Operando X-ray Absorption Research Articles

Direct monitoring of the potassium charge carrier in Prussian blue cathodes using potassium K-edge X-ray absorption spectroscopy

Monitoring the evolution of potassium carriers during the charge/discharge of Prussian blue batteries by operando potassium K-edge X-ray absorption spectroscopy.

Isolated PdO sites on SiO2-supported NiO nanoparticles as active sites for allylic alcohol selective oxidation

Silica-supported NiO nanoparticles as hosts for isolated PdO catalytic sites. Isolate PdO is confirmed as the species responsible for the chemoselective oxidation of cinnamyl alcohol to cinnamaldehyde by operando X-ray absorption spectroscopy.

Highly selective CO2 electrocatalytic reduction on nickel single-atom catalyst in a high-temperature shockwave method

Recently, the easily prepared catalysis used in CO2 electrochemical reduction with high selectivity and low cost has attracted more and more attention. Compared with the temperate wet chemical preparation method, which takes a long time and introduces some impurities, we found nickel metal catalysts can be easily and quickly synthesized on carbon black using the high-temperature shockwave method, which has the advantages of rapid heating up to 1300–1500 K within 0.5 s, no introduction of excess elements and extreme low unfavorable nitrogen dopants. Heating cycles and proportion of nitrogen source have been investigated on the catalyst morphology, particle size and electrocatalysis performance. Based on operando X-ray absorption and aberration-corrected scanning transmission electron microscopy (STEM) measurements, nickel single atom catalysts (Ni SACs) were prepared successfully and uniformly in modest heating cycles and proportion of nitrogen source. The Ni SACs exhibited remarkable performance in reducing carbon dioxide (CO2) to carbon monoxide (CO), with the current density of 27.5 mA/cm2 (−1.0 V vs reversible hydrogen electrode) and CO selectivity approaching 100 % in flow cell.

Iridium single atoms incorporated in Co3O4 efficiently catalyze the oxygen evolution in acidic conditions

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-Co3O4), 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-Co3O4 exhibits significantly higher mass activity and turnover frequency than those of benchmark IrO2 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.

Operando X-ray Absorption Spectroscopy Study of SnO2 Nanoparticles for Electrochemical Reduction of CO2 to Formate

Tin-based electrocatalysts exhibit a remarkable ability to catalyze CO2 to formate selectively. Understanding the size-property relationships and exploring the evolution of the active size still lack complete understanding. Herein, we prepared SnO2 nanoparticles (NPs) with a controllable size supported on commercial carbon spheres (SnO2/C-n, n = 1, 2, and 3) by a simple low-temperature annealing method. The transmission electron microscopy/scanning transmission electron microscopy images and fitting results of the small-angle X-ray scattering profile confirm the increased size of SnO2 NPs due to the increase of SnO2 loading. The catalytic performance of SnO2 has proved the size-dependent effect during the CO2 reduction reaction process. The as-prepared SnO2/C-1 displayed the maximum Faradic efficiency of formate (FEHCOO-) of 82.7% at -1.0 V versus reversible hydrogen electrode (RHE). In contrast, SnO2/C-2 and SnO2/C-3 with larger particle sizes achieved lower maximum FEHCOO- and larger overpotential. Moreover, we employed operando X-ray absorption spectroscopy to study the evolution of the oxidation state and local coordination environment of SnO2 under working conditions. In addition to the observed shifts of the rising edge of Sn K-edge X-ray absorption near-edge structure spectra to a lower energy side as the applied voltage decreases, the decreased coordination number of Sn in the Sn-O scattering path and the presence of Sn metal contribution in the extended X-ray absorption fine structure spectra verify the reduction of SnO2 to SnOx and metallic Sn.

Transition-Metal Catalysts for Methane Dehydroaromatization (Mo, Re, Fe): Activity, Stability, Active Sites, and Carbon Deposits

The most studied catalysts for methane dehydroaromatization (MDA)─Mo/ZSM-5─are not commercialized yet due to the rapid deactivation and insufficient activity. Catalytic systems based on Fe and Re are potential alternatives to Mo-containing zeolites. Here, we compare the catalytic performance of these catalysts as a function of metal type and its loading in ZSM-5 zeolite. The results show that the catalytic activity decreases in the order of Re/ZSM-5 > Mo/ZSM-5 > Fe/ZSM-5, while the catalyst stability decreases in the opposite order: Fe/ZSM-5 > Mo/ZSM-5 > Re/ZSM-5. The active metal species in the working catalysts were determined by operando X-ray absorption near-edge structure spectroscopy combined with mass spectrometry. We found that Re0 and Fe2+ species are the most likely active species for the catalytic dehydroaromatization of CH4 to aromatics in respective catalysts. Combining the pulse reaction technique with operando thermogravimetry analysis–mass spectrometry experiments, we demonstrate that the length of the induction period strongly correlates to the activity of the catalyst. The longer induction period of the Fe/ZSM-5 catalyst indicates the slow growth of hydrocarbon pool intermediates inside the zeolite pores and thus explains its poor catalytic performance. Finally, both the formation of hydrocarbon pool species and the activity of Fe/ZSM-5 can be improved by increasing the Fe loading, reaction pressure, and space velocity.

Engineering Single Atom Catalysts for Flow Production: From Catalyst Design to Reactor Understandings

ConspectusIn heterogeneous catalysis, the long-standing challenge is to achieve extremely high activity and chemoselectivity in liquid-phase organic transformations comparable to that of homogeneous or enzymatic processes. Single-atom catalysts (SACs) with atomically precise coordination are developed with the objectives to mimic the homogeneous pathways but face stability issues due to metal leaching or clustering. Additionally, the practical application of SACs in chemical production is hampered by the lack of standard preparation protocols and low conversion using laboratory batch reactors.This Account focuses on our recent studies in both catalyst design and reactor-level engineering for the flow synthesis of fine chemicals via SACs. At the catalyst and reaction level, we will discuss the intrinsic mechanism that controls reactivity and chemoselectivity in the SAC-catalyzed process and highlight examples where SACs outperform other catalytic approaches. Specifically, we reported the SAC-mediated preparation and late-stage functionalization of pharmaceutical drugs, including lonidamine, Tamiflu, cavosonstat, indomethacin, and many others by chemoselective transformations in a sequential or multicomponent manner. The ability of ultrahigh loading SACs in providing a multisite pathway for organic transformations involving two or more reactants is highlighted and contrasted with the single-site pathway in conventional SACs. Molecular-level understanding on the dynamic catalytic cycle obtained using operando X-ray absorption spectroscopies provides guidance for the design of more effective and leach resistant SACs. This also calls for the transformation of laboratory powder-based catalysts into industrially viable monolithic catalysts via formulation to further enhance the leach resistance. At the reactor level, we will highlight the importance of continuous-flow techniques in maximizing productivity and simplifying process transfer from laboratory to commercial production. Particularly, we discuss the use of fuel cell-type flow stacks for quantitative production of fine chemicals, including the synthesis of multifunctional anilines at a 5.8 g h–1 productivity using a Pt SAC module (3.2 mg Pt). Insight into multiscale reactor processes, for instance, the fluid diffusion kinetics, heat transfer, and influence of porosity and the gas–liquid–solid interface are provided by computational fluidic dynamics calculations as well as experimental techniques. In many cases, catalytic processes are more limited by mass diffusion than the intrinsic activity of the catalyst, calling for process optimization and engineering at the reactor level to enhance the productivity. Finally, we also identify technical barriers that need to be overcome in SAC research and offer our perspective on standardized and scalable protocols for mass production, the production cost analysis of typical SACs, high-throughput screening platforms, and novel flow reactor designs involving photochemical and electrochemical reactions for up-scaling chemical production by SACs.

Understanding the Stability of Manganese Chromium Antimonate Electrocatalysts through Multimodal In Situ and Operando Measurements.

Improving electrocatalyst stability is critical for the development of electrocatalytic devices. Herein, we utilize an on-line electrochemical flow cell coupled with an inductively coupled plasma-mass spectrometer (ICP-MS) to characterize the impact of composition and reactant gas on the multielement dissolution of Mn(-Cr)-Sb-O electrocatalysts. Compared to Mn2O3 and Cr2O3 oxides, the antimonate framework stabilizes Mn at OER potentials and Cr at both ORR and OER potentials. Furthermore, dissolution of Mn and Cr from Mn(-Cr) -Sb-O is driven by the ORR reaction rate, with minimal dissolution under N2. We observe preferential dissolution of Cr totaling 13% over 10 min at 0.3, 0.6, and 0.9 V vs RHE, with only 1.5% loss of Mn, indicating an enrichment of Mn at the surface of the particles. Despite this asymmetric dissolution, operando X-ray absorption spectroscopy (XAS) showed no measurable changes in the Mn K-edge at comparable potentials. This could suggest that modification to the Mn oxidation state and/or phase in the surface layer is too small or that the layer is too thin to be measured with the bulk XAS measurement. Lastly, on-line ICP-MS was used to assess the effects of applied potential, scan rate, and current on Mn-Cr-Sb-O during cyclic voltammetry and accelerated stress tests. With this deeper understanding of the interplay between oxygen reduction and dissolution, testing procedures were identified to maximize both activity and stability. This work highlights the use of multimodal in situ characterization techniques in tandem to build a more complete model of stability and develop protocols for optimizing catalyst performance.

Platinum-Iron(II) Oxide Sites Directly Responsible for Preferential Carbon Monoxide Oxidation at Ambient Temperature: An Operando X-ray Absorption Spectroscopy Study.

Operando X-ray absorption spectroscopy identified that the concentration of Fe2+ species in the working state-of-the-art Pt-FeOx catalysts quantitatively correlates to their preferential carbon monoxide oxidation steady-state reaction rate at ambient temperature. Deactivation of such catalysts with time on stream originates from irreversible oxidation of active Fe2+ sites. The active Fe2+ species are presumably Fe+2 O-2 clusters in contact with platinum nanoparticles; they coexist with spectator trivalent oxidic iron (Fe3+ ) and metallic iron (Fe0 ) partially alloyed with platinum. The concentration of active sites and, therefore, the catalyst activity strongly depends on the pretreatment conditions. Fe2+ is the resting state of the active sites in the preferential carbon monoxide oxidation cycle.

Manganese Molybdate Cathodes with Dual-Redox Centers for Aqueous Zinc-Ion Batteries: Impact of Electrolyte on Electrochemistry

Although bimetallic oxides have been shown to be beneficial as electrode materials in battery systems over their monometallic oxide counterparts, the implementation of metal molybdates with dual-redox centers has not been widely studied in aqueous rechargeable Zn-ion batteries. Manganese molybdate (MnMoO4) was synthesized via facile cosolvent coprecipitation and implemented as a cathode material for the first time in this system using 3 M ZnSO4 and 3 M ZnCl2 electrolytes to investigate the impact of the anion. In the two electrolytes, both manganese and molybdate metal centers were determined to be redox active using Mn and Mo K-edge operando X-ray absorption spectroscopies (XAS), with the corresponding voltage plateaus at 1.4 and 0.5 V, respectively. The difference in anions resulted in a preference regarding the active redox center, with the sulfate based preferring Mo redox and chloride based preferring Mn redox. Additionally, the redox reactions also differ in rate dependency, with Mn and Mo redox reactions preferring slow and fast current rates, respectively. In both systems, Mn redox was seen to be the more stable mechanism over prolonged cycling. The preference was related to the dissolution of the MnMoO4 material by applying Pearson’s hard–soft acid–base theory and considering the solubilities of the respective salts.

Clarification of underneath capacity loss for O3-type Ni, co free layered cathodes at high voltage for sodium ion batteries

Earth abundant O3-type NaFe0.5Mn0.5O2 layered oxide is regarded as one of the most promising cathodes for sodium ion batteries due to its low cost and high energy density. However, its poor structural stability and cycle life strongly impede the practical application. Herein, the dynamic phase evolution as well as charge compensation mechanism of O3-type NaFe0.5Mn0.5O2 cathode during sodiation/desodiation are revealed by a systemic study with operando X-ray diffraction and X-ray absorption spectroscopy, high resolution neutron powder diffraction and neutron pair distribution functions. The layered structure experiences a phase transition of O3 → P3 → OP2 → ramsdellite during the desodiation, and a new O3′ phase is observed at the end of the discharge state (1.5 V). The density functional theory (DFT) calculations and nPDF results suggest that depletion of Na+ ions induces the movement of Fe into Na layer resulting the formation of an inert ramsdellite phase thus causing the loss of capacity and structural integrity. Meanwhile, the operando XAS clarified the voltage regions for active Mn3+/Mn4+ and Fe3+/Fe4+ redox couples. This work points out the universal underneath problem for Fe-based layered oxide cathodes when cycled at high voltage and highlights the importance to suppress Fe migration regarding the design of high energy O3-type cathodes for sodium ion batteries.

High Current Density Oxygen Evolution in Carbonate Buffered Solution Achieved by Active Site Densification and Electrolyte Engineering.

High current density reaching 1 A cm-2 for efficient oxygen evolution reaction (OER) was demonstrated by interactively optimizing electrolyte and electrode at non-extreme pH levels. Careful electrolyte assessment revealed that the state-of-the-art nickel-iron oxide electrocatalyst in alkaline solution maintained its high OER performance with a small Tafel slope in K-carbonate solution at pH 10.5 at 353 K. The OER performance was improved when Cu or Au was introduced into the FeOx -modified nanostructured Ni electrode as the third element during the preparation of electrode by electrodeposition. The resultant OER achieved 1 A cm-2 at 1.53 V vs. reversible hydrogen electrode (RHE) stably for 90 h, comparable to those in extreme alkaline conditions. Constant Tafel slopes, apparent activation energy, and the same signatures from operando X-ray absorption spectroscopy among these samples suggested that this improvement seems solely correlated with enhanced electrochemical surface area caused by adding the third element.

Elucidating the optimized P2 type Na0.67Mn1−xCuxO2 cathode active material via operando XAS

Here we report on the structural and electrochemical properties of P2-type Na0.67Mn1−xCuxO2 (where x = 0.20–0.50) via various techniques. X-ray diffraction (XRD) reveals a reduction of the unit cell volume upon substitution of Cu elucidated through detailed Rietveld analysis. The cyclic voltammetry (CV) behavior is also affected by the Cu substitution indicating new redox reactions stemming from Cu substitution. Galvanostatic cycling measurements at room temperature show that when x = 0.35 in a P2-type Na0.67Mn1−xCuxO2 cathode active material, the best electrochemical performance is obtained. The Na-ion diffusion rate is found to be strongly dependent upon the environmental temperature. Changes in the valence state and the local structures of P2-type Na0.67Mn1−xCuxO2 during the charge/discharge are investigated through the operando X-ray absorption spectroscopy (XAS) technique.

De/protonation associated sustainable conversion reaction applicable to high-capacity zinc storage in mildly acidic aqueous system

We introduce an unexpected finding of the de/protonation associated conversion reaction occurred in K2V3O8 as a promising cathode material for zinc-ion batteries. The structure undergoes a conversion reaction between amorphous V5+2O5 and V3+OOH upon cycling when a cut-off voltage up to 1.9 V is applied. A combination of operando X-ray diffraction, in situ Raman spectroscopy, X-ray photoelectron spectroscopy, time-of-flight secondary-ion mass spectroscopy, and operando pH measurement analyses reveal that the reaction of the proton (H+) is indispensable for progression of the conversion reaction. The conversion reaction results in a large reversible capacity of 362 mAh g−1 (∼97 % of theoretical capacity) at 0.1 C (37 mA g−1) on discharge and an activity even at a rate of 7 C (187 mAh g−1), with the two-electron reaction by the V5+/3+ redox pair evidenced by operando X-ray absorption spectroscopy analysis. These findings underscore the importance of conversion reversibility associated with the proton reaction for a high cut-off voltage, contributing additional capacity to reach the theoretical capacity for cathode materials of zinc-ion batteries in mildly acidic aqueous systems.

Bimetallic Aluminum- and Niobium-Doped MCM-41 for Efficient Conversion of Biomass-Derived 2-Methyltetrahydrofuran to Pentadienes.

The production of conjugated C4-C5 dienes from biomass can enable the sustainable synthesis of many important polymers and liquid fuels. Here, we report the first example of bimetallic (Nb, Al)-atomically doped mesoporous silica, denoted as AlNb-MCM-41, which affords quantitative conversion of 2-methyltetrahydrofuran (2-MTHF) to pentadienes with a high selectivity of 91 %. The incorporation of AlIII and NbV sites into the framework of AlNb-MCM-41 has effectively tuned the nature and distribution of Lewis and Brønsted acid sites within the structure. Operando X-ray absorption, diffuse reflectance infrared and solid-state NMR spectroscopy collectively reveal the molecular mechanism of the conversion of adsorbed 2-MTHF over AlNb-MCM-41. Specifically, the atomically-dispersed NbV sites play an important role in binding 2-MTHF to drive the conversion. Overall, this study highlights the potential of hetero-atomic mesoporous solids for the manufacture of renewable materials.

Bimetallic Aluminum- and Niobium-Doped MCM-41 for Efficient Conversion of Biomass-Derived 2-Methyltetrahydrofuran to Pentadienes.

The production of conjugated C4-C5 dienes from biomass can enable the sustainable synthesis of many important polymers and liquid fuels. Here, we report the first example of bimetallic (Nb, Al)-atomically doped mesoporous silica, denoted as AlNb-MCM-41, which affords quantitative conversion of 2-methyltetrahydrofuran (2-MTHF) to pentadienes with a high selectivity of 91 %. The incorporation of AlIII and NbV sites into the framework of AlNb-MCM-41 has effectively tuned the nature and distribution of Lewis and Brønsted acid sites within the structure. Operando X-ray absorption, diffuse reflectance infrared and solid-state NMR spectroscopy collectively reveal the molecular mechanism of the conversion of adsorbed 2-MTHF over AlNb-MCM-41. Specifically, the atomically-dispersed NbV sites play an important role in binding 2-MTHF to drive the conversion. Overall, this study highlights the potential of hetero-atomic mesoporous solids for the manufacture of renewable materials.

In Situ Reconstruction NiO Octahedral Active Sites for Promoting Electrocatalytic Oxygen Evolution of Nickel Phosphate.

Electrochemical activation strategy is very effective to improve the intrinsic catalytic activity of metal phosphate toward the sluggish oxygen evolution reaction (OER) for water electrolysis. However, it is still challenging to operando trace the activated reconstruction and corresponding electrocatalytic dynamic mechanisms. Herein, a constant voltage activation strategy is adopted to in situ activate Ni2 P4 O12 , in which the break of NiONi bond and dissolution of PO4 3- groups could optimize the lattice oxygen, thus reconstructing an irreversible amorphous Ni(OH)2 layer with a thickness of 1.5-3.5nm on the surface of Ni2 P4 O12 . The heterostructure electrocatalyst can afford an excellent OER activity in alkaline media with an overpotential of 216.5 mV at 27.0 mA cm-2 . Operando X-ray absorption fine structure spectroscopy analysis and density functional theory simulations indicate that the heterostructure follows a nonconcerted proton-electron transfer mechanism for OER. This activation strategy demonstrates universality and can be used to the surface reconstruction of other metal phosphates.

High-Rate and Selective CO2 Electrolysis to Ethylene via Metal-Organic-Framework-Augmented CO2 Availability.

High-rate conversion of carbon dioxide (CO2 ) to ethylene (C2 H4 ) in the CO2 reduction reaction (CO2 RR) requires fine control over the phase boundary of the gas diffusion electrode (GDE) to overcome the limit of CO2 solubility in aqueous electrolytes. Here, a metal-organic framework (MOF)-functionalized GDE design is presented, based on a catalysts:MOFs:hydrophobic substrate materials layered architecture, that leads to high-rate and selective C2 H4 production in flow cells and membrane electrode assembly (MEA) electrolyzers. It is found that using electroanalysis and operando X-ray absorption spectroscopy (XAS), MOF-induced organic layers in GDEs augment the local CO2 concentration near the active sites of the Cu catalysts. MOFs with different CO2 adsorption abilities are used, and the stacking ordering of MOFs in the GDE is varied. While sputtering Cu on poly(tetrafluoroethylene) (PTFE) (Cu/PTFE) exhibits 43% C2 H4 Faradaic efficiency (FE) at a current density of 200mA cm- 2 in a flow cell, 49% C2 H4 FE at 1 A cm- 2 is achieved on MOF-augmented GDEs in CO2 RR. MOF-augmented GDEs are further evaluated in an MEA electrolyzer, achieving a C2 H4 partial current density of 220mAcm-2 for CO2 RR and 121mAcm-2 for the carbon monoxide reduction reaction (CORR), representing 2.7-fold and 15-fold improvement in C2 H4 production rate, compared to those obtained on bare Cu/PTFE.

Hollandite-type potassium titanium oxide with exceptionally stable cycling performance as a new cathode material for potassium-ion batteries

For the first time, we introduce hollandite-type K0.17TiO2, in which potassium ions are located at the center of a (2 × 2) tunnel structure, as a potential cathode material for potassium-ion batteries. Density functional theory calculation predicts the possibility of K+ insertion into the hollandite-type tunnel structure via a single-phase reaction. Operando X-ray diffraction and X-ray absorption spectroscopy analyses verify that potassium ions are de-/intercalated from/into the crystal structure of K0.17TiO2, accompanied by a Ti4+/Ti3+ redox reaction. The single-phase reaction is sustainable for long-term cycling, with exceptionally high operation voltage over 2.5 V. The hollandite-type K0.17TiO2 cathode delivers a reversible capacity of 60 mAh g−1 at 5 C (1.55 A g−1), with excellent capacity retention of over 98% of the initial capacity for 1000 cycles. This performance is related to the single-phase reaction with good structural stability. This work presents a facile approach that enables the use of a cathode with stable tunnel structure for potassium-ion batteries.

Role of Ni in PtNi Bimetallic Electrocatalysts for Hydrogen and Value-Added Chemicals Coproduction via Glycerol Electrooxidation.

Pt-based bimetallic electrocatalysts are promising candidates to convert surplus glycerol from the biodiesel industry to value-added chemicals and coproduce hydrogen. It is expected that the nature and content of the elements in the bimetallic catalyst can not only affect the reaction kinetics but also influence the product selectivity, providing a way to increase the yield of the desired products. Hence, in this work, we investigate the electrochemical oxidation of glycerol on a series of PtNi nanoparticles with increasing Ni content using a combination of physicochemical structural analysis, electrochemical measurements, operando spectroscopic techniques, and advanced product characterizations. With a moderate Ni content and a homogenously alloyed bimetallic Pt-Ni structure, the PtNi2 catalyst displayed the highest reaction activity among all materials studied in this work. In situ FTIR data show that PtNi2 can activate the glycerol molecule at a more negative potential (0.4 V RHE) than the other PtNi catalysts. In addition, its surface can effectively catalyze the complete C-C bond cleavage, resulting in lower CO poisoning and higher stability. Operando X-ray absorption spectroscopy and UV-vis spectroscopy suggest that glycerol adsorbs strongly onto surface Ni(OH) x sites, preventing their oxidation and activation of oxygen or hydroxyl from water. As such, we propose that the role of Ni in PtNi toward glycerol oxidation is to tailor the electronic structure of the pure Pt sites rather than a bifunctional mechanism. Our experiments provide guidance for the development of bimetallic catalysts toward highly efficient, selective, and stable glycerol oxidation reactions.