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TiO2 catalyst for soot oxidation with improved active oxygen generation and delivery abilities
Journal of Hazardous Materials 384 (2020) 121341
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CeO2 promoted Ag/TiO2 catalyst for soot oxidation with improved active oxygen generation and delivery abilities
T
Min June Kima, Geun-Ho Hana, Seong Ho Leea,b, Hyun Wook Junga, Jin Woo Choungc, ⁎ Chang Hwan Kimc, Kwan-Young Leea,b,d, a
Department of Chemical and Biological Engineering, Korea University, 145 Anam-ro, Seongbuk-Gu, Seoul, 02841, Republic of Korea Super Ultra Low Energy and Emission Vehicle (SULEEV) Center, Korea University, 145 Anam-ro, Seongbuk-Gu, Seoul, 02841, Republic of Korea c Advanced Catalysts and Emission-Control Research Lab, Research & Development Division, Hyundai Motor Group, Hyundaiyeonguso-Ro, Namyang-Eup, Hwaseong-Si, Gyeonggi-Do, 18280, Republic of Korea d Graduate School of Energy and Environment (KU-KIST Green School), Korea University, Seoul, 02841, Republic of Korea b
GRAPHICAL ABSTRACT
ARTICLE INFO
ABSTRACT
Editor: Xiaohong Guan
To evaluate the usability of TiO2 support for silver in soot oxidation, Ag/TiO2 and Ag/CeO2 catalysts were prepared and soot oxidation experiment was performed. The catalytic activity of silver was more enhanced on P25 and rutile than on anatase and CeO2 in tight contact, as evidenced from comparing the activities of supports and silver impregnated catalysts. The reasons for the difference in active metal enhancement were elucidated by various characterization methods (TEM, H2 TPR, and XPS), and it is verified that it resulted not from dispersion of silver but from oxidation ability. On the other hand, Ag/P25 showed poor activity in loose contact because of low contact between silver and soot. To improve the loose contact activity of the Ag/P25 catalyst, CeO2 was introduced for active oxygen delivery. The synthesized CeO2-Ag/P25 catalyst showed enhanced loose contact activity when compared with Ag/P25 and Ag/CeO2 due to synergistic effects from the active oxygen supply ability of silver on P25 and the oxygen transfer ability of loaded CeO2. Thermal stability of the catalyst was identified by recycling test to 800℃, and the maintained T20 and T50 demonstrated its durability for practical use in automobile exhaust removal.
Keywords: Soot oxidation Ceria promoted Ag/TiO2 Synergistic effect Active oxygen generation Active oxygen delivery
⁎
Corresponding author at: Department of Chemical and Biological Engineering, Korea University, 145 Anam-ro, Seongbuk-Gu, Seoul, 02841, Republic of Korea. E-mail address: [email protected] (K.-Y. Lee).
https://doi.org/10.1016/j.jhazmat.2019.121341 Received 17 May 2019; Received in revised form 23 September 2019; Accepted 27 September 2019 Available online 27 September 2019 0304-3894/ © 2019 Elsevier B.V. All rights reserved.
Journal of Hazardous Materials 384 (2020) 121341
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1. Introduction
Therefore, the development of soot oxidation catalysts that can initiate the combustion at low temperature is essential. CeO2 is a representative oxidation catalyst due to its high redox property and thermal stability (Wu et al., 2011; Abbasi et al., 2011; Montini et al., 2016; Gao et al., 2018; Xie et al., 2017). In soot oxidation, various CeO2-based catalysts have been reported. For instance, (1) controlling the structures of the catalysts (nanofiber (Kumar et al., 2012; Bensaid et al., 2013), nanocube (Sudarsanam et al., 2015), and 3DOM (Zhang et al., 2011; Zhai et al., 2019; Lee et al., 2019)) to increase the contact number with soot, (2) doping of transition metals (e.g., Mn, Cu, Co, and Zr) (Fu et al., 2010; Zhang et al., 2010; Aneggi et al., 2006) and rare earth metals (e.g., La, Pr, and Sm) (Harada et al., 2013; Rangaswamy et al., 2015; Jeong et al., 2019) to promote the redox ability, (3) impregnation of potassium to take advantage of its volatile property and to enhance the redox properties of CeO2 (Neyertz et al., 2014; Aneggi et al., 2008), and (4) addition of noble metals (e.g., Ag and Ru) were applied to improve the oxidation ability (Aouad et al., 2009; Kurnatowska et al., 2014). Among the promotion methods, Ag/CeO2 catalysts are known as the best ones for soot oxidation in oxygen atmosphere. However, a question still remains as to whether CeO2 is the best support in terms of silver’s role in soot oxidation. The roles of silver in soot oxidation were
Developing high fuel efficiency and low emissions for petroleumbased automobiles represents an important issue in recent years. Although diesel engines and gasoline direct injection engines (GDI) promote fuel efficiency, they result in high emissions of exhaust gas, NOx and particulate matter (PM) especially. Emission regulations (e.g., EURO 6, LEV III) have been reinforced at a rapid pace over the last two decades, and solving the problem is urgent in petroleum-based automobile industries. Accordingly, several researches about catalysts for automobile exhaust gas have been reported (Park et al., 2018; Lee et al., 2010, 2018). The main component of PM is a carbonaceous material which is also referred to as soot, and it induces various lung diseases in human beings through inhalation (Xing et al., 2016). For preventing the release of PM to the atmosphere, DPF (diesel particulate filter) and GPF (gasoline particulate filter) are equipped behind the engine. Accumulated PM is converted to CO2 by combustion for regeneration of filters. However, the stable structure of soot requires high combustion temperature, which is not favorable in general driving condition of vehicle. Hence, additional fuel injection is required to raise the temperature, but it results in reduced fuel efficiency (and increased emission of CO2).
Fig. 1. XRD patterns of the catalysts: (a) Full range and (b) magnified peaks from 35° to 42°.
Fig. 2. TEM images of (a), (b) Ag/anatase, (c), (d) Ag/P25, (e), (f) Ag/rutile, and (g) Ag/CeO2, and STEM image of (h) Ag/CeO2. 2
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Fig. 3. Particle size distributions of (a) Ag/anatase, (b) Ag/P25, (c) Ag/rutile, and (d) Ag/CeO2. Table 1 Ag and Ce contents, specific surface areas, and Ag sizes in the catalysts. Catalyst
Ag (wt.%)
Ce (wt.%)
Specific surface area (m2/g) b
Ag size (nm)
Ag/anatase Ag/P25 Ag/rutile Ag/CeO2 CeO2-Ag/P25
4.8 5.0 5.4 4.6 4.6
– – – 74 2.8
25 34 16 60 27
4.20 4.42 4.59 2.92 3.83
a b c
a
a
Table 2 Ag oxidation states of the catalysts.
c
Catalyst
Ag2+ (%)
Ag+ (%)
Ag0 (%)
Ag/anatase Ag/P25 Ag/rutile Ag/CeO2
27 56 56 0
50 22 36 0
23 22 8 100
Obtained by ICP-OES. Obtained by N2 adsorption-desorption isotherms. Obtained by TEM analysis.
Fig. 5. H2 TPR profiles of the catalysts.
classified into three major types. (1) Shimizu et al. reported that supported silver nanoparticles promoted a reducibility improvement of CeO2 at silver-ceria interfaces (Shimizu et al., 2010), (2) Liu et al.
Fig. 4. Ag 3d5/2 XPS spectra of Ag/TiO2 and Ag/CeO2 catalysts.
3
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Fig. 6. Total soot conversion. Reaction conditions: TGA, catalyst/soot = 10 (w/w), contact mode = tight contact, heating rate = 5℃/min, air flow =60 ml/min; (a) Pure TiO2, CeO2, and without catalyst and (b) Ag/TiO2 and Ag/CeO2 catalysts.
addressed in research regarding oxidation of carbonaceous materials (e.g., soot and CNT) by silver nanoparticles (Gardini et al., 2016; Yue et al., 2016). Hence, the superoxide formation capability of silver suggests the possibility of developing other supports which enhance the ability of silver to a greater extent than CeO2. Herein, we compared the soot oxidation activities of Ag/TiO2 and Ag/CeO2. Soot oxidation tests were performed in two contact modes: tight contact and loose contact. Improving the activity in loose contact mode is especially important because it is almost the same as in the case of activity under real conditions (Neeft et al., 1998). In Ag/TiO2 catalysts, the crystal phase of TiO2 affected the soot oxidation activity, and the best Ag/TiO2 catalyst demonstrated the possibility of replacing the Ag/CeO2 catalyst. However, the loose contact activity of Ag/TiO2 significantly decreased despite high tight contact activity. Thus, we solved the discrepancy between tight contact and loose contact by adding CeO2 in Ag/TiO2.
Table 3 T20 and T50 of the catalysts in tight contact and loose contact tests. Ⅰ.
Ⅱ.
Tight contact
T20 (℃)
T50 (℃)
anatase P25 rutile CeO2 w/o catalyst Ag/anatase Ag/P25 Ag/rutile Ag/CeO2 Loose contact Ag/P25 Ag/CeO2 CeO2-Ag/P25
506 492 487 368 574 347 322 322 322 T20 (℃) 500 495 468
541 525 517 390 606 360 327 330 327 T50 (℃) 567 548 514
2. Experimental 2.1. Catalyst synthesis 2.1.1. Preparation of Ag/TiO2 and Ag/CeO2 catalysts CeO2 support was prepared through calcination of Ce(NO3)3•6H2O (Sigma-Aldrich, 99%) at 500℃ for 4 h. Pure anatase (SigmaAldrich, < 25 nm particle size), pure rutile (Sigma-Aldrich, < 100 nm particle size), and P25 (Degussa, 21 nm average particle size) were used for supports. Before impregnation, TiO2 supports were calcinated at 500℃. Proper amounts of AgNO3 (Sigma-Aldrich, 99%) were dissolved in DI water to impregnate 5 wt.% Ag in the catalyst, and impregnation was performed by incipient wetness method. Impregnated catalysts were dried at 100℃ for 12 h and then calcinated at 500℃ for 4 h. 2.1.2. Preparation of CeO2 promoted Ag/P25 catalyst CeO2 was impregnated on prepared Ag/P25 to include 3 wt.% Ce in the catalyst. The impregnation was also performed by the incipient wetness method using an aqueous solution of Ce(NO3)3•6H2O (SigmaAldrich, 99%). Impregnated catalyst was dried at 100℃ for 12 h and then calcinated at 500℃ for 4 h. CeO2 impregnated Ag/P25 was denoted as CeO2-Ag/P25.
Fig. 7. Total soot conversion of Ag/P25 and Ag/CeO2 in tight contact and loose contact. Reaction conditions: TGA, catalyst/soot = 10 (w/w), heating rate = 5℃/min, and air flow =60 ml/min.
suggested that silver nanoparticles participated in soot oxidation as oxygen collectors that activated oxygen and transferred the active oxygen species to CeO2 (Liu et al., 2016), and (3) Corro et al. found that Ag/SiO2 exhibited enhanced soot oxidation activity that originated from superoxide formation by silver (Corro et al., 2013). Regarding the role of reducibility improvement and oxygen collector, silver should be present with CeO2 to promote soot oxidation. Meanwhile, superoxide formation indicates that silver can oxidize soot itself. This role was
2.2. Catalyst characterization Specific surface areas of the catalysts were measured by N2 adsorption-desorption experiments using a BELSORP-max instrument (BEL Japan Inc., Osaka, Japan). The catalysts were pretreated at 150℃ 4
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Scheme 1. Difference between tight contact and loose contact on Ag/P25.
Transmission electron microscopy (TEM) images of the catalyst were obtained using a Tecnai G2 11 F30 instrument (FEI Company, OR, USA). Samples were prepared by dispersing in ethanol and dropping onto Cu grids. Scanning transmission electron microscopy (STEM) was utilized to observe the CeO2-Ag/P25 and Ag/CeO2 catalyst using a Titan Themis3 Double Sc & Mono at the Korea Basic Science Institute (KBSI). To estimate the average particle size and distribution of silver, 50–100 A g particles were counted in each catalyst. X-ray photoelectron spectroscopy (XPS) analysis was carried out to verify the electronic state of silver using an ESCA2000 instrument (VG microtech: U.K.) with an AlKα X-ray anode source under 10−10 Torr. Hydrogen temperature-programmed reduction (H2-TPR) was carried out to measure the oxygen species in the catalysts using a Micromeritics AutoChem Ⅱ 2920 chemisorption analyzer. 50 mg of the catalyst was pretreated at 150℃ for 60 min under He flow. After cooling to 30℃, the sample was heated again to 800℃ under 50 ml/min of 10% H2/Ar flow.
Fig. 8. Total soot conversion of Ag/P25, Ag/CeO2, and CeO2-Ag/P25. Reaction conditions: TGA, catalyst/soot = 10 (w/w), contact mode = loose contact, heating rate = 5℃/min, and air flow =60 ml/min.
2.3. Soot oxidation tests Printex-U (Degussa, average particle size 25 nm) was used as a model compound of soot. Soot oxidation activities were evaluated under two contact modes: tight contact and loose contact. The soot (5.5 mg) and catalyst (55 mg) were mixed at a fixed ratio of 1/10. The mixture was rigorously ground in a mortar for 5 min to perform tight contact experiment. The mixture was prepared by spatula for 5 min to perform loose contact experiment. Oxidation experiments were done with a thermogravimetric analyzer (TA 5500, TA Instruments). The mixture was pretreated under N2 at 150℃ for 15 min to remove water on the catalyst and soot. The mixture (60 mg) was heated to 800℃ with a ramping rate of 5℃/min under 60 ml/min of air flow. The activity of catalyst was compared by T20 (temperature at which 20% of soot was
for 12 h under reduced pressure. Mass fractions of silver and cerium in the catalyst were measured by inductively coupled plasma optical emission spectroscopy (ICP-OES) using a JY Ultima2C (Jobin Yvon, France) dual ICP-OES spectrometer. For sample pretreatment, a mixture consisting of aqua regia and hydrofluoric acid was used as dissolving solvent. The analysis condition was 200℃ under reduced pressure. X-ray diffraction (XRD) analysis was performed to investigate the crystal phases of the catalysts using a SmartLab instrument (Rigaku). Scanning range was 20° to 80° with 2°/min scan rate. Cu-Kα irradiation (α =1.5406 Å) was used as the beam source.
Fig. 9. XRD patterns of Ag/P25, Ag/CeO2, and CeO2-Ag/P25: (a) Full range and (b) magnified peaks from 23° to 30°. 5
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Fig. 10. TEM and STEM images, EDX mappings of CeO2-Ag/P25.
practical condition, a temperature programmed oxidation (TPO) instrument was used. The soot (2 mg) and catalyst (20 mg) were mixed at a fixed ratio of 1/10. The mixture was prepared by spatula for 5 min to perform loose contact experiment and placed in a fixed-bed quartz reactor. 1% O2/He balance (100 ml/min) was fed to simulate GPF condition and 500 ppm NO/10% O2/He balance (100 ml/min) was fed to simulate DPF condition, respectively. The reaction temperature was heated to 800℃ with a ramping rate of 5℃/min. The concentrations of CO and CO2 in the outlet gas were measured by an infrared spectrometer (Nicloet iS 50, ThermoFisher Scientific). 3. Results and discussion 3.1. Ag/TiO2 and Ag/CeO2 catalysts for soot oxidation 3.1.1. XRD analysis Fig. 1(a) shows the XRD patterns of the Ag/TiO2 and Ag/CeO2 catalysts. Typical anatase, rutile, and CeO2 peaks are presented on Ag/ anatase, Ag/rutile, Ag/P25, and Ag/CeO2. Supports of Ag/anatase and Ag/rutile corresponded to pure anatase and pure rutile, respectively. Ag peaks (38.1°) were distinctly evident in Ag/rutile and Ag/CeO2. In the cases of Ag/P25 and Ag/anatase, the anatase peak of 37.8° is located closely within the vicinity of the Ag peak. Therefore, XRD patterns of 36.5° to 40.5° were magnified to observe Ag peaks in detail. In Fig. 1(b), the dotted line at 38.1° was drawn for detailed comparison of the Ag peaks. It appears that a small shoulder peak is slightly presented for Ag/ P25 and Ag/anatase, but the intensity was very weak compared with those of Ag/rutile and Ag/CeO2. The XRD patterns of the catalysts
Fig. 11. Ag 3d5/2 XPS spectra of Ag/P25 and CeO2-Ag/P25 catalysts.
converted) and T50 (temperature at which 50% of soot was converted). To evaluate thermal stability of CeO2-Ag/P25, recycling tests were performed. Temperature was cooled to room temperature after loose contact soot oxidation to 800℃. The catalyst (55 mg) was recovered and mixed with soot (5.5 mg) by spatula at a ratio of 1/10. The mixture was heated to 800℃ with a ramping rate of 5℃/min under 60 ml/min of air flow. Three recycling tests were performed by using same methods. To evaluate the soot oxidation activity of CeO2-Ag/P25 under 6
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Scheme 2. Difference between Ag/P25 and CeO2-Ag/P25 in loose contact.
composed of small nanoparticles smaller than 10 nm (Fig. 2(g)). However, the distinct shape of Ag was not observed in the bright field image of Ag/CeO2. To evaluate the dispersion of Ag in Ag/CeO2, HAADFSTEM image was obtained. Ag existed in the size range of 2–5 nm on the CeO2 support, as shown in Fig. 2(h). Additionally, the average particle size of silver was 2.92 nm, as determined from particle size distribution measurements. Comparing with Ag/TiO2 catalysts, Ag/CeO2 showed a narrow size distribution and small silver particle size. 3.1.3. XPS analysis Fig. 4 presents the Ag 3d5/2 XPS spectra of Ag/TiO2 and Ag/CeO2 catalysts. Ag 3d5/2 spectra typically appeared at different binding energies: that of metallic silver is 368.4 eV, and those of oxidized silver are 367.7 eV (Ag2O) and 367.4 eV (AgO) (Sandoval et al., 2015; Wen et al., 2011). The Ag 3d5/2 spectra were deconvoluted according to the locations of corresponding oxidation states, and Table 2 enumerates the oxidation state ratios. Ag/TiO2 and Ag/CeO2 catalysts showed distinct differences in oxidation states of Ag. Ag/TiO2 included AgOx (Ag2+ and Ag+) and metallic Ag (Ag0); on the other hand, Ag/CeO2 contained only metallic Ag. This may result from the unstable property of oxidized silver on CeO2, which is due to interaction between ionic silver and Ce3+ (Ag+ + Ce3+ → Ag + Ce4+) (Gao et al., 2017). Furthermore, changes in the oxidation degrees of silver on different TiO2 supports were observed. The XPS results revealed that silver species were more oxidized on rutile and P25 than on anatase. A similar effect was reported in CuO catalysts supported on TiO2 (Kang et al., 2013). Cu2+ species increased with higher rutile composition, and authors attributed the oxidation state changes to the intensity of interaction between CuO and TiO2 support. Ti species in anatase support interacted with Cu2+ ion strongly and reduced the ratio of Cu2+. The ratio of Ag2+ was lower on anatase than on rutile and P25; therefore, it was verified that Ag/ TiO2 catalysts also followed similar oxidation state tendencies.
Fig. 12. T20 and T50 of CeO2-Ag/P25 in soot oxidation recycling experiments.
3.1.4. H2 TPR analysis From H2 TPR characterization, oxygen species in the catalysts can be measured. As shown in Fig. 5, Ag/P25 and Ag/rutile showed similar H2 consumption profiles, in which a single peak existed at 55℃ and 62℃, respectively. The peak was attributed to oxygen included in silver oxide (AgOx) on P25 and rutile. Ag/anatase exhibited different H2 consumption trends in comparison with Ag/P25 and Ag/rutile. A lowtemperature peak at 94℃ was distributed in a wide range of temperatures, and peaks at 250 °C and 330 °C were also observed. The lowtemperature peak indicated the reduction of oxygen included in silver oxide on anatase, and high-temperature peaks may correspond to the reduction of anatase (Nanba et al., 2012). Considering the low-temperature peaks of Ag/TiO2 catalysts, silver on TiO2 contained some oxygen content that can be used for oxidation of reductants. Furthermore, the temperatures of peaks confirmed that the oxygen species of silver oxide on P25 and rutile were more active with respect to oxidation of reductant than the oxygen content of silver oxide on anatase. This result is related to oxidation states of silver verified in XPS analysis. It was reported that reduction of Ag2+ (AgO) occurred at a lower
Fig. 13. Ag 3d5/2 XPS spectra of fresh CeO2-Ag/P25 and spent CeO2-Ag/P25.
presented the possibility of highly dispersed Ag particles in Ag/anatase and Ag/P25; however, it is difficult to figure out the exact dispersion of Ag on TiO2 and CeO2 supports. Thus, TEM analysis was performed to accurately measure the Ag particle size in each catalyst. 3.1.2. TEM analysis TEM images of Ag/TiO2 and Ag/CeO2 catalysts are presented in Fig. 2. Ag size distributions were similar regardless of TiO2 supports. Most of the Ag particles were impregnated with sizes of 2–7 nm on TiO2 supports. The average particle sizes of Ag were 4.20 nm, 4.42 nm, and 4.59 nm in Ag/anatase, Ag/P25, and Ag/rutile, respectively (Fig. 3 and Table 1). Although the XRD results indicated the possibility of different Ag dispersions on anatase, P25, and rutile, the TiO2 crystal phase did not significantly affect Ag dispersions, as confirmed by the TEM images. CeO2 obtained from calcination of cerium nitrate precursor was 7
Journal of Hazardous Materials 384 (2020) 121341
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Fig. 14. T20 and T50 of CeO2-Ag/P25 and without catalyst under (a) GPF and (b) DPF conditions. Reaction conditions: TPO, catalyst/soot = 10 (w/w), heating rate = 5℃/min, and gas flow =100 ml/min of 1% O2/He balance for GPF and 100 ml/min of NO 500 ppm/10% O2/He balance for DPF.
temperature than reduction of Ag+ (Ag2O) (Skaf et al., 2014; Kim et al., 2013). In this research, Ag/P25 and Ag/rutile included more oxidized silver species compared with Ag/anatase (Table 2). Therefore, AgOx in Ag/P25 and Ag/rutile can exhibit reduction peaks at lower temperatures than Ag/anatase. In the case of Ag/CeO2, various peaks presented in the low-temperature region (90-250℃) and a high-temperature peak (740℃) was observed. CeO2 is well known for containing active oxygen species, and various researches characterized the H2 TPR related to CeO2-based catalyst. In pure CeO2, two peaks are generally observed: (1) Surface oxygen reduction at 350-650℃ and (2) bulk oxygen reduction at over 700℃ (Aneggi et al., 2014; Krishna et al., 2007). However, impregnating silver on CeO2 resulted in a peak shift to a lower temperature because silver promotes the reducibility of surface oxygen in CeO2; in contrast, the high-temperature peak remained nearly constant (Liu et al., 2016; Zhang et al., 2012). Considering the reported H2 TPR results for Ag/CeO2 catalysts, high-temperature peaks were originated from the bulk oxygen in CeO2, and low-temperature peaks resulted from the H2 consumption of surface oxygen (O2−) and more reducible oxygen (Ox-) influenced by silver on CeO2. XPS results confirmed that most of the silver contents on CeO2 are in metallic states, and therefore the low-temperature peaks were related to surface oxygen of CeO2. In comparison with Ag/P25 and Ag/rutile, Ag/CeO2 exhibited H2 consumption in a wider temperature range. Meanwhile, the low-temperature peaks suggest that Ag/P25 and Ag/rutile can supply active oxygen species to oxidation reactions at lower temperatures compared with Ag/CeO2.
was not a main factor which influenced soot oxidation activity, because Ag/CeO2 had more catalytic active sites than Ag/TiO2 catalysts considering the particle size of silver. Concerning the oxidation ability of silver, H2 TPR and XPS results presented different states of silver on TiO2 and CeO2 supports. Ag impregnated on P25 and rutile included more active oxygen, which can be reduced at a lower temperature compared with Ag on anatase and CeO2. Hence, soot oxidation activities were correlated with oxidation ability in low temperature Ag/TiO2 and Ag/CeO2 catalysts, as demonstrated by H2 TPR characterization. Skaf et al. reported similar results in carbon black oxidation (Skaf et al., 2014). Ag/CeO2 catalysts made by two preparation methods showed different catalytic activities, owing to the presence of Ag2+ species and lower reduction temperature in Ag/ CeO2 synthesized via impregnation method. 3.1.6. Soot oxidation activities in loose contact Ag/P25 and Ag/CeO2 catalysts were compared in loose contact. As shown in Fig. 7, the catalysts showed significantly decreased activities compared with tight contact and T50 of Ag/P25 catalyst was increased by 240℃ especially (Table 3). The large gap between tight contact and loose contact on Ag/TiO2 was observed in the previous report. At the beginning of soot oxidation in loose contact, silver in the vicinity of soot can participate in soot oxidation (Shimokawa et al., 2012). However, considerable amounts of impregnated silver cannot be used for soot oxidation in loose contact because of low contact ratio. Scheme 1 described the different contact modes on Ag/P25. Accordingly, the utilization possibility of Ag/TiO2 was low because of poor activity in loose contact regardless of the promoted oxidation ability of silver on TiO2 support in tight contact.
3.1.5. Soot oxidation activities in tight contact To evaluate the soot oxidation activities of supports, tight contact experiments were performed on TiO2 and CeO2 catalysts (Fig. 6(a) and Table 3). P25 and rutile showed 50% soot conversion at 525℃ and 517℃, and anatase showed 50% soot conversion at 541℃. CeO2 showed 50% soot conversion at 390℃; thus, it was found that the TiO2 catalysts had poor activities in comparison with CeO2. Because of low oxygen storage capacity and redox ability, 20% soot conversions were achieved above 480℃ for the TiO2 catalysts. On the other hand, Ag/P25 and Ag/rutile catalysts showed almost the same activities as Ag/CeO2, and the total conversion occurred at a lower temperature than in the case of Ag/CeO2 (Fig. 6(b) and Table 3). Considering the activities of supports (TiO2 and CeO2), it can be concluded that the soot oxidation activity of silver was promoted to a greater extent on P25 and rutile than on CeO2. Additionally, the effect of TiO2 crystalline phase on silver was verified because Ag/P25 and Ag/ rutile exhibited more improved activity than Ag/anatase. In TEM analysis, the average particle size of silver was smaller in Ag/CeO2 than those of Ag/TiO2 catalysts. Therefore, the particle size
3.2. CeO2-Ag/P25 catalyst for soot oxidation 3.2.1. Soot oxidation activity of CeO2-Ag/P25 in loose contact As described in Section 3.1, Ag/P25 showed the highest activity in tight contact, but the loose contact activity was decreased significantly. For developing the most useful catalyst under real conditions, we devised the improvement of oxygen delivery for the Ag/P25 catalyst. Therefore, we synthesized catalysts which have both high oxygen supply and transfer ability of generated active oxygen by adding CeO2 to Ag/P25. Fig. 8 shows that the CeO2 impregnated Ag/P25 catalysts exhibited higher activity than Ag/P25 and Ag/CeO2. In Table 3, CeO2Ag/P25 showed the conversion values of 20% and 50% at 468℃ and 514℃, respectively (30℃ lower than Ag/CeO2). 3.2.2. XRD, TEM, and XPS analyses XRD pattern of CeO2-Ag/P25 was obtained to compare the crystal 8
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phases with Ag/P25 and Ag/CeO2. In Fig. 9(a), CeO2-Ag/P25 exhibited the same XRD pattern as Ag/P25, and the CeO2 crystal phase was not detected despite cerium impregnation. This result may be explained by one of two reasons: (1) A small amount of CeO2 under the limit of detection or highly dispersed on the support, or (2) cerium substitution in the TiO2 lattice (mixed oxide phase). However, no mixed oxide phase of Ce-TiO2 existed, and the peak shift that resulted from Ce substitution was also not observed. Thus, it was presumed that impregnated cerium existed as small CeO2 particles with high dispersion on P25. In Fig. 10(a) and (b), silver particles on TiO2 supports were clearly observed, as seen in the TEM image of Ag/P25. However, CeO2 was not distinguished from TiO2 supports in both bright field and dark field images. Therefore, STEM EDX mapping was performed to reveal the location of CeO2 on TiO2. In Fig. 10(c) and (d), cerium contents (green color) were highly dispersed on TiO2, unlike silver nanoparticles (red color). This was correlated with the XRD pattern of CeO2-Ag/P25, within which the crystal phase of CeO2 did not present due to high dispersion. The STEM mapping confirmed that small CeO2 particles were impregnated adjacent to the silver nanoparticles on P25, and the structure indicated that the catalyst had a delivery function, as active oxygen generated from silver can be transferred to soot through CeO2. Fig. 11 shows the Ag 3d5/2 spectra of Ag/P25 and CeO2-Ag/P25. The peak position of Ag 3d5/2 was increased by 0.47 eV, which indicated the ratio of metallic Ag was higher in CeO2-Ag/P25 than in Ag/ P25. It resulted from the interaction between Ag and Ce (i.e., Ag+ + Ce3+ → Ag + Ce4+) (Gao et al., 2017). The verified Ce-Ag interaction evidenced the synergistic effect of Ag and CeO2 in CeO2-Ag/P25 catalyst can appear in soot oxidation. Scheme 2 described the difference in loose contact soot oxidation on Ag/P25 and CeO2-Ag/P25. The improved structure enabled silver nanoparticles to influence soot oxidation in loose contact by delivering active oxygen species through CeO2. This mechanism was comparably described in other reports. The rice-ball CeO2-Ag nanoparticle was a representative catalyst that had a similar oxidation mechanism (Yamazaki et al., 2011; Kayama et al., 2010). Oxygen diffused inside the silver nanoparticles, and then the silver nanoparticles generated active oxygen species and transferred them to CeO2; finally, soot reacted with the delivered active oxygen.
exhibited high soot oxidation activity after hydrothermal aging at 800℃ (Wang et al., 2019). CeO2-Ag/P25 also had thermal stability because Ag was closely surrounded by highly dispersed CeO2 and proved the practicability for commercial automobile exhaust catalyst, as evidenced in recycling test.
3.2.3. Thermal stability test Soot oxidation catalysts require thermal stability due to high temperatures behind automobile engines. To verify the thermal durability of CeO2-Ag/P25, recycling experiment to 800℃ was carried out four times. Fig. 12 shows the recycled soot oxidation results of CeO2-Ag/P25 catalyst: T20 and T50 were slightly decreased in comparison with the 1st oxidation test. To explain the reason of oxidation activity increase, we performed XPS analysis of spent catalyst. Fig. 13 shows the Ag 3d5/2 spectra of spent CeO2-Ag/P25 and fresh CeO2-Ag/P25. The peak position of Ag 3d5/2 spectra was decreased by 0.41 eV (from 367.99 eV to 367.58 eV), which indicated that Ag was more oxidized after reaction. Ag in CeO2-Ag/P25 was relatively metallic because of interaction between Ce and Ag compared with Ag in Ag/P25. After soot oxidation to 800℃, Ag in spent catalyst was more oxidized due to a factor which can affect oxidation state change. It may be thermal factor since fresh catalyst was calcined at 500℃. After soot oxidation, spent catalyst experienced high temperature to 800℃. As verified in soot oxidation results and characterizations of Ag/TiO2 and Ag/CeO2 catalysts, active oxygen generation ability increased with increase of Ag oxidation state. Consequently, it can be concluded that the change of Ag state affected the decrease of T20 and T50 in recycling test. With the activity improvement, thermal stability of CeO2-Ag/P25 prevented deactivation of the catalyst. The durability of CeO2-Ag/P25 resulted from resistance on sintering due to Ag-CeO2 interfacial effect. It was reported that Ag located on interface of CeO2 has large adhesion energy (Farmer and Campbell, 2010; Campbell and Mao, 2017). This property led to resistance on sintering of Ag on CeO2. Similarly, Ag/Co@Ce catalyst
Acknowledgements
3.2.4. Soot oxidation activity of CeO2-Ag/P25 under practical conditions Fig. 14 shows T20 and T50 of CeO2-Ag/P25 in DPF and GPF conditions. Despite low concentration of oxygen (1%), CeO2-Ag/P25 converted 20% of soot at 422℃ under GPF condition (185℃ lower than the case without catalyst) due to high active oxygen generation ability. Additionally, CeO2-Ag/P25 converted 20% of soot at 382℃ in DPF condition with NOx assisted soot oxidation. Totally, CeO2-Ag/P25 catalyst showed highly promoted soot oxidation activity in practical atmosphere. 4. Conclusions Silver was impregnated on TiO2 (anatase, P25, and rutile) and CeO2. The supports influenced the oxidation abilities of silver and Ag/P25 and Ag/rutile exhibited more enhancement of Ag functionality than cases of Ag/anatase and Ag/CeO2. Ag/P25 exhibited T50 at 327℃ (the same as Ag/CeO2) in tight contact. However, considerable amounts of silver cannot contact soot in loose contact (real condition), which resulted in the poor activity of Ag/P25, with T50 at 567℃. To solve the lack of oxygen delivery ability, we newly introduced CeO2 on Ag/P25. This modified catalyst had two enhanced abilities: (1) Active oxygen supply by silver on P25 and (2) satisfying delivery of the active oxygen through CeO2. Consequently, the CeO2-Ag/P25 catalyst exhibited the highest activity in loose contact even at low temperature, with T20 = 468℃ and T50 = 514℃, and the temperature was 30℃ lower than that by Ag/ CeO2 which is known as the best catalyst in soot oxidation. The thermal durability was corroborated by the recycling test, and T20 and T50 were not increased compared with fresh catalyst. Declaration of Competing Interest None.
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CeO2 promoted Ag/TiO2 catalyst for soot oxidation with improved active oxygen generation and delivery abilities
T
Min June Kima, Geun-Ho Hana, Seong Ho Leea,b, Hyun Wook Junga, Jin Woo Choungc, ⁎ Chang Hwan Kimc, Kwan-Young Leea,b,d, a
Department of Chemical and Biological Engineering, Korea University, 145 Anam-ro, Seongbuk-Gu, Seoul, 02841, Republic of Korea Super Ultra Low Energy and Emission Vehicle (SULEEV) Center, Korea University, 145 Anam-ro, Seongbuk-Gu, Seoul, 02841, Republic of Korea c Advanced Catalysts and Emission-Control Research Lab, Research & Development Division, Hyundai Motor Group, Hyundaiyeonguso-Ro, Namyang-Eup, Hwaseong-Si, Gyeonggi-Do, 18280, Republic of Korea d Graduate School of Energy and Environment (KU-KIST Green School), Korea University, Seoul, 02841, Republic of Korea b
GRAPHICAL ABSTRACT
ARTICLE INFO
ABSTRACT
Editor: Xiaohong Guan
To evaluate the usability of TiO2 support for silver in soot oxidation, Ag/TiO2 and Ag/CeO2 catalysts were prepared and soot oxidation experiment was performed. The catalytic activity of silver was more enhanced on P25 and rutile than on anatase and CeO2 in tight contact, as evidenced from comparing the activities of supports and silver impregnated catalysts. The reasons for the difference in active metal enhancement were elucidated by various characterization methods (TEM, H2 TPR, and XPS), and it is verified that it resulted not from dispersion of silver but from oxidation ability. On the other hand, Ag/P25 showed poor activity in loose contact because of low contact between silver and soot. To improve the loose contact activity of the Ag/P25 catalyst, CeO2 was introduced for active oxygen delivery. The synthesized CeO2-Ag/P25 catalyst showed enhanced loose contact activity when compared with Ag/P25 and Ag/CeO2 due to synergistic effects from the active oxygen supply ability of silver on P25 and the oxygen transfer ability of loaded CeO2. Thermal stability of the catalyst was identified by recycling test to 800℃, and the maintained T20 and T50 demonstrated its durability for practical use in automobile exhaust removal.
Keywords: Soot oxidation Ceria promoted Ag/TiO2 Synergistic effect Active oxygen generation Active oxygen delivery
⁎
Corresponding author at: Department of Chemical and Biological Engineering, Korea University, 145 Anam-ro, Seongbuk-Gu, Seoul, 02841, Republic of Korea. E-mail address: [email protected] (K.-Y. Lee).
https://doi.org/10.1016/j.jhazmat.2019.121341 Received 17 May 2019; Received in revised form 23 September 2019; Accepted 27 September 2019 Available online 27 September 2019 0304-3894/ © 2019 Elsevier B.V. All rights reserved.
Journal of Hazardous Materials 384 (2020) 121341
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1. Introduction
Therefore, the development of soot oxidation catalysts that can initiate the combustion at low temperature is essential. CeO2 is a representative oxidation catalyst due to its high redox property and thermal stability (Wu et al., 2011; Abbasi et al., 2011; Montini et al., 2016; Gao et al., 2018; Xie et al., 2017). In soot oxidation, various CeO2-based catalysts have been reported. For instance, (1) controlling the structures of the catalysts (nanofiber (Kumar et al., 2012; Bensaid et al., 2013), nanocube (Sudarsanam et al., 2015), and 3DOM (Zhang et al., 2011; Zhai et al., 2019; Lee et al., 2019)) to increase the contact number with soot, (2) doping of transition metals (e.g., Mn, Cu, Co, and Zr) (Fu et al., 2010; Zhang et al., 2010; Aneggi et al., 2006) and rare earth metals (e.g., La, Pr, and Sm) (Harada et al., 2013; Rangaswamy et al., 2015; Jeong et al., 2019) to promote the redox ability, (3) impregnation of potassium to take advantage of its volatile property and to enhance the redox properties of CeO2 (Neyertz et al., 2014; Aneggi et al., 2008), and (4) addition of noble metals (e.g., Ag and Ru) were applied to improve the oxidation ability (Aouad et al., 2009; Kurnatowska et al., 2014). Among the promotion methods, Ag/CeO2 catalysts are known as the best ones for soot oxidation in oxygen atmosphere. However, a question still remains as to whether CeO2 is the best support in terms of silver’s role in soot oxidation. The roles of silver in soot oxidation were
Developing high fuel efficiency and low emissions for petroleumbased automobiles represents an important issue in recent years. Although diesel engines and gasoline direct injection engines (GDI) promote fuel efficiency, they result in high emissions of exhaust gas, NOx and particulate matter (PM) especially. Emission regulations (e.g., EURO 6, LEV III) have been reinforced at a rapid pace over the last two decades, and solving the problem is urgent in petroleum-based automobile industries. Accordingly, several researches about catalysts for automobile exhaust gas have been reported (Park et al., 2018; Lee et al., 2010, 2018). The main component of PM is a carbonaceous material which is also referred to as soot, and it induces various lung diseases in human beings through inhalation (Xing et al., 2016). For preventing the release of PM to the atmosphere, DPF (diesel particulate filter) and GPF (gasoline particulate filter) are equipped behind the engine. Accumulated PM is converted to CO2 by combustion for regeneration of filters. However, the stable structure of soot requires high combustion temperature, which is not favorable in general driving condition of vehicle. Hence, additional fuel injection is required to raise the temperature, but it results in reduced fuel efficiency (and increased emission of CO2).
Fig. 1. XRD patterns of the catalysts: (a) Full range and (b) magnified peaks from 35° to 42°.
Fig. 2. TEM images of (a), (b) Ag/anatase, (c), (d) Ag/P25, (e), (f) Ag/rutile, and (g) Ag/CeO2, and STEM image of (h) Ag/CeO2. 2
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Fig. 3. Particle size distributions of (a) Ag/anatase, (b) Ag/P25, (c) Ag/rutile, and (d) Ag/CeO2. Table 1 Ag and Ce contents, specific surface areas, and Ag sizes in the catalysts. Catalyst
Ag (wt.%)
Ce (wt.%)
Specific surface area (m2/g) b
Ag size (nm)
Ag/anatase Ag/P25 Ag/rutile Ag/CeO2 CeO2-Ag/P25
4.8 5.0 5.4 4.6 4.6
– – – 74 2.8
25 34 16 60 27
4.20 4.42 4.59 2.92 3.83
a b c
a
a
Table 2 Ag oxidation states of the catalysts.
c
Catalyst
Ag2+ (%)
Ag+ (%)
Ag0 (%)
Ag/anatase Ag/P25 Ag/rutile Ag/CeO2
27 56 56 0
50 22 36 0
23 22 8 100
Obtained by ICP-OES. Obtained by N2 adsorption-desorption isotherms. Obtained by TEM analysis.
Fig. 5. H2 TPR profiles of the catalysts.
classified into three major types. (1) Shimizu et al. reported that supported silver nanoparticles promoted a reducibility improvement of CeO2 at silver-ceria interfaces (Shimizu et al., 2010), (2) Liu et al.
Fig. 4. Ag 3d5/2 XPS spectra of Ag/TiO2 and Ag/CeO2 catalysts.
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Fig. 6. Total soot conversion. Reaction conditions: TGA, catalyst/soot = 10 (w/w), contact mode = tight contact, heating rate = 5℃/min, air flow =60 ml/min; (a) Pure TiO2, CeO2, and without catalyst and (b) Ag/TiO2 and Ag/CeO2 catalysts.
addressed in research regarding oxidation of carbonaceous materials (e.g., soot and CNT) by silver nanoparticles (Gardini et al., 2016; Yue et al., 2016). Hence, the superoxide formation capability of silver suggests the possibility of developing other supports which enhance the ability of silver to a greater extent than CeO2. Herein, we compared the soot oxidation activities of Ag/TiO2 and Ag/CeO2. Soot oxidation tests were performed in two contact modes: tight contact and loose contact. Improving the activity in loose contact mode is especially important because it is almost the same as in the case of activity under real conditions (Neeft et al., 1998). In Ag/TiO2 catalysts, the crystal phase of TiO2 affected the soot oxidation activity, and the best Ag/TiO2 catalyst demonstrated the possibility of replacing the Ag/CeO2 catalyst. However, the loose contact activity of Ag/TiO2 significantly decreased despite high tight contact activity. Thus, we solved the discrepancy between tight contact and loose contact by adding CeO2 in Ag/TiO2.
Table 3 T20 and T50 of the catalysts in tight contact and loose contact tests. Ⅰ.
Ⅱ.
Tight contact
T20 (℃)
T50 (℃)
anatase P25 rutile CeO2 w/o catalyst Ag/anatase Ag/P25 Ag/rutile Ag/CeO2 Loose contact Ag/P25 Ag/CeO2 CeO2-Ag/P25
506 492 487 368 574 347 322 322 322 T20 (℃) 500 495 468
541 525 517 390 606 360 327 330 327 T50 (℃) 567 548 514
2. Experimental 2.1. Catalyst synthesis 2.1.1. Preparation of Ag/TiO2 and Ag/CeO2 catalysts CeO2 support was prepared through calcination of Ce(NO3)3•6H2O (Sigma-Aldrich, 99%) at 500℃ for 4 h. Pure anatase (SigmaAldrich, < 25 nm particle size), pure rutile (Sigma-Aldrich, < 100 nm particle size), and P25 (Degussa, 21 nm average particle size) were used for supports. Before impregnation, TiO2 supports were calcinated at 500℃. Proper amounts of AgNO3 (Sigma-Aldrich, 99%) were dissolved in DI water to impregnate 5 wt.% Ag in the catalyst, and impregnation was performed by incipient wetness method. Impregnated catalysts were dried at 100℃ for 12 h and then calcinated at 500℃ for 4 h. 2.1.2. Preparation of CeO2 promoted Ag/P25 catalyst CeO2 was impregnated on prepared Ag/P25 to include 3 wt.% Ce in the catalyst. The impregnation was also performed by the incipient wetness method using an aqueous solution of Ce(NO3)3•6H2O (SigmaAldrich, 99%). Impregnated catalyst was dried at 100℃ for 12 h and then calcinated at 500℃ for 4 h. CeO2 impregnated Ag/P25 was denoted as CeO2-Ag/P25.
Fig. 7. Total soot conversion of Ag/P25 and Ag/CeO2 in tight contact and loose contact. Reaction conditions: TGA, catalyst/soot = 10 (w/w), heating rate = 5℃/min, and air flow =60 ml/min.
suggested that silver nanoparticles participated in soot oxidation as oxygen collectors that activated oxygen and transferred the active oxygen species to CeO2 (Liu et al., 2016), and (3) Corro et al. found that Ag/SiO2 exhibited enhanced soot oxidation activity that originated from superoxide formation by silver (Corro et al., 2013). Regarding the role of reducibility improvement and oxygen collector, silver should be present with CeO2 to promote soot oxidation. Meanwhile, superoxide formation indicates that silver can oxidize soot itself. This role was
2.2. Catalyst characterization Specific surface areas of the catalysts were measured by N2 adsorption-desorption experiments using a BELSORP-max instrument (BEL Japan Inc., Osaka, Japan). The catalysts were pretreated at 150℃ 4
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Scheme 1. Difference between tight contact and loose contact on Ag/P25.
Transmission electron microscopy (TEM) images of the catalyst were obtained using a Tecnai G2 11 F30 instrument (FEI Company, OR, USA). Samples were prepared by dispersing in ethanol and dropping onto Cu grids. Scanning transmission electron microscopy (STEM) was utilized to observe the CeO2-Ag/P25 and Ag/CeO2 catalyst using a Titan Themis3 Double Sc & Mono at the Korea Basic Science Institute (KBSI). To estimate the average particle size and distribution of silver, 50–100 A g particles were counted in each catalyst. X-ray photoelectron spectroscopy (XPS) analysis was carried out to verify the electronic state of silver using an ESCA2000 instrument (VG microtech: U.K.) with an AlKα X-ray anode source under 10−10 Torr. Hydrogen temperature-programmed reduction (H2-TPR) was carried out to measure the oxygen species in the catalysts using a Micromeritics AutoChem Ⅱ 2920 chemisorption analyzer. 50 mg of the catalyst was pretreated at 150℃ for 60 min under He flow. After cooling to 30℃, the sample was heated again to 800℃ under 50 ml/min of 10% H2/Ar flow.
Fig. 8. Total soot conversion of Ag/P25, Ag/CeO2, and CeO2-Ag/P25. Reaction conditions: TGA, catalyst/soot = 10 (w/w), contact mode = loose contact, heating rate = 5℃/min, and air flow =60 ml/min.
2.3. Soot oxidation tests Printex-U (Degussa, average particle size 25 nm) was used as a model compound of soot. Soot oxidation activities were evaluated under two contact modes: tight contact and loose contact. The soot (5.5 mg) and catalyst (55 mg) were mixed at a fixed ratio of 1/10. The mixture was rigorously ground in a mortar for 5 min to perform tight contact experiment. The mixture was prepared by spatula for 5 min to perform loose contact experiment. Oxidation experiments were done with a thermogravimetric analyzer (TA 5500, TA Instruments). The mixture was pretreated under N2 at 150℃ for 15 min to remove water on the catalyst and soot. The mixture (60 mg) was heated to 800℃ with a ramping rate of 5℃/min under 60 ml/min of air flow. The activity of catalyst was compared by T20 (temperature at which 20% of soot was
for 12 h under reduced pressure. Mass fractions of silver and cerium in the catalyst were measured by inductively coupled plasma optical emission spectroscopy (ICP-OES) using a JY Ultima2C (Jobin Yvon, France) dual ICP-OES spectrometer. For sample pretreatment, a mixture consisting of aqua regia and hydrofluoric acid was used as dissolving solvent. The analysis condition was 200℃ under reduced pressure. X-ray diffraction (XRD) analysis was performed to investigate the crystal phases of the catalysts using a SmartLab instrument (Rigaku). Scanning range was 20° to 80° with 2°/min scan rate. Cu-Kα irradiation (α =1.5406 Å) was used as the beam source.
Fig. 9. XRD patterns of Ag/P25, Ag/CeO2, and CeO2-Ag/P25: (a) Full range and (b) magnified peaks from 23° to 30°. 5
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Fig. 10. TEM and STEM images, EDX mappings of CeO2-Ag/P25.
practical condition, a temperature programmed oxidation (TPO) instrument was used. The soot (2 mg) and catalyst (20 mg) were mixed at a fixed ratio of 1/10. The mixture was prepared by spatula for 5 min to perform loose contact experiment and placed in a fixed-bed quartz reactor. 1% O2/He balance (100 ml/min) was fed to simulate GPF condition and 500 ppm NO/10% O2/He balance (100 ml/min) was fed to simulate DPF condition, respectively. The reaction temperature was heated to 800℃ with a ramping rate of 5℃/min. The concentrations of CO and CO2 in the outlet gas were measured by an infrared spectrometer (Nicloet iS 50, ThermoFisher Scientific). 3. Results and discussion 3.1. Ag/TiO2 and Ag/CeO2 catalysts for soot oxidation 3.1.1. XRD analysis Fig. 1(a) shows the XRD patterns of the Ag/TiO2 and Ag/CeO2 catalysts. Typical anatase, rutile, and CeO2 peaks are presented on Ag/ anatase, Ag/rutile, Ag/P25, and Ag/CeO2. Supports of Ag/anatase and Ag/rutile corresponded to pure anatase and pure rutile, respectively. Ag peaks (38.1°) were distinctly evident in Ag/rutile and Ag/CeO2. In the cases of Ag/P25 and Ag/anatase, the anatase peak of 37.8° is located closely within the vicinity of the Ag peak. Therefore, XRD patterns of 36.5° to 40.5° were magnified to observe Ag peaks in detail. In Fig. 1(b), the dotted line at 38.1° was drawn for detailed comparison of the Ag peaks. It appears that a small shoulder peak is slightly presented for Ag/ P25 and Ag/anatase, but the intensity was very weak compared with those of Ag/rutile and Ag/CeO2. The XRD patterns of the catalysts
Fig. 11. Ag 3d5/2 XPS spectra of Ag/P25 and CeO2-Ag/P25 catalysts.
converted) and T50 (temperature at which 50% of soot was converted). To evaluate thermal stability of CeO2-Ag/P25, recycling tests were performed. Temperature was cooled to room temperature after loose contact soot oxidation to 800℃. The catalyst (55 mg) was recovered and mixed with soot (5.5 mg) by spatula at a ratio of 1/10. The mixture was heated to 800℃ with a ramping rate of 5℃/min under 60 ml/min of air flow. Three recycling tests were performed by using same methods. To evaluate the soot oxidation activity of CeO2-Ag/P25 under 6
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Scheme 2. Difference between Ag/P25 and CeO2-Ag/P25 in loose contact.
composed of small nanoparticles smaller than 10 nm (Fig. 2(g)). However, the distinct shape of Ag was not observed in the bright field image of Ag/CeO2. To evaluate the dispersion of Ag in Ag/CeO2, HAADFSTEM image was obtained. Ag existed in the size range of 2–5 nm on the CeO2 support, as shown in Fig. 2(h). Additionally, the average particle size of silver was 2.92 nm, as determined from particle size distribution measurements. Comparing with Ag/TiO2 catalysts, Ag/CeO2 showed a narrow size distribution and small silver particle size. 3.1.3. XPS analysis Fig. 4 presents the Ag 3d5/2 XPS spectra of Ag/TiO2 and Ag/CeO2 catalysts. Ag 3d5/2 spectra typically appeared at different binding energies: that of metallic silver is 368.4 eV, and those of oxidized silver are 367.7 eV (Ag2O) and 367.4 eV (AgO) (Sandoval et al., 2015; Wen et al., 2011). The Ag 3d5/2 spectra were deconvoluted according to the locations of corresponding oxidation states, and Table 2 enumerates the oxidation state ratios. Ag/TiO2 and Ag/CeO2 catalysts showed distinct differences in oxidation states of Ag. Ag/TiO2 included AgOx (Ag2+ and Ag+) and metallic Ag (Ag0); on the other hand, Ag/CeO2 contained only metallic Ag. This may result from the unstable property of oxidized silver on CeO2, which is due to interaction between ionic silver and Ce3+ (Ag+ + Ce3+ → Ag + Ce4+) (Gao et al., 2017). Furthermore, changes in the oxidation degrees of silver on different TiO2 supports were observed. The XPS results revealed that silver species were more oxidized on rutile and P25 than on anatase. A similar effect was reported in CuO catalysts supported on TiO2 (Kang et al., 2013). Cu2+ species increased with higher rutile composition, and authors attributed the oxidation state changes to the intensity of interaction between CuO and TiO2 support. Ti species in anatase support interacted with Cu2+ ion strongly and reduced the ratio of Cu2+. The ratio of Ag2+ was lower on anatase than on rutile and P25; therefore, it was verified that Ag/ TiO2 catalysts also followed similar oxidation state tendencies.
Fig. 12. T20 and T50 of CeO2-Ag/P25 in soot oxidation recycling experiments.
3.1.4. H2 TPR analysis From H2 TPR characterization, oxygen species in the catalysts can be measured. As shown in Fig. 5, Ag/P25 and Ag/rutile showed similar H2 consumption profiles, in which a single peak existed at 55℃ and 62℃, respectively. The peak was attributed to oxygen included in silver oxide (AgOx) on P25 and rutile. Ag/anatase exhibited different H2 consumption trends in comparison with Ag/P25 and Ag/rutile. A lowtemperature peak at 94℃ was distributed in a wide range of temperatures, and peaks at 250 °C and 330 °C were also observed. The lowtemperature peak indicated the reduction of oxygen included in silver oxide on anatase, and high-temperature peaks may correspond to the reduction of anatase (Nanba et al., 2012). Considering the low-temperature peaks of Ag/TiO2 catalysts, silver on TiO2 contained some oxygen content that can be used for oxidation of reductants. Furthermore, the temperatures of peaks confirmed that the oxygen species of silver oxide on P25 and rutile were more active with respect to oxidation of reductant than the oxygen content of silver oxide on anatase. This result is related to oxidation states of silver verified in XPS analysis. It was reported that reduction of Ag2+ (AgO) occurred at a lower
Fig. 13. Ag 3d5/2 XPS spectra of fresh CeO2-Ag/P25 and spent CeO2-Ag/P25.
presented the possibility of highly dispersed Ag particles in Ag/anatase and Ag/P25; however, it is difficult to figure out the exact dispersion of Ag on TiO2 and CeO2 supports. Thus, TEM analysis was performed to accurately measure the Ag particle size in each catalyst. 3.1.2. TEM analysis TEM images of Ag/TiO2 and Ag/CeO2 catalysts are presented in Fig. 2. Ag size distributions were similar regardless of TiO2 supports. Most of the Ag particles were impregnated with sizes of 2–7 nm on TiO2 supports. The average particle sizes of Ag were 4.20 nm, 4.42 nm, and 4.59 nm in Ag/anatase, Ag/P25, and Ag/rutile, respectively (Fig. 3 and Table 1). Although the XRD results indicated the possibility of different Ag dispersions on anatase, P25, and rutile, the TiO2 crystal phase did not significantly affect Ag dispersions, as confirmed by the TEM images. CeO2 obtained from calcination of cerium nitrate precursor was 7
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Fig. 14. T20 and T50 of CeO2-Ag/P25 and without catalyst under (a) GPF and (b) DPF conditions. Reaction conditions: TPO, catalyst/soot = 10 (w/w), heating rate = 5℃/min, and gas flow =100 ml/min of 1% O2/He balance for GPF and 100 ml/min of NO 500 ppm/10% O2/He balance for DPF.
temperature than reduction of Ag+ (Ag2O) (Skaf et al., 2014; Kim et al., 2013). In this research, Ag/P25 and Ag/rutile included more oxidized silver species compared with Ag/anatase (Table 2). Therefore, AgOx in Ag/P25 and Ag/rutile can exhibit reduction peaks at lower temperatures than Ag/anatase. In the case of Ag/CeO2, various peaks presented in the low-temperature region (90-250℃) and a high-temperature peak (740℃) was observed. CeO2 is well known for containing active oxygen species, and various researches characterized the H2 TPR related to CeO2-based catalyst. In pure CeO2, two peaks are generally observed: (1) Surface oxygen reduction at 350-650℃ and (2) bulk oxygen reduction at over 700℃ (Aneggi et al., 2014; Krishna et al., 2007). However, impregnating silver on CeO2 resulted in a peak shift to a lower temperature because silver promotes the reducibility of surface oxygen in CeO2; in contrast, the high-temperature peak remained nearly constant (Liu et al., 2016; Zhang et al., 2012). Considering the reported H2 TPR results for Ag/CeO2 catalysts, high-temperature peaks were originated from the bulk oxygen in CeO2, and low-temperature peaks resulted from the H2 consumption of surface oxygen (O2−) and more reducible oxygen (Ox-) influenced by silver on CeO2. XPS results confirmed that most of the silver contents on CeO2 are in metallic states, and therefore the low-temperature peaks were related to surface oxygen of CeO2. In comparison with Ag/P25 and Ag/rutile, Ag/CeO2 exhibited H2 consumption in a wider temperature range. Meanwhile, the low-temperature peaks suggest that Ag/P25 and Ag/rutile can supply active oxygen species to oxidation reactions at lower temperatures compared with Ag/CeO2.
was not a main factor which influenced soot oxidation activity, because Ag/CeO2 had more catalytic active sites than Ag/TiO2 catalysts considering the particle size of silver. Concerning the oxidation ability of silver, H2 TPR and XPS results presented different states of silver on TiO2 and CeO2 supports. Ag impregnated on P25 and rutile included more active oxygen, which can be reduced at a lower temperature compared with Ag on anatase and CeO2. Hence, soot oxidation activities were correlated with oxidation ability in low temperature Ag/TiO2 and Ag/CeO2 catalysts, as demonstrated by H2 TPR characterization. Skaf et al. reported similar results in carbon black oxidation (Skaf et al., 2014). Ag/CeO2 catalysts made by two preparation methods showed different catalytic activities, owing to the presence of Ag2+ species and lower reduction temperature in Ag/ CeO2 synthesized via impregnation method. 3.1.6. Soot oxidation activities in loose contact Ag/P25 and Ag/CeO2 catalysts were compared in loose contact. As shown in Fig. 7, the catalysts showed significantly decreased activities compared with tight contact and T50 of Ag/P25 catalyst was increased by 240℃ especially (Table 3). The large gap between tight contact and loose contact on Ag/TiO2 was observed in the previous report. At the beginning of soot oxidation in loose contact, silver in the vicinity of soot can participate in soot oxidation (Shimokawa et al., 2012). However, considerable amounts of impregnated silver cannot be used for soot oxidation in loose contact because of low contact ratio. Scheme 1 described the different contact modes on Ag/P25. Accordingly, the utilization possibility of Ag/TiO2 was low because of poor activity in loose contact regardless of the promoted oxidation ability of silver on TiO2 support in tight contact.
3.1.5. Soot oxidation activities in tight contact To evaluate the soot oxidation activities of supports, tight contact experiments were performed on TiO2 and CeO2 catalysts (Fig. 6(a) and Table 3). P25 and rutile showed 50% soot conversion at 525℃ and 517℃, and anatase showed 50% soot conversion at 541℃. CeO2 showed 50% soot conversion at 390℃; thus, it was found that the TiO2 catalysts had poor activities in comparison with CeO2. Because of low oxygen storage capacity and redox ability, 20% soot conversions were achieved above 480℃ for the TiO2 catalysts. On the other hand, Ag/P25 and Ag/rutile catalysts showed almost the same activities as Ag/CeO2, and the total conversion occurred at a lower temperature than in the case of Ag/CeO2 (Fig. 6(b) and Table 3). Considering the activities of supports (TiO2 and CeO2), it can be concluded that the soot oxidation activity of silver was promoted to a greater extent on P25 and rutile than on CeO2. Additionally, the effect of TiO2 crystalline phase on silver was verified because Ag/P25 and Ag/ rutile exhibited more improved activity than Ag/anatase. In TEM analysis, the average particle size of silver was smaller in Ag/CeO2 than those of Ag/TiO2 catalysts. Therefore, the particle size
3.2. CeO2-Ag/P25 catalyst for soot oxidation 3.2.1. Soot oxidation activity of CeO2-Ag/P25 in loose contact As described in Section 3.1, Ag/P25 showed the highest activity in tight contact, but the loose contact activity was decreased significantly. For developing the most useful catalyst under real conditions, we devised the improvement of oxygen delivery for the Ag/P25 catalyst. Therefore, we synthesized catalysts which have both high oxygen supply and transfer ability of generated active oxygen by adding CeO2 to Ag/P25. Fig. 8 shows that the CeO2 impregnated Ag/P25 catalysts exhibited higher activity than Ag/P25 and Ag/CeO2. In Table 3, CeO2Ag/P25 showed the conversion values of 20% and 50% at 468℃ and 514℃, respectively (30℃ lower than Ag/CeO2). 3.2.2. XRD, TEM, and XPS analyses XRD pattern of CeO2-Ag/P25 was obtained to compare the crystal 8
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phases with Ag/P25 and Ag/CeO2. In Fig. 9(a), CeO2-Ag/P25 exhibited the same XRD pattern as Ag/P25, and the CeO2 crystal phase was not detected despite cerium impregnation. This result may be explained by one of two reasons: (1) A small amount of CeO2 under the limit of detection or highly dispersed on the support, or (2) cerium substitution in the TiO2 lattice (mixed oxide phase). However, no mixed oxide phase of Ce-TiO2 existed, and the peak shift that resulted from Ce substitution was also not observed. Thus, it was presumed that impregnated cerium existed as small CeO2 particles with high dispersion on P25. In Fig. 10(a) and (b), silver particles on TiO2 supports were clearly observed, as seen in the TEM image of Ag/P25. However, CeO2 was not distinguished from TiO2 supports in both bright field and dark field images. Therefore, STEM EDX mapping was performed to reveal the location of CeO2 on TiO2. In Fig. 10(c) and (d), cerium contents (green color) were highly dispersed on TiO2, unlike silver nanoparticles (red color). This was correlated with the XRD pattern of CeO2-Ag/P25, within which the crystal phase of CeO2 did not present due to high dispersion. The STEM mapping confirmed that small CeO2 particles were impregnated adjacent to the silver nanoparticles on P25, and the structure indicated that the catalyst had a delivery function, as active oxygen generated from silver can be transferred to soot through CeO2. Fig. 11 shows the Ag 3d5/2 spectra of Ag/P25 and CeO2-Ag/P25. The peak position of Ag 3d5/2 was increased by 0.47 eV, which indicated the ratio of metallic Ag was higher in CeO2-Ag/P25 than in Ag/ P25. It resulted from the interaction between Ag and Ce (i.e., Ag+ + Ce3+ → Ag + Ce4+) (Gao et al., 2017). The verified Ce-Ag interaction evidenced the synergistic effect of Ag and CeO2 in CeO2-Ag/P25 catalyst can appear in soot oxidation. Scheme 2 described the difference in loose contact soot oxidation on Ag/P25 and CeO2-Ag/P25. The improved structure enabled silver nanoparticles to influence soot oxidation in loose contact by delivering active oxygen species through CeO2. This mechanism was comparably described in other reports. The rice-ball CeO2-Ag nanoparticle was a representative catalyst that had a similar oxidation mechanism (Yamazaki et al., 2011; Kayama et al., 2010). Oxygen diffused inside the silver nanoparticles, and then the silver nanoparticles generated active oxygen species and transferred them to CeO2; finally, soot reacted with the delivered active oxygen.
exhibited high soot oxidation activity after hydrothermal aging at 800℃ (Wang et al., 2019). CeO2-Ag/P25 also had thermal stability because Ag was closely surrounded by highly dispersed CeO2 and proved the practicability for commercial automobile exhaust catalyst, as evidenced in recycling test.
3.2.3. Thermal stability test Soot oxidation catalysts require thermal stability due to high temperatures behind automobile engines. To verify the thermal durability of CeO2-Ag/P25, recycling experiment to 800℃ was carried out four times. Fig. 12 shows the recycled soot oxidation results of CeO2-Ag/P25 catalyst: T20 and T50 were slightly decreased in comparison with the 1st oxidation test. To explain the reason of oxidation activity increase, we performed XPS analysis of spent catalyst. Fig. 13 shows the Ag 3d5/2 spectra of spent CeO2-Ag/P25 and fresh CeO2-Ag/P25. The peak position of Ag 3d5/2 spectra was decreased by 0.41 eV (from 367.99 eV to 367.58 eV), which indicated that Ag was more oxidized after reaction. Ag in CeO2-Ag/P25 was relatively metallic because of interaction between Ce and Ag compared with Ag in Ag/P25. After soot oxidation to 800℃, Ag in spent catalyst was more oxidized due to a factor which can affect oxidation state change. It may be thermal factor since fresh catalyst was calcined at 500℃. After soot oxidation, spent catalyst experienced high temperature to 800℃. As verified in soot oxidation results and characterizations of Ag/TiO2 and Ag/CeO2 catalysts, active oxygen generation ability increased with increase of Ag oxidation state. Consequently, it can be concluded that the change of Ag state affected the decrease of T20 and T50 in recycling test. With the activity improvement, thermal stability of CeO2-Ag/P25 prevented deactivation of the catalyst. The durability of CeO2-Ag/P25 resulted from resistance on sintering due to Ag-CeO2 interfacial effect. It was reported that Ag located on interface of CeO2 has large adhesion energy (Farmer and Campbell, 2010; Campbell and Mao, 2017). This property led to resistance on sintering of Ag on CeO2. Similarly, Ag/Co@Ce catalyst
Acknowledgements
3.2.4. Soot oxidation activity of CeO2-Ag/P25 under practical conditions Fig. 14 shows T20 and T50 of CeO2-Ag/P25 in DPF and GPF conditions. Despite low concentration of oxygen (1%), CeO2-Ag/P25 converted 20% of soot at 422℃ under GPF condition (185℃ lower than the case without catalyst) due to high active oxygen generation ability. Additionally, CeO2-Ag/P25 converted 20% of soot at 382℃ in DPF condition with NOx assisted soot oxidation. Totally, CeO2-Ag/P25 catalyst showed highly promoted soot oxidation activity in practical atmosphere. 4. Conclusions Silver was impregnated on TiO2 (anatase, P25, and rutile) and CeO2. The supports influenced the oxidation abilities of silver and Ag/P25 and Ag/rutile exhibited more enhancement of Ag functionality than cases of Ag/anatase and Ag/CeO2. Ag/P25 exhibited T50 at 327℃ (the same as Ag/CeO2) in tight contact. However, considerable amounts of silver cannot contact soot in loose contact (real condition), which resulted in the poor activity of Ag/P25, with T50 at 567℃. To solve the lack of oxygen delivery ability, we newly introduced CeO2 on Ag/P25. This modified catalyst had two enhanced abilities: (1) Active oxygen supply by silver on P25 and (2) satisfying delivery of the active oxygen through CeO2. Consequently, the CeO2-Ag/P25 catalyst exhibited the highest activity in loose contact even at low temperature, with T20 = 468℃ and T50 = 514℃, and the temperature was 30℃ lower than that by Ag/ CeO2 which is known as the best catalyst in soot oxidation. The thermal durability was corroborated by the recycling test, and T20 and T50 were not increased compared with fresh catalyst. Declaration of Competing Interest None.
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