Near-ambient Pressure X-ray Photoelectron Spectroscopy Research Articles

Differences in oxidation-reduction kinetics and mobility of Cu species in fresh and SO2-poisoned Cu-SSZ-13 catalysts

Fresh and SO2-poisoned Cu-SSZ-13 selective catalytic reduction (SCR) catalysts were studied using near-ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) in the presence of NH3, O2, and NO, as well as by in situ X-ray absorption spectroscopy (XAS) using H2, He, and CO. In contrast to the fresh catalyst, inhibited reduction of Cu-species and the absence of Cu-metal nanoparticles was found in the SO2-poisoned catalyst during heating/cooling in H2 and CO. High structural disorder and differences in the formation of Cu-carbonyl species were seen for the SO2-poisoned catalyst compared to the fresh catalyst. Suppressed oxidation-reduction and low mobility of Cu-species during exposure to NH3-SCR-related gases were observed in the SO2-poisoned catalyst, unlike in the fresh catalyst. These observations help explain catalyst deactivation during the standard NH3-SCR reaction. The formation of Cu-metal nanoparticles in the fresh catalyst revealed another possible deactivation pathway for the SCR-catalyst in combined LNT-SCR systems during fuel-rich periods.

In-Situ High-Temperature X-Ray Absorption Spectroscopy of Fe-N-C Electrocatalysts during Pyrolysis

Deeper understanding of the formation of active sites during the pyrolysis of iron-nitrogen-carbon (Fe-N-C) electrocatalysts for the oxygen reduction reaction (ORR) is required. Here, we present a nitrogen-doped carbon foam support treated at high temperature (1600 °C) to stabilise the material properties. Iron-containing precursors are then adsorbed onto the surface, followed by heat treatment. This changes the chemical state of the adsorbed iron, whilst the material properties of the support are effectively unchanged. The two-step synthesis method used to create these model electrocatalysts decouples porosity, microstructure, and conductivity from the electrochemistry, which can instead be attributed mainly to changes in the chemical environment of iron.Advanced characterisation techniques such as in-situ high-temperature X-ray absorption spectroscopy (HT-XAS) and high-temperature near ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) are employed to gain insights into the state of the adsorbed iron as it decomposes during pyrolysis to form active sites. Transmission electron microscopy (TEM) is used to track the microstructure for different types of precursor heated at different temperatures. Meanwhile, in-situ electrochemical XAS is used to understand how the chemical environment of the catalysts change during the ORR.The results are discussed by relating different chemical environment with the electrochemical ORR activity. Despite the two-step synthesis technique, these model Fe-N-C electrocatalysts are highly active. The load-cycling durability and start-stop durability are measured, and the degradation mechanisms are discussed.

Elucidating the Formation Mechanisms of Fe-N-C Electrocatalysts Using High-Temperature in Situ Characterisation Techniques

Pt-free cathode catalysts are attracting increased interest to replace platinum in polymer electrolyte membrane fuel cells (PEFCs). Heat-treated nitrogen-doped carbons decorated with iron have been the most widely studied alternative. However, lack of understanding of the formation of active sites and degradation mechanisms in such catalysts is hindering their progress. Here, we use a nitrogen-doped carbon foam support with optimised porosity, surface area, and conductivity which is then infiltrated with three different iron precursors: iron (II) acetate (FeAc); iron (III) chloride (FeCl3); and iron phthalocyanine (FePc). These are then pyrolyzed to form Fe-N-C-based ORR electrocatalysts. This approach takes advantage of an already optimized conductive, porous, high surface area and temperature-stable support. The stability of the support helps to separate the role of the different iron precursors from other factors in the generation of catalytic active sites. High temperature X-ray absorption fine structure (HT-XAFS) and high temperature X-ray photoelectron spectroscopy (HT-XPS) were employed to study changes in the iron precursors during the synthesis of Fe-N-C electrocatalysts. Furthermore, near ambient pressure XPS (NAP-XPS) and in-situ electrochemical XAFS were performed to study the interaction between the synthesized catalysts and oxygen. These results are then related to the electrochemical activity.

Probing Lithium-Ion Battery Electrolytes with Laboratory Near-Ambient Pressure XPS

In this article, we present Near Ambient Pressure (NAP)-X-ray Photoelectron Spectroscopy (XPS) results from model and commercial liquid electrolytes for lithium-ion battery production using an automated laboratory NAP-XPS system. The electrolyte solutions were (i) LiPF6 in EC/DMC (LP30) as a typical commercial battery electrolyte and (ii) LiTFSI in PC as a model electrolyte. We analyzed the LP30 electrolyte solution, first in its vapor and liquid phase to compare individual core-level spectra. In a second step, we immersed a V2O5 crystal as a model cathode material in this LiPF6 solution. Additionally, the LiTFSI electrolyte model system was studied to compare and verify our findings with previous NAP-XPS data. Photoelectron spectra recorded at pressures of 2–10 mbar show significant chemical differences for the different lithium-based electrolytes. We show the enormous potential of laboratory NAP-XPS instruments for investigations of solid-liquid interfaces in electrochemical energy storage systems at elevated pressures and illustrate the simplicity and ease of the used experimental setup (EnviroESCA).

1-Bromonaphthalene, by near-ambient pressure XPS

Near ambient pressure-x-ray photoelectron spectroscopy (NAP-XPS) is a less traditional form of XPS that allows samples to be analyzed at relatively high pressures, i.e., at greater than 5000 Pa. With NAP-XPS, liquids, biological samples, porous materials, and/or polymeric materials that outgas significantly can be studied. In this submission, we show survey, C 1s, Br 3p, and Br 3d NAP-XPS spectra of 1-bromonaphthalene. Small O 1s and N 1s signals from background gas (N2 and air) are also observed.

New Insights towards High-Temperature Ethanol-Sensing Mechanism of ZnO-Based Chemiresistors

In this work, we investigate ethanol (EtOH)-sensing mechanisms of a ZnO nanorod (NRs)-based chemiresistor using a near-ambient-pressure X-ray photoelectron spectroscopy (NAP-XPS). First, the ZnO NRs-based sensor was constructed, showing good performance on interaction with 100 ppm of EtOH in the ambient air at 327 °C. Then, the same ZnO NRs film was investigated by NAP-XPS in the presence of 1 mbar oxygen, simulating the ambient air atmosphere and O2/EtOH mixture at the same temperature. The partial pressure of EtOH was 0.1 mbar, which corresponded to the partial pressure of 100 ppm of analytes in the ambient air. To better understand the EtOH-sensing mechanism, the NAP-XPS spectra were also studied on exposure to O2/EtOH/H2O and O2/MeCHO (MeCHO = acetaldehyde) mixtures. Our results revealed that the reaction of EtOH with chemisorbed oxygen on the surface of ZnO NRs follows the acetaldehyde pathway. It was also demonstrated that, during the sensing process, the surface becomes contaminated by different products of MeCHO decomposition, which decreases dc-sensor performance. However, the ac performance does not seem to be affected by this phenomenon.

PtNi bimetallic structure supported on UiO-67 metal-organic framework (MOF) during CO oxidation

Supported bimetallic nanoparticles (BNPs) are promising catalysts, but study on their compositional and structural changes under reaction conditions remains a challenge. In this work, the structure of PtNi BNPs supported on UiO-67 metal–organic framework (MOF) catalyst (i.e., PtNi@UiO-67) was investigated by in situ by near ambient pressure X-ray photoelectron spectroscopy (NAP-XPS). The results showed differences in the reduction behaviour of Ni species in PtNi BNPs and monometallic Ni supported on UiO-67 catalysts (i.e., PtNi@UiO-67 and Ni@UiO-67), suggesting charge transfer between metallic Pt and Ni oxides in PtNi@UiO-67. Under CO oxidation conditions, Ni oxides segregated to the outer surface of the BNPs forming a thin layer of NiOx on top of the metallic Pt (i.e., a NiOx-on-Pt structure). This resulted in a core-shell structure which was confirmed by high-resolution scanning transmission electron microscopy (HR-STEM). Accordingly, the layer of NiOx on PtNi BNPs, which is stabilised by charge transfer from metallic Pt, was proposed as the possible active phase for CO oxidation, being responsible for the enhanced catalytic activity observed in the bimetallic PtNi@UiO-67 catalyst.

In situ Near-Ambient Pressure X-ray Photoelectron Spectroscopy Reveals the Influence of Photon Flux and Water on the Stability of Halide Perovskite.

For several years, scientists have been trying to understand the mechanisms that reduce the long‐term stability of perovskite solar cells. In this work, we examined the effect of water and photon flux on the stability of CH3NH3PbI3 perovskite films and solar cells using in situ near‐ambient pressure X‐ray photoelectron spectroscopy (NAP‐XPS), field emission scanning electron microscopy (FESEM), and current density–voltage (J–V) characterization. The used amount of water vapor (up to 1 mbar) had a negligible impact on the perovskite film. The higher the photon flux, the more prominent were the changes in the NAP‐XPS and FESEM data; also, a faster decline in power conversion efficiency (PCE) and a more substantial hysteresis in the J‐V characteristics were observed. Based on our results, it can be concluded that the PCE decrease originates from the creation of Frenkel pair defects in the perovskite film under illumination. The stronger the illumination, the higher the number of Frenkel defects, leading to a faster PCE decline and more substantial hysteresis in the J‐V sweeps.

Ambient Pressure X-ray Photoelectron Spectroscopy Investigation of Thermally Stable Halide Perovskite Solar Cells via Post-Treatment.

Long-term thermal stability is one limiting factor that impedes the commercialization of the perovskite solar cell. Inspired by our prior results from machine learning, we discover that coating a thin layer of 4,4'-dibromotriphenylamine (DBTPA) on top of a CH3NH3PbI3 layer can improve the stability of resultant solar cells. The passivated devices kept 96% of the original power conversion efficiency for 1000 h at 85 °C in a N2 atmosphere without encapsulation. Near-ambient pressure X-ray photoelectron spectroscopy (XPS) was employed to investigate the evolution of the composition and evaluate thermal and moisture stability by in situ studies. A comparison between pristine MAPbI3 films and DBTPA-treated films shows that the DBTPA treatment suppresses the escape of iodide and methylamine up to 150 °C under 5 mbar humidity. Furthermore, we have used attenuated total reflection Fourier transform infrared and XPS to probe the interactions between DBTPA and MAPbI3 surfaces. The results prove that DBTPA coordinates with the perovskite by Lewis acid-base and cation-π interaction. Compared with the 19.9% efficiency of the pristine sample, the champion efficiency of the passivated sample reaches 20.6%. Our results reveal DBTPA as a new post-treating molecule that leads not only to the improvement of the photovoltaic efficiency but also thermal and moisture stability.

Role of Water in Phase Transformations and Crystallization of Ferrihydrite and Hematite.

The oxides, hydroxides, and oxo-hydroxides of iron belong to the most abundant materials on earth. They also feature a wide range of practical applications. In many environments, they can undergo facile phase transformations and crystallization processes. Water appears to play a critical role in many of these processes. Despite numerous attempts, the role of water has not been fully revealed yet. We present a new approach to study the influence of water in the crystallization and phase transformations of iron oxides. The approach employs model-type iron oxide films that comprise a defined homogeneous nanostructure. The films are exposed to air containing different amounts of water reaching up to pressures of 10 bar. Ex situ analysis via scanning electron microscopy, transmission electron microscopy, selected area electron diffraction, and X-ray diffraction is combined with operando near-ambient pressure X-ray photoelectron spectroscopy to follow water-induced changes in hematite and ferrihydrite. Water proves to be critical for the nucleation of hematite domains in ferrihydrite, the resulting crystallite orientation, and the underlying crystallization mechanism.

Graphene-Capped Liquid Thin Films for Electrochemical Operando X-ray Spectroscopy and Scanning Electron Microscopy.

Electrochemistry is a promising building block for the global transition to a sustainable energy market. Particularly the electroreduction of CO2 and the electrolysis of water might be strategic elements for chemical energy conversion. The reactions of interest are inner-sphere reactions, which occur on the surface of the electrode, and the biased interface between the electrode surface and the electrolyte is of central importance to the reactivity of an electrode. However, a potential-dependent observation of this buried interface is challenging, which slows the development of catalyst materials. Here we describe a sample architecture using a graphene blanket that allows surface sensitive studies of biased electrochemical interfaces. At the examples of near ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) and environmental scanning electron microscopy (ESEM), we show that the combination of a graphene blanket and a permeable membrane leads to the formation of a liquid thin film between them. This liquid thin film is stable against a water partial pressure below 1 mbar. These properties of the sample assembly extend the study of solid–liquid interfaces to highly surface sensitive techniques, such as electron spectroscopy/microscopy. In fact, photoelectrons with an effective attenuation length of only 10 Å can be detected, which is close to the absolute minimum possible in aqueous solutions. The in-situ cells and the sample preparation necessary to employ our method are comparatively simple. Transferring this approach to other surface sensitive measurement techniques should therefore be straightforward. We see our approach as a starting point for more studies on electrochemical interfaces and surface processes under applied potential. Such studies would be of high value for the rational design of electrocatalysts.

FeMo sub-nanoclusters/single atoms for neutral ammonia electrosynthesis

Electrochemical N 2 reduction reaction (NRR) has long been regarded as a promising process to generate NH 3 under ambient conditions. Therefore, developing cost-effective and high-performance non-noble-metal catalysts for NRR is highly desirable. Inspired by the biological nitrogenase structure, we here designed and synthesized a catalyst with iron-molybdenum sub-nanoclusters and single atoms on porous nitrogen-doped carbon (FeMo/NC). The catalyst features porous structure beneficial to active site exposure and accessibility to electrolyte as well as FeMo sub-nanoclusters and single atoms enabling to activate N 2 molecular. In situ near-ambient pressure X-ray photoelectron spectroscopy tests reveal that during the process from vacuum to nitrogen saturation, N 2 was close to, adsorbed on and interacted with Fe and Mo in FeMo/NC. The Fe and Mo through electron transfer play a key role in activating the N 2 molecules. Therefore, when tested for NRR, FeMo/NC achieves the maximum Faradaic efficiency (FE) of 11.8 ± 0.8% at −0.25 V and NH 3 yield rate of 26.5 ± 0.8 μg h −1 mg cat. −1 at −0.3 V in neutral electrolyte. Moreover, the catalyst exhibits ignorable variations in the FE and a slight decrease in current density for 100,000 s. This work develops a non-precious bimetallic electrocatalyst with synergetic effect capability for efficient NH 3 production and provides a guideline for the design of efficient and robust catalysts with coexistence of sub-nanoclusters and single atoms. Inspired by the biological nitrogenase, we have designed a novel catalyst composed of FeMo sub-nanoclusters/single atoms on N-doped carbon that can effectively catalyse electroreduction of N 2 to generate NH 3 in neutral media. • FeMo sub-nanoclusters/single atoms are developed from the inspiration of biological nitrogenase. • The catalyst shows excellent activity toward neutral NH 3 electrosynthesis. • The catalyst also displays an outstanding stability during the electrolysis. • Near-ambient pressure X-ray photoelectron spectroscopy confirms that the coexistence of Fe and Mo activates the N 2 molecules.

Electronic Structure Evolution of Pd@Co Nanocatalysts Under Oxidation and Reduction Conditions and Preferential CO Oxidation

AbstractHerein, we present the surface electronic structure and morphological evolution under reduction and oxidation conditions for Pd@Co (PC) core‐shell nanoparticles with different Pd : Co ratio (PC=2 : 1, 1 : 1 and 1 : 2). Extensive measurements have been made with NAPXPS (near ambient pressure x‐ray photoelectron spectroscopy) under oxidising and reducing conditions, and ex‐situ HRTEM. It has been demonstrated that PC catalysts are thermally stable towards morphological changes, at least up to 575 K. Nonetheless, it shows a significant surface electronic structure changes under reaction environments, which are highly relevant to heterogeneous catalysis. As expected, high (low) population of metallic (oxidised) Co was observed, while retaining core shell structure under reduction (H2 and vacuum annealing) environment. Interestingly, the Pd−Co metallic interface helps to overcome the pyrophoric nature of cobalt and stabilised a significant amount of metallic Co at Pd−Co interface even in the presence of 0.1 mbar O2 up to 575 K. The presence of Pd−Co and Pd−Co@Co3O4 interfaces in reaction environment makes the catalyst dual functional. The proof of concept has been explored in terms of oxidation of CO in the presence of H2 or O2.

Roman coin, by near-ambient pressure XPS

Near ambient pressure-x-ray photoelectron spectroscopy (NAP-XPS) is a less traditional form of XPS that allows samples to be analyzed at relatively high pressures, i.e., at greater than 2500 Pa. With NAP-XPS, XPS can probe moderately volatile liquids, biological samples, porous materials, and/or polymeric materials that outgas significantly. In this submission, we show survey, O 1s/Ag 3p, Ca 3p/Ag 3d, and extended valence band (0–130 eV) NAP-XPS spectra of an ancient Roman coin at three different positions. A small N 1s signal from N2 background gas is also observed. On the obverse side, the coin bears the bust of Licinius I. On the reverse side, it bears the image of Jupiter. The Ag 3d region indicates different amounts of silver at different oxidation states in different positions.

Boosting CO2 hydrogenation via size-dependent metal–support interactions in cobalt/ceria-based catalysts

Metal–support interactions have a strong impact on the performance of heterogeneous catalysts. Specific sites at the metal–support interface can give rise to unusual high reactivity, and there is a growing interest in optimizing not only the properties of metal particles but also the metal–support interface. Here, we demonstrate how varying the particle size of the support (ceria–zirconia) can be used to tune the metal–support interactions, resulting in a substantially enhanced CO2 hydrogenation rate. A combination of X-ray diffraction, X-ray absorption spectroscopy, near-ambient pressure X-ray photoelectron spectroscopy, transmission electron microscopy and infrared spectroscopy provides insight into the active sites at the interface between cobalt and ceria–zirconia involved in CO2 hydrogenation to CH4. Reverse oxygen spillover from the support during treatment in hydrogen results in the generation of oxygen vacancies. Stabilization of cobalt particles by ceria–zirconia particles of intermediate size leads to oxygen spillover to the support during the CO2 and CO dissociation steps, followed by further hydrogenation of the resulting intermediates on cobalt. Metal–support interactions can effectively modify the catalytic properties of heterogeneous composites. Here, the authors report the possibility of controlling the interaction between cobalt and a ceria–zirconia support by changing the particle size of the latter, resulting in a superior CO2 hydrogenation system.

Clinoptilolite, a type of zeolite, by near ambient pressure-XPS

Near ambient pressure-x-ray photoelectron spectroscopy (NAP-XPS) is a less traditional form of XPS that allows samples to be analyzed at relatively high pressures, i.e., at greater than 2500 Pa. With NAP-XPS, XPS can probe moderately volatile liquids, biological samples, porous materials, and/or polymeric materials that outgas significantly. In this paper, we show the NAP-XPS survey; O 1s, Ca 2p, C 1s, K 2p, Al 2s, Al 2p, Si 2p, and Si 2s narrow scans; and the extended valence band spectrum of clinoptilolite, a natural zeolite that would be difficult to analyze by conventional XPS. A small N 1s signal from N2(g) is also observed in the survey spectrum. Signals in the narrow scans are fit to Gaussian–Lorentzian sum and Gaussian–Lorentzian product functions.

New Insight into Gas Sensing Properties of SnO2 and Cu2o/CuO Based Gas Sensors By Near Ambient Pressure XPS

This study presents an investigation of the sensing properties of SnO2 and Cu2O/CuO based nanostructured chemiresistors, n-type and p-type semiconductors, respectively. The chemiresistors were used for the detection of ethanol and nitrogen dioxide, reducing and oxidizing agents, respectively. To unravel the chemical processes connected with gas detection, an in-situ approach was applied. This approach was based on near ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) combined with simultaneous monitoring of sensor responses. The in-situ measurements were performed during exposure to the analytes at a total pressure of 0.05 – 1.05 mbar and 200 – 300 °C, and were correlated with chemiresistor response measurements carried out at a standard pressure and under an ambient atmosphere. The study enabled assessment of the relationship between the chemistry of surface processes on the sensitive layers and the "integral" response of the sensor devices and presented a complex view on the gas sensing mechanism and chemiresistors functionality. Figure 1

Polyethylene terephthalate by near-ambient pressure XPS

Near-ambient pressure x-ray photoelectron spectroscopy (NAP-XPS) is a less traditional form of XPS that allows samples to be analyzed at relatively high pressures, i.e., at greater than 2500 Pa. With NAP-XPS, XPS can probe moderately volatile liquids, biological samples, porous materials, and/or polymeric materials that outgas significantly. In this submission, we show survey, C 1s, and O 1s NAP-XPS spectra of polyethylene terephthalate, a common, widely used thermoplastic. The C 1s envelope was fit with different approaches, i.e., to three, four, and five Gaussian–Lorentzian sum (GLS) functions. Hartree–Fock orbital energy calculations of a model trimer served as a guide to an additional fit of the C 1s envelope. The best fit was obtained by adding an extra component to the four-component fit to compensate for adventitious carbon or additives in the polymer. The O 1s signal was well fit with two GLS peaks with a 1:1 area ratio representing the C—O and C=O moieties in PET.

Effects of background gas composition and pressure on 1,4-polymyrcene (and polytetrafluoroethylene) spectra in near-ambient pressure XPS

Near-ambient pressure XPS (NAP-XPS) is a less traditional form of XPS that allows samples to be analyzed at relatively high pressures, i.e., at 2500 Pa or greater. With NAP-XPS, one can analyze moderately volatile liquids, biological samples, porous materials, and/or polymeric materials that outgas significantly. In this submission, the authors show C 1s, O 1s, and survey NAP-XPS spectra from 1,4-polymyrcene. The C 1s and O 1s envelopes are fit with Gaussian–Lorentzian product, asymmetric Lorentzian, and Gaussian–Lorentzian sum functions. Water vapor and argon are used to control sample charging, and the corresponding signals from the gases are present in the survey spectra. The effect of background gas pressure on photoelectron attenuation is illustrated with a sample of polytetrafluoroethylene.

Ceria-Based Catalysts Studied by Near Ambient Pressure X-ray Photoelectron Spectroscopy: A Review

The development of better catalysts is a passionate topic at the forefront of modern science, where operando techniques are necessary to identify the nature of the active sites. The surface of a solid catalyst is dynamic and dependent on the reaction environment and, therefore, the catalytic active sites may only be formed under specific reaction conditions and may not be stable either in air or under high vacuum conditions. The identification of the active sites and the understanding of their behaviour are essential information towards a rational catalyst design. One of the most powerful operando techniques for the study of active sites is near ambient pressure X-ray photoelectron spectroscopy (NAP-XPS), which is particularly sensitive to the surface and sub-surface of solids. Here we review the use of NAP-XPS for the study of ceria-based catalysts, widely used in a large number of industrial processes due to their excellent oxygen storage capacity and well-established redox properties.