| Abstrakt: |
Heterogeneous (photo)catalysts are often complex mixtures of different nanostructured oxidic compounds. Chemical and electronic interactions within such combined materials may play a key role in improving the performance in technological applications but are difficult to investigate under technical conditions. This work presents a systematic study of the interactions between tungsten oxide clusters and the underlying rutile TiO2(110) surface in special consideration of point defects such as Ti3+interstitials. Using electron beam evaporation from WO3powder, stoichiometric (WO3)n, and oxygen-deficient (WO3–x)n, tungsten oxide clusters are produced simultaneously. Based on cluster coverage- and temperature-dependent X-ray photoelectron spectroscopy studies, the formation of a surface layer of stoichiometric and oxygen-deficient tungsten oxide clusters is shown, and the formation of mixed oxides can be excluded. For the first layer up to 7.1 WO3nm–2, stoichiometric clusters are dominant at the TiO2surface. The lack of W5+indicates an electron transfer from the clusters toward the substrate under formation of Ti3+interstitials. Furthermore, we found at elevated temperatures relevant for catalytic reactions that the tungsten oxide clusters are more stable on TiO2surfaces than on other substrates such as the silicon oxide layer of Si wafers. Up to 900 K, only slight changes were observed on titania. We observed an accumulation of Ti3+at the TiO2surface between 500 and 800 K in the case of high bulk Ti3+content in TiO2. As the Ti3+accumulation is accompanied by significant changes of the W 4f signals, we suggest an interaction between these sites under a possible generation of surface fields or an anionic [(WO3)n]z−-like cluster by electron transfer from Ti3+. |
