Oxygen-vacancy-promoted catalytic wet air oxidation of phenol from MnO_x–CeO₂

Abstract

Catalytic oxidation can be effectively promoted by the presence of oxygen vacancies on the catalyst surface. In this study, the effect of oxygen vacancies on the catalytic wet air oxidation (CWAO) of phenol was investigated with CeO₂ and MnO_x–CeO₂ as catalysts. CeO₂ and MnO_x–CeO₂ catalysts with different amounts of oxygen vacancies were obtained via hydrothermal methods and applied for the CWAO of phenol. It was found that CeO₂ and MnO_x–CeO₂ nanorods were much more active than the cubic nanorods. The physicochemical properties of the samples were characterized by TEM, XRD, BET, XPS, and H₂-TPR techniques. The results revealed that the presence of oxygen vacancies in CeO₂ and MnO_x–CeO₂ catalysts could increase the oxidizing ability of the catalysts surface. The addition of Mn could greatly improve the adsorption ability of CeO₂ and more efficiently oxidize phenol and its intermediates. The synergy between Mn and Ce could further improve the catalyst redox properties and produce a larger amount of active oxygen species, which is the reason why MnO_x–CeO₂ nanorods are the most active catalysts among the catalysts investigated in this study.

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1 Introduction

Phenol and its derivatives can be found as by-products from a variety of sources such as chemical, petrochemical, pharmaceutical and coal chemical industries.^1,2 Since phenol is highly toxic to aquatic life, it would result in a severe impact on water resources if phenol-contaminated streams are discharged without treatment.³ Biotreatment is often applied in water treatment; however, phenol can inhibit the activity of microbes, causing biotreatment to be impractical for phenolic wastewater.⁴ Catalytic wet air oxidation (CWAO) has been shown to be an effective and environmentally friendly way to remove refractory organic compounds, such as dyes, phenol, and succinic acid as normally no additions other than oxygen/air are needed for the process.^5–7 Under an oxygen or air atmosphere at relatively elevated temperatures and pressures, refractory organic compounds can be completely oxidized into CO₂ and H₂O or partially converted to other less toxic compounds by proper catalysts.^8,9 Clearly, developing a suitable catalyst essential for CWAO processes.

Among the various catalysts investigated for the CWAO of phenol in the reported literature, the cerium-incorporated manganese oxide composite (MnO_x–CeO₂) catalyst has been identified as one of the most promising catalysts.¹⁰ It was reported that MnO_x–CeO₂ is an effective catalyst for the CWAO of ammonia, poly(ethylene)glycol, acetic acid, pyridine, etc.^11,12 Arena reported a new “redox-precipitation” route to synthesis the MnO_x–CeO₂ catalyst resulting in improved catalytic activity.¹³ It was also reported that formic and acetic acids are the common intermediates during the CWAO of phenol, and that the adsorption of formic and acetic acids was driven by electrostatic interactions with the MnO_x–CeO₂ surface.^14,15 The surface reaction between the adsorbed intermediates and the activated oxygen species is the rate-determining step.¹⁶ Furthermore, it was reported that the interaction between Mn and Ce is beneficial for the CWAO of ammonia,¹¹ where a physical mixture of Mn₂O₃ and CeO₂ was far less active than that of Mn/Ce catalysts. The above information shows the merits of MnO_x–CeO₂ catalysts in the field of CWAO; however, less attention has been paid to the physicochemical properties of MnO_x–CeO₂ catalysts, such as the effect of oxygen vacancies on the CWAO of phenol.

Our previous study showed that O₂ activation ability can directly affect the performance of CO oxidation.¹⁷ The activation of oxygen can form superoxo (O²⁻) and/or peroxo (O₂²⁻) species, which are more oxidative than molecular oxygen (O₂). Active oxygen species can be formed on the surface of ceria in propane oxidation passing via a Mars and van Krevelen mechanism.¹⁸ Molecular oxygen prefers to fill the ceria oxygen vacancies to produce activated oxygen species.¹⁹ High oxygen vacancy concentrations can also assist the cycle transition between Ce³⁺ and Ce⁴⁺ ions (Ce³⁺ ↔ Ce⁴⁺), which is the key for many catalytic activities.

There are several ways to create oxygen vacancies. In the literature, both ceria (110) and (100) surfaces have been reported as having superior oxygen storage ability than the (111) surface.²⁰ However, the ceria (100) surface contains less oxygen vacancies than the (110) surface.²¹ This property of the ceria (110) surface causes the improved catalytic activity for CO oxidation and soot combustion.^22,23 Doping various metals into ceria is another way to create oxygen vacancies. As Mn and Ce have a tendency to form solid solutions,^24,25 doping Mn into the ceria lattice can greatly increase oxygen mobility and also help to produce oxygen vacancies on a catalyst's surface.²⁶ When the atomic ratio of Mn : Ce is less than 1, all the MnO_x–CeO₂ catalysts possess the fluorite cubic structures of CeO₂.²⁷ Thus, doping 50% Mn atoms into CeO₂ is expected to produce the maximum amount of oxygen vacancies.

In order to study the effect of oxygen vacancies on the CWAO of phenol, we synthesized CeO₂ nanorods and nanocubes using hydrothermal methods. To further confirm the effect of oxygen vacancies on the CWAO of phenol, 50% Mn in a molar ratio was doped into CeO₂ nanorods and nanocubes to form MnO_x–CeO₂ nanorods and nanocubes. Characterizations, including XRD, H₂ temperature-programmed reduction (H₂-TPR), XPS, and TEM, were performed to link the physicochemical properties and catalytic performance. The results revealed that the oxygen-vacancy-related properties as well as the synergy between Mn and Ce are the key for the high activities in the CWAO of phenol.

2 Experimental

2.1 Preparation of the catalyst

2.1.1 Chemicals. Cerium(III) nitrate hexahydrate (Ce(NO₃)₃·6H₂O), manganese(II) chloride tetrahydrate (MnCl₂·4H₂O), phenol (C₆H₆O), potassium permanganate (KMnO₄), manganous nitrate, 50% solution (Mn(NO₃)₂), and acetonitrile (C₃H₃N) were purchased from Sinopharm Chemical Reagent Co. Ltd. (Shanghai, China); sodium hydroxide (NaOH) and acetic acid glacial (CH₃COOH) were purchased from Guangdong Guanghua Sci-Tech Co. Ltd. (Shantou, China). The water used in all the experiments was deionized water produced using the Millipore Milli Q system. All the reagents were used as received.

2.1.2 Material synthesis.
2.1.2.1 Synthesis of CeO₂ nanorods and nanocubes. CeO₂ nanorods and nanocubes were prepared through a method similar to that reported in the literature.²⁸ To obtain CeO₂ nanorods, 1.736 g of Ce(NO₃)₃·6H₂O was dissolved in 10 mL of deionized water and stirred for 10 min. In a separate container, 19.2 g of NaOH was dissolved in 70 mL of deionized water, and stirred until the solution turned into a transparent color. The above two solutions were mixed and stirred for 30 min. Then, the mixed solution was transferred into a 100 mL Teflon-lined stainless autoclave and heated up to 100 °C for 24 h. After the solution was cooled to room temperature, the precipitates were separated by filtration and washed with deionized water several times. The obtained precipitates were dried at 60 °C overnight, and then calcined at 400 °C for 4 h. The procedure for the synthesis of CeO₂ nanocubes was exactly the same as for CeO₂ nanorods, except the hydrothermal condition was at 180 °C for 24 h.
2.1.2.2 Synthesis of MnO_x–CeO₂ nanorods and nanocubes. MnO_x–CeO₂ nanorods and nanocubes were synthesized through a modified method reported in the literature.^24,25 To obtain MnO_x–CeO₂ nanorods, 0.3957 g (50 mol% of Mn with respect to Ce) of MnCl₂·4H₂O and 0.868 g of Ce(NO₃)₃·6H₂O were dissolved into 5 mL and 10 mL of deionized water, respectively. Then the two solutions were mixed and stirred for 10 min. In another container, 14 g of NaOH was dissolved into 35 mL of deionized water. Then, the NaOH solution was mixed with the metal precursor solution. After stirring for 30 min, the slurry was transferred into a 100 mL Teflon-lined stainless autoclave and heated up to 120 °C for 24 h. Upon leaving the solution to cool down to room temperature, the precipitates were separated by filtration and washed by deionized water several times. The precipitates were then dried at 60 °C overnight and calcined at 400 °C for 4 h. MnO_x–CeO₂ nanocubes were synthesized using the same procedures as for the MnO_x–CeO₂ nanorods, except the hydrothermal condition was at 180 °C for 24 h.
2.1.2.3 Synthesis of MnO₂ catalysts. MnO₂ catalysts were prepared by a modified method according to the literature reported by Zhu et al.²⁹ In a typical experiment, 1.00 g of KMnO₄ and 0.50 g of 50 wt% Mn(NO₃)₂ solution were added into 64 mL of deionized water. After stirring for 10 min, the mixed solution was transferred into a 100 mL Teflon-lined stainless autoclave and kept at 160 °C for 12 h. After the solution was cooled to room temperature, the precipitates were washed and dried at 100 °C for 12 h.
2.1.2.4 Synthesis of Mn₃O₄ catalysts. Mn₃O₄ catalysts were synthesized by adding 3.22 g of 50 wt% Mn(NO₃)₂ solution and 1.78 g of MnCl₂·4H₂O into 10 mL of deionized water. In another container, 100 mL of 3 M NaOH solution was prepared, and then mixed with the metal precursor solution. After stirring for 1 h, the precipitates were washed using deionized water and dried at 100 °C for 12 h.

2.2 Catalyst characterization

X-ray diffraction (XRD) patterns for the cerium-based catalysts were collected on a Rigaku X-ray diffractometer and Cu Kα radiation (35 kV and 20 mA) was used as the X-ray source. The samples were scanned in the range of 10° to 80° at a scanning rate of 10° min⁻¹. X-ray photoelectron spectra (XPS) were obtained from the PHI Quantum 2000 Scanning ESCA Microprobe and the monochromatic Al Kα radiation (1846.6 eV) was used as the X-ray source. The binding energies of all the spectra were calibrated with the reference to the C 1s peak at 284.6 eV. N₂ adsorption–desorption isotherms were performed at 77 K using a Micromeritics ASAP 2020 instrument. All the samples (300 mg) were degassed at 200 °C for 2 h in vacuum before the measurements started. The specific surface areas were calculated by the Brunauer–Emmett–Teller (BET) method. Hydrogen temperature-programmed reduction (H₂-TPR) was recorded using a thermal conductivity detector (TCD). Typically, 80 mg of sample was pretreated in pure argon (Ar) at 200 °C for 2 h with a flow rate of 40 mL min⁻¹. After the temperature was cooled down to 50 °C, the gas was switched to 5% H₂/Ar with a flow rate of 40 mL min⁻¹. The temperature was then raised to 850 °C at a ramp of 10 °C min⁻¹. Transmission electron microscopy (TEM) was performed on the Tecnai F30 with an acceleration voltage of 300 kV. The specimen was prepared by dispersing the sample in ethanol solution and sonicating for 20 min. The dispersed sample was dripped on a carbon-coated copper grid and dried at room temperature. The catalyst composition was determined by inductively coupled plasma mass spectrometry (ICP-MS).

2.3 Catalytic activity measurements

The CWAO process was operated in a 100 mL temperature-programmed autoclave reactor with a magnetic-driven stirring mixer. A typical experiment was carried out as follows: 70 mL of phenol solution (1000 ppm) and 0.28 g of catalyst (4 g L⁻¹) were added into the reactor, and the reactor was purged using N₂ to remove the gaseous oxygen. The reactor was allowed to heat to the required temperature (100–180 °C) and the first sample was taken to evaluate the amount of phenol adsorbed. The desired amount of air (0.5–3.5 MPa) was then charged into the reactor, meanwhile, the time counting started. Samples were withdrawn periodically to monitor the progress of the reaction. The stirring speed was kept at 800 rotation per minute (rpm) at all times. The collected samples were filtered and the phenol concentrations were analyzed by Agilent 1100 high performance liquid chromatography (HPLC) instrument using a Dionex C₁₈ column. The wavelength of the UV detector was set to 254 nm. Acetonitrile and 0.2 wt% acetic acid solution were used as the mobile phases with a total flow rate of 1 mL min⁻¹. The reaction evolutions of phenol's intermediates were also analyzed by HPLC according to the methods reported by Yang.⁹ Total organic carbon (TOC) was measured by a Shimadzu TOC-L analyzer.

3 Results and discussion

3.1 Structural characterization

The TEM images of CeO₂ nanomaterials are displayed in Fig. 1. The crystal size for CeO₂ nanocubes and nanorods are 30 and 10 nm in diameter, respectively. The exposed surfaces for CeO₂ nanorods are the (110) and (100) surfaces, rather than the (100) surface for CeO₂ nanocubes. A clearer CeO₂ nanorods (110) surface image is also provided in Fig. S1.† It is obvious that CeO₂ nanorods have more “dark spots” and the surface seems “rougher” than that of the CeO₂ nanocubes. A similar phenomenon was also reported by Li, and it can be explained that the rougher surface of CeO₂ nanorods is due to the formed oxygen vacancy clusters.¹⁹ Thus, it could be expected that CeO₂ nanorods contain more oxygen vacancies than CeO₂ nanocubes. The TEM results also indicate that the ceria (110) surface has a tendency to form more oxygen vacancies than the (100) surface, which is consistent with previous studies.^20,21

Fig. 1 TEM images of the CeO₂ nanocubes (a and b) and nanorods (c and d).

The TEM images of MnO_x–CeO₂ nanocubes and nanorods are shown in Fig. 2. The average diameter of the MnO_x–CeO₂ nanocubes is about 17 nm, and the exposed surface is confirmed to be the (100) surface, where the inter-planar space is 0.27 nm. Fig. 2(c) and (d) are the typical images of MnO_x–CeO₂ nanorods, which have an average diameter of 10 nm. The predominant facets for the MnO_x–CeO₂ nanorods are the (111) and (100) surfaces. The surface of MnO_x–CeO₂ nanorods was also found to be “rougher” than the surface of MnO_x–CeO₂ nanocubes, which implies that MnO_x–CeO₂ nanorods have more surface reconstructions and oxygen vacancies. The existence of oxygen vacancy clusters can promote the transition from Ce⁴⁺ to Ce³⁺, which generates more active oxygen species.³⁰ This property of CeO₂ and MnO_x–CeO₂ nanorods is expected to play an important role in the CWAO of phenol.

Fig. 2 TEM images of the MnO_x–CeO₂ nanocubes (a and b) and nanorods (c and d).

In order to confirm the chemical compositions of the MnO_x–CeO₂ catalysts, line-scan analysis was conducted (Fig. 3). For both MnO_x–CeO₂ nanocubes and nanorods, Mn and Ce follow the same pattern, which suggests that they may have formed solid solutions. ICP-MS analysis was performed to further verify the elemental compositions of the MnO_x–CeO₂ nanocubes and nanorods. The results show that both MnO_x–CeO₂ nanocubes and nanorods contain 48 mol% of Mn. Therefore, it was proven that MnO_x–CeO₂ nanocubes and nanorods have similar chemical compositions.

Fig. 3 Line-scan analyses of the MnO_x–CeO₂ nanocubes (a and b) and nanorods (c and d).

The XRD patterns of CeO₂ and the MnO_x–CeO₂ nanomaterials are presented in Fig. 4. All the catalysts have the fluorite cubic structures of CeO₂ (PDF # 34-0394), in spite of there being some minor peaks of Mn₃O₄ (PDF # 18-0803) found for the MnO_x–CeO₂ nanocubes. This indicates that less Mn has doped into the ceria lattice for MnO_x–CeO₂ nanocubes. To clarify this phenomenon, magnified (111) diffraction peaks of MnO_x–CeO₂ nanomaterials with reference to CeO₂ nanorods are illustrated in Fig. 5.

Fig. 4 XRD patterns of the CeO₂ (a) and MnO_x–CeO₂ (b) nanomaterials.

Fig. 5 Magnified (111) diffraction peaks (a) and the effect of Mn doping on cerium lattice parameters (b).

It was found that the diffraction peak for MnO_x–CeO₂ nanorods had shifted to a higher Bragg angle compared with MnO_x–CeO₂ nanocubes. The Bragg diffraction of each (111) peak was recorded, and each corresponding lattice parameter ɑ was calculated (Fig. 5(b)). Indeed, the lattice parameter of the MnO_x–CeO₂ nanorods shrunk from 0.541 nm to 0.537 nm, whereas MnO_x–CeO₂ nanocubes only decreased to 0.539 nm. This further justifies that MnO_x–CeO₂ nanorods have formed a more homogeneous solid solution. Furthermore, the ionic radii of Mn ions were reported as follows: Mn⁴⁺: 0.053 nm, Mn³⁺: 0.065 nm, Mn²⁺: 0.083 nm.²⁶ The ionic radii of cerium ions were: Ce⁴⁺: 0.097 nm, Ce³⁺: 0.114 nm. Thus, if more Mn is doped into Ce for the MnO_x–CeO₂ nanorods, the average ionic radius is expected to be smaller than that of MnO_x–CeO₂ nanocubes, which might cause a decrease in the lattice parameters.

Moreover, Fig. 4 also illustrates that CeO₂ and MnO_x–CeO₂ nanocubes have greater intensity than the corresponding nanorods. This reveals that CeO₂ and MnO_x–CeO₂ nanorods have a higher degree of lattice defects than the corresponding cubic morphology.^8,28 This is consistent with the TEM results discussed earlier. The diffraction peaks of CeO₂ and MnO_x–CeO₂ nanorods are also relatively broader compared to the CeO₂ and MnO_x–CeO₂ nanocubes, suggesting that CeO₂ and MnO_x–CeO₂ nanorods have a smaller crystal size and would result in a higher specific surface area. This was further confirmed by the BET measurements, which show that the BET areas of CeO₂ and MnO_x–CeO₂ nanorods were 91.5 and 76.7 m² g⁻¹, respectively. The BET areas of CeO₂ and MnO_x–CeO₂ nanocubes were 15.4 and 39.2 m² g⁻¹, respectively. From the data shown above, it is apparent that the specific surface area was affected by the catalyst particle size. Generally, the smaller the particle size is, the larger BET surface area the catalyst possesses. As shown in Fig. 1 and 2, the particle size is increased in the following order: CeO₂ nanorods (10 nm) ≈ MnO_x–CeO₂ nanorods (10 nm) < MnO_x–CeO₂ nanocubes (17 nm) < CeO₂ nanocubes (30 nm). Therefore, the specific surface area is negatively related to the particle size: CeO₂ nanorods (91.5 m² g⁻¹) > MnO_x–CeO₂ nanorods (76.7 m² g⁻¹) > MnO_x–CeO₂ nanocubes (39.2 m² g⁻¹) > CeO₂ nanocubes (15.4 m² g⁻¹). Furthermore, as shown in Fig. 4, CeO₂ nanorods exhibited weaker XRD patterns than MnO_x–CeO₂ nanorods. This indicated that CeO₂ nanorods possessed poorer crystallinity and a smaller crystal size, although similar diameters were observed by TEM over the two catalysts. The poorer crystallinity and smaller crystal size enabled CeO₂ nanorods to have a larger BET surface area than MnO_x–CeO₂ nanorods.

3.2 Valence state and redox properties

The XPS spectrum of Ce 3d is displayed in Fig. 6. Typically, 10 peaks were deconvoluted into pairs of spin orbit doublets. Two peaks at 916.6 and 898.2 eV are attributed to the 3d_3/2, which correspond to the Ce⁴⁺ states. Two peaks at 903.4 and 885.2 eV are assigned to the 3d_5/2, which are the Ce³⁺ states. The other six peaks at 900.7, 882.5, 907.3, 888.5, 899.2, and 880.9 eV are the satellite peaks due to the “shake down” type processes.³¹ These results agree with what we found in the literature.³² The relative portion of Ce³⁺ and Ce⁴⁺ valence states are summarized in Table 1. It was found that CeO₂ nanorods have 6% more Ce³⁺ ions than CeO₂ nanocubes, whereas MnO_x–CeO₂ nanorods contain 11% more Ce³⁺ ions than MnO_x–CeO₂ nanocubes. The oxygen vacancy concentration is positively related to the concentration of Ce³⁺ ions, because oxygen vacancies can reduce the thermodynamic barrier for the transition from Ce⁴⁺ to Ce³⁺.³⁰ Thus, the XPS results are consistent with the XRD and TEM results, which helps to explain why CeO₂ and MnO_x–CeO₂ nanorods have rougher surfaces.

Fig. 6 Ce 3d core level XPS spectra of the CeO₂ and MnO_x–CeO₂ catalysts.

Table 1 Relative abundance of the different species obtained from the XPS data for each catalyst

Catalyst label	Ce^3+ (%)	Mn^4+ (%)	Mn^3+ (%)	Mn^2+ (%)	Oactive (%)
CeO2 nanocubes	32	—	—	—	16
CeO2 nanorods	38	—	—	—	20
MnCeOx nanocubes	19	14	36	50	18
MnCeOx nanorods	30	23	62	15	26

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The Mn 2p spectra of the MnO_x–CeO₂ catalysts are shown in Fig. 7. The three peaks of Mn 2p_3/2 are located at 640.8, 641.8, and 643.9 eV, corresponding to Mn²⁺, Mn³⁺, and Mn⁴⁺, respectively.^33–35 The Mn 2p_1/2 peak is centered at 653.3 eV, with a splitting energy of 11.45, based on the Mn 2p_3/2. By estimating the area of each Mn species, Table 1 shows that MnO_x–CeO₂ nanorods contain a larger portion of higher valence Mn species, whereas the corresponding Mn⁴⁺, Mn³⁺, and Mn²⁺ ion abundances are 23%, 62%, and 15%, respectively. Conversely, MnO_x–CeO₂ nanocubes prefer lower valence Mn species, and the corresponding portions of Mn⁴⁺, Mn³⁺, and Mn²⁺ are 14%, 36%, and 50%, respectively.

Fig. 7 Mn 2p core level XPS spectra of the MnO_x–CeO₂ catalysts.

It was also found that the Ce³⁺ concentration was suppressed when Mn was added. This effect was also reported by Larachi, where the addition of Mn caused an increase in surface Ce⁴⁺ concentration.³⁶ This phenomenon can be explained by the fact that electrons are transferred from Ce to Mn, which can decrease the concentration of Ce³⁺ and increase the concentration of Mn²⁺ or Mn³⁺. Compared to MnO_x–CeO₂ nanorods, the Ce³⁺ concentration of MnO_x–CeO₂ nanocubes was suppressed to a higher extent, and a much greater fraction of Mn²⁺ was observed. This also suggests that more electrons are transferred from Ce to Mn for MnO_x–CeO₂ nanocubes.

Noted that the Mn³⁺/Mn²⁺ that are incorporated into the CeO₂ lattice sites also contribute to the oxygen vacancies formation. MnO_x–CeO₂ nanocubes contained more Mn²⁺ species, but showed lower oxygen vacancies concentration than MnO_x–CeO₂ nanorods. Two possible reasons were responsible for the lower concentration of oxygen vacancies over MnO_x–CeO₂ nanocubes. First, some part of the Mn was not incorporated into the CeO₂ lattice for the MnO_x–CeO₂ nanocubes. Mn₃O₄ was detected by XRD, as shown in Fig. 4(b). It is well known that Mn₃O₄ is composed of both Mn³⁺ and Mn²⁺.^37,38 Some Mn³⁺/Mn²⁺ species come from the phase-segregated Mn₃O₄. These lower valence Mn species must not contribute to the formation of oxygen vacancies, since they were not in the CeO₂ lattice. Second, when Mn is doped into CeO₂, electrons were transferred from Ce to Mn and the Ce³⁺ fraction was decreased from 32% to 19%. Some oxygen vacancies over Mn³⁺/Mn²⁺ were produced at the expense of those over Ce³⁺ sites. Therefore, although a higher concentration of low valence Mn species was detected by XPS, the MnO_x–CeO₂ nanocubes still exhibited a lower concentration of oxygen vacancies. In the case of MnO_x–CeO₂ nanorods, electron transfer between Mn and Ce was also observed. However, the electron transfer was less significant than that over MnO_x–CeO₂ nanocubes. After the addition of Mn, the Ce³⁺ fraction was still up to 30%. Additionally, most of Mn was incorporated into the CeO₂ lattice for the MnO_x–CeO₂ nanorods, as proved by the XRD results. Thus, Mn³⁺/Mn²⁺ might have contributed to the formation of oxygen vacancies for the MnO_x–CeO₂ nanorods.

Fig. 8 shows the O 1s spectra for the CeO₂ and MnO_x–CeO₂ nanomaterials. The peaks appearing at 528.9 and 530 eV are the lattice oxygen of the CeO₂ and MnO_x–CeO₂ catalysts.³⁹ The peak at 531.4 eV is the adsorbed active oxygen species, the peroxo species (O₂²⁻), and the superoxo species (O₂⁻).⁴⁰ The remaining peak at 533 eV is attributed to the adsorbed surface hydroxyl and water species. For the CeO₂ catalysts, CeO₂ nanorods possess 20% surface active oxygen, where CeO₂ nanocubes have 16%. It is reported that oxygen vacancies can promote the formation of Ce³⁺, and that Ce³⁺ can induce the formation of active oxygen species.⁴¹ Thus, the cause of the higher active oxygen percentage for CeO₂ nanorods might be due to the oxygen vacancies formed on the CeO₂ nanorods surface. A similar trend was also observed for the MnO_x–CeO₂ catalysts, where MnO_x–CeO₂ nanorods contained 8% more active surface oxygen than MnO_x–CeO₂ nanocubes. It is also reported that active oxygen species can be created at the interface between the CeO₂ lattice and MnO_x.²⁶ It is reasonable that more active oxygen species are found in MnO_x–CeO₂ nanorods, due to the synergy of Mn/Ce (more –MnO–Ce– bonds) and the higher Ce³⁺ concentration.

Fig. 8 O 1s core level XPS spectra of the CeO₂ and MnO_x–CeO₂ catalysts.

The redox properties of the CeO₂ and MnO_x–CeO₂ catalysts were studied by H₂-TPR experiments, and the results are displayed in Fig. 9. It is widely accepted that the low temperature reduction peak of CeO₂ nanomaterials relates to the removal of surface capping oxygens, while the high temperature reduction peak is related to the bulk oxygen.^42,43 It was observed that CeO₂ nanorods are able to remove surface oxygen at 100 °C, while CeO₂ nanocubes start to reduce at 150 °C. This indicates that CeO₂ nanorods have more reducible surface Ce⁴⁺ ions compared with that of CeO₂ nanocubes, which is attributed to the oxygen vacancies formed on the CeO₂ nanorods surface, because oxygen vacancies can boost the transition from Ce⁴⁺ to Ce³⁺.

Fig. 9 H₂-TPR results of the CeO₂ (a) and MnO_x–CeO₂ (b) catalysts.

Doping Mn into the Ce lattice can improve the reducibility of the catalyst surface,²⁷ which can clearly be observed in Fig. 9(b). The reduction peak of MnO_x–CeO₂ nanorods starts at 50 °C, while MnO_x–CeO₂ nanocubes start to reduce at 150 °C. Four reduction peaks were obtained for each MnO_x–CeO₂ catalyst. For MnO_x–CeO₂ nanorods, the peak at 200 °C is attributed to the reduction of MnO₂ to Mn₂O₃. The second peak at 272 °C is assigned to the reduction of Mn₂O₃ to Mn₃O₄. The third peak at 360 °C is the mixed reduction of Mn₃O₄ to MnO and surface Ce⁴⁺ to Ce³⁺, while the high temperature reduction peak at 410 °C is the reduction of the bulk Ce⁴⁺ to Ce³⁺.^35,44

The reduction peaks for MnO_x–CeO₂ nanocubes followed a similar pattern as that of the MnO_x–CeO₂ nanorods. It was found that MnO₂ was first reduced to Mn₂O₃ at 220 °C, and then Mn₂O₃ was reduced to Mn₃O₄ at 315 °C. The third peak at 415 °C is due to the reduction of Mn₃O₄ to MnO. The reduction of surface Ce⁴⁺ was shifted to a high temperature around 500 °C due to the lower concentration of Ce³⁺ and the electron transfer from Ce to Mn. Compared to the reduction peaks of MnO_x–CeO₂ nanorods, all of the reduction peaks for MnO_x–CeO₂ nanocubes have shifted to higher temperatures. This is reasonable since more Mn has entered the Ce lattice for MnO_x–CeO₂ nanorods, which causes the formation of –Mn–O–Ce– bonds. It was reported that –Mn–O–Ce– bonds is more reducible than –Ce–O– or –Mn–O– bonds.²⁶ The presence of –Mn–O–Ce– bonds effectively lowered the overall reduction temperature for MnO_x–CeO₂ nanorods.

It is interesting that MnO_x–CeO₂ nanorods also possess more compacted reduction peaks than MnO_x–CeO₂ nanocubes. This might be due to the greater abundance of Mn⁴⁺ and Mn³⁺ of MnO_x–CeO₂ nanorods, because higher oxidation states of Mn are easier to be reduced. Another possible explanation is that oxygen vacancies in MnO_x–CeO₂ nanorods could also promote the transition of Mn from 4 + to lower valence states, since it is able to lower the thermodynamic barrier for Ce⁴⁺ → Ce³⁺.

3.3 Catalytic wet air oxidation of phenol

The catalytic performance of various oxides were investigated and the results are displayed in Table 2. It was found that cerium and manganese monoxides (Fig. S2†) exhibit much lower activities for the CWAO of phenol than the MnO_x–CeO₂ catalysts, which proves the effectiveness of Mn/Ce oxide composites. In order to study the effect of oxygen vacancies on the CWAO of phenol, CeO₂ nanorods and nanocubes were carefully investigated. The adsorption experiments showed that CeO₂ nanorods could absorb 5% of phenol, whereas CeO₂ nanocubes could only absorb 1%. Fig. 10 illustrates the reaction profiles of phenol and TOC conversions for CeO₂ nanomaterials at 140 °C and 2 MPa air. It was found that CeO₂ nanocubes were almost inert toward the CWAO of phenol, whereas phenol and total organic carbon (TOC) conversions were 11% and 8%, respectively. Conversely, CeO₂ nanorods were more active than CeO₂ nanocubes, whereas 51% of phenol conversion and 45% of TOC removal were achieved.

Table 2 Summary of the catalytic activity of different metal oxides on the CWAO of phenol. Phenol concentration: 1000 ppm; catalyst loading: 4 g L⁻¹; temperature: 140 °C; pressure: 2 MPa air; time: 1 h

Catalyst label	Phenol conversion (%)	TOC conversion (%)
CeO2 nanocubes	8	6
CeO2 nanorods	27	23
MnO2	79	67
Mn3O4	70	62
MnCeOx nanocubes	100	97
MnCeOx nanorods	100	98

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Fig. 10 The CWAO of phenol using CeO₂ nanorods and nanocubes, (a) phenol conversion, (b) TOC conversion. Concentration of phenol: 1000 ppm; catalyst loading: 4 g L⁻¹; temperature: 140 °C; pressure: 2 MPa air.

Compared to CeO₂ nanomaterials, MnO_x–CeO₂ catalysts give very rapid phenol oxidation. MnO_x–CeO₂ nanorods could completely convert phenol and remove 98% of TOC within 30 min, at 140 °C and 2 MPa air (Fig. 11). The comparable activity for MnO_x–CeO₂ nanocubes could only be achieved after 60 min. Furthermore, the adsorption experiment showed that MnO_x–CeO₂ nanorods could absorb 15% of phenol, whereas MnO_x–CeO₂ nanocubes only absorbed 7%. It was seen that the addition of Mn could enhance the adsorption ability of CeO₂ nanorods by more than 3 times. This is important, since adsorption is the primary step for the CWAO of phenol. Moreover, it was reported that high oxidation states of manganese oxide can also behave as active centers for phenol oxidation.³⁶ Thus, it could be postulated that the addition of Mn helped to adsorb phenol and its intermediates, then Mn⁴⁺ was reduced and the adsorbed organic compounds were oxidized.

Fig. 11 Phenol (a) and TOC (b) conversions on MnO_x–CeO₂ nanorods and nanocubes. Concentration of phenol: 1000 ppm; catalyst loading: 4 g L⁻¹; temperature: 140 °C; pressure: 2 MPa air.

Tests on the evolution of the small-molecule intermediates for MnO_x–CeO₂ catalysts were conducted, and the results are shown in Fig. 12. There are 4 major small-molecule intermediates: formic acid, acetic acid, oxalic acid, and hydroquinone. Other intermediates, including polymeric products, are also reported in the literature,⁴⁵ and can be calculated by subtracting the concentration of the remaining TOC and small-molecule intermediates. The initial TOC value for phenolic solution was about 850 ppm. For MnO_x–CeO₂ nanorods, the TOC conversion was 98%, and the remaining TOC was calculated to be 17 ppm. The total organic carbon concentration of formic acid, acetic acid, oxalic acid, and hydroquinone was also converted, and turned out to be 15 ppm. Thus, it implies that the small-molecule intermediates are responsible for 88% of the remaining TOC for MnO_x–CeO₂ nanorods. On the other hand, the small-molecule intermediates are only responsible for 32% of the remaining TOC for MnO_x–CeO₂ nanocubes. This means MnO_x–CeO₂ nanocubes have converted more phenol into large-molecule intermediates. In other words, MnO_x–CeO₂ nanocubes are less oxidative than MnO_x–CeO₂ nanorods.

Fig. 12 Evolution of the small-molecule intermediates in the CWAO of phenol over MnO_x–CeO₂ nanorods (a) and nanocubes (b).

The MnO_x–CeO₂ catalysts were also examined by comparing their catalytic activities at various temperatures (Fig. S3†). The results show that the catalytic activity of MnO_x–CeO₂ nanorods and nanocubes are both positively related to the reaction temperature. MnO_x–CeO₂ nanorods and nanocubes exhibit similar catalytic activity at 100 and 180 °C. However, MnO_x–CeO₂ nanorods become much more active than MnO_x–CeO₂ nanocubes at 140 °C. This can be due to the high reducibility of MnO_x–CeO₂ nanorods surface, which play a more important role at moderate temperature.

In order to study the effect of oxygen partial pressure in the CWAO of phenol on MnO_x–CeO₂ nanorods and nanocubes, different pressures were investigated (Fig. S4†) for reaction. The temperature was kept at 140 °C and the catalytic activity was measured after 30 min reaction. When the total air pressure was set to 0.5 MPa, MnO_x–CeO₂ nanorods could convert 83% of phenol and 71% of TOC, whereas MnO_x–CeO₂ nanocubes could only convert 64% of phenol and 62% of TOC. When the pressure was increased, both the MnO_x–CeO₂ nanorods and nanocubes exhibited higher phenol and TOC conversions. MnO_x–CeO₂ nanorods could completely remove phenol when the total pressure was greater than 2 MPa air. On the other hand, MnO_x–CeO₂ nanocubes could only achieve 95.7% phenol conversion at 3.5 MPa air. These phenomena emphasize the importance of the oxygen partial pressure to the performance of MnO_x–CeO₂ catalyst materials.

For the CWAO of phenol, the oxidizing ability of the catalyst plays a decisive role in the whole CWAO process. The TEM and XPS results indicated that the two nanorods catalysts presented a higher amount of oxygen vacancies, while the H2-TPR results suggested that both nanorod catalysts possessed higher reducibility. Generally, the lower temperature at which the reduction peak appears, the higher the oxidizing ability the catalyst possesses. Clearly, increasing the oxygen vacancies concentration can promote the oxidizing ability and thus enhance the catalytic performance for the CWAO of phenol.

4 Conclusions

CeO₂ and MnO_x–CeO₂ nanomaterials were synthesized and used for the CWAO of phenol, whereby both CeO₂ and MnO_x–CeO₂ nanorods displayed superior catalytic activity than the cubic counterparts. The characterizations revealed that the catalytic activity can benefit from the presence of oxygen vacancies, which improves the reducibility of the catalyst surface and boosts the productivity of active oxygen species. The incorporation of Mn into the CeO₂ lattice can increase the phenol adsorption ability and further promote the oxidizing ability of the catalyst. Overall, this study illustrates the importance of oxygen vacancies in cerium catalysts for the CWAO of phenol, which might provide clues for a later study on other catalysts in this field.

Acknowledgements

The authors would like to thank the financial supports from the National Key Technology Support Program of China (No. 2014BAC10B01). Prof Dr Gai would like to thank the support from key scientific and technological project of China's Shanxi Province (No. MH2014-10). The support by the Natural Science Foundation of China (21336009 and 21673187) and the Natural Science Foundation of Fujian Province of China (No. 2015J05031) is also acknowledged.

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Footnote

† Electronic supplementary information (ESI) available: HRTEM images of CeO₂ 110 surface, XRD patterns of MnO_x catalysts; phenol and TOC conversions on MnO_x–CeO₂ nanorods and nanocubes at various temperatures; phenol and TOC conversions on MnO_x–CeO₂ nanorods and nanocubes at various pressures. See DOI: 10.1039/c7ra04037g

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