Vanadia-Based Catalysts for the Sulfur Dioxide Oxidation Studied <i>In Situ</i> 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, <i>in situ</i> 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 <i>in situ</i> a vanadia-based SO₂ oxidation catalyst upon activation and reaction in an SO₂/O₂ gas mixture. The observations reveal that the vanadia phase dynamically redistributes on the support upon heating in an SO₂/O₂ 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 <i>in situ</i> studies with activity measurements leads to the proposal that the linked structures stabilize the catalyst in the active state