Pt on carbon nanofibers (CNF) is one of the most active catalysts for the selective hydrogenation of cinnamaldehyde (CALD), however the selectivity towards the desired cinnamyl alcohol (CALC) is low [1]. Several studies showed that catalysts with increasing Pt particle size result in a higher selectivity to CALC [2-6]. This was also found for Pt on carbon, however Pt particle sizes were varied by applying heat treatments [3, 4]. For these catalysts, heat-treatments also decreased the amount of oxygen surface groups, which can significantly affect the catalytic behavior [1]. Therefore we studied the hydrogenation of CALD over Pt on CNF which differed independently in their metal particle sizes and amount of oxygen surface groups. Pt was deposited on CNF containing oxygen surface groups, resulting in particle sizes of either 2-3 nm (synthesized using homogeneous deposition precipitation) or 3-5 nm (synthesized using atomic layer deposition). The oxygen surface groups were removed largely via heat-treatments in inert conditions, while Pt particle sizes remained unaltered. Pt on CNF with oxygen surface groups showed an expected particle size effect, i.e. larger particles resulted in the highest selectivity to CALC. After removal of the oxygen surface groups the order of selectivity changed, i.e. the highest selectivity to CALC was found for the smallest particles. A model explaining this effect will be discussed. On a non-polar, i.e. no oxygen containing support, the adsorption of the phenyl ring is favored resulting in the direction of the C=O double bond to the metal for small particles giving rise to high selectivities.

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