In Situ TEM Tracking of Disconnection Motion and Atomic Cooperative Reorientation in the Oriented Attachment of Pt Nanoparticles.

The oriented attachment (OA) of nanoparticles (NPs) is an important crystal growth mechanism in many materials. However, a comprehensive understanding of the atomic-scale alignment and attachment processes is still lacking. We conducted in situ atomic resolution studies using high-resolution transmission electron microscopy to reveal how two Pt NPs coalesce into a single particle via OA, which involves the formation of atomic-scale links and a grain boundary (GB) between the NPs, as well as GB migration. Density functional theory calculations showed that the system energy changes as a function of the number of disconnections during the coalescence process. Additionally, the formation and annihilation processes of disconnection are always accompanied by the cooperative reorientation motion of atoms. These results further elucidate the growth mechanism of OA at the atomic scale, providing microscopic insights into OA dynamics and a framework for the development of processing strategies for nanocrystalline materials.

Coordination Confined Thermolysis Synthesis of the Ni Single Atom Catalyst on the N-Doped Commercial Carbon for the Production of Syngas.

The electrochemical conversion of CO2 into controllable syngas (CO/H2) over a wide potential range is challenging. The main electrocatalysts are based on the noble metals Au (Ag) or heavy metal Pb. The development of alternative nonprecious catalysts is of paramount importance for practice. In this work, a simple coordination confined thermal pyrolysis method has been developed for the synthesis of Ni single-atom catalyst loaded onto nitrogen-doped commercial carbon. The catalyst is in the form of NiN3-C, which exhibits a high-performance electrocatalytic reduction of CO2 toward producing syngas with Faraday efficiencies of 62.28% of CO and 36.7% of H2. The Gibbs free energies of COOH* and H* on the NiN3-C structure were estimated by using density functional theory (DFT). The formation of COOH* intermediate is the speed-limiting step in the process, with ΔG COOH* being 0.7 eV, while H* is the speed-limiting step in the hydrogen evolution, respectively. This work provides a feasible method for the achievement of nonprecious catalysts for the resourceful use of CO2.

Surface-strain-enhanced oxygen dissociation on gold catalysts.

The excellent low-temperature oxidation performance and stability of nanogold catalysts have attracted significant interest. However, the main active source of the low-temperature oxidation of gold remains to be determined. In situ electron microscopy and mass spectrometry results show that nitrogen is oxidized, and the catalyst surface undergoes reconstruction during the process. Strain analysis of the catalyst surface and first-principles calculations show that the tensile strain of the catalyst surface affects the oxidation performance of gold catalysts by enhancing the adsorption ability and dissociation of O2. The newly formed active oxygen atoms on the gold surface act as active sites in the nitrogen oxidation reaction, significantly enhancing the oxidation ability of gold catalysts. This study provides evidence for the dissociation mechanism of oxygen on the gold surface and new design concepts for improving the oxidation activity of gold catalysts and nitrogen activation.

Stimuli-responsive cancer nanomedicines inhibit glycolysis and impair redox homeostasis.

The solid tumors are characterized with oxidative stress and metabolic reprogramming, which has been independently used for targeted tumor monotherapy. However, the potential of targeting metabolism-redox circuit in tumor therapy has long been neglected. Herein, we report a hybrid nanocarrier for concurrent targeting of glycolysis and redox balance in the current work. The nanocarriers are made of pH- and ATP-responsive zeolitic imidazolate framework (ZIF-8) as the porous core that was further coated with poloxamer 407 as the steric stabilizer. Two active cargos, glucose oxidase (GOx) and 3-bromopyruvate (3-BrPA) were co-loaded in the core of nanocarrier. GOx is well-known for its ability of producing hydrogen peroxide at the expense of glucose and oxygen. 3-BrPA can reduce oxygen and glucose consumption through glycolysis, which sensitized cancer cells to GOx-induced apoptosis. At the cellular level, the hybrid nanocarrier significantly impaired the redox balance in the liver hepatocellular carcinoma cell line (HepG2), as evidenced by the depletion of glutathione and boost of reactive oxygen species. The potency of hybrid nanocarrier in terms of suppressing HepG2 cell energy metabolism was proven by the exhaustion of ATP. As a consequence, cell viability was greatly reduced. The in vivo efficacy of hybrid nanocarriers was demonstrated in HepG2 tumor-bearing mice. The current work presents an approach of targeting metabolism-redox circuit for tumor treatment, which may enrich the available anti-tumor strategies. STATEMENT OF SIGNIFICANCE: Metabolic alterations and elevated reactive oxygen species (ROS) are two characteristics of cancer. The metabolic patterns of cancer cells are elaborately reprogrammed to enable the rapid propagation of cancer cells. However, the potential of targeting the metabolism-redox circuit in anti-tumor therapy has long been neglected. As a proof-of-concept, we report an engineered stimuli-responsive nanomedicine that can eradicate cancer cells via cooperative glycolysis inhibition and redox impairment. The current work presents an approach of targeting the metabolism-redox circuit for tumor treatment, which may enrich the available anti-tumor strategies.

Enhanced Adsorption of Aromatic Volatile Organic Compounds on a Perchloro Covalent Triazine Framework through Multiple Intermolecular Interactions.

Volatile organic compounds (VOCs) may have short- and long-term adverse health effects. Especially, aromatic VOCs including benzene, toluene, ethylbenzene, and xylene (BTEX) are important indoor air pollutants. Developing highly efficient porous adsorbents with broad applicability remains a major challenge. In this study, a perchlorinated covalent-triazine framework (ClCTF-1-400) is prepared for adsorbing BTEX. ClCTF-1-400 is confirmed as a partially oxidized/chlorinated microporous covalent triazine framework through a variety of characterization. It is found that ClCTF-1-400 is reversible VOCs absorbent with very high absorption capacities, which can adsorb benzene (693 mg g-1 ), toluene (621 mg g-1 ), ethylbenzene (603 mg g-1 ), o-xylene (500 mg g-1 ), m-xylene (538 mg g-1 ), and p-xylene (592 mg g-1 ) at 25 °C and their saturated vapor pressure (≈ 1 kPa). ClCTF-1-400 is of higher adsorption capacities for all selected VOCs than activated carbon and other reported adsorbents. The adsorption mechanism is also inferred through theoretical calculation and in-site Fourier Transform Infrared (FTIR) spectroscopy. The observed excellent BTEX adsorption performance is attributed to the multiple weak interactions between the ClCTF-1-400 frameworks and aromatic molecules through multiple weak interactions (CH… π and CCl… π). The breakthrough experiment demonstrates ClCTF-1-400 has the potential for real VOCs pollutant removal in air.

In Situ TEM Observation of the Atomic Transport Process during the Coalescence of Au Nanoparticles.

In practical applications, the coalescence of metal nanoparticles (NPs) is a major factor affecting their physical chemistry properties. Currently, due to a lack of understanding of the atomic-level mechanisms during the nucleation and growth stages of coalescence, the correlation between the different dynamic factors in the different stages of NP coalescence is unclear. In this study, we used advanced in situ characterization techniques to observe the formation of atomic material transport channels (Au chains) during the initiation of coalescence nucleation. We focused on the movement and migration states of Au atoms and discovered an atomic ordered arrangement growth mechanism that occurs after the completion of nucleation. Simultaneously, we used density functional theory to reveal the formation principle of Au chains. These findings improve our understanding of the atomic-scale coalescence process, which can provide a new perspective for further research on coalescence atomic dynamics.

Methane plasma-mediated phase engineering of Ni nanosheets for alkaline hydrogen evolution.

Engineering of the crystal structures of metallic nanomaterials is an alternative avenue to control the size and shape of nanocatalysts. However, the phase-controlled synthesis of Ni nanocatalysts is challenging because of its low reduction potential under mild conditions. We developed a room-temperature CH4 plasma conversion of Ni(OH)2 nanosheets to hexagonal close packed (hcp) Ni while maintaining a pristine shape. Increasing the temperature resulted in the formation of face-centered cubic (fcc) Ni. The hcp Ni nanosheets exhibited an overpotential of 85 mV at 10 mA cm-2 for an electrocatalytic hydrogen evolution reaction (HER) in alkaline solution, which was superior to that of the fcc (122 mV) counterpart. Density-functional-theory calculations demonstrated that during the HER, the d-band center of hcp Ni was closer to the Fermi level, which aided the formation of H2 molecules. This work could facilitate the synthesis of other metastable metals and metallic alloys with high efficiency for various applications.

Complete surface reconstruction of nanoporous gold during CH4 pyrolysis.

The catalytic activity and selectivity of metallic nanocatalysts can be controlled using physical and chemical methods to tune the exposed crystal facets. Nanoporous metals (NPMs) have unique bicontinuous structures, large specific surface areas, and high catalytic activities, and are widely used in the field of heterogeneous catalysis. However, owing to the complex surface topography of NPMs, it is difficult to regulate their exposed crystal facets over a large area. In this study, nanoporous gold (NPG) is successfully prepared with a complete regular surface that exposes the Au {111} and {100} facets through a methane pyrolysis reaction. The results of high-spatial and -temporal resolution in situ experiments and theoretical calculations indicate that C species significantly weaken the interaction between surface Au atoms with low coordination numbers and their surrounding atoms, which results in the migration and recombination of surface atoms. This research fundamentally clarifies the reconstruction mechanism of porous materials during methane pyrolysis and provides a theoretical basis for the targeted regulation of exposed NPM surfaces.

Accelerated Hydrogen "Spill-Over" Enhances Anode Performance of Tensile Strained Pd-Based Fuel Cell Electrocatalysts.

Development of efficient electrocatalysts usually relies on half-cell electrochemical tests for rapid material screening, which however are not always consistent with the associated full cell evaluation. This study designs a tensile-strained Pd anode and reveals that with a lower apparent activity toward the hydrogen oxidation reaction as compared to the unstrained one, it exhibits a surprisingly high activity in proton exchange membrane fuel cells (PEMFCs). With an ultralow Pd loading of 4.5 µg cm-2 , the tensile-strained Pd achieves a maximum power density of 1048 mW cm-2 , indicating a 30-fold improvement in power efficiency than that of commercial Pd/C, nearly four times of that of the unstrained one. This discrepancy can be ascribed to the hydrogen-rich surface in the H2 atmosphere of PEMFCs owing to the accelerated hydrogen "spill-over" in the tensile-strained Pd with a standout hydrogen storage property.

Accelerating Anode Reaction with Electro-oxidation of Alcohols over Ru Nanoparticles to Reduce the Potential for Water Splitting.

Generating hydrogen by water electrolysis is a promising and sustainable approach to the production of a green energy carrier, but the sluggish kinetics of the oxygen evolution reaction (OER) at anode leads to a high working potential. Replacing OER with electro-oxidation of organics driven at a low potential offers an effective way to accelerate the sluggish anode reaction, and thus increase hydrogen evolution in water-splitting. Herein, we have prepared a Ru nanoparticles on N-doped carbon nanotubes (Ru-NPs@NCNTs) to implement electro-oxidation of benzyl alcohol toward reducing the anodic potential in watersplitting. The potential of the anode reaction is remarkably decreased from 1.76 to 1.19 V vs RHE at a current density of 10 mA cm-2 with the assistance of a Ru-NPs catalyst. Furthermore, 100% selectivity and 95% yield of valuable benzaldehyde were achieved simultaneously. The Ru-NPs also exhibits good durability and wide applicability to other alcohols. The high performance of Ru-NPs is mainly attributed to the unique horizontal adsorption configuration of benzyl alcohol with surface atoms of the catalyst, shortening the distance between the •OH group and Ru atoms, and increasing the activation rate of the •OH group. This work presents a feasible strategy to boost water-splitting performance and concurrently produce value-added organics under mild conditions.

The Z-scheme transfer of photogenerated electrons for CO2 photocatalytic reduction over g-ZnO/2H-MoS2 heterostructure.

Effective separation of the photogenerated electrons and holes is critical to improve photocatalytic efficiency. To achieve this, we design a Z-scheme g-ZnO/2H-MoS2 heterostructure to spatially separate the photogenerated carriers promoting the reduction of CO2 on the surface of the heterostructure, through density functional theory (DFT) calculations. The g-ZnO/2H-MoS2 heterostructure has a narrow band gap, which is beneficial to speed up the transport of carriers. Simultaneously, the designed heterostructure forms a built-in electric field between the layers to cause band bending, which is very conducive to separate the photogenerated electrons on g-ZnO and the photogenerated holes on 2H-MoS2, and suppress their recombination effectively. Furthermore, the reaction mechanism of photocatalytic reduction of CO2 to CH4 on g-ZnO/2H-MoS2 is studied. The calculation results show that the Z-scheme charge transfer mechanism reduces the barrier of the potential energy control step compared with pristine g-ZnO and 2H-MOS2. Our calculations lay a theoretical foundation for designing and developing high performance photocatalysts for the photocatalytic reduction of CO2.

Coalescence and shape oscillation of Au nanoparticles in CO2 hydrogenation to methanol.

Recently, there has been renewed interest in Au nanoparticle (Au NP) catalysts owing to their high selectivity for CO2 hydrogenation to methanol. However, there is still limited knowledge on the main factors of the catalytic activity and product selectivity of Au NPs. To address this issue, we utilized in situ transmission electron microscopy to observe the evolution of Au NP catalysts during CO2 hydrogenation to methanol at 260 °C under ambient pressure. During the reaction, Au NPs sized ≤5 nm coalesced rapidly, forming stable Au NPs sized 5-10 nm with oscillating shapes. The first-principles calculations demonstrated that the adsorption of the reactant gas CO2 is the main factor in inducing the coalescence of Au NPs, and CO and/or H2O adsorption generated by the reaction caused the oscillation of the Au NP shape. Furthermore, the adsorption of various gas molecules resulted in continuous changes in the structure of the catalyst active center. In this study, the in situ observation of the dynamic evolution of the Au NP morphology is important in understanding the structural transformation of Au NP catalysts at the nanometer scale and determining the active site motifs under the reaction conditions. Moreover, this would allow us to further understand the size effect and the dynamic evolution behavior of the active center of Au NP catalysts, thereby providing a new idea for the development and application of new catalysts and strong theoretical support for heterogeneous catalytic reaction mechanisms.

Facet-Dependent Catalytic Performance of Au Nanocrystals for Electrochemical Nitrogen Reduction.

Nanostructured metal catalysts have attracted great interest due to their extraordinary performance for electrocatalysis including electrochemical nitrogen reduction (ENRR). However, their working mechanisms for ENRR are still not fully understood. Herein, seven monofaceted polyhedral Au nanocrystals were synthesized and systemically compared to elucidate the relation between Au crystal facets and NRR performance. It is found that polyhedra with high-index facets catalytically outperform those with low-index facets. Specifically, Au nanostars enclosed with (321) facets show a high NH3 production rate of 2.6 µg h-1 cm-2 (20 µg h-1 mg-2) and faradaic efficiency of 10.2% at -0.2 V, which are 3.1- and 5.1-folds larger than those of nanocubes enclosed with (100) facets. As revealed by theoretical investigation, a larger energy barrier for reduction of H+ to H* (ΔGH*) hinders occurrence of HER on the Au(321) surface, thus ensuring better NRR selectivity. Meanwhile, a lower energy barrier for formation of N2H2* on the catalyst surface and a larger energy barrier for decomposing the formed N2H2* back into N2 and 2H* jointly favor a higher NH3 production rate. This study provides mechanistic insights into ENRR and rational design of metal nanocrystals for electrocatalysis.

DFT study on Ag loaded 2H-MoS2 for understanding the mechanism of improved photocatalytic reduction of CO2.

Transition metal modified molybdenum disulfide to improve the performance of photocatalytic reduction of carbon dioxide has been receiving much attention. Herein, a novel high-efficiency photocatalytic composite Ag/2H-MoS2 has been constructed and simulated using density functional theory (DFT) for unveiling the mechanism of improved photocatalytic reduction of CO2 in our experimental research. Our calculations about the band structure and electronic and optical properties indicate that the loading of Ag atoms enhances the photocatalytic performance of 2H-MoS2 nanosheets by transferring the photogenerated electrons from the valence band of 2H-MoS2 to the loaded Ag atoms. Furthermore, 20 wt% Ag loaded 2H-MoS2 is the most suitable for the thermodynamic requirement of reducing CO2 to CH4 among the catalysts with different Ag loadings, and the formation of *CHO in 20 wt% Ag/2H-MoS2 is the potential-determining step, whose Gibbs free energy reduces from 2.830 eV of 2H-MoS2 to 0.925 eV. Meanwhile the thermochemical results predict the best path for reducing CO2 on such a photocatalyst as CO2 → *COOH → *CO → *CHO → *CH2O → *OCH3 → *CH3OH → CH4. The photocatalytic performance of pristine 2H-MoS2 in CO2 reduction is therefore significantly improved by loading silver. This research provides a theoretical reference for transition metal modified 2H-MoS2 nanosheets.

Dynamic co-catalysis of Au single atoms and nanoporous Au for methane pyrolysis.

Nanocatalysts and single-atom catalysts are both vital for heterogeneous catalysis. They are recognized as two different categories of catalysts. Nevertheless, recent theoretical works have indicated that Au nanoparticles/clusters release Au single atoms in CO oxidation, and they co-catalyze the oxidation. However, to date, neither experimental evidence for the co-catalysis nor direct observations on any heterogeneous catalysis process of single-atom catalysts are reported. Here, the dynamic process of nanoporous Au to catalyze methane pyrolysis is monitored by in situ transmission electron microscopy with high spatial-temporal resolutions. It demonstrates that nanoporous Au surfaces partially disintegrate, releasing Au single atoms. As demonstrated by DFT calculation, the single atoms could co-catalyze the reaction with nanoporous Au. Moreover, the single atoms dynamically aggregate into nanoparticles, which re-disintegrate back to single atoms. This work manifests that under certain conditions, the heterogeneous catalysis processes of nanocatalysts and single-atom catalysts are not independent, where their dynamic co-catalysis exists.

Effect of Temperature on the Composition of a Synthetic Hydrocarbon Aviation Lubricating Oil.

Synthetic hydrocarbon aviation lubricating oils (SHALOs) gradually degrade over time when subjected to high temperatures, resulting in their composition and properties varying over the operation lifetime. Therefore, understanding the SHALO degradation properties by elucidating the mechanism on a molecular level, as a function of high temperature, is of interest. A SHALO was subjected to thermal treatment (TT) at 180, 200, 230, 250, 270, or 300 °C for 2 h. The chemical compositions of six TT samples and one fresh oil were analyzed by fourier transform infrared F spectroscopy, advanced polymer chromatography, and gas chromatography/mass spectrometry. Furthermore, the physicochemical properties, such as kinematic viscosity, pour point, and acid number, of seven samples were determined. The oil samples were grouped by cluster analysis (CA) using a statistical method. The SHALO was identified to comprise 20 functional groups, including comb-like alkanes, long-chain diesters, amines, phenols, and other compounds. TT at <230 °C caused partial cracking of the SHALO base oils, with a concomitant change in the antioxidant content and type, and the polycondensation reactions were dominant. The observed antioxidant changes were not obvious from TT at >230 °C. A large number of small-molecule compounds were detected, including n-alkanes and olefins. TT at 250 °C was shown to be an important threshold for the kinematic viscosity, pour point, and acid number of the samples. Below 250 °C, the sample properties were relatively stable; but at elevated TT temperatures (>250 °C), the properties were observed to dramatically degrade. As the sample color was highly sensitive to temperature, the TT temperature induced rapid and significant color changes. The CA analysis results for the oil compounds at the molecular level were in good agreement with observed changes in the physicochemical properties at the macro level.

Anodically Triggered Aldehyde Cation Autocatalysis for Alkylation of Heteroarenes.

Alkylation of heteroarenes by using aldehydes is a direct approach to increase molecular complexity, which however often involves the use of stochiometric oxidant, strong acid, and high temperature. This study concerns an energy-efficient electrochemical alkylation of heteroarenes by using aldehydes under mild conditions without mediators. Interestingly, the graphite anode can trigger aldehyde cationic species, which act as the effective autocatalysts to react with a range of heteroarenes to produce the corresponding products with excellent regioselectivity and in high yields. Compared to the traditional electro-synthesis approaches, this electro-triggered reaction provides an electricity-saving and eco-friendly route to high-value chemicals.

The direct trifluoromethylsilylation and cyanosilylation of aldehydes via an electrochemically induced intramolecular pathway.

The initiator-free electrochemical trifluoromethylsilylation and cyanosilylation of aldehydes were developed in an undivided cell. A DFT study reveals that the direct cathodic activation of trimethylsilyl reagents significantly released the congestion around the 'Si' atom, allowing the Si-O bond affinity to form concerted anion intermediates with aldehydes. Thus, intramolecular -CF3 and -CN migration make the reactions much easier to carry out without initiators.

Anodic SnO2 porous nanostructures with rich grain boundaries for efficient CO2 electroreduction to formate.

Formic acid (HCOOH), the acidic form of formate, is an important hydrogen carrier which can be directly used in fuel cells. Development of earth-abundant element-based catalysts to convert carbon dioxide (CO2) into HCOOH or formate with high selectivity and high efficiency has been a vigorous research activity in recent years but remains an unsolved challenge. In this contribution, using one-step anodization, we prepare nanotubular SnO2 porous nanostructures with high surface area (90.1 m2 g-1), large porosity (0.74 cm3 g-1), and rich grain boundaries for electrochemical CO2 reduction (CO2RR). They exhibit stable 95% faradaic efficiency (FE) towards CO2RR and 73% FE for formate at -0.8 VRHE. The notable performance of such SnO2 nanostructures can be attributed to their unique structural and chemical properties, which provide active sites for CO2 adsorption and conversion, and easy access for CO2 to the active sites. The insights gained from the structure/property relationships might be beneficial for designing superior electrocatalysts for CO2 electroreduction into formate.

Self-Supported Nanoporous Au3 Cu Electrode with Enriched Gold on Surface for Efficient Electrochemical Reduction of CO2.

The key to the electrochemical conversion of CO2 lies in the development of efficient electrocatalysts with ease of operation, good conductivity, and rich active sites that fulfil the desired reaction direction and selectivity. Herein, an oxidative etching of Au20 Cu80 alloy is used for the synthesis of a nanoporous Au3 Cu alloy, representing a facile strategy for tuning the surface electronic properties and altering the adsorption behavior of the intermediates. HRTEM, XPS, and EXAFS results reveal that the curved surface of the synthesized nanoporous Au3 Cu is rich in gold with unsaturated coordination conditions. It can be used directly as a self-supported electrode for CO2 reduction, and exhibits high Faradaic efficiency (FE) of 98.12 % toward CO at a potential of -0.7 V versus the reversible hydrogen electrode (RHE). The FE is 1.47 times that over the as-made single nanoporous Au. Density functional theory reveals that *CO has a relatively long distance on the surface of nanoporous Au3 Cu, making desorption of CO easier and avoiding CO poisoning. The Hirshfeld charge distribution shows that the Au atoms have a negative charge and the Cu atoms exhibit a positive charge, which separately bond to the C atom and O atom in the *COOH intermediate through a bidentate mode. This affords the lowest *COOH adsorption free energy and low desorption energy for CO molecules.