(192b) Enhanced Sulfur Tolerance of Ni-Rh Alloys for Reforming and Methanation Reactions
The catalyst for hydrocarbon reforming is rapidly deactivated by sulfur compounds present in hydrocarbon fuels. The decreased activity of the catalyst induces the formation of higher olefins, paraffins and carbon deposits, which prevents the catalyst from reforming higher hydrocarbon sources. Among catalysts for hydrocarbon reforming, noble metals such Rh and Ru are known to minimize carbon formation because of their poor carbon solubility. Though Rh is an active catalyst for hydrocarbon reforming, it is deactivated quickly by sulfur compounds. The addition of Ni improves the sulfur tolerance of Rh catalysts. However, the mechanism that accounts for such an improvement is not clear. In this study, we use first principle methods to evaluate the formation and sulfur tolerance of Ni-Rh alloys. Results will be compared with experimental kinetic and characterization studies.
The overall hydrocarbon reforming reaction encompasses several reactions leading to H2, CH4, CO, CO2, and other products. The methane formation reaction is most sensitive to sulfur poisoning. Therefore, investigation of methane formation is initially chosen to elucidate the improved tolerance against sulfur poisoning with Ni addition to Rh. There are two routes for methane formation in hydrocarbon reforming: the methanation of the CO product by the H2 product and through C-C bond cleavage of higher hydrocarbons. The rate determining step for the first route is the dissociation reaction of CO on the catalyst surface. For two reasons, we have to look into the CO dissociation reaction in the presence of sulfur on the catalyst surface to understand the effect. The experimental XPS and TPR results suggest an intimate Ni-Rh interaction and EXAFS results indicate alloying of Ni and Rh in catalyst nanoparticles. Density functional theory (DFT) was used to model the (111) surface of Ni-Rh alloys at varying composition. Ni-Rh surfaces show weakened binding of S compared to the component pure Rh. Sulfur has less of an impact on the CO adsorption energy and dissociation barrier on Ni-Rh alloys, providing for a CO dissociation rate three orders of magnitude greater in the presence of sulfur over the alloys compared to the pure metals.