Catalytic Wet Air Oxidation Research Articles

Study of the catalytic wet air oxidation of p-hydroxybenzoic acid on a fresh ruthenium catalyst supported by different oxides

The catalytic wet air oxidation (CWAO) of p-hydroxybenzoic acid (p-HBA) was conducted in a batch reactor at 140 °C, and at a total air pressure of 50 bar over Ru-based catalysts. Four materials were selected as supports – TiO2, CeO2–TiO2, ZrO2–TiO2, and La2O3–TiO2 – all of which had mesopores in their texture and pollutant adsorption capacities. The supports were prepared by the sol-gel method, and then impregnated with 3 wt% of Ru precursor. Such characterization techniques as N2-sorption, XRD, XPS, H2–TPR, NH3-TPD, TEM, and HAADF-STEM were used to analyze the different solids. The correlation between catalytic activities and physicochemical properties was discussed. A significant specific surface area (SBET), a large amount of surface-active oxygen, and Lewis acidity sites were observed on cerium-containing catalysts (Ru/CeTi). Fresh Ru catalysts containing cerium showed higher activity than Ru/TiO2, Ru/ZrTi, and Ru/LaTi catalysts. It is assumed that the acidic sites and surface oxygen trap the p-HBA molecule, thus increasing the catalytic properties of the Ru particles which interact with the surface oxygen through the cerium redox process (Ce3+/Ce4+). As the presence of cerium increases surface-active oxygen, it inhibits the deposition of carbon on the surface of the Ru catalyst. The pseudo-second order (PSO) model adequately described the kinetic data of the p-HBA oxidation reaction using Ru catalysts.

Facile synthesis of platinum-loaded carbon nanosheets with high specific surface area for efficient CWAO of ammonia nitrogen: A low-cost alternative route for treating ammonia containing wastewater

Deep treatment is an important tool to solve the environmental pollution of ammonia-containing wastewater. Considering the stability and economy of industrial application, we propose a simple and economical synthesis strategy. The procedure is based on the carbonization of citrate at elevated temperatures to synthesize high specific surface area and pore-rich carbon nanoflowers, and carbon nanoflower-loaded Pt catalysts were constructed for the catalytic wet air oxidation (CWAO) of ammonia-containing wastewater. The results showed that the synergistic effect of Pt, edge carbon and carboxyl groups led to the increase of CWAO activity. During the reaction, Pt activated NH3 to put it in a high-energy state, and there was a positive synergistic effect between the oxygen-containing functional groups (carboxyl groups) on the surface of the porous carbon nanoflowers and the edge carbon exposed at the adjacent defect sites, which promoted the activation of O2 and the generation of reactive oxygen species. At 200 °C, the Pt1/PC catalysts showed high activity and hydrothermal stability for CWAO of NH3. The conversion of NH3 was 98.24 % within 4 h, and the selectivity of N2 was 100 %. The total nitrogen (TN) was reduced to below 15 mg‧L−1, which meets the Chinese emission standard for TN in petrochemical industry. Therefore, the novel catalyst proposed in this study effectively solved the problems of complicated catalyst synthesis steps and poor hydrothermal stability in ammonia CWAO studies. The catalyst has potential industrial applications for CWAO of ammonia-containing wastewater.

Synergistic catalytic wet oxidation of ammonia over carbon-supported low-loading Ru/Cu bimetallic catalyst

The manner of ammonia-containing wastewater in industrial production has always been an urgent issue that needs to be solved. Accordingly, a carbon-supported low-loading Ru/Cu bimetallic catalyst with high activity and hydrothermal stability was produced through facile disposal for catalytic wet air oxidation (CWAO). In the preparation process, Ru and Cu ions were loaded onto micro mesoporous carbon prepared through the calcination of potassium citrate by impregnation. Various characteristic techniques were utilized to characterize their structures and morphology, and results show that the calcination temperature with micro-mesopores’ characteristics could regulate the morphology of carbon. The results of catalytic experiments demonstrated that the obtained catalyst showed excellent catalytic performance in the process of CWAO. The Ru/Cu@C-700 catalyst achieved 99.6 % conversion of ammonia water at 800 ppm and 99.1 % selectivity to nitrogen at 200 °C and 2 MPa O2 in 4 h. The ammonia nitrogen content in the treated water was less than 15 ppm, meeting the standard for direct discharge. Moreover, no apparent deactivation phenomenon was observed after five cycles, indicating that the prepared catalyst can effectively prevent the leaching of ruthenium and copper nanoparticles. The low loading (less than 1 %) of the Ru/Cu bimetallic catalyst can significantly reduce costs and can synergistically enhance conversion and selectivity, providing an effective strategy for studying the CWAO of ammonia.

Chitosan – TiO2 Nanocomposite Films for Efficient Decolourisation of Sulphur Black Dye under Dark Conditions

ABSTRACT Sulphur dyes are a class of dyes known for their low cost and excellent fastness to light and wash. But, excess amounts of unused dyes, electrolytes and sulphide contents were found to cause serious hazards to health and environment. This work reports the synthesis of cost-effective, non-toxic, environment-friendly chitosan (CS) and chitosan-TiO2 (5 wt% (CST-5), 10 wt% (CST-10) and 15 wt% (CST-15)) polymer nanocomposite films by the facile solution casting method. CS and CST polymer nanocomposite films were tested for the decolourisation of sulphur black dye. The structural analysis of the synthesised films using X-ray Diffraction (XRD) displayed the semi-crystalline peaks of chitosan and the anatase phase of TiO2 with tetragonal-bcc structure. The morphological analysis using Atomic Force Microscopy (AFM) revealed that TiO2 had dispersed uniformly into the polymer matrix. UV–Vis DRS and PL revealed the wide indirect band gap energy and charge transfer characteristics, respectively, of the polymer nanocomposite films. Fourier Transform Infrared Spectroscopy (FTIR) and X-ray Photoelectron Spectroscopy (XPS) studies revealed the various bonds, functional groups and elemental properties of CS and TiO2-loaded CS films. Sulphur black dye decolourisation studies conducted under dark conditions at room temperature and atmospheric pressure revealed that CST-5 nanocomposite film exhibited an excellent decolourisation efficiency of 87.6% within 12 min of contact with the sulphur black dye solution. This swift and enhanced decolourisation was due to the synergistic effects of adsorption by chitosan and catalytic wet air oxidation (CWAO) by the catalyst TiO2. These nanocomposite films are potential materials for the treatment of wastewater and decolourisation of various toxic dyes.

Active Carbon from Date Stones for Phenol Oxidation in Trickle Bed Reactor, Experimental and Kinetic Study

The catalytic wet air oxidation (CWAO) of phenol has been studied in a trickle bed reactor
 using active carbon prepared from date stones as catalyst by ferric and zinc chloride activation (FAC and ZAC). The activated carbons were characterized by measuring their surface area and adsorption capacity besides conventional properties, and then checked for CWAO using a trickle bed reactor operating at different conditions (i.e. pH, gas flow rate, LHSV, temperature and oxygen partial pressure). The results showed that the active carbon (FAC and ZAC), without any active metal supported, gives the highest phenol conversion. The reaction network proposed accounts for all detected intermediate products of phenol oxidation that composed by several consecutive and parallel reactions. The parameters of the model estimated using experimental data obtained from a continuous trickle bed reactor at different temperatures (120-160 C) and oxygen partial pressures (8-12 bar). Simple power law as well as Langmuir-Hinshelwood (L-H) expressions accounting for the adsorption effects were checked in the modeling of the reaction network. A non-linear multi-parameter estimation approach was used to simultaneously evaluate the high number of model parameters. Approach by simple power law only succeeds in fitting phenol disappearance. Instead, when L-H expressions are incorporated for the intermediate reaction steps, the model accurately describes all the experimental concentration profiles, giving mean deviations below 10%.

The efficient degradation of high concentration phenol by Nitrogen-doped perovskite La2CuO4 via catalytic wet air oxidation: Experimental study and DFT calculation

Herein, N-doped perovskite La2CuO4 (LCO) (N-LCO(M)) was fabricated by sol–gel rapid calcination method using melamine (M) as N source/ligand, and applied for the catalytic wet air oxidation (CWAO) to degrade high-concentration phenol-containing simulated wastewater. The experimental results exhibited that N-LCO(M) illustrated enhanced phenol degradation efficiency. Thereinto, 87.9% COD degradation efficiency was achieved in simulated wastewater containing 8000 mg/L phenol, while for undoped LCO prepared with citric acid (CA) as ligand (LCO(CA)) was only 43.8%. The systematic characterizations and density functional theory (DFT) calculations were utilized to verify the above results. The results revealed that N doping favored the formation of more oxygen vacancies (*) and facilitated the charge transfer between Cu and O. More oxygen vacancies (*) reinforced the ability of N-LCO(M) to adsorb O2 to form reactive oxygen species (O*), and meanwhile, the charge transfer between Cu and O was manifested as the redox cycle of the catalytic active metal Cu2+/Cu1+. The redox cycle also promoted the formation of O2− and HO, making the more exhaustive degradation of phenol. This successfully proved that N doping was core to improving the activity and stability of N-LCO(M). Moreover, a possible mechanism of phenol degradation was proposed based on the experimental results and DFT calculations. Eventually, the toxicity of the intermediate products was further evaluated by quantitative structure–activity relationship (QSAR) analysis, showing the significantly reduced toxicity of the intermediate products. Overall, the proposed CWAO technique is a green and promising wastewater treatment method.

Catalytic performance and mechanism of PTFE modified NiCo2O4 in high-salt organic wastewater treatment during wet air oxidation at ambient pressure

High temperatures and high pressures restrict the application of catalytic wet air oxidation (CWAO). Thus, realizing the high performance of CWAO under ambient pressure (AP-CWAO) is attractive. In this study, hydrophobic modification by polytetrafluoroethylene (PTFE) was adopted to enhance the O2 transfer and salts resistibility of hollow microspheres NiCo2O4 (HM-NiCo2O4), for harvesting an efficient catalyst of AP-CWAO to treat high-salt wastewater. Under ambient pressure in 10 g/L NaCl solution, PTFE modified HM-NiCo2O4 (PHM-NiCo2O4) can catalyze O2 to realize about 70 % mineralization of bisphenol A (BPA, 100 mg/L) within 240 min at 80 ℃, while it is only about 50 % for HM-NiCo2O4. Density functional theory calculations and molecular dynamics simulations show that PTFE indeed can enhance O2 transfer and salt resistibility of HM-NiCo2O4. HO•, O2•-, and 1O2 all contribute to the degradation of BPA in presence of O2. These reactive oxygen species generate based on Mars-van Krevelen mechanism.

Developing Pt-M/C catalyst (M=Pb, Cu) for efficient catalytic wet air oxidation of phenol wastewater under mild conditions

Catalytic wet air oxidation (CWAO) is a promising process for degrading phenol in wastewater to CO2 and H2O. To achieve an active and stable Pt catalyst working at lower temperature, efficient catalysts are here developed by building a Pt-M (M=Pb, Cu) alloy structure where Pt is mainly in the form of Pt0 while M is mainly in the form of M2+. Firstly, carbon supported Pt-Pb/XC-72R and Pt-Pb/EC-300 catalysts are prepared for CWAO of phenol wastewater. It is found that Pt-Pb/XC-72R is highly active and stable for CWAO of phenol even at 100 °C. The initial TOC conversion is 91.1% and can be maintained higher than 75% in five consecutive cycles. Contrarily, Pt-Pb/EC-300 rapidly deactivates and TOC conversion decreased from 91.2% to 47.1% after five cycles. The characterization results indicate that two Pt-Pb alloy catalysts contained comparably high Pt0 concentration, which is responsible for the high activity. However, the Pb2+ concentration of Pt-Pb/XC-72R is much higher than that of Pt-Pb/EC-300, and higher Pb2+ concentration favors surface oxidation and keeps Pt0 from being oxidized during CWAO process, thus enhancing stability. For Pt-Pb/EC-300, both Pt0 and Pb0 are oxidized during the reaction, resulting in deactivation. To avoid using Pb in environmental considerations, the Pt-Pb is then replaced by Pt-Cu nanoparticles, where high catalytic activity and stability are maintained. More importantly, increasing Cu2+ can further enhance the catalytic performance. Tuning the second metal chemical state of Pt-based bimetallic alloy can be an effective approach to develop new catalysts for CWAO technology.

Life cycle assessment of high concentration organic wastewater treatment by catalytic wet air oxidation

There have been many studies on life cycle assessment in sewage treatment, but there are scarce few studies on the treatment of industrial wastewater in combination with advanced oxidation technology, especially in catalytic wet air oxidation (CWAO). There are no cases of using actual industrialized data onto life cycle assessment. This paper uses Simapro 9.0 software to establish a life cycle assessment model for the treatment of high-concentration organic wastewater by CWAO, and comprehensively explains the impact on the environment from three aspects: the construction phase, the operation phase and the demolition phase. In addition, sensitivity analysis and uncertainty analysis were performed. The results showed that the key factors affecting the environment were marine ecotoxicity, mineral resource consumption and global warming, the operation stage had the greatest impact on the environment, which was related to high power consumption during operation and emissions from the treatment process. Sensitivity analysis showed that electricity consumption has the greatest impact on abiotic depletion and freshwater aquatic ecotoxicity, and it also proved that global warming is mainly caused by pollutant emissions during operation phase. Monte Carlo simulations found slightly higher uncertainty for abiotic depletion and toxicity-related impact categories.

Kinetics and mechanism study of dyes degradation in electric field-promoting catalytic wet air oxidation process

Wet air oxidation (WAO) is a clean and eco-friendly technology for dyes removal, but the high operating temperature and pressure limit its practical application. In the present work, an electric field-promoting (EF-promoting) catalytic WAO process is developed to degrade dyes under room condition. The oxidation kinetics of four different types of dyes and their degradation pathways are studied. A kinetic model is constructed by including the exogenous electric field into the Langmuir-Hinshelwood-Hougen-Watson (LHHW) mechanism framework, and quantitative structure-activity relationship (QSAR) analysis is conducted to correlate the kinetic parameters to the physicochemical properties of the dyes. A negative linear relationship is found between the adsorption equilibrium constants of the dyes and their first ionization energies, and their surface reaction rate constants are positively linearly associated to Esum (ELUMO + EHOMO). The degradation pathways of the different dyes are proposed according to the degradation intermediates and the activities of the atoms within the dye molecules. The heteroatoms N and S, and the atom C connecting the aromatic rings are identified as the susceptible sites upon the electrophilic attack of O2. Bond cleavage at these sites gives rise to aromatic fragments which are eventually mineralized via carboxyl acids. The results of this work is helpful for guiding the design and operation of the EF-promoting catalytic WAO process into the treatment of various dye wastewaters.

Ru Catalysts Supported on Bamboo-like N-Doped Carbon Nanotubes: Activity and Stability in Oxidizing and Reducing Environment.

The catalysts with platinum-group metals on nanostructured carbons have been a very active field of research, but the studies were mainly limited to Pt and Pd. Here, Ru catalysts based on nitrogen-doped carbon nanotubes (N-CNTs) have been prepared and thoroughly characterized; Ru loading was kept constant (3 wt.%), while the degree of N-doping was varied (from 0 to 4.8 at.%) to evaluate its influence on the state of supported metal. Using the N-CNTs afforded ultrafine Ru particles (<2 nm) and allowed a portion of Ru to be stabilized in an atomic state. The presence of Ru single atoms in Ru/N-CNTs expectedly increased catalytic activity and selectivity in the formic acid decomposition (FAD) but had no effect in catalytic wet air oxidation (CWAO) of phenol, thus arguing against a key role of single-atom catalysis in the latter case. A remarkable difference between these two reactions was also found in regard to catalyst stability. In the course of FAD, no changes in the support or supported species or reaction rate were observed even at a high temperature (150 °C). In CWAO, although 100% conversions were still achievable in repeated runs, the oxidizing environment caused partial destruction of N-CNTs and progressive deactivation of the Ru surface by carbonaceous deposits. These findings add important new knowledge about the properties and applicability of Ru@C nanosystems.

Increasing biodegradability of a real amine-contaminated spent caustic problematic stream through WAO and CWAO oxidation using a high specific surface catalyst from petcoke

Different operating conditions of wet air oxidation and catalytic wet air oxidation have been studied for the treatment of highly concentrated methyldiethanolamine wastewater streams from amine units of acid gas recovery in petrol refineries. These units occasionally generate streams of high methyldiethanolamine content that require special actions to avoid undesirable impacts on the downstream biological process of the petrochemical wastewater treatment plant due to its inhibition effect. The wet air oxidation treatment achieved remarkable removals of methyldiethanolamine, sulfides, chemical oxygen demand and total organic carbon (99%, 95%, 65% and 38%, respectively). Likewise, activated petroleum coke materials from the own refinery plant were tested as catalysts in the process. These materials were prepared under different conditions (chemical activating agent and thermal carbonization process). The catalytic wet air oxidation treatment using an activated petroleum coke was able to remove the methyldiethanolamine at milder operation conditions keeping a similar performance in terms of wastewater treatment removals as compared to the non-catalytic experiments. This technology significantly increased the biodegradability of the treated effluents ranging from 25 to 70 % due to the formation of more biodegradable substrates (acetic acid and ammonium) for further biological treatment.

Spherical activated carbons derived from resin-microspheres for the adsorption of acetic acid

Separation of small-molecule carboxylic acids, especially acetic acid, from wastewater is challenging due to its high-water solubility, acidity, and low concentration. In this work, resin-derived spherical activated carbon (RSAC) was directly prepared from resin spheres through facile pre-oxidation and carbonization processes. RSAC was characterized by SEM, N2 physisorption, XRD, Raman, element analysis and XPS. RSAC showed high removal rates (> 95.8 %) for a variety of small-molecule carboxylic acids. The initial adsorption capacity of acetic acid over RSAC was 23.4 mg/g and 94 % of the removal rate was retained after five successive adsorption-desorption cycles for 1000 mg/L acetic acid. In addition, RSAC showed a high total organic carbon removal rate (> 80 %) for the high-salinity effluent of catalytic wet air oxidation (CWAO) process. The adsorption of acetic acid over RSAC complied with the pseudo-second-order model and Langmuir model ideally, through Vander Waals force and pore-filling effect. Furtherly, discarded resin microspheres were used to prepare spherical active carbon, which showed 88.2 % removal of acetic acid (1000 mg/L). The benign scheme of “treating wastes by wastes” from economical and environmental aspects was realized.

Efficient removal of antibiotic ciprofloxacin by catalytic wet air oxidation using sewage sludge-based catalysts: Degradation mechanism by DFT studies

In this work, the sewage sludge-derived activated carbon (SAC) loaded with iron nanoparticles (FeSAC) showed a highly effective catalytic activity in the degradation of the antibiotic ciprofloxacin by the CWAO reaction. The properties of FeSAC catalyst were studied by using N2 adsorption-desorption measurements at 77 K, scanning electron microscopy, X-ray fluorescence spectroscopy, X-ray photoelectron spectroscopy and thermogravimetric analysis. The CWAO reaction was evaluated at different temperatures (120–140 ºC), total pressure (10–30 bar) and catalyst doses (0.1–0.7 g/L) in a batch reactor. In this regard, temperature and catalyst dosage showed a significant impact on the removal of the tested antibiotic. By using a catalyst dose of 0.7 g/L, ciprofloxacin degradation and CO2 selectivity were higher than 99 % and 60 %, respectively, and were achieved within two hours at 140 °C and 20 bar. The loss of the active phase (Fe) of the catalyst in the reaction medium was measured, obtaining negligible values (less than 24 ppb). This catalyst showed high stability under the tested reaction conditions. In addition, a potential equation was proposed to correctly describe the evolution of ciprofloxacin degradation. The calculated activation energy of the CWAO process was 53.8 kJ/mol. Additionally, Density Functional Theory (DFT) calculations were performed to illustrate the degradation mechanism of ciprofloxacin, where the electronic energies indicated the compounds that are most difficult to degrade by CWAO. Finally, a proof of concept using an environmentally-relevant matrix was carried out, verifying the technical feasibility of the synthesized catalyst for its application with more complex matrices, consecutive reaction cycles and at a low treatment cost.

Preparation of Porous and Durable Metakaolin-Based Alkali-Activated Materials with Active Metal as Composites for Catalytic Wet Air Oxidation

Novel porous and durable metakaolin-based alkali-activated materials (MK-AAMs) with active metal as composites were produced to degrade bisphenol A (BPA) in catalytic wet air oxidation (CWAO). Two composite producing processes were employed. The first process consisted of mixing metakaolin (MK), a foaming agent and active metal oxide (CuO, MnO2) in a strongly alkaline solution of K2SiO3 and KOH. Paste was cured under microwave radiation to produce porous CuO and MnO2 composites. A porous blank MK-AAM was produced as described above but without active metal and was used as a reference as well. Cu(OH)2 composite was produced by refluxing a blank MK-AAM in 0.5 M CuSO4 solution for 24 h. The specific surface area (SSA) of the reference, CuO, MnO2, and Cu(OH)2 composites were 36, 53, 61, 89 m2/g, respectively. Mechanical durability was determined in terms of compressive strength and 2.8, 3.4, 3.2, 3.6 MPa were received, respectively. The activity of the reference and the composites were tested in CWAO at 1 MPa and 150 °C for 5 h by using an aqueous model solution of BPA. Under the optimal conditions for CWAO (pressure: 1 MPa; temperature: 150 °C; initial pH 5–6; c[catalyst]: 4.0 g/L) with Cu(OH)2 composite, the BPA and total organic carbon (TOC) conversions of 100% and 53% were reached. During 5 h oxidation, the composites degraded due to the combined effect of erosion (1.5 wt%) and active metal (Cu, Mn) leaching (1.1 wt%, 3.6 wt%). It was proposed that BPA can be degraded energy-efficiently via CWAO into less harmful compounds under mild reaction conditions without losing the desired properties of the composites.

Efficient mineralization of sugar industry wastewater by catalytic wet air oxidation as an eco‐friendly method and its kinetic modelling

AbstractIn this study, catalytic wet air oxidation using lanthanum cobalt oxide (LaCoO3) as catalyst was employed for the efficient treatment of synthetic sugar industry wastewater in a single process. A parametric study was performed to determine the optimum conditions. The results showed that reaction temperature and theoretical air percentage were the most effective parameters. Sugar derivatives were almost completely destroyed at the optimum conditions and total organic carbon (TOC) and chemical oxygen demand (COD) removals were determined as 91% and 87%, respectively, indicating a high mineralization degree, which was the main goal of advanced oxidation. The reaction kinetics were investigated by pseudo‐homogeneous and heterogeneous models based on two different parameters: sucrose and TOC concentration. The degree of fit showed that the reaction order was determined as two for the pseudo‐homogeneous approach. According to the surface concentration calculations for heterogeneous models, the presence of mass transfer limitations was only observed for oxygen as gas reactant. All heterogeneous models also fitted the reaction rate data accurately, but the Mars–van Krevelen was the selected model for sucrose and TOC oxidation with the best fit.

A highly hydrothermal stable copper-based catalyst for catalytic wet air oxidation of m-cresol in coal chemical wastewater

Catalytic wet air oxidation (CWAO) can degrade some refractory pollutants at a low cost to improve the biodegradability of wastewater. However, in the presence of high temperature and high pressure and strong oxidizing free radicals, the stability of catalysts is often insufficient, which has become a bottleneck in the application of CWAO. In this paper, a copper-based catalyst with excellent hydrothermal stability was designed and prepared. TiO2 with excellent stability was used as the carrier to ensure the long-term anchoring of copper and reduce the leaching of the catalyst. The one pot sol–gel method was used to ensure the super dispersion and uniform distribution of copper nanoparticles on the carrier, so as to ensure that more active centers could be retained in a longer period. Experiments show that the catalyst prepared by this method has good stability and catalytic activity, and the catalytic effect is not significantly reduced after 10 cycles of use. The oxidation degradation experiment of m-cresol with the strongest biological toxicity and the most difficult to degrade in coal chemical wastewater was carried out with this catalyst. The results showed that under the conditions of 140 °C, 2 MPa and 2 h, m-cresol with a concentration of up to 1000 mg·L−1 could be completely degraded, and the COD removal rate could reach 79.15%. The biological toxicity of wastewater was significantly reduced. The development of the catalyst system has greatly improved the feasibility of CWAO in the treatment of refractory wastewater such as coal chemical wastewater.

Effect of inorganic salt on the removal of typical pollutants in wastewater by RuO2/TiO2 via catalytic wet air oxidation

The treatment of high-salinity and high-organic wastewater is a tough task, with the removal of organic matter and the separation of salts often mutually restricting. Catalytic wet air oxidation (CWAO) coupled desalination technology (membrane distillation (MD), membrane bioreactor (MBR), ultrafiltration (UF), nanofiltration (NF), etc.) provides an effective method to simultaneously degrade the high-salinity (via desalination) and high-organic matters (via CWAO) in wastewater. In this work, five kinds of RuO2/TiO2 catalysts with different calcination temperatures were prepared for CWAO of maleic acid wastewater with a theoretical chemical oxygen demand (COD) value of 20,000 mg L−1. RuO2/TiO2 series catalysts demonstrated prominent salt resistance, with more than 80% TOC removal rates in the CWAO system containing 5 wt% Na2SO4; while RuO2/TiO2-350 showed the best degradation performance in both non-salinity and Na2SO4-containing conditions. Multiple characterization techniques, such as XRD, BET, XPS, NH3-TPD and TEM etc., verified the physicochemical structure of RuO2/TiO2 catalysts, and their influence on the degradation of pollutants. The calcination temperature was found to have a direct impact on the specific surface area, pore volume, oxygen vacancies and acid sites of catalysts, which in turn affected the ultimate catalytic activity. Furthermore, we also investigated the performance of the RuO2/TiO2-350 catalyst for the treatment of acids, alcohols and aromatic compounds with the addition of NaCl or Na2SO4, proving its good universality and excellent salt resistance in saline wastewater. Meanwhile, the relationship between the structure of three types of organic compounds and the degradation effect in the CWAO system was also explored.

Effective heterogeneous catalysis of micro-size H3PMo6W6O40@rht-MOF-1 for degradation of phenolic compounds under molecular oxygen

Since the huge damage to human health and the survival of animals and plants was caused by phenol and its derivatives, it is impending to develop efficient and durable heterogeneous catalysts of catalytic wet air oxidation (CWAO) for eliminating these phenolic compounds. The heterogeneous system POMs@rht-MOF-1 H3PMo12−xWxO40@rht-MOF-1 (1, x = 3; 2, x = 6; 3, x = 9) and the micro-size complex 2 (denoted by 2 (1 µm) have been designed and synthesized. FT-IR, PXRD, TGA, N2 adsorption-desorption isotherms and XPS were applied to characterize their structures. Upon complex 2 (1 µm) is employed as catalysts for degradation of phenol under molecular oxygen (O2) as oxidant, the catalytic degradation efficiency and TOC reduction up to 95 % and 91 % under optimized condition, respectively. The catalyst 2 (1 µm) could be reused 7 times with negligible reduction. An apparent Arrhenius activation energy of 33.13 KJ/mol and TOF value of 0.3325 (min−1) for phenol were obtained. When catalyst 2 (1 µm) is applied to catalyze mixed phenolic compounds, the degradation efficiency of 85 % is afforded. Furthermore, the catalyzed degradation reaction mechanism is verified via EPR analysis. This method affords an effective path for preparing heterogeneous catalysts and applying in removing the phenolic compounds in industrial wastewater.

Deactivation characteristics of Ce-modified Cu-based carbon materials for catalytic wet air oxidation of phenol wastewater

In this paper, Ce-modified Cu-based carbon materials (Cu-Ce/AC) were prepared by impregnation method using walnut shell as the carbon precursor, Cu-ammonia solution as Cu source and Ce as modifier. The catalytic wet air oxidation (CWAO) degradation of phenol wastewater was carried out in a self-assembled fixed-bed reactor. The stability of Cu-Ce/AC catalyst under different pH conditions was studied. This study combines XRD, BET, FT-IR, ICP-OES and XPS characterization techniques to study the main reasons for the deactivation of Cu-Ce/AC catalysts. The reaction mechanism of Cu-Ce/AC catalytic wet air oxidation degradation of phenol was clarified. The result indicates that although phenol was completely converted in the CWAO process, the acidic intermediate product produced were difficult to completely degraded. Therefore, the pH of the solution gradually decreased. In acidic environment, the interaction between active component Cu and carbon carrier is weak, resulting in a large amount of metal leaching into the solution. As a result, the COD conversion gradually decreases. The alkaline environment can effectively slow down the leaching of catalyst Cu while maintaining its high catalytic activity. In addition, the incompletely degraded intermediate products are easy to accumulate and deposit on the Cu-Ce/AC surface, block the micropores and cover the surface-active sites, which reduces the catalyst activity. The study also found that there is a redox cycle between Cu+, Cu2+ Ce3+ and Ce4+ during phenol degradation, which enables Cu and Ce to form a strong interaction. The redox cycle can also promote the formation of oxygen vacancy (h+), O2−·, and HO·, making the degradation of phenol more thorough. At the same time, the mutual conversion between metal lattice oxygen and surface chemisorbed oxygen can generate high concentration of oxygen defect sites, which can improve the oxygen storage capacity of Cu-Ce/AC. These oxygen vacancies also provide more reactive sites to further promote adsorption and catalytic oxidation degradation of phenol and intermediate products.