Research On The Hydrodechlorination Of2,4-dichlorophenol Over Pd Catalyst Supported On Titanium-based Materials And Its Mechanism

Chlorophenols are prevalent chlorinated organic contaminants in environment and cause serious threatens to human health. The exploration of effective approach to remove chlorophenols has become increasingly urgent and desirable in recent years. Catalytic hydrodechlorination (HDC), a technology that could be operated under mild conditions and realize resource reuse without formation of more toxic by-products, has been considered as a promising method in the abatement of chlorophenols released in both atmospheric and aqueous environment. TiO2is extensively applied in photo-catalysis because of its low cost and non-toxicity while titanate nanotube (TNT) prepared from TiO2is ideal for environmental catalysis for its high surface area, mesoporous structure and excellent ion-exchange ability.In this study, TNT was prepared via one-step hydrothermal reaction. Supported Pd catalysts were synthesized by deposition-precipitation (DP), traditional impregnation (Imp) and photo-deposition (PD) with both TiO2and TNT(or TNT200) as supports. Catalysts and supports were characterized by transmission electron microscope (TEM), X-ray diffraction (XRD), Brunauer-Emmett-Teller specific surface area measurement (BET), Fourier transform infrared spectroscopy (FT-TR) and X-ray photoelectron spectroscopy (XPS) etc to acquire a deeper understanding of the as-synthesized materials.2,4-dichlorophenol (2,4-DCP) was utilized as probe molecule to evaluate the activity of aforementioned catalysts for its liquid-phase catalytic HDC. Furthermore, the reaction kinetics and factors influencing the HDC process were also investigated. The Langmuir-Hinshelwood model was used to interpret the reaction mechanism of HDC for2,4-DCP over Pd catalyst supported on TNT200.When P25was applied as support, both deposition-precipitation and traditional impregnation realized the load of Pd on the support. Catalyst prepared by deposition-precipitation exhibited better Pd dispersion and higher activity in HDC reactions due to the stronger salt-support interactions during the preparation process.2-chlorophenol (2-CP) was the single intermediate and phenol was the final product when2,4-DCP underwent HDC process over both catalysts. HDC reaction was affected by initial pH, catalyst dosage and initial reactant concentration. Acidic condition facilitated the HDC process while the deactivation of catalysts was also more favored in this situation. The increase of the dosage of catalyst contributed to faster initial consumption rate of reactant while this rate almost remained constant if normalized by the amount of catalysts used, indicating the mass transport limitation exerts little impact on HDC reaction. The initial activity was enhanced with increasing initial reactant concentration within a range, namely from0.62-3.11mmol·L-1, while initial activity was insensitive to the further increase of initial reactant concentration. Under given reaction conditions, both catalysts demonstrated good performance in the catalytic HDC process for2,4-DCP.TNT, used as Pd catalyst support, was synthesized from one-step hydrothermal method and Pd(2)/TNT and Pd(2)/P25were synthesized via photo-deposition. With different initial pH and initial reactant concentration, Pd(2)/TNT always exhibited higher activity since larger amount of hydroxyl groups on its support, which was proved by both FT-IR and traditional acid-base titration, resulted in better dispersion of Pd particles.2-CP and phenol were formed in the HDC of2,4-DCP over Pd(2)/TNT and Pd(2)/P25. Selectivity to2-CP was influenced by the difference of support surface properties and initial pH. Under the same reaction conditions and conversion of2,4-DCP, Pd(2)/TNT was more favorable for the formation of2-CP and for any given catalyst, higher pH led to the accumulation of2-CP and higher selectivity to this intermediate. Analysis for the reaction kinetics revealed that step-by-step dechlorination is the dominating process for the catalytic HDC of2,4-DCP on both catalysts while the weight of concerted dechlorination increased on Pd(2)/P25, a phenomenon that might still be related to the difference of catalyst surface properties.The as-synthesized TNT remained the typical mesoporous structure even after calcination at200℃, which was denoted as TNT200. The traditional impregnation method was applied to prepare Pd(2)/TNT200. Comparison between catalyst and support demonstrated that the impregnation did not change the mesoporous structure of TNT200but reduced its surface area, pore size and pore volume due to the Pd blocking effect. No Pd or PdO diffraction peaks appeared in XRD spectrum but XPS confirmed that Pd on the surface of catalyst existed as both Pd0and Pd2+species and Pd2+was stabilized by the residual Cl introduced by the impregnation process. Other than2-CP and phenol, a small amount of cyclohexanone was also formed when the HDC of2,4-DCP occurred on Pd(2)/TNT200. Step-by-step dechlorination was also the main process because of large amount of surface hydroxyl groups on the catalyst. According to Langmuir-Hinshelwood model, the relationship between initial activity and initial reactant concentration could be perfectly explained by competitive adsorption mechanism. Further investigation of this model proved that catalyst activity in HDC reaction was directly influenced by the adsorption between catalyst and reactant.The research confirms that Pd catalysts supported on titanium material exhibited good performance in the HDC of2,4-DCP and the surface properties of catalyst exerted significant impacts on both reaction process and product distribution. The improvement of2,4-DCP HDC activity not only depended on the optimization of reaction conditions but also on the selection of catalyst with proper adsorption ability to reactant.