Operando X-ray Absorption Spectroscopy Studies Research Articles

Probing the Evolution of Palladium Species in Pd@MOF Catalysts during the Heck Coupling Reaction: An Operando X-ray Absorption Spectroscopy Study.

The mechanism of the Heck C-C coupling reaction catalyzed by Pd@MOFs has been investigated using operando X-ray absorption spectroscopy (XAS) and powder X-ray diffraction (PXRD) combined with transmission electron microscopy (TEM) analysis and nuclear magnetic resonance (1H NMR) kinetic studies. A custom-made reaction cell was used, allowing operando PXRD and XAS data collection using high-energy synchrotron radiation. By analyzing the XAS data in combination with ex situ studies, the evolution of the palladium species is followed from the as-synthesized to its deactivated form. An adaptive reaction mechanism is proposed. Mononuclear Pd(II) complexes are found to be the dominant active species at the beginning of the reaction, which then gradually transform into Pd nanoclusters with 13-20 Pd atoms on average in later catalytic turnovers. Consumption of available reagent and substrate leads to coordination of Cl- ions to their surfaces, which causes the poisoning of the active sites. By understanding the deactivation process, it was possible to tune the reaction conditions and prolong the lifetime of the catalyst.

Dual reactor for in situ/operando fluorescent mode XAS studies of sample containing low-concentration 3d or 5d metal elements.

Transition metal elements are the most important elements of heterogeneous catalysts used for chemical and energy transformations. Many of these catalysts are active at a temperature higher than 400 °C. For a catalyst containing a 3d or 5d metal element with a low concentration, typically their released fluorescence upon the K-edge or L-edge adsorption of X-rays is collected for the analysis of chemical and coordination environments of these elements. However, it is challenging to perform in situ/operando X-ray absorption spectroscopy (XAS) studies of elements of low-energy absorption edges at a low concentration in a catalyst during catalysis at a temperature higher than about 450 °C. Here a unique reaction system consisting two reactors, called a dual reactor system, was designed for performing in situ or operando XAS studies of these elements of low-energy absorption edges in a catalyst at a low concentration during catalysis at a temperature higher than 450 °C in a fluorescent mode. This dual-reactor system contains a quartz reactor for preforming high-temperature catalysis up to 950 °C and a Kapton reactor remaining at a temperature up to 450 °C for collecting data in the same gas of catalysis. With this dual reactor, chemical and coordination environments of low-concentration metal elements with low-energy absorption edges such as the K-edge of 3d metals including Ti, V, Cr, Mn, Fe, Co, Ni, and Cu and L edge of 5d metals including W, Re, Os, Ir, Pt, and Au can be examined through first performing catalysis at a temperature higher than 450 °C in the quartz reactor and then immediately flipping the catalyst in the same gas flow to the Kapton reactor remained up to 450 °C to collect data. The capability of this dual reactor was demonstrated by tracking the Mn K-edge of the MnOx/Na2WO4 catalyst during activation in the temperature range of 300-900 °C and catalysis at 850 °C.

The synergetic interaction between LiNO3 and lithium polysulfides for suppressing shuttle effect of lithium-sulfur batteries

LiNO3 has been widely used as an effective electrolyte additive in lithium-sulfur (Li-S) batteries to suppress the polysulfide shuttle effect. To better understand the mechanism of suppressed shuttle effect by LiNO3, herein we report a comprehensive investigation of the influence of LiNO3 additive on the formation process of the solid electrolyte interphase (SEI) layer on lithium anode of Li-S batteries by operando X-ray absorption spectroscopy (XAS). We observed that a compact and stable SEI layer composed of Li2SO3 and Li2SO4 on top of lithium anode is formed during the initial discharge process due to the synergetic effect of shuttled polysulfides and LiNO3, which can effectively suppress the subsequent reaction between polysulfides in electrolyte and lithium metal and thus result in the alleviation of polysulfide shuttle effect. In contrast, when using electrolyte without LiNO3, the shuttled polysulfides continuously react with lithium metal to form insulating Li2S on lithium anode, leading to the irreversible capacity loss. Our present operando XAS study provides a valuable insight into the important role of LiNO3 for the protection of lithium anodes, which will be beneficial for the further development of new electrolyte additives for high-performance Li-S batteries.

Operando X-ray Absorption Spectroscopy Study of Atomic Phase Reversibility with Wavelet Transform in the Lithium-Rich Manganese Based Oxide Cathode

Lithium-rich, Mn-based composites, such as Li(Li0.2Mn0.54Ni0.13Co0.13)O2, retain immense interest for researchers and practitioners as one of the most promising candidates for electric vehicle applications (EV/HEV/PHEV). Enabling the commercialization of this battery system requires reducing intrinsic drawbacks, that is, large capacity fading during first cycle or low-rate capability; it is of great importance to optimize the composite design of the battery cathode. Information about the electronic structure and atomic environment of the transition metals (Mn, Co, and Ni) provides crucial insight into the structure of high-energy density cathodes. In this study, we present precise characterization of the atomic environments of each transition metal by operando X-ray absorption spectroscopy using continuous Cauchy wavelet transform (CCWT) analysis during the first electrochemical cycle at the charge rate of 0.5 C. The CCWT interpretation provides continuous decomposition of extended X-ray absorption fine s...

Operando XAS study of the influence of CO and NO on methane oxidation by Pd/Al2O3

Methane oxidation on Pd/Al2O3 has been investigated using operando X-ray absorption spectroscopy (XAS) at the Pd K-edge. The influence of CO and NO on methane abatement has been addressed performing temperature programmed reaction runs while recording simultaneously XANES spectra. During CO oxidation Pd is reduced while methane conversion is shifted to higher temperature. NO strongly inhibits both CO and CH4 oxidation, despite the higher fraction of oxidized palladium.

In operando X-ray absorption spectroscopy study of charge rate effects on the atomic environment in graphene-coated Li-rich mixed oxide cathode

Li2O extraction from the electrode material is known to be a dominant mechanism of irreversible battery capacity loss in the first cycle. The extraction mechanism of the Li+ ions shows dependence on the charge rate. Here for the first time we report the difference in the electrochemical behavior at two different charge rates (0.125C vs 0.5C) observed using novel design transmission coin cells in the graphene-coated Li(Li0.2Mn0.54Ni0.13Co0.13)O2 cathode by in operando X-ray absorption spectroscopy (XAS). The results obtained from Mn, Co, and Ni atom XANES/EXAFS demonstrate that, whilst during fast charge Li2O extraction is localized to the lithium slab in the crystal structure, the delithiation is deeper at the slower charge rate, when Li+ ions are removed from both the transition metal and lithium layers. In the slow charge cell, NiO bond splitting resulting from the Jahn-Teller distortion were clearly identified at approximately 4.224V when a sudden rise in the bond length was observed in the EXAFS analysis of MnO and CoO bonds. The results demonstrate the power of using the novel cell design for transmission in operando XAS.

Rapid Mapping of Lithiation Dynamics in Transition Metal Oxide Particles with Operando X-ray Absorption Spectroscopy

Since the commercialization of lithium ion batteries (LIBs), layered transition metal oxides (LiMO2, where M = Co, Mn, Ni, or mixtures thereof) have been materials of choice for LIB cathodes. During cycling, the transition metals change their oxidation states, an effect that can be tracked by detecting energy shifts in the X-ray absorption near edge structure (XANES) spectrum. X-ray absorption spectroscopy (XAS) can therefore be used to visualize and quantify lithiation kinetics in transition metal oxide cathodes; however, in-situ measurements are often constrained by temporal resolution and X-ray dose, necessitating compromises in the electrochemistry cycling conditions used or the materials examined. We report a combined approach to reduce measurement time and X-ray exposure for operando XAS studies of lithium ion batteries. A highly discretized energy resolution coupled with advanced post-processing enables rapid yet reliable identification of the oxidation state. A full-field microscopy setup provides sub-particle resolution over a large area of battery electrode, enabling the oxidation state within many transition metal oxide particles to be tracked simultaneously. Here, we apply this approach to gain insights into the lithiation kinetics of a commercial, mixed-metal oxide cathode material, nickel cobalt aluminium oxide (NCA), during (dis)charge and its degradation during overcharge.

Transition metal dissolution and deposition in Li-ion batteries investigated by operando X-ray absorption spectroscopy

Operando X-ray absorption spectroscopy studies of transition metal dissolution in Li-ion batteries.

Conversion Reaction Mechanism in Complex Systems: A Triple X-Ray Absorption Spectroscopy Study of Tisnsb, a Very Efficient Negative Electrode Material for Li-Ion Batteries

The lithium-ion battery negative electrode material TiSnSb shows excellent electrochemical performance related to its high capacity (550 mA h g–1) and rate capability over several hundred cycles at relatively high rate (2 C).1 Such very good electrochemical performances are related to the highly reversible conversion reaction mechanism of this compound, and can be largely improved by specific electrode and electrolyte formulation.2–6 Using complementary information from operando X-ray diffraction (XRD) and 119Sn Mössbauer spectroscopy, it was determined that during the first discharge, TiSnSb undergoes a conversion process leading to the simultaneous formation of Li-Sb and Li-Sn intermetallics. The corresponding electrochemical equation was thus proposed for complete reaction of Li: TiSnSb + 6.5 Li = Ti + Li3Sb + 0.5 Li7Sn2 In this equation, however, the role of titanium was only hinted, since no specific information on this metal was available. Moreover, the mechanism by which the charge process occurs in TiSnSb is not yet fully understood due to the amorphous feature of the charged electrode. Different hypotheses have been proposed, including the presence of a mixture of binary phases, or the formation of a ternary alloy close to TiSnSb. At his time, however, the precise mechanism remains elusive. In order to shed more light on the role of titanium in this mechanism, and to better understand the interplay of the two p-group metal during the complex electrochemical discharge/charge mechanism, a complete operando X-ray absorption spectroscopy study of the system at the absorption K-edges of the three metals was performed. Operando EXAFS data at the Ti K-edge (Fig. 1), obtained on self-supported electrodes of TiSnSb formulated with CMC, provided clear evidence of the formation of Ti metal nanoparticles at the end of the discharge, confirming the supposed conversion mechanism. Operando EXAFS data obtained with fully formulated electrodes at the Sb and Sn edges (Fig. 2) show the formation of the lithiated phases, in line with the expected mechanism. However, the different reaction paths observed by comparing the Sn and the Sb spectra suggest that the two metals do not react simultaneously, and that Sn-rich intermediates are formed through the discharge and again during the following charge. These results are very important to corroborate the effective conversion mechanism in such system, and shed more light on the interplay of the two p-group metals in such complex system. References (1) Sougrati, M. T.; Fullenwarth, J.; Debenedetti, A.; Fraisse, B.; Jumas, J.-C.; Monconduit, L. J. Mater. Chem. 2011, 21, 10069. (2) Wilhelm, H.; Marino, C.; Darwiche, A.; Monconduit, L.; Lestriez, B. Electrochem. Commun. 2012, 24, 89. (3) Marino, C.; Sougrati, M. T.; Gerke, B.; Pöttgen, R.; Huo, H.; Ménétrier, M.; Grey, C. P.; Monconduit, L. Chem. Mater. 2012, 24, 4735. (4) Zhang, W.; Ghamouss, F.; Darwiche, A.; Monconduit, L.; Lemordant, D.; Dedryvère, R.; Martinez, H. J. Power Sources 2014, 268, 645. (5) Sivasankaran, V.; Marino, C.; Chamas, M.; Soudan, P.; Guyomard, D.; Jumas, J.-C.; Lippens, P.-E.; Monconduit, L.; Lestriez, B. J. Mater. Chem. 2011, 21, 5076. (6) Marino, C.; Darwiche, A.; Dupre, N.; Wilhelm, H.; Lestriez, B.; Martinez, H.; Dedryvère, R.; Zhang, W.; Ghamouss, F.; Lemordant, D.; Monconduit, L. J. Phys. Chem. C 2013, 117, 19302. Figure 1

Operando X-ray absorption spectroscopy study of supported Pt catalysts during NO reduction by hydrocarbons

To investigate the support effect on the hydrocarbon-NO-O2 reaction in automotive three-way catalytic reactions, operando X-ray absorption near edge structure (XANES) experiments were performed for reactions of C3H6 (or C3H8)-NO-O2 over Pt supported catalysts under reducing conditions. γ-Al2O3, ZrO2, CeO2-ZrO2 and La2O3 were used as the support oxides in this study. After oxidation pre-treatment, the oxidation state of the supported Pt was monitored by in situ Pt L3-edge XANES spectra while the temperature in the reaction mixture was ramped up and the catalytic reaction was followed by measuring the concentrations of C3H6 or C3H8, NO and O2. The operando experimental results were analyzed in terms of the catalytic reaction start-up temperature (20% HC conversion temperature) and the 50% Pt reduction temperature. A V-shape correlation was found between these two temperatures showing that an appropriate Pt reduction temperature should exist to lower the catalytic reaction start-up temperature. This result suggests that the creation of active metallic Pt sites together with the self-poisoning effect of adsorbed carbonaceous species on metallic Pt should determine the catalytic reaction start-up behavior of supported Pt catalysts. Besides the suitable Pt reduction temperature, the oxygen reactivity in the support oxide is also important for lowering the catalytic start-up temperature. The control of the Pt–support interactions to provide suitable Pt reducibility and also oxygen reactivity in oxide support are both important to achieve a lower catalytic reaction start-up temperature.

Operando X-ray Absorption Spectroscopy Study of Pt/γ-Al2O3 During the Total Oxidation of C3H6

Operando X-ray absorption fine structure (XAFS) investigations were performed on Pt/γ-Al2O3 during the total oxidation of C3H6 in reducing and oxidizing atmospheres. Study of the Pt valance state and catalytic conversion behavior as a function of temperature revealed that both the creation of metallic Pt sites and the activation of adsorbed species on Pt are important for the functioning of the catalytic reaction at low temperature.

Operando X-ray absorption spectroscopy studies on Pd-SnO2 based sensors

SnO2 gas sensors with palladium as additive in the range of 0.2 wt% and 3 wt% were studied by in situ X-ray absorption spectroscopy under idealized and real operating conditions. Simultaneously to the structural studies, measurements of the sensing properties were undertaken allowing for the determination of structure-function relationships. For this purpose a new in situ spectroscopic cell was designed which permitted on the one hand sensing on conventional screen printed 50 microm thick sensing layers and on the other hand structural analysis with X-rays provided by an insertion device at a 3rd generation synchrotron facility in fluorescence mode. Pd K-edge XANES and EXAFS results on gas sensors showed that palladium, present in an oxidized state, is finely dispersed if it is added in small quantities (0.2 wt%) while it forms clusters at higher concentrations (3 wt%). This is also reflected by the much easier reduction of palladium in the latter, higher concentrated ones. Under realistic sensing conditions (30-200 ppm H2; 10-50 ppm CO in dry and humid air at 200 and 300 degrees C) for the low additive concentration samples, no change in oxidation state was observed, i.e. palladium remained in its oxidized state. This has important consequences on the understanding and modeling of the gas sensing mechanism.