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.
The main chemical application of tungsten is in the form of catalysts.
- DeNOx catalysts for the removal of nitrogen oxides from combustion power plant stack gases by selective catalytic reduction with ammonia; the products are harmless nitrogen and water vapour. Typical DeNOx catalysts are honeycomb-shaped TiO2WO3V2O5 ceramics.
- Catalysts for hydrocracking, hydrodesulphuration and hydrodenitrification 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, polymerization, reforming, hydration and dehydration, hydroxylation, epoxidation, etc
Catalyst production usually starts with the very water soluble ammonium metatungstate [(NH4)6H2W12O40 . x H2O], tungstic acid [H2WO4] 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 (a "superacid" in the organic chemist’s terms).
Another example of catalytically active tungsten compounds is superfine tungsten carbide, mostly prepared by thermal decomposition of organic substances in the presence of a suitable tungsten compound.
For the manufacture of chemical products, commercially available tungsten compounds such as sodium tungstate, ammonium tungstates, tungstic oxide or tungstic acid are commonly used as raw materials. The following list gives a few examples.
- Inorganic pigments for ceramic glazes and enamels. Tungstic acid or tungsten oxide is used for bright yellow glazes. Tungsten bronzes, i.e. partly reduced alkali and alkaline earth tungstates, are available in many bright colours.
- Barium and zinc tungstate are examples for bright white pigments. Coloured organic dyes and pigments based on phosphotungstic acid and phosphotungsto-molybdic acid are made for paints, printing inks, plastic, rubber and other materials.
- Tungsten disulphide is a lubricant for temperatures above the application range of molybdenum disulphide. It has also been used to form a self-lubricating surface on razor blades.
- Organic tungsten compounds have been patented as viscosity stabilisers in lubricant oils.
In laboratories, tungsten is used in several applications, for example
- High purity sodium tungstate as a reagent in biochemical analysis
- Sodium metatungstate for the preparation of heavy liquids to be used for the separation of minerals by density in mineralogy or for density gradient centrifugation in biochemical analysis
- High purity tungsten granules as an accelerator in the determination of carbon and sulphur in metals by combustion in an induction furnace.