Roles of Oxygen Vacancies of CeO₂ and Mn-Doped CeO₂ with the Same Morphology in Benzene Catalytic Oxidation

Abstract

Mn-doped CeO₂ and CeO₂ with the same morphology (nanofiber and nanocube) have been synthesized through hydrothermal method. When applied to benzene oxidation, the catalytic performance of Mn-doped CeO₂ is better than that of CeO₂, due to the difference of the concentration of O vacancy. Compared to CeO₂ with the same morphology, more oxygen vacancies were generated on the surface of Mn-doped CeO₂, due to the replacement of Ce ion with Mn ion. The lattice replacement has been analyzed through XRD, Raman, electron energy loss spectroscopy and electron paramagnetic resonance technology. The formation energies of oxygen vacancy on the different exposed crystal planes such as (110) and (100) for Mn-doped CeO₂ were calculated by the density functional theory (DFT). The results show that the oxygen vacancy is easier to be formed on the (110) plane. Other factors influencing catalytic behavior have also been investigated, indicating that the surface oxygen vacancy plays a crucial role in catalytic reaction.

1. Introduction

Volatile organic compounds (VOCs) are a type of low-boiling point organic matter which contains aromatic hydrocarbons, straight-chain alkane and cycloalkanes, etc. [1]. VOCs can be produced in industrial process and daily life and bring harm to human health and environment. Hence, it is particularly urgent to limit VOC emissions [2]. There are many methods to remove VOCs, in which catalytic oxidation is considered to be the most effective method due to its destructive efficiency, lower operating temperature and less by-products [3,4]. At present, two types of common catalysts have been widely researched: one is precious metal type and the other is transition metal oxide type. The application of former is limited due to the cost, sintering risk and facile poisoning. In contrast, the transition metal oxide is attracting more attention owing to the high-stability, low cost and high oxidation activity at low temperatures [5,6,7]. In recently years, transition metal based oxides catalysts exhibited excellent catalytic behaviors towards the removal of VOCs, such as Mn-based [8,9,10] and Co-based [11,12] metal oxide catalysts.

Additionally, rare-earth oxides have also attracted wide attention over the VOCs catalytic oxidation. CeO₂, as typical rare earth oxide has been investigated in the field of heterogeneous catalysis and exhibits enhanced performances for VOC oxidation due to its outstanding redox property and oxygen storage capacity (OSC). It is worthy to note that the microstructure of CeO₂ is closely related with the catalytic activity, especially crystal plane defects. Trovarelli and Llorca [13] reported different planes of CeO₂ possessed different content of oxygen vacancy. The activity of CeO₂ for VOC oxidation is sensitive to the proportion of surface oxygen defects in the catalyst [14]. Once the surface active oxygen vacancies are covered by intermediates or carbon deposition, the catalytic behavior of CeO₂ will be poisoned [15,16]. In order to increase the content of active sites, transition metals are often used to be doped or composited with CeO₂ to produce more oxygen vacancies. Among the transition metals doped catalysts, Mn-based catalysts are widely used in the field of environmental catalysis [17,18]. It is reported that Mn-dopedCeO₂has been applied in the abatement of contaminants, such as the catalytic reduction of NO and diesel soot combustion, which exhibit excellent catalytic activity [19,20,21,22,23].

In this article, we have investigated the catalytic behavior of pure CeO₂ and Mn-doped CeO₂ with the same morphology through the complete catalytic oxidation of benzene. Their catalytic activities differences and crystal structures are analyzed in detail. Meanwhile, density function theory (DFT) was adopted to simulate crystal plane structure and calculate the formation energy of oxygen vacancies on the certain exposed plane. Although the effect of oxygen vacancy in pure CeO₂ on the catalytic property has been recognized, however the influence of dopants in oxygen vacancy for doped CeO₂ need to be described further. In addition, the difference of oxygen vacancy on the different exposed crystal plane has less been studied. Therefore, the research aims to explore the effect of crystal plane structure and dopant (Mn ion) dispersion on the formation of oxygen vacancy and analyze the roles of oxygen vacancy in the combustion of benzene further.

2. Materials and Methods

2.1. The Preparation of CeO₂-MnO_x Composite Oxides

The chemicals used in this work, including Ce(NO₃)₃·6H₂O (99%), Mn(NO₃)₂ solution (50%), NaOH (98%), and ethanol, were purchased from Beijing Chemicals Company (Beijing, China). Mn-doped CeO₂ with different morphologies (nanofiber: NF and nanocube: NC) were synthesized by a hydrothermal process. Firstly, Ce(NO₃)₃·6H₂O and Mn(NO₃)₂ in given amount (Ce/Mn mole ratio = 9/1) were dissolved in a 10 mL H₂O and mixed with a 6 M NaOH solution. The solution was stirred for 0.5 h at room temperature then transferred to an autoclave (100 mL) and gradually heated to a certain temperature, e.g., 120 °C for NF while 160 °C for NC, respectively. The reaction at the targeting temperature was kept for 24 h. After the reaction, the precipitates in autoclaves were collected by centrifugation, washed with distilled water and ethanol several times. The obtained materials, labeled as Ce-Mn-NF and Ce-Mn-NC respectively, were dried at 80 °C overnight and calcined at 550 °C for 4 h. Pure CeO₂with fiber and cube morphology were also prepared using the similar process as reference, labeled as CeO₂-NF and CeO₂-NC respectively.

2.2. Characterization

The crystal phases of the catalysts were characterized by X-ray diffraction (XRD) using PhilipsX’pert PRO analyzer (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°).

Scanning electron microscopy (SEM JC-Zeiss Merlin) and Transmission electron microscopy (TEM Tecnai G2 F20 U-TWIN) was used to observe the morphology and structure of samples. Aberration-corrected HAADF-STEM images, Mn L_2,3-edge and Ce M_4,5-edge electron energy loss spectroscopy (EELS) were performed from FEI Titan electron microscope equipped with a Gatan Enfnium camera system.

The surface composition and chemical states were determined by X-ray photoelectron spectroscopy (XPS ESCALAB250Xi). The binding energy (BE) was calibrated using the C1s line at 284.8 eV.

Raman spectra were measured with a Raman spectrometer (Renishaw inVia plus). The excitation source was an Ar⁺ ion laser (λ = 514.23 nm) and the laser power was 20 mW.

Reactive oxygen species were identified by the electron paramagnetic resonance technology (EPR) at room temperature on a Bruker EMX spectrometer (Bruker Corp., Billerica, MA, USA) at a frequency of 9.8 GHz and a magnetic field of 100 kHz.

Hydrogen temperature-programmed reduction (H₂-TPR) was performed on AutoChem 2920 chemisorption analyzer. Firstly, the sample (40–60 mesh) was heated to 150 °C and maintained at this temperature for 1h in 5% O₂ and 95% He mixture (30 mL/min), and then cooled to 50 °C under He flow. The samples were then heated again to 900 °C in 10% H₂ and 90% Ar mixture (50 mL/min). The heating rate is 10°C/min. The thermal conductivity detector signal was detected.

2.3. Catalytic Activity Tests

Activity tests for catalytic oxidation of benzene over CeO₂-NF, CeO₂-NC, Ce-Mn-NF and Ce-Mn-NC catalysts were performed in a continuous-flow fixed-bed reactor, containing 100 mg catalyst samples (40–60 mesh), respectively. The total reactant flow rate was 100 mL·min⁻¹, containing (100 ppm benzene + 20 vol% O₂ + N₂ (balance)). The weight hourly space velocity (WHSV) was typically 60,000 mL·g⁻¹·h⁻¹. The concentrations of Benzene, CO₂, and CO were analyzed on-line using a gas GC/MS (Hewlett-Packard 6890N and Hewlett-Packard 5973N). 5.0 vol% water vapor was introduced to the system by a water saturator at 34 °C. Catalytic activities were characterized by two parameters (T₅₀ and T₉₀). For a catalyst, T₅₀ and T₉₀ represent the temperature of 50 and 90% benzene conversion, respectively. The benzene conversion and CO₂ selectivity are calculated as following Equations (1) and (2). [Benzene]_in and [Benzene]_out are the inlet and outlet Benzene concentrations during catalytic reaction):

$$\mathrm{Benzene}\mathrm{conversion}(\%)=\frac{{\left[\mathrm{Benzene}\right]}_{in}-{\left[\mathrm{Benzene}\right]}_{out}}{{\left[\mathrm{Benzene}\right]}_{in}}\times 100\%$$

(1)

$${\mathrm{CO}}_2\mathrm{selectivity}(\%)=\frac{\left[{\mathrm{CO}}_2\right]}{\left(\left[\mathrm{CO}\right]+\left[{\mathrm{CO}}_2\right]\right)}\times 100\%$$

(2)

2.4. Details of DFT + U Calculation

First-principles calculations based on density functional theory (DFT) were carried out with the Vienna Ab-initio simulation package (VASP) and PBE functional. The interaction between core electrons and valence electrons was expressed by the projector-augmented wave (PAW) method. The cutoff energy of the plane-wave basis set was set to 550 eV. To guarantee the accuracy, the convergence criteria of energy and force were set to 10⁻⁵ eV and 0.05 eV·Å⁻¹, respectively. DFT + U with U = 5eV was applied to treat Ce 4f orbital. A model was built based on a 2 × 2 × 2 CeO₂ supercell. The Brillouin zone of (100) and (110) models were sampled by a 3 × 3 × 1 k-point set to acquire similar sampling densities. Before calculation, no more than half of the atomic layers from the bottom were fixed.

3. Results and Discussion

3.1. Characterization of Catalysts

The crystal structures of CeO₂-NF, CeO₂-NC, Ce-Mn-NF and Ce-Mn-NC was examined by XRD. As seen in Figure 1a, the diffraction peaks at 2θ = 28.5°, 33.0°, 47.4°, 56.4°, 59.2°, 69.5°, 76.6°, 79.1° and 88.6° clearly demonstrate the presence of cubic fluorite structure of CeO₂ (PDF# 81-0792). For Mn-doped CeO₂ (Ce-Mn-NF and Ce-Mn-NC), the patterns do not show any diffraction of manganese oxides, and only broad reflections are observed, which should be attributed to the formation of Ce-O-Mn solid solution. It is worthy to note that the characteristic diffraction peak of CeO₂ (2θ = 28.5°) in the Mn-doped CeO₂ moves to little bit higher value due to the decrease of the lattice parameter caused by the dopant Mn ions with smaller size, compared with the pure CeO₂ (Figure 1b).

The lattice parameters of all the samples have been calculated according to the XRD data and listed in

Table 1. Lattice parameters and extent of expansion of synthesized samples compared with those of pure CeO₂.

Samples	Lattice Parameters (a)	Extent of Deviation
	(Å)	(×10−4)
CeO2-NF	5.40864	10.06
Ce-Mn-NF	5.40842	11.94
CeO2-NC	5.41061	4.02
Ce-Mn-NC	5.40878	11.52

. The result shows that the lattice parameters of the Mn-doped CeO₂ are lower than those of pure CeO₂ with the same morphology. The ionic radius of Mnⁿ⁺ {Mn²⁺ (0.067nm), Mn³⁺ (0.066 nm) and Mn⁴⁺ (0.053 nm)} are all smaller than that of the Ce⁴⁺ (0.1098 nm), therefore, the incorporation of Mnⁿ⁺ into the CeO₂ lattice to form Ce-O-Mn solid solution could result in lattice contraction due to the doping of Mnⁿ⁺ and the consequent formation of the more oxygen vacancies.

The morphology and structure of samples were characterized by SEM and TEM, respectively. Figure 2a shows the SEM image of CeO₂-NF. It can be seen that CeO₂-NF with a fiber morphology has an average diameter of about 10 nm and length of about 150–300 nm. In the HRTEM of CeO₂-NF (Figure 2b), the exposed crystal planes (110) and (100) can be determined obviously according to the inter-planar distance 0.19 nm and 0.27 nm, respectively. Figure 2c shows the SEM image of CeO₂-NC. The size of CeO₂-NC possessing cubic morphology is about 12–25 nm. Through the HRTEM (Figure 2d), it can be acquired that the exposed crystal plane is only (100). (111) plane also exists inside the lattice. For Ce-Mn-NF and Ce-Mn-NC, the morphology has no change compared with pure CeO₂ with the same morphology except the change of sample size (Figure 2e–h). In order to analyze the element distribution of Mn-doped CeO₂, the HAADF mapping image of Ce-Mn-NF and Ce-Mn-NC were examined, respectively (Figure 2i,j). The result exhibits that both the Ce and Mn elements are homogeneously dispersed, which favors the oxygen vacancy formation.

Raman scattering was used to identify the solid solution phase, which can reflect indirectly the property of oxygen vacancy. Figure 3 shows visible Raman spectra of all the samples. In the pattern, a strong peak at ca. 460 cm⁻¹ and a weak one at ca. 610 cm⁻¹ are discerned, corresponding to the fluorite F_2g mode and a defect-induced mode (D mode), respectively [24,25]. The peaks at 275 and 1163 cm⁻¹ are assigned to second-order transverse and longitudinal vibration modes of cubic CeO₂ fluoride phase [26]. These bands at ca. 610 cm⁻¹are usually assigned to the presence of extrinsic oxygen vacancies created as charge compensating defects during solid solution formation [27]. For Ce-Mn-NF and Ce-Mn-NC, a band at ca. 610 cm⁻¹ corresponds to the second-order Raman mode attributes of O²⁻ vacancies formed by a low valence dopant (Mnⁿ⁺). For pure CeO₂, Raman band is also detected at ca. 610 cm⁻¹, which is also assigned to the presence of oxygen vacancies resulted from Ce⁴⁺ transforming to Ce³⁺. The ratio of integrated peak area of oxygen vacancy (~610 cm⁻¹) to that of main peak (460 cm⁻¹), defined as A₆₁₀/A₄₆₀, is used here to characterize the relative amount of oxygen vacancy among these samples. The A₆₁₀/A₄₆₀ ratio was calculated and ranked in the order of Ce-Mn-NF> Ce-Mn-NC> CeO₂-NF> CeO₂-NC (

Table 2. Physical and chemical properties of all samples.

Sample	A610/A460(%)	BE (eV)		Oβ/(Oα + Oβ)(%)	Ce3+/(Ce3+ + Ce4+) (%)	Mn 2p3/2 BE (eV)
		Oα	Oβ			Mn4+	Mn3+	Mn2+
CeO2-NF	13.9	529.2	531.5	21.1	8.74	----	----	----
Ce-Mn-NF	20.9	529.3	531.7	25.5	11.3	640.9	641.8	643.1
CeO2-NC	11.3	529.2	531.6	17.1	7.32	----	----	----
Ce-Mn-NC	19.4	529.1	531.5	22.0	9.71	640.8	641.8	643.2

), suggesting that Ce-Mn-NF exhibits the higher concentration of oxygen vacancies.

The oxidation state of catalyst surface species was examined by XPS analysis. Figure 4a presents the XPS spectra of the Ce3d core levels for all samples. The peaks V₀/ V_0′, V₁/V_1′and V₂/V_2′ refer to three pairs of spin-orbit doublets, which can be attributed to surface Ce⁴⁺ [28], while U₀/U_0′ andU₁/U_1′ can be ascribed to Ce³⁺ [29]. The relative amount of Ce³⁺ for all the samples can be calculated according to the Equation (3) and listed in Table 2. The result shows that the relative atomic ratio of Ce³⁺/(Ce³⁺ + Ce⁴⁺) in the Ce-Mn-NF is higher than that of CeO₂-NF. The amount of Ce³⁺ in the Ce-Mn-NC is also higher than that of CeO₂-NC. The data indicate that the introduction of Mn ion can increase the relative amount of Ce³⁺ in the Mn-doped CeO₂, which might arise from the charge rebalancing of oxygen vacancies and dopant Mnⁿ⁺ [30].

$${\mathrm{X}}_{\mathrm{C}{\mathrm{e}}^{3+}}=\frac{\raisebox{1ex}{${\mathrm{A}}_{{}_{\mathrm{C}{\mathrm{e}}^{3+}}}$}\!\left/ \!\raisebox{-1ex}{${\mathrm{S}}_{\mathrm{C}\mathrm{e}}$}\right.}{{\displaystyle \sum \raisebox{1ex}{${\mathrm{A}}_{(}{}_{\mathrm{C}{e}^{3+}+\mathrm{C}{\mathrm{e}}^{4+})}$}\!\left/ \!\raisebox{-1ex}{${\mathrm{S}}_{\mathrm{C}\mathrm{e}}$}\right.}}\times 100\%$$

(3)

where ${\mathrm{X}}_{\mathrm{C}{\mathrm{e}}^{3+}}$ 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 4b depicts the O 1s XPS spectra of the CeO₂-NF, CeO₂-NC, Ce-Mn-NF and Ce-Mn-NC samples. All catalysts exhibited dominant component of lattice oxygen (O_α) species, together with a shoulder of surface adsorbed oxygen (O_β) species on the surface vacancies. The binding energy of 529.0–530.0 eV corresponded to O_α and the binding energy of 531.0–532.0 eV was ascribed to O_β. The integral ratio of (O_β/O_α+ O_β) was applied to estimate the concentration of adsorbed oxygen species (Table 2). Ce-Mn-NF and Ce-Mn-NC samples possess more surface absorbed oxygen species compared with CeO₂-NF and CeO₂-NC, which indicate that Mn-doped CeO₂ contain more surface oxygen vacancy. Hence, it is inferred that the surface oxygen vacancy adsorbs and activates oxygen molecules to produce adsorbed oxygen species. This mechanism promotes the redox property of catalyst. A similar phenomenon was reported in the literature [31]. The O 1s XPS spectra results were in accordance with the XRD and Raman data.

Figure 4c presents the Mn 2p patterns of Ce-Mn-NF and Ce-Mn-NC. The binding energies of the Mn 2p_3/2 component appear at 641.8 eV and those for Mn 2p_1/2 appear at 653.5 eV. The spin orbit splitting is ΔE = 11.7 eV and the width is 3.62 eV. According to literature data [32], the observed binding energy is tented to correspond to Mn₂O₃. It should be noted, though, that the BEs of various Mn ions are very close to each other, rendering impossible the exact identification of oxidation states due to overlap of the energy ranges for various oxidation states of Mn [33]. In the Figure 4c, the Mn 2p_3/2 XPS spectra were fitted by the presence of Mn²⁺, Mn³⁺ and Mn⁴⁺. The binding energy of641.9–643 eV, 641.4–641.9 eV and 640.1–641.31 eV was ascribed to the peaks of Mn⁴⁺, Mn³⁺ and Mn²⁺ species [34], respectively.

The distribution feature of oxygen vacancy can also be reflected through electron energy loss spectroscopy (EELS). The patterns of CeO₂ and Mn-doped CeO₂ are shown in Figure 5a, in which Mn L_2,3-edge and Ce M_4,5-edge spectra can be distinguished obviously. Through the Mn L_2,3-edge spectrum, it can be obtained that Mn ion exhibits mainly trivalent [35], which is consistent with the XPS data of Mn element. According to the literature [36,37,38], it can be acquired that the formation of oxygen vacancies is coupled with the localization delocalization effect of Ce 4f electrons. Therefore, the Ce M_4,5-edge EELS spectra were used to further investigate the distribution conditions of Ce³⁺ and oxygen vacancies (Figure 5b). The proportions of Ce³⁺ ([Ce³⁺]) were calculated via the M₅/M₄ white-line ratios for every sample [39]. The data show that the relative proportions of Ce³⁺ in Ce-Mn-NF (0.876) and Ce-Mn-NC (0.837) are more than those of CeO₂ with the same morphology (CeO₂-NF: 0.775, CeO₂-NC: 0.765) due to Mnⁿ⁺ replacing some Ce⁴⁺, which indicate that abundant Ce³⁺ exists on the surface of Mn-doped CeO₂ and more oxygen vacancies are formed. The conclusion has also been identified by the results of Raman and XPS.

EPR is a sensitive to the paramagnetic species with unpaired electrons [40]. Thus, EPR spectra of the pure CeO₂and Mn-doped CeO₂ samples were collected to study the oxygen vacancies and the species of reactive oxygen (·O₂⁻, ¹O₂ and ·OH) [41,42]. As presented Figure 6a, pure-CeO₂ (CeO₂-NF and CeO₂-NC) exhibited two types of peaks of Ce³⁺ in terms of the surface (g = 1.965) and bulk (g = 1.947). After Mn doping, Mn-doped CeO₂ presented a strong EPR signal at g = 2.032, ascribed to the combination of Mn ion with one or two electrons [43,44]. The typical·O₂^- quartet spectrum, ¹O₂ triplet spectrum and ·OH spectrum can be detected for all the catalysts (Figure 6b–d). The intensities of peaks corresponding to ·O₂⁻ and ¹O₂ respectively in the Mn-doped CeO₂ catalyst are far higher than those of CeO₂. This may be due to more Ce³⁺ to Ce⁴⁺ transition on the catalyst surface of Mn-doped CeO₂, which provided many electrons to O₂ and accelerated the conversion O₂ to active oxygen species. It is beneficial to accelerate the catalytic oxidation of benzene. It is worthy to note that CeO₂ only exhibit ·OH characteristic quartet peaks, however Mn-doped CeO₂ contain other peaks attributed to oxidized DMPO (radical scavenger) besides ·OH peaks, which indicate that Mn-doped CeO₂ possess higher oxidization ability (Figure 6d). Therefore, it is easier to catalytic oxidize benzene for Mn-doped CeO₂. The EPR patterns of oxidized DMPO, labeled as DMPO-X can be seen in the Supplementary Materials.

The reducible surface oxygen species of CeO₂ and Mn-doped CeO₂were studied using H₂-TPR as illustrated in Figure 7. TheH₂-TPR measurement of CeO₂-NF exhibits two major reduction peaks at 478 °C and 725 °C. The two reduction peaks at 480 °C and 728 °C exist in the pattern of CeO₂-NC. According to the literature [45,46,47], the former low-temperature reduction (478 °C and 480 °C) is due to the removal of surface oxygen and the later high-temperature reduction (725 °C and 728 °C) is attributed to the release of oxygen species in bulk CeO₂. For Ce-Mn-NF, the TPR profile exhibits three overlapping peaks at lower temperature and one peak at higher temperature. The lower temperature peaks at 240°Cand 336 °Care assigned to the two-step reduction of Mn₂O₃ (Mn₂O₃→Mn₃O₄; Mn₃O₄→MnO) [48], while the reduction peak at 400 °C is corresponded to the reduction of surface oxygen in CeO₂. The higher temperature peak at 722 °C is attributed to the reduction of oxygen species in bulk CeO₂. The H₂-TPR pattern of Ce-Mn-NC is similar with that of Ce-Mn-NC. The lower temperature peaks (200 and 337 °C) correspond to the two-step reduction of Mn₂O₃. The peaks at 386 and 743 °C are attributed to the reduction of surface oxygen and bulk oxygen species in CeO₂, respectively. In addition, the reduction peaks of Mn-doped CeO₂ are shifted toward lower temperature compared with those of CeO₂, which indicate the reducibility of Mn-doped CeO₂ catalyst was increased due to Mn ion. Hence Mn-doped CeO₂ should possess higher catalytic activities, which is in accordance with the result of EPR.

3.2. Catalytic Oxidation Activity towards Benzene

The catalytic activities of Mn-doped CeO₂ and pure CeO₂ with the same morphology were evaluated by their oxidation performance towards benzene (Figure 8a). It can be seen that the catalytic activity decreases in the order of Ce-Mn-NF, Ce-Mn-NC, CeO₂-NF and CeO₂-NC, which is consistent with that of oxygen vacancy concentration. The oxygen vacancy is closely related with the catalytic activity. T₅₀ and T₉₀ on Ce-Mn-NF is 278 and 395 °C respectively, which are much lower than those on Ce-Mn-NC (304/450 °C) and pure phase oxide. The catalytic activities of the catalysts were also compared based on the yield of CO₂. From the Figure 8b, it can be also obtained that the catalytic properties of Mn-doped CeO₂ are enhanced compared with CeO₂ with the same morphology. Meanwhile benzene has almost been converted into CO₂ completely without the detection of any other gas. Water vapor is a common component of industrial waste gas, therefore the impact of water vapor on catalytic activity is requisite. The impact of 5.0 vol% water vapor introduction on catalytic activities of Ce-Mn-NC and Ce-Mn-NF at 400 °C was conducted (Figure 8c,d). As a result, the introduction of 5.0 vol% water vapor leads to benzene conversion dropping from 91% to 77% for Ce-Mn-NC and 83%to 67% for Ce-Mn-NF, respectively. After 10 h of continuous 5.0 vol% water vapor injection, the benzene percent conversion for Ce-Mn-NC and Ce-Mn-NF is completely recovered upon the cutting off of the water vapor feeding. The competitive adsorption between H₂O and benzene molecules onto the active sites is the main reason causing the decrease of benzene conversion. The result above indicated both of Ce-Mn-NC and Ce-Mn-NF having good water-resistant ability.

3.3. DFT Calculations

Through the analysis above, we have recognized that the oxygen vacancy has an important effect on their catalytic activities; however enough theory evidences are still needed. In this article, the density functional theory (DFT) is adopted to calculate the formation energy of oxygen vacancy so that identifying its role in the catalytic reaction further (Figure 9). The defect formation energy (E_f) of oxygen vacancy is defined as Equation (4):

$$E_f=E_{S\_V_O}+\frac{1}{2}{E}_{O_2}-E_S$$

(4)

where $E_{S\_V_O}$ and $E_S$ are the total energies of the supercell with and without an oxygen vacancy, and $E_{O_2}$ is the total energy of an O₂ molecule [49].

We compare the oxygen vacancy formation energies (E_f) on the (110) and (100) crystal planes of Mn-doped CeO₂. The calculated E_f corresponding to (110) plane is -0.48 eV. In contrast, the E_f on the (100) plane increases to 4.46 eV. The result means that the oxygen vacancy is easier to be formed on the (110) plane than (100) plane, which also indicate Mn ion is easier to be doped on the (110) plane. Given the central role of oxygen vacancy in catalyst [50,51], the Ce-Mn-NF [exposed (110) and (100) planes] is therefore expected to show better performance than Ce-Mn-NC [exposed (100) plane] and pure CeO₂ (CeO₂-NF and CeO₂-NC), which is consistent with the results of benzene catalytic degradation. In addition, the distances of Ce-O near the oxygen vacancy in the Mn-doped CeO₂, whether (110) or (100) planes, have only slight differences compared with those of CeO₂ which may be caused by the substitution of Mn ion. The distance data of Ce-O bond can be seen in Supplementary Materials.

3.4. Factor Influencing the Catalytic Activity

Through the analysis, it has been acquired that Mn-doped CeO₂performs higher activity than pure CeO₂ with the same morphology over catalytic oxidation of benzene. Oxygen vacancy is considered to be a critical factor in catalytic performances. For Mn-doped CeO₂, the existence of Ce-Mn solid solution results in the formation of more oxygen vacancies compared with CeO₂due to incorporate Mn into CeO₂ crystal lattice, which can provide more surface active sites to adsorb active oxygen species. This phenomenon has also been identified through the analysis of TPR, in which Mn-doped CeO₂ possesses more highly reducible surface species closely related with surface oxygen vacancy. In addition, exposed crystal plane can also influence the catalytic activity, which is also attributed to the oxygen vacancy due to the different formation energy of oxygen vacancy on the exposed crystal planes. Based on this, Ce-Mn-NF exhibits higher catalytic ability than Ce-Mn-NC.

As we known, the oxidation of organic molecules over transition metal oxide or mixed metal oxide catalysts involves two identical mechanisms: a Langmuir-Hinshelwood mechanism and a Mars-van Krevelen mechanism [52,53]. At lower temperature, the adsorbed oxygen species with higher activity can enhance the adsorption and oxidation of VOCs, which match with Langmuir-Hinshelwood mechanism. It mainly contains four steps: (1) benzene molecule is adsorbed on the exposed crystal plane to form π-complex of benzene due to the interaction of benzene ring with catalyst; (2) gas-phase oxygen molecule is activated on the crystal plane to adsorb in surface vacancies; (3) the attack of active surface oxygen species to benzene ring; (4) benzene ring breakage and the formation of final products. When the reaction temperature rises, the adsorbed organic molecules are oxidized by the oxygen of metal oxides, which is consistent with the Mars-van Krevelen mechanism. It involves a similar process: (1) adsorption of benzene molecule; (2) the attack of lattice oxygen released from the catalyst to benzene ring; (3) benzene ring breakage and deep oxidation to final products. In addition, there are also two key elements involved in this process: (ⅰ) oxygen vacancies formed are replenished by the surface active oxygen species; (ⅱ) active oxygen species are transported and delivered through the bulk oxygen vacancy (Figure 10). In summary, oxygen vacancy as transport medium, is the link of the whole reaction, therefore it has an important role in determining the catalytic activity.

4. Conclusions

Pure CeO₂ and Mn-doped CeO₂ with the same morphology (nanofiber and nanocube) were synthesized through hydrothermal method and the complete catalytic oxidation of benzene was examined. The results showed that Mn-doped CeO₂ possessed higher activity than pure CeO₂ with the same morphology due to the formation of more oxygen vacancies. In the Mn-doped CeO₂, Ce-Mn-NF exhibited better catalytic behavior than Ce-Mn-NC, which was closely related the exposed crystal planes. Ce-Mn-NF exposed (110) and (100) planes, while Ce-Mn-NC exposed only (100) plane. According to theoretical calculation, the formation energy of oxygen vacancy on the (110) plane is much less than that on (100), which indicate that oxygen vacancy is easier formed and more surface active species is adsorbed on the (110) plane resulting in the higher catalytic property of Ce-Mn-NF. In general, the effect of crystal plane on the activity is also attributed to the oxygen vacancy. Therefore, oxygen vacancy is a key factor in the process of catalytic reaction.