Interface gas-solid

In Section 2.2.5 we have considered adsorption and desorption, the processes which occur at a gas-solid interface and determine the surface coverage wifh an adsorbate. The same phenomena dictate the gas properties in the close vicinity of the surface. With increasing distance from fhe surface fheir influence vanishes due to intermolecular collisions in the gas phase. Therefore, one can define the gas boundary layer (GBL) as a gas slab where some properties are different from those in the gas interior. The higher the gas pressure, the more frequent are intermolecular collisions. Accordingly, the thickness of fhe GBL varies inversely with the pressure. 

Any optical signal from a gas is determined by an average over the ensemble of gas molecules and hence by their velocity distribution functions. In this section we shall consider kinetic properties of fhe GBL in an ideal monafomic 

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The characterization of surfaces undergoing corrosion phenomena at liquid-solid and gas-solid interfaces remains a challenging task. The use of STM for in situ studies of corrosion reactions will continue to shape the atomic-level understanding of such surface reactions. 

One of the most important advances in electrochemistry in the last decade was tlie application of STM and AFM to structural problems at the electrified solid/liquid interface [108. 109]. Sonnenfield and Hansma [110] were the first to use STM to study a surface innnersed in a liquid, thus extending STM beyond the gas/solid interfaces without a significant loss in resolution. In situ local-probe investigations at solid/liquid interfaces can be perfomied under electrochemical conditions if both phases are electronic and ionic conducting and this... 

The above discussion relates to diffusion-controlled transport of material to and from a carrier gas. There will be some circumstances where the transfer of material is determined by a chemical reaction rate at the solid/gas interface. If this process determines the flux of matter between the phases, the rate of transport across the gas/solid interface can be represented by using a rate constant, h, so that... 

In all of these systems, the rate of generation at the gas-solid interface is so rapid that only a small fraction is canied away from the particle surface by convective heat uansfer. The major source of heat loss from the particles is radiation loss to tire suiTounding atmosphere, and the loss per particle may be estimated using unity for both the view factor and the emissivity as an upper limit from tlris source. The practical observation is that the solids in all of these methods of roasting reach temperatures of about 1200-1800 K. 

To describe hypergolic heating, Anderson and Brown (A10) proposed a theoretical model based upon spontaneous exothermic heterogeneous reactions between the reactive oxidizer and a condensed phase at the gas-solid interface. In these studies, the least complex case was considered, i.e., the one in which the solid phase is instantaneously exposed to a stagnant (nonflowing) gaseous oxidizer environment. This situation can be achieved experimentally provided the sample to be tested is suddenly injected into the desired environment in a manner designed to minimize gas flow. 

The equations describing the possible nonreactive and/or reactive forms of energy transfer at the gas-solid interface ... 

Volume 19 Volume 20 Volume 21 Volume 22 Simple Processes at the Gas—Solid Interface Complex Catalytic Processes Reactions of Solids with Gases Reactions in the Solid State Additional Section... 

Section 8 deals with reactions which occur at gas—solid and solid—solid interfaces, other than the degradation of solid polymers which has already been reviewed in Volume 14A. Reaction at the liquid—solid interface (and corrosion), involving electrochemical processes outside the coverage of this series, are not considered. With respect to chemical processes at gas-solid interfaces, it has been necessary to discuss surface structure and adsorption as a lead-in to the consideration of the kinetics and mechanism of catalytic reactions. 

Shrouds are generally placed around grid holes to reduce the velocity at the gas-solids interface and reduce particle attrition. Shrouds simply consist of short pipes centered over the smaller grid holes which have been selected in size and number to operate at a hole velocity defined by Eq. (9). 

A great deal of success was attendant on the early application of PM-IRRAS to the gas/solid interface. Golden et ai (1981) reported the development of instrumentation, using conventional dispersive optics, able to record detailed infrared reflection-absorption spectra from molecules adsorbed on single-crystal Pt without any interference from the gas-phase species. 

The central issue which has to be addressed in any comprehensive study of electrode-surface phenomena is the determination of an unambiguous correlation between interfacial composition, interfacial structure, and interfacial reactivity. This principal concern is of course identical to the goal of fundamental studies in heterogeneous catalysis at gas-solid interfaces. However, electrochemical systems are far more complicated since a full treatment of the electrode-solution interface must incorporate not only the compact (inner) layer but also the boundary (outer) layer of the electrical double-layer. The effect of the outer layer on electrode reactions has been neglected in most surface electrochemical studies but in certain situations, such as in conducting polymers and... 

Alkali 10ns in aqueous solution are probably the most typical and most widely studied representatives of non-specific adsorption. The electrochemical term of non-specific adsorption is used to denote the survival of at least the primary hydration shell when an ion is interacting with a solid electrode. As pointed out previously, the generation of such hydrated ions at the gas-solid interface would be of great value because it would provide an opportunity to simulate the charging of the interfacial capacitor at the outer Helmholtz plane or perhaps even in the diffuse layer. 

A relation between rupture phenomena and (specific) surface energy 7 was postulated by Dupre 4 almost simultaneously with the hypothesis of Quincke. Let a cylindrical rod be broken in tension. After rupture, two new gas — solid interfaces of vr2 each are present, r being the radius of the rupture surface. Consequently, the work of rupture ought to contain a term 2 nr2 7 . Dupre did not indicate how to separate this term from the main component of the work of rupture, which is the work required to extend the rod to its maximum elongation (or strain). The modern development of Dupre s ideas is reviewed in Section III.3. below. 

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