摘要
选择性催化还原(SCR)是目前去除氮氧化物最有效的方法之一.V_2O_5/TiO_2催化剂被广泛应用于氨法选择性还原氮氧化物(NH_3-SCR)反应,但该催化剂存在工作温度高(300–400℃)及SO_2氧化率高引起设备腐蚀和管路堵塞等问题,开发低温SCR催化剂具有重要意义.过渡金属氧化物(如Fe_2O_3,MnO_x和CuO等)催化剂用于低温SCR先后见诸文献报道,但这些催化剂在SO_2和H_2O存在下易失活.近年来柱撑黏土(PILC)引起科学家广泛关注,Yang等首次将V_2O_5/TiO_2-PILC催化剂应用于NH_3-SCR反应,发现其催化活性高于传统V_2O_5/TiO_2催化剂.柱撑黏土基催化剂在NH_3-SCR反应中也显示出良好抗硫性能,但V_2O_5/TiO_2-PILC催化剂的抗硫机理至今尚未见深入研究.因此我们制备了一系列V_2O_5/TiO_2-PILC催化剂,采用原位漫反射红外等方法详细研究了其抗硫性能较好的原因.首先采用离子交换法制备出TiO_2-PILC载体,之后采用浸渍法制备了不同钒含量(质量分数x/%=0,3,4,5)的xV_2O_5/TiO_2-PILC催化剂.同时,制备了传统V_2O_5/TiO_2和V2O5-MoO_3/TiO_2催化剂作为对比.活性评价结果显示,4V/TiO_2-PILC催化剂具有最高的催化活性,其催化性能与传统钒钛催化剂相当.在160℃时,NO转化率可达80%以上.同时,4V/TiO_2-PILC催化剂还具有较宽的反应温度窗口,在260–500℃范围内,NO转化率保持在90%以上.向反应体系中加入0.05%SO_2和10%H_2O后,在低温(160℃以下)时所有催化剂的反应活性都有一定提高,可能是由于SO_2的加入提高了催化剂的表面酸性.继续升高温度,4V/TiO_2和4V6Mo/TiO_2催化剂活性均明显下降,而4V/TiO_2-PILC催化剂的活性则未出现明显下降.原位漫反射红外光谱结果显示,SO_2在三种催化剂表面的吸附以表面硫酸盐或亚硫酸盐物种以及离子态SO_4^(2–)物种形式存在,而在4V/TiO_2-PILC催化剂表面离子态SO_4^(2–)物种的量最少.X射线光电子能谱及O_2程序升温脱附结果显示,在4V/TiO_2-PILC催化剂上,表面吸附氧(Oads)的量最少.综合上述分析可以得出,在SO_2气氛下,离子态SO_4^(2–)物种在SCR催化剂表面的累积可能是导致其失活的主要原因,而离子态SO_4^(2–)物种的形成可能与催化剂表面吸附氧的量有关.
A titania pillared interlayered clay(Ti-PILC) supported vanadia catalyst(V2O5/TiO2-PILC) was prepared by wet impregnation for the selective catalytic reduction(SCR) of NO with ammonia. Compared to the traditional V2O5/TiO2 and V2O5-MoO3/TiO2 catalysts, the V2O5/TiO2-PILC catalyst exhibited a higher activity and better SO2 and H2O resistance in the NH3-SCR reaction. Characterization using TPD, in situ DRIFT and XPS showed that surface sulfate and/or sulfite species and ionic SO4^(2-)species were formed on the catalyst in the presence of SO2. The ionic SO4^(2-) species on the catalyst surface was one reason for deactivation of the catalyst in SCR. The formation of the ionic SO4^(2-) species was correlated with the amount of surface adsorbed oxygen species. Less adsorbed oxygen species gave less ionic SO4^(2-) species on the catalyst.
基金
supported by the National Natural Science Foundation of China(21277009,21577005)~~
关键词
选择性催化还原
二氧化钛柱撑粘土
氮氧化物
钒基催化剂
原位漫反射红外光谱
Selective catalytic reduction
TiO2-pillared clay
Nitrogen oxide
Vanadia catalyst
In situ diffuse reflectance infrared
Fourier transform spectroscopy