(720a) Low Temperature CO Oxidation with a Single Site Pt Catalyst Supported on the “29” Cuxo/Cu(111) Surface: Elucidating the Chemical Nature of the Pt Active Sites

Authors: 
Hensley, A., Washington State University
Therrien, A., Tufts University
Zhang, R., Washington State University
Sykes, E. C. H., Tufts University
McEwen, J. S., Washington State University
The ability to create highly dispersed, single site noble metal catalysts is of significant interest as such catalysts would be highly atom efficient and have the potential to significantly reduce the cost of catalysts for key reactions, such as low temperature CO oxidation and water-gas shift. While the presence and catalytic activity of various noble metal single site catalysts has been reported,[1-5] there is still disagreement over whether atomically dispersed metal atoms are indeed catalytically active or if the observed catalytic activity of single site catalysts is due to metal nanoparticles.[6, 7] Such disagreements create a crucial need for the development of well-defined single site supported catalysts with an accompanying experimentally accurate theoretical model in order to correctly determine the chemical nature of the catalytically active sites. To this end, we have studied low temperature CO oxidation on Pt single site catalysts supported on the â??29â? CuxO/Cu(111) surface. By first thoroughly characterizing the â??29â? CuxO/Cu(111) surface, we were able to determine the chemical nature of the Pt sites active for CO oxidation.

 In order to have a truly accurate model how and where the single Pt atoms bind to the â??29â? CuxO/Cu(111) surface, we must have an accurate model of the â??29â? CuxO/Cu(111) structure itself. This was accomplished by characterizing both the clean â??29â? CuxO/Cu(111) surface[8] and the adsorption of CO on the â??29â? CuxO/Cu(111). First, for the characterization of the clean â??29â? CuxO/Cu(111) surface, the comparison of experimental and theoretical and STM images showed that the CuxO layer was formed from 6 fused hexagonal rings, each with 6 O and 6 Cu atoms. Furthermore, we determined that there are 5 O adatoms present on the Cu(111) surface in the center of the CuxO rings. By developing a phase diagram for various CuxO species under the conditions used to create the â??29â? CuxO/Cu(111) surface, we showed this model for the â??29â? CuxO/Cu(111) surface was the most stable structure under the experimental conditions. Second, the â??29â? CuxO/Cu(111) surface structure was further verified through the adsorption of CO on said surface. Depending on the amount of CO exposed to the â??29â? CuxO/Cu(111) surface, 6 different CO ordered structures were observed experimentally to form. Through an extensive comparison with theoretical STM images, the nature of the ordered structures was elucidated. These comparisons confirm the structure of the â??29â? CuxO/Cu(111) surface.

With the exact structure known for the â??29â? CuxO/Cu(111) surface, we can then accurately characterize and model the binding of the catalytically active Pt sites to the oxide surface. Experimental STM images show that the Pt exists as single atoms on the â??29â? CuxO/Cu(111) surface. An electronic analysis of the Pt atoms on the â??29â? CuxO/Cu(111) surface, coupled with experimental x-ray photoelectron spectroscopy (XPS), show that these Pt atoms exist in the metallic Pt state. Furthermore, depending on the temperature used when Pt is added to the â??29â? CuxO/Cu(111) surface, experimental STM images show that there are two distinct Pt sites: an active CO oxidation site with Pt dosed below room temperature and an inactive CO oxidation site with Pt dosed at room temperature. By modeling the adsorption of CO on the single Pt sites on the â??29â? CuxO/Cu(111) surface, we determined that the binding strength of CO on the Pt single atoms is inversely proportional to the binding strength of Pt on the â??29â? CuxO/Cu(111) surface (i.e. CO is more weakly bound on Pt sites where the oxide binds the Pt atoms strongly). These results suggest that the catalytically active Pt single sites might be meta-stable, diffusing to more stable, yet inactive, sites as the temperature is raised. Overall, our careful elucidation of the chemical nature of the Pt single atom sites on the â??29â? CuxO/Cu(111) surface via both theoretical and experimental techniques can assist in the design and optimization of atom efficient Pt catalysts.

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