A Review of Low Temperature NH₃-SCR for Removal of NO_x

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

Influence of SiO₂ on M/TiO₂ (M = Cu, Mn, and Ce) Formulations for Low-Temperature Selective Catalytic Reduction of NO_<i>x</i> with NH₃: Surface Properties and Key Components in Relation to the Activity of NO_<i>x</i> 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 NO_<i>x</i> 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 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⁺ wasgreatly enhanced with an increase in TiO₂ content in the TiO₂–SiO₂ support, and these results are consistent with the H₂-TPR results in which the additional reduction peak evolved at 200 °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⁺/Ce^<i>n+</i> 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 °C. As bigger Ce⁴⁺ ions enter the lattice structure to proxy the Ti⁴⁺ ions with smaller ionic radii (2.48 and 2.15 Å, respectively), the lattice could become highly strained. The NO_<i>x</i> conversions and the apparent kinetic constant of the catalyst <i>k</i>_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