DFT Comparison of N-Nitrosodimethylamine Decomposition Pathways Over Ni and Pd

A DFT study on the adsorption and dissociation of N-Nitrosodimethylamine on a Ni8 nanocluster

N-Nitrosodimethylamine (NDMA, ONN(CH3)2) is a highly potent carcinogenic investigated by health authorities in some countries. In this manuscript, density functional theory (DFT) is applied to study the NDMA molecular and dissociative adsorption on a Ni8 nanocluster. Molecular adsorption is two times stronger than the NDMA adsorption on the Ni{111} surface. NDMA dissociative adsorption is found more stable than molecular adsorption by ≈1 eV. To dissociate the NDMA molecule into O and NN(CH3)2 fragments, an activation energy is calculated in 0.954 and 0.810 eV from the two most stable molecular configurations. However, to dissociate the NDMA molecule into ON and N(CH3)2 fragments, a smaller activation energy of 0.654 eV is calculated. With the inclusion of the London dispersion forces (optB88-vdW functional), NDMA molecular interactions are a bit stronger. However, the activation energies are slightly smaller. Meta-GGA functional SCAN has also, been applied. The inclusion of the implicit solvation model displays a NDMA weaker interaction with the Ni8 nanocluster. Dissociative adsorption is more stable than molecular adsorption, but the energy difference is a bit smaller, ≈0.850 eV. Present results show that the Ni8 nanoclusters are promising catalysts to NDMA elimination from water.

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.

Reductive destruction of multiple nitrated energetics over palladium nanoparticles in the H2-based membrane catalyst-film reactor (MCfR)

Nitrated energetics are widespread contaminants due to their improper disposal from ammunition facilities. Different classes of nitrated energetics commonly co-exist in ammunition wastewater, but co-removal of the classes has hardly been documented. In this study, we evaluated the catalytic destruction of three types of energetics using palladium (Pd⁰) nano-catalysts deposited on H₂-transfer membranes in membrane catalyst-film reactors (MCfRs). This work documented nitro-reduction of 2,4,6-trinitrotoluene (TNT), as well as, for the first time, denitration of hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) and pentaerythritol tetranitrate (PETN) over Pd⁰ at ambient temperature. The catalyst-specific activity was 20- to 90-fold higher than reported for other catalyst systems. Nitrite (NO₂^-) released from RDX and PETN also was catalytically reduced to dinitrogen gas (N₂). Continuous treatment of a synthetic wastewater containing TNT, RDX, and PETN (5 mg/L each) for more than 20 hydraulic retention times yielded removals higher than 96% for all three energetics. Furthermore, the concentrations of NO₂^- and NH₄⁺ were below the detection limit due to subsequent NO₂^- reduction with > 99% selectivity to N₂. Thus, the MCfR provides a promising strategy for sustainable catalytic removal of co-existing energetics in ammunition wastewater.

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.

Ruthenium Catalysts for the Reduction of N-Nitrosamine Water Contaminants

N-Nitrosamines have raised extensive concern due to their high toxicity and detection in treated wastewater and drinking water. Catalytic reduction is a promising alternative technology to treat N-nitrosamines, but to advance this technology pathway, there is a need to develop more-efficient and cost-effective catalysts. We have previously discovered that commercial catalysts containing ruthenium (Ru) are unexpectedly active in reducing nitrate. This study evaluated supported Ru activity for catalyzing reduction of N-nitrosamines. Experiments with N-nitrosodimethylamine (NDMA) show that contaminant is rapidly reduced on both commercial and in-house prepared Ru/Al₂O₃ catalysts, with the commercial material yielding an initial metal weight-normalized pseudo-first-order rate constant ( k₀) of 1103 ± 133 L·g_Ru^-1·h^-1 and an initial turnover frequency (TOF₀) of 58.0 ± 7.0 h^-1. NDMA is reduced to dimethylamine (DMA) and ammonia end-products, and a small amount of 1,1-dimethylhydrazine (UDMH) was detected as a transient intermediate. Experiment with a mixture of five N-nitrosamines spiked into tap water (1 μg L^-1 each) demonstrates that Ru catalysts are very effective in reducing a range of N-nitrosamine structures at environmentally relevant concentrations. Cost competitiveness and high catalytic activities with a range of contaminants provide a strong argument for developing Ru catalysts as part of the water purification and remediation toolbox.

Critical review of Pd-based catalytic treatment of priority contaminants in water

Catalytic reduction of water contaminants using palladium (Pd)-based catalysts and hydrogen gas as a reductant has been extensively studied at the bench-scale, but due to technical challenges it has only been limitedly applied at the field-scale. To motivate research that can overcome these technical challenges, this review critically analyzes the published research in the area of Pd-based catalytic reduction of priority drinking water contaminants (i.e., halogenated organics, oxyanions, and nitrosamines), and identifies key research areas that should be addressed. Specifically, the review summarizes the state of knowledge related to (1) proposed reaction pathways for important classes of contaminants, (2) rates of contaminant reduction with different catalyst formulations, (3) long-term sustainability of catalyst activity with respect to natural water foulants and regeneration strategies, and (4) technology applications. Critical barriers hindering implementation of the technology are related to catalyst activity (for some contaminants), stability, fouling, and regeneration. New developments overcoming these limitations will be needed for more extensive field-scale application of this technology.