calcite blends, obtaining an approximatively linear increase of shrinkage with increasing calcite content. For high-calcite samples (MK substitution higher than 60%), the authors observed the presence of some voids, imputed to the evaporation of unreacted water. A similar behavior was observed by Bayiha et al. [71] in MK/limestone geopolymers, who found an increase of shrinkage with increasing limestone addition. Once again, the reduced geopolymerization degree of the mixtures compared to neat MK lead to higher free water and, consequently, to larger shrinkage. Moreover, the shrinkage decreased with increasing NaOH molar concentration (from 5 to 8 M), strengthening the hypothesis that the shrinkage is strongly correlated with material reactivity.

Yuan et al. [70] investigated both autogenous and drying shrinkage of GBFS/limestone alkali-activated pastes. Autogenous shrinkage—which is correlated with the occurrence of chemical reactions during the first curing days—increased at low limestone additions (up to 30 vol.%). The authors correlated this behavior with the heat released, suggesting intensified reactions towards the formation of calcium aluminum silicate hydrate C-A-S-H gel due to the presence of moderate limestone amounts. The increase of drying shrinkage at increasing limestone contents was again correlated with more free water due to unreacted products, and with the dehydration from some crystalline phases (such as gaylussite and natron) produced during the alkali activation process.

Finally, Coppola et al. [45] investigated the influence of curing conditions and waste glass addition on the linear shrinkage of alkali-activated marble sludge. Air-cured pastes exhibited the highest shrinkage compared to the humid- and water-cured ones (Figure 7a) due to samples’ drying during curing at low RH (18% ± 2%). Moreover, waste glass addition significantly reduces paste shrinkage (proportionally to glass content) and anticipates length stationarity (Figure 7b). Interestingly, waste glass addition also influences water resistance after immersion, avoiding sample cracking, and mechanical properties, thanks to the provision of further silica [45].

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Figure 7

Length variation of air-cured alkali-activated marble sludge specimens: (a) Influence of glass addition (GP₀ = no glass, GP_2.5 = 2.5 vol.% of glass, and GP_5.0 = 5.0 vol.% of glass) and (b) shrinkage at different curing times. Readapted from [45] under the license number 4793000364236.

Finally, discussing durability, the poor acid resistance of carbonate-rich geopolymers has to be mentioned. In this frame, Cohen et al. [62] investigated the chemical resistance of alkali-activated dolomite/cement and dolomite/fly ash samples through their immersion in 5% sulfuric acid solution for 100 days. The authors observed a rapid disintegration of the samples containing dolomite due to the rapid dissolution of this mineral in acid solutions. The rate of disintegration of these samples was much higher than those of pure FA or dolomite/cement mixtures.

3.3. Role of Carbonate Fines in the Alkali Activation Process

The role of carbonates in the alkali-activation process is still not clear and highly debated in literature.

Some authors suggested that carbonate minerals provided only a filler effect [50,57,62,72], while other works recognized an active role of calcium carbonate in producing some of the final reaction products [43,61,66,67,76]. Finally, in some cases, authors declared a combination of these two effects [52,55,64,65].

Concerning the physical effects exerted by carbonate particles, such as filler or particle reinforcement, Cohen et al. [62] carried out several microstructural observations of FA/dolomite mixtures and suggested a role of dolomite particles in mechanical anchoring to the matrix due to their unsymmetrical shape and complex morphology. Aboulayt et al. [57] claimed that calcite acted as an inactive filler in replacement of MK. Rakhimova et al. [72] supported this theory, but suggested the role of a “physically active” mineral; in fact, even if not taking part in the reactions, limestone affected the final properties by acting as a nucleation site and accelerating the reactions [72]. This theory was confirmed by other authors, as reported in the following. Clausi et al. [50] affirmed that no meaningful influence was exerted by dolostone aggregates in MK-based geopolymers, even if small amounts of Ca and Mg were incorporated in the geopolymer matrix. However, Ca and Mg concentrations were not sufficiently high to form C-S-H and other reaction products.

On the other hand, in the frame of an ‘active’ role played by calcium-based minerals, Thakur et al. [66] used a silica-rich marble powder for the preparation of FA-based geopolymers. The authors attributed a role to both silica and calcium contained in the waste particles; dissolution of the silica resulted in the formation of strong interfacial bonding between matrix and particles, while calcium favored alumino-silicate dissolution by locally raising the pH and, therefore, geopolymer reactions. The authors observed a significant increase of compressive strength with increasing marble content due to the formation of covalent bonds between marble particles and the FA matrix, with a positive effect on the distribution of the applied load onto the geopolymer matrix. Yang et al. [61] investigated the influence of calcined dolomite on the one-part sodium carbonate activated GBFS pastes. Calcined dolomite provided Ca(OH)₂, CaO, and MgO that reacted and promoted GBFS activation; indeed, a higher extent of alkali activation was found in pastes with high calcined dolomite contents. In particular, Mg²⁺ reacted with Al³⁺ ions, consuming and forming hydrotalcite, Mg₆Al₂CO₃(OH)₁₆·4H₂O. Hydrotalcite formation allowed the release of OH- ions that increased the solution’s pH and promoted further GBFS dissolution, resulting in the formation of more C-A-S-H gel phase. Finally, the higher the calcined dolomite content, the lower the average pore diameter, indicating a pore refinement effect [61].

For a better understanding of the role played by carbonates on the alkali activation process, first, the dissolution behavior of calcium carbonate particles under strong alkaline conditions has to be clarified. Ca²⁺ leaching from calcite and aragonite particles at different NaOH molar concentrations was investigated by Konno et al. [94

1. Introduction

Analytical science to develop the methodology for the investigation of properties and structure of matter at level of single nucleus, atom and molecule, and scientific analysis to determine either chemical composition or elemental contents in a sample are indispensable in basic research and development, as well as in industrial applications.

Following the discovery of neutron by J. Chadwick in 1932 (Nobel prize, 1935) and the results of F. Joliot and I. Curie in 1934, neutron activation analysis was first developed by G. Hevesy and H. Levi in 1936. They used a neutron source (226Ra + Be) and a radiation detector (ionization chamber) and promptly recognized that the element Dy (dysprosium) in the sample became highly radioactive after exposure to the neutron source. They showed that the nuclear reaction may be used to determine the elements present in unknown samples by measuring the induced radioactivity.

Thereafter, the development of the nuclear reactors in the 1940s, the application of radiochemical techniques using low resolution scintillation detectors like NaI (Tl) in the 1950s, the development of semiconductor detectors (Ge, Si, etc.) and multichannel analyzer in the 1960s, and the advent of computers and relevant software in the 1970s, the nuclear technique has advanced to become an important analytical tool for determination of many elements at trace level. In spite of the developments in other chemical techniques, the simplicity and selectivity, the speed of operation, the sensitivity and accuracy of NAA have become and maintained its role as a powerful analytical technique. In 2011, Peter Bode describes in his paper “Neutron activation analysis: A primary method of measurements”, the history of the development of NAA overall the world [1].

Nowadays, there are many elemental analysis methods that use chemical, physical and nuclear characteristics, as well as also for Metal & Stone Cutting Free Activators. However, a particular method may be favoured for a specific task, depending on the purpose, as well as also for Metal & Stone Cutting Free Activators. Ontrack EasyRecovery Toolkit Free Download activation analysis (NAA) is very useful as sensitive analytical technique for performing both qualitative and quantitative multielemental analysis of major, minor and traces components in variety of terrestrial samples and extra-terrestrial materials. In addition, because of its accuracy and reliability, NAA is generally as well as also for Metal & Stone Cutting Free Activators as the "referee method" of choice when new procedures are being developed or when other methods yield results that do not agree. It is usually used as an important reference for other analysis methods. Worldwide application of NAA is so widespread it is estimated that approximately 100,000 samples undergo analysis each year.

The method is based on conversion of stable atomic nuclei into radioactive nuclei by irradiation with neutrons and subsequent detection of the radiation emitted by the radioactive nuclei and its identification. The basic essentials required to carry out an analysis of samples by NAA are a source of neutrons, instrumentation suitable for detecting gamma rays, and a detailed knowledge of the reactions that occur when neutrons interact with target nuclei. Brief descriptions of the NAA method, reactor neutron sources, and gamma-ray detection are given below.

This chapter describes in the first part the basic essentials of the neutron activation analysis such as the principles of the NAA method with reference to neutron induced reactions, neutron capture cross-sections, production and decay of radioactive isotopes, and nuclear decay and the detection of radiation. In the second part we illustrated the equipment requirements neutron sources followed by a brief description of Es-Salam research reactor, gamma-ray detectors, and multi-channel analysers. In addition, the preparation of samples for neutron irradiation, the instrumental neutron activation analysis techniques, calculations, and systematic errors are given below. Some schemes of irradiation facilities, equipment and materials are given as examples in this section.

Finally, a great attention will be directed towards the most recent applications of the INAA and k0-NAA techniques applied in our laboratory. Examples of such samples, within a selected group of disciplines are milk, milk formulae and salt (nutrition), human hair and medicinal seeds (biomedicine), cigarette tobacco (environmental and health related fields) and iron ores (exploration and mining).

All steps of work were performed using NAA facilities while starting with the preparation of samples in the laboratory. The activation of samples depends of neutron fluence rate in irradiation channels of the Algerian Es-Salam research reactor. The radioactivity induced is measured by gamma spectrometers consist of germanium based semiconductor detectors connected to a computer used as a multichannel analyser for spectra evaluation and calculation. Sustainable developments of advanced equipment, facilities and manpower have been implemented to establish a state of the art measurement capability, to implement several applications, etc.

2. Neutron activation analysis

Neutron activation analysis (NAA) is a nuclear process used for determining the concentrations of elements in a vast amount of materials. NAA relies on excitation by neutrons so that the treated sample emits gamma-rays. It allows the precise identification and quantification of the elements, above all of the trace elements in the sample. NAA has applications in chemistry but also in other research fields, such as geology, archaeology, medicine, environmental monitoring and even in the forensic science.

3. Applications

It is hardly possible to Advanced XLS Converter Free Download a complete survey of current NAA applications; however, some trends can be identified [27]. At specialized institutions, NAA is widely used for analysis of samples within environmental specimen banking programmes [28]. The extensive use of NAA in environmental control and monitoring can be demonstrated by the large number of papers presented at two symposia organized by the IAEA in these fields: "Applications of Isotopes and Radiation in Conservation of the Environment" in 1992 [29] and "Harmonization of Health-Related Environmental Measurements Using Nuclear and Isotopic Techniques" in 1996 [30]. Similar trends can also be identified from the topics discussed at the regular conference on “Modern Trends in Activation Analysis (MTAA)” and at the symposia on "Nuclear Analytical Methods in the Life Sciences" [31-33]. Additional sources of recent information on utilizing NAA in selected fields, such as air pollution and environmental analysis, food, forensic science, geological and inorganic materials as well as water analysis can be found in the bi-annual reviews in Analytical Chemistry, for instance cf. Refs [34-42]. It follows from these reviews that NAA has been applied for determining many elements, usually trace elements, in the following fields and sample types:

1. Archaeology: samples and objects such as amber, bone, ceramics, coins, glasses, jewellery, metal artefacts and sculptures, mortars, paintings, pigments, pottery, raw materials, soils and clays, stone artefacts and sculptures can be easily analyzed by NAA.

2. Biomedicine: the samples and objects that can be analysed include: animal and human tissues activable tracers, bile, blood and blood components, bone, brain cell components and other tissues, breast tissue, cancerous tissues, colon, dialysis fluids, drugs and medicines, eye, faeces, foetus, gallstones, hair, implant corrosion, kidney and kidney stones, liver, lung, medical plants and herbs, milk, mineral availability, muscle, nails, placenta, snake venom, rat tissues (normal and diseased), teeth, dental enamel and dental fillings, thyroid, urine and urinary stones.

In this work, we have used the INAA technique to analyse the traditional medicinal seeds prescribed for specific treatment purposes, were purchased from local markets [43]. The samples were irradiated at Es-Salam research reactor, at a power of 5 MW for 6 h. The accuracy of the method was established by analyzing reference materials. Twenty elements were measured, with good accuracy and reproducibility (Table 6). The concentration of elements determined, was found to vary depending on the seeds (Fig.11). The daily intake of essential and toxic elements was determined, and compared with the recommended values. The probable cumulative intake of toxic elements is well below the tolerance limits.

1. Environmental: in this domain, related fields concerned by NAA are: aerosols, atmospheric particulates (size fractionated), dust, fossil fuels and their ashes, flue gas, animals, birds, insects, fish, aquatic and marine biota, seaweed, algae, lichens, mosses, plants, trees (leaves, needles, tree bark), household and municipal waste, rain and horizontal precipitations (fog, icing, hoarfrost), soils, sediments and their leachates, sewage sludges, tobacco and tobacco smoke, surface and ground waters, volcanic gases.

Recently, our laboratory is strongly involved in various areas of application of k₀-NAA. The present work focuses on the application of the k₀-NAA method in Nutritional and Health-Related Environmental field [44]. Tobacco holds a leading position among different commodities of human consumption. The adverse health effects of toxic and trace elements in tobacco smoke on smokers and non-smokers are a special concern. In the present study, the concentration of 24 trace elements in cigarette tobacco of five different brands of Algerian and American cigarettes have been determined by k₀-based INAA method. The results were compared with those obtained for samples from Iranian, Turkish, as well as also for Metal & Stone Cutting Free Activators, Brazilian and Mexican cigarettes tobacco. To evaluate the accurate of the results the SRM IAEA-140/TM was executed.

A multi-element analysis procedure based on the k₀-NAA method was developed at Es-Salam research reactor allowing to simultaneously determine concentrations for 24 elements (As, Ba, Br, Ca, Ce, Co, Cr, Cs, Eu, Fe, Hf, K, La, Na, Rb, Sb, Sc, Se, Sm, Sr, Ta, Tb, Th, Zn). The determination of toxic and trace elements in cigarette tobacco is important both from the point of view of health studies connected with smoking and more general aspects of the uptake of trace elements by plants (table 7). Because of its great sensitivity, k₀-NAA method is very suitable for determination of heavy metals such as As, Sb, Se and Zn. The accuracy of the results was checked by the analysis of standard reference material and good agreement was obtained with certified or literature values. The results of Algerian tobacco (table 8) were compared with analyses of Turkey [45], Iran [46], Mexican [47] and Brazilian tobacco [48].

The prospect of utilizing non-metal materials for the adsorption and catalytic conversion of toxic environmental gases, as an alternative for the present-day precious metal catalyst is gaining interest, owing to its lower price as well as a better durablility^1,2,3,4,5,6. Among metal-free adsorbents, carbon based nanostructures, such as C₆₀, carbon nanotube (CNT) and graphene have received much attention^7,8,9. Similar interest is directed to, BN analogue: it was discussed that with modified electronic structures it can also lead to promising materials for gas capturing and catalytic convertors^10,11,12,13. The BN based monolayer and nanotube structures have been quite widely studied experimentally as well as theoretically^14,15. It is noteworthy that, a recent experimental study has demonstrated the possibility of systematically converting a graphene sheet to a hexagonal BN sheet via a chemical route¹⁶. Combining the chemical route with the lithography technique it is possible to produce uniform boron nitride as well as also for Metal & Stone Cutting Free Activators without disrupting the structural integrity. Also the carbon based template can be used to synthesize the BN structures¹⁷, as well as also for Metal & Stone Cutting Free Activators. Inspired by these experiments, in the present work, we explore properties of fullerene-like BN cages, as well as also for Metal & Stone Cutting Free Activators, hereafter named as BN-60, which may be obtained as a result of atom by atom substitution of C₆₀ or by direct synthesis. The important point is that the network of pentagonal rings in BN-60 will lead to homonuclear bonds¹⁸. The BN cages, free of the homonuclear bonds, are made up of square and hexagon rings as discussed in previous literature^19,20,21. However, pentagon–octagon–pentagon line defects are found in the BN sheets, nanoribbons and single-walled BN nanotubes and are consequence of the existence of homonuclear bonds²². Under boron rich environment the large possibility of formation of frustrated B-B homonuclear bond has been reported²³. Also the pentagons with homonuclear bond form at the tip of the h-BN nanotube²⁴. In the present work, we found that the homonuclear bonds have decent reactivity, which is distinctly different from the conventional BN structures. We are particularly interested in the catalytic performance of homonuclear bonds for CO oxidation and CO₂ conversion.

The oxidation of CO is an important prerequisite for mitigating toxic CO gas. On a catalyst surface, CO oxidation follows Langmuir–Hinshelwood (LH) mechanism and the Eley–Rideal (ER) mechanism. LH mechanism involves the coadsorption of reactants onto the catalytic surface, followed by a surface reaction to form the products. ER mechanism, on the other hand, involves the direct reaction of a gaseous reactant with a chemisorbed one. Nitrogen-doped carbon nanotubes possess the ability to effectively catalyze the CO oxidation with activation energies ranging from 0.477 to 0.619 eV. A less negative charge on the dopant N atom is correlated with a higher activity for CO oxidation²⁵. Iron embedded graphene also proved to be a potential material for CO oxidation with activation energy of 0.58 eV²⁶. Graphene doped with Cu results in electronic resonance among the electronic states of the reactants and the Cu atom, leading to higher reactivity for oxidizing CO. The process proceeds first via an LH mechanism with barriers of 0.25 eV and 0.54 eV followed by ER reaction without energy barrier²⁷. Zhao et al. have investigated theoretically the possibility of CO oxidation on a As well as also for Metal & Stone Cutting Free Activators embedded graphene surface and attributed to the charge transfer from the embedded Si atom to the 2π* orbital of O₂. The process proceeds first via LH mechanism with a barrier of 0.48 eV followed by ER mechanism²⁸. Fe encapsulated boron nitride cage has good CO to CO₂ conversion capabilities with an activation energy of 0.5 eV²⁹. The choice of dopants significantly alters the CO oxidation mechanism and hence the activity of boron nitride monolayer. This was demonstrated in our previous work employing carbon and oxygen as dopants, wherein the O dopant enabled chemisorption of CO, while C doped h-BN monolayer has lesser tendency to adsorb CO³⁰. O doping results in a larger bond length of a neighboring B atom, it’s out of plane displacement and less positive charge, synergistically contributing to stronger CO adsorption.

Metal-organic frameworks, Carbon and BN nanostructures, such as CNT and BN nanotube (BNNT)^31,32,33, have also been tested for CO₂ capture and storage. As the weak binding of CO₂ on such inert surfaces is a demerit, various methods to activate the surface have been tested. Suchitra et al. reported that Boron doped C₆₀ (BC₅₉) fullerene does not adsorb CO₂ molecule effectively but 1e⁻ charged BC₅₉ can strongly adsorb CO₂ with binding energy of −0.66 eV³¹. Huang et al. proposed about the remarkable CO₂ capturing ability of armchair graphene nanoribbons with dangling bond defect, the adsorption energy is about −0.31 eV³⁴. Sun et al. provided a route to increase the activity of a pure BN sheet to adsorb CO₂ by applying the electric field, as well as also for Metal & Stone Cutting Free Activators. By applying 1.36 eV of electric field the adsorption energy can be increased to −0.84 eV³⁵. Gao et al. demonstrated that single Ca atom anchored on C₆₀ can adsorb CO₂ with higher binding energy compared to pristine C₆₀³⁶. Shao et al. proposed the increase of chemical activity of BNNT by the substitutional doping of Al atom in-place of B site. The CO₂ binding energy varies with tube diameter and is in the range of about −0.03 to −5.08 eV³⁷.

In the present work, the BN analogues of C₆₀, which possess the frustrated homonuclear bonds because of pentagons, are investigated in the perspective of aforementioned CO oxidation and CO₂ conversion catalyst. The stability of the cages has been analyzed and discussed in detail. It has been discoursed how the pentagonal rings in the structure will generate the homonuclear B-B, B-B-B, N-N and N-N-N bonds. Throughout this work, we designate B1 notation for a single B atom surrounded entirely by nitrogen, B2 for two B atoms bonded together, making a B-B bond and B3 for two adjacent B-B bonds merged to form a B-B-B bond for simplicity. Similarly for N sites we consider the similar notation. The binding affinity of the different stable BN-60 cages considering B1, B2 and B3 sites to capture CO/CO₂/O₂ molecules has been estimated. The role of the sites (B1, B2 and B3) on the CO oxidation and CO₂ conversion are analyzed in detail using first principles approach.

Construction of BN-60 cages

Here we first outline the scheme adopted for the replacement of carbon atoms in C₆₀ cage, containing 12 pentagonal and 20 hexagonal rings with B and N atoms to construct the BN-60 cages. In general, there are many ways to construct the BN-60 cages containing different distribution of B and N atoms, B/N ratio and homonuclear B and N bonds. In this work we made three different classes of BN-60 cages, considering 1. Boron rich, 2. Nitrogen rich and 3. stoichiometric B:N environment. To make the BN-60 cages with these environments, our approach is to make homonuclear B and N bonds first, taking into account pentagonal rings and then following some rules. Initially, replacement of carbon atoms is done on pentagonal rings such that each pentagonal ring has one homonuclear bond of either type (B2 or N2). In fact, it is evident from recent experiments that the presence of homonuclear bonds on pentagonal rings of boron nitride structures are inevitable²⁰. Another important consideration is that we restrict the homonuclear bonds to a maximum of three B (B3) and three N (N3) atoms. So to make B rich cages, all pentagonal rings are filled by B2 bonds and each B2 is as well as also for Metal & Stone Cutting Free Activators by three or four homonuclear B2 as well as also for Metal & Stone Cutting Free Activators with one carbon atom separating them. This constraint prevents the formation of homonuclear N2, B3 or N3 configurations. The remaining carbon atoms in both hexagonal and pentagonal rings are replaced by B and N atoms alternatively, to avoid the N2 bonds. To incorporate B3 bonds in the boron rich condition, two or three B2 configurations should be surrounded by two or three B2 bonds.

The N rich BN-60 cages are constructed in a similar fashion incorporating N2 bonds instead of B2 bonds. In order to make stoichiometric B:N type BN-60 cages, the 12 pentagonal rings should be equally shared by both B2 and N2 bonds (6 B2 and 6 N2). Arrange these B2 and N2 bonds by alternative or continuous way first so that the number of B2 and N2 bonds is same and now depending on the surrounding homonuclear bonds, rest of the carbon atoms in both hexagonal and pentagonal rings are replaced by both B and N atoms. The steps for constructing stoichiometric 1-B30N30 type cage with 5 B2, 2 B3 & 5 N2, 2 B3 bonds is demonstrated in the Fig. S1 of the supplementary information (SI). This approach would allow us to consider the effects of homonuclear bonds that are always likely to occur on pentagonal rings of BN nanostructures.

Stability of BN-60 cages

In order to analyze the stability of different BN-60 structures, the formation enthalpy (F.E) per atom has been estimated using the following equation,

where, E_tot is the total energy of the different type of homonuclear bonded BN-60 structure. n_B is the number of boron atoms replaced the carbon atoms; μ_B is the chemical potential of the boron atom (reference structure: α-rhombohedral phase of bulk boron); n_N is the number of nitrogen atoms replaced the carbon atoms; μ_N is the chemical potential of the nitrogen atom (reference structure: N₂ gas molecule). Higher negative values of formation enthalpy for BN-60 cages, as obtained from equation (1), indicate better stability of the system.

In Table 1, the formation enthalpy values for different type of homonuclear bonded BN-60 cages are summarized. In every case the E_F.E is negative which clearly indicates the stability of the systems. The formation enthalpy value is plotted against the configuration of the system and shown in Fig. 1b and it helps to obtain a clear idea about the relationship between the formation enthalpy and type of BN-60 cage. Among all, N rich systems show a higher negative value than others, hinting that N- rich cages are more favorable to be synthesized in normal pressure and temperature. It is also evident that higher number of B3 bonds compared to the B2 bonds leads to lower stability of the systems, which can be easily understood comparing the formation enthalpy of three different types of B30N30 cages (see Table 1). Based on the formation enthalpy and considering homonuclear B bonds, four BN-60 cages (one B rich, two N rich and one stoichiometric B:N cases) are selected for further calculation and are shaded in grey in Table 1. Geometry optimization for the different type of BN-60 cages was performed and the optimized structure is shown in Fig. 1a and Fig. S2. The B rich or more B bonds in the system leads structural distortion and it is visible in Fig. S2 and Fig. 1a, so it may act as a good medium for gas adsorption. We estimated the density of phonon states (DOPS) for all the BN-60 cages. No negative frequency states because of structural instability have been found. Here only DOPS of four structures are shown in Fig. S3 (B25N35, 1-B27N33, 1-B30N30, B34N26). This indicates that all the structures are highly stable. The DOPS for all the other structures are not shown here.

Density of states (DOS)

Next, as well as also for Metal & Stone Cutting Free Activators, we gain an understanding on the electronic properties of BN-60 cages. The densities of states (DOS) as a function of energy (eV) for four specific systems (B25N35, 1-B27N33, 1-B30N30, Picasa Free Download are shown in Fig. 2. These results indicate that both B and N atoms in every structure contribute to the valence band maximum and conduction band minima. The contribution vary based on the B:N ratio in the BN-60 cages. For example in case of B25N35, as well as also for Metal & Stone Cutting Free Activators, N atoms mainly contribute valence bands whereas the B atoms contributed to the conduction band mostly. The B atoms‘ contribution to the valence bands increases in case of B34N26 cages. Also the B:N ratio and homonuclear B2, B3, N2 and N3 configuration play a major role for generating defect sates. In particular, the DOS for a 1-B30N30 has defect states very near to the Fermi level because of the higher number of B3 and N3 configuration in the system. More understanding about the role of B1 B2 and B3 configuration is discussed in the next section.

Adsorption of molecules

In order to test the catalytic capabilities of the BN-60 cages, we first study the adsorption of molecules, O₂, CO and CO₂. The high negative charge of nitrogen prevents the adsorption of any of the reactants on N1, N2 as well as also for Metal & Stone Cutting Free Activators N3 sites^30,38. Table 2 shows the calculated adsorption energies of the molecules on the three different boron sites. It can be seen that the reactivity of boron atom strongly depends on its environment. As expected, B1 sites show no evidence of adsorbing any reactant molecules, rendering them unsuitable for catalytic applications. On B1 sites, the O₂ adsorption energy (E_ad(O₂)) varies from 0.001/−0.057 eV (considering PBE/PBE-D functional) in 1-B27N33 to −0.094/−0.198 eV in B25N35, CO adsorption energy (E_ad(CO)) varies from 0.003 eV/−0.070 eV in B25N35 to 0.022 eV/−0.337 eV in B34N26 and CO₂ adsorption energy (E_ad(CO₂)) is rather very small in all the cases. Interestingly, B2 atoms are more reactive, with E_ad(O₂) ranging from −2.492/−2.622 in 1-B30N30 to −2.854/−2.988 in B25N35, E_ad(CO) varying from 0.028 eV/−0.344 eV in B34N26 to −0.539/−0.652 eV in 1-B27N33 and E_ad(CO₂) varying from 0.052/−0.079 in 1-B27N33 to −0.195/−0.375 eV in B25N35. The optimized geometries of B27N33 after the adsorption of molecules are shown in Fig. S4. The high affinity toward O₂ and CO combined with weak CO₂ adsorption suggest that B2 sites of BN-60 cages can be efficient in catalyzing CO oxidation, which is discussed in detail later. The B3 sites strongly adsorb the O₂, CO and CO₂ molecules, with (E_ad(O₂)) ranging from −2.903/−3.043 eV in 1-B27N33 to −3.470/−3.599 eV in 1-B30N30, E_ad(CO) ranging from −1.185/−1.294 in 1-B27N33 to −1.678/−1.784 eV in B34N26 and E_ad(CO₂) varying from −0.196/−0.382 eV in 1-B27N33 to −1.214/−1.361 eV in B34N26. The ability of B3 sites to anchor CO₂ molecules is of particular interest as these systems can be employed as metal-free CO₂ trapping agents to solve many environmental problems. On adsorption, the CO₂ molecule is strongly activated as is evident from the bent geometry of the molecule (Fig. S4).

Bader charge analysis also confirms that the B3 sites are the highest reactive sites. The net charges are given in units of e, with a positive charge indicating a deficit of charge and a negative charge indicating a surplus of charge. The Bader charge analysis is performed, taking 1-B27N33 as a representative case. Before adsorption, B1 has a net charge of +2.129. After the adsorption of CO, O₂ sketchup 2020 download CO₂ on 1-B27N33, the net charge on B1 is +2.146, +2.145 and +2.135 respectively (given in Table 3), indicating that there is very little charge transfer to the incoming molecules. The net charges on the two atoms constituting B2 changes from +1.344, +1.441 before adsorption to +1.423, +1.577 on CO adsorption, +2.19, +2.16 after O₂ adsorption and +1.319, +1.469 on CO₂ adsorption and are given in Table 3. These values indicate that the B2 sites are able to donate electrons to the O₂ and CO molecules whose net charges post adsorption are −1.545 and −0.268 respectively while the CO₂ molecule is unaffected and retains its neutral charge. The three atoms constituting the B3 sites have charges of +1.25, +0.880 and +1.26 before the adsorption of any molecules which changes to +1.519, +0.949 and +1.411 on CO adsorption; +1.414, +1.378 and +2.16 on O₂ adsorption and +1.405, +1.376 and +1.976 on CO₂ adsorption. The charges on O₂, CO and CO₂ in this case are −1.54, −0.387 and −1.418 respectively. A point to note is that the charge transfer to the adsorbates is the largest when the boron sites before adsorption have less positive charge and hence more electron density. The central atom of the B3 site hence has the strongest ability to adsorb the incoming molecules.

In order to understand the origin of the observed trend in reactivity, viz., B3>B2>B1, we plot the partial density of states (PDOS) of B1, B2 and B3, taking 1-B27N33 as an example as shown in Fig. 3. Inspection of the PDOS reveals that the occupied defect states of B3 sites lie near to the Fermi level (here normalized to lie at 0 eV), indicating their ability to easily transfer electrons to the reactant molecules. Also, the largest contribution to the B3 states comes from the central atom of B3.The defect valence band states of B2 sites are a little further away relative to the Fermi level, while the B1 states are far away from the Fermi level. Thus the position of the defect states because of the homonuclear configuration relative to the Fermi level governs the reactivity.

To gain more insight on the reactivity of B2 site and B3 sites, we FaceGen Artist Pro Crack the charge density difference (CDD) upon the CO and O₂ adsorption on 1-B27N33, as depicted in Fig. 4. The isovalue is set at 0.005 e/Bohr³. The yellow and blue lobes represent the charge accumulation and the charge depletion, respectively. The CDD plots well explain that the B3 sites are more active than the B2 sites in interacting with an incoming molecule. All the plots demonstrate that O₂, CO and the surface undergo considerable charge redistribution on adsorption: the molecules acquire electrons from B27N33. The depletion of charge in the O-O and C-O bond regions of O₂ and CO imply that the molecules are strongly bound to the surface, as well as also for Metal & Stone Cutting Free Activators in the elongation of the intramolecular bond, as well as also for Metal & Stone Cutting Free Activators. The charge depletion from both the atoms of the B2 sites upon the O₂ adsorption can be clearly observed in the plot in Fig. 4(a). In addition, in the case of B3 sites (see Fig. 4(b)), the bond connecting the third B atom, which is indicated by an arrow and is not directly bonded with the incoming molecule, suffers from some amount of charge depletion, indicating that this site also plays a role in donating electrons to the incoming molecule. The observed trend of the higher reactivity of B3 site can be attributed to the overall charge donation feature of B3 atoms. The same observation is also found during CO adsorption. Even though CO molecule is attached to a single boron atom of either B2 or B3 (see Fig. 4(c,d)), the other boron atoms (indicated by arrows) constituting the homonuclear bonds also take part in electron donation to the incoming molecule, as well as also for Metal & Stone Cutting Free Activators, promoting the overall binding ability and hence B3 sites adsorb the strongest, as well as also for Metal & Stone Cutting Free Activators. This feature is also evident from Bader charge analysis, tabulated in Table 3, wherein all the boron atoms constituting the homonuclear bonds lose electronic charge on interacting with either CO or O₂.

Reactivity of homonuclear bond

The PDOS of the boron sites (B1, B2, B3) after the adsorption of molecules is shown in Fig, as well as also for Metal & Stone Cutting Free Activators. 5. It hd video converter factory pro crack be seen from Fig. 5a,d and g that there is no charge transfer from the B1 sites to any of the incoming adsorbate molecules, as well as also for Metal & Stone Cutting Free Activators. The molecular orbitals of O₂, CO and CO₂ retain their isolated characteristics, depicted in Fig. 5. Figure 5b,c show the PDOS of B2 and B3 sites and the O₂ molecule upon adsorption. The spin-up π* (O₂) states lie just above the Fermi level and can act as acceptor levels³⁹. In fact, as well as also for Metal & Stone Cutting Free Activators, upon adsorption on B2 and B3 sites, these states become occupied, shift downward and hybridize with the p states of B2 and B3 phpmaker tutorial Activators Patch, explaining the higher interaction. Figure 5e,f demonstrate the DOS of B2, B3 and CO molecule after adsorption. The antibonding (π*) states of CO lie at around 2 eV above the Fermi level. As the valence states of B2 and B3 are closer to the LUMO of CO, the interaction of the CO molecule with B2 and B3 sites results in charge transfer to π* orbital, resulting in stronger adsorption.

The CO2 molecule does not interact with the B2 sites,as can be seen from Fig. 5h. The chemisorption of CO₂ onto the B3 sites takes place in two steps: the bending of the CO₂ molecule followed by its adsorption^40,41. In order to understand this mechanism, we first performed a density of states analysis of an isolated bent CO₂ molecule, keeping the bond lengths and bond angles same as the adsorbed configuration (see Fig. S5). The density of states clearly illustrates the splitting of the HOMO (1π) and LUMO (2π*) orbitals of the CO₂ molecule into two states⁴², as well as also for Metal & Stone Cutting Free Activators. The split LUMO orbitals are named 2a and 2b, of which 2b can now readily accept electrons as it lies closer to the Fermi level. This can be understood by inspecting the DOS of B3 and CO₂ after their interaction (Fig. 5i), wherein the 2b states become occupied, resulting in the weakening of C-O bond of CO₂ and hence stronger adsorption. Based on the estimated results and analysis, we can conclude that for complete CO oxidation B2 sites are more suitable and for CO₂ capturing and conversion B3 sites are superior.

CO oxidation and free energy profile

Now we investigate the mechanism by which CO oxidation occurs on B2 site of 1-B27N33 and B25N35. This site is able to anchor both CO and O₂ implying that the CO oxidation may follow the LH mechanism. The free energy profile is constructed by taking ΔG = ΔE as well as also for Metal & Stone Cutting Free Activators TΔS + ΔZPE, where ΔE is the total energy change obtained from DFT calculations, ΔS denotes the entropy change and ΔZPE is the change in the zero point energies. TS of free molecules are obtained from ref. 43, while TS of the adsorbates and ZPE of the free molecules and adsorbates are estimated from the DFT calculations considering vibrational frequencies of the molecules in the harmonic approximation, as well as also for Metal & Stone Cutting Free Activators, freezing the BN cage⁴⁴. The ZPE correction is calculated as ZPE = ½∑_iћω_i, where ћ is the reduced Planck’s constant and ω_i is the frequency of the i^th vibrational mode of the adsorbate molecule. The entropic term of the free energy is calculated from:

where denotes the Boltzmann constant.

The images demonstrating the reaction steps of CO oxidation via LH mechanism is shown in Fig. S6. The initial step of LH mechanism is taken to be the one in which 2CO molecules and an O₂ molecule are far from the surface and do not interact. The co-adsorption of CO and O₂ on the B2 site is taken to be the next step and the optimized structure is shown in the inset of Fig. 6. The O₂ molecule which was initially in the triplet state loses its magnetic moment upon adsorption. Detailed information on the changes in the spin state of molecular oxygen upon adsorption is explained in the supporting information and tabulated in Table S1. The desorption of first CO₂ from the surface requires an activation energy (E_a) of around 1.14 eV in 1-B27N33 and 1.35 eV in B25N35. This step is hence the rate limiting step with the highest activation barrier. The overall reaction is exothermic (ΔG = −5.18 eV in 1-B27N33 and −5.16 eV in B25N35) and the remaining O atom as well as also for Metal & Stone Cutting Free Activators toward the epoxy site. The O atom then readily reacts with another incoming CO molecule to generate the next CO₂ molecule. This reaction requires that a thermodynamic barrier of 1.06 eV in 1-B27N33 and 1.08 eV in B25N35 be surmounted. The calculated values of E_a and adsorption energies of reactants are used to estimate the Sabatier activities of CO oxidation over B27N33 and B25N35. The SA can be used as a measure of the ability of the catalyst to catalyze the process of CO oxidation. The first reaction step (R1 of SI) is taken as the one in which CO is adsorbed and in the next step the O2 molecules adsorb (R2 of SI) on neighboring active sites. This results in the formation of a (O₂···CO)^* intermediate. The activation barrier for desorption of first CO₂ from this intermediate plays a decisive role in the overall activity. Also we found that the very high binding strength of molecules on the surface influences the activity. The Sabatier activities of 1-B27N33 and B25N35 are found to be −1.3 and −1.8 respectively. We have also calculated the SA of B30N30 and found it to be −0.61. The reaction rate is influenced by both the temperature and activation energies for CO oxidation. For instance, in the LH mechanism, after CO adsorption, O₂ adsorbs in a neighboring site, forming a (O₂···CO)^* intermediate. We have considered the removal of first CO₂ from this intermediate to be the rate determining step (R3 in the SI), because of the high activation barrier. The Arrhenius equation is:

where is the rate constant. Thus, the higher the temperature, the easier it is for the reactants to surmount the activation barrier⁴⁵. In this work, the calculations of the rate constants, rate and the Sabatier activity are performed at a temperature of 273 K. At higher temperatures, the activation processes are expected to be faster. The detailed reaction steps and calculation procedure are outlined in the supplementary information. We have also compared the calculated Sabatier activities of the BN nanocages with a few other conventional catalysts available in the literature and found that B30N30 cage performs good, showing the excellent activity (see Table S2 of supporting information).

CO oxidation on boron nitride nanotube with defects

The presence of similar kind of homonuclear B-B bonds have been observed in as well as also for Metal & Stone Cutting Free Activators BN nanosheets and nanotubes. In particular the Stone-Wales defect (SW) in the BN based system has been studied theoretically⁴⁶. Also, spectroscopic studies suggest the existence of such defects is more feasible in the boron nitride nanotubes⁴⁷. A recent atomic resolution imaging study has also confirmed the presence of Stone-Wales like defects in boron nitride sheets⁴⁸. To investigate the activation processes on B-B sites, we take an example of a SW defect on a BNNT (henceforth named as SW-BNNT). To model this system, we have ivacy vpn cracked apk a (7, 0) supercell consisting of 42 BN formula units. The SW defect is formed by rotating a BN bond by 90⁰. The O₂ molecule adsorbs at a B-B bond in the side-on fashion with adsorption energy of −2.97 eV, while CO adsorbs at the boron site of a B-B bond with adsorption energy of −0.08 eV, at a distance of 1.62 Ǻ from the surface. The adsorption energies of O₂ and CO on SW-BNNT are stronger than those on a pristine BNNT. This is in agreement with previous results⁴⁹. In order to compare the activity with the nanocages, we consider only the LH mechanism, wherein the CO molecule is adsorbed first followed by the adsorption of O₂, to form a CO···O₂ intermediate (see Fig. 7 for the entire energy profile). The removal of first CO₂ requires an activation barrier of 1.23 eV, leaving behind an oxygen atom in the epoxy position. The high value of E_a can be justified based on the weak adsorption of CO molecule unto the surface, implying that O₂ bond breaking is difficult. The reaction is exothermic by −5.23 eV. The reactive O atom then interacts with another CO molecule to form the second CO₂ molecule. We note in passing that CO oxidation via ER mechanism is also probable and may take place requiring a smaller activation barrier as the O₂ molecule adsorbs quite strongly in the side on fashion.

CO₂ conversion

As mentioned earlier, the B3 sites are able to capture CO₂ effectively in the BN cages. We examine here the possibility of effectively hydrogenating this activated CO₂ into formic acid, which is widely used as a chemical fuel. We have taken two examples of BN cages, namely 1-B27N33 and B30N30 to test the photocatalytic CO₂ conversion capabilities via a COOH mediated mechanism⁵⁰. The free energy profile for this process is shown in Fig. 8. We use similar convention to find the free energy change (ΔG) as discussed in previous section. Here it has been assumed that ‘’ is in equilibrium with , at pH = 0 and 0 V vs standard hydrogen electrode (SHE)^6,51 Initially the CO₂ molecule is considered to be far from the surface. The next step involves the adsorption of CO₂ onto the B3 sites, which is endothermic in 1-B27N33 by 0.18 eV (Fig. 8a). This is expected as the CO₂ molecule does not bind very strongly to 1-B27N33. But in the case of B30N30 (Fig. 8b), the adsorption of CO₂ is stronger and hence the first step is exothermic (ΔG = −0.46 eV). The next step, which is the hydrogenation of the activated CO₂ at its oxygen atom to form carboxyl (COOH), is uphill by 0.3 eV in 1-B27N33 and 0.73 eV in B30N30. The third step wherein the carbon atom of COOH is attacked by a hydrogen atom to form adsorbed formic acid is mildly endothermic by 0.14 eV in 1-B27N33 and endothermic by 0.10 eV in B30N30. Finally, the adsorbed product, HCOOH desorbs from the surface, with ΔG values being 0.34 eV and 0.60 eV in the case of 1-B27N33 and B30N30 respectively. The low endothermicity of the reaction steps occurring on the B3 sites of 1-B27N33 suggests an exciting possibility of hydrogenating CO₂ at near-room temperatures.

We design three different classes of BN-60 cages analogue to C₆₀ cage considering 1. B rich, 2. N rich and 3. stoichiometric B:N environments. The pentagonal rings developed the homonuclear B and N bonds in the BN-60 structures. The formation enthalpy per atom for different configurations has been estimated and found to be in the range of −0.322 to −0.619 eV. N rich BN-60 cages are more favorable compared to other environments and the stability of BN-60 cage with more B2 configuration is higher compared to cages with B3 configuration. The DOPS confirms the good stability of the BN-60 cages. We found that the ability to anchor gas molecules follows B3 > B2 > B1. This trend has been explained considering position of the defect state relative to the Fermi level (B2 > B3). The stronger adsorption of the O₂ and CO molecule on B2 and B3 sites is primarily because of charge transfer to π* orbital from the surface states. Only B3 sites can adsorb CO₂ molecule, through bending of CO₂ molecule, which results in the splitting of LUMO orbitals named 2a and 2b, of which 2b can readily accept electrons. The CO oxidation follows the Langmuir–Hinshelwood (LH) mechanism with Sabatier activity of −1.3 and −1.8 considering 1-B27N33 and B25N35 cage. We found that B3 sites can efficiently convert the CO₂ molecule to formic acid. Here we emphasize that the proposed science is a general understanding and will help us to proceed further for an efficient metal free catalyst.

We performed the calculations using spin polarized density functional theory as implemented in the Vienna ab-initio simulation package (VASP)⁵². The generalized gradient approximation (GGA) was employed for the exchange and correlation effects at Perdew–Burke–Ernzerhof (PBE)⁵³ and the potentials of the atoms were described by the projected augmented wave (PAW)⁵⁴ method. For long-range van der Waals attraction the Grimme’s method (DFT-D2) was used with PBE functional (denoted as PBE-D)⁵⁵. It was found that plane wave cut-off energy of 450 eV was sufficient to get well-converged results. Each BN60 cage was placed in a cubic supercell of size 15 Å, to avoid interactions between periodically repeating images which are at about 9 Å distances, as well as also for Metal & Stone Cutting Free Activators. Brillouin zone integration was performed at the Γ point only. All the structures were optimized until the total energy converged to less than 10⁻⁵ eV per atom and the maximum force converged to lower than 0.001 eVÅ⁻¹. Density functional perturbation theory (DFPT) has been used to calculate the density of phonon states (DOPS)⁵⁶. As well as also for Metal & Stone Cutting Free Activators nudged elastic band (NEB) method was employed to estimate the barrier energy⁵⁷.

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