Ambient Pressure X-ray Photoelectron Spectroscopy Research Articles

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.

Enhancing catalytic performance and hot electron generation through engineering metal-oxide and oxide-oxide interfaces

Interfaces are of utmost importance in catalytic reactions, influencing reaction kinetics and electron transfer processes. However, investigations in combined interfaces of metal-oxide and oxide-oxide at heterogeneous catalysts still have challenges due to their complex structure. Herein, we synthesized well-defined Co3O4 and CeO2 cubes with distinct facets and investigated their catalytic performance when deposited on a Pt-thin film, focusing on the influence of metal-oxide and oxide-oxide interfaces. Catalytic measurements demonstrated that the CeO2/Pt interface significantly enhanced turnover frequency (TOF) and selectivity for partial methanol oxidation compared to Co3O4/Pt and bare Pt. Notably, the CeO2/Co3O4/Pt nanodevice exhibited improved partial oxidation selectivity, highlighting the role of the CeO2/Co3O4 interface in methyl formate production. Chemicurrent measurements demonstrate enhanced hot electron generation due to increased overall TOF and partial oxidation production. We also conducted near ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) analysis, revealing a higher concentration of Ce3+ ions and increased oxygen vacancies in the CeO2/Co3O4/Pt catalyst, suggesting oxygen migration from CeO2 to Co3O4, leading to methoxy species stabilization and promoting methyl formate formation.

Bifunctionality of Re Supported on TiO2 in Driving Methanol Formation in Low-Temperature CO2 Hydrogenation.

Low temperature and high pressure are thermodynamically more favorable conditions to achieve high conversion and high methanol selectivity in CO2 hydrogenation. However, low-temperature activity is generally very poor due to the sluggish kinetics, and thus, designing highly selective catalysts active below 200 °C is a great challenge in CO2-to-methanol conversion. Recently, Re/TiO2 has been reported as a promising catalyst. We show that Re/TiO2 is indeed more active in continuous and high-pressure (56 and 331 bar) operations at 125-200 °C compared to an industrial Cu/ZnO/Al2O3 catalyst, which suffers from the formation of methyl formate and its decomposition to carbon monoxide. At lower temperatures, precise understanding and control over the active surface intermediates are crucial to boosting conversion kinetics. This work aims at elucidating the nature of active sites and active species by means of in situ/operando X-ray absorption spectroscopy, Raman spectroscopy, ambient-pressure X-ray photoelectron spectroscopy (AP-XPS), and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). Transient operando DRIFTS studies uncover the activation of CO2 to form active formate intermediates leading to methanol formation and also active rhenium carbonyl intermediates leading to methane over cationic Re single atoms characterized by rhenium tricarbonyl complexes. The transient techniques enable us to differentiate the active species from the spectator one on TiO2 support, such as less reactive formate originating from spillover and methoxy from methanol adsorption. The AP-XPS supports the fact that metallic Re species act as H2 activators, leading to H-spillover and importantly to hydrogenation of the active formate intermediate present over cationic Re species. The origin of the unique reactivity of Re/TiO2 was suggested as the coexistence of cationic highly dispersed Re including single atoms, driving the formation of monodentate formate, and metallic Re clusters in the vicinity, activating the hydrogenation of the formate to methanol.

Adapting an Electron Microscopy Microheater for Correlated and Time-Resolved Ambient Pressure X-ray Photoelectron Spectroscopy

Adapting an Electron Microscopy Microheater for Correlated and Time-Resolved Ambient Pressure X-ray Photoelectron Spectroscopy

Stability and Reversible Oxidation of Sub-Nanometric Cu5 Metal Clusters: Integrated Experimental Study and Theoretical Modeling.

Sub-nanometer metal clusters have special physical and chemical properties, significantly different from those of nanoparticles. However, there is a major concern about their thermal stability and susceptibility to oxidation. In situ X-ray Absorption spectroscopy and Near Ambient Pressure X-ray Photoelectron spectroscopy results reveal that supported Cu5 clusters are resistant to irreversible oxidation at least up to 773 K, even in the presence of 0.15 mbar of oxygen. These experimental findings can be formally described by a theoretical model which combines dispersion-corrected DFT and first principles thermochemistry revealing that most of the adsorbed O2 molecules are transformed into superoxo and peroxo species by an interplay of collective charge transfer within the network of Cu atoms and large amplitude "breathing" motions. A chemical phase diagram for Cu oxidation states of the Cu5 -oxygen system is presented, clearly different from the already known bulk and nano-structured chemistry of Cu.

Operando Characterization of Copper–Zinc–Alumina Catalyst for Methanol Synthesis from Carbon Dioxide and Hydrogen by Ambient-Pressure Hard X-ray Photoelectron Spectroscopy

Operando Characterization of Copper–Zinc–Alumina Catalyst for Methanol Synthesis from Carbon Dioxide and Hydrogen by Ambient-Pressure Hard X-ray Photoelectron Spectroscopy

Surfactants Used in Colloidal Synthesis Modulate Ni Nanoparticle Surface Evolution for Selective CO2 Hydrogenation.

Colloidal chemistry holds promise to prepare uniform and size-controllable pre-catalysts; however, it remains a challenge to unveil the atomic-level transition from pre-catalysts to active catalytic surfaces under the reaction conditions to enable the mechanistic design of catalysts. Here, we report an ambient-pressure X-ray photoelectron spectroscopy study, coupled with in situ environmental transmission electron microscopy, infrared spectroscopy, and theoretical calculations, to elucidate the surface catalytic sites of colloidal Ni nanoparticles for CO2 hydrogenation. We show that Ni nanoparticles with phosphine ligands exhibit a distinct surface evolution compared with amine-capped ones, owing to the diffusion of P under oxidative (air) or reductive (CO2 + H2) gaseous environments at elevated temperatures. The resulting NiPx surface leads to a substantially improved selectivity for CO production, in contrast to the metallic Ni, which favors CH4. The further elimination of surface metallic Ni sites by designing multi-step P incorporation achieves unit selectivity of CO in high-rate CO2 hydrogenation.

Ambient-Pressure X-ray Photoelectron Spectroscopy Study of Acetic Acid Thermal Decomposition on Pd(111)

Acetic acid on Pd(111) is an excellent model system to study the effect of water on thermal decomposition of fatty acids and other oxygenates. Acetic acid is a simple molecule that has both methyl and acid groups, where its decomposition involves many of the key bond-breaking steps involved in more complex chemical decompositions. Here, we use ambient-pressure X-ray photoelectron spectroscopy and mass spectrometry supported by density functional theory (DFT) calculations of the core-level binding energies to study the effect of co-adsorbed water on the decomposition of acetic acid. The addition of water to acetic acid results in an increased acetic acid/acetate coverage and a decreased CO coverage. Moreover, when water is co-dosed with acetic acid, the gas phase composition from the reaction shows an approximately 30% increase in the CO2/CO ratio, suggesting a shift toward CO2 production in the presence of water. These observations are supported by previous DFT calculations, which show that the presence of co-adsorbed water lowers the barriers for O–H bond breaking but increases the barriers for C–O bond breakage, leading to increased CO2 production.

Revealing CO2 dissociation pathways at vicinal copper (997) interfaces

Size- and shape-tailored copper (Cu) nanocrystals can offer vicinal planes for facile carbon dioxide (CO2) activation. Despite extensive reactivity benchmarks, a correlation between CO2 conversion and morphology structure has not yet been established at vicinal Cu interfaces. Herein, ambient pressure scanning tunneling microscopy reveals step-broken Cu nanocluster evolutions on the Cu(997) surface under 1 mbar CO2(g). The CO2 dissociation reaction produces carbon monoxide (CO) adsorbate and atomic oxygen (O) at Cu step-edges, inducing complicated restructuring of the Cu atoms to compensate for increased surface chemical potential energy at ambient pressure. The CO molecules bound at under-coordinated Cu atoms contribute to the reversible Cu clustering with the pressure gap effect, whereas the dissociated oxygen leads to irreversible Cu faceting geometries. Synchrotron-based ambient pressure X-ray photoelectron spectroscopy identifies the chemical binding energy changes in CO-Cu complexes, which proves the characterized real-space evidence for the step-broken Cu nanoclusters under CO(g) environments. Our in situ surface observations provide a more realistic insight into Cu nanocatalyst designs for efficient CO2 conversion to renewable energy sources during C1 chemical reactions.

A laboratory-based multifunctional near ambient pressure X-ray photoelectron spectroscopy system for electrochemical, catalytic, and cryogenic studies.

A versatile multifunctional laboratory-based near ambient pressure x-ray photoelectron spectroscopy (XPS) instrument is presented. The entire device is highly customized regarding geometry, exchangeable manipulators and sample stages for liquid- and solid-state electrochemistry, cryochemistry, and heterogeneous catalysis. It therefore delivers novel and unique access to a variety of experimental approaches toward a broad choice of functional materials and their specific surface processes. The high-temperature (electro)catalysis manipulator is designed for probing solid state/gas phase interactions for heterogeneous catalysts including solid electrolyzer/fuel cell electrocatalysts at pressures up to 15mbar and temperatures from room temperature to 1000 °C. The liquid electrochemistry manipulator is specifically designed for in situ spectroscopic investigations of polarized solid/liquid interfaces using aqueous electrolytes and the third one for experiments for ice and ice-like materials at cryogenic temperatures to approximately -190 °C. The flexible and modular combination of these setups provides the opportunity to address a broad spectrum of in situ and operando XPS experiments on a laboratory-based system, circumventing the limited accessibility of experiments at synchrotron facilities.

Time evolution of surface species during the ALD of high-k oxide on InAs

Understanding the reaction mechanisms involved during the early stage of atomic layer deposition (ALD) of HfO2 on InAs is a key requirement for improving interfaces in III-V semiconductor-based devices. InAs is an excellent candidate to outperform silicon regarding speed and power consumption, and combined with HfO2, it gives promise for a new generation of ultra-fast MOSFETs. However, an improved interface quality and in-depth understanding of the involved surface species are needed. Here, we use in situ and operando ambient pressure XPS to follow in real-time the reaction mechanisms which control the ALD chemistry. Besides the removal of all unwanted oxide from the III-V, the same oxygen atoms are found to form HfOx already from the first half-cycle. In contrast to the standard ALD model, no hydroxyl groups are needed on the InAs surface. Furthermore, we observe an insertion reaction forming unexpected surface species. The second ALD half-cycle allows the immediate removal of all organic species leaving behind a uniform HfO2 layer partially terminated by hydroxyl groups. We find that prolonged exposure times upon both half-cycles guarantee a sharp InAs/HfO2 interface. Such an improved interface is an important step towards fast and sustainable III-V semiconductor-based electronics.

Redox Properties of TiO2 Thin Films Grown on Mesoporous Silica by Atomic Layer Deposition.

The redox properties of titania films grown by ALD on SBA-15, a silica-based mesoporous material, were characterized as a function of thickness (that is, the number of ALD cycles used). 29Si CP/MAS NMR helped to identify the nature of the surface species that form in the initial stages of deposition, and infrared absorption spectroscopy was used to follow the transition from silica to titania surfaces. The reducibility of the titania sites by CO and H2 was studied ex situ using EPR and in situ with ambient-pressure XPS. It was determined that the titania ALD films are amorphous and easier to reduce than crystalline titania and that the reduction is reversible. A transition in the nature of the surface was also observed, with unique mixed Si-O-Ti sites forming during the first few ALD cycles and a more typical titania surface progressively developing as the film grows in thickness.

CO2 Activation on Ni(111) and Ni(110) Surfaces in the Presence of Hydrogen

The structure sensitivity of CO2 activation in the presence of H2 has been identified by ambient-pressure X-ray photoelectron spectroscopy (APXPS) on Ni(111) and Ni(110) surfaces under identical reaction conditions. Based on the APXPS results and computer simulations, we propose that, around room temperature, the hydrogen-assisted activation of CO2 is the major reaction path on Ni(111), while the redox pathway of CO2 prevails on Ni(110). With increasing temperature, the two activation pathways are activated in parallel. While the Ni(111) surface is fully reduced to the metallic state at elevated temperatures, two stable Ni oxide species can be observed on Ni(110). Turnover frequency measurements indicate that the low-coordinated sites on Ni(110) promote the activity and selectivity of CO2 hydrogenation to methane. Our findings provide insights into the role of low-coordinated Ni sites in nanoparticle catalysts for CO2 methanation.

Dewetting Transition of CoO/Pt(111) in CO Oxidation Conditions Observed In Situ by Ambient Pressure STM and XPS

Dewetting Transition of CoO/Pt(111) in CO Oxidation Conditions Observed In Situ by Ambient Pressure STM and XPS

Photocatalytic setup for in situ and operando ambient-pressure X-ray photoelectron spectroscopy at MAX IV Laboratory.

The Ambient-Pressure X-ray Photoelectron Spectroscopy (APXPS) endstation at the SPECIES beamline at MAX IV Laboratory has been improved. The latest upgrades help in performing photo-assisted experiments under operando conditions in the mbar pressure range using gas and vapour mixtures whilst also reducing beam damage to the sample caused by X-ray irradiation. This article reports on endstation upgrades for APXPS and examples of scientific cases of insitu photocatalysis, photoreduction and photo-assisted atomic layer deposition (photo-ALD).

A spectroscopic study of the mechanism of Au(III) (hydro-)oxides in promoting plasmon-mediated photoelectrochemical water-oxidation.

To understand the roles of Au(III) (hydro-)oxides in promoting plasmon-mediated photoelectrochemical (PMPEC) water-oxidation, we employed in situ microphotoelectrochemical surface-enhanced Raman spectroscopy and ambient-pressure x-ray photoelectron spectroscopy to elucidate the correlations between the amount of surface Au(III) (hydro-)oxides and the photocurrent of PMPEC water-oxidation on Au. By applying preoxidation potentials, we made surface Au(III) (hydro-)oxides on a plasmonic Au photoanode. According to the charge of reductively stripping surface oxygenated species before and after PMPEC water-oxidation, we found that a negative shift of an onset potential, increase in photocurrent, and much less growth of surface (hydro-)oxides were correlated with each other as a result of the increase in the coverage of Au (III) (hydro-)oxides. These results suggest that the surface Au(III) (hydro-)oxides kinetically promoted water-oxidation by restricting the growth of surface (hydro-)oxides.

Activation and Conversion of Methane to Syngas over ZrO2/Cu(111) Catalysts near Room Temperature.

Enzymatic systems achieve the catalytic conversion of methane at room temperature under mild conditions. In this study, varying thermodynamic and kinetic parameters, we show that the reforming of methane by water (MWR, CH4 + H2O → CO + 3H2) and the water-gas shift reaction (WGS, CO + H2O → H2 + CO2), two essential processes to integrate fossil fuels toward a H2 energy loop, can be achieved on ZrO2/Cu(111) catalysts near room temperature. Measurements of ambient-pressure X-ray photoelectron spectroscopy and mass spectrometry, combined with density functional calculations and kinetic Monte Carlo simulations, were used to study the behavior of the inverse oxide/metal catalysts. The superior performance is associated with a unique zirconia-copper interface, where multifunctional sites involving zirconium, oxygen, and copper work coordinatively to dissociate methane and water at 300 K and move forward the MWR and WGS processes.

UV-enhanced environmental charge compensation in near ambient pressure XPS

In this work, we discuss the possibility of improving charge neutralization in near ambient pressure X-ray photoelectron spectroscopy by co-irradiating the sample with He I photons of 21.2 eV. This UV-enhanced neutralization of charges is a variation of the so-called environmental charge compensation, which uses the electrons produced by the photoionization of the ambient gas to neutralize the positive charges built at the sample surface. Adding an additional ionization source generates more charges at the sample but also larger amounts of electrons available for neutralization. The final surface charge equilibrium depends on different aspects of the experiment, such as the sample composition and geometry, the total ionization cross sections of the gas compared to the surface materials, the gas used, the luminosity and spot size of the sources used for photoionization, and the energy of the electrons present in the gas phase. Here we illustrate the efficiency of the UV-enhanced neutralization using three different dielectric samples with different geometries (a porous SiO2 monolith with an irregular surface, a flat mica sample, and a thin SiO2 film deposited onto a Si substrate), different X-ray spot sizes, and two different gases (N2 and Ar). The effect of biasing on the efficiency of the sample surface to attract electrons produced in the gas phase is also discussed.

Revealing the Role of CO during CO2 Hydrogenation on Cu Surfaces with In Situ Soft X-Ray Spectroscopy.

The reactions of H2, CO2, and CO gas mixtures on the surface of Cu at 200 °C, relevant for industrial methanol synthesis, are investigated using a combination of ambient pressure X-ray photoelectron spectroscopy (AP-XPS) and atmospheric-pressure near edge X-ray absorption fine structure (AtmP-NEXAFS) spectroscopy bridging pressures from 0.1 mbar to 1 bar. We find that the order of gas dosing can critically affect the catalyst chemical state, with the Cu catalyst maintained in a metallic state when H2 is introduced prior to the addition of CO2. Only on increasing the CO2 partial pressure is CuO formation observed that coexists with metallic Cu. When only CO2 is present, the surface oxidizes to Cu2O and CuO, and the subsequent addition of H2 partially reduces the surface to Cu2O without recovering metallic Cu, consistent with a high kinetic barrier to H2 dissociation on Cu2O. The addition of CO to the gas mixture is found to play a key role in removing adsorbed oxygen that otherwise passivates the Cu surface, making metallic Cu surface sites available for CO2 activation and subsequent conversion to CH3OH. These findings are corroborated by mass spectrometry measurements, which show increased H2O formation when H2 is dosed before rather than after CO2. The importance of maintaining metallic Cu sites during the methanol synthesis reaction is thereby highlighted, with the inclusion of CO in the gas feed helping to achieve this even in the absence of ZnO as the catalyst support.