Catalysis process mechanisms

Considering the stepwise mechanism, different possibilities arise depending on the relative values for the rate constants ki, k i and k2- The application of the steady state condition to the carbanion yields equation (6) for the rate constant of the basic catalysis process. 

Based on these observations the authors propose the following mechanism for the nickel-catalyzed hydroalumination (Scheme 2-4) During the catalysis process... 

At this point, we can undertake the study of chemisorption on a supported metal. Despite the importance of this process to catalysis, quantum-mechanical studies have been somewhat scarce. The problem was first investigated by Ruckenstein and Huang (1973), who formulated a general MO... 

Two-Step (Push-Pull, Ping-Pong) Mechanisms Two-step mechanisms are typical of chemical catalytic processes, as opposed to redox catalysis processes, that are discussed and exemplified in Section 6.2. The first step following the generation at the electrode of the active form of the catalyst, Q, is the formation of an adduct, C, with the substrate A (Scheme 2.11). C requires an additional electron transfer to regenerate the initial catalyst, P. There are then two main possibilities. One is when C is easier to reduce (or oxidize in oxidative processes) than P. The main route is then a homogeneous electron... 

Pradyot Patnaik, Ph.D., is Director of the Laboratory of the Interstate Environmental Commission at Staten Island, NY. He also teaches as an Adjunct Professor at the New Jersey Institute of Technology in Newark, NJ, and Community College of Philadelphia and does his research in the Center for Environmental Science at the City University of New York on Staten Island. His diverse interests include chemical processing, product development, catalysis, reaction mechanisms, environmental pollutants, and mass spectrometry. 

As it is generally the case with bifunctional catalysis processes, the balance between hydrogenating and acid functions determines for a large part the catalyst activity. This was quantitatively shown for series of bifunctional catalysts constituted by mechanical mixtures of a well dispersed Pt/Alumina catalyst and of mordenite samples differing by their acidity and their porosity (25). The balance between hydrogenating and acid functions was taken as nPt/nH+ the ratio between the number of accessible platinum atoms and the number of protonic sites determined by pyridine adsorption. 

The Wacker process was a major landmark and a great push towards the development of homogeneous catalysis. The mechanism of acetaldehyde formation differs fundamentally from the other oxidation processes as O2 itself is not directly involved. As is clear from Figure 28 the actual oxidant is Pd(II) which is reduced to Pd(0). The intimate pathway of the reaction involves nucleophilic attack and was the subject of much debate. 

Green and Rooney81 proposed an alternative mechanism (Scheme 11.13b) that also accounted for Z-N catalysis. The mechanism resembles a metathesis-like pathway by starting with a-elimination to give a metal-carbene hydride followed by cycloaddition with the alkene monomer to form a metallacyclobutane. Reductive elimination finally yields a new metal alkyl with two more carbon atoms in the growing chain. The Green-Rooney mechanism, although plausible overall, requires an a-elimination, a process that is difficult to demonstrate. 

Heterogeneous catalysis probes mechanisms of reactions that typically occur on metallic nanoclusters supported on insulators such as transition metal oxides. These studies relate directly to many commercial processes such as oil refining, hydrogen production, and food processing. 

The oxidation of alkylaromatic hydrocarbons proceeds particularly easily in the presence of both cobalt and bromide ions (a so-called cobalt-bromide catalysis ). Carboxylic acids are the final products of the reaction. For example, terephthalic acid is selectively formed from p-xylene, the whole process being used in the industrial production of the acid [Ik, 19]. Despite the large number of works on cobalt-bromide catalysis, its mechanism has long remained speculative. 

Since only the Pt atoms on the Pt particle surface can participate in the catalysis process, one way to maximize Pt atom utilization is to deposit an extremely thin layer of Pt on a non-Pt particle to form a core-shell structure with the Pt layer as the shell and the non-Pt particle as the core. The electron conductivity of the core particles is not important because they are covered by a highly electrical conducting Pt shell. In addition, when a submonolayer to several monolayers of Pt are made on certain core metal particles, the catalytic ability of Pt improves due to electronic interactions between the shell Pt atoms and the interior core metal atoms that increase the Pt 5d orbital vacancies and thus increases the n electron donation from O2 to Pt atoms (electronic effect), and due to the decrease in the Pt-Pt atomic distance (geometric effect). This mechanism is similar to the improved catalytic ability of PtM and PtM Ny alloys, where M and N represent different metals and X and y their atomic contents in the alloy. For the Pt/core, PtM or PtM Ny, if there is some leaching out of the non-Pt metals, their corresponding cations can replace the protons of the PFSA either in the catalyst layer or in the membrane to reduce its proton conductivity as well as the catalyst-PFSA-reactant three-phase boimdaries, and thus decrease the fuel cell performance. The shape and crystalline facet of the Pt nanoparticles can also affect the catalytic activity. 

While NMR does not easily lend itself to the direct analysis of many polymerization processes, the technique has been used to probe catalysis and mechanisms. NMR also provides information about reaction kinetics, as will be discussed more extensively in Secs III.B.2 and III.B.3. 

The mechanism of this cooperative catalysis process has been studied using high level DFT calculations. A similar transformation has been reported by Ogata and... 

Some work has to be done on the enolization of ketones with tertiary amino groups [105, 106]. The reactions of these compounds with iodine are complex processes which consume several moles of iodine per mole of ketone. Nevertheless, the pH-rate profile for enolization of 4-dimethylaminobutan-2-one appears to be sigmoid with the rare proportional to the concentration of the deprotonated form. The plateau rate constant and that for the enolization of 4-diethylamino-butanone and 5-ethylaminopentan-2-one are seven to eight powers of ten greater than the rate constants for the spontaneous enolization of simple aliphatic ketones. The results suggest that these compounds react with intramolecular catalysis. A mechanism as symbolized by 70 seems unlikely since the plateau rate constant for the enolization of 71 is similar to that for the enolization of 72 [105]. Instead, the kinetically equivalent process, involving reaction of the protonated form with hydroxide ion seems more likely. If this is correct, reaction via the six-membered cyclic transition state 73 is faster than via the... 

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