Structure Sensitivity of CO Oxidation on Co3O4: A DFT Study

The reaction mechanism of CO oxidation on the Co3O4 (110) and Co3O4 (111) surfaces is investigated by means of spin-polarized density functional theory (DFT) within the GGA+U framework. Adsorption situation and complete reaction cycles for CO oxidation are clarified. The results indicate that 1) the U value can affect the calculated energetic result significantly, not only the absolute adsorption energy but also the trend in adsorption energy; 2) CO can directly react with surface lattice oxygen atoms (O-2f/O-3f) to form CO2 via the Marsvan Krevelen reaction mechanism on both (110)-B and (111)-B; 3) pre-adsorbed molecular O-2 can enhance CO oxidation through the channel in which it directly reacts with molecular CO to form CO2 [O-2(a)+CO(g) CO2(g)+O(a)] on (110)-A/(111)-A; 4) CO oxidation is a structure-sensitive reaction, and the activation energy of CO oxidation follows the order of Co3O4 (111)-A(0.78 eV)>Co3O4 (111)-B (0.68 eV)>Co3O4 (110)-A (0.51 eV)>Co3O4 (110)-B (0.41 eV), that is, the (110) surface shows higher reactivity for CO oxidation than the (111) surface; 5) in addition to the O-2f, it was also found that Co3+ is more active than Co2+, so both O-2f and Co3+ control the catalytic activity of CO oxidation on Co3O4, as opposed to a previous DFT study which concluded that either Co3+ or O-2f is the active site.