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 KNiCa catalyst has also been compared to that of the supported Ni catalyst. Long-time catalytic measurements at 800°C show that the supported KNiCa catalyst has excellent catalyst stability for 140 h due to the promotional effect of surface carbonate species leading to surface enrichment of carbon dioxide, while the supported Ni catalyst is subjected to severe catalyst 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 that methane is activated on the surface Ni species and carbon dioxide interacts with alkaline promoters to form surface carbonates which hinder the formation of inactive coke or scavenge carbon from the surface Ni species. A study of deuterium isotope effects for the reforming reaction shows that there is almost no isotope effect on the supported KNiCa catalyst, suggesting that a CH4 dissociation step is not rate determining. In this work, mechanistic investgations reveal that reaction between the adsorbed carbon species and the dissociated oxygen atoms on Ni sites of catalyst surface leads to the production of carbon monoxide and the regeneration of metallic nickel species as a consequence, which is assumed to be a rate-determining step in the CO2 reforming. It is also proposed that the oxidation step of surface carbon with surface oxygen or adsorbed CO2 as surface carbonate species on the catalyst is important for maintaining catalyst stability of the supported KNiCa by the efficient removal of surface carbon species.