Chemisorbed intermediates reaction

A heterogeneous catalytic reaction, by definition, necessitates the participation of at least one chemisorbed intermediate (54) and involves a sequence of interlinked and interdependent (55,56) steps, which include the adsorption of reactant(s), one or more surface rearrangements, and the desorption of product(s). More than one area of the solid may be active in promoting reaction the activity of such regions may vary from one crystallographic... 

Below 0.45 V the chemisorbed intermediates formed on methanol adsorption are stable on any smooth platinum surface, with the steady-state current for methanol oxidation being extremely small. Above this potential, oxidation of methanol takes place at a rate that increases exponentially with potential, with the product being primarily C02. In addition, above potentials of approximately 0.6 V, the surface is steadily stripped of adsorbed carbon-containing species, with the loss of such species being complete near 0.8 V. It would seem likely on most surfaces that it is oxidation of COads or =C-OH in a sequential reaction pathway that leads to C02, but more active intermediates, such as CO adsorbed at less stable sites, such as those at the edges... 

While a knowledge of surface mobility is of great interest in physical adsorption, it becomes essential in chemisorption phenomena. For instance in calorimetric work a curve of differential heats of adsorption versus surface coverage will be horizontal if adsorption is localized but shows the customary slope from high to low values of the heat of adsorption if the adsorbed layer is mobile Furthermore if a chemisorbed intermediate takes part in a surface reaction (crystal growth, corrosion, catalysis), it is essential to know whether, after adsorption anywhere on the surface, it can migrate to a locus of reaction (dislocation, etch pit, active center). Yet here again, while Innumerable adsorption data have been scrutinized for their heat values, very few calculations have been made of the entropies of chemisorbed layers. A few can be found in the review of Kemball (4) and in the book of Trapnell (11). 

The unique features of the VPO catalysts in carrying out the reaction steps shown in Table 1 are (i) the ability to selectively activate n-butane during the rate-determining step, (ii) rapid oxidation of chemisorbed intermediates to maleic anhydride with high selectivity, and (iii) the lack of desorption of any intermediate contributing to high selectivity to maleic anhydride. Other cata-... 

Behavior and Characterization of Kinetically Involved Chemisorbed Intermediates in Electrocatalysis of Gas Evolution Reactions... 

The principal aims of this review are to indicate the role of chemisorbed intermediates in a number of well-known electrocatalytic reactions and how their behavior at electrode surfaces can be experimentally deduced by electrochemical and physicochemical means. Principally, the electrolytic gas evolution reactions will be covered thus, the extensive work on the important reaction of O2 reduction, which has been reviewed recently in other literature, will not be covered. Emphasis will be placed on methods for characterization of the adsorption behavior of the intermediates that are the kinetically involved species in the main pathway of the respective reactions, rather than strongly adsorbed by-products that may, in some cases, importantly inhibit the overall reaction. The latter species are, of course, also important as they can determine, in such cases, the rate of the overall reaction and its kinetic features, even though they are not directly involved in product formation. 

The mechanisms of the electron-transfer event in such systems, involving solvational reorganization of the reactant, have been treated in much detail in the literature of complex-ion chemistry in inorganic chemistry (25) and by Marcus (26), Hush (27), and Weaver (28) for corresponding redox processes conducted at electrodes. The details of these works are outside the scope of this article, but reviews (29,30) will be useful to the interested reader. Chemisorbed intermediates, produced in two- or multistep redox reactions, are not involved except with some organic redox systems such as quinones or nitroso compounds. 

NH, NHj, NH3, and H species are together larger than the free-site fraction so that Langmuir-Hinshelwood conditions, with only one significant chemisorbed intermediate, do not obtain. In fact, quite early work had already indicated 54) that, in technical catalysis for NH3 synthesis, it is the bonding of Nj (as N) to the catalyst surface which determines the overall rate of the reaction. Correspondingly (55), at moderate temperatures at W, NH3 decomposes giving imide and nitride species on the surface. The rate of decomposition of the nitride species (chemisorbed N) as an intermediate in the NH3 synthesis reaction at Fe was shown by Mittasch et al. (5(5) to be equal to that of NH3 production. 

The most important examples from both a fundamental and practical point of view are cathodic evolution from acidic or alkaline water, anodic evolution of O2 from similar solutions, and anodic Clj evolution from Cr ion in melts or in solution. Other related examples are anodic generation of Br2, I2, and (CN)2 from solutions of the corresponding anions, and an interesting case is the Kolbe reaction arising from discharge and decomposition of car-boxylate anions, followed by recombinative coupling of the resulting alkyl radicals. These processes intimately involve chemisorbed intermediates and... 

VI. Involvement of Chemisorbed Intermediates in Electrode Reactions, and Methods of Analysis... 

The involvement of chemisorbed intermediates in electrocatalytic reactions is manifested in various and complementary ways which may be summarized as follows (i) in the value of the Tafel slope dK/d In i related to the mechanism of the reaction and the rate-determining step (ii) in the value of reaction order of the process (iii) in the pseudocapacitance behavior of the electrode interface (see below), for a given reaction (iv) in the frequency-response behavior in ac impedance spectroscopy (see below) (v) in the response of the reaction to pulse and linear perturbations or in its spontaneous relaxation after polarization (see below) (vi) in certain suitable cases, also to the optical reflectivity behavior, for example, in reflection IR spectroscopy or ellipso-metry (applicable only for processes or conditions where bubble formation is avoided). It should be emphasized that, for any full mechanistic understanding of an electrode process, a number of the above factors should be evaluated complementarily, especially (i), (ii), and (iii) with determination, from (iii), whether the steady-state coverage by the kinetically involved intermediate is small or large. Unfortunately, in many mechanistic works in the literature, the required complementary information has not usually been evaluated, especially (iii) with 6(V) information, so conclusions remained ambiguous. 

In several papers by Tilak and Conway (126, 127) and more recently by Harrington and Conway (104), the behavior of potential relaxation at polarized electrodes has been worked out in detail for several reaction mechanisms involving significantly chemisorbed intermediates. 

We have indicated above that for a simple electron-transfer reaction, not involving a chemisorbed intermediate, or for such a step in a more complex process where the coverage, 9, by intermediates is small (say, <1%, when the discharge step producing the intermediate is rate controlling) the Tafel slope dVjd ni is simply... 

The models of these surface reactions (more details are given in Section 6.7) imply a number of assumptions (I) there is equilibrium in adsorption and desorption (2) there is only one rate-determining intermediate reaction between adsorbed species (3) the species on the surface are well mixed and (4) there is a thermal probability of a transition of the physisorbed to the chemisorbed state before subsequent reaction and diffusion. These assumptions are not independently proved but are justified by the degree of success of the models in predicting the kinetics. 

B.E. Conway and B.V. Tilak, Behavior and Characterization of Kinetically Involved Chemisorbed Intermediates in Electrocatalysis of Gas Evolution Reactions, In D.D. Eley, H. Pines, and P.B. Weisz (eds). Advances in Catalysis, vol. 38, Academic Press, New York (1992). 

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