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

Various NH3-SCO catalysts highly promising to reduce NH3 emission are summarized and classified considering of N-2-selectivity (S-N2) at T-100% (starting temperature for complete NH3 conversion) in previous scientific researches: noble metals (e.g. Ag degrees/Al2O3, Pd/Ru-modified zeolite with SN2>90% at T-100% of 150-300 degrees C), transition-metal oxides (e.g. Fe-based, Ce-V compounds, S-N2>90% at T-100% of 300-450 degrees C, except Cu-containing), molecular sieves (Fe- or/and Cu-modified ZSM-5 with similar to 100% S-N2 at T-100% = 400 degrees C, meso/micro-porous Cu-modified SSZ, Beta, SAPO with N-2-selectivity >95% at 250-350 degrees 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% S-N2 at T-100% = 150-250 degrees C); (ii) translation (rare-earth) metals modified oxides (Cu-CeOx, Cu-FeOx, etc., almost 100% at similar to 250 degrees C); (iii) CuO/Al2O3 and/or CuAl2O4, CuFe2O4, etc., >95% at 300-350 degrees C); (iv) Cu-doped Mg, Al, Fe, Zn complex metal-oxides (90% N-2-selectivity at similar to 450.C). The NH3-SCO catalytic oxidation products are strongly obeyed the differentiated reaction routes of NH3 dehydrogenation, N-2-, 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.

Automated summary

The article summarizes various NH3-SCO catalysts with promising potential to reduce NH3 emissions, classifying them based on N-2-selectivity at different temperatures. It specifically highlights several promising copper-based catalysts, discusses reaction paths of NH3-SCO catalytic oxidation products, and emphasizes the importance of controlling NH3 dehydrogenation pathways.