The Catalyst Film

As discussed below, the porosity and surface area of the catalyst film is controllable to a large extent by the sintering temperature during catalyst preparation. This, however, affects not only the catalytically active surface area AG but also the length, t, of the three-phase-boundaries between the solid electrolyte, the catalyst film and the gas phase (Fig. 4.7). 

Figure 4.7. Schematic representation of the location of electrocatalytically and catalytically active sites in a section perpendicular to the catalyst film-solid electrolyte interface.

The reference electrode-solid electrolyte interface must also be non-polarizable, so that rapid equilibration is established for the electrocatalytic charge-transfer reaction. Thus it is generally advisable to sinter the counter and reference electrodes at a temperature which is lower than that used for the catalyst film. Porous Pt and Ag films exposed to ambient air have been employed in most previous NEMCA studies.1,19... 

We start by considering a schematic representation of a porous metal film deposited on a solid electrolyte, e.g., on Y203-stabilized-Zr02 (Fig. 5.17). The catalyst surface is divided in two distinct parts One part, with a surface area AE is in contact with the electrolyte. The other with a surface area Aq is not in contact with the electrolyte. It constitutes the gas-exposed, i.e., catalytically active film surface area. Catalytic reactions take place on this surface only. In the subsequent discussion we will use the subscripts E (for electrolyte) and G (for gas), respectively, to denote these two distinct parts of the catalyst film surface. Regions E and G are separated by the three-phase-boundaries (tpb) where electrocatalytic reactions take place. Since, as previously discussed, electrocatalytic reactions can also take place to, usually,a minor extent on region E, one may consider the tpb to be part of region E as well. It will become apparent below that the essence of NEMCA is the following One uses electrochemistry (i.e. a slow electrocatalytic reaction) to alter the electronic properties of the metal-solid electrolyte interface E. 

We then concentrate on the meaning of UWr, that is, of the (ohmic-drop-free) potential difference between the catalyst film (W, for working electrode) and the reference film (R). The measured (by a voltmeter),... 

Negative current application, i.e., proton supply to the catalyst film causes up to 500% reversible enhancement to the rate of C2H4 oxidation. The catalytic rate increase is up to 2x104 times higher than the rate -I/F, of proton supply to the catalyst. 

Recalling that 0i=Q/Cjiraax and defining =z/L, where L is the thickness of the catalyst film, one can write equations (11.16) to (11.18) in the following dimensionless form ... 

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. 

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. 

In the preceding expression we include an effectiveness factor r to account for pore diffusion limitations of A. Hi fact, if the catalyst film thickness on the wall of the reactor is small enough that we can assume it planar, then the effectiveness factor becomes... 

Since the catalyst film coated is usually very thin, the catalyst may be considered to be only an active superficial surface and thus the gas/catalyst interface is assumed to coincide with the wall. 

Diffusion of the reactants through a boundary layer or film adjacent to the external surface of the catalyst (film diffusion or interphase diffusion)... 

Here L is the catalyst film thickness or nanoparticle size, k is the rate constant for depletion (reaction or desorption) of the promoting O species, Q max is its maximum possible surface concentration on the catalyst or nanoparticle surface, Ac is the metal-gas interface area of the film or nanoparticle, r is the promoted catalytic rate, and A is the Faradaic efficiency of the catalytic reaction being promoted. 

Fig. 4b demonstrates that under UHV conditions, electro-pumped Na is identical in behaviour and in chemical state with Na supplied by vacuum deposition firom a Na evaporation source. Spectrum (1) shows the XPS of the catalyst film when Na was vacuum-deposited. Heating to 400 K under open circuit conditions caused no change- spectrum (2). 

A high density of nucleation centers on a substrate can also be achieved by an accordingly dense arrangement of the catalyst particles in a CVD process. Examples include nanoparticles of iron or cobalt that are precipitated from solution on a silicon support or the pretreatment of the catalyst film with ammonia Likewise does the thermal decomposition of iron phthalocyanine generate enough iron particles for the subsequent growing nanotubes to interfere with each other, thus forcing them into a vertical orientation. 

The usual procedure for extracting the exchange current Iq is then to measure q as a function of I and to plot Inl vs q (Tafel plot). Such plots are shown in Figures 3 and 4 for Pt and Ag catalyst electrodes deposited on YSZ and acting as catalyst for C2H4 oxidation. Throughout the rest of this discussion, we omit the subscript "W" from q and simply write q, since the only overpotential of interest is that of the catalyst film. When Iql >100 mV, then the Butler-Volmer equation (16) reduces to its "high field approximation" form, i.e.. 

Figure 14 Effect of gaseous composition on the unpromoted (open-circuit) steady-state rate of C2H4 oxidation on Pt and on the NEMCA-induced catalytic rate when the catalyst film is maintained at Reprinted with permission from Academic Press.

The same strategy has been used to prepare flexible carbon nanotubes on a polymer support in a monomode reactor within 2 s [31b]. This technique enables the preparation to be performed without preheating of the catalyst film, under atmospheric operating conditions, with rapid synthesis and the ability to synthesize CNT on polymer substrates. 

An increase in voltammetric charge with increasing thickness of the catalyst film was observed, contributed by the inner charge, g , alone. 

Figure 18 depicts a galvanostatic transient experiment, showing the rate response to an applied current step (+300 aA for 120 min) during ethylene oxidation on an Ir02 catalyst/ The evolution of the catalyst film potential with respect to the reference electrode, Kwr, is also given. Initially the circuit is open (/ = 0) and the unpromoted catalytic rate is very slow. At f = 0 a constant current (/ = +300 tA) is applied between... 

Indirect bipolar (IB) polarization of the catalyst film in a ring-shaped electrochemical cell is realized by using the two gold electrodes as feeder electrodes. For advanced characterization of the cell, a third electric connection may be added. This latter, connected to the catalyst film itself, permits measurements also in the direct (monopolar) polarization mode, which is useful for the determination of the current bypass. 

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