Electrochemically Promoted Films

In order to estimate T P in actual electrochemical promotion experiments we use here typical values23 of the operating parameters (Table 11.2) to calculate J and D. The value of k is estimated on the basis of typical NEMCA galvanostatic transients which show that the lifetime of the promoting O5 species on the catalyst surface is typically 102 s at temperatures 350°-400°C. 

The surface diffusivity Ds is computed (conservatively) from the diffusivity measurements of Lewis and Gomer44 for O on Pt(lll) and Pt(110) near 400°C. They described their data via the equation  

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In a film, the cooperative effort of the different molecular motors, between consecutive cross-linked points, promotes film swelling and shrinking during oxidation or reduction, respectively, producing a macroscopic change in volume (Fig. 18). In order to translate these electrochemically controlled molecular movements into macroscopic and controlled movements able to produce mechanical work, our laboratory designed, constructed, and in 1992 patented bilayer and multilayer103-114 polymeric... 

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. 

Electrochemical promotion of the unpromoted Rh/YSZ film, via application of 1 or -1 V, leads to significant rate enhancement (tenfold increase in rCo2> four fold increase in rN2 (filled circles and diamonds in Fig. 2.3). This is a catalytic system which as we will see in Chapters 4 and 8 exhibits inverted volcano behaviour, i.e. the catalytic rate is enhanced both with positive and with negative potential. 

Since electrochemical promotion (NEMCA) studies involve the use of porous metal films which act simultaneously both as a normal catalyst and as a working electrode, it is important to characterize these catalyst-electrodes both from a catalytic and from an electrocatalytic viewpoint. In the former case one would like to know the gas-exposed catalyst surface area A0 (in m2 or in metal mols, for which we use the symbol NG throughout this book) and the value, r0, of the catalytic rate, r, under open-circuit conditions. 

S. Ladas, S. Bebelis, and C.G. Vayenas, Work Function Measurements on Catalyst Films subject to in-situ Electrochemical Promotion, Surf. Sci. 251/252, 1062-1068 (1991). 

D. Tsiplakides, J. Nicole, C.G. Vayenas, and C. Comninellis, Work function and catalytic activity measurements of an Ir02 film deposited on YSZ subjected to in situ electrochemical promotion,/. Electrochem. Soc. 145(3), 905-908 (1998). 

P.D. Petrolekas, S. Balomenou, and C.G. Vayenas, Electrochemical promotion of Ethylene Oxidation on Pt Catalyst Films deposited on Ce02, J. Electrochem. Soc. 

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). 

The electrochemically induced creation of the Pt(lll)-(12xl2)-Na adlayer, manifest by STM at low Na coverages, is strongly corroborated by the corresponding catalyst potential Uwr and work function O response to galvanostatic transients in electrochemical promotion experiments utilizing polycrystalline Pt films exposed to air and deposited on (T -AbCb. 3637 Early exploratory STM studies had shown that the surface of these films is largely composed of low Miller index Pt(lll) planes.5... 

This case has been already discussed in Chapter 2 (Fig. 2.3).69 The Rh film used is shown in Fig. 8.63 and exhibits inverted volcano behaviour,67 i.e. the rate of C02 and N2 formation is enhanced both with positive and with negative potentials. This is shown in Figure 8.65 and also in Figure 2.3 which depicts the rco2 and rN2 dependence on T of the unpromoted and electrochemically promoted Rh catalyst. The corresponding Tn2o vs T behaviour is shown in Figure 8.66. 

G. Pitselis, P. Petrolekas, and C.G. Vayenas, Electrochemical Promotion of NH3 decomposition over Fe catalyst films interfaced with K+ and Na+ conductors, Ionics 3, 110-117(1997). 

Figure 11.3. Schematic of the experimental setup used (a) to induce electrochemical promotion (via YSZ) on Ir02 and Ir02-Ti02 porous catalyst films (b) to compare the electrochemical promotion induced on Pt via YSZ and via Ti02 and (c) to compare the electrochemical promotion behaviour induced by varying UWR on a Rh porous catalyst film (left) and on a fully dispersed Rh catalyst supported on porous (80 m2/g) YSZ support.22...

Figure 11.4. Effect of the mole fraction, XIro2, of Ir02 in the Ir02-Ti02 catalyst film on the rate of C2H4 oxidation under open-circuit conditions (open circles) and under electrochemical promotion conditions (filled circles) via application of 1=200 pA T=380°C, Pc2h4=015 kPa, Po2=20 kPa. Triangles indicate the corresponding electrochemical promotion rate enhancement ratio p values.22,29...

Figure 11.5. Galvanostatic (constant current application) electrochemical promotion (NEMCA) transients during C2H4 oxidation on Ir02-Ti02 films deposited on YSZ T=380°C, Pc2H4=0.15 kPa, pO2=20 kPa.22,29...

Figure 11.6. Galvanostatic catalytic rate transients showing the equivalence of electrochemical promotion when using YSZ30 (a) or TiOj31 (b) as the Pt metal film support. See text for discussion.22 Reprinted with permission from Academic Press.

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).

Figure 11.8. Effect of po2 on the rate (TOF) of C2H4 oxidation on Rh supported on five supports of increasing d>. Catalyst loading 0.5wt%.22,27 Inset Electrochemical promotion of a Rh catalyst film deposited on YSZ Effect of potentiostatically imposed catalyst potential Uwr on the rate and TOF dependence on po2 at fixed Pc2H4-22,33 Reprinted with permission from Elsevier Science (ref. 27) and Academic Press (ref. 33).

The inset of Figure 11.8 shows the rate dependence on P02 (at the same PC2H4 and T) for the Rh film deposited on YSZ at various imposed potentials Uwr. The similarity between Figure 11.8 and the inset of Figure 11.8 is striking and underlines the equivalence of metal-support interactions and electrochemical promotion For low po2 values the rate is first order in P02 followed by a sharp decrease at a characteristic po2 value denoted by P02 ( Uwr ) which depends on the support (Fig. 11.8) or on the potential (inset of Fig. 11.8). Thereafter the rate becomes very low and negative order... 

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... 

Figure 11.12. Schematic of an electrochemically promoted metal catalyst film supported on a 02 conductor.23...

The significance of Equation (11.31), in conjunction with Figure 11.13 and the definitions of 11, P and J (Table 11.1) is worth emphasizing. In order to obtain a pronounced electrochemical promotion effect, i.e. in order to maximize p (=r/ro), one needs large II and r p values. The latter requires large J and small 0P values (Fig. 11.13). Small k and L values satisfy both requirements (Table 11.1). This implies that the promoting species must not be too reactive and the catalyst film must be thin. 

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 ... 

As in aqueous electrochemistry it appears that application of a potential between the two terminal (Au) electrodes leads to charge separation on the Pt film so that half of it is charged positively and half negatively8 thus establishing two individual galvanic cells. The Pt film becomes a bipolar electrode and half of it is polarized anodically while the other half is polarized cathodically. The fact that p is smaller (roughly half) than that obtained upon anodic polarization in a classical electrochemical promotion experiment can be then easily explained. 

Catalyst films used in electrochemical promotion (NEMCA) studies are usually prepared by using commercial metal pastes. Unfluxed pastes should be used, as fluxes may introduce unwanted side reactions or block electrocatalytic and catalytic sites. This action may obscure or even totally inhibit the electrochemical promotion effect. 

Catalyst films for electrochemical promotion studies should be thin and porous enough so that the catalytic reaction under study is not subject to internal mass-transfer limitations within the desired operating temperature. Thickness below 10 pm and porosity larger than 30% are usually sufficient to ensure the absence of internal mass-transfer limitations. Several SEM images of such catalyst films have been presented in this book. SEM characterization is very important in assessing the morphological suitability of catalyst films for electrochemical promotion studies and in optimizing the calcination procedure. 

Addresses of suppliers of catalyst pastes included in Table B.l are presented below. Other companies (e.g. Johnson-Matthey) may also supply similar products. The suitability of these products for preparing catalyst films for electrochemical promotion studies should be tested on the basis of the requirements already mentioned. A useful approach before proceeding with the study of new systems is to try to reproduce results of electrochemical promotion studies in model systems, such as ethylene oxidation on Pt, which has been thoroughly studied. It has to be pointed out that in general suppliers do not provide calcination procedures or the provided calcination procedures aim to the production of very dense and non-porous films not necessarily suitable for electrochemical promotion studies. 

Checking the absence of internal mass transfer limitations is a more difficult task. A procedure that can be applied in the case of catalyst electrode films is the measurement of the open circuit potential of the catalyst relative to a reference electrode under fixed gas phase atmosphere (e.g. oxygen in helium) and for different thickness of the catalyst film. Changing of the catalyst potential above a certain thickness of the catalyst film implies the onset of the appearance of internal mass transfer limitations. Such checking procedures applied in previous electrochemical promotion studies allow one to safely assume that porous catalyst films (porosity above 20-30%) with thickness not exceeding 10pm are not expected to exhibit internal mass transfer limitations. The absence of internal mass transfer limitations can also be checked by application of the Weisz-Prater criterion (see, for example ref. 33), provided that one has reliable values for the diffusion coefficient within the catalyst film. 

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