Stability and selectivity of Au/TiO2 and Au/TiO2/SiO2 catalysts in propeneepoxidation: An in situ FT-IR study

In situ FT-IR spectroscopy was used to study the surface species involved i n the selective epoxidation of propene with an O-2/H-2 mixture over a deact ivating 1 wt% Au/TiO2 catalyst and a non-deactivating 1 wt% Au/TiO2/SiO2 ca talyst. Propene adsorbs weakly on both catalysts via gamma -hydrogen bondin g with surface hydroxyl groups of the TiO2 or TiO2/SiO2 support. Propene ad sorption is completely reversible at temperatures between 300 and 400 R. Ir reversible adsorption of propene oxide (PO) was observed on both catalysts, yielding bidentate propoxy moieties. Similar propoxy species are formed af ter prolonged exposure of the catalysts to a propene/oxygen/hydrogen mixtur e. Deactivation of TiO2 catalysts is explained by the formation of these pr opoxy groups on Ti sites active and selective in propene epoxidation. Neigh boring acidic Ti sites are involved in coupling of PO onto these sites. Occ upation of selective sites with propoxy groups is limited on TiO2/SiO2 cata lysts. On this support, propoxy groups are located on acidic agglomerated T iOx sites and linked to an Si-OH functionality. These groups are not involv ed in the selective epoxidation of propene, which occurs over isolated tetr ahedral sites. Besides propoxy groups, surface compounds with absorptions in the C=O stret ching range (acetone and propanal) were observed on both catalysts. Formate and acetate species (implying C-C bond cleavage) were identified exclusive ly on the Au/TiO2 catalyst. The relevance of these species to the selectivi ty of the catalysts investigated is discussed. (C) 2001 academic Press.