Bifunctional roles of Nd2O3 on improving the redox property of CeO2–ZrO2–Al2O3 materials

Materials Chemistry and Physics 240 (2020) 122150

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Bifunctional roles of Nd2O3 on improving the redox property of CeO2–ZrO2–Al2O3 materials Shanshan Li a, Jishuang He b, Yi Dan a, **, Xia Li a, Yi Jiao b, Jie Deng b, Jianli Wang b, c, d, *, Yaoqiang Chen b, c, d, Long Jiang a a

State Key Laboratory of Polymer Materials Engineering of China (Sichuan University), Polymer Research Institute of Sichuan University, Chengdu, 610065, China Key Laboratory of Green Chemistry & Technology of Ministry of Education of China, College of Chemistry of Sichuan University, Chengdu, 610064, China Center of Engineering of Vehicular Exhaust Gases Abatement of Sichuan Province, Chengdu, 610064, China d Center of Engineering of Environmental Catalytic Material of Sichuan Province, Chengdu, 610064, China b c

H I G H L I G H T S

� Nd2O3 incorporation dramatically improved the reducibility of CZA material. � 7 wt% Nd2O3 modified CZA exhibits the best redox property. � Nd2O3 regulated Zr distribution and promoted to form homogeneous CZN solid solution. A R T I C L E I N F O

A B S T R A C T

Keywords: CeO2–ZrO2–Al2O3–Nd2O3 Redox property Bifunctional roles Zr-species distribution

A series of Nd2O3 was introduced into CeO2–ZrO2–Al2O3(CZA) materials to improve their redox property, and then their soot oxidation activities were characterized. Although Nd2O3 incorporation obviously decreased the specific surface area, 7 wt% Nd2O3 modified CZA (fresh CZAN7) also exhibited the best activity, the temperature for 50% soot conversion (T50) over which is 25 � C lower than that over fresh CZA due to the more surface oxygen vacancies and the faster mobility of oxygen species. It indicates that redox property of CZA plays a critical role than specific surface area in determining the soot oxidation activity. Meanwhile, it is interesting that 31 � C lower of T50 is detected over the aged CZAN7 (1000 � C/4h) than its fresh counterpart (600 � C/3h). Combined with the results of Powder X-ray diffraction (XRD), High-resolution transmission electron microscope (HRTEM) and Raman, a new sight about the bifunctional roles of Nd2O3 on improving the redox property of CZA has been put forward. Nd2O3 could directly incorporate into CeO2–ZrO2 lattice to form CeO2–ZrO2-Nd2O3 solid solution. Meanwhile, it could also regulate the distribution of Zr species and promote them incorporate into CeO2–ZrO2 lattice during thermal treatment. Then CZAN7 could exhibit more surface oxygen species, increased surface oxygen vacancies and better soot oxidation activity even being thermally aged.

1. Introduction CeO2–ZrO2 (CZ) materials have been widely used in oxidation re­ actions due to their advanced redox property since the formation of active oxygen species is strongly associated with the redox couple of Ce4þ ↔ Ce3þ [1,2]. However, a serious drawback of CZ is the poor thermal stability due to the phase separation and sintering of nano­ particles at high temperatures, which would significantly deactivate

their redox property [3]. Stimulated by “diffusion barrier” effects, Al2O3 was introduced into CZ and the resulting CeO2–ZrO2–Al2O3(CZA) has drawn widespread attentions [4,5]. By combining the individual ad­ vantages of CZ and Al2O3, CZA has not only larger surface area, but also enhanced thermal stability. However, some literatures have also reported that Al2O3 addition would prevent Zr4þ to incorporate into CeO2 lattice due to the strong interaction between ZrO2 and Al2O3. Thus, the zirconium concentration

* Corresponding author. Key Laboratory of Green Chemistry & Technology of Ministry of Education of China, College of Chemistry of Sichuan University, Chengdu, 610064, China. ** Corresponding author. E-mail addresses: [email protected] (Y. Dan), [email protected] (J. Wang). https://doi.org/10.1016/j.matchemphys.2019.122150 Received 9 July 2019; Received in revised form 7 September 2019; Accepted 9 September 2019 Available online 10 September 2019 0254-0584/© 2019 Elsevier B.V. All rights reserved.

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in CZ solid solution was found to be smaller than feed after Al2O3 addition, which leads to the deteriorated redox property of CZA compared to pure CZ [6–8]. As generally accepted, Al2O3 is stable and scarcely reduced in ordinary reductive condition. Consequently, the distribution of Zr-species in CZA system has a decisive effect in its redox property. And the homogeneous distribution of Zr-species in CeO2 lat­ tice could remarkably promote the redox property of CZ due to the formation of oxygen defects caused by lattice distortion and charge compensating [9–11]. To promote more Zr-species incorporate into CeO2 lattice, some modified methods have been put forward. By using proper metal pre­ cursors [12], optimizing precipitation mode [13], using different pre­ cipitants [14], CZA with homogeneous CZ structure and thus higher oxygen storage capacity could be achieved. Besides, selectively incorporating promoters is also an effective method. Lan et al. reported the positive effect of BaO in improving the redox property of CZA by regulating Zr species distribution [15]. They declared that the selective stabilization effect of BaO to Al2O3 would weaken the interaction between ZrO2 and Al2O3, thus indirectly pro­ mote more Zr-species incorporation into CeO2 lattice [15]. However, this improvement is limited by CZ composition since BaO–Al2O3 con­ tributes little to the redox property. Then it is curious if there exist other promoters which could display a “bifunctional” effect? On one hand, the promoter could improve the redox property of CZA by inserting into CeO2 lattice directly. On the other band, it could also regulate the dis­ tribution of Zr-species in CZA system, improve the solid solubility of Ce and Zr in CZ lattice, and thus indirectly enhance the redox property of CZA. Nd2O3, recognized as an effective promoter by extensive literatures, could directly improve the redox property of CZ by forming homoge­ neous CeO2–ZrO2-Nd2O3(CZN) solid solution [16–18]. However, there are little literatures about Nd2O3–Al2O3 system. Considering that, it could be inferred that Nd2O3 may selectively incorporate into CeO2 lattice and directly improve the redox property of CZA by crystal distortion and electrostatic neutrality principle. Besides, as reported, the incorporated Nd2O3 could also play a critical role in inhibiting ZrO2-­ phase segregation form the lattice of Ce-rich CZ solid solution, thus keeping homogeneous cubic structures [16,19]. Meanwhile, as Akira Morikawa et al. claimed, there exists a strong interaction between Nd2O3 and ZrO2 in Al2O3-containing system [20]. Then we wondered that if Nd2O3 could behave this “bifunctional” effects simultaneously? As we known, there are only few reports concerning the CeO2–ZrO2 –Al2O3–Nd2O3(CZAN) system. In this work, Nd2O3 will be introduced into the CZA system to study its impact on the redox property. Then, the soot oxidation reaction was chosen as probe reaction to further inves­ tigate the effects of Nd2O3 on its redox property.

CZA-f was further treated at 1000 � C for 4 h to get the aged materials, which were referred as CZA-a. CeO2–ZrO2–Al2O3–Nd2O3(CZAN) mate­ rials were prepared by similar process except that excess amount of Nd2O3(3 wt%, 5 wt%, 7 wt%, 9 wt%) was incorporated by mixing the Nd (NO3)3 (AR) with salt solution. The amount of Nd2O3 is based on the quality of CZA and the identification of Nd2O3 was explained in Fig. S1). The resulting fresh and aged materials were also donated as CZANx-f (x ¼ 3, 5, 7, 9) and CZANx-a (x ¼ 3, 5, 7, 9), respectively. 2.2. Soot oxidation activity measurements Printex-U (Degussa) with particle diameter of 25 nm was used as soot model. Materials and soot particles were mixed at a mass ratio of 20:1 with “tight contact”. Activities for soot oxidation over these materials were accessed in a self-assembled reaction apparatus equipped with temperature programmed oxidation (TPO) device. The soot-material mixtures were heated from room temperature to 650 � C in a flow of 10% O2/N2 with 10 � C/min. The outlet emissions form soot-TPO were transformed into CH4 by methanator and then detected by GC9790II chromatograph (Fuli Co. Ltd. Zhejiang, China) equipped with FID detector. 2.3. Characterization techniques N2 isothermal profiles of materials were carried out on Quantach­ rome automated surface area & pore size analyzer (Autosorb SI, Quan­ tachrome Institute, USA). Firstly, the samples were degassed at 300 � C for 5 h to clean the surface. Then the materials were cooled to 196 � C to measure the profiles. H2-Temperature-programmed reduction (H2-TPR) was performed using a gas chromatograph with a thermal conductivity detector (TP6076, Xianquan Co. Ltd. Tianjin, China). Sample (100 mg) was pre­ treated in He flow at 450 � C for 30 min to clean the surface. Then, the samples were cooled down to room temperature. After that, the reduc­ tion was carried out in a mixture of 5% H2/N2 with 10 � C/min, and the resulting profiles were donated as “1st”. In addition, to fully study the reducibility of these materials, the cycled H2-TPR was conducted on the same apparatus. Firstly, the sample (100 mg) was pre-reduced in 5% H2/ N2 flow at 550 � C for 30 min, then cooled down to room temperature. Next, the second H2-TPR profile (donated as “2nd”) was recorded in this flow from room temperature to 900 � C with 10 � C/min. It should be noted that a choice of the pre-reduction temperature of 550 � C is to remove the surface and sub-surface oxygen species [21–23]. After that, the sample was cooled down to room temperature. Then the sample was oxidized in 5%O2/N2 flow from room temperature to 900 � C and cooled down to room temperature again. Finally, the third H2-TPR (donated as “3rd”) was characterized again. Oxygen storage capacity (OSC) was measured by pulse injection method on a self-assembled instrument. Firstly, the sample (100 mg) was reduced at H2 flow at 550 � C for 40 min. Then after being cooled to 200 � C, a given amount of O2 was pulsed until no O2 consumption could be detected. X-ray photoelectron spectroscopy (XPS) were characterized on XSAM-800 electron spectrometer (Kratos, Britain) equipped with Mg Kα (1253.6 eV) X-ray source under ultra-high vacuum. The measurements were operated at 15 kV and 10 mA. The binding energy of the elements was calibrated by C 1s binding energy at 284.6 eV. Raman spectra were recorded by Renishaw in Via reflex Raman spectrometer (HORIBA JOBIN YVON, France) with a 532 nm excitation laser. The collected frequency range was 100–1000 cm 1 with 1 cm 1 resolution. Powder X-ray diffraction (XRD) patterns were characterized on Rigaku Ultima IV diffractometer (Rigaku, Japan). The experiment was operated employing Cu-Kα radiation (λ ¼ 1.5406 Å) with a 0.02� step size scanning in the range of 10� < 2θ < 90� . High-resolution transmission electron microscope (HRTEM) images

2. Experimental section 2.1. Materials preparation Chemicals including Ce(NO3)3⋅6H2O (CP), Al(NO3)3⋅9H2O (CP) and Nd(NO3)3 (CP) were purchased from Chengdu Kelong Chemical Re­ agents Factory (Chengdu, China). ZrOCO3 (CP) was purchased from YiXing Xinxing Zirconium Company Limited (JiangSu, China). NH3�H2O (AR) and (NH4)2CO3 (AR) were purchased form Chengdu Lucheng Chemical Reagents Factory (Chengdu, China). The CeO2–ZrO2–Al2O3(CZA) material, in which the mass ratio of CeO2: ZrO2: Al2O3 ¼ 24%: 36%: 40% was prepared by co-precipitation method. Firstly, a mixed salt solution was prepared by dissolving Ce (NO3)3⋅6H2O (CP), ZrOCO3 (CP) and Al(NO3)3⋅9H2O (CP). Next the aqueous solution was precipitated with a buffer solution (NH3⋅H2O (AR)/(NH4)2CO3(AR) equimolar ratio) under vigorous stirring. Then, the precipitate obtained was treated at 90 � C for 6 h, filtered and dried. Afterwards, the dry precipitation powders were calcined at 600 � C for 3 h to get the fresh materials, which were donated as CZA-f. Then the 2

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were carried out using a FEI Titan the Mis 200 transmission electron microscope (FEI, USA) equipped with Bruker super-X energy-dispersive spectrometer (EDS).

role since the soot oxidation is a typical gas-solid heterogeneous reaction [2]. In order to investigate the contact efficiency between soot and material, the textural property was determined by N2 adsorp­ tion–desorption. In Fig. 3(a), CZA-f and CZAN7-f exhibit similar IV-type isotherms (insert) (IUPAC definition) and H2-tye hysteresis. While, as presented in Fig. 3, Table 1 and Table S1, the volume of pore size larger than 25 nm (diameter of soot particle) in both CZA-f and CZAN7-f could be ignored (0.01 mL/g). Then it is difficult for the soot particles trans­ ferring into the inner pores of fresh material [24,25]. Then it is reasonable to infer that the specific surface area plays a minor effect in determining the soot oxidation. However, obvious differences in pore size distribution are observed after being thermally aged. According to IUPAC definition, the hysteresis loops in CZA-a could be kept as H2 type, while the hysteresis loop shape for CZAN7-a has transferred into H3 and H4 type, indicating its larger pores than CZA-a [26]. Therefore, the wider pore distribution in Fig. 3(c) and larger volume of pore size in the range of 25–80 nm (0.18 mL/g) (Table 1) with respect to CZA-a (0.01 mL/g) have also been detected. That is to say, increased soot-material interface may form since the soot particles could transfer into the inner pores of CZAN7-a, which may be one of the reasons for its enhanced activity. Besides, there are still some confusing issues: (i) why CZA-a exhibits an advanced soot oxidation activity than CZA-f in the case that no proof of increased soot-material interface has been ach­ ieved? (ii) a remarkable reconstruction of pore structure (shape and size) in CZAN7-a has occurred during the thermal process since 40% percent of large pores (>25 nm) has formed. And the reconstruction actually indicates the poor thermal stability of pore structures for CZAN7.

3. Results and analysis 3.1. The effects of Nd2O3 on the soot oxidation activity The TPO experiments for soot oxidation over CZA were carried out to investigate the effects of Nd2O3. As exhibited in Fig. 1(a), Nd2O3 incorporation improves the soot oxidation activity of CZA materials since all the Nd2O3-modified CZA show lower T50 (temperature for 50% soot conversion) and T90 (temperature for 90% soot conversion) values than pure CZA. The lowest T50 value is observed in CZAN7-f, which was decreased by 25 � C compared with that detected in CZA-f. That is to say, the addition of Nd2O3 could improve the soot oxidation activity of CZA and the optimum content is 7 wt%. After being thermally aged, similar trend has also been observed in Fig. 1(b) that CZAN7-a exhibits the best soot oxidation activity. And the T50 and T90 values for soot conversion over CZAN7-a have decreased by 22 � C and 35 � C than those over CZA-a, respectively. In addition, as shown in Fig. 2(a), the T50 over aged ma­ terials has decreased by 34 � C and 31 � C than their fresh counterparts for CZA and CZAN7, respectively. It seems contradictory with the most reported that a better soot oxidation activity is observed in the aged CZA materials than their corresponding fresh ones. And it seems worthy to discuss the phenomen in consideration of the practical use of these CZAN materials. As reported, the activity of soot oxidation is closely associated with the textural and redox property of CZ-based materials, which usually determins the contact efficiency bwtween soot and material and the formation of active oxygen species, respectively [2,24]. However, as widely accepted, obvious decrease of specific surface area combinging with less soot-material interface could be expected due to the thermal sintering when comparing with fresh and aged materials. Then it could be inferred that a structure modification of aged CZA materials may have occurred during thermal treatment, which may be beneficial to generate highly active oxygen species for soot oxidation.

3.3. The effects of Nd2O3 on the reducibility and oxygen storage capacity Except the contact interface between soot and material, the reduc­ ibility at low temperature is also very important for easy access of lattice oxygen species to the soot surface and their participation in catalytic soot oxidation activity [1,2]. Then the H2-TPR was characterized to suggest the involvement of the reducible species in soot oxidation. Clearly, a broad and asymmetric reduction peak in CZA-f (Fig. 4(a)) indicates the existence of different reducible oxygen species [27]. Generally, the reduction of homogeneous CZ solid solution usually oc­ curs concurrently on the surface and in the bulk [16]. However, the homogeneous structure was disturbed by the addition of Al2O3, thus a

3.2. The effects of Nd2O3 on the textural property The contact interface between soot and materials plays an important

Fig. 1. The soot oxidation activity of (a) fresh and (b) aged CZA materials. 3

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Fig. 2. The comparisons between (a) T50 and (b) T90 for soot oxidation over CZA and CZAN7.

Fig. 3. BJH pore size distribution and the corresponding cumulative pore volume of (a, b) fresh and (c, d) aged materials (The insert in the (a) and (c) is the isothermal profiles).

new heterogeneous distribution of Ce-rich nanoparticles would form due to the strong interaction between ZrO2 and Al2O3 [8]. Consequently, the reduction peaks above 800 � C in fresh CZA should be referred to the reduction of Ce-rich nanoparticles, while the peaks below 800 � C are

ascribed to the reduction of surface and subsurface oxygen of CZ solid solution [28]. Interestingly, as presented in Fig. 4(b), the high temperature reduction peak ascribed to Ce-rich nanoparticles disappears, indicating 4

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intense profiles are observed in CZA-a-3rd and CZAN7-a-3rd than others, which is consistent with the literatures that a higher-temperature reduction combining with a re-oxidation benefits the improvement of the reducibility of CZ-based materials [30,31]. As proposed by previous researches, the reduction capability of CZbased materials mainly relies on the surface area and the homogeneity of dopants distribution in CeO2 framework [32,33]. The larger surface area is beneficial to the adsorption of H2, which could be regarded as the first step of reduction. Meanwhile, there are more surface oxygen species on the CZ with larger surface area, thus promoting low-temperature reduction [21]. Besides, due to the resulting vacancies and defective structures, more labile oxygen is obtained when there is a truly ho­ mogenous distribution of dopants in the CeO2 lattice [21,34]. Consid­ ering the fact that CZAN7 exhibits smaller surface area than CZA (discussed in Section 3.2), it could be inferred that the incorporation of Nd2O3 may promote the formation of oxygen defects, which leads to faster mobility of lattice oxygen.

Table 1 Textural properties of the materials. Samples 2

1

Surface area (m ⋅g ) Total pore volume (mL⋅g 1)a Pore volume (mL⋅g 1)b Average pore diameter (nm) a b

CZA-f

CZAN7-f

CZA-a

CZAN7-a

190.1 0.70 0.01 14.8

176.3 0.72 0.01 16.4

99.3 0.40 0.01 17.4

79.4 0.45 0.18 22.8

Total pore volume obtained from N2 desorption branch. The pore volume of the pores with diameter above 25 nm.

a more homogenous CZ solid solution formed in the drive of temperature [8]. As a result, the OSC values of CZANx-a are larger than that of CZANx-f. Meanwhile, by adding Nd2O3, the maximum reduction tem­ perature of CZA decreases from 634 � C (CZA-a) to 588 � C (CZAN7-a), while a slight increase is observed for CZAN9-a (609 � C). In addition, as displayed in Fig. 5, CZAN7-a exhibits the highest OSC value of 247.68 μmol/g, which increases by 54.82 μmol/g compared with CZA-a. Namely, the optimum content of Nd2O3 is 7 wt% in terms of improving the reducibility of CZA. However, introduction of excessive amount of Nd2O3 declines the redox property as displayed in Figs. 4(b) and Fig. 5. That is to say, there exists a great accordance between redox property of CZA and the soot oxidation activity (Fig. 1). While, as easily be found in Fig. 1, the T90 of the soot oxidation over all materials are below 550 � C, which manifests the important role of upper surface oxygen species in soot combustion. Then to fully investi­ gate these surface oxygen species (reduction below 550 � C) and the mobility of lattice oxygen in CZA-a and CZAN7-a, cycled H2-TPR profiles were recorded. In Fig. 6 and Fig. 7, CZAN7-a-1st exhibits more intense reduction peak associating with surface oxygen species than CZA-a-1st, which is consistent with the better soot oxidation activity of CZAN7-a presented in Fig. 1. Besides, since the pre-reduction before the second H2-TPR is to remove the surface and sub-surface oxygen species, a decreased intensity of the integrated peak area with surface oxygen species in CZA-a-2nd with respect to that in CZA-a-1st is detected (Figs. 6(a) and 7). Surprisingly, the difference of the integrated peak area with surface oxygen species between CZAN7-a-1st and CZAN7-a2nd is limited. It indicates that the incorporation of Nd2O3 results in an increase in the oxygen mobility thus the spillover of bulk oxygen species could recover the surface oxygen species since the testing system was in anaerobic condition [29]. Besides, as shown in Figs. 6 and 7, more

Fig. 5. The OSC results of the materials.

Fig. 4. The 1st-H2-TPR profiles of (a) fresh and (b) aged materials. 5

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Fig. 6. The cycled H2-TPR profiles of (a) CZA-a and (b) CZAN7-a.

3.4.2. Raman results Raman spectroscopy was performed to investigate defect structures. In Fig. 9(a), CZA-f and CZAN7-f display similar peaks centered at ca. 463 cm 1 and 613 cm 1. The former corresponds to F2g vibration mode of cubic fluorite-type structure (Fm3m space group), and the latter is attributed to the defective structure in ceria lattices [41,42]. Generally, the concentration of oxygen vacancies in CZ solution solid could be evaluated by the ratio of ID/IF2g, where ID and IF2g are the maximum intensity of the defect band and the band of fluorite-type structure, respectively. Larger value of the ratio indicates higher concentration of oxygen defects sites [29,43,44]. Obviously, larger ID/IF2g values are observed in CZAN7-f (0.2021) and CZAN7-a (0.3448) than those in CZA-f (0.1320) and CZA-a (0.2524), respectively (Table 2). It indicates that Nd2O3 addition is beneficial to increase the oxygen defects in ceria lattices, and thus make more surface-active oxygen species and surface oxygen vacancies as manifested by XPS (Section 3.4.1). Meanwhile, it should also be noted that increased oxygen defects (increased ID/IF2g values) are observed for both aged CZA and CZAN7 with respect to their fresh ones, which is consistent with the sequence of Oβ/O, Ce3þ/Ce and the soot oxidation activity. It indicates that an atom rearrangement had occurred during thermal treatment. Consistently, the main Raman peak has shifted from ca. 460 cm 1 for fresh materials to 473 cm 1 for aged materials (Fig. 9(b)). Besides, peaks at 131, 263 and 305 cm 1 referring to tetragonal CZ are observed in CZA-a profile (Fig. 9(b)), demonstrating the phase inhomogeneity of CZ component in CZA-a. For CZA system, Zr-rich CZ solid solution could form due to the thermal sintering of the zirconia species which initially contact with Al2O3 after high-temperature treatment [6]. Combining with the textural results (Table 1), it is reasonable to infer that sharper tetragonal-CZ Raman peaks in CZAN7-a would be detected due to the smaller surface area of CZAN7-a to disperse these isolated Zr species. While surprisingly, CZAN7-a exhibits much weaker peaks at 131, 263 and 305 cm 1 (Fig. 9(b)), which may infer that Nd2O3 addition has regulated the distribution of Zr species.

Fig. 7. The peak areas of the reduction profiles of CZA-a and CZAN7-a.

3.4. The effects of Nd2O3 on the defective structure 3.4.1. XPS results For further investigating the surface species and surface oxygen va­ cancies, XPS characterization was performed. The Ce 3d and O 1s spectra are presented in Fig. 8, and the surface elemental composition of the materials are summarized in Table 2. As presented in Table 2, a higher Oβ/O ratio is observed for both CZAN7-f and CZAN7-a when compared to CZA-f and CZA-a, respectively. The finding further con­ firms the better low-temperature reducibility of CZAN7 detected in H2TPR due to the faster mobility of these chemisorbed oxygen species (Oβ) [35,36]. And the higher mobility of Oβ is also beneficial for the spillover phenomena at the soot surface. Meanwhile, a higher fraction of Ce3þ is also been detected in CZAN when compared to CZA (Table S2). It demonstrates that the incorporation of Nd2O3 makes part of Ce4þ transferred into Ce3þ and thus facilitates the formation of surface oxygen vacancies, which is essential for adsorption/dissociation of oxygen molecules during the reaction [37–39]. More importantly, increases in Ce/Zr ratios of both CZA-a and CZAN7-a when compared to their fresh counterpart are observed. And CZAN7-a even exhibits a higher value of 0.52, which is more closer to the theoretical value of 0.58, implying a higher elemental homogeneities in CZAN7-a [40].

4. Discussion 4.1. Relationships between soot oxidation activity and the redox property of CZA As generally accepted [25,45], soot oxidation activity is closely associated with the contact between soot and material, and the intrinsic activity of materials. Generally, the contact-interface is controlled by pore structures of material. It has been demonstrated that soot oxidation 6

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Fig. 8. The XPS profiles of (a)Ce 3d and (b)O 1s. Table 2 The defective structures derived from XPS and Raman. Samples CZA-f CZAN7-f CZA-a CZAN7-a a

Relative amount (%) Ce

Zr

Al

O

1.43 1.64 2.38 2.67

6.57 6.24 5.29 5.12

21.75 21.13 23.53 24.34

70.25 70.99 68.80 67.87

Ce3þ/Ce (%)

Oβ/O (%)

Ce/Zr

ID/IF2ga

11.65 18.62 15.37 21.13

35.40 40.82 35.25 41.32

0.22 0.26 0.45 0.52

0.1320 0.2021 0.2524 0.3448

It was calculated based on I613/I463.

Fig. 9. The Raman profiles of (a) fresh and (b) aged CZA and CZAN7 materials.

over ceria-based materials proceeds through a Mars-Van Krevelen mechanism, that is, oxygen species in the surface layer of ceria is transferred onto soot, and gaseous O2 fills up the vacancies created on the ceria in a subsequent step [46,47]. Then it could be inferred that the intrinsic activity of the materials is closely related with its low-temperature redox property and surface oxygen vacancies.

As analyzed in section 3.2, the pore diameters of CZA-f and CZAN7-f are smaller than the diameter of soot particle (25 nm), which inhibited the entrance of soot particle. Then the results of soot oxidation activity and textural property seem to be contradictory. However, as described in Section 3.3-3.4, the trend of OSC, Ce3þ and Oβ fraction follows the order of CZAN7 > CZA, which is consistent with the soot oxidation 7

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activity. Then it could be concluded that the redox property, the surface oxygen species and surface oxygen vacancies over the materials play a critical role than specific surface area in determining the soot oxidation activity. Meanwhile, the increased bulk oxygen vacancies manifested by the higher ID/IF2g value has formed in the drive of temperature, which benefits the formation of more surface-active species (including Ce3þ and Oβ). As a result, the aged materials exhibit a better soot oxidation with respect to corresponding fresh ones (Figs. 1 and 2). Moreover, an enlarged soot-material contact in CZAN7-a due to the re-construction of its pore structures also promoted to improve its soot oxidation activity. That is to say, the best soot oxidation activity of CZAN7-a is associated with the synergistic reaction of pore structure and reducibility.

are 0.97 Å, 0.995 Å and 0.84 Å, respectively, it could be speculated that (i) Nd2O3 may selectively incorporate into CZ framework to improve the redox property due to the larger ionic radius of Nd3þ than that of Zr4þ and Ce4þ; and (ii) Nd2O3 may combine with Al2O3 closely and mean­ while inhibit the entrance of Zr into CeO2 lattice, which would play a negative role in the redox property. Then combined with the higher OSC value and better reducibility of CZANx (Figs. 4–5), it could be inferred that Nd2O3 has selectively incorporated into CeO2 lattice to form CeO2–ZrO2-Nd2O3(CZN), thus enhanced redox property is detected due to more oxygen defects caused by the crystal distortion and electrostatic neutrality principle. After being thermally aged, CZ diffraction peak (Fig. 11) in CZA-a remarkably shifts to higher angles, which refers to the shrinkage of CZ lattice (Fig. 11(b)). Consistent with Raman results (Fig. 9(b)), it could be inferred that Zr atoms rearrangement has occurred during thermal treatment due to the smaller ionic radius of Zr4þ than Ce4þ, therefor, an increased Ce/Zr ratio has also been detected in XPS characterized (Section 3.4.1). Interestingly, the main CZ peaks for CZA-a and CZAN7-a locate at almost the same position. As the lower 2θ value of CZ in CZAN7-f (29.06� ) than that of CZA-f (29.24� ) (Fig. 10(b)), it may indi­ cate that (i) there are more Zr ions entered into the framework of CeO2 in the drive of temperature, (ii) Nd3þ has isolated from the CeO2 frame­ work, leading to cell shrinkage. In addition, it’s also worth noting that the peaks ascribed to Al2O3 in CZA-a have also been detected due to the thermal sintering. As presented in Fig. 11(c), an obvious cell shrinkage of Al2O3 is observed since the 2θ value for the peak (442) of Al2O3 increases from 67.04 of CZA-a to 67.30 for CZAN7-a, which is close to the theoretical value of pure Al2O3(67.31) (PDF 04–1770). Besides, as presented in Table 3, much bigger Al2O3 nanoparticles are observed in CZAN7-a (7.9 nm) than in CZA-a (5.9 nm) although the CZ crystals size in CZA and CZAN7 are almost the same. It manifests that there may also exist an atom migration associated with Al2O3 component after Nd2O3 incorporation. Considering the little amount of Nd2O3, we speculate that the shrinkage of Al2O3 cell may be caused by Ce and/or Zr arrangement. Although both Ce and Zr could work as the stabilizer of Al2O3, the interaction between ZrO2 and Al2O3 is rather stronger [50,51]. Consequently, the 2θ shift of Al2O3 may indicate the isolation of Zr4þ from its lattice due to the larger ionic radius of Zr4þ (0.84 nm) than that of Al3þ(0.57 nm). Consequently, smaller surface area and the larger Al2O3 crystal size are observed in

4.2. “Bi-functional”roles of Nd2O3 on improving the redox property of CZA Then it is confusing that why the aged materials exhibited an enhanced redox property than their fresh counterparts and why 7 wt% Nd2O3 incorporation could significantly improve the redox property of CZA. To elaborate these issues, XRD, TEM-Mapping and TEM-EDS were characterized, and the results are as follows. 4.2.1. XRD results In Fig. 10 (a), fresh materials exhibit similar characteristic of cubic CZ. No obvious peaks ascribed to γ-Al2O3 are observed in CZA-f. As claimed, Al2O3 could not be incorporated into CZ framework to form CZA solid solution. It can be deduced that Al2O3 is well dispersed or the grain size is too small to be detected in CZA-f [4,8,33]. However, it is interesting that obvious γ-Al2O3 peak in CZAN7-f is detected, which indicates that Nd2O3 addition promotes the growth of Al2O3 nano­ particles. Meanwhile, as reported, the introduction of Al2O3 would prevent Zr from incorporating into CeO2 lattice [8]. Accordingly, a lower 2θ value for the (111) crystal plane of cubic CZ in CZA-f (29.24� ) than pure CZ (29.37� ) (CeZrO4, PDF 54–0017) is observed [48,49]. From the amplifying patterns of Fig. 10(b), Table 3 and Table S3, it is observed that the peak ascribed to (111) crystal plane of cubic CZ shifts to lower angle with Nd2O3 addition from 29.24� (CZA-f) to 29.06� (CZAN7-f). Namely, an expansion of CeO2 lattice in CZAN7-f occurs according to Braggs law. Based on the observation that there are no Nd phases, combining with the fact that ionic radii of Ce4þ, Nd3þ and Zr4þ

Fig. 10. The XRD profiles of (a) CZA-f and CZAN7-f materials and (b) the amplifying patterns with narrower ranges of 2θ ¼ 26.5–31.5� . 8

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Materials Chemistry and Physics 240 (2020) 122150

Table 3 The structural property of CZA and CZAN7 materials. Sample CZA CZAN7 Ce(ZrO4) (PDF 73–0229) Al2O3(PDF 04–1770) a b

2θ(o)a

2θ(o)b

Crystal size of CZ (nm)

Crystal size of Al2O3 (nm)

Fresh

Aged

Fresh

Aged

Fresh

Aged

Fresh

Aged

29.24 29.06 29.37 –

29.30 29.30

– 66.70 – 67.31

67.06 67.30

4.2 4.0

6.8 7.1

– –

5.9 7.9

The 2θ refers to the (111) crystal plane of Ce(ZrO4). The 2θ refers to the (442) crystal plane of Al2O3.

Fig. 11. The XRD profiles of (a) CZA-a and CZAN7-a and the amplifying patterns with narrower ranges of (b) 2θ ¼ 27.5–31� and (c) 2θ ¼ 65–70� .

CZ with larger pore size and wider pore distribution could be achieved by enlarging the primary particles [3,35]. Then Fig. 11 illustrates the relationships among the specific surface area, the average pore radius, the nanoparticles’ crystal size of CZA materials modified by different Nd2O3 content. It is surprising that CZAN7-a exhibits the largest Al2O3 nanoparticles combing with the largest pore radius. Then considering the almost identical crystal size of CZ (Table 3), it could be inferred that the larger crystal size of Al2O3 caused by severe thermal sintering con­ tributes to the wider and larger pore structure for CZAN7. 4.2.2. TEM-EDS results TEM was carried out to investigate the microscopic morphology of the materials. Obviously, Fig. 13 displays a looser distribution of nanoparticles in CZAN7 due to larger pore radius (Fig. 3). In addition, special attention should be paid that nanoparticles with rod-shape, which refers to Al2O3, could only be detected in CZAN7-f (Fig. 13). It is consistent with XRD results that only CZAN7-f exhibits the charac­ teristic peak of γ-Al2O3 (Fig. 13). Most importantly, these rod-like Al2O3 nanoparticles have thermally sintered severely in CZAN7-a as shown in Fig. 13(c–d), as a result, larger crystal size of Al2O3 is characterized in Table 3 by XRD. In addition, HAADF-TEM mapping was also carried out to investigate the elemental distribution of Ce, Zr, Al and Nd. In Fig. 14(a), Zr and Al species are distributed in the region circled as “1” and “2”, where Ce species are absent. On the one hand, it refers that Ce and Zr species are uneven distributed in CZA-f. On the other hand, it manifests that Zr and Al species are interacted closely in CZA-f. After being thermally aged, there still exists the region donated as “1” and “2” in Fig. 14(b), where Ce

Fig. 12. The specific surface area, the average pore radius, the nanoparticles’ crystal size of CZA materials as a function of Nd2O3 content.

CZAN7-a than in CZA-a (as shown in Tables 1 and 3, respectively). Considering the better redox property (Figss. 4–5), it is reasonable that the introduction of Nd2O3 promotes the incorporation of Zr4þ into CeO2 lattice, and improves the reducibility of CZAN7-a materials by forming homogenous CZN solid solution and thus more defects structures. As discussed in section 3.2, Nd2O3 incorporation reduces the specific surface area but enlarges the pore size of CZA after being thermally aged (Fig. 3). As early as in 1997, Chen et al. has put forward that the for­ mation of nanopores is associated with the interstitial space among the nanoparticles [52]. Afterwards, literatures have also reported that the 9

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Materials Chemistry and Physics 240 (2020) 122150

Fig. 13. The TEM profiles of (a) CZA-f, (b) CZAN7-f, (c) CZA-a and (d) CZAN7-a.

better redox property of CZAN7-a could be characterized. TEM-EDS analysis was also performed to explore the micro-scale composition of CZA and CZAN7. Considering the fact that CZA mate­ rials are prepared by co-precipitation method, and the TEM information on the depth direction was inevitably extracted, it is difficult to completely discriminate the CZ and Al2O3 nano-particles. However, based on the results by Morikawa et al. the particle could be identified as Al2O3 if Al fraction of the analysis spot is higher than the average value, otherwise, the spot is regarded as CZ particle when the diameters of

and Zr species are heterogeneous distributed. In Fig. 15(a), the regions remarked as “1” and “2” are also observed with the heterogeneous distributed Ce and Zr in CZAN7-f. While it should also be noted that Al species in CZAN7-f are observed in the circled region “3”, where the Nd species are absent. It manifests the XRD results that Nd selectively incorporated into CeO2 lattice when being treated at 600 � C. More importantly, in Fig. 15(b), Ce, Zr and Nd are homogeneous distributed in the similar shape, indicating the formation of homogeneous CZN solid solution after being thermally treated. Then a

Fig. 14. The HAADF-TEM elemental mapping images of (a) CZA-f and (b) CZA-a (Elements distributions are uneven in the circle regions). 10

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Materials Chemistry and Physics 240 (2020) 122150

Fig. 15. The HAADF-TEM elemental mapping images of (a) CZAN7-f and (b) CZAN7-a (Elements distributions are uneven in the circle regions).

analysis points were narrowed down to the smallest values [4]. As a result, the red spots in Fig. 16 are recognized as Al2O3, while those spots in blue are inferred as CZ. Obviously in Table S4, the content of Nd2O3 in CZ is higher than that of Al2O3. Being consistent with XRD results, it could be inferred that Nd2O3 inclined to interact with CZ. Besides, particular attentions should be paid to the Zr/Al ratios of the analysis spots recognized as Al2O3. As shown in Table S3, the Zr/Al ratios in CZAN7 are lower than that in CZA although the value fluctuates to a large extent. It indicates that less Zr species interact with Al2O3, weak­ ening the stabilization effect of ZrO2 to Al2O3, thus more obvious γ-Al2O3 characteristic peaks referring to larger crystal size are only detected in CZAN7-f (Fig. 12). Considering the less amount of tetragonal phase manifested by Raman results (Fig. 9), it could be inferred that

more Zr ions have incorporated into the CeO2 lattice, which also illus­ trate the peak shift of CZ in XRD results discussed in section 4.2.1. 5. Conclusions In this work, a series of Nd2O3 was introduced into CZA system to improve its redox property and soot oxidation activity. It was found that 7 wt% Nd2O3 addition could significantly improve the redox property of CZA material. Then the bifunctional roles of Nd2O3 on improving the redox property of CZA material have been put forward. Nd2O3 could directly insert into CeO2 lattice, and meanwhile, it could also regulate the distribution of Zr-species, promote more Zr ions to incorporate into CeO2 lattice in the drive of temperature to form more homogeneous

Fig. 16. The TEM-EDS profiles of (a) CZA-f, (b) CZAN7-f, (c) CZA-a and (d) CZAN7-a. 11

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Materials Chemistry and Physics 240 (2020) 122150

CeO2–ZrO2-Nd2O3 ternary solid solution. As a result, Nd2O3 incorpora­ tion could increase the surface oxygen species and oxygen vacancies over CZA materials, which then lead to an enhancement of their redox property and soot oxidation activity.

[21] [22]

Acknowledgments

[23]

We gratefully acknowledge the National Key Research and Devel­ opment Program of China (2016YFC0204901and 2016YFC0204903) for the generous financial support to our research.

[24]

Appendix A. Supplementary data

[25]

Supplementary data to this article can be found online at https://doi. org/10.1016/j.matchemphys.2019.122150.

[26]

References

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