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

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 (6.7 nm), and the particle size gradually increased approaching almost the size of Au (8.2 nm) 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 C=C group of cinnamaldehyde was preferentially hydrogenated compared to the carbonyl (C=O) 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 C=O 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 C=O bond of CAL in the PdAu alloy particles-containing catalysts. (C) 2014 Elsevier B.V. All rights reserved.