What are SO2 and SO3

Experiment: chemisorption of sulfur dioxide on activated carbon
Experiment: Oxidation of sulfur dioxide by nitrogen oxides
Experiment: Oxidation of sulfur dioxide in the presence of vanadium oxide

Sulfuric acid is of great interest as a widely used industrial chemical. However, the production of concentrated sulfuric acid causes problems.
The oxidation of hydrogen sulfite ions HSO is easily possible3-, which is used in exhaust gas desulphurization using the limestone process. However, this does not result in concentrated sulfuric acid. But that's what you need in industry.
In the past, concentrated sulfuric acid was produced by heating sulfate hydrates ("vitriols"). The acid was also named after this process Vitriol oil.

Therefore one tried soon, sulfur trioxide SO3 as the anhydride of sulfuric acid and then mix it with water.
Sulfur oxidation is a two-step process. When sulfur-containing substances are burned, only sulfur dioxide is usually produced.

S + O2 ———> SO2    D H = - 297 kJ / mol

Although the further oxidation to sulfur trioxide is also strongly exothermic, it does not take place to any significant extent.

This is made more difficult by the fact that the oxidation of sulfur dioxide is a pronounced equilibrium reaction in which the reverse reaction begins at temperatures from 400-600 C. On the other hand, the reaction rate is still too slow at these temperatures because the reaction is inhibited.
Therefore one has to lower the activation energy and after catalysts for the oxidation of the gas SO2 search.

1 Chemisorption on activated carbon
This only plays a role to a small extent and then only for exhaust gas purification. The procedure is interesting from a theoretical point of view (-> experiment).

2 The historic lead chamber process
Nitrogen oxides as homogeneous catalysts mediate the reaction between oxygen and sulfur dioxide (-> experiment). Formally, the following applies:

The overall reaction is based on a more complicated mechanism that we do not want to present here.
The sulfuric acid formed in this way has the disadvantage that it is contaminated with nitrogen oxides.

3 The modern double contact process
Oxides of d group metals such as iron, chromium and vanadium act as heterogeneous catalysts. These metals can easily change their oxidation number and therefore act as oxygen carriers.
Today, sulfur dioxide is primarily produced by burning sulfur. It used to be won during ore arming for copper or iron extraction.
In practice, the gas mixture is allowed to react on a hot contact. There is an exchange of oxygen on the surface of the metal oxides (contact method; (-> experiment).

To everything SO2 to implement, the gas mixture is fed from the first reactor into a second reactor ("double contact process").

Further texts on the topic of catalysis