The catalytic activities and different behaviors for catalyst deactivation by forming surface hydrocarbon deposits were investigated on the Cl-free Ru/MnCO3 Fischer-Tropsch synthesis (FTS) catalysts in a low pressure fixed-bed reactor. Catalytic activity of LT-Ru(NO) was found to be a little higher than that of Ru(NO). The differences seem to stem mainly from ruthenium particle sizes and reduction behavior of adsorbed CO on the catalysts, which would be consistent with the results of CH4 formation from temperature-programmed reduction (TPR) of pre-adsorbed CO, as well as Ru particle sizes, as ascertained by H-2 and CO adsorptions and TEM measurements. The Ru particle sizes of used catalysts obtained after 120h-reaction were approximately close to those of fresh catalysts and no ruthenium aggregation occurred. The origin of catalyst deactivation was investigated in terms of the extent of hydrocarbon deposition. The adsorbed atomic carbon at the peak temperature around 250-350 degrees C was eliminated by H-2 treatment, but the more heavy hydrocarbons at peak temperature over 400 degrees C such as polymeric and graphitic carbons were not removed by any treatment. The nature of hydrocarbon deposition would be suggested by measuring the deposited carbon with the help of the characterizations, such as thermo-gravimetric analysis, temperature-programmed reduction and diffuse reflectance infrared Fourier transform (FT-IR) measurements. (C) 2012 Elsevier B.V. All rights reserved.