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

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: N[sbnd]H 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 N[sbnd]H 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 C[sbnd]O bond dissociation and the other is N[sbnd]H 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.