Natural photosynthesis is a thermodynamically uphill reaction (delta G positive) and photocatalytic degradation is thermodynamically downhill reaction (delta G negative). Some research papers reported degradation of pollutants with delta G positive.The natural photosynthesis follows a "Z-scheme" mechanism and it could be used in photocatalytic water splitting reaction because it is an uphill reaction. However, In the recent day, the "Z-scheme" has been used for the photocatalytic degradation of water pollutants and plenty of research papers have been published on Z-scheme photocatalyst system for degradation of pollutants. But the Z-scheme is majorly for uphill reaction (delta G positive). So, if we use Z-scheme in photocatalytic degradation reaction, then I felt some contradiction.
The "degradation" is the ill-defined term. Some reactions of water pollutant degradation might be thermodynamically unfavorable. In this case the use of Z-scheme is reasonable. Otherwise not.
Thank you Yurii, you have mentioned that some pollutants degradation might be thermodynamically unfavorable and use of Z-scheme is reasonable . But the recent studies are saying that most of the pollutants are degraded and it is thermodynamically favorable and they used Z-scheme term (delta G positive). Recently vast number of research papers are published on Z-scheme photocatalytic system for pollutants degradation. I have also used. So I have totally confused. Therefore can we say that the use of Z-scheme term is not appropriate and it cannot be used for pollutants degradation. Is that correct or not?. Otherwise can we say double semiconductor materials with enhanced charge separation which leads to higher photocatalytic degradation efficiency. please go through the attached paper
Thank you Fei Chen, can you please give some example? Heterojunction type system for phtocatalytic degradation of pollutants also a downhill reaction but Z-scheme is for uphill reaction. So I think we cannot use Z-scheme in photocatalytic degradation of pollutants. please go through the attached paper in my reply to Yurii.
Thanks for the reference, the paper is very interesting. I agree with the main statements of the author. This paper does not discuss "Recently vast number of research papers are published on Z-scheme photocatalytic system for pollutants degradation." PLease, provide me references. Hypothetically, I would agree with a use of Z-scheme for the overall downhill pollutant degradation for the cases when the activation energy of non-catalyzed reaction is too high or when the intermediate product formation is uphill..
Thanks for your information. Please go through the attached papers. I also would agree with a use of Z-scheme when the activation energy of non-catalyzed reaction is too high or when the intermediate product formation is uphill. Though the intermediate formation is uphill but overall of catalytic reactions are downhill reaction but the Z-scheme is uphill reaction. Catalyst are used to decrease the activation energy, but in Z-scheme we need extra energy to perform the electron transfer. Therefore I am still confusing.
Thanks for references. I don't have enough time to go through all details. Just a quick impression. The Z-schemes in these refs is applied for the reaction
O2 + H2O -> HO(.) + HO2(.), which is significantly uphill thermodynamically. Therefore you need an additional source of energy, e.g. light. Two photons are involved in the process. The rates of such processes depends on light intensity. Nobody looked at this dependence. The discussions of these Z-schemes in these paper are extremely naive. The main problem in such Z-schemes is the life-time of the hole-electron pairs, This is completely ignored. You are right that "Catalyst are used to decrease the activation energy, but in Z-scheme we need extra energy to perform the electron transfer." The right overall reactions for the Z-scheme is
O2 + H2O + 2 photons -> HO(.) + HO2(.), It proceeds through several elementary steps, which is called the reaction mechanism. I really doubt that it would be possible to write such a mechanism. My conclusion: the functioning of Z-schemes in all these publication is not proved experimentally and very questionable.
Thank you very much for spending your time and explanations.
I agree with Prof. Yurii and Prof. Viswanathan Sir's view. They have especially Prof. Yurii explained well. Hope you understand and Good Luck.
Dear Dr. Thillai Sivakumar Natarajan
Z-scheme mechanism is widely used in photocatalysts with heterojunction between two semiconductors. This is a pathway for charge transfer between CB and VB of two semiconductors. Please refer to the following papers.
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