Spherical Aberration-corrected Electron Microscopy Research Articles

Synergistic Active Heterostructure Design for Enhanced Two Electron Oxygen Reduction via Chemical and Electrochemical Reconstruction of Heterosulfides.

Transition metal sulfides, particularly heterostructures, represent a promising class of electrocatalysts for two electron oxygen reduction (2e- ORR), however, understanding the dynamic structural evolution of these catalysts during alkaline ORR remains relatively unexplored. Herein, NiS2/In2.77S4 heterostructure was synthesized as a precatalyst and through a series of comprehensive ex situ and in situ characterizations, including X-ray absorption spectroscopy, Raman spectroscopy, transient photo-induced voltage measurements, electron energy loss spectroscopy, and spherical aberration-corrected electron microscopy, it was revealed that nickel/indium (oxy)hydroxides (NiOOH/In(OH)3) could be evolved from the initial NiS2/In2.77S4 via both electrochemical and chemical-driven methods. The electrochemical-driven phase featured abundant bridging oxygen-deficient [NiO6]-[InO6] units at the interfaces of NiOOH/In(OH)3, facilitating a synergistic effect between active Ni and In sites, thus enabling an enhanced alkaline 2e- ORR capability than that of chemical-driven process. Remarkably, electrochemically induced NiOOH/In(OH)3 exhibited exceptional performance, achieving H2O2 selectivity of >90 % across the wide potential window (up to 0.4 V) with a peak selectivity of >99 %. Notably, within the three-electrode flow cell, a current density of 200 mA cm-2 was sustained over 20 h, together with an impressive Faradaic efficiency of ~90 % during the whole cycle process.

Synergistic effects of Pt single atomic site and Cu nanoclusters for catalytic oxidation of methyl mercaptan

Here, we report a catalyst design strategy, which uses hydrotalcite as a precursor to obtaining uniformly loaded nano copper clusters and further load single atom Pt to form a synergistic catalytic effect between precious metals and transition metals and is applied to the catalytic oxidation of CH3SH. The ability of adsorption-activation of molecular oxygen can be enhanced by constructing copper nanoclusters on the surface of metal oxides, and the catalytic oxidation path of methanethiol can be changed from MVK scheme to ER scheme. However, it is still not enough to obtain the ideal methanethiol conversion efficiency. Inspired by the organometallic chemistry of platinum, the element sulfur has a good bonding ability with platinum. We introduced platinum single atoms onto copper nanoclusters. Confirmation of atomically dispersed Pt by spherical aberration corrected electron microscopy and XAFS. Combined with the above characterization and calculation, it can be seen that the introduction of single-atom Pt is conducive to the adsorption of CH3SH, Cu nanoclusters can activate oxygen molecules, and the synergistic effect of the two active sites can significantly improve the catalytic oxidation activity of the catalyst for CH3SH. The oxidation of CH3SH occur via the Langmuir-Hinshelwood scheme (L-H scheme).

NH4Cl-assisted preparation of single Ni sites anchored carbon nanosheet catalysts for highly efficient carbon dioxide electroreduction

Single-atomic transition metal-nitrogen codoped carbon (M-N-C) are efficient substitute catalysts for noble metals to catalyze the electrochemical CO2 reduction reaction (CO2RR). However, the uncontrolled aggregations of metal and serious loss of nitrogen species constituting the M-Nx active sites are frequently observed in the commonly used pyrolysis procedure. Herein, single-atomic nickel (Ni)-based sheet-like electrocatalysts with abundant Ni-N4 active sites were created by using a novel ammonium chloride (NH4Cl)-assited pyrolysis method. Spherical aberration correction electron microscopy and X-ray absorption fine structure analysis clearly revealed that Ni species are atomically dispersed and anchored by N in Ni-N4 structure. The addition of NH4Cl optimized the mesopore size to 7–10 nm and increased the concentrations of pyridinic N (3.54 wt%) and Ni-N4 (3.33 wt%) species. The synergistic catalytic effect derived from Ni-N4 active sites and pyridinic N species achieved an outstanding CO2RR performance, presenting a high CO Faradaic efficiency (FECO) up to 98% and a large CO partial current density of 8.5 mA cm−2 at a low potential of –0.62 V vs. RHE. Particularly, the FECO maintains above 80% within a large potential range from –0.43 to –0.73 V vs. RHE. This work provides a practical and feasible approach to building highly active single-atomic catalysts for CO2 conversion systems.

The Effect of Cu Addition on the Precipitation Sequence in the Al-Si-Mg-Cr Alloy.

In this work, the effect of Cu additives and heat treatment on the precipitation sequence of an Al-Si-Mg-Cr alloy has been systematically studied by means of advanced spherical aberration-corrected electron microscopy. Cu atoms tend to gather at the interface between the precipitates and the matrix at the beginning of the aging process. Then, Cu atoms diffuse into the precipitates. Two types of GP zones are formed in the first stage of precipitation: one is the type I GP zone and the other is the type II GP zone. The type I GP zone βCu″ evolved into the Q' phase, while the type II GP zone evolved into the θ' phase during the aging process. The aging sequence of the Al-Si-Mg-Cr alloy can be determined as a supersaturated solid solution (SSSS) → GP zones → β″→ β'/B'(→β). The aging sequence of the Al-7%Si-0.3%Mg-0.3%Cr-1.5%Cu alloy can be determined as a supersaturated solid solution (SSSS)→GP zone→βCu″→Q' + θ'(→Q + θ).

Metal-Organic Framework-Derived Atomically Dispersed Co-N-C Electrocatalyst for Efficient Oxygen Reduction Reaction

In this work, an atomically dispersed cobalt-nitrogen-carbon (Co-N-C) catalyst is prepared for the oxygen reduction reaction (ORR) by using a metal-organic framework (MOF) as a self-sacrifice template under high-temperature pyrolysis. Spherical aberration-corrected electron microscopy is employed to confirm the atomic dispersion of high-density Co atoms on the nitrogen-doped carbon scaffold. The X-ray photoelectron spectroscopy results verify the existence of Co-N-C active sites and their content changes with the Co content. The electrochemical results show that the electrocatalytic activity shows a volcano-shaped relationship, which increases with the Co content from 0 to 0.99 wt.% and then decreases when the presence of Co nanoparticles at 1.61 wt.%. The atomically dispersed Co-N-C catalyst with Co content of 0.99 wt.% shows an onset potential of 0.96 V vs. reversible hydrogen electrode (RHE) and a half-wave potential of 0.89 V vs. RHE toward ORR. The excellent ORR activity is attributed to the high density of the Co-N-C sites with high intrinsic activity and high specific surface area to expose more active sites.

Hierarchical macro-mesoporous electrocatalysts with dual-active sites of Ru single atoms and monodispersed Ru–Mo nanoclusters for efficient hydrogen evolution

Rationally designing high-performance electrocatalysts with high density and multiactive sites is an effective strategy to accelerate the alkaline hydrogen evolution reaction (HER) kinetics, yet it remains challenging. Herein, a catalyst comprising numerous Ru–Mo-based single atoms and subnanoclusters incorporated into a hierarchical macro-mesoporous carbon framework with a P-doped skeleton (Ru–Mo/PC) was successfully developed via a simple template-assisted pyrolysis method. Spherical aberration correction electron microscopy and X-ray absorption fine structure measurements revealed that the dual-active sites consisted of Ru–C5 single atoms and ultrasmall Ru–Mo nanoclusters (mean diameter of 0.94 nm). In particular, the as-constructed RuMo1.5/PC catalyst exhibited a high activity towards the HER, requiring an overpotential of only 17.9 mV at 10 mA/cm2, which outperformed a commercial Pt/C catalyst in alkaline media. Electrochemical performance results also indicated that introducing moderate Mo species played a crucial role in further boosting the HER activity compared to Ru/PC. The present work provides a step forward in designing efficient and stable catalysts with high-density, atomically dispersed active sites for hydrogen production.

Enhanced oxygen reduction with carbon-polyhedron-supported discrete cobalt-nitrogen sites for Zn-air batteries

• A general strategy to construct discrete Co-N 4 sites confined in carbon polyhedron. • Porous carbon polyhedron with bumpy exterior boosts the exposure of active sites. • Zn acts as sacrificed “shield” to isolate the adjacent Co atoms from aggregating. • Co-N/ZIF-2 catalyst shows high ORR activity superior to commercial Pt/C. • Zn-air battery exhibits high power density of 260 mW cm −2 and stability in 200 h. The specific structure of metal atoms including coordination environment and dispersibility, together with carbon support architecture in metal-nitrogen-carbon (M−N−C) materials are of great importance for their capabilities to catalyze oxygen reduction reaction (ORR). Herein, we report a novel strategy to achieve highly exposed and atomically isolated Co-N 4 active sites within porous carbon polyhedron with bumpy exterior by pyrolysis of predesigned Zn-modified metal–organic frameworks (MOFs) together with tripolycyanmide, during which Zn is evaporated away at high temperature of 900 °C and leaves free vacancies for N to stabilize Co atoms. The bumpy exterior dramatically enlarges its specific surface area and improves the number of active sites. The local structure of single Co atoms coordinated with N is confirmed by combing spherical aberration correction electron microscopy and X-ray absorption fine structure analyses. Remarkably, the obtained Co-N 4 single sites exhibit a high ORR activity with a half-wave potential of 0.861 V that is superior to commercial Pt/C (0.819 V), as well as excellent performance and stability in Zn-air batteries with a peak power density of 260 mW cm −2 . Our work will endow the opportunities to propagate this approach to develop various single-metal-atom materials.

Synergistic effect of Ru-N4 sites and Cu-N3 sites in carbon nitride for highly selective photocatalytic reduction of CO2 to methane

Developing single-atom photocatalysts for selective conversion of CO2 to valuable fuel is of great attraction but remains challenging. In this work, ruthenium and copper single atoms are for the first time simultaneously incorporated into polymeric carbon nitride (PCN) through a simple preassembly-coprecipitation-pyrolysis process. The obtained PCN-RuCu sample exhibited much higher selectivity (95%) for CH4 production than the individual Ru or Cu decorated PCN during photocatalytic CO2 reduction under visible-light irradiation. The atomically dispersed Ru-N4 and Cu-N3 moieties were confirmed by spherical aberration-corrected electron microscopy and extended X-ray absorption fine structure spectroscopy. Density function theory (DFT) calculations revealed that the co-existence of Ru-N4 sites and Cu-N3 sites can effectively tune the electronic structure of PCN, making the Ru sites account for photogenerated electron-hole pairs and the Cu sites for CO2 hydrogenation. Moreover, the synergetic effect between Ru and Cu single atoms significantly promotes the consecutive hydrogenation processes of *CO species towards CH4 production. Our studies provide a new understanding of the mechanism for photocatalytic reduction of CO2 to CH4, and pave a new way to design photocatalysts for the selective production of solar fuels.

Control of Local Electronic Structure of Pd Single Atom Catalyst by Adsorbate Induction.

Aiming at regulating and controlling the localized electronic states while maintaining the metal atoms in the isolation form, an in situ adsorbate induced strategy is proposed at a programmed temperature to activate Zr-based metal-organic framework (MOF) supported single Pd atom catalyst. It is discovered that in situ treatment environments trigger the change of lattice parameters in MOF materials by reaction heat effect, observed by in situ X-ray diffraction, spherical aberration-corrected electron microscope, and X-ray adsorption fine structure (XAFS). The as-obtained electron-deficient Pd single atoms are critical to the high intrinsic activity (turnover frequency of 0.132 s-1 ) and selectivity of 93% with the long-term stability in the semihydrogenation of acetylene, which can be comparable to the state-of-the-art Pd catalysts. This superior catalytic behavior correlates with the reduced C2 H4 desorption energy and the activation barriers for the hydrogenation, confirmed by density functional theory calculation.

Photoinduction of palladium single atoms supported on defect-containing γ-AlOOH nanoleaf for efficient trans-stilbene epoxidation

Exploring the interaction between metal and support is of great importance in enhancing the catalytic performance and deepening the mechanistic understanding of single atom catalysis. Here we describe a photoinduction method to construct atomically dispersed palladium atoms supported over defect-containing boehmite (γ-AlOOH) nanoleaf with a palladium loading of 0.32 wt%. The existence of singly dispersed palladium atoms is confirmed by spherical aberration correction electron microscopy, extended X-ray absorption fine structure measurement, and CO-absorbed diffuse reflectance infrared Fourier transform spectroscopy. This catalyst shows excellent catalytic efficiency in trans-stilbene epoxidation to yield trans-stilbene oxide (TOF: 416 h−1; conversion: 84 %; selectivity: 99 %), along with excellent recyclability and scalability. In addition, various aromatic olefins were successfully transformed into the corresponding epoxides with high selectivity and excellent yield. DFT study further reveals that the high catalytic activity originates from under-coordinated single palladium atoms in the defect-containing γ-AlOOH in a unique coordination environment. This lays the foundation for the facile creation of single atom catalysts and sheds light on the possibility for scale-up production.

One-step synthesis of single palladium atoms in WO2.72 with high efficiency in chemoselective hydrodeoxygenation of vanillin

The pathway for efficient catalytic hydrodeoxygenation of biomass represents a powerful, yet challenging route for production of value-added liquid fuels. Herein, we describe a one-step synthetic approach to fabricate WO2.72 decorated with atomically dispersed palladium atoms that bond covalently to the nearby oxygen atoms. The presence of isolated palladium atoms is confirmed by spherical aberration correction electron microscopy, extended X-ray absorption fine structure measurement, and diffuse reflectance infrared Fourier transform spectroscopy. This catalyst manifests outstanding catalytic performance in hydrodeoxygenation of vanillin to yield 2-methoxy-4-methylphenol (MMP) efficiently and selectively, along with exceptional stability and scalability. Density functional theory (DFT) calculations indicate that this high activity results from the unique electronic structure of isolated palladium atoms confined in defective WO2.72. These findings may pave the way for the facile creation of single atom catalysts in a coordination-engineered strategy for the advance of single atom catalysis.

Low-Temperature Synthesis of Single Palladium Atoms Supported on Defective Hexagonal Boron Nitride Nanosheet for Chemoselective Hydrogenation of Cinnamaldehyde.

Metal-support interactions are of great importance in determining the support-activity in heterogeneous catalysis. Here we report a low-temperature synthetic strategy to create atomically dispersed palladium atoms anchored on defective hexagonal boron nitride (h-BN) nanosheet. Density functional theory (DFT) calculations suggest that the nitrogen-containing B vacancy can provide stable anchoring sites for palladium atoms. The presence of single palladium atoms was confirmed by spherical aberration correction electron microscopy and extended X-ray absorption fine structure measurement. This catalyst showed exceptional efficiency in chemoselective hydrogenation of cinnamaldehyde, along with excellent recyclability, sintering-resistant ability, and scalability. We anticipate this synthetic approach for the synthesis of high-quality SACs based on h-BN support is amenable to large-scale production of bench-stable catalysts with maximum atom efficiency for industrial applications.

Single Atomic Cerium Sites with a High Coordination Number for Efficient Oxygen Reduction in Proton-Exchange Membrane Fuel Cells

Fe–N–C electrocatalysts, as a representative of platinum group metal-free (PGM-free) catalysts, exhibit a comparable oxygen reduction reaction (ORR) activity but insufficient stability to that of commercial Pt/C in proton-exchange membrane fuel cells (PEMFCs), due to the unavoidable Fenton’s reactions. Herein, we report a hard-template approach to synthesize the rare-earth single-cerium-atom-doped metal–organic frameworks with a hierarchically macro–meso–microporous structure. Spherical aberration correction electron microscopy confirms the atomic dispersion of Ce sites. Additionally, X-ray absorption spectroscopy (XAS) was employed to further verify the coordination environment of Ce sites, which were stabilized by four-coordinated nitrogen atoms and six-oxygen atoms (Ce–N4/O6). The Ce sites were embedded in a hierarchically macro–meso–microporous N-doped carbon (Ce SAS/HPNC) catalyst, which exhibits a half-wave potential of 0.862 V in ORR and the highest power density of 0.525 W cm–2 under 2.0 bar H2/O2 in the fuel cell test.

Highly Active and Stable Palladium Single-Atom Catalyst Achieved by a Thermal Atomization Strategy on an SBA-15 Molecular Sieve for Semi-Hydrogenation Reactions.

Single-atom catalysts (SACs) have great potential to revolutionize heterogeneous catalysis, enabling fast and direct construction of desired products. Given their notable promise, a general and scalable strategy to access these catalyst systems is highly desirable. Herein, we describe a straightforward and efficient thermal atomization strategy to create atomically dispersed palladium atoms anchored on a nitrogen-doped carbon shell over an SBA-15 support. Their presence was confirmed by spherical aberration correction electron microscopy and extended X-ray absorption fine structure measurement. The nitrogen-containing carbon shells provide atomic diffusion sites for anchoring palladium atoms emitted from palladium nanoparticles. This catalyst showed exceptional efficiency in selective hydrogenation of phenylacetylene and other types of alkynes. Importantly, it showed excellent stability, recyclability, and sintering-resistant ability. This approach can be scaled up with comparable catalytic activity. We anticipate that this work may lay the foundation for rapid access to high-quality SACs that are amenable to large-scale production for industrial applications.

Selective Hydrogenation on a Highly Active Single-Atom Catalyst of Palladium Dispersed on Ceria Nanorods by Defect Engineering.

Single-atom catalysis represents a new frontier that integrates the merits of homogeneous and heterogeneous catalysis to afford exceptional atom efficiency, activity, and selectivity for a range of catalytic systems. Herein we describe a simple defect engineering strategy to construct an atomically dispersed palladium catalyst (Pdδ+, 0 < δ < 2) by anchoring the palladium atoms on oxygen vacancies created in CeO2 nanorods. This was confirmed by spherical aberration correction electron microscopy and extended X-ray absorption fine structure measurement. The as-prepared catalyst showed exceptional catalytic performance in the hydrogenation of styrene (99% conversion, TOF of 2410 h-1), cinnamaldehyde (99% conversion, 99% selectivity, TOF of 968 h-1), as well as oxidation of triethoxysilane (99% conversion, 79 selectivity, TOF of 10 000 h-1). This single-atom palladium catalyst can be reused at least five times with negligible activity decay. The palladium atoms retained their dispersion on the support at the atomic level after thermal stability testing in Ar at 773 K. Most importantly, this synthetic method can be scaled up while maintaining catalytic performance. We anticipate that this method will expedite access to single-atom catalysts with high activity and excellent resistance to sintering, significantly impacting the performance of this class of catalysts.

Direct Synthesis of Atomically Dispersed Palladium Atoms Supported on Graphitic Carbon Nitride for Efficient Selective Hydrogenation Reactions.

Heterogeneous catalysts with atomically precise metal sites have enabled unique insight into structure-property relationships in materials science. Herein, we report the construction and selective hydrogenation performance of a single-atom palladium catalyst by confining the palladium atoms into the six-fold N-coordinating cavities of graphitic carbon nitride (g-C3N4) through a facile spatial confinement-reduction approach under mild reducing conditions. Spherical aberration correction electron microscopy and extended X-ray absorption fine structure measurements confirm the presence of atomically dispersed palladium atoms stabilized by the g-C3N4 support. Its exceptional catalytic activity was demonstrated by the hydrogenation of styrene (98% conversion, 1.5 h) and furfural (conversion of 64% and selectivity of 99%, 4 h) and hydrodechlorination of 4-chlorophenol (99% conversion and 99% selectivity, 10 min). This palladium catalyst can be reused at least five times with negligible deterioration of its activity. Importantly, the palladium atoms retained their atomic dispersion following the thermal treatment. Moreover, this synthetic method can be scaled up while retaining similar catalytic activity. Fundamental insights are provided to elucidate how the material's structure significantly impacts the catalytic performance at the atomic scale.

Atomic-scale structure characteristics of antiferroelectric silver niobate

Antiferroelectric materials are a kind of functional material, which are widely used in electrostatic energy storage, energy conversion devices, and magnetoelectric coupling devices. As a typical lead-free antiferroelectric material, silver niobate has attracted much attention in recent years due to its excellent performance in energy storage. In this work, using the spherical aberration corrected electron microscopy technique, atomic-resolution images of pure silver niobate were obtained, which revealed typical microscopic physical characteristics of such complex antiferroelectric oxides: in such materials, all cations deviate from the average positions of the main lattice, and the displacement of each kind of cation varies periodically in two opposite directions, resulting in periodic wavy (1–10)c atomic planes, and the period of cation displacement is 15.6 Å. At the same time, the 90° antiferroelectric domain boundary and the antiphase domain wall defects are further revealed and analyzed.

Preassembly Strategy To Fabricate Porous Hollow Carbonitride Spheres Inlaid with Single Cu-N3 Sites for Selective Oxidation of Benzene to Phenol.

Developing single-atom catalysts with porous micro-/nanostructures for high active-site accessibility is of great significance but still remains a challenge. Herein, we for the first time report a novel template-free preassembly strategy to fabricate porous hollow graphitic carbonitride spheres with single Cu atoms mounted via thermal polymerization of supramolecular preassemblies composed of a melamine-Cu complex and cyanuric acid. Atomically dispersed Cu-N3 moieties were unambiguously confirmed by spherical aberration correction electron microscopy and extended X-ray absorption fine structure spectroscopy. More importantly, this material exhibits outstanding catalytic performance for selective oxidation of benzene to phenol at room temperature, especially showing phenol selectivity (90.6 vs 64.2%) and stability much higher than those of the supported Cu nanoparticles alone, originating from the isolated unique Cu-N3 sites in the porous hollow structure. An 86% conversion of benzene, with an unexpectedly high phenol selectivity of 96.7% at 60 °C for 12 h, has been achieved, suggesting a great potential for practical applications. This work paves a new way to fabricate a variety of single-atom catalysts with diverse graphitic carbonitride architectures.

A cocoon silk chemistry strategy to ultrathin N-doped carbon nanosheet with metal single-site catalysts

Development of single-site catalysts supported by ultrathin two-dimensional (2D) porous matrix with ultrahigh surface area is highly desired but also challenging. Here we report a cocoon silk chemistry strategy to synthesize isolated metal single-site catalysts embedded in ultrathin 2D porous N-doped carbon nanosheets (M-ISA/CNS, M = Fe, Co, Ni). X-ray absorption fine structure analysis and spherical aberration correction electron microscopy demonstrate an atomic dispersion of metal atoms on N-doped carbon matrix. In particular, the Co-ISA/CNS exhibit ultrahigh specific surface area (2105 m2 g−1) and high activity for C–H bond activation in the direct catalytic oxidation of benzene to phenol with hydrogen peroxide at room temperature, while the Co species in the form of phthalocyanine and metal nanoparticle show a negligible activity. Density functional theory calculations discover that the generated O = Co = O center intermediates on the single Co sites are responsible for the high activity of benzene oxidation to phenol.

One-Pot Pyrolysis to N-Doped Graphene with High-Density Pt Single Atomic Sites as Heterogeneous Catalyst for Alkene Hydrosilylation

Development of noble-metal single atomic site catalysts with high metal loading is highly required for many important chemical reactions but proves to be very challenging. Herein, we report a Na2CO3 salt-assisted one-pot pyrolysis strategy from EDTA–Pt complex to N-doped graphene with isolated Pt single atomic sites (Pt-ISA/NG) with Pt loading up to 5.3 wt %. The X-ray absorption fine structure analysis and spherical aberration-correction electron microscopy demonstrate an atomic dispersion of single Pt species on graphene support and stabilized by nitrogen in Pt–N4 structure. The Pt-ISA/NG catalyst exhibits high catalytic activity and reusability for anti-Markovnikov hydrosilylation of various terminal alkenes with industrially relevant tertiary silanes under mild conditions. In hydrosilylation of 1-octene, the Pt-ISA/NG catalyst delivers an overall turnover frequency of 180 h–1, which is a 4-fold enhancement compared with commercial Pt/C.