**1. Introduction**

The number of diesel-powered vehicles has increased rapidly in recent years due to their reduced fuel consumption and thus lower CO2 emission compared to petrol engines. However, diesel engines produce a high content of nitrogen oxides (NO*x*) and particulate matter (PM) in their exhaust [1]. These emissions have a negative impact on human health causing respiratory, cardiovascular, and lung diseases, as well as on the environment such as disruption of the natural growth of plants and pollution of air, water, and soil [2,3]. Even though it is likely that many diesel engines will be replaced by petrol or electric engines in the future, there will still be a great need for diesel exhaust cleaning for some time to come.

In order to remove soot from the exhaust, diesel particulate filters (DPFs) are widely used [1]. Conventional DPFs require periodic regeneration by increasing the temperature of the exhaust gases to the soot combustion temperature, which is approximately 600 ◦C [4]. This method results in an increase in fuel consumption and clogging of the DPF by ash resulting in a slow increase of back pressure in the exhaust [1].

The composition of the exhaust mixture also affects the catalytic activity. Oxygen and NO2 are generally used to oxidize diesel soot. NO contained in the raw exhaust gas is oxidized with excess oxygen into NO2. Therefore, the development of catalysts, which can produce highly reactive oxygen species from O2 molecules and NO2 from NO, is the key issue. The catalyst promotes NO to NO2 oxidation and NO2 is then transported via the gas phase over the soot particles, oxidizing carbon while being reduced back to NO [5,6].

The preferred solution for continuous regeneration of the DPF is a catalysed diesel particulate filter (C-DPF) [7]. The main requirements for the catalyst are a reduction in temperature at which soot combustion occurs and long-term thermal and chemical stability.

Ceria-based catalysts have been studied in depth for various environmental applications such as three-way catalysts (TWC) for automotive pollution control, fluid catalytic cracking (FCC), and fuel cells [8,9]. The high potential of ceria as a catalyst is due to its fast and reversible reduction to sub-stoichiometric phases (CeO2–CeO2−*x*) as well as the high mobility of oxygen ions in its crystal lattice [10]. However, the use of metal-doped oxide catalysts can improve the performance of the bare oxide due to the increased mobility of oxygen species or the facilitation of the redox mechanisms associated with oxygen release/adsorption [11].

The influence of doping elements on the catalytic properties of ceria has been reported on recently by many researchers. The catalytic activity of ceria can be enhanced by doping with isovalent (Ti4+, Zr4+, Hf4+, Sn4+, etc.) and aliovalent (Zn2+, La3+, Ag+, Eu3+, etc.) cations into the ceria lattice [12–15]. Furthermore, the beneficial influence of Rh, Pd, Cu, Au, and Ag supported CeO2 catalysts has been reported on elsewhere [16–18]. These reports showed increased electron mobility between the cerium buffer layer and support, favouring the formation of oxygen vacancies in CeO2. Rangaswamy et al. [19] studied CeO2–Sm2O3 and CeO2–La2O3 catalysts, which could oxidize 50% of diesel soot under loose contact mode at 517 and 579 ◦C, respectively.

Among the metal additives investigated so far, Ag-based materials are the most promising catalysts for oxidizing diesel soot at low temperatures. Aneggi et al. [20] reported the effect of Ag addition on various metal oxides (CeO2, ZrO2, Al2O3) during soot oxidation activity. They showed that Ag/CeO2 and Ag/ZrO2 catalysts have high soot oxidation activity in the temperature region around 300 ◦C under tight contact mode. Haneda et al. [21] also performed isotopic transient kinetic analysis on Ag/ZrO2 catalyst and concluded that the presence of Ag<sup>+</sup> sites in Ag/ZrO2 was responsible for the high soot oxidation activity. Ag and Fe doped Mn2O3 catalysts were examined by Kuwahara et al. to enhance soot oxidation under tight contact mode and showed the T50 (the temperature for 50% of soot combustion) at 290 ◦C and at 328 ◦C, respectively. Based on their measurements, the mechanism of soot oxidation was proposed to be by the activated lattice oxygen species in Ag doped Mn2O3 catalyst via the redox of Ag0/Ag2O species [22].

Machida et al. [9] investigated silver loading onto CeO2 and showed the enhancement of catalytic activity for soot oxidation because of the enhanced generation of superoxide. Shimizu et al. [23] showed that the presence of Ag metal nanoparticles on CeO2 significantly improved the reactivity of CeO2 lattice oxygen during soot decomposition under oxygen and under inert atmosphere. In addition, a dopant of silver in CeO2 may increase oxygen mobility due to a weakened Ce–O bond [24].

There are a number of methods of preparation of Ag doped CeO2 catalysts such as co-precipitation [25], impregnation [19,26], and liquid-phase chemical reduction [20]. We chose to use atomic layer deposition (ALD). The benefits of ALD compared to the other methods are extreme film thickness uniformity, precise thickness control, excellent step coverage, and high reproducibility. The thickness of the films can be easily controlled by controlling the number of deposition cycles. Furthermore, the fact that ALD operates via self-limiting surface reactions in consecutive cycles means that doping materials can be introduced with greater control and tuning than other deposition methods. ALD can be used to deposit catalytic coatings on high surface area porous powder supports or on geometrically complex structures [27] such as particulate filters in diesel engine exhaust systems.

In this study, we investigated the ALD of Ag2O and Ag doped CeO2 for catalytic applications in soot combustion under loose contact mode. The crystal structure, morphology, and composition properties of the deposited films were analysed. The effect of doping on the efficacy of soot combustion in annealing tests was also studied, paying particular attention to the doping concentration and oxidation state of silver in the CeO2 thin films.
