To improve the activity of Co/Al 2 O 3 catalysts in selective catalytic oxidation of ammonia (NH 3 -SCO), valence state and size of active centers of Al 2 O 3 -supported Co catalysts were adjusted by conducting H 2 reduction pretreatment. The NH 3 -SCO activity of the adjusted 2Co/Al 2 O 3 catalyst was substantially improved, outperforming other catalysts with higher Co-loading. Fresh Co/Al 2 O 3 catalysts exhibited multitemperature reduction processes, enabling the control of the valence state of the Co-active centers by adjusting the reduction temperature. Changes in the state of the Co-active centers also led to differences in redox capacity of the catalysts, resulting in different reaction mechanisms for NH 3 -SCO. However, in situ diffuse reflectance infrared Fourier transform spectra revealed that an excessive O 2 activation capacity caused overoxidation of NH 3 to NO and NO 2 . The NH 3 -SCO activity of the 2Co/Al 2 O 3 catalyst with low redox capacity was successfully increased while controlling and optimizing the N 2 selectivity by modulating the active centers via H 2 pretreatment, which is a universal method used for enhancing the redox properties of catalysts. Thus, this method has great potential for application in the design of inexpensive and highly active catalysts.