Palladium catalysts in the form of Pd nanoparticles supported on nanodiamonds have been studied in the hydrogenation of nitrobenzene, allyl alcohol, and cyclohexene and in the hydrogenating amination of propanal with 4-aminobenzoic acid. The ratio of two valence states of palladium, i.e., Pd2+ and Pd-0, in the catalysts has been determined by XPS. The dependence of hydrogenation reaction rate on electron density at the reaction site of nitrobenzene, allyl alcohol, cyclohexene, and 4-(propylideneamino)benzoic acid molecules has been studied using quantum chemical calculations (HF/6-31G, PCM).