CO Oxidation by Molecular and Atomic Oxygen on Ag(100): A Density Functional Theory Study

CO oxidation by molecular and atomic oxygen has been studied using Ag-38 and Ag-29 cluster models at the DFT-PW91PW91/[LANL2DZ, 6-31G(d)] level. The calculated results show that the adsorptions of O atom, O-2, and CO molecule on the silver (100)-surface are all of chemical type but relatively weak for CO and O-2 molecule with the adsorption energy of 8.7 and 2.7 kcal/rnol, respectively. Upon the adsorption on silver, O-2 molecule was partially activated and ready to participate in the CO oxidation reaction through an L-H mechanism. The reaction involves breaking of the O-O bond and forming, a C-O bond. The interactions between the 2s and 2p of C and O, and 4d of Ag orbitals facilitate the CO oxidation on silver. However, the CO oxidation by atomic oxygen slightly favors an E-R mechanism since the adsorption of CO is very weak and its calculated molecular properties identify itself mine like free CO. For CO3, it is a fairly stable intermediate, but it can be removed by CO or through dissociation, showing this reaction path is not favored thermodynamically. The activation energy for CO oxidation was calculated to be 6.9 and 2.1 kcal/thol by molecnlar and atomic oxygen, respectively. Such low energy barriers imply that both molecular oxygen and atomic oxygen are very reactive for CO oxidation on silver.