Solids interface reactions

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... 

We are specifically interested in the system in which a liquid-solid interface reaction has taken place. An example of this type of reaction is the chemical grafting of a rubber with a monomer at the interface. The function of the grafted rubber as an adhesive has been postulated (II, 29, 46, 64) but has never been proved. Since the grafted rubber is the key to bridging two incompatible polymers together, we devoted a major portion of our experimental work to the characterization of the grafted polymer as an adhesive at the interface. 

A method of covalently bonding heparin to a polymer substrate is presented. The synthetic route consists of coupling heparin covalently with polyisocyanatostyrene. This reaction was made possible by the fact that formamide is a room temperature solvent for sodium heparin and this allowed a liquid-solid interface reaction to take place. Lee-White clotting tests in vitro (in hydrogel tubes) showed these surfaces to be non-clotting after 24 hours whereas untreated controls and surfaces of GBH type clotted in approximately 25-35 minutes. 

We now consider the case where the total particle is being completely consumed. We choose as an example the case where species A must diffuse to the smface to react with solid B at the liquid-solid interface. Reactions of this type are typically zero-order in B and first-order in A. The rate of mass transfer to the surface is equal to the rate of surface reaction. 

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. 

A working model for the course of solid-solid interface reactions before the crystallization of a product has resulted from studies of thin film diffusion couples (75). It has generally been assumed that an amorphous "reaction layer" is formed at the interface as the solid reactants begin to interdiffuse. As this amorphous interface becomes thicker, the composition gradients in this layer are reduced, and nucleation of an intermediate crystalline product occurs. As this crystalline product layer grows, the two resulting interfaces repeat this process to form additional crystalline products. This is illustrated in Figure 3. 

The constant phase-angle model was also applied to other problems of fluid-solid interface reactions in rocks such as ... 

The usual situation, true for the first three cases, is that in which the reactant and product solids are mutually insoluble. Langmuir [146] pointed out that such reactions undoubtedly occur at the linear interface between the two solid phases. The rate of reaction will thus be small when either solid phase is practically absent. Moreover, since both forward and reverse rates will depend on the amount of this common solid-solid interface, its extent cancels out at equilibrium, in harmony with the thermodynamic conclusion that for the reactions such as Eqs. VII-24 to VII-27 the equilibrium constant is given simply by the gas pressure and does not involve the amounts of the two solid phases. 

This interface is critically important in many applications, as well as in biological systems. For example, the movement of pollutants tln-ough the enviromnent involves a series of chemical reactions of aqueous groundwater solutions with mineral surfaces. Although the liquid-solid interface has been studied for many years, it is only recently that the tools have been developed for interrogating this interface at the atomic level. This interface is particularly complex, as the interactions of ions dissolved in solution with a surface are affected not only by the surface structure, but also by the solution chemistry and by the effects of the electrical double layer [31]. It has been found, for example, that some surface reconstructions present in UHV persist under solution, while others do not. 

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 reaction kinetics approximation is mechanistically correct for systems where the reaction step at pore surfaces or other fluid-solid interfaces is controlling. This may occur in the case of chemisorption on porous catalysts and in affinity adsorbents that involve veiy slow binding steps. In these cases, the mass-transfer parameter k is replaced by a second-order reaction rate constant k. The driving force is written for a constant separation fac tor isotherm (column 4 in Table 16-12). When diffusion steps control the process, it is still possible to describe the system hy its apparent second-order kinetic behavior, since it usually provides a good approximation to a more complex exact form for single transition systems (see Fixed Bed Transitions ). 

In many important cases of reactions involving gas, hquid, and solid phases, the solid phase is a porous catalyst. It may be in a fixed bed or it may be suspended in the fluid mixture. In general, the reaction occurs either in the liquid phase or at the liquid/solid interface. In fixed-bed reactors the particles have diameters of about 3 mm (0.12 in) and occupy about 50 percent of the vessel volume. Diameters of suspended particles are hmited to O.I to 0.2 mm (0.004 to 0.008 in) minimum by requirements of filterability and occupy I to 10 percent of the volume in stirred vessels. 

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. 

The experimental and theoretical work reported in the literature will be reviewed for each of the five major types of ga s-liquid-particle operation under the headings Mass transfer across gas-liquid interface mass transfer across liquid-solid interface holdup and axial dispersion of gas phase holdup and axial dispersion of liquid phase heat transfer reaction kinetics. 

Since the free energy of a molecule in the liquid phase is not markedly different from that of the same species volatilized, the variation in the intrinsic reactivity associated with the controlling step in a solid—liquid process is not expected to be very different from that of the solid—gas reaction. Interpretation of kinetic data for solid—liquid reactions must, however, always consider the possibility that mass transfer in the homogeneous phase of reactants to or products from, the reaction interface is rate-limiting [108,109], Kinetic aspects of solid—liquid reactions have been discussed by Taplin [110]. 

Norris et al. [1254] discuss the application of several numerical methods to the determination of rate coefficients and of orders of solid state reactions of the contracting interface type. 

A reaction interface is the zone immediately adjoining the surface of contact between reactant and product and within which bond redistributions occur. Prevailing conditions are different from those characteristic of the reactant bulk as demonstrated by the enhanced reactivity, usually attributed to local strain, catalysis by products, etc. Considerable difficulties attend investigation of the mechanisms of interface reactions because this thin zone is interposed between two relatively much larger particles. Accordingly, many proposed reaction models are necessarily based on indirect evidence. Without wishing to appear unnecessarily pessimistic, we consider it appropriate to mention here some of the problems inherent in the provision of detailed mechanisms for solid phase rate processes. These difficulties are not always apparent in interpretations and proposals appearing in the literature. 

Another possibility is that one of the reactants is particularly mobile, this is apparent in certain solid—gas reactions, such as the reduction of NiO with hydrogen, which is a well-characterized nucleation and growth process [30,1166]. Attempts have been made to use the kinetic equations developed for interface reactions to elucidate the mechanisms of reactions between the crystalline components of rocks under conditions of natural metamorphism [1167,1168]. 

The catalytic activity of doped nickel oxide on the solid state decomposition of CsN3 decreased [714] in the sequence NiO(l% Li) > NiO > NiO(l% Cr) > uncatalyzed reaction. While these results are in qualitative accordance with the assumption that the additive provided electron traps, further observations, showing that ZnO (an rc-type semi-conductor) inhibited the reaction and that CdO (also an rc-type semi-conductor) catalyzed the reaction, were not consistent with this explanation. It was noted, however, that both NiO and CdO could be reduced by the product caesium metal, whereas ZnO is not, and that the reaction with NiO yielded caesium oxide, which is identified as the active catalyst. Detailed kinetic data for these rate processes are not available but the pattern of behaviour described clearly demonstrates that the interface reactions were more complicated than had been anticipated. 

Two product barrier layers are formed and the continuation of reaction requires that A is transported across CB and C across AD, assuming that the (usually smaller) cations are the mobile species. The interface reactions involved and the mechanisms of ion migration are similar to those already described for other systems. (It is also possible that solid solutions will be formed.) As Welch [111] has pointed out, reaction between solids, however complex they may be, can (usually) be resolved into a series of interactions between two phases. In complicated processes an increased number of phases, interfaces, and migrant entities must be characterized and this requires an appropriate increase in the number of variables measured, with all the attendant difficulties and limitations. However, the careful selection of components of the reactant mixture (e.g. the use of a common ion) or the imaginative design of reactant disposition can sometimes result in a significant simplification of the problems of interpretation, as is seen in some of the examples cited below. 

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... 

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