Factors Influencing Catalyst Performance

The nature of the adsorption of the reactant, intermediates, and products is central to whether a given system has any probability of functioning as an ammonia synthesis catalyst. The Langmuir expression (9.2.5) derived in the section on kinetics 

The preexponential factors for each of the reaction steps can, in principle, be estimated using either gas kinetics or transition state theory. Desorption occurs when the adsorbate-adsorbent bond acquires the required activation energy for desorption in the form of vibrational energy. To a first approximation the vibrational frequency can be assumed to be approximately 10 s for the temperature at which desorption proceeds at a significant rate. The frequency of bond rupture is given by 

TABLE 9.2. Bond Energies (kJ mol ) for M—N and M—H on Close-Packed Faces  

For molecules such as hydrogen, that dissociate on adsorption, the rate of desorption is controlled by atom recombination followed by molecular desorption. The rate of desorption is given by 

Under these circumstances the use of calculated preexponentials in kinetic model development is likely to lead to significant errors, in addition to those inherent in the use of calculated heats of adsorption. These problems are reflected in the attempt to model magnetite kinetics using observed heats of adsorption and estimated frequency factors, which gave rise to a calculated reaction rate a factor of 10 too low. The development of mechanistically sound, kinetic models will therefore remain dependent on the direct determination of the heats of adsorption, activation energies, and frequency factors for the forseeable future. 

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The object of the present study was to use in the above mentioned hydrogenations improved carbon supported catalysts, which could compete with the Pd black catalyst. Carbon materials are common supports, their surface properties can be modified easily and it is possible to prepare carbons with different proportion of micro-, meso- and macropores, which can be key factors influencing their performances. A highly mesoporous carbon was synthesised and used as support of Pd catalysts in the enantioselective hydrogenations. To our knowledge this is the first report on the use of highly mesoporous carbon for the preparation of Pd catalysts for liquid-phase hydrogenation. 

The factors that influence catalyst performance are numerous and only partially understood. What follows is a discussion of key catalyst design issues, starting with the catalytic surface and progressing through supported catalysts to shaped catalyst particles, successively incorporating new phenomena and variables as the complexity of the system increases. 

Two main factors influence catalyst activity chemical composition and surface area. But of no less importance for the performance are the heat transfer characteristics, which are governed by the size and shape of the particles. 

FACTORS INFLUENCING THE PERFORMANCE OF NAPHTHA HYDRODESULFURIZATION CATALYSTS... 

This chapter is concerned with highlighting some of the more notable advances that have come to light as a result of identifying key factors that influence catalyst performance, particularly those related to precatalyst structure. The importance of the initiator and the role played in chain fransfer is probed. Current mechanistic understanding is examined from both a spectroscopic and a computational viewpoint while efforts to prepare well-defined iron or cobalt alkyl catalysts are discussed. Efforts to heterogenize these homogeneous catalysts are briefly reviewed, as is their use in multi-component catalysis. 

Optimization of the initial conditions showed that the choice of base and its concentration significantly influenced catalyst performance use of 8.0 M KOH gave an improved TOF (3 h) of 860 h (Table 2, entry 1). Furthermore, an increase in the reaction temperature caused by the high base concentration (due to the salt effect) also contributed to the superior catalyst activity. Detailed investigations of the amount of base versus temperature further validated the beneficial effect of both factors on catalyst activity. 

An important aspect of the design of three phase bubble columns is the variation of catalyst distribution along the reactor height, and its effect on reactor performance. Many factors influence the degree of catalyst distribution, including gas velocity, liquid velocity, solid particle size, phase densities, slurry viscosity, and, to a lesser extent, column diameter, solid shape and chemical affinity between the solid and liquid phases. 

The extent to which the metallocene Iramework can be modified is extraordinary indeed, with from one to four different substituents on each ring or a covalent bridging group like 1,2-ethylene or dimethylsUyl to lock the rings into one particular conformation and open the active site to varying degree. All these factors have an influence on catalyst performance and polymers produced. An example... 

There are many factors which control catalyst performance. A pore structure, such as pore diameter and pore volume, is one of the important factors which strongly influence HDS activity and deactivation rates of the catalyst. While developing Cosmo CF catalyst, several test catalysts were investigated for their performance. 

A previous paper (1) discussed the heat mass transfer and pressure drop contributions of two specific channel structures. The paper showed that the channel shape can significantly influence the catalyst performance by effectively canceling the high surface area with a low cell shape factor. This paper extends this discussion to several other shapes and defines, within these sets, the limits on performance that they could reach under the same coating and testing conditions. 

The functional role of these components is discussed and their interaction reviewed from the materials technology standpoint. Aspects of catalyst performance and durability influenced by preparation factors are discussed with particular reference to factors (b), (c) and (d). 

Beside the catalyst composition, there are many other factors that will influence the performance of the catalyst, e.g. ... 

The nickel dispersion of the catalyst on alumina support was less than that on silica support. This may be due to the strong interaction between nickel and alumina and undeveloped support pore structure than that of silica support. However, high catalytic activity and resistance to carbon deposition were obtained on the nickel catalyst supported on alumina. This indicated that metal dispersion was not the decisive factor that influenced the catalyst performance. Actually, the catalytic performance of the catalysts were integrative effect of nickel loading, metal dispersion, support, promoter, preparation and activation. 

For polymer electrolyte membrane fuel cell (PEMFC) applications, platinum and platinum-based alloy materials have been the most extensively investigated as catalysts for the electrocatalytic reduction of oxygen. A number of factors can influence the performance of Pt-based cathodic electrocatalysts in fuel cell applications, including (i) the method of Pt/C electrocatalyst preparation, (ii) R particle size, (iii) activation process, (iv) wetting of electrode structure, (v) PTFE content in the electrode, and the (vi) surface properties of the carbon support, among others. ... 

Abthoff J, Zahn W, Loose G, Hirschmaim A (1994) Serial use of palladium for three-way-catalysts with high performance. Motor Z 55(5) 292 Amon DI, Hoagland DR (1940) Crop production in artificial culture solutions and in soil with special reference to factors influencing yields and absorption of inorganic nutrients. Soil Sci 50 463-484... 

Many engineering factors have a profound influence on the performance of catalysts in hydroformylation, and their effects are demonstrated in terms of reaction indices such as conversion and selectivity. More concretely, these factors influence the mass transfer rate, phase dispersion, emulsification and demulsification, catalyst distribution, product separation, etc. (Table 1). 

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