(472d) Energetics at Metal-Support Interfaces: Effect on Water Gas Shift Intermediates

Energetic Interactions at the Metal-Support
Interface: Effects on Water Gas Shift Intermediates

Metal-support
interactions often have a large impact on the catalytic properties, either by
direct participation of the support, indirect modification of electronic or
structural properties, or through special sites at the interface. For example,
the WGS activity of Au supported on TiO₂ is twenty times higher than
Au supported on γ-Al₂O₃.
We have found that density functional theory (DFT) based models of unsupported Pd and Pt fail to capture the
experimentally observed WGS kinetics on γ-Al₂O₃,
indicating that even "inert" supports play an active part in the catalysis.
Identifying the different factors that control reactivity at the metal-oxide
interface is thus essential for rational catalyst design and optimization.
However, there is no general approach to modeling metal-oxide interfaces. Multiple
interface models have been proposed in the literature, but they are mostly
limited to specific cases, and very often the implications of the model choice
are not discussed.

We
report detailed investigations of WGS intermediates on models of supported
quasi-two-dimensional transition metal nanowires. These calculations have been
performed in a consistent fashion within the framework of DFT. We
systematically vary the type of metal (Au, Ag, Pd, Pt), the oxide (MgO(100), α-Al₂O₃(0001)), and the
number of layers in the nanowire. We have found that electronic interactions
between the metal and the support are limited to near the interface (see Fig
1(a)), and metal-like behavior is recovered at the top of the nanowire with as
few as 2-3 layers. One-layer surfaces show different behavior due to quantum
size effects. This is also reflected in binding behavior of WGS intermediates
– adsorption energies on metal sites remote from the interface are
similar to those on the unsupported metal, Sites at the interface have a modest
stabilizing influence on the adsorbates, shown for
the case of H in Fig. 1(b-c). On the oxide, we find that binding of molecular
intermediates (CO, H₂O) is unaffected by the presence of the metal.
However, a dramatic metal-dependent stabilization of redox-active intermediates
(H, OH and COOH) is seen, caused due to electron tunneling between the metal and
the adsorbate. This is closely coupled with large
structural relaxations of the oxide surface, more pronounced in the case of
α-Al₂O₃(0001) than MgO(100). This behavior of odd-electron adsorbates
offers opportunities for tuning catalytic properties based on the choice of
oxide and the choice of metal. Isolating different energetic contributions that
affect the binding of such adsorbates and their
catalytic implications is the focus of our study.

In
Fig. 2, we compare potential energy diagrams of H₂O dissociation near
the interface of the 3-ML Pd-Al₂O₃ system to that of the
isolated metal and oxide. We see that H₂O dissociation is rapid on
the oxide and slow on the Pd, and only near the interface
can H and OH be separated. This illustrates the "dual" nature of the interface,
i.e., the combined metal oxide system can be catalytically active, even though
the individual components are inactive.