A Surface Spectroscopic Study of Supported Tungsten Oxide Catalysts Using XPS and SIMS/FAB

X-ray photoelectron spectroscopy (XPS) and secondary ion mass spectrometry by fast atom bombardment (SIMS/FAB) have been used to characterize the surface structure, surface properties, and depth profiles or five series of WO₃ supported on SiO₂, γ-Al₂O₃, TiO₂, ZrO₂and SnO₂.
The XPS data of five samples with WO₃ loading about equal to the close-packed monolayer coverage have shown that the tungsten content in the surface is 1-3 times greater than that in the bulk for WO₃ supported on γ-Al₂O₃, TiO₂, ZrO₂ and SnO₂. But for WO₃/SiO₂ the tungsten content in the surface is less than 1/3 of that in the bulk. These results reveal that WO₃ is highly dispersed on the supports γ-Al₂O₃, TiO₂, ZrO₂, and SnO₂ until the close-packed monolayer coverage is reached, while WO₃ is poorly dispersed on the SiO₂.
The SIMS/FAB results show that only molecular ions containing one W atom are observed in the SIMS/FAB spectra for WO₃/γ-Al₂O₃, WO₃/ZrO₂, WO₃/TiO₂ and WO₃/SnO₂ samples with WO₃ loading monolayer. However there are significant differences in the spectra for the samples with different supports. These differences may be attributed to the different coordination environments of the surface tungsten oxospecies.
The depth profiles of five samples with WO₃ loading equal to the theoretical monolayer coverage on above mentioned supports indicate that the SIMS intensities of tungsten species decline exponentially as the etching time increase for all samples except WO₃/SiO₂. It proves that WO₃ are dispersed as monolayer on the supports γ-Al₂O₃, TiO₂, ZrO-2 and SnO₂. For WO₃/SiO₂, the curves of SIMS intensities or W species against etching time at first rise and then decline. It may be due to the coexistence of the surface tungsten oxospecies and crystalline WO₃.
The above results suggest that the SIMS/FAB technique is very sensitive not only for the study of surface structure but also for the study of the dispersion of the active species on the support.