Electrochemical Generation of Catalytically Active Edge Sites in C₂N‐Type Carbon Materials for Artificial Nitrogen Fixation

The electrochemical nitrogen reduction reaction (NRR) to ammonia (NH₃) is a potentially carbon‐neutral and decentralized supplement to the established Haber–Bosch process. Catalytic activation of the highly stable dinitrogen molecules remains a great challenge. Especially metal‐free nitrogen‐doped carbon catalysts do not often reach the desired selectivity and ammonia production rates due to their low concentration of NRR active sites and possible instability of heteroatoms under electrochemical potential, which can even contribute to false positive results. In this context, the electrochemical activation of nitrogen‐doped carbon electrocatalysts is an attractive, but not yet established method to create NRR catalytic sites. Herein, a metal‐free C₂N material (HAT‐700) is electrochemically etched prior to application in NRR to form active edge‐sites originating from the removal of terminal nitrile groups. Resulting activated metal‐free HAT‐700‐A shows remarkable catalytic activity in electrochemical nitrogen fixation with a maximum Faradaic efficiency of 11.4% and NH₃ yield of 5.86 µg mg⁻¹cat h⁻¹. Experimental results and theoretical calculations are combined, and it is proposed that carbon radicals formed during activation together with adjacent pyridinic nitrogen atoms play a crucial role in nitrogen adsorption and activation. The results demonstrate the possibility to create catalytically active sites on purpose by etching labile functional groups prior to NRR

Theoretical Studies on Water Oxidation Catalysts - from Solvent to Interfaces

Water splitting contains two half-reactions, the water oxidation reaction and the hydrogen reduction reaction. In water oxidation, protons and electrons will be generated to offer two elemental components for production of fuels, such as H2 and CH3OH. To overcome the overpotential of the reaction, a large amount of water oxidation catalysts (WOCs) have been synthesized. In the second chapter, a variety of homogeneous and heterogenous catalysts have been introduced. The homogeneous catalysts include Ru-based catalysts, Ir-based catalysts, and first-row transition metal-based catalysts. Among these catalysts, a family of Ru(bda)L2 complexes was found in experiment to have a comparable turnover frequency (TOF) at acidic pH with photosystem II. A similar catalyst, Ru(tda)(py)2, was found to have an impressive TOF of 50 000 s-1 at pH 10.0. The heterogenous catalysts include heterogenous oxide and heterogenized molecular catalysts that catalyze the reaction using either electrochemical driving force or photoelectrochemical driving force. Understanding the details of the mechanism can help to design a better catalyst with high catalytic performance. For this purpose, several theoretical methods have been applied. Density functional theory (DFT) was employed to study the rate limiting reaction in implicit solvent. The empirical valence bond (EVB) method is a powerful tool for describing environment effects. This approach was used to get insight into the solvent and surface effects on the reaction pathway. Molecular dynamics (MD) and potential of mean force (PMF) methods are applied to perform simulations for large systems at long time-scales.       The Ru(bda)L2 catalysts have been found to have high TOF, up to 1000 s-1 and react via an I2M (Interaction of two metal centers) pathway. By using B3LYP-D3 functional to study the diradical coupling of the O-O bond formation, we found that there is no intrinsic barrier between the two RuV=O fragments of RuV=O complexes. On the basis of the study of the solvent role on the reaction using an EVB-MD model, the oxo of the RuV=O species was shown to be hydrophobic. The hydrophobic oxo explained why the Ru(bda)L2 complexes proceed the reaction via the I2M pathway. To study the full dimerization of two separated RuV=O species in fully explicit solvent, we calculated the diffusion of individual catalysts from MD simulations, association of pre-reactive dimer from PMF simulations, and the coupling reaction in explicit solvation using the EVB approach. The formation of the prereactive dimer was found to be the sole determining factor for the efficiency of the Ru(bda)L2 catalysts. In the study of four Ru complexes with different equatorial ligands, the secondary coordination environments, such as flexibility, hydrophilicity were proposed to be the affecting the different catalytic pathways. To make an efficient electrocatalyst, the Ru(bda)L2 catalyst has been modified by Sun and co-workers with pyrene groups at the axial L-ligands to be adhered on the CNT functionalized electrodes. A computational model of the RuV=O catalyst tethered on the CNT surface was built to study the O-O bond formation in heterogenous system. By using the same combination of MD, PMF, and EVB, we studied the full dimerization reaction of the catalyst at CNT-water interfaces with full dynamics. The reasons for the lower the TOF of the surface catalyst and methods to improve the lower TOF were addressed in this study. With the pH dependent Ru(tda)(py)2 complex, we used the same combination methods and proposed a conceptually new function of the dangling carboxylate – the oxide relay. The oxide relay provides a highly nucleophilic oxygen atom close to the oxo to facilitate the O-O bond formation at the first step, and a highly electrophilic center to react with the OH- even at neutral pH at the second step. The rate-limiting step is the O-O bond formation at high pH, OH- nucleophilic attack at neutral pH. In summary, several key properties of the water oxidation catalyzed by Ru-based complexes, such as solvent and surface effects, hydrophobicity, and oxide relay have been investigated in detail by using several computational techniques. Our studies can shed light on the design of molecular WOCs with high catalytic activity and will help the development of artificial photosysnthesis devices.QC 20190506</p

Theoretical Studies on Water Oxidation Catalysts - from Solvent to Interfaces

Water splitting contains two half-reactions, the water oxidation reaction and the hydrogen reduction reaction. In water oxidation, protons and electrons will be generated to offer two elemental components for production of fuels, such as H2 and CH3OH. To overcome the overpotential of the reaction, a large amount of water oxidation catalysts (WOCs) have been synthesized. In the second chapter, a variety of homogeneous and heterogenous catalysts have been introduced. The homogeneous catalysts include Ru-based catalysts, Ir-based catalysts, and first-row transition metal-based catalysts. Among these catalysts, a family of Ru(bda)L2 complexes was found in experiment to have a comparable turnover frequency (TOF) at acidic pH with photosystem II. A similar catalyst, Ru(tda)(py)2, was found to have an impressive TOF of 50 000 s-1 at pH 10.0. The heterogenous catalysts include heterogenous oxide and heterogenized molecular catalysts that catalyze the reaction using either electrochemical driving force or photoelectrochemical driving force. Understanding the details of the mechanism can help to design a better catalyst with high catalytic performance. For this purpose, several theoretical methods have been applied. Density functional theory (DFT) was employed to study the rate limiting reaction in implicit solvent. The empirical valence bond (EVB) method is a powerful tool for describing environment effects. This approach was used to get insight into the solvent and surface effects on the reaction pathway. Molecular dynamics (MD) and potential of mean force (PMF) methods are applied to perform simulations for large systems at long time-scales.       The Ru(bda)L2 catalysts have been found to have high TOF, up to 1000 s-1 and react via an I2M (Interaction of two metal centers) pathway. By using B3LYP-D3 functional to study the diradical coupling of the O-O bond formation, we found that there is no intrinsic barrier between the two RuV=O fragments of RuV=O complexes. On the basis of the study of the solvent role on the reaction using an EVB-MD model, the oxo of the RuV=O species was shown to be hydrophobic. The hydrophobic oxo explained why the Ru(bda)L2 complexes proceed the reaction via the I2M pathway. To study the full dimerization of two separated RuV=O species in fully explicit solvent, we calculated the diffusion of individual catalysts from MD simulations, association of pre-reactive dimer from PMF simulations, and the coupling reaction in explicit solvation using the EVB approach. The formation of the prereactive dimer was found to be the sole determining factor for the efficiency of the Ru(bda)L2 catalysts. In the study of four Ru complexes with different equatorial ligands, the secondary coordination environments, such as flexibility, hydrophilicity were proposed to be the affecting the different catalytic pathways. To make an efficient electrocatalyst, the Ru(bda)L2 catalyst has been modified by Sun and co-workers with pyrene groups at the axial L-ligands to be adhered on the CNT functionalized electrodes. A computational model of the RuV=O catalyst tethered on the CNT surface was built to study the O-O bond formation in heterogenous system. By using the same combination of MD, PMF, and EVB, we studied the full dimerization reaction of the catalyst at CNT-water interfaces with full dynamics. The reasons for the lower the TOF of the surface catalyst and methods to improve the lower TOF were addressed in this study. With the pH dependent Ru(tda)(py)2 complex, we used the same combination methods and proposed a conceptually new function of the dangling carboxylate – the oxide relay. The oxide relay provides a highly nucleophilic oxygen atom close to the oxo to facilitate the O-O bond formation at the first step, and a highly electrophilic center to react with the OH- even at neutral pH at the second step. The rate-limiting step is the O-O bond formation at high pH, OH- nucleophilic attack at neutral pH. In summary, several key properties of the water oxidation catalyzed by Ru-based complexes, such as solvent and surface effects, hydrophobicity, and oxide relay have been investigated in detail by using several computational techniques. Our studies can shed light on the design of molecular WOCs with high catalytic activity and will help the development of artificial photosysnthesis devices.QC 20190506</p

Theoretical Studies on Water Oxidation Catalysts - from Solvent to Interfaces

Water splitting contains two half-reactions, the water oxidation reaction and the hydrogen reduction reaction. In water oxidation, protons and electrons will be generated to offer two elemental components for production of fuels, such as H2 and CH3OH. To overcome the overpotential of the reaction, a large amount of water oxidation catalysts (WOCs) have been synthesized. In the second chapter, a variety of homogeneous and heterogenous catalysts have been introduced. The homogeneous catalysts include Ru-based catalysts, Ir-based catalysts, and first-row transition metal-based catalysts. Among these catalysts, a family of Ru(bda)L2 complexes was found in experiment to have a comparable turnover frequency (TOF) at acidic pH with photosystem II. A similar catalyst, Ru(tda)(py)2, was found to have an impressive TOF of 50 000 s-1 at pH 10.0. The heterogenous catalysts include heterogenous oxide and heterogenized molecular catalysts that catalyze the reaction using either electrochemical driving force or photoelectrochemical driving force. Understanding the details of the mechanism can help to design a better catalyst with high catalytic performance. For this purpose, several theoretical methods have been applied. Density functional theory (DFT) was employed to study the rate limiting reaction in implicit solvent. The empirical valence bond (EVB) method is a powerful tool for describing environment effects. This approach was used to get insight into the solvent and surface effects on the reaction pathway. Molecular dynamics (MD) and potential of mean force (PMF) methods are applied to perform simulations for large systems at long time-scales.       The Ru(bda)L2 catalysts have been found to have high TOF, up to 1000 s-1 and react via an I2M (Interaction of two metal centers) pathway. By using B3LYP-D3 functional to study the diradical coupling of the O-O bond formation, we found that there is no intrinsic barrier between the two RuV=O fragments of RuV=O complexes. On the basis of the study of the solvent role on the reaction using an EVB-MD model, the oxo of the RuV=O species was shown to be hydrophobic. The hydrophobic oxo explained why the Ru(bda)L2 complexes proceed the reaction via the I2M pathway. To study the full dimerization of two separated RuV=O species in fully explicit solvent, we calculated the diffusion of individual catalysts from MD simulations, association of pre-reactive dimer from PMF simulations, and the coupling reaction in explicit solvation using the EVB approach. The formation of the prereactive dimer was found to be the sole determining factor for the efficiency of the Ru(bda)L2 catalysts. In the study of four Ru complexes with different equatorial ligands, the secondary coordination environments, such as flexibility, hydrophilicity were proposed to be the affecting the different catalytic pathways. To make an efficient electrocatalyst, the Ru(bda)L2 catalyst has been modified by Sun and co-workers with pyrene groups at the axial L-ligands to be adhered on the CNT functionalized electrodes. A computational model of the RuV=O catalyst tethered on the CNT surface was built to study the O-O bond formation in heterogenous system. By using the same combination of MD, PMF, and EVB, we studied the full dimerization reaction of the catalyst at CNT-water interfaces with full dynamics. The reasons for the lower the TOF of the surface catalyst and methods to improve the lower TOF were addressed in this study. With the pH dependent Ru(tda)(py)2 complex, we used the same combination methods and proposed a conceptually new function of the dangling carboxylate – the oxide relay. The oxide relay provides a highly nucleophilic oxygen atom close to the oxo to facilitate the O-O bond formation at the first step, and a highly electrophilic center to react with the OH- even at neutral pH at the second step. The rate-limiting step is the O-O bond formation at high pH, OH- nucleophilic attack at neutral pH. In summary, several key properties of the water oxidation catalyzed by Ru-based complexes, such as solvent and surface effects, hydrophobicity, and oxide relay have been investigated in detail by using several computational techniques. Our studies can shed light on the design of molecular WOCs with high catalytic activity and will help the development of artificial photosysnthesis devices.QC 20190506</p

Dynamics and Reactions of Molecular Ru Catalysts at Carbon Nanotube–Water Interfaces

Immobilization of molecular catalysts to electrode surfaces can improve the recyclability and electron transfer rates. The drawback is that most experimental techniques and theoretical methods are not applicable. Here we present results from a study of a ruthenium water oxidation catalyst [Ru^VO­(bda)­L₂] in explicit water at a carbon nanotube water interface, forming the key O–O bond between two 128 atom catalysts, all fully dynamically. Our methodology is based on a recently developed empirical valence bond (EVB) model. We follow the key steps of the reaction including diffusion of the catalysts at the interface, formation of the prereactive dimer, and the bond formation between the two catalysts. On the basis of the calculated parameters, we compute the turnover frequency (TOF) at the experimental loading, in excellent agreement with the experiments. The key O–O bond formation was significantly retarded at the surface, and limiting components were identified that could be improved by catalyst modification

Group 7: Challenge: 3 - Defect detection in graphene sheets

Challenge 3 was focused on the identification of defects present within graphene sheets. Provided with electron microscopy images of sheets of graphene, the data-set was partitioned into a sample of perfect patches (regions of an image which do not contain defects), defect patches (regions of an image which contain a defect) and a data-set of images which are not edited or partitioned into smaller sections of analysis. The full image is 256 x 256 patches (Figure 1a). The blue (high electron density) corresponds to atoms and the green corresponds to background. In the full image patches, there is a perfect 48 x 48 patches and a defect 48 x 48 patches. By selecting and training an appropriate machine learning model, the goal was the identification of defect regions contained within a whole electron microscopy image of a graphene sheet

Whole basin modeling and parameter inversion of mining subsidence based on UAV photogrammetry technology

Taking Tangjiahui Mining Area in Inner Mongolia as the research object, the UAV photographic image data of August 2020 and March 2021 in this area were obtained, and DEM was produced. The subsidence basin in this area was obtained by subtracting the DEM data, and the denoising effects of different denoising methods were compared with MATLAB software. Based on the subsidence data of the whole basin, the subsidence coefficient and the main influence tangent of the subsidence basin are obtained by using the probability integral parameter inversion with method of simulated annealing(SA). Using this parameter to simulate the subsidence basin, it is calculated that the measurement error is 589 mm, accounting for 8.1% of the maximum subsidence value. Finally, the robust analysis of the parameters is made, and when the error in the measurement accounts for (1% to 10%) the maximum subsidence value, the result of parameter calculation is reliable. The results show that the BP neural network algorithm can effectively remove the noise in the basin and improve the accuracy of the subsidence basin. Based on SA and the data of the whole basin in the mining area, the probability integral parameters can be obtained effectively, which can compensate for the influence of the low accuracy of UAV photogrammetry technology

Why Is There a Barrier in the Coupling of Two Radicals in the Water Oxidation Reaction?

Two radicals can form a bond without an energetic barrier. However, the radical coupling mechanism in ruthenium-catalyzed water oxidation has been found to be associated with substantial activation energies. Here we have investigated the coupling reaction of [RuO­(bda)­L₂]⁺ catalysts with different axial L ligands. The interaction between the two oxo radical moieties at the Ru­(V) state was found to have a favorable interaction in the transition state in comparison to the prereactive complex. To further understand the existence of the activation energy, the activation energy has been decomposed into distortion energy and interaction energy. No correlation between the experimental rates and the calculated coupling barriers of different axial L was found, showing that more aspects such as solvation, supramolecular properties, and solvent dynamics likely play important roles in the equilibrium between the free Ru^VO monomer and the [Ru^VO···ORu^V] dimer. On the basis of our findings, we give general guidelines for the design of catalysts that operate by the radical coupling mechanism

Stabilized and Controlled Release of Radicals within Copper Formate-Based Nanozymes for Biosensing

Fenton-like radical processes are widely utilized to explain catalytic mechanisms of peroxidase-like nanozymes, which exhibit remarkable catalytic activity, cost-effectiveness, and stability. However, there is still a need for a comprehensive understanding of the formation, stabilization, and transformation of such radicals. Herein, a copper formate-based nanozyme (Cuf-TMB) was fabricated via a pre-catalytic strategy under ambient conditions. The as-prepared nanozyme shows comparable catalytic activity (K-m, 1.02 x 10(-5) mM(-1); K-cat, 3.09 x 10(-2) s(-1)) and kinetics to those of natural peroxidase toward H2O2 decomposition. This is attributed to the feasible oxidation by *OH species via the *O intermediate, as indicated by density functional theory calculations. The key .OH radicals were detected to be stable for over 52 days and can be released in a controlled manner during the catalytic process via in situ electron spin-resonance spectroscopy measurements. Based on the understanding, an ultrasensitive biosensing platform was constructed for the sensitive monitoring of biochemical indicators in clinic settings

Organic Polymer Dots Photocatalyze CO2 Reduction in Aqueous Solution

Developing low-cost and efficient photocatalysts to convert CO2 into valuable fuels is desirable to realize a carbon-neutral society. In this work, we report that polymer dots (Pdots) of poly[(9,9 ' -dioctylfluorenyl-2,7-diyl)-co-(1,4-benzo-thiadiazole)] (PFBT), without adding any extra co-catalyst, can photocatalyze reduction of CO2 into CO in aqueous solution, rendering a CO production rate of 57 mu mol g(-1) h(-1 )with a detectable selectivity of up to 100 %. After 5 cycles of CO2 re-purging experiments, no distinct decline in CO amount and reaction rate was observed, indicating the promising photocatalytic stability of PFBT Pdots in the photocatalytic CO2 reduction reaction. A mechanistic study reveals that photoexcited PFBT Pdots are reduced by sacrificial donor first, then the reduced PFBT Pdots can bind CO(2 )and reduce it into CO via their intrinsic active sites. This work highlights the application of organic Pdots for CO2 reduction in aqueous solution, which therefore provides a strategy to develop highly efficient and environmentally friendly nanoparticulate photocatalysts for CO2 reduction