Molecular Catalysis of Electrochemical Reactions.Mechanistic Aspects
Jean-Michel Savéant*
Laboratoire d’Electrochimie Mole´culaire,Unite´Mixte de Recherche Universite s CNRS7591,Universite´de Paris7s Denis Diderot,2place Jussieu,
75251Paris Cedex05,France
Received July24,2007
Contents
1.Introduction2348
1.1.“Electrocatalysis”and Molecular Catalysis2348
1.2.Redox and Chemical Catalysis.
Freely-Diffusing and Immobilized Catalysts.
Monolayers and Multilayers
2349
1.3.Synopsis and Connections with Other
Contributions in This Issue
2349
2.Electrochemical Techniques for Catalyst
Evaluation
2350
2.1.Preparative Scale Electrolysis and
Non-destructive Techniques
2350
2.2.Cyclic Voltammetry of Free-Diffusing Catalyst
Systems
2350
2.3.Rotating Disk Electrode Voltammetry of
Immobilized Catalyst Systems
2351
2.3.1.Monolayers2351
2.3.2.Multilayered Films2351
2.3.3.Rotating Ring-Disk Techniques2352
3.Contrasting Redox Catalysis with Chemical
Catalysis.Carbon-Halogen Bond Reductive
Cleavage
2352
3.1.Introduction2352
3.2.Kinetics and Stereochemistry of Vicinal
Dibromides Reductive Dehalogenation
2352
3.3.Vitamin B12and Reductive Dehalogenases2354
4.Applications of Homogeneous Redox Catalysis to
the Kinetic and Thermodynamic Character-
ization of Reaction Sequences:Involving
Short-Lived Intermediates
2355
5.Reduction of Dioxygen2357
5.1.Thermodynamics and Direct Electrochemistry2357
5.2.Redox Catalysis2358
5.3.Chemical Catalysis2359
5.4.Concluding Remarks2363
6.Oxidation of Water.Oxygen Evolution2363
6.1.Introduction.Thermodynamics,Direct
Electrochemistry,Redox Catalysis,
Electrocatalysis
2363
6.2.Photosystem II as a Source of Inspiration2364
6.3.Ruthenium Complexes2364
6.4.Manganese Complexes2365
6.5.Concluding Remarks2366
7.Reduction of Carbon Dioxide2366
7.1.Introduction.Thermodynamics2366
7.2.Electrochemistry at Inert Electrodes2366
7.3.Electrocatalysis2367
7.4.Redox Catalysis or“Quasi Redox”Catalysis2367
7.5.Chemical Catalysis:Families of Transition
Metal Complex Catalysts
2368
7.6.Chemical Catalysis.Mechanisms2370
7.7.Concluding Remarks2372
8.Conclusion2372
9.Acknowledgments2372
10.References2373 1.Introduction
1.1.“Electrocatalysis”and Molecular Catalysis Electrochemical reactions often require an important overpotential1to proceed at an appreciable rate.This is particularly true when the reaction does not merely consist of an outersphere electron transfer between the electrode and a reactant but gives rise to bond breaking and bond formation, involving more than one reactant and one product,which may trigger the uptake or release of additional electrons,as sketched in Scheme1.
This is a typical situation where catalysis of the electro-chemical reaction is sought in order to increase the reaction rate and hence the current at a potential as close as possible to the equilibrium potential.Several strategies can be envisaged to achieve this goal.The term“electrocatalysis”is traditionally used for reactions in which the electrode material s often,but not always,a metal s is chemi
cally involved in the catalytic process.2Although the chemical properties of the electrode material play an important role in governing the catalytic efficiency,geometric and crystal-lographic features,nature and number of defects,may also be of paramount significance.3The differences between the bulk properties of the metal and its surface properties are particularly important in this respect.It is therefore difficult, or even irrelevant,to analyze results and devise new catalytic systems on the basis of molecular concepts.
Another approach to catalyzing electrochemical reactions is to use molecules as catalysts.“Molecular catalysis”thus defined may involve catalyst molecules either homoge-neously dispersed in the solution bathing the electrode or
*E-mail:saveant@paris7.jussieu.fr.
Scheme
1 Chem.Rev.2008,108,2348–2378
2348
10.1021/cr068079z CCC:$71.00 2008American Chemical Society
Published on Web07/11/2008
immobilized in a monolayer or multilayered coating depos-ited on the electrode surface as sketched in
Figure 1.
In line with the general theme of this special issue,this review is mostly concerned with molecular catalysis.Nev-ertheless,discussion of examples of electrocatalysis will not be systematically excluded.Analysis of the similarities and differences between electrocatalysis and molecular catalysis of the same global reaction may indeed benefit from the understanding of the mechanism in each case.
1.2.Redox and Chemical Catalysis.
Freely-Diffusing and Immobilized Catalysts.Monolayers and Multilayers
Homogeneous catalysis is concerned with systems in which the catalyst diffuses freely in the solution that contains the substrate.
Testing molecular catalysts implies that the electrode material does not participate in the electrochemical reaction.In other words,the electrode plays the role of a heteroge-neous outersphere electron donor (or acceptor).The direct reduction (or oxidation)of the substrate requires a sizable overpotential to give rise to a significant current.The catalyst is usually one member of a reversible cou
ple,P/Q,the standard potential of which is located in between the standard potential of the global electrochemical reaction and the potential where the direct electrochemical reaction occurs.The closer the two standard potentials and the larger the current,the better the catalyst.As exemplified later,catalytic currents may be observed even when the homogeneous electron transfer between the active form of the catalyst is an outersphere reaction.As discussed in the next section,the very existence of “redox catalysis”derives from the fact that,instead of being confined within a two-dimensional space,the electrons to be transferred are then dispersed in a three-dimensional space.“Chemical catalysis”involves more
intimate interactions between the active form of the catalyst and the substrate,thus opening a route to more efficient and more specific catalytic processes.
In many,but not all,practical applications immobilization of the catalyst molecules onto the electrode surface,as sketched in Figure 1,is advantageous.Catalysis is expected to be more efficient at multilayered coatings than at mono-layer coatings,provided transport of electrons,substrate and product through the film does not become rate-limiting.It follows from the very nature of redox catalysis that it is possible in multilayered films but not at monolayer coatings,since redox catalysis implies that the electrons to be transferred be dispersed in a three-dimensional space.
1.3.Synopsis and Connections with Other Contributions in This Issue
Section 2discusses the techniques that may be used to evaluate a catalyst in terms of efficiency and selectivity.Section 3is devoted to the description of a particularly clear example illustrating the distinction between redox and chemical catalysis both in terms of efficiency and specificity.In the fourth section,we review the application of redox catalysis to the kinetic characterization of electron transfer mechanisms involving fast decaying intermediates.Sections 5–7are dedicated to selected examples where chemical catalysis is particularly important.The examples have been selected in relation with the activation of small molecules relevant to the present challenges of renewable energy and greenhouse effects:reduction of oxygen (section 5),oxygen evolution from the oxidation of water (section 6),reduction of carbon dioxide (section 7).The links that may exist with enzymes that catalyze the same reactions will be emphasized.These enzymes have served as sources of inspiration for the catalyst design or,conversely,the chemical catalyst may be used as a simplified model of the enzyme aiming at a better understanding of enzymatic mechanisms.In spite of its paramount importance,we let aside catalysis of
hydrogen
Figure 1.Schematic representation of the various types of molecular catalysis of electrochemical
reactions.
Jean-Michel Savéant received his education in the Ecole Normale Supérieure in Paris,where he became the Vice-Director of the Chemistry Department before moving to the University Denis Diderot (Paris 7)as a Professor in 1971.He is,since 1985,Directeur de Recherche au Centre National de la Recherche Scientifique in the same university.In 1988-1989he was a distinguished Fairchild Scholar at the California Institute of Technology.His current research interests involve all aspects of molecular and biomolecular electrochemistry as well as mechanisms and reactivity in electron transfer chemistry and biochemistry.Among many distinctions,Jean-Michel Savéant received the Faraday Medal of the Royal Chemical Society,the Olin Palladium Medal of the Electrochemical Society,la Medaglia Luigi Galvani della SocietàChimica Italiana and the Manuel Baizer Award of the Electrochemical Society.Jean-Michel Savéant is a member of the French Academy of Sciences and foreign associate of the National Academy of Sciences of the United States of America.
Molecular Catalysis of Electrochemical Reactions Chemical Reviews,2008,Vol.108,No.72349
evolution and uptake because it is treated in detail in a preceding Chemical Re V iews thematic issue“Hydrogen”. Numerous other examples,dealing with the organometallic catalysis of important org
anic reactions,can be found in the contribution of Anny Jutand,to this issue“Contribution of Electrochemistry to Organometallic Catalysis”.Reversing the concept of chemical catalysis of electrochemical reactions leads to the notion of using electrons(or holes)from an electrode to catalyze a chemical reaction.This important topic is treated in the contribution of Abdelaziz Houmam,“Elec-tron Transfer Initiated Reactions:Bond Formation and Bond Dissociation”.Homogeneous and supported catalysis of electrochemical reactions by enzymes are massively present in the contribution of Christophe Le´ger“Direct Electro-chemistry of Redox Enzymes as a Tool for Mechanistic Studies”.
2.Electrochemical Techniques for Catalyst Evaluation
2.1.Preparative Scale Electrolysis and
Non-destructive Techniques
Non-destructive techniques,such as cyclic voltammetry operated with a microelectrode,are particularly useful for a first evaluation of the catalyst:determination of the potential at which the catalytic process can be run and of the catalytic efficiency as measured by the current densityflowing through the electrode at this potential for a given concentration or partial pressure of the substrate.They may then be used by means of a more detailed kinetic analysis to unravel the mechanism of the catalytic reactio
n and suggest improve-ments.Gauging of the selectivity of the catalytic reaction requires moving to preparative-scale electrolysis and deter-mining the faradaic yields of each of the reaction products. Such sustained electrolyses are also necessary to estimate the stability of the catalyst by observing the variation of the preparative-scale current with time.Following the cyclic voltammetric response simultaneously is an additional way of observing the evolution of the catalyst in the course of electrolysis.
Preparative-scale evaluation is thus required to establish the actual performances and viability of catalytic systems beyond the rapid test that cyclic voltammetric allows. 2.2.Cyclic Voltammetry of Free-Diffusing Catalyst
Systems
Cyclic voltammetry has been and still is the most popular non-destructive technique applied to homogeneous catalysis systems,although other techniques might be used as well. Figure2summarizes the various characteristic shapes of the cyclic voltammetric responses expected for a catalytic reaction of the type shown in Figure1,in which the rate-determining step(rate constant k)isfirst order in substrate (bulk concentration:C A)and catalyst(bulk concentration: C P).The responses are governed b
y two(and only two) dimensionless parameters,taken as coordinates of the“kinetic zone diagram”shown in Figure2.4,5
The most familiar situation is that of a large excess of substrate over catalyst(right-hand“no substrate consump-tion”zone)where the cyclic voltammetric response passes from a reversible“no catalysis”response proportional to the square root of the scan rate to an S-shaped curve independent of the scan rate as the catalytic rate constant increases and/or the scan rate decreases.6–8The plateau current density, I p,is then an easy measure of the rate constant(D P:catalyst diffusion coefficient):
I
p
)FC
P√D P√kC A
The“pure kinetic”conditions thus achieved prevail when the catalytic reaction is fast as compared to , when the reaction layer within which the concentration profiles of the catalyst P and Q for
ms are confined is much thinner than the diffusion layer.At the left-hand end of diagram,consumption of the catalyst increasingly interferes up to the“total catalysis”situation where thefirst wave of the two-wave system that develops then is governed by the diffusion of the substrate.The peak current density,I p,
I
p
)0.609√FC A F V RT
is no longer a function of the catalytic rate constant,which instead governs the location of the peak potential:
E
p
)E
P⁄Q
0-0.409RT
F
+
RT
F
ln(RT F kC P2C A V)
in the cases where the electron transfer between the electrode and the catalyst couple is fast(standard potential:E P/Q0). How the variations of the experimental concentrations and rate parameters induce the passage from one kinetic situation to the other,in direction and magnitude,is summarized by the red compass rose on top of the diagram.
More complicated kinetic schemes may be encountered in practice as,for example,when two-electron stoichiom-etries,mixed kinetic control by successive steps,partial deactivation of the catalyst etc.are involved.Several of these schemes are analyzed in ref9.Any homogeneous catalytic mechanism may b
e analyzed by“digital simulation”10of the cyclic voltammetric responses by means offinite difference resolution of the set of diffusion-reaction partial derivative equations and initial and boundary conditions.11Two main commercial packages,Digisim12and DigiElech13are avail-able for this purpose.In the application of these powerful procedures to reaction schemes involving several successive and/or competing steps,a preliminarily dimensionless
analy-Figure2.Kinetic zone diagram showing the expected shapes of cyclic voltammetric responses as a function of the two dimension-less parameters taken as coordinates for a catalytic reactionfirst order in substrate(bulk concentration:C A)and catalyst(bulk concentration:C P).V:scan rate.
2350Chemical Reviews,2008,Vol.108,No.7Savéant
sis of the kinetic problem is advisable so as to determine the minimum number of governing dimensionless param-eters.This is a necessary step for a lucid assignment of the mechanism and a realistic estimate of grouped rate and equilibrium constants.Examples will be given in section 4.
2.3.Rotating Disk Electrode Voltammetry of Immobilized Catalyst Systems
2.3.1.Monolayers
There are many ways to attach monolayer or multilayered films containing redox centers onto electrode surfaces.14,15While cyclic voltammetry and other transient techniques could be used to investigate such systems,rotating disk electrode voltammetry (RDEV)has been mostly used instead.For monolayer coatings and a catalytic reaction that is first order in substrate and in catalyst,the current density (I )-potential (E )curve is 16
I I A
)I k I A
1+I k I A +exp [F RT
(E -E P⁄Q 0
)
]
which depends on a single dimensionless competition
parameter,I k /I A ,itself defined as the ration of two current densities:I k )Fk ΓP C A (ΓP ,total surface concentration of catalyst),which characterizes the catalytic reaction,and I )FC A D A /δ)0.62FC A D A 2/3ν-1/6ω1/2(δ,thickness of the diffusion layer;ν,kinematic viscosity;ω,rotation rate),which characterizes the mass transport of the substrate.The two limiting situations reached for I k /I A small and large are of the same type as the “pure kinetic”and “total catalysis”conditions,respectively,depicted in section 2.2for the cyclic voltammetry of homogeneous catalytic systems.The plateau current density may be expressed as
1I p )1Fk ΓP C A +
10.62FC A D A
2⁄3ν-1⁄6ω1⁄2
giving rise to a “Koutecky -Levich”plot of the variation of the inverse of the current density with the inverse of the square root of the rotation rate (Figure 3),from the intercept of which the catalytic rate constant,k ,can be estimated if the surface concentration of the catalyst,ΓP ,is known (if not,k ΓP nevertheless provides an interesting characterization of the catalytic properties of the electrode coating).
2.3.2.Multilayered Films
Expression of the governing parameters by means of characteristic current densities is also worthwhile in the case
of multilayered coatings.17Besides the catalytic reaction and the diffusion of the substrate in the bathing solution that are characterized by the same current densities as above (ΓP is now the total amount of catalyst contained in one unit surface area of film,C A is replaced by κC A ,κbeing the partiti
on coefficient of the substrate between the solution and the film),two other rate-limiting factors have to be taken into account,namely,the diffusion of the substrate through the film and electron hopping between the electrode surface and the redox centers in the film,characterized by the current densities,I S and I e :I S )F κC A D S /d f (d f ,film thickness;D S ,diffusion coefficient of the substrate in the film)and I e )FS Γp D e /d f 2(D e ,equivalent diffusion coefficient for electron hopping 18–22).The competition between these rate-limiting factors is represented by the kinetic zone diagram in Figure 4,which corresponds to the simple mechanism depicted in Figure 1.23–26The symbolic designation of each zone summarizes the nature of the kinetic control of the current,R for the catalytic reaction,S for substrate diffusion through the film,E for electron propagation.SR and ER apply to fast catalytic reactions and mean a combined control under the pure kinetic mode,in which the catalytic reaction and the diffusion of either the substrate or the electrons compensates each other,giving rise to a thin reaction layer,of thickness (√I s ⁄I k )d f and (√I e ⁄I k )d f .The expressions of the current in each zone of the kinetic zone diagram of Figures 4are available from Table 4.1of ref 26.Several more complicated processes have been treated according to the similar ap-proaches.These treatments and the resulting equations are given in the same reference.
The analyses and expressions of the current form the bases on which contemporary rationalization of electrochemical sensors is dealt with,as developed for example in refs 27and
28.
Figure 3.Rotating disk electrode voltammetry of a catalytic reaction first order in substrate and in catalyst at a monolayer coating.“Koutecky -Levich”
reaction rateplot.
Figure 4.Kinetic zone diagram characterizing the RDEV plateau currents for the reaction scheme in Figure 1.Full lines:substrate concentration profile.Dashed lines:concentration profile of the reduced form of the catalyst.Adapted with permission from Figure 5.5of ref 26.Copyright 1992J.Wiley and Sons.
Molecular Catalysis of Electrochemical Reactions Chemical Reviews,2008,Vol.108,No.72351
2.3.3.Rotating Ring -Disk Techniques
Generating a product or an intermediate at a disk electrode and collecting it at a ring electrode that concentrically surrounds the disk is an alternative to cyclic voltammetry.The product or intermediate is generated by fixing the disk potential at an appropriate value and scanning the ring potential so as to obtain its oxidative or reductive signature as a steady-state current -potential curve.This technique has been extensively and successfully applied in the determina-tion of the product,H 2O 2or H 2O in the catalytic reduction of oxygen (see section 5.3).
3.Contrasting Redox Catalysis with Chemical Catalysis.Carbon -Halogen Bond Reductive Cleavage 3.1.Introduction
As opposed to redox catalysis,where the catalyst acts as an outersphere electron transfer agent,chemical catalysis involves more intimate interactions between the active form of the catalyst and the substrate.A more precise picture of what is meant by “more intimate interactions”leads to distinguishing two situations.
One simply involves bonded interactions in the electron transfer transition state.In other words,electron transfer then possesses an inner-sphere character,which is expected to result in a lower activation energy than for an outersphere electron transfer of same driving force.More stereospecificity is also expected to result from bonded interactions in the transition state.The reduction of vicinal dibromides into the corresponding olefins offers a particularly clear example of this situation both in terms of kinetics and of stereochemistry.Another situation is when the “more intimate interactions”are so strong that an adduct is formed between the active form of the catalyst and the substrate.The potential energy profiles corresponding to the two situations are sketched in Figure 5.The formation of this intermediate is again expected to be faster than an outersphere electron transfer reaction;the more so the more stable the adduct.A problem that may then arise is the rate of decomposition of the adduct that ensures the formation of the products and the regeneration of the catalyst.Catalysis of the reduction of organic halides by vitamin B12offers an illustration of this type of chemical catalysis,in rela
tion with the mechanism by which reductive dehalogenases reduce dangerous polyhalide pollutants.We will focus on these two examples taken in the literature on organic halide reduction,noting that several other important
reactions pertaining to the same topic are discussed in Anny Jutand’s contribution to this issue.
3.2.Kinetics and Stereochemistry of Vicinal Dibromides Reductive Dehalogenation
An early investigation of the direct electrochemical reduction of these compounds in an aprotic medium (DMF)showed that the corresponding olefin is formed according to a two-electron stoichiometry (overall reaction in Scheme 2).29
Further studies 30–37revealed that the transfers of the two electrons are not concerted,although the debrominated radical formed in the first step of Scheme 2is reduced in the second step at a less negative potential than the potential at which the first reaction takes place.The current -potential responses obtained ,cyclic voltammetry (Figure 6)thus correspond to a two-electron stoichiometry,even though the two electrons come into the molecule successively.Each of the two successive electron transfers is coupled with
the
Figure 5.Redox and chemical catalysis.Potential energy profiles.
Scheme
2
Figure 6.Redox and chemical homogeneous catalysis of trans -1,2-dibromocyclohexane.(a)Cyclic voltammetry in dimethylfor-mamide of the direct electrochemical reduction at a glassy carbon electrode (top),of redox catalysis by fluorenone (middle),of chemical catalysis by an iron(I)porphyrin.(b)Catalysis rate constant as a function of the standard potential of the catalyst couple:green dots,aromatic anion radicals;downward brown triangles,Fe(I);upward red triangles,Fe(0);blue squares,Co(I);magenta dots,Ni(I)porphyrins.Adapted with permission from Figures 3and 4of ref 36.Copyright 1990American Chemical Society.2352Chemical Reviews,2008,Vol.108,No.7Savéant
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