Because VOx/TiO2 is widely used as a catalyst for the selective catalytic reduction of nitric oxide with NH3 (NH3-SCR), a number of studies have been performed to date. VOX/TiO2 is prepared by using various vanadium loading and TiO2 supports with different surface areas. The surface density of the vanadyl species is a factor in determining the optimal activity, depending on the VOX/TiO2. We studied the relationship between NH3-SCR activity and the VO, surface density of VOX/TiO2 using various vanadium loading and TiO2 supports. The optimal activity is observed when VOX/TiO2 has an approximate VOx surface density of 4.5 VO(x)nm(-2). In addition, the ideal vanadyl species for favorable SCR activity was examined using diffuse reflectance UV-visible (DRUV-vis) spectroscopy. In the VOX/TiO2 catalyst that provided the optimal VOx surface density and best SCR activity, polymeric tetrahedral VOx was the most abundant vanadium species. When the vanadium loading is determined by the manufacturing process of the catalyst, TiO2 with an appropriate surface area should be prepared in order to manufacture VOX/TiO2 with maximum activity. In contrast, when VOX/TiO2 is prepared using commercial TiO2, the vanadium loading of VOX/TiO2 that provides the maximum activity can be determined.(C) 2015 Elsevier B.V. All rights reserved.