(603b) Effects of Coverage Dependent Adsorbate-Adsorbate Interactions for CO Methanation On Transition Metal Surfaces

Authors: 
Medford, A., SLAC National Accelerator Laboratory
Lausche, A. C., University of Michigan
Norskov, J. K., SUNCAT Center for Interface Science and Catalysis, Stanford University and SLAC National Accelerator Laboratory
Studt, F., SLAC National Accelerator Laboratory



Recent
advances in electronic structure theory have enabled researchers to understand
the kinetics of surface reactions on the molecular level. Using
microkinetic-modeling techniques, trends in the reaction rates and
selectivities of heterogeneous catalysts can be predicted for a variety of
chemical reactions.

These
strategies, however, are based on a series of assumptions that may not be
appropriate under all reaction conditions. The mean field model, which
describes reaction kinetics with low coverages of reaction intermediates, may
represent one such example. It has been reported previously that the
dissociation of strongly bonded molecules (such as N2, O2,
and CO) typically takes place under conditions where the surface coverages of
intermediates are relatively high. In such cases, the trends in predicted rates
and selectivities may be influenced by interactions between surface adsorbed
reaction intermediates. Research described in this paper investigated the
effect of these interactions on the adsorption energies (and hence reaction
rates) of transition metal surfaces for the CO methanation reaction.

The
DFT-calculated adsorption and transition state energies of the relevant
intermediates were calculated for the 211 surfaces of Ag, Cu, Pd, Pt, and Rh. As
described in the literature, the binding energies for these intermediates on a
given surface can be scaled with the binding energies of carbon and oxygen.
Likewise, the energies of a given transition state species can be scaled with
the energies of the reaction products. By combining these scaling relations
with a microkinetic modeling technique, we calculated the reaction turnover
frequencies for CO methanation (both with and without interaction effects) for
a range of carbon and oxygen binding energies (Figure 1).

In
general, trends in the predicted methanation rates were unaltered by the
inclusion of adsorbate-adsorbate interactions. The highest methanation rates
were predicted for Ru, Co, and Ni, which is consistent with earlier
experimental and theoretical studies. Inclusion of adsorbate interactions
resulted in an increase in the predicted rates for materials with the strongest
binding energies of C and/or O (e.g., Fe and Re). These increases were
primarily due to an increase in the concentration of active sites via a
reduction in surface poisoning due to strongly bound intermediates.

In
general, the results of this analysis support the use of the mean field model
for determining qualitative trends in simple reactions like CO methanation.
Nevertheless, the inclusion of interaction effects is suggested to be
increasingly important in cases where reaction selectivities play an important
role.

Figure
1

- Theoretical turnover frequencies as a function of the carbon (ÆEC)
and oxygen (ÆEO) binding energies
with and without interaction
effects (T = 523 K, P = 1 bar, 1% CO in H2). Carbon and oxygen
binding energies for the (211) surfaces of selected transition metals are
depicted.