Homogeneous gas phase chemistry

Line A in Fig. 16.12 shows the OH concentrations predicted using only the homogeneous gas-phase chemistry of reactions (1)—(13). Clearly, much larger concentrations of OH are observed than can be rationalized on the basis of gas-phase chemistry alone. 

We are concerned with both homogeneous gas-phase chemistry and heterogeneous surface chemistry. Certainly in combustion, gas-phase chemistry is usually dominant. However, there may be good reason to be concerned with heterogeneous chemistry, for example, on the relatively cool walls of a combustion chamber. Moreover there are emerging materials-synthesis and surface-modification techniques that depend on flame-surface interactions. 

Explicit mechanisms attempt to include all nonmethane hydrocarbons believed present in the system with an explicit representation of their known chemical reactions. Atmospheric simulation experiments with controlled NMHC concentrations can be used to develop explicit mechanisms. Examples of these are Leone and Seinfeld (164), Hough (165) and Atkinson et al (169). Rate constants for homogeneous (gas-phase) reactions and photolytic processes are fairly well established for many NMHC. Most of the lower alkanes and alkenes have been extensively studied, and the reactions of the higher family members, although little studied, should be comparable to the lower members of the family. Terpenes and aromatic hydrocarbons, on the other hand, are still inadequately understood, in spite of considerable experimental effort. Parameterization of NMHC chemistry results when NMHC s known to be present in the atmosphere are not explicitly incorporated into the mechanism, but rather are assigned to augment the concentration of NMHC s of similar chemical nature which the... 

Fig. 5 Processes involving iron compounds in droplets and their interaction with gas-phase chemistry (see text). Both heterogeneous and homogeneous reactions are intimately involved in a complex manifold of reactions...

Chemistry is basically a practical science, and most of our knowledge about atmospheric chemical reactions has been established by laboratory experiments. Despite considerable advances in laboratory techniques it is often difficult to find experimental conditions whereby the reaction of interest can be sufficiently isolated from other concurrent ones so that its rate behavior can be unambiguously determined. Rate data thus are considered reliable only if two independent experimental techniques lead to the same results. As a consequence, the accumulation of dependable information on reactions deemed important to atmospheric chemistry has been a slow process. Nevertheless, a fairly large body of data now exists, at least for homogeneous gas-phase reactions. Tables A-4 and A-5 provide a compilation of reaction rate data as a reference for all subsequent discussions. The numbering of the reactions is used throughout this book. 

A wide variety of interrelated homogenous gas-phase, solution-phase, and heterogenous chemistry may ultimately result in oxidation of SO2 to sulfuric acid in DUV exposure tools. The three main possible reaction pathways for the oxidation of sulfur dioxide to sulfuric acid in the exposure chamber may include (i) direct oxidation of sulfur dioxide by stable atmospheric oxygen, (ii) catalyzed oxidation of sulfur dioxide by metal ions, and (iii) photochemical oxidation of sulfur dioxide by ozone and hydroxyl radical. 

The chemistry of organolead compounds in the environment has been reviewed elsewhere [93-96], but a few salient aspects are summarized here. First, tetra-alkyllead compounds are volatile Henry s Law constant of 4.7 x 10" and 6.9 X 10" (Pam )/mol for tetramethyl and tetraethyllead, respectively, according to Wang et al. [97]. However, only a small fraction of the Pb leaving an automobile as exhaust is in this form, e.g., typically 0.1-10% [95]. In addition to the relatively low emission factor from leaded gasoline combustion, tetra-alkyllead compounds are rapidly decomposed by homogeneous gas phase reactions such as photolysis, reaction with ozone, triplet atomic oxygen, or hydroxyl radical [98,99] with half-lives of less than 10 h in summer and 40 h in... 

In the previous chapters, we considered either homogeneous gas phase or aqueous phase chemistry. However, the condensed aqueous phase exists only in interaetion with the gas phase. As just mentioned, at the interface reactants meet from the aqueous and gaseous sites and build up high concentrations to provide favorable conditions for chemical reactions in recent years this has brought the term interfacial chemistry into fashion. 

Modeling of the gas phase chemistry and particle growth shows that homogeneous nucleation occurs early on in the process followed by heterogeneous condensation processes. Molecular dynamics computation have indicated that the iron oxide clusters will phase segregate and migrate toward the inside edge of the silica cluster consistent with experimental observation. 

This book covers homogeneous gas-phase kinetics important in the atmosphere, which has been almost established, and provides the solid scientific bases of oxidation of trace gases and oxidant formation. Nevertheless, unresolved problems remain, for example, unsatisfactory reproduction of observed OH/HO2 mixing ratio by model simulation under certain conditions, and oxidation mechanisms involving isoprene, terpenes and other biogenic hydrocarbons, and anthropogenic aromatic hydrocarbons. Therefore, descriptions of these topics are not completed in the book. Heterogeneous reaction chemistry is not covered well except for the chemistry on polar stratospheric clouds (PSCs) and reactive uptake coefficients of selected... 

An optimized mechanism for homogeneous combustion of C1-C3 species by Qin et al. (70 species, 14 irreversible and 449 reversible reactions) [2] was employed for modeling gas-phase chemistry. Thermodynamic data were included in the provided scheme. Surface and gas-phase reaction rates were evaluated with Surface-CHEMKIN [3] and CHEMKIN [4] respectively. Mixture-average difihi-sion was the transport model, using the CHEMKIN transport database [5]. 

Finally, microreactor CH4 and CO emissions during the start-up phase are plotted for Cases 11 and 24 in Fig. 8.19, considering a channel width (z-direction) of 1 cm. When neglecting gas-phase chemistry, the amount of unbumed CH4 emissions is overpredicted, since homogeneous reactions further consume CH4 (either via the direct gas-phase complete oxidation of CH4, or via the... 

In homogeneous catalysis, both the catalyst and the reactants are in the same phase, i.e. all are molecules in the gas phase, or, more commonly, in the liquid phase. One of the simplest examples is found in atmospheric chemistry. Ozone in the atmosphere decomposes, among other routes, via a reaction with chlorine atoms ... 

The chemistry of vinyl acetate synthesis from the gas-phase oxidative coupling of acetic acid with ethylene has been shown to be facilitated by many co-catalysts. Since the inception of the ethylene-based homogeneous liquid-phase process by Moiseev et al. (1960), the active c ytic species in both the liquid and gas-phase process has always been seen to be some form of palladium acetate [Nakamura et al, 1971 Augustine and Blitz, 1993]. Many co-catalysts which help to enhance the productivity or selectivity of the catalyst have appeared in the literature over the years. The most notable promoters being gold (Au) [Sennewald et al., 1971 Bissot, 1977], cadmium acetate (Cd(OAc)j) [Hoechst, 1967], and potassium acetate (KOAc) [Sennewald et al., 1971 Bissot, 1977]. 

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