Local Chemical Analysis of the Grain Surface, Cauliflowers, and Pores on Pt–Pd–Rh–Ru Gauzes after the Oxidation of NH3 at 1133 K

• Published in: Kinetics and catalysis 2021-09, Vol.62 (5), p.651-663
• Main Authors: Salanov, A. N., Chesnokova, N. M., Serkova, A. N., Isupova, L. A.
• Format: Article
• Language: English
• Subjects: Ammonia; Catalysis; Catalysts; Chemical analysis; Chemical composition; Chemistry; Chemistry and Materials Science; Crystal defects; Etch pits; Etching; Gauze; Grain boundaries; High temperature; Low temperature; Oxidation; Palladium; Physical Chemistry; Platinum; Rhodium; Ruthenium; Voids
• ISSN: 0023-1584; 1608-3210
• DOI: 10.1134/S0023158421050086

The local chemical composition of the most characteristic regions of a rough etched layer on the front side of the Pt–Pd–Rh–Ru gauze with a composition of 81, 15, 3.5, 0.5 wt %, respectively, after the oxidation of NH 3 with air at Т = 1133 K and a pressure of 3.6 bar was studied by scanning electron microscopy and energy-dispersive X-ray spectroscopy. The local chemical composition of the grain surface in the low etching region, on porous crystal agglomerates (cauliflowers) in the etched layer, and between cauliflowers at the bottom of pore voids was determined. Pt, Pd, Rh, Ru, C, O, and N were recorded on grains, cauliflowers, ​​and pore bottom, which had different defect concentrations and temperatures. The metal contents on the grains and at the bottom of pore voids coincided with the composition given by the gauze manufacturer, whereas cauliflowers had a higher Rh content (12.6 at %). Reliable quantitative data on the contents of O ab and N ab atoms in the subsurface layers of grains, cauliflowers, ​​and pore bottom were obtained for the first time: 18.6, 7.3, and 10.4 at % for О ab and 17.1, 16.8, and 33.6 at % for N ab , respectively. The maximum content of O ab atoms (18.6 at %) was recorded in defect regions with low temperature (grains in the low etching region), while increased content of N ab atoms (33.6 at %) was found in regions with low defect concentrations but elevated temperature (pore voids). These data on the O ab and N ab contents in regions with different degrees of defectiveness and temperatures made it possible to reveal for the first time the types of defects on which predominantly the O and N atoms penetrate into the catalyst during NH 3 oxidation. The O ab and N ab atoms accumulate in the subsurface layers of the Pt–Pd–Rh–Ru metal alloy during intercalation of O atoms on the grain boundaries and in etch pits and penetration of N atoms into the lattice of the alloy. The reported dissolution model of O and N atoms in the subsurface region of the catalyst will provide deeper understanding of the mechanism of NH 3 oxidation that forms NO oxide and of the related etching process.