dc.description.abstract | Bulk gold had been regarded as an inactive catalyst due to the 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 50 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/MgO and Au/MnO2 have high selectivity for CO oxidation and low conversion of CO. It was expected that by adding suitable amount of MgO or MnO2 into Au/TiO2, the catalyst may retain high CO conversion and suppress H2 conversion.
In this study, the promoter effect of various MOx-TiO2 mixed oxides was investigated and the support was prepared by incipient-wetness impregnation with aqueous solution of nitrate salt. TiO2 was supplied by Degussa Company (P-25). A series of preparation conditions of Au catalysts, prepared by deposition-precipitation (DP) method or photo-deposition (PD) method were discussed. The catalysts were characterized by inductively-coupled plasma-mass spectrometry (ICP-MS), X-ray diffraction (XRD), N2-sorption, transmission electron microscopy (TEM), high-resolution transmission microscopy (HRTEM), diffuse reflectance UV-vis spectroscopy (UV-vis DRS) and X-ray photoelectron spectroscopy (XPS). 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).
A high gold dispersion and narrow size distribution was obtained by DP method with different precipitating reagents, such as NaOH and NH4OH, for Au/TiO2 catalysts. The uptake of gold on TiO2 decreased as the pH value increased from 6 to 9. Au/TiO2 prepared at pH 8 has higher CO conversion and higher selectivity of CO oxidation than those prepared at other pH values. CO selectivity decreased with increasing reaction temperature on all catalysts. This phenomenon is more significant on Au/TiO2 prepared at pH 6 due to the uptake of more gold at low preparation pH value and hence the aggregation of gold particles during reaction.
Au/TiO2 catalysts prepared by PD method had narrow particle size distribution of gold particles within a few nanometers. The catalysts prepared by this method could produce very small gold particles (1.5 nm) on the support. The size of gold nanoparticles deposited on the support depends on irradiation time and light source. We have successfully prepared very small gold particles with low power of UV light at suitable irradiation time and a possible mechanism of reaction has been proposed. | en_US |