载体焙烧温度对NO_x储存还原催化剂K/Pt/TiO_2-ZrO_2结构与性能的影响

Effects of Support Calcination Temperature on the Structure and Properties of NO_x Storage and Reduction Catalyst K/Pt/TiO_2-ZrO_2

采用共沉淀法制备出TiO2-ZrO2复合氧化物载体,然后用分步浸渍法制备出K/Pt/TiO2-ZrO2催化剂,考察了载体焙烧温度对催化剂结构和储存氮氧化物性能的影响.X射线衍射结果表明,500℃焙烧载体后催化剂样品为无定形结构,650℃焙烧时开始出现ZrTiO4晶体,并随焙烧温度提高晶形越来越好.NH3程序升温脱附结果表明,500℃焙烧的载体有最大的酸量,但随焙烧温度升高,酸量显著下降,1000℃焙烧后,载体基本无酸性.比表面积和NOx储存量测定结果表明,样品对NOx的储存能力与比表面积之间无顺变关系,载体于500℃焙烧的样品对NOx的储存性能最差,而于800℃焙烧的样品储存性能最佳.原位漫反射傅里叶变换红外光谱结果表明,载体于500℃焙烧的样品中NOx以自由NO3-以及单齿或双齿硝酸根离子的形式存在,而在其它温度焙烧时,只检测到自由的NO3-物种.焙烧温度不仅影响载体的结构和酸碱性,而且影响载体与负载组分间的相互作用,载体表面羟基与K2CO3相互作用形成稳定的-OK基团对NOx储存不利,而高分散的K2CO3相则有利于将NOx物种以硝酸盐的形式储存起来.

The effects of support calcination temperature on the NOx storage capacity of the K/Pt/TiO2-ZrO2 catalyst were studied by X-ray diffraction （XRD）, NH3 temperature-programmed desorption （TPD）, N2 adsorption, and in situ diffuse reflectance infrared Fourier transform spectroscopy （DRIFT）. The XRD results indicate that the catalyst is amorphous with support calcined at 500 12. With the elevation of calcination temperature to 650 12, ZrTiO4 crystals are formed, whose crystallization becomes more and more complete at higher temperature up to 1 000 12. The results of NH3-TPD reveal that the support calcined at 500 12 possesses the biggest amount of acidic sites; however, it decreases remarkably as the calcination temperature increases. After calcination at 1 000 12 , the support nearly loses all the acidity. The NOx storage capacity of the samples is not proportional to the BET surface area of the supports. The sample with the support calcined at 500 ℃ shows the lowest NOx storage capacity, whereas the sample with the support calcined at 800 12 exhibits the highest NOx storage capacity. The results of DRIFT show that NOx is stored as NO3- and bidentate/monodentate nitrate species in the sample with the support calcined at 500 ℃ , whereas only NO3- species is detected in other samples with the support calcined at higher temperatures. As a result, the calcination temperature influences not only the structure and acidity of the support but also the interaction between the support and the supported components. The formation of the -OK group arising from the interaction between the surface hydroxyl groups of support and the K-containing phases is not favorable to NOx storage, while the highly dispersed K2CO3 phase facilitates the NOx storage as nitrate.