Water Adsorption and Dissociation on Copper/Nickel Bimetallic Surface Alloys: Effect of Surface Temperature on Reactivity

Water dissociation is the rate-determining step (RDS) in the industrially important water gas shift (WGS) reaction. Low temperature Cu catalysts are limited by a higher barrier to dissociation whereas Ni surfaces with lower barriers for this reaction are deactivated by carbon deposition due to CO dissociation. Density functional theory (DFT) calculations are performed on a series of overlayer and subsurface bimetallics starting with Ni(111) and Cu(111) to understand the synergistic catalytic activity of Cu/Ni bimetallics toward H2O dissociation which is the RDS. Surface parameters like surface energy, work function and density of states were calculated and were correlated with the change in reactivity. Transition state (TS) calculations showed that addition of Ni to Cu(111) surfaces decreased dissociation barriers while the scenario is reversed when Cu atoms replace Ni in Ni(111) surface with no linear relation with any calculated surface properties in both cases. Linear relations were found to correlate well the reaction energies with the activation energy barriers. Effects of surface temperature were included by determining the change in the barrier heights and barrier locations with lattice atom motion calculated from TS calculations. Dissociation probabilities calculated at different surface temperatures using semiclassical methods showed that increase in surface temperature increases dissociation probabilities where the extent of increase is strongly dependent on the change in barrier heights. Overall, Ni addition to Cu(111) surface proved beneficial while the Cu addition to Ni(111) surface proved detrimental to H2O dissociation.