Kai S. Exner

Controlling Selectivity in the Chlorine Evolution Reaction over RuO2-Based Catalysts†

In the industrially important Chlor-Alkali process, the chlorine evolution reaction (CER) over a ruthenium dioxide (RuO2) catalyst competes with the oxygen evolution reaction (OER). This selectivity issue is elucidated on the microscopic level with the single-crystalline model electrode RuO2(110) by employing density functional theory (DFT) calculations in combination with the concept of volcano plots. We demonstrate that one monolayer of TiO2(110) supported on RuO2(110) enhances the selectivity towards the CER by several orders of magnitudes, while preserving the high activity for the CER. This win-win situation is attributed to the different slopes of the volcano curves for the CER and OER.

Full Kinetics from First Principles of the Chlorine Evolution Reaction over a RuO2(110) Model Electrode

Current progress in modern electrocatalysis research is spurred by theory, frequently based on ab initio thermodynamics, where the stable reaction intermediates at the electrode surface are identified, while the actual energy barriers are ignored. This approach is popular in that a simple tool is available for searching for promising electrode materials. However, thermodynamics alone may be misleading to assess the catalytic activity of an electrochemical reaction as we exemplify with the chlorine evolution reaction (CER) over a RuO2 (110) model electrode. The full procedure is introduced, starting from the stable reaction intermediates, computing the energy barriers, and finally performing microkinetic simulations, all performed under the influence of the solvent and the electrode potential. Full kinetics from first-principles allows the rate-determining step in the CER to be identified and the experimentally observed change in the Tafel slope to be explained.

Do Nonclassical, Cyclically Delocalized 4N/5e Radical Anions and 4N/6e Dianions Exist?—One‐ and Two‐Electron Reduction of Proximate, Synperiplanar Bis‐Diazenes

Five and six electrons delocalized in the N,N,N,N plane are characteristic of the highly persistent 4N/5e radical anions (deep green) and σ-bis-homoaromatic 4N/6e dianions (red), respectively, which were generated by one- and two-electron reduction of rigid, proximate bis-diazenes (see reaction below). The longest wavelength UV/Vis absorptions of the radical anion and dianion are strongly dependent on the counterion.

Kinetics of Electrocatalytic Reactions from First-Principles: A Critical Comparison with the Ab Initio Thermodynamics Approach

Multielectron processes in electrochemistry require the stabilization of reaction intermediates (RI) at the electrode surface after every elementary reaction step. Accordingly, the bond strengths of these intermediates are important for assessing the catalytic performance of an electrode material. Current understanding of microscopic processes in modern electrocatalysis research is largely driven by theory, mostly based on ab initio thermodynamics considerations, where stable reaction intermediates at the electrode surface are identified, while the actual free energy barriers (or activation barriers) are ignored. This simple approach is popular in electrochemistry in that the researcher has a simple tool at hand in successfully searching for promising electrode materials. The ab initio TD approach allows for a rough but fast screening of the parameter space with low computational cost. However, ab initio thermodynamics is also frequently employed (often, even based on a single binding energy only) to comprehend on the activity and on the mechanism of an electrochemical reaction. The basic idea is that the activation barrier of an endergonic reaction step consists of a thermodynamic part and an additional kinetically determined barrier. Assuming that the activation barrier scales with thermodynamics (so-called Brønsted-Polanyi-Evans (BEP) relation) and the kinetic part of the barrier is small, ab initio thermodynamics may provide molecular insights into the electrochemical reaction kinetics. However, for many electrocatalytic reactions, these tacit assumptions are violated so that ab initio thermodynamics will lead to contradictions with both experimental data and ab initio kinetics. In this Account, we will discuss several electrochemical key reactions, including chlorine evolution (CER), oxygen evolution reaction (OER), and oxygen reduction (ORR), where ab initio kinetics data are available in order to critically compare the results with those derived from a simple ab initio thermodynamics treatment. We show that ab initio thermodynamics leads to erroneous conclusions about kinetic and mechanistic aspects for the CER over RuO2(110), while the kinetics of the OER over RuO2(110) and ORR over Pt(111) are reasonably well described. Microkinetics of an electrocatalyzed reaction is largely simplified by the quasi-equilibria of the RI preceding the rate-determining step (rds) with the reactants. Therefore, in ab initio kinetics the rate of an electrocatalyzed reaction is governed by the transition state (TS) with the highest free energy Grds#, defining also the rate-determining step (rds). Ab initio thermodynamics may be even more powerful, when using the highest free energy of an reaction intermediate Gmax(RI) rather than the highest free energy difference between consecutive reaction intermediates, ΔGloss, as a descriptor for the kinetics.

Zur Existenz nichtklassischer, cyclisch delokalisierter 4N/5e-Radikalanionen und 4N/6e-Dianionen – Ein- und Zweielektronen-Reduktion nahgeordneter, synperiplanarer Bisdiazene

Delokalisiert in der N,N,N,N-Ebene sind die funf bzw. sechs Elektronen der aus rigiden, nahgeordneten Bisdiazenen durch Ein- bzw. Zweielektronen-Reduktion erhaltenen, auserst bestandigen 4N/5e-Radikalanionen (tiefgrun) und σ-bishomoaromatischen 4N/6e-Dianionen (rot) (siehe unten). Deren langstwellige Absorption hangt stark vom Gegenion ab.

Microscopic Insights into the Chlorine Evolution Reaction on RuO2(110): a Mechanistic Ab Initio Atomistic Thermodynamics Study

AbstractThe frequently discussed mechanisms for the chlorine evolution reaction (CER)—Volmer–Tafel, Volmer–Heyrovsky, and Krishtalik—are assessed for the case of RuO2 within a mechanistic ab initio thermodynamics approach, employing the concept of Gibbs energy loss. The CER over the fully O-covered RuO2(110) surface, the stable surface configuration under CER conditions, is shown to proceed via the Volmer–Heyrovsky mechanism, i.e., the adsorption and discharge of the chloride ion are followed by the direct recombination of this surface species with a chloride ion from the electrolyte solution. The weak adsorption of the chloride ion on the fully O-covered RuO2(110) surface constitutes the elementary reaction step with highest Gibbs energy loss which has its origin in a too strong ruthenium–oxygen bond. Therefore, the activity of the model catalyst RuO2(110) can be enhanced by weakening the surface metal–oxygen bond such as realized with a monolayer of PtO2 coated on RuO2(110). Graphical Abstractᅟ

Highly efficient photometathesis in a proximate, synperiplanar diazene-diazene oxide substrate: retention of optical purity, mechanistic implications

In a specifically designed proximate and almost perfectly synperiplanar diazene–diazene oxide substrate, metathesis is the exclusive photoreaction and occurs with retention of optical purity, providing support for the [π2 + π2]photocycloaddition pathway (tetrazetidine oxide intermediate).