Semiconductor photocatalysis could be an effective means to combat nitrogen oxides (NO_x) based air pollution through mineralisation of NO_x to nitrate. However, most of the typically TiO₂-based catalysts employed show a much higher reactivity towards NO than NO₂, leading to an accumulation of this unwanted and toxic intermediate. By grafting the photocatalyst with small amounts (≤0.1 at%) of isolated iron(III) ions, the reactivity towards NO₂ is increased by the factor of 9, bringing it up to par with the NO-reactivity and alleviating the problem with intermediate accumulation. Consequently, the observed selectivity of the reaction is dramatically increased from less than 40% to more than 90%. The paper also discusses possible mechanisms for this very beneficial behavior.