Temperature CO Oxidation in Air

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. 

Except for H2 oxidation and hydrocarbon hydrogenations, most reactions are remarkably structure-sensitive over supported Au catalysts. One typical reaction is CO oxidation, which is remarkably sensitive to the junction perimeter between Au particles and support, the type of support and the size of the Au particles. 

---

In summary, external recycle reactors are expensive and their usefulness is limited. They can be practical for simple chemical systems where no condensation can occur and neither high pressure nor high temperature is needed. For example Carberry et al (1980) preferred an external recycle reactor over a spinning basket reactor for the study of CO oxidation in dry air at atmospheric pressure. 

The catal5fsts were tested for CO oxidation in a flow reactor using a 2.5 % CO in dry air mixture at a fixed flow rate of 200 seem. Thirty milligrams of the catalyst were used for each experimental run. The reaction was conducted at 298, 323, 373 and 473 K with 75 minutes duration at each temperature. The carbon monoxide conversion to carbon dioxide was monitored by an online gas chromatogr h equipped with a CTR-1 column and a thermal conductivity... 

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. 

Coball(lI) hydroxide exists in two allolropic forms, a blue or-Co (OH) and a pink /l-Co(OH) . The hydroxide is prepared by precipitation from u cobaltous salt solution by an alkali hydroxide. When the alkali is in excess the pink ft form is produced—the blue a-furni is produced when the cobalt salt is in excess. The salt slowly oxidizes in air at mom temperature and changes to hydrated cobaltic oxide, Co-Oi - H 0. The hydroxide is practically insoluble in H 0 and in bases, but highly soluble in mineral and organic acids. The commercial salt is used as Ihe starting material in the preparation of drying agents. 

SiC was made accidentally by E.G. Acheson in 1891. He recognized its abrasive power and named it carborundum [carbo(n) and (co)rundum (AI2O3)]. SiC is formed from coke and SiC>2 by sublimation in an electric furnace. Its hardness on the Mohs scale is 9.5, next to that of diamond (10). Carborundum has excellent abrasive power because of its hardness and the tendency to fracture to give sharp cutting edges. SiC is chemically stable, and is oxidized in air only above 1,000°C. (3-SiC is useful as a high-temperature semiconductor. SiC has been found as the mineral moissanite and in a meteorite found in Colorado. 

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]. 

Reducing gas used for the treatment of metals in industry can be produced from propane in the presence of air or O2 over a supported metal catalyst with high selectivity towards CO+H2 formation. This process is performed at temperatures higher than 800°C. There are several different reactions for high temperature propane oxidation in the presence of air [1]. 

The conversion for propane oxidation in air at temperatures in the range studied for the catalyst samples listed in Table 1 was total. CO, H2, CH4, CO2 and H2O were the only detectable products on all catalysts. 

Water Gas Shift Reaction (WGSR). The WGSR was studied over shale samples which had been previously decharred and silicated. After de-charring at 700K in a 10% O2 stream, the sample was exposed to 40% CO2 at HOOK for 12 hours. Upon completion of silication the temperature was adjusted to the desired value and the shale was either oxidized (in air or CO2) or reduced (in H2 or CO). This was followed by WGSR experiments in which various concentrations of CO, H2O, CO2 and H2 were fed to the reactor. 

Although Chen et al. focused on CO oxidation in gas turbine exhausts with noble metal catalysts, much of the deactivation data that they presented is also relevant to oxidation of VOCs in other air pollution control applications. They reported that 100 to 200 ppm SO2 in the exhaust will require 150 to 200 C higher catalyst temperatures for the same CO conversion as that without SO2. However, above -350 C the effect of SO2 disappears with these catalysts because the CO reaction rate becomes mass-transfer controlled. The inhibition by SO2 is attributed to the strong adswption of the sulfur compounds on both the catalyst and carrier, limiting adsorption of CO. These adsorbed sulfur compounds can be removed with time and high temperatures in the absence of SO2, restoring catalyst activity. 

Pd/Zr02 catalysts were prepared from the glassy PdZrs alloy by oxidation either in situ, (i.e. in the reactant gas) or in air. For the in situ oxidation (catalyst PdZr-i), the alloy was exposed for ca. 90 h to the reactant gas mixture (CO O2 = 1 1) at 280°C. The oxidation in air (catalyst PdZr-a) was carried out at tlie same temperature for 20 h. Before use, the alloy-derived catalysts were reduced in pure... 

---