Structural Properties of Ag/TiO₂ Catalysts for Acrolein Hydrogenation

We have successfully employed Ag/TiO2 catalysts (metal loading, 7 wt %) in the gas-phase hydrogenation of acrolein, the α,β-unsaturated aldehyde which is the most difficult to hydrogenate at the carbonyl group. The relations of the structural characteristics and surface state of these catalysts with respect to their activity and selectivity have been studied by surface analytical techniques (XPS, UPS, ISS) and magnetic resonance (ESR). The catalysts, consisting of titania-supported silver nanoparticles of less than 3 nm mean size, have been formed by various pretreatment procedures including low-temperature reduction at 473 K (LTR) and high-temperature reduction at 700 or 773 K (HTR). The unexpected finding of smaller silver nanoparticles of ≈1.5 nm mean diameter upon high-temperature reduction as compared to ≈3 nm mean diameter upon low-temperature reduction points to growth inhibition by Ti suboxide overlayers (several atomic layers thick) due to strong metal−support interaction, being the more pronounced the higher the reduction temperature applied. This interaction also leads to a truncated particle morphology deviating from spherical shape. The effect of both increasing particle coverage by TiOx and decreasing particle size, as obtained by high-temperature reduction, results in a decrease of the catalyst activity and selectivity to allyl alcohol. This behavior and the absence of Ti3+ at the very catalyst surface point to the fact that, different from the case of catalysts such as Pt/TiO2, TiOx/Ti3+ species do not act as special sites for carbonyl group activation with our Ag/TiO2 catalysts. With the LTR catalyst, the specific activity was 1 order of magnitude higher, compared to the HTR catalyst, and was accompanied by the formation of allyl alcohol with a much larger selectivity (LTR, 42%; HTR, 27%)