Electrochemically converting nitrate, a widespread water pollutant, back to valuable ammonia is a green and delocalized route for ammonia synthesis, and can be an appealing and supplementary alternative to the Haber-Bosch process. However, as there are other nitrate reduction pathways present, selectively guiding the reaction pathway towards ammonia is currently challenged by the lack of efficient catalysts. Here we report a selective and active nitrate reduction to ammonia on Fe single atom catalyst, with a maximal ammonia Faradaic efficiency of ~ 75% and a yield rate of up to ~ 20,000 μg h−1 mgcat.−1 (0.46 mmol h−1 cm−2). Our Fe single atom catalyst can effectively prevent the N-N coupling step required for N2 due to the lack of neighboring metal sites, promoting ammonia product selectivity. Density functional theory calculations reveal the reaction mechanisms and the potential limiting steps for nitrate reduction on atomically dispersed Fe sites.

Insight into the nature of transient reaction intermediates and mechanistic pathways involved in heterogeneously catalyzed chemical reactions is obtainable from a number of surface spectroscopic techniques. Carrying out these investigations under actual reaction conditions is preferred but remains challenging, especially for catalytic reactions that occur in water. Here, we report the direct spectroscopic study of the catalytic hydrodechlorination of 1,1-dichloroethene in H2O using surface-enhanced Raman spectroscopy (SERS). With Pd islands grown on Au nanoshell films, this reaction can be followed in situ using SERS, exploiting the high enhancements and large active area of Au nanoshell SERS substrates, the transparency of Raman spectroscopy to aqueous solvents, and the catalytic activity enhancement of Pd by the underlying Au metal. The formation and subsequent transformation of several adsorbate species was observed. These results provide the first direct evidence of the room-temperature catalytic hydrodechlorination of a chlorinated solvent, a potentially important pathway for groundwater cleanup, as a sequence of dechlorination and hydrogenation steps. More broadly, the results highlight the exciting prospects of studying catalytic processes in water in situ, like those involved in biomass conversion and proton-exchange membrane fuel cells.

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Nitrate (NO3 −) is an ubiquitous groundwater contaminant and is detrimental to human health. Bimetallic palladium-based catalysts have been found to be promising for treating nitrate (and nitrite, NO2 −) contaminated waters. Those containing indium (In) are unusually active, but the mechanistic explanation for catalyst performance remains largely unproven. We report that In deposited on Pd nanoparticles (NPs) (“In-on-Pd NPs”) shows room-temperature nitrate catalytic reduction activity that varies with volcano-shape dependence on In surface coverage. The most active catalyst had an In surface coverage of 40%, with a pseudo-first order normalized rate constant of k cat ∼ 7.6 L gsurface-metal −1 min−1, whereas monometallic Pd NPs and In2O3 have nondetectible activity for nitrate reduction. X-ray absorption spectroscopy (XAS) results indicated that In is in oxidized form in the as-synthesized catalyst; it reduces to zerovalent metal in the presence of H2 and reoxidizes following NO3 − contact. Selectivity in excess of 95% to nontoxic N2 was observed for all the catalysts. Density functional theory (DFT) simulations suggest that submonolayer coverage amounts of metallic In provide strong binding sites for nitrate adsorption and they lower the activation barrier for the nitrate-to-nitrite reduction step. This improved understanding of the In active site expands the prospects of improved denitrification using metal-on-metal catalysts.

Groundwater contaminated by hazardous chlorinated compounds, especially chlorinated ethenes, continues to be a significant environmental problem in industrialized nations. The conventional treatment methods of activated carbon adsorption and air-stripping successfully remove these compounds by way of transferring them from the water phase into the solid or gas phase. Catalysis is a promising approach to remove chlorinated compounds completely from the environment, by converting them into safer, non-chlorinated compounds. Palladium-based materials have been shown to be very effective as hydrodechlorination catalysts for the removal of chlorinated ethenes and other related compounds. However, relatively low catalytic activity and a propensity for deactivation are significant issues that prevent their widespread use in groundwater remediation. Palladiumon-gold bimetallic nanoparticles, in contrast, were recently discovered to exhibit superior catalyst activity and improved deactivation resistance. This new type of material is a significant next-step in the development of a viable hydrodechlorination catalysis technology.

Perfluorooctanesulfonate (PFOS) is a persistent organic pollutant that is bioaccumulative and toxic. While its use in most countries has been restricted to certain industrial applications due to environmental and health concerns, chrome plating and semiconductor manufacturing facilities are industrial point sources of PFOS-containing wastewater. Current remediation technologies are ineffective at treating these highly concentrated industrial effluents. In this work, UiO-66 metal–organic frameworks (MOFs) of several defect concentrations were studied as sorbents for the removal of PFOS from concentrated aqueous solutions. PFOS sorption isotherms indicated that defective UiO-66, prepared with HCl as a modulator, had a maximum Langmuir sorption capacity of 1.24 mmol/g, which was ∼2× greater than powdered activated carbon (PAC), but ∼2× less than that of a commercial ion-exchange resin. Defective UiO-66 adsorbed PFOS 2 orders of magnitude faster than the ion-exchange resin. Large pore defects (∼16 and ∼20 Å) within the framework were critical to the increased adsorption capacity due to higher internal surface area and an increased number of coordinatively unsaturated Zr sites to bind the PFOS head groups. Of the common co-contaminants in chrome plating wastewaters, chloride ions have a negligible effect on PFOS sorption, while sulfate and hexavalent chromium anions compete for cationically charged adsorption sites. These materials were also effective adsorbents for the shorter-chain homologue, perfluorobutanesulfonate (PFBS). The enhanced PFOS and PFBS adsorptive properties of UiO-66 highlight the advantage of structurally defective MOFs as a water treatment approach toward environmental sustainability.

Concern over water contamination by per/polyfluoroalkyl substances (PFAS) has highlighted the lack of effective treatment approaches. Photocatalysis offers advantages of using ambient conditions for reaction, air as the oxidant, and light as the energy source, but identifying photoactive materials is challenging. Herein, we report that boron nitride (BN) degrades PFOA upon irradiation with 254 nm light. The ability of BN to degrade PFOA photocatalytically has previously been unreported and is unexpected, because its band gap is too large for light absorption. On the basis of scavenger results, we suggest that PFOA degrades in the presence of BN via a hole-initiated reaction pathway similar to the TiO 2 case and involves superoxide/hydroperoxyl and hydroxyl radicals. We surmised that defects allow BN to absorb in the UVC range and to photogenerate reactive oxygen species. Sealed batch studies indicated BN was ∼2 and ∼4 times more active than TiO 2 , before and after ball milling the material, respectively. BN can be reused, showing no decrease in activity over three cycles. BN was active for the photocatalytic degradation of GenX, another PFAS of concern. These findings present fresh opportunities for materials design and for the re-evaluation of other wide band gap semiconductors for PFAS photocatalytic degradation.

An unintended consequence of industrial nitrogen fixation through the Haber–Bosch process is nitrate (NO3 –) and nitrite (NO2 –) contamination of ocean, ground, and surface waters from fertilizer runoff. Transition-metal catalysts, particularly those based on Pd, are effective in removing NO3 –/NO2 – through reduction to N2 or NH4 +. Pd is regarded as the most effective metal for NO3 –/NO2 – reduction, and as such, few studies have thoroughly explored the performance of other transition metals as a function of varying reaction conditions. In this work, we investigated the NO2 – reduction properties of alumina-supported Rh using Pd as a benchmark, where we varied the bulk solution pH to probe the effect of reaction conditions on the catalytic chemistry. Pd expectedly showed a high reduction activity (289 L/g-surface-metal/min) and a high N2 selectivity (>99% at 20% conversion) at low pH and near inactivity at high pH. Surprisingly, the Rh catalyst, while inactive at low pH, showed moderate activity (22 L/g-surface-metal/min) and high NH4 + selectivity (>90% at 20% conversion) at high pH. Hydrazine (N2H4) was also detected as a reaction intermediate when NH4 + was formed. Microkinetic models built with energetics from density functional theory reveal that Rh catalysts are poisoned by NO* at low pH because of the rapid dissociative adsorption of protonated nitrite (HNO2) under acidic conditions, which was confirmed by in aqua surface-enhanced Raman spectroscopy. NO* poisoning of the Rh surface lessens at increased solution pH because NO2 – does not dissociate as readily compared to HNO2, which explains why Rh exhibits higher activity in basic solutions. The microkinetic models further elucidate the competition between N2H4 and NH3/NH4 + formation as a function of pH, where we find that hydrogen surface coverage dictates product selectivity. These results update the common view that only Pd-based catalysts are effective for NO2 – reduction and suggest unexplored avenues for nitrogen chemistry.