Ce-Cr Composite Metal Oxide Doped with Multi-walled Carbon Nanotubes for Catalytic Combustion of Aromatic Solvents

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Catalytic combustion has been proven to be an effective method for the efficient removal of VOCs from industrial sources. Herein, the applicability of Ce-Cr composite metal oxide doped with multi-walled carbon nanotubes (MWCNTs) for catalytic combustion of aromatic solvent oil was evaluated. Using varying mass proportions of MWCNTs, a series of MWCNTs-doped of chromium-cerium composite oxide catalysts (Ce 0.3 Cr 0.7 - x wt%MWCNTs) were synthesized by the citric acid sol-gel method and subsequent air pyrolysis. The effect of MWCNTs doping on the catalytic combustion was investigated. The measured complete combustion temperature of the low-concentration aromatic solvent oil indicated that the catalytic performance of the Ce-Cr composite oxide was improved by the doping of MWCNTs in range of 0.5-2.0 wt%, reaching its best at 1.5 wt%. It is revealed that Ce 0.3 Cr 0.7 -1.5wt%MWCNTs possessed abundant honeycomb-like pore structures, significantly enhancing the adsorption and activation of the aromatic solvent oil during the reaction. The excellent repeatability and stability of Ce 0.3 Cr 0.7 -1.5wt% MWCNTs were also be verified. This study provides a feasible approach for the design of effective catalysts for the decomposition of VOCs. VOCs Catalytic combustion Aromatic solvent oil Ce-Cr composite metal oxide MWCNT promoters Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 Figure 9 1 Introduction In recent years, with the continuous development of industrial production in China, the types and quantities of air pollutants have increased. Air pollution has become one of the most significant environmental problems in the country. Industrial emissions, particularly VOCs, are a major source of air pollution, and their health hazards have drawn significant attention. VOCs are generally defined as gases with vapor pressures greater than 0.5 kPa at 25°C. They include aromatic compounds, chlorinated hydrocarbons, alcohols, fats, ketones, and straight-chain hydrocarbons [ 1 ]. VOCs pose a significant threat to human health due to their carcinogenic, teratogenic, and mutagenic properties. They can also cause respiratory irritation and damage to the central nervous system [ 2 ]. The introduction of China's "Emission Standards for Pollutants from the Petrochemical Industry" in 2015 signifies the recognition of oil, gas, and VOC recovery and treatment as crucial elements of environmental protection in China. Currently, both in China and globally, the main methods for the recovery and treatment of VOCs and oil fumes include adsorption [ 3 ], absorption [ 4 ], membrane separation [ 5 ], condensation [ 6 , 7 ], catalytic combustion [ 8 ], and combinations of these methods. Among these technologies, catalytic combustion offers advantages such as lower ignition temperature, faster purification rates, energy efficiency, and ease of operation, making it an effective approach for the prevention and control of VOC emissions. Among the available techniques, catalytic oxidation of VOCs is highly favored as it allows for the treatment of emissions of a wide range of organic compounds under mild operating conditions. Highly active supported catalysts, in which the active catalyst component is supported on a low-cost carrier, are typically prepared by loading transition metals onto such supports. The catalyst activity is not only related to the specific surface area, acidity, and chemical properties of the carrier but is also influenced by the active components of the loaded metals. Rahou et al. [ 9 ] prepared a series of Mn-Ce catalysts doped with multi-walled carbon nanotubes using the sol-gel method. These catalysts were applied in the catalytic combustion of toluene and exhibited excellent catalytic oxidation activity with T 90 (the temperature corresponding to a 90% conversion rate) of 295°C at an initial toluene concentration of 1000 ppm and a gas hourly space velocity of 120 L/(g·h). The catalytic mechanism on the catalyst surface was analyzed through kinetic studies. Using the wet impregnation method, Zhao et al. [ 10 ] prepared a series of Ce x Cr 1− x HZSM-5 catalysts, which exhibited different catalytic activities for the combustion of aromatic solvent oils in dependence on the ratio of Ce:Cr. The authors determined a T 90 of 308°C for the aromatic solvent oils and elucidated the excellent catalytic activity of the active components cerium and chromium in this combustion process. Li et al. [ 11 ] synthesized the metal-organic framework material UiO-66 (Ce) using a hydrothermal method and prepared a cerium (IV) oxide catalyst through a one-step thermal decomposition method. While T 90 decreased in this experiment to 206°C, there was a mismatch between the raw material cost and the energy consumption of the catalytic combustion process. Although there have been extensive studies on catalysts for the catalytic combustion of VOCs, most researchers have focused on the oxidation of individual VOCs in gases. More research is needed on the catalytic oxidation of gas streams containing mixtures of VOCs in low concentrations, as found in actual chemical factory environments. Transition metals are cost-effective and offer excellent economic benefits. Carbon nanotubes are primarily used in the field of electrocatalysis and have limited applications in the field of catalytic combustion. In this study, using MWCNTs in varying mass ratios, a series of Ce 0.3 Cr 0.7 - x wt%MWCNTs catalysts were prepared by the citric acid sol-gel method. The results showed that Ce 0.3 Cr 0.7 -1.5wt%MWCNTs exhibited excellent catalytic activity in the catalytic oxidation of low-concentration aromatic solvent oils and achieved a T 90 of 248°C. The effect of the doping amount of MWCNTs on the catalytic activity of the catalysts for the oxidation of these oils was investigated. The catalysts were characterized by X -ray diffraction, Fourier transform infrared spectroscopy, X -ray photoelectron spectroscopy, scanning electron microscopy, and nitrogen adsorption-desorption tests. The catalytic oxidation mechanism of the Ce 0.3 Cr 0.7 - x wt%MWCNTs catalysts was proposed for this process. 2 Experimental 2.1 Synthesis of Ce 0.3 Cr 0.7 - x wt%MWCNTs The purchased MWCNTs had a purity of 95%, and further purification was not necessary for this research. First, a certain amount of MWCNTs was weighed and placed in a 50-mL centrifuge tube. Then, 8 mL concentrated sulfuric acid and 24 mL concentrated nitric acid were added to the tube, which was then sealed and placed in an ultrasonic bath for 3 h. After the ultrasonic treatment, the tube was centrifuged for 5 min at a speed of 12,000 rpm. The supernatant was removed, the centrifuged MWCNTs were rinsed with deionized water, and the centrifugation and washing process was repeated. Then, the MWCNTs were placed in a hot-air drying oven at 80°C and dried overnight. The catalyst was synthesized using the citric acid sol-gel method [ 12 ], as shown in Fig. 1 . First, 7.68 g anhydrous citric acid was weighed as the complexing agent for the sol-gel process. According to a molar ratio of 7:3, 2.6053 g of Ce(NO 3 ) 3 ·6H 2 O and 5.6021 g of Cr(NO 3 ) 3 ·9H 2 O were accurately weighed and added to 100 mL deionized water. The mixture was vigorously stirred at 40°C until the solids were completely dissolved. Then, 10 mL of an aqueous 25 wt% ammonia solution was added dropwise to the solution. The temperature was then raised to 80°C to evaporate the water present in the solution and form a gel-like substance. Subsequently, treated MWCNTs were added to the solution in proportions of 0.5 wt%, 1.0 wt%, 1.5 wt%, and 2.0 wt% relative to the total mass. The mixture was stirred for an additional 10 min and then subjected to 3 h of ultrasound treatment in an ultrasonic bath. Finally, the sample was transferred to a crucible and dried overnight at 130°C in a hot-air drying oven. The resulting honeycomb-like solid was then transferred to a ceramic boat and placed in a muffle furnace for calcination at 450°C for 3 h. After cooling to room temperature, the catalyst was removed and kept for further use. The obtained catalysts were labeled Ce 0.3 Cr 0.7 -0.5wt%—2wt%MWCNTs. 2.2 Catalyst characterization The powder X-ray diffraction (XRD) patterns in the 2θ range from 10° to 80° were obtained using a diffractometer (Bruker D8 QUEST) equipped with a Cu-Kα X-ray source at a scanning rate of 1°/min. Fourier transform infrared (FT-IR) spectroscopy was conducted on a Thermo Fisher Scientific Nicolet 5700 instrument, using a DTGS detector and an air-cooled light source Evo-Glo beam splitter. The measurements were performed at a resolution of 4 cm − 1 in a scanning range from 4000 to 500 cm − 1 using 64 scans. Nitrogen adsorption-desorption isotherms were obtained at a temperature of 77 K (SDT Q600, Micromeritics, USA). The specific surface areas were calculated using the Brunauer-Emmett-Teller (BET) equation. Scanning electron microscopy (SEM) was conducted with a scanning electron microscope (SU8010, Hitachi, Japan). X-ray photoelectron spectroscopy (XPS) analysis was performed using an Al Kα excitation source on an ESCALAB250 instrument from Thermo Fisher Scientific, USA. The measurements were conducted at a constant pass energy (1486.6 eV), and all the obtained results were calibrated to the binding energy of C 1s (284.6 eV). 2.3 Catalyst activity Aromatic solvent oil was subjected to catalytic combustion in a fixed-bed quartz microreactor with a diameter of 2.2 cm. The aromatic solvent oil flow rate was set at 1 L/min using a flowmeter, and the concentration of aromatic solvent oil was 120 ppm. After mixing, dry air was used for transportation. Subsequently, a catalyst mass of 0.5 g was used to test catalytic combustion activity. A handheld VOC detector was employed to measure the inlet and outlet concentrations of VOCs, carbon monoxide (CO), and carbon dioxide (CO 2 ) before and after the reaction. The aromatic solvent oil conversion rate \(\:{x}_{Solvent\:oil}\) (%) was determined by monitoring the inlet and outlet concentrations, which can be expressed by the following equation: $$\:{x}_{Solvent\:oil}\left(\text{%}\right)=\frac{{c}_{Solvent\:oil,in}-{c}_{Solvent\:oil,out}}{{c}_{Solvent\:oil,in}}$$ where \(\:{c}_{Solvent\:oil,in}\:\) is the inlet concentration of aromatic solvent oil, and \(\:{c}_{Solvent\:oil,out}\) is the outlet concentration of aromatic solvent oil. 3 Results and discussion 3.1 Catalytic activities The catalytic activity of four different catalysts in the combustion of aromatic hydrocarbon solvent oils is shown in Fig. 2 . Ce 0.3 Cr 0.7 -1.5wt%MWCNTs exhibited higher activity than the other three catalysts, resulting in lower T 50 and T 90 (the temperatures corresponding to 50% and 90%). Furthermore, the generation of CO 2 was consistent with the conversion of aromatic solvent oil, while the generation of CO was contrary to that (Fig. 9 ). The summarized data of aromatic solvent oil combustion activity for each catalyst are presented in Table 1 . According to Fig. 2 and Table 1 , the addition of MWCNTs can significantly enhance the catalytic activity of the catalyst. However, the catalyst with a doping amount of 1.5 wt% exhibited the best performance, and increasing the amount of MWCNTs did not further enhance catalytic activity, which was consistent with the results of SEM analysis. Secondly, the MWCNT-free composite metal oxide exhibited the lowest activity. The catalyst doped with 2 wt% MWCNTs showed higher activity under low-temperature conditions ( T< 225 ℃) compared to the catalyst with 1 wt% content, but the opposite was observed under high-temperature conditions. At low conversion rates (<10%), the catalyst exhibited a longer adsorption process for gases, and significant desorption phenomena started to occur at ~ 150 ℃. Table 1 T 50 , T 90 of Ce 0.3 Cr 0.7 - x wt%MWCNTs Catalyst 0wt%MWCNTs 0.5wt%MWCNTs 1wt%MWCNTs 1.5wt%MWCNTs 2wt%MWCNTs T 50 , ℃ 257 251 215 187 210 T 90 , ℃ 300 260 245 268 3.2 XRD analysis The XRD spectra of prepared catalysts are shown in Fig. 3 a. The characteristic peaks of all the samples were within the range of 10° to 80°, and the peak positions were similar. The XRD pattern in Fig. 3 b indicated that the composite metal oxide catalyst without doped MWCNTs was amorphous. Except for the catalyst with the lowest doping level, the other three catalyst samples exhibited characteristic peaks of cubic fluorite-type CeO 2 at 28.54°, 33.08°, 47.48°, and 56.29°, corresponding to the (1 1 1), (2 0 0), (2 2 0), and (3 1 1) crystal planes, respectively, and matching the standard CeO 2 peaks (JCPDS Card No. 78–0694) [ 13 ]. The catalyst samples exhibited characteristic peaks of Cr 2 O 3 at 24.498°, 36.192°, 41.478°, and 65.094°, corresponding to the (1 1 0), (-1 1 0), (1 2 0), and (-2 1 1) crystal planes, respectively. These peaks matched the standard peaks of Cr 2 O 3 (JCPDS 34-85-0730) [ 14 ]. After modification with different proportions of MWCNTs, the intensity of the characteristic peaks of the catalysts changed accordingly. The XRD spectrum of the metal oxide composite without the inclusion of MWCNTs displayed amorphous characteristics and poor crystallinity. In the sample doped with 0.5 wt% MWCNTs, the peaks were not distinct, indicating that different proportions of MWCNTs result in the deformation of the CeO 2 lattice. For the Ce 0.3 Cr 0.7 - x wt%MWCNTs catalyst, neither diffraction peaks corresponding to CeO 2 at 2θ of 59.094°, 69.421°, 76.708°, 79.084°, and 88.438° nor peaks corresponding to Cr 2 O 3 at 2θ of 33.606°, 36.192°, 39.765°, 44.191°, and 50.215° were observed. This may suggest that the surface of CeO 2 was covered by Cr 2 O 3 or that some MWCNTs have entered the CeO 2 and Cr 2 O 3 lattice, forming a ternary solid solution [ 15 ]. The modification with MWCNTs has improved the catalytic performance of the chromium-cerium composite oxide catalysts by facilitating the interaction between different components. A multi-component solid was formed, which led to the generation of more active sites and enhanced the synergistic effects among the metals. 3.3 XPS analysis To investigate the valence distribution of chromium and cerium in the catalyst, XPS was performed on the most effective catalyst, Ce 0.3 Cr 0.7 -1.5wt%MWCNTs. The O 1s XPS spectrum in Fig. 4 a can be divided into three components, corresponding to lattice oxygen, surface adsorbed oxygen, and adsorbed water and hydroxyl species. Among these, the peak of lattice oxygen (O 2− ) was located at a binding energy of 529.68 eV, denoted as O Latt , that of surface adsorbed oxygen (O 2 − , O 2 2− , or O − ) was situated at 530.88 eV, denoted as O Ads , and adsorbed water or hydroxyl species had a binding energy of 532.68 eV [ 16 ]. In the catalytic combustion of aromatic hydrocarbon solvent oils, lattice oxygen and surface adsorbed oxygen contributed to the synergistic reaction of chromium and cerium. Surface-adsorbed oxygen, particularly peroxide ions (O 2 2− ) and superoxide ions (O 2 ⁻), possess weaker oxygen bond energies and readily coordinate with other atoms to form active species in catalytic combustion reactions [ 13 ]. As shown in Fig. 4 b, the Ce 3d XPS spectrum of Ce 0.3 Cr 0.7 -1.5wt%MWCNTs indicated the presence of different Ce valences, well corresponding to Ce 4+ at binding energies of 882.94, 888.07, 907.73, and 916.48 eV and Ce 3+ at binding energies of 898.02 or 901.4 eV. Generally, the higher the relative proportion of Ce 3+ to Ce 4+ , the more likely Ce 3+ may lose electrons and get oxidized to Ce 4+ , which is advantageous for improving lattice defects and for redox reactions between Ce 3+ and Ce 4+ [ 17 ]. In the case of the Ce 0.3 Cr 0.7 -1.5wt%MWCNTs catalyst, an increase of 0.02 in the ratio of Ce 3+ to Ce 4+ was observed compared to the values reported by Liu et al. [ 18 ]. As shown in Fig. 4 c, the Cr 2p XPS spectrum of Ce 0.3 Cr 0.7 -1.5wt%MWCNTs indicated different valence states, and the binding energies of 579.46 eV and 576.47 eV corresponded well to Cr 4+ and Cr 3+ , respectively [ 19 , 20 ]. Quasi-in-situ XPS results indicated that Ce 3+ and Cr 3+ are the primary active centers. As shown in Fig. 5 , the oxidation process of aromatic solvent oil initially involves an adsorption process, where oxygen molecules and oil/gas molecules are adsorbed on the catalyst surface. The catalyst then facilitates the transformation of oxygen molecules into O 2− through electron transfer, leading to the oxidation reaction of the substrate. 3.4 FT-IR spectroscopy analysis Based on the FT-IR results (Fig. 6 ), the four catalysts exhibited similar characteristic peaks. The peak at 779 cm − 1 corresponded to the out-of-plane vibration of Cr (Ce)-O bonds, while the peak at 2832 cm − 1 was assigned to C-H stretching vibrations [ 21 , 22 ]. The peaks at 1367, 1581, and 1595 cm − 1 can be attributed to C = O stretching vibrations [ 23 ]. Additionally, the peak at 3430 cm − 1 was caused by the stretching vibrations of the -OH groups of water molecules adsorbed on the material surface. The FT-IR spectra revealed the presence of different oxygen-containing groups, namely Ce-O, Cr-O, and C = O groups, indicating the presence of a large number of carbon defect sites (carboxyl and carbonyl groups) on the catalyst surface. These defects can serve as active sites for the catalytic oxidation of aromatic hydrocarbon solvents [ 16 ]. 3.5 SEM analysis The surface morphology of Ce 0.3 Cr 0.7 -0.5-2wt%MWCNTs samples was analyzed by SEM, and the images are shown in Fig. 7 . The catalyst with a doping level of 0.5 wt% exhibited larger pore structures with an uneven distribution. The catalysts with doping levels of 1 wt% and 1.5 wt% had higher pore densities and more uniform and dense distributions of pores, indicating that the addition of MWNTs favors the dispersion of the catalyst. The catalyst with a doping level of 2 wt% showed agglomeration between the oxide particles, evident by a significant decrease in the number of pores, which corroborated the results of specific surface area measurements. Furthermore, under the conditions of SEM imaging, no MWCNTs were observed, indicating that the MWCNTs were deeply embedded within the composite oxide catalyst and in a highly dispersed state. 3.6 Catalyst stability Considering economic benefits and requirements in industrial applications, stability and reuse of catalysts are very important aspects. As shown in Fig. 8 a, repeated experiments with Ce 0.3 Cr 0.7 -1.5wt%MWCNTs catalyst under the same conditions of sequential temperature increase demonstrated the excellent repeatability of Ce 0.3 Cr 0.7 -1.5wt%MWCNTs. As shown in Fig. 8 b, the catalyst maintained a conversion rate of 100% over a prolonged 30-h operation, showcasing outstanding long-term stability. 3.7 Analysis of the main products The concentrations of CO and CO 2 , as the prime combustion products, were also investigated as shown in Fig. 9 . The composite metal oxide catalyst without MWCNT addition exhibited high selectivity for CO 2 under low-temperature conditions. CO started to generate at ~ 220°C, and its concentration peaked at 240°C and remained at ~ 300 ppm with subsequent temperature increases. But in the presence of catalysts doped with MWCNTs, CO concentration sharply diminished after reaching the maximum. Apparently, the incorporation of MWCNTs was beneficial for the conversion of CO to CO 2 at high temperatures (Fig. 9 a). All samples showed a consistent rising pattern in outlet CO 2 concentration with temperature (Fig. 9 b). 4 Conclusions In this study, a series of composite metal oxide catalysts, Ce 0.3 Cr 0.7 - x wt%MWCNTs ( x = 0.5-2.0, in wt%), were prepared using a sol-gel method with Ce and Cr salt solution under addition of MWCNTs in different mass percentages as metal precursors, followed by a one-step pyrolysis process under ambient conditions. The results of the catalytic combustion experiment using aromatic solvent oil as VOC indicates that Ce 0.3 Cr 0.7 -1.5wt%MWCNTs is a promising candidate for industrial applications due to its high catalytic activity, long-term reactivity, recyclability, and economic benefits. This study provides a simple and practical method for designing stable catalysts for the catalytic combustion of VOCs in real-world environments at low temperatures. Abbreviations MWCNT: multi-walled carbon Nanotube VOC: volatile organic compound MWCNTs-doped of chromium-cerium composite oxide catalyst: Ce 0.3 Cr 0.7 - x wt%MWCNT XRD: X-ray diffraction FT-IR: Fourier transform infrared BET : Brunauer emmett teller SEM : scanning electron microscopy XPS: X-ray photoelectron spectroscopy Declarations Ethics and Consent to Participate Not applicable Consent for Publication Not applicable Competing Interest declaration There are no Competing Interests. Author Contribution Wei Yuan: Conceptualization; funding acquisition; methodology; project administration; resources; writing –original draft. Yuheng Yang: writing –original draft; Formal analysis; investigation. Xinli Bao & Yuying Zhao: writing –original draft; validation. Yinlu Sun: Funding acquisition; project administration; supervision; writing – review and editing. Funding National Natural Science Foundation of China (grant no. 21406101), Natural Science Foundation of Liaoning Province (grant no. JKZ0091), and Key Research and Development Plan of Liaoning Province (grant no. 2020JH2/10300061). Availability of data and materials All data included in this study are available upon request by contact with the corresponding author. Acknowledgments We would like to acknowledge the financial support from the National Natural Science Foundation of China (grant no. 21406101), the Natural Science Foundation of Liaoning Province (grant no. JKZ0091), and the Key Research and Development Plan of Liaoning Province (grant no. 2020JH2/10300061). References Dobre T, Parvulescu OC, Iavorschi G, Stroescu M, Stoica A (2014) Volatile organic compounds removal from gas streams by adsorption onto activated carbon. Ind Eng Chem Res 53(9):3622-3628 Ma JW, Li L (2024) VOC emitted by biopharmaceutical industries: Source profiles, health risks, and secondary pollution. J Environ Sci 135:570-584 Shen XQ, Du XS, Yang DF, Ran JY, Yang ZQ, Chen YR (2021) Influence of physical structures and chemical modification on VOCs adsorption characteristics of molecular sieves. J Environ Chem Eng 9(6):106729 Lin CC, Chien KS (2008) Mass-transfer performance of rotating packed beds equipped with blade packings in VOCs absorption into water. Sep Purif Technol 63(1):138-144 Shen BW, Zhao S, Yang XQ, Carta M, Zhou HL, Jin WQ (2022) Relation between permeate pressure and operational parameters in VOC/nitrogen separation by a PDMS composite membrane. Sep Purif Technol 280:119974 Jiang QF, Zhu Q, Duan WQ, Wan SQ, Guo T, Li HB, Feng HS, Du W, Gu JY (2023) Thermodynamic design and experimental study of a condensation recovery system for VOCs. Appl Therm Eng 236 (Part D):121822 Tang FY, Gao JP, Wu X, Hu GH, Yao H, Zhu XJ (2024) Performance investigation on a precision air conditioning system with a condensation heat recovery unit under varying operating conditions. Appl Therm Eng 236 (Part B):121664 Wen M, Dong F, Tang ZC, Zhang JY (2022) Engineering order mesoporous CeCoO x catalyst via in-situ confined encapsulation strategy for VOCs catalytic combustion. Mol Catal 519:112149 Rahou S, Benadda-Kordjani A, Ivanova S, Odriozola JA, Chebout R, Mahzoul H, Zouaoui N (2023) Toluene combustion on MnOx, CeO 2 , and Mn-Ce-O solids prepared via citrate complexation, and citrate and urea combustion methods. J nanopart res (An interdisciplinary forum for nanoscale science and technology):258962318 DOI:10.1007/s11051-023-05759-6 Zhao YY (2023) Development and performance study of catalyst for catalytic oxidation of aromatic solvent oil at low temperature. A master’s degree from Liaoning university DOI:10.27209/d.cnki.glniu.2023.000230 (In Chinese) Li ZJ, Ma C, Qi M, Li YL, Qu YC, Zhang Y, Zhou LL, Yun J (2023) CeO 2 from pyrolysis of MOFs for efficient catalytic combustion of VOCs. Mol Catal 535:112857 Polthep S, Boontawee L, Toshiyuki Y, Chawalit N (2016) Lanthanum-doped mesostructured strontium titanates synthesized via sol-gel combustion route using citric acid as complexing agent. Mater Chem Phys 181:422-431 Chen X, Yu EQ, Cai SC, Jia HP, Chen J, Liang P (2018) In situ pyrolysis of Ce-MOF to prepare CeO 2 catalyst with obviously improved catalytic performance for toluene combustion. Chem Eng J 344:469-479 He J, Lu JQ, Xie GQ, Qian L, Chen KF, Zhang XL, Luo MF (2009) Characterization of CrO x -Y 2 O 3 catalysts for fluorination of 2-chloro-1,1,1-trifluoroethane. Indian J Chem 48:489-497 Kan JW, Deng L, Li B, Huang Q, Zhu SM, Shen SB, Chen YW (2017) Performance of co-doped Mn-Ce catalysts supported on cordierite for low concentration chlorobenzene oxidation. Appl Catal A: General 530:21-29 Su CJ, Li Z, Mao MQ, Ye WH, Zhong JP, Ren QM, Cheng HR, Huang HM, Fu ML, Wu JL, Hu J, Ye DQ, Xu HH (2022) Unraveling specific role of carbon matrix over Pd/quasi-Ce-MOF facilitating toluene enhanced degradation. J Rare Earth 40(11):1751-1762 Xu HD, Zhang QL, Qiu CT, Lin T, Gong MC, Chen YQ (2012) Tungsten modified MnOx–CeO 2 /ZrO 2 monolith catalysts for selective catalytic reduction of NO x with ammonia. Chem Eng Sci 76:120-128 Liu C. (2020) Preparation of Mn-Ce catalyst and its performance in catalytic combustion. A master’s degree from Liaoning Petrochemical University https://link.cnki.net/doi/10.27023/d.cnki.gfssc.2020.000083 doi:10.27023/d.cnki.gfssc.2020.000083 Salvi AM, Castle JE, Watts JF, Desimoni E (1995) Peak fitting of the chromium 2p XPS spectrum. Appl Surf Sci 90(3):333-341 Luo H, Su HZ, Dong CF, Li XG (2017) Passivation and electrochemical behavior of 316L stainless steel in chlorinated simulated concrete pore solution. Appl Surf Sci 400:38-48 Mahendraprabhu K, Elumalai P (2016) Effect of citric acid on formation of oxides of Cu and Zn in modified sol-gel process: A comparative study. J Chem Sci 128(5):831-837 Sun H, Liu ZG, Chen S, Quan X (2015) The role of lattice oxygen on the activity and selectivity of the OMS-2 catalyst for the total oxidation of toluene. Chem Eng J 270:58-65 Catauro M, TranquilloBarrinoBlancoI, PoggettoFD, Naviglio D (2018) Drug release of hybrid materials containing Fe(II) citrate synthesized by sol-gel technique. Materials 11(11): 2270 DOI:10.3390/ma11112270 Additional Declarations No competing interests reported. Cite Share Download PDF Status: Posted Version 1 posted You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. Our growing team is made up of researchers and industry professionals working together to solve the most critical problems facing scientific publishing. Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-5365890","acceptedTermsAndConditions":true,"allowDirectSubmit":true,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":381714386,"identity":"ceff7b50-79f2-4575-87a2-90630d2dff29","order_by":0,"name":"Wei Yuan","email":"","orcid":"","institution":"Liaoning University","correspondingAuthor":false,"prefix":"","firstName":"Wei","middleName":"","lastName":"Yuan","suffix":""},{"id":381714387,"identity":"770799aa-7f28-4c7d-bd71-c4965c57fc4c","order_by":1,"name":"Yuheng Yang","email":"","orcid":"","institution":"Liaoning University","correspondingAuthor":false,"prefix":"","firstName":"Yuheng","middleName":"","lastName":"Yang","suffix":""},{"id":381714388,"identity":"aefd8df5-a1dc-4829-bd0b-b8804c496831","order_by":2,"name":"Xinli Bao","email":"","orcid":"","institution":"Liaoning University","correspondingAuthor":false,"prefix":"","firstName":"Xinli","middleName":"","lastName":"Bao","suffix":""},{"id":381714389,"identity":"9322d478-0ee5-4dee-9cb2-aa9b169576c9","order_by":3,"name":"Yuying Zhao","email":"","orcid":"","institution":"Liaoning University","correspondingAuthor":false,"prefix":"","firstName":"Yuying","middleName":"","lastName":"Zhao","suffix":""},{"id":381714390,"identity":"deec8003-3f72-4f2a-80fe-12748e839a95","order_by":4,"name":"Yinlu Sun","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAAsklEQVRIiWNgGAWjYLCCBxUWYFqCeC0JZyRI1ZLYRooW+Rm5xyQS50nkGRxgPnibh8Euj6AWgxt5aRKJ2ySKDQ6wJVvzMCQXE9YikWMG0pK44QCPmTQPw4HEBsIOA2mZA9LC/404LQw3QFoawLawEafF4MwbY4sEYAjMPMxmbDnHIJkIh7XnGN74UGOT2He8+eGNNxV2RDiMgYEFEh3MYEuJUA9S+4E4daNgFIyCUTBiAQC04zemYSDI4wAAAABJRU5ErkJggg==","orcid":"","institution":"Liaoning University","correspondingAuthor":true,"prefix":"","firstName":"Yinlu","middleName":"","lastName":"Sun","suffix":""}],"badges":[],"createdAt":"2024-10-31 08:23:15","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-5365890/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-5365890/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":69808153,"identity":"e97b2242-25fd-47f6-8d9b-21797470f08f","added_by":"auto","created_at":"2024-11-25 12:17:03","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":182967,"visible":true,"origin":"","legend":"\u003cp\u003eSynthesis steps of the Ce\u003csub\u003e0.3\u003c/sub\u003eCr\u003csub\u003e0.7\u003c/sub\u003e-\u003cem\u003ex\u003c/em\u003ewt%MWCNTs catalysts\u003c/p\u003e","description":"","filename":"1.png","url":"https://assets-eu.researchsquare.com/files/rs-5365890/v1/81e5fd7d3b6ef9bfb0f055c0.png"},{"id":69809270,"identity":"22302ae6-a09a-4f0a-b711-cd3014a91665","added_by":"auto","created_at":"2024-11-25 12:25:03","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":138101,"visible":true,"origin":"","legend":"\u003cp\u003eConversion of aromatic solvent oil by the different catalysts at temperatures between 80 and 380 ℃\u003c/p\u003e","description":"","filename":"2.png","url":"https://assets-eu.researchsquare.com/files/rs-5365890/v1/29446009b5158547e5808420.png"},{"id":69808154,"identity":"021108e8-4384-4669-a639-e3569ba2afec","added_by":"auto","created_at":"2024-11-25 12:17:03","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":135077,"visible":true,"origin":"","legend":"\u003cp\u003eXRD patterns of (a) Ce\u003csub\u003e0.3\u003c/sub\u003eCr\u003csub\u003e0.7\u003c/sub\u003e-0.5—2wt%MWCNTs and (b) Ce\u003csub\u003e0.3\u003c/sub\u003eCr\u003csub\u003e0.7\u003c/sub\u003e-1.5wt%MWCNTs and Ce\u003csub\u003e0.3\u003c/sub\u003eCr\u003csub\u003e0.7\u003c/sub\u003e\u003c/p\u003e","description":"","filename":"3.png","url":"https://assets-eu.researchsquare.com/files/rs-5365890/v1/6e29770fae40e5c8db939a2e.png"},{"id":69808155,"identity":"947a9905-78b7-440b-8440-1aedbb5207a9","added_by":"auto","created_at":"2024-11-25 12:17:03","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":144349,"visible":true,"origin":"","legend":"\u003cp\u003e(a) O 1s, (b) Ce 3d, and (c) Cr 2p XPS patterns of Ce\u003csub\u003e0.3\u003c/sub\u003eCr\u003csub\u003e0.7\u003c/sub\u003e-1.5wt%MWCNTs\u003c/p\u003e","description":"","filename":"4.png","url":"https://assets-eu.researchsquare.com/files/rs-5365890/v1/a8c877b38a7557ea09d4b9c5.png"},{"id":69809727,"identity":"87d69adb-42a7-48f0-a6fd-35874530d4ac","added_by":"auto","created_at":"2024-11-25 12:33:03","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":231978,"visible":true,"origin":"","legend":"\u003cp\u003ePotential mechanism for aromatic solvent oil combustion over Ce\u003csub\u003e0.3\u003c/sub\u003eCr\u003csub\u003e0.7\u003c/sub\u003e-1.5wt%MWCNTs\u003c/p\u003e","description":"","filename":"5.png","url":"https://assets-eu.researchsquare.com/files/rs-5365890/v1/72fb45eb9b3fed068901b9f6.png"},{"id":69808156,"identity":"b2dc3f4d-9aa0-486a-bde2-d46ea57c21e0","added_by":"auto","created_at":"2024-11-25 12:17:03","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":157923,"visible":true,"origin":"","legend":"\u003cp\u003eFT-IR spectra of as-synthesized catalysts\u003c/p\u003e","description":"","filename":"6.png","url":"https://assets-eu.researchsquare.com/files/rs-5365890/v1/67b8da1f62dc86dec6df89a0.png"},{"id":69808161,"identity":"e4be12ec-ffbf-4eb3-a60d-b07e8b9c05a2","added_by":"auto","created_at":"2024-11-25 12:17:03","extension":"png","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":526883,"visible":true,"origin":"","legend":"\u003cp\u003eSEM images of (a, b) Ce\u003csub\u003e0.3\u003c/sub\u003eCr\u003csub\u003e0.7\u003c/sub\u003e,\u003csub\u003e \u003c/sub\u003e(c, d) Ce\u003csub\u003e0.3\u003c/sub\u003eCr\u003csub\u003e0.7\u003c/sub\u003e0.5wt%MWCNTs, (e, f) Ce\u003csub\u003e0.3\u003c/sub\u003eCr\u003csub\u003e0.7\u003c/sub\u003e1wt%MWCNTs, (g, h) Ce\u003csub\u003e0.3\u003c/sub\u003e Cr\u003csub\u003e0.7\u003c/sub\u003e1.5wt%MWCNTs and (i, j) Ce\u003csub\u003e0.3\u003c/sub\u003eCr\u003csub\u003e0.7\u003c/sub\u003e2wt%MWCNTs\u003c/p\u003e","description":"","filename":"7.png","url":"https://assets-eu.researchsquare.com/files/rs-5365890/v1/4f44f37c30135bba4ad05690.png"},{"id":69808159,"identity":"f8310eea-13eb-4af8-986c-9329b42740e4","added_by":"auto","created_at":"2024-11-25 12:17:03","extension":"png","order_by":8,"title":"Figure 8","display":"","copyAsset":false,"role":"figure","size":68628,"visible":true,"origin":"","legend":"\u003cp\u003e(a) Cycling test with sequential temperature increase and (b) 30-h catalytic stability test of Ce\u003csub\u003e0.3\u003c/sub\u003eCr\u003csub\u003e0.7\u003c/sub\u003e-1.5wt%MWCNTs Stability of Ce\u003csub\u003e0.3\u003c/sub\u003eCr\u003csub\u003e0.7\u003c/sub\u003e-1.5wt%MWCNTs\u003c/p\u003e","description":"","filename":"8.png","url":"https://assets-eu.researchsquare.com/files/rs-5365890/v1/732209e17fb4d6ac01192f84.png"},{"id":69809272,"identity":"3fbce598-b7ca-4e8c-8874-8c50840f7b04","added_by":"auto","created_at":"2024-11-25 12:25:03","extension":"png","order_by":9,"title":"Figure 9","display":"","copyAsset":false,"role":"figure","size":200568,"visible":true,"origin":"","legend":"\u003cp\u003eMain combustion of aromatic solvent oil (a) CO outlet concentrations and (b) CO\u003csub\u003e2\u003c/sub\u003e outlet concentrations at different temperatures for Ce0.3Cr0.7-0-2wt%MWCNTs\u003c/p\u003e","description":"","filename":"9.png","url":"https://assets-eu.researchsquare.com/files/rs-5365890/v1/46941d829a2d48284ef04b88.png"},{"id":83098515,"identity":"57cd13cd-2824-4922-96ab-dee14a2772a4","added_by":"auto","created_at":"2025-05-20 04:16:39","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":2434441,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-5365890/v1/0f1c5d23-d33e-437e-b7a4-e7dde6ea0ed0.pdf"}],"financialInterests":"No competing interests reported.","formattedTitle":"Ce-Cr Composite Metal Oxide Doped with Multi-walled Carbon Nanotubes for Catalytic Combustion of Aromatic Solvents","fulltext":[{"header":"1 Introduction","content":"\u003cp\u003eIn recent years, with the continuous development of industrial production in China, the types and quantities of air pollutants have increased. Air pollution has become one of the most significant environmental problems in the country. Industrial emissions, particularly VOCs, are a major source of air pollution, and their health hazards have drawn significant attention. VOCs are generally defined as gases with vapor pressures greater than 0.5 kPa at 25\u0026deg;C. They include aromatic compounds, chlorinated hydrocarbons, alcohols, fats, ketones, and straight-chain hydrocarbons [\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e]. VOCs pose a significant threat to human health due to their carcinogenic, teratogenic, and mutagenic properties. They can also cause respiratory irritation and damage to the central nervous system [\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e]. The introduction of China's \"Emission Standards for Pollutants from the Petrochemical Industry\" in 2015 signifies the recognition of oil, gas, and VOC recovery and treatment as crucial elements of environmental protection in China. Currently, both in China and globally, the main methods for the recovery and treatment of VOCs and oil fumes include adsorption [\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e], absorption [\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e], membrane separation [\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e], condensation [\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e, \u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e], catalytic combustion [\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e], and combinations of these methods. Among these technologies, catalytic combustion offers advantages such as lower ignition temperature, faster purification rates, energy efficiency, and ease of operation, making it an effective approach for the prevention and control of VOC emissions. Among the available techniques, catalytic oxidation of VOCs is highly favored as it allows for the treatment of emissions of a wide range of organic compounds under mild operating conditions.\u003c/p\u003e \u003cp\u003eHighly active supported catalysts, in which the active catalyst component is supported on a low-cost carrier, are typically prepared by loading transition metals onto such supports. The catalyst activity is not only related to the specific surface area, acidity, and chemical properties of the carrier but is also influenced by the active components of the loaded metals. Rahou et al. [\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e] prepared a series of Mn-Ce catalysts doped with multi-walled carbon nanotubes using the sol-gel method. These catalysts were applied in the catalytic combustion of toluene and exhibited excellent catalytic oxidation activity with \u003cem\u003eT\u003c/em\u003e\u003csub\u003e90\u003c/sub\u003e (the temperature corresponding to a 90% conversion rate) of 295\u0026deg;C at an initial toluene concentration of 1000 ppm and a gas hourly space velocity of 120 L/(g\u0026middot;h). The catalytic mechanism on the catalyst surface was analyzed through kinetic studies. Using the wet impregnation method, Zhao et al. [\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e] prepared a series of Ce\u003csub\u003e\u003cem\u003ex\u003c/em\u003e\u003c/sub\u003eCr\u003csub\u003e1\u0026minus;\u003cem\u003ex\u003c/em\u003e\u003c/sub\u003eHZSM-5 catalysts, which exhibited different catalytic activities for the combustion of aromatic solvent oils in dependence on the ratio of Ce:Cr. The authors determined a \u003cem\u003eT\u003c/em\u003e\u003csub\u003e90\u003c/sub\u003e of 308\u0026deg;C for the aromatic solvent oils and elucidated the excellent catalytic activity of the active components cerium and chromium in this combustion process. Li et al. [\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e] synthesized the metal-organic framework material UiO-66 (Ce) using a hydrothermal method and prepared a cerium (IV) oxide catalyst through a one-step thermal decomposition method. While \u003cem\u003eT\u003c/em\u003e\u003csub\u003e90\u003c/sub\u003e decreased in this experiment to 206\u0026deg;C, there was a mismatch between the raw material cost and the energy consumption of the catalytic combustion process. Although there have been extensive studies on catalysts for the catalytic combustion of VOCs, most researchers have focused on the oxidation of individual VOCs in gases. More research is needed on the catalytic oxidation of gas streams containing mixtures of VOCs in low concentrations, as found in actual chemical factory environments.\u003c/p\u003e \u003cp\u003eTransition metals are cost-effective and offer excellent economic benefits. Carbon nanotubes are primarily used in the field of electrocatalysis and have limited applications in the field of catalytic combustion. In this study, using MWCNTs in varying mass ratios, a series of Ce\u003csub\u003e0.3\u003c/sub\u003eCr\u003csub\u003e0.7\u003c/sub\u003e-\u003cem\u003ex\u003c/em\u003ewt%MWCNTs catalysts were prepared by the citric acid sol-gel method. The results showed that Ce\u003csub\u003e0.3\u003c/sub\u003eCr\u003csub\u003e0.7\u003c/sub\u003e-1.5wt%MWCNTs exhibited excellent catalytic activity in the catalytic oxidation of low-concentration aromatic solvent oils and achieved a \u003cem\u003eT\u003c/em\u003e\u003csub\u003e90\u003c/sub\u003e of 248\u0026deg;C. The effect of the doping amount of MWCNTs on the catalytic activity of the catalysts for the oxidation of these oils was investigated. The catalysts were characterized by \u003cem\u003eX\u003c/em\u003e-ray diffraction, Fourier transform infrared spectroscopy, \u003cem\u003eX\u003c/em\u003e-ray photoelectron spectroscopy, scanning electron microscopy, and nitrogen adsorption-desorption tests. The catalytic oxidation mechanism of the Ce\u003csub\u003e0.3\u003c/sub\u003eCr\u003csub\u003e0.7\u003c/sub\u003e-\u003cem\u003ex\u003c/em\u003ewt%MWCNTs catalysts was proposed for this process.\u003c/p\u003e"},{"header":"2 Experimental","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e\n \u003ch2\u003e2.1 Synthesis of Ce\u003csub\u003e0.3\u003c/sub\u003eCr\u003csub\u003e0.7\u003c/sub\u003e-\u003cem\u003ex\u003c/em\u003ewt%MWCNTs\u003c/h2\u003e\n \u003cp\u003eThe purchased MWCNTs had a purity of 95%, and further purification was not necessary for this research. First, a certain amount of MWCNTs was weighed and placed in a 50-mL centrifuge tube. Then, 8 mL concentrated sulfuric acid and 24 mL concentrated nitric acid were added to the tube, which was then sealed and placed in an ultrasonic bath for 3 h. After the ultrasonic treatment, the tube was centrifuged for 5 min at a speed of 12,000 rpm. The supernatant was removed, the centrifuged MWCNTs were rinsed with deionized water, and the centrifugation and washing process was repeated. Then, the MWCNTs were placed in a hot-air drying oven at 80\u0026deg;C and dried overnight.\u003c/p\u003e\n \u003cp\u003eThe catalyst was synthesized using the citric acid sol-gel method [\u003cspan class=\"CitationRef\"\u003e12\u003c/span\u003e], as shown in Fig. \u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003e. First, 7.68 g anhydrous citric acid was weighed as the complexing agent for the sol-gel process. According to a molar ratio of 7:3, 2.6053 g of Ce(NO\u003csub\u003e3\u003c/sub\u003e)\u003csub\u003e3\u003c/sub\u003e\u0026middot;6H\u003csub\u003e2\u003c/sub\u003eO and 5.6021 g of Cr(NO\u003csub\u003e3\u003c/sub\u003e)\u003csub\u003e3\u003c/sub\u003e\u0026middot;9H\u003csub\u003e2\u003c/sub\u003eO were accurately weighed and added to 100 mL deionized water. The mixture was vigorously stirred at 40\u0026deg;C until the solids were completely dissolved. Then, 10 mL of an aqueous 25 wt% ammonia solution was added dropwise to the solution. The temperature was then raised to 80\u0026deg;C to evaporate the water present in the solution and form a gel-like substance. Subsequently, treated MWCNTs were added to the solution in proportions of 0.5 wt%, 1.0 wt%, 1.5 wt%, and 2.0 wt% relative to the total mass. The mixture was stirred for an additional 10 min and then subjected to 3 h of ultrasound treatment in an ultrasonic bath. Finally, the sample was transferred to a crucible and dried overnight at 130\u0026deg;C in a hot-air drying oven. The resulting honeycomb-like solid was then transferred to a ceramic boat and placed in a muffle furnace for calcination at 450\u0026deg;C for 3 h. After cooling to room temperature, the catalyst was removed and kept for further use. The obtained catalysts were labeled Ce\u003csub\u003e0.3\u003c/sub\u003eCr\u003csub\u003e0.7\u003c/sub\u003e-0.5wt%\u0026mdash;2wt%MWCNTs.\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec4\" class=\"Section2\"\u003e\n \u003ch2\u003e2.2 Catalyst characterization\u003c/h2\u003e\n \u003cp\u003eThe powder X-ray diffraction (XRD) patterns in the 2\u0026theta; range from 10\u0026deg; to 80\u0026deg; were obtained using a diffractometer (Bruker D8 QUEST) equipped with a Cu-K\u0026alpha; X-ray source at a scanning rate of 1\u0026deg;/min. Fourier transform infrared (FT-IR) spectroscopy was conducted on a Thermo Fisher Scientific Nicolet 5700 instrument, using a DTGS detector and an air-cooled light source Evo-Glo beam splitter. The measurements were performed at a resolution of 4 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e in a scanning range from 4000 to 500 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e using 64 scans. Nitrogen adsorption-desorption isotherms were obtained at a temperature of 77 K (SDT Q600, Micromeritics, USA). The specific surface areas were calculated using the Brunauer-Emmett-Teller (BET) equation. Scanning electron microscopy (SEM) was conducted with a scanning electron microscope (SU8010, Hitachi, Japan). X-ray photoelectron spectroscopy (XPS) analysis was performed using an Al K\u0026alpha; excitation source on an ESCALAB250 instrument from Thermo Fisher Scientific, USA. The measurements were conducted at a constant pass energy (1486.6 eV), and all the obtained results were calibrated to the binding energy of C 1s (284.6 eV).\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec5\" class=\"Section2\"\u003e\n \u003ch2\u003e2.3 Catalyst activity\u003c/h2\u003e\n \u003cp\u003eAromatic solvent oil was subjected to catalytic combustion in a fixed-bed quartz microreactor with a diameter of 2.2 cm. The aromatic solvent oil flow rate was set at 1 L/min using a flowmeter, and the concentration of aromatic solvent oil was 120 ppm. After mixing, dry air was used for transportation. Subsequently, a catalyst mass of 0.5 g was used to test catalytic combustion activity. A handheld VOC detector was employed to measure the inlet and outlet concentrations of VOCs, carbon monoxide (CO), and carbon dioxide (CO\u003csub\u003e2\u003c/sub\u003e) before and after the reaction. The aromatic solvent oil conversion rate \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{x}_{Solvent\\:oil}\\)\u003c/span\u003e\u003c/span\u003e (%) was determined by monitoring the inlet and outlet concentrations, which can be expressed by the following equation:\u003c/p\u003e\n \u003cdiv id=\"Equa\" class=\"Equation\"\u003e\n \u003cdiv class=\"mathdisplay\" id=\"FileID_Equa\" name=\"EquationSource\"\u003e$$\\:{x}_{Solvent\\:oil}\\left(\\text{%}\\right)=\\frac{{c}_{Solvent\\:oil,in}-{c}_{Solvent\\:oil,out}}{{c}_{Solvent\\:oil,in}}$$\u003c/div\u003e\u003c/div\u003e\u003cp\u003ewhere \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{c}_{Solvent\\:oil,in}\\:\\)\u003c/span\u003e\u003c/span\u003eis the inlet concentration of aromatic solvent oil, and \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{c}_{Solvent\\:oil,out}\\)\u003c/span\u003e\u003c/span\u003e is the outlet concentration of aromatic solvent oil.\u003c/p\u003e\u003c/div\u003e"},{"header":"3 Results and discussion","content":"\u003cdiv id=\"Sec7\" class=\"Section2\"\u003e\n \u003ch2\u003e3.1 Catalytic activities\u003c/h2\u003e\n \u003cp\u003eThe catalytic activity of four different catalysts in the combustion of aromatic hydrocarbon solvent oils is shown in Fig. \u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003e. Ce\u003csub\u003e0.3\u003c/sub\u003eCr\u003csub\u003e0.7\u003c/sub\u003e-1.5wt%MWCNTs exhibited higher activity than the other three catalysts, resulting in lower \u003cem\u003eT\u003c/em\u003e\u003csub\u003e50\u003c/sub\u003e and \u003cem\u003eT\u003c/em\u003e\u003csub\u003e90\u003c/sub\u003e (the temperatures corresponding to 50% and 90%). Furthermore, the generation of CO\u003csub\u003e2\u003c/sub\u003e was consistent with the conversion of aromatic solvent oil, while the generation of CO was contrary to that (Fig. \u003cspan class=\"InternalRef\"\u003e9\u003c/span\u003e). The summarized data of aromatic solvent oil combustion activity for each catalyst are presented in Table \u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003e. According to Fig. \u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003e and Table \u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003e, the addition of MWCNTs can significantly enhance the catalytic activity of the catalyst. However, the catalyst with a doping amount of 1.5 wt% exhibited the best performance, and increasing the amount of MWCNTs did not further enhance catalytic activity, which was consistent with the results of SEM analysis. Secondly, the MWCNT-free composite metal oxide exhibited the lowest activity. The catalyst doped with 2 wt% MWCNTs showed higher activity under low-temperature conditions (\u003cem\u003eT\u0026lt;\u003c/em\u003e225 ℃) compared to the catalyst with 1 wt% content, but the opposite was observed under high-temperature conditions. At low conversion rates (\u0026lt;10%), the catalyst exhibited a longer adsorption process for gases, and significant desorption phenomena started to occur at ~\u0026thinsp;150 ℃.\u003c/p\u003e\n \u003cdiv class=\"gridtable\"\u003e\u0026nbsp;\u0026nbsp;\u003ctable id=\"Tab1\" border=\"1\" style=\"margin-right: calc(39%); width: 61%;\"\u003e\n \u003ccaption language=\"En\"\u003e\n \u003cdiv class=\"CaptionNumber\"\u003eTable 1\u003c/div\u003e\n \u003cdiv class=\"CaptionContent\"\u003e\n \u003cp\u003e\u003cem\u003eT\u003c/em\u003e\u003csub\u003e50\u003c/sub\u003e, \u003cem\u003eT\u003c/em\u003e\u003csub\u003e90\u003c/sub\u003e of Ce\u003csub\u003e0.3\u003c/sub\u003eCr\u003csub\u003e0.7\u003c/sub\u003e-\u003cem\u003ex\u003c/em\u003ewt%MWCNTs\u003c/p\u003e\n \u003c/div\u003e\n \u003c/caption\u003e\n \u003cthead\u003e\n \u003ctr\u003e\n \u003cth align=\"left\" style=\"width: 9.224%;\"\u003e\n \u003cp\u003eCatalyst\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\" style=\"width: 17.4231%;\"\u003e\n \u003cp\u003e0wt%MWCNTs\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\" style=\"width: 19.3265%;\"\u003e\n \u003cp\u003e0.5wt%MWCNTs\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\" style=\"width: 17.4231%;\"\u003e\n \u003cp\u003e1wt%MWCNTs\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\" style=\"width: 19.3265%;\"\u003e\n \u003cp\u003e1.5wt%MWCNTs\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\" style=\"width: 17.2767%;\"\u003e\n \u003cp\u003e2wt%MWCNTs\u003c/p\u003e\n \u003c/th\u003e\n \u003c/tr\u003e\n \u003c/thead\u003e\n \u003ctbody\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\" style=\"width: 9.224%;\"\u003e\n \u003cp\u003e\u003cem\u003eT\u003c/em\u003e\u003csub\u003e50\u003c/sub\u003e, ℃\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\" style=\"width: 17.4231%;\"\u003e\n \u003cp\u003e257\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\" style=\"width: 19.3265%;\"\u003e\n \u003cp\u003e251\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\" style=\"width: 17.4231%;\"\u003e\n \u003cp\u003e215\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\" style=\"width: 19.3265%;\"\u003e\n \u003cp\u003e187\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\" style=\"width: 17.2767%;\"\u003e\n \u003cp\u003e210\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\" style=\"width: 9.224%;\"\u003e\n \u003cp\u003e\u003cem\u003eT\u003c/em\u003e\u003csub\u003e90\u003c/sub\u003e, ℃\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" style=\"width: 17.4231%;\"\u003e\u0026nbsp;\u003c/td\u003e\n \u003ctd align=\"char\" style=\"width: 19.3265%;\"\u003e\n \u003cp\u003e300\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\" style=\"width: 17.4231%;\"\u003e\n \u003cp\u003e260\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\" style=\"width: 19.3265%;\"\u003e\n \u003cp\u003e245\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\" style=\"width: 17.2767%;\"\u003e\n \u003cp\u003e268\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003c/tbody\u003e\n \u003c/table\u003e\n \u003c/div\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec8\" class=\"Section2\"\u003e\n \u003ch2\u003e3.2 XRD analysis\u003c/h2\u003e\n \u003cp\u003eThe XRD spectra of prepared catalysts are shown in Fig. \u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003ea. The characteristic peaks of all the samples were within the range of 10\u0026deg; to 80\u0026deg;, and the peak positions were similar. The XRD pattern in Fig. \u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003eb indicated that the composite metal oxide catalyst without doped MWCNTs was amorphous. Except for the catalyst with the lowest doping level, the other three catalyst samples exhibited characteristic peaks of cubic fluorite-type CeO\u003csub\u003e2\u003c/sub\u003e at 28.54\u0026deg;, 33.08\u0026deg;, 47.48\u0026deg;, and 56.29\u0026deg;, corresponding to the (1 1 1), (2 0 0), (2 2 0), and (3 1 1) crystal planes, respectively, and matching the standard CeO\u003csub\u003e2\u003c/sub\u003e peaks (JCPDS Card No. 78\u0026ndash;0694) [\u003cspan class=\"CitationRef\"\u003e13\u003c/span\u003e]. The catalyst samples exhibited characteristic peaks of Cr\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e at 24.498\u0026deg;, 36.192\u0026deg;, 41.478\u0026deg;, and 65.094\u0026deg;, corresponding to the (1 1 0), (-1 1 0), (1 2 0), and (-2 1 1) crystal planes, respectively. These peaks matched the standard peaks of Cr\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e (JCPDS 34-85-0730) [\u003cspan class=\"CitationRef\"\u003e14\u003c/span\u003e]. After modification with different proportions of MWCNTs, the intensity of the characteristic peaks of the catalysts changed accordingly. The XRD spectrum of the metal oxide composite without the inclusion of MWCNTs displayed amorphous characteristics and poor crystallinity. In the sample doped with 0.5 wt% MWCNTs, the peaks were not distinct, indicating that different proportions of MWCNTs result in the deformation of the CeO\u003csub\u003e2\u003c/sub\u003e lattice. For the Ce\u003csub\u003e0.3\u003c/sub\u003eCr\u003csub\u003e0.7\u003c/sub\u003e-\u003cem\u003ex\u003c/em\u003ewt%MWCNTs catalyst, neither diffraction peaks corresponding to CeO\u003csub\u003e2\u003c/sub\u003e at 2\u0026theta; of 59.094\u0026deg;, 69.421\u0026deg;, 76.708\u0026deg;, 79.084\u0026deg;, and 88.438\u0026deg; nor peaks corresponding to Cr\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e at 2\u0026theta; of 33.606\u0026deg;, 36.192\u0026deg;, 39.765\u0026deg;, 44.191\u0026deg;, and 50.215\u0026deg; were observed. This may suggest that the surface of CeO\u003csub\u003e2\u003c/sub\u003e was covered by Cr\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e or that some MWCNTs have entered the CeO\u003csub\u003e2\u003c/sub\u003e and Cr\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e lattice, forming a ternary solid solution [\u003cspan class=\"CitationRef\"\u003e15\u003c/span\u003e]. The modification with MWCNTs has improved the catalytic performance of the chromium-cerium composite oxide catalysts by facilitating the interaction between different components. A multi-component solid was formed, which led to the generation of more active sites and enhanced the synergistic effects among the metals.\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec9\" class=\"Section2\"\u003e\n \u003ch2\u003e3.3 XPS analysis\u003c/h2\u003e\n \u003cp\u003eTo investigate the valence distribution of chromium and cerium in the catalyst, XPS was performed on the most effective catalyst, Ce\u003csub\u003e0.3\u003c/sub\u003eCr\u003csub\u003e0.7\u003c/sub\u003e-1.5wt%MWCNTs.\u003c/p\u003e\n \u003cp\u003eThe O 1s XPS spectrum in Fig. \u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003ea can be divided into three components, corresponding to lattice oxygen, surface adsorbed oxygen, and adsorbed water and hydroxyl species. Among these, the peak of lattice oxygen (O\u003csup\u003e2\u0026minus;\u003c/sup\u003e) was located at a binding energy of 529.68 eV, denoted as O\u003csub\u003eLatt\u003c/sub\u003e, that of surface adsorbed oxygen (O\u003csub\u003e2\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e, O\u003csub\u003e2\u003c/sub\u003e\u003csup\u003e2\u0026minus;\u003c/sup\u003e, or O\u003csup\u003e\u0026minus;\u003c/sup\u003e) was situated at 530.88 eV, denoted as O\u003csub\u003eAds\u003c/sub\u003e, and adsorbed water or hydroxyl species had a binding energy of 532.68 eV [\u003cspan class=\"CitationRef\"\u003e16\u003c/span\u003e]. In the catalytic combustion of aromatic hydrocarbon solvent oils, lattice oxygen and surface adsorbed oxygen contributed to the synergistic reaction of chromium and cerium. Surface-adsorbed oxygen, particularly peroxide ions (O\u003csub\u003e2\u003c/sub\u003e\u003csup\u003e2\u0026minus;\u003c/sup\u003e) and superoxide ions (O\u003csub\u003e2\u003c/sub\u003e⁻), possess weaker oxygen bond energies and readily coordinate with other atoms to form active species in catalytic combustion reactions [\u003cspan class=\"CitationRef\"\u003e13\u003c/span\u003e]. As shown in Fig. \u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003eb, the Ce 3d XPS spectrum of Ce\u003csub\u003e0.3\u003c/sub\u003eCr\u003csub\u003e0.7\u003c/sub\u003e-1.5wt%MWCNTs indicated the presence of different Ce valences, well corresponding to Ce\u003csup\u003e4+\u003c/sup\u003e at binding energies of 882.94, 888.07, 907.73, and 916.48 eV and Ce\u003csup\u003e3+\u003c/sup\u003e at binding energies of 898.02 or 901.4 eV. Generally, the higher the relative proportion of Ce\u003csup\u003e3+\u003c/sup\u003e to Ce\u003csup\u003e4+\u003c/sup\u003e, the more likely Ce\u003csup\u003e3+\u003c/sup\u003e may lose electrons and get oxidized to Ce\u003csup\u003e4+\u003c/sup\u003e, which is advantageous for improving lattice defects and for redox reactions between Ce\u003csup\u003e3+\u003c/sup\u003e and Ce\u003csup\u003e4+\u003c/sup\u003e [\u003cspan class=\"CitationRef\"\u003e17\u003c/span\u003e]. In the case of the Ce\u003csub\u003e0.3\u003c/sub\u003eCr\u003csub\u003e0.7\u003c/sub\u003e-1.5wt%MWCNTs catalyst, an increase of 0.02 in the ratio of Ce\u003csup\u003e3+\u003c/sup\u003e to Ce\u003csup\u003e4+\u003c/sup\u003e was observed compared to the values reported by Liu et al. [\u003cspan class=\"CitationRef\"\u003e18\u003c/span\u003e]. As shown in Fig. \u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003ec, the Cr 2p XPS spectrum of Ce\u003csub\u003e0.3\u003c/sub\u003eCr\u003csub\u003e0.7\u003c/sub\u003e-1.5wt%MWCNTs indicated different valence states, and the binding energies of 579.46 eV and 576.47 eV corresponded well to Cr\u003csup\u003e4+\u003c/sup\u003e and Cr\u003csup\u003e3+\u003c/sup\u003e, respectively [\u003cspan class=\"CitationRef\"\u003e19\u003c/span\u003e, \u003cspan class=\"CitationRef\"\u003e20\u003c/span\u003e].\u003c/p\u003e\n \u003cp\u003eQuasi-in-situ XPS results indicated that Ce\u003csup\u003e3+\u003c/sup\u003e and Cr\u003csup\u003e3+\u003c/sup\u003e are the primary active centers. As shown in Fig. \u003cspan class=\"InternalRef\"\u003e5\u003c/span\u003e, the oxidation process of aromatic solvent oil initially involves an adsorption process, where oxygen molecules and oil/gas molecules are adsorbed on the catalyst surface. The catalyst then facilitates the transformation of oxygen molecules into O\u003csup\u003e2\u0026minus;\u003c/sup\u003e through electron transfer, leading to the oxidation reaction of the substrate.\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec10\" class=\"Section2\"\u003e\n \u003ch2\u003e3.4 FT-IR spectroscopy analysis\u003c/h2\u003e\n \u003cp\u003eBased on the FT-IR results (Fig. \u003cspan class=\"InternalRef\"\u003e6\u003c/span\u003e), the four catalysts exhibited similar characteristic peaks. The peak at 779 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e corresponded to the out-of-plane vibration of Cr (Ce)-O bonds, while the peak at 2832 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e was assigned to C-H stretching vibrations [\u003cspan class=\"CitationRef\"\u003e21\u003c/span\u003e, \u003cspan class=\"CitationRef\"\u003e22\u003c/span\u003e]. The peaks at 1367, 1581, and 1595 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e can be attributed to C\u0026thinsp;=\u0026thinsp;O stretching vibrations [\u003cspan class=\"CitationRef\"\u003e23\u003c/span\u003e]. Additionally, the peak at 3430 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e was caused by the stretching vibrations of the -OH groups of water molecules adsorbed on the material surface. The FT-IR spectra revealed the presence of different oxygen-containing groups, namely Ce-O, Cr-O, and C\u0026thinsp;=\u0026thinsp;O groups, indicating the presence of a large number of carbon defect sites (carboxyl and carbonyl groups) on the catalyst surface. These defects can serve as active sites for the catalytic oxidation of aromatic hydrocarbon solvents [\u003cspan class=\"CitationRef\"\u003e16\u003c/span\u003e].\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec11\" class=\"Section2\"\u003e\n \u003ch2\u003e3.5 SEM analysis\u003c/h2\u003e\n \u003cp\u003eThe surface morphology of Ce\u003csub\u003e0.3\u003c/sub\u003eCr\u003csub\u003e0.7\u003c/sub\u003e-0.5-2wt%MWCNTs samples was analyzed by SEM, and the images are shown in Fig. \u003cspan class=\"InternalRef\"\u003e7\u003c/span\u003e. The catalyst with a doping level of 0.5 wt% exhibited larger pore structures with an uneven distribution. The catalysts with doping levels of 1 wt% and 1.5 wt% had higher pore densities and more uniform and dense distributions of pores, indicating that the addition of MWNTs favors the dispersion of the catalyst. The catalyst with a doping level of 2 wt% showed agglomeration between the oxide particles, evident by a significant decrease in the number of pores, which corroborated the results of specific surface area measurements. Furthermore, under the conditions of SEM imaging, no MWCNTs were observed, indicating that the MWCNTs were deeply embedded within the composite oxide catalyst and in a highly dispersed state.\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec12\" class=\"Section2\"\u003e\n \u003ch2\u003e3.6 Catalyst stability\u003c/h2\u003e\n \u003cp\u003eConsidering economic benefits and requirements in industrial applications, stability and reuse of catalysts are very important aspects. As shown in Fig. \u003cspan class=\"InternalRef\"\u003e8\u003c/span\u003ea, repeated experiments with Ce\u003csub\u003e0.3\u003c/sub\u003eCr\u003csub\u003e0.7\u003c/sub\u003e-1.5wt%MWCNTs catalyst under the same conditions of sequential temperature increase demonstrated the excellent repeatability of Ce\u003csub\u003e0.3\u003c/sub\u003eCr\u003csub\u003e0.7\u003c/sub\u003e-1.5wt%MWCNTs. As shown in Fig. \u003cspan class=\"InternalRef\"\u003e8\u003c/span\u003eb, the catalyst maintained a conversion rate of 100% over a prolonged 30-h operation, showcasing outstanding long-term stability.\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec13\" class=\"Section2\"\u003e\n \u003ch2\u003e3.7 Analysis of the main products\u003c/h2\u003e\n \u003cp\u003eThe concentrations of CO and CO\u003csub\u003e2\u003c/sub\u003e, as the prime combustion products, were also investigated as shown in Fig. \u003cspan class=\"InternalRef\"\u003e9\u003c/span\u003e. The composite metal oxide catalyst without MWCNT addition exhibited high selectivity for CO\u003csub\u003e2\u003c/sub\u003e under low-temperature conditions. CO started to generate at ~\u0026thinsp;220\u0026deg;C, and its concentration peaked at 240\u0026deg;C and remained at ~\u0026thinsp;300 ppm with subsequent temperature increases. But in the presence of catalysts doped with MWCNTs, CO concentration sharply diminished after reaching the maximum. Apparently, the incorporation of MWCNTs was beneficial for the conversion of CO to CO\u003csub\u003e2\u003c/sub\u003e at high temperatures (Fig. \u003cspan class=\"InternalRef\"\u003e9\u003c/span\u003ea). All samples showed a consistent rising pattern in outlet CO\u003csub\u003e2\u003c/sub\u003e concentration with temperature (Fig. \u003cspan class=\"InternalRef\"\u003e9\u003c/span\u003eb).\u003c/p\u003e\n\u003c/div\u003e"},{"header":"4 Conclusions","content":"\u003cp\u003eIn this study, a series of composite metal oxide catalysts, Ce\u003csub\u003e0.3\u003c/sub\u003eCr\u003csub\u003e0.7\u003c/sub\u003e-\u003cem\u003ex\u003c/em\u003ewt%MWCNTs (\u003cem\u003ex\u003c/em\u003e\u0026thinsp;=\u0026thinsp;0.5-2.0, in wt%), were prepared using a sol-gel method with Ce and Cr salt solution under addition of MWCNTs in different mass percentages as metal precursors, followed by a one-step pyrolysis process under ambient conditions. The results of the catalytic combustion experiment using aromatic solvent oil as VOC indicates that Ce\u003csub\u003e0.3\u003c/sub\u003eCr\u003csub\u003e0.7\u003c/sub\u003e-1.5wt%MWCNTs is a promising candidate for industrial applications due to its high catalytic activity, long-term reactivity, recyclability, and economic benefits. This study provides a simple and practical method for designing stable catalysts for the catalytic combustion of VOCs in real-world environments at low temperatures.\u003c/p\u003e"},{"header":"Abbreviations","content":"\u003cp\u003eMWCNT: multi-walled carbon Nanotube\u003c/p\u003e\n\u003cp\u003eVOC: volatile organic compound\u003c/p\u003e\n\u003cp\u003eMWCNTs-doped of chromium-cerium composite oxide catalyst: Ce\u003csub\u003e0.3\u003c/sub\u003eCr\u003csub\u003e0.7\u003c/sub\u003e-\u003cem\u003ex\u003c/em\u003ewt%MWCNT\u003c/p\u003e\n\u003cp\u003eXRD:\u0026nbsp;X-ray diffraction\u003c/p\u003e\n\u003cp\u003eFT-IR: Fourier transform infrared\u003c/p\u003e\n\u003cp\u003eBET : Brunauer emmett teller\u003c/p\u003e\n\u003cp\u003eSEM : scanning electron microscopy\u003c/p\u003e\n\u003cp\u003eXPS: X-ray photoelectron spectroscopy\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eEthics and Consent to Participate\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eNot applicable\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eConsent for Publication\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eNot applicable\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eCompeting Interest declaration\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThere are no Competing Interests.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAuthor Contribution\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eWei Yuan:\u0026nbsp;Conceptualization; funding acquisition; methodology; project administration; resources; writing\u0026nbsp;–original draft.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eYuheng Yang:\u0026nbsp;writing\u0026nbsp;–original draft; Formal analysis; investigation.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eXinli Bao \u0026amp; Yuying Zhao:\u0026nbsp;writing\u0026nbsp;–original draft; validation.\u003c/p\u003e\n\u003cp\u003eYinlu Sun:\u0026nbsp;Funding acquisition; project administration; supervision; writing\u0026nbsp;–\u0026nbsp;review and editing.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eFunding\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eNational Natural Science Foundation of China (grant no. 21406101), Natural Science Foundation of Liaoning Province (grant no. JKZ0091), and Key Research and Development Plan of Liaoning Province (grant no. 2020JH2/10300061).\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAvailability of data and materials\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eAll data included in this study are available upon request by contact with the corresponding author.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAcknowledgments\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eWe would like to acknowledge the financial support from the National Natural Science Foundation of China (grant no. 21406101), the Natural Science Foundation of Liaoning Province (grant no. JKZ0091), and the Key Research and Development Plan of Liaoning Province (grant no. 2020JH2/10300061).\u0026nbsp;\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eDobre T, Parvulescu OC, Iavorschi G, Stroescu M, Stoica A (2014) Volatile organic compounds removal from gas streams by adsorption onto activated carbon. Ind Eng Chem Res 53(9):3622-3628\u003c/li\u003e\n\u003cli\u003eMa JW, Li L (2024) VOC emitted by biopharmaceutical industries: Source profiles, health risks, and secondary pollution. J Environ Sci 135:570-584\u003c/li\u003e\n\u003cli\u003eShen XQ, Du XS, Yang DF, Ran JY, Yang ZQ, Chen YR (2021) Influence of physical structures and chemical modification on VOCs adsorption characteristics of molecular sieves. J Environ Chem Eng 9(6):106729\u003c/li\u003e\n\u003cli\u003eLin CC, Chien KS (2008) Mass-transfer performance of rotating packed beds equipped with blade packings in VOCs absorption into water. Sep Purif Technol 63(1):138-144\u003c/li\u003e\n\u003cli\u003eShen BW, Zhao S, Yang XQ, Carta M, Zhou HL, Jin WQ (2022) Relation between permeate pressure and operational parameters in VOC/nitrogen separation by a PDMS composite membrane. Sep Purif Technol 280:119974\u003c/li\u003e\n\u003cli\u003eJiang QF, Zhu Q, Duan WQ, Wan SQ, Guo T, Li HB, Feng HS, Du W, Gu JY (2023) Thermodynamic design and experimental study of a condensation recovery system for VOCs. Appl Therm Eng 236 (Part D):121822\u003c/li\u003e\n\u003cli\u003eTang FY, Gao JP, Wu X, Hu GH, Yao H, Zhu XJ (2024) Performance investigation on a precision air conditioning system with a condensation heat recovery unit under varying operating conditions. Appl Therm Eng 236 (Part B):121664\u003c/li\u003e\n\u003cli\u003eWen M, Dong F, Tang ZC, Zhang JY (2022) Engineering order mesoporous CeCoO\u003csub\u003ex\u003c/sub\u003e catalyst via in-situ confined encapsulation strategy for VOCs catalytic combustion. Mol Catal 519:112149\u003c/li\u003e\n\u003cli\u003eRahou S, Benadda-Kordjani A, Ivanova S, Odriozola JA, Chebout R, Mahzoul H, Zouaoui N (2023) Toluene combustion on MnOx, CeO\u003csub\u003e2\u003c/sub\u003e, and Mn-Ce-O solids prepared via citrate complexation, and citrate and urea combustion methods. J nanopart res (An interdisciplinary forum for nanoscale science and technology):258962318 DOI:10.1007/s11051-023-05759-6\u003c/li\u003e\n\u003cli\u003eZhao YY (2023) Development and performance study of catalyst for catalytic oxidation of aromatic solvent oil at low temperature. A master\u0026rsquo;s degree from Liaoning university DOI:10.27209/d.cnki.glniu.2023.000230 (In Chinese)\u003c/li\u003e\n\u003cli\u003eLi ZJ, Ma C, Qi M, Li YL, Qu YC, Zhang Y, Zhou LL, Yun J (2023) CeO\u003csub\u003e2\u003c/sub\u003e from pyrolysis of MOFs for efficient catalytic combustion of VOCs. Mol Catal 535:112857\u003c/li\u003e\n\u003cli\u003ePolthep S, Boontawee L, Toshiyuki Y, Chawalit N (2016) Lanthanum-doped mesostructured strontium titanates synthesized via sol-gel combustion route using citric acid as complexing agent. Mater Chem Phys 181:422-431\u003c/li\u003e\n\u003cli\u003eChen X, Yu EQ, Cai SC, Jia HP, Chen J, Liang P (2018) In situ pyrolysis of Ce-MOF to prepare CeO\u003csub\u003e2\u003c/sub\u003e catalyst with obviously improved catalytic performance for toluene combustion. Chem Eng J 344:469-479\u003c/li\u003e\n\u003cli\u003eHe J, Lu JQ, Xie GQ, Qian L, Chen KF, Zhang XL, Luo MF (2009) Characterization of CrO\u003csub\u003ex\u003c/sub\u003e-Y\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e catalysts for fluorination of 2-chloro-1,1,1-trifluoroethane. Indian J Chem 48:489-497\u003c/li\u003e\n\u003cli\u003eKan JW, Deng L, Li B, Huang Q, Zhu SM, Shen SB, Chen YW (2017) Performance of co-doped Mn-Ce catalysts supported on cordierite for low concentration chlorobenzene oxidation. Appl Catal A: General 530:21-29\u003c/li\u003e\n\u003cli\u003eSu CJ, Li Z, Mao MQ, Ye WH, Zhong JP, Ren QM, Cheng HR, Huang HM, Fu ML, Wu JL, Hu J, Ye DQ, Xu HH (2022) Unraveling specific role of carbon matrix over Pd/quasi-Ce-MOF facilitating toluene enhanced degradation. J Rare Earth 40(11):1751-1762\u003c/li\u003e\n\u003cli\u003eXu HD, Zhang QL, Qiu CT, Lin T, Gong MC, Chen YQ (2012) Tungsten modified MnOx\u0026ndash;CeO\u003csub\u003e2\u003c/sub\u003e/ZrO\u003csub\u003e2\u003c/sub\u003e monolith catalysts for selective catalytic reduction of NO\u003csub\u003ex\u003c/sub\u003e with ammonia. Chem Eng Sci 76:120-128\u003c/li\u003e\n\u003cli\u003eLiu C. (2020) Preparation of Mn-Ce catalyst and its performance in catalytic combustion. A master\u0026rsquo;s degree from Liaoning Petrochemical University https://link.cnki.net/doi/10.27023/d.cnki.gfssc.2020.000083 doi:10.27023/d.cnki.gfssc.2020.000083\u003c/li\u003e\n\u003cli\u003eSalvi AM, Castle JE, Watts JF, Desimoni E (1995) Peak fitting of the chromium 2p XPS spectrum. Appl Surf Sci 90(3):333-341\u003c/li\u003e\n\u003cli\u003eLuo H, Su HZ, Dong CF, Li XG (2017) Passivation and electrochemical behavior of 316L stainless steel in chlorinated simulated concrete pore solution. Appl Surf Sci 400:38-48\u003c/li\u003e\n\u003cli\u003eMahendraprabhu K, Elumalai P (2016) Effect of citric acid on formation of oxides of Cu and Zn in modified sol-gel process: A comparative study. J Chem Sci 128(5):831-837\u003c/li\u003e\n\u003cli\u003eSun H, Liu ZG, Chen S, Quan X (2015) The role of lattice oxygen on the activity and selectivity of the OMS-2 catalyst for the total oxidation of toluene. Chem Eng J 270:58-65\u003c/li\u003e\n\u003cli\u003eCatauro M, TranquilloBarrinoBlancoI, PoggettoFD, Naviglio D (2018) Drug release of hybrid materials containing Fe(II) citrate synthesized by sol-gel technique. Materials 11(11): 2270 DOI:10.3390/ma11112270\u003c/li\u003e\n\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":true,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true},"keywords":"VOCs, Catalytic combustion, Aromatic solvent oil, Ce-Cr composite metal oxide, MWCNT promoters","lastPublishedDoi":"10.21203/rs.3.rs-5365890/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-5365890/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eVolatile organic compounds (VOCs) pose risk to eco-environment and human health. Catalytic combustion has been proven to be an effective method for the efficient removal of VOCs from industrial sources. Herein, the applicability of Ce-Cr composite metal oxide doped with multi-walled carbon nanotubes (MWCNTs) for catalytic combustion of aromatic solvent oil was evaluated. Using varying mass proportions of MWCNTs, a series of MWCNTs-doped of chromium-cerium composite oxide catalysts (Ce\u003csub\u003e0.3\u003c/sub\u003eCr\u003csub\u003e0.7\u003c/sub\u003e-\u003cem\u003ex\u003c/em\u003ewt%MWCNTs) were synthesized by the citric acid sol-gel method and subsequent air pyrolysis. The effect of MWCNTs doping on the catalytic combustion was investigated. The measured complete combustion temperature of the low-concentration aromatic solvent oil indicated that the catalytic performance of the Ce-Cr composite oxide was improved by the doping of MWCNTs in range of 0.5-2.0 wt%, reaching its best at 1.5 wt%. It is revealed that Ce\u003csub\u003e0.3\u003c/sub\u003eCr\u003csub\u003e0.7\u003c/sub\u003e-1.5wt%MWCNTs possessed abundant honeycomb-like pore structures, significantly enhancing the adsorption and activation of the aromatic solvent oil during the reaction. The excellent repeatability and stability of Ce\u003csub\u003e0.3\u003c/sub\u003eCr\u003csub\u003e0.7\u003c/sub\u003e-1.5wt% MWCNTs were also be verified. This study provides a feasible approach for the design of effective catalysts for the decomposition of VOCs.\u003c/p\u003e","manuscriptTitle":"Ce-Cr Composite Metal Oxide Doped with Multi-walled Carbon Nanotubes for Catalytic Combustion of Aromatic Solvents","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2024-11-25 12:16:58","doi":"10.21203/rs.3.rs-5365890/v1","editorialEvents":[{"type":"communityComments","content":0}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"01863b35-5257-49f6-9629-b6238b605169","owner":[],"postedDate":"November 25th, 2024","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[],"tags":[],"updatedAt":"2025-05-20T04:08:31+00:00","versionOfRecord":[],"versionCreatedAt":"2024-11-25 12:16:58","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-5365890","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-5365890","identity":"rs-5365890","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}