Nature of the Catalyst Support

The effect that a solvent exerts on the rate of a heterogeneously catalyzed reaction depends on the reaction substrate, the catalytically active species and the nature of the catalyst support. 2 Because of this complexity, attempts to derive a general equation relating reaction rate with some property or properties of the solvent have been imsuccessful. 2.l4 The primary reason for this failure is that the solvents are not present merely as diluents in which the reactants are dissolved, they are also capable of interacting with the catalyst surface so they can also modify the extent to which different types of substrates are adsorbed. 

Nature of the Catalyst Support. - Of equal importance to the ionic character of the impregnating solution is the ionic exchange type and capacity of the catalyst-support surface. This is, of course, directly related to the chemical structure of the support surface. To say that information in the catalyst literature on the chemical structure of support surfaces is sparse is almost an understatement. Evaluations of this important catalyst-preparation parameter are almost without exception entirely overlooked in the published literature on catalyst preparations. A very limited amount of information can be found in a review of 1970. This review is, however, primarily concerned with the physical rather than the chemical structure of catalyst supports. The chemical reactivity of an oxide support surface would appear to depend upon the extent of its hydroxylation. This in turn depends upon the chemical type of the support, the way it was made, and particularly upon its previous thermal history. A few generalizations can be made, as follows. 

The distributions of cis and trans structures in the array of products are subject to wide variations that are characteristic of each of the transition metal catalysts. The nature of the catalyst support is of minor concern as long as the support is not an active catalyst in its own right. A single sequence of reaction steps common to all transition metals is envisaged. Product variability is associated with significant differences in the relative rates of successive steps in the reaction path. Productcontrolling steps may occur early or late. 

The papers included in this symposium cover the full gamut of problems that had to be addressed. The physical and chemical stability of the catalysts had to be significantly improved over known catalysts in order to meet the 50,000 mile life requirement prescribed by the regulations. The effects of catalyst poisons such as lead, sulfur, phosphorus, etc. were also critical in relation to the limits of deposition that could be tolerated while maintaining catalyst effectiveness. The nature of the catalyst support or substrate became significant in relation to its interaction with the metallic components of the catalyst—adherence, distribution, and reactivity at high temperature. 

As described above, the basic catalyst characterisation involves two main steps the investigation on the porous nature of the catalyst support (physical properties) and on the properties of the active sites that are dispersed on the support surface (see Table 1). 

The dissociation of CO2 depends on the nature of the catalyst support. On an acidic or inert support such as Si02, the CO2 chemisorption and dissociation occurs on a transition metal surface and is dominated by electron transfer, requiring the formation of an anionic C02 precursor [142] ... 

The reaction of 2-propanol to propanone and propene over a series of alkali-metal-doped catalysts with use of microwave irradiation has been studied by Bond et al. [90], The nature of the carbon support was shown to affect the selectivity of the catalyst. Under microwave irradiation the threshold reaction temperature (i. e. the lowest temperature at which the reaction proceeded) was substantially reduced this was explained in terms of hot spots (Sect. 10.3.3) formed within the catalyst bed. 

In another example, a polymer-supported chromium porphyrin complex was supported on ArgoGel Cl and then employed for the ring-opening polymerization of 1,2-cyclohexene oxide and C02 [95], This complex showed higher activity than a C02-soluble equivalent, and the solid nature of the catalyst meant that recycling of the catalyst was much easier. 

Supported aqueous phase (Chapter 3, Section 3.6, Chapter 5, Section 5.2.5) and supported ionic liquid phase catalysts, Chapter 7, Section 7.3) are probably not suitable for use with higher alkenes because the liquid feed slowly dissolves some of the water or ionic liquid changing the nature of the catalyst and leading to catalyst leaching. 

One way in which cobalt dispersion can be increased is the addition of an organic compound to the cobalt nitrate prior to calcination. Previous work in this area is summarized in Table 1.1. The data are complex, but there are a number of factors that affect the nature of the catalyst prepared. One of these is the cobalt loading. Preparation of catalysts containing low levels of cobalt tends to lead to high concentrations of cobalt-support compounds. For example, Mochizuki et al. [37] used x-ray photoelectron spectroscopy (XPS) and temperature-programmed reduction (TPR) to identify cobalt silicate-like species in their 5% Co/Si02 catalysts modified with nitrilotriacetic acid (NTA). The nature of the support also has... 

We begin with the structure of a noble metal catalyst. The emphasis is on the preparation of rhodium on aluminum oxide and the nature of the metal-support interaction. Next we focus on a promoted surface in a review of potassium on noble metals. This section illustrates how single crystal techniques have been applied to investigate to what extent promoters perturb the surface of a catalyst. The third study deals with the sulfidic cobalt-molybdenum catalysts used in hydrotreating reactions. Here we are concerned with the composition and structure of the catalytically active... 

The series 3-6 in Table II constitute a strong support for this approach to elimination mechanisms. The slopes of the Taft relationships p ) change with the nature of the catalysts and could be correlated with their other intensive properties, which were determined independently (Fig. 3). The... 

The actual selectivity depends on the nature of the catalyst. For example, the following data were reported for n-hexane transformed over platinum and palladium supported on the same alumina 44) (pulse system, hydrogen carrier gas, T = 520°C) ... 

In 2005, Rossi and coworkers investigated the effect of the carbon support on the activity of Au/C catalysts in the oxidation of glucose under atmospheric pressure of oxygen at 323 K and a pH of 9.5 [121]. Au nanoparticles (in a range of 1-6 nm) dispersed on carbon XC72R (specific area 254 m g pore volume 0.19 mL g ) were more active than Au nanoparticles dispersed on carbon X40S (specific area 1,100 m g pore volume 0.37 mL g ). However, if the nature of the carbon support played a role on the catalyst activity, authors pointed out that the major role was played by the size of the Au nanoparticles, the smaller particles being the most active. 

Although generalization of the present results to other catalyst systems is tempting, the evidence currently available is undoubtedly insufficient to justify such an extrapolation. It is clear, however, from both the present and previous work in this laboratory, that the chlorine atoms interact with the surface of the catalyst. There is also strong evidence to support the contention that the effect produced by the introduction of TCM into the feedstream can be primarily attributed to a modification of the catalyst surface and not to a gas phase process. The present work demonstrates that, at least with praseodymium oxide, the oxychloride is produced on addition of TCM and further, the oxychloride is largely responsible for the beneficial effects. Since the enhancements observed with TCM have been shown (2, 4-7) to be related to the nature of the catalyst, it is conceivable that these effects, while dependent on the formation of the oxychloride, are also a function of the thermodynamic stability of the oxychloride. Further work is in progress. 

Similar results have been obtained using silica-supported metals [53,64], although the shape of the isotherm and the extent of retention appear to be dependent upon the physical nature of the catalyst (Fig. 7). 

Table I. Dependence of the Degree of Nickel Reduction to Metal and the Size of Crystallites on the Nature of a Catalyst Support (45)...

Details about preparation and characterization of dispersed microcrystals can be found in review chapters [322] and will not be dealt with here. All investigations indicate that the properties of microcrystals differ considerably from those of bulk metals (and from those of adatoms and thin films as well) [328], and that they can also be influenced by the nature and texture of the support. In particular, micro-deposits of precious metals on various inert supports (Ti, Ta, Zr, Nb, glassy carbon etc.) exhibit enhanced electrocatalytic effects as evaluated per metal atom, while the mechanism of H2 evolution remains the same [329], and the enhancement increases as the crystallite size decreases [326, 331] (Fig. 17). However, while this is the case with Rh, Pt, Os and Ir, Pd shows only an insignificant increase, whereas for Ru even a drastic decrease is observed [315, 332]. Thus, the effect of crystal size on the catalytic activity appears to depend on the nature of the catalyst (without any relation with the crystal structure group) [330]. 

In order to achieve the goal of reducing sulfur levels in fuels, there is a clear need for understanding the mechanism of the reaction (Chapter 4) in conjunction with the nature of the catalyst and support. Most of the work has been carried out with the traditional cobalt-molybdenum catalyst supported on alumina. This system is a time-tested and effective. 

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