Experimente und Simulationen

Contents Acknowledgements 3 Zusammenfassung 4 Abstract 5 Acronyms 11 Nomenclature 12 1 Heterogeneous catalysis 13 1.1 Kinetics 13 1.2 Size effects 14 1.3 CO oxidation 16 1.4 Organization of the dissertation 17 2 Experimental 19 2.1 Molecular beam apparatus 19 2.2 Sample preparation 23 3 Steady state and rate-determining step 27 3.1 CO2 production at steady state as a function of xCO and T 27 3.2 Establishment of the homogeneous mean field model 28 3.3 Rate-determining step 32 3.4 Summary 36 4 Diffusion under reaction conditions 39 4.1 Experimental 39 4.2 Size, xCO and T dependence 40 4.3 Establishment of the reaction-diffusion model 45 4.4 Simulations with the reaction-diffusion model 50 4.5 Summary 57 5 Diffusion and bistability 59 5.1 Global reaction rates at steady state 59 5.2 Local reaction rates at steady state 61 5.3 Global reaction rates under transient conditions 63 5.4 Summary 67 6 Bistability and stochastic phenomena 69 6.1 Global reaction rates under transient conditions 69 6.2 Establishment of the heterogeneous mean field model 72 6.3 Simulations with the heterogeneous mean field model 73 6.4 Establishment of the stochastic model 74 6.5 Simulations with the stochastic model 77 6.6 Summary 78 7 Outlook: Applicability of the methods to other systems 79 7.1 Diffusion under reaction conditions 79 7.2 Diffusion and bistability 80 7.3 Bistability and stochastic phenomena 80 Bibliography 83 Publications 92 A Programs 95Thesis This work proposes new methods, combining experiments and modeling, to study three fundamental aspects of the kinetics of heterogeneous catalytic reactions on supported catalysts: (i) diffusion under reaction conditions, (ii) the interplay between diffusion and kinetic bistability and (iii) the effect of spontaneous fluctuations on a kinetic bistability. Heterogeneous catalysis has an outstanding economical impact. However, the relevant phenomena at the atomic scale remain largely unclear. One reason for this lack of understanding is the complexity of the materials as well as of the reactions occurring at the surface. Another reason is often the lack of suitable experiments to address these difficulties. We therefore propose a series of new approaches concerning the study of diffusion under reaction conditions, the interplay between diffusion and kinetic bistability and the effect of spontaneous fluctuations on a kinetic bistability, always combining molecular beam experiments and modeling. In a first step, the rate-determining steps (RDSs) of CO oxidation on Pd catalysts are calculated for a large temperature range and different experimental conditions. We find that under all conditions, COad governs the activity. We then concentrate on size effects. For that, three different samples prepared by physical vapor deposition (PVD) and electron-beam lithography (EBL) are used. Angle-resolved mass spectrometry and a reaction- diffusion model enable us to follow the diffusion of Oad under reaction conditions and to determine the relevance of local reaction rates on the overall kinetics, for different experimental conditions and surface temperatures between 440 and 490 K. Other parameters such as the influence of the support, the local desorption of products and the morphology of the particle are investigated in detail. We then propose to study the influence of diffusion on a kinetic bistability. We find that, in the present case and despite the large gradient in reaction rate and coverage, switching of the bistability is synchronized over the whole particle. We attribute this phenomenon to the fast diffusion of COad, on which the RDS depends under all conditions. Finally, we concentrate on the bistability. Two models are elaborated: an heterogeneous surface model, which takes edge and step sites into account and a stochastic model, which takes spontaneous fluctuations into account. It is found that only a combination of both models can reproduce the experiments quantitatively. Id est, two concurrent size effects have to be taken into account simultaneously in order to understand the experimental results.These In dieser Arbeit wird ein neuer Ansatz, bestehend aus einer Kombination von Molekularstrahlexperimenten und numerischer Modellierung, zur Erforschung der Kinetik trägerfixierter Katalysatoren vorgestellt und damit die folgenden grundlegenden Phänomene der heterogenen Katalyse diskutiert: (i) Oberflächendiffusion unter Reaktionsbedingungen, (ii) Zusammenhänge zwischen Oberflächendiffusion und kinetischen Bistabilitäten und (iii) Einfluss spontaner Fluktuationen auf kinetische Bistabilitäten. Obwohl heterogene Katalysatoren in vielen industriellen Prozessen eine herausragende Bedeutung haben, sind die zugrunde liegenden Vorgänge auf atomarer Ebene oft völlig unklar. Dies hängt mit der hohen Komplexität realer Katalysatoren und den damit verbundenen experimentellen Schwierigkeiten bei der Untersuchung solcher Systeme zusammen. In dieser Arbeit werden daher mehrere neue Ansätze vorgeschlagen, mit denen durch die Kombination von Molekularstahlexperimenten mit numerischer Modellierung die Untersuchung fundamentaler Prozesse der Katalyse wie die Zusammenhänge zwischen Oberflächendiffusion und spontanen Fluktuationen mit kinetischen Bistabilitäten ermöglicht wird. Zuerst wurden die geschwindigkeitsbestimmenden Schritte (rate-determining step, RDS) der CO-Oxidation auf der Oberfläche von Pd-Katalysatoren für verschiedene Temperaturen und experimentelle Bedingungen berechnet. Es hat sich hierbei gezeigt, dass unter allen Bedingungen die Aktivität des Katalysators durch die Geschwindigkeit der CO-Adsorption bestimmt wird. Anschließend wurde der Einfluss der Pd-Partikelgröße untersucht. Hierzu wurden Modellkatalysatoren mit verschiedenen Partikelgrößen verwendet, die durch PVD und EBL präpariert wurden. Mittels winkelaufgelöster Massenspektrometrie und eines Reaktions-Diffusions-Modells (RD) war es möglich, die Oberflächendiffusion des adsorbierten Sauerstoffs und den Einfluss der lokalen Reaktionskinetik auf die globale Reaktionskinetik für verschiedene experimentelle Bedingungen im Temperaturbereich zwischen 440 und 490 K zu betrachten. Zusätzlich konnten Zusammenhänge zwischen Katalysatorträger, lokaler Desorption der Produkte, Partikelmorphologie und Reaktionskinetik im Detail untersucht werden. Weiterhin wurde der Einfluß der Oberflächendiffusion auf kinetische Bistabilitäten studiert und gezeigt, dass in diesem System das Umschalten zwischen beiden bistabilen Reaktionspunkten trotz großer Unterschiede der lokalen Reaktionsgeschwindigkeiten und der Oberflächenbedeckung auf den Partikeln immer synchron auf dem gesamten Partikel stattfindet. Dies ist mit der schnellen Oberflächendiffusion von adsorbiertem CO verknüpft, dessen Oberflächenbedeckung unter allen Bedingungen den geschwindigkeitsbestimmenden Schritt kontrolliert. Schließlich wurden die Bistabilitäten selbst genauer untersucht. Zwei verschiedene Modelle wurden hierbei verwendet: ein heterogenes Oberflächenmodel, welches Kantenplätze und Stufen der Partikel explizit berücksichtigt, und ein stochastisches Modell, welches spontane Fluktuationen annimmt. Es konnte gezeigt werden, dass nur durch die Kombination der beiden Modelle die Molekularstrahlexperimente quantitativ reproduziert werden können. Das bedeutet, dass zwei konkurrierende Größeneffekte berücksichtigt werden müssen, um die experimentellen Ergebnisse verstehen zu können

Controlling the adsorption kinetics via nanostructuring : Pd nanoparticles on TiO2 nanotubes

Activity and selectivity of supported catalysts critically depend on transport and adsorption properties. Combining self-organized porous oxide films with different metal deposition techniques, we have prepared novel Pd/TiO(2) catalysts with a new level of structural control. It is shown that these systems make it possible to tune adsorption kinetics via their nanostructure. Self-organized TiO(2) nanotubular arrays (TiNTs) prepared by electrochemical methods are used as a support, on which Pd particles are deposited. Whereas physical vapor deposition (PVD) in ultrahigh vacuum (UHV) allows us to selectively grow Pd particles at the tube orifice, Pd/TiNT systems with homogeneously distributed Pd aggregates inside the tubes are available by particle precipitation (PP) from solution. Both methods also provide control over particle size and loading. Using in-situ infrared reflection absorption spectroscopy (IRAS) and molecular beam (MB) methods, we illustrate the relation between the nanostructure of the Pd/TiNT systems and their adsorption kinetics. Control over the metal nanoparticle distribution in the nanotubes leads to drastic differences in adsorption probability and saturation behavior. These differences are rationalized based on differences in surface and gas phase transport resulting from their nanostructure. The results suggest that using carefully designed metal/TiNT systems it may be possible to tailor transport processes in catalytically active materials

Airborne ultrasound surface motion camera: application to seismocardiography

International audienceThe recent achievements in the accelerometer-based seismocardiography field indicate a strong potential for this technique to address wide variety of clinical needs. Recordings from different locations on the chest can give a more comprehensive observation and interpretation of wave propagation phenomena than a single-point recording, can validate existing modeling assumptions (such as the representation of the sternum as a single solid body), and provide better identifiability for models using richer recordings. Ultimately, the goal is to advance our physiological understanding of the processes to provide useful data to promote cardiovascular health. Accelerometer-based multichannel system is a contact method and laborious for use in practice, also even ultralight accelerometers can cause non-negligible loading effects. We propose a new contactless ultrasound imaging method to measure thoracic and abdominal surface motions, demonstrating that it is adequate for typical seismocardiogram use. The developed method extends non-contact surface-vibrometry to fast 2D mapping by originally combining multi-element airborne ultrasound arrays, a synthetic aperture implementation and pulsed-waves. Experimental results show the ability of the developed method to obtain 2D seismocardiographic maps of the body surface 30×40 cm 2 in dimension, with a temporal sampling rate of several hundred Hz, using ultrasound waves with the central frequency of 40 kHz. Our implementation was validated in-vivo on eight healthy human participants. The shape and position of the zone of maximal absolute acceleration and velocity during the cardiac cycle were also observed. This technology could potentially be used to obtain more complete cardio-vascular information than single-source SCG in and out of clinical environments, due to enhanced identifiability provided by distributed measurements, and observation of propagation phenomena

Sensitivity of CO oxidation toward metal oxidation state in ceria-supported catalysts: an operando DRIFTS-MS study

The oxidation of carbon monoxide has been studied on pristine CeO2, Rh–CeO2, and Pt–CeO2 powders prepared in one step by solution combustion synthesis (SCS). The reaction was cycled between an oxygen-rich and a CO-rich feed with regard to the stoichiometric conditions. CO2 production was monitored by mass spectrometry, while the surface species were probed by operando DRIFTS (Diffuse Reflectance Infrared Fourier Transform Spectroscopy). Whereas the reaction starts above 150 °C on CeO2 and Rh–CeO2 and does not depend on the state of the surface (oxidized or reduced), the reaction on Pt–CeO2 shows strong dependency on the initial state and substantial activity is achieved at much lower temperatures with the CO-rich feed. We relate this result to the change in the oxidation state of Pt via strong interaction with ceria

Local reaction rates and surface diffusion on nanolithographically prepared model catalysts: Experiments and simulations

Combining molecular beam methods and angular resolved mass spectrometry, we have studied the angular distribution of desorbing products during CO oxidation on a planar Pd/silica supported model catalyst. The model catalyst was prepared by means of electron beam lithography, allowing individual control of particle size, position, and aspect ratio, and was characterized by atomic force microscopy and scanning electron microscopy before and after reaction. In the experiment, both oxygen and CO rich regimes were investigated using separate molecular beams for the two reactants. This allows exploration of diffusion effects of reactants on the particles and of shadowing and backscattering phenomena. A reaction-diffusion model was developed in order to extract information about local reaction rates on the surface of the catalyst nanoparticles. The model takes into account the structural parameters of the catalyst as well as the backscattering of the reactants and products from the support. It allows a quantitative description of the experimental data and provides a detailed understanding of temperature and reactant flux dependent effects. Moreover, information on the surface mobility of oxygen under steady-state reaction conditions could be obtained by comparison with the experimental results

Tree height in tropical forest as measured by different ground, proximal, and remote sensing instruments, and impacts on above ground biomass estimates

Tree height is an important structural trait, critical in forest ecology and for above ground biomass estimate, and difficult to accurately measure in the field especially in dense forests, such as the tropical ones. The accuracy of height measurements depend on several factors including forest status, the experience of the observer, and the equipment used, with large subjectivity, heterogeneity and uncertainty in results, that can propagate when tree height is used in models. A comparison of Terrestrial Laser Scanning, Airborne Lidar Scanning, and stereo-photogrammetry (with imagery acquired by a RGB camera mounted on Unmanned Aerial Vehicle) approaches for estimating tree height was here performed, also with reference to ground methods. In fact, all those technique may increase the possibility of precise tree height measures, while reducing manual effort in comparison to more traditional ground techniques. The research was carried out in a dense tropical forest in Ghana; differences in measured heights as well as their impact on above ground biomass estimation were analyzed. All the different methods were characterized by pros and cons: the obtained results indicate that in dense forests, where sight occlusion problems occur, ground traditional techniques can lead to overestimation, while with the other mentioned techniques underestimation can occur, but in variable amount according to the considered instrument. The different height measures caused a remarkable variation in the estimated biomass of this tropical forest: more accurate height measurements are needed to reduce the uncertainty in biomass mapping efforts at any scale. Possibly, the simultaneous use of different methods can help in correctly estimate height uncertainty and reach a convergent and accurate result