Effect of Dispersion State and Surface Properties of Supported Vanadia on the Activity of V2O5/TiO2 Catalysts for the Selective Catalytic Reduction of NO by NH3

Abstract

The effect of the dispersion state and surface properties of supported vanadia on the selective catalytic reduction (SCR) activity of NO over V 2 O 5 /TiO 2 catalysts was studied by various experimental techniques. The experimental monolayer dispersion capacity of V 2 O 5 on anatase (6.86 VO x /nm 2 ) 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 (Brönsted acid site on each vanadium ion). The TOF decreased rapidly at high VO x 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 Brönsted 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 V 2 O 5 /TiO 2 catalysts was related to the dispersion state, acidity, and reducibility of the vanadia species. 采用多种物理化学手段研究了不同负载量 V 2 O 5 /TiO 2 催化剂的 VO x 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, V 2 O 5 在锐钛矿 TiO 2 表面的实测单层分散容量约为 1.14 mmol V/100 m 2 TiO 2 , 与“嵌入模型”的估算值相符, 表明分散态的钒离子应键合在 TiO 2 表面的八面体空位上. 随着 V 2 O 5 负载量的增加, V 2 O 5 /TiO 2 催化剂上 NO 转化频率 (TOF) 先急剧增加, 至 0.70 mmol V/100 m 2 TiO 2 (略超过分散容量的一半) 时达到极大 (约 8.3 × 10 −3 s −1 ), 然后又急剧下降; 同时, 孤立 VO x 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 VO x 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 Brönsted 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 Brönsted 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 V 2 O 5 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 Brönsted 酸位 (V–O(H) –V) 也是 SCR 反应活性中心之一, 不同负载量 V 2 O 5 /TiO 2 催化剂上 SCR 活性与表面 VO x 物种的分散状态、表面酸性和钒离子可还原性密切相关. Isolated vanadia species interact with their nearest neighbor vanadia species through bridging V-O-V bonds to form polymeric vanadia species. The Brönsted acid sites on bridging V-O(H)-V are also active sites in the SCR reaction.