Decomposition of pyrrole or pyridine with iron catalysts supported on carbons has been studied with a cylindrical quartz-made pulse reactor, into which liquid pyrrole or pyridine is injected as a pulse, in order to develop a novel hot gas cleanup method of removing the nitrogen present in tar as N-2. The catalyst is mainly prepared by heating FeO OH precipitated onto powdery cellulose from FeCl3 solution. Nanoscale iron catalysts with the average particle sizes of 25-30 nm can promote decomposition reactions of the N-containing heterocyclic compounds in inert gas at >500 degrees C and provide N-2 yields of 40-45% after the almost complete decomposition of pyrrole at 600 degrees C or pyridine at 650-700 degrees C. Iron catalyst with the mean size of 100-500 nm, prepared from Fe(NO3)(3) impregnated with a commercial activated carbon, is also active for the decomposition of the heterocyclic N-compounds, but it is almost inactive for N-2 formation, the yields being as low as less than 2% in all cases. The increase in the number of pulses lowers the catalytic activity of iron nanoparticles for N-2 formation, whereas in situ H-2 treatment at 500 degrees C after reaction can restore it to the almost original state. On the basis of the above-mentioned results, it is likely that the activity of iron catalyst for N-2 formation from pyrrole or pyridine is very sensitive to the iron particle size, and the in situ H-2 treatment is effective for recovery of the decreased catalytic performance. (C) 2012 Elsevier B.V. All rights reserved.