Imaging Beam-Sensitive Materials by Electron Microscopy

Electron microscopy allows the extraction of multidimensional spatiotemporally correlated structural information of diverse materials down to atomic resolution, which is essential for figuring out their structure-property relationships. Unfortunately, the high-energy electrons that carry this important information can cause damage by modulating the structures of the materials. This has become a significant problem concerning the recent boost in materials science applications of a wide range of beam-sensitive materials, including metal-organic frameworks, covalent-organic frameworks, organic-inorganic hybrid materials, 2D materials, and zeolites. To this end, developing electron microscopy techniques that minimize the electron beam damage for the extraction of intrinsic structural information turns out to be a compelling but challenging need. This article provides a comprehensive review on the revolutionary strategies toward the electron microscopic imaging of beam-sensitive materials and associated materials science discoveries, based on the principles of electron-matter interaction and mechanisms of electron beam damage. Finally, perspectives and future trends in this field are put forward.

Synergism of Iron and Platinum Species for Low-Temperature CO Oxidation: From Two-Dimensional Surface to Nanoparticle and Single-Atom Catalysts

CO oxidation is one of the most studied reactions in heterogeneous catalysis. It is present in air cleaning and automotive emission control. It also participates in the removal of CO from streams of hydrogen used in fuel cells. Because of the competitive adsorption of CO and O₂ over active sites, the use of Pt-based catalysts for low-temperature CO oxidation remains a challenge. Recently, great progress has been made with catalysts containing Pt-Fe species because of the contribution of Fe species to O₂ activation. The structure-activity relationship and reaction mechanisms have been investigated with various Pt-Fe catalysts. In this Perspective, we give a summary of the recent advances of low-temperature CO oxidation over Pt-Fe catalysts with a focus on the synergistic effect of Pt and Fe species in the CO and O₂ activation of catalytic reactions. Future prospects for the preparation of highly effective Pt-Fe catalysts are also proposed.

Facile synthesis of covalent organic framework derived Fe-COFs composites as a peroxidase-mimicking artificial enzyme

An artificial catalyst (Fe-COFs) with peroxidase-like activity was successfully synthesized at room temperature and applied to catalyze the reaction between TMB and H₂O₂. This catalytic system can be used not only to detect residual hydrogen peroxide (H₂O₂) in milk efficiently, but also to degrade rhodamine B (RhB) in waste water.

High-throughput, combinatorial synthesis of multimetallic nanoclusters

Multielement nanomaterials hold great promise for various applications due to their widely tunable surface chemistry, yet it remains challenging to efficiently study this multidimensional space. Conventional approaches are typically slow and depend on serendipity, while a robust and general synthesis is still lacking among increasingly complex compositions. We report a high-throughput technique for combinatorial compositional design (formulation in solution phases) and rapid synthesis (within seconds) of ultrafine multimetallic nanoclusters with a homogeneous alloy structure. We synthesized and screened the PtPdRhRuIrFeCoNi compositional space using scanning droplet cell electrochemistry, with two promising catalysts quickly identified and further verified in a rotating disk setup. The reported high-throughput approach establishes a facile and reliable pipeline to significantly accelerate material discovery in multimetallic nanomaterials.

Multimetallic nanoclusters (MMNCs) offer unique and tailorable surface chemistries that hold great potential for numerous catalytic applications. The efficient exploration of this vast chemical space necessitates an accelerated discovery pipeline that supersedes traditional “trial-and-error” experimentation while guaranteeing uniform microstructures despite compositional complexity. Herein, we report the high-throughput synthesis of an extensive series of ultrafine and homogeneous alloy MMNCs, achieved by 1) a flexible compositional design by formulation in the precursor solution phase and 2) the ultrafast synthesis of alloy MMNCs using thermal shock heating (i.e., ∼1,650 K, ∼500 ms). This approach is remarkably facile and easily accessible compared to conventional vapor-phase deposition, and the particle size and structural uniformity enable comparative studies across compositionally different MMNCs. Rapid electrochemical screening is demonstrated by using a scanning droplet cell, enabling us to discover two promising electrocatalysts, which we subsequently validated using a rotating disk setup. This demonstrated high-throughput material discovery pipeline presents a paradigm for facile and accelerated exploration of MMNCs for a broad range of applications.

Achieving Efficient Alkaline Hydrogen Evolution Reaction over a Ni5 P4 Catalyst Incorporating Single-Atomic Ru Sites

Developing efficient electrocatalysts for alkaline water electrolysis is central to substantial progress of alkaline hydrogen production. Herein, a Ni₅ P₄ electrocatalyst incorporating single-atom Ru (Ni₅ P₄ -Ru) is synthesized through the filling of Ru³⁺ species into the metal vacancies of nickel hydroxides and subsequent phosphorization treatment. Electron paramagnetic resonance spectroscopy, X-ray-based measurements, and electron microscopy observations confirm the strong interaction between the nickel-vacancy defect and Ru cation, resulting in more than 3.83 wt% single-atom Ru incorporation in the obtained Ni₅ P₄ -Ru. The Ni₅ P₄ -Ru as an alkaline hydrogen evolution reaction catalyst achieves low onset potential of 17 mV and an overpotential of 54 mV at a current density of 10 mA cm^-2 together with a small Tafel slope of 52.0 mV decade^-1 and long-term stability. Further spectroscopy analyses combined with density functional theory calculations reveal that the doped Ru sites can cause localized structure polarization, which brings the low energy barrier for water dissociation on Ru site and the optimized hydrogen adsorption free energy on the interstitial site, well rationalizing the experimental reactivity.

Dynamic Atom Clusters on AuCu Nanoparticle Surface during CO Oxidation

Supported alloy nanoparticles are prevailing alternative low-cost catalysts for both heterogeneous and electrochemical catalytic processes. Gas molecules selectively interacting with one metal element induces a dynamic structural change of alloy nanoparticles under reaction conditions and largely controls their catalytic properties. However, such a multicomponent dynamic-interaction-controlled evolution, both structural and chemical, remains far from clear. Herein, by using state-of-the-art environmental TEM, we directly visualize, in situ at the atomic scale, the evolution of a AuCu alloy nanoparticle supported on CeO₂ during CO oxidation. We find that gas molecules can "free" metal atoms on the (010) surface and form highly mobile atom clusters. Remarkably, we discover that CO exposure induces Au segregation and activation on the nanoparticle surface, while O₂ exposure leads to the segregation and oxidation of Cu on the particle surface. The as-formed Cu₂O/AuCu interface may facilitate CO-O interaction corroborated by DFT calculations. These findings provide insights into the atomistic mechanisms on alloy nanoparticles during catalytic CO oxidation reaction and to a broad scope of rational design of alloy nanoparticle catalysts.

Multimetallic High-Index Faceted Heterostructured Nanoparticles

Multimetallic heterostructured nanoparticles with high-index facets potentially represent an important class of highly efficient catalysts. However, due to their complexity, they are often difficult to synthesize. Herein, a library of heterostructured, multimetallic (Pt, Pd, Rh, and Au) tetrahexahedral nanoparticles was synthesized through alloying/dealloying with Bi in a tube furnace at 900-1000 °C. Electron microscopy and selected area diffraction measurements show that the domains of the heterostructured nanoparticles are epitaxially aligned. Although nanoparticles formed from Au alone exhibit low-index facets, Pt and Au form PtAu heterostructured nanoparticles with high-index facets, including domains that are primarily made of Au. Furthermore, the alloying/dealloying of Bi occurs at different rates and under different conditions within the heterostructured nanoparticles. This influences the types of architectures observed en route to the final high-index state, a phenomenon clearly observable in the case of PdRhAu nanoparticles. Finally, scanning probe block copolymer lithography was used in combination with this synthetic strategy to control nanoparticle composition in the context of PtAu nanoparticles (1:4 to 4:1 ratio range) and size (15 to 45 nm range).

In situ observations of the structural dynamics of platinum-cobalt-hydroxide nanocatalysts under CO oxidation

The structures, compositions and chemical states of metal catalysts are prone to dynamic changes in response to reaction conditions. In this work, a combination of in situ X-ray absorption fine structure spectroscopy and diffuse reflectance infrared Fourier transform spectroscopy has been used to monitor the temperature-dependent structural dynamics in bimetallic Pt-Co(OH)₂ nanocatalysts during CO oxidation. Alloying with electron-donating Co promotes the catalytic activity of metallic Pt for CO oxidation at low temperature. At elevated temperatures under an oxidation atmosphere, O₂ drives the segregation of the Pt-Co alloy into cobalt oxide and platinum metal, with the extent of alloying sharply decreasing from ∼30% at 300 K to 0 at 473 K. Reduction at high temperature could recover the formation of the Pt-Co alloy with the same alloying extent. The observed structural dynamics could be well correlated with the kinetic behavior of the catalysts. This work highlights the importance of tracking the dynamic structural changes of working catalysts for a correct understanding of their catalytic behavior.

Flickering nanometre-scale disorder in a crystal lattice tracked by plasmonic flare light emission

The dynamic restructuring of metal nanoparticle surfaces is known to greatly influence their catalytic, electronic transport, and chemical binding functionalities. Here we show for the first time that non-equilibrium atomic-scale lattice defects can be detected in nanoparticles by purely optical means. These fluctuating states determine interface electronic transport for molecular electronics but because such rearrangements are low energy, measuring their rapid dynamics on single nanostructures by X-rays, electron beams, or tunnelling microscopies, is invasive and damaging. We utilise nano-optics at the sub-5nm scale to reveal rapid (on the millisecond timescale) evolution of defect morphologies on facets of gold nanoparticles on a mirror. Besides dynamic structural information, this highlights fundamental questions about defining bulk plasma frequencies for metals probed at the nanoscale.

Dynamic restructuring of metal nanoparticle surfaces greatly influences their catalytic, electronic transport, and chemical binding functionalities. Here, the authors show that non-equilibrium atomic-scale lattice defects can be detected in nanoparticles by using nano-optics at the sub-5nm scale.

Effect of Hydrotalcites Interlayer Water on Pt-Catalyzed Aqueous-Phase Selective Hydrogenation of Cinnamaldehyde

The heterogeneous hydrogenation of α,β-unsaturated compounds requires understanding of the structure-activity relationship of metallic catalysts in consideration of solvent-mediated processes. In this work, a CoAl hydrotalcites (CoAl-HTs)-supported Pt nanoparticle catalyst is employed to study the effect of solvent water and HTs interlayer water on the aqueous-phase selective hydrogenation of cinnamaldehyde (CALD). Pt/Co₂Al₁-HTs catalyst displays 5075 h^-1 of specific reaction rate and 89% of C═O hydrogenation selectivity at 80 °C under 20 bar of H₂. Combined results of isotope-labeling experiments and theoretical DFT calculations demonstrate that the water-mediated hydrogen-exchange pathway exists in the reaction with a relatively lower-energy barrier in comparison to the direct H₂-dissociated hydrogenation pathway. The results also reveal that the interlayer water species of HTs support participate in the hydrogen-exchange reaction. Based on the H₂-D₂ exchange results, these HTs interlayer water species can promote H₂ activation and dissociation processes and thus accelerate the CALD hydrogenation reaction even under solvent-free conditions.

Atomic-scale engineering of metal–oxide interfaces for advanced catalysis using atomic layer deposition

Two main routes to optimization of metal–oxide interfaces: reducing metal particle size and oxide overcoating.

Well-Defined Materials for Heterogeneous Catalysis: From Nanoparticles to Isolated Single-Atom Sites

The use of well-defined materials in heterogeneous catalysis will open up numerous new opportunities for the development of advanced catalysts to address the global challenges in energy and the environment. This review surveys the roles of nanoparticles and isolated single atom sites in catalytic reactions. In the second section, the effects of size, shape, and metal-support interactions are discussed for nanostructured catalysts. Case studies are summarized to illustrate the dynamics of structure evolution of well-defined nanoparticles under certain reaction conditions. In the third section, we review the syntheses and catalytic applications of isolated single atomic sites anchored on different types of supports. In the final part, we conclude by highlighting the challenges and opportunities of well-defined materials for catalyst development and gaining a fundamental understanding of their active sites.

Electronic Metal-Support Interaction-Modified Structures and Catalytic Activity of CeOx Overlayers in CeOx /Ag Inverse Catalysts

Electronic metal-support interactions (EMSIs) of oxide-supported metal catalysts strongly modifies the electronic structures of the supported metal nanoparticles. The strong influence of EMSIs on the electronic structures of oxide overlayers on metal nanoparticles employing cerium oxides/Ag inverse catalysts is reported herein. Ce₂ O₃ overlayers were observed to exclusively form on Ag nanocrystals at low cerium loadings and be resistant to oxidation treatments up to 250 °C, whereas CeO₂ overlayers gradually developed as the cerium loading increased. Ag cubes enclosed by {001} facets with a smaller work function exert a stronger EMSI effect on the CeO_x overlayers than Ag cubes enclosed by {111} facets. Only the CeO₂ overlayers with a fully developed bulk CeO₂ electronic structure significantly promote the catalytic activity of Ag nanocrystals in CO oxidation, whereas cerium oxide overlayers with other electronic structures do not. These results successfully extend the concept of EMSIs from oxide-supported metal catalysts to metal-supported oxide catalysts.

Rational Design of Multisite Trielement Ru-Ni-Fe Alloy Nanocatalysts with Efficient and Durable Catalytic Hydrogenation Performances

The co-decomposition of non-noble metals into Ru nanoparticles (NPs) would provide multiple active centers as well as synergistically alter the reaction pathway, enhancing the catalytic hydrogenation performance. Herein, a facile route for synthesizing trielement Ru-Ni-Fe alloy NPs was proposed. The catalytic hydrogenation performance of NPs was measured using p-nitrophenol as a model. The synergistic effect of these three elements (Ru, Ni, and Fe) and synergistic catalysis of multiple crystal faces greatly improved the catalytic hydrogenation performance of Ru₄₄Ni₂₈Fe₂₈ alloy NPs. Ru with more vacant orbitals showed a strong coordination with BH₄^- for the generation of active H species. Ni played a major role in transporting electrons and active H species, increasing the accessibility of catalytically active sites. Fe could cooperate with BH₄^- to produce active H species and promote electrons transfer. Ru₄₄Ni₂₈Fe₂₈ alloy NPs could be reused and applied for the fabrication of films at the oil-water (ethyl acetate-water) interface. The densely packed Ru₄₄Ni₂₈Fe₂₈ NP films were good Raman substrates for monitoring the complete conversion of 4-nitrothiophenol into 4-aminothiophenol. The rational design of Ru₄₄Ni₂₈Fe₂₈ will broaden the application range of Ru-based catalysts and provide new insights into the rational design of other multisite alloy catalysts.

Amphiphilic Mesoporous Sandwich-Structured Catalysts for Selective Hydrogenation of 4-Nitrostyrene in Water

Selective catalytic hydrogenation of substituted nitro compounds (NCs) of hydrophobic nature in aqueous solution using transition-metal-based catalysts is highly desirable yet fairly challenging. Herein, we propose the idea of amphiphilic mesoporous catalysts for selective hydrogenation of hydrophobic NCs in aqueous solution. The amphiphilic catalyst Co@Co-N-C@SBA-15 with a sandwich-like structure is constructed by a one-step solvent-free melting coating method. The catalyst has an external hydrophilic silica support that facilitates catalyst dispersion in water. It has unique Co-N-C catalytic layers uniformly coated in the inner mesopore surfaces of the silica support, which enhance the selective adsorption and activation of hydrophobic NCs. It has a high surface area (448.2 m²/g) and a uniform mesopore size (∼7.0 nm) for fast mass transportation. It possesses ultrafine metallic Co nanoparticles uniformly anchored within the N-doped carbon (N-C) layers for easy magnetic separation. These features make the catalyst excellent for the selective hydrogenation of 4-nitrostyrene to form 4-aminostyrene, with a high conversion of 98.0% in 1.0 h, a superior selectivity of 98.8%, and a good stability under mild conditions. A comprehensive study confirms the excellence of the amphiphilic mesoporous catalysts compared with other control catalysts. The Co-N sites are the intrinsic active sites. They can selectively adsorb and activate the nitro groups other than the vinyl groups, leading to superior selectivity. Water as the solvent results in the best performance compared with typical organic solvents probably because of an enhanced water-mediated hydrogen spillover and transfer.

Synthetically Tuned Pd-Based Intermetallic Compounds and their Structural Influence on the O2 Dissociation in Benzylamine Oxidation

Intermetallic compounds (IMCs) have diverse electronic and geometrical properties to offer. However, the synthesis of intermetallic nanoparticles is not always easy; developing new methodologies that are conventional for many systems can be challenging, especially when incorporating highly electropositive metals to reduce to IMCs using solution synthesis methodologies. In this study, we report a comprehensive approach to access nanocrystalline Pd_xM_y (M = Cu, Zn, Ga, Ge, Sn, Pb, Cd, In) intermetallic (IM) via the coreduction method employing sodium borohydride as the reductant. A combination of diffraction, spectroscopic, and microscopic techniques were performed to characterize the formed nanoparticles in terms of their phase composition, purity, particle size distribution, and surface oxidation properties of metals, respectively. IMCs of Pd with the elements such as Cu, Zn, Ga, and Ge exhibited higher catalytic activity that with elements such as In, Sn, Pb, and Cd. The DFT studies on these compounds revealed that the adsorption of benzylamine at the Pd site and the dissociative adsorption of O₂ on the IM surface play a significant effect on catalytic activity. Among them, PdCu IM exhibited an excellent conversion of benzylamine (94.0%), with 92.2% of dibenzylimine selectivity compared to other IMCs. Moreover, PdCu exhibited decent recyclability and activity for the oxidation of different substituted primary amines.

A symbiotic hetero-nanocomposite that stabilizes unprecedented CaCl2-type TiO2 for enhanced solar-driven hydrogen evolution reaction

Symbiotic hetero-nanocomposites prevail in many classes of minerals, functional substances and/or devices. However, design and development of a symbiotic hetero-nanocomposite that contains unachievable phases remain a significant challenge owing to the tedious formation conditions and the need for precise control over atomic nucleation in synthetic chemistry. Herein, we report a solution chemistry approach for a symbiotic hetero-nanocomposite that contains an unprecedented CaCl2-type titania phase inter-grown with rutile TiO2. CaCl2 structured TiO2, usually occurring when bulk rutile-TiO2 is compressed at an extreme pressure of several GPa, is identified to be a distorted structure with a tilt of adjacent ribbons of the c-axis of rutile. The structural specificity of the symbiotic CaCl2/rutile TiO2 hetero-nanocomposite was confirmed by Rietveld refinement, HRTEM, EXAFS, and Raman spectra, and the formation region (TiCl4 concentration vs. reaction temperature) was obtained by mapping the phase diagram. Due to the symbiotic relationship, this CaCl2-type TiO2 maintained a high stability via tight connection by edge dislocations with rutile TiO2, thus forming a CaCl2/rutile TiO2 heterojunction with a higher reduction capacity and enhanced charge separation efficiency. These merits endow symbiotic CaCl2/rutile TiO2 with a water splitting activity far superior to that of the commercial benchmark photocatalyst, P25 under simulated sunlight without the assistance of a cocatalyst. Our findings reported here may offer several useful understandings of the mechanical intergrowth process in functional symbiotic hetero-nanocomposites for super interfacial charge separation, where interfacial dislocation appears to be a universal cause.

Catalytic Enhancement of CO Oxidation on LaFeO3 Regulated by Ruddlesden-Popper Stacking Faults

The influence of planar defects, in the form of stacking faults, within perovskite oxides on catalytic activity has received little attention because controlling stacking-fault densities presents a major synthetic challenge. Furthermore, stacking faults in ceramics are not thought to appreciably impact surface chemistry, which partly explains why their direct effect on catalysis is generally ignored. Here, we show that Ruddlesden-Popper (RP) stacking faults in otherwise stoichiometric LaFeO₃ can be broadly controlled by modulating the ceramic synthesis route. Electronic structure calculations along with electron microscopy and spectroscopy show that energetically favorable RP faults occur both near the surface and in bunches and enhance CO oxidation kinetics. Density functional theory (DFT) + U shows that subsurface RP faults strengthen the adsorption and co-adsorption of CO, O, and O₂, which could lower the apparent activation energy of CO oxidation on faulted catalysts compared to that on their pristine counterparts. Our work suggests that planar defects should be considered a new and useful feature in hierarchal nanoscale design of future catalysts.

Hydrangea-like α-Ni1/3Co2/3(OH)2 Reinforced by Ethyl Carbamate "Rivet" for All-Solid-State Supercapacitors with Outstanding Comprehensive Performance

Improving the self-conductivity and structural stability of electrode materials is a key strategy to improve the energy density, rate performance, and cycle life of supercapacitors. Controlled intercalation of ethyl carbamate (CH₃CH₂OCONH₂) as the rivet between Ni-Co hydroxide layers can be used to obtain sufficient ion transport channels and robust structural stability of hydrangea-like α-Ni_1/3Co_2/3(OH)₂ (NC). Combining the improved electronic conductivity offered by the coexistence of Ni²⁺ and Co²⁺ optimizing itself electronic conductivity and the addition of carbon nanotubes (CNTs) as the electron transport bridge between the active material and the current collector and the large specific surface area (296 m² g^-1) reducing the concentration polarization, the capacitance retention ratio of NC-CNT from 0.2 to 20 A g^-1 is up to 93.4% and its specific capacitance is as high as 1228.7 F g^-1 at 20 A g^-1. The large total hole volume (0.40 cm³ g^-1) and wide crystal plane spacing (0.71 nm) provide an adequate space to withstand structure deformation during charge/discharge processes and enhance the structural stability of the NC material. The capacitance fading ratio of NC-CNT is only 4.5% at 10 A g^-1 for 10 000 cycles. The aqueous supercapacitor (NC-CNT//AC) and all-solid-state supercapacitor (PVA-NC-CNT//PVA-AC) exhibit high energy density (35.2 W h kg^-1 at 100.0 W kg^-1 and 35.4 W h kg^-1 at 100.7 W kg^-1), ultrahigh rate performance (the specific capacitances at 20 A g^-1 are 92.8 and 87.2% compared to that at 0.5 A g^-1), and long cycling life span (the specific capacitances after 100 000 cycles at 10 A g^-1 are 91.5 and 90.8% compared with that of their initial specific capacitances), respectively. Therefore, hydrangea-like NC could be a promising material for advanced next-generation supercapacitors.

Remarkable active-site dependent H2O promoting effect in CO oxidation

The interfacial sites of supported metal catalysts are often critical in determining their performance. Single-atom catalysts (SACs), with every atom contacted to the support, can maximize the number of interfacial sites. However, it is still an open question whether the single-atom sites possess similar catalytic properties to those of the interfacial sites of nanocatalysts. Herein, we report an active-site dependent catalytic performance on supported gold single atoms and nanoparticles (NPs), where CO oxidation on the single-atom sites is dramatically promoted by the presence of H₂O whereas on NPs’ interfacial sites the promoting effect is much weaker. The remarkable H₂O promoting effect makes the Au SAC two orders of magnitude more active than the commercial three-way catalyst. Theoretical studies reveal that the dramatic promoting effect of water on SACs originates from their unique local atomic structure and electronic properties that facilitate an efficient reaction channel of CO + OH.

The issue that whether single-atom sites possess similar catalytic properties to the interfacial sites of nanocatalysts remains unresolved. Here, the authors demonstrate a large H₂O promotional effect on CO oxidation over Au single-atom sites due to their unique local atomic structure and electronic properties.

In situ redox growth of mesoporous Pd-Cu2O nanoheterostructures for improved glucose oxidation electrocatalysis

Interfaces of metal-oxide heterostructured electrocatalyst are critical to their catalytic activities due to the significant interfacial effects. However, there are still obscurities in the essence of interfacial effects caused by crystalline defects and mismatch of electronic structure at metal-oxide nanojunctions. To deeply understand the interfacial effects, we engineered crystalline-defect Pd-Cu₂O interfaces through non-epitaxial growth by a facile redox route. The Pd-Cu₂O nanoheterostructures exhibit much higher electrocatalytic activity toward glucose oxidation than their single counterparts and their physical mixture, which makes it have a promising potential for practical application of glucose biosensors. Experimental study and density functional theory (DFT) calculations demonstrated that the interfacial electron accumulation and the shifting up of d bands center of Cu-Pd toward the Fermi level were responsible for excellent electrocatalytic activity. Further study found that Pd(3 1 0) facets exert a strong metal-oxide interface interaction with Cu₂O(1 1 1) facets due to their lattice mismatch. This leads to the sinking of O atoms and protruding of Cu atoms of Cu₂O, and the Pd crystalline defects, further resulting in electron accumulation at the interface and the shifting up of d bands center of Cu-Pd, which is different from previously reported charge transfer between the interfaces. Our findings could contribute to design and development of advanced metal-oxide heterostructured electrocatalysts.

Three-dimensional nickel vanadium layered double hydroxide nanostructures grown on carbon cloth for high-performance flexible supercapacitor applications

This study reports the synthesis of ultrathin Ni-V layered double hydroxide nanosheets on carbon cloth (NVL@CC) through adopting a facile and cost-effective method for flexible supercapacitor applications. The as-synthesized NVL@CC possesses a uniform, mechanically strong and highly ordered porous network with connected pores, ensuring high specific capacitance and enhanced cyclability. A high specific capacity of 1226 C g^-1 (2790 F g^-1) was obtained at 1 A g^-1, and it remained at 430 C g^-1 (1122 F g^-1) even at a higher current density of 10 A g^-1. A hybrid supercapacitor (HSC) was assembled with the NVL@CC electrode as the positive electrode and activated carbon coated carbon cloth as the negative electrode (NVL@CC//AC HSC). The devices showed an excellent energy density of 0.69 mW h cm^-3 at a power density of 2.5 mW cm^-3 with 100% of the original capacitance being retained at a current density of 5 mA cm^-2. Furthermore, the devices exhibited an energy density of 0.24 mW h cm^-3 even at a higher power density of 214.4 mW cm^-3, surpassing the performances observed for many recently reported flexible supercapacitors. Importantly, the electrochemical performance of the solid-state flexible supercapacitors showed a negligible change upon bending and twisting of the devices. The devices showed no decay in specific capacitance and coulombic efficiency up to 5000 charge-discharge cycles, confirming the excellent cycle life of the HSC device. The performance of NVL@CC indicates the great potential of the material for future flexible energy storage devices.

Cu2O-Supported Atomically Dispersed Pd Catalysts for Semihydrogenation of Terminal Alkynes: Critical Role of Oxide Supports

Atomically dispersed catalysts have demonstrated superior catalytic performance in many chemical transformations. However, limited success has been achieved in applying oxide-supported atomically dispersed catalysts to semihydrogenation of alkynes under mild conditions. By utilizing various metal oxides (e.g., Cu2O, Al2O3, ZnO, and TiO2) as supports for atomically dispersed Pd catalysts, we demonstrate herein the critical role of the oxidation state and coordinate environment of Pd centers in their catalytic performance, thus leading to the discovery of an “oxide-support effect” on atomically dispersed metal catalysts. Pd atomically dispersed on Cu2O exhibits far better catalytic activity in the hydrogenation of alkynes, with an extremely high selectivity toward alkenes, compared to catalysts on other oxides. Pd species galvanically displace surface Cu(I) sites on Cu2O to create two-coordinated Pd(I), which is a critical step for the activation and heterolytic splitting of H2 into Pd-H− and O-H+ species for the selective hydrogenation of alkynes. Moreover, the adsorption of alkenes on H2-preadsorbed Pd(I) is relatively weak, preventing deeper hydrogenation and increased selectivity during semihydrogenation. We demonstrate that the local coordinate environment of active metal centers plays a crucial role in determining the catalytic performance of an oxide-supported atomically dispersed catalyst.

Defect engineering of metal-oxide interface for proximity of photooxidation and photoreduction

Close proximity between different catalytic sites is crucial for accelerating or even enabling many important catalytic reactions. Photooxidation and photoreduction in photocatalysis are generally separated from each other, which arises from the hole-electron separation on photocatalyst surface. Here, we show with widely studied photocatalyst Pt/[Formula: see text] as a model, that concentrating abundant oxygen vacancies only at the metal-oxide interface can locate hole-driven oxidation sites in proximity to electron-driven reduction sites for triggering unusual reactions. Solar hydrogen production from aqueous-phase alcohols, whose hydrogen yield per photon is theoretically limited below 0.5 through conventional reactions, achieves an ultrahigh hydrogen yield per photon of 1.28 through the unusual reactions. We demonstrated that such defect engineering enables hole-driven CO oxidation at the Pt-[Formula: see text] interface to occur, which opens up room-temperature alcohol decomposition on Pt nanoparticles to [Formula: see text] and adsorbed CO, accompanying with electron-driven proton reduction on Pt to [Formula: see text].

Highly Efficient Polydopamine-coated Poly(methyl methacrylate) Nanofiber Supported Platinum⁻nickel Bimetallic Catalyst for Formaldehyde Oxidation at Room Temperature

We fabricated one fibrous-membrane type of flexible and lightweight supported catalyst via loading platinum–nickel nanoparticles (PtNi NPs) directly on the polydopamine-coated polymethylmethacrylate electrospun-fibers (PMMA@PDA). The polymer support with the PDA layer provided numerous active sites, leading to well-monodispersed and sized PtNi NPs on the nanofibers. Through the rational design of PtNi NPs, the resultant catalyst system exhibited 90% conversion for decomposing HCHO (10 ppm) at room temperature with only a low dosage (0.02 g), retaining the high activity for 100 h. This superior catalytic performance stems from the formate oxidation, which was the key intermediate during HCHO decomposition, and was promoted by the existence of a sufficient Pt–OH–Ni interface in the PtNi NPs with an appropriate Pt/Ni ratio of 1:5. Such tailored Pt-based nanoparticles ideally work together with the polymer nanofibers as a support for catalytic reaction. Compared with classical catalysts, our system can handle a comparable efficiency with much lower air resistance and remarkably lower dosage. Furthermore, the membrane-like morphology provides easy handling and minimizes the leaching of catalyst nanoparticles.

Roles of N-Vacancies over Porous g-C3N4 Microtubes during Photocatalytic NO x Removal

The development of catalysts that effectively activate target pollutants and promote their complete conversion is an admirable objective in the environmental photocatalysis field. In this work, graphitic carbon nitride (g-C₃N₄) microtubes with tunable N-vacancy concentrations were controllably fabricated using an in situ soft-chemical method. The morphological evolution of g-C₃N₄, from the bulk to the porous tubular architecture, is discussed in detail with the aid of time-resolved hydrothermal experiments. We found that the NO removal ratio and apparent reaction rate constant of the g-C₃N₄ microtubes were 1.8 and 2.6 times higher than those of pristine g-C₃N₄, respectively. By combining detailed experimental characterization and density functional theory calculations, the effects of N-vacancies in the g-C₃N₄ microtubes on O₂ and NO adsorption activation, electron capture, and electronic structure were systematically investigated. These results demonstrate that surface N-vacancies act as specific sites for the adsorption activation of reactants and photoinduced electron capture, while enhancing the light-absorbing capability of g-C₃N₄. Moreover, the porous wall structures of the as-prepared g-C₃N₄ microtubes facilitate the diffusion of reactants, and their tubular architectures favor the oriented transfer of charge carriers. The intermediates formed during photocatalytic NO removal processes were identified by in situ diffuse reflectance infrared Fourier transform spectroscopy, and different reaction pathways over pristine and N-deficient g-C₃N₄ are proposed. This study provides a feasible strategy for air pollution control over g-C₃N₄ by introducing N-vacancy and porous tubular architecture simultaneously.

Wet-Chemistry Strong Metal-Support Interactions in Titania-Supported Au Catalysts

Classical strong metal-support interactions (SMSI), which play a crucial role in the preparation of supported metal nanoparticle catalysts, is one of the most important concepts in heterogeneous catalysis. The conventional wisdom for construction of classical SMSI involves in redox treatments at high-temperatures by molecular oxygen or hydrogen, sometimes causing sintered metal nanoparticles before SMSI formation. Herein, we report that the aforementioned issue can be effectively avoided by a wet-chemistry methodology. As a typical example, we demonstrate a new concept of wet-chemistry SMSI (wcSMSI) that can be constructed on titania-supported Au nanoparticles (Au/TiO₂-wcSMSI), where the key is to employ a redox interaction between Au^δ+ and Ti³⁺ precursors in aqueous solution. The wcSMSI is evidenced by covering Au nanoparticles with the TiO _x overlayer, electronic interaction between Au and TiO₂, and suppression of CO adsorption on Au nanoparticles. Owing to the wcSMSI, the Au-TiO _x interface with an improved redox property is favorable for oxygen activation, accelerating CO oxidation. In addition, the oxide overlayer efficiently stabilizes the Au nanoparticles, achieving sinter-resistant Au/TiO₂-wcSMSI catalyst in CO oxidation.

Surface Compositions of Oxide Supported Bimetallic Catalysts: A Compared Study by High-Sensitivity Low Energy Ion Scattering Spectroscopy and X-Ray Photoemission Spectroscopy

It is well known that there is a critical relationship between the surface composition and catalytic performance for a bimetallic catalyst. However, in most cases, the surface composition is obviously different from that of the bulk. Moreover, the surface is normally reconstructed under reaction conditions. In this personal account, our recent progresses in determining the surface compositions of oxide supported bimetal catalysts by high-sensitivity low energy ion scattering spectroscopy (HS-LEIS) and X-ray photoemission spectroscopy (XPS) are summarized. Phase diagrams of the surface compositions under various conditions as a function of the bulk composition are established and compared. It is found that oxidation induces de-alloying and enrichment of PdO, CuO, SnO₂ on the surface, while H₂ reduction results in re-alloying. The addition of the second component not only modifies the nature of the active site, but also varies the dispersion of the active components. The support effects are discussed. The compared studies reveal that HS-LEIS can achieve a more reliable surface composition for oxide supported catalysts.

Nonpyrolyzed Fe-N Coordination-Based Iron Triazolate Framework: An Efficient and Stable Electrocatalyst for Oxygen Reduction Reaction

Pyrolyzed base-metal-based metal-organic frameworks (MOFs) with FeN_x coordination are emerging as nonprecious metal catalysts for electrochemical oxygen reduction reaction (ORR). However, surprisingly, nonpyrolyzed MOFs involving Fe-N coordination have not been explored for the ORR. This study concerns the catalytic performance of a semiconducting nonpyrolyzed iron triazolate framework (FeTa₂ ) for ORR in alkaline electrolyte. The FeTa₂ catalyst is studied as composites with different amounts of conductive Ketjenblack carbon (KB). The performance of these FeTa₂ -x KB (x denotes the KB/FeTa₂ weight ratio) composites by onset and half-wave potentials of ORR appears to be superior to most previously documented nonpyrolyzed MOFs. Characterization by elemental analysis, FTIR spectroscopy, XPS, and cyclic voltammetry suggest that N-Fe^III -OH^- sites at the surface of FeTa₂ function as the catalytic active sites. This FeTa₂ also shows very stable activity during ORR, as supported by accelerated durability test of the FeTa₂ -x KB sample (20 000 cycles, ca. 90 h). The framework structure of FeTa₂ remains intact during the durability test, which would help to explain its excellent catalytic durability. This would be the first study demonstrating efficient and stable ORR catalysis by a nonpyrolyzed Fe-N coordination-based MOF material.

Surface-modified Pt1Ni1–Ni(OH)2 nanoparticles with abundant Pt–Ni(OH)2 interfaces enhance electrocatalytic properties

Pt₁Ni₁–Ni(OH)₂ NPs with abundant Pt–Ni(OH)₂ interfaces exhibit a rather high activity and stability for the MOR in alkaline electrolytes.