Alumina‐Supported Ni–Au: Surface Synergistic Effects in Catalytic Hydrodechlorination

Abstract

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 H2 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 H2 uptake and low HDC activity to generate 2‐chlorophenol (2‐CP) as the sole product. H2 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 H2 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.

Sites for the selective hydrogenation of ethyne to ethene on supported NiO/Au catalysts

“…The similar shape was observed for NiO particles immobilized on TiO 2 , ZnO, and ZrO 2 ( Table 1). The obtained results are in agreement with the shape of nickel particles immobilized by impregnation from the aqueous solution of Ni(NO 3 ) 2 [9].…”

“…The rate-determining step in hydrogenation is usually a dissociative adsorption of H 2 that takes place at Ni 0 sites not at Ni 2+ [4]. Keane and Medina showed that large NiO clusters are reduced much easier to Ni 0 in comparison with small NiO clusters that strongly interact with surface groups of the support [9,27]. Therefore, the 7 nm NiO of the sample №s 8 and 11 possess higher activity in hydrogenation in comparison with the 3 and 4.5 nm NiO of the sample № 3 and № 5 ( Table 1).…”

“…For example, Keane linked the increase of the rate of 2,4-dichlorophenol hydrodeclorination on Au/Ni/Al 2 O 3 with decoration of nickel clusters with gold ones [9]. It was proposed that the Au component served to adsorb and activate the C-Cl bond whereas the H 2 dissociated at the Ni 0 .…”

“…Nano-sized gold is an effective catalyst for oxidation [6], isomerization [7], cyclization [8], hydrodechlorination [9], and other reactions [10,11]. Composites based on bare gold clusters demonstrate high activity and/or selectivity in hydrogenation of 1,3-butadiene [2], 1-propyne [12], and ethynylbenzene [13] under the mild conditions.…”

“…Composites based on bare gold clusters demonstrate high activity and/or selectivity in hydrogenation of 1,3-butadiene [2], 1-propyne [12], and ethynylbenzene [13] under the mild conditions. Bimetallic goldnickel catalysts also may possess the enhanced catalytic properties in conversion of different hydrocarbons: n-butane [14], allylbenzene [5,7], and 2,4-dichloropfenol [9]. Thus, the application of gold and gold-nickel clusters in selective hydrogenation is very promising.…”