Low-temperature Selective Catalytic Reduction Performance Research Articles

Synthesis of Co/Ni-Doped mesoporous spinel ferrite microspheres for enhanced Low-Temperature SCR performance

The iron-based catalysts commonly used in the selective catalytic reduction (SCR) process cannot meet the increasingly stringent low-temperature NOx emission targets. In this study, mesoporous microspherical MFe2O4 (M=Co, Ni) spinel ferrite catalysts were synthesized by hydrothermal method and applied to the SCR of ammonia (NH3-SCR) reaction. Compared to Fe3O4, both NiFe2O4 and CoFe2O4 catalysts showed higher SCR catalytic activity at low temperature. The NiFe2O4 catalyst achieved a NO conversion of 95.5 % at 240 °C, while CoFe2O4 exhibited a high NO conversion of 97.6 % at 180 °C and a N2 selectivity of 99.6 %. Various characterization experiments revealed that the introduction of Co and Ni resulted in the formation of a specific mesoporous microsphere morphology in CoFe2O4 and NiFe2O4 catalysts, along with the generation of a large number of oxygen vacancies and acidic sites on the surface. These features facilitate the sequential adsorption and activation of NH3 and NOx species, resulting in excellent catalytic activity at low temperatures. Detailed in situ diffuse reflectance Fourier transform infrared spectroscopy (DRIFTS) measurements showed that on NiFe2O4 the reaction follows the Eley–Rideal (E–R) mechanism, whereas it proceeds through the Langmuir–Hinshelwood (L–H) and Eley–Rideal (E–R) mechanisms on the CoFe2O4 catalyst. This work provides a feasible strategy for enhancing the low-temperature SCR reaction over iron-based catalysts; moreover, the elucidation of the complete reaction mechanism offers a solid foundation for future improvements in denitrification catalysts.

Revealing different lead species of PbCl2, Pb(NO3)2, PbSO4 and PbCO3 poisoning effects on Mn-Ce/CuX catalyst for low-temperature NH3-SCR of NO

For the sintering flue gas in steel industry, selective catalytic reduction (SCR) of NOx using NH3 at low temperature in the presence of heavy metals is still a challenge. In this study, the toxicity effects of lead salt species (PbCl2, Pb(NO3)2, PbSO4, PbCO3) on Cu-exchanged zeolite X doped with Mn-Ce catalysts (MCCX) were investigated. All catalysts exhibited poor low-temperature SCR performance within 75–225 °C after Pb poisoning, and the poisoning effects of PbCl2, Pb(NO3)2, PbSO4 and PbCO3 on MCCX catalyst varied. The PbSO4 had the most serious poisoning on MCCX catalyst, while PbCO3 had poisoning effect on catalysts even at 300 °C. Lead species poisoning had bad impact on the structure of zeolite X on MCCX catalyst, and caused some agglutination of Cu species. Likewise, the Pb species might primarily occupy the active site of Mn and Cu species and influence the redox property of catalysts, thereby obstructing the redox and oxidation cycles on catalysts surface. In addition, the PbCl2 and PbNO3 species predominantly affected the Brønsted acid sites and occupied the Cu or Mn active sites, inhabiting the absorption and conversion of nitrites and nitrates on the catalyst surface. Moreover, PbSO4 and PbCO3 species mainly impacted the adsorption of NO + O2 and the conversion of nitrites and nitrates on the catalysts.

Boosting the low-temperature activity of fast-synthesized Cu-SAPO-18 for NH3-SCR by hydrothermal treatment

Cu-SAPO-18 catalysts attracts increasing attention in the reaction of selective catalytic reduction (SCR) of nitrogen oxides (NOx), but the catalyst synthesis process is complicated and time-consuming. Also, it is urgent to enhance the low-temperature catalytic conversion of NOx for practical applications. Herein, we reported a fast-synthesis method to prepare the Cu-SAPO-18 catalysts in 32–80 h, and the catalysts showed an excellent catalytic performance of over 80 % NO conversion at 260–480 °C. The crystalline of the catalysts was improved in short crystallization time and high crystallization temperature, accelerating the NH3-SCR reaction. The low-temperature activity of all catalysts was improved after hydrothermal treatment, and the temperature range of over 80 % NO conversion was extended to 190–515 °C for the optimized catalyst, which was due to the increasing active Cu2+ species (ZCuOH and Z2Cu, where Z represents an anionic site on the zeolite framework). The redox cycle of Cu2+/Cu+ was accelerated and more Lewis acid sites on the catalyst enhanced the NO/NH3 adsorption and activation, promoting the low-temperature catalytic activity. Furthermore, after hydrothermal treatment, part of Z2Cu species converted to ZCuOH species, which exhibited stronger NO adsorption energies (−2.06 eV) than that of Z2Cu (−1.38 eV) based on theoretical calculation. Both fresh and aged catalysts showed remarkable SO2/H2O resistance. This work reveals the effects of hydrothermal treatment on the Z2Cu and ZCuOH species, which is helpful to prepare the fast-synthesized zeolite catalysts with excellent low-temperature SCR performance.

Speeding up low-temperature SCR with reactants-coupling dual catalytic sites

Selective catalytic reduction (SCR) of NOx with NH3 over V2O5-based catalysts has been widely used to control NOx emissions from power plants, but its application is largely limited in other industrial boilers because of insufficient catalytic efficiency at low temperatures. Here we resolve this problem by mechanistically designing a highly active Ce-W/TiO2 catalyst with reactants-coupling dual catalytic sites. The results demonstrate that W and Ce are highly dispersed on the surfaces of Ce-W/TiO2 to form dual catalytic sites, which act as acid-base pairs favorable for NH3 and NO adsorption, respectively, thereby allowing the reaction of adsorbed NH3 on the W site with oxidized NO species (NO3−) adsorbed on the adjacent Ce site to proceed along a low-activation-energy pathway via Langmuir-Hinshelwood mechanism. Compared with Ce/TiO2, a W-Ce synergy gives rise to more abundant Ce3+ on Ce-W/TiO2, which facilitates the activation of O2 during SCR process, thus improving the low-temperature SCR performance. Therefore, the design of the dual catalytic sites based on the properties of reactants provides an alternative approach to developing highly active SCR catalysts.

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

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.

Effect of preferential exposure of anatase TiO2 {0 0 1} facets on the performance of Mn-Ce/TiO2 catalysts for low-temperature selective catalytic reduction of NOx with NH3

Titanium dioxide with preferentially exposed {001} facets (TiO2-NS) was synthesized by hydrothermal method using HF as a crystal plane regulator. The obtained TiO2-NS was used as the carrier of the low temperature SCR catalysts Mn-Ce/TiO2-NS which was prepared by impregnation method. Meanwhile, Mn-Ce/TiO2-NP catalyst was prepared as a comparison by the same method using titanium dioxide with priority to {101} facets (TiO2-NP) as the carrier. The effect of preferential exposure of anatase TiO2 {001} facets on the low-temperature selective catalytic reduction performance of Mn-Ce/TiO2 was studied. The support TiO2 (TiO2-NS and TiO2-NP) and the SCR catalysts were characterized by means of Scanning electron microscopy (SEM), Transmission electron microscope (TEM), X-ray diffraction (XRD), Brunauer-Emmett-Teller (BET) surface measurement, X-ray photoelectron spectroscopy (XPS), Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES) and NH3-temperature programmed desorption (NH3-TPD). The results showed that the preferentially exposed {001} facets of TiO2 could increase the specific surface area and the concentration of surface adsorbed oxygen of the catalyst, and enhance the surface acidity. The denitration test indicated that Mn-Ce/TiO2-NS catalyst showed excellent NO conversion. In particular, the catalytic efficiency of Mn-Ce/TiO2-NS was about 30% higher than that of Mn-Ce/TiO2-NP in the low temperature window within 100–160 °C.

The promotion effect of Fe on CeZr2O x catalyst for the low-temperature SCR of NO x by NH3

Fe was used as the modifier to improve the performance of CeZr2O x 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 CeZr2O x , 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 Fe2CeZr2O x 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 Fe2CeZr2O x compared with CeZr2O x . By means of in situ DRIFT technique, it could be deduced that the SCR reaction process over Fe2CeZr2O x was accelerated mainly through theL–H pathway due to the advanced adsorption of NH3 and NO x species, along with the promoted NO conversion to NO2. Therefore, the modification of CeZrO x catalyst by Fe had a significant stimulation effect on its low-temperature SCR performance.

Mechanistic Investigation of the Promotion Effect of Bi Modification on the NH3–SCR Performance of Ce/TiO2 Catalyst

In this study, Bi was used as the modifier to elevate the performance of Ce/TiO2 catalyst for selective catalytic reduction (SCR) of NOx with NH3. Experimental results indicated that the CeBi/TiO2 catalyst with a Bi/Ce molar ratio of 0.15 exhibited excellent low-temperature SCR performance and SO2/H2O resistance compared with the Ce/TiO2 catalyst. Characterization results revealed that the introduction of a proper amount of Bi to Ce/TiO2 catalyst could generate more Ce3+ and chemisorbed oxygen species on its surface, along with the enhanced reducibility and surface acidity. From the results of an in situ DRIFT study, the formation of more adsorbed NH3 and NO2 species could be detected, as a result, greatly facilitating the low-temperature NH3–SCR reaction over CeBi/TiO2-0.15 catalyst through the Langmuir–Hinshelwood route.

Why the Low-Temperature Selective Catalytic Reduction Performance of Cr/TiO2 Is Much Less than That of Mn/TiO2: A Mechanism Study

Although Mn/TiO2 and Cr/TiO2 had many similar physicochemical properties, the low-temperature selective catalytic reduction (SCR) performance of Mn/TiO2 was much better than that of Cr/TiO2. In this work, the physicochemical properties of Mn/TiO2 and Cr/TiO2 were characterized. Meanwhile, the mechanism of NO reduction over Mn/TiO2 and Cr/TiO2 was compared using the transient reaction. Furthermore, the kinetic parameters of NO reduction over Mn/TiO2 and Cr/TiO2 were obtained from the steady-state kinetic study. The Eley–Rideal mechanism contributed to NO reduction over both Mn/TiO2 and Cr/TiO2. As the SCR reaction through the Eley–Rideal mechanism needed to overcome the binding of activated NH3 species with the interface and the binding of activated NH3 with Cr/TiO2 was much stronger than that with Mn/TiO2, the rate constant of the SCR reaction over Cr/TiO2 through the Eley–Rideal mechanism was much lower than that over Mn/TiO2. Meanwhile, the rate of the catalytic oxidation of NH3 to NO (i.e., C–O reactio...

A study on chemisorbed oxygen and reaction process of Fe–CuOx/ZSM-5 via ultrasonic impregnation method for low-temperature NH3-SCR

Fe–CuOx/ZSM-5 catalyst was prepared for low-temperature selective catalytic reduction (SCR) of NOx with ammonia by ultrasonic impregnation method. The catalyst were characterized by HRTEM, XRD, H2-TPR, XPS, in situ DRIFTS and tested for their activities in a fixed-bed reactor. The NOx conversion over Fe(0.67)–Cu/ZSM-5 sample reached 98% within the temperature range from 180 to 360°C. Ultrasonic impregnation can promote active component to highly disperse into the channel of ZSM-5, greatly improving low-temperature SCR performance compared to conventional impregnation according to characterization of XRD, HRTEM. XPS results showed that chemisorbed oxygen had catalytic oxidation activity against NO and NH3, the proportion of chemisorbed oxygen was affected by molar ration of iron and copper. When the molar ratio of iron and copper was 2:1, the maximum chemisorbed oxygen by XPS and the minimum reduction temperature of 263°C by H2-TPR were obtained. In situ DRIFTS results revealed that catalyst had oxidation activity against NO and NH3 under oxygen-free condition due to action of chemisorbed oxygen, reaction rate of SCR is much faster than the reaction between adsorbed ammonia and oxygen, nitrate as a kind of intermediate was produced by reaction of NH3 and NO in the SCR process.

Low temperature SCR of NO with catalysts prepared by modified ACF loading Mn and Ce: effects of modification method

Achievement of a higher NOx conversion ratio in selective catalytic reduction (SCR) at low temperature is challenging. In this work, pure activated carbon fibres (ACFs) were modified with different ratios of H2O (g), NaOH, CO2 and HNO3, respectively (named as modified ACF). The chemical and physical properties of modified ACFs were identified by Brunauer–Emmett–Teller, X-ray diffraction, Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy methods. The NOx conversion ratio of ACF was improved from 56.1% to 82.4% at 80°C after modification with 30% (mass ratio) NaOH. These modified ACFs were further loaded with the mixture of MnO2 and CeO2 in the form of metal salt solutions (named as Mn0.5Ce0.5O2/modified ACF). The NOx conversion ratio of 30% SHACF remained similar at 80°C but was increased from 60.0% to 98.5% at 360°C after loading with Mn and Ce, which showed the best performance in SCR of NOx at low temperature. It could be seen that ACF delivered higher performance in low temperature SCR after being modified with the aforementioned reactants and further loading with metals. Based on chemical and physical characterization and the performance of the catalysts, the reasons for different performances of these catalysts in low temperature SCR are discussed.

In situ DRIFTS investigation on the SCR of NO with NH3 over V2O5 catalyst supported by activated semi-coke

Abstract 3 wt.% of V2O5 is loaded onto the activated semi-coke (V2O5/ASC) via impregnation method and used for low temperature selective catalytic reduction (SCR) of NOx with NH3. The prepared V2O5/ASC catalyst yields an over 90% NO conversion rate with excellent N2 selectivity at 250 °C with a space velocity of 12,000 h−1. The adsorption state of different species and reaction behaviors under various conditions are systematically examined with in situ diffuse reflectance infrared Fourier transform spectroscopy (in situ DRIFTS). It is evidenced that NH3 and NO are absorbed on different active sites over the V2O5/ASC catalyst. At the reaction temperature (200 °C) in this study, NH3 is mainly absorbed on the Lewis acid sites provided by V2O5, and NO is mainly absorbed on the active sites originated from the support ASC. In addition, the NH3-SCR process takes place according to two pathways, including reaction between the coordinated NH3 and gaseous NO (E–R mechanism), and reaction between the absorbed NO2 and coordinated NH3 (L–H mechanism). The latter one plays a primary role for the improved low-temperature SCR performance of V2O5/ASC catalyst.