Effect of α-Fe 2O 3 surface coating on reconstruction of platinum–rhodium catalysts during oxidation of ammonia

Abstract

Surface coating of α-Fe 2O 3 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-Fe 2O 3, 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-Fe 2O 3-coated surfaces involves “cauliflower”-like excrescences similar to those observed on corresponding materials without coatings. The reconstructions on α-Fe 2O 3-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-Fe 2O 3 coating. The boundary zones between the α-Fe 2O 3 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-Fe 2O 3 to more inactive Fe 3O 4. The progressing degradation of α-Fe 2O 3 to Fe 3O 4 shows that the temperature in the hotspots must exceed some 1400 °C or that reducing conditions prevail locally at the surface.