Effects of carbon vacancies on the CO oxidation on Cu double atom catalyst supported by graphene

Abstract

The activity of double metal atoms is closely related to their chemical coordination environments. In this work, CO oxidation on Cu double atom catalyst loaded on defective graphene with triple carbon vacancy (Cu2-TG) and quadruple carbon vacancy (Cu2-QG) has been comprehensively studied through DFT calculations. Empty Cu-3d orbital is created through charge transfer from Cu atom to carbon atom of graphene, which is parallel to the basal plane of graphene for one Cu atom and nearly perpendicular to the basal plane for the other Cu atom in Cu2-TG. While the empty Cu-3d orbital is parallel to the basal plane for both Cu atoms in Cu2-QG. The delocalized orientation of empty Cu-3d orbital in Cu2-TG makes it more convenient to accept electrons and more active. This is verified through the larger adsorption energies of -1.74 eV and -1.68 eV for O2 and CO molecules on Cu2-TG, as well as the smaller values of -0.20 eV and -0.40 eV on Cu2-QG, respectively. Because of the weak interactions between Cu2-QG and gas molecules, only Cu2-TG is further considered to catalyze the CO oxidation. The maximum barrier of CO oxidation along ER path is 1.75 eV on Cu2-TG, while that is only 0.39 eV along LH path, which indicates a smooth reaction progress on Cu2-TG. The electronic structures show that the Cu atom can accept electrons through its empty 3d orbital and lose electrons through its filled 3d orbital with an “acceptance and feedback” mechanism. Cu2-TG can move the 5σCO orbital downshift significantly, which causes the 5σCO/1πCO orbitals very close to 5σO2/1πO2 orbitals and facilitates the formation of OC-OO bond along LH path. These results indicate that the synergistic effects between the two Cu atoms and the triple carbon vacancy make Cu2-TG to be an efficient CO oxidation catalyst.