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CO Poisoning and CO Hydrogenation on the Surface of Pd Hydrogen Separation Membranes

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journal contribution
posted on 05.07.2017, 00:00 by Casey P. O’Brien, Ivan C. Lee
To understand how CO inhibits hydrogen transport across Pd membranes, a 25-μm-thick Pd foil membrane was monitored by infrared-reflection absorption spectroscopy (IRAS) during exposure to H2/CO gas mixtures while the rate of hydrogen permeation across the membrane was measured simultaneously in the 373–533 K temperature range. As the coverage of CO on the membrane surface increases with increasing CO concentration and decreasing temperature, the rate of hydrogen permeation across the membrane decreases. However, in addition to adsorbing on the membrane surface, CO reacts with H2 to form surface-adsorbed methylene (CH2) species and methane in the gas phase. The coverage of methylene increases with decreasing temperature and, therefore, the strong poisoning effect of CO at low temperatures may be due to both CO and methylene species blocking H2 dissociation sites on the membrane surface. The activity and selectivity of the Pd membrane for CO methanation is much higher than expected from previous studies and from the high activation barrier for CO dissociation on Pd. It is possible that the high concentration of defect sites on the polycrystalline Pd foil surface and hydrogen facilitate dissociation of CO at low temperatures. This work demonstrates that spectroscopic observation of membranes under realistic permeation conditions is critical for understanding surface poisoning mechanisms and for rational design of membranes that are resistant to poisoning.