Chemical catalysis definition

So far, catalytic systems in which the mediator plays the role of both catalyst and electron carrier have been considered. Figure 4.21 shows an example where these two roles are dissociated.21 The catalyst, in the sense of a chemical catalyst, is the Co(II) porphyrin embedded in the Nafion (a trademark of Dupont) film, while the electron are shuttled by the ruthenium hexamine 3 + /2+ couple attached electrostatically to the Nafion backbone. The catalytic reaction now involves two successive steps, as expected for a chemical catalysis process (see Sections 4.2.1 and 4.3.1), calling for the definition of two characteristic currents. One has the same... 

Charge transfer resistance, 1056 Charge transfer overpotential, 1231 Charge transfer, partial. 922. 954 Charges in solution, 882 chemical interactions, 830 Charging current. 1056 Charging time, 1120 Chemical catalysis, 1252 Chemical and electrochemical reactions, differences, 937 Chemical equilibrium, 1459 Chemical kinetics, 1122 Chemical potential, 937, 1058 definition, 830 determination, 832 of ideal gas, 936 interactions, 835 of organic adsorption. 975 and work function, 835... 

The reach of hydrophobicity in the model can severely complicate its synthesis. This criterion makes sense in the case of the task of achieving as close a correspondence as possible between the model and the simulated enzyme. However, a more particular objective is often assigned for the synthesis of mimics. It consists of synthesizing an effective catalyst simulating a definite (useful) enzyme property. Therefore, it seems more desirable to replace hydrophobic tub synthesis by more accessible methods, developed in chemical catalysis, particularly in coordination and heterogeneous catalysis. 

The subsequent deconstruction of biopolymers into single, low-molecular-weight chemicals, allows the identification and definition of catalyst to become clearer. Chemical catalysis has been used to improve many of these... 

Specific acid catalysis as illustrated in the mechanism of Eqs. 4-7 is the most common form of catalysis encountered in environmental systems. However, we may ask whether the reaction of hydrogen peroxide with thiourea is truly acid-catalyzed. According to some of the early definitions of chemical catalysis, this reaction would not be categorized as a catalytic, since the proton is a stoichiometric reactant, but in accord with the IUPAC definition the pathway involving H +... 

Although the definition of catalyst cannot be directly extrapolated for that corresponding to electrocatalyst, the concepts underlying the definition can be effectively used. In a catalyzed reaction, there is a specific interaction between the catalyst and some species involved in the chemical reaction. The specific interaction is then the key point in the catalysis definition and, thus, the electrocatalyst can be defined as an electrode material that interacts specifically with some species involved in the reaction and remains unaltered after the reaction. This definition allows a good distinction between electrocatalyzed and non-electrocatalyzed reactions the existence of specific surface interactions is characteristic of an electrocatalyzed reaction. 

Ammonia decomposition over Fe, Cu, Ag, Au, and Pt Hydrolysis of starch to glucose catalyzed by acids Mixture of coal gas and air makes a platinum wire white hot Measurements on the rate of H2O2 decomposition Selective oxidation of ethanol to acetic acid over platinum Comprehensive paper on the H2 + O2 reaction on platinum foils, including reaction rates, deactivation, reactivation, and poisoning Definition of catalysis, catalyst, and catalytic force First quantitative analysis of reaction rates Systematic studies on the concentration dependence of reaction rates First concise monograph on chemical kinetics Definition of order of reaction Arrhenius equation k = u exp (-Ea/RT)... 

A catalyst is a substance that iacreases the rate of approach to equiUbrium of a chemical reaction without being substantially consumed itself. A catalyst changes the rate but not the equiUbrium of the reaction. This definition is almost the same as that given by Ostwald ia 1895. The term catalysis was coiaed ia ca 1835 by Ber2eHus, who recognized that many seemingly disparate phenomena could be described by a single concept. For example, ferments added ia small amounts were known to make possible the conversion of plant materials iato alcohol and there were numerous examples of both decomposition and synthesis reactions that were apparendy caused by addition of various Hquids or by contact with various soHds. 

A recent definition of catalysis that is based on dier-modynamics was advanced by the Subcommittee on Chemical Kinetics, Physical Chemistry Division, lUPA ... 

Another definition of acids and bases is due to G. N. Lewis (1938). From the experimental point of view Lewis regarded all substances which exhibit typical acid-base properties (neutralisation, replacement, effect on indicators, catalysis), irrespective of their chemical nature and mode of action, as acids or bases. He related the properties of acids to the acceptance of electron pairs, and bases as donors of electron pairs, to form covalent bonds regardless of whether protons are involved. On the experimental side Lewis definition brings together a wide range of qualitative phenomena, e.g. solutions of BF3, BC13,... 

In 1835 Berzelius coined the term catalysis to describe the influence of certain substances on the nature of diverse reactions, the substances themselves apparently being unchanged by the reaction. He imbued these materials with a catalytic force capable of awakening the potential for chemical reaction between species that would normally be nonreactive at a given temperature. In more modern terms the following definition is appropriate. 

In the absence of aging (deactivation) of the catalyst, a phenomenological definition of catalysis would be enhancement of chemical reactions or a change of their rate under the influence of substances — catalysts — which several times enter into transient chemical interactions with reaction participants and then, after each cycle of transient interactions, regenerate their chemical identity [1]. 

The term catalysis was coined by Berzelius over 150 years ago when he had noticed changes in substances when they were brought in contact with small amounts of certain species called "ferments". Many years later in 1895 Ostwald came up with the definition that we use until today A catalyst is a substance that changes the rate of a chemical reaction without itself appearing into the products. This means that according to Ostwald a catalyst can also slow down a reaction The definition used today reads as follows A catalyst is a substance which increases the rate at which a chemical reaction approaches equilibrium without becoming itself permanently involved. 

The clay fraction, which has long been considered as a very important and chemically active component of most solid surfaces (i.e., soil, sediment, and suspended matter) has both textural and mineral definitions [22]. In its textural definition, clay generally is the mineral fraction of the solids which is smaller than about 0.002 mm in diameter. The small size of clay particles imparts a large surface area for a given mass of material. This large surface area of the clay textural fraction in the solids defines its importance in processes involving interfacial phenomena such as sorption/desorption or surface catalysis [ 17,23]. In its mineral definition, clay is composed of secondary minerals such as layered silicates with various oxides. Layer silicates are perhaps the most important component of the clay mineral fraction. Figure 2 shows structural examples of the common clay solid phase minerals. 

The homogeneous catalytic reaction occurs in the multi-component liquid phase P. The chemical constituents of the liquid phase include H, e", atoms, ions, and molecules etc. which are dissolved/solvated in one or more molecular or ionic solvents. Primary examples of the ions and molecules present are the dissolved organic and organometallic reagents, intermediates and products. By definition, all the molecular and ionic species involved directly in the homogeneous catalysis are soluble in this liquid phase P. The set of all dissolved species in the phase will be denoted by Eq. (3). 

To develop a unifying view of iron center catalysis, properties of the iron center in individual enzymes must be determined. Obviously, the definitive solution for the structure is atomic resolution of the active enzyme and postulated intermediates determined by diffraction or nuclear magnetic resonance (NMR) spectroscopy. Just as obviously, these methods are limited by enormous time, effort, and instrumentation requirements as well as by practical and theoretical considerations. This point is emphasized by the paucity of available protein structures. In addition to the strictly structural details of the iron center, chemical and physical properties are required and, in some cases, these results augment diffraction or NMR structural studies. Discussed below are a few of the more common processes by which this information is obtained. 

The phenomenon under consideration was studied systematically in the beginning of the 19th century. In 1815, Davy performed experiments that dealt with catalytic combustion on platinum gauzes. The term catalysis , however, was introduced by Berzelius in 1836. He first defined a catalyst (Berzelius, 1836) as a compound, which increases the rate of a chemical reaction, but which is not consumed during the reaction. This definition was later amended by Ostwald (1853-1932) in 1895 to involve the possibility that small amounts of the catalyst are lost in the reaction or that the catalytic activity is slowly decreased A catalyst is a substance that increases the rate of approach to equilibrium of a chemical reaction without being substantially consumed in the reaction. It was more than a century after Berzelius first definition that Marcel Prettre s introduced the notion of yield The catalyst is a substance that increases the rate of a chemical transformation without modifying the yield, and that is found intact among the final products of the reaction. ... 

Berzelius introduced the term catalysis as early as 1836 to explain various decomposition and transformation reactions. He later referred to the special power that some substances (catalysts) have for influencing the affinity of chemical substances. According to the Ostwald definition of catalyst (1895), it was assumed that the catalyst remained unchanged in the course of the reaction but now it is known that it is involved in chemical bonding with the reactants during the catalytic cycle. Thus, catalysis is a process in which the rate of a reaction is enhanced under... 

RNA is the only macromolecule known to have a role both in the storage and transmission of information and in catalysis, which has led to much speculation about its possible role as an essential chemical intermediate in the development of life on this planet. The discovery of catalytic RNAs, or ribozymes, has changed the very definition of an enzyme, extending it beyond the domain of proteins. Proteins nevertheless remain essential to RNA and its functions. In the modem cell, all nucleic acids, including RNAs, are complexed with proteins. Some of these complexes are quite elaborate, and... 

The phenomenon of catalysis is a subject of chemical kinetics, as follows from Ostwald s definition of catalysis. Therefore, the accumulation of data on the kinetics of concrete catalytic reactions favors the progress of the theory of catalysis. 

Process intensification can be considered to be the use of measures to increase the volume-specific rates of reaction, heat transfer, and mass transfer and thus to enable the chemical system or catalyst to realize its full potential (2). Catalysis itself is an example of process intensification in its broadest sense. The use of special reaction media, such as ionic liquids or supercritical fluids, high-density energy sources, such as microwaves or ultrasonics, the exploitation of centrifugal fields, the use of microstructured reactors with very high specific surface areas, and the periodic reactor operation all fall under this definition of process intensification, and the list given is by no means exhaustive. 

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