Ambient Pressure X-ray Photoelectron Spectroscopy Research Articles

Removal and Reoccurrence of LLZTO Surface Contaminants under Glovebox Conditions.

The reactivity of Li6.4La3Zr1.4Ta0.6O12 (LLZTO) solid electrolytes to form lithio-phobic species such as Li2CO3 on their surface when exposed to trace amounts of H2O and CO2 limits the progress of LLZTO-based solid-state batteries. Various treatments, such as annealing LLZTO within a glovebox or acid etching, aim at removing the surface contaminants, but a comprehensive understanding of the evolving LLZTO surface chemistry during and after these treatments is lacking. Here, glovebox-like H2O and CO2 conditions were recreated in a near ambient pressure X-ray photoelectron spectroscopy chamber to analyze the LLZTO surface under realistic conditions. We find that annealing LLZTO at 600 °C in this atmosphere effectively removes the surface contaminants, but a significant level of contamination reappears upon cooling down. In contrast, HCl(aq) acid etching demonstrates superior Li2CO3 removal and stable surface chemistry post treatment. To avoid air exposure during the acid treatment, an anhydrous HCl solution in diethyl ether was used directly within the glovebox. This novel acid etching strategy delivers the lowest lithium/LLZTO interfacial resistance and the highest critical current density.

Compression of a Stearic Acid Surfactant Layer on Water Investigated by Ambient Pressure X-ray Photoelectron Spectroscopy.

We present a combined Langmuir-Pockels trough and ambient pressure X-ray photoelectron spectroscopy (APXPS) study of the compression of stearic acid surfactant layers on neat water. Changes in the packing density of the molecules are directly determined from C 1s and O 1s APXPS data. The experimental data are fit with a 2D model for the stearic acid coverage. Based on the results of these proof-of-principle experiments, we discuss the remaining challenges that need to be overcome for future investigations of the role of surfactants in heterogeneous chemical reactions at liquid-vapor interfaces in combined Langmuir-Pockels trough and APXPS measurements.

Temperature-Dependent Selectivity and Detection of Hidden Carbon Deposition in Methane Oxidation.

Reaction products in heterogeneous catalysis can be detected either on the catalyst surface or in the gas phase after desorption. However, if atoms are dissolved in the catalyst bulk, then reaction channels can become hidden. This is the case if the dissolution rate of the deposits is faster than their formation rate. This might lead to the underestimation or even overlooking of reaction channels such as, e.g., carbon deposition during hydrocarbon oxidation reactions, which is problematic as carbon can have a significant influence on the catalytic activity. Here, we demonstrate how such hidden deposition channels can be uncovered by carefully measuring the product formation rates in the local gas phase just above the catalyst surface with time-resolved ambient pressure X-ray photoelectron spectroscopy. As a case study, we investigate methane oxidation on a polycrystalline Pd catalyst in an oxygen-lean environment at a few millibar pressure. By ramping the temperature between 350 and 525 °C, we follow the time evolution of the different reaction pathways. Only in the oxygen mass-transfer limit do we observe CO production, while our data suggests that carbon deposition also happens outside this limit.

Large Language Model-Informed X-ray Photoelectron Spectroscopy Data Analysis

X-ray photoelectron spectroscopy (XPS) remains a fundamental technique in materials science, offering invaluable insights into the chemical states and electronic structure of a material. However, the interpretation of XPS spectra can be complex, requiring deep expertise and often sophisticated curve-fitting methods. In this study, we present a novel approach to the analysis of XPS data, integrating the utilization of large language models (LLMs), specifically OpenAI’s GPT-3.5/4 Turbo to provide insightful guidance during the data analysis process. Working in the framework of the CIRCE-NAPP beamline at the CELLS ALBA Synchrotron facility where data are obtained using ambient pressure X-ray photoelectron spectroscopy (APXPS), we implement robust curve-fitting techniques on APXPS spectra, highlighting complex cases including overlapping peaks, diverse chemical states, and noise presence. Post curve fitting, we engage the LLM to facilitate the interpretation of the fitted parameters, leaning on its extensive training data to simulate an interaction corresponding to expert consultation. The manuscript presents also a real use case utilizing GPT-4 and Meta’s LLaMA-2 and describes the integration of the functionality into the TANGO control system. Our methodology not only offers a fresh perspective on XPS data analysis, but also introduces a new dimension of artificial intelligence (AI) integration into scientific research. It showcases the power of LLMs in enhancing the interpretative process, particularly in scenarios wherein expert knowledge may not be immediately available. Despite the inherent limitations of LLMs, their potential in the realm of materials science research is promising, opening doors to a future wherein AI assists in the transformation of raw data into meaningful scientific knowledge.

Investigating methanol decomposition at low temperatures on polycrystalline Pt using near ambient pressure XPS (NAP-XPS)

In this work, methanol was frozen on the surface of polycrystalline Pt and the decomposition of methanol was monitored from its frozen state to 195 K. This works shows that methanol spontaneously decomposes to CO on Pt even at 160 K. CO was observed to adsorb onto the top and bridge site of Pt based on the C 1s and O 1s binding energy assignment from the NAP-XPS measurement. At low temperatures, CO is preferred to bind onto the top site with a bridge to top coverage ratio of 0.4, while CO is found to be predominantly on the bridge site at room temperature, with a CO bridge to top ratio of 4. The difference in the energy of the CO binding sites were found to be 6.4 (±1.0) kJ/mol, which is in good agreement with literature.

Interfacial Nanostructure and Hydrogen Bond Networks of Choline Chloride and Glycerol Mixtures Probed with X-ray and Vibrational Spectroscopies.

The molecular distribution at the liquid-vapor interface and evolution of the hydrogen bond interactions in mixtures of glycerol and choline chloride are investigated using X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy. Nanoscale depth profiles of supersaturated deep eutectic solvent (DES) mixtures up to ∼2 nm measured by ambient-pressure XPS show the enhancement of choline cation (Ch+) concentration by a factor of 2 at the liquid-vapor interface compared to the bulk. In addition, Raman spectral analysis of a wide range of DES mixtures reveals the conversion of gauche-conformer Ch+ into the anti-conformer in relatively lower ChCl concentrations. Finally, the depletion of Ch+ from the interface (probing depth = 0.4 nm) is demonstrated by aerosol-based velocity map imaging XPS measurements of glyceline and water mixtures. The nanostructure of liquid-vapor interfaces and structural rearrangement by hydration can provide critical insight into the molecular origin of the deep eutectic behavior and gas-capturing application of DESs.

Operando study of HfO2 atomic layer deposition on partially hydroxylated Si(111)

The introduction of atomic layer deposition (ALD), to the microelectronics industry has introduced a large number of new possible materials able to be deposited in layers with atomic thickness control. One such material is the high-κ oxide HfO2; thermally stable and ultrathin HfO2 films deposited by ALD are a significant contender to replace SiO2 as the gate oxide in capacitor applications. We present a mechanistic study of the first deposition cycle of HfO2 on the Si(111) surface using tetrakis(dimethylamido) hafnium (TDMAHf) and water as precursors using operando ambient pressure x-ray photoelectron spectroscopy. Here, we show that the hydroxylation of the clean Si(111) surface by residual water vapor, resulting in a 0.3 monolayer coverage of hydroxyls, leads to instantaneous full surface coverage of TDMAHf. The change in the atomic ratio of Hf to C/N found during the first deposition half-cycle, however, does not match the assumed immediate ligand loss through reaction with surface hydroxyls. One would expect an immediate loss of ligands, indicated by a Hf:N ratio of approximately 1:3 as TDMAHf deposits onto the surface; however, a Hf:N ratio of 1:3.6 is observed. The partial hydroxylation on the Si(111) surface leads to binding through the TDMAHf ligand N atoms resulting in both N and CH3 being found remaining on the surface post water half-cycle. Although there is evidence of ligand exchange reactions occurring at Si–OH sites, it also seems that N binding can occur on bare Si, highlighting the complexity of the substrate/precursor reaction even when hydroxyls are present. Moreover, the initial low coverage of Si–OH/Si–H appears to severely limit the amount of Hf deposited, which we hypothesize is due to the specific geometry of the initial arrangement of Si–OH/Si–H on the rest- and adatoms.

Metastable nickel–oxygen species modulate rate oscillations during dry reforming of methane

AbstractWhen a heterogeneous catalyst is active, it forms metastable structures that constantly transform into each other. These structures contribute differently to the catalytic function. Here we show the role of different metastable oxygen species on a Ni catalyst during dry reforming of methane by combining environmental scanning electron microscopy, near ambient pressure X-ray photoelectron spectroscopy, on-line product detection and computer vision. We highlight the critical role of dissociative CO2 adsorption in regulating the oxygen content of the catalyst and in CH4 activation. We also discover rate oscillations during dry reforming of methane resulting from the sequential transformation of metastable oxygen species that exhibit different catalytic properties: atomic surface oxygen, subsurface oxygen and bulk NiOx. The imaging approach allowed the localization of fluctuating surface regions that correlated directly with catalytic activity. The study highlights the importance of metastability and operando analytics in catalysis science and provides impetus towards the design of catalytic systems.

Importance of standard terminology in surface chemical analysis: ISO 18115‐1:2023, general terms and terms used in spectroscopy

The International Standard ISO 18115‐1 on terminology in surface chemical analysis has been revised with clarifications, modifications, and deletions to more than 70 terms and with the addition of more than 50 terms in response to trends, issues, and needs identified by the surface analysis community. This revision adds terminology and concepts associated with emerging methods such as atom probe tomography, near ambient pressure XPS, and hard X‐ray photoelectron spectroscopy. It includes 25 new and revised terms to ensure that the description of resolution is consistent across all surface analysis methods. The 630 terms in the document cover words or phrases used in describing the samples, instruments, and concepts involved in surface chemical analysis. The terms have been collated into subject specific sections to ensure that related terms can be found easily.

Solar light driven atomic and electronic transformations in a plasmonic Ni@NiO/NiCO3 photocatalyst revealed by ambient pressure X-ray photoelectron spectroscopy

This work employs ambient pressure X-ray photoelectron spectroscopy (APXPS) to delve into the atomic and electronic transformations of a core–shell Ni@NiO/NiCO3 photocatalyst – a model system for visible light active plasmonic photocatalysts used in water splitting for hydrogen production.

Mechanism of selective catalytic reduction on VOx/TiO2 catalyst by in situ ambient pressure X-ray photoelectron spectroscopy

Unveiling the relationship between NH3 adsorption/activation behaviors and SCR reaction mechanism is imperative for rational catalyst design, but is is still not established because of lacking methodology of in situ spectroscopies. Here, vanadia-based SCR catalysts were used to identify the electron transfer and dynamic evolutions of active sites in SCR reaction by in situ AP-XPS, Raman, and DRIFTS spectra. The activation of NH3 occurred at V5+O sites, and then they reduced to V4+OH species by proton-coupled electron transfer process at high temperature (350 °C). Reducibility of vanadium dominated the catalytic reaction and both lattice and surface oxygen can oxidize the V4+OH to fulfill redox cycles. Whereas, at low temperature (150 °C), the adsorptions of NH3 on Brønsted acid sites was the rate-determine step of NH3 activations. NO was stored as nitrate/nitrite with the help of surface chemisorbed oxygen. The NHx species reacted with adsorbed nitrite/nitrate to generate the NHxNOx intermediate that subsequently decomposed into N2 and H2O.

Operando Ambient Pressure X Ray Photoelectron Spectroscopy for Li-Ion Battery Electrode/Electrolyte Interface Probing

Electrochemical reactions occur at the electrode/electrolyte interface. In lithium-ion batteries, the reversible Li ion insertion/desertion from the interfaces of electrodes gives the capacity of the battery. The surface phase transition and component chemical states change during the lithiation/delithiation in turn hugely effect the further battery capacity and reversibility. In the past decades, many ex-situ characterizations gave some insights of electrode surface behavior after cycling. However, the surface chemistry of electrode relies decisively on the real battery operation condition, e.g. the passive layer formed on the electrode surface changes dramatically with the absence of electrolyte. Therefore it is emerging and necessary to develop operando study for electrode/electrolyte interface probing during battery cycling.X-ray photoelectron spectroscopy (XPS) as a surface sensitive technique detects the photoelectrons ejected from core level with certain pass energy. The binding energy of the electrons give the information of chemical composition and chemical states of the materials. Ambient pressure XPS, developed rapidly in recent years after since it gives possibility of the participation of gas and/ liquid phase, enabling the observation of structure dynamics during chemical reactions, thereby unprecedentedly extended the application of the technique. A method called dip-and-pull is used for operando liquid/solid interface probing. With a typical three-electrode system, the electrode is pulled up from the electrolyte with still contact to liquid to maintain the circuit. A thin film of electrolyte formed with a shape of meniscus at the interface of electrode, electrolyte and atmosphere is probed through. The probing of the electrochemical active surface through the liquid can be achieved with an optimized liquid layer and surface condition.In this study Lithium cobalt oxide (LCO), as the most widely used positive electrode for Li-ion battery with rock salt layered structure optimized for reversible Li intercalation/deintercalation, is used for interface dynamics investigation. As Li is deintercalated from LCO structure, the charge transfer from Li+ to Co-O environment causes a phase change. It was commonly assumed that the charge due to the deintercalation of Li from structure is fully transferred to Co3+, i.e. the loss of one electron contributes to one Co4+ from Co3+ oxidation. However, it is recently reported that the real condition maybe more complicated. The charge transfer from Li+ to Oa- (a<2) might take place especially during high voltage due to the hybrid fermi level of O and Co which might increase the mobility of Oa- and further results in cobalt reduction and oxygen evolution reaction. But the mechanism of structure evolution under corresponding potentials are very unclear. In this study, with the operando probing with APXPS, it is discovered that a native phase of Co2+ may play an unignorable role in the charge transfer, giving some new understandings of phase transition during lithiation/delithiation (see figure 1). With the development of APXPS for battery probing, combining with software data treatment, a clearer picture of phase evolution is possible to be given by time-resolved probing with APXPS. Potentially hidden intermediate states during electrochemical reactions which cannot be given from ex-situ probing are resolved. Figure 1

Multimodal in Situ synchrotron Analysis Reveals the Effect of the Oxygen Evolution Reaction on Corrosion of a Ni-Cr-Mo Alloy

Nanometer-thin and spontaneously formed oxide films, or passive films, govern the corrosion resistance of many advanced alloys [1]. Our current understanding of the structure and chemistry of the passive film is primarily based on ex situ surface analysis using X-ray Photoelectron Spectroscopy (XPS) [2] and Scanning Tunneling Microscopy (STM) [3]. XPS has historically been limited to Ultra High Vacuum (UHV) conditions, so the results do not represent the true passive film/electrolyte interface. To observe corrosion initiation and progression and obtain a fundamental understanding of corrosion mechanisms, there is a need for techniques that can combine in situ capabilities with detailed chemical and structural information from the surface region. We have previously used Ambient Pressure (AP) XPS to study passive film growth [4] [5] and a combination of X-ray Reflectivity (XRR), Grazing Incidence X-ray Diffraction (GI-XRD), X-ray Fluorescence (XRF) to study the passivity breakdown of duplex stainless steel [6].Ni-based alloys are known for their excellent mechanical properties and corrosion resistance and are used in many demanding industrial environments. Ni is commonly alloyed with Cr and Mo, where Cr is known to improve the corrosion resistance due to the formation of a Cr2O3 oxide film on the surface, while the role of Mo is debated. Commonly used electrochemical techniques for studying passivity breakdown of other alloys may not be applicable to Ni-Cr-Mo alloys because the measured electrochemical current is not only due to corrosion reactions [7]. Ni and Mo are good catalysts for the Oxygen Evolution Reaction (OER) [8, 9], which adds another dimension of complexity to the material system since OER is known to be coupled with dissolution and degradation in other metallic systems [10]. A fundamental holistic understanding is missing regarding how the chemistry and structure of the passive film on Ni-Cr-Mo alloys evolve in realistic aqueous conditions and how that correlates with the onset of dissolution, which determines the breakdown of passivity.Here we present a comprehensive study combining several synchrotron-based techniques to study the surface region of a Ni-Cr-Mo alloy in NaCl solutions in situ during electrochemical polarization, as shown in Figure 1 a). XRR and AP-XPS were used to investigate the thickness and chemistry of the passive film. GI-XRD was used to determine the change in the metal lattice underneath the passive film. XRF was used to quantify the dissolution of alloying elements. X-ray Absorption Near Edge Structure (XANES) was used to study the chemical state of the dissolved species in the electrolyte and the chemical state of corrosion products formed on the surface. The combination of these techniques allowed us to study the corrosion process and detect the passivity breakdown in situ and correlate it to the onset of OER.Growth of the passive film and enrichment of Mo6+ oxide was observed in the passive range below 800 mV vs. Ag/AgCl, followed by a drastic increase in the electrochemical current coupled with the formation of a thick film of MoO3, and Cr(OH)3, as seen from the AP-XPS data in Figure 1 b). The current increase at potentials above 800 mV vs. Ag/AgCl coincided with the dissolution of Ni2+, Cr3+, and Mo6+, as seen in Figure 1 c). Quantitative analysis revealed that a substantial part of the measured current was due to oxygen evolution, as shown in Figure 1 d). The experimental techniques and the unique information they provide will be discussed, as well as the role of OER on the passivity breakdown of the Ni-Cr-Mo alloy. D. D. Macdonald, Pure and Applied Chemistry, 71 (6), 951-978 (1999). H. Strehblow and P. Marcus, CORROSION TECHNOLOGY-NEW YORK AND BASEL-, 22 1 (2006). V. Maurice and P. Marcus, Progress in Materials Science, 95 132-171 (2018). A. Larsson, K. Simonov, J. Eidhagen, A. Grespi, X. Yue, H. Tang, A. Delblanc, M. Scardamaglia, A. Shavorskiy, J. Pan, and E. Lundgren, Appl Surf Sci, 611 155714 (2023). M. Långberg, C. Örnek, J. Evertsson, G. S. Harlow, W. Linpé, L. Rullik, F. Carlà, R. Felici, E. Bettini, U. Kivisäkk, E. Lundgren, and J. Pan, npj Materials Degradation, 3 (1), 22 (2019). E. Bettini, C. Leygraf, and J. Pan, Int J Electrochem Sc, 8 11791-11804 (2013). H. Liao, X. Zhang, S. Niu, P. Tan, K. Chen, Y. Liu, G. Wang, M. Liu, and J. Pan, Applied Catalysis B: Environmental, 307 121150 (2022). A. Lončar, D. Escalera-López, S. Cherevko, and N. Hodnik, Angewandte Chemie International Edition, 61 (14), e202114437 (2022). Figure 1

Palladium single atoms from melting nanoparticles on WO3−x for boosted hydrogen evolution reaction

Surface-supported isolated atoms in single-atom catalysts (SACs) grant maximum utilization of metals in heterogeneous catalysis. Herein, we report a feasible pyrolysis strategy to synthesize Pd single atoms by thermally melting Pd nanoparticles on an oxygen-vacancy-rich tungsten-oxide matrix at reduction atmosphere. Near ambient pressure X-ray photoelectron spectroscopy was used to monitor the formation of zero-valence Pd single atoms and the increased metallic feature of WO3−x substrate. Accordingly, the as-obtained zero-valence Pd single-atom catalyst exhibits a markedly boosted HER activity with a low overpotential (η10 = 70 mV) at the current density of 10 mA/cm2 and a small Tafel slope (b = 68 mV/dec), nearly 150 mV and a 3.0-fold enhancement than those of Pd nanoparticles (η10 = 220 mV, b = 133 mV/dec) under the same conditions. In addition, quasi in situ XPS results suggest the hydrogen spillover effect is more likely to occur on Pd single atoms during the electrochemical process. Our work may pave an interesting route for the rational design of highly-efficient single-atom catalysts, and the elucidation of corresponding enhanced reaction mechanisms by the utilization of advanced characterization techniques.

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.