Gas-solid interface reaction

HiPCO-SWCNTs were oxidized in a UV-03 gas-solid interface reaction and subsequently assembled on a rigid oligo(phenylenethynylene) self-assembled monolayer (SAM). In a chemical assembly , based on condensation between the carboxylic acid functionalities of the 03-oxidized SWNTs and the amine functionalities of the SAMs, SWCNT-amides were formed in ordered arrays [116]. 

The reaction is a gas-solid interface reaction which requires two independent types [79] of electronically different sites. One site should be electron-rich to activate molecular oxygen (site A) the other site should be electron-deficient to react with activated oxygen atoms... 

Next, we discuss the concept of phonon-assisted reactions. In relation to thermal reactions, they can be assisted by phonon-mode softening leading to large-amplitude overdamped oscillations. In the case of a photochemical reaction, a strong electron-phonon coupling can assist in polymerization. Then some non-linear spectroscopic studies are discussed which illuminate on the dynamics of photopolymerization process. Then follows a discussion of results on reaction in a different kind of molecular assembly, the Langmuir-Blodgett films. Finally, some gas-solid interface reactions which produce polymers in a doped state are discussed. 

Gas-solid interface reaction can provide a simple method of producing ordered polymers. In our laboratory, we have used reactions with AsF, to produce polymers. The advantage of this method is that it produces electroactive polymers (enhanced electrical conductivity) in a doped state. The polymer formed, however, generally contains disorder as revealed by their x-ray... 

Hurt DM. Principles of reactor design gas-solid interface reactions. Ind Eng Chem 35 522 528, 1943. 

For iron catalyst of ammonia synthesis with Fes04 as precursor, from a mechanistic point of view complex, the simple overall reduction reaction is that it is a gas-solid interface reaction. 

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. 

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... 

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. 

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. 

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... 

Inspired by these Surface Science studies at the gas-solid interface, the field of electrochemical Surface Science ( Surface Electrochemistry ) has developed similar conceptual and experimental approaches to characterize electrochemical surface processes on the molecular level. Single-crystal electrode surfaces inside liquid electrolytes provide electrochemical interfaces of well-controlled structure and composition [2-9]. In addition, novel in situ surface characterization techniques, such as optical spectroscopies, X-ray scattering, and local probe imaging techniques, have become available and helped to understand electrochemical interfaces at the atomic or molecular level [10-18]. Today, Surface electrochemistry represents an important field of research that has recognized the study of chemical bonding at electrochemical interfaces as the basis for an understanding of structure-reactivity relationships and mechanistic reaction pathways. 

A net flow of electrons occurs across the metal/solution interface in a normal electrode reaction. The term electrocatalysis is applied to working electrodes that deliver large current densities for a given reaction at a fixed overpotential. A different, though indirectly related, effect is that in which catalytic events occur in a chemical reaction at the gas/solid interface, as they do in heterogeneous catalysis, though the arrangement is such that the interface is subject to a variation in potential and the rate depends upon it... 

That products of intermediate oxidation level can be detected in the photocatalytic reactions of hydrocarbons and fossil fuels is also consistent with a surface bound radical intermediate . Photocatalytic isotope exchange between cyclopentane and deuterium on bifunctional platinum/titanium dioxide catalysts indicates the importance of weakly adsorbed pentane at oxide sites. The platinum serves to attract free electrons, decreasing the efficiency of electron-hole recombination, and to regenerate the surface oxide after exchange. Much better control of the exchange is afforded with photoelectrochemical than thermal catalysis > ) As before, hydrocarbon oxidations can also be conducted at the gas-solid interface... 

This chapter sets out the basic formulation and governing equations of mass-action kinetics. These equations describe the time evolution of chemical species due to chemical reactions in the gas phase. Chapter 11 is an analogous treatment of heterogeneous chemical reactions at a gas-solid interface. A discussion of the underlying theories of gas-phase chemical reaction rates is given in Chapter 10. 

Very many rate processes that proceed at a gas-solid interface obey the Arrhenius equation, which expresses the variation of the specific reaction rate constant k with temperature... 

Most detailed studies of water photodissociation on SrTi03 and Ti02 have concentrated on photoelectrochemical cells (PEC cells) operating under conditions of optimum efficiency, that is with an external potential applied between the photoanode and counterelectrode. We have become interested in understanding and improving reaction kinetics under conditions of zero applied potential. Operation at zero applied potential permits simpler electrode configurations (11) and is essential to the development of photochemistry at the gas-semiconductor interface. Reactions at the gas-sold, rather than liquid-solid, interface might permit the use of materials which photocorrode in aqueous electrolyte. 

Most reactions of interest to chemists take place in either solution or at the gas-solid interface. At the atomic level, much less is known about the reaction dynamics in such systems than about the dynamics of gas-phase reactions. In the gas phase one may follow the detailed evolution from reactants to products without disturbing collisions with other molecules, at least in the low pressure limit. Contrary to that, in solution, where reactants and products are continually perturbed by collisions with solvent molecules, it is much more complicated to follow a chemical reaction. 

Chapter 8 provides a unified view of the different kinetic problems in condensed phases on the basis of the lattice-gas model. This approach extends the famous Eyring s theory of absolute reaction rates to a wide range of elementary stages including adsorption, desorption, catalytic reactions, diffusion, surface and bulk reconstruction, etc., taking into consideration the non-ideal behavior of the medium. The Master equation is used to generate the kinetic equations for local concentrations and pair correlation functions. The many-particle problem and closing procedure for kinetic equations are discussed. Application to various surface and gas-solid interface processes is also considered. 

The temporal evolution of any system is described by kinetic equations. In the lattice-gas models the evolution of the gas-solid interface is made up of the evolution of occupancies of each of its sites, which can change at the expense of the surface reactions and migration of solid body atoms. The model allows to trace the evolution of all the sites of the system at times x, T>Tm, where tm is the minimum characteristic time interval relating to the most rapid elementary process of changing the occupancy state of the lattice sites. 

---