- Email: [email protected]
In-situ modified the surface of Pt-doped perovskite catalyst for soot oxidation
Journal of Hazardous Materials 383 (2020) 121210
Contents lists available at ScienceDirect
Journal of Hazardous Materials journal homepage: www.elsevier.com/locate/jhazmat
In-situ modified the surface of Pt-doped perovskite catalyst for soot oxidation Lirong Zenga, Lan Cuib, Caiyun Wanga, Wei Guoa, Cairong Gonga, a b
T
⁎
Insitute of New Energy, School of Materials Science and Engineering, Tianjin University, 300072, PR China Center of Analysis, Tianjin University, Tianjin 300072, PR China
G R A P H I C A L A B S T R A C T
A R T I C LE I N FO
A B S T R A C T
Editor: Danmeng Shuai
In-situ modification is studied in this work on LaCo1-xPtxO3 for soot oxidation. A series of perovskite catalysts LaCo1-xPtxO3 (by sol-gel method) are modified with 30% H2O2. XRD, TEM, SEM, BET, XPS, Raman, in-situ DRIFTS, H2-TPR and TGA are used to investigate physicochemical properties of catalysts. TGA results show that all doped catalysts have a lower temperature of soot conversion, especially LaCo0.94Pt0.06O3 (T50 = 437 °C). The T50 of the catalyst with modification by H2O2 solution decreases at least 20 °C compared with the doped catalysts. A highly symmetrical structure and an obvious amorphous layer about 3–5 nm are observed in the modified catalysts. According to the XPS study, the symmetrical structure benefits to the movement of oxygen vacancy thus catalyst captures more adsorbed oxygen (about 95%). And the amorphous surface could adsorb more oxygen species. In addition, all catalysts show excellent aging resistance performance. The reaction mechanism of catalyst for soot oxidation is presented in the end.
Keywords: Surface modification Symmetrical structure Amorphization Aging resistance
1. Introduction With the rapid development of economy, the number of motor vehicles kept increasing. Diesel engines duo to their high efficiency and remarkable fuel-saving effect have been applied widely on motor vehicles (Huang et al., 2018). However, particles matter (PM), nitric oxide
⁎
(NOx), carbon monoxide (CO) and hydrocarbon (HC) emission by diesel vehicle were harm for the environment and human health (Matarrese et al., 2017; Andana et al., 2016; Shang et al., 2017; Cheng et al., 2017). Particularly, the emission of PM was about dozens of times than gasoline engine (Dhal et al., 2018). To address this problem, many posttreatment technologies have been developed in global over the years
Corresponding author. E-mail address: [email protected] (C. Gong).
https://doi.org/10.1016/j.jhazmat.2019.121210 Received 11 June 2019; Received in revised form 9 September 2019; Accepted 10 September 2019 Available online 11 September 2019 0304-3894/ © 2019 Elsevier B.V. All rights reserved.
Journal of Hazardous Materials 383 (2020) 121210
L. Zeng, et al.
Fig. 1. The active evaluation of catalysts: (a) the soot conversion of the doped catalysts under air; (b)the soot conversion of the modified catalysts under air; (c) the soot conversion of LCP0.06-4 (air or 2000 ppmNO/air); (d) the kinetics of the catalytic reaction under air (Ozawa plots and Ea); (e) the soot cycle of LCP0.06-4 (air).
without catalyst, which exceeded the normal exhaust temperature range (200 °C˜500 °C) of diesel engines (Fino et al., 2016). Therefore, the adoption of catalyst was essential to ensure normal operation of DPF at the lower temperature. Up to now, researches of catalysts for soot oxidation were mainly focused on the noble metals and metal oxides. Noble metal catalysts (such as Pt, Pd, Au etc.) were applied in industrial production due to the good catalytic activity and selectivity (Xie et al., 2017; Andana et al., 2018; Wu et al., 2019b; Xiong et al., 2019; Wu et al., 2019c; Xiong et al., 2018). Moreover, Pt-based catalysts were among the most sulfurresistant soot combustion catalysts, which could tolerate much more SO2 in comparison with the non-noble metal samples (Liu et al., 2013, 2015). However, the cost of noble metal had to be considered. The metal oxides with multifarious valence and strong redox could wildly apply in removing soot particles. For example, Ji et al. (Ji et al., 2019) studied the catalytic activity of Mn3O4 catalysts with Special shapes via facile hydrothermal and co-precipitation methods. They found that the
Table 1 The catalytic activities of catalysts. Catalysts
T10 (°C)
T50 (°C)
T90 (°C)
LC LCP0.02 LCP0.04 LCP0.06 LCP0.1 LCP0.06-2 LCP0.06-4
395 390 391 390 400 355 344
450 449 444 437 447 411 403
498 487 482 477 486 448 435
such as diesel particulate filter (DPF) (van Setten et al., 2001), lean NOx trap (LNT) (Agathokleous et al., 2019) and NOx selective catalytic reduction (SCR) (Wu et al., 2019a). DPF that removed soot particles (the basis of PM) was considered as the most potential and efficient technology due to the regeneration (Białobok et al., 2007). However, the temperature of DPF system must over 600 °C to remove soot particles 2
Journal of Hazardous Materials 383 (2020) 121210
L. Zeng, et al.
Table 2 Comparison of diesel soot combustion catalysts of different nature tested in realistic laboratory conditions. Catalysts
T50 (°C)
Atmosphere
Heating rate (°C/min)
Reference
Ce0.5Pr0.5O2 LaSr25AgFe LaSr25AgMn LCSC C40nw Ce0.8Eu0.1La0.1 RuxCe1-xO2-y Pd/LS LCP0.06 LCP0.06-2 LCP0.06-4
444 509 449 451 448 446 487 463 437 411 403
O2/N2 1%O2/He 1%O2/He O2 O2/He air air O2 air air air
10 10 10 4 10 10 3 4 10 10 10
(Guillén-Hurtado et al., 2015) (Hernández et al., 2018) (Hernández et al., 2018) (Ai et al., 2018b) (Comelli et al., 2018) (Vinodkumar et al., 2018) (Ledwa et al., 2018) (Ai et al., 2018a) This article This article This article
La0.7Ag0.3MnO3 perovskite-like catalyst exhibited a lower soot ignition temperature (160 °C) due to the partial substitution of Ag which affected valence of B-site ions. The catalyst activity had proved to controlled by substitution amount of the A, B sites, perovskite catalyst thus had a great potential in removing soot particles. However, the perovskite via traditional method (sol-gel method etc.) exhibited a larger particle size and a severe agglomeration, which resulted in a noneffective contact with the active sites of catalyst (Lebid and Omari, 2013; Köferstein and Ebbinghaus, 2013). It was necessary to study how to enhance activity of perovskite for soot oxidation. As we all know, the performance of catalyst was closely related to surface state of catalysts. Surface modifications thus had been widely studied to promote the activity of catalyst in different research field. For example, Tsvetkov et al. (2016) modified the surface of La0.8Sr0.2CoO3 through V, Nb, Zr, Co, Ti, Hf and Al chloride solution, which enhanced oxidative activity of surface and stability. Lam et al. (2017) modified the surface of La0.8Sr0.2CoO3 catalyst with H2O2 aqueous solution and enhanced the surface area of catalyst. In addition, they reported that Ba2+ and Sr2+ ions would selectively precipitate from Ba0.5Sr0.5Co0.8Fe0.2O3 lattices during in-situ decomposition of H2O2 aqueous solution, resulting in a certain A-site defect (Su et al., 2015). However, no widely report that perovskite with modification was applied to soot oxidation. And no report on in-situ catalytic decomposition of B-sites (active sites) of perovskite by H2O2. In this work, we synthesized a series of LaCo1-xPtxO3 (x = 0.02, 0.04, 0.06, 0.1) perovskite catalysts via sol-gel method. The physicochemical properties of catalysts were evaluated by XRD, SEM, TEM, BET, XPS, in-situ DRIFTS, Raman and H2-TPR. The catalytic activity was tested by TGA. It was found that the catalytic activity of LaCo1xPtxO3 for soot oxidation was outstanding and increased with the amount of Pt (when x < 0.1). Meanwhile, the surface of LaCo1-xPtxO3 catalysts is modified with 30% H2O2 aqueous solution. The activity of catalysts was improved obviously after modification. Fig. 2. The H2-TPR profiles of catalysts: (a) doped catalysts; (b) modified catalysts.
2. Experimental 2.1. Synthesis of catalyst
hexagonal nanoplates Mn3O4 had superior activity on soot combustion (Tm = 407.7 °C). Liu et al. (Liu et al., 2017) prepared a ceria-based catalyst (via incipient wetness method) which showed a great catalytic activity for soot oxidation (the conversion of CO2 was about 100%). However, the catalysts showed a lower thermal stability at high temperature. Perovskite oxides with high stability, good catalytic activity and flexible structure regulation overcame the shortcomings of the ceria-based catalysts. For example, Hong group (Hong and Lee, 2000) reported a series of perovskite type oxide (LaMO3 (M = Co, Fe, Mn)) prepared by sol-gel method and the LaCoO3 showed the best catalytic activity (Tm = 400 °C). In addition, Li et al. (2010) investigated the activity of LaCoO3 perovskite type oxide catalysts with different metals doped (A = K; B = Fe). Moreover, La0.9K0.1Co0.9Fe0.1O3-δ showed a highest activity and stability (700 °C). Urán et al. (2019) reported
The LaCo1-xPtxO3(x = 0.02, 0.04, 0.06, 0.1) catalysts were synthesized via sol-gel method. The detailed process was as follows: Lanthanum nitrate (La(NO3)3·6H2O), Cobalt nitrate (Co(NO3)3·6H2O) and Platinum nitrate (Pt(NO3)2) were dissolved in deionized water by stoichiometry. And then added citric acid and ethylenediaminetetraacetic acid (EDTA) into the solution and adjusted the pH to 6–7 with ammonia (25%). The solution was heated at 120℃ for 24 h to obtain dry gel. And then the gel heated at 400℃ for 3 h to remove excess water and organics. The LaCo0.94Pt0.06O3 catalyst was subsequently obtained at 800℃ for 5 h and denoted as LCP0.02, LCP0.04, LCP0.06 and LCP0.1 respectively. For comparison, pure LaCoO3 was synthesized in same way and noted as LC. The modified catalysts were prepared by adding 30% H2O2 aqueous 3
Journal of Hazardous Materials 383 (2020) 121210
L. Zeng, et al.
Fig. 3. XRD patterns of catalysts: (a) (b) doped catalysts; (c) (d) modified catalysts.
Based on the TGA data, the absolute temperature (Tη) corresponding to the fixed η (25%, 50%, 70%) were obtained. The Ea could be calculated according to the slope which could be obtained through the relationship between log Ф and Tη−1 after least-squares fit.
solution into beaker containing 100 mg of LCP0.06 catalyst. Solution was kept the at room temperature for 10 h to completely decompose H2O2 and then heated at 80 °C to obtain dry catalyst powder. Repeated the above steps two or four times to obtain the final product and noted as LCP0.06-2, LCP0.06-4 respectively. In addition, the aged samples were obtained through heating catalysts at 800 °C for 50 h under air atmosphere and recorded with series of “A”.
2.3. Characterization of catalyst Temperature-programmed reduction with H2 (H2-TPR) were tested to evaluate reducibility of catalyst. 50 mg catalysts were heated to 400 °C under Ar for 1 h. Then the catalysts were heated to 900 °C under 10% H2 balanced Ar with 10 °C/min. X-ray diffraction (XRD) was performed to test phase structure using Cu Kα radiation (λ = 015418 nm) on the Bruker-D8 Advance. The diffraction patterns were collected from 20° to 80° with a scanning rate of 5°/min. The transmission electron microscopy (TEM) and scanning electron microscope (SEM) were used to obtain morphology by jem-2100f and s4800 from Japan. Nitrogen adsorption–desorption isotherms were carried under the N2 at −196 °C on a NOVA-2000 instrument (Conta, US). And the pore sizes were obtained through the BJH method, the pore volume was calculated at a pressure of 0.986. The X-ray photoelectron spectra (XPS) was measured on KratosAXIS ULT RADLD equipment with monochromatic Al Kα X-ray source (1487.6 eV). The reference value of binding energy was 284.8 eV (C 1s) of carbon. In-situ diffusion reflectance infrared fourier transform spectroscopy (in-situ DRIFTS) was detected by Nicolet iS50 FTIR spectrometer with MCT detector at 32 scans with a spectral resolution of 4 cm−1. The catalyst, Degussa carbon black and KBr (10:1:20) were intermixed in agate mortar to obtain sample. And then the sample was heated to 344 °C and 403 °C respectively under air atmosphere.
2.2. Catalyst activity evaluation The activity of catalyst was evaluated by Thermogravimetric Analysis (TGA). Firstly, the catalyst and Degussa carbon black were intermixed in agate mortar (catalyst to carbon 10:1, loose contact). And then 30 mg samples were put into U-shaped reactor and heated to 800 °C under air or 2000 ppm NO/air with 100 mL/min. Data was recorded by computer in whole process. The soot conversion rate (η) was calculated by the following formula:
η=
Winitial − W0 * 100% Winitial − Wend
(1)
Where Winitial is the initial weight of samples, Wend is the weight after the heating and W0 is the weight of each temperature point. The activation energy (Ea) for soot catalytic oxidation was investigated by TGA on basis of the Ozawa method (Ozawa, 1970). 30 mg samples were put into U-shaped reactor and heated to 800 °C under air with 100 mL/min. Different heating rates Φ (2.5, 5, 7, 9, 10 ℃/min) were selected to calculate the Ea according to the following formula:
log Φ = B − 0.4567(
Ea ) RTη
(2)
Where B is a constant and R is the ideal gas constant (8.314 J/(mol · K)). 4
Journal of Hazardous Materials 383 (2020) 121210
L. Zeng, et al.
Fig. 4. TEM (left) and HRTEM (right) images of catalysts: (a) LC; (b) LCP0.06; (c) LCP0.06-2; (d) LCP0.06-4.
TGA under the air atmosphere. In Fig. 1a, LCP0.06 exhibits the greatest catalytic activity for removing soot at lower temperature (T50 =437 °C, which is lower than the T50 in previous studies (Table 2)). Moreover, the T50 of all doped catalysts is lower than the undoped catalyst (LC, T50 =450 °C). It indicates that the partial replacement of Co in the B site of perovskite with Pt benefits to improve the activity of the catalyst. The T50 rises to 447 °C when x is 0.1 (Table 1). In Fig. 1b and in Table 1, the conversion temperature shows a significant reduction (at least 20 °C) when the catalyst is modified two times with 30% H2O2. And the T50
Raman spectra was tested on inVia reflex equipment with a CCD detector using a visible laser (λ = 532 nm) and the scanning scope was 100–3200 cm−1.
3. Result and discussion 3.1. The catalytic activity of catalysts The activity of catalysts for removing soot particles is evaluated by 5
Journal of Hazardous Materials 383 (2020) 121210
L. Zeng, et al.
Fig. 6. The (a) TGA and (b) T50 images of the aged LC, LCP0.06 and LCP0.06-4 catalysts. Fig. 5. (a) N2 adsorption-desorption isotherms and (b) pore size distribution of catalysts.
Table 3 BET specific surface area (SBET) and XPS data for catalysts. Catalysts
SBET (m2/g)a
Oα/(Oα+Oβ) (%)b
Co2+/ (Co2++Co3+) (%)b
LC LCP0.02 LCP0.04 LCP0.06 LCP0.1 LCP0.06-2 LCP0.06-4 A-LC A-LCP0.06 A-LCP0.06-4
4.355 / / 3.920 4.607 23.846 43.151 17.817 9.863 14.575
56.93 58.59 60.89 61.79 61.24 72.89 95.71 54.79 61.26 95.62
38.03 40.80 40.83 44.73 43.79 48.48 53.90 36.22 43.70 53.67
a b
Obtained from the nitrogen physisorption at −196℃. Obtained from the XPS data.
Fig. 7. XRD patterns of different aged catalysts.
still decreases about 8 °C when the times of modification increases to 4. As we all know, the surface property of catalyst is the decisive factors affecting its catalytic activity (Jian et al., 2009). It indicates that H2O2 solution activates the surface of the catalyst resulting in removing soot effectively. To investigate the influence of NOx on soot oxidation, the TGA measurement is performed under 2000 ppmNO/air (Fig. 1c). Obviously, the NO plays an active role in soot oxidation. The temperature of soot oxidation shifts to a lower range, especially the T50 decreases 26 °C when NO is introduced. The kinetics of the catalytic reaction also is considered. Fig. 1d shows the Ozawa plots to obtain the Ea which is respectively 105.5, 107.4 and 109.2 kJ/mol at 25%, 50% and 70% soot conversion and the average value of Ea is 107.3 kJ/mol which is a lower level. It indicates that the LCP0.06-4 as a great catalyst has a higher
intrinsic activity for the catalyst in the soot oxidation. Moreover, the soot cycles test is used to evaluate the stability of catalyst (Fig. 1e). The result shows that LCP0.06-4 catalyst has an excellent stability for soot oxidation. The T50 only decreases 10 °C after 4 cycles. Fig. 2 shows H2-TPR graph of all catalysts to evaluate the reducibility. Two reduction peaks are observed in the range of 350 °C–500 °C and 500 °C–650 °C for the LC catalyst (Fig. 2a). According to the pervious literature (Wu et al., 2018; Ma et al., 2011), the first peak about 350 °C–500 °C is attributes to Co3+→Co2+ (or adsorbed oxygen (Oα)), the second peak about 500 °C–650 °C belongs to Co2+→Co (or lattice oxygen (Oβ)). For the doped catalysts, the temperature of the peaks decreases with the increasing of Pt. It indicates 6
Journal of Hazardous Materials 383 (2020) 121210
L. Zeng, et al.
Fig. 8. XPS spectra of catalysts: (a) Co2p of doped catalysts; (b) Co2p of modified catalysts; (c) Co2p of aged catalysts; (d) O1 s of doped catalysts; (e) O1 s of modified catalysts; (f) O1 s of aged catalysts.
perovskite structure (Fig. 3c). However, the diffraction peaks at 32.5°–33.5° merge into one peak, which indicates that the perovskite lattice has a high symmetry (rhombohedral phase transformed into cubic phase) after modification. The surface morphology of catalysts is observed via SEM (Fig. S1). As shown in Fig. S1a and Fig. S1b, the LCP0.06 exists more pores than the LC. The results of EDS (Fig. S1-b3) show that the content of atom is close to the theoretical value (Pt = 6%). In addition, the modified catalysts exhibit similar pore structure while the surface roughness increases. And the roughness increases with the times of modification. The SEM-EDS mapping of LCP0.06-4 reveals that all elements shows a high dispersity (Fig. S2). The microstructure of catalysts is observed via TEM in Fig. 4. The particle sizes of the LC and the LCP0.06 range from 100 nm to 300 nm (Fig. 4-a1 and -b1), and decrease to 50–100 nm after modification with H2O2 (Fig. 4-c1 and -d1). In addition, an amorphous layer about 3–5 nm appears on the surface of modified catalysts (Fig. 4-c2 and -d2). However, the exposed crystal plane (110) of doped catalyst has no change after modification. It indicates that the surface modification only effects the surface morphology of perovskite. According to the previous report (Pahalagedara et al., 2012), the improvement of performance after modification may be attributed to the amorphous layers on surface of perovskite. In order to explore the effect of surface modification on surface area of catalysts, N2 adsorption-desorption isotherms and pore size distribution are detected (Fig. 5) and the SBET data is listed in Table 3. All catalysts exhibit a type Ⅳ isotherm and a H3 hysteresis loop (Fig. 5a). Obviously, the catalysts exhibit a micropore structure before modification of perovskite (Fig. 5b). The structure turned to a mesoporous structure completely with the times of modification increasing to 4. In Table 3, the surface area of doped catalysts shows the following order: LCP0.06 < LC < LCP0.1, which is not consistent with the order of activity of the catalysts. It indicates that the surface area of doped catalysts is not a factor affecting catalytic activity. However, the surface area of modified catalysts evidently increases compared with the LCP0.06, which is consistent with the order of activity of the modified catalysts.
that the reducibility of LaCoO3 improves with the partial replacement. This phenomenon may be explained by synergistic effect between Co3+ and Pt2+ which limits the mobility of Oβ at high-temperature (Wu et al., 2018). However, the temperature increases to a higher temperature (at least 5 °C) when x = 0.1. It implies that the interaction between Co3+ and Pt2+ is too strong to enhance the reducibility of the catalyst. No peaks of Pt are observed in our work due to a small amount of Pt. Anuj Bisht et al. (Bisht et al., 2018) reported that the reducibility of La0.8Sr0.2Co0.95Pt0.05O3 without reduction peaks of Pt, which is similar to our result. In Fig. 2b, four peaks (Pt4+→Pt2+ (177 °C), Pt2+→Pt (222 °C) (Bisht et al., 2015), Co3+→Co2+ or Oα (336 °C) and Co2+→Co or Oβ (505 °C) (Wu et al., 2018; Ma et al., 2011)) are detected. Compares to the doped catalyst, the peaks of Co3+→Co2+ or Oα of modified catalysts decrease 62 °C. However, the peaks of Co2+→Co or Oβ increase 39 °C. It indicates that the Co3+ is easier reduced to Co2+ at a lower temperature in the lattice of modified catalyst. Moreover, the result proves that Oα could easier appear at a lower temperature. The Oα is a strong evidence for the performance of catalysts (Wang et al., 2016). Therefore, it implies that the modified catalysts have a better reducibility. In addition, the unchanged reduction temperature of the modified catalysts indicates that the times of modification have no effect on reducibility of catalysts. Two peaks of Pt ion are observed at 177 °C and 222 °C, respectively, which indicates that the Pt ions transform to Pt in two steps (Pt4+→Pt2+→Pt). Meanwhile, it implies that the Pt ions is easier detected due to the interaction between H2O2 and perovskite. 3.2. The morphology and structural characterizations The XRD patterns of catalysts are showed in Fig. 3. All doped catalysts exhibit an ideal perovskite structure (PDF#48-0123) with a space group of R-3c (Fig. 3a). The diffraction peaks shift to a lower angle with the increasing of Pt (Fig. 3b), which indicates that a lattice expansion of LaCo1-xPtxO3 occurs due to a larger ionic radius of the Pt2+ (0.086 nm) compares to the Co3+ (0.062 nm) (Wang et al., 2014). No phase such as PtO2 is detected, which could confirm Pt atoms are introduced into the lattice of LaCoO3. In addition, all modified catalysts remain the ideal 7
Journal of Hazardous Materials 383 (2020) 121210
L. Zeng, et al.
In addition, the surface area of the doped catalysts increases about 5.943 m2/g (Table 3). The surface area of the A-LCP0.06-4 decreases 28.576 m2/g, and the T50 only increases 2 °C, which excludes the effect of BET on activity of the catalysts after age. 3.4. Surface states of catalysts The XPS is performed to evaluate the surface chemical state of the catalysts (Fig. 8 and Fig. S3). In Fig. S3, the peaks at 77.1˜77.9 eV and 73.8 eV are attributed to Pt4+ and the peaks at 76.4 eV and 73.1 eV are attributed to Pt2+ (Bisht et al., 2015). It indicates that the interaction between Co ion and Pt ion leads to Pt2+→Pt4+. And the detailed reaction process could follow the formula: Pt 2 + + 2Co3 + → Pt 4 + + 2Co2 +. Moreover, no Pt0 is detected, which further proves that Pt atoms are introduced into the perovskite lattice. For all Co 2p spectra of catalysts, the Co3+ (peaks at 779.3 eV and 794.4 eV), the Co2+ (peaks at 781 eV and 796.5 eV), and the shake-up satellites (Li et al., 2010; Wu et al., 2018; An et al., 2018) are obviously observed. Generally speaking, the Co2+ could create more OV or the Pt4+ in order to maintain the electric neutrality (Wu et al., 2018). The OV would move to the surface of catalyst and form the defect sites which capture oxygen species to form the active oxygen species (the main factor affecting catalytic activity). The detailed process is summed up in two formulas (Pt 2 + + 2Co3 + → Pt 4 + + 2Co2 + and x − + + 2 3 Co − OV + 2 O2 → Co − Ox ). Therefore, the changes of the catalytic activity can be explained by the number of Co2+. The ratio of the Co2+/(Co2++Co3+) is calculated (Table 3) to evaluate the number of Co2+, which increases with the substitution of Pt (except x = 0.1). After modification, the ratio increases at least 3.75%. However, the ratio reduces slightly for the aged catalysts, and follows the order: ALCP0.06-4 > A-LCP0.06 > A-LC. Besides, the O1s spectra are detected (Fig. 8).The peaks at 528.6 eV and 531.5 eV can be attributed to the Oβ and the Oα, respectively (Liang et al., 2017). Moreover, the ratio of the Oα/(Oα+Oβ) (Table 3) is used to evaluate the number of Oα which relates to the activity of catalyst (Urán et al., 2019; Liang et al., 2017). Obviously, the trend of ratio of the Oα/(Oα+Oβ) is similar with the ratio of the Co2+/(Co2++Co3+). The phenomenon can be explained by the Co2+ creates more the OV, which moves to the surface of catalyst and captures more oxygen species to form Oα. A large amount of Oα is adsorbed on the surface of the modified catalysts before or after age (Table 3). Especially, the ratio of the Oα/(Oα+Oβ) of the LCP0.06-4 reaches 95.71%, which indicates that the surface of modified catalyst can easily adsorb oxygen species to form Oα. The correlation among the Co2+/(Co2++Co3+), the Oα/(Oα+Oβ) and the T50 are analyzed in Fig. S4. The activity of catalyst increases with the ratio of the Co2+/(Co2++Co3+) and the Oα/(Oα+Oβ), which confirms that the number of Co2+ and Oα can lead to the change of the catalytic activity.
Fig. 9. Scheme of the reaction mechanism for soot combustion over perovskite catalysts: (Ⅰ) LCP0.06 catalyst; (Ⅱ) LCP0.06-4 catalyst; (Ⅲ) A-LCP0.06-4 catalyst.
3.3. Thermal stability test of catalysts The catalysts were heated to 800 °C for 50 h under air atmosphere to evaluate the practicability. All aged catalysts show a similar activity trend compared with the fresh catalysts (Fig. 6): A-LCP0.06-4 (T50 = 405 °C) > A-LCP0.06 (T50 = 440 °C) > A-LC (T50 = 462 °C). The T50 of A-LCP0.06-4 and A-LCP0.06 only increases 2–3 °C, and the T50 of ALC increases 12 °C compares to the fresh catalysts. It indicates that the thermal stability of catalyst is evidently promoted after introducing the Pt into the B-sites of LaCoO3, or modifying the catalyst. Fig. 7 shows XRD pattern of the aged catalysts. Both the A-LCP0.06 and the A-LC maintain a pure perovskite phase (PDF#48-0123). However, some impurities (La2O3, CoO and Pt) are observed in A-LCP0.06-4, which indicates that the H2O2 reacts with the surface of perovskite, resulting in an unstable structure of modified catalyst. After adding active sites at adjacent region of precious metal which shows a certain promoting effect on the activity of perovskite (Ai et al., 2018a). It may result in the A-LCP0.06-4 catalyst has no obvious deactivation.
4. Discussion A series of the perovskite catalysts LaCo1-xPtxO3 (x = 0, 0.02, 0.04, 0.06, 0.1) (via sol-gel method) were modified with 30% H2O2. All catalysts show an ideal perovskite structure (Fig. 3). Interestingly, the asymmetry structure gradually transforms to a symmetry structure after modifying the catalysts (Fig. 3). The surface of the modified catalysts becomes rough (Fig. S1) which is confirmed by the TEM, that the increased roughness is attributed to the amorphization (about 3–5 nm) of surface (Fig. 4). On the one hand, the doped catalyst could create the Co2+ which leads to the increasing of the OV (Ai et al., 2018a; An et al., 2018). Subsequently, the OV would move to the surface of catalyst and form the defect sites which capture oxygen species and form the Oα. Actually, the Oα is oxygen species such as Ox− (x = 1 or 2) absorbed on the surface of catalyst, which is mentioned in the previous report (Wang 8
Journal of Hazardous Materials 383 (2020) 121210
L. Zeng, et al.
et al., 2016; Li et al., 2012). The Ox-species as the reactivity descriptor is used to explain the mechanism of soot oxidation frequently (Wang et al., 2018a,b). And the reduction-oxidation process of oxygen species O2(g ) ↔ O(*ads) ( the surfa is about as follows: − 2− . The Ox- species (x = 1 or ce oxygen in the defect sites) ↔ O2( ↔ O ads ) 2(ads ) 2) can react quickly with soot to remove it, which means that the catalyst with large amounts of the Ox- species (Oα) has a higher activity for soot oxidation. For the modified catalyst, the surface of LCP0.06-4 enriches a majority of the Oα (over 95%) (Table 3), which indicates that the amorphous surface can easier absorb the oxygen species to form the Oα. It implies that the amorphous layer plays another critical role in promoting the catalytic performance, resulting in that the modified catalyst exhibits an excellent performance of soot oxidation compared with the doped catalyst. On the other hand, the catalyst with same element content and a different structure (symmetry structure or asymmetric structure) exhibits a difference evident activity. The high symmetry catalyst (LCP0.06-4) shows an excellent activity by accelerating the movement of the OV aggregated on the surface of the modified catalyst. The surface, with a large amount of the OV, would capture more the oxygen species to form Oα which promotes the activity of catalyst (The Oα has close correlation with the activity of catalyst, which has been discussed above). Moreover, the OV transport process of the soot combustion is faster than the decomposition of oxygen gas (Liu et al., 2017, 2016). Besides, the effect of surface areas on the catalytic reaction also is crucial. The BET results indicate that the surface areas of the catalysts increase obviously (from 4.607 to 43.151 m2/g) after modification with H2O2. And the modified catalysts show the mesoporous structure which is not detected in the doped catalysts. The mesoporous structure is more conducive to enhance the surface areas of catalysts. The larger surface areas could expose more active sites to ensure the effective contact between soot and catalysts. It could affect the performance of the modified catalysts. The in-situ DRIFTS spectra and the Raman spectra are used to deduce the reaction processes of soot. In the in-situ DRIFTS spectra (Fig. S5), the transformation process of soot at the characteristic temperature (T10 (344℃) and T50 (403℃) is detected. The peaks of the BO6 (590 cm−1), the coordinated COX (1066 cm−1), the carbonates (1450 cm−1) and the CO2 (2337 cm−1, 2371 cm−1) are observed. The BO6 reveals the structure of catalyst is the perovskite oxide, which agrees well with XRD. The existence of coordinated COX and carbonates reveals the transformation from C (soot) to COX. Moreover, the peaks of CO2 are observed no matter whether the spectra are detected at the T10 or at T50, which implies that the soot is completely converted to CO2 in the process of catalytic oxidation. In the Raman spectra (Fig. S6), the peaks of La F2g (149 cm−1), the peaks of Co(Pt)O6 (460 cm−1, 521 cm−1, 640 cm−1, 670 cm−1) and the peaks of soot (1357 cm−1, 1600 cm−1) are observed. The existence of La F2g and Co(Pt)O6 confirms the perovskite structure, which agrees with XRD and in-situ DRIFTS. The soot peaks of the LCP0.06-4/C are detected, which can′t observed after the TGA test (1 st). It indicates that the soot transforms into CO2 completely. The mechanism of the whole reaction is showed in Fig. 9. The activity of the aged catalysts shows a similar trend with the fresh catalysts in Fig. 6 (A-LCP0.06-4 > A-LCP0.06 > A-LC). The ALCP0.06-4 catalyst presents some impurities such as Pt, CoO and La2O3, which proves that the modified catalyst exhibits an unstable structure after aging (Fig. 7). However, the activity of A-LCP0.06-4 and the number of Oα have no evident fluctuation (Figs. 6b and 8). The phenomenon can be attributed to the precious metal which has a certain promoting on the soot oxidation of perovskite (Ai et al., 2018a). Moreover, we exclude the effect of BET on the activity of the aged catalyst (Table 3). According to the discussion of the OV transport mechanism, the scheme of the reaction mechanism for the A-LCP0.06-4 catalyst is summarized in Fig. 9(Ⅲ).
5. Conclusion In summary, the perovskite catalysts (via sol-gel method) are modified with 30% H2O2 solution and applied to soot oxidation. The XRD result shows that the modified catalysts have a high symmetry structure which benefits to the movement of OV. The TEM images indicate that the surface of the modified catalyst has an amorphous layer which can easier capture the surface oxygen species to form the adsorbed oxygen. After modification, the catalyst exhibited an excellent activity (T50 =403 °C). Meanwhile, the reducibility of modified catalyst is superior to other catalysts. According to the XPS result, there is a close connection between the adsorbed oxygen and the performance of catalyst. The performance for soot oxidation increases with the adsorbed oxygen. For the aged catalyst, no obvious deactivation phenomenon is observed which can be attributed to the Pt particles. Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.jhazmat.2019.121210. References Agathokleous, E., Belz, R.G., Calatayud, V., De Marco, A., Hoshika, Y., Kitao, M., Saitanis, C.J., Sicard, P., Paoletti, E., Calabrese, E.J., 2019. Predicting the effect of ozone on vegetation via linear non-threshold (LNT), threshold and hormetic dose-response models. Sci. Total Environ. 649, 61–74. Ai, L., Wang, Z., Gao, Y., Cui, C., Wang, B., Liu, W., Wang, L., 2018a. Effect of surface and bulk palladium doping on the catalytic activity of La2Sn2O7 pyrochlore oxides for diesel soot oxidation. J. Mater. Sci. 54, 4495–4510. Ai, L., Wang, Z., Cui, C., Liu, W., Wang, L., 2018b. Catalytic oxidation of soot on a novel active Ca-Co dually-doped lanthanum tin pyrochlore oxide. Materials (Basel) 11. An, S.R., Song, K.H., Lee, K.Y., Park, K.T., Jeong, S.K., Kim, H.J., 2018. Fe-doped LaCoO3 perovskite catalyst for NO oxidation in the post-treatment of marine diesel engine’s exhaust emissions. Korean J. Chem. Eng. 35, 1807–1814. Andana, T., Piumetti, M., Bensaid, S., Russo, N., Fino, D., Pirone, R., 2016. Nanostructured ceria-praseodymia catalysts for diesel soot combustion. Appl. Catal. B-Environ. 197, 125–137. Andana, T., Piumetti, M., Bensaid, S., Veyre, L., Thieuleux, C., Russo, N., Fino, D., Quadrelli, E.A., Pirone, R., 2018. Nanostructured equimolar ceria-praseodymia for NOx-assisted soot oxidation: insight into Pr dominance over Pt nanoparticles and metal–support interaction. Appl. Catal. B-Environ. 226, 147–161. Białobok, B., Trawczyński, J., Rzadki, T., Miśta, W., Zawadzki, M., 2007. Catalytic combustion of soot over alkali doped SrTiO3. Catal. Today 119, 278–285. Bisht, A., Sihag, A., Satyaprasad, A., Mallajosyala, S.S., Sharma, S., Metal, P., 2018. Supported and Pt4+ doped La1−xSrxCoO3: non-performance of Pt4+ and reactivity differences with Pt metal. Catal. Lett. 148, 1965–1977. Bisht, A., Zhang, P., Shivakumara, C., Sharma, S., 2015. Pt-doped and Pt-supported La1–xSrxCoO3: comparative activity of Pt4+and Pt0 toward the CO poisoning effect in formic acid and methanol electro-oxidation. Catal. Lett. 119, 14126–14134. Cheng, L., Men, Y., Wang, J., Wang, H., An, W., Wang, Y., Duan, Z., Liu, J., 2017. Crystal facet-dependent reactivity of α-Mn2O3 microcrystalline catalyst for soot combustion. Appl. Catal. B-Environ. 204, 374–384. Comelli, N.A., Ruiz, M.L., Aparicio, M.S.L., Merino, N.A., Cecilia, J.A., RodríguezCastellón, E., Lick, I.D., Ponzi, M.I., 2018. Influence of the synthetic conditions on the composition, morphology of CuMgAl hydrotalcites and their use as catalytic precursor in diesel soot combustion reactions. Appl. Clay Sci. 157, 148–157. Dhal, G.C., Dey, S., Mohan, D., Prasad, R., 2018. Simultaneous abatement of diesel soot and NOX emissions by effective catalysts at low temperature: an overview. Catal. Rev.-Sci. Eng. 60, 437–496. Fino, D., Bensaid, S., Piumetti, M., Russo, N., 2016. A review on the catalytic combustion of soot in diesel particulate filters for automotive applications: from powder catalysts to structured reactors. Appl. Catal. A-Gen. 509, 75–96. Guillén-Hurtado, N., García-García, A., Bueno-López, A., 2015. Active oxygen by Ce–Pr mixed oxide nanoparticles outperform diesel soot combustion Pt catalysts. Appl. Catal. B-Environ. 174–175, 60–66. Hernández, W.Y., Lopez-Gonzalez, D., Ntais, S., Zhao, C., Boréave, A., Vernoux, P., 2018. Silver-modified manganite and ferrite perovskites for catalyzed gasoline particulate filters. Appl. Catal. B-Environ. 226, 202–212. Hong, S.S., Lee, G.D., 2000. Simultaneous removal of NO and carbon particulates over lanthanoid perovskite-type catalysts. Catal. Today 63, 397–404. Huang, X., Pan, H., Chen, K., Yin, Z., Hong, J., 2018. Facile synthesis of porous spherical La0.8Sr0.2Mn1−xCuxO3 (0≤x≤0.4) and nanocubic La0.8Sr0.2MnO3 with high catalytic activity for CO. Crystengcomm 20, 7020–7029. Ji, F., Men, Y., Wang, J., Sun, Y., Wang, Z., Zhao, B., Tao, X., Xu, G., 2019. Promoting diesel soot combustion efficiency by tailoring the shapes and crystal facets of nanoscale Mn3O4. Appl. Catal. B-Environ. 242, 227–237. Jian, L., Zhen, Z., Jie, L., Xu, C., Duan, A., Jiang, G., Wang, X., Hong, H., 2009. Catalytic combustion of soot over the highly active (La0.9K0.1CoO3) x/nmCeO2 catalysts. J.
9
Journal of Hazardous Materials 383 (2020) 121210
L. Zeng, et al.
electrochemical performance. J. Power Sources 274, 1024–1033. Tsvetkov, N., Lu, Q., Sun, L., Crumlin, E.J., Yildiz, B., 2016. Improved chemical and electrochemical stability of perovskite oxides with less reducible cations at the surface. Nat. Mater. 15, 1010–1016. Urán, L., Gallego, J., Li, W.Y., Santamaría, A., 2019. Effect of catalyst preparation for the simultaneous removal of soot and NOx. Appl. Catal. A-Gen. 569, 157–169. van Setten, B.A.A.L., Makkee, M., Moulijn, J.A., 2001. Science and technology of catalytic diesel particulate filters. Catal. Rev.-Sci. Eng. 43, 489–564. Vinodkumar, T., Mukherjee, D., Subrahmanyam, C., Reddy, B.M., 2018. Investigation on the physicochemical properties of Ce0.8Eu0.1M0.1O2−δ (M = Zr, Hf, La, and Sm) solid solutions towards soot combustion. New J. Chem. 42, 5276–5283. Wang, L., Fang, S., Feng, N., Wan, H., Guan, G., 2016. Efficient catalytic removal of diesel soot over Mg substituted K/La0.8Ce0.2CoO3 perovskites with large surface areas. Chem. Eng. J. 293, 68–74. Wang, X., Qi, X., Chen, Z., Jiang, L., Wang, R., Wei, K., 2014. Studies on SO2 tolerance and regeneration over perovskite-type LaCo1–xPtxO3 in NOx storage and reduction. J. Phys. Chem. C 118, 13743–13751. Wang, H., Luo, S., Zhang, M., Liu, W., Wu, X., Liu, S., 2018a. Roles of oxygen vacancy and O− in oxidation reactions over CeO2 and Ag/CeO2 nanorod model catalysts. J. Catal. 368, 365–378. Wang, H., Jin, B., Wang, H., Ma, N., Liu, W., Weng, D., Wu, X., Liu, S., 2018b. Study of Ag promoted Fe2O3@CeO2 as superior soot oxidation catalysts: the role of Fe2O3 crystal plane and tandem oxygen delivery. Appl. Catal. B-Environ. 237, 251–262. Wu, Y., Li, L., Chu, B., Yi, Y., Qin, Z., Fan, M., Qin, Q., He, H., Zhang, L., Dong, L., Li, B., Dong, L., 2018. Catalytic reduction of NO by CO over B-site partially substituted LaM0.25Co0.75O3 (M = Cu, Mn, Fe) perovskite oxide catalysts: the correlation between physicochemical properties and catalytic performance. Appl. Catal. A-Gen. 568, 43–53. Wu, X., Feng, Y., Du, Y., Liu, X., Zou, C., Li, Z., 2019a. Enhancing DeNOx performance of CoMnAl mixed metal oxides in low-temperature NH3-SCR by optimizing layered double hydroxides (LDHs) precursor template. Appl. Surf. Sci. 467–468, 802–810. Wu, Q., Xiong, J., Zhang, Y., Mei, X., Wei, Y., Zhao, Z., Liu, J., Li, J., 2019b. Interactioninduced self-assembly of Au@La2O3 core–Shell nanoparticles on La2O2CO3 nanorods with enhanced catalytic activity and stability for soot oxidation. ACS Catal. 9, 3700–3715. Wu, Q., Jing, M., Wei, Y., Zhao, Z., Zhang, X., Xiong, J., Liu, J., Song, W., Li, J., 2019c. High-efficient catalysts of core-shell structured Pt@transition metal oxides (TMOs) supported on 3DOM-Al2O3 for soot oxidation: the effect of strong Pt-TMO interaction. Appl. Catal. B-Environ. 244, 628–640. Xie, Y., Galvez, M.E., Matynia, A., Da Costa, P., 2017. Experimental investigation on the influence of the presence of alkali compounds on the performance of a commercial Pt–Pd/Al2O3 diesel oxidation catalyst. Clean Technol. Environ. Policy 20, 715–725. Xiong, J., Mei, X., Liu, J., Wei, Y., Zhao, Z., Xie, Z., Li, J., 2019. Efficiently multifunctional catalysts of 3D ordered meso-macroporous Ce0.3Zr0.7O2-supported PdAu@CeO2 coreshell nanoparticles for soot oxidation: synergetic effect of Pd-Au-CeO2 ternary components. Appl. Catal. B-Environ. 251, 247–260. Xiong, J., Wu, Q., Mei, X., Liu, J., Wei, Y., Zhao, Z., Wu, D., Li, J., 2018. Fabrication of spinel-type PdxCo3–xO4 binary active sites on 3D ordered meso-macroporous Ce-ZrO2 with enhanced activity for catalytic soot oxidation. ACS Catal. 8, 7915–7930.
Phys. Chem. C 113, 17114–17123. Köferstein, R., Ebbinghaus, S.G., 2013. Synthesis and characterization of nano-LaFeO3 powders by a soft-chemistry method and corresponding ceramics. Solid State Ion. 231, 43–48. Lam, K., Gao, Y., Wang, J., Ciucci, F., 2017. H2O2Treated La0.8Sr0.2CoO3-δ as an efficient catalyst for oxygen evolution reaction. Electrochim. Acta 244, 139–145. Lebid, M., Omari, M., 2013. Synthesis and electrochemical properties of LaFeO3 oxides prepared via sol–gel method. Arab. J. Sci. Eng. 39, 147–152. Ledwa, K.A., Pawlyta, M., Kępiński, L., 2018. RuxCe1-xO2-y nanoparticles deposited on functionalized γ-Al2O3 as a thermally stable oxidation catalyst. Appl. Catal. BEnviron. 230, 135–144. Li, Z., Meng, M., Li, Q., Xie, Y., Hu, T., Zhang, J., 2010. Fe-substituted nanometric La0.9K0.1Co1−xFexO3−δ perovskite catalysts used for soot combustion, NOx storage and simultaneous catalytic removal of soot and NOx. Chem. Eng. J. 164, 98–105. Li, Z., Meng, M., Zha, Y., Dai, F., Hu, T., Xie, Y., Zhang, J., 2012. Highly efficient multifunctional dually-substituted perovskite catalysts La1−xKxCo1−yCuyO3−δ used for soot combustion, NOx storage and simultaneous NOx-soot removal. Appl. Catal. BEnviron. 121–122, 65–74. Liang, H., Mou, Y., Zhang, H., Li, S., Yao, C., Yu, X., 2017. Sulfur resistance and soot combustion for La0.8K0.2Co1-yMnyO3 catalyst. Catal. Today 281, 477–481. Liu, S., Wu, X., Weng, D., Li, M., Fan, J., 2013. Sulfation of Pt/Al2O3 catalyst for soot oxidation: high utilization of NO2 and oxidation of surface oxygenated complexes. Appl. Catal. B-Environ. 138–139, 199–211. Liu, S., Wu, X., Weng, D., Ran, R., 2015. Ceria-based catalysts for soot oxidation: a review. J. Rare Earths 33, 567–590. Liu, M., Wu, X., Liu, S., Gao, Y., Chen, Z., Ma, Y., Ran, R., Weng, D., 2017. Study of Ag/ CeO2 catalysts for naphthalene oxidation: balancing the oxygen availability and oxygen regeneration capacity. Appl. Catal. B-Environ. 219, 231–240. Liu, S., Wu, X., Liu, W., Chen, W., Ran, R., Li, M., Weng, D., 2016. Soot oxidation over CeO2 and Ag/CeO2: factors determining the catalyst activity and stability during reaction. J. Catal. 337, 188–198. Ma, Z., Gao, X., Yuan, X., Zhang, L., Zhu, Y., Li, Z., 2011. Simultaneous catalytic removal of NOx and diesel soot particulates over La2−xAxNi1−yByO4 perovskite-type oxides. Catal. Commun. 12, 817–821. Matarrese, R., Morandi, S., Castoldi, L., Villa, P., Lietti, L., 2017. Removal of NOx and soot over Ce/Zr/K/Me (Me = Fe, Pt, Ru, Au) oxide catalysts. Appl. Catal. B-Environ. 201, 318–330. Ozawa, T., 1970. Kinetic analysis of derivative curves in thermal analysis. J. Therm. Anal. Calorim. 2, 301–324. Pahalagedara, L., Sharma, H., Kuo, C.-H., Dharmarathna, S., Joshi, A., Suib, S.L., Mhadeshwar, A.B., 2012. Structure and oxidation activity correlations for carbon blacks and diesel soot. Energy Fuels 26, 6757–6764. Shang, Z., Sun, M., Che, X., Wang, W., Wang, L., Cao, X., Zhan, W., Guo, Y., Guo, Y., Lu, G., 2017. The existing states of potassium species in K-doped Co3O4 catalysts and their influence on the activities for NO and soot oxidation. Catal. Sci. Technol. 7, 4710–4719. Su, C., Xu, X., Chen, Y., Liu, Y., Tadé, M.O., Shao, Z., 2015. A top-down strategy for the synthesis of mesoporous Ba0.5Sr0.5Co0.8Fe0.2O3−δ as a cathode precursor for buffer layer-free deposition on stabilized zirconia electrolyte with a superior
10
Contents lists available at ScienceDirect
Journal of Hazardous Materials journal homepage: www.elsevier.com/locate/jhazmat
In-situ modified the surface of Pt-doped perovskite catalyst for soot oxidation Lirong Zenga, Lan Cuib, Caiyun Wanga, Wei Guoa, Cairong Gonga, a b
T
⁎
Insitute of New Energy, School of Materials Science and Engineering, Tianjin University, 300072, PR China Center of Analysis, Tianjin University, Tianjin 300072, PR China
G R A P H I C A L A B S T R A C T
A R T I C LE I N FO
A B S T R A C T
Editor: Danmeng Shuai
In-situ modification is studied in this work on LaCo1-xPtxO3 for soot oxidation. A series of perovskite catalysts LaCo1-xPtxO3 (by sol-gel method) are modified with 30% H2O2. XRD, TEM, SEM, BET, XPS, Raman, in-situ DRIFTS, H2-TPR and TGA are used to investigate physicochemical properties of catalysts. TGA results show that all doped catalysts have a lower temperature of soot conversion, especially LaCo0.94Pt0.06O3 (T50 = 437 °C). The T50 of the catalyst with modification by H2O2 solution decreases at least 20 °C compared with the doped catalysts. A highly symmetrical structure and an obvious amorphous layer about 3–5 nm are observed in the modified catalysts. According to the XPS study, the symmetrical structure benefits to the movement of oxygen vacancy thus catalyst captures more adsorbed oxygen (about 95%). And the amorphous surface could adsorb more oxygen species. In addition, all catalysts show excellent aging resistance performance. The reaction mechanism of catalyst for soot oxidation is presented in the end.
Keywords: Surface modification Symmetrical structure Amorphization Aging resistance
1. Introduction With the rapid development of economy, the number of motor vehicles kept increasing. Diesel engines duo to their high efficiency and remarkable fuel-saving effect have been applied widely on motor vehicles (Huang et al., 2018). However, particles matter (PM), nitric oxide
⁎
(NOx), carbon monoxide (CO) and hydrocarbon (HC) emission by diesel vehicle were harm for the environment and human health (Matarrese et al., 2017; Andana et al., 2016; Shang et al., 2017; Cheng et al., 2017). Particularly, the emission of PM was about dozens of times than gasoline engine (Dhal et al., 2018). To address this problem, many posttreatment technologies have been developed in global over the years
Corresponding author. E-mail address: [email protected] (C. Gong).
https://doi.org/10.1016/j.jhazmat.2019.121210 Received 11 June 2019; Received in revised form 9 September 2019; Accepted 10 September 2019 Available online 11 September 2019 0304-3894/ © 2019 Elsevier B.V. All rights reserved.
Journal of Hazardous Materials 383 (2020) 121210
L. Zeng, et al.
Fig. 1. The active evaluation of catalysts: (a) the soot conversion of the doped catalysts under air; (b)the soot conversion of the modified catalysts under air; (c) the soot conversion of LCP0.06-4 (air or 2000 ppmNO/air); (d) the kinetics of the catalytic reaction under air (Ozawa plots and Ea); (e) the soot cycle of LCP0.06-4 (air).
without catalyst, which exceeded the normal exhaust temperature range (200 °C˜500 °C) of diesel engines (Fino et al., 2016). Therefore, the adoption of catalyst was essential to ensure normal operation of DPF at the lower temperature. Up to now, researches of catalysts for soot oxidation were mainly focused on the noble metals and metal oxides. Noble metal catalysts (such as Pt, Pd, Au etc.) were applied in industrial production due to the good catalytic activity and selectivity (Xie et al., 2017; Andana et al., 2018; Wu et al., 2019b; Xiong et al., 2019; Wu et al., 2019c; Xiong et al., 2018). Moreover, Pt-based catalysts were among the most sulfurresistant soot combustion catalysts, which could tolerate much more SO2 in comparison with the non-noble metal samples (Liu et al., 2013, 2015). However, the cost of noble metal had to be considered. The metal oxides with multifarious valence and strong redox could wildly apply in removing soot particles. For example, Ji et al. (Ji et al., 2019) studied the catalytic activity of Mn3O4 catalysts with Special shapes via facile hydrothermal and co-precipitation methods. They found that the
Table 1 The catalytic activities of catalysts. Catalysts
T10 (°C)
T50 (°C)
T90 (°C)
LC LCP0.02 LCP0.04 LCP0.06 LCP0.1 LCP0.06-2 LCP0.06-4
395 390 391 390 400 355 344
450 449 444 437 447 411 403
498 487 482 477 486 448 435
such as diesel particulate filter (DPF) (van Setten et al., 2001), lean NOx trap (LNT) (Agathokleous et al., 2019) and NOx selective catalytic reduction (SCR) (Wu et al., 2019a). DPF that removed soot particles (the basis of PM) was considered as the most potential and efficient technology due to the regeneration (Białobok et al., 2007). However, the temperature of DPF system must over 600 °C to remove soot particles 2
Journal of Hazardous Materials 383 (2020) 121210
L. Zeng, et al.
Table 2 Comparison of diesel soot combustion catalysts of different nature tested in realistic laboratory conditions. Catalysts
T50 (°C)
Atmosphere
Heating rate (°C/min)
Reference
Ce0.5Pr0.5O2 LaSr25AgFe LaSr25AgMn LCSC C40nw Ce0.8Eu0.1La0.1 RuxCe1-xO2-y Pd/LS LCP0.06 LCP0.06-2 LCP0.06-4
444 509 449 451 448 446 487 463 437 411 403
O2/N2 1%O2/He 1%O2/He O2 O2/He air air O2 air air air
10 10 10 4 10 10 3 4 10 10 10
(Guillén-Hurtado et al., 2015) (Hernández et al., 2018) (Hernández et al., 2018) (Ai et al., 2018b) (Comelli et al., 2018) (Vinodkumar et al., 2018) (Ledwa et al., 2018) (Ai et al., 2018a) This article This article This article
La0.7Ag0.3MnO3 perovskite-like catalyst exhibited a lower soot ignition temperature (160 °C) due to the partial substitution of Ag which affected valence of B-site ions. The catalyst activity had proved to controlled by substitution amount of the A, B sites, perovskite catalyst thus had a great potential in removing soot particles. However, the perovskite via traditional method (sol-gel method etc.) exhibited a larger particle size and a severe agglomeration, which resulted in a noneffective contact with the active sites of catalyst (Lebid and Omari, 2013; Köferstein and Ebbinghaus, 2013). It was necessary to study how to enhance activity of perovskite for soot oxidation. As we all know, the performance of catalyst was closely related to surface state of catalysts. Surface modifications thus had been widely studied to promote the activity of catalyst in different research field. For example, Tsvetkov et al. (2016) modified the surface of La0.8Sr0.2CoO3 through V, Nb, Zr, Co, Ti, Hf and Al chloride solution, which enhanced oxidative activity of surface and stability. Lam et al. (2017) modified the surface of La0.8Sr0.2CoO3 catalyst with H2O2 aqueous solution and enhanced the surface area of catalyst. In addition, they reported that Ba2+ and Sr2+ ions would selectively precipitate from Ba0.5Sr0.5Co0.8Fe0.2O3 lattices during in-situ decomposition of H2O2 aqueous solution, resulting in a certain A-site defect (Su et al., 2015). However, no widely report that perovskite with modification was applied to soot oxidation. And no report on in-situ catalytic decomposition of B-sites (active sites) of perovskite by H2O2. In this work, we synthesized a series of LaCo1-xPtxO3 (x = 0.02, 0.04, 0.06, 0.1) perovskite catalysts via sol-gel method. The physicochemical properties of catalysts were evaluated by XRD, SEM, TEM, BET, XPS, in-situ DRIFTS, Raman and H2-TPR. The catalytic activity was tested by TGA. It was found that the catalytic activity of LaCo1xPtxO3 for soot oxidation was outstanding and increased with the amount of Pt (when x < 0.1). Meanwhile, the surface of LaCo1-xPtxO3 catalysts is modified with 30% H2O2 aqueous solution. The activity of catalysts was improved obviously after modification. Fig. 2. The H2-TPR profiles of catalysts: (a) doped catalysts; (b) modified catalysts.
2. Experimental 2.1. Synthesis of catalyst
hexagonal nanoplates Mn3O4 had superior activity on soot combustion (Tm = 407.7 °C). Liu et al. (Liu et al., 2017) prepared a ceria-based catalyst (via incipient wetness method) which showed a great catalytic activity for soot oxidation (the conversion of CO2 was about 100%). However, the catalysts showed a lower thermal stability at high temperature. Perovskite oxides with high stability, good catalytic activity and flexible structure regulation overcame the shortcomings of the ceria-based catalysts. For example, Hong group (Hong and Lee, 2000) reported a series of perovskite type oxide (LaMO3 (M = Co, Fe, Mn)) prepared by sol-gel method and the LaCoO3 showed the best catalytic activity (Tm = 400 °C). In addition, Li et al. (2010) investigated the activity of LaCoO3 perovskite type oxide catalysts with different metals doped (A = K; B = Fe). Moreover, La0.9K0.1Co0.9Fe0.1O3-δ showed a highest activity and stability (700 °C). Urán et al. (2019) reported
The LaCo1-xPtxO3(x = 0.02, 0.04, 0.06, 0.1) catalysts were synthesized via sol-gel method. The detailed process was as follows: Lanthanum nitrate (La(NO3)3·6H2O), Cobalt nitrate (Co(NO3)3·6H2O) and Platinum nitrate (Pt(NO3)2) were dissolved in deionized water by stoichiometry. And then added citric acid and ethylenediaminetetraacetic acid (EDTA) into the solution and adjusted the pH to 6–7 with ammonia (25%). The solution was heated at 120℃ for 24 h to obtain dry gel. And then the gel heated at 400℃ for 3 h to remove excess water and organics. The LaCo0.94Pt0.06O3 catalyst was subsequently obtained at 800℃ for 5 h and denoted as LCP0.02, LCP0.04, LCP0.06 and LCP0.1 respectively. For comparison, pure LaCoO3 was synthesized in same way and noted as LC. The modified catalysts were prepared by adding 30% H2O2 aqueous 3
Journal of Hazardous Materials 383 (2020) 121210
L. Zeng, et al.
Fig. 3. XRD patterns of catalysts: (a) (b) doped catalysts; (c) (d) modified catalysts.
Based on the TGA data, the absolute temperature (Tη) corresponding to the fixed η (25%, 50%, 70%) were obtained. The Ea could be calculated according to the slope which could be obtained through the relationship between log Ф and Tη−1 after least-squares fit.
solution into beaker containing 100 mg of LCP0.06 catalyst. Solution was kept the at room temperature for 10 h to completely decompose H2O2 and then heated at 80 °C to obtain dry catalyst powder. Repeated the above steps two or four times to obtain the final product and noted as LCP0.06-2, LCP0.06-4 respectively. In addition, the aged samples were obtained through heating catalysts at 800 °C for 50 h under air atmosphere and recorded with series of “A”.
2.3. Characterization of catalyst Temperature-programmed reduction with H2 (H2-TPR) were tested to evaluate reducibility of catalyst. 50 mg catalysts were heated to 400 °C under Ar for 1 h. Then the catalysts were heated to 900 °C under 10% H2 balanced Ar with 10 °C/min. X-ray diffraction (XRD) was performed to test phase structure using Cu Kα radiation (λ = 015418 nm) on the Bruker-D8 Advance. The diffraction patterns were collected from 20° to 80° with a scanning rate of 5°/min. The transmission electron microscopy (TEM) and scanning electron microscope (SEM) were used to obtain morphology by jem-2100f and s4800 from Japan. Nitrogen adsorption–desorption isotherms were carried under the N2 at −196 °C on a NOVA-2000 instrument (Conta, US). And the pore sizes were obtained through the BJH method, the pore volume was calculated at a pressure of 0.986. The X-ray photoelectron spectra (XPS) was measured on KratosAXIS ULT RADLD equipment with monochromatic Al Kα X-ray source (1487.6 eV). The reference value of binding energy was 284.8 eV (C 1s) of carbon. In-situ diffusion reflectance infrared fourier transform spectroscopy (in-situ DRIFTS) was detected by Nicolet iS50 FTIR spectrometer with MCT detector at 32 scans with a spectral resolution of 4 cm−1. The catalyst, Degussa carbon black and KBr (10:1:20) were intermixed in agate mortar to obtain sample. And then the sample was heated to 344 °C and 403 °C respectively under air atmosphere.
2.2. Catalyst activity evaluation The activity of catalyst was evaluated by Thermogravimetric Analysis (TGA). Firstly, the catalyst and Degussa carbon black were intermixed in agate mortar (catalyst to carbon 10:1, loose contact). And then 30 mg samples were put into U-shaped reactor and heated to 800 °C under air or 2000 ppm NO/air with 100 mL/min. Data was recorded by computer in whole process. The soot conversion rate (η) was calculated by the following formula:
η=
Winitial − W0 * 100% Winitial − Wend
(1)
Where Winitial is the initial weight of samples, Wend is the weight after the heating and W0 is the weight of each temperature point. The activation energy (Ea) for soot catalytic oxidation was investigated by TGA on basis of the Ozawa method (Ozawa, 1970). 30 mg samples were put into U-shaped reactor and heated to 800 °C under air with 100 mL/min. Different heating rates Φ (2.5, 5, 7, 9, 10 ℃/min) were selected to calculate the Ea according to the following formula:
log Φ = B − 0.4567(
Ea ) RTη
(2)
Where B is a constant and R is the ideal gas constant (8.314 J/(mol · K)). 4
Journal of Hazardous Materials 383 (2020) 121210
L. Zeng, et al.
Fig. 4. TEM (left) and HRTEM (right) images of catalysts: (a) LC; (b) LCP0.06; (c) LCP0.06-2; (d) LCP0.06-4.
TGA under the air atmosphere. In Fig. 1a, LCP0.06 exhibits the greatest catalytic activity for removing soot at lower temperature (T50 =437 °C, which is lower than the T50 in previous studies (Table 2)). Moreover, the T50 of all doped catalysts is lower than the undoped catalyst (LC, T50 =450 °C). It indicates that the partial replacement of Co in the B site of perovskite with Pt benefits to improve the activity of the catalyst. The T50 rises to 447 °C when x is 0.1 (Table 1). In Fig. 1b and in Table 1, the conversion temperature shows a significant reduction (at least 20 °C) when the catalyst is modified two times with 30% H2O2. And the T50
Raman spectra was tested on inVia reflex equipment with a CCD detector using a visible laser (λ = 532 nm) and the scanning scope was 100–3200 cm−1.
3. Result and discussion 3.1. The catalytic activity of catalysts The activity of catalysts for removing soot particles is evaluated by 5
Journal of Hazardous Materials 383 (2020) 121210
L. Zeng, et al.
Fig. 6. The (a) TGA and (b) T50 images of the aged LC, LCP0.06 and LCP0.06-4 catalysts. Fig. 5. (a) N2 adsorption-desorption isotherms and (b) pore size distribution of catalysts.
Table 3 BET specific surface area (SBET) and XPS data for catalysts. Catalysts
SBET (m2/g)a
Oα/(Oα+Oβ) (%)b
Co2+/ (Co2++Co3+) (%)b
LC LCP0.02 LCP0.04 LCP0.06 LCP0.1 LCP0.06-2 LCP0.06-4 A-LC A-LCP0.06 A-LCP0.06-4
4.355 / / 3.920 4.607 23.846 43.151 17.817 9.863 14.575
56.93 58.59 60.89 61.79 61.24 72.89 95.71 54.79 61.26 95.62
38.03 40.80 40.83 44.73 43.79 48.48 53.90 36.22 43.70 53.67
a b
Obtained from the nitrogen physisorption at −196℃. Obtained from the XPS data.
Fig. 7. XRD patterns of different aged catalysts.
still decreases about 8 °C when the times of modification increases to 4. As we all know, the surface property of catalyst is the decisive factors affecting its catalytic activity (Jian et al., 2009). It indicates that H2O2 solution activates the surface of the catalyst resulting in removing soot effectively. To investigate the influence of NOx on soot oxidation, the TGA measurement is performed under 2000 ppmNO/air (Fig. 1c). Obviously, the NO plays an active role in soot oxidation. The temperature of soot oxidation shifts to a lower range, especially the T50 decreases 26 °C when NO is introduced. The kinetics of the catalytic reaction also is considered. Fig. 1d shows the Ozawa plots to obtain the Ea which is respectively 105.5, 107.4 and 109.2 kJ/mol at 25%, 50% and 70% soot conversion and the average value of Ea is 107.3 kJ/mol which is a lower level. It indicates that the LCP0.06-4 as a great catalyst has a higher
intrinsic activity for the catalyst in the soot oxidation. Moreover, the soot cycles test is used to evaluate the stability of catalyst (Fig. 1e). The result shows that LCP0.06-4 catalyst has an excellent stability for soot oxidation. The T50 only decreases 10 °C after 4 cycles. Fig. 2 shows H2-TPR graph of all catalysts to evaluate the reducibility. Two reduction peaks are observed in the range of 350 °C–500 °C and 500 °C–650 °C for the LC catalyst (Fig. 2a). According to the pervious literature (Wu et al., 2018; Ma et al., 2011), the first peak about 350 °C–500 °C is attributes to Co3+→Co2+ (or adsorbed oxygen (Oα)), the second peak about 500 °C–650 °C belongs to Co2+→Co (or lattice oxygen (Oβ)). For the doped catalysts, the temperature of the peaks decreases with the increasing of Pt. It indicates 6
Journal of Hazardous Materials 383 (2020) 121210
L. Zeng, et al.
Fig. 8. XPS spectra of catalysts: (a) Co2p of doped catalysts; (b) Co2p of modified catalysts; (c) Co2p of aged catalysts; (d) O1 s of doped catalysts; (e) O1 s of modified catalysts; (f) O1 s of aged catalysts.
perovskite structure (Fig. 3c). However, the diffraction peaks at 32.5°–33.5° merge into one peak, which indicates that the perovskite lattice has a high symmetry (rhombohedral phase transformed into cubic phase) after modification. The surface morphology of catalysts is observed via SEM (Fig. S1). As shown in Fig. S1a and Fig. S1b, the LCP0.06 exists more pores than the LC. The results of EDS (Fig. S1-b3) show that the content of atom is close to the theoretical value (Pt = 6%). In addition, the modified catalysts exhibit similar pore structure while the surface roughness increases. And the roughness increases with the times of modification. The SEM-EDS mapping of LCP0.06-4 reveals that all elements shows a high dispersity (Fig. S2). The microstructure of catalysts is observed via TEM in Fig. 4. The particle sizes of the LC and the LCP0.06 range from 100 nm to 300 nm (Fig. 4-a1 and -b1), and decrease to 50–100 nm after modification with H2O2 (Fig. 4-c1 and -d1). In addition, an amorphous layer about 3–5 nm appears on the surface of modified catalysts (Fig. 4-c2 and -d2). However, the exposed crystal plane (110) of doped catalyst has no change after modification. It indicates that the surface modification only effects the surface morphology of perovskite. According to the previous report (Pahalagedara et al., 2012), the improvement of performance after modification may be attributed to the amorphous layers on surface of perovskite. In order to explore the effect of surface modification on surface area of catalysts, N2 adsorption-desorption isotherms and pore size distribution are detected (Fig. 5) and the SBET data is listed in Table 3. All catalysts exhibit a type Ⅳ isotherm and a H3 hysteresis loop (Fig. 5a). Obviously, the catalysts exhibit a micropore structure before modification of perovskite (Fig. 5b). The structure turned to a mesoporous structure completely with the times of modification increasing to 4. In Table 3, the surface area of doped catalysts shows the following order: LCP0.06 < LC < LCP0.1, which is not consistent with the order of activity of the catalysts. It indicates that the surface area of doped catalysts is not a factor affecting catalytic activity. However, the surface area of modified catalysts evidently increases compared with the LCP0.06, which is consistent with the order of activity of the modified catalysts.
that the reducibility of LaCoO3 improves with the partial replacement. This phenomenon may be explained by synergistic effect between Co3+ and Pt2+ which limits the mobility of Oβ at high-temperature (Wu et al., 2018). However, the temperature increases to a higher temperature (at least 5 °C) when x = 0.1. It implies that the interaction between Co3+ and Pt2+ is too strong to enhance the reducibility of the catalyst. No peaks of Pt are observed in our work due to a small amount of Pt. Anuj Bisht et al. (Bisht et al., 2018) reported that the reducibility of La0.8Sr0.2Co0.95Pt0.05O3 without reduction peaks of Pt, which is similar to our result. In Fig. 2b, four peaks (Pt4+→Pt2+ (177 °C), Pt2+→Pt (222 °C) (Bisht et al., 2015), Co3+→Co2+ or Oα (336 °C) and Co2+→Co or Oβ (505 °C) (Wu et al., 2018; Ma et al., 2011)) are detected. Compares to the doped catalyst, the peaks of Co3+→Co2+ or Oα of modified catalysts decrease 62 °C. However, the peaks of Co2+→Co or Oβ increase 39 °C. It indicates that the Co3+ is easier reduced to Co2+ at a lower temperature in the lattice of modified catalyst. Moreover, the result proves that Oα could easier appear at a lower temperature. The Oα is a strong evidence for the performance of catalysts (Wang et al., 2016). Therefore, it implies that the modified catalysts have a better reducibility. In addition, the unchanged reduction temperature of the modified catalysts indicates that the times of modification have no effect on reducibility of catalysts. Two peaks of Pt ion are observed at 177 °C and 222 °C, respectively, which indicates that the Pt ions transform to Pt in two steps (Pt4+→Pt2+→Pt). Meanwhile, it implies that the Pt ions is easier detected due to the interaction between H2O2 and perovskite. 3.2. The morphology and structural characterizations The XRD patterns of catalysts are showed in Fig. 3. All doped catalysts exhibit an ideal perovskite structure (PDF#48-0123) with a space group of R-3c (Fig. 3a). The diffraction peaks shift to a lower angle with the increasing of Pt (Fig. 3b), which indicates that a lattice expansion of LaCo1-xPtxO3 occurs due to a larger ionic radius of the Pt2+ (0.086 nm) compares to the Co3+ (0.062 nm) (Wang et al., 2014). No phase such as PtO2 is detected, which could confirm Pt atoms are introduced into the lattice of LaCoO3. In addition, all modified catalysts remain the ideal 7
Journal of Hazardous Materials 383 (2020) 121210
L. Zeng, et al.
In addition, the surface area of the doped catalysts increases about 5.943 m2/g (Table 3). The surface area of the A-LCP0.06-4 decreases 28.576 m2/g, and the T50 only increases 2 °C, which excludes the effect of BET on activity of the catalysts after age. 3.4. Surface states of catalysts The XPS is performed to evaluate the surface chemical state of the catalysts (Fig. 8 and Fig. S3). In Fig. S3, the peaks at 77.1˜77.9 eV and 73.8 eV are attributed to Pt4+ and the peaks at 76.4 eV and 73.1 eV are attributed to Pt2+ (Bisht et al., 2015). It indicates that the interaction between Co ion and Pt ion leads to Pt2+→Pt4+. And the detailed reaction process could follow the formula: Pt 2 + + 2Co3 + → Pt 4 + + 2Co2 +. Moreover, no Pt0 is detected, which further proves that Pt atoms are introduced into the perovskite lattice. For all Co 2p spectra of catalysts, the Co3+ (peaks at 779.3 eV and 794.4 eV), the Co2+ (peaks at 781 eV and 796.5 eV), and the shake-up satellites (Li et al., 2010; Wu et al., 2018; An et al., 2018) are obviously observed. Generally speaking, the Co2+ could create more OV or the Pt4+ in order to maintain the electric neutrality (Wu et al., 2018). The OV would move to the surface of catalyst and form the defect sites which capture oxygen species to form the active oxygen species (the main factor affecting catalytic activity). The detailed process is summed up in two formulas (Pt 2 + + 2Co3 + → Pt 4 + + 2Co2 + and x − + + 2 3 Co − OV + 2 O2 → Co − Ox ). Therefore, the changes of the catalytic activity can be explained by the number of Co2+. The ratio of the Co2+/(Co2++Co3+) is calculated (Table 3) to evaluate the number of Co2+, which increases with the substitution of Pt (except x = 0.1). After modification, the ratio increases at least 3.75%. However, the ratio reduces slightly for the aged catalysts, and follows the order: ALCP0.06-4 > A-LCP0.06 > A-LC. Besides, the O1s spectra are detected (Fig. 8).The peaks at 528.6 eV and 531.5 eV can be attributed to the Oβ and the Oα, respectively (Liang et al., 2017). Moreover, the ratio of the Oα/(Oα+Oβ) (Table 3) is used to evaluate the number of Oα which relates to the activity of catalyst (Urán et al., 2019; Liang et al., 2017). Obviously, the trend of ratio of the Oα/(Oα+Oβ) is similar with the ratio of the Co2+/(Co2++Co3+). The phenomenon can be explained by the Co2+ creates more the OV, which moves to the surface of catalyst and captures more oxygen species to form Oα. A large amount of Oα is adsorbed on the surface of the modified catalysts before or after age (Table 3). Especially, the ratio of the Oα/(Oα+Oβ) of the LCP0.06-4 reaches 95.71%, which indicates that the surface of modified catalyst can easily adsorb oxygen species to form Oα. The correlation among the Co2+/(Co2++Co3+), the Oα/(Oα+Oβ) and the T50 are analyzed in Fig. S4. The activity of catalyst increases with the ratio of the Co2+/(Co2++Co3+) and the Oα/(Oα+Oβ), which confirms that the number of Co2+ and Oα can lead to the change of the catalytic activity.
Fig. 9. Scheme of the reaction mechanism for soot combustion over perovskite catalysts: (Ⅰ) LCP0.06 catalyst; (Ⅱ) LCP0.06-4 catalyst; (Ⅲ) A-LCP0.06-4 catalyst.
3.3. Thermal stability test of catalysts The catalysts were heated to 800 °C for 50 h under air atmosphere to evaluate the practicability. All aged catalysts show a similar activity trend compared with the fresh catalysts (Fig. 6): A-LCP0.06-4 (T50 = 405 °C) > A-LCP0.06 (T50 = 440 °C) > A-LC (T50 = 462 °C). The T50 of A-LCP0.06-4 and A-LCP0.06 only increases 2–3 °C, and the T50 of ALC increases 12 °C compares to the fresh catalysts. It indicates that the thermal stability of catalyst is evidently promoted after introducing the Pt into the B-sites of LaCoO3, or modifying the catalyst. Fig. 7 shows XRD pattern of the aged catalysts. Both the A-LCP0.06 and the A-LC maintain a pure perovskite phase (PDF#48-0123). However, some impurities (La2O3, CoO and Pt) are observed in A-LCP0.06-4, which indicates that the H2O2 reacts with the surface of perovskite, resulting in an unstable structure of modified catalyst. After adding active sites at adjacent region of precious metal which shows a certain promoting effect on the activity of perovskite (Ai et al., 2018a). It may result in the A-LCP0.06-4 catalyst has no obvious deactivation.
4. Discussion A series of the perovskite catalysts LaCo1-xPtxO3 (x = 0, 0.02, 0.04, 0.06, 0.1) (via sol-gel method) were modified with 30% H2O2. All catalysts show an ideal perovskite structure (Fig. 3). Interestingly, the asymmetry structure gradually transforms to a symmetry structure after modifying the catalysts (Fig. 3). The surface of the modified catalysts becomes rough (Fig. S1) which is confirmed by the TEM, that the increased roughness is attributed to the amorphization (about 3–5 nm) of surface (Fig. 4). On the one hand, the doped catalyst could create the Co2+ which leads to the increasing of the OV (Ai et al., 2018a; An et al., 2018). Subsequently, the OV would move to the surface of catalyst and form the defect sites which capture oxygen species and form the Oα. Actually, the Oα is oxygen species such as Ox− (x = 1 or 2) absorbed on the surface of catalyst, which is mentioned in the previous report (Wang 8
Journal of Hazardous Materials 383 (2020) 121210
L. Zeng, et al.
et al., 2016; Li et al., 2012). The Ox-species as the reactivity descriptor is used to explain the mechanism of soot oxidation frequently (Wang et al., 2018a,b). And the reduction-oxidation process of oxygen species O2(g ) ↔ O(*ads) ( the surfa is about as follows: − 2− . The Ox- species (x = 1 or ce oxygen in the defect sites) ↔ O2( ↔ O ads ) 2(ads ) 2) can react quickly with soot to remove it, which means that the catalyst with large amounts of the Ox- species (Oα) has a higher activity for soot oxidation. For the modified catalyst, the surface of LCP0.06-4 enriches a majority of the Oα (over 95%) (Table 3), which indicates that the amorphous surface can easier absorb the oxygen species to form the Oα. It implies that the amorphous layer plays another critical role in promoting the catalytic performance, resulting in that the modified catalyst exhibits an excellent performance of soot oxidation compared with the doped catalyst. On the other hand, the catalyst with same element content and a different structure (symmetry structure or asymmetric structure) exhibits a difference evident activity. The high symmetry catalyst (LCP0.06-4) shows an excellent activity by accelerating the movement of the OV aggregated on the surface of the modified catalyst. The surface, with a large amount of the OV, would capture more the oxygen species to form Oα which promotes the activity of catalyst (The Oα has close correlation with the activity of catalyst, which has been discussed above). Moreover, the OV transport process of the soot combustion is faster than the decomposition of oxygen gas (Liu et al., 2017, 2016). Besides, the effect of surface areas on the catalytic reaction also is crucial. The BET results indicate that the surface areas of the catalysts increase obviously (from 4.607 to 43.151 m2/g) after modification with H2O2. And the modified catalysts show the mesoporous structure which is not detected in the doped catalysts. The mesoporous structure is more conducive to enhance the surface areas of catalysts. The larger surface areas could expose more active sites to ensure the effective contact between soot and catalysts. It could affect the performance of the modified catalysts. The in-situ DRIFTS spectra and the Raman spectra are used to deduce the reaction processes of soot. In the in-situ DRIFTS spectra (Fig. S5), the transformation process of soot at the characteristic temperature (T10 (344℃) and T50 (403℃) is detected. The peaks of the BO6 (590 cm−1), the coordinated COX (1066 cm−1), the carbonates (1450 cm−1) and the CO2 (2337 cm−1, 2371 cm−1) are observed. The BO6 reveals the structure of catalyst is the perovskite oxide, which agrees well with XRD. The existence of coordinated COX and carbonates reveals the transformation from C (soot) to COX. Moreover, the peaks of CO2 are observed no matter whether the spectra are detected at the T10 or at T50, which implies that the soot is completely converted to CO2 in the process of catalytic oxidation. In the Raman spectra (Fig. S6), the peaks of La F2g (149 cm−1), the peaks of Co(Pt)O6 (460 cm−1, 521 cm−1, 640 cm−1, 670 cm−1) and the peaks of soot (1357 cm−1, 1600 cm−1) are observed. The existence of La F2g and Co(Pt)O6 confirms the perovskite structure, which agrees with XRD and in-situ DRIFTS. The soot peaks of the LCP0.06-4/C are detected, which can′t observed after the TGA test (1 st). It indicates that the soot transforms into CO2 completely. The mechanism of the whole reaction is showed in Fig. 9. The activity of the aged catalysts shows a similar trend with the fresh catalysts in Fig. 6 (A-LCP0.06-4 > A-LCP0.06 > A-LC). The ALCP0.06-4 catalyst presents some impurities such as Pt, CoO and La2O3, which proves that the modified catalyst exhibits an unstable structure after aging (Fig. 7). However, the activity of A-LCP0.06-4 and the number of Oα have no evident fluctuation (Figs. 6b and 8). The phenomenon can be attributed to the precious metal which has a certain promoting on the soot oxidation of perovskite (Ai et al., 2018a). Moreover, we exclude the effect of BET on the activity of the aged catalyst (Table 3). According to the discussion of the OV transport mechanism, the scheme of the reaction mechanism for the A-LCP0.06-4 catalyst is summarized in Fig. 9(Ⅲ).
5. Conclusion In summary, the perovskite catalysts (via sol-gel method) are modified with 30% H2O2 solution and applied to soot oxidation. The XRD result shows that the modified catalysts have a high symmetry structure which benefits to the movement of OV. The TEM images indicate that the surface of the modified catalyst has an amorphous layer which can easier capture the surface oxygen species to form the adsorbed oxygen. After modification, the catalyst exhibited an excellent activity (T50 =403 °C). Meanwhile, the reducibility of modified catalyst is superior to other catalysts. According to the XPS result, there is a close connection between the adsorbed oxygen and the performance of catalyst. The performance for soot oxidation increases with the adsorbed oxygen. For the aged catalyst, no obvious deactivation phenomenon is observed which can be attributed to the Pt particles. Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.jhazmat.2019.121210. References Agathokleous, E., Belz, R.G., Calatayud, V., De Marco, A., Hoshika, Y., Kitao, M., Saitanis, C.J., Sicard, P., Paoletti, E., Calabrese, E.J., 2019. Predicting the effect of ozone on vegetation via linear non-threshold (LNT), threshold and hormetic dose-response models. Sci. Total Environ. 649, 61–74. Ai, L., Wang, Z., Gao, Y., Cui, C., Wang, B., Liu, W., Wang, L., 2018a. Effect of surface and bulk palladium doping on the catalytic activity of La2Sn2O7 pyrochlore oxides for diesel soot oxidation. J. Mater. Sci. 54, 4495–4510. Ai, L., Wang, Z., Cui, C., Liu, W., Wang, L., 2018b. Catalytic oxidation of soot on a novel active Ca-Co dually-doped lanthanum tin pyrochlore oxide. Materials (Basel) 11. An, S.R., Song, K.H., Lee, K.Y., Park, K.T., Jeong, S.K., Kim, H.J., 2018. Fe-doped LaCoO3 perovskite catalyst for NO oxidation in the post-treatment of marine diesel engine’s exhaust emissions. Korean J. Chem. Eng. 35, 1807–1814. Andana, T., Piumetti, M., Bensaid, S., Russo, N., Fino, D., Pirone, R., 2016. Nanostructured ceria-praseodymia catalysts for diesel soot combustion. Appl. Catal. B-Environ. 197, 125–137. Andana, T., Piumetti, M., Bensaid, S., Veyre, L., Thieuleux, C., Russo, N., Fino, D., Quadrelli, E.A., Pirone, R., 2018. Nanostructured equimolar ceria-praseodymia for NOx-assisted soot oxidation: insight into Pr dominance over Pt nanoparticles and metal–support interaction. Appl. Catal. B-Environ. 226, 147–161. Białobok, B., Trawczyński, J., Rzadki, T., Miśta, W., Zawadzki, M., 2007. Catalytic combustion of soot over alkali doped SrTiO3. Catal. Today 119, 278–285. Bisht, A., Sihag, A., Satyaprasad, A., Mallajosyala, S.S., Sharma, S., Metal, P., 2018. Supported and Pt4+ doped La1−xSrxCoO3: non-performance of Pt4+ and reactivity differences with Pt metal. Catal. Lett. 148, 1965–1977. Bisht, A., Zhang, P., Shivakumara, C., Sharma, S., 2015. Pt-doped and Pt-supported La1–xSrxCoO3: comparative activity of Pt4+and Pt0 toward the CO poisoning effect in formic acid and methanol electro-oxidation. Catal. Lett. 119, 14126–14134. Cheng, L., Men, Y., Wang, J., Wang, H., An, W., Wang, Y., Duan, Z., Liu, J., 2017. Crystal facet-dependent reactivity of α-Mn2O3 microcrystalline catalyst for soot combustion. Appl. Catal. B-Environ. 204, 374–384. Comelli, N.A., Ruiz, M.L., Aparicio, M.S.L., Merino, N.A., Cecilia, J.A., RodríguezCastellón, E., Lick, I.D., Ponzi, M.I., 2018. Influence of the synthetic conditions on the composition, morphology of CuMgAl hydrotalcites and their use as catalytic precursor in diesel soot combustion reactions. Appl. Clay Sci. 157, 148–157. Dhal, G.C., Dey, S., Mohan, D., Prasad, R., 2018. Simultaneous abatement of diesel soot and NOX emissions by effective catalysts at low temperature: an overview. Catal. Rev.-Sci. Eng. 60, 437–496. Fino, D., Bensaid, S., Piumetti, M., Russo, N., 2016. A review on the catalytic combustion of soot in diesel particulate filters for automotive applications: from powder catalysts to structured reactors. Appl. Catal. A-Gen. 509, 75–96. Guillén-Hurtado, N., García-García, A., Bueno-López, A., 2015. Active oxygen by Ce–Pr mixed oxide nanoparticles outperform diesel soot combustion Pt catalysts. Appl. Catal. B-Environ. 174–175, 60–66. Hernández, W.Y., Lopez-Gonzalez, D., Ntais, S., Zhao, C., Boréave, A., Vernoux, P., 2018. Silver-modified manganite and ferrite perovskites for catalyzed gasoline particulate filters. Appl. Catal. B-Environ. 226, 202–212. Hong, S.S., Lee, G.D., 2000. Simultaneous removal of NO and carbon particulates over lanthanoid perovskite-type catalysts. Catal. Today 63, 397–404. Huang, X., Pan, H., Chen, K., Yin, Z., Hong, J., 2018. Facile synthesis of porous spherical La0.8Sr0.2Mn1−xCuxO3 (0≤x≤0.4) and nanocubic La0.8Sr0.2MnO3 with high catalytic activity for CO. Crystengcomm 20, 7020–7029. Ji, F., Men, Y., Wang, J., Sun, Y., Wang, Z., Zhao, B., Tao, X., Xu, G., 2019. Promoting diesel soot combustion efficiency by tailoring the shapes and crystal facets of nanoscale Mn3O4. Appl. Catal. B-Environ. 242, 227–237. Jian, L., Zhen, Z., Jie, L., Xu, C., Duan, A., Jiang, G., Wang, X., Hong, H., 2009. Catalytic combustion of soot over the highly active (La0.9K0.1CoO3) x/nmCeO2 catalysts. J.
9
Journal of Hazardous Materials 383 (2020) 121210
L. Zeng, et al.
electrochemical performance. J. Power Sources 274, 1024–1033. Tsvetkov, N., Lu, Q., Sun, L., Crumlin, E.J., Yildiz, B., 2016. Improved chemical and electrochemical stability of perovskite oxides with less reducible cations at the surface. Nat. Mater. 15, 1010–1016. Urán, L., Gallego, J., Li, W.Y., Santamaría, A., 2019. Effect of catalyst preparation for the simultaneous removal of soot and NOx. Appl. Catal. A-Gen. 569, 157–169. van Setten, B.A.A.L., Makkee, M., Moulijn, J.A., 2001. Science and technology of catalytic diesel particulate filters. Catal. Rev.-Sci. Eng. 43, 489–564. Vinodkumar, T., Mukherjee, D., Subrahmanyam, C., Reddy, B.M., 2018. Investigation on the physicochemical properties of Ce0.8Eu0.1M0.1O2−δ (M = Zr, Hf, La, and Sm) solid solutions towards soot combustion. New J. Chem. 42, 5276–5283. Wang, L., Fang, S., Feng, N., Wan, H., Guan, G., 2016. Efficient catalytic removal of diesel soot over Mg substituted K/La0.8Ce0.2CoO3 perovskites with large surface areas. Chem. Eng. J. 293, 68–74. Wang, X., Qi, X., Chen, Z., Jiang, L., Wang, R., Wei, K., 2014. Studies on SO2 tolerance and regeneration over perovskite-type LaCo1–xPtxO3 in NOx storage and reduction. J. Phys. Chem. C 118, 13743–13751. Wang, H., Luo, S., Zhang, M., Liu, W., Wu, X., Liu, S., 2018a. Roles of oxygen vacancy and O− in oxidation reactions over CeO2 and Ag/CeO2 nanorod model catalysts. J. Catal. 368, 365–378. Wang, H., Jin, B., Wang, H., Ma, N., Liu, W., Weng, D., Wu, X., Liu, S., 2018b. Study of Ag promoted Fe2O3@CeO2 as superior soot oxidation catalysts: the role of Fe2O3 crystal plane and tandem oxygen delivery. Appl. Catal. B-Environ. 237, 251–262. Wu, Y., Li, L., Chu, B., Yi, Y., Qin, Z., Fan, M., Qin, Q., He, H., Zhang, L., Dong, L., Li, B., Dong, L., 2018. Catalytic reduction of NO by CO over B-site partially substituted LaM0.25Co0.75O3 (M = Cu, Mn, Fe) perovskite oxide catalysts: the correlation between physicochemical properties and catalytic performance. Appl. Catal. A-Gen. 568, 43–53. Wu, X., Feng, Y., Du, Y., Liu, X., Zou, C., Li, Z., 2019a. Enhancing DeNOx performance of CoMnAl mixed metal oxides in low-temperature NH3-SCR by optimizing layered double hydroxides (LDHs) precursor template. Appl. Surf. Sci. 467–468, 802–810. Wu, Q., Xiong, J., Zhang, Y., Mei, X., Wei, Y., Zhao, Z., Liu, J., Li, J., 2019b. Interactioninduced self-assembly of Au@La2O3 core–Shell nanoparticles on La2O2CO3 nanorods with enhanced catalytic activity and stability for soot oxidation. ACS Catal. 9, 3700–3715. Wu, Q., Jing, M., Wei, Y., Zhao, Z., Zhang, X., Xiong, J., Liu, J., Song, W., Li, J., 2019c. High-efficient catalysts of core-shell structured Pt@transition metal oxides (TMOs) supported on 3DOM-Al2O3 for soot oxidation: the effect of strong Pt-TMO interaction. Appl. Catal. B-Environ. 244, 628–640. Xie, Y., Galvez, M.E., Matynia, A., Da Costa, P., 2017. Experimental investigation on the influence of the presence of alkali compounds on the performance of a commercial Pt–Pd/Al2O3 diesel oxidation catalyst. Clean Technol. Environ. Policy 20, 715–725. Xiong, J., Mei, X., Liu, J., Wei, Y., Zhao, Z., Xie, Z., Li, J., 2019. Efficiently multifunctional catalysts of 3D ordered meso-macroporous Ce0.3Zr0.7O2-supported PdAu@CeO2 coreshell nanoparticles for soot oxidation: synergetic effect of Pd-Au-CeO2 ternary components. Appl. Catal. B-Environ. 251, 247–260. Xiong, J., Wu, Q., Mei, X., Liu, J., Wei, Y., Zhao, Z., Wu, D., Li, J., 2018. Fabrication of spinel-type PdxCo3–xO4 binary active sites on 3D ordered meso-macroporous Ce-ZrO2 with enhanced activity for catalytic soot oxidation. ACS Catal. 8, 7915–7930.
Phys. Chem. C 113, 17114–17123. Köferstein, R., Ebbinghaus, S.G., 2013. Synthesis and characterization of nano-LaFeO3 powders by a soft-chemistry method and corresponding ceramics. Solid State Ion. 231, 43–48. Lam, K., Gao, Y., Wang, J., Ciucci, F., 2017. H2O2Treated La0.8Sr0.2CoO3-δ as an efficient catalyst for oxygen evolution reaction. Electrochim. Acta 244, 139–145. Lebid, M., Omari, M., 2013. Synthesis and electrochemical properties of LaFeO3 oxides prepared via sol–gel method. Arab. J. Sci. Eng. 39, 147–152. Ledwa, K.A., Pawlyta, M., Kępiński, L., 2018. RuxCe1-xO2-y nanoparticles deposited on functionalized γ-Al2O3 as a thermally stable oxidation catalyst. Appl. Catal. BEnviron. 230, 135–144. Li, Z., Meng, M., Li, Q., Xie, Y., Hu, T., Zhang, J., 2010. Fe-substituted nanometric La0.9K0.1Co1−xFexO3−δ perovskite catalysts used for soot combustion, NOx storage and simultaneous catalytic removal of soot and NOx. Chem. Eng. J. 164, 98–105. Li, Z., Meng, M., Zha, Y., Dai, F., Hu, T., Xie, Y., Zhang, J., 2012. Highly efficient multifunctional dually-substituted perovskite catalysts La1−xKxCo1−yCuyO3−δ used for soot combustion, NOx storage and simultaneous NOx-soot removal. Appl. Catal. BEnviron. 121–122, 65–74. Liang, H., Mou, Y., Zhang, H., Li, S., Yao, C., Yu, X., 2017. Sulfur resistance and soot combustion for La0.8K0.2Co1-yMnyO3 catalyst. Catal. Today 281, 477–481. Liu, S., Wu, X., Weng, D., Li, M., Fan, J., 2013. Sulfation of Pt/Al2O3 catalyst for soot oxidation: high utilization of NO2 and oxidation of surface oxygenated complexes. Appl. Catal. B-Environ. 138–139, 199–211. Liu, S., Wu, X., Weng, D., Ran, R., 2015. Ceria-based catalysts for soot oxidation: a review. J. Rare Earths 33, 567–590. Liu, M., Wu, X., Liu, S., Gao, Y., Chen, Z., Ma, Y., Ran, R., Weng, D., 2017. Study of Ag/ CeO2 catalysts for naphthalene oxidation: balancing the oxygen availability and oxygen regeneration capacity. Appl. Catal. B-Environ. 219, 231–240. Liu, S., Wu, X., Liu, W., Chen, W., Ran, R., Li, M., Weng, D., 2016. Soot oxidation over CeO2 and Ag/CeO2: factors determining the catalyst activity and stability during reaction. J. Catal. 337, 188–198. Ma, Z., Gao, X., Yuan, X., Zhang, L., Zhu, Y., Li, Z., 2011. Simultaneous catalytic removal of NOx and diesel soot particulates over La2−xAxNi1−yByO4 perovskite-type oxides. Catal. Commun. 12, 817–821. Matarrese, R., Morandi, S., Castoldi, L., Villa, P., Lietti, L., 2017. Removal of NOx and soot over Ce/Zr/K/Me (Me = Fe, Pt, Ru, Au) oxide catalysts. Appl. Catal. B-Environ. 201, 318–330. Ozawa, T., 1970. Kinetic analysis of derivative curves in thermal analysis. J. Therm. Anal. Calorim. 2, 301–324. Pahalagedara, L., Sharma, H., Kuo, C.-H., Dharmarathna, S., Joshi, A., Suib, S.L., Mhadeshwar, A.B., 2012. Structure and oxidation activity correlations for carbon blacks and diesel soot. Energy Fuels 26, 6757–6764. Shang, Z., Sun, M., Che, X., Wang, W., Wang, L., Cao, X., Zhan, W., Guo, Y., Guo, Y., Lu, G., 2017. The existing states of potassium species in K-doped Co3O4 catalysts and their influence on the activities for NO and soot oxidation. Catal. Sci. Technol. 7, 4710–4719. Su, C., Xu, X., Chen, Y., Liu, Y., Tadé, M.O., Shao, Z., 2015. A top-down strategy for the synthesis of mesoporous Ba0.5Sr0.5Co0.8Fe0.2O3−δ as a cathode precursor for buffer layer-free deposition on stabilized zirconia electrolyte with a superior
10