Site Heterogeneity and Broad Surface-Binding Isotherms: A Cornerstone of Modern Catalysis

Learning the science of heterogeneous catalysis and electrocatalysis always starts with the simple case of a flat uniform surface. It has of course been recognized for a century that real catalysts are more complicated. Still, this idealized model is often used as the starting point for current models of catalysis, with non-ideal adsorption introduced later as a correction. This Perspective argues that surface heterogeneity and a range of substrate-surface binding energies are a central feature of the increasingly complex catalysts of the 21st century that should be part of the foundation of catalysis models. Modern catalysis employs nanoscale materials whose surfaces have substantial step, edge, corner, impurity, and other defect sites. Catalysts increasingly have both metallic and non-metallic elements MnXm, including metal oxides, chalcogenides, pnictides, carbides, doped carbons, etc. The surfaces of such catalysts are typically not simple crystal facets of the bulk phase underneath, they are often non-stoichiometric, amorphous, dynamic, and impure. Little is known about the different types of active and inactive sites, the energies of substrate absorption, and the reactivities of surface intermediates. Examples of broad, non-Langmuirian isotherms for H binding to surfaces are presented here, mostly involving solid/solution interfaces. The presence of non-ideal adsorption isotherms challenges the underlying assumptions of linear free energy relationships (LFERs) including Brønsted/Bell-Evans-Polanyi (BEP) relations, volcano plots, Tafel slopes, and the Butler-Volmer equation. This Perspective encourages the community to measure and compute binding energies and reaction kinetics for complex nanoscale materials, and to include the resulting intuition in even first-order models of catalysis. The diversity of surface sites and binding energies should not be viewed as a complexity to be minimized but rather as an inherent strength of these catalysts. The diversity makes many catalysts inherently a high-throughput screen wrapped in a nanoscale package. The challenge is how to understand and harness the diversity, to identify and control the catalysts and catalytic conditions to optimize the desired catalytic pathway.