以釕摻雜層狀鈣鈦礦氧化物作為乙醇氧化蒸氣重組產氫反應的觸媒

Abstract

現今，我們使用的能源，大多還是取自於化石燃料，但卻造成了嚴重的環境問題，如空氣汙染及全球暖化，其中，氫氣為最重要的可再生替代能源之一。乙醇氧化蒸氣重組反應(Oxidative steam reforming of ethanol，OSRE)為一種有效的產氫方式，其反應包括全氧化反應，部分氧化反應，水煤氣反應及其它副反應。 本論文，著重於研究以釕金屬部分取代層狀鈣鈦礦氧化物La2Ti2O7中的鈦位置時，對於其結構及催化活性於不同氧化還原環境下之影響。以溶膠-凝膠法製備La2Ti2-xRuxO7 (x=0.1~0.4)系列樣品。並以粉末繞射結構精算，得知金屬釕為混合填佔到鈦的位置，其取代並不具有位置的選擇性。根據X光粉末繞射(PXRD)圖譜並經過晶格大小的計算，得到樣品的相寬介於0.0~0.4。以即時(In-situ)同步輻射X光粉末繞射來觀測程序升溫還原(TPR)實驗中結構的變化，可發現La2Ti1.7Ru0.3O7在約730 °C時，其結構會從單斜晶相轉為正交晶相，此結果說明了其結構在不同氧化還原環境下會呈不同的相態，並可能伴隨氧空缺的產生。X光光電子能譜(XPS)及程序升溫還原分析顯示，當x≧0.2時，Ru於La2Ti2-xRuxO7結構中的氧化數大於4價，且Ru3d束縛能隨著摻雜量增加而往高能量偏移。因此，觸媒之催化活性可藉由結構中含高價態釕離子而得到提升。 在La2Ti2-xRuxO7 (x=0.1~0.4)此系列中，La2Ti1.7Ru0.3O7此觸媒在C/O=0.6及外部溫度為400 °C時，有優異的催化活性(乙醇轉換率XEtOH~100%；氫氣選擇率 ~100%)。La2Ti1.7Ru0.3O7觸媒吸附在鋯酸鑭(La2Zr2O7)載體上，經120小時的長時間OSRE反應下，其效率維持穩定，且由拉曼光譜可知，反應後的觸媒無碳沉積。結果顯示，La2Ti2-xRuxO7 (x = 0.1~0.4)此固溶體觸媒，在乙醇氧化蒸氣重組反應的催化上十分具有潛力。

Most of the energy resources we using today are coming from the burning of fossil fuels, which causes serious environmental problems, such as air pollution and global warming. Hydrogen is one of the most important renewable alternatives. The oxidative steam reforming of ethanol (OSRE) contains total oxidation, partial oxidation, water gas shift reaction etc, which could be a valuable hydrogen production technology. In this work, we have studied the structure and catalytic properties of substituted Ru ions on the B-site of layered perovskite La2Ti2O7 in different redox atmosphere. The series of La2Ti2-xRuxO7 (x=0.1~0.4) solid solution were synthesized by sol-gel method. The Rietveld Refinements indicated that the ruthenium ions were mixed occupied with titanium without site preference. According to the powder X-ray diffraction (PXRD) and cell refinement, the phase width is between 0.0~0.4. In-situ temperature programmed reduction (TPR) experiment of La2Ti1.7Ru0.3O7 was carried out by using the synchrotron PXRD, which revealed a phase transition from monoclinic to orthorhombic structure at temperature ~730 °C .The structure will be changed during redox process which might also induce oxygen vacancies. The X-ray photoelectron spectra (XPS) and TPR analysis indicate that the oxidation state of Ru ions in La2Ti2-xRuxO7 is deduced to higher than tetravalent as x≧0.2. The position of Ru3d binding energy is gradually shifted to higher energy as the doping amount increases, which affect the catalytic performance. Within the series of La2Ti2-xRuxO7 (x=0.1~0.4), the catalyst, La2Ti1.7Ru0.3O7, exhibits excellent activity (XEtOH ~100%； ~100%) at C/O=0.6 and furnace temperature=400 °C. The time-on-stream OSRE experiment of the catalyst, La2Ti1.7Ru0.3O7, remains stable at least for 120 hours by using La2Zr2O7 as support. There is no detectable coke deposition over the used catalyst by Raman spectroscopy. The results shows that the solid solution of La2Ti2-xRuxO7 (x=0.1~0.4) is a promising catalyst for hydrogen production with OSRE.