Mechanistic Study of Hydrogen-Driven Deoxydehydration over Ceria-Supported Rhenium Catalyst Promoted by Au Nanoparticles

The structure and activity of ReO_x-Au/CeO₂ catalysts for deoxydehydration (DODH) of polyols to alkenes with H₂ were investigated in detail. Based on X-ray diffraction (XRD) and transmission electron microscopy (TEM), the sizes of Au particles are in a similar scale to CeO₂ support particles and the number of Au particles is much smaller than that of CeO₂ support particles. Nevertheless, the catalytic activity and temperature-programmed reduction (TPR) data of the physical mixture of Re/CeO₂ and Au/CeO₂ indicate that Re species on all the CeO₂ particles can be reduced with H₂ and can work as a catalytic center. The H₂ activation ability of the catalyst with larger (∼12 nm) Au particles (^impAu; "imp" means impregnation for loading) is lower than that with smaller (∼3 nm) Au particles (^dpAu; "dp" means deposition-precipitation for loading), and the DODH reaction rate over ReO_x-^impAu/CeO₂ is limited by the H₂ activation rate. The CeO₂-supported ^dpAu particles have also higher activity in C=C hydrogenation, C=C migration, and cis/trans isomerization of diols. The C=C hydrogenation and C=C migration are side reactions in DODH, and in the DODH reaction of polyols, such as glycerol and erythritol, ReO_x-^impAu/CeO₂ shows higher yield of DODH products than ReO_x-^dpAu/CeO₂. In contrast, in the case of diols, the selectivity decrease by these side reactions is small, and ReO_x-^dpAu/CeO₂ is a better catalyst than ReO_x-^impAu/CeO₂ because of the higher activity. In addition, the cis/trans isomerization activity of ReO_x-^dpAu/CeO₂ enables the DODH of trans-1,2-cyclohexanediol, which is usually unreactive in DODH, into cyclohexene via isomerization to cis-1,2-cyclohexanediol and the subsequent DODH.