Solid electrolytes electrochemical promotion

Nasicon solid electrolyte electrochemical promotion with, 440 sodium ion conductor, 440 NEMCA, see electrochemical promotion NEMCA coefficient, 152,319... 

When the promoter is consumed at a faster rate, which is still, however, smaller than that of the catalytic reactant, then the promoter is termed sacrificial promoter [13,14]. This is the case, as we will see, in electrochemical promotion utilizing conducting solid electrolytes. The promoting species is introduced via a Faradaic process on the catalyst surface at a rate of I/2F, where / is the applied current and F is Faraday s constant. At steady state, I/2F also equals the rate of consumption of the sacrificial promoter species on the catalyst surface. 

A. Gorkovenko, S. Jaffe, U.S. Patent Appl. 2006/0154144 Al, 2006. Novel enhanced electrochemical cells with solid-electrolyte interphase promoters. 

When using YSZ as the solid electrolyte, the promoting ionic species (0 ) are generated in an electrochemical step at the catalyst-gas-solid electrolyte interface (three-phase boundaries, tpb) [34]... 

Thus, as will be shown in this book, the effect of electrochemical promotion (EP), or NEMCA, or in situ controlled promotion (ICP), is due to an electrochemically induced and controlled migration (backspillover) of ions from the solid electrolyte onto the gas-exposed, that is, catalytically active, surface of metal electrodes. It is these ions which, accompanied by their compensating (screening) charge in the metal, form an effective electrochemical double layer on the gas-exposed catalyst surface (Fig. 1.5), change its work function and affect the catalytic phenomena taking place there in a very pronounced, reversible, and controlled manner. 

Detailed and shorter39 45 reviews of the electrochemical promotion literature prior to 1996 have been published, mainly addressed either to the catalytic or to the electrochemical community. Earlier applications of solid electrolytes in catalysis, including solid electrolyte potentiometry and electrocatalysis have been reviewed previously. The present book is the first on the electrochemical activation of catalytic reactions and is addressed both to the electrochemical and catalytic communities. We stress both the electrochemical and catalytic aspects of electrochemical promotion and hope that the text will be found useful and easy to follow by all readers, including those not frequently using electrochemical, catalytic and surface science methodology and terminology. 

Most of the electrocatalysts we will discuss in this book are in the form of porous metal films deposited on solid electrolytes. The same film will be also used as a catalyst by cofeeding reactants (e.g. C2H4 plus 02) over it. This idea of using the same conductive film as a catalyst and simultaneously as an electrocatalyst led to the discovery of the phenomenon of electrochemical promotion. 

Promotion We use the term promotion, or classical promotion, to denote the action of one or more substances, the promoter or promoters, which when added in relatively small quantities to a catalyst, improves the activity, selectivity or useful lifetime of the catalyst. In general a promoter may either augment a desired reaction or suppress an undesired one. For example, K or K2O is a promoter of Fe for the synthesis of ammonia. A promoter is not, in general, consumed during a catalytic reaction. If it does get consumed, however, as is often the case in electrochemical promotion utilizing O2 conducting solid electrolytes, then we will refer to this substance as a sacrificial promoter. 

Electrochemical promotion or NEMCA is the main concept discussed in this book whereby application of a small current (1-104 pA/cm2) or potential ( 2 V) to a catalyst, also serving as an electrode (electrocatalyst) in a solid electrolyte cell, enhances its catalytic performance. The phenomenology, origin and potential practical applications of electrochemical promotion, as well as its similarities and differences with classical promotion and metal-support interactions, is the main subject of this book. 

A complete classification of electrochemical promotion (EP) studies on the basis of the type of solid electrolyte used is given in Table 4.1 of Chapter 4 together with the corresponding references. These studies are further discussed in detail in Chapters 8 to 10. 

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. 

The implications of Equation (4.30) for solid state electrochemistry and electrochemical promotion in particular can hardly be overemphasized It shows that solid electrolyte cells are both work function probes and work function controllers for their gas-exposed electrode surfaces. 

Table 4.1. Electrochemical promotion studies classification based on the type of solid electrolyte...

Massive oxygen backspillover from the solid electrolyte onto the catalyst surface takes place under electrochemical promotion conditions. 

The first indication that NEMCA is due to electrochemically induced ion backspillover from solid electrolytes to catalyst surfaces came together with the very first reports of NEMCA Upon constant current application, i.e. during a galvanostatic transient, e.g. Fig. 5.2, the catalytic rate does not reach instantaneously its new electrochemically promoted value, but increases slowly and approaches asymptotically this new value over a time period which can vary from many seconds to a few hours, but is typically on the order of several minutes (Figure 5.2, galvanostatic transients of Chapters 4 and 8.)... 

It also shows that electrochemical promotion is due to electrochemically controlled migration (backspillover) of ions (acting as promoters) from the solid electrolyte to the gas-exposed catalytically active catalyst-electrode surface. 

Work function, a quantity of great importance in surface science and catalysis, plays a key role in solid state electrochemistry and in electrochemical promotion. As will be shown in Chapter 7 the work function of the gas-exposed surface of an electrode in a solid electrolyte cell can be used to define an absolute potential scale in solid state electrochemistry. 

The solid electrolyte is always visible to the XPS through microcracks of the metal films. As already discussed, some porosity of the metal film is necessary to guarantee enough tpb and thus the ability to induce electrochemical promotion. In order, however, to have sufficient signal from species adsorbed on the metal it is recommended to use films with relatively small porosity (crack surface area 10-25% of the superficial film surface area). 

Reversible sodium backspillover as the origin of electrochemical promotion when using Na+ conductors, such as P"-A1203, as the solid electrolyte has been confirmed by the in situ XPS work of Lambert and coworkers.56 61... 

The significant point is that PEEM, as clearly presented in Figures 5.45 to 5.47, has shown conclusively that follows reversibly the applied potential and has provided the basis for space-and time-resolved ion spillover studies of electrochemical promotion. It has also shown that the Fermi level and work function of the solid electrolyte in the vicinity of the metal electrode follows the Fermi level and work function of the metal electrode, which is an important point as analyzed in Chapter 7. 

Can one use STM to study spillover/backspillover phenomena and to confirm the origin of electrochemical promotion The answer is positive and the experimental setup used for the first demonstration of electrochemically controlled spillover/backspillover between a catalyst-electrode (Pt) and a solid electrolyte (p"-Al203) is shown in Figure 5.48.78,79 A polished Pt(lll) single crystal (lOmmxlOmmxlmm) was mounted on an appropriately carved polycrystalline p"-Al203 sample (20mmx20mmx3mm). 

Electrochemically induced and controlled Na backspillover is the origin of electrochemical promotion on metals interfaced with p"-Al203 solid electrolytes. 

These rules were gradually established on the basis of experimental observations on the plethora of electrochemical promotion studies outlined in Tables 4.1 to 4.3 of Chapter 4 and described in more detail in Chapters 8 to 10 of this book. They correspond to some 60 reactions, using a variety of metals and solid electrolytes. There is every reason to believe that these mles apply not only to electrochemical promotion but also to chemical promotion in general. There is already strong experimental evidence for this as discussed below. 

It will also be shown that the absolute electrode potential is not a property of the electrode but is a property of the electrolyte, aqueous or solid, and of the gaseous composition. It expresses the energy of solvation of an electron at the Fermi level of the electrolyte. As such it is a very important property of the electrolyte or mixed conductor. Since several solid electrolytes or mixed conductors based on ZrC>2, CeC>2 or TiC>2 are used as conventional catalyst supports in commercial dispersed catalysts, it follows that the concept of absolute potential is a very important one not only for further enhancing and quantifying our understanding of electrochemical promotion (NEMCA) but also for understanding the effect of metal-support interaction on commercial supported catalysts. 

Equations (7.11) and (7.12) provide a firm basis for understanding the effect of Electrochemical Promotion but also provide an additional, surface chemistry, meaning to the emf of solid electrolyte cells in addition to its usual Nerstian one. 

The oxidation of H2 at room temperature on Pt black electrodes deposited on Nafion 117 was the first electrochemical promotion study utilizing a solid polymer electrolyte.35... 

There is an additional important observation to be made in Fig. 9.25 regarding the magnitude of the relaxation time constant, x, upon current imposition Electrochemical promotion studies involving both solid electrolytes and aqueous alkaline solutions have shown that x (defined as the time required for the catalytic rate increase to reach 63% of its final steady-state value upon current application) can be estimated from ... 

An important question frequently raised in electrochemical promotion studies is the following How thick can a porous metal-electrode deposited on a solid electrolyte be in order to maintain the electrochemical promotion (NEMCA) effect The same type of analysis is applicable regarding the size of nanoparticle catalysts supported on commercial supports such as Zr02, Ti02, YSZ, Ce02 and doped Zr02 or Ti02. What is the maximum allowable size of supported metal catalyst nanoparticles in order for the above NEMCA-type metal-support interaction mechanism to be fully operative ... 

Both questions have been recently addressed via a surface diffusion-reaction model developed and solved to describe the effect of electrochemical promotion on porous conductive catalyst films supported on solid electrolyte supports.23 The model accounts for the migration (backspillover) of promoting anionic, O5, species from the solid electrolyte onto the catalyst surface. The... 

II. Ease of electrical connection Here the main problem is that of efficient electrical current collection, ideally with only two electrical leads entering the reactor and without an excessive number of interconnects, as in fuel cells. This is because the competitor of an electrochemically promoted chemical reactor is not a fuel cell but a classical chemical reactor. The main breakthrough here is the recent discovery of bipolar or wireless NEMCA,8 11 i.e. electrochemical promotion induced on catalyst films deposited on a solid electrolyte but not directly connected to an electronic conductor (wire). 

Most of the electrochemical promotion studies surveyed in this book have been carried out with active catalyst films deposited on solid electrolytes. These films, typically 1 to 10 pm in thickness, consist of catalyst grains (crystallites) typically 0.1 to 1 pm in diameter. Even a diameter of 0.1 pm corresponds to many (-300) atom diameters, assuming an atomic diameter of 3-10 10 m. This means that the active phase dispersion, Dc, as already discussed in Chapter 11, which expresses the fraction of the active phase atoms which are on the surface, and which for spherical particles can be approximated by ... 

The electrochemical promotion of 1-butene isomerization to 2-butene (cis-and trans-) using Nafion as the solid electrolyte and finely dispersed Pd deposited on carbon as the electrode has been described in section 9.2.2.14,15 Faradaic efficiency, A, values up to 28 were obtained in this remarkable study. The Pd dispersion is near complete on the high surface area C support.14,15... 

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