3.1. 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
5Zr
2K
2/rGO presents the most evenly dispersed metal oxide particles compared to Ce
5Zr
1K
1/rGO and Ce
5Zr
3K
3/rGO. By using Nano measurer software [
27], the average particle size of Ce
5Zr
1K
1/rGO, Ce
5Zr
2K
2/rGO, and Ce
5Zr
3K
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 Ce
5Zr
2K
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
5Zr
2K
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
5Zr
2K
2/rGO was relatively wide among the CeZrK/rGO catalysts, indicating the crystallite size of Ce
5Zr
2K
2/rGO was smaller than that of Ce
5Zr
1K
1/rGO and Ce
5Zr
3K
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 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 CeO
2-ZrO
2-K
2O 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
5Zr
1K
1/rGO, Ce
5Zr
2K
2/rGO, and Ce
5Zr
3K
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.
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
5Zr
1K
1/rGO, Ce
5Zr
2K
2/rGO, and Ce
5Zr
3K
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
5Zr
2K
2/rGO (151.5 m
2/g) and Ce
5Zr
3K
3/rGO (152.4 m
2/g) were slightly larger than that of Ce
5Zr
1K
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
5Zr
1K
1/rGO, Ce
5Zr
2K
2/rGO, Ce
5Zr
3K
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
5Zr
2K
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.
The FTIR spectra of CeZrK/rGO catalysts and GO shown in
Figure 7 reveals numerous O-containing functional groups such as (O-H) (3423 cm
−1) and (C=O) (1735 cm
−1) of COOH, (O-H) of tertiary C-OH (1384 cm
−1), and (H
2O) (1637 cm
−1) [
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.
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
Table 2 and
Table 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
5Zr
2K
2/rGO exhibits a higher concentration of Ce
3+ ions, which indicates that Ce
5Zr
2K
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 presents two types of surface oxygen in the CeZrK/rGO catalysts. The O
I (529.8–530.0 eV) is the characteristic of the surface lattice oxygen (O
2−) in cerium oxide, and the O
II (531.5-531.8 eV) indicates the presence of chemisorbed oxygen on the surface of oxygen (O
−, O
2−, O
22−) 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
5Zr
2K
2/rGO had a much higher O
II/O
I peak area ratio than Ce
5Zr
1K
1/rGO and Ce
5Zr
3K
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
5Zr
2K
2/rGO catalyst contained more oxygen vacancies which is beneficial to improve the OSC of CeO
2, and the Ce
5Zr
2K
2/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
5Zr
2K
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
5Zr
2K
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
5Zr
3K
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
5Zr
2K
2/rGO presents the lowest reduction temperature in the H
2-TPR profiles.
3.2. 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
5Zr
1K
1/rGO, Ce
5Zr
2K
2/rGO, Ce
5Zr
3K
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.
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
5Zr
1K
1/rGO, Ce
5Zr
2K
2/rGO, and Ce
5Zr
3K
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
5Zr
2K
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
5Zr
3K
3/rGO > Ce
5Zr
2K
2/rGO > Ce
5Zr
1K
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
5Zr
2K
2/rGO > Ce
5Zr
1K
1/rGO > Ce
5Zr
3K
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.
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
5Zr
2K
2/rGO and Ce
5Zr
3K
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
5Zr
2K
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.
Table 5 and
Table 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
5Zr
2K
2/rGO. Generally, Ce
5Zr
2K
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.