X-ray Absorption Spectroscopy Investigation Research Articles

Mitigating Phosphate Anion Poisoning of Cathodic Pt/C Catalysts in Phosphoric Acid Fuel Cells

The effect of phosphate anion poisoning of cathodic Pt-based catalysts in phosphoric acid fuel cells was investigated by observing the kinetics of the oxygen reduction reaction (ORR) in the presence of varied concentrations of H3PO4. In an attempt to reduce phosphate anion poisoning, a Pt-based Ni alloy catalyst was synthesized. Detailed electrochemical and X-ray absorption spectroscopy investigations have been carried out on both Pt and PtNi electrocatalysts under in situ conditions. It was found that the phosphate poisoning effect on the ORR is much less severe on PtNi/C than on Pt/C. The Δμ X-ray absorption near-edge spectroscopy analysis suggests that the adsorbed phosphate anions actually remain to higher potential on PtNi than on Pt, and this inhibits OH adsorption from water activation and the conversion from OH to O, thus allowing more sites for the ORR. Due to the presence of phosphate anions on the atop sites of PtNi/C, OH cannot occupy such sites now either due to steric hindrance or just lower...

Variation in the iron oxidation states of magnetite nanocrystals as a function of crystallite size: The impact on electrochemical capacity

We have investigated magnetite (Fe3O4) as an electroactive battery electrode material, where a linear relationship was observed between Fe3O4 crystallite size and capacity, with a negative slope. In order to better understand this novel relationship, we report here the Rietveld refinement and X-ray absorption spectroscopy (XAS) investigation of nanosized Fe3O4 as a function of crystallite size (7–26nm). Rietveld refinement established that the Fe3O4 samples were phase pure, while the extended X-ray absorption fine structure (EXAFS) and X-ray absorption near edge structure (XANES) provided insight into the local geometries and electronic structure of the iron centers, including oxidation state assignment. From our current and recent studies, we suggest that the surface of the Fe3O4 crystallites is rich in Fe3+, thus as the Fe3O4 crystallite size decreases, the electrochemical capacity increases, due to a net enrichment of Fe3O4 in Fe3+.

Electrochemical and XAS investigation of oxygen reduction reaction on Pt-TiO2-C catalysts

Pt-TiO2-C composites with different titanium oxide loading were synthesized by photo-deposition and chemical vapor deposition methods. The changes in their electronic properties improve the electrochemical activity toward the oxygen reduction reaction (ORR) compared to the Pt-C catalyst synthesized at the same conditions. The platinum samples were physically characterized by means of Transmission Electron Microscopy (TEM), Small Angle X-ray Scattering (SAXS), X-ray Absorption Spectroscopy (XAS) and X-ray Photo-electron Spectroscopy (XPS). Their electrochemical activity was also investigated by cyclic and linear voltammetry techniques. TEM analysis shows homogeneously dispersed platinum nanoparticles with an average particle size of 2 nm in all the synthesized samples. Form factor (morphology model) and particle size were determined by SAXS, the data adjusted to spherical Pt nanoparticles in both synthesis methods. XAS studies at the Pt L3-edge shows a close interaction of Pt with the support material, i.e. C or TiO2. XPS analysis reveals surface modifications that induce electronic changes on Pt-TiO2-C. Significant differences in the ORR electrochemical activity were correlated to the TiO2 loading and the synthesis procedure.

In-Situ XAS Investigation of the Effect of Electrochemical Reactions on the Structure of Graphene in Aqueous Electrolytes

In-situ X-ray Absorption Spectroscopy (XAS), Raman Spectroscopy, AFM and XPS have been used to investigate the effect of reactions occurring in aqueous electrolytes on the structure of a single-layer graphene produced by CVD. It was found that defects are readily and irreversibly produced by application of electrode voltages. The defects and the products were identified also by new features in the XAS spectra. Our findings show the poor stability of the CVD graphene, which could be a challenge in applications such as super-capacitors, fuel-cells, batteries and photo-catalysis.

Selenide Retention by Mackinawite

The isotope (79)Se may be of great concern with regard to the safe disposal of nuclear wastes in deep geological repositories due to its long half-life and potential mobility in the geosphere. The Se mobility is controlled by the oxidation state: the oxidized species (Se(IV)) and (Se(VI)) are highly mobile, whereas the reduced species (Se(0) and Se(-II)) form low soluble solids. The mobility of this trace pollutant can be greatly reduced by interacting with the various barriers of the repository. Numerous studies report on the oxidized species retention by mineral phases, but only very scarce studies report on the selenide (Se(-II)) retention. In the present study, the selenide retention by coprecipitation with and by adsorption on mackinawite (FeS) was investigated. XRD and SEM analyses of the samples reveal no significant influence of Se on the mackinawite precipitate morphology and structure. Samples from coprecipitation and from adsorption are characterized at the molecular scale by a multi-edge X-ray absorption spectroscopy (XAS) investigation. In the coprecipitation experiment, all elements (S, Fe, and Se) are in a low ionic oxidation state and the EXAFS data strongly point to selenium located in a mackinawite-like sulfide environment. By contacting selenide ions with FeS in suspension, part of Se is located in an environment similar to that found in the coprecipitation experiment. The explanation is a dynamical dissolution-recrystallization mechanism of the highly reactive mackinawite. This is the first experimental study to report on selenide incorporation in iron monosulfide by a multi-edge XAS approach.

Ni speciation in a New Caledonian lateritic regolith: A quantitative X-ray absorption spectroscopy investigation

Changes in Ni speciation in a 64m vertical profile of a New Caledonian saprolitic–lateritic regolith developed over ultramafic rocks under tropical weathering conditions were investigated by EXAFS spectroscopy. Quantitative analysis of the EXAFS spectra by linear combination-least squares fitting (LC-LSF) using a large set of model compound spectra showed that Ni hosted in primary silicate minerals (olivine and serpentine) in the bedrock is incorporated in secondary phyllosilicates (serpentine) and Fe-oxides (goethite) in the saprolite unit and mainly in goethite in the laterite unit. A significant concentration of Ni (up to 30% of total Ni) is also hosted by Mn-oxides in the transition laterite (i.e. the lowest part of the laterite unit which contains large amounts of Mn-oxides). However, the amount of Ni associated with Mn-oxides does not exceed 20% of the total Ni in the overlying laterite unit. This sequence of Ni species from bedrock to laterite yields information about the behavior of Ni during tropical weathering of ultramafic rocks. The different Ni distributions in phyllosilicates in the bedrock (randomly distributed) and in the saprolite unit (clustered) indicate two generations of Ni-bearing phyllosilicates. The first, which formed at higher temperature, is related to serpentinization of oceanic crust, whereas the second one, which formed at lower temperature, is associated with post-obduction weathering of ultramafic rocks. In addition, the observed decrease in the proportion of Ni hosted by Mn-oxides from the transition laterite to the upper lateritic horizons indicates dissolution of Mn-oxides during the last stages of differentiation of the lateritic regolith (i.e. lateritization). Finally, the ubiquitous occurrence of Ni-bearing goethite emphasizes the major role of this phase in Ni speciation at the different weathering stages and suggests that goethite represents the major host for Ni in the final tropical weathering stages of New Caledonian ultramafic rocks.

In Situ Electrochemical X-ray Absorption Spectroscopy of Oxygen Reduction Electrocatalysis with High Oxygen Flux

An in situ electrochemical X-ray absorption spectroscopy (XAS) cell has been fabricated that enables high oxygen flux to the working electrode by utilizing a thin poly(dimethylsiloxane) (PDMS) window. This cell design enables in situ XAS investigations of the oxygen reduction reaction (ORR) at high operating current densities greater than 1 mA in an oxygen-purged environment. When the cell was used to study the ORR for a Pt on carbon electrocatalyst, the data revealed a progressive evolution of the electronic structure of the metal clusters that is both potential-dependent and strongly current-dependent. The trends establish a direct correlation to d-state occupancies that directly tracks the character of the Pt-O bonding present.

Nickel distribution and speciation in rapidly dehydroxylated goethite in oxide-type lateritic nickel ores: XAS and TEM spectroscopic (EELS and EFTEM) investigation

The distribution of Ni in four lateritic Ni-goethites that were rapidly dehydroxylated to form hematite by shock heating at 340/400°C and 800°C for 30 min was investigated using synchrotron X-ray diffraction (SXRD), TEM spectroscopy (EELS and EFTEM) and synchrotron X-ray absorption spectroscopy (XAS). The Ni K-edge EXAFS results for non-heated samples showed three distinct Ni–Fe shells, including two edge-sharing (R Ni–Fe ∼ 3.01 and R Ni–Fe ∼ 3.22 Å) and a double corner-sharing (R Ni–Fe ∼ 3.52 Å) complex for most of the samples. These interatomic distances are indicative of Ni substituting for Fe in goethite, which has resulted in an expansion in the goethite structure along the a-axis direction and a contraction along the b-axis direction. Ravensthorpe Ni goethite was considerably different from the other goethites, with two Ni–O interatomic lengths (R Ni–O ∼ 2.04 and 2.46 Å), an edge-sharing (R Ni–Fe ∼3.04 Å) and a corner-sharing (R Ni–Fe ∼ 3.56 Å) complex. The R Ni–O ∼ 2.46 Å bond length is not indicative of Ni substituting for Fe in goethite, nor is it associated with single or multiple scattering events in NiO, Ni(OH)2 or Ni substituting for Fe in Fe oxides. The corresponding Ni K-edge EXAFS results for 340/400°C and 800°C heated samples (i.e. hematite) were very similar. Four distinct metal neighbours correspond to Fe/Ni in face-sharing (R Ni–Fe ∼ 2.87–2.91 Å) and three different corner-sharing complexes (R Ni–Fe ∼ 3.37– 3.41 Å, R Ni–Fe ∼ 3.62– 3.64 Å and R Ni–Fe ∼ 3.92– 4.09 Å) represent Ni substituting for Fe in hematite. The fourth shell is indicative of an inner sphere surface complex. EFTEM maps for Ni in goethite are consistent with the formation of a surface complex as they provide evidence for clustering of Ni on the surface of neoformed hematite crystals. There was no evidence from EXAFS or SXRD supporting the formation of discrete Ni phases (e.g. NiO) as a result of shock heating. Therefore, for Ni-goethites subjected to shock heating at 800°C (i.e. high-temperature dehydroxylation to hematite), most of the Ni is retained in the structures of the neoformed hematites, whereas some of the Ni migrates to the surface of the neoformed hematite where it forms a surface complex. During acid dissolution (e.g. heap leaching) of oxide-type lateritic Ni ores, Ni on the hematite surface is more accessible to acid solutions; therefore, these results may provide a basis for more efficient extraction methods for Ni in oxide-type lateritic Ni ores, as well as providing information on the possible redistribution of Ni in heated goethite-rich soils.

X-ray Absorption and Micro X-ray Fluorescence Spectroscopy Investigation of Copper and Zinc Speciation in Biosolids

Despite its pivotal role in determining the risks and time frames associated with contaminant release, metal speciation remains a poorly understood aspect of biosolids chemistry. The work reported here used synchrotron-based spectroscopy techniques to investigate the speciation of copper and zinc in a range of Australian biosolids. High resolution element mapping of biosolids samples using micro X-ray fluorescence spectroscopy revealed considerable heterogeneity in key element associations, and a combination of both organic and inorganic copper and zinc binding environments. Linear combination fitting of K-edge X-ray absorption spectra indicated consistent differences in metal speciation between freshly produced and stockpiled biosolids. While sulfide minerals play a dominant role in metal binding in freshly dewatered biosolids, they are of lesser importance in dried biosolids that have been stockpiled. A degree of metal binding with iron oxide minerals was apparent but the results did not support the hypothesis that biosolids metals are chiefly associated with iron minerals. This work has potential implications for the long-term stability of metals in biosolids and their eventual fate following land application.

Optimizing the synthesis of cobalt-based catalysts for the selective growth of multiwalled carbon nanotubes under industrially relevant conditions

An industrially applicable cobalt-based catalyst was optimized for the production of multiwalled carbon nanotubes (CNTs) from ethene in a hot-wall reactor. A series of highly active Co–Mn–Al–Mg spinel-type oxides with systematically varied Co:Mn ratios was synthesized by precipitation and calcined at different temperatures. The addition of Mn drastically enhanced the catalytic activity of the Co nanoparticles resulting in an extraordinarily high CNT yield of up to 249 g CNT/g cat. All quaternary catalysts possessed an excellent selectivity towards the growth of CNTs. The detailed characterization of the obtained CNTs by electron microscopy, Raman spectroscopy and thermogravimetry demonstrated that a higher Mn content results in a narrower CNT diameter distribution, while the morphology of the CNTs and their oxidation resistance remains rather similar. The temperature-programmed reduction of the calcined precursors as well as in situ X-ray absorption spectroscopy investigations during the growth revealed that the remarkable promoting effect of the Mn is due to the presence of monovalent Mn(II) oxide in the working catalyst, which enhances the catalytic activity of the metallic Co nanoparticles by strong metal-oxide interactions. The observed correlations between the added Mn promotor and the catalytic performance are of high relevance for the production of CNTs on an industrial scale.

X-ray Absorption Spectroscopy Investigation on Activity and Durability of De-Alloyed Pt3Co Cathodic Catalysts

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X-ray Absorption Spectroscopy Investigation on High Activity Dealloyed PtCo3 Cathodic Catalysts

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Space-Resolved, in Operando X-ray Absorption Spectroscopy Investigations on both the Anode and Cathode in a DMFC

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X-ray absorption and emission spectroscopic investigation of Mn doped ZnO films

The electronic structure of (Zn,Mn)O films with different Mn concentrations has been investigated by element-selective soft X-ray absorption and emission spectroscopy. The band gap narrowing of (Zn,Mn)O with increase of Mn concentration (<20% Mn) is attributed to the Mn doping and sp– d exchange interactions. According to analysis of the O Kα and resonant Mn L 2,3 X-ray emission spectra, the splitting of Mn 3 d subbands is related to Mn-derived states. It indicates that ferromagnetic coupling in (Zn,Mn)O can be taken into account to be carrier-induced. The presence of antiferromagnetism in the heavier Mn-doped films can be explained in terms of the existence of MnO secondary phases.

EXAFS−XANES Evidence of in Situ Cesium Reduction in Cs−Ru/C Catalysts for Ammonia Synthesis

We present here a X-ray absorption spectroscopy (XAS) investigation on the local chemical order and electronic structure of Cs and Ba, promoters of the Ru/C catalysts for ammonia synthesis that attracted interest because of highly increased productivity. The role of the promoters is still largely unclear, although indirect evidence for Cs partial reduction has been obtained by this and other groups. Our XAS analysis with in situ H(2) reduction directly supports the partial Cs reduction in the promoted Ru/C catalysts, depending on the presence of Ru and on the graphitization degree of the support. Higher coordination of Ba was observed with respect to Cs in the reduced samples, without evidence of heavy atoms (Ru, Cs, and Ba) in the surroundings. Because of the strong electropositive nature of Cs, direct experimental evidence of its partial reduction is of outstanding significance also for other applications.

Modified Naples yellow in Renaissance majolica: study of Pb–Sb–Zn and Pb–Sb–Fe ternary pyroantimonates by X-ray absorption spectroscopy

X-Ray absorption spectroscopy investigations combined with ab initio structural simulations of ternary forms (Pb–Sb–Zn and Pb–Sb–Fe) of the pigment Naples yellow (Pb2Sb2O7) allowed to achieve a detailed insight into the incorporation route of Zn and Fe ions inside the pigment lattice. The study of newly synthesised yellow pyroantimonates provided direct evidence that Zn and Fe ions enter the pyrochlore octahedral sites replacing Sb ions. Experimental results were discussed considering, also, the information provided by Raman spectroscopy used in this study as a sensitive probe of the pyrochlore lattice distortions in modified yellow pyroantimonates. Finally, by exploiting the non-invasiveness of the X-ray absorption spectroscopy technique, the same structural arrangement has been unambiguously observed for Zn cations in a modified Naples yellow pigment of a Renaissance ceramic shard.

Soft X-ray absorption spectroscopy investigations of 1111 and 122 iron pnictides

Abstract Soft X-ray absorption and emission spectroscopy was used to investigate the electronic structure of BaFe 2 As 2 and oxygen-vacated PrFeAsO iron pnictides. The effect of carrier doping on the conduction band was investigated. Iron–arsenic hybridized spectral features reveal that Fe–As bonding is involved in the process of electron addition near the Fermi-level for PrFeAsO. X-ray spectra reveals an increase in the Fe density of states at the PrFeAsO Fermi-level as the doping decreases. This doping dependent electronic behavior indicates the possibility of a magnetic instability in undoped PrFeAsO.

XAS and DFT Investigation of Mononuclear Cobalt(III) Peroxo Complexes: Electronic Control of the Geometric Structure in CoO2 versus NiO2 Systems

The geometric and electronic structures of two mononuclear [(L)CoO2](+) complexes, [(12-TMC)CoO2](ClO4) (1) and [(14-TMC)CoO2](ClO4) (2), have been evaluated using Co K-edge X-ray absorption spectroscopy (XAS) and extended X-ray absorption fine structure (EXAFS) and correlated with density functional theory (DFT) calculations to evaluate the differences in the geometric and electronic structures due to changes in the TMC chelate ring size. Co K-edge XAS shows that both 1 and 2 are Co(III) species. Co K-edge EXAFS data show that both 1 and 2 are side-on O2-bound cobalt(III) peroxide complexes. A combination of EXAFS and DFT calculations reveals that while the constrained 12-TMC ring in 1 allows for side-on O2 binding to the Co center with ease, the 14-TMC chelate in 2 has to undergo significant distortion of the ring to overcome steric hindrance posed by the four cis-methyl groups of the chelate to allow side-on O2 binding to the Co center. The Ni analogue of 2, [(14-TMC)NiO2](+), has been shown to form an end-on-bound nickel(II) superoxide species. The electronic and geometric factors that determine the different electronic structures of 2 and [(14-TMC)NiO2](+) are evaluated using DFT calculations. The results show that while the sterics of the cis-14-TMC chelate contribute to the geometry of O2 binding and result in an end-on-bound Ni(II)O2(-) complex in [(14-TMC)NiO2](+), the higher thermodynamic driving force for oxidation of Co(II) overcomes this steric constraint, resulting in stabilization of a side-on-bound Co(III)O2(2-) electronic structure in 2.

Electronic and magnetic properties of the graphene–ferromagnet interface

This paper presents our work on the investigation of the surface structure and the electronic and magnetic properties of the graphene layer on the lattice-matched surface of a ferromagnetic material, Ni(111). Scanning tunneling microscopy imaging shows that perfectly ordered epitaxial graphene layers can be prepared by elevated temperature decomposition of hydrocarbons, with domains larger than the terraces of the underlying Ni(111). In some exceptional cases, graphene films do not show rotational alignment with the metal surface, leading to moiré structures with small periodicities. We discuss the crystallographic structure of graphene with respect to the Ni(111) surface relying both on experimental results and on recent theoretical studies. X-ray absorption spectroscopy investigations of empty valence-band states demonstrate the existence of interface states, which originate from the strong hybridization between the graphene π and Ni 3d valence-band states with the partial charge transfer of the spin-polarized electrons to the graphene π* unoccupied states. The latter leads to the appearance of an induced magnetic moment of carbon atoms in the graphene layer, which is unambiguously confirmed by both x-ray magnetic circular dichroism and spin-resolved photoemission. Further angle-resolved photoemission investigations indicate a strong interaction between graphene and Ni(111), showing considerable modification of the valence-band states of Ni and graphene due to strong hybridization. A detailed analysis of the Fermi surface of the graphene/Ni(111) system shows very good agreement between experimental and calculated two-dimensional maps of the electronic states around the Fermi level, supporting the idea of spin-filtering. We analyze our spectroscopic results relying on the currently available band structure calculations for the graphene/Ni(111) system and discuss the perspectives of the realization of graphene/ferromagnet-based devices.

EXAFS as Powerful Analytical Tool for the Investigation of Organic–Inorganic Hybrid Materials

AbstractThe potential and application of X‐ray absorption spectroscopy (XAS) for structural investigations of organic–inorganic hybrid materials, with a special emphasis on systems consisting of inorganic building blocks (clusters) embedded into polymer backbones, is extensively reviewed. In the first part of the paper, the main features of organic–inorganic hybrid materials, their classification, the synthetic approaches for their preparation, and their applications are concisely presented, whereas the particular issues related to their characterization are discussed in more detail. In the second section of the paper, the principles and the theoretical background of the XAS method, including experimental design, data reduction, evaluation, analysis, and interpretation are described and discussed. Examples of potentialities of the method for the short‐range structural investigation of inorganic nanostructures in hybrids are provided, and the state‐of‐the‐art in the field of hybrid materials is reviewed. In the third part, six different case studies belonging to our past and present experience in this field are presented and discussed, with a particular focus on their XAS investigation.