<p>Ammonia-fueled internal combustion engines commonly exhibit significant unburned ammonia and ammonia slip, which critically impede the realization of their clean-combustion potential. The selective catalytic oxidation of ammonia (NH₃-SCO) is considered an effective method for reducing emissions of unburned ammonia released from engines fueled by ammonia. In this study, Cu-based catalysts on CeO₂, Al₂O₃, and ZrO₂ were synthesized via the incipient-wetness impregnation technique, and their NH₃-SCO activity was systematically evaluated using a simulated gas reaction system coupled with multiple physicochemical characterizations. The results demonstrated that Cu/CeO₂ achieves 90% NH₃ conversion in the temperature range of approximately 300–325&#xa0;°C, showing superior activity compared with Cu/Al₂O₃ and Cu/ZrO₂. For Cu/CeO₂ and Cu/ZrO₂, the Cu²⁺ species predominate and serve as the primary catalytic centers in NH₃-SCO, whereas Cu/Al₂O₃ exhibits a higher fraction of Cu⁺. The smaller crystallites in Cu/CeO₂ facilitated higher dispersion of the active species, whereas the excessive hydroxyl oxygen in Cu/ZrO₂ inhibited catalytic activity. The enhanced Cu-support interaction in Cu/CeO₂ contributes to higher concentrations of surface O_ads and improves the redox behavior of CuO. The combined effect of these properties enhances NHx oxidation during the reaction, resulting in the superior catalytic performance observed for Cu/CeO₂.</p> Graphical Abstract <p></p>