Using W or Fe as surface additive or as co-dopant with N of TiO2, we modified the selectivity of aromatic hydrocarbon(s) photo-oxidation to increase the yield to aldehyde(s). Among the variables that influence the photo-process, the acidity of the surface sites seems key to interpret the "selective" product obtained. Can such "conclusions" be generally applied for TiO2 photo-selective oxidation of HCs or organics?
Dear Moisés,
thanks for the information. As you indicated, diferent adsorption modes of the molecule could be somehow correlated with different adsorption centers at the catalysts surface; if the active centeres differ in protonation capability this would imply different acidity. Any specific correlation between acidity and "degree of oxidation"?. Will read carefully the paper and learn about this possibility. Regards.
Dear Moisés, when ready, please send me a copy of the article. Although we are not experts (only work on it in recent dates, using very specific systems and mainly looking at from the catalyst point of view) in analyzing the photo-selectivity, it is a field of increasing interest and rarely studied.
Hello Markcos
The above discussions are logical. I wish to add one aspect, namely the oxidation/reduction potentials needed.
Depending on band edge positions of the attached species onto TiO2, the oxidation or reduction reactions can be engineered. For instance, if a dye is attached to TiO2, and the VB of the dye is at certain potential then it may oxidize specie which are higher (more negative potential) only, whereas species needing more positive potentials will not be oxidized by that dye.
Whereas, if TiO2 is used, with UV radiation, it is expected to photo-oxidize almost all organic species (at least thermodynamically speaking).
Therefore, thermodynamics has a role to do, in addition to kinetics as well.
Dear Hikmat,
thanks for you contribution; I agree that "enough" chemical potential to carry out the (+)/e(-) reactions is a necessary conditions to fulfill, previously to any kinetic/thermodynamic additional condition which would drive selectivity. As you said, using TiO2 (and UV) we comply with it (in almost all cases) and the question is what variables may control selectivity. Although not usually mentioned in the open literature, the photo-oxidation of a lot of organic compounds renders several products (including CO2) and I was wondering if we can highlight a few variables that may play a dominant role. In our view, a simple variable is acidity and, in the cases we studied (doping or surface modification of titania), this means that we induced new surface sites on TiO2. Such issue could be analyzed "thermodynamically" (structure of the active site; energy “local” issues) or kinetically (as Moisés mentioned previously) but I am not aware of any source which will provide a comprehensive analysis serving as a guide of researchers.
Moisés,
thanks for the new info. Will read it. We can keep contact by email for further ideas and collaboration. I will be really happy.
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