Critical effect of carbon vacancies on the reverse water gas shift reaction over vanadium carbide catalysts

Experimental and theoretical evidences show that carbon vacancies determine the catalytic behavior of vanadium carbides in the CO2 conversion to CO via the Reverse Water Gas Shift (RWGS) reaction. Two VCx samples, one mostly containing stoichiometric VC and the other being C deficient, mainly V8C7, were synthesized, characterized, and studied. The samples show different CO2 adsorption heats, which correlate with those calculated using Density Functional Theory (DFT) on suitable models. The sample containing more V8C7 shows a higher CO2 conversion and CO selectivity and a lower apparent activation energy, being a stable catalyst over long-time tests. DFT calculations confirm that C vacancies in V8C7 are responsible for the observed catalytic behavior, allowing reactants to adsorb more strongly and lowering the energy barrier for both H-2 and CO2 dissociation steps. The present work highlights the importance of such native point defects in the transition metal carbides surface chemistry and catalytic properties.