The contact process is a major industrial process for manufacturing sulfuric acid. Sulfur is burned in air to give sulfur dioxide, which is then oxidized to sulfur trioxide with a vanadium pentoxide or platinum catalyst. Arsenic, which poisons the catalyst, must first be removed. The sulfur trioxide is absorbed in concentrated sulfuric acid to yield oleum, which is diluted with water to the concentration required. Direct reduction of sulfur trioxide with water is too violent.