# Synthesis and Oxygen Storage Capability of CeO2 Powders for Enhanced Photocatalytic Degradation of Acid Orange 7

Authors: Yaohui Xu; Chi Deng; Changxue Dong; Qin Wang; Nunan Gao
Journal: International Journal of Photoenergy
 (2022)

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## Abstract

Acid Orange 7 (AO7) is one of the most common azo dyes; however, its strong azo bond makes them difficult to biologically degrade. We sought to degrade AO7 dye using CeO2 as a promising alternative photocatalyst. CeO2 powders were synthesized with alternative monoethanolamine (MEA), diethanolamine (DEA), and triethanolamine (TEA) as a precipitant by solvothermal process combined calcination in air. Compared to the oxygen storage capability of Blank-CeO2 (0.186 mmol O2/g), that of MEA-CeO2 synthesized in the presence of MEA as a precipitant increased by 21.0%, while that of DEA-CeO2 and TEA-CeO2 synthesized in the presence of DEA and TEA as precipitants were decreased. Importantly, such MEA-CeO2 exhibited the highest photocatalytic activity than Blank-, DEA-, and TEA-CeO2 in degradation of AO7 under simulated sunlight illumination, and the removal rate of AO7 by MEA-CeO2 could reach 98.3% within 100 min.


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## Body

## 1. Introduction

Azo dyes, containing plural azo and aryl groups, are the most widely synthetic dyes, used for dyeing and printing of various fibers [1–3], as well as coloring of paints, plastics and rubber, etc. [4–6]. Under special conditions, azo dyes can decompose and produce more than twenty kinds of carcinogenic aromatic amines, which could change the DNA structure of the human body to cause lesions and induce cancer by activation [7–12]. Therefore, the azo wastewater must be treated harmlessly before discharge [13–15]. So far, numerous approaches have been engaged to remove azo dyes, such as electrocatalysis by destroying color groups [16, 17], biodegradation by mineralizing colorless organic intermediates [18, 19], and chemical oxidation by photocatalytic catalyst [20–22]. Among these techniques, photocatalytic degradation is a promising process of choice [23–26]. Titanium dioxide (TiO2) is considered to be the most popular photocatalyst due to its environmental friendliness and biological inertness [27–29].Ceria (CeO2) is a typical rare earth oxide and can serve as catalyst [30], catalyst carrier [31], UV absorbent [32], fuel cell electrolyte [33], automobile exhaust absorbent [34], electronic ceramics [35], etc. These applications of CeO2 benefit generally from its excellent redox property of Ce3+↔Ce4+ and high oxygen storage capacity (OSC). It is accepted that the OSC of CeO2 is well associated with the presence of oxygen vacancies, as well as their photocatalytic activity [36–38]. However, the researches into the photocatalytic degradation of azo dyes on CeO2 are just beginning. For example, Aboutaleb and El-Salamony [39] prepared pure CeO2 and Fe-doped CeO2 nanocomposites by the precipitation and modified Sol-Gel auto combustion methods and investigated their photocatalytic activity in Congo Red azo dye under the visible light irradiation. For 1 g/L catalyst loading with 25 mg/L Congo Red solution, the degradation rates were 87, 82, and 48% for Fe-CeO2(p), pure CeO2, and Fe-CeO2 (sg) after visible irradiation for 180 min, respectively. Krishnan et al. [40] prepared Sn-doped 1 : 2 CeO2-Fe2O3 nanocomposite with different Sn contents by the thermal decomposition method for efficient degradation of Methylene Blue (MB) and Methyl Orange (MO) dyes under visible light. The as-synthesized nanocomposite with 5%-Sn achieved nearly complete degradation for 10-50 mg/L MO solution, and the maximum degradation efficiency obtained for MB solution was 93.54-94.65% for 10-30 mg/L MB solution. Mirzazadeh and Lashanizadegan [41] prepared CdO/CeO2/RGO composite by a hydrothermal process for the sonocatalytic degradation of Rhodamine B (Rho B) and MO under ultrasonic irradiation. The highest degradation efficiencies of Rho B and MO were 97% and 85% within 150 min, when the initial concentration of dyes was 1.2 g/L with 20 mg/L catalyst. Despite these progresses in the synthesis of pure CeO2 and CeO2-based photocatalyst, it is still challenging to further improve their photocatalytic activity.Herein, a series of CeO2 powders were synthesized solvothermally at 120°C followed by calcination at 500°C in air using Ce(NO3)3·6H2O as a cerium source and using alternative monoethanolamine (MEA), diethanolamine (DEA), and triethanolamine (TEA) as a precipitant. The relative OSC of the as-synthesized CeO2 was quantified by O2 temperature-programmed desorption (O2-TPD) measurements, and the photocatalytic activity for Acid Orange 7 (AO7) was determined.


## 2. Experimental

### 2.1. Starting Materials

Ce(NO3)3·6H2O (99.95%) and triethanolamine (TEA, >99.0%) were supplied by Aladdin Co. Ltd. Monoethanolamine (MEA, 99.0%) was supplied by Shanghai Xianding Biological Science and Technology Co. Ltd. Diethanolamine (DEA, 99.0%) was supplied by Shanghai Maclin Biochemical Technology Co. Ltd. Acid Orange 7 (AO7, >97.0%) was supplied by Tokyo Chemical Industry Co. Ltd., and ethanol (≥99.7%) was supplied by Chengdu Kelong Chemical Co. Ltd. All reagents were commercially obtained and used as received without further purification.


### 2.2. Sample Preparation

A solvothermal procedure was employed to synthesize CeO2 precursors. Briefly, 6.0 mmol Ce(NO3)3·6H2O and an appropriate precipitant (MEA, DEA, or TEA) were dissolved into 20 mL ethanol, the value of nitrogen to cerium ratio (N : Ce (mol.)) was 0.3, and then, ethanol was added to make a final volume of 25 mL. The as-formed solution was decanted into a 50 mL Teflon-lined autoclave and maintained for 24 h at 120°C. Subsequently, the resulting precipitates were washed with distilled water and ethanol and dried at 60°C for 24 h. Finally, the as-formed powders were calcined in air at 500°C for 2 h. These final CeO2 products were labeled as MEA-CeO2, DEA-CeO2, and TEA-CeO2 synthesized with TEA, MEA, and DEA as a precipitant, respectively.For comparison, the CeO2 sample was synthesized following the same procedure as control, however in the absence of precipitants (MEA, DEA, and TEA), labeled as Blank-CeO2.


### 2.3. Characterization

The phases of the precursors and the final products were examined by X-ray diffraction (XRD, DX-2700). The morphologies of the final products CeO2 were examined using a field-emission Scanning Electron Microscope (SEM, Hitachi S-4800). The Brunauer-Emmett-Teller (BET) specific surface areas (SBET) of the final products CeO2 were obtained from nitrogen adsorption measurements (Quantachrome, Quadrasorb SI). Raman spectra were measured using a Horiba Jobin Yvon LabRam Aramis Raman spectrometer with a He-Cd laser of 325 nm.


### 2.4. O2-TPD and H2-TPR Measurements

The OSCs of the final products CeO2 were estimated by O2 temperature-programmed desorption (O2-TPD) measurements, which were carried out in a plug-flow microreactor system (TP5000) with a thermal conductivity detector (TCD), and the amount of O2 desorption during the process was measured by the TCD. The as-synthesized CeO2 powders (~100 mg) were activated with air stream for 30 min at 400°C, changed into Helium (He) and cooled, changed into air stream for 30 min at 120°C, then purged with He stream to remove the excess O2, and finally was conducted surface oxygen desorption at the flow of He (10 mL/min), while the temperature was raised to ~930°C (10°C/min).H2 temperature-programmed reduction (H2-TPR) measurements were still carried out in the same TP5000 system, and the amount of H2 uptake during the reduction was measured by the TCD. The as-synthesized CeO2 powders (~50 mg) were activated with 5% O2/He stream for 1 h at 500°C, cooled to room temperature in 5% O2/He stream, and changed into He stream to remove the excess O2. Finally, the H2-TPR experiments were performed in a 5% H2/He stream (30 mL/min) with a heating rate of 10°C/min.


### 2.5. Evaluation of Photocatalytic Activity

The photocatalytic activities of the as-synthesized CeO2 powders as catalysts were evaluated by degradation of AO7 dye using an offline photocatalytic reactor attached to a 300 W pulsed Xenon lamp. Briefly, 200 mg of the as-synthesized CeO2 powders was dispersed into 200 mL of 20 mg/L AO7 solution. Before illumination, the mixture was stirred for 60 min in dark to establish adsorption-desorption equilibrium between CeO2 and AO7 molecules. Subsequently, the mixture was exposed to the simulated sunlight illumination originating from a 300 W pulsed Xenon lamp. The absorbance of the supernatant was measured at the maximum absorption wavelength (485 nm) for AO7 dye using a Hitachi U-3900 spectrophotometer, and the removal rate (η, %) was estimated as follows:
(1)η%=A0−AtA0×100,where A0 is the absorbance value of the initial AO7 solution (20 mg/L) and At is the absorbance value of the AO7 solution at a given time t.


## 2.1. Starting Materials

Ce(NO3)3·6H2O (99.95%) and triethanolamine (TEA, >99.0%) were supplied by Aladdin Co. Ltd. Monoethanolamine (MEA, 99.0%) was supplied by Shanghai Xianding Biological Science and Technology Co. Ltd. Diethanolamine (DEA, 99.0%) was supplied by Shanghai Maclin Biochemical Technology Co. Ltd. Acid Orange 7 (AO7, >97.0%) was supplied by Tokyo Chemical Industry Co. Ltd., and ethanol (≥99.7%) was supplied by Chengdu Kelong Chemical Co. Ltd. All reagents were commercially obtained and used as received without further purification.


## 2.2. Sample Preparation

A solvothermal procedure was employed to synthesize CeO2 precursors. Briefly, 6.0 mmol Ce(NO3)3·6H2O and an appropriate precipitant (MEA, DEA, or TEA) were dissolved into 20 mL ethanol, the value of nitrogen to cerium ratio (N : Ce (mol.)) was 0.3, and then, ethanol was added to make a final volume of 25 mL. The as-formed solution was decanted into a 50 mL Teflon-lined autoclave and maintained for 24 h at 120°C. Subsequently, the resulting precipitates were washed with distilled water and ethanol and dried at 60°C for 24 h. Finally, the as-formed powders were calcined in air at 500°C for 2 h. These final CeO2 products were labeled as MEA-CeO2, DEA-CeO2, and TEA-CeO2 synthesized with TEA, MEA, and DEA as a precipitant, respectively.For comparison, the CeO2 sample was synthesized following the same procedure as control, however in the absence of precipitants (MEA, DEA, and TEA), labeled as Blank-CeO2.


## 2.3. Characterization

The phases of the precursors and the final products were examined by X-ray diffraction (XRD, DX-2700). The morphologies of the final products CeO2 were examined using a field-emission Scanning Electron Microscope (SEM, Hitachi S-4800). The Brunauer-Emmett-Teller (BET) specific surface areas (SBET) of the final products CeO2 were obtained from nitrogen adsorption measurements (Quantachrome, Quadrasorb SI). Raman spectra were measured using a Horiba Jobin Yvon LabRam Aramis Raman spectrometer with a He-Cd laser of 325 nm.


## 2.4. O2-TPD and H2-TPR Measurements

The OSCs of the final products CeO2 were estimated by O2 temperature-programmed desorption (O2-TPD) measurements, which were carried out in a plug-flow microreactor system (TP5000) with a thermal conductivity detector (TCD), and the amount of O2 desorption during the process was measured by the TCD. The as-synthesized CeO2 powders (~100 mg) were activated with air stream for 30 min at 400°C, changed into Helium (He) and cooled, changed into air stream for 30 min at 120°C, then purged with He stream to remove the excess O2, and finally was conducted surface oxygen desorption at the flow of He (10 mL/min), while the temperature was raised to ~930°C (10°C/min).H2 temperature-programmed reduction (H2-TPR) measurements were still carried out in the same TP5000 system, and the amount of H2 uptake during the reduction was measured by the TCD. The as-synthesized CeO2 powders (~50 mg) were activated with 5% O2/He stream for 1 h at 500°C, cooled to room temperature in 5% O2/He stream, and changed into He stream to remove the excess O2. Finally, the H2-TPR experiments were performed in a 5% H2/He stream (30 mL/min) with a heating rate of 10°C/min.


## 2.5. Evaluation of Photocatalytic Activity

The photocatalytic activities of the as-synthesized CeO2 powders as catalysts were evaluated by degradation of AO7 dye using an offline photocatalytic reactor attached to a 300 W pulsed Xenon lamp. Briefly, 200 mg of the as-synthesized CeO2 powders was dispersed into 200 mL of 20 mg/L AO7 solution. Before illumination, the mixture was stirred for 60 min in dark to establish adsorption-desorption equilibrium between CeO2 and AO7 molecules. Subsequently, the mixture was exposed to the simulated sunlight illumination originating from a 300 W pulsed Xenon lamp. The absorbance of the supernatant was measured at the maximum absorption wavelength (485 nm) for AO7 dye using a Hitachi U-3900 spectrophotometer, and the removal rate (η, %) was estimated as follows:
(1)η%=A0−AtA0×100,where A0 is the absorbance value of the initial AO7 solution (20 mg/L) and At is the absorbance value of the AO7 solution at a given time t.


## 3. Results and Discussions

XRD analysis was employed to identify the phase composition and crystallographic structure of the precursors and the final products. Figure1(a) shows XRD patterns of the precursors synthesized using the solvothermal process at 120°C for 24 h with alternative MEA, DEA, or TEA as a precipitant. As observed, all broad peaks had a good match with the standard CeO2 pattern (JCPDS NO. 34-0394), and no additional phases for impurities such as cerium carbonate (e.g., Ce2(CO3)3 or Ce2(CO3)OH) were detected, which indicated that CeO2 could be successfully obtained by the solvothermal process. Moreover, compared with the JCPDS card of 34-0394, all the identified peaks were assigned to the face-centered cubic fluorite structure of CeO2 and found no observable differences in the relative intensity of the peaks, which suggested that there was no preferential orientation or preferential crystal growth. After, followed by calcination in air at 500°C for 2 h, all the identified peaks in Figure 1(b) were still assigned to the cubic CeO2 (JCPDS No. 34-0394), no impurity phases were detected, and the intensities of the corresponding diffraction peaks were comparable. Compared to the relative intensities of the diffraction peaks in Figure 1(a), these of CeO2 in Figure 1(b) were improved obviously, indicating that the crystallinity of CeO2 synthesized by the solvothermal process was improved during calcination.Figure 1
XRD patterns of (a) the precursors synthesized solvothermally at 120°C for 24 h and (b) the final products synthesized followed by calcination in air at 500°C for 2 h.After O2 chemisorption at 400°C with air stream on CeO2, O2-TPD experiments were performed in He stream, and the O2-TPD spectra of Blank-, MEA-, DEA-, and TEA-CeO2 powders are showed in Figures 2(a)–2(d), respectively. As observed, it could be clearly observed that the oxygen desorption was rapid at the early stages of the process (120-170°C), reached a state of stable deoxidation, and maintained until ~600°C. The oxygen desorption peaks at 120-600°C could be attributed to the desorption of the adsorption oxygen in surface/subsurface lattice oxygen. Moreover, the amounts of oxygen desorption began to decrease gradually above ~600°C, which may be attributed to the adsorption oxygen in bulk lattice oxygen. Further analysis of the reason was conducted by H2-TPR analysis as discussed later.Figure 2
O2-TPD profiles of (a) Blank-, (b) MEA-, (c) DEA-, and (d) TEA-CeO2 powders synthesized by solvothermal process combined calcination in air.To understand the decrease in amounts of oxygen desorption after 600°C in Figure2(b), H2-TPR analyses were performed. Figures 3(a) and 3(b) show the profiles of Blank-CeO2 and MEA-CeO2 powders synthesized by solvothermal process combined calcination in air, respectively. As observed in Figures 3(a) and 3(b), it can be clearly found that there were two obvious peaks of hydrogen reduction appearing at ~510 and~790°C, implying the existence of two kinds of oxygen species at various coordination environments. According to Reference [42], the low temperature peaks (light yellow area, 120-600°C) could be assigned to the reduction of surface Ce4+ cation, while the high temperature peaks (light green area, 600-930°C) could be assigned to the reduction of bulk Ce4+ cation. In other words, there were two kinds of oxygen species on these CeO2 and adsorption oxygen in surface and bulk lattice oxygen. Therefore, the decrease in amounts of oxygen desorption after 600°C in Figure 2(b) could be explained by the following. The adsorption oxygen in CeO2 surface and subsurface was gradual while the lattice oxygen in CeO2 bulk cannot be transferred to the CeO2 surface in time. Finally, all lattice oxygen in CeO2 bulk was transmitted to the CeO2 surface and desorbed out; the O2-TPD profiles would overlap with the baseline as well as H2-TPR profiles. It was worth noting that the peak intensities of MEA-CeO2 in Figure 3(b), whether the low temperature peaks (light yellow area) or high temperature peaks (light green area), were much higher than that of Blank-CeO2. It indicated that the reaction of H2 with MEA-CeO2 was more violent during the entire temperature range of 120-930°C.Figure 3
H2-TPR profiles of (a) Blank- and (b) MEA-CeO2 powders synthesized by solvothermal process combined calcination in air.The relative OSC could be quantified using the amount of O2 desorption per gram of CeO2 by measuring the corresponding peak area of O2-TPD profile, and the values of quantified OSC are showed in Figure 4. As observed, it was found that the OSC decreased following the order: MEA-CeO2 (0.225 mmol/g)>Blank-CeO2 (0.186 mmol/g)>DEA-CeO2 (0.162 mmol/g)>TEA-CeO2 (0.138 mmol/g). Compared to the OSC of Blank-CeO2, that of MEA-CeO2 synthesized in the presence of MEA as precipitant increased by 21.0%, while that of DEA-CeO2 and TEA-CeO2 synthesized in the presence of DEA and TEA as precipitants were decreased. Differences in OSC may be attributed to differences in particle size, morphology, and specific surface area of final products CeO2; further analysis was conducted by SEM analysis and nitrogen adsorption measurements as discussed later.Figure 4
Relative OSC (mmol O2/g) of Blank-, MEA-, DEA-, and TEA-CeO2 powders calculated by measuring the corresponding peak area of O2-TPD profiles in Figures 2(a)–2(d).Raman scattering is a widely used technique for structural analysis of CeO2 due to its sensitivity to significant structural changes, such as the formation of oxygen vacancies [43]. Figure 5 shows the Raman spectra of the Blank-, MEA-, DEA-, and TEA-CeO2 powders. The Raman spectra of each sample showed two peaks at about 450 and 580 cm-1. The peak at about 450 cm-1 was attributed to the F2g vibration mode of the O atoms around each Ce4+ cation, while the band at 580 cm-1 was known to be associated with the oxygen vacancies and had been widely observed in CeO2-x [44]. Furthermore, the relative oxygen vacancy concentration could be determined from the Raman spectra by the ratio of the Raman band intensity at ~580 cm-1 to that at ~450 cm-1 (I580/I450) [45]. The oxygen vacancy concentrations of Blank-, MEA-, DEA-, and TEA-CeO2 powders, which were the corresponding values of I580/I450, were estimated as 0.39, 0.47, 0.34, and 0.29, respectively. The order of relative concentrations of oxygen vacancy (MEA-CeO2 (0.47)>Blank-CeO2 (0.39)>DEA-CeO2 (0.34)>TEA-CeO2 (0.29)) is the same as that of OSC in Figure 4. This result indicates that the MEA promoted the oxygen vacancies of CeO2, and the higher value might be beneficial for their OSC.Figure 5
Raman spectra with 325 nm laser excitation for Blank-, MEA-, DEA-, and TEA-CeO2 powders.Figures6(a)–6(d) show the SEM images of Blank-, MEA-, DEA-, and TEA-CeO2 powders synthesized by solvothermal process combined calcination in air, respectively. As observed in Figure 6(a), the Blank-CeO2 sample displayed near spherical aggregate particles with two diameters of ~150 nm and ~50 nm. After adding precipitant (MEA, DEA, and TEA) in the solvothermal process, the as-synthesized final products CeO2 still displayed two sizes of aggregate particles, but the smaller particles got smaller (see Figures 6(b)–6(d)). Moreover, these size values of Blank-, MEA-, DEA-, and TEA-CeO2 particles were demonstrated by statistical analysis; the size distribution histograms are showed in Figures 7(a)–7(d). Furthermore, the specific surface areas of Blank-, MEA-, DEA-, and TEA-CeO2 powders were determined using the Brunauer-Emmett-Teller method, and SBET decreased following the order: MEA-CeO2 (58.4 m2/g)>Blank-CeO2 (42.6 m2/g)>DEA-CeO2 (35.8 m2/g)>TEA-CeO2 (32.1 m2/g). The ordering of SBET was consistent with that of OSC, indicating that SBET of CeO2 powders was the major factors in their OSC.Figure 6
SEM images of (a) Blank-, (b) MEA-, (c) DEA-, and (d) TEA-CeO2 particles synthesized by solvothermal process combined calcination in air.
(a)(b)(c)(d)Figure 7
Size distribution histograms of (a) Blank-, (b) MEA-, (c) DEA-, and (d) TEA-CeO2 particles.
(a)(b)(c)(d)The adsorption capacities of final CeO2 products in the dark and the photocatalytic activities upon simulated sunlight were investigated by the removal of AO7 dye. From Figure 8, it could be observed that the removal rates of AO7 within 60 min reached 12.3, 26.6, 6.4, and 4.4% for Blank-, MEA-, DEA-, and TEA-CeO2 powders in the dark, respectively. In fact, the adsorption reactions in the dark were mostly completed within 40 min, and no significant changes were observed from 40 to 60 min, indicating that the adsorption-desorption equilibriums between AO7 molecules and CeO2 were established within the first 40 min of adsorption reactions. After illumination upon simulated sunlight for 220 min, the removal rate of AO7 was 99.8% MEA-CeO2, which was much higher than 89.5% for Blank-CeO2, while only 68.4 and 46.7% for DEA- and TEA-CeO2, respectively. It was worth noting that the photocatalytic reaction on MEA-CeO2 was completed within 100 min, the removal rate was 98.3%, no significant changes were observed from 100 to 220 min, and the removal rate of MEA-CeO2 increased by 44.9% compared to that of Blank-CeO2 within 100 min. It indicated that the as-synthesized MEA-CeO2 was more suitable as a promising alternative photocatalyst for the removal of AO7 dye. In addition, the photostability of the as-synthesized MEA-CeO2 was also evaluated through recycling experiments for photodegradation of AO7 upon illumination with a 300 W Xenon lamp. As shown in Figure 9, after four photodegradation cycles, MEA-CeO2 showed no significant change in the photocatalytic activity within 5 h, indicating that the MEA-CeO2 serving as photocatalyst were stable during photocatalysis.Figure 8
AO7 adsorption in the dark and AO7 degradation upon illumination with a 300 W Xenon lamp in the presence of Blank-, MEA-, DEA-, and TEA-CeO2 powders synthesized by solvothermal process combined calcination in air (CeO2=1.0g/L;AO7=20mg/L;V=200mL; room temperature; without pH preadjustment).Figure 9
Cycling runs of photocatalytic degradation of AO7 dye with MEA-CeO2 particles under a 300 W Xenon lamp.


## 4. Conclusions

CeO2 powders with enhanced OSC and enhanced photocatalytic activity were synthesized by a solvothermal method at 120°C for 24 h followed by calcination at 500°C for 2 h, in which MEA served as an alternative precipitant with N:Cemol.=0.3. The OSC decrease following the order was MEA-CeO2 (0.225 mmol/g)>Blank-CeO2 (0.186 mmol/g)>DEA-CeO2 (0.162 mmol/g)>TEA-CeO2 (0.138 mmol/g). Compared to Blank-CeO2, the OSC of MEA-CeO2 synthesized in the presence of MEA as a precipitant increased by 21.0%. Moreover, the removal rate of AO7 was 99.8% for MEA-CeO2, which was much higher than 89.5% for Blank-CeO2, while only 68.4 and 46.7% for DEA- and TEA-CeO2, respectively. The photocatalytic reaction on MEA-CeO2 was mostly completed within 100 min, the removal rate was 98.3%, and the removal rate of MEA-CeO2 increased by 44.9% compared to that of Blank-CeO2 within 100 min. Moreover, the MEA-CeO2 serving as photocatalyst were stable during the four recycling experiments for photodegradation of AO7 with a 300 W Xenon lamp, indicating that the as-synthesized MEA-CeO2 was more suitable as a promising alternative photocatalyst for the removal of AO7 dye.



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*Source: 8594451-2022-03-17.xml*