The Role of Rh and Mg Oxides in the Etching of Industrial Pt–Pd–Rh–Ru Gauze in the Course of Catalytic Oxidation of Ammonia with Air at 1133 K

Scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS) at microscope probe electron energies (E0) of 5 and 20 keV in a range of magnification factors from ×50 to ×100 k were used to characterize the morphology, microstructure and chemical compositions of surface and subsurface regions of etching structures in the industrial Pt−Pd−Rh−Ru gauze with concentrations of 81, 15, 3.5, and 0.5 wt %, respectively, which was used in the oxidation of NH3 with air at a pressure of 3.6 bar and a temperature of 1133 K. After NH3 oxidation on the Pt−Pd−Rh−Ru gauze, we observed the microgranular etching structure with grains about 10 µm in size, oxide particles with a diameter of ~80 nm on grain boundaries, and crystal facets 25−50 nm in height on the grain surfaces in the region of weak etching. The oxide particles consisted of 25−50 nm cores with the MgRh2O4 spinel composition and 25−50 nm thick SiO2 shells. These particles were formed during the diffusion of Rh atoms along grain boundaries from the alloy bulk to the surface, the formation of Rh2O3 particles, and their reactions with MgOх and SiOх impurities from the gas flow of reagents. These processes led to the formation of etch pits, increased the intensity of NH3 oxidation and temperature, and accelerated the evaporation of volatile oxides PtO2(gas), RhO2(gas), etc., into the gas phase, which resulted in the catalyst etching. In the region of strong etching, the etching layer of porous crystal cauliflower agglomerates with different shapes and a size of ~10 µm was observed. The agglomerates had a close phase composition and a low concentration of defects up to a depth of ~500 nm. During the NH3 oxidation on Pt–Pd–Rh–Ru gauzes, etching structures were formed upon evaporation of volatile oxides PtO2(gas), RhO2(gas), etc., on hot regions of the surface and their subsequent condensation on cold fragments of the catalyst.