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The effect of varying the particle size of poly(vinylpyrrolidone)-capped colloidal platinum nanocatalysts, synthesized in the 1–10 nm size range, on the activity and selectivity during the aqueous-phase hydrogenation of cyclohexenone was investigated. For all particle sizes utilized, products observed during the reaction in order of increasing selectivity were cyclohexenol, cyclohexanol, and cyclohexanone. Selectivity for the unsaturated alcohol (cyclohexenol) was found to be highest for the smallest-sized nanocatalyst, whereas selectivity for the saturated ketone (cyclohexanone) was found to be highest for the largest-sized nanocatalyst. Activities for cyclohexenone hydrogenation were observed to increase with increasing particle size, yet apparent activation energies were determined to be similar for 2.9–7.1 nm nanocatalysts, likely indicating that the same surface reaction is kinetically favored on these nanocatalyst surfaces. Attenuated total reflectance infrared (ATR-IR) spectroscopic measurements on adsorbed cyclohexenone suggest the η 1 (C O) form is more prevalent on the smaller sized nanocatalysts, while the η 2 (C C) and η 4 (C O, C C) forms are more preferred on the larger sized nanocatalyst. The findings presented herein provide further evidence that structure sensitivity in ketone hydrogenation reactions is linked with catalyst surface structure and adsorbate orientation. |
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