常压等离子体还原的Ni/-γAl_2O_3催化剂的程序升温脱附研究

TPD Studies on Ni/γ-Al_2O_3 Catalysts Reduced by Atmosphere Plasma

用程序升温脱附(TPD)手段考察了常规焙烧还原(GR)、焙烧后等离子体还原(PR)、未焙烧等离子体直接还原(PDR)三种方法制备的Ni/-γAl2O3催化剂的H2和CO2的吸附-脱附性能,并用X射线衍射和N2吸附方法进行了表征.结果表明,H2的化学吸附发生在活性组分Ni上,而CO2的化学吸附则主要发生在Al2O3载体的强碱性中心.等离子体还原(PR、PDR)的催化剂对H2和CO2的化学吸附量大大增加,且H2的脱附温度分别降低了55和69℃.以H2的化学吸附量为基础计算得到PR和PDR催化剂的分散度分别为32%和58%,分别是GR催化剂的1.23和2.23倍.等离子体还原的催化剂的典型特征是具有良好的分散性、更多的强碱中心以及较低的H2脱附温度.造成这些特征的原因是等离子体使催化剂在较低的温度和较短的时间内还原,最大程度地保持了载体的比表面积,改善了活性组分的分散度.

Ni/-γAl2O3 catalysts prepared by three different ways, general reduction （GR）, plasma reduction after calcination （PR）, and plasma direct reduction without calcination （PDR）, were investigated by temperatureprogrammed desorption （TPD） with H2 and CO2, X-ray diffraction, and N2 adsorption. The results showed that H2 was adsorbed mainly on the Ni active sites, whereas CO2 was adsorbed mainly on the strong alkaline sites of the support. Compared with that on the GR catalyst, H2 and CO2 adsorbed on the PR and PDR catalysts increased a lot, and the desorption temperature of H2 decreased by 55 and 69 ℃, respectively. An estimation based on assumption of H2 showed that the dispersions of GR, PR, and PDR catalysts were 26%, 32%, and 58%, respectively. The catalysts reduced by plasma had better dispersion, more alkaline sites, and lower H2 desorption temperature. Plasma reduction that was performed at a low temperature for a short time could preserve the structure of the support and improve the dispersion of the active sites, which were responsible for the good performance of these catalysts.