Effect of Au Doping of Ni/Al2O3 Catalysts Used in Steam Reforming of Methane: Mechanism, Apparent Activation Energy, and Compensation Effect

Investigation was made of the effect of addition of Au to 15%Ni/Al2O3 catalysts used in the steam reforming of methane. The addition of Au was performed by impregnation of reduced Ni catalysts. The catalysts were characterized by X-ray diffraction (XRD) and diffuse reflectance infrared Fourier transform spectroscopy of adsorbed CO (DRIFTS-CO). DRIFTS-CO showed that Au modified the surface structure of Ni. Au blocked the low coordination Ni sites with high electron density and high activity for CH4 activation. Furthermore, it modified the high coordination Ni, causing geometric and electronic effects, changing the CO adsorption from bridge to linear form, and increasing the electron density of Ni-Au, relative to Ni at sites such as Ni(1 1 1). Addition of Au to the Ni catalysts had a strong influence on the activation of CH4. A compensation effect was observed between the apparent activation energy (Eaapp) and the natural logarithm of the pre-exponential factor (ln(k0)), obtained by fitting of the Arrhenius equation to the rate of the overall reaction. Greater Au addition decreased the catalytic activity, with linear increase of Eaapp with increase of ln(k0). These results suggested that the intermediates formed by decomposition of CH4, in the step determining the overall velocity, presented low interaction with the Ni surface. The proposed mechanisms also suggested that the activation of CH4 required two Ni neighbor sites.