Methanol Steam Reforming And CO Preferential Oxidation: Monolithic Microfibrous Entrappment Of Pd-ZnO/Al₂O₃ And Pt-Co/Al₂O₃ Catalysts

Polymer electrolyte membrane fuel cells (PEMFCs) have received great attention as an attractive alternative of the primary and secondary batteries especially for portable and some aeronautical applications. However, effective and efficient solutions are urgently required to overcome the difficult problems associated with hydrogen and distribution. One promising way is to use microreactor to liberate the hydrogen on demand from higher energy density H-containing compounds but this still remains significant challenge. Methanol steam reforming (MSR) is considered to be the most favorable process of hydrogen production process due to the ability to produce gas with high hydrogen concentration (～75%). One of the strategies in this effort is to develop novel microreaction technology to meet the fundamental criteria needed for a miniature fuel cell power system, meanwhile, to avoid complex combination of multiple processes for required low CO levels. The goal of this study was to demonstrate our microfibrous structured catalytic packing technology toward miniature methanol fuel processor and CO cleanup for portable fuel cell supplies. A thin-sheet nickel microfibrous entrapped Pd-ZnO/Al2O3 catalyst packing, with low Pd-loading of 3 wt%(without including the mass of nickel fiber), was developed for high efficiency methanol steam reforming. The microfibrous entrapped Pd-ZnO/Al2O3 catalyst composites were studied with respect to the catalytically relevant physiochemical properties. A prototype fuel processor system integrating MSR reformer and CO cleanup train based on such microfibrous structured catalytic packings was designed and demonstrated in a longer-term test.High-performance Pd-ZnO/Al2O3 catalysts with low Pd loading (e.g.,3 wt%; a third that literature catalyst) has been developed for MSR by carefully optimizing Pd/Zn ratio. For example, formation of PdZn alloy was observed definitely within H2 atmosphere at 450℃on the catalyst with Pd-loadings of 3 wt% only at Pd/Zn ratio of 0.1 thereby leading to significant reduction of the CO formation while retaining high methanol conversion. A Pt-Co/Al2O3 (Pt:1 wt%; Pt/Co:50) catalyst was prepared for CO PROX according to the literature. A highly void and tailorable sinter-locked microfibrous carrier consisting of 3.5 vol% 8μm diameter Ni fibers is used to entrap 35 vol% 150-250μm catalyst particulates of Pd-ZnO/Al2O3 (Pd:5 wt%; Pd/Zn:0.2) for MSR and Pt-Co/Al2O3 for CO PROX. The microfibrous entrapment provided unique combination of large void volume, high surface area, entirely open structure, high heat/mass transfer, high permeability, high thermal conductivity, and unique form factors. By taking these beneficial properties, the MSR catalyst utilization efficiency is significantly enhanced by a 4-fold improvement of the weight hourly space velocity (WHSV), compared to the single Pd-ZnO/Al2O3 particulates as keeping the methanol conversion at >98%. The microfibrous entrapped Pt-Co/Al2O3 catalyst packings can drive the CO from 2% down to <50 ppm at 150℃with O2/CO ratio of 1 using a GHSV of 32000 h-1.The microfibrous structured catalytic packings for miniature methanol fuel processor consisting of a methanol steam reformer and a subsequent CO cleanup train has been investigated experimentally. This test rig is capable of producing roughly 1700 seem PEMFC-grade H2 (equivalent to～163 W of electric power) in a longer-term test. In this test, the MSR reactor delivers >97% methanol conversion throughout the entire 1200 h test; the CO cleanup train placed in line after 800 h MSR illustrates the capability to decrease the CO concentration from～3.5% to～1% at PROX-1 reactor and then to less than 20 ppm at PROX-2 reactor until to the end of test.