Effect of noble metal on selective hydrogenation of cinnamaldehyde over cobalt-based catalyst

gamma-Al2O3 was used as a support, and Co/gamma-Al2O3 catalyst was prepared by wet impregnation from precursor CoCl2 together with KOH. H2PtCl6, PdCl2 and RuCl3 were used to modify the cobalt-based catalyst, and the catalyst precursor was dried at 210 degreesC and calcined at 450 degreesC for 3 h. Before used, the catalyst samples were reduced at 450 degreesC by hydrogen flow (30 ml/min) for 3 h. The selective hydrogenation of cinnamaldehyde (V(EtOH)/V(PhCH:CHCHO) = 9) was performed in a stainless steel autoclave with magnetic stirring. After covering, the air was replaced by hydrogen. The activity and selectivity of Co/gamma-Al2O3 increased with the Co-loading when it is less than 9% and only showed a small increment when it is more than 9%. Selective hydrogenation of cinnamaldehyde over Co/Al2O3 catalyst modified by noble matels was studied. The activity of Co-Pt/Al2O3 catalyst is straightly increased with Pt content when it is lower than 0.5% and was 5 times that of Co/Al2O3 when Pt loading reaches 0.5%. The activity of Co-Pd/Al2O2 and Co-Ru/Al2O3 increased, but their selectivity decreased to 0.3% and 83.8%, respectively. Only the selectivity of Co-Pt/Al2O3 was higher than 95% when the Pt loading is 0.5%. It is due to the interaction of Co and Pt. The XRD result showed that the Co3O4 in the modified catalyst was reduced to alpha-Co-0. The TPR result showed that the presence of small amount of noble metals promotes the reduction Of Co3O4 in the catalyst, and the reduction temperature Of Co3O4 decreased with noble metal loading obviously. The XPS result indicated that the Co3O4 in Co-Pt/Al2O3 with small amount of Pt can be reduced to Co-0, but can only be reduced to Co-0 partially in Co-Pd/Al2O3 and Co-Ru/Al2O3, and the other Co species is CoO. So, the Co-0 amount on the catalyst surface can not be correlated with the activity.