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.

The selective catalytic reduction (SCR) of NOx with NH3 to harmless N2 and H2O plays a crucial role in reducing highly undesirable NOx acid gas emissions from large utility boilers, industrial boilers, municipal waste plants, and incinerators. The supported V2O5–WO3/TiO2 catalysts have become the most widely used industrial catalysts for these SCR applications since introduction of this technology in the early 1970s. This Perspective examines the current fundamental understanding and recent advances of the supported V2O5–WO3/TiO2 catalyst system: (i) catalyst synthesis, (ii) molecular structures of titania-supported vanadium and tungsten oxide species, (iii) surface acidity, (iv) catalytic active sites, (v) surface reaction intermediates, (vi) reaction mechanism, (vii) rate-determining-step, and (viii) reaction kinetics.

A Perspective on the Selective Catalytic Reduction (SCR) of NO with NH3 by Supported V2O5–WO3/TiO2 Catalysts

Ceria-based catalysts for low-temperature selective catalytic reduction of NO with NH3

A CuO–CeO2–TiO2 catalyst for selective catalytic reduction of NOx with NH3 (NH3-SCR) at low temperatures was prepared by a sol–gel method and characterized by X-ray diffraction, Brunner–Emmett–Teller surface area, ultraviolet–visible spectroscopy, H2 temperature-programmed reduction, scanning electron microscopy and in situ diffuse reflectance infrared Fourier transform spectroscopy (in situ DRIFTS). The CuO–CeO2–TiO2 ternary oxide catalyst shows excellent NH3-SCR activity in a low-temperature range of 150–250 °C. Lewis acid sites generated from Cu2+ are the main active sites for ammonia activation at low temperature, which is crucial for low temperature NH3-SCR activity. The introduction of ceria results in increased reducibility of CuO species and strong interactions between CuO particles with the matrix. The interactions between copper, cerium and titanium oxides lead to high dispersion of metal oxides with increased active oxygen and enhanced catalyst acidity. Homogeneously mixed metal oxides facilita...

DRIFT Study of CuO–CeO2–TiO2 Mixed Oxides for NOx Reduction with NH3 at Low Temperatures

Deactivation of alkali metals and arsenic and regeneration methods are studied on commercial V 2 O 5 –WO 3 /TiO 2 for the SCR reaction using experiments and DFT calculations. The poisoning of alkali metals is found to decrease the amount of Bronsted acid sites and the reducibility of active V 5+ sites. Arsenic decreases the amount of Lewis acid sites and the stability of Bronsted acid sites and increases N 2 O formation. After the catalysts are poisoned by both alkali metals and arsenic, the activity and N 2 selectivity are significantly suppressed. Diluted H 2 SO 4 effectively removes alkali metals from the poisoned catalysts. Half of the amount of arsenic can be removed using a 4% H 2 O 2 solution; however, some V 2 O 5 and surface sulfates are also eliminated from the catalysts. The activity of the regenerated catalysts is almost recovered at high temperatures. From the DFT results on the V 2 O 5 /TiO 2 (0 0 1) plane, potassium and arsenic significantly alter the electronic structures of the V orbitals and broaden the band gap of the models. Interactions between potassium and arsenic are also found. Potassium covers the active sites of the models that are constructed by V 2 O 5 and As 2 O 5 , which further decreases the number of acid sites. Potassium causes V and As orbitals to move to lower energies and inhibits the reactivity of the model.

Deactivation and regeneration of a commercial SCR catalyst: Comparison with alkali metals and arsenic

Alkali metals were found to be poisonous to the Ce‐Ti oxide catalyst. The NO conversion of the catalyst decreased by more than 80 % at temperatures from 250 to 450 °C with the doping of either 3.71 atom % K or 3.74 atom % Na. A combined experimental and theoretical study was used to reveal the mechanism of the deactivation caused be the alkali metals. The results indicated that the doping of alkali metal atoms on the catalyst surface greatly decreases the surface acidity and reducibility. XPS characterization revealed that the interaction between the alkali atoms and the oxygen atom of cerium oxide inhibits the transformation between Ce3+ and Ce4+, which effects a decrease in the proportion of Ce3+. The theoretical results show that the alkali atoms strongly interact with the cerium oxygen and titanium oxygen, which causes the degradation of reducibility and surface acidity, respectively. This causes a serious deactivation of the Ce‐Ti oxide catalyst.

The Influence of Alkali Metals on the Ce‐Ti Mixed Oxide Catalyst for the Selective Catalytic Reduction of NOx

Relationship between the molecular structure of V2O5/TiO2 catalysts and the reactivity of SO2 oxidation

Experimental and theoretical studies on the influence of water vapor on the performance of a Ce-Cu-Ti oxide SCR catalyst

Numerical simulation of selective catalytic reduction of NO and SO2 oxidation in monolith catalyst

In this study, we investigated the effect of potassium on the activity and regeneration of potassium-poisoned SCR catalysts. With the addition of potassium species, the NO conversion rate of the catalysts continuously decreased. After washing the poisoned catalysts with an H2SO4 solution or doping them with CeO2, the activity of the catalysts was improved to different extents. Acid washing almost completely removed the surface potassium species, freeing acidic sites to adsorb NH3, but it also potentially removed some of the active components, such as vanadia. CeO2 doping, on the other hand, added active components. Combining these two methods, the poisoned catalysts were washed with an H2SO4 solution and then doped with 5 wt.% CeO2. It was found that the level of activity could be restored to that of a fresh catalyst, and a conversion rate of over 90% was observed for NO between 300°C and 450°C, as the added CeO2 compensated for the active components lost during SCR reactions. Consequently, the above hybrid method shows high potential for regenerating commercial SCR catalysts.

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Peidong Ji

Papers

The Influence of Alkali Metals on the Ce‐Ti Mixed Oxide Catalyst for the Selective Catalytic Reduction of NOx

Relationship between the molecular structure of V2O5/TiO2 catalysts and the reactivity of SO2 oxidation

Experimental and theoretical studies on the influence of water vapor on the performance of a Ce-Cu-Ti oxide SCR catalyst

Numerical simulation of selective catalytic reduction of NO and SO2 oxidation in monolith catalyst

Regeneration of Potassium Poisoned Catalysts for the Selective Catalytic Reduction of NO with NH3