Poisoning surface selective

One promising extension of this approach Is surface modification by additives and their Influence on reaction kinetics. Catalyst activity and stability under process conditions can be dramatically affected by Impurities In the feed streams ( ). Impurities (promoters) are often added to the feed Intentionally In order to selectively enhance a particular reaction channel (.9) as well as to Increase the catalyst s resistance to poisons. The selectivity and/or poison tolerance of a catalyst can often times be Improved by alloying with other metals (8,10). Although the effects of Impurities or of alloying are well recognized In catalyst formulation and utilization, little Is known about the fundamental mechanisms by which these surface modifications alter catalytic chemistry. 

As already mentioned, the poison formation reaction is potential-dependent, and the poisoning rate for the basal planes is Pt(llO) > Pt(lOO) > Pt(lll) [Sun et al., 1994 Iwasita et al., 1996]. The case of Pt(lll) is special, since the poisoning has been associated with the presence of defects on the surface. Selective covering of the defects on the Pt(l 11) electrode by some adatoms prevents the formation of CO on the electrode surface [Macia et al., 1999, 2001 Smith et al., 2000]. 

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). 

Thus it has been concluded that the donor is fixed on the catalyst surface, selectively poisoning the non-specific centers and simultaneously increasing the reactivity of the isospecific centers. 

SELECTIVITY POISONS The Selectivity of a solid surface for catalyzing one reaction with respect to another is not well understood. However, it is known that some materials in the reactant stream will adsorb on the surface and then catalyze other undesirable reactions, thus lowering the selectivity. The very small quantities of nickel, copper, iron, etc., in petroleum stocks may act as poisons in this way. When such stocks are cracked, the metals deposit on the catalyst and act as dehydrogenation catalysts. This results in increased yields of hydrogen and coke and lower yields of gasoline. 

The ability of the poison to block the active catalytic sites can be studied by determining a decrease in the catalytically active component on the surface. Selective chemisorption changes may be employed53 to detect changes in the accessible metal. Alternately, the covering of active species may be inferred from its decrease in detection from surface-sensitive techniques such as ESCA O. The approach is that the ESCA or Auger spectra for the active component decreases as it becomes covered with the poison. The escape depth for the emission from the active elements is limited and... 

In many cases the effect of poisoning on catalytic surfaces can be represented as a linear function of the fraction of total surface poisoned ( non-selective poisoning). 

When the organometallic is transformed into adatoms these adatoms are expected to be present on some specific crystallographic positions of the particles and this leads to surface selective poisoning. 

The AIREs have a higher surface selectivity and sensitivity thus, the interface/surface reactions can be selectively monitored with less interference from the bulk solution. Generally, in situ FTIRS studies are concerned with the dissociative adsorption and oxidation of organic molecules, the formation, adsorption and oxidation of intermediates, the nature of adsorbed species and their interaction with catalysts, the determination of reaction selectivity, and also effects of catalyst composition, size, and morphology. The dual-path mechanism and active/poisoning intermediates in the electrooxidation of small organic molecules (SOMs) have been well characterized by in situ FTIRS methods. ... 

Anotlier important modification metliod is tire passivation of tire external crystallite surface, which may improve perfonnance in shape selective catalysis (see C2.12.7). Treatment of zeolites witli alkoxysilanes, SiCl or silane, and subsequent hydrolysis or poisoning witli bulky bases, organophosphoms compounds and arylsilanes have been used for tliis purjDose [39]. In some cases, tire improved perfonnance was, however, not related to tire masking of unselective active sites on tire outer surface but ratlier to a narrowing of tire pore diameters due to silica deposits. 

Unlike ion-selective electrodes using glass membranes, crystalline solid-state ion-selective electrodes do not need to be conditioned before use and may be stored dry. The surface of the electrode is subject to poisoning, as described earlier for a Ck ISE in contact with an excessive concentration of Br. When this happens, the electrode can be returned to its original condition by sanding and polishing the crystalline membrane. 

Catalytic Properties. In zeoHtes, catalysis takes place preferentially within the intracrystaUine voids. Catalytic reactions are affected by aperture size and type of channel system, through which reactants and products must diffuse. Modification techniques include ion exchange, variation of Si/A1 ratio, hydrothermal dealumination or stabilization, which produces Lewis acidity, introduction of acidic groups such as bridging Si(OH)Al, which impart Briimsted acidity, and introducing dispersed metal phases such as noble metals. In addition, the zeoHte framework stmcture determines shape-selective effects. Several types have been demonstrated including reactant selectivity, product selectivity, and restricted transition-state selectivity (28). Nonshape-selective surface activity is observed on very small crystals, and it may be desirable to poison these sites selectively, eg, with bulky heterocycHc compounds unable to penetrate the channel apertures, or by surface sdation. 

A selective poison is one that binds to the catalyst surface in such a way that it blocks the catalytic sites for one kind of reaction but not those for another. Selective poisons are used to control the selectivity of a catalyst. For example, nickel catalysts supported on alumina are used for selective removal of acetjiene impurities in olefin streams (58). The catalyst is treated with a continuous feed stream containing sulfur to poison it to an exacdy controlled degree that does not affect the activity for conversion of acetylene to ethylene but does poison the activity for ethylene hydrogenation to ethane. Thus the acetylene is removed and the valuable olefin is not converted. 

Nickel. As a methanation catalyst, nickel is presently preeminent. It is relatively cheap, it is very active, and it is the most selective to methane of all the metals. Its main drawback is that it is easily poisoned by sulfur, a fault common to all the known active methanation catalysts. The nickel content of commercial nickel catalysts is 25-77 wt %. Nickel is dispersed on a high-surface-area, refractory support such as alumina or kieselguhr. Some supports inhibit the formation of carbon by Reaction 4. Chromia-supported nickel has been studied by Czechoslovakian and Russian investigators. 

In selecting reference electrodes for practical use, one should apply two criteria that of reducing the diffusion potentials and that of a lack of interference of RE components with the system being studied. Thus, mercury-containing REs (calomel or mercury-mercuric oxide) are inappropriate for measurements in conjunction with platinum electrodes, since the mercury ions readily poison platinum surfaces. Calomel REs are also inappropriate for systems sensitive to chloride ions. 

An elegant complementary test to mercury poison is the use of dienes as selective poisons for homogeneous catalysts, due to their strong coordination to metal centres yielding inert catalytic complexes. In addition, their interaction with metal surfaces is weak. If the presence of diene (dienemetal =1 1) inhibits the catalytic process and Hg test does not, homogeneity can be strongly supported. 

The selectivity in a system of parallel reactions does not depend much on the catalyst size if effective diffusivities of reactants, intermediates, and products are similar. The same applies to consecutive reactions with the product desired being the final product in the series. In contrast with this, for consecutive reactions in which the intermediate is the desired product, the selectivity much depends on the catalyst size. This was proven by Edvinsson and Cybulski (1994, 1995) for. selective hydrogenations and also by Colen et al. (1988) for the hydrogenation of unsaturated fats. Diffusion limitations can also affect catalyst deactivation. Poisoning by deposition of impurities in the feed is usually slower for larger particles. However, if carbonaceous depositions are formed on the catalyst internal surface, ageing might not depend very much on the catalyst size. 

In order to improve the selectivity toward the formation of 1,3-PDO, we studied the influence of metal salt additives. While the addition of calcium or copper salts exhibited a moderate influence, the presence of iron salts played a significant role on the rate and selectivity of the reaction (Figure 35.1). The metal additives reduced noticeably the activity of the rhodium catalysts suggesting that they acted as a surface poison, but they modified the selectivity of the glycerol hydrogenolysis, probably through selective diol chelation. 

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