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ToF-SIMS and the catalyst surface

December 17, 2013

In heterogeneous catalysis, everything is related to the surface…

Today, I want to point out to an interesting review that is published in Appl. Catal. A. In this overview, Prof. Lu-Tao Weng, from the Hong Kong University of Science and Technology presents an important surface analysis technique that indeed deserves the best attention of the heterogeneous catalysis community.

ToF-SIMS apparatus at the SUCH facilities, IMCN

ToF-SIMS apparatus at the SUCH facilities, IMCN

Time-of-Flight Secondary Ion Mass Spectroscopy (ToF-SIMS), unlike XPS for example, is underexploited in the field heterogeneous catalysts characterization. And this looks weird to me. ToF-SIMS is based on the mass analysis of atoms and molecules removed from the surface by sputtering. In its “static” mode, the obtained mass spectra are characteristic of the virgin surface. Of the outermost surface, I should say. The sampling depth is typically of 1 nm. Tof-SIMS offers richer molecular information as compared to XPS. At the same time, detection limit is lower, surface sensitivity is higher, imaging capability is higher… All these advantages are clearly described in a critical way by Weng in his review. Also the reasons for the fact that ToF-SIMS has remained relatively confidential are discussed.

Several practical examples are reviewed to demonstrate the power of ToF-SIMS. Among others, our own results are reported. These deal with for example the characterization of V2O5/TiO2 catalysts used in the abatement of volatile organic pollutants or the characterization of MoO3/SiO2-Al2O3 catalysts used in the metathesis of light olefins. Recently, we have also published papers including ToF-SIMS data on silica-alumina or rhenium oxide-silica-alumina catalysts. In each case, ToF-SIMS was decisive in the conclusions we draw: intimacy of the mixed oxides, degree of condensation of the supported active phase, surface contamination, etc.

tof-simsTo illustrate briefly the power of ToF-SIMS, I want to mention the case of MoO3/SiO2-Al2O3 catalysts prepared by flame spray pyrolysis. These samples consist in non-porous silica-alumina decorated with Mo oxide on the surface. At low Mo loading one would expect mainly the presence of well-dispersed MoOx species. At higher loading, oligomeric and polymeric molybdates (or even crystals) are expected. The degree of condensation cannot be probed by XPS. The classical approach is then to use Raman spectroscopy to identify isolated, polymeric and crystalline species. With Raman, only isolated molybdates are detected up to 5 wt.% MoO3 loading. Such loading is indeed well below the theoretical monolayer coverage. One could hastily conclude that the dispersion is excellent and that only monomeric species are stabilized at the surface of the catalyst… Clusters containing 2 or 3 Mo atoms were however detected by ToF-SIMS in these samples. These are unambiguous evidence of the existence of Mo-O-Mo bonds, thus at least oligomeric species, at the catalyst surface. Interestingly, the set of data could be processed in a semi-quantitative way to come up with a “degree of MoOx condensation” value for each sample. This was then clearly (anti)correlated to the olefin metathesis activity, supporting the idea that isolated molybdates are the active metathesis sites.

To conclude, I want to mention that our institute hosts a technical platform for surface analysis. This structure is called “SUCH” and benefits from the expertise of two experienced engineers, helping our researchers and external stakeholders to take the maximum out of surface characterization tools like XPS and ToF-SIMS. Do not hesitate contacting them 😉

DD

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