Synthesis And Catalytic Activity Of Transition Metal Nitrides For NO Dissociation And Reduction Reactions

Transition metal compounds,including nitrides,carbides and phosphides,have unique physical and chemical properties,which combine the characteristic properties of three different classes of materials:covalent solids,ionic crystals,and transition metals. Nevertheless,transition metal nitrides,carbides and phosphides have received far less attention in redox reactions.In the present work,in order to expand their application field, transition metal nitrides,carbides and phosphides were prepared by temperature-programmed reaction method,and the approach of enhancing the thermal stability of nitride and their activities for NO dissociation and reduction reactions were investigated.The main results presented in the dissertation have been summarized as followings:1.Amongst these metal nitrides(VN,Mo₂N,Fe₃N),Co₄N showed the highest activity for NO dissociation.Co₄N was prepared onγ-Al₂O₃ support and,in comparison with Co₄N, their microstructural and redox properties were investigated.The results indicated that, compared with Co₄N,Co₄N/γ-Al₂O₃ was not only more thermally stable,but also more active for low-temperature NO dissociation.2.The deactivation mechanism for NO dissociation reaction over cobalt nitrides and the activity for NO reduction over Co₄N/γ-Al₂O₃ indicated that there were two kinds of deactivation mechanisms in the reactions.Heavy accumulation of surface oxygen generated during NO dissociation caused temporary deactivation of the nitrides at low temperatures.By reduction with H₂,the deactivated samples can be regenerated.Surface oxygen gradually diffused into the nitride lattices at high temperatures,which caused the bulk oxidation and permanent deactivation of nitrides.A catalytic NO dissociation and reduction cycle was established in NO/CO system below 200℃,at which surface oxygen from NO dissociation can not enter the nitride lattice.3.Based on the MS technique,the reaction process of NO reduction with H₂ over Co₄N/γ-Al₂O₃ catalyst and the effect of reaction temperatures and feed gas compositions on product concentration were investigated.The results indicated that,in 1%NO/1%H₂/He step-response at 300℃,the whole reaction process can be classified into three stages.In initial state,NO and H₂ were dissociative and reactive,there were a large amount of N₂ and H₂O,and a little NH₃ in products.Due to the first reduction of surface oxygen by H₂, more active sites were provided and hence the activity of NO dissociation was higher at initial state.Then,the concentration of N₂,H₂O and NH₃ became steady,and the steady state occurred.When the active sites were covered gradually by O^*,the active sites decreased and the NO^* reacting with NO^* or N^* to form N₂O became easy,and deactivated state occurred.Reaction temperatures and feed gas compositions had an important effect on product concentration over Co₄N/γ-Al₂O₃ catalyst in NO+H₂ reaction.4.A series of transition metal carbides and phosphides were prepared by temperature-programmed reaction method,and their activities for NO dissociation and reduction reactions were investigated.The results indicated that molybdenum carbide and iron phosphide showed good thermal stability.Through selecting appropriate temperature, reducing agent and feed gas composition,a catalytic NO dissociation and reduction cycle was established over molybdenum carbide and iron phosphide catalysts at 400℃in NO/CO system with 3:1 CO/NO ratio.