Understanding Solid-Gas Reaction Mechanisms by Operando Soft X-Ray Absorption Spectroscopy at Ambient Pressure.

Ambient-pressure operando soft X-ray absorption spectroscopy (soft-XAS) was applied to study the reactivity of hydroxylated SnO ₂ nanoparticles toward reducing gases. H ₂ was first used as a test case, showing that the gas phase and surface states can be simultaneously probed: Soft-XAS at the O K-edge gains sensitivity toward the gas phase, while at the Sn M _4,5 -edges, tin surface states are explicitly probed. Results obtained by flowing hydrocarbons (CH ₄ and CH ₃ CHCH ₂ ) unequivocally show that these gases react with surface hydroxyl groups to produce water without producing carbon oxides and release electrons that localize on Sn to eventually form SnO. The partially reduced SnO _{2 -  x} layer at the surface of SnO ₂ is readily reoxidized to SnO ₂ by treating the sample with O ₂ at mild temperatures (>200 °C), revealing the nature of "electron sponge" of tin oxide. The experiments, combined with DFT calculations, allowed devising of a mechanism for dissociative hydrocarbon adsorption on SnO ₂ , involving direct reduction of Sn sites at the surface via cleavage of C-H bonds and the formation of methoxy- and/or methyl-tin species at the surface.
Competing Interests: The authors declare no competing financial interest.