Synthesis Of Pd-Cu/XC-72R Catalyst And Its Nitrate Catalytic Reduction Efficiency And Mechanism In Water

Nitrate anions?NO₃^-?can be widely found in surface water and especially in groundwater in China.Nitrate in drinking water can cause serious health problems in a direct or indirect way.Given that,the Standard for Drinking Water Quality of China?GB5749-2006?has established the maximum contaminant level for nitrate at10.0 mg/L as N.Heterogeneous catalytic denitrification technology,which was first reported by Vorlop and Tacke in 1989,can efficiently reduce nitrate to nitrogen gas over a bimetallic catalyst with no biological risk.Therefore,this method is considered to be the most promising denitrification process.However,the technology still faces the key problems of the catalyst development,which should have both high catalytic activity and N₂ selectivity.Focus on the above problems,the effects of support,bimetallic nanocrystal structure,particle size of Pd on the nitrate and/or nitrite catalytic reduction have been systematically investigation in the dissertation.Furtherly,highly catalytic active and N₂ selective Pd-Cu bimetallic catalyst have been synthesized and used for the nitrate catalytic reduction.Four typical materials,?-Al₂O₃,?-Al₂O₃,SBA-15 and XC-72R were selected as the supports,the monometallic Pd and bimetallic Pd-Cu catalysts with different nanocrystal structures were synthesized using impregnation and monodispersion method.The structures and properties of the catalysts were characterized using TEM,XRD and N₂-adsorption and desorption methods.The results showed that,the nanocrystal structure of the Pd-Cu catalyst prepared by impregnation and solvent reduction method was heterostructure and alloyed structure,respectively.And the results of catalytic experiments show that,the Pd-Cu bimetallic catalyst prepared by impregnation method showed a significantly higher nitrate removal rate and N₂selectivity,compared to which prepared by monodispersion method,implying that Pd-Cu catalyst with heterogeneous structure has higher catalytic activity and N₂selectivity.In addition,the effects of the support on the catalyst properties is mainly derived from the particle size of the metal nanoparticles and the support's properties.The mass transfer process of substrate and product inside the support pores is the main factor influencing the catalytic reaction activity and selectivity.Among these supports,XC-72R showed an excellent mass transfer effect due to its small particle size and high dispersion,and the catalyst with equal particle size supported on XC-72R showed the best comprehensive performance in terms of catalytic activity and N₂ selectivity.Based on the above results,the XC-72R was used as the support,and a series of Pd/XC-72R catalysts with different particle size range from 2⁸ nm were synthesized using liquid phase reducing method.The experiments results showed that the mass catalytic activity(A_mass)of Pd/XC-72R catalyst has a volcanic curve relationship with Pd nanometer size,and the A_mass reaches the maximum value of30.60 mg·g^-1_cat·min^-1 when the Pd particle size is 2.7 nm.The intrinsic catalytic activity?TOF?of the catalyst increases correspondingly with the increase of Pd particle size.Mechanism analysis showed a positive correlation between the TOF and the ratio of Pd bulk atoms,implying that the?-PdH has a higher reducing activity during the catalytic reaction.In addition,the ammonia selectivity of the Pd/XC-72R catalyst decreases when the Pd particle size increases.Therefore,the catalytic activity and N₂ selectivity of catalytic NO₃^--N and NO₂^--N reduction can be effectively optimized by adjusting the size of Pd nanoparticles.Based on the monometallic Pd/XC-72R catalyst,Pd-Cu/XC-72R catalysts were synthesized using surface reduction reaction method.The nitrate catalytic reduction experiment results showed that the mass ratio of Pd and Cu in Pd-Cu/XC-72R catalyst significantly affected the performances of the catalytic reaction,a 99.99%nitrate?c₀=50 mg/L?removal rate was obtained at the condition of mass ratio of6:1,catalyst dosage of 0.5 g/L,reaction time of 120 min,with an optimum catalytic activity of 1.495 mg/g_cat·min and N₂ selectivity of 89.04%.Reaction thermodynamics and kinetics results showed that the apparent activation energy of nitrate catalytic reduction reaction was 45.24 kJ/mol,and the nitrate reduction kinetics followed Langmuir-Hinshelwood mechanism,the kinetics model could be described as ln?c₀/c_t?/?c₀-c_t?+0.0425=0.0822×w_cat×p _H2^0.85×t/?c₀-c_t?.Influencing factors studies showed that the optimum reaction pH condition for catalytic nitrate reduction over Pd-Cu/XC-72R was 5.5.The inhibition effect of common anions on the catalytic reaction followed the order of SO₄^2-?SO₃^2-<<F^-<HCO₃^-<Cl^-,and the coexistence of Fe²⁺can cause an increase of ammonia selectivity.The catalytic activity sequence of Pd-Cu/XC-72R bimetallic catalyst for catalytic oxyanion reduction was ClO₃^-<NO₃^-<Cr₂O₇^2-<BrO₃^-.