The Co-based catalysts were prepared with different cobalt acetate solutions.Effects of pH value were studied deeply on Fischer-Tropsch synthesis (FTS) through a semi-batch reactor.Among all impregnation solutions (water,butanol,amyl alcohol,acetic acid,nitric acid and ammonium nitrate),the catalyst prepared by NH4NO3 solution showed the highest catalytic activity due to its small particle size and high reduction degree.However,the catalyst with the smallest particle size derived from water as impregnation solution exhibited low activity as well as high methane selectivity since it was difficult to be reduced and inactive in FTS.According to FT-IR spectra results,the low intensity of absorbed CO on the catalyst prepared from water solution resulted in low FTS activity.Whereas,the high activity of catalysts prepared from NH4NO3 solution could be explained by the high intensity of absorbed CO on the catalysts.The cobalt species on the catalysts prepared under lower pH conditions exhibited smaller particle size distribution as well as lower CO conversion than those prepared at higher pH value.