Ce(III) and Ce(IV) (re)distribution and fractionation in a laterite profile from Madagascar: Insights from in situ XANES spectroscopy at the Ce LIII-edge

The distribution of trivalent and tetravalent cerium, Ce(III) and Ce(IV) respectively, in a lateritic profile from Madagascar, has been characterized by X-ray-absorption near-edge structure (XANES) spectroscopy at the Ce LIII-edge on the LUCIA beamline (SOLEIL synchrotron, France). XANES spectra were acquired on bulk-rock samples as well as on specific lateritic minerals or polymineral zones (in-situ measurements) of the tonalite bedrock and the three overlying weathered horizons (C-, B- and A-horizons). Geochemically, the bedrock, and the A- and C-horizons show similar rare earth element content (REE = 363–405 mg/kg). They also display the same positive Ce-anomaly (CeCN/Ce∗ = 1.12–1.45), which is therefore likely to be inherited from the bedrock. In the B-horizon, the higher REE content (REE = 2194 mg/kg) and the larger Ce-anomaly (CeCN/Ce∗ = 4.26) are consistent with an accumulation zone caused by the evaporation of groundwater during the dry season. There is a good agreement between the Ce(III)/Cetotal ratio (XCe(III)) deduced from the positive Ce-anomaly (bulk-rock geochemical data) and that derived from XANES spectroscopy on the same bulk-rock samples (BR-XCe(III)-XANES) in the bedrock, and the C- and B-horizons. In the A-horizon, XANES measurements on bulk rock and minerals revealed a higher BR-XCe(III)-XANES (up to 100%) compared to the XCe(III) deduced from geochemical data (XCe(III) = 79%). The preservation of a positive Ce-anomaly in the A-horizon suggests that the Ce mobilization and redistribution during weathering occurred with no significant Ce fractionation from other trivalent REE. Remarkably, the only investigated sample where cerianite is observed belongs to the B-horizon. Within this horizon, Ce oxidation state varies depending on the microstructural position (porosity, cracks, clay-rich groundmass). The highest Ce(IV) concentrations are measured in cerianite (and aluminophosphates) localized in pores at the vicinity of Mn-rich domains (XCe(III)-XANES = 30–51%). Therefore, Ce fractionation from other REE is attributed to a Ce oxidation and precipitation potentially assisted by oxyhydroxide scavenging. In the C-horizon, Ce(III) and Ce(IV) are mainly distributed in REE-minerals of the rhabdophane group found in pores and cracks. The similarity between the Ce(III) proportion of rhabdophane grains (XCe(III)-XANES = 74–89%) with that of the bedrock (BR-XCe(III)-XANES = 79%) suggests no significant fractionation of Ce(III) and Ce(IV) between solution and mineral during the successive stages of primary REE-mineral alteration, transport in solution and secondary precipitation in the incipient stages of weathering. Overall, our novel spectroscopic approach shows that Ce is not necessarily oxidized nor fractionated from other REE during weathering in lateritic conditions. This implies that like Ce(III), Ce(IV) can be mobilized in aqueous fluids during weathering, possibly thanks to complexation with organic molecules, and can precipitate together with Ce(III) in secondary REE-bearing minerals. The corollary is that (paleo)redox reconstructions in soils and/or sediments based on Ce-anomaly in weathered rocks or minerals must be interpreted with caution.

Colloidal gold immobilized on mesoporous silica as a highly active and selective catalyst for styrene epoxidation with H2O2

Colloidal gold nanoparticles were synthesized by different procedures affording suspensions with two different mean sizes (2 and 5 nm). Au catalysts were prepared by sol immobilization onto several silica frameworks with different 2D and 3D mesoporosities. The catalysts were tested in styrene oxidation reactions showing excellent efficiency and selectivity. The effect of nanoparticle size and mesoporous framework on the physical and catalytic properties of the final materials was studied. The most selective catalyst was prepared from the 5 nm Au nanoparticles and the more interconnected silica framework (3D mesoporosity).

Absorption and fluorescence spectroscopy of rhodamine 6G in titanium dioxide nanocomposites

A comparison has been made between the spectroscopic properties of the laser dye rhodamine 6G (R6G) in mesostructured titanium dioxide (TiO2) and in ethanol. Steady-state excitation and emission techniques have been used to probe the dye-matrix interactions. We show that the TiO2-nanocomposite studied is a good host for R6G, as it allows high dye concentrations, while keeping dye molecules isolated, and preventing aggregation. Our findings have important implications in the context of solid state dye-lasers and microphotonic device applications. (C) 2003 Elsevier B.V. All rights reserved.

Postsynthesis stabilization of free-standing mesoporous silica films

Mixed ammonia-water vapor postsynthesis treatment provides a simple and convenient method for stabilizing mesostructured silica films. X-ray diffraction, transmission electron microscopy, nitrogen adsorption/desorption, and solid-state NMR (C-13, Si-29) were applied to study the effects of mixed ammonia-water vapor at 90 degreesC on the mesostructure of the films. An increased cross-linking of the silica network was observed. Subsequent calcination of the silica films was seen to cause a bimodal pore-size distribution, with an accompanying increase in the volume and surface area ratios of the primary (d = 3 nm) to secondary (d = 5-30 nm) pores. Additionally, mixed ammonia-water treatment was observed to cause a narrowing of the primary pore-size distribution. These findings have implications for thin film based applications and devices, such as sensors, membranes, or surfaces for heterogeneous catalysis.

Sol-gel sulfonic acid silicas as catalysts

Silica-supported sulfonic acids are a class of solid Brønsted acid catalysts that generally comprise organo-sulfonic acid groups tethered to silica surfaces. Methodologies to prepare organically modified silica have been widely developed in separation science and the techniques for their preparation are well documented. The application of this chemistry to prepare pure Brønsted sulfonic acid functionalized mesoporous silicas has stimulated significant research effort in this area, since these materials are interesting alternatives to commercially available sulfonated polymer resins, such as Amberlyst–15 and Nafion-H (sulfonated polystyrene and perfluorinated sulfonic acid resins respectively), which suffer from low surface areas and thermal stability. This chapter presents an overview of the preparation of mesostructured silica supported sulfonic acids, their catalytic applications and reviews the approaches taken to tune catalyst performance in organic transformations.

Protocol optimization for the mild detemplation of mesoporous silica nanoparticles resulting in enhanced texture and colloidal stability

Porosity development of mesostructured colloidal silica nanoparticles is related to the removal of the organic templates and co-templates which is often carried out by calcination at high temperatures, 500-600 °C. In this study a mild detemplation method based on the oxidative Fenton chemistry has been investigated. The Fenton reaction involves the generation of OH radicals following a redox Fe3+/Fe2+ cycle that is used as catalyst and H2O2 as oxidant source. Improved material properties are anticipated since the Fenton chemistry comprises milder conditions than calcination. However, the general application of this methodology is not straightforward due to limitations in the hydrothermal stability of the particular system under study. The objective of this work is three-fold: 1) reducing the residual Fe in the resulting solid as this can be detrimental for the application of the material, 2) shortening the reaction time by optimizing the reaction temperature to minimize possible particle agglomeration, and finally 3) investigating the structural and textural properties of the resulting material in comparison to the calcined counterparts. It appears that the Fenton detemplation can be optimized by shortening the reaction time significantly at low Fe concentration. The milder conditions of detemplation give rise to enhanced properties in terms of surface area, pore volume, structural preservation, low Fe residue and high degree of surface hydroxylation; the colloidal particles are stable during storage. A relative particle size increase, expressed as 0.11%·h-1, has been determined.

The Redox Potentials of n-type Colloidal Semiconductor Nanocrystals

Thesis (Ph.D.)--University of Washington, 2016-08

Mesoscale Optoelectronic Design of Wire-Based Photovoltaic and Photoelectrochemical Devices

The overarching theme of this thesis is mesoscale optical and optoelectronic design of photovoltaic and photoelectrochemical devices. In a photovoltaic device, light absorption and charge carrier transport are coupled together on the mesoscale, and in a photoelectrochemical device, light absorption, charge carrier transport, catalysis, and solution species transport are all coupled together on the mesoscale. The work discussed herein demonstrates that simulation-based mesoscale optical and optoelectronic modeling can lead to detailed understanding of the operation and performance of these complex mesostructured devices, serve as a powerful tool for device optimization, and efficiently guide device design and experimental fabrication efforts. In-depth studies of two mesoscale wire-based device designs illustrate these principles—(i) an optoelectronic study of a tandem Si|WO3 microwire photoelectrochemical device, and (ii) an optical study of III-V nanowire arrays.

The study of the monolithic, tandem, Si|WO3 microwire photoelectrochemical device begins with development and validation of an optoelectronic model with experiment. This study capitalizes on synergy between experiment and simulation to demonstrate the model’s predictive power for extractable device voltage and light-limited current density. The developed model is then used to understand the limiting factors of the device and optimize its optoelectronic performance. The results of this work reveal that high fidelity modeling can facilitate unequivocal identification of limiting phenomena, such as parasitic absorption via excitation of a surface plasmon-polariton mode, and quick design optimization, achieving over a 300% enhancement in optoelectronic performance over a nominal design for this device architecture, which would be time-consuming and challenging to do via experiment.

The work on III-V nanowire arrays also starts as a collaboration of experiment and simulation aimed at gaining understanding of unprecedented, experimentally observed absorption enhancements in sparse arrays of vertically-oriented GaAs nanowires. To explain this resonant absorption in periodic arrays of high index semiconductor nanowires, a unified framework that combines a leaky waveguide theory perspective and that of photonic crystals supporting Bloch modes is developed in the context of silicon, using both analytic theory and electromagnetic simulations. This detailed theoretical understanding is then applied to a simulation-based optimization of light absorption in sparse arrays of GaAs nanowires. Near-unity absorption in sparse, 5% fill fraction arrays is demonstrated via tapering of nanowires and multiple wire radii in a single array. Finally, experimental efforts are presented towards fabrication of the optimized array geometries. A hybrid self-catalyzed and selective area MOCVD growth method is used to establish morphology control of GaP nanowire arrays. Similarly, morphology and pattern control of nanowires is demonstrated with ICP-RIE of InP. Optical characterization of the InP nanowire arrays gives proof of principle that tapering and multiple wire radii can lead to near-unity absorption in sparse arrays of InP nanowires.

Synthesis of Ordered Mesoporous Carbon Materials by Dry Etching

A novel synthesis method for ordered mesoporous carbons is presented. The inverse replication of a silica template was achieved using the carbonization of sucrose within mesoporous KIT-6. Instead of liquid acid etching, as in classical nanocasting, a novel dry chlorine etching procedure for template removal is presented for the first time. The resultant ordered mesostructured carbon material outperforms carbons obtained by conventional hard templating with respect to high specific micro- and mesopore volumes (0.6 and 1.6 cm3 g−1, respectively), due to the presence of a hierarchical pore system. A high specific surface area of 1671 m2 g−1 was achieved, rendering this synthesis route a highly convenient method to produce ordered mesoporous carbons.

Mesoporous mixed Nb2O5-ZrO2 catalysts for dehydration of glucose into 5-hydroxymethylfurfural

Biomass is the world’s most important renewable carbon source, whose major component, carbohydrates, can be valorized by transformation into biofuels and high value-added chemicals. Among the latter, 5-hydroxymethylfurfural (HMF), obtained by C6 carbohydrates dehydration, is a versatile and key intermediate for the production of a large spectrum of biobased chemicals. Different catalytic systems have been evaluated for HMF production, mostly based on heterogeneous catalysis as alternative to the use of conventional mineral acids [1]. Moreover, niobium oxide has shown interesting properties as acid catalyst for dehydration of sugars [2-3]. On the other hand, the high surface area and large pore size of mesoporous solids make them suitable for many catalytic processes. In the present work, the dehydration of glucose to HMF has been evaluated by using different mesoporous mixed Nb2O5-ZrO2 in a biphasic water–Methyl Isobutyl Ketone (MIBK) solvent system to avoid the HMF degradation. Different experimental parameters, such as reaction temperature and time, as well as the addition of CaCl2 have been studied in order to maximize the HMF yield.N2 adsorption-desorption isotherms have corroborated the mesostructured character of catalysts, being all isotherms of Type IV according to the IUPAC classification. BET surface area decreases for catalysts with higher Zr content (Table 1). Likewise, pore volume and average pore diameter values diminish after Zr incorporation. Concerning the acid properties, a clear correlation between Nb and acidity can be observed, in such a way that total acidity, as deduced from NH3-TPD, decreases when the Zr content rises, and consequently the amount of Nb is reduced.These mesoporous Nb-Zr catalysts have been tested in the dehydration of glucose to HMF at 175 ºC under batch operation in aqueous solution, using MIBK as co-solvent. It can be observed that both glucose conversion and HMF yield increase with the Nb content, being maximum (90% and 36%, respectively) after 90 minutes for Nb2O5. This trend changes when CaCl2 is added to the reaction medium, improving the catalytic performance of mixed oxides and ZrO2, but Nb2O5 maintains similar results than without salt addition. This could be justified by the interaction between CaCl2 and Lewis acid sites, since zirconium oxide possesses a higher amount of this acid sites type.