Metal oxide-confined interweaved titania nanotubes M/TNT (M = Mn, Cu, Ce, Fe, V, Cr, and Co) for the selective catalytic reduction of NOx in the presence of excess oxygen

Abstract

A series of metal oxide confined titania nanotube M/TNT (M = Mn, Cu, Ce, Fe, V, Cr and Co) catalytic formulations were successfully synthesized using hydrothermal method followed by wet impregnation technique. The resulting materials were investigated for the low temperature Selective Catalytic Reduction (SCR) of NOx with NH3 in the presence of excess oxygen (10 vol.%). The prepared catalysts exhibit remarkable deNOx potential at temperatures as low as 100 °C and in a wide temperature range from 100 to 350 °C. Compared to manganese oxide supported on conventional TiO2, the synthesized Mn/TNT catalyst exhibits superior activity. Interestingly, the surface texture and tubular morphology of Mn/TNT catalyst greatly promotes the NOx conversion in the temperature regime of 100–300 °C. Our BET results of the as-prepared catalysts demonstrated that this is an effective synthesis for the generation of high specific surface area (421 m2/g) titania nanotubes. High resolution transmission electron microscopy (HR-TEM) results revealed the formation of tubular structure and multi-walled layer construction in the pristine titania nanotube (TNT), Mn/TNT and Ce/TNT samples. The relative atomic percentage value of Mn4+/Mn3+ and Ce3+/Ce4+ characterized by deconvoluted XPS spectra was considerably high for the Mn/TNT (Mn4+/Mn3+ = 2.15) and Ce/TNT catalysts (Ce3+/Ce4+ = 0.79), respectively. The existence of abundant surface Mn4+ species apparently contributes to the remarkable low-temperature SCR activity over our Mn/TNT catalyst. The H2-TPR results are in good accordance with our XPS results, with MnO2 being the prevailing phase and the increased reduction potentials of manganese appear to be the reason for the high deNOx activity of the Mn/TNT catalyst at low-temperatures. Vanadia based catalyst (V/TNT) formulations exhibit monolayer isolated species present over the support where V5+ is the dominant oxidation state. Titania nanotubes confined vanadium oxide catalyst demonstrates a broad operation temperature window attributed to the high dispersion of active species on the support.