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Eingeladener Vortrag
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Cr3+-doped MgF2 systems are synthesised by a novel non-aqueous soft chemistry route using different Cr precursors and varying Cr loading. These systems have been characterized by X-ray powder diffraction (XRD), BET surface area, TPD of ammonia, FT-IR pyridine adsorption analysis and X-ray photoelectron spectroscopy, and tested for their catalytic activity in dismutation of CCl2F2 and CF3CHClF. Catalysts synthesised starting from CrO3 or (CH3CO2)7Cr3(OH)2 showed better catalytic activity than those prepared with CrCl3. FT-IR pyridine adsorption studies reveal that the catalytic activity is highly correlated with the Lewis acid strength. In TPD of NH3 catalysts prepared from CrO3 and (CH3CO2)7Cr3(OH)2, precursors showed a similar behaviour, different from that of catalysts prepared from CrCl3.
The dismutation of CCl2F2 was used to probe the effect of halogenation of chromia by Cl/F exchange reactions to find out the difference between the halogenated inactive and active catalysts. The heterogeneous reactions were performed in a continuous flow Ni reactor and also under simulated reaction conditions in a reactor where after the reaction X-ray photoelectron spectroscopy (XPS) and X-ray excited Auger electron spectroscopy (XAES) analyses are possible without air exposure of the catalyst, i.e., under so-called "in situ" conditions. The Cr(III) 2p XP spectra, which revealed multiplet splitting features and satellite emission, were used for chemical analysis by using a simple evaluation procedure which neglects this inherent complexity. Chemical analysis was also applied by using chemical state plots for Cr 3s in order to cross-check Cr 2p related results. Both ex and in situ XPS show that as soon as Cr2O3 is exposed to CCl2F2 at 390°C fluorination as well as chlorination takes place at the catalyst surface. When the XPS surface composition reaches approximately 4 at. % fluorination and 6 at. % chlorination, maximum catalytic activity was obtained. Application of longer reaction times did not change significantly the obtained surface composition of the activated chromia. The fluorination and chlorination of chromia was further investigated by various HF and HCl treatments. The activated chromia samples and the Cr2O3, Cr(OH)3, CrF2OH, CrF3·H2O, α-CrF3, β-CrF3, and CrCl3 reference samples with well-known chemical structures were also characterized by X-ray absorption near edge structure (XANES), time-of-flight secondary ion mass spectroscopy (TOF-SIMS), pyridine-FTIR, wet chemical (F and Cl) analysis, X-ray powder diffraction (XRD), and surface area (BET) analysis. The results suggest that the formation of chromium oxide chloride fluoride species, e.g., chromium oxide halides, at the surface is sufficient to provide catalytic activity. The presence of any CrF3 and/or CrCl3 phases on the activated chromia samples was not found.
Cr3+-doped MgF2 systems are synthesised by a novel non-aqueous soft chemistry route using different Cr precursors and varying Cr loading. These systems have been characterized by X-ray powder diffraction (XRD), BET surface area, TPD of ammonia, FT-IR pyridine adsorption analysis and X-ray photoelectron spectroscopy, and tested for their catalytic activity in dismutation of CCl2F2 and CF3CHClF. Catalysts synthesised starting from CrO3 or (CH3CO2)7Cr3(OH)2 showed better catalytic activity than those prepared with CrCl3. FT-IR pyridine adsorption studies reveal that the catalytic activity is highly correlated with the Lewis acid strength. In TPD of NH3 catalysts prepared from CrO3 and (CH3CO2)7Cr3(OH)2, precursors showed a similar behaviour, different from that of catalysts prepared from CrCl3.
The dismutation of CHClF2 to CHF3 and CHCl3 was used to probe the effect of γ-Al2O3 fluorination on fluorine-against-chlorine (Cl/F) exchange reactions. X-ray photoelectron spectroscopy (XPS), X-ray excited Auger electron spectroscopy (XAES) and X-ray absorption near edge structure (XANES) spectroscopy were employed to probe the modifications at the surface of γ-Al2O3 accompanying the initial stages of fluorination. The results suggest that fluorine uptake by γ-Al2O3 starts initially at the surface of the oxide particles, with the formation of a fluorine species that is characterised by a single bond and a high effective charge q. Catalytic Cl/F-exchange is only observed after accumulation of higher fluorine concentrations, for which insertion of fluorine into the sub-surface region of the oxide crystallites has commenced. Quantitative XPS shows that the fluorination level must exceed approximately 10 atom% of the anions to achieve catalytic activity. The spectroscopic evidence suggests that the formation of aluminium oxofluorides is sufficient to provide catalytic activity. The presence of an AlF3 phase is not a condition for the observation of catalytic activity, even though defect-rich AlF3 is ultimately the endpoint of the fluorination process and is itself a Cl/F exchange catalyst.
The formation of magnesium fluoride sols and xerogels according to the fluorolytic sol–gel synthesis based on the reaction of Mg(OMe)2 with non-aqueous HF has been thoroughly investigated by X-ray scattering (WAXS/XRD), TEM, SAXS, DLS and 19F MAS NMR spectroscopy. Mechanistic insights were gained by following the reaction progress and formation of intermediate phases of the fluorination of magnesium alkoxides. For F:Mg ratios of 0.3 and 0.4 the formation of two crystalline phases was observed containing the recently obtained compound [Mg6F2(OCH3)10(CH3OH)14] hexanuclear dicubane units. The stoichiometric reaction yields magnesium fluoride nanoparticles with crystallite sizes below 5 nm, which show broad reflections in the X-ray diffraction pattern. Metal fluoride sols prepared by this way undergo tremendous changes over the first several weeks after synthesis. Immediately after the fluorination, particles of about 120 nm—probably agglomerates—are formed, which break apart in the course of about one month of ageing and low-viscous, transparent sols with particles of about 12 nm are obtained. At the same time structural re-organisation processes within the magnesium fluoride particles are observed by an increase of the (110) reflection in WAXS.
Time-optimized 15N quantitative NMR (qNMR) is described for the precise determination of Lewis and Brønsted site concentrations per gram catalyst, enabling a measurement time reduction of more than a factor of 10. The proof-of-principle is demonstrated for amorphous aluminum hydroxide fluorides with their tunable Lewis to Brønsted functionalities and relevance to highly active and selective heterogeneous catalysis using pyridine as a widely accepted probe molecule for Lewis and Brønsted sites. Two different Lewis centers were found, and reliable site concentrations down to the micromole per gram region can be determined. Furthermore, 1H{15N} REDOR NMR for the acid proton at the Brønsted site yields a distance of the 15N atom of bonded pyridine to this acid proton of (0.112 ± 0.002) nm, such that fast motion/diffusion of the Brønsted site-bonded pyridine at the surface can be excluded.
This study reports three new crystalline aluminum isopropoxide oxide fluorides with molar ratios of Al:F equal to 1:1 and 1:1.25. These are the first three representatives isolated without the incorporation of external donor molecules. Compound 1 Al4F4(µ4-O)(µ-O'Pr)5[H(O'Pr)2] contains a tetranuclear unit consisting of two different five fold coordinated AlFO4-units, with F exclusively in the terminal position. Compound 2, Al4F4(µ4-O)(µ-O'Pr)5[H(O'Pr)2]·Al5F5(µ5-O)(µ-O'Pr)8, contains both a tetranuclear unit (as in 1) and a pentanuclear Al-unit. Al-atoms in the latter are five- and six fold coordinated. Compound 3, Al16F20(µ4-O)4(µ-O'Pr)20·2('PrOH), exhibits a slightly higher fluorination degree and contains an oligomeric chain of four F-linked tetranuclear Al-units. In addition to X-ray structure analysis, compound 1 was characterized by different solid state MAS NMR techniques, including 27Al triple quantum MAS NMR and 1H, 1H→13C CP, 19F and 27Al MAS NMR. On the basis of the collected data, a reliable decomposition of 27Al single pulse MAS NMR spectra and an unambiguous assignment of the resonances to the respective structural AlFO4-units are given. The new crystalline aluminum isopropoxide oxide fluorides are direct evidence of the fluorolytic sol–gel mechanism previously discussed.
Dielectric, anti-reflective or high reflective systems consist of low and high refractive index layers. Common systems are oxides. The preparation of low refractive index MgF2-films of optical quality by means of an anhydrous low temperature solgel synthesis is presented. The MgF2-sol is prepared by spin-coating on silicon and glass substrates. Various film thicknesses between 20 nm and 435 nm have been deposited. It has been shown that the thickness increase is proportional to the number of coating steps. The deposited MgF2-films consist of 10 nm to 20 nm large nanoparticles and have smooth surfaces with an average roughness (Ra) of (1.7 ± 0.3) nm. The optical constants n and k of the films are in agreement with the literature data of bulk-MgF2.
A series of crystalline aluminum hydroxy fluorides in cubic pyrochlore structure AlFx(OH)3-x·H2O with variable F-content x were investigated by solid-state NMR by applying different magnetic fields up to 21.1 T. Distinguishable octahedral species AlFx(OH)6-x (x = 1-6) were identified in the crystalline aluminum hydroxy fluorides. The subsequent analysis of the highfield 27Al MAS NMR data allows the derivation of the trend analysis graphs giving correlations between the 27Al chemical shifts and the quadrupolar frequencies and the F-content x in AlFx(OH)6-x. Clear trends were obtained for both, which are, along with the 19F MAS chemical-shift trend analysis presented earlier, valuable tools for the interpretation of MAS NMR spectra of amorphous AlFx(OX)3-x compounds (X = H, alkyl). Following the dehydration of the pyrochlores by solid-state NMR eventually reveals a remarkable influence of the incorporated solvent molecules (H2O) on the 19F chemical shift. On that basis, a new chemical-shift trend analysis for 19F chemical shifts in correlation with x in AlFx(OH)6-x units for proton-poor substances (in the Al, F, O, H system) was determined. By using this correlation, high-surface AlF3 has a mean bulk Al:F ratio similar to that found for ACF, namely, AlF2.8(O/OH)0.2.
Thin films of MgF2 have been deposited on silicon wafers in optical quality with a novel sol-gel deposition process at temperatures as low as 100 °C. The properties of the fluoride layers are compared to bulk MgF2 with respect to the optical constants. By employing spectroscopic mapping ellipsometry, the uniformity of the thickness of fluoride layers is confirmed to be within acceptable limits for optical applications. The optical constants are close to the bulk data. High-quality low-index films of suitable and homogeneous thickness have already been produced on a laboratory scale.
Static TOF-SIMS results strongly underpin earlier conclusions from ESCA, XANES, and tracer studies that the accumulation of fluorine and chlorine at the surface of chromia by a heterogeneous reaction with a chlorofluorocarbon compound results in the formation of mixed chromium oxide halide species and not in the nucleation of CrCl3 and/or CrF3 phases.
A successful mechanochemical synthesis of strontium terephthalate trihydrate is described for the first time. The dehydration of Sr(C8H4O4)·3H2O occurs at about 100 °C and results in a well-defined strontium terephtalate, Sr(C8H4O4), thermally stable up to 550 °C. Both compounds are not described so far in the literature. Their structures were solved by ab initio structure determination and subsequent Rietveld refinement of the powder diffraction data. Further methods like DTA-TG, MAS NMR and FT-IR spectroscopy, and BET measurements were used to characterize these compounds.
A comprehensive spectroscopic characterization of all known crystalline AlF3 phases (α-, β-, η-, κ-, θ-AlF3) is presented for the first time in this study. Beside their X-ray diffraction powder patterns, which were already published in the literature, 27Al and 19F MAS NMR, FT IR and XPS spectroscopic techniques were applied for all phases in a consistent manner. For all phases prepared the utilization of 27Al satellite transition (SATRAS) NMR allowed to determine the quadrupolar parameters of the aluminium sites including their distributions.
In addition, η-AlF3 was isolated with high phase purity and characterized following a new preparation path different from those known so far in the literature.
Improved Durability of Wood Treated with Nano Metal Fluorides against Brown-Rot and White-Rot Fungi
(2022)
Low-water soluble metal fluorides such as magnesium fluoride (MgF2) and calcium Fluoride (CaF2) were evaluated for decay protection of wood. Initially, the biocidal efficacy of nano metal fluorides (NMFs) against wood destroying fungi was assessed with an in-vitro agar test. The results from the test showed that agar medium containing MgF2 and CaF2 was more efficient in preventing fungal decay than stand-alone MgF2 or CaF2. These metal fluorides, in their nanoscopic form synthesized using fluorolytic sol-gel synthesis, were introduced into the sapwood of Scots pine and beech wood and then subjected to accelerated ageing by leaching (EN 84). MAS 19F NMR and X-ray micro CT images showed that metal fluorides were present in treated wood, unleached and leached. Decay resistance of Scots pine and beech wood treated with NMFs was tested against Wood destroying fungi Rhodonia placenta and Trametes versicolor in accordance with EN 113. Results revealed that mass losses were reduced to below 3% in wood treated with the combination of MgF2 and CaF2.
It is concluded that NMFs provide full protection to wood even after it has been leached and can be used as wood preservatives in outdoor environments.
The fluorolytic sol–gel synthesis is applied with the intention to obtain two different types of core–shell nanoparticles, namely, SrF2–CaF2 and CaF2–SrF2. In two separate fluorination steps for core and shell formation, the corresponding metal lactates are reacted with anhydrous HF in ethylene glycol. Scanning transmission electron microscopy (STEM) and dynamic light scattering (DLS) confirm the formation of particles with mean dimensions between 6.4 and 11.5 nm. The overall chemical composition of the particles during the different reaction steps is monitored by quantitative Al Kα excitation X-ray photoelectron spectroscopy (XPS). Here, the formation of stoichiometric metal fluorides (MF2) is confirmed, both for the core and the final core–shell particles. Furthermore, an in-depth analysis by synchrotron radiation XPS (SR-XPS) with tunable excitation energy is performed to confirm the core–Shell character of the nanoparticles. Additionally, Ca2p/Sr3d XPS intensity ratio in-Depth profiles are simulated using the software Simulation of Electron Spectra for Surface Analysis (SESSA). In principle, core–shell like particle morphologies are formed but without a sharp interface between calcium and strontium containing phases.
Surprisingly, the in-depth chemical distribution of the two types of nanoparticles is equal within the error of the experiment. Both comprise a SrF2-rich core domain and CaF2-rich shell domain with an intermixing zone between them. Consequently, the internal morphology of the final nanoparticles seems to be independent from the synthesis chronology.