Partial Thermal Condensation Mediated Synthesis of High-Density Nickel Single Atom Sites on Carbon Nitride for Selective Photooxidation of Methane into Methanol.

Direct selective transformation of greenhouse methane (CH4) to liquid oxygenates (methanol) can substitute energy-intensive two-step (reforming/Fischer-Tropsch) synthesis while creating environmental benefits. The development of inexpensive, selective, and robust catalysts that enable room temperature conversion will decide the future of this technology. Single-atom catalysts (SACs) with isolated active centers embedded in support have displayed significant promises in catalysis to drive challenging reactions. Herein, high-density Ni single atoms are developed and stabilized on carbon nitride (NiCN) via thermal condensation of preorganized Ni-coordinated melem units. The physicochemical characterization of NiCN with various analytical techniques including HAADF-STEM and X-ray absorption fine structure (XAFS) validate the successful formation of Ni single atoms coordinated to the heptazine-constituted CN network. The presence of uniform catalytic sites improved visible absorption and carrier separation in densely populated NiCN SAC resulting in 100% selective photoconversion of (CH4) to methanol using H2O2 as an oxidant. The superior catalytic activity can be attributed to the generation of high oxidation (NiIII═O) sites and selective C─H bond cleavage to generate •CH3 radicals on Ni centers, which can combine with •OH radicals to generate CH3OH.

Microwave-Enhanced Crystalline Properties of Zinc Ferrite Nanoparticles.

Two series of ZnFe2O4 mixed cubic spinel nanoparticles were prepared by a coprecipitation method, where a solution of Fe3+ and Zn2+ was alkalised by a solution of NaOH. While the first series was prepared by a careful mixing of the two solutions, the microwave radiation was used to enhance the reaction in the other series of samples. The effect of the microwave heating on the properties of the prepared particles is investigated. X-ray powder diffraction (XRD), 57Fe Mössbauer spectroscopy and magnetometry were employed to prove the cubic structure and superparamagnetic behavior of the samples. The particle size in the range of nanometers was investigated by a transmission electron microscopy (TEM), and the N2 adsorption measurements were used to determine the BET area of the samples. The stoichiometry and the chemical purity were proven by energy dispersive spectroscopy (EDS). Additionally, the inversion factor was determined using the low temperature Mössbauer spectra in the external magnetic field. The microwave heating had a significant effect on the mean coherent length. On the other hand, it had a lesser influence on the size and BET surface area of the prepared nanoparticles.

Single-Phase Precursors for the Preparation of Spinel Ferrites via Oxalate Route: the Study of Cobalt Ferrite Synthesis.

This work explores the benefits of single-phase oxalate precursors for the preparation of spinel ferrites by thermal decomposition. A direct comparison between the genuine oxalate solid solution and the physical mixture of simple oxalates is presented using the case study of cobalt ferrite preparation. The mixing of metal cations within a single oxalate structure could be verified prior to its thermal decomposition by several non-destructive experimental techniques, namely Mössbauer spectroscopy, X-ray powder diffraction (XRD) and energy-dispersive X-ray spectroscopy. In situ XRD experiments were conducted to compare the decomposition processes of the solid solution and the physical mixture. Additionally, the decomposition products of the FeCo oxalate solid solution were studied ex situ by means of N2 adsorption, Mössbauer spectroscopy and XRD. The results obtained for different reaction temperatures demonstrate the possibilities to easily control the physical properties of the prepared oxides.

P- and F-co-doped Carbon Nitride Nanocatalysts for Photocatalytic CO2 Reduction and Thermocatalytic Furanics Synthesis from Sugars.

A new P- and F-co-doped amorphous carbon nitride (PFCN) has been synthesized via sol-gel-mediated thermal condensation of dicyandiamide. Such synthesized P- and F-co-doped carbon nitride displayed a well-defined mesoporous nanostructure and enhanced visible light absorption region up to infrared with higher BET surface area of 260.93 m2 g-1 ; the highest recorded value for phosphorus-doped carbon nitride materials. Moreover, the formation mechanism is delineated and the role of templates was found to be essential not only in increasing the surface area but also in facilitating the co-doping of P and F atoms. Co-doping helped to narrow the optical band gap to 1.8 eV, thus enabling an excellent photocatalytic activity for the aqueous reduction of carbon dioxide into methanol under visible-light irradiation, which is fifteen times higher (119.56 µmol g-1 h-1 ) than the bare carbon nitride. P doping introduced Brønsted acidity into the material, turning it into an acid-base bifunctional catalyst. Consequently, the material was also investigated for the thermal conversion of common carbohydrates into furanics.

Formation of Cobalt Ferrites Investigated by Transmission and Emission MA¶ssbauer Spectroscopy.

This study focuses on cobalt and iron ordering within a ferrite structure CoxFe3-xO4, formed during a solid-state reaction of ?-Fe2O3 and CoCl2. A unique combination of transmission and emission Mössbauer spectroscopy was employed to inspect selectively the positions of iron and cobalt atoms in the structure. The comparison of transmission and emission spectra allowed the determination of tetrahedral and octahedral positions occupation. The presented method of combining the two Mössbauer spectroscopy techniques is suitable for any compounds containing both iron and cobalt atoms. Additional information concerning the samples composition and morphology were obtained by X-ray powder diffraction and scanning electron microscopy. An increased level of Co atoms incorporation into the structure of ferrite was revealed when higher amounts of Co entered the reaction.

Preparation of Magnetite by Thermally Induced Decomposition of Ferrous Oxalate Dihydrate in the Combined Atmosphere.

This study presents an investigation of thermal decomposition of ferrous oxalate dihydrate in the combined atmosphere of inert and conversion gases to find an optimal route for a simple magnetite preparation. Homogenized precursor was isothermally treated inside the stainless-steel cells at 8 equidistant temperatures ranging from 300 to 650 °C for 1, 6, and 12 hours. The enclosure of samples inside the cells with the combined atmosphere eliminates the necessity of the inert gas to flow over the treated samples. Structural, magnetic, and morphological aspects of the prepared materials were examined by the combination of experimental techniques, such as Mössbauer spectroscopy, X-ray powder diffraction, and scanning electron microscopy.