预处理条件对Ag/SBA-15催化剂上甲苯催化氧化反应性能的影响

甲苯是一种典型的挥发性有机污染物.近年来,催化氧化法是一种广泛使用并具有开发潜力的有效去除挥发性有机污染物的方法,而贵金属催化剂一直是首选.介孔二氧化硅材料SBA-15具有规则孔道和较高的比表面积因而在催化领域中具有较大的应用潜力.本课题组已对一系列的SBA-15负载的纳米银催化剂的制备和低温催化氧化性能进行了研究,本文则通过研究不同预处理处理气氛对Ag/SBA-15催化剂结构以及甲苯催化氧化性能的系统研究,获得纳米银催化剂结构与甲苯催化氧化性能间的构效关系,对新催化剂的结构优化以及甲苯催化净化的低温催化剂开发具有重要的科学意义.研究表明,处理气氛明显影响了银物种和氧物种的状态,进而影响了催化剂的催化活性,先氧气(500℃)后氢气(300℃)处理的O500-H300催化剂对甲苯的反应活性明显优于在500℃氧气处理样品O500及氢气处理样品H500.由X-射线衍射和O2-程序升温脱附(TPD)可知,氧气500 oC处理使催化剂上形成大颗粒银粒子和氧化银粒子,以及大量次表层氧物种.氢气处理使催化剂形成较大的银粒子,由于未经过氧气处理,该催化剂上并没有次表层氧的生成.先氧气处理再氢气处理后催化剂上形成高分散的小粒径银粒子以及次表层氧物种,这表明低温氢气处理可以降低银粒子的尺寸并使催化剂上的银粒子得到再分散,同时不会影响次表层氧物种的形成.从催化剂的甲苯吸附和TPD实验中看出,大尺寸银粒子对甲苯具有较强的吸附性能,从而有利于甲苯在低温的催化氧化,但是在高温反应中没有优势;小尺寸银粒子虽然对甲苯的吸附能力不强,但是对分子氧有较好的吸附作用,进而增强自身与甲苯的相互作用,而且也促进了分子氧的活化,预处理中形成的次表层氧有效增强了甲苯和银粒子的相互作用,因此,先氧气后氢气处理的O500-H300样品在反应中显示出最好的甲苯催化活性.

Effect of pretreatment conditions on catalytic activity of Ag/SBA-15 catalyst for toluene oxidation

The catalytic oxidation of toluene over Ag/SBA-15 synthesized under different pretreatment condi-tions, including O2 at 500 ℃ (denoted O500), H2 at 500 ℃ (H500), and O2 at 500 ℃ followed by H2 at 300 ℃ (O500-H300) was studied. The pretreated samples were investigated by N2 physisorption, X-ray diffraction, and ultraviolet-visible diffuse reflectance. The pretreatment atmosphere greatly influences the status of the Ag and O species, which in turn significantly impacts the adsorption and catalytic removal of toluene. Ag2O and amorphous Ag particles, as well as a large amount of subsur-face oxygen species, are formed on O500, and the subsurface oxygen enhances the interaction be-tween Ag species and toluene, so O500 shows good activity at higher temperature. However, its activity at lower temperature is not as high as expected, with a reduced presence of Ag2O and lower adsorption capacity for toluene. H2 pretreatment at 500 ℃ is conducive to the formation of large Ag particles and yields the largest adsorption capacity for toluene, so H500 exhibits the best activity at lower temperatures; however, because of poor interaction between Ag and toluene, its activity at higher temperature is modest. The O500-H300 sample exhibits excellent catalytic activity during the whole reaction process, which can be attributed to the small and highly dispersed Ag nanoparti-cles as well as the existence of subsurface oxygen.