Lee, Seung-Jae; (2002) Development of supported gold catalysts for low temperature CO oxidation. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Recently supported gold catalysts have been reported to exhibit surprisingly high activities in low-temperature CO oxidation. The remarkable catalytic behaviour shown by supported gold depends on it being formed into very small particles, which are not obtained from conventional impregnation but from deposition-precipitation or co-precipitation. Most active gold catalysts produced without a carrier usually fail to acquire shapes and hardness desirable for practical use. This study is to investigate the preparation of gold catalysts on several types of supports having a large surface area in order to reduce the use of expensive gold and expand application of the supported gold catalysts. The chemical, physical and catalytic properties of Au-Mn co-precipitates were examined varying preparation conditions such as Au loading, washing methods and drying temperature and atmosphere. Gold existed only in metallic form, and fresh samples contained manganese carbonate and MnOx such as MnO2. The carbonate was converted to manganese oxides after a light-off test. Au particle sizes were in the range of 4.5 - 6.5 nm before the light-off test. Small Au particles influenced the activity of the freshly dried catalysts. After treatment at 400 °C, catalytic activity improved and a strong interaction between gold particles and MnOx support was evident. The concentration of sodium ions also affected the activity of the samples after heat treatment. Preparation methods modified from Au-Mn co-precipitation were explored in order to produce co-precipitates on alumina pellet and powder support. The catalytic activity of the pellet samples was not improved for the Au-Mn co-precipitates, probably due to non-associated Au with Mn compounds. However, the increase of pH of the manganese solution and the use of a high concentration of chlorine free gold solution showed better catalytic performance in the pellet samples. Similar activity was observed in powder samples. Furthermore, washing with water or sodium hydroxide solution was helpful to enhance the activity of the gold catalysts. The effects of pH and the ageing time of HAuCl4 solution were examined for Au/γ-Al2O3 and Au/TiO2 catalysts prepared by impregnation. HAuCl4 solution was hydrolysed by adding Na2CO3 solution prior to impregnation of γ-Al2O3 powder, γ- Al2O3 monolith and TiO2 powder. The hydrolysis of AuCl4 complex was observed with UV/vis. spectroscopy and the concentration of AuCl4 complex changed with pH and ageing time. Au particle size and Au loading of AU/γ-Al2O3 powder catalysts were influenced by speciation of Au complexes, and thus CO conversion was maximised at a sample having high Au loading and small Au particle size. The activity of Au/γ- Al2O3 powder catalysts was not affected by whether dry or wet impregnation was utilised, but the activity increased with using excess oxygen rather than stoichiometric CO and O2. Au/γ-Al2O3 monolith catalyst was deactivated under the presence of water vapour in a reactant feed stream by a partly irreversible process. The hydrolysis of AuCl4 complex did not influence the activity of Au/TiO2 powder catalysts, but water vapour in the reaction gas suppressed CO oxidation of Au/TiO2 powder catalysts. Supported Au catalysts were prepared also in plate form by using anodised metal oxide films. The A1 and Ti oxide films were prepared on A1 and Ti foils by anodic oxidation in sulphuric acid, and then the anodised foils were used as a support for gold catalysts. The current and temperature of sulphuric acid during anodisation affected the thickness of anodic A1 oxide (5-45 μm), but anodic Ti oxides were below 3 μm probably due to the very high temperature of the sulphuric acid. The activity of Au/Al2O3/Al catalysts was observed only at low space velocity and increased with increasing the thickness of A1 oxide film. However, the activity of Au/TiO2/Ti catalysts was not observed due to very low Au loading and thin TiO2 film.

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