Hydrothermal and sulfur aging of CeTi/CeWTi catalysts for selective catalytic reduction of NOx with NH3

The CeO2-TiO2(CeTi)and CeO2/WO3-TiO2(CeWTi)catalysts were prepared by sol-gel method.The asprepared catalysts were hydrothermally treated at 760℃for 48 h in air containing 10 vol%H2O to obtain the hydrothermal aged catalysts.The sulfur aged catalysts were treated at 400℃with 100 ppm SO2,10%water vapor,air balance for 48 h and catalysts.The powder X-ray diffraction(XRD)and Raman results indicate that the crystallization of hydrothermal aged catalysts is more serious than sulfur aged catalysts.In addition,tungsten species can stabilize the CeTi catalyst from grain growth.According to the results of in situ diffuse reflectance infrared Fourier transform spectra(DRIFTS),temperatureprogrammed desorption of ammonia(NH3-TPD),H2 temperature-programmed reduction(H2-TPR)and ammonia oxidation,the aging process leads to loss of surface area,redox properties,surface acidities and surface ceria concentration,especially for the hyd rothermal aging.The NH3-NO/NO2 SCR perfo rmances of sulfur aged catalysts are better than that of hydrothermal aged catalysts.Compared with CeTi catalyst,the addition of tungsten inhibits the crystallization of catalyst.So that more acid sites and active sites are retained.This is also the reason why tungsten addition improves the NH3-NO/NO2 SCR performance of CeTi catalyst.

Low-temperature activity and mechanism of WO_3-modified CeO_2-TiO_2 catalyst under NH_3-NO/NO_2 SCR conditions

The CeO2‐TiO2(CeTi)and CeO2/WO3‐TiO2(CeWTi)catalysts were prepared by a sol‐gel precipitation method and their NH3‐NO/NO2 selective catalytic reduction(SCR)performance was studied.N2O formation and effect of oxygen concentration on SCR performance over CeWTi catalyst were also investigated while varying the NO2/NOx ratio.Results indicate that fast SCR behavior of CeWTi catalyst has the best NH3‐NO/NO2 SCR performance due to the catalyst reoxidation rate by NO2 higher than by O2.Compared with CeTi catalyst,CeWTi catalyst exhibits higher de‐NOx performance under NH3‐NO/NO2 SCR conditions.As the CeTi and CeWTi catalysts exhibit similar redox property,addition of WO3 provides more acid sites which accelerate the reaction between NH4NO3 and NO to get a superior low‐temperature activity.Amount of N2O formation shows a peak at 250 oC mainly derived from NH4NO3 decomposition.

离心喷雾条件对CeWTi脱硝催化剂性能的影响

以离心喷雾干燥技术为基础,采用浸渍法制备了CeO2-WO3/TiO2(CeWTi)脱硝催化剂,研究了制备过程中离心喷雾温度对喷雾速率及催化剂粉体形貌、粒度、比表面积和脱硝性能的影响,分析了喷雾温度影响的机制。结果表明,CeWTi催化剂的离心喷雾速率、粒度均随离心喷雾温度的升高而显著增大,但其焙烧前粉体的粒度却无显著变化,仅为1μm左右。这是由于喷雾干燥后主要形成Ce(NO3)3·4H2O和Ti(W)O2的团聚体,并未形成Ce-W-Ti-O的固溶体。因此,在测试粒度时由于Ce(NO3)3·4H2O的溶解,测得粒度不是喷雾粉体的实际粒度,而是钛钨粉的粒度。

柴油车用铈基SCR蜂窝催化剂制备工艺及性能研究

采用正交实验考察浆液浓度、铝溶胶用量、研磨时间及球料比对CeWTi催化剂涂覆效果的影响,对成型后的蜂窝催化剂进行XRD、BET及SEM表征,并研究了催化剂上载量对其脱硝性能的影响。结果表明,采用浆液浓度为45%,铝溶胶为20%,球磨时间为30min,球料比为8∶1,浆液粘度为4.7Pa·s的工艺条件涂覆成型的蜂窝CeWTi催化剂在200℃时NOx转化率为80%,250℃~500℃NOx转化率高达93%以上;催化剂的NOx和NH3转化率与涂覆量成正相关,但其N2选择性与涂覆量无明显对应关系。此外,比表面也是影响蜂窝催化剂SCR性能的主要因素之一。

pH值对浸渍法制备的铈钨钛脱硝催化剂的影响

通过调变分步浸渍法制备过程中的溶液pH值,制备了一系列的铈钨钛复合金属氧化物催化剂,以研究它们在NH3-SCR(NH3-Selective catalytic reduction)反应中的脱硝表现。重点考察了催化剂制备过程中溶液的pH值的变化对脱硝催化剂的影响。XRD表征结果显示,提高pH值有利于氧化钨在催化剂表面的高分散。XPS分析、NH3-TPD分析及NH3-SCR活性测试的结果显示,随着制备过程中pH值的增大,催化剂的表面W原子浓度、Ce^3+/Ce^4+比、Oα/Oβ比、表面酸量或NOx转化率都呈现先上升然后趋缓(或略降低)的变化趋势。高分散的WO3物种与CeO2物种之间的相互作用及表面W原子浓度一定程度的升高被认为是催化剂脱硝活性上升的主要原因。在pH=9的条件下制得的铈钨钛复合金属氧化物催化剂展现了最优的SCR脱硝活性,并在300℃~450℃范围内NOx转化率保持在90%以上。