Mechanistic Study of Selective Catalytic Reduction of NOx with NH3 over Mn-TiO2: A Combination of Experimental and DFT Study

Mn-TiO2 oxide catalyst has been studied intensively for selective catalytic reduction (SCR) of NO with NH3 due to its extraordinarily good low-temperature performance. However, the mechanism of SCR on Mn-TiO2 still remains unclear, especially with regard to the decomposition pathway of the NH2NO intermediate and the reason for the decreasing N-2 selectivity with the increasing of temperature. In this work, we attempt to provide a molecular level understanding of these questions via a combination of DFT and experimental study. A complete catalytic cycle of the SCR reaction was proposed based on a model in which Mn is doped into the TiO2(101) surface by quantum chemical DFT+U calculations. In situ DRIFTS experiments were performed to provide evidence to the important intermediates as proposed in the reaction mechanism. The doping Mn enhances NH3 adsorption and activation due to its lower conduction band. NH2NO can decompose into N-2 and H2O fast via a concerted H migration step. The decreasing selectivity with rising temperature can be explained by the deep oxidation of NH3. This study provides atomic-scale insights into the catalytic cycle and the important role of doping Mn in NH3 SCR reaction on Mn-TiO2 catalysts, which is of significance for the design of high activity low-temperature SCR catalysts.