Low-temperature steam reforming of jet fuel in the absence and presence of sulfur over Rh and Rh-Ni catalysts for fuel cells

This work is a comparative study on low-temperature steam reforming of jet fuel over Rh and Rh-Ni loaded on CeO₂-modified Al₂O ₃ support in the absence and presence of different amounts of organic sulfur. Rh loaded on CeO₂-Al₂O₃ support can promote reforming of sulfur-free or desulfurized jet fuel at <520 °C with >97% conversion to syngas and CH₄. However, monometallic Rh/CeO₂-Al₂O₃ catalyst deactivates by S poisoning. During the reforming of liquid fuel with >10 ppmw S, catalytic activity rapidly decreases when the amount of sulfur in the fuel flown over the catalyst reaches the level corresponding to a S _fuel:Rh_surf atomic ratio of 0.28-0.30 (for which the amount of surface Rh is based on H₂ and CO pulse chemisorption analysis). Methane formation is even more sensitive (than conversion) to sulfur poisoning. At a S_fuel:Rh_surf ratio of 0.15, methane selectivity over the Rh/CeO₂-Al₂O₃ catalyst begins to decline. Addition of Ni by co-impregnation into the Rh/CeO ₂-Al₂O₃ catalyst leads to much higher sulfur tolerance. Ni acts as a protective and sacrificial metal for Rh in the Rh-Ni/CeO₂-Al₂O₃ catalyst. Ni surface saturation of sulfur was found to occur at a S_fuel:Ni_surf ratio of 0.59-0.60, corresponding to a S_fuel:Rh_surf ratio of 1.1 for 2% Rh-10% Ni/CeO₂-Al₂O₃. The bimetallic Rh-Ni/CeO₂-Al₂O₃ catalyst allows for successful low-temperature reforming of a JP-8 jet fuel containing 22 ppm sulfur for 72 h with >95% conversion. TPR and XPS analysis reveals close Rh-Ni metal-metal interactions. The presence of Ni increases the temperature for Rh reduction in TPR, whereas Rh helps maintain Ni in a reduced state in an oxidative atmosphere.