Catalysts, hydrogenation factors influencing activity

There are also reports indicating that the surface area and porosity of carbons do not affect either the active-phase dispersion or the catalytic activity. A very important factor influencing active-phase dispersion is the precursor used to prepare it. Rodrfguez-Reinoso et al. [14] used two different iron precursors (iron nitrate in aqueous solution and iron pentacarbonyl in organic solution) to prepare iron catalysts supported on activated carbons with different pore size distributions. They obtained an increase in iron dispersion with the support surface area for the nitrate series, but a high and unaffected dispersion was found for the pentacarbonyl series. These catalysts were used in the CO hydrogenation reaction, where no important differences in catalytic behavior were found for catalysts in both series. 

CO hydrogenation activity was found to depend on both the amount of available surface cobalt metal and the environment of the reduced cobalt. Factors influencing these cobalt properties included the type of support, the type of cobalt species present on a given support, and the presence of additives. Due to the interrelationship of these properties, a detailed examination of the activity and structure of each catalyst is needed in order to determine the importance of each factor for CO hydrogenation activity. 

Most synthetically useful catalytic processes are run over metal catalysts which, as outlined in Fig. 8.2, can be composed of a single metallic component or a mixture of metals. Either of these types can be supported or unsupported. Metal catalysts are used primarily for hydrogenations, hydrogenolyses, isomerizations and oxidative dehydrogenations. They are rather easily prepared in a pure form and can be characterized without too much difficulty. Because of this, metal catalysts are generally preferred for basic research. Such materials have been used to obtain almost all of the fundamental information on which the various theories of catalysis have been derived. A general discussion of catalytically active metals and the factors influencing their activity is presented in Chapter 11 while Chapter 12 deals with the preparation and properties of the various types of unsupported or bulk metal catalysts. The preparation and properties of the supported metal catalysts are presented in Chapter 13. 

The nature of the solvent in liquid-phase alkyne hydrogenations and the extent to which it can influence the competitive adsorption factors needed to attain selectivity should also be considered. The semihydrogenation of 1-octyne over a series of Pd/Nylon-66 catalysts of varying metal load gave 1-octene with a selectivity of 100% over a wide range of metal loads when the reaction was run in heptane.38 n-propanol, however, reaction selectivity increased with decreasing metal load. Apparently the alcohol interacted with the catalyst to modify the active sites and influenced the relative adsorption characteristics of the acetylenic and olefinic species. This can affect reaction selectivity particularly if reactant diffusion assumes some importance in the reaction. 

Zeolites are especially suitable as support materials for active components such as metals and rare earths. With rare earths, the activity of the catalyst and its stability towards steam and heat can be increased. Suitable metals are effective catalysts for hydrogenations and oxidations, whereby the shape selectivity of the carrier is retained. Important factors influencing the reactions of such bifiinctional catalysts are the location of the metal, the particle size, and the metal-support interaction. 

Lower activities were attributed to reduced stability of the active species under polymerization conditions. The lower molecular weights of their products were explained to be the result of increased hydrogen transfer rates. Variations within the heterocyclic components of the ligand showed that both steric and electronic factors influence polymerization behavior of such catalysts. 

Currently, hydrogen and methanol are the main fuels used in PEMFCs. The electrochemical oxidation of hydrogen on a wide range of metal surfaces is very facile, but in the acidic environment in which the catalyst must work only Pt and Pd show reasonable activity and stability. Many factors influence the activity and stability of a Pt/C catalyst, of which the dispersion of Pt particles seems to be particularly important. In general, to achieve the maximum number of active sites, Pt particles are finely dispersed on an inert support. The relationship between Pt particle size and surface area can be calculated by the following simple relationship (assuming spherical particles) [3] ... 

The support basicity was found to be the major factor influencing the nickel catalyst hydrogenation properties (Fig. 2). The activity of nickel catalysts in toluene hydrogenation decreased with increasing number of basic sites. 

It is clear that the influence of surface geometry upon catalytic activity is extremely complex and many more studies are required before any definitive relationship between catalytic activity and metal particle size can be established. Such studies will require to take cognisance of such factors as the perturbation of surface structure due to the formation of carbidic residues, as noted by Boudart [289] and by Thomson and Webb [95], and by the modification of catalytic properties on adsorption, as noted by Izumi et al. [296—298] and by Groenewegen and Sachtler [299] in studies of the modification of nickel catalysts for enantioselective hydrogenation. Possible effects of the support, as will be discussed in Sect. 6.3, must also be taken into account. 

Rules 1 and 2 may be accepted as a generalization based primarily on the results obtained over platinum catalysts. However, there have been known many examples of the exception to this rule,153 since the stereochemistry of hydrogenation may be influenced by many factors, such as the solvent, the temperature, the hydrogen pressure, and the basic or acidic impurity associated with catalyst preparation, as well as the activity of the catalyst, and since the effects of these factors may differ sensitively with the catalyst employed and by the structure of the ketone hydrogenated. 

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