| 摘要: | 一直以來，金都被視為不具活性的金屬物質，然而，奈米級的金顆粒卻表現出很高的活性。金觸媒的重要應用為在較低溫度下，進行選擇性CO催化氧化反應(PROX)，此反應重要的應用背景是為燃料電池提供適合的氫氣來源，以甲醇蒸氣重組或水煤氣反應產生的氫氣中，含有大量 CO，若將其直接應用於燃料電池會毒化白金電極。採用承載於適當的擔體上的高分散性奈米金作為觸媒，在氫氣流中進行選擇性CO氧化反應，是解决這一問題的有效途徑，此類觸媒具有很高的催化活性，可有效降低一氧化碳濃度至50 ppm，避免白金電極被毒化。 本研究探討了承載於不同比例鈦/鐵擔體的金觸媒對反應活性的影響，以初濕含浸法製備FeOx-TiO2擔體，在 pH值為7、反應溫度為65℃的條件下，利用沉積沉澱法將奈米金顆粒擔載於金屬氧化物上。另外，對於不同的pH值、照光時間、光源強度等條件對光沉積法製備Au/TiO2觸媒性質的影響，也進行了深入研究。分別以感應耦合電漿質譜分析儀(ICP)，X光繞射分析儀(XRD)、穿透式電子顯微鏡(TEM)、X光電子能譜儀(XPS)等儀器來鑑定其觸媒特性。反應則以固定床反應器填充觸媒，並以進料體積比CO: O2: H2: He = 1.33: 1.33: 65.33: 32.01 進行反應。 Au/TiO2 具有很高CO轉化率和H2轉化率，添加適量的Fe2O3 到Au/TiO2上，可以提高CO的轉化率並有效提高PROX的選擇性。實驗結果表明FeOx/TiO2的最佳莫耳比為1 : 9，在80℃下，Au/FeOx-TiO2 (1 : 9) 的CO轉化率可達100%，CO選擇率則為48%。當TiO2加入Fe2O3，不僅可以增加奈米Au與擔體間的相互作用力，也可以增加金顆粒的穩定性，進而提高觸媒的活性。 以光沉積法製備的Au/TiO2，可以成功獲得極小的金顆粒，粒徑大小約為1.5奈米。在 pH值為10、照光時間為10分鐘的製備條件下，其最佳的CO轉化率為95%，而CO選擇率則為47%。隨著照光時間的增加，金顆粒會產生聚集現象，使得活性大幅降低。Au/TiO2 (pH 10、照光時間10分鐘)經過60小時反應測試後，其CO轉化率仍維持在90%以上，選擇率也維持在42%以上，此結果證實此觸媒具有較好的壽命。 關鍵字：金，TiO2，Fe2O3，金觸媒，選擇性一氧化碳氧化，光沉積法。 Bulk gold had been known as an inactive catalyst. However, nano gold particles appear to be extremely active. An example of catalytic application is that gold catalysts are able to selectively oxidize carbon monoxide in a hydrogen stream at low temperature, which is important for producing clean hydrogen for fuel cells. When hydrogen-rich fuel is produced from water gas shift reaction, the Pt anodes in fuel cell at low temperatures are poisoned by CO. Highly dispersed gold on suitable metal oxide exhibits high activity to oxidize CO in hydrogen stream to reduce CO concentration less than 50 ppm. In this study, a series of Au catalysts supported on FeOx-TiO2 with various Fe contents were prepared. FeOx-TiO2 support was prepared by incipient-wetness impregnation with aqueous solution of Fe(NO3)3 on TiO2. Gold catalyst with nominal loading of 1 wt. % was prepared by deposition-precipitation at pH 7 and 65°C. Au/TiO2 catalysts were prepared by photo-deposition (PD) method for various preparation parameters, such as pH value, power of UV light and irradiation time. The catalysts were characterized by ICP, XRD, TEM and XPS. The catalytic performance of these catalysts was investigated by preferential oxidation of carbon monoxide in hydrogen stream (PROX). The reaction was carried out in a fixed bed reactor with feed of CO: O2: H2: He = 1.33: 1.33: 65.33: 32.01 (volume ratio). Au/TiO2 catalyst had both high CO oxidation activity and high H2 oxidation activity. Adding suitable amount of Fe2O3 on Au/TiO2 could enhance CO conversion to a higher extent and suppress H2 oxidation conversion. Au/FeOx-TiO2 with molar ratio (10:90) is found to be the best catalyst when compared with other catalysts for PROX and it showed high CO conversion (100%) and selectivity (48%) at 80°C for fuel cell application. Supported gold catalysts, the amorphous nature of iron oxide along with TiO2 not only enhance electronic interaction but also stabilize the nano-size gold particles, thereby enhancing the catalytic activity. Au/TiO2 catalysts prepared by PD method had narrow particle size distribution and the particle size of Au was around 1.5 nm. Among all the catalysts, Au/TiO2 catalyst prepared at pH 10 and irradiated for 10 min showed the highest CO conversion (95%) and CO selectivity (47%) at 80°C for fuel cell operation. When the irradiation time increased from 10 to 60 min, the small gold particles aggregated to form larger gold particles. The catalytic activity of Au/TiO2 decreased with an increase in gold particle size. In long time test, The CO conversion of Au/TiO2 (prepared at pH 10 and 10 min irradiation time) was greater than 90% and CO selectivity also maintained above 42% at 80°C for 60 h. The results clearly demonstrated that the catalyst was stable for a long time. Key words: Au, TiO2, Fe2O3, gold catalyst, preferential oxidation of CO in H2 stream, photo-deposition. |
