Ambient Pressure X-ray Photoelectron Spectroscopy Research Articles

In situ AP-XPS study on reduction of oxidized Rh catalysts under CO exposure and catalytic reaction conditions

Reduction of oxidized Rh catalysts under carbon monoxide (CO) exposure and reaction conditions were studied by using ambient-pressure x-ray photoelectron spectroscopy. First, Rh powders pressed into a pellet were deeply oxidized and the reduction process under 100 mTorr CO environment was monitored in situ at different temperatures. The oxidized Rh surfaces are composed of Rh2O3 and RhO2, the latter of which is more segregated near the surface. Both oxide species are reduced simultaneously to the metallic state; kinetic analyses indicate that the activation energy of the reduction of the Rh oxides is 1.68 eV, which is a little larger than those for Pd oxides, probably due to a stronger Rh–O interaction. Reduction of oxidized Rh nano-particles deposited on SiO2 under two reaction conditions (CO + O2 and CO + NO + O2) was observed with increasing temperature. It was found that the reduction temperature shifts to the higher temperature in the presence of NO, even though the S-factors are almost the same. The NO molecule more strongly prevents the reduction of oxidized Rh catalyst compared to O2.

Avoiding Sabatier’s conflict in bifunctional heterogeneous catalysts for the WGS reaction

Summary A new strategy for designing efficient bifunctional heterogeneous catalysts for the water-gas-shift (WGS) reaction (CO + H2O → CO2 + H2) is presented. We avoid conflicting tasks that require ∗OH or ∗O from dissociated water to adsorb on and then desorb from the substrate of a catalyst. Instead, the CO on metal directly obtains OH from water that is connected by weak hydrogen bonds to the substrate. The task of the substrate is to efficiently channel ∗H on its surface and release them as H2 gas. Experimental and theoretical results show that bifunctional catalysts with weakly reactive substrates have significantly higher CO conversion rates compared with highly reactive substrates that follow either the LH-associative or LH-redox process.

Another step of the APXPS evolution taken at MAX IV synchrotron.

The article by Zhu et al. [J. Synchrotron Rad. (2021), 28, 624–636] published in this journal issue presents the setup and specifications of the new soft X-ray beamline HIPPIE, dedicated to ambient-pressure X-ray photoelectron spectroscopy at the MAX IV Laboratory.

In situ ambient pressure XPS study of Pt/Cu(111) single-atom alloy in catalytically relevant reaction conditions

The presence of multiple reactant gases as well as reaction intermediates in a heterogeneous catalytic reaction results in a complex interaction between different components of the catalyst with each gas, which can alter the surface and chemical state of the catalyst differently than in the presence of an individual gas alone. In this study, we used in situ ambient pressure x-ray photoelectron spectroscopy to study the surface state of Pt/Cu(111) single-atom alloy model system in two catalytically relevant reaction conditions: CO2 hydrogenation and CO oxidation. We found that the activation of CO2 results in the formation of CO, which adsorbs on Pt sites at up to 400 K. In the presence of CO2 and H2, Pt catalyzes the reverse water–gas shift reaction, which produces more CO and further stabilizes surface Pt atoms at 450 K. On the other hand, in CO oxidation condition, the presence of O2 results in the formation of a thick Cu2O layer at higher temperatures, and Pt atoms are no longer detected in the surface and subsurface layers. When O2 is introduced to the sample before CO, the formation of a complete Cu2O layer that covers all Pt atoms occurs immediately at room temperature. However, when CO is introduced at room temperature before O2, the presence of adsorbed CO on Pt sites stabilizes the surface Pt atoms and prevents the formation of a complete Cu2O layer, thus exposing the Pt atoms in ‘holes’ in the Cu2O layer.

Oxidation of Cu2O(111) by NO2: an ambient pressure x-ray photoelectron spectroscopy study

With NO x persisting as a problematic pollutant, understanding the interactions and reactivity of NO x species at atmospherically relevant pressures with surfaces remains essential. Synchrotron-based ambient pressure x-ray photoelectron spectroscopy (APXPS) and near-edge x-ray absorption spectroscopy fine structure (NEXAFS) were used to investigate the interactions of NO2 with a Cu2O(111) surface. The NEXAFS data reveals a pressure dependence of the oxidation of the Cu2O surface to CuO by NO2. APXPS results show NO2 chemi- and physisorption on the Cu2O surface and decomposition to NO and O, or to N and O, with the amount of the species also being pressure dependent. The combination of the two spectroscopic techniques shows a comprehensive picture of NO2 adsorption with implications for catalysis, gas sensing, and gas filtration materials.

A combined laboratory and synchrotron in-situ photoemission study of the rutile TiO2 (110)/water interface

In-situ analysis of the TiO2/water interface via near ambient pressure–x-ray photoelectron spectroscopy (NAP–XPS) is demonstrated in both a lab based system (NAP-cell configuration) and synchrotron endstation (backfill configuration). Ultra-thin wetting layers of liquid water (∼10 nm) are formed on a rutile TiO2 surface with minimal contamination present in addition to unique insight during the growth of the liquid films as indicated via NAP–XPS, in-situ sample temperature and background vapour pressure monitoring. Chemical changes at the solid/liquid interface are also demonstrated via healing of Ti3+ surface defect states. Photon depth profiling of the as grown liquid layers indicate that the formed films are ultra-thin (∼10 nm) and likely to be continuous in nature. This work demonstrates a novel and flexible approach for studying the solid/liquid interface via NAP–XPS which is readily integrated with any form of NAP–XPS system, thereby making a critical interface of study available to a wide audience of researchers for use in operando electrochemical and photocatalytic research.

Detailed Characterization of MoOx-Modified Rh Metal Particles by Ambient-Pressure XPS and DFT Calculations

Characterization of Mo species on Rh metal particles in Rh–MoOx/C catalyst for C–O hydrogenolysis reactions was carried out with in situ ambient-pressure X-ray photoelectron spectroscopy (AP-XPS) a...

Operando characterisation of alumina-supported bimetallic Pd–Pt catalysts during methane oxidation in dry and wet conditions

Near ambient pressure x-ray photoelectron spectroscopy (NAP-XPS) was used to study the chemical states of a range of alumina-supported monometallic Pd and bimetallic Pd–Pt nanocatalysts, under methane oxidation conditions. It has been suggested before that for optimal complete methane oxidation, palladium needs to be in an oxidised state. These experiments, combining NAP-XPS with a broad range of characterisation techniques, demonstrate a clear link between Pt presence, Pd oxidation, and catalyst activity under stoichiometric reaction conditions. Under oxygen-rich conditions this behaviour is less clear, as all of the palladium tends to be oxidised, but there are still benefits to the addition of Pt in place of Pd for complete oxidation of methane.

A rechargeable all-solid-state sodium peroxide (Na2O2) battery with low overpotential

Na–O2 batteries have been attracting attention owing to their intrinsically high theoretical energy density. Several Na–O2 systems can produce various discharge products with different electrochemical performances. For example, sodium superoxide (NaO2) batteries have a low overpotential, and sodium peroxide (Na2O2) batteries have a high capacity. Studies of Na2O2 batteries are relatively scarce, owing to the difficulty of forming pure Na2O2 discharge products. A pure Na2O2 battery system is highly desirable for fully exploring the formation and decomposition of Na2O2 in Na–O2 batteries and evaluating their potential. This model of a Na2O2 battery should also be compatible with in situ characterization. To this end, we constructed a simple rechargeable all-solid-state Na2O2 battery. Using a nanoporous gold film as the cathode and Na–β″-Al2O3 as a solid electrolyte, we assembled a Na–O2 battery that can produce and decompose Na2O2. The all-solid-state Na–O2 battery is a simple model for conducting in situ ambient-pressure x-ray photoelectron spectroscopy (APXPS) investigations. The battery can be cycled at a low overpotential (≈450 mV). Qualitative and quantitative analyses of the APXPS and Raman results demonstrated that Na2O2 was the main discharge product and its transformation occurred during the charge and discharge periods. The operando investigation of this type of all-solid-state Na2O2 battery can help in the comprehensive exploration of the potential of Na–O2 batteries.

Effect of thermal annealing on gas separation performance and aggregation structures of block polyimide membranes

Block polymers with thermal annealing are attractive routes to produce 6FDA-based polyimide membranes with enhanced gas separation performance. However, some 6FDA-based polyimide membranes exhibit different gas separation behavior trends under the thermal annealing process. In this work, two 6FDA-DETDA/DMMDA block polyimide membranes with different 6FDA-DETDA block lengths were prepared and annealed at three temperatures (150 °C, 250 °C and 350 °C). To investigate the thermal annealing process and its influence on the thermal cross-linking behavior and aggregation structures, in situ XRD and ambient-pressure X-ray photoelectron spectroscopy (AP-XPS) were employed. Based on the orbital-weighted Fukui function calculated by Time-dependent density functional theory (TD-DFT) and AP-XPS spectra, it's found that free radicals cross-linking that induced by 350 °C annealing occurred in imide rings, and resulted in the destroyed imide rings and decreased average interchain distances in both block polyimides. However, the massive 6FDA-DETDA block hindered imide rings from destruction and resulted in a slight molecular filtration, which benefited the gas transport of small kinetic diameter gases. As a result, 350 °C annealed membrane of bPI-2 exhibited enhanced gas separation performance. We expect that this work will shed light on the effect of thermal annealing on the gas separation behavior of block polyimides and offer a way to tune gas separation performance by thermal annealing.

HIPPIE: a new platform for ambient-pressure X-ray photoelectron spectroscopy at the MAX IV Laboratory.

HIPPIE is a soft X-ray beamline on the 3 GeV electron storage ring of the MAX IV Laboratory, equipped with a novel ambient-pressure X-ray photoelectron spectroscopy (APXPS) instrument. The endstation is dedicated to performing in situ and operando X-ray photoelectron spectroscopy experiments in the presence of a controlled gaseous atmosphere at pressures up to 30 mbar [1 mbar = 100 Pa] as well as under ultra-high-vacuum conditions. The photon energy range is 250 to 2200 eV in planar polarization and with photon fluxes >1012 photons s-1 (500 mA ring current) at a resolving power of greater than 10000 and up to a maximum of 32000. The endstation currently provides two sample environments: a catalysis cell and an electrochemical/liquid cell. The former allows APXPS measurements of solid samples in the presence of a gaseous atmosphere (with a mixture of up to eight gases and a vapour of a liquid) and simultaneous analysis of the inlet/outlet gas composition by online mass spectrometry. The latter is a more versatile setup primarily designed for APXPS at the solid-liquid (dip-and-pull setup) or liquid-gas (liquid microjet) interfaces under full electrochemical control, and it can also be used as an openport for ad hoc-designed non-standard APXPS experiments with different sample environments. The catalysis cell can be further equipped with an IR reflection-absorption spectrometer, allowing for simultaneous APXPS and IR spectroscopy of the samples. The endstation is set up to easily accommodate further sample environments.

Upgrade of the SPECIES beamline at the MAX IV Laboratory.

The SPECIES beamline has been transferred to the new 1.5 GeV storage ring atthe MAX IV Laboratory. Several improvements have been made to the beamline and its endstations during the transfer. Together the Ambient Pressure X-ray Photoelectron Spectroscopy and Resonant Inelastic X-ray Scattering endstations are capable of conducting photoelectron spectroscopy in elevated pressure regimes with enhanced time-resolution and flux and X-ray scattering experiments with improved resolution and flux. Both endstations offer a unique capability for experiments at low photon energies in the vacuum ultraviolet and soft X-ray range. In this paper, the upgrades on the endstations and current performance of the beamline are reported.

Thermocatalytic CO2 Conversion over a Nickel-Loaded Ceria Nanostructured Catalyst: A NAP-XPS Study.

Despite the increasing economic incentives and environmental advantages associated to their substitution, carbon-rich fossil fuels are expected to remain as the dominant worldwide source of energy through at least the next two decades and perhaps later. Therefore, both the control and reduction of CO2 emissions have become environmental issues of major concern and big challenges for the international scientific community. Among the proposed strategies to achieve these goals, conversion of CO2 by its reduction into high added value products, such as methane or syngas, has been widely agreed to be the most attractive from the environmental and economic points of view. In the present work, thermocatalytic reduction of CO2 with H2 was studied over a nanostructured ceria-supported nickel catalyst. Ceria nanocubes were employed as support, while the nickel phase was supported by means a surfactant-free controlled chemical precipitation method. The resulting nanocatalyst was characterized in terms of its physicochemical properties, with special attention paid to both surface basicity and reducibility. The nanocatalyst was studied during CO2 reduction by means of Near Ambient Pressure X-ray Photoelectron Spectroscopy (NAP-XPS). Two different catalytic behaviors were observed depending on the reaction temperature. At low temperature, with both Ce and Ni in an oxidized state, CH4 formation was observed, whereas at high temperature above 500 °C, the reverse water gas shift reaction became dominant, with CO and H2O being the main products. NAP-XPS was revealed as a powerful tool to study the behavior of this nanostructured catalyst under reaction conditions.

Near ambient-pressure X-ray photoelectron spectroscopy study of CO2 activation and hydrogenation on indium/copper surface

Indium-based catalysts exhibit excellent performance for CO2 hydrogenation to methanol, yet their nature and chemical evolution under reaction conditions are still elusive, thus hindering an understanding of their reaction mechanism. In this work, near ambient-pressure X-ray photoelectron spectroscopy (NAP-XPS) is employed to investigate the chemical properties and the catalytic role of indium/copper model catalysts under CO2 hydrogenation conditions. We found that the deposition of In on the surface of a Cu foil led to the formation of Cu-In alloy, whereas upon CO2 exposure, In was partially oxidized to In2O3-x and Cu remains metallic. Due to the presence of In2O3-x, CO2 was activated on the surface of In/Cu samples mainly in the form of carbonate. In addition, compared with the pure In foil reference, both the fraction of oxygen vacancies and the coverage density of carbonate were higher on the In/Cu samples, indicating the promotion effect of Cu-In alloy in the activation of CO2. These results reveal the evolution of the active sites of indium/copper catalysts and inspire the design of advanced In-based bimetallic catalysts for CO2 hydrogenation.

Electrochemical reduction of CO2 in ionic liquid: Mechanistic study of Li–CO2 batteries via in situ ambient pressure X-ray photoelectron spectroscopy

In-depth mechanistic understanding of CO 2 reduction reaction (CRR) and CO 2 evolution reaction (CER) is the prerequisite to develop stable and efficient Li–CO 2 batteries. In this study, we investigate the redox reaction of CO 2 on the porous carbon surface in ionic liquid electrolyte via synchrotron-based in situ ambient pressure X-ray photoelectron spectroscopy (APXPS) technique. We used ionic liquid to facilitate CO 2 capture and stabilize CRR intermediate anions owing to its strong solvation for Li + . We demonstrate that pure CO 2 reduction is not electrochemically active at room temperature on porous carbon electrode and its kinetics can be remarkably improved by H 2 O, which could be responsible for the discrepancy on CRR kinetics in the literature. However, in the presence of H 2 O, the formed LiOH gradually converted to Li 2 CO 3 leading to severe electrolyte degradation in charging reaction. With the assistance of O 2 in the ionic liquid -based electrolyte, we show, for the first time, CRR yields a low-valence amorphous carbon and Li 2 O 2 /Li 2 O in the Li–CO 2 batteries. The formed amorphous carbon, Li 2 O 2 and Li 2 O exhibit higher rechargeability and faster kinetics compared with Li 2 CO 3 -oxidation. These new findings provide direct evidence of CRR mechanism and insights in resolving the discrepancy on CRR kinetics in Li–CO 2 batteries. • Synchrotron-based in-situ APXPS is developed to probe the Li–CO 2 reaction chemistry in ionic liquid electrolyte. • Pure CO 2 reduction is not electrochemically active at room temperature and its kinetics can be significantly improved by H 2 O. • CO 2 reduction in ionic liquid yields amorphous carbon and Li 2 O 2 /Li 2 O which are more reversible than Li 2 CO 3 .

In Situ Characterization of Catalysis and Electrocatalysis Using APXPS

Taking 50 years of effort, ambient pressure X-ray photoelectron spectroscopy (APXPS) has become a useful characterization tool for studies in heterogeneous catalysis and electrocatalysis. This achievement is realized not by mere improvement of instrumentation but through interactions between the desire to explore new scientific phenomena and the advancement of new techniques. Here we review a brief history of APXPS development and its applications in catalysis and electrocatalysis. With the selected recent case stories, we would like to illustrate the intertwining between tool driven and science driven breakthroughs. Looking forward from here, we also discuss new demands and current frontiers of APXPS investigations. It is a dynamic environment that we need for understanding real behavior of catalyst under varying conditions and of electrocatalysts with on-demand energy. Therefore, a precise control of reaction condition and efficient detection with improved spatial and temporal resolution are essential. Finally, designing reactors that are customized or dedicated for a given study is among future development of the field.

In Situ probing of solid/liquid interfaces of potassium–oxygen batteries via ambient pressure X-ray photoelectron spectroscopy: New reaction pathways and root cause of battery degradation

Direct probing of metal–oxygen chemistry at the solid/liquid interface is essential to unravel the reaction mechanism of metal–air batteries. Ambient pressure X-ray photoelectron spectroscopy (APXPS) is an effective tool to study metal–oxygen reactions but has been limited to solid-state environments. Here, for the first time, we demonstrate in situ probing of potassium oxygen reduction/evolution reaction (ORR/OER) via APXPS in ionic liquid-based air batteries. We reveal the formation and oxidation of K2O and K2O2 in K–O2 batteries, highlighting new reaction pathways in addition to the widely established one-electron process in K–O2 batteries. Importantly, we show that the formation and subsequent oxidation of K2O2 trigger irreversible cell chemistry, which could be responsible for capacity decay in K–O2 batteries. We further show that common impurities in air such as H2O and CO2 reduce chemical reversibility of ORR and OER. This study reveals root causes of irreversible cell chemistry in K–air batteries via direct probing of ionic liquid-based air batteries via APXPS, which can be widely applied to study the chemistry of solid/liquid interfaces of metal–air batteries.

Application of near-ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) in an in-situ analysis of the stability of the surface-supported metal-organic framework HKUST-1 in water, methanol and pyridine atmospheres

Surface-supported metal-organic frameworks HKUST-1 (Hong Kong University of Science and Technology) were used as a model system for a development of a near ambient pressure (NAP) XPS based approach to investigate interaction with atmospheres of water, methanol or pyridine at pressures ranging from 1 to 4 mbar. The films were grown on a gold substrate functionalized with a COOH-terminated self-assembled monolayer using liquid-phase epitaxy in a step-by-step fashion. Measurement protocols were developed and optimised for different gases in order to obtain spectra of similar quality in terms of signal intensity, noise and shape. Peak shapes were found to depend on the efficiency of charge compensation. Reference measurements in argon proved to be a useful strategy not only for the evaluation of the Cu(II)-fraction in pristine samples, but also to identify the contributions by the respective gas atmosphere to the C 1s and O 1s photoelectron spectra. Reduced copper was found during the exposition of HKUST-1 to water vapour and pyridine, but this effect was not observed in case of methanol. Additionally, it was established that there are no changes in relative Cu(II) percentage with increasing exposure time. This indicates that saturation was reached already at the lowest time of gas exposure. A detailed elucidation of the mechanism of Cu(II) reduction to Cu(I) in HKUST-1 mediated by water and pyridine is part of ongoing work and not in the scope of the present paper.

Identification of the surface species in electrochemical promotion: ethylene oxidation over a Pt/YSZ catalyst.

The electrochemical promotion of the C2H4 + O2 total oxidation reaction over a Pt catalyst, interfaced to yttrium stabilized zirconia (YSZ), has been studied at 0.25 mbar and T = 650 K using near ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) as an in situ method. The electrochemical promoter effect is linked to the presence of a several layers thick graphitic overlayer that forms on the Pt surface in the presence of C2H4. Our NAP-XPS investigation reveals that electrochemical pumping of the Pt/YSZ catalyst, using a positive potential, leads to the spillover of oxygen surface species from the YSZ support onto the surface of the Pt electrode. Based on the XP spectra, the spillover species on Pt is identical to oxygen chemisorbed from the gas-phase.

Extending the Polyol Reduction Process into the Second Dimension: Oxide Thin Film Reduction

The formation of extended metal thin films (<5 nm) or monolayers on oxide surfaces, for applications in (electro-)catalysis, has never been achieved due to the high interfacial energy of the metal/oxide interface that always results in a 3D growth of the deposited metal. To realize 2D growth, the outermost surface of the oxide must be reduced prior to metal deposition in the same system. Here, we demonstrate that the polyol method, typically used for metal nanoparticles synthesis, can be used for the reduction of oxide thin films. The reduction of the oxide layer upon heating in ethylene glycol was electrochemically monitored in situ by measuring the open circuit potential and confirmed by cyclic voltammetry and near ambient pressure X-ray photoelectron spectroscopy. The reduction of oxide thin films could be verified for nanoparticles of Sn, Ni and Sb-doped SnO2 in accordance with thermodynamic calculations. This method will enable the formation of metal thin films and monolayers on oxide substrates for applications in (electro-)catalysis.