Catalysis often involves a Bronsted-Evans-Polanyi (BEP) relationship between reaction energy and the activation barrier. This correlation between thermodynamics and kinetics, coupled with Sabatier’s Principle, leads to volcano relationships. A single site catalyst, therefore, needs to optimally bind the reactive intermediates to maximize the overall rate of reaction. A descriptor-based approach can be used to screen catalysts for optimal binding. The use of BEP and scaling relations further simplifies and speeds up the computational screening process.<br/>Here, we analyze, through microkinetic models, the case for catalytic cascades comprising two different sites. A cascade, in this paradigm, can be an electrochemical cell, supported catalysts or a two-site bifunctional catalyst. Such a cascade involves activation on one type of site, followed by diffusion of an intermediate to react on a second site. Through microkinetic modeling, we show that, for two sites following the same BEP relationship, the cascade cannot exceed the rate of the optimal single site catalyst. A simple graphical argument is used to explain this result. Using the microkinetic model, we discuss the origins and implications of the limitation of dual site cascades. We discuss the underlying assumptions in the microkinetic model and their impact on our analysis. We explore scenarios where these assumptions may break down and can lead to potential applications of heterogeneous catalytic cascades. Furthermore, by using different reaction chemistries, we discuss possible ways to improve activity and selectivity on different multi-component systems. Examples include increasing activity by tuning electronic metal-support interactions and selective catalysis using surface modifiers like thiol self-assembled monolayers (SAMs).<br/> <br/>

Kinetic modeling of dual-site catalysts: Limitations and prospects

254th National Meeting (2017)

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