Effect of morphology on the catalytic oxidation of dichloromethane over CeO<sub>2</sub> catalysts

Jiaqing Miao; Lei Qian; Chang Wu; Erhao Gao; Wei Wang; Jiali Zhu; Shuiliang Yao; Zhuliang Wu; Jing Li

doi:10.4491/eer.2024.622

Environ Eng Res > Volume 30(5); 2025 > Article

Miao, Qian, Wu, Gao, Wang, Zhu, Yao, Wu, and Li: Effect of morphology on the catalytic oxidation of dichloromethane over CeO2 catalysts

Research

Environmental Engineering Research 2025; 30(5): 240622.

Published online: February 1, 2025

DOI: https://doi.org/10.4491/eer.2024.622

Effect of morphology on the catalytic oxidation of dichloromethane over CeO₂ catalysts

Jiaqing Miao¹, Lei Qian¹, Chang Wu¹, Erhao Gao^1,², Wei Wang^1,², Jiali Zhu^1,², Shuiliang Yao^1,², Zhuliang Wu^1,^2,^†

, Jing Li^1,^2,^†

¹School of Environmental Science and Engineering, Changzhou University, Changzhou 213164, China

²Key Laboratory of Advanced Plasma Catalysis Engineering for China Petrochemical Industry, Jiangsu 213164, China

^†Corresponding author: E-mail: lijing_831@cczu.edu.cn (J.L.), wuzuliang@cczu.edu.cn (Z.L.W.), Tel: +0086-0519-86330086 (J.L.), +0086-0519-86330086 (Z.L.W.), ORCID: 0000-0003-4742-7012 (J.L.), 0000-0003-0156-8696 (Z.L.W.)

Received October 31, 2024 Revised January 12, 2025 Accepted January 30, 2025

This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/3.0/) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

The catalytic oxidation of CVOCs has emerged as a prominent and challenging research frontier in environmental science. This study systematically investigated the influence of CeO₂ morphology on the oxidation of dichloromethane (DCM), a representative and recalcitrant CVOC. Through solvothermal synthesis, we successfully fabricated CeO₂ catalysts with four distinct morphologies: cubes, nanorods, nanosheets, and nanospheres. These nanostructured catalysts were evaluated for their catalytic performance in DCM oxidation, revealing significant morphology-dependent activity. CeO₂-Nanosheets emerged as the most effective catalyst, achieving a T₉₀ of 384°C, with the highest CO₂ yield and the lowest chlorine-containing by-products. It maintained stable performance over 48 h with minimal decline in DCM conversion and CO₂ selectivity. Characterization revealed that the nanosheet morphology possessed several advantageous features: high concentrations of reactive oxygen species, abundant Ce³⁺ active sites, enhanced redox capacity and optimal surface acidity. These characteristics collectively contributed to the exceptional catalytic activity and selectivity. In situ DRIFTS experiments were conducted to elucidate the reaction mechanism, identifying three key intermediates in the oxidation process: methoxy, formaldehyde and formate species. These findings not only provide a detailed understanding of the reaction pathway but also offer valuable insights for optimizing catalyst design.

Keywords: Catalytic oxidation, CeO2 catalyst, Dichloromethane degradation, Morphology effect, Reaction mechanism

Graphical Abstract

Keywords: Catalytic oxidation, CeO2 catalyst, Dichloromethane degradation, Morphology effect, Reaction mechanism

1 Introduction

Chlorinated volatile organic compounds (CVOCs) are characterized by high toxicity, high stability and difficult to degrade. Most of the CVOCs released into the atmosphere tend to accumulate in the environment, causing long-term and widespread environmental pollution [1–3]. Pharmaceutical, electronics, steel coking and waste incineration industries are the main sources of CVOCs emissions [4, 5]. In recent years, the treatment of CVOCs has received much attention. Plasma, photo-catalysis and thermal-catalysis technologies for the treatment of CVOCs have been reported [6–9]. Catalytic oxidation technology has become one of the most promising technologies for CVOCs treatment due to its advantages of less secondary pollution, wide applicability and high treatment efficiency [10–14].

Transition metal oxides, including CeO₂, are considered as important catalysts for the catalytic oxidation of CVOCs due to their excellent redox capacity and rapid oxygen storage/release ability [15–17]. The morphology of metal oxide catalyst is an important factor affecting the catalytic reaction. By regulating the morphology of the catalyst, its physicochemical properties can be effectively tuned to improve the catalytic activity, selectivity and stability [18, 19]. For example, Jiang et al. [20] prepared MnO₂ with different morphologies by alkaline hydrolysis oxidation and applied them to the catalytic oxidation of chlorobenzene. The results showed that the adjustment of morphology facilitated the transfer and diffusion of chlorobenzene and the corresponding intermediates, which led to the enhancement of the activity. Fu et al. [21] prepared a series of hollow spherical catalysts with controllable microscopic surface structure for the catalytic oxidation of chlorobenzene using a simple two-step method. Among them, the nanorod-stacked Mn_1.2VO_X catalyst showed the best catalytic activity and stability.

Recently, studies on the morphology modulation of CeO₂ catalysts have gradually emerged. By controlling the parameters of the hydrothermal method, different morphologies of CeO₂ were prepared for the catalytic reaction of CO₂ methanation, among which the CeO₂ nanoparticle catalyst showed the best catalytic activity and stability [22]. Cao et al. [23, 24] synthesized Pd/CeO₂ catalysts with hollow nanospheres, octahedrons, cubes and nanorods for catalytic dehydrogenation of formic acid. The results showed that the hollow nanosphere catalyst was highly active, which could be attributed to its large specific surface area and the maximum number of Ce³⁺ and oxygen vacancies. Morphology-based optimization can lead to the development of more efficient and sustainable catalysts, significantly reducing energy consumption and enhancing oxidation efficiency. As a result, studies that investigate the relationship between CeO₂ morphology and catalytic performance offer critical insights into designing advanced catalysts tailored for CVOCs oxidation.

On the other hand, the study of anti-chlorine poisoning during the catalytic oxidation of CVOCs is another hot spot. Due to the adsorption of inorganic chlorine at the active sites, the catalyst will inevitably deactivate [25, 26]. Dai et al. [27] proposed two mechanisms to solve the rapid deactivation problem of the catalysts. The first method is to dope other metals (e.g., Mn, Cu, Fe, Co, Ru) to improve the redox properties of the catalysts and to promote the desorption of chlorine species adsorbed on the catalyst surface, but it is prone to promote the formation of polychlorinated by-products. The second method is acidic modification by V, Mo, W, P or HZSM-5 to introduce Brønsted acidic sites, so that the inorganic chlorine adsorbed on the surface of the catalyst reacted with the H to promote its desorption into gaseous HCl [28]. However, this affects the oxidizing ability of the catalyst and easily leads to carbon deposition and poor CO₂ selectivity. Nowadays, the oxygen vacancy concentration and acidity can be adjusted by preparing catalysts with different morphologies.

As one of the typical CVOCs, dichloromethane (DCM) is usually considered as a representative of halogenated hydrocarbons. The study of catalytic degradation of DCM is of great significance for the effective treatment of CVOCs. Based on the above background, CeO₂ catalysts with different morphologies of cubes, nanorods, nanosheets and nanospheres were prepared by solvothermal method and evaluated for the catalytic oxidation of DCM. The structural features, redox properties and surface acidity of CeO₂ with different morphologies were investigated by characterizations such as BET, XRD, H₂-TPR, XPS and NH₃-TPD, and the catalytic mechanism was analyzed in depth.

2 Experimental Methods

2.1. Catalyst Preparation

CeO₂ catalysts with different morphologies were prepared by solvothermal method. The preparation process was as follows.

CeO₂ in cube-like morphology

3 g of Ce(NO₃)₃·6H₂O and 8.8 g of NaOH were dissolved in 80 mL of deionized water with stirring. After stirring vigorously for 1 h, the mixed milky slurry was transferred to a Teflon-lined autoclave and heated at 180°C for 24 h. After cooling to 25°C, the precipitates were washed with deionized water and ethanol, and then dried at 110°C for 12 h. The solid product was calcined in air at 350°C for 3 h to obtain CeO₂-Cubes.

CeO₂ in nanorod-like morphology

3 g of Ce(NO₃)₃·6H₂O and 34 g of NaOH were dissolved in 80 mL of deionized water under stirring. After 1 h of vigorous stirring, the mixed milky slurry was transferred to a Teflon-lined autoclave and heated at 100°C for 48 h. After cooling to 25°C, the precipitates were washed with deionized water and ethanol, and then dried at 110°C for 12 h. The solid product was calcined in air at 350°C for 3 h to obtain CeO₂-Nanorods.

CeO₂ in nanosheet-like morphology

2.76 g of Ce(CH₃COO)₃ was dissolved in 80 mL of anhydrous methanol with stirring. After stirring vigorously for 1 h, the mixed milky slurry was transferred to a Teflon-lined autoclave and heated at 180°C for 20 h. After cooling to 25°C, the precipitates were washed with deionized water and ethanol, and then dried at 110°C for 12 h. The solid product was calcined in air at 400°C for 4 h to obtain CeO₂-Nanosheets.

CeO₂ in nanophere-like morphology

1 g of Ce(CH₃COO)₃ and 15 g of urea were dissolved in 80 mL of methanol with stirring. After stirring vigorously for 1 h, the mixed milky slurry was transferred to a Teflon-lined autoclave and heated at 150°C for 16 h. After cooling to 25°C, the precipitates were washed with deionized water and ethanol, and then dried at 80°C for 24 h. The solid product was calcined in air at 500°C for 4 h to obtain CeO₂-Nanospheres.

It is worth noting that although the synthesis of CeO₂ with different morphologies is well documented and theoretically reproducible, successful reproduction relies heavily on the careful optimization of the synthetic parameters.

2.2. Catalyst Characterization

The crystalline structure of catalysts were characterized by X-ray diffraction (XRD) using an X-ray diffractometer (Rigaku Ultima IV, Japan) with Cu-α radiation (40 kV, 30 mA), a scanning range of 20–80 kV and a step size of 0.02°. The specific surface area of catalysts was measured at liquid nitrogen temperature using N₂ adsorption and desorbed analyzer (Micromeritics ASAP 2460, USA). All samples were pretreated at 100°C under vacuum for 2 h before measurement. The morphology of the catalyst was observed by scanning electron microscopy (SEM) with an accelerating voltage of 3 kV. The instrument model used was Hitachi Regulus 8100 (Japan). X-ray photoelectron spectroscopy (XPS) was determined by an X-ray photoelectron spectrometer (Thermo Scientific K-Alpha, USA) to study the valence distribution of surface elements, especially the chemical states of Ce and O species. All spectra were corrected by the C1s peak at 284.8 eV.

H₂ temperature programmed reduction (TPR) experiments were conducted on a Micromeritics AutoChem II 2920 (USA) to investigate the reducibility of CeO₂. The sample was weighed above 80 mg in a U-shaped quartz tube, heated from room temperature to 300°C at 10°C/min, dried and pretreated, then purged with He flow (30 mL/min) for 1 h. The sample was then cooled to 50°C, and passed through 10% H₂/He mixture (30 mL/min) for 1 h. After purged with He flow (30 mL/min) for 0.5 h, the sample was heated to 800°C at 10°C/min, and the gas was detected by TCD.

NH₃-TPD experiments were carried out on a Micromeritics AutoChem II 2920 (USA) to assess the acidity of CeO₂. 0.1 g sample was weighed into a reaction tube, heated from room temperature to 300°C at 10°C/min, dried and pretreated. He flow (30 mL/min) was purged for 1h, then cooled to 50°C and saturated with 10% NH₃/He mixture (30 mL/min) for 1h. The sample was then switched to He flow (30 mL/min) and purged for 1 h to remove surface adsorbents. Finally, the sample was heated in He atmosphere at 10°C/min to 800°C. The off-gas was detected by TCD.

The in situ DRIFTS technique was utilized to detect the changes of surface groups of catalysts during the catalytic oxidation process. First, the samples were placed in the in situ reaction bath and pretreated with He at 300°C for 0.5 h, After cooling to room temperature, the infrared spectra were recorded as background. A gas stream containing 1000 ppm DCM (DCM/O₂/He) was introduced and the spectra were recorded at the desired temperature.

2.3. Catalytic Activity Evaluation

The catalytic oxidation of DCM was evaluated in a continuous flow microreactor (6 mm inner diameter) containing 0.2 g of catalyst. All catalysts were sieved to 40–60 mesh prior to testing. The activity tests were performed at 150–450°C under atmospheric pressure. A schematic of the catalytic evaluation system was shown in Fig. S1. The total gas flow rate was controlled at 100 mL/min using mass flow meters. Typical reaction gases consisted of 1000 ppm DCM, 20% O₂ and balanced N₂ at a gas hourly space velocity (GHSV) of 30000 h⁻¹. The liquid DCM was delivered via a syringe pump to the vaporization chamber. To ensure that the reactants were existed in the pipeline in gaseous form, the entire reaction pipeline was heated to 80°C. The effluent gasses (such as CH₃Cl, CH₂Cl₂, CHCl₃, CO and CO₂) were analyzed on-line by a gas chromatography (GC, GC9700II, Fuli).

The conversion of DCM (η_DCM) and the selectivity of COx (S_CO_{_x}) were calculated according to the following equations:

(1)

η_{DCM} = \frac{C_{in} - C_{out}}{C_{in}} \times 100 %

(2)

S_{C O_{x}} = \frac{C_{C O_{x}}}{C_{in} - C_{out}} \times 100 %

where, C_in and C_out were the inlet and outlet concentrations of DCM, respectively. C_COx (x =1, 2) were the concentrations of CO and CO₂ in the exhaust gas.

3 Results and Discussion

3.1. Catalytic Performance

Fig. 1(a) showed the effect of CeO₂ catalysts with different morphologies on DCM conversion. The catalytic activities of CeO₂ catalysts with different morphologies varied greatly. Among them, CeO₂-Cubes catalyst exhibited the worst catalytic activity. The conversion of DCM started to increase when the reaction temperature was higher than 275°C, and the conversion was only 55% (±4%) at 450°C. In contrast, the catalytic activities of CeO₂-Nanorods, CeO₂-Nanosheets and CeO₂-Nanospheres catalysts were better. T₅₀ and T₉₀ values were offset in the direction of lower temperatures. According to the DCM conversion curves, the catalytic activities of different CeO₂ catalysts were in the order of CeO₂-Nanosheets (T₉₀=384°C) > CeO₂-Nanorods (T₉₀=406°C) > CeO₂-Nanospheres (T₉₀=470°C) > CeO₂-Cubes (T₉₀>500°C).

The selectivity of CO₂ during the catalytic oxidation process was shown in Fig. 1(b). CeO₂-Nanosheets showed the best CO₂ selectivity, with more than 70% (±2%) at 225°C, whereas CeO₂-Nanorods and CeO₂-Nanospheres were less selective than the former. The worst CeO₂-Cubes catalyst did not start to produce CO₂ until 300°C, with a maximum selectivity of only 50% (±4%). Therefore, the morphology of CeO₂ had a great impact on the catalytic oxidation of DCM, with CeO₂-Nanosheets catalyst presenting the best catalytic activity.

The distribution of other detected products during the catalytic oxidation of DCM over different CeO₂ catalysts was further investigated (Fig. 1(c)–(f)). For the four different forms of CeO₂ catalysts, the concentration of CO increased with the increase of reaction temperature. Meanwhile, chlorine-containing by-products only appeared above 300°C, and only CHCl₃ was detected. Table S1 listed the concentrations of products produced by the different forms of CeO₂ catalysts. CeO₂-Cubes catalyst produced the lowest CO product of 98 (±6) ppm and the highest CHCl₃ product of 35 (±34) ppm due to the lowest activity. Compared with CeO₂-Nanorods and CeO₂-Nanospheres, CeO₂-Nanosheets catalyst produced the lowest CO and CHCl₃ products of 129 (±5) ppm CO and 5 (±2) ppm CH₃Cl, respectively. Overall, CeO₂-Nanosheets catalyst had the best performance in generating the target products.

It should be emphasized that the error bars accompanying each data point in the figures and table represented the standard error, illustrating the variability observed over multiple experiments. The shorter error bars for CeO₂-Nanosheets catalyst highlighted its exceptional consistency and reliability, demonstrating a high level of reproducibility and minimal experimental variability. In contrast, the longer error bars for CeO₂-Cubes catalyst indicated greater fluctuations in its catalytic activity, likely due to its relatively lower catalytic efficacy.

The stability of the four catalysts was tested at 400°C for 48 h (Fig. 2). CeO₂-Cubes catalyst was the most unstable in terms of activity, with a significant decrease in DCM conversion from 25.1% to 12.5% over 48 h. CeO₂-Nanorods and CeO₂-Nanosphere catalysts showed a decrease in DCM conversion of more than 15%. In contrast, the DCM conversion of CeO₂-Nanosheets catalyst remained above 88% for 48 h, only dropping from 95.6% to 88.8%. This indicated that the chlorine resistance of CeO₂ catalysts with different morphologies also varied, which was related to the surface reactive oxygen species and acidic sites. It has been reported in the literature [29] that the acid sites on the catalyst surface can effectively promote the desorption of chloride species, thus greatly improving the stability of the catalyst. In addition, the four catalysts showed no significant decrease in CO₂ selectivity, which was related to their excellent oxygen storage and release capabilities, and the oxygen vacancies consumed on the surface were quickly replenished by oxygen, thus ensuring that their oxidizing ability was not significantly weakened.

3.2. Structural and Morphological Analysis

The N₂ adsorption-desorption curve and pore size distributions of CeO₂ catalysts with different morphologies were showed in Fig. S2, from which it could be seen that the different CeO₂ catalysts had similar sorption curves. When the relative pressure P/P₀<0.4, all catalysts showed similar microporous characteristics. Meanwhile, all catalysts exhibited type IV isotherms with H3-type hysteresis loops in the range of relative pressure P/P₀=0.4–1.0.

The texture properties of CeO₂ catalysts with different morphologies were listed in Table S2. It could be seen that the specific surface area, pore volume and pore diameter of the four CeO₂ catalysts did not differ much, indicating that the specific surface area was not a key factor affecting the catalytic oxidation performance.

In addition, the crystal structures of CeO₂ catalysts with different morphologies were analyzed by XRD. All catalysts exhibited distinct reflections at 2θ of 28.6°, 33.3°, 47.5°, 56.5°, 59.2°, 69.4°, 76.7° and 79.0° and all detected characteristic diffraction peaks were consistent with the face-centered cubic fluorite structure of CeO₂ (PDF#34-0394) (Fig. 3(a)). Based on the intensities and widths of these characteristic peaks (Fig. 3(b)), these peaks were clearest and had the narrowest half-peak widths for CeO₂-Cubes catalyst, suggesting that the crystallinity of CeO₂-Cubes catalyst was the highest. The peak intensities were similar for CeO₂-Nanosheets and CeO₂-Nanospheres catalysts. However, CeO₂-Nanosheets catalyst had a wider half-peak width, indicating a large degree of lattice distortion in its structure. Thus, it was more favorable to expose more oxygen vacancies [30].

The morphologies of different CeO₂ catalysts were observed by SEM. According to the SEM images, the prepared CeO₂ catalysts with different morphologies achieved the expected results. As illustrated in Fig. 4, CeO₂-Cubes catalyst was mainly composed of many cubes. CeO₂-Nanorods catalyst exhibited a predominant nanorod shape, while CeO₂-Nanospheres catalyst was characterized by spherical stacks. In case of CeO₂-Nanosheets catalyst, a typical nanosheet structure with a relatively smooth surface and a few small fragments were observed.

3.3. Chemical Properties Analysis

Fig. 5 exhibited the XPS spectra of CeO₂ catalysts with different morphologies. In Fig. 5(a), two fitted peaks appeared on the O 1s orbitals of all catalysts, where O_lat and O_ads represented lattice oxygen and surface adsorbed oxygen, respectively. The peaks of O_lat were located in the range of 528.3–529.1 eV, and the peaks of O_ads were located in the range of 530.8–531.8 eV [8]. The relative ratios of O_ads/(O_ads+O_lat) for CeO₂ catalysts with different morphologies were listed in Table S3, which were in the order of CeO₂-Nanosheets (47.8%) > CeO₂-Nanorods (44.1%) > CeO₂-Nanospheres (42.4%) > CeO₂-Cubes (37.1%). A high O_ads/(O_ads+O_lat) ratio indicates that a greater proportion of oxygen is in an accessible and reactive state at the catalyst surface, facilitating faster and more efficient oxidation cycles. The results were consistent with the order of their catalytic activities.

The XPS spectra of Ce 3d orbitals for CeO₂ catalysts with different morphologies were shown in Fig. 5(b). Among them, v’ and u’ were the characteristic peaks of Ce³⁺, and the rest of the peaks belonged to Ce⁴⁺. Generally, Ce³⁺ is considered to be more favorable for the VOCs oxidation reaction [9]. A higher Ce³⁺/(Ce³⁺+Ce⁴⁺) ratio reflects an increased proportion of cerium in the reduced state, enhancing the ability to participate in oxygen activation. As analyzed in Table S3, the sequence of Ce³⁺/(Ce³⁺+Ce⁴⁺) ratios was CeO₂-Nanosheets (26.6%) > CeO₂-Nanorods (22.9%) > CeO₂-Nanospheres (21.7%) > CeO₂-Cubes (18.9%), which was the same as that for O_ads/(O_ads+O_lat) ratios. CeO₂-Nanosheets catalyst had the highest O_ads/(O_ads+O_lat) and Ce³⁺/(Ce³⁺+Ce⁴⁺) ratios, indicating the strongest catalytic oxidation ability.

The NH₃-TPD curves of CeO₂ catalysts with different morphologies were analyzed in Fig. 5(c). The acidic sites of the catalysts were mainly divided into weak acidic sites (<250°C), medium strong acidic sites (250 to 350°C) and strong acidic sites (>350°C). The catalyst showed distinct NH₃ desorption peaks in the temperature range from 50 to 200°C, which was attributed to the desorption of NH₃ adsorbed on Ce⁴⁺/Ce³⁺ and surface acidic hydroxyl groups [31]. The peaks above 200°C might be attributed to the different crystal planes of CeO₂ having different atomic arrangements and coordination environments, which provided different acidity. For example, the (111) crystal plane of CeO₂ had a higher acidity than the (100) crystal plane, and this plane-dependent acidity could provide medium to strong acidic sites for the catalysts [32].

It was noteworthy that the desorption peaks for different morphologies of CeO₂ were different. Specifically, two weak NH₃ desorption peaks appeared at 92°C and 537°C for CeO₂-Cubes catalyst, indicating that there were fewer acidic sites. Conversely, the NH₃ desorption peaks of CeO₂-Nanorods catalyst were significantly broader and stronger, suggesting a larger proportion of medium and strong acidic sites alongside a smaller amount of weak acidic sites. CeO₂-Nanosheets catalyst exhibited a larger NH₃ desorption peak at 99°C, along with two smaller peaks at 230°C and 406°C, indicating the presence of numerous weak acidic sites as well as a number of medium and strong acidic sites. One large NH₃ desorption peak appeared at 110°C for CeO₂-Nanospheres catalyst, suggesting an abundance of weak acidic sites and the scarcity of medium and strong acidic sites.

According to the desorption peak areas, the order of total acid sites was as follows: CeO₂-Nanospheres (3.21) >CeO₂-Nanorods (3.00) > CeO₂-Nanosheets (2.93) > CeO₂-Cubes (0.69) (Table S4). Combined with the experimental results, the activity of CeO₂-Nanospheres catalysts was not the best even though it had the highest acidity. This might be due to the fact that the increase in the number of acidic sites on the surface affected the amount and migration of surface reactive oxygen species. Therefore, a moderate amount of acidity as well as excellent oxidation properties were more favorable for the degradation of CVOCs.

To investigate the redox properties of CeO₂ catalysts with different morphologies, the four catalysts were characterized by H₂-TPR. In Fig. 5(d), the reduction peaks of CeO₂-Cubes, CeO₂-Nanorods, CeO₂-Nanospheres and CeO₂-Nanosheets catalysts appeared at 477, 458, 439 and 460°C, respectively, indicating the reduction of Ce⁴⁺ species in each catalyst. Notably, CeO₂-Nanosheets catalyst exhibited the lowest initial reduction temperature and the largest peak area, suggesting its excellent low-temperature redox property [33]. In addition, the H₂-TPR desorption peak areas were compared in the following order: CeO₂-Nanosheets (3.88) > CeO₂-Nanorods (2.85) > CeO₂-Nanospheres (2.60) > CeO₂-Cubes (1.87) (Table S4). The redox capability played a crucial role in the deep oxidation of CVOCs. CeO₂ catalysts with higher redox capacity can activate CVOCs at lower temperatures and improve CO₂ selectivity.

3.4. Structure-activity Discussion

By systematically comparing the performances of CeO₂ catalysts with different morphologies (cubes, nanorods, nanosheets and nanospheres) in the catalytic oxidation of DCM, it was found that the catalytic activities were clearly hierarchical: CeO₂-Nanosheets > CeO₂-Nanorods > CeO₂-Nanospheres > CeO₂-Cubes. Meanwhile, CeO₂-Nanosheets exhibited the highest CO₂ selectivity, reaching more than 70% at 225°C, while CeO₂-Nanorods, CeO₂-Nanospheres and CeO₂-Cubes showed lower selectivity. Several physicochemical properties that might affect the catalytic oxidation of DCM were summarized in Fig. 6. The results of XPS and H₂-TPR showed that the morphology structure affected the amount and migration of reactive oxygen species on the catalyst surface. It could be seen that the catalytic activities (DCM conversion and CO₂ selectivity) of CeO₂-Cubes, CeO₂-Nanorods, CeO₂-Nanosheets and CeO₂-Nanorods catalysts were related to their surface reactive oxygen species and Ce³⁺ concentration with a good linear relationship. In addition, the moderate amount of acidic sites favored the stability of the catalyst, but was not decisively related to the amount of acidic sites. According to the literature, surface reactive oxygen species and surface acidic sites together played roles in CVOCs degradation reaction [34, 35]. CeO₂-nanosheets catalyst possessed a high concentration of reactive oxygen species and Ce³⁺ active sites, coupled with excellent redox capacity and appropriate acidity, all of which collectively contributed to its superior catalytic performance.

3.5. Mechanism Discussion

Fig. 7 showed a set of temperature-dependent DRIFTS spectra of the best performing CeO₂-Nanosheets catalyst for the catalytic oxidation of DCM in the temperature range of 25–450°C. When the catalyst was at low temperature, the conversion of DCM had not yet begun, and therefore a small amount of DCM was adsorbed on the catalyst surface.

The peak at 1269 cm⁻¹ was attributed to the bending vibration of -CH₂ on DCM [36]. As the temperature increased, DCM was desorbed or catalytically oxidized, and the bending vibration of -CH₂ disappeared gradually. When the temperature was higher than 100°C, some new peaks were generated, and the peak intensity increased gradually with the increase of temperature. The peaks at 2939 cm⁻¹ corresponded to the asymmetric and symmetric stretching vibration of methoxy (CH₃O-) group, respectively [37]. The peaks at 1731 and 1488 cm⁻¹ could be attributed to C=O and -CH₂ vibration of formaldehyde (CH₂O) [38]. The peak at 1425–1467 cm⁻¹ could be attributed to the C-H bending vibration of chloromethoxy (CH₂Cl-O-) group, which might result from dehydrochlorination of DCM by nucleophilic attack on the oxygen vacancies [39]. The peaks at 1702 and 1496 cm⁻¹ were attributed to C=O stretching vibration of formate (HCOO-) [40]. The peak at 1365 cm⁻¹ was attributed to the C-H deformation vibration of formate. The peaks at 1588 cm⁻¹ were attributed to the asymmetric and symmetric stretching vibration of formate [41]. These results indicated that methoxy and formate were the main intermediates in the catalytic oxidation of DCM over CeO₂-Nanosheets catalyst.

The possible adsorption and oxidation mechanism of DCM on CeO₂-Nanosheets catalyst was displayed in Fig. 8. Firstly, DCM adsorbed on the oxygen vacancies of CeO₂-Nanosheets catalyst, and the C-Cl bond broke simultaneously to directly generate dimethoxy. With the participation of gaseous oxygen, it was gradually converted to formate, or a mixture of formate (COOH-) and methoxy (CH₃O-) was formed simultaneously through Cannizzaro type disproportionation reaction. Finally, the above groups could be completely degraded to the final products (CO₂/CO, Cl₂/HCl and H₂O). Meanwhile, Ce³⁺ was gradually oxidized to Ce⁴⁺ as gaseous O₂ adsorbed and activated on the surface oxygen vacancies to form reactive oxygen species. The reactive oxygen species combined with Ce⁴⁺ to form lattice oxygen, thus filling the vacancies.

4 Conclusions

Different morphologies (cubes, nanorods, nanosheets and nanospheres) of CeO₂ catalysts were prepared by solvothermal methods, and their catalytic performance and catalytic mechanism for DCM degradation were deeply analyzed. The results showed that the morphology of CeO₂ had a great influence on the catalytic oxidation of DCM. Among them, CeO₂-Nanosheets catalyst presented the highest catalytic activity with a T₉₀ of 384°C, generating the most CO₂ and the least chlorine-containing by-products. Meanwhile, there was almost no decrease in its DCM conversion and CO₂ selectivity in the 48 h stability test. The characterization results showed that CeO₂-Nanosheets catalyst had the highest O_ads/(O_ads+O_lat) and Ce³⁺/(Ce³⁺+ Ce⁴⁺) ratios, as well as the best redox ability and suitable surface acidity. In situ DRIFTS experiments showed that methoxy, formaldehyde and formate were the main intermediates of DCM oxidation on CeO₂-Nanosheets catalyst. This study offers in-depth insights into the development of efficient, sustainable and scalable transition metal oxide catalysts through morphology modulation. By employing these advanced catalysts, it demonstrates the effectiveness of catalytic oxidation of CVOCs under practical operating conditions, ensuring minimal secondary emissions and underscoring the environmental advantages of this approach. The proposed method and findings hold significant potential for extension to other types of CVOCs and catalytic systems, offering a versatile framework for the development of high-performance catalysts tailored to specific applications.

Supplementary Information

eer-2024-622-supplementary.pdf

Notes

Acknowledgements

This work was supported by the National Natural Science Foundation of China (12075037).

Conflict-of-Interest Statement

The authors declare that they have no conflict of interest.

Author Contributions

J.Q.M. (Master student) and L.Q. (Master student) took charge of investigation, scientific experiments, data curation, formal analysis and writing. C.W. (Master student) took charge of data supplement. E.H.G. (Associate Professor) and W.W. (Lecturer) did a formal analysis. J.L.Z. (Lecturer) and S.L.Y. (Professor) took charge of investigation. J.L. (Associate Professor) revised the manuscript. Z.L.W. (Professor) provided supervision, theoretical foundation, and experimental guidance.

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Fig. 1

(a) Conversion of DCM and (b) selectivity of CO₂ for different morphologies of CeO₂ catalysts. DCM concentration = 1000 ppmv, GHSV = 30000 h⁻¹. Distribution of by-products during the catalytic oxidation of DCM by different CeO₂ catalysts: (c) CeO₂-Cubes, (d) CeO₂-Nanorods, (e) CeO₂-Nanosheets, (f) CeO₂-Nanospheres.

Fig. 2

Stability test of different catalysts at 400°C. (a) CeO₂-Cubes, (b) CeO₂-Nanorods, (c) CeO₂-Nanosheets, (d) CeO₂-Nanospheres.

Fig. 3

(a) XRD patterns and (b) enlarged view of CeO₂ catalysts with different morphologies.

Fig. 4

SEM images of (a) CeO₂-Cubes, (b) CeO₂-Nanorods, (c) CeO₂-Nanosheets, (d) CeO₂-Nanospheres catalysts.

Fig. 5

XPS spectra of CeO₂ catalysts with different morphologies: (a) O 1s, (b) Ce 3d). (c) NH₃-TPD and (d) H₂-TPR profiles of CeO₂ catalysts with different morphologies.