## 4 Discussion
It could be described as Pt\({}_{\text{n}}\)O\({}_{\text{m}}\) clusters, where m \(<\) n and the platinum state is intermediate between the Pt\({}^{0}\) and Pt\({}^{2+}\). Note that these structures do not give a separate peak on the hydrogen consumption curve in TPR-H\({}_{2}\) experiments but result in the broadened peak in the region up to 200 \({}^{\circ}\)C. However, oxygen in these structures affects the charge state of platinum. The formation of Pt-O-Ti bonds can be proposed in good agreement with results of EXAFS analysis. The decrease of catalytic activity after annealing at high temperatures is a common effect. In our case it could be explained by sintering of platinum nanoparticles, leading to decreased amount of active sites. In case of TiO\({}_{2}\) supports platinum surface could be passivated by migration of TiO\({}_{2}\) on the Pt particles (SMSI effect). For samples studied in the present work annealing at 600 and 800 \({}^{\circ}\)C leads to TiO\({}_{2}\) phase transition from anatase to rutile. This process is accompanied by the decrease of surface area of the support which, in turn, leads to reduction and growth of Pt particles. In the case of the Pt/TiO\({}_{2}\)-P25 catalyst, platinum particles are weakly bonded to the surface. During heating under reaction conditions, platinum easily reduces and sinters, leading to the decrease of catalytic activity. In the case of the Pt/TiO\({}_{2}\)-PLA sample, these processes are less pronounced due to stronger interaction between platinum and TiO\({}_{2}\). As it was shown by XPS and TEM, platinum particles preferentially stabilize on rutile surfaces. As TiO\({}_{2}\) rutile is more stable in the PLA support, reduction and sintering of platinum are slowed down and the higher catalytic activity is maintained. Previously it was shown that in the course of the catalytic reaction of NH\({}_{3}\) with O\({}_{2}\) a transition from the oxidized to partially reduced platinum species takes place [30; 50]. TPR-H\({}_{2}\) data shows that the reduction processes of Pt/Ti-P25-400 catalysts and Pt/Ti-PLA-400 are different. Therefore, it is of interest to compare the catalysts in their reduced state after the catalytic cycle, i.e., after exposure to the reaction medium. The XPS spectra of the Pt/Ti-PLA-400 and Pt/Ti-P25-400 catalysts before and after their exposure to the reaction medium were compared. Figure 12 shows Pt4f spectra collected for the samples after the first TPR-NH\({}_{3}\)+O\({}_{2}\) cycle. The spectra reveal that, despite the higher degree of oxidation of platinum in the initial Pt/Ti-P25-400 catalyst, the degree of reduction of this catalyst in the reaction medium is much stronger. The fraction of the ionic form of platinum with E\({}_{\text{b}}\) (Pt4f\({}_{7/2}\)) = 72.4-72.7 eV (Pt\({}^{2+}\)) becomes lower (Table 4), and Pt\({}^{4+}\) ions with E\({}_{\text{b}}\) (Pt4f\({}_{7/2}\)) = 74.3 eV completely disappear. The Pt4f spectra of the Pt/Ti-PLA-400 and Pt/Ti-P25-400 catalysts after exposure to the reaction atmosphere show higher degree of similarity and consist of two main doublets with E\({}_{\text{b}}\) (Pt4f\({}_{7/2}\)) = 70.7-70.8 eV (Pt\({}^{8-}\)) and 72.4-72.7 eV (Pt\({}^{2+}\)). However, the ratio between these platinum species is different. Pt\({}^{2+}\)/Pt\({}^{8-}\) ratio is 0.5 for the Pt/Ti-PLA-400 catalyst, while it is 0.4 for the Pt/Ti-P25-400 catalyst (Table 4). Based on the characterization data, the higher activity of the Pt/Ti-P25-400 catalyst during the first catalytic cycle can be explained as follows. The Pt/Ti-P25-400 catalyst contains highly dispersed clusters with a high Pt oxidation state. These clusters are characterized by two types of oxygen with reduction temperatures of \(-2\) and 24 \({}^{\circ}\)C;. In the Pt/Ti-PLA-400 catalyst, there is only one type of oxygen with the reduction temperature of 4 \({}^{\circ}\)C. Furthermore, the evolution of water during the reduction is observed at different temperatures.