Au/CeO2觸媒在水氣轉移反應上的應用

Abstract

本研究以沉澱沉積法製備Au/CeO2，期望製備在低溫(200℃)水氣轉移反應中，具有高CO轉化率之觸媒。嘗試研究如下變因：(1) 鹼液中處理CeO2表面的影響；(2)金溶液酸鹼度的影響；(3) Au/CeO2前處理的影響；(4)保存Au/CeO2的方式。研究中並對觸媒的活性變化作鑑定和探討，鑑定方式包括：AA、UV-Vis、HRTEM、XRD。 研究結果顯示以表面修飾至pH值為6到8的CeO2最為適合，未經表面修飾或是修飾至pH值為10的CeO2所製成的觸媒會造成轉化率些微的下降；金溶液則以調至pH值為9所製成的Au/CeO2效果最佳，因為此時可使氯化金酸中的氯離子完全被OH-取代；Au/CeO2若在200oC下經一氧化碳前處理，因能維持小的金顆粒而有較高的催化活性；最後，製備好的Au/CeO2需儲存於冰箱的冷凍庫中，以免發生活性衰退的情形。 應用較佳的Au/CeO2，在總體積流速33.33ml/min(298K)，反應溫度200℃，及以下兩種進料莫耳比例：(1) CO/H2O/N2 = 2.65/41.17/56.18；(2) CO/H2O/H2/CO2= 2.65/41.17 /44.94/11.24，分別測試催化活性。在(1)條件中，CO轉化率達98%，已接近理論平衡轉化率；但在(2)條件中，轉化率僅約73%，顯示觸媒的催化活性在未來仍具改善的空間。

The purpose of this research is to investigate Au/CeO2 which is prepared by deposition-precipitation method (DP method) with high CO conversion under low temperature water-gas-shift reaction (WGSR). Preparation factors listed below are studied: (1) CeO2 surface modification by NaOH(aq); (2) pH value of gold solution for deposition; (3) pretreatment of Au/CeO2 before water-gas-shift reaction; (4) Au/CeO2 storage effect. Additionally, the properties of the catalysts are characterized by AA, UV-Vis, HRTEM, XRD. The experimental results indicate that CeO2 with surface modification to pH=6 ~ 8 possesse the better performance; however, the CO conversion would decrease when the CeO2 with surface modification to pH=10 or without surface modification. The better pH value for the gold solution for deposition is 9, because the four Cl－ in the solution could be ideally replaced by OH-. Au/CeO2 pretreated before reaction by CO at 200oC has the best CO conversion, because of the smallest particle size of gold particles. Finally, Au/CeO2 needs to store under low temperature without light (i.e., in a refrigerator) to protect against decay. For the tests of water-gas-shift reaction, the total flow rate and reaction temperature were carried out at 33.33ml/min and 200℃. Two mole flow rate ratios at reactor inlet : (1)CO/H2O/N2 = 2.65/41.17/56.18；(2) CO/H2O/H2/CO2=2.65/41.17/ 44.94/11.24 were used. For the best performance Au/CeO2 catalyst prepared in this research, CO conversion of condition (1) was 98%, which is approaching the equilibrium conversion obtained from thermodynamics simulation; however, CO conversion of condition (2) could only reach 73%. This implies there is still much room to improve the performance of the catalyst in the future.