NEMCA Time Constant

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 general observation in NEMCA studies with O2 conductors is that the magnitude ofx can be predicted by  

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Equations 4.31 and 4.32 also suggest another important fact regarding NEMCA on noble metal surfaces The rate limiting step for the backspillover of ions from the solid electrolyte over the entire gas exposed catalyst surface is not their surface diffusion, in which case the surfacediffusivity Ds would appear in Eq. 4.32, but rather their creation at the three-phase-boundaries (tpb). Since the surface diffusion length, L, in typical NEMCA catalyst-electrode film is of the order of 2 pm and the observed NEMCA time constants x are typically of the order of 1000 s, this suggests surface diffusivity values, Ds, of at least L2/t, i.e. of at least 4 10 11 cm2/s. Such values are reasonable, in view of the surface science literature for O on Pt(l 11).1314 For example this is exactly the value computed for the surface diffusivity of O on Pt(lll) and Pt(100) at 400°C from the experimental results of Lewis and Gomer14 which they described by the equation ... 

Figure 20 shows a typical galvanostatic transient. Positive current application (/ = 400 pk) causes an 88-fold increase in catalytic rate (p = 88). The rate increase is 770 times larger than the rate I/2F of 0 supply to the catalyst A = 770). The NEMCA time constant r is 40 s, in good qualitative agreement with the parameter 2FN/I = 18 s. 

Figure 4.13. NEMCA Rate and catalyst potential response to step changes in applied current during C2H4 oxidation on Pt T=370°C, p02=4.6 kPa, Pc2h4=0.36 kPa. The experimental (t) and computed (2FNG/I) rate relaxation time constants are indicated on the figure. See text for discussion. ro=1.5-10 8 mol O/s, Ar=38.5-10 8 mol O/s, I/2F=5.2-10 12 mol O/s, pmax=26, Amax=74000, Ng=4.240 9 mol Pt.4 Reprinted with permission from Academic Press.

Then let us examine the rate relaxation time constant x, defined as the time required for the rate increase Ar to reach 63% of its steady state value. It is comparable, and this is a general observation, with the parameter 2FNq/I, (Fig. 4.13). This is the time required to form a monolayer of oxygen on a surface with Nq sites when oxygen is supplied in the form of 02 This observation provided the first evidence that NEMCA is due to an electrochemically controlled migration of ionic species from the solid electrolyte onto the catalyst surface,1,4,49 as proven in detail in Chapter 5 (section 5.2), where the same transient is viewed through the use of surface sensitive techniques. 

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.)... 

Figure 5.5. (a) Dependence of the NEMCA relaxation time constant x on 2FNc/I for C2H4 epoxidation on Agu and (b) for CO, C2H4 and CH3OH oxidation on Pt and Ag.12 Adapted from ref. 11 and reprinted from ref. 12 with permission from the American Chemical Society and from Elsevier Science respectively. 

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 ... 

Figure 2 also shows this point At steady-state the rate, r< , of consumption of the promoting O species via reaction with C2H4, has to equal its rate of formation I/2F. Consequently, since A=Ar/(I/2F) and Ar=r, it follows A=r/r =TOF/TOF where TOF is the turnover frequency of the catalytic reaction in the NEMCA-promoted state and TOF is the turnover frequency of the reaction of the promoting oxygen species with ethylene. It thus follows for the experiment of Fig. 2 that TOFc=TOF/A=1.3xlO s. This implies that that average lifetime of the promoting species on the catalyst surface is TOF =770 s in excellent qualitative agreement with the catalytic rate relaxation time constant upon current interruption (Fig. 2). This observation provides strong support for the oxygen backspillover mechanism of electrochemical promotion.

Figure 5.2. NEMCA and its origin on Pt/YSZ catalyst electrodes. Transient effect of the application of a constant current (a, b) or constant potential UWR (c) on (a) the rate, r, of C2H4 oxidation on Pt/YSZ (also showing the corresponding UWR transient)3 (b) the 02 TPD spectrum on Pt/YSZ4,7 after current (1=15 pA) application for various times t. (c) the cyclic voltammogram of Pt/YSZ4,7 after holding the potential at UWR = 0.8 V for various times t.

Data collected in Table 3 can be used to evaluate the proposed NEMCA effect in the oxidation of methane since the CH4/O2 ratio was held constant and the only variables that are changing are the potential and residence time. According to the theory a significant change in the methane conversion should be observed as the voltage on the cell is changed. However the methane conversion values are all clustered around 7.0 0.3 % and can be considered identical within the error of the measurement. Thus the cell under consideration provides no evidence of the NEMCA behavior. 

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