A Single Model for\nthe Thermodynamics and Kinetics\nof Metal Exsolution from Perovskite Oxides

Exsolution has emerged\nas an outstanding route for producing\noxide-supported\nmetal nanoparticles. For <i>AB</i>O<sub>3</sub>-perovskite\noxides, various late transition-metal cations can be substituted into\nthe lattice under oxidizing conditions and exsolved as metal nanoparticles\nafter reduction. A consistent and comprehensive description of the\npoint-defect thermodynamics and kinetics of this phenomenon is lacking,\nhowever. Herein, supported by hybrid density-functional-theory calculations,\nwe propose a single model that explains diverse experimental observations,\nsuch as why substituent transition-metal cations (but not host cations)\nexsolve from perovskite oxides upon reduction; why different substituent\ntransition-metal cations exsolve under different conditions; why the\nmetal nanoparticles are embedded in the surface; why exsolution occurs\nsurprisingly rapidly at relatively low temperatures; and why the reincorporation\nof exsolved species involves far longer times and much higher temperatures.\nOur model’s foundation is that the substituent transition-metal\ncations are reduced to neutral species <i>within</i> the\nperovskite lattice as the Fermi level is shifted upward within the\nbandgap upon sample reduction. The calculations also indicate unconventional\ninfluences of oxygen vacancies and <i>A</i>-site vacancies.\nOur model thus provides a fundamental basis for improving existing,\nand creating new, exsolution-generated catalysts.