Electrochemical characterization of Au/Pd catalysts

Gold-based catalysts have attracted more and more interest in recent years due to their peculiar activity in catalytic reactions, such as low-temperature oxidation of CO, hydrochlorination of alkyne, liquid phase oxidation of alcohols and polyols [1]. However, concerning selective liquid phase oxidation, despite the peculiar selectivity shown, gold catalysts suffer from a severe limitation consisting in the use of a basic environment. This drawback has a strong impact on the use of gold and promoted the evolution of monometallic into bimetallic systems. In the present work, an electrochemical characterization of homemade Au/Pd bimetallic catalysts is performed. Cyclic Voltammetry (CV) and Electrochemical Impedance Spectroscopy (EIS) demonstrate the presence of an electronic interaction between the two metals providing a strong support in the determination of the nature of the synergy between Au and Pd in the liquid phase oxidation of alcohols [2]. Fig.1 shows the voltammetric patterns, recorded in 0.1M H2SO4, for Au, Pd and alloyed Au/Pd nanoparticles deposited for drop casting on a glassy carbon electrode (GC) used as inert support. The nanoalloy presents a larger oxidation peak due to the oxidation of the alloy, which comprises the two oxidation peaks of the single metals, while the reduction peak (at 0.63V) has an intermediate behaviour between Au and Pd. This behaviour indicates that the oxide formed on the alloy is more stable with respect to the gold one but it is really less stable than the palladium one. Charge transfer resistance (RCT) obtained from impedance data are in the order Pd > Alloy >> Au, confirming the intermediate behaviour shown in the voltammetric characterization. Correlating these results to the catalytic behaviour observed in the glycerol selective oxidation of Pd, Au and AuPd catalysts we can conclude that the activities of the different systems seems to be connected not only to the redox behavior. In fact, the higher activity of the AuPd bimetallic system in the liquid phase oxidation of glycerol seems to be due to a compromise between the stability of the oxidic species (decreased with respect to Pd) and the facility of hydride formation (increased with respect to Au). References [1] L. Prati, A. Villa, Gold Catalysis: Preparation, Characterization and Applications, Pan Stanford Publishing Pte. Ltd.: Singapore City, Singapore, (2015). [2] V. Pifferi, C. Chan-Thaw, S. Campisi, A. Testolin, A. Villa, L. Falciola and L. Prati, Molecules, 21, (2016), 261.