Advances in selective catalytic oxidation of ammonia (NH3–SCO) to dinitrogen in excess oxygen: A review on typical catalysts, catalytic performances and reaction mechanisms

Abstract

Various NH3–SCO catalysts highly promising to reduce NH3 emission are summarized and classified considering of N2–selectivity (SN2) at T100% (starting temperature for complete NH3 conversion) in previous scientific researches: noble metals (e.g. Ag°/Al2O3, Pd/Ru–modified zeolite with SN2>90% at T100% of 150–300 °C), transition–metal oxides (e.g. Fe–based, Ce–V compounds, SN2>90% at T100% of 300–450 °C, except Cu–containing), molecular sieves (Fe– or/and Cu–modified ZSM–5 with ∼100% SN2 at T100% = 400 °C, meso/micro–porous Cu–modified SSZ, Beta, SAPO with N2–selectivity >95% at 250–350 °C). The promising Cu–based catalysts gratifyingly include: (i) excellent noble–metals doped–CuO (CuO/RuOx, CuO–RuOx/Al–ZrOx, Ag–CuO, etc., almost 100% SN2 at T100% = 150–250 °C); (ii) translation (rare–earth) metals modified oxides (Cu–CeOx, Cu–FeOx, etc., almost 100% at ∼250 °C); (iii) CuO/Al2O3 and/or CuAl2O4, CuFe2O4, etc., >95% at 300–350 °C); (iv) Cu–doped Mg, Al, Fe, Zn complex metal–oxides (90% N2–selectivity at ∼450 °C). The NH3–SCO catalytic oxidation products are strongly obeyed the differentiated reaction routes of NH3 dehydrogenation, N2–, Langmuir–Hinshelwood, Eley-Rideal and internal-SCR mechanisms. Important ideas for hierarchical–controlling NH3 dehydrogenation should be given more attentions on improving both activity and selectivity that mainly determined by the surface oxygen coverage and also reaction temperature. Finally, future trends are urgently addressed on operating in a low–broad temperature range with simulating exhaust mixture-gases, imperatively considering the possible changes on designing the integrated structured catalysts and on testing large–scale–up effects typical for real flue gases.