Low-Temperature CO Oxidation in

One particular application for which supported Au catalysts may find a niche market is in fuel cells [4,50] and in particular in polymer electrolyte fuel cells (PEFC), which are used in residential electric power and electric vehicles and operate at about 353 73 K. Polymer electrolyte fuel cells are usually operated by hydrogen produced from methane or methanol by steam reforming followed by water-gas shift reaction. Residual CO (about 1 vol.%) in the reformer output after the shift reaction poisons the Pt anode at a relatively low PEFC operating temperature. To solve this problem, the anode of the fuel cell should be improved to become more CO tolerant (Pt-Ru alloying) and secondly catalytic systems should be developed that can remove even trace amounts of CO from H2 in the presence of excess CO2 and water. 

For successfid operation a selective CO oxidation catalyst in a reformer-PEFC system must be operated at ca. 353-373 K in a complex feed consisting of CO, O2, H2, CO2, H2O and N2, and be capable of reducing CO concentrations from about 1% to below 50 ppm - this is equivalent to a CO conversion of at least 99.5% [4, 54, 60]. In addition, this conversion must be achieved with the addition of equimolar O2 (twice the stoichiometric amount) and the competitive oxidation of H2 must be minimized. This is expressed as selectivity, which is defined as the percentage of the oxygen fed consumed in the oxidation of CO for commercial operation a selectivity of 50% is acceptable, since at this selectivity rninimal H2 is oxidized to water. 

Catalyst supports Cal (K) Components of feed gas %) T(K) SV (h mLg ) Conversion (%) Selectivity (%) Reference 

Mechanism for CO Oxidation Over Supported Gold Nanopartides 

1 Mechanisms Involving Junction Perimeter Between Cold and the Metal-Oxide Supports 

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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 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-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. 

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. 

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... 

Since Haruta s initial report (Sanchez et al., 1997) of the unexpected activity of supported gold catalysts for low-temperature CO oxidation, there has been a resurgence of research and interest in gold-mediated catalysis. Supported gold clusters have since been found to be active in a... 

Gold nanoparticles have received significant attention in recent years because of their unique catalytic activity [37 3]. Supported gold nanoparticles in the range of 2-5 nm are effective as catalysts for a variety of reactions including selective oxidation of propane to propylene oxide [44] and low-temperature CO oxidation [45]. Numerous experimental studies have focused on understanding the effect of particle size in gold catalysts however, the picture is often complicated by the lack of a monodisperse size distribution. 

Lambert reviews the role of alkali additives on metal films and nanoparticles in electrochemical and chemical behavior modihcations. Metal-support interactions is the subject of the chapter by Arico and coauthors for applications in low temperature fuel cell electrocatalysts, and Haruta and Tsubota look at the structure and size effect of supported noble metal catalysts in low temperature CO oxidation. Promotion of catalytic activity and the importance of spillover are discussed by Vayenas and coworkers in a very interesting chapter, followed by Verykios s examination of support effects and catalytic performance of nanoparticles. In situ infrared spectroscopy studies of platinum group metals at the electrode-electrolyte interface are reviewed by Sun. Watanabe discusses the design of electrocatalysts for fuel cells, and Coq and Figueras address the question of particle size and support effects on catalytic properties of metallic and bimetallic catalysts. 

Effects of Size and Contact Structure of Supported Noble Metal Catalysts in Low-Temperature CO Oxidation... 

Also, Marsh and co-workers [145] showed that gold on cobalt oxide particles, supported on a mechanical mixture of zirconia-stabilised ceria, zirconia and titania remains active in a gas stream containing 15 ppm SO2. Haruta and co-workers [207] found that although the low-temperature CO oxidation activity of Ti02-supported Au can be inhibited by exposure to SO2, the effect on the activity for the oxidation of H2 or propane is quite small. Venezia and co-workers [208] reported that bimetallic Pd-Au catalysts supported on silica/alumina are resistant to sulphur poisoning (up to 113 ppm S in the form of dibenzothiophene) in the simultaneous hydrogenation of toluene and naphthalene at 523 K. 

Catalytic CO oxidation has lately drawn considerable attention due to the growing applications for air purification, pollution control in automobiles, and incinerator exhaust gases. In addition to many different metal oxide catalysts, a wide variety of precious metal catalysts have been studied for low-temperature CO oxidation. Among them, it is noteworthy that Au nanoparticles deposited on oxide supports, such as AI2O3, SiOz, TiOz, MnOx, FezOs, and NiO, are very active for CO oxidation at room temperature [1-4]. Although Pd/SnOz and Pt/SnOz were known to be active for the low-temperature oxidation of CO, they often required complicated pretreatments and relatively long induction periods [5-7]. A Pd/CeOz-TiOz catalyst was also recently reported to exhibit high catalytic activity for CO oxidation at low temperature [8]. 

In recent years, the unexpected observation of highly active Au as a low temperature CO oxidation catalyst59,60 has initiated extensive research activity into the use of supported gold for liquid phase oxidation reactions. In general, the adsorption characteristics and catalytic properties of Au depend crucially on particle size, which can be controlled by the preparation method and the support.61-66 The crucial question involving gold catalysis, which as yet has not been fully answered, is the concept of why Au nanoparticles exhibit such radically different behaviour than bulk Au.67-70... 

In this paper, we present a novel synthesis of TiOi support by precipitation using supercritical CO2 as an antisolvent. We found that supercritical treated supports can remarkablely enhance the catalytic activity of gold nanoparticles for low temperature CO oxidation. 

Low Temperature CO Oxidation Over Alloy Type Sn-Pt/Si02 Catalysts. Figure 19 shows selected Temperature Programmed Reaction (TPRe) results obtained in the oxidation of CO over Sn-Pt/Si02 catalysts with different... 

Structure of Alloy Type Sn-Pt/Si02 Catalysts used in Low Temperature CO Oxidation. Both Mossbauer and FTIR spectroscopy provided sufficient proof of surface reconstruction during the low temperature CO oxidation. However, the above reconstruction appeared to be reversible as the reversible interconversion of PtSn Sn -I- Pt was demonstrated by both spectroscopic techniques. This reversibility can only be achieved if the segregation described above is within the supported nanoparticle, i.e., when surface reactions involved in CO oxidation do not result in formation of separate Pt and tin-oxide phases on the silica support. 

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