Ab initio molecular orbital study of adsorption of oxygen, nitrogen, and ethylene on silver-zeolite and silver halides

An ab initio molecular orbital study is undertaken on the adsorption of N-2 O-2, and C2H4 (adsorbate) on Ag-zeolite and Ag halides (adsorbent). Geometry optimization is performed at the HF/3-21G level, while MP2/3-21G with natural bond orbital calculations are performed to obtain energies, atomic charges, orbital energies, and orbital populations (occupancies). The bonding of adsorbate to adsorbent is discussed in the context of sigma-donation (i.e., overlap of the 2p orbitals of the adsorbate molecule with the 5s orbital of Ag) and d-pi* backdonation (i.e., overlap of the 4d(yz) orbitals of Ag with the 2p* antibonding orbitals of the adsorbate). For all adsorbate-adsorbent pairs, the ratio of sigma-donation to d-pi* backdonation is approximately 3:1. Results on occupancy analysis indicate that a considerable electron redistribution from the 4d(z2) orbitals to the 4d(yz) orbitals occurs in Ag during adsorption and that this redistribution has possibly enhanced the d-pi* backdonation. Net charge and energy gap (Delta epsilon) analyses indicate that it is slightly easier for N-2 than O-2 to adsorb, whereas a comparison of N-2 and O-2 adsorption from calculations of the energies of adsorption is inconclusive. However, a fair agreement is obtained in comparison of theory and experiment for energy of adsorption of N-2 and C2H4 on Ag-zeolite. The dispersion energies of adsorption, based on the MP2 correlation energies, are nearly the same for all adsorption pairs, i.e., approximately 4-5 kcal/mol.