Poison curve types

The phosphorus deactivation curve is typical type C, and, according to the Wheeler model, this is associated with selective poisoning of pore mouths. Phosphorus distribution on the poisoned catalyst is near the gas-solid interface, i.e. at pore mouths, which confirms the Wheeler model of pore mouth poisoning for type C deactivation curves. Thus we may propose that in the fast oxidative reactions with which we are dealing, transport processes within pores will control the effectiveness of the catalyst. Active sites at the gas-solid interface will be controlled by relatively fast bulk diffusional processes, whereas active sites within pores of 20-100 A present in the washcoat aluminas on which the platinum is deposited will be controlled by the slower Knudsen diffusion process. Thus phosphorus poisoning of active sites at pore mouths will result in a serious loss in catalyst activity since reactant molecules must diffuse deeper into the pore structure by the slower Knudsen mass transport process to find progressively fewer active sites. 

This equation shows that for fast reaction in which a poison is distributed homogeneously, activity will fall less than linearily with poison concentration. This type of poisoning might be called anti-selective. Physically this occurs because the reaction uses more of the internal surface of the less active poisoned catalyst. That is, the slower reaction on a poisoned catalyst penetrates deeper into the catalyst pellet. In Fig. 8 we plot the poisoning curves for these two limiting cases ho very large (Curve B) and ho very small (Curve A). For intermediate values of ho, intermediate curves would be obtained. We next consider the... 

Fig. 8. Types of poisoning curves to be expected for porous catalysts. Curve A is for a nonporous catalyst. Curve B is for homogeneous adsorption of poison (eq. 75). Curves C and D are for preferential adsorption of poison near the pore mouth. For curve C, ho 10 and for curve D, ho = 100 (eqs. 77, 78).

Curve Symbol Catalyst system Temp. °C Poison type Ref. 

Fig. 21. (A) Potential-relaxation profiles, with arrest, giving the type of curves in (C). (Original plots based on 1(X) points.) (B) Corresponding t versus log i/[l — exp(-2t F/RT)] profiles for chemisorption of H on Pt in the HER from acid solutions. (C) Pseudocapacilance profiles for chemisorption of H on Pi in the HER from acidic solution as a function of overpotential. The series of curves of decreasing arises from progressive adventitious poisoning during extended periods of electrolysis. (From Ref. 136.)...

Many such poisoning titrations have been performed over the years and the results vary widely with the poison chosen and the catalyst used [377]. Some other examples are listed in Table 7. Clearly, some poisons are more selective than others. In Figures 29 and 30, one can see the typical shape of the titration curve as different types of poisons were added incrementally to a polymerization reaction. The activity, on a relative basis, is plotted against the poison stoichiometry (number of poison molecules added per Cr atom). All of the curves have the classical poisoning profile. Benzene was a mild poison, whereas oxygen was a severe one. 

On the other hand, lead poisoning deactivation curves have non-selective characteristics (Type A). These result when the poison is less strongly chemisorbed, and it tends to suffer many collisions with the alumina washcoat structure before chemisorption. Consequently, lead is found deep inside the washcoat structure, as is demonstrated by electron probe microanalysis, and the more accessible metal sites are left active to gaseous reactants by the faster bulk transport processes. 

Figure 4 shows the total conversion of ethanol as a function of temperature as measured by gas chromatography. Except for the silica catalysts, the platinum catalysts exhibit equal or lower light-ofif temperatures than the supported catalysts with palladium as active material (compare with Figure 7). The platinum on alumina and platinum on titania catalysts are more active than the other catalyst combinations. The conversion curves for the Pd and Pt on ceria catalysts practically coincide, which implies that ceria would be a more suitable support material for a palladium catalyst than for a platinum catalyst. The activities of the silica catalysts are low. This observation is consistent with recent results in another research project using the same type of silica sol (Zwinkels et al, 1994). According to these experiments, it is crucial to reduce the alkali content to a very low level in the support, since sodium increases the mobility of silica, which poisons the active platinum and palladium sites. Platinum is apparently more sensitive to this phenomenon than palladium.

Figure 7.23 Types of relationships encountered with porous catalysts. Curve A is the result for nonselective deactivation for equations (vi) and (xvi). Curve B is for uniform deposition of poison with (po large and antiselective deactivation, equation (v). Curves C and D are for selective deactivation, equation (xiv), with Q = 10 and 100, respectively. [After A. Wheeler, Advan. Catal., 3, 249 with permission of Academic Press, New York, NY, (1951).]...

Another type of analysis is based on the pattern of the concentration front in the bed, essentially the reverse image of the breakthrough curve, particularly when this front passes through the bed with a eonstant velocity. Some of this we mentioned in Chapter 4. Such constant pattern waves are not limited to ion exchange or adsorption, for they are often encountered in the poisoning of fixed-bed reactors, in either isothermal [A. Wheeler and A.J. Robell, J. CataL, 13, 299 (1969) H.W. Haynes, Jr., Chem. Eng. ScL, 25, 1615 (1970)] or nonisothermal operation [H.S. Weng, G. Eigenberger and J.B. Butt, Chem. Eng. Sci., 30, 1341 (1975) T.H. Price and J.B. Butt, Chem. Eng. Sci., 32, 393 (1977)]. 

This method was first reported by Heinemann et al. [1] for the polymerization of ethylene with the catalysts depicted in Figure 3.15 in the presence of clays with different types of organic modifications. Figure 3.16 compares the ethylene uptake curves with homogeneous and clay-supported MBI catalyst. Lower activity and more stable polymerization rate were observed for the clay-supported system. As water present on the clay surface acts as a catalyst poison [71], the low polymerization activity observed for the clay-supported system was attributed to water traces remaining on the organoclay surface. 

Once the mathematical problem has been formulated and solved, the numerical results should be compared to experimental results. In PEM fuel cell modeling, experimental data of the polarization curve are normally used for comparison reasons. The polarization curve is a measure of the performance of the PEM fuel cell. There are two types of performance measures, a full cell polarization curve and a half-cell polarization curve. In many cases, which is also the case in CO poisoning, mass transport is simulated in only half of the cell. One of the most widely used experimental datasets for CO poisoning was collected by Lee et al. [96]. In their study, they investigated the performance of the cell exposed to CO and studied the behavior of the performance depending on the electrode used. Baschuk and Li [21] compared their numerical results to the results from Lee et al. [96]. The comparison is shown in Figure 7.7 and Figure 7.8. 

Let us first concentrate on the 6-values of the fractions in the concentrated (6") and dilute (6 ) phases. Graphs of V x) and b x) curves for some of the initial distribution functions listed in Table 1 are collected in Fig. 8. The drawn curves, which represent 6"(x) for different values of [Pg.16]

In summary, the PbB decay rates reported under experimental and epidemiological survey conditions indicate relatively short biological half-lives. The values of the half-lives vary with the type of study, e.g., length of survey and number of measurement points. For example, the lead-poisoned workers of Hryhorczuk et al (1985) showed a median PbB decay half-life of 619 days when followed for more than 5 years. The PbB curves for these subjects probably included more of the slow decay component than in any of the other reports. With an increase in lead exposure, adults appear to require ca. 60 days to return to exposure steady state, i.e., a rise in the PbB curve followed by a plateau. 

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