Carbonate-mediated Mars–van Krevelen mechanism for CO oxidation on cobalt-doped ceria catalysts: facet-dependence and coordination-dependence
Carbonate intermediates have been reported to play an active role in CO oxidation over ceria-based catalysts in recent experimental studies. However, the detailed CO oxidation mechanism involving carbonate intermediates over ceria-based catalysts remains obscure. In this work, we carried out systematic density functional theory calculations corrected by on-site Coulomb interactions (DFT+U) to investigate the complete CO oxidation mechanism involving carbonate intermediates over cobalt-doped CeO2 catalysts, aiming to unravel how the carbonate participates in CO oxidation and shed light on the underlying factors that control the carbonate-mediated reaction mechanism. A novel carbonate-mediated Mars–van Krevelen (M–vK) mechanism was proposed, in which the carbonate acts as an active intermediate rather than a spectator and can react with CO to form CO2. This carbonate-mediated M–vK mechanism is facet-dependent because it is predominant on the (110) surface whereas the conventional M–vK mechanism is more favorable on (111) and (100) surfaces. The origin of facet-dependence was discussed by analyzing the geometric and electronic structures. It is found that the negatively charged bent CO2− intermediate formed on the (110) surface plays a critical role in the carbonate-mediated M–vK mechanism, whereas the formation of a neutral linear CO2 intermediate on (111) and (100) surfaces hinders the carbonate-mediated M–vK mechanism. The surface oxygen vacancy hinders the formation of carbonate intermediates, indicating that the carbonate-mediated M–vK mechanism is also vacancy-dependent. The formation of carbonate intermediates on different metal (Ti, V, W, Mo and Re) doped CeO2(110) surfaces was studied and the results indicate that the coordination environment of the dopant species is a key factor that determines the carbonate-mediated M–vK mechanism. This study provides atomic-scale insights into the reaction mechanism involving carbonate intermediates and the structure–mechanism relationship for CO oxidation over cobalt–ceria catalysts.