Bimetallic Fe–Ni exsolution from A site deficient SrTiO3: insight into the reciprocal role of metal active centers

Abstract

In this work, we examine the redox behavior and exsolution kinetics of Ni and Fe in A-site-deficient Sr_0.95Ti_1−x+yFe_xNi_yO_3±δ, combining TPR, XAS, electron microscopy and EPR to elucidate dopant-specific contributions to defect chemistry and metal nucleation. In particular, TPR data indicated that Fe predominantly governs the formation and reducibility of oxygen-vacancy-associated defects, generating reduced Fe species at comparatively low temperature, whereas Ni dictates the overall reduction extent. In fact, as assessed by XAS, Ni-only samples exhibit relatively low conversion to Ni(0), while co-doping with Fe boosts Ni exsolution. Moreover, it turned out that even a tiny Ni amount promotes iron reduction, as unveiled by EXAFS. Along this line, STEM analyses confirmed the surface segregation of well-anchored Fe–Ni co-exsolved nanoparticles for co-doped perovskites, while scarce exsolution was observed for Fe-doped samples. EPR completed the picture corroborating that Fe species, incorporated in the perovskite as highly stable Fe(III) substitutional defects, are less reducible and less mobile than Ni ones, which instead appear mostly as intercalated sites. Such Fe defects become mobile and undergo surface segregation only when Ni is incorporated in the lattice. These structure–property relationships directly translate into enhanced performance in the RWGS reaction of co-doped perovskites, where Fe drives vacancy-mediated redox processes while Ni promotes H₂ dissociation. Overall, the results indicate that the methodological approach developed here can be extended to other exsolvable dopants (e.g., Co), offering a pathway toward the rational design of exsolved catalysts with tunable structures and compositions for advanced energy-conversion applications.