Galvanostatic transient

Hickling", in attempting to study the corrosion of steels under thin film conditions that simulate atmospheric exposure, took into account the time-dependence of polarisation measurements, and developed a technique using galvanostatic transients. 

This is the essence of NEMCA. The reader can check the validity of the sacrificial promoter concept in all NEMCA galvanostatic transients of this book. 

Figure 4.14 shows a similar galvanostatic transient obtained during C2H4 oxidation on Rh deposited on YSZ.50 Upon application of a positive current 1=400 pA with a concomitant rate of O2 supply to the catalyst I/2F=2.M0 9 mol O/s the catalytic rate increases from its open-circuit value r0=1.8 10 8 mol O/s to a new value r= 1.62-1 O 6 mol O/s which is 88 times larger than the initial unpromoted rate value. The rate increase Ar is 770 times larger than the rate of supply of O2 ions to the Rh catalyst surface. 

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. 

Figure 4.17 shows another galvanostatic transient obtained on Pt/p"-AI2O3 at 375°C. The reaction under study is the reduction of NO by H2, a reaction of significant technological interest52 ... 

It is instructive at this point to make some comparisons between the galvanostatic transients obtained with O2 conductors (Figs. 4.13 and 4.14) and those obtained with Na+ conductors (Figs. 4.15 and 4.17) and also to explain some of the salient and distinguishing features of the latter including the dotted straight lines marked eq. (4.25) on Figs. 4.15 and 4.17. 

This is easy to understand In the former case the backspillover species (O2 ) is also a reactant in the catalytic reaction. Thus as its coverage on the catalyst surface increases during a galvanostatic transient its rate of consumption with C2H4 also increases and at steady state its rate of consumption equals its rate of creation, I/2F. This means that the backspillover O2 species reacts with the fuel (e.g. C2H4) at a rate which is A times slower than the rate of reaction of more weakly bonded chemisorbed oxygen formed via gaseous chemisorption. 

In the latter case, however, galvanostatic transients such as the ones shown in Figures 4.15 and 4.17 should not be considered useless, as they... 

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 NEMCA time constant, t, is defined1,4 as the time required for the rate increase Ar to reach 63% of its steady-state value during a galvanostatic transient, such as the one shown in Fig. 4.13 and 4.14. Such rate transients can usually be approximated reasonably well by ... 

A more rigorous mathematical analysis is presented in section 5.3, but the reader is invited to check the validity of Eq. (5.3) with all galvanostatic transients presented in this book. 

ANALYSIS OF RATE TIME CONSTANTS DURING GALVANOSTATIC TRANSIENTS... 

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.)... 

This observation immediately rules out the possibility that NEMCA is an electrocatalytic phenomenon causing only a local acceleration of the catalytic rate at the three-phase-boundaries (tpb) metal-solid electrolyte-gas. In such a case the rate increase would obviously be instantaneous during a galvanostatic transient. 

It was quickly observed that the catalytic rate response during galvanostatic transients can be reasonably well approximated by the response of a first order system, i.e. by ... 

Time Constants During Galvanostatic Transients and Faradaic Efficiency... 

Thus by simply measuring tD, rj and NG one can estimate A. The thus computed values are always in excellent qualitative agreement with those accurately computed from Eq. (4.19). The interested reader can check this with all galvanostatic transients presented in this book. 

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... 

In Figure 4.14 we have seen a typical galvanostatic transient of this system. Positive current application (1=400 pA) causes a 88-fold increase in catalytic rate (p=88). The rate increase is 770 times larger than the rate I/2F ofO2 supply to the catalyst(A=770). The NEMCA time constantt is 40s in good qualitative agreement with the parameter 2FNG/I=18s. 

Figure 8.10. Potentiostatic and galvanostatic transient during C2H4 oxidation on IrCVYSZ 17 Pc2h4=0.26 kPa pO2=20 kPa T=390°C A 100.

Figure 8.40 shows some typical galvanostatic transients, one for I>0 and another for I<0. In the former case (Fig. 8.40a) both rates of C2H40 and C02 formation increase but both exhibit an initial "overshooting". 

As already shown in Figure 6.3b the system exhibits remarkable electrophilic promotional behaviour with p values up to 20.64 This is also shown in Fig. 8.60 which depicts a galvanostatic transient. Application of a negative current between the Pt catalyst-working electrode and the Au counter electrode causes a sharp increase in all reaction rates. In the new steady state of the catalyst (achieved within lhr of current application) the catalytic rate increase of C02 and N2 production is about 700%, while lesser enhancement (250-400%) is observed in the rates of CO and N20 production. The appearance of rate maxima immediately after current application can be attributed to the reaction of NO with previously deposited carbon.64... 

Figure 8.68 shows a typical galvanostatic transient under oxidizing gaseous conditions. The reaction rate is enhanced by a factor of 20 (p=21) and the faradaic efficiency A (=Ar/(I/2F)) is 1880. The behaviour is clearly electrophobic (dr/dV Xi) and strongly reminiscent of the case of C2H4 oxidation on Pt/YSZ (Fig. 4.13) with some small but important differences ... 

Figure 9.9 shows typical galvanostatic transients. Computed dipole moments of Na/Pt are again in reasonable agreement with the literature value ofl.75xl0 29 C m (5.3 Debye)13. The promotion indexP is up to 250 under... 

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 catalyst surface is typically 102 s at temperatures 350°-400°C. 

Figure 12.5. Ethylene oxidation on Pt finely dispersed on Au supported on YSZ.7 Effect of the current 1 on x 1, where x is the time constant measured during a galvanostatic transient experiment with I as the applied current x is obtained by fitting either r/r0=exp(-t/x) or l-exp(-t/x) to the experimental data depending on the sign of the current and whether the reaction is electrophilic or electrophobic, (a) Positive values of I for electrophilic (squares, T=371°C, pO2=18.0 kPa, Pc2H4=0-6 kPa) and electrophobic behavior (circle, T=421°C, p02=l 4.8 kPa, Pc2H4 CU kPa) (b) negative currents, electrophilic behavior (T=421°C, p02=14.8 kPa, pC2H4=0.1 kPa. Reprints with permission from Academic Press.

Why are NEMCA galvanostatic transients so slow (compared with electrocatalysis)... 

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