Josef Kopp
@upol.cz
Department of Experimental Physics
Palacký University Olomouc
Scopus Publications
Scopus Publications
Petr Novák, Tereza Schlattauerová, Vít Procházka, Josef Kopp, and Vlastimil Vrba
Springer Science and Business Media LLC
AbstractThe 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.
Josef Kopp, Karolína Kalusová, Vít Procházka, and Petr Novák
Springer Science and Business Media LLC
Abstractβ-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.
Pawan Kumar, Peter Antal, Xiyang Wang, Jiu Wang, Dhwanil Trivedi, Ondřej František Fellner, Yimin A. Wu, Ivan Nemec, Vinicius Tadeu Santana, Josef Kopp,et al.
Wiley
AbstractDirect 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.
Vítězslav Heger, Josef Kopp, Vít Procházka, and Petr Novák
Springer Science and Business Media LLC
AbstractRecently, 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.
Soňa Lisníková, Petr Novák, and Josef Kopp
Springer Science and Business Media LLC
AbstractA 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.
Jan Kočiščák, Petr Novák, Aleš Stejskal, Josef Kopp, and Vít Procházka
Elsevier BV
Martin Ochmann, Vlastimil Vrba, Josef Kopp, Tomáš Ingr, Ondřej Malina, and Libor Machala
MDPI AG
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.
Xuan Thang Cao, Subodh Kumar, Ivan Nemec, Josef Kopp, and Rajender S. Varma
Frontiers Media SA
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.
Soňa Lisníková, Josef Kopp, Vlastimil Vrba, and Petr Novák
Wiley
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 N 2 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.
Josef Kopp, Petr Novák, Soňa Lisníková, Vlastimil Vrba, and Vít Procházka
Wiley
Subodh Kumar, Manoj B. Gawande, Josef Kopp, Stepan Kment, Rajender S. Varma, and Radek Zbořil
Wiley
A novel 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-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 enhancing 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 the Brønsted acidity into the material, turning it into an acid-base bi-functional catalyst. Consequently, the material was also investigated for the thermal conversion of common carbohydrates into furanics.
Vit Prochazka,, Anezka Burvalova, Vlastimil Vrba, Josef Kopp, and Petr Novak
Slovenian Chemical Society
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.
Petr Novak, Vit Prochazka, Ales Stejskal, Josef Kopp, and Jiri Pechousek
Elsevier BV
Josef Kopp, Petr Novak, Josef Kaslik, and Jiri Pechousek
Slovenian Chemical Society
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.