The formation of Ce3+ ions in CeO2 is key for its applications. We present density functional theory (DFT) studies of doping of the CeO2 (110) surface with +5 cations, Ta and Nb, as a route to formation of Ce3+. We use DFT corrected for on-site Coulomb interactions (DFT+U, U=5eV) and hybrid DFT (HSE06 functional). With both methods we find formation of Ce3+ ions due to the extra electron from the dopant residing on a single Ce site, confirming the DFT+U description of these systems. Doping with Nb and Ta activates the surface to NO2 reduction – there is no interaction with the undoped surface but the Ce3+ ion in the doped surface interacts strongly with NO2.