Farrel W. Lytle

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.

Neutron diffraction and exafs evidence for TiO5 units in vitreous K2O · TiO2 · 2SiO2

A combination of neutron diffraction and X-ray absorption spectroscopic techniques has been used to investigate the Ti coordination in vitreous K2O· TiO2 · 2SiO2. In the former case, the Tizz.sbnd;O bond lengths have been studied using the technique of isotopic substitution. XANES (X-ray absorption near-edge structure) data indicate that there is little or no evidence for a centre of inversion symmetry at the Ti site in the glass. TiO bond distances derived from neutron diffrraction and EXAFS (extended X-ray absorption fine structure) agree to within 0.03 A and it is concluded that the Ti is present as TiO5 tetragonal pyramids containing one non-bridging oxygen atom associated witth a titanyl bond (Ti=0 = 1.65 A). The bridging TiO bond length is 1.96 A and TiTi correlations occur at 3.4 A and 6.1 A.

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.

Investigation of the structural forms of potassium in coke by electron microscopy and X-ray absorption spectroscopy

Abstract An investigation of the chemical and structural forms of potassium in coal, coke, and potassium-enriched coke has been made. The principal analytical techniques used were X-ray absorption spectroscopy, computer controlled scanning electron microscopy, and scanning transmission electron microscopy. The potassium in the bituminous coals investigated was contained in the clay mineral illite, which was transformed to a predominantly amorphous or glassy potassium aluminosilicate during coking at temperatures of 1050–1100 °C. In cokes artificially enriched in potassium and annealed at 1260 °C under argon, two principal forms of potassium were identified: potassium in K -enriched aluminosilicates and silicates, and potassium dispersed throughout the carbonized macerals and bonded to carbon. Although the aluminosilicates and quartz act as effective potassium receptors, approximately half of the added potassium was dispersed through and bonded to the carbonized macerals. Both potassium forms were predominantly amorphous in structure. Analysis of the electronic structure and local atomic environment by X-ray absorption spectroscopy showed that these amorphous phases exhibited some similarities to chemically similar crystalline phases such as leucite (KAlSi 2 O 6 ) and intercalated potassium-graphite (KC 8 ). X-ray absorption spectroscopy measurements on samples extracted from a blast furnace experiencing high alkali levels indicated that the forms of potassium in the blast-furnace coke were qualitatively similar to those in the laboratory prepared cokes, and suggested a possible mechanism for alkali attack on refractories.

Investigation of alkali and alkaline earth coal gasification catalysts by EXAFS spectroscopy

Abstract EXAFS spectroscopy and several supplementary techniques have been used to investigate a variety of coal and polymer chars containing alkali and alkaline earth catalysts, including Ca, K and Rb. In-situ EXAFS measurements were performed on Rb-loaded chars during gasification. At 450 °C in 90% N 2 -10% O 2 , the Rb XANES exhibited a doublet structure similar to that of Rb 2 O and Rb 2 CO 3 . EXAFS and other techniques show that the Ca in lignites, both naturally occurring and ion-exchanged, is essentially molecularly dispersed and bonded to carboxyl groups. Calcite-like features are induced in both the near-edge structure and radial structure functions of chars with increasing pyrolysis time and temperature. The structure of K is found to differ significantly in polymer, lignite and bituminous coal chars.

Further EXAFS examination of the state of calcium in pyrolysed char

EXAFS spectroscopy has been used to examine the state of calcium in chars newly prepared by rapid and slow pyrolysis methods from a North Dakota lignite and a Ca-loaded phenol-formaldehyde polymer. Both EXAFS and XANES regions of the spectra showed systematic changes with increasing severity of the pyrolysis treatment that related to the catalytic properties of calcium in these chars. In rapidly pyrolysed chars, the calcium state remained highly dispersed and only subtly different from its state in the unpyrolysed lignite. No spectral evidence was found for the formation of CaO or other discrete Ca compounds in these chars. For chars prepared by slow pyrolysis, however, significant fractions of the calcium had transformed to bulk CaO. This latter finding differs from our earlier EXAFS results in which CaO was not found in pyrolysed chars that had been prepared over 3 years before. This difference is attributed to hydrolysis and reaction of CaO to Ca(OH)2 and CaCO3 during storage of these aged chars.

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.

Investigation of atomic structures of calcium in ash and deposits produced during the combustion of lignite and bituminous coal

Abstract X-ray absorption fine structure spectroscopy was used to investigate the molecular structure of calcium in ash and deposit samples generated in combustion tests on a Texas lignite and an Illinois No. 6 coal. The tests were carried out in a 50 lb/hr −1 coal combustion facility. For samples extracted from the flame region and rapidly cooled, the calcium is contained in an aluminosilicate glass phase. In both waterwall and superheater deposits, the calcium tends to be predominantly in the form of CaSO 4 in the initial layers of the deposits, and in aluminosilicate glass in the outer layer. Approximate percentages of calcium in the two phases are derived by computer analysis of the near-edge spectra. A simple model for the combustion reactions of calcium is suggested on the basis of this data.

An Investigation of Uranium L-Edges of Metamict and Annealed Betafite

The uranium site in naturally occurring metamict minerals of the pyrochlore group (A/sub 1-2/B/sub 2/O/sub 6/Y/sub 0-1/) has been investigated using x-ray absorption spectroscopy (XAS). Pyrochlore structures are common phases in proposed polycrystalline waste forms. Betafite, a member of the pyrochlore group (B = 2Ti greater than or equal to Nb+Ta), exhibits U-O bond lengths of 1.94 and 2.37 A for the metamict state, and 2.03 and 2.51 A for the crystalline (annealed) state. The U-O bond lengths decrease (approx. 0.1 A) and there is a disruption in the second nearest neighbor periodicidy as material is converted from the crystalline to the metamict state.

X-Ray absorption spectroscopy of uranium at low ppm levels in coral skeletal aragonite

Abstract We performed X-ray absorption spectroscopy of uranium in three coral specimens and present results of analysis of U L-II edge XANES (X-ray absorption near edge structure) spectra. The XANES of two specimens, Pavona gigantea (collected live at Champion Island, Galapagos) and Pavona clavus (collected live from Bartolme, Galapagos) were most similar to XANES of model compounds of U6+. In contrast, the XANES of a specimen of Montastrea annularis (Pleistocene age from an exposed reef terrace on Marie Galante, Guadeloupe) showed some features typical of spectra of model compounds of U4+. These data and interpretations suggest that uranium is incorporated into coral skeletons chiefly in its oxidized U6+ form. The uranium may be subject to postmortem subaerial reduction to U4+, or may be replaced or enhanced by addition of U4+ compounds.