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«The Location of Noble Metal Clusters on Multicomponent and Multifunctional Aerosol-Made Heterogeneous Catalysts A dissertation submitted to ETH ...»

-- [ Page 1 ] --

Diss. ETH No. 19617

The Location of Noble Metal Clusters on

Multicomponent and Multifunctional

Aerosol-Made Heterogeneous Catalysts

A dissertation submitted to

ETH ZÜRICH

for the degree of

Doctor of Sciences

presented by

Robert Büchel

Dipl. Ing. ETH

born on March 11th, 1982

citizen of Altstätten SG, Switzerland

accepted on the recommendation of

Prof. Dr. S. E. Pratsinis, examiner

Prof. Dr. A. Baiker, examiner

Zürich 2011 "Wenn ich mal ungeduldig werde, denke ich an die Geduld der Erde."

Johann Wolfgang von Goethe c Robert Büchel

ACKNOWLEDGMENT

This Ph.D. project was carried out in close collaboration of the Particle Technology Laboratory (PTL) and the Institute for Chemical- and Bioengineering at ETH Zürich.

I am deeply grateful to my advisors Prof. Dr. Sotiris E. Pratsinis and Prof. Dr. Alfons Baiker, for supervising my Ph.D. project and for encouraging me with valuable scientific advice and critical feed back. I thank the members of the Particle Technology Laboratory for providing an environment for scientific working and providing multifaceted ideas. A special thanks goes to Dr. Björn Schimmöller and Dr. Reto Strobel for their help with the setup of the new sulfur testing reactor and for inputs and discussions of ambiguous catalytic results. I also like to thank the members of the Baiker group namely Dr. Urakawa Atsushi, Dr. Eva Rödel, and Dr.

Nobutaka Maeda for critical discussions about NSR catalysts and their helpful support. For the kind introduction to FTIR I thank Dr. Davide Ferri and Dr. Jun Huang who helped me to measure stoichiometry factors using DRIFTS measurements.

For hints and tips for data analysis with MATLAB many thanks to our "programming" team Beat Büsser, Maximilian Eggersdorfer and Arto Gröhn. For good start into the PTL group I thank Dr. Frank Ernst and i ii Acknowledgment Dr. Karsten Wegner for the continuous knowledge transfer and support.

This work would not have been possible without the help of students helping to advance my projects: Oliver Waser who did his diploma thesis, Michele Bolla, Alessandro Karpf, and Leonardo Meiler who did their bachelor thesis investigating NSR catalysts; Philipp Furler, George Sotiriou, Justus Lambrecht, Jonas Wegmann, Daniel Mesitschek, Michelle Müller, David Vles Lukas Keller, Akbar Fashori doing a their semester project under my supervision; Stephanie Berger, Nora Hild, Juuso Jokiniemi making an internship at PTL and Richard Häfeli, Georg Balmer who worked as help-assistants.

Special thanks to Dr. Frank Krumeich for visualizing the nanoparticles with electron microscopy, and the electron microscopy center (EMZ) for providing the infrastructure. For technical and constructive help I was always supported by René Plüss. Computer and network problems were always competently solved by Sascha Jovanovic or Justina Palmer. Many thanks also to Agnes Rupacher for her devoted help with administrative work.

I wish to acknowledge financial support by the Swiss Federal Institute of Technology (TH Research Grant TH-09 06-2) and the European Research Council/European Community (under FP7). I thank for the contribution of platinum chemicals by Johnson Matthey PLC.

Finally, I would like to express my thanks to both of my parents and my sister Barbara who motivated and supported me in times, when experiments did not work as planned.

CONTENTS

–  –  –

List of Publications 202 Summary Noble metals are important in catalysis. They typically determine the catalysts activity and selectivity. To gain maximum activity out of the noble metals, they are finely dispersed on a support. Although the support is chemically inert, it plays a crucial role in the catalyst performance as it can promote the catalyst performance, enhance or inhibit spillover phenomena.

With a two nozzle Flame Spray Pyrolysis unit (FSP) a variety of material combination was prepared. With the two flames individual particles can be made in each of the two flames, which then mix continuously in the upper part of the flame. By this means, the formation of mixed oxides can be avoided and the noble metal position controlled: By adding noble metal to one of two flames, selective deposition of the noble metal was achieved.

The angle between the two flames changed the meeting distance of the two flames. The physical properties and catalytic performance was tested under different production angle between the flames for Pt/Ba/CeO2 with selected Pt location. When Pt was deposited on CeO2 more PtOx was formed, having a lower performance due to higher C3 H6 light off temperatures. When Pt was deposited selectively on CeO2 a constant light off temperature at 325 ◦ C was observed. For Pt deposited on the Ba sites, the light off temperature depended on the angle between the two flames: the earlier the Pt/BaCO3 particles mixed with CeO2 the higher was the light of temperature. This showed that close contact of Pt to CeO2 can be a disadvantage and this is also reflected in the catalyst performance.

Here, catalysts for nitrogen oxides (NOx ) storage and reduction were investigated, a technology used in mobile applications for NOx abatement under lean exhaust conditions. These catalysts work under alternating cycles; one of them is fuel lean the other fuel rich. In the fuel lean cycle NOx is stored on a storage material, typically Ba or K, and in the form of metal nitrates. In the subsequent fuel rich cycle, the storage material is Summary ix regenerated and the released NOx reduced to N2. When Pt was in close contact with CeO2 the C3 H6 reduction activity was reduced and therefore the catalyst performance dropped. For other supports, like γ-Al2 O3, this is not the case; Enhanced NO2 formation was observed with Pt/Ba/Al2 O3 catalysts when the Pt was deposited on the Al-sites. It was found that the forward spill-over was not limiting the catalyst’s performance. However, Pt deposited on Ba promoted the regeneration of the nitrates and steady state and continuous storage and reduction was observed. A close contact of the noble metal to the storage material enhances the regeneration of the nitrates. When depositing preferentially Pt by FSP the other catalysts properties changed only minimally. This was shown by measuring the specific surface area (SSA), X-ray diffraction (XRD), elemental analysis with laser ablation-inductive coupled plasma-mass spectroscopy (LA-ICP-MS) and scanning transmission electron microscopy combined with energy dispersive X-ray spectroscopy (STEM-EDX). The latter also showed that the noble metal was effectively preferentially deposited. The preferential location of the noble metal, however, could only be catalytically differentiated when the performance was below 80% conversion per cycle. At higher temperatures all chemical processes were enhanced and the difference between the catalysts became minimal.





High storage performance was observed when using potassium (K) as storage material. The NOx storage was very fast and regeneration very efficient: at 300 ◦ C about 80% of NOx inlet was reduced during more than 50 cycles and at 400 ◦ C no effluent NOx was measured. When sintering the particles at elevated temperature (600 ◦ C) the original high storage activity was lost partially, however, it still performed with 40% NOx conversion.

The crystal structure of the K2 CO3 was initially amorphous but slowly underwent crystallization. With Raman spectroscopy this amorphous to crystalline transition could be accelerated using a 25 mW power laser beam.

This measurement also proved that the initial K2 CO3 was amorphous as x Summary the signal was irreversible.

This results motivated to investigate K2 CO3 /Al2 O3 systems further by adding palladium (Pd). Palladium is known for enhancing the NOx reduction at lower temperature. Different mono metallic Pt or Pd catalysts were made but also bimetallic combinations of Pt and Pd. For the bimetallic metal catalysts the metals could be sprayed together, or, in different flames. Different metal weight loadings were tested at 250 and 300 ◦ C. For Pt, higher loading resulted in higher NOx conversion with increasing temperature. For Pd this was also true at 300 ◦ C, but at 250 ◦ C higher loading did not increase the catalyst activity. Combinations of Pd with Pt lowered the catalysts activity at 250 ◦ C, as Pd inhibited the Pt performance. The advantage of Pd became evident only at elevated temperatures, e.g. 350 ◦ C in Pd/Pt/Ba/CeO2 system where Pt on CeO2 and Pd on BaCO3 showed the highest performance.

The NSR catalysts are easily poisoned by sulfur dioxide (SO2 ). To investigate the sulfation process a new test bench was built. Combination of rhodium (Rh) and Pt in Rh/Pt/Ba/Al2 O3 catalysts were investigated as Pt is tolerant against SO2 poisoning for NO oxidation while Rh was found to be SO2 tolerant for NOx reduction. Therefore Pt and Rh were combined. Mono- and bimetallic Rh/Pt combinations were tested without and in the presence of 25 ppm SO2. All catalysts performed well without SO2. With SO2, however, the Pt catalysts were immediately poisoned and the presence of Rh was essential for a good NOx reduction performance.

The Rh alone catalysts had the lowest dispersion but performed best. Carbon monoxide Diffuse Reflectance Infrared Fourier Transformation (CODRIFT) spectrometry showed distinct difference between Pt and Rh when deposited either on Al2 O3 or BaCO3. Bimetallic catalysts containing Pt and Rh showed similar performance as Rh-alone catalysts.

Zusammenfassung Edelmetalle sind in der Katalyse wichtig: Ihr Einsatz definiert häug die Aktivität und die Selektivität einer Reaktion hin zum gewünschten Produkt. Das Aufbringen sehr fein verteilter Edelmetall-Partikel auf einem Trägermaterial erhöht die Aktivität pro Edelmetallmasse, da die katalytisch aktive spezifische Oberfläche erhöht wird. Das Trägermaterial selbst nimmt nicht an der chemischen Reaktionen teil, kann aber die Effizienz des Katalysators massgeblich beeinflussen, da es die Umgebung des Edelmetalls bestimmt und oft den Oxidationszustand des Edelmetalls bestimmt.

Mit der zwei Flammen Spray Pyrolyse (FSP) Einheit konnten verschiedene Mehrkomponentenverbindungen hergestellt werden. Die beiden Flammen erlauben es zwei Materialien unabhängig voneinander zu produzieren. So kann die Entstehung von Mischoxiden gezielt vermieden werden. Die beiden Partikel beladenen Gas-Ströme werden kontinuierlich in der Luft, oberhalb der Flammen, gemischt. Weiter eröffnet dieses Verfahren die Möglichkeit Edelmetalle gezielt auf ein Trägermaterial zu applizieren: Wurde das Edelmetall in den Ausgangstoff (Precursor) der Einen der beiden Flammen zugeführt, so wurde das Edelmetall selektiv auf dem entsprechenden mitproduzierten Material gebildet. Abhängig von der Ausrichtung beider Flammen zueinander kann die Flugbahn der Partikeln, und somit die Zeit bis zur Vermischung beider Partikelströme, variiert werden. Die bestimmende Grösse war der Winkel zwischen den beiden Flammen. Grössere Winkel erhöhen die Distanz bis sich die Partikel der beiden Flammen vermischten.

Um diesen Effekt zu zeigen, wurden Katalysatoren aus Pt/Ba/CeO2 mit präferentieller Pt-Position unter verschiedenen Winkeln hergestellt und ihre physikalischen und katalytischen Eigenschaften getestet: Wenn Pt mit Ce produziert wurde, entstand vermehrt PtOx mit einer C3 H6 Anspringtemperatur (light off temperature) von 325 ◦ C, welche unabhängig vom Winkel zwischen den beiden Flammen war. Für Pt, welcher mit dem Ba

xii Zusammenfassung

produziert wurde, war die Anspringtemperatur abhängig vom Winkel zwischen den beiden Flammen: Wenn sich die Pt/BaCO3 Partikel früher mit dem CeO2 mischten, erhöhte sich die Anspringtemperatur. Der Kontakt von Pt zu CeO2 kann deshalb ein Nachteil sein, welches sich auch in der Effizienz des Katalysators niederschlägt. In dieser Arbeit werden Katalysatoren zur Stickstoffoxid (NOx ) Speicherung und Reduktion untersucht. Anwendung findet diese Technologie in der Abgasnachbehandlung von sauerstoffreichem Abgas (z.B. Magermotoren). Die Katalysatoren arbeiten unter wechselnden Bedingungen: Brennstoffarme und brennstoffreiche Konditionen wechseln sich zyklisch ab. In brennstoffarmer Umgebung wird NOx auf einem Speichermaterial (meist Ba oder K) als Metall-Nitrat eingelagert.

Im darauf folgenden brennstoffreichen Zyklus, wird das Speichermaterial regeneriert und das freigesetzte NOx zu N2 reduziert. Ist Platin in Kontakt mit einer CeO2 -Oberfläche, verschlechtert sich die C3 H6 Verbrennung und somit auch die Leistung des Katalysators. Bei anderen Trägermaterialien, wie beispiels/-weise γ-Al2 O3, ist dies nicht der Fall. Für Pt/Ba/Al2 O3 Katalysatoren wurde eine erhöhte NO2 Umsatzrate gemessen, wenn Pt auf den Al2 O3 Partikeln abgeschieden wurde. Der Vorwärts Spill-Over limitierte nicht die Leistung des Katalysators. Anderseits unterstütze der Kontakt von Pt zu Ba die Ba(NO3 )2 Regeneration (von Ba(NO3 )2 zu BaO bzw BaCO3 ) und es stellte sich bald ein lang anhaltender stationäre Zustand zwischen Speicherung und Regeneration ein. Mit der Flammensynthese kann das Edelmetall präferentiell abgeschieden werden ohne die anderen Katalysatoreigenschaften zu verändern. Dies wurde durch folgende Messungen bestätigt: Spezifische Oberflächen-Messung (SSA), Röntgenbeugung (XRD), Elementaranalyse (ICP-MS) und Elektronenmikroskopie (STEM, TEM). Mit der Elektronenmikroskopie kombiniert mit Energiedispersiver Röntgenspektroskopie (EDX) konnte die präferentielle Positionierung visuell bestätigt werden. Der Unterschied zwischen Katalysatoren mit präferentieller Edelmetallablagerung konnte nur bei Umsätzen unter 80% Zusammenfassung xiii zugeordnet werden, da bei höheren Umsätzen (z.B. bei erhöhter Temperatur) die chemischen Prozesse beschleunigt werden und die Unterschiede somit nahezu verschwinden. Kalium (K) wies eine erhöhte NOx Speicherkapazität und eine verbesserte Regenerationsfähigkeit im Vergleich zu Barium auf. Auch konnte die Reaktionsgeschwindigkeit des NOx Speicherungsvorgangs durch den Einsatz von K beschleunigt werden. Bei 300 ◦ C wurde über mehr als 50 Zyklen hinweg ca. 80% der NOx Masse im Einlass umgesetzt.



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