Papers by Zara Cherkezova-Zheleva
Metals, Jun 1, 2024
This article is an open access article distributed under the terms and conditions of the Creative... more This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY
Inorganics, Apr 15, 2024
This article is an open access article distributed under the terms and conditions of the Creative... more This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY
Molecules, Jan 10, 2024
This article is an open access article distributed under the terms and conditions of the Creative... more This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY
Proceedings of the 9th World Congress on Mechanical, Chemical, and Material Engineering
Metals, Jun 1, 2024
This article is an open access article distributed under the terms and conditions of the Creative... more This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY
Environmental Science & Technology, Jul 17, 2023
Fuel Processing Technology, Sep 1, 2015
Activated carbons with different textural and chemical surface characteristics were synthesized f... more Activated carbons with different textural and chemical surface characteristics were synthesized from waste biomass and low rank coals, and furthermore used as a host matrix for cobalt species, varying the preparation and modification methods. The obtained activated carbons and modified samples were characterized by complex of various physicochemical methods, such as: low temperature physisorption of nitrogen, XRD, EPR, XPS, UV-Vis and TPR with hydrogen. Boehm method was applied for qualitative and quantitative determination of oxygen-containing groups on the carbon surface before and after cobalt deposition. The catalytic properties of cobalt modifications were tested in methanol decomposition. The dominant effect of activated carbon texture over the surface chemistry on the state and catalytic behaviour of cobalt species was discussed.
were prepared by coprecipitation and thermal treatment in nitrogen and studied as photocatalysts ... more were prepared by coprecipitation and thermal treatment in nitrogen and studied as photocatalysts for oxidative degradation of Malachite Green dye in aqueous solution under UV-A light irradiation. The phase composition and magnetic behaviour of the prepared materials were established by powder X-ray diffraction analysis and Mössbauer spectroscopy. Synthesized ferrite/AC composites exhibit a superparamagnetic and collective magnetic excitation behaviour. A degradation degree of Malachite Green dye, used as model pollutant, reached 93% for Mn 0.5 Fe 2.5 O 4 /AC and 99% for Co 0.5 Fe 2.5 O 4 /AC and Cu 0.5 Fe 2.5 O 4 /AC composites, which shows that these materials are applicable as photocatalysts for purification of waters containing above-mentioned dye. Photocatalytic results indicate that Co 0.5 Fe 2.5 O 4 /AC manifested the highest photocatalytic activity. Comparison of the photocatalytic properties of the synthesized ferrite/activated carbon materials proved that the rate constants over the tested materials are increasing in the following order: 7.1×10-3 min-1 (Mn 0.5 Fe 2.5 O 4 /AC) < 24.1×10-3 min-1 (Cu 0.5 Fe 2.5 O 4 /AC) < 28.4×10-3 min-1 (Co 0.5 Fe 2.5 O 4 /AC).
World Academy of Science, Engineering and Technology, International Journal of Environmental and Ecological Engineering, Mar 23, 2015
Zenodo (CERN European Organization for Nuclear Research), Mar 23, 2023
This article is an open access article distributed under the terms and conditions of the Creative... more This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY
Journal of Solid State Chemistry, May 1, 2021
ZnO thin films were prepared by sol-gel dip coating method onto glass substrates. The effects of ... more ZnO thin films were prepared by sol-gel dip coating method onto glass substrates. The effects of aging time of the starting solution (2, 10 and 30 days) and the number of coats (2, 5 and 10 coatings) on structural, morphological and optical properties were investigated. Photocatalytic efficiency was also assessed. X-ray diffraction analysis indicates that all the films exhibit a Zincite-type structure with a preferred grains orientation along the [002] direction. The preferred orientation factor (POF) increases with aging time while the crystallite size decreases. The field emission scanning electron microscopy observations reveals nanorods morphology. The length of ZnO nanorods increase with increasing number of layers whereas their length decreases as a function of aging time while adopting a random orientation. A high optical transparency is observed for all ZnO thin films, ranging from 90 up to 96%. Methylene Blue (MB) dye photocatalytic degradation was found increases with aging time, reaching almost 94% after 10 h under UV irradiation. The apparent reaction rate (K app) obtained by Langmuir-Hinshelwood model increases with increasing aging time from, from 0.218 h À 1 for 2 days to reach a steady state around 0.270 h À 1. Nevertheless, a small variation of K app was recorded when varying the number of coats; 0.223-0.226 h À 1. & 2016 Elsevier Ltd and Techna Group S.r.l. All rights reserved. nanoparticles [16-18], nanoplatelets [19], or thin films [20-22]. This interest has been fueled by low production cost, control over shape and size, non-toxicity and availability. The dye photocatalytic activity mechanism can be summarized in five different steps [23-25]: (i) when ZnO films are irradiated with appropriate photon energy (hν ZE g) to generate electronhole (h þ /e À) pairs within ZnO films; (ii) the holes (h þ) react with water molecules adhering to ZnO films surface and form hydroxyl radicals such as (OH •) and H þ via a valence band process. (iii) generated electrons (e-) in the conduction band react with dissolved O 2 to produce superoxide (O • À 2) radicals; (vi) the superoxide products react with H 2 O forming the peroxide H 2 O 2 molecules which produce hydroxyl radicals • OH; (v) these reactive radicals have a very short life time and decompose partially the organic dye into CO 2 and H 2 O. The generated electron-holes pairs can be combined within the material or diffuse to the surface where they may recombine, be trapped in metastable surface states or captured by the adsorbed molecules on the semiconductor surface [26]. During photocatalytic Contents lists available at ScienceDirect
Physica Status Solidi A-applications and Materials Science, Jan 14, 2022
Журнал физической химии, 2009
Hyperfine Interactions, Dec 19, 2019
In this paper, the physico chemical properties of Fe-based amorphous alloys containing critical r... more In this paper, the physico chemical properties of Fe-based amorphous alloys containing critical raw materials (CRM) were investigated in order to reuse them for photocatalytic degradation of azo dyes. The iron-based metallic glasses were prepared by melt spinning method. Their chemical composition is:
Hyperfine Interactions, Dec 20, 2017
Biogenic composites are interesting as green and sustainable precursors for preparation of advanc... more Biogenic composites are interesting as green and sustainable precursors for preparation of advanced materials of various applications. Biomaterials from Leptothrix group bacteria cultivated in two feeding media of silicon-iron-glucose-peptone (SIGP) and Lieske were studied. Iron-containing biogenic powders and biofilms on silica gel covered aluminum plates were prepared. They were studied to elucidate the effect of the plates in the process of growing biogenic iron nanotubes. The cultivation period was varied from 4 to 30 days. Biomass phase composition and physicochemical properties were studied by Mössbauer spectroscopy at room and low temperature, as well as by means of powder XRD, FTIRS, and SEM methods. Mössbauer analysis offered us a unique possibility to register iron oxide species at the different steps of biogenic material formation. Tetrahedrally coordinated iron species were registered at an early stage of biofilm formation. So, important results on the mechanism of biomineralization process are obtained. The reaction of CO oxidation on prepared biomaterials was studied using in situ DRIFTS. Comparative analysis of the obtained materials and examination of spent samples showed differences in their phase composition, stability and dispersity. Changes of the phase composition were observed during catalytic tests with biomasses obtained in Lieske feeding media. No differences in biogenic powder and biofilm composition and dispersity were registered when This article is part of the Topical Collection on
Hyperfine Interactions, Feb 22, 2016
Series of iron-based biogenic materials prepared by cultivation of Leptothrix group of bacteria i... more Series of iron-based biogenic materials prepared by cultivation of Leptothrix group of bacteria in different feeding media (Sphaerotilus-Leptothrix group of bacteria isolation medium, Adler, Lieske and silicon-iron-glucose-peptone) were studied. Control samples were obtained in the same conditions and procedures but the nutrition media were not infected with bacteria, i.e. they were sterile. Room and low temperature Mössbauer spectroscopy, powder X-ray diffraction (XRD), and infrared spectroscopy (IRS) were used to reveal the composition and physicochemical properties of biomass and respective control samples. Comparative analysis showed differences in their composition and dispersity of present phases. Sample composition included different ratio of nanodimensional iron oxyhydroxide and oxide phases. Relaxation phenomena such as superparamagnetism or collective magnetic excitation behaviour were registered for some of them. The experimental data showed that the biogenic materials were enriched in oxyhydroxides of high dispersion. Catalytic behaviour of a selected biomass and abiotic material were studied in the reaction of CO oxidation. In situ diffuse-reflectance (DR) IRS was used to monitor the phase transformations in the biomass and CO conversion. Keywords Biogenic nanosized iron oxides • Leptothrix genus of bacteria • Mössbauer spectroscopy • X-ray diffraction • IR spectroscopy This article is part of the Topical Collection on
Journal of physics, Mar 1, 2010
The aim of the study is to obtain the exact state of iron oxide catalyst active phase in reaction... more The aim of the study is to obtain the exact state of iron oxide catalyst active phase in reaction conditions, as well as the correlation between the active phase and catalytic properties of iron-containing catalysts. In situ Mössbauer spectroscopy is the major investigation technique. It is established that the change of reaction conditions (temperature and gas reaction mixture) lead to redistribution of the relative weight of spectra components and influence mainly tetrahedrally and octahedrally coordinated cations in Fe 3 O 4 phase. It was concluded, that the active sites of the catalyst in studied reaction are probably pairs of Fe 3+ +Fe 2+-(Fe 2.5+) ions, i.e. the mixed valance iron ions. The obtained catalytic activity can be explained with combination of the natural thermo-activated and catalytically induced electron exchange and better synchronizing of oxidation and reduction steps of the catalytic reaction.
International Journal of Applied Glass Science, Jan 30, 2023
The high temperature synthesis of glasses in the system (100‐x)(0.16Na2O/0.10CaO/0.74SiO2)/xFe2O3... more The high temperature synthesis of glasses in the system (100‐x)(0.16Na2O/0.10CaO/0.74SiO2)/xFe2O3, x = 5 ÷ 20 mol% is reported. For Fe2O3 concentrations ≤15 mol%, glasses are formed while the sample with 20 mol% crystallizes during cooling the melt. X‐ray diffraction shows the crystallization of magnetite. The microstructure of the glass‐crystalline sample is investigated by optical microscopy and scanning electron microscopy and two types of iron‐rich crystals corresponding to magnetite and hematite are detected. The refractive indices as determined by the Becke line method are in the 1.567 ‐ 1.639 range and. increase with increasing Fe2O3 concentration. The structure is characterized using Infra‐red spectroscopy. The presence of symmetric stretching, asymmetric stretching and bending vibrations of Si‐O‐Si is detected and attributed to the occurrence of SiO4 tetrahedral units with varying numbers of nonbridging oxygens. Also, the increasing Fe2O3 concentration results in occurrence of Fe‐O‐Si bonds indicating the glass network depolymerization due to Fe2O3 addition. In all samples, the presence of Fe3+ and Fe2+ and the existence of iron ions in tetrahedral and octahedral coordination, as well as a very small amount of Fe0 and the precipitation of hematite and magnetite in the glass‐crystalline sample is revealed by Mössbauer spectroscopy.
Applied Biochemistry and Biotechnology, 2017
Mixed cobalt-copper ferrite-type materials having different compositions were prepared using a co... more Mixed cobalt-copper ferrite-type materials having different compositions were prepared using a coprecipitation technique and mechanochemical or microwave treatment. Various physicochemical methods, such as powder X-ray diffraction, Mössbauer and FTIR spectroscopy, single point BET method, porosity investigations, and temperatureprogrammed reduction, were used for characterization of the obtained ferrite-type samples. The photocatalytic activities of the so synthesized cobalt-copper ferrite-type materials were examined in oxidative degradation of Reactive Black 5 (RB5) dye as model contaminant in aqueous solution under UV-A light illumination. Photocatalytic activity tests established that microwave treatment of coprecipitated Co 0.25 Cu 0.25 Fe 2.5 O 4 sample leads to a higher degree of degradation of RB5 dye (98%) compared to that of coprecipitated (96%) and mechanochemically treated (73%) materials. Mechanochemical treatment enhanced the conversion degree of RB5 dye after 120 min of illumination: 77 and 78% for Co 0.5 Cu 0.5 Fe 2 O 4 and Co 0.4 Cu 0.1 Fe 2.5 O 4 , respectively, compared with matching coprecipitated samples (48% and 52%), accordingly. An enhancement of photocatalytic efficiency of prepared cobalt-copper ferrites upon decreasing the cobalt content was established. The photocatalytic properties of obtained mixed cobalt-copper ferrite-type samples were affected by the degree of incorporation of metal ions in the magnetite-type structure and by applied preparation methods.
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Papers by Zara Cherkezova-Zheleva