ANSTO :: Browsing by Author "Luca, V"

Browsing by Author "Luca, V"

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Aerosol-assisted production of mesoporous titania microspheres with enhanced photocatalytic activity: the basis of an improved process
(American Chemical Society, 2010-06) Araujo, PZ; Luca, V; Bozzano, PB; Bianchi, HL; Arturo Soler-Illia, GJDA; Blesa, MA
An aerosol-based process was used to prepare mesoporous TiO2 microspheres (MTM) with an average diameter in the range of 0.5−1 μm. The structural characteristics and photocatalytic properties of the synthesized materials were determined. As-prepared MTM materials and those heated in air from 400 to 600°C exhibited mesoporous texture with a narrow size distribution and an inorganic framework that consisted of 4−13 nm anatase crystallites. Pore volumes for the MTM materials were in the range of 0.17−0.34 cm3 g−1. Microspheres heated to 400°C presented a locally ordered mesopore structure and possessed X-ray diffraction d spacings between 9.8 and 17.3 nm. Heating above 400°C resulted in a loss of the mesoscopic order, a decrease of the surface area, retention of the porosity, and an increase of the anatase nanoparticle size to 13 nm. The accessibility of the pore volume was measured by monitoring the uptake of gallic acid (GA) using Fourier transform IR. The MTM materials made excellent catalysts for the photodegradation of GA, with the performance being higher than that of an equivalent sample of Degussa P25. The present MTM materials are advantageous in terms of their ease of separation from the aqueous phase, and hence a novel photocatalytic process is proposed based on separate adsorption and photocatalytic decomposition steps with an improved and more rational use of both catalyst and sunlight. © 2010, American Chemical Society
Anodically synthesized titania films for lithium batteries: effect of titanium substrate and surface treatment
(Elsevier, 2009-05-01) Lindsay, MJ; Skyllas-Kazacos, M; Luca, V
A number of titania films have been produced through anodising high purity titanium from different suppliers in either the as-received state or following polishing and etching. Anodising was carried out galvanostatically for a period of 10 min in 0.2 M H2SO4. The performance of the films was then evaluated as potential anode materials for lithium batteries. Raman spectroscopy showed these films had spectra characteristic of anatase with the presence of some rutile whilst the spectra of the lithiated state was characteristic of the orthorhombic phase of LixTiO2. The surface condition in particular was found to have an effect on the electrochemical performance and properties of the films most notably on capacity fade. Whilst the electrodes produced from as-received titanium demonstrated stable cycle capacities after the initial few cycles, those on polished and etched substrates faded over 50 cycles. The best performing films offered a capacity of at least 48 μAh cm−2 over the 50 cycles. All the electrodes examined however did show signs of the film having being damaged as a result of electrochemical cycling. With the wide range of anodising parameters that can be altered there is considerable scope for optimising the electrochemical performance of films produced through such a technique. © 2009, Elsevier Ltd.
Anomalous lattice parameter increase in alkali earth aluminium substituted tungsten defect pyrochlores
(Elsevier, 2009-03) Thorogood, GJ; Kennedy, BJ; Peterson, VK; Elcombe, MM; Kearley, GJ; Hanna, JV; Luca, V
The structures of the defect pyrochlores AAl(0.33)W(1.67)O(6) where A = K, Rb or Cs have been investigated using X-ray and neutron powder diffraction methods as well as the ab initio modelling program VASP The three cubic pyrochlores exhibit a non-linear increase in lattice parameter with respect to ionic radius of the A cation as a consequence of displacive disorder of the A-type cations. Solid state Al-27 MAS NMR studies of this pyrochlore system reveal shifts in the delta-21-22 ppm range that are indicative of pseudo-5 coordinate Al environments and emanate frorn distorted Al octahedral with one abnormally long Al. O bond. Solid state K-39, Rb-85, Rb-87 and Cs-133 MAS and static NMR Studies reflect the local cation disorder demonstrated in the structural Studies. © 2008, Elsevier Ltd.
Cation disorder in NaW2O6+δ·nH2−zO post-ion exchange with K, Rb, Sr, and Cs
(Pergamon-Elsevier Science Ltd, 2011-06-01) Thorogood, GJ; Kennedy, BJ; Avdeev, M; Peterson, VK; Hanna, JV; Luca, V
The structure of the defect pyrochlore NaW2O6+delta center dot nH(2-z)O after ion exchange with K, Rb, Sr or Cs for Na has been investigated using thermal analysis, solid-state nuclear magnetic resonance, laboratory X-ray and neutron diffraction methods. Neutron diffraction studies show that both the A-type cations (Na+, K+, Rb+, and/or Cs+) and the water molecules reside within the channels that form in the 111 direction of the W2O6 framework and that these strongly interact. The analytical results suggest that the water and A-type cations compete for space in the tunnels within the W2O6 pyrochlore framework, with the total number of water molecules and cations being approximately constant in the six samples investigated. The interplay between the cations and water explains the non-linear dependence of the a lattice parameter on the choice of cation. It appears that the ion-exchange capacity of the material will be controlled by the amount of water initially present in the sample. Crown Copyright (C) 2011 Published by Elsevier Ltd. All rights reserved.
Comparison of size-dependent structural and electronic properties of anatase and rutile nanoparticles
(American Chemical Society, 2009-04-23) Luca, V
Size-dependent variations in the electronic and structural properties of anatase and rutile nanoparticles have been compared. The anatase nanoparticles of the present study were prepared by hydrothermal ripening of an anatase sol and had diameters in the range 2−130 nm whereas the rutile nanoparticles were prepared by calcination of sol−gel derived rutile and had diameters in the range 3.6−60 nm. The hydrothermally ripened anatase nanoparticles had similar surface structures as deduced from the XANES as previously reported sol−gel anatase materials prepared through calcination (Luca et al., J. Phys. Chem. B 1998, 102, 10650). The optical band gap (Eg) of the anatase nanoparticles as deduced from their electronic absorption spectra showed some variation with size but Eg was not a smooth function of crystallite size, as would be dictated by the effective mass model for both types of anatase nanoparticles. In distinct contrast to the anatase nanoparticles, rutile nanoparticles showed a smooth size dependent variation in optical band gap in line with the dictates of the effective mass model. However, the XANES of the rutile nanoparticles was not dependent on size as it was for both the calcined and hydrothermally ripened anatase materials where the pre-edge XANES and EXAFS revealed a high concentration of distorted surface atoms with reduced coordination. The results suggest that sol−gel anatase nanoparticles consist of a core−shell structure in which the core is bulk-like and the shell interphase is less ordered with a high degree of Ti under-saturation. On the other hand, if such an interphase region was present at all in rutile nanoparticles, it was so thin as to avoid detection by XANES. The unique surface structure of anatase nanoparticles derived from sol−gel preparation methods is probably responsible for the lack of a clear quantum confinement effect. © 2009, American Chemical Society
Crystallite size dependence of lithium intercalation in nanocrystalline rutile
(American Chemical Society, 2009-07-30) Milne, NA; Skyllas-Kazacos, M; Luca, V
An electrochemical investigation has been undertaken into the dependence of particle size on the mechanism of lithium intercalation in rutile. A thoroughly characterized suite of rutile nanoparticles with diameters in the broad size range 4−50 nm was used in the study. Variation in the cyclic voltammetry of rutile over cycling is presented for the first time and numerous new electrode reactions identified. While a crystallite size effect was witnessed during fast voltammetry experiments, this effect disappeared as the rate was reduced and no variation in capacity was witnessed in galvanostatic cycling experiments. The intercalation potential was seen to increase with decreasing crystallite size, the opposite of what was predicted from quantum confinement. Enhanced lithium intercalation into the smaller nanoparticles has been ascribed to the slightly expanded structure of smaller nanocrystals relative to larger ones taking the dimensions of the tunnels into which Li inserts beyond a threshold value. This lattice expansion or volume dilation with decreasing crystallite size is due to the expected increase in surface tension with decreasing crystallite size. This would correspondingly increase the intercalation potential as the crystallite size decreased. © 2009, American Chemical Society
Cyclic voltammetric experiment - simulation comparisons of the complex mechanism associated with electrochemical reduction of Zr4+ in LiCl-KCl eutectic molten salt
(The Electrochemical Society, 2012-11-29) Fabian, CP; Luca, V; Le, TH; Bond, AM; Chamelot, P; Massot, L; Caravaca, C; Hanley, TL; Lumpkin, GR
Nuclear energy increasingly represents an important option for generating largely clean CO2-free electricity and zirconium is a fission product that is expected to be present in irradiated fuels. The present investigation addresses the electrochemical reduction of Zr4+ to Zro in LiCl - KCl eutectic molten salt in the temperature range 425–550°C using cyclic voltammetry (CV), square-wave voltammetry (SWV) and bulk electrolysis. Simulations of the CV data indicate that the initial reduction proceeds through surface confined steps: Zr4+* + 2e− ↔ Zr2+* and Zr2+* + 2e− ↔ Zr* processes (* adsorbed species) followed by a peak-shaped complex diffusion controlled step that consists of a combination of closely spaced processes associated with the reactions Zr4+ + 4e− → Zro and Zr4+ + 3e− → Zr+*. Zr+*, probably in the form of ZrCl* is then further reduced to Zro* at even more negative potentials. The simulations provide the first quantitative analysis of the thermodynamics and kinetics of the Zr4+ reduction in the LiCl-KCl eutectic. © 2012, The Electrochemical Society.
Delineating the first few seconds of supramolecular self-assembly of mesostructured titanium oxide thin films through time-resolved small angle x-ray scattering
(American Chemical Society, 2008-10-07) Luca, V; Bertram, WK; Sizgek, GD; Yang, B; Cookson, DJ
The early stages of evaporation induced self-assembly of titanium oxide mesophases from a precursor solution containing TiCl4 and the Pluronic triblock copolymer F-127 in HCl-water-ethanol solution have been studied using time-resolved SAXS techniques. Two experimental protocols were used to conduct these experiments. In one of these, the precursor solution was pumped around a closed loop as solvent was allowed to evaporate at a constant humidity-controlled rate. In the second protocol, a film of precursor solution was measured periodically as it dried completely to a residue under a stream of dry air. This permitted the detailed monitoring of changes in solution chemistry as a function of the elimination of volatile components followed by the actual drying process itself. The SAXS data were modeled in terms of two Guinier radii for soft nanoparticles while a broad Gaussian feature in the scatter profiles was accounted for by particle-article scattering interference due to close packing. For the initial precursor solution, one Guinier radius was found to be about 17 (A) over circle while the other ranged from 4 to 11 (A) over circle. Changing the rate of evaporation affected the two radii differently with a more pronounced effect on the smaller particle size range. Analysis gave an interparticle distance in the range 55-80 (A) over circle for the initial precursor solution which decreased steadily at both of the humidities investigated as evaporation proceeded and the particle packing increased. These results represent the first attempts to monitor in a precise fashion the growth of nano building blocks during the initial stages of the self-assembly process of a titanium oxide mesophase. © 2008, American Chemical Society
Electrochemical behavior of zirconium in LiCl - KCl eutectic molten salt
(IOP Publishing, 2010-04-25) Fabian, CP; Caravaca, C; Griffith, CS; Luca, V; Lumpkin, GR
Nuclear fission energy increasingly represents a potentially important option for generating clean, CO2-free electricity and the nuclear community is considering advanced reactor and fuel cycle technologies. Pyroelectrochemical processes based on molten salt electrochemistry have become important for the potential recycle of metal and other advanced nuclear fuels. Zirconium is one of the troublesome fission products that is expected to be present in such used fuels and hence it is important to acquire an understanding of its electrochemistry in molten salts. The present investigation addresses the electrochemical reduction of Zr4+ in LiCl - KCl eutectic molten salt using transient techniques such as cyclic voltammetry (CV), constant-current chronopotentiometry (CP) and square wave voltammetry (SWV) at 425, 475 and 525 °C. The CV data clearly indicate reduction of Zr4+ proceeds through an initial adsorption process followed by two or three reaction steps dependent on the temperature. Results show three previously unobserved reaction steps at 425 and 475 °C, possibly due to improved pretreatment of the salt. The first cathodic peak is observed at -1.18 V (vs. Ag/AgCl). The second and third peaks are observed at -1.26 and -1.33 V (vs. Ag/AgCl), respectively with a separation of 60-70 mV. The CV data at 525 °C show only two cathodic peaks at about -1.1 and -1.25 V (vs. Ag/AgCl)which may indicate the effect of temperature on the mechanism of the electrochemical process. The overall reaction mechanism assessed by these electrochemical techniques will be presented. Moreover, electrodeposition of zirconium on aluminium metal will also be presented along with XRD analysis of the salt residue and zirconium-aluminium alloy. Finally, the kinetics of the electrochemical process have been studied using Electrochemical Impedance Spectroscopy and the results analyzed using the Randles-Ershler equivalent circuit model to determine the charge-transfer resistance (Rct). © 2010 The Electrochemical Society
Hierarchically porous carbon–zirconium carbide spheres as potentially reusable transmutation targets
(Elsevier, 2015-08-01) Scales, N; Chen, J; Hanley, TL; Riley, DP; Lumpkin, GR; Luca, V
The preparation of hierarchically porous phase-pure carbon–zirconium carbide spheres with surface areas upwards of 70 m2/g and diameters in the 1–2 mm range has been achieved. The zirconium carbide beads were prepared through carbothermal reduction of polyacrylonitrile-zirconium composites prepared via three different routes including infiltration of a zirconium precursor into preformed polyacrylonitrile (PAN) beads and two one-pot co-precipitation methods. Depending on the route used to prepare the composites, relatively high surface area phase-pure zirconium carbides with the radial macroporous internal structure of the PAN template could be prepared. The adsorption of the elements U, Mo, Cs and Sr by the zirconium carbide beads was studied as a function of pH in the range 1–13 and target element concentrations of 0.025 mmol/L. The as-prepared hierarchical zirconium carbide beads demonstrated almost quantitative uptake of Mo below pH 4 with uptake decreasing to close to zero and above pH 10.5. On the other hand, U uptake was negligible below pH 4 and became quantitative in the range 4–12. Cesium was not extracted to any significant extent in the pH range studied and Sr only to a limited extent above pH 10. The engineered hierarchical porosities, neutron transparency, thermal stability and anticipated chemical stability of the present zirconium carbide spheres makes them promising candidates as reusable transmutation targets for Minor Actinide incineration. © 2015 Elsevier Inc.
Hybrid inorganic-organic adsorbents, Part 1: synthesis and characterization of mesoporous zirconium titanate frameworks containing coordinating organic functionalities
(American Chemical Society, 2010-12-22) Griffith, CS; Reyes, MDL; Scales, N; Hanna, JV; Luca, V
A series of functional hybrid inorganic−organic adsorbent materials have been prepared through postsynthetic grafting of mesoporous zirconium titanate xerogel powders using a range of synthesized and commercial mono-, bis-, and tris-phosphonic acids, many of which have never before been investigated for the preparation of hybrid phases. The hybrid materials have been characterized using thermogravimetric analysis, diffuse reflectance infrared (DRIFT) and 31P MAS NMR spectroscopic techniques and their adsorption properties studied using a 153Gd radiotracer. The highest level of surface functionalization (molecules/nm2) was observed for methylphosphonic acid (3 molecules/nm2). The level of functionalization decreased with an increase in the number of potential surface coordinating groups of the phosphonic acids. Spectral decomposition of the DRIFT and 31P MAS NMR spectra showed that each of the phosphonic acid molecules coordinated strongly to the metal oxide surface but that for the 1,1-bis-phosphonic acids and tris-phosphonic acids the coordination was highly variable resulting in a proportion of free or loosely coordinated phosphonic acid groups. Functionalization of a porous mixed metal oxide framework with the tris-methylenephosphonic acid (ATMP-ZrTi-0.33) resulted in a hybrid with the highest affinity for 153Gd3+ in nitric acid solutions across a wide range of acid concentrations. The ATMP-ZrTi-0.33 hybrid material extracted 153Gd3+ with a Kd value of 1 × 104 in 0.01 M HNO3 far exceeding that of the other hybrid phases. The unfunctionalized mesoporous mixed metal oxide had negligible affinity for Gd3+ (Kd < 100) under identical experimental conditions. It has been shown that the presence of free or loosely coordinated phosphonic acid groups does not necessarily translate to affinity for 153Gd3+. The theoretical cation exchange capacity of the ATMP-ZrTi-0.33 hybrid phase for Gd3+ has been determined to be about 0.005 mmol/g in 0.01 M HNO3. This behavior and that of the other hybrid phases suggests that the surface-bound ATMP ligand functions as a chelating ligand toward 153Gd3+ under these acidic conditions. © 2010, American Chemical Society
Hydrolytic stability of mesoporous zirconium titanate frameworks containing coordinating organic functionalities
(American Chemical Society, 2013-04-12) de los Reyes, M; Majewski, PJ; Scales, N; Luca, V
The hydrolytic stability of lanthanide and actinide selective mono- and polyphosphonate-functionalized mesoporous zirconium titanium oxide adsorbents has been investigated in nitric acid solutions. Hydrolytic degradation of the surfaces, as measured through the fractional loss of phosphorus and elements of the oxide framework, increased by more than an order of magnitude as the nitric acid concentration was increased from 0 to 2 mol/L. The unfunctionalized parent oxide suffered considerable dissolution in 2 mol/L acid over a period of 72 h. Under identical conditions, the fractional Zr and Ti release was reduced to 1 × 10–2 for monophosphonate functionalized hybrids and reached as low as 1 × 10–6 for trisphosphonate functionalized variants. The bisphosphonates showed intermediate values. The leaching of P, Zr and Ti was found to be incongruent with the Zr leaching to a lesser extent implying enhanced stability of the Zr–O–P bond. Quantitative analysis of the dissolution kinetics indicated a parabolic dissolution model with a rate constant in the range of 0.5–1.5 mg g–1 min–1/2 for the elemental leaching of P, Ti, and Zr. The leaching of Zr from the mesoporous matrix was relatively more complex than for the other elements with evidence of a leaching mechanism involving two processes. ToF-SIMS and DRIFT analysis demonstrated that after leaching in 2 M HNO3 for 24 h, a significant proportion of grafted ligands remained on the surface. The oxide functionalized with amino trismethylenephosphonic acid, which had previously shown excellent 153Gd3+ selectivity, was demonstrated to have outstanding stability, with low fractional elemental losses and preservation of mesoporous texture even after leaching for 24 h in 2 M HNO3. This suggests this particular hybrid to be worthy of additional study. © 2013, American Chemical Society.
Mesoporous zirconium titanium oxides. Part 3. Synthesis and adsorption properties of unfunctionalized and phosphonate-functionalized hierarchical polyacrylonitrile-f-127-templated beads
(American Chemical Society, 2009-10-06) Sizgek, GD; Griffith, CS; Sizgek, E; Luca, V
A method is presented for the preparation of zirconium titanate mixed oxides in bead form having hierarchical pore structure. This method entailed the use of both preformed polyacrylonitrile (PAN) polymer beads and surfactants as templates. The templates were removed by calcination at temperatures below about 500°C, resulting in mixed oxide beads with trimodal pore size distributions and interconnected pores. The pore size distributions as determined using nitrogen adsorption−desorption showed clear maxima at 4.5 and 45 nm length scales and also clear evidence of microporosity. The macroporous framework morphology was a replica of the PAN beads with radial structure. The mesoporous framework possessed wormhole-like pores with pore size of about 6 nm that was consistent with the F-127 triblock copolymer template used. The mixed oxide beads exhibited surface areas of 215 and 185 m2/g after calcination at 500 and 600°C. Thermal stability up to 650°C is unprecedented for bulk systems. The adsorption properties were characterized using uranyl as the target cation and the mass transport in the beads with the present hierarchical architectures has been shown to be exceptional. The beads were functionalized with 4-amino,1-hydroxy,1,1-bis-phosphonic acid (HABDP) and amino-tris-methylene phosphonic acid (ATMP) and the adsorption properties for the extraction of uranyl sulfate complexes from acidic solution examined. Of the two molecules investigated, ATMP functionalization resulted in the best extraction efficiency with equilibrium uptake of about 90% of uranium available in solution between pH 1 and 2. The beads could potentially be utilized as catalysts, catalyst supports, adsorbents, and separation materials. © 2009, American Chemical Society
Microcrystalline hexagonal tungsten bronze. 1. Basis of ion exchange selectivity for cesium and strontium
(American Chemical Society, 2009-07-06) Griffith, CS; Luca, V; Hanna, JV; Pike, KJ; Smith, ME; Thorogood, GJ
The structural basis of selectivity for cesium and strontium of microcrystalline hexagonal tungsten bronze (HTB) phase NaxWO3+x/2·zH2O has been studied using X-ray and neutron diffraction techniques, 1D and 2D 23Na magic angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy, and radiochemical ion exchange investigations. For the HTB system, this study has shown that scattering techniques alone provide an incomplete description of the disorder and rapid exchange of water (with tunnel cations) occurring in this system. However, 1D and 2D 23Na MAS NMR has identified three sodium species within the HTB tunnels—species A, which is located at the center of the hexagonal window and is devoid of coordinated water, and species B and C, which are the di- and monohydrated variants, respectively, of species A. Although species B accords with the traditional crystallographic model of the HTB phase, this work is the first to propose and identify the anhydrous species A and monohydrate species C. The population (total) of species B and C decreases in comparison to that of species A with increasing exchange of either cesium or strontium; that is, species B and C appear more exchangeable than species A. Moreover, a significant proportion of tunnel water is redistributed by these cations. Multiple ion exchange investigations with radiotracers 137Cs and 85Sr have shown that for strontium there is a definite advantage in ensuring that any easily exchanged sodium is removed from the HTB tunnels prior to exchange. The decrease in selectivity (wrt cesium) is most probably due to the slightly smaller effective size of Sr2+; namely, it is less of a good fit for the hexagonal window, ion exchange site. The selectivity of the HTB framework for cesium has been shown unequivocally to be defined by the structure of the hexagonal window, ion exchange site. Compromising the geometry of this window even in the slightest way by either (1) varying the cell volume through changes to hydration or sodium content or (2) introducing disorder in the a−b plane through isomorphous substitution of molybdenum is sufficient to reduce the selectivity. Indeed, it is our hypothesis that this applies for all cations which are strongly bound by the HTB framework. © 2009, American Chemical Society
Microcrystalline hexagonal tungsten bronze. 2. Dehydration dynamics
(American Chemical Society, 2009-07-06) Luca, V; Griffith, CS; Hanna, JV
Low-temperature (25−600°C) thermal transformations have been studied for hydrothermally prepared, microcrystalline hexagonal tungsten bronze (HTB) phases AxWO3+x/2·zH2O as a function of temperature, where A is an exchangeable cation (in this case Na+ or Cs+) located in hexagonal structural tunnels. Thermal treatment of the as-prepared sodium- and cesium-exchanged phases in air were monitored using a conventional laboratory-based X-ray diffractometer, while thermal transformations in vacuum were studied using synchrotron X-ray and neutron diffraction. Concurrent thermogravimetric, diffuse reflectance infrared (DRIFT), and 23Na and 133Cs magic angle spinning (MAS) NMR spectroscopic studies have also been undertaken. For the cesium variant, cell volume contraction occurred from room temperature to about 350°C, the regime in which water was “squeezed” out of tunnel sites. This was followed by a lattice expansion in the 350−600°C temperature range. Over the entire temperature range, a net thermal contraction was observed, and this was the result of an anisotropic change in the cell dimensions which included a shortening of the A−O2 bond length. These changes explain why Cs+ ions are locked into tunnel positions at temperatures as low as 400°C, subsequently inducing a significant reduction in Cs+ extractability under low pH (nitric acid) conditions. The changing Cs+ speciation as detected by 133Cs MAS NMR showed a condensation from multiple Cs sites, presumably associated with differing modes of Cs+ hydration in the tunnels, to a single Cs+ environment upon thermal transformation and water removal. While similar lattice contraction was observed for the as-prepared sodium variant, the smaller radius of Na+ caused it to be relatively easily removed with acid in comparison to the Cs+ variant. From 23Na MAS NMR studies of the parent material, complex Na+ speciation was observed with dehydrated and various hydrated Na+ species being identified, and a subsequent dynamic interchange within this speciation was observed upon thermal treatment. © 2009, American Chemical Society
One-pot preparation and uranyl adsorption properties of hierarchically porous zirconium titanium oxide beads using phase separation processes to vary macropore morphology
(American Chemical Society, 2010-11-16) Drisko, GL; Kimling, MC; Scales, N; Ide, A; Sizgek, E; Caruso, RA; Luca, V
A simple and engineering friendly one-step process has been used to prepare zirconium titanium mixed oxide beads with porosity on multiple length scales. In this facile synthesis, the bead diameter and the macroporosity can be conveniently controlled through minor alterations in the synthesis conditions. The precursor solution consisted of poly(acrylonitrile) dissolved in dimethyl sulfoxide to which was added block copolymer Pluronic F127 and metal alkoxides. The millimeter-sized spheres were fabricated with differing macropore dimensions and morphology through dropwise addition of the precursor solution into a gelation bath consisting of water (H2O beads) or liquid nitrogen (LN2 beads). The inorganic beads obtained after calcination (550°C in air) had surface areas of 140 and 128 m2 g−1, respectively, and had varied pore architectures. The H2O-derived beads had much larger macropores (5.7 μm) and smaller mesopores (6.3 nm) compared with the LN2-derived beads (0.8 μm and 24 nm, respectively). Pluronic F127 was an important addition to the precursor solution, as it resulted in increased surface area, pore volume, and compressive yield point. From nonambient XRD analysis, it was concluded that the zirconium and titanium were homogeneously mixed within the oxide. The beads were analyzed for surface accessibility and adsorption rate by monitoring the uptake of uranyl species from solution. The macropore diameter and morphology greatly impacted surface accessibility. Beads with larger macropores reached adsorption equilibrium much faster than the beads with a more tortuous macropore network. © 2010, American Chemical Society
Pore size and volume effects on the incorporation of polymer into macro- and mesoporous zirconium titanium oxide membranes
(American Chemical Society, 2009-12) Drisko, GL; Cao, L; Kimling, MC; Harrisson, S; Luca, V; Caruso, RA
Macro- and mesoporous hybrid materials have applications in the fields of drug delivery, catalysis, biosensing, and separations. The pore size requirements must be well-understood to maximize the performance (e.g., load capacity and accessibility) of such materials. Hybrid materials were prepared by coating five distinct macroporous commercial membranes with zirconium titanium oxide through sol−gel chemistry. Calcination of these templated materials produced oxide membranes which had a suite of macropore and mesopore architectures, pore volumes, and surface areas. These differences in physical properties were used to conduct a fundamental study on the relationship between the pore size and volume and the polymer incorporation. Metal oxide membranes were postsynthetically modified with poly(ethyleneimine) (PEI) ranging in molecular weight from 1300 to 1000000 Da (1.2−11 nm in hydrodynamic diameter). The incorporation of the polymer from a 9 wt % solution at pH 10 was highly dependent on the pore size and pore volume. As the surface area increased, loading capacity decreased, indicating that much of the increased internal surface, due to small pore diameters (≤8 nm), was inaccessible to the macromolecules. Exclusion of PEI from small mesopores was apparent even for the lowest molecular weight polymer. A high maximum loading of 1.25 mg m−2 of 600000−1000000 Da PEI was achieved in the metal oxide with the largest minimum mesopore diameter. Thus, mesopore diameter and pore volume must be considered when designing a mesoporous solid support. © 2009, American Chemical Society
Reaction mechanisms of La3+ and Zr4+ in LiCl-KCl eutectic molten salt - cyclic and square wave voltammetry data simulations
(IOP Publishing, 2011-05-01) Fabian, CP; Chamelot, P; Massot, L; Caravaca, C; Luca, V; Lumpkin, GR
Nuclear energy increasingly represents an important option for generating clean CO2-free electricity. This has generated a nuclear renaissance with increased interest in both innovative nuclear reactors and fuel cycle technologies. The innovative reactor concepts proposed have mainly involved closed fuel cycles with implied recycling of irradiated fuel. Pyroelectrochemical processes based on a molten alkali chloride/fluoride electrolyte system show considerable promise, and as such have received considerable attention for fuel reprocessing. Lanthanum and zirconium are two fission products that are expected to be present in irradiated fuels. The present investigation addresses the electrochemical reduction of La3+ and Zr4+ in LiCl - KCl eutectic molten salt using transient techniques such as cyclic voltammetry (CV), square wave voltammetry (SWV) and chronopotentiometry (CP) from 425 to 550° C, at two concentration levels. Experimental CV data from 50 to 500 mV/sec appears to indicate that the lanthanum reaction mechanism involve a single step reduction process, i.e., La3+ + 3e- ↔ La. Hence a theoretical simulation of this experimental data should be straightforward as verification of the reaction mechanism. However, zirconium experimental CV data also from 50 to 500 mV/sec appears to indicate that its reduction proceeds through an ab initio sluggish charge-transfer step coupled with an adsorption/underpotential deposition: Zr4+ + 2e- ↔ Zr2+ followed by a second and faster step where two electrons further are transferred: Zr2+ + 2e- ↔ Zr. A theoretical simulation of the experimental voltammograms has been also conducted to verify this reaction mechanism. We show the parameter Λ in their potential separation, ∆Epp and scan rate plot to indicate the reversibility of their electrochemical process, and evaluate their standard electrochemical rate constant. Deconvolution of the SWV data for lanthanum and zirconium confirms that the reduction of La3+ proceeds through three-electron one-step reaction mechanism while that of Zr4+ proceeds through two two-electron step reactions independent of temperature and concentrations levels. © 2011 The Electrochemical Society
Size matters: incorporation of poly(acrylic acid) and small molecules into hierarchically porous metal oxides prepared with and without templates
(American Chemical Society, 2010-09-07) Drisko, GL; Imperia, P; de los Reyes, M; Luca, V; Caruso, RA
Template synthesis of metal oxides can create materials with highly controlled and reproducible pore structures that can be optimized for particular applications. Zirconium titanium oxides (25:75 mol %) with three different pore structures were synthesized in order to relate polymer loading capacity to macropore architecture. Sol−gel chemistry was used to prepare the materials in conjunction with (i) agarose gel templating, (ii) no template, and (iii) stearic acid templating. The three materials possessed high surface areas (212−316 m2 g−1). Surface modification was performed postsynthetically using propionic acid (a monomer), glutaric acid (a dimer), and three molecular weights of poly(acrylic acid) (2000, 100000, and 250000 g mol−1). Higher loading (mg g−1) was observed for the polymers than for the small molecules. Following surface modification, a perceptible decrease in surface area and mesopore volume was noted, but both mesoporosity and macroporosity were retained. The pore architecture had a strong bearing on the quantity and rate of polymer incorporation into metal oxides. The templated pellet with hierarchical porosity outperformed the nontemplated powder and the mesoporous monolith (in both loading capacity and surface coverage). The materials were subjected to irradiation with 60Co γ-rays to determine the radiolytic stability of the inorganic support and the hybrid material containing the monomer, dimer, and polymer. The polymer and the metal oxide substrate demonstrated notable radiolytic stability. © 2010, American Chemical Society
Sol-gel tungsten oxide/titanium oxide multilayer nanoheterostructured thin films: structural and photoelectrochemical properties
(American Chemical Society, 2007-12-20) Luca, V; Blackford, MG; Finnie, KS; Evans, PJ; James, M; Lindsay, MJ; Skyllas-Kazacos, M; Barnes, PRF
Multilayer structures of alternating thin titanium and tungsten oxide layers having dimensions of similar to 20 nm have been fabricated from titanium alkoxide and various tungstate precursor solutions using the dip coating technique. Single, double, and triple layer titanate and tungstate thin films were deposited on silicon substrates, and these films were initially annealed at 400°C. Structural and microstructural aspects of the films were investigated using a variety of techniques, including X-ray reflectometry, grazing incidence X-ray absorption spectroscopy (GIXAS), cross-sectional transmission electron microscopy (TEM), and secondary ion mass spectrometry. The dimensions of the films and the character of the interfaces were principally gauged by cross-sectional TEM and X-ray reflectometery. All films were continuous on a local scale and had relatively low surface roughness. At the treatment temperature of 400°C, only the tungsten oxide component showed appreciable crystallinity. The multilayer films had relatively diffuse interfaces, even after annealing in air at this temperature. At these temperatures, easily measurable diffusion of tungsten into the titanium oxide component was observed, whereas the diffusion of titanium into the tungsten oxide component occurred to a lesser degree. At higher temperatures, interdiffusion of components was found to be significant. TEM, X-ray diffraction, and Ti K-edge GIXAS measurements indicated that annealing at 400°C generated films in which the titanate component remained amorphous while the tungstate component crystallized in the tetragonal modification Of WO3, which is normally stable only at high temperatures. Grazing incidence X-ray absorption spectroscopy allowed the degree of distortion of the tungsten oxygen polyhedra to be monitored as a function of depth into the film. The photoelectrochernical activity of the multilayer film electrodes was investigated, and the activity for water photo-oxidation was assessed. The photoelectrochernical response was greatest when crystalline WO3 was bounded on both sides by amorphous TiO2 layers. In this bounded state, WO3 had unique structural characteristics. © 2007, American Chemical Society

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