Ambient Pressure X-ray Photoelectron Spectroscopy Research Articles

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.

Pt‐Promoted In2O3 Reduction and Reconstruction at the Pt/In2O3 Interface in CO2 Hydrogenation Atmosphere

AbstractAs an essential path toward the elucidation of the reaction mechanism, the capture of the catalytically active phase and its evolution has been the primary goal of mechanistic studies. Here, the physicochemical properties on In2O3 (111) and Pt/In2O3 (111) thin film model catalysts were tracked in CO2 hydrogenation atmosphere under various pressures and temperatures by ambient pressure X‐ray photoelectron spectroscopy (APXPS). The redox behaviours of surface Pt and In2O3, the formation and interconversion of reaction intermediates, and the structural dynamics at the topmost surface, as a consequence of the outward diffusion of InOx species, nucleation of surface Pt nanostructures and the formation of PtInOx species, were captured and analyzed. The reconstruction at the Pt/In2O3 (111) interface was also observed by time‐of‐flight secondary ion mass spectrometry (ToF‐SIMS) and scanning electron microscopy (SEM) analysis. In‐situ spectroscopic and structural analysis on a well‐defined metal/metal‐oxide interface offers a powerful means to probe the mechanistic details of the heterogeneous processes.

NAP-XPS Probes the Electronic Structure of the Mg–Al–Cl Layered Double Hydroxide upon Controlled Hydration

The hydration of [Mg2Al(OH)6]+[Cl–, yH2O] LDH was investigated by volumetric water adsorption, X-ray diffraction, Fourier Transformed Infrared (FTIR) sepctroscopy, and Near Ambient Pressure X-ray Photoelectron Spectroscopy (NAP-XPS). It was demonstrated that water uptake into interlayer region can be quantified by NAP-XPS. The LDH’s electronic structure is assessed for increasing relative humidity (RH) up to 0.044. The Cl 2p level appears essentially unaffected by water uptake, but as supported by DFT calculation, H-bonds between chloride and water induce Cl 3p orbitals splitting. At RH = 0.7, LDH is covered by a water film that contains Mg and Cl species but no Al. Soft X-ray radiolysis of hydrated Cl species induces the formation of oxidized Cl species.

Direct observation of hydration of a Nafion membrane using APXPS and AIMD simulation.

The hydration of perfluorinated sulfonic-acid ionomers is the most important phenomenon that determines their transport and electrical properties. To bridge the gap between the macroscopic electrical properties and the microscopic water-uptake mechanism, we investigated the hydration process of a Nafion membrane using ambient-pressure x-ray photoelectron spectroscopy (APXPS) from vacuum up to ∼90% relative humidity at room temperature. The O 1s and S 1s spectra provided a quantitative analysis of the water content (λ) and the transformation of the sulfonic acid group (-SO3H) to its deprotonated type (-SO3 -) during the water-uptake process. Taking advantage of a specially designed two-electrode cell, the conductivity of the membrane was determined by electrochemical impedance spectroscopy before APXPS measurements with the same conditions, thereby establishing the connection between the electrical properties and the microscopic mechanism. By means of ab initio molecular dynamics simulations based on density functional theory, the core-level binding energies of O- and S-containing species in the Nafion + H2O system were obtained.

Tuning CO2 Hydrogenation Selectivity through Reaction-Driven Restructuring on Cu-Ni Bimetal Catalysts.

Tuning the selectivity of CO2 hydrogenation is of significant scientific interest, especially using nickel-based catalysts. Fundamental insights into CO2 hydrogenation on Ni-based catalysts demonstrate that CO is a primary intermediate, and product selectivity is strongly dependent on the oxidation state of Ni. Therefore, modifying the electronic structure of the nickel surface is a compelling strategy for tuning product selectivity. Herein, we synthesized well dispersed Cu-Ni bimetallic nanoparticles (NPs) using a simple hydrothermal method for CO selective CO2 hydrogenation. A detailed study on the monometallic (Ni and Cu) and bimetallic (CuxNi1-x) catalysts supported on γ-Al2O3 was performed to increase CO selectivity while maintaining the high reaction rate. The Cu0.5Ni0.5/γ-Al2O3 catalyst shows a high CO2 conversion and more CO product selectivity than its monometallic counterparts. The surface electronic and geometric structure of Cu0.5Ni0.5 bimetallic NPs was studied using ambient pressure X-ray photoelectron spectroscopy (AP-XPS) and in situ diffuse reflectance infrared Fourier-transform spectroscopy under reaction conditions. The Cu core atoms migrate toward the surface, resulting in the restructuring of the Cu@Ni core-shell structure to a Cu-Ni alloy during the reaction and functioning as the active site by enhancing CO desorption. A systematic correlation is obtained between catalytic activity from a continuous fixed-bed flow reactor and the surface electronic structural details derived from AP-XPS results, establishing the structure-activity relationship. This investigation contributes to providing a strategy for controlling CO2 hydrogenation selectivity by modifying the surface structure of bimetallic NP catalysts.