Combining Benzyl Alcohol Oxidation Saturation Kinetics and Hammett Studies as Mechanistic Tools for Examining Supported Metal Catalysts

Understanding and quantifying how the active sites in supported metal catalysts can be modified are critical for rationally designing catalysts. This problem is particularly complex for reactions that occur at the metal-support interface (MSI) because of the multiple chemistries associated with the metal and the support. In this study, we used the oxidation of substituted benzyl alcohol over Au/TiO2 and Au/Al2O3 to probe MSI chemistry. Substituents impacted substrate binding, deprotonation, and the rate-limiting transfer of a hydride from benzyl alcohol to Au, as shown by a combination of Michaelis-Menten (M-M) saturation kinetics and kinetic isotope effects. Hammett studies performed with a single substrate versus those done with two substrates together in competition experiments showed significant differences, which were attributable to stronger competitive adsorption on the support by more electron-rich alcohols. The M-M analysis showed that alcohol substitution impacts substrate binding and deprotonation equilibria, which in turn affect the number of active alkoxides adsorbed at the MSI. Hammett slopes should therefore be measured under saturating conditions using one substrate at a time. The Hammett slopes measured for heterogeneous systems in this manner agree well with the KIE-Hammett slope relationship determined in homogeneous systems, which provide information on the early or late nature of the transition state. Our results show that the combination of Michaelis-Menten and Hammett techniques for benzyl alcohol oxidation provides mechanistic information associated with the MSI chemistry of supported Au catalysts as well as information on active site electronics.