Decomposition of ammonia: The effect of syngas components on the activity of zeolite Hβ supported iron catalyst

Abstract

Zeolite beta (SiO2/Al2O3=38) supported iron catalyst (Fe/HZβ) was used to investigate the effects of syngas components on ammonia decomposition reaction at 700°C and 1 atmosphere. While 97.3% of NH3 conversion was achieved over Fe/HZβ catalyst under nitrogen atmosphere, conversions decreased tremendously upon the addition of syngas components. Conversion loss with H2 was attributed to the competing reaction steps: NH bond breaking and N2 association. The number of active sites filled with N atoms was thought to be decreased under less favorable dehydrogenation atmosphere where minimum NH bond breaking might take place. In the presence of CO, its inhibition effect was interpreted as competing reactions on the same active sites: one for CO bond dissociation and the other is NH bond dissociation. On the other hand, neither H2 nor CO alone did lead to a permanent activity loss. With coexistence of CO and H2, a dramatic loss of activity to a level of 12.5% and a severe coking were observed. This was attributed to the metal dusting corrosion. This was verified both by XRD measurement and accumulated carbon inside the reactor. Upon CO2 addition to H2-CO mixture, reverse water gas shift reaction was prominent. Interestingly, NO2 was detected in the product stream as an indication of the oxidation reaction between NH3 and H2O. In the presence of CO2, hematite phase (α-Fe2O3) appeared with the loss of polycrystalline α-iron phase. CH4 addition to CO-H2-CO2 mixture seemed to be effective in further reducing NH3 concentration in syngas. A well-known reaction between CH4 and NH3 to give HCN was seen to be responsible for the unexpected increase in NH3 conversion. Finally, H2S in syngas did not seem to poison the active sites but was exposed to a similar decomposition giving elemental sulfur.