In aebi (1984), the most used reference to assay Catalase activity, it is said the same thing: "In the ultraviolet range H202 shows a continual increase in absorption with decreasing wavelength".

Now why the kinetic is measured at 240 nm wavelength, for this, you have to consider many things:

- First, maybe to avoid interferences with absorbances of other substances in biological tissues or that of the products of H2O2 degradation (like in the example of NADH which absorbs at two wavelengths 260 and 340 nm, and the pic is at 260. But, as its product NAD+ can also absorb at 260 but not at 340, so the ideal is to measur the abs. at 340 nm only). (Link 1 and 2)

- Or also it's maybe to avoid at minimum the H2O2 degradation (as H2O2 is sensitive to UV rays). (Link 3)

- And finally, you have to consider another thing, which is the molar extinction coefficient (ε). In case of taking measure of H2O2 in a low wavelength, here you need to decrease also H2O2 concentration because of the increasing ε, and that could be also a limitating factor for enzyme activity, I mean lack of substrate (and you can't dilute your catalse more than it's recommanded, otherwise the subunits will dissociate).

All what I wrote are only suggestions. Maybe some other researchers will have the right answer.

https://www.google.dz/imgres?imgurl=http%3A%2F%2Fwww.perrin33.com%2Fenzym%2Fautres%2Fcofacteursorg_2_f4.png&imgrefurl=http%3A%2F%2Fwww.perrin33.com%2Fenzym%2Fautres%2Fcofacteursorg_2.php&docid=qPA6ZRyaZHfreM&tbnid=ltTyL2SxYGthIM%3A&vet=1&w=607&h=348&bih=702&biw=1304&q=absorbance%20of%20NADPH&ved=0ahUKEwjikLaJsrfSAhWLPxoKHZYbCqYQMwgmKA4wDg&iact=mrc&uact=8#h=348&imgdii=CEL07t6BH548sM:&imgrc=ltTyL2SxYGthIM:&vet=1&w=607

https://www.macalester.edu/~kuwata/classes/2010-11/Chem%20222/Seinfeld%20O2%20Spectrum.pdf

http://www.h2o2.com/technical-library/physical-chemical-properties/radiation-properties/default.aspx?pid=65

I didn't work on H2O2 before, but I did agree with Amine. It is reasonable. Usually, people will choose the maximum absorbance wavelength, if possible. However, if the above three cases occur for H2O2, then you need to find a better wavelength. In the end, I want to introduce an interesting paper in our group about point 3, but in HPLC system. We usually quantify analyte using either peak height or peak area, but never peak width. If the signal of analyte can saturate the detector, then width-based quantification will be a good option. Again, this is not for your question, but for fun. Here is the link: Width Based Characterization of Chromatographic Peaks. Beyond Height and Area http://pubs.acs.org/doi/abs/10.1021/acs.analchem.6b04858