Impurity Segregation and Nanoparticle Reorganization of Indium Doped MgO Cubes

Conversion of MgO nanocrystal surfaces into ceramic interfaces: Exsolution of BaO as photoluminescent interface probes

Ion exsolution can be instrumental to engineer intergranular regions inside ceramic microstructures. BaO admixtures that were trapped inside nanometer-sized MgO grains during gas phase synthesis undergo annealing-induced exsolution to generate photoluminescent surface and interface structures. During their segregation from the bulk into the grain interfaces, the BaO admixtures impact grain coarsening and powder densification, effects that were compared for the first time using an integrated characterization approach. For the characterization of the different stages the materials adopt between powder synthesis and compact annealing, spectroscopy measurements (UV-Vis diffuse reflectance, cathodo- and photoluminescence [PL]) were complemented by an in-depth structure characterization (density measurements, X-ray diffraction [XRD], and electron microscopy). Depending on the Ba²⁺ concentration, isolated impurity ions either become part of low-coordinated surface structures of the MgO grains where they give rise to a characteristic bright PL emission profile around λ = 500 nm, or they aggregate to form nanocrystalline BaO segregates at the inner pore surfaces to produce an emission feature centered at λ = 460 nm. Both types of PL emission sites exhibit O₂ gas adsorption-dependent PL emission properties that are reversible with respect to its pressure. The here-reported distribution of BaO segregates between the intergranular region and the free pore surfaces inside the MgO-based compacts underlines that solid-based exsolution strategies are well suited to stabilize nanometer-sized segregates of metal oxides that otherwise would coalesce and grow in size beyond the nanoscale.

Catalytic activity, water formation, and sintering: Methane activation over Co- and Fe-doped MgO nanocrystals

Microstructure, structure, and compositional homogeneity of metal oxide nanoparticles can change dramatically during catalysis. Considering the different stabilities of cobalt and iron ions in the MgO host lattice [M. Niedermaier et al., J. Phys. Chem. C 123, 25991 (2019)], we employed MgO nanocube powders with or without transition metal admixtures for the oxidative coupling of methane (OCM) reaction to analyze characteristic differences in catalytic activity and sintering behavior. Undoped MgO nanocrystals exhibit the highest C₂ selectivity and retain the nanocrystallinity of the starting material after 24 h time on stream. For the Co-Mg-O nanoparticle powder, which exhibits the highest activity and CO_x selectivity and where OCM-induced coarsening is strongest, we found that the Co²⁺ ions remain homogeneously distributed over the MgO lattice. Trivalent Fe ions migrate to the surface of Fe-Mg-O nanoparticles where they form a magnesioferrite phase (MgFe₂O₄) with a characteristic impact on catalytic performance: Fe-Mg-O is initially less selective than MgO despite its lower activity. An increase in C₂ selectivity and a decrease in the CO₂/CO ratio with time on stream are attributed to the increasing fraction of coarsened particles that become depleted in redox active Fe. Surface water is a by-product of the OCM reaction, favors mass transport across the particle surfaces, and serves as a sintering aid during catalysis. The characteristic changes in size and morphology of MgO, Co-doped, and Fe-doped MgO particles can be consistently explained by activity and C₂ selectivity trends. The original morphology of the nanocubes as a starting material for the OCM reaction does not impact the catalytic activity.