A series of Pd-only catalysts supported on activated carbon pretreated with HNO(3) or EDTA-2Na for liquid phase selective hydrogenation 3,4-dichloronitrobenzene (DCNB) to corresponding 3,4-dichloroaniline (DCAN) are investigated. Characterization of the catalysts is performed by N(2) adsorption/desorption, TPD, XRD and TPD techniques. The selectivity to DCAN is not only correlate with the Pd particle average size, but also correlate with distribution of Pd particles sizes. The surface groups of activated carbon can influence the average size and distribution of Pd particles. Pretreated activated carbon with HNO(3), forms various oxygen-containing groups on the surface, results in large average size but wide-distributed Pd particles. On the contrast, pretreated activated carbon with EDTA-2Na results in large average size and narrow-distributed of Pd particles simultaneously. More than 60% Pd particles sizes are distributed in the 20-30 nm range of the 2% Pd loading catalyst pretreated with 1 M EDTA-2Na, of which the selectivity to DCAN is up to 99.61%. (C) 2011 Elsevier B.V. All rights reserved.