Activated dissociation of CO2 on Rh(111) and CO oxidation dynamics

The activated dissociative chemisorption of CO, on Rh(111) is characterized theoretically with the aid of detailed balance simulations of the CO2 product state distributions measured in thermally driven CO oxidation experiments. A two-parameter formulation of the physisorbed complex - microcanonical unimolecular rate theory (PC-MURT), a local hot spot model for the CO2 gas-surface reactivity, is used to calculate CO, dissociative sticking coefficients, as well as the angular yields, mean translational energies, and rovibrational energy distributions of desorbing CO, product from CO oxidation. Measured CO2 product angular yields and infrared chemiluminescence from CO oxidation are consistent with two parallel mechanisms for activated CO, dissociative chemisorption on Rh(111): a direct mechanism and an indirect mechanism involving a thermalized intermediate. The PC-MURT describes the direct mechanism that is favored at low coverage and helps to reveal the indirect mechanism that can be a substantial reaction pathway (e.g., 65%) under other conditions. For the direct mechanism, simulations of diverse experimental data indicate that the reaction threshold energy for CO2 dissociative chemisorption is E-0 = 73 kJ/mol, two surface oscillators participate in the dissociative transition state, and molecular rotation is approximately a spectator to the dissociation dynamics. Accordingly, an experimentally consistent activation energy for CO oxidation via Langmuir-Hinshelwood reaction kinetics is E-a' similar to 99 kJ/mol.