Resistance to sulfidation and catalytic performance of titanium–tin solid solutions in SO2 + CO and NO + SO2 + CO reactions Original Research Article

The resistance of TiO2–SnO2 solid solutions with different ratios to the sulfidation in the reduction of sulfur dioxide (SO2) and/or nitric oxide (NO) by carbon monoxide (CO) has been studied by temperature-programmed sulfiding (TPS), X-ray powder diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). The catalytic performance has been related to these resistance results. A surface chemisorbed sulfur species which can be written as SnS2 is found rather than bulk tin sulfides in the solid solution catalysts after reactions. The resistance of TiO2–SnO2 solid solutions is rationalized by the formation of solid solutions (not mechanical mixtures) of TiO2 and SnO2, by the excellent stability of TiO2 in sulfur-containing gases, and by the bigger relative electronegativity of Sn than Ti. This leads to an extended electronic effect together with the lower stability of tin sulfides than titanium sulfides. The Ti0.88Sn0.12O2 catalyst can be sulfided neither by the SO2 + CO reaction nor by TPS of H2S. As the tin amount increased, TiO2–SnO2 solid solutions might separate into two microscopic agglomeration groups: one is rich in Ti4+ which cannot be sulfided like TiO2, the other is rich in Sn4+ which can be easily sulfided like pure SnO2. The higher tin content means that more Sn4+-rich agglomerate is available, which would suggest that: (1) the solid solution is more oxidative; and (2) the solid solution is more prone to be sulfided. Under the two interactions, the highest SO2 and NO conversions were obtained at a balanced composition of a weight ratio of 1:1 of TiO2 and SnO2 for the SO2 + CO and NO + SO2 + CO reactions. SO2 in the flue gas is a great promoter to the latter reaction.