Heterogeneous catalysis by metals and metal oxides

The well-known Fischer and Tropsch reaction involves the conversion of coal to hydrocarbons according to the equations  

Under the given conditions, only the first reaction, in which methane is produced, is thermodynamically possible, because this reaction is the only one in which the Gibbs free energy is lowered. However, changing the temperature and pressure makes it possible to shift individual equilibria in the 

Among the above mentioned reactions, the third one is very important, in which methanol is produced using heterogeneous, copper-zinc oxide catalysts at 250 °C and 50 atm. Methanol can then be converted to acetic acid. 

There have been many discussions and speculations about the reaction mechanism of CO reduction by hydrogen. The chemical industry mostly uses heterogeneous catalysts, metals and metal oxides. Such catalysts are stable and universal (can be applied to various reactions), and the products are easy to separate. But, also homogeneous catalysts have some advantages lower temperatures and pressures are needed, and the catalysts are more selective in leading the reaction to a desired product. In addition, spectroscopic methods are easily applied, enabling studies of the reaction mechanisms and their desired corrections.  

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Gas Phase. The decomposition of gaseous ozone is sensitive not only to homogeneous catalysis by light, trace organic matter, nitrogen oxides, mercury vapor, and peroxides, but also to heterogeneous catalysis by metals and metal oxides. 

Busca, G. (2009). Use of infiared spectroscopy methods in the field of heterogeneous catalysis by metal oxides, in S. Jackson and J. Hargreaves (eds.). Metal Oxide Catalysis, Vol. 1, WUey-VCH, New York, pp. 95-175. 

The thermal substitution of CO is catalyzed by several supported transition metals and their oxides, for example, PdO (see Heterogeneous Catalysis by Metals and Oxide Catalysts... 

With the advent of synthetic methods to produce more advanced model systems (cluster- or nanoparticle-based systems either in the gas phase or on planar surfaces), we come to the modern age of surface chemistry and heterogeneous catalysis. Castleman and coworkers demonstrate the large influence that charge, size, and composition of metal oxide clusters generated in the gas phase can have on the mechanism of a catalytic reaction. Rupprechter (Chap. 15) reports on the stmctural and catalytic properties of planar noble metal nanocrystals on thin oxide support films in vacuum and under high-pressure conditions. The theme of model systems of nanoparticles supported on planar metal oxide substrates is continued with a chapter on the formation of planar catalyst based on size-selected cluster deposition methods. In a second contribution from Rupprecther (Chap. 17), the complexities of surface chemistry and heterogeneous catalysis on metal oxide films and nanostructures, where the extension of the bulk structure to the surface often does not occur and the surface chemistry is often dominated by surface defects, are discussed. 

Our own group is also involved in the development of domino multicomponent reactions for the synthesis of heterocycles of both pharmacologic and synthetic interest [156]. In particular, we recently reported a totally regioselective and metal-free Michael addition-initiated three-component substrate directed route to polysubstituted pyridines from 1,3-dicarbonyls. Thus, the direct condensation of 1,3-diketones, (3-ketoesters, or p-ketoamides with a,p-unsaturated aldehydes or ketones with a synthetic equivalent of ammonia, under heterogeneous catalysis by 4 A molecular sieves, provided the desired heterocycles after in situ oxidation (Scheme 56) [157]. A mechanistic study demonstrated that the first step of the sequence was a molecular sieves-promoted Michael addition between the 1,3-dicarbonyl and the cx,p-unsaturated carbonyl compound. The corresponding 1,5-dicarbonyl adduct then reacts with the ammonia source leading to a DHP derivative, which is spontaneously converted to the aromatized product. 

In the majority of catalytic reactions discussed in this chapter it has been possible to rationalize the reaction mechanism on the basis of the spectroscopic or structural identification of reaction intermediates, kinetic studies, and model reactions. Most of the reactions involve steps already discussed in Chapter 21, such as oxidative addition, reductive elimination, and insertion reactions. One may note, however, that it is sometimes difficult to be sure that a reaction is indeed homogeneous and not catalyzed heterogeneously by a decomposition product, such as a metal colloid, or by the surface of the reaction vessel. Some tests have been devised, for example the addition of mercury would poison any catalysis by metallic platinum particles but would not affect platinum complexes in solution, and unsaturated polymers are hydrogenated only by homogeneous catalysts. 

Bond (1987) covers the basic principles of catalysis, adsorption on solid surfaces, chemisorption at metal and oxide surfaces, the kinetics of catalyzed reactions, the quantitative aspects of catalysis by metals and the structure, preparation and use of heterogeneous catalysts. The book also discusses the application of catalysts in different fields including energy conservation, production of hydrocarbon feedstocks, bifunctional catalysts in petroleum industry, oxidation catalysts in the petrochemical industry, heavy inorganic industry, hydrogenation of multiple bonds and catalysts used in atmospheric pollution control. 

It is necessary to note that many hydrocarbon transformations on heterogeneous catalysts proceed with intermediate formation of the same species although, they give rise to different products depending on the natnre of the catalyst or conditions of the process. Eor example, dehydrogenation and isomerization of alkanes involve stages of the formation of coordinated to the metal surfaces species in the case of catalysis by metals (see below. Figure III.3). Carbe-nium ions are formed if a metal oxide is used as a catalyst. 

Although the oxidation of tertiary phosphines by these catalytic processes has minimal useful application, it needs to be considered as a problematic side reaction in homogeneous catalysis. Much effort is being currently expended to immobilize platinum metal phosphine complexes on heterogenized tertiary phosphine supports, and irreversible oxidation at phosphorus on these supports effectively destroys the supported catalyst. Recent observations that the compound Rh6(CO)i6 catalyzes the oxidation of tertiary phosphines correlate with the report that phosphine oxidation occurs with molecular oxygen on Rh6(CO)i6 bound to diphenylphosphino-functionalized poly(styrenedivinylbenzene). Thus, in order to use these phosphinated polymer-supported rhodium catalysts, one needs either to rigorously exclude oxygen, or to find a way to inhibit the simultaneous catalyzed phosphine oxidation. 

Understanding of the modification from bulk liquid water behavior when water is introduced into pores of porous media or confined in the vicinity of metallic surfaces is important in technological problems such as oil recovery from natural reservoirs, mining, heterogeneous catalysis, corrosion inhibition, and numerous other electrochemical processes. Water in porous materials such as Vycor glass, silica gel, and zeolites has been actively under investigation because of their relevance in catalytic and separation processes. In particular, the structure of water near layer-like clay minerals [11,12], condensed on hydroxylated oxide surface [13], confined in various types of porous silica [14-22] or in carbon powder [23] has been studied by neutron and/or x-ray diffraction. 

We extend our imderstanding of the concepts of chemical bonding and reactivity learned in Chapter 3 on metals and Chapter 4 on zeolites to catalysis over metal oxides and metal sulfides in Chapter 5. The featmes that lead to the generation of surface acidity and basicity are described via simple electrostatic bonding theory concepts that were initially introduced by Pauling. The acidity of the material and its application to heterogeneous catalysis are sensitive to the presence of water or other protic solvents. We explicitly examine the effects of the reaction medium in which the reaction is carried out. In addition, we compare and contrast the differences between liquid and solid acids. We subsequently describe the influence of covalent contributions to the bonding in oxides and transition to a discussion on the factors that control selective oxidation. 

Heterogeneous catalysis by insoluble metals and metal combinations is complex and poorly reproducible. Of the insoluble catalysts, Ni and its oxides are the most troublesome, being frequent contaminants in strong NaOH solutions that have been in contact with high-Ni stainless steel used in piping and tankage. 

Hydrogenations and oxidations form two important classes of catalytic reactions. In heterogeneous catalysis, the metals from the groups Vin and IB of the periodic system, as well as oxides or sulfides, catalyze such reactions. In view of their unique reaction mechanisms, acid-catalyzed reactions are considered as a separate class, while a fourth category is formed by reactions that are catalyzed by coordination complexes or organometallic complexes in solution, as in homogeneous catalysis. Heterogeneous catalytic reactions will be the focus, however. 

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