Rangus Mojca, Mazaj Matjaž, Dražić Goran, Popova Margarita, Tušar Nataša Novak
National Institute of Chemistry, Hajdrihova 19, SI-1001 Ljubljana, Slovenia.
Institute of Organic Chemistry with Centre of Phytochemistry, Bulgarian Academy of Sciences, Acad. G. Bontchev str., bl. 9, 1113 Sofia, Bulgaria.
Materials (Basel). 2014 May 30;7(6):4243-4257. doi: 10.3390/ma7064243.
Iron-functionalized disordered mesoporous silica (FeKIL-2) is a promising, environmentally friendly, cost-effective and highly efficient catalyst for the elimination of volatile organic compounds (VOCs) from polluted air via catalytic oxidation. In this study, we investigated the type of catalytically active iron sites for different iron concentrations in FeKIL-2 catalysts using advanced characterization of the local environment of iron atoms by a combination of X-ray Absorption Spectroscopy Techniques (XANES, EXAFS) and Atomic-Resolution Scanning Transmission Electron Microscopy (AR STEM). We found that the molar ratio Fe/Si ≤ 0.01 leads to the formation of stable, mostly isolated Fe sites in the silica matrix, while higher iron content Fe/Si > 0.01 leads to the formation of oligonuclear iron clusters. STEM imaging and EELS techniques confirmed the existence of these clusters. Their size ranges from one to a few nanometers, and they are unevenly distributed throughout the material. The size of the clusters was also found to be similar, regardless of the nominal concentration of iron (Fe/Si = 0.02 and Fe/Si = 0.05). From the results obtained from sample characterization and model catalytic tests, we established that the enhanced activity of FeKIL-2 with the optimal Fe/Si = 0.01 ratio can be attributed to: (1) the optimal concentration of stable isolated Fe in the silica support; and (2) accelerated diffusion of the reactants in disordered mesoporous silica (FeKIL-2) when compared to ordered mesoporous silica materials (FeSBA-15, FeMCM-41).
铁功能化无序介孔二氧化硅（FeKIL-2）是一种很有前景的、环境友好、经济高效的催化剂，可通过催化氧化从污染空气中消除挥发性有机化合物（VOCs）。在本研究中，我们结合X射线吸收光谱技术（XANES、EXAFS）和原子分辨率扫描透射电子显微镜（AR STEM）对铁原子的局部环境进行先进表征，研究了FeKIL-2催化剂中不同铁浓度下的催化活性铁位点类型。我们发现，摩尔比Fe/Si≤0.01会导致在二氧化硅基质中形成稳定的、大多为孤立的铁位点，而较高的铁含量Fe/Si>0.01会导致形成多核铁簇。STEM成像和EELS技术证实了这些簇的存在。它们的尺寸范围从一纳米到几纳米不等，并且在整个材料中分布不均匀。还发现，无论铁的标称浓度（Fe/Si = 0.02和Fe/Si = 0.05）如何，簇的尺寸都相似。根据样品表征和模型催化测试的结果，我们确定，Fe/Si = 0.01最佳比例的FeKIL-2活性增强可归因于：（1）二氧化硅载体中稳定孤立铁的最佳浓度；（2）与有序介孔二氧化硅材料（FeSBA-15、FeMCM-41）相比，反应物在无序介孔二氧化硅（FeKIL-2）中的扩散加速。
