Homogeneous redox catalysis of electrochemical reactions: Part I. Introduction

In redox homogeneous catalysis the catalyst couple merely plays the role of an electron carrier by contrast with chemical catalysis which involves the transitory formation of catalyst-substrate adduct. A detailed kinetic analysis of redox catalysis in the case of an EC-type electrode reaction is given involving the following reaction sequence

$\begin{array}{l}B\stackrel{k}{\to }C\\ \begin{array}{c}A+1e?B\\ B\to C\end{array}\}directelectrodereductionofthesubstrate\end{array}$ The Hush-Marcus relationship between heterogeneous and homogeneous electron transfer rate constants is used to select the range of the parameter variations and to predict the magnitude of the catalytic efficiency as a function of the potential separation between the catalyst reduction and the direct substrate reduction, the ratio of the catalyst over the substrate concentration and the electrochemical standard rate constant of the substrate. Two situations are of particular interest for the kinetic analysis of the experimental data: (a) kinetic control of the direct substrate reduction by the follow-up chemical reaction with diffusion by charge transfer with activation or diffusion control of the solution electron exchange, the rate determining step of the catalytic sequence being A+Q→P+B. In these conditions, kinetic information on the substrate reduction process is obtainable which could not have been derived from a direct electrochemical analysis.     

Molecular approach to catalysis of electrochemical reaction in porous films

Current attempts to bridge the fields of what is conventionally called ‘electrocatalysis’ and of molecular catalysis of electrochemical reactions are surveyed and discussed. It amounts in many cases to ‘heterogenize’ molecular catalytic systems. Information on the meso- to nanostructures of the resulting catalytic films forms the basis of the understanding of new modes of transport of the reactants (catalysts, substrates and cosubstrates, products) that may govern the mechanistic competitions and consequently selectivity. Efforts to adapt benchmarking procedures developed in homogeneous molecular catalysis (catalytic Tafel plots) should be encouraged, taking into account, as additional factors, the transport of electrons and reacting species (including gases) through the catalytic system.     

Homogeneous chemical catalysis of electrochemical reactions: Competition with catalyst consumption

Chemical catalysis as opposed to redox catalysis involves an electrophilic-nucleophilic reaction between the substrate and the reduced form of the catalyst instead of an electron transfer reaction between these two species. The adduct thus formed then decomposes regenerating the catalyst. Decomposition may, however, occur along a competing pathway leading to a side-product with progressive consumption of the catalyst. The kinetics of the competition is analyzed in the framework of polarography, rotating disc electrode voltammetry and controlled potential coulometry. Two main cases are considered corresponding respectively to a one- and a two-electron catalysis. The effect of the initial concentration on the shifting of the system from one case to the other is discussed. This is finally compared to what occurs when redox analysis replaces chemical catalysis in the competition with the catalyst consumption.     

Non-Faradaic electrochemical activation of catalysis

The use of fuel cells for carrying out oxidation reactions with cogeneration of electrical power and chemicals led, upon cofeeding oxygen and fuel at the anode, to the discovery of the effect of non-Faradaic electrochemical modification of catalytic activity or electrochemical promotion of catalysis. This phenomenon has been studied already for more than 70 catalytic reactions, including oxidations, reductions and isomerizations and using a variety of metal catalysts, and solid electrolytes. In this work we summarize the main features of electrochemical promotion and discuss critically its currently accepted sacrificial promoter mechanism which involves electrochemically controlled migration (spillover-backspillover) of promoting species from the electrolyte to the catalytically active metal-gas interface. It is shown that the spillover ionic species (e.g., O(delta-), Na(delta+)) form an overall neutral double layer at the catalyst-gas interface which alters the catalyst work function and the binding energies of coadsorbed reactants and intermediates, thus causing very pronounced and reversible alterations in the catalytic activation energy and catalytic rate and selectivity. Recent efforts for the practical utilization of electrochemical promotion are also briefly discussed.     

对映选择催化 Diels-Alder 反应

本文综述了对映选择催化 Diels-Alder 反应的进展。     

Homogeneous redox catalysis of electrochemical reactions: Part III. Rate determining electron transfer. Kinetic characterization of follow-up chemical reactions

Electrochemical systems involving moderately fast charge transfers and very fast irreversible follow-up chemical reactions usually escape from kinetic and mechanistic characterization through the standard use of electrochemical techniques. It is shown that these difficulties can be overcome using an indirect approach which involves the homogeneous redox catalysis of the considered electrochemical reaction. A procedure for determining the rate constant of such fast follow-up reaction is presented. It is illustrated by the determination of the cleavage rate constant of the chlorobenzene anion radical in DMF which reaches a value on the order of 10⁷ s^?1.     

Single-atom catalysis for organic reactions

Metal-based catalysis, including homogeneous and heterogeneous catalysis, plays a significant role in the modern chemical industry. Heterogeneous catalysis is widely used due to the high efficiency, easy catalyst separation and recycling. However, the metal-utilization efficiency for conventional heterogeneous catalysts needs further improvement compared to homogeneous catalyst. To tackle this, the pursing of heterogenizing homogeneous catalysts has always been attractive but challenging. As a recently emerging class of catalytic material, single-atom catalysts (SACs) are expected to bridge homogeneous and heterogeneous catalytic process in organic reactions and have arguably become the most active new frontier in catalysis field. In this review, a brief introduction and development history of single-atom catalysis and SACs involved organic reactions are documented. In addition, recent advances in SACs and their practical applications in organic reactions such as oxidation, reduction, addition, coupling reaction, and other organic reactions are thoroughly reviewed. To understand structure-property relationships of single-atom catalysis in organic reactions, active sites or coordination structure, metal atom-utilization efficiency (e.g., turnover frequency, TOF calculated based on active metal) and catalytic performance (e.g., conversion and selectivity) of SACs are comprehensively summarized. Furthermore, the application limitations, development trends, future challenges and perspective of SAC for organic reaction are discussed.     

Synergistic catalysis of anthrone Diels-Alder reactions

[reaction: see text] The combination of an amine base and Lewis acid (Li+) leads to synergistic catalysis of the Diels-Alder reaction of anthrone with methyl crotonate and some other less reactive dienophiles. These cycloadditions either do not occur with the individual catalysts used separately or they are greatly accelerated by the combined catalysts. DMSO solvent allows the use of LiCl as the Lewis acid source and can provide greater control of subsequent conversion to Michael adduct.     

Aligned Pt-diamond core-shell nanowires for electrochemical catalysis

Heavily boron-doped diamond electrode has been applied as a robust substrate for Pt based catalyst. However, by simply applying a planar electrode the effective surface area of the catalyst is limited. In this article we for the first time prepared vertically aligned Pt-diamond core-shell nanowires electrode in a convenient and scalable method (up to 6-inch wafer size). The diamond nanowires are first fabricated with reactive ion etching with metal nanoparticles as etching masks. The following Pt deposition was achieved by DC sputtering. Different amounts of Pt were coated on to the nanowires and the morphology of the core-shell wires is characterized by SEM and TEM. The catalytic oxygen/hydrogen adsorption/desorption response are characterized by cyclic voltammetry. The results show that the active Pt surface area is 23 times higher than a planar Pt electrode, and 4.3 times higher than previously reported on Pt nanoparticles on diamond by electro-deposition. Moreover, this highly active surface is stable even after 1000 full surface oxidation and reduction cycles.     

Water-wire catalysis in photoinduced acid-base reactions

The pronounced ability of water to form a hyperdense hydrogen (H)-bond network among itself is at the heart of its exceptional properties. Due to the unique H-bonding capability and amphoteric nature, water is not only a passive medium, but also behaves as an active participant in many chemical and biological reactions. Here, we reveal the catalytic role of a short water wire, composed of two (or three) water molecules, in model aqueous acid-base reactions synthesizing 7-hydroxyquinoline derivatives. Utilizing femtosecond-resolved fluorescence spectroscopy, we tracked the trajectories of excited-state proton transfer and discovered that proton hopping along the water wire accomplishes the reaction more efficiently compared to the transfer occurring with bulk water clusters. Our finding suggests that the directionality of the proton movements along the charge-gradient H-bond network may be a key element for long-distance proton translocation in biological systems, as the H-bond networks wiring acidic and basic sites distal to each other can provide a shortcut for a proton in searching a global minimum on a complex energy landscape to its destination.     

Superparamagnetic nanoparticle-supported catalysis of Suzuki cross-coupling reactions

Emulsion polymerization was examined as a novel route for the synthesis of core/shell superparamagnetic nanoparticles consisting of a highly crystalline gamma-Fe2O3 core and a very thin polymeric shell wall. These nanoparticles were used as soluble supports for immobilizing Pd catalysts to promote Suzuki cross-coupling reactions. Recovery of catalysts was facilely achieved by applying a permanent magnet externally. Isolated catalysts were reused for new rounds of reactions without significant loss of their catalytic activity.     

Nanoconfinement effects in electrochemical reactions

Nanostructuring materials in the aims to enhance its catalytic activity has long been indispensable in electrocatalyst development. In particular, nanoporous electrodes with numerous pores in the nanoscale, are widely utilized owing to its enlarged surface area as well as activated surface characteristics. In the geometrical point of view, nanocavities of nanoporous electrodes offer unique spatial environment that confine reactant molecules, resulting in enhanced interaction between the reactant molecule and the electrode surface. Such electrocatalytic effects stemming from the morphology of nanoporous electrodes have been denoted as nanoconfinement effects. This review introduces the concept of nanoconfinement effects in electrochemical systems, the recent progress, and perspectives in this field.     

Physicochemical principles of ecological catalysis of gas-phase reactions

In this article we summarize and correlate results obtained in numerous studies of heterogeneous catalytic gasphase reactions with low reactant concentrations — mostly studies performed under the direction of the author at the L. V. Pisarzhevskii Institute of Physical Chemistry, National Academy of Sciences of the Ukraine. Physicochemical principles are defined for the catalysis of such reactions in the presence of solid catalysts. Examples are cited to illustrate the use of these relationships in developing effective catalysts and processes of ecological catalysis, particularly for catalytic treatment of gas emissions and the development of essentially zero-waste technologies in resource-conserving and energy-efficient versions.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 29, No. 6, pp. 482–500, November–December, 1993.     

Double- and triple-cobalt catalysis in multicomponent reactions

The combination of different types of cobalt-catalyzed transformations in one-pot procedures is described. One of the key building blocks, a boron-functionalized isoprene derivative (boroprene), led to the realization of four-component reaction sequences comprising the cobalt-catalyzed Diels-Alder and a 1,4-hydrovinylation reaction. Eventually, a reaction sequence including a cobalt-catalyzed Diels-Alder reaction, a cobalt-catalyzed 1,4-hydrovinylation, an allylboration, and a cobalt-catalyzed Alder-ene reaction led to a five-component one-pot reaction sequence in which five carbon-carbon bonds were formed in excellent regio- and diastereoselectivity to generate complex products in good overall yields.     

Turnover numbers, turnover frequencies, and overpotential in molecular catalysis of electrochemical reactions. Cyclic voltammetry and preparative-scale electrolysis

The search for efficient catalysts to face modern energy challenges requires evaluation and comparison through reliable methods. Catalytic current efficiencies may be the combination of many factors besides the intrinsic chemical properties of the catalyst. Defining turnover number and turnover frequency (TOF) as reflecting these intrinsic chemical properties, it is shown that catalysts are not characterized by their TOF and their overpotential (η) as separate parameters but rather that the parameters are linked together by a definite relationship. The log TOF-η relationship can often be linearized, giving rise to a Tafel law, which allows the characterization of the catalyst by the value of the TOF at zero overpotential (TOF(0)). Foot-of-the-wave analysis of the cyclic voltammetric catalytic responses allows the determination of the TOF, log TOF-η relationship, and TOF(0), regardless of the side-phenomena that interfere at high current densities, preventing the expected catalytic current plateau from being reached. Strategies for optimized preparative-scale electrolyses may then be devised on these bases. The validity of this methodology is established on theoretical grounds and checked experimentally with examples taken from the catalytic reduction of CO(2) by iron(0) porphyrins.     

Dynamic allosteric control of noncovalent DNA catalysis reactions

Allosteric modulation of catalysis kinetics is prevalent in proteins and has been rationally designed for ribozymes. Here, we present an allosteric DNA molecule that, in its active configuration, catalyzes a noncovalent DNA reaction. The catalytic activity is designed to be modulated by the relative concentrations of two DNA regulator molecules, one an inhibitor and the other an activator. Dynamic control of the catalysis rate is experimentally demonstrated via three cycles of up and down regulation by a factor of over 10. Unlike previous works, both the allosteric receptor and catalytic core are designed, rather than evolved. This allows flexibility in the sequence design and modularity in synthetic network construction.     

Manganese compounds in the catalysis of organic reactions

The review summarizes published data on the use of manganese compounds as catalysts in oxidation of alkanes, alcohols, aldehydes, sulfides, and amines and carbometalation, cross coupling, telomerization, chlorination, hydrosilylation, hydrohydrazination, and other reactions.