Preparation of Ce–Mn Composite Oxides with Enhanced Catalytic Activity for Removal of Benzene through Oxalate Method

Abstract

The catalytic activities of CeO₂-MnO_x composite oxides synthesized through oxalate method were researched. The results exhibited that the catalytic properties of CeO₂-MnO_x composite oxides were higher than pure CeO₂ or MnO_x. When the Ce_at/Mn_at ratio was 3:7, the catalytic activity reached the best. In addition, the activities of CeO₂-MnO_x synthesized through different routes over benzene oxidation were also comparative researched. The result indicated that the catalytic property of sample prepared by oxalate method was better than others, which maybe closely related with their meso-structures. Meanwhile, the effects of synergistic interaction and oxygen species in the samples on the catalytic ability can’t be ignored.

1. Introduction

Volatile organic compounds (VOCs) produced by industrial manufacturing are an important class of air pollutants. Among the VOCs, aromatic compounds are one of the major hazardous pollutants, in which benzene is considered to be one of the representing aromatic materials extensively applied in the industry. The complete catalytic oxidation of benzene is often studied as a model reaction, characteristic of the catalytic combustion of VOCs due to its chemical stability [1,2,3,4,5]. The selection of catalysts is important to catalytic degradation of benzene. At present, both classes of catalysts i.e., noble metals and transition metal oxides, have been widely studied for the degradation of VOCs [6,7,8,9]. However, the usage of noble-metal-based catalysts is limited due to high cost, low thermal stability and sensitivity to poisoning. Transition metal oxide-based catalysts are suitable alternative because of higher thermal stability and lower price [10]. In certain cases, transition metal oxides can be actually more active than noble metal catalysts [11].

Ceria (CeO₂), as a typical rare earth oxide, was investigated in heterogeneous catalysis field due to its high oxygen storage capacity. It can provide active oxygen species to ensure the catalytic reaction. More recently, CeO₂-based composite oxides were employed for VOCs removal and obtain satisfied results, especially Ce-Mn composites [12,13,14]. CeO₂-MnO_x can be applied as heterogeneous catalysts for the abatement of contaminants in the liquid and gas phases, such as the catalytic reduction of NO and oxidation of acrylic acid and formaldehyde, which exhibit much higher catalytic activity than those of pure MnO_x and CeO₂ [15,16,17,18].

In our previous work [19], Mn element was also doped or mixed with CeO₂ to obtain Ce-Mn composites through hydrothermal method. Their catalytic behaviours over benzene oxidation were researched, among which, all of Ce-Mn composites exhibited higher activity than MnO_x and CeO₂. However, the best conversion temperature of benzene oxidation over Ce-Mn composite was ca. 375 °C, which could not be applied in the moderate or lower temperature range (100–200 °C) [19], even if supporting noble metal species [20]. Therefore, we need to prepare catalysts with better performance through adjusting microstructure. In this article, we report a series of Ce-Mn composites synthesized through different routes. Their catalytic activities over benzene oxidation are researched comparatively so as to acquire more active catalyst in the temperature range of 100 and 200 °C. Their microstructures are also analyzed in detail so that understanding the elements influencing the activity of samples.

2. Experimental

2.1. The Preparation of Ce-Mn Composite Oxides

The chemicals used in this work, including Ce(NO₃)₃·6H₂O (99%), Mn(CH₃COO)₂·4H₂O, Na₂CO₃, C₂H₂O₄·2H₂O, NaOH (98%), and ethanol, were purchased from Beijing Chemicals Company (Beijing, China). A series of Ce-Mn composite oxides were synthesized by carbonate method. Briefly, Ce(NO₃)₃·6H₂O and Mn(CH₃COO)₂·4H₂O in appreciate amounts were dissolved in a 100 mL H₂O and mixed with a 0.24 M C₂H₂O₄·2H₂O solution under strong stirring. Then, the mixture was stirred for another 1 h. The precipitates were collected by centrifugation, washed with distilled water and ethanol several times. The obtained materials, labeled as Ce_xMn_1−x (where x refers to the Ce/(Ce + Mn) atomic ratio) were dried at 80 °C overnight and calcined at 450 ℃ for 2 h with a heating rate of 2 °C·min⁻¹. Pure CeO₂ and MnO_x were also prepared using the similar process as reference. In order to compare the catalytic activity, Ce-Mn composite oxides were also synthesized through carbonate method and hydrothermal method [19,21].

2.2. Characterization Technique

The crystal phase of the materials was characterized on X-ray diffraction (XRD, Philips, Amsterdam, The Netherlands) equipped with a Cu Kα radiation source (λ = 0.154187 nm) at a scanning rate of 0.03 °/s (2θ from 10° to 90°). The assignment of the crystalline phases was based on the ICSD data base (CeO₂ no. 81-0792; Mn₃O₄ no. 80-0382; Mn₂O₃ no. 89-4836). The morphology images of catalysts were recorded on a scanning electron microscopy (SEM, JEOL JSM-6700F, Tokyo, Japan) operating at 15 kV and 10 μA. The microstructures of catalysts were examined using transmission electron microscopy (TEM, JEOL JEM-2010F) with an accelerating voltage of 200 kV.

The BET specific surface area (S_BET) was measured by physical adsorption of N₂ at the liquid nitrogen temperature using an Autosorb-1 analyzer (Quantachrome, Boynton Beach, FL, USA). Before measurement, the samples were degassed at 300 °C for 4 h under vacuum. Surface composition was determined by X-ray photoelectron spectroscopy (XPS, VG Scientific, Waltham, MA, USA) using an ESCALab220i-XL electron spectrometer from VG Scientific with a monochromatic Al Kα radiation. The binding energy (BE) was referenced to the C1s line at 284.8 eV from adventitious carbon.

Hydrogen temperature-programmed reduction (H₂-TPR) was performed with a U-type quartz reactor equipped with Automated Catalyst Characterization System (Autochem 2920, MICROMERITICS). A 50 mg sample (40–60 mesh) was loaded and pretreated with a 5% O₂ and 95% He mixture (30 mL/min) at 150 °C for 1 h and cooled to 50 °C under He flow. The samples were then heated to 900 °C at a rate of 10 °C/min under the flow of a 10% H₂ and 90% Ar mixture (30 mL/min).

2.3. Catalytic Activity Tests

Activity tests for catalytic oxidation of benzene over Ce_xMn_1−x composite catalysts were performed in a continuous-flow fixed-bed reactor under atmospheric pressure, containing 100 mg of catalyst samples (40–60 mesh). A standard reaction gas containing 1000 ppm benzene and 20% O₂ in N₂ was fed with a total flow rate of 100 mL/min. The weight hourly space velocity (WHSV) was typically 60,000 mL·g⁻¹·h⁻¹. The reactants and the products were analyzed on-line using a GC/MS 6890N gas chromatograph (Hewlett-Packard, Palo Alto, CA, USA) interfaced to a 5973N mass selective detector (Hewlett-Packard, Palo Alto, USA) with a HP-5MS capillary column (30 m × 0.25 mm × 0.25 μm) and another GC (GC112A, Shangfen, Shanghai, China) with a carbon molecule sieve column. The conversion of benzene (X_benzene, %) was calculated as follows:

$$X_{\mathrm{benzene}}=\frac{C_{\mathrm{benzene}\left(\mathrm{in}\right)}-C_{\mathrm{benzene}\left(\mathrm{out}\right)}}{C_{\mathrm{benzene}\left(\mathrm{in}\right)}}\times 100\%$$

(1)

where, C_{benzene (in)} (ppm) and C_{benzene (out)} (ppm) are the concentrations of benzene in the inlet and outlet gas, respectively.

3. Results and Discussion

3.1. Catalytic Oxidation Activity of Ce_xMn_1−x Composite Oxides for Benzene

The catalytic performance of CeO₂, MnO_x and Ce_xMn_1−x catalysts was evaluated through the oxidation of benzene. The catalytic conversion of benzene as a function of the temperature, 100–400 °C, is shown in Figure 1a. It can be acquired that MnO_x exhibits the least active followed by CeO₂. With the Mn element adding into CeO₂, the activity increases monotonically up to a Mn content of 70 at.% and Ce_0.3Mn_0.7 is the most active among all catalysts achieving complete benzene conversion at ca. 200 °C. The MnO_x and CeO₂ catalysts synthesized through oxalate route also achieve full conversion at ca. 300 °C. In addition, the activities of Ce_0.3Mn_0.7 synthesized through different routes over benzene oxidation are comparative researched in order to identify the advantage of oxalate route (Figure 1b). The result indicates that the sample exhibits higher activity than that of corresponding samples synthesized by hydrothermal or carbonate routes, which is probably related with the microstructure of catalyst. For the purposes of comparison, the reaction temperatures T_10%, T_50%, T_90% (corresponding to the benzene conversion = 10%, 50%, 90%) used to evaluate the performances of the catalysts are summarized in

Table 1. Catalytic activities of CeO₂, MnO_x and Ce_xMn_1−x catalysts.

Catalyst	Benzene Oxidation Activity
	T10% (°C)	T50% (°C)	T90% (°C)
CeO2	145	200	257
Ce0.7Mn0.3	153	200	220
Ce0.5Mn0.5	128	185	218
Ce0.3Mn0.7	117	156	190
MnOx	194	241	273
Ce0.3Mn0.7 (Hydrothermal)	200	265	350
Ce0.3Mn0.7 (Carbonate)	150	220	250

.

3.2. Characterization of Ce_xMn_1−x Catalysts

Figure 2a shows the XRD patterns of the samples in the angular range 10–70 2θ. The diffraction peaks at 2θ = 28.5, 33.0, 47.4, 56.4 and 59.2 in the XRD profile of the pure cerium oxide clearly demonstrate the presence of cubic fluorite structure of CeO₂ (JCPDS 81-0792). For pure MnO_x, the XRD pattern exhibits complex diffraction peaks which can be considered as a mixture of crystalline Mn₃O₄ (JCPDS 80-0382) and Mn₂O₃ (JCPDS 89-4836). The data demonstrates that manganese oxide is a multivalent framework manganese (2⁺ and 3⁺), which indicates the existence of various manganese oxides. For Ce_xMn_1−x catalyst, the XRD patterns exhibit more complex. The XRD patterns of the Ce_xMn_1−x mixed oxide (x ≥ 0.5) do not show any diffraction of manganese oxides, and only broad reflections attributed to CeO₂ are observed, which is possible to be related with the formation of solid solution between MnO_x and CeO₂. The diffraction patterns of Ce_xMn_1−x mixed oxides at x ≤ 0.3 show crystallization of Mn₃O₄ and Mn₂O₃ except that of CeO₂.

The formation of solid solution between MnO_x and CeO₂ can be further evidenced by the fact that the characteristic diffraction peak of CeO₂ in the composite oxides is slightly shifted to higher values of the Bragg angles, compared with the pure CeO₂ (Figure 2b). Since the ionic radius of Mn²⁺ (0.091 nm) and Mn³⁺ (0.066 nm) are both smaller than that of the Ce⁴⁺ (0.1098 nm), the incorporation of Mn²⁺ or Mn³⁺ into the CeO₂ lattice to form CeO₂-MnO_x solid solution would result in remarkable decrease in the lattice parameter of CeO₂ in the Ce_xMn_1−x composite oxide. Meanwhile the O vacancy is also easier to form in order to balance charge due to Ce⁴⁺ replaced by Mn²⁺ or Mn³⁺ in the Ce_xMn_1−x catalyst. The oxygen vacancy is beneficial to catalytic activities of Ce_xMn_1−x catalyst [19].

The SEM images of as-prepared Ce_xMn_1−x oxide catalysts are presented in Figure 3a. For pure oxide CeO₂, it can be seen that the catalyst is composed of many thin flakes, which are overlapped together to form butterfly-like structure (inset picture). The thickness of every flake is about 200 nm and the length can extend to several micrometers. In the SEM image of MnO_x, a lot of grains with ellipsoid-like morphology are seen clearly and the size is ca.10 μm. At the surface of every grain, deep ravines are also obviously observed (inset picture). For Ce_xMn_1−x composite catalysts, the morphology changes gradually with the Mn content increasing. When the Mn theoretical content reaches 50% (Ce_0.5Mn_0.5), a few of bulk-like particles with size of several micrometers can be detected except thin flakes. When the theoretical content of Mn ion reaches to 70% (Ce_0.3Mn_0.7), a large number of grains possessing layered structure can be observed. In addition, it can be acquired that Ce, Mn and O elements are dispersed together homogeneously through element distribution over Ce_xMn_1−x composite catalysts.

Through the discussion over catalytic activities of Ce_xMn_1−x, it has been acquired that Ce_0.3Mn_0.7 presents the highest activity and preparation route is also important to the property of catalyst. Therefore, the TEM images of Ce_0.3Mn_0.7 prepared through different technology routes are shown so as to analyze their microstructures (Figure 3b). In the TEM image of Ce_0.3Mn_0.7 synthesized by oxalate route, the catalyst is composed of thin flakes. At the surface of flake, some mesoporous structures can be observed and the size of mesoporous is ca. 2 nm. As we known, the oxalate chains chelated in the corresponding precursors are easy to be decomposed into CO_x and H₂O with calcination in air, which can leave behind large numbers of voids due to the release of gaseous CO_x and H₂O [21]. Meanwhile, some primary nanoparticles are assembled together thereby forming porous structure, which is beneficial to absorb and desorb the gas due to the formation of massive active sites and decrease of mass transfer effect [22,23]. For Ce_0.3Mn_0.7 synthesized by carbonate route, some grains with dumbbell shape can be seen, which are composed of some stacked nanoparticles with the size of 1–2 nm. In the TEM of Ce_0.3Mn_0.7 prepared through hydrothermal method, a lot of nanorods with the diameter of ca. 10 nm and the length of 300–400 nm are observed, which is also composed of some assembled nanoparticles.

In addition, the N₂ adsorption–desorption isotherms and the pore size distribution of the as-prepared catalysts are displayed in Figure S1. The data show that the isotherms of the as-prepared materials possess type IV characteristics with well-developed H3 type hysteresis loops. The result indicates that the Ce_xMn_1−x composite catalysts possess porous structure, which is consistent with the result of SEM. The porous structure can facilitate the adsorption and diffusion of reactive molecules, thus greatly reducing limitations of inter-phase mass transfer and enhancing their catalytic activities [21].

The oxidation state of catalyst surface species was examined by XPS analysis. Figure 4 exhibits XPS patterns of Ce 3d, Mn 2p, Mn 3s and O 1s for samples, respectively. In the Ce 3d spectrum of support (Figure 4a), six peaks labeled as V₀ (882.1 eV), V₁ (888.7 eV), V₂ (898.1 eV), V₀′ (900.7 eV), V₁′ (907.1 eV) and V₂′ (916.3 eV) can be identified as characteristic of Ce⁴⁺ 3d final states [24,25]. The high BE doublet (V₂/V₂′) is attributed to the final state of Ce(IV)3d⁹4f⁰O2p⁶, doublet V₁/V₁′ is originated from the state of Ce(IV)3d⁹4f¹O2p⁵, and doublet V₀/V₀′ corresponds to the state of Ce(IV)3d⁹4f²O2p⁴. The character peaks of Ce³⁺ are also observed at 903.4/884.7 eV and 897.6/879.3 eV labeled as U₁/U₁′ and U₀/U₀′, respectively [26]. The amount of Ce³⁺ is estimated to be 11.05, 10.89, 10.05 and 5.65% for CeO₂, Ce_0.7Mn_0.3, Ce_0.5Mn_0.5 and Ce_0.3Mn_0.7, which can be calculated according to the Equation (2). Therefore, Ce species in the Ce_xMn_1−x composite oxides exist mainly in tetravalent oxidation state.

$$X_{C{e}^{3+}}=\frac{\raisebox{1ex}{$A_{{}_{C{e}^{3+}}}$}\!\left/ \!\raisebox{-1ex}{$S_{Ce}$}\right.}{{\displaystyle \sum \raisebox{1ex}{$A_{(}{}_{C{e}^{3+}+C{e}^{4+})}$}\!\left/ \!\raisebox{-1ex}{$S_{Ce}$}\right.}}\times 100\%$$

(2)

where $X_{C{e}^{3+}}$ is the percentage content of Ce³⁺, A is the integrate area of characteristic peak in the XPS pattern, S is sensitivity factors (S = 7.399).

Figure 4 shows the Mn 2p XPS spectra of Ce_xMn_1−x composite oxides, in which Mn 2p doublet can be distinguished. The binding energies of the Mn 2p_3/2 component appear at 641.7 eV and those for Mn 2p_1/2 appear at 653.2 eV. The BE values of the Mn 2p_3/2 (641.7 eV) and spin–orbit splitting (11.7 eV) are well matching with the reported values of the trivalent manganese [27]. The shoulder of the Mn 2p_3/2 component at 640.7 eV is attributed to Mn²⁺ species [28]. The XPS results do not provide any evidence for the presence of Mn⁴⁺ species (642.2–643 eV) [29,30]. In order to determine the chemical states of Mn further, Mn 3s XPS spectra of Ce_xMn_1−x are analyzed (Figure 4c). The spin−orbit splitting value (ΔEs) between the two doublets was 5.44 eV for all samples, closing to the value of 5.1 for the standard sample of α-Mn₂O₃. The ΔEs of MnO is about 6.3 eV, indicating that the oxidation status of Mn is predominantly tervalent [31,32]. The average oxidation state of Mn was calculated using formula A_OS = 8.95 − 1.13 × ΔEs [33]. The data was calculated to be 2.80 that fall in between the average oxidation state of Mn (+2.67) in Mn₃O₄ and the state of Mn (+3) in Mn₂O₃. Therefore, the element Mn in the Ce_xMn_1−x catalysts is existed in the form of Mn₃O₄ and Mn₂O₃, which is consistent with the data of XRD. In addition, the peak of Mn 2p3/2 (641.5 eV) for Ce_xMn_1−x composites is shifted to lower binding energy comparing with pure MnO_x, which may be a consequence of interaction between CeO₂ and MnO_x [19]. It is worthy to note that the pattern and the intensity of Ce_0.7Mn_0.3 is similar and less weaker compared with other Ce_xMn_1−x composites. The corresponding XPS spectrum can be seen in the Figure S2.

The XPS O1s spectra (Figure 4d) show a main peak at a binding energy of 529.1–529.9 eV, corresponding to lattice oxygen of CeO₂ and MnO_x phases(O²⁻; denoted as O_α) [27,30]. A broad shoulder at the higher binding energy region (531.3–531.8 eV) is ascribed to defective oxides or oxygen species of the surface carbonates and hydroxide (denoted as O_β) [18,34]. It is worthy to note that the peak corresponding to lattice oxygen in Ce_xMn_1−x composite catalyst with higher Mn content tends to shift toward higher BE value than that of pure CeO₂ (from 529.1 eV to 529.6 eV), which suggests that the environments of oxygen change with increasing Mn content. This appearance is also attributed to the interaction between CeO₂ and MnO_x. In addition, the content of O_α is calculated according to the Equation (3) and listed in

Table 2. XPS results of Ce_xMn_1−x samples.

Sample	O		Oα/(Oα + Oβ) (%)	Ce (%)	Mn (%)
	Oα	Oβ		Ce3+/(Ce3+ + Ce4+)	Mn2+/(Mn2+ + Mn3+)
CeO2	529.0	531.3	77.16	11.05	-
Ce0.7Mn0.3	529.1	531.3	81.01	10.89	16.23
Ce0.5Mn0.5	529.1	531.3	82.04	10.05	15.12
Ce0.3Mn0.7	529.2	531.5	83.18	5.65	13.42
MnOx	529.6	531.6	78.47	-	10.69

. The data shows that the Ce_0.3Mn_0.7 sample possesses more lattice oxygen species, confirming that the mobility and availability of lattice oxygen species are enhanced due to the synergistic effects of CeO₂ and MnO_x in Ce_0.3Mn_0.7 [35].

$$X_{O_{\alpha }}=\frac{\raisebox{1ex}{$A_{{}_{{}_{O_{\alpha }}}}$}\!\left/ \!\raisebox{-1ex}{$S_O$}\right.}{{\displaystyle \sum \raisebox{1ex}{$A_{(}{}_{O_{\alpha }+O_{\beta })}$}\!\left/ \!\raisebox{-1ex}{$S_O$}\right.}}\times 100\%$$

(3)

where $X_{O_{\alpha }}$ is the percentage content of O_α, A is the integrate area of characteristic peak in the XPS pattern, S is sensitivity factors (S = 0.711).

In order to check the redox properties of the new series of Ce_xMn_1−x systems, TPR of all the catalysts were carried out. Figure 5a shows the H₂-TPR profiles of CeO₂, MnO_x and Ce_xMn_1−x composite oxides. Similar to previous findings [36,37,38], pure CeO₂ exhibits two reduction peaks at around 405 °C and about 719 °C. The former low-temperature reduction is due to the removal of surface oxygen and the later high-temperature reduction is related to the oxygen species in bulk CeO₂. The H₂-TPR profile of pure MnO_x shows two strong reduction peaks at 167 °C and 330 °C, respectively, with an area ratio of the lower to the higher temperature hydrogen consumption of about 1:2.42. The actual hydrogen consumption of two reduction peaks is 0.92343 and 1.88118 mmol/g and the corresponding ratio is 1:2.04. As we known, Mn₂O₃ proceed two-step reduction, the low-temperature reduction peak represented the reduction of Mn₂O₃ to Mn₃O₄ and the high-temperature reduction peak referred to the further reduction of Mn₃O₄ to MnO [39]. The theoretical hydrogen consumption ratio is 1:2 as seen in Formula (4) and (5), which is less than the fitted value (2.42) and actual data (2.04). It indicates that the hydrogen gas is more consumed at higher temperature, which is attributed to the extra existence of Mn₃O₄ phase. This is in agreement with the XRD data, which show that the crystalline phase of pure MnO_x corresponds to Mn₂O₃ and Mn₃O₄.

$$3M{n}_2{O}_3+H_2\to 2M{n}_3{O}_4+H_{2}O$$

(4)

$$M{n}_3{O}_4+H_2\to 3MnO+H_{2}O$$

(5)

In contrast to pure CeO₂ and MnO_x, the reduction profiles of Ce_xMn_1−x catalysts are more complicated. For Ce_xMn_1−x, the TPR profiles consist of three overlapping peaks at lower temperature and one peak at higher temperature. According to the reduction characteristics of pure MnO_x and CeO₂, it can be deduced that the lower temperature peaks (182/351 °C, 189/340 °C, 169/343 °C) are assigned to the two-step reduction of Mn₂O₃. The peaks at 693 °C or 714 °C are attributed to the oxygen species in bulk CeO₂. It is worthy to note that another obvious peak at lower temperature (295 °C, 283 °C or 232 °C) as shown in Figure 5b is possible to be caused by the synergistic effect between Mn²⁺/Mn³⁺ and Ce⁴⁺, which is related with CeO₂-MnO_x solid solution. It can facilitate the mobility of the oxygen species in the Ce_xMn_1−x composite oxide. Therefore, their catalytic activities over benzene are higher than pure CeO₂ and MnO_x. Additionally, the reduction temperature is lower and the peak corresponding to the oxygen species in bulk CeO₂ disappear in the TPR pattern of Ce_0.3Mn_0.7 compared with those of Ce_0.7Mn_0.3/Ce_0.5Mn_0.5. This indicates that the reduction of the manganese oxide and the cerium oxide in Ce_0.3Mn_0.7 is promoted more obvious, which results in the highest activity over benzene in the Ce_xMn_1−x composite oxides. In the view of hydrogen consumption, the same conclusion can be also obtained. Ce_0.3Mn_0.7 consumed the most hydrogen gas (4.88447 mmol/g) among Ce_xMn_1−x composite oxides indicating that Ce_0.3Mn_0.7 possesses more oxygen species, which are beneficial to benzene oxidation reaction. Therefore, Ce_0.3Mn_0.7 presents higher activity compared with other Ce_xMn_1−x.

3.3. Factors Influencing the Catalytic Activity

Through the analysis, it has been acquired that Ce_0.3Mn_0.7 performs excellently in catalytic oxidation of benzene among the Ce_xMn_1−x catalysts. Many factors are considered to determine catalytic performances. Firstly, the existence of Ce-Mn solid solution results in the formation of oxygen vacancies due to incorporate Mn into CeO₂ crystal lattice, which can enhance their activities. Secondly, the large numbers of active sites will be introduced after removal of oxalate chains, because of the formation of porous structure which can also facilitate the adsorption and diffusion of organic molecules, thus reducing limitations of interphase mass transfer and promoting their catalytic activities. Thirdly, the better reducibility at low temperature also plays a great role in the catalytic activity. Through the analysis of TPR, it has been acquired that the reductions of Ce_xMn_1−x catalysts start at the lower temperature compared with CeO₂, which indicate that the catalysts possess more highly reducible surface species such as absorbed oxygen. Additionally, the existence of special peak caused by the strong interaction between Ce and Mn compared with MnO_x also result in the enhancement of reducibility. Finally, the oxidation of organic molecules over transition metal oxide or mixed metal oxide catalysts involves two identical mechanisms: a Langmuir–Hinshelwood mechanism at lower temperature and a Mars–van Krevelen mechanism with increasing reaction temperature [28,40]. At lower temperature, the adsorbed oxygen species with higher activity can enhance the adsorption and oxidation of VOCs. With the temperature rising, the adsorbed organic molecules are oxidized by the oxygen of metal oxides, which can be replenished by gas phase oxygen. Therefore, the adsorbed oxygen species will have an important role to play in determining its catalytic activity. As displayed in Table 2, the Ce_0.3Mn_0.7 exhibits a higher content of Ce⁴⁺ and Mn³⁺. The high-valence of cerium and manganese ions are preferred to adsorb more active oxygen species to attend in the reaction, thereby Ce_0.3Mn_0.7 possessed higher activity.

4. Conclusions

A series of Ce_xMn_1−x composite oxides, CeO₂ and MnO_x were synthesized through oxalate method and the complete catalytic oxidation of benzene were examined. The results indicated that Ce_xMn_1−x catalysts exhibited better activities comparing with pure CeO₂ or MnO_x, among which the catalytic activity reached the best when the Ce_at/Mn_at optimum ratio was 3:7. In order to identify the advantage of oxalate route, Ce-Mn composite oxides were also synthesized through carbonate method and hydrothermal method. The results indicated that the samples prepared by oxalate route exhibited higher activities, which were probably related with the microstructure of catalyst. Additionally, the influence of oxygen vacancy and synergistic effect in the benzene catalytic oxidation can’t be also ignored.