Alumina-supported Ni-Au: Surface synergistic effects in catalytic hydrodechlorination

Catalytic gas-phase hydrodechlorination (HDC) of 2,4-dichlorophenol (2,4-DCP) has been investigated over Ni/Al2O3 and Au/Al2O3 prepared by impregnation, and Au-Ni/Al2O3 prepared by reductive deposition of Au onto Ni. Catalyst activation by temperature-programmed reduction is examined and the activated catalysts are characterized in terms of H-2 chemisorption, XRD and TEM-energy dispersive X-ray (EDX) measurements. Ni/Al2O3 (< 1-10 nm) and Au/Al2O3 (< 1-15 nm) exhibit a relatively narrow metal size distribution while Au-Ni/Al2O3 bore larger particles (1-30 nm) with variable surface Ni/Au ratios. Au/Al2O3 exhibits low H-2 uptake and low HDC activity to generate 2-chlorophenol (2-CP) as the sole product. H-2 chemisorption on Au-Ni/Al2O3 was approximately five times lower than that recorded for Ni/Al2O3 but both catalysts delivered equivalent initial HDC activities. Ni/Al2O3 exhibits an irreversible temporal deactivation where partial dechlorination to 2-CP is increasingly favored over full dechlorination to phenol. In contrast, thermal treatment of Au-Ni/Al2O3 in H-2 after reaction elevates HDC activity with a preferential full HDC to phenol. This response is linked to a surface reconstruction resulting in a more homogeneous combination of Ni and Au. This result was also achieved by a direct treatment of Au-Ni/Al2O3 with HCl. A parallel/consecutive kinetic model is used to quantify the catalytic HDC response.