铁盐对制备MIL-100(Fe)的影响及其光催化性能

三价铁盐和二价亚铁盐作为合成MIL-100(Fe)的铁源,对所合成产物的结晶度、形貌以及尺寸具有重要影响。以二价亚铁盐作为原料,利用室温水相合成法可获得八面体形状、高结晶度、高比表面积MIL-100(Fe)纳米颗粒;以三价铁盐作为原料,只能获得尺寸更小、低结晶度的Fe-BTC金属-有机聚合物纳米颗粒。采用X射线衍射仪、扫描电子显微镜、傅里叶红外光谱仪、紫外-可见漫反射光谱仪、紫外-可见分光光度计等对MIL-100(Fe)和Fe-BTC纳米颗粒的晶体结构、形貌、光吸收以及对罗丹明B(RhB)吸附和光催化降解性能进行测试表征。结果表明:Fe²⁺被弱碱性溶液缓慢氧化成Fe³⁺,进而形成MIL-100(Fe)的无机次级结构单元μ_3-OFe(Ⅲ)O_6,是合成高结晶度MIL-100(Fe)的关键。以FeCl_3为原料时,由于Fe³⁺与BTC^3-快速反应,不利于形成μ_3-OFe(Ⅲ)O_6次级结构单元,因此产物为Fe-BTC纳米颗粒聚合物。Fe-BTC纳米颗粒粒径更小,且聚集态的纳米颗粒表面具有大孔或介孔结构,更有利于吸附物种以及光降解物种的扩散,因此,Fe-BTC对RhB的吸附和光催化降解性能优于MIL-100(Fe)。

Effects of iron salts on synthesis of MIL-100(Fe) and photocatalytic activity

The iron salts (ferric and ferrous salts) play a critical role for the synthesis of morphology and size controllable, and highly crystalline MIL-100(Fe). The use of ferrous salt results in octahedral, highly crystalline MIL-100(Fe) with high surface areas. In contrast, the use of ferric salt only results in smaller Fe-BTC metal-organic polymers with low crystallinity. The crystal structure, morphology, photo-absorption property, absorption and photocatalytic-degradation properties of MIL-100(Fe) and Fe-BTC toward RhB, were fully characterized by XRD, SEM, FT-IR, UV-Vis DRS, and UV-Vis spectroscopy. The results indicate that the slow oxidation of Fe 2+ to Fe 3+ is key to the synthesis of highly crystalline MIL-100(Fe) since this process facilitates the formation of μ 3-OFe(Ⅲ)O 6, which is inorganic secondary building unit (SBU) of MIL-100(Fe). When FeCl 3 used as iron source, the fast dynamic reaction of Fe 3+ with BTC 3- is not beneficial to the formation of μ 3-OFe(Ⅲ)O 6 SBU;therefore, Fe-BTC nanoparticle polymer is obtained. The smaller size of Fe-BTC nanoparticless, and meso-/macroporous structure in the aggregated nanoparticle surface, which facilitates the diffusion of RhB as well as the photodegradated forms. Fe-BTC shows better absorption capacity and photocatalytic activity toward RhB compared to MIL-100(Fe).