Elucidating and Predicting the Chemisorption Properties On Catalytic Alloy Surfaces. Applications to PdAu Alloys

It is well-established that alloying can lead to significant improvements in the activity as well as the selectivity for various catalytic systems than either metal alone [1].  The effects of alloying can systematically be broken down into ensemble, ligand, and lattice effects.  Ensemble effects describe interactions between an adsorbate and metal atoms of the ensemble that it is directly bound to – as the composition of the ensemble changes, so does the binding energy.  Ligand effects describe the influence of metal atoms neighboring the adsorption site and are weaker than ensemble effects.  Lattice effects describe the change in binding energies due to strain induced by the expansion or contraction of the underlying lattice as its composition changes.  Although much work, both experimental [2] and computational [3], has been performed relating to alloy surfaces, there is still a lack of understanding of the atomic-level fundamentals of how specific alloy surfaces and ensembles interact with specific adsorbates.

We have used density functional theory (DFT) calculations carried out using the Vienna ab-initio Simulation Package [4] to follow the changes in binding for simple adsorbates containing hydrogen, carbon, nitrogen, and oxygen for changes in the Pd/Au alloy compositions and configurations.  The adsorbates include C, CH, CH₂, CH₃, CH₃C, CH₃CH, CH₃CH₂, CN, CO, H, N, NH, NH₂, NO, O, and OH which demonstrate different characteristic modes and sites for adsorption.   The atop, bridge, fcc-hollow, and hcp-hollow adsorption sites were considered for all of the adsorbates considered.  Ensemble and ligand effects were studied by replacing one or more Pd atoms in the top two layers of the slab with Au.  Calculations were first carried out on a periodic four layer slab with the metal atoms frozen to their positions in the optimized Pd(111) slab and the adsorbates allowed to fully relax.  A second set of calculations was then carried out where the top two layers of the slab also allowed to relax.  This was done to deconvolute the electronic alloy effects from the geometric alloy effects.

Ensemble effects were found to be the largest of the three alloy effects, with the binding energy becoming weaker by 0.25 – 1.40 eV for each Au atom incorporated into the ensemble.  The magnitude of the ensemble effect was found to be greater for binding sites with higher coordination numbers and decreases as substituents are added to the adatom of the adsorbate.  Surface ligand effects were significantly weaker, with the binding energy weakening by less than 0.28 eV for each Au atom substituted into the surface layer.  Subsurface ligand effects were weaker still, with the binding energy weakening by less than 0.09 eV for each Au atom substituted into the subsurface layer.

It was found that in the absence of surface relaxation, the ensemble and ligand effects are very well approximated by a pair-wise model.  The model takes the form

The first sum is over all Au atoms in the ensemble (those metal atoms that are directly bound to the adsorbate) and accounts for the ensemble effects.  In the second sum, the index i runs over all metal atoms in the ensemble and the index j runs over all Au ligands that are nearest neighbors of atom i – this term accounts for the ligand effects.  The maximum deviation of this model from DFT results was 0.04 eV.

In order to understand the electronic effects underlying these alloy effects, we have performed an analysis using the Quasiatomic Orbital method of Qian and coworkers [5].  This method reduces the full plane wave calculation to a tight binding representation by determining a minimal set of quasiatomic orbitals that closely correspond to the atomic orbitals.  From this analysis, we are able to obtain tight binding quantities such as orbital occupancy, bond order, and hopping and overlap integrals.  We are then able to elucidate a theoretical framework describing the electronic mechanism behind the alloy effects.

An understanding of how the composition and the specific atomic configuration of specific surface alloys and ensembles influence chemisorption properties and reactivity will provide important insights as to how alloys influence catalysis and how they may be designed to enhance catalytic performance. This study provides fundamental understanding to these effects and suggests an accurate coarse-grained chemical model that can be employed in ab initio based  kinetic Monte Carlo simulations of catalytic performance.

1.      R.A. van Santen, M. Neurock, "Molecular Heterogeneous Catalysis", Wiley, Weinheim (2006).

2.      M. Chen, D. Kumar, C.-W. Yi, D.W. Goodman, Science 310 291 (2005).

3.      V. Pallassana, M. Neurock, J. Catal. 191 301 (2000).

4.      G. Kresse, Phys. Rev. B 54 11169 (1996).

5.      X. Qian, J. Li, L. Qi, C.-Z. Wang, T.-L. Chan, Y.-X. Yao, K.-M. Ho, S. Yip, Phys. Rev. B 78 245112 (2008).