Jun ware glaze colours: An X-ray absorption spectroscopy study

Jun ware is stoneware created in the late Northern Song dynasty (12^th century) with a blue glaze combining transparent-blue and whitish-opaque submillimetric areas.The glaze has a glass nanostructure with lime-rich droplets in a silica-rich matrix resulting from a high temperature liquid-liquid phase separation. Calcium-rich opaque and calcium-poor transparent areas are combined. Iron is more oxidised in the calcium rich areas (≈17–20% Fe²⁺) than in the calcium poor areas (≈60–70% Fe²⁺) of the glaze. Therefore, iron is oxidised in the lime-rich droplets and reduced in the silica-rich matrix. The sky-like appearance of the glaze is due to the combination of the light absorption in the transparent-dark-blue Fe²⁺ rich areas and scattering in the white-yellowish Fe³⁺ rich areas.Copper appears mainly oxidised but in the red areas a few small copper nanoparticles are present and iron appears more oxidised. The result indicates the simultaneous reduction of copper and oxidation of iron.     

Studies of Cu-ZSM-5 by X-ray absorption spectroscopy and its application for the oxidation of benzene to phenol by air

The oxidation of benzene to phenol has been successfully carried out in air over Cu-ZSM-5 at moderate temperatures. Several parameters such as Cu loading, calcination temperature and co-exchanged metal ions influence the nature of the catalyst. At low Cu loadings, the catalyst is more selective to phenol while at high Cu loadings CO₂ is the major product. In situ H₂-TPR XAFS studies reveal that at low Cu loadings, Cu exists as isolated pentacoordinated ions, with 4 equatorial oxygens at 1.94 Å and a more distant axial oxygen at 2.34 Å. At higher loadings, monomeric as well as dimeric Cu species exist, with a Cu–Cu distance of 2.92 Å. This suggests that the isolated Cu sites are the active sites responsible for phenol formation. When the catalyst was calcined at 450 °C, the activity peaked in the first hour and then slowly deactivated, but when the calcination temperature was increased to 850 °C, the activity slowly increased until it reached a plateau. Analysis of the XANES region during in situ H₂-TPR shows that at lower calcination temperatures two reduction peaks are present, corresponding to Cu²⁺ → Cu⁺ and Cu⁺ → Cu⁰. At high calcination temperatures, only a small fraction of the Cu undergoes the two-step reduction and most of the Cu remains in the +2 state. Slow deactivation of the catalyst calcined at 450 °C is due to migration of the Cu ions to inaccessible sites in the zeolite; at high calcination temperatures the Cu is tightly bound to the framework and is unable to migrate. EXAFS analysis of the sample calcined at 850 °C reveals two Cu–Si(Al) scattering paths at 2.83 Å. Doping the catalyst with other metals, in particular Ag and Pd, further improves the activity and selectivity of the reaction. The addition of water to the reaction increases the selectivity of the reaction by displacing the product from the active site.     

Catalysts at work: From integral to spatially resolved X-ray absorption spectroscopy

Spectroscopic studies on heterogeneous catalysts have mostly been done in an integral mode. However, in many cases spatial variations in catalyst structure can occur, e.g. during impregnation of pre-shaped particles, during reaction in a catalytic reactor, or in microstructured reactors as the present overview shows. Therefore, spatially resolved molecular information on a microscale is required for a comprehensive understanding of theses systems, partly in ex situ studies, partly under stationary reaction conditions and in some cases even under dynamic reaction conditions.Among the different available techniques, X-ray absorption spectroscopy (XAS) is a well-suited tool for this purpose as the different selected examples highlight. Two different techniques, scanning and full-field X-ray microscopy/tomography, are described and compared. At first, the tomographic structure of impregnated alumina pellets is presented using full-field transmission microtomography and compared to the results obtained with a scanning X-ray microbeam technique to analyse the catalyst bed inside a catalytic quartz glass reactor. On the other hand, by using XAS in scanning microtomography, the structure and the distribution of Cu(0), Cu(I), Cu(II) species in a Cu/ZnO catalyst loaded in a quartz capillary microreactor could be reconstructed quantitatively on a virtual section through the reactor. An illustrating example for spatially resolved XAS under reaction conditions is the partial oxidation of methane over noble metal-based catalysts. In order to obtain spectroscopic information on the spatial variation of the oxidation state of the catalyst inside the reactor XAS spectra were recorded by scanning with a micro-focussed beam along the catalyst bed. Alternatively, full-field transmission imaging was used to efficiently determine the distribution of the oxidation state of a catalyst inside a reactor under reaction conditions. The new technical approaches together with quantitative data analysis and an appropriate in situ catalytic experiment allowed drawing important conclusions on the reaction mechanism, and the analytical strategy might be similarly applied in other case studies. The corresponding temperature profiles and the catalytic performance were measured by means of an IR-camera and mass spectrometric analysis. In a more advanced experiment the ignition process of the partial oxidation of methane was followed in a spatiotemporal manner which demonstrates that spatially resolved spectroscopic information can even be obtained in the subsecond scale.     

Mechanism of anodic oxidation of molybdenum in nearly-neutral electrolytes studied by electrochemical impedance spectroscopy and X-ray photoelectron spectroscopy

Anodic oxidation of molybdenum in weakly acidic, nearly neutral and weakly alkaline electrolytes was studied by voltammetric and electrochemical impedance spectroscopic measurements in a wide potential and pH range. Current vs. potential curves were found to exhibit two pseudo-Tafel regions suggesting two parallel pathways of the dissolution process. Electrochemical impedance spectra indicated the presence of at least two reaction intermediates. X-ray photoelectron spectroscopic (XPS) results pointed to the formation of an oxide containing Mo(IV), Mo(V) and Mo(VI), the exact ratio between different valence states depending on potential and pH of the solution. A physico-chemical model of the processes is proposed and a set of kinetic equations for the steady-state current vs. potential curve and the impedance response are derived. The model is found to reproduce quantitatively the current vs. potential curves and impedance spectra at a range of potentials and pH and to agree qualitatively with the XPS results. Subject to further improvement, the model could serve as a starting point for the optimization of the electrochemical fabrication of functional molybdenum oxide coatings.     

X-ray absorption near edge structure and X-ray photoelectron spectroscopy studies of chloride in passive oxide films

X-ray absorption near edge structure (XANES), utilizing both electron yield and X-ray fluorescence detectors, and X-ray photoelectron spectroscopy (XPS) were used to follow chloride uptake by oxide-covered aluminum in 0.1 M NaCl solutions. The aluminum samples were polarized at selected potentials below (less positive than) the pitting potential. The electron yield XANES and XPS showed multiple peaks. The XPS chloride spectra showed two distinct sets of doublets. One doublet is related to chloride on the surface and the second is related to chloride incorporated in the oxide film. The XANES results also showed two peaks which are attributed to chloride on the surface and in the bulk of the oxide.     

Characterization of coke deposited on catalysts by carbon K-edge near edge X-ray absorption fine structure (NEXAFS) spectroscopy

C K-edge NEXAFS spectra could distinguish between the structures of coke on the catalysts used in fluid catalytic cracking, hydrotreatment and dry reforming of methane from the characteristic ;π^* features. A particular advantage of NEXAFS spectroscopy was found in the characterization of highly aromatic coke that could not be analyzed by cross-polarization ¹³C-NMR due to the lack of magnetization transfer from ¹H to ¹³C. The NEXAFS results elucidated two different structures of coke on Ni catalysts used in dry reforming of methane; one with a graphitic structure formed at lower temperatures and the other with a non-graphitic structure formed at higher temperatures. This revised version was published online in June 2006 with corrections to the Cover Date.     

Synthesis and characterisation by X-ray absorption spectroscopy of a suite of seven mesoporous catalysts containing metal ions in framework sites

MCM-41 type mesoporous silicas have been prepared in which one or more of the following elements are accommodated in framework sites: titanium, iron, chromium, vanadium, manganese, boron and aluminium. XRD and FTIR are used as aids to characterisation, which is achieved chiefly — and to a degree that arrives at valence states, bond lengths and coordination numbers of the metal ion — by X-ray absorption spectroscopy (XAS). Ti-containing MCM-41, as well as the Fe-, V- and Cr-containing variants, yield self-consistent, XAS-based, structural data of the respective metal-ion sites. Some of these (especially those containing Ti) are exceptionally good catalysts for the selective oxidation of large organic molecules such as limonene and norbornene.     

Axial variation of the oxidation state of Pt-Rh/Al2O3 during partial methane oxidation in a fixed-bed reactor: An in situ X-ray absorption spectroscopy study

Probing the structure of materials in situ is of central importance in heterogeneous catalysis. Mostly, this is done in an integral manner, that is without spatial resolution. However, at high conversion in a catalyst bed prominent concentration and/or temperature profiles may exist which can result in significant spatial variation of the catalyst structure. In the present study, X-ray absorption spectroscopy combined with on-line mass spectrometry was used to monitor the structural changes of a Pt-Rh/Al₂O₃ catalyst in a fixed-bed reactor during partial oxidation of methane. The reaction ignited at 310 °C and integral X-ray absorption spectroscopy showed that the Rh-Pt-particles were reduced at the same time. However, monitoring with a beam of 1 mm × 0.6 mm size along the axial position of the catalyst bed uncovered that Rh and Pt were still in oxidized state in the entrance region, whereas they were in reduced state in the zone at the end of the catalyst bed. The gradual transition from the reduced to the oxidized state was found to shift towards the bed entrance if the temperature was slightly increased.An erratum to this article can be found at .     

X-ray absorption fine structure spectroscopy and X-ray diffraction study of cementitious materials derived from coal combustion by-products

Cementitious materials derived from coal combustion by-products have been investigated by means of X-ray diffraction (XRD) and S and Ca K-edge X-ray absorption fine structure (XAFS) spectroscopy. The XRD analysis revealed that these materials are a complex mixture of a small amount of quartz [SiO₂] and three calcium-bearing compounds: hannebachite [CaSO₃·1/2H₂O], gypsum [CaSO₄·2H₂O] and ettringite [(Ca₆(Al(OH)₆)₂(SO₄)₃·26H₂O)]. Analysis of the S XAFS data focused on deconvolution of the X-ray absorption near-edge structure (XANES) regions of the spectra. This analysis established that sulfate and sulfite are the two major sulfur forms, with a minor thiophenic component contained in unburned carbon in the fly ash. Increasing sulfate and decreasing sulfite correlated well with increasing gypsum and ettringite and decreasing hannebachite content in the samples. Different calcium compounds were identified primarily through simple comparison of the Ca K-edge XANES and radial structure functions (RSFs) of the cementitious samples with those of reference compounds. Because of the complex coordination chemistry of calcium in these materials, it was difficult to obtain detailed local atomic environment information around calcium beyond the first CaO peak. Analysis of the extended X-ray absorption fine structure (EXAFS) and the RSF gave average CaO distances in the range 2.44-2.5 Å, with each calcium atom surrounded roughly by eight oxygen atoms. In certain samples, the average CaO distances were close to that in ettringite (2.51 Å), suggesting that these samples have higher ettringite content. The results of S and Ca K-edges XAFS and the XRD data were in reasonable agreement.     

Electrochemical kinetics and X-ray absorption spectroscopy investigations of select chalcogenide electrocatalysts for oxygen reduction reaction applications

Transition metal-based chalcogenide electrocatalysts exhibit a promising level of performance for oxygen reduction reaction applications while offering significant economic benefits over the state of the art Pt/C systems. The most active materials are based on Ru_xSe_y clusters, but the toxicity of selenium will most likely limit their embrace by the marketplace. Sulfur-based analogues do not suffer from toxicity issues, but suffer from substantially less activity and stability than their selenium brethren. The structure/property relationships that result in these properties are not understood due to ambiguities regarding the specific morphologies of Ru_xS_y-based chalcogenides. To clarify these properties, an electrochemical kinetics study was interpreted in light of extensive X-ray diffraction, scanning electron microscopy, and in situ X-ray absorption spectroscopy evaluations. The performance characteristics of ternary M_xRu_yS_z/C (M = Mo, Rh, or Re) chalcogenide electrocatalysts synthesized by the now-standard low-temperature nonaqueous (NA) route are compared to commercially available (De Nora) Rh- and Ru-based systems. Interpretation of performance differences is made in regards to bulk and surface properties of these systems. In particular, the overall trends of the measured activation energies in respect to increasing overpotential and the gross energy values can be explained in regards to these differences.     

Uptake of Se(IV/VI) oxyanions by hardened cement paste and cement minerals: An X-ray absorption spectroscopy study

The uptake of selenate (Se^VIO₄²⁻) or selenite (Se^IVO₃²⁻) by hardened cement paste (HCP) and important constituents of the cement matrix such as calcium silicate hydrate (C-S-H), portlandite (CH), ettringite (AFt) and monosulfate (AFm) was investigated using X-ray absorption spectroscopy (XAS). The XAS measurements were conducted on samples with Se loadings ranging between 1200 and 8800 ppm. X-ray absorption near edge structure (XANES) spectroscopy shows that redox reactions do not influence uptake processes in the cementitious systems. The EXAFS (extended X-ray absorption fine structure) spectra of Se(IV) and Se(VI) bound to CH, AFt, AFm and C-S-H are similar to those of SeO₄²⁻ and SeO₃²⁻ in solution, indicating a “solution-like” coordination environment upon uptake by the cement minerals. Similarly, the spectra of Se(IV)- and Se(VI)-treated HCP samples reveal the absence of backscattering atoms at short distances. These results suggest that the coordination sphere of the SeO₄²⁻ and SeO₃²⁻ entities is maintained upon immobilization by HCP and cement minerals and non-specific interactions dominate at the given Se loadings.     

Characterization of pure and sulfided NiMoO4 catalysts using synchrotron-based X-ray absorption spectroscopy (XAS) and temperature-programmed reduction (TPR)

This study aims at characterizing the properties of pure and sulfided NiMoO₄ catalysts using synchrotron-based near-edge X-ray absorption fine structure (NEXAFS) and temperature-programmed reduction (TPR). Mo L_II-edge and M_III-edge NEXAFS spectra indicate that on reaction with H₂S, the Mo component of NiMoO₄ gets partially reduced with the formation of MoS₂ type species. For the β-phase of NiMoO₄, the sulfidation of Mo is more extensive than for the α-phase, making the former a better precursor for catalysts of hydrodesulfurization (HDS) reactions. The Ni L_II-edge features are relatively insensitive to the changes accompanying the partial sulfidation of NiMoO₄. The sulfidation of the Ni component is confirmed by analysis of the Ni K-edge extended X-ray absorption fine structure (EXAFS) spectra which show the formation of Ni–S bonds (bond length ～2.48 Å) and a NiMoS_x phase. The S K-edge NEXAFS spectra show the presence of at least two types of sulfur species, one associated with a formal oxidation state of 2- and another associated with a formal oxidation state of 6+. We attribute the former to the presence of metal–sulfur bonds (MoS_x and NiS_y). The latter is associated with the formation of S–O bonds (SO ₄ ^2- ). The formation of sulfates is also supported by the O K-edge NEXAFS spectra. The partially sulfided NiMoO₄ catalysts (both α- and β-isomorphs) have a much lower thermal stability in a reducing environment than pure NiMoO₄ and MoS₂. The sulfided molybdates react with H₂ in TPR producing H₂O and H₂S at temperatures above 400 K.     

X-ray absorption near-edge structure and photoelectron spectroscopy of single-walled carbon nanotubes modified by a HBr solution

X-ray absorption near-edge structure (XANES) spectroscopy and photoelectron spectroscopy (PES) have been used to investigate single-walled carbon nanotubes (SWNTs) modified by immersion in a HBr solution at room temperature. After treatment XANES spectra of SWNTs show a new pronounced feature, which has been assigned to new bonds between the sidewall of the SWNTs and Br atoms. This investigation demonstrates the unique capabilities of the XANES spectroscopy as a tool to achieve structural and bonding information of carbon nanotubes induced by chemical processes.     

Surface analysis of emulsion adhesives by fast atom bombardment mass spectrometry and X-ray photoelectron spectroscopy

This paper reports an investigation by fast atom bombardment mass spectrometry (fabms) and X-ray photoelectron spectroscopy (xps) of the nature of surfaces of films cast from various emulsion adhesives against air and against polystyrene or polytetrafluoroethylene (ptfe) surfaces. An objective was to ascertain the extent to which the nature of the surfaces of films derived from emulsion adhesives depends upon the nature of the substrate against which the film is cast. The emulsion adhesives contained copolymers of vinyl acetate with vinyl neodecanoate (vaw), with ethylene (vae1, vae2, vae3) and with n-butyl acrylate (vaba). FABMS spectra show that the compositions of the air-dried surfaces of the films from the vavv, vae1, and vae2 emulsions are essentially those of the respective copolymers, that the surfaces of films from the vaba adhesives comprise mainly copolymer, but there is some dodecylbenzenesulphonate surfactant also present, and that the composition of the surface of films from the vae3 emulsion is dominated by potassium ions. The fabms spectra indicate no differences between the surfaces of vae3 adhesive films cast against polystyrene and ptfe on the one hand, and the surface of vae3 adhesive dried in air on the other. The fabms spectra clearly demonstrate that interfacial failure occurs between vae3 adhesive films and polystyrene and ptfe substrates when the films are separated from the substrates. XPS spectra show that the air- and polymer-cast surfaces of films from vae3 and vaba emulsions are very similar, and that the surfaces of films from the vae3 emulsions contain 5–6% of potassium together with significant levels of carboxylate moieties.     

Quantitative evaluation of sp/sp hybridization ratio in cluster-assembled carbon films by in situ near edge X-ray absorption fine structure spectroscopy

We present a near edge X-ray absorption fine structure spectroscopy characterization of nanostructured carbon films containing carbynoid species. By a careful data analysis and normalization of the spectra at the carbon K-edge we have quantitatively evaluated the extent of valence sp hybridization of the films. A sp/sp² ratio between 10% and 25% has been obtained. This result allowed the evaluation of the ratio between the sp and sp² Raman cross section at different excitation laser wavelengths.     

Study of the distribution of the molecular orientation in thick polyethylene samples by X-ray diffraction, infra-red dichroism and Raman spectroscopy

The molecular orientation in thick polyethylene samples has been studied by wide-angle X-ray diffraction, i.r. dichroism and Raman spectroscopy. The original specimens, with dimensions of the order of a centimetre, were cut to obtain 1 mm thick platelets on which the measurements were made. The mean coefficient of the second-order Legendre polynomial, P₂, was calculated from X-ray diffraction and from the 1894 cm⁻¹ i.r. band for the crystalline phase, from the 909 cm⁻¹ i.r. band for the vinyl end groups and from the 1130 and 1060 cm⁻¹ Raman bands for the all-trans C-C conformers. The fourth-order coefficient, P₄, was also determined from X-ray diffraction and Raman spectroscopy for a series of cylindrical rods of draw ratios (λ) ranging from 6 to 20. An excellent correlation is observed between the P₂ coefficients measured from different X-ray reflections and from the 1894 cm⁻¹ i.r. band. The Raman spectroscopy results show that the all-trans bonds located in the amorphous phase are aligned perpendicular to the extrusion direction for the λ = 6 rod, and gradually reorient towards the fibre axis for λ values up to 20, while the P₂ and P₄ coefficients calculated for the crystalline phase remain constant at λ ≥ 12. The variation of the orientation through the thickness of the samples was investigated for the cylindrical rods and for an H-shaped moulding produced by extrusion and rolling. Minor differences in the degree of molecular orientation were detected between the centre and the surface of the rods, whereas important variations were measured for the H-shaped sample.     

Characterization of free carbon forms in β-SiC nanopowders by temperature-programmed oxidation and Raman spectroscopy

Free carbon appears in β-SiC nanopowders as two-dimensional and ultrathin structures exposed predominantly on particle surfaces. These structures range from graphitic to lamella and amorphous, with more than one type being present at once. Their relative proportions and total percentage are quantified based on CO₂ evolution traces resulting from temperature-programmed oxidation (TPO). The TPO peak parameters are representative of the carbon nanostructure. The temperature of the peak maximum relates to the degree of graphitic order, whereas an apparent activation energy to structure homogeneity. Among the Raman parameters (for a 514-nm excitation wavelength), the area ratio of the D and G + D′ lines closely correlates with the relative proportion and structural perfection of the graphitic form. Furthermore, the area ratio of the β-SiC and carbon Raman lines is a strong function of the free carbon content in the range 0.1 to 5 wt%. The established correlations provide a guide to the consistent implementation of both techniques to characterize mixed carbon forms in β-SiC nanopowders.     

Surface oxygen species and their reactivities in the mild oxidation of ethylene on cerium oxide studied by FT-IR spectroscopy

Surface oxidation of ethylene on cerium oxide was studied at mild temperatures (293–473K) by usingin situ FT-IR spectroscopy, and isotopic technique. Ethylene oxidation took place even at around 330 K on a well outgassed cerium oxide independent of the presence of gaseous O₂. At mild temperatures, the surface adsorbed product was mainly formate species. Adsorbed Superoxide (O ₂ ^– ) species was definitely observed at temperatures up to 373 K when gaseous oxygen was present. However isotopic experiment confirmed that the Superoxide was not the active form of oxygen due to the mild oxidation of ethylene. The principal oxygen species participating in the mild oxidation of ethylene was surface lattice oxygen which is supposed to be in O^–-like form created by an outgassing at high temperatures. The mild oxidation of ethylene could be also initiated by surface peroxide (O ₂ ^2– ) and O^– species which were formed via the adsorption of O₂ on a partially reduced cerium oxide.     

Ruthenium as oxidation catalyst: bridging the pressure and material gaps between ideal and real systems in heterogeneous catalysis by applying DRIFT spectroscopy and the TAP reactor

Two supported Ru catalysts were prepared by the chemical vapor deposition of Ru₃(CO)₁₂ on MgO and SiO₂ (MOCVD). TEM, XRD, and static H₂ chemisorption measurements confirmed that the Ru particle size was about 2 nm on both supports. Using in situ DRIFT (diffuse reflectance infrared Fourier transform) spectroscopy at atmospheric pressure it was found that the adsorption of CO on the reduced samples is clearly influenced by the supports whereas the adsorption of CO on the oxidized Ru catalysts is essentially independent of the support. O₂ chemisorption measurements showed that a thin RuO₂ surface layer was formed on both catalysts under oxidizing conditions at room temperature. The observed C–O stretching frequencies were found to be in good agreement with HREELS and LEED data reported for the RuO₂(1 1 0) single crystal surface. The catalytic activity was assessed under high-vacuum conditions using the TAP (temporal analysis of products) reactor by co-feeding CO and O₂. These conditions ensured that heat and mass transfer limitations were absent. Both supported Ru catalysts were found to be highly active and stable under the CO oxidation conditions even down to room temperature. The deactivation of the catalysts observed at room temperature was reversible and independent of the support. The turnover frequencies (number of CO₂ molecules per metal surface site per second) derived from steady-state measurements are in good agreement with data reported for the RuO₂(1 1 0) single crystal surface under UHV conditions. Based on the results of the DRIFTS (diffuse reflectance infrared Fourier transform spectroscopy) and the kinetic measurements supported RuO₂ is identified as the catalytically active phase. In addition, the turnover frequencies are in good agreement with data reported for Ru/SiO₂ at atmospheric pressure. Thus, both the materials and the pressure gap were bridged successfully.     

Phase distribution and corresponding piezoelectric responses in a morphotropic phase boundary Pb(Mg1/3Nb2/3)O3-PbTiO3 single crystal revealed by confocal Raman spectroscopy and piezo-response force microscopy

Pb(Mg_1/3Nb_2/3)O₃-xPbTiO₃ (PMN-xPT) single crystals exhibit ultrahigh piezoelectric property and have already been applied in various industrial fields. Here, spatial distribution of coexistent phases and corresponding local piezoelectric responses in a morphotropic phase boundary (MPB) PMN-xPT single crystal are thoroughly investigated by confocal Raman spectroscopy coupled with piezo-response force microscopy (PFM). Different from previous studies on MPB crystals, spatial distribution of coexistent phases is exhibited intuitively in an image obtained by confocal Raman mapping. Via in-situ PFM measurements, domain morphology of each coexistent phase is observed. Moreover, local piezoelectric responses of each coexistent phase are obtained by switching spectroscopy PFM measurements, indicating that Ma and Mc phases behave like PNRs and polar matrix in PMN-xPT crystals Our work clarifies the contribution from different phases to the giant piezoelectric performance, which can deepen the understanding on the ultrahigh piezoelectricity in PMN-xPT crystals and provide the key point for developing novel high piezoelectricity materials.