An investigation of alumina-supported catalysts for the selective catalytic oxidation of ammonia in biomass gasification

Alumina-supported catalysts containing different transition metals (Ni, Cu, Cr, Mn, Fe and Co) were prepared and tested for their activity in the selective oxidation of ammonia reaction at high temperatures (between 700 and 900 °C) using a synthetic gasification gas mixture. The catalysts were also characterised for their acidic properties by infrared studies of pyridine and ammonia adsorption and reaction/desorption. The Ni/Al2O3 and Cr/Al2O3 catalyst displayed the highest selective catalytic oxidation (SCO) activity in that temperature range with excellent N2 selectivities. FT-IR studies of adsorbed pyridine and NH3 indicate that Lewis acid sites dominate and that NH3 adsorption on these sites is likely to be the first step in the SCO reaction. FT-IR studies on less active catalysts, particularly on Cu/Al2O3 allowed the detection of oxidation intermediates, amide (NH2), and possibly hydrazine and imido and nitroxyl species. The amide and hydrazine intermediate gives credence to a proposed SCO mechanism involving a hydrazine intermediate, while the proposed imide, N–H, and/or nitroxyl, HNO species could be intermediates in incomplete oxidation of NH3 to N2O.