Why Must Oxygen Atoms Be Activated from Hollow Sites to Bridge Sites in Catalytic CO Oxidation

There is no doubt that oxidation involving oxygen is one of the most important catalytic reactions: oxygen is a very active species and participates in many catalytic reactions, such as CO oxidation, epoxidation and fuel reforming. Taking catalytic CO oxidation as an example, which is very important not only technologically (in car exhaust emission control, CO{sub 2} lasers, and air purification) but also scientifically, the elementary steps of CO oxidation at low and medium pressures are believed to be as follows: (1) O{sub 2} molecules dissociate on a metal surface, resulting in O atom chemisorption; (2) CO molecules adsorb on the metal surface and then react with the chemisorbed O atoms, forming CO{sub 2} (the Langmuir-Hinshelwood mechanism); and (3) the product, CO{sub 2}, desorbs from the metal surface. The authors carried out Density Functional Theory calculations for CO oxidation on Rh(111). A generalized gradient approximation was utilized in the calculations. The electronic wave functions were expanded in a plane wave basis set and the ionic cores were described with ultrasoft pseudopotentials. The surface was modeled by a p(2 x 2) unit cell with a slab of three layers of Th(111). The vacuum region between slabs was 10 {angstrom}. A cutoffmore » energy of 300 eV and 2 x 2 x 1 k-point sampling within the surface Brillouin zone were used.« less