High-temperature water gas shift reaction on Ni–Cu/CeO2 catalysts: effect of ceria nanocrystal size on carboxylate formation
Thermally stable CeO2 nanospheres of various controllable sizes were successfully synthesized via a PVP-assisted hydrothermal method to study the effect of ceria crystal size in high-temperature water gas shift reaction. The intrinsic properties of ceria crystal size effect was explored using X-ray Diffraction (XRD), Field Emission Scanning Electron Microscope (FESEM), Brunauer, Emmett and Teller Surface area (BET), X-ray Photon Spectroscopy (XPS), Carbon monoxide-Temperature Programmed Reduction-Mass Spectrometry (CO-TPR-MS), and in situ Diffuse Reflectance Infra-red Fourier Transform Spectroscopy (DRIFTS) techniques. The XRD, FESEM and BET results indicate that the ceria with the largest particle size and the smallest crystal size of 12 nm shows a high specific surface area of 50m2 g−1 after calcination at 700 °C. After impregnation, high metal dispersion (15%) and a high amount of surface lattice oxygen are observed on the Ni–Cu bimetallic catalyst supported on ceria with the largest particle size. This Ni–Cu/CeO2 catalyst presents high reaction rates with low apparent activation energy as compared to other Ni–Cu/CeO2 catalysts, revealing the important effect of ceria crystal and Ni–Cu alloy sizes. A further study shows that the high amount of carboxylate species on the 5Ni5Cu/CeO2 catalyst with the biggest ceria crystal size could be the inhibitor or the real intermediate species. In addition, the reaction mechanism strongly depends on the Ni–Cu surface composition.