scholarly journals Strikingly Distinctive NH3-SCR Behavior over Cu-SSZ-13 in the Presence of NO2

2021 ◽  
Author(s):  
Yulong Shan ◽  
Guangzhi He ◽  
Jinpeng Du ◽  
Yu Sun ◽  
Zhongqi Liu ◽  
...  
Keyword(s):  
Active Sites ◽  
Density Functional ◽  
Catalytic Reduction ◽  
Acid Sites ◽  
Zeolite Catalysts ◽  
Nh3 Scr ◽  
Highly Active ◽  
Small Pore ◽  
Standard Scr ◽  
Catalyst Systems

Abstract Commercial Cu-exchanged small-pore SSZ-13 (Cu-SSZ-13) zeolite catalysts are highly active for the selective catalytic reduction (SCR) of NOx with NH3, but distinct from other catalyst systems, their activity is unexpectedly inhibited in the presence of NO2. Here, we combined kinetic experiments, in-situ/operando X-ray absorption spectroscopy, and density functional theory (DFT) calculations to obtain direct evidence that under reaction conditions, strong oxidation by NO2 forces Cu ions to exist mainly as fixed framework Cu2+ species (fw-Cu2+), which impede the formation of dynamic binuclear Cu+ species that serve as the main active sites for the standard SCR (SSCR) reaction. As a result, the SSCR reaction is significantly inhibited by NO2 in the zeolite system, and the NO2-involved SCR reaction occurs with an energy barrier higher than that of the SSCR reaction on dynamic binuclear sites. Moreover, the NO2-involved SCR reaction tends to occur at the Brønsted acid sites (BAS) rather than the fw-Cu2+ sites. This work clearly explains the strikingly distinctive selective catalytic behavior in the zeolite system.

scholarly journals Identification of the Mechanism of NO Reduction with Ammonia (SCR) on Zeolite Catalysts

2020 ◽  
Author(s):  
Konstantin Khivantsev ◽  
Ja-Hun Kwak ◽  
Nicholas R. Jaegers ◽  
Miroslaw A. Derewinski ◽  
János Szanyi
Keyword(s):  
Active Sites ◽  
No Reduction ◽  
Catalytic Reduction ◽  
Molecular Nitrogen ◽  
Acid Sites ◽  
Zeolite Catalysts ◽  
Catalytic Cycle ◽  
Rate Limiting Step ◽  
Catalytically Active

<p>Cu/Zeolites catalyze selective catalytic reduction of nitric oxide with ammonia. Although the progress has been made in understanding the rate-limiting step of reaction which is reoxidation of Cu(I)(NH<sub>3</sub>)<sub>2</sub> with oxygen to restore the catalytically active Cu(II) site, the exact NO reduction chemistry remained unknown. Herein, we show that nitrosyl ions NO<sup>+</sup> in the zeolitic micropores are the true active sites for NO reduction. They react with ammonia even at below/room temperature producing molecular nitrogen through the intermediacy of N<sub>2</sub>H<sup>+</sup> cation. Isotopic experiments confirm our findings. No copper is needed for this reaction to occur. However, when NO<sup>+</sup> reacts, “freed up” Bronsted acid site gets occupied by NH<sub>3</sub> to form NH<sub>4</sub><sup>+</sup> – and so the catalytic cycle stops because NO<sup>+</sup> does not form on NH<sub>4</sub>-Zeolites due to their acid sites being already occupied. Therefore, the role of Cu(II) in Cu/Zeolite catalysts is to produce NO<sup>+</sup> by the reaction: Cu(II) + NO à Cu(I) + NO<sup>+ </sup>which we further confirm spectroscopically. The NO<sup>+</sup> then reacts with ammonia to produce nitrogen and water. Furthermore, when Cu(I) gets re-oxidized the catalytic cycle then can continue. Thus, our findings are critical for understanding complete SCR mechanism.</p>

scholarly journals Identification of the Mechanism of NO Reduction with Ammonia (SCR) on Zeolite Catalysts

2020 ◽  
Author(s):  
Konstantin Khivantsev ◽  
Ja-Hun Kwak ◽  
Nicholas R. Jaegers ◽  
Miroslaw A. Derewinski ◽  
János Szanyi
Keyword(s):  
Active Sites ◽  
No Reduction ◽  
Catalytic Reduction ◽  
Molecular Nitrogen ◽  
Acid Sites ◽  
Zeolite Catalysts ◽  
Catalytic Cycle ◽  
Rate Limiting Step ◽  
Catalytically Active

<p>Cu/Zeolites catalyze selective catalytic reduction of nitric oxide with ammonia. Although the progress has been made in understanding the rate-limiting step of reaction which is reoxidation of Cu(I)(NH<sub>3</sub>)<sub>2</sub> with oxygen to restore the catalytically active Cu(II) site, the exact NO reduction chemistry remained unknown. Herein, we show that nitrosyl ions NO<sup>+</sup> in the zeolitic micropores are the true active sites for NO reduction. They react with ammonia even at below/room temperature producing molecular nitrogen through the intermediacy of N<sub>2</sub>H<sup>+</sup> cation. Isotopic experiments confirm our findings. No copper is needed for this reaction to occur. However, when NO<sup>+</sup> reacts, “freed up” Bronsted acid site gets occupied by NH<sub>3</sub> to form NH<sub>4</sub><sup>+</sup> – and so the catalytic cycle stops because NO<sup>+</sup> does not form on NH<sub>4</sub>-Zeolites due to their acid sites being already occupied. Therefore, the role of Cu(II) in Cu/Zeolite catalysts is to produce NO<sup>+</sup> by the reaction: Cu(II) + NO à Cu(I) + NO<sup>+ </sup>which we further confirm spectroscopically. The NO<sup>+</sup> then reacts with ammonia to produce nitrogen and water. Furthermore, when Cu(I) gets re-oxidized the catalytic cycle then can continue. Thus, our findings are critical for understanding complete SCR mechanism.</p>

scholarly journals Copper-Iron Bimetal Ion-Exchanged SAPO-34 for NH3-SCR of NOx

Catalysts ◽  
2020 ◽  
Vol 10 (3) ◽  
pp. 321 ◽  
Author(s):  
Tuan Doan ◽  
Phong Dam ◽  
Khang Nguyen ◽  
Thanh Huyen Vuong ◽  
Minh Thang Le ◽  
...  
Keyword(s):  
Active Sites ◽  
Catalytic Reduction ◽  
Catalytic Performance ◽  
Physical Structure ◽  
Acid Sites ◽  
Tetraethylammonium Hydroxide ◽  
Nh3 Scr ◽  
Nox Conversion ◽  
Temperature Window ◽  
Scr Of Nox

SAPO-34 was prepared with a mixture of three templates containing triethylamine, tetraethylammonium hydroxide, and morpholine, which leads to unique properties for support and production cost reduction. Meanwhile, Cu/SAPO-34, Fe/SAPO-34, and Cu-Fe/SAPO-34 were prepared through the ion-exchanged method in aqueous solution and used for selective catalytic reduction (SCR) of NOx with NH3. The physical structure and original crystal of SAPO-34 are maintained in the catalysts. Cu-Fe/SAPO-34 catalysts exhibit high NOx conversion in a broad temperature window, even in the presence of H2O. The physicochemical properties of synthesized samples were further characterized by various methods, including XRD, FE-SEM, EDS, N2 adsorption-desorption isotherms, UV-Vis-DRS spectroscopy, NH3-TPD, H2-TPR, and EPR. The best catalyst, 3Cu-1Fe/SAPO-34 exhibited high NOx conversion (> 90%) in a wide temperature window of 250–600 °C, even in the presence of H2O. In comparison with mono-metallic samples, the 3Cu-1Fe/SAPO-34 catalyst had more isolated Cu2+ ions and additional oligomeric Fe3+ active sites, which mainly contributed to the higher capacity of NH3 and NOx adsorption by the enhancement of the number of acid sites as well as its greater reducibility. Therefore, this synergistic effect between iron and copper in the 3Cu-1Fe/SAPO-34 catalyst prompted higher catalytic performance in more extensive temperature as well as hydrothermal stability after iron incorporation.

scholarly journals Identification of Main Active Sites and the Role of NO2 on NOx Reduction with CH4 over In/BEA Catalyst: A Computational Study

Catalysts ◽  
2020 ◽  
Vol 10 (5) ◽  
pp. 572
Author(s):  
Erhao Gao ◽  
Hua Pan ◽  
Li Wang ◽  
Yao Shi ◽  
Jun Chen
Keyword(s):  
Active Sites ◽  
Density Functional ◽  
Catalytic Reduction ◽  
Computational Study ◽  
Nox Reduction ◽  
Initial Step ◽  
Acid Sites ◽  
Strong Interactions ◽  
Reduction Of Nox ◽  
No2 Adsorption

The main active sites and the catalytic process in selective catalytic reduction of NOx by CH4 (CH4-SCR) on In/BEA catalyst were investigated by density functional theory (DFT) using a periodic model. The [InO]+ and [InOH]2+ moieties were constructed in the channel of periodic BEA zeolite representing the Lewis and Brønsted acid sites. The electronic structures [InO]+ and [InOH]2+ were analyzed, and it was found that the [InO]+ group were the main active sites for CH4 activation and NO/NO2 adsorption in the CH4-SCR process. CH4 molecules could be activated on the O site of the [InO]+ group in In/BEA, which was resulted from the strong interactions between the C-p orbital of the CH4 molecule and the O-p orbital of the [InO]+ group. CH4 activation was the initial step in CH4-SCR on In/BEA catalyst. NO2 molecules were essential in the SCR process, and they could be produced by NO reacting with gaseous O2 or the O atom of the [InO]+ group. The presence of NO2 could facilitate the key intermediate nitromethane (CH3NO2) formation and lower the reaction barrier in the SCR process.

scholarly journals Promoting Effect of Mn on In Situ Synthesized Cu-SSZ-13 for NH3-SCR

Catalysts ◽  
2020 ◽  
Vol 10 (12) ◽  
pp. 1375
Author(s):  
Jinpeng Du ◽  
Jingyi Wang ◽  
Xiaoyan Shi ◽  
Yulong Shan ◽  
Yan Zhang ◽  
...  
Keyword(s):  
Low Temperature ◽  
Active Sites ◽  
Catalytic Reduction ◽  
Acid Sites ◽  
Promoting Effect ◽  
Nh3 Scr ◽  
Temperature Programmed ◽  
Diffuse Reflectance Infrared ◽  
Redox Ability

The effect of Mn impregnation on the NH3-SCR (selective catalytic reduction of NOx by NH3) activity of in situ synthesized Cu-SSZ-13 was investigated in this work. It was found that Mn addition could efficiently improve the low-temperature activity of Cu-SSZ-13. The optimal amount of Mn was 5 wt.%, and NOx conversion was improved by more than 20% over a temperature range of 120 °C to 150 °C. SEM (scanning electron microscopy), XRD (X-ray diffraction), N2 adsorption-desorption, H2-TPR (temperature programmed reduction of H2), NH3-TPD (temperature programmed desorption of NH3) and in situ DRIFTS (diffuse reflectance infrared Fourier transform spectroscopy) experiments were conducted to investigate the changes in the zeolite structure, active sites, acid sites and reaction mechanism. The impregnated MnOx species caused a decline in the crystallinity of Cu-SSZ-13 but markedly improved the redox ability. Nitrate and nitrite species were observed in the Mn-modified Cu-SSZ-13, and the formation of these species was thought to cause the observed increase in low-temperature NH3-SCR activity. The results show that the addition of Mn is a promising method for promoting the low-temperature catalytic activity of Cu-SSZ-13.

scholarly journals The Deactivation Mechanism of the Mo-Ce/Zr-PILC Catalyst Induced by Pb for the Selective Catalytic Reduction of NO with NH3

Nanomaterials ◽  
2021 ◽  
Vol 11 (10) ◽  
pp. 2641
Author(s):  
Chenxi Li ◽  
Jin Cheng ◽  
Qing Ye ◽  
Fanwei Meng ◽  
Xinpeng Wang ◽  
...  
Keyword(s):  
Selective Catalytic Reduction ◽  
Catalytic Reduction ◽  
Surface Acidity ◽  
Catalyst Activity ◽  
Acid Sites ◽  
Nh3 Scr ◽  
Highly Active ◽  
Negative Effect ◽  
Redox Ability ◽  
Deactivation Mechanism

As a heavy metal, Pb is one component in coal-fired flue gas and is widely considered to have a strong negative effect on catalyst activity in the selective catalytic reduction of NOx by NH3 (NH3-SCR). In this paper, we investigated the deactivation mechanism of the Mo-Ce/Zr-PILC catalyst induced by Pb in detail. We found that NO conversion over the 3Mo4Ce/Zr-PILC catalyst decreased greatly after the addition of Pb. The more severe deactivation induced by Pb was attributed to low surface area, lower amounts of chemisorbed oxygen species and surface Ce3+, and lower redox ability and surface acidity (especially a low number of Brønsted acid sites). Furthermore, the addition of Pb inhibited the formation of highly active intermediate nitrate species generated on the surface of the catalyst, hence decreasing the NH3-SCR activity.

scholarly journals Influence of Impurity Dissolution on Surface Properties and NH3-SCR Catalytic Activity of Rare Earth Concentrate

Minerals ◽  
2019 ◽  
Vol 9 (4) ◽  
pp. 246
Author(s):  
Zhang ◽  
Zhu ◽  
Zhang ◽  
Li ◽  
Luo ◽  
...  
Keyword(s):  
Rare Earth ◽  
Active Sites ◽  
Catalytic Effect ◽  
Catalytic Reduction ◽  
Weak Acid ◽  
Active Components ◽  
Catalyst Sample ◽  
Nh3 Scr ◽  
O2 Concentration ◽  
Alkali Leaching

Impurity removal and modification of rare earth concentrate powder were conducted by roasting weak acid-weak alkali leaching to obtain the active components of denitrification catalysts. NH3 selective catalytic reduction catalyst samples were prepared by mixing and kneading with pseudo-γ-Al2O3 boehmite as carrier. The results showed that the Ce7O12 content in the active component samples increased and dispersed more evenly. The grain size of the samples was refined, the specific surface area increased, and the active sites exposed more. Ce coexists in the form of Ce3+ and Ce4+. Fe coexists in the form of Fe3+ and Fe2+, but Fe3+ is abundant. Some Ce, La, Nd, Pr, Fe, Mn, and other components formed solid melts during preparation, which increased the synergistic catalytic effect. The denitrification efficiency of the catalyst sample was 92.8% under the conditions of reaction temperature 400 °C, NO content was 600 ppm, NH3/NO ratio was 1.5, and O2 concentration was 4%.

scholarly journals Preparation of Mesoporous Mn–Ce–Ti–O Aerogels by a One-Pot Sol–Gel Method for Selective Catalytic Reduction of NO with NH3

Materials ◽  
2020 ◽  
Vol 13 (2) ◽  
pp. 475
Author(s):  
Yabin Wei ◽  
Shuangling Jin ◽  
Rui Zhang ◽  
Weifeng Li ◽  
Jiangcan Wang ◽  
...  
Keyword(s):  
Selective Catalytic Reduction ◽  
Lewis Acid ◽  
Catalytic Reduction ◽  
Sol Gel ◽  
Acid Sites ◽  
Lewis Acid Sites ◽  
One Pot ◽  
Gel Method ◽  
Nh3 Scr ◽  
Composite Aerogels

Novel Mn–Ce–Ti–O composite aerogels with large mesopore size were prepared via a one-pot sol–gel method by using propylene oxide as a network gel inducer and ethyl acetoacetate as a complexing agent. The effect of calcination temperature (400, 500, 600, and 700 °C) on the NH3–selective catalytic reduction (SCR) performance of the obtained Mn–Ce–Ti–O composite aerogels was investigated. The results show that the Mn–Ce–Ti–O catalyst calcined at 600 °C exhibits the highest NH3–SCR activity and lowest apparent activation energy due to its most abundant Lewis acid sites and best reducibility. The NO conversion of the MCTO-600 catalyst maintains 100% at 200 °C in the presence of 100 ppm SO2, showing the superior resistance to SO2 poisoning as compared with the MnOx–CeO2–TiO2 catalysts reported the literature. This should be mainly attributed to its large mesopore sizes with an average pore size of 32 nm and abundant Lewis acid sites. The former fact facilitates the decomposition of NH4HSO4, and the latter fact reduces vapor pressure of NH3. The NH3–SCR process on the MCTO-600 catalyst follows both the Eley–Rideal (E–R) mechanism and the Langmuir–Hinshelwood (L–H) mechanism.

scholarly journals The Keggin Structure: An Important Factor in Governing NH3–SCR Activity Over the V2O5–MoO3/TiO2 Catalyst

2018 ◽  
Vol 148 (4) ◽  
pp. 1228-1235 ◽  
Author(s):  
Rui Wu ◽  
Ningqiang Zhang ◽  
Xiaojun Liu ◽  
Lingcong Li ◽  
Liyun Song ◽  
...  
Keyword(s):  
Low Temperature ◽  
Catalytic Reduction ◽  
Physicochemical Characteristics ◽  
Acid Sites ◽  
Keggin Structure ◽  
Lewis Acid Sites ◽  
Nh3 Scr ◽  
Strong Acidity ◽  
Ft Ir ◽  
Brønsted And Lewis Acid

Abstract Heteropolyacids and their salts have been effectively used in selective catalytic reduction because of the Keggin structure and extraordinarily strong acidity. Catalysts with and without the Keggin structure were synthesized to further investigate the effects of heteropolyoxometallate on low temperature NH3–SCR. XRD, BET, Raman, H2–TPR, NH3–TPD, FT-IR, and SO2–TPD techniques were used to characterize the physicochemical characteristics of the catalysts. Results indicate that catalysts with the Keggin structure had more surface Brönsted and Lewis acid sites, and these catalysts had significantly improved performances in the SCR reaction and in SO2 poisoning resistance. Graphical Abstract

scholarly journals New Insight into the In Situ SO2 Poisoning Mechanism over Cu-SSZ-13 for the Selective Catalytic Reduction of NOx with NH3

Catalysts ◽  
2020 ◽  
Vol 10 (12) ◽  
pp. 1391
Author(s):  
Yu Qiu ◽  
Chi Fan ◽  
Changcheng Sun ◽  
Hongchang Zhu ◽  
Wentian Yi ◽  
...  
Keyword(s):  
Selective Catalytic Reduction ◽  
Active Sites ◽  
Catalytic Reduction ◽  
Zeolite Catalysts ◽  
So2 Poisoning ◽  
Negative Effect ◽  
Temperature Programmed ◽  
Diffuse Reflectance Infrared ◽  
Sulfate Species

To reveal the nature of SO2 poisoning over Cu-SSZ-13 catalyst under actual exhaust conditions, the catalyst was pretreated at 200 and 500 °C in a flow containing NH3, NO, O2, SO2, and H2O. Brunner−Emmet−Teller (BET), X-ray diffraction(XRD), thermo gravimetric analyzer (TGA), ultraviolet Raman spectroscopy (UV Raman), temperature-programmed reduction with H2 (H2-TPR), temperature-programmed desorption of NO+O2 (NO+O2-TPD), NH3-TPD, in situ diffuse reflectance infrared Fourier transform spectroscopy (in situ DRIFTS), and an activity test were utilized to monitor the changes of Cu-SSZ-13 before and after in situ SO2 poisoning. According to the characterization results, the types and generated amount of sulfated species were directly related to poisoning temperature. Three sulfate species, including (NH4)2SO4, CuSO4, and Al2(SO4)3, were found to form on CZ-S-200, while only the latter two sulfate species were observed over CZ-S-500. Furthermore, SO2 poisoning had a negative effect on low-temperature selective catalytic reduction (SCR) activity, which was mainly due to the sulfation of active sites, including Z2Cu, ZCuOH, and Si-O(H)-Al. In contrast, SO2 poisoning had a positive effect on high-temperature SCR activity, owing to the inhibition of the NH3 oxidation reaction. The above findings may be a useful guideline to design excellent SO2-resistant Cu-based zeolite catalysts.

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