▪ Abstract  The hydrogen economy is fast approaching as petroleum reserves are rapidly consumed. The fuel cell promises to deliver clean and efficient power by combining hydrogen and oxygen in a simple electrochemical device that directly converts chemical energy to electrical energy. Hydrogen, the most plentiful element available, can be extracted from water by electrolysis. One can imagine capturing energy from the sun and wind and/or from the depths of the earth to provide the necessary power for electrolysis. Alternative energy sources such as these are the promise for the future, but for now they are not feasible for power needs across the globe. A transitional solution is required to convert certain hydrocarbon fuels to hydrogen. These fuels must be available through existing infrastructures such as the natural gas pipeline. The present review discusses the catalyst and adsorbent technologies under development for the extraction of hydrogen from natural gas to meet the requirements for the proton exchange membrane (PEM) fuel cell. The primary market is for residential applications, where pipeline natural gas will be the source of H2 used to power the home. Other applications including the reforming of methanol for portable power applications such as laptop computers, cellular phones, and personnel digital equipment are also discussed. Processing natural gas containing sulfur requires many materials, for example, adsorbents for desulfurization, and heterogeneous catalysts for reforming (either autothermal or steam reforming) water gas shift, preferential oxidation of CO, and anode tail gas combustion. All these technologies are discussed for natural gas and to a limited extent for reforming methanol.

Sulfur removal is important for a fuel cell that uses a hydrocarbon fuel, such as natural gas, liquefied petroleum gas, and gasoline, to prevent the downstream sulfur poisoning of catalysts in the fuel processor and in the fuel cell anode. Although most sulfur species are removed prior to reforming, the reducing environment of the reforming stage (such as autothermal reforming) converts residual sulfur to hydrogen sulfide (H2S). H2S in the reformate must be removed to ensure longevity of the catalysts in downstream processing and in the anode chamber of fuel cell systems. A unique modified ZnO sample with a different morphology has been prepared and comparatively studied together with a commercially available ZnO sample under various conditions. Extremely low H2S outlet concentrationsas low as 20 parts per billion by volume (ppbv)have been observed over the modified ZnO sample for extended periods of times. The sulfur-trap capacity (the amount of H2S trapped before breakthrough) also is dependent on space velocity, temperature, steam concentration, CO2 concentration, and particle size. Higher capacity is observed at higher H2S inlet concentration of 8 ppmv, compared to lower inlet concentrations of 1−4 ppmv. The trap capacity decreases monotonically as the temperature increases. Steam in the reformate inhibits the capture of H2S by ZnO; it seems to shift the equilibrium of the reaction ZnO(s) + H2S(g) ⇔ ZnS(s) + H2O(g) to the left, toward ZnO and H2S. The effect of steam seems to be reversible. Increasing the CO2 concentration in the feed up to 12 vol % decreases the capacity of ZnO for the capture of H2S.

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This work is part of our efforts to explore more-effective ways to remove hydrogen sulfide (H 2 S) for fuel cell applications. Various absorbents (ZnO, SnO 2 , coprecipitated NiO-MoO 3 , supported CuO-ZnO, V 2 O 5 -ZnO, and ZnO supported on γ-alumina) were tested for H 2 S removal. The absorbents that were wash-coated onto the monolith were compared with particulate traps in the inlet H 2 S concentration range of 0.5-8 parts per million by volume (ppmv). The monolith provides much-higher dynamic capacity (the amount of H 2 S trapped before breakthrough) under the same conditions. The ZnO-based monolith demonstrated the best performance. An extremely low H 2 S outlet concentration (as low as 20 parts per billion, by volume (20 ppbv)) was observed over ZnO-based monolith samples for extended periods of time, under various conditions relevant for the desulfurization of gas products from the autothermal reforming of hydrocarbon fuels for a proton-exchange membrane fuel cell. The capacity of the H 2 S trap is dependent on the monolith characteristics (active component loading per cubic inch, and the number of cells per square inch) and operating conditions (including inlet H 2 S concentration, space velocity, and temperature). Wash-coating of ZnO that was chemically modified by an ammonium carbonate treatment onto a monolith with 400 cells per square inch gave the best H 2 S absorbence with higher dynamic capacity.

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