The presence of chemically active oxide supports in catalytic systems may result in substantial changes in reactivity as compared to catalysts supported on inert oxides. In particular, the support can participate in surface reactions, providing active surface species for further elementary reaction steps on the metal particles. The coupling of reaction sites on the support and on the metal particles may critically modify the reaction mechanisms, the overall activity and the selectivity towards different reaction pathways. Specific cases are scarce, however, in which a microscopic understanding of metal-support interaction has been achieved.
In the present study we explore the role of metal-support interaction for two well-defined Pd model catalysts based on the one hand on chemically active Fe3O4/Pt(111) and on the other hand on inert Al2O3/NiAl(110) thin oxide films. Specifically we compare methanol adsorption, dissociation and oxidation on these two catalyst systems applying a broad range of surface science techniques, which include molecular beam methods, time-resolved infrared reflection adsorption spectroscopy and TPD.
On Fe3O4/Pt(111) the formation of two types of methoxy surface species has been observed by IRAS. The methanol decomposition is in significant contrast to molecular methanol adsorption on Al2O3/NiAl(110). Desorbing methoxy species from Fe3O4/Pt(111) were shown to decompose into small amounts of formaldehyde. Moreover these methoxy groups exhibit an exceptionally high thermal stability - up to 500 K - and can spill-over to supported Pd particles at elevated temperatures. As a consequence the Fe3O4/Pt(111) support acts as a methoxy reservoir opening up new reaction channels on the Pd particles. In TR-IRAS experiments it has been shown that further dissociation of methoxy groups on Fe3O4/Pt(111) supported Pd particles results in a significantly higher rate of carbon accumulation on the metal particles as compared to the Al2O3/NiAl(110) supported catalyst. The formation of Pd oxides at the metal-support interface [1] reduces the carbon accumulation rate on Pd particles supported on Fe3O4/Pt(111) by hampering the spillover of methoxy species across the metal-support interface.
[1] T. Schalow et al., Angew. Chem. Int. 2005, 44, 7601 |