Steam reforming and related reactions in hydrogen-permselective membrane reactor

Abstract

Selective and efficient permeation of H2 was accomplished by a thin Pd or Pd-Ag alloy membrane supported on porous alumina ceramics. Such membranes were prepared by deposition of Pd or Pd and Ag on the outer surface of the support cylinder by an electroless plating technique. It was found that either steam or CO2 reforming of CH4 was promoted beyond equilibrium by use of a membrane reactor with the Pd/ceramic composite membrane at temperatures as low as 773 K. In spite of a similar type of reforming, a quite different pattern of reaction products was obtained in the membrane reactor: H2 and CO2 from steam reforming, and H2 and CO (syngas) from CO2 reforming. Nickel catalyzed carbon-free steam reforming, and the level of CH4 conversion depended on the efficiency of the membrane for H2 permeation since the rate of H2 production was sufficiently higher than that of H2 permeation. On the contrary, carbon deposition extensively took place in CO2 reforming on Ni catalyst, while alumina-supported noble metals showed far less selectivities for carbon deposition than did Ni even in the membrane reactor. Differing from the activity sequence observed in the conventional reaction system, it was found in membrane reactor that it decreased in the order: Rh∼Pt>Pd>Ru>Ir, due to contamination of the active surface. Among the noble metal catalysts tested, Pt supported on Al2O3-La2O3 and Al2O3-CeO2 was effective without significant carbon deposition to enable prolonged activity. The membrane reactor can provide a partial oxidation process with utilization of air instead of pure O2 to produce H2 or syngas.