OXIDATION-STATE OF GERMANIUM PROMOTER ON A PALLADIUM CARBON CATALYST AND ITS ROLE IN HYDROGENATION REACTIONS

The oxidation state and location of germanium promoter in a carbon-sup ported palladium catalyst were studied by X-ray photoelectron spectros copic, X-ray diffraction, electrochemical and atomic emission spectros copic methods. During promotion with an aqueous GeO2 solution at room temperature, germanium deposited exclusively on palladium particles vi a the ionization of adsorbed hydrogen. Two types of germanium species were identified. Up to a Ge/Pd(s)=0.6 atomic ratio only strongly bound adatoms with a partial positive charge (Ge(n+)) were formed. At highe r promoter/palladium ratios a rapid bulk alloy formation was also obse rved (weakly bound species, Ge0). 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 slurr y revealed that the increase in the oxidation state of germanium durin g hydrogenation reactions was negligible. Germanium deposition onto pa lladium suppressed the hydrogen adsorption and absorption. Accordingly , the activity of the bimetallic catalysts decreased with increasing p romoter/palladium ratio in the liquid phase hydrogenation of organic s ubstrates having various reducible functional groups. An anomalous inc rease in activity and a substantial change in selectivity was observed in the hydrogenation of C=C double bonds conjugated with hydroxyl, ca rbonyl or phenyl groups, while this phenomenon was absent in the reduc tion of substrates having a C=C double bond in conjugation with carbox yl, nitrile or ester functional groups. The role of Ge(n+) acidic cent ers could be excluded and an ensemble effect of Ge adatoms resulting i n changes in the adsorption of substrates and/or products was suggeste d.