A Surface Science Study of Model Catalysts. 2. Metal−Support Interactions in Cu/SiO₂Model Catalysts

Abstract

The thermal stability of wet-chemically prepared Cu/SiO2 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 °C. Annealing in ultrahigh vacuum up to 620 °C does not result in sintering of the Cu particles. When a 5 nm thin SiO2 layer on top of a Si(100) substrate is used as SiO2 model support, interdiffusion of Cu into the Si substrate takes place. For a 400−500 nm thick SiO2 layer, Cu silicide islands are formed by reaction between Cu and SiO2, 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.

Reaction of nanometer-sized Cu particles with a SiO2 substrate

“…19,20 If the SiO 2 layer is not thick enough, the direct reaction between Cu and SiO 2 may compete with Cu diffusion through the SiO 2 layer to the SiO 2 /Si interface where silicide is formed, as described by Dallaporta et al 21 For Cu particles on a 5 nm SiO 2 layer on top of a Si͑100͒ wafer, we have clearly observed Cu interdiffusion in the Si substrate after UHV annealing up to 620°C. 4 Due to the wet-chemical preparation, Cu is fairly strongly attached to SiO 2 since no sintering of the Cu particles is observed during UHV annealing, and we believe that the silicide formation starts at the metal-support interface. Strongly bonded Cu may catalyze the Si-O bond breaking, since Cu also catalyzes the reverse reactions of Si-O formation.…”

“…Strongly bonded Cu may catalyze the Si-O bond breaking, since Cu also catalyzes the reverse reactions of Si-O formation. 13,14,[16][17][18] Metal evaporation as applied by others 9,21 may result in a weaker metal-support interaction 4 and, therefore, not result in a reaction between Cu and SiO 2 during UHV annealing. We note that also impurities on the sample surface or constituents of the residual gas may help to reduce SiO 2 .…”

“…This system serves as a model catalyst to provide detailed information on surface and interface phenomena such as sintering, wetting, metal sublimation, encapsulation of the metal particles by the support, metal interdiffusion, and reaction of the metal with the support. 3,4 Supported Cu catalysts are applied in, e.g., the water-gas shift reaction and methanol synthesis. 5 Cu is also of great importance in the microelectronics industry, especially since the recent introduction by IBM of a new technology using Cu wires to link transistors in chips.…”

Interaction of nanostructured metal overlayers with oxide surfaces

“…47 gives the schematic description of the processes which could happen at metal/SiO 2 /Si model systems. The typical chemical interactions described in Section 2.3.1, which include redox reaction, alloy formation, encapsulation, and interdiffusion, have been observed at metal-SiO 2 interfaces [100,593].…”

Comparison of Two Preparation Methods in the Redox Properties of Pd/CeO2/Ta/Si Model Catalysts: Spin Coating Versus Sputter Deposition

“…Using such a flat model catalyst has advantages on the parallel use of other surface science tools. For example, according to the work by van van den Oetellar [12], the MSI (metal-support interaction) was verified by the Cu/SiO 2 model catalysts prepared by spin coating. In addition, Brookshier and Goodman [13] reported the control of CuO particle size on SiO 2 by using spin coating method.…”