Resistance to sulfidation and catalytic performance of titanium–tin solid solutions in SO 2 + CO and NO + SO 2 + CO reactions

Abstract

The resistance of TiO 2–SnO 2 solid solutions with different ratios to the sulfidation in the reduction of sulfur dioxide (SO 2) 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 SnS 2 is found rather than bulk tin sulfides in the solid solution catalysts after reactions. The resistance of TiO 2–SnO 2 solid solutions is rationalized by the formation of solid solutions (not mechanical mixtures) of TiO 2 and SnO 2, by the excellent stability of TiO 2 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 Ti 0.88Sn 0.12O 2 catalyst can be sulfided neither by the SO 2 + CO reaction nor by TPS of H 2S. As the tin amount increased, TiO 2–SnO 2 solid solutions might separate into two microscopic agglomeration groups: one is rich in Ti 4+ which cannot be sulfided like TiO 2, the other is rich in Sn 4+ which can be easily sulfided like pure SnO 2. The higher tin content means that more Sn 4+-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 SO 2 and NO conversions were obtained at a balanced composition of a weight ratio of 1:1 of TiO 2 and SnO 2 for the SO 2 + CO and NO + SO 2 + CO reactions. SO 2 in the flue gas is a great promoter to the latter reaction.