Effect of Particle Size and Crystal Surface of CeO2 on the Catalytic Combustion of Benzene

In this study, three kinds of CeO2 were synthesized, and supported PdOx (x = 0,1) catalysts were prepared for benzene catalytic combustion. The samples were characterized by XRD, N2 adsorption/desorption, HRTEM, XPS and H2-TPR. The results show that three kinds of CeO2 with different structures can be formed by different preparation methods. This is mainly reflected in the differences in pore structure, particle size and crystal plane. CeO2-DC obtained from directly calcined Ce(NO3)3·6H2O had the largest pore volume and pore diameter and smallest particle size. CeO2-DC was mainly exposed to the (200) plane. Combined with the results of the ability test, it could be concluded that when Pd2+ and Pd0 exist at the same time, the activity increases with an increase in the proportion of Pd2+. Meanwhile, the structure of CeO2 affects the formation of oxygen vacancies, thereby affecting the adsorption and degradation of benzene. This article reveals that the particle size, crystal planes, oxygen vacancies and proportion of Pd2+ have a great impact on the catalytic combustion of benzene and allow a more comprehensive understanding of the structure–activity relationship, which can guide us to design high-efficiency catalysts targeted to obtain suitable CeO2-based catalysts for the catalytic combustion of benzene.


Introduction
Volatile organic compounds (VOCs) are a main component of air pollution and have been increasing rapidly in recent years. Most VOCs not only harm the environment but also threaten human health due to their toxicity and carcinogenicity [1][2][3]. Aromatic hydrocarbons, as a kind of VOCs, are considered to cause great harm to the environment and are usually toxic and carcinogenic [4,5]. Benzene is a kind of carcinogen, which can be determined to be carcinogenic. Toluene, ethylbenzene and xylene are all possible carcinogens [6]. Among the aromatic hydrocarbons, benzene, toluene, ethylbenzene and xylene (BTEX) constitute the majority of the total industrial emissions [7]. Therefore, it is very urgent to develop an effective and suitable method to reduce the total amount of VOCs. The main methods for dealing with VOCs include adsorption, membrane separation, absorption and oxidative approaches (thermal incineration, biological degradation, photocatalytic decomposition, nonthermal plasma oxidation and catalytic combustion) [8][9][10][11][12][13]. Catalytic combustion has become the most widely used technology because of its characteristics of high catalytic efficiency, low energy consumption and low secondary pollution [14][15][16].
Catalytic combustion is a typical gas-solid catalytic reaction. In the reaction, VOCs need to be adsorbed on the surface of the catalyst, and then a catalytic degradation reaction is carried out. Supports with a high specific surface area (S BET ) and rich pore structure can improve the adsorption property and the catalytic performance of VOCs [8]. Thus, it is important to identify excellent supports where: D = Average thickness of particle perpendicular to crystal plane (nm); K = Scherrer constant; γ = X-ray wavelength (0.154 nm); B = Full width at half maxima (FWHM); θ = Diffraction angle.
A Tristar II 3020 apparatus (Micromeritics Company, Norcross, GA, USA) was used to determine the nitrogen adsorption and desorption isotherms at −195.8 • C. The Brunauer-Emmett-Teller (BET) model was used to measure the S BET . Meanwhile, the Barrett-Joyner-Halenda (BJH) method was used to calculate the pore size and average pore diameter. Before the adsorption process, the samples were kept under vacuum at 200 • C for 4 h.
The morphology of the supports and the catalysts was obtained using high-resolution transmission electron microscopy (HRTEM, JEOL-2010F, Tokyo, Japan) operated at 200 kV. The chemical composition of the Pd/CeO 2 catalyst was detected using energy-dispersive X-ray spectroscopy (EDS) with an Oxford INCA instrument (OXFORD instruments, Abingdon, UK). The valence states and the elemental proportions of samples were analyzed by X-ray photoelectron spectroscopy (XPS) (Thermo Fisher Scientific, Waltham, MA, USA) on a Thermo ESCALAB 250 with Al Kα (hν = 1486.8 eV) as the excitation source. C1s was used as the internal reference to calibrate electron energies.
Temperature-programmed reduction (H 2 -TPR) experiments were carried out in a CHEMBET-3000 apparatus prior to the measurement, and 200 mg catalyst was put into the quartz reactor and pretreated in air at 300 • C for 0.5 h. After being cooled to room temperature, the catalyst was raised from room temperature to 900 • C under H 2 flow (5 vol.% in Ar, 40 mL/min) at a heating rate of 15 • C/min. A gas chromatograph (TCD) (Shimadzu, GC-14C, Kyoto, Japan) was used to record and analyze data.

Catalytic Activity Evaluation
The catalytic activity of the CeO 2 supports and Pd/CeO 2 catalysts is shown in Figure 1. The results show that the performance of three different CeO 2 samples was obviously different, which indicated that the ability of CeO 2 was closely related to the preparation method. The catalytic combustion activity of benzene followed the sequence CeO 2 -DC > CeO 2 -MOF > CeO 2 -P. The conversion rate of CeO 2 -DC for benzene reached 91.7% at 320 • C. After loading Pd, the catalytic combustion ability of benzene was significantly improved. All the catalysts showed better catalytic ability than single CeO 2 , indicating that the active component of PdO x plays an important role in benzene catalytic combustion. Catalytic activity decreases in the following order: Pd/CeO 2 -DC > Pd/CeO 2 -MOF > Pd/CeO 2 -P. The complete conversion temperatures of benzene using Pd/CeO 2 -MOF and Pd/CeO 2 -P were 280 and 300 • C, respectively. Pd/CeO 2 -DC showed the best performance of catalytic combustion, which could completely convert benzene at 260 • C. The catalytic activity data of the samples and related catalysts for benzene catalytic combustion are compared in Table S1. Although the Pd/CeO 2 catalyst synthesized by Guo et al. can achieve 90% benzene catalytic combustion at 187 • C, they used a higher content of Pd (2 wt.%) than Pd/CeO 2 -DC [34]. Therefore, considering the comprehensive cost, Pd/CeO 2 -DC has high application value. Temperature-programmed reduction (H2-TPR) experiments were carried out in a CHEMBET-3000 apparatus prior to the measurement, and 200 mg catalyst was put into the quartz reactor and pretreated in air at 300 °C for 0.5 h. After being cooled to room temperature, the catalyst was raised from room temperature to 900 °C under H2 flow (5 vol.% in Ar, 40 mL/min) at a heating rate of 15 °C/min. A gas chromatograph (TCD) (Shimadzu, GC-14C, Kyoto, Japan) was used to record and analyze data.

Catalytic Activity Evaluation
The catalytic activity of the CeO2 supports and Pd/CeO2 catalysts is shown in Figure 1. The results show that the performance of three different CeO2 samples was obviously different, which indicated that the ability of CeO2 was closely related to the preparation method. The catalytic combustion activity of benzene followed the sequence CeO2-DC > CeO2-MOF > CeO2-P. The conversion rate of CeO2-DC for benzene reached 91.7% at 320 °C. After loading Pd, the catalytic combustion ability of benzene was significantly improved. All the catalysts showed better catalytic ability than single CeO2, indicating that the active component of PdOx plays an important role in benzene catalytic combustion. Catalytic activity decreases in the following order: Pd/CeO2-DC > Pd/CeO2-MOF > Pd/CeO2-P. The complete conversion temperatures of benzene using Pd/CeO2-MOF and Pd/CeO2-P were 280 and 300 °C, respectively. Pd/CeO2-DC showed the best performance of catalytic combustion, which could completely convert benzene at 260 °C. The catalytic activity data of the samples and related catalysts for benzene catalytic combustion are compared in Table S1. Although the Pd/CeO2 catalyst synthesized by Guo et al. can achieve 90% benzene catalytic combustion at 187 °C, they used a higher content of Pd (2 wt.%) than Pd/CeO2-DC [34]. Therefore, considering the comprehensive cost, Pd/CeO2-DC has high application value.

Durability Test
The temperature of the conversion rate at 52% was selected for the life test of the catalyst. Figure  2 shows the results of the durability test of Pd/CeO2-DC at 190 °C. After 100 h of reaction, the activity

Durability Test
The temperature of the conversion rate at 52% was selected for the life test of the catalyst. Figure 2 shows the results of the durability test of Pd/CeO 2 -DC at 190 • C. After 100 h of reaction, the activity of the catalyst remained at approximately 49%, with no obvious decrease. It exhibited exceptional stable catalytic activity. Industrial chlorine-containing VOCs and water were widely distributed in waste gases. Figure 2 shows that when 3 vol.% water was added, its catalytic activity decreased because of competitive adsorption [38]. After introducing chlorobenzene at 100 ppm into the system, the catalytic activity further decreased. However, after water vapor and chlorobenzene were removed, the activity of the catalyst returned to its original level. The results show that the Pd/CeO 2 -DC catalyst had a certain ability to resist chlorine poisoning that may be due to the interaction between PdO x and CeO 2 -DC, which can timely release the combustion products of chlorobenzene and prevent chlorine poisoning. Meanwhile, it exhibited good resistance to moisture conditions. In addition, the catalyst maintained its catalytic activity over a long reaction time of 100 h. of the catalyst remained at approximately 49%, with no obvious decrease. It exhibited exceptional stable catalytic activity. Industrial chlorine-containing VOCs and water were widely distributed in waste gases. Figure 2 shows that when 3 vol.% water was added, its catalytic activity decreased because of competitive adsorption [38]. After introducing chlorobenzene at 100 ppm into the system, the catalytic activity further decreased. However, after water vapor and chlorobenzene were removed, the activity of the catalyst returned to its original level. The results show that the Pd/CeO2-DC catalyst had a certain ability to resist chlorine poisoning that may be due to the interaction between PdOx and CeO2-DC, which can timely release the combustion products of chlorobenzene and prevent chlorine poisoning. Meanwhile, it exhibited good resistance to moisture conditions. In addition, the catalyst maintained its catalytic activity over a long reaction time of 100 h.    [26]. The XRD spectra of the precursor Ce-MOF are shown in Figure S1. From Figure 3, it can be found that the intensity and FWHM of the diffraction peak are different and decrease in the following order: CeO2-P > CeO2-MOF > CeO2-DC. According to the Scherrer formula, the average particle size of the samples can be calculated as follows: CeO2-P (34 nm) > CeO2-MOF (11 nm) > CeO2-DC (8 nm). This result indicates that CeO2-DC prepared by direct calcining had the smallest particle size. As shown in Figure 3, it can be concluded that the crystallinity of CeO2-P is the highest and that of CeO2-DC is the lowest. Combined with their particle size, it can be inferred that the smaller the particle size of CeO2, the poorer the crystallinity. It is well known that the lattice defects of CeO2 increase with decreasing particle size, so the smaller particle size of CeO2 will lead to the formation of surface lattice defects and the generation of reactive oxygen species [33]. However, no diffraction peak for the PdOx phase could be found, which may be attributed to the low loading content and the high dispersion of Pd.   [26]. The XRD spectra of the precursor Ce-MOF are shown in Figure S1. From Figure 3, it can be found that the intensity and FWHM of the diffraction peak are different and decrease in the following order: CeO 2 -P > CeO 2 -MOF > CeO 2 -DC. According to the Scherrer formula, the average particle size of the samples can be calculated as follows: CeO 2 -P (34 nm) > CeO 2 -MOF (11 nm) > CeO 2 -DC (8 nm). This result indicates that CeO 2 -DC prepared by direct calcining had the smallest particle size. As shown in Figure 3, it can be concluded that the crystallinity of CeO 2 -P is the highest and that of CeO 2 -DC is the lowest. Combined with their particle size, it can be inferred that the smaller the particle size of CeO 2 , the poorer the crystallinity. It is well known that the lattice defects of CeO 2 increase with decreasing particle size, so the smaller particle size of CeO 2 will lead to the formation of surface lattice defects and the generation of reactive oxygen species [33].

XRD Analysis
However, no diffraction peak for the PdO x phase could be found, which may be attributed to the low loading content and the high dispersion of Pd.   Figure 4 shows the morphology and crystal plane structure of CeO2 supports and their Pd-based catalysts. Figure 4a-c show that all CeO2 synthesized by different preparation methods have the morphology of nanoparticles and show an irregular shape. The particle sizes of CeO2-P, CeO2-DC and CeO2-MOF were 30-60, 3-10 and 10-30 nm, respectively. It was found that the particle size of CeO2-DC was significantly smaller than that of CeO2-P and CeO2-MOF, which was consistent with the results of XRD analysis. The fine structure of three different Pd/CeO2 was observed when the resolution was further improved. We can observe the lattice fringes of PdOx and CeO2, as shown in  (200) crystal planes. The exposed crystal surface may affect the formation of oxygen vacancies, and different Pd/CeO2 should have different chemical properties [33]. Through catalytic activity testing, we can assume that the (200) crystal plane of CeO2 may play an important role in benzene catalytic combustion. The morphology of the recovered catalyst is shown in Figure S2. It can be found that the morphology of the catalyst has no obvious change after use, indicating that the catalyst has good stability.

HRTEM Analysis
In addition, the EDS spectrum of Pd/CeO2-DC had Pd, O and Ce signals, as shown in Figure 4g, indicating that the active components of PdOx have been successfully loaded and highly dispersed in CeO2-DC and the particle size of PdOx nanoparticles is about 2-3 nm.  Figure 4 shows the morphology and crystal plane structure of CeO 2 supports and their Pd-based catalysts. Figure 4a-c show that all CeO 2 synthesized by different preparation methods have the morphology of nanoparticles and show an irregular shape. The particle sizes of CeO 2 -P, CeO 2 -DC and CeO 2 -MOF were 30-60, 3-10 and 10-30 nm, respectively. It was found that the particle size of CeO 2 -DC was significantly smaller than that of CeO 2 -P and CeO 2 -MOF, which was consistent with the results of XRD analysis. The fine structure of three different Pd/CeO 2 was observed when the resolution was further improved. We can observe the lattice fringes of PdO x and CeO 2 , as shown in Figure 4d [33]. Through catalytic activity testing, we can assume that the (200) crystal plane of CeO 2 may play an important role in benzene catalytic combustion. The morphology of the recovered catalyst is shown in Figure S2. It can be found that the morphology of the catalyst has no obvious change after use, indicating that the catalyst has good stability.

HRTEM Analysis
In addition, the EDS spectrum of Pd/CeO 2 -DC had Pd, O and Ce signals, as shown in Figure 4g, indicating that the active components of PdO x have been successfully loaded and highly dispersed in CeO 2 -DC and the particle size of PdO x nanoparticles is about 2-3 nm.

N 2 Adsorption/Desorption Analysis
The pore size distribution and S BET of the catalyst have a great influence on the activity of catalytic combustion. It can be measured by N 2 adsorption/desorption, and the results are shown in Figure 5. It can be clearly seen in Figure 5a that all the samples showed a type IV isotherm, which indicates that there are mesoporous structures in the materials [39,40]. CeO 2 -P and CeO 2 -MOF showed a H3 hysteresis loop appearing at P/P 0 = 0.4-1.0, indicating that there are slit-shaped pores in the sample [41]. Additionally, CeO 2 -DC exhibited a H1 hysteresis loop, indicating that there is a cylindrical order in the sample. The pore size distribution curve is shown in Figure 5b. From the pore size distribution curves, it can be clearly found that the average pore size of CeO 2 -DC is approximately 11.0-12.7 nm and the V P is 0.23 cm 3 /g. Meanwhile, the pore size does not change after loading PdO x nanoparticles, but the V P decreases slightly. Table 1 summarizes the physical properties of samples according to the N 2 adsorption/desorption isotherms. From Table 1, we can conclude that the S BET of three different CeO 2 is arranged as follows: CeO 2 -MOF > CeO 2 -DC > CeO 2 -P. After the introduction of PdO x nanoparticles, V P decreased in varying degrees, and some catalysts formed a microporous structure, which may be due to the blockage of the pore channels by PdO x nanoparticles. However, the S BET of CeO 2 -DC increases after loading with PdO x nanoparticles, which may be due to the fact that PdO x nanoparticles are mainly distributed on the surface of CeO 2 -DC. The results of EDS in Figure 4g also show that PdO x is mainly distributed on the surface of CeO 2 -DC and highly dispersed. Pd/CeO 2 -DC has the highest S BET (80.4 m 2 /g) and the largest V P (0.21 cm 3 /g) among all catalysts. It is well known that higher S BET and V P can provide more active sites and promote catalytic activity. The results are consistent with the experimental results of the catalytic combustion performance.

N2 Adsorption/Desorption Analysis
The pore size distribution and SBET of the catalyst have a great influence on the activity of catalytic combustion. It can be measured by N2 adsorption/desorption, and the results are shown in Figure 5. It can be clearly seen in Figure 5a that all the samples showed a type IV isotherm, which indicates that there are mesoporous structures in the materials [39,40]. CeO2-P and CeO2-MOF showed a H3 hysteresis loop appearing at P/P0 = 0.4-1.0, indicating that there are slit-shaped pores in the sample [41]. Additionally, CeO2-DC exhibited a H1 hysteresis loop, indicating that there is a cylindrical order in the sample. The pore size distribution curve is shown in Figure 5b. From the pore size distribution curves, it can be clearly found that the average pore size of CeO2-DC is approximately 11.0-12.7 nm and the VP is 0.23 cm 3 /g. Meanwhile, the pore size does not change after loading PdOx nanoparticles, but the VP decreases slightly. Table 1 summarizes the physical properties of samples according to the N2 adsorption/desorption isotherms. From Table 1, we can conclude that the SBET of three different CeO2 is arranged as follows: CeO2-MOF > CeO2-DC > CeO2-P. After the introduction of PdOx nanoparticles, VP decreased in varying degrees, and some catalysts formed a microporous structure, which may be due to the blockage of the pore channels by PdOx nanoparticles. However, the SBET of CeO2-DC increases after loading with PdOx nanoparticles, which may be due to the fact that PdOx nanoparticles are mainly distributed on the surface of CeO2-DC. The results of EDS in Figure 4g also show that PdOx is mainly distributed on the surface of CeO2-DC and highly dispersed. Pd/CeO2-DC has the highest SBET (80.4 m 2 /g) and the largest VP (0.21 cm 3 /g) among all catalysts. It is well known that higher SBET and VP can provide more active sites and promote catalytic activity. The results are consistent with the experimental results of the catalytic combustion performance.

XPS Analysis
The valence states of the elements on the surfaces of materials can be investigated using the XPS technique. Figure 6a shows the Pd 3d spectra of the catalysts. It was reported that the complex

XPS Analysis
The valence states of the elements on the surfaces of materials can be investigated using the XPS technique. Figure 6a shows the Pd 3d spectra of the catalysts. It was reported that the complex spectrum of Pd 3d can be decomposed into four peaks associated with the two spin orbitals. The peaks of 339.3-342.9 and 335.3-337.1 eV can be allocated to Pd 3d 3/2 and Pd 3d 5/2 , respectively [30]. It is obvious that Pd 0 and Pd 2+ were detected in the prepared Pd/CeO 2 -DC and Pd/CeO 2 -MOF catalysts, but only Pd 2+ was detected in the prepared Pd/CeO 2 -P catalyst. The percentage of Pd 2+ species in the catalyst was obtained by calculating the fitted area of Pd 2+ /(Pd 2+ + Pd 0 ). The concentration of Pd 2+ in catalysts is reduced in the following order: Pd/CeO 2 -P (100%) > Pd/CeO 2 -DC (71.1%) > Pd/CeO 2 -MOF (57.1%). Many researchers found that Pd 2+ plays an important role in hydrocarbon oxidation, indicating that Pd 2+ is more active than Pd 0 in the reaction [42]. However, it is worth noting that compared with other catalysts, Pd/CeO 2 -P exhibited relatively low activity, indicating that single Pd 2+ has a negative effect on benzene catalytic combustion [43].
(21.9%) > CeO2-P (17.8%). After loading PdOx, the concentration of Ce 3+ on the CeO2 surface increased, which indicates that there is a strong interaction between PdOx and CeO2. Pd/CeO2-DC had the highest proportion of Ce 3+ /(Ce 3+ + Ce 4+ ). The proportion of Ce 3+ /(Ce 3+ + Ce 4+ ) decreased in the order: Pd/CeO2-DC (24.8%) > Pd/CeO2-MOF (23.1%) > Pd/CeO2-P (18.1%). This was consistent with the higher activity of Pd/CeO2-DC. The corresponding results are listed in Table 2. It has been reported that Ce 3+ can promote the formation of oxygen vacancies [35]. Therefore, a higher ratio of Ce 3+ means more oxygen vacancies on the surface of the catalyst, which can promote the interaction between PdOx and CeO2, which is crucial for redox performance and catalytic activity. It is well known that CeO2 has good oxygen storage performance and oxygen in the CeO2 phase can be moved. Due to the strong interaction between PdOx and CeO2, Pd/CeO2-DC can transfer oxygen from CeO2 to PdOx, resulting in the appearance of more Ce 3+ and Pd 2+ . Combined with the data of activity evaluation, it can be found that the difference in activity of Pd/CeO2 catalysts with the same preparation method and the same Pd loading amount is mainly due to the different structure of CeO2. CeO2 prepared by different methods has different particle sizes and exposed crystal planes, which leads to different binding abilities between PdOx and CeO2, resulting in the change of the valence state of Pd and CeO2 on the catalyst surface, thus affecting the catalytic activity of the catalysts.    Figure 6b shows the Ce 3d spectra of the catalysts. Ce mainly exists in the form of Ce 3+ and Ce 4+ [44]. According to the equation of Ce 3+ /(Ce 3+ + Ce 4+ ), the ratio of Ce 3+ on the catalyst was calculated based on the peak area. To compare the concentration of Ce 3+ on the supports and catalysts, the concentration of Ce 3+ on the CeO 2 support was calculated. The results show that the proportion of Ce 3+ /(Ce 3+ + Ce 4+ ) on supports is reduced in the following order: CeO 2 -DC (22.5%) > CeO 2 -MOF (21.9%) > CeO 2 -P (17.8%). After loading PdO x , the concentration of Ce 3+ on the CeO 2 surface increased, which indicates that there is a strong interaction between PdO x and CeO 2 . Pd/CeO 2 -DC had the highest proportion of Ce 3+ /(Ce 3+ + Ce 4+ ). The proportion of Ce 3+ /(Ce 3+ + Ce 4+ ) decreased in the order: Pd/CeO 2 -DC (24.8%) > Pd/CeO 2 -MOF (23.1%) > Pd/CeO 2 -P (18.1%). This was consistent with the higher activity of Pd/CeO 2 -DC. The corresponding results are listed in Table 2. It has been reported that Ce 3+ can promote the formation of oxygen vacancies [35]. Therefore, a higher ratio of Ce 3+ means more oxygen vacancies on the surface of the catalyst, which can promote the interaction between PdO x and CeO 2 , which is crucial for redox performance and catalytic activity. It is well known that CeO 2 has good oxygen storage performance and oxygen in the CeO 2 phase can be moved. Due to the strong interaction between PdO x and CeO 2 , Pd/CeO 2 -DC can transfer oxygen from CeO 2 to PdO x , resulting in the appearance of more Ce 3+ and Pd 2+ . Combined with the data of activity evaluation, it can be found that the difference in activity of Pd/CeO 2 catalysts with the same preparation method and the same Pd loading amount is mainly due to the different structure of CeO 2 . CeO 2 prepared by different methods has different particle sizes and exposed crystal planes, which leads to different binding abilities between PdO x and CeO 2 , resulting in the change of the valence state of Pd and CeO 2 on the catalyst surface, thus affecting the catalytic activity of the catalysts.

H 2 -TPR Analysis
To study the redox ability of different CeO 2 and their Pd/CeO 2 catalysts, H 2 -TPR was used. As shown in Figure 7, CeO 2 exhibits one or two reduction peaks at temperatures below 600 • C, which are attributed to the reduction of CeO 2 surface oxygen and subsurface oxygen. When the temperature is higher than 800 • C, the CeO 2 lattice oxygen is reduced. The total hydrogen consumption follows the sequence CeO 2 -DC > CeO 2 -MOF > CeO 2 -P. The results show that CeO 2 -DC has the best redox capacity, which is consistent with the catalytic activity. After loading PdO x , a new peak appeared near 100 • C, and the peak disappeared between 423 and 523 • C, which indicates that there is a strong interaction between PdO x and CeO 2 which promotes the reduction of CeO 2 . The peak at low temperature is caused by the reduction of PdO x and the co-reduction of oxygen adsorbed on the CeO 2 surface. Obviously, although Pd/CeO 2 -DC shows a small acromion below 100 • C, neither Pd/CeO 2 -P nor Pd/CeO 2 -MOF has an acromion, which can be attributed to the strong binding with Pd-O-Ce, resulting in a higher PdO x reduction temperature. Therefore, the first reduction temperature of Pd/CeO 2 -DC is the lowest (70 • C); that is, the PdO x species on CeO 2 -DC is relatively easy to reduce, which leads to maintaining the palladium metal state and catalytic oxidation of benzene activity. It has been reported that the reducibility of catalysts is closely related to their catalytic activity, and catalysts with lower reduction temperatures usually show higher catalytic activity.

Conclusions
In this study, three kinds of CeO2 were obtained by purchase, calcining of Ce(NO3)3·6H2O and thermal decomposition of Ce-MOF, which exhibited different pore structures, particle sizes and crystal planes. CeO2-DC had the largest pore volume and the smallest particle size, which promotes the formation of oxygen vacancies. Meanwhile, the Pd/CeO2 catalysts were synthesized by hightemperature liquid reduction, and Pd/CeO2-DC showed the best catalytic performance, excellent durability and resistance to poisoning in the catalytic combustion of benzene due to the good properties of CeO2-DC, which could completely combust benzene at 260 °C. Moreover, the research indicates that the difference in binding ability between the exposed surfaces of CeO2 and Pd can affect catalytic activity and that CeO2 with the (200) crystal plane may play an important role in the catalytic combustion of benzene. Furthermore, the increase in the Pd 2+ proportion can promote the catalytic activity when Pd 0 and Pd 2+ exist at the same time. These characteristics indicate that the catalyst has great potential in industrial applications. However, we must realize that although this catalyst has shown high catalytic activity in the laboratory, the industrial environment is more complex and changeable. Therefore, further research in environments with high concentrations and multiple mixed VOCs is urgently required.
Supplementary Materials: The following are available online at www.mdpi.com/xxx/s1, Figure S1: The XRD diffraction pattern of the precursor Ce-MOF-120 of CeO2-MOF, Figure S2: The morphology of recovered Pd/CeO2-DC catalyst, Table S1: The catalytic activity data of the samples and related catalysts for benzene catalytic combustion.

Conflicts of Interest:
The authors declare no conflict of interest.

Conclusions
In this study, three kinds of CeO 2 were obtained by purchase, calcining of Ce(NO 3 ) 3 ·6H 2 O and thermal decomposition of Ce-MOF, which exhibited different pore structures, particle sizes and crystal planes. CeO 2 -DC had the largest pore volume and the smallest particle size, which promotes the formation of oxygen vacancies. Meanwhile, the Pd/CeO 2 catalysts were synthesized by high-temperature liquid reduction, and Pd/CeO 2 -DC showed the best catalytic performance, excellent durability and resistance to poisoning in the catalytic combustion of benzene due to the good properties of CeO 2 -DC, which could completely combust benzene at 260 • C. Moreover, the research indicates that the difference in binding ability between the exposed surfaces of CeO 2 and Pd can affect catalytic activity and that CeO 2 with the (200) crystal plane may play an important role in the catalytic combustion of benzene. Furthermore, the increase in the Pd 2+ proportion can promote the catalytic activity when Pd 0 and Pd 2+ exist at the same time. These characteristics indicate that the catalyst has great potential in industrial applications. However, we must realize that although this catalyst has shown high catalytic activity in the laboratory, the industrial environment is more complex and changeable. Therefore, further research in environments with high concentrations and multiple mixed VOCs is urgently required.