Hydrogenation of cinnamaldehyde in the presence of PdAu/C catalysts prepared by the reverse “water-in-oil” microemulsion method

Abstract

Liquid phase hydrogenation of cinnamaldehyde was studied on carbon supported (Vulcan XC72) – PdAu catalysts with Au/Pd atomic ratio varying from 0.1 up to 2.1. Colloid-based method, namely the reverse “water-in-oil” microemulsion technique was used to prepare the catalysts. Accordingly, pre-formed metal nanoparticles of controlled size with a narrow size distribution were deposited on carbon support. The BET, XRD, XPS, SEM, TEM, and HRTEM techniques were employed to characterize PdAu/C catalysts and their monometallic 2%Pd/C and 2%Au/C counterparts. The metal particles were spherically shaped, nearly monodispersed and well distributed throughout the carbon support. The particles of low Au-content (Au/Pd<0.8) were of smaller size than Pd (6.7nm), and the particle size gradually increased approaching almost the size of Au (8.2nm) at Au/Pd=2.1. It was found that the content of Au influenced both activity and selectivity of PdAu/C catalysts for cinnamaldehyde hydrogenation. The CC group of cinnamaldehyde was preferentially hydrogenated compared to the carbonyl (CO) group on the Pd/C catalyst. At Au/Pd<0.8 the effect of Au-content was relatively weak as reflected in the small decrease in the activity with only slight increase in the reactivity to CO hydrogenation. The effect of Au manifested distinctly on Au-rich catalysts (Au/Pd>1) giving strongly reduced activity to saturated aldehyde formation accompanied by the selectivity preference to the carbonyl group reduction. These activity/selectivity effects induced by the gold were discussed to be related with the microstructure of metal particles determining their surface composition. Moreover, both geometric and electronic modifications of Pd-sites due to the Pd–Au interactions should be also taken into account. These interactions may also provide new active sites which are able to activate the CO bond of CAL in the PdAu alloy particles-containing catalysts.