Ruthenium Catalysts for the Reduction of N-Nitrosamine Water Contaminants

Highly effective electrocatalytic reduction of N-nitrosodimethylamine on Ru/CNT catalyst

N-Nitrosodimethylamine (NDMA) is a commonly identified carcinogenic and genotoxic pollutant in water. In this study, we prepared Ru catalysts supported on carbon nanotube (Ru/CNT) and studied the electrocatalytic reduction of N-nitrosamines on Ru/CNT electrode in a three-electrode system. The results show that Ru-based catalyst exhibits a high activity of 793.3 μmol L^-1 gCat^-1 h^-1 for electrochemical reduction of NDMA. Reaction mechanism study discloses that the electrocatalytic reduction of NDMA is accomplished by both direct electron reduction and atomic H* mediated indirect reduction pathways. Further product analysis indicates that NDMA is finally reduced to dimethylamine (DMA) and ammonia. The reduction efficiency of NDMA strongly relies on cathode potential, initial NDMA concentration and solution pH. To verify the universality of Ru/CNT electrode, electrocatalytic reduction of three dialkyl N-nitrosamines with different alkyl groups was performed and Ru catalyst has high catalytic activities for the three N-nitrosamines, while the catalytic efficiency differs with their structures. Simultaneous electrochemical reduction of the three N-nitrosamines indicates that the reduction rates of N-nitrosamines follow the same order in the multiple-component system as that in the single-component system. Catalyst recycling results demonstrate that after 5 consecutive recycling runs Ru/CNT electrode remains almost identical catalytic activity to the fresh catalyst, manifesting the high catalytic stability of Ru/CNT electrode.

Facet and d-band center engineering of CuNi nanocrystals for efficient nitrate electroreduction to ammonia

Electrocatalytic nitrate reduction offers a sustainable route to ammonia synthesis and wastewater treatment. However, the nitrate-to-ammonia conversion remains inefficient due to the sluggish kinetics and diverse side reactions. Herein, well-faceted CuNi nanocrystals with Ni-rich surfaces and favorable d-band centres were synthesized with the assistance of γ-cyclodextrin via a solvothermal process. When used as catalysts for nitrate electroreduction, they delivered an ammonia yield of 1.374 mmol h^-1 mg^-1 (0.5496 mmol h^-1 cm^-2) at -0.3 V with the faradaic efficiency and selectivity reaching 94.5% and 65.0%, respectively, surpassing pure Cu or Ni nanocrystals and most reported catalysts. Such excellent performances originated from the optimal geometric and electronic structures and special element distribution, which optimized the adsorption behaviors and accelerated the reaction kinetics. A NO₃^--NO₂^--NH₃ pathway was proposed with the chemical process following the initial electron transfer process as the rate-determining step. This work sheds light on the design of efficient catalysts to achieve carbon neutrality through simultaneous geometric and electronic structure modulation.

Atomic Hydrogen in Electrocatalytic Systems: Generation, Identification, and Environmental Applications

Electrochemical reduction has emerged as a viable technology for the removal of a variety of organic contaminants from water. Atomic hydrogen (H*) is the primary species generated in electrochemical reduction processes. In this work, identification and quantification for H* are reviewed with a focus on methods used to generate H* at different positions. Additionally, we present recently developed proposals for the surface chemistry mechanisms of H* on the most commonly used cathodes as well as the use of H* in standard electrochemical reactors. The proposed reaction pathways in different H* systems for environmental applications are also discussed in detail. As shown in this review, the key hurdles facing H* reduction technologies are related to i) the establishment of systematic and practical synthetic methods; ii) the development of effective identification approaches with high specificity; and, iii) an in-depth exploration of the H* reaction mechanism to better understand the reaction process of H*.

Photodegradation of N-nitrosodimethylamine under 365 nm Light Emitting Diode Irradiation

The photodegradation of NDMA has been extensively investigated under the irradiation of low-pressure or medium-pressure Hg lamps and xenon lamp. However, NDMA photolysis remains unknown under 365 nm ultraviolet light-emitting diode (UV-LED) irradiation. This study conducted a comprehensive investigation on NDMA photodegradation by 365 nm UV-LED illumination. The quantum yield of NDMA photolysis under 365 nm UV-LED irradiation was determined to be 0.0312 ± 0.0047. The influence of pH on NDMA photodegradation was found to be wavelength dependent. Compared with distilled and deionized water (DDW), tap water inhibited NDMA photodegradation, but secondary wastewater effluent did not. Based on the quantification of NDMA photolysis products and pH influence, the photooxidation of the excited NDMA in the nonprotonated form was proposed to be a major pathway for NDMA photodegradation under the irradiation of UV-LED lamp at 365 nm. This study further enhances our knowledge on NDMA photodegradation. PRACTITIONER POINTS: Quantum yield of NDMA photolysis at 365 nm was determined to be 0.0312 ± 0.0047. The influence of pH on NDMA photodegradation was wavelength dependent. NDMA photodegradation was inhibited in tap water compared with that in DDW. NDMA photodegradation in SWE was similar to that in DDW. Excited nonprotonated NDMA photooxidation is a major degradation pathway.

Rapid detoxification of Microcystin-LR by selective catalytic hydrogenation of the Adda moiety using TiO2-supported Pd catalysts

The hepatotoxicity of Microcystin-LR (MC-LR) is mainly caused by its Adda moiety. In this study, we used TiO₂-supported Pd catalysts to selectively hydrogenate the CC bonds in the Adda moiety, achieving rapid detoxification of MC-LR in water under ambient conditions. MC-LR was removed within 5 min by catalytic hydrogenation on Pd(1.0)/TiO₂ with a catalyst dosage normalized rate constant of 1.3 × 10^-2 L mg_cat^-1 min^-1, significantly more efficient than other catalytic treatment methods. The reactions proceeded in a highly selective manner towards catalytic hydrogenation at the CC bond of the Mdha moiety and subsequently the conjugated double bond of the Adda moiety, yielding two intermediates and one final product. Upon catalytic hydrogenation for 30 min on Pd(0.07)/TiO₂, the toxicity of MC-LR (assessed by protein phosphatase 2A activity assay) drastically decreased by 90.8%, demonstrating effective detoxification. The influence of catalyst support, Pd content, initial MC-LR concentration, reaction pH, and catalytic stability were examined. Surface adsorption and the cationic Pd played a crucial role in the reaction kinetics. Our results suggest that catalytic hydrogenation is a highly effective and safe strategy for detoxifying MC-LR by selective reactions.

An Organic Chemist's Guide to N-Nitrosamines: Their Structure, Reactivity, and Role as Contaminants

N-Nitrosamines are a class of compounds notorious both for the potent carcinogenicity of many of its members and for their widespread occurrence throughout the human environment, from air and water to our diets and drugs. Considerable effort has been dedicated to understanding N-nitrosamines as contaminants, and methods for their prevention, remediation, and detection are ongoing challenges. Understanding the chemistry of N-nitrosamines will be key to addressing these challenges. To facilitate such understanding, we focus in this Perspective on the structure, reactivity, and synthetic applications of N-nitrosamines with an emphasis on alkyl N-nitrosamines. The role of N-nitrosamines as water contaminants and the methods for their detection are also discussed.

Water-Mediated Reduction of Aqueous N-Nitrosodimethylamine with Pd

Pd-catalyzed reduction has emerged as a promising treatment strategy to remove the recalcitrant disinfection byproduct N-nitrosodimethylamine (NDMA). However, the reaction pathways remain unexplored, and questions remain about how water solvent influences NDMA reduction mechanisms and selectivity. Here, we compute the energies and barriers of all relevant elementary steps in NDMA reduction by H₂ on Pd(111) using density functional theory. We further calculate water-assisted H-shuttling for all hydrogenation reactions explicitly and include water solvation for all elementary reactions implicitly. We parametrize microkinetic models to predict product formation rates and selectivities over a wide range of NDMA concentrations. We show that H₂O-mediated H-shuttling lowers the reaction barriers for all hydrogenation reactions involved in NDMA reduction while implicit solvation has negligible impact on the reaction and activation energies. We further conduct batch experiments with SiO₂-supported Pd nanoparticles and compare them with the microkinetic models. The predicted rates, selectivity, and apparent activation energy from the model parametrized with both explicit H₂O-mediated H-shuttling and implicit solvation correspond well with experimental observations. Models that ignore water as an H-shuttle or solvent fail to recover the experimental rates and apparent activation energy. We identified the rate-determining steps of the reaction and show the reaction flow pathways of the complicated reaction network. Finally, we demonstrate that water-mediated H-shuttling changes the rate-determining steps and reaction flows of elementary reactions.

Simultaneous Adsorption and Electrochemical Reduction of N-Nitrosodimethylamine Using Carbon-Ti4O7 Composite Reactive Electrochemical Membranes

This study focused on synthesis and characterization of Ti₄O₇ reactive electrochemical membranes (REMs) amended with powder-activated carbon (PAC) or multiwalled carbon nanotubes (MWCNTs). These composite REMs were evaluated for simultaneous adsorption and electrochemical reduction of N-nitrosodimethylamine (NDMA). The carbon-Ti₄O₇ composite REMs had high electrical conductivities (1832 to 2991 S m^-1), where carbon and Ti₄O₇ were in direct electrical contact. Addition of carbonaceous materials increased the residence times of NDMA in the REMs by a factor of 3.8 to 5.4 and therefore allowed for significant electrochemical NDMA reduction. The treatment of synthetic solutions containing 10 μM NDMA achieved >4-log NDMA removal in a single pass (liquid residence time of 11 to 22 s) through the PAC-REM and MWCNT-REM with the application of a -1.1 V/SHE cathodic potential, with permeate concentrations between 18 and 80 ng L^-1. The treatment of a 6.7 nM NDMA-spiked surface water sample, under similar operating conditions (liquid residence time of 22 s), achieved 92 to 97% removal with permeate concentrations between 16 and 40 ng L^-1. Density functional theory calculations determined a probable reaction mechanism for NDMA reduction, where the rate-limiting step was a direct electron transfer reaction.

Hydrogenation of dioctyl phthalate over a Rh-supported Al modified mesocellular foam catalyst

The solvent free hydrogenation of DOP to DEHHP over an Al modified MCF supported Rh catalyst.

Copper Corrosion Products Catalyzed Reduction of N-Nitrosodimethylamine with Iron

Copper corrosion products (Cu(OH)₂, Cu₂O, CuO and Cu₂CO₃(OH)₂) were applied to catalyze the reduction of N-Nitrosodimethylamine (NDMA) with iron. All the copper corrosion products showed catalytic abilities. Lower pH values and DO concentrations facilitated NDMA reduction in most cases. 1,1-dimethylhydrazine (unsymmetrical dimethylhydrazine, UDMH) and dimethylamine (DMA) formed during the degradation of NDMA. There were also some undetected products. Catalytic hydrogenation was proposed as the mechanism. The catalytic systems did not promote the formation of hydrogen atoms. The dissolved copper ions in these systems were too sparse to enhance the reaction. The smooth iron surface and formation of Cu₂O in each catalytic system explained the enhancement of NDMA removal. Different surface morphologies and states of Cu₂O accounted for the differences in NDMA removal and kinetics between the reaction systems. This technique could be an alternative for NDMA reduction and could broaden the application of copper corrosion products.