Gas - Particle Reactions

Lane and Katz reported in 1977 that the dark reaction of BaP deposited on the surface of glass Petri dishes with air containing 200 ppb of ozone was fast, with a half-life of —38 min. Katz and co-workers (1979) exposed nine PAHs on thin-layer chromatography plates of cellulose in the dark to 200 ppb of O, in air and found pronounced differences in their reactivities, e.g., half-lives of 36 min for BaP, 2.9 h for BaA, 7.6 h for BeP, and 53 h for benzol b ]fluoranthene. Subsequently, in good agreement with Lane and Katz, a half-life of -1 h was determined for BaP deposited on glass fiber filters and exposed (passively in a controlled atmosphere) to 200 ppb of 03 in the dark (Pitts et al., 1980). 

Pitts et al. (1986) exposed five individual PAHs, pyrene, fluoranthene, benz[a]anthracene, BeP, and BaP, deposited on glass fiber and Teflon-impregnated glass fiber filter (TIGF) substrates passively for 3 h in the dark in a 360-L Teflon environmental chamber to 50-300 ppb of 03 in air at several relative humidities. These experimental conditions more nearly resemble the actual exposure of ambient particles to 03 (in the dark) during transport than do exposures in Hi-Vol flow systems. Consistent with earlier studies, BaP, BaA, 

FIGURE 10.29 Typical photoproducts observed (A) in the irradiation (A = 300 nm) in air of naphthalene and 1-methylnaphthalene adsorbed on silica and formed by a Type I electron transfer (superoxide) mechanism (Barbas et al., 1993) and (B) in the irradiation in air of acenaphthylene (A = 350 nm) adsorbed on silica formed by a Type II singlet oxygen mechanism (Barbas et al., 1994) (adapted from Dabestani, 1997). 

Fresh ambient particulate POM sampled near a freeway was also exposed in this passive system for 3 h in the dark to 200 ppb of 03 in air at 1% RH. Concentrations of specific PAHs determined in the ambient particles and their percent reacted were similar for samples collected on both kinds of filters (glass fiber and TIGF). Again, under passive exposure conditions to approximately ambient levels of 03, BaP and BaA were found to be significantly more reactive than BeP. 

Results of a study of the ozonolysis of primary combustion aerosols by Van Vaeck and Van Cauwen-berghe (1984a) are illustrated in Fig. 10.30. Shown are percent conversion profiles as a function of time for the decay of several 5- and 6-ring PAHs in diesel exhaust 

---

Guo and Kamens (1991) describe a system for studying gas-particle reactions on the surfaces of combustion aerosols in which they report a half-life of 80 h for high loadings of particle-bound BaP in wood smoke particles reacting with 200 ppb of NOz in air. 

In addition to the numerous elemental characterisations of ambient particles, we have recently seen more applications in fundamental physico-chemistry. This includes laboratory studies on gas-particle reactions, e.g. sea salt or soil dust with nitric acid. Environmental SEM (ESEM) or environmental TEM (ETEM) does offer excellent future prospects in this field. In ESEM and ETEM, it is possible to obtain high quality images and to do some chemical analysis while the gaseous environment around the sample is controlled i.e. vacuum in the neighbourhood of the sample is not necessary, the relative humidity can be varied and a temperature around — 30 °C can be maintained). Ice nucleation properties of individual atmospheric particles have also been studied recently. Both ESEM and ETEM are relatively new techniques, of which the potential has not fully been explored. 

Gas-phase reactions play a fundamental role in nature, for example atmospheric chemistry [1, 2, 3, 4 and 5] and interstellar chemistry [6], as well as in many teclmical processes, for example combustion and exliaust fiime cleansing [7, 8 and 9], Apart from such practical aspects the study of gas-phase reactions has provided the basis for our understanding of chemical reaction mechanisms on a microscopic level. The typically small particle densities in the gas phase mean that reactions occur in well defined elementary steps, usually not involving more than three particles. 

Trimoleciilar reactions require the simultaneous encounter of tliree particles. At the usually low particle densities of gas phase reactions they are relatively unlikely. Examples for trimoleciilar reactions are atom recombination reactions... 

Naphthaleneamine. 1-Naphthylamine or a-naphth5iamine/7i5 -i2- can be made from 1-nitronaphthalene by reduction with iron—dilute HCl, or by catalytic hydrogenation it is purified by distillation and the content of 2-naphthylamine can be reduced as low as 8—10 ppm. Electroreduction of 1-nitronaphthalene to 1-naphthylamine using titania—titanium composite electrode has been described (43). Photoinduced reduction of 1-nitronaphthalene on semiconductor (eg, anatase) particles produces 1-naphthylamine in 77% yield (44). 1-Naphthylamine/7J4-J2-. can also be prepared by treating 1-naphthol with NH in the presence of a catalyst at elevated temperature. The sanitary working conditions are improved by gas-phase reaction at... 

Spherical, Fine-Particle Titanium Dioxide. Spherical, fine-particle titanium dioxide that has no agglomeration and of mono-dispersion can be manufactured by carrying out a gas-phase reaction between a tetraalkyl titanate vapor and methanol vapor in a carrier gas to form an initial fine particle, which can then be hydrolyzed with water or steam (572). 

Although they are termed homogeneous, most industrial gas-phase reactions take place in contact with solids, either the vessel wall or particles as heat carriers or catalysts. With catalysts, mass diffusional resistances are present with inert solids, the only complication is with heat transfer. A few of the reactions in Table 23-1 are gas-phase type, mostly catalytic. Usually a system of industrial interest is liquefiea to take advantage of the higher rates of liquid reactions, or to utihze liquid homogeneous cat ysts, or simply to keep equipment size down. In this section, some important noncatalytic gas reactions are described. 

Rigopoulos, Stelios and Alan G. Jones, 2001. Dynamic Modelling of a Bubble Column for Particle Fonuation via a Gas-Liquid Reaction. Chemical Engineering Science (in press). 

Operations such as blending, solids-suspension, dissolving, heat transfer and liquid-liquid extraction are typical of systems requiring high flow relative to turbulence, while gas-liquid reactions and some liquid-liquid contacting require high turbulence relative to flow. The case of (1) 100% of suspension—requires head to keep particles suspended and (2) 100% uniformity of distribution of particles—requires head for suspension plus flow for dis-tiibution. 

Gas-phase reactions catalyzed by solid catalysts are normally carried out in gas-particle operation in either fixed or fluidized beds. The possibility of using gas-liquid-particle operations for such reactions is, however, of interest in certain cases, particularly if the presence of a liquid medium for the transfer of heat or mass is desirable. 

A number of such processes were established before the second World War in Germany, Japan, and France for the production of hydrocarbon mixtures in the liquid fuel range (P2). This way of manufacturing automotive fuels is now uneconomical in most areas, but related processes may be utilized for the production of various chemicals, such as paraffinic waxes or oxygenated compounds. (The manufacture of methanol from carbon monoxide and hydrogen, usually by catalytic reaction in fixed-bed gas-particle operation, is an important process of this type.)... 

Commercial Fischer-Tropsch processes have been based exclusively on gas-particle operations, mainly in fixed beds (P2). The chemical reactions are highly exothermic, however, and accurate temperature control is therefore difficult to achieve in a fixed bed. Good temperature control is important because of the temperature sensitivity of the chemical reactions taking place, and several attempts have therefore been made to develop processes based on other types of operation. 

The above rate equations were originally largely developed from studies of gas—solid reactions and assume that particles of the solid reactant are completely covered by a coherent layer of product. Various applications of these models to kinetic studies of solid—solid interactions have been given. 

Figure 2. MSssbauer Spectrum and Corresponding Computer Simulation for Sample 2 Under Water-Gas Shift Reaction Conditions at 613 K. A) situ MSssbauer spectrum of sample 2 at 613 K B) Computer-simulated spectrum C) Distribution of particle radii D) Relative volume fractions as a function of radius (A). For the computer simulation, the following pareimeters were used 0-1.25, mean radius = 65A, k-8 x 10 ergs/cm3. The Klebsch-Gordon coefficients used were 3 3 1.

MSssbauer spectroscopy and magnetic susceptibility were used to demonstrate that magnetite particles supported on a Grafoil substrate sinter very slowly under water-gas shift reaction conditions. 

---