Initiation heterogeneous reactions

Mixing of H2SO4 solution with leady oxide initiates heterogeneous reactions, which lead to the formation of lead hydroxides, crystalline basic lead sulfates and orthorhombic PbO. These reactions can be expressed by the following chemical equations ... 

Modelling plasma chemical systems is a complex task, because these system are far from thennodynamical equilibrium. A complete model includes the external electric circuit, the various physical volume and surface reactions, the space charges and the internal electric fields, the electron kinetics, the homogeneous chemical reactions in the plasma volume as well as the heterogeneous reactions at the walls or electrodes. These reactions are initiated primarily by the electrons. In most cases, plasma chemical reactors work with a flowing gas so that the flow conditions, laminar or turbulent, must be taken into account. As discussed before, the electron gas is not in thennodynamic equilibrium... 

Heterogeneous reaction (Section 6 1) A reaction involving two or more substances present in different phases Hydro genation of alkenes is a heterogeneous reaction that takes place on the surface of an insoluble metal catalyst Heterolytic cleavage (Section 4 16) Dissociation of a two electron covalent bond in such a way that both electrons are retained by one of the initially bonded atoms Hexose (Section 25 4) A carbohydrate with six carbon atoms High density lipoprotein (HDL) (Section 26 11) A protein that carries cholesterol from the tissues to the liver where it is metabolized HDL is often called good cholesterol Histones (Section 28 9) Proteins that are associated with DNA in nucleosomes... 

The Clemmensen reduction can be formulated to proceed by a sequence of one-electron and proton transfer reactions. It is a heterogenous reaction, taking place at the zinc surface. Initially an electron is transferred from zinc to the carbonyl group of ketone 1, leading to a radical species 3, which is presumed to react further to a zinc-carbenoid species 4 ... 

Step (2) is supported by the observed slowing down of the initial decompn stages when N02 is added. It is also supported by Levy s studies of the decompn of ethyl nitrate (Ref 15b). At the higher temps, steps (3) and (4) remove both RCH20 and N02 so rapidly that there is essentially no back reaction (step 2). Thus the reaction is first order (step 1) over its entire course. It would appear steps (3) and (4) are heterogenous reactions (at least at lower temps) that are favored by the combination of added surface and increased pressure (upper dashed line in Fig 13). The reacceleration... 

In an emulsion polymerization, the reaction mixture is initially heterogeneous due to the poor solubility of the monomer in the continuous phase. In order for a reaction to take advantage of the desirable Smith-Ewart kinetics [96], the monomer and initiator must be segregated with the initiator preferentially dissolved in the continuous phase and not the monomer phase. Because of the kinetics of an emulsion polymerization, high molecular weight polymer can be produced at high rates. The polymer which results from an emulsion polymerization exists as spherical particles typically smaller than one pm in diameter. However, due to the high solubility of most vinyl monomers in C02, emulsion polymerization in C02 probably will not be a very useful process for commercially important monomers. 

A related development that had profound impact on heterogeneous reactions is the use of microwave (MW) irradiation techniques for the acceleration of organic reactions. Since the appearance of initial reports on the application of microwaves for chemical synthesis in polar solvents [11], the approach has blossomed into a useful... 

From the measurements published in the paper it cannot be inferred that the concentration-time curves can safely be extrapolated to zero time. The authors do not communicate details as to how well the beginning of the reaction could be defined. It should be pointed out that reaction kinetics derived by these means are subject to the uncertainties inherent in the extrapolation method. This holds particularly for a rapidly accelerated reaction. Furthermore, the possibility that the decomposition was initiated heterogeneously could not be excluded with certainty. These objections have to be considered when regarding the following kinetic results. 

Solid-state polymerization of D3, initiated by 7-irradiation, has been known for a long time. The reaction proceeds according to the cationic polymerization mechanism.3,5 Solid-state anionic polymerization of hexaphenylcyclotrisi-loxane in the presence of KOH or potassium oligosiloxanolate was recently reported. The crystalline structure of the polymers obtained in high yield in the heterogenous reaction was determined.1... 

Write an equation of the form of Equation (8) for each reversible reaction. To obtain the initial fluid phase concentration mj, it is necessary to know the analytical concentrations in the solution at the start of the heterogeneous reaction as well... 

Then the fraction of the total irreversible process due to each of the reactions can be determined. Like ANg, av is a sum °f terms. For the same initial mass of solid NgjCD ANg i i are obtained from individual rate curves. Unless each ANg,i depends on its 5 in the same way, the relative importance of the various heterogeneous reactions changes with the total extent of reaction. In any event, it is necessary to express each ANs,i as a function of av. 

Lactam polymerizations (nonassisted as well as assisted) are usually complicated by heterogeneity, usually when polymerization is carried out below the melting point of the polymer [Fries et al., 1987 Karger-Kocsis and Kiss, 1979 Malkin et al., 1982 Roda et al., 1979]. (This is probably the main reason why there are so few reliable kinetic studies of lactam polymerizations.) An initially homogeneous reaction system quickly becomes heterogeneous at low conversion, for example, 10-20% conversion (attained at a reaction time of no more than 1 min) for 2-pyrrolidinone polymerization initiated by potassium t-butoxide and A-benzoyl-2-pyrrolidinone. The (partially) crystalline polymer starts precipitating from solution (which may be molten monomer), and subsequent polymerization occurs at a lower rate as a result of decreased mobility of /V-acyl lactam propagating species. 

This new technique incorporates a catalyzed short contact time (SCT) substrate into a shock tube. Fig. 13. These SCT reactors are currently used in industry for a variety of applications, including fuel cell reformers and chemical synthesis.The combination of a single pulse shock tube with the short contact time reactor enables the study of complex heterogeneous reactions over a catalyst for very well defined regimes in the absence of transport effects. These conditions initiate reaction in a real environment then abruptly terminate or freeze the reaction sequence. This enables detection of intermediate chemical species that give insight into the reaction mechanism occurring in the presence of the chosen catalyst. There is no limitation in terms of the catalyst formulations the technique can study. 

Heterogeneous reactions that do not require nucleation of a new phase. That is, all the phases involved in the reactions are initially present. Many of these reactions can be quantified well if the boundary conditions are simple. The following are some examples. 

Moreover, as neither the concept of surface initiated homogeneous-heterogeneous reaction (11) can be invoked to explain our results, it can be stated that the methane partial oxidation reaction proceeds via a surface catalysed process which likely involves specific catalyst requirements. However, by comparing the HCHO productivity of the different catalytic systems previously proposed (9) with that of our 5% V205/Si02 catalyst, it emerges that our findings constitute a relevant advancement in this area (23),... 

Liquid phase hydrogenation catalyzed by Pd/C is a heterogeneous reaction occurring at the interface between the solid catalyst and the liquid. In our one-pot process, the hydrogenation was initiated after aldehyde A and the Schiff s base reached equilibrium conditions (A B). There are three catalytic reactions A => D, B => C, and C => E, that occur simultaneously on the catalyst surface. Selectivity and catalytic activity are influenced by the ability to transfer reactants to the active sites and the optimum hydrogen-to-reactant surface coverage. The Langmuir-Hinshelwood kinetic approach is coupled with the quasi-equilibrium and the two-step cycle concepts to model the reaction scheme (1,2,3). Both A and B are adsorbed initially on the surface of the catalyst. Expressions for the elementary surface reactions may be written as follows ... 

Transport of the gas to the surface and the initial interaction. The first step in heterogeneous reactions involving the uptake and reaction of gases into the liquid phase is diffusion of the gas to the interface. At the interface, the gas molecule either bounces off or is taken up at the surface. These steps involve, then, gaseous diffusion, which is determined by the gas-phase diffusion coefficient (Dg) and the gas-surface collision frequency given by kinetic molecular theory. 

Arctic at polar sunrise. The mechanism likely involves regeneration of photochemically active bromine via heterogeneous reactions on aerosol particles, the snow-pack, and/or frozen seawater. The source of the bromine is likely sea salt, but the nature of the reactions initiating this ozone loss remains to be identified. For a review, see the volume edited by Niki and Becker (1993) and an issue of Tellus (Barrie and Platt, 1997). 

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