Partial Thermal Condensation Mediated Synthesis of High-Density Nickel Single Atom Sites on Carbon Nitride for Selective Photooxidation of Methane into Methanol Pawan Kumar, Peter Antal, Xiyang Wang, Jiu Wang, Dhwanil Trivedi, et al. Small, 2024 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.
Nickel–iron and zinc–iron bimetal oxalates: preparation, characterization and thermal decomposition to spinel ferrites Soňa Lisníková, Petr Novák, Josef Kopp Chemical Papers, 2024 A systematic investigation of Ni and Zn spinel ferrites preparation via oxalate route, involving a detailed characterization of synthesized precursors, in situ study of thermally induced decomposition reactions and analyses of the prepared ferrites is presented. Although the oxalate route in general is rather well known, the detailed investigations of the decomposition reactions of the well-characterized bimetal oxalate precursors have been mostly omitted by the authors. The formation of the solid solution, i.e., the incorporation of both metals into the single oxalate crystal structure, is essential for the subsequent decomposition reaction and synthesis of pure spinel ferrites. The optimally prepared precursor decomposes in a single reaction step at relatively low temperatures, evading the undesirable sintering, and allowing the preparation of microporous/mesoporous ferrites with relatively high BET areas.
Polymorphism of anhydrous iron(II) oxalate Vítězslav Heger, Josef Kopp, Vít Procházka, Petr Novák Chemical Papers, 2024 Recently, iron(II) oxalate has experienced a renewed interest due to their newly found application in lithium-ion batteries. Lithium is expected to be embedded between the oxalate sheets, dramatically increasing the need to understand the oxalate structure. Despite being known for decades, the discrepancies still exist regarding the anhydrous iron(II) oxalate. In this work, we explore the dehydration process of both α-FeC2O4·2H2O and β-FeC2O4·2H2O polymorphs at different heating rates and calcination temperatures by X-ray powder diffraction, Mössbauer spectroscopy and scanning electron microscopy. After dehydration, iron(II) oxalates formed two polymorphs with different XRD patterns: α-FeC2O4 with sharp and narrow diffraction lines and β-FeC2O4 with very broadened lines, which were attributed to the monoclinic structure with space group P21/n.
Thermally induced solid-state reaction of Fe2(SO4)3 with NaCl or KCl: a route to β-Fe2O3 synthesis Josef Kopp, Karolína Kalusová, Vít Procházka, Petr Novák Chemical Papers, 2023 β-Fe2O3 is a rare crystalline polymorph of the ferric oxide family with an interesting application potential, e.g., in photocatalysis. In this study, the effect of different alkali salts addition, namely NaCl and KCl, on the preparation of β-Fe2O3 via thermally induced solid-state reaction was investigated. Two series of samples were prepared by calcining two different mixtures, Fe2(SO4)3 + NaCl (molar ratio 1:3) and Fe2(SO4)3 + KCl (molar ratio 1:3) at temperatures from 350 to 700 °C. Although the addition of either alkali salt led the preparation of β-Fe2O3 particles in wide temperature range up to 650 °C, differences in the overall phase composition and β-Fe2O3 purity were observed between the two series. The addition of KCl to Fe2(SO4)3 allowed the preparation of pure β-Fe2O3 (≥ 95%) in relatively wide temperature range of 450‒600 °C, while in the case of NaCl, pure β-Fe2O3 (≥ 95%) was found only in samples calcined at 500 °C and 550 °C. Other phases could be identified as additional ferric oxide polymorphs, γ-Fe2O3 and α-Fe2O3. The in situ XRD results suggest that, in the case of NaCl + Fe2(SO4)3 reaction, simultaneous formation of β-Fe2O3 and α-Fe2O3 may be possible between 350 and 500 °C, depending on the reaction conditions.
Lamb–Mössbauer factor of powders determined by Mössbauer spectroscopy with resonant detector Petr Novák, Tereza Schlattauerová, Vít Procházka, Josef Kopp, Vlastimil Vrba Chemical Papers, 2023 The Lamb–Mössbauer factor is a crucial material parameter for the proper quantitative analysis of Mössbauer experiments. We report on a method for determining the Lamb–Mössbauer factor of powdered samples. It utilizes a resonant Mössbauer spectrometer together with a customized sample preparation, which ensures a homogeneous thickness of the powdered absorbers. Compared with other methods of Lamb‒Mössbauer factor determination, the presented approach is direct and requires only a single Mössbauer measurement. To demonstrate this method, the Lamb–Mössbauer factor of iron(II) oxalate dihydrate samples with varying thickness was measured. The resulting value of the Lamb–Mössbauer factor was 0.38 ± 0.03. The presented approach can be used for a large variety of powdered materials.
Microwave-Enhanced Crystalline Properties of Zinc Ferrite Nanoparticles Martin Ochmann, Vlastimil Vrba, Josef Kopp, Tomáš Ingr, Ondřej Malina, et al. Nanomaterials, 2022 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.
Solvent-Directed Morphological Transformation in Covalent Organic Polymers Xuan Thang Cao, Subodh Kumar, Ivan Nemec, Josef Kopp, Rajender S. Varma Frontiers in Materials, 2022 Synthesis of bi-functional covalent organic polymers in two distinctive morphologies has been accomplished by simply switching the solvent from DMF to DMSO when 1,3,5-tribenzenecarboxyldehyde and 2,5-diaminobenzene sulfonic acid were reacted via Schiff base condensation reaction to afford covalent organic polymers (COPs) encompassing flower (F-COPDMF)- and circular (C-COPDMSO)-type morphologies. Chemical and morphological natures of the synthesized COPs were compared by characterization using TEM, SEM, XRD, FT-IR, and XPS analysis techniques. Besides diverse morphology, both the polymeric materials were found to comprise similar chemical natures bearing protonic acid–SO3H and Lewis base–C=N functionalities. Subsequently, both the COPs were evaluated for the synthesis of hydroxymethylfurfural (HMF) by the dehydration of fructose to investigate their morphology-dependent catalytic activity.
