Dissertation / PhD Thesis

PUBDB-2022-07167

http://join2-wiki.gsi.de/foswiki/pub/Main/Artwork/join2_logo100x88.png Growth, single-particle, and ensemble X-ray diffraction from model catalysts under operandoreaction conditions

Runge, H. (Corresponding author)DESY* ; Stierle, A. (Thesis advisor)XFEL.EU*DESY* ; Vartaniants, I. (Thesis advisor)XFEL.EU*DESY*

2021

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Please use a persistent id in citations: urn:nbn:de:gbv:18-ediss-99235  doi:10.3204/PUBDB-2022-07167

Abstract: This thesis provides a growth study of α-Al2O3(0001) supported Pt particles and subsequent structure monitoring using ensemble and single-particle X-ray diffraction (XRD) during catalytically driven CO oxidation on PtRh particles.The growth study provided two different particle morphologies dependent on the type of annealing after epitaxial growth at 830 °C using molecular beam epitaxy (MBE) in ultra high vacuum (UHV): annealing of particles at 1100 °C in UHV yielded (111)-oriented, flat particles with a height of about 4 nm and an average diameter of about 20 nm (surface coverage about 40 %). In turn, upon exposure to air at 1200 °C, the nanoparticles undergo a redistribution of the deposited material with the coverage significantly decreased to 7 %, likely via the formation of metastable PtOx species. The transformation results in a broad height and diameter distribution with particles covering sizes from few nanometers to diameters of above 100 nm. These particles do not exhibit a flat morphology, instead they are closer to their equilibrium shape.Monitoring a sample with particles of flat morphology using ensemble XRD during catalytically enhanced, CO oxidation detected no shape changes at a temperature of 370 °C with a stoichiometric ratio of oxygen and carbon monoxide near ambient pressure. However, at 450 °C and over-stoichiometric ratio of oxygen to carbon monoxide, the flat particles were subject to non-reversible intra-particle mass transport increasing height while decreasing diameter of the particles which was pronounced for larger particles. The formation of a superficial rhodium oxide layer and Ostwald-rippening were not detected.A single-particle XRD study was carried out at the synchrotron end station ESRF ID01 on a single, SrTiO3-supported Pt60Rh40 particle with (111)-orientation, a height of 55 nm, and a diameter of about 120 nm. Exposure to catalytically enhanced CO oxidation (stoichiometric ratio of O2 and CO) at 425 °C did not inflict significant shape changes of the particle. Differences in the diffraction patterns for different gas dosing steps indicate partially reversible changes in surface strain fields. Analysis of the particle’s diffraction signal indicates outward displacement of the topmost 4-5 atomic layers caused by an expansion of the fourth or fifth layer of at least 9% under Argon dosing, contrary to findings reported by other researchers. Again, no rhodium oxide layers were detected under oxidizing conditions.

Note: Dissertation, University of Hamburg, 2021

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Contributing Institute(s):

1. Nanolab (FS-NL)

Research Program(s):

1. 632 - Materials – Quantum, Complex and Functional Materials (POF4-632) (POF4-632)
2. 6G3 - PETRA III (DESY) (POF4-6G3) (POF4-6G3)

Experiment(s):

1. DESY NanoLab: Sample Preparation
2. DESY NanoLab: Surface Spectroscopy
3. PETRA Beamline P07 (PETRA III)
4. PETRA Beamline P22 (PETRA III)
5. PETRA Beamline P23 (PETRA III)
6. PETRA Beamline P21.2 (PETRA III)

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