Title: A micro- and spectroscopic study of heterogeneous photocatalytic materials
Other Titles: Een micro- en spectroscopische studie van heterogene fotokatalytische materialen
Authors: Laurier, Katrien; S0194678
Issue Date: 25-Nov-2013
Abstract: The public concern regarding the sustainability of our planet has seen a significant growth during the last decades. As a result, the global concept of green and sustainable chemistry was introduced in our society and entails the search for possibilities to reduce and prevent pollution from industrial and domestic processes. The use of solar light energy asa renewable energy source is such an interesting possibility. In this PhD work, light energy is used to induce reactions in the presence of a solid catalyst, which is categorized as heterogeneous photocatalysis. This PhD work focuses on two objectives of photocatalysis. The first three experimental chapters cover different topics within the environmental application, whereas the last experimental chapter uses photocatalysis to postsynthetically modify semiconductors.The aim of the first part was to develop solid materials that are visible light responsive and show photocatalytic activity under visible lightillumination. Therefore, we investigated the possibility of using iron(III) oxide materials since iron(III) oxide shows an inherent visible light absorption. However, due to the high electron/hole recombination ratein the nanosecond time scale in pure iron(III) oxide particles, they can not be used directly. To prevent this recombination, we incorporated iron(III) oxide based clusters, more specific Fe3- μ 3-oxo clusters, inside a metal-organic framework. In these small Fe3- μ 3-oxo clusters the surface area of the photocatalytic active centers is increased resulting in a reasonable chance that the photogenerated electrons and holes reach the surface where they can react with adsorbed molecules before recombination occurs. A series of MOFs which contain such Fe3- μ 3-oxo clusters (MIL-100(Fe), MIL-88B(Fe) and amino-substituted MIL-88B(Fe) and MIL-101(Fe)) were investigated for the photocatalytic degradation of rhodamine 6G. Upon visible light illumination, successful photocatalytic degradation of rhodamine 6G was observed. Moreover, two amorphous materials containing such Fe3- μ 3-oxo clusters (Basolite F300 and Fe(III)-aminogel) were tested and displayed significant photocatalytic activity as well. From this, it was concluded that crystallinity over extended length scales is not required to yield an effective photocatalyst, but only a local order around the Fe3- μ 3-oxo clusters is sufficient. Furthermore, amino-substitution of the linker has no significant effect on the photocatalytic activity of the Fe(III)-MOFs.In the next part, further examination of these Fe(III)-MOFs containing Fe3- μ 3-oxo clusters was conducted to better understand the photochemical mechanism(s) involved. The photocatalytic properties of the following Fe(III)-MOFs were confirmed through redox reactions: MIL-100(Fe), Basolite F300, MIL-88B(Fe) and aminosubstituted MIL-88B(Fe) and MIL-101(Fe). On the one hand N,N,N’,N’-tetramethyl-p-phenylene-diamine as electron donor and on the other hand methyl viologen dichloride as electron acceptor were added to the MOF suspension and upon illumination the corresponding blue colored radicals were formed. These results suggested that at least a fraction of the photogenerated holes and electrons in theFe(III)-MOFs under study was able to escape recombination and both charge carriers were accessible at the surface of the MOFs. Furthermore, thegeneration of long-lived species in MIL-88B(Fe) was proven via transient absorption spectroscopy. These experiments indicate that the electron/hole recombination is actually delayed to the microsecond time scale when iron(III) oxide based clusters are incorporated into the rigid networkof a MOF.Besides in liquid environment, photocatalysis can also be used for gaseous reactions. Therefore, the third part was focused on diminishing the emission of nitrogen oxides in the atmosphere by reducing them into nitrogen en water via the selective photocatalytic reduction with ammonia (photo-NH3-SCR). The commercial TiO2 photocatalyst PC500 showed a significant higher photocatalytic efficiency compared to P25 and displayed also a higher photocatalytic activity in comparison with the thermal photocatalyst Cu-ZSM-5 at 150 °C. This indicates that photocatalysis can offer asolution for processes which occur at lower temperatures. The parameters of contact time and water content were also investigated. Furthermore,mesoporous TiO2 was tested and the sample which was calcined at 600 °C displayed and even higher efficiency than the highly active PC500. When using the photo-NH3-SCR, unreacted ammonia can remain in the outlet gas.This so called NH3-slip is harmful and should be degraded. It was foundthat ammonia can be oxidized via the selective photocatalytic oxidation(photo-SCO) over three commercial TiO2 samples (P25, PC500 and UV100). PC500 displayed again the highest activity at 150 °C under UV illumination as nearly complete conversion of NH3 into nitrogen was observed.The superior activity of PC500 was ascribed to its high adsorption affinity. Furthermore, evidence was provided that the photo-SCO over titanium dioxide proceeds via the internal SCR mechanism.Finally, photocatalysis was utilized for the post-synthetic modification of metal deposition on zinc oxide. Illuminating ZnO crystals with UV light resulted in the reduction of silver ions to silver metal. Via optical and fluorescent microscopy, it was observed that the photocatalytic silver reduction does not occur homogeneously but more rapidly in certainzones. Further illumination resulted in the remarkable growth of well seperated large dendritic silver nanostructures. Such a dendritic silver structure was not one ontinuous metal structure but rather a heterogeneous structure with multiple silver nano-features because it generated luminescence. As a result of these nano-features, these dendritic structures could be excellent SERS substrates which was onfirmed through Raman mapping experiments. The Raman enchancement was reasonably homogeneous throughout the whole structure which is important for sensing applications. In summary, multiple aspects of photocatalysis were examined is this work.Firstly, a series of Fe(III)-MOFs were identified for their applicationin water purification and the investigation to better understand the photodynamic processes in these materials was started. Secondly, the photocatalytic degradation of nitrogen oxide and ammonia over different titanium dioxide samples was examined. Finally, photocatalysis was used to deposit metal particles on zinc oxide resulting in the remarkable growth of dendritic nanostructures which are excellent SERS substrates.
Publication status: published
KU Leuven publication type: TH
Appears in Collections:Molecular Imaging and Photonics
Centre for Surface Chemistry and Catalysis

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