Pd、Pt及其双金属催化剂的表面特性和催化活性

Surface Character and Catalytic Activity of Pd, Pt and Pd-Pt Bimetallic Catalysts

发现由ESCA技术测得的Pd-Pt/Al_2O_3催化剂的表相Pt原子浓度低于体相浓度,Pd/Pt比低者其表相Pd原子浓度高于体相浓度,EPMA测定结果表明,表相Pd和Pt原子浓度均高于体相浓度。经CO处理后其表相Pd原子浓度降低,而Pt原子浓度却有所增高,可见Pt原子向催化剂表相的富集作用更为明显。从吸附的CO红外光谱也可证明,金属向催化剂表相的富集作用以CO在Pd-Pt/Al_2O_3上与Pt的键合能力比与Pd的键合能力强。H_2的低温和中温TPD峰面积与催化剂的Pd/Pt比和除H_2活性有着密切关系。

The metal atom concentrations on the surface layers of Pd/Al_2O_3, Pt/Al_2O_3 and Pd-Pt/Al_2O_3 cata-lysts were determined by ESCA method. It was found that the Pt atom concentration on the surface layer was lower than that of bulk for Pd-Pt bimetallic catalysts. The Pd atom concentrations on the surface layer of the supported catalysts increased with decreasing Pd/Pt ratio. EPMA determination showed the surface concentrations of Pd and Pt atoms were higher than that of bulk. When the Pd-Pt bimetallic catalysts were treated under CO atmosphere, the Pd and Pt surface concentration of the supported catalysts enhanced obviously. It is suggested that surface aggregation of metal atoms may occur on the supported catalysts. TPD profiles show that areas of low and medium temperature H_2 adsorption peaks on the Pd-Pt bimetallic catalysts are larger than that of Pd or Pt monometallic catalyst. It is interested to note that close relationships exist among the areas of low and medium temperature TPD peaks of adsorbed H_2, the Pd/Pt ratio of catalysts, and the hydrogen removal activity of Pd, Pt and Pd-Pt catalysts.