Monitoring Catalytic reactions on Nanoparticles by solid state NMR: a joint experimental and theoretical approach
Final Report Abstract
Multi nuclear solid-state, gas-phase NMR, DNP enhanced solid-state-NMR and other physicochemical techniques for the study of catalytically active metal nanoparticles (Ru, Pd, Au, RuCu) and their interactions with substrates were developed and employed to a series of novel MNP catalysts. Those were prepared by the French partner from organometallic precursors and partially surface covered with ancillary ligands (phosphines and N-heterocyclic carbenes) and stabilized by organic linkers or embedding in a polymer to protect against Ostwald ripening respectively agglomeration of the MNPs. Particular emphasis was given to the interaction of hydrogen/deuterium with the active metal sites on the surface of the MNPs. In the case of Ru-MNPs it was shown by selective deuteration experiments on aza-compounds that this state is of particular importance for the catalytic performance. Ru and Pt-MNPs were investigated and compared with respect to their reactivity towards isotope exchange processes of hydrogen and deuterium, both between gas phase and the metal surface. The particles were also tested as alkane deuteration catalysts for which the deuteration of cyclopentane is chosen as a benchmark reaction. The H/D exchange rates between a D2 gas phase and surface hydrides were qualitatively compared by time depended 1H gas phase NMR. Hydrogen induced side reactions of the dppb ligands are characterized by GC-MS. For a more detailed characterization of the particles, 2H solid state NMR under MAS conditions and static measurements at low temperatures were performed. The values of the observed quadrupolar splittings were assigned with the help of previous theoretical studies, to distinguish deuteride binding geometries. Based on these experiments a detailed kinetic model of the different processes on the MNPs was developed and programmed. The activation of nitrogen in Haber-Bosch type processes occurring on the surface of Ru-MNPs in the course of the ammonia synthesis were studied by 13C and 15N solid-state NMR after adsorption of 15N labeled NH3 on the MNPs. The resulting signals are in perfect agreement with pure 15N and 13C labeled urea and clearly evidence the formation of organic compounds on the surface of metallic nanoparticles. The effect of ligand coordination on the hydrogenation efficacy was studies of 31P and 13C solid-state NMR measurements on MNPs with secondary phosphine oxides (SPOs). They exhibited a strong correlation between the ligand system and the hydrogenation efficiency. Employing deuterium as spin-label, H/D exchange experiments were performed and the modified particles were investigated in Darmstadt employing 2H low-temperature static NMR and MAS techniques. These experiments gave information on the hydrogen binding sites on these particles as well as on the existence of dynamics and fast isotope exchange with OH groups on the surface. Based on this work in the next step Au MNPs were investigated by 31P and 13C CP MAS solid state NMR experiments in order to get a deeper understanding on the coordination of the ligand systems and their influence on the reactivity. The 31P chemical shift parameters (isotropic chemical shift iso and the chemical shift anisotropy CSA ) enabled us to probe important differences in the polarity and strength of the P−O bond of the SPOs coordinated to the nanoparticle surface depending on the type of substituents in the ligand, which are reflected in the catalytic activity and selectivity of these AuNPs. Nanoparticles prepared with aryl-substituted SPOs present a strong polarity of the P=O bond and showed high catalytic activity and very high chemo selectivity in the hydrogenation of substituted aldehydes. This ability is lost in the case of NPs ligated by aliphatic phosphine oxides, which contain a lower polarity of the P=O bond or the presence of POH bonds which may slow the heterolytic cleavage of dihydrogen. Finally, we were recently able to perform first Dynamic Nuclear Polarization (DNP) NMR experiments on supported Pt NPs, for which the signals of surface CO sites are enhanced by nearly one order of magnitude, which opens up a new field of surface characterization on systems containing low amounts of surface molecules or molecules with less sensitive spins such as 13C or 15N in natural abundance without the need of expensive or chemically costly 15N isotope labeling or enrichments.
Publications
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From molecular complexes to complex metallic nanostructures – 2H solidstate NMR studies of ruthenium containing hydrogenation catalysts. Chem. Phys. Chem. (2013), 14, 3026-3033
T. Gutmann, I.del Rosal, B. Chaudret, R. Poteau,H.-H. Limbach, G. Buntkowsky
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Investigation of the surface-chemistry of phosphine- stabilized ruthenium nanoparticles – An advanced solid-state NMR study. Phys. Chem. Chem. Phys., (2013), 15, 8627 – 8638
T. Gutmann, E. Bonnefille, H. Breitzke, P.-J. Debouttière, K. Philippot, G. Buntkowsky, B. Chaudret
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Secondary Phosphine Oxides as Pre-Ligands for Nanoparticle Stabilization. Catalysis Science & Technology, (2013), 3, 595-599
E. Rafter, T. Gutmann, F. Löw, G. Buntkowsky, K. Philippot, B. Chaudret and P.W. N. M. van Leeuwen
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Solid-State NMR Concepts for the Investigation of supported Transition Metal Catalysts and Nano-particles. Solid State NMR, (2013), 55, 1-11
T. Gutmann, A. Grünberg, N. Rothermel, M. Werner, M. Srour, S. Abdulhussain, S. Tan, Y. Xu, H. Breitzke and G. Buntkowsky
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Regioselective and Stereospecific Deuteration of Bioactive Aza Compounds by the Use of Ruthenium Nanoparticles. Angew. Chem. Int. Ed., (2014), 53, 230-234
G. Pieters, C. Taglang, E. Bonnefille, T. Gutmann, C. Puente, J.-C. Berthet, C. Dugave, B. Chaudret, B. Rousseau
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Tin-decorated ruthenium nanoparticles: a way to tune selectivity in hydrogenation reaction. Nanoscale, (2014), 6, 9806-9816
E. Bonnefille, F. Novio, T. Gutmann, R. Poteau, P. Lecante, J. C. Jumas, K. Philippot, B. Chaudret
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Air-Stable Gold Nanoparticles Ligated by Secondary Phosphine Oxides as Catalyst for the Chemoselective Hydrogenation of Substituted Aldehydes: a Remarkable Ligand Effect. J. Am. Chem. Soc., (2015), 137, 7718-7727
I. Cano, M. Huertos, A. Chapman, G. Buntkowsky, T. Gutmann, P. Groszewicz, P. van Leeuwen
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Solid-state NMR Studies of Supported Transition Metal Catalysts and Nanoparticles, in Modern Magnetic Resonance, Springer International Publishing AG, Cham, G.A. Webb (ed.), 2017, 1-21
Torsten Gutmann, Gerd Buntkowsky
