SI methane hydrate confined in C8-grafted SBA-15: A highly efficient storage system enabling ultrafast methane loading and unloading

Confinement of water and methane in mesopores of hydrophobized SBA-15 is demonstrated to promote methane hydrate formation. In comparison to as-synthesized SBA-15, hydrophobization by C8 grafting accelerates the kinetics of methane storage in and delivery from the hydrate. C8 grafting density was determined at 0.5 groups nm-2 based on TGA and quantitative NMR spectroscopy. Multinuclear 1H-1H DQSQ and 1H-1H RFDR NMR provided spectroscopic evidence for the occurrence of C8 chains inside the mesopores of SBA-15, by showcasing close spatial proximity between the grafted C8 chains and pore-intruded water species. X-ray diffraction demonstrates formation of Structure I hydrate on SBA-15 C8. At 7.0 MPa and 248 K, the water-to-hydrate conversion on hydrophobized SBA-15 C8 reaches 96 pct. as compared to only 71 pct. on a pristine SBA-15 sample with comparable pore size, pore volume and surface area. The clathrate loading amounted to 14.8 g g-1. 2D correlation NMR spectroscopy (1H-13C CP-HETCOR, 1H-1H RFDR) reveals hydrate formation occurs within pores of SBA-15 C8 as well as in interparticle volumes. Following the initial crystallization of SBA-15 C8-supported methane hydrate taking several hours, a pressure swing process at 248 K allows to desorb and re-adsorb methane from the structure within minutes and without thawing the frozen water structure. Fast loading and unloading of methane was achieved in 19 subsequent cycles without losses in kinetics. The ability to harvest the gas and regenerate the structure without the need to re-freeze the water represents a 50 pct. energy gain with respect to melting and subsequently recrystallizing the hydrate at 298 K and 248 K, respectively. After methane desorption, a small amount of residual methane hydrate in combination with an amorphous yet locally ordered ice phase is observed using 13C and 2H NMR spectroscopy

Strongly Reducing (Diarylamino)benzene-Based Covalent Organic Framework for Metal-Free Visible Light Photocatalytic H2O2 Generation

Photocatalytic reduction of molecular oxygen is a promising route toward sustainable production of hydrogen peroxide (H2O2). This challenging process requires photoactive semiconductors enabling solar energy driven generation and separation of electrons and holes with high charge transfer kinetics. Covalent organic frameworks (COFs) are an emerging class of photoactive semiconductors, tunable at a molecular level for high charge carrier generation and transfer. Herein, we report two newly designed two-dimensional COFs based on a (diarylamino)benzene linker that form a Kagome (kgm) lattice and show strong visible light absorption. Their high crystallinity and large surface areas (up to 1165 m(2)center dot g(-1)) allow efficient charge transfer and diffusion. The diarylamine (donor) unit promotes strong reduction properties, enabling these COFs to efficiently reduce oxygen to form H2O2. Overall, the use of a metal-free, recyclable photocatalytic system allows efficient photocatalytic solar transformations.DFG, 390540038, EXC 2008: Unifying Systems in Catalysis "UniSysCat"EC/H2020/665501/EU/[PEGASUS]², giving wings to your career./PEGASUS-2EC/H2020/834134/EU/Water Forced in Hydrophobic Nano-Confinement: Tunable Solvent System/WATUSOEC/H2020/647755/EU/First principle molecular dynamics simulations for complex chemical transformations in nanoporous materials/DYNPO

Quantitative Determination of 99Tc(IV) Eigencolloïds by a Novel Column Precipitation Chromatography Technique

status: publishe

Fast technetium eigencolloid determination : preparative CPC combined with suspension liquid scintillation

Both medical and environmental studies concerned with the solubility and the complexation chemistry of technetium have encountered colloidal Tc(IV)-forms. Although the existence of the Tc colloids has been proven by various techniques [1-6], their determination still remains an issue. Recently a Column Precipitation Chromatography (CPC) technique was developed which enabled the quantitative determination of technetium eigencolloids. Based on this technique, a solid phase extraction (SPE)–like methodology was developed that can be used in combination with suspension liquid scintillation to provide a fast analysis of the eigencolloid content of a sample. The CPC technique is a thorough analysis methodology for the quantitative determination of the eigencolloid content of a sample containing reduced technetium species. This technique requires a relative long elution scheme and fractionation of the eluate. The fractionation also implies a relatively long counting time to determine the eigencolloid activity of a sample. Currently an SPE-like analysis methodology was developed which combines a good estimate of the eigencolloid content with fast analysis times. To construct a methodology providing both features a specialised extraction apparatus was constructed and a quantitative suspension liquid scintillation technique was developed. This combination enables the eigencolloid determination within a short experimental time (15 min) and a limited counting time (60 min).status: publishe

Structure Elucidation of Tc(IV) Pyrogallol Complexes

The redox-sensitive fission product technetium-99 (Tc) is of great interest in nuclear waste disposal studies because of its potential for contaminating the geosphere due to its very long half-life (2.13105 year) and high mobility under oxidising conditions, where technetium forms pertechnetate (TcO4-). Under suitable reducing conditions, e.g. in the presence of an iron(II) containing solid phase which can act as an electrondonor, the solubility can be limited by the reduction of pertechnetate followed by the formation of a surface precipitate [1]. However, by association with mobile humic substances (HS) or other associating/complexing species, the solubility of reduced Tc species may be drastically enhanced [2]. The elucidation of the identity and geometrical structure of the species causing this enhanced solubility often remains a difficult issue. EXAFS/XANES analysis can be very helpful to determine the molecular surroundings of reduced Tc organic complexes or Tc(IV) colloid-HS-associations. The interpretation of the EXAFS spectra is however not a straightforward process due to the strong influence of multiple scattering paths on the spectra.. Contrary to single scattering analysis which can be easily performed on the experimental EXAFS spectra, multiple scattering analysis can only be done, based on good approximations of the possible geometrical structures of the species under consideration. As the structure of humic substances is not exactly known and would be too complex for modelling, pyrogallol was used as a model compound for phenolic functional groups in natural organic matter. Detailed multiple scattering analysis of EXAFS data of Tc(IV)/pyrogallol solutions based on DFT modelled reference structures revealed the existence of a stable Tc-pyrogallol complex which is readily formed at pH 11 upon reduction of pertechnetate by hydrazine or dithionite in presence of pyrogallol. This initial complex serves as precursor for a pH dependent series of pyrogallol complexes exhibiting a reasonable stability towards technetium colloid or precipitate formation when lowering the pH from 11 to 2 after synthesis. The occurrence of stable, readily formed Tc-complexes with a humic substance model compound potentially changes current knowledge about Tc(IV) behaviour in natural systems containing free organic matter. This observation is an indication for an extra competition between the formation of immobile Tc(IV) precipitates upon reduction of TcO4- or potentially mobile organic Tc(IV) complexes and previously discovered Tc(IV) eigencolloids stabilised by dissolved HS through colloid-colloid interactions [3, 4]. As the slow reduction of TcO4- in the presence of phenolic functional groups at high pH is expected to occur during storage lifetime this competition could have important implications for the current knowledge of the migration behaviour of Tc. 1. Cui, D.Q. and T.E. Eriksen, Reduction of pertechnetate in solution by heterogeneous electron transfer from Fe(II)-containing geological material. Environmental Science & Technology, 1996. 30(7): p. 2263-2269. 2. Maes, A., et al., Quantification of the interaction of Tc with dissolved boom clay humic substances. Environmental Science & Technology, 2003. 37(4): p. 747-753. 3. Maes, A., et al., Evidence for the Formation of Technetium Colloids in Humic Substances by X-Ray Absorption Spectroscopy. Environmental Science & Technology, 2004. 38(7): p. 2044-2051. 4. Geraedts, K., et al., Evidence for the existence of Tc(IV) - humic substance species by X-ray absorption near-edge spectroscopy. Radiochimica Acta, 2002. 90(12): p. 879-884.status: publishe

Structure elucidation and occurrence of Tc(IV) pyrogallol complexes

The redox-sensitive fission product technetium-99 (Tc) is of great interest in nuclear waste disposal studies because of its potential for contaminating the geosphere due to its very long half-life (2.13x1e5 year) and high mobility under oxidising conditions, where technetium forms pertechnetate (TcO4-). Under suitable reducing conditions, e.g. in the presence of an iron(II) containing solid phase which can act as an electrondonor, the solubility can be limited by the reduction of pertechnetate followed by the formation of a surface precipitate [1]. However, by association with mobile humic substances (HS) or other associating/complexing species, the solubility of reduced Tc species may be drastically enhanced [2]. The identification and elucidation of the geometrical structure of the species causing this enhanced solubility often remains a difficult issue. EXAFS/XANES analysis can be very helpful to determine the molecular surroundings of reduced Tc organic complexes or Tc(IV) colloid-HS-associations. The interpretation of the EXAFS spectra is however not a straightforward process due to the strong influence of multiple scattering paths on the spectra. Contrary to single scattering analysis, multiple scattering analysis can only be done, based on good approximations of the local geometry around the primary scatterer in the candidate species under consideration. As the structure of humic substances is not exactly known, model compounds mimicing the functional properties of natural organic matter, e.g. pyrogallol, … were used. Detailed multiple scattering analysis of EXAFS data of Tc(IV)/pyrogallol solutions based on DFT modelled reference structures revealed the existence of a stable Tc-pyrogallol complex which is readily formed at pH 11 upon reduction of pertechnetate by hydrazine or dithionite in presence of pyrogallol. This initial complex serves as precursor for a pH dependent series of complexes in the pH range 11 - 2. The complexes exhibit a reasonable stability towards technetium colloid or precipitate formation both as function of pH and time. The occurrence of stable, readily formed Tc-complexes with a humic substance model compound potentially changes current knowledge about Tc(IV) behaviour in natural systems containing dissolved organic matter. This observation is an indication for an extra competition between the formation of immobile Tc(IV) precipitates and previously discovered Tc(IV) eigencolloids stabilised by dissolved HS through colloid-colloid interactions [2]. As the slow reduction of TcO4- in the presence of phenolic functional groups at high pH is expected to occur during storage lifetime this competition could have important implications for the current knowledge of the migration behaviour of Tc. [1] Cui, D.Q. and T.E. Eriksen, Reduction of pertechnetate in solution by heterogeneous electron transfer from Fe(II)-containing geological material. Environmental Science & Technology, 1996. 30(7): p. 2263-2269. [2] Maes, A., et al., Evidence for the Formation of Technetium Colloids in Humic Substances by X-Ray Absorption Spectroscopy. Environmental Science & Technology, 2004. 38(7): p. 2044-2051.status: publishe

Technetium hydrolysis and eigencolloid formation

The chemistry of reduced technetium species has been subject to investigations initiated from both the medical community and the nuclear industry. Studies concerned with medical nuclear imaging applications (planar scintigraphy, …) extensively focus on the complexation chemistry of Tc [1]. Research initiated by the nuclear industry on the other hand is mainly interested in: a) the separation of 99Tc (t½ = 2.14 x 105 years, - emitter) from high level nuclear waste streams [2], a potential application for the recycling of nuclear waste, and b) the geochemical behaviour of 99Tc, an important issue with regard to the safety of the geological disposal facilities for long-term storage of nuclear waste [3, 4]. 99Tc has been identified as one of the critical radionuclides that could impair the long-term safety of these facilities. Both medical and environmental studies concerned with the solubility and the complexation chemistry of technetium have encountered colloidal Tc(IV)-forms. Although the existence of the Tc colloids has been proven by various techniques [5-10], their quantitative determination still remains problematic. Tc(IV)-oxide colloids, radiolytically formed by γ-irradiation of aqueous TcO4- solutions, were visualised for the first time by transmission electron spectroscopy [7]. This experiment confirmed the existence of nano-sized (2 – 130 nm ) Tc(IV) colloids in reducing aqueous technetium solutions. Recently X-ray Absorption Fine Structure (EXAFS) spectroscopy also provided evidence for the existence of colloidal and/or polymeric Tc(IV) species in mixed chloride/sulphate media [10, 11] and lab-scale natural systems containing humic substances [12, 13]. Henry et al. [14] developed a partial charge model to capable of predicting the reactivity of cations in aqueous solutions towards condensation and complexation via the their hydrolysis behaviour. This PCM model is based on the electronegativity equalization theorem, initially formulated by Sanderson [15] and afterwards theoretically supported by the work of Parr et al. [16] . The PCM framework was successfully applied to technetium to explain the pH dependent hydrolysis behaviour and reactivity of Tc(IV) that was experimentally determined throughout the previous decades and resulted in the definition of a pH region where Tc(IV) eigencolloid formation is possible. The combination of the theoretical hydrolysis behaviour with recent experimental results obtained from Colloid Precipitation Chromatography (CPC) has enabled the formulation of a more detailed hypothesis on the formation of eigencolloids from condensation of mono-nuclear hydrolysed Tc(IV) species into binuclear and polynuclear Tc(IV) speciesstatus: publishe

ESRF Experiment - 26-01-1003 - Rb+ cations as templating species in zeolite formation

status: publishe

ESRF Experiment - CH 2303 - Tc(IV) eigencolloid formation and characterisation

status: publishe

Se(IV) reduction by FeS2 and FeS - solid phase reaction products

Only a very limited amount of information is available concerning the interaction between Se oxyanions and sulphidic mineral phases such as pyrite, troilite and mackinawite. However, it is a well-known fact that in sediments Se is closely correlated with pyrite [1, 2], where it can substitute for sulphur and thus form FeSe, FeSe2 or mixed FeSxSey phases which control its solubility [3, 4]. In the present study, the solid phase reaction products of selenite oxyanions with FeS and FeS2 at pH 7-8 are investigated using X-ray Absorption Near-Edge Spectroscopy and Extended X-ray Absorption Fine Structure (XANES-EXAFS) to elucidate the selenium speciation and, thus, the underlying geochemical reaction mechanisms. Reference spectra were collected from Se(IV) solution species and reduced Se solid phases such as amorphous and crystalline elemental selenium, and FeSe. It was shown that the energy position of the XANES absorption edge is indicative for the Se valence state, as observed before [5]. Also, the XANES region can be used to identify reduced Se solid phases. Se K-edge EXAFS spectra and Fourier-transformed Radial Structure Functions (RSFs) could be fitted very well using two coordination shells only. Comparing spectra taken from the solid phase of experiments in which FeS2 was contacted with SeO32, with the reference spectra, the presence of amorphous elemental selenium was reveiled, providing direct evidence that FeS2 acts as a redox mediator for oxidised Se species and thus inevitably controls the redox speciation of Se under geochemical conditions revelant for geological disposal of high-level nuclear waste. A similar sample in which FeS was contacted with SeO32-, showed the formation of FeSe as end product. Sulphide minerals subjected to HCl-based pretreatment did not show any difference with respect to the spectra obtained after equilibration with SeO32-, but an influence on the reaction kinetics was noted.status: publishe