Next Article in Journal
Eco-Friendly Conversion of Waste Zeolite Dust into Dual Oil/Water Affinity Sorbents via HPGR-Based Agglomeration–Deagglomeration
Previous Article in Journal
Development of a Tool to Support the Sustainable Management of Urban Living Labs as Platforms for Co-Creation
 
 
Search for Articles:
Title / Keyword
Author / Affiliation / Email
Journal
Article Type
 
 
Advanced Search
 
Section
Special Issue
Volume
Issue
Number
Page
 
Journals
Sustainability
Volume 17
Issue 10
10.3390/su17104358
sustainability-logo
Submit to this Journal Review for this Journal Propose a Special Issue
► Article Menu

Article Menu

share Share announcement Help format_quote Cite question_answer Discuss in SciProfiles
first_page
settings
Order Article Reprints
Font Type:
Arial Georgia Verdana
Font Size:
Aa Aa Aa
Line Spacing:
Column Width:
Background:
Article

High Performance Ru-CNx/CeO2 Catalyst for Catalytic Wet Oxidation of N-Methyldiethanolamine in Water

by
Yuantao Han
1,
Yuchuan Ye
1,
Wanjin Yu
2,
Shaohong Zang
3,
Lili Ji
3,
Shijie Li
3 and
Liuye Mo
1,*
1
School of Petrochemical & Environment, Zhejiang Ocean University, Zhoushan 316022, China
2
State Key Lab Fluorinated Greenhouse Gases Replacement Control and Treatment, Zhejiang Research Institute of Chemical Industry, Hangzhou 310023, China
3
National Engineering Research Center for Marine Aquaculture, Zhejiang Ocean University, Zhoushan 316022, China
*
Author to whom correspondence should be addressed.
Sustainability 2025, 17(10), 4358; https://doi.org/10.3390/su17104358
Submission received: 1 April 2025 / Revised: 25 April 2025 / Accepted: 9 May 2025 / Published: 12 May 2025
(This article belongs to the Section Pollution Prevention, Mitigation and Sustainability)
Browse Figures
Review Reports Versions Notes

Abstract

:
The synthesis of high performance catalysts for the catalytic wet oxidation (CWO) of N-methyldiethanolamine (MDEA) in water remains a challenge, and is a topic of considerable importance in relation to sustainability. In this paper, a Ru-CNx/CeO2 catalyst was synthesized through a modified impregnation process for the CWO of MDEA, exhibiting a high activity of 80% COD removal at 180 °C and 2.5 MPa. EPR, Raman, and XPS characterizations revealed that the CNx species facilitated the reduction in Ru4+ to Ru0 species and enhanced the Ru–Ce interaction to form a high-density Ru-O-Ce structure with Ce3+ sites, which strongly correlate to the generation of oxygen vacancies. The oxygen vacancies enabled the adsorption and activation of the oxygen, generating active species (h+, ·O2−, and ·OH) that effectively oxidized the MDEA during the catalytic reaction.

Graphical Abstract

1. Introduction

N-Methyldiethanolamine (MDEA) is widely utilized as an absorbent for the desulfurization and decarbonization of Claus tail gas, natural gas, oil field gas, and coal gas, owing to its low corrosiveness and minimal regeneration energy requirements [1,2]. However, the discharge of MDEA-containing wastewater poses significant environmental concerns because of its high chemical oxygen demand (COD) and resistance to biodegradation [3]. This wastewater with refractory organic MDEA presents a major challenge for conventional biochemical treatment systems and necessitates effective pretreatment strategies for its safe disposal.
The common means to treat the MDEA wastewater include biological, physical, and chemical methods [4,5,6]. Nonetheless, the methyl groups (CH3) in MDEA molecules exhibit biological resistance, rendering them difficult to degrade via direct biotreatment, with up to 28 days required to achieve a 96% total organic carbon (TOC) removal [7]. Thermal persulfate treatment of MDEA-contaminated wastewater has shown 85.31% removal within two hours, with the activation energy required for the thermal activation of persulfate reaching 87.11 kJ/mol [8], but its large-scale industrial application is constrained by high energy consumption and unstable treatment efficiency. Photocatalytic treatment of MDEA-contaminated wastewater has shown that the TOC removal efficiency does not exceed 50%, as the remaining TOC is attributed to the refractory intermediates formed during the reaction [6,9]. From the above-reported literature, it can be seen that there is a lack of effective methods for treating high concentrations of MDEA in wastewater.
Catalytic wet oxidation (CWO) employs oxygen as an oxidant to produce reactive species including superoxide radicals (·O2−) and hydroxyl radicals (·OH) with −0.33 eV and 2.8 eV redox potential, which participate in advanced oxidation processes to convert harmful organic substances into environmental benign compounds under catalytic conditions [10,11,12,13]. The CWO technique has been widely applied to treat wastewater with a high concentration of organic pollutants for its advantages of excellent degradability with low energy consumption and high treatment capacity [14,15,16]. The catalytic activity of a catalyst is crucial for an effective CWO technique. Noble metals (Ru, Pt, Pd, Au, Rh, and Ir)-based catalysts have shown excellent activities and chemical stabilities in the CWO reactions [17,18,19,20,21]. Ru-based catalysts have attracted significant interest owing to their cost effectiveness and excellent catalytic activity. The CWO of BPA (bisphenol A) using Ru-ZrO2 catalyst at 100 °C and 20 bar pressure achieved a 90% conversion of BPA within 3 h. This enhanced performance is attributed to between Ru and zirconia, enhancing its catalytic activity [22]. The Ru-ZrO2/CeO2 catalyst demonstrated exceptional catalytic performance in the CWO of phenol, maintaining stable removal efficiencies of 100% for phenol and 96% for TOC throughout a continuous 100 h operation at 300 °C [23]. Maleic acid subjected to treatment with the Ru/CeO2 catalyst via CWO exhibited complete mineralization under operating conditions of 160 °C and 20 bar [24]. The high concentrations of oxygen vacancies and the interaction between the metal and the support on the Ru/CeO2 catalyst account for its excellent catalytic performance, resulting in 90% BPA degradation at 180 °C and 30 bar [25].
The aforementioned literature review shows that Ru/CeO2 catalysts exhibit excellent catalytic performances in CWO. However, the synergistic effect of oxygen vacancies and Ru species on the Ru/CeO2 catalyst has not been used for the breakdown of MDEA. Additionally, the g-C3N4 is widely recognized for its attraction in the field of catalysis due to its high activity and low cost [26,27]. Renji et al. [28] prepared a g-C3N4/TiO2-Ag2O composite catalyst by calcination and hydrothermal method, resulting in 94.5% degradation of Rhodamine B (RhB) degradation within 60 min via photocatalysis. The enhanced catalytic activity is ascribed to the synergistic interaction between Ag/Ag2O and the heterojunctions formed within the g-C3N4/TiO2 composite. Karu et al. [29] reported that the Pd/g-C3N4 catalyst demonstrates excellent catalytic activity in reducing p-nitrophenol in wastewater by sodium boron hydride. This enhancement is attributed to the synergistic effect between Pd and g-C3N4, which prevents the oxidation of Pd nanoparticles and promotes the stability of the catalyst.
Notably, few studies have been reported on the application of the CWO method for MEDA wastewater treatment, despite its considerable superiority over alternative treatment methods. Zhao et al. studied the removal of MDEA by CWO on the catalyst of activated carbon (AC), achieving 98% conversion at 230 °C, a pressure of 5.0 MPa, and a residence time of 90 min [30]. Previous studies have demonstrated that the redox between Ce4+ and Ce3+ in CeO2 facilitates the formation of oxygen vacancies, thereby enhancing metal-support interactions and improving the catalytic activity and stability [31]. Furthermore, it has been demonstrated that the incorporation of g-C3N4 into a metal or metal oxide catalyst strengthens the interactions between catalytic components, thereby enhancing catalytic efficacy [32]. Moreover, ruthenium-based catalysts have been employed in CWO processes due to their comparatively lower cost relative to other noble metals and their superior catalytic performance [33]. Inspired by the literature, in this study, a series of Ru-CNx/CeO2 catalysts were synthesized through the modified impregnation method for the CWO of MDEA under mild reaction conditions, aligning with sustainability goals by minimizing energy consumption while maintaining high degradation efficiency of recalcitrant organic contaminants.

2. Experimental

2.1. Catalyst Preparation

The 3%Ru-XCNx/CeO2 catalysts were prepared using the modified impregnation method developed by our group [34], in which 3% and X (X = 0 wt.%, 5 wt.%, 10 wt.%, and 20 wt.%) mean the nominal weight percents of Ru and CNx. The catalyst of 3%Ru-10%CNx/CeO2 catalyst, which was abbreviated as Ru-CNx/CeO2, was adopted to describe the preparation procedure. Specifically, 1.16 g of melamine (Sinopharm Chemical Co., Ltd., Shanghai, China) and 6.20 mL of Ru (NO)3 aqueous solution (Sino-platinum Metals Co., Ltd., Kunming, China) with Ru concentration of 0.01 g/mL and 15.0 g of deionized water were mixed together to form a suspension solution. Then, the above solution was impregnated onto 4.94 g of Ce(OAC)3·xH2O (Aladdin Industrial Corporation, Shanghai, China) powder with evenly stirring and drying at 80 °C for around 12 h. Finally, the sample was calcined at 550 °C for 4 h in a nitrogen atmosphere. The preparation procedure of Ru-CNx/CeO2 is shown in Figure S1. For comparison, the 3%Ru/CeO2 and the 10%CNx/CeO2 abbreviated as Ru/CeO2 and CNx/CeO2 were also prepared just as the procedure of the Ru-CNx/CeO2 except no melamine and Ru (NO)3 aqueous solution were added, respectively.
The authentic loadings of Ru and CNx on the synthesized Ru-CNx/CeO2 catalyst are 1.16 and 5.3 wt.%, which were measured by ICP-OES and TG-DTA as described in Supporting Information. Although the real loadings of Ru and CNx on the catalysts are different from the nominated loadings, the designed loadings are still adopted in the names of the prepared catalysts.

2.2. Catalyst Characterization

The X-ray powder diffraction (XRD) patterns of the prepared catalysts were acquired on a Rigaku Miniflex600 (Rigaku Corporation, Tokyo, Japan) (Cu Kα,40 kV, 15 mA) scanning within a 2θ range between 20 and 90° with 0.02° steps and 10 s acquisition per step at room temperature. Furthermore, the crystal sizes were calculated by the Scherrer formula.
Scanning electron microscopy (SEM) analysis was conducted on a FEI Quanta 650 microscope (FEI Company, Hillsboro, OR, USA) with an energy-dispersive X-ray spectroscopy (EDS) system.
The specific surface areas of the catalysts were determined using a Micromeritics ASAP 2460 instrument (Micromeritics Instrument Corporation, Norcross, GA, USA) through the Brunner–Emmett–Teller (BET) method with N2 adsorption at 77 K within the pressure range of 0.05–0.25 P/P0.Prior to measurement, the samples were degassed in a vacuum at 200 °C for 6 h.
Transmission electron microscopy (TEM) images were taken on a FEI Talos F200S instrument (FEI Company, Hillsboro, OR, USA). Prior to TEM measurements, the samples were put in ethanol and dispersed with ultrasonication for 15 min. Then, a few drops of the resulting suspension were dropped onto ultrathin copper grids and air-dried before imaging.
Electron Paramagnetic Resonance (EPR) characterizations were conducted using a JES-FA300 instrument (JEOL Ltd., Tokyo, Japan). For the analysis, 0.05 g of a catalyst was ultrasonically dispersed in 10 mL of methanol, followed by mixing 200 μL of the suspension with 200 μL of 50 mM DMPO or TEMPO solution for radical detection under 30 and 180 °C. Moreover, the oxygen vacancies on the samples were directly measured on the instrument at 30 and 180 °C without the addition of DMPO or TEMPO solution.
The UV-Raman characterizations were performed on a Lab Ram HR Evolution Raman spectrometer (HORIBA Ltd., Kyoto, Japan) equipped with a 325 nm. Each spectrum was recorded over 10 scans at a spectral resolution of 0.50 cm−1. The laser exposure time and accumulation number were set at 5 s and 10 times. A silicon single crystal with a peak at 520.7 cm−1 was used to calibrate the UV Raman spectrometer.
The X-ray photoelectron spectroscopy (XPS) spectra were acquired on a Thermo Fisher Nexsa spectrometer (Thermo Fisher Scientific, Waltham, MA, USA) equipped with an Al Kα radiation source (12 kV, 6 mA). The C1s peak of adventitious carbon at 284.8 eV was adopted to calibrate the binding energies.

2.3. Catalytic Wet Oxidation (CWO) of MDEA

Commonly, 100 mg of a catalyst and 100 mL of a pre-formed MDEA aqueous solution (with a concentration of 1000 ppm/L) were introduced into the slurry reactor (WATTCAS). Unless otherwise stated, the reactions were conducted under the following conditions: PO2 = 2.5 MPa, reaction temperature = 180 °C, reaction time = 30 min, and stirring speed = 900 r/min. Following the reaction, the mixture was cooled to room temperature and the solid catalyst was separated by filtration before analysis.
The catalytic activities of the catalysts were evaluated based on the removal of COD and TOC. For COD measurements, 2 mL of pure water (as a blank) and the samples were separately introduced into colorimetric tubes. The subsequent step involves the introduction of 1 mL of COD detection powder reagent, followed by 3 mL of COD catalyst powder into a tube. This mixture is then digested in reactor LH-TX6 (Lu Heng Biological, Hangzhou, China) at a temperature of 165 °C for 20 min. After cooling to room temperature, the COD measurements were performed on the Analyzer LH-T725 (Lu Heng Biological).
The total organic carbon (TOC) of the MDEA-containing water was measured using a Multi-C/N-3100 (Analytik Jena GmbH, Jena, Germany) with a column temperature of 750 °C and oxygen (99.9%) as the carrier gas.
The removal of COD and TOC (X%) were determined using the following formula:
X % = C 0 − C C 0 × 100 %
where C0 stands for the initial value of COD/TOC in MDEA-containing water (mg/L), while C denotes the COD/TOC value in the MDEA-containing water (mg/L) after reaction. The average value was obtained from six experimental data points under the same conditions.

3. Results and Discussion

3.1. XRD Results

The crystal structures and phase compositions of the prepared samples were investigated by XRD. As illustrated in Figure 1, the XRD patterns of CeO2, Ru/CeO2, CNx/CeO2, and Ru-CNx/CeO2 catalysts show characteristic diffraction peaks at 28.6, 33.2, 47.6, and 56.6°, corresponding to the (111), (200), (220), and (311) crystal planes of the CeO2 with a cubic fluorite-type structure (PDF#75-0076) [35,36]. The CNx sample exhibits two distinct peaks at 12.7 and 27.7°, corresponding to (002) and (100) plane of the C3N4 phase (PDF#87-1523) [37]. It is noteworthy that no diffraction CNx or Ru species peaks are observed in the Ru/CeO2, CNx/CeO2, and Ru-CNx/CeO2 samples, which may be attribute to the high dispersion of Ru and CNx species, amorphous phase, or too low loading in the catalysts [21,38].
The crystal sizes of CeO2 in the Ru/CeO2, CNx/CeO2, and Ru-CNx/CeO2 catalysts are presented in Table 1, and determined as 9.9, 8.8, 8.0, and 6.3 nm, respectively. Evidently, the Ru-CNx/CeO2 catalyst exhibits the smallest CeO2 crystal size, indicating that the species CNx on the CeO2 supports may inhibit the growth of CeO2 crystals [39].

3.2. SEM and BET Surface Area (SA) Results

The SEM technique is widely used for characterizing and analyzing catalyst morphology. Figure 2a–d exhibits the SEM images of CeO2, Ru/CeO2, CNx/CeO2, and Ru-CNx/CeO2 samples, respectively. Figure 2a shows that the pure CeO2 catalyst mainly presents a nanosheet structure. The Ru/CeO2 catalyst has a more uniform, thinner, and smaller nanosheet than the pristine CeO2 sample owing to the introduction of Ru, as shown in Figure 2b. Meanwhile, the CNx/CeO2 catalyst also displays nanosheet with clean surfaces, as depicted in Figure 2c. Interestingly, Figure 2d shows that the Ru-CNx/CeO2 catalyst has a nanosheet structure similar to that of the CeO2 sample, but with a thinner thickness (shown in the white frame). The above results demonstrate that the introduction of Ru and CNx preserves the nanosheet morphology of CeO2 sample while significantly reducing the nanosheets thickness.
The SA and pore volume of the catalysts were measured using the N2 adsorption/desorption isotherms. As illustrated in Figure S2a, the physisorption isotherms of all samples are classified as type IV, indicating that all the samples show mesoporous structures. The values of the SA, pore volume, and average pore diameter of the samples are summarized in Table 1. Specifically, the SA of the samples decrease significantly from 98.1 to around 60 m2/g after the CNx or Ru components were supported on the CeO2. The SA of Ru/CeO2, CNx/CeO2, and Ru-CNx/CeO2 samples fall between 50 and 60 m2/g. Figure S2b displays that the pore volume of the CeO2 sample is 0.17 cm3/g, and the average pore size is 6.8 nm. The pore volume of the Ru/CeO2 catalyst is drastically decreased to 0.07 cm3/g with an average pore size of 5.5 nm. However, the pore volume values of CNx/CeO2 and Ru-CNx/CeO2 samples are 0.14 and 0.15 cm3/g, respectively, which are similar to that of the CeO2 sample. The average pore sizes of the CNx/CeO2 and Ru-CNx/CeO2 samples are 11.1 and 12.5 nm, respectively. The increase in pore size can be attributed to the pyrolysis effect of melamine evolving gases during thermal pretreatment to form a larger pore structure [40]. The above results indicate that the introduction of CNx to the samples can effectively prevent a decrease in catalyst pore volume.

3.3. TEM and Mapping Results

Figure 3 depicts the TEM images and elemental mappings of Ru-CNx/CeO2 catalyst. Figure 3a reveals that the Ru-CNx/CeO2 sample displays a morphology of thin nanosheets, demonstrating agreement with the SEM results. The HR-TEM image of Ru-CNx/CeO2 sample in Figure 3b shows lattice planes of 0.32 and 0.31 nm that are assigned to the planes of CeO2 (111) and RuO2 (110) [41,42]. Figure 3c displays the particle size distribution of CeO2 for the Ru-CNx/CeO2 sample. As observed, the majority of CeO2 particles are in the range of 4.2 to 6.7 nm, with an average particle size of 5.4 nm, which matches well with the Scherrer equation result (6.3 nm). Nevertheless, the EDS mapping images and corresponding elemental analysis reveal a uniform distribution of C, N, Ru, O, and Ce elements on the surface of the Ru-CNx/CeO2 catalyst (Figure 3d,e). Notably, Ru and CNx species are not observed in the TEM images, possibly due to their high dispersion and low concentrations. These results are in agreement with the XRD results.

3.4. UV Raman Results

The lattice distortions and structural defects are analyzed using UV Raman spectroscopy. Figure 4a displays the UV Raman spectra of the different samples. The peaks at 1340 and 1598 cm−1 are observed on the Ru-CNx/CeO2 and CNx/CeO2 catalysts, which are attributed to the carbon species with D (defect) and G (graphitization) [43,44]. Additionally, the peak near 461 cm−1 is observed clearly on all the prepared catalysts, corresponding to the F2g Raman vibrational mode of CeO2 [45,46]. Figure 4b displays the magnified spectral feature centered at 461 cm−1 associated with the synthesized catalyst. The Ru-CNx/CeO2 catalyst exhibits the most pronounced Raman shift from 461 to 454 cm−1, arising from the formation of Ru-O-Ce structures that induce lattice distortion in the CeO2 matrix, accompanied by a red shift in the F2g vibrational mode [34].

3.5. EPR Results

EPR is a powerful technique for characterizing the oxygen vacancies (Ov) in a catalyst [47]. Figure 5 presents the EPR spectra for the Ru/CeO2 and Ru-CNx/CeO2 catalysts. Notably, both catalysts exhibit prominent signals at g = 2.0004 which are attributed to Ov [48]. The Ru-CNx/CeO2 catalyst exhibits a stronger signal compared to the Ru/CeO2 catalyst, indicating a higher concentration of Ov present on the Ru-CNx/CeO2 catalyst. Consequently, the incorporation of CNx significantly augments the Ov content on the catalyst surface, thereby enhancing oxygen adsorption and activation capacities to generate more reactive oxygen species [49,50].
The generation of active species is regarded as the critical step in the CWO process [51]. In the reaction, the holes (h+), as a strong oxidizing species, can directly attack organic pollutants or promote the formation of ·OH [52]. In addition, ·O2− and ·OH species act as oxidizing agents, either directly oxidizing organic pollutants or indirectly generating reactive species, which enhances the degradation of organic [53,54]. The 2,2,6,6-tetramethylpiperidinooxy (TEMPO) and 5,5-diethyl-1-pyridine N-oxide (MDPO) are always used to study and detect radical reactions as radical scavengers [55,56,57]. Figure 6 shows that the signals of TEMPO-h+, MDPO-·O2−, and MDPO-·OH species over the Ru-CNx/CeO2 catalyst are stronger at 180 °C than those at room temperature. The results demonstrate that the h+ (holes) and ·O2− and ·OH species exist on the Ru-CNx/CeO2 catalyst at the reaction temperature of 180 °C.

3.6. XPS Results

The full XPS survey spectrum of the Ru-CNx/CeO2 catalyst, as depicted in Figure 7a, confirms the presence of C, N, Ru, O, and Ce elements within the composite. This analytical evidence substantiates the successful incorporation of CNx and Ru active species into the catalyst structure. The peak of C1s at 288.8, 286.4, and 284.8 eV are attributed to the N–C=N bonds of the triazine ring, C–O=C bonds, and C=C group of CNx, as shown in Figure 7b [58]. Additionally, the N1s (Figure 7c) can be categorized into three peaks at 397.4, 399.1, and 400.7 eV, corresponding to the sp2-hybridized nitrogen (C-N-C), sp3-hybridized nitrogen (N-[C]3), and amino functional groups with a hydrogen atom (C-NH), respectively [59]. Table 2 presents the surface element concentrations and ratios on the Ru-CNx/CeO2 and Ru/CeO2 catalysts. The C/N ratio of the Ru-CNx/CeO2 catalyst attains a value of 7.24, which substantially exceeds that of C3N4 (0.75). This indicates that the CNx phase is not a pure C3N4 phase. Evidently, the decomposed product from melamine on the Ru-CNx/CeO2 catalyst differs significantly from that of the pure melamine. The experimental results suggest that the existence of Ru and Ce constituents significantly disrupts the thermal decomposition behavior of melamine.
Due to signal interference between Ru3d and C1s, the Ru3p peaks were adopted to be analyzed as depicted in Figure 7d. The characteristic peaks of Ru0 (463.4, 463.9 eV) and Ru4+ (460.8 and 461.4 eV) are present on the Ru/CeO2 and Ru-CNx/CeO2 catalysts [60]. Furthermore, the Ru0 and Ru4+ species on the Ru-CNx/CeO2 catalyst exhibit a pronounced shift toward higher binding energies relative to those observed on the Ru/CeO2 catalyst, implying a markedly enhanced Ru species-support interaction upon the incorporation of CNx [61]. The surface compositions of Ru on the Ru-CNx/CeO2 catalyst and the Ru/CeO2 catalyst listed in Table 2 are 3.78 and 2.73 at.%, indicating that the dispersion of Ru on the Ru-CNx/CeO2 catalyst is significantly promoted by the addition of CNx. Furthermore, the ratio of the Ru0/(Ru0 + Ru4+) on the Ru-CNx/CeO2 catalyst is 0.72, which is significantly higher than that of Ru/CeO2 (0.42) catalyst. The results indicate that CNx promotes the reduction in Ru species during the thermal pretreatment process, resulting in more Ru0 species in the catalyst [62].
The O1s profiles of the catalysts are depicted in Figure 7e. The peaks at 529.1 and 529.4 eV are ascribed to surface lattice oxygen (OL). Additionally, peaks at 531.1 and 531.6 eV correspond to adsorbed oxygen species (OA) [63,64]. The higher binding energies of OL and OA on the Ru-CNx/CeO2 catalyst suggest a stronger ability for oxygen adsorption and activation [61]. It is noteworthy that OA/OL ratios on the catalyst increase from 0.45 to 0.72 with the addition of CNx. This indicates that the Ru-CNx/CeO2 composite exhibits a higher absorbed oxygen content and an enhanced capacity for oxygen adsorption from the surrounding environment, thereby promoting the formation of reactive oxygen species and improving catalytic performance [65,66].
The four characteristic peaks corresponding to Ce4+ are observed at 916.6, 907.5, 900.9, 907.5, 916.6 and 888.7, 898.3, and 882.4 eV, while the peaks at 885.2 and 903.4 eV are attributed to Ce3+ (Figure 7f) [67,68]. It is well known that the Ce3+ species contribute to the formation of oxygen vacancy on the CeO2 surface. Furthermore, an elevated surface concentration of Ce3+ directly correlates with a greater abundance of surface oxygen vacancies. Quantitative analysis shows that the Ce3+/Ce4+ ratios for the Ru-CNx/CeO2 and Ru/CeO2 catalytic systems were determined to be 0.25 and 0.17, respectively. These results imply that the Ru-CNx/CeO2 catalyst obtains more Ce3+ species and more oxygen vacancies, resulting in a stronger ability of oxygen adsorption and activation [42,69,70]. These results are agreement with the EPR results.

3.7. Catalytic Performances of the Prepared Catalysts

Figure 8 shows the catalytic activities of the prepared catalysts under reaction conditions at 2.5 MPa and 170 °C. The 10%CNx/CeO2 catalyst shows only 8% conversion of COD. The COD conversions of Ru/CNx and Ru/CeO2 catalysts are 10 and 20%, respectively. Notably, the introduction of CNx species leads to a significant increase in the activity of the Ru-CNx/CeO2 catalyst, which increases dramatically to 55%. It indicates that the doping of CNx species markedly improves the catalytic performance.
Subsequently, the impact of CNx loading on the performances of Ru-XCNx/CeO2 catalysts is investigated as depicted in Figure 8. The catalytic activities of Ru-XCNx/CeO2 catalysts exhibit a volcano-type trend depending on the loading of CNx. With the increase of CNx loading from 5 to 10%, the COD removal of Ru-XCNx/CeO2 catalyst increases from 45 to 55%. However, the activity of the catalyst progressively declines to 35% with a further increase in CNx loading from 10 to 20%. It demonstrates that excessive CNx loading may cover the active sites of Ru leading to a decrease in activity [61]. Among them, the Ru-CNx/CeO2 catalyst demonstrates the highest COD removal efficiency of 55%, proving that it possesses the strongest synergistic effect of CNx and Ru species.

3.8. Effect of Reaction Temperature

Reaction temperature is a critical factor in the CWO of organic compounds. According to the law of Arrhenius, the rate of the reaction increases with temperature [71]. Hence, the influence of the reaction temperature from 120 to 200 °C on the removal of COD and TOC over the Ru-CNx/CeO2 was investigated. As shown in Figure 9a,b, the removal of COD and TOC increases from 4 to 90% and 1 to 82% with the temperature increasing from 120 to 200 °C. Nevertheless, when the temperature is elevated from 180 to 200 °C, the removal efficiency of TOC and COD exhibits no substantial improvement. This phenomenon is likely attributable to the generation of recalcitrant intermediate compounds during the reaction process, thereby limiting further degradation [72].

3.9. Effect of Reaction Pressure

The oxygen pressure always has a strong relationship with the performance of the catalyst in the CWO reaction. Therefore, the influence of the oxygen pressure on the catalytic performance of Ru-CNx/CeO2 catalyst was investigated, as illustrated in Figure 10. The efficiency of COD removal significantly increases from 58 to 74% as the oxygen pressure is from 0.1 to 1.0 MPa. Notably, a further increase in oxygen pressure from 1.0 to 3.0 MPa, slightly improves the COD removal efficiency from 74 to 80%. Generally, the high oxygen pressures, i.e., high dissolved oxygen concentrations, facilitate the breakdown of organics during the CWO. Nevertheless, the above results indicate that excessively high dissolved oxygen concentrations do not significantly influence the degradation of MDEA, suggesting that the dissolved oxygen concentrations are above the stoichiometric concentration needed for the reaction as the oxygen pressure is >1.0 Mpa [30].

3.10. Effect of Inorganic Salt

In practical applications, inorganic salts always exist in wastewater. And an excessive amount of salts can inhibit the pollutant removal efficiency owing to their significant capacity for radical scavenging [73]. Additionally, reactive species can directly interact with inorganic ions, resulting in byproduct formation and hindering the degradation of contaminants [74]. Therefore, the effect of NaCl and Na2SO4 salts on the catalytic activity of the Ru-CNx/CeO2 catalyst was investigated. Figure 11a shows that the NaCl concentration has a notable influence on COD removal. The COD removal decreases from 78 to 55% as the NaCl concentration increases from 0 to 9%. Regarding Na2SO4, the COD removal is also rapidly decreased to 65% in the presence of 1% Na2SO4 concentration. However, the COD removal slightly decreases as the Na2SO4 concentration further increases to 9%. Although the inorganic salts in the MDEA solution inhibit the removal of COD, the Ru-CNx/CeO2 catalyst remains active with 55 and 65% removal of COD in the presence of high NaCl and Na2SO4 concentrations of 9%.

3.11. Effect of pH

The solution’s pH is a critical parameter influencing the efficiency of a catalyst to oxidize pollutants [75]. Therefore, the pH effect on the removal of COD was performed in a range between 3 and 13 under 2.5 MPa and 180 °C. It is observed that the degradation of MDEA increases from 45 to 80% as the pH rises from 3 to 10. Specifically, when pH is below 5, the overabundance of H+ ions may deplete OH−, significantly suppressing the generation of ·OH and ultimately resulting in a noticeable reduction in the oxidation efficiency of MDEA. In contrast, for a pH between 7 and 10, the increased OH− may promote ·OH generation, which could enhance the degradation efficiency of MDEA [76]. However, the degradation efficiency markedly diminishes as the pH increases to 12, as depicted in Figure 12a. This phenomenon is likely attributable to the scavenging of free radicals under conditions of elevated alkalinity [77]. Notably, the highest degradation efficiency is obtained at pH = 10, reaching 80%. Figure 12b shows that the pH values of the solutions significantly decrease after the reaction, indicating that the acidic intermediates are generated and the amine group is destroyed during the CWO [78]. The aforementioned findings conclusively demonstrate that the Ru-CNx/CeO2 catalyst exhibits applicability across a broad spectrum of pH values in aqueous solutions, a critical characteristic for industrial implementation.

3.12. Ru-CNx/CeO2 Reusability and Stability

The stability of the Ru-CNx/CeO2 catalyst was evaluated through five cycles under the reaction conditions of T = 180 °C and P = 2.5 Mpa, as shown in Figure 13. The catalytic activity is gradually decayed with the reuse times. And the removal of MDEA over the Ru-CNx/CeO2 catalyst decreases from 80 to 60% after five consecutive reactions. In order to investigate the cause of the decrease in the catalyst activity, the used Ru-CNx/CeO2 catalyst was characterized further by N2 adsorption—desorption isotherm, TG and XPS techniques. The SA and pore size of the used Ru-CNx/CeO2 catalyst decrease from 53.0 m2/g and 12.5 nm to 47.2 m2/g and 9.6 nm, as depicted in Table 1. The TG-DTG analysis indicates that the weight loss of the used Ru-CNx/CeO2 is 3% at 525 °C (Figure S3). This is attributed to the decomposition of organic matter covering the catalyst surface during the reactions. In addition, the XPS results for the used Ru-CNx/CeO2 catalyst are summarized in Table 2. The Ce3+/Ce4+ ratio of the used Ru-CNx/CeO2 catalyst remains stable, only slightly decreasing from 0.25 to 0.24. However, the ratio of Ru0+/(Ru4+ + Ru0+) drops from 0.72 to 0.61 after five reaction cycles. Furthermore, the OA/OL ratio decreases from 0.72 to 0.51.
The above characterization results demonstrate that the catalyst surface of the Ru-CNx/CeO2 catalyst becomes progressively covered by the organic matter and part of Ru0 is oxidized to Ru4+ species during the reactions, which results in a decrease in the oxygen adsorption capacity. Additionally, the leaching of the active metal of Ru into the water was determined by ICP-OES analysis after the fifth cycle, as shown in the Supporting Information, Table S1. The concentrations of Ru in the reaction mixtures after the first and third cycles are 2.66 and 0.34 mg/L. Notably, after five cycles, the Ru content in the reaction mixture increases to 0.54 mg/L, possibly due to intensified mechanical wear of the catalyst during prolonged operation, which exacerbates the leaching of Ru. These results indicate a significant leaching of Ru during the reaction process, contributing to the observed decline in catalytic performance. The above factors collectively result in a decline in the catalytic activity of Ru-CNx/CeO2 catalyst after five reaction cycles.

3.13. Discussion

3.13.1. The Promoted Effect of the CNx

The catalytic activity measurements reveal that the CNx species are crucial for the excellent catalytic performance of the Ru-CNx/CeO2 catalyst. To explore the origin of the CNx effect, a series of characterization techniques were used, as described in the Results Section. The XRD and TEM results exhibit that the addition of CNx prevents the growth of CeO2 and Ru species crystals/particles during the pretreatment of the Ru-CNx/CeO2 catalyst. And the TEM and XPS analyses confirm that the Ru species are uniformly dispersed across the support. It is widely recognized that a diminutive particle or crystal dimensions play a pivotal role in enhancing the catalytic efficacy of a catalyst [34,79]. In addition, the introduction of CNx promotes the formation of thinner nanosheets and richer pore volume for the Ru-CNx/CeO2 catalysts, enhancing the MDEA contact and adsorption onto the catalyst, which facilitates the catalytic oxidation of MDEA. The XPS results show that a large proportion of the Ru3+ species are reduced to Ru0 by the melamine or CNx during the thermal pretreatment. The Ru0 species facilitate the adsorption of oxygen molecules, leading to the formation of active adsorbed oxygen species that participate in catalytic oxidation reactions [80,81]. Additionally, the CNx promotes electron transfer, making the species of Ru binding energy shift to high binding energy, which suggests a strong interaction between Ru species and support [82]. The UV Raman and ESR results demonstrate that the doping of the CNx species enhances the Ru–Ce interactions and promotes the formation of a more Ru-O-Ce structure [34], which closely relates to the oxygen vacancies. A significant amount of Ce3+, i.e., oxygen vacancies, are formed on the Ru-CNx/CeO2 catalyst, which significantly improves the oxygen adsorption to form OA species. Therefore, oxygen adsorption plays a critical role in facilitating the activation of molecular oxygen (O2) to generate reactive oxygen species (ROS). These ROS, such as ·OH and ·O2−, directly participate in the oxidative degradation of organic pollutants [12]. The above effects of the CNx species mainly account for the excellent catalytic performance of the Ru-CNx/CeO2 catalyst for the CWO reaction of MDEA.

3.13.2. The Active Species on the Ru-CNx/CeO2 Catalyst

It is widely accepted that triethanolamine (TEOA), 1,4-benzoquinone (BQ) and isopropanol (IPA) can effectively quench the active species of holes (h+), ·O2−, and ·OH [53,83,84], respectively. For the purpose of distinguishing the active species on the catalyst, a small amount of TEOA, BQ, and IPA were added to the reaction solutions, respectively. Figure 14 shows that the degradation efficiencies of MDEA decrease significantly to 37%, 2%, and 1% with the addition of BQ, TEOA, and IPA to the reaction system. This demonstrates that h+, ·O2−, and ·OH are the main contributors to MDEA conversion.
Meanwhile, the EPR results indicate that the Ru-CNx/CeO2 catalyst contains abundant Ov and the interaction between CNx and Ru/CeO2 promotes the adsorption of O2. Therefore, the mechanism of the radical process is deduced. Under high-temperature conditions, oxygen vacancies are generated on Ru-CNx/CeO2. The h+ sites react with H2O/OH− to produce ·OH [52], and O2 can be adsorbed by oxygen vacancies on the Ru-CNx/CeO2 catalyst, forming ·O2− through electron transfer. The active species h+, ·O2−, and ·OH could further oxidize and degrade MDEA [30,85].

4. Conclusions

In this research, Ru-XCNx/CeO2 catalysts were synthesized through the modified impregnation method for the CWO of MDEA. It was found that the CNx species significantly promoted the catalytic activity of Ru-CNx/CeO2 catalyst, achieving the highest COD conversion of 80% at 180 °C for 0.5 h and PO2 = 2.5 MPa. The CNx species enhance the dispersion of the Ru species and the CeO2 crystals with high ratios of Ru0/Ru4+ and Ce3+/Ce4+. Furthermore, the doping of CNx promotes more Ru ions to enter the CeO2 lattice forming a high content of Ru-O-Ce structures. The structure can effectively create rich oxygen vacancies. Oxygen vacancies are critical in enhancing the adsorption and activation of MDEA. Concurrently, they facilitate the adsorption and dissociation of O2 and H2O, generating reactive species (h+, ·O2−, and ·OH) that exhibit robust oxidative activity, thereby driving the efficient oxidative degradation of MDEA. The innovative design of Ru-XCNx/CeO2 catalysts not only advances the efficiency of MDEA degradation through the doping of CNx but also aligns with sustainability goals by enabling energy-efficient wastewater treatment.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/su17104358/s1, Figure S1: Synthesis process of the Ru-CNx/CeO2; Figure S2: (a) N2 adsorption–desorption isotherms (b) BJH pore size distributions. Figure S3: TG-DTG analysis of Ru-CNx/CeO2 Fresh (a) and Used (b). Table S1: ICP-OES Ru leaching in water.

Author Contributions

Data curation, Y.H.; Writing—original draft, Y.H.; Writing—review & editing, Y.H., Y.Y., W.Y., S.Z., L.J., S.L. and L.M. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The original contributions presented in this study are included in the article/supplementary material. Further inquiries can be directed to the corresponding author.

Acknowledgments

We sincerely thank Huigang Wang from Zhejiang Normal University and Sufang He from Kunming University Science and Technology for the Raman and the ICP characterizations.

Conflicts of Interest

The authors declare no conflict of interest.

References

  1. Pal, P.; AbuKashabeh, A.; Al-Asheh, S.; Banat, F. Role of aqueous methyldiethanolamine (MDEA) as solvent in natural gas sweetening unit and process contaminants with probable reaction pathway. J. Nat. Gas. Sci. Eng. 2015, 24, 124–131. [Google Scholar] [CrossRef]
  2. Ghanbarabadi, H.; Gohari, F.K.Z. Optimization of MDEA concentration in flow of input solvent to the absorption tower and its effect on the performance of other processing facilities of gas treatment unit in Sarakhs refinery. J. Nat. Gas. Sci. Eng. 2014, 20, 208–213. [Google Scholar] [CrossRef]
  3. Randall, T.L.; Knopp, P.V. Detoxification of specific organic substances by wet oxidation. Water Pollut. Control. Fed. 1980, 52, 2117–2130. [Google Scholar]
  4. Yang, L.; Chen, Z.; Zhang, D.; Liu, Y.; Han, Y.; Shen, J. Adsorption of dimethylamine from aqueous solution by manganese dioxide. Water Sci. Technol. 2011, 63, 45–50. [Google Scholar] [CrossRef] [PubMed]
  5. Fürhacker, M.; Pressl, A.; Allabashi, R. Aerobic biodegradability of methyldiethanolamine (MDEA) used in natural gas sweetening plants in batch tests and continuous flow experiments. Chemosphere 2003, 52, 1743–1748. [Google Scholar] [CrossRef]
  6. Samira Molareza, G.; Ahmadi, M.; Zinati Zadeh, A.A. Photochemical oxidation of methyldiethanolamine (MDEA) in aqueous solution by UV/K2S2O8 process. Int. J. Ind. Chem. 2016, 7, 1–8. [Google Scholar] [CrossRef]
  7. Wang, B.; Tian, K.; Xiong, X.; Ren, H. Treatment of overhaul wastewater containing N-methyldiethanolamine (MDEA) through modified Fe–C microelectrolysis-configured ozonation: Investigation on process optimization and degradation mechanisms. J. Hazard. Mater. 2019, 369, 655–664. [Google Scholar] [CrossRef] [PubMed]
  8. Li, Y.; Yue, D.; Wang, B.; Ren, H. Degradation of MDEA in aqueous solution in the thermally activated persulfate system. Environ. Technol. 2017, 38, 730–736. [Google Scholar] [CrossRef]
  9. Ali, R.; ABU, B.W.; Mislan, S.S.; Sharifuddin, M.A. Photodegradation of n-methyldiethanolamine over ZnO/SnO2 coupled photocatalysts. Sci. Iran 2010, 17, 124–130. [Google Scholar]
  10. Jiang, Y.; Li, S.; Fan, X.; Tang, Z. Recent advances on aerobic photocatalytic methane conversion under mild conditions. Nano Res. 2023, 16, 12558–12571. [Google Scholar] [CrossRef]
  11. Petrović, M.; Rančev, S.; Đorđević, M.P.; Najdanović, S.; Velinov, N.; Vučić, M.R.; Bojić, A. Electrochemically synthesized Molybdenum oxides for enhancement of atmospheric pressure non-thermal pulsating corona plasma induced degradation of an organic compound. Chem. Eng. Sci. 2021, 230, 116209. [Google Scholar] [CrossRef]
  12. Sun, W.; Wei, H.; Yang An, L.; Jin, C.; Wu, H.; Xiong, Z.; Pu, C.; Sun, C. Oxygen vacancy mediated La1-xCexFeO3-δ perovskite oxides as efficient catalysts for CWAO of acrylic acid by A-site Ce doping. Appl. Catal. B Environ. 2019, 245, 20–28. [Google Scholar] [CrossRef]
  13. Silva, J.A. Advanced Oxidation Process in the Sustainable Treatment of Refractory Wastewater: A Systematic Literature Review. Sustainability 2025, 17, 3439. [Google Scholar] [CrossRef]
  14. Le Phuong, T.; Besson, M. Catalytic wet air oxidation using supported Pt and Ru catalysts for treatment of distillery wastewater (cognac and sugarcane vinasses). Energies 2019, 12, 3974. [Google Scholar] [CrossRef]
  15. Luck, F. Wet air oxidation: Past, present and future. Catal. Today 1999, 53, 81–91. [Google Scholar] [CrossRef]
  16. Luck, F. A review of industrial catalytic wet air oxidation processes. Catal. Today 1996, 27, 195–202. [Google Scholar] [CrossRef]
  17. Kim, K.; Ihm, S. Heterogeneous catalytic wet air oxidation of refractory organic pollutants in industrial wastewaters: A review. J. Hazard. Mater. 2011, 186, 16–34. [Google Scholar] [CrossRef]
  18. Rabelo, A.C.; Rosario, A.V.; Trapp, M.A.; Forim, M.; Pereira, E.C. Catalytic wet air oxidation of Methyl Orange onto Pt and Pt–TiO2. J. Nanosci. Nanotechnol. 2016, 16, 10040–10047. [Google Scholar] [CrossRef]
  19. Yang, S.; Besson, M.; Descorme, C. Catalytic wet air oxidation of succinic acid over Ru and Pt catalysts supported on CexZr1−xO2 mixed oxides. Appl. Catal. B Environ. 2015, 165, 1–9. [Google Scholar] [CrossRef]
  20. Bernardi, M.; Le Du, M.; Dodouche, I.; Descorme, C.; Deleris, S.; Blanchet, E.; Besson, M. Selective removal of the ammonium-nitrogen in ammonium acetate aqueous solutions by catalytic wet air oxidation over supported Pt catalysts. Appl. Catal. B Environ. 2012, 128, 64–71. [Google Scholar] [CrossRef]
  21. Zhang, W.; Sun, W.; Zhang, Y.; Yu, D.; Piao, W.; Wei, H.; Liu, X.; Sun, C. Effect of inorganic salt on the removal of typical pollutants in wastewater by RuO2/TiO2 via catalytic wet air oxidation. Chemosphere 2023, 312, 137194. [Google Scholar] [CrossRef] [PubMed]
  22. Bensouilah, R.; Hammedi, T.; Ouakouak, A.; Ghorbel, A.; Ksibi, Z. Comparative study of the efficiency of different noble metals supported on zirconium oxide in the catalytic wet air oxidation of bisphenol-A solution. Chem. Phys. Lett. 2020, 761, 138022. [Google Scholar] [CrossRef]
  23. Wang, J.; Zhu, W.; Yang, S.; Wang, W.; Zhou, Y. Catalytic wet air oxidation of phenol with pelletized ruthenium catalysts. Appl. Catal. B Environ. 2008, 78, 30–37. [Google Scholar] [CrossRef]
  24. Oliviero, L.; Barbier Jr, J.; Duprez, D.; Wahyu, H.; Ponton, J.W.; Metcalfe, I.S.; Mantzavinos, D. Wet air oxidation of aqueous solutions of maleic acid over Ru/CeO2 catalysts. Appl. Catal. B Environl 2001, 35, 1–12. [Google Scholar] [CrossRef]
  25. Roy, S.; Barman, P. Unraveling the Structure-Dependent Activity of Ru/CeO2 Catalysts for Oxidative Degradation of Plastic Chemical Bisphenol A by Catalytic Wet Oxidation. Res. Sq. 2024, 1–28. [Google Scholar] [CrossRef]
  26. Li, Y.; Gu, M.; Shi, T.; Cui, W.; Zhang, X.; Dong, F.; Cheng, J.; Fan, J.; Lv, K. Carbon vacancy in C3N4 nanotube: Electronic structure, photocatalysis mechanism and highly enhanced activity. Appl. Catal. B Environ. 2020, 262, 118281. [Google Scholar] [CrossRef]
  27. Dong, G.; Jacobs, D.L.; Zang, L.; Wang, C. Carbon vacancy regulated photoreduction of NO to N2 over ultrathin g-C3N4 nanosheets. Appl. Catal. B Environ. 2017, 218, 515–524. [Google Scholar] [CrossRef]
  28. Rajendran, R.; Vignesh, S.; Sasireka, A.; Suganthi, S.; Raj, V.; Baskaran, P.; Shkir, M.; AlFaify, S. Designing Ag2O modified g-C3N4/TiO2 ternary nanocomposites for photocatalytic organic pollutants degradation performance under visible light: Synergistic mechanism insight. Colloid. Surf. A Physicochem. Eng. Asp. 2021, 629, 127472. [Google Scholar] [CrossRef]
  29. Saravanakumar, K.; Priya, V.S.; Balakumar, V.; Prabavathi, S.L.; Muthuraj, V. Noble metal nanoparticles (Mx= Ag, Au, Pd) decorated graphitic carbon nitride nanosheets for ultrafast catalytic reduction of anthropogenic pollutant, 4-nitrophenol. Environ. Res. 2022, 212, 113185. [Google Scholar] [CrossRef]
  30. Zhao, J.; Wei, Y.; Liu, Z.; Zhang, L.; Cui, Q.; Wang, H. Study on heterogeneous catalytic wet air oxidation process of high concentration MDEA-containing wastewater. Chem. Eng. Process 2022, 171, 108744. [Google Scholar] [CrossRef]
  31. Montini, T.; Melchionna, M.; Monai, M.; Fornasiero, P. Fundamentals and catalytic applications of CeO2-based materials. Chem. Rev. 2016, 116, 5987–6041. [Google Scholar] [CrossRef] [PubMed]
  32. Wang, H.; Guan, J.; Li, J.; Li, X.; Ma, C.; Huo, P.; Yan, Y. Fabricated g-C3N4/Ag/m-CeO2 composite photocatalyst for enhanced photoconversion of CO2. Appl. Surf. Sci. 2020, 506, 144931. [Google Scholar] [CrossRef]
  33. Zheng, Q.; Ren, K.; Qin, L.; Wang, X.; Ma, J. Efficient removal of piperazine by catalytic wet air oxidation using RuNiCe/γ-Al2O3-activated carbon. Environ. Sci. Water Res. Technol. 2024, 10, 1653–1665. [Google Scholar] [CrossRef]
  34. Ye, Y.; Xu, J.; Gao, L.; Zang, S.; Chen, L.; Wang, L.; Mo, L. CuO/CeO2 catalysts prepared by modified impregnation method for ethyl acetate oxidation. Chem. Eng. J. 2023, 471, 144667. [Google Scholar] [CrossRef]
  35. Balestrieri, M.; Colis, S.; Gallart, M.; Schmerber, G.; Ziegler, M.; Gilliot, P.; Dinia, A. Photoluminescence properties of rare earth (Nd, Yb, Sm, Pr)-doped CeO2 pellets prepared by solid-state reaction. J. Mater. Chem. C 2015, 3, 7014–7021. [Google Scholar] [CrossRef]
  36. Wu, C. Solvothermal synthesis of N-doped CeO2 microspheres with visible light-driven photocatalytic activity. Mater. Lett. 2015, 139, 382–384. [Google Scholar] [CrossRef]
  37. Sharma, P.; Sasson, Y. A photoactive catalyst Ru–gC3N4 for hydrogen transfer reaction of aldehydes and ketones. Green. Chem. 2017, 19, 844–852. [Google Scholar] [CrossRef]
  38. Huang, Z.; Cai, X.; Zang, S.; Li, Y.; Zheng, D.; Li, F. Strong metal support effect of Pt/g-C3N4 photocatalysts for boosting photothermal synergistic degradation of benzene. Int. J. Mol. Sci. 2023, 24, 6872. [Google Scholar] [CrossRef]
  39. Madhavi, V.; Kumar, P.J.; Kondaiah, P.; Hussain, O.M.; Uthanna, S. Effect of molybdenum doping on the electrochromic properties of tungsten oxide thin films by RF magnetron sputtering. Ionics 2014, 20, 1737–1745. [Google Scholar] [CrossRef]
  40. Rao, R.; Huang, Y.; Zhang, H.; Hu, C.; Dong, X.; Fang, W.; Zhou, Q.; Chen, Z.; Fang, S.; Jin, D. A simple melamine-assisted cellulose pyrolysis synthesis of magnetic and mesoporous N-doped carbon composites with excellent adsorption of Congo red. Sep. Purif. Technol. 2024, 347, 127678. [Google Scholar] [CrossRef]
  41. Li, H.; Pan, Y.; Wu, L.; He, R.; Qin, Z.; Luo, S.; Yang, L.; Zeng, J. IrO2 deposited on RuO2 as core-shell structured RuO2@ IrO2 for oxygen evolution reaction in electrochemical water electrolyzer. Mol. Catal. 2023, 551, 113619. [Google Scholar] [CrossRef]
  42. Huang, H.; Dai, Q.; Wang, X. Morphology effect of Ru/CeO2 catalysts for the catalytic combustion of chlorobenzene. Appl. Catal. B Environ. 2014, 158, 96–105. [Google Scholar] [CrossRef]
  43. Barathi, D.; Rajalakshmi, N.; Ranjith, R.; Sangeetha, R.; Meyvel, S. Controllable synthesis of CeO2/g-C3N4 hybrid catalysts and its structural, optical and visible light photocatalytic activity. Diam. Relat. Mater. 2021, 111, 108161. [Google Scholar] [CrossRef]
  44. Yu, J.; Wang, S.; Low, J.; Xiao, W. Enhanced photocatalytic performance of direct Z-scheme gC3N4–TiO2 photocatalysts for the decomposition of formaldehyde in air. Phys. Chem. Chem. Phys. 2013, 15, 16883–16890. [Google Scholar] [CrossRef]
  45. Li, L.; Zhang, C.; Chen, F.; Xiang, Y.; Yan, J.; Chu, W. Facile fabrication of hollow structured Cu-Ce binary oxides and their catalytic properties for toluene combustion. Catal. Today 2021, 376, 239–246. [Google Scholar] [CrossRef]
  46. Wu, Z.; Li, M.; Howe, J.; Meyer III, H.M.; Overbury, S.H. Probing defect sites on CeO2 nanocrystals with well-defined surface planes by Raman spectroscopy and O2 adsorption. Langmuir 2010, 26, 16595–16606. [Google Scholar] [CrossRef]
  47. Han, L.; Jing, F.; Luo, X.; Zhong, Y.; Wang, K.; Zang, S.; Teng, D.; Liu, Y.; Chen, J.; Yang, C. Environment friendly and remarkably efficient photocatalytic hydrogen evolution based on metal organic framework derived hexagonal/cubic In2O3 phase-junction. Appl. Catal. B Environ. 2021, 282, 119602. [Google Scholar] [CrossRef]
  48. Zou, Y.; Dong, L.; Yan, S.; Liu, J.; Mu, L.; Li, L.; Hu, Y.; Qi, H.; Mao, S.; Chen, Z. Activity enhancement of Ru/CeO2 for N-alkylation of amines with alcohols through tailoring metal-support interaction. J. Catal. 2024, 429, 115241. [Google Scholar] [CrossRef]
  49. Chen, S.; Wang, H.; Kang, Z.; Jin, S.; Zhang, X.; Zheng, X.; Qi, Z.; Zhu, J.; Pan, B.; Xie, Y. Oxygen vacancy associated single-electron transfer for photofixation of CO2 to long-chain chemicals. Nat. Commun. 2019, 10, 788. [Google Scholar] [CrossRef]
  50. Wang, H.; Yong, D.; Chen, S.; Jiang, S.; Zhang, X.; Shao, W.; Zhang, Q.; Yan, W.; Pan, B.; Xie, Y. Oxygen-vacancy-mediated exciton dissociation in BiOBr for boosting charge-carrier-involved molecular oxygen activation. J. Am. Chem. Soc. 2018, 140, 1760–1766. [Google Scholar] [CrossRef]
  51. Li, L.; Chen, P.; Gloyna, E.F. Generalized kinetic model for wet oxidation of organic compounds. Aiche J. 1991, 37, 1687–1697. [Google Scholar] [CrossRef]
  52. You, C.; Wang, C.; Cai, M.; Liu, Y.; Zhu, B.; Li, S. Improved photo-carrier transfer by an internal electric field in BiOBr/Nrich C3N5 3D/2D S-scheme heterojunction for efficiently photocatalytic micropollutant removal. Acta Phys.-Chim. Sin. 2024, 40, 2407014. [Google Scholar] [CrossRef]
  53. Li, G.; Chai, S.; Zhang, G.; Liu, J.; Zhang, Y.; Lv, Y.; Wang, Y.; Zhao, Y. Deactivation characteristics of Ce-modified Cu-based carbon materials for catalytic wet air oxidation of phenol wastewater. J. Environ. Chem. Eng. 2022, 10, 108228. [Google Scholar] [CrossRef]
  54. Papailias, I.; Todorova, N.; Giannakopoulou, T.; Ioannidis, N.; Boukos, N.; Athanasekou, C.P.; Dimotikali, D.; Trapalis, C. Chemical vs thermal exfoliation of g-C3N4 for NOx removal under visible light irradiation. Appl. Catal. B Environ. 2018, 239, 16–26. [Google Scholar] [CrossRef]
  55. Li, S.; Rong, K.; Wang, X.; Shen, C.; Yang, F.; Zhang, Q. Design of carbon quantum dots/CdS/Ta3N5 S-scheme heterojunction nanofibers for efficient photocatalytic antibiotic removal. Acta Phys.-Chim. Sin. 2024, 40, 2403005. [Google Scholar] [CrossRef]
  56. Li, S.; You, C.; Rong, K.; Zhuang, C.; Chen, X.; Zhang, B. Chemically bonded Mn0. 5Cd0. 5S/BiOBr S-scheme photocatalyst with rich oxygen vacancies for improved photocatalytic decontamination performance. Adv. Powder Mater. 2024, 3, 100183. [Google Scholar] [CrossRef]
  57. Dong, K.; Shen, C.; Yan, R.; Liu, Y.; Zhuang, C.; Li, S. Integration of plasmonic effect and S-scheme heterojunction into Ag/Ag3PO4/C3N5 photocatalyst for boosted photocatalytic levofloxacin degradation. Acta Phys. Chim. Sin 2024, 40, 2310013. [Google Scholar] [CrossRef]
  58. Pan, Z.; Zhao, M.; Zhuzhang, H.; Zhang, G.; Anpo, M.; Wang, X. Gradient Zn-doped poly heptazine imides integrated with a van der Waals homojunction boosting visible light-driven water oxidation activities. ACS Catal. 2021, 11, 13463–13471. [Google Scholar] [CrossRef]
  59. Jiang, J.; Ou-yang, L.; Zhu, L.; Zheng, A.; Zou, J.; Yi, X.; Tang, H. Dependence of electronic structure of g-C3N4 on the layer number of its nanosheets: A study by Raman spectroscopy coupled with first-principles calculations. Carbon 2014, 80, 213–221. [Google Scholar] [CrossRef]
  60. Xiao, C.; Goh, T.; Qi, Z.; Goes, S.; Brashler, K.; Perez, C.; Huang, W. Conversion of levulinic acid to γ-valerolactone over few-layer graphene-supported ruthenium catalysts. ACS Catal. 2016, 6, 593–599. [Google Scholar] [CrossRef]
  61. Yuan, C.; Zhang, Y.; Zong, Z.; Zhou, S.; Cui, H.; Tan, H. In situ growth of an ultrathin Cu/gC3N4 coating over SBA-15 for catalytic wet air oxidation of pollutants under extremely mild conditions. Green. Chem. 2024, 26, 6601–6615. [Google Scholar] [CrossRef]
  62. Gao, X.; Zhu, S.; Dong, M.; Wang, J.; Fan, W. Ru/CeO2 catalyst with optimized CeO2 morphology and surface facet for efficient hydrogenation of ethyl levulinate to γ-valerolactone. J. Catal. 2020, 389, 60–70. [Google Scholar] [CrossRef]
  63. Yan, L.; Liu, Y.; Zha, K.; Li, H.; Shi, L.; Zhang, D. Deep insight into the structure–activity relationship of Nb modified SnO2–CeO2 catalysts for low-temperature selective catalytic reduction of NO by NH3. Catal. Sci. Technol. 2017, 7, 502–514. [Google Scholar] [CrossRef]
  64. Sellick, D.R.; Aranda, A.; García, T.; López, J.M.; Solsona, B.; Mastral, A.M.; Morgan, D.J.; Carley, A.F.; Taylor, S.H. Influence of the preparation method on the activity of ceria zirconia mixed oxides for naphthalene total oxidation. Appl. Catal. B Environ. 2013, 132, 98–106. [Google Scholar] [CrossRef]
  65. Lv, Z.; Feng, J.; Zhao, R.; Shen, J.; Yang, W. Visible-light-driven photocatalytic degradation of ibuprofen by Cu-doped tubular C3N4: Mechanisms, degradation pathway and DFT calculation. Chemosphere 2024, 358, 142106. [Google Scholar] [CrossRef]
  66. Huang, C.; Zhang, Y.; Li, X.; Cao, H.; Guo, Y.; Zhang, C. Mn-incorporated Co3O4 bifunctional electrocatalysts for zinc-air battery application: An experimental and DFT study. Appl. Catal. B Environ. 2022, 319, 121909. [Google Scholar] [CrossRef]
  67. Khawaji, M.; Chadwick, D. Au–Pd NPs immobilised on nanostructured ceria and titania: Impact of support morphology on the catalytic activity for selective oxidation. Catal. Sci. Technol. 2018, 8, 2529–2539. [Google Scholar] [CrossRef]
  68. Jiang, K.; Dong, L.; Shen, Q.; Wu, W.; Wu, X.; Mei, J.; Yang, S. Chlorobenzene Oxidation over Phosphotungstic-Acid-Coated Cerium Oxide: Synergistic Effect of Phosphotungstic and Cerium Oxide and Inhibition Mechanism of Sulfur Dioxide. Sustainability 2024, 16, 2245. [Google Scholar] [CrossRef]
  69. Wang, Z.; Huang, Z.; Brosnahan, J.T.; Zhang, S.; Guo, Y.; Guo, Y.; Wang, L.; Wang, Y.; Zhan, W. Ru/CeO2 catalyst with optimized CeO2 support morphology and surface facets for propane combustion. Environ. Sci. Technol. 2019, 53, 5349–5358. [Google Scholar] [CrossRef]
  70. Wang, F.; Li, C.; Zhang, X.; Wei, M.; Evans, D.G.; Duan, X. Catalytic behavior of supported Ru nanoparticles on the (100), (110), and (111) facet of CeO2. J. Catal. 2015, 329, 177–186. [Google Scholar] [CrossRef]
  71. Chang, C.J.; Li, S.S.; Ko, C.M. Catalytic wet oxidations of phenol-and p-chlorophenol-contaminated waters. J. Chem. Technol. Biotechnol. 1995, 64, 245–252. [Google Scholar] [CrossRef]
  72. Delgado, J.J.; Chen, X.; Pérez-Omil, J.A.; Rodríguez-Izquierdo, J.M.; Cauqui, M.A. The effect of reaction conditions on the apparent deactivation of Ce–Zr mixed oxides for the catalytic wet oxidation of phenol. Catal. Today 2012, 180, 25–33. [Google Scholar] [CrossRef]
  73. Mao, Y.; Liang, J.; Jiang, L.; Shen, Q.; Zhang, Q.; Liu, C.; Zheng, H.; Liao, Y.; Cao, X.; Dong, H. Removal of microorganic pollutants in high salinity wastewater by comproportionation system of Fe (VI)/Fe (III): Enhancement of chloride and bicarbonate. Water Res. 2022, 214, 118182. [Google Scholar] [CrossRef] [PubMed]
  74. Muthukumaran, S.; Song, L.; Zhu, B.; Myat, D.; Chen, J.; Gray, S.; Duke, M. UV/TiO2 photocatalytic oxidation of recalcitrant organic matter: Effect of salinity and pH. Water Sci. Technol. 2014, 70, 437–443. [Google Scholar] [CrossRef]
  75. Kumari, M.; Saroha, A.K. Performance of various catalysts on treatment of refractory pollutants in industrial wastewater by catalytic wet air oxidation: A review. J. Environ. Manag. 2018, 228, 169–188. [Google Scholar]
  76. Ao, X.; Wang, W.; Sun, W.; Lu, Z.; Li, C. Degradation and transformation of norfloxacin in medium-pressure ultraviolet/peracetic acid process: An investigation of the role of pH. Water Res. 2021, 203, 117458. [Google Scholar] [CrossRef]
  77. Yadav, B.R.; Garg, A. Efficacy of fresh and used supported copper-based catalysts for ferulic acid degradation by wet air oxidation process. Ind. Eng. Chem. Res. 2012, 51, 15778–15785. [Google Scholar] [CrossRef]
  78. Zhou, L.; Cao, H.; Descorme, C.; Xie, Y. Phenolic compounds removal by wet air oxidation based processes. Front. Env. Sci. Eng. 2018, 12, 1. [Google Scholar] [CrossRef]
  79. Ye, Y.; Gao, L.; Xu, J.; Wang, L.; Mo, L.; Zhang, X. Effect of CuO species and oxygen vacancies over CuO/CeO2 catalysts on low-temperature oxidation of ethyl acetate. J. Rare Earth 2023, 41, 862–869. [Google Scholar] [CrossRef]
  80. Promhuad, P.; Sawatmongkhon, B.; Theinnoi, K.; Wongchang, T.; Chollacoop, N.; Sukjit, E.; Tunmee, S.; Tsolakis, A. Effect of Metal Oxides (CeO2, ZnO, TiO2, and Al2O3) as the Support for Silver-Supported Catalysts on the Catalytic Oxidation of Diesel Particulate Matter. ACS Omega 2024, 9, 19282–19294. [Google Scholar] [CrossRef]
  81. Pu, T.; Setiawan, A.; Mosevitzky Lis, B.; Zhu, M.; Ford, M.E.; Rangarajan, S.; Wachs, I.E. Nature and reactivity of oxygen species on/in silver catalysts during ethylene oxidation. ACS Catal. 2022, 12, 4375–4381. [Google Scholar] [CrossRef]
  82. Heydariyan, Z.; Monsef, R.; Salavati-Niasari, M. Insights into impacts of Co3O4-CeO2 nanocomposites on the electrochemical hydrogen storage performance of g-C3N4: Pechini preparation, structural design and comparative study. J. Alloy Compd. 2022, 924, 166564. [Google Scholar] [CrossRef]
  83. Afzal, M.Z.; Zu, P.; Zhang, C.; Guan, J.; Song, C.; Sun, X.; Wang, S. Sonocatalytic degradation of ciprofloxacin using hydrogel beads of TiO2 incorporated biochar and chitosan. J. Hazard. Mater. 2022, 434, 128879. [Google Scholar] [CrossRef] [PubMed]
  84. Gupta, P.; Pandey, K.; Verma, N. Augmented complete mineralization of glyphosate in wastewater via microbial degradation post CWAO over supported Fe-CNF. Chem. Eng. J. 2022, 428, 132008. [Google Scholar] [CrossRef]
  85. Geng, K.; Wu, Y.; Jiang, G.; Liu, K.; Jiang, L. RuC@ g-C3N4 (H+)/TiO2 visible active photocatalyst: Facile fabrication and Z-scheme carrier transfer mechanism. Mol. Catal. 2018, 458, 33–42. [Google Scholar] [CrossRef]
Sustainability 17 04358 g001
Figure 1. XRD patterns of the prepared samples.
Figure 1. XRD patterns of the prepared samples.
Sustainability 17 04358 g001
Sustainability 17 04358 g002
Figure 2. SEM images of the prepared samples: (a) CeO2, (b) Ru/CeO2, (c) CNx/CeO2, and (d) Ru-CNx/CeO2.
Figure 2. SEM images of the prepared samples: (a) CeO2, (b) Ru/CeO2, (c) CNx/CeO2, and (d) Ru-CNx/CeO2.
Sustainability 17 04358 g002
Sustainability 17 04358 g003
Figure 3. TEM (a), HRTEM (b), and size distribution (c), and EDS mapping images (d), and elemental analysis of Ru-CNx/CeO2 catalyst (e).
Figure 3. TEM (a), HRTEM (b), and size distribution (c), and EDS mapping images (d), and elemental analysis of Ru-CNx/CeO2 catalyst (e).
Sustainability 17 04358 g003
Sustainability 17 04358 g004
Figure 4. Raman spectra of (a) the prepared samples and (b) magnified peaks around 461 (cm−1).
Figure 4. Raman spectra of (a) the prepared samples and (b) magnified peaks around 461 (cm−1).
Sustainability 17 04358 g004
Sustainability 17 04358 g005
Figure 5. EPR spectra of Ru/CeO2 and Ru-CNx/CeO2 catalysts.
Figure 5. EPR spectra of Ru/CeO2 and Ru-CNx/CeO2 catalysts.
Sustainability 17 04358 g005
Sustainability 17 04358 g006
Figure 6. The EPR spectra (a) TEMPO for h+, (b) DMPO for ·O2−, and (c) DMPO for ·OH.
Figure 6. The EPR spectra (a) TEMPO for h+, (b) DMPO for ·O2−, and (c) DMPO for ·OH.
Sustainability 17 04358 g006
Sustainability 17 04358 g007
Figure 7. XPS spectra of (a) full scan spectrum, (b) C1S, (c) N1s for Ru-CNx/CeO2 catalyst, (d) Ru3p (e) O1s and (f) Ce3d for Ru-CNx/CeO2 and Ru/CeO2 catalysts.
Figure 7. XPS spectra of (a) full scan spectrum, (b) C1S, (c) N1s for Ru-CNx/CeO2 catalyst, (d) Ru3p (e) O1s and (f) Ce3d for Ru-CNx/CeO2 and Ru/CeO2 catalysts.
Sustainability 17 04358 g007
Sustainability 17 04358 g008
Figure 8. The catalytic performances of various catalysts.
Figure 8. The catalytic performances of various catalysts.
Sustainability 17 04358 g008
Sustainability 17 04358 g009
Figure 9. Effect of reaction temperature on the catalytic performance of Ru-CNx/CeO2 catalyst (a) the removal of COD, and (b) the removal of TOC.
Figure 9. Effect of reaction temperature on the catalytic performance of Ru-CNx/CeO2 catalyst (a) the removal of COD, and (b) the removal of TOC.
Sustainability 17 04358 g009
Sustainability 17 04358 g010
Figure 10. Effect of oxygen pressure on the catalytic performance of Ru-CNx/CeO2 catalyst.
Figure 10. Effect of oxygen pressure on the catalytic performance of Ru-CNx/CeO2 catalyst.
Sustainability 17 04358 g010
Sustainability 17 04358 g011
Figure 11. The effect of the concentration of inorganic salts on the removal of COD of the Ru-CNx/CeO2 catalyst, (a) NaCl concentration, and (b) Na2SO4 concentration (reaction temperature 180 °C, PO2 = 2.5 MPa, reaction time 30 min).
Figure 11. The effect of the concentration of inorganic salts on the removal of COD of the Ru-CNx/CeO2 catalyst, (a) NaCl concentration, and (b) Na2SO4 concentration (reaction temperature 180 °C, PO2 = 2.5 MPa, reaction time 30 min).
Sustainability 17 04358 g011
Sustainability 17 04358 g012
Figure 12. The effect of pH on the catalytic performance of Ru-CNx/CeO2: (a) COD removal, and (b) pH of the solution after reaction.
Figure 12. The effect of pH on the catalytic performance of Ru-CNx/CeO2: (a) COD removal, and (b) pH of the solution after reaction.
Sustainability 17 04358 g012
Sustainability 17 04358 g013
Figure 13. Stability of the Ru-CNx/CeO2 catalyst.
Figure 13. Stability of the Ru-CNx/CeO2 catalyst.
Sustainability 17 04358 g013
Sustainability 17 04358 g014
Figure 14. Radicals scavenging experiments.
Figure 14. Radicals scavenging experiments.
Sustainability 17 04358 g014
Table 1. Specific surface areas, crystal sizes, and cell parameters of the various samples.
Table 1. Specific surface areas, crystal sizes, and cell parameters of the various samples.
CatalystsSA
(m2/g)		Pore Volume
(cm3/g)		Average
Pore Size
(nm)		CeO2 Crystal Size a (nm)
CeO2 b98.10.176.89.9
Ru/CeO256.00.075.58.8
CNx/CeO260.00.1411.18.0
Ru-CNx/CeO253.00.1512.56.3
Ru-CNx/CeO2 c47.20.159.68.3
a Crystal sizes were determined using the Scherrer equation. b The CeO2 sample was prepared by calcining Ce (OAC)3·xH2O at 550 °C for 4 h in air. c The used catalyst.
Table 2. XPS characterization of the different samples.
Table 2. XPS characterization of the different samples.
SampleSurface Composition (at.%) Atomic Concentration (at.%)
CNRuOCeRu0/(Ru0 + Ru4+)OA/OLCe3+/Ce4+
Ru/CeO2//2.7343.6313.020.380.450.17
Ru-CNx/CeO243.075.953.7836.8010.310.720.720.25
Ru-CNx/CeO2 a34.092.533.3145.7814.290.610.510.24
a The used catalyst.
Disclaimer/Publisher’s Note: The statements, opinions and data contained in all publications are solely those of the individual author(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury to people or property resulting from any ideas, methods, instructions or products referred to in the content.

Share and Cite

MDPI and ACS Style

Han, Y.; Ye, Y.; Yu, W.; Zang, S.; Ji, L.; Li, S.; Mo, L. High Performance Ru-CNx/CeO2 Catalyst for Catalytic Wet Oxidation of N-Methyldiethanolamine in Water. Sustainability 2025, 17, 4358. https://doi.org/10.3390/su17104358

AMA Style

Han Y, Ye Y, Yu W, Zang S, Ji L, Li S, Mo L. High Performance Ru-CNx/CeO2 Catalyst for Catalytic Wet Oxidation of N-Methyldiethanolamine in Water. Sustainability. 2025; 17(10):4358. https://doi.org/10.3390/su17104358

Chicago/Turabian Style

Han, Yuantao, Yuchuan Ye, Wanjin Yu, Shaohong Zang, Lili Ji, Shijie Li, and Liuye Mo. 2025. "High Performance Ru-CNx/CeO2 Catalyst for Catalytic Wet Oxidation of N-Methyldiethanolamine in Water" Sustainability 17, no. 10: 4358. https://doi.org/10.3390/su17104358

APA Style

Han, Y., Ye, Y., Yu, W., Zang, S., Ji, L., Li, S., & Mo, L. (2025). High Performance Ru-CNx/CeO2 Catalyst for Catalytic Wet Oxidation of N-Methyldiethanolamine in Water. Sustainability, 17(10), 4358. https://doi.org/10.3390/su17104358

Note that from the first issue of 2016, this journal uses article numbers instead of page numbers. See further details here.

Article Metrics

Back to TopTop