Influence of the nature of the precursor salts on the properties of Rh–Ge/TiO2 catalysts for citral hydrogenation

Four sets of Rh–Ge/TiO2 bimetallic catalysts were prepared by surface redox reaction (i.e., catalytic reduction method) between hydrogen activated on a Rh parent catalyst and a Ge salt dissolved in aqueous solution. The four sets of catalysts differ depending on the presence or absence of chloride ions in the Rh and Ge precursor salts used (i.e., RhCl3 vs. Rh(NO3)3, GeCl4 vs. GeO2). Samples were reduced either at a lower temperature (300 °C) or at a temperature chosen to induce a strong metal–support interaction (SMSI) effect (500 °C). Catalysts were characterized by elemental analysis, transmission electronic microscopy (TEM), and Fourier transform infrared (FTIR) spectroscopy of adsorbed CO and evaluated for their activity for the gas phase dehydrogenation of cyclohexane and selective hydrogenation of citral. Regardless of the nature of the Rh and Ge precursor salts, the catalytic reduction method causes the Ge to be in intimate contact with the Rh particles, favoring the citral hydrogenation toward unsaturated alcohols (UA: nerol and geraniol). For low Ge loadings, the bimetallic effect can be combined with the SMSI effect. It was observed that the UA selectivity is directly correlated to the ratio R (R = ∑A(COads on oxidized Rh⩾1+ species)/∑A(COads on total exposed Rh species)) determined by FTIR. A better UA selectivity is obtained when bimetallic catalysts possess a surface in a predominantly oxidized state, a situation that is enhanced when chlorinated rhodium and germanium precursors are used.