Industrial catalysts properties

The typical industrial catalyst has both microscopic and macroscopic regions with different compositions and stmctures the surfaces of industrial catalysts are much more complex than those of the single crystals of metal investigated in ultrahigh vacuum experiments. Because surfaces of industrial catalysts are very difficult to characterize precisely and catalytic properties are sensitive to small stmctural details, it is usually not possible to identify the specific combinations of atoms on a surface, called catalytic sites or active sites, that are responsible for catalysis. Experiments with catalyst poisons, substances that bond strongly with catalyst surfaces and deactivate them, have shown that the catalytic sites are usually a small fraction of the catalyst surface. Most models of catalytic sites rest on rather shaky foundations. 

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. 

Catalysis by Metals. Metals are among the most important and widely used industrial catalysts (69,70). They offer activities for a wide variety of reactions (Table 1). Atoms at the surfaces of bulk metals have reactivities and catalytic properties different from those of metals in metal complexes because they have different ligand surroundings. The surrounding bulk stabilizes surface metal atoms in a coordinatively unsaturated state that allows bonding of reactants. Thus metal surfaces offer an advantage over metal complexes, in which there is only restricted stabilization of coordinative... 

In the bulk form, vanadium oxides display different oxidation states and V—O coordination spheres and exhibit a broad variety of electronic, magnetic, and structural properties [96, 97], which make these materials attractive for many industrial applications. Prominent examples range from the area of catalysis, where V-oxides are used as components of important industrial catalysts for oxidation reactions [98] and environment pollution control [99], to optoelectronics, for the construction of light-induced electrical switching devices [100] and smart thermo-chromic windows. In view of the importance of vanadium oxides in different technological applications, the fabrication of this material in nanostructured form is a particularly attractive goal. 

Archaea or Archaebacteria, which live in sulphurous waters around undersea volcanic vents. An extraordinarily stable enzyme which functions even at 135 °C and survives at pH 3.2-12.7 has been identified [142]. This enzyme has been termed STABLE (stalk-associated archaebacterial endoprotease). It is suggested that such exceptional stability may be attributable to unusually large Mr and tight folding of the protein chain. Suggested uses include washing powders and detergents, as well as industrial catalysts. It is even proposed that such remarkable properties may have contributed to the early evolution of life on earth [142]. 

We will describe first the different methods of immobilization of catalysts, and highlight their advantages and disadvantages and their fields of application. We will then examine the properties of such supported complexes for the major classes of catalytic reactions. We will focus mainly on those studies where at least some characterization of the supported catalyst is given, unless the catalytic properties of the described system are outstanding the review is therefore far from being exhaustive. Finally, where possible, we will mention tests of recyclability, which are essential for the supported complex to be as a potential industrial catalyst. 

None of the above properties and characteristics act independently. When one among them is changed with a view to improvement, the others are also modified and not necessarily in the direction of an overall improvement. As a result, industrial catalysts are never ideal. Fortunately, however, the ideal is not altogether indispensable. Certain properties, such as activity and reproducibility, are always necessary, but selectivity, for example, has hardly any meaning in reactions such as ammonia synthesis, and the same holds true for thermal conductivity in an isothermal reaction. Stability is always of interest but becomes less important in processes that include continuous catalyst regeneration. Regenerability must be optimized in this case. 

The oxidation of propene to acrolein has been one of the most studied selective oxidation reaction. The catalysts used are usually pure bismuth molybdates owing to the fact that these phases are present in industrial catalysts and that they exhibit rather good catalytic properties (1). However the industrial catalysts also contain bivalent cation molybdates like cobalt, iron and nickel molybdates, the presence of which improves both the activity and the selectivity of the catdysts (2,3). This improvement of performances for a mixture of phases with respect to each phase component, designated synergy effect, has recently been attributed to a support effect of the bivalent cation molybdate on the bismuth molybdate (4) or to a synergy effect due to remote control (5) or to more or less strong interaction between phases (6). However, this was proposed only in view of kinetic data obtained on a prepared supported catalyst. 

Spinel oxides are important industrial catalysts, possessing good thermal and chemical stability and an ability to maintain catalytic activity at high temperatures. The study reported here is concerned with the properties ofZn(II)-Cr(III) spinel oxides prepared from a carbonate containing LDH precursor. For this study the spinel oxide was separated, by treatment with dilute mineral acid, from the oxide mixture (ZnO and ZnC O, ) obtained by the thermal decomposition of LDH. For comparison, Zn(II)-Cr(lII) spinel oxides were also prepared by more standard synthesis methods. Structural and textural properties were evaluated, using PXRD, TG/MS, FT-IR and N2 adsorption. 

As a catalyst for propylene oxidation, Bi203 itself has fairly low activity and yields primarily the products of complete oxidation. Pure molybdenum trioxide has an even lower activity, but is fairly selective. In combination, however, remarkable activity and selectivity for propylene oxidation is obtained. Although industrial catalysts contain silica and phosphate as well as Bi203 and Mo03, many fundamental studies have employed catalysts containing only bismuth and molybdenum oxides in an attempt to determine the structure of the catalytically active phase. As a result of such studies, it is now known that bismuth molybdate catalysts display their superior properties only if the catalyst composition lies within the composition range of Bi/Mo = f to f (atomic ratio). 

Theoretical models based on first principles, such as Langmuir s adsorption model, help us understand what is happening at the catalyst surface. However, there is (still) no substitute for empirical evidence, and most of the papers published on heterogeneous catalysis include a characterization of surfaces and surface-bound species. Chemists are faced with a plethora of characterization methods, from micrometer-scale particle size measurement, all the way to angstrom-scale atomic force microscopy [77]. Some methods require UHV conditions and room temperature, while others work at 200 bar and 750 °C. Some methods use real industrial catalysts, while others require very clean single-crystal model catalysts. In this book, I will focus on four main areas classic surface characterization methods, temperature-programmed techniques, spectroscopy and microscopy, and analysis of macroscopic properties. For more details on the specific methods see the references in each section, as well as the books by Niemantsverdriet [78] and Thomas [79]. 

These various aspects of the catalyst are related through cause and effect. The properties sought in the industrial catalyst by the user flow from its intrinsic physico-chemical characteristics and both industrial... 

Perfecting an industrial catalyst is thus the culmination of a long and complicated process that requires a knowledge as broad as possible of the methods relative to the preparation of catalysts, to the study of catalytic and mechanical properties, and to the determination of the physicochemical characteristics... 

Introduction to Industrial Polyethylene Properties, Catalysts, Processes by Dennis P. Malpass. Published 2010. ISBN 978-0-470-625989. 

The CoMo/A1203 catalyst system has been studied by a variety of techniques in attempts to relate structural features of the system to catalytic performance. It is a remarkable property of this catalyst that despite high metal loading (about 15-20 wt.% based on oxides), poor crystallinity is developed. Consequently, traditional methods provide little information about the local structure of Mo and Co in these industrial catalysts and about structural changes with the preparative method. It is obvious that X-ray absorption studies are appropriate means for their characterization. 

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