NO reduction with CO over a highly dispersed Mn/TiO2 catalyst at low temperature: a combined experimental and theoretical study

Abstract

A highly dispersed Mn/TiO2 catalyst, which has high efficiency for NO conversion with CO and almost completed N2 selectivity at a low-temperature range (350–550 K), was investigated using experimental and DFT theoretical calculation. The characterization results illustrated that the catalyst assembled with nanoparticles and the Mn doping into the TiO2 surface lattice led to the formation of Mn–O–Ti configuration, which enhanced the dispersion of Mn on the body of TiO2. The DFT study mapped out the complete catalytic cycle, including reactants adsorption, oxygen vacancy generation, N2O intermediates formation, N2 formation in Eley−Rideal (ER), Langmuir−Hinshelwood, and termolecular Eley−Rideal mechanisms. With thermodynamic and kinetic analysis combined with experimental results, the ER reaction process was considered to be the fundamental mechanism over the highly dispersed Mn/TiO2 catalyst. The calculation results indicated that N2O was a significant intermediate. However, the rapid N2O reduction process led to high N2 selectivity. The rate-limiting step was the deoxygenation step of NO−MnOv/TiO2 from N−O bond scission. The active site Mn−Ov pair embedded in Mn/TiO2 was responsible not only for the formation of N−Mn/TiO2 in the ER-1 step but also for the N2O deoxygenation process to make the final product N2 in the ER-2 step. The synergetic effect between Mn 3d electron and the oxygen vacancy of TiO2 were responsible for the catalytic activity of Mn/TiO2.