Surface chemical reaction mechanism

FORTRAN computer program that predicts the species, temperature, and velocity profiles in two-dimensional (planar or axisymmetric) channels. The model uses the boundary layer approximations for the fluid flow equations, coupled to gas-phase and surface species continuity equations. The program runs in conjunction with CHEMKIN preprocessors (CHEMKIN, SURFACE CHEMKIN, and TRAN-FIT) for the gas-phase and surface chemical reaction mechanisms and transport properties. The finite difference representation of the defining equations forms a set of differential algebraic equations which are solved using the computer program DASSL (dassal.f, L. R. Petzold, Sandia National Laboratories Report, SAND 82-8637, 1982). 

Recently, electron spin resonance (ESR) and other spectroscopic techniques have been applied to the study of redox reactions of Fe and Mn oxides (Kung and McBride, 1988 McBride, 1989a,b). Identification and quantification of adsorbed reactants, intermediates, and products by spectroscopic techniques could substantially improve our understanding of surface chemical reaction mechanisms. 

To illustrate this point, consider the following surface chemical reaction mechanism ... 

Hildebrand, M. Self-OTgaruzed nanosbuctures in surface chemical reactions Mechanisms and mesoscopic modeling. Chaos 12, 144 (2002)... 

These considerations led us to think about how to describe the surface chemical reaction mechanisms. This requires knowledge of the structure of molecules and the... 

The importance of surface science is most often exliibited in studies of adsorption on surfaces, especially in regards to teclmological applications. Adsorption is the first step in any surface chemical reaction or film-growdi process. The mechanisms of adsorption and the properties of adsorbate-covered surfaces are discussed in section Al.7.3. 

Wlien a surface is exposed to a gas, the molecules can adsorb, or stick, to the surface. Adsorption is an extremely important process, as it is the first step in any surface chemical reaction. Some of die aspects of adsorption that surface science is concerned with include the mechanisms and kinetics of adsorption, the atomic bonding sites of adsorbates and the chemical reactions that occur with adsorbed molecules. 

Rates of reductive dissolution of transition metal oxide/hydroxide minerals are controlled by rates of surface chemical reactions under most conditions of environmental and geochemical interest. This paper examines the mechanisms of reductive dissolution through a discussion of relevant elementary reaction processes. Reductive dissolution occurs via (i) surface precursor complex formation between reductant molecules and oxide surface sites, (ii) electron transfer within this surface complex, and (iii) breakdown of the successor complex and release of dissolved metal ions. Surface speciation is an important determinant of rates of individual surface chemical reactions and overall rates of reductive dissolution. 

Similarly, inner-sphere and outer-sphere mechanisms can be postulated for the reductive dissolution of metal oxide surface sites, as shown in Figure 2. Precursor complex formation, electron transfer, and breakdown of the successor complex can still be distinguished. The surface chemical reaction is unique, however, in that participating metal centers are bound within an oxide/hydroxide... 

When chemisorption is involved, or when some additional surface chemical reaction occurs, the process is more complicated. The most common combinations of surface mechanisms have been expressed in the Langmuir-Hinshelwood relationships 36). Since the adsorption process results in the net transfer of molecules from the gas to the adsorbed phase, it is accompanied by a bulk flow of fluid which keeps the total pressure constant. The effect is small and usually neglected. As adsorption proceeds, diffusing molecules may be denied access to parts of the internal surface because the pore system becomes blocked at critical points with condensate. Complex as the situation may be in theory,... 

The dissolved form of O decays to the final electroinactive product via a volume chemical reaction occnrring in the diffusion layer with the volume rate constant (kv), whereas the adsorbed form participates in the surface chemical reaction confined to the electrode surface, characterized by a surface rate constant (kg). These two chemical reactions proceed with different rates due to significant differences between the chenucal nature of dissolved and adsorbed forms of O. Obviously, the mechanisms (2.172)-(2.174) and (2.177) are only limiting cases of the general mechanism (2.178). 

Rate determining step (cont.) electrocatalysis and, 1276 methanol oxidation, 1270 in multistep reactions, 1180 overpotential and, 1175 places where it can occur, 1260 pseudo-equilibrium, 1260 quasi equilibrium and, 1176 reaction mechanism and, 1260 steady state and, 1176 surface chemical reactions and, 1261 Real impedance, 1128, 1135 Reciprocal relation, the, 1250 Recombination reaction, 1168 Receiver states, 1494 Reddy, 1163... 

Most catalytic reactions involve a number of species of atoms and molecules. To deduce the mechanism of the reaction and the forces between the various species and between the species and the surface is obviously a complex procedure, but the problem is simplified by a study of the adsorption phenomena of a single species of atom or molecule. Such studies have shown that when some molecules are adsorbed on some adsorbents, the molecular bond is broken and is replaced by two bonds with the adsorbent the admole is changed to two adatoms. A surface chemical reaction has taken place and the adatoms are said to be chemisorbed. If at sufficiently low temperatures this reaction does not take place, the adsorbed molecules are not broken up into two adatoms, and the admoles are said to be physisorbed. Above a certain temperature the rate of the reaction is sufficiently rapid to be appreciable at higher temperatures, the rate is very rapid. Such reactions have led to the concept of an activation energy, that is, the energy which must be given to an admole to convert it into adatoms. Even if an admole is not completely dissociated, it is to be expected that the strength of the molecular bond has been weakened as a result of the adsorption hence it is likely that the probability of reaction with other adsorbed species will be quite different from what it is between the two species in free space. 

An alternate approach is to specify an elementary chemical reaction mechanism at the surface. In this case one can have reactions between gas-phase species and surface species, as well as reactions between adsorbed species. At this level of specification, surface reaction mechanisms often become very complex, including dozens of elementary reactions. Such complex surface chemistry reaction mechanisms have been used in models for many CVD systems, for example. 

Chemical Reaction Mechanisms and Kinetics. CVD chemistry is complex, involving both gas-phase and surface reactions. The role of gas-phase reactions expands with increasing temperature and partial pressure of the reactants. At high reactant concentrations, gas-phase reactions may eventually lead to gas-phase nucleation that is detrimental to thin-film growth. The initial steps of gas-phase nucleation are not understood for CVD systems, not even for the nucleation of Si from silane, which has a potential application in bulk Si production (97). In addition to producing film precursors, gas-phase reactions can have adverse effects by forming species that are potential impurity sources. 

The science of catalysis covers a large spectrum of phenomena. We observe—with some pride and joy—that this volume presents eight topics which, like the rainbow, form an almost systematic and complete sweep of the major classes of topics in catalysis. It spans from the most classical mechanistic study (P. W. Selwood), to a presentation of a hard practical application (M. Shelef et al). As we sweep across, we cover characterization studies of catalyst solids in terms of electronic (G. M. Schwab), surface chemical (H. A. Benesi and B. H. C. Winquist), as well as physicochemical and structural (F. E. Massoth) parameters, chemical reaction mechanisms and pathways (G. W. Keulks et al., and B. Gorewit and M. Tsutsui), and a topic on reactor behavior (V. Hlavacek and J, Votruba), which takes us from the single catalyst particle to the macroscopic total reactor operation. 

The rate-controlling step in reductive dissolution of oxides is surface chemical reaction control. The dissolution process involves a series of ligand-substitution and electron-transfer reactions. Two general mechanisms for electron transfer between metal ion complexes and organic compounds have been proposed (Stone, 1986) inner-sphere and outer-sphere. Both mechanisms involve the formation of a precursor complex, electron transfer with the complex, and subsequent breakdown of the successor complex (Stone, 1986). In the inner-sphere mechanism, the reductant... 

In the absence of mechanical activation, the XPS spectrum is different from that obtained in the presence of mechanical activation. Thus, sulfur compounds that are formed on the surface (162 eV) appear in the spectra that are not present in the spectra for static immersion. Cutting produces a change in the surface chemistry and the reaction product formed appears to be similar to that obtained by mechanical activation. Thus, mechanical activity plus increased surface temperatures on a solid surface tend to promote surface chemical reactions. A lubricant interacting with metal oxide can produce entirely different reaction products from that of a lubricant interacting with rubbing metal surfaces. 

The second major section will focus on those special centers of minerals thought to be of importance to their catalytic activity, with an emphasis on the known and possible effects of electronic excitation on the population and mode of action of these centers. Metastable states constitute a hidden variable in defective solids, a non-negligible one for non-stoichiometric ones. With regard to concepts of mineral catalysis, the only systems for which extensive spectroscopic information on mineral catalytic centers has been definitively coupled to the mechanism of a well understood surface chemical reaction is exchange on binary oxides. Existing data for the... 

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