Theoretical and experimental exploration of NiM(111) (M = Fe, Co, Cu, Zn) bimetallic catalysts for the water-gas shift reaction

Abstract

Ni-based bimetallic catalysts have been widely used in the water-gas shift (WGS) reaction, but the synergistic effect and reaction mechanism over bimetallic catalysts are rather limited. In this work, the WGS reaction mechanism and the main side-reaction (coke formation) over the NiM catalysts (NiFe(111), NiCo(111), NiCu(111) and NiZn(111) surfaces) are investigated via the density functional theory (DFT) approach. The results show that a second metal element (Fe, Co, Cu and Zn) leads to the dispersion of active Ni atoms on the NiM surfaces and M sites can also act as active sites, which increase the energy barriers for coke formation. Binary linear regression of site size and electron factors to adsorption energy of H₂O and CO reveals that the ligand effect plays a more important role in determining adsorption strength than the strain effect on NiFe(111) and NiCo(111), while for NiCu(111) and NiZn(111) surfaces, the adsorption energies are strongly affected by strain rather than ligand effects. The WGS mechanism is further calculated over NiCo (111) and NiCu(111), which present both less carbon deposition and better H₂O dissociation ability than the others. It is illustrated that on NiCo(111), the redox path is the most favorable, and the CO association with O is the rate determining step while on NiCu(111), the dominant pathway is the carboxyl path with the association of CO and OH to form COOH as the rate determining step. In addition, experimental studies verify that the NiCo and NiCu samples exhibit the optimum catalytic activity and resistance against carbon deposition, which accords well with the theoretical prediction. This study provides guidance for the rational design of Ni-based bimetallic catalysts for the WGS reaction based on the chemical nature and electronic environment of the doped metal.