Catalytic behavior of supported KNiCa catalyst and mechanistic consideration for carbon dioxide reforming of methane

Carbon dioxide reforming of methane to synthesis gas has been investigated using a KNiCa catalyst loaded on a highly siliceous NaZSM-5 zeolite support which was promoted with alumina. The catalytic behavior of the supported K NiCa catalyst has also been compared to that of the supported Ni catalyst. Long-time catalytic measurements at 800 degrees C show that the supported K NiCa catalyst has excellent catalyst stability for 140 h due to the promoti onal effect of surface carbonate species leading to surface enrichment of c arbon dioxide, while the supported Ni catalyst is subjected to severe catal yst deactivation due to extensive coke deposition less than 40 h on stream. Pulse reaction, thermogravimetric analysis, isotope experiment, and X-ray absorption spectroscopy have been performed for understanding the detailed chemistry and the mechanistic aspects of the CO2 reforming. Pulse reaction and thermogravimetric analysis on the supported KNiCa catalyst indicate tha t methane is activated on the surface Ni species and carbon dioxide interac ts with alkaline promoters to form surface carbonates which hinder the form ation of inactive coke or scavenge carbon from the surface Ni species. A st udy of deuterium isotope effects for the reforming reaction shows that ther e is almost no isotope effect on the supported KNiCa catalyst, suggesting t hat a CH4 dissociation step is not rate determining. In this work, mechanis tic investgations reveal that reaction between the adsorbed carbon species and the dissociated oxygen atoms on Ni sites of catalyst surface leads to t he production of carbon monoxide and the regeneration of metallic nickel sp ecies as a consequence, which is assumed to be a rate-determining step in t he CO2 reforming. It is also proposed that the oxidation step of surface ca rbon with surface oxygen or adsorbed CO2 as surface carbonate species on th e catalyst is important for maintaining catalyst stability of the supported KNiCa by the efficient removal of surface carbon species. (C) 2000 Academi c Press.