Intrinsic kinetics of the internal steam reforming of CH4 over a Ni-YSZ-cermet catalyst-electrode

The kinetics of steam reforming of methane were studied on a Ni-ZrO2(Y2O3)-cermet film, at temperatures of 1073-1173 K and methane and steam partial pressures of up to 60 and 5 kPa, respectively. It was found that the reaction exhibits Langmuir-Hinselwood kinetic behavior, corresponding to competitive adsorption of methane tin the form of active carbon species, C-ad) and H2O (in the form of adsorbed oxygen species, O-ad) on the catalytic surface. Kinetic results concerning methane consumption were explained in the frame of a mechanistic scheme involving two rate-limiting steps. These steps correspond (i) to the activated adsorption of CH4 for the production of active carbon species, C-ad, and (ii) to the surface reaction of the adsorbed C-ad with the O-ad species, originating from the adsorption of H2O, for the production of CO. The relative magnitude of the turnover frequencies of these two rate-limiting steps affects considerably the apparent activation energy of the reaction at different P-CH4 and P-H2O values as well as the tendency of the reaction system to generate graphitic carbon on the catalytic surface. Regarding CO2 formation rate, the kinetic results were explained by considering as the rate-determining step the surface reaction of the adsorbed COad with adsorbed oxygen species.