Poisoned reactions

Moisture measurements are important in the process industries because moisture can foul products, poison reactions, damage equipment, or cause explosions. Moisture measurements include both absolute-moisture methods and relative-humidity methods. The absolute methods are those that provide a primaiy output that can be directly calibrated in terms of dew-point temperature, molar concentration, or weight concentration. Loss of weight on heating is the most familiar of these methods. The relative-humidity methods are those that provide a primaiy output that can be more direc tly calibrated in terms of percentage of saturation of moisture. 

When the poisoning reaction is analyzed under potential control, the formation rate is dependent on the electrode potential. The hrst experiments that clearly showed that the poison formation reaction was potential-dependent were performed by Clavilier using pulsed voltammetry [Clavilier, 1987] (Fig. 6.15). In this technique, a short pulse at high potential is superimposed on a normal voltammetric potential... 

The qualitative voltammetric behavior of methanol oxidation on Pt is very similar to that of formic acid. The voltammetry for the oxidation of methanol on Pt single crystals shows a clear hysteresis between the positive- and negative-going scans due to the accumulation of the poisoning intermediate at low potentials and its oxidation above 0.7 V (vs. RHE) [Lamy et al., 1982]. Additionally, the reaction is also very sensitive to the surface stmcture. The order in the activity of the different low index planes of Pt follows the same order than that observed for formic acid. Thus, the Pt(l 11) electrode has the lowest catalytic activity and the smallest hysteresis, indicating that both paths of the reaction are slow, whereas the Pt( 100) electrode displays a much higher catalytic activity and a fast poisoning reaction. As before, the activity of the Pt(l 10) electrode depends on the pretreatment of the surface (Fig. 6.17). 

A different mechanism seems to operate in the case of poison formation from methanol [Herrero et al., 1993]. In this case, modification of the Pt(lll) surface by Bi deposition only causes a linear decrease in the amount of poison formed, indicating the existence of a mere third-body effect. Complete inhibition of the poisoning reaction is achieved for > 0.23, i.e., before the surface is completely covered. This suggests the existence of ensemble requirements for this reaction, which need enough free contiguous Pt sites to take place. 

The frara-effect in co-ordination chemistry is well known however, what has not been examined in any detail is the effect of trons-molecules in heterogeneous catalytic hydrogenation. In this paper we will show that trans molecules hydrogenate more slowly than other isomers and can poison reactions of species that would be expected to be more strongly bound. However, if a c/s/frans-mixture is used this strong adsorption can be disrapted. 

In the design of commercial scale heterogeneous catalytic reactors, the activity of the catalyst will almost invariably change with time. We now wish to focus our attention on the implications of poisoning reactions for efficient use of catalyst surface areas. Since reactant molecules must interact with unpoisoned catalyst sites before reaction can occur, the poisoning process may have two effects on the reaction rate one observes. 

If the species giving rise to the poisoning reaction ipust make several collisions with the catalyst... 

In order to demonstrate the selective effect of pore-mouth poisoning, it is instructive to consider the two limiting cases of reaction conditions corresponding to large and small values of the Thiele modulus for the poisoned reaction. For the case of active catalysts with small pores, the arguments of all the hyperbolic tangent terms in equation 12.3.124 will become unity and... 

Be careful using food supplements prescribed by a therapist who is not sufficiently knowledgeable about MCS and the problems engendered by an impaired detoxification process due to damaged detoxification paths in the liver, which often occurs with MCS patients (see entry 27). In consultation with your therapist, start by taking smaller than normal doses and slowly build up to a higher dose, to avoid a pile-up and poisonous reactions. The best way to take supplements is to get treated by an environmental health specialist. 

Rate of the exchange reaction per unit volume of catalyst, mol m 3 s 1 Rate of HN03 poisoning reaction, mol m 2 1 Reaction time, s... 

The main reaction i.e, benzene hydrogenation occuring inside porous Ni-catalyst pellets is accompanied by poisoning reaction in which the thiophene presented in the feed stream reacts irtevcrsibly with the catalytic active sites. An analysis was made assuming isothermal behaviour [7], the same effective diffusivity for reactant and poison and that the steady-state continuity equation represents a good approximation at all times [8,9]. Under these conditions the mass balances for benzene, thiophene and catalyst activity are... 

Evidently, however, another species arises in a side, self-poisoning, reaction and extensively covers the surface, inhibiting the progress of the above main reaction in the sequence of steps shown (89-91) In situ IR spectroscopy shows that this species is principally chemisorbed CO, bridged or linearly bonded to surface metal atoms. Its behavior is similar to that observed with CO directly chemisorbed at a Pt electrode from the gas phase. However, the mechanism of its catalytic formation from HCOOH is unclear. It is well known that CO can be formed from HCOOH by dehydration, but such conditions do not obtain at a Pt electrode in excess liquid water. Hence a catalytic pathway for adsorbed CO formation has to be considered. The species C=0 or C—OH are not to be regarded as the kinetically involved intermediates in the main reaction sequence (Section IV). Because the poisoning species seems to be formed in the presence of coadsorbed, H steps such as... 

Deactivation Kinetics A separable form of rate equation was used for the poisoning reaction, first order with poison concentration and active sites, respectively. Similar forms have been used elsewhere (Richardson, 1971 Baiker et al -, 1966 Zrncevic and Gomzi, 1983) From previous studies done on the thiophene... 

Hence the effect of poisoning is proportional to — a that is, it is less than a linear effect. This situation, corresponding to a slow poisoning reaction and a fast (large OJ main reaction, was termed antiselectivepoisoning by Wheeler. 

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