The Selective Oxidation of Carbon Monoxide in Hydrogen

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... 

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. 

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. 

Transition metal carbides (mainly of W and Mo) have been shown to be effective catalysts in some chemical reactions that are usually catalyzed by noble metals such as Pt and Pd (ref.1). Their remarkable physical properties added to lower cost and better availability could make them good candidates for substitute materials to noble metals in automobile exhaust catalysis. Hence, for this purpose, we have prepared several catalysts of tungsten carbide and W,Mo mixed carbides supported on y alumina with different Mo/W atom ratios. The surface composition has been studied by XPS while the quantitative determination of catalytic sites has been obtained by selective chemisorption of hydrogen and of carbon monoxide. The catalytic performances of these catalysts have been evaluated in the simultaneous conversion of carbon monoxide, nitric oxide and propane from a synthetic exhaust gas. 

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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