Selective catalytic oxidation of ammonia over MnOx–TiO2 mixed oxides

Abstract

The selective catalytic oxidation of ammonia to nitrogen (NH₃–SCO) was investigated over MnO_x–TiO₂. The physicochemical properties of MnO_x–TiO₂ were characterized by XRD, O₂-TPD, NH₃-TPD, H₂-TPR and XPS, and the reaction mechanism was studied by in situ DRIFTS. The addition of Mn into TiO₂ accelerated the support phase transformation and the formation of Mn–O–Ti bonds. MnO_x(0.25)–TiO₂ showed the best performance in NH₃–SCO, for which complete conversion of NH₃ was obtained at 200 °C with the temperature window (180–300 °C) for N₂ yield > 80%. The formation of Mn–O–Ti provided abundant oxygen vacancies which promoted the adsorption and dissociation of molecular oxygen to form active oxygen species. The finely dispersed MnO_x species on the support favored NH₃ adsorption. N₂O was produced over the whole temperature range while NO_x was produced only at high temperatures (>250 °C). N₂O was formed from the combination of two HNO species at low temperatures, whereas it was formed from NH₃ and nitrate/nitrite species reaction at high temperatures.