Acceptor-doped, redox-active perovskite oxides such as La0.8Sr0.2FeO3 (LSF) are active for ethane oxidation to COx but show poor selectivity to ethylene. This article reports molten Li2CO3 as an effective “promoter” to modify LSF for chemical looping–oxidative dehydrogenation (CL-ODH) of ethane. Under the working state, the redox catalyst is composed of a molten Li2CO3 layer covering the solid LSF substrate. The molten layer facilitates the transport of active peroxide (O22−) species formed on LSF while blocking the nonselective sites. Spectroscopy measurements and density functional theory calculations indicate that Fe4+→Fe3+ transition is responsible for the peroxide formation, which results in both exothermic ODH and air reoxidation steps. With >90% ethylene selectivity, up to 59% ethylene yield, and favorable heat of reactions, the core-shell redox catalyst has an excellent potential to be effective for intensified ethane conversion. The mechanistic findings also provide a generalized approach for designing CL-ODH redox catalysts.

Synergy between the reactant activation by catalytically active Ni and the CeO2–TiO2/Ce2Ti2O7 stoichiometric redox cycle for dramatically enhanced solar fuel production.

In this paper, we propose a facile and efficient strategy for synthesizing mesoporous BaSnO3 with a surface area as large as 67 m(2)/g using a peroxo-precursor decomposition procedure. As far as we know, this is the largest surface area reported in literature for BaSnO3 materials and may have a potential to greatly promote the technological applications of this kind of functional material in the area of chemical sensors, NOx storage, and dye-sensitized solar cells. The structure evolution of the mesoporous BaSnO3 from the precursor was followed using a series of techniques. Infrared analysis indicates large amount of protons and peroxo ligands are contained in the peroxo-precursor. Although the crystal structure of the precursor appears cubic according to the analysis of X-ray diffraction data, Raman and Mössbauer spectroscopy results show that the Sn atom is offset from the center of [SnO6] octahedron. After calcination at different temperatures, the precursor gradually transforms into BaSnO3 by release of water and oxygen, and the distortion degree of [SnO6] octahedral decreases. However, a number of oxygen vacancies are generated in the calcined samples, which are further confirmed by the physical property measurement system, and they would lower the local symmetry to some content. The concentration of the oxygen vacancies reduces simultaneously as the calcination temperature increases, and their contributions to the total heat capacity of the sample are calculated based on theoretical analysis of heat capacity data in the temperature region below 10 K.

A series of BaFe 1-x Sn x O 3-δ catalysts were prepared by sol-gel method and tested for N 2 O decomposition to shed light on the effect of B-site substitution on the catalytic behavior of perovskite catalysts. 119 Sn and 57 Fe Mössbauer results confirmed that the 5-fold coordinated Fe 3+ cations with one adjacent oxygen vacancy (Fe 3+-O 5) were the main active centers for N 2 O decomposition. Doping of Sn cations can significantly improve the percentage of Fe 3+-O 5 from 30% (x = 0) to 68% (x = 0.8). More importantly, the valence state of Fe could be gradually reduced due to weakening of Fe-O bond with increasing the Sn content, which was attributed to the stronger force of Sn than Fe in Fe-O-Sn structure to draw the oxygen anion and expansion of unit cell volume. Such change of Fe chemical state favored the oxygen mobility of the catalyst, leading to reduction of activation energy for N 2 O decomposition from ca. 241 (x = 0) to 178 kJ mol-1 (x = 0.8). BaFe 0.2 Sn 0.8 O 3-δ catalyst exhibited the highest intrinsic rate of 1.49 s-1 (550 o C), nearly 4 times larger than BaFeO 3-δ (0.43 s-1).

Rational design of perovskite-type redox catalysts is still challenging due to the limited understanding of the correlation between structural distortion and lattice oxygen activity. Herein, a series of model catalysts with a composition of LaFe 0.8 M 0.2 O 3 (M = Al, Ga, Fe, and Sc) were designed to shed light on the structure− activity relationship. Combined experimental results and DFT calculations verify that the tilting degree of the FeO 6 octahedron rather than the Fe−O bond length plays a dominant role in modulating the oxygen activity. Reducing the octahedral tilting via doping redox-inert cations with a smaller radius can greatly enhance the Fe−O bond covalency and reduce the oxygen vacancy formation energy. This not only contributes to the activation of reactants but also accelerates the oxygen mobility. Consequently, LaFe 0.8 Al 0.2 O 3 with the smallest FeO 6 octahedral tilting (6.6°tilting) manifests superior syngas productivity that is 70% higher than that of LaFe 0.8 Sc 0.2 O 3 (9.5°tilting) with simultaneously better CO 2 conversion for chemical looping dry reforming of methane. This discovery highlights the importance of geometric effect in adjusting the oxygen activity, paving an avenue for developing prospective redox catalysts for chemical looping processes and reactions proceeding via a Mars−van Krevelen mechanism.

Suppressing coke deposition over reduced oxygen carriers, the key to breaking competing effects between oxygen supply and methane-to-syngas selectivity, is an important but challenging task for chemical looping partial oxidation technology. We report that A-site engineering of La1–x Sr x Fe0.8Al0.2O3 oxides significantly adjusts the oxygen capacity, which nearly triples from 1.0 mmol/g (x = 0.1) to 2.7 mmol/g (x = 0.5) with CO selectivity maintaining above 94%. Characterization results show that doping of Sr at the La-site induces dynamic crystal reconstruction from perovskite to Fe0 and La n SrFe n‑x Al x O3n+1 (n = 1 and 2) oxides with Ruddlesden–Popper (RP) structure, possessing good methane activation and oxygen transport property, respectively. Spontaneously growth of RP oxides around Fe0 rapidly delivers lattice oxygen from perovskite oxide to Fe0. Density functional theory calculations further suggest that introduction of Sr notably reduces oxygen vacancy formation energy, leading to better oxygen donating ability. The synergy between dynamic structure evolution and oxygen mobility modulation greatly improves methane-to-syngas performance, which provides meaningful guidance to design advanced catalysts with better oxygen capacity for specific catalytic transformations.

The oxidative dehydrogenation (ODH) of alkanes with N2O or CO2 is an attractive pathway to produce alkenes and decrease environmental issues. However, the sole production of alkenes from the ODH process is significantly important but remains a great challenge due to the uncontrollable oxygen species for overoxidized products. Here, we report a sulfate-modified NiAl mixed oxide derived from layered double hydroxide that exhibits ∼100% ethylene selectivity at 10% ethane conversion using N2O as a representative soft oxidant, which surpasses the previously reported catalysts. Extensive characterizations and theoretical calculations demonstrate that the sulfate modifier favors the formation of more Ni3+ species and promotes the isolation of electrophilic oxygen (O−) species through electronic and steric effects. Different from the conventional cognition of electrophilic oxygen species leading to overoxidation, we identify that the increase of the proportion of isolated electrophilic oxygen species determines the higher selectivity of ethylene. The isolated oxygen species are favorable for the effective breakage of C–H bonds of ethane to selectively produce ethylene with facile desorption, while the adjacent ones strongly bind ethylene and excessively break the C–H bonds to overoxidation. More importantly, the sulfate-modification strategy for increasing ethylene selectivity can be extendable to not only the CO2-assisted ODH reaction but also to other catalysts.

It is of great significance to improve the syngas selectivity of Fe-based oxygen carriers (OCs), because of their sufficient lattice oxygen, low cost, and environmental compatibility in chemical looping partial oxidation of CH4. In this work, it was found that the addition of Y could remarkably increase CO selectivity of Fe2O3/Al2O3 to 98% with a CH4 conversion of ∼90%. X-ray diffraction (XRD) and transmission electron microscopy (TEM) characterizations combined with Mössbauer spectroscopy illustrated that the incorporation of Y led to the Fe species gradually transferring from Fe2O3 into the garnet structure (Y3Fe2Al3O12), a newly formed phase, which was found to be highly active for syngas generation. Density functional theory (DFT) calculations demonstrated that such a high CO selectivity of confined Fe species in garnet originated from enhanced oxygen vacancy formation energy (E ov), compared with Fe2O3, which resulted from the lattice oxygen shared by not only reducible Fe ions but also nonreducible Al and Y ones in a garnet structure. Therefore, our work provides a meaningful guidance of new materials screening for methane partial oxidation in the chemical looping process.