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

In order to oxidize CO, which is diluted in H2, 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. defect-free Y-type zeolite membrane was synthesized on the inner surface of a porous α-alumina support tube by a hydrothermal process. The membrane was ion-exchanged and calcined in air and, finally, reduced in a flow of H2, to give a Pt-loaded Y-type zeolite membrane (PtY membrane). A mixture of H2, CO (10 000 ppm) and O2 (0–12 000 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 H2/CO separation factor of approximately 10. When the O2 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 H2 and CO are able to enter the Y-type zeolitic pores without size discrimination, the H2/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. γ-Al2O3 layer was formed on the outer surface of a porous α-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 γ-Al2O3 layer using a silica sol. The SiO2 membrane showed a H2/CO separation factor of 100. Thus, the CO concentration of 50 000 ppm on the feed side was decreased, with no feed of O2, to 500 ppm at 523 K on the surface of the Rh/γ-Al2O3 layer. When O2 was added to the feed, CO was oxidized in the Rh/γ-Al2O3 layer, and a corresponding decrease in CO concentration on the permeate side was observed.