Sulphur dioxide, heterogeneous oxidation

Oxides of metals such as aluminium, calcium, chromium, iron, lead, or vanadium may also serve as catalysts for the heterogeneous oxidation of sulphur dioxide. These oxides may also adsorb sulphur dioxide (4]. 

Because of the rapid developments in the field of heterogeneous catalysis, the material reviewed here is exclusively dedicated to selective oxidations. No attention is given to total oxidations or combustion processes (including the problem of automotive exhaust gases). There is one exception, however the oxidation of sulphur dioxide to trioxide. Work on vanadate catalysts for this reaction is close to research on selective catalysts and therefore included. 

The oxidation of sulphur dioxide to trioxide is one of the oldest heterogeneous catalytic processes. The classic catalyst based on V2Os has therefore been the subject of numerous investigations which are amply reviewed by Weychert and Urbaneck [346]. These authors conclude that none of the 34 rate equations reported is applicable over a wide range of process conditions. Generally, these equations have the form of a power expression, in which the reverse reaction is taken into account within the limits imposed by chemical equilibrium, viz. 

The choice of an appropriate model is heavily dependent on the intended application. In particular, the science of the model must match the pollutant(s) of concern. If the pollutant of concern is fine PM, the model chemistry must be able to handle reactions of nitrogen oxides (NOx), sulphur dioxide (SO2), volatile organic compounds (VOC), ammonia, etc. Reactions in both the gas and aqueous phases must be included, and preferably also heterogeneous reactions taking place on the surfaces of particles. Apart from correct treatment of transport and diffusion, the formation and growth of particles must be included, and the model must be able to track the evolution of particle mass as a function of size. The ability to treat deposition of pollutants to the surface of the earth by both wet and dry processes is also required. 

It is quite ambitious for a scientist to describe a natural phenomenon in terms of a specific reaction. The situation in the atmospheric environment is however more complicated as a variety of reactions are occurring simultaneously and a certain species may take part in different reactions affecting the relative equilibria. Most data are coming from laboratory work and experimental conditions are definitely different from the ones observed in the troposphere. As an example the mechanism of oxidation of sulphur dioxide, in gas phase is usually reported occur to a large extent through free radicals. If the presence of humidity and of particulated matter is considered, specifically in the lower part of the troposphere, definitely also heterogeneous reactions play a very important role. I feel that experiments carried on in the atmosphere yield more consistent results to elucidate the chemistry of the atmospheric environment. 

During fossil fuel combustion, sulphur is oxidized to sulphur dioxide, however, a small amount of sulphur trioxide is also formed. Sulphur trioxide is formed in combustion products by the dissociation of sulphate, by the reaction of sulphur dioxide with oxygen in the flame and by a heterogeneous... 

Heterogeneous reactions on solid particles may also play a role in the removal of sulphur dioxide from the atmosphere. In atmospheric photochemical reactions, such particles may function as nucleation centres. Thus, they act as catalysts and grow in size by accumulating reaction products. The final result would be the production of an aerosol with a composition unlike that of the original particle. Little research has been done on the role that solid particles play in the oxidation of sulphur dioxide under conditions like those found in the atmosphere. Soot particles, which consist of elemental carbon contaminated with polycyclic aromatic hydrocarbons produced in the incomplete combustion of carbonacetous fuels, have been shown to catalyse the oxidation of sulphur dioxide to sulphates. 

A common feature of heterogeneous catalysis is an increase in rate with increasing subdivision of the catalytic material. This arises from increasing accessibility of the surface, and reduction in difTusional constraints between reactants and catalytic sites. In general, continued subdivision will eventually lead to a levelling off in the reaction rate at a value dictated solely by adsorption and chemical processes. For simple, selective reactions (olefin hydrogenation, oxidation of sulphur dioxide), the ratio of the reaction rate with a practical catalyst form to the maximum rate attainable by mechanical subdivision is often referred to as the effectiveness factor. 

Heterogeneous Reaction of solids and liquids. Burning of coal. Roasting of ores. Gas-liquid absorption with reaction. Production of bio-mass Ammonia synthesis. Cracking of crude oil. Oxidation of sulphur dioxide. Gas-liquid absorption with reaction (often promoters added)... 

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