Influence of SiO2 on MiTiO2 (M = Cu, Mn, and Ce) Formulations for Low-Temperature Selective Catalytic Reduction of NOx with NH3: Surface Properties and Key Components in Relation to the Activity of NOx Reduction

A series of M/TiO2 and M/TiO2-SiO2 (with M = Mn, Cu, and Ce) catalysts were prepared by adopting a wet-impregnation method and investigated for the selective catalytic reduction (SCR) of NOx in the temperature range of 100-500 degrees C with excess (10 vol %) oxygen in the feed at industrially relevant conditions. Our X-ray diffraction (XRD) results suggest that the growth of the crystalline TiO2 phase is strongly inhibited because of SiO2 migration into the TiO2 lattice. The increase of SiO2 molar content in the TiO2-SiO2 support led to the decrease in the anatase phase of the titania peak intensity of the XRD spectrum and also exhibited a lower crystallinity of TiO2 with no phase transition of anatase to rutile. Our X-ray photoelectron spectroscopy (XPS) depth profile analysis illustrated that the surface atomic ratio of Cu1+/ Cu2+ was greatly enhanced with an increase in TiO2 content in the TiO2-SiO2 support, and these results are consistent with the H-2-TPR results in which the additional reduction peak evolved at 200 degrees C for the copper-loaded titania-rich (Cu/TiO2) catalyst. The high activity of the Cu-based TiO2 formulations has been assigned to the enhancement in the formation of Cu1+ active sites, existence of surface Cu2+, Cu1+ species, and the increment of reduction potentials of the surface copper species. The Ce3+/Ce4+ and Ce3+/Cen+ atomic ratio (1.14 and 0.53, respectively) in the Ce/TiO2 catalyst calculated from deconvoluted XPS spectra are much higher than that of Ce/TiO2-SiO2 (1:1) and Ce/TiO2-SiO2 (3:1). The existence of the higher Ce3+ surface species over CeO2/TiO2 illustrates the increment of surface oxygen vacancies and thus facilitates the adsorption of oxygen species or activates reactants in the SCR reaction. The relative atomic percentage value of Mn4+/Mn3+ characterized by deconvoluted XPS was significantly high (Mn4+/Mn3+ = 1.98) for the Mn/TiO2 compared to Mn/TiO2-SiO2 catalysts (Mn4+/Mn3+ = 1.23, 1.75). When ceria was supported on pure TiO2, the low-temperature reduction peak was broad and less defined, and the reducibility in the low temperature range was much less pronounced. On the other hand, the addition of ceria to titania with strong reciprocal interaction is generally perceived as a shift in the bulk reduction temperature to lower values, to about 500-650 degrees C. As bigger Ce4+ ions enter the lattice structure to proxy the Ti4+ ions with smaller ionic radii (2.48 and 2.15 angstrom, respectively), the lattice could become highly strained. The NOx conversions and the apparent kinetic constant of the catalyst k(ac) over the Cu, Mn, Ce-loaded on different support TiO2 and TiO2-SiO2 (3:1 and 1:1) catalysts measured under steady-state conditions demonstrated higher activity of the Ti-rich materials.