*3.3. Catalytic Activity of Ag2O, CeO2, and Ag-doped CeO2 catalysts*

The evaluation of the catalytic activity of Ag2O, CeO2, and Ag doped CeO2 thin films deposited at 200 ◦C on stainless steel foil was carried out to show the effectiveness of the catalysts for soot combustion applications. The annealing tests of carbon soot, which was generated from diesel fuel, was carried out under ambient air environment inside an oven in the temperature range 300–490 ◦C for 2 h.

Figure 12 shows histograms of soot conversion vs. annealing temperature for catalytic and non-catalytic combustion. The conversion is defined as:

$$\mathbb{C}(\%) = \frac{M\_0 - M}{M\_0} \times 100\% \tag{1}$$

where is *M*<sup>0</sup> is the initial soot mass, and *M* is the amount of soot left on the catalyst after burning by heating up to a given *T* value. Weight loss values were obtained by weighing the samples before and after the annealing test which continued for 2 h.

**Figure 12.** Conversion of oxidized soot on silver oxide, cerium oxide, and silver doped cerium oxide in ratio CeO2:Ag 10:1, 20:1, 30:1 thin films deposited on stainless steel foil at 200 ◦C vs. annealing temperature over 2 h. The measurement uncertainty is approximately 5%.

The conversion of oxidized soot was demonstrated on non-catalysed reference steel foil, and on cerium oxide, silver oxide, and Ag doped CeO2 thin films deposited on stainless steel foil at 200 ◦C in loose contact mode (Figure 12). As expected, complete soot conversion on the uncoated reference sample was only achieved at 600 ◦C. All the catalysts were effective in promoting combustion at temperatures below 490 ◦C. Although the soot was well oxidized on pure CeO2 and Ag2O thin films themselves, Ag loading into CeO2 thin films caused a significant enhancement of soot oxidation rate, in accordance with previous reports [28]. It is noteworthy that the soot oxidation activity of Ag doped CeO2 was different depending on the dopant concentration. The sample having the maximum silver loading CeO2:Ag 10:1 and pure Ag2O showed the lowest oxidation temperature of 300 ◦C and complete combustion of the soot was achieved below 390 and 410 ◦C under real-world loose contact conditions, respectively. Table 4 lists the characteristic temperatures, where the ignition temperature (*T*i) is the temperature at which the combustion began, and the final temperature (*T*f) is the temperature at which the soot was completely oxidized.


**Table 4.** Catalytic performance for soot oxidation.

The catalysts with silver doping concentration of Ce2O:Ag 20:1 and 30:1 showed lower performance for 100% soot oxidation at *T*<sup>f</sup> = 490 ◦C. This indicates that the highest concentration of Ag<sup>+</sup> ions, which is contained mostly in CeO2:Ag 10:1 catalyst, can effectively promote 100% soot oxidation at *T*<sup>f</sup> = 390 ◦C due to the oxygen species formed on Ag+ sites. Zou et al. [46] proposed that during silver oxide decomposition, its released oxygen migrates to soot surfaces to form carbon–oxygen intermediates, which are subsequently oxidized further. Finally, the adsorbed oxygen on the silver promotes the regeneration process. The redox reaction of Ag<sup>+</sup> and the active oxygen species prevail in the reaction at low combustion temperatures. In addition, the silver oxide contributes to the adsorption of reactants to form complex π bonds that are significant for the formation of peroxide and superoxide species [47]. It is worth mentioning that higher concentration of Ce3+ contained in CeO2:Ag 10:1 catalyst can also form more active oxygen vacancies, which promote the activation of adsorbed oxygen to form superoxides in the lattice. These types of oxygen react with soot efficiently [48].

Because the melting point of Ag2O oxides is relatively low, compared with CeO2 (2400 ◦C), the stability of Ag/CeO2 catalysts during the soot oxidation is an important factor from the practical point of view. In order to gain information on the stability of Ag/CeO2, repetitive activity tests were carried out in loose contact mode at annealing temperature 430 ◦C. Figure 13 shows the conversion of oxidized soot after five replicate trials, and the observed results showed acceptable reproducibility with relative standard deviation of less than 5%. The pure Ag2O catalyst lost its catalytic activity immediately after the first trial from 100% to 50% of oxidized soot. Further use of Ag2O catalyst (in the third, fourth, and fifth trials) led to significant restructuring of the film and total loss of catalytic properties. For this reason, bulk Ag2O cannot be used in catalytic systems operating at higher temperature (above 300 ◦C).

**Figure 13.** Repetitive soot oxidation in the presence of silver oxide, cerium oxide, and silver doped cerium oxide in ratio CeO2:Ag 10:1, 20:1, and 30:1 thin films deposited on stainless steel foil at 200 ◦C vs. annealing temperature over 2 h at 430 ◦C.

It appears that CeO2/Ag 10:1 lost 10% of its activity in the third combustion test, indicating the deactivation of Ag/CeO2. However, during further trials this catalyst showed stable results of 90% oxidized soot. We can assume that Ag ions have a strong interaction with the CeO2 catalyst and remain stable after durability tests. Other catalysts such as CeO2:Ag 20:1, 30:1 and pure CeO2 do not lose activity, since the combustion curves obtained from the first to the fifth tests were very similar.
