Asymmetrically coordinated single-atom catalysts: from synthetic strategy to structure-activity relationship

Developing metal-nitrogen-doped carbon single-atom catalysts (M-NC SACs) with high loadings for the electrochemical CO2 reduction reaction (eCO2RR) remains challenging owing to the risk of metal aggregation. Herein, the study presents a facile strategy for synthesizing M-NC SACs using metal-chelating ligands, eliminating the need for additional processing steps. Specifically, using ethylenediaminetetraacetic acid as a strong metal-chelating ligand, the formation of Ni nanoparticles is effectively prevented and a high loading of ≈2.7 wt.% is achieved, leading to the development of high-loading Ni SACs. The resulting catalysts exhibit a high CO faradaic efficiency (FECO) of 96.6% and CO partial current density of -120.2 mA cm-2 and retain a FECO over 90% in a broad potential range of -0.4 to -0.9 V versus the reversible hydrogen electrode. Furthermore, theoretical calculations indicate that the asymmetric Ni-N3C1 local coordination structure within the catalyst reveals an optimal balance between *COOH formation and *CO desorption, which enhances the activity for eCO2RR to CO. This study offers an efficient strategy to suppress metal nanoparticle formation while simultaneously improving the metal loading in M-NC SACs.

Propane Dehydrogenation on Single-Site [PtZn4] Intermetallic Catalysts

The high cost of noble metal catalysts has been a major factor limiting their industrial applications. It is thus of strong interest to develop catalysts with minimum metal loading. Here, we designed and prepared a single-atom ruthenium catalyst through a cascade anchoring strategy to maximize the efficiency of Ru atoms for acetylene hydrochlorination. The single-atom catalyst supported on commercial activated carbon (AC) exhibits excellent catalytic activity with acetylene conversion of 95.4% at an acetylene gas hourly space velocity (GHSV) of 720 h-1 and almost no deactivation during a 600 h catalyst lifetime test. In conjunction with a series of experimental characterizations of the catalyst, including aberration-corrected scanning transmission electron microscopy (Ac-STEM), X-ray photoelectron spectroscopy (XPS), and extended X-ray absorption fine spectroscopy (EXAFS), density functional theory (DFT) study shows that RuN4 sites are likely responsible for acetylene hydrochlorination catalytic activity. This work provides a strategy to design efficient single-atom catalysts for acetylene hydrochlorination and helps us to gain deeper understanding of single-atom catalytic mechanisms.

Carbon-based material-supported single-atom catalysts for energy conversion

The (11)B(p,alpha)(8)Be* nuclear reaction was assessed for its ability to quantitatively map the in vivo subcellular distribution of boron within gliosarcomas treated with a boronated neutron capture therapy agent. Intracranial 9L gliosarcomas were produced in Fischer 344 rats. Fourteen days later, the majority of the rats were treated with f-boronophenylalanine and killed humanely 30 or 180 min after intravenous injection. Freeze-dried tumor cryosections were imaged using the (11)B(p,alpha)(8)Be* nuclear reaction and proton microbeams obtained from the nuclear microprobe at Lawrence Livermore National Laboratory. The (11)B distributions within cells could be imaged quantitatively with spatial resolutions down to 1.5 microm, minimum detection limits of 0.8 mg/kg, and acquisition times of several hours. These capabilities offer advantages over alpha-particle track autoradiography, electron energy loss spectroscopy, and secondary ion mass spectrometry (SIMS) for quantification of (11)B in tissues. However, the spatial resolution, multi-isotope capability, and analysis times achieved with SIMS are superior to those achieved with (11)B(p,alpha)(8)Be* analysis. When accuracy in quantification is crucial, the (11)B(p,alpha)(8)Be* reaction is well suited for assessing the microdistribution of (11)B. Otherwise, SIMS may well be better suited to image the microdistribution of boron associated with neutron capture therapy agents in biological tissues.

Direct conversion of bulk metal or nanoparticles into metal single atoms under thermal pyrolysis conditions is a highly efficient and promising strategy to fabricate single-atom catalysts (SACs). Usually, nitrogen-doped carbon is used as the anchoring substrate to capture the migrating metal ion species at high temperatures, and stable isolated SACs with nitrogen coordination are formed during the process. Herein, we report unexpected oxygen-coordinated metal single-atom catalysts (Fe-, Co-, Ni-, Mn-SACs) with high loadings (above 10 wt %) through direct transformation of metal oxide nanoparticles (Fe-, Co-, Ni-, Mn-NPs) in an inert atmosphere at 750 °C for 2 h. The atomic dispersion of metal single atoms and their coordinated structures were confirmed by aberration-corrected scanning transmission electron microscopy and X-ray absorption fine structures. In addition, the dynamic process of nanoparticles to atoms was directly observed by in situ transmission electron microscopy. The as-prepared Fe SAC exhibited high activity and superior selectivity for catalytic oxidation of benzene to phenol with hydrogen peroxide.

Metal oxides with conversion and alloying mechanisms are more competitive in suppressing lithium dendrites. However, it is difficult to simultaneously regulate the conversion and alloying reactions. Herein, conversion and alloying reactions are regulated by modulation of the zinc oxide bandgap and oxygen vacancies. State-of-the-art advanced characterization techniques from a microcosmic to a macrocosmic viewpoint, including neutron diffraction, synchrotron X-ray absorption spectroscopy, synchrotron X-ray microtomography, nanoindentation, and ultrasonic C-scan demonstrated the electrochemical gain benefit from plentiful oxygen vacancies and low bandgaps due to doping strategies. In addition, high mechanical strength 3D morphology and abundant mesopores assist in the uniform distribution of lithium ions. Consequently, the best-performed ZnO-2 offers impressive electrochemical properties, including symmetric Li cells with 2000h and full cells with 81% capacity retention after 600 cycles. In addition to providing a promising strategy for improving the lithiophilicity and mechanical strength of metal oxide anodes, this work also sheds light on lithium metal batteries for practical applications.

Supported metal clusters with the integrated advantages of single-atom catalysts and conventional nanoparticles held great promise in the electrocatalytic carbon dioxide reduction (ECO2R) operated at low overpotential and high current density. However, its precise synthesis and the understanding of synergistically catalytic effects remain challenging. Herein, we report a facile method to synthesize the bimetallic Cu and Ni clusters anchored on porous carbon (Cu/Ni-NC) and achieve an enhanced ECO2R. The aberration-corrected high-angle annular dark-field scanning transmission electron microscopy and synchrotron X-ray absorption spectroscopy were employed to verify the metal dispersion and the coordination of Cu/Ni clusters on NC. As a result of this route, the target Cu/Ni-NC exhibits excellent electrocatalytic performance including a stable 30 h electrolysis at 200 mA cm-2 with carbon monoxide Faradaic efficiency of ∼95.1% using a membrane electrode assembly electrolysis cell. Combined with the in situ analysis of the surface-enhanced Fourier transform infrared spectroelectrochemistry, we propose that the synergistic effects between Ni and Cu can effectively promote the H2O dissociation, thereby accelerate the hydrogenation of CO2 to *COOH and the overall reaction process.

A versatile approach for quantification of surface site fractions using reaction kinetics: The case of CO oxidation on supported Ir single atoms and nanoparticles

Atomically dispersed catalysts refer to substrate-supported heterogeneous catalysts featuring one or a few active metal atoms that are separated from one another. They represent an important class of materials ranging from single-atom catalysts (SACs) and nanoparticles (NPs). While SACs and NPs have been extensively reported, catalysts featuring a few atoms with well-defined structures are poorly studied. The difficulty in synthesizing such structures has been a critical challenge. Here we report a facile photochemical method that produces catalytic centers consisting of two Ir metal cations, bridged by O and stably bound to a support. Direct evidence unambiguously supporting the dinuclear nature of the catalysts anchored on α-Fe2O3 is obtained by aberration-corrected scanning transmission electron microscopy (AC-STEM). Experimental and computational results further reveal that the threefold hollow binding sites on the OH-terminated surface of α-Fe2O3 anchor the catalysts to provide outstanding stability against detachment or aggregation. The resulting catalysts exhibit high activities toward H2O photooxidation.

Isotopic engineering provides a powerful route to control phonon behavior in crystalline solids, enabling fundamental studies of lattice dynamics and heat transport at the atomic scale. Here, we directly visualize isotope-dependent phonon propagation in epitaxial Cr2O3 using aberration-corrected scanning transmission electron microscopy (STEM) combined with monochromated, high-energy-resolution electron energy-loss spectroscopy (EELS). Guided by ab initio phonon calculations, we demonstrate that optical phonon modes above 70 meV are predominantly oxygen-derived and exhibit measurable redshifts upon substitution of natural 16O by enriched 18O. Spatially resolved vibrational spectrum imaging reveals isotope-enriched tracer layers within Cr2O3 thin films, correlating isotope concentration with phonon intensity variations and vibrational energy shifts. At the nanometer and atomic scales, vibrational EELS mapping uncovers coherent phonon propagation across isotopic interfaces, consistent with theoretical phonon density of states and dispersion relations. These results establish vibrational EELS as a quantitative probe for isotope-dependent phonon transport in materials, opening new possibilities for studying energy dissipation and lattice dynamics.

Grain refinement of Mg-Ca alloys by native MgO particles

An Atomic-Scale Perspective of the Challenging Microstructure of YBa2Cu3O7−x Thin Films

Comprehensive characterization of the irradiation effects of glassy carbon

In lithium ion batteries (LIBs), the layered cathode materials of composition LiNi1- x - y Cox Mny O2 are critical for achieving high energy densities. A high nickel content (>80%) provides an attractive balance between high energy density, long lifetime, and low cost. Consequently, Ni-rich layered oxides cathode active materials (CAMs) are in high demand, and the importance of LiNiO2 (LNO) as limiting case, is hence paramount. However, achieving perfect stoichiometry is a challenge resulting in various structural issues, which successively impact physicochemical properties and result in the capacity fade of LIBs. To better understand defect formation in LNO, the role of the Ni(OH)2 precursor morphology in the synthesis of LNO requires in-depth investigation. By employing aberration-corrected scanning transmission electron microscopy, electron energy loss spectroscopy, and precession electron diffraction, a direct observation of defects in the Ni(OH)2 precursor preparedis reported and the ex situ structural evolution from the precursor to the end product is monitored. During synthesis, the layered Ni(OH)2 structure transforms to partially lithiated (non-layered) NiO and finally to layered LNO. The results suggest that the defects observed in commercially relevant CAMs originate to a large extent from the precursors, hence care must be taken in tuning the co-precipitation parameters to synthesize defect-free Ni-rich layered oxides CAMs.