Sulfur dioxide heterogeneous oxidation

Finally, atmospheric chemical transformations are classified in terms of whether they occur as a gas (homogeneous), on a surface, or in a liquid droplet (heterogeneous). An example of the last is the oxidation of dissolved sulfur dioxide in a liquid droplet. Thus, chemical transformations can occur in the gas phase, forming secondary products such as NO2 and O3 in the liquid phase, such as SO2 oxidation in liquid droplets or water films and as gas-to-particle conversion, in which the oxidized product condenses to form an aerosol. 

Sulfur is widely distributed as sulfide ores, which include galena, PbS cinnabar, HgS iron pyrite, FeS, and sphalerite, ZnS (Fig. 15.11). Because these ores are so common, sulfur is a by-product of the extraction of a number of metals, especially copper. Sulfur is also found as deposits of the native element (called brimstone), which are formed by bacterial action on H,S. The low melting point of sulfur (115°C) is utilized in the Frasch process, in which superheated water is used to melt solid sulfur underground and compressed air pushes the resulting slurry to the surface. Sulfur is also commonly found in petroleum, and extracting it chemically has been made inexpensive and safe by the use of heterogeneous catalysts, particularly zeolites (see Section 13.14). One method used to remove sulfur in the form of H2S from petroleum and natural gas is the Claus process, in which some of the H2S is first oxidized to sulfur dioxide ... 

As mentioned before, heterogeneous sulfur dioxide oxidation reactions... 

Halstead, J. A., R. Armstrong, B. Pohlman, S. Sibley, and R. Maier, "Nonaqueous Heterogeneous Oxidation of Sulfur Dioxide, . /. Phys. Chem., 94, 3261-3265 (1990). 

Sulfide ores are very common, and sulfur is a by-product of the extraction of a number of metals, especially copper, from their ores. Sulfur is also commonly found in petroleum, and extracting it chemically has been made inexpensive and safe by the use of heterogeneous catalysts. One method is the Claus process, in which some of the H2S that occurs in oil and natural gas wells is first oxidized to sulfur dioxide ... 

Heterogeneous catalysis also occurs in the oxidation of gaseous sulfur dioxide to gaseous sulfur trioxide. This process is especially interesting because it illustrates both positive and negative consequences of chemical catalysis. 

However, this reaction is very slow in the absence of a catalyst. One of the mysteries during early research on air pollution was how the sulfur dioxide produced from the combustion of sulfur-containing fuels is so rapidly converted to sulfur trioxide in the atmosphere. It is now known that dust and other particles can act as heterogeneous catalysts for this process (see Section 15.9). In the preparation of sulfur trioxide for the manufacture of sulfuric acid, either platinum metal or vanadium(V) oxide (V205) is used as a catalyst, and the reaction is carried out at approximately 500°C, even though this temperature decreases the value of the equilibrium constant for this exothermic reaction. 

The heterogeneous oxidation can be significant, but in the presence of liquid water there is, as we will see, an important heterogeneous process for the production of sulfate from SO2. Sulfur dioxide is not especially soluble in water, but subsequent equilibria increase the partitioning into cloud water ... 

When we consider of the heterogeneous oxidation of SO2, it has to include not only the oxidation of the SO2 within the droplet phase, but the transfer of further SO2 into the droplet and the overall depletion of the gas in the air mass as a whole. In general the overall oxidation in the remote atmosphere is rather slow and takes 2-4 d, but under some conditions it can be much faster. The depletion rates of sulfur dioxide in volcanic plumes can sometimes be very fast with residence times of as little as 15 min in moist plumes, where catalytic mechanisms similar to urban air masses probably occur. 

Two processes are used commercially to produce sulfuric acid, the contact process, where the oxidation of sulfur dioxide to sulfur trioxide is accomplished with air heterogeneously using a solid catalyst, and the chamber process, where the oxidation step is accomplished homogeneously with a gaseous catalyst. Of the two, the contact process is used to produce more than 95% of the supply of sulfuric acid at present, both in Europe and in North America. 

Sulfur dioxide (SO ) and nitrogen oxides (NO ) are oxidized to sulfate and nitrate aerosols either homogeneously rn the gas phase or heterogeneously in atmospheric microdroplets and hydrometeors Gas-phase production of nitric acid appears to be the dominant source of aerosol nitrate because the aqueous phase reactions of NO (aq) are slow at the nitrogen oxide partial pressures typically encountered in the atmosphere (5,i5). Conversely, field studies indicate that the relative importance of homogeneous and heterogeneous SO2 oxidation processes depends on a variety of climatological factors such as relative humidity and the intensity of solar radiation (4, -1 ). 

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. 

However, in the years after Ostwald, his former students and collaborators dominated catalysis in Germany. We can mention Bodenstein who put the study of the kinetics of heterogeneous catalysis essentially in its modern state. The Ostwald school made little contribution to mechanism, but it adhered to a view surprisingly close to that of Faraday. The famous paper of Bodenstein and Fink of 1907 interpreted the kinetics of the oxidation of sulfur dioxide on platinum in terms of the diffusion of sulfur dioxide or oxygen through a polymolecular layer of adsorbed material. In extreme cases, accord with the observed inhibition by sulfur trioxide would have required adsorbed layers so thick that they could have been pared with a razor. 

Usher, C.R., Al-Hosney, H., Carlos-Cuellar, S., Grassian, V.H. A laboratory study of the heterogeneous uptake and oxidation of sulfur dioxide on mineral dust particles. J. Geophys. Res. 107,4713 (2002)... 

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