Promotional Transients

A simple model for the accumulation of O promoters was developed recently on the basis of the following assumptions  

The formation rate of the promoter is proportional to the fraction of available surface sites at the tpb  

The migration of the promoter over the gas-exposed catalyst surface is rapid in comparison to its average lifetime at that surface  

The consumption rate of the promoter is of first order in promoter coverage. 

The first assumption implies that the current efficiency for promoter formation is a decreasing function of time during polarization. The competing reaction is oxygen evolution at the tpb. The second assumption implies that the concentration of the promoter at the catalyst/gas interface is uniform and equal to that at the tpb. With thin catalyst films this is a reasonable assumption, while for high catalyst loadings a finite surface diffusivity of the promoter has to be considered. The third assumption may be best satisfied close to open-circuit conditions, i.e. at low promoter coverage. 

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While Au promoted reduction of ceria to a lower temperature than Pt, both 5% loaded catalysts led to reduction of the ceria surface shell by heating in H2 to 200 °C. However, 20 times the amount of Au/ceria catalyst was required to achieve the same lightoff curve as was obtained with Pt/ceria, indicating that the Pt/ceria catalyst was much more active (Table 78). As previously observed with Pt, increasing the loading of Au did increase the CO conversion rate, as shown in Table 79. To help explain the observed differences in the water-gas shift rates observed for the two different promoters, transient formate decomposition tests in the presence of steam432 were... 

It has been proposed that protonation or complex formation at the 2-nitrogen atom of 14 would enhance the polarization of the r,6 -7i system and facilitate the rearrangement leading to new C-C bond formation. The equilibrium between the arylhydrazone and its ene-hydrazine tautomer is continuously promoted to the right by the irreversible rearomatization in stage II of the process. The indolization of arylhydrazones on heating in the presence of (or absence of) solvent under non-catalytic conditions can be rationalized by the formation of the transient intermediate 14 (R = H). Under these thermal conditions, the equilibrium is continuously pushed to the right in favor of indole formation. Some commonly used catalysts in this process are summarized in Table 3.4.1. 

The mechanism for the transformation of 5 to 4 was not addressed. However, it seems plausible that samarium diiodide accomplishes a reduction of the carbon-chlorine bond to give a transient, resonance-stabilized carbon radical which then adds to a Smni-activated ketone carbonyl or combines with a ketyl radical. Although some intramolecular samarium(n)-promoted Barbier reactions do appear to proceed through the intermediacy of an organo-samarium intermediate (i.e. a Smm carbanion),10 ibis probable that a -elimination pathway would lead to a rapid destruction of intermediate 5 if such a species were formed in this reaction. Nevertheless, the facile transformation of intermediate 5 to 4, attended by the formation of the strained four-membered ring of paeoniflorigenin, constitutes a very elegant example of an intramolecular samarium-mediated Barbier reaction. 

Although the term non-Faradaic process has been used for many decades to describe transient electrochemical processes where part of the current is lost in charging-discharging of metal-electrolyte interfaces, in all these cases the Faradaic efficiency, A, is less than 1 (100%). Furthermore such non-Faradaic processes disappear at steady state. Electrochemical promotion (NEMCA) must be very clearly distinguished from such transient non-Faradaic processes for two reasons ... 

How can we confirm this sacrificial promoter model By simply looking at the r vs t transient behaviour of Figure 4.13 or of any galvanostatic NEMCA experiment upon current interruption (1=0). 

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. 

There is an important point to be made regarding UWr vs t transients such as the ones shown in Fig. 4.15 when using Na+ conductors as the promoter donor. As will be discussed in the next section (4.4) there is in solid state electrochemistry an one-to-one correspondence between potential of the working electrode (UWr) and work function (O) of the gas exposed (catalytically active) surface of the working electrode (eAUwR=AO, eq. 4.30). Consequently the UWr vs t transients are also AO vs t transients. 

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 ... 

Figure 4.26. Transient response of the rate of CO2 formation and of the catalyst potential during NO reduction by CO on Pt/p"-Al2C>396 upon imposition of fixed current (galvanostatic operation) showing the corresponding (Eq. 4.24) Na coverage on the Pt surface and the maximum measured (Eq. 4.34) promotion index PINa value. T=348°C, inlet composition Pno = Pco = 0.75 kPa. Reprinted with permission from Academic Press.

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 maximum 0os value is thus computed for the transient of Fig. 5.6b to be 0os 0.5. This, in view of the definition of the promotion index, PI, (Chapter 4) and the observed p value (p 16.5) gives a PIQ5- value of the order of 30, in good qualitative agreement with PI0s- values for other Pt catalyzed oxidations.5... 

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... 

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... 

Nevertheless both transient rate analysis24,71 and XPS24 have shown that in both cases the electrochemical promotion mechanism is identical with that obtained with YSZ, i.e. electrochemically controlled migration (back-spillover) ofO2 onto the gas-exposed catalyst-electrode surface.24,71... 

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... 

As shown in Fig. 9.25, upon current interruption rH2, ro and Urj,e return to their open circuit values, showing the reversibility of the effect. It is worth noting that the rate transient parallels, to a large extent, the catalyst potential. This shows the important role of catalyst potential in describing electrochemical promotion. 

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.

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. 

A typical electrochemical promotion experiment includes kinetic measurements under open and closed circuit conditions as well as study of the effect of catalyst potential or work function on catalytic rate and selectivity under steady state and transient conditions. In kinetic measurements one should change the partial pressure of each reactant while... 

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