Donald R. Sandstrom

Investigation of TiO2SiO2 glasses by X-ray absorption spectroscopy☆

The coordination and nearest-neighbor bond distances of Ti in a series of TiO2SiO2 glasses have been quantitatively determined using a combination of XANES and EXAFS measurements about the Ti K-edge at 4966 eV. The glasses covering the range 0.012 to 14.7 wt% TiO2 were prepared by flame hydrolysis of predetermined mixtures of SiCl4 and TiCl4 vapors. At TiO2 concentrations below ∼0.05 wt%, Ti is found in a rutile-like octahedral coordination. With increased TiO2 content in the glass, a two-site model applies, in which Ti is found predominately in a fourfold site. About ∼9 wt% TiO2, t6he sixfold/fourfold ratio increases appreciably and eventually at ∼15 wt% TiO2, crystalline TiO2 segregates out as a second phase. The average TiOSi bond angle in these glasses was estimated to be ∼159 ° which is slightly larger than the most probable value of 152 degrees for SiOSi in vitreous SiO2. Within the accuracy of the edge shift measurements all Ti in the glass is in 4+ valence. Finally, various physical properties such as density, optical transparency and thermal expansion are correlated in light of the new structural data for this interesting binary silicate glass system.

Coordination of Ti in TiO2SiO2 glass by X-ray absorption spectroscopy☆

Abstract Evidence from both the extended X-ray absorption fine structure (EXAFS) and near-edge regions of the Ti K-edge X-ray absorption spectrum for TiO2SiO2 glasses has been examined to determine the coordination geometry of the Ti4+ ion. It is concluded that this geometry is primarily a four-fold coordination, with a small fraction of the Ti4+ ions having six-fold coordination and a longer TiO bond length. The ratio of six-fold to four-fold coordination is seen to increase with increased TiO2 content in the glass.

Structural genesis of Pt on SiO2: determination by X-ray absorption spectroscopy

Abstract Extended X-ray absorption fine structure (EXAFS) and X-ray absorption near-edge structure (XANES) were used to structurally characterize in situ a 1% Pt on Cab-O-Sil catalyst. We report data and structural parameters determined for a wide range of catalyst conditions varying from the dried, impregnated support to the sintered catalyst. Most of the work is on the active, clean catalyst and with chemisorbed O 2 and benzene. Specific new results include (1) evaluation of support interaction by measuring the PtO bonds to the support and by relating this to a residual, temperature-invariant static disorder, (2) the effect of chemisorbed O 2 or benzene, and (3) discovery of a temperature-driven change in the structure of the catalyst cluster believed to be due to making and breaking PtO bonds to the support. The effect was present either in H 2 or He. Sequential Fourier filtering in the data analysis with appropriately constructed phase shifts was used to uncover the minority-species-to-Pt bonds from the PtPt bonds which predominate in the data.

Determination of thermal amplitude of surface atoms in a supported Pt catalyst by EXAFS spectroscopy

A temperature sequence of EXAFS measurements was carried out on a highly dispersed Pt/SiO2 catalyst to determine the temperature dependence of atomic structure in the very small supported clusters. A one‐shell EXAFS model was fit to the first neighbor oscillations in the catalyst data to determine bond lengths and relative thermal motion (disorder) for an average cluster environment. A Debye model of disorder was employed in the EXAFS analysis. The disorder measured for the catalyst was determined to be 1.3 to 2 times the value determined for bulk Pt over all temperatures. Asymmetry of the radial distributions of nearest neighbors in the largely surface coordinated clusters and the increased atomic disorder lead to underestimates of the nearest‐neighbor distance R1 at higher temperatures. The apparent bond distance contraction with temperature is manifest in the data as a retardation in the phase of the first neighbor oscillations. It is demonstrated that model EXAFS functions employing asymmetric interat...

Ni2+ coordination in aqueous NiCl2 solutions: Study of the extended x‐ray absorption fine structure

Extended x‐ray absorption fine structure measurements are reported for aqueous NiCl2 solutions of molar concentration 2.78 and 3.74. Analysis of the Fourier filtered spectra by a multiparameter fitting procedure reveals a first coordination sphere consisting of six water molecules with an Ni–O bond distance of 2.07 A, and no first sphere Ni–Cl coordination. Measurement of an outer coordination sphere consisting of both Cl− ions and water molecules is also reported. The relationship of these results to previously postulated ordered structures in these solutions is discussed.

Study of Ni(II) coordination in aqueous solution by EXAFS analysis

Extended x‐ray absorption fine structure (EXAFS) spectroscopy has been applied to the study of water coordination of the Ni(II) ion in aqueous solution. High resolution x‐ray spectra were measured for Ni(NO3) ⋅6H2O salt and its 0.1M aqueous solution using a synchrotron radiation source. Analysis of the spectra by Fourier transform reveals first sphere coordination in agreement with x‐ray diffraction results. In addition, a second coordination sphere is detected and a tentative distance from the Ni(II) ion to the second sphere water molecules of 3.2±0.1 A is measured.

Electron‐Bombardment Desorption: Carbon Monoxide Adsorbed on Tungsten

Experiments are described in which the surface properties of an electron‐bombarded tungsten (211) surface are studied by the technique of surface mass spectrometry. In most of the work the tungsten ribbon was in a controlled carbon monoxide atmosphere at a pressure between 10−9 and 10−6 torr. Electrons bombarding this surface release an O+ ion current which has two distinctly separate components coming from different molecular sorption states. The smaller of these components has been identified as direct desorption from the α‐phase carbon monoxide. The other was observed to depend in a complex manner upon both the sorption of gas on the surface and the prior history of electron bombardment. These results can only be explained fully by assuming that the electron bombardment produces an intermediate adsorbed state having a large cross section for O+ ion desorption under subsequent electron bombardment. This so‐called “intermediate” state is stable always building up to the same equilibrium density under pro...

Study of Ti in solvent-refined coal by X-ray absorption spectroscopy

Abstract The chemical environment of titanium in solvent-refined coal (SRC-I and SRC-II) has been studied using X-ray absorption spectroscopy. Evidence that organic complexes of Ti are produced in the liquefaction process from mineral forms of Ti present in the feed coal is presented, based on the pre-edge structure of the X-ray absorption spectra.

Structural Evidence for Solutions from EXAFS Measurements

Physical evidence for the structure of solutions has been obtained by x-ray scattering,1,2,3,4 neutron scattering,5,6,7,8 Raman scattering,9,10 transport measurements,11 and NMR12 techniques. In addition, extended x-ray absorption fine structure spectroscopy (EXAFS spectroscopy) has been applied to this problem. It is the sensitivity to local structure that makes EXAFS especially suitable for systems like this, for which no long range order is expected. Also, the element specificity of EXAFS means that the radial distribution functions deduced from EXAFS analysis contain only the relationship between atoms of the x-ray absorbing element and its neighbors. In contrast, neutron and x-ray scattering methods result in an average correlation function for the sample as a whole, unless special techniques such as isotopic substitution in the scattering of neutrons6 or anomalous scattering of x-rays13 are employed to distinguish the correlations between specific pairs of elements.

Multiple scattering effects in EXAFS: Comparison between theory and experiment for Pt metal

Extended x‐ray absorption fine structure (EXAFS) model functions have been calculated for the LIII edge in Pt metal. All single, double, and triple scattering contributions were taken into account, using a recently developed multiple scattering formalism. Theoretical values for the scattering amplitude, phase, and Debye–Waller factor and the ‘‘universal curve’’ for the electron mean free path have been used. Comparison to experimental data gives an estimate of the limitations in the current formalism, and shows that multiple scattering effects are important in Pt only at radial distances corresponding to fourth shell nearest neighbors.