氣膠一步合成法製備中孔洞矽質材料擔載金屬觸媒及其丙酮催化探討

Abstract

摘 要 對於空氣污染防治之新型觸媒開發與應用一直是熱門的研究課題，有關金屬觸媒催化揮發性有機化合物(Volatile Organic Compounds, VOCs)的研究不勝枚舉，其中最具效果的為金、鉑、鈀等貴重金屬觸媒，但是由於該類貴重金屬的價格昂貴，因此相對價格便宜的其他類金屬觸媒即不斷的被開發並應用於各種領域上。 本研究旨在利用氣膠輔助揮發誘導自組裝製程(Aerosol Assisted Evaporation Induced Self-Assembly, Aero-EISA)直接製備非貴重金屬觸媒，Aero-EISA製程有別於傳統的溶膠膠凝法或水熱法等水相合成法，為一具有快速合成、縮短製備時間的特性，而目前文獻上還未有以Aero-EISA製備金屬觸媒後應用於催化VOCs的探討。因此本研究內容主要為利用Aero-EISA一步合成中孔洞矽質顆粒的金屬觸媒，除了探討各類金屬觸媒物理與化學的特性外，並同時對丙酮VOCs進行催化效率的研究比較。 在單金屬觸媒Metal-MSPs(Mesoporous Silica Particles)的比較上，Ce-MSPs對於丙酮的催化效率高於Mn, Cu, Fe, Al-MSPs，其主要原因為CeO2具有Ce3+/Ce4+雙氧化態的特性，可直接促使進行氧化還原反應(Redox)，此有助於氧的傳送(transport oxygen)、提升氧儲存容量(oxygen storage capacity, OSC)以及於氧化反應中能供給更多的氧。又針對Ce金屬濃度的比較方面，Si/Ce莫耳比為25之Ce-MSPs(25)因為具有951m2/g的高比表面積與3.72wt.%最佳化的Ce金屬添加量，使得CeO2能均勻分佈在MSPs顆粒上，因此對於丙酮的催化效率為最高；而Ce-MSPs(10)雖然有最高9.76wt.%的Ce添加量，但是因為過多的Ce金屬卻造成觸媒的比表面積降低至615m2/g，同時孔徑分佈與孔洞結構也變得最差，因此即使增加Ce活性金屬，也仍無法同步提高丙酮的催化效率。 以Ce金屬為主之雙金屬觸媒Ce/Mn, Ce/Cu, Ce/Al-MSPs與單金屬觸媒Ce-MSPs(25)在丙酮濃度1000ppmv、GHSV=15000h-1的催化效率比較上，顯示四種觸媒在高溫(>250°C)的催化效率都在90%以上且相差不多，原因為Ce金屬在高溫時具有良好的觸媒催化活性；然而在低溫(<200°C)的反應條件下，Ce/Al-MSPs對於丙酮的催化效率明顯高於其他觸媒，其中Ce/Al-MSPs(50/25)在150°C時的催化效率可達80%以上，而其他不含Al金屬的觸媒均低於20%，主要是因為Al金屬在低溫時能發揮顯著的吸附活性，而如此Ce與Al金屬的協合效應(synergetic effect)，說明Ce/Al-MSPs為一具有研究潛力的低溫觸媒；同時也說明了Al金屬的添加量、觸媒的比表面積與CeO2的顆粒大小為決定雙金屬觸媒催化丙酮效率的重要影響因素。 本研究延伸探討比較Aero-EISA一步合成Ce/Al-MSPs(50/25)及先以Aero-EISA製成Al-MSPs擔體，再加上Ce金屬含浸之二步合成法所製得之Ce-Al-MSPs(25)，以及以商用沸石ZSM-5為擔體，再用含浸法製備成Ce-ZSM-5之三種觸媒在150°C與200°C低溫下進行丙酮催化效率的比較；實驗結果顯示，Ce/Al-MSPs(50/25)具有高比表面積與集中的孔徑分佈，CeO2分佈最均勻，以及具有最多的氧儲存容量(oxygen storage capacity, OSC)與氧空缺(oxygen vacancy)，經由27Al-NMR的分析結果顯示，Ce/Al-MSPs(50/25)為Framework Aluminum(FA)的四配位鋁結構，而另外二種觸媒則還包含了Extra-framework Aluminum(EFA)的六配位鋁型式，因此Ce/Al-MSPs(50/25)在三種觸媒之中催化丙酮的效率最高；並且在GHSV=5000h-1、丙酮濃度1000ppmv的長效實驗中，能維持150°C、24小時74%以及200°C、72小時85%的穩定催化效率。

關鍵字：揮發性有機化合物(VOCs)、中孔洞矽材料、金屬觸媒、丙酮、二氧化鈰、氣膠輔助揮發誘導自組裝製程

Abstract There are many studies investigated on the VOCs abatement and noble metal-based catalysts such as Au, Pt, Pd-catalysts show good activity. However, the industrial application of noble metal-based catalysts is limited by their price. Thus many different species of transition metals have been developed and applied in many fields. This study intends to synthesize non-noble metal catalysts via one step aerosol assisted evaporation induced self-assembly (Aero-EISA) process. Unlike conventional sol-gel or hydrothermal method which required tedious and time-consumed steps for manufacturing metal catalysts, the one step Aero-EISA process has the advantage of time-saving and can be designed as a continuous process for easy mass production. There is no report available on synthesizing metal catalysts by Aero-EISA and applying them to the destruction of VOCs. This study intends to present results on the physical and chemical characterization of metal mesoporous silica particles (metal-MSPs) by one step Aero-EISA process and further investigation for the catalytic oxidation of acetone. The results showed that Ce-MSPs is the best metal catalytst for the acetone destruction among various metals of Ce, Mn, Cu, Fe, Al-MSPs. This is because CeO2 has the ability to transport oxygen, high oxygen-storge capacity (OSC) and more oxygen available in the oxidation reaction by the Ce3+/Ce4+ redox couple state. Among the five different Ce loading amounts, Ce-MSPs(25) showed the best catalytic activity due to its relatively high specific surface area of 951 m2/g and optimal Ce metal content of 3.72 wt.%. The Ce-MSPs(10) had a higher Ce loading of 9.76 wt.%, but its smaller specific surface area of 615 m2/g, poor pore size distribution, and less-ordered pore structure resulted in a relatively lower acetone removal as compred to the Ce-MSPs(25). For tests on the monometallic Ce-MSPs(25) and bimetallic Ce/Mn, Ce/Cu, Ce/Al-MSPs under acetone concentration of 1000ppmv and gas hourly space velocity (GHSV) of 15000h-1, it was demonstrated that all of the samples had similar acetone removal efficiencies at the temperature above 250°C, which was due to the good oxidation activity of Ce metal in higher temperature range. But at low temperature of 150-200°C, the Ce/Al-MSPs had much higher acetone removals than all other cataltsts due to the good absoption ability of Al metal in lower temperature range. The synergetic effect of Ce, Al metal was observed for bimetallic Ce/Al-MSPs on the acetone removal as compared to the monometallic Ce-MSPs or Al-MSPs catalysts. The Al loading amount, specific surface area, and the CeO2 particle size played important roles on the low temperature catalytic oxidation of acetone. This study also compared the acetone oxidation efficiencies between Ce/Al-MSPs(50/25) (synthesized by one step Aero-EISA), Ce-Al-MSPs(25) (synthesized by combining the one step Aero-EISA and wet metal impregnation method) and Ce-ZSM-5 (synthesized by Ce metal wet impregnation on ZSM-5 commercial product) under 150, 200°C low temperature. The results indicated that Ce/Al-MSPs(50/25) had the best acetone oxidation performance due to the high specific surface area, uniform pore size as well as wide spread of CeO2 particles. Moreover, the highest amount of CeO2, oxygen storage capacity, oxygen vacancy and tetrahedral coordination framework Aluminum structure also resulted in the highest acetone oxidation and the best stability. The Ce/Al-MSPs(50/25) exhibited good stability of 74% acetone removal within 24h test period at 150°C and 85% acetone removal within 72h test period at 200°C. Keywords: Volatile organic compounds (VOCs); mesoporous silica materials; metal catalyst; acetone; ceria; aerosol assisted evaporation induced self-assembly (EISA) process