Smart material concept: reversible microstructural self-regeneration for catalytic applications

Abstract

a This paper presents a proof-of-concept study and demonstrates the next generation of a "smart" catalyst material, applicable to high temperature catalysis and electro-catalysis such as gas processing and as a catalyst for solid oxide cells. A modified citrate-gel technique was developed for the synthesis of La x Sr 1À1.5x Ti 1Ày Ni y O 3Àd . This method allowed the synthesis of single phase materials with a high specific surface area, after the first calcination step at temperatures as low as 650 C. Up to 5 at% of nickel could be incorporated into the perovskite structure at this low calcination temperature. X-ray powder diffraction and microscopy techniques have proven the exsolution of nickel nanoclusters under low oxygen partial pressure. The amount of exsoluted nickel nanoparticles was sensitive to surface finishing, whereby much more exsoluted nanoparticles were observed on pre-treated and polished surfaces as compared to original ones. Increasing A-site deficiency leads to a larger number of nickel particles on the surface, indicating a destabilizing influence of the A-site vacancies on the B-site metal cations.Repetitive redox cycles prove that the nickel exsolution and re-integration is a fully reversible process.These materials working in a cyclic and repetitive way may overcome the drawbacks of currently used conventional catalysts used for high temperature systems and overcome major degradation issues related to catalyst poisoning and coarsening-induced aging.

Structural Reversibility and Nickel Particle stability in Lanthanum Iron Nickel Perovskite‐Type Catalysts

“…This indicates that the reincorporation temperature is not strongly affected by the Ni content for x ≤0.2. A comparison with the higher temperatures reported for Ni incorporation into La‐doped strontium titanates (another class of PMOs in which Ni exhibits reversible segregation), suggests that the reincorporation temperature is influenced rather by the nature of the other A‐ and B‐site cations in the structure . This behavior is essentially identical to that of precious‐metal‐substituted LaFeO 3 .…”

“…Conversely, the self‐regenerating properties of PMOs demonstrated for precious metals have been scarcely studied with Ni. Recent findings suggest that this self‐regeneration behavior can also be applied to Ni perovskites, at least for high‐temperature processes . We have shown that a careful selection of the PMO composition produces suitable anodes for solid oxide fuel cells with the capability to segregate and reincorporate Ni at relevant temperatures (>800 °C).…”

Exsolution of Nickel Nanoparticles from Mixed‐Valence Metal Oxides: A Quantitative Evaluation by Magnetic Measurements

“…Eminent examples of the strategic use of nanomaterials include biomedical devices, inks, advanced filters, and semiconductors . The wide diffusion of the nanomaterials is mainly due to their unique features such as the nanoconfinement, high surface‐to‐volume ratio, and a large specific surface area . In catalysis, an extensive reactive surface area allows for a significant increment of the active sites and of their exposure to the reactants .…”

Structural Reversibility of LaCo1‐xCuxO3 Followed by In Situ X‐ray Diffraction and Absorption Spectroscopy

“…Attempts to rationalize the mechanism of this self‐regeneration concept and to provide guidance for further development were made by theory, while electron microscopy was used to critically assess the extent and practical relevance of this property . Recently, the segregation‐dissolution approach was demonstrated for characteristic anode materials of SOFC with nickel as the active metal . The boundary conditions necessary to achieve the self‐regeneration of La−Fe−Ni−O mixed oxides claimed earlier were defined using temperature programmed reduction, X‐ray diffraction, X‐ray absorption spectroscopy and electron microscopy …”

“…[15] Recently, the segregation-dissolution approach was demonstrated for characteristic anode materials of SOFC with nickel as the active metal. [16] The boundary conditions necessary to achieve the self-regeneration of LaÀFeÀNiÀO mixed oxides claimed earlier [17] were defined using temperature programmed reduction, X-ray diffraction, X-ray absorption spectroscopy and electron microscopy. [18] Cobalt exhibits the most reducible perovskite-type oxide among the simplest and most common LaMnO 3 , LaFeO 3 and LaCoO 3 oxidation catalysts.…”