Preparation, Characterization And Performance Of Sulfur Tolerance Pt/Ce_0.8Gd_0.2O_1.9 Catalyst For Steam Reforming Of High Energy Density Hydrocarbon Fuels

Fuel cells have been emerged as promising devices for clean and efficient generation of power for global energy needs. Proton-exchanged membrane fuel cell (PEMFC) is a preferred fuel cell technology for many transportation applications. Until a widespread hydrogen refueling infrastructure exists, on-board reformation technologies are needed to convert high energy density commercial grade liquid hydrocarbon fuels into hydrogen for PEMFC. A significant challenge conflicting these efforts is the development of high efficiency fuel processing catalysts that have to be active, selective, durable, and tolerant to sulfur and coke formation.To accomplish this goal, a high sulfur tolerant Pt catalyst has been developed for fuel processor that is being developed for use with PEMFC, using a fluorite-type Ce_0.8Gd_0.2O_1.9 (CGO) oxide as support. Sulfur tolerance of the catalyst was tested in steam reforming of iso-octane at 750 °C and a WHSV (iso-octane) of 1.0 h^-1 with a H₂O/C molar ratio of 3, using thiophene as sulfur source.It was found that catalyst calcination temperature was crucial to ensure the maintenance of the sulfur tolerance. The catalyst that was calcined at 800 °C maintained its activity and selectivity for the entire 100 h test using iso-octane with 300 ug/g or more sulfur, whereas the catalyst that was calcined at 600 °C lost its activity slowly in this course due to both sulfur poisoning and its poor thermal stability.It was also found that the CGO oxide preparation method also influenced sulfur tolerance of the catalysts. The catalyst whose support was synthesized by citric acid sol-gel method exhibited the best sulfur tolerance. Compared with that, the catalyst whose support was synthesized by oxalic acid coprecipitation method or hexamethylenetetramine coprecipitation method exhibited a little worse sulfur tolerance, because of both poorer thermal stability and weaker interaction effect between Pt and CGO oxide at the interface.Temperature-Programmed reduction (H2-TPR) and X-ray diffraction (XRD) analyses, together with comparison of the activity of Pt/CGO with Ni/CGO and Pt/Al₂O₃, showed that a strong interaction effect between Pt and CGO oxide at the interface existed while being enhanced and consolidated with the calcination treatment at 800 °C, which made Pt immune to sulfur poison and active to convert thiophene sulfur into H2S. The results on in-situ diffuse reflectance infrared Fourier transform spectroscopy (in-situ DRIFTS) of CO adsorption showed that a strong electronic interaction effect between Pt and CGO oxide at the interface existed, which gave rise to an unique electron-deficient site on Pt surface when the Pt/CGO catalyst was calcined at 800°C. It was the other reason that made Pt immune to sulfur.In addition, sulfur balance measurements together with diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and microcoulomb titration for surface sulfur-containing species on the used catalyst suggested that thiophene sulfur was completely converted into H₂S during steam reforming of iso-octane, probably complying with a redox mechanism.