负载Pd密偶催化剂的载体效应  

Effect of Support on Close Coupled Supported Palladium Catalysts

作　　者：李想[1] 孟明[1] 刘咏[1] 罗金勇[1] 

机构地区：[1]天津大学化工学院催化科学与工程系,天津300072

出　　处：《催化学报》2007年第9期835-840,共6页

基　　金：国家高技术研究发展计划(863计划;2006AA06Z348);国家自然科学基金(20676097)

摘　　要：采用尿素水解法或吸附沉淀法制备了金属氧化物载体,并用浸渍法负载0.5%Pd制得了Pd/Sn0.4Zr0.6O2,Pd/ZrO2,Pd/SnO2,Pd/SnO2-Al2O3和Pd/Al2O3催化剂.采用原位漫反射红外光谱、拉曼光谱、X射线光电子能谱和程序升温还原等方法对催化剂结构进行了表征,探讨了不同载体对表面PdOx物种化学吸附性质和氧化还原性能的影响,并与样品的丙烷氧化活性相关联.漫反射红外光谱表明,在Pd/SnO2-Al2O3中,Sn对Al2O3表面的Pd原子簇起到稀释作用,促进了Pd的分散,使得其CO线式吸附强度明显高于Pd/Al2O3,但Pd过高的分散度不仅减少了表面Pd-PdO活性位对的数目,而且使反应中间物种Pd-OH之间脱水困难,因而阻塞了活性位,降低了其循环氧化还原活性;而在Sn0.4Zr0.6O2复合氧化物载体中,SnO2有效地阻止了四方晶相ZrO2向稳态单斜晶相转变,且复合载体的比表面积较ZrO2和SnO2有所增加,其表面PdOx物种的分散度适中.此外,Sn0.4Zr0.6O2复合氧化物负载的Pd的价态介于Pd0与Pd2+之间,表面氧空位较多,促进了丙烷中C-H键的活化,使比表面积较低的Pd/Sn0.4Zr0.6O2具有最好的催化丙烷氧化能力,相反比表面积较高的Pd/SnO2-Al2O3活性很差,说明分散度适中且具有较低氧化态的PdOx(0<x<1)微晶是丙烷氧化的主要活性相,而高分散PdOx物种对丙烷氧化活性不利.The ZrO2, SnO2, and SnO2-ZrO2 oxides were prepared by the urea-hydrolysis method, and the SnO2-Al2O3 was prepared by the adsorption-precipitation method. A series of catalysts with 0.5 % Pd supported on these oxides were prepared by the impregnation method. X-ray photoelectron spectroscopy, in-situ diffuse reflectance FT-IR （DRIFTS）, laser Raman spectroscopy, and temperature-programmed reduction were employed to characterize the catalysts. The effect of support on the ehemisorption and redox ability of the supported palladium oxide species （PdO~） was investigated and correlated with the C3H8 oxidation performance of the catalyst. The DRIFTS results indicate that the dilution effect of Pd cluster on Al2O3 by Sn in Pd/SnO2-Al2O3 catalyst increases both the dispersion of Pd and the intensity of linear CO adsorption. However, the dilution effect probably decreases the amount of Pd-PdO active site pairs and inhibits the condensation of palladium hydroxides （Pd- OH）, which blocks the active sites and thus decreases the catalytic activity. In Sno.4Zr0.6O2 support, SnO2 prevents the transformation of tetragonal phase ZrO2, and the surface area of Sn0.4Zr0.6O2 binary oxide is larger than that of SnO2 and ZrO2. As a result, the PdO, crystallites possess a moderate dispersion on Sn0.4Zr0.6O2 support. Additionally, the interaction between Pd and Sn0.4Zr0.6O2 support leads to the oxidation state of Pd between Pd^0 and PdO, making the PdO, more favorable to activating C-H bonds. The results of activity evalua- tion reveal that Pd/Sn0.4Zr0.6O2 with lower surface area shows the best C3H8 oxidation activity, whereas Pd/SnO2-Al2O3 with higher surface area gives very low oxidation activity. This indicates that the highly dispersed PdOχspecies appear to be responsible for the detrimental effect on the C3H8 oxidation process, whereas the PdOχ （χ 〈 1） crystalline with moderate dispersion and low oxidation state is the main active component.

关 键 词：钯 载体效应 原位漫反射红外光谱 拉曼光谱 丙烷 氧化 

分 类 号：O643.36[理学—物理化学]