TY - JOUR A1 - Krishna Murthy, J. A1 - Gross, U. A1 - Rüdiger, St. A1 - Ünveren, Ercan A1 - Unger, Wolfgang A1 - Kemnitz, E. T1 - Synthesis and characterization of chromium(III)-doped magnesium fluoride catalysts JF - Applied physics A N2 - 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 CF3–CHClF. 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. KW - Lewis acid KW - Heterogeneous catalysis KW - Metal fluoride KW - Doped systems PY - 2005 UR - http://www.sciencedirect.com/science/journal/0926860X DO - https://doi.org/10.1016/j.apcata.2004.12.004 SN - 0947-8396 VL - 282 IS - 1-2 SP - 85 EP - 91 PB - Springer CY - Berlin AN - OPUS4-7442 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER - TY - JOUR A1 - Ünveren, Ercan A1 - Kemnitz, E. A1 - Lippitz, Andreas A1 - Unger, Wolfgang T1 - Surface Characterization of Chromia for Chlorine/Fluorine Exchange Reactions JF - The journal of physical chemistry / B N2 - 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. PY - 2005 DO - https://doi.org/10.1021/jp045902r SN - 1520-6106 SN - 1520-5207 SN - 1089-5647 VL - 109 IS - 5 SP - 1903 EP - 1913 PB - Soc. CY - Washington, DC AN - OPUS4-10531 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER - TY - JOUR A1 - Murthy, J.K. A1 - Gross, U. A1 - Rüdiger, S. A1 - Ünveren, Ercan A1 - Unger, Wolfgang A1 - Kemnitz, E. T1 - Synthesis and characterization of chromium(III)-doped magnesium fluoride catalysts JF - Applied catalysis / A N2 - 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 CF3–CHClF. 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. KW - Lewis acid KW - Heterogeneous catalysis KW - Metal fluoride KW - Doped systems PY - 2005 SN - 0926-860X SN - 1873-3875 VL - 282 IS - 1-2 SP - 85 EP - 91 PB - Elsevier CY - Amsterdam AN - OPUS4-11953 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER - TY - JOUR A1 - Böse, Olaf A1 - Unger, Wolfgang A1 - Kemnitz, E. A1 - Schroeder, S. L. M. T1 - Active sites on an oxide catalyst for F/Cl-exchange reactions: X-ray spectroscopy of fluorinated y-Al2O3 JF - Physical chemistry, chemical physics N2 - 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. PY - 2002 DO - https://doi.org/10.1039/b110792e SN - 1463-9076 SN - 1463-9084 VL - 4 IS - 12 SP - 2824 EP - 2832 PB - The Royal Soc. of Chemistry CY - Cambridge AN - OPUS4-6940 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER - TY - JOUR A1 - Noack, J. A1 - Emmerling, Franziska A1 - Kirmse, H. A1 - Kemnitz, E. T1 - Sols of nanosized magnesium fluoride: formation and stabilisation of nanoparticles JF - Journal of materials chemistry N2 - 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. KW - Surface modification KW - Colloidal particles KW - Gold nanoparticles KW - Gel synthesis KW - Small-angle scattering KW - Oxide KW - Polyelectrolytes KW - Aggregation KW - Dispersion PY - 2011 DO - https://doi.org/10.1039/c1jm11943e SN - 0959-9428 SN - 1364-5501 VL - 21 IS - 38 SP - 15015 EP - 15021 PB - Royal Society of Chemistry CY - Cambridge AN - OPUS4-26183 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER - TY - JOUR A1 - Hemmann, F. A1 - Scholz, G. A1 - Scheurell, K. A1 - Kemnitz, E. A1 - Jäger, Christian T1 - Time-optimized 15N qNMR determination of Lewis and Brønsted site concentrations and chemical bonding of pyridine at Brønsted sites in amorphous aluminium hydroxide fluoride JF - The journal of physical chemistry / C N2 - 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. KW - Catalysis KW - Lewis/Brønsted acid sites KW - Acid site concentration KW - Pyridine KW - Quantitative NMR KW - 15N KW - REDOR PY - 2012 DO - https://doi.org/10.1021/jp212045w SN - 1932-7447 SN - 1089-5639 VL - 116 IS - 19 SP - 10580 EP - 10585 PB - Soc. CY - Washington, DC AN - OPUS4-27724 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER - TY - JOUR A1 - König, R. A1 - Scholz, G. A1 - Veiczi, M. A1 - Jäger, Christian A1 - Troyanov, S.I. A1 - Kemnitz, E. T1 - New crystalline aluminium alkoxide oxide fluorides: evidence of the mechanism of the fluorolytic sol-gel reaction JF - Dalton transactions N2 - 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. KW - Aluminium fluorine KW - MAS NMR KW - Qudrupolar nuclei KW - Crystalline aluminium isopropoxide fluoride PY - 2011 DO - https://doi.org/10.1039/c1dt10514k SN - 1477-9226 SN - 1477-9234 SN - 1364-5447 VL - 40 IS - 34 SP - 8701 EP - 8710 PB - RSC CY - Cambridge AN - OPUS4-26245 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER - TY - CONF A1 - König, R. A1 - Scholz, G. A1 - Kemnitz, E. A1 - Jäger, Christian A1 - Pawlik, Alf A1 - van Rossumm, B. A1 - Oschkinat, H. T1 - Crystalline aluminium hydroxy fluorides: Structural insights obtained by high field solid state NMR and 27Al chemical shift trend analysis T2 - Gesellschaft Deutscher Chemiker T2 - Gesellschaft Deutscher Chemiker CY - Regensburg, Germany DA - 2008-09-22 PY - 2008 AN - OPUS4-17956 LA - deu AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER - TY - CONF A1 - König, R. A1 - Scholz, G. A1 - Kemnitz, E. A1 - Jäger, Christian A1 - Pawlik, Alf A1 - van Rossumm, B. A1 - Oschkinat, H. T1 - Crystalline aluminium hydroxyl fluorides: Structural insights obtained by high-field solid state NMR and 27Al chemical shift trend analysis T2 - Gesellschaft Deutscher Chemiker T2 - Gesellschaft Deutscher Chemiker CY - Regensburg, Germany DA - 2008-09-22 PY - 2008 AN - OPUS4-17957 LA - deu AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER - TY - JOUR A1 - Conrad, R. A1 - Klose, M. A1 - Noll, Matthias A1 - Kemnitz, D. A1 - Bodelier, P.L.E. T1 - Soil type links microbial colonization of rice roots to methane emission JF - Global change biology N2 - Most of the methane (CH4) emission from rice fields is derived from plant photosynthates, which are converted to CH4. Rice cluster I (RC-1) archaea colonizing the rhizosphere were found to be the methanogens responsible for this process. Hence, RC-1 methanogens seem to play a crucial role in emission of the greenhouse gas CH4. We determined the community composition and activity of methanogens colonizing the roots of eight different rice cultivars after growth on both Italian rice soil and river bank soil, which contained different communities of methanogenic archaea. The community composition was analyzed by terminal restriction fragment length polymorphism and cloning/sequencing of the archaeal 16S rRNA gene and the mcrA gene coding for a subunit of the methyl coenzyme M reductase. When grown on rice field soil, the methanogenic community of the different rice cultivars was always dominated by RC-1 methanogens. In contrast, roots were colonized by Methanomicrobiales when grown on river bank soil, in which RC-1 methanogens were initially not detectable. Roots colonized with Methanomicrobiales compared with RC-1 exhibited lower CH4 production and CH4 emission rates. The results show that the type of methanogens colonizing rice roots has a potentially important impact on the global CH4 cycle. KW - Gene sequence KW - Methane emission KW - Methanomicrobiales KW - Methyl coenzyme M reductase KW - Ribosomal RNA KW - Rice cluster I KW - Rice cultivar KW - Rice field soil KW - Rice root KW - Terminal restriction fragment length polymorphism PY - 2008 DO - https://doi.org/10.1111/j.1365-2486.2007.01516.x SN - 1354-1013 SN - 1365-2486 VL - 14 IS - 3 SP - 657 EP - 669 PB - Blackwell Science CY - Oxford AN - OPUS4-16517 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER -