The effect of metal-support interactions on the hydrogenation of benzaldehyde and benzyl alcohol

Both rates (per g catalyst) and turnover frequencies for benzaldehyde hydrogenation were markedly enhanced on Pt/TiO2 after a high-temperature reduction (HTR) step, compared to either Pt/TiO2 after a low-temperature reduction (LTR) step, Pt/SiO2 or Pt/Al2O3. All these catalysts consisted of dispersed 1-nm Pt crystallites and ail gave similar activation energies of 8 +/- 2 kcal/mole. More importantly, Pt/TiO2 (HTR) retained a selectivity to benzyl alcohol of 100% up to conversions of 80%, whereas significant amounts of toluene and benzene began to be formed at conversions above 20% with the other Pt catalysts, including Pt powder. The specific activity for pure benzyl alcohol hydrogenation was similar to that for pure benzaldehyde on PtAl2O3 and Pt/TiO2 (LTR), but it was greatly suppressed on Pt/TiO2 (HTR). Thus, in addition to a higher formation rate of benzyl alcohol and less competitive benzyl alcohol adsorption compared to benzaldehyde, the lower intrinsic rate constant for benzyl alcohol hydrogenation further enhances the maintenance of high selectivity to benzyl alcohol at high conversions. A simple Langmuir-Hinshelwood model invoking the addition of the second H atom to benzaldehyde as the rate-determining step fit the data well. (C) 1997 Academic Press.