Understanding the Effect of Steps, Strain, Poisons, and Alloying: Methane Activation on Ni Surfaces
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Catalysis Letters Vol. 105, Nos. 1–2, November 2005 ( 2005) DOI: 10.1007/s10562-005-7998-9
Understanding the effect of steps, strain, poisons, and alloying: Methane activation on Ni surfaces Frank Abild-Pedersen, Jeff Greeley, and Jens K. Nørskov* Center for Atomic-scale Materials Physics, Department of Physics, Technical University of Denmark, DK-2800, Kongens Lyngby, Denmark
Received 10 May 2005; accepted 8 August 2005
It is shown that a single parameter characterizing the electronic structure of a transition metal surface, the d-band center (ed), can be used to provide a unified description of a range of phenomena in heterogeneous catalysis. Using methane activation on Ni surfaces as an example, we show that variations in ed can be used to quantitatively describe variations in the activation energy when the surface structure is changed, when the coverage of carbon is changed, when the surface is strained, when the surface is alloyed, and when the surface is poisoned by sulfur. The d-band center is, therefore, a very general descriptor of the reactivity of a surface. KEY WORDS: Density Functional Theory; steps; strain; defects; promoters; surface alloys; d-band center; methane; nickel.
1. Introduction
2. Calculational methods
One of the principal goals of surface science has long been to develop a fundamental understanding of the reactivity of transition metal surfaces [1–4]. The realization of such an understanding could potentially provide concepts for the interpretation of kinetic data on heterogeneous catalysts. Unfortunately, given the immense complexity of catalytic systems, these types of principles have proven extremely difficult to develop. The d-band model of Hammer and Nørskov [5,6] has recently been shown to have significant explanatory power for the analysis of adsorbate binding energies and transition state energies on a variety of welldefined metal surfaces. One of the fundamental descriptors of the theory, the surface d-band center (ed), correlates well with these reactivity parameters for a variety of systems, including pure metals and surface alloys with homogeneous overlayers [7–13], alloys with heterogeneous surface layers [14,15], distorted surfaces (with, e.g., steps and strain) [16,17], and surfaces with poisons and promoters [18]. However, to our knowledge, no single study that simultaneously correlates a significant number of these effects has been undertaken. In the present work, we report on the results of just such a study. We show that, when it is properly defined, ed can be used to quantitatively predict trends in methane activation energies on a variety of surfaces with different geometries, strain levels, promoters, poisons, and alloy compositions. Thus, we demonstrate that ed has a more general descriptive power than was previously known.
Using the Dacapo total energy code [19], we perform periodic, self-consistent, Density Functional Theory (DFT) calculations of the dissociative adsorption of methane on pure and modified nickel surfaces. On the (111) surfaces, we use a 3-layer, (22
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