This scenario can be possible because of various reasons. The structure and functional groups present in the catalyst and the cationic dyes can be one of the reasons. The natural pH of the dyes also play a crucial role. The interactions between the functional groups of catalyst and the dye is resonsible for the adsorption studies, whereas the band gap engineering and the nature of light used can be the driving force for the photocatalytic reaction. If the catalyst synthesized can easily adosrb a particular dye on to its surface, adsorption happens, if not, and if the heterojunction formed is suitable enough, then irradiation of light can degrade the dye molecules. So the exact reason can only be understood with respect to the particular catalysts studied, dyes used, and the experimental conditions followed for the study.

If you can give me more information about the catalyst and the dyes used I can be more specific, but in general adsorption can be favored because of functional group interactions between catalyst and adsorbate, or by the pH of the solution or the electrical charge. The susceptibility to degradation depends mostly on the structure of the adsorbate. for more information you can contact me via email.

You can manipulate adsorption by adjusting the pH. It's important to ensure that there is electrostatic repulsion between the dye and the nanoparticles. To achieve this, you should know the point of zero charge (pHzc) of the nanoparticles and the pKa of the dye. However, be cautious, as some dyes may degrade in basic media.

The behavior of a photocatalyst in adsorbing one cationic dye while degrading another is influenced by various factors, including the structural characteristics and functional groups of both the catalyst and the dyes. Effective adsorption requires favorable interactions between the catalyst's functional groups and the dye, while photocatalytic degradation depends on the catalyst's band gap and the nature of the light used for irradiation.

In addition to previous answers, molecular size, and steric hindrance also play important roles, as they can affect how easily dye molecules access the catalyst's active sites, influencing both adsorption and degradation efficiency. Larger dye molecules may struggle to reach these sites, resulting in reduced adsorption and lower degradation efficiency. Bulky groups or complex structures can further obstruct the dye's approach, hindering interaction with the catalyst. The impact of molecular size and steric hindrance is discussed in section 3.1.4 of the following paper, which I highly recommend for a deeper understanding of these dynamics: http://dx.doi.org/10.1016/j.jclepro.2024.141850