The performance of activated carbon (AC) modified by different methods was compared for its catalytic degradation of ortho-chlorophenol (o-CP). For the chemically treated AC, the catalytic effect of AC-NH3 center dot H2O was superior to the other catalysts examined, having an o-CP removal efficiency of 82.2% at 330 degrees C. For the metal-modified catalysts, AC-V and AC-Co showed similar removal performances of 93.2% at 330 degrees C. N-2 adsorption-desorption isotherms, Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), and gas chromatography-mass spectrometry (GC-MS) analyses were used to characterize the reaction products, and different reaction mechanisms were proposed for both AC-NH3 center dot H2O and AC-V according to the results. Complete oxidative degradation of o-CP was achieved by AC-V, with AC-NH3 center dot H2O leading to the formation of additional dioxins. It can be deduced that a risk of dioxin synthesis and escape during the regeneration process is possible when nitrogen-modified carbon is used in selective catalytic reduction (SCR) denitrification reactions, especially in the presence of chlorine atoms, benzene rings, and oxygen.