Preferential Carbon Monoxide Oxidation

A detailed computational model was developed for several different iimovative designs for the preferential carbon monoxide (CO) oxidation reactor using a kinetic mechanism and reaction sequence derived from a micro-kinetic model and literature data for the specific adsorption coefficients and kinetic parameters for a platinum-based catalyst. 

To understand the behaviour of preferential oxidation catalysts, the operating principle requires explanation. The common feature of these catalysts is the preferential adsorption of carbon monoxide at low temperature. When the reaction temperature increases, the carbon monoxide coverage decreases and reaction with oxygen (when it is present in the gas phase) takes place. At even higher temperatures and lower coverage of active sites with carbon monoxide, hydrogen oxidation occurs in parallel. Thus, an operating window exists for preferential oxidation catalysts. 

Surface Blockage by Impurity Coverage In this case, some impurity becomes adsorbed on reaction sites, preventing adsorption of the desired reactant. This is commonly seen in low-temperature fuel cells when a CO impurity is present. Carbon monoxide preferentially adsorbs on platinum catalyst sites in the anode and has a very high polarization for oxidative removal. 

For example, carbon dioxide from air or ethene nitrogen oxides from nitrogen methanol from diethyl ether. In general, carbon dioxide, carbon monoxide, ammonia, hydrogen sulfide, mercaptans, ethane, ethene, acetylene (ethyne), propane and propylene are readily removed at 25°. In mixtures of gases, the more polar ones are preferentially adsorbed). 

From the results of other authors should be mentioned the observation of a similar effect, e.g. in the oxidation of olefins on nickel oxide (118), where the retardation of the reaction of 1-butene by cis-2-butene was greater than the effect of 1-butene on the reaction of m-2-butene the ratio of the adsorption coefficients Kcia h/Kwas 1.45. In a study on hydrogenation over C03O4 it was reported (109) that the reactivities of ethylene and propylene were nearly the same (1.17 in favor of propylene), when measured separately, whereas the ratio of adsorption coefficients was 8.4 in favor of ethylene. This led in the competitive arrangement to preferential hydrogenation of ethylene. A similar phenomenon occurs in the catalytic reduction of nitric oxide and sulfur dioxide by carbon monoxide (120a). 

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. 

To further reduce the carbon monoxide, a preferential oxidation reactor or a carbon monoxide selective methanation reactor is used. The term selective oxidation is also used for preferential oxi-... 

As mentioned earlier, reformate from a fuel processor often needs addition processing to reduce the carbon monoxide levels. Researchers at the Stevens Institute of Technology are developing a microscale preferential oxidation (FrOx) reactor to decrease the carbon monoxide level in the reformate stream to below 100 ppm. As part of their research, they used advanced computational fluid dynamic modeling. 

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... 

In this chapter, recent results are discussed In which the adsorption of nitric oxide and its Interaction with co-adsorbed carbon monoxide, hydrogen, and Its own dissociation products on the hexagonally close-packed (001) surface of Ru have been characterized using EELS (13,14, 15). The data are interpreted In terms of a site-dependent model for adsorption of molecular NO at 150 K. Competition between co-adsorbed species can be observed directly, and this supports and clarifies the models of adsorption site geometries proposed for the individual adsorbates. Dissociation of one of the molecular states of NO occurs preferentially at temperatures above 150 K, with a coverage-dependent activation barrier. The data are discussed in terms of their relevance to heterogeneous catalytic reduction of NO, and in terms of their relationship to the metal-nitrosyl chemistry of metallic complexes. 

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. 

The removal of low concentrations of carbon monoxide from the pre-cleaned reformate of hydrocarbon and ethanol reformers is commonly performed by oxidation with air. Owing to the lower carbon monoxide concentration achieved by the low temperatures of methanol reforming, in this case the reformate goes directly to the preferential CO oxidation (PrOx) ... 

Preferential Carbon Monoxide Oxidation 1 [PrOx 1] MEMS-like Reactor Applied to Studies of the PrOx Reaction in Micro Channels... 

Preferential Carbon Monoxide Oxidation 2 [PrOx 2] Single-plate Reactor Based on MEMS Technology... 

Preferential Carbon Monoxide Oxidation 3 [PrOx 3] Integrated Micro Structure Heat Exchanger for PrOx Applied in a 20 kW Fuel Processor... 

Preferential Carbon Monoxide Oxidation 4 [PrOx 4] Stack-like Reactor Applied to PrOx... 

Preferential Carbon Monoxide Oxidation 5 [PrOx 5] Integrated Heat Exchanger/Reactor for PrOx... 

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

Schuessler et al. [85] of XCELLSiS (later BALLARD) presented an integrated methanol fuel processor system based on autothermal reforming, which coupled fuel/water evaporation with exothermic preferential oxidation (PrOx) of carbon monoxide. The reactor technology was based, in contrast to most other approaches, on a sintering technique. 

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],... 

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