Au/CeO2觸媒在水氣轉移反應上的應用

Abstract

本論文研究目標為製備在低溫(約200℃)水氣轉移反應(WGSR)反應條件下，具有高CO轉化率的觸媒。 根據文獻回顧的選擇與比較後，發現Au/CeO2觸媒具備上述的優勢，是最好的選擇之一。本研究以沉澱沉積法(deposition-precipitation)製備Au/CeO2，嘗試改變以下幾種製備條件變因：(1)CeO2製備方式；(2)金溶液酸鹼度；(3)金溶液調pH值的時間；(4)NaOH(aq)滴加方式，以製備出較佳的觸媒。研究中並對這些觸媒的活性變化作鑑定和探討，除了反應活性測試外，鑑定方式包括：AA、ICP、BET表面積、HRTEM、XRD和XPS。 就以上各變因而言，反應測試結果顯示以經過表面修飾至pH值為6的CeO2最為適合，未經表面修飾或是修飾至pH值為10的CeO2製成的觸媒皆因為氯離子置換率不足或是奈米金結塊(agglomeration)問題會造成轉化率的下降；金溶液酸鹼度則是以調至pH值為9的觸媒效果最佳，這是製備方式為「計算恰可取代該濃度金溶液中AuCl4－四個氯離子的0.1 N NaOH(aq)量」時的pH值，因為此添加量恰可使氯化金酸分子中的氯離子完全被取代；調整金溶液時間則是以6h最佳，時間過短時氯離子置換仍不完全，過長又會造成Au(OH)3之間因脫水反應結成大顆粒，而不利反應；NaOH(aq)的滴加方式中，「一次加入」比「慢慢加入」適合，因為一次加入的方式在滴加之初便提供了足夠置換所有氯離子的量，而且以調配時間方向而言也比較充足。 本研究使用兩種反應條件，室溫下總流速皆是33.33ml/min，反應時溫度200℃：(1) CO/H2O/N2 = 2.65/41.17/56.18；(2) CO/H2O/H2/CO2=2.65/41.17 /44.94/11.24，在(1)條件中，本研究實驗結果已經接近100%，即熱力學的平衡極限，但在(2)條件中，本實驗XCO最高僅達約75%，在觸媒製程設計上仍有可以努力的空間。

The purpose of this research is to investigate catalysts with high CO conversion under low temperature water-gas shift reaction (WGSR). According to the literature review, Au/CeO2 was one of the best choices since it could reach high CO conversion under low temperature WGSR. Therefore, Au/CeO2 is prepared by deposition-precipitation method (DP method) and several preparation factors listed below are systematically studied: (1) The preparation method of CeO2; (2) The pH value of gold solution; (3) The pH adjusting time of gold solution; (4) The method of the addition of NaOH(aq) into gold solution. Additionally, the performances as well as the properties of the catalysts which are prepared with different preparation conditions are all measured by WGSR and characterized by AA, ICP, BET surface area, HRTEM, XRD and XPS. The experimental results indicate that the CeO2 with surface modification to pH=6 possesses the better performance; however, the CO conversion would decrease when the CeO2 with surface modification to pH=10 or without surface modification, and this is because the incomplete replacement of Cl－or the agglomeration of gold nanoparticles occurred in the latter two cases. The better pH value for the gold solution is found to be 9 and this is the pH value corresponding to the state that the four Cl－ in the gold solution could be ideally replaced by the calculated amount of 0.1N NaOH(aq). The proper pH adjusting time of gold solution is found to be 6h. The insufficient pH adjusting time would result in the incomplete replacement of Cl－; nevertheless, longer pH adjusting time would result in the aggregation of gold species. Both of the above-mentioned conditions possess negative effects toward the reactions. As to the method of the addition of NaOH(aq) into gold solution, “Add NaOH(aq) into the gold solution at once in the beginning” is better than “Add NaOH(aq) into the gold solution gradually to keep specific pH value”. This is because more time and sufficient NaOH(aq) obtained in the former case would benefit to the occurance of completely replacement. In this research, two reaction condition were applied:(1)CO/H2O/N2 = 2.65 /41.17/56.18；(2) CO/H2O/H2/CO2=2.65/41.17/44.94/11.24, total flow rate and reaction temperature of both conditions were 33.33ml/min and 200℃. The CO conversion of condition (1) was close to 100%, which is approaching the equilibrium conversion obtained from thermodynamics simulation; however, the CO conversion of condition (2) could only reach 75%, and this implies there is still much room to improve the performance in terms of the catalyst preparation process design.