Mechanistic Investigation into the Effect of Sulfuration on the FeW Catalysts for the Selective Catalytic Reduction of NOx with NH3.

Abstract

Iron tungsten (FeW) catalyst is a potential candidate for the selective catalytic reduction (SCR) of NOx with ammonia because of its excellent performance in a wide operating window. Sulfur poisoning effects in SCR catalysts have long been recognized as a challenge in development of efficient catalysts for applications. In this paper, the impact of sulfuration on catalyst structure, NH3-SCR reaction performance and mechanism was systematically investigated through spectroscopic and temperature-programmed approaches. The sulfuration inhibited the SCR activity at low temperatures (<300 °C), while no evident effect was observed at high temperatures (≥300 °C). After sulfuration for FeW oxides catalyst, the organic-like with covalent S═O bonds sulfate species were mainly formed over the FeW catalysts. Combining TPD with in situ DRIFTS results, it was found that the Lewis and the Brønsted acidity were enhanced by the interaction between metal species and sulfate species due to the strong electron withdrawing effect of the S═O double bonds. The in situ DRIFTS study showed that the formation of NO2 was hindered, leading to the "fast-SCR" pathway was partly cut off by the sulfuration process and thereby the loss of SCR activity at low temperatures. However, the Langmuir-Hinshelwood reaction pathway between adsorbed NH3/NH4+ species and nitrate species was facilitated and dominated at high temperatures, making the as-synthesized FeW catalysts resistant to SO2 poisoning.