| Summary: | Platinum alloy gauzes are employed for the high-temperature oxidation of NH<sub>3</sub> to NO used in the industrial production of HNO<sub>3</sub> for application in agricultural fertilizers. To enhance the efficiency of NH<sub>3</sub> oxidation, various Pt–Pd–Rh alloys are employed for the production of such catalytic gauzes. To understand the role of these metals in NH<sub>3</sub> oxidation, scanning electron microscopy, energy-dispersive spectroscopy and X-ray diffraction were applied to investigate the morphology, composition and structure of Pt, Pd and Rh foils after annealing in O<sub>2</sub> and oxidation of NH<sub>3</sub> with air at 1133 K. After annealing in O<sub>2</sub> and NH<sub>3</sub> oxidation, the metallic microgranular structure was detected on Pt(poly), whereas oxide layers of Rh<sub>2</sub>O<sub>3</sub> and PdO were observed on Rh(poly) and Pd(poly). At the onset of NH<sub>3</sub> oxidation (<i>t</i> = 1 h), fibrous metal-oxide agglomerates of nanofibers formed on these oxide layers. The long-term (5–10 h) oxidation of NH<sub>3</sub> led to the formation of a continuous layer of pyramidal crystals on Rh(poly) and palladium “cauliflowers” on Pd(poly). The highly exothermic reaction of NH<sub>3</sub> with oxygen on metals and PdO or Rh<sub>2</sub>O<sub>3</sub> initiates strong catalytic etching forming grains and facets on Pt, fibrous metal-oxide agglomerates, pyramidal crystals and metallic “cauliflowers” on Rh and Pd.
|
|---|