Examples of Surface Reactions

As a second example of surface-reaction-rate control, consider the slightly more complicated case of a bimolecular reaction occurring on the same sites, S, of a surface. Overall, A -I- B C -I- D, and the individual steps are... 

Although surface organometallic chemistry is still in its infancy, there are already several examples of surface reactions leading to well-defined surface complexes (Table l-I). It appears that these reactions obey the same principles as those encountered in molecular chemistry nucleophilic attack at the ligands, electrophilic attack of the metal-carbon bond, oxidative addition, Lewis acid-base adduct formation, redox reactions, disproportionation, and the cooperative effect of dual acid-base sites in an insertion reaction. 

Surface science has tlirived in recent years primarily because of its success at providing answers to frmdamental questions. One objective of such studies is to elucidate the basic mechanisms that control surface reactions. For example, a goal could be to detennine if CO dissociation occurs prior to oxidation over Pt catalysts. A second objective is then to extrapolate this microscopic view of surface reactions to the... 

Surface science studies of corrosion phenomena are excellent examples of in situ characterization of surface reactions. In particular, the investigation of corrosion reactions with STM is promising because not only can it be used to study solid-gas interfaces, but also solid-liquid interfaces. 

The F H- H — H —> F—H + H reaction is a common example of a reaction easily studied by classical trajectory analysis. The potential surface we are interested in is that for FH2. This potential surface may have many extrema. One of them corresponds to an isolated Fluorine atom and a stable H2 molecule these are the reactants. Another extremum of the surface corresponds to an isolated hydrogen atom and the stable H-Fmolecule these are the products. Depending on how the potential surface was obtained there may or may not be an extremum corresponding to stable H2F, but at the least you would expect an extremum corresponding to the transition state of the reaction being considered. 

Constmction of multilayers requires that the monolayer surface be modified to a hydroxylated one. Such surfaces can be prepared by a chemical reaction and the conversion of a nonpolar terminal group to a hydroxyl group. Examples of such reactions are the LiAlH reduction of a surface ester group (165), the hydroboration—oxidation of a terminal vinyl group (127,163), and the conversion of a surface bromide using silver chemistry (200). Once a subsequent monolayer is adsorbed on the "activated" monolayer, multilayer films may be built by repetition of this process (Fig. 8). 

The example of consecutive, irreversible heterogeneous catalytic reaction of the type A —> B — C has been solved in a more general way by Thomas et al. (16). The authors considered scheme (III) with the listed values of the rate constants of surface reactions along with the constants of adsorption and desorption of the reactant A and of the product C. 

They varied only the values of the adsorption and desorption rate constants of the reaction intermediate B, and by using the simplest Langmuir kinetics, they calculated time-concentration curves of compounds A, B, and C shown in Fig. 5. Also from this example, which does not consider any step as clearly rate determining, it is evident how very different concentration versus time plots can be obtained for the same sequence of surface reactions if adsorption and desorption of the intermediate B proceed by different rates, which are, however, comparable with the rate of surface reactions. In particular, the curves in the first and second columns of Fig. 5 simulate the parallel formation of substances B and C, at least... 

The quantitative solution of the problem, i.e. simultaneous determination of both the sequence of surface chemical steps and the ratios of the rate constants of adsorption-desorption processes to the rate constants of surface reactions from experimental kinetic data, is extraordinarily difficult. The attempt made by Smith and Prater 82) in a study of cyclohexane-cyclohexene-benzene interconversion, using elegant mathematic procedures based on the previous theoretical treatment 28), has met with only partial success. Nevertheless, their work is an example of how a sophisticated approach to the quantitative solution of a coupled heterogeneous catalytic system should be employed if the system is studied as a whole. 

The examples of the model studies presented show how the meshing of modern surface techniques with reaction kinetics can provide valuable Insights Into the mechanisms of surface reactions and serve as a useful complement to the more traditional techniques. Close correlations between these two areas holds great promise for a better understanding of the many subtleties of heterogeneous catalysis. 

An example of a reaction cavity is illustrated by the van der Waals surface plots derived from the X-ray structures of di-2,6-dihydroxy-2,6-diphenylcyclohexanone 51 and its photodecarbonylation products, the cis- and tra i-2,6-diphenylcyclopen-tane-l,2-diols 52 and 53 (Scheme While the two products are formed in... 

Sorption can significantly diminish the mobility of certain dissolved components in solution, especially those present in minor amounts. Sorption, for example, may retard the spread of radionuclides near a radioactive waste repository or the migration of contaminants away from a polluting landfill (see Chapters 21 and 32). In acid mine drainages, ferric oxide sorbs heavy metals from surface water, helping limit their downstream movement (see Chapter 31). A geochemical model useful in investigating such cases must provide an accurate assessment of the effects of surface reactions. 

In a second example, we calculate how pH affects sorption onto hydrous ferric oxide, expanding on our discussion (Section 10.4) of Dzombak and Morel s (1990) surface complexation model. We start as before, setting the dataset of surface reactions, suppressing the ferric minerals hematite (Fe203) and goethite (FeOOH), and specifying the amount of ferric oxide [represented in the calculation by Fe(OH)3 precipitate] in the system... 

Hydrolysis reactions occur by nucleophilic attack at a carbon single bond, involving either the water molecule directly or the hydronium or hydroxyl ion. The most favorable conditions for hydrolysis, e.g. acidic or alkaline solutions, depend on the nature of the bond which is to be cleaved. Mineral surfaces that have Bronsted acidity have been shown to catalyze hydrolysis reactions. Examples of hydrolysis reactions which may be catalyzed by the surfaces of minerals in soils include peptide bond formation by amino acids which are adsorbed on clay mineral surfaces and the degradation of pesticides (see Chapter 22). 

Table I. Examples of Sub-Reactions Needed to Describe Overall Metal Adsorption Using Surface Coordination Models.

In a similar fashion, the introduction of angle-dependent electron densities into the EAM suggests that this formalism may be successfully extended to chemical reactions. This would allow the study, for example, of the reaction of a metal-ligand cluster with a metal surface. This would enhance the applicability of the EAM, and would increase the realm of processes which computer simulations can effectively model. 

Numerous variables influence the yields and selectivity of surface reactions, for example (i) nature and loading of the metal salt or organometallic precursor adsorbed on the inorganic oxide (ii) nature of the inorganic oxide (iii) physical and chemical properties of the surface as such or after addition of some reactants (e.g., alkali or acids) (iv) nature and composition of the gaseous phase (v) temperature, pressure and reaction time [6]. 

Although the division of surface reaction mechanisms into LH or ER dates to the early days of catalysis, ER/HA surface reactions have only been demonstrated recently and only for strongly reactive atomic gas phase species, e.g., H, O. There are many differences between the ER/HA mechanism and the LH mechanism that can be used to separate them experimentally. For example, ER/HA reactions of reactive incident atoms are very exothermic relative to the equivalent LH reaction, typically by several eV. Much of this released energy should end up in the gas-phase product molecule. ER/HA are direct non-activated reactions whose final state properties depend on the initial conditions of the incoming atom and not Ts. This is of course the exact opposite of LH properties. 

A corollary to the conclusion of the above discussion is that the number of lattice oxygen available for reaction at the active site depends not only on the atomic details of the site, but also on the rate of lattice diffusion compared with the rate of surface reaction. If the rate of lattice diffusion is slow relative to surface reaction, then the number of lattice oxygen atoms available for the reaction during the residence of an intermediate could be estimated from the atomic structure of the site, as in the examples above. On the other hand, if lattice diffusion is fast, a lattice oxygen atom will be replenished immediately after its removal. Then the behavior of the catalyst no longer depends as much on the stoichiometry of the active site, but on the degree of reduction of the active site at steady state, which would determine the heat of removal (or the reactivity) of the lattice oxygen. 

Another example of a reaction nearly independent of pressure in this way is the catalytic decomposition of hydrogen iodide on the surface of a heated gold wire. X The initial pressure of the gas can be varied from 100 mm. to 400 mm. with a resulting change in the absolute rate of reaction which amounts to about 45% only instead of 400%. 

Systematically speaking, so-called internal oxidation reactions of alloys (A,B) are extreme cases of morphological instabilities in oxidation. Internal oxidation occurs if oxygen dissolves in the alloy crystal and the (diffusional) transport of atomic oxygen from the gas/crystal surface into the interior of the alloy is faster than the countertransport of the base metal component (B) from the interior towards the surface. In this case, the oxidation product BO does not form a stable oxide layer on the alloy surface. Rather, BO is internally precipitated in the form of small oxide particles. The internal reaction front moves parabolically ( Vo into the alloy. Examples of internal reactions are discussed quantitatively in Chapter 9. 

The kinetic equations are useful as a fitting procedure although their basis - the homogeneous system - in general does not exist. Thus they cannot deal with segregation and island formation which is frequently observed [27]. Computer simulations incorporate fluctuation and correlation effects and thus are able to deal with segregation effects but so far the reaction systems under study are oversimplified and contain only few aspects of a real system. The use of computer simulations for the study of surface reactions is also limited because of the large amount of computer time which is needed. Especially MC simulations need so much computer time that complicated aspects (e.g., the dependence of the results on the distribution of surface defects) in practice cannot be studied. For this reason CA models have been developed which run very fast on parallel computers and enable to study more complex aspects of real reaction systems. Some examples of CA models which were studied in the past years are the NH3 formation [4] and the problem of the universality class [18]. However, CA models are limited to systems which are suited for the description by a purely parallel ansatz. 

Figure 4.69 Examples of oxidation reactions of organic molecules at the surface of photoexcited titanium dioxide in polar solvents in the presence of molecular oxygen...

In contrast to spatial distribution, the equilibrium energy distribution of adsorbed particles cannot be violated to any substantial degree by reaction since energy is rapidly transferred between adsorbed particles and solids. Therefore, the activated complex method may be applied to rates of surface reactions. For this we consider the activated complex (transition state) of a surface reaction as a likeness of adsorbed particle (21). But, assuming that each adsorbed particle occupies only one site, it is necessary, even in the simplest kinetic model, to consider that activated complexes are able to occupy not only one, but also several surface sites (21). For example, the usual picture of a reaction between two particles adsorbed on neighboring sites involves, in fact, the notion that the activated complex occupies both sites. When the activated complex occupies several sites, this does not create any difficulty for the theory since the surface concentration of activated complexes is an infinitesimal quantity, and so the possibility of overlapping the required sites is excluded. 

The biochemical reduction of sulfate to sulfide by bacteria of the genus Desulfovibrio in anoxic waters is a significant process in terms of the chemistry of natural waters since sulfide participates in precipitation and redox reactions with other elements. Examples of these reactions are discussed later in this paper. It is appropriate now, however, to mention the enrichment of heavy isotopes of sulfur in lakes. Deevey and Nakai (13) observed a dramatic demonstration of the isotope effect in Green Lake, a meromictic lake near Syracuse, N. Y. Because the sulfur cycle in such a lake cannot be completed, depletion of 32S04, with respect to 34S04, continues without interruption, and 32S sulfide is never returned to the sulfate reservoir in the monimolimnion. Deevey and Nakai compared the lake to a reflux system. H2S-enriched 32S diffuses to the surface waters and is washed out of the lake, leaving a sulfur reservoir depleted in 32S. The result is an 34S value of +57.5% in the monimolimnion. 

Redox Reactions. Aquatic organisms may alter the particular oxidation state of some elements in natural waters during activity. One of the most significant reactions of this type is sulfate reduction to sulfide in anoxic waters. The sulfide formed from this reaction can initiate several chemical reactions that can radically change the types and amounts of elements in solution. The classical example of this reaction is the reduction of ferric iron by sulfide. The resultant ferrous iron and other transition metals may precipitate with additional sulfide formed from further biochemically reduced sulfate. Iron reduction is often accompanied by a release of precipitated or sorbed phosphate. Gardner and Lee (21) and Lee (36) have shown that Lake Mendota surface sediments contain up to 20,000 p.p.m. of ferrous iron and a few thousand p.p.m. of sulfide. The biochemical formation of sulfide is undoubtedly important in determining the oxidation state and amounts of several elements in natural waters. 

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