A series of Pd catalysts supported on various silicas including mesoporous silica and non-porous silica was prepared in order to elucidate activity controlling factors for the selective hydrogenation of 1-nitrohexane to N-hexyl hydroxylamine ((n)Hex-NHOH). The Pd on silica (5 wt.%) catalysts were characterized by XRD, N-2 adsorption/desorption, and CO chemisorption. The hydrogenation was done in isopropyl alcohol under 1 bar of H-2 at room temperature. The Pd catalysts supported on non-porous silica or silicas that possessed large pores efficiently produced (n)Hex-NHOH in high yields, presumably because the influence of pore diffusion was small. Additionally, the catalytic activity strongly depended on the Pd dispersion: the activity generally decreased with the increased Pd dispersion (or the decreased Pd particle size). On the other hand, the yield of (n)Hex-NHOH was little affected by the initial oxidation state of surface Pd before hydrogenation. (C) 2011 Elsevier B.V. All rights reserved.