The Mechanism of Electrochemical Promotion

Electrochemical promotion of catalysis, similarly to usual (chemical) promotion and to metal-support interactions in heterogeneous catalysis, is related to spillover-backspillover phenomena. The latter can be described as the mobility of adsorbed species from one phase on which they easily adsorb (donor) to another phase where they do not directly adsorb (acceptor). By this mechanism a seemingly inert material can acquire catalytic activity. Spillover may lead to an improvement of catalytic activity or selectivity and also to an increase in lifetime of the catalyst. 

As a consequence of the occurrence of spillover in heterogeneous catalysis, the usual kinetic models can no longer be applied in a direct way. Generation of new surface sites or change in surface concentrations lead to new terms in the rate equations. A general 

Equation (7a) describes the dissociative adsorption and recombination of oxygen on a donor D. The transfer between the donor D and acceptor A is given by Eq. (7b). The spillover oxygen (0) is a mobile species present on the acceptor surface without being associated with a particular surface site. The mobile spillover species may interact with a particular surface site B (Eq. 7c) forming an active site C. Equation (7d) represents the deactivation of the active site C by interaction with a reactant R and the formation of product P. Kinetic models based on this mechanism are well supported by experiment.  

In an electrochemical promotion experiment a potential step is applied to the electrochemical cell. In the ideal case of a perfect reference electrode having invariant potential, the applied change in the ohmic drop free potential difference between the catalyst (working electrode) and the reference electrode, I wr. is equal to the change in the inner (Galvani) potential of the catalyst, (  

This difference is measurable. Changing the potential of the catalyst modifies its Fermi level, Ef, or in other terms, the electrochemical potential of the electrons in the catalyst, e ( = ) This latter is defined as the difference between the zero energy state of the electrons (taken at ground state at infinite distance from the solid) and the energy of a conduction electron in the bulk of the catalyst. It is common practice to count this energy difference in two conceptually different ways. One of them is common in electrochemistry, the other is common in surface science. 

Figute 5. Schematic representation of the mechanism of eleC trochemical promotion under anodic cunent application via bade spillover of charged promoting species (O.  

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Also the similarity between the remote control spillover mechanism of Fig. 3.5 and the mechanism of electrochemical promotion (NEMCA) already outlined in Figure 1.4c and thoroughly proven in Chapter 5, should be noted. In electrochemical promotion the solid electrolyte is the donor phase and the conductive catalyst is the acceptor phase, using Delmon s terminology. 

Figure 11.7. XPS confirmation of O5 backspillover as the mechanism of electrochemical promotion on Pt films deposited on YSZ (a) and on Ti02 (b). Adapted from refs.31,32. In both cases A is the open-circuit Ols spectrum, B is the 01 s spectrum under anodic (I>0, AUWr>0) polarization and C is the difference spectrum.22,31,32 Reprinted with permission from the American Chemical Society (a, ref. 32) and from Academic Press (b, 31).

Electrochemical Activation of Catalysis contains a very full and detailed treatment of the mechanisms of electrochemical promotion. It is likely to remain the standard work on this remarkable new technology for who other than the present authors will write a book with such a background of authority in the field ... 

In this Chapter, an overview is given of recent developments in this field with special attention to the investigation of the mechanism of electrochemical promotion and to the development of new bipolar cell configurations aiming at future industrial applications. 

Similar is the behaviour with Ag electrodes deposited on YSZ as shown in Figures 5.40 and 5.41.68 The oxygen ion backspillover mechanism of electrochemical promotion is confirmed quite conclusively. 

Similar observations confirming the reversible Na spillover-backspillover mechanism of electrochemical promotion have been made by Lambert and coworkers using AES (Auger Electron Spectroscopy).61... 

The good qualitative agreement between eUwR variation and O0 variation shown in Figure 11.11 for the various supports used, underlines again the common promotional mechanism of electrochemically promoted and metal-support interaction promoted metal catalysts. 

R. Imbihl, J. Janek The groups of Professors Imbihl and Janek have made important contributions in the use of PEEM, work function measurement and XPS (Chapter 5) to establish the O2 backspillover mechanism of electrochemical promotion under UHV conditions. 

It was only the use of larger metal particles deposited on these supports, up to 1 pm in size, comparable to the spatial resolution of XPS, that enabled researchers to understand the origin of electrochemical promotion and, at the same time, to discover the backspillover mechanism of metal-support interactions [137,138]. 

Although the use of TPD and cyclic voltammetry described above suffice to clarify the origin of electrochemical promotion with conductors, we survey here some of the key results obtained with work function measurements, AC impedance spectroscopy, XPS, PEEM, and STM, together with some additional results obtained from TPD in conjunction with rigorous quantum mechanical calculations. For experimental details, results with other techniques of Figure 14 and a more detailed analysis of the results surveyed here, the reader is referred to a recent book [14] and to the original papers. 

These results indicate that, at least under certain conditions, the mechanism of metal-support interactions may be identical to that of electrochemical promotion, i.e., the charge exchanged between the metal particles and the support material, caused by the difference in the electrochemical potential of the two solids, may be oxygen ions, as in the case of electrochemical promotion. This, of course, could be complementary, or in addition, to electrons or positive holes or other charge carriers, depending on the support material and gas-phase conditions. 

An alternative interpretation of the phenomenon of metal-support interactions induced by doping of semiconductive carriers with aliovalent cations is based on the theory of electrochemical promotion or the NEMCA effect. According to this interpretation, the charge carriers transported from the carrier to the metal particles are oxygen ions, which diffuse to the surface of the metal particles, thus altering the surface work function and, subsequently, chemisorptive and catalytic parameters. Work is currently in progress to elucidate the mechanism of induction of metal-support interactions by carrier doping. 

Figure 2 also shows this point At steady-state the rate, r< , of consumption of the promoting O species via reaction with C2H4, has to equal its rate of formation I/2F. Consequently, since A=Ar/(I/2F) and Ar=r, it follows A=r/r =TOF/TOF where TOF is the turnover frequency of the catalytic reaction in the NEMCA-promoted state and TOF is the turnover frequency of the reaction of the promoting oxygen species with ethylene. It thus follows for the experiment of Fig. 2 that TOFc=TOF/A=1.3xlO s. This implies that that average lifetime of the promoting species on the catalyst surface is TOF =770 s in excellent qualitative agreement with the catalytic rate relaxation time constant upon current interruption (Fig. 2). This observation provides strong support for the oxygen backspillover mechanism of electrochemical promotion.

The model of electrochemical promotion regards the phenomenon as catalysis in presence of an electrically controlled double layer formed by spillover-backspillover mechanism at the gas-exposed catalyst surface. This shows strong analogy with catalyst-support interactions... 

In this Chapter, the progress recently made in the field of electrochemical promotion (EP) of catalytic gas reactions is reviewed. The phenomenon consists of electrochemical polarization of metal or metal oxide electrodes interfaced with solid electrolytes which result in a pronounced increase in the catalytic reaction rate. The effect is also termed non-Faradaic electrochemical modification of catalytic activity (NEMCA effect), since the rate increase may exceed the ionic current by several orders of magnitude. The promotion is not limited to the electrochemically polarized interface between catalyst and solid electrolyte, but extends to the entire catalyst surface exposed to the reactive gas. In fact, one of the major challenges in the field of electrochemical promotion is to elucidate the exact mechanism by which the promoting effect propagates from one interface to the other. 

As shown in Fig. 24, the mechanism of the instability is elucidated as follows At the portion where dissolution is accidentally accelerated and is accompanied by an increase in the concentration of dissolved metal ions, pit formation proceeds. If the specific adsorption is strong, the electric potential at the OHP of the recessed part decreases. Because of the local equilibrium of reaction, the fluctuation of the electrochemical potential must be kept at zero. As a result, the concentration component of the fluctuation must increase to compensate for the decrease in the potential component. This means that local dissolution is promoted more at the recessed portion. Thus these processes form a kind of positive feedback cycle. After several cycles, pits develop on the surface macroscopically through initial fluctuations. 

The reader already familiar with some aspects of electrochemical promotion may want to jump directly to Chapters 4 and 5 which are the heart of this book. Chapter 4 epitomizes the phenomenology of NEMCA, Chapter 5 discusses its origin on the basis of a plethora of surface science and electrochemical techniques including ab initio quantum mechanical calculations. In Chapter 6 rigorous rules and a rigorous model are introduced for the first time both for electrochemical and for classical promotion. The kinetic model, which provides an excellent qualitative fit to the promotional rules and to the electrochemical and classical promotion data, is based on a simple concept Electrochemical and classical promotion is catalysis in presence of a controllable double layer. 

Consequently one of the key experimental observations of electrochemical promotion obtains a firm theoretical quantum mechanical confirmation The binding energy of electron acceptors (such as O) decreases (increases) with increasing (decreasing) work function in a linear fashion and this is primarily due to repulsive (attractive) dipole-dipole interactions between O and coadsorbed negative (positive) ionically bonded species. These interactions are primarily through the vacuum and to a lesser extent through the metal . 

Thioglycosides can also be activated by a one-electron transfer reaction from sulfur to the activating reagent tris-(4-bromophenyl)ammoniumyl hexachloroanti-monate (TBPA+) [102,103]. The use of this promoter was inspired by an earlier report where activation was achieved under electrochemical conditions to give an intermediate S-glycosyl radical cation intermediate [104], and the reactivity and mechanism have also been explored [105,106]. 

The mechanism of the enantioselective 1,4-addition of Grignard reagents to a,j3-unsaturated carbonyl compounds promoted by copper complexes of chiral ferrocenyl diphosphines has been explored through kinetic, spectroscopic, and electrochemical analysis.86 On the basis of these studies, a structure of the active catalyst is proposed. The roles of the solvent, copper halide, and the Grignard reagent have been examined. 

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