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Abstract
We
propose and assess a criterion for the application of Brønsted–Evans–Polanyi (BEP)
relations for dissociation reactions at surfaces. A theory-to-theory
comparison with density functional theory calculations is presented
on different reactions, metal catalysts, and surface terminations.
In particular, the activation energies of CH, CO, and trans-COOH dissociation reactions
on (100), (110), (111), and (211) surfaces
of Ni, Cu, Rh, Pd, Ag, and Pt are considered. We show that both the
activation energy and the reaction energy can be decomposed into two
contributions that reflect the influence of reactant and products
in determining either the activation energy or the reaction energy.
We show that the applicability of the BEP relation implies that the
reaction energy and activation energy correlate to these two contributions
in the range of conditions to be described by the BEP relation. A
lack of correlation between these components for the activation energy
is related to a change in the character of the transition state (TS)
and this turns out to be incompatible with a BEP relation because
it results in a change of the slope of the BEP relation. Our analysis
reveals that these two contributions follow the same trends for the
activation energy and for the reaction energy when the path is not
characterized either by the formation of stable intermediates or by
the change of the binding mechanism of the reactant. As such, one
can assess whether a BEP relation can be applied or not for a set
of conditions only by means of thermochemical calculations and without
requiring the identification of the TS along the reaction pathway.
We provide evidence that this criterion can be successfully applied
for the preliminary discrimination of the applicability of the BEP
relations. For instance, on the one hand, our analysis provides evidence
that the two contributions are fully anticorrelated for the trans-COOH dissociation
reactions on different metals and
surfaces, thus revealing that the reaction is characterized by a change
in the TS character. In this situation, no BEP relation can be used
to describe the activation energy trend among the different metals
and surfaces in full agreement with our DFT calculations. On the other
hand, our criterion reveals that the TS character is not expected
to change for CH dissociation reactions both for the same facet, different
metals and for same metal, different facets, in good agreement with
the DFT calculations of the activation energy. The formation of multiple
stable intermediates along the reaction pathways and the change of
the binding mechanism of one of the reactants are demonstrated to
affect the validity of the criterion. As a whole, our findings make
possible an assessment of the applicability of the BEP relation and
paves the way toward its use for the exploration of complex reaction
networks for different metals and surfaces.
The activation energy to reaction is a key quantity that controls catalytic activity. Having used ab inito calculations to determine an extensive and broad ranging set of activation energies and enthalpy changes for surface-catalyzed reactions, we show that linear relationships exist between dissociation activation energies and enthalpy changes. Known in the literature as empirical Brønsted-Evans-Polanyi (BEP) relationships, we identify and discuss the physical origin of their presence in heterogeneous catalysis. The key implication is that merely from knowledge of adsorption energies the barriers to catalytic elementary reaction steps can be estimated.