Metal-support effects on acetone hydrogenation over platinum catalysts

Abstract

Acetone hydrogenation was studied over Pt TiO 2 , Pt η- Al 2O 3 , Pt SiO 2 , Pt powder, and Pt Au catalysts to test the hypothesis that the metal-support effect responsible for higher CO hydrogenation rates over certain metal/TiO 2 catalysts represents a phenomenon capable of activating carbonyl bonds in general. Compared with the other catalysts, the high-temperature reduced (HTR) Pt TiO 2 samples had turnover frequencies more than 500 times higher than those of unsupported Pt and Pt SiO 2 catalysts, and the specific activity (per g Pt) of the Pt TiO 2 catalyst was 10 times that of a Pt SiO 2 catalyst with comparable dispersion. Complete hydrogenation to C 3H 8 and H 2O occurred only on large, unsupported Pt crystallites; however, partial hydrogenation to isopropyl alcohol appeared to be structure insensitive and activation energies were similar over all catalysts, as were pressure dependencies, which associates the higher activity with a larger preexponential factor. Only one Langmuir-Hinshelwood model provided a rate expression consistent with experimental results—that which assumed competitive adsorption of H 2 and acetone on the same sites and addition of the second H atom as the rate-determining step. This model is consistent with previous TPD, IR, and EELS studies and is also substantiated by theoretical calculations based on the bond-order conservation method. The much higher activities over Pt TiO 2 catalysts are attributed to an increase in the active site concentration in the Pt-titania interface region. These special sites are presumed to be defects on the titania surface near the Pt that can activate the carbonyl bond in the presence of atomic hydrogen provided by the Pt. This model is very similar to that proposed to explain higher CO hydrogenation activities over TiO 2-supported Pt.