4 Slides About: Electrocatalysis

Slides:



Advertisements
Similar presentations
4 Slides About: Electrocatalysis Created by Matt Whited, Jillian Dempsey, Mike Norris, Abby O'Connor, and Anne Ryter, and posted on VIPEr on July 19, 2012,
Advertisements

Modeling in Electrochemical Engineering
Lecture 6a Cyclic Voltammetry.
Evans Diagrams.
Chapter 4 Electrochemical kinetics at electrode / solution interface and electrochemical overpotential.
Cyclic voltammetry: a fingerprint of electrochemically active species
Introduction to Electroanalytical Chemistry
Electrochemistry Why are we doing this experiment?
Introduction to electrochemistry - Basics of all techniques -
VOLTAMMETRY A.) Comparison of Voltammetry to Other Electrochemical Methods 1.) Voltammetry: electrochemical method in which information about an analyte.
Controlled potential microelectrode techniques—potential sweep methods
Electrochemistry for Engineers LECTURE 4 Lecturer: Dr. Brian Rosen Office: 128 Wolfson Office Hours: Sun 16:00.
INTRODUCTION TO ELECTROCHEMICAL CELLS AND BASIC ELECTROANALYTICAL MEASUREMENTS ANDREA MARDEGAN JAN 17th 2013.
Experimental techniques Linear-sweep voltammetry At low potential value, the cathodic current is due to the migration of ions in the solution. The cathodic.
Chapter 19 Electrochemistry
Voltaic Cells Chapter 20.
Cyclic Voltammetry for the Detection of Dopamine in vivo
Fundamentals of Electrochemistry It’s shocking!. Electroanalytical Chemistry: group of analytical methods based upon electrical properties of analytes.
Midterm Exam 1: Feb. 2, 1:00- 2:10 PM at Toldo building, Room 100.
Summer Course on Exergy and Its Applications EXERGY ANALYSIS of FUEL CELLS C. Ozgur Colpan July 2-4, 2012 Osmaniye Korkut Ata Üniversitesi.
§7.11 Polarization of electrode
16-1 Voltammetry Electrochemistry techniques based on current (i) measurement as function of voltage (E appl ) Working electrode §(microelectrode) place.
Electrochemistry Introduction
Electrochemical Cell An electrochemical cell uses chemistry to produce electricity.
Principles of Reactivity: Electron Transfer Reactions Chapter 20.
ELECTROCHEMICAL CELLS
1 Chapter Eighteen Electrochemistry. 2 Electrochemical reactions are oxidation-reduction reactions. The two parts of the reaction are physically separated.
Electrochemical Methods Dr M.Afroz Bakht. Potentiometry Potentiometry is a method of analysis used in the determination of concentration of ions or substances.
Chapter 26 – Electricity from Chemical Reactions.
CHAPTER 11 ELEMENTS OF ELECTROCHEMISTRY Introduction to Analytical Chemistry.
Influence of product adsorption on catalytic reaction determined by Michaelis-Menten kinetics Šebojka Komorsky-Lovrić and Milivoj Lovrić Department of.
25.10 Voltammetry Voltammetry: the current is monitored as the potential of the electrode is changed. Chronopotentiometry: the potential is monitored as.
Chapter 4 Electrochemical kinetics at electrode / solution interface and electrochemical overpotential.
Fuel Cells. What is a Fuel Cell? Quite simply, a fuel cell is a device that converts chemical energy into electrical energy, water, and heat through electrochemical.
Best seen broken into four categories Part 1: Ion Selective Electrodes Part 3: Step Voltammetry Part 2: Amperometric Sensors Part 4: Cyclic Voltammetry.
ELECTROCHEMICAL CELLS. ELECTROCHEMISTRY The reason Redox reactions are so important is because they involve an exchange of electrons If we can find a.
Consider the reduction half reaction: M z+ + ze → M The Nernst equation is E = E ө + (RT/zF) ln(a) When using a large excess of support electrolyte, the.
Lecture 5:Enzymes Ahmad Razali Ishak
Chemical Kinetics. The branch of Physical chemistry which deals with the rate of reactions is called chemical kinetics. The study of chemical kinetics.
In voltaic cells, oxidation takes place at the anode, yielding electrons that flow to the cathode, where reduction occurs. Section 1: Voltaic Cells K What.
Bulk Electrolysis: Electrogravimetry and Coulometry
Electrochemical Methods: Intro Electrochemistry Basics Electrochemical Cells The Nernst Equation Activity Reference Electrodes (S.H.E) Standard Potentials.
Introduction to Electroanalytical Chemistry
Voltammetry and Polarography
Lecture 7a Cyclic Voltammetry.
Electrochemistry: Introduction Electrochemistry at your finger tips
Electrochemistry: Introduction Electrochemistry at your finger tips
Energy Flow in the Life of a Cell
Energy and Life Ch. 5.
An electrochemical cell uses chemistry to produce electricity.
Collision Theory Rates of reactions.
Probing electron transfer mechanisms
Electrochemistry.
Chapter 6 Chemical kinetics.
Figure A hemispherical working electrode in cross section
Complex Anode Kinetics Chronocoulometry Evidence
Electrochemistry Oxidation-Reduction
Kinesthetic Learning: Cyclic Voltammetry Mechanisms
The Role of Catalysis in Next Generation Direct Hydrocarbon Solid Oxide Fuel Cell Anodes Steven McIntosh, Department of Chemical Engineering, University.
2.4. Chronoamperometry measurement of currents as a function of time a kind of ‘controlled-potential voltammetry’ or ‘controlled-potential micro electrolysis.
towards more negative values towards more positive values Second-order irreversible chemical reaction following a reversible electron transfer:
A Chemical Reaction Interposed Between Two Electron Transfers ECE the number of electrons exchanged in the two electron transfers; n2/n1 the.
Voltametric techniques Chapter 2 Prof. Rezvani.
2. Electrochemical techniques complementary to cyclic voltammetry.
Cyclic Voltammetry Dr. A. N. Paul Angelo Associate Professor,
Electrochemical Cells
Standard Electrode Potentials
Chapter 15: Chemical Kinetics
Presentation transcript:

4 Slides About: Electrocatalysis Created by Matt Whited, Jillian Dempsey, Mike Norris, Abby O'Connor, and Anne Ryter, and posted on VIPEr on July 19, 2012, Copyright 2012. This work is licensed under the Creative Commons Attribution-NonCommercial-ShareAlike License. To view a copy of this license visit http://creativecommons.org/about/license/

Cyclic Voltammetry Refresher A complex that can be reversibly oxidized/reduced shows symmetric “duck-shaped” curves for reduction (potential decreasing) and oxidation (potential increasing) Potential Co(II)→ Co(I) Time Cyclic voltammetry is a useful method for probing the electrochemical properties of molecules (esp. inorganic ones with readily accessible redox couples). The basic experiment involves a linear ramp of potential versus time like linear sweep voltammetry, with the distinction that you move back and forth between two potentials (see potential vs. time plot). A variety of scan rates can be used, corresponding to a different slope for the lines in the potential vs. time plot. The readout shows current flowing vs. applied potential (versus a reference, SCE in this case). When an electron transfer process occurs (reduction of the metal complex at the cathode or oxidation at the anode), current flows and a bump in the plot is observed. Note that the duck-like shape of the plot (where the peak cathodic current and peak cathodic current are not observed at the same potential) is due to mass transport issues at the electrode. For a nice reversible one-electron transfer (like the Co(II)/Co(I) couple here), symmetric reduction and oxidation waves are observed. CV involves a linear potential sweep and provides a reading of current versus applied potential Co(II)←Co(I) Figure adapted from Valdez et al., PNAS, DOI:10.1073/pnas.1118329109

Electrochemical Reactions If a complex undergoes an irreversible chemical reaction after reduction or oxidation, then a return wave is not observed. This is known as an EC mechanism, since the Electrochemical step is followed by a Chemical reaction. E C Ex: LnCoIII–Cl + e– [LnCoII–Cl]– LnCoII + Cl– Co(III)→ Co(II) As we know, not all complexes are equally happy after being reduced or oxidized. Often, the reduced/oxidized species will undergo a chemical reaction at some rate. In this case, when a Co(III) chloride is reduced to a Co(II) chloride anion, the resulting complex is unstable with respect to chloride dissociation. Since the Co(II) complex undergoes a chemical reaction after reduction (an EC mechanism), the original Co(II) chloride anion is not around anymore when the sweep is reversed and so no current is measured for its oxidation. This leads to a “goose-” or “slug-type” shape. It is possible that oxidation of the chemically generated species would be observed at more positive potential (assuming it doesn’t all diffuse away from the electrode or react further), but that is not observed in this case and is beyond the scope of this LO. no Co(III)←Co(II) return wave

Electrocatalytic Reactions If an electrochemically oxidized or reduced complex undergoes a chemical reaction that regenerates the starting material, a catalytic response may be observed. Co(II)→ Co(I) Now let’s put the pieces together. If the electrochemically generated species (a reduced cobalt complex in this case) can undergo a reaction that regenerates the original complex, then we’ll see a catalytic response. The wave will grow much larger than any other redox events in the CV because the same complexes are being reduced over and over as they react with protons to generate hydrogen and regenerate the original Co(II) complex. In this figure, it is important to note that the measured current increases as the acid concentration increases. This is simple kinetics: protons are involved in the rate-limiting step of the reaction we observe, so an increase in proton concentration leads to a greater measured current due to increased rate of reaction. If we’re really sophisticated, then we can determine the order of the reaction in [H+], but that is also beyond the scope of this LO. Since protons are involved in the reaction, the electrochemical response (proportional to reaction rate) increases as the acid concentration increases

Electrocatalysis and Overpotential Electrocatalysts will normally operate at the same potential where they are reduced or oxidized, but this may overshoot the required thermodynamic potential for the reaction: Thermodynamic potential for reduction of TsOH in acetonitrile: –0.24 V vs. SCE only 40 mV overpotential!! As you may have noted in the previous slide, catalysis occurs at the same place where Co(II) is reduced to Co(I). This is no coincidence, since electron transfer sets off the catalytic cycle. However, this means that catalysis will occur at a potential that is defined by the catalyst and not by the thermodynamics of proton reduction. The “overpotential” basically describes the extra applied potential required to pull off the reaction relative to the thermodynamic requirements. Any extra applied potential is generally lost as heat. For this series of catalysts, the tetraphenyl version looks great in terms of overpotential (40 mV is quite small), but note that the rate constant stinks. This is also no coincidence, as the sentence at the bottom of the slide indicates. Normally, across a series of catalysts that operate by the same mechanism, the ones with greater overpotentials will react much faster because they end up with a ton of excess energy at their disposal for carrying out catalysis. Note that catalysts with smaller overpotential typically operate less efficiently!! This is a common problem for a set of electrocatalysts operating by the same mechanism.