Intrinsic Kinetics of the Internal Steam Reforming of CH₄over a Ni−YSZ−Cermet Catalyst−Electrode

The kinetics of steam reforming of methane were studied on a Ni−ZrO 2 (Y 2 O 3 )−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 (in the form of active carbon species, C ad ) and H 2 O (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 CH 4 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 H 2 O, 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 CH 4 and P H 2 O values as well as the tendency of the reaction system to generate graphitic carbon on the catalytic surface. Regarding CO 2 formation rate, the kinetic results were explained by considering as the rate-determining step the surface reaction of the adsorbed CO ad with adsorbed oxygen species.