On highly active partially oxidized platinum in carbon monoxide oxidation over supported platinum catalysts

摘要

In situ X-ray absorption spectroscopy identified the role of oxidized platinum species in generating a high-activity state in the oxidation of carbon monoxide over Pt/Al_2O_3, Pt/TiO_2 and Pt/SiO_2. Two activity regimes were identified: low-activity at high carbon monoxide concentration and low temperature, and high activity at low carbon monoxide concentration and high temperature. The change between the two regimes occurred suddenly; there was ignition when going from low to high activity, and extinction from high to low activity. Ignition only occurred when the concentration of oxygen was higher than that of carbon monoxide. The catalyst was metallic and covered with carbon monoxide at low activity and partially oxidic at high activity. Carbon monoxide poisoned the catalyst at low activity. The partially oxidized platinum catalysts showed high activity. The partially oxidized catalyst had a lower oxygen coordination number and shorter Pt-O bond length as expected for bulk PtO_2, suggesting that a strongly defected platinum oxide was formed with a possibly square planar coordination. The catalyst structure showed a dynamic behavior and the amount of oxide depended on the concentration of reactants in the gas phase. Smaller particles of Pt/Al_2O_3 reached high activity at lower temperature than the larger particles. Pt/TiO_2 reached high activity at the lowest temperature while Pt/SiO_2 required the highest temperature. Parallel with the temperature of ignition, Pt/SiO_2 was the most oxidized while Pt/TiO_2 was the least oxidized.