Liquid-Phase Hydrogenation of Cinnamaldehyde: Enhancing Selectivity of Supported Gold Catalysts by Incorporation of Cerium into the Support

The gold nanocrystals supported over CeO2-containing mixed-metal oxides were synthesized by a reduction deposition approach followed by calcination. The zerovalent Au-0 was obtained from the reduction of Au3+ ion by the hydrolysis of sucrose in an aqueous solution. The thermal post-treatment led to supported gold catalysts, in which Au nanoparticles with adjustable sizes were anchored onto the mixed oxides. The incorporation of cerium (Ce) into the support remarkably enhanced the selectivity toward C=C bond (hydrocinnamaldehyde, HCAL, ca. 83%) in cinnamaldehyde hydrogenation than the catalyst with no cerium (ca. 42%) under a high conversion (above 91%). The enhancement of selectivity to HCAL could be attributed to the decreasing sizes of Au and/or CeO2, the morphology effect of Au, and the interaction of Au and CeO2 components in the support, revealed by XRD, HRTEM, and XPS. The increasing Ce3+ amount in the catalysts leads to more oxygen vacancies. The surface electron density of Au diminishes due to the presence of oxygen vacancies. The morphological and electronic aspects of Au particles result in favorable adsorption of C=C bond versus C=O bond. A control experiment showed that the Au/CeO2 catalyst has a relatively low activity and selectivity under the identical reaction conditions. This finding indicated that a better dispersion and decreased size of CeO2 in the mixed oxides could be the key factors to enhancing the selectivity of supported Au catalysts.