Low-Temperature CO Oxidation

Oscar H. Laguna, Luis F. Bobadilla, Willinton Y. Hernandez, and Miguel Angel Centeno 

Perovskites (general formula ABO3) are mixed oxides with a well-reported good catalytic activity in CO oxidation reactions at low temperatures. Despite that a detailed description of the structural features of perovskites has been presented in previous chapters, some relevant aspects will be briefly summarized herein in order to allow an easy understanding of the close relationship between some structural aspects of these materials and their performances in CO oxidation reactions. 

The catalytic properties of the perovskite-type oxides depend on aspects such as the morphology, the particle size, and the crystalline structure, among others. The chemical composition also results determinant, Just as it is the nature of the A and B site ions and their valence states [8]. The A site ions, in contrast to B site ones, are generally proposed to be catalytically inactive, although their nature influence the stability of the solid [9,10]. 

Nevertheless, substitution of either or both A and B cations with other hetero-valent cations or varying the oxygen content of the structure may enhance the catalytic activity due to the formation of structural defects such as cationic or anionic vacancies (e.g., oxygen vacancies) in the network and/or the modification in the oxidation state of the transition metal cation (B site ion), generating 

Perovskites and Related Mixed Oxides Concepts and Applications, First Edition. 

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Effect of ozone pretreatment on low temperature CO oxidation catalysts... 

The objective of this study was to develop a low temperature CO oxidation catalyst that continually removes low concentrations of CO from the atmospheres of space stations. CO is a major contaminant in spacecraft environments. Since... 

Au/metal oxides are active for low temperature CO oxidation. The activity of the catalysts is very sensitive to catalyst preparation. All the Au/metal oxides tested for room temperature CO oxidation deactivated substantially with time. The deactivation... 

Tian, F. and Ding, Y. (2006) Low temperature CO oxidation over unsupported nanoporous gold. 

Grunwaldt J-D, Maciejewski M, Becker OS, et al. 1999. Comparative Study of Au/Ti02 and AU/Z1O2 catalysts for low-temperature CO oxidation. J Catal 186 458-469. 

Lopez N, Janssens TVW, Clausen BS, et al. 2004a. On the origin of the catal3dic activity of gold nanoparticles for low-temperature CO oxidation. J Catal 223 232-235. 

Au-Pd alloys with compositions close to that of the bulk components and that particle sizes were ca. 25 to 50 nm in diameter. The catalysts that were effective for H2O2 synthesis were found to be wholly inactive for CO oxidation at ambient temperature, and catalysts that were effective for low temperature CO oxidation were inactive for H2O2 synthesis. This shows that selective oxidation reactions active may utilize very different sites than those for the oxidation of CO. 

Y. Z. Yuan, A. P. Kozlova, K. Asakura, H. L. Wan, K. Tsai, and Y. Iwasawa, Supported Au catalysts prepared from Au phosphine complexes and As-precipitated metal hydroxides Characterization and low-temperature CO oxidation, J. Catal. 170(1), 191-199 (1997). 

J. D. Grunwaldt, C. Kiener, C. Wogerbauer, and A. Baiker, Preparation of supported gold catalysts for low-temperature CO oxidation via size-controlled gold colloids, J. Catal. 181(2), 223—232 (1999). 

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. 

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. 

The results of X-ray photoelectron spectroscopy (XPS) and X-ray absorption fine structure (XAFS) proved that the oxidation state of Au was important for low temperature CO oxidation over Au catalysts supported on Fe203, Ti02 and A1203. It was proposed that oxidized Au is more active than metallic Au however, this conclusion was not directly evidenced [95]. 

Catalyst coking may involve carbonaceous species such as partially hydrogenated fragments (QHy) and may be initiated on metal or than acidic-oxide sites [1]. Three types of carbonaceous deposits may be formed on say Pt [2], which may be differentiated by temperature-programmed oxidation. SnO,-promoted Pt catalysts are important in reforming of alkanes [3] and low temperature CO oxidation [4]. Of course Sn02 is an n-type semiconductor and certainly in photoelectrolysis one expects metal-oxide electron transfers across the junction [51, but the nature of the Pt-SnOt interaction in catalytic systems remains unclear. 

Monitoring pollutants in a variety of composition ranges in motor vehicle and chemical process exhaust gases is a major area of research in pollution abatement technology. Low-temperature CO oxidation catalysts are needed for zero emission vehicles, CO gas sensors, selective oxidation of CO in H2 rich streams in fuel cell applications,1,2 and in closed-cycle C02 lasers used for remote sensing in space applications.3"5 Effective oxidation of CO during... 

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