Effect of Nickel Diameter on the Rates of Elementary Steps Involved in CO2 Reforming of CH4 Over Ni/Al2O3 Catalysts

The effect of Ni diameter on the rates of individual steps involved in CH4-CO2 reforming was examined on Ni/Al2O3 catalysts in the diameter range of ca. 4-22 nm. It was revealed that CO2 dissociation to give CO and O-ads was improved by decreasing the diameter. This may result in increasing the number of adsorbed oxygen species per active site of catalyst, which may consequently enhance the oxidation of CHx,ads to CHxOads. Furthermore subsequent rate-determining step, CHxOads -> CO + x/2H(2), was also promoted by decreasing the diameter. The enhancement of these elementary steps was considered a cause for the increase of turnover frequency with decreasing nickel diameter. Gasification of deposited carbons by CO2, i.e., the reverse Boudouard reaction, was also improved by decreasing the size of nickel. Although suppression of coking on fine nickel may primarily be due to the retardation of graphite nucleation, improvement of the reverse Boudouard reaction may also contribute to the carbon-free reforming to a certain degree. [GRAPHICS] .