- 學位論文
氯殘留對Au/γ-Al2O3觸媒在CO氧化反應特性之影響
Effects of residual chlorine and acidity on the CO oxidation over the Au/γ-Al2O3 prepared by impregnation
摘要
含浸法製備的Au/γ-Al2O3觸媒活性較沈澱法觸媒差，本研究測試利用高溫氫氣處理法和濕式鹼處理法兩種前處理法，來改善觸媒的活性。在高溫氫氣處理法部分，在氫氣773-1273K溫度下處理，對觸媒活性沒有明顯改善，將反應後觸媒測試pH值皆呈現酸性，由DRIFTS分析發現CO吸附在金氧化物上，因此推測高溫氫氣還原沒辦法將金完全還原出來；而在濕式鹼處理法中，先前本實驗室發現低活性的含浸法觸媒可以用鹼液浸泡來提升活性，進一步探討以NaBH4溶液的處理，發現鹼當量數在OH-/Au=4.5時，可以有效提升觸媒活性。較低或較高當量數的鹼溶液處理後觸媒的觸媒活性較低，但仍優於未經鹼處理觸媒的活性，而活化後觸媒的pH值皆高於處理前的觸媒。 若將經過773K氫氣還原處理的觸媒再以濕式鹼處理進行觸媒改質，活性也有顯著提高。反應後觸媒與將觸媒用去離子水均勻混合後的pH值有提升，而鹼處理後的活化，若再以含浸方式，添加HCl或HNO3，活性有明顯的下降，測試pH值皆成酸性。XANES測試如發現在white-line的部分，添加HCl或HNO3的觸媒均有提高，表示Au電子密度降低，推測為擔體表面酸性所導致；而DRITS則發現，添加HCl或HNO3的觸媒，在CO吸附在supoort的位置上有藍移現象，表示support呈現酸性。因此，證實表面酸性也是影響觸媒活性的因素之一。 若將不同前處理的觸媒測試pH值，在高溫氫氣前處理的觸媒呈酸性，濕式處理法的觸媒呈鹼性，比較兩者的活性，濕式鹼處理的觸媒活性優於高溫氫氣處理。把濕式鹼處理後鍛燒的觸媒再加入HCl，觸媒呈現酸性，活性也相對降低。因此，觸媒呈現在pH=7-8時，會有較好的活性。
並列摘要
An incipient-wetness impregnation method results in a less active Au/γ-Al2O3 catalyst than a deposition-precipitation method. In this study, we tried to enhance the CO oxidation activity of the Au/γ-Al2O3 prepared by an incipient wetness method via treatment with H2 at high temperature (HTR) and/or treatment using excess basic solution. The HTR treatment was not successful even with a reduction temperature as high as 1273 K. DRIFTS analysis indicates that CO adsorbed on partially oxidized Au sites. The acidity of catalysts after HTR was examined and all of them were acidic. On the other hand, a wet treatment with basic solution successfully enhanced the activity and a NaBH4 solution at OH-/Au = 4.5 is effective. The acidity of catalysts after wet treatment turns to neutral or slightly basic. DRIFTS analysis indicates that CO adsorbed on redued Au sites. The catalyst after 773K HTR is further treated by excess basic solution, the activity is significantly improved and catalyst becomes slightly basic. The active catalyst after NH4OH wet treatment is further loaded with HCl or HNO3 by incipient-wetness impregnation, and the activity is suppressed. XANES analyses show that the white-line is higher after the addition of HCl or HNO3. It implies that Au has lower charge density after HCl or HNO3 addition. DRIFTS analysis indicates that the catalyst with HCl or HNO3 has a blue shift in CO absorbed on the support. These results suggest that surface acidity affect the charge density of Au which consequently affect the CO oxidation activity. The effect of residual Cl may be related to this surface acidity effect.