Reversibility limitations of metal exsolution reactions in niobium and nickel co-doped strontium titanate

Abstract

Metal exsolution reactions enable the preparation of metal-oxide nano-composites from oxide parent materials in a single thermal reduction step. In this process, reducible metals are released from the doped oxide and nucleate in the form of finely dispersed, supported nanoparticles. A reversible exsolution and re-dissolution reaction could provide an effective way to regenerate catalysts, where the surface structure and functionality dynamically adapt to the ambient gas environment. However, the reversibility of exsolution reactions is often limited. We investigate reversibility limitations in the niobium and nickel co-doped perovskite SrTi_0.95−xNb_0.05Ni_xO_3−δ with varying Ni doping concentrations between x = 0.005–0.1. Combined morphological, structural and chemical analyses of the material response upon consecutive thermal treatments in reducing and oxidizing environments reveal a non-correlated bulk and surface response of the material upon redox treatment. While the bulk structural changes are mostly reversible, no re-dissolution of the exsolved surface nanoparticles is detected for the investigated time–temperature window (T = 800 °C, t = 5 h for reduction and reoxidation, respectively). Instead, a modification in the nanoparticle distribution and an increased surface wetting of the support by the exsolved metal species are observed upon reoxidation of the nanoparticles.