To identify an effective catalyst for hydrodechlorination (HDC) reaction in an aqueous solvent, three types of modified Pt/SiO2 catalysts were prepared using the water-repellent organosilane reagents of butyldimethylchlorosilane (BDMS), dimethyloctylchlorosilane (DMOS), and dimethyloctadecylchlorosilane (DMODS). The catalysts were characterized and their HDC activity toward para-chloroacetophenone (CLAP) was examined in an aqueous solvent. The BET specific surface area, mesopore volume, platinum surface area, and high oxidation species of platinum surface of Pt/SiO2-DMODS slightly decreased after the modification. Elemental analysis showed that 0.272 groups/nm(2) of the DMODS substituent were tethered on the catalyst surface. Notably, in a water (35 mL)/ethanol (5 mL) mixed solvent under 1 MPa of hydrogen at 373 K for 60 min, the HDC reaction over the modified catalysts readily took place. From the difference in the extent of HDC product yields for the reactions, it was clear that the HDC activity of the catalysts decreased in the order Pt/SiO2-DMODS approximate to Pt/SiO2-DMOS > Pt/SiO2-BDMS >> Pt/SiO2. Furthermore, the turnover frequency (TOF) of Pt/SiO2-DMODS (10.7 min(-1)) was found to be more than fifty times that of Pt/SiO2 (0.2 min(-1)). These results indicate that the catalytic activity was significantly improved by the surface modification of the Pt/SiO2 catalyst with the water-repellent organosilane reagents. The high HDC activity of the modified catalysts is believed to result from the formation of a hydrophobic space on the catalytic support surface by the water-repellent organosilyl substituents. Therefore, more reactants in the aqueous solvent can interact with this space, leading to a significant increase in the number of collisions between the reactants on the active site. The effects of the solvent composition and reaction temperature on the HDC activity of Pt/SiO2-DMODS were also examined. (c) 2013 Elsevier B.V. All rights reserved.