468785 Unraveling the Nature of Boundary Sites of Metal-on-Oxide Catalysts

Friday, November 18, 2016: 1:30 PM
Imperial B (Hilton San Francisco Union Square)
Prateek Mehta1, Jeffrey P. Greeley2, W. Nicholas Delgass2 and William F. Schneider1, (1)Chemical and Biomolecular Engineering, University of Notre Dame, Notre Dame, IN, (2)School of Chemical Engineering, Purdue University, West Lafayette, IN

The rise of density functional theory (DFT) over the past few decades has enabled great progress in rational catalyst design, especially for extended surfaces of transition metals [1]. However, insights from such metal-only models do not always translate to real catalysts, which usually consist of metal nanoparticles dispersed on high surface area supporting phases. In this contribution, we examine how the inclusion of a support phase can lead to deviations in reactivity compared to a metal-only model. Specifically, we investigate the adsorption behavior of simple adsorbates at the boundary of several late transition metals supported on MgO(100). We model the system as quasi one dimensional metal nanowires on the oxide, similar to the model proposed by Molina et al [2], shown in Fig. 1. Our results show that the role of the oxide is highly variable and strongly depends on the choice of metal and adsorbate. For some metal-adsorbate combinations, the presence of the support results in enhanced adsorption at the perimeter sites compared to the unsupported metal nanowire. In other cases, adsorption is destabilized when the metal is supported by the MgO. We have traced the origins of these stabilization/destabilization effects to a combination of multiple structural and electronic perturbations, including steric interactions, a shift in the metal d-band center, and redox energy gained by charge transfer facilitated by the oxide. Moreover, we show that in many cases, while the oxide has a destabilizing effect on the adsorption of an atomic adsorbate, A, it has a stabilizing effect on the adsorption of its hydrogenated form, AHx. We use these results to discuss possible implications for scaling relationships developed on metal-only models.

[1] J. K. Norskov, Abild-Pedersen, F., Studt, F., & Bligaard, T. (2011). PNAS, 108(3), 937-943.

[2] Molina, L., & Hammer, B. (2003). Phys. Rev. Lett., 90(20), 206102.

Figure 1.  Structural model of a Pd nanowire supported on MgO(100). An oxygen atom adsorbed at the metal-oxide boundary is also shown.

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