Hydrogenation electrocatalysis

Santos, E., Quaino, P., and Schmickler, W. 2012. Theory of electrocatalysis Hydrogen evolution and more. Phys. Chem. Chem. Phys. 14 11224-11233. 

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]. 

Santos E, Schmickler W. 2007b. Electrocatalysis of hydrogen oxidation—Theoretical foundations. Angew Chem Int Ed 46 8262-8265. 

Electrol3dic hydrogen evolution in acid solutions. J Electroanal Chem 39 163-184. Trasath S. 1977. Electrocatalysis of hydrogen evoluhon. Adv Elechochem Electrochem Eng 10 213. 

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. 

Ross PN, Kinoshita K, Scarpellino AJ, Stonehart P. 1975a. Electrocatalysis on binary alloys I. Oxidation of molecular hydrogen on supported Pt-Rh alloys. J Electroanal Chem 59 177-189. 

As the reader might have noticed, many conclusions in electrocatalysis are based on results obtained with electrochemical techniques. In situ characterization of nanoparticles with imaging and spectroscopic methods, which is performed in a number of laboratories, is invaluable for the understanding of PSEs. Identification of the types of adsorption sites on supported metal nanoparticles, as well as determination of the influence of particle size on the adsorption isotherms for oxygen, hydrogen, and anions, are required for further understanding of the fundamentals of electrocatalysis. 

Chen SL, Kucemak A. 2004b. Electrocatalysis under conditions of high mass transport investigation of hydrogen oxidation on single submicron Pt particles supported on carbon. J Phys ChemB 108 13984-13994. 

Adzic RR, Markovic NM. 1982. Structural effects in electrocatalysis Oxygen and hydrogen peroxide reduction on single crystal gold electrodes and the effects of lead ad-atoms. J Electroanal Chem 138 443-447. 

Anson FC, Ni CL, Saveant JM. 1985. Electrocatalysis at redox polymer electrodes with separation of the catalytic and charge propagation roles. Reduction of dioxygen to hydrogen peroxide as catalyzed by cobalt(II) tetrakis(4-A-methylpyridyl)porphyrin. J Am Chem Soc 107 3442. 

The basic characteristics of electrocatalysis will be demonstrated on several examples, in the first place on the electrode processes of hydrogen,... 

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

Identification of Peaks for Hydrogen Adsorption on the Disordered Low Index Planes. Besides the major objective for studying electrocatalysis on single crystal stepped surfaces mentioned above, these studies offer a wealth of information on the behaviour of polycrystalline surfaces, of preferentially oriented surfaces and, as we suggested recently (12), of disordered low-index surface. 

Fig. 18b.9. Example cychc voltammograms due to (a) multi-electron transfer redox reaction two-step reduction of methyl viologen MV2++e = MV++e = MV. (b) ferrocene confined as covalently attached surface-modified electroactive species—peaks show no diffusion tail, (c) follow-up chemical reaction A and C are electroactive, C is produced from B through irreversible chemical conversion of B, and (d) electrocatalysis of hydrogen peroxide decomposition by phosphomolybdic acid adsorbed on a graphite electrode.

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. 

DMFCs and direct ethanol fuel cells (DEFCs) are based on the proton exchange membrane fuel cell (PEM FC), where hydrogen is replaced by the alcohol, so that both the principles of the PEMFC and the direct alcohol fuel cell (DAFC), in which the alcohol reacts directly at the fuel cell anode without any reforming process, will be discussed in this chapter. Then, because of the low operating temperatures of these fuel cells working in an acidic environment (due to the protonic membrane), the activation of the alcohol oxidation by convenient catalysts (usually containing platinum) is still a severe problem, which will be discussed in the context of electrocatalysis. One way to overcome this problem is to use an alkaline membrane (conducting, e.g., by the hydroxyl anion, OH ), in which medium the kinetics of the electrochemical reactions involved are faster than in an acidic medium, and then to develop the solid alkaline membrane fuel cell (SAMFC). 

The theory of electrocatalysis is still in its infancy. It was developed first for the hydrogen evolution reaction in the second half of the 1900s. The grounds can be traced back in a seminal paper by Floriuti and Polanyi [25]. Accordingly, for a simple one-electron electrode reaction ... 

A complete theory of electrocatalysis leading to volcano curves has been developed only for the process of hydrogen evolution and can be found in a seminal paper by Parsons in 1958 [26]. The approach has shown that a volcano curve results irrespective of the nature of the rate-determining step, although the slope of the branches of the volcano may depend on the details of the reaction mechanism. 

Pt is, of course, not a good electrocatalyst for the O2 evolution reaction, although it is the best for the O2 reduction reaction. However, also with especially active oxides of extended surface area, the theoretical value of E° has never been observed. For this reason, the search for new or optimized materials is a scientific challenge but also an industrial need. A theoretical approach to O2 electrocatalysis can only be more empirical than in the case of hydrogen in view of the complexity of the mechanisms. However, a chemical concept that can be derived from scrutiny of the mechanisms mentioned above is that oxygen evolution on an oxide can be schematized as follows [59] ... 

Trasatti, S. (1990) Electrode kinetics and electrocatalysis of hydrogen and oxygen electrode reactions. 1. Introduction, in Electrochemical Hydrogen Technologies (ed. 

Trasatti, S. (1992) Electrocatalysis of hydrogen evolution progress in cathode activation, in Advances in Electrochemical Science and Engineering (eds H. Gerischer and C.W. Tobias), VCH Verlag GmbH, Weinheim. 

Wendt, H. and Plzak, V. (1990) Electrode kinetics and electrocatalysis of hydrogen and oxygen electrode reactions. 2. Electrocatalysis and electrocatalysts for cathodic evolution and anodic oxidation of hydrogen, in Electrochemical Hydrogen Technologies (ed. H. Wendt), Elsevier, Amsterdam, Chapter 1. 2. 

---