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Learning the science of heterogeneous catalysis and electrocatalysis always starts with the simple case of a flat, uniform sur-face with an ideal adsorbate. It has of course been recognized for a century that real catalysts are more complicated. For the increasingly complex catalysts of the 21st century, this Perspective argues that surface heterogeneity and non-ideal binding isotherms are central features, and their implications need to be incorporated in current thinking. A variety of systems are described herein where catalyst complexity leads to broad, non-Langmuirian surface isotherms for the binding of hydrogen atoms – and this occurs even for ideal, flat Pt(111) surfaces. Modern catalysis employs nanoscale materials whose surfaces have substantial step, edge, corner, impurity, and other defect sites, and they increasingly have both metallic and non-metallic elements MnXm, including metal oxides, chalcogenides, pnictides, carbides, doped carbons, etc. The surfaces of such catalysts are often not crystal facets of the bulk phase underneath, and they typically have a variety of potential active sites. Catalytic surfaces in operando are often non-stoichiometric, amorphous, dynamic, and impure, and often vary from one part of the surface to another. Understanding of the issues that arise at such nanoscale, multi-element catalysts is just beginning to emerge. Yet these catalysts are widely discussed using Brønsted/Bell-Evans-Polanyi (BEP) relations, volcano plots, Tafel slopes, the Butler-Volmer equation, and other linear free energy relations (LFERs), which all depend on the implicit assump-tion that the active sites are all “similar” and that surface adsorption is close to ideal. These assumptions underly the ubiqui-tous intuition based on the Sabatier Principle, that the fastest catalysis will occur when key intermediates have free energies of adsorption that are not too strong nor too weak. Current catalysis research often aims to minimize the complexity of non-ideal isotherms through experimental and computational design (e.g., the use of single crystal surfaces), and these studies are the foundation of the field. In contrast, this Perspective argues that the heterogeneity of binding sites and binding energies is an inherent strength of these catalysts. This diversity makes many nanoscale catalysts inherently a high-throughput screen wrapped in a tiny package. Only by making the heterogeneity part of the foundation of catalysis models, sorting the types of active sites and dissecting non-ideal binding isotherms, will modern catalysis learn to harness the inherent diversity of real catalysts. Controlling rather than avoiding diversity is needed to optimize complex modern catalysts and catalytic condi-tions. 

The mechanism of proton-coupled electron transfer at the surface of titanium-substituted polyoxovanadate-alkoxide clusters can be tuned by judicious selection of substrate. 

Abstract. Rate-driving force relationships, known as Brønsted-Evans Polanyi (BEP) relations, are central to many methods for predicting the performance of heterogeneous catalysts and electrocatalysts. Methods such as Tafel plots and ‘volcano’ analyses often assume the effect of adsorbate coverage on reaction rate across different materials is constant and known. Here we use UV-visible spectroscopy to test these assumptions, by measuring rates of net hydrogen atom transfer from colloidal cerium oxide nanoparticles (nanoceria) to organic reagents at varying surface CeO–H bond strengths and surface coverages. The resulting rate constants follow a linear BEP relationship, ∆log(k)=[alpha]∆log(Keq), across two sizes of nanoceria, two organic reagents, and a ~10 kcal mol-1 range of CeO–H bond strengths. Interestingly, the Brønsted slope is only 0.2, demonstrating that the rate constants are far less insensitive to CeO–H bond strength, than would commonly be assumed for a heterogeneous nanomaterial. Furthermore, we observe a Brønsted slope >1 when altering the reaction driving force via the organic reagent bond strength instead of that of CeO–H. The implications of these Brønsted slopes for either concerted or stepwise mechanisms are discussed. To our knowledge, these are the first solution-phase measurements of BEP relationships for hydrogen coverage on a (nano)material. 

Rhenium complexes with aliphatic PNP pincer ligands have been shown to be capable of reductive N 2 splitting to nitride complexes. However, the conversion of the resulting nitride to ammonia has not been observed. Here, the thermodynamics and mechanism of the hypothetical N–H bond forming steps are evaluated through the reverse reaction, conversion of ammonia to the nitride complex. Depending on the conditions, treatment of a rhenium( iii ) precursor with ammonia gives either a bis(amine) complex [(PNP)Re(NH 2 ) 2 Cl] + , or results in dehydrohalogenation to the rhenium( iii ) amido complex, (PNP)Re(NH 2 )Cl. The N–H hydrogen atoms in this amido complex can be abstracted by PCET reagents which implies that they are quite weak. Calorimetric measurements show that the average bond dissociation enthalpy of the two amido N–H bonds is 57 kcal mol −1 , while DFT computations indicate a substantially weaker N–H bond of the putative rhenium( iv )-imide intermediate (BDE = 38 kcal mol −1 ). Our analysis demonstrates that addition of the first H atom to the nitride complex is a thermochemical bottleneck for NH 3 generation.