SIMS methodology for probing the fate and dispersion of catalytically active molecules

• Pyrolytic effect on catalytic molecules is assessed with SIMS in event-by-event mode.
• Co-N4 moiety is supported to be the structure of the catalytically active site.
• Local chemical environment of the active sites is revealed by SIMS.
• SIMS shows dispersion of the active sites as a function of pyrolysis temperature.

Secondary ion mass spectrometry (SIMS) with 520 keV Au4004+ projectiles operating in event-by-event bombardment/detection mode was applied to investigate pyrolized mixtures of catalyst (vitamin B12) and carbon supports (carbon black). The specimens are alternative cathode materials to platinum for fuel cells. The goal was to understand the effect of temperature on the chemical environment of the catalytically active sites formed on the carbon supports during pyrolysis. The Au4004+ projectiles probed the samples one-at-a-time in a stochastic manner and the resulting secondary ions from the individual impacts were recorded separately. The coincidental methodology allows one to compile mass spectra comprising selectively the secondary ions from the impacts on the active sites, extracting the local molecular information of the active sites within a nanodomain (∼103 nm3). The detection of CoN4Cx− suggests Co-N4 be the moiety retained from the pyrolized vitamin B12 in the presence of the carbon black. Variations in the local chemical environment of the active sites as a function of pyrolysis temperature were also observed. Additionally, we could quantify the dispersion of the active sites among the carbon supports, a measure providing a direction for future device optimization.

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