# TiO2 Modified Geopolymers for the Photocatalytic Dye Decomposition
## 3 Results and Discussion
### Photocatalytic Activity
On the other hand, DLS demonstrated that the average particle size for the pure CG300 photocatalyst during the preparation of its incorporated counterpart is the highest among the pure photocatalysts (Figure 2), which makes available less-active sites for the reaction. The results of the DLS are inversely proportional to the activities of the photocatalysts; therefore, the higher the DLS-average particle size, the lower the photocatalytic activity. The photocatalytic activity of the incorporated photocatalyst GP-CG300 was negatively affected either by incorporation into the geopolymer structure or by self shielding. Furthermore, the photocatalytic activity of the incorporated photocatalysts could also be influenced by other material characteristics, such as the specific surface area (see Table A4) of the TiO\({}_{2}\) additives. The stability of incorporated photocatalysts, which is one of the most important aspects of heterogeneous photocatalysts in general, was tested in the three consecutive cycles for GP-CG100 material, under the above-mentioned illumination conditions. The catalyst was recovered by filtration between each cycle and at the same time the appropriate volume of fresh RhB dye solution was used. After 120 min of illumination in each cycle, the photocatalytic activity reached 19.5, 18.0 and 18.5% showing good stability, with respect to the experimental error in X determination, for the GP-CG100 photocatalyst. A detailed analysis of photocatalysts after each cycle will be explored in our future work. In summary, the properties of raw TiO\({}_{2}\) powder materials such as the average particle size, mineralogical composition, etc., are suitable for the incorporated photocatalysts GP-CG100 and GP-P25 and the parent geopolymer works well for them as a supporting material, which leads to the higher photocatalytic activity of these materials in comparison with the GP-CG300 material. Furthermore, our future work will be focused on the identification of the key properties and/or their relationship for the preparation of photoactive geopolymer-based photocatalyst with incorporated TiO\({}_{2}\) powder and the photocatalytic activity of block samples, which would be more relevant to their possible practical usage. For comparison, the kinetic data according to the work of [PERSON] et al. [54] were evaluated. The pseudo first-order rate constants (see Table 5) for each photocatalyst were estimated from the slopes of linear plots ln (A\({}_{0}\)/A) vs. time. With regard to pure photocatalysts, the highest rate constant (0.27 min\({}^{-1}\)) was observed for TiO\({}_{2}\) P25. For CG100 and CG300 photocatalysts, the reaction was approximately 10 times slower, with rate constants of 0.033 min\({}^{-1}\) and 0.029 min\({}^{-1}\), respectively (Table 5). As mentioned above, this may  
Figure 9: The FTIR spectra of pure CG100 and CG300 photocatalysts and those incorporated into the parent geopolymer matrix. relate to the properties of the photocatalysts (the mineralogical composition, the average particle size, etc. ); the decrease in the rate constants of the CG100 and CG300 photocatalysts is in good agreement with their decrease in photocatalytic activity, (X) in comparison with the P25 photocatalyst. Furthermore, the rate constant for the mechanical mixture of the parent geopolymer with the addition of 3 wt.% of TiO\({}_{2}\) P25 was 0.012 min\({}^{-1}\), which is approximately 4.4% of the rate constant of the pure TiO\({}_{2}\) P25 photocatalyst and thus, in good agreement with its content in the mixture. This may be the result of the shielding effect [64]. The rate constant for the parent geopolymer matrix has not been evaluated as this material exhibits very low photocatalytic activity (X \(\approx\) 1.4%). The rate constants for the photocatalysts incorporated into the parent geopolymer matrix were significantly lower in comparison with their pure counterparts.