In chemical processes, catalysts are used to modify the mechanisms of chemical reactions. By providing an energetically favourable pathway, catalysts accelerate reactions which would normally be too slow or would not even take place. After the reaction, the catalyst remains essentially unchanged.
Main chemical applications of tungsten in the form of catalysts are:
- DeNOx catalysts for the removal of nitrogen oxides from stack gases of combustion power plants, chemical plants, cement plants or diesel engines by selective catalytic reduction with ammonia or urea (Ad Blue); the products are harmless nitrogen and water vapour. Typical DeNOx catalysts are honeycomb-shaped TiO2-WO3-V2O5 ceramics.
- Catalysts for hydrocracking, hydrodesulphurisation (HDS), hydrodenitrogenation (HDN) and hydrodearomatisation (HAD) of mineral oil products, where tungsten and nickel oxides on ceramic carriers are used. These catalysts help to increase the yield of gasoline and other light hydrocarbons in crude oil processing and to make the products more environmentally friendly by reducing the contents of aromatic hydrocarbons, sulphur and nitrogen compounds.
- Other tungsten containing catalysts for various applications in the chemical industry, for example dehydrogenation, isomerisation, polymerisation, reforming, hydration and dehydration, hydroxylation, epoxidation, etc.
Catalyst production usually starts with the highly water soluble ammonium metatungstate, tungstic acid or ammonium paratungstate. In the finished catalyst, tungsten is mostly present in the form of tungsten oxide or sulphide, or in the form of phosphotungstic acid.
- Another example of a catalytically active tungsten compound is superfine tungsten carbide, mostly prepared by thermal decomposition of organic substances in the presence of a suitable tungsten compound. It is mainly used in oxidation/reduction reactions.