Deactivation waves

Theoretical and experimental results on deactivation have been summarized in two reviews by Butt (.1,2). Previous work of particular interest to the present study has been done by Blaum (3) who used a one-dimensional two-phase model to explore the dynamic behavior of a deactivating catalyst bed. Butt and cowotkers (b,5,6)have performed deactivation studies in a short tubular reactor for benzene hydrogenation for both adiabatic and nonadiabatic arrangements. They experimentally observed both the standing (6) and travelling (4) deactivation wave. Hlavacek... 

Several mechanisms have been proposed to explain the activation of carbon surfaces. These have Included the removal of surface contaminants that hinder electron transfer, an Increase In surface area due to ralcro-roughenlng or bulld-up of a thin porous layer, and an Increase In the concentrations of surface functional groups that mediate electron transfer. Electrode deactivation has been correlated with an unintentional Introduction of surface contaminants (15). Improved electrode responses have been observed to follow treatments which Increase the concentration of carbon-oxygen functional groups on the surface (7-8,16). In some cases, the latter were correlated with the presence of electrochemical surface waves (16-17). However, none of the above reports discuss other possible mechanisms of activation which could be responsible for the effects observed. 

Sanders In the preparations that you use, you are already getting waves before depolarization. Haven t you actually started to deactivate the process somewhat in these single cells ... 

Recently, it has been shown that ultrasonic agitation during hydrogenation reactions over skeletal nickel can slow catalyst deactivation [122-124], Furthermore, ultrasonic waves can also significantly increase the reaction rate and selectivity of these reactions [123,124], Cavitations form in the liquid reaction medium because of the ultrasonic agitation, and subsequently collapse with intense localized temperature and pressure. It is these extreme conditions that affect the chemical reactions. Various reactions have been tested over skeletal catalysts, including xylose to xylitol, citral to citronellal and citronellol, cinnamaldehyde to benzenepropanol, and the enantioselective hydrogenation of 1-phenyl-1,2-propanedione. Ultrasound supported catalysis has been known for some time and is not peculiar to skeletal catalysts [125] however, research with skeletal catalysts is relatively recent and an active area. 

Current-voltage profiles for chlorine evolution, obtained at various times between periods of square-wave potential cycling (1.35 to — 0.32 V versus SCE, 60 s cycle-1), all at the same 40 at. % Ru electrode, are shown in Fig. 5.4. It can be seen that the activity of these electrodes increases at the beginning of the deactivation period, as revealed by the decrease in the overpotential at approximately 5 h. This may arise, at least in part, from... 

To further understand and characterise the oxide deactivation process, a.c. impedance studies were carried out, primarily with a 30 at.% Ru/Ti electrode, at various stages during deactivation. These data were compared to those obtained for freshly formed Ru/Ti oxide films, ranging in Ru content from 5 to 40 at.%. Impedance data were collected at the oxide OCP (approximately 0.9 V versus SCE) in fresh NaCI solutions. Under these conditions, no chlorine reactions can occur and the OCP is defined by the equilibria of the redox states on the Ru oxide surface. Deactivation was generally accomplished by square-wave potential cycling, using overpotentials versus the chlorine/chloride potential of 1.59 to — 0.08 V (60 s cycle-1) in 5 M NaCI + 0.1 M HC1 solutions at room temperature. 

Fig. 5.8 Time dependence of impedance of 30 at.% Ru electrodes during deactivation in 5 M NaCI + 0.1 M HCI at room temperature. The electrodes were subjected to square-wave potential cycling from 1.35 to -0.32 V versus SCE at 60s cycle-1. Impedance was measured in 5 M NaCI at room temperature at OCP. 

Using, for example, cyclic voltammetry, the cathodic peak current (normalized to its value in the absence of RX) is a function of the competition parameter, pc = ke2/(ke2 + kin), as detailed in Section 2.2.6 under the heading Deactivation of the Mediator. The competition parameter can be varied using a series of more and more reducing redox catalysts so as eventually to reach the bimolecular diffusion limit. km is about constant in a series of aromatic anion radicals and lower than the bimolecular diffusion limit. Plotting the ratio pc = keij k,n + km) as a function of the standard potential of the catalysts yields a polarogram of the radical whose half-wave potential provides the potential where ke2 = kin, and therefore the value of... 

The electrode will promote the reduction of dioxygen, the reduction wave shifting from —0.5 V on bare gold to —0.05 V on the FePc film, but the intensity of the wave decays on repeated scanning indicating some deactivation process. The deactivation is caused by a combination of blocking by peroxide intermediates and actual loss of iron centres from the film. 

When dehydration occurs as a consecutive reaction, its effect on polarographic curves can be observed only, if the electrode process is reversible. In such cases, the consecutive reaction affects neither the wave-height nor the wave-shape, but causes a shift in the half-wave potentials. Such systems, apart from the oxidation of -aminophenol mentioned above, probably play a role in the oxidation of enediols, e.g. of ascorbic acid. It is assumed that the oxidation of ascorbic acid gives in a reversible step an unstable electroactive product, which is then transformed to electroinactive dehydroascorbic acid in a fast chemical reaction. Theoretical treatment predicted a dependence of the half-wave potential on drop-time, and this was confirmed, but the rate constant of the deactivation reaction cannot be determined from the shift of the half-wave potential, because the value of the true standard potential (at t — 0) is not accessible to measurement. 

A somewhat different application of foam is in blast and fire suppression during attempts to deactivate, or intentionally destroy, suspected terrorist bombs. When such a device, or even a small explosive device used to destroy the suspected item, explodes, severe damage can be caused by the compression wave emanating from the blast or from the fireball that may follow the blast. In Section 9.6 it was mentioned that foam blankets have been developed for blast noise and pressure wave... 

The quantitative treatment of absorption, electronic excitation, and deactivation of the excited state falls within the domain of wave mechanics. 

In aprotic medium, on the other hand, pyrimidine gives a reversible diffusion-controlled le wave at a very negative potential, with formation of a radical anion which is deactivated via two pathways rapid formation of the anionic, probably 4,4 -, dimer, with a rate constant of 8 x 105 L mol-1 sec-1, and proton abstraction from residual water in the medium at a much lower rate constant, 7 L mol-1 sec-1 98). This is rapidly followed by a further le reduction to produce, ultimately, 3,4-dihydropyrimidine 98). In the presence of acid there is also a le reduction wave corresponding to formation of a free radical which, as in aqueous medium, dimerizes, most likely to 4,4 -Z w-(3,4-dihydropyrimidine). Examination of the mechanism of reduction in acetonitrile in the presence of acids supported the conclusion that reduction of pyrimidine in aqueous medium is preceded by its protonation98). 

Other significant spectral changes are also observed. The total splitting increases from 8.3 cm-1 at B=0 T to 24 cm-1 at 5= 12 T. Moreover, due to the field induced mixings of the wave functions, the radiative allowedness of the transitions from the T substates to the ground state is strongly redistributed. The emission from the lowest B-field disturbed substate 1(B) becomes dominant, while the transitions 11(B) —> 0 and III(B) — 0 lose intensity. This is also displayed in the emission decay time of substate I at 1.5 K, which becomes as short as 12 ps at 12 T, while it amounts to 85 ps at zero-field (see next section). Due to this B-field induced increase of radiative allowedness, it also becomes possible to tune magnetically other important properties like the mechanisms of vibrational deactivation [78-82]. 

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