Surface heterogeneity reactions

Chemical vapour deposition (CVD) processes have become a very important group of film-formation methods. Basically CVD is a material synthesis in which constituents of the vapour phase react chemically to form thin solid films as a solid-phase reaction product which condenses on the substrate. The reaction should take place very near to or on the substrate surface (heterogeneous reactions) and not in the gas phase to avoid powdery deposits. Activation of the reaction can be performed by various means such as the application of heat, high-frequency electrical fields, light or X-ray radiation, electric arc, electron bombardment or catalytic action of the substrate surface. A marked influence of the process parameters such as sub-... 

The particulate surface heterogeneous reactions may result In primary and/or secondary processes. Most of the examples of algal associated processes suggest secondary reactions resulting from exudates In the aqueous phase. 

Photochemical reactions (chapter A3.13) and heterogeneous reactions on surfaces (chapter A3.10) are discussed in separate chapters. 

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... 

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

Heterogeneous catalytic studies should also be concerned with the significance of adsorption and desorption rates and equilibria of the reactants, intermediates and products. Yang and Hougen (1950) tabulated the expressions for surface catalyzed reactions controlled by various steps. 

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

Surface area effects in mass transfer or heterogeneous reactions... 

SURFACE AREA EFEECTS IN MASS TRANSFER OR HETEROGENEOUS REACTIONS 5 1... 

Heterogeneous reaction (Section 6.1) A reaction involving two or more substances present in different phases. Hydrogenation of alkenes is a heterogeneous reaction that takes place on the surface of an insoluble metal catalyst. 

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 ... 

In the case of coupled heterogeneous catalytic reactions the form of the concentration curves of analytically determined gaseous or liquid components in the course of the reaction strongly depends on the relation between the rates of adsorption-desorption steps and the rates of surface chemical reactions. This is associated with the fact that even in the case of the simplest consecutive or parallel catalytic reaction the elementary steps (adsorption, surface reaction, and desorption) always constitute a system of both consecutive and parallel processes. If the slowest, i.e. ratedetermining steps, are surface reactions of adsorbed compounds, the concentration curves of the compounds in bulk phase will be qualitatively of the same form as the curves typical for noncatalytic consecutive (cf. Fig. 3b) or parallel reactions. However, anomalies in the course of bulk concentration curves may occur if the rate of one or more steps of adsorption-desorption character becomes comparable or even significantly lower then the rates of surface reactions, i.e. when surface and bulk concentration are not in equilibrium. 

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... 

The chemical properties of oxide surfaces have been studied by several methods, including oxygen exchange. This method has been used to investigate the mechanisms of heterogeneous reactions for which oxides are active catalysts [36]. The dimerization step does not necessarily precede desorption and Malinin and Tolmachev [634], in one of the few reviews of decomposition kinetics of solid metal oxides, use this criterion to distinguish two alternative reaction mechanisms, examples being... 

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