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A rapid dual flame synthesis is developed for one-step preparation and modification of double modified ZnO catalysts with carbon doping and oxygen vacancies. The growth mechanism and nanostructure evolution are also investigated. The double modified ZnO with carbon doping and oxygen vacancies exhibies excellent performance in CO 2 reduction to syngas, with a high current density of 14.02 mA/cm2 at −1.1 V vs. RHE and a maximum Faradaic efficiency of 71% for CO production. The maximum adjustable range of the CO/H 2 ratio from 0.5 to 2.7. In a flow cell, the double modified ZnO exhibits a large current density of 126.8 mA/cm2 at −1.2 V vs. RHE. The maximum Faradaic efficiency for CO 2 reduction to CO is 74.8% at 50 mA/cm2. Double modified ZnO maintains stable potential and Faradaic efficiency for over 5 h of electrolysis. Operando X-ray absorption spectroscopy and density functional theory calculations reveal that the active site is the Zn–Zn configuration, and that carbon dopant and oxygen vacancy promote CO 2 activation and CO desorption, and reduce reaction overpotential of CO 2 RR. • The objective is to efficiently convert CO 2 to syngas using renewable energy. • Carbon doping and oxygen vacancy are employed to modify the electrocatalyst. • Dual flame synthesis is proposed for fabricating the double modified catalysts. • Double modifications can enhance catalytic activity and alter selectivity. • Operando XAS reveals that Zn sites serve as the active sites for CO 2 reduction. [ABSTRACT FROM AUTHOR]

Metal‐organic frameworks (MOFs) have exhibited encouraging catalytic activity for the oxygen evolution reaction (OER), a crucial process for water electrolysis to produce green hydrogen. Nonetheless, distinguishing the source of catalytic activity and establishing the structure‐composition‐property relationships of MOFs during OER processes remain challenging. Here, for the first time, operando X‐ray absorption spectroscopy (XAS) is utilized to monitor the structural evolution and identify the active components of ferrocene‐based MOFs (Ni‐Fc) for OER. Ligand‐defect‐rich Ni‐Fc is synthesized via the co‐deposition method. After electrochemical activation, Ni‐Fc exhibits superior electrocatalytic activity (228 mV at 10 mA cm−2 in 0.1 m KOH), which is highly competitive compared with state‐of‐the‐art electrocatalysts. Operando XAS analysis and ex‐situ characterizations reveal the structural reconstruction of Ni‐Fc into amorphous NiFe‐catalysts (a‐NiFe) during the activation process, and further into real catalytic phases (a‐NiFe‐C) under catalytic potential greater than 1.45 V (vs RHE). In catalytic phases, in‐situ formed deprotonated and oxygen‐defected Ni oxyhydroxide analogues act as catalytic sites, while Fe hydroxide analogues derived from ligands optimize the electronic structure of Ni sites for improving OER activity. Density functional theory (DFT) analysis indicates a reduced energy barrier in a‐NiFe‐C compared to pristine MOFs, supporting the improved catalytic activity of the latter. [ABSTRACT FROM AUTHOR]

The efficiency of the synthesis of renewable fuels and feedstocks from electrical sources is limited, at present, by the sluggish water oxidation reaction. Single-atom catalysts (SACs) with a controllable coordination environment and exceptional atom utilization efficiency open new paradigms toward designing high-performance water oxidation catalysts. Here, using operando X-ray absorption spectroscopy measurements with calculations of spectra and electrochemical activity, we demonstrate that the origin of water oxidation activity of IrNiFe SACs is the presence of highly oxidized Ir single atom (Ir5.3+) in the NiFe oxyhydroxide under operating conditions. We show that the optimal water oxidation catalyst could be achieved by systematically increasing the oxidation state and modulating the coordination environment of the Ir active sites anchored atop the NiFe oxyhydroxide layers. Based on the proposed mechanism, we have successfully anchored Ir single-atom sites on NiFe oxyhydroxides (Ir0.1/Ni9Fe SAC) via a unique in situ cryogenic-photochemical reduction method that delivers an overpotential of 183 mV at 10 mA ⋅ cm-2 and retains its performance following 100 h of operation in 1 M KOH electrolyte, outperforming the reported catalysts and the commercial IrO2 catalysts. These findings open the avenue toward an atomic-level understanding of the oxygen evolution of catalytic centers under in operando conditions.

The rational design of high-performance electrocatalysts requires a detailed understanding of dynamic changes in catalyst properties, including oxidation states, surface area, and morphology under realistic working conditions. Oxide-derived Cu catalysts exhibit a remarkable selectivity toward multicarbon products for the electrochemical CO reduction reaction (CORR), but the exact role of the oxide remains elusive for explaining the performance enhancements. Here, we used operando X-ray absorption spectroscopy (XAS) coupled with simultaneous measurements of the catalyst activity and selectivity by gas chromatography (GC) to study the relationship between oxidation states of Cu-based catalysts and the activity for ethylene (C2H4) production in a CO gas-fed cell. By utilizing a custom-built XAS cell, oxidation states of Cu catalysts can be probed in device-relevant settings and under high current densities (>80 mA cm-2) for the CORR. By employing an electrochemical oxidation process, we found that the Cu oxidation states and specific ion species do not correlate with C2H4 production. The difference in the CORR activity is also investigated in relation to electrochemical surface area (ECSA) changes. While the hydrogen evolution reaction (HER) activity is positively correlated to the ECSA changes, the increased C2H4 activity is not proportional to the ECSA. Ex situ characterization from microscopic techniques suggests that the changes in the C2H4 activity and selectivity may arise from a morphological transformation that evolves into a more active structure. These comprehensive results give rise to the development of a cell regeneration method that can restore the performance of the Cu catalyst without cell disassembly. Our study establishes a basis for the rational design of highly active electrocatalysts for broad-range reactions in a gas-fed device.

In this study, ex situ and operando X‐ray absorption spectroscopy (XAS) is employed to shed light on structure and degradation mechanism of Fe‐based catalysts for the oxygen reduction reaction (ORR) in proton exchange membrane fuel cells (PEMFCs). Ex situ XAS on pristine Fe0.07Zr0.93O2‐δ/C catalyst confirms the incorporation of Fe3+ in the ZrO2 structure and clearly exclude any significant presence of Fe−N−C‐type structures. The edge shift in data on in‐house aged samples demonstrates a mixed oxidation state of Fe (Fe3+ and Fe2+), consistent with Fe demetalation from the ZrO2 structure. Furthermore, a more symmetric coordination in the pre‐edge shape points towards the formation of oxidic Fe clusters upon aging. Fe demetalation is inferred also from the edge shift to higher energy (presence of Fe3+) in operando XAS data at 0.3 V, due to Fe phases not electrically polarizable/reducible at the applied voltage. Electrochemical data exclude any correlation between extent of aging and type of test, also for a commercial Fe−N−C catalyst by Pajarito Powder. The observed faster aging for Fe0.07Zr0.93O2‐δ compared to Fe−N−C is attributed to an improved mass transport to/from active sites, manifest also in very similar initial current densities at 0.3 V, despite much higher catalyst activity for Fe−N−C. [ABSTRACT FROM AUTHOR]

A hybrid catalyst-bonded membrane device using gaseous reactants for a carbon monoxide reduction (COR) reaction in the cathode chamber, an aqueous electrolyte for an oxygen evolution reaction (OER) in the anode chamber, and an anion exchange membrane (AEM) for product separation was modeled, constructed, and tested. The Cu electrocatalyst was electrodeposited onto gas diffusion layers (GDLs) and was directly bonded to AEM by mechanical pressing in the hybrid device. The impacts of relative humidity at the cathode inlet on the selectivity and activity of COR were investigated by computational modeling and experimental methods. At a relative humidity of 30%, the Cu-based catalyst in the hybrid device exhibited a total operating current density of 87 mA cm-2 with a -2.0 V vs Ag/AgCl reference electrode, a Faradaic efficiency (FE) for C2H4 generation of 32.6%, and an FE for a liquid-based carbon product of 42.6%. Significant improvements in the partial current densities for COR were observed in relation to planar electrodes or flooded gas diffusion electrodes (GDEs). In addition, a custom test bed was constructed to characterize the oxidation states of the Cu catalysts in real time along with product analysis though the backside of the GDLs via operando X-ray absorption (XAS) measurements.

Abstract In this study, ex situ and operando X‐ray absorption spectroscopy (XAS) is employed to shed light on structure and degradation mechanism of Fe‐based catalysts for the oxygen reduction reaction (ORR) in proton exchange membrane fuel cells (PEMFCs). Ex situ XAS on pristine Fe0.07Zr0.93O2‐δ/C catalyst confirms the incorporation of Fe3+ in the ZrO2 structure and clearly exclude any significant presence of Fe−N−C‐type structures. The edge shift in data on in‐house aged samples demonstrates a mixed oxidation state of Fe (Fe3+ and Fe2+), consistent with Fe demetalation from the ZrO2 structure. Furthermore, a more symmetric coordination in the pre‐edge shape points towards the formation of oxidic Fe clusters upon aging. Fe demetalation is inferred also from the edge shift to higher energy (presence of Fe3+) in operando XAS data at 0.3 V, due to Fe phases not electrically polarizable/reducible at the applied voltage. Electrochemical data exclude any correlation between extent of aging and type of test, also for a commercial Fe−N−C catalyst by Pajarito Powder. The observed faster aging for Fe0.07Zr0.93O2‐δ compared to Fe−N−C is attributed to an improved mass transport to/from active sites, manifest also in very similar initial current densities at 0.3 V, despite much higher catalyst activity for Fe−N−C.

The construction and understanding of synergy in well‐defined dual‐atom active sites is an available avenue to promote multistep tandem catalytic reactions. Herein, we construct a dual‐hetero‐atom catalyst that comprises adjacent Cu‐N4 and Se‐C3 active sites for efficient oxygen reduction reaction (ORR) activity. Operando X‐ray absorption spectroscopy coupled with theoretical calculations provide in‐depth insights into this dual‐atom synergy mechanism for ORR under realistic device operation conditions. The heteroatom Se modulator can efficiently polarize the charge distribution around symmetrical Cu‐N4 moieties, and serve as synergistic site to facilitate the second oxygen reduction step simultaneously, in which the key OOH*‐(Cu1‐N4) transforms to O*‐(Se1‐C2) intermediate on the dual‐atom sites. Therefore, this designed catalyst achieves satisfied alkaline ORR activity with a half‐wave potential of 0.905 V vs. RHE and a maximum power density of 206.5 mW cm−2 in Zn‐air battery. [ABSTRACT FROM AUTHOR]

High-entropy-alloy (HEA) nanocrystals hold immense potential for catalysis, offering virtually unlimited alloy combinations through the inclusion of at least five constituent elements in varying ratios. However, general and effective strategies for synthesizing libraries of HEA nanocrystals with controlled surface atomic structures remain scarce. In this study, a transferable strategy for developing a library of facet-controlled seed@HEA nanocrystals through seed-mediated growth is presented. The synthesis of seed@HEA core-shell nanocrystals incorporating up to ten different metallic elements, with control over the number of solid-solution HEA atomic layers is demonstrated. Epitaxial HEA growth on nanocrystal seeds with low-index and high-index facets leads to the formation of seed@HEA catalyst library with composition- and facet-dependent catalytic activities in both electrocatalysis and photocatalysis. In situ synchrotron X-ray absorption spectroscopy and density-functional theory calculations are employed to identify surface active sites of the HEA, rationalizing the high level of catalytic activities achieved. This work enables facet engineering in the multi-elemental chemical space and unveils the critical needs for their future development toward catalysis., (© 2024 Wiley‐VCH GmbH.)

Accelerating the alkaline hydrogen evolution reaction (HER), which involves the slow cleavage of HO-H bonds and the adsorption/desorption of hydrogen (H*) and hydroxyl (OH*) intermediates, requires developing catalysts with optimal binding strengths for these intermediates. Here, the unconventional hexagonal close-packed (HCP) high-entropy alloy (HEA) atomic layers are prepared composed of five platinum-group metals to enhance the alkaline HER synergistically. The breakthrough is made by layer-by-layer heteroepitaxial deposition of subnanometer RuRhPdPtIr HEA layers on the HCP Ru seeds, despite the thermodynamic stability of Rh, Pd, Pt, and Ir in a face-centered cubic (FCC) structure except for Ru. The synchrotron X-ray absorption spectroscopy (XAS) confirms the atomic mixing and coordination environment of HCP RuRhPdPtIr HEA. Most importantly, they exhibit notable improvements in both electrocatalytic activity and durability for the HER in an alkaline environment, as compared to their FCC RuRhPdPtIr counterparts. Electrochemical measurements, operando XAS analysis, and density functional theory unveil that the binding strengths of H* and OH* intermediates on the active Pt and Ir sites can be weakened and strengthened to a moderate level, respectively, by mixing non-active Ru, Rh, and Pd atoms with Pt and Ir atoms within the HCP HEA with strong synergistic electronic effects., (© 2024 The Author(s). Advanced Science published by Wiley‐VCH GmbH.)

This study provides evidence of the enhancement of the oxygen reduction reaction (ORR) activity of Pt/C and Pd-core Pt-shell catalysts upon ionic liquid modification. Under high potential at 1.15 V (vs. reversible hydrogen electrode), the lower coverage of the oxide species on the Pt surface modified with [7-methyl-1,5,7-triazabicyclo[4.4.0]dec-5-ene (MTBD)][bis(trifluoromethylsulfonyl)imide (NTf2)] ionic liquid (IL) was confirmed by electrochemical analysis. Operando X-ray absorption spectra (XAS) of the ionic liquid-modified Pd-core Pt-shell catalyst displayed smaller 5d orbital vacancies than the naked catalyst. Shrinkage of Pt-Pt bond length was also displayed, indicating the suppression of oxidation. Thus, we conclude that the IL-modified systems are effective in avoiding the adsorption of oxide species on the Pt surface and improving the oxygen reduction activity.

X‐ray absorption spectroscopy (XAS) is one of the powerful operando tools to track structural variations in heterogeneous catalysts. The nature of active sites in catalyst research is of great relevance, especially given the growing importance of energy storage using CO2 as feedstock and the need for dynamic availability of electric power. Due to the pressure/temperature prerequisite of catalyst performance, the characterization of catalyst structure during catalysis under such high‐pressure reaction conditions is important to further improve catalyst design at a molecular level. Investigating catalysts in a controlled reaction atmosphere, while probing with X‐rays, offers an excellent opportunity for developing infrastructure at the synchrotron. Herein, a mobile setup with a robust spectroscopic cell for in situ and operando XAS applications, including a high‐pressure gas dosing equipment for such catalytic systems, is presented. The in situ/operando cell is operational for both the transmission and the fluorescence XAS mode at up to 50 bar and 450 °C. The setup comes with a protective box with Kapton windows, which holds the cell and serves as a miniature fume hood, and on‐line product analysis. Furthermore, the gas dosing equipment is compact, light‐weighted and can be easily transported to different synchrotrons and allows an optimum pre‐mix of gas flows and pressure build‐up. Methanol and Fischer‐Tropsch syntheses are used as examples for the highly flexible instrumentation. [ABSTRACT FROM AUTHOR]

Identifying real active sites and understanding the mechanism of oxygen evolution reaction (OER) are still a big challenge today for developing efficient electrochemical catalysts in renewable energy technologies. Here, using a combined in situ/operando experiments and theory, the catalytic mechanism of the ordered OER active Co and Ir ions in Sr2CoIrO6−δ is studied, which exhibits an unprecedented low overpotential 210 mV to achieve 10 mA cm–2, ranking the highest performance among perovskite‐based solid‐state catalysts. Operando X‐ray absorption spectroscopies as a function of applied voltage indicates that Ir4+ ion is gradually converted into extremely high‐valence Ir5+/6+, while the part of Co3+ ion is transferred into Co4+ under OER process. Density functional theory calculations explicitly reveal the order Co‐O‐Ir network as an origin of ultrahigh OER activity. The work opens a promising path to overcome the sluggish kinetics of OER bottleneck for water splitting via proper arrangements of the multi‐active sites in catalyst. [ABSTRACT FROM AUTHOR]

The efficiency of the synthesis of renewable fuels and feedstocks from electrical sources is limited, at present, by the sluggish water oxidation reaction. Single-atom catalysts (SACs) with a controllable coordination environment and exceptional atom utilization efficiency open new paradigms toward designing high-performance water oxidation catalysts. Here, using operando X-ray absorption spectroscopy measurements with calculations of spectra and electrochemical activity, we demonstrate that the origin of water oxidation activity of IrNiFe SACs is the presence of highly oxidized Ir single atom (Ir5.3+) in the NiFe oxyhydroxide under operating conditions. We show that the optimal water oxidation catalyst could be achieved by systematically increasing the oxidation state and modulating the coordination environment of the Ir active sites anchored atop the NiFe oxyhydroxide layers. Based on the proposed mechanism, we have successfully anchored Ir single-atom sites on NiFe oxyhydroxides (Ir0.1/Ni9Fe SAC) via a unique in situ cryogenic-photochemical reduction method that delivers an overpotential of 183 mV at 10 mA ⋅ cm-2 and retains its performance following 100 h of operation in 1 M KOH electrolyte, outperforming the reported catalysts and the commercial IrO2 catalysts. These findings open the avenue toward an atomic-level understanding of the oxygen evolution of catalytic centers under in operando conditions. [ABSTRACT FROM AUTHOR]

x%Ni/CeO 2 (x = 3, 6, 9, 12 and 18 wt%) catalysts were synthesized for CO 2 methanation reaction. The catalysts were prepared via sol-gel method with co-adding of citric acid to improve Ni dispersion. 18 wt%Ni/CeO 2 exhibited the highest catalytic performance. An in-situ experiment during CO 2 methanation for monitoring the oxidation state changes of Ni and Ce was conducted by TRXAS. The oxidation states of Ni2+ and Ce4+ were reduced upon increasing the reduction temperature. Moreover, Nio and Ce3+ remained unchanged during reaction. The operando XAS experiment was performed to investigate the electronic state changes of Ni species during fluctuating conditions. The state of Nio remained unchanged upon H 2 dropout and re-purging, which indicated that Ni species were completely re-reduced after H 2 was added into the feed stream again. The completely reduced behavior of Ni active species was caused by highly dispersed Ni by the addition of citric acid. [Display omitted] • CeO 2 -supported nickel catalyst was synthesized by sol-gel method with co-adding of citric acid. • 18 wt%Ni/CeO 2 catalyst exhibited the highest CO 2 methanation catalytic activity. • Added Ni was incorporated into CeO 2 lattice and those excess amount was dispersed on CeO 2 surface. • The role of citric acid was to disperse Ni by chelating process. • High dispersion of Ni species resulted in high stability of catalyst under fluctuating condition. [ABSTRACT FROM AUTHOR]

Direct conversion of carbon dioxide into multicarbon liquid fuels by the CO2 electrochemical reduction reaction (CO2RR) can contribute to the decarbonization of the global economy. Here, well‐defined Cu2O nanocubes (NCs, 35 nm) uniformly covered with Ag nanoparticles (5 nm) were synthesized. When compared to bare Cu2O NCs, the catalyst with 5 at % Ag on Cu2O NCs displayed a two‐fold increase in the Faradaic efficiency for C2+ liquid products (30 % at −1.0 VRHE), including ethanol, 1‐propanol, and acetaldehyde, while formate and hydrogen were suppressed. Operando X‐ray absorption spectroscopy revealed the partial reduction of Cu2O during CO2RR, accompanied by a reaction‐driven redispersion of Ag on the CuOx NCs. Data from operando surface‐enhanced Raman spectroscopy further uncovered significant variations in the CO binding to Cu, which were assigned to Ag−Cu sites formed during CO2RR that appear crucial for the C−C coupling and the enhanced yield of liquid products. [ABSTRACT FROM AUTHOR]

Nickel‐based oxide catalysts are widely used for the oxygen evolution reaction (OER) in alkaline water electrolysis because of their low cost and high activity. In particular, the LiNiO2 catalyst shows high activity. Therefore, to elucidate the fundamental relationship between the local structure, catalyst activity, and stability of LiNiO2, we investigated the cation mixing effect by mixing sites of lithium and nickel ions in the LiNiO2‐based catalysts. Lower degrees of cation mixing lead to higher intrinsic OER activity but lower long‐term stability. The X‐ray absorption spectra (XAS) displayed a strong hybridization state of the Ni 3d and O 2p orbitals, which is the origin of the different catalytic activity behaviors. Meanwhile, operando XAS studies combined with potentiostatic stability tests and inductively coupled plasma optical emission spectrometry (ICP‐OES) demonstrated the Li ion loss during the OER process. Thus, the instability of LiNiO2 originates from de‐intercalation of Li ions and this irreversible structure change deteriorates the performance. Hindering the lithium diffusion path by cation mixing is a useful strategy for maintaining performance. This strategy could provide a novel design principle for compatible high activity and long‐lasting catalysts by reasonable structure mediation. [ABSTRACT FROM AUTHOR]

• A set of highly complementary operando techniques (Advanced operando X-ray absorption spectroscopy (XAS), X-ray diffraction (XRD) and Raman spectroscopy) sensitive to potential-dependent alteration of highly porous oxidic precursor materials used for the electrochemical reduction of CO 2 into value-added products such as hydrocarbons and higher alcohols. • This complementary and holistic approach of 'bulk'- and surface-sensitive techniques demonstrates that the electro-reduction of porous Cu x O foams into metallic Cu is completed before CO 2 RR products formation sets in. • There are substantial differences in the particular potential dependence of the oxide reduction when comparing the 'bulk' with the respective 'surface' processes. • It is valuable in comparison with the bulk-sensitive XAS and XRD techniques which both indicate oxide-metal transitions that are 'delayed' on the potential scale with respect to what is observed in the surface-sensitive Raman spectroscopy. Copper oxides have recently emerged as promising precursor catalyst materials demonstrating enhanced reactivity and selectivity towards C2 and C3 products like ethylene, ethanol, and n-propanol generated from the direct electro-reduction reaction of CO 2 (denoted as CO 2 RR). Advanced operando X-ray absorption spectroscopy (XAS), X-ray diffraction (XRD) and Raman spectroscopy were employed to probe the potential-dependent changes of the chemical states of Cu species in the Cu oxide foams (referred to as Cu x O) before and during the CO 2 RR. This complementary and holistic approach of 'bulk'- and surface-sensitive techniques demonstrates that the electro-reduction of Cu x O foams into metallic Cu is completed before hydrocarbon (e.g., ethylene, ethane) and alcohol (e.g., ethanol, n-propanol) formation sets in. There are, however, substantial differences in the potential dependence of the oxide reduction when comparing the 'bulk' with the respective 'surface' processes. Only in the very initial stage of the CO 2 RR, the reduction of the Cu oxide precursor species is temporarily superimposed on the production of CO and H 2. Complementary identical location (IL) SEM analysis of the Cu x O foams prior to and after the CO 2 RR reveals a significant alteration in the surface morphology caused by the appearance of smaller Cu nanoparticles formed by the reduction process of Cu x O species. [ABSTRACT FROM AUTHOR]

We report the effects of alloying Cu into silica supported Pd nanoparticles on the catalytic activity, surface composition, and particle structure during CO oxidation Pd is highly active for CO oxidation but is inhibited by relevant competitive reagents (e.g. NO) at low temperatures (< 150 °C). By alloying Cu into Pd nanoparticles, NO inhibition is suppressed without any CO oxidation activity loss. Infrared spectroscopy illustrates the formation of surface nitrosyl (NO-Pd) is eliminated in the PdCu alloy catalyst, preventing CO oxidation inhibition by NO. X-ray absorption spectroscopy studies show that Pd in the monometallic catalyst forms a surface oxide after light-off. Conversely, in the PdCu catalyst, Pd remains metallic during reaction, independent of temperature, whereas the Cu oxidizes when the catalyst becomes active, after light-off. Furthermore, DRIFTS studies show that the Cu segregates to the surface as the catalyst becomes active for diesel oxidation, presumably becoming the O 2 adsorption site. [Display omitted] • PdCu alloys are more resistant than monometallic Pd to NO poisoning during CO oxidation. • FT-IR results show that the presence of Cu prevents surface nitrosyl formation on PdCu alloys. • Cu migrates to the surface of the catalyst nanoparticles during CO oxidation. • XAS measurements show Cu prevents the formation of Pd surface oxide during CO oxidation independent of reaction conditions. [ABSTRACT FROM AUTHOR]

The nature of the active phase (metallic vs. oxidic, metal phase vs. concentrated hydride/ dilute solid solution with hydrogen) in heterogeneous catalysis by supported metals is still a matter of high debate. Here, we have monitored for the first time oxide-supported Pd nanocatalysts (average particle size 4.5 nm) during both CO oxidation (in H 2 -free atmosphere) and preferential oxidation of CO in H 2 excess (PROX) by operando X-ray absorption spectroscopy. Under our conditions, the CO conversion in the absence of H 2 is around 30% at 150 °C and reaches 100% at 200 °C, whereas in the presence of H 2 the conversion reaches a maximum of 15% (at 250 °C), in agreement with our previous works using a conventional flow-fixed bed reactor. The active phase for CO oxidation below 200 °C is metallic Pd, whereas it is a solid solution of Pd with hydrogen during PROX below 300 °C. This work provides a direct evidence of the presence of subsurface/bulk hydrogen as a probable cause of the low PROX performance of supported Pd catalysts. [ABSTRACT FROM AUTHOR]

Operando assessment of electronic properties near the Fermi level is essential to gain new insights into the mechanisms of electrochemical reactions as well as for the development of more efficient electrocatalysts. However, the high vacuum ambient needed for X-ray absorption measurements at low energies has made studies under electrochemical conditions quite challenging. Here, we describe an out-of-chamber setup with a new electrochemical cell that allowed us to performed operando X-ray absorption studies at the low energy of the Pd L3 edge. Using the new electrochemical cell, we were able to probe, for the first time, the changes in electronic properties of carbon-supported Pd nanoparticles induced by the electrochemical oxidation of the Pd surface. Our results demonstrate that the oxidation process produces an increase in the Pd 4d band vacancy, which indicates that charge is transferred from the metal to the adsorbed oxygenated species. [ABSTRACT FROM AUTHOR]

Heterostructured compounds with rationally engineered interfaces are widely reported as efficient oxygen evolution reaction (OER) electrocatalysts, but the mechanism of the intrinsic activities still unclear. Herein, a typical model catalyst of NiSe 2 /FeSe 2 heterostructure with a low overpotential of 230 mV at 10 mA cm−2 and long-term durability is fabricated by selenizing Ni-Fe Prussian-blue analog (PBA) templates. In-situ/operando Raman spectroscopy and synchrotron-based X-ray absorption spectroscopy (XAS) are employed to reveal the real catalytic process. It is attested that the dynamic surface reconstruction into Ni-O-Fe configurations, and the high-valence NiIII-O-FeIII moieties in Ni–FeOOH is responsible for the high oxygen evolution. Density functional theory (DFT) simulations further confirm that the electron transfer driven by strong Ni-Fe synergistic effect in NiSe 2 /FeSe 2 heterostructure not only improves the density of Ni/Fe active sites, but also modulates the nature active sites. This works may deepen our understanding of intrinsic activities of metal compound heterostructures and the correlation between dynamic surface structural evolution and catalytic activity. [Display omitted] • The interfacial component coupling effects for NiSe 2 /FeSe 2 heterostructure are comprehensively proposed. • The dynamic evolution at the surfaces of NiSe 2 /FeSe 2 using viable operando XAS as well as In situ Raman Spectra. • Both Ni and Fe atoms are catalytic sites and the high-valence NiⅢ-O-FeⅢ bridge-mode acts as the real activity for OER. • The dual-metal-centered synergistic mechanism of NiSe 2 /FeSe 2 during the OER process is identified by DFT calculations. [ABSTRACT FROM AUTHOR]

A dynamic structural behavior of Pt nanoparticles on the ceria surface under reducing/oxidizing conditions was found at moderate temperatures (<500 °C) and exploited to enhance the catalytic activity of Pt/CeO2-based exhaust gas catalysts. Redispersion of platinum in an oxidizing atmosphere already occurred at 400 °C. A protocol with reducing pulses at 250-400 °C was applied in a subsequent step for controlled Pt-particle formation. Operando X-ray absorption spectroscopy unraveled the different extent of reduction and sintering of Pt particles: The choice of the reductant allowed the tuning of the reduction degree/particle size and thus the catalytic activity (CO>H2>C3H6). This dynamic nature of Pt on ceria at such low temperatures (250-500 °C) was additionally confirmed by in situ environmental transmission electron microscopy. A general concept is proposed to adjust the noble metal dispersion (size, structure), for example, during operation of an exhaust gas catalyst. [ABSTRACT FROM AUTHOR]

Low Pt-based alloy catalysts are regarded as an efficient strategy in achieving high activity for the oxygen reduction reaction (ORR) in proton-exchange membrane fuel cells (PEMFCs). However, the desired durability for the low Pt-based catalysts, such as the Pt 1 Co 3 catalyst, has still been considered a great challenge for PEMFCs. In this study, we investigate sub-2.5 nm Pt x Co y alloy catalysts with varying Co content and Pt 1 Co 3 @Pt core-shell (CS) nanostructure catalysts obtained through a simple displacement reaction. The Pt 1 Co 3 @Pt&#95;H catalysts showed a high mass activity (MA) of 1.46 A/mg Pt at 0.9 V and 14% MA loss after 10k accelerated degradation test (ADT) cycles, which suggested the improved stability compared with Pt 1 Co 3 catalysts (52% MA loss). To clarify the degradation mechanism, operando high-energy resolution fluorescence detection X-ray absorption spectroscopy (XAS) was applied in addition to conventional advanced measurement techniques, including operando conventional XAS, to analyze the electronic state and structure changes during operation potentials. We found that introducing Co improves the catalysts' activity mainly from the strain effect, but an excessive amount of Co leads to increased Pt-oxidation, which accelerates the degradation of the catalysts. The Pt 1 Co 3 @Pt&#95;H catalyst shows high tolerance to Pt-oxidation, benefiting both the stability and activity. Our findings demonstrate an in-depth understanding of the degradation mechanism and the importance of designing PtCo CS nanostructures with optimal Co content for enhanced performance in PEMFCs.

In polymer electrolyte type fuel cells, the platinum-based catalysts are applied for the oxygen reduction reaction. However, the specific adsorption from the sulfo group in perfluorosulfonic acid ionomers has been considered to passivate the active sites of the platinum. Herein, we present platinum catalysts covered by an ultrathin two-dimensional nitrogen-doped carbon shell (CN x ) layer to protect the platinum from the specific adsorption of perfluorosulfonic acid ionomers. Such coated catalysts were obtained by the facile polydopamine coating method, which is available to tune the thickness of the carbon shell by tuning the polymerization time. The coated catalysts that possess a CN x with a thickness of 1.5 nm demonstrated superior ORR activity and comparable oxygen diffusivity when compared to the commercial Pt/C. These results were supported by the changes in the electronic statements observed in the X-ray photoelectron spectroscopy (XPS) and CO stripping analyses. Furthermore, the oxygen coverage, CO displacement charge, and operando X-ray absorption spectroscopy (XAS) tests were employed to identify the protection effect of CN x in coated catalysts compared with the Pt/C catalysts. In summary, the CN x could not only suppress the oxide species generation but also prevent the specific adsorption of the sulfo group in the ionomer.

Highlights: [•] Addition of Mo and Co to a Pt-based aqueous phase reforming catalyst increased reaction site time yields by factors of 5. [•] PtMo had lower hydrogen selectivity (65%) relative to Pt and PtCo (>85%) at a conversion of 65%. [•] Operando X-ray absorption spectroscopy show Pt-rich structures with PtCo bimetallic character under reaction conditions. [•] Scanning transmission electron microscopy (STEM) reveals three distinct particle configurations on used samples. [•] Reaction environment results in Co leaching and particle configurations observed in the STEM. [Copyright &y& Elsevier]

Operando X-ray absorption spectroscopy is used to measure simultaneous changes in catalyst structure and changes in catalytic activity versus time during the liquid phase hydrogenation of furfural over supported copper catalysts. This approach allows the size of the copper nanoparticles to be monitored continuously versus time-on-stream, such that these changes in dispersion can be accounted for in the calculation of turnover frequency. It is shown that sintering of the copper nanoparticles is the predominant mode of catalyst deactivation for a Cu/γ-Al2O3 catalyst throughout its time-on-stream, leading to irreversible loss of catalytic activity. In contrast, this mode of deactivation is eliminated by atomic layer deposition of an alumina overcoat; however, deposition of carbonaceous deposits in the small pores of the overcoat leads to deactivation that is reversible upon calcination of the catalyst. [ABSTRACT FROM AUTHOR]

-Highlights: [•] Active site for standard SCR in Cu-SSZ-13 is isolated Cu exchanged in 6-ring. [•] Under ambient conditions, the precursor to the active Cu is hydrated Cu(II). [•] Above Cu:Altot ∼0.2 in Cu-SSZ-13 with Si:Altot =4.5, Cu x O y species begin to form. [•] Kinetically-relevant steps occur on isolated Cu, not H+ or Cu x O y . [ABSTRACT FROM AUTHOR]

The turnover rate (TOR, normalized to sites measured by CO chemisorption before reaction) and selectivity for the aqueous phase reforming of glycerol have been determined for Pt/C and PtMo/C catalysts. While the TOR of PtMo/C is higher than that of Pt/C by about 4 times at comparable conversion, the selectivity to C-O bond cleavage is higher, thus reducing the H yield at high conversion. Under reaction conditions on Pt/C, CO is observed as the most abundant Pt surface species with a fractional coverage of about 0.6 using operando X-ray absorption spectroscopy. Since there is little CO in the effluent (CO:CO ratios > 100:1, when CO is detected), it is thought that surface CO is converted to H and CO by the water gas shift reaction. DFT calculations suggest that the role of metallic Mo is to alter the electronic properties of Pt lowering the binding energy of CO and reducing the activation energies of dehydrogenation and C-O bond cleavage. Because the activation energy for C-O cleavage is lowered more than for dehydrogenation, the selectivity for C-O bond cleavage is increased, ultimately lowering the H yield compared to Pt/C. [ABSTRACT FROM AUTHOR]

In-depth insights into the structure-activity relationships and complex reaction mechanisms of oxygen evolution reaction (OER) electrocatalysts are indispensable to efficiently generate clean hydrogen through water electrolysis. We introduce a convenient and effective sulfur heteroatom tuning strategy to optimize the performance of active Ni and Fe centers embedded into coordination polymer (CP) catalysts. Operando monitoring then provided the mechanistic understanding as to how exactly our facile sulfur engineering of Ni/Fe-CPs optimizes the local electronic structure of their active centers to facilitate dioxygen formation. The high OER activity of our optimized S-R-NiFe-CPs outperforms the most recent NiFe-based OER electrocatalysts. Specifically, we start from oxygen-deprived O d -R-NiFe-CPs and transform them into highly active Ni/Fe-CPs with tailored sulfur coordination environments and anionic deficiencies. Our operando X-ray absorption spectroscopy analyses reveal that sulfur introduction into our designed S-R-NiFe-CPs facilitates the formation of crucial highly oxidized Ni 4+ and Fe 4+ species, which generate oxygen-bridged Ni IV -O-Fe IV moieties that act as the true OER active intermediates. The advantage of our sulfur-doping strategy for enhanced OER is evident from comparison with sulfur-free O d -R-NiFe-CPs, where the formation of essential high-valent OER intermediates is hindered. Moreover, we propose a dual-site mechanism pathway, which is backed up with a combination of pH-dependent performance data and DFT calculations. Computational results support the benefits of sulfur modulation, where a lower energy barrier enables O-O bond formation atop the S-Ni IV -O-Fe IV -O moieties. Our convenient anionic tuning strategy facilitates the formation of active oxygen-bridged metal motifs and can thus promote the design of flexible and low-cost OER electrocatalysts.

Ir and its oxide are the only available oxygen evolution reaction (OER) electrocatalysts with reasonably high activity and stability for commercial proton-exchange membrane electrolyzers. However, the establishment of structure–performance relationships for the design of better Ir-based electrocatalysts is hindered by their uncontrolled surface reconstruction during OER in acidic media. Herein, we monitor the structural evolution of two model Ir nanocrystals (one with a flat surface enclosed by (100) facets and the other with a concave surface containing numerous high-index planes) under acidic OER conditions. Operando X-ray absorption spectroscopy measurements reveal that the promotion of surface IrO x formation during the OER by the concave Ir surface with high-index planes results in a gradual OER activity increase, while a decrease in activity and limited oxide formation are observed for the flat Ir surface. After the activation process, the Ir concave surface exhibits ~ 10 times higher activity than the flat surface. Density functional theory computations reveal that Ir high-index surfaces are thermodynamically preferred for the adsorption of oxygen atoms and the formation of surface oxides under OER conditions. Thus, our work establishes a structure–performance relationship for Ir nanocrystals under operating conditions, providing new principles for the design of nanoscale OER electrocatalysts. Surface structure–controlled model Ir nanocrystals with flat and concave surfaces were designed as electrocatalysts for the oxygen evolution reaction under acidic conditions. The Ir concave surface containing high-index planes exhibited progressive activation during the oxygen evolution reaction due to the facile formation of active IrO x , while the flat surface showed a gradual decrease in activity. [Display omitted] • Surface structure–controlled model Ir nanocrystals with flat and concave surfaces. • The structural evolution of Ir catalysts under acidic oxygen evolution reaction. • The activation process of Ir concave surface containing high-index planes under OER. • The deactivation process of Ir flat surface during acidic OER. • The important role of high-index planes for promoting OER activity in acidic media. [ABSTRACT FROM AUTHOR]

[Display omitted] • A facile approach to prepare metallic Ni/NC hybrid composites is presented. • The dynamic surface reconfiguration is identified by operando/ex-situ XAS technique. • Difference EXAFS spectra (diffEXAFS) method is developed to study the surface evolution. • The key active species of NiOOH and irreversible Ni(OH) 2 transformation is elucidated. Developing cost-effective electrocatalyst for oxygen evolution reaction (OER) and elucidating the structural transformation is urgent but remains a significant challenge. Here, we report a scalable approach to prepare well-defined Ni nanoparticles supported by nitrogen-doped carbon (Ni/NC), which possess superior catalytic activity and durability for OER, especially for the sample pyrolyzed at 600 °C. Moreover, the dynamic surface reconstruction of as-prepared catalysts during OER was detailedly investigated by ex-situ characterizations and operando X-ray absorption spectroscopy combined with the difference method that is a novel technique to probe the subtle structural changes. Our findings demonstrate that a surface oxidation to Ni(OH) 2 on Ni nanoparticles occurs at the beginning of OER, and then a reversible phase transitions between Ni(OH) 2 and NiOOH depends on the applied potential. Thus, this work develops a new method for preparing low-cost Ni-based electrocatalyst and sheds light on the direct observation of surface structure evolution during the OER. [ABSTRACT FROM AUTHOR]

• MO x H y (M = Ni, Fe, and NiFe) film electrocatalyst fabricated on carbon substrate. • NiFeO x H y film exhibits excellent alkaline OER activity. • Operando XAS reveals the highly oxidized Fe which is active site in NiFeO x H y. • UPS confirms the electron transferring effect from FeO x H y to NiO x H y. Nickel-iron (oxy)hydroxide (NiFeO x H y) is a state-of-the-art non-precious electrocatalyst for oxygen evolution reaction (OER) in alkaline media. However, the origin of the synergy between Ni and Fe remains unclear. Here, operando X-ray absorption and ultraviolet photoelectron spectroscopy were used to prove the charging-discharging behavior of Ni and Fe when they are adjacent. Fe oxidation and Ni reduction were simultaneously observed during electrocatalytic OER in NiFeO x H y compared to unary metal (oxy)hydroxide. The higher Fermi level of FeO x H y compared to NiO x H y enables charge transfer from Fe to Ni domain near the boundaries. From the experimental results, the combination of Ni and Fe results in superior oxygen-evolving activity by exposing high valent Fe coupled with a dischargeable Ni matrix. [Display omitted] [ABSTRACT FROM AUTHOR]

Ni-rich NMC is an attractive Li-ion battery cathode due to its combination of energy density, thermal stability, and reversibility. While higher delivered energy density can be achieved with a more positive charge voltage limit, this approach compromises sustained reversibility. Improved understanding of the local and bulk structural transformations as a function of charge voltage, and their associated impacts on capacity fade are critically needed. Through simultaneous operando synchrotron X-ray diffraction (XRD) and X-ray absorption spectroscopy (XAS) of cells cycled at 3-4.3 or 3-4.7 V, this study presents an in-depth investigation into the effects of voltage window on local coordination, bulk structure, and oxidation state. These measurements are complemented by ex situ X-ray fluorescence (XRF) mapping and scanning electrochemical microscopy mapping (SECM) of the negative electrode, X-ray photoelectron spectroscopy (XPS) of the positive electrode, and cell level electrochemical impedance spectroscopy (EIS). Initially, cycling between 3 and 4.7 V leads to greater delivered capacity due to greater lithium extraction, accompanied by increased structural distortion, moderately higher Ni oxidation, and substantially higher Co oxidation. Continued cycling at this high voltage results in suppressed Ni and Co redox, greater structural distortion, increased levels of transition metal dissolution, higher cell impedance, and 3× greater capacity fade.

The surface chemistry of perovskite-type lanthanum transition metal oxides is an important parameter in high-temperature electrochemical devices such as solid oxide fuel cells (SOFCs). Strontium segregation on the surface of lanthanum strontium cobalt oxides (LSC) may lead to poor oxygen reduction kinetics. However, previous studies have not sufficiently addressed the oxide surface chemistry, including the oxidation change of LSC under cathodic polarization. In this study, operando total-reflection X-ray fluorescence absorption and X-ray fluorescence spectroscopy were performed to investigate cathodic polarization at high temperature (773 K) and at 1 atm pressure. Dense thin-film electrodes of La 0.6 Sr 0.4 CoO 3 (LSC 113) on Zr 0.92 Y 0.08 O 1.96 electrolytes with a gadolinium-doped ceria interlayer were examined as model SOFC cathodes. This may be the first time that predominant surface reduction of cobalt is directly observed by using LSC 113 under cathodic polarization. This phenomenon is related to strontium enrichment on the LSC surface. (La 0.5 Sr 0.5) 2 CoO 4 decoration on LSC 113 enhanced the oxygen reduction kinetics compared with that of bare LSC 113. This is due to the suppression of cobalt surface reduction and Sr enrichment in the LSC 113 phase. • Operando total-reflection X-ray absorption analysis is performed for SOFC cathodes. • Predominant surface reduction of cobalt in La 0.6 Sr 0.4 CoO 3 was confirmed under polarization. • (La 0.5 Sr 0.5) 2 CoO 4± δ decoration suppresses Co reduction on the surface and enhances the kinetics. [ABSTRACT FROM AUTHOR]

Alloy clusters are attractive electrocatalysts that have adjustable activity through the synergistic effects of alloy components. However, the synthesis of atomically-precise alloy clusters is still a challenge. Here, we report a molecular template method in which phosphomolybdic acid adsorbs foreign noble metal ions, and then anneals into a series of Mo-based alloy oxide clusters (PtMo, IrMo, AuMo and PdMo), where oxygen is the terminal oxygen outside the cluster. The as-prepared alloy clusters have tunable atomic ratio and uniform size around 1 nm. Taking PtMo clusters as an example, their electrocatalytic nitrogen reduction reaction (NRR) has been investigated in depth. Among all compositions, PtMo-6 cluster (Pt:Mo atomic ratio 1:2) exhibits the highest NRR activity with a Faradaic efficiency of 14.37% and a NH 3 yield rate of 65.3 μg h−1 mg−1 cat. in 0.1 M KOH, surpassing the known Mo-based catalysts. The combination of operando X-ray absorption spectroscopy and DFT calculations reveals that the oxide cluster in working state is the partially reduced. An unusual synergy is discovered, namely that Pt acts as a proton donor, providing *H to promote the hydrogenation of N 2 on the adjacent Mo atom. This work provides a simple and universal method for the precise synthesis of alloy clusters and new guidance for designing highly efficient NRR catalysts. A molecule template method is developed to prepare Mo-based alloy clusters (PtMo, IrMo, AuMo and PdMo) with uniform particle size of 1–2 nm and controllable atomic ratio. Among all PtMo compositions, PtMo-6 cluster exhibits the highest electrocatalytic activity towards nitrogen reduction reaction. The combination of X-ray absorption spectroscopy and DFT calculations reveals the synergistic mechanism of NRR on PtMo-6 clusters, that is, Pt acts as a proton donor, providing *H (* represents active site) to promote the hydrogenation of N 2 on nearby Mo atoms. Image 1 • A molecule template method is developed to prepare Mo-based alloy clusters. • Partially oxidized PtMo cluster exhibits high electrocatalytic activity towards nitrogen reduction reaction. • Operando observation of nitrogen reduction reaction on PtMo cluster by X-ray absorption spectroscopy. • An unusual synergy is discovered, that is, Pt providing *H to promote the hydrogenation of N 2 on Mo. [ABSTRACT FROM AUTHOR]

Na x Mn 1/3 Fe 1/3 Ni 1/3 O 2 (x = 2/3 and 1) layered oxides are synthesized and applied as positive electrode materials for sodium batteries. The crystal structure of the material changes from the O3 single phase to P2/O3 mixed phases as the Na content decreases from 1 to 2/3. The mixed-phases Na 2/3 Mn 1/3 Fe 1/3 Ni 1/3 O 2 shows superior cycling performance compared to the single-phase NaMn 1/3 Fe 1/3 Ni 1/3 O 2 due to different redox process and structural change that are demonstrated by operando X-ray absorption spectroscopy (XAS). The Na 2/3 Mn 1/3 Fe 1/3 Ni 1/3 O 2 experiences redox reactions of Ni3+|Ni4+ and Fe3+|Fe4+ with inactive Mn ions during the charge/discharge processes, while the NaMn 1/3 Fe 1/3 Ni 1/3 O 2 undergoes deeper redox reactions from Ni2+|Ni3+|Ni4+ and Fe3+|Fe4+ that Mn ions are irreversibly oxidized to Mn4+ in the 1st charge process. In addition, the mixed-phases material has smaller changes in the transition metal oxygen bond lengths during cycling, corresponding to less distortions of TM O 6 units in the crystal structure. The better reversibility of the redox reactions and the occurrence of less structural changes are both responsible for the enhanced cycling performance obtained from the mixed-phases material compared to the single-phase material. These results strengthen the understanding of interactions of transition metals in the layered cathode and provide guidelines for designing positive electrode materials for sodium batteries. Image 1 •.●Na x Mn 1/3 Fe 1/3 Ni 1/3 O 2 (x = 2/3 and 1) layered oxides have been synthesized. •.●The mixed-phases Na 2/3 Mn 1/3 Fe 1/3 Ni 1/3 O 2 shows superior cycling performance. •.●The mixed-phases Na 2/3 Mn 1/3 Fe 1/3 Ni 1/3 O 2 exhibits less structural distortions. •.●The mixed-phases Na 2/3 Mn 1/3 Fe 1/3 Ni 1/3 O 2 has more reversible redox reactions. [ABSTRACT FROM AUTHOR]