Synchrotron Radiation-based X-ray Absorption Research Articles

Synchrotron Radiation Based X-ray Absorption Spectroscopy: Fundamentals and Applications in Photocatalysis.

Photocatalysis is one of the most promising green technologies to utilize solar energy for clean energy achievement and environmental governance. There is a knotty problem to rational designing high-performance photocatalyst, which largely depends on an in-depth insight into their structure-activity relationships and complex photocatalytic reaction mechanisms. Synchrotron radiation based X-ray absorption spectroscopy (XAS) is an important characterization method for photocatlayst to offer the element-specific key geometric and electronic structural information at the atomic level, on this basis, time-resolved XAS technique has a huge impact on mechanistic understanding of photochemical reaction owing to their powerful ability to probe, in real-time, the electronic and geometric structures evolution within photocatalysis reactions. This review will focus on the fundamentals of XAS and their applications in photocatalysis. The detailed applications obtained from XAS is described through the following aspects: 1) identifying local structure of photocatalyst; 2) uncovering in situ structure and chemical state evolution during photocatalysis; 3) revealing the photoexcited process. We will provide an in depth understanding on how the XAS method can guide the rational design of highly efficient photocatalyst. Finally, a systematic summary of XAS and related significance is made and the research perspectives are suggested.

Dynamic Transformation of Nano-MoS2 in a Soil-Plant System Empowers Its Multifunctionality on Soybean Growth.

Molybdenum disulfide (nano-MoS2) nanomaterials have shown great potential for biomedical and catalytic applications due to their unique enzyme-mimicking properties. However, their potential agricultural applications have been largely unexplored. A key factor prior to the application of nano-MoS2 in agriculture is understanding its behavior in a complex soil-plant system, particularly in terms of its transformation. Here, we investigate the distribution and transformation of two types of nano-MoS2 (MoS2 nanoparticles and MoS2 nanosheets) in a soil-soybean system through a combination of synchrotron radiation-based X-ray absorption near-edge spectroscopy (XANES) and single-particle inductively coupled plasma mass spectrometry (SP-ICP-MS). We found that MoS2 nanoparticles (NPs) transform dynamically in soil and plant tissues, releasing molybdenum (Mo) and sulfur (S) that can be incorporated gradually into the key enzymes involved in nitrogen metabolism and the antioxidant system, while the rest remain intact and act as nanozymes. Notably, there is 247.9 mg/kg of organic Mo in the nodule, while there is only 49.9 mg/kg of MoS2 NPs. This study demonstrates that it is the transformation that leads to the multifunctionality of MoS2, which can improve the biological nitrogen fixation (BNF) and growth. Therefore, MoS2 NPs enable a 30% increase in yield compared to the traditional molybdenum fertilizer (Na2MoO4). Excessive transformation of MoS2 nanosheets (NS) leads to the overaccumulation of Mo and sulfate in the plant, which damages the nodule function and yield. The study highlights the importance of understanding the transformation of nanomaterials for agricultural applications in future studies.

Atomically Dispersed Mg-N-C Material Supported Highly Crystalline Pt3Mg Nanoalloys for Efficient Oxygen Reduction Reaction.

Single-atom or atomically dispersed metal materials have emerged as highly efficient catalysts, but their potential as excellent supports has rarely been reported. In this work, we prepared Mg-N-C materials derived from annealing of a Mg-based metal-organic framework (MOF). By introducing Pt, Mg-N-C not only serves as a platform for anchoring Pt nanoparticles but also facilitates the integration of Mg into the Pt face-centered cubic lattice, resulting in the formation of highly crystalline Pt3Mg nanoalloys via the metal-support interfacial interaction. Synchrotron radiation-based X-ray absorption spectroscopy (XAS) enables us to study the interfacial interaction and the surface electronic structure of this intricate system. The formation of Pt3Mg nanoalloys induces a downshift of the Pt d-band (gaining d-charge), as revealed by the decrease in the Pt L3-edge white-line (WL) area under the curve. This downshift can weaken the binding of oxygen reduction reaction (ORR) intermediates, hence improving the ORR performance.

High Valence State Sites as Favorable Reductive Centers for High-Current-Density Water Splitting.

Electrochemical water splitting is a key process for producing sustainable and clean hydrogen. Typically, high valence state sites are favorable for oxidation evolution reaction, while low valence states are suitable for hydrogen evolution reaction (HER) due to efficiency and stability issues. However, we propose a high valence state of Co3+in Ni9.5Co0.5-S-FeOxhybrid as active center for efficient and stable HER, while structural analogues with relatively low chemical states show much worse performance. The Ni9.5Co0.5-S-FeOxcatalyst can drive alkaline HER with an ultra-low overpotential of 175mV for 1000mAcm-2at the industrial working temperature of 60oC, with an excellent stability over 300h. Moreover, this material can work as a favorable bi-functional catalyst for both OER and HER, with a low cell voltage being 1.730V to achieve 1000mAcm-2for overall water splitting over 300h. Synchrotron radiation based X-ray absorption spectroscopy (XAS) clearly identifies the high valence Co3+sites, while in-situ XAS during HER and theoretical calculations reveal the favorable electron capture capability at Co3+and suitable H adsorption/desorption energy around the high valence sites, which can accelerate the HER reaction. The understanding of high valence state to drive the reductive reaction may pave the way for the rational design of energy-related catalysts.

Comparison of sedimentary iron speciation obtained by sequential extraction and X-ray absorption spectroscopy

Iron (Fe) speciation in marine sediments and suspended particulate matter (SPM) are widely used to investigate the role of Fe minerals in marine biogeochemical cycling and to reconstruct paleo-redox conditions. A traditional method for the determination of Fe speciation are sequential extractions, in which operationally defined Fe pools are dissolved by specific extraction solutions. More recently, many studies make use of synchrotron radiation-based X-ray absorption near edge spectroscopy (XANES), especially if little sample material is available or a high spatial resolution is required. However, few studies have systematically compared Fe speciation obtained by the two methods. Here, we report Fe speciation for marine sediments (both wet and freeze-dried) and SPM from contrasting marine environments (western Baltic Sea and Amazon shelf) obtained by sequential extraction and XANES. The two methods yield comparable results regarding the approximate proportion of reactive Fe ((oxyhydr)oxide, carbonate and sulfide minerals) in the total Fe pool and the extent to which reactive Fe minerals have been converted to pyrite. However, methodological issues associated with both approaches complicate a direct comparison of specific mineral groups. Pyrite concentrations obtained by XANES are lower than those obtained by extraction, which is attributed to an underestimation by XANES due to self-absorption effects and an overestimation by the chromium reduction method due to dissolution of reduced sulfur phases that are not associated with Fe. As substantial amounts of pyrite are oxidized during freeze-drying and sample storage, we recommend to analyze wet sediments in modern marine environments. The XANES spectra of Fe (oxyhydr)oxide and Fe(III)-containing (ferric) clay minerals are similar, which is why these components cannot be differentiated by XANES. Similarly, sequential extraction schemes are known to dissolve a variable proportion of ferric clay minerals along with Fe carbonates and Fe (oxyhydr)oxides. Further investigating the role of ferric clay minerals in the low-temperature marine Fe cycle may therefore require the use of additional methods. Our findings may help to assess the transferability of XANES-based Fe speciation for small sediment and SPM samples to those reported in more traditional studies on Fe biogeochemical cycling and paleo-redox conditions.

Electronic Modulation of Metal-Organic Frameworks Caused by Atomically Dispersed Ru for Efficient Hydrogen Evolution.

Designing excellent electrocatalysts for the hydrogen evolution reaction (HER) is extremely significant in producing clean and sustainable hydrogen fuel. Herein, a rational strategy is developed to fabricate a promising electrocatalyst by introducing atomically dispersed Ru into a cobalt-based metal-organic framework (MOF), Co-BPDC (Co(bpdc)(H2 O)2 , BPDC: 4,4'-Biphenyldicarboxylic acid). The obtained CoRu-BPDC nanosheet arrays exhibit remarkable HER performance with an overpotential of 37mV at a current density of 10mAcm-2 in alkaline media, which is superior to most of the MOF-based electrocatalysts and comparable to the commercial Pt/C. Synchrotron radiation-based X-ray absorption fine structure (XAFS) spectroscopy studies verify that the isolated Ru atoms are dispersed in Co-BPDC nanosheets with the formation of five-coordinated Ru-O5 species. XAFS spectroscopy combined with density functional theory (DFT) calculations unravels that atomically dispersed Ru can modulate the electronic structure of the as-obtained Co-BPDC, contributing to the optimization of binding strength for H* and the enhancement of HER performance. This work opens a new avenue to rationally design highly-active single-atom modified MOF-based HER electrocatalysts via modulating electronic structures of MOF.

The differential effect of amorphous μ-S and orthorhombic ɑ-S8 on chalcopyrite bioleaching by Acidithiobacillus ferrooxidans

Elemental sulfur (S0), as one of the most important intermediates formed during chalcopyrite bioleaching, can exist in a variety of allotropic forms, among which amorphous μ-S and orthorhombic ɑ-S8 are the most common forms. Although previous studies have focused on the effect of S0 on chalcopyrite bioleaching, how S0 allotropes affect chalcopyrite bioleaching is unclear. Therefore, in this work, the differential effect of ɑ-S8 and μ-S on chalcopyrite bioleaching by Acidithiobacillus ferrooxidans (A. ferrooxidans) was studied at the molecular and atomic levels by combining synchrotron radiation-based X-ray absorption near edge structure (XANES) spectroscopy, comparative transcriptome analysis and density functional theory (DFT) calculations, accompanied by the determination of solution behaviors and mineral surface morphologies. The results showed that µ-S was favorable for bacterial adsorption and that the addition of µ-S could promote the copper extraction rates by A. ferrooxidans, while the addition of ɑ-S8 inhibited the copper extraction rates. The XANES results showed that both cases with the addition of ɑ-S8 and µ-S could affect the formation and evolution of secondary minerals during bioleaching by A. ferrooxidans. The comparative transcriptome analysis showed that the gene expression of A. ferrooxidans was much more affected by the addition of µ-S, with more genes upregulated than ɑ-S8. The DFT results indicated that both ɑ-S8 and µ-S could adsorb on the CuFeS2 (001) surface easily, and µ-S could make the CuFeS2 surface obviously change, while ɑ-S8 hindered the adsorption of bacterial cells. Of note, µ-S could enhance the interaction between the CuFeS2 (001) surface and bacterial cells with stronger bonding and more transferred electrons than ɑ-S8.

Thermal transport properties for unveiling the mechanism of BiSbTe alloys in thermoelectric generation: A glance from synchrotron radiation Bi L3-XAFS

For thermal to electric energy conversion, designing a high efficiency thermoelectric material entails simultaneously optimizing multiple properties of the material. Although it may appear simple to increase electrical power while minimizing thermal losses, the complicated link between these parameters makes optimization difficult, necessitating a more sophisticated approach. The one-pot zone melting method was used to fabricate undoped and Sb doped Bi2Te3 (Bi2−xSbxTe3; x = 0.0, 0.2, 0.6, and 1.0) in pellets form. X-ray diffraction (XRD), Raman and selected area electron diffraction (SAED) revealed rhombohedral polycrystalline nature and a unique high intensity of the Eg2 optical mode according to the nanosheet nature of the alloys as observed in the scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images. Besides, High Resolution TEM (HRTEM) micrographs depicted twin boundary effect with an angle of 120° for Bi1.8Sb0.2Te3 alloys, which may enhance the electronic transport and the thermoelectric properties of the fabricated compounds. To obtain selective and detailed local structural information of Bi2−xSbxTe3 matrix, synchrotron radiation-based X-ray absorption spectroscopy (XAS) measurements were performed at around Bi L3-edge. A significant influence of the Sb/Bi substitution on the local/atomic structure was observed through the gradual elongation of the average in-plane Bi–Sb bond distance. The combination of different off-line structural characterization techniques such as XRD, Raman, SEM, and HRTEM with synchrotron based XAS technique and Laser-Beam Deflection Spectroscopy (BDS) is necessary to fully describe the samples nature and to get average crystal, morphological and local/electronic structural information for unveiling the BiSbTe mechanism in thermoelectric generation.

Ultrafine Co-MoS2 monolayer catalyst derived from oil-soluble single-molecule polyoxometalates for slurry phase hydrocracking

Herein, a novel single-molecule confinement-sulfurization strategy was proposed to construct ultrafine Co-MoS2 monolayer catalysts by using oil-soluble single-molecule polyoxometalates (POMs), (DODA)6Co2Mo10, as precursors. Well-defined Co2Mo10-POM, acting as molecular pre-assembling platform, is introduced for the atomically precise construction of bimetallic sites. Highly efficient conversion of VR under severe condition is realized with (DODA)6Co2Mo10. The origin of prominent catalytic performance can be attributed to the in-situ formation of ultrafine single-layered Mo-based nanosheets with abundant exposed CoMoS active sites. By combining synchrotron radiation-based X-ray absorption spectroscopy and density functional theory calculations, we further discover that there is clearly intimate interaction between Mo and Co atoms and the dissociation of molecular hydrogen at CoMoS phases is energetically preferred, leading to remarkable hydrogenation and coke suppression activity. This work highlights an idea on the rational design and targeted synthesis of ultrafine bimetallic catalysts for slurry phase HCK with dramatically synergistic catalysis.

Nanocatalytic Materials for Energy-Related Small-Molecules Conversions: Active Site Design, Identification and Structure-Performance Relationship Discovery.

The catalytic conversion of energy-related small-molecules is a critical process in the fields of chemical production, environmental restoration, and energy conversion and storage. Over the years, numerous nanocatalytic materials have been explored in efforts to substantially boost the inherently sluggish catalytic processes. Despite achievements, the lack of fundamental insights into the design and identification of active sites and the structure-performance relationship has been one of the main obstacles to further improvement in catalytic performance. With the development of first-principles density functional theory (DFT) calculations and state-of-art spectroscopic techniques, the pace of research has started to move forward again.In this Account, we illustrate our recent representative attempts to gain fundamental insights into the rational development of efficient nanocatalytic materials and thus boost the typical electrochemical and mechanochemical conversions of energy-related small-molecules, including for the hydrogen evolution reaction (HER), oxygen reduction reaction (ORR), and ammonia synthesis. DFT calculations and advanced spectroscopic techniques, such as synchrotron radiation-based X-ray absorption spectroscopy (XAS, hard and soft X-ray), were properly adopted for this purpose.Specifically, to achieve a fast-electrochemical hydrogen evolution process, Ir active sites with balanced hydrogen adsorption/desorption behaviors were first computationally designed via orbital modulation and experimentally identified, and they showed significantly enhanced catalytic activity toward HER in acidic media. For the electrochemical reduction of oxygen, well-designed Zn-N2 active sites and quinone functional groups were introduced into the different carbon matrixes and structurally identified by the XAS technique, utilizing hard and soft X-rays, respectively. Both experimental and DFT studies revealed that Zn-N2 active sites with their unique structure can greatly activate the adsorbed oxygen species, leading to a highly efficient and selective four-electron oxygen reduction pathway, while the quinone functional groups are able to modify the activation mode and alter it into a selective two-electron oxygen reduction pathway for H2O2 production.In another study, inspired by the dissociation of stable nitrogen molecules on the surface of Fe, dynamic strained Fe active sites were designed for mechanochemical ammonia synthesis. Combined XAS and Mössbauer spectroscopy revealed the formation of a short-range Fe4N structure by the Fe active sites and dissociated nitrogen during the ball milling process, facilitating robust hydrogenation and ammonia production under mild conditions.Thanks to the theoretical methods and advanced spectroscopic techniques, fundamental insights into the design and identification of active sites and understanding of the structure-performance relationship can be easily obtained using such tools, which will guide the development of nanocatalytic materials and boost the conversions of energy-related small-molecules for various applications.

Sulfur in humin as a redox-active element for extracellular electron transfer

Humin (HM), an insoluble humic substance that occurs as an organo-mineral complex, mediates extracellular electron transfer (EET) among microorganisms in the geosphere. Although sulfur (S)-containing functional groups have been posited as possible redox-active groups in HM, there has been no direct evidence in this regard. Here, we characterized the redox-active S in HM from different natural sources using synchrotron radiation-based X-ray absorption spectroscopy in partial fluorescence yield mode. The S content in HM is less than 0.9%, with the oxidation states ranged widely from −2 to +6. About 9% to 19% of total S in HM was redox variable, corresponding to 1.2 × 10−5 to 17.1 × 10−5 Eq/g-HM redox capacity. The redox capacity of HM driven by S is comparable to or larger than the electron donating capacity of HM observed in anaerobic microbial dechlorination of pentachlorophenol, suggesting that S is one of the significant redox-active elements in HM. These findings provide new insights into the role of S cycling in biogeochemical processes and microbial energy networks.

Identification of the Active-Layer Structures for Acidic Oxygen Evolution from 9R-BaIrO3 Electrocatalyst with Enhanced Iridium Mass Activity.

Iridium-based perovskites show promising catalytic activity for oxygen evolution reaction (OER) in acid media, but the iridium mass activity remains low and the active-layer structures have not been identified. Here, we report highly active 1 nm IrOx particles anchored on 9R-BaIrO3 (IrOx/9R-BaIrO3) that are directly synthesized by solution calcination followed by strong acid treatment for the first time. The developed IrOx/9R-BaIrO3 catalyst delivers a high iridium mass activity (168 A gIr-1), about 16 times higher than that of the benchmark acidic OER electrocatalyst IrO2 (10 A gIr-1), and only requires a low overpotential of 230 mV to reach a catalytic current density of 10 mA cm-2geo. Careful scanning transmission electron microscopy, synchrotron radiation-based X-ray absorption spectroscopy, and X-ray photoelectron spectroscopy analyses reveal that, during the electrocatalytic process, the initial 1 nm IrOx nanoparticles/9R-BaIrO3 evolve into amorphous Ir4+OxHy/IrO6 octahedrons and then to amorphous Ir5+Ox/IrO6 octahedrons on the surface. Such high relative content of amorphous Ir5+Ox species derived from trimers of face-sharing IrO6 octahedrons in 9R-BaIrO3 and the enhanced metallic conductivity of the Ir5+Ox/9R-BaIrO3 catalyst are responsible for the excellent acidic OER activity. Our results provide new insights into the surface active-layer structure evolution in perovskite electrocatalysts and demonstrate new approaches for engineering highly active acidic OER nanocatalysts.

Highly efficient CoNiP nanoboxes on graphene oxide for the hydrolysis of ammonia borane

Efficient and low-cost catalysts for the hydrolysis of Ammonia borane (AB) are highly required for the hydrogen economy. Here we report a highly efficient CoNiP nanobox on graphene oxide (GO) for the hydrolysis of AB. By modulating the electronic structure of Co in the catalyst with small amount of doped Ni (1.7 wt%), the hydrolysis activity can be significantly enhanced up to 4 times of the original activity. The CoNiP/GO catalyst can thus achieve a remarkable TOF value of 134.6 (H2) mol/((Cat-metal)mol·min), which is one of the best values ever reported for noble metal-free catalysts. The catalyst also shows a low activation energy of 44.12 kJ/mol and a good durability of 84.6% after five cycles. Synchrotron radiation based X-ray absorption spectroscopy reveals that the high performance can be mainly attributed to the slightly modulated chemical state of Co, which is suitable for the hydrolysis reaction. The favorable nanobox morphology and GO support also contribute to the high performance.

XAFS and XRD study on Fe, Ni, and Ge in iron meteorite NWA 859

Iron meteorites record the evolutionary and cosmochemical processes of their parent bodies. Fe–Ni phases in iron meteorites show complex textures from various thermal histories of parent bodies as well as the phase relationships and crystal chemistry of Fe–Ni metal. Synchrotron radiation-based X-ray absorption fine structure spectra and X-ray diffraction were applied herein to the study of iron meteorite NWA 859 Fe, Ni, and Ge contents at the K-edge, since they are effective techniques in identifying crystal structures in iron meteorites. The bond distances of Fe and Ni in tetrataenite and kamacite were detected. Field-emission scanning electron microscopy and energy-dispersive spectroscopy were used to observe the petrological and chemical characteristics of the main minerals, kamacite and tetrataenite, and the trace mineral schreibersite. The tetrataenite phase and body-centered cubic kamacite formed a Widmanstatten pattern and cloudy zone. The extended X-ray absorption fine structure (EXAFS) analyses of NWA 859 and a single-crystal diffraction of tetrataenite show that it has a near-face-centered cubic (FCC) tetragonal structure with 12 nearest-neighboring Ni, Fe, and Ge atoms at distances of rNi-(Ni, Fe) = 2.5170(13) A, rFe-(Ni, Fe) = 2.534(3) A, and rGe-(Ni, Fe) = 2.524(5) A, respectively. Moreover, the X-ray absorption near-edge structure (XANES) spectra suggest that the Ge in tetrataenite exhibits a specific local structure with coordination number 12, suggesting that a new local structure of Ge-(Ni, Fe) was first discovered in extraterrestrial material, forming a stable tetragonal structure at approximately 688–618 K. X-ray absorption fine structure (XAFS) is an efficient technique that could provide us further information about local atomic structures and forming conditions in extraterrestrial materials without damage.

Intranasal exposure to ZnO nanoparticles induces alterations in cholinergic neurotransmission in rat brain

The neurotoxicity of inhaled ZnO nanoparticles (NPs) and the underlying mechanisms remain largely unknown. In this study, ZnO NPs (30 ± 6 nm) were intranasally instilled to rats via a single dose (13 mg Zn/kg BW), with ZnSO4 as the ionic control, and analysis 7-days post exposure. The hippocampus was found to be the main target for Zn accumulation for both ZnO NPs and ZnSO4. Synchrotron radiation based X-ray absorption fine structure (XAFS) analysis showed that no particulate ZnO was found, suggesting the occurrence of dissolution and transformation of ZnO NPs. Multi-omics analysis, including transcriptomics, proteomics and metabolomics, demonstrated that cholinergic neurotransmission was the main biological process affected following both treatments. The release of the key neurotransmitter acetylcholine (ACh) was increased by enhanced ACh synthesis, upregulation of vesicular ACh transporter, and suppression of the activity of ACh hydrolysis enzyme (AChE), either by direct Zn-AChE interaction or a transcriptional down-regulation mechanism. In addition, ZnO NPs and ZnSO4 induced similar molecular consequences and exhibited the same Zn chemical speciation (100 % of Zn complexes) in the hippocampal region evidenced by XAFS analysis, suggesting that the observed biological effects were mainly derived from Zn2+ released from the ZnO NPs. This study not only evidences a new pathway for the impact of ZnO NPs on the brain, but also identifies the origin of the impact as ionic Zn, which provides the basis for safe-by-design of ZnO NPs.

Advances in Synchrotron Radiation-based X-ray Absorption Spectroscopy to Characterize the Fine Atomic Structure of Single-atom Nanozymes.

Single-atom nanozymes (SAzymes) with high atomic utilization, excellent catalytic activities, and selectivity have recently attracted significant interest. Usually, they contain only isolated metal atoms embedded in host matrices. However, traditional measuring instruments are extremely difficult to obtain their useful structural information due to ultra-low metal loading, amorphous structure, coordination with light-weight surface atoms and/or co-existing of other metal elements. Synchrotron radiation-based X-ray absorption fine structure spectroscopy (XAFS) has demonstrated its usefulness for this type of catalyst. In this mini-review, we have summarized the recent progress using XAFS to characterize the fine atomic structure of these nanozymes. The synthetic strategies of SAzymes, the principle of XAFS, delicate structural information by XAFS, and the applications of SAzymes have been presented. Furthermore, the outlook and challenges in this active research field have also been discussed. We expect that the help of XAFS can offer a wealth of opportunities to design and develop more efficient SAzymes and apply them to various fields.

Understanding Photoelectrochemical Water Oxidation with X-ray Absorption Spectroscopy

Synchrotron radiation-based X-ray absorption spectroscopy (XAS) has been demonstrated to be a powerful and effective characterization tool in comprehensively and deeply understanding the photoelect...

Design of assembled composite of Mn3O4@Graphitic carbon porous nano-dandelions: A catalyst for Low–temperature selective catalytic reduction of NOx with remarkable SO2 resistance

The biggest challenge for selective catalytic reduction (SCR) of NO by NH3 at low temperatures is to develop robust catalysts with high resistance to SO2. Mn-based catalysts have attracted great attention for their excellent activity, but the poor SO2 resistance limits their application. Herein, we report a novel assembled composite featuring Mn3O4 nanoparticles embedded in defect-abundant graphitic carbon frame (Mn3O4@G-A) via a confinement–pyrolysis–oxidation strategy derived from MOF. The Mn3O4@G-A exhibits excellent activity (reaching over 90 % NO conversion in 100, 000 h−1 GHSV at 160 °C) and significantly improved SO2 resistance in comparison to pristine Mn3O4, and all the performance tests were conducted with 5.6 vol.% water vapor. By combining synchrotron radiation-based X-ray absorption spectroscopy and density functional theory calculation, we further discovered the positive role and the origin of graphitic carbon for improving SO2 resistance. Over the Mn3O4@G-A sample, the electrons can be transferred from graphitic carbon to Mn3O4 nanoparticles, which can regulate the electronic properties and oxidizing ability of Mn3O4 nanoparticles. The proper oxidizing ability of Mn3O4@G-A can inhibit the oxidation of SO2 to SO3, thus further avoid the formation of NH4HSO4 and improve the SO2 resistance. Our work here not only solves the current problem of poor SO2 resistance of Mn-based catalysts, but also provides new insights into the design and synthesis of MOF-derived graphitic carbon-containing catalysts for SCR applications.

A Mechanism Study of Structural Transition of Lithium Vanadium Phosphate during the 1st Charge/Discharge in Comparison to Ti-Doped One

Battery is one of the most attractive energy storage systems for its usefulness in energy conversion in these days. A great attention has been paid to the Li-ion batteries, and a lot of attempts to develop the system exhibiting higher energy density and higher safety have been tried for many years. However, the safety of Li-ion battery has been problems continuously while the energy density has developed largely. The safety issues of Li-ion battery generally come from thermal runaway, mainly derived from oxygen evolution from cathode materials. Phosphate materials including frames of PO4 polyanions such as LiFePO4 are regarded as a suitable cathode material in Li-ion battery in terms of the point of view. Lithium vanadium phosphate (LVP) has high expectations as a cathode material because of its higher operating voltage (~3.6V) and theoretical capacity(197mAh/g) compared to LiFePO4. While monoclinic LVP has a higher energy density, large hysteresis comes from asymmetric charge/discharge profile, and capacity fading are the main problems when three lithium ions are extracted by raising the voltage up to 4.8V. To overcome these limitations, fine studies about the mechanism have been carried out by a few groups. Although the mechanism studies were carried out by L. Nazar group about structural transition [1] and by Lee and Park about the possible pathway of lithium ion in the structure [2], representatively, the reasons for capacity fading on LVP should be studied more. In this work, we investigated the structural changes of LVP during the first charge and discharge process of voltage window of 3.0V to 4.8V by using Galvanostatic Intermittent Titration Technique (GITT), synchrotron radiation-based X-ray Absorption Spectroscopy (XAS) and in situ X-ray diffraction (XRD). The irreversible structural transitions between charge and discharge process are one of the possible reason for the capacity fading. Also, we conducted comparison study on the electrochemical and structural properties between LVP and Li3Ti0.03V1.97(PO4)3/graphene (LVP-Ti) to overcome limitations of LVP. In situ XRD patterns show four consecutive two-phase transitions on LVP during the charge process. During the reinsertion of lithium ion, the patterns of LVP look like a combined process of single solid solution and two-phase transitions between full charged LVP (FC-LVP) and Li2V2(PO4)3 (Li2 phase) without forming Li1V2(PO4)3. In GITT data, a two phase transition behavior which was previously reported as a solid solution behavior during the Li-ion insertion from full charged LVP to Li2 phase in the discharge process is observed. From these data, it can be said that transforming behavior from FC-LVP to Li2 phase is closer to two-phase reaction. In LVP-Ti case, the charge capacity was 185.84 mAh/g and the discharge capacity was 171.20 mAh/g in the first cycle, which were higher than those of LVP, 178.84 mAh/g and 162.99 mAh/g. And in situ XRD patterns show four consecutive two-phase transitions as in LVP. During the phase transition in charge/discharge, LVP-Ti faces less lattice mismatch than LVP, resulting in better electrochemical performance, and this phenomenon is originated from smaller particle size and enduring strain. The XAS result shows that the XANES peaks of LVP-Ti change gradually which is in line with the XRD result as same as in LVP case. However, the larger peak area of the pre-edge for LVP-Ti suggesting that more electrons were released upon charging. This means that the higher capacities of LVP-Ti is contributed by further utilization of the V redox reaction. [1] Yin, S-C., Hiltrud Grondey, Pierre Strobel, M. Anne, and Linda F. Nazar. "Electrochemical property: structure relationships in monoclinic Li3-yV2(PO4)3." Journal of the American Chemical Society 125, no. 34 (2003): 10402-10411. [2] Lee, S., & Park, S. S. (2012). Atomistic simulation study of monoclinic Li3V2(PO4)3 as a cathode material for lithium ion battery: structure, defect chemistry, lithium ion transport pathway, and dynamics. The Journal of Physical Chemistry C, 116(48), 25190-25197.

Surface deep profile synchrotron studies of mechanically modified top-down silicon nanowires array using ultrasoft X-ray absorption near edge structure spectroscopy

Atomic, electronic structure and composition of top-down metal-assisted wet-chemically etched silicon nanowires were studied by synchrotron radiation based X-ray absorption near edge structure technique. Local surrounding of the silicon and oxygen atoms in silicon nanowires array was studied on as-prepared nanostructured surfaces (atop part of nanowires) and their bulk part after, first time applied, in-situ mechanical removal atop part of the formed silicon nanowires. Silicon suboxides together with disturbed silicon dioxide were found in the composition of the formed arrays that affects the electronic structure of silicon nanowires. The results obtained by us convincingly testify to the homogeneity of the phase composition of the side walls of silicon nanowires and the electronic structure in the entire length of the nanowire. The controlled formation of the silicon nanowires array may lead to smart engineering of its atomic and electronic structure that influences the exploiting strategy of metal-assisted wet-chemically etched silicon nanowires as universal matrices for different applications.