TECHNISCHE UNIVERSITÄT MÜNCHEN Department Chemie, Lehrstuhl für Technische Chemie II Controlled Catalysis by the Electronic Charge Transfer at Metal-Support Interfaces: A Study of Gallium Nitride Supported Platinum Nanoparticles Sonja Agnes Wyrzgol Vollständiger Abdruck der von der Fakultät für Chemie der Technischen Universität München zur Erlangung des akademischen Grades eines Doktors der Naturwissenschaften (Dr. rer. nat.) genehmigten Dissertation. Vorsitzender: Univ.-Prof. Dr. T. Brück Prüfer der Dissertation: 1.Univ.-Prof. Dr. J. A. Lercher 2.Univ.-Prof. Dr. M. Tromp, Ph.D. Die Dissertation wurde am 03.07.2013 bei der Technischen Universität München eingereicht und von der Fakultät für Chemie am 08.10.2013 angenommen. „Tu erst das Notwendige, dann das Mögliche und plötzlich schaffst du das Unmögliche…!“ Franz von Assisi Acknowledgment In the following part, I would like to thank all the people, who helped and supported me in all respects. First of all, I would like to thank Prof. Dr. Johannes A. Lercher, who gave me the opportunity to be a part of his group and provided this interesting and interdisciplinary topic to me. Thank you for your confidence allowing me to guide my research work, as well as to participate on scientific conferences, for your critical advice, and for your support in discussions and financials. Exactly all these points helped me to extend my expertise in various areas like chemistry, engineering, and physics. My special thanks go to Prof. Martin Stutzmann (Walter Schottky Institute) for facilitating fruitful collaboration and discussion. Thank you for your scientific contribution and your financial support at the end of my thesis. Moreover, I am grateful to apl. Prof. Dr. Andreas Jentys, who agreed to supervise me during the last half of my thesis. Thank you for your interest in my work, your active cooperation, and advice. Additionally, I would like to thank my project partner, Susanne Schäfer (Walter Schottky Institute). I am convinced that our special teamwork led to successful research and gave a personal impact on our work. Thank you for your contribution, motivation, and critical view. I enjoyed our internal meetings, our trips to the annual IGSSE Forum, winter school, MRS Meeting, and especially our beamtimes at BESSY II. We learned a lot about each other that enforces our friendship. I would like to acknowledge the International Graduate School of Science and Engineering (IGSSE) of the TUM Graduate School for project funding and the possibility of a three-month stay abroad. I am grateful to Xuebing Li for his assistance and help during the first period of my work. I would like to thank Dr. Habil. Stefan Vajda for his cooperation and support during my stay at the Argonne National Laboratory. Thank you for your motivation, interest in my work, and scientific discussion. Moreover, I am grateful to my colleagues, who shared the office with me. The personal occupation changed continuously during my work and I would like to thank everyone for his or her personal contribution and support. Richard, Manuela, Frederik, Dechao, Manuel, Claudia, Despina, Linus, Christian, Jieli, Sebastian, Max, Pamina, Sarah, and Dani, thank you for the nice office climate. Thank you, Sarah and Dani, for your motivating and constructive support and your friendship. Many thanks go to Sungsik Lee, Marcel Di Vece, Byeongdu Lee, and Sönke Seifert (Argonne National Laboratory, Yale University) for their supporting help at the APS beamline regarding practical work, experiments, and discussion. Especially, thank you, Sönke, for the trip to Chicago. I would like to acknowledge all the people, who were involved in measurements at BESSY II of the Helmholtz-Zentrum Berlin. Thank you, Michael Hävecker, Detre Teschner, Raoul Blume, Tulio Rocha, Alexander Klyushin, Rosa Arrigo, and Axel Knop-Gericke (Fritz- Haber-Institut der Max-Planck-Gesellschaft) for your subject-specific and technical assistance at the setup, as well as your contribution to data evaluation and discussion. Special thanks go to Roberta Caterino, Sebastian Schöll, Moritz Hauf, Matthias Sachsenhauser, Markus Hofstetter, Martin Schmid, and Ian Sharp (Walter Schottky Institute, Helmholtz-Zentrum München, and Lawrence Berkeley National Laboratory) for their help during the beamtime. Additionally, I would like to thank Marianne Hanzlik (Institute of Electron Microscopy, TUM) for introduction into transmission electron microscopy and her support during measurements. I am grateful to Thomas Michel (Lehrstuhl I für Technische Chemie, TUM) for introduction into atomic force microscopy. I would like to thank Florian Schweinberger and Martin Tschurl (Chair of Physical Chemistry, TUM) for discussion on micro-structured reactor technology and photochemistry. Many thanks go to Prof. Gary L. Haller (Yale University) for scientific discussion. Special thanks go to the team of the workshop for fine mechanics, to Otto Straßer, Albert Englert, Reinhold Merz, Roland Schwarz, Marc Schönberger, Harald Wunder, Robert Göring, and Michael Wanninger. Thank you for the constructions of reactors and setup parts, and your endless patience regarding optimization and repairing. I am grateful to all students, who I supervised during his or her practical, Bachelor´s, or Master´s thesis. Thank you, Sarah Konrad, Irmgard Buchberger, Pankaj Madkikar, and Iulius Markovits, for your research contribution and interest. Especially, thank you, Iulius, for your cooperation as student assistant and your decision doing your Master´s thesis on this topic. I would like to acknowledge Prof. Dr. Günther Rupprechter for giving me the opportunity to do my stay abroad at the Institute of Material Chemistry of Vienna University of Technology. I would like to thank all group members for their kind response and especially Karin Föttinger and Andreas Haghofer for experimental introduction in sum frequency generation vibrational spectroscopy and interest in my work. Moreover, I am grateful to Stefanie Maier, Bettina Federmann, Karen Schulz, Helen Brenner, Katharina Thies, Joana Figueiredo (Walter Schottky Institute), and Silke Schnaubelt (Department of Physics, TUM) for their administrative support. Thank you, Steffi, for your help organizing meetings and appointments, and especially for your kindness. I would like to acknowledge Xaver Hecht, Martin Neukamm, and Andreas Marx for their support regarding technical, safety, and computing issues. Especially, thank you, Xaver, for your assistance in any technical problems and your availability whenever I needed your help. Additionally, I am grateful to all senior scientists, postdoctoral, and doctoral candidates of the group of Lehrstuhl II für Technische Chemie for their support and teamwork. Finally, I would like to thank my family and my boyfriend, who supported me in every intention and pushed me not to give up. Ich danke euch, Mama und Papa, dass ihr mich immer unterstützt habt und mir die Freiheit gegeben habt, das zu lernen was ich wollte. Danke euch, Peter und Edith, für eure Ehrlichkeit und Aufmunterung, aber auch für eure Kritik. Ich danke dir, Alex, für deine Liebe und Unterstützung. Ohne euch wäre ich niemals so weit gekommen. Table of Contents Table of Contents 1 General Introduction ........................................................................................................ 1 1.1 Motivation .................................................................................................................... 1 1.2 Scope of the Thesis ...................................................................................................... 3 1.3 Metal Nanoparticles in Catalysis .................................................................................. 4 1.3.1 Size- and Shape-Selectivity ................................................................................... 5 1.3.2 Physical Properties .............................................................................................. 13 1.3.3 Oxidation, Reduction, and Sintering of Platinum Nanoparticles ........................... 16 1.4 Gallium Nitride as Wide Bandgap Semiconducting Support ....................................... 21 1.4.1 Chemical and Physical Properties ........................................................................ 23 1.4.2 Native Surface Oxide and Surface Cleaning ........................................................ 30 1.4.3 Metal-Semiconductor Interface ........................................................................... 31 1.5 References .................................................................................................................. 35 2 Experimental Methods ................................................................................................... 41 2.1 Catalyst Preparation ................................................................................................... 41 2.1.1 Chemicals and Materials ..................................................................................... 42 2.1.2 Platinum Nanoparticle Synthesis ......................................................................... 42 2.1.3 Spin-coating ........................................................................................................ 45 2.2 Ex situ Characterization .............................................................................................. 47 2.2.1 Transmission Electron Microscopy (TEM) .......................................................... 47 2.2.2 Atomic Force Microscopy (AFM) ....................................................................... 50 2.3 In situ Characterization ............................................................................................... 55 2.3.1 Grazing Incidence Small-angle X-ray Scattering (GISAXS) ................................ 57 2.3.2 Grazing Incidence X-ray Absorption Spectroscopy (GIXAS) .............................. 62 2.3.3 X-ray Photoelectron Spectroscopy (XPS) ............................................................ 66 2.4 Catalytic Performance ................................................................................................ 76 2.4.1 Ethene Hydrogenation as Probe Reaction ............................................................ 76 2.4.2 Batch-Operated External Recycle Reactor ........................................................... 88 2.5 References .................................................................................................................. 96 3 Combined TPRx, in situ GISAXS and GIXAS Studies of Model Semiconductor- Supported Platinum Catalysts in the Hydrogenation of Ethene......................................... 102 3.1 Introduction .............................................................................................................. 102 3.2 Experimental ............................................................................................................ 104 3.2.1 Combined TPRx and in situ GISAXS/GIXAS ................................................... 104 3.2.2 Sample Preparation ........................................................................................... 106 3.2.3 In situ Experiments............................................................................................ 108 3.3 Results and Discussion ............................................................................................. 109 3.3.1 Particle Size Analysis by TEM, AFM, and GISAXS ......................................... 109 3.3.2 In situ Characterization under Reduction Conditions and at Elevated Temperatures .................................................................................................................. 111 3.3.3 Hydrogenation of Ethene with Deuterium .......................................................... 115 3.4 Conclusion ............................................................................................................... 120 Table of Contents 3.5 References ................................................................................................................ 122 4 Platinum Nanoparticles on Gallium Nitride Surfaces: The Effect of Semiconductor Doping on Nanoparticle Reactivity ...................................................................................... 126 4.1 Introduction .............................................................................................................. 126 4.2 Experimental ............................................................................................................ 127 4.2.1 Sample Preparation ........................................................................................... 127 4.2.2 High-Pressure X-ray Photoemission Spectroscopy ............................................ 129 4.3 Results ..................................................................................................................... 134 4.4 Discussion ................................................................................................................ 143 4.5 Conclusion ............................................................................................................... 147 4.6 References ................................................................................................................ 147 5 Optoelectronic Control of the Catalytic Reactivity of GaN-Supported Pt Nanoparticles ........................................................................................................................ 151 5.1 Introduction .............................................................................................................. 151 5.2 Results and Discussion ............................................................................................. 152 5.3 Conclusion ............................................................................................................... 157 5.4 Methods ................................................................................................................... 158 5.4.1 Model Catalyst Preparation ............................................................................... 158 5.4.2 Transmission Electron and Atomic Force Microscopy (TEM, AFM) ................. 159 5.4.3 Batch-operated Recycle Reactor Setup for Ethene Hydrogenation ..................... 159 5.4.4 Kinetic Data Evaluation .................................................................................... 160 5.4.5 Charge transfer to Pt nanoparticles on GaN ....................................................... 161 5.5 References ................................................................................................................ 163 6 Summary & Zusammenfassung ................................................................................... 166 6.1 Summary .................................................................................................................. 166 6.2 Zusammenfassung .................................................................................................... 170 7 Outlook .......................................................................................................................... 176 7.1 Outlook .................................................................................................................... 176 7.2 References ................................................................................................................ 180 8 Curriculum Vitae .......................................................................................................... 181 9 List of Publications ....................................................................................................... 183 10 List of Presentations ..................................................................................................... 184 Chapter 1 Chapter 1 General Introduction Abstract The general introduction includes the motivation and scope of this work, as well as the basic theory regarding nanometer-sized metal particles in catalysis and gallium nitride as supporting material. The effects of the particle size and shape on catalysis are discussed with their physical properties. The potential of gallium nitride as support is described for fundamental applications based on its chemical and physical properties. Finally, characteristics of the metal-semiconductor interface are summarized to understand strong metal-support interactions. Chapter 1 1 General Introduction 1.1 Motivation Heterogeneous catalyzed reactions are essentially relevant for the chemical industry, facilitating processes with high efficiency regarding high product selectivity, low energy consumption, and low waste production.[1] The concept of environmentally compatible technologies using sustainable methods and raw materials is described by Green Chemistry.[2] Nearly 90% of fine chemicals are produced using large-scale catalysis.[3] For this purpose, solid catalysts are rationally developed to achieve a cost-effective catalytic activity with properties tuned especially for their corresponding function in the reaction. The catalytic properties can be influenced by the use of various materials containing small particles of suitable metals and supporting substrates, which interact with the catalytically active particles geometrically or electronically.[4] While the metal particles exhibit a well-defined particle size and geometry with high surface-to-volume ratios, as well as high fractions of low-coordinated sites, the support shows a stabilizing effect against particle sintering and modifies the particle reactivity due to metal-support interactions.[3, 5-9] Strong metal-support interactions (SMSI) were investigated in the past for e.g. titania-based catalysts, where reduction at high temperatures altered the catalytic activity of metal particles due to the particle encapsulation by reduced titania species, changing their electronic properties.[4, 10, 11] However, the indirect effect of the electronic impact of the support on the catalytic activity of the particles is still difficult to assess and to quantify. A detailed and systematic study of defined metal-support interfaces, their charge transfer behavior, and their consequence on the catalytic reactivity is required to understand the electronic interplay for an advanced catalyst development. Using semiconducting supports with suitable energies of the valence and conduction bands, as well as the bandgap energy provide the potential for charge transfer between metal particles and the underlying support.[8, 12] Exactly such materials are utilized in heterogeneous photocatalysis, which are currently excessively studied regarding e.g. the use of direct sunlight converting solar energy into chemical energy with high efficiency. Selected photocatalysts were developed for the destruction of organic pollutants and toxic compounds in industrial wastewaters, the activation of C-H bonds for the selective oxidation of intermediates for fine chemicals, the recycling of the greenhouse gas carbon dioxide back into fuels, or for the overall water splitting for hydrogen evolution.[13-17] Titania-based materials 1 Chapter 1 are mainly used as photocatalysts, but its wide bandgap (3.0-3.2 eV) allows only light absorption in ultraviolet range and therefore, this material is not suitable for applications with sunlight. Recent studies concentrated on bandgap engineering for narrowing the bandgap by doping or by introduction of surface structure disorders.[18, 19] Alternative photocatalysts were developed for the visible light-driven hydrogen production from water using an oxynitride solid solution of gallium nitride and zinc oxide loaded with ruthenium oxide or rhodium- chromium mixed oxide particles.[20, 21] The particle-semiconductor contact increased the photocatalytic efficiency due to the trapping of optically excited electrons by the particles, hindering a fast charge carrier recombination.[22] The ability of the particle-semiconductor interface for the transfer of photo-generated charge carriers shows the possibility to control catalytic properties of metal particles by the optical, above bandgap excitation of the underlying support. The study of metal catalysts that facilitate a controlled charge transfer over the metal-support interface in both directions is essentially required to verify the influence of electronic changes of the catalytically active species on their reactivity. For this purpose, the semiconducting support has to be able to transport negative as well as positive charge carriers to the particle in contact. Gallium nitride surfaces have been the most suitable wide bandgap semiconductors (bandgap of 3.4 eV), which are commercially available with n- and p-type conductivity, providing the same crystal structure for both types of doping.[23] In addition to that, gallium nitride is frequently used in the optoelectronic technology and its high thermal stability provides the device application at high powers and temperatures.[24-26] Catalytically active metal particles can be chemically prepared from platinum, where controlled synthesis methods are used to yield size- and shape-selected particles with defined exposed metal surfaces. An identical set of such particles on both n- and p-type gallium nitride surfaces allow for the exclusion of particle effects as origin of a modified catalytic reactivity under optical excitation. Previous studies showed the possible transfer of electrons through the platinum-gallium nitride interface for sensor applications, e.g. the detection of small amounts of hydrogen or the detection of an electron current generated by the exothermic CO oxidation.[27, 28] Such catalyst systems seem to be well-suitable to investigate the electronic control of catalytic properties and the possibilities and limitations of catalysis on demand. 2