Study On The Hydrodechlorination Of Trichloroethylene Over Transition Mental Phosphides Catalysts

Trichloroethylene （TCE） is one of the most widely distributed volatile organic compounds that have been used as dry cleaning agent and metal-degreasing agent in industry. Thus, there is substantial interest in the development of efficient, economic methods for remediating ground water and soils contaminated with TCE. The catalytic hydrodechlorination （HDC） of TCE is a safe method and can meet the requirements of economic, environmental and the ecological balance. Over the past years, transition metal phosphides were employed as hydroprocessing catalyst using in hydrodenitrogenation （HDN） and hydrodesulfurization （HDS） and exhibited excellent reactivity. While up to now, little has been reported of the transition metal phosphide catalysts used in the HDC reactions. In this thesis, MoP/γ-Al₂O₃ was prepared by sol-gel-temperature programmed reduction （SG-TPR） method for the first time and compared its activity for HDC decomposition of TCE to those by impregnation and mechanical mixing method. The properties of the catalysts were evaluated by several different characterization techniques such as Nitrogen adsorption-desorption, X-ray diffraction （XRD）, X-ray photoelectron spectroscopy （XPS） and Transmission electron microscopy （TEM）. Hydrogen temperature programmed reduction experiments （H2-TPR）, NH3 temperature programmed desorption （NH3-TPD）, differential scanning calorimetry thermogravimetric analyses （DSC-TG）.Firstly, we prepared several transition metal phosphides by temperature programmed reduction, and employed them to the TCE HDC reaction. The results proved that MoP and M₂P had better catalytic activity than WP, Fe₂P and Co₂P.Secondly, we prepared MoP/γ-Al₂O₃ catalysts by several different methods and found that MoP/γ-Al₂O₃ prepared by SG-TPR method showed higher surface area （417.85m2/g） and catalytic activity than those prepared by impregnation and mechanical mixing method for the HDC decomposition of TCE. Also, we found 15wt% MoP/γ-Al₂O₃ prepared by SG-TPR method had the highest catalytic activity when the space velocity was 1500h^-1 and the temperature was 450℃. Then, the properties of the MoP/γ-Al₂O₃ catalyst were evaluated by XRD, XPS, H₂-TPR, NH₃-TPD. The results are as follows:（1） The MoP phase can be obtained at 650℃ by temperature programmed reduction. It was difficult to identify a MoP phase on alumina support catalysts prepared by impregnation method as the loadings were below 15wt%, and the MoP/γ-Al₂O₃ catalyst only showed weak characteristic peaks of MoP at 15wt% loadings because the MoP highly dispersed on the support. Due to the interaction of the MoP and alumina support, some peaks of MoP/γ-Al₂O₃ catalyst prepared by impregnation method at loadings more than 15wt% can’t match with the standard XRD characteristic peaks of MoP. We can’t get the crystal structure of the MoP in MoP/γ-Al₂O₃ prepared by SG-TPR at 650℃.（2） The XPS spectra indicate the presence Mo and P species with one or more states at the surface of the 15wt% MoP/γ-Al₂O₃ catalyst prepared by SG-TPR. It can be inferred that Mo exists as Mo⁶⁺, Mo⁵⁺, Mo⁴⁺, Mo³⁺, Mo0, and P exists as P⁰, P^3-,P⁵⁺ in this catalyst.（3） The NH₃-TPD indicates that many weak acid centers exist at the surface of the bulk MoP, MoP/γ-Al₂O₃ prepared by SG-TPR and impregnation method. The higher amount of the surface acid center, the higher hydrogenation dechlorination activity of the catalyst.Finally, we prepared Ni/γ-Al₂O₃, Ni3P/γ-Al₂O₃, Ni₂P/γ-Al₂O₃, NiP/γ-Al₂O₃ by SG-TPR method. Ni₃P/γ-Al₂O₃ shows the highest catalytic activity among these.