Search for: All records

The Hume-Rothery rules governing solid-state miscibility limit the compositional space for new inorganic material discovery. Here, we report a non-equilibrium, one-step, and scalable flame synthesis method to overcome thermodynamic limits and incorporate immiscible elements into single phase ceramic nanoshells. Starting from prototype examples including (NiMg)O, (NiAl)O_x, and (NiZr)O_x, we then extend this method to a broad range of Ni-containing ceramic solid solutions, and finally to general binary combinations of elements. Furthermore, we report an “encapsulated exsolution” phenomenon observed upon reducing the metastable porous (Ni_0.07Al_0.93)O_xto create ultra-stable Ni nanoparticles embedded within the walls of porous Al₂O₃nanoshells. This nanoconfined structure demonstrated high sintering resistance during 640 h of catalysis of CO₂reforming of methane, maintaining constant 96% CH₄and CO₂conversion at 800 °C and dramatically outperforming conventional catalysts. Our findings could greatly expand opportunities to develop novel inorganic energy, structural, and functional materials.

 

Carbon dioxide-assisted coupling of methane offers an approach to chemically upgrade two greenhouse gases and components of natural gas to produce ethylene and syngas. Prior research on this reaction has concentrated efforts on catalyst discovery, which has indicated that composites comprised of both reducible and basic oxides are especially promising. There is a need for detailed characterization of these bifunctional oxide systems to provide a more fundamental understanding of the active sites and their roles in the reaction. We studied the dependence of physical and electronic properties of Ca-modified ZnO materials on Ca content via X-ray photoelectron and absorption spectroscopies, electron microscopy, and infrared spectroscopic temperature-programmed desorption (IRTPD). It was found that introduction of only 0.6 mol% Ca onto a ZnO surface is necessary to induce significant improvement in the catalytic production of C2 species: C2 selectivity increases from 5% on unmodified ZnO to 58%, at similar conversions. Evidence presented shows that this selectivity increase resultsfrom the formation of an interface between the basic CaO and reducible ZnO phases. The basicity of these interface sites correlates directly with catalytic activity over a wide composition range, and this relationship indicates that moderate CO2 adsorption strength is optimal for CH4 coupling. These results demonstrate, for the first time to our knowledge, a volcano-type relationship between CO2-assisted CH4 coupling activity and catalyst surface basicity, which can inform further catalyst development. 

Atomically dispersed catalysts have been shown highly active for preferential oxidation of carbon monoxide in the presence of excess hydrogen (PROX). However, their stability has been less than ideal. We show here that the introduction of a structural component to minimize diffusion of the active metal center can greatly improve the stability without compromising the activity. Using an Ir dinuclear heterogeneous catalyst (DHC) as a study platform, we identify two types of oxygen species, interfacial and bridge, that work in concert to enable both activity and stability. The work sheds important light on the synergistic effect between the active metal center and the supporting substrate and may find broad applications for the use of atomically dispersed catalysts. 

Hard carbon (HC) is the most promising anode for the commercialization of sodium‐ion batteries (NIBs); however, a general mechanism for sodium storage in HC remains unclear, obstructing the development of highly efficient anodes for NIBs. To elucidate the mechanism of sodium storage in the pores, operando synchrotron small‐angle X‐ray scattering, wide‐angle X‐ray scattering, X‐ray absorption near edge structure, Raman spectroscopy, and galvanostatic measurements are combined. The multimodal approach provides mechanistic insights into the sodium pore‐filling process for different HC microstructures including the pore sizes that are preferentially filled, the extent to which different pore sizes are filled, and how the defect concentration influences pore filling. It is observed that sodium in the larger pores has an increased pseudo‐metallic sodium character consistent with larger sodium clusters. Furthermore, it is shown that the HCs prepared at higher pyrolysis temperatures have a larger capacity from sodium stored in the pores and that sodium intercalation between graphene layers occurs simultaneously with the pore filling in the plateau region. Opportunities are outlined to improve the performance of HC anodes by fully utilizing the pores for sodium storage, helping to pave the way for the commercialization of sodium ion batteries.