Adsorption of carbon monoxide on Ag(I)-ZSM-5 zeolite: An ab initio density functional theory study

Adsorption of carbon monoxide on different Ag+-exchange sites of Ag–ZSM-5 zeolite has been investigated using density functional theory. The coordination and local geometry of the Ag+ ion in Ag–ZSM-5 as well as adsorption structures and energies of CO adsorbed on these sites are explored extensively. The structure of Ag+-exchange sites, location of the Al atom on the T site, and number of the Al atoms contained in the sites are considered in the theoretical calculations. The calculated results show that the Agsingle bondO coordination number of two is strongly preferred before and after CO adsorption. The Agsingle bondO bond lengths are in a broad range of 2.2–2.9 Å, and the Agsingle bondC bond lengths for CO adsorbed on Ag–ZSM-5 zeolite are calculated to be 2.0–2.2 Å. Both Agsingle bondO and Agsingle bondC bond lengths for CO–Ag–ZSM-5 complex are longer than those for CO–Cu–ZSM-5 complex. The calculated adsorption energy of CO adsorbed on the I2 sites is between 28.5 and 29.6 kcal/mol, and that on the Z5, Z6, M5 and M6 sites containing one Al atom on the T position is between 11.3 and 18.9 kcal/mol whereas the calculated adsorption energy of CO adsorbed on the M7 site containing one Al atom is 19.9 kcal/mol. The introduction of the two Al atoms to the Ag+-exchange site results in a reduction of CO adsorption energy. In general, the adsorption energy of CO on Ag–ZSM-5 is lower than that on Cu–ZSM-5. The predicted coordination of the Ag+ ion, bond lengths of Agsingle bondO and Agsingle bondC as well as adsorption energy are in accord with available experimental results.