Platinized tin oxide: A low-temperature oxidation catalyst.

摘要

Some combinations of noble metal and reducible metal oxide possess a synergistic level of activity for the catalytic oxidation of CO at lower temperatures and partial pressures of O2 than can be obtained by either component alone. The most promising candidates, Au on MnO2 and Pt on SnO 2, were investigated for the effects on activity of noble metal loading, of reductive pretreatment at elevated temperature, and of reaction gas mixture composition. Optimum activity was obtained at a noble metal loading of 28% Au/MnO2 and 17% Pt/SnO2. Both catalysts benefitted from reductive CO pretreatment at 125°C. Both catalysts showed enhanced activity when the reaction gas mixture contained excess O2, and reduced activity under excess CO. Au/MnO2 was severely inhibited by added CO2; Pt/SnO2 was not.; A method for coating Pt/SnO2 onto various substrates was developed. The SnO2 layer was applied via thermal decomposition of tin(II) 2-ethylhexanoate. The decomposition was investigated using DRIFTS. The effects of surface area and pore size distribution of both substrate and of the SnO 2 layer on the CO oxidation activity of the final catalyst coating were investigated. Low-surface area, high-pore diameter substrates provided the highest activity catalysts coatings. The activity also increased as the total SnO2 loading and surface area were increased. Activity correlated with total Pt-SnO2 periphery.; Small amounts of H2O in the reaction gas mixture enhanced the low-temperature, CO-oxidation activity of Pt/SnO2; large amounts destroyed the activity unless the catalysts was mildly heated. The activity-enhancing effect of water is due to the formation of surface hydroxyl groups. Hydroxyl groups may function in the dissociative adsorption of O2 or as oxidizing agents themselves. Silylation of the hydroxyl groups of Pt/SnO 2 destroyed the CO-oxidation activity. Changes in the catalyst surface from silylation were analyzed using DRIFTS.; The oxidation of formaldehyde, acetaldehyde, the C1 to C 3 alcohols, and the C1 to C5 hydrocarbons over Pt/SnO2 was investigated and compared to that over Pt. Differences in product mixtures were attributed to the presence of higher concentrations of surface oxygen provided by SnO2. Possible mechanisms are discussed.