Scanning Electron Microscopy Study of Platinum Catalyst Gauzes Treated in Air, Ammonia and NH3 in Air
Ammonia oxidation with air on platinum catalyst gauzes is widely used in chemical industry for synthesis of nitric acid. It is well known that during this process the gauzes undergo deep structural rearrangement of surface layers (catalytic etching) leading to a loss of platinum and decrease of catalytic activity. To elucidate the contributions of individual reactions of О2 and NH3 with the platinum surface in the catalytic etching of platinum catalyst gauzes during the NH3 oxidation, we carried out detailed investigation of the surface microstructure of platinum catalyst gauzes treated in air, ammonia, and in the reaction medium (NH3+O2). The platinum catalyst gauzes used in the study were made from a polycrystalline wire with d ≈ 82 μm, with the chemical composition (in wt.%) 81% Pt, 15% Pd, 3.5% Rh and 0.5% Ru. A laboratory flow reactor made of a quartz tube with the inner diameter of 11.2 mm was used at the feed (ca.10% NH3 in air) flow rate 880–890 l/h, the gauze temperature 860±5 °C and total pressure ca.3.6 bar. The surface microstructure was studied using a scanning electron microscope (SEM) JSM-6460 LV (Jeol) in the modes of secondary electrons (SE) and backscattered electrons (BSE) at beam energy 20 keV.