The liquid-phase hydrogenation of 1-heptyne over Pd–Au/TiO2 catalysts prepared by the combination of incipient wetness impregnation and deposition–precipitation

TiO2-supported Pd–Au catalysts were prepared with Pd (0.5 wt%) by impregnation and then Au (1.0 wt%) by deposition–precipitation (Au/Pd/TiO2) and vice versa (Pd/Au/TiO2). For the former sample, the state of dispersion of Pd on TiO2 changed during the loading of Au, resulting in the formation of small Au–Pd alloy particles and thus changing the electronic properties of Pd species. The alloy formation was not clearly detected on the Pd/Au/TiO2, and the electronic properties of Pd were not so different from those of monometallic Pd/TiO2. All the catalysts were selective to 1-heptene (>95%) until close to the complete conversion of 1-heptyne. However, the selectivity of 1-heptene drastically decreased to 0% (which was further hydrogenated to heptane) in 120 min for the Au/Pd/TiO2 while it still remained >60% for the Pd/Au/TiO2 similar to the monometallic Pd/TiO2 catalyst. The rate of 1-heptene hydrogenation (in the second step) did not depend on the Pd dispersion but was found to be greatly enhanced as the Pd species became electron-rich in the Au–Pd alloy particles. The modification of Pd species by Au was less significant for the first step of hydrogenation of 1-heptyne to 1-heptene.