CO selective oxidation in H2-rich gas over Ag nanoparticles—effect of oxygen treatment temperature on the activity of silver particles mechanically mixed with SiO2

In order to elucidate the roles of pretreatment with oxygen at high temperatures and the interaction between Ag nanoparticles and SiO2 in low-temperature CO selective oxidation, a mechanical mixture of Ag nanoparticles and SiO2 powder was prepared and pretreated with oxygen at different temperatures. The samples calcined at 300 and 400 °C have poor activities for CO oxidation, while oxygen reacts totally with H2 at temperatures of 160 and 120 °C, respectively. It becomes catalytically active after treatment with oxygen at 500 °C, and the activity increases with the treatment temperature up to 700 °C, reaching nearly the same level of CO conversion as that of the 8%Ag/SiO2 prepared by the incipient wetness impregnation method and treated with He at 500 °C. XRD and TEM results show that calcinations under oxygen atmosphere between 500 and 700 °C help dispersing the silver nanoparticles on the surface of SiO2, as well as induce an interaction between the silver nanoparticles and SiO2. Moreover, the ability of CO adsorption at low temperatures is obviously enhanced. However, further increasing the pretreatment temperature decreases the catalytic activity due to the heat-induced aggregation of silver nanoparticles on the surface of SiO2 and the corresponding decrease of the CO adsorption ability, but higher than that of the initial Ag–SiO2 mixture. CO adsorbed on the Ag nanoparticles dispersed on the surface of silica shows a high activity toward CO selective oxidation, and it is proved that the sub-surface oxygen species plays a very important role in this reaction.