Formation and Decomposition of Sulfite and Sulfate Species on Pt/Pd Catalysts: An SO₂ Oxidation and Sulfur Exposure Study

SO 2 oxidation studies were conducted with Pd/Pt mono- and bimetallic, Al 2 O 3 -supported catalysts. Preliminary reactor studies showed that catalysts with higher Pt content were more active in terms of apparent SO 2 to SO 3 oxidation based on outlet gas-phase measurements. Diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) studies showed that higher Pd content catalysts were more active for oxidizing surface sulfur species at low temperatures. Due to this apparent disconnect, the work herein focused on investigating how oxidation products formed on the monometallic catalyst surface influence the catalysts’ apparent activity for the SO 2 oxidation reaction. In addition to precious metal type, oxidation state and gas environment were shown to influence SO 2 uptake and oxidation activity as well as surface sulfur oxidation product type and formation amount. In the presence of oxygen, Pd uptakes less SO 2 than Pt, but inhibiting species, such as palladium sulfates, are stabilized on the catalyst surface up to higher temperatures. This work provides evidence that Pd/Pt catalyst SO 2 oxidation characteristics, as well as the potential for sulfur inhibition, are governed by the sulfur species formed on the surface.