Co-function mechanism of multiple active sites over Ag/TiO2 for formaldehyde oxidation

Ag-based catalysts are shown to be efficient for formaldehyde (HCHO) oxidation at low temperature, while the role of active Ag species in this reaction remains a controversial issue. In this work, we prepared Ag/TiO 2 catalysts with TiO 2 of different crystal structures (anatase or rutile) as supports and tested their performances in HCHO oxidation. We observed that the crystal structure of the TiO 2 support had a significant influence on the activity of the Ag/TiO 2 catalysts, and Ag/A (anatase) exhibited dramatically superior activity compared with Ag/R (rutile). More interestingly, we observed a clear inflection point in the HCHO conversion curve for the Ag/R catalyst. Combining the results of BET, XRD, TEM, XAFS, H 2 -TPR characterization and DFT calculations, we show that HCHO oxidation on Ag/TiO 2 catalysts is a co-function process of multiple active sites including surface oxygen, Ag 2 O, and metallic Ag species. The low-temperature non-renewable surface oxygen and Ag 2 O species mainly contribute to HCHO oxidation in the low-temperature range, while metallic Ag species are primarily responsible for the reaction at high temperature since metallic Ag is capable of activation of oxygen at high temperature. The relative contents of surface oxygen, Ag 2 O and metallic Ag species are different on the Ag/A and Ag/R catalysts. The dispersion degree of metallic Ag species also differs on the Ag/A and Ag/R, therefore their capacities for oxygen activation are quite dissimilar. The above factors combine to result in distinct catalytic behaviors for Ag/A and Ag/R in HCHO oxidation, as well as the appearance of an inflection point.