Novel manganese oxide confined interweaved titania nanotubes for the low-temperature Selective Catalytic Reduction (SCR) of NOx by NH3

Abstract

A series of manganese confined titania nanotubes (Mn/TNT) prepared by alkaline hydrothermal synthesis technique and were investigated for the Selective Catalytic Reduction (SCR) of NOx with NH3 in the presence of excess (10vol.%) oxygen. Remarkably, the surface texture and tubular morphology of Mn/TNT-H catalyst greatly promote the NOx conversions in the temperature regime of 100–300°C. The existence of abundant surface Mn4+ species apparently contributes to the remarkable low-temperature SCR activity, and additionally the increased surface area, high dispersion and numerous Lewis acid sites distribution contribute to broaden temperature window over Mn/TNT-H catalyst with 0.25 Mn/Ti atomic ratio. The impregnation of MnOx over TNT-SA catalyst occurs bare minimum because of the low specific surface area and low pore volume, and this leads to poor deNOx activity. The Mn4+/Mn3+ fraction deduced by XPS found to be considerably high for the Mn(0.25)/TNT-H catalyst (Mn4+/Mn3+=2.15) while in other catalysts the fraction is typically in the range of 0.36–1.34. Raman scattering reveals a redshift of 13cm−1 from 640cm−1 (TiO2-Hombikat) to 627cm−1 in Mn/TNTs-H sample, which indicates the formation of a new phase or structural difference. In comparison with the Mn supported on TiO2 (Hombikat) nanoparticles, the Mn species on the developed tubular Mn/TNT-H were less pronounced to coagulate under the reaction temperatures. It is interesting to note that the relative atomic ratios of Mn4+/Ti and Mn4+/Mn3+ as well as the existence of surface Mn4+ species in the titania nanotube appear to be directly correlated with the deNOx performance of the catalysts.