Steam reforming of logistic fuels to produce hydrogen for fuel cells: Development of sulfur tolerant catalyst.

摘要

Hydrogen generation from logistic fuels such as diesel, gasoline and jet fuel is a better alternative to natural gas reforming because of higher hydrogen density of the fuel, making this opportunity appropriate for onboard hydrogen generation in vehicles. However, logistic fuel reforming leads to the deactivation of reforming catalyst due to coking, as well as sulfur poisoning. A systematic study of catalyst deactivation was required for the development of a coke resistant and sulfur tolerant catalyst for reforming of logistic fuels.; In the first part of the research, we tested the performance of a Pd/ZrO2 catalyst coated on a metal foil at various steam to carbon ratios (S/C), temperatures (T) and sulfur content (S) of the fuel were investigated for the steam reforming of n-hexadecane using a tubular reactor. Here n-hexadecane is used as the surrogate for diesel fuel. A decrease in time dependent hydrogen yield indicated catalyst deactivation, with the effects of reaction conditions on the rate of deactivation determined through a statistically designed experiment. A first order kinetic model, with first order deactivation rate, was used to obtain best fit values for the reaction rate constant (k 0) and the deactivation rate constant (kd) as a function of S/C, temperature and sulfur loading. Palladium-catalyzed steam reforming of n-hexadecane was adequately modeled using first order kinetics, the reaction rate was enhanced by increased temperatures and S/C ratios, but was decreased by the presence of sulfur. Catalyst deactivation was more rapid in the presence of sulfur, at low S/C ratios, and at lower temperatures.; In the second part of the research, metal oxide additives (CuO and ZnO) were used as modifiers to Pd/ZrO2 catalyst. Different catalyst formulations were tested and catalyst performance was again evaluated using ko and kd. It was found that, 5% CuO and 2.5% ZnO formulation provided a more stable catalyst under the selected reaction conditions; however, this catalyst had lower activity than the Pd/ZrO2 catalyst for steam reforming of n-hexadecane.; It was required to find an improved catalyst material which is active for steam reforming of n-hexadecane and stable in the presence of sulfur containing compounds. In the third part of the research different powered catalysts were prepared using impregnation method and tested for sulfur tolerance and coke resistance. It was found that 0.5%Rh-10%Ni/Al2O3 catalyst formulation was sulfur tolerant for 150 hrs at S/C=5, T=800°C and S=25ppm. In order to understand the catalyst deactivation mechanism, the used catalysts were characterized using TEM, XRD, ICP, TPR, TPO and TPD using ammonia. It was found that sintering, coke formation and sulfur absorption are the main mechanisms of deactivation.