Enhancement of CO oxidation by use of H 2-selective membranes impregnated with noble-metal catalysts

Abstract

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 α-alumina support tube by a hydrothermal process. The membrane was ion-exchanged and calcined in air and, finally, reduced in a flow of H 2, to give a Pt-loaded Y-type zeolite membrane (PtY membrane). A mixture of H 2, CO (10 000 ppm) and O 2 (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 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 γ-Al 2O 3 layer was formed on the outer surface of a porous α-Al 2O 3 support tube using a boehmite sol. The layer was then impregnated with Rh using an aqueous solution of RhCl 3, and calcined in air at 573 K . A microporous silica layer was then formed on the surface of the Rh-impregnated γ-Al 2O 3 layer using a silica sol. The SiO 2 membrane showed a H 2/CO separation factor of 100. Thus, the CO concentration of 50 000 ppm on the feed side was decreased, with no feed of O 2, to 500 ppm at 523 K on the surface of the Rh/ γ-Al 2O 3 layer. When O 2 was added to the feed, CO was oxidized in the Rh/ γ-Al 2O 3 layer, and a corresponding decrease in CO concentration on the permeate side was observed.