Gas phase mechanism

Investigations of the plasma chemical decomposition of tantalum-containing fluoride solutions indicated no significant differences in the process and product parameters compared to the corresponding decomposition of niobium-containing fluoride solution [529, 532]. The particle diameter, shape and specific surface area of both niobium oxide and tantalum oxide powders attest to a gas-phase mechanism of the interaction, with sequential condensation and agglomeration of the oxides. 

Figure 8.3 Proposed reaction mechanism for methanol synthesis on Pd and comparison with gas-phase mechanism surface intermediates are speculative and associated energies are estimates...

We can, however, form alkoxide ions that are monosolvated by a single alcohol group, via the Riveros reaction [Equation (7)]. When the monosolvated methoxide is reacted with acrylonitrile, the addition process reaction (8a), is the major pathway, because there is a molecule of solvent available to carry off the excess energy. The proton transfer pathway, reaction (8b), becomes endothermic, because the methoxide-methanol hydrogen bond, at about 29 kcal/mol, must be broken in order to yield the products. Thus, one can observe either the unique gas phase mechanism in the gas phase, reaction (6b), or the solution phase mechanism in the gas phase, reaction (8a), and the only difference is in the presence of the first molecule of solvent. 

As noted above, it should be realized that understanding the activity on a surface is not the only issue needed to fully characterize the catalytic system. There must be an understanding of how the surface reactions produce intermediates that can desorb and initiate gas phase reactions close to the surface. To that extent, the author is developing a catalytic shock tube technique to probe the interface of the catalyst surface and the immediate gas phase layer, bridging the gap between surface mechanisms and gas phase mechanisms. 

BAC-MP4 method Coupled-cluster calculations Gas-phase thermochemistry Equilibrium Gas-phase mechanisms... 

The oxidation of the methyl radicals formed by DMTC pyrolysis is well understood compared with the tin chemistry. Gas-phase mechanisms describing this chemistry are readily available [61-63]. These reactions lead to the formation of other reactive species that can attack DMTC, including H, O, OH, and HO2. The OH radical in particiflar is a very efficient H-abstractor, and will therefore quickly react with DMTC ... 

In contrast to this, the group of Jensen and others have proposed rather detailed chemical mechanisms for GaN (and GaAs) formation, with Arrhenius-type parameters used to define temperature-dependent rate constants [83-86]. These mechanisms were applied in CFD simulations in order to study the actual species concentrations under growth conditions [83-87]. Scheme 1 gives a highly simplified summary of the gas-phase mechanism. In the following we will briefly discuss theoretical investigations of the reactions shown here. 

Because the oxidants for S02 are generated in the VOC-NO. system discussed in Chapter 6, the overall gas-phase mechanism for the oxidation of S02 to h2so4 is quite complex. The reader is referred to the mechanism in the RADM model (Regional zlcid Deposition Model) for a treatment of the VOC-NO -S02 chemistry (Stockwell et al., 1990 Gao et al., 1996). It should be noted that an earlier version of this mechanism is given in some of the examples included with the OZIPR model discussed in Chapter 16 and whose applications are included with this book. 

When introduced, chromatographic techniques completely revolutionized analysis of reaction mixtures and have proved particularly important for kinetic studies of complex gaseous reactions. A complete revision of most gas phase reactions proved necessary, because it was soon discovered that many intermediates and minor products had not been detected previously, and a complete re-evaluation of gas phase mechanisms was essential. 

Other radiative association reactions leading to formic acid, ethanol, and sundry species are discussed in Leung, Herbst, and Huebner (1984). Calculations of radiative association rate coefficients for ion-molecule systems with large numbers of atoms will be necessary to extend gas phase mechanisms to the syntheses of still larger species. Unfortunately, such calculations are often rendered difficult by the lack of suitable thermodynamic, structural, and vibrational data on the product ions which are needed as input into the calculations. A somewhat easier approach is to estimate the radiative association rate coefficient from higher temperature laboratory three-body rates (Smith et al. 1983). Even so, this approach cannot be used for most reactions of interest involving more complex reactants because of a paucity of laboratory measurements. It is clear that more laboratory work will always be needed ... 

Gas-Phase Mechanism Synergistic Effect Based on Halogen-Antimony Compounds.79... 

GAS-PHASE MECHANISM SYNERGISTIC EFFECT BASED ON HALOGEN-ANTIMONY COMPOUNDS... 

The literature contains numerous speculations as to the mechanism. Some involve a condensed phase mechanism, as, for example, in the case of cellulose in which it is suggested the formation in situ of antimony chloride which may react with cellulose to alter the course of thermal decomposition and/or form a heavy vapor tending to extinguish the flame. 28 Some involve a physical gas-phase mechanism such as formation in the flame of nonvolatile, antimony-containing solid or... 

S02 is oxidized to sulfuric acid both by homogeneous gas-phase reactions and by multiphase processes when a precursor gas is dissolved in water and then subsequently oxidized. The routes to atmospheric sulfuric acid production have been studied extensively for decades. The most important gas-phase mechanism is oxidation by OH radicals ... 

The Gas-Phase Mechanism of C-H Activation by the Iridium(iii) Complex Cations... 

COSMO EM- Several gas phase mechanisms. Each time step or time None... 

These examples already illuminate the difficulties encountered by gas phase mechanisms even at the level of intermediate sized molecules such as NHj, HCN, HNC or HjCO. With each additional reaction and possible associated branching ratio the uncertainties grow. Despite these diflficnilties it seems, however, that molecules such as HD, CN, HCO, HCN, HNC, NH3 and probably also H CO are preferentially formed through ion-molecule reactions. It seems, however, certain that in the formation of ions and/or radicals the final step in the formation scheme has to occur through s-phase reaction rather than surface reaction. 

The condensed phase strategy includes the described mechanism of removal of heat and the enhancement of the decomposition temperature as in heat resistant fibres. In Table 8.1 the condensed phase and the gas phase mechanisms are compared. 

The UCR EPA chamber is a new large indoor environmental chamber constructed at the University of California at Riverside (UCR) under United States EPA funding for the purpose of evaluating gas-phase and secondary aerosol mechanisms for ground-level air pollution. The major characteristics of this chamber, the results of its initial characterization for gas-phase mechanism evaluation, and examples of initial gas-phase mechanism evaluation experiments, are described. It is concluded that the chamber has lower or at most comparable background effects than other chambers previously used for mechanism evaluation, and can provide useful mechanism evaluation data at NOx levels as low as 2 ppb. Future research directions to utilize the capabilities of this chamber are discussed. 

The future research directions for the UCR EPA chamber will of course depend on funding availability and the interests of the funding agencies, but we would expect the research would be designed to take full advantage of its capabilities. In any case, we expect to continue the ozone reactivity and gas-phase mechanism studies that are currently underway, with the mechanism evaluation being focused on aromatics, whose mechanism are the most... 

As discussed in more detail by Cocker (2004), we will also utilize the chamber for well-characterized experiments needed to develop and evaluate models for secondary organic aerosol (SOA) formation. The temperature control capabilities of this chamber permit systematic studies of temperature and humidity effects that are not possible with existing outdoor chambers or with indoor chambers using blacklights (whose intensities are temperature-dependent). The established capability of this chamber for gas phase mechanism evaluation is an important characteristic in this regard, since the model cannot properly simulate SOA formation if it does not also properly simulate the formation of the SOA precursors from the gas-phase reactions. 

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