Ag-Cu based catalysts for the selective ammonia oxidation into nitrogen and water vapour

Abstract

XRD, BET, H2-TPR, UV–vis-DRS, XPS and XAFS were used to characterize a series of Ag and/or Cu – Ag (1–5%), Cu (10–15%) or Ag-Cu (1–1, 1–10, 1.5–10, 5–5% of metal) – supported on γ-Al2O3. Correlation between physicochemical properties, catalytic activity and selectivity in NH3-SCO were thoroughly investigated. Silver species mainly in the form of Ag2O on the Ag/Al2O3 catalysts led to enhanced activity together with drop in N2 selectivity with increasing silver loading up to 5%. A mixture of CuO and CuAl2O4 formed on the Cu/Al2O3 catalysts. Easily reducible highly dispersed CuOx promoted the activity of the catalysts, while bulk CuOx and CuAl2O4 decreased N2 selectivity up to 500 °C. The activity of all Cu-containing materials was inferior to Ag-containing ones. Thus, the gap between the high conversion temperature over Cu/Al2O3 and low N2 selectivity over Ag/Al2O3 was bridged by applying the Ag-Cu/Al2O3 catalysts, with the optimum loading of 1.5 and 10 wt.% for silver and copper, respectively. NH3-TPD, NH3-TPSR and in situ FTIR were used to determine the in situ selective catalytic reduction (i-SCR) mechanism over 1.5% Ag/Al2O3, 10% Cu/Al2O3 and 1.5% Ag-10% Cu/Al2O3 respectively. The i-SCR mechanism involved the partial oxidation of NH3 into NOx species, along with adsorbed NOx species interacting with adsorbed ammonia (NHx species) and being reduced to reaction products.