負載銀之Y型沸石與二氧化鈰觸媒的製備與其應用於甲醛氧化反應的研究

Abstract

甲醛是常見的揮發性有機物之一，主要來源來自於工業廢氣的排放、居家的建築材料以及建築裝飾油漆，並且對於人體產生健康的危害。因此如何減少生活中的甲醛以降低對於人體健康危害的風險變成最重要的課題。甲醛氧化反應因為具有高效率並且可以將甲醛轉化成無害的水及二氧化碳，因此被視為最具可行性的甲醛移除方法。在甲醛氧化反應當中，一般以鉑、鈀與金為主的觸媒，價格昂貴且可以在較低溫的環境有效地進行反應；以銀為基礎的觸媒雖然價格較為便宜，但卻需要較高的反應溫度才可進行。本篇論文是以含浸法的方式將銀負載於不同的載體上，而載體選擇上有Na-Y型沸石、H-Y型沸石及二氧化鈰奈米粒子。利用氮氣等溫吸附脫附(N2 adsorption–desorption)、電感耦合電漿體質譜(ICP-MS)、X光粉末繞射(XRD)、X光電子光譜(XPS)、X光吸收光譜(XAS)、掃描穿透式電子顯微鏡附X射線能量散佈分析(STEM/EDS)、高分辨透射電子顯微鏡(HRTEM)、傅里葉轉換紅外光譜(FTIR)、氫氣程溫還原分析(H2-TPR)、原位擴散反射紅外光谱(in-situ DRIFTS)與熱分析-質譜儀(TA-MS)來鑑定觸媒材料的性質結構、化學態的變化以及負載銀的顆粒大小，以便去探討觸媒材料的特性與甲醛反應的活性的關聯性。 以銀負載於Y型沸石之觸媒為例，將以Na離子交換之Y型沸石、鍛燒步驟、還原步驟以及製備硝酸銀溶液的溶液選擇進行探討。觸媒的最佳銀金屬負載量為6 wt%。製備觸媒的含浸溶液可為水與乙腈，分別適用於具有親水性及疏水性的沸石表面特性上。6%Ag/NaY40觸媒經由723K鍛燒處理後，於473 K下進行氫氣還原反應，所獲得的觸媒標示為6Ag/NaY40-723-R。該觸媒在400 ppm的甲醛濃度下，反應溫度為373 K，反應72小時候依舊可維持趨近於100%的甲醛轉化率，二氧化碳選擇轉化率為95%以上。在環境的相對溼度為50%狀態下，水氣的存在仍可促進銀負載於Y型沸石之觸媒在甲醛氧化反應的進行並提升其反應活性。在420 ppm的甲醛與920 ppm的甲苯共存環境下，反應溫度提高至473K時，依舊可維持約98%的甲醛轉化率。從XRD, TEM and Ag L3-XANES的分析結果可知，6Ag/NaY40-723-R具有最佳的催化效果來自於表面的金屬銀奈米粒子具有高度分散性。 以銀負載於二氧化鈰奈米粒子之觸媒為例，鍛燒溫度的不同及氫氣還原的前處理會影響觸媒在甲醛氧化反應的催化活性。6%Ag/CeO2觸媒經由573K鍛燒處理後，於473 K下進行氫氣還原反應，所獲得的觸媒標示為6Ag/CeO2-573-R，該觸媒在400 ppm的甲醛濃度下，反應溫度為333K，反應72小時候依舊可維持95%的甲醛轉化率，二氧化碳選擇轉化率為98%以上。Ag/CeO2觸媒的催化活性與銀的氧化態與粒子大小有關。金屬銀粒子均勻分散於二氧化鈰奈米粒子表面上，有利於氧氣分子的吸附而產生表面活性氧化物，進而促使甲醛氧化反應可在低溫下進行。 另一方面，二氧化鈰奈米粒子表面的羥基群可以穩定甲醛分子吸附在觸媒表面上以及藉由中間產物的去氫反應來促進甲醛氧化反應的進行。

Formaldehyde (HCHO) is one of the most common VOCs emitted from industrial exhaust stream, building materials and decorative paint, and it can cause serious health problems. It is important to reduce the concentration of HCHO in the outdoor and indoor air to improve public health risk. Among the methods for HCHO removal, catalytic oxi-dation of HCHO is regarded as the most promising method due to its higher efficiency and resulting harmless products of CO2 and H2O. Compared to Pt, Pd and Au-based cat-alysts, Ag-based catalysts were significantly less expensive but showed good perfor-mance in HCHO oxidation at relatively high temperature. In this thesis, Ag-based cata-lysts with different supports (sodium-form zeolite Y, proton-form zeolite Y and CeO2 na-noparticles) were fabricated through impregnation method and used as the catalysts in HCHO oxidation. The catalytic activities were correlated with the structure, chemical state, and particle size of supported Ag catalysts, which were characterized systematically by N2 adsorption–desorption, ICP-MS, XRD, XPS, XAS, STEM/EDS, HRTEM, FTIR, H2-TPR, in-situ DRIFTS, and TA-MS. In the case of zeolite Y supported Ag catalysts, factors examined include Na content in zeolite Y, thermal pre-treatment, Ag loading, and the solvent to dissolve the silver salt. Water and acetonitrile were found to be suitable solvents for impregnating Ag on zeolite Y with more hydrophilic and hydrophobic surfaces, respectively. The 6 wt% Ag loading was found to give the optimal catalytic activity. 6%Ag/NaY40 calcined at 723 K in air and then reduced at 473 K in 10% H2/N2 stream showed excellent activities and stabilities for HCHO oxidation at 373 K, achieving ca. 100% conversion of 400 ppm HCHO and CO2 selectivity was maintained above 95%. The addition of moisture (Relative Humidity of 50%) to the reaction mixture considerably enhanced the conversion of HCHO oxida-tion over Ag/zeolite Y catalysts. The HCHO conversion over 6Ag/NaY-723-R catalyst could be achieved about 98% at 473 K under 400 ppm HCHO and 920 ppm toluene co-existence. XRD, TEM and Ag L3-XANES results showed that the excellent performance of the catalysts is ascribed to high dispersion of Ag nanoparticles. In the case of Ag-supported on CeO2 nanoparticles (NPs), the pre-treatments of Ag/CeO2 catalysts by calcining at different temperatures and H2 reduction were found to significantly influence their catalytic activities in HCHO oxidation. The 6wt%Ag/CeO2 catalyst calcined at 573 K and then reduced at 473 K with 10%H2/N2 showed 95% con-version of 400 ppm formaldehyde at 333 K under high gas hourly space velocity (GHSV = 24400 h−1) and CO2 selectivity is maintained above 98%. The catalytic activities were correlated with the oxidation state and particle size of silver. Metallic Ag particles well dispersed on CeO2 NPs could adsorb oxygen readily and generate surface active oxygen species, which contributed to the oxidation of HCHO at low temperature. The retention of surface hydroxyl groups on the surface of CeO2 NPs also promoted the catalytic per-formance for HCHO oxidation probably by stabilizing the adsorbed HCHO molecules and removing proton from surface intermediates.