In the selective catalytic reduction (SCR) of NO with NH3 over activated carbon (AC), deactivation occurs over time in the presence of SO2. This work distinguishes the multiple roles of SO2 in the gas phase versus the solid deposition product and clarifies the effects of the physicochemical properties of AC on NO conversion. The deposition products were detected using temperature-programmed desorption (TPD) coupled with mass spectrum (MS) analysis and Fourier transform infrared (FTIR) spectrometry. The results showed that the activated carbon loses less de-NOx activity when it has more CO- and CO2 containing groups with decomposition temperatures over 900 K. The Raman spectra revealed that the disorder of the microcrystalline structure of the graphite has a positive linear correlation with NO conversion regardless of the presence of functional groups. The deposition products were analyzed by Gaussian-Lorentz deconvolution of the TPD spectra, and it was discovered that the sulfur-containing species included sulfate and strongly adsorbed SO2/SO3; the NH3-containing species included NH4HSO4 and freely adsorbed NH3; and the ratios of SO2/SO3, NH4HSO4 and NH3 were approximately 31 mol %, 42 mol %, and 26 mol %, respectively. NH4HSO4 does not notably inhibit NO conversion, even with a high loading amount. The inhibitory effect of gaseous SO2 on NO conversion is reversible, and this inhibitory effect is greater than that caused by the loss of functional groups. Increasing the disorder of the microcrystalline structure of the graphite and reducing the gaseous SO2 were identified as ways to improve activated carbon activity for NO conversion.