摘要
A new compound based on immobilizing of Pd6(RuL3)8(BF4)28 (L=2-(pyridin-3-yl)-1H-imidazo [4,5-f][1,10]-phenanthroline) cage (MOC-16) on g-C3N4 was synthesized. Infrared spectrum and powder X-ray diffraction were used to characterize structure of hybrid MOC-16/g-C3N4, as well as UV-vis absorption spectrum and X-ray photoelectron spectroscopy were carried out to unveil photocatalytic mechanism. With the introduction of MOC-16, the absorption edge of MOC-16/g-C3N4 in UV-vis spectrum extended apparently to long-wavelength region compared with pristine g-C3N4. H2 evolution yielded with MOC-16/g-C3N4 in aqueous solution containing TEOA was much higher than that with RuL3/g-C3N4, Pd/RuL3/g-C3N4 and mixture of MOC-16 and g-C3N4, showing that the octahedral cage structure with high-efficient electron transfer and the interface interaction between MOC-16 and g-C3N4 were significant for improvement of H2 evolution.
可见光驱动的光催化水制氢是一种太阳能转化策略,各种异质结半导体和均相分子器件在光催化领域取得了很大的进展.本课题组曾开发了一种含多个吸光中心和催化中心的金属-有机自组装分子笼Pd6(RuL3)8(BF4)28(MOC-16),并研究了其光催化分解水产氢性能.尽管该笼子定向电子通过多个独立的通道转移实现了高效制氢,但仍存在均相催化剂的典型缺点,即在光催化过程中,笼子分解后钯纳米颗粒团聚致使催化剂失活.而二维片层结构的石墨相氮化碳(g-C3N4)具有大量的共轭π电子和终止边缘以构成氢键,是构建杂化材料的理想基底.在此基础上,本文采用简单的制备方法将MOC-16分立固定在g-C3N4基质上,得到异质体系MOC-16/g-C3N4,相比其均相组分,该新型催化剂具有更优良的光催化分解水产氢性能.本文设计了空白的催化产氢实验,比较了MOC-16中各部分及分子笼在催化产氢体系中的作用,并采用X射线衍射(XRD),傅里叶红外光谱,透射电镜,紫外可见光谱(UV-Vis),瞬态光电流响应(i-t)、X射线光电子能谱(XPS)等手段研究了MOC-16和g-C3N4之间的相互作用和杂化材料MOC-16/g-C3N4的光催化产氢机理.10 wt% MOC-16/g-C3N4表现出最高的产氢速率2021 μmol g^–1 h^–1,并优于空白对照组的产氢效果,循环15 h时的TON(Pd)为517, TOF(Pd)值约36 h^–1,与MOC-16均相催化剂相比,催化剂MOC-16/g-C3N4在产氢效率和稳定性上有明显提升.形貌结构表征显示, MOC-16不与g-C3N4形成新的共价键,也不改变g-C3N4原有形貌结构, MOC-16以配合物形式均匀分散在g-C3N4基底材料上. UV-Vis结果表明, MOC-16/g-C3N4的紫外-可见吸收峰结合了两种组分的吸收峰,杂化材料的可见光区的吸收峰延伸至700 nm左右.随着MOC-16负载量增大,杂化材料MOC-16/g-C3N4的吸光范围越大. i-t结果进一步表明, MOC-16和g-C3N4之间存在有效的电子转移. XPS结果显示,杂化前后, MOC-16中Pd价态未发生改变,但峰位置发生位移, Pd 3d的电子结合能分别从343.1和338.0移动到342.6和337.3 eV,进一步表明杂化后MOC-16和g-C3N4间存在相互作用.然而经过三轮连续循环产氢后,部分二价钯被还原为零价,表明固定在g-C3N4表面的MOC-16在光催化过程中光生电子不断流向Pd,电子消耗缓慢导致Pd-N键的断裂.我们提出了MOC-16/C3N4复合光催化剂可能的光催化产氢机制. MOC-16的LUMO和HOMO能级分别为–0.95和1.55V (vs.NHE), g-C3N4的导带在–1.09至–1.3 V,价带在1.53至1.4 V之间,所以光生电子从g-C3N4转移到MOC-16在热力学上是可行的.光生电子转移到MOC-16分子的Pd上, Pd作为助催化剂为H2的产生提供活性位点,而TEOA作为牺牲试剂,则在g-C3N4表面消耗光生空穴.
基金
supported by the National Natural Science Foundation of China(21875293,21821003,21890380,21720102007,21572280)
the Natural Science Foundation of Guangdong Province(2016A030313268)
the STP Project of Guangzhou(201804010386,201707010114)
the Fundamental Research Funds for the Central Universities(17lgzd18,17lgzd01)
the Research Fund Program of Key Laboratory of Fuel Cell Technology of Guangdong Province~~
