X-ray Photoelectron Spectroscopy Experiments Research Articles

Quantitative alignment parameter estimation for analyzing X-ray photoelectron spectra.

The interpretation of X-ray photoelectron spectroscopy (XPS) data relies on measurement models that depend on several parameters, including the photoelectron attenuation length and X-ray photon flux. However, some of these parameters are not known, because they are not or cannot be measured. The unknown geometrical parameters can be lumped together in a multiplicative factor, the alignment parameter. This parameter characterizes the ability of the exciting light to interact with the sample. Unfortunately, the absolute value of the alignment parameter cannot be measured directly, in part because it depends on the measurement model. Instead, a proxy for the experimental alignment is often estimated, which is closely related to the alignment parameter. Here, a method for estimating the absolute value of the alignment parameter based on the raw XPS spectra (i.e. non-processed photoelectron counts), the geometry of the sample and the photoelectron attenuation length is presented. The proposed parameter estimation method enables the quantitative analysis of XPS spectra using a simplified measurement model. All computations can be executed within the open and free Julia language framework PROPHESY. To demonstrate feasibility, the alignment parameter estimation method is first tested on simulated data with known acquisition parameters. The method is then applied to experimental XPS data and a strong correlation between the estimated alignment parameter and the typically used alignment proxy is shown.

Highly efficient electrocatalysis dechlorination of Dichloromethane over Single-Atom Cu/Co3O4-β spinel nanofibers

Electrochemical selective dechlorination of dichloromethane (DCM) to chloromethane is a promising and sustainable strategy of resources. However, the current lack of high-performance electrocatalysts and complicated dechlorination mechanisms pose significant challenges. In this study, we report a series of Cu single-atom catalysts (SACs) supported on Co3O4-β spinel with a tube-in-tube nanofiber structure, denoted as Cux SACs/Co3O4-β, where x represents the theoretical Cu loading (0.26 wt%, 0.30 wt% and 0.50 wt%). These catalysts effectively dechlorinate DCM, producing high-value hydrocarbons like chloromethane and methane. Notably, Cu0.30 SACs/Co3O4-β exhibited a high chloromethane production rate (2931 μmol g−1h−1) and selectivity (76%) at −2.98 V vs (Ag/AgCl/Me4NCl) due to the synergy of cobaltosic oxide (Co3O4) spinel and Cu single atom. X-ray photoelectron spectroscopy and electron paramagnetic resonance experiments confirmed the presence of additional surface oxygen vacancies in Cu0.30 SACs/Co3O4-β, which improved the electrocatalysis performance for DCM dechlorination. Furthermore, a concerted dechlorination mechanism was suggested based on the transfer coefficient α. This research paves the way for the development of novel spinel electrocatalysts and the advancement of dechlorination reactions.

Modulate the metallic Sb state on ultrathin PdSb-based nanosheets for efficient formic acid electrooxidation

Incorporation of oxophilic metals into Pd-based nanostructures has shown great potential in small molecule electrooxidation owing to their superior anti-poisoning capability. However, engineering the electronic structure of oxophilic dopants in Pd-based catalysts remains challenging and their impact on electrooxidation reactions is rarely demonstrated. Herein, we have developed a method for synthesizing PdSb-based nanosheets, enabling the incorporation of the Sb element in a predominantly metallic state despite its high oxophilic nature. Moreover, the Pd90Sb7W3 nanosheet serves as an efficient electrocatalyst for the formic acid oxidation reaction (FAOR), and the underlying promotion mechanism is investigated. Among the as-prepared PdSb-based nanosheets, the Pd90Sb7W3 nanosheet exhibits a remarkable 69.03% metallic state of Sb, surpassing the values observed for the Pd86Sb12W2 (33.01%) and Pd83Sb14W3 (25.41%) nanosheets. X-ray photoelectron spectroscopy (XPS) and CO stripping experiments confirm that the Sb metallic state contributes the synergistic effect of their electronic and oxophilic effect, thus leading to an effective electrooxidation removal of CO and significantly enhanced FAOR electrocatalytic activity (1.47 A mg−1; 2.32 mA cm−1) compared with the oxidated state of Sb. This work highlights the importance of modulating the chemical valence state of oxophilic metals to enhance electrocatalytic performance, offering valuable insights for the design of high-performance electrocatalysts for electrooxidation of small molecules.

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.

Engineering a Heterophase Interface by Tailoring the Pt Coverage Density on an Amorphous Ru Surface for Ultrasensitive H2S Detection.

Amorphous/crystalline heterophase engineering is emerging as an attractive strategy to adjust the properties and functions of nanomaterials. Here, we reveal a heterophase interface role by precisely tailoring the crystalline Pt coverage density on an amorphous Ru surface (cPt/aRu) for ultrasensitive H2S detection. We found that when the atomic ratio of Pt/Ru increased from 10 to 50%, the loading modes of Pt changed from island coverage (1cPt/aRu) to cross-linkable coverage (3cPt/aRu) and further to dense coverage (5cPt/aRu). The differences in coverage models further regulate the chemical adsorption of H2S on Pt and the electronic transformation process on Ru, which can be proved by ex situ X-ray photoelectron spectroscopy experiments. Notably, a special cross-linkable coverage 3cPt/aRu on ZnO shows the best gas-sensitive performance, in which the operating temperature reduces from 240 to 160 °C compared with pristine ZnO and the selectivity coefficient for H2S gas improves from ∼1.2 to ∼4.6. This is mainly benefit from the maximized exposure of the amorphous/crystalline heterophase interface. Our work thus provides a new platform for future applications of amorphous/crystalline heterogeneous nanostructures in gas sensors and catalysis.

Investigation on the mechanism of Nb and Si co-doping on low SO3 generation from V-based catalyst during NH3-SCR process

Generally, SO3 tends to be formed in the process of selective catalytic reduction (SCR), and SO3 has negative effects on atmospheric environmental production and the safe operation of coal-fired power plants. In this study, a series of Nb and Si codoped V-based catalysts were prepared via the impregnation method, and Nb and Si species were applied to reduce the SO3 generation ratio from the catalyst. Moreover, the controlled condensation method was used to collect the SO3, which formed during the NH3-SCR reaction process. The results indicated that the Nb and Si codoped catalysts exhibited the best NO reduction performance with the lowest SO3 generation ratio. The SO3 generation ratio and the NO conversion ratio for the VW2NbTi-10Si catalyst were 0.39% and 92%, respectively. The physicochemical properties of the catalysts were analysed by a series of characterization techniques. The X-ray diffraction (XRD) results revealed that the Nb and Si species could inhibit the crystallization of TiO2, and the X-ray photoelectron spectroscopy (XPS) results indicated that Nb and Si could decrease the ratio of Oα and V5+ on the surface of the catalyst. From the CO2 and NH3 temperature programmed desorption (CO2 and NH3-TPD) results, the Nb and Si species could reduce the number of alkaline sites on the catalyst surface, and the Si species could significantly enhance the acidity of weak and strong acid sites over the catalyst. Combined with the results of N2-adsorption and desorption, CO2-TPD, H2 temperature programmed reduction (H2-TPR) and XPS experiments, the addition of Nb and Si species was beneficial to inhibiting the adsorption and oxidation of SO2 on the surface of catalysts, resulting in a significant decrease in SO3 emissions. Therefore, in-situ diffuse reflective infrared Fourier transform spectroscopy (DRIFTS) analysis demonstrated that the Nb and Si species could restrict the reaction between SO2 and V species, decreasing the formation of the intermediate product VOSO4. Furthermore, the enhancement of reaction temperature and NH3 volumetric fraction would significantly promote the formation of SO3. However, the variations in the volumetric fraction of O2 and NO expressed ignorable effects on the SO3 generation ratio.

Bimetallic NiFe Nanoparticles Supported on CeO2 as Catalysts for Methane Steam Reforming.

Ni-Fe nanocatalysts supported on CeO2 have been prepared for the catalysis of methane steam reforming (MSR) aiming for coke-resistant noble metal-free catalysts. The catalysts have been synthesized by traditional incipient wetness impregnation as well as dry ball milling, a green and more sustainable preparation method. The impact of the synthesis method on the catalytic performance and the catalysts' nanostructure has been investigated. The influence of Fe addition has been addressed as well. The reducibility and the electronic and crystalline structure of Ni and Ni-Fe mono- and bimetallic catalysts have been characterized by temperature programmed reduction (H2-TPR), in situ synchrotron X-ray diffraction (SXRD), X-ray photoelectron spectroscopy (XPS), and Raman spectroscopy. Their catalytic activity was tested between 700 and 950 °C at 108 L gcat-1 h-1 and with the reactant flow varying between 54 and 415 L gcat-1 h-1 at 700 °C. Hydrogen production rates of 67 mol gmet-1 h-1 have been achieved. The performance of the ball-milled Fe0.1Ni0.9/CeO2 catalyst was similar to that of Ni/CeO2 at high temperatures, but Raman spectroscopy revealed a higher amount of highly defective carbon on the surface of Ni-Fe nanocatalysts. The reorganization of the surface under MSR of the ball-milled NiFe/CeO2 has been monitored by in situ near-ambient pressure XPS experiments, where a strong reorganization of the Ni-Fe nanoparticles with segregation of Fe toward the surface has been observed. Despite the catalytic activity being lower in the low-temperature regime, Fe addition for the milled nanocatalyst increased the coke resistance and could be an efficient alternative to industrial Ni/Al2O3 catalysts.

Suppressing Universal Cathode Crossover in High-Energy Lithium Metal Batteries via a Versatile Interlayer Design.

The universal cathode crossover such as chemical and oxygen has been significantly overlooked in lithium metal batteries using high-energy cathodes which leads to severe capacity degradation and raises serious safety concerns. Herein, a versatile and thin (≈25 μm) interlayer composed of multifunctional active sites was developed to simultaneously regulate the Li deposition process and suppress the cathode crossover. The as-induced dual-gradient solid-electrolyte interphase combined with abundant lithiophilic sites enable stable Li stripping/plating process even under high current density of 10 mA cm-2 . Moreover, X-ray photoelectron spectroscopy and synchrotron X-ray experiments revealed that N-rich framework and CoZn dual active sites can effectively mitigate the undesired cathode crossover, hence significantly minimizing Li corrosion. Therefore, assembled lithium metal cells using various high-energy cathode materials including LiNi0.7 Mn0.2 Co0.1 O2 , Li1.2 Co0.1 Mn0.55 Ni0.15 O2 , and sulfur demonstrate significantly improved cycling stability with high cathode loading.

When copper is not enough: Advantages and drawbacks of using copper in de-NOx reactions over lanthanum manganite perovskite structures

Using a Cu-doped lanthanum manganite perovskite structure (LaCu0.5Mn0.5O3-δ), we show how Pd addition following different synthesis approaches (sol-gel vs. impregnation) allows to overcome the oxidation limitation of Cu in realistic de-NOx reaction mixtures involving NO, CO, O2 and water. The beneficial interplay of Cu with the oxygen-deficient perovskite resulting from in situ exsolution during humid de-NOx treatments is strongly connected with the metallic Cu state enhancing the N2O chemistry. The beneficial catalytic properties of metallic Cu can be preserved by adjustment of the Pd amount, as evidenced by combined in situ X-ray photoelectron spectroscopy and catalytic experiments following the main reactions of the de-NOx reaction network, including water-gas shift reaction and CO oxidation. The concept of impregnation with a noble metal and simultaneous exsolution with Cu yields a reactive Cu(Pd)/perovskite interface and provides a first step towards the economization of the noble metal content at the surface in future works.

Effect of a Single Platinum Atom within a Small Metal Oxide Cluster: Reaction of DMMP with Size-Selected Pt1Zr2O7 Supported on HOPG.

Chemical warfare agents (CWAs) are a persistent threat facing civilians and military personnel across the modern geopolitical landscape. The development of the next generation of protective and sensing materials stands to benefit from an improved fundamental understanding of the interaction of CWA molecules with the active components of such candidate materials. The use of model systems in well-controlled environments offers a route to glean such information and has been applied here to investigate the fundamental interaction of a nerve agent simulant molecule, dimethyl methylphosphonate (DMMP), with a small cluster model of a single atom catalyst (SAC) active site. The cluster models, Pt1Zr2O7, were prepared by depositing mass-selected cluster anions synthesized in the gas phase onto a 100 K highly oriented pyrolytic graphite (HOPG) substrate surface prepared in ultra-high vacuum (UHV) at sub-monolayer coverage. Upon deposition, the cluster anions lost their charge to the electrically conductive surface to yield free-standing neutral clusters. The HOPG-supported clusters were characterized by X-ray photoelectron spectroscopy (XPS) to determine the oxidation states and chemical environment of the metal atoms present within the clusters. The reactivity of the clusters with DMMP was investigated via temperature-programmed desorption/reaction (TPD/R) and XPS experiments in which the clusters were exposed to DMMP and incrementally heated to higher temperatures. In contrast to two other HOPG-supported clusters, (ZrO2)3 and Pt1Ti2O7, recently investigated in our laboratory, Pt1Zr2O7 decomposed DMMP to primarily evolve a methane species, which was completely absent for the other clusters.

BPA degradation using biogenic manganese oxides produced by an engineered Escherichia coli with a non-blue laccase from Bacillus sp. GZB

Bisphenol A (BPA), an endocrine disruptor that is often found in a variety of environmental matrixes, poses a serious health risk. One of the most effective methods for completely degrading BPA is biological oxidation. This study used a non-blue laccase to develop an engineer Escherichia coli strain for the synthesis of biogenic manganese oxides (BMO). The recombinant strain LACREC3 was utilized for the efficient production of BMO. The LACREC3 strain developed the erratic clumps of BMO after prolonged growth with Mn2+, as shown by scanning electron microscopy (SEM) and energy-dispersive X-ray (EDS) tests. After 12 days of incubation under liquid culture conditions, a total of 51.97 ± 0.56% Mn-oxides were detected. The Brunauer-Emmett-Teller (BET) surface areas, X-ray diffraction (XRD), Fourier transform infrared (FT-IR), and X-ray photoelectron spectroscopy (XPS) experiments were further used to characterize the purified BMO. Data revealed that Mn(IV)-oxides predominated in the structure of BMO, which was amorphous and weakly crystalline. The BPA oxidation assay confirmed the high oxidation efficiency of BMO particle. BMO degraded 96.16 ± 0.31% of BPA in total over the course of 60 min. The gas chromatography and mass spectroscopy (GC-MS) identified BPA-intermediates showed that BPA might break down into less hazardous substances that were tested by Photobacterium Phosphoreum in an acute toxicity experiment. Thus, employing BMO generated by a non-blue laccase, this study introduces a new biological technique of metal-oxidation and organic-pollutant degradation.

Suppression of anthracite dust by a composite of oppositely-charged ionic surfactants with ultra-high surface activity: Theoretical calculation and experiments

In this study, we developed composite anionic–cationic (AC) surfactants with ultra-high surface activity using sodium alpha olefin sulfonate and octadecyltrimethylammonium chloride to improve the wettability and dust suppression efficiency of coal dust. Moreover, we investigated the synergistic suppression mechanisms of AC surfactant compounds at a molecular level. The thermodynamics of the self-assembly suggested that the surfactant molecules in the composite solution form a flexible alternating anion–cation bilayer. The analysis of the critical micelle concentration (CMC), surface tension (24.121 mN·m−1), contact angle (35.14°) and settling time (18 s) of the composite surfactant solution proved the superior performance of our composite surfactants compared to the monomeric alternatives, resulting in enhanced wettability owing to an ultra-high surface activity. Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS) experiments revealed an increased number of hydrophilic groups on the surface of the composite surfactant-treated anthracite, explaining the microscopic mechanism behind the high wettability induced by the composite surfactant. The specific surface area and pore size distribution of anthracite before and after dust suppressant treatment were calculated, and it was concluded that the dust suppressant was adsorbed on the surface and in the pores of the anthracite. In addition, the microscopic external morphology of the anthracite was shown by scanning electron microscopy (SEM), and it was found that the dust suppressant showed filling and aggregation behavior. Wind erosion and evaporation resistance experimental results show that dust suppressant-treated coal dust has enhanced water retention and wind resistance. Our findings indicate that ionic surfactants with oppositely charged compounds are promising high-efficiency dust suppressants, laying the foundation for safe coal mining and providing a theoretical background for a broad range of applications, such as haze control.

Heterogenization of Heteropolyacid with Metal-Based Alumina Supports for the Guaiacol Gas-Phase Hydrodeoxygenation.

Because of the global necessity to decrease CO2 emissions, biomass-based fuels have become an interesting option to explore; although, bio-oils need to be upgraded, for example, by catalytic hydrodeoxygenation (HDO), to reduce oxygen content. This reaction generally requires bifunctional catalysts with both metal and acid sites. For that purpose, Pt-Al2O3 and Ni-Al2O3 catalysts containing heteropolyacids (HPA) were prepared. HPAs were added by two different methods: the impregnation of a H3PW12O40 solution onto the support and a physical mixture of the support with Cs2.5H0.5PW12O40. The catalysts were characterized by powder X-ray diffraction, Infrared, UV-Vis, Raman, X-ray photoelectron spectroscopy and NH3-TPD experiments. The presence of H3PW12O40 was confirmed by Raman, UV-Vis and X-ray photoelectron spectroscopy, while the presence of Cs2.5H0.5PW12O40 was confirmed by all of the techniques. However, HPW was shown to strongly interact with the supports, especially in the case of Pt-Al2O3. These catalysts were tested in the HDO of guaiacol, at 300 °C, under H2 and at atmospheric pressure. Ni-based catalysts led to higher conversion and selectivity to deoxygenated compound values, such as benzene. This is attributed to both a higher metal and acidic contents of these catalysts. Among all tested catalysts, HPW/Ni-Al2O3 was shown to be the most promising, although it suffered a more severe deactivation with time-on-stream.

Preparation of cinnamic hydroxamic acid collector and study on flotation characteristics and mechanism of scheelite

In this paper, using methyl cinnamate as raw material, the new cinnamic hydroxamic acid collector (CIHA) was synthesized by the hydroxylamine method. The collector performance of hydroxamic acid was investigated for scheelite and gangue calcite, and the flotation separation test of scheelite and calcite was carried out with CIHA as the collector. The interaction mechanism between hydroxamic acid and scheelite minerals has also been investigated through zeta potential, Fourier transform infrared spectroscopy (FTIR) experiments, X-ray photoelectron spectroscopy (XPS) experiments, and density functional theory (DFT) calculation. The single mineral flotation test and artificially mixed ore showed that CIHA had an excellent collection effect and selectivity. Zeta potential, FTIR, and XPS showed that CIHA was adsorbed on the scheelite surface by strong chemical adsorption. The active group of CIHA was analyzed through quantum chemical calculation. It was speculated that CO and NO bonds could synthesize a five-membered chelated hydroxamic acid group with Ca element chelate on scheelite surface, changing hydrophobicity and making it more likely to emerge from the pulp.

Molecular Dynamics Simulation of Methane Adsorption and Diffusion: A Case Study of Low-Rank Coal in Fukang Area, Southern Junggar Basin

Adsorption and diffusion are the key factors affecting coalbed methane (CBM) accumulation, resource assessment and production prediction. To study the adsorption and diffusion mechanism of Fukang low-rank coal at the microscopic level, samples of Fukang low-rank coal were collected, and the elemental composition, carbon type distribution and functional group type of the Fukang low-rank coal structure were determined by elemental analysis (Ea), Fourier-transform interferometric radiometer (FTIR), X-ray photoelectron spectroscopy (XPS) and 13C nuclear magnetic resonance (13C NMR) experiments to construct a 2D molecular structure of the coal and a 3D macromolecular structure model. The adsorption and diffusion characteristics of methane were researched by giant regular Monte Carlo (GCMC) and molecular dynamics (MD) simulation methods. The results showed that the excess adsorption amount of methane increased and then decreased with the increase in pressure. The diffusion of methane showed two stages with increasing pressure: a sharp decrease in the diffusion coefficient from 0.5 to 5.0 MPa and a slow decrease in the diffusion coefficient from 5.0 to 15.0 MPa. The lower the pressure, the larger the effective radius of the CH4 and C atoms, and the higher the temperature, the more pronounced the diffusion and the larger the effective radius.

2D Nb<sub>2</sub>CT<sub><i>x</i></sub> MXene/MoS<sub>2</sub> heterostructure construction for nonlinear optical absorption modulation

Two-dimensional (2D) nonlinear optical mediums with high and tunable light modulation capability can significantly stimulate the development of ultrathin, compact, and integrated optoelectronics devices and photonic elements. 2D carbides and nitrides of transition metals (MXenes) are a new class of 2D materials with excellent intrinsic and strong light-matter interaction characteristics. However, the current understanding of their photo-physical properties and strategies for improving optical performance is insufficient. To address this issue, we rationally designed and <italic>in situ</italic> synthesized a 2D Nb₂C/MoS₂ heterostructure that outperforms pristine Nb₂C in both linear and nonlinear optical performance. Excellent agreement between experimental and theoretical results demonstrated that the Nb₂C/MoS₂ inherited the preponderance of Nb₂C and MoS₂ in absorption at different wavelengths, resulting in the broadband enhanced optical absorption characteristics. In addition to linear optical modulation, we also achieved stronger near infrared nonlinear optical modulation, with a nonlinear absorption coefficient of Nb₂C/MoS₂ being more than two times that of the pristine Nb₂C. These results were supported by the band alinement model which was determined by the X-ray photoelectron spectroscopy (XPS) experiment and first-principal theory calculation. The presented facile synthesis approach and robust light modulation strategy pave the way for broadband optoelectronic devices and optical modulators.

Oxygen Reduction Reaction on Pt-Y/C Catalysts: Activity and Long-Term Stability Study

Pt-Y/C catalysts were prepared by a modified formic acid method and their structural characteristics and activity for oxygen reduction reaction (ORR) were evaluated. X-ray diffraction analysis showed that no alloy was formed between the metals and X-ray photoelectron spectroscopy (XPS) experiments showed that yttrium is presented as Y2O3, Y(OH)3 and Y-O-Pt species. CO stripping voltammetry and in situ X-ray absorption spectroscopy confirmed the interaction between Pt nanoparticles and yttrium species. The Pt-Y/C 7:3 material showed the higher specific activity (103 μA cmPt -2) for ORR. Following repetitive potential cycling, the activity for the ORR of the Pt-Y/C 7:3 catalyst declined in a similar proportion to the Pt/C material. Transmission electron microscopy analysis after stability for both catalysts showed that Pt/C particle size slightly increased and that for Pt-Y/C 7:3 remained the same. XPS made in ultrathin catalyst layer, agreeing with energy-dispersive X-ray spectroscopy analysis, revealed that the intensities of the Y 3d peaks were suppressed relative to the initial one and that yttrium in the form of Y2O3 and Y(OH)3 were dissolved, while the species Y-O-Pt still remain in the catalyst, maintaining the higher activity.

Determining surface-specific Hubbard-U corrections and identifying key adsorbates on nickel and cobalt oxide catalyst surfaces.

NiO is a popular transition metal oxide (TMO) with high thermal and chemical stability and Co3O4 is a relatively more reducible TMO due to weaker metal-oxygen bonds. Both are often used as catalysts in a variety of chemical transformations. Density functional theory (DFT) and X-ray photoelectron spectroscopy (XPS) are used to investigate catalysis on TMO surfaces, yet both techniques have their own limitations. The accuracy of DFT highly depends on the choice of Hubbard U correction. The bulk-property optimized U value of 5.3 eV for NiO and different U values for Co3O4, without any consensus, are often used in the literature to simulate surface catalysis. However, U values optimized using bulk properties often fail to reproduce surface-adsorbate interactions on TMOs. Similarly, there exists arbitrariness in assigning observed XPS shifts to different surface species on these metal oxides. Hence, a synergistic application of XPS and DFT+U is implemented to determine the surface specific U values for NiO and Co3O4, and to identify adsorbed surface moieties corresponding to experimentally observed XPS shifts. For the NiO (100) surface, the U value of ∼2 eV is able to reproduce the experimentally observed XPS O1s core level binding energy shifts correctly, instead of the bulk property optimized and commonly used U value of 5.3 eV. Using this surface specific U value of 2 eV, the experimentally observed XPS shifts are assigned. Similarly, for Co3O4 (100) surface, ∼3 eV of U value could successfully predict the experimentally observed XPS shifts and corresponding adsorbates. The surface adsorbates and configurations suggested in this work will help analyze experimental XPS data and the surface specific U values will ensure accurate predictions of adsorption and reaction energetics on these catalysts.

In Situ Synchrotron-Based Studies of IrO2(110)–TiO2(110) under Harsh Acidic Water Splitting Conditions: Anodic Stability and Radiation Damages

In situ stability studies of an IrO2(110)–TiO2(110) model electrode are carried out under acidic water electrolysis conditions, employing synchrotron-based techniques including surface X-ray diffraction (SXRD) and X-ray reflectometry (XRR) with a photon energy of 21.5 keV. These experiments are complemented by ex situ scanning electron microscopy (SEM), scanning tunneling microscopy (STM), and X-ray photoelectron spectroscopy (XPS) experiments. Even at an anodic current density of 250 mA·cm–2 during electrochemical water splitting, the IrO2(110)–TiO2(110) model electrode turned out to be stable against Ir dissolution. However, radiation-induced damages of the IrO2(110) film are observed: Part of the IrO2(110) film delaminates upon heavy exposure to the synchrotron beam, while subsequently the uncovered TiO2(110) is subject to further (photon-induced) corrosion. We propose that the X-ray photons induce oxygen vacancy formation by displacing O2– ions of TiO2 from regular to interstitial sites, while the potential drop across the TiO2(110) substrate leads to a migration of interstitial O2– ions from interface toward bulk TiO2. This reduction step at the interface between IrO2(110) and TiO2(110) weakens the adhesion of the epitaxially grown IrO2(110) film to the TiO2(110) substrate so that the strained IrO2(110) film is partially delaminated. Higher X-ray photon energies of 60–90 keV mitigate this degradation process.

In Situ Study of Hydrogen Permeable Electrodes for Electrolytic Ammonia Synthesis Using Near Ambient Pressure XPS.

Hydrogen permeable electrodes can be utilized for electrolytic ammonia synthesis from dinitrogen, water, and renewable electricity under ambient conditions, providing a promising route toward sustainable ammonia. The understanding of the interactions of adsorbing N and permeating H at the catalytic interface is a critical step toward the optimization of this NH3 synthesis process. In this study, we conducted a unique in situ near ambient pressure X-ray photoelectron spectroscopy experiment to investigate the solid-gas interface of a Ni hydrogen permeable electrode under conditions relevant for ammonia synthesis. Here, we show that the formation of a Ni oxide surface layer blocks the chemisorption of gaseous dinitrogen. However, the Ni 2p and O 1s XPS spectra reveal that electrochemically driven permeating atomic hydrogen effectively reduces the Ni surface at ambient temperature, while H2 does not. Nitrogen gas chemisorbs on the generated metallic sites, followed by hydrogenation via permeating H, as adsorbed N and NH3 are found on the Ni surface. Our findings suggest that the first hydrogenation step to NH and the NH3 desorption might be limiting under the operating conditions. The study was then extended to Fe and Ru surfaces. The formation of surface oxide and nitride species on iron blocks the H permeation and prevents the reaction to advance; while on ruthenium, the stronger Ru-N bond might favor the recombination of permeating hydrogen to H2 over the hydrogenation of adsorbed nitrogen. This work provides insightful results to aid the rational design of efficient electrolytic NH3 synthesis processes based on but not limited to hydrogen permeable electrodes.