When CO is removed from H
2 by selective oxidation using a catalytically active membrane, the CO is oxidized during its permeation through the catalytic layer to the permeate side. Since the oxidation rate of CO is proportional to the inverse of its concentration, the rejection of CO using an H
2-selective microporous layer which is overlaid on the zeolite layer is also effective. In this study, Y-type zeolite membranes with different thicknesses were prepared on the outer surface of porous α-Al
2O
3 tubes by hydrothermal synthesis. The membranes were modified to different extents by ion-exchange with Pt ions. Gas permeation properties of the membranes were then determined using single-component H
2 and CO, as well as mixtures of H
2, CO, O
2 and Ar. Thus, the effects of the membrane thickness, the amount of Pt loading and the size of micropores on CO oxidation could be evaluated. For the Pt/NaY membrane prepared by hydrothermal synthesis for 12 h and ion-exchanged using a 2.0-mmol L
–1 solution of [Pt(NH
3)
4]Cl
2, the H
2 permeance and the H
2/CO separation factor were 1.1 × 10
–6 mol m
–2 Pa
–1 s
–1 and 1.9 at 473 K. The thickness of the zeolite layer had no detectable effect on CO oxidation. When the membrane was ion-exchanged with a 10.0-mmol L
–1 solution of [Pt(NH
3)
4]Cl
2, the CO/H
2 mole fraction ratio on the permeate side was minimized to 0.007 at 493 K for a feed of H
2:CO:O
2:Ar = 4:2:1:93 on a molar basis. The membranes were further modified with SiO
2 using dip-coating and chemical vapor deposition techniques. The latter technique led to a membrane, the outer surface of which was coated with an H
2-selective microporous SiO
2 layer. This resulted in a decrease in CO concentration in the zeolite layer. Thus the CO oxidation proceeded effectively in the catalytically active zeolite layer, and the CO/H
2 mole fraction ratio on the permeate side was 0.01 at 523 K for a feed of H
2:CO:O
2:Ar = 4:2:1:93 on a molar basis.
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