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The importance of the low-temperature selective catalytic reduction (LT-SCR) of NOx by NH3 is increasing due to the recent severe pollution regulations being imposed around the world. Supported and mixed transition metal oxides have been widely investigated for LT-SCR technology. However, these catalytic materials have some drawbacks, especially in terms of catalyst poisoning by H2O or/and SO2. Hence, the development of catalysts for the LT-SCR process is still under active investigation throughout seeking better performance. Extensive research efforts have been made to develop new advanced materials for this technology. This article critically reviews the recent research progress on supported transition and mixed transition metal oxide catalysts for the LT-SCR reaction. The review covered the description of the influence of operating conditions and promoters on the LT-SCR performance. The reaction mechanism, reaction intermediates, and active sites are also discussed in detail using isotopic labelling and in situ FT-IR studies.

TiO2 supported transition metal oxides were investigated for the direct decomposition of nitrogen oxides (NO x ) at lower temperatures. The results showed that the catalytic performance strongly depends on the kind of transition metal oxide deposited on the TiO2. Among the various catalysts examined, Mn/TiO2 and Co/TiO2 exhibited relatively high NO x conversion at lower temperatures in the presence of 3 vol % O2. The oxygen in the reaction stream had a positive impact on the NO x decomposition over the Mn/TiO2 catalyst. We have not observed any TiO2 phase conversion from anatase to rutile in the Mn/TiO2 during the NO x decomposition reaction at different temperatures (100–350 °C). NO x decomposition activity was shown to be governed by the surface labile oxygen rather than the gas phase oxygen. The Mn/TiO2 catalyst exhibited a good resistance to 10 vol % H2O and 100 ppm of SO2.

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 o C with excess (10 vol.%) oxygen in the feed at industrially relevant conditions. Our XRD results suggest that the growth of crystalline TiO 2 phase is strongly inhibited due to the SiO 2 migration into the TiO 2 lattice. The increase of SiO 2 molar content in TiO 2 -SiO 2 support led to the decrease in anatase phase of titania peak intensity of XRD spectrum and also exhibiting a lower crystallinity of TiO 2 with no phase transition of anatase to rutile. Our XPS depth profile analysis illustrate that the surface atomic ratio of Cu 1+ / Cu 2+ greatly enhanced with increase in TiO 2 content in the TiO 2 -SiO 2 support, and these results are consistent with the H 2 -TPR results where the additional reduction peak evolved at 200 o C for copper loaded titania-rich (Cu/TiO 2 ) catalyst. The high activity of 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 surface copper species. The Ce 3+ / Ce 4+ and Ce 3+ /Ce n+ atomic ratio (1.14 and 0.53, respectively) in 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 higher Ce 3+ surface species over CeO 2 /TiO 2 illustrates the increment of surface oxygen vacancies and thus facilitates for 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 supports TiO 2 and TiO 2 -SiO 2 (3:1 and 1:1) catalysts measured under steady-state conditions results demonstrated higher activity of the Ti-rich materials.

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