Search

Ammonia selective catalytic reduction (NH 3 -SCR) of nitrogen oxides is an effective and well-established technology for NO x removal, but current commercial denitrification catalysts based on V 2 O 5 -WO 3 /TiO 2 have some obvious disadvantages, including narrow operating temperature windows, toxicity, poor hydrothermal stability, and unsatisfied SO 2 /H 2 O tolerance. To overcome these drawbacks, it is imperative to investigate new types of highly efficient catalysts. In order to design catalysts with outstanding selectivity, activity, and anti-poisoning ability, core-shell structured materials have been widely applied in the NH 3 -SCR reaction, which exhibits numerous advantages including the large surface area, the strong synergy interaction of core-shell materials, the confinement effect, and the shielding effect from the shell layer to protect the core. This review summarizes recent developments of core-shell structured catalysts for NH 3 -SCR, including basic classification, synthesis methods, and a detailed description of the performance and mechanisms of each type of catalyst. It is hoped that the review will stimulate future developments in NH 3 -SCR technology, leading to novel catalyst designs with improved denitrification performance., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2023 Elsevier Ltd. All rights reserved.)

In this study, it was found that the CeVO 4 -CeO 2 nanocomposite possessed remarkably selective catalytic reduction (SCR) performance and wider active temperature scope. And, the promotion principle was explored based on BET, XRD, XPS, H 2 -temperature-programmed reduction, NH 3 -temperature-programmed desorption, and in situ diffuse reflectance infrared Fourier transform (DRIFT) techniques. The characterization outcomes manifested that the CeVO 4 -CeO 2 nanocomposite could inhibit its crystallinity and enhance the concentrations of chemisorbed oxygen species and Ce 3+ , which was advantageous to the SCR process. Moreover, the in situ DRIFT technique manifested that the NH 3 -SCR reaction over Ce 0.75 V 0.25 O y was enhanced effectively through the mechanism of L-H.

CO pretreatment was found to effectively improve the SCR performance of CeO 2 , with over 90% at about 300 °C. The larger specific area and the decrease of CeO 2 crystallization indicated the modification of the surface structure after CO pretreatment. Abundant Ce 3+ species and active oxygen, better reducibility, and the higher surface adsorption capacity were mainly responsible for its enhanced SCR performance. DRIFT analysis revealed the presumed coexistence of two reaction routes that the L-H mechanism was related to the reaction temperature, while the reaction rate of E-R route was almost directly proportional to the NO concentration at a certain temperature, based on the kinetic calculation. In addition, the CO-pretreated CeO 2 also exhibited a better poisoning tolerance for SO 2 and H 2 O and excellent thermal stability and circularity. Graphical abstract The process of NH 3 -SCR reaction over CeO 2 -CO catalyst.

Fe was used as the modifier to improve the performance of CeZr2Ox catalyst for the selective catalytic reduction (SCR) reaction with ammonia. Experimental results revealed that proper Fe species (Fe/Ce molar ratio = 2) could effectively improve the SCR activity of CeZr2Ox, along with the elevated resistance to SO2 and H2O. The characterization results including BET, XRD, H2-TPR and NH3-TPD confirmed that Fe species in Fe2CeZr2Ox catalyst could inhibit the formation of ZrO2 crystals and lead to higher reducibility and surface acidity. In addition, more Ce3+ and surface active oxygen species could be detected on Fe2CeZr2Ox compared with CeZr2Ox. By means of in situ DRIFT technique, it could be deduced that the SCR reaction process over Fe2CeZr2Ox was accelerated mainly through theL-H pathway due to the advanced adsorption of NH3 and NOx species, along with the promoted NO conversion to NO2. Therefore, the modification of CeZrOx catalyst by Fe had a significant stimulation effect on its low-temperature SCR performance. [ABSTRACT FROM AUTHOR]

The different poisoning mechanisms of Cl and P on Mn/TiO 2 catalyst for selective catalytic reduction of NO x with NH 3 were investigated in this study. It was found that the poisoning effect of P was stronger than that of Cl, and the poisoning effect of Cl was very weak when the reaction temperature was over 250 °C. And the presence of Cl or P would weaken the poisoning effect of SO 2 and H 2 O. From the characterization results of temperature programmed reduction (TPR) and X-ray photoelectron spectroscopy (XPS), the loading of P and Cl would lead to the decrease of reducible species and the loss of Mn 4+ and chemisorbed oxygen species, as a result, inhibiting the oxidation of NO to NO 2 .The results of temperature programmed desorption (TPD) study indicated that the presence of Cl only had a weak inhibition effect on the adsorption of NH 3 and NO x species over Mn/TiO 2 catalyst, while the addition of P on Mn/TiO 2 catalyst would greatly suppress the adsorption of NH 3 and NO x species. From the results of in situ DRIFT study, it could be found that the NH 3 –SCR reaction over Mn/TiO 2 catalyst was mainly controlled by L–H mechanism at lower temperature range (≤ 200 °C) and E–R mechanism at higher temperature range (≥ 250 °C). For Mn/TiO 2 Cl, the deactivation was mainly caused by the prohibited NO oxidation; while for Mn/TiO 2 P, the deactivation was mainly due to the seriously suppressed NO oxidation and NH 3 adsorption. [ABSTRACT FROM AUTHOR]

CeO2–CuO/Al2O3catalysts were prepared by three different methods and their activities for selective catalytic reduction (SCR) of NO with NH3were investigated. As can be seen from the experimental results, the catalyst prepared by the single-step sol–gel (SG) method showed the best SCR activity and resistance to SO2and H2O. In order to investigate the relationship between the preparation method and the performance of SCR catalysts, the catalysts were characterized by using Brunauer–Emmett–Teller, X-ray diffraction, temperature programmed reduction with hydrogen, temperature programmed desorption with ammonia, X-ray photoelectron spectroscopy, Fourier transform infrared and thermo-gravimetric analysis techniques. It was found that the excellent performance of CeO2–CuO/Al2O3catalyst prepared by the single-step SG method should be resulted from its large surface area, low crystallinity, high oxygen storage capacity, high NH3adsorption capacity, high concentration of surface chemisorbed oxygen, weak sulphation process and weak water absorption. [ABSTRACT FROM PUBLISHER]

Na has a serious deactivation effect on Mn/TiO 2 catalyst for selective catalytic reduction of NO with NH 3 . In this study, it was found that Sb had a promotion effect on the Na resistance of MnOx/TiO 2 catalyst. Then the promotion mechanism was investigated based on the characterization results of BET, XRD, H 2 -TPR, NH 3 -TPD, XPS and in situ DRIFT. The results of H 2 -TPR and NH 3 -TPD showed that the addition of Sb on MnOx/TiO 2 could enhance the redox ability and surface acidity respectively, and the results of DRIFT indicated that the addition of Sb on MnOx/TiO 2 could generate strong Lewis acid sites on the catalyst surface for NH 3 adsorption. Therefore, the doping of Sb has a promotion effect on the Na resistance of Mn/TiO 2 catalyst. [ABSTRACT FROM AUTHOR]

Heavy metals have been proven to be of deactivation effect on SCR catalyst. In this paper, the poisoning effect of heavy metals (Zn and Pb) on Mn/TiO 2 catalyst was investigated based on experimental and theoretical study. It was found that both of Zn and Pb had a poisoning effect on Mn/TiO 2 catalyst and the poisoning effect of Zn was more obvious. The characterization results indicated that the doping of heavy metals on Mn/TiO 2 surface would lead to a great decrease of reducibility, surface acidity and NO adsorption ability. The results of XPS characterization revealed the decrease of Mn 4+ atomic concentration and chemisorbed oxygen species caused by the doping of heavy metals. Based on the above mentioned unfavorable properties, the doping of heavy metals would cause a serious deactivation of Mn/TiO 2 catalyst. From the results of DFT calculations, it could be concluded that the doping of heavy metals on Mn/TiO 2 catalyst resulting in a great decrease of NH 3 adsorption capacity and NO adsorption ability, which agrees well with the experimental results. [ABSTRACT FROM AUTHOR]

Ce/TiO 2 is a prospective catalyst used for low temperature selective catalytic reduction (SCR) of NO with NH 3 . In this paper, the poisoning effect of Zn and Pb on the SCR performance of Ce/TiO 2 catalyst was investigated. Experimental results indicated that both Zn and Pb had a poisoning effect on Ce/TiO 2 catalyst, and Pb exhibited more poisoning effect than Zn in most of the reaction temperature range. Combined with the characterization results, we proposed that the enlargement of TiO 2 nanoparticles, the reduced chemisorbed oxygen content and surface acidity all contribute to the deactivation of Zn–Ce/TiO 2 and Pb–Ce/TiO 2 . [ABSTRACT FROM AUTHOR]

Mn/TiO 2 catalyst is of high activity for low temperature selective catalytic reduction (SCR) of NO with NH 3 . And the deposition of alkali metal would lead to the deactivation of Mn/TiO 2 catalyst. In this paper, the poisoning effect of Na and K on Mn/TiO 2 was investigated based on experimental and theoretical study. It was found that K had a stronger poisoning effect than that of Na. The bad performance of K–Mn/TiO 2 may be due to its small surface area, high crystallinity, weak surface acidity, low content of Mn 4+ and chemisorbed oxygen, and bad redox ability. The interpretation of the experimental results is supported by DFT calculations. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • Sb doping could improve the SCR activity and SO 2 resistance effectively for Cu/TiO 2 catalyst. • The addition of Sb on Cu/TiO 2 catalyst could promote the adsorption of NH 3 and NOx species. • Sb in CuSb/TiO 2 -0.4 catalyst could enhance the reactivities of adsorbed reactants. Abstract The Cu/TiO 2 catalysts modified by Sb were compounded through the sol-gel way and put into use for the selective catalytic reduction (SCR) of NOx with NH 3. Experimental results suggested that CuSb/TiO 2 -0.4 exhibited the excellent NOx conversion and SO 2 tolerance. Subsequently, the relationship between surface physicochemical properties and catalytic performance was also investigated by different characterization methods. From characterizations, the introduction of Sb for Cu/TiO 2 could promote the generation of more Cu2+, surface adsorbed oxygen species and surface acid sites on CuSb/TiO 2 -0.4 catalyst. Furthermore, Sb species on CuSb/TiO 2 -0.4 could contribute to the formation of more surface ad-NOx and ad-NH 3 species, which would enhance the NH 3 -SCR process effectively through both the Langmuir-Hinshelwood (L-H) and Eley-Rideal (E-R) mechanisms. [ABSTRACT FROM AUTHOR]

Fe was used as the modifier to improve the performance of CeZr2Ox catalyst for the selective catalytic reduction (SCR) reaction with ammonia. Experimental results revealed that proper Fe species (Fe/Ce molar ratio = 2) could effectively improve the SCR activity of CeZr2Ox, along with the elevated resistance to SO2 and H2O. The characterization results including BET, XRD, H2-TPR and NH3-TPD confirmed that Fe species in Fe2CeZr2Ox catalyst could inhibit the formation of ZrO2 crystals and lead to higher reducibility and surface acidity. In addition, more Ce3+ and surface active oxygen species could be detected on Fe2CeZr2Ox compared with CeZr2Ox. By means of in situ DRIFT technique, it could be deduced that the SCR reaction process over Fe2CeZr2Ox was accelerated mainly through theL-H pathway due to the advanced adsorption of NH3 and NOx species, along with the promoted NO conversion to NO2. Therefore, the modification of CeZrOx catalyst by Fe had a significant stimulation effect on its low-temperature SCR performance. [ABSTRACT FROM AUTHOR]

A series of MnEuOx catalysts were prepared by the co-precipitation method and used for selective catalytic reduction of NOx with NH 3 . It was found that the modification of MnOx catalyst by Eu with a Eu/Mn molar ratio of 0.1 could greatly enhance its SCR activity and broaden its operating temperature window, accompanied by good resistance to SO 2 and H 2 O. From the characterization results of N 2 adsorption, XRD, XPS, H 2 -TPR, it could be concluded that the larger surface area resulted from the inhibited crystallization of MnOx, the enrichment of surface Mn 4+ and chemisorbed oxygen, and the higher reducibility all made a contribution to the high SCR activity of MnEuOx-0.1 catalyst. Furthermore, the results of in situ DRIFTS study revealed the NH 3 -SCR reactions over MnOx and MnEuOx-0.1 were mainly controlled by L-H mechanism (<150 °C) and E-R mechanism (>150 °C) respectively, and the contribution of E–R mechanism became more important with increasing reaction temperature. The enhanced NO oxidation and NH 3 adsorption should be mainly contribute to the distinguished SCR performance of MnEuOx catalyst in the lower temperature range (<150 °C) and higher temperature range (>150 °C) respectively. [ABSTRACT FROM AUTHOR]

A series of Mn/TiWO x catalysts were prepared and used in NH 3 -SCR reaction for NO x emission control. The experimental results showed that the SCR activities of Mn/TiWO x catalysts are higher than that of Mn/TiO 2 catalyst. After that, several characterization techniques including XRD, H 2 -TPR, NH 3 -TPD, XPS were used to investigate the effect of TiWO x support on the SCR performance of Mn/TiWO x (1:1) catalyst. The stronger reducibility and NH 3 adsorption capacity, and more surface chemisorbed oxygen of Mn/TiWO x (1:1) should all contribute to its better SCR performance. Moreover, Mn/TiWO x (1:1) catalyst shows higher N 2 selectivity, better SO 2 tolerance and stability than Mn/TiO 2 catalyst. The results of in situ DRIFT study revealed that the NH 3 -SCR reactions over Mn/TiO 2 and Mn/TiWO x (1:1) were all followed the Langmuir–Hinshelwood mechanism, in which the SCR reaction takes place between the adsorbed NH 3 species and NO x species. [ABSTRACT FROM AUTHOR]

The effect of F and Cl on Mn/TiO 2 catalyst for selective catalytic reduction of NO with NH 3 was investigated in this study. It was found that the doping of F or Cl has a deactivation effect on Mn/TiO 2 catalyst. And the poisoning effect of Cl is more serious than that of F. The characterization results showed that the decrease of BET surface area, the increase of crystallinity, the reduced reducibility and surface acidity, and the decreased concentrations of surface Mn 4 + and chemisorbed oxygen should be responsible for the deactivation of Mn/TiO 2 catalyst by the doping of F or Cl. [ABSTRACT FROM AUTHOR]