Self-poisoning process

Methanol oxidation is a self-poisoning process, in which the intermediate adsorbate CO, formed from the dissociation of methanol, poisons the catalytic sites of Pt. The mechanism of methanol oxidation is as follows ... 

The formation of (XX) requires the elimination of one fluorine atom per four molecules of amine. The fluorine was expelled as F, and since the enzyme reaction is retarded by F, the process is self-poisoning. As might have been expected the reaction was not complete it stopped at 30 per cent completion and F was detected on the walls of the containing vessel. 

FTS is a process which is plagued by the self-poisoning of the catalyst, essentially... 

FTS is a process which is plagued by the self-poisoning of the catalyst, essentially by coking. A homogeneous catalyst would in principle be free of such troubles and therefore, several years after 1974, an intensive search was conducted for homogeneous FTS catalysts. However, when it appeared that the most efficient synthesis pathway comprised initiation by CO dissociation, which required large ensembles of active atoms [8], the research in this direction was stopped. 

One can illustrate the reactivity effect of an active modifier such as molybdenum by the change in behavior with respect to surface processes in which CO is an important reactant. Table 1 [8] shows that the self-poisoning... 

The over-oxidation of Pt active sites occurs if the rate of the surface chemical reaction is lower than the rate of oxygen supply [3,4]. We propose that in most instances the self-poisoning of the catalyst is the reason for the low rate of surface chemical reaction. Consequently, the over-oxidation of Pt is not the reason, but the result of the deactivation process. Note that most of the authors, who claimed that catalyst deactivation was due to over-oxidation of the active sites, had never measured the oxidation state of their catalyst [e.g. 3,4,18]. We propose that there is a strong correlation between the role of promoter, the oxidation state of the catalyst and the catalyst deactivation, as it will be shown in the following section. 

It seems reasonable to assume that an oxidation-reduction dynamic equilibrium is established on the catalyst surface during the reduction step and that appreciable restructuring of the catalyst surface can take place. Since the kinetics of the process are controlled by pore diffusion, particle size and shape once again play an important role in determining the initial activity of the catalyst. In fact, rate measurements conducted by the authors in the laboratory with prereduced catalysts of various particle sizes show that the initial activity of the larger catalyst particles is considerably lower than that of the smaller particles. Consequently, another corrective factor, EFsp, which takes into account the influence of the catalyst particle size on this inherent self-poisoning effect, must be included in the rate equation for use in converter calculations. Values of EFsp for various catalyst size ranges are reported in Table 6.11. 

In the case of direct methanol fuel cells, compared with oxygen reduction, methanol oxidation accounts for the main activation loss because this process involves six-electron transfer per methanol molecule and catalyst self-poison when Pt alone was used from the adsorbed intermediate products such as COads-From the thermodynamic point of view, methanol electrooxidation is driven due to the negative Gibbs free energy change in the fuel cell. On the other hand, in the real operation conditions, its rate is obviously limited by the sluggish reaction kinetics. In order to speed up the anode reaction rate, it is necessary to develop an effective electrocatalyst with a high activity to methanol electrooxidation. Carbon-supported (XC-72C, Cabot Corp.) PtRu, PtPd, PtW, and PtSn were prepared by the modified polyol method as already described [58]. Pt content in all the catalysts was 20 wt%. 

The effect of diffusion was also investigated by means of computer calculations. A run at rather extreme conditions, 450 °C, 317 atm, and space velocity = 32 100 vol/h, had an effectiveness factor of 0.5 and 0.8 at 10 and 20% into the catalyst bed respectively. In the main part of the bed, the effectiveness factor was very close to 1.0 as in the majority of the runs. Flow conditions in the experiment were in the viscous region or in the transition region between viscous and turbulent flow. The non-ideal conditions due to flow and wall effects were believed to have minor effects. The fact that the catalyst was reduced in the 3-6 mm size range affect the intrinsic activity due to the self-poisoning by water diffusing out of the pores during the reduction process. 

In summary, the experimental observations of minima in the kinetic studies are not in question. These are clearly the result of the type of experiment involved. In interpreting the results, however, it needs to be recognized that these observations are the consequence of the intermixing of two distinctly different kinetic processes. This intermixing leads to the observed minima. The role, if any, of self-poisoning in influencing the kinetics, either overall or growth, must be superposed on the rapid transformation of a folded crystallite to an extended one.(6c,6d)... 

In addition to the CIR process the cosmetic industry has instituted a second, important, self-regulatory procedure the voluntary reporting of adverse reactions, which is intended to provide data on the type and incidence of adverse reactions noted by consumers or by their medical advisors. This reporting procedure creates early awareness of problems handled outside hospital emergency facilities or centers for acute poisoning. 

As noted earlier, microtubule elongation has been characterized largely with respect to the involvement of guanine nucleotides and the modes of drug inhibition of microtubule formation. There have also been a number of important studies on the influence of microtubule-associated proteins and solution variables on the kinetics and thermodynamics of microtubule self-assembly. Of these, the characterization of the so-called mitotic spindle poisons has been particularly complex because of the variety of agents and the diversity of systems studied. For this reason, we shall concentrate on the other factors affecting the elongation process. 

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