摘要
利用吡啶吸附-IR、氨吸附及TPD考察了添加碱金属元素锂以后,对于γ-Al_2O_3表面酸中心的类型、强度和数目的影响。发现所用γ-Al_2O_3表面酸中心主要为Lewis酸中心;锂的加入对于表面酸中心的类型无大的影响;然而,随着锂含量的增加,γ-Al_2O_3表面酸性较强的酸中心数目下降,而同时其总的酸中心数目并未下降,反而略有增加。表明锂的作用不是除去表面所有的酸中心,而是通过与表面强酸中心的作用,使其向弱酸中心过渡,从而改变γ-Al_2O_3表面的酸强度分布。临氢反应原位热重实验表明,锂的加入增强了γ-Al_2O_3表面抗积炭的能力,同时,锂对于Pt/Al_2O_3、Pt-Sn/Al_2O_3在烷烃脱氢反应中的表面积炭也有明显的抑制作用。
The effect of alkali metal lithium on the type, strength and number of acid sites of γ-Al2O3 was studied by pyridine-IR, and adsorption and TPD of ammonia. It is found that almost all of the acid sites on the surface of γ-Al2O3 are Lewis type. The number of the acid sites with strong acidity decreases when the Li content increases, however, the total number of acid sites increases slightly with the increasing of Li amount. Therefore, the role of Li is to modify and convert the strong acid sites into weak ones rather than to neutralize or eliminate all the acid sites on γ-Al2O3. Furthermore, the results of in-situ thermogravi-metry show that the rate of carbon deposition on the surface of γ-Al2O3 can be slowed down by the addition of lithium.
