ESR spectroscopy as a powerful tool for monitoring the exsolution process in Cu-doped SrTiO3 catalytic system

Supported nanoparticle systems have received increased attention over the last decades because of their potential for high activity levels in catalytic conversions though, due to their nanoscale nature, they tend to exhibit problems with long-term durability. Mostly in the frame of perovskite-type, the recent discovery of the so-called redox exsolution has addressed many of these challenges, providing a relatively simple, single-step, synthetic pathway to produce supported metal NPs combining high stability against agglomeration and poisoning, and superior catalytic properties [1]. This approach represents an “inside-out” route where the guest elements (e. g. Cu, Co, Ni ions), via usage of high temperature and low oxygen partial pressure, get reduced to elemental state and nucleate on the surface as NPs [2]. Since the process occurs under reducing conditions, by exposing the material to an oxidative atmosphere, the reaction can be reversed, and the metal NPs can be redissolved in the lattice [3]. The concentration, position, and type of defects in the matrix play an important role in the effectiveness of exsolution [4]. Therefore, the knowledge of defect chemistry of the oxide and redox behavior of metals are mandatory to understand and monitor the process. In this context, the present work aims at the preparation and characterization of Cu-doped SrTiO3 systems exploiting redox exsolution, with a special focus on monitoring, during the process, the evolution of the perovskite defectivity as well as the variation of metal oxidation state and coordination environment by Electron Spin Resonance (ESR). In detail, quasi-in-situ measurements have been carried out to monitor the modifications of the paramagnetic defects during the exsolution in terms of change in coordination and oxidation state of Cu2+ ions and as regards the generation/evolution/annihilation of O and Sr vacancies (VO and VSr) [5], [6]. To this end, the sample has been heated up to 900° C under Ar/H2 reducing atmosphere to promote the exsolution. The process was divided into several steps, at the end of which the sample was quenched at the temperature of liquid N2 to “freeze” the process and the ESR spectra recorded at 130 K. Changes in the Cu2+ signal intensity has been retrieved as well as the formation of VO, VSr and Ti3+ species. Afterwards the reduced powders underwent re-oxidation in O2 to assess the reversibility of the procedure and spectra were acquired. XPS, UV-DRS and HR-TEM analysis before and after exsolution were also performed to corroborate the modification of defect structure. This work represents the beginning of a new way of monitoring the redox exsolution process in technologically relevant materials by using non complicated techniques.