Importance of Fuel Cell Tests for Stability Assessment - Suitability of Titanium Diboride as an Alternative Support Material

Carbon corrosion is a severe issue limiting the long-term stability of carbon- supported catalysts, in particular in the highly dynamic conditions of automotive applications. (Doped) oxides have been discussed as suitable alternatives to replace carbon, but often suffer from poor electron conductivity. That is why non-oxide ceramics, such as tungsten carbide and titanium nitride, have been discussed recently. Titanium diboride has also been proposed, due to its promising activity and stability in an aqueous electrochemical cell. In this work, Pt nanoparticles were deposited onto μm- sized TiB2 particles with improved grain size, manufactured into porous gas diffusion electrodes and tested in a realistic polymer electrolyte membrane (PEM) fuel cell environment. In contrast to the model studies in an aqueous electrochemical cell, in the presence of oxygen and high potentials at the cathode side of a real fuel cell, TiB2 becomes rapidly oxidized as indicated by intensely colored regions in the membrane-electrode assembly (MEA). Moreover, already the electrode manufacturing process led to the formation of titanium oxides, as shown by X-ray diffraction measurements. This demonstrates that Cyclic Voltammetry (CV) measurements in an aqueous electrochemical cell are not sufficient to prove stability of novel materials for fuel cell applications

Improved carrier concentration control in Zn-doped Ca_5Al_2Sb_6

Ca_5Al_2Sb_6 is an inexpensive, Earth-abundant compound that exhibits promising thermoelectric efficiency at temperatures suitable for waste heat recovery. Inspired by our previous study of p-type Ca_(5−x)Na_xAl_2Sb_6, this work investigates doping with Zn^(2+) on the Al^(3+) site (Ca_5Al_(2−x)Zn_xSb_6). We find Zn to be an effective p-type dopant, in contrast to the low solubility limit and poor doping efficiency of Na. Seebeck coefficient measurements indicate that the hole band mass is unaffected by the dopant type in the high-zT temperature range. Band structure and density of states calculations are employed in order to understand the carrier concentration-dependent effective mass. Ca_5Al_(2−x)Zn_xSb_6 has a low lattice thermal conductivity that approaches the predicted minimum value at high temperature (1000 K) due to the complex crystal structure and high mass contrast

Cobalt Catalysts for Alkene Hydrosilylation under Aerobic Conditions without Dry Solvents or Additives

[EN] Alkene hydrosilylation is typically performed with Pt catalysts, but inexpensive base-metal catalysts would be preferred. Here, we report a simple method for the use of air-stable cobalt catalysts for anti-Markovnikov alkene hydrosilylation that can be used under aerobic conditions without dry solvents or additives. These catalysts can be generated from low-cost commercially available materials. In addition, these catalysts possess good catalytic ability for both hydrosilanes and hydroalkoxysilanes. Finally, a mechanistic study demonstrates that the silane and the catalyst generate a Co-H species in the course of the reaction, which has been observed by in situ Raman spectroscopy.Program Severo Ochoa SEV-2016-0683 is gratefully acknowledged. S. G. T. and P. O.-B. thank MINECO for a FPU Ph.D. fellowship FPU16/02117 and a Ramon y Cajal contract RYC-2014-16620, respectively. Authors would like to thank Prof. Avelino Corma for discussion on the work and support. Authors would like to thank Ms. Adelina Munoz, Dr. Alejandro Vidal, and Ms. Carmen Clemente for the Raman, EPR, and ESI-MS measurements, respectively. Authors are also grateful for the use of analytical facilities at the X-ray Unit of RIAIDT (Universidad de Santiago de Compostela).Gutiérrez-Tarriño, S.; Concepción Heydorn, P.; Oña-Burgos, P. (2018). Cobalt Catalysts for Alkene Hydrosilylation under Aerobic Conditions without Dry Solvents or Additives. European Journal of Inorganic Chemistry. 45:4867-4874. https://doi.org/10.1002/ejic.201801068S4867487445Marciniec, B. (s. f.). Hydrosilylation of Alkenes and Their Derivatives. Advances In Silicon Science, 3-51. doi:10.1007/978-1-4020-8172-9_1Nakajima, Y., & Shimada, S. (2015). Hydrosilylation reaction of olefins: recent advances and perspectives. RSC Advances, 5(26), 20603-20616. doi:10.1039/c4ra17281gSun, J., & Deng, L. (2015). Cobalt Complex-Catalyzed Hydrosilylation of Alkenes and Alkynes. ACS Catalysis, 6(1), 290-300. doi:10.1021/acscatal.5b02308Wang, C., Teo, W. J., & Ge, S. (2016). Cobalt-Catalyzed Regiodivergent Hydrosilylation of Vinylarenes and Aliphatic Alkenes: Ligand- and Silane-Dependent Regioselectivities. ACS Catalysis, 7(1), 855-863. doi:10.1021/acscatal.6b02518Markó, I. E., Stérin, S., Buisine, O., Mignani, G., Branlard, P., Tinant, B., & Declercq, J.-P. (2002). Selective and Efficient Platinum(0)-Carbene Complexes As Hydrosilylation Catalysts. Science, 298(5591), 204-206. doi:10.1126/science.1073338Markó, I. E., Stérin, S., Buisine, O., Berthon, G., Michaud, G., Tinant, B., & Declercq, J.-P. (2004). Highly Active and Selective Platinum(0)-Carbene Complexes. Efficient, Catalytic Hydrosilylation of Functionalised Olefins. Advanced Synthesis & Catalysis, 346(12), 1429-1434. doi:10.1002/adsc.200404048Marciniec, B., Posała, K., Kownacki, I., Kubicki, M., & Taylor, R. (2012). New Bis(dialkynyldisiloxane)triplatinum(0) Cluster: Synthesis, Structure, and Catalytic Activity in Olefin-Hydrosilylation Reactions. ChemCatChem, 4(12), 1935-1937. doi:10.1002/cctc.201200319Bernhammer, J. C., & Huynh, H. V. (2013). Platinum(II) Complexes with Thioether-Functionalized Benzimidazolin-2-ylidene Ligands: Synthesis, Structural Characterization, and Application in Hydroelementation Reactions. Organometallics, 33(1), 172-180. doi:10.1021/om400929t2003 minerals.usgs.gov/minerals/pubs/commodity/platinum/Du, X., & Huang, Z. (2017). Advances in Base-Metal-Catalyzed Alkene Hydrosilylation. ACS Catalysis, 7(2), 1227-1243. doi:10.1021/acscatal.6b02990Tondreau, A. M., Atienza, C. C. H., Weller, K. J., Nye, S. A., Lewis, K. M., Delis, J. G. P., & Chirik, P. J. (2012). Iron Catalysts for Selective Anti-Markovnikov Alkene Hydrosilylation Using Tertiary Silanes. Science, 335(6068), 567-570. doi:10.1126/science.1214451Mitchener, J. C., & Wrighton, M. S. (1981). Photogeneration of very active homogeneous catalysts using laser light excitation of iron carbonyl precursors. Journal of the American Chemical Society, 103(4), 975-977. doi:10.1021/ja00394a060Chalk, A. J., & Harrod, J. F. (1967). Homogeneous Catalysis. IV. Some Reactions of Silicon Hydrides in the Presence of Cobalt Carbonyls. Journal of the American Chemical Society, 89(7), 1640-1647. doi:10.1021/ja00983a020A. Schroeder, M., & S. Wrighton, M. (1977). Pentacarbonyliron(0) photocatalyzed reactions of trialkylsilanes with alkenes. Journal of Organometallic Chemistry, 128(3), 345-358. doi:10.1016/s0022-328x(00)92207-1Reichel, C. L., & Wrighton, M. S. (1980). Photochemistry of cobalt carbonyl complexes having a cobalt-silicon bond and its importance in activation of catalysis. Inorganic Chemistry, 19(12), 3858-3860. doi:10.1021/ic50214a058Seitz, F., & Wrighton, M. S. (1988). Photochemical Reaction of [(CO)4Co(SiEt3)] with Ethylene: Implications for Cobaltcarbonyl-Catalyzed Hydrosilation of Alkenes. Angewandte Chemie International Edition in English, 27(2), 289-291. doi:10.1002/anie.198802891Seitz, F., & Wrighton, M. S. (1988). Die photochemische Reaktion von [(CO)4Co(SiEt3)] mit Ethylen und ihre Bedeutung für die Katalyse der Hydrosilylierung von Alkenen durch Carbonylcobalt-Komplexe. Angewandte Chemie, 100(2), 281-283. doi:10.1002/ange.19881000217Hojilla Atienza, C. C., Tondreau, A. M., Weller, K. J., Lewis, K. M., Cruse, R. W., Nye, S. A., … Chirik, P. J. (2012). High-Selectivity Bis(imino)pyridine Iron Catalysts for the Hydrosilylation of 1,2,4-Trivinylcyclohexane. ACS Catalysis, 2(10), 2169-2172. doi:10.1021/cs300584bBart, S. C., Lobkovsky, E., & Chirik, P. J. (2004). Preparation and Molecular and Electronic Structures of Iron(0) Dinitrogen and Silane Complexes and Their Application to Catalytic Hydrogenation and Hydrosilation. Journal of the American Chemical Society, 126(42), 13794-13807. doi:10.1021/ja046753tWu, J. Y., Stanzl, B. N., & Ritter, T. (2010). A Strategy for the Synthesis of Well-Defined Iron Catalysts and Application to Regioselective Diene Hydrosilylation. Journal of the American Chemical Society, 132(38), 13214-13216. doi:10.1021/ja106853yMarciniec, B., Kownacka, A., Kownacki, I., & Taylor, R. (2014). Hydrosilylation cross-linking of silicon fluids by a novel class of iron(0) catalysts. Applied Catalysis A: General, 486, 230-238. doi:10.1016/j.apcata.2014.08.037Chen, J., Cheng, B., Cao, M., & Lu, Z. (2015). Iron-Catalyzed Asymmetric Hydrosilylation of 1,1-Disubstituted Alkenes. Angewandte Chemie International Edition, 54(15), 4661-4664. doi:10.1002/anie.201411884Chen, J., Cheng, B., Cao, M., & Lu, Z. (2015). Iron-Catalyzed Asymmetric Hydrosilylation of 1,1-Disubstituted Alkenes. Angewandte Chemie, 127(15), 4744-4747. doi:10.1002/ange.201411884Cheng, B., Liu, W., & Lu, Z. (2018). Iron-Catalyzed Highly Enantioselective Hydrosilylation of Unactivated Terminal Alkenes. Journal of the American Chemical Society, 140(15), 5014-5017. doi:10.1021/jacs.8b01638Atienza, C. C. H., Diao, T., Weller, K. J., Nye, S. A., Lewis, K. M., Delis, J. G. P., … Chirik, P. J. (2014). Bis(imino)pyridine Cobalt-Catalyzed Dehydrogenative Silylation of Alkenes: Scope, Mechanism, and Origins of Selective Allylsilane Formation. Journal of the American Chemical Society, 136(34), 12108-12118. doi:10.1021/ja5060884Mo, Z., Liu, Y., & Deng, L. (2013). Anchoring of Silyl Donors on a N-Heterocyclic Carbene through the Cobalt-Mediated Silylation of Benzylic CH Bonds. Angewandte Chemie, 125(41), 11045-11049. doi:10.1002/ange.201304596Chen, C., Hecht, M. B., Kavara, A., Brennessel, W. W., Mercado, B. Q., Weix, D. J., & Holland, P. L. (2015). Rapid, Regioconvergent, Solvent-Free Alkene Hydrosilylation with a Cobalt Catalyst. Journal of the American Chemical Society, 137(41), 13244-13247. doi:10.1021/jacs.5b08611Lipschutz, M. I., & Tilley, T. D. (2012). Synthesis and reactivity of a conveniently prepared two-coordinate bis(amido) nickel(ii) complex. Chemical Communications, 48(57), 7146. doi:10.1039/c2cc32974cBuslov, I., Becouse, J., Mazza, S., Montandon-Clerc, M., & Hu, X. (2015). Chemoselective Alkene Hydrosilylation Catalyzed by Nickel Pincer Complexes. Angewandte Chemie International Edition, 54(48), 14523-14526. doi:10.1002/anie.201507829Buslov, I., Becouse, J., Mazza, S., Montandon-Clerc, M., & Hu, X. (2015). Chemoselective Alkene Hydrosilylation Catalyzed by Nickel Pincer Complexes. Angewandte Chemie, 127(48), 14731-14734. doi:10.1002/ange.201507829Greenhalgh, M. D., Frank, D. J., & Thomas, S. P. (2014). Iron-Catalysed Chemo-, Regio-, and Stereoselective Hydrosilylation of Alkenes and Alkynes using a Bench-Stable Iron(II) Pre-Catalyst. Advanced Synthesis & Catalysis, 356(2-3), 584-590. doi:10.1002/adsc.201300827K. Brandstadt S. Cook B. T. Nguyen A. Surgenor R. Taylor M. Tzou 2013Docherty, J. H., Peng, J., Dominey, A. P., & Thomas, S. P. (2017). Activation and discovery of earth-abundant metal catalysts using sodium tert-butoxide. Nature Chemistry, 9(6), 595-600. doi:10.1038/nchem.2697Noda, D., Tahara, A., Sunada, Y., & Nagashima, H. (2016). Non-Precious-Metal Catalytic Systems Involving Iron or Cobalt Carboxylates and Alkyl Isocyanides for Hydrosilylation of Alkenes with Hydrosiloxanes. Journal of the American Chemical Society, 138(8), 2480-2483. doi:10.1021/jacs.5b11311Schuster, C. H., Diao, T., Pappas, I., & Chirik, P. J. (2016). Bench-Stable, Substrate-Activated Cobalt Carboxylate Pre-Catalysts for Alkene Hydrosilylation with Tertiary Silanes. ACS Catalysis, 6(4), 2632-2636. doi:10.1021/acscatal.6b00304Constable, E. C., Housecroft, C. E., Jullien, V., Neuburger, M., & Schaffner, S. (2006). Structural characterisation of a 1:1 cobalt(II) – 2,2′:6′,2″-Terpyridine complex. Inorganic Chemistry Communications, 9(5), 504-506. doi:10.1016/j.inoche.2006.01.018Indumathy, R., Radhika, S., Kanthimathi, M., Weyhermuller, T., & Unni Nair, B. (2007). Cobalt complexes of terpyridine ligand: Crystal structure and photocleavage of DNA. Journal of Inorganic Biochemistry, 101(3), 434-443. doi:10.1016/j.jinorgbio.2006.11.002Mizuno, K., Imamura, S., & Lunsford, J. H. (1984). An EPR study of [CoIIL2]2+, [CoIILL’]2+ and [CoIIILL’O2-]2+ (L = 2,2’,2"-terpyridine; L’ = 2,2’-bipyridine) complexes in zeolite Y. Inorganic Chemistry, 23(22), 3510-3514. doi:10.1021/ic00190a015Cibian, M., & Hanan, G. S. (2015). Geometry and Spin Change at the Heart of a Cobalt(II) Complex: A Special Case of Solvatomorphism. Chemistry - A European Journal, 21(26), 9474-9481. doi:10.1002/chem.201500852G. R. Eaton S. S. Eaton D. P. Barr R. T. Weber Quantitative EPR Springer-Verlag Wien Austria 2010Evans, D. F. (1959). 400. The determination of the paramagnetic susceptibility of substances in solution by nuclear magnetic resonance. Journal of the Chemical Society (Resumed), 2003. doi:10.1039/jr9590002003Evans, D. F., Fazakerley, G. V., & Phillips, R. F. (1971). Organometallic compounds of bivalent europium, ytterbium, and samarium. Journal of the Chemical Society A: Inorganic, Physical, Theoretical, 1931. doi:10.1039/j19710001931C. W. Garland J. W. Nibler D. P. Shoemaker Experiments in Physical Chemistry 2003Sprengers, J. W., de Greef, M., Duin, M. A., & Elsevier, C. J. (2003). Stable Platinum(0) Catalysts for Catalytic Hydrosilylation of Styrene and Synthesis of [Pt(Ar-bian)(η2-alkene)] Complexes. European Journal of Inorganic Chemistry, 2003(20), 3811-3819. doi:10.1002/ejic.200300088Bleith, T., & Gade, L. H. (2016). Mechanism of the Iron(II)-Catalyzed Hydrosilylation of Ketones: Activation of Iron Carboxylate Precatalysts and Reaction Pathways of the Active Catalyst. Journal of the American Chemical Society, 138(14), 4972-4983. doi:10.1021/jacs.6b02173Park, E. S., Ro, H. W., Nguyen, C. V., Jaffe, R. L., & Yoon, D. Y. (2008). Infrared Spectroscopy Study of Microstructures of Poly(silsesquioxane)s. Chemistry of Materials, 20(4), 1548-1554. doi:10.1021/cm071575

Elucidation of the reaction mechanism upon lithiation and delithiation of Cu0.5TiOPO4

he reaction mechanism of Cu0.5TiOPO4 upon lithiation and delithiation was elucidated by XAS, 31P-NMR, XRD, EDX, and electrochemical methods. The material reacts with a combined insertion and conversion process, in which first copper is extruded irreversibly by forming LiTiOPO4. Afterwards, Ti4+ is reduced reversibly in an insertion reaction followed by a conversion reaction. The conversion reaction leads to amorphization of the sample while titanium is reduced to oxidation states below 2+.ISSN:2050-7488ISSN:2050-749

Mechanism of the Iron(II)-Catalyzed Hydrosilylation of Ketones: Activation of Iron Carboxylate Precatalysts and Reaction Pathways of the Active Catalyst

A detailed mechanistic study of the catalytic hydrosilylation of ketones with the highly active and enantioselective iron­(II) boxmi complexes as catalysts (up to >99% ee) was carried out to elucidate the pathways for precatalyst activation and the mechanism for the iron-catalyzed hydrosilylation. Carboxylate precatalysts were found to be activated by reduction of the carboxylate ligand to the corresponding alkoxide followed by entering the catalytic cycle for the iron-catalyzed hydrosilylation. An Eyring-type analysis of the temperature dependence of the enantiomeric ratio established a linear relationship of ln­(<i>S</i>/<i>R</i>) and <i>T</i>^–1, indicating a single selectivity-determining step over the whole temperature range from −40 to +65 °C (ΔΔ<i>G</i>^‡_sel, 233 K = 9 ± 1 kJ/mol). The rate law as well as activation parameters for the rate-determining step were derived and complemented by a Hammett analysis, radical clock experiments, kinetic isotope effect (KIE) measurements (<i>k</i>_H/<i>k</i>_D = 3.0 ± 0.2), the isolation of the catalytically active alkoxide intermediate, and DFT-modeling of the whole reaction sequence. The proposed reaction mechanism is characterized by a rate-determining σ-bond metathesis of an alkoxide complex with the silane, subsequent coordination of the ketone to the iron hydride complex, and insertion of the ketone into the Fe–H bond to regenerate the alkoxide complex

Iron Achieves Noble Metal Reactivity and Selectivity: Highly Reactive and Enantioselective Iron Complexes as Catalysts in the Hydrosilylation of Ketones

Chiral iron alkyl and iron alkoxide complexes bearing boxmi pincers as stereodirecting ligands have been employed as catalysts for enantioselective hydrosilylation reactions with unprecedented activity and selectivity (TOF = 240 h^–1 at −40 °C, ee up to 99% for alkyl aryl ketones), which match the performance of previously established noble-metal-based catalysts. This shows the potential of earth-abundant metals such as iron for replacing platinummetals without any drawbacks for the reaction design