SO₂ adsorption on silica supported iridium

The interaction of SO₂ with Ir/SiO₂ was studied by simultaneous in situ diffuse reflectance infrared Fourier transform spectroscopy and mass spectrometry, exposing the sample to different SO₂ concentrations ranging from 10 to 50 ppm in the temperature interval 200-400 °C. Evidences of adsorption of sulfur species in both absence and presence of oxygen are found. For a pre-reduced sample in the absence of oxygen, SO₂ disproportionates such that the iridium surface is rapidly saturated with adsorbed S while minor amounts of formed SO₃ may adsorb on SiO₂. Adding oxygen to the feed leads to the oxidation of sulfide species that either (i) desorb as SO₂ and/or SO₃, (ii) remain at metal sites in the form of adsorbed SO₂, or (iii) spillover to the oxide support and form sulfates (SO₄^2-). Notably, significant formation of sulfates on silica is possible only in the presence of both SO₂ and O₂, suggesting that SO₂ oxidation to SO₃ is a necessary first step in the mechanism of formation of sulfates on silica. During the formation of sulfates, a concomitant removal/rearrangement of surface silanol groups is observed. Finally, the interaction of SO₂ with Ir/SiO₂ depends primarily on the temperature and type of gas components but only to a minor extent on the inlet SO₂ concentration.