The sulfur trioxide is produced by burning sulfur to produce sulfur dioxide , which was then oxidized to sulfur trioxide with a catalyst .
When they reach temperatures above 300 Â C , sulfuric acid begins to decompose into sulfur trioxide and water , both in the gas phase . Sulfur trioxide is highly reactive and dissociates into sulfur dioxide and atomic oxygen , which oxidizes traces of carbon monoxide to form carbon dioxide .