WWW.ABSTRACT.XLIBX.INFO
FREE ELECTRONIC LIBRARY - Abstract, dissertation, book
 
<< HOME
CONTACTS



Pages:   || 2 | 3 | 4 | 5 |   ...   | 13 |

«Shape and Catalytic Mechanism of RuO2 Particles at CO Oxidation Reaction Conditions: First-Principles Based Multi-Scale Modeling DISSERTATION Tongyu ...»

-- [ Page 1 ] --

TECHNISCHE UNIVERSITÄT MÜNCHEN

Department Chemie

Lehrstuhl für Theoretische Chemie

Shape and Catalytic Mechanism of RuO2 Particles at CO

Oxidation Reaction Conditions:

First-Principles Based Multi-Scale Modeling

DISSERTATION

Tongyu Wang

TECHNISCHE UNIVERSITÄT MÜNCHEN

Lehrstuhl für Theoretische Chemie

Shape and Catalytic Mechanism of RuO2 Particles at CO Oxidation Reaction Conditions: First-Principles Based Multi-Scale Modeling Tongyu Wang Vollständiger Abdruck der von der Fakultät für Chemie der Technischen Universität München zur Erlangung des akademischen Grades eines Doktor der Naturwissenschaften (Dr. rer. nat) genehmigten Dissertation.

Vorsitzender: Univ.-Prof. Dr. Ville Kaila

Prüfer der Dissertation:

1. Univ.-Prof. Dr. Karsten Reuter

2. Univ.-Prof. Dr. Sebastian Günther Die Dissertation wurde am 17.09.2015 bei der Technischen Universität München eingereicht und durch die Fakultät für Chemie am 14.10.2015 angenommen.

i Abstract A central objective of heterogeneous catalysis research is to establish structuremorphology-activity relationships. However, due to experimental difficulties in determining the crystal habit under reaction conditions, such relationships are yet scarcely built up. The hitherto primarily feasible access are so-called ex-situ studies, which neglect the attested possibly strong dynamical evolution of operating catalysts and the significant effect that this can have on the catalytic function. In this situation, independent information from first-principles-based theory can be an enormous asset.

This route is pursued in this thesis, and CO oxidation on RuO2 nanoparticles is chosen as a prototypical system. The hitherto almost exclusively studied lowest-energy RuO2(110) facet was recently shown to deactivate into a RuO2(100)-c(2x2) structure under O-rich reaction conditions. In contrast, increasing activity of the nanoparticles upon changing to more reducing conditions has been reported. This could be due to the stabilization of an O-deficient RuO2 terminations or the evolution to a different crystal shape while on stream. To scrutinize the latter hypothesis, the surface structure and composition of low-index RuO2 facets at different oxidation conditions is first investigated using density-functional theory (DFT) calculations combined within ab initio thermodynamics. The resulting DFT-based Wulff construction of the equilibrium shape of RuO2 particles in an oxygen environment allows for a detailed discussion of the dependence ofthe oxidizing pretreatment used in recent powder catalyst studies. The analysis specifically indicates an incomplete particle shape equilibration in previously used low temperature calcination. We suggest that the equilibrated particles could be active CO oxidation catalysts with long-term stability in oxidizing feed and then represent an interesting alternative to a previously suggested the core-shell concept.

Moreover, the obtained results point at a prominent role of the hitherto little studied apical facets for the long-term catalytic activity. To scrutinize this we perform DFTbased kinetic Monte Carlo simulations of CO oxidation at the (111) facet of RuO2.

Comparing the detailed insight into elementary processes, steady-state surface coverages and catalytic activity to equivalent published simulation data for the frequently studied RuO2(110) facet, qualitative differences are identified in virtually every aspect ranging from binding energetics over lateral interactions to the interplay of elementary processes at the different active sites. Nevertheless, particularly at ii technologically relevant elevated temperatures, near-ambient pressures and nearstoichiometric feeds both facets exhibit almost identical catalytic activity. This can be an important aspect in understanding the long-term steady-state activity of RuO2 nanoparticles observed in experiments. Furthermore, these findings challenge the traditional definition of structure sensitivity based on macroscopically observable turnover frequencies, in which the role of electronic effects due to a different degree of under-coordination of surface atoms, as well as the role of geometric effects due to different bonding configurations are considered as the most important factors for metal catalysts. For oxide surface our study suggests that the spatial distribution of the active sites and concomitant diffusion limitations constitute a third important factor.

Our theoretical study on RuO2 polycrystalline powders builds up an appealing link between detailed surface science work on single crystals and the supported nanoparticles in real catalysis. This ansatz offers a general route to the exploration of important factors such as particle size, shape, composition and catalytic activity without the additional complexity introduced by a support material. Our work thus serves as an important step along the long-term track of systematically bridging between singlecrystal model and real catalysis.

iii

Zusammenfassung

Eine der Hauptaufgaben in der Forschung über heterogene Katalyse ist es das Verhältnis zwischen der Struktur, Morphologie und Aktivität von Katalysatorpartikeln zu etablieren. Bislang sind solche Korrelationen kaum erarbeitet, unter anderem auch durch experimentelle Limitierungen die Kristallform unter laufenden Reaktionsbedingungen zu bestimmen. Primär kann dies bisher nur in sogenannten exsitu Untersuchungen erfolgen, die aber eine eventuell starke dynamische Veränderung aktiver Katalysatoren unter Reaktionsbedingungen nicht berücksichtigen. In dieser Situation birgt die unabhängige Information von ab initio basierter Theorie und Modellierung ein enormes Potenzial.





Dieser Ansatz wird in dieser Arbeit verfolgt. Speziell wird die CO Oxidation an RuO2 Nanoteilchen als ein prototypisches Modellsystem gewählt. Neuere Arbeiten zeigen eine Deaktivierung der erschöpfend untersuchten, niederenergetischen RuO2(110) Facette unter Sauerstoff-reichen Reaktionsbedingungen durch Mikrofacettierung in eine RuO2(100)-c(2x2) Struktur. Im Gegensatz hierzu wurde eine zunehmende Aktivität der RuO2 Nanoteilchen unter reduzierenden Reaktionsbedingungen beobachtet. Dies könnte auf eine Stabilisierung O-defizienter RuO2 Terminierungen oder eine dynamische Veränderung der Kristallform zurückgeführt werden. Um diese Hypothesen zu prüfen wird die Oberflächenstruktur und die Zusammensetzung aller niedrigindizierten RuO2 Facetten in unterschiedlichen Oxidationsbedingungen zunächst mit Hilfe der Dichtefunktionaltheorie (DFT) in Kombination mit thermodynamischen Methoden untersucht. Die resultierende DFT-basierte Wulff Konstruktion der Gleichgewichtsform von RuO2 Teilchen in einer Sauerstoffumgebung ermöglicht eine detaillierte Diskussion über die Abhängigkeit der oxidativen Vorbehandlung in zuletzt durchgeführten experimentellen Arbeiten an polykristallinen Pulvern. Die Analyse deutet insbesondere auf eine unvollständige Equilibrierung der Teilchenform bei der teilweise verwendeten Kalzinierung bei niedriger Temperatur. Equilibrierte Teilchen könnten hingegen aktive CO Oxidationskatalysatoren mit langfristiger Stabilität unter Sauerstoffreichen Bedingungen sein und dann eine interessante Alternative zum bisher vorgeschlagenen Core-Shell Konzept darstellen.

–  –  –

untersuchen führen wir eine DFT-basierte kinetische Monte-Carlo-Studie zur COOxidation an der (111) Facette von RuO2 durch. Im detaillierten Vergleich der Elementarprozesse, der Bedeckungen im stationären Zustand, und der katalytische Aktivität mit früher veröffentlichten Simulationsdaten für die extensiv untersuchte RuO2(110) Facette zeigen sich qualitative Unterschiede in praktisch jedem Aspekt von Bindungsenergien über laterale Wechselwirkungen bis hin zum Zusammenspiel der elementaren Prozesse an den verschiedenen aktiven Zentren. Dennoch weisen beide Facetten fast identische katalytische Aktivität auf, vor allem bei technisch-relevanten erhöhten Temperaturen, bei (fast) atmosphärischen Drücken und grob stöchiometrischen Partialdrücken. Dieser Umstand könnte ein grundlegendes Puzzlestück zum Verständnis der experimentell beobachteten Langzeit-Aktivität von RuO2 Partikeln sein. Darüber hinaus sind diese Ergebnisse im Hinblick auf die traditionelle Definition einer Struktursensitivität interessant, die allein auf makroskopisch beobachtbaren Umsatzfrequenzen basiert. Bisher werden hierfür elektronische Effekte aufgrund einer unterschiedlichen Unterkoordinierung der Oberflächenatome, sowie geometrische Effekte aufgrund der unterschiedlichen Bindungskonfigurationen als die wichtigsten Faktoren für Metallkatalysatoren betrachtet. Für Oxide weist unsere Studie auf einen dritten wichtigen Faktor in Form der räumliche Verteilung der aktiven Stellen bei gleichzeitigen Diffusionsbeschränkungen hin.

Unsere theoretische Untersuchung zu polykristallinen RuO2 Pulvern errichtet eine zukunftsweisende Verbindung zwischen detaillierten Studien im Surface Science Kontext an Einkristallen und den geträgerten Nanopartikeln in der realen Katalyse.

Dieser Ansatz bietet einen allgemeinen Weg zur Erforschung von Faktoren wie Teilchengröße, Form, Zusammensetzung und katalytischer Aktivität, ohne die durch ein Trägermaterial zusätzlich eingeführte Komplexität. Unsere Arbeit dient somit als einen wichtiger Schritt in Richtung der langfristig verfolgten systematischen Brückenbildung zwischen Einkristall-Modellkatalysatoren und realer Katalyse.

v

Contents

Abstract

Zusammenfassung

Contents

1 Introduction

I. Theoretical background

2 Multi-scale modeling methods: Electronic regime

2.1 Electronic structure theory

2.1.1 Adiabatic Born-Oppenheimer approximation

2.1.2 Direct solution of the electronic schrödinger equation

2.1.3 Hartree-Fock approximation

2.2 Density-functional theory

2.2.1 Thomas-Fermi model

2.2.2 Hohenberg–Kohn theorems

2.2.3 Kohn-Sham density-functional theory

2.2.4 Approximations to the exchange-correlation functionals

2.3 Calculating the electronic ground-state

2.3.1 Iterative approach to solving the Kohn-Sham equations

2.3.2 Basis sets and boundary conditions

2.3.3 Pseudopotentials

2.4 Transition state location

2.4.1 Drag method

2.4.2 Nudged Elastic Band (NEB) method

3 Multi-scale modeling methods: Mesoscopic regime

3.1 Ab initio thermodynamics

3.1.1 Surfaces at reaction conditions

3.1.2 Calculating Gibbs free energies

3.1.3 Surface free energy plots

3.2 First-principles kinetic Monte Carlo (1p-kMC) simulations

vi 3.2.1 1p-kMC algorithm

3.2.2 First-principles rate constants

3.3 The kmos framework

II RuO2 particles at oxidation reaction conditions

4 Ab initio Wulff construction for RuO2 nanoparticles under oxidizing conditions

4.1 Introduction

4.2 Computational details

4.3 Results and discussion

4.3.1 Surface free energies of RuO2(111) terminations

4.3.2 Wulff construction for RuO2 nanoparticles

4.3.3 Comparison with experimental data

4.4 Conclusions

5 First-principles kinetic Monte Carlo simulations for CO Oxidation at RuO2(111)

5.1 Introduction

5.2 Methodology

5.2.1 Lattice model and elementary processes

5.2.2 First-principles rate constants and computational details

5.2.3 1p-kMC simulation setup

5.3 Results and discussion

5.3.1 CO and O adsorption geometries and energetics

5.3.2 Coverage and turnover frequency at catalytically active conditions............... 70

5.4 Conclusions

6 Summary and outlook

7 Appendix

8 References

List of publications

List of abbreviations

Acknowledgements

Introduction

1. Introduction ___________________________________________________________________ 2

1.1 Scientific background Gasoline-engine exhaust, especially CO, is a serious threat to the environment and public health, so environmentally benign technologies are required to eliminate this poisonous gas. An efficient way of removing CO is catalytic oxidation of CO to CO2.

Although this reaction has been an extensively studied model in heterogeneous catalysis research for many decades, increasing atomic-scale characterization and understanding in recent years reveals an unexpected complexity that is now strongly debated in the literature.1-4 Much of this debate concerns the oxidization of the predominantly employed late transition metal catalysts5-7 in the O-rich environments under near-ambient technological operation. The really active state of the working catalyst is not the metal in its original state, but possibly a formed bulk oxide, or an oxidized film of a core-shell structured nanoparticle.8-11 CO oxidation at Ru catalysts has been high up on the research agenda of oxide formation due to an enigmatic huge activity difference of Ru. This particular system has the lowest reactivity in ultra-high vacuum (UHV), while shows the highest reactivity under near-ambient conditions.5, 7, 11-13 Particular interest in CO oxidation over Ru was originally triggered by Cant, Hicks and Lennon7 working on supported catalysts. Under UHV conditions their Ru catalyst exhibited by far the lowest activity among the studied late transition metals, while under reactant pressures of atmospheres, the situation was reversed, and a superior activity of Ru compared to Pt, Rh or Pd was measured.



Pages:   || 2 | 3 | 4 | 5 |   ...   | 13 |


Similar works:

«Dokumentation Technische Parameter Modell Agrammon Thomas Kupper, Harald Menzi Version 20.03.2013 1. Einleitung Die verwendeten Emissionsraten und Annahmen zur Wirkung verschiedener Einflussgrössen beruhen soweit möglich auf wissenschaftlichen Untersuchungen in der Schweiz. Wo solche fehlten, wurden Daten aus dem Ausland beigezogen. Sie wurden soweit möglich und sinnvoll auf die von der UNECE (Wirtschaftskommission der Vereinten Nationen für Europa) vorgeschlagenen Werte abgestimmt (vgl....»

«NASA/TM—2002-211716 An Introduction to Tensors for Students of Physics and Engineering Joseph C. Kolecki Glenn Research Center, Cleveland, Ohio September 2002 The NASA STI Program Office... in Profile • CONFERENCE PUBLICATION. Collected Since its founding, NASA has been dedicated to papers from scientific and technical the advancement of aeronautics and space conferences, symposia, seminars, or other science. The NASA Scientific and Technical meetings sponsored or cosponsored by...»

«Loughborough University Institutional Repository Tourism destination preferences of low-cost airline users in the East Midlands This item was submitted to Loughborough University's Institutional Repository by the/an author.DAVISON, L.J. and RYLEY, T., 2010. Tourism destination prefCitation: erences of low-cost airline users in the East Midlands. Journal of Transport Geography, 18 (3), pp.458-465.Additional Information: • This is the author's version of a work that was accepted for publication...»

«CA11: Redruth, Camborne and Gwennap Key Landscape Characteristics1 • Rolling slate and siltstone landscape with small scale inland but more open on the north coast.• Post industrial mining landscape with many visible mining relics, including old engine houses and revegetating spoil heaps with remnant surviving or developing woodland, heath or wetland.• Extensive Lowland Heathland with bracken and scrub along coastal strip • Pastoral landscape of improved and rough grazing with extensive...»

«RISK MANAGEMENT AND CONTROL GUIDANCE FOR SECURITIES FIRMS AND THEIR SUPERVISORS A Report by the Technical Committee of the International Organization of Securities Commissions May 1998 TABLE OF CONTENTS I. Introduction II. The Role of Risk Management and Controls III. Firm and Supervisory Considerations IV. Elements of a Risk Management and Control System. 13 Appendix A Significant Risk Management and Control Papers. 19 Appendix B Risk Management and Control Self-Assessment Grid. 27 -1I....»

«Simulation in Produktion und Logistik Entscheidungsunterstützung von der Planung bis zur Steuerung Wilhelm Dangelmaier, Christoph Laroque & Alexander Klaas (Hrsg.) Paderborn, HNI-Verlagsschriftenreihe 2013 Simulationsbasiertes Assistenzsystem zur Bewertung, Auswahl und Konfiguration von PPS-Methoden für mittelständische Produktionsdienstleister – Anforderungen, Systemkonzeption und Anwendungsperspektiven Simulation-based assistance system for evaluation, selection and configuration of...»

«XLIV Annual Symposium on Optical Materials for High-Power Lasers 23-26 September 2012 Abstract Book spie.org/ld Location Laser-Induced Damage Issues: Other Laser-Induced National Institute of Standards Damage Related Issues: Photonic bandgap materials and Technology (NIST) Measurement protocols High-power fiber lasers Boulder, Colorado, USA Materials characterization Fibers for high-power laser applications Fundamental mechanisms Conference High-power/ultrafast lasers Contamination of optical...»

«What is Time Management? Time management is the managing of your time so that time is used to your advantage and it gives you a chance to spend your most valuable resource in the way you choose. Time management is a skill that can be learned which involves techniques for prioritizing activities and using time effectively while eliminating disruptions and time wasters. Time Management is an endless series of decisions, small and large, that gradually change the shape of your life. No one has...»

«Eurex Clearing Response to ESMA Consultation Paper on Draft Technical Standards for the Regulation on OTC Derivatives, CCPs and Trade Repositories Frankfurt am Main, August 3rd, 2012 Eurex Clearing response to ESMA Consultation Paper on ‘Draft Technical Standards for the Regulation on OTC Derivatives, CCPs and Trade Repositories’ issued in June 2012 A. Introduction Eurex Clearing is a globally leading central counterparty (CCP). We offer fully automated and straight-through post trade...»

«Challenges of User-Centered Applications Visualization on and Interaction with Arbitrary Display Environments Vom Fachbereich Informatik der Technischen Universit¨t Kaiserslautern a zur Erlangung des akademischen Grades Doktor der Ingenieurswissenschaften (Dr. -Ing.) genehmigte Dissertation von Dipl. -Ing. Peter Scott Olech Datum der wissenschaftlichen Aussprache: 23. Juli 2013 Dekan: Prof. Dr. Arnd Poetzsch-Heffter Vorsitzender des Pr¨fungsausschusses: u Prof. Dr. Karsten Berns 1....»

«Earth & E-nvironment 3:115-158 University of Leeds Press The Effects of Varied Management Techniques on the Recreation of Lowland Heathland Habitats in Devon Katie S. Fortnam School of Earth & Environment, University of Leeds, Leeds, W. Yorkshire LS2 9JT; Tel: 0113 3436461 Abstract Lowland heathland habitats have decreased extensively in recent years, making it a priority habitat for conservation. Governmental targets have now been set to create, restore and expand heathland in the UK. For...»

«Maimonides and the Protective Function of Mezuza By David Silverberg Towards the end of Parashat Eikev (11:13-21), we read the second paragraph of the daily shema prayer, which, like the first paragraph (presented earlier in the Book of Devarim – 6:4-9), concludes with the mitzva of mezuza: “You shall inscribe them upon the doorposts of your home and in your gates.” The obligation to inscribe “them” obviously refers to these two Biblical passages, which the Torah here requires that...»





 
<<  HOME   |    CONTACTS
2016 www.abstract.xlibx.info - Free e-library - Abstract, dissertation, book

Materials of this site are available for review, all rights belong to their respective owners.
If you do not agree with the fact that your material is placed on this site, please, email us, we will within 1-2 business days delete him.