A carbon nanodot modified Cu–Mn–Ce/ZSM catalyst for the enhanced microwave-assisted degradation of gaseous toluene

Abstract

In this work, carbon nanodots (CNDs) were firstly synthesized by hydrothermal treatment of natural biomass (dead leaves of a plane tree) without any synthetic chemicals. The concentration of the fabricated carbon nanodot solution was ca. 13 g L⁻¹ and the mass yield of CNDs was ca. 5.6%. The characterization results indicated that the prepared CNDs with sizes of 2–7 nm contained rich surface functional groups such as O and N-containing groups and possessed blue fluorescence properties. Using a conventional impregnation approach, the fabricated carbon nanodot solution was used to further modify a Cu–Mn–Ce/ZSM (CMCZ) catalyst. The CND modified Cu–Mn–Ce/ZSM (CND–CMCZ) catalyst was subsequently evaluated for the catalytic oxidation of gaseous toluene under microwave heating. The experimental results demonstrated that the catalyst modified with CNDs with the features of analogous humic acids clearly enhanced the adsorptive capacity of gaseous toluene and the rate of temperature increase of the catalyst during the catalytic reaction compared with the unmodified Cu–Mn–Ce/ZSM catalyst. The improved adsorptive capacity of gaseous toluene could be due to the enhanced specific surface area and the role of the surface functional groups of the composite catalyst, while the “hot spots” effect of the CNDs could significantly contribute towards the improved rate of temperature increase of the composite catalyst. Owing to these advantages, 75% of the gaseous toluene was degraded within 80 min at 150 °C using the CND–CMCZ catalyst, which was almost 1.9 times that of the unmodified catalyst. At a lower reaction bed temperature (e.g., 150 °C), the obtained catalytic performance could be due to a synergistic role between the carbon nanodots and Cu–Mn–Ce/ZSM. Although the degradation efficiency of gaseous toluene was further improved by increasing the reaction bed temperature (200 °C and 250 °C), the role of the CND modification for improving catalytic performance was obviously reduced. This could be ascribed to the structure and surface functional group damage of the carbon nanodots at higher catalytic temperatures. This work demonstrated the possibility of developing CND modified catalysts for low-temperature catalytic oxidation of volatile organic compounds (VOCs).

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1. Introduction

Because of the many adverse effects on the environment and human health after long exposure, toxic volatile organic compounds (VOCs) have attracted great attention in the fields of science and technology.^1–3 In recent years, many techniques have been developed to treat waste gas containing VOCs before they are able to be released into the environment, such as adsorption,^4,5 biological treatment,⁶ and catalytic oxidation.⁷ Among them, catalytic oxidation has become a popular technique for the treatment of toxic and complex VOCs.^8,9 As a core of catalytic oxidation techniques, the catalyst is of critical importance for achieving a high treatment efficiency of VOCs. For this, many catalysts such as MnO_x/γ-alumina and MnCuO_x/TiO₂ catalysts have been developed to treat VOCs, exhibiting superior treatment performances.^10,11 To further improve the treatment efficiency, coupling catalytic oxidation with other techniques such as microwave heating has also been widely explored for the removal of VOCs in many literature references.^12,13 Microwaves, as electromagnetic waves with a high thermal effect, dipole polarization and versatile capacity, have been widely applied in environmental remediation.^14,15 Microwave heating can shorten the time of catalytic reaction bed temperature adjustment, thus improving the catalytic treatment efficiency of VOCs.¹⁶ In our previous studies, a molecular sieve supported copper, manganese and cerium (Cu–Mn–Ce/ZSM) catalyst has been developed and evaluated for the catalytic oxidation of VOCs under microwave heating assistance.¹⁷ Our studies have demonstrated that the fabricated Cu–Mn–Ce/ZSM catalyst has a strong microwave absorption capability, resulting in the effective removal of VOCs.¹⁸ Even so, conventional catalysts usually require higher catalytic reaction temperatures (350–600 °C) to generate more catalytic active sites on the catalysts’ surface, which may not be favorable for large-scale application in the treatment of VOCs. Therefore, development of a low-cost catalyst with high catalytic activity and low-temperature catalytic ability is highly desired.

J. A. Menéndez et al. have illustrated that biomass char possesses a dual function as a catalyst and microwave receptor in the process of a catalytic oxidation reaction.¹⁹ Based on this, carbon material modified catalysts have been widely investigated for the removal of VOCs by coupling catalytic oxidation with microwave heating.^13,20 Recently, carbon nanodots (CNDs) have attracted great research interest because of their abundance, benignity, easy synthesis, low cost, rich surface functional groups, and excellent fluorescence properties.^21,22 Owing to many superior properties, CNDs have been investigated in the areas of photocatalysis, photovoltaics, and fluorescent detection.²³ Due to their small sizes (<10 nm) and rich surface functional groups, CNDs possess very different physical and chemical properties to bulk carbon materials.^24,25 In particular, to the best of our knowledge, no reports have illustrated the effects of carbon nanodot modification of traditional catalysts on VOC treatment efficiency under microwave heating.

Herein, we report a simple hydrothermal method for the synthesis of CNDs using dead leaves from a plane tree as a reaction precursor without any synthetic chemicals. The prepared CNDs were employed to further modify the Cu–Mn–Ce/ZSM catalyst by a conventional impregnation approach. The resulting CND modified Cu–Mn–Ce/ZSM catalyst was evaluated for the catalytic oxidation of gaseous toluene under microwave heating. For comparison, the unmodified Cu–Mn–Ce/ZSM catalyst was also tested under the same experimental conditions.

2. Materials and methods

2.1 Preparation of carbon nanodots (CNDs)

The CND sample was prepared by a simple hydrothermal approach similar to previously reported methods.²³ In a typical process, dead leaves from a plane tree were firstly collected from the campus of Xi'an university of architecture and technology (east longitude 107° 40′–109° 49′ and north latitude 33° 39′–34° 45′), and then washed with deionized water and dried at 80 °C for 12 h. Subsequently, 50 g dried leaves were put directly into 500 mL of deionized water, and then the mixture was transferred into a Teflon-lined autoclave and heated up to 180 °C for 3 h in an oven. After the hydrothermal reaction, the autoclave was cooled to room temperature and the obtained product was centrifuged for 10 min at 12 000 r min⁻¹, and the CND solution with an excellent water solubility (pH = 4.16 ± 0.05) was collected for further use. The concentration of the fabricated CND solution was ca. 13 g L⁻¹ and the mass yield was about 5.6%.

2.2 Preparation of the CNDs modified Cu–Mn–Ce/ZSM catalyst

The Cu–Mn–Ce/ZSM catalyst was firstly prepared using the conventional impregnation method as reported by Y. Zhang et al.²⁶ The resulting Cu–Mn–Ce/ZSM catalyst was thermally treated at 400 °C for 4 h in a muffle furnace. The obtained Cu–Mn–Ce/ZSM catalyst was denoted as CMCZ, with spherical particle sizes of 3–4 mm. The calcined CMCZ (5.0 g) was then dispersed in 100 mL of diluted CND solution (10 mL of original CND solution was diluted to 100 mL using deionized water (pH = 7.04 ± 0.04)) for 6 h at 100 r min⁻¹ in a shaker. After that, the CND modified CMCZ was dried at 80 °C for 12 h in a vacuum oven. After the above process, the loading amount of CNDs on CMCZ was about 19 mg g⁻¹. The CND modified CMCZ was further expressed as CND–CMCZ.

2.3 Characterizations

The fluorescence of the CNDs was recorded on a 3D-spectrofluorometer (Jasco FP-6500) with fluorescence excitation–emission (Ex–Em) matrix (EEM) spectroscopy. The surface functional groups of the CNDs were measured by Fourier transform infrared spectrometry (FT-IR, SHIMADZU IRPrestige-21). The composition of the CNDs was further analyzed by X-ray photoelectron spectroscopy (XPS, Thermo Scientific K-Alpha). The specific surface area and pore structure of the catalysts were recorded on a Specific Surface Area Analyzer (V-Sorb 2800P).

2.4 Catalytic oxidation of gaseous toluene under microwave heating

The catalytic oxidation of gaseous toluene was carried out in a fixed bed reactor under microwave heating and a continuous flow mode, where the quartz reactor with a 30 mm inner diameter was vertically inserted in a microwave device and 110 g of catalyst were installed prior to the experiment. The microwave device was a modified domestic microwave oven (2.45 GHz) with adjustable output power (max 700 W), which was controlled by a variable transformer.²⁷ In our experiment, gaseous toluene with an initial concentration of 2000 mg m⁻³ and a flow rate of 0.08 m³ h⁻¹ was pumped into the reactor from the bottom under microwave irradiation, and the relative gas hourly space velocity (GHSV) was 600 h⁻¹. The treated gas was purified with methanol and NaOH solutions in turn before being discharged. Gaseous toluene concentrations in the inlet and outlet were collected and detected by gas chromatography (GC-FID, Aglient 6890N), and the reaction bed temperature was measured using a thermocouple probe. For comparison, the experiment without microwave irradiation was also conducted under the same experimental conditions.

3. Results and discussion

3.1 Characteristics of the CNDs

After diluting 10 mL of original CND solution to 200 mL, the obtained CND solution was yellow, and exhibited blue fluorescence under UV light (main wavelength of 365 nm) irradiation (photograph in Fig. 1A), which is in agreement with the reported results.²⁸ The UV-vis absorption spectrum indicates that the absorption bands of the CNDs are located at around 270 nm (Fig. 1A), which is consistent with the reported results.²⁹ It has been reported that the excitation jump of electron-donating groups and the discrete energy level structure of CNDs with sizes of 1–10 nm can generate fluorescence easily under UV irradiation.³⁰ The above optical results indicated that the prepared CNDs possessed nanometer level sizes and electron-donating functional groups.²² Transmission electron microscopy (TEM) analysis further confirmed that the sizes of the CNDs were 2–7 nm (Fig. 1B). Fig. 1C shows the excitation–emission matrix (EEM) spectrum of the CNDs, and a clear isoheight peak (Ex/Em 325/450 nm) appears in the area of excitation wavelength 300–360 nm and emission wavelength 410–480 nm, indicating CNDs with abundant analogous humic substances.^31,32 The peak shown in Fig. 1C is uniform and proportional, which reveals the homogeneity and simplicity of humic substances,³³ further demonstrating the size uniformity of the fabricated CNDs in this work.

Fig. 1 (A) UV-Vis absorption spectrum and fluorescence photograph of the CND liquid compared with its original color. (B) Transmission electron microscopy (TEM) image of the CNDs. (C) Excitation–emission matrix (EEM) spectrum of the CNDs.

The surface functional groups of the CNDs were analyzed by FT-IR and the corresponding spectrum is shown in Fig. 2A. As shown, different absorption peaks can be observed at 3417 cm⁻¹, 2941 cm⁻¹, 1639 cm⁻¹, 1419 cm⁻¹ and 1107 cm⁻¹, respectively, indicating that the synthesized CNDs possess complex surface functional groups, which is in accordance with reported results.^34–36 The peak at 2941 cm⁻¹ is associated with the stretching vibration of a C–H bond, and the wide peak located at 3417 cm⁻¹ could be the stretching vibration of an O–H bond or a N–H bond, and the peak centered at 1107 cm⁻¹ could be the stretching vibration of a C–O bond or a C–N bond.^37,38 Therefore, carboxylic, phenolic and/or aminic functional groups could be present in the CNDs.^39,40 The peak located at 1639 cm⁻¹ could be considered to be the deformation vibration of a N–H bond or the stretching vibration of a C=O bond, and the peak at 1419 cm⁻¹ could be the stretching vibration of a C–N bond, so amide groups could also be present in the prepared CNDs. Moreover, it was found that some weak absorption peaks appeared between 1200 cm⁻¹ and 1600 cm⁻¹, and these peaks verified that the CNDs could have contain a structure with an aromatic ring, which is consistent with the fluorescence of the CNDs.^30,41 Overall, the FT-IR results confirm that the fabricated CNDs possess rich O and N-containing surface functional groups, which may be beneficial for quickly increasing the catalytic reaction temperature using the CNDs as microwave acceptors.⁴² The surface functional groups of the CNDs were further analyzed by X-ray photoelectron spectroscopy (XPS). The surface XPS survey spectrum shows that the CNDs are mainly composed of C, N and O (Fig. 2B), confirming the nitrogen doping of the CNDs.⁴³ Fig. 2B further reveals that the CNDs contain a lot of carbon and oxygen as well as a small amount of nitrogen, and the relative atomic ratio of C, O and N was calculated to be 73.31%, 25.12% and 1.57%, respectively. Fig. 2C and D show the high resolution XPS C 1s and O 1s spectra of the CNDs. As shown, the C 1s spectrum (Fig. 2C) exhibits four peaks at 284.3, 285.5, 286.2 and 288.0 eV, corresponding to C–C, C–N, C–O, and C=O bonds, respectively. The O 1s spectrum shown in Fig. 2D displays two peaks at 531.4 and 532.8 eV, which are attributed to C–OH/C–O–C and C=O bonds, respectively. The above results further indicate that the CNDs are rich with O and N-containing functional groups, consistent with the FT-IR results. The presence of rich O and N-containing functional groups in the CNDs is very favorable for improving the interaction between the CNDs and the CMCZ catalyst, thus forming a stable composite catalyst.⁴⁴

Fig. 2 (A) FT-IR spectrum, (B) XPS spectrum, (C) high resolution C 1s spectrum and (D) high resolution O 1s spectrum.

3.2 Catalytic performance

In order to investigate the influence of the CND modification on the catalytic performance of the Cu–Mn–Ce/ZSM (CMCZ) catalyst, the adsorption breakthrough curve of gaseous toluene and the rate of temperature increase curve of the CND–CMCZ catalyst were carried out. For comparison, the unmodified CMCZ catalyst was also measured under the same experimental conditions. The measured results are shown in Fig. 3. The adsorption breakthrough curves (Fig. 3A) of CMCZ and CND–CMCZ revealed that the adsorption breakthrough time of gaseous toluene for CMCZ and CND–CMCZ was 25 and 38 min, respectively, and that the loading of CNDs apparently improved the adsorptive capacity of the catalyst within 30–80 min. The increase of adsorption ability is beneficial for improving the degradation efficiency of gaseous toluene on the catalyst surface. The improved adsorptive capacity of CND–CMCZ could be due to the enhanced specific surface area which was nearly 1.5 times higher than that of CMCZ (60.36 m² g⁻¹). The average pore size of the catalyst decreased from 17.18 nm to 7.71 nm after modification with the CNDs. The role of the surface functional groups of the CNDs could also be a reason for the improved adsorptive capacity of CND–CMCZ, which deserves further investigation in the future. Under the same experimental conditions, it was found in Fig. 3B that the reaction bed temperature increase of the CND–CMCZ catalyst was obviously faster than the CMCZ catalyst (especially after 50 s under microwave irradiation), and the time taken for the temperature to rise from room temperature to 100 °C was shortened by almost 45 s. The enhanced reaction bed temperature rate increase of CND–CMCZ can be ascribed to the CNDs which contain rich surface functional groups able to act as microwave acceptors to transform the microwave energy into heat (the CNDs acts as “hot spots”),⁴⁵ thus quickly increasing the reaction bed temperature. Carbon-based materials as microwave acceptors have been widely studied for improving microwave treatment efficiency in the literature.^46,47 The concept demonstrated in this work may be beneficial for the scale-up application of the CND modified catalyst for VOC treatment.

Fig. 3 (A) Adsorption breakthrough curves of gaseous toluene for CMCZ and CND–CMCZ catalysts. (B) Rate of temperature increase curves of CMCZ and CND–CMCZ catalysts under microwave irradiation at a power of 43.5 W.

In order to eliminate the adsorption effect, the CMCZ and CND–CMCZ catalysts were preliminarily adsorbed with gaseous toluene for 100 min (based on Fig. 3A) to ensure adsorption equilibrium of the two catalysts, and then the catalytic oxidation reaction was carried out under microwave irradiation. Fig. 4 shows the degradation efficiency of gaseous toluene using the CMCZ and CND–CMCZ catalysts at different reaction temperatures. As shown in Fig. 4A, when the reaction bed temperature is set at 150 °C, nearly 75% degradation efficiency of gaseous toluene can be achieved using the CND–CMCZ catalyst within 80 min, which is almost 1.9 times that of the CMCZ catalyst, indicating a significant influence of the CND modification on the catalytic performance at the relatively low temperature of 150 °C. Compared with MnO_x/γ-alumina and MnCuO_x/TiO₂ catalysts which have high degradation efficiencies for VOCs at nearly 350 °C, the reaction temperature of toluene oxidation was significantly decreased by using the CND–CMCZ catalyst, which required moderate reaction conditions and lower energy consumption. Increasing the reaction bed temperature to 200 °C, the degradation efficiency of gaseous toluene is improved to nearly 80% using the CND–CMCZ catalyst, while 70% degradation efficiency can be achieved using the CMCZ catalyst (Fig. 4B). When the reaction bed temperature is increased to 250 °C, the degradation efficiency of gaseous toluene can reach ca. 90% for both the CMCZ and CND–CMCZ catalysts (Fig. 4C). The above results indicate that a high reaction bed temperature is favorable for improving the catalytic performance of the catalyst. However, the effect of the CND modification on the catalytic performance of catalyst becomes gradually less significant with increasing reaction bed temperature. Many studies have demonstrated that carbon-based materials for the microwave treatment of VOCs can not only be used as microwave acceptors to absorb microwave energy, but also generate catalytic active sites as catalysts.⁴⁸ In this work, at a relatively low reaction bed temperature of 150 °C, the significantly improved degradation efficiency of gaseous toluene could be due to the synergistically catalytic role of carbon nanodots and Cu–Mn–Ce/ZSM. Moreover, CNDs could provide a greater contribution to the improved catalytic performance because the Cu–Mn–Ce/ZSM catalyst has fewer catalytically active sites at a lower reaction bed temperature.^42,49 The role of the “hot spots” of CNDs can generate more catalytic active sites even at a low reaction bed temperature, which is beneficial for developing a low-temperature catalyst for the catalytic degradation of VOCs. At higher reaction bed temperatures (e.g., 200 °C and 250 °C), the degradation efficiency of gaseous toluene using both the CMCZ and CND–CMCZ catalysts is obviously enhanced, however, the effect of the CNDs on the catalytic performance of the catalyst diminishes gradually with increasing reaction bed temperature. This is because higher reaction bed temperatures are favorable for generating more catalytic active sites on the Cu–Mn–Ce/ZSM catalyst, thus improving the degradation efficiency of gaseous toluene. However, further carbonization of CNDs at higher temperatures may destroy the nanodot structure, including the surface functional groups, leading to a decreased influence of the carbon nanodots on the catalytic performance. Therefore, the high degradation efficiency of gaseous toluene at higher reaction bed temperatures is mainly due to the catalytic role of Cu–Mn–Ce/ZSM, while the degradation efficiency of gaseous toluene using the CND–CMCZ catalyst at a lower reaction bed temperature (e.g., 150 °C) is ascribed to the synergistic role of carbon nanodots and Cu–Mn–Ce/ZSM.

Fig. 4 Catalytic oxidation performance of gaseous toluene using CMCZ and CND–CMCZ catalysts at different reaction temperatures. (A) 150 °C, (B) 200 °C, (C) 250 °C.

4. Conclusions

This work has demonstrated a green hydrothermal method for the synthesis of fluorescent carbon nanodots (CNDs) using dead leaves from a plane tree as a reaction precursor without any synthetic chemicals. The modification of the CNDs with rich surface O and N-containing functional groups enhances the adsorptive capacity and rate of temperature increase of the catalyst, thus improving the catalytic performance at a lower reaction bed temperature (e.g., 150 °C), which can be ascribed to the synergistic role of CNDs and Cu–Mn–Ce/ZSM. However, it is believed that the Cu–Mn–Ce/ZSM significantly contributes towards the improved catalytic performance of the catalysts at higher reaction bed temperatures. Our findings demonstrate the possibility of developing a carbon nanodot modified catalyst for the low-temperature catalytic oxidation of volatile organic compounds.

Acknowledgements

The research was funded by the project of technology transfer promotion engineering from the industrial technology innovation plan of Xi'an City, Shaanxi Province (Project no. CX12176-④).

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