The Role of Etch Pits in Catalytic Etching of Platinum Catalyst Gauzes in Ammonia Oxidation
Detailed investigation of the surface microstructure of platinum catalyst gauzes after 50 hours of treatment in the NH3+O2 reaction mixture revealed fragments of wires with different etching degrees demonstrating development of catalytic etching. On fragments with minimum etching, pores, cavities and etch pits were observed at grain boundaries, whereas crystallite plains with the height ~ 100 nm and many etch pits with the size of 50-150 nm were observed on the surface of the grains. A continuous layer of crystallites with the size of 3-16 m having internal penetrating pores with the diameter of 1-5 m was observed on the fragment with maximum etching. We believe that the NH3 oxidation on metal atoms located on surface defects (grain boundaries, dislocations) results in weakening of bonds between these atoms. As a result, the mobility of the atoms increases, they migrate and are gradually incorporated into crystal plains on the grain surfaces.