The Experts below are selected from a list of 16536 Experts worldwide ranked by ideXlab platform
Dongyuan Zhao - One of the best experts on this subject based on the ideXlab platform.
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one pot generation of mesoporous carbon supported nanocrystalline Calcium Oxides capable of efficient co2 capture over a wide range of temperatures
Physical Chemistry Chemical Physics, 2011Co-Authors: Na Hao, Gongkui Xiao, Liying Liu, Paul A Webley, Dongyuan ZhaoAbstract:Ordered mesoporous carbon-supported Calcium oxide materials have been rationally synthesized for the first time. Large specific surface area, high content of nanosized Calcium Oxides can be easily obtained and tuned. The structure, porosity and the particle size evolution as a function of Calcium content and carbonization temperature are extensively characterized and well correlated with their CO2 sorption properties. The composite materials are of significance for CO2 physisorption at ambient temperatures with high capacity and selectivity over N2. Meanwhile, the nanocrystalline Calcium Oxides are highly active for CO2 chemisorption, with tuneable and high CO2 capacity at 200–500 °C. An almost complete initial conversion and fast reaction kinetics at a low temperature (450 °C) and low CO2 pressure can be achieved within minutes. Cyclic stability is also substantially improved due to the confinement effect of the CaO nanoparticles within the mesopores. These materials would be suitable for CO2 separation over a wide range of temperatures.
Ulrich Gernet - One of the best experts on this subject based on the ideXlab platform.
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electrosynthesis of biomimetic manganese Calcium Oxides for water oxidation catalysis atomic structure and functionality
Chemsuschem, 2016Co-Authors: Diego Gonzalezflores, Ivelina Zaharieva, Jonathan Heidkamp, Petko Chernev, Elias Martinezmoreno, Chiara Pasquini, M R Mohammadi, Katharina Klingan, Ulrich GernetAbstract:Water-oxidizing Calcium-manganese Oxides, which mimic the inorganic core of the biological catalyst, were synthesized and structurally characterized by X-ray absorption spectroscopy at the manganese and Calcium K edges. The amorphous, birnesite-type Oxides are obtained through a simple protocol that involves electrodeposition followed by active-site creation through annealing at moderate temperatures. Calcium ions are inessential, but tune the electrocatalytic properties. For increasing Calcium/manganese molar ratios, both Tafel slopes and exchange current densities decrease gradually, resulting in optimal catalytic performance at Calcium/manganese molar ratios of close to 10 %. Tracking UV/Vis absorption changes during electrochemical operation suggests that inactive Oxides reach their highest, all-Mn(IV) oxidation state at comparably low electrode potentials. The ability to undergo redox transitions and the presence of a minor fraction of Mn(III) ions at catalytic potentials is identified as a prerequisite for catalytic activity.
Mohammad Mahdi Najafpour - One of the best experts on this subject based on the ideXlab platform.
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synthetic manganese Calcium Oxides mimic the water oxidizing complex of photosynthesis functionally and structurally
Energy and Environmental Science, 2011Co-Authors: Ivelina Zaharieva, Mohammad Mahdi Najafpour, Mathias Wiechen, Michael Haumann, Philipp KurzAbstract:In the worldwide search for sustainable energy technologies, water oxidation by abundant low-cost materials is of key importance. In nature, this process is efficiently catalyzed by an intricate manganese–Calcium (Mn4Ca) complex bound to the proteins of photosystem II (PSII). Recently synthetic manganese–Calcium Oxides were found to be active catalysts of water oxidation but at the atomic level their structure has remained elusive. To investigate these amorphous catalysts, extended-range X-ray absorption spectroscopy (XAS) at the K-edges of both manganese and Calcium was performed. The XAS results reveal striking similarities between the synthetic material and the natural Mn4Ca complex. The oxidation state of manganese in the active Oxides was found to be close to +4, but MnIII ions are present as well at a level of about 20%. Neighboring Mn ions are extensively interconnected by two bridging oxygens, a characteristic feature of layered manganese Oxides. However, the Oxides do not exhibit long-range order, as opposed to canonical, but catalytically inactive MnIII- or MnIV-Oxides. Two different Ca-containing motifs were identified. One of them results in the formation of Mn3CaO4 cubes, as also proposed for the natural paragon in PSII. Other Calcium ions likely interconnect oxide-layer fragments. We conclude that these readily synthesized manganese–Calcium Oxides are the closest structural and functional analogs to the native PSII catalyst found so far. Evolutionary implications are considered. From the differences to inactive manganese Oxides, we infer structural features facilitating the catalysis of water oxidation in both the protein-bound Mn4Ca complex of PSII and in the synthetic Oxides.
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Amorphous Manganese-Calcium Oxides as a Possible Evolutionary Origin for the CaMn_4 Cluster in Photosystem II
Origins of Life and Evolution of Biospheres, 2011Co-Authors: Mohammad Mahdi NajafpourAbstract:In this paper a few Calcium-manganese Oxides and Calcium-manganese minerals are studied as catalysts for water oxidation. The natural mineral marokite is also studied as a catalyst for water oxidation for the first time. Marokite is made up of edge-sharing Mn^3+ in a distorted octahedral environment and eight-coordinate Ca^2+ centered polyhedral layers. The structure is similar to recent models of the oxygen evolving complex in photosystem II. Thus, the oxygen evolving complex in photosystem II does not have an unusual structure and could be synthesized hydrothermally. Also in this paper, oxygen evolution is studied with marokite (CaMn_2O_4), pyrolusite (MnO_2) and compared with hollandite (Ba_0.2Ca_0.15K_0.3Mn_6.9Al_0.2Si_0.3O_16), hausmannite (Mn_3O_4), Mn_2O_3.H_2O, CaMn_3O_6.H_2O, CaMn_4O_8.H_2O, CaMn_2O_4.H_2O and synthetic marokite (CaMn_2O_4). I propose that the origin of the oxygen evolving complex in photosystem II resulted from absorption of Calcium and manganese ions that were precipitated together in the archean oceans by protocyanobacteria because of changing pH from ~5 to ~8-10. As reported in this paper, amorphous Calcium-manganese Oxides with different ratios of manganese and Calcium are effective catalysts for water oxidation. The bond types and lengths of the Calcium and manganese ions in the Calcium-manganese Oxides are directly comparable to those in the OEC. This primitive structure of these amorphous Calcium-manganese compounds could be changed and modified by environmental groups (amino acids) to form the oxygen evolving complex in photosystem II.
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mixed valence manganese Calcium Oxides as efficient catalysts for water oxidation
Dalton Transactions, 2011Co-Authors: Mohammad Mahdi NajafpourAbstract:Incorporation of Calcium to mixed-valence manganese Oxides improved the water oxidation activity of these manganese Oxides
Ivelina Zaharieva - One of the best experts on this subject based on the ideXlab platform.
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electrosynthesis of biomimetic manganese Calcium Oxides for water oxidation catalysis atomic structure and functionality
Chemsuschem, 2016Co-Authors: Diego Gonzalezflores, Ivelina Zaharieva, Jonathan Heidkamp, Petko Chernev, Elias Martinezmoreno, Chiara Pasquini, M R Mohammadi, Katharina Klingan, Ulrich GernetAbstract:Water-oxidizing Calcium-manganese Oxides, which mimic the inorganic core of the biological catalyst, were synthesized and structurally characterized by X-ray absorption spectroscopy at the manganese and Calcium K edges. The amorphous, birnesite-type Oxides are obtained through a simple protocol that involves electrodeposition followed by active-site creation through annealing at moderate temperatures. Calcium ions are inessential, but tune the electrocatalytic properties. For increasing Calcium/manganese molar ratios, both Tafel slopes and exchange current densities decrease gradually, resulting in optimal catalytic performance at Calcium/manganese molar ratios of close to 10 %. Tracking UV/Vis absorption changes during electrochemical operation suggests that inactive Oxides reach their highest, all-Mn(IV) oxidation state at comparably low electrode potentials. The ability to undergo redox transitions and the presence of a minor fraction of Mn(III) ions at catalytic potentials is identified as a prerequisite for catalytic activity.
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biogenic manganese Calcium Oxides on the cell walls of the algae chara corallina elemental composition atomic structure and water oxidation catalysis
European Journal of Inorganic Chemistry, 2014Co-Authors: Andreas Scholer, Ivelina Zaharieva, Mathias Wiechen, Philipp Kurz, Sebastian Zimmermann, Annemarie Manke, Christoph Plieth, Holger DauAbstract:Chara corallina freshwater algae produce brown deposits of manganese Oxides on their cell wall surfaces when growing in manganese-rich media. We report on the formation, topology, composition, atomic structure, and catalytic activities of these biogenic manganese Oxides (BMOs). The deposits are volcano shaped and exhibit 3–5 μm craters in their centers. Microfocus X-ray irradiation and detection of characteristic X-ray fluorescence lines allowed elemental mapping at 5 μm spatial resolution and the identification of the volcano-shaped deposits as a Mn–Ca oxide. X-ray absorption spectroscopy (XAS) revealed a high-valent MnIII/IV oxide. The structural analysis involved XAS spectra collected for a single volcano at room temperature and for single cells at 20 K. On the basis of the XAS data, the Oxides were identified as members of the birnessite family of layered manganese Oxides containing di-μ-oxido-bridged MnIII/IVO6 octahedra as central building units. The deposits share structural motifs with synthetic water-oxidizing Mn–Ca Oxides and with the Mn4Ca complex of photosystem II, the biological water-oxidation catalyst. Model reactions demonstrate low, but clearly detectable, activity of the manganese deposits for water-oxidation catalysis. The biogenic manganese Oxides on the cell walls of Chara corallina thus represent an intriguing object to study how manganese-based catalysts for water oxidation are formed in a biological environment. The formation of BMOs in relation to cellular ion transport and the possibility of BMOs to fulfill a detoxification function in plants were also examined.
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synthetic manganese Calcium Oxides mimic the water oxidizing complex of photosynthesis functionally and structurally
Energy and Environmental Science, 2011Co-Authors: Ivelina Zaharieva, Mohammad Mahdi Najafpour, Mathias Wiechen, Michael Haumann, Philipp KurzAbstract:In the worldwide search for sustainable energy technologies, water oxidation by abundant low-cost materials is of key importance. In nature, this process is efficiently catalyzed by an intricate manganese–Calcium (Mn4Ca) complex bound to the proteins of photosystem II (PSII). Recently synthetic manganese–Calcium Oxides were found to be active catalysts of water oxidation but at the atomic level their structure has remained elusive. To investigate these amorphous catalysts, extended-range X-ray absorption spectroscopy (XAS) at the K-edges of both manganese and Calcium was performed. The XAS results reveal striking similarities between the synthetic material and the natural Mn4Ca complex. The oxidation state of manganese in the active Oxides was found to be close to +4, but MnIII ions are present as well at a level of about 20%. Neighboring Mn ions are extensively interconnected by two bridging oxygens, a characteristic feature of layered manganese Oxides. However, the Oxides do not exhibit long-range order, as opposed to canonical, but catalytically inactive MnIII- or MnIV-Oxides. Two different Ca-containing motifs were identified. One of them results in the formation of Mn3CaO4 cubes, as also proposed for the natural paragon in PSII. Other Calcium ions likely interconnect oxide-layer fragments. We conclude that these readily synthesized manganese–Calcium Oxides are the closest structural and functional analogs to the native PSII catalyst found so far. Evolutionary implications are considered. From the differences to inactive manganese Oxides, we infer structural features facilitating the catalysis of water oxidation in both the protein-bound Mn4Ca complex of PSII and in the synthetic Oxides.
Philipp Kurz - One of the best experts on this subject based on the ideXlab platform.
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biogenic manganese Calcium Oxides on the cell walls of the algae chara corallina elemental composition atomic structure and water oxidation catalysis
European Journal of Inorganic Chemistry, 2014Co-Authors: Andreas Scholer, Ivelina Zaharieva, Mathias Wiechen, Philipp Kurz, Sebastian Zimmermann, Annemarie Manke, Christoph Plieth, Holger DauAbstract:Chara corallina freshwater algae produce brown deposits of manganese Oxides on their cell wall surfaces when growing in manganese-rich media. We report on the formation, topology, composition, atomic structure, and catalytic activities of these biogenic manganese Oxides (BMOs). The deposits are volcano shaped and exhibit 3–5 μm craters in their centers. Microfocus X-ray irradiation and detection of characteristic X-ray fluorescence lines allowed elemental mapping at 5 μm spatial resolution and the identification of the volcano-shaped deposits as a Mn–Ca oxide. X-ray absorption spectroscopy (XAS) revealed a high-valent MnIII/IV oxide. The structural analysis involved XAS spectra collected for a single volcano at room temperature and for single cells at 20 K. On the basis of the XAS data, the Oxides were identified as members of the birnessite family of layered manganese Oxides containing di-μ-oxido-bridged MnIII/IVO6 octahedra as central building units. The deposits share structural motifs with synthetic water-oxidizing Mn–Ca Oxides and with the Mn4Ca complex of photosystem II, the biological water-oxidation catalyst. Model reactions demonstrate low, but clearly detectable, activity of the manganese deposits for water-oxidation catalysis. The biogenic manganese Oxides on the cell walls of Chara corallina thus represent an intriguing object to study how manganese-based catalysts for water oxidation are formed in a biological environment. The formation of BMOs in relation to cellular ion transport and the possibility of BMOs to fulfill a detoxification function in plants were also examined.
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synthetic manganese Calcium Oxides mimic the water oxidizing complex of photosynthesis functionally and structurally
Energy and Environmental Science, 2011Co-Authors: Ivelina Zaharieva, Mohammad Mahdi Najafpour, Mathias Wiechen, Michael Haumann, Philipp KurzAbstract:In the worldwide search for sustainable energy technologies, water oxidation by abundant low-cost materials is of key importance. In nature, this process is efficiently catalyzed by an intricate manganese–Calcium (Mn4Ca) complex bound to the proteins of photosystem II (PSII). Recently synthetic manganese–Calcium Oxides were found to be active catalysts of water oxidation but at the atomic level their structure has remained elusive. To investigate these amorphous catalysts, extended-range X-ray absorption spectroscopy (XAS) at the K-edges of both manganese and Calcium was performed. The XAS results reveal striking similarities between the synthetic material and the natural Mn4Ca complex. The oxidation state of manganese in the active Oxides was found to be close to +4, but MnIII ions are present as well at a level of about 20%. Neighboring Mn ions are extensively interconnected by two bridging oxygens, a characteristic feature of layered manganese Oxides. However, the Oxides do not exhibit long-range order, as opposed to canonical, but catalytically inactive MnIII- or MnIV-Oxides. Two different Ca-containing motifs were identified. One of them results in the formation of Mn3CaO4 cubes, as also proposed for the natural paragon in PSII. Other Calcium ions likely interconnect oxide-layer fragments. We conclude that these readily synthesized manganese–Calcium Oxides are the closest structural and functional analogs to the native PSII catalyst found so far. Evolutionary implications are considered. From the differences to inactive manganese Oxides, we infer structural features facilitating the catalysis of water oxidation in both the protein-bound Mn4Ca complex of PSII and in the synthetic Oxides.
