Unexpected Oxidation of Co Nanoparticles Under H 2 : X‐Ray‐Induced Water Radiolysis in NAP‐XPS Studies
Sagar Sharma, Michele De Rocco, Martine Trentesaux, Jean‐Jacques Gallet, Fabrice Bournel, Ahmed Naitabdi, Anne‐Sophie Mamede, Jean‐François Paul, Héloise TissotABSTRACT
Cobalt nanoparticles exposed to 1 mbar H 2 during near‐ambient‐pressure x‐ray photoelectron spectroscopy oxidize instead of reducing, a result that contradicts thermodynamic expectations. Using beam‐free references, flux‐dependent operando measurements, depth‐resolved spectra, and thermal cycling, we show that x‐ray radiolysis of trace water (∼10 − 7 mbar) generates oxidizing species that dominate the surface chemistry of Co/SiO 2 . This radiolytic pathway produces a reproducible kinetic steady state in which ∼37% Co(II) persists even at 775 K under H 2 , and it drives complete oxidation under CO due to the absence of a hydrogen‐based reduction channel. Depth‐dependent data confirm that this oxidation is surface‐localized and directly controlled by photon flux, meaning the beam selectively modifies the layer that NAP‐XPS is intended to probe. These results establish photon flux, background water, and thermal history as active reaction parameters and define general constraints for operando NAP‐XPS on reducible, water‐reactive metals.