A DFT plus U study of the Mars Van Krevelen mechanism of CO oxidation on Au/TiO2 catalysts

Density Functional Theory calculations were carried out to study CO oxidation by lattice oxygen atoms of Au-10/TiO2 (001) catalyst. The variation in Delta G degrees with temperature and the barriers associated with the elementary reactions were calculated. The Mars Van Krevelen (MvK) mechanism starts with the CO oxidation by lattice oxygen atoms located at the periphery of Au-10 on Au-10/TiO2 system. The variation in Delta G degrees indicates that the process is spontaneous at all the temperatures studied and requires 53 kJ/mol activation barrier. The CO2 desorption, as result of this oxidative process, leaves a surface oxygen vacancy at the perimeter of Au-10/TiO2 which is refilled in the subsequent O-2 adsorption step. The variation in Delta G degrees for adsorption of O-2 showed that the adsorption process is spontaneous until 420 K. Dissociation of O-2 into O + O is an exothermic process and activated by a small activation barrier (30 kJ/mol). The second cycle of CO oxidation is even more probable, having a lower activation barrier and high exothermicity. While the Delta G degrees for CO oxidation by lattice oxygen atoms is negative on Au-10/TiO2, it is positive from 100 to 650 K on Au-3/TiO2 and clean TiO2 (001) surfaces, respectively. Thus, CO oxidation by MvK mechanism on Au/TiO2 catalysts depends upon the size/dispersion of the Au cluster. Since, Au cluster facilitates CO oxidation by MvK mechanism, therefore, MvK mechanism is expected to take place together with the Langmuir-Hinshelwood mechanism on Au/TiO2 catalysts. (C) 2016 Elsevier B.V. All rights reserved.