טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
M.Sc Thesis
M.Sc StudentRochkind Malka
SubjectVisible Light Activity of Bismuth-Oxide Photocatalysts
DepartmentDepartment of Chemical Engineering
Supervisor Professor Yaron Paz


Abstract

Removal of organic pollutants is one of the most important challenges of our society. One possible solution is photocatalytic degradation by semiconductors such as TiO2, ZnO and WO3. There is a growing interest in studying new photocatalysts, among which are bismuth oxyhalide (BiOX) compounds, considered to be promising due to their remarkable photocatalytic performance. These properties are closely related with their layered structure, which promotes the separation of electron-hole pairs. In this study, we focused on the UV - active BiOCl, synthesized at room temperature using a co-precipitation method which yielded high specific surface area.

            Two model systems comprising of BiOCl as the photocatalyst were studied. In the first system the degradation of a dye stuff, rhodamine B, under UV and visible light was followed. A comparison was made between a sensitization mechanism and photocatalytic mechanism. Care was made to perform action spectra experiments, as well as to observe the effect of electron scavengers, superoxide scavengers, hole scavengers and lack of oxygen on the kinetics of degradation. Monitoring the cleavage of the chromophore of the dye versus the formation of N-de-ethylated intermediate products assisted in analyzing the prevailing mechanisms.  It was shown that the N-de-ethylated degradation occurs through a reductive pathway through the formation of superoxide radicals. In the absence of oxygen, reduction may occur by a direct (electron transfer) mechanism. In contrast, an oxidative mechanism is responsible for the cleavage of the chromophore, both directly by attacking the dye molecule and indirectly by attacking the N-de-ethylated intermediates.  

In the second part of this work, bio-mimetic systems based on synthetic derivatives of Flavin attached to the surface of BiOCl were prepared and characterized. The loaded particles were used to study their ability to promote the photoinduced degradation of contaminants under visible light. The concept was based on injection of electrons from the LUMO of the sensitizer to the conduction band of the photocatalyst, leading to formation of active species on the surface of the semiconductor. These active species induce degradation of toxic species present in the solution. Salicylic acid, which does not absorb visible light was chosen as our model contaminant. 

The prepared BiOCl-flavin composites were characterized by FTIR, P-NMR and UV-Vis DRS. Flavins with different functional groups and linkers were used and the effects of these groups on the photocatalytic activity were studied. The BiOCl-flavins composites enabled the degradation of salicylic acid under visible light. A flavin containing a conjugated linker was found to be the most effective. The turnover number with respect to the flavin molecules was calculated to be larger than 9, suggesting that the composite system exhibits catalytic properties. The stability of the flavinated compounds was tested. It was found that the flavins were very stable in the presence of other contaminants. However, in the absence of contaminants, they lost in their stability and eventually were partially degraded. At any case, the results indicated that their stability was improved upon anchoring to the surface.