Oxidation state of germanium promoter on a palladium/ carbon catalyst and its role in hydrogenation reactions

Abstract

The oxidation state and location of germanium promoter in a carbon-supported palladium catalyst were studied by X-ray photoelectron spectroscopic, X-ray diffraction, electrochemical and atomic emission spectroscopic methods. During promotion with an aqueous GeO 2 solution at room temperature, germanium deposited exclusively on palladium particles via the ionization of adsorbed hydrogen. Two types of germanium species were identified. Up to a Ge/Pd 8 = 0.6 atomic ratio only strongly bound adatoms with a partial positive charge (Ge n+ ) were formed. At higher promoter/palladium ratios a rapid bulk alloy formation was also observed (weakly bound species, Ge 0). The rate of bulk alloy formation was dependent on the particle size of palladium and on the initial Ge/Pd ratio. Measurement of the open circuit potential of the catalyst slurry revealed that the increase in the oxidation state of germanium during hydrogenation reactions was negligible. Germanium deposition onto palladium suppressed the hydrogen adsorption and absorption. Accordingly, the activity of the bimetallic catalysts decreased with increasing promoter/palladium ratio in the liquid phase hydrogenation of organic substrates having various reducible functional groups. An anomalous increase in activity and a substantial change in selectivity was observed in the hydrogenation of C=C double bonds conjugated with hydroxyl, carbonyl or phenyl groups, while this phenomenon was absent in the reduction of substrates having a C=C double bond in conjugation with carboxyl, nitrile or ester functional groups. The role of Ge n+ acidic centers could be excluded and an ensemble effect of Ge adatoms resulting in changes in the adsorption of substrates and/or products was suggested.