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

Abstract

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 Al2O3, 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.