Selective catalytic oxidation of ammonia to nitrogen over ceria–zirconia mixed oxides

The selective catalytic oxidation of ammonia to nitrogen (NH3-SCO) has been studied over ceria–zirconia mixed oxides (Ce1−xZrxO2). The addition of Zr into ceria leaded to the phase transition from the cubic fluorite structure to the tetragonal structure and the generation of oxygen vacancies. The Ce1−xZrxO2 (0.2 ⩽ x ⩽ 0.8) mixed oxide catalysts exhibited certain amount of moderate acid sites. Particularly, the Ce0.4Zr0.6O2 catalyst achieved the largest amount of moderate acid sites and highest proportion of Ce3+ (oxygen vacancies). Meanwhile, it showed the best NH3 oxidation activity, and the complete conversion temperature was about 360 °C. In addition, numerical results also indicated that the higher zirconium content in Ce1−xZrxO2 catalysts improved the N2 selectivity which was about 100% with x > 0.4. The formation of N2O was the main reason for low N2 selectivity over these catalysts (x ⩽ 0.4) proved by TPSR experiments. Meanwhile, the reaction mechanism of NH3 with lattice oxygen was observed to be different from the gaseous oxygen, and the gaseous oxygen species was much more active than lattice oxygen for NH3 oxidation.

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► It was the first time to report the Ce1−xZrxO2 catalysts for SCO of ammonia to N2.
► The Ce1−xZrxO2 (x = 0.2–0.6) catalysts showed high activity for ammonia oxidation.
► The phase transformation and formed moderate acid sites favored the SCO of ammonia.
► The Zr-rich (tetragonal) phase (x > 0.4) improved the N2 selectivity of the catalyst.