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, H2 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 Au to both Ni/support samples lowered the temperature requirements for NiII 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 Au, 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.