Catalytic etching of industrial Pt−Pd−Rh−Ru gauzes during high-temperature oxidation of ammonia with air

High-temperature oxidation of NH₃ to NO on platinum alloy gauzes is employed for industrial production of HNO₃. The annual world output of HNO₃ reaches 70−80 million tons. About 80% of the produced acid is used to obtain agricultural mineral fertilizers. The oxidation of NH₃ on platinum alloy gauzes is accompanied by the formation of etching layers, which deteriorate the strength and activity of the gauzes and increase the catalyst losses. Such etching layers are being studied intensely to find ways for enhancing the efficiency of catalysts applied in the industrial oxidation of NH₃. This study deals with the morphology, microstructure and chemical composition of the etching structures on the industrial Pt−Pd−Rh−Ru gauzes with the composition 81, 15, 3.5, 0.5 wt.%, which were used in the oxidation of NH₃ with air at T = 1133 К and pressure 3.6 bar in industrial and laboratory reactors. The etching layer detected on such gauzes included “cauliflower”-type porous crystal agglomerates with the size of 10−50 µm, various crystal fragments and the wire surface with a high concentration of defects. The etching layers have an increased specific surface area, stable crystal structure and phase composition, elevated concentration of absorbed O_ab and N_ab atoms (20−25 at.%) in subsurface layers of the catalyst, and nonuniform distribution of temperature regions. The highly exothermic NH₃ oxidation reaction results in the emergence of “hotspot”-type etching sites, which form temperature gradients both on the surface and in the layer of agglomerates. The formation of such gradients can lead to the mass transfer of metals from “hot” to “cold” regions of the catalyst during the surface diffusion of metal atoms as well as upon evaporation and condensation of PtO₂-type volatile oxides leading to deep surface etching with the formation of a rough layer of “cauliflowers”.

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