光催化去除燃燒尾氣中的氮氧化物

Abstract

由於近年來嚴重的空氣污染問題，如PM2.5，提升了大眾對於空氣品質的重視。氮氧化物NOx (為一氧化氮NO與二氧化氮NO2的合稱)是其中一項對環境及人類都極具傷害力的污染物，因其本身具有毒性，也會導致上述PM2.5之產生。光催化能利用地球上最豐沛的太陽能，被視為是一項永續綠色科技，因此越來越多的科學家投入於使用光催化程序去除氮氧化物之研究。 本研究在不同的氣體反應物組成以及溫度下探討兩種類型之光反應，分別是：光催化氧化以及光選擇性催化還原。將實驗結果結合了熱力學的理論計算，提出了光催化去除氮氧化物的反應路徑。得出光氧化反應在低溫下為光反應之主體而光選擇催化還原在逐漸增高的溫度下慢慢將其取而代之。 使用丁烷作為還原劑並搭配紫外光燈作為光源，研究二氧化鈦/α型氧化鋁(TiO2/α-Al2O3)及二氧化鈦/γ型氧化鋁(TiO2/γ-Al2O3)之氮氧化物去除效率，發現反應溫度在110-200°C時能夠有效降低二氧化氮的選擇率。進一步在實驗工廠規模之光反應器內通入燃燒煤炭所獲得之真實尾氣，在反應溫度120°C獲得68-75 %的氮氧化物去除效率。與傳統光反應器不同，填充床式光反應器能有效利用光能及熱能。隨後，利用掃描式電子顯微鏡搭配能量散射X光能譜儀(SEM&EDS)及X光光電子能譜儀(XPS)確認了觸媒的中毒失活是因為其表面與吸附的二氧化硫(SO2)反應形成硫酸鹽(SO42-)並佔據觸媒活性位置，不過在5 % H2/N2的還原條件下將觸媒加熱至500°C維持一小時能使其脫硫再生。 使用溶熱法合成具有高比例(001)暴露面的奈米平板狀二氧化鈦。合成之觸媒以一系列方法進行鑑定，包含：紫外－可見光光譜儀(UV-vis)、X光繞射儀(XRD)、掃描式電子顯微鏡(SEM)、穿隧式電子顯微鏡(TEM)、螢光光譜儀(PL)、以及X光光電子能譜儀(XPS)。表徵鑑定揭示了從鈦酸四丁酯(TBOT)至TiO2 nanosheet的轉換過程，並且得知在(TBOT)與氫氟酸的F/Ti原子比例為1.5時經由180°C及24小時的溶熱處理下能得到最佳光轉化效率之TiO2 nanosheet。由於其最適化的(001)及(010)晶面比例此條件下合成之觸媒不僅能用來去除氮氧化物，可有潛力用在其他光催化反應。 本研究為首次探討輔以熱力學解釋光催化去除氮氧化物之反應途徑，之後更是成功使用實驗工業規模之光反應器去除真實尾氣，並在最後提升光觸媒之光利用效率。在此之前，半工廠級或更大規模之光反應器只被用在液相中的非均相光催化。本研究成果提供光催化去除氮氧化物及其他氣相中的光催化非均相反應新的契機。

Serious air pollution such as PM2.5 and smog has raised public awareness of air quality. The pollutant, NOx (NO+NO2), is one of the most detrimental pollutants both to the environment and human beings because it not only possesses toxicity but also acts as the precursor to form PM2.5 and smog. Photocatalysis is a sustainable technology to utilize solar energy which is the most abundant energy source available on Earth. Thus, an increasing number of scientists have investigated the use of photocatalytic processes to remove NOx. In this study, two types of photocatalytic NO removal, including photo-assisted selective catalytic reduction (photo-SCR) and photo-oxidation were investigated at different simulated gas compositions under elevated temperatures. The reaction pathway of photocatalytic NOx removal is proposed coupled with the theoretical thermodynamic calculations. Photo-oxidation dominates at lower temperatures while photo-SCR gradually takes control when the temperature is increased. The feasibility of photo-SCR is studied for the industrial application. The photo-SCR is systemically examined on the TiO2-coated α-Al2O3 and γ-Al2O3 spherical support photocatalyst by using C4H10 as a reductant under UV-light irradiation. The formation of NO2 is almost suppressed in the reaction temperature range of 110-200 °C. Furthermore, NOx removal efficiency was between 68 and 75% at 120 °C for real flue gas in the pilot-scale photoreactor. Unlike traditional photoreactors, our packed-bed photoreactor efficiently utilizes both light and heat energy. Scanning electron microscopy and X-ray photoelectron spectroscopy techniques reveal that SO2 is adsorbed onto the active sites of the photocatalyst forming a sulfate (SO42-) which causes deactivation. Nevertheless, spent TiO2/γ-Al2O3 photocatalyst can be regenerated via desulfurization in 5% H2/N2 1h at 500 °C. In order to achieve a higher efficiency of photocatalyst, TiO2 nanosheets with dominantly exposed (001) reactive facets are synthesized using the solvothermal method. The TiO2 nanosheets are characterized by ultraviolet-visible spectroscopy, X-ray diffraction, high-resolution transmission electron microscopy, photoluminescence, and X-ray photoelectron spectroscopy. In addition, the transformation of titanium n-butoxide precursor to TiO2 nanosheet is revealed. TiOF2 is the main intermediate during this process. Moreover, using titanium n-butoxide and hydrofluoric acid with an F/Ti atomic ratio of 1.5 (denoted as FT1.5) at 453 K for 24 h demonstrates the highest efficiency in terms of photocatalysis. The nitrogen-balance of photo-SCR experiment is above 90% that exhibits excellent performance by using FT1.5 in NOx removal. The synthesized TiO2 nanosheet exhibits great potential to be applied in not only NOx abatement but also other photocatalytic reactions due to its optimal ratio of (001) and (010) facets. This work represents the first attempt at scaling up the photo-SCR process for use with real flue gas, leading to a significant improvement of TiO2 photocatalysts. Prior to this work, pilot-scale or larger photoreactors were only applied in liquid-solid phase heterogeneous photocatalysis. The results indicate a promising future for photocatalytic NOx abatement and all other photocatalytic heterogeneous reactions in gas-solid phase.