Ag-incorporated macroporous CeO2 catalysts for soot oxidation: Effects of Ag amount on the generation of active oxygen species
Graphical abstract
Introduction
Recently, regulations of automobile emissions have been continuously strengthened, and diesel vehicles are regulated to reduce various pollutants such as NOx, CO, hydrocarbons, and soot in exhaust gases [[1], [2], [3], [4], [5], [6], [7]]. Soot (small-size carbon particles), a hazardous material, is primarily generated by incomplete fuel combustion and is removed by the diesel particulate filter (DPF) in the purification system. Since the soot could be stacked in the DPF, it should be continuously oxidized and removed from the filter by the purification system to maintain optimal filtration ability [4,8]. Therefore, several kinds of oxidation catalysts for the DPF have been studied to improve combustion efficiency by thinly coating an oxidation catalyst on the filter. Many studies have reported that CeO2-based catalysts, which are used for various oxidation reactions, have remarkable activity for soot oxidation due to their ability to switch between Ce3+ and Ce4+ states resulting in high oxygen storage capacity (OSC) performance [[9], [10], [11], [12]]. Hence, CeO2 could store O2 in an oxidizing atmosphere and release it in reduction conditions due to the OSC and redox properties [9,10].
In the soot oxidation using a CeO2 catalyst, the reduction of the CeO2 lattice oxygen leads to the formation of vacant sites which are then filled with gaseous O2 [13]. Since a redox cycle of CeO2 occurs during the soot oxidation, it has been known that the OSC and redox properties of CeO2 are important for soot oxidation [[14], [15], [16], [17]]. In contrast, several studies proposed that the bulk OSC of CeO2 is not crucial factor, but the generation of active oxygen and transfer to soot is rather more important for the soot oxidation activity [9,16,18,19] implying that the active oxygen species (Oxn−) could exist as peroxide (O−) or superoxide (O2−). Machida et al. suggested that the reactive oxygen (O2−) formed from gas phase O2 is adsorbed at the three-phase boundary between soot, reduced CeO2, and the gas phase [9]. In addition, Liu et al. reported that O2 is adsorbed on the CeO2 surface and the various Oxn− species are continuously consumed and generated on ceria surface through the following consecutive reduction of oxygen by CeO2 surface oxygen vacancies [20]: (i) O2 may migrate to the ceria surface which is in contact with soot and (ii) converted into Oxn− simultaneously through several electronic states by reduction, which is O2 → O2− → 2O− → O2− [20,21]. Among the various oxygen species, O2− has higher activity than O− and O2− species for soot oxidation [9,[21], [22], [23]]. Hence, the vacancies in CeO2 promote the migration of bulk oxygen towards the reaction site, and CeO2 needs to have sufficient surface oxygen vacancies to promote the generation of active oxygen species (Oxn−). However, the excessive surface oxygen vacancies could induce the generation of less active oxygen species. Wang et al. reported that excessive surface oxygen vacancies can lead to the formation of less active species (like O2−) rather than O2−, which results in decrease of the catalysts’ redox stability and activity [21]. In contrast, if the concentration of surface oxygen vacancies is too low, the total Oxn− generation process will be hindered resulting in a slowed reaction [20,21]. Consequently, to improve soot oxidation activity, it is important to have a high proportion of O2− in the successive O2 reduction steps, and this requires an appropriate amount of surface oxygen vacancies.
Ag metal has been known to remarkably promote the formation of O2− oxygen species and the supplementation of active oxygen [9,22,[24], [25], [26]]. The atomic oxygen is reduced by surface oxygen vacancies and in this process, Ag has been proven effective for gaseous O2 dissociation and CeO2 bulk oxygen utilization, improving the O2−, O−, and O2− generation of the Ag/CeO2 and resulting in enhanced soot oxidation activity. Therefore, the ratio of active Oxn− species (O−, O2− and especially O2−) on the ceria surface is one of the most important factors responsible for soot oxidation over Ag-loaded CeO2 [10,21,27]. Meanwhile, a few studies have been conducted on the difference in activity of CeO2 in soot oxidation depending on the amount of Ag. Aneggi et al. reported that lower oxidation temperatures were found between 5 wt.% and 10 wt.% loading of Ag for CeO2, ZrO2, and Al2O3 supports [24]. Additionally, the onset temperature of soot oxidation can be lowered as the amount of Ag on CeO2 increases [9], and the Tmax for soot oxidation as a function of the Ag amount in the Ag/CeO2 catalysts were reported [26]. However, to the best of our knowledge, the explanations on the difference in activity of various amounts of Ag-loaded CeO2 due to correlations between surface oxygen vacancies with the generation of active oxygen species have not been reported. There should be an appropriate amount of Ag on the CeO2 and surface oxygen vacancies concentration that optimizes the activity and formation rate of Oxn−. In this work, to solve the above issues, as well as to improve the ceria-soot contact, macroporous structured CeO2 was chosen as a support since the macroporous structure catalysts indicated improved soot oxidation efficiency due to enhanced ceria-soot contact [17,[28], [29], [30]]. Prior to this study, Ag-loaded macroporous CeO2 catalysts have not yet been applied to soot oxidation; thus, herein, Ag-loaded macroporous CeO2 catalysts were applied to the soot oxidation reaction in this study.
Based on the macroporous CeO2 (M-CeO2), a series of Ag-loaded M-CeO2 catalysts (Ag(x)_M-CeO2) with similar morphologies and different Ag loadings were synthesized and examined. Through characterization and comparison of the activity of the catalysts for soot oxidation, the appropriate amount of active oxygen species and surface oxygen vacancies for a given amount of Ag were verified. The results suggest a correlation between the optimum amount of Ag and the surface oxygen vacancies of CeO2, resulting in active oxygen species generation and activity improvement.
Section snippets
Preparation of catalysts
The macroporous structure catalysts were synthesized using PMMA as templates by modifying the methods found in the literature [29,31]. A PMMA template was synthesized through a series of polymerization processes using methyl methacrylate (MMA, Sigma Aldrich) and potassium persulfate (Sigma Aldrich) as the precursors. MMA (240 ml) and deionized water (560 ml) were mixed in a 1000-ml four-neck and round-bottom flask, and 1.62 g of potassium persulfate (Sigma Aldrich) was added to the solution.
Textural properties of the samples
The morphology and pore structure of the samples were examined by high-resolution scanning electron microscopy (HRSEM). Fig. 1a shows SEM image of Printex U and the image demonstrates that the Printex U particle size is larger than approximately 20 nm and varied from 20 nm to 150 nm since it is not uniform size and could agglomerate with other particles. Due to this large particle combination, it could be difficult for soot to migrate into inner pores of catalysts with small pore diameters.
Conclusion
This study shows that macroporous CeO2 (M-CeO2) demonstrates improved activity in soot oxidation when compared to mesoporous CeO2 catalyst. This is because the enlarged pore size of the M-CeO2 catalyst could make soot particles pass through into the inner pore of the catalyst easily, and consequently overall contact between catalyst and soot was increased. With the Ag incorporation, Ag(x)_M-CeO2 showed enhanced soot oxidation activity in all Ag loading amounts (2–20 wt.%). Especially, the
Acknowledgement
This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (NRF-2016R1A5A1009592).
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