Scanning Electron Microscopy Study of Rhodium, Palladium and Platinum Foils Treated in NH3-Air Flow
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 determine the role of individual metals: Pt, Pd and Rh in the catalytic etching of platinum catalyst gauzes during the NH3 oxidation, we carried out detailed investigation of the surface microstructure of platinum, palladium and rhodium catalysts treated in the reaction medium (NH3+O2). Polycrystalline Pt, Pd and Rh foils with the size of 10 х 5 х 0.04 mm were used as the catalyst samples. Each sample was assembled into a package with four platinum gauzes required to maintain standard conditions of the NH3 oxidation process. The platinum catalyst gauzes were made from a polycrystalline wire (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 about 3.6 bar. The surface microstructure was studied using a scanning electron microscope (SEM) JSM-6460 LV (Jeol) in the mode of secondary electrons at beam energy 25 keV.