Catalytic Hydrodehalogenation Of Emerging Pollutants Didofenac And Tetrabromobisphenol A Over Supported Pd Catalysts

Halogenated organic compounds, due to their potential impacts on the aqueous ecosystems and human health, were considered as one of the main representative pollutants. Nowadays, there is a rising concern with the occurrence and persistence of several new halogenated organic compounds including active pharmaceutical ingredients, disinfection byproducts, perfluorooctane sulphonates and brominated flame retardants. Therefore, development of effective treatment methods to abate these potential pollutants is highly demanded.Liquid phase catalytic hydrogenation has been recognized as an environmentally friendly, cost-competitive and effective technique to degrade halogenated organic pollutants. It has a strong capability to transform halogenated pollutants into environment friendly products. Meantime, synthesizing effective catalysts with controllable structure is also a very important aspect in catalytic hydrogenation. In the present study, diclofenac and tetrabromobisphenol A (TBBPA) were selected as target pollutants in the hydrodechlorination and hydrodebromination system. We prepared different supported Pd catalysts to study the catalytic hydrogenation mechanism of diclofenac and tetrabromobisphenol A systemically.For the catalytic reduction of diclofenac, palladium catalysts supported on Al2O3, activated carbon (AC), SiO2 and CeO2 were prepared using the impregnation and deposition-precipitation methods. Characterization results indicated that the Pd catalyst supported on CeO2 had a higher Pd dispersion than those supported on Al2O3, AC and SiO2. The binding energy of Pd 3d5/2 in Pd/CeO2 was higher than Pd/Al2O3 with a similar Pd loading amount. Additionally, for Pd/CeO2 prepared by the deposition-precipitation method the binding energy of Pd 3d5/2 slightly decreased with Pd loading amount. As for catalytic diclofenac reduction, Pd/SiO2 exhibited a nearly negligible catalytic activity, whereas diclofenac concentration decreased by 100,86, and 29% within 50 min of reaction on Pd/CeO2, Pd/Al2O3, and Pd/AC, respectively, indicative of a catalytic activity order of Pd/CeO2>Pd/Al2O3>Pd/AC>>Pd/SiO2. The hydrodechlorination of diclofenac on Pd/CeO2 could be well described using the Langmuir-Hinshelwood model. Diclofenac hydrodechlorination processed via a combined stepwise and concerted pathway, and increasing Pd loading amount in Pd/CeO2 favoured the concerted pathway. In comparison with original diclofenac, catalytic hydrodechlorination of diclofenac led to markedly decreased toxicity to Daphnia magna.For the catalytic reduction of TBBPA, Pd catalysts supported on TiO2, CeO2, Al2O3 and SiO2 were prepared by the impregnation, deposition-precipitation and photo-deposition methods. The catalysts were characterized by N2 adsorption-desorption isotherm, X-ray diffraction, transmission electron microscopy, measurement of zeta potential, CO chemisorption, and X-ray photoelectron spectroscopy. The results showed that at an identical Pd loading amount (2.0 wt.%) Pd particle size in dp-Pd/TiO2 was much smaller than those in im-Pd/TiO2 and pd-Pd/ToO2. Pd particle size of the dp-Pd/TiO2 catalyst increased with Pd loading amount. Additionally, Pd particles in the dp-Pd/TiO2 catalysts were positively charged due to the strong metal-support interaction, whereas the cationization effect was gradually attenuated with the increase of Pd loading amount. For the liquid phase catalytic hydrodebromination (HDB) of TBBPA, tri-bromobisphenol A (tri-BBPA), di-bromobisphenol A (di-BBPA), and mono-bromobisphenol A (mono-BBPA) were identified as the intermediate products, indicative of a stepwise debromination process. The catalytic HDB of TBBPA followed the Langmuir-Hinshelwood model, reflecting an adsorption enhanced catalysis mechanism. At an identical Pd loading amount, the Pd catalyst supported on TiO2 exhibited a much higher catalytic activity than those on other supports. Furthermore, dp-Pd/TiO2 was found to be more active than im-Pd/TiO2 and pd-Pd/TiO2.In order to improve the catalytic activity, we prepared supported bimetallic catalysts by the impregnation method. The promotion of monometallic catalysts by addition of a second metal is described in terms of two main aspects:electronic effects and geometric effects. To distinguish the electronic effects and geometric effects, we prepared two series of Pd-Au/Al2O3 catalysts with different Pd/Au ratio. Characterization results indicated that catalysts with a low Au content (Pd/Au ratio>40) has a similar Pd particle size in Pd/Al2O3. However, catalysts with a high Au content (Pd/Au ratio<20) exhibits a reasonable quality of Pd-Au alloying. Catalytic hydrodechlorination of diclofenac and 2,4-dichlorophenol (2,4-DCP) was studied over these catalysts. Comparing with the monometallic palladium, increasing Au content led to enhanced catalytic activity of Pd-Au/Al2O3 catalysts (Pd/Au ratio>40). For Pd-Au/Al2O3 catalysts (Pd/Au ratio<20), catalytic efficiency first increased then decreased with the increase of Au content.