We have studied the effect of the catalyst load, initial concentration and light intensity upon the MO degradation with WO3-P25 composite systems. We have tried the LH mechanism and attributed pseudo-first order kinetic to it and half order kinetics, but the models don not fit the experimental data very well.
The reaction mechanism might be very complex and include tens of "elementary" steps. Commonly, it takes much time and very significant efforts to get some ideas on the reaction mechanism. Your question is probably not about the mechanism but about a kinetic model, which is actually based on the reaction mechanism (often simplified). What you have to do is to write a sequence of possible reactions in your system (reaction mechanism), derive the reaction rate law (or use a digital fitting of your data to the proposed mechanism), and see whether your mechanism is consistent with your data. This is a pretty complicated job, which requires an experience in chemical kinetics.
I agree with Yurii.
How did you measure the degradation kinetics? In the case of simple decoloration, its rate may be affected by the degradation products.
Anyway, I recommend you to read the article of J.-M. Herrmann - https://doi.org/10.1016/j.jphotochem.2010.05.015
@Lipeng Xin the temperature was kept at 25 C in all cases, precisely so the temperature won't effect the degradation.
@Yurii V Geletii yes, you are right my question is about a kinetic model, in our case we have used a simplistic Langmuir-Hinshelwood mechanism (r=k*K*C/1+K*C) and implemented it into the model.
@ Vasily Lebedev the degradation was measured via spectrofotometer (absorbance), the simple decoloration was measured, and in this case the intermiedates cannot be measured.
Thank you all for your answers!
Your reaction does not proceed via " a simplistic Langmuir-Hinshelwood mechanism ." I wrote what you are supposed to do.
Vasily's comment is correct. You do not follow the kinetics of "methyl ORANDE" consumption, but follow the kinetics of decoloration. Colorless reaction products might be involved.
The point is that you can use the pure LH equation only in the first steps of reactions. After that you should modify it somehow - for example, as r=k*K*C/(1+K*C+sum(Ki*Ci)), where Ki and Ci for the intermediates. To do that correctly, you should perform all the things, that Yurii describe in the first post. Only after these modifications you can try to fit your data well, but it is a very complicated work. You can find some details in Guettai's work https://doi.org/10.1016/j.desal.2005.04.049 , for example.
Another way is to determine the order of observable decoloration rate by the first dozens of percents decolorated or - better, I suppose - just to use the pseudo-first order as well-proven for this system, and to calculate the PCA of raw and loaded P25 with some error estimations. It should works for a simple samples comparison, especially if you will check your experimental conditions according to the Herrmann's article.
Couple of questions - what the lightsource did you use? Was it the aerobic experiment or not? And, the most interesting thing for me, what was the phase of WO3?
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