Unraveling the Unique Strong Metal-Support Interaction in Titanium Dioxide Supported Platinum Clusters for the Hydrogen Evolution Reaction.

Strong metal-support interaction (SMSI) is crucial to modulating the nature of metal species, yet the SMSI behaviors of sub-nanometer metal clusters remain unknown due to the difficulties in constructing SMSI at cluster scale. Herein, we achieve the successful construction of the SMSI between Pt clusters and amorphous TiO2 nanosheets by vacuum annealing, which required a relatively low temperature that avoids the aggregation of small clusters. In situ scanning transmission electron microscopy observation is employed to explore the SMSI behaviors, and the results reveal the dynamic rearrangement of Pt atoms upon annealing for the first time. The originally disordered Pt atoms become ordered as the crystallizing of the amorphous TiO2 support, forming an epitaxial interface between Pt and TiO2. Such SMSI state can remain stable in oxidation environment even at 400 °C. Further investigations prove that the electron transfer from TiO2 to Pt occupies the Pt 5d orbitals, which is responsible for the disappeared CO adsorption ability of Pt/TiO2 after forming SMSI. This work not only opens a new avenue for constructing SMSI at cluster scale but also provides in-depth understanding on the unique SMSI behavior, which would stimulate the development of supported metal clusters for catalysis applications.

Cobalt Single Atoms Enabling Efficient Methanol Oxidation Reaction on Platinum Anchored on Nitrogen-Doped Carbon.

Developing efficient platinum (Pt)-based electrocatalysts with high tolerance to CO poisoning for the methanol oxidation reaction is critical for the development of direct methanol fuel cells. In this work, cobalt single atoms are introduced to enhance the electrocatalytic performance of N-doped carbon supported Pt (N-C/Pt) for the methanol oxidation reaction. The cobalt single atoms are believed to play a critical role in accelerating the prompt oxidation of CO to CO2 and minimizing the CO blocking of the adjacent Pt active sites. Benefitting from the synergistic effects among the Co single atoms, the Pt nanoparticles, and the N-doped carbon support, the Co-modified N-C/Pt (Co-N-C/Pt) electrocatalyst simultaneously delivers impressive electrocatalytic activity and durability with lower onset potential and superb CO poisoning resistance as compared to the N-C/Pt and the commercial Pt/C electrocatalysts.

Cu3(BTC)2 nanoflakes synthesized in an ionic liquid/water binary solvent and their catalytic properties.

Low-dimensional metal-organic frameworks (MOFs) exhibit enhanced properties compared with three-dimensional (3D) geometry MOFs in many fields. In this work, we demonstrate the synthesis of Cu3(BTC)2 (BTC = benzene-1,3,5-tricarboxylate) nanoflakes in a binary solvent of ionic liquid (IL) and water. Such a MOF architecture has a high surface area and abundant unsaturated coordination metal sites, making them attractive for adsorption and catalysis. For example, in catalyzing the oxidation reactions of a series of alcohols, the Cu3(BTC)2 nanoflakes exhibit a high performance that is superior to Cu3(BTC)2 microparticles synthesized in a conventional solvent. Experimental and theoretical studies reveal that the IL accelerates the crystallization of Cu3(BTC)2, while water plays a role in stripping the Cu3(BTC)2 blocks that are formed at an early stage through its attack on the crystal plane of Cu3(BTC)2. Such an in situ crystallization-exfoliation process that uses an IL/water solvent opens a new route for producing low-dimensional MOFs.

Single-Atom Electrocatalysts for Multi-Electron Reduction of CO2.

The multi-electron reduction of CO2 to hydrocarbons or alcohols is highly attractive in a sustainable energy economy, and the rational design of electrocatalysts is vital to achieve these reactions efficiently. Single-atom electrocatalysts are promising candidates due to their well-defined coordination configurations and unique electronic structures, which are critical for delivering high activity and selectivity and may accelerate the explorations of the activity origin at atomic level as well. Although much effort has been devoted to multi-electron reduction of CO2 on single-atom electrocatalysts, there are still no reviews focusing on this emerging field and constructive perspectives are also urgent to be addressed. Herein recent advances in how to design efficient single-atom electrocatalysts for multi-electron reduction of CO2 , with emphasis on strategies in regulating the interactions between active sites and key reaction intermediates, are summarized. Such interactions are crucial in designing active sites for optimizing the multi-electron reduction steps and maximizing the catalytic performance. Different design strategies including regulation of metal centers, single-atom alloys, non-metal single-atom catalysts, and tandem catalysts, are discussed accordingly. Finally, current challenges and future opportunities for deep electroreduction of CO2 are proposed.

Highly Electrocatalytic Ethylene Production from CO2 on Nanodefective Cu Nanosheets.

The electrochemical synthesis of chemicals from carbon dioxide, which is an easily available and renewable carbon resource, is of great importance. However, to achieve high product selectivity for desirable C2 products like ethylene is a big challenge. Here we design Cu nanosheets with nanoscaled defects (2-14 nm) for the electrochemical production of ethylene from carbon dioxide. A high ethylene Faradaic efficiency of 83.2% is achieved. It is proved that the nanoscaled defects can enrich the reaction intermediates and hydroxyl ions on the electrocatalyst, thus promoting C-C coupling for ethylene formation.

Highly Efficient Electroreduction of CO2 to Methanol on Palladium-Copper Bimetallic Aerogels.

Electrochemical reduction of CO2 to CH3 OH is of great interest. Aerogels have fine inorganic superstructure with high porosity and are known to be exceptional materials. Now a Pd-Cu bimetallic aerogel electrocatalyst has been developed for conversion of CO2 into CH3 OH. The current density and Faradaic efficiency of CH3 OH can be as high as 31.8 mA cm-2 and 80.0 % over the Pd83 Cu17 aerogel at a very low overpotential (0.24 V). The superior performance of the electrocatalyst results from efficient adsorption and stabilization of the CO2 radical anion, high Pd0 /PdII and CuI +Cu0 /CuII ratios, and sufficient Pd/Cu grain boundaries of aerogel nanochains.

Pickering emulsions stabilized by a metal-organic framework (MOF) and graphene oxide (GO) for producing MOF/GO composites.

Herein we demonstrate the formation of a novel kind of Pickering emulsion that is stabilized by a Zr-based metal-organic framework (Zr-MOF) and graphene oxide (GO). It was found that the Zr-BDC-NO2 and GO solids assembling at the oil/water interface can effectively stabilize the oil droplets that are dispersed in the water phase. Such a Pickering emulsion offers a facile route for fabricating Zr-MOF/GO composite materials. After removing water and oil by freeze drying from Pickering emulsions, the Zr-MOF/GO composites were obtained and their morphologies, structures and interaction properties were characterized by scanning electron microscopy, transmission electron microscopy, X-ray diffraction and Fourier transform infrared spectrometry, respectively. The influences of the concentration of GO and Zr-MOF on the emulsion microstructures and the properties of the MOF/GO composites were studied. Based on experimental results, the mechanisms for the emulsion formation by Zr-MOF and GO and the as-synthesized superstructures of the Zr-MOF/GO composite were proposed. It is expected that this facile and tunable route can be applied to the synthesis of different kinds of MOF-based or GO-based composite materials.

Interfacial assembly and hydrolysis for synthesizing a TiO2/metal-organic framework composite.

Herein we propose an interfacial assembly and hydrolysis route for fabricating TiO2/UiO-67 composites. The UiO-67 assembles at the water-oil interface serving as a stabilizer of the emulsion. TiO2 nanoparticles are loaded on UiO-67 by hydrolysis of the precursor TBT (tetra-n-butyl titanate) at the water-oil interface. By such a strategy, hollow capsules structured by UiO-67 and decorated by ultra-small TiO2 nanoparticles were produced. The newly-constructed composite combines the CO2 adsorption properties of UiO-67 and the photocatalytic activity of TiO2, showing high activity for the photocatalytic reduction of CO2 to formic acid. Such a composite with a novel structure provides a promising route for the preparation of new compound materials.

Micellization of long-chain ionic liquids in deep eutectic solvents.

The aggregation behavior of the ionic liquid (IL) 1-alkyl-3-methylimidazolium chloride with different alkyl chain lengths in a deep eutectic solvent (DES, composed of choline chloride and glycerol in a 1 : 2 mole ratio) was studied for the first time. The critical micellar concentration, micellar size and intermolecular interactions in IL/DES solutions were investigated by different techniques including the fluorescence probe technique, small angle X-ray scattering and Fourier transform infrared spectroscopy. The solvophobic effect dominates the micellization of CnmimCl in DES and the intermolecular hydrogen-bond interaction plays a positive role to promote micelle formation. The micellar solutions were utilized for the synthesis of the water-unstable metal-organic framework Cu3(BTC)2 (BTC = 1,3,5-benzenetricarboxylate) at room temperature. X-Ray diffraction, scanning electron microscopy, transmission electron microscopy and nitrogen adsorption-desorption isotherms confirm the formation of crystalline Cu3(BTC)2 nanocrystals with mesoporous structures. The morphologies and porosity properties of Cu3(BTC)2 nanocrystals can be modulated by varying the concentration of CnmimCl.

Metal-Organic Framework for Emulsifying Carbon Dioxide and Water.

Forming emulsions of carbon dioxide (CO2 ) and water can largely expand the utility of CO2 . Herein we propose for the first time the utilization of a metal-organic framework (MOF) for emulsifying CO2 and water. Owing to the hybrid composition, MOF particles can easily assemble at the CO2 /water interface to create a rigid protective barrier around the dispersed droplet. The MOF-stabilized CO2 and water emulsion has exceptional stability compared to those emulsions stabilized by surfactants or other solids. Moreover, the CO2 and water emulsion stabilized by MOF is "tunable" due to the designable features of MOFs and adjustable character of CO2 . Such a novel kind of emulsion composed of CO2 , water, and MOF provides a facile route for constructing MOF superstructures with many advantages. The macroporous networks and hollow capsules of different kinds of MOFs have been successfully derived from CO2 and water emulsions.

Concentrated Solar Light Photoelectrochemical Water Splitting for Stable and High-Yield Hydrogen Production.

Photoelectrochemical water splitting is a promising technique for converting solar energy into low-cost and eco-friendly H2 fuel. However, the production rate of H2 is limited by the insufficient number of photogenerated charge carriers in the conventional photoelectrodes under 1 sun (100 mW cm-2 ) light. Concentrated solar light irradiation can overcome the issue of low yield, but it leads to a new challenge of stability because the accelerated reaction alters the surface chemical composition of photoelectrodes. Here, it is demonstrated that loading Pt nanoparticles (NPs) on single crystalline GaN nanowires (NWs) grown on n+ -p Si photoelectrode operates efficiently and stably under concentrated solar light. Although a large number of Pt NPs detach during the initial reaction due to H2 gas bubbling, some Pt NPs which have an epitaxial relation with GaN NWs remain stably anchored. In addition, the stability of the photoelectrode further improves by redepositing Pt NPs on the reacted Pt/GaN surface, which results in maintaining onset potential >0.5 V versus reversible hydrogen electrode and photocurrent density >60 mA cm-2 for over 1500 h. The heterointerface between Pt cocatalysts and single crystalline GaN nanostructures shows great potential in designing an efficient and stable photoelectrode for high-yield solar to H2 conversion.

Toward Electrocatalytic Methanol Oxidation Reaction: Longstanding Debates and Emerging Catalysts.

The study of direct methanol fuel cells (DMFCs) has lasted around 70 years, since the first investigation in the early 1950s. Though enormous effort has been devoted in this field, it is still far from commercialization. The methanol oxidation reaction (MOR), as a semi-reaction of DMFCs, is the bottleneck reaction that restricts the overall performance of DMFCs. To date, there has been intense debate on the complex six-electron reaction, but barely any reviews have systematically discussed this topic. To this end, the controversies and progress regarding the electrocatalytic mechanisms, performance evaluations as well as the design science toward MOR electrocatalysts are summarized. This review also provides a comprehensive introduction on the recent development of emerging MOR electrocatalysts with a focus on the innovation of the alloy, core-shell structure, heterostructure, and single-atom catalysts. Finally, perspectives on the future outlook toward study of the mechanisms and design of electrocatalysts are provided.

Fast and Durable Alkaline Hydrogen Oxidation Reaction at the Electron-Deficient Ruthenium-Ruthenium Oxide Interface.

The slow hydrogen oxidation reaction (HOR) kinetics under alkaline conditions remain a critical challenge for the practical application of alkaline exchange membrane fuel cells. Herein, Ru/RuO2 in-plane heterostructures are designed with abundant active Ru-RuO2 interface domains as efficient electrocatalysts for the HOR in alkaline media. The experimental and theoretical results demonstrate that interfacial Ru and RuO2 domains at Ru-RuO2 interfaces are the optimal H and OH adsorption sites, respectively, endowing the well-defined Ru(100)/RuO2 (200) interface as the preferential region for fast alkaline hydrogen electrocatalysis. More importantly, the metallic Ru domains become electron deficient due to the strong interaction with RuO2 domains and show substantially improved inoxidizability, which is vital to maintain durable HOR electrocatalytic activity. The optimal Ru/RuO2 heterostructured electrocatalyst exhibits impressive alkaline HOR activity with an exchange current density of 8.86 mA cm-2 and decent durability. The exceptional electrocatalytic performance of Ru/RuO2 in-plane heterostructure can be attributed to the robust and multifunctional Ru-RuO2 interfaces endowed by the unique metal-metal oxide domains.

Atomically dispersed chromium coordinated with hydroxyl clusters enabling efficient hydrogen oxidation on ruthenium.

Overcoming the sluggish kinetics of alkaline hydrogen oxidation reaction (HOR) is challenging but is of critical importance for practical anion exchange membrane fuel cells. Herein, abundant and efficient interfacial active sites are created on ruthenium (Ru) nanoparticles by anchoring atomically isolated chromium coordinated with hydroxyl clusters (Cr1(OH)x) for accelerated alkaline HOR. This catalyst system delivers 50-fold enhanced HOR activity with excellent durability and CO anti-poisoning ability via switching the active sites from Ru surface to Cr1(OH)x-Ru interface. Fundamentally different from the conventional mechanism merely focusing on surface metal sites, the isolated Cr1(OH)x could provide unique oxygen species for accelerating hydrogen or CO spillover from Ru to Cr1(OH)x. Furthermore, the original oxygen species from Cr1(OH)x are confirmed to participate in hydrogen oxidation and H2O formation. The incorporation of such atomically isolated metal hydroxide clusters in heterostructured catalysts opens up new opportunities for rationally designing advanced electrocatalysts for HOR and other complex electrochemical reactions. This work also highlights the importance of size effect of co-catalysts, which should also be paid substantial attention to in the catalysis field.

SnS2/polypyrrole for high-efficiency photocatalytic oxidation of benzylamine.

Here, SnS2/polypyrrole (PPy) was synthesized, which shows high catalytic activity for the photocatalytic oxidation of benzylamine under mild conditions (at 25 °C, in air and without adding an additional sacrificial reagent, redox mediator and photosensitizer).

Lattice-Confined Ir Clusters on Pd Nanosheets with Charge Redistribution for the Hydrogen Oxidation Reaction under Alkaline Conditions.

Electrocatalysts with high activity and long-term stability for the hydrogen oxidation reaction (HOR) under alkaline conditions is still a major challenge for anion exchange membrane fuel cells (AEMFCs). Herein, a heterostructured Ir@Pd electrocatalyst with ultrasmall Ir nanoclusters (NCs) epitaxially confined on Pd nanosheets (NSs) for catalyzing the sluggish alkaline HOR is reported. Apparent charge redistribution occurs across the heterointerface, and both experimental and theoretical results suggest that the electrons transfer from Pd to Ir, which consequently greatly weakens the hydrogen binding on Pd. More interestingly, the interfacial epitaxy results in the formation of Ir/IrO2 Janus nanostructures, where the partially oxidized Ir species away from the interface further optimize the hydroxyl adsorption behavior. The unique Ir@Pd heterostructure eventually shows an optimal balance between hydrogen and hydroxyl adsorption, and hence exhibits impressive HOR activity with an exchange current density of up to 7.18 mA cm-2 in 0.1 m KOH solution. In addition, the Ir@Pd electrocatalyst exhibits negligible activity degradation owing to the confinement effect of the unique epitaxial interface.

Improved photocatalytic performance of covalent organic frameworks by nanostructure construction.

Here, we demonstrate for the first time the construction of covalent organic framework (COF) capsules with nanostructured surfaces, which combine advantages of highly accessible surface area, excellent light absorbance, and efficient separation of photogenerated electron-hole pairs. The COF capsules exhibit high activity and selectivity for photocatalytic oxidation under mild conditions.

Hierarchically macro-meso-microporous metal-organic framework for photocatalytic oxidation.

The macro-meso-microporous and defective metal-organic framework constructed by transition metal Zn and 2,2'-bipyridine-5,5'-carboxylate was synthesized in CO2-expanded solvent. It shows high photocatalytic activity and selectivity for the oxidation of amines to imines under mild conditions, i.e., air as an oxidant, room temperature, and involving no photosensitizer or cocatalyst.

CO2 controls the oriented growth of metal-organic framework with highly accessible active sites.

The production of 2D metal-organic frameworks (MOFs) with highly exposed active surfaces is of great importance for catalysis. Here we demonstrate the formation of MOF nanosheets by utilizing CO2 as a capping agent to control the oriented growth of MOF. This strategy has many advantages over the conventional methods. For example, it is template-free and proceeds at mild temperature (35 °C), CO2 can be easily removed by depressurization, and the properties of the MOF nanosheets can be well adjusted by changing CO2 pressure. Such a simple, rapid, efficient and adjustable route produces MOF nanosheets with ultrathin thickness (∼10 nm), small lateral size (∼100 nm) and abundant unsaturated coordination metal sites on surfaces. Owing to these unique features, the as-synthesized MOF nanosheets exhibit superior activity for catalyzing the oxidation reactions of alcohols.

Improved photocatalytic performance of metal-organic frameworks for CO2 conversion by ligand modification.

Here we demonstrate that the utilization of 2,4,6-tris(4-pyridyl)pyridine (tpy) for metal-organic framework modification can greatly improve the photocatalytic performance for CO2 reduction. The electron-donating nature of tpy enables the charge transfer effect, which induces strong CO2 binding affinity, facilitates *COOH formation and promotes CO2-to-CO conversion.