How do nitrogen oxides catalyze sulfur dioxide oxidation in sulfuric acid production?

In the historical chamber process for producing sulfuric acid, gaseous nitrogen oxides act as catalysts to facilitate the oxidation of sulfur dioxide into sulfur trioxide, which is subsequently combined with water to form the final product.12

The catalytic mechanism involves the formation of nitrosylsulfuric acid, a crystalline intermediate produced when nitrogen oxides react with sulfur dioxide and other components within the reaction chamber. Because the direct reaction between sulfur dioxide and oxygen is relatively slow, the presence of nitrogen oxides provides an alternative pathway that allows the reaction to proceed at a rate suitable for industrial production.32

Due to the highly corrosive nature of the gaseous mixture involved in this catalytic cycle, the process was traditionally carried out in large, lead-lined chambers. While historically significant, this method has been largely replaced in modern industry by the contact process.12