scholarly journals Methanol conversion on borocarbonitride catalysts: Identification and quantification of active sites

2020 ◽  
Vol 6 (26) ◽  
pp. eaba5778 ◽  
Author(s):  
Xuefei Zhang ◽  
Pengqiang Yan ◽  
Junkang Xu ◽  
Fan Li ◽  
Felix Herold ◽  
...  
Keyword(s):  
Active Sites ◽  
Density Functional ◽  
Catalytic Mechanism ◽  
Reaction Pathway ◽  
Methanol Conversion ◽  
Kinetic Measurements ◽  
Catalytic Systems ◽  
X Ray Absorption ◽  
Further Development

Borocarbonitrides (BCNs) have emerged as highly selective catalysts for the oxidative dehydrogenation (ODH) reaction. However, there is a lack of in-depth understanding of the catalytic mechanism over BCN catalysts due to the complexity of the surface oxygen functional groups. Here, BCN nanotubes with multiple active sites are synthesized for oxygen-assisted methanol conversion reaction. The catalyst shows a notable activity improvement for methanol conversion (29%) with excellent selectivity to formaldehyde (54%). Kinetic measurements indicate that carboxylic acid groups on BCN are responsible for the formation of dimethyl ether, while the redox catalysis to formaldehyde occurs on both ketonic carbonyl and boron hydroxyl (B─OH) sites. The ODH reaction pathway on the B─OH site is further revealed by in situ infrared, x-ray absorption spectra, and density functional theory. The present work provides physical-chemical insights into the functional mechanism of BCN catalysts, paving the way for further development of the underexplored nonmetallic catalytic systems.

The catalytic mechanism of the Au@TiO2−x/ZnO catalyst towards a low-temperature water-gas shift reaction

2020 ◽  
Vol 10 (3) ◽  
pp. 768-775
Author(s):  
Ning Liu ◽  
Pan Yin ◽  
Ming Xu ◽  
Yusen Yang ◽  
Shaomin Zhang ◽  
...  
Keyword(s):  
Density Functional ◽  
Catalytic Mechanism ◽  
Density Functional Theory Calculation ◽  
Water Gas Shift ◽  
Water Gas Shift Reaction ◽  
Functional Theory ◽  
Shift Reaction ◽  
Water Gas ◽  
Theory Calculation

A redox mechanism towards the water-gas shift reaction was certified based on in situ/operando experiments and density functional theory calculation studies.

scholarly journals Photocatalytic degradation of trace CHCl3 in drinking water by nano TiO2

2020 ◽  
Author(s):  
T.-L. Hsiung ◽  
L.-W. Wei ◽  
H.-L. Huang ◽  
H. Paul Wang
Keyword(s):  
Drinking Water ◽  
Photocatalytic Degradation ◽  
Active Sites ◽  
Disinfection Byproducts ◽  
Active Species ◽  
Nano Tio2 ◽  
Edge Structure ◽  
Tio2 Photocatalysts ◽  
X Ray Absorption

Abstract Toxic disinfection byproducts such as trihalomethanes (CHCl3) are frequently found after chlorination for drinking water. Nano TiO2 which has been widely used for photocatalytic degradation of organic pollutants in wastewater, however, has relatively low effectiveness in the treatments of trace CHCl3. To engineer capable TiO2 photocatalysts, an understanding of their photoactive sites is of great importance and interest. By in situ X-ray absorption near edge structure (XANES) spectroscopy, photoactive sites such as A1 (4969 eV), A2 (4971 eV) and A3 (4972 eV) can be distinguished asfour-, five-, and six- coordinated Ti species, respectively in the nano-TiO2 (8.5 and 4.6 nm for TiO2 on SBA-15), TiO2 clusters (TiO2-SiO2), and highly atomic dispersed Ti (Ti-MCM-41) photocatalysts. It appears that the reactivity for the photocatalytic degradation of trace CHCl3 in drinking water lacks an expected relationship with the crystalline phase, band gap absorption edge, nor the particle size of the TiO2-based photocatalysts. Notably, the A2 sites being the main photocatalytic active species of the TiO2 may be accountable for the main (about 95%) photocatalytic degradation of trace CHCl3 in drinking water (7.2 ppm CHCl3/gTiO2∙hr). This work reveals that the A2 active sites of a TiO2-based photocatalyst are responsible for the photocatalytic reactivity, especially in photocatalytic degradation of CHCl3 in drinking water.

scholarly journals How Stable Are 2H-MoS2 Edges Under Hydrogen Evolution Reaction Conditions?

2021 ◽  
Author(s):  
Nawras Abidi ◽  
Audrey Bonduelle-Skrzypczak ◽  
Stephan Steinmann
Keyword(s):  
Hydrogen Evolution ◽  
Hydrogen Evolution Reaction ◽  
Active Sites ◽  
Density Functional ◽  
External Source ◽  
Operating Conditions ◽  
Adsorbed Hydrogen ◽  
Functional Theory ◽  
Acidic Conditions

MoS<sub>2</sub>, have emerged as a promising class of electrocatalysts for the production of H<sub>2</sub> via the hydrogen evolution reaction (HER) in acidic conditions.<div>The edges of MoS<sub>2</sub> are known for their HER activity, but their precise atomistic nature and stability under HER conditions is not yet known. In contrast to other typical uses of MoS<sub>2</sub> as a catalyst, under HER there is no external source of sulfur. Therefore, the sulfidation of the edges can only decrease under operating conditions and the thermodynamics of the process are somewhat ill-defined. Our results suggest that the 50%S S-edge may be active for HER via the Volmer-Tafel mechanism and is, despite a high H coverage, stable with respect to H<sub>2</sub>S release. </div><div>At the 50%S Mo-edge, the adsorbed hydrogen opens the way for H<sub>2</sub>S release, leading to the 0%S Mo-edge, which was previously investigated and found to be HER active. HER being a water-based process, we also considered the effect of the presence of H<sub>2</sub>O and the in-situ formation of OH. For the 50%S Mo-edge, H<sub>2</sub>O is only very weakly adsorbed and OH formation is unfavorable. Nevertheless, OH assists the loss of sulfur coverage, leading to OH-based HER active sites. In contrast, OH is strongly adsorbed on the 50%S S-edge. By explicitly considering the electrochemical potential using grand-canonical density functional theory, we unveil that the Volmer-Heyrovsky mechanism on sulfur sites is still accessible in the presence of surface OH at the 50%S S-edge. However, the 50%S S-edge is found to be mildly unstable with respect to H<sub>2</sub>S in the presence of water/OH. Hence, we suggest that the 50%S S-edge evolves over time towards a 0%S S-edge, covered by surface OH that will block permanently the active sites. </div>

scholarly journals A Cobalt-Iron Double-Atom Catalyst for the Oxygen Evolution Reaction

2019 ◽  
Author(s):  
Lichen Bai ◽  
Chia-Shuo Hsu ◽  
Duncan Alexander ◽  
Hao Ming Chen ◽  
Xile Hu
Keyword(s):  
Oxygen Evolution ◽  
Active Site ◽  
Oxygen Evolution Reaction ◽  
Active Sites ◽  
Single Atom ◽  
X Ray ◽  
Highly Active ◽  
Cobalt Iron ◽  
X Ray Absorption

Single atom catalysts exhibit well-defined active sites and potentially maximum atomic efficiency. However, they are unsuitable for reactions that benefit from bimetallic promotion such as the oxygen evolution reaction (OER) in alkaline medium. Here we show that a single atom Co precatalyst can be in-situ transformed into a Co-Fe double atom catalyst for OER. This catalyst exhibits one of the highest turnover frequencies among metal oxides. Electrochemical, microscopic, and spectroscopic data including those from operando X-ray absorption spectroscopy, reveal a dimeric Co-Fe moiety as the active site of the catalyst. This work demonstrates double-atom catalysis as a promising approach for the developed of defined and highly active OER catalysts.

scholarly journals In situ X-ray absorption spectroscopic studies of TiO2 photocatalytic active sites for degradation of trace CHCl3 in drinking water

2021 ◽  
Vol 28 (6) ◽  
Author(s):  
T.-L. Hsiung ◽  
L.-W. Wei ◽  
H.-L. Huang ◽  
H. Paul Wang
Keyword(s):  
Drinking Water ◽  
Photocatalytic Degradation ◽  
Active Sites ◽  
Spectroscopic Studies ◽  
Band Gap Energy ◽  
Disinfection Byproducts ◽  
Edge Structure ◽  
X Ray ◽  
X Ray Absorption

Toxic disinfection byproducts such as trihalomethanes (e.g. CHCl3) are often found after chlorination of drinking water. It has been found that photocatalytic degradation of trace CHCl3 in drinking water generally lacks an expected relationship with the crystalline phase, band-gap energy or the particle sizes of the TiO2-based photocatalysts used such as nano TiO2 on SBA-15 (Santa Barbara amorphous-15), TiO2 clusters (TiO2–SiO2) and atomic dispersed Ti [Ti-MCM-41 (Mobil Composition of Matter)]. To engineer capable TiO2 photocatalysts, a better understanding of their photoactive sites is of great importance and interest. Using in situ X-ray absorption near-edge structure (XANES) spectroscopy, the A1 (4969 eV), A2 (4971 eV) and A3 (4972 eV) sites in TiO2 can be distinguished as four-, five- and six- coordinated Ti species, respectively. Notably, the A2 Ti sites that are the main photocatalytic species of TiO2 are shown to be accountable for about 95% of the photocatalytic degradation of trace CHCl3 in drinking water (7.2 p.p.m. CHCl3 gTiO2 −1 h−1). This work reveals that the A2 Ti species of a TiO2-based photocatalyst are mainly responsible for the photocatalytic reactivity, especially in photocatalytic degradation of CHCl3 in drinking water.

scholarly journals Time-resolved in situ visualization of the structural response of zeolites during catalysis

2020 ◽  
Vol 11 (1) ◽  
Author(s):  
Jinback Kang ◽  
Jerome Carnis ◽  
Dongjin Kim ◽  
Myungwoo Chung ◽  
Jaeseung Kim ◽  
...  
Keyword(s):  
Active Sites ◽  
Density Functional ◽  
Three Dimensional ◽  
Structural Response ◽  
Active Regions ◽  
Structural Deformation ◽  
Time Resolved ◽  
X Ray ◽  
Future Design

AbstractZeolites are three-dimensional aluminosilicates having unique properties from the size and connectivity of their sub-nanometer pores, the Si/Al ratio of the anionic framework, and the charge-balancing cations. The inhomogeneous distribution of the cations affects their catalytic performances because it influences the intra-crystalline diffusion rates of the reactants and products. However, the structural deformation regarding inhomogeneous active regions during the catalysis is not yet observed by conventional analytical tools. Here we employ in situ X-ray free electron laser-based time-resolved coherent X-ray diffraction imaging to investigate the internal deformations originating from the inhomogeneous Cu ion distributions in Cu-exchanged ZSM-5 zeolite crystals during the deoxygenation of nitrogen oxides with propene. We show that the interactions between the reactants and the active sites lead to an unusual strain distribution, confirmed by density functional theory simulations. These observations provide insights into the role of structural inhomogeneity in zeolites during catalysis and will assist the future design of zeolites for their applications.

Comparative DFT study on the α-glycosidic bond in reactive species of galactosyl diphosphates

2009 ◽  
Vol 63 (5) ◽  
Author(s):  
Juraj Kóňa ◽  
Igor Tvaroška
Keyword(s):  
Density Functional ◽  
Catalytic Mechanism ◽  
Activation Barrier ◽  
Reaction Pathway ◽  
Glycosidic Bond ◽  
Stationary Points ◽  
Correct Prediction ◽  
Sugar Phosphate ◽  
Dft Methods ◽  
Mp2 Method

AbstractCorrect prediction of the structure and energetics along the reaction pathway of the formation or dissociation of the glycosidic bond in sugar phosphates is crucial for the understanding of catalytic mechanism and for the determination of transition state structures of sugar-phosphate processing enzymes. The performance of seven density functional theory (DFT) methods (BLYP, B3LYP, MPW1PW91, MPW1K, MPWB1K, M05 and M05-2X) and two wave function methods (HF and MP2) was tested using four structural models with the activated sugar-phosphate α-glycosidic linkage. The models were chosen based on the crystal structure of the retaining glycosyltransferase LgtC complex with methyl α-d-galactopyranose diphosphate and its 2-fluoro derivative. Results of the MP2 method were used as a benchmark for the other methods. Two structural trends were observed in the calculations: predicted length of the activated C1-O1 glycosidic bond of 1.49–1.63 Å was significantly larger than values of a standard C1-O1 glycosidic bond in crystal structures of carbohydrates (1.39–1.48 Å), and the calculated value depended on the DFT method used. The MPW1K, M05 and M05-2X functionals provided results in closest agreement with those from the MP2 method, the difference being less than 0.02 Å in the calculated glycosidic bond lengths. On the contrary, the BLYP and B3LYP functionals failed to predict sugar diphosphate in the (-sc) conformation as a stable structure. Instead, the only stationary points localized along the C1-O1 dissociation coordinate were oxocarbenium ions at the distance of approximately 2.8 Å. The M05-2X, MPW1K and MPWB1K functionals gave the most reasonable prediction of the thermochemical kinetic parameters, where the formation of the oxocarbenium ion has a slightly endothermic character (0.4–1.7 kJ mol−1) with an activation barrier of 7.9–9.2 kJ mol−1.

scholarly journals Direct Evidence of Cobalt Oxyhydroxide Formation on a La0.2Sr0.8CoO3 Perovskite Water Splitting Catalyst

2021 ◽  
Author(s):  
Anthony Boucly ◽  
Luca Artiglia ◽  
Emiliana Fabbri ◽  
Dennis Palagin ◽  
Dino Aergerter ◽  
...  
Keyword(s):  
Photoelectron Spectroscopy ◽  
Active Sites ◽  
Direct Evidence ◽  
Ambient Pressure ◽  
Spectral Feature ◽  
Ex Situ ◽  
Oxide Surface ◽  
Catalytic Systems ◽  
Cobalt Oxyhydroxide

Abstract Understanding the mechanism of oxygen evolution reaction (OER) on perovskite materials is of great interest to tailor the synthesis of better catalyst materials. Despite the huge amount of literature reports, the complexity of catalytic systems and scarce in situ and operando surface sensitive spectroscopic tools render the detection of active sites and the understanding of the reaction mechanisms challenging. Here, we carried out and compared in situ and ex situ ambient pressure X-ray photoelectron spectroscopy experimental procedures on a La0.2Sr0.8CoO3 perovskite OER catalyst. Experimental results show that segregated surface strontium, which is present in the as prepared sample, is leached into the electrolyte after immersion, leading to surface cobalt active sites enrichment. Such cobalt-enriched oxide surface evolves into a new phase, whose spectral feature is detected in situ, during/after OER. With the help of theoretical simulations, such species is assigned to cobalt oxyhydroxide, providing a direct evidence of its crucial role in the reaction.

Elucidating the structure of a high-spin σ-phenyliron(iii) species in a live FeCl3–PhZnCl reaction system

2018 ◽  
Vol 54 (12) ◽  
pp. 1481-1484 ◽  
Author(s):  
Zhiliang Huang ◽  
Dongchao Zhang ◽  
Jyh-Fu Lee ◽  
Aiwen Lei
Keyword(s):  
Density Functional ◽  
Reaction System ◽  
Paramagnetic Resonance ◽  
Functional Theory ◽  
System P ◽  
First Time ◽  
X Ray Absorption ◽  
Electron Paramagnetic

Characterization of σ-aryliron(iii) species in a live reaction system: an unknown sextet Ph(THF)FeCl2 species was well-characterized in a live FeCl3–PhZnCl reaction system for the first time by Raman, in situ IR, electron paramagnetic resonance (EPR), X-ray absorption spectroscopic (XAS) and density functional theory (DFT) calculations.

scholarly journals Spectral Decomposition of X-ray Absorption Spectroscopy Datasets: Methods and Applications

Crystals ◽  
2020 ◽  
Vol 10 (8) ◽  
pp. 664 ◽  
Author(s):  
Andrea Martini ◽  
Elisa Borfecchia
Keyword(s):  
Absorption Spectroscopy ◽  
Multivariate Curve Resolution ◽  
Beam Position ◽  
X Ray ◽  
External Variables ◽  
Spectral Profiles ◽  
X Ray Absorption ◽  
Further Development ◽  
Selection Of

X-ray absorption spectroscopy (XAS) today represents a widespread and powerful technique, able to monitor complex systems under in situ and operando conditions, while external variables, such us sampling time, sample temperature or even beam position over the analysed sample, are varied. X-ray absorption spectroscopy is an element-selective but bulk-averaging technique. Each measured XAS spectrum can be seen as an average signal arising from all the absorber-containing species/configurations present in the sample under study. The acquired XAS data are thus represented by a spectroscopic mixture composed of superimposed spectral profiles associated to well-defined components, characterised by concentration values evolving in the course of the experiment. The decomposition of an experimental XAS dataset in a set of pure spectral and concentration values is a typical example of an inverse problem and it goes, usually, under the name of multivariate curve resolution (MCR). In the present work, we present an overview on the major techniques developed to realize the MCR decomposition together with a selection of related results, with an emphasis on applications in catalysis. Therein, we will highlight the great potential of these methods which are imposing as an essential tool for quantitative analysis of large XAS datasets as well as the directions for further development in synergy with the continuous instrumental progresses at synchrotron sources.

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