Characterization of platinum alloy used in ammonia oxidation. Morphology and microstructure of Pt–Pd–Rh–Ru gauzes after the oxidation of NH3 with air at 1133 K

Abstract

Scanning electron microscopy and X-ray diffraction analysis were used to characterize the morphology and microstructure of a fresh industrial Pt–Pd–Rh–Ru alloy gauze (81, 15, 3.5, 0.5 wt.%, respectively) as well as the back and frontal sides of such gauze after oxidation of NH3 with air at T = 1133 K and pressure 3.6 bar. The fresh unused gauze has a smooth surface, whereas the back and frontal sides of the spent one have etched layers consisting of microcrystals and porous “cauliflowers”, respectively. For these gauzes, the specific surface area derived from SEM images has different values −26, 52, 260 cm2/g, whereas the fcc lattice parameter (a) and the size of coherent scattering region (D) have close values − 3.903, 3.900, 3.902 Å and 51, 74, 39 nm, respectively. The obtained a and D values testify to the absence of both the dissolution of detected C, O, N atoms in the alloy lattice and the significant merging of subgrains during the catalytic etching initiated by NH3 oxidation. The etch pits associated with dislocations were detected in the concentration of 1.0 × 108−6.0 × 108 cm−2 on both sides of the spent gauze. The highly exothermic NH3 oxidation leads to “hotspot” etching sites on etch pits, which cause temperature gradients. Metal transfer from “hot” to “cold” regions of the catalyst leads to the formation of a rough “cauliflower” layer.