Preparation and water-gas shift performance of zinc oxide supported dispersed gold catalysts
Master Thesis
2007
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University of Cape Town
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Abstract
Two deposition-precipitation style methods of preparing zinc oxide supported dispersed gold materials for use as water-gas shift catalysts were examined, with some of the better formulated materials being tested for catalytic activity, and compared to World Gold Council Au/TiO₂ reference material and a commercial copper-based WGS catalyst (Cu/ZnO/AlO₃ - C 18-7 from Sud-Chemie). Materials Synthesis: The classical deposition-precipitation synthesis from the group of Haruta (Tsubota et al., 1995) - where the support is added to a pH adjusted solution of HAuCl₄ and the system aged at constant pH and temperature - was examined, using ZnO as the support. Gold uptake by the support was confirmed to decrease with ageing pH, tending to zero as the IEPS of ZnO (~ 9) is approached. Such behaviour is both qualitatively and quantitatively consistent with theory, which proposes that the magnitude and polarity of the charge on the support surface will determine the effective carrying capacity of that surface for an (an)ionic solution phase gold species. Decreasing post-calcination (120°C) gold crystallite size with increasing ageing pH [as reported by Haruta (1997)] was also observed (figure 11.2) - but it is not clear whether this resulted from pH dependent crystallization dynamics, from crystallite size being simply determined by the amount of deposited gold (which clearly decreases with increasing pH), or from chloride induced sintering during heat treatment (with chloride uptake by the support decreasing with increasing pH [Kung et al., 2003)). Nevertheless, gold deposition at pH 8 produced highly dispersed gold crystallites around 3.5 nm in diameter. It emerged that an inherent trade-off exists with this, the classical depositionprecipitation method, in that acidic ageing pH promotes a high degree of gold uptake by the support, but produces large gold crystallites, and vice versa. To overcome this, a modified method - where HAuCI₄ and the base (ammonium carbonate) were simultaneously added dropwise to a slurry of the support, maintaining a constant pH of 8 (Fu et al., 2003b) - was investigated. This method was attractive because it is claimed to simultaneously achieve total gold uptake and post-calcination Au crystallite size in the range 5 - 6 nm. Since it was not clear from the published description whether a constant pH was maintained across the ageing period (practiced here as MDP1 ), or if the pH was rather allowed to drift (practiced here as MDP2), both alternatives were investigated. When a constant pH was maintained across the ageing period (MDP1 ), gold uptake by the support was found to reach a maximum (of ~ 60 %) when operating at a pH of ~ 8. The degree of gold uptake was found to be independent of both gold loading and support surface area. Furthermore, the degree of gold uptake achieved using this variation was increased to unity by allowing the pH to drift during the ageing period (after being initially held constant at 8 during HAuCI₄ addition) [= MDP2], instead of being maintained at a constant value via addition of nitric acid (as is done in MDP1). In terms of the size of the gold crystallites produced, after calcination in air at 400°C, a mean diameter of 3.8 ± 1.5 nm was observed for a sample 1.9 wt % in Au, increasing slightly with increasing gold loading [to 4.6 ± 1. 7 nm by 5.1 wt %Au].
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Includes bibliographical references (pages 99-107).
Reference:
Barkhuizen, D. 2007. Preparation and water-gas shift performance of zinc oxide supported dispersed gold catalysts. University of Cape Town.