The descriptor-based approach in heterogeneous catalysis simplifies the daunting task of studying catalysts’ performance for complex reaction networks by employing scaling relations based on reactivity descriptors derived from density functional theory (DFT) simulations.1 Within this approach, catalysts can be evaluated qualitatively through a simple descriptor, such as an adsorption energy or d-band center, that is associated with an active site. Although the resulting volcano curve makes it possible to study activity trends and identify promising materials, it imposes a maximum achievable activity approximated by only a few candidates.
In order to widen the candidate pool for various catalytic processes and possibly even overcome the theoretical limit of monofunctional catalysts, we extend the descriptor-based approach to materials that consist of two or more distinct sites that catalyze different reaction steps independently; such materials are often referred to as “bifunctional catalysts”. Using DFT and microkinetic modeling, we aim to understand the interplay of active sites on bifunctional catalyst surfaces by using CO oxidation as probe reaction. Although a similar approach was taken earlier by Savchenko and colleagues, interfacial interactions among active sites have been limited to spillover effects only.2 We present here a more general method to evaluate bifunctionality, where all possible mechanisms are considered.
Our results indicate that there are theoretical limits for the achievable activity improvement, which depend on the slope of the underlying scaling relation. For steep scaling lines activity improvements through bifunctional mechanisms are feasible when the diffusion barrier between active sites is within 0.2 to 0.5 eV. When CO oxidation was allowed to occur over certain bifunctional catalysts consisting of step and terrace sites, oxygen was found to activate on a step and diffuse to a terrace site where it reacts with CO. In addition, interfacial reactions on bimetallic systems were shown to follow the same Brønsted-Evans-Polanyi relations obtained for single metallic sites, as well as provide a significant contribution to the overall bifunctional activity. However, bifunctional catalysts do not necessarily outperform monofunctional systems. More specifically, for CO oxidation we found that the overall activity is not significantly altered when bifunctional catalysts are considered, but equally active bifunctional catalysts may be tailored from less active and cheaper components.
(1) Nørskov, J. K.; Bligaard, T.; Rossmeisl, J.; Christensen, C. H. Nat. Chem. 2009, 1, 37–46.
(2) Savchenko, V.; Dadayan, K. React. Kinet. Catal. Lett. 1995, 55, 33–40.