Promotion of formaldehyde degradation by electro-Fenton: Controlling the distribution of ·OH and formaldehyde near cathode to increase the reaction probability

The promotion mechanism of different nitrogen doping types on the catalytic activity of activated carbon electro-Fenton cathode: Simultaneous promotion of H2O2 generation and phenol degradation ability

Doping with nitrogen atoms can improve the catalytic activity of activated carbon cathodes in electro-Fenton systems, but currently there is a lack of understanding of the catalytic mechanism, which limits the further development of high-performance activated carbon cathodes. Here, a multi-scale exploration was conducted using density functional theory and experimental methods to investigate the mechanism of different nitrogen doping types promoting the redox performance of activated carbon cathodes and the degradation of phenol. The density functional theory results indicate that the introduction of nitrogen atoms enhances the binding ability between carbon substrates and oxygen-containing substances, promotes the localization of surrounding electrons, and makes it easier for O2 to bind with protons and catalyze the hydrogenation reaction of *OOH. Due to its weak binding ability with oxygen-containing substances, AC is difficult to form H2O2, resulting in a tendency towards the 4e-ORR pathway. The binding energy between graphite-N carbon substrate and pyridine-N carbon substrate with *OOH is closer to the volcano top, so graphite n and pyridine n can better promote the selectivity of activated carbon for 2e-ORR. In addition, the calculation results also indicate that pyrrole-N and graphite-N are more capable of catalyzing the reaction energy barrier between ·OH and phenol. Finally, the simulation results were used to guide the modification of nitrogen doped activated carbon and experimental verification was carried out. The degradation results of phenol confirmed the efficient synergistic effect between different types of nitrogen doping, and the NAC-800 electrode exhibited efficient and stable characteristics. This work provides a guiding strategy for further developing stable and highly selective activated carbon cathode materials.

Clarifying the Direct Generation of •OH Radicals in Photocatalytic O2 Reduction: Theoretical Prediction Combined with Experimental Validation

This work systematically studied thermocatalytic and photocatalytic pathways of formaldehyde degradation and H-assisted O2 reduction over a Pt13/anatase-TiO2(101) composite via DFT calculations together with constrained molecular dynamics (MD) simulations. We show that photocatalytic O2 reduction on Pt/TiO2 can directly generate •OH radicals (*O2 → *OOH → •OH) via two hydrogenation steps with small barriers, and the product selectivity (*H2O2 or •OH) is decided by the relative position between catalyst Fermi level and •OH/*H2O2 redox potential (theoretical determination of 0.07 V referencing to the SHE). Such a novel reaction channel was furthermore validated at the liquid-solid interface via constrained MD simulations and experimental electron paramagnetic resonance detections, and a wide range of H resources, e.g., *HCHO, *HCO, *H (H+ + e-), can always drive the direct •OH generation. The additional portion of e--triggered •OH radicals are prone to diffuse into solution or the TiO2 surface and furthermore cooperate with the conventional h+-driven photooxidations.

Preparation of Perovskite-Type LaMnO3 and Its Catalytic Degradation of Formaldehyde in Wastewater

Formaldehyde (HCHO) is identified as the most toxic chemical among 45 organic compounds found in industrial wastewater, posing significant harm to both the environment and human health. In this study, a novel approach utilizing the Lanthanum-manganese complex oxide (LaMnO3)/peroxymonosulfate (PMS) system was proposed for the effective removal of HCHO from wastewater. Perovskite-Type LaMnO3 was prepared by sol-gel method. The chemical compositions and morphology of LaMnO3 samples were analyzed through thermogravimetric analysis (TG), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and transmission electron microscopy (TEM). The effects of LaMnO3 dosage, PMS concentration, HCHO concentration, and initial pH on the HCHO removal rate were investigated. When the concentration of HCHO is less than 1.086 mg/mL (5 mL), the dosage of LaMnO3 is 0.06 g, and n(PMS)/n(HCHO) = 2.5, the removal rate of HCHO is more than 96% in the range of pH = 5-13 at 25 °C for 10 min. Compared with single-component MnO2, the perovskite structure of LaMnO3 is beneficial to the catalytic degradation of HCHO by PMS. It is an efficient Fenton-like oxidation process for treating wastewater containing HCHO. The LaMnO3 promoted the formation of SO4•- and HO•, which sequentially oxidized HCHO to HCOOH and CO2.

Optimization Strategy for Formaldehyde Removal by Carbon Cathode Electro-Fenton: Enhancement of Formaldehyde and Oxygen Co-adsorption by Rational Nitrogen Doping Types

This study investigates the effect of N-doped coal-based activated carbon cathode on formaldehyde-oxygen coadsorption. Further investigation investigates the effect of formaldehyde-oxygen coadsorption on H2O2 generation and formaldehyde removal in an electro-Fenton system. Nitrogen doping enhances formaldehyde and oxygen coadsorption by modulating competitive adsorption. Density Functional Theory (DFT) calculations confirm pyrrole nitrogen favors formaldehyde, and graphite nitrogen favors oxygen adsorption. N-doped activated carbon adsorbs 0.36 mg of formaldehyde and 0.1 mg of oxygen in 120 min and removes 82.43% of formaldehyde after electro-Fenton treatment. N-doped activated carbon enhances the synergistic adsorption of formaldehyde and oxygen. In the synergistic adsorption process, the amount of formaldehyde adsorbed is greater than the amount of oxygen adsorbed. This improves the removal efficiency of formaldehyde by electro-Fenton technology. It provides a new method for electro-Fenton removal of organic pollutants.

Mechanisms of efficient indoor formaldehyde removal via electro-Fenton: Synergy in ·OH generation and utilization through a modified carbon cathode

Indoor formaldehyde poses a significant carcinogenic risk to human health, making its removal imperative. Electro-Fenton degradation has emerged as a promising technology for addressing this concern. In the electro-Fenton system, ·OH is identified as the primary active species responsible for formaldehyde removal. Hence, its generation and utilization are pivotal for the system's effectiveness and economy. Experimental and quantum chemical methods were employed to investigate the effects and mechanisms of nitrogen doping on various aspects influencing ·OH generation and utilization. Results indicate that nitrogen doping synergistically enhances the generation and utilization of ·OH, leading to an improved formaldehyde removal efficiency in nitrogen-doped cathodic systems. The dominant nitrogen type influencing ·OH generation and utilization varies across different stages. Pyridinic nitrogen facilitates H2O2 adsorption through hydrogen bonding, while pyrrolic and graphitic nitrogen contribute to formaldehyde adsorption and catalyze the conversion of H2O2 to ·OH. Both pyridinic nitrogen and pyrrolic nitrogen boost the degradation of formaldehyde by ·OH. In comparison to the unmodified system, the modified system with NAC-GF/700C as cathode exhibits remarkable improvements. The formaldehyde removal efficiency has increased twofold, and energy consumption reduced by 73.45%. Furthermore, the system demonstrates excellent cyclic stability. These advancements can be attributed to the activation temperature, which leads to the appropriate types and high content of nitrogen elements in NAC-GF/700C. The research represents an important step towards more economical and efficient electro-Fenton technology for indoor formaldehyde removal.

Size Control of Carbon Xerogel Spheres as Key Factor Governing the H2O2 Selectivity in Metal-Free Bifunctional Electro-Fenton Catalysts for Tetracycline Degradation

Carbon xerogel spheres co-doped with nitrogen and eco-graphene were synthesized using a typical solvothermal method. The results indicate that the incorporation of eco-graphene enhances the electrochemical properties, such as the current density (JK) and the selectivity for the four transferred electrons (n). Additionally, nitrogen doping has a significant effect on the degradation efficiency, varying with the size of the carbon xerogel spheres, which could be attributed to the type of nitrogenous group doped in the carbon material. The degradation efficiency improved in the nanometric spheres (48.3% to 61.6%) but decreased in the micrometric-scale spheres (58.6% to 53.4%). This effect was attributed to the N-functional groups present in each sample, with N-CNS-5 exhibiting a higher percentage of graphitic nitrogen (35.7%) compared to N-CMS-5 (15.3%). These findings highlight the critical role of sphere size in determining the type of N-functional groups present in the sample. leading to enhanced degradation of pollutants as a result of the electro-Fenton process.

Facile Synthesis of Carbon-Based Inks to Develop Metal-Free ORR Electrocatalysts for Electro-Fenton Removal of Amoxicillin

The electro-Fenton process is based on the generation of hydroxyl radicals (OH•) from hydroxide peroxide (H2O2) generated in situ by an oxygen reduction reaction (ORR). Catalysts based on carbon gels have aroused the interest of researchers as ORR catalysts due to their textural, chemical and even electrical properties. In this work, we synthesized metal-free electrocatalysts based on carbon gels doped with graphene oxide, which were conformed to a working electrode. The catalysts were prepared from organic-gel-based inks using painted (brush) and screen-printed methods free of binders. These new methods of electrode preparation were compared with the conventional pasted method on graphite supports using a binder. All these materials were tested for the electro-Fenton degradation of amoxicillin using a homemade magnetite coated with carbon (Fe3O4/C) as a Fenton catalyst. All catalysts showed very good behavior, but the one prepared by ink painting (brush) was the best one. The degradation of amoxicillin was close to 90% under optimal conditions ([Fe3O4/C] = 100 mg L-1, -0.55 V) with the catalyst prepared using the painted method with a brush, which had 14.59 mA cm-2 as JK and a H2O2 electrogeneration close to 100% at the optimal voltage. These results show that carbon-gel-based electrocatalysts are not only very good at this type of application but can be adhered to graphite free of binders, thus enhancing all their catalytic properties.

Silica wastes derived multifunctional coatings for formaldehyde photodegradation and autonomous indoor humidity buffering

To maintain a comfortable and healthy indoor environment without large amounts of energy consumption is of great importance. The progress of multifunctional indoor coatings with formaldehyde photodegradation and humidity buffering capability is necessary. From the viewpoints of circular economy, the preparation of effective photocatalysts (denoted as sFCC/GCN-x and ESF/GCN-y) via the decoration of recycling industrial wastes (i.e., spent fluid catalytic cracking catalysts (sFCC) and enhancement silica fume (ESF)) onto graphitic carbon nitride (GCN) by using a simple route is reported. The obtained results show that the prepared sFCC/GCN-0.15 and ESF/GCN-0.15 photocatalysts have the rate constants of formaldehyde degradation of 0.0075 and 0.0082 min-1, respectively, which are superior to that of pristine GCN (0.0044 min-1) under visible-light irradiation. The enhanced transfer kinetics of photogenerated electrons and declined recombination of electron-hole pairs may account for the surpassing photocatalytic performance. Results obtained from electron paramagnetic resonance spectra and Mott-Schottky plots indicate that the formation of ･O2- via the reaction of O2 with electrons generated on the conduction band is the major reaction pathway to photodegrade formaldehyde under visible light. To further assess the real applications of prepared photocatalysts, the sFCC/GCN-0.15 and ESF/GCN-0.15 are used to fabricate the multifunctional coatings (denoted as s- and E-coatings) with sFCC and ESF as the main compositions. Experimentally, the E-coatings could reach the formaldehyde degradation efficiency of ca. 84.5% after 3 h of visible light irradiation and excellent humidity buffering ability (293.8 g m-2) which is at least 10-folds higher than commercial coatings (28.9 g m-2). This notable progress of humidity buffering capacity on E-coatings can be attributed to their surface textural properties. Most importantly, this study exemplifies the valorization of inorganic silica wastes to produce sustainable and multifunctional coatings which may offer the practical and cost-effective applications in the indoor living space.

Strategies for promoting the degradation of phenol by electro-Fenton: Simultaneously promoting the generation and utilization of H2O2

The use of the electro-Fenton process to continuously generate H2O2 and efficiently degrade organic pollutants is considered a promising technology. The ratio of generation of H2O2 is usually regarded as the critical step; however, how the H2O2 is utilized is also of particular importance. Herein, activated carbon was activated at different temperatures and used to explore the effect of nitrogen doping on the production and utilization of H2O2 in the electro-Fenton-based degradation of organic pollutants. The experimental results indicate that nitrogen-doped activated carbon simultaneously promotes the generation and utilization of H2O2, which is attributed to the regulation of the competition between phenol and O2 adsorption by the doped nitrogen. Nitrogen doping not only improves 2e-ORR selectivity but also aggregates phenol near the cathode to balance the concentrations of phenol and ·OH. Density functional theory (DFT) calculations further confirmed that pyrrole-N as a dopant promoted the adsorption of phenol, while pyridine-N was more favorable for O2 adsorption. The unique balance of nitrogen types possessed by modified activated carbon NAC-750 permits the efficient synergistic generation and utilization of H2O2 in a balanced manner during the degradation of phenol. This work provides a new direction for the rational nitrogen-doping modification of activated carbon for the electro-Fenton-based degradation of organic pollutants.

Synergistic mechanism of formaldehyde adsorption by intrinsic defects and carboxyl groups on the surface of carbon materials

The adsorption of formaldehyde on the original carbon material is limited. Determining the synergistic adsorption of formaldehyde by different defects on the carbon material is necessary for comprehensively understanding the mechanism of formaldehyde adsorption on the surface of the carbon material. The synergistic effect of intrinsic defects and oxygen-containing functional groups on formaldehyde adsorption on the surface of carbon materials was simulated and verified by experiments. Based on the density functional theory, the adsorption of formaldehyde on different carbon materials was simulated by quantum chemistry. The synergistic adsorption mechanism was studied by energy decomposition analysis, IGMH, QTAIM, and charge transfer, and the binding energy of hydrogen bonds was estimated. The results showed that the energy for the adsorption of formaldehyde adsorbed by the carboxyl group on the vacancy defect was the highest, at -11.86 kcal/mol, the hydrogen bond binding energy was -9.05 kcal/mol, and a larger charge transfer was recorded. The mechanism of synergy was studied comprehensively, and the simulation results were verified at multiple scales. This study provides valuable insights into the effect of carboxyl groups on the adsorption of formaldehyde by activated carbon.

Effect of oxygen functional groups on competitive adsorption of benzene and water on carbon materials: Density functional theory study

It is important to study the effect of oxygen-containing functional groups on the competitive adsorption mechanism of benzene and water on the surface of carbon materials, and to directional modification of activated carbon to improve its selective adsorption of benzene in air. In this study, the adsorption characteristics of benzene and water on original and linked ester, carboxyl, hydroxyl, carbon materials linked by ether groups were calculated by quantum chemical simulation based on density functional theory. The types and proportions of weak interactions in the adsorption process were calculated by energy decomposition analysis, and the adsorption mechanism of carbon materials for water and benzene was described. The influence and contribution of oxygen-containing functional groups on the adsorption of benzene and water were further analyzed by van der Waals potential and electrostatic potential, respectively, so as to determine the difference in the adsorption effect of different types of oxygen-containing functional groups on the two molecules. It was found that the carboxyl group has a great influence on the hydrophilicity of carbon materials, and the electrostatic potential distribution before and after linking the carboxyl group changed significantly. Therefore, they can attract each other with water through hydrogen bonds and occupy the surface adsorption sites of carbon materials, thereby inhibiting the adsorption of benzene on carbon materials. On the contrary, due to its hydrophobic properties, the ether group will free up adsorption space for the adsorption of benzene on the surface of the carbon material, which is beneficial to the adsorption of benzene. The adsorption experiments were carried out, and the results were consistent with the simulation. This study provides an idea for preparing efficient carbonaceous adsorbent of benzene and reducing benzene pollution in industry.

Ultrasound-Promoted Abatement of Formaldehyde in Liquid Phase with Electrospun Nanostructured Membranes: The Synergy of Combined AOPs

The present work investigates the effect of ultrasounds in the performance of combined advanced oxidation processes (AOPs) on the degradation of formaldehyde (HCHO)-polluted aqueous solutions for potential application in wastewater treatment. Different heterogeneous nanostructured catalysts based on TiO₂ and FeSO₄ for photocatalysis and the Fenton process were employed after electrospray deposition on electrospun nanofibrous membranes. Such systems were tested, without the use of any added hydrogen peroxide, by varying the combinations among the selected AOPs in a batch reactor configuration. The results show that, in the absence of a Fenton reaction, ultrasounds provided a significantly increased formaldehyde photocatalytic abatement, probably by increasing the concentration of active species through a different set of reactions while providing a favorable mass transfer regime by the cavitational effect. Due to the faster kinetics of the photo-Fenton process, thanks to its partial homogeneous nature, such a beneficial effect is more limited for the sono-photo-Fenton configuration. On the other hand, the employment of a sono-photocatalytic-Fenton process revealed a synergic effect that provided the best results, reducing the formaldehyde concentration to less than 99% after 240 min. Further analysis showed that, due to a mutual influence, only a tailored TiO₂/FeSO₄ ratio on the membranes was able to display the best performance.