X-ray photoelectron spectroscopy and electron microscopy of Pt$z.sbnd;Rh gauzes used for catalytic oxidation of ammonia

Scanning electron microscopy, X-ray photoelectron spectroscopy (XPS) and sputtering techniques are used to study platinum-rhodium gauzes working in industrial burners for ammonia oxidation. Electron microscopy shows that during the combustion, two distinct crystalline systems develop, namely, small rhodium-rich crystals and large platinum-rich geometric crystals with well-developed facets. Chemical analysis of the surface by XPS associated with ion sputtering shows that the catalyst deactivation may be caused by the segregation of rhodium at the surface, resulting from the volatilization of platinum oxide at high temperature after the oxidation of Rh and Pt. The small rhodium-rich crystals are identified as Rh2O3 and the oxidation ratio (Rh2O3Rh + Rh2O3) varies with the location of the gauzes in the catalytic pack: it is 0.2 at the head of the reactor and reaches up to 0.8 in the tail.