Catalytic ozonation is an attractive method for low-temperature denitrification in recent years. However, the research concerning catalytic ozonation of NO by inadequate ozone (the molar ratio of O-3/NO < 1) was rarely reported. In this study, black-TiO2 prepared by a sol-gel method was tested for denitrification by catalytic ozonation with O-3/NO molar ratio of 0.6, presenting a 21.2% increase in NOx removal as compared to O-3 alone. Interestingly, black-TiO2 also performed an 8.6% improvement in NO conversion in comparison to TiO2 catalyst. Moreover, the result of ion chromatography (IC) suggested that HNO3 was the main product of this catalytic ozonation-removal process over black-TiO2. Black-TiO2 and TiO2 were characterized by XRD, UV-vis, Raman, XPS and H-2-TPR, suggesting that large numbers of Vo-OH (surface hydroxyl on oxygen vacancy) were formed on black-TiO2. UV-vis spectra of salicylic acid and p-benzoquinone revealed that a large number of additional HO2 center dot radicals were generated in O-3 + H2O + black-TiO2 system, compared with the main production of (OH)-O-center dot radicals in O-3 + H2O + TiO2 system. A mechanism for catalytic ozonation denitrification over black-TiO2 was proposed. Both (OH)-O-center dot radicals and HO2 center dot radicals were produced, especially, the Vo-OH promoted the formation of HO2 center dot radicals, which promoted deep oxidation from NO to HNO3. Therefore, this work provides a promising approach to NOx removal with a small amount of ozone. We expect that this study can shed some lights on the catalyst design of catalytic ozonation, which contributes to the route design for denitrification. (C) 2018 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.