Silica- and titania-supported Ni-Au: Application in catalytic hydrodechlorination

The catalytic gas-phase (473 K) hydrodechlorination (HDC) of 2,4-dichlorophenol has been investigated over Ni/SiO2, Ni/TiO2, Ni-Au/SiO2, and Ni/Au-TiO2 (Ni loading ca. 5 wt%; bulk Ni/Au atomic ratio = 10). The samples were prepared by either (co-)impregnation or (co-)deposition-precipitation. The catalyst samples were characterized in terms of BET surface area, TPR, H-2 chemisorption, and TEM-EDX measurements. The impregnated Ni/SiO2 and Ni/TiO2 samples had a similar narrow Ni particle size distribution (1-6 nm). The addition of An to both Ni/support samples lowered the temperature requirements for Ni-II reduction and suppressed H2 chemisorption. The impregnated Ni-Au/SiO2 (10-150 nm) and Ni-Au/TiO2 (2-95 nm) were characterized by a wider range of particle sizes than the monometallic nickel catalysts and a variable surface Ni/Au atomic ratio (< 1-40). In comparison, Ni-Au/TiO2 prepared by deposition-precipitation exhibited a narrower particle size range (2-60 nm) and a more uniform Ni and Au surface distribution (Ni/Au atomic ratio of < 1-15). The titania-supported catalysts delivered significantly higher specific HDC rates and distinct HDC selectivities than the silica systems, with catalytic responses discussed in terms of metal-support and reactant-surface interactions. The incorporation of An, regardless of the support or method of preparation, resulted in higher HDC activity, an effect attributed to a surface Ni-Au synergism. Hydrogen thermal treatment of the bimetallic catalysts after reaction resulted in appreciable surface reconstruction, notably a more homogeneous combination of Ni and Au in smaller particles, which enhanced HDC performance. (C) 2007 Elsevier Inc. All rights reserved.