Electron Transfer in Oxide–Oxide Cocatalysts: Interaction of Tungsten Oxide Clusters with Ti³⁺ States in Rutile TiO₂

Abstract

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 WO3 powder, 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 WO3 nm–2, stoichiometric clusters are dominant at the TiO2 surface. 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 TiO2 surfaces 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 TiO2 surface 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+.

Conversion of methanol on rutile TiO2(110) and tungsten oxide clusters: 2. The role of defects and electron transfer in bifunctional oxidic photocatalysts

“…[50][51][52][53][54] We have thoroughly characterized the produced system combining X-ray photoelectron spectroscopy (XPS), LEED and probe molecule temperature-programmed desorption (TPD) spectroscopy in another recent work. 46 Since we use the same equipment, materials and devices, we have identical conditions to previous experiments. At this point, we refer to the original publication 46 for a detailed description and the accompanying work regarding the thermal chemistry of methanol on this system.…”

“…46 Since we use the same equipment, materials and devices, we have identical conditions to previous experiments. At this point, we refer to the original publication 46 for a detailed description and the accompanying work regarding the thermal chemistry of methanol on this system. 32 Here, we just summarize the major findings relevant for this work: up to 7.1 WO 3 nm À2 , a layer of stoichiometric clusters (W oxidation state +VI) is formed in direct interaction with the titania surface.…”

“…Again, the system is described elsewhere in more detail. 46 Here, identical conditions, devices and evaporating materials were used. Therefore, the surface characterization may be directly transferred from that publication.…”

“…Therefore, the surface characterization may be directly transferred from that publication. 32,46 Before use in TPRS experiments, the clusters were initially heated to 880 K (2 K s À1 , keeping 880 K for 1 minute) to remove possible residual contaminants from the cluster evaporation. Based on XPS and TPRS experiments, a significant role of surface hydroxylation by water adsorption from the chamber background can be excluded.…”

“…We shall combine these results with our recent finding, that an electron transfer between TiO 2 point defects and tungsten oxide clusters is possible under accumulation of surface near Ti 3+ sites and formation of partially anionic clusters. 46…”