Dual site contiguity study for CO2 catalytic activation and CO2 reforming of CH4 over Ni and NiM2 catalysts with MgO-spinel support

Abstract

In catalytic CO2 reforming of CH4 reaction, CO2 is activated by the basic sites and CH4 by the metallic sites on the catalysts. The contiguity of the dual sites, i.e., how the two kinds of sites neighbor to each other and how the adsorption species form and interact with each other, determines the activity, selectivity, and stability of the reaction. Following our earlier studies, catalysts with different ratios of Mg/Al and the different second metal (Co, Cu, Fe, or Mn) to the primary metal Ni were prepared using the coprecipitation method. Extensive characterizations showed the catalysts had basic sites of various basicity strength and metallic particles of different sizes. The pulse adsorption experiments showed that the ability of CO2 activation on the basic sites was varying with not only the site basicity itself (pushing effect) but also the pulling effect from the neighbor metallic sites. On the other hand, the ability of CH4 dissociation on the metallic site was determined by the metallic sites themselves and the basic sites that burned the carbon and made the metallic sites recovered. The correlation between the turnover frequency of CO2 and CH4 based on the number of metallic sites indicated that the size of metallic particles was the main factor that determined the catalyst performance once the two kinds of sites were made neighbor to each other with proper Mg/Al ratio. Studies with the catalysts reduced at a higher temperature confirmed these observations. Further analysis of the catalyst deactivation revealed that the decrease in H2/CO ratio (selectivity) in the CO2 reforming of CH4 may be caused by the C–Hx species from the incomplete dissociation of CH4 on the weakened metallic sites and their interaction with the activated CO2 on the basic sites.