Electrocatalysis correlations

More than a decade ago, Hamond and Winograd used XPS for the study of UPD Ag and Cu on polycrystalline platinum electrodes [11,12]. This study revealed a clear correlation between the amount of UPD metal on the electrode surface after emersion and in the electrolyte under controlled potential before emersion. Thereby, it was demonstrated that ex situ measurements on electrode surfaces provide relevant information about the electrochemical interface, (see Section 2.7). In view of the importance of UPD for electrocatalysis and metal deposition [132,133], knowledge of the oxidation state of the adatom in terms of chemical shifts, of the influence of the adatom on local work functions and knowledge of the distribution of electronic states in the valence band is highly desirable. The results of XPS and UPS studies on UPD metal layers will be discussed in the following chapter. Finally the poisoning effect of UPD on the H2 evolution reaction will be briefly mentioned. 

Moreover, the conductivity, and hence the catalytic decomposition of hydrogen peroxide, has been observed to influence the stability of the oxygen electrode. The stability of phthalocyanine catalysts is a decisive factor for the practical applicability of organic catalysts in fuel cells operating in an acid medium. This is therefore a very important observation. The observed disturbance of the delocalization of the n electrons (rubiconjugation) in Fe-polyphthalocyanines, in addition to the correlation between conductivity on the one hand, and electrocatalysis and catalytic decomposition of hydrogen peroxide on the other, leads to a special model of the electroreduction of oxygen on phthalocyanines. The model... 

If correlations do exist for simple metals, predictions are much more difficult for composite materials. On the other hand, cathode activation has two aims (i) to replace active but expensive materials with cheaper ones, and (ii) to enhance the activity of cheaper materials so as to approach or even surpass that of the more expensive catalysts. In the case of pure metals there is little hope to find a new material satisfying the above requirements since in the volcano curve each metal has a fixed position which cannot be changed. Therefore, activation of pure metals can only be achieved by modifying its structure so as to enhance the surface area (which has nothing to do with electrocatalysis in a strict sense), and possibly to influence the mechanism and the energetic state of the intermediate in the wanted direction. This includes the preparation of rough surfaces but also of dispersed catalysts. Examples will be discussed later. 

Much research effort has been directed at understanding the mechanism of electrocatalysis. Various empirical attempts have been made to correlate the reaction rate with other quantities. Phe-... 

High-temperature superconducting oxides are of interest for electrocatalysis since they represent materials that are comprehensively characterized by various physical methods. They therefore hold promise for obtaining new correlative relationships between catalytic activity and the bulk properties of materials. 

The activity, stability, and tolerance of supported platinum-based anode and cathode electrocatalysts in PEM fuel cells clearly depend on a large number of parameters including particle-size distribution, morphology, composition, operating potential, and temperature. Combining what is known of the surface chemical reactivity of reactants, products, and intermediates at well-characterized surfaces with studies correlating electrochemical behavior of simple and modified platinum and platinum alloy surfaces can lead to a better understanding of the electrocatalysis. Steps, defects, and alloyed components clearly influence reactivity at both gas-solid and gas-liquid interfaces and will understandably influence the electrocatalytic activity. 

Electrocatalysts for cathodic hydrogen evolution or its oxidation and catalysts for chemical hydrogenation are essentially the same platinum and the transition metals of group 10 of the periodic table. Hence, for catalysis and electrocatalysis the same correlation of catal5dic activity in terms of exchange current density (mA/cm ) and... 

Efforts to understand the catalytic activities of metals have a long history. Much of the early work was phenomenological, and correlations were established between the reaction rate and various properties such as work functions, hydrogen adsorption energies, and unfilled d orbitals [60-63]. All these attempts had limited success and contributed little to our understanding of electrocatalysis. 

Chapters 7-12 focus on the electrocatalysis of carbon-based non-precious metal catalysts. The unique properties and fuel cell applications of various carbon based catalysts are intensively discussed in these chapters. Chapter 7 summarizes the fundamental studies on the electrocatalytic properties of metallomacrocyclic and other non-macrocyclic complexes. Chapter 8 and 9 review the progress made in the past 5 years of pyrolyzed carbon-supported nitrogen-coordinated transition metal complexes. Chapter 10 gives a comprehensive discussion on the role of transitional metals in the ORR electrocatalysts in acidic medium. Chapter 11 introduces modeling tools such as density functional theory (DPT) and ah initio molecular dynamics (AIMD) simulation for chemical reaction studies. It also presents a theoretical point of view of the ORR mechanisms on Pt-based catalysts, non-Pt metal catalysts, and non-precious metal catalysts. Chapter 12 presents an overview on recent progresses in the development of carbon-based metal-free ORR electrocatalysts, as well as the correlation between catalyst structure and their activities. 

The reduction of the above A -hetero-cyclic nitro compounds on Au modified by ad-atoms may by used as model reactions for studying the surface geometric and electronic structure in electrocatalysis. By using Au, Ag, or Pt single-crystal electrodes, it could be possible to correlate the catalytic activity with the superlattice structures of the ad-atoms. 

In this chapter, we will describe first the main methods utilized to fabricate technical electrodes, because it is well known that they can influence their electrochemical behavior. Next, we will give some insights on the electrocatalysis and mechanistics aspects of HOR, necessary to understand properly the studies made with technical electrodes, which we will discuss later. Finally, we will try to correlate the electrochemical behavior of model electrodes with those shown for technical electrodes on the basis of their chemical composition. 

The successful development of volcano plots is intimately tied both to the identification of appropriate descriptors and to the relation of these descriptors to the catalytic properties of interest. Many approaches, of varying levels of complexity, are available to identify suitable descriptors and to incorporate these into volcano plots. Some classes of volcanoes have been developed in an empirical or semiempirical manner, by essentially correlating experimental reaction rates with experimentally or computationally determined values of catalytic parameters. An early example of a purely empirical volcano, in the field of electrocatalysis, was... 

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