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 o f N-2 O-2, and C2H4 (adsorbate) on Ag-zeolite and Ag halides (adsorben t). Geometry optimization is performed at the HF/3-21G level, while MP 2/3-21G 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 th e context of sigma-donation (i.e., overlap of the 2p orbitals of the a dsorbate 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 orb itals 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 redistribu tion from the 4d(z2) orbitals to the 4d(yz) orbitals occurs in Ag duri ng 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, wherea s a comparison of N-2 and O-2 adsorption from calculations of the ener gies of adsorption is inconclusive. However, a fair agreement is obtai ned in comparison of theory and experiment for energy of adsorption of N-2 and C2H4 on Ag-zeolite. The dispersion energies of adsorption, ba sed on the MP2 correlation energies, are nearly the same for all adsor ption pairs, i.e., approximately 4-5 kcal/mol.