Catalysts competitive adsorption

From the study of the influencing of single reactions by products and by other added substances and from the analysis of mutual influencing of reactions in coupled systems, the following conclusions can be drawn concerning adsorption of the reaction components. (1) With the exception of crotyl alcohol on the platinum-iron-silica gel catalyst, all the substances present in the coupled system, i.e. reactants, intermediate products, and final products, always adsorbed on the same sites of the catalytic surface (competitive adsorption). This nonspecificity was established also in our other studies (see Section IV.F.2) and was stated also by, for example, Smith and Prater (32), (2) The adsorption of starting reactants and the desorption of the intermediate and final products appeared in our studies always as faster, relative to the rate of chemical transformations of adsorbed substances on the surface of the catalyst. 

Figure 9.13. Preparation of a bifunctional Pt/Al203 catalyst. The alumina support is impregnated with an aqueous solution of hexachloroplatinic acid (H2PtCl6) and HCl.The competitive adsorption between C and...

Water exerts both a deactivating and inhibiting influence on Cu and Fe samples, while the reaction over Co is only inhibited. The deactivation of Fe- and Cu-ZSM-5 is clearly due to migration and the sintering of the active component in H2O atmospheres [34]. The Co-ZSM-5 catalyst is much more hydrotheimally stable in wet gas conditions [34,35]. The inhibition by water can be accounted for in a similar way as for CO via competitive adsorption on active sites, like in selective NO reduction studies [34]. For N2O decomposition this yields an expression like eq. (12). At 793 K Kn amounts to about 0.7 kPa". ... 

It is believe that the HDS sites (rim sites and edge sites) are different than the olefin hydrogenation sites (rim sites) opening an opportunity for the development of selective HDS catalysts [45 171. Another concept to exploit in catalyst development is the competitive adsorption, by which the sulfur compounds inhibit olefins hydrogenation [48]. 

On co-adsorbing phenol and methanol, the protonation of methanol occurs on the active acid sites as the labile protons released from the phenol reacted with methanol. Thus protonated methanol became electrophilic methyl species, which undergo electrophilic substitution. The ortho position of phenol, which is close to the catalyst surface, has eventually become the substitution reaction center to form the ortho methylated products (Figure 3). This mechanism was also supported by the competitive adsorption of reactants with acidity probe pyridine [79]. A sequential adsorption of phenol and pyridine has shown the formation of phenolate anion and pyridinium ion that indicated the protonation of pyridine. 

In chapter 12 we discussed a model for a surface-catalysed reaction which displayed multiple stationary states. By adding an extra variable, in the form of a catalyst poison which simply takes place in a reversible but competitive adsorption process, oscillatory behaviour is induced. Hudson and Rossler have used similar principles to suggest a route to designer chaos which might be applicable to families of chemical systems. They took a two-variable scheme which displays a Hopf bifurcation and, thus, a periodic (limit cycle) response. To this is added a third variable whose role is to switch the system between oscillatory and non-oscillatory phases. 

The theoretical calculations described have recently been supported by an extraordinary kinetic analysis conducted by Vanrysellberghe and Froment of the HDS of dibenzothiophene (104). That work provides the enthalpies and entropies of adsorption and the equilibrium adsorption constants of H2, H2S, dibenzothiophene, biphenyl, and cyclohexylbenzene under typical HDS conditions for CoMo/A1203 catalysts. This work supports the assumption that there are two different types of catalytic sites, one for direct desulfurization (termed a ) and one for hydrogenation (termed t). Table XIV summarizes the values obtained experimentally for adsorption constants of the various reactants and products, using the Langmuir-Hinshelwood approach. As described in more detail in Section VI, this kinetic model assumes that the reactants compete for adsorption on the active site. This competitive adsorption influences the overall reaction rate in a negative way (inhibition). 

The observation that no hydrogenolysis of the C-X bond takes place as long as either nitro compounds or reaction intermediates are present can be explained by the strong adsorption of these molecules, thereby preventing the interaction of the C-Cl bond with the catalyst. The mode of action of the modifiers is less clear. It could be due to a modification of the catalytic properties of the Raney nickel or also to a competitive adsorption between the effective modifiers and the... 

Amidine derivatives are effective dehalogenation inhibitors for the chemoselective hydrogenation of aromatic halonitro compounds with Raney nickel catalysts. The best modifiers are unsubstituted or N-alkyl substituted formamidine acetates and dicyandiamide which are able to prevent dehalogenation even of very sensitive substrates. Our results indicate that the dehalogenation occurs after the nitro group has been completely reduced i.e. as a consecutive reaction from the halogenated aniline. A possible explanation for these observations is the competitive adsorption between haloaniline, nitro compound, reaction intermediates and/or modifier. The measurement of the catalyst potential can be used to determine the endpoint of the desired nitro reduction very accurately. 

The transitory poisoning by scavengers is explained by competitive adsorption of a halogen-containing species on catalyst sites that are needed for the oxidation of CO and hydrocarbons. In the case of EDB it is thermodynamically probable that HBr 33), or Br2 is the actual adsorbed species (66). The possible interactions of EDB and EDC with TEL and the resulting loss in noble metal surface area on the one hand, and catalyst activity on the other, are very complex (66). 

Catalyst inhibition is traditionally associated with biocatalytic processes, but can also apply to homogeneous and heterogeneous catalysis. Competitive inhibition is analogous to competitive adsorption in gas/solid heterogeneous catalysis, where two molecules from the gas phase compete for the same active site on the catalyst surface. A competitive inhibitor is any chemical species I which can bind to the same site as the substrate, or to another site on the enzyme (an allosteric site). The overall reaction scheme is then given by Eqs. (2.58)-(2.60), where El indicates an enzyme-inhibitor complex. 

Optimal operating conditions and catalysts Acetylation of phenyl ethers was generally carried out in the absence of solvents, which makes easier the recovery of the acetylated product from the reaction mixture. On the other hand, because of the high melting point of substrate and acetylated products, solvents were always used in the acetylation of 2-methoxynaphthalene. Flow reactors (e.g. fixed bed tubular reactors), in which the detrimental effect of competitive adsorption of substrate and products on the acetylation yield is lower than in the batch reactors, should be preferred. However although the set up of fixed bed reactors for liquid phase reactions is relatively simple, their substitution to the batch reactors, which are the only system used in academic organic chemistry, remains essentially limited to commercial units. 

Derouane, E. G., Crehan, G., Dillon, C. J., Bethell, D., He, H., Derouane Abd-Hamid, S. B. Zeolite catalysts as solid solvents in fine chemicals synthesis-2. Competitive adsorption of the reactants and products in the Friedel-Crafts acetylations of anisole and toluene, J. Catal., 2000, 194, 410-423. 

Prins summarizes advances in understanding of the reactions in catalytic hydrodenitrogenation (HDN), which is important in hydroprocessing of fossil fuels. Hydroprocessing is the largest application in industrial catalysis based on the amount of material processed. The chapter addresses the structures of the oxide precursors and the active sulfided forms of catalysts such as Ni-promoted Mo or W on alumina as well as the catalytically active sites. Reaction networks, kinetics, and mechanisms (particularly of C-N bond rupture) in HDN of aliphatic, aromatic, and polycyclic compounds are considered, with an evaluation of the effects of competitive adsorption in mixtures. Phosphate and fluorine promotion enhance the HDN activity of catalysts explanations for the effect of phosphate are summarized, but the function of fluorine remains to be understood. An account of HDN on various metal sulfides and on metals, metal carbides, and metal nitrides concludes this chapter. 

Water may perturb directly the activity of the catalysts by competitive adsorption on active sites. In that case, the inhibition will be a function of the adsorption strength. It was reported in previous papers [7,8] that the inhibition by oxygenated compounds is moderate. Water caused... 

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