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Role of the nanoscale in catalytic CO oxidation by supported Au and Pt nanostructures

Sergey N. Rashkeev1,2, Andrew R. Lupini1, Steven H. Overbury3,
Stephen J. Pennycook1,2, and Sokrates T. Pantelides2,1

Phys. Rev. B, 76, 035438 (2007)

Although Au is inactive in bulk form, at the nanoscale it becomes one of the most active catalysts known, catalyzing the oxidation of CO to CO2 even below room temperature. The cause of the activity of nanosized Au has been a mystery for many years, with several explanations proposed. Images of the catalyst prepared by deposition/precipitation onto nanocrystalline anatase show many individual Au atoms in the as-deposited state, but after reduction to the active form, the atoms are much less visible. It is known that the melting point of Au nanoparticles reduces with diameter, and reaches room temperature at approximately the size range observed. Therefore it seems likely that the smallest nanoparticles may be in a liquid-like state. Most of the reduced nanoparticles are 1-2 nm in diameter, with a high fraction just 1 or 2 monolayers thick.

First-principles calculations revealed the key feature for the activity of the nanoparticles relative to bulk Au is the presence of low-coordination sites. With reducing coordination number, the energy required to desorb O2 exceeds the activation barrier for the reaction for a coordination number less than five. This means that an O2 molecule can reside on the surface long enough to react with an adsorbed CO molecule. At sites with coordination number six or greater, desorption will occur on average before reaction proceeds, and the surface of the Au nanoparticle will become inactive for the reaction. In this situation, only perimeter sites can participate in the reaction. This is the underlying reason for the vast change in catalytic behavior for nanoscale Au. In the case of Pt nanoclusters, the increased binding energy also occurs with reducing size, but in this case it quenches the reaction because the molecules are pinned too strongly to the surface. Part of the reason for the low activation barrier on the Au nanoparticle is the “looseness” of the Au-Au bonds, which allows the molecules to move during the reaction and find a configuration that minimizes the activation barrier.


Z-contrast images of Au clusters on anatase (left) after deposition, with well resolved atoms, and (right) after activation, with “loose” gold bonds resulting in blurred images.
   
 

1Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA
2Department of Physics and Astronomy, Vanderbilt University, Nashville, TN 37235, USA
3Chemical Sciences Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA

   
 

 Oak Ridge National Laboratory