Electrochemical Promotion Galvanostatic Transients

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...

This equation would enable one to predict the UWR vs t behaviour in galvanostatic transients such as those of Fig. 4.15 if the value of the Na dipole moment on Pt were exactly known. The surface science literature (Chapter 2) suggests pNa=5.2 D for the initial dipole moment of Na on Pt(lll). This value has been used, in conjunction with Eq. (4.25) to draw the lines labeled Eq. (4.25) ] in Fig. 4.15 and in subsequent figures throughout this book. As shown in Fig. 4.15 there is very good qualitative agreement between Eq. (4.25) and the initial Uwr vs t transient. This supports the approach and underlines the similarities between electrochemical and classical promotion. 

One of the most important, but not too surprising experimental observations after the discovery of electrochemical promotion is that the work function, O, of the gas exposed catalyst-electrode surfaces changes significantly (up to 2 eV) during galvanostatic transients such as the ones shown in Figures 4.13, 4.14, 4.15 and 4.17 as well as at steady-state and in fact that, over wide experimental conditions, it is (Fig. 4.21)54 ... 

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.)... 

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... 

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.

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 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. 

Another important parameter in electrochemical promotion studies is the characteristic rate relaxation time, t, needed for the catalytic rate to reach steady state upon imposition of a constant current (galvanostatic transient). As one would expect and as experiment has clearly shown [13,14], t is always of the order of 2FNg/I (Figure 2) ... 

The NEMCA rate relaxation time constant, x, is defined [9,14] as the time required for the catalytic rate increase to reach 63% of its final steady-state value in galvanostatic transient experiments, such as the one depicted in Fig. 2. As shown in this Figure, T is of the order of 2FN/I. This is a general observation in electrochemical promotion studies utilizing YSZ ... 

Catalytic rate transients due to electrochemical promotion may be of great importance for a better understanding of the phenomenon. The time dependence of reaction rate following a galvanostatic step, as depicted in Figure 3, depends on the formation rate of the promoting species and on the rate of their migration to the gas-exposed catalyst surface, hut may also depend on their reactivity when they are consumed in a chemical reaction sacrificial promoter ). 

Figure 13. Typical polarization and relaxation transients of catalyst potential, (Fwr-// ), and ethylene combustion reaction rate, r, on Ir02/YSZ catalyst. Galvanostatic anodic polarization with / = 10 pA during 100 min. (a), (b), (c) and (d) designate the subsequent transient steps. Catalyst loading 77 pg IrCh. Feed composition at pm = 100 kPa PC2H4 = 12.5 Pa, - 1.25 kPa, balance helium. Flow rate 200 mL min" STP 7 =375°C. Reprinted from J. Electroanal. Chem., Q. F6ti, V. Stankovii, I. BolzoneUa, and Ch. Comninel-lis. Transient Behavior of Electrochemical Promotion of Gas-Phase Catalytic Reactions, (2002), in press, with permission from Elsevier Science,...

It was concluded that a dynamic approach considering the applied current as key parameter is well adapted for the interpretation of galvanostatic electrochemical promotion of ethylene combustion over Ir02 catalysts. Both, the transient and the steady-state behavior of the system, were satisfactorily described by the proposed model, which assumes free surface site dependent formation, rapid spreading-out (backspillover), and first order rate consumption of the O promoter. 

Figure 23 shows galvanostatic transients of ethylene oxidation obtained with IrOi - T1O2 samples of different composition. Upon increasing the TiOj content, the magnitude of the rate enhancement factor, p, decreases, and the reaction rate tends to return very slowly towards its initial unpromoted value (permanent electrochemical promotion). 

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