Promotion index

The promotional propensity of a promoter, p, can be quantified by defining20 a promotional index, PIp, from ... 

Thus the promotional index PIp is positive for promoters and negative for poisons. In the latter case the definition of PIp coincides with that of the toxicity defined by Lamy-Pitara, Bencharif and Barbier several years ago.21 In the case of pure site-blocking it is PIp=-l. Values of PI02- up to 150 and of PINas+ up to 6000 have been measured as we will see in Chapter 4. 

The table also gives the catalyst material, the operating temperature range, the maximum (for A>1) or minimum (for A<-1) measured A value and the maximum (for p>l) or minimum for (prate enhancement, p, value. It also provides the maximum measured promotion index, PIi5 value. An asterisk in the p column indicates that electrochemical promotion causes also a change in product selectivity. 

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.

Figure 4.27 presents steady-state potentiostatic r vs 0Na results during NO reduction by H2 on Pt/p"-Al203f2 PInb values well in excess of 4000 are obtained for 0Na values below 0.002. This is due to the tremendous propensity of Na to induce NO dissociation on transition metal surfaces. Since Plj is often found to be strongly dependent on 0, (Figs. 4.26 and 4.27), it is also useful to define a differential promotion index pij from ... 

As previously noted, determination of the promotion index PI0s- of the promoting oxide ions O6 requires knowledge of the coverage of O6 on the... 

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 promotional index, Pip [Eq. (2)]. After the establishment, via the use of surface spectroscopy (XPS [87,88], UPS [89], TPD [90], PEEM [91], STM [92], work function measurements [93]) but also electrochemistry (cyclic voltammetry [90], potential programmed reduction [94], AC impedance spectroscopy [43,95]), that electrochemical promotion is due to the potential-controlled migration (reverse spillover or backspillover) [13] of promoting ionic species (0 , Na", H, F ) from the solid electrolyte to the gas-exposed catalyst surface, it became clear that electrochemical promotion is functionally very similar to classical promotion and that the promotional index PI, already defined in Eq. (2), can be used interchangeably, both in classical and in electrochemical promotion. 

The maximum A values measured so far in NEMCA studies utilizing YSZ are 3x10 3,5 shown in Table 1, which lists the catalytic reactions studied so far under EP conditions using conducting electrolytes. When the promoting ion does not react at all with any of the reactants (e.g., Na ) then, in principle, "infinite" values are expected. In practice A is always measurable (Table 2) due to a very slow consumption of Na to form surface oxides and carbonates. Nevertheless, in all cases the catalytically important parameter is the promotion index Pj defined below and also shown in Tables 1 and 2. 

The promotion index P values shown in Table 1 have been computed on the basis of the approximate equation ... 

Nevertheless, a far more important parameter in this case is the promotion index Pj (Section 4.2) which takes values up to 250 and down to -30 for the case of Na promotion and poisoning, respectively, of CO oxidation on Pt (Table 2 and Figure 18). As noted in Section 4 (Figures 16 and 17) and also shown on Table 2, p values up to infinity and down to zero have been recently obtained for the cases of NO reduction by C2H4 on Pt and benzene hydrogenation on Pt. Also the use of P"-Al203 as a Na donor in the case of ethylene epoxidation, in conjunction with the use of chlorinated hydrocarbon moderators, leads to ethylene oxide selectivity up to 88 percent (Figure 30). 

Agent Promotion index Tensile strength (MPa) Elongation at break (%) Notched Izod impact strength 0/cm)... 

From a catalytic point of view, a very important parameter which can be obtained via NEMCA is the promotion index Pj of the doping species defined from ... 

From a catalytic viewpoint, the promotion index P/ is the most important phenomenological parameter for quantifying the promoting or poisoning effect of a given coadsorbed species i (e.g., O ", F, Na", on the rate of a catalytic reaction. It is defined by... 

NO reduction by C2H4 on Pt/j8"-Al203 was investigated at temperatures between 250 and 400°C by Harkness and Lambert both under atmospheric pressure and under high-vacuum conditions. The reaction exhibits pronounced electrophilic behavior (Fig. 87). The authors concluded that this is due to enhanced oxygen chemisorption rather than enhanced NO dissociation. The results are very spectacular in that a formally infinite p value was obtained that is, was immeasurably low on the unpromoted surface 9 = 0, Fig. 87). At higher Na coverages (0.03 to 0.3), the differential promotion index p, is up to 500. 

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