Co_xFe_{1- x}Al₂O_{4+ δ} composite oxides supported Pt nanoparticles as efficient and recyclable catalysts for the liquid-phase selective hydrogenation of cinnamaldehyde

Co_xFe_{1- x}Al₂O_{4+ δ} composite oxides were prepared using a sol-gel method with glucose as hard template. Pt nanoparticles supported on Co_xFe_{1- x}Al₂O_{4+ δ} were employed for the liquid-phase selective hydrogenation of cinnamaldehyde (CAL) to yield the desired cinnamyl alcohol (COL). The activity increased with the x value; while the selectivity to COL almost kept above 90% despite of different x values. Upon investigation of kinetic behaviour with Pt/Co_0.5Fe_0.5Al₂O_{4+ δ} as a model catalyst, the initial activity (in terms of TOF, defined as the converted CAL molecules per surface Pt atoms per second) of 4.1 s⁻¹ was furnished. As for the selectivity to COL, it almost kept around 95% at whatever CAL conversions despite of the reaction temperatures. Pt/Co_0.5Fe_0.5Al₂O_{4+ δ} catalyst can also be recycled for at least 10 times without obvious loss in activity or selectivity toward COL. This suggests that the active sites on the Pt/Co_0.5Fe_0.5Al₂O_{4+ δ} catalyst preferentially adsorb and activate CAL via the terminal carbonyl groups. XPS analyses, H₂-TPR, and CO-IR studies reveal that the intimate contact of Pt nanoparticles with FeO_x or CoO_x plays an important role in determining the catalytic performance of the Pt/Co_xFe_{1- x}Al₂O_{4+ δ} catalysts. The Pt ^{δ +}/Pt⁰ ratio increased with the x value so that the Pt/CoAl₂O₄ was the most active among the Pt/Co_xFe_{1- x}Al₂O_{4+ δ} catalysts.