Insights into the deactivation mechanism of metal carbide catalysts for dry reforming of methane via comparison of nickel-modified molybdenum and tungsten carbides†
For the first time, Ni-modified Mo2C and WC catalysts with various Ni/M (M = Mo and W) molar ratios were compared for dry reforming of CH4 with CO2 (DRM). Although Ni–Mo2C and Ni–WC catalysts with high Ni/M molar ratios (1/2 and 1/9) showed similar catalytic performance during the test period, the Ni/M molar ratio (1/21) catalysts deactivated rapidly after 5 h, whereas the former remained active and stable over a 12 h test period. The results of X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) indicated that bulk oxidation of Mo2C to MoO2 may be the main reason for deactivation of Ni–Mo2C (1/21), as had been recognized by most researchers. However, Ni–WC (1/21) remained resistant to oxidation. It was found that Ni–W oxide could be more easily carburized than Ni–Mo oxide, which might explain the reason why Ni–WC (1/21) catalyst had much better ability to keep control of the crystal structure than the Ni–Mo2C (1/21) catalyst in the DRM reaction. It can be noted that TEM images and XRD data indicated that severe sintering of Ni–Mo2C particles occurred during the high-temperature DRM reaction. As for Ni–WC particles, there was no obvious size change after reaction, which could also contribute for maintaining their stable catalytic performance. Based on the results, particle sintering was hypothesized as a reason for carbide catalyst deactivation in the DRM reaction.