The gas-phase catalytic hydrodechlorination (HDC) of 1,3-dichlorobenzene (1,3-DCB) and hydrodebromination (HDB) of 1,3-dibromobenzene (1,3-DBB) have been Studied (T = 573 K) over Ni/SiO2. The catalyst was 100% selective in terms of hydrodehalogenation where HDC far exceeded HDB, an effect that is attributed to substituent inductive effects. Catalytic activity declined with time oil stream with a concomitant increase in partial dehalogenation (to the mono-haloarene). A precontact of the catalyst with the haloarene resulted in a complete loss of any hydrogenation activity. The pre- and postreaction catalyst samples have been characterized by TPR, TEM, H-2 chemisorption, TPD, XRD, and XANES/EXAFS. Appreciable Ni particle growth during catalysis (particularly HDB) has been observed with Ni particle faceting, which is attributed to a halide-induced metal agglomeration/reconstruction; there was no evidence of any bulk nickel halide formation. The possible effect of nickel sintering on hydrodehalogenation was considered by inducing particle growth by thermal means where the resultant sintering did not result in any significant loss of catalytic activity. Catalyst deactivation is linked to a restructuring of the Ni sites during reaction that serves to disrupt H-2 chemisorption/desorption dynamics: the used samples exhibit a lower H-2 uptake with weaker surface interactions. (C) 2004 Elsevier Inc. All rights reserved.