Catalytic Etching of Polycrystalline Rhodium during High-Temperature Oxidation of Ammonia with Air at 1133 K

Abstract The role of Rh2O3 oxides and the reaction of NH3 molecules with the rhodium oxide oxygen in the formation of an etching layer on Rh during NH3 oxidation with air on polycrystalline rhodium (Rh(poly)) at 1133 K has been identified using X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), and energy-dispersive X-ray spectroscopy (EDS) methods. Vigorous oxidation of rhodium and the reaction of NH3 molecules with the rhodium oxide oxygen simultaneously occur during NH3 oxidation on Rh(poly). According to XPS, Rh2O3 oxide layers are formed during NH3 oxidation on Rh(poly). The XRD data yield similar values of the fcc cell 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 the presence of oxide layers on metallic Rh containing subgrains with a size of 39−55 nm on the rhodium grain biundaries. The initial stage of NH3 oxidation (t ≤ 1 h) is characterized by the formation of porous oxide agglomerates with a size of ~0.7 μm and the growth of Rh2O3 crystalline fibers with a diameter of 50−100 nm on the rhodium grain biundaries. In the case of a long-term oxidation of NH3 (t ≥ 10 h), lamellar oxide agglomerates with a size of 0.5−1.0 μm are formed on the surface; Rh2O3 crystalline pyramids with a height of 1−2 μm grow on these agglomerates. The formation of these etching structures during NH3 oxidation can occur through the gas phase via the vapor–liquid–crystal (VLC) mechanism.