The invention relates to a photocatalyst containing titanium dioxide, to a
method for using it and to its application. A sulphurous titanium dioxide
hydrate precipitate is precipitated from an acid titanium oxysulphate
solution at a temperature below the boiling point of the solution, e.g.
in the range from 70 to 100.degree. C., using crystal nuclei and without
addition of base. The precipitate is separated, washed and calcinated.
The photocatalytic titanium dioxide thus obtained has a specific area in
the range from 100 to 250 m.sup.2/g and a 0.3 to 5% sulphur
concentration. Catalytic activity has been confirmed in asetal dehyde
decomposition and in anionic (SNC.sup.-).sub.2 radical formation.