Ultrafine Pd nanoparticles@g-C3N4 for highly efficient dehalogenation of chlorinated environmental pollutant: Structure, efficacy and mechanisms

Highlights: • Ultrafine Pd NPs embedded inside the well-ordered structure of g-C₃N₄ was prepared. • This catalyst had a rapid dehalogenation ability and strong catalytic activity. • NaBH₄ hydrolysis was controlled by the acid-catalyzed and Pd NPs-catalyzed pathways. • A synergistic effect between Pd nanocatalyst and g-C₃N₄ support was demonstrated. • Catalyst activity was due to the ultrafine Pd NPs and its electron-deficient nature. Catalytic hydrodechlorination represents one of the most promising strategies for the remediation of chlorinated environmental pollutants. However, the wide application of this approach in the practical settings is hampered by the efficiency and stability of catalysts as well as the danger of using molecular hydrogen as hydrogen source. By combining the advantages of both palladium nanocatalyst and graphitic carbon nitride (g-C₃N₄) as support, here we report the preparation of a hybrid material with ultrafine Pd nanoparticles (NPs, ca. 2 nm) finely embedded inside the well-ordered geometric structure of g-C₃N₄ by making use of the unique interaction between metal and nitrogen functional groups, and for the first time apply this catalyst for the remediation of chlorinated environmental pollutant. This catalyst (Pd NPs@g-C₃N₄) exhibited significantly better catalytic performance than any catalysts reported previously with catalytic activity as high as ca. 101 mmol• g⁻¹_Pd• min⁻¹ and the ability to achieve rapid complete dechlorination of 1.0 mM 4-chlorophenol (4-CP) within 2 min by using a very low dosage of NaBH₄ (2 equivalent) as hydrogen source. The distinctive catalytic dehalogenation property of Pd NPs@g-C₃N₄ was attributed to i) the ultrafine Pd NPs that activated molecular hydrogen into numerous reactive species (e.g., atomic hydrogen) and ii) the electron-deficient nature of Pd nanocatalyst due to the presence of g-C₃N₄ support that greatly facilitated the electron transfer on the catalyst surface. This hybrid Pd NPs@g-C₃N₄ catalyst displayed a rapid dechlorination ability, wide pH applicability, good stability and recyclability. This study provides a promising alternative catalyst for highly efficient dehalogenation of chlorinated environmental pollutants and can shed light onto developing other powerful catalysts for the remediation of these recalcitrant contaminants in water. This work provides new insights into the preparation of ultrafine metal catalyst by using nitrogen functional groups and the possible effect of support to the electronic structure of Pd nanocatalyst.