The gas-phase hydrodechlorination (HDC) of chlorobenzene (CB), 1,2-dichlorobenzene (1,2-DCB), and 1,3-dichlorobenzene (1,3-DCB) was investigated over a 5% w/w Pd/SiO2 and a series of SiO2-supported Yb-Pd (5% w/w Pd and Yb/Pd = 2/3 mol/mol) catalysts. The Pd/SiO2 catalyst was prepared by Pd(C2H3O2)(2) impregnation, supported Yb synthesized by contacting SiO2 with Yb powder in liquid NH3 and the bimetallic catalysts prepared by two stepwise routes: Pd(C2H3O2)(2) impregnation of Yb/SiO2 (Pd-Yb/SiO2-step) or Pd impregnation preceding Yb introduction (Yb-Pd/SiO2-step) and a single-step simultaneous introduction of Pd and Yb from a {(DMF)(10)Yb-2[Pd(CN)(4)](3)}(infinity) precursor (Yb/Pd/SiO2-sim). Under identical reaction conditions the following specific initial CB HDC rate sequence was established: Pd-Yb/SiO2-step (0.21 mol(Cl) h(-1) m(-2)) Pd/SiO2 (0.24 mol(Cl) h(-1) m(-2)) < Yb/Pd/SiO2-sim (0.41 mol(Cl) h(-1) m(-2)) approximate to Yb-Pd/SiO2)-step (0.46 mol(Cl) h(-1) m(-2)): reaction over Yb/SiO2 resulted in a negligible conversion. Yb acts as a CB HDC promoter through a Surface synergism with Pd: the extent of this promotion depends on the nature of the catalyst precursor and the sequence of metal(s) introduction to the support. The promotional effect of Yb extends to DCB HDC, where Yb-Pd/SiO2-step outperforms Yb/Pd/SiO2-sim (with a specific HDC rate 25 times greater than that delivered by Pd/SiO2) and to catalytic hydrogenation (benzene -> cyclohexane). The prereaction and postreaction catalyst samples were characterized in terms of BET area, TPR, TEM-EDX, H-2 chemisorption/TPD, XRD. and XPS measurements. The role of Yb as a promoter is discussed in terms of electron donation and impact on Pd dispersion but is attributed to the action of YbH2, which serves as an additional source of surface reactive hydrogen: Yb activation of the C-Cl bond(s) for hydrogenolytic attack is also considered. HDC activity decreased with time-on-stream. an effect that we link to deleterious HCl/catalyst interactions that modify surface composition. leading to a disruption in H-2 uptake/release: XPS and TEM-EDX were used to characterize the residual surface Cl post-HDC. (c) 2006 Elsevier Inc. All rights reserved.