AVS 51st International Symposium
    Surface Science Monday Sessions
       Session SS1-MoA

Paper SS1-MoA5
Towards an Understanding of the Silver Catalysed Ethylene Epoxidation Process

Monday, November 15, 2004, 3:20 pm, Room 210B

Session: Simulation and Theory of Adsorption
Presenter: A. Michaelides, University of Cambridge, United Kingdom
Authors: A. Michaelides, University of Cambridge, United Kingdom
K. Reuter, Fritz-Haber-Institut der Max-Planck Gesellschaft, Germany
M. Scheffler, Fritz-Haber-Institut der Max-Planck Gesellschaft, Germany
Correspondent: Click to Email
A number of recent studies indicate that, under moderate pressures of oxygen, some transition metal catalysts are covered in thin oxide overlayers. For oxidation catalysis, it has been suggested that such "surface-oxide" layers are catalytically active, and that this role is not performed by the pure metal surfaces as was traditionally assumed. This contemporary picture can be traced back to Ag catalysis, where it has been believed for 30 years that exposure of oxygen to Ag{111} leads to the formation of an ultra-thin surface-oxide. Extensive experimental and theoretical work has been carried out for oxygen on Ag, motivated mainly by the desire to understand silver's unique ability as a partial oxidation catalyst for ethylene [see, for eg., refs. 1-4]. However, density functional theory results, presented here, augmented with thermodynamic calculations, indicate that previous conclusions are significantly incomplete and that the structure of this original surface-oxide must be reconsidered. Indeed novel oxide overlayers are identified, and, predicted to be stable under the oxygen pressures at which the industrial ethylene epoxidation reaction is carried out. Moreover, we find that under these conditions finite variations in the stoichiometry of the surface oxides can occur with practically no change in free energy. It is suggested that this is one of the essential hallmarks of an effective red-ox catalyst. The first phase diagrams of Ag in contact with gaseous environments of both oxygen and ethylene are also be presented, as are reaction mechanisms for the formation of ethylene-epoxide. @FootnoteText@ @footnote 1@C.T. Campbell, J. Catal. 94, 436 (1985). @footnote 2@C.I. Carlisle, M.-L. Bocquet, J. Cerda, D.A. King, P. Sautet, Phys. Rev. Lett. 84, 3899 (2000). @footnote 3@A. Michaelides, M.-L. Bocquet, P. Sautet, A. Alavi, D.A. King. Chem. Phys. Lett. 367, 344 (2002). @footnote 4@W.X. Li, C. Stampfl, M. Scheffler, Phys. Rev. B 68, 165412 (2003).