Membranes for Hydrogen Purification: An Important Step toward a Hydrogen-Based Economy
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Membranes for Hydrogen Purification: An Important Step toward a HydrogenBased Economy Tina M. Nenoff and Richard J. Spontak, Guest Editors, and Christopher M. Aberg Abstract Production of pure molecular hydrogen is essential to the realization of the proposed “hydrogen economy” that could ultimately provide hydrogen as a clean, renewable source of energy; eliminate the industrialized world’s dependence on petroleum; and reduce the generation of greenhouse gases linked to global warming. A crucial step in obtaining pure hydrogen is separating it from other gaseous compounds—mainly CO2—that often accompany hydrogen in industrial chemical reactions. Advanced membrane technology may prove to be the key to the successful, economical production of molecular hydrogen. Size-sieving glassy polymer membranes can separate H2 on the basis of its small size. Alternatively, reverse-selective rubbery polymers can expedite the passage and, hence, removal of CO2 due to its relatively high solubility in such membranes alone or in conjunction with dissociative chemical reactions. Transition-metal membranes and their alloys can adsorb H2 molecules, dissociate the molecules into H atoms for transport through interstitial sites, and subsequently recombine the H atoms to form molecular H2 again on the opposite membrane side. Microporous amorphous silica and zeolite membranes comprising thin films on a multilayer porous support exhibit good sorption selectivity and high diffusion mobilities for H2, leading to high H2 fluxes. Finally, carbon-based membranes, including carbon nanotubes, may be viable for H2 separation on the basis of selective surface flow and molecular sieving. A wide variety of materials challenges exist in hydrogen purification, and the objective of this issue of MRS Bulletin is to address those challenges and their potential solutions from basic principles. Keywords: adsorption, carbon, diffusion, energy, film, hydrogen, membrane, metal, polymer, silica, zeolite.
Introduction Although hydrogen composes 80% of all known matter in the universe—excluding the elusive “dark matter” whose nature is currently not known—it is not available in its free molecular form on MRS BULLETIN • VOLUME 31 • OCTOBER 2006
Earth. Here, it is tied up in the water that covers two-thirds of the planet, in the valuable hydrocarbon deposits that lie under the Earth’s surface, and in countless other compounds. To obtain molecular
hydrogen as a source of fuel requires the development of methods capable of separating hydrogen from carbon, oxygen, nitrogen, and other elements to which it is chemically bound. In some cases, such as in steam reforming of natural gas, described by the reaction → CO ⫹ 4H , CH4 ⫹ 2H2O ← 2 2
(1)
the separation of hydrogen from carbon and oxygen is accomplished on an industrial scale. However, molecular hydrogen is accompanied by CO2 as a reaction product, which requires another separation step to produce a stream of pure hydrogen. The challenges of producing hydrogen on an industrial scale are tremendous, but so are th
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