Poisoning models

For Fe, Zn and Pb that may cause chemical poisoning, model poisoned catalysts were prepared by dipping catalysts into aqueous solutions of metal nitrates at various concentrations, and the catalyst carrying the nearest amount of each element was selected for the selectivity measurement. The uniform distribution of the loaded metals in the catalyst layer was confirmed by EPMA line analysis. While B. E. T. surface areas of the model poisoned catalysts differ little, the amoimt of CO adsorption decreases with the increase in the concentration of the poisonous metal, and it is noteworthy that the amount greatly decreases by loading a trace amount of Zn or Pb. 

Fig. 5. Activity of the model-poisoned catalyst. Catalyst the same as in Fig. 4 (d). Reaction conditions and symbols are the same as in Fig. 1 except for GHSV (a) 30000 h , (b) 90000 h. ...

A Pt-Rh three way catalyst used in natural gas-fueled engine systems for 21,000 h showed specific deactivation characteristics, including a decrease in the selectivity of NO reduction, which can neither be reproduced by heat treatment nor explained by physical poisoning such as the blockage of micropores. Through chemieal analyses, EPMA, and activity tests of the used catalyst and model-poisoned catalysts, it was found that the activities of Rh on the used eatalyst were decreased by chemical poisoning due to Pb, causing a decrease in the NO reduction selectivity, and that the absolute rates of NO reduction and other reactions are considerably reduced by a decrease in the effective surface area of the catalyst due to accumulated compounds on the wash coat surface, in addition to thermal effects. 

Measurements were carried out with the model poison triethyl phosphate at 520-800 K. 

The second case was the shell model poisoning or diffusion controlled mechanism. They used the results of Haynes to calculate the rate of reaction ... 

The typical industrial catalyst has both microscopic and macroscopic regions with different compositions and stmctures the surfaces of industrial catalysts are much more complex than those of the single crystals of metal investigated in ultrahigh vacuum experiments. Because surfaces of industrial catalysts are very difficult to characterize precisely and catalytic properties are sensitive to small stmctural details, it is usually not possible to identify the specific combinations of atoms on a surface, called catalytic sites or active sites, that are responsible for catalysis. Experiments with catalyst poisons, substances that bond strongly with catalyst surfaces and deactivate them, have shown that the catalytic sites are usually a small fraction of the catalyst surface. Most models of catalytic sites rest on rather shaky foundations. 

Beginning with the 1975 U.S. automobiles, catalytic converters were added to nearly all models to meet the more restrictive emission standards. Since the lead used in gasoline is a poison to the catalyst used in the converter, a scheduled introduction of unleaded gasoline was also required. The U.S. petroleum industry simultaneously introduced unleaded gasoline into the marketplace. 

Latent cancer is calculated to be the primary risk from a nuclear accident (this may be due to the conservatism in the low-dose models). At Chernobyl, most of the deaths were from fire and impact. Chemical process risk depends on the chemicals being processed. Experience shows that processing poisons poses the highest risk to public and workers. 

Interest in the ZGB model arises due to its rieh and eomplex irreversible eritieal behavior. In faet, in two dimensions and for the asymptotie regime (t — cxd), the system reaehes a stationary state whose nature solely depends on the parameter 7 - For Fa < Fia = 0.3874 (Fa > F2A = 0.5250) the surfaee beeomes irreversibly poisoned by B (A) speeies, while for Fia < Fa < F2A a steady state with sustained produetion of AB is observed. Fig. 2 shows plots of the rate of AB produetion (J ab) and the surfaee eoverage with A ( a) and B ( s) speeies versus Fa. So, just at Fja and F2A the ZGB model exhibits IPTs between the reaetive regime and poisoned states, whieh are of seeond and first order, respeetively. Experimental evidenee of a first-order transition-like behavior has been reported for the eatalytie oxidation of earbon monoxide on Pt(210) and Pt(lll) [19], as shown, e.g., in Fig. 3. 

Negleeting CO desorption, as in the standard ZGB model, the CO-poi-soned state is irreversible sinee there is no possibility of removing CO from the surfaee. So, CO desorption has to be eonsidered in order to avoid the fully CO-poisoned state. The adsorption and desorption of X then drives the system from a state with high eoneentration of adsorbed CO to the reaetive state and baek. This proeess ean be understood with the aid of Fig. 8. At low X eoverage only the reaetive state is stable. Inereasing X eoverage eauses site bloeking and eonsequently the adsorption of both CO and O2 is redueed. 

Eight variants of the DD reaction mechanism, described by Eqs. (21-25) have been simulated. The simplest approach is to neglect B2 desorption in Eq. (22) and the reaction between AB species (Eq. (25)). For this case, an IPT is observed at the critical point Tib, = 2/3. Thus this variant of the model has a zero-width reaction window and the trivial critical point is given by the stoichiometry of the reaction. For Tb2 < T1B2 the surface becomes poisoned by a binary compound of (A -I- AB) species and the lattice cannot be completely covered because of the dimer adsorption requirement of a... 

The phase diagram of the MM model is quite simple for I"a< — 1/2 (7a > 7]a) the catalyst becomes poisoned by A (B) species, respectively. Thus one has a first-order IPT where 7ia = 1/2 is a trivial critical point given by the stoichiometry of the reaction. In contrast to the ZGB model. 

The MM model with one species desorption (say B) has also been studied [90]. Due to desorption, the B-poisoned state is no longer observed and the system undergoes a second-order IPT between a reactive regime and an A-poisoned state. The behavior of the MM model with one species desorption is similar to another variant of the MM model which incorporates the Eley-Rideal mechanism [57]. 

P. Meakin. Simple models for heterogeneous catalysis with a poisoning transition. J Chem Phys 95 2903-2910, 1990. 

J. W. Evans, T. R. Ray. Interface propagation and nucleation phenomena for discontinuous poisoning transitions in surface reaction models. Phys Rev E 50 4302 314, 1994. 

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