ZIF-8的高温热解: 锌(Ⅱ)单原子中心从四面体构型到类卟啉构型的演变

Evolution of Zn(Ⅱ) single atom catalyst sites during the pyrolysis-induced transformation of ZIF-8 to N-doped carbons

高温热解金属有机骨架(ZIFs)是一种合成含类卟啉单原子碳材料的有效方式.理解煅烧产物中单原子位点在热解过程中的构型变化,对实现高性能单原子催化材料具有重要意义.因此,本工作系统地研究了在ZIF-8煅烧过程中(500~900℃)锌单原子中心的构型演变.同步辐射吸收谱结果表明:ZIF-8前驱体中四面体构型的锌单子中心在600℃时开始转变为类卟啉锌中心(锌中心凸出卟啉平面);并随着温度升高至900℃,锌中心逐渐接近N4卟啉平面.由于Zn N4中心位点的几何构型变化改变了锌中心原子的电子密度分布,因此其在降解H2O2中表现不同的催化活性(800℃煅烧得到的锌单原子产物性能最优).本工作为理解ZIF煅烧中单原子的构型演变以及其催化性能提供了新思路.

The pyrolysis of zeolitic imidazolate frameworks(ZIFs) is becoming a popular approach for the synthesis of catalysts comprising porphyrin-like metal single atom catalysts(SACs) on N-doped carbons(M-N-C).Understanding the structural evolution of M-N-C as a function of ZIF pyrolysis temperature is important for realizing high performance catalysts.Herein,we report a detailed investigation of the evolution of Zn single atom catalyst sites during the pyrolysis of ZIF-8 at temperatures ranging from 500 to 900℃.Results from Zn L-edge and Zn K-edge X-ray absorption spectroscopy studies reveal that tetrahedral ZnN4 centers in ZIF-8 transform to porphyrin-like ZnN4 centers supported on N-doped carbon at temperatures as low as 600℃.As the pyrolysis temperature increased in the range 600-900℃,the Zn atoms moved closer to the N4 coordination plane.This subtle geometry change in the ZnN4 sites alters the electron density on the Zn atoms(formally Zn2+),strongly impacting the catalytic performance for the peroxidase-like decomposition of H2 O2.The catalyst obtained at 800℃(Zn-N-C-800) offered the best performance for H2 O2 decomposition.This work provides valuable new insights about the evolution of porphyrin-like single metal sites on N-doped carbons from ZIF precursors and the factors influencing SAC activity.