Effect of the supports on catalytic activity of Pd catalysts for liquid-phase hydrodechlorination/hydrogenation reaction.

Carbon nanotubes (CNTs), activated carbon (AC), graphene, and aluminum oxide (Al2O3) supported 5% Pd catalysts were prepared by the conventional impregnation method, and catalytic activity was tested in the hydrogenation of 4-chlorophenol (4-CP) and nitrobenzene (NB) under ambient conditions (313 K and atmospheric pressure). It was found that catalytic activity was greatly affected by the supports. Moreover, Pd/CNTs catalyst exhibited much higher catalytic activity than the other three supported Pd catalysts. The mechanism of this phenomenon was studied through catalyst characterization (ICP-MS, Brunauer-Emmett-Teller [BET], TEM, and SEM). It was found that the mean particle size of Pd nanoparticles for Pd/CNTs (4.3 nm) was smaller than that for Pd/AC (6.9 nm), Pd/Al2O3 (5.0 nm), and Pd/graphene (5.2 nm). Moreover, the actual loading amounts of Pd and BET surface areas were not the main reasons for the different catalytic activity of the four supported Pd catalysts. Above all, the smaller Pd particles of Pd/CNTs enabled the Pd/CNTs catalyst to exhibit much higher catalytic activity for the hydrogenation reactions.