Showing papers on "Chemical state published in 2015"

Monitoring the Chemical State of Catalysts for CO2 Electroreduction: An In Operando Study

Abstract: A major concern of electrocatalysis research is to assess the structural and chemical changes that a catalyst may itself undergo in the course of the catalyzed process. These changes can influence not only the activity of the studied catalyst but also its selectivity toward the formation of a certain product. An illustrative example is the electroreduction of carbon dioxide on tin oxide nanoparticles, where under the operating conditions of the electrolysis (that is, at cathodic potentials), the catalyst undergoes structural changes which, in an extreme case, involve its reduction to metallic tin. This results in a decreased Faradaic efficiency (FE) for the production of formate (HCOO–) that is otherwise the main product of CO2 reduction on SnOx surfaces. In this study, we utilized potential- and time-dependent in operando Raman spectroscopy in order to monitor the oxidation state changes of SnO2 that accompany CO2 reduction. Investigations were carried out at different alkaline pH levels, and a strong co...

Catalyst Chemical State during CO Oxidation Reaction on Cu(111) Studied with Ambient-Pressure X-ray Photoelectron Spectroscopy and Near Edge X-ray Adsorption Fine Structure Spectroscopy.

Abstract: The chemical structure of a Cu(111) model catalyst during the CO oxidation reaction in the CO+O2 pressure range of 10-300 mTorr at 298-413 K was studied in situ using surface sensitive X-ray photoelectron and adsorption spectroscopy techniques [X-ray photoelectron spectroscopy (XPS) and near edge X-ray adsorption fine structure spectroscopy (NEXAFS)]. For O2:CO partial pressure ratios below 1:3, the surface is covered by chemisorbed O and by a thin (∼1 nm) Cu2O layer, which covers completely the surface for ratios above 1:3 between 333 and 413 K. The Cu2O film increases in thickness and exceeds the escape depth (∼3-4 nm) of the XPS and NEXAFS photoelectrons used for analysis at 413 K. No CuO formation was detected under the reaction conditions used in this work. The main reaction intermediate was found to be CO2(δ-), with a coverage that correlates with the amount of Cu2O, suggesting that this phase is the most active for CO oxidation.

Chemistry of Supported Palladium Nanoparticles during Methane Oxidation

Abstract: Time-resolved in situ, energy-dispersive X-ray absorption spectroscopy and mass spectrometry are used to correlate changes in the chemical state of alumina- and ceria-supported palladium nanopartic...

Chemical States of Overcharged LiCoO2 Particle Surfaces and Interiors Observed Using Electron Energy-Loss Spectroscopy

Abstract: Deterioration mechanisms of LiCoO2 electrode materials for lithium ion batteries remain unclear. Using electron energy-loss spectroscopy and transmission electron microscopy, this study investigated chemical states of LiCoO2 particles on first overcharging. We present a scheme for quantification of the Li/Co atomic ratio. Using quantitative Li mapping and comprehensive probing of Li–K, Co–M2,3, Co–L3, and O–K edges, we observed that overcharging causes the progression of Co3+/Co2+ reduction with oxygen extraction from the particle surface to the interior. A gradual change in the chemical composition at and around the particle surfaces after charging of 60% revealed the presence of Co3O4-like and CoO-like phases at surface regions. We also observed nanocracks with deficient Li ions. These results are key factors affecting degradation on overcharging.

Graphene as an anti-corrosion coating layer

Abstract: Graphene, a single layer of carbon atoms arranged in an aromatic hexagonal lattice, has recently drawn attention as a potential coating material due to its impermeability, thermodynamic stability, transparency and flexibility Here, the effectiveness of a model system, a graphene covered Pt(100) surface, for studying the anti-corrosion properties of graphene, has been evaluated Chemical vapour deposition techniques were used to cover the single crystal surface with a complete layer of high-quality graphene and the surface was characterised after exposure to corrosive environments with scanning tunnelling microscopy (STM) and Raman spectroscopy Graphene covered Pt samples were exposed to: (i) ambient atmosphere for 6 months at room temperature and 60 °C for 75 min, (ii) Milli-Q water for 14 hours at room temperature and 60 °C for 75 min, and (iii) saltwater (0513 M NaCl) for 75 min at room temperature and 60 °C STM provides atomic resolution images, which show that the graphene layer and the underlying surface reconstruction on the Pt(100) surface remain intact over the majority of the surface under all conditions, except exposure to saltwater when the sample is kept at 60 °C Raman spectroscopy shows a broadening of all graphene related peaks due to hybridisation between the surface Pt d-orbitals and the graphene π-bands This hybridisation also survives exposure to all environments except saltwater on the hot surface, with the latter leading to peaks more representative of a quasi free-standing graphene layer A mechanism explaining the corrosive effect of hot saltwater is suggested Based on these experiments, graphene is proposed to offer protection against corrosion in all tested environments, except saltwater on a hot surface, and Raman spectroscopy is proposed as a useful method for indirectly assessing the chemical state of the Pt surface

Effects of Surface and Bulk Silver on PrMnO3+δ Perovskite for CO and Soot Oxidation: Experimental Evidence for the Chemical State of Silver

Abstract: Unambiguous evidence has been obtained to explain the presence and effects of both framework and extra-framework silver on catalytic properties of Pr(Ag)MnO3+δ perovskite type materials, using various tools such as XPS, HR-TEM, O2-TPD, and H2-TPR analysis. Three types of Ag-incorporated PrMnO3 perovskite samples were synthesized by means of Ag partial substitution in perovskite lattice and Ag dispersion on the surface of the synthesized perovskite phase, using two different calcination temperatures of 200 and 550 °C. The amount of silver used was 1 wt % (0.000225 mol), in all three catalysts. On the basis of extensive characterization studies, it was clearly explained that the partially substituted Ag for Pr is present in the lattice along with Pr at the “A” site of the ABO3 perovskite structure. The Ag surface incorporated PrMnO3+δ sample calcined at 550 °C shows both surface metallic silver and partially substituted Ag in perovskite lattice, whereas Ag2O nanoparticles were observed on the surface in the...

X‐Ray Spectroscopic Investigation of Chlorinated Graphene: Surface Structure and Electronic Effects

Abstract: Chemical doping of graphene represents a powerful means of tailoring its electronic properties. Synchrotron-based X-ray spectroscopy offers an effective route to investigate the surface electronic and chemical states of functionalizing dopants. In this work, a suite of X-ray techniques is used, including near edge X-ray absorption fi ne structure spectroscopy, X-ray photoemission spectroscopy, and photoemission threshold measurements, to systematically study plasma-based chlorinated graphene on different substrates, with special focus on its dopant concentration, surface binding energy, bonding confi guration, and work function shift. Detailed spectroscopic evidence of C‐Cl bond formation at the surface of single layer graphene and correlation of the magnitude of p-type doping with the surface coverage of adsorbed chlorine is demonstrated for the fi rst time. It is shown that the chlorination process is a highly nonintrusive doping technology, which can effectively produce strongly p-doped graphene with the 2D nature and long-range periodicity of the electronic structure of graphene intact. The measurements also reveal that the interaction between graphene and chlorine atoms shows strong substrate effects in terms of both surface coverage and work function shift.

Comprehensive and quantitative analysis for controlling the physical/chemical states and particle properties of nanodiamonds for biological applications

Abstract: The physical/chemical states and properties of nanodiamonds subjected to thermal annealing and air oxidation, which are indispensable processes for the preparation of fluorescent nanodiamonds, were investigated. Specifically, the weight loss, particle size, crystal quality, chemical bonding states of carbon and oxygen, zeta potential, dispersibility, and fluorescent and optically detected magnetic resonance (ODMR) properties were determined using X-ray diffraction (XRD) analysis, transmission electron microscopy (TEM), elemental analysis, dynamic light scattering, Raman analysis, X-ray photoelectron spectroscopy (XPS), IR spectroscopy, and a home-made fluorescence and ODMR microscope. The study focused on small-sized nanodiamonds (∼50 nm), which are applicable for biological research. The obtained results should be useful for controlling the mutually-related physical/chemical states and properties of diamond nanoparticles.

Non-equilibrium oxidation states of zirconium during early stages of metal oxidation

Abstract: The chemical state of Zr during the initial, self-limiting stage of oxidation on single crystal zirconium (0001), with oxide thickness on the order of 1 nm, was probed by synchrotron x-ray photoelectron spectroscopy. Quantitative analysis of the Zr 3d spectrum by the spectrum reconstruction method demonstrated the formation of Zr1+, Zr2+, and Zr3+ as non-equilibrium oxidation states, in addition to Zr4+ in the stoichiometric ZrO2. This finding resolves the long-debated question of whether it is possible to form any valence states between Zr0 and Zr4+ at the metal-oxide interface. The presence of local strong electric fields and the minimization of interfacial energy are assessed and demonstrated as mechanisms that can drive the formation of these non-equilibrium valence states of Zr.

Tuning the properties of copper-based catalysts based on molecular in situ studies of model systems

Abstract: Studying catalytic processes at the molecular level is extremely challenging, due to the structural and chemical complexity of the materials used as catalysts and the presence of reactants and products in the reactor's environment. The most common materials used on catalysts are transition metals and their oxides. The importance of multifunctional active sites at metal/oxide interfaces has been long recognized, but a molecular picture of them based on experimental observations is only recently emerging. The initial approach to interrogate the surface chemistry of catalysts at the molecular level consisted of studying metal single crystals as models for reactive metal centers, moving later to single crystal or well-defined thin film oxides. The natural next iteration consisted in the deposition of metal nanoparticles on well-defined oxide substrates. Metal nanoparticles contain undercoordinated sites, which are more reactive. It is also possible to create architectures where oxide nanoparticles are deposited on top of metal single crystals, denominated inverse catalysts, leading in this case to a high concentration of reactive cationic sites in direct contact with the underlying fully coordinated metal atoms. Using a second oxide as a support (host), a multifunctional configuration can be built in which both metal and oxide nanoparticles are located in close proximity. Our recent studies on copper-based catalysts are presented here as an example of the application of these complementary model systems, starting from the creation of undercoordinated sites on Cu(111) and Cu2O(111) surfaces, continuing with the formation of mixed-metal copper oxides, the synthesis of ceria nanoparticles on Cu(111) and the codeposition of Cu and ceria nanoparticles on TiO2(110). Catalysts have traditionally been characterized before or after reactions and analyzed based on static representations of surface structures. It is shown here how dynamic changes on a catalyst's chemical state and morphology can be followed during a reaction by a combination of in situ microscopy and spectroscopy. In addition to determining the active phase of a catalyst by in situ methods, the presence of weakly adsorbed surface species or intermediates generated only in the presence of reactants can be detected, allowing in turn the comparison of experimental results with first principle modeling of specific reaction mechanisms. Three reactions are used to exemplify the approach: CO oxidation (CO + 1/2O2 → CO2), water gas shift reaction (WGSR) (CO + H2O → CO2 + H2), and methanol synthesis (CO2 + 3H2 → CH3OH + H2O). During CO oxidation, the full conversion of Cu(0) to Cu(2+) deactivates an initially outstanding catalyst. This can be remedied by the formation of a TiCuOx mixed-oxide that protects the presence of active partially oxidized Cu(+) cations. It is also shown that for the WGSR a switch occurs in the reaction mechanism, going from a redox process on Cu(111) to a more efficient associative pathway at the interface of ceria nanoparticles deposited on Cu(111). Similarly, the activation of CO2 at the ceria/Cu(111) interface allows its facile hydrogenation to methanol. Our combined studies emphasize the need of searching for optimal metal/oxide interfaces, where multifunctional sites can lead to new efficient catalytic reaction pathways.

Simultaneous detection of electronic structure changes from two elements of a bifunctional catalyst using wavelength-dispersive X-ray emission spectroscopy and in situ electrochemistry.

Abstract: Multielectron catalytic reactions, such as water oxidation, nitrogen reduction, or hydrogen production in enzymes and inorganic catalysts often involve multimetallic clusters. In these systems, the reaction takes place between metals or metals and ligands to facilitate charge transfer, bond formation/breaking, substrate binding, and release of products. In this study, we present a method to detect X-ray emission signals from multiple elements simultaneously, which allows for the study of charge transfer and the sequential chemistry occurring between elements. Kβ X-ray emission spectroscopy (XES) probes charge and spin states of metals as well as their ligand environment. A wavelength-dispersive spectrometer based on the von Hamos geometry was used to disperse Kβ signals of multiple elements onto a position detector, enabling an XES spectrum to be measured in a single-shot mode. This overcomes the scanning needs of the scanning spectrometers, providing data free from temporal and normalization errors and therefore ideal to follow sequential chemistry at multiple sites. We have applied this method to study MnOx-based bifunctional electrocatalysts for the oxygen evolution reaction (OER) and the oxygen reduction reaction (ORR). In particular, we investigated the effects of adding a secondary element, Ni, to form MnNiOx and its impact on the chemical states and catalytic activity, by tracking the redox characteristics of each element upon sweeping the electrode potential. The detection scheme we describe here is general and can be applied to time-resolved studies of materials consisting of multiple elements, to follow the dynamics of catalytic and electron transfer reactions.

Surface−Enhanced Polymerization via Schiff-Base Coupling at the Solid–Water Interface under pH Control

Abstract: On-surface polymerization realized at the solid–liquid interface represents a promising route to obtain stable and conductive organic layers with tunable properties. We present here spectroscopic evidence of π-conjugated polymer formation at the interface between an iodine-modified Au(111) and an aqueous solution. Schiff-base coupling has been used to drive the reaction by changing the pH. Scanning tunneling microscopy (STM) investigations show that the substrate acts as a template driving the formation of 1D ordered nanostructures. All the chemical states of the molecules on the surface have been identified and their evolution as a function of the pH has been monitored by synchrotron radiation X-ray photoelectron spectroscopy (XPS), demonstrating that two polymeric phases, undistinguishable by STM, exist on the surface: intermediate state and π-conjugated final product. The I/Au(111) substrate enhances the formation of π-conjugated polymers, as established comparing their production on the surface and in...

Electrodeposition and pyrolysis of Mn/polypyrrole nanocomposites: a study based on soft X-ray absorption, fluorescence and photoelectron microspectroscopies

Abstract: Electrodeposition of manganese/polypyrrole (Mn/PPy) nanocomposites has been recently shown to be a technologically relevant synthesis method for the fabrication of Oxygen Reduction Reaction (ORR) electrocatalysts. In this study we have grown such composites with a potentiostatic anodic/cathodic pulse-plating procedure and characterised them by a multi-technique approach, combining a suite of in situ and ex situ spectroscopic methods with electrochemical measurements. We have thus achieved a sound degree of molecular-level understanding of the hybrid co-electrodeposition process consisting of electropolymerisation of polypyrrole with incorporation of Mn. By in situ Raman spectroscopy we followed the formation of MnOx and the polymer by monitoring the build-up and development of the relevant vibrational bands. The compositional and chemical-state distribution of the as-deposited material has been investigated ex situ by soft X-ray fluorescence (XRF) mapping and micro-absorption spectroscopy (micro-XAS). XRF shows that the spatial distribution of Mn is consistent in a rather wide range of current densities (c.d.s), while micro-XAS reveals a mixture of Mn valencies, with higher oxidation states prevailing at higher c.d.s. Pyrolysis of electrodeposits, desirable for obtaining more durable and active catalysts, has been followed in situ by photoelectron microspectroscopy, allowing to assess the evolution of: (i) the electrodeposit morphology, resulting in a uniform distribution of nanoparticles; (ii) the chemical state of manganese, changing from a mixture of valences to a final state consisting of Mn(III) and Mn(IV) oxides and (iii) the bonding nature of nitrogen, from initially N-pyrrolic to a combination of pyridinic and Mn–N/graphitic.

Chemical State Analysis of Phosphorus Performed by X-ray Emission Spectroscopy.

Abstract: An experimental and theoretical study of phosphorus electronic structure based on high energy resolution X-ray emission spectroscopy was performed. The Kα and Kβ emission spectra of several phosphorus compounds were recorded using monochromatic synchrotron radiation and megaelectronvolt (MeV) proton beam for target excitation. Measured spectra are compared to the results of ab initio quantum chemical calculations based on density functional theory (DFT). Clear correlation between energy position of the Kα emission line and the phosphorus formal oxidation state as well as DFT-calculated number of valence electrons is obtained; measured energy shifts are reproduced by the calculations. Chemical sensitivity is increased further by looking at the Kβ emission spectra probing directly the structure of occupied molecular orbitals. Energies and relative intensities of main components are given together with the calculated average atomic character of the corresponding molecular orbitals involved in transitions.

Growth characteristics of Ti-based fumaric acid hybrid thin films by molecular layer deposition.

Abstract: Ti-based fumaric acid hybrid thin films were successfully prepared using inorganic TiCl4 and organic fumaric acid as precursors by molecular layer deposition (MLD). The effect of deposition temperature from 180 °C to 350 °C on the growth rate, composition, chemical state, and topology of hybrid films has been investigated systematically by means of a series of analytical tools such as spectroscopic ellipsometry, atomic force microscopy (AFM), high resolution X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FTIR). The MLD process of the Ti-fumaric acid shows self-limiting surface reaction with a reasonable growth rate of ∼0.93 A per cycle and small surface roughness of ∼0.59 nm in root-mean-square value at 200 °C. A temperature-dependent growth characteristic has been observed in the hybrid films. On increasing the temperature from 180 °C to 300 °C, the growth rate decreases from 1.10 to 0.49 A per cycle and the XPS composition of the film's C : O : Ti ratio changes from 8.35 : 7.49 : 1.00 to 4.66 : 4.80 : 1.00. FTIR spectra indicate that the hybrid films show bridging bonding mode at a low deposition temperature of 200 °C and bridging/bidentate mixed bonding mode at elevated deposition temperatures of 250 and 300 °C. The higher C and O amounts deviating from the ideal composition may be ascribed to increased organic incorporation into the hybrid films at lower deposition temperature and temperature-dependent density of reactive sites (-OH). The composition of hybrid films grown at 350 °C shows a dramatic decrease in C and O elemental composition (C : O : Ti = 1.97 : 2.76 : 1.00) due to the thermal decomposition of the fumaric acid precursor. The produced by-product H2O changes the structure of the hybrid films, resulting in the formation of more Ti-O bonds at high temperatures. The stability of the hybrid films against chemical and thermal treatment, and long-term storage by vacuum-packing was explored carefully. It is found that the ultrathin hybrid film can be transformed into TiO2 nanoparticles via various post deposition annealing processes with different topographies. Finally, the charge trapping ability of the hybrid film is confirmed by fabricating a charge trapping memory capacitor in which the hybrid film was inserted as a charge trapping layer.

Processes of Removing Zinc from Water using Zero-Valent Iron

Abstract: Zero-valent iron has received considerable attention for its potential application in the removal of heavy metals from water. This paper considers the possibility of removal of zinc ions from water by causing precipitates to form on the surface of iron. The chemical states and the atomic concentrations of solids which have formed on the surface of zero-valent iron as well as the type of the deposited polycrystalline substances have been analyzed with the use of X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD), respectively. The BET surface area, the pH at point of zero charge (pHPZC), the ORP of the solutions, and the pH and chemical concentrations in the solutions have also been measured. Furthermore, the paper also considers the possibility of release of zinc from the precipitates to demineralised water in changing physicochemical and chemical conditions. In a wide range of pH values, Zn x Fe3 - x O4 (where x ≤ 1) was the main compound resulting from the removal of zinc in ionic form from water. In neutral and alkaline conditions, the adsorption occurred as an additional process.

The solid state conversion reaction of epitaxial FeF2(110) thin films with lithium studied by angle-resolved X-ray photoelectron spectroscopy

Abstract: The phase evolution and morphology of the solid state FeF2 conversion reaction with Li has been characterized using angle-resolved X-ray photoelectron spectroscopy (ARXPS). An epitaxial FeF2(110) film was grown on a MgF2(110) single crystal substrate and exposed to atomic lithium in an ultra-high vacuum chamber. A series of ARXPS spectra was taken after each Li exposure to obtain depth resolved chemical state information. The Li–FeF2 reaction initially proceeded in a layer-by-layer fashion to a depth of ∼1.2 nm. Beyond this depth, the reaction front became non-planar, and regions of unreacted FeF2 were observed in the near-surface region. This reaction progression is consistent with molecular dynamics simulations. Additionally, the composition of the reacted layer was similar to that of electrochemically reacted FeF2 electrodes. An intermediary compound FexLi2−2xF2, attributed to iron substituted in the LiF lattice, has been identified using XPS. These measurements provide insight into the atomistics and phase evolution of high purity FeF2 conversion electrodes without contamination from electrolytes and binders, and the results partially explain the capacity losses observed in cycled FeF2 electrodes.

Chemical state of phosphorus in amorphous Ni–Fe–P electroplates

Abstract: High-quality Fe–Ni–P coatings on copper plates were prepared by electrodeposition from solutions containing simultaneously Ni(II) and Fe(II) cations and hypophosphite-anions. The structure of Fe–Ni–P deposits was compared to Fe–P and Ni–P coatings prepared under similar conditions. All as-prepared deposits showed amorphous or very fine nanocrystalline structure as no Bragg peaks of any crystalline phase were detected. Quantitative analysis of valence-to-core X-ray emission (vtc-XES) spectra of amorphous coatings showed that majority of phosphorus atoms are chemically bonded to the atoms of iron and nickel. These results prove that the structure of amorphous coatings formed during electrodeposition does not contain a significant amount of elemental (or so-called “free” or “intermediate”) phosphorus. Thermal treatment in vacuum of Fe–Ni–P coatings (as well as Fe–P and Ni–P) resulted in their complete crystallization. Comparison of vtc-XES spectra of original and thermally treated samples allows one to conclude that the concentration of phosphorus chemically bonded to metals increases only slightly (by 1–2 at.%) further confirming very low concentration (or the absence) of elemental phosphorus in the original structure.

Directly probing the effect of the solvent on a catalyst electronic environment using X-ray photoelectron spectroscopy

Abstract: The electronic environment of the metal centre of a catalyst dissolved in ionic liquids has a determining effect on its catalytic efficiency in chemical reactions. However, the electronic environment of the ionic liquid-based metal centres can be influenced by not only their chemical state but also the solute–solvent interaction. In this work, we demonstrate that the anion of an ionic liquid can significantly influence the electronic environment of a metal centre. The metal centre electronic environment can be monitored by measuring the typical electron binding energies by X-ray photoelectron spectroscopy (XPS). The correlation of the electronic environment of the metal centre with reaction performance provides a possibility to design and control a chemical reaction. In this work, we also illustrate a strategy for tuning the electronic environment of metal centres, by the selection of particular ionic liquid anions, to design a catalytic system and consequently to finally control the reaction performance of a model Suzuki cross coupling reaction.

Growth and composition of nanostructured and nanoporous cerium oxide thin films on a graphite foil

Abstract: The morphology and composition of CeOx films prepared by r.f. magnetron sputtering on a graphite foil have been investigated mainly by using microscopy methods. This study presents the formation of nanocrystalline layers with porous structure due to the modification of a carbon support and the formation of cerium carbide crystallites as a result of the deposition process. Chemical analyses of the layers with different thicknesses performed by energy dispersive X-ray spectroscopy, electron energy loss spectroscopy and X-ray photoelectron spectroscopy have pointed to the reduction of the cerium oxide layers. In the deposited layers, cerium was present in mixed Ce(3+) and Ce(4+) valence. Ce(3+) species were located mainly at the graphite foil-CeOx interface and the chemical state of cerium was gradually changing to Ce(4+) going to the layer surface. It became more stoichiometric in the case of thicker layers except for the surface region, where the presence of Ce(3+) was associated with oxygen vacancies on the surface of cerium oxide grains. The degree of cerium oxide reduction is discussed in the context of particle size.