You haven't specified which catalyst system (and what oxidant) you are interested in. For the commercially used vanadium catalysts with air as oxidant see for example DOI: 10.1021/i260044a020 or http://eportfolio.lib.ksu.edu.tw/~T093000207/repository/fetch/(6)Oxidation%20of%20sulfur%20dioxide%20to%20sulfur%20trioxide%20over%20supported%20vanadium%20catalyst.pdf. A lot more information is available on the net for this and for another systems.
I suggest that you read article number, 8.4 (pages: 478-490 including an example) from Elements of Chemical Reaction Engineering 3rd Edition by H. Scot Folgler.