DRIFT, XPS and XAS Investigation of Au–Ni/Al2O3 Synergetic Catalyst for Allylbenzene Isomerization

Abstract

Au/Al 2 O 3 and Ni/Al 2 O 3 prepared by deposition-precipitation and impregnation methods, respectively, and Au-Ni/Al 2 O 3 prepared by impregnation of the Au/ Al 2 O 3 system with nickel have been studied by physicochemical methods. The catalysts were tested in allylbenzene isomerization at 443 K. The isomerization on Au/Al 2 O 3 proceeds with a rather low rate, while monometallic Ni/Al 2 O 3 is inactive. Unlike monometallic catalysts, Au-Ni/Al 2 O 3 demonstrates a strong synergetic effect: the isomerization rate on the bimetallic sample exceeds the sum of the rates over the individual metals by an order of magnitude. The catalysts structure was estimated by XPS, DRIFT, and XAS techniques. It was revealed that nickel in Au-Ni/Al 2 O 3 and Ni/Al 2 O 3 exist as Ni 2? cations with the coordination state close to that in the precursor, gold in Au/Al 2 O 3 exist as Au 0 nanoclusters, whereas the Au 0 nanoclusters co-exist with Au 3? cations in the bimetallic Au-Ni/Al 2 O 3 sample. Reasons of the synergetic catalytic effect for Au-Ni/Al 2 O 3 are discussed in terms of the formation of new positively charged Au catalytic centers.

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

“…The addition of tin to the Pt/SiO 2 catalyst produced a remarkable positive shift in both the product distribution and TOF in the liquid-phase hydrogenation of unsaturated aldehyde to corresponding alcohols [43,44]. The simultaneous increase in both activity and selectivity indicates the formation of a new type of mixed (M x+ )/(Pt) 0 clusters, different in nature when Our investigations of model Au, Ni/Au, and Ni catalysts by XPS, DRIFT, and XAS techniques showed that nickel in both Ni/Au and Ni catalysts exists as NiO, gold in the Au catalyst exists as Au 0 nanoclusters, whereas the Au 0 nanoclusters co-exist with Au 3+ cations in the NiO/Au samples [7]. Then, the first probable explanation (I) of synergistic activity between NiO and Au in ethyne hydrogenation (Table 1) could be due to the electron transfer from the electron-rich Au 0 particles to the electron-deficient NiO that give new Au δ+ catalytic sites.…”

“…An increase in the surface positive charge on Au due to electron transfer [7] improves the electron acceptor properties of gold and thereby should lead to stronger adsorption of ethyne or ethene on Au δ+ than on Au 0 and, therefore, increase the probability of the chemical reaction. Table 2 shows that positive charge on the flat as well as on the Au 12 + (2D) and Au 12 + (3D) cluster leads to an increase in the heat of adsorption of ethene in π-complex up to 132 and 147 kJ/mol, respectively.…”

“…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.…”

“…It was found that synergistic gain on NiO/Au clusters was maximized at 357 K, and Au/Ni ratio equaled 1:1 [15]. According to the analysis of oxidation state of metals in gold-contained catalysts [7], it was proposed that synergistic effect could be caused by the formation of new Au δ+ catalytic sites. The goals of the present research were: (1) to study the regularities of catalytic action of Au and NiO/Au clusters (Au/ Ni01:1), supported on Al 2 O 3 , TiO 2 , ZrO 2 , and ZnO, in the hydrogenation of ethyne-ethene mixture at 357 K; (2) to study the impact of clusters shape and size to the catalysis by gold, and (3) to determine the origin of the active gold sites by density functional theory (DFT).…”