A Surface Science Study of Model Catalysts. 2. Metal-Support Interactions in Cu/SiO_2 Model Catalysts

摘要

The thermal stability of wet-chemically prepared Cu/SiO_2 model catalysts containing nanometer-sized Cu particles on silica model supports was studied upon heating in hydrogen and ultrahigh vacuum. The surface and interface phenomena that occur are determined by the metal-support interactions. Heating in hydrogen results in a reduction of the metal-support interaction and sintering occurs via Cu particle migration at 350 ℃. Annealing in ultrahigh vacuum up to 620 ℃ does not result in sintering of the Cu particles. When a 5 nm thin SiO_2 layer on top of a Si(100) substrate is used as SiO_2 model support, interdiffusion of Cu into the Si substrate takes place. For a 400-500 nm thick SiO_2 layer, Cu silicide islands are formed by reaction between Cu and SiO_2, which can be regenerated by exposure to air at room temperature for several hours. The importance of the preparation procedure for the strength of the metal-support interaction is discussed. This paper shows that the use of model catalysts to study surface and interface phenomena is relevant for technical heterogeneous metal catalysis.