Self-poisoning

The kinetics of oxidation over noble metals is dramatically different and much more complex. Every chemical species has an inhibiting effect on the rate of oxidation of another species. Carbon monoxide is a particularly strong self-poison, so that its oxidation kinetics usually proceeds at a negative order with respect to CO concentration. The kinetics also... 

Fig. 19. Comparison of first-order kinetics with highly self-poisoned kinetics.

Experimental evidence illustrating the effect that hydrides of nickel or its alloys with copper have on the catalytic activity of the respective metals is to be found in papers which discuss catalytic consequences of the special pretreatment of these metal catalysts with hydrogen during their preparation. One must also look very carefully into cases where self-poisoning has been reported as appearing in reactions of hydrogen with other reactants. 

In several papers dealing with catalytic reactions involving hydrogen and unsaturated hydrocarbons the observed self-poisoning of nickel or its alloys has been quite properly attributed to the presence of carbonaceous... 

Czeizel AE. 1994. Phenotypic and cytogenetic studies in self-poisoned patients. Mutat Res 313 175-181. 

Table 3), Enantioselective reaction was of order 0 7 in hydrogen by the initial rate method (over the range 2 to 50 bar, 293 K, cinchonine modifier) and 0 2 in pyruvate (0 1 to 3.0 M, 293 K, 10 bar pressure, cinchonine modifier) Enantiomeric excess was independent of reactant concentrations within these ranges Reactions exhibited self-poisoning so that complete conversion was not achieved within 20 h reaction time. As the quantity of cinchonine modifier added to the catalyst was increased from zero to 1 gram per gram so the... 

The reduction of this ester over Pd differed from the corresponding reaction over Pi in every important particular. Enantiomeric excess was low (high over Pt) and in the reverse sense (e g cinchonidine modification provided an S-excess in the product over Pd but an R-excess over Pt) Enantioselective reactions underwent self-poisoning over Pd (proceeded to completion over Pt), were of non-integral order (integral over Pt) and proceeded more slowly than reaction over unmodified catalyst (enhanced rate over Pt) Enantioselective reaction was solvent-specific over Pd (not over Pt) and was favoured by low catalyst reduction temperature (high reduction temperature for Pt)... 

Steady state measurements of NO decomposition in the absence of CO under potentiostatic conditions gave the expected result, namely rapid self-poisoning of the system by chemisorbed oxygen addition of CO resulted immediately in a finite reaction rate which varied reversibly and reproducibly with changes in catalyst potential (Vwr) and reactant partial pressures. Figure 1 shows steady state (potentiostatic) rate data for CO2, N2 and N2O production as a function of Vwr at 621 K for a constant inlet pressures (P no, P co) of NO and CO of 0.75 k Pa. Also shown is the Vwr dependence of N2 selectivity where the latter quantity is defined as... 

Macia MD, Herrero E, Fehu JM, Aldaz A. 1999. Fomtic acid self-poisoning on bismuth-modified Pt(755) and Pt(775) electrodes. Electrochem Commun 1 87-89. 

A comprehensive kinetic model addressing all the findings has not been developed. Some of the reported rate equations consider the self-poisoning effect of the reactant compounds, some other that effect of ammonia, and so on so forth. The reported data is dispersed with a variety of non-comparable conditions and results. The adsorption of the poisoning compounds has been modeled assuming one or two-sites on the catalyst surface however, the applicability of these expressions also needs to be addressed to other reacting systems to verity its reliability. The model also needs of validated adsorption parameters, difficult to measure under the operating conditions. 

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. 

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