Effect of α-Fe2O3 surface coating on reconstruction of platinum-rhodium catalysts during oxidation of ammonia

Surface coating of α-Fe2O3 has been shown to affect the reconstruction of platinum-rhodium catalysts during oxidation of ammonia at 900 °C and atmospheric pressure. Surfaces of wire-formed Pt-Rh specimens with 0-30 and 100 wt.% Rh were coated with thin layers of α-Fe2O3, deposited by thermal decomposition of iron(III) nitrate or by atomic layer chemical vapor deposition. Scanning electron microscopy, electron microprobe analysis, and powder X-ray diffraction were used to examine the catalyst wires before and after use in ammonia oxidation. The reconstruction on Pt-Pt/10 wt.% Rh and Rh catalysts with α-Fe2O3-coated surfaces involves “cauliflower”-like excrescences similar to those observed on corresponding materials without coatings. The reconstructions on α-Fe2O3-coated catalysts of Pt/20 wt.% Rh and Pt/30 wt.% Rh carry the same characteristics. However, for these alloys the reconstruction process becomes much faster and the resulting patterns more extensive than for corresponding materials without α-Fe2O3 coating. The boundary zones between the α-Fe2O3 cover and the liberated Pt-Rh surfaces appear to stand out as spots (hotspots in the thermal sense) with enhanced catalytic activity. A certain decrease in activity and selectivity with time is observed for all tested specimens. This is attributed to gradual degradation of α-Fe2O3 to more inactive Fe3O4. The progressing degradation of α-Fe2O3 to Fe3O4 shows that the temperature in the hotspots must exceed some 1400 °C or that reducing conditions prevail locally at the surface.