(670a) Strong Interaction of Au with FeOx Surfaces Evidenced in the Water-Gas Shift Reaction, and Probed by Aberration-Corrected Electron Microscopy and Scanning Tunneling Microscopy

Authors: 
Overbury, S. H., Oak Ridge National Laboratory
Flytzani-Stephanopoulos, M., Tufts University
Si, R., Brookhaven National Lab
Allard, Jr., L. F., Oak Ridge National Laboratory
Deng, W., Oak Ridge National Laboratory
Rim, K. T., Columbia University
Zhai, Y., Tufts University
Ricks, B., Tufts University
Frenkel, A., Yeshiva University
Flynn, G., Columbia University


Gold-iron oxide catalysts have been found equally active to gold-ceria catalysts for the water-gas shift reaction and more stable than the latter at temperatures higher than 250°C.[1] Systematic studies of the evolution of gold species in iron oxide and their structural stability under different reaction gas compositions and temperatures were carried out in this work by in-situ XAS, TPR, high resolution aberration-corrected electron microscopy, and scanning tunneling microscopy of Au on Fe3O4(111) surfaces.

High Angle-Annular Dark Field (HA-ADF) microscopy with an aberration corrected microscope has been combined with scanning probe microscopy to explore the interaction of Au particles with iron oxide catalysts. Both techniques provide sufficient resolution to see single atoms within the catalyst. HA-ADF microscopy was performed on high surface area catalysts, the same as are used in reactive studies and following various ex situ and in situ sample treatments. We have succeeded in obtaining images while heating the catalyst and have been able to observe directly the effects of exposure to the environment of a catalytic reactor. The goal of the study was to identify the active components within the catalyst and observe how they transforms during preparation steps or during catalytic operation.

By comparing HAADF images of samples used in reaction studies, it was determined that small (~2nm) metallic Au particles are the active agent for catalyzing the CO oxidation reaction while highly dispersed cationic Au species are the active sites for the water gas shift reaction. A crucial aspect is that the starting catalyst was leached to produce a state in which no discrete Au nanoparticles were present on the surface of the supporting iron oxide crystallites. Subsequent microscopy was able to detect Au cations and 1-2 nm sized Au nanoparticles (NPs) embedded within the oxide support crystallites, revealing the phase interpenetration that results from co-impregnation methods of catalyst preparation. Voids within the iron oxide support particles were observed, with surfaces decorated with Au atoms. A technique of through-particle focusing permitted analysis of the 3-d spatial relationship of the Au NPs. Interestingly, embedded NPs had decreased lattice constants, compared to bulk Au or compared to comparably sized particles on the external surfaces. Thermal treatments resulted in the collapse of the voids and the migration of Au to the external surfaces of the iron oxide, a process captured by sequences of images recorded with time at temperature. Below 400°C, gold exists as individually dispersed Au atoms (cations), while at higher temperatures it is aggregated to form metallic Au only a few nanometers across.[2] Hence, sintering of gold is suppressed by a strong interaction between gold and iron oxide.

Scanning tunneling microscopy (STM) measurements were performed of Au deposited in UHV onto a (2 x 2) Fe3O4(111) surface. Gold forms two electrically distinct types of nanoparticles on an iron oxide surface upon annealing multilayer Au/Fe3O4(111) at 400 C. I(V) curves taken via STS measurements show that large gold nanoparticles (~ 8 nm) exhibit a metallic electronic structure and thus are likely neutral. Single gold adatoms appear to be strongly bonded to the oxygen sites of the surface, and tunneling electrons are observed to flow predominantly from the STM tip to the Au adatoms and into the oxygen sites of the surface. The site-specific adsorption of the gold adatoms on oxygen surface atoms and the size sensitive nature of the electronic structure suggest that Au adatoms are likely positively charged.[3] When this Au/Fe3O4(111) system is dosed with CO at -13°C, adsorption of CO molecules normal to the surface atop the gold adatom sites takes place. CO adsorption on the large Au nanoparticles (~ 8nm) could not be confirmed by STM. These observations indicate that nonmetallic, positively charged, Au species may play a key role in reactions of CO on Au/iron oxide surfaces.

A portion of this work was supported by a DOE-BES/HFI grant to Tufts University; and another portion of this research was sponsored by the Division of Chemical Sciences, Geosciences, and Biosciences, Office of Basic Energy Sciences, U.S. Department of Energy, under contract DE-AC05-00OR22725 with Oak Ridge National Laboratory, managed and operated by UT-Battelle, LLC

[1] W. Deng, C. Carpenter, N. Yi, M. Flytzani-Stephanopoulos, Top. Catal. 2007, 44, 199.

[2] L.F. Allard, A. Borisevich, W. Deng, R. Si, M. Flytzani-Stephanopoulos, S.H. Overbury, "Evolution of gold structure during thermal treatment of Au/FeOx catalysts revealed by aberration-corrected electron microscopy," J. Electron Microscopy, in press.

[3] K.T.Rim, D. Eom, G.W. Flynn, R. Si, M. Flytzani-Stephanopoulos et al., J. Phys. Chem.C, in press.

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