Factors determining gasoline soot abatement over CeO2–ZrO2-MnOx catalysts under low oxygen concentration condition

Journal of the Energy Institute xxx (xxxx) xxx

Contents lists available at ScienceDirect

Journal of the Energy Institute journal homepage: http://www.journals.elsevier.com/journal-of-the-energyinstitute

Factors determining gasoline soot abatement over CeO2eZrO2-MnOx catalysts under low oxygen concentration condition Peng Yao a, Jishuang He b, Xue Jiang a, Yi Jiao a, *, Jianli Wang b, Yaoqiang Chen a, b, ** a b

Institute of New Energy and Low-Carbon Technology, Sichuan University, Chengdu 610064, Sichuan, China College of Chemistry, Sichuan University, Chengdu 610064, Sichuan, China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 18 March 2019 Received in revised form 8 May 2019 Accepted 13 May 2019 Available online xxx

Soot oxidation under a low concentration O2 (0.5% O2/N2) was investigated using CeO2eZrO2-MnOx mixed oxides with varied amounts of MnOx, in order to gain low temperature catalytic activity and find out the main factors affecting the soot oxidation. The catalytic activity was remarkably improved over these catalysts compared to that of non-catalyst in such a low concentration of O2. In particular, CeO2 eZrO2-MnOx with 10% MnOx doping (M10-CZ) showed the highest catalytic activity with its T50 values of 340
C under tight contact condition. The results of N2 adsorption-desorption and X-ray diffraction (XRD) indicated that the textural and structural properties were not positive correlation with soot oxidation, are not the main factors affecting the catalytic activity of CeO2eZrO2 and CeO2eZrO2-MnOx catalysts. The results of oxygen storage capacity (OSC), hydrogen-temperature programmed reduction (H2-TPR), O2 temperature program desorption (O2-TPD), UV Raman spectroscopy (UV Raman) and X-ray photoelectron spectroscopy (XPS) testified that redox ability, oxygen storage capacity, oxygen desorption capacity at low temperature and surface active oxygen species are more important for soot oxidation. The enhancements of the catalytic behavior after MnOx addition can be due to the improving of the adsorbed, activation and mobility of reactive oxygen species. In this work, these factors about generation and movement of reactive oxygen species are crucial for soot oxidation in a low oxygen concentration condition. © 2019 Energy Institute. Published by Elsevier Ltd. All rights reserved.

Keywords: CeO2eZrO2-MnOx mixed oxides Soot oxidation Surface oxygen species Oxygen storage capacity Redox properties

1. Introduction Soot particles emitted from gasoline direct injection (GDI) engines have drawn a growing concern in recent years for its serious harm on environment and human health [1]. Different from diesel soot particles, gasoline soot particles are more toxic for its smaller diameter and further quantity [1,2]. The amount of soot particles from GDI engines' exhaust can be efficiently reduced by gasoline particulate filter (GPF). Moreover, a catalyzed filter (cGPF) is used to realize continuously regeneration at low temperature and avoid uncontrollable exotherm, so as to prolong the service life of GPF system [1,3,4]. In the past decades, various catalysts have been investigated for soot combustion, such as precious metals [5e10], transition metals oxides [11e15] and rare earth metals oxides [16e21], which exhibited excellent catalytic performances for soot combustion. Precious metals are the most active catalysts, but the expensive prices limit its application to some extent. As one of the most reactive rare earth metals oxides, CeO2-based catalysts have outstanding oxygen storage and release capability through its facial cycle conversion between Ce3þ and Ce4þ oxidation states [22]. Additionally, relatively lower cost makes them more attractive, especially CeO2eZrO2 catalysts [23]. Many researchers found that the incorporation of Zr4þ ions into a ceria lattice could increase the oxygen storage capacity (OSC), oxygen mobility which is helpful for improving the catalytic performance and thermal stability [18,24e29]. Moreover, the combination of Ce4þ ion and Zr4þ ion favors the formation of structural defects which is conducive to soot oxidation [24e29]. In addition, transition metal oxide MnOx with the advantages of environment friendly, low cost and efficient low-temperature oxidation activity, was also introduced into CeO2-based

* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (Y. Jiao), [email protected] (Y. Chen). https://doi.org/10.1016/j.joei.2019.05.005 1743-9671/© 2019 Energy Institute. Published by Elsevier Ltd. All rights reserved.

Please cite this article as: P. Yao et al., Factors determining gasoline soot abatement over CeO2eZrO2-MnOx catalysts under low oxygen concentration condition, Journal of the Energy Institute, https://doi.org/10.1016/j.joei.2019.05.005

2

P. Yao et al. / Journal of the Energy Institute xxx (xxxx) xxx

catalysts for diesel soot oxidation in recent years [14,30e33]. Reddy et al. [12] found that the Mn doped ceria solid solutions showed superior catalytic activity (T50 ¼ ~665 K) and thermal stability towards soot oxidation compared to pure ceria. Fu et al. [30] found that MnOx(0.4)CeO2 catalyst presented more O-vacancies and better redox properties, and thus showed higher catalytic activities for soot oxidation. Wu et al. [31e33] showed that the incorporation of Mn into the ceria lattice could accelerate the mobility of active oxygen and promote the evolution of lattice oxygen favoring for soot oxidation. However, despite most of the pure CeO2 and MnOx, MnOx-CeO2 and CeO2eZrO2 catalysts have a relative high catalytic activity under diesel exhaust atmosphere, which contains almost 6e16% O2 and 400e1300 ppm NOx, there is a great difficult to remove the soot emitted from gasoline exhaust atmospheres. Gasoline exhaust atmosphere is very difference from that of diesel, which contains only a low amount of oxygen (0.2e0.6%) and nearly no NOx due to the upstream three-way catalysts (TWC) [11,21]. Therefore, the development of excellent catalysts suited for soot oxidation under gasoline exhaust atmospheres and research on the main factors affecting the catalytic activity is of great significance. In this study, we have synthesized a series of CeO2eZrO2-MnOx mixed oxides with different MnOx content by co-precipitation method, and investigated the influence of MnOx doping on the catalytic activity for soot oxidation, in order to obtain an excellent catalyst which can be used in an extremely low concentration of oxygen conditions. In addition, we have also analyzed these catalysts by some characterization techniques in order to find out the main factors affecting the catalytic activity for soot oxidation under the low oxygen concentration condition. 2. Experimental 2.1. Catalysts preparation CeO2eZrO2-MnOx mixed oxides with different amount of MnOx (0, 10, 15, 100 wt%) were prepared by co-precipitation method. Mn(NO3)2 aqueous solution (50 wt%), Ce(NO3)3$6H2O (95% pure) and ZrOCO3 (99.95% pure) were used as the precursors. These precursors were first dissolved in distilled water and then mixed with a buffer solution (pH  9.0) consisting of NH3$H2O and (NH4)2CO3 at 60
C to obtain the precipitates. After filtration, washing and drying overnight, then calcined at 600
C in air for 3 h, the final products were obtained. All of the CeO2eZrO2-MnOx mixed oxides with a Ce/Zr molar ratio of 40:60 and a weight ratio of MnOx: CeO2eZrO2 ¼ X (X ¼ 0:100, 10:90, 15:85, 100:0), denoting as CZ, M10-CZ, M15-CZ, M in turn. 2.2. Catalyst characterization N2 adsorption-desorption isotherms at 196
C were obtained by an automatic surface analyzer (Quantachrome Quadrasorb SI Instrument, USA). Prior to the measurements, the samples were degassed at 300
C under vacuum for 3 h. The specific surface area and the average pore radius of catalysts were determined by a multipoint BET equation and BJH methods, respectively. Structural features of the catalysts were carried out with X-ray diffraction (XRD) by a Rigaku DX-2500 diffractometer (Rigaku, Japan) with CuKa radiation (l ¼ 0.15406 nm). The X-ray tube was operated at 40 kV and 40 mA. The XRD patterns of samples were recorded in the range of 10
 2q  90
with a scanning step size of 0.03
. H2 temperature-programmed reduction (H2-TPR) measurements were carried out on a TP-5076 instrument (Xianquan Co. Ltd. Tianjin, China). Prior to the test, the samples were pretreated by N2 at 450
C for 1 h, then cooled down to room temperature, and followed by turning the flow of 5 vol% H2/N2 into the system with a flow rate of 25 mL/min. Finally, the samples were heated from room temperature to 900
C at a rate of 10
C/min. The consumption signals of H2 were monitored by a TCD detector. The oxygen storage capacity (OSC) of catalysts was measured by O2 pulse injection on a self-assembled reaction system equipped with a GC-9790 gas chromatograph (Fuli Co. Ltd. Zhejiang, China). Prior to the measurement, the sample was pretreated at 550
C for 1 h under a flow of high purity H2 (30 mL/min). After that, the sample was pulsed with quantitative amount of pure O2 at 200
C every 3 min to obtain the breakthrough curve, high purity helium (He) was used as carrier gas during the O2 pulses. O2 temperature program desorption (O2-TPD) was measured by using a self-assembled reaction system. The samples (100 mg) were pretreated at 450
C for 1 h with a flow of He (25 mL/min), then cool down to 60
C naturally, and then subsequently treated by 5% O2/N2 mixture gas for 1 h. Finally, the samples were heated from 60
C to 900
C at a rate of 10
C/min. The desorption signals of O2 was monitored by a TCD detector. Raman spectra were collected by a LabRAM HR laser Raman spectrograph (HORIBA Jobin Yvon, France) with a neodymium-doped YAG laser (excitation wavelength 532 nm, laser power 73.5 mW; 20  30 s transits per sample, spectral window 100-1000 cm1, spectral resolution 1 cm1). The XPS measurements of the materials were carried out on a Thermo Scientific Escalab 250 spectrometer (resolution 0.1 eV) with Al Ka (1486.8 eV) radiation as the excitation source. The pressure in the analysis chamber was kept below 2  107 Pa. The binding energy (Eb) values were reported relative C1s line at 284.6 eV. 2.3. Catalytic activity measurement Special black 6 (Degussa) with the particle size of 17 nm, surface area 300 m2/g, volatile 18%, was used as the model soot. For each test, 100 mg catalyst was mixed with 10 mg soot by a spatula in the mortar for 5 min to realize “loose contact” conditions. “Tight contact” conditions can be achieved by crushed together for 5 min in the mortar. To avoid reaction run away, the mixture was diluted by 110 mg SiC powder. Then, these catalysts were tested by temperature-program oxidation (TPO) in a continuous flow fixed-bed reactor set in a quartz tube (20 mm). The reaction gas containing 0.5% O2 balanced with N2 was passed through the fixed-bed reactor at a flow rate of 500 mL/min, and the inlet gas temperature was monitored by a K-type thermocouple at the front end of the reaction bed. The reaction temperature was raised from room temperature to 800
C (5
C/min). The CO2 emissions from the soot oxidation process were continuously detected by a COx analyzer coupled with IR detector in a 1050E on-line CO2 gas determinator. Please cite this article as: P. Yao et al., Factors determining gasoline soot abatement over CeO2eZrO2-MnOx catalysts under low oxygen concentration condition, Journal of the Energy Institute, https://doi.org/10.1016/j.joei.2019.05.005

P. Yao et al. / Journal of the Energy Institute xxx (xxxx) xxx

3

The soot-TG analysis was carried out on a thermogravimetric analyzer (HCT-2, Beijing) to obtain soot conversion. 1 mg soot was mixed with 10 mg catalyst in both contact modes. Then the temperature program was start from 150 to 800
C in a flow of 0.5%O2/N2 (50 mL/min) at a rate of 5
C/min. The weight loss was used to calculate the soot conversion. T50, T90 means the temperature about 50%, 90% of soot was converted. The weighing error of the catalyst is less than 0.5%, and the error of temperature measurement is less than 1
C, while the error of flow measurement is less than 1 mL/min. Moreover, the repetitive experiments of each group catalysts were carried out, and the experimental results showed that the repeatability were good, the error of the repetitive experiments were less than 2% in each temperature spots. 3. Results and discussion

3.1. Catalytic activity 3.1.1. Soot-TPO and soot-TG results The soot-TPO profiles over CZ, M, M10-CZ and M15-CZ were presented in Fig. 1. It can be seen that the peak temperature of soot combustion in tight contact mode over CZ, M, M10-CZ and M15-CZ are ~390
C, ~410
C, ~370
C, ~375
C, respectively, which is almost 300
C lower than that of blank sample, indicating that the as-prepared catalysts have the excellent catalytic properties even under such a low oxygen concentration. In addition, the doping of MnOx into CZ also has positive effects on the catalytic oxidation compared with pure CZ or/and MnOx samples. However, this advantage is not very significant under the loose contact condition, which may be due to the weak contact between catalysts and soot. For blank sample, the observation of the CO2 production in the temperature range of 300e500
C belongs to the combustion of VOCs adsorbed on soot, and the amounts of these CO2 is about 18%, which is almost equal to the data provided by Special black 6 (Degussa). Similarly, the production of CO2 under low temperature of these catalysts may also belong to the combustion of VOCs adsorbed on soot at loose contact condition. Fig. 2 compares the soot conversion of these samples by soot-TG test, and Table 1 summarizes T50 and T90 values under tight and loose contact conditions. Clearly, the introduction of catalysts has an obvious promotion effect for soot oxidation in both contact conditions. Moreover, M10-CZ and M15-CZ catalysts show better soot conversion compared with pure MnOx and CZ catalysts. M10-CZ shows the highest activity among the catalysts investigated under both contact conditions and the accelerative effect is even more pronounced under tight contact. Comparatively, all samples show much higher catalytic activity for soot oxidation under tight contact condition, which is attributed to the effective contact of soot and catalysts. However, the main factors affecting the catalytic activity for soot oxidation under such low oxygen condition and the difference among these catalysts should be investigated.

3.2. Catalyst characterization 3.2.1. Textural and structural properties The BET surface areas (SBET), average pore radius and pore volumes measured by N2 adsorption-desorption are listed in Table 2. It can be seen that the SBET of MCZ is significantly larger than pure CZ or M. It is obviously that the addition of MnOx is conducive to the textural properties of MCZ, which may be ascribed to the synergistic effect between MnOx and CeO2eZrO2. The strong interaction inhibited the grown of crystalline grain and then made MCZ samples obtain much larger SBET values [30,34]. However, there seems to be no linear relation between the SBET and the activity for soot oxidation. It was reported by some researchers [35e37] that the SBET has a liner relation with the soot catalytic oxidation when the value is less than 60 m2/g. In this work, the surface areas of MCZ and CZ catalysts are much larger than 60 m2/g, and the SBET difference between MCZ or CZ and pure MnOx are markedly higher than that of the soot oxidation activity. Therefore, the SBET has some influences on the contact efficiency between soot and catalysts, but is not the major factor that influences the soot oxidation activity. Furthermore, the pore diameter of MCZ and CZ catalysts are <10 nm, which is much smaller than the soot diameter (17 nm). Therefore, it is difficult for soot to enter the inner surface of the catalysts so as to the incomplete utilization of SBET values, which have been reported by many researchers [36,37]. The appropriate pore diameter of MnOx (33 nm) may be good to its catalytic activity, although its surface area is small (21.6 m2/g).

Fig. 1. Evolutions of soot-TPO test with the samples (Reaction conditions: [O2] ¼ 0.5%, N2 balance, Flow rate ¼ 500 mL/min, a: tight contact, b: loose contact).

Please cite this article as: P. Yao et al., Factors determining gasoline soot abatement over CeO2eZrO2-MnOx catalysts under low oxygen concentration condition, Journal of the Energy Institute, https://doi.org/10.1016/j.joei.2019.05.005

4

P. Yao et al. / Journal of the Energy Institute xxx (xxxx) xxx

Fig. 2. Evolutions of soot-TG test of the samples (Reaction conditions: [O2] ¼ 0.5%, N2 balance, Flow rate ¼ 50 mL/min, a: soot without catalyst, b: tight contact, c: loose contact).

Table 1 T50, T90 of the prepared catalysts under tight and loose contact conditions. Samples

Tight contact T50 (
C)

CZ M10-CZ M15-CZ M

363 340 350 372

± ± ± ±

1 1 1 1

Loose contact T90 (
C) 436 396 411 428

± ± ± ±

T50 (
C)

1 1 1 1

540 520 540 583

± ± ± ±

T90 (
C)

1 1 1 1

623 605 623 670

± ± ± ±

1 1 1 1

Table 2 Textural properties of the prepared catalysts. Samples

Surface area (m2/g)

Pore volume (mL/g)

Average pore radius (nm)

CZ M10-CZ M15-CZ M

110.2 145.9 139.8 21.6

0.26 0.34 0.35 0.17

4.7 4.8 5.0 16.5

All samples were calcined at 600
C for 3 h in air.

Structural properties of the catalysts were studied by XRD measurement, and the XRD patterns are shown in Fig. 3. The XRD results are aimed at weighing the structural impact on soot oxidation activity. The results display that the diffraction peaks of CZ and MCZ catalysts are symmetrical centered at 2q ¼ 29.2
, 33.8
, 48.4
, 58.1
(JCPDS card No. 38-1439) without other miscellaneous peaks, indicating the formation of homogeneous Ce0.4Zr0.6O2 solid solutions. For samples containing Mn, Mn2O3 and Mn3O4 species have been detected for pure MnOx, but no characteristic peaks of MnOx were observed in the MCZ samples. This phenomenon may be caused by two reasons. On the one hand, the diffraction main peak of MCZ samples shift toward higher diffraction angle compared with CeeZr sample, indicating the smaller radius Mnnþ entered into the lattice of Cenþ resulting in the contraction of its unit cell, and formed CeO2eZrO2-MnOx solid solutions. On the other hand, a part of MnOx may highly dispersed on the surface of CZ, so as to the crystallites of MnOx is not large enough to be detected [33,38]. Besides, the peak intensity of MCZ samples are more weaker and wider than that of CZ sample, implying much smaller crystalline grain size and crystallinity of MCZ samples. The results may be due to the highly dispersion of MnOx on the surface of CZ and (or) the formation of CeO2eZrO2-MnOx solid solutions inhibit the growth of CZ crystalline, which agrees well with the SBET results. Therefore, there is a synergistic effect between MnOx and CZ. However, the influences of structure properties of the as-prepared catalysts on the soot oxidation activity is similar to textural properties, is not the most important factor. 3.2.2. Redox properties The redox abilities of the samples were determined by H2-TPR and the results are shown in Fig. 4. Generally, for pure CeO2, there are always two reduction peaks observed during the reduction process according to many reports [19,39e41]. The peak temperature at 450 ~ 550
C belongs to the reduction of Ce4þ on the surface, while the one at around 800
C belongs to the reduction of Ce4þ on the bulk. For the CZ sample, there is only one asymmetric reduction peak can be detected which starts from 350
C to 630
C, illustrating that the CZ sample has better reduction property than pure ceria. In other words, the reduction properties of the CZ sample become easier and the reduction ability become stronger compared with the pure ceria. The results could be attributed to the co-reduction of both the surface and bulk ceria with the doping of Zr into the lattice of ceria [42]. For pure MnOx sample, there are two reduction peaks centered at 400
C and 550
C, which belong to the reduction of Mn4þ/Mn8/3þ and Mn8/3þ/Mn2þ, respectively [15,43,44]. It is worth noting that there is only one broad coreduction peak observed from the M10-CZ and M15-CZ catalysts and the onset temperature is also shifted to a much lower temperature. It indicates that the synergistic effect between MnOx and CZ has facilitated the mobility of lattice oxygen through the formation of Please cite this article as: P. Yao et al., Factors determining gasoline soot abatement over CeO2eZrO2-MnOx catalysts under low oxygen concentration condition, Journal of the Energy Institute, https://doi.org/10.1016/j.joei.2019.05.005

P. Yao et al. / Journal of the Energy Institute xxx (xxxx) xxx

5

Fig. 3. XRD profiles of the samples.

Fig. 4. H2-TPR profiles of the samples.

CeO2eZrO2-MnOx uniform solid solutions, the creation of more defects through the mismatch of Cenþ and Mnnþ, then greatly improved the redox ability of M and CZ samples [45]. Besides, the highly dispersed MnOx species with outstanding redox ability on the surface of CZ may be another reason for the excellent redox ability of MCZ samples, which could increase the number of interface between CZ and MnOx and the contact possibility of soot with the active MnOx species and then improve the soot oxidation activity. Moreover, it can clearly be observed that M10-CZ sample exhibits the best low temperature redox ability (<550
C). Many literatures [45,46] have reported that low temperature redox ability is more important for soot oxidation, which is in accordance with the soot oxidation catalytic activity of the samples. MnOx exhibits larger H2 consumption peaks and lower reduction temperature than CZ, but weaker activity for soot oxidation, which may be caused by the poor contact efficiency resulting from lower SBET. In general, the low temperature redox ability is one of the key influencing factors on soot oxidation under the same contact condition. 3.2.3. Oxygen storage capacity The oxygen storage capacity (OSC) of the samples is shown in Fig. 5. Obviously, the OSC of CZ sample greatly increased with MnOx addition by forming MCZ solid solutions. A high value of OSC is usually accompanied with more oxygen vacancies and can allow the catalysts

Please cite this article as: P. Yao et al., Factors determining gasoline soot abatement over CeO2eZrO2-MnOx catalysts under low oxygen concentration condition, Journal of the Energy Institute, https://doi.org/10.1016/j.joei.2019.05.005

6

P. Yao et al. / Journal of the Energy Institute xxx (xxxx) xxx

to utilize molecule oxygen maximumly and release oxygen species easily [22]. Si et al. [47] found that the lattice strains in CZ showed a linear relationship with the OSC values. Mamontov et al. [48] also reported that there is a direct correlation between the concentration of vacancyinterstitial oxygen defects and OSC values. Especially, the M10-CZ sample exhibits the highest OSC value among these catalysts, which could be assigned to the increasing content of structure oxygen vacancies or defects through the mismatch of Cenþ and Mnnþ when doping of MnOx into ceria lattice [22,49,50]. Many scholars have reported that the oxygen storage ability, like the redox ability, is also closely connected with the soot oxidation activity [15,35,51]. Larger oxygen storage ability could release much oxygen to participate in soot oxidation reactions when gas oxygen is insufficient in the reaction atmosphere. However, MnOx sample shows higher OSC value but lower catalytic activity for soot oxidation, which could be attributed to the lower SBET values. For a solid-solid-gas three phase reaction, the contact efficiency between soot and catalysts is crucially important. The lower SBET have affected the contact efficiency between soot and MnOx catalyst and the mobility of active oxygen species, resulting in the ineffective use of these active oxygen species. So we can reasonably speculate that the OSC values would be essential for soot oxidation, especially under a relative low oxygen concentration when the contact efficiency is unrestricted. 3.2.4. O2-TPD results O2-temperature program desorption (O2-TPD) is a useful technology to investigate the type of oxygen species of metal oxides. As shown in Fig. 6, there are two weak desorption peaks centered at 100
C and 850
C for MnOx, which respectively belong to the desorption of 2 superoxide ion O in MnO2 [52]. The weak peaks of MnOx may be 2 weakly bounded to the surface of MnO2 and lattice oxygen ion O attributed to the low SBET value which leads to a small amount of oxygen species adsorption [8]. For CZ and MCZ samples, there is only one desorption peak (form 80e300
C/500
C), which is generally attributed to desorption of O2 from the surface oxygen species and lattice oxygen species [53]. Obviously, M10-CZ and M15-CZ samples exhibit a much border peak from 80 to 500
C. That is to say, the doping of MnOx significantly enhances the amount of desorption oxygen species especially from 150 to 500
C through the formation of much structure defects, which may benefit to the adsorption of gas oxygen species, as testified by Raman results. It has been well known that the surface oxygen species are much more reactive than bulk or gas oxygen species and then can greatly improve the catalytic activity. Therefore, the oxygen species desorption at low temperature (below 500
C) may have large contribution to soot oxidation. The desorption amount of oxygen species at temperature below 500
C is in accordance with the soot-TPO results. This result agrees well with the previous study [54]. 3.2.5. Raman spectra According to precious literatures [18,30,55], oxygen vacancies and defect sites play an important role on the soot oxidation reaction. Fig. 7 shows the UV Raman spectra result which was used to further confirm these active species of the samples. The Raman spectra of CZ, M10-CZ and M15-CZ displayed three major distinct peaks. Two main bands at 465 and 620 cm1 are assigned to the characteristic of the F2g mode of the CeO2 fluorite like structure [56,57] and oxygen vacancies or lattice defects [58,59], respectively. Moreover, the characteristic peak centered at 1190 cm1 belongs to second-order longitudinal optical mode [60,61]. Generally, the peak area ratio of 620 cm1/465 cm1 (A620/A465) can be used to describe the relative amount of oxygen vacancies, and the results are also shown in Fig. 7. It is obviously observed that the intensity of the major distinct peak at 620 cm1 and the concentration of oxygen vacancies increased with the introduction of MnOx, indicating the doping of MnOx can cause structural rearrangement which is consistent with the XRD result. In addition, M10-CZ exhibits the largest amount of oxygen vacancies, which is benefit for the adsorption and activation of gas oxygen and improving the mobility of lattice oxygen. The results are well consistent with soot catalytic oxidation activity.

Fig. 5. OSC values of the samples.

Please cite this article as: P. Yao et al., Factors determining gasoline soot abatement over CeO2eZrO2-MnOx catalysts under low oxygen concentration condition, Journal of the Energy Institute, https://doi.org/10.1016/j.joei.2019.05.005

P. Yao et al. / Journal of the Energy Institute xxx (xxxx) xxx

7

Fig. 6. O2-TPD curves of the samples.

Fig. 7. UV Raman spectra and the concentration of surface oxygen vacancies of the samples.

Therefore, the increase of oxygen vacancies after doping MnOx may contribute to the enhancement of oxygen concentration observed from the structural rearrangement, which may be another reason for the excellent catalytic activity under a relative low oxygen concentration. 3.2.6. XPS results XPS analysis was used to investigate the surface chemical states and properties of the samples. The spectra of the Ce3d, Mn2p and O1s are shown in Fig. 8. For the Ce3d spectra, the peaks labeled with V and U that assigned to Ce3d3/2 and Ce3d5/2 spin orbit component can be observed after deconvolution according to the literature [62,63]. The peaks marked as V, V2, V3 and U, U2, U3 are characteristic peaks of Ce4þ3d3/2 and Ce4þ3d5/2, respectively. The other two peaks labelled as V1 and U1 belongs to Ce3þ 3d3/2 and Ce3þ 3d5/2, respectively. After calculating the surface concentrations of Ce3þ and Ce4þ, the results are listed in Table 3. Interestingly, the concentration of surface Ce4þ increases with the doping of MnOx. Fu et al. [64] reported the transformation from Ce4þ to Ce3þ could create more Frenkel-type oxygen vacancies, which is beneficial for the activation of surface oxygen species. In addition, the formation of Frenkel-type oxygen vacancies through the process of Ce4þ to Ce3þ or the doping of foreign elements can facilitate on the soot oxidation via promoting the mobility of lattice oxygen and activation of surface oxygen species, which has been certified by many researchers [59,64]. Therefore, this result indicating that more Ce4þ could accelerate the formation of surface oxygen vacancies through the conversion of Ce4þ to Ce3þ during the reaction and then promote the adsorbed and activation of gas oxygen. For the Mn2p spectra, the binding energy located at 641.1 eV and 642.8 eV are corresponding to Mn3þ (or Mn2þ) and Mn4þ, respectively. The binding energy increase with the MnOx addition indicates that strong interaction exist between Mn and CZ. This interaction may enhance the oxygen storage and redox properties among the synthesized catalysts according to OSC and H2-TPR results [59,65,66]. The Please cite this article as: P. Yao et al., Factors determining gasoline soot abatement over CeO2eZrO2-MnOx catalysts under low oxygen concentration condition, Journal of the Energy Institute, https://doi.org/10.1016/j.joei.2019.05.005

8

P. Yao et al. / Journal of the Energy Institute xxx (xxxx) xxx

Fig. 8. XPS analysis for the samples, (a) Ce3d spectra, and (b) Mn2p and (c) O1s.

Table 3 Surface compositions and charge states of Ce, Mn and O species derived from XPS analyses. Samples

Ce3þ/Ce

Ce4þ/Ce

Mn4þ/Mn

(O2 2 þ O2  )/OT

CZ M10-CZ M15-CZ M

0.21 0.14 0.16 y

0.79 0.86 0.84 y

y 0.362 0.357 0.24

0.28 0.35 0.32 0.34

corresponding O1s spectra of these catalysts show three characteristic peaks: lattice oxygen species (O2) at 528.6~529.7 eV and active oxygen species (O2 2 and O2  ) at 529.5~531 eV as well as 530.8~532.5 eV [5], respectively. The relative concentration of active oxygen species is counted through (O2 2 þ O2  )/OT (OT ¼ O2þO2 2 þO2  ) and the results are also presented in Table 3. From the results we can clearly observe that the concentration of active oxygen species increase with the doping of MnOx, which would give rise to unusual catalytic properties for the deep oxidation reaction [67]. Although MnOx shows higher surface oxygen species value, its soot oxidation activity is poor. It may be attributed to the severe aggregation of soot on the MnOx surface since the low SBET values, which inhibits the mobility of active species. Therefore, the active oxygen species (surface oxygen O2 2 and adsorbed oxygen O2  ) are one of the key factors determining soot oxidation activity when the contact efficiency is unrestricted. 3.3. Influential factors for soot combustion This catalytic performance difference among these catalysts may come from two major factors: one is intrinsic catalytic activity which may attribute to texture and structure properties, microstructure, redox ability, oxygen storage capacity, oxygen species desorption capacity, surface oxygen vacancies; another is the contact efficiency between catalysts and reactants (soot, O2) [46]. During the experimental operation, MnOx exhibits much lower SBET (21.6 m2/g) and poor contact efficiency than CZ and MCZ, resulting in the poor soot oxidation activity. Therefore, contact efficiency is the main factor affecting the soot combustion for MnOx catalyst. However, all the CZ and MCZ catalysts were investigated at the same contact conditions and O2 concentration when comparing, so the difference of activity mainly comes from the nature of catalysts. From the textual and structure parameters, no linear relation is found by comparing activity against the SBET and XRD diffraction peak among CZ and MCZ catalysts. Moreover, some properties associated with generation and movement of reactive oxygen species, including the low-temperature reducibility, oxygen storage capacity, low temperature oxygen  desorption capacity, the amount of oxygen vacancies and active oxygen species (O2 2 þ O2 )/OT are more important for soot combustion, which is in accordance with the catalytic activity of these CZ and MCZ catalysts. Above all, these factors about generation and movement of reactive oxygen species are crucial for soot oxidation in a low oxygen concentration. 4. Conclusion In this work, a series of CeO2eZrO2-MnOx catalysts with various amount of MnOx were synthesized via co-precipitation method, for the purpose of investigating the major factors to influence the catalytic activity of soot oxidation under a low oxygen concentration. The doping of MnOx can obviously influence the textural, structural and redox properties as well as oxygen storage capacity of CZ sample. Among these samples, M10-CZ exhibited the best soot catalytic activity with the T50 of 340
C and 520
C under tight and loose contact conditions, respectively, which can be attributed to the excellent redox ability, oxygen storage capacity and oxygen desorption capacity at low temperature along with much more active surface oxygen species and it's mobility, and the amount of oxygen vacancies. Acknowledgement The research was performed in Institute of New Energy and Low-Carbon Technology and College of Chemistry, Sichuan University. This work was supported by Sichuan Science and Technology Program (2019YFS0498) and National Engineering Laboratory for Mobile Source Emission Control Technology (NELMS2018A09). Please cite this article as: P. Yao et al., Factors determining gasoline soot abatement over CeO2eZrO2-MnOx catalysts under low oxygen concentration condition, Journal of the Energy Institute, https://doi.org/10.1016/j.joei.2019.05.005

P. Yao et al. / Journal of the Energy Institute xxx (xxxx) xxx

9

Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.joei.2019.05.005.

References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34] [35] [36] [37] [38] [39] [40] [41] [42] [43] [44] [45] [46]

J.M. Richter, R. Klingmann, S. Spiess, K.F. Wong, Application of catalyzed gasoline particulate filters to GDI vehicles, SAE Int. J. Engines 01 (1244) (2012) 1361e1370. T. Johnson, Vehicular emissions in review, SAE Int. J. Engines 01 (0919) (2016) 1258e1275. S.X. Miao, L. Luo, Y. Liu, Z.S. Zhan, Development of a gasoline particulate filter for China 6(b) emission standards, SAE Tech. Pap. 24 (2017) 0135. T. Johnson, Vehicular emissions in review, SAE Int. J. Engines 01 (2014) 1491. Y.C. Wei, Z. Zhao, J. Liu, S.T. Liu, C.M. Xu, A.J. Duan, G.Y. Jiang, Multifunctional catalysts of three-dimensionally ordered macroporous oxide- supported Au@Pt core-shell nanoparticles with high catalytic activity and stability for soot oxidation, J. Catal. 317 (2014) 62e74. D. Gardini, J.M. Christensen, C.D. Damsgaard, A.D. Jensen, J.B. Wagner, Visualizing the mobility of silver during catalytic soot oxidation, Appl. Catal., B 183 (2016) 28e36. Y.X. Gao, A.Q. Duan, S. Liu, X.D. Wu, W. Liu, M. Li, S.G. Chen, X. Wang, D. Weng, Study of Ag/CexNd1-xO2 nanocubes as soot oxidation catalysts for gasoline particulate filters: balancing catalyst activity and stability by Nd doping, Appl. Catal., B 203 (2017) 116e126. S. Liu, X.D. Wu, W. Liu, W.M. Chen, R. Ran, M. Li, D. Weng, Soot oxidation over CeO2 and Ag/CeO2: factors determining the catalyst activity and stability during reaction, J. Catal. 337 (2016) 188e198. J. Oi-Uchisawa, S.D. Wang, T. Nanba, A. Ohi, A. Obuchi, Improvement of Pt catalyst for soot oxidation using mixed oxide as a support, Appl. Catal., B 44 (2003) 207e215. S. Lee, H. Lee, C. Song, J. Park, Experimental study on fundamental effect of H2 for catalytic soot oxidation with Pt/CeO2 using a flow reactor system, J. Energy Inst. (2018), https://doi.org/10.1016/j.joei.2018.08.009. W.N. Yang, S.M. Wang, K.Z. Li, S. Liu, L.N. Gan, Y. Peng, J.H. Li, Highly selective a-Mn2O3 catalyst for cGPF soot oxidation: surface activated oxygen enhancement via selective dissolution, Chem. Eng. J. 364 (2019) 448e451. P. Venkataswamy, D. Jampaiah, K.N. Rao, B.M. Reddy, Nanostructured Ce0.7Mn0.3O2-d and Ce0.7Fe0.3O2-d solid solutions for diesel soot oxidation, Appl. Catal., A 488 (2014) 1e10. C. Rao, R. Liu, X.H. Feng, J.T. Shen, H.G. Peng, X.L. Xu, X.Z. Fang, J.J. Liu, X. Wang, Three-dimensionally ordered macroporous SnO2-based solid solution catalysts for effective soot oxidation, Chin. J. Catal. 39 (2018) 1683e1694. Q. Tang, J. Du, B. Xie, Y. Yang, W.C. Yu, C.Y. Tao, Rare earth metal modified three dimensionally ordered macroporous MnOx-CeO2 catalyst for diesel soot combustion, J. Rare Earth 36 (2018) 64e71. D. Jampaiah, V.K. Velisoju, P. Venkataswamy, V.E. Coyle, A. Nafady, B.M. Reddy, S.K. Bhargava, Nanowire morphology of mono- and bidoped a-MnO2 catalysts for remarkable enhancement in soot oxidation, ACS Appl. Mater. Interfaces 9 (2017) 32652e32666.
nez-Man ~ ogil, A. GarcíarGarcía, Lattice oxygen activity in ceria-praseodymia mixed oxides for soot oxidation in catalysed J.C. MartMnez-Munuera, M. Zoccoli, J. Gime gasoline particle filters, Appl. Catal., B 245 (2019) 706e720. G. Vairamuthu, S. Sundarapandian, C. Kailasanathan, B. Thangagiri, Experimental investigation on the effects of cerium oxide nanoparticle on Calophyllum inophyllum (Punnai) biodiesel blended with diesel fuel in DI diesel engine modified by nozzle geometry, J. Energy Inst. 89 (2016) 668e682. M. Piumetti, S. Bensaid, N. Russo, D. Fino, Investigations into nanostructured ceria-zirconia catalysts for soot combustion, Appl. Catal., B 180 (2016) 271e282. M. Piumetti, S. Bensaid, N. Russo, D. Fino, Nanostructured ceria-based catalysts for soot combustion: investigations on the surface sensitivity, Appl. Catal., B 165 (2015) 742e751. S. Liu, X.D. Wu, J. Tang, P.Y. Cui, X.Q. Jiang, C.G. Chang, W. Liu, Y.X. Gao, M. Li, D. Weng, An exploration of soot oxidation over CeO2-ZrO2 nanocubes: do more surface oxygen vacancies benefit the reaction? Catal. Today 281 (2017) 454e459.
ndez, M.N. Tsampas, C. Zhao, A. Boreave, F. Bosselet, P. Vernoux, La/Sr-based perovskites as soot oxidation catalysts for gasoline particulate filters, Catal. Today W.Y. Herna 258 (2015) 525e534. J. Kullgren, K. Hermansson, P. Broqvist, Supercharged low-temperature oxygen storage capacity of ceria at the nanoscale, J. Phys. Chem. Lett. 4 (2013) 604e608. P. Fang, J.Q. Lu, X.Y. Xiao, M.F. Luo, Catalytic combustion study of soot on Ce0.7Zr0.3O2 solid solution, J. Rare Earth 26 (2008) 250e253.
nez-Man ~ ogil, A. García-García, Opportunities for ceria-based mixed oxides versus commercial platinum-based catalysts in the soot combustion reaction. J. Gime Mechanistic implications, Fuel Process. Technol. 129 (2015) 227e235. D. Devaiah, L.H. Reddy, S.E. Park, B.M. Reddy, Ceria-zirconia mixed oxides: synthetic methods and applications, Catal. Rev. 60 (2018) 177e277. F.F. Dai, M. Meng, Y.Q. Zha, Z.Q. Li, T.D. Hu, Y.N. Xie, J. Zhang, Performance of Ce substituted hydrotalcite-derived mixed oxide catalysts Co2.5Mg0.5Al1-x%Cex%O used for soot combustion and simultaneous NOx-soot removal, Fuel Process. Technol. 104 (2012) 43e49. E. Aneggi, C.D. Leitenburg, A. Trovarelli, On the role of lattice/surface oxygen in ceria-zirconia catalysts for diesel soot combustion, Catal. Today 181 (2012) 108e115. S. Bensaid, N. Russo, D. Fino, CeO2 catalysts with fibrous morphology for soot oxidation: the importance of the soot-catalyst contact conditions, Catal. Today 216 (2013) 57e63. P.A. Kumar, M.D. Tanwar, S. Bensaid, N. Russo, D. Fino, Soot combustion improvement in diesel particulate filters catalyzed with ceria nanofibers, Chem. Eng. J. 207e208 (2012) 258e266. X.T. Lin, S.J. Li, H. He, Z. Wu, J.L. Wu, L.M. Chen, D.Q. Ye, M.L. Fu, Evolution of oxygen vacancies in MnOx-CeO2 mixed oxides for soot oxidation, Appl. Catal., B 223 (2018) 91e102. X.D. Wu, S. Liu, D. Weng, F. Lin, R. Ran, MnOx-CeO2-Al2O3 mixed oxides for soot oxidation: activity and thermal stability, J. Hazard Mater. 187 (2011) 283e290. Q. Liang, X.D. Wu, D. Weng, H.B. Xu, Oxygen activation on Cu/Mn-Ce mixed oxides and the role in diesel soot oxidation, Catal. Today 139 (2008) 113e118. W.J. Shan, N. Ma, J.L. Yang, X.W. Dong, C. Liu, L.L. Wei, Catalytic oxidation of soot particulates over MnOx-CeO2 oxides prepared by complexation- combustion method, J. Nat. Gas Chem. 19 (2010) 86e90. X.D. Wu, F. Lin, H.B. Xu, D. Weng, Effects of adsorbed and gaseous NOx species on catalytic oxidation of diesel soot with MnOx-CeO2 mixed oxides, Appl. Catal., B 96 (2010) 101e109. E. Aneggi, C.D. Leitenburg, G. Dolcetti, A. Trovarelli, Promotional effect of rare earths and transition metals in the combustion of diesel soot over CeO2 and CeO2-ZrO2, Catal. Today 114 (2006) 40e47.
pez, M. Makkee, J.A. Moulijn, Potential rare earth modified CeO2 catalysts for soot oxidation I. Characterisation and catalytic activity with O2, K. Krishna, A. Bueno-Lo Appl. Catal., B 75 (2007) 189e200. X.D. Wu, S. Liu, D. Weng, F. Lin, Textural-structural properties and soot oxidation activity of MnOx-CeO2 mixed oxides, Catal. Commun. 12 (2011) 345e348. P. Venkataswamy, K.N. Rao, D. Jampaiah, B.M. Reddy, Nanostructured manganese doped ceria solid solutions for CO oxidation at lower temperatures, Appl. Catal., B 162 (2015) 122e132. X.F. Tang, Y.G. Li, X.M. Huang, Y.D. Xu, H.Q. Zhu, J.G. Wang, W.J. Shen, MnOx-CeO2 mixed oxide catalysts for complete oxidation of formaldehyde: effect of preparation method and calcination temperature, Appl. Catal., B 62 (2006) 265e273. H. Muroyama, S. Hano, T. Matsui, K. Eguchi, Catalytic soot combustion over CeO2-based oxides, Catal. Today 153 (2010) 133e135. E.T. Saw, U. Oemar, M.L. Ang, H. Kus, S. Kawi, High temperature water gas shift reaction on Ni-Cu/CeO2 catalysts: effect of ceria nanocrystal size on carboxylate formation, Catal. Sci. Technol. 6 (2016) 5336e5349. B. Zhao, G.F. Li, C.H. Ge, Q.Y. Wang, R.X. Zhou, Preparation of Ce0.67Zr0.33O2 mixed oxides as supports of improved Pd-only three-way catalysts, Appl. Catal., B 96 (2010) 338e349. D. Terribile, A. Trovarelli, C.D. Leitenburg, A. Primavera, G. Dolcetti, Catalytic combustion of hydrocarbons with Mn and Cu-doped ceria-zirconia solid solutions, Catal. Today 47 (1999) 133e140. F. Kapteijn, L. Singoredjo, A. Andreini, J.A. Moulijn, Activity and selectivity of pure manganese oxides in the selective catalytic reduction of nitric oxide with ammonia, Appl. Catal., B 3 (1994) 173e189. D. Delimaris, T. Ioannides, VOC oxidation over MnOx-CeO2 catalysts prepared by a combustion method, Appl. Catal., B 84 (2008) 303e312. L. Xiong, P. Yao, S. Liu, S.S. Li, J. Deng, Y. Jiao, Y.Q. Chen, J.L. Wang, Soot oxidation over CeO2-ZrO2 based catalysts: the influence of external surface and low-temperature reducibility, Mol. Catal. 467 (2019) 16e23.

Please cite this article as: P. Yao et al., Factors determining gasoline soot abatement over CeO2eZrO2-MnOx catalysts under low oxygen concentration condition, Journal of the Energy Institute, https://doi.org/10.1016/j.joei.2019.05.005

10

P. Yao et al. / Journal of the Energy Institute xxx (xxxx) xxx

[47] R. Si, Y.W. Zhang, S.J. Li, B.X. Lin, C.H. Yan, Urea-based hydrothermally derived homogeneous nanostructured Ce1-xZrxO2 (x ¼ 0-0.8) solid solutions: a strong correlation between oxygen storage capacity and lattice strain, J. Phys. Chem. B 108 (2004) 12481e12488. [48] E. Mamontov, T. Egami, R. Brezny, M. Koranne, S. Tyagi, Lattice defects and oxygen storage capacity of nanocrystalline ceria and ceria-zirconia, J. Phys. Chem. B 104 (2000) 11110e11116. [49] Z. Wang, G.L. Shen, J.Q. Li, H.D. Liu, Q. Wang, Y.F. Chen, Catalytic removal of benzene over CeO2-MnOx composite oxides prepared by hydrothermal method, Appl. Catal., B 138e139 (2013) 253e259. [50] K. Tikhomirov, O. Krocher, M. Elsener, A. Wokaun, MnOx-CeO2 mixed oxides for the low-temperature oxidation of diesel soot, Appl. Catal., B 64 (2006) 72e78. [51] R.D. Monte, J. Kaspar, Nanostructured CeO2-ZrO2 mixed oxides, J. Mater. Chem. 15 (2005) 633e648. [52] X.D. Wu, H.N. Yu, D. Weng, S. Liu, J. Fan, Synergistic effect between MnO and CeO2 in the physical mixture: electronic interaction and NO oxidation activity, J. Rare Earth 31 (2013) 1141e1147. [53] S. Kaliaguine, A. Van Neste, V. Szabo, J.E. Gallot, M. Bassir, R. Muzychuk, Perovskite-type oxides synthesized by reactive grinding: Part I. Preparation and characterization, Appl. Catal., A 209 (2001) 345e358. [54] G.C. Zou, Y. Xu, S.J. Wang, M.X. Chen, W.F. Shangguan, The synergistic effect in Co-Ce oxides for catalytic oxidation of diesel soot, Catal. Sci. Technol. 5 (2015) 1084e1092.
drine, Revisiting active sites in heterogeneous catalysis: their structure and their dynamic behavior, Appl. Catal., A 474 (2014) 40e50. [55] J.C. Ve [56] T. Sato, T. Komanoya, Selective oxidation of alcohols with molecular oxygen catalyzed by Ru/MnOx/CeO2 under mild conditions, Catal. Commun. 10 (2009) 1095e1098.
pez, A. García-García, Combined removal of diesel soot particulates and NOx over CeO2-ZrO2 mixed oxides, J. Catal. 259 (2008) 123e132. [57] I. Atribak, A. Bueno-Lo [58] C. Li, X. Gu, Y. Wang, Y. Wang, Y. Wang, X. Liu, G. Lu, Synthesis and characterization of mesostructured ceria-zirconia solid solution, J. Rare Earth 27 (2009) 211e215. [59] G.F. Li, Q.Y. Wang, B. Zhao, R.X. Zhou, The promotional effect of transition metals on the catalytic behavior of model Pd/Ce0.67Zr0.33O2 three-way catalyst, Catal. Today 158 (2010) 385e392. [60] Z.L. Wu, M.J. Li, J. Howe, H.M. Meyer, S.H. Overbury, Probing defect sites on CeO2 nanocrystals with well-defined surface planes by Raman spectroscopy and O2 adsorption, Langmuir 26 (2010) 16595e16606. [61] A. Alinezhadchamazketi, A.A. Khodadadi, Y. Mortazavi, A. Nemati, Catalytic evaluation of promoted CeO2-ZrO2 by transition, alkali, and alkaline-earth metal oxides for diesel soot oxidation, J. Environ. Sci. 25 (2013) 2498e2506. [62] H.L. Zhang, J.L. Wang, Y. Cao, Y.J. Wang, M.C. Gong, Y.Q. Chen, Effect of Y on improving the thermal stability of MnOx-CeO2 catalysts for diesel soot oxidation, Chin. J. Catal. 36 (2015) 1333e1341. [63] H.L. Zhang, J.L. Wang, Y.H. Zhang, Y. Jiao, C.J. Ren, M.C. Gong, Y.Q. Chen, A study on H2-TPR of Pt/Ce0.27Zr0.73O2 and Pt/Ce0.27Zr0.70La0.03Ox for soot oxidation, Appl. Surf. Sci. 377 (2016) 48e55. [64] M. Zhang, M.L. Fu, J.L. Wu, B.C. Huang, H. Liang, D.Q. Ye, Characteristic of surface oxygen species and catalytic property on MnOx-CeO2 for soot combustion, J. Chin Soc. Rare Earth 29 (2011) 303e309. [65] H.Y. Chen, A. Sayari, A. Adnot, F. Larachi, Composition-activity effects of Mn-Ce-O composites on phenol catalytic wet oxidation, Appl. Catal., B 32 (2001) 195e204. [66] L.M. Shi, W. Chu, F.F. Qu, J.Y. Hu, M.M. Li, Catalytic performance for methane combustion of supported Mn-Ce mixed oxides, J. Rare Earth 26 (2008) 836e840. [67] J. Xiong, Q.Q. Wu, X.L. Mei, J. Liu, Y.C. Wei, Z. Zhao, D. Wu, J.M. Li, Fabrication of spinel-type PdxCo3-xO4 binary active sites on 3D ordered meso-macroporous Ce-Zr-O2 with enhanced activity for catalytic soot oxidation, ACS Catal. 8 (2018) 7915e7930.

Please cite this article as: P. Yao et al., Factors determining gasoline soot abatement over CeO2eZrO2-MnOx catalysts under low oxygen concentration condition, Journal of the Energy Institute, https://doi.org/10.1016/j.joei.2019.05.005