Katarina Norén

Rethinking Arsenate Coordination at the Surface of Goethite

A fundamental precept of geochemistry is that arsenate coordinates at mineral surfaces in a predominately bridging-bidentate fashion. We show that this is incorrect for the model system, arsenate adsorbed at the surface of goethite (alpha-FeOOH), using a combination of XRD, EXAFS, and IR spectroscopic results. We report the crystal structure of pentaamminecobalt(III) arsenate, which consists of monodentate-coordinated metal-arsenato complexes that have Co-As distances of only 3.25 A. This result implies that metal-arsenic distances are not diagnostic for the coordination mode of arsenate. We show that the K-edge EXAFS spectra of pentaamminecobalt(III) arsenate and arsenate-goethite surface complexes are strikingly similar, which suggests that arsenate could be coordinated at the goethite surface in a monodentate fashion. Refinements of the k(3)-weighted EXAFS spectra of arsenate adsorbed on goethite results in values of CN(As-Fe) between 0.8-1.1 (+/-0.7), and there is no evidence that the coordination mode of arsenate changes as a function of pH or arsenate surface coverage. We report IR spectra from the first simultaneous IR and potentiometric titration of arsenate adsorbed on deuterated goethite (alpha-FeOOD) in D(2)O, and we show for the first time the As-O stretching bands of arsenate-goethite surface complexes. We deduce that arsenate-goethite surface complexes are un-, singly, or doubly protonated, depending on pH, from a principal component analysis of the As-O stretching region and an interpretation of the Type-B OH stretching region. In summary, our cumulative results show that arsenate coordinates at the water-goethite interface in a predominately monodentate fashion. Furthermore, we find no evidence for bridging-bidentate coordination, which is a finding that impacts oxoanion bioavailability and challenges theories of mineral dissolution and surface complexation.

Scrutinizing negative thermal expansion in MOF-5 by scattering techniques and ab initio calculations

Complementary experimental techniques and ab initio calculations were used to determine the origin and nature of negative thermal expansion (NTE) in the archetype metal-organic framework MOF-5 (Zn(4)O(1,4-benzenedicarboxylate)(3)). The organic linker was probed by inelastic neutron scattering under vacuum and at a gas pressure of 175 bar to distinguish between the pressure and temperature responses of the framework motions, and the local structure of the metal centers was studied by X-ray absorption spectroscopy. Multi-temperature powder- and single-crystal X-ray and neutron diffraction was used to characterize the polymeric nature of the sample and to quantify NTE over the large temperature range 4-400 K. Ab initio calculations complement the experimental data with detailed information on vibrational motions in the framework and their correlations. A uniform and comprehensive picture of NTE in MOF-5 has been drawn, and we provide direct evidence that the main contributor to NTE is translational transverse motion of the aromatic ring, which can be dampened by applying a gas pressure to the sample. The linker motion is highly correlated rather than local in nature. The relative energies of different framework vibrations populated in MOF-5 are suggested by analysis of neutron diffraction data. We note that the lowest-energy motion is a librational motion of the aromatic ring which does not contribute to NTE. The libration is followed by transverse motion of the linker and the carboxylate group. These motions result in unit-cell contraction with increasing temperature.

Copper doped TiO2 nanoparticles characterized by X-ray absorption spectroscopy, total scattering, and powder diffraction – a benchmark structure–property study

Metal functionalized nanoparticles potentially have improved properties e.g. in catalytic applications, but their precise structures are often very challenging to determine. Here we report a structural benchmark study based on tetragonal anatase TiO2 nanoparticles containing 0-2 wt% copper. The particles were synthesized by continuous flow synthesis under supercritical water-isopropanol conditions. Size determination using synchrotron PXRD, TEM, and X-ray total scattering reveals 5-7 nm monodisperse particles. The precise dopant structure and thermal stability of the highly crystalline powders were characterized by X-ray absorption spectroscopy and multi-temperature synchrotron PXRD (300-1000 K). The combined evidence reveals that copper is present as a dopant on the particle surfaces, most likely in an amorphous oxide or hydroxide shell. UV-VIS spectroscopy shows that copper presence at concentrations higher than 0.3 wt% lowers the band gap energy. The particles are unaffected by heating to 600 K, while growth and partial transformation to rutile TiO2 occur at higher temperatures. Anisotropic unit cell behavior of anatase is observed as a consequence of the particle growth (a decreases and c increases).

The structures of T6, T3R3 and R6 bovine insulin: combining X-ray diffraction and absorption spectroscopy.

The crystal structures of three conformations, T(6), T(3)R(3) and R(6), of bovine insulin were solved at 1.40, 1.30 and 1.80 Å resolution, respectively. All conformations crystallized in space group R3. In contrast to the T(6) and T(3)R(3) structures, different conformations of the N-terminal B-chain residue PheB1 were observed in the R(6) insulin structure, resulting in an eightfold doubling of the unit-cell volume upon cooling. The zinc coordination in each conformation was studied by X-ray absorption spectroscopy (XAS), including both EXAFS and XANES. Zinc adopts a tetrahedral coordination in all R(3) sites and an octahedral coordination in T(3) sites. The coordination distances were refined from XAS with a standard deviation of <0.01 Å. In contrast to the distances determined from the medium-resolution crystal structures, the XAS results were in good agreement with similar coordination geometries found in small molecules, as well as in other high-resolution insulin structures. As the radiation dose for XRD experiments is two orders of magnitude higher compared with that of XAS experiments, the single crystals were exposed to a higher degree of radiation damage that affected the zinc coordination in the T(3) sites in particular. Furthermore, XANES spectra for the zinc sites in T(6) and R(6) insulin were successfully calculated using finite difference methods and the bond distances and angles were optimized from a quantitative XANES analysis.

An in situ sample environment reaction cell for spatially resolved x-ray absorption spectroscopy studies of powders and small structured reactors

An easy-to-use sample environment reaction cell for X-ray based in situ studies of powders and small structured samples, e.g., powder, pellet, and monolith catalysts, is described. The design of the cell allows for flexible use of appropriate X-ray transparent windows, shielding the sample from ambient conditions, such that incident X-ray energies as low as 3 keV can be used. Thus, in situ X-ray absorption spectroscopy (XAS) measurements in either transmission or fluorescence mode are facilitated. Total gas flows up to about 500 mln/min can be fed while the sample temperature is accurately controlled (at least) in the range of 25-500 °C. The gas feed is composed by a versatile gas-mixing system and the effluent gas flow composition is monitored with mass spectrometry (MS). These systems are described briefly. Results from simultaneous XAS/MS measurements during oxidation of carbon monoxide over a 4% Pt/Al2O3 powder catalyst are used to illustrate the system performance in terms of transmission XAS. Also, 2.2% Pd/Al2O3 and 2% Ag - Al2O3 powder catalysts have been used to demonstrate X-ray absorption near-edge structure (XANES) spectroscopy in fluorescence mode. Further, a 2% Pt/Al2O3 monolith catalyst was used ex situ for transmission XANES. The reaction cell opens for facile studies of structure-function relationships for model as well as realistic catalysts both in the form of powders, small pellets, and coated or extruded monoliths at near realistic conditions. The applicability of the cell for X-ray diffraction measurements is discussed.

Study of methane oxidation over alumina supported Pd–Pt catalysts using operando DRIFTS/MS and in situ XAS techniques

Graphical Abstract Abstract Methane oxidation over Pd–Pt/ model catalysts calcined at three different conditions is investigated using operando diffuse reflectance infrared Fourier transform spectroscopy and mass spectrometry, and in situ X-ray absorption spectroscopy while cycling the feed gas stoichiometry between lean (net-oxidising) and rich (net-reducing) conditions. When calcined in air, alloy Pd–Pt nanoparticles are present only on catalysts subjected to elevated temperature () whereas calcination at lower temperature (500 ) leads to segregated Pt and Pd nanoparticles on the support. Here, we show that the alloy Pd–Pt nanoparticles undergo reversible changes in surface structure and composition during transient methane oxidation exposing a PdO surface during lean conditions and a metallic Pd–Pt surface (Pd enriched) under rich conditions. Alloyed particles seem more active for methane oxidation than their monometallic counterparts and, furthermore, an increased activity for methane oxidation is clearly observed under lean conditions when PdO has developed on the surface, analogous to monometallic Pd catalysts. Upon introducing rich conditions, partial oxidation of methane dominates over total oxidation forming adsorbed carbonyls on the noble metal particles. The carbonyl spectra for the three samples show clear differences originating from different surfaces exposed by alloyed vs. non-alloyed particles. The kinetics of the noble metal oxidation and reduction processes as well as carbonyl formation during transient methane oxidation are discussed.

The BALDER Beamline at the MAX IV Laboratory

X-ray absorption spectroscopy (XAS) includes well-established methods to study the local structure around the absorbing element - extended X-ray absorption fine structure (EXAFS), and the effective oxidation number or to quantitatively determine the speciation of an element in a complex matrix - X-ray absorption near-edge structure (XANES). The increased brilliance and intensities available at the new generation of synchrotron light sources makes it possible to study, in-situ and in-operando, much more dilute systems with relevance for natural systems, as well as the micro-scale variability and dynamics of chemical reactions on the millisecond time-scale. The design of the BALDER beamline at the MAX IV Laboratory 3 GeV ring has focused on a high flux of photons in a wide energy range, 2.4-40 keV, where the K-edge is covered for the elements S to La, and the L 3-edge for all elements heavier than Sb. The overall design of the beamline will allow large flexibility in energy range, beam size and data collection time. The other focus of the beamline design is the possibility to perform multi-technique analyses on samples. Development of sample environment requires focus on implementation of auxiliary methods in such a way that techniques like Fourier transform infrared (FTIR) spectroscopy, UV-Raman spectroscopy, X-ray diffraction and/or mass spectrometry can be performed simultaneously as the XAS study. It will be a flexible system where different instruments can be plugged in and out depending on the needs for the particular investigation. Many research areas will benefit from the properties of the wiggler based light source and the capabilities to perform in-situ and in-operando measurements, for example environmental and geochemical sciences, nuclear chemistry, catalysis, materials sciences, and cultural heritage.

High performance emission spectrometer at Balder/MAX IV beamline

The emission spectrometer at Balder/MAX IV beamline is presented. Its unique features are described. Comparison is given with other types of curved crystals analyzers.