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. H 2 reduction at 900 °C induces the aggregation of Pd metal particles (>20 nm) on γ-Al 2 O 3 with a defective spinel structure, while MgAl 2 O 4 suppresses aggregation, yielding Pd metal nanoparticles with a size of around 10 nm. Interestingly, when the Pd catalyst is supported on MgAlMnO 4, where parts of the Al 3+ ions are replaced by Mn 3+ 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 C 3 H 6 and CO, the reduced Pd/MgAl 2 O 4 and Pd/MgAlMnO 4 catalysts exhibit considerably higher activities compared with that of the reduced Pd/Al 2 O 3 . Among these catalysts, the reduced Pd/MgAlMnO 4 shows superior N 2 selectivity compared with that of the reduced Pd/MgAl 2 O 4 . The core–shell structure in the reduced Pd/MgAlMnO 4 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 N 2 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.