Converting Poisonous Sulfate Species to an Active Promoter on TiO₂ Predecorated MnO_<i>x</i> Catalysts for the NH₃-SCR Reaction

Abstract

MnO_<i>x</i>-based catalysts possess excellent low-temperature NH₃ selective catalytic reduction (NH₃-SCR) activity, but the poor SO₂/sulfate poisoning resistance and the narrow active-temperature window limit their application for NO_<i>x</i> removal. Herein, TiO₂ nanoparticles and sulfate were successively introduced into MnO_<i>x</i>-based catalysts to modulate the NH₃-SCR activity, and the active-temperature window (NO conversion above 80%, <i>T</i>₈₀) was significantly broadened to 100-350 °C (SO₄^2--TiO₂@MnO_<i>x</i>) compared to that of the pristine MnO_<i>x</i> catalyst (ca. <i>T</i>₈₀: 100-268 °C). Combined with advanced characterizations and control experiments, it was clearly shown that the poisonous effects of sulfate on the MnO_<i>x</i> catalyst could be efficiently inhibited in the presence of TiO₂ species due to the interaction between sulfate and TiO₂ to form a solid superacid (SO₄^2--TiO₂) species as NH₃ adsorption sites for the low-temperature process. Furthermore, such solid superacid (SO₄^2--TiO₂) species could weaken the redox ability to inhibit the excessive oxidation of NH₃ and thus enhance the high-temperature activity significantly. This work not only puts forward the TiO₂ predecoration strategy that converts sulfate to a promoter to broaden the active temperature window but also experimentally proves that the requirement of redox ability and acidity in the MnO_<i>x</i>-based NH₃-SCR catalyst was dependent on the reaction temperature range.