Insight into Photoactive Sites for the Ethylene Oxidation on Commercial HZSM-5 Zeolites with Iron Impurities by UV Raman, X-ray Absorption Fine Structure, and Electron Paramagnetic Resonance Spectroscopies

Abstract

This study investigated the chemical nature of impurity iron species and photoactive sites in the commercially available HZSM-5 zeolites with different Si/Al ratios. The samples were characterized by X-ray fluorescence spectroscopy, atomic absorption spectroscopy, X-ray absorption near-edge structure spectroscopy, extended X-ray absorption fine structure spectroscopy, electron paramagnetic resonance spectroscopy, UV Raman spectroscopy, and photocatalytic degradation of ethylene. The results revealed that the photocatalytic activity of the HZSM-5 originated from the isolated tetrahedrally coordinated iron−oxo species present in the zeolite framework. The formation of such active isolated iron−oxo species was strongly influenced by the chemical compositions (e.g., Si/Al ratio, and iron content) and the preparation methods of zeolite. This could explain the difference in photoactivity of the different types of HZSM-5 and FeZSM-5 zeolites. Possible mechanisms of the photocatalysis on the zeolites were also discussed.

Photocatalytic Degradation and Selective Oxidation of Typical Gas Phase Molecules with TS-1 and HZSM-5 Zeolites

“…基于分子结构差异和沸石孔窗的择形优势, 国外研究小 组较早就重视到多组分液相体系内特定污染物选择性 氧化的突出价值, 设立了几类成功的选择性光催化体系 并获得了较佳的转化效果 [27,28] . 然而, 本课题组在以有 机污染物有效降解为目的的光催化反应中却发现, 沸石 分子筛对典型有机分子的降解速度及矿化效率与常规 半导体光催化体系相比仍存较大差距 [17,18] . 因此, 不同 本征物化结构的催化材料(如氧化物半导体与沸石分子 筛)可能匹配和促进不同类型的光催化进程, 该构-效反 应机制可指导和构建不同终端产物的光催化反应新体 系.…”

“…中心不易被水分子吸附, 使得催化剂的活性中心与反应 物充分接触而显示高活性. 商品 HZSM-5 沸石的光催化 活性与孤立四配位状态的 Fe 3＋ 物种直接相关 [17,18] , Fe 3＋ 能被紫外光激发而形成电荷激发态: [35] . 孤 立 Ti 4＋ 有 Ti＝O 键及四面体配位两种形式而区别于 TiO 2 中钛离子的八配位, 其光响应发生蓝移, 这使得钛原子 光催化氧化-还原能力增强 [36] .…”

Modern Spectroscopic Methods Applied to Nanoscale Porous Materials

“…38 The chemical nature of impurity iron species and photoactive sites in commercially available HZSM-5 zeolites of different Si/Al ratios was studied by Raman spectroscopy using two different excitation wavelengths in the UV range. 39 Resonance Raman conditions are achieved at 244 nm due to O 2À ÀFe 3þ ! O 2 ÀFe 2þ ligand to metal charge-transfer transition.…”

“…10,13,[18][19][20][21][22][23][24][25][26][27][28][29][30][31][32][33] In addition, FTIR and Raman spectroscopy are very sensitive to atomic bonding and thus gives the opportunity to study active sides and interactions between embedded functional molecules and framework atoms in porous materials. 5,12,[33][34][35][36][37][38][39][40][41][42] Raman spectroscopy compares favourably to FTIR spectroscopy because of the better signal-to-noise ratio in the low-energy spectral range and the better spatial resolution when a microscope is used. Besides, Raman spectroscopy does not require any special sample preparation, which makes this technique a truly non-invasive and non-destructive method for structural analysis.…”

“…The method is especially advantageous in studying active centres in functionalized by appropriate doping porous materials; however, its proper utilization requires a synchrotron radiation. 15,39,52,60,61 Mößbauer spectroscopy is based on recoilless resonance absorption/emission of gamma-rays in solids. Its utilization for structural analysis stems from the fact that the principle energy levels of atomic nuclei are strongly influenced by the positions of the surrounding atoms.…”