Poisoning adsorption coefficient

The model followed single-site, nondissociative, Langmuir-Hinshelwood poisoning. This resulted in the same adsorption coefficients for deactivation and start-of-cycle kinetics. 

There is actually a whole spectrum of possible phenomena, between very strong irreversible poisoning and normal competition between molecules for a given site (this competition leading to a decrease of activity). Inhibitors, as defined above, correspond to a behaviour intermediate between those just mentioned. The sensitivity to inhibitors can thus be expressed either as in the case of true poisons, or as in the case of competition. In the latter formulation, for example, the value or relative value, of the adsorption coefficient could characterize the inhibitor. 

The minimum concentration required to eliminate the catalytic activity is one possible measure of the sensitivity of a catalyst to a poison. Sensitivity to poisoning is most properly defined by the amount, np, of the poison adsorbed on a unit amount of catalysts which causes a given fractional decrease of the catalytic activity (a = (a0 - ap)/a0), where aQ and ap are the activities in the absence or presence of poison, respectively) ot/rtp is a measure of the sensitivity to poisoning. One may also use the ratio of a to the concentration cp of the poison in the feed, namely a/cp, but this is less precise, as this depends on the adsorption coefficient of the poison. 

In Eq. (9.263), ti3 corresponds to the reaction rate at inhnite time when the catalyst is maximally poisoned. This value depends on the ratio between the adsorption coefficient of the poison multiplied by poison concentration and the parameter W+1. It should be kept in mind that W = 6i/Oy reflecting the ratio between the coverage ofthe adsorbed intermediate and the fraction ofvacant sites. For low coverage 0 and thus the final catalyst, activity... 

Catalysts are porous and highly adsorptive, and their performance is affected markedly by the method of preparation. Two catalysts that are chemically identical but have pores of different size and distribution may have different activity, selectivity, temperature coefficient of reaction rate, and response to poisons. The intrinsic chemistry and catalytic action of a surface may be independent of pore size, but small pores appear to produce different effects because of the manner and time in which hydrocarbon vapors are transported into and out of the interstices. 

Due to the fact that protein adsorption in fluidized beds is accomplished by binding of macromolecules to the internal surface of porous particles, the primary mass transport limitations found in packed beds of porous matrices remain valid. Protein transport takes place from the bulk fluid to the outer adsorbent surface commonly described by a film diffusion model, and within the pores to the internal surface known as pore diffusion. The diffusion coefficient D of proteins may be estimated by the semi-empirical correlation of Poison [65] from the absolute temperature T, the solution viscosity rj, and the molecular weight of the protein MA as denoted in Eq. (16). 

K , equilibrium constant of main reaction Ki adsorption constant of i component ki kinetic coefficient of poisoning rate, [eqn.(8)] k2 kinetic coefficient of deactivation rate, [eqn. (9)] kgi mass transfer coefficient of i component ko pre-exponential factor in eqn. (6)... 

It is well known from catalysis that electropositive (e.g. Na, Cs, K) and electronegative (e.g. S, O, C, Cl) adatoms decrease or increase the reaction rate and thus poison or promote the reaction, respectively [153-155]. Alkali-metal influenced adsorption on transition metals was reviewed by Bonzel [154]. Coadsorption of alkali metals and H, or D, on Al(lOO) revealed that the sticking coefficient and dissociation rate are extremely weak ( 10 at all alkali coverages [156]). Upon exposing alkali-covered metal substrates to a beam of atomic H or D, alkali hydride formation was observed. 

Though the problem of surface poisoning is of great importance in catalysis and H - embrittlement (Berkowitz et al., 1976), there is a big gap between results of experimental and theoretical work on this subject. Some experimental facts are C, 0 and CO strongly influence H adsorption on Ru (Feulner and Menzel, 1985)J 0 decreases the overall H sorption kinetics and the sticking coefficient (Fromm and Wulz, 1984) S on Pd inhibits desorption of H probably by affecting the surface mobility of H (Bucur, 1981) SO2,... 

CH, HCHO and HCOOH were found in reformate generated from methanol (Narusawa et al. 2003). All three small organic molecules are present in minute quantities from either steam reforming or autothermal reforming. No adverse effect was observed from CH adsorption on platinum electrodes. On the other hand, both HCHO and HCOOH had a negative impact on fuel cell performance. It is estimated that the poisoning coefficient for HCHO is about 0.1 times that for CO, whereas the poisoning coefficient for HCOOH is only 0.004 times that for CO. 

These adsorptions cause aging of the catalyst. A poison is characterized by its toxicity coefficient, which is the strongly negative slope of the line of the rate ratio in the presence and absence of poison, depending on its concentration in the inlet gas. 

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