- 學位論文
Au/CeO2 觸媒的鑑定及水氣轉移反應上的應用
Characterization of Au/CeO2 and the Application in Water-Gas Shift Reaction
摘要
本研究嘗試探討Au/CeO2 觸媒在不同前處理程序下的水氣轉移反應，分別以一氧化碳、氫氣及水氣在200℃水氣反應的溫度下前處理觸媒，期望找出最佳的前處理程序使其在實際應用上有更佳的表現。 本研究利用紫外-可見光(UV-Vis)光譜，由量測奈米金約在550nm波長處的振動吸收峰(surface plasmon bands)來估計金屬金的顆粒大小。研究結果顯示，此振動吸收峰的最高波長位置(lmax)和顆粒大小有密切的關係，吸收峰波長越長則顆粒越大。此外，吸收峰的吸收度(absorbance)和金在觸媒表面上的價態有關，吸收度越大則還原態的金越多。 本研究使用水氣轉移反應(WGSR)來測試Au/CeO2觸媒的活性，以高解析穿透式電子顯微鏡(HRTEM)觀察金在觸媒表面分佈情形，以紫外-可見光(UV-Vis)光譜衡量表面金顆粒大小，及以擴散反射式傅立葉紅外光譜儀(DRIFTS)偵測觸媒表面吸附物質。研究結果顯示，觸媒在200℃下經一氧化碳處理後活性最佳，而以水氣處理表現最差。UV-Vis鑑定顯示其活性大小和金觸媒上的振動波長有強烈的關係，以水氣處理的觸媒在反應後的波長最長，因此估計是因表面形成了大的金顆粒，而導致催化活性的下降；然而，金顆粒的成長並不是發生在前處理過程，而是發生在之後的水氣轉移反應。DRIFTS鑑定顯示此金顆粒的成長可能是擔體CeO2表面吸附的甲酸鹽類物質(formate species)所引起，該甲酸鹽類在以水氣前處理及反應程序後之觸媒上殘留的最多。 本研究結論出水氣轉移反應前先通以一氧化碳前處理Au/CeO2，會使觸媒擁有較佳的反應活性。
並列摘要
The effect of Au/CeO2 pretreatment in CO, H2 or H2O atmosphere on the catalytic activity for water gas shift reaction (WGSR, a critical step in producing H2 from hydrocarbons) has been investigated. It was discovered that gold particle size is an essential property for gold catalytic activity. In order to determine the size of nano-particles of gold metal, the particle sizes were predicted by using plasmon band around 550 nm in UV-Vis spectra and by using the theoretical calculations in this research. The correlations of lmax (wavelength of maximum Plasmon band) and particle size were established. It is concluded that the higher lmax is measured, the larger gold particle size on the CeO2 surface should be. In addition, the higher absorbance is from the more reduced gold on the surface. The activities of catalysts were examined by using WGSR tests at 200 ℃, and by HRTEM for observing gold particles, diffuse reflectance UV-Vis spectroscopy for measuring nano-gold plasmon bands and average particle sizes, and diffuse reflectance infrared Fourier Transform spectroscopy (DRIFTS) for detecting surface adsorbates. It was found that different pretreatments caused considerable different reaction activity for WGSR, and the lowest activity was from the pretreatment in H2O at 200 ℃. The study of diffuse reflectance UV-Vis spectroscopy indicated that the reaction activities for WGS were strongly correlated to the wavelength of plasmon bands. The highest wavelength (red shift) from Au/CeO2 after pretreatment under H2O and after WGSR suggests that the lowest catalytic activity was due to the largest gold nano-particles on CeO2 surface. However, the particle growing of nano-gold was not at H2O pretreatment step; rather, it was after the injection of CO into the reactor for WGSR. DRIFTS studies indicated that the particle growing of gold species was induced by the formate species on CeO2. The most active Au/CeO2 was from the pretreatment in CO atmosphere before WGSR at 200℃.