Effect of Microwave Activation on the Activity of Multiphase Heterogeneous CuO/CuFe2O4 Mixed Catalysts for NH3–SCR of NO

Marine main engines commonly operate at low speed and low load to achieve economical cruising, which lowers the exhaust temperature. Developing catalysts for the selective catalytic reduction of NO with ammonia (NH3–SCR) that are active at low temperature and over a wide temperature window is therefore essential. In this work, multiphase heterojunction CuO/CuFe2O4 spinel catalysts were synthesized by a microwave-assisted hydrothermal method. Performance testing showed that the CF-MW-5 catalyst (Cu:Fe = 1.07:2, 5 min of microwave treatment) gave the best performance. Notably, in a continuous 60 h test at 230 °C that included 33 h under 200 ppm of SO2 and 5 vol % H2O, its NO removal efficiency never fell below 86%. Comprehensive characterization showed that the microwave-assisted synthesis was accompanied by partial reduction of Cu2+ and Fe3+ ions and by enrichment of the surface in adsorbed-oxygen and vacancy-related species, providing abundant active sites for the SCR reaction. Crucially, a structure–activity analysis shows that the catalytic activity scales neither with the total surface acidity nor with the total active-oxygen fraction─both of which increase monotonically with irradiation time─nor with the surface area, but instead follows a volcano-type relationship that closely mirrors the surface Cu+/Fe2+ redox-couple density and the population of low-temperature (weak) acid sites. After normalizing the apparent rate constants to unit surface area, CF-MW-5 still exceeds the untreated sample by a factor of 1.69, showing this improvement to be intrinsic rather than merely textural. We propose that these coupled Cu+/Fe2+ sites sustain the redox cycle while acting as Lewis acid centers that govern low-temperature NH3 activation, together rationalizing the wide operating window; the elementary reaction pathway is offered as a hypothesis consistent with these data rather than as an established mechanism. This work therefore moves beyond synthesis optimization to establish which surface descriptors do, and which do not, control low-temperature NH3–SCR over microwave-engineered spinel catalysts.