Vanadia-Based Catalysts for the Sulfur Dioxide Oxidation Studied In Situ by Transmission Electron Microscopy and Raman Spectroscopy

The industrial production of sulfuric acid involves the oxidation of sulfur dioxide, which is catalyzed by a silica-supported phase consisting of V2O5 species dissolved in a pyrosulfate melt with Na, K, and Cs added as promoters. As the molten phase is only present during the catalytic reaction and solidifies at room temperature, in situ studies are necessary to address the working state of the SO2 oxidation catalyst. Here we combine transmission electron microscopy (TEM) and Raman spectroscopy to study in situ a vanadia-based SO2 oxidation catalyst upon activation and reaction in an SO2/O-2 gas mixture. The observations reveal that the vanadia phase dynamically redistributes on the support upon heating in an SO2/O-2 mixture. Surprisingly, the vanadia phase can disperse into partially crystalline islands on convex surfaces of the silica support and into a molten state on concave areas of the support. The presence of Cs was found to lower the temperature for the pyrosulfate formation and stabilize vanadium in the active V-v state by forming linked structures at low temperature. Combining these in situ studies with activity measurements leads to the proposal that the linked structures stabilize the catalyst in the active state.