## 3 Results and Discussion
In the case of the alumina underlay, the cover was seemingly more homogenous, and this may indicate that the alumina surface is more convenient for the purpose of immobilization of catalytic ceria nano-surfaces than roughened silica glass. It is expected that the catalytic properties of ceria on alumina and silica underlays are about the same as the catalytic property of the nanoparticles synthesized at the same temperature, which is evaluated further in [29]. Figure 1:0 Results of the Brunauer–Emmett–Teller (BET) analysis, capturing the dependence of the surface area on the final temperature of the used thermal treatment. Figure 1:1 Thin ceria layers prepared on alumina underlays (**a**) and on roughened silica glass (**b**) obtained by SEM and EDS. Synthesized nanoparticle fractions were evaluated on catalytic activity on model contaminant 2,4,6-TCP in a water environment. In order to rule out only the surface absorption effect, an experiment was carried out. A decreasing catalytic activity was expected to be observed with the growth of crystallite sizes. Therefore, because of their significant crystallite size differences, only the TT-25, TT-600 and TT-1100 samples were examined. For the UV-VIS measurement, each sample had to be centrifuged to get a liquid phase free from the nanoparticles. For this purpose, the SIGMA 2k15 centrifuge was used with the setup of 15,300 rpm. The first measurement took place 30 min after the experiment was set up, and then after 1, 5, 8, 12 and 14 days. After each measurement, the entire measured sample was returned to the centrifugation vial and placed for a sonic bath treatment, redispersing the separated nanoparticles to suppress their losses to a minimum. All samples were then stored in the dark under standard laboratory conditions. The results of the measurements can be seen in Figure 1:2 No detailed mechanism of the reaction was examinated in this study, and it is described in the literature for Fenton-like catalysts [30; 31], where it is considered to be based on radical generation caused by the transition of oxidative states, in the case of CeO\({}_{2}\) by Ce\({}^{3+}\)/Ce\({}^{4+}\). The molar amount of nanoparticles added to the system exceeded the molar amount of TCP by almost 72 times. According to our experience and measurements, such a surplus of nanoparticles should reliably shift the UV-VIS signal of TCP to lower energies, signaling the presence of the deprotonated form. As can be seen in Figure 8a, 30 min after the experiment started, the TT-25 sample signal showed an expected shift to higher wavelengths and increased its intensity. UV-VIS signals of all other samples also shifted to lower energies, but the absorbance decreased as the samples' surface areas diminished. The absorbance of the TT-25 sample remained unchanged during the entire experiment's duration, while all other samples' absorbances increased. As can be seen in Figure 8b, 14 days after the experiment had begun, the TT-600 sample showed a higher absorbance value, but still not as high as the TT-25 sample, while the TT-1100 sample still had the lowest absorbance value. The absolute concentration of TCP in the samples was not essential in the case of this experiment. This experiment just aimed to demonstrate that the surface area reduction of the used nanoparticles led to decreased catalytic activity. It is essential to say that we did not observe any unexpected reactions or by-products. We only observed a limited function of the treated nanoparticles, which means that the higher the temperature of nanoparticles' thermal treatment was, the higher the amount of catalyst that was needed to reach the same result as with the use of the non-treated ones.