Sulfur tolerance of an inexpensive limonite catalyst for high temperature decomposition of ammonia

Catalytic performance of limonite in the decomposition of 2000 ppm NH3 in the presence of a small concentration of H2S has been studied with a cylindrical quartz reactor at a high space velocity of 45000 h− 1 to examine the sulfur tolerance of the limonite catalyst. It is not poisoned apparently by 50–500 ppm H2S at 750 °C, but the remarkable sulfur poisoning is observed at 2000 ppm H2S. In the coexistence of 100 ppm H2S, conversion of NH3 to N2 decreases gradually with time after 10 h at 650 °C, whereas the limonite maintains the high conversion of almost 100% for 50 h at 750 °C. The SEM-EDX measurements after reaction show that the surface composition at 750 °C is richer in metallic Fe, which may be produced by the reaction of NH3 and/or H2 with FeS formed. Commercially-available FeS promotes the decomposition of NH3 diluted with inert gas at 750 °C, and the FeS is partly transformed into H2S and α-Fe in this process. Possible mechanisms for the decomposition of NH3 in the presence of H2S with the limonite are discussed on the basis of the results of some model experiments.