Due to the crucial role of a catalyst in growth of carbon nanotubes (CNTs) by a chemical vapor deposition, it is important to understand the activation and deactivation of a catalyst for controlling production of CNTs. Using molecular benzene as the carbon source and Fe-Co/gamma-Al2O3 as catalyst, multi-CNTs were synthesized with a thermal analyzer coupled a mass spectrometer in different atmospheres of N-2 and H-2. Very different thermogravimetry behaviors (TG-DTG) were observed during CNT growth, indicating that CNTs experienced very different activation and deactivation processes. In a N-2 atmosphere, a catalyst particle was activated jumpily, then about half of catalyst surface was covered by the produced CNTs generated on one side of a catalyst, leading to trigger deactivation immediately following activation; in a H-2 atmosphere, a catalyst was activated gradually, and deactivation developed gradually, the total CNT yield in H-2 was higher than in N-2, suggesting H-2 suppressed the deactivation development of a catalyst by cleaning the amorphous carbon over a catalyst. In combination of the mass spectral measurements, the activation and deactivation mechanism of a catalyst in an inert and reducing reaction atmospheres gas were revealed. (C) 2014 Elsevier B.V. All rights reserved.