The strong metal–support interaction (SMSI) prompts the reductive treatment-induced encapsulation of platinum group metal (PGM) nanoparticles by reducible oxide catalyst supports and therefore often leads to negative outcomes, such as a decrease in catalytic activity. Herein, the SMSI effect was found to benefit a model reaction of automotive exhaust gas purification, namely the catalytic reduction of NO with C₃H₆ and CO over a Pd/Ca₂AlMnO_5+δ. The reductive treatment of Pd/Ca₂AlMnO_5+δ induced epitaxial growth of the (111) plane of the MnO–CaO solid solution on the (111) plane of the Pd metal core to produce Mn-doped CaO shells. Experimental results and theoretical calculations indicated that the bimetallic oxide surface of the Mn-doped CaO(111) plane exhibited excellent catalytic activity for NO reduction despite the absence of a Pd metal surface. Thus, this study not only demonstrates that transition-metal sites can exhibit high catalytic activity, similar to that of a Pd metal surface, but also provides a design guideline for environmental catalysts.