Secondary ion mass spectrometry (SIMS) with Bi₃ ⁺ primary ions as a sensitive probe of surface structures of heterogeneous catalysts

We prepared mesoporous Al₂O₃- or SiO₂-supported Fe-oxide nanoparticles and their geometric and chemical properties were studied. For Fe-oxide/SiO₂, transmission electron microscopy (TEM) images and X-ray diffraction (XRD) patterns clearly show structure of Fe-oxide with particle sizes of 7–15 nm. However, TEM and XRD did not show clear evidence for the structure of Fe-oxide on Al₂O₃, even though a higher dispersity of Fe-oxide on Al₂O₃ was suggested by elemental mapping techniques. Catalytic activity of Al₂O₃-supported Fe-oxide was much higher than that of the SiO₂-based catalyst for CO oxidation and NO reduction by CO, implying a higher effective surface area of Fe-oxide on Al₂O_3, also in good agreement with Fe-oxide being more highly dispersed on Al₂O₃. SIMS shed more light on the catalyst structure. The Fe⁺/Al⁺ ratio from Fe-oxide/Al₂O₃ was much higher than the Fe⁺/Si⁺ ratio from Fe-oxide/SiO₂, in line with our observations from TEM and catalytic activity that Fe-oxide is more highly dispersed on Al₂O₃ than on SiO₂. Moreover, not only Al⁺ and Fe⁺ but also mixed cluster ions of Fe⁺ and Al⁺ such as FeAlO⁺, Al₂FeOH₂ ⁺ and Al₂FeO₃ ⁺ were observed from the surface of Fe-oxide on Al₂O₃. The presence of these ions comments on chemical bond formation between Fe-oxide and Al₂O₃ at the interface of nanostructured overlayers and substrates. The Fe-oxide/SiO₂ surface produced much less pronounced SIMS emission of FeSiO⁺ compared to the Fe-oxide/Al₂O₃ surface, suggesting lower dispersion and much larger size of the Fe-oxide particles on SiO₂. Detailed analyses of SIMS spectra can provide deep insight into complex nanostructures.