Catalytic etching of polycrystalline rhodium during high-temperature oxidation of ammonia with air at 1133 K

X-ray diffraction analysis (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS) were used to reveal the role of Rh 2 O 3 oxides and reaction of NH 3 molecules with rhodium oxide oxygen in the formation of etching layer on Rh during NH 3 oxidation with air over polycrystalline rhodium (Rh(poly)) at 1133 K. The oxidation of NH 3 on Rh(poly) is accompanied simultaneously by the intense oxidation of rhodium and the reaction of NH 3 molecules with Rh oxide oxygen. According to XPS, NH 3 oxidation on Rh(poly) leads to the formation of oxide layers with the composition Rh 2 O 3 . XRD data give close values for FCC lattice parameter a of metallic rhodium (3.802−3.803 Å) and coherent scattering region (CSR) D (39−55 nm) for all the samples. These data indicate oxide layers on metallic Rh, which contains subgrains with the size of 39−55 nm. In the initial step of NH 3 oxidation ( t ≤ 1 h), porous oxide agglomerates ca. 0.7 µm in size are formed and crystal Rh 2 O 3 fibers 50−100 nm in diameter grow. A long-term oxidation of NH 3 ( t ≥ 10 h) results in the formation of plate-like 0.5−1.0 µm oxide agglomerates, on which crystal Rh 2 O 3 pyramids 1−2 µm in height grow. The formation of these etching structures during NH 3 oxidation can occur via the gas phase according to the vapor-liquid-solid (VLS) mechanism.