Selective oxidation of carbon monoxide in hydrogen

Although the target conditions for this process are closely delimited, the system is in fact quite complex because of the greater variety of reactants and possible products. It may be helpful if before examining the literature we attempt to define those qualities that a successful catalyst ought to have. 

There should not be independent adsorption sites for each reactant they should adsorb competitively, carbon monoxide much more strongly than hydrogen. 

The hydrogen and water should not inhibit the reaction of carbon monoxide it would be beneficial if they promoted it, for example by limiting formation of carbonate ion. 

The oxidation of hydrogen should not be assisted by either of the carbon oxides or other intermediate products (COf-, HCOO-, etc.). 

Carbon monoxide should not be re-formed by the water-gas shift. If all these conditions were met, high selectivity should be attained. 

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The selective oxidation of carbon monoxide in hydrogen on Au/a-Fe2C>3 (haematite) catalysts shows characteristics that differ from those of the oxidation of carbon monoxide by itself (Chapter 6). The presence of hydrogen, or perhaps more importantly, of the water formed from it, decreases the rate of activity loss by inhibiting formation of carbonate ion, and causes the rate of carbon monoxide oxidation to increase a little.7,18,21,22... 

Hasegawa Y, Kusakabe K, and Morooka S. Selective oxidation of carbon monoxide in hydrogen-rich mixtures by permeation through a platinum-loaded Y-type zeolite membrane. J Membr Sci 2001 190 1-8. 

Park, J.W., Jeong, J.H., Yoon, W.L. and Rhee, Y.W. (2004) Selective oxidation of carbon monoxide in hydrogen-rich stream over Cu/Ce/Y-Al203 catalysts promoted with cobalt in a fuel processor for proton exchange membrane fuel cells. J. Power Sources, 132, 18—28. 

Snytnikov, P.V., Stadnichenko, A.I., Semin, G.L., Belyaev, V.D., Boronin, A.I. and Sobyanin, V.A. (2007) Copper-cerium oxide catalysts for the selective oxidation of carbon monoxide in hydrogen-containing mixtures I. Catalytic activity. Kinet. Catal., 48 (3), 439-447. 

G. Avgouropoulos, T. loannides, C. Papadopoulou, J. Batista, S. Hocevar, and H. K. Matralis, A comparative study of Pt/gamma-A1203, Au/alpha-Fe203 and CuO-Ce02 catalysts for the selective oxidation of carbon monoxide in excess hydrogen, Catal. Today 75, 157-167 (2002). 

Avgouropoulos, G., Ioannides, T., Matralis, H., Batista, J., and Hocevar, S. CuO-Ce02 mixed oxide catalysts for the selective oxidation of carbon monoxide in excess hydrogen. Catalysis Letters, 2001, 73, 33. 

V. Cominos, V. Hessel, C. Hofmann, et at Selective oxidation of carbon monoxide in a hydrogen-rich fuel cell feed using a catalyst coated microstructured reactor. Catal. Today 2005, 110, 140-153. 

The plant consists of a pre-reforming step which converts C2+ into methane, which also reduces the coking risk downstream (Figure 4.50). The actual reforming step, the water gas shift reaction, follows and also a step for the selective oxidation of carbon monoxide before the hydrogen-rich gas enters the fuel cell. A steam evaporator is also included in the set-up. 

Another measure to reduce the detrimental effect of carbon monoxide on the anode performance is the addition of a small amount of air during normal operation, which is commonly termed bleed air . It oxidises the carbon monoxide adsorbed on the active sites of a selective oxidation catalyst layer [26] at the anode (see Section 4.1.2). However, similar to the oxygen addition performed for the preferential oxidation of carbon monoxide in a dedicated clean-up reactor (see Section 3.10.2), addition of air to the hydrogen containing reformate generates safety issues. 

The oxidation of carbon monoxide at around room temperature is the most famous reaction known for gold catalysts. Haruta s group discovered in 1987 [1, 2] that gold is a unique catalyst for this reaction when gold metal particles are smaller than 5 nm and supported on oxides. Since then, extensive and intensive fundamental works have been published, and expanding new applications, from air purification (gas masks, gas sensors, indoor air quality control) to hydrogen purification for fuel cells (PROX, preferential selective oxidation of CO in the presence of Hj) have been developed. 

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