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In this present study, the zirconia-modified TiO2 having different crystallite sizes of 11, 13 and 16 nm as support for the cobalt catalysts were investigated. The different TiO2 crystallites derived from the sol-gel method was obtained by varying the water:alkoxide molar ratios from 165 to 4. Then, the TiO2 supports were used to yield various Co/TiO2 catalysts. It was found that the sizes of cobalt oxides species dispersed on TiO2 were corresponding to TiO2 sizes. Based on XPS study, the binding energy of Co 2p3/2 was not affected by the size of TiO2. The catalytic activity was measured via CO hydrogenation under methanation condition. In case of unmodified TiO2 support, the smaller TiO2 crystallites (Co/TiO2_11 nm) exhibited higher activity than other larger ones. Moreover, the zirconia modification also resulted in increased activity for all samples. For both cases, it can be attributed to the increased cobalt dispersion with small crystallite size of TiO2 support coupled with the zirconia modification as measured by CO chemisorption.