Stabilisierte Einzelatome in der Katalyse : Einfluss stickstoffhaltiger Trägermaterialien am Beispiel kovalenter Triazin-basierter Netzwerke

Iemhoff, Andree; Palkovits, Regina (Thesis advisor); Rose, Marcus Sören (Thesis advisor)

Aachen : RWTH Aachen University (2022)
Dissertation / PhD Thesis

Dissertation, RWTH Aachen University, 2022

Abstract

In this thesis, covalent triazine frameworks (CTFs) are investigated for the stabilization of late transition metal atoms. Specifically, the influence of reductive conditions on immobilized complexes and their thermodynamically favoured agglomeration towards particles is explored. The application in the catalytic dehydrogenation of formic acid (FA) is demonstrated, which serves as an additional analytical tool to comment on the speciation of the immobilized metal species. After evaluation of different synthetic approaches towards CTFs with special focus on surface functionalization, ruthenium and iridium complexes are immobilized in the porous supports and tested in the catalytic dehydrogenation of formic acid. The stabilization of iridium species in atomic dispersion in CTFs against reducing conditions is demonstrated, while immobilized ruthenium complexes agglomerate to nanoparticles in such conditions. Using Ir(acac)(COD) immobilized in solvothermally-synthesized CTFs as an example, an increase in activity from 16.800 to 24.400 molFA molIr-1 h-1 after reductive pre-treatment in hydrogen at 400 °C is reported. Furthermore, after this pre-treatment the best selectivies with less than 30 ppm CO side product in the gas phase as well as improved stability during five recycling runs is shown. The chemical nature of the iridium sites after reduction is examined by an array of analytical methods. Firstly, the atomic dispersion is confirmed by electron microscopy. Secondly, indirect evidence of hydrogenolytic removal of organic ligands is obtained from temperature programmed reduction coupled with mass spectrometry and can be connected to results showing highly cationic iridium sites from photoelectron spectroscopy and x-ray absorption spectroscopy. Lastly, a higher electron density at the metal sites by donation from the macroligand upon reduction is unraveled by in-situ near edge x-ray absorption spectroscopy at the carbon and nitrogen K-edges.

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