Towards a new step-forward in the Electrochemical Promotion of Catalysis: Development of highly stable Ni nanoparticles by exsolution
Résumé
Catalytic reactions may be promoted in a controlled and reversible manner in electrochemical devices via electrochemical promotion of catalysis (EPOC) [1]. This phenomenon is based on the modification of the local electronic density of the surface of a catalyst coated on a dense electrolyte via the supply of O2- anion. The latter impact the catalytic performances of the catalyst behaving as electronic promoters. So far, the main technological issue of EPOC is related to the use of continuous metallic coatings interfaced on dense solid electrolyte supports. On account of that, the metallic dispersion of the catalyst/electrodes, and therefore their catalytic activity, are usually far lower than that of commercially available dispersed catalysts. Hence, one of the main challenges of the EPOC phenomenon is to combine dispersed catalysts at the nanometric scale with the concept of electrochemical activation via EPOC. In this sense, within the last decade, a powerful phenomena known as exsolution has been identified as a versatile tool allowing a control of the reactivity and durability of metallic particle [2]. Upon the use of ABO3 perovskite materials, certain metals can be inserted within the lattice on the B sites under oxidizing conditions. Upon reductive thermal treatment, the aforementioned metal exsolves, allowing the in situ growth of metallic nanoparticles. Fine tuning of the experimental condition allows the formation of well anchored and well distributed nanoparticles. The non-stoichiometry of the perovskite (i.e. A site deficiency) appears as one of the key parameter driving the exsolution of metal cations to its surface [2]. Recent progress have shown that the exsolution process can be driven electrochemically in fuel cell type devices.
The aim of this work is to prove, for the very first time, that stable and well dispersed metallic nanoparticles on the surface of a perovskite may be obtained via exsolution and their activity could be enhanced via EPOC.
The perovskite chosen was a A site deficient lanthanum calcium titanate (LCT) doped with 6 % of nickel on the B site (i.e. La0.43Ca0.37Ni0.06Ti0.94O3-δ; LCNT6). It was prepared via a citrate sol-gel synthesis [3] with a calcination temperature of 1200 °C leading to a specific surface area of 7 m2·g-1. A reduction was then performed in 5% H2 (i.e. 900 °C, 2 h) in order to exsolve Ni from LCT lattice. The material was then tested for CO oxidation allowing to probe its catalytic performance (see Figure 1). One can observe that the reductive treatment clearly enhanced the catalytic activity of LCNT6. Moreover, in order to study the stability of the Ni nanoparticles formed via exsolution, a hydrothermal treatment was applied (i.e. 900 °C, 24 h, 10% H2O) followed by a subsequent reduction (i.e. 900 °C, 2 h, 5% H2). As observed in Figure 1, the activity of LCNT6 was not affected by this treatment suggesting the good thermal stability of Ni particles regarding aggregation. To further study the stability of the exsolved nanoparticles aging in presence of water and oxygen was performed (i.e. 900 °C, 24 h, 10% H2O, 20% O2) followed by a reduction (i.e. 900 °C, 2 h, 5% H2). The performance of LCNT6 is not affected either by a hydrothermal treatment in presence of oxygen which proves Ni nanoparticles formed via exsolution to possess very good stability properties.
Following the successful formation of stable Ni particles via exsolution on the surface of LCT, electrochemical promotion of catalysis was investigated on this material for CO oxidation.
[1] P. Vernoux et al., Chem. Rev., 113, 8192 (2013)
[2] D. Neagu et al., Nature Chemistry 5, 916 (2013)
[3] N.K.Monteiro et al., International Journal of Hydrogen Energy, 37, 9816, (2012)