Review of Sulfur Promotion Effects on Metal Oxide Catalysts for NOx Emission Control

Sulfur poisoning is a common problem causing catalyst deactivation during the industrial catalytic process. However, for selective catalytic reduction of NOx with ammonia (NH3-SCR) technology, the formation of sulfate species from SO2 in flue gas could remarkably improve NH3-SCR performance by adjusting the surface acidity of the metal oxides. This Review systematically summarizes recent advances concerning sulfur promotion roles of metal oxide catalysts for the NH3-SCR reaction. First, the effects of sulfation treatment on the properties of sulfate species are discussed and compared in detail. The synthesis procedures and the critical parameters of gas-phase and liquid-phase methods are introduced. Second, it is tentative to conclude how the sulfation treatments coordinate acid and metal sites and propose a comprehensive overview of the reaction mechanisms over sulfated catalysts. Subsequently, it summarizes the effective methods to construct appropriate acidity and reducibility of sulfated metal oxides by exposing crystal planes and creating specific structures, including CeO2-, Fe2O3-, V2O5-, and CuO-based catalysts. The corresponding reaction pathways are also deliberated over specific catalysts. Finally, current scientific issues and prospects of sulfated catalysts are put forward, and other acidification methods are supposed to improve the surface acidity effectively. For future NH3-SCR technology, the experimental and mechanistic discussions in this Review will be beneficial to understand reaction chemistry and design highly efficient NH3-SCR catalysts.