**4. Conclusions**

We have demonstrated that ALD can be used to deposit Ag2O and Ag doped CeO2 catalysts at 200 ◦C with (hfac)Ag(PMe3), Ce(thd)4, and O3 as precursors. The growth rate stabilized to a steady-state Ag2O GPC of 0.28 Å/cycle after 100 ALD cycles. The silver doping concentration was finely tuned by setting the CeO2:Ag cycle ratio to 10:1, 20:1, and 30:1. With increasing Ag concentration in the CeO2 thin films, the overall GPC decreased from 0.32 down to 0.21 Å/cycles. AFM and SEM analyses showed that higher concentration of Ag (CeO2:Ag 10:1) inhibited CeO2 growth so that the Ag nuclei were not covered with CeO2 and so the next Ag cycle nucleated more easily on top of the Ag and could therefore form bigger crystals. XRD and XPS analyses showed that the Ag+ oxidation state dominated for CeO2:Ag 10:1 catalysts and stoichiometric CeO2 tended to be reduced from Ce4+ to Ce3+ ions.

The performance of soot combustion with Ag2O and Ag doped CeO2 with different silver concentrations was compared at operating temperatures 300–500 ◦C. The annealing test showed that higher concentration of Ag+ in CeO2:Ag 10:1 catalytic films was as effective as Ag2O films; effectively promoting 100% soot oxidation at *T*<sup>f</sup> = 390 ◦C due to the oxygen species formed on Ag<sup>+</sup> sites. It is worth mentioning that the higher concentrations of Ce3+ in the CeO2:Ag 10:1 catalyst films can also form more active oxygen, which then reacts with soot to yield carbon dioxide.

In repetitive tests in loose contact mode, the Ag doped CeO2 catalysts showed stable performance after a small initial decrease (down to approximately 90% of the initial performance for the case of CeO2:Ag 10:1 films) whereas the performance of the pure Ag2O films decreased by 50% after the first test and continued to decrease thereafter.

Overall, the results show that Ag-doped CeO2 films grown by ALD are effective and stable as catalysts for soot oxidation.

**Author Contributions:** Conceptualization, T.V.I. and A.B., D.C.C.; Methodology, T.V.I., T.H., D.C.C.; Investigation, T.V.I., T.H., A.B., D.C.C.; Writing—Original Draft Preparation, T.V. Ivanova, D.C.C; Supervision, D.C.C.

**Funding:** This research was funded by TEKES, the Finnish Agency for Technology and Innovation (70006/13), European Regional Development Fund (project CZ.1.05/2.1.00/03.0086), and Ministry of Education Youth and Sports of Czech Republic (project LO1411 (NPU I)).

**Acknowledgments:** Acknowledgment is given to Monika Stupavská and Pavel Souˇcek for acquiring XPS and XRD data.

**Conflicts of Interest:** The authors declare no conflict of interest.
