Au/ZrO2 catalysts for LT-WGSR: Active role of sulfates during gold deposition

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Abstract

The effect of the addition of various amounts of sulfates to a zirconia support and its possible role during the Au deposition–precipitation step was examined. The high activity showed by the Au/ZrO2 catalysts in the WGS reaction was enhanced by the action of sulfates on the support. SO42− addition to zirconia brings a higher gold dispersion due to (i) the larger surface area and (ii) the positive role of SO42− groups that determine the deposition of Au in the form of highly dispersed non-metallic gold clusters in close contact with the support.

Introduction

Catalysis by gold nanoparticles is a topic of current interest, as proved by the exponential growth of the papers on this subject [1]. In fact gold supported on oxides or carbon, once considered catalytically inert, is now firmly established as an effective catalyst. The catalogue of reactions that it can catalyze is really wide. In particular, supported gold particles are effective catalysts for low-temperature CO oxidation, selective oxidation of propene to propene oxide, water-gas shift reaction, NO reduction, selective hydrogenation of acetylene (or butadiene) [2].

The relationship between activity, microstructure and nature of the catalytically active gold sites is, up to now, not fully understood. There is still a big debate on the role played by the preparation method and by the acidity of the support and its modification with surface species like sulfates on the nature of the resulting gold species. Experimental data on selective oxidation [3] indicate that there is a limiting diameter size (1.5–2 nm) that discriminates between active samples from the inactive ones. This is the same limiting size observed for both “active” and “inactive” supports, as BN, SiO2 and C on which Au is dispersed. It appears related to an intrinsic modification of the electronic structure of non-metallic gold nanoclusters with respect to the metallic nanoparticles. As for the water-gas shift reaction, cationic gold or highly dispersed non-metallic gold clusters in close contact with the support are usually considered active sites [4]. Very recently, an unprecedented reactivity in the HCOOH decomposition into H2 and CO2 of Au-based catalysts for fuel cells designed for portable use has been reported [5]. It has been shown that the reactivity derives from highly dispersed and stable Au species, undetectable by transmission electron microscopy. The primary role of the support is to have a high concentration of nucleation sites for gold and to avoid coalescence and agglomeration. Characteristics such as surface area, presence of surface hydroxyl groups, density of defects, modification of the surface acidity and of the predominant crystalline phase can influence its adsorption ability [1]. We have recently focused our attention on gold based catalysts supported on zirconia [6], [7]. The choice of zirconia as support is due to its intrinsic chemical and physical characteristics that can be adjusted by choosing different precursors and synthesis conditions. Moreover, the addition of dopants, in particular sulfates, increases surface acidity, retards crystallization and enhances the surface area [8]. ZrO2 has also been found to be a very suitable support for gold [9], [10], [11]. We have optimised a method for dosing low coordination sites exposed at the surface of gold supported on different oxides [12]. This procedure is based on the combined use of pulsed CO chemisorption measurements and FTIR spectroscopy of adsorbed CO, both techniques being applied in well controlled experimental conditions. It has been shown that the amount of low coordinated gold sites exposed at the surface of some Au/ZrO2 systems [7] is up to 10 times higher than the one observed for standard Au/TiO2 by WGC (World Gold Council). In fact, a very good relationship between catalytic activity and chemisorption data has been evidenced [13], indicating that the chemisorption test is suitable for a preliminary evaluation of the Au/ZrO2 systems used for the LT-WGSR. We have recently demonstrated [13] that gold supported on sulfated zirconia is more active than the samples on plain zirconia in the LT-WGSR. This higher activity is probably due to a larger surface area of sulfated zirconia that leads to a better dispersion of gold on the surface. It would be very interesting to clarify the role of SO42− in the delicate phase of gold deposition on the support and subsequently on dispersion and catalytic activity. The goal of the present work is to examine the effects of the addition of various amounts of sulfates to a zirconia support and to check their role during the Au deposition–precipitation step.

Section snippets

Catalyst preparation

Zirconia was prepared by a two-step synthesis technique. Zr(OH)4 was prepared by precipitation from ZrOCl2·8H2O at constant pH (pH = 8.6) and then aged under reflux conditions for 20 h at 363 K, washed free from chloride (AgNO3 test) and dried at 383 K overnight. The hydroxide was then sulfated with (NH4)2SO4 (Merck) by incipient wetness impregnation in order to obtain a 1, 2, 4, 8 wt% amount of sulfates respectively on the final support. Sulfated zirconium hydroxides were then calcined in air (30 

Catalysts characterization

Sulfated zirconia supports and gold samples were analyzed by ion exchange chromatography to determine the amount of sulfate groups and the results are reported in Table 1. As it can be seen, tests of sulfates carried out on the sulfated zirconium hydroxides calcined at 923 K show that the SO42− wt% found is much lower than the expected value. In particular, the difference between the nominal and the actual value increases with the rise of the amount of anions on the support. It is already known

Conclusions

The high activity showed by gold on zirconia catalysts in the water-gas shift reaction was enhanced by the action of sulfates on the support. Such SO42− promotion is carried on in a well defined concentration range. We have demonstrated that sulfates addition to zirconia brings a twofold advantage: (i) a higher gold dispersion due to higher surface area and (ii) a higher gold dispersion due to the positive role of SO42− groups that address the deposition of Au in the form of highly dispersed

Acknowledgments

We thank Prof. Giuseppe Cruciani for XRD data and Mrs. Tania Fantinel for technical assistance. Financial support to this work by MIUR (Rome-Cofin 2006) is gratefully acknowledged.

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