| The effect of metal-support interactions (MSI) on the activity and selectivity of Au and Pt catalysts was examined by kinetic studies using Au and Pt dispersed on different supports. Au catalysts were found to be active for CO oxidation at room temperature, and after a high-temperature reduction (HTR) step followed by calcination at 673 K and a low-temperature reduction (LTR) step, TiO{dollar}sb2{dollar}-supported Au was found to be almost 100-fold more active than a Pd/Al{dollar}sb2{dollar}O{dollar}sb3{dollar} catalyst at 300 K.; Benzene and toluene hydrogenation over Pt/SiO{dollar}sb2{dollar}, Pt/Al{dollar}sb2{dollar}O{dollar}sb3{dollar}, Pt/SiO{dollar}sb2{lcub}cdot{rcub}{dollar}Al{dollar}sb2{dollar}O{dollar}sb3{dollar} and Pt/TiO{dollar}sb2{dollar} was studied. The activation energy and the apparent reaction orders on H{dollar}sb2{dollar} and benzene (or toluene) were almost independent of the support. The turnover frequency was found to vary with the support, and a more acidic material like SiO{dollar}sb2{lcub}cdot{rcub}{dollar}Al{dollar}sb2{dollar}O{dollar}sb3{dollar} gave a more active catalyst than a nonacidic oxide such as SiO{dollar}sb2{dollar} or Al{dollar}sb2{dollar}O{dollar}sb3{dollar}. The model previously proposed for Pd is also applicable to these Pt catalysts. This model attributes the enhanced activity with acidic supports to an additional contribution from the metal-support interfacial region in which the aromatic molecules adsorb on acid sites and react with hydrogen migrating from the metal surface. Among nine reaction sequences examined to describe the reactions over Pt surfaces, only the model invoking the addition of the first H atom as the rate-determining step along with a concurrent surface decomposition reaction of the aromatic could explain the benzene reaction and also describe the toluene reaction.; Acetophenone hydrogenation was studied over Pt catalysts. Pt/SiO{dollar}sb2{lcub}cdot{rcub}{dollar}Al{dollar}sb2{dollar}O{dollar}sb3{dollar} converted acetophenone almost completely to ethylbenzene. The other Pt catalysts--Pt/SiO{dollar}sb2{dollar}, Pt/Al{dollar}sb2{dollar}O{dollar}sb3{dollar}, and Pt/TiO{dollar}sb2{dollar}--were more selective to phenylethanol, and Pt/TiO{dollar}sb2{dollar} (HTR) was the most selective of all--producing over 90% phenylethanol at conversions up to 20%. The high selectivity to phenylethanol over Pt/TiO{dollar}sb2{dollar} (HTR) can be explained by a model invoking special metal-support interfacial sites that interact with the oxygen end of the carbonyl bond and activate this bond toward hydrogen. This study shows that the support can have a significant effect on selectivity and that the concept of MSI has potential for use in the synthesis of specialty chemicals. |
