NH3-selective Catalytic Reduction Reaction Research Articles

Deactivation induced by metal sulfate over MnCeOx catalyst in NH3-SCR reaction at low temperature

Sulfate adsorption is one of the factors that cause the poisoning of catalyst in the low-temperature NH3 selective catalytic reduction reaction (NH3-SCR). In this paper, by controlling the co-adsorption time of SO2 and O2 at 150 °C, a range of sulfated MnCeOx catalysts with different contents of metal sulfate species were prepared to reveal the influence of metal sulfate species content on the catalytic performances and reaction mechanisms at low temperature. The catalytic activity below 250 °C rapidly decreases with increasing metal sulfate species content. The results of characterizations shed light on the reduction of specific surface area, Mn4+ and Ce3+ content, and redox ability of MnCeOx owing to the formation of metal sulfate species. Further experiments reveal that metal sulfate species preferentially absorb on MnOx domains rather than on CeO2 domains, and the adsorbed metal sulfate species can suppress the Eley-Rideal and the Langmuir–Hinshelwood reaction mechanisms over the sulfated MnCeOx. All the above results are detrimental to the activity of sulfated MnCeOx in the low-temperature NH3-SCR reaction.

Kinetic modeling of NH3-selective catalytic reduction (SCR) over a mildly hydrothermally aged commercial Cu-SSZ-13 catalyst

Cu-SSZ-13 is often studied as a selective catalytic reduction (SCR) of NOx catalyst for diesel after-treatment systems. Mild hydrothermal aging (HTA) is commonly known to cause changes in catalyst performance. In this study, a kinetic model of SCR over a Cu-SSZ-13 catalyst that includes mild HTA and its impact on the SCR reaction was developed. The migration of Cu species during HTA, and how this change in Cu distribution affects low-temperature and high-temperature NH3 oxidation and SCR reaction mechanisms, were considered. Flow reactor test results were used for parameter estimation and verification. Different characterization techniques, including H2 temperature-programmed reduction (H2-TPR), NO + NH3-TPR, NH3 temperature-programmed desorption (NH3-TPD), and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), were used to characterize and quantify catalyst site densities. The model can simulate the migration of Cu species and the NOx reduction performance changes during mild hydrothermal aging as a function of aging temperature and time.

Characteristics of deactivation and thermal regeneration of Nb-doped V2O5–WO3/TiO2 catalyst for NH3–SCR reaction

This study investigated the effect of Nb doping into V2O5–WO3/TiO2 (VWT) catalyst for removing NOxvia the SCR (selective catalytic reduction) by NH3. The experimental results exhibited that Nb can improve the reactivity of the VWT catalyst at low temperatures. The addition of Nb also enhanced the tolerance to SO2 and H2O. The de-NOx efficiency of the V2O5–WO3–Nb2O5/TiO2 (VWNbT) catalyst was increased up to 12% over that of the VWT catalyst at 240 °C when the catalyst was poisoned for 24 h. The prepared catalysts were characterized by FT−IR, XRD, XPS, and N2 physisorption, elemental analysis. The results showed that the ammonium bisulfate (ABS) was less formed in the VWNbT than in the VWT. Moreover, evolved gas analysis was performed to examine the thermal decomposition behavior of the poisoned catalyst, and confirmed that the ABS deposited on the catalyst was sufficiently decomposed between about 300 and 400 °C. In particular, to most effectively recover the characteristics and activity of the catalysts, thermal treatment at a temperature of 400 °C is suitable.

Insights into the poisoning mechanisms of zinc compounds on V2O5-WO3/TiO2 catalyst for NH3-SCR: Effect of the anion species

V2O5-WO3/TiO2 is commercial catalyst for controlling NOx emissions from municipal solid waste incineration by NH3 Selective Catalytic Reduction (NH3-SCR). However, the flue gas contains a large amount of zinc compounds, which would lead to the deactivation of the catalyst. Therefore, in this work we studied the effect of different zinc species (ZnSO4, ZnO and ZnCl2) on a V2O5-WO3/TiO2 catalyst, investigating the deactivation mechanism. The different compounds negatively affected the activity in the order ZnCl2 >ZnO>ZnSO4, indicating an important influence of the anions. The characterization results points to the evidence that Zn2+ would interact with the active V-OH and OV structure, reducing the content of active Brønsted acid sites and influencing the V5+⇄V4+ redox cycle, as well as inhibiting the activation of NH3. Cl- would interact with V-OH, possibly forming V-Cl and also suppress the formation and mobility of surface adsorbed oxygen, which further exacerbates the negative effect. On the other hand, SO42- would weaken the poisoning effect of Zn2+ by a strong anion/cation interaction and likely form V-O-SO3OH by reaction with V-OH, which is proposed to be an active Brønsted acid site able to active NH3 in the NH3-SCR reaction. Finally, the introduction of Zn2+ increased the adsorption of NOx species to form a stable and inactive nitro species, the formation of which is inhibited by the presence of SO42-.

First principle study on the interactions of NH3, NOx and O2 with Fe3O4 (1 1 1) surfaces

The interactions of NH3, NOx and O2 with two Fe3O4 (111) surfaces named Fetet- and Feoct-tet-terminated, respectively, were investigated by first principle calculation. The results indicated that the Feoct-tet-terminated surface had a stronger effect on the adsorption and activation of these small molecules overall. Specifically, NH3, NO and O2 tend to be stably adsorbed on the octahedral Fe site on the Feoct-tet-terminated surface. The energy released by O2 adsorption is the most among them, and it is easy to form abundant surface adsorbed oxygen on the catalyst surface, which further co-adsorb or combine with NO to form nitrate and nitrite species with various coordination structures. The chelated and bridged bidentate-coordinated nitrate species with the highest adsorption energies are important reaction intermediates in NH3 selective catalytic reduction (NH3-SCR). In addition, due to the horizontal adsorption configuration, NH3 is significantly affected by the nearest lattice oxygen, which makes the oxidative dehydrogenation reaction more likely to occur, although the adsorption energy of NH3 on Feoct-tet-terminated surface is relatively lower. These results provide a basis for understanding the interactions of NH3, NOx and O2 on Fe3O4 (111) surfaces in NH3-SCR reaction.

Like Cures like: Detoxification Effect between Alkali Metals and Sulfur over the V2O5/TiO2 deNOx Catalyst.

The V2O5/TiO2 (VTi) catalyst has been widely employed for the NH3 selective catalytic reduction (NH3-SCR) reaction, and sulfur (S) and alkali metals (K) were usually considered as poisons during this reaction. In this work, the synergistic effect of S and K over the VTi catalyst for the NH3-SCR reaction was analyzed and discussed. It is surprisingly observed that the synergistic effects of S and K exhibited a detoxification effect on the NH3-SCR reaction. That is, although the VTi catalyst exhibited moderate resistance to S poisoning and unsatisfactory resistance to K deactivation, the SCR activity was restored to close to fresh VTi when K and S coexisted. This detoxification effect also could occur between other alkali metals (e.g., Ca and Na) and sulfur. X-ray photoelectron spectroscopy and charge density difference studies both indicate that the introduction of K could significantly affect the electronic structure of V, but this toxic effect was recovered by the further addition of S because of the strong interaction between S and K. Therefore, this detoxification effect can occur in the practical reaction atmosphere, which alleviates the alkali metal poisoning of commercial catalysts.

Numerical Investigation on the Intraphase and Interphase Mass Transfer Limitations for NH3-SCR over Cu-ZSM-5

A systematic modeling approach was scrutinized to develop a kinetic model and a novel monolith channel geometry was designed for NH3 selective catalytic reduction (NH3-SCR) over Cu-ZSM-5. The redox characteristic of Cu-based catalysts and the variations of NH3, NOx concentration, and NOx conversion along the axis in porous media channels were studied. The relative pressure drop in different channels, the variations of NH3 and NOx conversion efficiency were analyzed. The model mainly considers NH3 adsorption and desorption, NH3 oxidation, NO oxidation, and NOx reduction. The results showed that the model could accurately predict the NH3-SCR reaction. In addition, it was found that the Cu-based zeolite catalyst had poor low-temperature catalytic performance and good high-temperature activity. Moreover, the catalytic reaction of NH3-SCR was mainly concentrated in the upper part of the reactor. In addition, the hexagonal channel could effectively improve the diffusion rate of gas reactants to the catalyst wall, reduce the pressure drop and improve the catalytic conversion efficiencies of NH3 and NOx.

Enhancing low-temperature NH3-SCR performance of Fe–Mn/CeO2 catalyst by Al2O3 modification

In the work, supported catalysts of FeOx and MnOx co-supported on aluminum-modified CeO2 was synthesized for low-temperature NH3-selective catalytic reduction (NH3-SCR) of NO. Impressively, the SCR activity of the obtained catalyst is markedly influenced by the adding amount of Al and the appropriate Ce/Al molar ratio is 1/2. The activity tests demonstrate that Fe–Mn/Ce1Al2 catalyst shows over 90% NO conversion at 75–250 °C and exhibits better SO2 resistance compared to Fe–Mn/CeO2. Fe–Mn/Ce1Al2 shows the expected physicochemical characters of the ideal catalyst including the larger surface and increased active reaction active sites by controlling the amount of Al doping. Also, the better catalytic activity is well correlated with the present advantaged surface adsorption oxygen species, Mn4+ species, Ce3+ species and the enhanced reducibility of Fe–Mn/Ce1Al2, which is superior to the Fe–Mn/CeO2 catalyst. More importantly, we further demonstrate that the amount and strength of surface acid sites are improved by Al-doping and more active intermediates (monodentate nitrate) is generated during NH3-SCR reaction. This work provides certain insight into the rational creation of simple and practical denitration catalyst environmental purification.

Toward rational design of a novel hierarchical porous Cu-SSZ-13 catalyst with boosted low-temperature NOx reduction performance

Nitrogen oxide (NOx), one of the main air pollutants exhausted from diesel vehicles, is harmful to the environment and human health and has been selectively catalytically reduced to N2 by Cu-exchanged zeolites with chabazite structures (CHA, Cu-SSZ-13). Unfortunately, the conventional Cu-SSZ-13 catalyst still has disadvantages in real applications, such as diffusion limits and micropore blocking due to formation of ammonium sulfates, especially at low reaction temperatures. Here, a highly crystalline hierarchical porous Cu-SSZ-13 zeolite (termed Cu-SSZ-13-HP) with enriched mesopores was rationally designed and prepared via a dual-template method for the first time. The novel Cu-SSZ-13-HP catalyst displayed boosted low-temperature activity (temperature of 70% conversion below 150 °C) and broadened active temperature window (between 175 and 500 °C, NOx conversion above 90%) compared with conventional Cu-SSZ-13 with only micropores. Importantly, Cu-SSZ-13-HP also exhibited superior hydrothermal stability and enhanced sulfur dioxide (SO2) tolerance, even when exposed to 100 ppm of SO2 for 4 h. The in situ diffuse reflectance infrared Fourier transform spectroscopy revealed that the NH3 selective catalytic reduction reaction might conform to the Langmuir–Hinshelwood mechanism in which ammonia (NH3) and NOx were activated by the acid and redox sites, respectively. The Cu-SSZ-13-HP catalyst developed by the dual-template strategy might be a good candidate for low-temperature reduction of NOx exhaust from diesel vehicles.

Tailored Alkali Resistance of DeNOx Catalysts by Improving Redox Properties and Activating Adsorbed Reactive Species

SummaryIt is still challenging to develop strongly alkali-resistant catalysts for selective catalytic reduction of NOx with NH3. It is generally believed that the maintenance of acidity is the most important factor because of neutral effects of alkali. This work discovers that the redox properties rather than acidity play decisive roles in improving alkali resistance of some specific catalyst systems. K-poisoned Fe-decorated SO42−-modified CeZr oxide (Fe/SO42−/CeZr) catalysts show decreased acidity but reserve the high redox properties. The higher reactivity of NHx species induced by K poisoning compensates for the decreased amount of adsorbed NHx, leading to a desired reaction efficiency between adsorbed NHx and nitrate species. This study provides a unique perspective in designing an alkali-resistant deNOx catalyst via improving redox properties and activating the reactivities of NHx species rather than routinely increasing acidic sites for NHx adsorption, which is of significance for academic interests and practical applications.

Doping effect of cations (Zr4+, Al3+, and Si4+) on MnOx/CeO2 nano-rod catalyst for NH3-SCR reaction at low temperature

Thermally stable Zr4+, Al3+, and Si4+ cations were incorporated into the lattice of CeO2 nano-rods (i.e., CeO2-NR) in order to improve the specific surface area. The undoped and Zr4+, Al3+, and Si4+ doped nano-rods were used as supports to prepare MnOx/CeO2-NR, MnOx/CZ-NR, MnOx/CA-NR, and MnOx/CS-NR catalysts, respectively. The prepared supports and catalysts were comprehensively characterized by transmission electron microscopy (TEM), high-resolution TEM, X-ray diffraction, Raman and N2-physisorption analyses, hydrogen temperature-programmed reduction, ammonia temperature-programmed desorption, in situ diffuse reflectance infrared Fourier-transform spectroscopic analysis of the NH3 adsorption, and X-ray photoelectron spectroscopy. Moreover, the catalytic performance and H2O+SO2 tolerance of these samples were evaluated through NH3-selective catalytic reduction (NH3-SCR) in the absence or presence of H2O and SO2. The obtained results show that the MnOx/CS-NR catalyst exhibits the highest NOx conversion and the lowest N2O concentration, which result from the largest number of oxygen vacancies and acid sites, the highest Mn4+ content, and the lowest redox ability. The MnOx/CS-NR catalyst also presents excellent resistance to H2O and SO2. All of these phenomena suggest that Si4+ is the optimal dopant for the MnOx/CeO2-NR catalyst.

Promotional effect of SO2 on Cr2O3 catalysts for the marine NH3-SCR reaction

Cr2O3 catalysts for the marine NH3-selective catalytic reduction (NH3-SCR) reaction have been synthesized by precipitation, and their catalytic performances were evaluated. The experimental results revealed that the as-synthesized Cr2O3 catalysts presented an enhanced NO removal efficiency when combined with SO2. Notably, the reaction efficiency increased from 61% to 94% with 100 ppm SO2 and from 61% to 82% with 500 ppm SO2 at 280 °C. The lifetime test confirmed the promotional effect of SO2 and the good sulfur and H2O resistance of Cr2O3, and the deactivation by SO2 proved to be irreversible. The NO removal efficiency remained at 89% after 28 h of reaction with 100 ppm SO2 and 3% H2O. The catalyst was characterized and the results confirmed eskolaite (Cr2O3) was successfully synthesized, the SCR reaction followed a Langmuir-Hinshelwood mechanism, and the increased SCR efficiency could mainly be attributed to the acid sites and Oa on the catalyst surface.

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.

Catalytic performance of highly dispersed WO3 loaded on CeO2 in the selective catalytic reduction of NO by NH3

AbstractThe influence of tungsten trioxide (WO3) loading on the selective catalytic reduction (SCR) of nitric oxide (NO) by ammonia (NH3) over WO3/cerium dioxide (CeO2) was investigated. The NO conversion first rose and then declined with increasing WO3 loading. It was found that the crystalline WO3 in the 1.6WO3/CeO2 sample could be removed in 25 wt% ammonium hydroxide at 70 °C, which improved the catalytic activity of the sample. The obtained samples were characterized by X-ray diffraction, Raman spectroscopy, X-ray photoelectron spectroscopy, hydrogen (H2) temperature programmed reduction, NH3 temperature programmed desorption, and in situ diffuse reflectance infrared Fourier transform spectroscopy. The results revealed that the dispersed WO3 promoted the catalytic activity of WO3/CeO2 while the crystalline WO3 inhibited catalytic activity. The oxygen activation of CeO2 was inhibited by the coverage of WO3, which weakened NO oxidation and adsorption of nitrate species over WO3/CeO2. In addition, the NH3 adsorption performance on CeO2 was improved by modification with WO3. NH3 was the most stable adsorbed species under NH3 SCR reaction conditions. In situ DRIFT spectra suggested that the NH3 SCR reaction proceeded via the Eley-Rideal mechanism over WO3/CeO2. Thus, when the loading of WO3 was close to the dispersion capacity, the effects of NH3 adsorption and activation were maximized to promote the reaction via the Eley-Rideal route.

Support effect of the supported ceria-based catalysts during NH3-SCR reaction

To investigate how the physicochemical properties and NH3-selective catalytic reduction (NH3-SCR) performance of supported ceria-based catalysts are influenced as a function of support type, a series of CeO2/SiO2, CeO2/γ-Al2O3, CeO2/ZrO2, and CeO2/TiO2 catalysts were prepared. The physicochemical properties were probed by means of X-ray diffraction, Raman spectroscopy, Brunauer-Emmett-Teller surface area measurements, X-ray photoelectron spectroscopy, H2-temperature programmed reduction, and NH3-temperature programmed desorption. Furthermore, the supported ceria-based catalysts' catalytic performance and H2O + SO2 tolerance were evaluated by the NH3-SCR model reaction. The results indicate that out of the supported ceria-based catalysts studied, the CeO2/γ-Al2O3 catalyst exhibits the highest catalytic activity as a result of having a high relative Ce3+/Ce4+ ratio, optimum reduction behavior, and the largest total acid site concentration. Finally, the CeO2/γ-Al2O3 catalyst also presents excellent H2O + SO2 tolerance during the NH3-SCR process.

Novel hollow microspheres MnxCo3−xO4 (x = 1, 2) with remarkable performance for low-temperature selective catalytic reduction of NO with NH3

Hollow microspheres MnCo2O4 and CoMn2O4 have been synthesized by a facile solvothermal route followed by pyrolysis of the carbonate counterparts and carbon microspheres, using carbon microspheres as the template. The NH3-selective catalytic reduction reaction was used to test the catalytic activity. The obtained hollow microspheres are composed of numerous primary particles with sizes of tens of nanometers, giving a porous shell. The obtained CoMn2O4 microsphere shows better low-temperature catalytic activity and N2 selectivity than MnCo2O4 microsphere in the NH3-selective catalytic reduction reaction. The X-ray photoelectron spectroscopy results demonstrate that CoMn2O4 microsphere has a relatively higher number content of Mn3+ and chemisorbed oxygen species. The temperature-programmed desorption by ammonia and in situ diffuse reflectance infrared Fourier transform spectroscopy results indicate that the CoMn2O4 microsphere possesses stronger Lewis acid strength than the MnCo2O4 microsphere. Additionally, the CoMn2O4 microsphere also presented outstanding stability, H2O resistance and SO2 tolerance. The NH3-SCR reaction mechanism is proposed that NH3(g) is adsorbed on the surface of Lewis acid sites and Bronsted acid sites in the shape of NH4 + ions and gaseous NH3. Besides, the adsorption of NO could exist in the form of gaseous or oxide ions NO2 − on the surface of catalysts.The adsorbed NH3 species could react with NO2 − species easily to produce NH4NO2, which subsequently to produce the innocuous N2 and H2O.

The role of pore diffusion in determining NH3 SCR active sites over Cu/SAPO-34 catalysts

Cu/SAPO-34 catalysts have been extensively investigated recently for NOx selective catalytic reduction (SCR) reactions. Due to the small pore size (4.3Å) of the zeolites, it is a natural concern that the reaction rates might be dictated by the pore diffusion regime rather than the intrinsic kinetics. In this work, a series of Cu/SAPO-34 catalysts, with identical Cu ion active site loadings but varying zeolite particle sizes (1–9μm), were evaluated. A variety of characterization techniques (XRD, BET, H2 TPR, and NH3 TPD) have confirmed that the same copper species and surface chemistry existed in these samples despite the different SAPO-34 particle sizes. While the Thiele modules governing the pore diffusion effects were vastly different, up to nine times, these samples shared a similar turnover frequency (TOF) and apparent activation energy (44.8±3.0kJ/mol) for the NH3 SCR reaction. These results unequivocally conclude that under the representative SCR reaction conditions investigated in this work, the reaction rates on Cu/SAPO-34 powder catalysts are kinetically controlled and pore diffusion plays little role in disguising the intact intrinsic activity per single-site copper.

Role of Brønsted acidity in NH3 selective catalytic reduction reaction on Cu/SAPO-34 catalysts

In the present work, a series of Cu/SAPO-34 catalysts with varying number of Brønsted acid sites has been prepared by ion exchanging with potassium to investigate the role of Brønsted acidity in the NH3-SCR reaction. The impact of potassium on the structural and acidic properties of Cu/SAPO-34 catalysts has been studied. No significant changes in both the SAPO-34 structure and the copper species were observed in the presence of potassium. With increasing potassium loading, the NH3-SCR activity of the Cu-K/SAPO-34 catalysts decreased in accordance with the decreasing Brønsted acidity. The quantification method of Brønsted acid sites in the Cu-(K)/SAPO-34 was developed by using IR spectroscopy, which was confirmed by acid site generation mechanism and NH3-TPD results. A linear relationship was found between the amounts of Brønsted OH groups determined by DRIFTS and the NH3 desorbed at high temperatures from NH3-TPD.

Enhanced Activity of Ti-Modified V2O5/CeO2 Catalyst for the Selective Catalytic Reduction of NOx with NH3

A novel V2O5/CeTiOx catalyst showed excellent catalytic performance in the selective catalytic reduction (SCR) of NOx with NH3. The addition of Ti into V2O5/CeO2 enhanced catalytic activity, N2 selectivity, and resistance against SO2 and H2O. These catalysts were also characterized by N2 adsorption, XRD, XPS, and H2-TPR. The lower crystallinity, more reduced species, better dispersion of surface vanadium species, and more acid sites due to the modification of V2O5/CeO2 with TiO2 all improved the NH3–SCR activity significantly. Based on in situ DRIFTS, it was concluded that the NH3–SCR reaction over V2O5/CeTiOx and V2O5/CeO2 mainly followed the Eley–Rideal mechanism.

Low temperature NH3–SCR over Zr and Ce pillared clay based catalysts

A series of Zr and Ce pillared clay loaded with manganese oxide were prepared by wet-impregnation for the low temperature selective catalytic reduction (SCR) of NO. Various preparation methods for the MnOx/Zr–Ce–PILCs catalysts were studied with respect to the effects on NOx conversions. The MnOx(12%)/Zr–Ce–PILC(30) catalyst prepared by the simultaneous Zr–Ce pillaring with a Zr:Ce molar ratio of 5:5 was demonstrated to be optimal in NH3–SCR reaction at low temperature, reaching 96% of NOx conversion at 200°C. Various techniques were used to characterize these catalysts. N2 physisorption results suggested that the simultaneous Zr–Ce pillaring enlarged the specific surface area and pore volume. The XRD and XPS results illustrated that MnO2, Mn2O3 and Mn3O4 all existed on the surfaces of the catalysts. The results of H2–TPR NH3–TPD for the catalysts demonstrated that the MnOx(12%)/Zr–Ce–PILC(30) catalyst had high redox characteristic and surface acidity. The shift of Mn and O elements to lower binding energies indicated more active for these elements in MnOx(12%)/Zr–Ce–PILC(30). This demonstrated that Zr–Ce–PILC was a kind of potential support, which contributed to the high SCR activity of MnOx(12%)/Zr–Ce–PILC(30).