Chemisorbed intermediate products

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... 

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. 

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. 

Several years later Craxford and Rideal (54) published some interesting results regarding the question of formation of carbides as intermediate products of the synthesis. Higher hydrocarbons should be formed by the reaction of carbides with molecular hydrogen, while methane should be the result of a reaction of carbide with chemisorbed hydrogen (see Sec. III). 

It is then necessary to bring the reactant molecules to the active centre by a process of mass transport which can sometimes be rate limiting (Section 5.2.1). Having been chemisorbed they must then react at the reacting centre, and how they do this will occupy us for much of the rest of the book. The product molecules remain on the surface or may be ejected from the surface as it is formed in the former case it its desorption may be the slow step, and if it does not desorb quickly its further reaction may lead to undesired products. Speedy removal of the product from the neighbourhood of the surface by another mass transport step is often important if it is an intermediate product that is wanted. Conversion of reactants or products into strongly held residues, or adventitious poisons in the feedstock, can block the active centre and lead to deactivation. In their absence the reaction should continue indefinitely. 

At low temperature, the dissociation of adsorbed NO species occmring over reduced perovskite and yielding N2O and N2 was recognized as the rate determining step for catalytie reduction of NO by CO. The dimeric species of NO, such as N2O2, can be an intermediate, the formation of which involves the N-N bond formation and N-0 bond eleavage[40]. Two parallel reaetions for chemisorbed NO dissociation occurring over a redueed surfaee with N2O and N2 as the respective products were assumed ... 

Pre-treatment of the cathode produces a finely divided copper surface and this catalyses the hydrogenolysis of phenylhydroxylamine to aniline which becomes the principal reaction product. The reaction probably involves promotion of nitrogen-oxygen bond cleavage in the chemisorbed phenylhydroxylamine intermediate. 

It was found that the electrocatalytic activity strongly depends on the nature of the electrode it decreases in the order Rh > Ru > Ir > Pd and Pt for the transition-metal electrodes and in the order Cu > Ag > Au for the coinage metals. It was concluded that the rate-determining step on Ru, Rh, Ir, Pt, Cu, and Ag is the reduction of nitrate to nitrite. It was assumed that chemisorbed nitric oxide is the key surface intermediate in the nitrate reduction. It was suggested that ammonia and hydroxylamine are the main products on transition-metal electrodes. This is in agreement with the known mechanism for NO reduction, which forms N2O or N2 only if NO is present in the solution. On Cu the production of gaseous NO was found, which was explained by the weaker binding of NO to Cu as compared to the transition metals. 

If other gases than the reactants and products of the catalytic reaction are present and if these foreign atoms are chemisorbed much more weakly than the intermediates of the catalytic reaction, the presence of the foreign gas will be of no consequence for the kinetic of the catalytic reaction. In this case the foreign gas behaves as an inert. 

The best example of a study of this type of intermediate is found in the oxidation of CO over a nickel-nickel oxide catalyst 24). The latter term is used because there is doubt as to the specific nature of the catalyst surface. The spectrum in Fig. 14 was obtained during the oxidation of CO over nickel-nickel oxide at 35° C. The band at 4.56 u is tentatively attributed to an intermediate complex having the structure Ni- 0 C rr.O. The bands at 6.5 and 7.2 u are due to C02 chemisorbed on the catalyst surface. This C02 is considered to be adsorbed product rather than as a reaction intermediate because these bands remain after the reaction is completed. The 4.56- u band in Fig. 14 is attributed to the asymmetrical 0—C—0 vibration rather than to the C—O vibration of chemisorbed CO. This interpretation implies that there should be a second band due to the symmetrical vibration. The symmetrical 0—C—O vibration of C02 produces a Raman band at 7.2 ju. The symmetrical 0—C—0 vibration of Ni - -O—C=0 would be expected to produce an infrared band near 6 or 7 u- Thus far this band has not been observed. This failure is not considered a serious obstacle to the structure assignment,... 

In the course of catalytic oxidation, the production rate of intermediates that finally generate CO2 and H2O is also limited by rx. Some intermediates are the chemisorbed species that form surface donors and acceptors, and the other intermediates are the excited species. Both the generation rate of carriers originated from the surface states and the production rate of the excited species is governed by rx- Thus the dependence of CTL intensity on flow velocity should agree with that of rx in the diffusion-controlled region. 

Table VIII lists the total bond energies of reactants and products as well as conceivable intermediates in the gas-phase (D) and chemisorbed (D + Q) states on Pt(lll), Pd(lll), Ni(lll). Tables IX and X summarize the activation barriers of the conceivable elementary steps leading to CH4 and CH3OH, respectively.

The increase of the sticking probability at higher kinetic energies was attributed to a direct access of the chemisorbed molecular states [81, 83, 86, 87], which is also sketched in Fig. 9. Now, such an one-dimensional sketch of the potential energy surface along some suitable reaction coordinate is certainly very helpful for a compact presentation of the energetics of reaction intermediates and products. Furthermore, it can be used as a basis for a kinetic modelling of a reaction. However, in... 

Polanyi represents the reaction thus suppose that there are four valencies proceeding from four atoms of the solid surface, and the reacting atoms Ay By CtD are attracted, perhaps chemisorbed, to these four atoms of the surface. We have a surface intermediate compound involving all four atoms, in which the spacings may differ widely from those of the unstrained forms of AB and CD. With suitable disposition of the atoms and the forces adsorbing them, the reaction may proceed very easily, particularly if the end products AC and BD are not strongly adsorbed on the surface, and so evaporate as soon as they are formed. 

Research activity on methanol has been vigorous because of its commercial importance as an alternative feedstock in fuel cells. When CH3OH is chemisorbed on a catalytic surface at ambient temperatures, it is usually present as a methoxy intermediate the latter then undergoes extensive decomposition to yield a product distribution that depends upon the temperature. A tabulation of products generated under various experimental conditions such as metal catalyst and decomposition temperature is given in Table 1 HREELS... 

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