| dc.description.abstract | Bulk gold had been known as an inactive catalyst due to the smooth surface of gold which inhibits chemisorption of reactant molecules. Since Haruta and co-workers reported that the nano-gold catalysts could achieve CO oxidation efficiently below ambient temperature, several applications for gold catalysts have been under attention recently. One of the important cases in these applications for gold catalysts is preferential CO oxidation in hydrogen-rich stream (PROX). Hydrogen has been recognized as a good energy carrier since the development of fuel cell. When hydrogen-rich fuel is produced from methanol or gasoline by partial oxidation and/or steam reforming combined with water gas shift reaction, the Pt anodes in fuel cell at these low temperatures are poisoned by CO, reducing the overall fuel cell performance. Gold catalyst has been confirmed as a catalyst to oxidize CO in hydrogen stream to reduce CO concentration less than 5 ppm. Develop a catalyst which has high CO conversion and low H2 conversion is the target of this study. Au/TiO2 has high conversion of CO and low selectivity of CO oxidation; Au/CoOx and Au/ZnO have high selectivity for CO oxidation and low conversion of CO. It was expected that by adding suitable amount of CoOx or ZnO into Au/TiO2, the catalyst may retain high CO conversion and suppress H2 conversion. In this study, multiple metallic oxides are prepared by impregnation method. Hydrochloro-auric acid is the gold precursor used to load on the support by deposition-precipitation method at pH 7 and 65 °C. The catalysts were calcined at 180 °C for 4 h. The gold particles have high dispersion and thermal stability. In the fuel cell operating temperature range (50–100 °C), gold catalysts can remove CO almost completely. The catalysts were characterized by inductively coupled plasma-mass spectrometry, X-ray diffraction, transmission electron microscope and high-resolution transmission electron microscopy, X-ray photoelectron spectroscopy. The PROX reaction was carried out in a fixed bed reactor with a feed containing 65.33% H2, 32.01% He, 1.33% CO and 1.33% O2 (vol. %) at 30000 h-1 (GHSV). According to literature, TiO2 has been widely used in the synthesis of supported gold catalysts and active for selective CO oxidation. But its activity decreases obviously when the temperature reaches above 80 °C. 1 wt % Au/TiO2 has the CO conversion 86 % and CO selectivity 43% at temperatures around 80 °C. In thermodynamic aspect, hydrogen will compete with CO for oxygen at high temperature. The method to improve the CO selectivity is the key point. Two metal oxides, CoOx and ZnO have been regarded as additives into TiO2 support which was prepared by incipient-wetness impregnation method. The effects of pH in preparation and Co or Zn/Ti ratios of the catalyst on the catalytic properties of the catalysts were investigated. The high catalytic activity of Au/CoOx-TiO2 over Au/TiO2 has been attributed to the synergistic effect of gold particle size, optimum of combination metallic and electron-deficient gold species, as well as to Au-support interactions. Moreover, the results of this study have demonstrated that Au/ZnO-TiO2 (Zn/Ti = 5/95) catalyst prepared at pH 6 is a better catalyst than Au/ZnO and Au/TiO2 for PROX reaction. By adding suitable amount of promoters and by choosing suitable pH value during DP process to deposit gold, one is able to obtain a catalyst which has the best performance in PROX reaction.
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