Experimental Evaluation on the Catalytic Activity of a Novel CeZrK/rGO Nanocomposite for Soot Oxidation in Catalyzed Diesel Particulate Filter

: A nanostructured solid solution catalyst CeZrK/rGO for soot oxidation in catalyzed diesel particulate ﬁlter was synthesized using the dipping method. The reduced graphene oxide (rGO) was used as the catalyst carrier, and CeO 2 , ZrO 2 , and K 2 O were mixed with the molar ratio of 5:1:1, 5:2:2 and 5:3:3, which were referred to as Ce 5 Zr 1 K 1 /rGO, Ce 5 Zr 2 K 2 /rGO, and Ce 5 Zr 3 K 3 /rGO, respectively. The structure, morphology and catalytic activity of the CeZrK/rGO nanocomposites were thoroughly investigated and the results show that the CeZrK/rGO nanocomposites have nanoscale pore structure (36.1–36.9 nm), high-dispersion quality, large speciﬁc surface area (117.2–152.4 m 2 /g), small crystallite size (6.7–8.3 nm), abundant oxygen vacancies and superior redox capacity. The 50% soot conversion temperatures of Ce 5 Zr 1 K 1 /rGO, Ce 5 Zr 2 K 2 /rGO, and Ce 5 Zr 3 K 3 /rGO under tight contact condition were decreased to 352 ◦ C, 339 ◦ C and 358 ◦ C respectively. The high catalytic activity of CeZrK/rGO nanocomposites can be ascribed to the following factors: the doping of Zr and K ions causes the nanocrystalline phase formation in CeZrK solid solutions, reduces the crystallite size, generates abundant oxygen vacancies and improves redox capacity; the rGO as a carrier provides a large speciﬁc surface area, thereby improving the contact between soot and catalyst. desorption tests, X-ray powder diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectrometry (FTIR), Raman spectra and H 2 -temperature programmed reduction (TPR) methods. The catalytic performance of the catalyst was evaluated using thermogravimetric analysis.


Introduction
The diesel engine has been widely used in transportation and various non-road construction machinery due to a high thermal efficiency [1]. However, its exhausted emissions can cause serious environmental pollution and endanger human health at the same time, especially for soot which can enter the blood, trigger genetic mutations and create health risks [2,3]. In recent years, with the increasingly stringent emission regulations, the control technologies of the soot emission in diesel engines have become the focus of governments and scholars of various countries [4].
In the current scenario, soot is mainly caused by the incomplete combustion of hydrocarbon compounds inside the engine under high temperature and oxygen deficiency. In the post-treatment system, it gets into contact with the catalyst and reacts at high temperatures to form CO 2 . Diesel particulate filter (DPF) is an effective tool to control soot from diesel engine. Generally, the exhaust temperature ranges from 150 • C to 450 • C, while the oxidation temperature of soot exceeds 550 • C. It is, therefore, necessary to raise the temperature for soot combustion, but excessive soot combustion temperature will damage the filter material [5]. Therefore, a superior solution is to reduce the oxidation reaction temperature of soot using catalysts. Catalyzed diesel particulate filter (CDPF) does not need an extra heat source and complex control system, and thus it greatly reduces fuel consumption Processes 2021, 9, 674 2 of 18 and engine cost. As the catalytic oxidation of soot is a reaction between O 2 , soot and the catalyst, the contact between soot and the catalyst and the activity of the catalyst are two key factors affecting the soot oxidation efficiency [6], which is a critical issue in CDPF regeneration. Accordingly, the design and preparation of catalysts with large contact area and high intrinsic activity are significant for the CDPF.
The noble metal catalysts have a superior intrinsic catalytic activity, but the price is too expensive, limiting their applications in CDPF [7]. It was established that the oxides of rare earth metal, like Ce 0.5 Zr 0.5 O 2 [8], LaFeO 3 [9], LaMn 0.8 Pt 0.2 O 3 [10], can catalyze the oxidation of soot. Among them, CeO 2 presents superior oxygen storage capacity (OSC) and catalytic performance over other traditional metal oxide catalysts in oxidizing soot at low temperature [11]. The introduction of external cations can cause the lattice distortion of CeO 2 and increase oxygen vacancies, improving the redox capacity and catalytic activity of CeO 2 [12]. It was reported by Atribak et al. [13] and Gross et al. [14] that the addition of Zr and K ions can enhance high-temperature stability and oxygen storage capacity of the catalysts, respectively. Thus, it can be inferred that the doping of these two elements in the catalyst can improve soot oxidation.
It is important to select a suitable substrate to increase the surface area, improve adsorption and provide more active sites on the catalyst. Graphene exists as a 2D nanomaterial with a honeycomb hexagonal structure composed of Sp 2 hybridized carbon atoms (Olabi et al., 2020). Due to excellent physical and chemical properties, graphene has attracted extensive attention. The super-large specific surface area and special pore structure of graphene enable the metal nanoparticles loaded onto graphene to get a good dispersion, thus ensuring sufficient contact area between the active components of the catalyst and soot. Tan et al. [15] found that the maximum adsorption capacity of NiAl-LDH/rGO composite with a sandwich structure toward uranium is up to 277.8 mg/g. The high adsorption capacity has to do with the specific surface area and special pore structure characteristics of rGO. Due to the negatively charged nature of graphene and positively charged nature of the metal cation, the combination between rGO and metal oxide particles can be easily achieved by electrostatic interaction, which is conducive to the uniform dispersion of metal oxide particles on rGO [16]. Some researchers have pointed out that graphene-based materials can stabilize active sites and inhibit surface migration of metal nanoparticles, guaranteeing high catalytic stability even under severe operating conditions [17][18][19]. Therefore, the usage of graphene as the matrix of catalysts is a feasible strategy.
Still, the weak thermal-oxidative stability of bare graphene makes it easy to decompose during combustion. This impact is negative on the catalytic activity. Consequently, intellectual capital and resources are being committed to upturn the thermal stability of graphene by surface modification. Generally speaking, modified graphene is to compound other substances with active components through covalent or non-covalent interactions so that graphene has other special properties while maintaining its original properties [20]. Li et al. [21] portrayed that zirconium phosphate nanoparticles immobilized on the surface of graphene oxide (GO) yielded superior thermal stability and flame retardancy to the phenolic foams, and this is essentially ascribed to the harmonious impacts of zirconium phosphate nanoparticles and GO. Wu et al. [22] prepared graphene sheets using the hydrogen arc discharge exfoliation method. The materials exhibit a remarkable thermal stability from GO with an oxidization resistance temperature of 601 • C. The research performed by Bora et al. [23], Yuan et al. [24] and Zong et al. [25] found that rGO exhibited a higher thermal stability than GO due to the removal of thermally labile oxygen functional groups during the reduction process. Salavagione et al. [4] report a strategy for the synthesis of vinylalcohol/rGO nanocomposites and found the thermal stability was improved by more than 100 • C.
This study aims to develop a catalyst with high catalytic activity, i.e., CeO 2 -ZrO 2 -K 2 O/rGO (referred to as CeZrK/rGO), for soot oxidation in the CDPF. The structure and morphology of the catalyst were thoroughly characterized using field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), N 2 adsorption-Processes 2021, 9, 674 3 of 18 desorption tests, X-ray powder diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectrometry (FTIR), Raman spectra and H 2 -temperature programmed reduction (TPR) methods. The catalytic performance of the catalyst was evaluated using thermogravimetric analysis.

Catalyst Preparation
The schematic diagram of the catalyst's preparation process is shown in Figure 1. All materials were of analytical grade and used without further purification. Graphene oxide was prepared by the Hummer method in the oxidation-reduction method [26]. Under low temperature stirring conditions, some amount of concentrated sulfuric acid and strong oxidant were sequentially added to the graphite powder, diluted with water and hydrogen peroxide, heated and dried, and then subjected to ultrasound exfoliation to obtain graphene oxide. Ce(NO 3 ) 3 ·6H 2 O, ZrO(NO 3 ) 2 ·6H 2 O, and KNO 3 were dissolved in deionized water by magnetic stirring to generate the salt solution containing the metallic ions of Ce, Zr, and K. Then, 60 mg of GO was put into 30 mL of deionized water to generate the GO solution under the ultrasonic treatment for 0.5 h. The prepared solutions were mixed and then processed by the ultrasonic treatment for 0.5 h. The mixed solution was heated in a water bath at 80 • C for 1 h. Ammonia hydroxide was added into the heated solution and gently stirred until pH 10 ± 0.1 was reached. The obtained solution was hydrothermally treated at 160 • C for 12 h. After that, the solution was washed with deionized water by repeated centrifugations until the pH was adjusted to neutral. The solution was further freeze-dried for 18 h in a FD-1A-80 vacuum freeze drier to obtain the precursor. It was calcined at 200 • C for 2 h in a tube furnace under argon atmosphere to realize CeZrK/rGO. Processes 2021, 9, x FOR PEER REVIEW 3 of 18 morphology of the catalyst were thoroughly characterized using field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), N2 adsorptiondesorption tests, X-ray powder diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectrometry (FTIR), Raman spectra and H2-temperature programmed reduction (TPR) methods. The catalytic performance of the catalyst was evaluated using thermogravimetric analysis.

Catalyst Preparation
The schematic diagram of the catalyst's preparation process is shown in Figure 1. All materials were of analytical grade and used without further purification. Graphene oxide was prepared by the Hummer method in the oxidation-reduction method [26]. Under low temperature stirring conditions, some amount of concentrated sulfuric acid and strong oxidant were sequentially added to the graphite powder, diluted with water and hydrogen peroxide, heated and dried, and then subjected to ultrasound exfoliation to obtain graphene oxide. Ce(NO3)3·6H2O, ZrO(NO3)2·6H2O, and KNO3 were dissolved in deionized water by magnetic stirring to generate the salt solution containing the metallic ions of Ce, Zr, and K. Then, 60 mg of GO was put into 30 mL of deionized water to generate the GO solution under the ultrasonic treatment for 0.5 h. The prepared solutions were mixed and then processed by the ultrasonic treatment for 0.5 h. The mixed solution was heated in a water bath at 80 °C for 1 h. Ammonia hydroxide was added into the heated solution and gently stirred until pH 10 ± 0.1 was reached. The obtained solution was hydrothermally treated at 160 °C for 12 h. After that, the solution was washed with deionized water by repeated centrifugations until the pH was adjusted to neutral. The solution was further freeze-dried for 18 h in a FD-1A-80 vacuum freeze drier to obtain the precursor. It was calcined at 200 °C for 2 h in a tube furnace under argon atmosphere to realize CeZrK/rGO.

Experimental Apparatus
The morphology and composition of catalysts were investigated by FESEM (Czech Nova NanoSEM 230, FEI Company, Hillsboro, OR, USA) equipped with energy dispersive X-ray (EDX). The TEM images were acquired by an FEI Tecnai G2 F20 at an accelerating voltage of 200 kV. The crystal structure of the catalysts was observed using a Rigaku D/Max 2550 XRD (Tokyo, Japan). The static adsorption and desorption tests were carried out on a Micromeritics ASAP2020 to obtain N 2 adsorption/desorption isotherms. The specific surface area and the pore size distribution were calculated from the adsorption branch and desorption branch of the isomers based on Brunauer-Emmett-Teller (BET) and Barrett-Joyner-Halenda (BJH) methods, respectively. The XPS spectra was recorded on an ESCALAB 250Xi XPS in the analysis chamber of 2 × 10 −9 Pa using Al Kα radiation (1486.6 eV), and the obtained binding energies were referenced to C 1s line set at 284.9 eV. The FT-IR spectra was obtained on a Nicolet iS50 FT-IR (Thermo Fisher Scientific, Waltham, MA, USA) using potassium bromide (KBr) beamsplitter. The Raman spectra were measured using a Horiba Jobin-Yvon HR800 Raman spectrometer (Kyoto, Japan) with a 532 nm line of Ar + ion laser as the excitation source at room temperature. The TG 209 F3 Tarsus (Netzsch, Selb, Germany) was used to carry out the thermogravimetric analysis, under the temperature range between room temperature and 900 • C with a heating rate of 10 • C·min −1 . The TPR measurements were carried out on an Auto Chem II 2920 instrument equipped with a thermal conductivity detector (TCD). Each time, 40 mg of the sample was heated from room temperature to 800 • C at a heating rate of 10 • C·min −1 . A mixed gas consisting of 10% H 2 in Ar at a fixed flow rate of 20 mL·min −1 was used as reductant.
In the experiment, the soot and the cerium-based catalyst were mixed in loose contact and close contact, respectively. In close contact conditions, the soot particles and the catalyst powder were mixed and ground in an agate mortar for 30 min. In loose contact conditions, a stainless-steel spatula is used to uniformly mix soot particles with the same mass ratio and catalyst powder for 3-4 min. Under the two working conditions, the masses of soot and cerium-based catalyst in the crucible were 4 mg and 40 mg. A thermogravimetry technique using Printex-U soot (Degussa, Frankfurt, Germany) as the model reactant was utilized to assess the catalytic soot activity of the catalysts. The catalytic activity of the catalysts for diesel soot combustion were realized using simulated air (21% O 2 + 79% N 2 ) or pure N 2 under different contact modes. The thermogravimetric analysis was performed in a TG 209 F3 Tarsus under the temperature range between room temperature and 650 • C with a heating rate of 10 • C·min −1 .

Characterization Studies
The SEM and TEM techniques were employed to characterize the morphology and size of the catalysts as shown in Figure 2a,c,e and Figure 2b,d,f, respectively. It can be seen from Figure 2a that the substrate material of graphene exists in the form of a translucent sheet with wrinkles and folds. Looking at Figure 2c,e, it is apparent that metal oxide particles are uniformly and highly dispersed on graphene. It can be found from TEM images that Ce 5 Zr 2 K 2 /rGO presents the most evenly dispersed metal oxide particles compared to Ce 5 Zr 1 K 1 /rGO and Ce 5 Zr 3 K 3 /rGO. By using Nano measurer software [27], the average particle size of Ce 5 Zr 1 K 1 /rGO, Ce 5 Zr 2 K 2 /rGO, and Ce 5 Zr 3 K 3 /rGO were 9.65 nm, 7.42 nm, and 9.39 nm, respectively, which were smaller than that of CeZrO x catalysts (15-20 nm) [28]. It has been shown that graphene inhibits the surface migration of metal oxide nanoparticles and reduces the size of metal oxide nanoparticles [29]. To obtain the distributed information of Ce, Zr, and K elements in the Ce5Zr2K2/rGO catalysts, the EDX elemental mapping is shown in Figure 3. The catalysts presented a uniform and orderly element distribution, and elements Ce, Zr, K, C, and O were highly dispersed on the catalysts [19]. It can be further verified in TEM images that the rGO carrier was loaded with well-dispersed spherical metal oxide nanoparticles. The catalytic oxidation of soot belonged to the heterogeneous catalytic reaction, and thus the contact between soot and catalyst was one of the most important influencing factors on the catalytic activity and efficiency. Therefore, the well-dispersed metal oxide nanoparticles promoted the catalytic oxidation of soot. To obtain the distributed information of Ce, Zr, and K elements in the Ce 5 Zr 2 K 2 /rGO catalysts, the EDX elemental mapping is shown in Figure 3. The catalysts presented a uniform and orderly element distribution, and elements Ce, Zr, K, C, and O were highly dispersed on the catalysts [19]. It can be further verified in TEM images that the rGO carrier was loaded with well-dispersed spherical metal oxide nanoparticles. The catalytic oxidation of soot belonged to the heterogeneous catalytic reaction, and thus the contact between soot and catalyst was one of the most important influencing factors on the catalytic activity and efficiency. Therefore, the well-dispersed metal oxide nanoparticles promoted the catalytic oxidation of soot.
The XRD patterns of CeO 2 and Ce 5 Zr 2 K 2 /rGO catalysts are depicted in Figure 4. It was observed that all the catalysts show a typical cubic structured CeO 2 peak. The XRD of CeZrK/rGO catalysts showed a series of reflections at 29 • (111), 33 • (200), 47 • (220), 56 • (331), 59 • (222) and 69 • (400) indexed to typical cubic spinel CeO 2 (PDF#89-8436). The doping with Zr 4+ and K + irons caused the shifting of diffraction peak towards lower angle side since Ce 4+ was partially replaced by Zr 4+ and K + ions, which led to the slight changes in cell parameter and crystallite size [30,31]. The peaks of CeZrK/rGO catalysts were wider than those of CeO 2 . Meanwhile, the half-peak width of Ce 5 Zr 2 K 2 /rGO was relatively wide among the CeZrK/rGO catalysts, indicating the crystallite size of Ce 5 Zr 2 K 2 /rGO was smaller than that of Ce 5 Zr 1 K 1 /rGO and Ce 5 Zr 3 K 3 /rGO. There were no obvious characteristic diffraction peaks observed for graphene-related phases. The complete exfoliation of graphene in the hybrid was achieved as a result of efficiently avoiding the restacking of the graphene [19]. Meanwhile, no other obvious diffractions The XRD patterns of CeO2 and Ce5Zr2K2/rGO catalysts are depicted in Figure 4. It was observed that all the catalysts show a typical cubic structured CeO2 peak. The XRD of CeZrK/rGO catalysts showed a series of reflections at 29° (111), 33° (200), 47° (220), 56° (331), 59° (222) and 69° (400) indexed to typical cubic spinel CeO2 (PDF#89-8436). The doping with Zr 4+ and K + irons caused the shifting of diffraction peak towards lower angle side since Ce 4+ was partially replaced by Zr 4+ and K + ions, which led to the slight changes in cell parameter and crystallite size [30,31]. The peaks of CeZrK/rGO catalysts were wider than those of CeO2. Meanwhile, the half-peak width of Ce5Zr2K2/rGO was relatively wide among the CeZrK/rGO catalysts, indicating the crystallite size of Ce5Zr2K2/rGO was smaller than that of Ce5Zr1K1/rGO and Ce5Zr3K3/rGO. There were no obvious characteristic diffraction peaks observed for graphene-related phases. The complete exfoliation of graphene in the hybrid was achieved as a result of efficiently avoiding the restacking of the graphene [19]. Meanwhile, no other obvious diffractions were found in Zr-related and K-related phases, indicating that the Zr-related and K-related phases were non-crystalline states or presented as a part of CeO2-ZrO2-K2O solid solution.   The XRD patterns of CeO2 and Ce5Zr2K2/rGO catalysts are depicted in Figure 4. It was observed that all the catalysts show a typical cubic structured CeO2 peak. The XRD of CeZrK/rGO catalysts showed a series of reflections at 29° (111), 33° (200), 47° (220), 56° (331), 59° (222) and 69° (400) indexed to typical cubic spinel CeO2 (PDF#89-8436). The doping with Zr 4+ and K + irons caused the shifting of diffraction peak towards lower angle side since Ce 4+ was partially replaced by Zr 4+ and K + ions, which led to the slight changes in cell parameter and crystallite size [30,31]. The peaks of CeZrK/rGO catalysts were wider than those of CeO2. Meanwhile, the half-peak width of Ce5Zr2K2/rGO was relatively wide among the CeZrK/rGO catalysts, indicating the crystallite size of Ce5Zr2K2/rGO was smaller than that of Ce5Zr1K1/rGO and Ce5Zr3K3/rGO. There were no obvious characteristic diffraction peaks observed for graphene-related phases. The complete exfoliation of graphene in the hybrid was achieved as a result of efficiently avoiding the restacking of the graphene [19]. Meanwhile, no other obvious diffractions were found in Zr-related and K-related phases, indicating that the Zr-related and K-related phases were non-crystalline states or presented as a part of CeO2-ZrO2-K2O solid solution.  As adsorption and diffusion are the critical processes in a heterogeneous catalytic reaction, the pore structure of the catalyst has a great impact on catalytic performance [32]. Figure 5 displays the N 2 adsorption isotherms and pore size distribution of the as-prepared catalysts. A similar IV type isotherm with H3-type hysteresis loops on IUPAC classification suggested that the CeZrK/rGO catalysts had mesoporous distribution characteristic [33]. From Figure 5b,d,f, it can be seen that the most probable pore size of Ce 5 Zr 1 K 1 /rGO, Ce 5 Zr 2 K 2 /rGO, and Ce 5 Zr 3 K 3 /rGO were 36.9 nm, 36.8 nm, and 36.1 nm, respectively, which were larger than that of soot (>20 nm). At this time, most of the soot particles stayed on the external surface, and some entered the pores, which increased the contact area between the catalyst and the PM to a certain extent, thereby improving the catalytic activity. sification suggested that the CeZrK/rGO catalysts had mesoporous distribution characteristic [33]. From Figure 5b,d,f, it can be seen that the most probable pore size of Ce5Zr1K1/rGO, Ce5Zr2K2/rGO, and Ce5Zr3K3/rGO were 36.9 nm, 36.8 nm, and 36.1 nm, respectively, which were larger than that of soot (>20 nm). At this time, most of the soot particles stayed on the external surface, and some entered the pores, which increased the contact area between the catalyst and the PM to a certain extent, thereby improving the catalytic activity.  Table 1 lists the average crystallite size and the specific surface area (SBET) of the catalysts. The average crystallite sizes was computed from XRD based on the Scherrer equation. The average crystallite size of Ce5Zr1K1/rGO, Ce5Zr2K2/rGO, and Ce5Zr3K3/rGO were 8.3 nm, 6.9 nm, and 6.7 nm, respectively, which were smaller than that of CeO2 because  Table 1 lists the average crystallite size and the specific surface area (S BET ) of the catalysts. The average crystallite sizes was computed from XRD based on the Scherrer equation. The average crystallite size of Ce 5 Zr 1 K 1 /rGO, Ce 5 Zr 2 K 2 /rGO, and Ce 5 Zr 3 K 3 /rGO were 8.3 nm, 6.9 nm, and 6.7 nm, respectively, which were smaller than that of CeO 2 because the lattice distortion caused by the doping of Zr and K ions inhibited the crystal growth of CeO 2 phase [34]. The S BET can be obtained from N 2 adsorption isotherms based on the multi-point BET. The as-prepared catalysts had a higher specific surface area compared to that of CeZrK catalysts (17 m 2 /g) in Ref. [35] and other Ce-based catalysts (55-82 m 2 /g) in Ref. [36]. Meanwhile, among the CeZrK/rGO catalysts, the specific surface area of Ce 5 Zr 2 K 2 /rGO (151.5 m 2 /g) and Ce 5 Zr 3 K 3 /rGO (152.4 m 2 /g) were slightly larger than that of Ce 5 Zr 1 K 1 /rGO (117.2 m 2 /g). It has proved that graphene-based catalysts have a large surface area and superior pore structure, which effectively guarantees the high catalytic activity of the CeZrK/rGO catalysts [37,38].  Figure 6 presents the Raman profiles of CeZrK/rGO catalysts and GO. The typical Raman peaks of D band and G band appeared around 1350 cm −1 and 1594 cm −1 , respectively. The D band represents the structural defect caused by oxygen-containing functional groups on the C base, and the G band represents the E 2g symmetric mode of sp 2 hybrid C [39]. For the Raman profiles of the CeZrK/rGO catalysts, no bands occurred at 400~500 cm −1 assigned to CeO 2 [29], showing good dispersion of the CeO 2 particle on the graphene layers. The intensity ratio of D band to G band (I D /I G ) is an indicator of disorder degree and the average size of the in-plane sp 2 regions in graphite [39]. The I D /I G for Ce 5 Zr 1 K 1 /rGO, Ce 5 Zr 2 K 2 /rGO, Ce 5 Zr 3 K 3 /rGO, and GO were 1.10, 1.28, 1.21, and 0.99, respectively. The higher I D /I G means that there may be smaller sp 2 regions and unrepaired defect sites in Ce 5 Zr 2 K 2 /rGO. The defect sites on the surface of rGO can efficiently inhibit the agglomeration of metal oxide nanoparticles to reduce the particle size and improve the dispersion characteristics [40], which is consistent with the above results observed in SEM and TEM.
multi-point BET. The as-prepared catalysts had a higher specific surface area compared to that of CeZrK catalysts (17 m 2 /g) in Ref. [35] and other Ce-based catalysts (55-82 m 2 /g) in Ref. [36]. Meanwhile, among the CeZrK/rGO catalysts, the specific surface area of Ce5Zr2K2/rGO (151.5 m 2 /g) and Ce5Zr3K3/rGO (152.4 m 2 /g) were slightly larger than that of Ce5Zr1K1/rGO (117.2 m 2 /g). It has proved that graphene-based catalysts have a large surface area and superior pore structure, which effectively guarantees the high catalytic activity of the CeZrK/rGO catalysts [37,38].  Figure 6 presents the Raman profiles of CeZrK/rGO catalysts and GO. The typical Raman peaks of D band and G band appeared around 1350 cm −1 and 1594 cm −1 , respectively. The D band represents the structural defect caused by oxygen-containing functional groups on the C base, and the G band represents the E2g symmetric mode of sp 2 hybrid C [39]. For the Raman profiles of the CeZrK/rGO catalysts, no bands occurred at 400~500 cm −1 assigned to CeO2 [29], showing good dispersion of the CeO2 particle on the graphene layers. The intensity ratio of D band to G band (ID/IG) is an indicator of disorder degree and the average size of the in-plane sp 2 regions in graphite [39]. The ID/IG for Ce5Zr1K1/rGO, Ce5Zr2K2/rGO, Ce5Zr3K3/rGO, and GO were 1.10, 1.28, 1.21, and 0.99, respectively. The higher ID/IG means that there may be smaller sp 2 regions and unrepaired defect sites in Ce5Zr2K2/rGO. The defect sites on the surface of rGO can efficiently inhibit the agglomeration of metal oxide nanoparticles to reduce the particle size and improve the dispersion characteristics [40], which is consistent with the above results observed in SEM and TEM.  [41]. Just on these O-containing groups of GO, during the synthesis of the catalyst, the mixed metal ions (Ce 4+ , Zr 4+ , K + ) are first adsorbed uniformly on the surface of negatively charged GO sheets via electrostatic attractions. Compared to GO, the FTIR spectra of CeZrK/rGO catalysts was greatly reduced or even disappeared at 1735 cm −1 and 1384 cm −1 , implying that GO was deoxidized and reduced to rGO, which was consistent with the phenomenon reported in Ref. [42]. Moreover, the infrared peak changed from 1637 cm −1 to 1654 cm −1 and 1560 cm −1 due to the stretching vibration of C=C. The band at 1228 cm −1 emerged from C-O-C stretch epoxy bands on rGO surface. electrostatic attractions. Compared to GO, the FTIR spectra of CeZrK/rGO catalysts was greatly reduced or even disappeared at 1735 cm −1 and 1384 cm −1 , implying that GO was deoxidized and reduced to rGO, which was consistent with the phenomenon reported in Ref. [42]. Moreover, the infrared peak changed from 1637 cm −1 to 1654 cm −1 and 1560 cm −1 due to the stretching vibration of C=C. The band at 1228 cm −1 emerged from C-O-C stretch epoxy bands on rGO surface. The oxidation states of the main elements Ce, Zr, O and C, including an estimate of the atomic ratio in the CeZrK/rGO catalysts, were achieved using the XPS measurement technique and the corresponding result is depicted in Figure 8. The atomic surface concentrations, the relative percentages, and the binding energy of Ce, Zr, O and C elements in the CeZrK/rGO catalysts are summarized in Tables 2 and 3, respectively. The u and v in the XPS spectra of Ce 3d refer to the 3d3/2 and 3d5/2 spin-orbit components, respectively [43]. The peaks of u, u″, u‴ and v, v″, v‴ are attributed to Ce 4+ ions, whereas u′ and v′ are attributed to Ce 3+ ions. Therefore, it can be inferred that the CeZrK/rGO catalysts had both Ce 4+ and Ce 3+ ions. The Ce 3+ /Ce 4+ ratios of the CeZrK/rGO catalysts were calculated by the ratio of the sum of the integrated areas of Ce 3+ (u′, v′) to the sum of the integrated area of Ce 4+ (u, v, u″, v″, u‴, v‴) as shown in Table 4. It was found that Ce5Zr2K2/rGO exhibits a higher concentration of Ce 3+ ions, which indicates that Ce5Zr2K2/rGO contained more oxygen vacancies [29]. The XPS spectra of Zr 3d in Figure 8b shows that there is no difference in the binding energies of Zr element in the CeZrK/rGO catalysts. The binding energies of Zr 3d5/2 and Zr 3d3/2 were 182.6 eV and 185 eV, respectively, which can prove the existence of Zr +4 in the CeZrK/rGO catalysts [44]. The XPS spectra of O 1s in Figure 8c presents two types of surface oxygen in the CeZrK/rGO catalysts. The OI (529.8-530.0 eV) is the characteristic of the surface lattice oxygen (O 2− ) in cerium oxide, and the OII (531.5-531.8 eV) indicates the presence of chemisorbed oxygen on the surface of oxygen (O − , O2−, O2 2− ) belonging to the defect-oxygen or hydroxyl-like group [19]. In general, the surface-adsorbed oxygen was derived from the adsorption of gaseous O2 on the oxygen vacancies of the oxide catalyst. It is clear that Ce5Zr2K2/rGO had a much higher OII/OI peak area ratio than Ce5Zr1K1/rGO and Ce5Zr3K3/rGO, which effectively increased the concentration of surfaceadsorbed oxygen. Compared to the lattice oxygen, chemical adsorbed oxygen (O Ⅱ ) had higher mobility, and thus it played a more important role in the soot oxidation reaction. The oxidation states of the main elements Ce, Zr, O and C, including an estimate of the atomic ratio in the CeZrK/rGO catalysts, were achieved using the XPS measurement technique and the corresponding result is depicted in Figure 8. The atomic surface concentrations, the relative percentages, and the binding energy of Ce, Zr, O and C elements in the CeZrK/rGO catalysts are summarized in Tables 2 and 3, respectively. The u and v in the XPS spectra of Ce 3d refer to the 3d 3/2 and 3d 5/2 spin-orbit components, respectively [43]. The peaks of u, u", u"' and v, v", v"' are attributed to Ce 4+ ions, whereas u and v are attributed to Ce 3+ ions. Therefore, it can be inferred that the CeZrK/rGO catalysts had both Ce 4+ and Ce 3+ ions. The Ce 3+ /Ce 4+ ratios of the CeZrK/rGO catalysts were calculated by the ratio of the sum of the integrated areas of Ce 3+ (u , v ) to the sum of the integrated area of Ce 4+ (u, v, u", v", u"', v"') as shown in Table 4. It was found that Ce 5 Zr 2 K 2 /rGO exhibits a higher concentration of Ce 3+ ions, which indicates that Ce 5 Zr 2 K 2 /rGO contained more oxygen vacancies [29]. The XPS spectra of Zr 3d in Figure 8b shows that there is no difference in the binding energies of Zr element in the CeZrK/rGO catalysts. The binding energies of Zr 3d 5/2 and Zr 3d 3/2 were 182.6 eV and 185 eV, respectively, which can prove the existence of Zr +4 in the CeZrK/rGO catalysts [44]. The XPS spectra of O 1s in Figure 8c  2− ) belonging to the defect-oxygen or hydroxyl-like group [19]. In general, the surface-adsorbed oxygen was derived from the adsorption of gaseous O 2 on the oxygen vacancies of the oxide catalyst. It is clear that Ce 5 Zr 2 K 2 /rGO had a much higher O II /O I peak area ratio than Ce 5 Zr 1 K 1 /rGO and Ce 5 Zr 3 K 3 /rGO, which effectively increased the concentration of surface-adsorbed oxygen. Compared to the lattice oxygen, chemical adsorbed oxygen (O II ) had higher mobility, and thus it played a more important role in the soot oxidation reaction. Figure 8d displays the deconvolution of C 1s peaks in the CeZrK/rGO catalysts. The main peaks are centred in 284.5-284.6 eV, 285.5 eV, and 288.4 eV, which represents graphite structure, C-O, and -O-C=O, respectively. The C-O and -O-C=O bonds in graphene indicate that graphene can provide abundant active sites for the directional connection of CeO 2 -ZrO 2 particles. Additionally, the XPS failed to detect the presence of K element. It can be explained that the binding energy of K and C were mainly in the range of 291-296 eV and of 277-295 eV, respectively, and the content of C in the as-prepared catalysts was much larger than that of K, which may have interfered with the detection of K element. Generally, among the three CeZrK/rGO catalysts, the Ce 5 Zr 2 K 2 /rGO catalyst contained more oxygen vacancies which is beneficial to improve the OSC of CeO 2 , and the Ce 5 Zr 2 K 2 /rGO catalyst had a higher chemically adsorbed oxygen concentration. eV and of 277-295 eV, respectively, and the content of C in the as-prepared catalysts was much larger than that of K, which may have interfered with the detection of K element. Generally, among the three CeZrK/rGO catalysts, the Ce5Zr2K2/rGO catalyst contained more oxygen vacancies which is beneficial to improve the OSC of CeO2, and the Ce5Zr2K2/rGO catalyst had a higher chemically adsorbed oxygen concentration.     Figure 9 shows the result of the H 2 -TPR analyses administered on GO and CeZrK/rGO catalysts to examine their redox behavior. It was shown that the reduction peak of GO appears at 663 • C, while no obvious reduction peak was observed for the CeZrK/rGO catalysts at 550-650 • C, indicating that the oxygen-containing functional groups on the GO surface are deoxidized during the preparation [45], which agrees with the FTIR results shown in Figure 7. Three obvious reduction peaks ranging from 250 • C to 600 • C were observed for the CeZrK/rGO catalysts. The shoulder peak at 360 • C corresponds to adsorbent oxygen, and the reduction peak at 465 • C corresponds to the reduction of the outermost layer of Ce 4+ to Ce 3+ . The reduction peak ranging from 700 • C to 850 • C can be attributed to the reduction of CeO 2 (Ce 4+ inner layer) and the reduction of lattice oxygen (bulk reduction) [46]. Curve b has a higher peak intensity ratio (surface/bulk reduction) than that of other curves, which indicates the enhanced oxygen mobility within the lattice of Ce 5 Zr 2 K 2 /rGO. Due to the synergistic effect of Ce and Zr, a shoulder peak occurs around 301 • C. Moreover, the peak at around 465 • C in curve b moves to low temperature, indicating that the reducibility of Ce 5 Zr 2 K 2 /rGO improved. The reduction peak in curve c becomes flat due to the aggregation of excess metal oxides, which could impact negatively on the catalytic performance of Ce 5 Zr 3 K 3 /rGO. In general, the doping of Zr and K ions into CeO 2 with graphene as the carrier can effectively promoted the formation of a solid solution. The graphene can enhance the redox capacity of cerium oxide at low temperatures [47]. All the CeZrK/rGO catalysts showed a superior redox capacity, and this reflects on the improved catalytic oxidation activity of the soot. Meanwhile, the redox capacity of the CeZrK/rGO catalysts was affected by the metal doping ratio. Among the CeZrK/rGO catalysts, Ce 5 Zr 2 K 2 /rGO presents the lowest reduction temperature in the H 2 -TPR profiles. Figure 9 shows the result of the H2-TPR analyses administered on GO and CeZrK/rGO catalysts to examine their redox behavior. It was shown that the reduction peak of GO appears at 663 °C, while no obvious reduction peak was observed for the CeZrK/rGO catalysts at 550-650 °C, indicating that the oxygen-containing functional groups on the GO surface are deoxidized during the preparation [45], which agrees with the FTIR results shown in Figure 7. Three obvious reduction peaks ranging from 250 °C to 600 °C were observed for the CeZrK/rGO catalysts. The shoulder peak at 360 °C corresponds to adsorbent oxygen, and the reduction peak at 465 °C corresponds to the reduction of the outermost layer of Ce 4+ to Ce 3+ . The reduction peak ranging from 700 °C to 850 °C can be attributed to the reduction of CeO2 (Ce 4+ inner layer) and the reduction of lattice oxygen (bulk reduction) [46]. Curve b has a higher peak intensity ratio (surface/bulk reduction) than that of other curves, which indicates the enhanced oxygen mobility within the lattice of Ce5Zr2K2/rGO. Due to the synergistic effect of Ce and Zr, a shoulder peak occurs around 301 °C. Moreover, the peak at around 465 °C in curve b moves to low temperature, indicating that the reducibility of Ce5Zr2K2/rGO improved. The reduction peak in curve c becomes flat due to the aggregation of excess metal oxides, which could impact negatively on the catalytic performance of Ce5Zr3K3/rGO. In general, the doping of Zr and K ions into CeO2 with graphene as the carrier can effectively promoted the formation of a solid solution. The graphene can enhance the redox capacity of cerium oxide at low temperatures [47]. All the CeZrK/rGO catalysts showed a superior redox capacity, and this reflects on the improved catalytic oxidation activity of the soot. Meanwhile, the redox capacity of the CeZrK/rGO catalysts was affected by the metal doping ratio. Among the CeZrK/rGO catalysts, Ce5Zr2K2/rGO presents the lowest reduction temperature in the H2-TPR profiles.

Evaluation on Catalytic Activity for Soot Oxidation
The catalytic activity of the CeZrK/rGO catalysts for soot oxidation were evaluated under the tight contact and loose contact modes in simulated air (21% O2 + 79% N2) by thermogravimetric analysis. Figure 10 shows the normalized soot conversion under the tight contact mode as the function of temperature over the as-prepared catalysts. To provide a better comparison for the catalytic activity, T50 was defined as the temperature at which 50% soot conversion is achieved and Tm was defined as the temperature at which the maximum rate of soot combustion occurs [48]. The results show that the T50 of Ce5Zr1K1/rGO, Ce5Zr2K2/rGO, Ce5Zr3K3/rGO, and CeO2 were 352 °C, 339 °C, 358 °C, and

Evaluation on Catalytic Activity for Soot Oxidation
The catalytic activity of the CeZrK/rGO catalysts for soot oxidation were evaluated under the tight contact and loose contact modes in simulated air (21% O 2 + 79% N 2 ) by thermogravimetric analysis. Figure 10 shows the normalized soot conversion under the tight contact mode as the function of temperature over the as-prepared catalysts. To provide a better comparison for the catalytic activity, T 50 was defined as the temperature at which 50% soot conversion is achieved and T m was defined as the temperature at which the maximum rate of soot combustion occurs [48]. The results show that the T 50 of Ce 5 Zr 1 K 1 /rGO, Ce 5 Zr 2 K 2 /rGO, Ce 5 Zr 3 K 3 /rGO, and CeO 2 were 352 • C, 339 • C, 358 • C, and 407 • C, respectively. It can be inferred that the CeZrK/rGO catalysts had higher catalytic activity than CeO 2 . The difference of T 50 among the CeZrK/rGO catalysts was small due to the tight contact between soot and the catalyst. Under this condition, the difference in the structure and morphology of the CeZrK/rGO catalysts had a small impact on the catalytic activity. 407 °C, respectively. It can be inferred that the CeZrK/rGO catalysts had higher catalytic activity than CeO2. The difference of T50 among the CeZrK/rGO catalysts was small due to the tight contact between soot and the catalyst. Under this condition, the difference in the structure and morphology of the CeZrK/rGO catalysts had a small impact on the catalytic activity. The loose contact mode is the most realistic approach because soot is mixed with the catalyst without any force exertion [49]. Figure 11 shows the normalized soot conversion under the loose contact mode as a function of temperature with and without as-prepared catalysts. The T50 for Ce5Zr1K1/rGO, Ce5Zr2K2/rGO, and Ce5Zr3K3/rGO were 390 °C, 383 °C, and 432 °C, respectively, which were far less than those of CeO2 (523 °C), rGO (532 °C) and catalyst-free (604 °C). The catalytic activity of rGO on soot oxidation was presented due to the fact that rGO contains residual structural defects, which improve oxygen reduction reactions. The higher catalytic activity of Ce5Zr2K2/rGO was closely related to its superior dispersion quality, small particle size (Figure 2d), large specific surface area (Table 1), and abundant oxygen vacancies ( Figure 8 and Table 3) and redox capacity ( Figure  9). It should be noted that the slope of the curve before 300 °C was arranged in the order of Ce5Zr3K3/rGO > Ce5Zr2K2/rGO > Ce5Zr1K1/rGO, which is consistent with the order of the specific surface area (Table 1) of the three CeZrK/rGO catalysts. The slope of the curve after 300 °C was arranged in the order of Ce5Zr2K2/rGO > Ce5Zr1K1/rGO > Ce5Zr3K3/rGO, which is consistent with the order of oxygen vacancy concentration (Ce 3+ /Ce 4+ ) and the concentration of chemically adsorbed oxygen (O Ⅱ /O Ⅰ ) ( Table 4). This may be due to the fact that the specific surface area had a greater influence on the activity in the low temperature region, while the oxygen vacancy concentration had a greater influence on the activity in the medium and high temperature region. In order to avoid the influence of graphene on experimental data, the weight loss of modified graphene under air atmosphere was investigated. As shown in Figure 12, the weight loss was 2.3 (wt)% and mainly occurred at 520 °C, which had little impact on the TG experimental data.  The loose contact mode is the most realistic approach because soot is mixed with the catalyst without any force exertion [49]. Figure 11 shows the normalized soot conversion under the loose contact mode as a function of temperature with and without as-prepared catalysts. The T 50 for Ce 5 Zr 1 K 1 /rGO, Ce 5 Zr 2 K 2 /rGO, and Ce 5 Zr 3 K 3 /rGO were 390 • C, 383 • C, and 432 • C, respectively, which were far less than those of CeO 2 (523 • C), rGO (532 • C) and catalyst-free (604 • C). The catalytic activity of rGO on soot oxidation was presented due to the fact that rGO contains residual structural defects, which improve oxygen reduction reactions. The higher catalytic activity of Ce 5 Zr 2 K 2 /rGO was closely related to its superior dispersion quality, small particle size (Figure 2d), large specific surface area (Table 1), and abundant oxygen vacancies ( Figure 8 and Table 3) and redox capacity ( Figure 9). It should be noted that the slope of the curve before 300 • C was arranged in the order of Ce 5 Zr 3 K 3 /rGO > Ce 5 Zr 2 K 2 /rGO > Ce 5 Zr 1 K 1 /rGO, which is consistent with the order of the specific surface area (Table 1) of the three CeZrK/rGO catalysts. The slope of the curve after 300 • C was arranged in the order of Ce 5 Zr 2 K 2 /rGO > Ce 5 Zr 1 K 1 /rGO > Ce 5 Zr 3 K 3 /rGO, which is consistent with the order of oxygen vacancy concentration (Ce 3+ /Ce 4+ ) and the concentration of chemically adsorbed oxygen (O II /O I ) ( Table 4). This may be due to the fact that the specific surface area had a greater influence on the activity in the low temperature region, while the oxygen vacancy concentration had a greater influence on the activity in the medium and high temperature region. In order to avoid the influence of graphene on experimental data, the weight loss of modified graphene under air atmosphere was investigated. As shown in Figure 12, the weight loss was 2.3 (wt)% and mainly occurred at 520 • C, which had little impact on the TG experimental data. Processes 2021, 9, x FOR PEER REVIEW 13 of 18 Figure 11. Normalized soot conversion versus temperature of CeZrK/rGO catalysts and free-catalyst under loose contact mode in simulated air (21% O2 + 79% N2). The research on the surface oxygen species is necessary for the catalytic oxidation reaction. Therefore, the catalytic performance of the CeZrK/rGO catalysts in pure N2 was performed as shown in Figure 13. Due to the absence of gaseous O2, soot can only be oxidized by active adsorbed oxygen (O − , O2 -) and lattice oxygen (O 2− ) from the catalyst surface. It can be seen from DTG profiles that there was a trough for the CeZrK/rGO catalysts at 318~322 °C, which is attributed to the soot oxidation by adsorbed oxygen. The troughs for Ce5Zr2K2/rGO and Ce5Zr3K3/rGO at 513 °C and 521 °C, respectively, correspond to the soot oxidation by lattice oxygen. In comparison with the peak strength and the peak temperature, Ce5Zr2K2/rGO showed a better catalytic performance among the CeZrK/rGO catalysts, which can be derived from its abundant reactive oxygen species and reasonable metal doping concentration, improving the mobility of adsorbed oxygen and lattice oxygen.    The research on the surface oxygen species is necessary for the catalytic oxidation reaction. Therefore, the catalytic performance of the CeZrK/rGO catalysts in pure N2 was performed as shown in Figure 13. Due to the absence of gaseous O2, soot can only be oxidized by active adsorbed oxygen (O − , O2 -) and lattice oxygen (O 2− ) from the catalyst surface. It can be seen from DTG profiles that there was a trough for the CeZrK/rGO catalysts at 318~322 °C, which is attributed to the soot oxidation by adsorbed oxygen. The troughs for Ce5Zr2K2/rGO and Ce5Zr3K3/rGO at 513 °C and 521 °C, respectively, correspond to the soot oxidation by lattice oxygen. In comparison with the peak strength and the peak temperature, Ce5Zr2K2/rGO showed a better catalytic performance among the CeZrK/rGO catalysts, which can be derived from its abundant reactive oxygen species and reasonable metal doping concentration, improving the mobility of adsorbed oxygen and lattice oxygen.  The research on the surface oxygen species is necessary for the catalytic oxidation reaction. Therefore, the catalytic performance of the CeZrK/rGO catalysts in pure N 2 was performed as shown in Figure 13. Due to the absence of gaseous O 2 , soot can only be oxidized by active adsorbed oxygen (O − , O 2 -) and lattice oxygen (O 2− ) from the catalyst surface. It can be seen from DTG profiles that there was a trough for the CeZrK/rGO catalysts at 318~322 • C, which is attributed to the soot oxidation by adsorbed oxygen. The troughs for Ce 5 Zr 2 K 2 /rGO and Ce 5 Zr 3 K 3 /rGO at 513 • C and 521 • C, respectively, correspond to the soot oxidation by lattice oxygen. In comparison with the peak strength and the peak temperature, Ce 5 Zr 2 K 2 /rGO showed a better catalytic performance among the CeZrK/rGO catalysts, which can be derived from its abundant reactive oxygen species and reasonable metal doping concentration, improving the mobility of adsorbed oxygen and lattice oxygen. Processes 2021, 9, x FOR PEER REVIEW 14 of 18 Figure 13. DTG plots of soot combustion in tight contact mode over CeZrK/rGO catalysts in a pure N2 atmosphere. Tables 5 and 6 compare the catalytic performance of various catalysts with high catalytic activity, including perovskite catalysts, noble metal catalysts, and CeZrK/rGO catalysts for soot oxidation [9,19,30,[50][51][52][53][54][55][56][57][58][59]. It is clear that the CeZrK/rGO catalysts by doping metal oxide particles onto graphene have higher catalytic activity than other catalysts, especially for Ce5Zr2K2/rGO. Generally, Ce5Zr2K2/rGO is a promising catalyst for the catalytic oxidation of soot in the CDPF owing to the advantages of easy synthesis, low cost, and high catalytic activity.  Tables 5 and 6 compare the catalytic performance of various catalysts with high catalytic activity, including perovskite catalysts, noble metal catalysts, and CeZrK/rGO catalysts for soot oxidation [9,19,30,[50][51][52][53][54][55][56][57][58][59]. It is clear that the CeZrK/rGO catalysts by doping metal oxide particles onto graphene have higher catalytic activity than other catalysts, especially for Ce 5 Zr 2 K 2 /rGO. Generally, Ce 5 Zr 2 K 2 /rGO is a promising catalyst for the catalytic oxidation of soot in the CDPF owing to the advantages of easy synthesis, low cost, and high catalytic activity.

Conclusions
Today the environmental pollution [62][63][64][65] and energy crisis [66][67][68] have to learn another new problem. In this study, a new nanostructured solid solution catalyst CeZrK/rGO was prepared for the catalytic oxidation of soot by loading CeO 2 -ZrO 2 -K 2 O nanoparticles on the surface of rGO. Some major conclusions can be summarized as follows: 1.
The use of rGO as the carrier of CeZrK/rGO catalysts provides a large specific surface area and special pore structure, increasing the contact between soot and catalyst compared to the Ce-based catalysts without rGO.

2.
The doping of Zr and K ions in the CeZrK/rGO catalysts leads to the lattice distortion of CeO 2 , which can increase oxygen vacancies and active oxygen species, control the crystallite growth of CeO 2 , and ensure the dispersion quality of active sites.

3.
The characterization studies show that the CeZrK/rGO catalysts have nanoscale pore structure, superior dispersion quality, small particle size, large specific surface area, as well as sufficient oxygen vacancies. 4.
The T 50 related to soot conversion of Ce 5 Zr 1 K 1 /rGO, Ce 5 Zr 2 K 2 /rGO, and Ce 5 Zr 3 K 3 /rGO under tight contact and loose contact conditions were 352 • C, 339 • C, 358 • C and 390 • C, 383 • C, 432 • C, respectively, which are smaller than those of some typical rare earth, perovskite, and precious metal catalysts, such as CeO 2 , 3DOM La 0.8 Ce 0.2 FeO 3 , Pt/Al 2 O 3 , etc. It is indicated that the CeZrK/rGO catalysts have high catalytic activity for soot oxidation, especially for Ce 5 Zr 2 K 2 /rGO.  Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.

Data Availability Statement:
The data presented in this study are available in the main text of the article.