CO Oxidation Reaction

CO oxidation reaction. The spectral changes in Cluster C are followed hy Cluster B reduction with a rate constant that is similar to the steady-state value. On the other hand, the rate of formation of the characteristic EPR signal for the CO adduct at Cluster A is much slower. Its rate constant matches that for acetyl-CoA synthesis, hut is several orders of magnitude slower than CO oxidation. Therefore, it was proposed that the following steps are involved in CO oxidation (1) CO hinds to Cluster C, (2) EPR spectral changes in Cluster C are accompanied hy oxidation of CO to CO2 hy Cluster C, (3) Cluster C reduces Cluster B, and (4) Cluster B couples to external electron acceptors (133). 

Isocyanides [RNC] (174, 175) are isoelectronic with CO and have been extensively used as CO analogs in studies of heme proteins (176-180). W-Butyl isocyanide (N-BIC) behaves as a CO analog at both the CODH and ACS active sites (181). N-BIC competes with CO in the CO oxidation reaction, is a sluggish reductant, and causes EPR spectral changes at Clusters A, B, and C similar to those elicited by CO. 

Additionally, NO is reduced by H2 and by hydrocarbons. To enable the three reactions to proceed simultaneously - notice that the two first are oxidation reactions while the last is a reduction - the composition of the exhaust gas needs to be properly adjusted to an air-to-fuel ratio of 14.7 (Fig. 10.1). At higher oxygen content, the CO oxidation reaction consumes too much CO and hence NO conversion fails. If however, the oxygen content is too low, all of the NO is converted, but hydrocarbons and CO are not completely oxidized. An oxygen sensor (l-probe) is mounted in front of the catalyst to ensure the proper balance of fuel and air via a microprocessor-controlled injection system. 

Room temperature CO oxidation has been investigated on a series of Au/metal oxide catalysts at conditions typical of spacecraft atmospheres CO = 50 ppm, COj = 7,000 ppm, H2O = 40% (RH) at 25 C, balance = air, and gas hourly space velocities of 7,000- 60,000 hr . The addition of Au increases the room temperature CO oxidation activity of the metal oxides dramatically. All the Au/metal oxides deactivate during the CO oxidation reaction, especially in the presence of CO in the feed. The stability of the Au/metal oxide catalysts decreases in the following order TiOj > FejO, > NiO > CO3O4. The stability appears to decrease with an increase in the basicity of the metal oxides. In situ FTIR of CO adsorption on Au/Ti02 at 25 C indicates the formation of adsorbed CO, carboxylate, and carbonate species on the catalyst surface. 

Recent studies [193] of the CO oxidation activity exhibited by highly dispersed nano-gold (Au) catalysts have reached the following conclusions (a) bilayer structures of Au are critical (b) a strong interaction between Au and the support leads to wetting and electron rich Au (c) oxidative environments deactivate Au catalyst by re-ox-idizing the support, which causes the Au to de-wet and sinter. Recent results have shown that the direct intervention of the support is not necessary to facilitate the CO oxidation reaction therefore, an Au-only mechanism is sufficient to explain the reaction kinetics. 

Bottcher A, Niehus H, Schwegman S, Over H, Ertl G. 1997. CO oxidation reaction over oxygen-rich Ru(OOOl) Surfaces. J Phys Chem B 101 11185-11191. 

For the same reason, Ru(OOOl) modihcation by Pt monolayer islands results in a pronounced promotion of the CO oxidation reaction at potentials above 0.55 V, which on unmodified Ru(OOOl) electrodes proceeds only with very low reaction rates. The onset potential for the CO oxidation reaction, however, is not measurably affected by the presence of the Pt islands, indicating that they do not modify the inherent reactivity of the O/OH adlayer on the Ru sites adjacent to the Pt islands. At potentials between the onset potential and a bending point in the j-E curves, COad oxidation proceeds mainly by dissociative H2O formation/ OHad formation at the interface between the Ru(OOOl) substrate and Pt islands, and subsequent reaction between OHad and COad- The Pt islands promote homo-lytic H2O dissociation, and thus accelerate the reaction. At potentials anodic of the bending point, where the current increases steeply, H2O adsorption/OHad formation and COad oxidation are proposed to proceed on the Pt monolayer islands. The lower onset potential for CO oxidation in the presence of second-layer Pt islands compared with monolayer island-modified Ru(OOOl) is assigned to the stronger bonding of a double-layer Pt film (more facile OHad formation). 

Blume R, Havecker M, Zafeiratos S, Teschner D, Kleimenov E, Knop-Gericke A, Schlogl R, Barinov A, Dudin P, Kiskinova M. 2006. Catalytically active states of Ru(OOOl) catalyst in CO oxidation reaction. J Catal 239 354. 

Ertl and his colleagues in 1997 reported detailed STM data for the oxidation of CO at Pt(l 11) surfaces, with quantitative rates extracted from the atomically resolved surface events.27 The aim was to relate these to established macroscopic kinetic data, particularly since it had been shown that no surface reconstruction occurred and the reaction was considered to obey the Langmuir-Hinshelwood mechanism, where it is assumed that the product (C02) is formed by reaction between the two adsorbed reactants, in this case O(a) and CO(a). Nevertheless, it was well known that for many features of the CO oxidation reaction at Pt(lll) there is no mechanism that is consistent with all features of the kinetics the inherent problem is that in general a reaction mechanism cannot be uniquely established from kinetics because of the possible contribution of intermediates or complications for which there might be no direct experimental evidence. 

In connection with practical situations where CO oxidation is important, we must also consider the perennial question of how to connect the low pressure results onto those at high pressure. Qualitatively this has been done for the CO oxidation reaction but it would still be worthwhile to attempt a numerical prediction of high pressure results based on low-pressure rate parameters. A very nice paper modeling steady-state CO oxidation data over a supported Pt catalyst at CO and O2 pressures of several torr has very recently appeared (.25). Extension of this work to other systems in warranted and, even though unresolved questions continue to exist, every indication is that the high and low pressure data can be reliably modeled with the same rate parameters if no adsorption - desorption equilibria are assumed. 

H2 production technologies based on natural gas. Operating the reaction at relatively lower temperature, between 300 and 450 °C could minimize the CO formation because the equilibria for WGS and CO oxidation reactions are thermodynamically more favorable at lower temperatures. In order to achieve this goal, highly selective catalysts that are specific for reforming via acetaldehyde formation rather than ethanol decomposition to CH4 and/or ethylene are required. The success in the development of ethanol-based H2 production technology therefore relies on the development of a highly active, selective and stable catalyst. 

Would the preferential CO oxidation reaction be needed if the proton-exchange membrane fuel cell (PEMFC) with Pt anode catalyst were able to work at temperatures higher than about 403 K ... 

Figure 10.19 A histogram of Au cluster density after the indicated treatment normalized to the cluster density after nucleation at room temperature. The Au coverage in each of the experiments was 0.4 ML Au. The first three columns compare the cluster density on Ti-free Si02, TiOJt(8%)-Si02, and Ti0x(17%)-Si02thin films, respectively, after a 850 K anneal. The fourth column shows the normalized Au cluster density of a Ti0x(17%)-Si02thin film after a CO oxidation reaction (CO/02 = 2 1,60 torr, 370 K, and 120 min). (Reprinted from Min, B.K. et al., J. Phys. Chem. B, 108, 14609-14615, 2004. Copyright 2004. With permission from American Chemical Society.)...

A. Martinez-Arias, J. M. Coronado, R. Cataluna, J. C. Conesa, and J. C. Soria, Influence of mutual platinum-dispersed ceria interactions on the promoting effect of ceria for the CO oxidation reaction in a R/Ce02/Al203 catalyst, J. Phys. Chem. B 102,4357 365 (1998). 

CO oxidation catalyst, 45 CO oxidation reaction, 50 Co silicide nanoparticles, 155 CO stretching frequencies,... 

Table 15.5 CO oxidation reaction on catalysts made of Pd nanowires and particles in FSM-16 and HMM-1 .

Table 1.2 indicates that alloying platinum with tin led to important changes in the product distribution an increase in the AA chemical yield and a decrease in the AAL and CO2 chemical yields. The presence of tin seems to allow, at lower potentials, the activation of water molecules and the oxidation of AAL species into AA. In the same manner, the amount of CO2 decreased, which can be explained by the need for several adjacent platinum atoms (three or four) to realize the dissociative adsorption of ethanol into CO species, via breaking the C-C bond. In the presence of tin, dilution of platinum atoms can limit this reaction. The effect of tin, in addition to the activation of water molecules, may be related to some electronic effects (ligand effects) on the CO oxidation reaction [38]. 

Figure 7-17 Conversions of hydrocarbons, CO (oxidation reactions), and NO (reduction reactions) versus air-fuel ratio. The engine should operate near the stoichiometric ratio to obtain maximimi conversions in all reactions, and cars are tuned to operate within this window of composition (dashed lines).

The CO oxidation reaction occurs rapidly at room temperature and below. As an example, on a Au(lll) surface at 250 K with onear unity exposed to a constant CO pressure of Pco = 2 x 10 Torr, the reaction rate expressed as turn-over frequency (TOF [molecules CO2 (Au atom s)" )/ is approximately 2.5 x 10-3 immediately after the reaction has been initiated and then declines at a relatively constant rate reaching a value of 3 x 10" after the reaction has proceeded approximately 800 seconds. Reaction order... 

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