Catalytic hydrodechlorination of chloroaromatic gas streams promoted by Pd and Ni: the role of hydrogen spillover.

Catalytic hydrodechlorination (HDC) is an effective means of detoxifying chlorinated waste. Involvement of spillover hydrogen is examined in gas phase dechlorination of chlorobenzene (CB) and 1,3-dichlorobenzene (1,3-DCB) over Pd and Ni. The catalytic action of single component Pd and Ni, Pd/Al(2)O(3), Ni/Al(2)O(3) and physical mixtures with Al(2)O(3) has been considered. Catalyst activation is characterized in terms of temperature programmed reduction, the supported nano-scale metal phase by transmission electron microscopy and hydrogen/surface interactions by chemisorption/temperature programmed desorption. Pd/Al(2)O(3) generated significantly greater amounts of spillover hydrogen (by a factor of over 40) compared with Ni/Al(2)O(3). Hydrogen spillover on Pd/Al(2)O(3) far exceeded the chemisorbed component, whereas chemisorbed and spillover content was equivalent for Ni/Al(2)O(3). Inclusion of Al(2)O(3) with Ni and Ni/Al(2)O(3) increased spillover with an associated increase in specific HDC rate (up to a factor of 10) and enhanced selectivity to benzene from 1,3-DCB. HDC rate delivered by Pd and Pd/Al(2)O(3) was largely unaffected by the addition of Al(2)O(3). This can be attributed to the higher intrinsic HDC performance of Pd that results in appreciable HDC activity under conditions where Ni/Al(2)O(3) was inactive. Spillover was partially recovered (post TPD) for the Ni samples but the loss was irreversible in the case of Pd.