High-temperature oxidation of NH3 with air to NO oxide on catalytic platinum alloy gauzes is used in the industrial production of nitric acid. Scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS) with the probe electron energy of 5 and 20 keV were used to characterize chemical composition of the etching structure on Pt(poly) after NH3 oxidation with air at a pressure of 3.6 bar and 1133 К. Pt, Fe, Si, Al, Mg, O, N and C were detected in the etching structures, which contain oxide particles, crystal platinum pyramids and etch pits (grooves). Pt and О were detected in high concentrations (22.2–69.0 at %), while other elements—in low ones (0.2–8.4 at %). Oxide particles ca. 100 nm in size with the composition MgFe2O4 and a spinel structure are formed from Mg, Fe impurities during NH3 oxidation. Nucleation and growth of platinum pyramids, etch pits and deep etch grooves proceed during the capture of Pt and PtO2 from gas phase by MgFe2O4 particles, migration of Pt atoms through the oxide particles to the metal-oxide boundary, and their attachment to the Pt crystal lattice. The formation of etching structure can proceed by the vapor-liquid-solid (VLS) mechanism.
Salanov et al. (Mon,) studied this question.