Permanent Electrochemical Promotion

As was seen in the previous section, galvanostatic transients may have a complex structure indicating a multi-step mechanism of promotion. In this section, the slow parts of complex galvanostatic transients will be discussed. In order to get a large contribution of the slow step to the total effect, application of high polarizing currents is preferable. 

The proposed model of two-stage process is well supported by the cyclic voltammetric experiments presented in Section III.4. The fast reversible stage, attributed to formation of an electric double layer at the catalyst/gas interface via backspillover of promoters, has been discussed in detail in Section III.5. To explain the slow irreversible pretreatment. This phenomenon is called permanent electrochemical promotion or permanent NEMCA effect. The similarity between the regions of rate increase and decrease indicates that similar mechanisms are involved during current application and interruption, but the enhancement of the open-circuit rate indicates that the electrochemical promotion of the Ir02 catalyst is not reversible. This behavior of an oxide catalyst is different from that of a metal catalyst for which the electrochemical promotion is usually reversible.  

Eleetroehemieal promotion of RUO2 catalysts has also been studied. It is well known that the higher oxides of Ru (RUO3 and RUO4) are less stable than the higher oxides of Ir and thus it was expeeted that the eleetroehemieal promotion of RUO2 would be reversible. Indeed, EP measurements earried out with RUO2 catalyst have shown that the catalyst restores its initial activity after current interruption.  

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In order to address both fundamental and application aspects of electrochemical promotion in this Chapter, the combustion of ethylene over Ir02 or RUO2 catalysts and the reduction of NO with propylene over Rh catalysts —all deposited on yttria-stabilized zirconia (YSZ) solid electrolyte— have been chosen as model catalytic systems. While the literature of electrochemical promotion deals mainly with metal catalysts, our laboratory has a long experience with promotion of metal oxide catalysts, such as Ir02 and RUO2. In fact, ethylene combustion with Ir02/YSZ film catalyst was the first catalytic system found to exhibit the phenomenon of permanent electrochemical promotion, which manifests itself as a shift in the steady-state open-circuit activity due to polarization, and is attributed to a change in... 

When the factor y is unity, the electrochemical promotion is termed reversible. Several cases of irreversible modification have been reported from our laboratory. This effect has also been termed permanent electrochemical promotion or permanent NEMCA effect. 

Figure 23 shows galvanostatie transients of ethylene oxidation obtained with Ir02 - Ti02 samples of different composition. Upon increasing the TiOa 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). 

For prolonged polarization times, the catalytic reaction rate after current interruption can remain higher than the value before current application [3]. This behavior has been reported as permanent-electrochemical promotion of catalysis (P-EPOC). This phenomenon was mainly observed over catalysts which have multiple valence states and the ability to form oxides, i.e., Pt/PtOx or Rh/Rh203. 

Permanent Electrochemical Promotion for Environmental Applications, Fig. 1 Polarization and relaxation transients of the rate of C2H4 combustion on I1O2/YSZ catalyst due to an anodic current application (300 mA) for two different times (a) short polarization to give reversible EPOC and (b) long polarization time to give P-EPOC. T = 380 °C, P02 = 17 kPa, Pc2H4 = 0.14 kPa (Reprint ref 3)... 

Permanent Electrochemical Promotion for Environmental Applications, Table 1 Exhaustive list of P-EPOC... 

Permanent Electrochemical Promotion for Environmental Applications, Fig. 2 (a) Oxygen species produced electrochemically under anodic polarization. Storage at three different locations (1) at the PtA SZ interface, (2) at the gas-exposed catalyst surface via backspillover and (3) in the bulk platinum at the vicinity of the PtAfSZ interface. Leaving (via O2 evolution and/or... 

Falgairete C, Jaccoud A, Foti G, Comninellis C (2008) The phenomenon of permanent electrochemical promotion of catalysis (P-EPOC). J Appl Electrochem 38 1075-1082... 

One of the most interesting and potentially important from a practical viewpoint aspect of electrochemical promotion is the permanent NEMCA effect first discovered and studied by Comninellis and coworkers at Lausanne. 

Positive potentials lead to p values up to 20. (Figure 4.52). Negative currents also enhance the rate and selectivity but to a lesser extent (Fig. 8.64). Permanent NEMCA behaviour is also observed with positive currents at lower temperatures (Fig. 4.52). Overall, however, electrochemical promotion is not as pronounced as in the case where propene is used. This can be attributed to the much stronger electron donor character of C3H6 relative to CO which, as already noted in this chapter, behaves predominantly as an electron acceptor. Thus positive potentials weaken CO bonding to the surface while they enhance CjH6 chemisorption. 

Particulate matter combustion of, electrochemically promoted, 525 Dinex process, 525 European and US standards, 526 Permanent NEMCA characteristics, 176 potential for catalyst preparation, 177 Perovskites... 

Sonentie S, Xia C, Falgairete C, li YD, Comninellis C (2010) Investigation of the permanent electro-ehemieal promotion of catalysis (P-EPOC) by electrochemical mass spectrometry (EMS) measunanents. Electrochem Commun 12 323-326... 

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