AVS 51st International Symposium
    Surface Science Tuesday Sessions
       Session SS1-TuM

Paper SS1-TuM5
High Pressure Scanning Tunneling Microscopy Study on CO Poisoning of Ethylene Hydrogenation on Metal Single Crystals

Tuesday, November 16, 2004, 9:40 am, Room 210B

Session: Catalytic Reactions: The Role of Surface Steps and Structure
Presenter: D.C. Tang, Lawrence Berkeley National Laboratory
Authors: G.A. Somorjai, University of California, Berkeley
D.C. Tang, Lawrence Berkeley National Laboratory
M. Salmeron, Lawrence Berkeley National Laboratory
K.S. Hwang, University of California, Berkeley
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The nature of catalyst poisoning under reaction conditions is an important question in catalysis science@footnote 1@@super ,@@footnote 2@. During a reaction the catalyst surface is covered with a dense layer of adsorbates in equilibrium with the gas phase. This layer consists of reactants, inactive spectator molecules, and active poisoning adsorbates@footnote 3@. For the reaction to occur the surface should remain flexible, atomic rearrangements should be possible, and adsorbate mobility should be high enough so that favorable adsorption sites can be accessed. Using a high pressure scanning tunneling microscope we monitored the co-adsorption of hydrogen, ethylene and carbon dioxide on rhodium (111) and platinum(111) crystal faces in the mTorr pressure range at 300 K in equilibrium with the gas phase. We show that poisoning occurs when surface mobility is suppressed and the adsorbate species become locked into static ordered structures. During the catalytic hydrogenation of ethylene to ethane in the absence of CO the metal surfaces are covered by an adsorbate layer that is very mobile on the time scale of STM imaging. We found that the addition of CO poisons the hydrogenation reaction and induces ordered structures on the single crystal surfaces. Several ordered structures were observed upon CO addition to the surfaces pre-covered with hydrogen and ethylene: domains of c(4 x 2)-CO+C@sub 2@H@sub 3@, previously unobserved (4 x 2)-CO+3C@sub 2@H@sub 3@, and (2 x 2)-3CO on Rh(111), and a (@sr@19 x @sr@19)R23.4° on Pt(111). A mechanism for CO poisoning of ethylene hydrogenation on the metal single crystals was proposed, in which CO blocks surface metal sites and reduces adsorbate mobility to limit adsorption and reaction rate of ethylene and hydrogen. @FootnoteText@ @footnote 1@ Bartholomew, C. H. App. Catal. A 212, 17-60 (2001).@footnote 2@ Kumbilieva, K., Kiperman, S. L. & Petrov, L. Kinet. Catal. 36, 73-79 (1995).@footnote 3@ Zaera, F. Prog. Surf. Sci. 69, 1-98 (2001).