Mechanistic investigations on NO reduction with CO over Mn/TiO2 catalyst at low temperatures

Abstract

A series of titania-supported transition metal oxide catalysts were evaluated for NO reduction with CO as reductant at low temperature (200°C) in the presence of excess oxygen. Among the investigated systems, MnOx/TiO2 has been found to be the preeminent catalyst. In situ FT-IR and transient studies were carried out in order to identify the chemisorbed species which could act as catalytic intermediates, and in consequence to propose the low temperature reduction of NO pathway over the Mn/TiO2. The NO adsorption and NO+O2 co-adsorption of in situ FT-IR experiments revealed the formation of surface reaction intermediate N2O species along with the formation of monodentate and bidentate nitrates, whereas CO adsorption on the catalyst leads to the formation of carbonate species. Interestingly, in the case of NO and CO co-adsorption over MnOx/TiO2, the formation of CO2 (CO oxidation with gas phase oxygen) is inhibited due to the surface reaction competition between NO and CO. The formation of N2O as an intermediate was evident from the occurrence of peaks at 1286 and 1335cm−1 region. There is no formation of NO2 as a surface intermediate or gas phase stable product in the present in situ Fourier transform infrared spectroscopy (FT-IR) and transient mass spectroscopic studies, respectively. Remarkably, a neat absorption peak at 2178cm−1 ascribed to the isocyanate (–NCO) species was not observed during in situ infrared spectroscopic studies of NO+CO and CO+NO co-adsorptions. Based upon this evidence it is proposed that the formation of NCO− species from the reaction between CO and Nads is completely forbidden over the Mn/TiO2 catalysts. These results indicate that the reaction mechanism follows a different pathway for our catalyst, from that of the other metal based catalysts. The role of lattice oxygen in the reaction mechanism is substantiated by isotopic labeling and transient analysis studies. Lewis acid sites act as the active sites for the reaction.