In-situ spectroscopies for catalysis

Research output: Book/ReportPh.D. thesis

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Abstract

Nitrogen oxides (NOx) are the main air pollutants that are harmful to the environment. NH3-Selective Catalytic Reduction (NH3-SCR) is a mature technology to remove NOx from exhaust. Cu-exchanged zeolites, such as Cu-chabazite (CHA) are applied as catalysts in the automotive industry.
Cu exchanged zeolites or zeotypes are already used for many redox reactions. It is useful to study the distribution of copper species on different Cu exchanged zeolites or zeotypes. We have successfully synthesized different Cu exchanged zeolite or zeotype catalysts and tested their copper distribution and redox performance by Electron paramagnetic resonance (EPR). We found that different Cu molecular sieves have different copper distributions, and Cu-CHA has the highest amount of site B, resulting in the highest redox performance. Copper species are not fixed in molecular sieves, especially when the sample is exposed to gas phase redox reactants. Migration is helpful, and the distribution and migration of copper species can give us some ideas on how to improve the high redox catalytic performance of the catalyst.
EPR was employed to investigate the effect of synthesis method of Cu-CHA and the importance of Na+ being available in synthesis gel. The EPR results indicate that the addition of Na+ promotes the formation of "paired" Al in the zeolite structure, but at the same time promotes the formation of extra-framework aluminium and limits the ion exchange capacity.
A study was carried out the effects of sulfur poisoning and regeneration on the distribution of copper species in Cu-CHA with different copper loading. After sulfur poisoning, a large amount of active copper sites disappears to form CuSOx, which maybe the reason for the decrease of redox ability. The rate of reduction of Cu by NO and NH3 decreased, whereas the rate of oxidation increased. Some Cu sites survived the sulfation and were still catalytically active, particularly Cu close to 2 Al in the framework, Z2Cu, but other Cu sites such as ZCuOH were very sensitive to SO2 and lost the catalytic activity. This work provides ideas for the development of efficient and sulfur-resistant SCR catalysts for practical applications by characterizing the effect of sulfur species building up in the cage structure of Cu-CHA.
In-situ UV-vis is a good spectroscopy to investigate the effect of Ni and Co loading on the ethene absorption ability of FAU zeolite. Na-FAU didn´t show any ability for ethene absorption. The Ni loading on Na-FAU shows the best performance for ethene absorption, as these samples can promote the formation of monoene carbon cations on the Brønsted and Lewis (Ni(II) acid sites in zeolites), while minimizing the formation of dienyl carboxylates. The loading of Co on Na-FAU didn't improve the ethene absorption property.
Original languageEnglish
PublisherDTU Chemistry
Number of pages216
Publication statusPublished - 2023

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