Nitrate (NO3(-)) and nitrite (NO2(-)) anions are often found in groundwater and surface water as contaminants globally, especially in agricultural areas due to nitrate-rich fertilizer use. One popular approach to studying the removal of nitrite/nitrate from water has been their degradation to dinitrogen via Pd-based reduction catalysis. However, little progress has been made towards understanding how the catalyst structure can improve activity. Focusing on the catalytic reduction of nitrite in this study, we report that Au NPs supporting Pd metal ("Pd-on-Au NPs") show catalytic activity that varies with volcano-shape dependence on Pd surface coverage. At room temperature, in CO2-buffered water, and under H2 headspace, the NPs were maximally active at a Pd surface coverage of 80%, with a first-order rate constant (k(cat) = 576 L g(Pd)(-1) min(-1)) that was 15x and 7.5x higher than monometallic Pd NPs (~4 nm; 40 L g(Pd)(-1) min(-1)) and Pd/Al2O3 (1 wt% Pd; 76 L g(Pd)(-1) min(-1)), respectively. Accounting only for surface Pd atoms, these NPs (576 L g(surface-Pd)(-1) min(-1)) were 3.6x and 1.6x higher than monometallic Pd NPs (160 L g(surface-Pd)(-1) min(-1)) and Pd/Al2O3 (361 L g(surface-Pd)(-1) min(-1)). These NPs retained ~98% of catalytic activity at a chloride concentration of 1 mM, whereas Pd/Al2O3 lost ~50%. The Pd-on-Au nanostructure is a promising approach to improve the catalytic reduction process for nitrite and, with further development, also for nitrate anions.

Pollutants in the form of heavy metals, fertilizers, detergents, and pesticides have seriously reduced the supply of pure drinking water and usable water. Gold metal has intriguing potential to deal with the water pollution problem, as recent research on several fronts is advancing the concept of nanoscale gold as the basis for cost-effective nanotechnology-based water treatment. Nano-gold has special properties, such as enhanced catalytic activity, visible surface plasmon resonance color changes, and chemical stability, that make it more useful than other materials. This Perspective article highlights the current use of gold nanoparticles for the efficient removal and the selective and sensitive detection of a variety of pollutants in water. The challenges in further developing nano-gold to address water contamination are discussed, which should stimulate future research into improved removal and detection of undesirable chemical compounds.

SignificanceChloroform is a common groundwater contaminant that is very difficult to remove. Chemically converting it into a less toxic form through heterogeneous catalysis is an attractive approach over conventional physical removal methods if it can be done economically. In this study, we explore the efficacy of supported precious metal catalysts for chloroform hydrodechlorination. We find that Pd/Al 2 O 3 is catalytically active for this reaction (6.4 L/g Pd /min) at room temperature, atmospheric pressure, in buffered water, and in the presence of hydrogen gas, and that Pd deposited on commercial Au/ Al 2 O 3 shows activities as high as 22.4 L/g Pd /min, suggestive of some Pd metal located on top of Au domains. The primary reaction product is methane, with selectivity values exceeding 90%. Surface-enhanced Raman spectroscopy shows evidence of chloroform adsorption and dechlorination on the catalyst surface under aqueous conditions. The results highlight the potential of ambient-condition reductive catalysis to remove chloroform from water.

BACKGROUND: Chloroform (CF), a common groundwater contaminant, can be degraded in deionized water reductively using Pd and Pd-Au catalysts under mild conditions (room temperature, atmospheric pressure) via hydrodechlorination (HDC). However, the performance of these catalysts under field-like conditions is unknown. This study evaluates the lab-scale performance and optimal operating conditions for flow reactors using