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- ItemProbing Oxide Reduction and Phase Transformations at the Au-TiO2 Interface by Vibrational Spectroscopy(Bussum : Baltzer, 2017-8-17) Pougin, Anna; Lüken, Alexander; Klinkhammer, Christina; Hiltrop, Dennis; Kauer, Max; Tölle, Katharina; Havenith-Newen, Martina; Morgenstern, Karina; Grünert, Wolfgang; Muhler, Martin; Strunk, JenniferBy a combination of FT-NIR Raman spectroscopy, infrared spectroscopy of CO adsorption under ultrahigh vacuum conditions (UHV-IR) and Raman spectroscopy in the line scanning mode the formation of a reduced titania phase in a commercial Au/TiO2 catalyst and in freshly prepared Au/anatase catalysts was detected. The reduced phase, formed at the Au-TiO2 interface, can serve as nucleation point for the formation of stoichiometric rutile. TinO2n−1 Magnéli phases, structurally resembling the rutile phase, might be involved in this process. The formation of the reduced phase and the rutilization process is clearly linked to the presence of gold nanoparticles and it does not proceed under similar conditions with the pure titania sample. Phase transformations might be both thermally or light induced, however, the colloidal deposition synthesis of the Au/TiO2 catalysts is clearly ruled out as cause for the formation of the reduced phase.
- ItemAddressing the Reproducibility of Photocatalytic Carbon Dioxide Reduction(Weinheim : Wiley-VCH Verlag, 2019) Marx, Maximilian; Mele, Andrea; Spannenberg, Anke; Steinlechner, Christoph; Junge, Henrik; Schollhammer, Philippe; Beller, MatthiasReproducibility of photocatalytic reactions, especially when conducted on small scale for improved turnover numbers with in situ formed catalysts can prove challenging. Herein, we showcase the problematic reproducibility on the example of attractive photocatalytic CO2 reduction utilizing [FeFe] hydrogenase mimics. These Fe complexes, well-known for their application in proton reduction reactions, were combined with a heteroleptic Cu photosensitizer and produced CO/H2/HCO2H mixtures of variable constitution. However, the reactions indicated a poor reproducibility, even when conducted with well-defined complexes. Based on our experience, we make suggestions for scientists working in the field of photocatalysis on how to address and report the reproducibility of novel photocatalytic reaction protocols. In addition, we would like to highlight the importance of studying reproducibility of novel reaction protocols, especially in the fields of photocatalytic water splitting and CO2 reduction, where TONs are widely used as the comparable measure for catalytic activity. © 2019 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.
- ItemCrystal structure of bis(η5-cyclopenta-dienyl)(2, 3-diethylbutane-1, 4-diyl)-hafnium(IV)(Chester : International Union of Crystallography, 2015) Burlakov, Vladimir V.; Baumann, Wolfgang; Arndt, Perdita; Spannenberg, Anke; Rosenthal, UweThe title compound, [Hf(C5H5)2(C8H16)], proves a structural motif of hafnacyclopentane besides the coordination of two cyclopentadienyl ligands in an [eta]5-fashion. The hafnacyclopentane ring has a twist conformation and is substituted by two ethyl groups in the [beta],[beta]'-positions, which are trans orientated to each other. One cyclopentadienyl ring and one ethyl group are each disordered over two positions with site-occupancy ratios of 0.679 (15):0.321 (15) and 0.702 (18):0.298 (18), respectively.
- ItemSynthesis of Single Atom Based Heterogeneous Platinum Catalysts: High Selectivity and Activity for Hydrosilylation Reactions(Washington, DC : ACS Publ., 2017) Cui, Xinjiang; Junge, Kathrin; Dai, Xingchao; Kreyenschulte, Carsten; Pohl, Marga-Martina; Wohlrab, Sebastian; Shi, Feng; Brückner, Angelika; Beller, MatthiasCatalytic hydrosilylation represents a straightforward and atom-efficient methodology for the creation of C-Si bonds. In general, the application of homogeneous platinum complexes prevails in industry and academia. Herein, we describe the first heterogeneous single atom catalysts (SACs), which are conveniently prepared by decorating alumina nanorods with platinum atoms. The resulting stable material efficiently catalyzes hydrosilylation of industrially relevant olefins with high TON (≈105). A variety of substrates is selectively hydrosilylated including compounds with sensitive reducible and other functional groups (N, B, F, Cl). The single atom based catalyst shows significantly higher activity compared to related Pt nanoparticles.
- ItemCooperative catalytic methoxycarbonylation of alkenes: Uncovering the role of palladium complexes with hemilabile ligands(Cambridge : RSC, 2018) Dong, Kaiwu; Sang, Rui; Wei, Zhihong; Liu, Jie; Dühren, Ricarda; Spannenberg, Anke; Jiao, Haijun; Neumann, Helfried; Jackstell, Ralf; Franke, Robert; Beller, MatthiasMechanistic studies of the catalyst [Pd2(dba)3/1,1′-bis(tert-butyl(pyridin-2-yl)phosphanyl)ferrocene, L2] for olefin alkoxycarbonylation reactions are described. X-ray crystallography reveals the coordination of the pyridyl nitrogen atom in L2 to the palladium center of the catalytic intermediates. DFT calculations on the elementary steps of the industrially relevant carbonylation of ethylene (the Lucite α-process) indicate that the protonated pyridyl moiety is formed immediately, which facilitates the formation of the active palladium hydride complex. The insertion of ethylene and CO into this intermediate leads to the corresponding palladium acyl species, which is kinetically reversible. Notably, this key species is stabilized by the hemilabile coordination of the pyridyl nitrogen atom in L2. The rate-determining alcoholysis of the acyl palladium complex is substantially facilitated by metal-ligand cooperation. Specifically, the deprotonation of the alcohol by the built-in base of the ligand allows a facile intramolecular nucleophilic attack on the acyl palladium species concertedly. Kinetic measurements support this mechanistic proposal and show that the rate of the carbonylation step is zero-order dependent on ethylene and CO. Comparing CH3OD and CH3OH as nucleophiles suggests the involvement of (de)protonation in the rate-determining step.
- ItemEnzyme Activity by Design: An Artificial Rhodium Hydroformylase for Linear Aldehydes(Weinheim : Wiley-VCH, 2017-9-13) Jarvis, Amanda G.; Obrecht, Lorenz; Deuss, Peter J.; Laan, Wouter; Gibson, Emma K.; Wells, Peter P.; Kamer, Paul C. J.Artificial metalloenzymes (ArMs) are hybrid catalysts that offer a unique opportunity to combine the superior performance of natural protein structures with the unnatural reactivity of transition-metal catalytic centers. Therefore, they provide the prospect of highly selective and active catalytic chemical conversions for which natural enzymes are unavailable. Herein, we show how by rationally combining robust site-specific phosphine bioconjugation methods and a lipid-binding protein (SCP-2L), an artificial rhodium hydroformylase was developed that displays remarkable activities and selectivities for the biphasic production of long-chain linear aldehydes under benign aqueous conditions. Overall, this study demonstrates that judiciously chosen protein-binding scaffolds can be adapted to obtain metalloenzymes that provide the reactivity of the introduced metal center combined with specifically intended product selectivity.
- Item3,3′-Dimethyl-1,1′-methylenediimidazolium tetrabromidocobaltate(II)(Chester : IUCr, 2018) Peppel, Tim; Spannenberg, AnkeThe title compound, (C9H14N4)[CoBr4], was obtained as single crystals directly in very low yield as a side product in the reaction of 1,1′-bis(1-methylimidazolium)acetate bromide and CoBr2. The title compound consists of an imidazolium-based dication and a tetrabromidocobaltate(II) complex anion, which are connected via C—H...Br interactions in the crystal. The dihedral angle between the imidazolium rings in the cation is 72.89 (16)°. The CoII ion in the anion is coordinated tetrahedrally by four bromide ligands [Co—Br = 2.4025 (5)–2.4091 (5) Å and Br—Co—Br = 106.224 (17)–113.893 (17)°]. The compound exhibits a high melting point (>300°C) and is a light-blue solid under ambient conditions.
- ItemSynthetic strategies to bicyclic tetraphosphanes using P1, P2 and P4 building blocks(London : Soc., 2015) Bresien, Jonas; Faust, Kirill; Hering-Junghans, Christian; Rothe, Julia; Schulz, Axel; Villinger, AlexanderDifferent reactions of Mes* substituted phosphanes (Mes* = 2,4,6-tri-tert-butylphenyl) led to the formation of the bicyclic tetraphosphane Mes*P4Mes* (5) and its unknown Lewis acid adduct 5·GaCl3. In this context, the endo–exo isomer of 5 was fully characterized for the first time. The synthesis was achieved by reactions involving “self-assembly” of the P4 scaffold from P1 building blocks (i.e. primary phosphanes) or by reactions starting from P2 or P4 scaffolds (i.e. a diphosphene or cyclic tetraphosphane). Furthermore, interconversion between the exo–exo and endo–exo isomer were studied by 31P NMR spectroscopy. All compounds were fully characterized by experimental as well as computational methods.
- ItemA second polymorph of 3,4-bis(6-bromopyridin-3-yl)-1,2,5-thiadiazole(Chester : International Union of Crystallography, 2016) Becker, Lisanne; Altenburger, Kai; Spannenberg, Anke; Arndt, Perdita; Rosenthal, UweThe title compound, C12H6Br2N4S, a second polymorph in the triclinic space group P-1, is presented. As in the earlier reported monoclinic polymorph in the space group C2/c [Becker et al. (2016[Becker, L., Reiss, F., Altenburger, K., Spannenberg, A., Arndt, P., Jiao, H. & Rosenthal, U. (2016). Chem. Eur. J. In the press. doi: 10.1002/chem.201601337.]). Chem. Eur. J. In the press], the thiadiazole ring is planar with an r.m.s. deviation of 0.004 Å. The five-membered ring is tilted with respect to the two pyridyl substituents by 23.16 (7) and 49.47 (9)°. In the crystal, molecules are linked by a weak non-bonding Br⋯N interaction [3.056 (3) Å]. Furthermore, a column of molecules is established along the b axis by π–π stacking interactions between the pyridine rings [centroid–centroid distances = 3.7014 (16) and 3.5934 (15) Å]. Additionally, a short intermolecular Br⋯Br contact [3.3791 (6) Å] and Br⋯π-aryl contacts [3.6815 (11)–3.7659 (12) Å] towards the thiadiazole and pyridine rings are found.
- ItemNb-modified Ce/Ti oxide catalyst for the selective catalytic reduction of NO with NH3 at low temperature(Basel : MDPI, 2018) Mosrati, Jawaher; Atia, Hanan; Eckelt, Reinhard; Lund, Henrik; Agostini, Giovanni; Bentrup, Ursula; Rockstroh, Nils; Keller, Sonja; Armbruster, Udo; Mhamdi, MouradRecently, great attention has been paid to Ceria-based materials for selective catalytic reduction (SCR) with NH3 owing to their unique redox, oxygen storage, and acid-base properties. Two series of bimetallic catalysts issued from Titania modified by Ce and Nb were prepared by the one-step sol-gel method (SG) and by the sol-gel route followed by impregnation (WI). The resulting core-shell and bulk catalysts were tested in NH3-SCR of NOx. The impregnated Nb5/Ce40/Ti100 (WI) catalyst displayed 95% NOx conversion at 200 °C (GHSV = 60,000 mL·g−1·h−1, 1000 ppm NOx, 1000 ppm NH3, 5% O2/He) without forming N2O. The catalysts were characterized by various methods including ICP-OES, N2-physisorption, XRD, Raman, NH3-TPD, DRIFTS, XPS, and H2-TPR. The results showed that the introduction of Nb decreases the surface area and strengthens the surface acidity. This behavior can be explained by the strong interaction between Ceria and Titania which generates Ce-O-Ti units, as well as a high concentration of amorphous or highly dispersed Niobia. This should be the reason for the excellent performance of the catalyst prepared by the sol-gel method followed by impregnation. Furthermore, Nb5/Ce40/Ti100 (WI) has the largest NH3 adsorption capacity, which is helpful to promote the NH3-SCR reaction. The long-term stability and the effect of H2O on the catalysts were also evaluated.