Insights into the catalytic activity of trimetallic Al/Zn/Cu surfaces for the water gas shift reaction

Abstract

Abstract In this study, we evaluated the performance of Al/Zn/Cu trimetallic catalysts for the water gas shift (WGS) reaction by Density Functional Theory (DFT) calculations. A previous DFT-based study comparing the activity of a large series of trimetallic surfaces towards the catalysis of water dissociation showed that the (AlZn)s@Cu(111) surface is likely the most active catalysts for the WGS reaction. Note that, the water dissociation is the rate-determining step of the WGS reaction on copper surfaces. Therefore, in this work we carried out a systematic study of all possible WGS reaction steps on such catalyst model surface. The most plausible WGS reaction mechanism on the trimetallic surface was inferred by comparing the activation energies, reaction energies and rate constants computed for its different reaction steps. The latter demonstrated that the WGS evolves on this trimetallic surface following an associative mechanism through the carboxyl intermediary, which is dehydrogenated on the surface, assisted by a hydroxyl, to produce CO2. The other WGS reaction product, this is H2, is obtained by the combination of hydrogen atoms from the water dissociation. The activation energy barriers obtained for the WGS reaction steps on that trimetallic surface are always lower than the adsorption energy of the correspondent reactants, indicating that desorption cannot compete with the catalytic process and also, that the trimetallic Al/Zn/Cu surface should be very reactive for the WGS reaction catalysis. Overall the results of this study allowed us to suggest that the active phase of commonly employed commercial catalyst based on Cu/ZnO/Al2O3 might embody a trimetallic alloy of Al/Zn/Cu.