The design of nanoporous perovskite oxides is considered an efficient strategy to develop performing, sustainable catalysts for the conversion of methane. The dependency of nanoporosity on the oxygen defect chemistry and the catalytic activity of perovskite oxides towards CH 4 and CO oxidation was here studied. A novel colloidal synthesis route for nanoporous, high-temperature stable SrTi 0.65 Fe 0.35 O 3-δ with specific surface area (SSA) ranging from 45 to 80 m 2 /g and pores from 10 to 100 nm was developed. High-temperature investigations by in situ synchrotron XRD and TG-MS combined with H 2 -TPR and Mössbauer spectroscopy showed that the porosity improved the release of surface oxygen and the oxygen diffusion, whereas the release of lattice oxygen depended more on the state of the iron species and strain effects in the materials. Regarding catalysis, light-off tests showed that low-temperature CO oxidation significantly benefitted from the enhancement of the SSA, whereas high-temperature CH 4 oxidation is influenced more from the dioxygen release. During isothermal long-term catalysis tests however the continuous oxygen release from large SSA materials promoted both CO and CH 4 conversion. Hence, if SSA maximization turned out to efficiently improve low-temperature and long-term catalysis applications, the role of both reducible metal center concentration and crystal structure cannot be completely ignored, as they also contribute to the perovskite oxygen release properties. TOC GraphicsScheme 1: Graphical representation of the synthesis of the SrTi 0.65 Fe 0.35 O 3-δ perovskites showing the dependency of the materials porosity and crystallite size from the type of polyol (ethylene glycol, glycerol, and 1,6 hexandiol) used

In this paper, we show how the composition of bimetallic Fe−Ni exsolution can be controlled by the nature and concentration of oxygen vacancies in the parental matrix and how this is used to modify the performance of CO 2 -assisted ethane conversion. Mesoporous A-sitedeficient La 0.4 Sr 0.6−α Ti 0.6 Fe 0.35 Ni 0.05 O 3±δ (0 ≤ α ≤ 0.2) perovskites with substantial specific surface area (>40 m 2 /g) enabled fast exsolution kinetics (T < 500 °C, t < 1 h) of bimetallic Fe−Ni nanoparticles of increasing size (3−10 nm). Through the application of a multitechnique approach we found that the A-site deficiency determined the concentration of oxygen vacancies associated with iron, which controlled the Fe reduction. Instead of homogeneous bimetallic nanoparticles, the increasing Fe fraction from 37 to 57% led to the emergence of bimodal Fe/Ni 3 Fe systems. Catalytic tests showed superior stability of our catalysts with respect to commercial Ni/Al 2 O 3 . Ethane reforming was found to be the favored pathway, but an increase in selectivity toward ethane dehydrogenation occurred for the systems with a low metallic Fe fraction. The chance to control the reduction and growth processes of bimetallic exsolution offers interesting prospects for the design of advanced catalysts based on bimodal nanoparticle heterostructures.

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Nanoporosity is clearly beneficial for the performance of heterogeneous catalysts. Although exsolution is a modern method to design innovative catalysts, thus far it is predominantly studied for sintered matrices. A quantitative description of the exsolution of Ni nanoparticles from nanoporous perovskite oxides and their effective application in the biogas dry reforming is here presented. The exsolution process is studied between 500 and 900 °C in nanoporous and sintered La 0.52 Sr 0.28 Ti 0.94 Ni 0.06 O 3±𝜹 . Using temperature-programmed reduction (TPR) and X-ray absorption spectroscopy (XAS), it is shown that the faster and larger oxygen release in the nanoporous material is responsible for twice as high Ni reduction than in the sintered system. For the nanoporous material, the nanoparticle formation mechanism, studied by in situ TEM and small-angle X-ray scattering (SAXS), follows the classical nucleation theory, while on sintered systems also small endogenous nanoparticles form despite the low Ni concentration. Biogas dry reforming tests demonstrate that nanoporous exsolved catalysts are up to 18 times more active than sintered ones with 90% of CO 2 conversion at 800 °C. Time-on-stream tests exhibit superior long-term stability (only 3% activity loss in 8 h) and full regenerability (over three cycles) of the nanoporous exsolved materials in comparison to a commercial Ni/Al 2 O 3 catalyst.

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