Catalysis of CO oxidation

In addition, CO was found to be a poisoning adsorbate during the oxidation of methanol and other small organic molecules [9]. An important although not unique aspect of the catalysis of methanol oxidation in direct methanol fuel cells (DMFC) is related to the catalysis of CO oxidation. Therefore, methanol and CO oxidation reactions are both discussed in several reviews [10-12]. 

CO oxidation catalysis is understood in depth because potential surface contaminants such as carbon or sulfur are burned off under reaction conditions and because the rate of CO oxidation is almost independent of pressure over a wide range. Thus ultrahigh vacuum surface science experiments could be done in conjunction with measurements of reaction kinetics (71). The results show that at very low surface coverages, both reactants are adsorbed randomly on the surface CO is adsorbed intact and O2 is dissociated and adsorbed atomically. When the coverage by CO is more than 1/3 of a monolayer, chemisorption of oxygen is blocked. When CO is adsorbed at somewhat less than a monolayer, oxygen is adsorbed, and the two are present in separate domains. The reaction that forms CO2 on the surface then takes place at the domain boundaries. 

T.I. Politova, G.G. Gal vita, V.D. Belyaev, and V.A. Sobyanin, Non-Faradaic catalysis the case of CO oxidation over Ag-Pd electrode in a solid oxide electrolyte cell, Catal. Lett. 44, 75-81 (1997). 

Recent years, the authors have innovatively proposed a method by using the aqueous ammonia liquor containing hexamine cobalt (II) complex to scrub the NO-containing flue gases[6-9], since several merits of this complex have been exploited such as (1) activation of atmospheric O2 to a peroxide to accelerate the O2 solubility, (2) coordination of NO, as NO is a stronger ligand than NH3 and H2O of Co( II) complexes to enhance the NO absorption and (3), catalysis of NO oxidation to further improve the absorption both of O2 and NO. Thus, a valuable product of ammonium nitrate can be obtained. 

Another interesting case - which immediately illustrates how opportunistic the concept of reaction orders for catalytic reactions may be - is that of CO oxidation, an important subreaction in automotive exhaust catalysis ... 

Within the general mechanism for the oxidation of Ci molecules, proposed by Bagotzsky, formic acid is one of the simplest cases, since it requires only the transfer of two electrons for the complete oxidation to CO2 [Bagotzky et al., 1977]. In fact, it has the same oxidation valency as CO both require two electrons for complete oxidation to CO2. When compared with CO, the reaction mechanism of formic acid is more complex although the catalysis of the oxidation reaction is much easier. In fact, formic acid can be readily oxidized at potentials as low as 0.2 V (vs. RHE). Its reaction mechanism takes place according to the well-established dual path mechanism [Capon and Parsons, 1973a, b] ... 

For Se and Te, oxidation of the adatom takes place at potentials higher than that of CO oxidation. The adatom is always in its reduced state, and no bifunctional catalysis through the transfer of oxygen from the adatom to the CO molecule can take place. 

W. Liu, and M. Flytzani-Stephanopoulos, Transition metal-promoted oxidation catalysis by fluorite oxides A study of CO oxidation over Cu—Ce02, Chem. Eng. J. 64, 283—294 (1996). 

The oxidation of CO by Oj over group VIII metal catalysts has been the subject of a large body of ultrahigh vacuum surface science and high pressure catalysis work due to its importance in pollution control. Currently, the removal of CO as CO2 from automobile exhaust is accomplished by catalytic converters which employ a supported Pt, Pd, and Rh catalyst. The importance of CO oxidation has led to numerous recent studies of the kinetics of this reaction on supported metal catalysts and transient kinetic studies on polycrystalline foils , which have sought to identify and quantify the parameters of the elementary mechanistic steps in CO oxidation. 

Supported Au catalysts have been extensively studied because of their unique activities for the low temperature oxidation of CO and epoxidation of propylene (1-5). The activity and selectivity of Au catalysts have been found to be very sensitive to the methods of catalyst preparation (i.e., choice of precursors and support materials, impregnation versus precipitation, calcination temperature, and reduction conditions) as well as reaction conditions (temperature, reactant concentration, pressure). (6-8) High CO oxidation activity was observed on Au crystallites with 2-4 nm in diameter supported on oxides prepared from precipitation-deposition. (9) A number of studies have revealed that Au° and Au" play an important role in the low temperature CO oxidation. (3,10) While Au° is essential for the catalyst activity, the Au° alone is not active for the reaction. The mechanism of CO oxidation on supported Au continues to be a subject of extensive interest to the catalysis community. 

There have been many attempts to relate bulk electronic properties of semiconductor oxides with their catalytic activity. The electronic theory of catalysis of metal oxides developed by Hauffe (1966), Wolkenstein (1960) and others (Krylov, 1970) is base d on the idea that chemisorption of gases like CO and N2O on semiconductor oxides is associated with electron-transfer, which results in a change in the electron transport properties of the solid oxide. For example, during CO oxidation on ZnO a correlation between change in charge-carrier concentration and reaction rate has been found (Cohn Prater, 1966). 

This section has attempted to delineate the possible ways in which CO activation can be achieved by discrete metal complexes in order that homogeneous catalysis of CO reactions can be better understood. Principal means of activation are by significant bond order reduction and/or development of reactive charge distributions on the coordinated carbonyl. Oxidation or reduction of the CO ligand will transpire at carbon, and the primary mode of attack at that site will be by nucleophiles. 

Currently, low-temperature CO oxidation over Au catalysts is practically important in connection with air quality control (CO removal from air) and the purification of hydrogen produced by steam reforming of methanol or hydrocarbons for polymer electrolyte fuel cells (CO removal from H2). Moreover, reaction mechanisms for CO oxidation have been studied most extensively and intensively throughout the history of catalysis research. Many reviews [4,19-28] and highlight articles [12, 29, 30] have been published on CO oxidation over catalysts. This chapter summarizes of the state of art of low temperature CO oxidation in air and in H2 over supported Au NPs. The objective is also to overview of mechanisms of CO oxidation catalyzed by Au. 

In Wang et al. s synthesis of asimilobin (Scheme 10-23), the key diol 123 was also prepared by Sharpless AD in high enantioselectivity. Oxidation of 123 furnished the trans-threo-tram fow-THF intermediate 124 in catalysis of Co(modp)2. The C2 symmetrical diol 124 was desymmetralized and converted... 

Whether the porphyrin apoprotein radical shown in reactions above has a role in catalyzing lipid in oxidation is still being debated. Current evidence suggests that the heme protein radical is required for electron transfer in the ferryl iron-heme complexes (157) and that it may co-oxidize proteins or other molecules (163, 178), but is probably not involved in direct catalysis of lipid oxidation (144). 

Gustafson J, Westerstrom R, Mikkelsen A, Torrelles X, Balmes O, Bovet N, et al. (2008). Sensitivity of catalysis to surface structure the example of CO oxidation on Rh under reahstic conditions. Phys Rev B, 78, 045423... 

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