抄録
Catalytic decomposition of NH3 to inert N2 at high temperatures has been studied with a fixed bed quartz reactor to develop a novel hot gas cleanup method of removing NH3 from coal gasification products. The decomposition efficiency of 2000 ppm NH3 diluted with He at 1073 K is 17 % with a commercial α-Fe2O3 pretreated with H2 at 773 K. On the other hand, low-valued iron ore (limonite) containing a large amount of α-FeOOH is quite active; the iron achieves the complete decomposition at 823-1223 K and maintains the high efficiency of 100 % at 1023 K. Since syngas is the major component in an actual coal gasification process, NH3 decomposition under flowing 20% CO - 10% H2 is also carried out. The coexistence of the syngas lowers the catalytic activity of the iron at 1023 K, and the efficiency decreases to 45 % after 4 h reaction. On the other hand, the addition of 10 % CO2 or 10 % H2O to the syngas restores the catalytic performance to almost the same level as that in inert He. The XRD measurements after H2 pretreatment or after NH3 decomposition reveal the formation of metallic Fe with the average crystallite size of 15 nm or Fe4N, respectively. In other words, the pretreatment converts α-FeOOH in the ore to nanoparticles of metallic Fe, which is subsequently transformed to Fe4N as intermediate species. These observations strongly suggest that the decomposition of NH3 proceeds through a cycle mechanism involving α-Fe and the nitride species.
