Selective catalytic reduction with NH3 (NH3-SCR) is the most efficient technology to reduce the emission of nitrogen oxides (NOx) from coal-fired industries, diesel engines, etc. Although V2O5-WO3(MoO3)/TiO2 and CHA structured zeolite catalysts have been utilized in commercial applications, the increasing requirements for broad working temperature window, strong SO2/alkali/heavy metal-resistance, and high hydrothermal stability have stimulated the development of new-type NH3-SCR catalysts. This review summarizes the latest SCR reaction mechanisms and emerging poison-resistant mechanisms in the beginning and subsequently gives a comprehensive overview of newly developed SCR catalysts, including metal oxide catalysts ranging from VOx, MnOx, CeO2, and Fe2O3 to CuO based catalysts; acidic compound catalysts containing vanadate, phosphate and sulfate catalysts; ion exchanged zeolite catalysts such as Fe, Cu, Mn, etc. exchanged zeolite catalysts; monolith catalysts including extruded, washcoated, and metal-mesh/foam-based monolith catalysts. The challenges and opportunities for each type of catalysts are proposed while the effective strategies are summarized for enhancing the acidity/redox circle and poison-resistance through modification, creating novel nanostructures, exposing specific crystalline planes, constructing protective/sacrificial sites, etc. Some suggestions are given about future research directions that efforts should be made in. Hopefully, this review can bridge the gap between newly developed catalysts and practical requirements to realize their commercial applications in the near future.

Selective Catalytic Reduction of NOx with NH3 by Using Novel Catalysts: State of the Art and Future Prospects.

A superior Ce-W-Ti mixed oxide catalyst prepared by a facile homogeneous precipitation method showed excellent NH3-SCR activity and 100% N2 selectivity with broad operation temperature window and extremely high resistance to space velocity, which is a very promising catalyst for NOx abatement from diesel engine exhaust. The excellent catalytic performance is associated with the highly dispersed active Ce and promotive W species on TiO2. The introduction of W species could increase the amount of active sites, oxygen vacancies, and Bronsted and Lewis acid sites over the catalyst, which is also beneficial to improve the low temperature activity by facilitating “fast SCR” reaction and enhance both of the high temperature activity and N2 selectivity simultaneously by inhibiting the unselective oxidation of NH3 at high temperatures.

A superior Ce-W-Ti mixed oxide catalyst for the selective catalytic reduction of NOx with NH3

A series of MoO3-doped CeO2/TiO2 catalysts prepared by the impregnation method were investigated for the selective catalytic reduction of NO,c by NH3(NH3-SCR). It was found that CeO2-MoO3/TiO2 catalyst is much more active than CeO2/TiO2 for NH3-SCR and the optimum MoO3 loading is 5%. The mechanistic cause of the promoting effect of MoO3 on the activity of CeO2/TiO2 catalyst for NH3-SCR was studied using in situ diffuse reflectance infrared transform spectroscopy (DRIFTS). The results revealed that the highly dispersed molybdenum on CeO2-MoO3/TiO2 catalyst not only resulted in more Brensted acid sites formed on the catalyst surface, but also reduced the thermal stability of the inactive nitrate specie, leaving more active sites available for the adsorption of NH3, both of which are favorable for the promotion of SCR activity. (C) 2013 Elsevier B.V. All rights reserved.

Promoting effect of MoO3 on the NOx reduction by NH3 over CeO2/TiO2 catalyst studied with in situ DRIFTS

The amorphous Ce–Ti mixed oxides were reported to be catalysts for selective catalytic reduction of NOx with NH3, in which Ce and not Ti acts as their solvent in spite of the fact that Ce is low in content. The amorphous catalysts were characterized by X-ray powder diffraction (XRD) and transmission electron microscopy (TEM) equipped with selective area electron diffraction (SAED). The Ce–Ti amorphous oxide shows higher activity than its crystalline counterpart at lower temperatures. Moreover, the presence of small CeO2 crystallites as for the impregnated sample is deleterious to activity. The Ce–O–Ti short-range order species with the interaction between Ce and Ti in atomic scale was confirmed for the first time to be the active site using temperature programmed reduction with H2 (H2–TPR), in situ FTIR spectra of NO adsorption, X-ray photoelectron spectroscopy (XPS), and X-ray absorption fine-structure (XAFS). Lastly, the Ce–O–Ti structure was directly observed by field-emission TEM (FETEM).

Ce–Ti Amorphous Oxides for Selective Catalytic Reduction of NO with NH3: Confirmation of Ce–O–Ti Active Sites

Novel cerium-tungsten mixed oxide catalyst for the selective catalytic reduction of NO(x) with NH3.

Preparation and characterization of CeO2/TiO2 catalysts for selective catalytic reduction of NO with NH3

A Ce–Cu–Ti oxide catalyst for the selective catalytic reduction of NO with NH3

The effect of the Sb and Nb additives on the V 2 O 5 /TiO 2 catalyst for the selective catalytic reduction (SCR) of NO with NH 3 was investigated. The experimental results show that either Nb or Sb can improve the activity of V 2 O 5 /TiO 2 catalyst. Higher Nb loading led to higher N 2 selectivity. The co-doping of Sb and Nb showed higher improving effect than the single doping of Sb or Nb. The V 2 O 5 /TiO 2 catalyst doped with Sb and Nb had a better H 2 O resistance than the V 2 O 5 /TiO 2 catalyst. The addition of Sb and Nb also enhance the resistance of the V 2 O 5 /TiO 2 catalyst to K 2 O poisoning. The catalysts were characterized by BET, XRD, TEM, and XPS. The results showed that the active components of V, Sb, and Nb were well interacting with each other. The coexistence of Sb and Nb will enhance the redox ability and surface acidity and thus promote the SCR performance.

Yin-cheng Fu

Papers

Preparation and characterization of CeO2/TiO2 catalysts for selective catalytic reduction of NO with NH3

A Ce–Cu–Ti oxide catalyst for the selective catalytic reduction of NO with NH3

The co-effect of Sb and Nb on the SCR performance of the V2O5/TiO2 catalyst

Investigation of the effect of Cu addition on the SO2-resistance of a CeTi oxide catalyst for selective catalytic reduction of NO with NH3

Theoretical and experimental study on the deactivation of V2O5 based catalyst by lead for selective catalytic reduction of nitric oxides