Catalytic oxidation of ammonium ion in water with ozone over metal oxide catalysts

• Co3O4 was an excellent catalyst for ozonation of NH4+ in water.
• Selectivity to N2 over Co3O4 was 88%.
• Metal oxide catalysts with low ΔH°f showed high selectivity to gaseous products.
• Cl− in water was involved in catalyst cycle over Co3O4.

Oxidative decomposition of NH4+ (10 mmol L−1) with O3 in water was studied at 333 K over a variety of metal oxide catalysts without pH control of the solution. Although MgO and NiO had the highest catalytic activities, large amounts of undesired NO3− formed due to low selectivity to gaseous products as well as high activity. Co3O4, which was slightly less active than MgO and NiO, was the best catalyst in terms of activity, selectivity to gaseous products, and dissolution degree among the metal oxide catalysts studied. Over Co3O4, NH4+ was selectively oxidized to N2 with 88% selectivity in water, and the dissolution degree of Co3O4 was less than 1%. Fe2O3, SnO2, Mn3O4, CuO, MgO, and Al2O3 were less selective to gaseous products or much less active for the reaction. The selectivities to gaseous products were strongly related to the standard enthalpy changes of formation per mol of oxygen atom (ΔH°f) of the metal oxides. The metal oxide catalysts with low ΔH°f, like Co3O4, showed high selectivity to gaseous products probably due to the low surface density of the active oxygen formed from O3 on the catalysts. Chloride ions (Cl−) present in the reaction solution significantly accelerated the reaction rate for NH4+ decomposition with O3 in the presence of Co3O4. This was due to the involvement of Cl− in the catalytic cycle. For instance, ClO−, which may form by the reaction of Cl− with O3 over Co3O4, could further oxidize NH4+.

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