Catalytic Combustion Of Methane On Structural Catalysts Supported On Metal Foams

H₂-O2proton-exchange membrane fuel cells （PEMFCs） have drawn wide interests due tothe high efficiency and low emissions. The hydrogen needed by PEMFCs can be producedfrom a variety of feedstocks, including oxygenates from biomass, through steam reforming（SR）. However, the systematic evaluation on the performance of SR-PEMFC system withdifferent oxygenate feedstocks is rarely reported. One of the goals of this study is to comparethe performances of SR-PEMFC systems with different oxygenate feedstocks on the aspectsof hydrogen production and overall energy utilization efficiency. By a thermodynamicanalysis of a SR-PEMFC system, ten oxygenated fuels were investigated as feedstocks ofhydrogen production. Among the fuels studied, methanol offers the widest operational regimeand the highest energy efficiency. Polyols, represented by ethylene glycol and glycerol, alsopossess high energy efficiencies. Narrow operating windows and low energy efficiencies werediscovered for glucose and acetic acid. In general, heavy fuels with high oxygenation degreeare inappropriate in SR process for hydrogen production. The results would offer a guidelinetoward a rational selection of raw materials of a renewable SR-PEMFC system based onunderstanding the features of oxygenate fuels in SR.To achieve such a renewable SR-PEMFC system, a micro-reformer integrated with acombustion heat-exchanger was designed. The laminated heat exchanger was used to capturethe combustion heat of by-products in reformate, mainly methane, CO and unreacted H₂, toevaporate the reactants and steam. Pd catalysts supported by structural metal foams wereprepared to catalyze the combustion reactions. Using Al₂O₃collosol as binder, Pd/Al₂O₃catalysts were coated on copper and nickel foams through impregnation of Pd precursor. Cucould diffuse from foam skeleton to binder layer, which poisons the Pd catalysts and reducesthe activity of catalytic combustion of methane, because the concentration of Pd on catalystsurface was diluted and the oxidation degree of Pd was enhanced due to penetration of Cu.However, an improved activity was observed over the Ni foam supported Pd/Al₂O₃catalyst. The methane conversion at450oC increased by about2folds over Ni foam supportedPd/Al₂O₃compared with its counterpart in powder form. The structural catalyst was stable inmethane combustion for60h at a space velocity of30000ml·gcat^-1·h^-1. The high activities forcatalytic combustion of hydrogen and carbon monoxide was also proved over Ni foamsupported catalysts. The enhanced activity could be attributed to the doping of Ni from foamskeletons via diffusion, which increased the concentration of Pd on surfaces and benefited tothe crystallizaiton of PdO.