Electrochemical reaction mechanism
Encyclopedia
In chemistry, an electrochemical reaction mechanism
is the step by step sequence of elementary steps, involving at least one outer sphere electron transfer
, by which an overall chemical change occurs .
and the movement of electrons between an electrode and substrate are special to electrochemical processes. Electrochemical mechanisms are important to all redox chemistry including corrosion
, redox active photochemistry
including photosynthesis
, other biological systems often involving electron transport chain
s and other forms of homogeneous and heterogeneous electron transfer. Such reactions are most often studied with standard three electrode techniques such as cyclic voltammetry
(CV), chronoamperometry
, and bulk electrolysis as well as more complex experiments involving rotating disk electrode
s and rotating ring-disk electrode
s. In the case of photoinduced electron transfer
the use of time-resolved spectroscopy
is common.
; sometimes EO and ER are used to represent oxidations and reductions respectively. The C represents a chemical reaction which can be any elementary reaction step and is often called a "following" reaction. In coordination chemistry common C steps which "follow" electron transfer are ligand
loss and association. The ligand loss or gain is associated with a geometric change in the complexes coordination sphere
.
The reaction above would be called an EC reaction.
s (UME) capable of very high scan rates of 0.5 to 5.0 V/s. Plots of forward and reverse peak ratios against modified forms of the scan rate often identify the rate of the chemical reaction. It has become a common practice to model such plots with electrochemical simulations. The results of such studies are suspect since simulation require excellent experimental data, better than that routinely obtained and reported. Further more the parameters of such studies are rarely reported and often include an unreasonably high variable to data ratio. A better practice is to look for a simple well documented relationship between observed results and implied phenomenon; or to investigate the phenomenon through an alternate technique such as chronoamperometry
or those involving a rotating electrode.
is a catalytic process involving oxidation or reduction through the direct transfer of electrons. The electrochemical mechanisms of electrocatalytic processes are a common research subject for various fields of chemistry and associated sciences. This is important to the development of water oxidation and fuel cells catalysts. For example half the water oxidation reaction is the reduction of protons to hydrogen, the subsequent half reaction.
This reaction requires some form of catalyst to avoid a large overpotential
in the delivery of electrons. A catalyst can accomplish this reaction through different reaction pathways, two examples are listed below for the homogeneous catalysts [MLn]2+.
Pathway 1 is described as a ECECC while pathway 2 would be described as an ECC. If the catalyst was being considered for solid support pathway 1 which requires a single metal center to function would be a viable candidate. In contrast a solid support system which separates the individual metal centers would render a catalysts that operates through pathway 2 useless since it requires a step which is second order in metal center. Determining the reaction mechanism is much like other methods with some techniques unique to elctrochemistry. In most cases electron transfer can be assumed to be much faster than the chemical reactions. Unlike stoichiometric reactions where the steps between the starting materials and the rate limiting step dominate in catalysis
the observed reaction order is usually dominated by the steps between the catalytic resting state and the rate limiting step.
. If a species has been deposited on the electrode during a potential sweep then on the return return sweep a stripping wave is usually observed.
While the nucleation wave may be pronounced or difficult the detect the stripping wave is usually very distinct. Often these phenomenon can be avoided by reducing the concentration of the complex in solution. Neither these physical state changes involve a chemical reaction mechanism but they are worth mentioning here since the resulting data is at times confused with some chemical reaction mechanisms.
Reaction mechanism
In chemistry, a reaction mechanism is the step by step sequence of elementary reactions by which overall chemical change occurs.Although only the net chemical change is directly observable for most chemical reactions, experiments can often be designed that suggest the possible sequence of steps in...
is the step by step sequence of elementary steps, involving at least one outer sphere electron transfer
Electron transfer
Electron transfer is the process by which an electron moves from an atom or a chemical species to another atom or chemical species...
, by which an overall chemical change occurs .
Overview
Elementary steps like proton coupled electron transferProton coupled electron transfer
Proton-coupled electron transfer is a reaction mechanism that is thought to be common in redox reactions. It involves the concerted transfer of an electron and proton to or from a substrate....
and the movement of electrons between an electrode and substrate are special to electrochemical processes. Electrochemical mechanisms are important to all redox chemistry including corrosion
Corrosion
Corrosion is the disintegration of an engineered material into its constituent atoms due to chemical reactions with its surroundings. In the most common use of the word, this means electrochemical oxidation of metals in reaction with an oxidant such as oxygen...
, redox active photochemistry
Photochemistry
Photochemistry, a sub-discipline of chemistry, is the study of chemical reactions that proceed with the absorption of light by atoms or molecules.. Everyday examples include photosynthesis, the degradation of plastics and the formation of vitamin D with sunlight.-Principles:Light is a type of...
including photosynthesis
Photosynthesis
Photosynthesis is a chemical process that converts carbon dioxide into organic compounds, especially sugars, using the energy from sunlight. Photosynthesis occurs in plants, algae, and many species of bacteria, but not in archaea. Photosynthetic organisms are called photoautotrophs, since they can...
, other biological systems often involving electron transport chain
Electron transport chain
An electron transport chain couples electron transfer between an electron donor and an electron acceptor with the transfer of H+ ions across a membrane. The resulting electrochemical proton gradient is used to generate chemical energy in the form of adenosine triphosphate...
s and other forms of homogeneous and heterogeneous electron transfer. Such reactions are most often studied with standard three electrode techniques such as cyclic voltammetry
Cyclic voltammetry
Cyclic voltammetry or CV is a type of potentiodynamic electrochemical measurement. In a cyclic voltammetry experiment the working electrode potential is ramped linearly versus time like linear sweep voltammetry. Cyclic voltammetry takes the experiment a step further than linear sweep voltammetry...
(CV), chronoamperometry
Chronoamperometry
Chronoamperometry is an electrochemical technique in which the potential of the working electrode is stepped and the resulting current from faradic processes occurring at the electrode is monitored as a function of time...
, and bulk electrolysis as well as more complex experiments involving rotating disk electrode
Rotating disk electrode
A rotating disk electrode is a hydrodynamic working electrode used in a three electrode system. The electrode rotates during experiments inducing a flux of analyte to the electrode. These working electrodes are used in electrochemical studies when investigating reaction mechanisms related to...
s and rotating ring-disk electrode
Rotating ring-disk electrode
A rotating ring-disk electrode is double working electrode used in hydrodynamic voltammetry, very similar to a rotating disk electrode . The electrode actually rotates during experiments inducing a flux of analyte to the electrode...
s. In the case of photoinduced electron transfer
Photoinduced electron transfer
Photoinduced electron transfer is an electron transfer which occurs when certain photoactive materials interact with light. - Breadth :Such materials include semiconductors that can be photoactivated like many solar cells, biological systems such as those used in photosynthesis, and small...
the use of time-resolved spectroscopy
Time-resolved spectroscopy
In physics and physical chemistry, time-resolved spectroscopy is the study of dynamic processes in materials or chemical compounds by means of spectroscopic techniques. Most often, processes are studied that occur after illumination of a material, but in principle, the technique can be applied to...
is common.
Formalism
When describing electrochemical reactions an "E" and "C" formalism is often employed. The E represents an electron transferElectron transfer
Electron transfer is the process by which an electron moves from an atom or a chemical species to another atom or chemical species...
; sometimes EO and ER are used to represent oxidations and reductions respectively. The C represents a chemical reaction which can be any elementary reaction step and is often called a "following" reaction. In coordination chemistry common C steps which "follow" electron transfer are ligand
Ligand
In coordination chemistry, a ligand is an ion or molecule that binds to a central metal atom to form a coordination complex. The bonding between metal and ligand generally involves formal donation of one or more of the ligand's electron pairs. The nature of metal-ligand bonding can range from...
loss and association. The ligand loss or gain is associated with a geometric change in the complexes coordination sphere
Coordination sphere
In coordination chemistry, the coordination sphere refers to a central atom or ion and an array of molecules or anions, the ligands, around.Molecules that are attached noncovalently to the ligands are called the second coordination sphere....
.
- [MLn]2+ + e- → [MLn]+ E
- [MLn]+ → [ML(n-1)]+ + L C
The reaction above would be called an EC reaction.
Characterization
The production of [ML(n-1)]+ in the reaction above by the "following" chemical reaction produces a species directly at the electode that could display redox chemistry anywhere in a CV plot or none at all. The change in coordination from [MLn]+ to [ML(n-1)]+ often prevents the observation of "reversible" behavior during electrochemical experiments like cyclic voltammetry. On the forward scan the expected diffusion wave is observed, in example above the reduction of [MLn]2+ to [MLn]1+. However on the return scan the corresponding wave is not observed, in the example above this would be the wave corresponding to the oxidation of [MLn]1+ to [MLn]2+. In our example there is no [MLn]1+ to oxidize since it has been converted to [ML(n-1)]+ through ligand loss. The return wave can sometimes be observed by increasing the scan rates so the following chemical reaction can be observed before the chemical reaction takes place. This often requires the use of ultramicroelectrodeUltramicroelectrode
An Ultramicroelectrode is a working electrode used in a three electrode system. The small size of UME give them relatively large diffusion layers and small overall currents. These features allow UME to achieve useful steady-state conditions and very high scan rates with limited distortion...
s (UME) capable of very high scan rates of 0.5 to 5.0 V/s. Plots of forward and reverse peak ratios against modified forms of the scan rate often identify the rate of the chemical reaction. It has become a common practice to model such plots with electrochemical simulations. The results of such studies are suspect since simulation require excellent experimental data, better than that routinely obtained and reported. Further more the parameters of such studies are rarely reported and often include an unreasonably high variable to data ratio. A better practice is to look for a simple well documented relationship between observed results and implied phenomenon; or to investigate the phenomenon through an alternate technique such as chronoamperometry
Chronoamperometry
Chronoamperometry is an electrochemical technique in which the potential of the working electrode is stepped and the resulting current from faradic processes occurring at the electrode is monitored as a function of time...
or those involving a rotating electrode.
Electrocatalysis
ElectrocatalysisElectrocatalyst
An electrocatalyst is a catalyst that participates in electrochemical reactions. Catalyst materials modify and increase the rate of chemical reactions without being consumed in the process. Electrocatalysts are a specific form of catalysts that function at electrode surfaces or may be the...
is a catalytic process involving oxidation or reduction through the direct transfer of electrons. The electrochemical mechanisms of electrocatalytic processes are a common research subject for various fields of chemistry and associated sciences. This is important to the development of water oxidation and fuel cells catalysts. For example half the water oxidation reaction is the reduction of protons to hydrogen, the subsequent half reaction.
- 2H+ + 2e- → H2
This reaction requires some form of catalyst to avoid a large overpotential
Overpotential
Overpotential is an electrochemical term which refers to the potential difference between a half-reaction's thermodynamically determined reduction potential and the potential at which the redox event is experimentally observed. The term is directly related to a cell's voltage efficiency...
in the delivery of electrons. A catalyst can accomplish this reaction through different reaction pathways, two examples are listed below for the homogeneous catalysts [MLn]2+.
- Pathway 1
- [MLn]2+ + e- → [MLn]+ E
- [MLn]+ + H+ → [HMLn]2+ C
- [HMLn]2+ + e- → [HMLn]+ E
- [HMLn]+ + H+ → [H2MLn]2+ C
- [H2MLn]2+ → [MLn]2+ + H2 C
- Pathway 2
- [MLn]2+ + e- → [MLn]+ E
- [MLn]+ + H+ → [HMLn]2+ C
- 2[HMLn]2+ → [MLn]2+ + H2 C
Pathway 1 is described as a ECECC while pathway 2 would be described as an ECC. If the catalyst was being considered for solid support pathway 1 which requires a single metal center to function would be a viable candidate. In contrast a solid support system which separates the individual metal centers would render a catalysts that operates through pathway 2 useless since it requires a step which is second order in metal center. Determining the reaction mechanism is much like other methods with some techniques unique to elctrochemistry. In most cases electron transfer can be assumed to be much faster than the chemical reactions. Unlike stoichiometric reactions where the steps between the starting materials and the rate limiting step dominate in catalysis
Catalysis
Catalysis is the change in rate of a chemical reaction due to the participation of a substance called a catalyst. Unlike other reagents that participate in the chemical reaction, a catalyst is not consumed by the reaction itself. A catalyst may participate in multiple chemical transformations....
the observed reaction order is usually dominated by the steps between the catalytic resting state and the rate limiting step.
"Following" physical transformations
During potential variant experiments common to go through a redox couple in which the major species is transformed from a species that is soluble in the solution to one that is insoluble. This results in nucleation process in which a new species plates out on the working electrodeWorking electrode
The working electrode is the electrode in an electrochemical system on which the reaction of interest is occurring. The working electrode is often used in conjunction with an auxiliary electrode, and a reference electrode in a three electrode system...
. If a species has been deposited on the electrode during a potential sweep then on the return return sweep a stripping wave is usually observed.
- [MLn]+(solvated) + e- → [MLn]0(solid) nucleation
- [MLn]0(solid) → e- + [MLn]+(solvated) stripping
While the nucleation wave may be pronounced or difficult the detect the stripping wave is usually very distinct. Often these phenomenon can be avoided by reducing the concentration of the complex in solution. Neither these physical state changes involve a chemical reaction mechanism but they are worth mentioning here since the resulting data is at times confused with some chemical reaction mechanisms.