Structure, of catalysts

STM has particularly great potential for in situ chemical studies. While our present knowledge of the atomic structure of catalyst surfaces is largely limited to those structures which are stable in ultra-high vacuum before and after reaction, STM may provide an insight into both adsorbate and catalyst surface structure in situ during the reaction. The following issues to be characterized by STM may be most relevant to characterization of catalysts and catalysis ... 

The development of experimental methods over the last 50 years has been at the forefront of new strategies that emerged, driven by the need to obtain molecular information relevant to the structure of catalyst surfaces and the dynamics of surface reactions. The ultimate aim was in sight with the atomic resolution that became available from STM, particularly when this was coupled with chemical information from surface-sensitive spectroscopies. 

In HRTEM, very thin samples can be treated as weak-phase objects (WPOs) whereby the image intensity can be correlated with the projected electrostatic potential of crystals, leading to atomic structural information. Furthermore, the detection of electron-stimulated XRE in the electron microscope (energy dispersive X-ray spectroscopy, or EDX, discussed in the following sections) permits simultaneous determination of chemical compositions of catalysts to the sub-nanometer level. Both the surface and bulk structures of catalysts can be investigated. 

VII. Cathodoluminescence Imaging for Elucidation of Electronic Structures of Catalysts... 

Here, following the works of J.H. De Boer (Delft, The Netherlands, see elsewhere [1,2]), by texture one means the individual geometrical structure of catalysts, supports, and other porous systems (PSs) at the level of pores, particles and their ensembles (i.e., on a supramolecular level scale of 1 nm and larger). In a more complete interpretation, texture includes morphology of porous space and the skeleton of a condensed (solid or sometimes liquid) phase, the shape, size, interconnectivity, and distribution of individual supramolecular elements of the system particles and pores (or voids) between particles, various phases, etc. In turn, texturology also involves general laws of texture formation and methods for its characterization [3],... 

In the following decades, researchers in catalysis turned their efforts to controlhng molecular structure as well as size. The catalyst zeolite paved the way. In the late 1960s, researchers at Mobil Oil Co. were able to s)uithesize zeolite by deliberately designing and preparing the structure of catalysts at the atomic and molecular levels. The resulting nanostructured crystalline material (ZSM-5)—with a 10-atom ring and pore size of 0.45-0.6 nm—enabled the control of selectivity in petrochemical processes at the... 

Nevertheless, one must be aware of the possible discrepancy between the observation in an electron microscope and the real structure of catalysts. The discrepancy may come from (1) using model catalyst, which is different from the real one, (2) electron beam induced effects and (3) conditions in an electron microscope that is very different from the ambient of the activated catalysts. For ex situ investigation, it is important to work with good designed model systems whose catalytic activities can be experimentally measured and care must be taken when relating the observed structure with reactivity of catalysts. 

On the basis of the core structure of catalyst 11 (Figure 6.14), the Jacobsen group constructed a new optimization parallel library of 70 Schiff base compounds incorporating seven amino acids with bulky a-substituents and 10 new salicylal-dehyde derivatives [196]. Each library member was evaluated for enantioselectivity... 

Heterogeneous catalysis is a dynamic process and gas-catalyst reactions occur at the atomic level. In gas-catalyst reactions, the dynamic atomic structure of catalysts under operating conditions therefore plays a pivotal role in catalytic properties. Direct observations of the microstructural evolution and active sites of... 

Most of the adsorbents used in the adsorption process are also useful to catalysis, because they can act as solid catalysts or their supports. The basic function of catalyst supports, usually porous adsorbents, is to keep the catalytically active phase in a highly dispersed state. It is obvious that the methods of preparation and characterization of adsorbents and catalysts are very similar or identical. The physical structure of catalysts is investigated by means of both adsorption methods and various instrumental techniques derived for estimating their porosity and surface area. Factors such as surface area, distribution of pore volumes, pore sizes, stability, and mechanical properties of materials used are also very important in both processes—adsorption and catalysis. Activated carbons, silica, and alumina species as well as natural amorphous aluminosilicates and zeolites are widely used as either catalyst supports or heterogeneous catalysts. From the above, the following conclusions can be easily drawn (Dabrowski, 2001) ... 

Somorjai, G. A., Introduction to Surf ace Chemistry and Catalysis, Wiley, New York, 1994. (Undergraduate level. This in-depth treatment of surface chemistry and catalysis brings the experience and perspectives of a pioneer in the field to the general audience. The book is meant to be an introductory-level description of modern developments in the area for students at the junior level. However, it is also an excellent source of the current literature and contains numerous, extensive tables of data on kinetic parameters, surface structure of catalysts, and so on. Chapter 3, Thermodynamics of Surfaces, and Chapter 7, Catalysis by Surfaces, cover information relevant to the present chapter. Chapter 8 discusses applications in tribology and lubrication (not discussed in this chapter).)... 

Further developments in the fundamental approach to the electronic structure of catalysts were made possible by the development of the quantum mechanical treatments of solids which followed the work of Sommerfeld, Bloch and others. Similarly, Pauling s resonating valence bond treatment has lent further impetus to consideration of metallic catalysts. 

The electronic structure of catalysts has been shown to be important in adsorption, and since adsorption is a necessary step in heterogeneous catalytic reactions, it would be expected that changes in the electronic structure would influence the rate of reaction. 

Fig. 3. Schematic representation of the influence of reactant structure, of catalyst nature and of temperature on the elimination mechanism. Numbers in parentheses denote the rate-determining steps on Scheme 1.

Chiral C2-symmetric boron bis(oxazolines) act as enantioselective catalysts in the reduction of ketones promoted by catecholborane.321 DFT calculations indicate that the stereochemical outcome is determined by such catalysts being able to bind both the ketone and borane reducing agent, activating the latter as a hydride donor, while also enhancing the electrophilicity of the carbonyl. X-ray structures of catalyst-catechol complexes are also reported. 

Unfortunately, there has hardly been any ab initio work on the structure of catalyst/polymer interface. On the other hand, there is a bulk of literature on the structures of catalyst/water interfaces. 

Determination of the Structure of Catalyst Supports by Spectroscopy with Particular Reference to Spillover and Hydrogen Diffusion. - The adsorption of gas at a metal and the subsequent diffusion of that gas or some of its atoms onto the surface of a support is known as spillover. The process is a critical step in a number of catalytic reactions and it can be exploited in the... 

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