(561c) 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).