Operando X-ray Absorption Near-edge Structure Research Articles

Reduction of Cofed Carbon Dioxide Modifies the Local Coordination Environment of Zeolite-Supported, Atomically Dispersed Chromium to Promote Ethane Dehydrogenation.

The reduction of CO2 is known to promote increased alkene yields from alkane dehydrogenations when the reactions are cocatalyzed. The mechanism of this promotion is not understood in the context of catalyst active-site environments because CO2 is amphoteric, and even general aspects of the chemistry, including the significance of competing side reactions, differ significantly across catalysts. Atomically dispersed chromium cations stabilized in highly siliceous MFI zeolite are shown here to enable the study of the role of parallel CO2 reduction during ethylene-selective ethane dehydrogenation. Based on infrared spectroscopy and X-ray absorption spectroscopy data interpreted through calculations using density functional theory (DFT), the synthesized catalyst contains atomically dispersed Cr cations stabilized by silanol nests in micropores. Reactor studies show that cofeeding CO2 increases stable ethylene-selective ethane dehydrogenation rates over a wide range of partial pressures. Operando X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine-structure (EXAFS) spectra indicate that during reaction at 650 °C the Cr cations maintain a nominal 2+ charge and a total Cr-O coordination number of approximately 2. However, CO2 reduction induces a change, correlated with the CO2 partial pressure, in the population of two distinct Cr-O scattering paths. This indicates that the promotional effect of parallel CO2 reduction can be attributed to a subtle change in Cr-O bond lengths in the local coordination environment of the active site. These insights are made possible by simultaneously fitting multiple EXAFS spectra recorded in different reaction conditions; this novel procedure is expected to be generally applicable for interpreting operando catalysis EXAFS data.

High-Density Cobalt Single-Atom Catalysts for Enhanced Oxygen Evolution Reaction.

Single atom catalysts (SACs) possess unique catalytic properties due to low-coordination and unsaturated active sites. However, the demonstrated performance of SACs is limited by low SAC loading, poor metal-support interactions, and nonstable performance. Herein, we report a macromolecule-assisted SAC synthesis approach that enabled us to demonstrate high-density Co single atoms (10.6 wt % Co SAC) in a pyridinic N-rich graphenic network. The highly porous carbon network (surface area of ∼186 m2 g-1) with increased conjugation and vicinal Co site decoration in Co SACs significantly enhanced the electrocatalytic oxygen evolution reaction (OER) in 1 M KOH (η10 at 351 mV; mass activity of 2209 mA mgCo-1 at 1.65 V) with more than 300 h stability. Operando X-ray absorption near-edge structure demonstrates the formation of electron-deficient Co-O coordination intermediates, accelerating OER kinetics. Density functional theory (DFT) calculations reveal the facile electron transfer from cobalt to oxygen species-accelerated OER.

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.

Tailoring the adsorption behavior of superoxide intermediates on nickel carbide enables high-rate Li–O2 batteries

Probing the relationship between the adsorption of superoxide species and the kinetics of Li–O2 chemistry is critical for designing superior oxygen electrodes for the Li–O2 battery, yet the modulation essence, especially at the atomic level, remains little understood. Herein, we reveal that the adsorption behaviors of superoxide species can be effectively regulated via a core-induced interfacial charge interaction, and we find that moderate adsorption strength can enable superior rate capability in a Li–O2 battery. More importantly, operando X-ray absorption near-edge structure and surface-enhanced Raman spectroscopy provide tools to monitor in situ the evolution of the superoxide intermediates and the electronic states of the catalyst's metal sites during the discharge and charge processes, and correlate these with the surface adsorption states. The concept of tuning adsorption behavior through interfacial charge engineering could open up new opportunities to further advance the development of the Li–O2 battery and beyond.

Interpreting the Operando X-ray Absorption Near-Edge Structure of Supported Cu and CuPd Clusters in Conditions of Oxidative Dehydrogenation of Propane: Dynamic Changes in Composition and Size

Supported subnano-cluster catalysts are highly dynamic, developing true active sites only under the pressures and temperatures of reaction conditions. Operando X-ray absorption near-edge structure (XANES) spectroscopy can track changes in the oxidation state and the local environment of cluster atoms, providing insight into the development of these active sites. While bulk metal, oxide, and hydroxide standards are often used for fitting experimental XANES spectra to obtain average oxidation states, we recently showed that computed cluster standards of relevant compositions are a more suitable basis, producing more accurate fits. Here, we theoretically interpret the operando XANES of supported Cu3Pd and Cu4 clusters during temperature-programmed reaction (TPRx) of oxidative dehydrogenation of propane. We use an expanded basis set including both globally optimized computed clusters and bulk standards. Not only can we track reversible composition/oxidation state change with temperature, but also the irreversible growth of the bulk fraction upon heating, which we attribute to cluster sintering. This has important implications for the mechanism of the catalyzed reaction and the nature of the available active sites. We propose that operando XANES provides most significant insight into the nature of supported cluster catalysts in reaction conditions when interpreted using mixed computed cluster and bulk standards.

Revisited Ti2Nb2O9 as an Anode Material for Advanced Li-Ion Batteries.

Ti2Nb2O9 with a tunnel-type structure is considered as a perspective negative electrode material for Li-ion batteries (LIBs) with theoretical capacity of 252 mAh g-1 corresponding to one-electron reduction/oxidation of Ti and Nb, but only ≈160 mAh g-1 has been observed practically. In this work, highly reversible capacity of 200 mAh g-1 with the average (de)lithiation potential of 1.5 V vs Li/Li+ is achieved for Ti2Nb2O9 with pseudo-2D layered morphology obtained via thermal decomposition of the NH4TiNbO5 intermediate prepared by K+→ H+→ NH4+ cation exchange from KTiNbO5. Using operando synchrotron powder X-ray diffraction (SXPD), single-phase (de)lithiation mechanism with 4.8% unit cell volume change is observed. Operando X-ray absorption near-edge structure (XANES) experiment revealed simultaneous Ti4+/Ti3+ and Nb5+/Nb4+ reduction/oxidation within the whole voltage range. Li+ migration barriers for Ti2Nb2O9 along [010] direction derived from density functional theory (DFT) calculations are within the 0.15-0.4 eV range depending on the Li content that is reflected in excellent C-rate capacity retention. Ti2Nb2O9 synthesized via the ion-exchange route appears as a strong contender to widely commercialized Ti-based negative electrode material Li4Ti5O12 in the next generation of high-performance LIBs.

Probing Active Sites in CuxPdy Cluster Catalysts by Machine-Learning-Assisted X-ray Absorption Spectroscopy

Size-selected clusters are important model catalysts because of their narrow size and compositional distributions, as well as enhanced activity and selectivity in many reactions. Still, their structure-activity relationships are, in general, elusive. The main reason is the difficulty in identifying and quantitatively characterizing the catalytic active site in the clusters when it is confined within subnanometric dimensions and under the continuous structural changes the clusters can undergo in reaction conditions. Using machine learning approaches for analysis of the operando X-ray absorption near-edge structure spectra, we obtained accurate speciation of the CuxPdy cluster types during the propane oxidation reaction and the structural information about each type. As a result, we elucidated the information about active species and relative roles of Cu and Pd in the clusters.

Interpreting the Operando XANES of Surface-Supported Subnanometer Clusters: When Fluxionality, Oxidation State, and Size Effect Fight

X-ray absorption near edge structure (XANES) spectroscopy is widely used for operando catalyst characterization. We show that, for highly fluxional supported nanoclusters, the customary extraction ...

Elucidating the Electrocatalytic CO2 Reduction Reaction over a Model Single-Atom Nickel Catalyst.

Designing effective electrocatalysts for the carbon dioxide reduction reaction (CO2 RR) is an appealing approach to tackling the challenges posed by rising CO2 levels and realizing a closed carbon cycle. However, fundamental understanding of the complicated CO2 RR mechanism in CO2 electrocatalysis is still lacking because model systems are limited. We have designed a model nickel single-atom catalyst (Ni SAC) with a uniform structure and well-defined Ni-N4 moiety on a conductive carbon support with which to explore the electrochemical CO2 RR. Operando X-ray absorption near-edge structure spectroscopy, Raman spectroscopy, and near-ambient X-ray photoelectron spectroscopy, revealed that Ni+ in the Ni SAC was highly active for CO2 activation, and functioned as an authentic catalytically active site for the CO2 RR. Furthermore, through combination with a kinetics study, the rate-determining step of the CO2 RR was determined to be *CO2 - +H+ →*COOH. This study tackles the four challenges faced by the CO2 RR; namely, activity, selectivity, stability, and dynamics.

Ex Situ and Operando Characterization of Energy Storage Materials By Benchtop XAFS and XES

Advanced x-ray spectroscopies permit the interrogation of a material’s local electronic structure in an element-specific manner. In particular, these techniques reveal the speciation and ligand environment of an electroactive element and can provide direct insight into the state-of-charge and state-of-health of a battery. Despite their utility, the scientific impact of advanced x-ray spectroscopies is necessarily constrained by the availability of instrumentation. For x-ray absorption fine structure (XAFS) and x-ray emission spectroscopy (XES), studies are traditionally performed at synchrotron x-ray facilities. These facilities frequently serve to push the forefront of science and, as a result, necessarily operate under an access model which excludes projects which require routine analytical characterization, rapid feedback, or regular access. Over the last four years, our group at the University of Washington has developed several families of laboratory-based instruments to expand the accessibility of advanced x-ray spectroscopies. For the study of transition metal chemistry, we have constructed two Rowland circle spectrometers based upon related designs. Both spectrometers provide energy resolutions comparable to that observed at a synchrotron endstation. Their designs are also highly efficient, allowing studies to be completed with commercial x-ray tube sources in time scales relevant for materials research programs. For many such studies, the instrumentation can generate satisfactory spectra in a matter of minutes, permitting operando studies of battery materials at charge rates in excess of 1C. From the standpoint of instrumentation, a brief review of the present designs is presented with emphasis on recent advances in extended-XAFS capabilities. Differences between the spectrometer designs and the advantages of each will also be highlighted. Regarding materials inquiry, a survey of the above instrument’s application to energy storage research is provided. This will include XAFS analyses of two systems of nanoparticle compounds which were engineered to be catalysts or pseudocapacitors. These results will facilitate a review of the information attainable from the underlying technique. The rich electronic structure of vanadyl phosphate cathode materials, as well a suite of vanadium oxides, is probed via valence-to-core XES (VTC-XES). Finally, operando x-ray absorption near-edge structure (XANES) results obtained from a pouch cell containing a nickel-rich NMC cathode at a variety of charge rates are presented.

Enhanced Stability and CO/Formate Selectivity of Plasma-Treated SnO x/AgO x Catalysts during CO2 Electroreduction.

CO2 electroreduction into useful chemicals and fuels is a promising technology that might be used to minimize the impact that the increasing industrial CO2 emissions are having on the environment. Although plasma-oxidized silver surfaces were found to display a considerably decreased overpotential for the production of CO, the hydrogen evolution reaction (HER), a competing reaction against CO2 reduction, was found to increase over time. More stable and C1-product-selective SnOx/AgOx catalysts were obtained by electrodepositing Sn on O2-plasma-pretreated Ag surfaces. In particular, a strong suppression of HER (below 5% Faradaic efficiency (FE) at −0.8 V vs the reversible hydrogen electrode, RHE) during 20 h was observed. Ex situ scanning electron microscopy (SEM) combined with energy-dispersive X-ray spectroscopy (EDS), quasi in situ X-ray photoelectron spectroscopy (XPS), and operando X-ray absorption near-edge structure spectroscopy (XANES) measurements showed that our synthesis led to a highly roughened surface containing stable Snδ+/Sn species that were found to be key in the enhanced activity and stable CO/formate (HCOO–) selectivity. Our study highlights the importance of roughness, composition, and chemical state effects in CO2 electrocatalysis.

Operando X-ray Absorption Spectroscopy and Emission Kβ1,3 Study of the Manganese Redox Activity in High-Capacity Li4Mn2O5 Cathode

The electrochemical performance of nanostructured Li4Mn2O5 (or rather the 0.93Li3.6–xMn2.4O5.4–0.07Li2O composite) displaying an outstanding charge capacity of 350 mA h/g was recently reported. Interestingly, the removal of lithium from Li4Mn2O5 is found to take place beyond the oxidation limit of +4 for Mn in an octahedral environment. To characterize the nature of this extra capacity, we have approached the study of the redox chemistry and local structure of manganese in Li4Mn2O5 via a combination of X-ray absorption and emission spectroscopies at the manganese K-edge. To support our results, we have thoroughly characterized the composition of the materials at several potential values by inductively coupled plasma and online electrochemical mass spectrometry. Additionally, operando X-ray absorption near-edge structure studies, in excellent agreement with ex situ data, were carried out for the charge and discharge of the battery. Our results unequivocally rule out the participation of the Mn4+/Mn5+ redox couple and indicate the participation of oxygen in the electrochemistry. After the first charge, the battery cycles reversibly between the charged and discharged states, where the lithium exchange is mainly compensated by anionic redox.

Molecularly Tailored Nickel Precursor and Support Yield a Stable Methane Dry Reforming Catalyst with Superior Metal Utilization

Syngas production via the dry reforming of methane (DRM) is a highly endothermic process conducted under harsh conditions; hence, the main difficulty resides in generating stable catalysts. This can, in principle, be achieved by reducing coke formation, sintering, and loss of metal through diffusion in the support. [{Ni(μ2-OCHO)(OCHO)(tmeda)}2(μ2-OH2)] (tmeda = tetramethylethylenediamine), readily synthesized and soluble in a broad range of solvents, was developed as a molecular precursor to form 2 nm Ni(0) nanoparticles on alumina, the commonly used support in DRM. While such small nanoparticles prevent coke deposition and increase the initial activity, operando X-ray Absorption Near-Edge Structure (XANES) spectroscopy confirms that deactivation largely occurs through the migration of Ni into the support. However, we show that Ni loss into the support can be mitigated through the Mg-doping of alumina, thereby increasing significantly the stability for DRM. The superior performance of our catalytic system is a direct consequence of the molecular design of the metal precursor and the support, resulting in a maximization of the amount of accessible metallic nickel in the form of small nanoparticles while preventing coke deposition.

Unleashing the Power and Energy of LiFePO4-Based Redox Flow Lithium Battery with a Bifunctional Redox Mediator.

Redox flow batteries, despite great operation flexibility and scalability for large-scale energy storage, suffer from low energy density and relatively high cost as compared to the state-of-the-art Li-ion batteries. Here we report a redox flow lithium battery, which operates via the redox targeting reactions of LiFePO4 with a bifunctional redox mediator, 2,3,5,6-tetramethyl-p-phenylenediamine, and presents superb energy density as the Li-ion battery and system flexibility as the redox flow battery. The battery has achieved a tank energy density as high as 1023 Wh/L, power density of 61 mW/cm2, and voltage efficiency of 91%. Operando X-ray absorption near-edge structure measurements were conducted to monitor the evolution of LiFePO4, which provides insightful information on the redox targeting process, critical to the device operation and optimization.

Operando XANES study of simulated transient cycles on a Pd-only three-way catalyst

A model Pd-only three-way catalyst has been subjected to simulated driving conditions of natural gas and gasoline operation in an operando reactor cell for X-ray absorption spectroscopy that included alternated, but longer than real oscillations, rich and lean periods and a high temperature surge (850–900 °C). The X-ray absorption near edge structure (XANES) spectra indicated that metallic palladium is observed in the whole temperature range investigated (up to 900 °C) and irrespective of the air/fuel ratio. In both natural gas and gasoline cycles, the XANES data show that the PdO reduced in the rich periods cannot be restored in the lean periods. With this background, activity for methane abatement in the high temperature regime is greatly affected by the oxidation state of palladium rather than by the change of air/fuel ratio. In the case of propene oxidation, while Pd also remains predominantly in the reduced state, activity is dictated by the oxygen concentration in the feedstock. Comparison between the two hydrocarbons demonstrates that the oxidation state of Pd may be responsible for observed methane emissions under realistic operating circumstances. Moreover, the experiments demonstrate that reduced Pd may be continuously present during operation in agreement with observations on real catalytic converters. Although this may be the average oxidation state of Pd, more advanced probes are certainly necessary to capture variations of oxidation state under the fast oscillatory conditions needed to imitate real operation.