Notably, the ex-solution approach requires only a single-step heat treatment in a reducing environment to transform a doped metal oxide into a supported metal catalyst. Specifically, the reduction of host oxides doped with a metallic element causes the spontaneous precipitation of metal nanoparticles over the oxide surface, during which the oxide forms a socket around the nanoparticles and induces a strong metal-support interaction through the build-up of strain at the metal-oxide interface. Owing to these structural advantages, supported metal catalysts prepared by the ex-solution process exhibit significantly enhanced surface activity as well as thermal and chemical stability, showing much promise for applications as highperformance catalysts in solid oxide cell/ electrolysis, [3][4][5] three-way catalyst, [6][7][8] and chemical looping fields. [9][10][11][12] However, ex-solution-based heterogeneous catalysts suffer from critical issues such as the low surface area of host metal oxides, the limited dopant solubility in metal oxides, and the incomplete conversion of doped cations into metal nanoparticles. The critical issue in preparing ex-solved catalysts arises from the high-temperature processing required to incorporate dopants and precipitate the catalytic nanoparticles over a reservoir oxide, thereby causing grain growth and sintering in the host materials and reducing the active surface area of the oxides. Recently, Gorte and co-workers addressed this obstacle by fabricating thin perovskite films via atomic layer deposition, [13] which has a short diffusion length and therefore enables the dopants to readily ex-solve without structural degradation in the support. Another major challenge is that metals with high stability against ionization during calcination are not readily doped into an oxide matrix. Instead, these metals segregate as particles or form a second phase during synthesis, showing limited solubility in parent materials. As a strategy to retain these metal species in an oxidized state, Irvine's group reported on Ba-stabilized Pt oxides as a Pt precursor that can be easily incorporated into the perovskite lattice, which facilitates the ex-solution of Pt with a controllable content. [14] Overall, these works have greatly contributed to improving the viability of the ex-solution approach.Ex-solution catalysts, in which a host oxide is decorated with confined metallic nanoparticles, have exhibited breakthrough activity in various catalytic reactions. However, catalysts prepared by conventional ex-solution processes are limited by the low surface area of host oxides, the limited solubility of dopants, and the incomplete conversion of doped cations into metal catalysts. Here, the design of the host oxide structure is reconceptualized using a metal-organic framework (MOF) as an oxide precursor that can absorb a large quantity of ions while also promoting ex-solution at low temperatures (400-500 °C). The MOF-derived metal oxide host can readily incorporate metal cations, from which catalytic nanoparti...