No abstract

No 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.

Work functions of Pd-based bimetallic surfaces, including mainly M/Pd(111), Pd/M, and Pd/M/Pd(111) (M = 4d transition metals, Cu, Au, and Pt), are studied using density functional theory. We find that the work function of these bimetallic surfaces is significantly different from that of parent metals. Careful analysis based on Bader charges and electron density difference indicates that the variation of the work function in bimetallic surfaces can be mainly attributed to two factors: (1) charge transfer between the two different metals as a result of their different intrinsic electronegativity, and (2) the charge redistribution induced by chemical bonding between the top two layers. The first factor can be related to the contact potential, i.e., the work function difference between two metals in direct contact, and the second factor can be well characterized by the change in the charge spilling out into vacuum. We also find that the variation in the work functions of Pd/M/Pd(111) surfaces correlates very well with the variation of the d-band center of the surface Pd atom. The findings in this work can be used to provide general guidelines to design new bimetallic surfaces with desired electronic properties.