Ambient Pressure X-ray Photoelectron Spectroscopy Research Articles

Understanding of a Pt thin-film H2 sensor under working conditions using AP-XPS and XAFS

Abstract The operating principle of a Pt thin-film H2 gas sensor was investigated using a combination of surface sensitive ambient-pressure X-ray photoelectron spectroscopy and bulk sensitive X-ray absorption fine structure techniques, which provided chemical and structure information under working conditions, coupled with electric resistivity measurements. It is shown that the sensor response was in a linear relation with both coverages of H and O atoms on the Pt surface. Moreover, the bulk structure of Pt remains unchanged under H2­ exposure. These observations support that the resistivity change is associated with electron scattering in the near-surface region.

Low-Temperature Hydrogenation of CO2 to Methanol in Water on ZnO-Supported CuAu Nanoalloys.

Optimizing processes and materials for the valorization of CO2 to hydrogen carriers or platform chemicals is a key step for mitigating global warming and for the sustainable use of renewables. We report here on the hydrogenation of CO2 in water on ZnO-supported CuAu nanoalloys, based on ≤ 7 mol% Au. CuxAuy/ZnO catalysts were characterized using 197Au Mössbauer, in-situ X-ray absorption (Au LIII- and Cu K-edges), and ambient pressure X-ray photoelectron spectroscopy (APXP), together with X-ray diffraction and high-resolution electron microscopy. At 200 °C, the conversion of CO2 showed a significant increase by 34 times (from 0.1 to 3.4 %) upon increasing Cu93Au7 loading from 1 to 10 wt.%, while maintaining methanol selectivity at 100%. Limited CO selectivity (4-6%) was observed upon increasing temperature up to 240 °C but associated with a ~3-fold increase in CO2 conversion. Based on APXPS during CO2 hydrogenation in an H2O-rich mixture, Cu segregates preferentially to the surface in mainly metallic state, while slightly charged Au submerges deeper into the subsurface region. These results and detailed structural analyses are topics of present contribution.

Niedrige Temperatur Hydrierung von CO2 zu Methanol in Wasser auf ZnO‐geträgerten CuAu‐Nanolegierungen

AbstractDie Optimierung von Prozessen und Materialien für die Valorisierung von CO2 zu Wasserstoffträgern oder Plattformchemikalien ist ein wichtiger Schritt zur Eindämmung der globalen Erwärmung und zur nachhaltigen Verwendung von erneuerbaren Energien. Wir berichten hier über die Hydrierung von CO2 in Wasser an ZnO‐geträgerten CuAu‐Nanolegierungen, basierend auf ≤7 mol % Au. Die CuxAuy/ZnO‐Katalysatoren wurden mit Hilfe von 197Au‐Mössbauer, In situ‐Röntgenabsorption (Au LIII‐ und Cu K‐Kanten) und Umgebungsdruck‐Röntgenphotoelektronen (APXPS) spektroskopische Methoden zusammen mit Röntgenbeugung und hochauflösender Elektronenmikroskopie charakterisiert. Bei 200 °C stieg der CO2‐Umsatz bei einer Erhöhung der Cu93Au7‐Beladung von 1 auf 10 Gew.‐% signifikant um das 34‐fache (von 0.1 auf 3.4 %), wobei die Methanolselektivität bei 100 % blieb. Eine begrenzte CO‐Selektivität (4–6 %) wurde bei einer Temperaturerhöhung bis auf 240 °C beobachtet, die jedoch mit einem ≈3‐fachen Anstieg der CO2‐Umwandlung einherging. Auf der Grundlage von APXPS, während der CO2‐Hydrierung in einem H2O‐reichen Gemisch segregiert Cu bevorzugt an der Oberfläche in einem hauptsächlich metallischen Zustand, während leicht negative‐geladenes Au tiefer in den unterirdischen Bereich eintaucht. Diese Ergebnisse und detaillierte Strukturanalysen sind Gegenstand des vorliegenden Beitrags.

Revealing the effect of metal-support interactions at the Ni/In2O3(111) interface on the selective CO2 hydrogenation

In2O3-supported Ni catalysts exhibit remarkable catalytic activity and selectivity in CO2 hydrogenation to methanol, but the underlying mechanisms and metal-oxide interactions during the reaction remain elusive. Herein, we investigate the Ni-In2O3 interaction by physical vapor deposition of Ni onto well-defined In2O3(111) thin films. In-situ near ambient-pressure X-ray photoelectron spectroscopy (NAP-XPS) was employed to probe the CO2 hydrogenation processes on these Ni/In2O3(111) model systems. Our results reveal that the small Ni clusters supported on the In2O3(111) surface at low Ni coverages exhibit cationic states. The chemical bonding and associated electron transfer at the Ni/In2O3(111) interface play crucial roles in the activation of H2 and CO2. Importantly, reaction intermediates (CO3*, OH, and HCOO*) are readily formed and desorbed under CO2 hydrogenation conditions. Our study highlights the significance of metal-support interactions on the selectivity of CO2 hydrogenation. These findings provide valuable insights into the rational design of advanced In2O3-based catalysts for CO2 hydrogenation.

Impact of Varying the Photoanode/Catalyst Interfacial Composition on Solar Water Oxidation: The Case of BiVO4(010)/FeOOH Photoanodes.

Photoanodes used in a water-splitting photoelectrochemical cell are almost always paired with an oxygen evolution catalyst (OEC) to efficiently utilize photon-generated holes for water oxidation because the surfaces of photoanodes are typically not catalytic for the water oxidation reaction. Suppressing electron-hole recombination at the photoanode/OEC interface is critical for the OEC to maximally utilize the holes reaching the interface for water oxidation. In order to explicitly demonstrate and investigate how the detailed features of the photoanode/OEC interface affect interfacial charge transfer and photocurrent generation for water oxidation, we prepared two BiVO4(010)/FeOOH photoanodes with different Bi:V ratios at the outermost layer of the BiVO4 interface (close to stoichiometric vs Bi-rich) while keeping all other factors in the bulk BiVO4 and FeOOH layers identical. The resulting two photoanodes show striking differences in the photocurrent onset potential and photocurrent density for water oxidation. The ambient pressure X-ray photoelectron spectroscopy results show that these two BiVO4(010)/FeOOH photoanodes show drastically different Fe2+:Fe3+ ratios in FeOOH both in the dark and under illumination with water, demonstrating the immense impact of the interfacial composition and structure on interfacial charge transfer. Using computational studies, we reveal the effect of the surface Bi:V ratio on the hydration of the BiVO4 surface and bonding with the FeOOH layer, which in turn affect the band alignments between BiVO4 and FeOOH. These results explain the atomic origin of the experimentally observed differences in electron and hole transfer and solar water oxidation performance of the two photoanodes having different interfacial compositions.

Molecular elucidation of CO2 methanation over a highly active, selective and stable LaNiO3/CeO2-derived catalyst by in situ FTIR and NAP-XPS

The CO2 methanation mechanism over the highly active (TOF=75.1 h−1), selective (>92%) and stable 10% LaNiO3/CeO2-derived catalyst is still unresolved. The surface of the catalyst is monitored under hydrogenation (H2), oxidizing (CO2) and CO2 methanation (H2 +CO2) conditions by near ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) using synchrotron radiation. Meanwhile, the main reaction intermediates are identified by in situ FTIR analysis. NAP-XPS experiments confirm that LaNiO3 perovskite reduction leads to the ex-solution of Ni0 nanoparticles and Ni2+CeO2−x and Ni2+La2O3 interfaces conformation, favouring the CO2 adsorption and the H2 dissociation/transfer. In situ FTIR experiments combined with the C1s spectra (NAP-XPS) suggest that the CO2 activation occurs on CeO2−x (oxygen vacancies and OH–) at low temperatures, in the form of bicarbonates; whereas, mono-/bidentate carbonates are formed on different strength La2O3 sites at increasing temperatures. These species are consecutively reduced to formates, as the main reaction intermediate, and methane by the H spilled from Ni0 nanoparticles near to NiOCeO2−x and NiOLa2O3 interfaces.

Atomic-Scale Visualization of Heterolytic H2 Dissociation and COx Hydrogenation on ZnO under Ambient Conditions.

Studying catalytic hydrogenation reactions on oxide surfaces at the atomic scale has been challenging because of the typical occurrence of these processes at ambient or elevated pressures, rendering them less accessible to atomic-scale techniques. Here, we report an atomic-scale study on H2 dissociation and the hydrogenation of CO and CO2 on ZnO using ambient pressure scanning tunneling microscopy, ambient pressure X-ray photoelectron spectroscopy, and density functional theory (DFT) calculations. We directly visualized the heterolytic dissociation of H2 on ZnO(101̅0) under ambient pressure and found that dissociation reaction does not require the assistance of surface defects. The presence of CO or CO2 on ZnO at 300 K does not impede the availability of surface sites for H2 dissociation; instead, CO can even enhance the stability of coadsorbed hydride species, thereby facilitating their dissociative adsorption. Our results show that hydride is the active species for hydrogenation, while hydroxyl cannot hydrogenate CO/CO2 on ZnO. Both AP studies and DFT calculations showed that the hydrogenation of CO2 on ZnO is thermodynamically and kinetically more favorable compared to that of CO hydrogenation. Our results point toward a two-step mechanism for CO hydrogenation, involving initial oxidation to CO2 at step sites on ZnO followed by reaction with hydride to form formate. These findings provide molecular insights into the hydrogenation of CO/CO2 on ZnO and deepen our understanding of syngas conversion and oxide catalysis in general.

Application of ambient pressure X-ray photoelectron spectroscopy to catalysis

Application of ambient pressure X-ray photoelectron spectroscopy to catalysis

NAP-XPS study of surface chemistry of CO and ethanol sensing with WO3 nanowires-based gas sensor

The nanowires prepared on a commercial Al2O3-based sensor platform were studied by near ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) upon detecting CO and ethanol. Our findings indicated that the ex-situ prepared WO3 NWs-based gas sensor might not be suitable for NAP-XPS studies, as the surfaces tend to accumulate significant carbon contamination, which remains on the surface even after long-term annealing in mbar range oxygen atmosphere at 593 K. To address this challenge, we developed a methodology involving exposure of the sensor to NOx, which effectively cleans the surface and enables the detection of subtle chemical changes upon exposure to various carbon-containing analytes. Using NAP-XPS analysis, we investigated the gas sensing mechanisms of WO3 NWs for CO and ethanol at different temperatures, revealing that it might combine several mechanisms commonly observed in metal-oxide chemiresistors. It is shown that the response to CO rather arises from the quick interaction of CO molecules with the ionosorbed oxygen. In the case of ethanol sensing, the main driving force for the sensor macroscopic electrical response at low temperatures is the adsorption of ethanol molecules. In contrast, at high temperatures, it combines the adsorption of ethoxy groups with the strong reducing effect of EtOH on the WO3 surface.

Hydrogen-induced Sulfur Vacancies on the MoS2 Basal Plane Studied by Ambient Pressure XPS and DFT Calculations.

Sulfur vacancy on an MoS2 basal plane plays a crucial role in device performance and catalytic activity; thus, an understanding of the electronic states of sulfur vacancies is still an important issue. We investigate the electronic states on an MoS2 basal plane by ambient-pressure X-ray photoelectron spectroscopy (AP-XPS) and density functional theory calculations while heating the system in hydrogen. The AP-XPS results show a decrease in the intensity ratio of S 2p to Mo 3d, indicating that sulfur vacancies are formed. Furthermore, low-energy components are observed in Mo 3d and S 2p spectra. To understand the changes in the electronic states induced by sulfur vacancy formation at the atomic scale, we calculate the core-level binding energies for the model vacancy surfaces. The calculated shifts for Mo 3d and S 2p with the formation of sulfur vacancy are consistent with the experimentally observed binding energy shifts. Mulliken charge analysis indicates that this is caused by an increase in the electronic density associated with the Mo and S atoms around the sulfur vacancy as compared to the pristine surface. The present investigation provides a guideline for sulfur vacancy engineering.

Dynamic Structural Evolution of CeO2 in CuO−CeO2 Catalyst Revealed by In Situ Spectroscopy

AbstractThe oxides and active metals at the interface synergistically activate reactants and thus promote the reaction, but the interface structure often changes dynamically during the reaction. In the conventional supported catalysts, the metals at the interface have been extensively studied, while the structural evolution of oxides is often overlooked due to the interference of the bulk phase signal. In this work, CeO2−CuO inverse catalysts are designed to reveal the dynamic structure evolution of CeO2 in the CeO2−CuO system during the water gas shift (WGS) reaction by in situ Raman, in situ XRD, quasi in situ XPS, and near ambient pressure XPS (NAP‐XPS). CeO2 is partially concealed in the CuO phase in the un‐pretreated catalyst and gradually exposed to the surface, forming an inverse CeOx/Cu structure during the reducing process. This structure exhibits a high catalytic activity in the WGS reaction and remains durable under the reductive conditions. When the inverse CeOx/Cu structure is exposed to the non‐redox conditions, the reconfiguration of the reduced oxide is observed which is caused by the oxygen migration of CeO2. This work explores the structure evolution of CeO2 in CeO2−CuO inverse catalyst under different conditions by in situ characterization technique and provides a reference for monitoring the dynamic changes of oxide structure.

Ambient Pressure X-ray Photoelectron Spectroscopy Study of Oxidation Phase Transitions on Cu(111) and Cu(110).

The surface structure effect on the oxidation of Cu has been investigated by performing ambient-pressure X-ray photoelectron spectroscopy (APXPS) on Cu(111) and Cu(110) surfaces under oxygen pressures ranging from 10-8 to 1 mbar and temperatures from 300 to 750 K. The APXPS results show a subsequential phase transition from chemisorbed O/Cu overlayer to Cu2 O and then to CuO on both surfaces. For a given temperature, the oxygen pressure needed to induce initial formation of Cu2 O on Cu(110) is about two orders of magnitude greater than that on Cu(111), which is in contrast with the facile formation of O/Cu overlayer on clean Cu(110). The depth profile measurements during the initial stage of Cu2 O formation indicate the distinct growth modes of Cu2 O on the two surface orientations. We attribute these prominent effects of surface structure to the disparities in the kinetic processes, such as the dissociation and surface/bulk diffusion over O/Cu overlayers. Our findings provide new insights into the kinetics-controlled process of Cu oxidation by oxygen.

Oxidation of NiCr and NiCrMo Alloys at Low Temperatures

Oxidation of Ni-Cr and Ni-Cr-Mo was studied in operando with near ambient pressure x-ray photoelectron spectroscopy in the Cabrera-Mott regime. The oxidation temperature was 200°C—a severely diffusion-limited regime. The near-surface alloy is Cr-enriched after the reduction of native oxide in vacuum, and especially so for Ni-15Cr-6Mo. Mo-cations are integrated into the oxide and Mo(VI) dominates at the surface. The surface chemistry-driven promotion of chromia by Mo predicted by theory is negated by the limited surface diffusion of reactants. Preoxidation processing is proposed to control the oxide properties for the use of Ni-Cr superalloys at low temperatures.

Stability of Iridium Single Atoms on Fe3O4(001) in the mbar Pressure Range.

Stable single metal adatoms on oxide surfaces are of great interest for future applications in the field of catalysis. We studied iridium single atoms (Ir1) supported on a Fe3O4(001) single crystal, a model system previously only studied in ultra-high vacuum, to explore their behavior upon exposure to several gases in the millibar range (up to 20 mbar) utilizing ambient-pressure X-ray photoelectron spectroscopy. The Ir1 single adatoms appear stable upon exposure to a variety of common gases at room temperature, including oxygen (O2), hydrogen (H2), nitrogen (N2), carbon monoxide (CO), argon (Ar), and water vapor. Changes in the Ir 4f binding energy suggest that Ir1 interacts not only with adsorbed and dissociated molecules but also with water/OH groups and adventitious carbon species deposited inevitably under these pressure conditions. At higher temperatures (473 K), iridium adatom encapsulation takes place in an oxidizing environment (a partial O2 pressure of 0.1 mbar). We attribute this phenomenon to magnetite growth caused by the enhanced diffusion of iron cations near the surface. These findings provide an initial understanding of the behavior of single atoms on metal oxides outside the UHV regime.

Relating Selective Electrochemical Nitrate Reduction to Electronic Structure by AP-XPS

Ammonia produced by the Haber-Bosch process fixes nitrogen by sourcing hydrogen from methane steam reforming—today’s most carbon intensive industrial process. Alternatively, ammonia can be produced electrochemically from industrial waste nitrogen sources (e.g. nitric oxides, and nitrate), sourcing protons from water and electrons as reducing agents. However, reduction of such oxidized nitrogen species involves complex reaction mechanisms where the stability of critical reaction intermediates (e.g. nitric oxide) dictates selectivity towards ammonia as a final product. Inspired by Hammer and Nørskov’s d-band theory, we utilize synchrotron-based ambient-pressure X-ray photoelectron spectroscopy (AP-XPS) to explore the adsorption affinity, and ability of a surface to dissociate, nitric oxide for a series of transition metals (Fe, Co, Ni, Cu). Nitric oxide adsorption affinity and dissociation are then linked to transition metal electronic structure and contextualized further with ex-situ electrochemical selectivity measurements and density functional theory (DFT) calculations.

Facet-Dependent Selectivity of Rutile IrO2 for Oxygen and Chlorine Evolution Reactions

Water electrolysis is a promising route for sustainable production of hydrogen as an energy storage medium and valuable precursor for industrial chemical syntheses such as ammonia and methanol. Direct electrolysis of seawater circumvents costly desalination and purification steps to reduce the price of renewable hydrogen to achieve cost parity with carbon-intensive steam reforming. However, the significant concentration of chloride salts in seawater poses a challenge to selectivity of seawater electrolysis. In aqueous chloride electrolytes, the chlorine evolution reaction (CER) is kinetically favored over the oxygen evolution reaction (OER). Rutile-type iridium dioxide (IrO2) is a state-of-the-art electrocatalyst for water electrolysis but is also a benchmark chlorine evolution electrocatalyst in the industrial chlor-alkali process. Understanding OER/CER selectivity is needed to make seawater electrolysis a viable energy conversion technology in the future.In this work, we seek to understand the relationship between crystallographic facet and competitive reaction pathway between OER and CER on epitaxial, rutile IrO2 thin films. We investigate facet-dependent OER and CER activity on a series of single-crystalline IrO2 thin films using a rotating disk electrode geometry. The OER/CER selectivity, reaction rate order, and reaction intermediate electroadsorption affinities are explored to add fundamental insight into the reactivity of rutile IrO2 surface. To gain further insight into OER intermediate adsorption affinities, we paired electrochemical measurements with surface-sensitive ambient pressure x-ray photoelectron spectroscopy (AP-XPS). Moving forward, our facet-dependent study of OER/CER selectivity of rutile IrO2 can be used to design selective seawater electrocatalysts for cost-effective and sustainable hydrogen production.

Interrogation of the Interfacial Energetics at a Tantalum Nitride/Electrolyte Heterojunction during Photoelectrochemical Water Splitting by Operando Ambient Pressure X-ray Photoelectron Spectroscopy

Interrogation of the Interfacial Energetics at a Tantalum Nitride/Electrolyte Heterojunction during Photoelectrochemical Water Splitting by <i>Operando</i> Ambient Pressure X-ray Photoelectron Spectroscopy

Oxidation and Reduction of Polycrystalline Cerium Oxide Thin Films in Hydrogen.

This study investigates the oxidation state of ceria thin films' surface and subsurface under 100 mTorr hydrogen using ambient pressure X-ray photoelectron spectroscopy. We examine the influence of the initial oxidation state and sample temperature (25-450 °C) on the interaction with hydrogen. Our findings reveal that the oxidation state during hydrogen interaction involves a complex interplay between oxidizing hydride formation, reducing thermal reduction, and reducing formation of hydroxyls followed by water desorption. In all studied conditions, the subsurface exhibits a higher degree of oxidation compared to the surface, with a more subtle difference for the reduced sample. The reduced samples are significantly hydroxylated and covered with molecular water at 25 °C. We also investigate the impact of water vapor impurities in hydrogen. We find that although 1 × 10-6 Torr water vapor oxidizes ceria, it is probably not the primary driver behind the oxidation of reduced ceria in the presence of hydrogen.

Adaptivity of depth distribution of two metals in Pd-Ag/HOPG catalyst to external conditions in the course of mild CO oxidation

Bimetallic Pd-Ag nanoparticles supported on surface of highly oriented pyrolytic graphite have been studied by mass-spectrometry and near ambient pressure (NAP) X-ray photoelectron spectroscopy (XPS) techniques in the stoichiometric reaction mixture of CO oxidation at a total pressure of 0.25 mbar in a range of temperatures 25–300 °C to establish the correlation between their catalytic properties and evolution of the surface composition therein. Under the reaction mixture at room temperature, Pd segregation on the surface took place, which became more apparent upon a temperature increase to 150 °C. The intensity of the Pd-CO adsorption state identified in Pd3d X-ray photoelectron spectra gradually decreased from its maximum at room temperature to zero at 200 °C. Simultaneously, the Pd/Ag atomic ratio on the surface also dropped down due to the progressive CO desorption. Mass-spectrometry data showed that the reaction did not proceed at a temperature below 150 °C. Indeed, a CO2 (reaction product) signal appeared in Mass-spectrometry only above 150 °C, when the Pd-CO adsorption state in XPS essentially disappeared. Moreover, the thermal stability of Pd-Ag nanoparticles under the experimental conditions applied has been analyzed using scanning tunneling microscopy, which is also discussed in the paper.

Unraveling surface structures of gallium promoted transition metal catalysts in CO2 hydrogenation

Gallium-containing alloys have recently been reported to hydrogenate CO2 to methanol at ambient pressures. However, a full understanding of the Ga-promoted catalysts is still missing due to the lack of information about the surface structures formed under reaction conditions. Here, we employed near ambient pressure scanning tunneling microscopy and x-ray photoelectron spectroscopy to monitor the evolution of well-defined Cu-Ga surfaces during CO2 hydrogenation. We show the formation of two-dimensional Ga(III) oxide islands embedded into the Cu surface in the reaction atmosphere. The islands are a few atomic layers in thickness and considerably differ from bulk Ga2O3 polymorphs. Such a complex structure, which could not be determined with conventional characterization methods on powder catalysts, should be used for elucidating the reaction mechanism on the Ga-promoted metal catalysts.