Halogenated organic pollutants are of major environmental concern due to their harmful impacts on human health and aquatic ecosystems. Such compounds contaminate aquatic systems through disinfection by-products, pharmaceuticals, surfactants, and solvents. Pd-based nanoparticulate catalysts have been shown to effectively treat halogenated organics by reductive hydrogenation, but present many challenges that inhibit their practical implementation, including the scarcity of palladium, its expensive cost, and non-selective reactivity due to homolytic H2 dissociation (targeting functional groups other than carbon-halogen bonds, thus hindering their effectiveness for dehalogenation). Single atom catalysts (SACs) have the ability to address each of these issues. Each metal atom in SACs is exposed to catalyze the reaction of interest, yielding an atomic efficiency of 100%; this makes Pd-based catalysts more sustainable and economically feasible than Pd nanoparticles, where only surface Pd atoms participate in the reaction and leave inner atoms hidden and wasted. Additionally, SAC architecture allows for high catalytic selectivity for the dehalogenation reaction due to heterolytic H2 dissociation. A Pd SAC (Pd1/SiC) was synthesized using a facile wet chemical method via surface modification of a silicon carbide substrate and subsequent photoreduction with a palladium precursor using UV-C light. Visual confirmation of single atom dispersion was shown using high-angle annular dark-field scanning TEM (HAADF-STEM). Valence and coordination environment was probed using X-ray absorption spectroscopy (XAS) and X-ray photoelectron spectroscopy (XPS) to confirm the absence of Pd-Pd bonds. Pd1/SiC outperformed Pd nanoparticles for the degradation of many halogenated organic compounds, including halogenated phenols, 2,4-dichlorophenoxyacetic acid (disinfection by-products), and tetrabromobisphenol A (a brominated flame retardant), reaching complete degradation within 10 minutes. In simulated water matrices with natural organic matter (NOM), which contains many C=C bonds, Pd1/SiC additionally performed well, targeting only the carbon-halogen bonds of the target compounds. These results indicate that Pd-based SACs are a viable option for water decontamination of halogenated organics, exhibiting excellent performance and degradation efficiency, while upholding sustainability and cost-effectiveness of Pd-based materials.

Palladium single atom catalyst for the selective degradation of halogenated organic pollutants

ACS Fall 2021

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