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Single-atom catalysts with full utilization of metal centers can bridge the gap between molecular and solid-state catalysis. Metal-nitrogen-carbon materials prepared via pyrolysis are promising single-atom catalysts but often also comprise metallic particles. Here, we pyrolytically synthesize a Co–N–C material only comprising atomically dispersed cobalt ions and identify with X-ray absorption spectroscopy, magnetic susceptibility measurements and density functional theory the structure and electronic state of three porphyrinic moieties, CoN 4 C 12 , CoN 3 C 10,porp and CoN 2 C 5 . The O 2 electro-reduction and operando X-ray absorption response are measured in acidic medium on Co–N–C and compared to those of a Fe–N–C catalyst prepared similarly. We show that cobalt moieties are unmodified from 0.0 to 1.0 V versus a reversible hydrogen electrode, while Fe-based moieties experience structural and electronic-state changes. On the basis of density functional theory analysis and established relationships between redox potential and O 2 -adsorption strength, we conclude that cobalt-based moieties bind O 2 too weakly for efficient O 2 reduction. Nitrogen-doped carbon materials with atomically dispersed iron or cobalt are promising for catalytic use. Here, the authors show that cobalt moieties have a higher redox potential, bind oxygen more weakly and are less active toward oxygen reduction than their iron counterpart, despite similar coordination.

Photocatalytic two-electron oxygen reduction to produce high-value hydrogen peroxide (H 2 O 2 ) is gaining popularity as a promising avenue of research. However, structural evolution mechanisms of catalytically active sites in the entire photosynthetic H 2 O 2 system remains unclear and seriously hinders the development of highly-active and stable H 2 O 2 photocatalysts. Herein, we report a high-loading Ni single-atom photocatalyst for efficient H 2 O 2 synthesis in pure water, achieving an apparent quantum yield of 10.9% at 420 nm and a solar-to-chemical conversion efficiency of 0.82%. Importantly, using in situ synchrotron X-ray absorption spectroscopy and Raman spectroscopy we directly observe that initial Ni-N 3 sites dynamically transform into high-valent O 1 -Ni-N 2 sites after O 2 adsorption and further evolve to form a key *OOH intermediate before finally forming HOO-Ni-N 2 . Theoretical calculations and experiments further reveal that the evolution of the active sites structure reduces the formation energy barrier of *OOH and suppresses the O=O bond dissociation, leading to improved H 2 O 2 production activity and selectivity. Here, the authors explore how Ni single-atom sites on carbon nitride evolve under photocatalytic conditions. They show that this evolution plays a pivotal role in enhancing photocatalytic H 2 O 2 production.

Transition metal chalcogenides have been identified as low-cost and efficient electrocatalysts to promote the hydrogen evolution reaction in alkaline media. However, the identification of active sites and the underlying catalytic mechanism remain elusive. In this work, we employ operando X-ray absorption spectroscopy and near-ambient pressure X-ray photoelectron spectroscopy to elucidate that NiS undergoes an in-situ phase transition to an intimately mixed phase of Ni 3 S 2 and NiO, generating highly active synergistic dual sites at the Ni 3 S 2 /NiO interface. The interfacial Ni is the active site for water dissociation and OH* adsorption while the interfacial S acts as the active site for H* adsorption and H 2 evolution. Accordingly, the in-situ formation of Ni 3 S 2 /NiO interfaces enables NiS electrocatalysts to achieve an overpotential of only 95 ± 8 mV at a current density of 10 mA cm −2 . Our work highlighted that the chemistry of transition metal chalcogenides is highly dynamic, and a careful control of the working conditions may lead to the in-situ formation of catalytic species that boost their catalytic performance. Transition metal chalcogenides are effective and economical electrocatalysts for the hydrogen evolution reaction in alkaline media, yet active sites and catalytic mechanisms remain unclear. Here the authors use operando spectroscopy to study the in-situ conversion of NiS to highly active Ni 3 S 2 /NiO dual-site catalysts for the alkaline hydrogen evolution reaction.

Hydrogen evolution reaction (HER) in neutral media is of great practical importance for sustainable hydrogen production, but generally suffers from low activities, the cause of which has been a puzzle yet to be solved. Herein, by investigating the synergy between Ru single atoms (RuNC) and RuSe x cluster compounds (RuSe x ) for HER using ab initio molecular dynamics, operando X-ray absorption spectroscopy, and operando surface-enhanced infrared absorption spectroscopy, we establish that the interfacial water governs neutral HER. The rigid interfacial water layer in neutral media would inhibit the transport of H 2 O*/OH* at the electrode/electrolyte interface of RuNC, but the RuSe x can promote H 2 O*/OH* transport to increase the number of available H 2 O* on RuNC by disordering the interfacial water network. With the synergy of RuSe x and RuNC, the resulting neutral HER performance in terms of mass-specific activity is 6.7 times higher than that of 20 wt.% Pt/C at overpotential of 100 mV. Understanding the slow kinetics of hydrogen evolution reaction in neutral media is of fundamental importance for the rational design of high-performance electrocatalysts for hydrogen energy. Here, by studying Ru single atom and RuSe x cluster, the authors report how the rate of hydrogen evolution reaction activity in neutral media is governed by interfacial water.

Achieving active and stable oxygen evolution reaction (OER) in acid media based on single-atom catalysts is highly promising for cost-effective and sustainable energy supply in proton electrolyte membrane electrolyzers. Here, we report an atomically dispersed Ru 1 -N 4 site anchored on nitrogen-carbon support (Ru-N-C) as an efficient and durable electrocatalyst for acidic OER. The single-atom Ru-N-C catalyst delivers an exceptionally intrinsic activity, reaching a mass activity as high as 3571 A g metal −1 and turnover frequency of 3348 O 2 h −1 with a low overpotential of 267 mV at a current density of 10 mA cm −2 . The catalyst shows no evident deactivation or decomposition after 30-hour operation in acidic environment. Operando synchrotron radiation X-ray absorption spectroscopy and infrared spectroscopy identify the dynamic adsorption of single oxygen atom on Ru site under working potentials, and theoretical calculations demonstrate that the O-Ru 1 -N 4 site is responsible for the high OER activity and stability. Monitoring catalyst structural changes under working conditions is crucial for understanding how catalysts operate. Here, authors examine single-atom Ru electrocatalyst by operando synchrotron spectroscopies to identify the catalytic mechanism during the acidic oxygen evolution reaction.

Copper electrocatalysts have been shown to selectively reduce carbon dioxide to hydrocarbons. Nevertheless, the absence of a systematic study based on time-resolved spectroscopy renders the functional agent—either metallic or oxidative Copper—for the selectivity still undecidable. Herein, we develop an operando seconds-resolved X-ray absorption spectroscopy to uncover the chemical state evolution of working catalysts. An oxide-derived Copper electrocatalyst is employed as a model catalyst to offer scientific insights into the roles metal states serve in carbon dioxide reduction reaction (CO 2 RR). Using a potential switching approach, the model catalyst can achieve a steady chemical state of half-Cu(0)-and-half-Cu(I) and selectively produce asymmetric C 2 products - C 2 H 5 OH. Furthermore, a theoretical analysis reveals that a surface composed of Cu-Cu(I) ensembles can have dual carbon monoxide molecules coupled asymmetrically, which potentially enhances the catalyst’s CO 2 RR product selectivity toward C 2 products. Our results offer understandings of the fundamental chemical states and insights to the establishment of selective CO 2 RR. A systematic time-resolved study can provide key insights on selective carbon dioxide electro-reduction. Here, the authors report operando seconds-resolved X-ray absorption spectroscopy to uncover the chemical state evolution of working catalysts in a carbon dioxide electroreduction process.

Designing active and stable electrocatalysts with economic efficiency for acidic oxygen evolution reaction is essential for developing proton exchange membrane water electrolyzers. Herein, we report on a cobalt oxide incorporated with iridium single atoms (Ir-Co 3 O 4 ), prepared by a mechanochemical approach. Operando X-ray absorption spectroscopy reveals that Ir atoms are partially oxidized to active Ir >4+ during the reaction, meanwhile Ir and Co atoms with their bridged electrophilic O ligands acting as active sites, are jointly responsible for the enhanced performance. Theoretical calculations further disclose the isolated Ir atoms can effectively boost the electronic conductivity and optimize the energy barrier. As a result, Ir-Co 3 O 4 exhibits significantly higher mass activity and turnover frequency than those of benchmark IrO 2 in acidic conditions. Moreover, the catalyst preparation can be easily scaled up to gram-level per batch. The present approach highlights the concept of constructing single noble metal atoms incorporated cost-effective metal oxides catalysts for practical applications. Designing active, stable and cost-effective catalysts for the acidic oxygen evolution reaction remains a challenge. Here, the authors report iridium single atoms incorporated cobalt oxides, showing distinctly enhanced performance in the acid.

High-temperature superconductivity was discovered in the pressurized nickelate La 3 Ni 2 O 7 which has a unique bilayer structure and mixed valence state of nickel. The properties at ambient pressure contain crucial information of the fundamental interactions and bosons mediating superconducting pairing. Here, using X-ray absorption spectroscopy and resonant inelastic X-ray scattering, we identified that Ni 3 d x 2 − y 2 , Ni 3 d z 2 , and ligand oxygen 2 p orbitals dominate the low-energy physics with a small charge-transfer energy. Well-defined optical-like magnetic excitations soften into quasi-static spin-density-wave ordering, evidencing the strong electronic correlation and rich magnetic properties. Based on an effective Heisenberg spin model, we extract a much stronger inter-layer effective magnetic superexchange than the intra-layer ones and propose two viable magnetic structures. Our findings emphasize that the Ni 3 d z 2 orbital bonding within the bilayer induces novel electronic and magnetic excitations, setting the stage for further exploration of La 3 Ni 2 O 7 superconductor. It was recently found that a certain nickelate compound, La 3 Ni 2 O 7 , at moderately high pressures has a superconducting phase that persists to above liquid nitrogen temperatures. Here, by studying the parent phase at ambient pressure, Chen et al uncover rich magnetic properties and show the vital role of the strong bonding of the inter-layer Ni orbitals in the magnetic and electronic excitations.

Direct functionalization of methane in natural gas remains a key challenge. We present a direct stepwise method for converting methane into methanol with high selectivity (~97%) over a copper-containing zeolite, based on partial oxidation with water. The activation in helium at 673 kelvin (K), followed by consecutive catalyst exposures to 7 bars of methane and then water at 473 K, consistently produced 0.204 mole of CH₃OH per mole of copper in zeolite. Isotopic labeling confirmed water as the source of oxygen to regenerate the zeolite active centers and renders methanol desorption energetically favorable. On the basis of in situ x-ray absorption spectroscopy, infrared spectroscopy, and density functional theory calculations, we propose a mechanism involving methane oxidation at CuII oxide active centers, followed by CuI reoxidation by water with concurrent formation of hydrogen.

A general approach to promote IR light-driven CO 2 reduction within ultrathin Cu-based hydrotalcite-like hydroxy salts is presented. Associated band structures and optical properties of the Cu-based materials are first predicted by theory. Subsequently, Cu 4 (SO 4 )(OH) 6 nanosheets were synthesized and are found to undergo cascaded electron transfer processes based on d - d orbital transitions under infrared light irradiation. The obtained samples exhibit excellent activity for IR light-driven CO 2 reduction, with a production rate of 21.95 and 4.11 μmol g −1 h −1 for CO and CH 4 , respectively, surpassing most reported catalysts under the same reaction conditions. X-ray absorption spectroscopy and in situ Fourier-transform infrared spectroscopy are used to track the evolution of the catalytic sites and intermediates to understand the photocatalytic mechanism. Similar ultrathin catalysts are also investigated to explore the generality of the proposed electron transfer approach. Our findings illustrate that abundant transition metal complexes hold great promise for IR light-responsive photocatalysis. This study demonstrates the effectiveness and generality of utilizing ultrathin Cu-based hydrotalcite-like hydroxy salts as catalysts for infrared light-driven CO 2 reduction based on their d-d orbital transition mechanism.