Heterogeneous catalysis surface reactions

The Eley-Rideal mechanism for gas-solid heterogeneous catalysis envisions reaction between a molecule adsorbed on the solid surface and one that is still in the gas phase. Consider a reaction of the form... 

Transition metals play an important role in heterogeneous catalysis where reactions occur on the surfaces of metal or oxide crystals. Typical of these metals are V or Mo which exist in oxides with tetrahedral, tetragonal pyramidal, or octahedral coordination and which can change their oxidation states with minimal changes in their coordination environment. As in the case of soil minerals (Section 13.4.1), bond valences can be used to determine the bonding strength of the anions on the surface, by noting how far the valence sums around the surface ions fall short of 2.00 vu. 

Substances which increase the rate of a chemical reaction without themselves being used up or incorporated into the finished product are called catalysts. In heterogeneous catalysis the reaction takes place on the surface of a solid support. The activity of the catalyst in this case is determined by the structure and size of the surface area as well as the way the catalyst is produced. Catalysts are not limited to immobilization on solids, they can also be introduced as homogeneous catalysts in solution. Between heterogeneous and homogeneous catalysts there is the possibility to evenly distribute small particles (dispersions) of catalyst in a liquid phase. 

Textural properties are important in the field of catalyst design for heterogeneous catalysis Surface area and pore size determine the accessibility to active sites and this is often related to catalytic activity and selectivity in catalysed reactions Therefore textural properties are often a target of catalyst design. 

Heterogeneous catalysis describes reactions in which the catalyst and the reactants are in different phases. In these reactions the catalyst is most often an insoluble solid and the reactants are in the gaseous or liqtiid/ solution phase. A key feature of this type of catalysis is that the reactants must adsorb to the catalyst s surface. Large catalyst surfaces, then, ensure that the desired reaction occurs rapidly. 

The study of dissociative chemisorption of gas-phase molecules on metal surfaces is important to the understanding of a myriad of processes such as heterogeneous catalysis, electrode reactions, and corrosion, to name but a few (179-181). Recent advances in molecular beam, laser, and surface detection technologies have made it possible to study the reactions of monoenergetic molecular beams with clean, well-characterized metal surfaces. 

Sohds are exposed to the environment via their surfaces. Adsorption and reactions at the surface of various materials used in technology and everyday life are responsible for many important desired and undesired phenomena, including coating, passivation, corrosion, and heterogeneous catalysis. Surface interactions also offer a way to create novel advanced materials with tailored structural, electronic, magnetic and chemical properties. High-level quantum chemistry methods, together with the rich arsenal of experimental surface science techniques... 

The increased specific surface area is very important in heterogeneous catalysis since reactions take place at the gas-solid or liquid-solid interface. For example, the dehydrogenation rate of butane over VN significantly improves with the increasing... 

The other case is that of slow adsorption of the substance, with fast chemical reaction at the surface. Such process is known as chemical adsorption or chemisorption. Here adsorbed molecules are glued to the surface by chemical forces of the same type as the forces of valence bonds. In order for these forces to reveal themselves, a molecule should come into a deformed state and overcome the activation barrier. Therefore the process of chemical adsorption requires a certain activation energy. Sometimes chemical adsorption is called activated adsorption. Chemical adsorption is closely related to the process of heterogeneous catalysis. The reaction rate of chemical adsorption is given by the expression ... 

In heterogeneous catalysis the reaction occurs on the interphase boundary. Adsorption of reagents on the catalyst surface possessing collective electronic properties is followed by their activation. However, not all active centers located on the catalyst surface are equivalent. It is possible that only one of the set is capable of catalyzing the desired reaction, whereas others are inactive and even responsible for some side reactions [14]. 

Heterogeneous catalysis entails reactions between organic molecules and the surface of inorganic materials. Although a great deal of work has been carried out, and continues, to characterize the surfaces of materials, we remain far from being able to predict surface properties for all but the simplest. Even... 

In heterogeneous catalysis, the reaction takes place at the interface between the catalyst and the less dense phase. Adsorption is defined as the preferential concentration of gas molecules at a fresh solid surface, caused by the existence of a field force that attracts molecules of the contacting fluid. Two major types of adsorption have been recognized, namely, physical adsorption and chemisorption [5, 6]. 

At a surface, the normal components of vectorial fluxes are scalar. Differences in temperatures and chemical potentials across or with a surface can therefore drive a chemical reaction in heterogeneous catalysis. Chemical reactions can similarly drive heat and mass transport into and through a surface. This has not been studied before. A chemical reaction will lead to changes in the concentrations (see eq 14.12) and thereby modify the chemical potentials. In this manner a chemical reaction may also modify heat and mass transport in homogeneous systems. This will be discussed further in subsection 14.3.2. 

Studies of inelastic scattering are of considerable interest in heterogeneous catalysis. The degree to which molecules are scattered specularly gives information about their residence time on the surface. Often new chemical species appear, whose trajectory from the surface correlates to some degree with that of the incident beam of molecules. The study of such reactive scattering gives mechanistic information about surface reactions. 

Studies of surfaces and surface properties can be traced to the early 1800s [1]. Processes that involved surfaces and surface chemistry, such as heterogeneous catalysis and Daguerre photography, were first discovered at that time. Since then, there has been a continual interest in catalysis, corrosion and other chemical reactions that involve surfaces. The modem era of surface science began in the late 1950s, when instmmentation that could be used to investigate surface processes on the molecular level started to become available. 

The microscopic understanding of tire chemical reactivity of surfaces is of fundamental interest in chemical physics and important for heterogeneous catalysis. Cluster science provides a new approach for tire study of tire microscopic mechanisms of surface chemical reactivity [48]. Surfaces of small clusters possess a very rich variation of chemisoriDtion sites and are ideal models for bulk surfaces. Chemical reactivity of many transition-metal clusters has been investigated [49]. Transition-metal clusters are produced using laser vaporization, and tire chemical reactivity studies are carried out typically in a flow tube reactor in which tire clusters interact witli a reactant gas at a given temperature and pressure for a fixed period of time. Reaction products are measured at various pressures or temperatures and reaction rates are derived. It has been found tliat tire reactivity of small transition-metal clusters witli simple molecules such as H2 and NH can vary dramatically witli cluster size and stmcture [48, 49, M and 52]. 

Other reactions at surfaces (heterogeneous catalysis and reduction reactions)... 

Volume 109 Dynamics of Surfaces and Reaction Kinetics in Heterogeneous Catalysis. 

Of these, the most extensive use is to identify adsorbed molecules and molecular intermediates on metal single-crystal surfaces. On these well-defined surfaces, a wealth of information can be gained about adlayers, including the nature of the surface chemical bond, molecular structural determination and geometrical orientation, evidence for surface-site specificity, and lateral (adsorbate-adsorbate) interactions. Adsorption and reaction processes in model studies relevant to heterogeneous catalysis, materials science, electrochemistry, and microelectronics device failure and fabrication have been studied by this technique. 

Nonstoichiometric oxide phases are of great importance in semiconductor devices, in heterogeneous catalysis and in understanding photoelectric, thermoelectric, magnetic and diffusional properties of solids. They have been used in thermistors, photoelectric cells, rectifiers, transistors, phosphors, luminescent materials and computer components (ferrites, etc.). They are cmcially implicated in reactions at electrode surfaces, the performance of batteries, the tarnishing and corrosion of metals, and many other reactions of significance in catalysis. ... 

One problem with heterogeneous catalysis is that the solid catalyst is easily poisoned. Foreign materials deposited on the catalytic surface during the reaction reduce or even destroy its effectiveness. A major reason for using unleaded gasoline is that lead metal poisons the Pt-Rh mixture in the catalytic converter. 

Models and theories have been developed by scientists that allow a good description of the double layers at each side of the surface either at equilibrium, under steady-state conditions, or under transition conditions. Only the surface has remained out of reach of the science developed, which cannot provide a quantitative model that describes the surface and surface variations during electrochemical reactions. For this reason electrochemistry, in the form of heterogeneous catalysis or heterogeneous catalysis has remained an empirical part of physical chemistry. However, advances in experimental methods during the past decade, which allow the observation... 

Normally in heterogeneous catalysis compensation effect behaviour is obtained either for the same reaction upon using differently prepared catalysts of the same type, or with the same catalyst upon using a homologous set of reactants. In the case of electrochemical promotion (Figs. 4.38 and 4.39) one has the same catalyst and the same reaction but various potentials, i.e. various amounts of promoter on the catalyst surface. 

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