Electrocatalytic reaction processes

In this section, a number of electrocatalytic processes will be discussed where surface chemical bonding plays a central role in the reaction mechanism. The selection of reactions is far from complete and not representative of the wide range of technologically important electrocatalytic processes. The selection is biased towards the areas of electrochemical energy conversion and fuel cell electrochemistry, which have been catalyzing a renewed interest in the field of electrochemistry. 

Unlike classical electroanalytical chemistry, recent work on the selected reactions has been focusing on the surface electrochemistry aspects, that is, the molecular basis of the interfacial processes. Of chief interest is insight in the molecular mechanisms as well as the nature, formation conditions, and reactivity of the surface intermediates. The development and application of in-situ surface-sensitive spectroscopic methods, especially using synchrotron radiation, has been aiding this objective. 

In addition to more sophisticated experimental methods, modem theoretical-computational tools, such as state-of-the-art DFT calculations, have greatly improved our understanding of basic electrocatalytic processes. Ever growing computational resources and more and more sophisticated algorithms raise computational methods from the role of a complementary tool to experiments to a viable alternative to experiments. 

With a basic mechanism at hand, a rational approach for designing catalysts with desired properties becomes possible. However, despite progress in the direct observation of surface intermediates using high pressure, realistic in situ spectroscopic methods and deeper insight into basic reaction processes, the capability of rationally designing an electrocatalytic surface with a set of desired properties has not yet fully been achieved. 

The oxygen/water half-cell reaction has been one of the most challenging electrode systems for decades. Despite enormous research, the detailed reaction mechanism of this complex multi-step process has remained elusive. Also elusive has been an electrode material and surface that significantly reduces the rate-determining kinetic activation barriers, and hence shows improvements in the catalytic activity compared to that of the single-noble-metal electrodes such as Pt or Au. 

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More complicated mechanisms of the same category are encountered in SrnI reactions (Section 2.5.6) where the electrocatalytic reaction, which corresponds to a zero-electron stoichiometry, is opposed to two-electron consuming side reactions (termination step in the chain process). 

In this chapter, we will first discuss thermodynamic and kinetic concepts of electrified interfaces and point out some distinct features of electrochemical reaction processes. Subsequently, we will relate these concepts to chemical bonding of adsorbates on electrode surfaces. Finally, a discussion of the surface electrocatalytic mechanism of some important technological electrochemical reactions will highlight the importance of understanding chemical bonding at electrified surfaces. 

The general electrochemical behavior of surface-bound molecules is treated in Sect. 6.4. The response of a simple electron transfer reaction in Multipulse Chronoamperometry and Chronocoulometry, CSCV, CV, and Cyclic Staircase Voltcoulometry and Cyclic Voltcoulometry is also presented. Multielectronic processes and first- and second-order electrocatalytic reactions at modified electrodes are also discussed extensively. 

An alternative application of cobalt Mb has been reported by Willner and coworkers (92). They immobilized the reconstituted cobalt Mb on the functionalized electrodes and generated cobalt(I)-Mb by the electrochemical reduction of cobalt(II)-Mb. The hydrogenation of acetylenedicarboxylic acid smoothly occurred on the functionalized electrode, and the electrocatalytic reaction in H20 and D20 reveals a clear isotope effect, kH/kD = 2.7, indicating that the hydride transfer from cobalt(III)-hydride in Mb is the key reactive process. 

When an electrocatalytic reaction involves a primary step of molecular dissociative chemisorption, for example, a c,e mechanism, then the electrocatalysis arises more directly, in the same way as for many regular catalytic processes that involve such a step of dissociative chemisorption. In this type of electrocatalytic reaction, the dissociated adsorbed fragments, for example, adsorbed H in H2 oxidation, become electrochemically ionized or oxidized in one or more charge-transfer steps following the initial dissociation. The rate... 

The involvement of chemisorbed intermediates in electrocatalytic reactions is manifested in various and complementary ways which may be summarized as follows (i) in the value of the Tafel slope dK/d In i related to the mechanism of the reaction and the rate-determining step (ii) in the value of reaction order of the process (iii) in the pseudocapacitance behavior of the electrode interface (see below), for a given reaction (iv) in the frequency-response behavior in ac impedance spectroscopy (see below) (v) in the response of the reaction to pulse and linear perturbations or in its spontaneous relaxation after polarization (see below) (vi) in certain suitable cases, also to the optical reflectivity behavior, for example, in reflection IR spectroscopy or ellipso-metry (applicable only for processes or conditions where bubble formation is avoided). It should be emphasized that, for any full mechanistic understanding of an electrode process, a number of the above factors should be evaluated complementarily, especially (i), (ii), and (iii) with determination, from (iii), whether the steady-state coverage by the kinetically involved intermediate is small or large. Unfortunately, in many mechanistic works in the literature, the required complementary information has not usually been evaluated, especially (iii) with 6(V) information, so conclusions remained ambiguous. 

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... 

Catalysis and Electrocatalysis at Nanoparticle Surfaces reflects many of the new developments of catalysis, surface science, and electrochemistry. The first three chapters indicate the sophistication of the theory in simulating catalytic processes that occur at the solid-liquid and solid-gas interface in the presence of external potential. The first chapter, by Koper and colleagues, discusses the theory of modeling of catalytic and electrocatalytic reactions. This is followed by studies of simulations of reaction kinetics on nanometer-sized supported catalytic particles by Zhdanov and Kasemo. The final theoretical chapter, by Pacchioni and Illas, deals with the electronic structure and chemisorption properties of supported metal clusters. 

One major complication that distinguishes electrocatalytic reactions from catalytic reactions at metal-gas or metal-vacuum interfaces is the influence of the solvent. Modeling the role of the solvent in electrode reactions essentially started with the pioneering work of Marcus [68]. Originally these theories were formulated to describe relatively simple electron-transfer reactions, but more recently also ion-transfer reactions and bond-breaking reactions have been incorporated [69-71]. Moreover, extensive molecular dynamics simulations have been carried out to obtain a more molecular picture of the role of the solvent in charge-transfer processes, either in solution or at metal-solution interfaces. 

Figure 13. (a) Formation of mixed SAM on Au electrode, (b) immobilization of Fc-D, (c) immobilization of thiolated capture probe with bifunctional linker (succinimidyl 4-(/V-maleimidomethyl)cyclohexane-l-carboxylate (SMCC)), (d) hybridization with target, (e) hybridization with biotinylated detection probe, (f) association with avidin-alkaline phosphatase, (g) description of the process of the electrocatalytic reaction of />-aminophenol (p-AP) via electronic mediation of ferrocenyl dendrimer (Adapted from Ref. [160])... 

For a Pt(lll) surface, with a surface density of 1.5 x 1015 atoms cm-2, the current density corresponding to a TOF of Is-1 is 0.18 mA cm-2 for a one-electron charge-transfer process. Such exchange current densities based on the real electrocatalyst surface area are quite typical for a decent electrocatalyst for H2 evolution or oxidation. Thus, one may conclude that the order of magnitude of the TOFs of catalytic and electrocatalytic reactions are quite similar. In the latter... 

The MCFC anodes are made from a porous sintered nickel with a thickness of 0.8-1.0 mm and a porosity of 55-70% with a mean pore diameter of 5pm. This porosity range provides adequate interconnected pores for mass transport of gaseous reactants and adequate surface area for the anodic electrocatalytic reactions. Because the anode kinetics is faster than that of the cathode, less active surface area is sufficient for the anodic process. Partial flooding of the comparatively thick anode is therefore acceptable at the anode interface. 

At the electrochemical interface, adsorption of either charged or neutral molecules and charge transfer processes may occur simultaneously. Electroadsorption and electrodesorption processes play a key role in electrocatalytic reactions [2],... 

For electrocatalytic reactions, the charge transfer process to or from the charged particles has to be considered. The formation of a reactant-surface site complex is involved together with a fast charge transfer that produce a product species with the re-formation of the active surface site. In most cases, the formation of the complex is rapid and the subsequent charge transfer process is slow. It is important to know whether the re-formation of the catalytic site is fast or slow. The pathway will depend on the height of the activation barriers for the individual steps and in most of the cases the latter is the rate-determining step. The rate expression for the case of an electrocatalytic reaction can, a priori, be... 

There are two types of problems in the analysis of electrocatalytic reactions with mixed control kinetics reactant adsorption and combined considerations of mass and charge transfer processes in the current vs. potential profiles. The dependence of the current density, j, with the overpotential, x, can be expressed under r values larger than 0.12 V (in absolute values) through the Tafel expression corrected by the mass transfer effects ... 

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