Catalytic Etching of Platinoid Gauzes during the Oxidation of Ammonia by Air: Etching of the Frontal Surface of the Platinoid Gauze in the Course of NH3 Oxidation at 1133 K

The morphology, microstructure, and chemical composition of the surface and near-surface layers of polycrystalline wire of an industrial platinoid gauze composed of Pt (81 wt %), Pd (15 wt %), Rh (3.5 wt %), and Ru (0.5 wt %) are investigated by scanning electron microscopy and energy dispersive X-ray spectroscopy. After oxidizing NH3 with air at T = 1133 K under a pressure of 3.6 bar for 50 h, a continuous rough layer of the cauliflower-type agglomerates formed during catalytic etching is detected on the frontal surface of the gauze. On the surface of wire fragments from 100 to 150 μm in size with a smooth micrograined structure, nanometer-size etch pits are detected at a concentration of 1.0 × 108–6.0 × 108 cm–2, which may be etching sites of the hotspot type. The growth of etch pits and the formation of crystalline terraces on the grain surface are caused by the surface diffusion of metal atoms. The continuous etching layer contains porous crystalline agglomerates (cauliflowers) with a linear size of 3 to 18 μm (mean size about 10 μm) at a concentration of 4.9 × 105 cm–2. Pores with a diameter of 0.1 to 1.7 μm are detected on the surface of agglomerates at a concentration of 1.3 × 107 cm–2. The specific surface area of the platinoid gauze, which is calculated from microscopic images taking into account the surface area of agglomerates and pores, is about 260 cm2/g. In the process of highly exothermic oxidation of NH3 with oxygen, on the surface of agglomerates with a low concentration of defects and in pore voids 5–15 μm in width and up to 10 μm in depth with an increased specific surface area and a high concentration of defects, vapor of volatile oxides and metals that are formed at hot regions of the bottom of pore voids can be condensed on the overlaying cold regions of the surface of agglomerates and single crystals. These processes give rise to the formation of a continuous etching layer of porous crystalline agglomerates, massive single crystals, and deep pore voids. The formed etching layer substantially increases the specific surface area of the catalyst, which leads to an increase in the volumetric rate of NH3 oxidation that accelerates the etching process.