Rates of Catalytic Reactions

Fig. 10. The Thiele plot accounting for the influence of intraparticle mass transport on rates of catalytic reaction. The dimensionless terms Tj and ( ) are the...

This holds for noncatalytic reactions both isolated and in competitive system, as well as for isolated catalytic reactions. The rate of catalytic reaction in competitive (and generally in any coupled) system depends, however, on the concentrations of all the compounds present in the system, insofar as they are adsorbed on the same active centers on which the given reaction is taking place. 

On his return to Princeton after the war, Hugh Taylor organized catalytic research at the Frick Chemical Laboratory. He applied high vacuum technique, liquid air cryoscopy to the study of adsorptive characteristics of catalysts, correlating rates of catalytic reactions and rates of adsorption. He introduced the concept of activated adsorption and defended it against all comers. ... 

In sections 4.5.5 and 4.5.6 we have seen how the catalyst potential and work function affect the rates of catalytic reactions. We discussed in particular the r vs O dependence first at the local level (i.e. for small, e.g. 0.1-0.2 eV variations in eUwRand O) and then at the global level (i.e. for eUWRand variations as wide as the experimentally available eUWR range, typically 1.5-2 eV). 

These rules are not limited to electrochemical promotion only. To the best of our knowledge they are also in good qualitative agreement with the results of classical chemical promotion (electropositive or electronegative promoters) on the rates of catalytic reactions. Several examples are shown in this chapter. 

Effect of Ring Size (in Number of C Atoms) on the Relative Rates of Catalytic Reactions... 

RTD Models. The next class of models relied on the RTD to calculate conversions. But since the rate of catalytic reaction of an element of gas depends on the amount of solid in its vicinity, the effective rate constant is low for bubble gas, high for emulsion gas. Thus any model that simply tries to calculate conver-... 

The studies of well defined systems consists of spectroscopic studies of individual molecules and measurements of the rate of catalytic reactions on single crystal surfaces, as well as structure and reactivity of well-defined catalyst models. 

Since the classification is essentially based on rates of catalytic reactions relative to rates of diffusion of redox carriers, there are oxidation reactions that are intermediate between the two limiting cases. We note that neither the molecular size nor the polarity of reactant molecules is the principal characteristic determining the type of catalysis. Although oxide ions migrate rapidly in the bulk, bulk type II catalysis is not observed for oxidation catalyzed by Bi-Mo oxides. In this case the rate-limiting step is a surface reaction. 

By applying the considerations on the absolute rate of catalytic reactions, given in IV, 2, a better understanding of some of the observations can be obtained. We refer in particular to the equations... 

The adsorption of ammonia leads to a variety of chemically distinct species in most cases. Different types of sites are responsible for the formation of these surface species. Any correlation between rates of catalytic reactions and quantities of adsorbed ammonia may, therefore, be misleading and the characterization of active sites becomes ambiguous. Furthermore, ammonia is a very strong... 

Rates of catalytic reactions are obtained by measurement of the conversion of a key component, often the rate limiting reactant, in laboratory reactors and relating this to the amount of catalyst used and the amount or flow rate of reactants used, to obtain an intrinsic quantity, mols-1 amount-1. For practical application the mass or volume of a catalyst is most relevant as the amount but, for comparitive studies the amount of active phase on a supported catalyst, its specific surface area or the number of active sites may be preferred. In the latter case this yields the turnover frequency (TOF) [3], which is quite relevant for fundamental studies. The number of active sites is, however, usually hard to determine and the mass of the catalyst W will be used, resulting in a rate dimensions of mol s 1 kg-1. Other quantities are easily derived from this. 

A ratio of Cas to the rate of catalytic reaction under steady-state conditions (r) gives a rough estimation for the reaction time scale. For typical heterogeneous catalytic processes applied for the production of bulk chemicals and petrochemicals, this value is estimated to be 10-2—101 s (Fig. 5). The changes of the reaction rate caused by the side processes of catalyst modification can take considerably longer. This is attributed to the higher capacity of substances in the catalyst bulk phase that can be involved in side interactions and to a slow rate of side processes in comparison with the stages of the catalytic cycle. 

Periodical interruption in feeding the reactant, which leads to the formation of nonreactive surface species, increases the rate of catalytic reaction. For example, for the dehydration of alcohols or deamination of primary amines on acid-base catalysts [15] the process can be generally described by one of the two simple schemes ... 

As seen from eqs. 1 to 3, the rate of production and, consequently, the rate of catalytic reaction are related to the quantity of catalyst Q which may, for example, be expressed as the mass (mc), volume (Kc) or the surface area (Ac) of the solid catalyst. The numerical values of the corresponding specific reaction rates (rm, rv, r ) naturally differ from each other depending on the quantity to which they are related. 

During investigation of the NEMCA effect the rates of catalytic reactions were found to depend on catalyst work function, , via the equation In(r/r0) = aty/k T where a is a reaction-specific constant and kB is the... 

Metal oxides are ubiquitous in catalysis and are key components in several catalytic reactions. They function directly as catalytic reactive centers or serve as high surface area supports to disperse active metal centers or as promoters to enhance the rate of catalytic reactions. Many commercial catalysts consist of zero valent metal atoms dispersed finely on a high surface area metal oxide support such as silica or alumina. 

Effects of coke on adsorption, diffusion, selectivities of catalytic reactions, and rates of catalytic reactions. 

In the presence of catalysts, heterogeneous catalytic cracking occms on the surface interface of the melted polymer and solid catalysts. The main steps of reactions are as follows diffusion on the surface of catalyst, adsorption on the catalyst, chemical reaction, desorption from the catalyst, diffusion to the liquid phase. The reaction rate of catalytic reactions is always determined by the slowest elementary reaction. The dominant rate controller elementary reactions are the linking of the polymer to the active site of catalyst. But the selectivity of catalysts on raw materials and products might be important. The selectivity is affected by molecular size and shape of raw materials, intermediates and products [36]. 

Proper substrate binding allows for the hound (closed) state (EzS) to be in dynamic equilibrium with free substrate. Upon domain closure, catalytic reaction can occur to transform the bound state (EzS) to an energetically less stable state than the open state of the protein (F.zP) by altering the interactions between the protein and the bound molecule. Note that the upper limit on the rate of catalytic reaction should, therefore, be fixed by the rate of domain movements. Since the open state is more energetically favored, the product will desorb to return the enzyme to the open state. 

The benefits of nonuniform activity distributions (site density) or diffusive properties (porosity, tortuosity) within pellets on the rate of catalytic reactions were first suggested theoretically by Kasaoka and Sakata (Ml). This proposal followed the pioneering experimental work of Maatman and Prater (142). Models of nonuniform catalyst pellets were later extended to more general pellet geometries and activity profiles (143), and applied to specific catalytic reactions, such as SO2 and naphthalene oxidation (144-146). Previous experimental and theoretical studies were recently discussed in an excellent review by Lee and Aris (147). Proposed applications in Fischer-Tropsch synthesis catalysis have also been recently reported (50-55,148), but the general concepts have been widely discussed and broadly applied in automotive exhaust and selective hydrogenation catalysis (142,147,149). 

Carbonaceous deposits result from the transformation of reactants, reaction products, impurities of the feed, etc., on acid sites through various successive bimolecular steps condensation, hydrogen transfer, etc. Therefore, their rate of formation depends on the following parameters which usually affect the rate of catalytic reactions, namely ... 

All statements refer to rate of catalytic reaction (catalytic activity) or to amount of chemisorption unless otherwise specifled. Numbers refer to rates in arbitrary units unless otherwise specified. means increase, increases, or increased, and ], means decrease, etc. 

O conductor, Na-p"-Al203, a Na conductor, CaZro.9lno.i03-a, a H" conductor [22], or Ti02, a mixed electronic-ionic conductor [23]). Catalyst, counter and reference electrode preparation and characterization details have been presented in detail elsewhere [9,14] together with the gas analysis system for on-line monitoring of the rates of catalytic reactions via gas chromatography, mass spectrometry and infrared spectroscopy. 

The use of power law kinetics to describe the rates of catalytic reactions. This is an empirically oriented approach which considerably limits the generdity of the rate equations obtained and makes it valid only in the region of parameters in which the empirical power law kinetics was obtained. 

As discussed above, the transport properties of porous catalyst particles of ca 3 to 100 pm are extremely important for the selectivity of catalytic reactions in which the desired initial products are liable to further reaction to undesired material. The ratio of the rate of catalytic reaction to that of transport within the pore system of catalyst particles is represented by Thiele s modulus [1], which is proportional to the pore length and to the square root of the diameter of the pores. Accordingly reducing the size of the catalyst particles is more elfective than increasing the diameter of the pores. 

It was the purpose of the preceding considerations to develop a basis for the analysis of the rates of catalytic reactions in zeolites. 

Likewise, the change in the rate of catalytic reaction, e.g., the para-hydrogen conversion with alloy composition is indicative of charge transfer concurrent with adsorption (128). 

Although this research work, carried on for many years, resulted in a vast amount of valuable data, a closer study of the catalytic behavior of metallic compounds may be said to have been initiated by Schwab and Rienacker, who, from 1936 onwards, made extensive researches to investigate the decomposition of formic acid on a large variety of metals and metal alloys. These workers primarily attempted to find an answer to the question whether or not the rate of catalytic reactions on metals in general is influenced by alloying these metals with others in such a way that the electron concentration increases. 

It was shown above that the activation energy is related to . There is almost no information available on as a function of activation energies and rates of catalytic reactions over silver. According to Hayes (130) the activation energy for NaO decomposition on alloys of silver with various -decreasing metals will be low. Sosnovsky (175) has investigated the catalytic activities (E and K0) for different planes of silver crystals, with respect to the decomposition of formic acid. E and Ko were found to increase with plane indexes. The relation between and the rate of ethylene oxidation to ethylene oxide was not established. 

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