Combined effects Na and SO₂ in flue gas on Mn-Ce/TiO₂ catalyst for low temperature selective catalytic reduction of NO by NH₃ simulated by Na₂SO₄ doping

Highlights: • Sodium sulfate (Na₂SO₄) was deposited on Mn-Ce/TiO₂ catalyst to simulate the co-existing of sodium and SO₂ in the flue gas. • Na₂SO₄ had strong and fluctuant influence on Mn-Ce/TiO₂ catalyst's performance in SCR of NOx with NH₃, due to the combined effect of the deactivation of sodium salts and the enhanced performance of ceria with surface sulfation. • The changes of the surface chemical species and acid sites on the Na₂SO₄ deposited catalysts could be considered as the main reasons for the fluctuation changes with the catalytic activity. - Abstract: A series of Mn-Ce/TiO₂ catalysts were synthesized through an impregnation method and used for low temperature selective catalytic reduction (SCR) of NOx with ammonia (NH₃). Na₂SO₄ was added into the catalyst to simulate the combined effects of alkali metal and SO₂ in the flue gas. Experimental results showed that Na₂SO₄ had strong and fluctuant influence on the activity of Mn-Ce/TiO₂, because the effect of Na₂SO₄ included pore occlusion and sulfation effect simultaneously. When Na₂SO₄ loading content increased from 0 to 1 wt.%, the SCR activities of Na₂SO₄-doped catalysts decreased greatly. With further increasing amount of Na₂SO₄, however, the catalytic activity increased gradually. XRD results showed that Na₂SO₄ doping could induce the crystallization of MnOx phases, which were also confirmed by TEM and SEM results. BET results showed that the surface areas decreased and a new bimodal mesoporous structure formed gradually with the increasing amount of Na₂SO₄. XPS results indicated that part of Ce⁴⁺ and Mn³⁺ were transferred to Ce³⁺ and Mn⁴⁺ due to the sulfation after Na₂SO₄ deposition on the surface of the catalysts. When the doped amounts of Na₂SO₄ increased, NH₃-TPD results showed that the Lewis acid sites decreased and the Brønsted acid sites of Mn-Ce/TiO₂ increased quickly, which could be considered as another reason for the observed changes in the catalytic activity. The decreased Mn and Ce atomic concentration, the changes of their oxidative states, and the variation in acidic properties on the surface of Na₂SO₄-doped catalysts could be the reasons for the fluctuant changes of the catalytic activity.