The international workshop on ambient pressure X-ray photoelectron spectroscopy is organized annually by a large facility (synchrotron/national laboratory) hosting such a technique. The workshop brings together academic staff, researchers and students to discuss the application and development of ambient pressure X-ray photoelectron spectroscopy in a wide variety of scientific topics including catalysis, electrochemistry, materials science, and environmental science. The workshop is also open to scientists making use of other in situ/operando techniques and working in the field of theoretical calculations. The main goals are to highlight new scientific discoveries, discuss about technical upgrades and foster the discussion among research facilities making use of in situ/operando techniques.
The workshop features talks from plenary and invited speakers as well as from contributed speakers, a poster session, and a technical discussion session. Participants have the opportunity to share and learn about the latest scientific discoveries, technical developments of APXPS instrumentation, and discuss the future directions in the field.
This year the workshop will be hosted by the Paul Scherrer Institute, at the University of Applied Sciences in Windisch, Switzerland.
Aqueous solution-vapor interfaces govern important phenomena in the environment and atmosphere, including the uptake and release of trace gases by aerosols and carbon dioxide sequestration by the oceans. A detailed understanding of these processes requires the investigation of liquid-vapor interfaces with chemical sensitivity and interface specificity under ambient conditions, i.e., temperatures above 270 K and water vapor pressures in the millibar to tens of millibar pressure range. It is thus appropriate that these interfaces were the first to be investigated using ambient pressure XPS by the Siegbahn group 50 years ago. This talk will discuss opportunities and challenges for investigations of liquid-vapor interfaces using X-ray photoelectron spectroscopy and describe some recent experiments that have focused on the propensity of certain ions and the role of surfactants at the liquid/vapor interface.
A wide collection of invaluable probes in material science is provided by synchrotron-based hard X-ray techniques: for example, hard X-ray absorption spectroscopy (XAS) is an irreplaceable tool in the investigation of local atomic and electronic structures of materials.  Moreover, operando XAS experiments with hard X-rays are well-established, and almost every experimental condition can be reached to simulate realistic reaction environments.  On the contrary, in the soft X-ray regime, the applications of XAS (soft-XAS) in material science were often limited to a “surface science” approach, i.e., to the study of clean surfaces in high-vacuum conditions. In fact, the low penetration depth of X-rays with energies lower than 1 keV and the severe vacuum limitation have somehow hindered the development of operando experiments. However, soft-XAS is capable to give invaluable information for a complete understanding of the mechanisms of phenomena taking place at material surfaces and interfaces, such as catalysis, intercalation, electrochemistry, etc. Unexpectedly, these subjects have been mainly approached with XPS, by developing the electron energy able to work at pressure up to 1 bar . Despite these latest developments, performing XPS close to ambient pressure remains experimentally challenging. In this framework, soft-XAS represent an interesting alternative to access in operando condition the electronic structure of the working catalyst at ambient pressure with great surface sensitivity and with an experimental apparatus relatively simple. During the presentation I will present the instrumental solutions that are adopted to measure soft x ray absorption spectroscopy at ambient pressure and the possibilities that are now offered at the APE-HE beamline at the Elettra synchrotron . Afterwards I will present few examples of application of the soft XAS to the study of working catalyst ranging from the study of cathode fuel cells  to the investigation of the mechanism of tin oxide based sensors  to the study of the catalyst promoting the polymerization of polyolefin .
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Perovskite-type materials are being intensively investigated as cathodes for solid oxide fuel cells (SOFC) applications. The basic function of the cathode in SOFCs is to incorporate O2 from air as oxygen ions into its crystal lattice and transport them to the electrolyte through oxygen vacancy skipping mechanisms. Strontium doped lanthanum cobaltites (SrxLa1-xCoO3-δ) can undergo reversible redox transitions that involve conversion of O2 molecules into oxide ions at the material surface, followed by fast oxide ion conduction at temperatures as low as 300°C. The addition of strontium to the lanthanum cobaltite network provides cobalt ions with redox flexibility that can compensate the formation of oxygen vacancies. In addition, the inclusion of strontium to the network can induce phase transitions, from tetragonal to cubic, that can also impact vacancy generation thermodynamics. We have studied SrxLa1-xCoO3-δ for the whole 0≤x≤1 range by means of AP-XPS and AP-XAS studies and simulating the working conditions of cathodes at SOFCs by alternative cycles of oxygen dosing and vacuum annealing. We have found that
oxygen K edge XAS spectroscopy is particularly sensitive to the oxygen insertion/extraction in the perovskite network. The O K-edge spectrum exhibits a clear spectroscopic feature associated with Co4+, at approx. 528 eV, that has allowed us to study the dependency of the perovskite activity with composition. We observe better reversibility and kinetics for samples with x≤0.3, which suggests that SrxLa1-xCoO3-δ with small strontium content will be better candidates as cathodes for SOFC. Figure 1 shows the O K edge XAS for La0.1Sr0.9CoO3-δ and La0.3Sr0.7CoO3-δ thin films at 350°C in UHV, under 100 mtorr of O2 and back in UHV as an example of a reversible and an irreversible sample.
At pressures above 0.1 mbar, gas-phase NEXAFS spectra recorded in typical AP-XPS endstations or cells are significantly affected by photon absorption within the gas along the path to the detection volume. The Figure shows experimental C K-edge spectra of CO$_2$ measured in TEY mode at the beamline entrance (Aperture) and 15 mm from the entrance (Detector). It illustrates the effects which range from line broadening to peak splitting and critically depend on pressure and the distance between beamline entry and detector. If the attenuation of the photon beam exceeds the level of a few percent separating the NEXAFS signal of a solid sample in a gas environment from the gas-phase signal cannot be achieved by simple subtraction.
We present experimental data and model spectra for various gases and discuss strategies of removing the gas-phase signal from NEXAFS spectra of solid samples.
To track the chemical state and structural properties of catalytically active materials under realistic reaction conditions, in order to understand charge transfer at the liquid-solid interface, CO2 activation, and surface carbon intermediate species, reaction cells are critical for operando near ambient soft X-ray photoelectron and absorption spectroscopy (NAP-XPS/XAS) experiments. The main challenges with developing operando NAP-XPS/XAS cells for photocatalytic and electrocatalytic reactions are that these reactions are typically performed in liquid phase at the solid–liquid interface and that the catalyst must be exposed to UV-visible irradiation during reaction (and apply an external potential in both electrocatalysis and photoelectrocatalysis). The presence of the liquid phase alone poses a challenge due to the high absorbing background, but external excitation with UV-visible light or electrons imposes additional limitations on cell design. In order to address these challenges while probing catalyst surface during the reaction, we have developed back-side illuminated operando reaction cells for synchrotron-based NP-XPS AND NEXAFS studies. Our design concept is based on a modular design and uses non-metal (PEEK material) body, and replaceable membranes which can be either of X-ray transparent silicon nitride or of water permeable polymer membrane materials (e.g. Nafion). These membrane materials are particularly suitable for liquid flow or electrochemical cells and enable measuring photoelectrons emitted from the membranes or from catalyst material deposited at the solid-liquid interface outside the cell. The preliminary results of in-situ NAP-XPS and NEXAFS measurements (from B07 beamline@Diamond) on electrochemical reduction with a Cu catalyst and oxidation with an IrOx catalyst will be presented. The developed system is highly modular and can be used in the laboratory or directly at the beamline for operando XPS/XAS measurements (Figure 1).
High Entropy Oxides (HEOs) are a recent class of materials where the configurational entropy of mixing is thought to be the main term in the Gibbs free energy of formation. Although the role of configurational entropy as a stabilization term for these compounds is still debated, these materials display a number of promising properties such as anode material in Li-ion batteries, or as large k dielectric material, fast ionic conductors, or as catalytic materials. The “high entropy paradigm” requires that at least five different cations are mixed in equimolar fractions in the oxide structure, and the intrinsic multicomponent nature of these materials renders the individuation of the atomic mechanisms leading to the relevant functional properties of the HEOs a particularly difficult task. Here we show that Operando X-ray Absorption Spectroscopy represents an invaluable tool in this respect, specifically where the interfacial properties are involved, and in particular in the energy region of the soft X-rays. Examples are thus given in the field of catalysis and electrochemistry (anode material in Li-ion batteries).
Establishing relationships between structure and performance can enable a ‘smart design’ approach in the development of next generation catalysts, processes and the advanced materials used, for example, in energy storage and conversion. Heterogeneous catalysts and energy materials are often dynamic in nature with respect to structure, composition and local electronic environment. Structural evolution under operating conditions then places further demands on structure determination; to be meaningful this must be achieved under relevant pressures, temperatures and feed compositions.
We present the first results from a prototype microreactor for soft XAS measurements at the VerSoX B07 beamline, at elevated pressure and temperature (tested up to 0.9 bar and 400 °C) using the Total Electron Yield detection mode to probe the first few nanometres of a sample. We have selected the Fischer-Tropsch (FT) reaction as our test case; the combination of high temperature and pressures of toxic, flammable gases, the undoubted mechanistic complexity and a product slate comprising both gases and liquids makes this an ambitious but highly rewarding study. There has been a resurgent interest in FT chemistry in recent years, driven partly by the potential to generate fuels and chemicals from above ground sources of carbon (e.g. biomass, waste and CO2) via synthesis gas.
Figure 1. Photograph of the prototype microreactor and Co L edge NEXAFS showing the in-situ reduction of Co3O4 nanoparticles acquired at 0.9 bar H2 at 350 C.
Interfaces between water and materials are ubiquitous and are crucial in materials sciences and in biology, where investigating the interaction of water with the surface under ambient conditions is key to shedding light on the main processes occurring at the interface. Magnesium oxide is a popular model system to study the metal oxide-water interface, where, for sufficient water loadings, theoretical models have suggested that reconstructed surfaces involving hydrated Mg(II) metal ions may be energetically favored. In this presentation, our group’s recent efforts to apply an innovative approach using ambient pressure operando near edge X-ray near structure spectroscopy (AP-NEXAFS) to study the chemical processes occurring at the interphase will be summarized. In particular, by combining experimental and theoretical surface-selective ambient pressure X-ray absorption spectroscopy with multivariate curve resolution and molecular dynamics, we evidence in real time the occurrence of Mg2+ solvation at the interphase between MgO and solvating media such as water and methanol. Further, we show that the Mg2+ surface ions undergo a reversible solvation process, we prove the dissolution/redeposition of the Mg(II) ions belonging to the MgO surface and demonstrate the formation of octahedral intermediate solvated species. The unique surface, electronic, and structural sensitivity of the developed technique may be beneficial to access often elusive properties of low-Z metal ion intermediates involved in interfacial processes of chemical and biological interest.
We have recently developed a fluorescence-yield wavelength-dispersive soft X-ray absorption spectroscopy (XAS) technique, by which the XAS data is recorded without scanning the monochromator , and the real-time observation of surface chemical reactions under near ambient pressure conditions up to ~5000 Pa has been realized .
In the technique, the wavelength-dispersed X rays illuminate the sample, where the wavelength (photon energy) continuously changes as a function of position, and the fluorescence soft X rays generated at each position on the sample are separately focused by an imaging optics consisting of two spherical mirrors onto each position at the detector. Accordingly, the fluorescence-yield soft X-ray absorption spectrum is obtained without scanning the monochromator. The sample area is separated by two 3 mm × 3 mm Si3N4 windows with a thickness of 200 nm to prevent the reaction gases from flowing into the beamline and imaging optics.
Moreover, the developed technique has been combined with a depth-resolved technique, in which a set of XAS data is simultaneously obtained at different probing depths by correcting the fluorescence soft x rays at different emission angles. By using the time- and depth-resolved XAS technique, we observed the oxidation reaction proceeding from the surface to inside in real time (without halting the reaction), and clarified the time evolution of the depth profile of the chemical species .
Recent results for the real-time observation of the surface chemical reactions with depth-resolved analyses will be presented after the introduction to the developed technique.
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The HIPPIE beamline at MAX IV Laboratory has recently commissioned its Fourier Transform Infrared Spectroscopy (FTIR) setup. Using an IRRAS (Infrared Reflection Absorption Spectroscopy) geometry, HIPPIE allows simultaneous vibrational and photoemission spectroscopy in the Ambient Pressure (AP) reaction cell. Using variable polarisation or using polarisation modulation to identify or suppress gas phase spectral contributions, IRRAS can provide a surface sensitive probe of adsorbates. The vibrational modes of surface species can be a used as a fingerprint of surface species, which can sometimes be difficult to identify with any certainty using XPS alone. Therefore this multimodal approach of combining AP-XPS and AP-IRRAS promises to provide significantly more information regarding surface chemistry under reaction conditions than either method can separately.
This talk will primarily introduce the IRRAS method to those in the AP-XPS community who are new to vibrational spectroscopy, focusing on the value it can add to typical AP-XPS experiments. This is becoming particularly topical due to several existing and upcoming instruments that incorporate AP-XPS and AP-IRRAS, both using lab sources and synchrotron radiation. We will introduce the setup at HIPPIE, which is now available to general users, including discussion of its capabilities, limitations and future possibilities.
The above discussion will draw heavily on a case study: the interaction of CO with Cu(111) and Cu(211) surfaces under oxidation conditions. Time-resolved AP-IRRAS and AP-XPS measurements at HIPPIE were used to examine surface species that form, highlighting how the assignment of reaction intermediates can be aided using both methods simultaneously. This experiment additionally compares how the surfaces behave with and without being alloyed with Sn.
For the last ten years XPS under near ambient pressure conditions (NAP-XPS) has gained significant attention in the XPS community. The technique allows for standard analysis of samples under pressures up to about 50 mbar. This opens XPS to liquids, solid-liquid interfaces, gas-solid-interfaces, gas-liquid-interfaces. New fields like operando studies on electrochemical systems, corrosion experiments, analysis of food samples, but also studies of biological samples have been added to the XPS portfolio. The background gas pressure in such experiments is beneficial for the analysis of materials, because it avoids beam damages and degradation due to UHV conditions and also enables true non-destructive analysis of all types of degassing samples and insulators.
In this presentation, we demonstrate the enormous potential of laboratory NAP-XPS for investigations of solid-liquid interfaces in electrochemical energy storage systems at elevated pressures, also illustrating the ease of use of the setup used.
We show different examples with increasing level of complexity from solid liquid interface studies, like obtaining relevant results on Silicon in different organic solvents without the need of highly sophisticated setups, all the way to complex experiments such follow the effects of metal corrosion in organic acid.
Most sophisticated experiments so far have been operando electrochemistry in a classical three-electrode setup. A versatile setup is presented, allowing for studies of solid-electrolyte interfaces for example in Lithium-ion batteries as a simple laboratory experiment. First experiments on a V2O5 cathode in 1 molar LiPF6 in EC/DMC electrolyte solution show the operando intercalation of Lithium into the cathode and the related changes in its chemical compositions. A control experiment after air exposure of the intercalated cathode demonstrate need for inert environments during measurements.
Investigating reaction intermediates, oxidation states, solid-liquid interfaces and buried interfaces under near ambient pressure conditions is highly desired in materials science applications. Ambient pressure X-ray photoelectron spectroscopy (APXPS) is a powerful method to investigate the chemical nature of surfaces and interfaces and has undergone a tremendous improvement in the last years. The development of the HiPP analysers allowed to overcome the one bar pressure regime without using pressure separating membranes [1,2]. The virtual cell approach implemented in Scienta Omciron’s BAR XPS system, allows for sub-second gas exchange rates at sample surfaces and thereby allowing to study time dynamics . Successful investigation of solid-liquid interfaces [4,5] is achieved by a sophisticated pre-lens design in which efficient pumping between two close-by apertures allows dragging out corrosive gases or moisture, which would otherwise be detrimental to the instrument.
During the past decade, increased attention has been shown to laboratory based APXPS system solutions, which is motivated by the 24/7 access capability and possibility for highly customized sample environments. Drawing on extensive experience in the fields of photoelectron spectroscopy, UHV technology, and system design, Scienta Omicron has designed the HiPPLab as an easy-to-use system that encourages user creativity through flexibility, modularity and an innovate chamber design. It combines a state-of-the-art HiPP analyser with a new high flux, variable focus X-ray source. Multiple options complement the HiPPLab offer, these include glovebox, preparation chamber, electrochemical cells, dip&pull method, options for photo-induced electrochemistry, laser heating, mass-spectroscopy, and UV-light source and many more. Using automated gas-flow controllers, experiments can be conducted in a controlled way.
The HiPP-3 analyser features a 2D detector allowing for spatial resolved measurements with customer proven results down to 2.8 µm resolution . The swift acceleration mode allows for high electron transmission without applying a sample bias.
In this presentation, we will give an overview on our APXPS product portfolio and present application examples.
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To produce clean transportation fuels without emission of CO2, a possible route is the synthesis of hydrocarbons from CO and H2 via the Fischer-Tropsch process . In the industrial process, cobalt is the catalyst of choice [2-5]. Although this process has been investigated intensively, open questions still remain. Examples are the oxidation state of cobalt during the actual chemical reaction, and the coverages of reactants, products, and possible impurities on the surface of the catalyst. Near-ambient-pressure X-ray photoelectron spectroscopy (NAP-XPS) is an excellent method to try to answer these two questions.
In this talk, I will present the results we obtained using NAP-XPS during the Fischer-Tropsch synthesis reaction on the Co(0001) model catalyst surface. I will show that the oxidation behavior of cobalt depends on the structure of the sample, the water partial pressure, and the H2-to-CO ratio. We found that CO is more effective than H2 to reduce cobalt. This behavior can be explained by the different adsorption and dissociation sites of H2, CO, and H2O on Co(0001).
Furthermore, I will discuss the role of adsorbates on the surface. We found that at least 70% of carbon present on Co(0001) during Fischer-Tropsch synthesis is in carbidic form. We can also distinguish two different hydrocarbon peaks, one resulting from hydrocarbon impurities in the CO gas feed, and one being the product of the chemical reaction.
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In-Situ Studies of Tailored Nanoparticle Exolsution on Catalyst Surfaces
Driven by Electrochical Potenial, Gas Composition and Gas Pressure
R. Rameshan a, L. Lindenthal a, F. Schrenk a, T. Ruh a,b, A. Nenning c, A.K. Opitz c, C. Rameshan a,d
a Chair of Physical Chemistry, Montanuniversity Leoben, Austria
b Institute of Applied Physics, TU Wien, Austria
c Institute of Chemical Technologies and Analytics, TU Wien, Austria
d Institute of Materials Chemistry, TU Wien, Austria
In heterogeneous catalysis, excellent performance can be achieved by uniformly dispersed, ideally catalytically highly active (nano)particles on a surface. Our aim is to tailor and control the formation of nanoparticles on a surface during reaction by either applying an electrochemical potential (bias) and/or adjusting the gas phase (pO2).
We use Perovskite-type oxide catalysts for our research, as they can incorporate catalytically highly active guest elements as dopants. When reducing conditions (electrical bias or pO2) are applied, the dopants are exsolved from the oxide lattice and form catalytically active and morphologically stable nanoparticles on the surface, thus cause increasing the catalytical activity of the material.
Whether the process of exsolution can be characterized by one parameter independent of the driving force – for instance electrochemical polarization (bias) or gas phase composition (pO2) – remains an open question. Different surface structures dependent on the driving force will be explored.
We thoroughly characterised Nd0.6Ca0.4Fe0.9Co0.1O3-δ using different methods: In-situ Near Ambient Pressure X-ray Photoelectron Spectroscopy (NAP-XPS) studies are supported by Electrochemical Impedance Spectroscopy (EIS) and Scanning Electron Microscopy (SEM). Furthermore, “high pressure” (160 mbar) measurements performed at a synchrotron (DESY) were used to further explore the questions about exsolution driving forces. Especially the switching behaviour of iron to iron oxide as well as cobalt to cobalt oxide at different electrochemical, gas phase and pressure conditions was compared.
How these conditions affect the particle size and distribution was investigated by SEM. We could show that the exsolution behaviour of these (nano)particles can be described by only using a calculated pO2 value (derived from combining bias and gas phase); however, the size of the obtained (nano)particles differs depending on how exsolution was triggered (by bias or gas phase). Moreover, we present our first attempt of investigating the pressure dependence of exsolution.
This project has received funding from the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation programme (grant agreement n° 755744 / ERC - Starting Grant TUCAS).
Catalysis depends on the availability of active sites and the binding strength of adsorbates to these sites. Recently, catalysis in confined space has been suggested to offer enhanced selectivity and reactivity [1,2], while simultaneously reducing the likelihood of catalyst poisoning or degradation . 2D materials on metal substrates have been extensively studied for fundamental confined catalysis experiments because they can be prepared in a structurally well-defined form and can be characterized in great detail by surface science techniques, providing solid experimental reference data for theoretical studies. However, metal-oxide surfaces are generally more catalytically active than their metal counterparts , and only recently has a well-ordered 2D/metal oxide interface been prepared. Our group has synthesized a “Cu2O-like” thin film (~3-4 Å thick) confined in-between a hexagonal boron nitride (h-BN) overlayer and a Cu(111) substrate via O2 intercalation and oxidation of the Cu substrate. Experimental characterization combined with theoretical simulations unraveled the oxide structure, providing an ideal model system for studies of molecular adsorption and diffusion in confined space. To probe the stability of the heterostructure, we attempted to reduce the confined oxide via H2 intercalation through exposure to varying H2 partial pressures (10-5 to 0.1 mbar) and temperatures (room temperature up to 200 °C) while simultaneously collecting APXPS data. Following exposure to 0.1 mbar at 200 °C, we observe a decrease in the O 1s core level intensity for the peaks corresponding to the “Cu2O-like” oxide. Furthermore, a slight shift occurs in the B 1s and N 1s core levels to higher binding energy while the peak shape remains intact, suggesting that H2 was able to intercalate and reduce the surface oxide without destroying the h-BN overlayer. Ultimately, we demonstrate the reversible oxidation of the confined Cu(111) surface beneath h-BN.
With a rise in the number of lab-based APXPS systems, these instruments afford an opportunity to continue the development of multimodal capabilities for more comprehensive information of reactions at surfaces. I will discuss the methods of obtaining multimodal data from infrared reflection absorption spectroscopy (IRRAS) and environmental transmission electron microscopy (ETEM) under the same reaction environments as the lab-based APXPS system at the Center for Functional Nanomaterials at Brookhaven National Laboratory. In situ polarization-dependent IRRAS measurements have been used to confirm the reaction of CO with a Cu2(111) surface to form CO2. The uncommon IRRAS measurements of a single crystal transition metal oxide surface allow for insights into the geometry of the adsorbates. The combination of APXPS and IRRAS determine that a C 1s binding energy commonly assigned to carbonates is actually CO2. This study has implications for catalysis and also metal oxide XPS studies, where in this case the adsorbed CO and CO2 have binding energies higher than other systems. While IRRAS provides more insight into chemical environments on surfaces, ETEM can offer complementary structural information. A 50 micrometer heater on a Nano-Chip used in ETEM was adapted to a gas cell for APXPS measurements. Proof-of-concept measurements show that the heater functions identical to ETEM experiments. The gas lines in the cell enable locally high pressures above the heater, estimated to be 1 mbar with the potential for higher pressures. The rapid temperature increase of the microheater (≤1 s) also enables time resolved measurements. The reduction of an oxidized Pd film was followed with 500 ms resolution of the Pd 3d5/2 core level. This timescale matches the timescale of ETEM measurements (≥10 ms) of identical processes. Using this Nano-Chip in APXPS offers chemical information complementary to structural changes seen in ETEM. The rapid heating enables new opportunities in time-resolved APXPS. Overall, both the ETEM heater and IRRAS offer ways of combining additional information to yield a deeper understanding of surface reactions beyond the metal oxide chemistry demonstrated here.
The interaction of alkali ions with multilayer graphene is critical in many applications, for example in energy storage devices. This requires a detailed understanding of ion interactions with carbonaceous layers. The mechanism of ion intercalation into graphene can be different from that observed for hard graphite. We investigated the vertical alkali ion (Na, K, Cs) distribution on multilayer graphene deposited onto SiO2 in vacuum and in the presence of water vapor using Standing Wave Ambient Pressure Photoemission Spectroscopy. It was found that Cs, K, and Na ions do not intercalate into multilayer graphene under vacuum conditions. The most likely reasons for this behavior are the reversibility of the process due to large inter-sheet spacing or lack of time for intercalation. When exposed to water vapor, Na ions intercalate soft carbon whereas Cs ion do not. This is a clear indication for the difference in the intercalation mechanisms on hard graphite and soft graphene.
Ni is known to be a catalyst for the Boudouad reaction
CO+CO $\rightarrow$ CO2 + C
which is favored by high pressure and thus escaped so far direct investigation under operando conditions. We report here on a Near Ambient Pressure XPS study performed at Soleil Synchrotron using the Tempo Beamline.
A bare Ni(111) sample has been exposed to CO at a pressure PCO∼2 mbar. Graphene growth occurred already at 550 K , a temperature significantly lower than the one (670 K) at which growth of graphene by segregation of dissolved carbon occurs.
It has recently been shown  that the space between graphene layer and the substrate may act as a nano-reactor cavity where the activation barrier for CO oxidation is effectively reduced. We show here that this is the case also for the Boudouard reaction.
Exposing single layer graphene on Ni(111) to CO at 3.7 mbar, CO intercalates under the layer causing its detachment from the substrate. The so-obtained high local CO coverage under graphene cover enables the formation of CO2 via the Boudouard reaction catalyzed by the Ni(111) surface already at 340 K . The carbon produced by the reaction is used to transform residual carbide into graphene. Moreover, under such conditions a chemisorbed CO species forms above the graphene film, thus paving the use of supported graphene for catalysis.
We also investigated the effect of the presence of vacancies obtained by low energy ion bombardment.
We find that CO intercalates at a rate which is comparable to the one observed in absence of defects and reacts via the Boudouard reaction producing additional carbon atoms and CO2.
While the former attach to the graphene layer and extend it over areas previously covered by carbide, the CO2 molecules bind to the graphene vacancies thus mending the defects. The so-formed complexes give rise to a peak at 533.4 eV which persists upon evacuating the vacuum chamber at room temperature and which we assign to a covalently bonded species containing C and O .
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Graphene’s popularity has boomed since its isolation in 2004 . We can find graphene in multiple domains of modern technology, from sensing and catalysis to functional materials, electronic devices, and energy-related technologies. However, most of these studies lack the characterization of such systems under realistic conditions. With this contribution, we present two recently published studies [2, 3] highlighting the necessity of APXPS studies for implementing 2D materials in modern technology.
First, we address the stability of hydrogen functionalized graphene under mbar pressures . Hydrogen adsorption on graphene is widely used to open a bandgap on the graphene electronic structure, which has clear advantages for its implementation in electronic devices . However, until now, no studies have addressed their stability under mbar pressures. Our study shows that H-dimers motifs on a graphene surface are selectively removed by room temperature oxygen exposure. Interestingly, DFT calculations indicate that the dimer configuration can attack the O=O double bound already at room temperature leading to water or H$_2$O$_2$ formation. Moreover, these results present a new and exciting role for graphene in catalysis studies: as an adsorption template for studying how configurational placement of reactants is linked to catalytic function.
The second part addresses the lack of knowledge on the kinetics of confined reactions under 2D materials . Enhanced activity or selectivity of catalyst materials placed in confined environments has already been studied for a variety of systems . However, until recently, most studies focused on ex-situ characterization or studies under steady-state conditions. Our work demonstrates how we can use short pulses of changing gas composition to repeatedly intercalate and de-intercalate molecules into the space between graphene flakes and their metal substrate. In more detail, we study CO and H$_2$ oxidation below iridium-supported graphene flakes. We show that hydrogen rapidly mixes into the oxygen structure below the graphene and converts it into a mixed OH-H$_2$O phase. In contrast, we find that CO exposure only leads to oxygen removal and little CO intercalation. Finally, we demonstrate how H$_2$ mixed into CO can be used as an intercalation promoter to change the undercover chemistry. Our work is a clear example of how APXPS provides a detailed kinetic perspective on the intercalation process and the undercover reaction not accessible until now.
Altogether, we hope our studies will inspire future APXPS catalysis studies of reactions above and below 2D materials.
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 Chem. Soc. Rev., 46, 1842-1874 (2017)
Epoxides are cyclic ethers used to manufacture plastics, solvents, and surfactants[1,2]. The simplest epoxide, ethylene oxide (EO), is produced through the direct partial oxidation of ethylene over silver at 220-280°C. In this process EO is formed by the addition of a single oxygen atom across ethylene’s C=C double bond, and while combustion is thermodynamically favorable, EO can be produced with 90% selectivity. Despite the technological importance of such a process to selectively produce propylene oxide (PO), no selective direct oxidation route to PO is known. A major focus in efforts to understand why has been uncovering what drives EO selectivity.
APXPS has revealed the active catalyst contains two types of oxygen, nucleophilic and electrophilic, which are distinguishable by their O1s binding energies (BE) of ca. 528 eV and 530 eV, respectively[3-5]. Only electrophilic oxygen is known to produce EO. Its atomic structure has been debated[7-9]. This ambiguity has hampered mechanistic understanding. By combining APXPS with density functional theory (DFT) calculations we demonstrated electrophilic oxygen is related to trace sulfur impurities that can produce O-SO3 adsorbed on unreconstructed silver. DFT shows this species has the spectroscopic fingerprints of electrophilic oxygen, and it is predicted to participate in EO formation. APXPS appears to confirm the DFT results, showing surface the coverage of SOx species correlates with EO selectivity. This knowledge has made it possible to gain insights into propylene epoxidation.
DFT suggests adsorbed O-SO3 can participate in PO formation. APXPS reveals, however, that O-SO3 is not present under propylene epoxidation conditions. We found it can be formed via SO2 dosing, a process that increases PO selectivity with increasing O-SO3 coverage. The formed O-SO3 is not, however, stable and requires a continuous sulfur source to maintain the SOx surface coverage.
By combining APXPS and DFT were we able to solve the nature of electrophilic oxygen thought to be responsible for EO production. Extending this study to propylene epoxidation allowed us to identify the steady-state coverage of electrophilic oxygen as a key factor limiting PO selectivity, and, moreover, develop a strategy to circumvent this limitation.
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Supported metal catalysts for the water-gas shift reaction show strong structure-activity dependence. The direct relationships between spectral features and their function in these catalysts have been challenging to draw, due to the complexity of real catalytic systems. Discussions remain about the nature and reactivity of the active species, whether they are atomically dispersed(AD) or nanoparticles(NP). In this work, we provide an overview of how reactivity correlates with the specific electronic structure/morphology of the supported metal. WGS over Pt/CeO2 powder catalysts containing AD species and NPs having various diameters in the absence and presence of alkali promotor was monitored by means of in situ ambient pressure X-ray photoelectron spectroscopy (APXPS) and scanning transmission electron microscopy (STEM). Theoretical modelling of platinum NPs arches over spectroscopy and microscopy, thus enables deconvolution of XPS using morphology as a guide. This brings the direct quantification of the different types of platinum sites under working condition. Catalytic tests, carried out in parallel and reproducing the reaction conditions adopted during spectroscopic measurements, provide relevant information about the catalytic performance, thus bridging the pressure gap between techniques.
We show a triangular connection between morphology, electronic structure and performance of actual Pt/CeO2 catalysts. The apparent activation energy depends negatively on the fraction of corner platinum sites and such dominant active sites are metallic in nature. Other sites (bulk, terrace and edge) on nanoparticles and AD species are spectators. We further predict the intrinsic activation energies of corner sites located at small to large NPs. This work proposes a suitable approach to straightly correlate structure and activity by means of a smart combination of in situ and ex situ characterization techniques with theoretical calculations. The approach is generally applicable and enables the understanding of reaction mechanisms in heterogeneous industrial catalysts.
Herein, we operated the fast gas pulse set-up coupled with APXPS at HIPPIE beamline to map the transient evaluation of CO oxidation reaction on Pt(111) single crystal surface with sub-millisecond time resolution. To elaborate the reaction mechanism, two strategies were designed and implemented: one pulse, two pulses experiment.
In the one pulse experiment, CO was pulsed onto the sample surface joining in a constant O2 flow to react at the surface under reaction temperature. Mapping of CO pulse reveals reaction only happened on the rising edge, and CO2 production rendered an upward trend with a decrease in frequency of event from 40 HZ to 2.5 HZ. The diversity of reactivities at different stage of a pulse and at different periods can be correlated to the corresponding surface species of each time window (before pulse, rising edge, top site, falling edge).
The reactant gases, CO and O2 were pulsed to the sample surface alternatively via manipulating time delay of the two gas pulses, which can create oscillating local gas environments in the two pulses experiments. Thus, the oscillation of the local gas environment led to the fluctuation of surface species, which drove CO oxidation reaction away from steady state into nonequilibrium state.
Here we present results from a study of low-temperature CO oxidation performed on different model Au/CeO2 (111) catalysts. We prepared stoichiometric CeO2(111) surfaces decorated with gold nanoparticles and varying amounts of step edges. The prepared samples were investigated in UHV using synchrotron radiation photoelectron spectroscopy (SRPES) and scanning tunneling mi-croscopy (STM). This study helped to figure out how the one monolayer-high ceria step edges affect the metal–substrate interaction between Au and the CeO2(111) surface. It was found that the concentration of ionic Au+ species on the ceria surface increases with the increasing number of ceria step edges and does not correlate with the concentration of Ce3+ ions, which are supposed to form due to the interaction with gold nanoparticles, indicating an additional channel of Au+ formation on the surface of CeO2(111). The study of CO oxidation on the highly stepped Au/CeO2(111) model sample performed by combining near-ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) and near-ambient pressure scanning tunneling microscopy (NAP-STM) demonstrated the high catalyst stability in CO. However, it underwent substantial chemical and morphological changes in CO oxidation operational conditions. Already at 300 K, almost all Au+ was reduced into metallic gold. In addition, gold nanoparticles begin to grow using a mechanism that involves the disintegration of small gold nanoparticles in favor of large ones. With increasing temperature, the catalyst quickly transformed into a system of primarily large Au particles that con-tained no ionic gold species.
Catalysis plays a central role in the design of efficient processes for energy production and environmental remediation. One of the main obstacles for finding sustainable catalysts is related to the relatively poor understanding of the material properties, especially under working conditions. This presentation illustrates a characteristic example where ambient pressure soft X-ray photoelectron (APXPS) and absorption (XAS) spectroscopies were applied to identify critical parameters governing the reactivity of cobalt for CO Preferential Oxidation in H2-rich mixtures (COPrOx), an important catalytic reaction involved in the purification of H2. At first, CoO was identified as the surface oxidation state with the highest activity for COPrOx, by combining in situ spectroscopy and first-principles calculations . Nevertheless, it appears that CoO is unstable under reaction conditions and is readily oxidized to the less active Co3O4. In an attempt to stabilize Co2+ oxidation state, cobalt was impregnated with V  and Mn  promoters. The nature of the catalytic active sites of the promoted catalysts during COPrOx was established by operando APXPS, while the stability of the CoO surface under reaction-relevant conditions was verified by in situ XAS at 1 bar. The promotion effect of the two metals on cobalt will be discussed in terms of enhanced redox stability of CoO surface based on by spectroscopic evidence. In addition, the influence of the spatial distribution of the promoter on the stability of CoO will be discussed. Furthermore, deeper insights regarding the surface composition and electronic structure of the promoted catalysts were deduced from theoretical simulation of XPS peak intensities and L3-absorption edges using SESSA  and CTM4XAS  software, respectively. Overall, our results correlate surface state and catalytic performance of cobalt-based COPrOx catalysts and validate the application of in situ and operando spectroscopies to provide the concept for designing better performing catalysts.
L. Zhong, et al., Correlation between Reactivity and Oxidation State of Cobalt Oxide Catalysts for CO Preferential Oxidation, ACS Catal. 9 (2019) 8325–8336.
L. Zhong, et al. Improving the Catalytic Performance of Cobalt for CO Preferential Oxidation by Stabilizing the Active Phase through Vanadium Promotion, ACS Catal. 11 (2021) 5369–5385.
L. Zhong, et al., Effect of manganese promotion on the activity and selectivity of cobalt catalysts for CO preferential oxidation, Appl. Catal. B Environ. 297 (2021) 120397.
W. Smekal, et al, Simulation of electron spectra for surface analysis (SESSA): A novel software tool for quantitative Auger-electron spectroscopy and X-ray photoelectron spectroscopy, Surf. Interface Anal. 37 (2005) 1059–1067.
E. Stavitski, et al, The CTM4XAS program for EELS and XAS spectral shape analysis of transition metal L edges, Micron. 41 (2010) 687–694.
Bicomponent heterogeneous catalysts, often composed of noble metals (NMs) supported on transition metal oxides (TMOs), attract much interest due to their prominent performance in catalytic processes. Results from Prof. Gabor Somorjai’s group  demonstrate a 500-fold higher reactivity of catalytic CO oxidation over Pt/Co3O4 catalysts than other oxide-supported Pt nanoparticles (Fig.1, left). Typically, prior to catalytic reaction, pre-treatment under an H2 atmosphere at 573 K is conducted, which produces metallic Pt and a partially reduced CoO substrate. More importantly, oxide moieties originating from the support further migrate over the metallic nanoparticles, and can totally cover the underlying NM. This newly-formed catalyst is known as the inverse catalyst and hints at the atomic scale nature of active sites which can exhibit the so-called strong metal-support interactions (SMSI).
Herein, employing ambient pressure X-ray photoelectron spectroscopy (AP-XPS) on CoO/Au(111) model catalysts, we observed: (1) CoOx of monolayer thickness wetting onto Au(111) substrate after metallic bilayer Co island exposed to O2 ; (2) Reversible adsorption of CO on partially oxidized CoO (Fig.1, right); (3) Reduction of CoO after annealing in CO atmosphere. Due to the charge transfer between ultrathin CoO and electronegative Au substrate, the Co(II) atoms in direct contact with Au support have a unique electronic structure and we believe are responsible for the observed reactivity of the supported monolayer CoOx layer. We are currently pursuing investigations using ambient pressure scanning tunneling microscopy (AP-STM) and theoretical calculations to achieve more fundamental insights into SMSI effects at the atomic scale on CoO/Au(111) model systems.
Hydrogen spillover is the surface migration of activated hydrogen atoms from a metal catalyst particle, on which molecular hydrogen dissociates, onto the catalyst support. It is of high importance for heterogeneous catalysis, hydrogen storage materials, and fuel cell technology. Its occurrence on metal oxide surfaces is established , yet questions remain about how far from the metal center spillover is reaching. Achieving spatial understanding of this process remains a challenging quest since the relevant materials are usually complex nanomaterials. We chose the approach to translate the complex structure of metal oxide catalysts into planar model representations . This approach allows the use of surface sensitive techniques that otherwise would not provide sufficient spatial information.
By employing X-ray photoelectron emission spectroscopy (XPS), we were able to gather direct evidence that spillover is occurring over several microns across the oxide surface . In this short communication, the propagation of hydrogen across the metal oxide originating from deposited platinum will be presented in a temperature-resolved chemical states of the oxide surface. These findings and the derived understanding at which temperatures hydrogen spillover in affecting the surface will help to improve and exploit the process of hydrogen spillover for application.
 Prins R. Chem. Rev., 2012, 112(5), 2714-2738
 Karim W., Spreafico C., Kleibert A., Gobrecht J., Vandevondele J, Ekinci Y., van Bokhoven J. A. Nature, 2017, 541(7635), 68-71.
 Beck A., Frey H., Kleibert A., van Bokhoven J. A. in preparation
Operando characterization of working catalysts, requiring the simultaneous measurement of catalytic performance, is crucial to identify the relevant catalyst structure/composition and how molecules interact with interfaces . Three examples of model and technological catalysts illustrate what can be learnt from synchrotron based spectroscopic and microscopic studies.
i) Operando APXPS/SFG/MS:
CO oxidation on Pt/ZrO2 prepared by atomic layer deposition (ALD) was examined by sum frequency generation (SFG) spectroscopy and ambient pressure X-ray photoelectron spectroscopy (AP-XPS @MAX IV), combined with mass spectrometry (MS) . Complemented by Density Functional Theory (DFT), we show that the reaction onset is determined by a delicate balance between CO disproportionation and oxidation.
ii) In situ SPEM/PEEM:
H2 oxidation on polycrystalline Rh was studied by scanning photoelectron microscopy (SPEM @ELETTRA) and photoemission electron microscopy (PEEM), which allow local surface analysis and visualising the heterogeneity of ongoing reactions on a µm-scale . This revealed an anisotropy of surface oxidation, yielding an oxidation map. In situ PEEM imaging of ongoing H2 oxidation directly compares the local reactivity of metallic and oxidised Rh, revealing a high transient activity of Rh surface oxide, providing a direct imaging of a structure-activity relation for plenty surface structures. In a follow-up SPEM study , an unknown coexistence of four different states was observed: an active steady state, an inactive steady state and multifrequential oscillating states.
iii) Operando APXPS/XANES/MS:
Turning from model systems to applied catalysis, AP-XPS and X-ray absorption near edge structure (XANES @SLS/PSI) were employed to characterize Ni/ZrO2 and Ni/MgO-ZrO2 upon H2 pretreatment and during Partial Oxidation of Methane (POM) to Syngas at 750 ⁰C (activity monitored by inline MS). During POM (partial) Ni re-oxidation occurred, although Niº is often suggested as active phase, but the Ni oxidation state was sensitive to feed gas changes.
The insights by monitoring ongoing reactions may stimulate new ways of catalyst design.
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2. V. Pramhaas et al., ACS Catalysis 11 (2021) 208–214
3. P. Winkler et al., Nature Communications 12 (2021) 69
4. P. Winkler et al., Nature Communications 12 (2021) 6517
5. J. Asencios et al., in preparation
Acknowledgements: Work supported by the Austrian Science Fund (FWF) and the Austrian Academy of Sciences (ÖAW).
The Sabatier reaction (CO2 + 4 H2 ⟶ CH4 + 2 H2O) is of growing interest in the context of limiting anthropogenic CO2 emissions. Despite its low reaction temperature (200-400°C), the activation of CO2 remains difficult to achieve and requires the formulation of highly active catalysts. Among them, Ni, Co or Ru based catalysts supported on CeO2 are often considered for the reaction but still suffer from low activity at low temperature and/or rapid deactivation . An amelioration of the catalyst formulation is therefore needed to envisage industrialization of the CO2 methanation process.
We have recently developed a synthesis method to produce Ni-doped CeO2 nanoparticles (NPs) based on Schiff base metal complexes [2,3]. This method produces nanoparticles with well-defined composition of Ni0.04Ce0.96O2, a particle size distribution ranging between 5–7 nm and a considerably higher methanation activity in comparison to classical Ni/CeO2 prepared with same nominal Ni loading. A first NAP-XPS study conducted at the TEMPO beamline of SOLEIL synchrotron (France) showed a better reducibility of cerium (Ce4+ → Ce3+) between 200 and 700 °C under different gases encountered in methanation conditions (H2, H2O, CO2) for the doped sample in comparison to pure CeO2 prepared via the same synthesis method. Ce3+ species is accompanied by the formation of oxygen vacancies in ceria lattice and better ability to activate CO2 for its gradual hydrogenation .
To gain insight on the Ni oxidation state evolution, the surface state of the NPs was studied under reducing conditions (1 bar of H2) by NEXAFS at the APE-HE beamline of ELETTRA synchrotron (Italy). It was found that the reduction of Ni2+ ions in 1 bar H2 is hindered while reduction of Ce4+ is promoted on Ni0.04Ce0.96O2 NPs, as compared to pure NiO and CeO2 reference samples. Interestingly, reduction of Ce4+ is accompanied by further oxidation of Ni2+ into Niδ+ (2<δ<3). This is quite unexpected observation, for nickel oxide treated in 1 bar H2, and indicates an electronic interaction between Ni and Ce ions which was also observed under reaction conditions (CO2/H2 = 1:4). Theoretical simulation of Ni L-edge spectra suggested that Ni atoms are tetrahedrally coordinated into ceria lattice, in contrast to the familiar octahedral symmetry of bulk NiO.
 H. Yang, C. Zhang, P. Gao, H. Wang, X. Li, L. Zhong, W. Wei, Y. Sun, Catal. Sci. Technol., 2017, 7, 4580-4598
 W. Derafa, F. Paloukis, B. Mewafy, W. Baaziz, O. Ersen, C. Petit, G. Corbel, S. Zafeiratos, RSC Adv., 2018, 8, 40712-40719
 M. Barreau, D. Chen, J. Zhang, V. Papaefthimiou, C. Petit, D. Salusso, E. Borfecchia, S. Turczyniak, K. Sobczak, S. Mauri, L. Braglia, P. Torelli, S. Zafeiratos, Mater. Today Chem., 2022, 26, 101011
 M. Boaro, S. Colussi, A. Trovarelli, Front. Chem., 2019, 7 , 28
Noxious NO gas is a harmful environmental pollutant and toxic to many life forms, however as it stands, prominent reactions such as combustion reactions taking place in engines, generate NO by-products and require removal from the exhaust gas to prevent unnecessary exposure. Current technology lacks a sufficient catalyst to effectively breakdown the kinetically hindered NO molecule without interaction of a reducing agent, i.e. NH3, urea, hydrocarbon, or expending a surplus of energy. The advent of new catalyst materials diverging from a traditional metal and metal oxide may prove advantageous, as is the case for perovskite materials, and more ecologically friendly options that have the ability to perform metal-free catalysis. One such promising candidate is nitrogen-doped graphene (N-Gr). Despite numerous studies of N-Gr examining adsorption properties and reaction chemistry, a fundamental understanding of the role of the active sites in the NO decomposition reaction is currently not well understood. We explored the role of two primary active sites (graphitic N, pyridinic N) in N-Gr films composed of 1) purely graphitic N and 2) a mixture of graphitic N and pyridinic N sites. We tracked the electronic and vibrational profiles of reaction intermediates and products using simultaneous in situ ambient pressure X-ray photoemission spectroscopy (APXPS) and infrared reflection-absorption spectroscopy (IRRAS), respectively. We found that the incorporation of pyridinic N sites lowers the temperature of intermediate formation and potentially promotes different reaction intermediates due to different bonding environments between pyridinic N and graphitic N sites leading to higher reactivity.
The MAX phases are novel structural and functional ceramics with a layered ternary carbides structure discovered in the ‘60s.(1) They are so-called MAX because of their composition: namely, Mn+1AXn, where M is an early transition metal, A is mainly a group IIIA or IVA (i.e., groups 13 or 14) element, X is C and/or N, and n= 1 - 4. The applications of solid MAX phases are narrow due to their metallic, high-temperature stabilities, and superior mechanical properties, but when they are exfoliated into 2D MXenes they exhibit quite different electronic, magnetic, optical, and electrochemical properties that are rarely seen in their original MAX phases.(2)
These newly discovered 2D metal carbides (MXenes), and their 3D parents (MAX phase), hold promises to possess interesting catalytic properties (3) due to a bouquet of particular properties such as good electronic conductivity, hydrophilicity, resistance to chemical attack and oxidation under high-temperature conditions. These properties are strongly related to their very rich surface functionality formed during wet-etching synthesis of MXenes from MAX phases, when functional groups such as −O,−OH, and −F are formed on the surface.
Yet, little is known about the dynamics of the surface and structural changes that these systems undergo during catalysis and lead to efficient catalyst function. Recently, we investigated at the X07DB-In Situ Spectroscopy beamline at the Swiss light source synchrotron the dynamics of the surface and subsurface rearrangements of the MAX phase (Ti3SiC2) and MXene (Ti3C2Tz) under steady state O2 and/or CH4 environment at different temperatures (between room temperature and 350 ˚C). In this way, the impact of reduction and oxidation conditions was assessed. Sensitive surface and subsurface in situ XPS spectra were obtained at different kinetic photoelectron energies (300 and 600 eV). The influence of water on surface dynamics was also studied. As expected, the surface is “alive” and reversible surface modifications were observed, indicating the presence of a possible “memory” phenomenon.
The most important outcome of this study was the appearance of OH surface species in the presence of CH4 onto the MXene surface. One possible explanation for this finding is that CH4 might be activated on the metal species, then one C–H bond is broken and the resulting H is immediately attached to the O species on the surface. Furthermore, a good stability of the of MAX phase and MXene throughout the AP-XPS experiments was confirmed. These are valuable findings for further understanding of the MAX phase and MXene behaviour in catalytic processes.
All-solid-state lithium-ion battery (ASSLIB) is a promising next generation rechargeable battery because of its high safety and reliability. Understanding of the electrochemical reactions and accompanying structural changes is important to develop high-performance materials and cell structures. Because ex-situ measurements may result in misinterpretation due to the variation of samples and undesired side reactions during sample transfer, in-situ/operando measurements of the same position of the same sample are essential for comprehensive understanding. Recently, we developed in-situ/operando XPS and HAXPES apparatuses equipped with a bias application system and observed the electrochemical lithiation/delithiation reactions of an amorphous silicon thin film electrode sputter-deposited on a solid electrolyte sheet. Upon lithiation, not only lithium silicides but also lithium oxides, lithium silicates and lithium carbonates were formed due to the insertion of lithium into the silicon electrode and native oxide, followed by side reactions of those surface species with residual gasses in the vacuum chamber. Although lithium silicides reversible responded to the successive delithiation, lithium oxides, lithium silicates and lithium carbonates maintained at the surface as irreversible species. Interestingly, a drastic shift of lithium silicide peak was observed in the successive delithiation after preceding lithiation up to certain level. This is attributed to the phase transition of a crystalline lithium silicides to an amorphous phase. Further details and a few other works will be presented.
 R. Endo, T. Ohnishi, K. Takada, and T. Masuda, “In Situ Observation of Lithiation and Delithiation Reactions of a Silicon Thin Film Electrode for All-Solid-State Lithium-Ion Batteries by X‑ray Photoelectron Spectroscopy”, J. Phys. Chem. Lett. 2020, 11, 6649−6654.
 R. Endo, T. Ohnishi, K. Takada, T. Masuda, “Instrumentation for tracking electrochemical reactions by x-ray photoelectron spectroscopy under conventional vacuum conditions”, Journal of Physics Communications, 2021, 5, 015001.
 R. Endo, T. Ohnishi, K. Takada, T. Masuda, “Electrochemical Lithiation and Delithiation in Amorphous Si Thin Film Electrodes Studied by Operando X‑ray Photoelectron Spectroscopy”, J. Phys. Chem. Lett. 2022, 13, 7363-7370.
Electrocatalytic generation of chemical fuels such as hydrogen enables storing intermittent renewable energies, and perovskite oxides are among the most attractive candidate materials to catalyze the kinetically limiting half reaction, the oxygen evolution reaction (OER). OER activity is typically correlated to electronic and atomic structure parameters. But the catalyst surface – i.e. where the reaction happens – changes during the reaction. To design next-generation electrocatalysts, a detailed operando understanding of the relationships between catalytic activity, stability and atomic-level surface properties during the reaction is required.
In my talk I will address two essential ingredients to achieve this next-level understanding: Firstly, epitaxial thin films are a direct route for single crystalline model electrocatalysts that can be fabricated with atomically-tailored surface composition. These offer the ideal platform to derive structure-property-function relationships, track the evolution of the surface properties with applied potential and enable direct comparison to the surfaces investigated in density functional theory. Secondly, I will discuss application and challenges of surface-sensitive operando characterization in a liquid medium, with specific attention to atomically-defined thin film model electrode surfaces. Information from the outermost surface of a catalyst can be obtained through a standing-wave approach1,2 or extraction of a surface-only signal from careful thickness-dependent studies.3
The key example will be LaNiO3 thin films, which are atomically flat both before and after application as electrocatalysts for the OER during water electrolysis. We selectively tuned the surface cationic composition in epitaxial growth. The Ni-termination is approximately twice as active for the OER as the La-termination.1 Using a suite of ex situ, in situ and operando spectroscopy tools, we found that the Ni-rich surface undergoes a surface transformation towards a catalytically active Ni hydroxide-type surface.1 If LaNiO3 surfaces are exposed to the CO2-containing atmospheres, however, surface carbonate groups decrease the activity as evidenced by APXPS.4
Our work thus demonstrates tunability of surface transformation pathways by modifying a single atomic layer at the surface. It also highlights the need of and summarizes pathways for the exploration of the three-step relationship between as-prepared surface, transformation under applied potential, and electrocatalytic activity.
1. Baeumer, C. et al. Nat. Mater. 20, 674–682 (2021).
2. Martins, H. P. et al. J. Phys. D. Appl. Phys. 54, 464002 (2021).
3. Baeumer, C. J. Appl. Phys. 129, 170901 (2021).
4. Baeumer, C. et al. J. Mater. Chem. A 9, 19940–19948 (2021).
The chemical states of the catalyst surface play vital roles in the various kinds of catalytic re-actions, such as oxygen evolution reaction, oxygen reduction reaction and nitrogen reduction reac-ton, which will dominate the active sites and the reaction dynamic. Numerous research efforts have been devoted to revealing the active species on the catalyst surface to design the catalyst with great performance. Most innovative spectroscopy techniques are developed and applied to better under-stand the real-time changes during catalytic reactions nowadays. Several different electrochemical cell designs combined with an ambient pressure X-ray photoelectron spectroscopy (APXPS) end-station have been reported to study the chemical state of the catalyst surface during the electro-chemical reaction. Here, we report an electrochemical cell system inspired by the original design from the BESSY II ISISS beamline coupled with the APXPS endstation at the BL 24A of NSRRC to study the surface evolution of the catalyst under different pH environments during electrochemi-cal reactions.[1, 2]
In acidic environment, nafion electrodes with a low-loading amount of Pt catalyst are prepared by the e-gun deposition method to study the species evolution during a redox reaction. The ratio of divalent specie on Pt catalyst surface gradually increases during the whole anodic process. In con-trast, the tetravalent specie appears as the applied potential exceeds the threshold voltage of OER. While stepwise oxidation is observed during the anodic polarization, the hysteresis of oxide species is found on the surface with the incomplete reduction of divalent specie, which could be due to the heavier surface oxidation from higher anodic polarization. Additionally, in-operando measurement of valence band spectra shows the decline in electron density near Fermi energy, suggesting the im-portance of a slightly oxidized surface on the Pt OER catalyst.
In an alkaline environment, the zinc substitution iron cobaltite-based spinel structure catalysts (ZnxFe1-xCo2O4) are prepared onto the anion exchange membrane (AEM) to study species evolution during the OER reaction. Our results help propose the important intermediate, Co4+ and superoxide species, during oxygen production. More detailed results will be presented in this meeting.
Keywords : Electrochemical Ambient Pressure X-ray Photoelectron Spectroscopy (EC-APXPS), Oxygen Evolution Reaction (OER), Platinum catalyst, Anion Exchange Membrane, Nafion Mem-brane.
 R. Arrigo, M. Hävecker, M. E. Schuster, C. Ranjan, E. Stotz, A. Knop-Gericke, R. Schlögl, Angew. Chem. Int. Ed. 2013, 52 (44), 11660-11664.
 R. Mom, L. Frevel, J.-J. Velasco-Vélez, M. Plodinec, A. Knop-Gericke, R. Schlögl, J. Am. Chem. Soc. 2019, 141 (16), 6537-6544.
The valorization of carbon dioxide is one of the main research topics in the current time, since it is highly relevant for the development of sustainable energy cycles within the scope of net-zero CO_2-emissions for the future. The reaction of CO_2 on silver catalysts can take place either electrochemically (mainly to CO) or thermochemically (mainly to methanol). It is fundamentally interesting to determine the similarities and differences between the molecular activation processes and intermediates defining the product selectivity in each of these reaction environments.
We analyzed the thermochemical reaction (CO_2 + 3H_2 → CH_3 OH+H_2 O on Ag) using the solid/gas endstation at the In Situ Spectroscopy (ISS) beamline and the electrochemical reaction (CO_2 →CO+0.5 O_2 in electrolytic cell with Ag cathode) using the solid/liquid dip-and-pull method at the PHOENIX beamline, both situated at the Swiss Light Source (SLS).
Understanding the response of a model reaction catalyst (here Ag foil) is crucial before increasing the catalyst complexity. The response of the Ag foil surface states to different gases and educt/product gas mixtures was evaluated as a function of temperature and time. Time resolved data are generated by using transient photoelectron emission measurements (TPM). By fast XPS scans during iterated gas switching, time-dependent measurements allow the detection of short-lived species and the determination of reaction kinetics.
The evolution of the electrochemical system over time on the other hand is characterized by pulling the working electrode out of the electrolyte under potential control, preserving the operational surface condition and present surface contaminations accumulated during CO_2-reduction. Performance improvements achieved e.g. by electrochemical reconditioning can then be directly related to changes of the surface species.
To obtain the best possible correlation between model experimental results and processes under industrially relevant conditions, the performed APXPS experiments are combined with ex situ reactor/electrochemical cell studies. Methanol synthesis from the state-of-the-art Cu-based catalyst is industrially performed at ≈50 bar and ≈250 °C, which can be mimicked in a high-pressure lab-scale reactor.
For the economically viable electrochemical conversion of CO_2, current densities ≫100 mA/cm^2 geometric surface area are required. The results obtained by in situ APXPS CO_2-reduction (≤1 mA/cm_geom^2) are thus compared to ex situ liquid-electrolyte electrolyzer cells (≤50 mA/cm_geom^2) and gas diffusion electrode containing bipolar-membrane cells (>100 mA/cm_geom^2 ) in the presented project.
Interactions of N2 and H2O at transition metal oxide surfaces are of fundamental interest for gaining insight into electrocatalytic nitrogen reduction reaction (NRR) mechanisms. N2/H2O interactions at the polycrystalline vanadium oxide/vapor interface were monitored at room temperature and N2 partial pressures between 10-9 Torr and 10-1 Torr using Near-Ambient Pressure X-ray Photoelectron Spectroscopy (NAP-XPS). The oxide film was predominantly V(IV), with significant V(III) and V(V) components. Such films have been previously demonstrated to be NRR active at pH 7. There is little understanding, however, of the detailed nature of N2-surface interactions. XPS measurements were acquired at room temperature in environments of both pure N2 and equal pressures of N2 and H2O vapor, up to a N2 partial pressure of 10-1 Torr. In the absence of H2O, broad N 1s features were observed at binding energies of 401 eV and 398.7 eV with relative intensity ratios of ~ 3:1, respectively. These features remained upon subsequent pumpdown to 10-9 Torr, indicating that adsorbed nitrogen is stable at room temperature in the absence of equilibrium with gas phase N2. In the presence of equal pressures of N2 and H2O vapor, the 401 eV N 1s feature was reduced in intensity by ~ 50% at 10-1 Torr N2 partial pressure, with the feature at 398.7 eV binding energy barely observable. DFT calculations show that the above NAP-XPS data demonstrating stable N2-surface binding in the absence of N2 overpressure are consistent with N2 binding at V(IV) or V(III) sites, but not at V(V) sites, and further show that N2/H2O binding is competitive. SCF-HF calculations suggest that the two N 1s XPS features correspond to "shake" and normal transitions at 401 eV and 398.7 eV, respectively, for N2 bonded end-on to the surface. The shake feature involves a charge transfer from V 3d to N2 pi* in addition to N 1s ionization. The difference in binding energies of the two features, ~ 2.3 eV, strongly suggests N2 -V(III) binding. The data presented demonstrate the ability of NAP-XPS, in concert with theory, to provide atomic-level insight concerning interfacial reactions relevant to electrocatalysis.
This research used resources of the Advanced Light Source, which is a DOE Office of Science User Facility, under Contract No. DE-AC02-05CH11231. Work at UNT was supported in part by the NSF through grants DMR-2112864 (JAK, TRC), and via NSF support for the UNT CASCaM HPC cluster via Grant CHE-1531468. PSB was supported by the Geosciences Research Program, Office of Basic Energy Sciences, U.S. DOE through its Geosciences program at PNNL.
Highly selective, rapid ion transport across membranes is essential in biological ion channels and electrochemical and water purification membranes. Ion exchange membrane (IEM) based technologies play a critical role in meeting increasing global demands for energy and water because charged groups tethered to the polymeric chains enable selective permeation of ions based on their charge/valence.1 In 1911, permselectivity of charged membranes was postulated for the first time by Prof. Frederick G. Donnan, regarding the formation of an electrical potential at the membrane/solution interface.2 The so-called Donnan potential was found to be responsible for the selective transport of ions. Despite rich literature across various fields, many fundamental molecular interactions underpinning ion selectivity in IEMs are poorly understood. In addition, the direct measurement of Donnan potential has been thought to be unmeasurable3,4 and has never been accomplished in the over 100 years since the original Donnan theory was proposed.
In this talk, I will present the direct measurement of the Donnan potential of an IEM equilibrated with aqueous salt solutions using tender (hv = 4 keV) ambient pressure X-ray photoelectron spectroscopy (APXPS).5,6 Our results directly reveal the dependence of the membrane’s Donnan potential on external salt concentration and counter-ion valence, as suggested by Donnan himself.7 In addition, by comparing our experimental results with well-known thermodynamic models, we show that the classic Donnan model assuming ideal behavior fails to predict experimentally measured Donnan potentials, while an improved model (Donnan/Manning) incorporating some of the thermodynamic non-idealities shows reasonable correlation. The ability to discern the various models is highly important, since current models fail to capture all non-idealities (e.g., mobile ion association, ionic cross-linking, non-electrostatic interactions, etc.). We anticipate that our methodology will be an important step toward better understanding permselectivity in membranes that are important in cellular processes as well as in energy storage and conversion and water purification applications.
1 Xu, T. J. Membr. Sci. 263, 1-29, doi:10.1016/j.memsci.2005.05.002 (2005).
2 Donnan, F. G. & Harris, A. B. CLXXVII. Journal of the Chemical Society, Transactions 99, 1554-1577, doi:10.1039/CT9119901554 (1911).
3 Strathmann, H. in Membrane Science and Technology Vol. 9 (ed Heiner Strathmann) Ch. 3 - Preparation and Characterization of Ion-Exchange Membranes, 89-146 (Elsevier, 2004).
4 Strathmann, H. in Encyclopedia of Separation Science (ed Ian D. Wilson) 1707-1717 (Academic Press, 2000).
5 Axnanda, S. et al. Sci. Rep. 5, 9788, doi:10.1038/srep09788 (2015).
6 Favaro, M. et al. Nat. Commun. 7, 12695, doi:10.1038/ncomms12695 (2016).
7 Aydogan Gokturk, P. et al. Nat. Commun. 13, 5880, doi:10.1038/s41467-022-33592-3 (2022).
Ion-exchange membranes (IEMs) are a critical component of electrochemical conversion and storage devices, ranging from electrolyzers to batteries, where the IEM not only separates different media, but also ensures the selective transfer of specific ions from one medium to another in order to sustain the electrochemical processes of interest. The ion selectivity and conductivity, and the mechanical/chemical stability are key properties of the IEM, directly influencing the device performance and lifetime. These properties are often interdependent, and their optimization for the targeted reaction environment is a crucial challenge in the development of IEMs, and hence of efficient devices. A rational optimization requires a molecular level understanding of the processes occurring in the membrane, particularly during device operation. In this talk, we will describe an in situ ambient pressure hard X-ray photoelectron spectroscopy (AP-HAXPES) investigation performed at the SpAnTeX end-station (BESSY II), of selected commercial CEM and AEM to characterize their chemical properties close to the vapor pressure of water at room temperature and under operating conditions. First, we characterized the Nafion® 115 (N115) and the Fumasep® PEEK-reinforced FAA-3-PK-75 (FAA-3-75) membranes under hydrated conditions by means of AP-HAXPES. Second, we used a customized hybrid gas/liquid phase electrolyzer to investigate the aforementioned membranes under working conditions. In a typical experiment, a membrane was placed between a compartment containing a NaCl aqueous solution (1 M) and a Pt electrode on one side, and an Au electrode and the analysis chamber on the other side. This configuration enabled the polarization of the membranes by applying a potential difference between both electrodes. We observed the unambiguous sodium and chlorine fingerprints for N115, and FAA-3-75, respectively. Interestingly, these fingerprints were independent of the applied potential – whether negative or positive – and were observable already at equilibrium conditions, i.e. OCP. Therefore, the ion out-diffusion is likely predominantly mediated by water diffusion and not by the electric field. Furthermore, although a polarization is still effective upon potential switch as indicated by the core level binding energy shift, the respective amount of out-diffused species does not change significantly, thereby pointing to the irreversible accumulation of ions at the membrane surface. Our work illustrates the potential of AP-HAXPES as a tool to investigate the chemical properties of IEMs in electrochemical configurations and under operating conditions. Our findings suggest that commercial membranes do not exhibit optimal ionic selectivity, which in turns plays a crucial role in the electrochemical performance and lifetime of the applied device.
 M. Favaro et al., Surf. Sci. 2021, 713, 121903
The degradation of technical alloys including Ni-based superalloys and steels remains a technological challenge, and depends on the protective oxide layer. Our work focusses on the oxidation of Ni-based superalloys where the a protective oxide layer with chromia provides the corrosion and degradation resistance.  The majority of mechanistic studies are centered on transport through the oxide leaving significant gaps in the mechanistic understanding of the initial reaction steps of alloy oxidation. Our presentation will give comprehensive view of the oxidation processes on Ni-Cr-(W,Mo) surfaces. 
We use operando and in-situ experiments to capture the initial oxidation steps at the pristine alloy surface combining NAP-XPS, XPEEM, XPS, and STM experiments and DFT calculation. This work straddles the complexity gap by using single crystal thin films  as well as polycrystalline cast alloys with a wide composition range Ni-5Cr to Ni-30Cr, and the ternaries Ni-15Cr-6W, and Ni-15Cr-6Mo (all (at%)). The use of thin films is achieves a variation in alloy composition with relative ease, and offers NiCr(100) and NiCr(111) surfaces. 
Our work identified using NAP-XPS the surface chemical reactions central to understand the role of minor alloying elements (MAE) such as W, and M where a relatively small addition of a specific element has an outsized impact on oxidation.  With NAP-XPS experiments  in combination with DFT it was shown that MAE is tied to the formation of preferential adsorption sites at the W-Cr bridge site thus favoring the nucleation of the coveted chromia over NiO. Variation in oxidation parameters p(O2) (10-9 to 10-2 mbar) and T (200 °C to 600 °C) isolate kinetic limiters, which include an enhanced bulk diffusion through alloying with W, and an unexpectedly short diffusion length for O-atoms for T<300°C.  Polycrystalline, large grain (up to 1 mm) cast alloys gives access to a wide range of surface orientation,  far beyond the low index surfaces of model systems. The relevance of crystallographic orientation on oxide nucleation and chemistry will be discussed based on XPEEM, and STM experiments.
 V. Maurice, P. Marcus. Prog. Mater. Sci., 95, 132-171 (2018)
 C. Volders, V.A. Angelici, I. Waluyo, A. Hunt, L. Árnadóttir, P. Reinke. npj Materials Degradation, 6, 52 (2022)
 G. Ramalingam, P. Reinke. Journal of Applied Physics, 120, 225302 (2016)
 W.H. Blades, M.R. Barone, P. Reinke. npj Materials Degradation, 5, 1-10 (2021)
 K. Lutton Cwalina, C.R. Demarest, A.Y. Gerard, J.R. Scully. Curr. Opin. Solid State Mater. Sci., 23, 129-141 (2019)
 R. Ramanathan, G. Ramalingam, J.H. Perepezko, P. Reinke, P.W. Voorhees. ACS Applied Materials & Interfaces, 10, 9136-9146 (2018)
 K. Gusieva, K.L. Cwalina, W.H. Blades, G. Ramalingam, J.H. Perepezko, P. Reinke, J.R. Scully. J. Phys. Chem. C, 122, 19499-19513 (2018)
The electrodeposition of metal ions onto an electrode is influenced by several phenomena, such as diffusion, ion-water interactions, and adsorption. Probing these underlying aspects is technically challenging due to the lack of techniques that are only sensitive to the electrode-electrolyte interface. Here, we have used a novel X-ray spectroscopy method to overcome this issue, where interface-sensitive X-ray absorption spectra are obtained by separation of a frequency modulated X-ray current (AC) signal from the continuous electrochemical current (DC). Using this approach, the electrode-electrolyte interface was followed during copper electrodeposition. The detection of O K-edge and Cu L-edges spectra enabled the observation of the surface structure of the electrode, as well as the near-surface Cu2+ ions concentration and the interfacial water structure, providing a very complete picture of the deposition process. We find that the Cu2+ ions are reduced via an atom transfer mechanism, where a Cu2O or CuOH intermediate is formed rather than the simple Cu+. This result highlights the complexity of interfacial electrochemistry, and the need to resolve it in molecular-level detail.
Layered Ni-rich transition metal oxide materials have been considered as the most promising cathode utilized in Li-ion batteries, e.g., LiNi0.8 Mn0.1 Co0.1 O2 (NMC 811). However, one of the drawbacks of NMC 811 is its high air sensitivity, leading to a degradation layer forming on the surface, and a lower cycling performance. Since the degradation mechanism is not fully understood, in this work, we use ambient pressure photoelectron spectroscopy (APPES)  to investigate the surface sensitivity of NMC 811 towards CO2 and H2O in situ, aiming to determine the factor triggering the degradation. Before gas exposure, NMC 811 surface was studied in UHV. The changes in surface chemical composition were monitored as a function of time and gas pressure. Results show that carbonate compounds will form on the surface when NMC 811 is exposed to CO2 at around10-3 mbar and start to disappear in UHV after CO2 exposure. More interestingly, the photon beam can accelerate the formation of carbonate on NMC particles surface. The same measurements were finished with H2O exposure as well. Results indicate that lithium hydroxide is formed, where active surface oxygen can be the possible explanation. However, this reaction is reversible in UHV as well.
[ R. Jung et al., "Effect of Ambient Storage on the Degradation of Ni-Rich Positive Electrode Materials (NMC811) for Li-Ion Batteries," Journal of The Electrochemical Society, vol. 165, no. 2, pp. A132-A141, 2018, doi: 10.1149/2.0401802jes.
 E. Kokkonen et al., "Upgrade of the SPECIES beamline at the MAX IV Laboratory," Journal of Synchrotron Radiation, vol. 28, no. 2, pp. 588-601, 2021-03-01 2021, doi: 10.1107/s1600577521000564.
 M. Yoon et al., "Reactive boride infusion stabilizes Ni-rich cathodes for lithium-ion batteries," Nature Energy, vol. 6, no. 4, pp. 362-371, 2021-04-01 2021, doi: 10.1038/s41560-021-00782-0.
Pt catalyst particles on reducible oxide supports often change their activity significantly at elevated temperatures due to the strong metal-support interaction (SMSI), which induces the formation of an encapsulation layer around the noble metal particles. However, the impact of oxidizing and reducing treatments at elevated pressures on this encapsulation layer remains controversial, partly due to the ‘pressure gap’ between surface science studies and applied catalysis. In the present work, we employ synchrotron-based near-ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) to study the effect of O$_2$ and H$_2$ on the SMSI-state of well-defined oxide-supported Pt catalysts at pressures from UHV up to 0.1 mbar. On a TiO$_2$(110) support, we can either selectively oxidize the support or both the support and the Pt particles by tuning the O$_2$ pressure. We find that the growth of the encapsulating oxide overlayer is inhibited when Pt is in an oxidic state. Our experiments show that the Pt particles remain embedded in the support once encapsulation has occurred.
Cu-based catalysts are a cornerstone of technological efforts to build sustainable energy cycles. They show promising activity towards CO2 reduction, in heterogeneous as well as electrocatalytic reaction pathways. The ability of the Cu surface to adsorb CO2 in a chemisorbed state and accommodate oxygen atoms is favourable to breaking the stable C=O double bond - the first and most important step towards catalytic conversion into, e.g. methanol.
Therefore, understanding the chemical composition of the catalyst surface in situ is essential to unraveling the underlying reaction mechanism.
Crystalline Cu surfaces are frequently used as model catalysts for fundamental studies on this reaction. At pressures > 0.01 mbar, they have been found to catalyze CO2 dissociation. Atomic sites with lower coordination, such as steps (7-fold) and kinks (6-fold), may increase catalytic activity but the role of these active sites has not been studied comprehensively. Most previous studies have only considered a few, typically high symmetry, Cu surfaces.
In the work presented here, we employ a curved model catalyst with variable surface structure to investigate CO2 adsorption and dissociation at near-ambient pressures. Using XPS to characterize different reactant and product species, we study the reaction on the highly symmetric (111) surface, as well as increasingly corrugated surfaces. We follow the reaction from onset to saturation, covering a wide pressure and temperature range.
We observe how the coverage of competing surface species evolves over time, as CO2 adsorbs into both physisorbed and chemisorbed states and eventually dissociates into CO gas and atomically adsorbed O. Gradual surface oxidation prevents further CO2 adsorption and leads to eventual saturation of the surface. The step-density dependent differences are quantified by kinetic modelling.
In this talk, I will discuss how the different surface sites (terraces, steps, kinks) control the chemical composition of the surface coverage as well as the kinetics of the reaction in different ways and reveal new insights into the reaction mechanism.
Ceria catalysts present a great potential for the selective hydrogenation of alkynes to alkenes and the hydrogenation of carbon dioxide to methanol. Recent works suggest that the type of surface and subsurface hydrogen may play an important role, affecting both the activity and the selectivity in hydrogenation reactions. Interaction with hydrogen go typically through two routes: homolytic dissociation to form two hydroxyls and heterolytic route to form hydride and a proton, although other pathways are possible. The hydrides are stabilized by oxidizing the cerium atoms next to the oxygen vacancies.
Using simultaneous X-ray photoelectron spectroscopy, X-ray absorption spectroscopy, and grazing incidence resonant X-ray scattering measurements at ambient conditions, we aim to understand both the structural and chemical changes occurring during reduction, oxidation, and interaction with hydrogen. We measured anomalous/resonant X-ray scattering at Ce L5 edge to distinguish between Ce+4 or Ce+3 species revealing dramatic changes in shape of the form factor. X-ray photoelectron spectroscopy shows that the surface was most oxidized when annealed in the H2 atmosphere, suggesting formation of hydrides. Ceria can expand through the process, and this volumetric change can be observed in diffraction measurements. Correlation of all the simultaneously acquired spectral and diffraction data gains novel insights of the ceria-H2 system.
Sensing of small gas molecules in human breath (e.g. H2, NH3, CH4) is expected to be a non-invasive and simple method for health diagnosis. Recently, a sensor using a Pt thin film of about 10 nm thickness has been developed to recognize ppm-order hydrogen gas from the change in its electrical resistance . Previously, we have performed operando analysis using ambient-pressure X-ray photoelectron spectroscopy (AP-XPS) to elucidate the basic principle of the sensor response on the Pt thin film surface, and found a direct relationship between the surface chemical state and the sensor response . Then, we have focused on an alloyed Pt-Rh thin-film sensor that is insensitive to ammonia and selective for hydrogen, and performed an operando AP-XPS analysis to clarify the principle of the sensor response and selectivity [3, 4]. All AP-XPS experiments were carried out at an undulator beamline BL-13B at the Photon Factory, Japan. The experiments were performed at room temperature (298 K). Two electric contacts were attached onto the sample surface, and the input voltage was about 100 mV. The background gases (H2, O2, and NH3) were introduced to the chamber up to 0.1 Torr. Evolutions of Rh 3d and Pt 4f core-levels were tracked in various gas atmospheres by AP-XPS, and the electrical resistance was measured simultaneously. It was found that the sensor response and the surface chemical state including elemental distribution changed depending on the background gas atmosphere. Initially, the surface is mainly covered by Rh oxide. When the hydrogen gas was exposed to the surface, Rh oxide was reduced to the metallic state, resulting in a decrease in resistance. Besides, it is found that Rh oxide inhibits the dissociative adsorption of ammonia, which acts as the key to chemical selectivity.
 Tanaka et al., Sens. Actuators B Chem., 258, 913, (2018).
 Toyoshima et al., Chem. Commun., 56, 10147 (2020).
 Tanaka et al., IEEE Trans. Electron Devices, 66, 5393, (2019).
 Toyoshima et al., J. Phys. Chem. Lett., 13, 8546 (2022).
When measuring dielectric materials by XPS in ambient pressure conditions, the build-up of charge at the surface can be partially eliminated by the so-called environmental charge compensation. The charge compensation is achieved by the electrons resulting from the photoionization of the gas phase. In this work we have explored how to enhance the environmental charge compensation by irradiating the sample simultaneously with UV (HeI, 21.2 eV) photons during the XPS measurement. UV sources, already present in many laboratory-based spectrometers for the performance of UV-photoelectron spectroscopy (UPS), constitute a potentially interesting ionization source, with the ability to provide gas-phase electrons within an energy range of up to some tens of eV. The elevated high cross sections for ionization of UV photons should guarantee higher electronic currents coming from the photoionization of the gas as compared with the illumination with X-rays alone. On the other hand, the large cross section for ionization would also result in higher charge levels at the substrate, so it is not straightforward to predict which of the two effects (higher charge or higher compensating electron current from the gas) would dominate. The final equilibrium achieved at the surface between out-going and in-going currents will depend on several aspects, such as the chemistry of the surface and gas phase, the sample geometry, size of the irradiation beam and photon density, among others. We have illustrated the effect of a combined X-ray and UV irradiation by measuring three types of dielectric samples with different composition and geometries: a meso-porous 3D SiO2 monolith with an irregular surface, a flat mica sample, and thin SiO2 layers deposited onto doped Si wafers. Additionally, we have also explored the relevance of the irradiation spot size for the minimization of differential charging, by itself and in combination with UV irradiation. We will discuss the influence of these parameters in terms of how they affect both lateral and vertical inhomogeneous charging. The effect of gas and sample composition will also be discussed.