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

Paper SS1-TuM4
Controlling the Catalytic Reactivity and Selectivity of Ni Surfaces by Step Blocking

Tuesday, November 16, 2004, 9:20 am, Room 210B

Session: Catalytic Reactions: The Role of Surface Steps and Structure
Presenter: R.T. Vang, University of Aarhus, Denmark
Authors: R.T. Vang, University of Aarhus, Denmark
K. Honkala, Technical University of Denmark
S. Dahl, Haldor Topsøe A/S, Denmark
E.K. Vestergaard, University of Aarhus, Denmark
J. Schnadt, University of Aarhus, Denmark
E. Lægsgaard, University of Aarhus, Denmark
B.S. Clausen, Haldor Topsøe A/S, Denmark
J.K. Nørskov, Technical University of Denmark
F. Besenbacher, University of Aarhus, Denmark
Correspondent: Click to Email
Step or defect sites have been shown to dominate the reactivity of catalytic surfaces for the dissociation of a number of diatomic molecules, but so far no studies have addressed the influence of steps in the decomposition of more complex molecules. In most catalytic processes involving hydrocarbon molecules, selectivity (between, e. g., C-H and C-C bond breaking) is crucial for the overall efficiency of the catalyst. In this study we have used STM and DFT calculations to investigate the dissociative adsorption of a simple molecule, CO, as well as a more complex molecule, ethylene (C@sub 2@H@sub 4@), on Ni(111). The STM studies reveal that both molecules decompose at the step edges at a much higher rate than on the (111) facets. These observations are supported by DFT calculations showing a much lower activation barrier for decomposition at a step site compared to a terrace site. Furthermore, the steps are shown to have a crucial influence on the selectivity of the Ni(111) surface towards ethylene decomposition, in the sense that the step effect is much more pronounced for C-C bond breaking than for C-H bond breaking. We also demonstrate how we can control the number of active step sites and thus the reactivity by depositing small amounts of Ag, which from STM studies are shown to block all the step sites on the Ni(111) surface. Finally we exploit this new principle of step control by synthesizing a new high-surface area supported NiAg alloy catalyst. We show in flow reactor tests that the NiAg catalyst has a much lower activity for ethane hydrogenolysis than a similar Ni catalyst, thus confirming that we can block C-C bond breaking by step control.