Catalytic poisoning

The palladium may be recovered by heating the spent catalyst to redness in order to remove organic impurities this treatment may reduce some of the barium sulphate to barium sulphide, which acts as a catalytic poison. The palladium is then dissolved out with aqua regia and the solution evaporated the residue is dissolved in hot water and hydrochloric acid to form palladium chloride. 

J. W. Evans, M. S. Miesch. Characterizing kinetics near a first-order catalytic poisoning transition. Phys Rev Lett 66 833-836, 1991. 

It is only since 1980 that in situ spectroscopic techniques have been developed to obtain identification of the adsorbed intermediates and hence of reliable reaction mechanisms. These new infrared spectroscopic in situ techniques, such as electrochemically modulated infrared reflectance spectroscopy (EMIRS), which uses a dispersive spectrometer, Fourier transform infrared reflectance spectroscopy, or a subtractively normalized interfacial Fourier transform infrared reflectance spectroscopy (SNIFTIRS), have provided definitive proof for the presence of strongly adsorbed species (mainly adsorbed carbon monoxide) acting as catalytic poisons. " " Even though this chapter is not devoted to the description of in situ infrared techniques, it is useful to briefly note the advantages and limitations of such spectroscopic methods. 

The electrodes in the direct methanol fuel cell (DMFC) (i.e. the anode for oxidising the fuel and the cathode for the reduction of oxygen) are based on finely divided Pt dispersed onto a porous carbon support, and the electro-oxidation of methanol at a polycrystalline Pt electrode as a model for the DMFC has been the subject of numerous electrochemical studies dating back to the early years ot the 20th century. In this particular section, the discussion is restricted to the identity of the species that result from the chemisorption of methanol at Pt in acid electrolyte. This is principally because (i) the identity of the catalytic poison formed during the chemisorption of methanol has been a source of controversy for many years, and (ii) the advent of in situ IR culminated in this controversy being resolved. 

Figure 3.35 shows the potential dependence of the integrated band intensity of the linear CO observed in the experiment described above and the corresponding variation in the methanol oxidation current. The latter was monitored as a function of potential after the chemisorption of methanol under identical conditions to those employed in the IRRAS experiments. As can be seen from the figure the oxidation of the C=Oads layer starts at c. 0.5 V and the platinum surface is free from the CO by c. 0.65 V. The methanol oxidation current shows a corresponding variation with potential, increasingly sharply as soon as the CO is removed strong evidence in support of the hypothesis that the adsorbed CO layer established at 0.4 V acts as a catalytic poison for the electro-oxidation of methanol. 

The work of Kunimatsu and Kita (1987) is very powerful evidence in favour of linearly adsorbed CO being the catalytic poison for methanol oxidation at a smooth platinum electrode in acid solution and has resulted in this hypothesis being generally accepted. However, there is some conflict between the IR results and those obtained by Vielstich and colleagues using chronocoulometry, ECTDMS and DEMS. 

In contrast to a mixture of redox couples that rapidly reach thermodynamic equilibrium because of fast reaction kinetics, e.g., a mixture of Fe2+/Fe3+ and Ce3+/ Ce4+, due to the slow kinetics of the electroless reaction, the two (sometimes more) couples in a standard electroless solution are not in equilibrium. Nonequilibrium systems of the latter kind were known in the past as polyelectrode systems [18, 19]. Electroless solutions are by their nature thermodyamically prone to reaction between the metal ions and reductant, which is facilitated by a heterogeneous catalyst. In properly formulated electroless solutions, metal ions are complexed, a buffer maintains solution pH, and solution stabilizers, which are normally catalytic poisons, are often employed. The latter adsorb on extraneous catalytically active sites, whether particles in solution, or sites on mechanical components of the deposition system/ container, to inhibit deposition reactions. With proper maintenance, electroless solutions may operate for periods of months at elevated temperatures, and exhibit minimal extraneous metal deposition. 

Sometimes the rate of a catalysed reaction is reduced by the presence of a small amount of same substance (may be as impurities in the reactants). Such a substance, which destroys the activity of a catalyst, is called poison and the process is called catalytic poisoning. For example, in reaction... 

Poisoning of the catalyst by presence of a catalytic poison may be either due to chemical reaction between catalyst and poison (e.g. Fe + H2S Fe + H2) or poison may render surface of the catalyst unavailable for adsorption of reactants. 

Catalytic poisoning, 19 60 Catalytic precipitation, 19 63-64 Catalytic processes... 

Intrinsic to interpreting catalytic poisoning and promotion in terms of electronic effects is the inference that adsorption of an electropositive impurity should moderate or compensate for the effects of an electronegative impurity. Recent experiments have shown this to be true in the case of CO2 methanation where the adsorption of sulfur decreases the rate of methane formation significantly. The adsorption of potassium in the presence of sulfur indicates that the potassium can neutralize the effects of sulfur. 

Although the PO WW ER system can treat concentrated and dilnte aqueous wastes, treatment of dilute aqueous waste may require increased energy (however, brine disposal costs will be lower). Also, the PO WW ER system can treat a broad range of mixed aqneons waste streams, but the specific characteristics of the wastewater to be treated can affect the performance of the system. In addition, the pH and ionic strength of the waste stream, contaminant loading, nature of the contaminants, foaming, and catalytic poisons can all affect system performance. 

In spite of much effort, the nature of the active sites on acid—base inorganic catalysts is still not completely understood. However, the work on this problem has shown how complicated the surface structure may be and that several types of active centres may be simultaneously present on the surface the question is then which type plays the major role in a particular reaction. Also, the catalytic activity may be influenced to a large extent by impurities present in the feed (catalytic poisons) or by-products of the reaction. The last point is often not taken into account and it will be discussed specially in Sect. 1.2.6. First, the models of surface sites on the most important and best-studied catalysts will be described. 

In the presence of catalytic poisons, that is substances which are strongly adsorbed on the catalyst surface, the rate equation, eqn. (3), has to be expanded by adding the term KPPP to the denominator... 

By changing flow rate and the composition of the gas mixture at the inlet of the cycle, we vary the composition of the reacting gas mixture and determine the corresponding variation in reaction rate. We may also add different amounts of the reaction products, inert gases, or catalytic poisons to the initial gas mixture. 

Uses of Gels Gels have found several applications. Silica gel is used in laboratory and in industry and is also used to support the platinum catalyst, when this is used in the contact process of H2S04 manufacture. It is resistant to catalytic poisoning. Solidified alcohol (gel) is used as fuel in picnic stoves and is made from alcohol and calcium acetate. 

A substance which destroys the activity of the catalyst to accelerate a reaction, is called a catalytic poison and the process is called catalytic poisoning. 

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