Study On The Mechanism Of The Hydrodechlorination Of 4-CP By The Ultrafine Palladium Nanocatalyst Supported By G-C₃N₄

Chlorophenols have acute toxicity,resistance to biodegradation and strong bioaccumulation property.Studies have shown that they have serious harm to human health.Therefore,their disposal has attracted many people’s attention,and it is urgent to study technologies for the safe treatment of chlorophenols pollutants.Catalytic hydrodechlorination represents one of the most promising strategies for the remediation of chlorinated environmental pollutants,because it requires only mild reaction conditions,has recyclable products and no secondary pollution,and is suitable for a wide range of chlorinated compounds.However,the wide application of this approach in the practical settings is hampered by the ef ficiency and stability of catalysts as well as the danger of using molecular hydrogen as hydrogen source.Graphitic carbon nitride（g-C₃N₄）possesses large surface area,excellent thermal and chemical stability.g-C₃N₄ is rich in nitrogen functional groups which can serve as anchor sites to disperse and stabilize the metal nanoparticles（NPs）.By combining the advantages of both palladium nanocatalyst and g-C₃N₄ as support,here we report the preparation of a hybrid material with ultrafine Pd NPs（,ca.2 nm）（Pd NPs@g-C₃N₄）finely embedded inside the well-ordered geometric structure of g-C₃N₄ by making use of the unique interaction between metal and nitrogen functional groups.The catalysts were characterized in detail by i nductive coupled plasma-mass spectroscopy（ICP-MS）,transmission electron microscopy（TEM）,Fourier transform infrared spectra（FTIR）,X-ray photoelectron spectroscopy（XPS）,and powder X-ray diffraction（XRD）.For the first time,we apply this catalyst for the remediation of chlorinated environmental pollutant.The major operating variables affecting HDC efficiency such as Pd loading amount,p H,Na BH ₄ dosages,catalyst dosages,and reaction temperature were systematically studied to determine the optimum reaction conditions.The optimum reaction conditions:25°C,p H=2.0,2.0m M Na BH₄,0.5 wt.%Pd loading amount,0.1 g Pd NPs@g-C₃N₄.This catalyst exhibited significantly better catalytic performance than any catalysts reported previously with catalytic activity as high as c a.101 mmol·g^-1Pd·min^-1 and the ability to achieve rapid complete dechlorination of 1.0 m M 4-chlorophenol（4-CP）within 2min by using a very low dosage of Na BH₄（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 and accelerated the electron transfer from electron donor（e.g.Na BH ₄）to the surface of the catalyst,thus significantly enhancing the catalytic activity of the catalyst.This hybrid Pd NPs@g-C₃N₄ catalyst displayed a rapid dechlorination abilit y(rate constant was 0.0167 s^-1),wide p H applicability（p H 2～9）,good stability and recyclability（After five time recycles,the dechlorination efficiency could still achieve ca.85%）.This study provides a promising alternative catalyst for highly effici ent 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.