Properties of the Catalyst

the correlation between the properties of the catalyst and its catalytic character  

the relationship between preparative procedure, the bulk and surface properties, and the catalytic character. 

Since the early studies of tin-antimony oxides have already been excellently reviewed (6, 7) only those reports that have a major bearing on more recent investigations and are directly related to the objectives of this article will be cited in this work. It should also be noted that the sections and subsections in this article represent an attempt to assess the available data on specific aspects of the material according to the objectives. However, some deviation from rigorous adhesion to individual themes has been inevitable when considering the interdependence of certain properties. 

It would seem reasonable to presume that the preparation of tin-antimony oxides by coprecipitation would lead to a more intimate mixture of tin and antimony than would be achieved by solid state reactions between the respective oxides, and it would also seem reasonable that the close proximity 

It is interesting that this mechanism, which was suggested during one of the early studies of tin-antimony oxides, has received so little subsequent attention, since it is a process that could be applied to some of the mechanisms of catalytic oxidation proposed in later studies. 

Conclusions from the Literature Reports 10.5.3.1 Properties of the Catalyst 

As shown above, several heterogeneous catalysts are capable of an efficient activation of bromobenzene and of aryl chlorides. The results obtained with palladium supported on carbon and on metal oxides revealed that the best catalyst performance was achieved if the following rules are observed (i) Pd should be highly dispersed on the support surface, (ii) It should be present as Pd(ll) (oxide or hydroxide). The classical pre-reduction in hydrogen at elevated temperature decreases the activity significantly. Low activity of several reported Pd/C catalysts can be explained in this manner, (iii) The catalysts should not be dried before use. Some water content is found to be advantageous (153). 

Taking this into account, the influence of the support is of minor importance for reactions of aU aryl bromides. Similar good results can be obtained with activated carbon, MgO, Ti02, AI2O3, Si02 etc. Differences found in the literature are probably due to different Pd dispersion, palladium reduction degree or water content. For the activation of non-activated aryl chlorides a fine-tuning of the support and the reaction conditions may be necessary. 

A similar mode of operation, i.e. delivery of highly active palladium and efficient prevention of Pd agglomeration can be assumed for other heterogeneous catalyst systems that effectively activate aryl chlorides, like layered double hydroxides (LDH) or chosen polymers. Recent reports on interesting catalytic results using polymers as support unfortunately do not give clear and sufficient experimental results on mechanistic aspects or on metal leaching [163, 171). 

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Promoters. Many industrial catalysts contain promoters, commonly chemical promoters. A chemical promoter is used in a small amount and influences the surface chemistry. Alkali metals are often used as chemical promoters, for example, in ammonia synthesis catalysts, ethylene oxide catalysts, and Fischer-Tropsch catalysts (55). They may be used in as Httie as parts per million quantities. The mechanisms of their action are usually not well understood. In contrast, seldom-used textural promoters, also called stmctural promoters, are used in massive amounts and affect the physical properties of the catalyst. These are used in ammonia synthesis catalysts. 

Whereas changing catalyst volume or residence time rarely yields compHcations, changing temperature or pressure could iatroduce sintering. The properties of the catalyst should be measured both before and after deactivation and inlet and outlet streams should be analyzed by chromatography (qv) or spectrometry. 

If a catalyst is coking up or falling apart in a short time in the recycle reactor then idow will decrease and becomes unknown after a time. In this case is best to improve the life time or the mechanical properties of the catalyst before making tests in the recycle reactor. 

The PSD is an indicator of the fluidization properties of the catalyst. In general, fluidization improves as the fraction of the 0-40 micron particles is increased however, a higher percentage of 0-40 micron particles will also result in greater catalyst losses. 

The tests that reflect physical properties of the catalyst are surface area, average bulk density, pore volume, and particle size distribution. 

Catalyst circulation is largely influenced by the physical layout of the unit and the fluidization properties of the catalyst. Some cat crackers circulate with ease regardless of the catalyst s physical properties. However, in other designs, the unit can experience circulation difficulties with minor changes in catalyst properties. 

Unfortunately, direct experimental proof of this mechanism is not available. Kinetic data (see below) offer none since the interpretation need not be unique—more than one mechanism can serve equally well. In addition, the physical properties of the catalyst are obviously contributing, and, in the extreme, diffusion can be rate-controlling, completely obscuring chemical mechanisms. 

Varying the semiconducting properties of the catalyst crystal affects the rate of ammonium perchlorate decomposition. 

The above results show consistency with the known properties of the catalysts except for aluminium chloride, the tendency of which to dimerize would lead one... 

It is clear that in case (a) the rate, r, of the catalytic reaction (e.g. CO oxidation) will not be affected while in case (b) the rate increase, Ar, will at most equal I/nF (e.g. direct reaction of O2 with CO). In case (c), however, the new species introduced electrochemically onto the catalyst surface will interact with coadsorbed reactants and will change the catalytic properties of the catalyst surface in an a priori unpredictable manner, which is nevertheless not subject to Faraday s law. Thus in cases (a) and (b) there will be no NEMCA but in case (c) it is entirely logical to anticipate it. Even in case (b) one may anticipate NEMCA, if the product remains on the surface and has some catalytic or promotional properties. 

The band gap energy of modified catalysts decreased down to 1.6 eV, and the basic structure and physical properties of the catalysts were not changed during modification process. All of the synthesized Ti02 were anatase structure but commercial Ti02 were contained 30% rutile structure. However, the catalytic activity of modified catalysts using two different Ti02 were almost the same in this reaction conditions. 

The purpose of this work was to increase the A3 selectivity at low conversion through a catalyst modification. Previous studies of phenol alkylation with methanol (the analogue reaction) over oxides and zeolites showed that the reaction is sensitive to acidic and basic properties of the catalysts [3-5]. It is the aim of this study to understand the dependence of catalyst structure and acidity on activity and selectivity in gas phase methylation of catechol. Different cations such as Li, K, Mg, Ca, B, incorporated into y-Al203 can markedly modify the polarisation of the lattice and consequently influence the acidic and basic properties of the surface [5-8] which control the mechanism of this reaction. 

The valence band structure of very small metal crystallites is expected to differ from that of an infinite crystal for a number of reasons (a) with a ratio of surface to bulk atoms approaching unity (ca. 2 nm diameter), the potential seen by the nearly free valence electrons will be very different from the periodic potential of an infinite crystal (b) surface states, if they exist, would be expected to dominate the electronic density of states (DOS) (c) the electronic DOS of very small metal crystallites on a support surface will be affected by the metal-support interactions. It is essential to determine at what crystallite size (or number of atoms per crystallite) the electronic density of sates begins to depart from that of the infinite crystal, as the material state of the catalyst particle can affect changes in the surface thermodynamics which may control the catalysis and electro-catalysis of heterogeneous reactions as well as the physical properties of the catalyst particle [26]. 

Selectivity is determined by a number of factors, such as intrinsic properties of the catalyst complex (metal and ligands), reaction conditions (concentration, temperature, pressure), and reactor configuration (solvent, reactor, process). Here, we will focus on the catalyst properties. 

While the previously described techniques were measuring the nanoscopic and microscopic properties of the catalyst pellets, respectively, fluid transport within... 

Furthermore, specific experiments carried out in our labs showed that the formation of NH3 depended also on the amounts of stored NO,, on temperature and on H2 concentration [56], Accordingly, the match of data collected on the systems with different Ba loading evidenced that the reduction process is complex and that it can lead to nitrogen and/or ammonia depending on the formulation and properties of the catalysts and on the adopted experimental conditions. 

Mg/Me (Me=Al, Fe) mixed oxides prepared from hydrotalcite precursors were compared in the gas-phase m-cresol methylation in order to find out a relationship between catalytic activity and physico-chemical properties. It was found that the regio-selectivity in the methylation is considerably affected by the surface acid-basic properties of the catalysts. The co-existence of Lewis acid sites and basic sites leads to an enhancement of the selectivity to the product of ortho-C-alkylation with respect to the sole presence of basic sites. This derives from the combination of two effects, (i) The H+-abstraction properties of the basic site lead to the generation of the phenolate anion, (ii) The coordinative properties of Lewis acid sites, through their interaction with the aromatic ring, make the mesomeric effect less efficient, with predominance of the inductive effect of the -O species in directing the regio-selectivity of the C-methylation into the ortho position. 

This section discusses the techniques used to characterize the physical properties of solid catalysts. In industrial practice, the chemical engineer who anticipates the use of these catalysts in developing new or improved processes must effectively combine theoretical models, physical measurements, and empirical information on the behavior of catalysts manufactured in similar ways in order to be able to predict how these materials will behave. The complex models are beyond the scope of this text, but the principles involved are readily illustrated by the simplest model. This model requires the specific surface area, the void volume per gram, and the gross geometric properties of the catalyst pellet as input. 

In this case the reaction rate will depend not only on the system temperature and pressure but also on the properties of the catalyst. It should be noted that the reaction rate term must include the effects of external and intraparticle heat and mass transfer limitations on the rate. Chapter 12 treats these subjects and indicates how equation 8.2.12 can be used in the analysis of packed bed reactors. 

The physical properties of the catalyst (specific surface area, porosity, effective thermal conductivity, effective diffusivity, pellet density, etc.). 

It is well known that the performance of the air gas-diffusion electrode is influenced not only by the activity of the catalyst, but also by all transport processes taking place in its porous structure. In addition, the transport hindrances in the electrode are function not only of its overall structure, but also of the porous structure and the surface properties of the catalyst. Methods for diagnostic of the activity and the transport properties of air gas-diffusion electrodes were proposed [9]. 

An increase in the zeolite crystallites size would very likely produce substantial changes in the physicochemical properties of the catalyst and consequently on the selectivity for hydroisomerisation. Since the effect of the zeolite crystallites size in the nanoscale range cannot be predicted theoretically, n-hexadecane hydroisomerization was carried out on PtHBEA catalysts with different zeolite crystallites sizes. 

The effect of microwave irradiation on the catalytic properties of a silver catalyst (Ag/Al203) in ethane epoxidation was studied by Klimov et al. [91]. It was found that on catalyst previously reduced with hydrogen the rates of both epoxidation and carbon dioxide formation increased considerably on exposure to a microwave field. This effect gradually decreased or even disappeared as the catalyst attained the steady state. It was suggested that this was very likely because of modification of electronic properties of the catalyst exposed to microwave irradiation. 

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