Surface reactions Poisoning

Toxic heavy metals and ions, eg, Pb, Hg, Bi, Sn, Zn, Cd, Cu, and Fe, may form alloys with catalytic metals (24). Materials such as metallic lead, ziac, and arsenic react irreversibly with precious metals and make the surface unavailable for catalytic reactions. Poisoning by heavy metals ordinarily destroys the activity of a precious-metal catalyst (8). 

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... 

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. 

H. C. Kang, W. H. Weinberg. Interface roughening and kinetics of poisoning in a surface reaction. J Chem Phys 700 1630-1633, 1994. 

As shown by Re. 1—2, methanol oxidation to carbon dioxide is a six-electron reaction. This reaction, however, does not proceed by a simple single step. On the contrary, it is considered as a complex multi-step reaction involving several intermediates and by-product which may be different depending on the catalysts, media, temperature and other conditions. Although the details are yet to be found, it is widely agreed that some intermediates or by-products accumulate on the surface and poison the catalyst to decrease its activity. 

The importance of catalyst stability is often underestimated not only in academia but also in many sectors of industry, notably in the fine chemicals industry, where high selectivities are the main objective (1). Catalyst deactivation is inevitable, but it can be retarded and some of its consequences avoided (2). Deactivation itself is a complex phenomenon. For instance, active sites might be poisoned by feed impurities, reactants, intermediates and products (3). Other causes of catalyst deactivation are particle sintering, metal and support leaching, attrition and deposition of inactive materials on the catalyst surface (4). Catalyst poisons are usually substances, whose interaction with the active surface sites is very strong and irreversible, whereas inhibitors generally weakly and reversibly adsorb on the catalyst surface. Selective poisons are sometimes used intentionally to adjust the selectivity of a particular reaction (2). 

One or more steps may form a dead end in the form of an intermediate formed through an elementary reaction and consumed exclusively by the reverse of this step. Although the dead-end will not contribute to the overall reaction rate, the step may affect the kinetics if the intermediate is strongly adsorbed on the surface. The poisonous effect of H2O in ammonia synthesis is an example. 

Fig. 12.4. Stationary-state solutions and limit cycles for surface reaction model in presence of catalyst poison K3 = 9, k2 = 1, k3 = 0.018. There is a Hopf bifurcation on the lowest branch p = 0.0237. The resulting stable limit cycle grows as the dimensionless partial pressure increases and forms a homoclinic orbit when p = 0.0247 (see inset). The saddle-node bifurcation point is at...

We have been able to identify two types of structural features of platinum surfaces that influence the catalytic surface reactions (a) atomic steps and kinks, i.e., sites of low metal coordination number, and (b) carbonaceous overlayers, ordered or disordered. The surface reaction may be sensitive to both or just one of these structural features or it may be totally insensitive to the surface structure, The dehydrogenation of cyclohexane to cyclohexene appears to be a structure-insensitive reaction. It takes place even on the Pt(l 11) crystal face, which has a very low density of steps, and proceeds even in the presence of a disordered overlayer. The dehydrogenation of cyclohexene to benzene is very structure sensitive. It requires the presence of atomic steps [i.e., does not occur on the Pt(l 11) crystal face] and an ordered overlayer (it is poisoned by disorder). Others have found the dehydrogenation of cyclohexane to benzene to be structure insensitive (42, 43) on dispersed-metal catalysts. On our catalyst, surfaces that contain steps, this is also true, but on the Pt(lll) catalyst surface, benzene formation is much slower. Dispersed particles of any size will always contain many steplike atoms of low coordination, and therefore the reaction will display structure insensitivity. Based on our findings, we may write a mechanism for these reactions by identifying the sequence of reaction steps ... 

Selective poisoning occurs with very active catalysts. Initially, the exterior surface is poisoned and then, as more poison is added, an increasing depth of the interior surface becomes poisoned and inaccessible to reactant. If the reaction rate in the unpoisoned portion of catalyst happens to be chemically controlled, the reaction... 

Catalytic reactions at a metal surface involve a subtle and delicate balance of adsorption forces. Too weak an adsorption and the catalyst will have low activity, too strong and the surface becomes poisoned by adsorbed reactants or products. Consequently, quite small changes in the nature of a metal surface may result in significant variations in catalytic properties. Structure sensitivity is known to exist. There is good evidence that the selectivity and activity of a metal catalyst are affected by changes in structure and/or electronic properties. 

Is it known that the rate of hydrogenolysis reactions are extremely sensitive to effects of alloying, surface contamination, poisoning, etc. Consequently, in all cases where supported metals are used there must be concern as to whether apparent particle size effects are due to structure sensitivity or to some minor contamination effect. In the few cases where clean single crystal surfaces have been used there is evidence of a structure effect.338 However, the maximum change in activity between different crystal faces seems to be about a factor of 10. For Ni single crystals the (100) surface is more active than the (111) surface. A similar conclusion has been reached for oriented Ni powder samples.339... 

In heterogeneous catalysis, chemisorption of reagents is a preliminary step of the surface reaction. Thus every discussion about catalytic modification (activity, selectivity, and lifetime of the catalyst), induced by poison deposition, has to be carried out in parallel with a study of the poison... 

After the type and strength of interaction of a potential poison with the catalyst surface has been studied and the number of adsorption sites estimated, its effect on the rate of a given catalytic reaction can be studied. Any kind of catalytic reactor may in principle be used for these studies, that is, static as well as dynamic methods are suitable, and the various forms of pulse techniques are applicable. The real distinction between the two types of poisoning experiments that have been performed lies in the fact that the poison is either fed together with the reactant and is present in the gas phase throughout the run or the surface is poisoned by irreversibly preadsorbing the poison while the gas phase is kept free of it. If the poison is present in the gas phase, a larger number of modes of interaction with different surface sites may be possible than for the... 

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