Preparation of a catalyst for the oxidation of sulfur dioxide to sulfur trioxide

A process for the preparation of a catalyst for the oxidation of sulfur dioxide to sulfur trioxide, said catalyst consisting essentially of a silica-based carrier on which is supported an active substance containing vanadium and alkali metal compounds, said process comprising:

• dissolving vanadium pentoxide in an alkali solution, acidifying the solution with sulfuric acid, mixing the acidified solution with the carrier, molding the mixture, drying and calcining, wherein
• (a) the vanadium pentoxide solution used additionally contains a dissolved alkali metal silicate, silica sol or mixtures thereof
• (b) this solution is brought to ph<2 by adding the solution to the required amount of sulfuric acid, the temperature being kept below 35.degree. C., and
• (c) the acidified solution is immediately mixed with the solid silica-based carrier to give a moist crumbly material, which is then immediately molded, with the proviso that,
• (d) the moist crumbly material contains from 2 to 6% by weight of V.sub.2 O.sub.5 and from 35 to 45% by weight of H.sub.2 O and has a total SiO.sub.2 content of from 15 to 50% by weight, from 5 to 25% by weight of the total silica having been introduced into the moist crumbly material together with the solution from (a).