SO2 promoted alkali metal doped Ag/Al2O3 catalysts for CH4-SCR of NOx

Abstract

A study of the lean NOx reduction activity in the presence of SO2 and water over alkali promoted Ag/Al2O3 catalysts has been done using methane as a reductant. The alkali doped materials are synthesized by the co-impregnation method. Their promotional behavior, existence of several silver species and improved adsorption properties have been thoroughly investigated by various techniques: XRD, XANES, TEM, UV–Vis DRS, SO2 TPD and NO TPD. The evaluated samples exhibited high surface area around 220m2g−1. TEM results demonstrated the presence of highly dispersed nano sized silver particles on surface, where the addition of alkali metals slightly enhanced the crystallization of silver. Moreover, standard XRD profiles of fresh and used samples indicate the high durability and mechanical strength of catalysts. These findings are in line with the time-on-stream studies. The XANES results revealed that the edge spectra of prepared materials are similar to that of the reference Ag2SO4. From XANES and UV–Vis DRS, the presence of crystalline Ag0 and Ag+ species were identified. Poor activity of Na promoted sample is attributed to absence of suitable amount of ionic silver compounds. However, the synthesized alkali doped materials showed the promotional deNOx conversions in the presence of SO2 and H2O stream. Among the investigated samples K–Ag/Al2O3 and Cs–Ag/Al2O3 exhibited higher NOx conversions and thermal stability. The higher SCR of NOx was explained by the NO adsorption properties identified from the NO TPD studies.