【摘要】 采用多种物理化学手段研究了不同负载量V2O5/TiO2催化剂的VOx物种分散状态、表面酸性、可还原性及其选择性催化还原(SCR)NO性能.结果表明,V2O5在锐钛矿TiO2表面的实测单层分散容量约为1.14mmol V/100m2TiO2,与"嵌入模型"的估算值相符,表明分散态的钒离子应键合在TiO2表面的八面体空位上.随着V2O5负载量的增加,V2O5/TiO2催化剂上NO转化频率(TOF)先急剧增加,至0.70mmol V/100m2TiO2(略超过分散容量的一半)时达到极大(约8.3×10-3s-1),然后又急剧下降;同时,孤立VOx物种可能倾向于分散在相邻的八面体空位上,且通过V-O-V化学键相连形成聚合的VOx物种,V-O-V键所占比例增加而V-O-Ti键所占比例减小,催化剂表面单位钒离子的Brnsted酸中心量增加,故催化剂的TOF急剧增加.随着负载量进一步增加,虽然催化剂表面单位钒离子的Brnsted酸中心量仍缓慢增加,但V-O-Ti键所占比例减少,导致钒离子的可还原性下降,另外,分散容量以上时晶相V2O5的形成也导致钒离子表面利用率下降,从而导致催化剂的TOF下降.桥式Brnsted酸位(V-O(H)-V)也是SCR反应活性中心之一,不同负载量V2O5/TiO2催化剂上SCR活性与表面VOx物种的分散状态、表面酸性和钒离子可还原性密切相关.更多还原

【Abstract】 The effect of the dispersion state and surface properties of supported vanadia on the selective catalytic reduction(SCR) activity of NO over V2O5/TiO2 catalysts was studied by various experimental techniques.The experimental monolayer dispersion capacity of V2O5 on anatase(6.86 VOx/nm2) measured by XRD was almost the same as the concentration of surface vacant sites of anatase estimated by the incorporation model,and it was suggested that isolated vanadia species tend to be dispersed on adjacent octahedral vacant sites.An increase of the NO turnover frequency(TOF) at 300 °C to a maximum(8.3 × 10-3 s-1) at a coverage near half a monolayer was related to the increase of the amount of weak acid sites(Brnsted acid site on each vanadium ion).The TOF decreased rapidly at high VOx coverages because of a decrease of the reducibility of vanadia species and a decrease of the ratio of exposed vanadia species on the surface.The Brnsted acid sites on bridging V-O(H)-V and terminal V-OH of polymeric vanadia species were all active sites in the SCR reaction.The SCR activity of the V2O5/TiO2 catalysts was related to the dispersion state,acidity,and reducibility of the vanadia species.更多还原