Catalytic Etching of Industrial Pt–Pd–Rh–Ru Gauzes During High-Temperature Oxidation of Ammonia with Air

High-temperature oxidation of ammonia on Pt alloy gauzes to NO oxide is used for the industrial production of nitric acid. The world annual production of HNO3 reaches 70–80 million tons. About 80% of the produced acid is used to manufacture mineral fertilizers utilized in agriculture. In the process of oxidation of NH3 on Pt alloy gauzes, etched layers are formed, which reduce the activity and strength of the gauzes and increase catalyst losses. To improve the efficiency of catalysts used in the industrial oxidation of NH3, etched structures on the surface of catalysts are being actively studied. This paper presents the results of a study of the morphology, microstructure, and chemical composition of etched structures on industrial Pt–Pd–Rh–Ru gauzes with a composition of 81, 15, 3.5, and 0.5 wt %, used in the oxidation of NH3 with air at T = 1133 K and a pressure of 3.6 bar in industrial and laboratory reactors. The used gauzes were found to contain a etched layer, including porous crystalline agglomerates with dimensions of 10–50 µm and other crystalline structures. The etched layers are characterized by an increased specific surface area and a stable crystalline structure and phase composition, as well as an increased concentration of absorbed atoms Oab and Nab (20–25 at %) in the near-surface layers of the catalyst. Due to the formation of catalyst areas with different temperatures, the mass transfer of metals from “hot” to “cold” areas occurs both during the surface diffusion of metal atoms and the evaporation and condensation of volatile oxides such as PtO2 and others. As a result of these processes, deep etching of the catalyst occurs with the formation of a rough layer of large crystalline agglomerates.