(344c) Comparison of Multi-Adsorbate Interaction Models for NO Catalytic Oxidation

Nitrogen
oxides (NO_x) are pollutants generated during combustion in air.
Catalytic oxidation of NO to NO₂ is a critical component of all NO_x
remediation strategies. NO oxidation is catalyzed by supported Pt, and
experiments reveal that turnover rates are highest on Pt single crystals [1].
Under catalytic conditions, the Pt surface is covered with O and NO. The
binding energies of these species are coverage-dependent, and molecular models
that properly capture this coverage dependence are necessary to capture
experimentally observed rates, rate orders, and apparent activation energies.

Our
group has used a cluster expansion (CE) approach to describe coverage-dependent
O adsorption on Pt (111) [2] and, combined with Monte Carlo simulations and the
observed Brønsted-Evans-Polanyi relationship between adsorption energy and
surface reaction barriers, developed NO oxidation kinetic models that recover
most observed behavior [3]. One of the basic assumptions of the models used so
far was that NO coverage is negligible and oxygen is the only species that
adsorbs on the catalyst surface. In this work, we use a density functional
theory (DFT) based cluster expansion that incorporates co-adsorption of O and
NO on Pt (111) to obtain kinetic parameters for NO oxidation conditions using
Grand Canonical Monte Carlo (GCMC) simulations. We compare results to an
oxygen-only model and to mean-field models that incorporate the coverage
dependence approximately.

We
find that the complexity of the model needed to simulate NO oxidation on Pt
(111) depends on the temperature and the concentration of NO in the reaction
system. At temperatures below ~ 523 K, we observe a presence of NO on the
catalyst surface that affects the obtained kinetic parameters and therefore an
oxygen only model does not capture this feature. For temperatures beyond 573 K,
both, oxygen only and the NO-O dual adsorption model produce similar results.

References

1.     
A. D. Smeltz, W. N. Delgass, F. H.
Ribeiro, Langmuir 26 (21), (2010) 16578-16588.

2.     
D. J. Schmidt, W. Chen, C.
Wolverton, W. F. Schneider, J. Chem. Theory Comput. 8, (2012) 264-273.

3.     
C. Wu, D. J. Schmidt, C.
Wolverton, W. F. Schneider, J. Catal. 286, (2012) 88-94.