Selective catalytic reduction of NO with NH₃ on SO^-2 ₄/TiO₂ superacid catalyst

SO^-2₄/TiO₂ superacid is studied for selective catalytic reduction (SCR) of NO with NH₃. Since SO^-2 ₄/TiO₂ is formed on TiO₂ under SCR reaction conditions when SO₂ is present, the results also elucidate the role of TiO₂ support (and SO₂) in the SCR reaction. SO^-2 ₄/TiO₂ exhibits a considerable activity, exceeding that of V₂O₅/TiO₂ at temperatures above 400^°C, and reaches a peak activity at 500-525^°C. TGA, XPS, and IR analyses of the catalyst indicate that SO^-2 ₄ ions are formed on TiO₂ and the structure of the TiO₂ surface sulfate under the SCR reaction conditions is bridged bidentate, Ti₂SO₃(OH). XPS and chemisorption measurements along with SCR kinetic results indicate that the reaction takes place by an Eley-Rideal mechanism, on Brønsted-acid sites on SO^-2 ₄/TiO₂ surface. At 400^°C. about 70% of the SO^-2 ₄ ions are occupied by ammonia, and about a half of the chemisorbed ammonia is active for reaction with NO.