Reactions, chemical catalysis

Surrounding Medium Effects on Chemical Reactions Catalysis... 

CV is extensively used for the study of multi-electron transfer reactions, adsorbed species on the electrode surface, coupled chemical reactions, catalysis, etc. Figure 18b.9 shows some of the examples. 

Chemical Synthesis The traditional tools of chemical synthesis in use today are organic and inorganic synthesis and catalysis. Synthesis is the efficient conversion of raw materials such as minerals, petroleum, natural gases, coal, and biomass into more useful molecules and products catalysis is the process by which chemical reactions are either accelerated or slowed by the addition of a substance that is not changed in the chemical reaction. Catalysis-based chemical syntheses account for 60% of today s chemical products and 90% of current chemical processes (Collins, 2001). 

Kinetic study of a simple chemical reaction Catalysis Today 34(1997)401—409. 

The above-mentioned Arrhenius law also indicates a third option to accelerate a chemical reaction - catalysis. The presence of a catalytically active substance lowers the activation energy barrier and thus allows a higher reaction rate at constant... 

Investigation of chemical reactions (catalysis, syntheses, polymerisation)... 

The physical chemist is very interested in kinetics—in the mechanisms of chemical reactions, the rates of adsorption, dissolution or evaporation, and generally, in time as a variable. As may be imagined, there is a wide spectrum of rate phenomena and in the sophistication achieved in dealing wifli them. In some cases changes in area or in amounts of phases are involved, as in rates of evaporation, condensation, dissolution, precipitation, flocculation, and adsorption and desorption. In other cases surface composition is changing as with reaction in monolayers. The field of catalysis is focused largely on the study of surface reaction mechanisms. Thus, throughout this book, the kinetic aspects of interfacial phenomena are discussed in concert with the associated thermodynamic properties. 

P. G. Ashmore, Catalysis and Inhibition of Chemical Reactions, Butterworths, London, 1963. 

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. 

Recently, in situ studies of catalytic surface chemical reactions at high pressures have been undertaken [46, 47]. These studies employed sum frequency generation (SFG) and STM in order to probe the surfaces as the reactions are occurring under conditions similar to those employed for industrial catalysis (SFG is a laser-based teclmique that is described in section A 1.7.5.5 and section BT22). These studies have shown that the highly stable adsorbate sites that are probed under vacuum conditions are not necessarily tlie same sites that are active in high-pressure catalysis. Instead, less stable sites that are only occupied at high pressures are often responsible for catalysis. Because the active... 

This example illustrates a subtle control of a chemical reaction by a delicate manipulation of tire stereochemical environment around a metal centre dictated by tire selection of tire ligands. This example hints at tire subtlety of nature s catalysts, tire enzymes, which are also typically stereochemically selective. Chiral catalysis is important in biology and in tire manufacture of chemicals to regulate biological functions, i.e., phannaceuticals. 

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. 

Enzymatic Catalysis. Enzymes are biological catalysts. They increase the rate of a chemical reaction without undergoing permanent change and without affecting the reaction equiUbrium. The thermodynamic approach to the study of a chemical reaction calculates the equiUbrium concentrations using the thermodynamic properties of the substrates and products. This approach gives no information about the rate at which the equiUbrium is reached. The kinetic approach is concerned with the reaction rates and the factors that determine these, eg, pH, temperature, and presence of a catalyst. Therefore, the kinetic approach is essentially an experimental investigation. 

Ca.ta.lysts, A catalyst has been defined as a substance that increases the rate at which a chemical reaction approaches equiHbrium without becoming permanently involved in the reaction (16). Thus a catalyst accelerates the kinetics of the reaction by lowering the reaction s activation energy (5), ie, by introducing a less difficult path for the reactants to foUow. Eor VOC oxidation, a catalyst decreases the temperature, or time required for oxidation, and hence also decreases the capital, maintenance, and operating costs of the system (see Catalysis). 

The chemical reaction catalyzed by triosephosphate isomerase (TIM) was the first application of the QM-MM method in CHARMM to the smdy of enzyme catalysis [26]. The study calculated an energy pathway for the reaction in the enzyme and decomposed the energetics into specific contributions from each of the residues of the enzyme. TIM catalyzes the interconversion of dihydroxyacetone phosphate (DHAP) and D-glyceraldehyde 3-phosphate (GAP) as part of the glycolytic pathway. Extensive experimental studies have been performed on TIM, and it has been proposed that Glu-165 acts as a base for deprotonation of DHAP and that His-95 acts as an acid to protonate the carbonyl oxygen of DHAP, forming an enediolate (see Fig. 3) [58]. 

The basic function of lysis processes is to split molecules to permit further treatment. Hydrolysis is a chemical reaction in which water reacts with another substance. In the reaction, the water molecule is ionized while the other compound is split into ionic groups. Photolysis, another lysis process, breaks chemical bonds by irradiating a chemical with ultraviolet light. Catalysis uses a catalyst to achieve bond cleavage. 

A catalyst is defined as a substance that influences the rate or the direction of a chemical reaction without being consumed. Homogeneous catalytic processes are where the catalyst is dissolved in a liquid reaction medium. The varieties of chemical species that may act as homogeneous catalysts include anions, cations, neutral species, enzymes, and association complexes. In acid-base catalysis, one step in the reaction mechanism consists of a proton transfer between the catalyst and the substrate. The protonated reactant species or intermediate further reacts with either another species in the solution or by a decomposition process. Table 1-1 shows typical reactions of an acid-base catalysis. An example of an acid-base catalysis in solution is hydrolysis of esters by acids. 

Catalysis A process by which the rate of a chemical reaction is increased by a substance (namely enzymes in biochemical reactions) that remains chemically unchanged at the end of the reaction. 

In this chapter shock modification of powders (their specific area, x-ray diffraction lines, and point defects) measurements via analytical electron microscopy, magnetization and Mossbauer spectroscopy shock activation of catalysis, solution, solid-state chemical reactions, sintering, and structural transformations enhanced solid-state reactivity. 

Chemical reaction sources catalysis, reaction with powerful oxidants, reaction of metals with halocarhons, thermite reaction, thermally unstahle materials, accumulation of unstahle materials, pyrophoric materials, polymerization, decomposition, heat of adsorption, water reactive solids, incompatihle materials. 

Phase-transfer catalysis (Section 22.5) Method for increasing the rate of a chemical reaction by transporting an ionic reactant from an aqueous phase where it is solvated and less reactive to an organic phase where it is not solvated and is more reactive. Typically, the reactant is an anion that is carried to the organic phase as its quaternary ammonium salt. 

It is important to make the distinction between the multiphasic catalysis concept and transfer-assisted organometallic reactions or phase-transfer catalysis (PTC). In this latter approach, a catalytic amount of quaternary ammonium salt [Q] [X] is present in an aqueous phase. The catalyst s lipophilic cation [Q] transports the reactant s anion [Y] to the organic phase, as an ion-pair, and the chemical reaction occurs in the organic phase of the two-phase organic/aqueous mixture [2]. 

The oxidation methods described previously are heterogeneous in nature since they involve chemical reactions between substances located partly in an organic phase and partly in an aqueous phase. Such reactions are usually slow, suffer from mixing problems, and often result in inhomogeneous reaction mixtures. On the other hand, using polar, aprotic solvents to achieve homogeneous solutions increases both cost and procedural difficulties. Recently, a technique that is commonly referred to as phase-transfer catalysis has come into prominence. This technique provides a powerful alternative to the usual methods for conducting these kinds of reactions. 

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