同步辐射研究高温氧化对金属-载体相互作用的影响

Synchrotron radiation study of effects of high temperature oxidation on metal-support interactions in catalysts

金属-载体相互作用(Metal-Support Interaction,MSI)通过改变金属颗粒的形貌、组分和电子密度,从而改变催化剂的活性、稳定性和选择性。负载型单原子和团簇催化剂在不同温度与气氛下处理,金属和载体的相互作用会随之发生变化。本文研究单原子和纳米颗粒共存的Pt/TiO2和Pt/SiO2催化剂在空气中升温煅烧时对金属-载体相互作用的影响。利用高分辨透射电子显微镜(High-Resolution Transmission Electron Microscopy,HRTEM)、球差校正高角环形暗场扫描透射电子显微镜(High Angle Annular Dark Field-Scanning Transmission Electron Microscope,HAADF-STEM)和同步辐射X射线吸收精细结构谱(X-ray Absorption Fine Structure,XAFS)、漫反射傅里叶变换红外光谱(Diffuse Reflaxions Infrared Fourier Transformations Spectroscopy,DRIFT)等技术,探究两种催化剂的不同结构演变行为。结果表明:经过700℃高温煅烧后,Pt/TiO2中的Pt物种全部表现为单原子,而在Pt/SiO2中则团聚成更大的颗粒。金属氧化物的还原性决定了载体锚定Pt原子的能力:Pt单原子与TiO2可以通过共价金属-载体相互作用(Covalent Metal-Support Interaction,CMSI)固定因而利于Pt原子分散,而Pt与SiO2不能形成CMSI,故导致颗粒烧结。

[Background]Metal-Support Interaction(MSI)modulates the activity,stability,and selectivity of catalysts by changing the morphology,composition,and electron density of metal particles.Such interactions in supported single-atom and cluster catalysts will be subjected to changes when the catalysts are treated at different temperatures and under different atmospheres.[Purpose]This study aims to explore the effects of high temperature treatment on metal-support interactions of Pt/TiO2and Pt/SiO2catalysts with the coexistence of single atoms and clusters when heating and calcined in air at high temperature.[Methods]The structural evolutions of these two catalysts were investigated by a combination of high-resolution transmission electron microscopy(HRTEM),aberration-corrected high-angle annular dark-field transmission electron microscopy(AC-HAADF-STEM),synchrotron radiation technique X-ray absorption fine structure spectroscopy(XAFS),diffuse reflection Fourier transform infrared spectroscopy(DRIFT).XAFS characterization was conducted at Beijing Synchrotron Radiation Facility(BSRF)1W1B beamline station to obtain information about the valence state and coordination environment of Pt/TiO2at different calcination temperatures to confirm that Pt mono-atoms were immobilized on the support by Covalent Metal-Support Interaction(CMSI).[Results]After calcination at 700℃,all Pt species are presented as Pt single atoms in Pt/TiO2,while they agglomerate into larger particles in Pt/SiO2.The reducibility of metal oxides determines the ability of the support for anchoring single atoms of Pt:single atoms of Pt in TiO2can be stabilized via the CMSI,and thus facilitate the dispersion of Pt atoms.On the contrast,CMSI cannot be formed between Pt and SiO2,resulting in particle sintering at high temperatures.[Conclusions]The formation of CMSI effect between metals and supports can be used as a general method for the preparation of reducible oxide supported high-loading,thermally stable single-atom catalysts.