A theoretical study of surface-structural sensitivity of the reverse water-gas shift reaction over Cu(hkl) surfaces

The surface-structural sensitivity of the reverse water-gas shift (RWGS) reaction (CO2 + H2 → CO + H2O) over the Cu(1 1 1), Cu(1 0 0), and Cu(1 1 0) surfaces has been studied by first-principle density functional calculations together with the UBI-QEP approach. Cluster models of the surface have been employed to simulate the adsorption of CO2, H2, H, O, OH, CO, and H2O on the Cu(hkl) surfaces at low coverage. This sensitivity is determined by the difference in the activation barriers. It can be noticed that the most likely rate-determining step in RWGS reaction is the CO2 dissociative adsorption, namely CO2,g → COs + Os. The trend in the calculated activation barriers for the reaction of CO2 dissociative adsorption follows the order of Cu(1 1 0) < Cu(1 0 0) < Cu(1 1 1), suggesting that the most efficient crystal surface for catalyzing RWGS reaction by copper is Cu(1 1 0), and the more densely packed Cu(1 1 1) surface is the least active among the Cu(hkl) surfaces studied here. As expected, the activation barriers for the recombinative reactions over Cu(hkl) are in the order of Cu(1 1 0) > Cu(1 0 0) > Cu(1 1 1), just opposite to the dissociative reactions. The interesting thing is that there is a good correlation between the adsorption bond length and the adsorption energy: The preferred adsorption site is the one with the shortest adsorption bond length. The present calculations are in good agreement with experimental observations.