Hydrogen diffusive purification

A striking property of many interstitial metal hydrides is the high rate of hydrogen diffusion through the solid at slightly elevated temperatures. This mobility is utilized in the ultra-purification of H2 by diffusion through a palladium-silver alloy tube. 

Electrolysis, and thermochemical and photochemical decomposition of water followed by purification through diffusion methods are expensive processes to produce hydrogen. 

H. Palladium. Palladium (mp 1,SS2°C), is soft and ductile but work-hardens. At elevated temperatures, the diffusion of hydrogen through palladium is rapid, which forms the basis of a method for the purification of hydrogen. The best performance is obtained from a palladium-silver alloy containing about 18% silver because, unlike pure Pd, this alloy does not undergo a phase transition in the presence of hydrogen. Palladium is not as inert as platinum and is attacked by sulfuric and nitric acids. 

Non-hydrocarbon, pure H2 or cracked ammonia (25% N2, 75% H2, and residual NH3) can be fed directly to the anode. Due to the high diffusion rate of hydrogen, in the case of NH3 feed, results only a very small decrease in the cell s potential, at medium current densities. Gas purification is necessary for H2 from carbon-containing sources (US DOE, 2002 Larminie et al., 2003). 

Monsanto and Ube (Japan) developed membrane processes for purification of hydrogen gas mixtures. This process is based on the selective diffusion of hydrogen through semi-permeable membranes in the form of hollow fibers. The Monsanto PRISM separator process (owned by Air Products as of 2004) uses a polysulfone fiber whereas Ube uses an aromatic polyimide fiber.46... 

Both Pd and Pt are capable of absorbing large volumes of molecular hydrogen, and Pd is used for the purification of H2 by diffusion. 

Method of purification By scrubbing with various solutions (see especially the Girbitol absorption process). For very pure hydrogen, by diffusion through palladium. 

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