Electrocatalytic trends

The aim of this chapter is to review our understanding of the fundamental processes that yield improved electrocatalytic properties of bimetallic systems. Three classes of bimetallic systems will be discussed bulk alloys, surface alloys, and overlayer(s) of one metal deposited on the surface of another. First, we describe PtjM (M=Ni, Co, Fe, Cr, V, and Ti) bulk alloys, where a detailed and rather complete analysis of surface structure and composition has been determined by ex situ and in situ surface-sensitive probes. Central to our approach to establish chemisorption and electrocatalytic trends on well-characterized surfaces are concepts of surface segregation, relaxation, and reconstruction of near-surface atoms. For the discussion on surface alloys, the emphasis is on Pd-Au, a system that highlights the importance of surface segregation in controlling surface composition and surface activity. For exploring adsorption and catalytic properties of submonolayer and overlayer structures of one metal on the surface of another, we summarize the results for Pd thin metal films deposited on Pt single-crystal surfaces. For all three systems, we discuss electrocatalytic reactions related to the development of materials... 

The examples snmmarized in the chapter were chosen to demonstrate the emergence of in sitn electrochemical snrface science and its parallels with traditional UHV-based snrface science. Even though we emphasize a strong link between metal surface phenomena in vacuum and electrochemical environments, there are substantial differences between these two environments the presence of spectator species from snpporting electrolyte on electrodes, even in the absence of the fuel, sets the electrochemical interface apart from the same interface in UHV environments. This phenomenon drives the kinetics of electrochemical reactions by controlling the number of active sites. Our examples reveal the surface science of electrocatalysis on bimetallic surfaces is still in its infancy, but we can recognize electrocatalytic trends that form the basis for the predictive ability to tailor active sites with desirable reactivity. 

BUzanac BB, Stamenkovic V, Markovic NM (2007) Electrocatalytic trends on IB group metals the oxygen reduction reaction. Z Phys Chem 221 1379... 

Once we have developed our basic model and shown how it may be used to estab-hsh trends in electrochemical reactivity, we will take the further step of applying it to the identification of new bimetallic electrocatalysts. We will introduce simple procedures to rapidly screen bimetallic alloys for promising electrocatalytic properties, and we will demonstrate the importance of including estimates of the alloys stabihty in the screening procedure. Finally, we will give examples of successful apphcation of this method to specific problems in the area of electrocatalyst development. 

A remarkable progress has been made in the last several years in electrocatalysis on single crystal surfaces. This parallels the progress in surface science and it has been partly stimulated by developments in that field, mostly regarding the preparation and characterization of surfaces. New advances in preparation of surfaces outside of high vacuum, achieved in electrocatalytic studies, also helped this trend. 

Only the first deprotonation product of the cationic nucleobase radical needed to be considered to obtain the appropriate trends in potential, which were verified by testing whether electrocatalytic oxidation was observed with metal complexes of appropriate potential. 

One factor that may be important, but not systematically investigated, is the influence of the Pt electrocatalyst-support interactions on the electrocatalytic activity for O2 reduction. In Figure 14, an attempt to incorporate the pHzpc as a qualitative measure of the importance of carbon surface chemistry and metal-support interaction on the electrocatalytic activity of Pt is reported. The trend of the data in Figure 14 suggests that the specific activity for oxygen reduction increases as the pHzpc of the surface becomes more basic this effect may be related to the parallel increase of the particle size with the pHzpc of the catalyst. At this stage, one... 

Measurements performed with FePPIX as a function ofpH yielded similar trends to those found for FeTsPc, that is, (set shifted to more negative values as the solution became more alkaline. Plots of i]im as a function of the coverage of FePPIX, T Feppix> recorded at two different values of co (see Figure 3.49) were found to be close to linear for TFeppix < Tpeppix (about 0.7 nmol cm-2) and reached a plateau for TFePPix > r ppix, which indicates that only the outermost layer of the catalyst is electrocatalytically active. 

The main trends of the investigations are determined by the main factors influencing the electrochemical behavior of electrocatalytic systems. These factors and the corresponding trends are summarized in the following schemes ... 

The two fundamental trends (1 and 2) in the research are the subject of further subdivision. The role of the nature (the chemical composition) of the electrode (catalyst) in the electrocatalytic transformation is evident it follows from the very concept of catalysis. 

There should be increased study of bimetalUc surfaces to establish stmctural trends in surface behavior across the periodic table and the correlation between these trends and electrocatalytic reactivity. 

In a catalytic reaction, all steps do not equally depend on the surface structure. So, for example, the rate of simple desorption processes is often not markedly affected by the structure of the surface. In catalysis, therefore, reactions are classified into "structure sensitive" and "structure insensitive" [5], usually on the basis of the variation of reactivity with particle size. As an example, the electrocatalytic oxygen reduction at platinum (which is of importance for fuel cells) will be mentioned, where a decrease of specific activity with increasing particle size was reported [6,7]. In a theoretical analysis [8], the kinetics was treated on the (111), (10 0), and (211) facets of several transition metals, and the results were combined with simple models for the geometries of catalytic nanoparticles. Thus, the experimentally observed trend could be well reproduced. 

This trend is common for electrocatalytic reactions involving MPc complexes where CoPc, FePc, and MnPc show better catalytic activities than the other MPc complexes. 

Even though some progress has been made towards understanding electrocatalytic process and screening electrocatalysts from DFT, the method has difficulty in providing quantitative numbers for detailed reaction steps. On one hand, methodological improvements are required to describe the electron transfer at solid-liquid interface, the band structure, and the excited states effectively, which is currently limitation of DFT. On another hand, the model systems in DFT studies are somewhat too simplified to model the real catalysts effectively. For instance, the real catalysts are powders, which may behave differently with size. Recently, efforts have been made to model the nanoparticles with the size of the real catalysts (<5 nm), showing indeed different behaviors from the extended surfaces even in term of trend (Fig. 3), a common model used in DFT studies [24, 25]. Thus, theoretical... 

Treglazov 1, Leonova L, Dobrovolsky Y, Ryabov A, Vakulenko A, Vassiliev S (2005) Electrocatalytic effects in gas sensors based on low-temperature superprotonics. Sens Actuators B 106 164-169 TuUer HL, Moon PK (1988) Fast ion conductors future trends. Mater Sci Eng B 1 171-191... 

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