Evolution of Gold Structure During Thermal Treatment of Au/FeOx Catalysts Revealed by Aberration-Corrected Electron Microscopy

Allard, L.F., A. Borisevich, W. Deng, R. Si, M. Flytzani-Stephanopoulos and S.H. Overbury., 2009
Fusion TEM holder tip
Image courtesy of J. Electron Microsc.

Abstract

High-resolution aberration-corrected electron microscopy was performed on a series of catalysts derived from a parent material, 2 at.% Au/Fe(2)O(3) (WGC ref. no. 60C), prepared by co-precipitation and calcined in air at 400 degrees C, and a catalyst prepared by leaching surface gold from the parent catalyst and exposed to various treatments, including use in the water-gas shift reaction at 250 degrees C. Aberration-corrected JEOL 2200FS (JEOL USA, Peabody, MA) and Vacuum Generators HB-603U STEM instruments were used to image fresh, reduced, leached, used and re-oxidized catalyst samples. A new in situ heating technology (Protochips Inc., Raleigh, NC, USA), which permits full sub-Angström imaging resolution in the JEOL 2200FS was used to study the effects of temperature on the behavior of gold species. A remarkable stability of gold to redox treatments up to 400 degrees C, with atomic gold decorating step surfaces of iron oxide was identified. On heating the samples in vacuum to 700 degrees C, it was found that monodispersed gold began to sinter to form nanoparticles above 500 degrees C. Gold species internal to the iron oxide support material was shown to diffuse to the surface at elevated temperature, coalescing into discrete nanocrystals. The results demonstrate the value of in situ heating for understanding morphological changes in the catalyst with elevated temperature treatments.

Impact Statement

Follows structural evolution of Au as a function of thermal treatment in situ. Identifies and distinguishes structural effects produced ex situ via redox, and catalytic treatments of Au on Fe2O3 with atomic-scale resolution using a JEOL 2200FS Cs corrected STEM.