Abstract: A gas mixture comprising molecular oxygen and 15 to 60 vol-% SO2 flows through a first catalyst layer which contains a catalyst containing vanadium pentoxide, and directly subsequently through a second catalyst layer which contains a catalyst containing iron. With an inlet temperature of 350 to 600° C., the gas mixture is introduced into the first catalyst layer which contains a granular V2O5 catalyst and 20 to 80 wt-% catalytically inactive inert material. Directly subsequently, the gas mixture is introduced into the second catalyst layer with a temperature of 500 to 750° C. Preferably, the catalyst of the second catalyst layer contains 3 to 30 wt-% arsenic oxide. There is produced an SO3-containing product gas with a volume ratio of SO2: SO3 of not more than 0.1.

Abstract: A catalyst for reacting SO2 with molecular oxygen to form SO3 is suited for a continuous operation at temperatures of 700° C. and above, when the same consists of a carrier and an active component connected with the carrier, the active component consists of 10 to 80 wt-% iron, the carrier has a BET surface of 100 to 2000 m2/g and an SiO2 content of at least 90 wt-%, and the weight ratio carrier:active component is 1:1 to 100:1.