Efficient Water–Gas Shift Catalysts for H2O and CO Dissociation Using Cu–Ni Step Alloy Surfaces

H2O dissociation is the rate-limiting step for the water–gas shift (WGS) reaction. Density functional theory (DFT) was used to study H2O and CO dissociation on a series of step (211) bimetallic alloy surfaces consisting of Cu and Ni metals. The water dissociation process was more facile on (211) step surfaces than on (111) flat surfaces. But CO dissociation on the (211) step surfaces was less reactive than that on its close-packed (111) counterparts. Depending on the initial-state structures, the transition states for H2O dissociation were located at different sites available on a (211) step surface. The introduction of Cu atoms on a bare Ni(211) surface (Ni-based alloy) decreased the reactivity of H2O dissociation, whereas the introduction of Ni atoms on a bare Cu(211) surface (Cu-based alloy) increased the reactivity. At a given molecular temperature, rate constant values were calculated. They showed that bare Ni(211) and Cu-based alloy surfaces exhibited a significantly higher rate constant for H2O dissociation than bare Cu(211) and Ni-based alloy surfaces. Different surface-based properties such as surface energy, work function, d-band center energy, and H adsorption energy were calculated and qualified as descriptors for H2O dissociation on the alloy surfaces. The linear relationship between the activation energy barrier and the reaction energy (Bronsted–Evans–Polanyi (BEP) relationship) held good for H2O and CO dissociation processes on all of the alloy surfaces. The transition state toward a given dissociation pathway is modified by the motion of top-layer lattice atom over which H2O gets adsorbed. The effect of surface temperature on the reactivity, which was calculated using semiclassical methods, showed that the reactivity increased with surface temperature on all of the alloy surfaces. C2 dimer formation, the first step of coke formation on any catalyst surface, was less reactive for Cu-based alloy surfaces. Overall, considering all of the aspects of H2O and CO dissociation and C2 dimer formation, Cu-based (211) step alloy surfaces showed improved performance as the WGS reaction catalyst.