Influence of SiO 2 on M/TiO 2 (M = Cu, Mn, and Ce) Formulations for Low-Temperature Selective Catalytic Reduction of NO x with NH 3 : Surface Properties and Key Components in Relation to the Activity of NO x Reduction

A series of M/TiO 2 and M/TiO 2 –SiO 2 (with M = Mn, Cu, and Ce) catalysts were prepared by adopting a wet–impregnation method and investigated for the selective catalytic reduction (SCR) of NO x in the temperature range of 100–500 °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 TiO 2 phase is strongly inhibited because of SiO 2 migration into the TiO 2 lattice. The increase of SiO 2 molar content in the TiO 2 –SiO 2 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 TiO 2 with no phase transition of anatase to rutile. Our X-ray photoelectron spectroscopy (XPS) depth profile analysis illustrated that the surface atomic ratio of Cu 1+ / Cu 2+ wasgreatly enhanced with an increase in TiO 2 content in the TiO 2 –SiO 2 support, and these results are consistent with the H 2 -TPR results in which the additional reduction peak evolved at 200 °C for the copper-loaded titania-rich (Cu/TiO 2 ) catalyst. The high activity of the Cu-based TiO 2 formulations has been assigned to the enhancement in the formation of Cu 1+ active sites, existence of surface Cu 2+, Cu 1+ species, and the increment of reduction potentials of the surface copper species. The Ce 3+ /Ce 4+ and Ce 3+ /Ce n+ atomic ratio (1.14 and 0.53, respectively) in the Ce/TiO 2 catalyst calculated from deconvoluted XPS spectra are much higher than that of Ce/TiO 2 –SiO 2 (1:1) and Ce/TiO 2 –SiO 2 (3:1). The existence of the higher Ce 3+ surface species over CeO 2 /TiO 2 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 Mn 4+ /Mn 3+ characterized by deconvoluted XPS was significantly high (Mn 4+ /Mn 3+ = 1.98) for the Mn/TiO 2 compared to Mn/TiO 2 –SiO 2 catalysts (Mn 4+ /Mn 3+ = 1.23, 1.75). When ceria was supported on pure TiO 2, 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 °C. As bigger Ce 4+ ions enter the lattice structure to proxy the Ti 4+ ions with smaller ionic radii (2.48 and 2.15 Å, respectively), the lattice could become highly strained. The NO x conversions and the apparent kinetic constant of the catalyst k ac over the Cu, Mn, Ce-loaded on different support TiO 2 and TiO 2 –SiO 2 (3:1 and 1:1) catalysts measured under steady-state conditions demonstrated higher activity of the Ti-rich materials.