The Study Of Liquid Catalytic Reduction Of N-nitrosodimethylamine Over Bimetallic Pd-Ni Catalysts

N-nitrosodimethylamine (NDMA) has been widely detected as disinfection byproducts in drinking water and wastewater effluent, and NDMA is defined as "Probable Human Carcinogen" by the U.S. Environmental Protection Agency (EPA). The frequent detection of NDMA in source water raises serious concerns for its potential threat to public health. Therefore, there is a considerable need for developing rapid and effective methods to remove NDMA from drinking water.Compared with traditional removal technologies, liquid catalytic hydrogeneration has a lot of advantages in fast reaction rate, mild conditions, high product selectivity, and no secondary pollution, it exhibits great potential in water treatment. In this paper, Al2O3 was selected as the support, and Pd-Ni loaded catalysts were prepared, the preparation The as-prepared catalysts were characterized by general characterization techniques, such as X-ray diffraction(XRD), transmission electron microscopy(TEM), X-ray photoelectron spectroscopy(XPS), N2 adsorption-desorption, inductive coupled plasma emission spectrometer(ICP), zeta potential, and CO chemisorptions etc. Based on the experimental and characterization results, we obtained the mechanism of catalytic hydrogegeration. The main research contents and conclusions as follows:Catalytic reduction of N-nitrosodimethylamine (NDMA) was investigated over γ-Al2O3 supported bimetallic Pd-Ni catalysts (3%(PdxNi1-x)). NDMA could be reduced to dimethylamine over 3%(Pd0.8Ni0.2) with a metal-loading-normalized pseudo-first-order rate constant of 836±21L gme-1 h-1, which was more than 10 times larger than previous values achieved by other researchers. Characterization results showed that the reducibility of PdO (or NiO) was improved by addition of Ni (or Pd) in 3%(PdxNi1-x); Pd-Ni ensembles were formed in 3%(PdxNi1-x) and there was an electronic transfer from Pd to Ni; metal dispersion was affected by the formation of Pd-Ni ensembles and a volcano curve of metal dispersion via Pd/Ni ratio was observed. The activity profiles demonstrated that TOF had a significant positive relationship with metal dispersion of 3%(PdxNi1-x), indicating high metal dispersion favor NDMA reduction. NDMA reduction over 3%(Pd0.8Ni0.2) catalyst could be described by Langmuir-Hinshelwood model, reflecting an adsorption controlled reduction mechanism. The reduction mechanism of NDMA over 3%(Pd0.8Ni0.2) catalyst was proposed to be that NDMA molecule could adsorb onto Pd or Ni/NiO and then be activated. Meanwhile, the adsorbed H2 was activated by Pd and "spillover" to the surface of Ni/NiO, and then the N-N bond in NDMA was cleaved by active H. Finally, DMA and nitrosyl radical were formed and the latter was further reduced to ammonium.To explore a higher activity of NDMA reduction, Pd-Ni/CeO2-TiO2 catalysts were also prepared. Characterization results showed that the doped titanium could effectively inhibit the growth of cerium oxide crystal particles, and particle sizes of CeO2-TiO2 were affected by the Ce/Ti ratio. Moreover, the doped titanium could effectively enhance the metal dispersion of Pd-Ni/CeO2-TiO2 catalysts. Pd-Ni/Ce4Ti presented a higher metal dispersion and smaller metal particle size. Experimental results showed the catalytic activities of Pd-Ni/CeO2-TiO2 catalysts were listed in the order:Pd-Ni/Ce4Ti> Pd-Ni/Ce9Ti> Pd-Ni/Ce2Ti> Pd-Ni/Ce1Ti. Metal dispersion was affected by Ce/Ti ratio, and then enhanced the catalytic activity.