Electrocatalysis energies

The rates of C2H4, C2H6 and C02 formation depend exponentially on UWr and O according to equation (4.49) with a values of 1.0, 0.75 and 0.4, respectively, for I>0, and of 0.15, 0.08 and 0.3, respectively for I<0. Linear decreases in activation energy with increasing have been found for all three reactions.54 It should be emphasized, however, that, due to the high operating temperatures, A is near unity and electrocatalysis, rather than NEMCA, plays the dominant role. 

In the electron transfer theories discussed so far, the metal has been treated as a structureless donor or acceptor of electrons—its electronic structure has not been considered. Mathematically, this view is expressed in the wide band approximation, in which A is considered as independent of the electronic energy e. For the. sp-metals, which near the Fermi level have just a wide, stmctureless band composed of. s- and p-states, this approximation is justified. However, these metals are generally bad catalysts for example, the hydrogen oxidation reaction proceeds very slowly on all. sp-metals, but rapidly on transition metals such as platinum and palladium [Trasatti, 1977]. Therefore, a theory of electrocatalysis must abandon the wide band approximation, and take account of the details of the electronic structure of the metal near the Fermi level [Santos and Schmickler, 2007a, b, c Santos and Schmickler, 2006]. 

Medvedev IG. 2004. To a theory of electrocatalysis for the hydrogen evolution reaction The hydrogen chemisorption energy on the transition metal alloys within the Anderson-Newns model. Russ J Electrochem 40 1123-1131. 

This last equation contains the two essential activation terms met in electrocatalysis an exponential function of the electrode potential E and an exponential function of the chemical activation energy AGj (defined as the activation energy at the standard equilibrium potential). By modifying the nature and structure of the electrode material (the catalyst), one may decrease AGq, thus increasing jo, as a result of the catalytic properties of the electrode. This leads to an increase in the reaction rate j. 

Stonehart P. 1994. The role of electrocatalysis in solid polymer electrolyte fuel cells. In Drake JAG, editor. Electrochemistry and Clean Energy. Cambridge The Royal Society of Chemistry. 

Enzymes are efficient catalysts for cathodic and anodic reactions relevant to fuel cell electrocatalysis in terms of overpotential, active site activity, and substrate/reaction specificity. This means that design constraints (e.g., fuel containment and anode-cathode separation) are relaxed, and very simple devices that may take up ambient fuel or oxidant from their environment are possible. While operation is generally confined to conditions close to ambient temperature, pressure, and pH, and power densities over about 10 mW cm are rarely achieved, enzyme fuel cells may be particularly useM in niche environments, for example scavenging trace H2 released into air, or sugar and O2 from blood. Thus, trace or unusual fuels become viable for energy production. 

Self-sustainable production of hydrogen, chemicals, and energy from renewable alcohols by electrocatalysis. ChemSusChem, 3 (7), 851-855. 

In the short to medium term, renewable fossil based energy will remain important. C02 sequestration and the consequential importance of hydrogen imply a large interest in hydrogen production technologies, its storage and subsequent conversion. Electrocatalysis may be expected to be of increasing importance. 

For the longer term, there is the solar energy conversion challenge. Once solar radiation is efficiently captured it will be stored in the form of hydrogen or electricity, with major challenges again for electrocatalysis. 

In recent years, electrochemical charge transfer processes have received considerable theoretical attention at the quantum mechanical level. These quantal treatments are pivotal in understanding underlying processes of technological importance, such as electrode kinetics, electrocatalysis, corrosion, energy transduction, solar energy conversion, and electron transfer in biological systems. 

Several important energy-related applications, including hydrogen production, fuel cells, and CO2 reduction, have thrust electrocatalysis into the forefront of catalysis research recently. Electrocatalysis involves several physiochemical environmental dfects, which poses substantial challenges for the theoreticians. First, there is the electric potential which can aifect the thermodynamics of the system and the kinetics of the electron transfer reactions. The electrolyte, which is usually aqueous, contains water and ions that can interact directly with a surface and charged/polar adsorbates, and indirectly with the charge in the electrode to form the electrochemical double layer, which sets up an electric field at the interface that further affects interfacial reactivity. 

The concept of electrocatalysis and its relation to chemical surface bonding of reactive intermediates is closely related to that of heterogeneous catalysis. Following the previous section, simple Gibbs energy curves can illustrate the essential ideas of how adsorption of intermediates and their associated Gibbs energy affect the rate of an inner-sphere reaction. 

The development of a consistent theory for a dissociative electron transfer is a recent challenge in the field of theoretical electrocatalysis. Progress in this field of electrochemistry has involved the use of an harmonic Morse curves [25] instead of harmonic approximations. Applying the principles of the theory of the activated complex to adiabatic dissociative electron transfer reactions, the work of Saveant resulted in the following expressions [24] for the Gibbs energy of activation... 

Bond-breaking reaction, 1518 potential energy curves, 1519 bond activation, 1519 George-Griffith model, 1519 Wiss—Marcus model, 1519 Bond strength, in electrocatalysis, 1287 Boulders, electrodeposition, 1336 Bowden, 1402... 

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