Design of Pd–Metal Oxide Heterojunction Interfaces through Mg–Al–Mn-Based Spinel Oxides for Efficient NO Reduction

Precise control of metal-metal oxide interfaces is crucial for developing efficient heterogeneous catalysts, yet strategies for tuning these junctions remain limited. This study shows that the heterojunction interface between Pd metal and a metal oxide can be controlled by adjusting the composition of Mg-Al-Mn spinel oxides. H₂ reduction at 900 °C induces the aggregation of Pd metal particles (>20 nm) on γ-Al₂O₃ with a defective spinel structure, while MgAl₂O₄ suppresses aggregation, yielding Pd metal nanoparticles with a size of around 10 nm. Interestingly, when the Pd catalyst is supported on MgAlMnO₄, where parts of the Al³⁺ ions are replaced by Mn³⁺ ions, a core-shell structure forms, consisting of Pd-Mn alloy particles (≈10 nm) encapsulated by a thin Mn oxide layer. In NO reduction with C₃H₆ and CO, the reduced Pd/MgAl₂O₄ and Pd/MgAlMnO₄ catalysts exhibit considerably higher activities compared with that of the reduced Pd/Al₂O₃. Among these catalysts, the reduced Pd/MgAlMnO₄ shows superior N₂ selectivity compared with that of the reduced Pd/MgAl₂O₄. The core-shell structure in the reduced Pd/MgAlMnO₄ disassembles during the catalytic reaction; that is, a spontaneous migration of Mn ions slightly exposes the Pd metal surface. This unique structure, formed via the core-shell structure, contributes to high catalytic activity and N₂ selectivity. These results demonstrate that precise composition control of spinel oxides enables the rational design of Pd-oxide interfaces, offering a versatile strategy for developing thermally stable and highly selective catalysts for NO reduction and related reactions.