Water-Gas Shift Reaction on Ni–W–Ce Catalysts: Catalytic Activity and Structural Characterization

Abstract

The water-gas shift reaction (WGS, CO + H2O → H2 + CO2) was studied over a series of W–Ce, Ni–Ce, and Ni–W–Ce mixed-metal oxide catalysts. The structure of the catalysts and the WGS reaction intermediates were characterized using in situ techniques including X-ray diffraction (XRD), X-ray absorption near edge structure (XANES), scanning transmission electron microscopy (STEM), and diffuse reflectance infrared Fourier-transform spectroscopy (DRIFTS). XANES showed the existence of Ni2+ and W6+ inside the ceria lattices. The coexistence of Ni and W inside of ceria led to a large lattice strain, not seen for Ni–Ce and W–Ce, that facilitated the reduction of Ni–W–Ce and gave this oxide special catalytic properties. A Ni0.2W0.1Ce0.7O2 catalyst displayed the highest catalytic activity among all the mixed oxides, followed by a Ni0.2W0.2Ce0.6O2 catalyst. Besides high activity, the Ni–W–Ce catalysts also displayed the effective suppression of the methanation reaction (CO + 3H2 → CH4 + H2O) under WGS conditions compared to W-free Ni–Ce catalysts. The introduction of W in the lattice of Ni–Ce favored the formation of O vacancies that facilitated the dissociation of water, preventing the dissociation of CO and the formation of methane. Because of the special chemical properties of Ni–W–Ce, monodentate formates and carbonates, which could be chemically active species for the WGS reaction, appear on the surface of these catalysts. Synergistic interactions between Ni and W give Ni–W–Ce unique structural and chemical properties not seen for W–Ce or Ni–Ce mixed-metal oxides.