106 results on '"Alexandre I. Rykov"'
Search Results
2. Enhanced oxygen reduction with single-atomic-site iron catalysts for a zinc-air battery and hydrogen-air fuel cell
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Yuanjun Chen, Shufang Ji, Shu Zhao, Wenxing Chen, Juncai Dong, Weng-Chon Cheong, Rongan Shen, Xiaodong Wen, Lirong Zheng, Alexandre I. Rykov, Shichang Cai, Haolin Tang, Zhongbin Zhuang, Chen Chen, Qing Peng, Dingsheng Wang, and Yadong Li
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Science - Abstract
Development of fuel cells and metal-air batteries is hindered by electrocatalyst performance, which can be enhanced with uniform and atomically dispersed active sites. Here the authors report an iron-based electrocatalyst for oxygen reduction in cathodes for a zinc-air battery and a hydrogen-air fuel cell.
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- 2018
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3. Synthesis of Iron-Carbide Nanoparticles: Identification of the Active Phase and Mechanism of Fe-Based Fischer–Tropsch Synthesis
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Huabo Zhao, Jin-Xun Liu, Ce Yang, Siyu Yao, Hai-Yan Su, Zirui Gao, Mei Dong, Junhu Wang, Alexandre I. Rykov, Jianguo Wang, Yanglong Hou, Wei-Xue Li, and Ding Ma
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Reaction mechanism ,Chemical engineering ,Chemistry ,Active phase ,Nanoparticle ,Fischer–Tropsch process ,General Chemistry ,Fe based ,Mechanism (sociology) ,Carbide - Abstract
Despite the extensive study of the Fe-based Fischer–Tropsch synthesis (FTS) over the past 90 years, its active phases and reaction mechanisms are still unclear due to the coexistence of metals, oxi...
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- 2021
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4. Highly active and controllable MOF-derived carbon nanosheets supported iron catalysts for Fischer-Tropsch synthesis
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Jing Lv, Xiaoxue Han, Shouying Huang, Qiao Zhao, Junhu Wang, Mei-Yan Wang, Yue Wang, Alexandre I. Rykov, Jiajia Chen, and Xinbin Ma
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Materials science ,Dopant ,chemistry.chemical_element ,Fischer–Tropsch process ,02 engineering and technology ,General Chemistry ,010402 general chemistry ,021001 nanoscience & nanotechnology ,01 natural sciences ,0104 chemical sciences ,Catalysis ,symbols.namesake ,chemistry ,Chemical engineering ,X-ray photoelectron spectroscopy ,symbols ,General Materials Science ,0210 nano-technology ,Raman spectroscopy ,Pyrolysis ,Carbon ,Monoclinic crystal system - Abstract
N-doped carbon materials as catalyst supports have shown great superiority because of rich porosity, tunable metal-support interaction and electronic promotion effect. Controllable fabrication of carbon materials with specific morphology and different configurations provides wider possibilities for applications in catalysis. Herein, N-doped carbon nanosheets (CNS) were successfully synthesized by using monoclinic ZIF-8 nanosheets as templates. Compared with commercial active carbon and cubic ZIF-8 particles-derived carbon particles, the derived CNS exhibit superior graphitization degree and electronic effect. Consequently, the promoted Fe carburization improved the intrinsic activity for Fe/CNS catalysts in FTS. Further investigation on pyrolysis temperature achieved tunable configuration and content of N dopant. XPS, Raman, Mossbauer spectroscopy, TEM, H2-TPR etc. were combined to elucidate the promotion effect of CNS on dispersion, electronic properties and formation of Fe carbides. By correlating the composition and structure to catalytic performance in FTS, we illustrated the significance of carbon materials in Fe supported catalysts.
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- 2021
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5. Magnetic Field Manipulation of Tetrahedral Units in Spinel Oxides for Boosting Water Oxidation
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Xiao Lyu, Yanan Zhang, Zhengwei Du, Hao Chen, Sicheng Li, Alexandre I. Rykov, Chen Cheng, Weina Zhang, Ling Chang, Wang Kai, Junhu Wang, Liang Zhang, Qiang Wang, Chengxi Huang, and Erjun Kan
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Biomaterials ,General Materials Science ,General Chemistry ,Biotechnology - Abstract
Magnetic field enhanced electrocatalysis has recently emerged as a promising strategy for the development of a viable and sustainable hydrogen economy via water oxidation. Generally, the effects of magnetic field enhanced electrocatalysis are complex including magnetothermal, magnetohydrodynamic and spin selectivity effects. However, the exploration of magnetic field effect on the structure regulation of electrocatalyst is still unclear whereas is also essential for underpinning the mechanism of magnetic enhancement on the electrocatalytic oxygen evolution reaction (OER) process. Here, it is identified that in a mixed NiFe
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- 2022
6. Identification of the Electronic and Structural Dynamics of Catalytic Centers in Single-Fe-Atom Material
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Ying-Rui Lu, Qihua Xiong, Zehua Hu, Shibo Xi, Yanqiang Huang, Chang Su Cao, Junhu Wang, Jun Li, Ting-Shan Chan, Weizheng Cai, Hao Ming Chen, Tao Zhang, Shu Miao, Wei Xu, Hongbin Yang, Xuning Li, Jiyong Zhao, Sung Fu Hung, Hai Xiao, Bin Liu, Alexandre I. Rykov, and Esen E. Alp
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Reaction mechanism ,Materials science ,General Chemical Engineering ,02 engineering and technology ,Electronic structure ,010402 general chemistry ,Electrocatalyst ,Photochemistry ,01 natural sciences ,Biochemistry ,Catalysis ,symbols.namesake ,Materials Chemistry ,Environmental Chemistry ,Moiety ,biology ,Biochemistry (medical) ,Active site ,General Chemistry ,021001 nanoscience & nanotechnology ,0104 chemical sciences ,biology.protein ,symbols ,Electron configuration ,0210 nano-technology ,Raman spectroscopy - Abstract
Summary The lack of model single-atom catalysts (SACs) and atomic-resolution operando spectroscopic techniques greatly limits our comprehension of the nature of catalysis. Herein, based on the designed model single-Fe-atom catalysts with well-controlled microenvironments, we have explored the exact structure of catalytic centers and provided insights into a spin-crossover-involved mechanism for oxygen reduction reaction (ORR) using operando Raman, X-ray absorption spectroscopies, and the developed operando 57Fe Mossbauer spectroscopy. In combination with theoretical studies, the N-FeN4C10 moiety is evidenced as a more active site for ORR. Moreover, the potential-relevant dynamic cycles of both geometric structure and electronic configuration of reactive single-Fe-atom moieties are evidenced via capturing the peroxido (∗O2−) and hydroxyl (∗OH−) intermediates under in situ ORR conditions. We anticipate that the integration of operando techniques and SACs in this work shall shed some light on the electronic-level insight into the catalytic centers and underlying reaction mechanism.
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- 2020
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7. Paramagnetic Conducting Metal–Organic Frameworks with Three‐Dimensional Structure
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Alexandre I. Rykov, Zhijun Chen, Daoben Zhu, Xiaoyu Wu, Wei Xu, Xiang Hao, Yimeng Sun, Ye Zou, Yi Qiu, Liyao Liu, Bo Guan, and Junliang Sun
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Materials science ,010405 organic chemistry ,Band gap ,business.industry ,Analytical chemistry ,General Medicine ,General Chemistry ,010402 general chemistry ,01 natural sciences ,Catalysis ,0104 chemical sciences ,Paramagnetism ,Semiconductor ,Electrical resistivity and conductivity ,Seebeck coefficient ,Thermoelectric effect ,Antiferromagnetism ,Absorption (electromagnetic radiation) ,business - Abstract
Three-dimensional (3D) well-crystallized metal-organic frameworks (MOFs), M-THBQ (M = Fe, Co, Mn, THBQ = tetrahydroxybenzoquinone), are synthesized and characterized. Their structures are determined as cubic cell in the group of Pm-3 from powder X-ray diffraction data, and their properties of electronic, magnetic and spectroscopic are also investigated. They are all semiconductors, and Fe-THBQ exhibits the air-stable n-type thermoelectric characteristic as its Seebeck coefficient reaches -130 μV K-1, and the electrical conductivity is 2.7 x 10-4 S cm-1 at 300 K. Additional, M-THBQ are paramagnetic, and the value of Weiss constant of Fe-THBQ is -219.37 K, indicating the existence of robust intramolecular antiferromagnetic exchanges. Meanwhile, they display strong absorption bands in the range of 220 to 1000 nm, suggest M-THBQ could have the potential to become photoabsorbers, and Fe-THBQ exhibits a narrow bandgap of 0.63 eV according to the ultraviolet absorption edge spectrum. The successful preparation of M-THBQ enriches the family of 3D magnetic conductive MOFs.
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- 2020
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8. The new WEB-accessible online database of the Mössbauer effect data center
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Duorong Liu, Wenhui Zhou, Rile Ge, Zoltán Klencsár, Junhu Wang, Alexandre I. Rykov, and Tao Zhang
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Nuclear and High Energy Physics ,Information retrieval ,Computer science ,business.industry ,Online database ,Data center ,Physical and Theoretical Chemistry ,Condensed Matter Physics ,business ,Atomic and Molecular Physics, and Optics - Abstract
The web-accessible online database (WAD) of the Mössbauer Effect Data Center (MEDC) is one of the worldwide available information services provided by MEDC to the scientific community. It is based on the uniquely wide scope Mössbauer spectroscopy database that has been compiled and maintained by MEDC since the 1960’s. Following enhancements applied to the capabilities of the MEDC core database in connection with the development of a new database management software named “MEDC DBM”, the development of a new web-accessible online database (MEDC WAD) was started in 2019. Here we introduce the current state and the main features of the newly developed MEDC WAD system with emphasis put on its novel and rather unique attributes that can effectively aid the scientific research process in the field of Mössbauer spectroscopy.
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- 2021
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9. The synergistic effect between Ni sites and Ni-Fe alloy sites on hydrodeoxygenation of lignin-derived phenols
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Oliver Y. Gutiérrez, Qiao Han, Yujun Zhao, Shengping Wang, Johannes A. Lercher, Mooeez Ur Rehman, Junhu Wang, Xinbin Ma, and Alexandre I. Rykov
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Cyclohexane ,Process Chemistry and Technology ,Cyclohexanol ,02 engineering and technology ,010402 general chemistry ,021001 nanoscience & nanotechnology ,01 natural sciences ,Catalysis ,0104 chemical sciences ,Reaction rate ,chemistry.chemical_compound ,chemistry ,Hydrogenolysis ,Phenol ,0210 nano-technology ,Bimetallic strip ,Hydrodeoxygenation ,General Environmental Science ,Nuclear chemistry - Abstract
The catalytic hydrodeoxygenation (HDO) of lignin-derived phenolic compounds is a critical step in the upgrading of bio-oil. Here, bimetallic Ni-Fe nanoparticles supported on mesoporous carbon spheres (MCSs) were fabricated and applied in HDO of phenol. In comparison with monometallic Ni and Fe catalysts, the bimetallic Ni-Fe catalyst exhibited better performance for phenol HDO due to the formation of Ni-Fe alloy phase identified by X-ray powder diffraction (XRD) and Mossbauer spectroscopy techniques. Among several explored ratios, the catalysts with Ni/Fe ratio of 3/1 presented the highest cyclohexane yield. The reaction occurred in two consecutive steps: the hydrogenation of phenol to cyclohexanol and the further hydrogenolysis of cyclohexanol to cyclohexane. Kinetic studies showed that the hydrogenolysis of cyclohexanol controlled the overall reaction rate of phenol HDO due to the lower reaction rate of this step. Indeed, the turnover frequency (TOF) values of cyclohexanol normalized by surface metallic Ni sites exhibited a linear correlation with Ni-Fe alloy sites. The alloying of iron in the bimetallic Ni-Fe catalysts significantly enhanced the adsorption strength of cyclohexanol, which is the reason of the high activity of the Ni-Fe alloy particles. Thus, Fe-containing sites adsorb the hydroxyl species while Ni sites perform the H2 activation, their synergistic effect plays a key role in phenol HDO process.
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- 2019
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10. Improving Syngas Selectivity of Fe2O3/Al2O3 with Yttrium Modification in Chemical Looping Methane Conversion
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Xiaodong Wang, Junhu Wang, Yu Kang, Lin Li, Ming Tian, Jian Lin, Chuande Huang, Baolin Hou, Xiaoli Pan, and Alexandre I. Rykov
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Materials science ,010405 organic chemistry ,Compatibility (geochemistry) ,chemistry.chemical_element ,General Chemistry ,Yttrium ,010402 general chemistry ,01 natural sciences ,Oxygen ,Catalysis ,Methane ,0104 chemical sciences ,chemistry.chemical_compound ,Chemical engineering ,chemistry ,Lattice oxygen ,Selectivity ,Syngas - Abstract
It is of great significance to improve the syngas selectivity of Fe-based oxygen carriers (OCs), because of their sufficient lattice oxygen, low cost, and environmental compatibility in chemical lo...
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- 2019
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11. Abrading bulk metal into single atoms
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Gao-Feng Han, Feng Li, Alexandre I. Rykov, Yoon-Kwang Im, Soo-Young Yu, Jong-Pil Jeon, Seok-Jin Kim, Wenhui Zhou, Rile Ge, Zhimin Ao, Tae Joo Shin, Junhu Wang, Hu Young Jeong, and Jong-Beom Baek
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Biomedical Engineering ,General Materials Science ,Bioengineering ,Electrical and Electronic Engineering ,Condensed Matter Physics ,Atomic and Molecular Physics, and Optics - Abstract
Single-atom catalysts have recently attracted considerable attention because of their highly efficient metal utilization and unique properties. Finding a green, facile method to synthesize them is key to their widespread commercialization. Here we show that single-atom catalysts (including iron, cobalt, nickel and copper) can be prepared via a top-down abrasion method, in which the bulk metal is directly atomized onto different supports, such as carbon frameworks, oxides and nitrides. The level of metal loading can be easily tuned by changing the abrasion rate. No synthetic chemicals, solvents or even water were used in the process and no by-products or waste were generated. The underlying reaction mechanism involves the mechanochemical force in situ generating defects on the supports, then trapping and stably sequestering atomized metals.
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- 2021
12. Investigation on the demetallation of Fe-N-C for oxygen reduction reaction: The influence of structure and structural evolution of active site
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Xinlong Xu, Xiaoming Zhang, Zhichong Kuang, Zhangxun Xia, Alexandre I. Rykov, Shansheng Yu, Junhu Wang, Suli Wang, and Gongquan Sun
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Process Chemistry and Technology ,Catalysis ,General Environmental Science - Published
- 2022
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13. The effect of tuning the coordination sphere of iron complexes for the oxygen reduction reaction in acidic media
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Tianlong Zheng, Xiaojiang Wang, Junhu Wang, Xuning Li, Yizhao Tang, Alexandre I. Rykov, Qinggang He, Jun Cheng, Xue Zhang, Xiaoyu Li, and School of Chemical and Biomedical Engineering
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Coordination sphere ,Materials science ,Catalysts ,Renewable Energy, Sustainability and the Environment ,Inorganic chemistry ,Materials Chemistry ,Electrochemistry ,Chemical engineering [Engineering] ,Electrolytic Reduction ,Oxygen reduction reaction ,Condensed Matter Physics ,Surfaces, Coatings and Films ,Electronic, Optical and Magnetic Materials - Abstract
As a type of important non-precious catalyst for the oxygen reduction reaction (ORR), the regulating role of a metal centre in metal-macrocycles and other complexes for activity has been extensively studied. However, a common guideline to explain the effect of peripheral coordinated-ligands has not been reached. Herein, a series of organic iron complexes (denoted as FeL, L = TAA, Pc, TPP, Corrole, Tim and Salen) were synthesized as ORR catalysts and an explicit relationship of structure-activity was constructed. The kinetic current density for these compounds was identified to follow the order of FeTAA > FePc > FeTPP > FeCorrole > FeTim > FeSalen. An electron-transfer number close to 4 was derived for all these complexes except for FeTim and FeSalen, implying a near complete reduction of oxygen to water. X-ray absorption near edge structure spectroscopy (XANES) and Mössbauer spectroscopy were used to probe the nature of the distinct activities by investigating the iron-centre electron structures. Density function theory (DFT) calculations were carried out to study the charge redistribution across the iron complexes. Novel activity descriptors including the charge and spin densities on the Fe site were proposed and validated by available experimental data, presenting a strategy to design highly-active nonprecious metal complex catalysts with specific supporting ligands. Published version Q.H. acknowledges the financial support from the National Natural Science Foundation of China (grant nos. 21978260, 21676241, and U1732111).
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- 2021
14. Insight into the role of Ni–Fe dual sites in the oxygen evolution reaction based on atomically metal-doped polymeric carbon nitride
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Rui Si, Junhu Wang, Huanqiao Li, Miao Shu, Xinlong Xu, Alexandre I. Rykov, Suli Wang, Shansheng Yu, Chuchu Wu, Gongquan Sun, Xiaoming Zhang, and Xia Zhangxun
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Tafel equation ,Materials science ,Renewable Energy, Sustainability and the Environment ,Inorganic chemistry ,Oxygen evolution ,02 engineering and technology ,General Chemistry ,Overpotential ,021001 nanoscience & nanotechnology ,Catalysis ,Metal ,Metal doped ,chemistry.chemical_compound ,chemistry ,visual_art ,visual_art.visual_art_medium ,General Materials Science ,Density functional theory ,0210 nano-technology ,Carbon nitride - Abstract
The oxygen evolution reaction (OER) plays a critical role in efficient conversion and storage of renewable energy sources, whereas the active sites for the most representative electrocatalysts, Ni–Fe compounds, remain under debate. In this work, we have developed polymeric carbon nitride (PCN) with atomically dispersed N-coordinated Ni–Fe sites to investigate the OER process. The Ni–Fe dual sites consist of adjacent Ni and Fe atoms coordinated with N atoms in the PCN matrix. NiFe-codoped PCN exhibits higher electrocatalytic activity than monometal-doped catalysts, showing a lower overpotential (310 mV at 10 mA cm−2) and smaller Tafel slope (38 mV dec−1) in 1 M KOH, indicating that Ni–Fe dual-metal sites significantly favor the OER process. According to density functional theory calculations based on the oxidized-NiFe@PCN model, it was found that adjacent Ni and Fe atoms co-participate in the OER process for NiFe-codoped PCN, leading to a much lower energy barrier (0.10 or 0.22 eV for U = 1.58 V), while the effect of electronic modification of the single metal active sites by the other component (Ni sites by Fe sites or vice versa) contributes less to activity enhancement, thus leading to a rational explanation on the synergistic effect of the NiFe-based OER catalysts.
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- 2019
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15. In situ encapsulation of iron(0) for solar thermochemical syngas production over iron-based perovskite material
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Aiqin Wang, Jian Wu, Chun-Ran Chang, Xiaodong Wang, Jian Lin, You-Tao Chen, Ming Tian, Junhu Wang, Baolin Hou, Chuande Huang, Alexandre I. Rykov, and Xiaoli Pan
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Materials science ,Oxide ,chemistry.chemical_element ,General Chemistry ,Biochemistry ,Oxygen ,Methane ,Catalysis ,lcsh:Chemistry ,chemistry.chemical_compound ,chemistry ,Chemical engineering ,lcsh:QD1-999 ,Aluminium ,Materials Chemistry ,Environmental Chemistry ,Syngas ,Perovskite (structure) ,Carbon monoxide - Abstract
Methane-to-syngas conversion plays an important role in industrial gas-to-liquid technologies, which is commercially fulfilled by energy-intensive reforming methods. Here we present a highly selective and durable iron-based La0.6Sr0.4Fe0.8Al0.2O3-δ oxygen carrier for syngas production via a solar-driven thermochemical process. It is found that a dynamic structural transformation between the perovskite phase and a Fe0@oxides core–shell composite occurs during redox cycling. The oxide shell, acting like a micro-membrane, avoids direct contact between methane and fresh iron(0), and prevents coke deposition. This core–shell intermediate is regenerated to the original perovskite structure either in oxygen or more importantly in H2O–CO2 oxidant with simultaneous generation of another source of syngas. Doping with aluminium cations reduces the surface oxygen species, avoiding overoxidation of methane by decreasing oxygen vacancies in perovskite matrix. As a result, this material exhibits high stability with carbon monoxide selectivity above 95% and yielding an ideal syngas of H2/CO ratio of 2/1. Iron-based oxides are promising oxygen carriers for thermochemical syngas production, but can be prone to deactivation during the reaction. Here an iron-based catalyst is shown to transform reversibly between perovskite and core–shell structures during methane-to-syngas conversion, accounting for its high stability toward coke deposition.
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- 2018
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16. Unique role of Mössbauer spectroscopy in assessing structural features of heterogeneous catalysts
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Kaiyue Zhu, Mingyuan Zheng, Xuefeng Zhu, Alexandre I. Rykov, Jifeng Pang, Ming Tian, Xiaodong Wang, Junhu Wang, Yanqiang Huang, Jiayi Liu, Weishen Yang, Bin Liu, Xuning Li, Tao Zhang, and School of Chemical and Biomedical Engineering
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biology ,Chemistry ,Process Chemistry and Technology ,Inorganic chemistry ,Chemical engineering [Engineering] ,Active site ,Catalytic Reactions ,02 engineering and technology ,010402 general chemistry ,021001 nanoscience & nanotechnology ,Electrocatalyst ,01 natural sciences ,Catalysis ,0104 chemical sciences ,Mössbauer Spectroscopy ,Chemical engineering ,Oxidation state ,Phase (matter) ,Mössbauer spectroscopy ,Photocatalysis ,biology.protein ,Reactivity (chemistry) ,0210 nano-technology ,General Environmental Science - Abstract
Their wide availability in nature, low cost, high reactivity, and low toxicity make Fe-based catalysts versatile in various catalysis fields, including photocatalysis, Fenton-like reaction, electrocatalysis, Li-ion batteries (LIBs), Fischer–Tropsch synthesis (FTS), biomass conversion, N2O decomposition and etc. Mossbauer spectroscopy, a powerful technique that is able to give account of structural features for all iron species taking part in the catalysis process, is considered to be a crucial technique for determining catalyst phase, identifying active site, and investigating correlations between catalytic behavior and the coordination structure of catalysts, which are highly desirable for clarifying the catalytic mechanisms. Each kind of Fe-based materials could be functionalized in the most suitable catalysis field, wherever Mossbauer technique may play a unique role. For instance, Fe-N-C based materials are extensively investigated as electrocatalysts for oxygen reduction reaction and Mossbauer spectroscopy application in this field has been utilized to identify the chemical nature of the active site on the Fe-N-C catalyst. Iron carbides are considered as the most active phase for FTS and Mossbauer technique is widely applied in determining the chemical phase of catalysts. Fe-based silicates, phosphates or polyanionic compounds are recognized as promising cathode materials for LIBs, for which Mossbauer technique has been mainly applied for tracking of the oxidation state and coordination environment change of Fe between charged and discharged states of the batteries. Similar phenomena can also be found in other catalysis fields. To give a clear understanding of which field is most suitable for a certain Fe-based catalyst and the best role of the Mossbauer technique in a certain catalysis field associated with the investigation of the mechanism, in this review, the recent advances of applying Mossbauer technique in catalysis are thoroughly summarized, including results from environmental catalysis and energy catalysis. Remarkable cases of study are highlighted and brief insight into applying Mossbauer technique for various Fe-based materials in their special catalysis field is presented. Finally, the trends for future potential applications of Mossbauer technique are discussed.
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- 2018
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17. Further development of the database of the Mössbauer Effect Data Center
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Tao Zhang, Wenhui Zhou, Junhu Wang, Rile Ge, Zoltán Klencsár, Alexandre I. Rykov, and Duorong Liu
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Nuclear and High Energy Physics ,Database ,010308 nuclear & particles physics ,Computer science ,business.industry ,Frame (networking) ,Web access ,Condensed Matter Physics ,computer.software_genre ,01 natural sciences ,Atomic and Molecular Physics, and Optics ,Information resource ,0103 physical sciences ,Information system ,Data center ,Physical and Theoretical Chemistry ,010306 general physics ,business ,computer - Abstract
The Mössbauer spectroscopy database compiled and maintained by the Mössbauer Effect Data Center (MEDC) is a unique, wide-scope Mössbauer-spectroscopy related information resource, which forms the basis of information services provided by the Mössbauer Effect Data Center to the worldwide scientific community. The Mössbauer Effect Reference and Data Journal (MERDJ) and the Mössbauer Web Access Database (MWAD), both published by MEDC, are widely known examples of the services that rely on the MEDC database. In recent years a further improvement of these services, especially that of MWAD, has been envisaged, and as a first step of the corresponding process the further development of the MEDC database was started. In the present work we introduce the main features of the MEDC database and the steps that have been already taken in the frame of its further development. Implications of the work regarding the associated services are also presented.
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- 2020
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18. Iron Single Atom Catalyzed Quinoline Synthesis
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Shibo Xi, Zhongxin Chen, Ming Joo Koh, Cuibo Liu, Xin Zhou, Runlai Li, Jingting Song, Wenhui Zhou, Junhu Wang, Linxin Zhong, Xinwen Peng, Haohan Li, Jia Liu, Alexandre I. Rykov, Xing Li, Hai-Sen Xu, Kian Ping Loh, Rile Ge, Xiaoxu Zhao, and Lu Wang
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Materials science ,Mechanical Engineering ,Quinoline ,chemistry.chemical_element ,Combinatorial chemistry ,Catalysis ,chemistry.chemical_compound ,chemistry ,Suzuki reaction ,Mechanics of Materials ,Atom ,General Materials Science ,Selectivity ,Carbon ,Derivative (chemistry) ,Reusability - Abstract
The production of high-value chemicals by single-atom catalysis is an attractive proposition for industry owing to its remarkable selectivity. Successful demonstrations to date are mostly based on gas-phase reactions, and reports on liquid-phase catalysis are relatively sparse owing to the insufficient activation of reactants by single-atom catalysts (SACs), as well as, their instability in solution. Here, mechanically strong, hierarchically porous carbon plates are developed for the immobilization of SACs to enhance catalytic activity and stability. The carbon-based SACs exhibit excellent activity and selectivity (≈68%) for the synthesis of substituted quinolines by a three-component oxidative cyclization, affording a wide assortment of quinolines (23 examples) from anilines and acetophenones feedstock in an efficient, atom-economical manner. Particularly, a Cavosonstat derivative can be synthesized through a one-step, Fe1 -catalyzed cyclization instead of traditional Suzuki coupling. The strategy is also applicable to the deuteration of quinolines at the fourth position, which is challenging by conventional methods. The synthetic utility of the carbon-based SAC, together with its reusability and scalability, renders it promising for industrial scale catalysis.
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- 2021
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19. Zinc-modulated Fe–Co Prussian blue analogues with well-controlled morphologies for the efficient sorption of cesium
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Weimin Cong, Alexandre I. Rykov, Hailian Tang, Xiaodong Wang, Kaixin Zhu, Junhu Wang, Ayyakannu Sundaram Ganeshraja, Qiaohui Fan, Jiayi Liu, Wei Xu, Changzi Jin, Xuning Li, and Rile Ge
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Prussian blue ,Materials science ,Renewable Energy, Sustainability and the Environment ,Inorganic chemistry ,Oxide ,chemistry.chemical_element ,Sorption ,02 engineering and technology ,General Chemistry ,Zinc ,010402 general chemistry ,021001 nanoscience & nanotechnology ,01 natural sciences ,0104 chemical sciences ,Ion ,Crystal ,chemistry.chemical_compound ,Adsorption ,chemistry ,Organic chemistry ,General Materials Science ,Absorption (chemistry) ,0210 nano-technology - Abstract
Prussian blue analogues (PBAs) with tunable compositions and morphologies have demonstrated great potential in many applications. We successfully synthesized a series of KFexZn1−x[Co(CN)6] (FexZn1−x–Co) PBAs with well-controlled compositions and morphologies and used them as adsorbents for the removal of Cs+ ions. The increase of Zn : Fe ratio had a significant influence on the final morphology and improved the sorption capacity for Cs. X-ray diffraction and X-ray absorption fine structure spectra were used to confirm that the Cs+ ions were inserted into the crystal channels rather than simply adsorbed on the surface of the PBAs. Based on the quantitative correlation between the concentration of ions released from the PBAs and the Cs+ ions adsorbed, the mechanism of Cs+ sorption in the FexZn1−x–Co PBAs was studied and a Zn2+-modulated Cs+ sorption model, which illustrated the difference in sorption behavior between the FexZn1−x–Co PBAs, was proposed and confirmed by FTIR spectra, extended X-ray absorption fine structure spectra and 57Fe Mossbauer spectra. The results indicated that the FexZn1−x–Co PBAs are excellent candidates for the removal of radioactive 137Cs from nuclear waste.
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- 2017
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20. Hydrazine drastically promoted Fenton oxidation of bisphenol A catalysed by a FeIII–Co Prussian blue analogue
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Junhu Wang, Alexandre I. Rykov, and Xuning Li
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Prussian blue ,Bisphenol A ,biology ,Reducing agent ,Process Chemistry and Technology ,Hydrazine ,Active site ,02 engineering and technology ,General Chemistry ,010402 general chemistry ,021001 nanoscience & nanotechnology ,Photochemistry ,01 natural sciences ,Catalysis ,0104 chemical sciences ,chemistry.chemical_compound ,chemistry ,Mössbauer spectroscopy ,biology.protein ,Degradation (geology) ,0210 nano-technology ,Nuclear chemistry - Abstract
Herein, for the first time it has been demonstrated that hydrazine (Hz) could significantly promote the bisphenol A (BPA) degradation in the Fenton reaction catalysed by Fe[Co(CN)6] 2H2O Prussian blue analogue (FeIII–Co PBA). Results indicate that the dramatic enhancement of BPA degradation could be partly attributed to the induced homogeneous Fenton reaction by the enhanced dissolution of FeIII–Co PBA. Meanwhile, the Hz coordinated iron site (H2NH2N–Fe), which is evolved from the original water coordinated iron site (H2O–Fe), was identified as the main active site. A possible reaction pathway involving the proposed active iron species was proposed.
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- 2016
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21. Enhanced oxygen reduction with single-atomic-site iron catalysts for a zinc-air battery and hydrogen-air fuel cell
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Alexandre I. Rykov, Rongan Shen, Qing Peng, Chen Chen, Dingsheng Wang, Xiaodong Wen, Weng-Chon Cheong, Shu Zhao, Wenxing Chen, Zhongbin Zhuang, Lirong Zheng, Yadong Li, Shichang Cai, Yuanjun Chen, Juncai Dong, Haolin Tang, and Shufang Ji
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inorganic chemicals ,Materials science ,Hydrogen ,Kirkendall effect ,Science ,General Physics and Astronomy ,chemistry.chemical_element ,02 engineering and technology ,010402 general chemistry ,01 natural sciences ,General Biochemistry, Genetics and Molecular Biology ,Article ,Catalysis ,Active center ,Zinc–air battery ,lcsh:Science ,Multidisciplinary ,General Chemistry ,021001 nanoscience & nanotechnology ,Sulfur ,0104 chemical sciences ,chemistry ,Chemical engineering ,lcsh:Q ,0210 nano-technology ,Platinum ,Carbon - Abstract
Efficient, durable and inexpensive electrocatalysts that accelerate sluggish oxygen reduction reaction kinetics and achieve high-performance are highly desirable. Here we develop a strategy to fabricate a catalyst comprised of single iron atomic sites supported on a nitrogen, phosphorus and sulfur co-doped hollow carbon polyhedron from a metal-organic framework@polymer composite. The polymer-based coating facilitates the construction of a hollow structure via the Kirkendall effect and electronic modulation of an active metal center by long-range interaction with sulfur and phosphorus. Benefiting from structure functionalities and electronic control of a single-atom iron active center, the catalyst shows a remarkable performance with enhanced kinetics and activity for oxygen reduction in both alkaline and acid media. Moreover, the catalyst shows promise for substitution of expensive platinum to drive the cathodic oxygen reduction reaction in zinc-air batteries and hydrogen-air fuel cells., Development of fuel cells and metal-air batteries is hindered by electrocatalyst performance, which can be enhanced with uniform and atomically dispersed active sites. Here the authors report an iron-based electrocatalyst for oxygen reduction in cathodes for a zinc-air battery and a hydrogen-air fuel cell.
- Published
- 2018
22. Fe Co3−O4 nanocages derived from nanoscale metal–organic frameworks for removal of bisphenol A by activation of peroxymonosulfate
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Junhu Wang, M.A. Ahmed, Bo Zhang, Zhaohui Wang, Alexandre I. Rykov, and Xuning Li
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Bisphenol A ,Prussian blue ,Chemistry ,Process Chemistry and Technology ,Radical ,Inorganic chemistry ,02 engineering and technology ,010402 general chemistry ,021001 nanoscience & nanotechnology ,01 natural sciences ,Catalysis ,0104 chemical sciences ,chemistry.chemical_compound ,Nanocages ,X-ray photoelectron spectroscopy ,Degradation (geology) ,Metal-organic framework ,0210 nano-technology ,General Environmental Science - Abstract
Here we report a facile strategy to synthesize porous FexCo3−xO4 nanocages by heating Prussian blue analogues FeyCo1−y[Co(CN)6]0.67 nH2O nanospheres with tunable size and morphology. The iron doping amount had significant influence on the final morphology and the most uniform nanocages were obtained from x = 0.8. The catalytic performance of the nanocages was thoroughly evaluated by activation of peroxymonosulfate (PMS) for removal of bisphenol A (BPA) in water. The influence of different process parameter on the BPA degradation efficiency was examined and the catalytic stability was tested. The BPA degradation pathway was proposed based on GC–MS and LC–MS results. The involved radicals were identified through radical scavenging experiments and electron paramagnetic resonance spectroscopy. Mossbauer and XPS techniques were applied to illustrate the catalytic mechanism and B-site CoII on the surface of FexCo3−xO4 nanocages was determined as the main factor for PMS activation. Results indicate that porous FexCo3−xO4 nanocages are available to serve as alternative environmentally friendly catalysts for pollutants removal by activation of PMS.
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- 2016
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23. Structural and Magnetic Properties of 57 Fe‐Doped TiO 2 and 57 Fe/Sn‐Codoped TiO 2 Prepared by a Soft‐Chemical Process
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Junhu Wang, Xin Liu, Yanjie Wang, Alexandre I. Rykov, Kiyoshi Nomura, Changzi Jin, and Tao Liu
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010302 applied physics ,Anatase ,Aqueous solution ,Inorganic chemistry ,chemistry.chemical_element ,02 engineering and technology ,021001 nanoscience & nanotechnology ,01 natural sciences ,Inorganic Chemistry ,symbols.namesake ,Ferromagnetism ,chemistry ,Rutile ,0103 physical sciences ,Mössbauer spectroscopy ,symbols ,0210 nano-technology ,Tin ,Raman spectroscopy ,Titanium - Abstract
A series of 57Fe-doped TiO2 and 57Fe/Sn-codoped TiO2 materials were prepared by a soft-chemical solution process with various 57Fe concentrations from 0.1 to 1.0 at.-% and a fixed Sn concentration of 2.0 at.-%. In this process, an aqueous H2O2 solution reacts with iron and titanium or iron, tin, and titanium precursor compounds to produce peroxopolymetallic acids. The structures and magnetic properties of the products were characterized by magnetic-moment measurements, X-ray diffraction, and Raman and Mossbauer spectroscopy. The results showed that all samples consisted of a rutile phase (and a trace amount of anatase in 57Fe-TiO2). All samples exhibited hysteresis loops in the M–H curves, which demonstrated the presence of room-temperature ferromagnetism. In addition, their magnetic properties decreased with increasing degree of 57Fe doping. Oxygen vacancies were considered to be responsible for the presence of ferromagnetism.
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- 2016
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24. Graphene encapsulated FexCoy nanocages derived from metal–organic frameworks as efficient activators for peroxymonosulfate
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Alexandre I. Rykov, Junhu Wang, Bo Zhang, Xuning Li, and Yanjie Zhang
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Prussian blue ,Graphene ,Inorganic chemistry ,chemistry.chemical_element ,02 engineering and technology ,010402 general chemistry ,021001 nanoscience & nanotechnology ,01 natural sciences ,Catalysis ,0104 chemical sciences ,law.invention ,chemistry.chemical_compound ,Nanocages ,chemistry ,Catalytic oxidation ,Chemical engineering ,law ,Metal-organic framework ,0210 nano-technology ,Cobalt oxide ,Cobalt - Abstract
A series of nitrogen-doped graphene encapsulated FexCoy bimetallic (FexCoy@C) nanocages were newly fabricated via a one-step thermal decomposition of Prussian blue analogue FeyCo1−y[Co(CN)6]0.67·nH2O nanospheres at different temperatures. The as-synthesized FexCoy@C nanocages were systematically characterized and applied as effective catalysts to activate peroxymonosulfate (PMS) for the removal of bisphenol A (BPA). The catalytic activity of Fe3Co7@C-650 obtained at 650 °C was found to be the highest and also much higher than that of metallic cobalt, cobalt oxide and cobalt–iron spinels. The nitrogen-doped graphene shells were demonstrated to be effective in preventing the release of metal ions, thus resulting in a better stability. Furthermore, the BPA degradation pathway was proposed based on GC-MS and LC-MS results. The involved radicals were identified through electron paramagnetic resonance and radical scavenger experiments. Based on the systematic characterization of the catalyst before and after the catalytic oxidation reaction, the overall mechanism of PMS activation over FexCoy@C nanocages was proposed. Results indicate that FexCoy@C nanocages are available to serve as promising materials for environmental remediation by activation of PMS.
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- 2016
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25. Excellent photo-Fenton catalysts of Fe–Co Prussian blue analogues and their reaction mechanism study
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Alexandre I. Rykov, Junhu Wang, Jiayi Liu, Xuning Li, Hongxian Han, Xin Liu, and Changzi Jin
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Reaction mechanism ,Prussian blue ,Valence (chemistry) ,Singlet oxygen ,Process Chemistry and Technology ,Radical ,Inorganic chemistry ,Photochemistry ,Catalysis ,Rhodamine ,chemistry.chemical_compound ,chemistry ,Mössbauer spectroscopy ,General Environmental Science - Abstract
Two kinds of Fe–Co Prussian blue analogues (Fe–Co PBAs) with different iron valence state, Fe3[Co(CN)6]2·12H2O and Fe[Co(CN)6]·2H2O, were developed as the photo-Fenton catalysts for in-depth investigation of the heterogeneous Fenton reaction mechanism. These two catalysts showed exceptionally high efficiencies for the degradation of Rhodamine B. The efficient redox cycling of iron species in the Fe–Co PBAs photo-Fenton process was deeply explored by Mossbauer spectroscopy. The excellent photo-Fenton activities of these Fe–Co PBAs were ascribed to the existence of highly dispersed water coordinated iron sites and abundant vacancies in the metal-organic-frameworks. Singlet oxygen was identified by radical scavenger experiments and electron paramagnetic resonance spectroscopy, which plays a key role in the photo-Fenton process. Based on these comprehensive experimental results, a novel radical reaction mechanism was proposed. This work not only opens up a new avenue for the application of Fe–Co PBAs, but also gives a deeper insight into the mechanisms of heterogeneous Fenton reactions.
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- 2015
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26. Crystal structure refinement of the electron-transfer-active potassium manganese hexacyanoferrates and isomorphous potassium manganese hexacyanocobaltates
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Alexandre I. Rykov, Xuning Li, and Junhu Wang
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Prussian blue ,Inorganic chemistry ,chemistry.chemical_element ,Quadrupole splitting ,Manganese ,Crystal structure ,Condensed Matter Physics ,Resonance (chemistry) ,Electronic, Optical and Magnetic Materials ,Inorganic Chemistry ,Electron transfer ,Crystallography ,chemistry.chemical_compound ,chemistry ,Mössbauer spectroscopy ,Materials Chemistry ,Ceramics and Composites ,Singlet state ,Physical and Theoretical Chemistry - Abstract
We report on the crystal structure refinements in the novel electron-transfer-active Prussian Blue analogs (PBA) KMn 4 II [ Co 1 − x III Fe x III ( CN ) 6 ] 3 · n H 2 O ( n ≃ 12 ). The series of novel PBA with the end members of KMn 4 [ Co ( CN ) 6 ] 3 · 11.8 H 2 O and KMn 4 [ Fe ( CN ) 6 ] 3 · 10.5 H 2 O have been synthesized for the first time, all showing a number of extra-reflections incompatible with ordinary face-centered cell of the Fm-3m symmetry group. We have analyzed the Rietveld patterns for x = 0 , 0.53 , 1 and found that the extra-reflections could be well fitted using several primitive (P) cell symmetries. The best fitting quality was obtained using the noncentrosymmetric space group (S.G.) P 4 ¯ 3 m (Z=1) with the origin of coordinate system shifted into a zeolitic site. In this structure model, the Co–CN–Mn entities are bent owing to the charge introduced by the K+ insertion that induces also the electron transfer between Mn and Fe. Using Mossbauer spectroscopy the electron transfer activity is identified with the appearance of unsplit resonance at the isomer shift of typically −0.15 mm/s evidencing the low-spin state for Fe3+ and Fe2+ species. In the same P 4 ¯ 3 m phases doped with Fe 2 + 57 into the Mn site, a sequence of discrete values of quadrupole splitting (0 mm/s, 0.9 mm/s, 1.8 mm/s) is observed and attributed to different conformations of the polyhedra, in which the ground states are orbital triplet, doublet and singlet, respectively.
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- 2015
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27. Prussian blue/TiO2 nanocomposites as a heterogeneous photo-Fenton catalyst for degradation of organic pollutants in water
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Mingrun Li, Junhu Wang, Huangzhao Wei, Dionysios D. Dionysiou, Xin Liu, Chenglin Sun, Virender K. Sharma, Songhua Yu, Alexandre I. Rykov, Changzi Jin, and Xuning Li
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chemistry.chemical_compound ,Prussian blue ,Nanocomposite ,chemistry ,Radical ,Titanium dioxide ,Inorganic chemistry ,Rhodamine B ,Redox ,Catalysis ,Isophorone - Abstract
Nowadays, a lot of research focuses on accelerating FeII/FeIII redox cycles to increase the pseudo first-order rates of the Fenton reaction. Here, Prussian blue/titanium dioxide nanocomposites (PB/TiO2 NPs) were designed as heterogeneous photo-Fenton catalysts to increase the FeII recovery in degrading organic contaminants in water for the first time. The PB/TiO2 NPs were characterized by various analytical techniques to obtain the optimum ratio of PB and TiO2 for efficient degradation of organics. The performance of the catalysts was tested by following the removal of rhodamine B dye, salicylic acid, m-cresol, and isophorone under various conditions (pH, ratios of PB and TiO2, H2O2, and temperature). Formation of the intermediates of iron (FeII/FeIII) in the studied system using Mossbauer spectroscopy was explored for the first time and presents important insights into the relevant catalytic phenomena. The generation of ˙OH radicals in the reaction system was identified using electron paramagnetic resonance spectroscopic techniques. Results demonstrated that the developed PB/TiO2 NPs were stable and could degrade organic contaminants in water efficiently.
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- 2015
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28. A structural phase transition coupled to the Fe3+ spin-state crossover in anhydrous RbMn[Fe(CN)6]
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Jun Wang, Alexandre I. Rykov, Tao Zhang, and Kiyoshi Nomura
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Nuclear and High Energy Physics ,Phase transition ,Spin states ,Ligand ,Chemistry ,Inorganic chemistry ,Condensed Matter Physics ,Atomic and Molecular Physics, and Optics ,Crystallography ,Spin crossover ,Phase (matter) ,Mössbauer spectroscopy ,Anhydrous ,Physical and Theoretical Chemistry ,Linkage isomerism - Abstract
Linkage isomerism is the coexistence of iso-compositional molecules or solids differing by connectivity of the metal to a ligand. In a crystalline solid state, the rotation is possible for asymmetric ligands, e.g., for cyanide ligand. Here we report on our observation of a phase transition in anhydrous RbMn[Fe(CN)6] (nearly stoichiometric) and on the effect of linkage isomerism ensuing our interpretation of the results of Mossbauer study in which we observe the iron spin state crossover among two phases involved into this transition. The anhydrous RbMn[Fe(CN)6] can be prepared via prolonged thermal treatment (1 week at at 80 °C) of the as-synthesized hydrated RbMn[Fe(CN)6]·H2O. The latter compound famous for its charge-transfer phase transition is a precursor in our case. As the temperature is raising above 80 °C (remaining below 100 °C) we observe RbMn[Fe(CN)6] that inherited its F-43 m symmetry from RbMn[Fe(CN)6]·H2O transforming to a phase of the Fm-3 m symmetry. In the latter, more than half of Fe3 + ions are in high-spin state. We suggest a plausible way to explain the spin-crossover that is to allow the linkage isomerism by rotation of the cyanide ligands.
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- 2012
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29. Influence of hydroxyapatite on maghemite-to-hematite phase transfer of FeO x -hydroxyapatite composite
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Junhu Wang, Kunfeng Zhao, Changzi Jin, Fuhai Yu, Alexandre I. Rykov, Tao Zhang, and Xin Liu
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Nuclear and High Energy Physics ,Materials science ,Magnetism ,Maghemite ,Mineralogy ,Hematite ,engineering.material ,Condensed Matter Physics ,Atomic and Molecular Physics, and Optics ,law.invention ,X-ray absorption fine structure ,Octahedron ,Chemical engineering ,law ,visual_art ,Mössbauer spectroscopy ,visual_art.visual_art_medium ,engineering ,Thermal stability ,Calcination ,Physical and Theoretical Chemistry - Abstract
FeO x -hydroxyapatite (FeO x -HAP) composites with different FeO x contents were prepared, and compared with pure FeOx, the FeO x -HAP composites exhibit strongly magnetic behavior in an external magnetic field even after 600 °C calcination. The combination of 57Fe Mossbauer and Fe K-edge XAFS indicates that HAP can stabilize the size and crystal phase of γ-Fe2O3 during heat treatment. Even after 600 °C calcination, the interaction imposed by HAP could produce large amounts of distorted octahedral coordination Fe sites in the interior lattice and then result in strong magnetism. The thermally stable γ-Fe2O3-HAP composites may provide a new opportunity for developing efficient supported crystal-dependent catalysts.
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- 2012
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30. Vibration DOS of 57Fe and Zn doped rutile Sn(Sb) oxides
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Alexandre I. Rykov, Zoltán Németh, Kiyoshi Nomura, and Yoshitaka Yoda
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Nuclear and High Energy Physics ,Materials science ,Doping ,Inorganic chemistry ,Analytical chemistry ,Inelastic scattering ,Condensed Matter Physics ,Atomic and Molecular Physics, and Optics ,Spectral line ,Ion ,Condensed Matter::Materials Science ,Ferromagnetism ,Rutile ,Mössbauer spectroscopy ,Density of states ,Condensed Matter::Strongly Correlated Electrons ,Physical and Theoretical Chemistry - Abstract
Sn oxides co-doped with Zn, Sb and 57Fe were prepared by sol-gel method, and especially the doping effect of non-magnetic Zn ions was studied. The bulk saturation magnetization is in accordance with the intensity of the magnetic component in Mossbauer spectra. The nuclear inelastic scattering (NIS) spectra of these compounds were measured in SPring 8. The vibration density of states (VDOS) of 57Fe doped Sn(Sb) oxides showed that the softening peaks around 15–20 meV appeared by doping less than 10% Zn ions. The clusters of non-magnetic ZnFe2O4 may be most probably formed under the limit of XRD detections. The results suggest that the strengthening of ferromagnetism, which appears in the dilute Zn doping, may occur due to the spin arrangement of dilute Fe3 + through magnetic defects rather than the formation of magnetic iron oxides.
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- 2011
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31. Orbital order in layered manganites probed with 57Fe Mössbauer spectroscopy
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Kiyoshi Nomura, Yutaka Ueda, and Alexandre I. Rykov
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Nuclear and High Energy Physics ,Condensed matter physics ,Chemistry ,Doping ,Rare earth ,Quadrupole splitting ,Condensed Matter Physics ,Atomic and Molecular Physics, and Optics ,Ion ,Crystallography ,Order (biology) ,Mössbauer spectroscopy ,Mossbauer spectra ,Physical and Theoretical Chemistry - Abstract
In mixed-valence manganites, the doped iron species play the roles of simultaneous probes and ruiners for the orbital order occurring in the Mn-O electronic subsystem. We investigated the Mn3 + -based undoped system RBaMn2O5.5 and the half-doped systems RBaMn2O6, and RBaMn2O5 substituting a tiny portion of Mn3 + sites with Fe3 + . Single-site Mossbauer spectra were observed at room temperature in the charge-orbitally ordered state, i.e., below TCOO, for R=Y, Sm, Gd. Size of the rare earth ion strongly influences the quadrupole splitting in RBaMn2O6, but not in RBaMn2O5.
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- 2011
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32. Manganite charge and orbitally ordered and disordered states probed by Fe substitution into Mn site in , and (, Gd, Sm, Nd, Pr, La)
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Alexandre I. Rykov, Kiyoshi Nomura, and Yutaka Ueda
- Subjects
Mössbauer effect ,Condensed matter physics ,Chemistry ,chemistry.chemical_element ,Crystal structure ,Quadrupole splitting ,Manganese ,Condensed Matter Physics ,Manganite ,Electronic, Optical and Magnetic Materials ,Inorganic Chemistry ,Charge ordering ,Crystallography ,Mössbauer spectroscopy ,Materials Chemistry ,Ceramics and Composites ,Physical and Theoretical Chemistry ,Perovskite (structure) - Abstract
The layered manganese oxides Ln BaMn 1.96 Fe 0.04 O y ( Ln = Y , Gd, Sm, Nd, Pr, La) have been synthesized for y = 5 , 5.5 and 6. In the oxygen-saturated state ( y = 6 ) they exhibit the charge and orbital order at ambient temperature for Ln = Y , Gd, Sm, but unordered e g -electronic system for Ln = La , Pr, Nd. Fourfold increase of quadrupole splitting was observed owing to the charge and orbital ordering. This is in agreement with the jumplike increase in distortion of the reduced perovskite-like cell for the charge and orbitally ordered manganites compared to the unordered ones. Substitution of 2% of Mn by Fe suppresses the temperatures of structural and magnetic transitions by 20–50 K. Parameters of the crystal lattices and the room-temperature Mossbauer spectra were studied on 40 samples whose structures were refined within five symmetry groups: P 4 / mmm , P 4 / nmm , Pm - 3 m , Icma and P 2 / m . Overwhelming majority of the Fe species are undifferentiated in the Mossbauer spectra for most of the samples. Such the single-component spectra in the two-site structures are explained by the preference of Fe towards the site of Mn(III) and by the segmentation of the charge and orbitally ordered domains.
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- 2009
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33. Anisotropic phonon density of states: the application of Rietveld and Mössbauer texture analysis in aligned powders
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Kiyoshi Nomura, Y. Ueda, Alexandre I. Rykov, and Makoto Seto
- Subjects
Materials science ,Condensed matter physics ,Basis (linear algebra) ,Rietveld refinement ,Mössbauer spectroscopy ,Neutron ,Cuprate ,Texture (crystalline) ,Inelastic scattering ,Anisotropy ,General Biochemistry, Genetics and Molecular Biology - Abstract
Since it is not always feasible to synthesize single crystals of novel materials, the orientation of layered polycrystals has become an attractive basis for studying the angular dependence of inelastic scattering of X-rays or neutrons. Utilizing Rietveld analysis, the anisotropic properties of layered structures in novel manganites and cuprates have been studied with oriented powders instead of single crystals. The phonon density of states (DOS) and atomic thermal displacement are anisotropic in theA-site-ordered manganites LnBaMn2Oyfor the seriesy= 5 andy= 6 (Ln = Y, La, Sm and Gd). This article establishes the angular dependence of the DOS on texture of arbitrary strength, links the textures observed by X-ray and γ-ray techniques, and solves the problem of disentanglement of the Goldanskii–Karyagin effect and texture in Mössbauer spectra.
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- 2009
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34. A 'copolymer-co-morphology' conception for shape-controlled synthesis of Prussian blue analogues and as-derived spinel oxides
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Kaiyue Zhu, Weishen Yang, D. L. Nagy, M.A. Ahmed, Junhu Wang, Luhua Jiang, Alexandre I. Rykov, Gongquan Sun, Csilla Bogdán, Xuning Li, and Yuan Lizhi
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chemistry.chemical_classification ,Prussian blue ,Materials science ,Spinel ,Nanotechnology ,02 engineering and technology ,Polymer ,Thermal treatment ,engineering.material ,010402 general chemistry ,021001 nanoscience & nanotechnology ,Electrocatalyst ,01 natural sciences ,0104 chemical sciences ,Nanomaterials ,chemistry.chemical_compound ,Chemical engineering ,chemistry ,Mössbauer spectroscopy ,Copolymer ,engineering ,General Materials Science ,0210 nano-technology - Abstract
The morphologically and compositionally controlled synthesis of coordination polymers and spinel oxides is highly desirable for realizing new advanced nanomaterial functionalities. Here we develop a novel and scalable strategy, containing a "copolymer-co-morphology" conception, to shape-controlled synthesis of various types of Prussian blue analogues (PBAs). Three series of PBAs MyFe1-y[Co(CN)6]0.67·nH2O (MyFe1-y-Co, M = Co, Mn and Zn) with well-controlled morphology have been successfully prepared through this strategy. Using MnyFe1-y-Co PBAs as the model, by increasing the relative content of Mn, flexible modulation of the morphology could be easily realized. In addition, a series of porous MnxFe1.8-xCo1.2O4 nano-dices with well-inherited morphologies and defined cation distribution could be obtained through a simple thermal treatment of the PBAs. All these results demonstrate the good universality of this novel strategy. When evaluated as an electrocatalyst, the octahedral-site Mn(III)/Mn(IV) content in MnxFe1.8-xCo1.2O4, mainly determined by sensitive (57)Fe Mössbauer in combination with X-ray photoelectron spectroscopic techniques, was discovered to be directly correlated with the oxygen reduction/evolution reaction (ORR/OER) activity.
- Published
- 2016
35. XAFS and CEMS study of dilute magneto-optical semiconductor, Fe doped TiO2 films
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Alexandre I. Rykov, Kenji Sakurai, Hiromi Eba, Tetsuya Hasegawa, and Kiyoshi Nomura
- Subjects
Materials science ,Extended X-ray absorption fine structure ,Metals and Alloys ,Analytical chemistry ,Surfaces and Interfaces ,Magnetic semiconductor ,XANES ,Surfaces, Coatings and Films ,Electronic, Optical and Magnetic Materials ,Pulsed laser deposition ,X-ray absorption fine structure ,Rutile ,Torr ,Materials Chemistry ,Thin film - Abstract
TiO2 films doped with 6% Fe were prepared by pulsed laser deposition (PLD) under different oxygen pressures, and characterized by X-ray absorption fine spectra (XAFS) and conversion electron Mossbauer spectra (CEMS). The edge energy and spectrum profiles of Fe– and Ti K X-ray absorption showed only Fe 3+ and Ti 4+ states for rutile TiO2 films prepared under 10 −1 Torr, the metallic Fe and Ti 4+ for rutile TiO2 films prepared in 10 −6 Torr, and the metallic Fe and the average valance of less than “4+” for Ti in TinO2n−x films prepared by the PLD under 10 −8 Torr. The metallic Fe clusters are also found in the TEM images of TinO2n−x film. Magnetic property of Fe doped TiO2 films prepared by PLD at high vacuum (10 −6 and 10 −8 Torr) is considered to originate mainly from the magnetic metal iron clusters.
- Published
- 2007
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36. Room-temperature magnetism in chemically oxygenated conducting oxide Sr2FeCoO6
- Author
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Alexander P. Nemudry, Alexandre I. Rykov, Kiyoshi Nomura, and T. Mitsui
- Subjects
Colossal magnetoresistance ,Magnetism ,Chemistry ,Metals and Alloys ,Analytical chemistry ,Oxide ,chemistry.chemical_element ,Surfaces and Interfaces ,Oxygen ,Surfaces, Coatings and Films ,Electronic, Optical and Magnetic Materials ,chemistry.chemical_compound ,Ferromagnetism ,Magnet ,Mössbauer spectroscopy ,Materials Chemistry ,Superparamagnetism - Abstract
Using chemical oxidation technique, we prepared a new half-metal Sr2FeCoO6. Mossbauer study has presented a simultaneous test of stability of the oxygen content in this material. Caused by the oxygen leak, the Mossbauer spectra were progressively changed in the sample kept at room temperature. The spectra were measured within 3 months alternately with and without field of 4 kOe of a permanent magnet. The strong magnetizing effect of such a moderate field is conceptually well explained by nanostructuring, assisted by the unexpected effect of the superparamagnetic fraction permanently increasing with the “leak-time”. © 2007 Elsevier B.V. All rights reserved.
- Published
- 2007
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37. ChemInform Abstract: Crystal Structure Refinement of the Electron-Transfer-Active Potassium Manganese Hexacyanoferrates and Isomorphous Potassium Manganese Hexacyanocobaltates
- Author
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Xuning Li, Alexandre I. Rykov, and Junhu Wang
- Subjects
Electron transfer ,Aqueous solution ,Chemistry ,Potassium ,Inorganic chemistry ,chemistry.chemical_element ,General Medicine ,Manganese ,Crystal structure - Abstract
The compounds KMn4 [Fe1-xCox(CN)6] 3·12H2O (0 ≤ x ≤ 1) are synthesized from aqueous solutions containing excess K3[Fe(CN)6] and K3[Co(CN)6], Mn(NO3)2, and excess KCl (50 °C, 10 min).
- Published
- 2015
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38. Frequency Spectra of Quantum Beats in Nuclear Forward Scattering of 57Fe: The Mössbauer Spectroscopy with Superior Energy Resolution
- Author
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I. A. Rykov, Alexandre I. Rykov, X. Zhang, and Kiyoshi Nomura
- Subjects
Physics ,Nuclear and High Energy Physics ,Zeeman effect ,Field (physics) ,Condensed Matter Physics ,Atomic and Molecular Physics, and Optics ,Spectral line ,symbols.namesake ,Quantum beats ,Mössbauer spectroscopy ,Quadrupole ,symbols ,Physical and Theoretical Chemistry ,Atomic physics ,Spectral resolution ,Hyperfine structure - Abstract
Frequency spectra of quantum beats (QB) in nuclear forward scattering (NFS) are analysed and compared to Mossbauer spectra. Lineshape, number of lines, sensitivity to minor sites, and other specific properties of the frequency spectra are discussed. The most characteristic case of combined magnetic and quadrupole interactions is considered in detail for 57Fe. Pure magnetic Zeeman splitting corresponds to a eight-line spectrum of QB, six of which show the same energy separation as the six lines in Mossbauer spectra. Two other lines (called 2′ and 3′) are the lower-energy satellites of the lines 2 and 3. As the quadrupole interaction EQ appears, the satellites remain unsplit in the quantum beat frequency spectra, as well as the first (zero-frequency) and the 6th (largest frequency) lines. Each of the lines 3 and 5 generates a doublet split by 2EQ, and the lines 2 and 4 generate triplets. In QB frequency spectra (QBFS) of thin absorbers of GdFeO3 we demonstrate the enhanced spectral resolution compared to Mossbauer spectra. Small particle size in an antiferromagnet (Fe2O3) was found to affect the QBFS via enhancement of the intensity around zero-frequencies. An asymmetric hyperfine field distribution mixes up into the hybridization with dynamical beats, which enlarges the frequencies of the low-lying QBFS lines and makes their shifts relatively large compared to the shift of the highest-frequency line.
- Published
- 2006
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39. Mössbauer study on the CMR double perovskite AFe0.5Mo0.5O3 with A=(Ba,Sr) or (Sr,Ca): Chemical pressure effect
- Author
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Motomi Katada, Alexandre I. Rykov, Tadashi Yokoyama, Kentaro Nomura, A. Vértes, T. Yamakoshi, T. Mitsui, and Zoltán Homonnay
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Phonon ,Chemistry ,Health, Toxicology and Mutagenesis ,Public Health, Environmental and Occupational Health ,Analytical chemistry ,Inelastic scattering ,Pollution ,Spectral line ,Analytical Chemistry ,Crystallinity ,Nuclear magnetic resonance ,Nuclear Energy and Engineering ,Mössbauer spectroscopy ,Radiology, Nuclear Medicine and imaging ,Hyperfine structure ,Spectroscopy ,Superparamagnetism ,Mossbauer spectrometry - Abstract
The double perovskites, AFe0.5Mo0.5O3with A=(Ba,Sr) or (Sr,Ca), were prepared by a sol-gel method, and the substitution effect at site A was studied by Mossbauer spectrometry. In the Mossbauer spectra of the double perovskite (Ba, Sr)Fe0.5Mo0.5O3, the isomer shifts decreased fromδ=0.72 mm/s to δ=0.4 mm/s and the internal magnetic fields increased with the increase of the Sr content. The Ba-rich samples were shown to contain superparamagnetic components under the same preparation conditions. Better crystallinity and larger hyperfine fields were obtained when 5% of the Sr-content of SrFe0.5Mo0.5O3was substituted by Ca as compared with substitution by Ba. Phonon density of states (DOS) of SrFe0.5Mo0.5O3substituted with Ca or Ba were obtained by nuclear inelastic scattering. The peaks of phonon DOS were shifted, depending on chemical compression/expansion of the lattice. The chemical pressure effect could be observed in the Mossbauer spectra and the phonon DOS spectra.
- Published
- 2005
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40. Low-energy excitations in brownmillerites and related oxides
- Author
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Alexandre I. Rykov, T. Mitsui, Makoto Seto, and Kiyoshi Nomura
- Subjects
Materials science ,Mössbauer effect ,Condensed matter physics ,Synchrotron radiation ,engineering.material ,Inelastic scattering ,Condensed Matter Physics ,Electronic, Optical and Magnetic Materials ,Amorphous solid ,symbols.namesake ,Chemical bond ,engineering ,symbols ,Brownmillerite ,Electrical and Electronic Engineering ,Debye ,Perovskite (structure) - Abstract
It is generally believedthat the glasses andcrystals d iffer very much from each other by their vibrational spectra. However, the crystalline systems with frozen disorder may exhibit the vibrational density of states (DOS) peaked in low-energy region quite resembling to the Bose peak typically observedin glasses. Common origin of these features in disordered crystalline and amorphous systems is discussed in the frameworks of two theoretical concepts: the soft potential and the broken ladder models. The experimental results are presented for large series of standard, oxygen deficient, layered perovskite-based systems and systems containing the metal-oxygen chains. They were studied with using the nuclear ð 57 FeÞ inelastic scattering of synchrotron radiation. The largest excess of vibrational density of states with respect to the Debye-like one was foundin the brownmillerite CaSrFeCoO 5 with the peak energy 7:5 meV: The peak in E � 2 -weightedDOS gðEÞ=E 2 exceeds the Debye behavior ðgðEÞpE 2 Þ by C4 times at room temperature and
- Published
- 2004
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41. Commensurability of higher order between vortex and crystal lattice in RBa2Cu3O7 (R=Y, Nd)
- Author
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Th. Wolf, A.A. Zhukov, Christoph Meingast, Setsuko Tajima, H. Küpfer, Alexandre I. Rykov, P.A.J. de Groot, and G. Ravikumar
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Superconductivity ,Physics ,Condensed matter physics ,Energy Engineering and Power Technology ,Crystal structure ,Condensed Matter Physics ,Electronic, Optical and Magnetic Materials ,Magnetization ,Lattice constant ,Electrical resistivity and conductivity ,Condensed Matter::Superconductivity ,Lattice (order) ,Cuprate ,Electrical and Electronic Engineering ,Type-II superconductor - Abstract
Commensurate states, investigated by magnetization, are observed in very pure RBa 2 Cu 3 O 7 (R=Y, Nd) single crystals if the intervortex distance a c becomes equal to an integer number k of the crystal lattice constant d ( a c = kd ). Incommensurate states at higher temperatures and fields develop, split up and become finally commensurate with k /2 or even with a smaller fraction k / n of the crystal lattice constant ( na c = kd ). These additional states of higher order ( n >1) are related to a superstructure of the vortex lattice with respect to the crystal lattice.
- Published
- 2004
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42. [Untitled]
- Author
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Alexandre I. Rykov, Y. Yoda, Kiyoshi Nomura, Yoji Kobayashi, Makoto Seto, T. Mitsui, and T. Sawada
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Materials science ,Mössbauer effect ,Phonon ,Health, Toxicology and Mutagenesis ,Public Health, Environmental and Occupational Health ,Analytical chemistry ,Synchrotron radiation ,engineering.material ,Inelastic scattering ,Pollution ,Analytical Chemistry ,symbols.namesake ,Nuclear Energy and Engineering ,Excited state ,symbols ,engineering ,Brownmillerite ,Radiology, Nuclear Medicine and imaging ,Atomic physics ,Spectroscopy ,Debye model ,Perovskite (structure) - Abstract
Resonant nuclear inelastic scattering (NIS) spectra of (Sr,Ca)(Fe,Co)O3- and (Ba,Ca)(Fe,Co)O3- were measured with the energy resolution of 3.5 MeV by detecting the 6.3 keV Fe K X-rays following after Mossbauer effect on 57 Fe transition excited by the monochromatized 14.41 keV synchrotron radiation. Both series of oxides annealed in air and in CO2 were examined. Soft phonon peak arising at ~8 MeV in NIS spectra of (Sr0.5Ca0.5)(Fe0.5Co0.5)O3- and (Ba0.5Ca0.5)FeO3- was found to be correlated with the oxygen ordering into brownmillerite structure, whereas more complex multi-phonon structure was observed in the spectra of less oxygen-deficient oxides such as (Sr0.95Ca0.05)(Fe0.5Co0.5)O3- and (Sr0.7Ca0.3)(Fe0.5Co0.5)O3- showing rather random distrubution of the oxygen vacancies. The changes in phonon DOS after CO2 absorbtion were also detected. Lam-Mossbauer factors of these oxides were calculated on the base of Debye Model.
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- 2003
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43. Recent Advances in the Application of Mößbauer Spectroscopy in Heterogeneous Catalysis
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Tao Zhang, Alexandre I. Rykov, Kuo Liu, and Junhu Wang
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law ,Chemistry ,Ionic bonding ,Synchrotron radiation ,Nanotechnology ,Inelastic scattering ,Heterogeneous catalysis ,Spectroscopy ,Hyperfine structure ,Synchrotron ,law.invention ,Catalysis - Abstract
Mosbauer spectroscopy is a high-resolution spectroscopic technique suitable for investigating solid catalysts that contain nuclei which exhibit the Mosbauer effect. As integral part of a solid, the Mosbauer nucleus plays the role of a probe that interacts, by virtue of its magnetic and electric moments, with the surrounding fields created by unpaired electrons and ionic charges. The Mosbauer effect can be used to quantitatively measure these hyperfine interactions with unprecedented energy resolution, and a series of gamma-resonance techniques are based on this effect. These techniques allow the determination of the catalyst phase, the particle size, the structure, and the oxidation state in the bulk and at the surface of the catalyst, and they provide additional information that is difficult to acquire with other techniques. In this review, the principles of Mosbauer spectroscopy, the observed effects and hyperfine interactions, and in situ techniques are presented in compact form. The search for correlations between catalytic behavior and the structure and composition of catalysts motivate Mosbauer measurements under in situ conditions. The recent advances made by application of Mosbauer spectroscopy in catalysis are summarized in detail, including results from energy catalysis, environmental catalysis, aerospace catalysis, petrochemistry, and photocatalysis. Finally, brief insight into biological catalysis is given by reviewing the latest applications of synchrotron radiation nuclear gamma-resonance elastic and inelastic scattering spectroscopies that have recently been implemented at synchrotron rings of the third generation.
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- 2015
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44. Magnetic penetration depth and reversible magnetization in single crystals of Bi2Sr2CaCu2O8+δ grown by TSFZ method
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Tsuyoshi Tamegai and Alexandre I. Rykov
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Materials science ,Condensed matter physics ,Energy Engineering and Power Technology ,Field dependence ,Condensed Matter Physics ,Electronic, Optical and Magnetic Materials ,Superfluidity ,Magnetization ,Surface conductivity ,Condensed Matter::Superconductivity ,Electrical and Electronic Engineering ,Dislocation ,Penetration depth ,Critical field ,Microwave - Abstract
We analyze the reversible magnetization in Bi2Sr2CaCu2O8+δ using several theoretical concepts, starting from the London model. The field dependence of the empirical “superfluid density” d M/ d ln B is fitted with using the theory of circular cell. Additionally, we take into account the pinning mechanism associated with dislocation structures naturally existing in the crystals grown by travelling-solvent-floating-zone (TSFZ) technique. This allows us to explain: (i) the rapid variation of d M/ d ln B at small fields; (ii) the reduction of thermal fluctuation term with respect to the theoretical prediction for decoupled pancakes. In the light of this reduction we derive the curves Hc1(T) and λ2(0)/λ2(T) from the reversible magnetization similar to the ones obtained by direct measurements of Hc1 by micro-Hall AC technique, and with microwave surface impedance, respectively.
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- 2001
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45. Superconducting Gap and Strong In-Plane Anisotropy in UntwinnedYBa2Cu3O7−δ
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Kyle Shen, Setsuko Tajima, Donglai Feng, Ruixing Liang, Donghui Lu, Alexandre I. Rykov, Changyoung Kim, Z. X. Shen, Andrea Damascelli, N. P. Armitage, D. A. Bonn, Walter Hardy, and Filip Ronning
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Superconductivity ,Physics ,Condensed matter physics ,Resolution (electron density) ,General Physics and Astronomy ,Synchrotron radiation ,Particle accelerator ,02 engineering and technology ,Electronic structure ,021001 nanoscience & nanotechnology ,01 natural sciences ,Square lattice ,Spectral line ,law.invention ,law ,0103 physical sciences ,010306 general physics ,0210 nano-technology ,Anisotropy - Abstract
With significantly improved sample quality and instrumental resolution, we clearly identify in the ( pi,0) photoemission spectra from YBa(2)Cu(3)O(6.993), in the superconducting state, the long-sought "peak-dip-hump" structure. This advance allows us to investigate the large a-b anisotropy of the in-plane electronic structure including, in particular, a 50% difference in the magnitude of the superconducting gap that scales with the energy position of the hump feature. This anisotropy, likely induced by the presence of the CuO chains, raises serious questions about attempts to quantitatively explain the YBa(2)Cu(3)O(7-delta) data from various experiments using models based on a perfectly square lattice.
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- 2001
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46. Angle-Resolved Photoemission Study of Insulating and Metallic Cu-O Chains inPrBa2Cu3O7andPrBa2Cu4O8
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Yuh Yamada, Setsuko Tajima, M. Goto, A. Fujimori, T. Yoshida, Nobuyoshi Yamada, Shigeru Horii, M. Tagami, Y. Yamada, Alexandre I. Rykov, Z.-X. Shen, Y. Siohara, A. Ino, Takashi Mizokawa, Changyoung Kim, S. Uchida, Hiroshi Eisaki, K. Tomimoto, Izumi Hirabayashi, and K. Yoshida
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Metal ,symbols.namesake ,Charge ordering ,Materials science ,Condensed matter physics ,Spectral weight ,visual_art ,Fermi level ,visual_art.visual_art_medium ,symbols ,General Physics and Astronomy ,Spectral line - Abstract
We compare the angle-resolved photoemission spectra of the hole-doped Cu-O chains in ${\mathrm{PrBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7}$ (Pr123) and in ${\mathrm{PrBa}}_{2}{\mathrm{Cu}}_{4}{\mathrm{O}}_{8}$ (Pr124). While, in Pr123, a dispersive feature from the chain takes a band maximum at ${k}_{b}$ (momentum along the chain) $\ensuremath{\sim}\ensuremath{\pi}/4$ and loses its spectral weight around the Fermi level, it reaches the Fermi level at ${k}_{b}\ensuremath{\sim}\ensuremath{\pi}/4$ in Pr124. Although the chains in Pr123 and Pr124 are approximately 1/4 filled, they show contrasting behaviors: While the chains in Pr123 have an instability to charge ordering, those in Pr124 avoid it and show an interesting spectral feature of a metallic coupled-chain system.
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- 2000
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47. Anomalously Large Oxygen-Ordering Contribution to the Thermal Expansion of UntwinnedYBa2Cu3O6.95Single Crystals: A Glasslike Transition near Room Temperature
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Alexandre I. Rykov, Christoph Meingast, Setsuko Tajima, Volker Pasler, and Peter Nagel
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Materials science ,chemistry ,Condensed matter physics ,Thermal ,General Physics and Astronomy ,chemistry.chemical_element ,Kinetic energy ,Glass transition ,Oxygen ,Energy (signal processing) ,Thermal expansion - Abstract
We present high-resolution thermal expansion data from 5--500 K of untwinned ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{x}$ (Y123) single crystals for $x\ensuremath{\sim}6.95$ and $x\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}7.0$. Large contributions to the thermal expansivities due to O ordering are found for $x\ensuremath{\sim}6.95$, which disappear below a kinetic glasslike transition near 280 K. The kinetics at this glass transition is governed by an energy barrier of $0.98\ifmmode\pm\else\textpm\fi{}0.07\mathrm{eV}$, in very good agreement with other O-ordering studies. Using thermodynamic arguments, we show that O ordering in the Y123 system is particularly sensitive to uniaxial stress along the chain axis and that the lack of well-ordered chains in Nd123 and La123 is most likely a consequence of a chemical-pressure effect.
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- 2000
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48. Angular-dependent muon-spin rotation and torque magnetometry on the mixed state of the high-temperature superconductorYBa2Cu3O7−δ
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Alexandre I. Rykov, T.M Riseman, Stephen Lee, S. J. Johnson, Feodor Y. Ogrin, Setsuko Tajima, I. M. Savić, P. G. Kealey, E. M. Forgan, S. H. Lloyd, Hugo Keller, C. Ager, Christof M. Aegerter, and S. Romer
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Superconductivity ,Physics ,High-temperature superconductivity ,Effective mass (solid-state physics) ,Condensed matter physics ,Magnetometer ,law ,Condensed Matter::Superconductivity ,Torque ,Muon spin spectroscopy ,Anisotropy ,Single crystal ,law.invention - Abstract
Muon-spin rotation $(\ensuremath{\mu}\mathrm{SR})$ and torque magnetometry have been used to probe the anisotropy of an untwinned single crystal of ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}.$ The absence of twin planes allows the ratios of all three principal components of the superconducting effective mass tensor to be determined. The values for the in-plane anisotropy are in good agreement with values obtained by other techniques. The out-of-plane anisotropies as measured by $\ensuremath{\mu}\mathrm{SR}$ are also found to be in good agreement with other microscopic measurements, but somewhat lower than those resulting from torque measurements on this and other crystals.
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- 2000
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49. Evidence for vortex pinning by surface irregularities in untwinned YBaCuO crystals
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Alexandre I. Rykov, Bernard Plaçais, Ch. Simon, Setsuko Tajima, P. Mathieu, N. Lütke-Entrup, Y. Simon, Christophe Goupil, and Alain Pautrat
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Flux pinning ,Materials science ,Condensed matter physics ,Energy Engineering and Power Technology ,Tourbillon ,Condensed Matter Physics ,Curvature ,Magnetic flux ,Electronic, Optical and Magnetic Materials ,Vortex ,Condensed Matter::Superconductivity ,Electrical and Electronic Engineering ,Penetration depth ,Pinning force ,Ohmic contact - Abstract
A direct method of induction in a small pick up coil shows that in untwinned YBaCuO crystals, the subcritical current flows close to the surfaces of the sample. By studying the frequency dependence of the ac penetration depth, we have also shown that the two-modes through Mathieu–Simon (MS) model of pinning by surface irregularities is able to account for the observed spectrum. The Campbell model of weak bulk pinning can be completely ruled out. Above the first order transition line, the critical current is zero and the ac response is that of an ohmic system. This can be interpreted by the curvature of the vortices close to the surface. This shows the irrelevance of weak bulk pinning models for clean untwinned YBaCuO crystals that were often proposed as model systems for this case.
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- 2000
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50. Superconductivity-induced effects on phononic and electronic Raman scattering in twin-freeYBa2Cu3O7−xsingle crystals
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Akio Yamanaka, Alexandre I. Rykov, Setsuko Tajima, and Mikhail F. Limonov
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Brillouin zone ,Superconductivity ,Physics ,symbols.namesake ,Condensed matter physics ,Phonon ,Condensed Matter::Superconductivity ,Doping ,symbols ,Orthorhombic crystal system ,Anisotropy ,Omega ,Raman scattering - Abstract
Raman scattering spectra have been investigated in twin-free ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}x}$ single crystals with various oxygen content. Superconductivity-induced effects are found for all ${A}_{g}$ phonons in the overdoped crystals, but they rapidly weaken with reducing oxygen content. The 340 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ phonon line is analyzed in detail by means of the Green function, as well as the standard Fano formula. The results provide much evidence for the nodal structure of a superconducting gap. The polarization dependence, which is significant in the highly doped crystals, illustrates the x-y anisotropy of the electronic response $\ensuremath{\chi}(\ensuremath{\omega})=\ensuremath{-}R(\ensuremath{\omega})+i\ensuremath{\rho}(\ensuremath{\omega})$ and implies a difference of the superconducting gap amplitudes ${\ensuremath{\Delta}}_{x}\ensuremath{\ne}{\ensuremath{\Delta}}_{y}$ in the ${\mathbf{k}}_{x}$ and ${\mathbf{k}}_{y}$ directions in the two-dimensional Brillouin zone for orthorhombic ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}x}.$ The gap energy difference $({\ensuremath{\Delta}}_{y}\ensuremath{-}{\ensuremath{\Delta}}_{x})$ decreases as the doping level is deduced.
- Published
- 2000
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