Study On CeO₂/TiO₂ Catalysts Doped With Mo For Selective Catalytic Reduction Of NO_x With Ammonia

Selective catalytic reduction?SCR?with ammonia is widely recognized as an effective and economical technology to remove NO_x in the industry,the technical core is the development of SCR catalysts.At present,commercial catalysts were faced with many problem in our country,such as dependence on import,high cost,high deactivation rate and toxicity of vanadia to the environment,etc.Therefore,we must develop a high stability,strong anti-toxic properties,low cost and environmentally friendly SCR catalyst system.The SCR reaction was systematically investigated by using CeO₂-MoO₃/TiO₂ catalysts in this dissertation.Firstly,CeO₂-MoO₃/TiO₂ catalysts prepared by three methods were investigated.The catalyst prepared by a single step sol-gel method showed the best SCR activity and resistance to 10%H₂O and 1000 ppm SO₂.The samples were characterized by BET,XRD,XPS,H₂-TPR and TEM.The results implied that large surface area,highly dispersed active species of Ce and Mo,presence of more Ce³⁺and chemisorbed oxygen,strong interaction among ceria,molybdenum and titania,and good redox ability might contribute to the superior de NO_x performance of the CeO₂-MoO₃/TiO₂ catalyst prepared by a single step sol-gel method.Based on the catalysts prepared by a step sol-gel method,the operating parameters were studied.The experimental studies demonstrated that,CeO₂-MoO₃/TiO₂ with a CeO₂:WO_3:TiO₂mass ratio of 20:10:100 exhibited the best activity toward NH₃-SCR reaction over a broad temperature range of 250-450?at a space velocity of 90,000 h^-1 and a[NH₃]/[NO]molar ratio of 1.The characterization results indicated that Mo doping could enlarge the BET surface area and total pore volume of the catalyst and inhibit the transformation of crystalline phase of TiO₂ carrier,thereby markedly increasing the concentration of Ce³⁺and chemisorbed oxygen on catalyst surface.These could contribute to increase redox ability,thereby promoting SCR reaction.Surface sulfation modifications by SO₂,H₂O and H₂SO₄ for CeO₂-MoO₃/TiO₂ catalysts were undertaken.From activity tests and characterization results,it was found that surface sulfation could reduce the NO removal efficiency of catalysts.The increase of crystallinity of active components,the sulfation of catalyst active phase and the decrease of redox ability were proved to be the main reasons for the catalyst deactivation during SCR reaction.The reaction kinetics were studied on the selective catalytic reduction of NO with NH₃over the CeO₂-MoO₃/TiO₂ catalyst.The result showed that the reaction was a first-order reaction with NO,a zero-order reaction with NH₃ and nearly quarter-order with O₂.The reaction followed the Eley-Rideal mechanism model.The activation energy of CeO₂-MoO₃/TiO₂catalysts was 47.39 kJ/mol.