Preferential oxidation of carbon

Li, W., Gracia, F.J. and Wolf, E.E. (2003) Selective combinatorial catalysis challenges and opportunities the preferential oxidation of carbon monoxide. Catal. Today, 81, 437. 

Alayoglu S, Nilekar AU, Mavrikakis M et al (2008) Ru-Pt core-shell nanoparticles for preferential oxidation of carbon monoxide in hydrogen. Nat Mater 7 333-338... 

However, most fuel cell systems can tolerate methane concentrations up to at least 1% in the reformate, no special purification reactions are required. In contrast, hence, removing small residual amounts of carbon monoxide from pre-purifled reformate applying the methanation reaction may be considered as an alternative to the preferential oxidation of carbon monoxide, provided that the CO concentration is low enough to have no significant impact on the hydrogen yield. However, no applications of methanation for CO clean-up in micro structured devices appear to have been reported, hence the issue is not discussed in depth. Finally, during hydrocarbon reforming all hydrocarbon species (saturated and unsaturated) smaller than the feed molecule may be formed. 

Figure 2.60 Integrated reactor/heat exchanger for the preferential oxidation of carbon monoxide developed by Eindhoven University and IMM [89] (source IMM).

Comparison Between Coated Micro Structures and a Conventional Monolith Applied to Preferential Oxidation of Carbon Monoxide... 

Another comparison of micro structures with monoliths was carried out by Chen et al. [38] for the preferential oxidation of carbon monoxide (see Section 2.6.2). 

R 20] The fuel processing system consists of a fuel evaporator, a reformer, a reactor for the preferential oxidation of carbon monoxide and a catalytic burner (Figure 4.48) [95],... 

Watanabe, M., Uchida, H., Igarashi, H., and Suzuki, M. Development of Pt/ZSM-5 catalyst with high CO selectivity for preferential oxidation of carbon monoxide in a reformed gas. Chemistry Letters, 1995, 24, 21. 

Marino, F, Descorme, C., and Duprez, D. Noble metal catalysts for the preferential oxidation of carbon monoxide in the presence of hydrogen (PROX). Applied Catalysis. B, Environmental, 2004, 54, 59. 

Chin, P., Sun, X., Roberts, G.W., and Spivey, JJ. Preferential oxidation of carbon monoxide with iron-promoted platinum catalysts supported on metal foams. Applied Catalysis. A, General, 2006, 302, 22. 

G. W. Roberts, P. Chin, X. L. Sun, J. J. Spivey, Preferential oxidation of carbon monoxide with Pt/Fe monolithic catalysts interactions between external transport and the reverse water-gas-shift reaction. Appl. Catal. B 2003, 46, 601-611. 

Both the preferential oxidation of carbon monoxide and the hydrogen oxidation, which occurs in parallel as an undesired side reaction, are highly exothermic. Preferential oxidation... 

Formation of carbon monoxide over the catalyst by the reverse water-gas shift reaction (RWGS) in an oxygen-deficient atmosphere is frequently observed especially under conditions of partial load, because most catalysts for preferential oxidation of carbon monoxide have some activity for WGS and its reverse reaction. Therefore oversizing the reactor bears the danger of impaired conversion and the same applies for partial load of the reactor unfortunately. Because the concentration of carbon monoxide that is tolerated by low-temperature fuel cells is usually in the range below 100 ppm or less, even low catalytic activity for reverse shift becomes an issue. 

When oxygen is added to the feed, some catalysts convert carbon monoxide to carbon dioxide with very high selectivity vdiile the methane formation starts not before the oxygen is completely consumed [150]. Therefore preferential oxidation and selective methanation could be operated in a combined manner. Care has to be taken when choosing the catalyst for preferential oxidation of carbon monoxide, because some formulations are very sensitive to light hydrocarbon contained in the reformate as reported by Kraaij et al. [151] who observed rapid deactivation. 

Ouyang et al. [147] studied the preferential oxidation of carbon monoxide in silicon reactors of the smallest scale fabricated by photolithography and deep reactive ion etching. The reactors had two gas inlets for reformate and air, a premixer, a single reaction channel, and an outlet zone where the product flow was cooled. The chaimels were sealed by anodic bonding with a Pyrex glass plate. Full conversion of carbon monoxide was achieved between 170 and 300° C reaction temperature. 

Adachi et al. [168] developed a model for a natural gas fuel processor composed of an ATR designed as metallic foam monolith coated with catalyst and two-stage WGS reactors also designed as foam monoliths followed by two-stage ceramic monoliths for the preferential oxidation of carbon monoxide as shown in Figure 14.27. Figure 14.28 shows the course of temperature and gas composition of feed and reformate as calculated for... 

Dudfield et al. [ 161 ] combined a 20 kW methanol reformer with two oil-cooled reactors for the preferential oxidation of carbon monoxide switched in series. The remaining concentration of carbon monoxide in the product was lower than 10 ppm for more than 2 h at a feed concentration of 1.6% carbon monoxide. 

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. 

Numerical simulations of the reaction system for preferential oxidation of carbon monoxide was performed by Ouyang et cd. who applied CHEMKIN software and a network of 8 species in the gas phase, 8 surface species and 28 elementary reaction steps, not provided here in detail [117]. The simulation described the experimental performance of the reactor of Ouyang et al. very well. It revealed that the oxidation of carbon monoxide occurred by reaction between adsorbed CO and O H species and not by the reaction between adsorbed CO and O species, because the latter reaction rate was ten orders of magnitude lower. Thus a simplified mechanism of the reaction network could be formulated according to Ouyang et al. as follows ... 

The reaction rate of preferential oxidation of carbon monoxide increases with increasing pressure. For a platinum/alumina catalyst Kahlich et al. determined that the reaction rate was proportional to p [114]. 

However, sintering of dispersed metal may take place at much lower temperatures, examples are the sintering of platinum under the conditions of the water-gas shift (see Section 4.5.1) and sintering of gold when applied as a catalyst for the preferential oxidation of carbon monoxide at temperatures well below 150 °C (see Section 4.5.2)... 

The catalyst most frequently applied for the preferential oxidation of carbon monoxide is platinum on alumina, which contains low amounts of platinum in the range below 1 wt.% [102, 322], to reduce the cost of the catalyst. 

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