Perferential Oxidation of CO in H2 stream on Au/MgO-TiO2

• Main Authors: Zhi-Yuan Chen, 陳致遠
• Other Authors: 內容為英文
• Format: Others
• Language: en_US
• Published: 2007
• Online Access: http://ndltd.ncl.edu.tw/handle/76651258249420826920

碩士 === 國立中央大學 === 化學工程與材料工程研究所 === 95 === Hydrogen has been recognized as a good energy carrier. 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 used as a catalyst to oxidize CO in hydrogen to reduce CO concentration less than 50 ppm.1 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 has high selectivity of CO oxidation and low conversion of CO. It was expected that by adding suitable amount of MgO on Au/TiO2, the catalyst may keep high CO conversion and suppress H2 conversion. In this study, varous MgO-TiO2 mixed oxides were prepared by incipient-wetness impregnation of aqueous solution of Mg(NO3)2. The materials were then calcined at 573 K for 4 h. Au catalysts were prepared by deposition-precipitation (DP) method using HAuCl4 as a precursor at pH 8-9 at 338 K. The catalysts were characterized by XRD, TEM, XPS and ICP-MS. The reaction was carried out at a fixed bed reactor with a feed containing 65.33%H2．32.01%He．1.33%CO and 1.33%O2 at 30000 h-1(GHSV). All the catalysts can decrease CO concentration less than 50 ppm providing that O2/CO ratio is 1.5. In order to magnify the difference for comparison , O2/CO ratio in the feed was kept at 1 and reaction temperature was kept below 373 K. Au/MgO-TiO2 has better performance than Au/TiO2 and Au/MgO, especially at 353 and 373 K. The size of gold particle in these catalysts was smaller than 4 nm as evidenced by TEM. The catalyst with Ti/Mg molar ratio of 4 and prepared at pH 8 had the highest CO conversion and selectivity at 353 K, the operation temperature of fuel cell. The results of this study demonstrated that adding suitable amount of MgO can suppress H2 oxidation without sacrificing CO oxidation. It is also in accordance with literature. MnO2 and Fe2O3 are promoter for gold catalysts. Fe3+ and Mn4+ were used as the promoters for 1% Au/MgO-TiO2(2:8) in this study. It was found very difficult to reproduce the preparation of small Au particles deposited on MnO2-MgO-TiO2 various ratio. Nevertheless, addition of MnO2 to MgO-TiO2 resulted in good reproducibility of manufacturing catalysts with a high dispersive Au phase with a narrow particle size distribution. The average of these gold catalysts is narrow at between 1 and 3 nm. The XRD patterns in figures have not appeared Au peaks. According the two phenomenon, the Au particles can deposited with small and well dispersed on MnO2-MgO-TiO2 various ratio. The addition of Fe2O3 promoter in the Au/MgO-TiO2 (2:8) prepared at pH 8 could increase the catalytic conversion at low temperatures with increasing oxygen supply. Nevertheless, when the catalyst with 9 mol% and 16.7 mol% Fe2O3 promoter has loaded on the support and subjected to PROX at high temperatures, CO conversion decreases with increasing temperature. It may be due to the non-availability of oxygen supply from Fe2O3, which would be inactive or it reacted with hydrogen and hydroxyl group at high temperatures. It may be due to the non-availability of oxygen supply from Fe2O3, which would be inactive or it reacted with hydrogen and hydroxyl group at high temperatures. The catalysts, Au/MgO-TiO2 (2:8) prepared by deposition-precipitation method at pH 8 reported high conversion and selectivity for PROX.