Enhancement of CO oxidation by use of H2-selective membranes impregnated with noble-metal catalysts

In order to oxidize CO, which is diluted in H-2, two types of catalytic membranes have been developed. In this study, the characteristics of the membranes are briefly summarized for a typical reaction temperature of 523 K, and the permeation and oxidation mechanism of those membrane reactors are analyzed. (i) A defect-free Y-type zeolite membrane was synthesized on the inner surface of a porous alpha-alumina support tube by a hydrothermal process. The membrane was ion-exchanged and calcined in air and, finally, reduced in a flow of I-h, to give a Pt-loaded Y-type zeolite membrane (PtY membrane). A mixture of H-2, CO (10000 ppm) and O-2 (0-12000 ppm) was permeated through the PtY membrane at 523 K. The total pressure on the feed and permeate sides was maintained at atmospheric pressure, and the permeate side was swept with an inert gas. The PtY membrane rejected CO at a H-2/CO separation factor of approximately 10. When the O-2 feed rate exceeded the CO feed rate of the stoichiometry, the CO concentration on the permeate side, 1000 ppm, was decreased to less than 8 ppm. Since H-2 and CO are able to enter the Y-type zeolitic pores without size discrimination, the H-2/CO separation factor can be attributed to the slow diffusivity of CO in the membrane. Thus, CO has a longer residence time in the pores, and the oxidation proceeds effectively. (ii) A gamma-Al2O3 layer was formed on the outer surface of a porous alpha-Al2O3 support tube using a boehmite sol. The layer was then impregnated with Rh using an aqueous solution of RhCl3, and calcined in air at 573 K. A microporous silica layer was then formed on the surface of the Rh-impregnated gamma-Al2O3 layer using a silica sol. The SiO2 membrane showed a H-2/CO separation factor of 100. Thus, the CO concentration of 50000 ppm on the feed side was decreased, with no feed of 02, to 500 ppm at 523 K on the surface of the Rh/gamma-Al2O3 layer. When O-2 was added to the feed, CO was oxidized in the Rh/gamma-Al2O3 layer, and a corresponding decrease in CO concentration on the permeate side was observed. (C) 2002 International Association for Hydrogen Energy. Published by Elsevier Science Ltd. All rights reserved.