Operando Spectroscopy Research Articles

Molecular Views on Mechanisms of Brønsted Acid-Catalyzed Reactions in Zeolites.

The Brønsted acidity of proton-exchanged zeolites has historically led to the most impactful applications of these materials in heterogeneous catalysis, mainly in the fields of transformations of hydrocarbons and oxygenates. Unravelling the mechanisms at the atomic scale of these transformations has been the object of tremendous efforts in the last decades. Such investigations have extended our fundamental knowledge about the respective roles of acidity and confinement in the catalytic properties of proton exchanged zeolites. The emerging concepts are of general relevance at the crossroad of heterogeneous catalysis and molecular chemistry. In the present review, emphasis is given to molecular views on the mechanism of generic transformations catalyzed by Brønsted acid sites of zeolites, combining the information gained from advanced kinetic analysis, in situ, and operando spectroscopies, and quantum chemistry calculations. After reviewing the current knowledge on the nature of the Brønsted acid sites themselves, and the key parameters in catalysis by zeolites, a focus is made on reactions undergone by alkenes, alkanes, aromatic molecules, alcohols, and polyhydroxy molecules. Elementary events of C-C, C-H, and C-O bond breaking and formation are at the core of these reactions. Outlooks are given to take up the future challenges in the field, aiming at getting ever more accurate views on these mechanisms, and as the ultimate goal, to provide rational tools for the design of improved zeolite-based Brønsted acid catalysts.

Anchoring hydroxyl intermediate on NiCo(OOH)x nanosheets to enable highly efficient electrooxidation of benzyl alcohols

AbstractSelective oxidation of alcohols to the carboxylic acid plays an important role in the production of valuable chemicals. Herein, NiCo hydroxide (NiCo(OOH)x) nanosheets with a thickness of ~4 nm were controllably fabricated in a facile way efficient for the electrooxidation of benzyl alcohol (BAL) to benzoic acid (BAD). Mechanistic studies confirmed the hydroxyl active intermediate (OH*) generated on the surface of NiCo(OOH)x nanosheets through operando electrochemical impedance spectroscopy and the electron paramagnetic resonance spectroscopy analysis during the electrooxidation process. Importantly, we discovered that the OH* was crucial to boost the selective oxidation of BAL by attacking the hydroxyl end group to achieve carboxylic acid. The early onset potential of 1.16 VRHE in the presence of BAL was below 1.23 VRHE for oxygen evolution reaction. The rotating ring‐disk electrode further revealed that the electrocatalytic BAL oxidation reaction occurred earlier in the oxygen evolution reaction. In 150 mM BAL, NiCo(OOH)x nanosheets generated a high BAD selectivity of >99% and full conversion of BAL in 3 h. The experimental and theoretical calculation revealed that the dominant pathway of BAL oxidation was the interaction between nucleophilic BAL with OH* to form BAL‐OH* directly by dehydrogenates, thereby producing BAD with high efficiency. This research sheds mechanism insights on the electrocatalytic alcohols oxidation reaction.

Oxygen-defects evolution to stimulate continuous capacity increase in Co-free Li-rich layered oxides

Though oxygen defects are associated with deteriorated structures and aggravated cycling performance in traditional layered cathodes, the role of oxygen defects is still ambiguous in Li-rich layered oxides due to the involvement of oxygen redox. Herein, a Co-free Li-rich layered oxide Li1.286Ni0.071Mn0.643O2 has been prepared by a co-precipitation method to systematically investigate the undefined effects of the oxygen defects. A significant O2 release and the propagation of oxygen vacancies were detected by operando differential electrochemical mass spectroscopy (DEMS) and electron energy loss spectroscopy (EELS), respectively. Scanning transmission electron microscopy-high angle annular dark field (STEM-HAADF) reveals the oxygen vacancies fusing to nanovoids and monitors a stepwise electrochemical activation process of the large Li2MnO3 domain upon cycling. Combined with the quantitative analysis conducted by the energy dispersive spectrometer (EDS), existed nano-scale oxygen defects actually expose more surface to the electrolyte for facilitating the electrochemical activation and subsequently increasing available capacity. Overall, this work persuasively elucidates the function of oxygen defects on oxygen redox in Co-free Li-rich layered oxides.

Thickness-dependent β/γ-NiOOH transformation of Ni-MOFs in oxygen evolution reaction

In-situ generated hydroxides and oxyhydroxides have been gradually recognized as the real active sites of metal organic frameworks (MOFs) towards the oxygen evolution reaction (OER). Nevertheless, the fine phase evolution process is still less of concern and elusive. Herein, we successfully construct serious of Ni-MOFs with differing thickness ranging from 1 nm to 270 nm through a competitive coordination strategy. Utilizing the operando resonance spectroscopy, an accurate phase inversion behavior of the Ni-MOFs has been witnessed during the OER, and demonstrated as a thickness-dependent process. Ultrathin architecture is identified as a positive factor to promote the exchange between MOFs ligands and applied electrolyte during the electrochemical test. Hence, after removing most of the internal ligands, 2D MOFs favor to be transformed as β-NiOOH, which is more active than the generated γ-NiOOH in bulky system. The density function theory (DFT) calculation alongside with the in-situ electrochemistry tests further affirm the β/γ-NiOOH transformation behavior. This work offers a new understanding of the electrocatalytic mechanism from the view point of phase evolution in 2D MOFs.

Hybrid molecular graphene transistor as an operando and optoelectronic platform

Lack of reproducibility hampers molecular devices integration into large-scale circuits. Thus, incorporating operando characterization can facilitate the understanding of multiple features producing disparities in different devices. In this work, we report the realization of hybrid molecular graphene field effect transistors (m-GFETs) based on 11-(Ferrocenyl)undecanethiol (FcC11SH) micro self-assembled monolayers (μSAMs) and high-quality graphene (Gr) in a back-gated configuration. On the one hand, Gr enables redox electron transfer, avoids molecular degradation and permits operando spectroscopy. On the other hand, molecular electrode decoration shifts the Gr Dirac point (VDP) to neutrality and generates a photocurrent in the Gr electron conduction regime. Benefitting from this heterogeneous response, the m-GFETs can implement optoelectronic AND/OR logic functions. Our approach represents a step forward in the field of molecular scale electronics with implications in sensing and computing based on sustainable chemicals.

Understanding the effect of MXene in a TMO/MXene hybrid catalyst for the oxygen evolution reaction

Very recently, it has been reported that mixed transition metal oxide (TMO)/MXene catalysts show improved performance over TMO only catalysts for the oxygen evolution reaction (OER). However, the reasoning behind this observation is unknown. In this work mixed Co(OH)2/Ti3C2Tx were prepared and characterized for the OER using ex situ and operando spectroscopy techniques in order to initiate the understanding of why mixed TMO/MXene materials show better performances compared to TMO only catalysts. This work shows that the improved electrocatalysis for the composite material compared to the TMO only catalyst is due to the presence of higher Co oxide oxidation states at lower OER overpotentials for the mixed TMO/MXene catalysts. Furthermore, the presence of the MXene allows for a more mechanically robust film during OER, making the film more stable. Finally, our results show that small amounts of MXene are more advantageous for the OER during long-term stability measurements, which is linked to the formation of TiO2. The sensitivity of MXene oxidation ultimately limits TMO/MXene composites under alkaline OER conditions, meaning mass fractions must be carefully considered when designing such a catalyst to minimize the residual TiO2 formed during its lifetime.

Revealing the effect of electrolyte coordination structures on the intercalation chemistry of batteries

In-depth understanding of the electrolyte-dependent intercalation chemistry in batteries through direct operando/in situ characterizations is crucial for the development of the high-performance batteries. Herein, taking the Al/graphite battery as a model system, the effect of electrolyte coordination structure on the intercalation processes has been investigated over the batteries with either 1-hexyl-3-methylimidazolium chloride (HMICl)-AlCl3 or 1-ethyl-3-methylimidazolium chloride (EMICl)-AlCl3 ionic liquid electrolyte using operando X-ray photoelectron spectroscopy (XPS) and X-ray diffraction. With a weaker anion-cation interaction in HMI-based electrolyte, the XPS-derived atomic ratio between co-intercalated N and intercalated Al is 0.9, which is lower than 1.6 for EMI-based electrolyte. Attributed to the additional de-solvation process, the batteries with the HMI-based electrolyte show a lower ionic diffusion rate, capacity, and cycling performance, which agree with the operando characterization results. Our findings highlight the critical role of the electrolyte coordination structure on the (co–)intercalation chemistry.

Analysis of Iodide Transport on Methyl Ammonium Lead Iodide Perovskite Solar Cell Structure Through Operando Hard X-ray Photoelectron Spectroscopy

To analyze the dynamic of ion species through a MAPbI3–xClx (HaP) solar cell, an operando hard X-ray photoelectron spectroscopy (HAXPES) study was proposed. Stacked Au/Ag/AZO/PCBM/HaP/NiO/ITO/glass structures, where AZO, PCBM, NiO, and ITO stand for Al-doped zinc oxide, phenyl-C61-butyric acid methyl ester, nickel oxide, and indium tin oxide, respectively, were evaluated. Experimental core level analysis under an applied bias voltage highlighted that a certain threshold voltage (above the open-circuit voltage (VOC)) is required to trigger the iodide (I–) migration from the HaP to the adjacent electron and hole transport layers. Study also demonstrated that the diffusion of I– in AZO/PCBM over time is unidirectional and follows the gradient descent in concentration. We observed that the irreversibility of the I– transport leads to the absorption of I– by the Ag layer to form a silver iodide (AgI) compound. Furthermore, this work also pointed out the efficiency of an ITO layer when deposited between Ag and AZO to serve as the ion diffusion barrier layer. Finally, the results demonstrated that the presence of ITO restrains the AgI formation and contributes to improving cell performances.

State of the Surface During CO Hydrogenation over Ni(111) and Ni(211) Probed by Operando X-ray Photoelectron Spectroscopy

The state of the surface near-region during CO hydrogenation of Ni(111) and Ni(211) single crystal surfaces was investigated using various gas mixtures between 150 and 500 mbar, 200 and 325 °C, by operando X-ray photoelectron spectroscopy. We report how higher temperatures and hydrogen content correlate with a movement of CO away from the on-top configurations and toward multicoordinated sites of the nickel surface and how a nickel carbide is formed in the surface near region, particularly at high partial pressures of CO and lower temperatures. The presence of the carbide affects the CO bonding and was observed to be reduced during hydrogen-rich conditions and temperatures above 250 °C.

CO2 Methanation over Nickel Catalysts: Support Effects Investigated through Specific Activity and Operando IR Spectroscopy Measurements

Renewed interest in CO2 methanation is due to its role within the framework of the Power-to-Methane processes. While the use of nickel-based catalysts for CO2 methanation is well stablished, the support is being subjected to thorough research due to its complex effects. The objective of this work was the study of the influence of the support with a series of catalysts supported on alumina, ceria, ceria–zirconia, and titania. Catalysts’ performance has been kinetically and spectroscopically evaluated over a wide range of temperatures (150–500 °C). The main results have shown remarkable differences among the catalysts as concerns Ni dispersion, metallic precursor reducibility, basic properties, and catalytic activity. Operando infrared spectroscopy measurements have evidenced the presence of almost the same type of adsorbed species during the course of the reaction, but with different relative intensities. The results indicate that using as support of Ni a reducible metal oxide that is capable of developing the basicity associated with medium-strength basic sites and a suitable balance between metallic sites and centers linked to the support leads to high CO2 methanation activity. In addition, the results obtained by operando FTIR spectroscopy suggest that CO2 methanation follows the formate pathway over the catalysts under consideration.

Operando Laboratory X-ray Absorption Spectroscopy and UV–Vis Study of Pt/TiO2 Photocatalysts during Photodeposition and Hydrogen Evolution Reactions

Photocatalytic hydrogen (H2) production is a promising route for alternative energetics. Understanding structure–activity relationships is a crucial step towards the rational design of photocatalysts, which requires the application of operando spectroscopy under relevant working conditions. We performed an operando investigation on a catalytic system during the photodeposition of Pt on TiO2 and photostimulated H2 production, using simultaneous laboratory X-ray absorption spectroscopy (XAS), UV–Vis spectroscopy, and mass spectrometry. XAS showed a progressive increase in Pt fluorescence for Pt deposited on TiO2 for over an hour, which is correlated with the signal of the produced H2. The final Pt/TiO2 catalyst contained Pt(0) particles. The electronic features corresponding to the Pt4+ species in the UV–Vis spectrum of the solution disappear as soon as UV radiation is applied in the presence of formic acid, which acts as a hole scavenger, resulting in the presence of Pt(0) particles in solution.

Spatial-Temporal Dynamics at the Interface of 3D-Printed Photocurable Thermoset Resin Layers

Additive manufacturing (AM) is used to fabricate polymeric materials into complex three-dimensional (3D) structures. As the 3D structure is built by sequential layer-by-layer deposition of filaments dispensed from a translating nozzle (in the case of extrusion-based printing), defects often form at the filament-filament interface. The out-of-equilibrium structural development that occurs during the printing process is difficult to directly measure by quantitative means, limiting our understanding of the physical mechanisms at play. Here, we utilize in operando X-ray photon correlation spectroscopy (XPCS) measurements with microbeam capability to probe the real-time structural evolution at the filament-filament interface during extrusion 3D printing. We investigate the solidification of a dual-cure (UV/thermal) acrylate/epoxy resin during multilayer 3D printing as a rational model by tracking the nanoscale motion of filler particles embedded in the resin. The spatially and temporally resolved dynamics (on length scales from several nm to a few hundreds of nm and time scales of 10–3 < t < 103 seconds) are measured during the deposition of a single filament as well as during the deposition of a second layer on top of the cured underlayer. The addition of a second layer introduces structural perturbations at the interface and results in accelerated interfacial dynamics compared to those of the cured underlayer. However, as time proceeds, the local dynamical heterogeneity disappears, and the evolution of the dynamics progresses uniformly within the entire interfacial region. The homogeneity across the interface results from the formation of an interpenetrated epoxy network that spans across the first and second filaments. This homogeneous interface is responsible for the isotropic tensile properties of a 3D-printed sample that are independent of print direction and nearly the same as the bulk (non-3D-printed) sample. The XPCS microrheology approach provides insight into the dynamics-process-property relationship at the printed filament interfaces.

Trace Cobalt Doping and Defect Engineering of High Surface Area α‐Ni(OH)2 for Electrocatalytic Urea Oxidation

Owing to the intrinsically sluggish kinetics of urea oxidation reaction (UOR) involving a six‐electron transfer process, developing efficient UOR electrocatalyst is a great challenge remained to be overwhelmed. Herein, by taking advantage of 2‐Methylimidazole, of which is a kind of alkali in water and owns strong coordination ability to Co2+ in methanol, trace Co (1.0 mol%) addition was found to induce defect engineering on α‐Ni(OH)2 in a dual‐solvent system of water and methanol. Physical characterization results revealed that the synthesized electrocatalyst (WM‐Ni0.99Co0.01(OH)2) was a kind of defective nanosheet with thickness around 5–6 nm, attributing to the synergistic effect of Co doping and defect engineering, its electron structure was finely altered, and its specific surface area was tremendously enlarged from 68 to 172.3 m2 g−1. With all these merits, its overpotential to drive 10 mA cm−2 was reduced by 110 mV. Besides, the interfacial behavior of UOR was also well deciphered by operando electrochemical impedance spectroscopy.

Enhanced alkaline/seawater hydrogen evolution reaction performance of NiSe2 by ruthenium and tungsten bimetal doping

The exploration of high-efficiency and stable electrocatalysts for alkaline and seawater hydrogen evolution reaction (HER) is the key to realize energy conversion, but there is still a significant challenge owing to the slow HER kinetics in alkaline and seawater systems. In this study, we prepared nickel foamed-supported Ru, W co-doped NiSe2 (Ru, W–NiSe2/NF) by a brief two-step hydrothermal strategy and the prepared Ru, W–NiSe2/NF displays exceptional HER property, requiring only a low overpotential of 100 and 353 mV to reach 10 mA cm−2 in 1 M KOH and natural seawater, respectively, far superior to Ru–NiSe2/NF, W–NiSe2/NF and NiSe2/NF. Electrochemical surface area (ECSA) and operando electrochemical impedance spectroscopy (EIS) verify the abundant active sites and superior electron transfer rate of Ru, W–NiSe2, which optimized the HER kinetics in alkaline solution and natural seawater. The ECSA normalization and TOF results indicated that Ru, W co-doping increased the intrinsic activity of NiSe2. This study revealed the impact of bimetallic doping on the intrinsic activity of NiSe2, and provided a practical strategy for designing and developing the HER electrocatalysts with excellent performance.

Zhang-Rice singlets state formed by two-step oxidation for triggering water oxidation under operando conditions

The production of ecologically compatible fuels by electrochemical water splitting is highly desirable for modern industry. The Zhang-Rice singlet is well known for the superconductivity of high-temperature superconductors cuprate, but is rarely known for an electrochemical catalyst. Herein, we observe two steps of surface reconstruction from initial catalytic inactive Cu1+ in hydrogen treated Cu2O to Cu2+ state and further to catalytic active Zhang-Rice singlet state during the oxygen evolution reaction for water splitting. The hydrogen treated Cu2O catalyst exhibits a superior catalytic activity and stability for water splitting and is an efficient rival of other 3d-transition-metal catalysts. Multiple operando spectroscopies indicate that Zhang-Rice singlet is real active species, since it appears only under oxygen evolution reaction condition. This work provides an insight in developing an electrochemical catalyst from catalytically inactive materials and improves understanding of the mechanism of a Cu-based catalyst for water oxidation.

Understanding Rh‐catalysed Hydroformylation with Phosphite Ligands through Catalyst Speciation Analysis by Operando FlowNMR Spectroscopy

AbstractPhosphites are industrially important ligands for the Rh‐catalysed hydroformylation of olefins because they produce more active catalysts than phosphines, which is mainly due to their strong π‐acceptor properties facilitating CO dissociation from the metal centre. Herein, the effect of three prominent phosphite ligands (triphenylphosphite, Alkanox and BiPhePhos) on catalyst speciation during turnover is investigated using multi‐nuclear operando FlowNMR spectroscopy. The quantitative catalyst distribution maps derived explain activity trends across a range of catalytic reaction conditions that show how phosphites produce more active catalysts, including reduced formation of inactive Rh0 dimers in the absence of substrate during pre‐activation and at the end of the reaction. Additionally, [(alkanox)Rh(acac)] complexes have been found to be of high stability which can be exploited for post‐catalytic Rh recovery by simple addition of 2,4‐pentanediones.

Shape-Dependent CO2 Hydrogenation to Methanol over Cu2O Nanocubes Supported on ZnO.

The hydrogenation of CO2 to methanol over Cu/ZnO-based catalysts is highly sensitive to the surface composition and catalyst structure. Thus, its optimization requires a deep understanding of the influence of the pre-catalyst structure on its evolution under realistic reaction conditions, including the formation and stabilization of the most active sites. Here, the role of the pre-catalyst shape (cubic vs spherical) in the activity and selectivity of ZnO-supported Cu nanoparticles was investigated during methanol synthesis. A combination of ex situ, in situ, and operando microscopy, spectroscopy, and diffraction methods revealed drastic changes in the morphology and composition of the shaped pre-catalysts under reaction conditions. In particular, the rounding of the cubes and partial loss of the (100) facets were observed, although such motifs remained in smaller domains. Nonetheless, the initial pre-catalyst structure was found to strongly affect its subsequent transformation in the course of the CO2 hydrogenation reaction and activity/selectivity trends. In particular, the cubic Cu particles displayed an increased activity for methanol production, although at the cost of a slightly reduced selectivity when compared to similarly sized spherical particles. These findings were rationalized with the help of density functional theory calculations.

Understanding the failure process of sulfide-based all-solid-state lithium batteries via operando nuclear magnetic resonance spectroscopy

The performance of all-solid-state lithium metal batteries (SSLMBs) is affected by the presence of electrochemically inactive (i.e., electronically and/or ionically disconnected) lithium metal and solid electrolyte interphase (SEI), which are jointly termed inactive lithium. However, the differentiation and quantification of inactive lithium during cycling are challenging, and their lack limits the fundamental understanding of SSLMBs failure mechanisms. To shed some light on these crucial aspects, here, we propose operando nuclear magnetic resonance (NMR) spectroscopy measurements for real-time quantification and evolution-tracking of inactive lithium formed in SSLMBs. In particular, we examine four different sulfide-based solid electrolytes, namely, Li10GeP2S12, Li9.54Si1.74P1.44S11.7Cl0.3, Li6PS5Cl and Li7P3S11. We found that the chemistry of the solid electrolyte influences the activity of lithium. Furthermore, we demonstrate that electronically disconnected lithium metal is mainly found in the interior of solid electrolytes, and ionically disconnected lithium metal is found at the negative electrode surface. Moreover, by monitoring the Li NMR signal during cell calendar ageing, we prove the faster corrosion rate of mossy/dendritic lithium than flat/homogeneous lithium in SSLMBs.

Homogeneous Carbon Dot-Anchored Fe(III) Catalysts with Self-Regulated Proton Transfer for Recyclable Fenton Chemistry.

Fenton chemistry has been widely studied in a broad range from geochemistry, chemical oxidation to tumor chemodynamic therapy. It was well established that Fe3+/H2O2 resulted in a sluggish initial rate or even inactivity. Herein, we report the homogeneous carbon dot-anchored Fe(III) catalysts (CD-COOFeIII) wherein CD-COOFeIII active center activates H2O2 to produce hydroxyl radicals (•OH) reaching 105 times larger than that of the Fe3+/H2O2 system. The key is the •OH flux produced from the O-O bond reductive cleavage boosting by the high electron-transfer rate constants of CD defects and its self-regulated proton-transfer behavior probed by operando ATR-FTIR spectroscopy in D2O and kinetic isotope effects, respectively. Organic molecules interact with CD-COOFeIII via hydrogen bonds, promoting the electron-transfer rate constants during the redox reaction of CD defects. The antibiotics removal efficiency in the CD-COOFeIII/H2O2 system is at least 51 times large than the Fe3+/H2O2 system under equivalent conditions. Our findings provide a new pathway for traditional Fenton chemistry.

Probing the Nature of Zinc in Copper-Zinc-Zirconium Catalysts by Operando Spectroscopies for CO2 Hydrogenation to Methanol.

Active Zn species in Cu-based methanol synthesis catalysts have not been clearly identified yet due to their complex nature and dynamic structural changes during reactions. Herein, atomically dispersed Zn on ZrO2 support is established in Cu-based catalysts by separating Zn and Zr components from Cu (Cu-ZnZr) via the double-nozzle flame spray pyrolysis (DFSP) method. It exhibits superiority in methanol selectivity and yield compared to those with Cu-ZnO interface and isolated ZnO nanoparticles. Operando X-ray absorption spectroscopy (XAS) reveals that the atomically dispersed Zn species are induced during the reaction due to the strengthened Zn-Zr interaction. They can suppress formate decomposition to CO and decrease the H2 dissociation energy, shifting the reaction to methanol production. This work enlightens the rational design of unique Zn species by regulating coordination environments and offers a new perspective for exploring complex interactions in multi-component catalysts.