High NO Conversion Research Articles

Formation mechanism of yolk–shell LaMnO3 microspheres prepared by P123-template and oxidation of NO

The yolk-shell LaMnO3 perovskite microspheres were fabricated by a novel, simple and mild soft template approach. A series of template-P123 concentrations (0-6.12 mmol∙L-1) were employed to optimize the most complete spheres. When the concentration of P123 is 3.0 mmol∙L-1, the obtained yolk-shell microspheres with a diameter of 200–700 nm were constructed by nanoparticles. The possible formation mechanism of the yolk-shell microspheres was revealed step by step via XRD, SEM, TEM, EDS and HRTEM. Molecules of P123 were suitably mixed with solvents for double shelled vesicles through self-assembly, which interacted with metal complexes to form P123-metal vesicles. After the removal of P123 and citric acid by calcination at 700 °C, the yolk-shell LaMnO3 microspheres with through-channels were obtained. Through-channels on the surface were due to citric acid and the solid core was attributed to the shrink of inner vesicles. Prepared yolk-shell microsphere samples possessed a larger surface area and a higher maximum NO conversion value of 78% at 314 °C for NO oxidation, compared with samples without the yolk-shell structure.

Denitration of the gas-fired boiler flue gas based on chemical-looping combustion

The denitration performances of transition metal oxide oxygen carriers (OCs) including CuO/Al2O3, MnO2/Al2O3 and NiO/Al2O3 for the gas-fired boiler flue gas were investigated. The results showed that NO was able to be reduced to N2 through a direct reaction with the reduced OCs, and at the same time, the reduced OCs were converted to the oxidized OCs. Then, the oxidized OCs were able to be regenerated to the reduced OCs by introducing reducing gases. As such, the continuous denitration was achieved based on a chemical-looping combustion (CLC) denitration mechanism. Among the reduced CuO/Al2O3, MnO2/Al2O3 and NiO/Al2O3, the reduced CuO/Al2O3 showed the best denitration activity. The NO denitration based on the reduced CuO/Al2O3 and CO also included the catalytic mechanism, whereas the CLC mechanism was dominant. The denitration performance of CO as a reducing gas was significantly better than that of C2H5OH, CH3OH, H2 or NH3. 8% CO2 in the flue gas inhibited the denitration below 250 °C, whereas 15% H2O in the flue gas improved the denitration above 125 °C. O2 in the flue gas deteriorated the denitration through reducing the proportion of the reduced species in the OC, whereas a high NO conversion could be obtained through increasing CO concentration to make λ less than 1.0. For the denitration of a simulating flue gas based on the reduced CuO/Al2O3 and CO, the temperature needed for a NO conversion of 90% was as low as 125 °C at 10,000 h−1.

Simultaneous removal of NO and Hg0 over Nb-Modified MnTiOx catalyst

Removal of NO and Hg0 simultaneously was achieved on Nb-modified MnTiOx catalyst. Experimental results revealed that high NO conversion (>95%) and Hg0 oxidation (>95%) in 200–300 °C were available on Mn0.15Nb0.02TiOx catalyst with a GHSV of 108,000 h−1. The excellent catalytic performance of Mn0.15Nb0.02TiOx catalyst should be originated from its high reducibility and massive surface chemisorbed oxygen species. Moreover, the introduction of Nb into MnTiOx catalyst greatly enhanced its resistance to SO2 and H2O. Neither gaseous Hg0 nor HgO formed on catalyst surface could affect the NH3-SCR reaction. NO had a promotion effect on Hg0 oxidation, while NH3 was detrimental to Hg0 oxidation owing to the inhibited adsorption of Hg0 by NH3. Under SCR reaction conditions, an inhibition impact on Hg0 oxidation could be detected, but this negative effect could be avoided by using more catalyst sample.

Rational construction of direct Z-scheme doped perovskite/palygorskite nanocatalyst for photo-SCR removal of NO: Insight into the effect of Ce incorporation

V-Ti oxide catalyst has been widely employed in selective catalytic reduction (SCR) of NOx in industry. However, considerable cost and high reaction temperatures are commonly required. Alternatively, photo-SCR working at low temperature has attracted widespread interest. In the present work, Pr1−xCexFeO3/palygorskite (Pal) nanocomposite was synthesized by an in situ sol-gel method. Results indicate that Pr1−xCexFeO3 remain as a perovskite solid solution uniformly immobilized on the surface of Pal when x is less than 0.05. CeO2 co-precipitates with Pr1−xCexFeO3 when x is beyond 0.05, leading to an enhanced charge separation induced by the formation of intimate direct Z-scheme heterostructure of Pr1−xCexFeO3/CeO2, which is unraveled by the photoluminescence spectroscopy and Mott-Schottky analysis and further verified by the DFT calculations. Under visible light irradiation, the influence of the doping amount of Ce (from 0.05 to 0.5) on the NO conversion and N2 selectivity was investigated. The Pr0.7Ce0.3FeO3/Pal was found to achieve the highest NO conversion rate of 92% and 99% N2 selectivity. In addition, the SO2 and H2O resistance experiment show that the doping of Ce could benefit the formation of cerium sulphate which prevents NH3 from being sulfated by SO2 while Pal prevents the catalysts from being corroded by H2O, respectively.

Preparation of Mn-Ce/TiO2 Catalysts and its Selective Catalytic Reduction of NO at Low-temperature

The catalytic performance of NO removal was studied over Mn-Ce/TiO2 catalysts prepared by rheological phase method under different preparation parameters, such as preparation method, manganese precursor, Mn/(Mn + Ce) molar ratio and calcination conditions. It was found that manganese nitrate and manganese acetate were more conducive to high denitration efficiency than manganese chloride. The highest NO conversion of 92% is achieved at the Ce/(Mn + Ce) molar ratio of 0.15, which is much higher than that of the pure manganese constituent. The increase of calcining temperature favored the crystallization of active components and leads to the decline of catalytic activity.

Low temperature selective catalytic reduction of nitric oxide with urea over activated carbon supported metal oxide catalysts

ABSTRACTSelective catalytic reduction of nitrogen oxides (SCR) with loaded urea is a method for removing NO under oxygen-rich and low-temperature conditions, which can solve the inhibitory effect of oxygen on the catalyst and the slip of ammonia. In present study, a series of activated carbon (wo-AC, co-AC, cs-AC and nu-AC) supported metal (Mn, Fe, Co, Cu and Zn) oxide catalysts with loading urea were prepared by ultrasonic assisted impregnation. The catalysts were used for NO removal at 50–120°C and characterized by XRD, SEM, GFAAS, EDS, XPS, BET and FTIR techniques. The effects of activated carbon type, loaded active element, metal oxides loading, temperature fluctuation on catalytic activity and the catalytic stability were also studied in this paper. The results indicated that nutshell-based activated carbon was more suitable as a carrier than other activated carbons, and urea-10Mn/nu-AC catalyst yielded a higher NO conversion than other catalysts. Besides, for used activated carbons, the larger specific surface area, more micropores distribution and the larger number of hydroxyl group and cyano terminal group are beneficial to the catalytic process. Moreover, the downward trend of NO conversion with increasing temperature suggested the adsorption of reactant gases played a crucial role in the catalytic process of urea-SCR.

Effect of the Presence of Ceria in the NSR Catalyst on the Hydrothermal Resistance and Global DeNOx Performance of Coupled LNT–SCR Systems

The global performance of coupled LNT–SCR systems, addressed to high NOx-to-N2 conversion, minimal ammonia slip and null N2O production, as well as the hydrothermal resistance of single NSR and SCR monolith catalysts and their coupling is discussed. Pt–Ba/Al2O3 and Pt–Ce–Ba/Al2O3 were washcoated on cordierite monoliths as NSR catalysts, and Cu/CHA was washcoated on similar monoliths as SCR catalysts. Both monoliths were coupled in two subsequent reactors to conform the LNT–SCR system. Previously to washcoating, the fresh powder catalysts and after severe hydrothermal aging were fully characterized by N2 adsorption–desorption isotherms at 77 K, X-ray diffraction, NH3 temperature-programmed desorption, and H2 chemisorption to relate textural and chemical characteristics with the DeNOx performance. The Cu/CHA catalyst shows an excellent hydrothermal resistance for the NH3–SCR reaction. Incorporation of ceria to the model Pt–BaO/Al2O3is beneficial for the NO-to-NOx oxidation and NO2 storage, improving NO conversion at low temperature and reducing the NH3 slip. However, addition of ceria is detrimental for the hydrothermal resistance of the NSR catalyst. However, this detrimental effect is minimized when the NSR catalyst is coupled with the Cu/CHA monolith downstream of the NSR catalyst, achieving the coupled LNT–SCR device high NO conversion and minimal NH3 slip with superior N2 selectivity for an extended temperature windows, including as low as 220 °C, and maintaining performance even after severe hydrothermal aging.

Facile synthesis of Mn-based nanobelts with high catalytic activity for selective catalytic reduction of nitrogen oxides

The development of low-temperature SCR catalysts is of vital importance due to their potential application in downstream of the desulfurizer and electrostatic precipitator, where the flue gas temperature is usually bellow 473 K. In this study, various Mn-based nanobelts were synthesized by a facile redox-precipitation route and were further used as low-temperature SCR catalysts. In particular, the as-prepared ɑ-MnO2 nanobelts with exposed (110) surface were shown to be single crystallites with widths of 10–20 nm and lengths of 50–200 nm. The ɑ-MnO2 with porous and hierarchical nanostructure interweaved by ɑ-MnO2 nanobelts presented large specific surface area and exhibited high NO conversions and wide operating temperature window, reaching up to above 90% NO conversion within the testing range of 393–453 K. An Eley-Rideal (E-R) reaction mechanism is proposed with Brønsted acid sites involved in the SCR reaction for ɑ-MnO2. The possible growth mechanism of ɑ-MnO2 nanobelts was also tentatively discussed.

Di-metal-doped sulfur resisting perovskite catalysts for highly efficient H2-SCR of NO.

Lanthanide perovskite catalysts doped with limited palladium (to improve activity) and cerium (to improve sulfur resistance) were prepared using sol-gel method. In different B sites, lanthanide perovskites were studied at harsh conditions for H2 selective catalytic reduction of NO. The activity sequence was as follows: LaCeMnPd > LaCeCoPd > LaCeFePd. LaCeMnPd had a high NO conversion of 96.6% at only 150°C. And it also had a surprising SO2 resistance in different SO2 concentrations. After cutting out SO2, NO conversion recovered rapidly to its original level, indicating that the slight deactivation was reversible. In addition, the effect of gas hourly space velocity, H2/NO ratio, O2, and SO2 concentration was studied. And XRD, energy-dispersive X-ray, SEM, XPS, H2-temperature programmed reduction, and NH3-temperature programmed desorption were performed to characterize the catalysts. Graphical abstract ᅟ.

Low-temperature flue gas denitration with transition metal oxides supported on biomass char

Several transition metals (Mn, Ce, V and Fe) were loaded on nitric acid modified biomass char (BC) using an impregnation method for the selective catalytic reduction (SCR) of NO with NH3 at low-temperature. The series of prepared catalysts were characterized by BET, SEM, FT-IR and XRD. Results showed the sequence of NO conversion within the temperature of 125–225 °C was Mn/BC > Ce/BC > V/BC > Fe/BC > BC, and Mn/BC exhibited the highest NO conversion of 87.6% at 200 °C. BC supports provided high surface area, which could contribute to the better dispersion of transition oxides on biomass char, revealing rich oxygen containing groups. Besides, the BC worked not only as a promising support, but also provided high activity adsorption sites for NH3 and conducted as oxidizing agent for NO due to the existence of graphite crystallite structure. Owing to the existence of “fast SCR” reaction process, the transition metal oxides supported on BC catalysts exhibited superior denitration efficiency in low temperature.

Selective catalytic reduction of NOx by NH3 at low temperature over manganese oxide catalysts supported on titanate nanotubes

We report the NO conversion with NH3 at low temperature over MnOx-supported on titanate nanotubes. The effect of SO2 and water on catalytic activity was also analyzed. For the catalytic activity tests, three catalysts with 1, 3, and 5 wt.% of MnOx were synthesized. A high NO conversion (92%) at 180 °C is reported for the 1Mn/NTiO2 catalyst. The further addition of MnOx to the support improves the catalytic activity but NO conversion (95%) was shifted to a higher temperature (from 180 to 220 °C). However, the presence of SO2 (50 ppm) and water (5 vol.%) diminishes the NO conversion up to 60%, which remains after 300 min. We found that addition of MnOx increases the Lewis acid sites concentration of the nanotubes. As the Lewis acid sites increase (from 1.86 to 3.56 µmol m−2), the Mn4+/Mn3+ ratio on the surface of the nanotubes decreases (from 4 to 1.6), which indicates that the surface of the catalysts is deficient in electrons. We concluded that a high Lewis acid sites concentration and a low Mn4+/Mn3+ ratio, hence a surface deficient in electrons, improves the catalytic activity to remove NO on the Mn/NTiO2 catalysts.

Single-phase SO2-resistant to poisoning Co/Mn-MOF-74 catalysts for NH3-SCR

Single-phase Co/Mn-MOF-74 solid materials were synthesized successfully via the hydrothermal method. The effect of Co/Mn atom ratio on the structure of Co/Mn-MOF-74 solids and their catalytic activity for low-temperature NH3-SCR of NOx were investigated. Very high NO conversion values (>96.0%) at 180–240 °C were obtained using the Mn0.66Co0.34-MOF-74 chemical composition (optimal content of Co). Furthermore, Mn0.66Co0.34-MOF-74 showed outstanding resistance to SO2 poisoning and the incorporation of Co was shown to cause the weakening of the SO2 adsorption strength on the catalyst surface, thus to enhance catalyst's SO2 resistance to poisoning. These features were probed by in situ DRIFTS and catalytic performance studies. The present work suggests that Mn0.66Co0.34-MOF-74 could be likely considered as a prospective industrial NH3-SCR NOx control catalyst.

Different exposed facets VOx/CeO2 catalysts for the selective catalytic reduction of NO with NH3

VOx/CeO2 catalysts, employing CeO2 nanocubes (NCs), nanorods (NRs), and nanopolyhedrons (NPs) with predominately exposed {100}, {110}, and {111} facets as the supports, were prepared by an incipient wetness impregnation method. The catalysts were used in the selective catalytic reduction (SCR) of NOx with NH3. The SCR performance showed that V-CeO2-NPs could achieve significantly higher NO conversion than V-CeO2-NCs and V-CeO2-NRs in the entire temperature range. The characterization results confirmed that the redox and acidic properties of VOx/CeO2 catalysts were closely related to the exposed facets of the CeO2 supports. The excellent SCR activity of V-CeO2-NPs should be attributed to its appropriate redox ability and abundant surface acid sites, which are associated with the predominately exposed {111} facets of CeO2-NPs.

Experimental study of the combustion and NO emission behaviors during cofiring coal and biomass in O2/N2 and O2/H2O

AbstractOxy‐H2O is considered to be a new generation of carbon capture and storage technology. The combustion and NO emission behaviors during co‐combustion in O2/N2 and O2/H2O were studied simultaneously in an isothermal thermogravimetric system, including the influence of blending ratio, coal type, biomass type, and oxygen concentration. The results show that replacing N2 by H2O accelerates the burning rate of a blend significantly and reduces the NO emission obviously. In O2/N2 and O2/H2O, blending biomass improves the burning rate and reduces the NO emission. The improvement of H2O on burning rate becomes weak with increasing blending ratio. There is an interactive effect on NO emission during co‐combustion in both atmospheres. For cofiring bituminous coal and poplar wood, the NO yield decreases with increasing blending ratio, whereas the NO conversion ratio increases in both atmospheres. As different coal ranks blend with a certain biomass, the blend with a higher coal rank results in a more obvious improvement of burning rate and a higher NO conversion ratio in both atmospheres. As a coal blends with different biofuels, a higher volatile biofuel leads to a higher burning rate in O2/N2, whereas this behavior is not obvious in O2/H2O. Blending coal with a higher nitrogen biofuel results in a lower NO conversion ratio in both atmospheres. In both atmospheres, the burning rate and NO conversion ratio of a blend both increase, as the oxygen concentration increases from 21% to 30%. The results provide direct guidance for the application of co‐combustion in O2/H2O.

The Role of Fe2O3 Species in Depressing the Formation of N2O in the Selective Reduction of NO by NH3 over V2O5/TiO2-Based Catalysts

Promotion of 2.73% Fe2O3 in an in-house-made V2O5-WO3/TiO2 (VWT) and a commercial V2O5-WO3/TiO2 (c-VWT) has been investigated as a cost effective approach to the suppression of N2O formation in the selective catalytic reduction of NO by NH3 (NH3-SCR). The promoted VWT and c-VWT catalysts all gave a significantly decreased N2O production at temperatures >400 °C compared to the unpromoted samples. However, such a promotion led to the loss in high temperature NO conversion, mainly due to the oxidation of NH3 to N-containing gases, particularly NO. Characterization of the unpromoted and promoted catalysts using X-ray diffraction (XRD), NH3 adsorption-desorption, and Raman spectroscopy techniques could explain the reason why the promotion showed much lower N2O formation levels at high temperatures. The addition of Fe2O3 to c-VWT resulted in redispersion of the V2O5 species, although this was not visible for 2.73% Fe2O3/VWT. The iron oxides exist as a highly-dispersed noncrystalline α-Fe2O3 in the promoted catalysts. These Raman spectra had a new Raman signal that could be tentatively assigned to Fe2O3-induced tetrahedrally coordinated polymeric vanadates and/or surface V-O-Fe species with significant electronic interactions between the both metal oxides. Calculations of the monolayer coverage of each metal oxide and the surface total coverage are reasonably consistent with Raman measurements. The proposed vanadia-based surface polymeric entities may play a key role for the substantial reduction of N2O formed at high temperatures by NH3 species adsorbed strongly on the promoted catalysts. This reaction is a main pathway to greatly suppress the extent of N2O formation in NH3-SCR reaction over the promoted catalysts.

Mn-Ce-V-WOx/TiO2 SCR Catalysts: Catalytic Activity, Stability and Interaction among Catalytic Oxides

A series of Mn-Ce-V-WOx/TiO2 composite oxide catalysts with different molar ratios (active components/TiO2 = 0.1, 0.2, 0.3, 0.6) have been prepared by wet impregnation method and tested in selective catalytic reduction (SCR) of NO by NH3 in a wide temperature range. These catalysts were also characterized by X-ray diffraction (XRD), Transmission Electron Microscope (TEM), in situ Fourier Transform infrared spectroscopy (in situ FTIR), H2-Temperature programmed reduction (H2-TPR) and X-ray photoelectron spectroscopy (XPS). The results show the catalyst with a molar ratio of active components/TiO2 = 0.2 exhibits highest NO conversion value between 150 °C to 400 °C and good resistance to H2O and SO2 at 250 °C with a gas hourly space velocity (GHSV) value of 40,000 h−1. Different oxides are well dispersed and interact with each other. NH3 and NO are strongly adsorbed on the catalyst surface and the adsorption of the reactant gas leads to a redox cycle with the valence state change among the surface oxides. The adsorption of SO2 on Mn4+ and Ce4+ results in good H2O and SO2 resistance of the catalyst, but the effect of Mn and Ce are more than superior water and sulfur resistance. The diversity of valence states of the four active components and their high oxidation-reduction performance are the main reasons for the high NO conversion in this system.

A Rational Design for Enhanced Catalytic Activity and Durability: Strongly Coupled N-Doped CrOx/Ce0.2Zr0.8O2 Nanoparticle Composites

As a classic catalyst for NO oxidation, CrOx/Ce0.2Zr0.8O2 has been widely researched to improve its intrinsic catalytic activity and stability under complex flue gas environments. Some strategies, such as nanosize reduction, composite catalysts, and transition metal or rare earth ion doping, have been reported to enhance the catalytic properties. However, the commercialization of CrOx/Ce0.2Zr0.8O2 is greatly hindered by its poor stability under complex flue gas environments. Herein, we reveal a new route to fabricate N-doped CrOx/Ce0.2Zr0.8O2 nanoparticles, which exhibit not only higher NO conversion but also H2O and SO2 tolerance. The morphology and structure were analyzed via X-ray diffraction, transmission electron microscope, et al., investigating the enhancement of N doping. Additionally, the formation of the Ce–O–N–Zr chemical bond and the possible catalytic mechanism were examined by in situ diffuse reflectance infrared Fourier transform spectroscopy, which provided insight into both the fabricatio...

Comparison of NO conversion over Cu/M (M = TiO2, Al2O3, ZSM-5, carbon nanotubes, activated carbon) catalysts assisted by plasma

NO conversion using dielectric barrier discharge (DBD) plasma with different catalysts was investigated. Cu/TiO2, Cu/Al2O3, Cu/ZSM-5, Cu/CNTs and Cu/AC catalysts were prepared by incipient wetness impregnation with 4 wt% Cu loaded. BET, SEM, XRD, XPS, H2-TPR, NH3-TPD were used to measure and evaluate various catalysts. Compared with the plasma only process, the combination of plasma with different catalysts significant improved the NO conversion rate. The highest NO conversion rate of 76% could be achieved with Cu/ZSM-5 catalyst at the energy density of 184 J/L. The results indicate that Cu species over the support ZSM-5 can produce high concentrations of CuO, largest specific surface area, largest amounts of acidity and lowest reduction temperature (209 °C), these factors conducive to plasma-catalyst reaction. Besides, Cu/TiO2 and Cu/Al2O3 catalysts exhibited excellent NO conversion at the low energy density because of the low thermal stability. After plasma-catalyst reaction, the reducibility of CuO and Cu2O over supports decreased and the peak position of acid sites shifted to lower temperature.

Operando Spectroscopic Studies of Cu\u2013SSZ-13 for NH3\u2013SCR deNOx Investigates the Role of NH3 in Observed Cu(II) Reduction at High NO Conversions

The small pore zeolite chabazite (SSZ-13) in the copper exchanged form is a very efficient material for the selective catalytic reduction by ammonia (NH3) of nitrogen oxides (NOx) from the exhaust of lean burn engines, typically diesel powered vehicles. The full mechanism occurring during the NH3–SCR process is currently debated with outstanding questions including the nature and role of the catalytically active sites. Time-resolved operando spectroscopic techniques have been used to provide new level of insights in to the mechanism of NH3–SCR, to show that the origin of stable Cu(I) species under SCR conditions is potentially caused by an interaction between NH3 and the Cu cations located in eight ring sites of the bulk of the zeolite and is independent of the NH3–SCR of NOx occurring at Cu six ring sites within the zeolite.

Effects of Ni modification on NO‐CO reaction with MnOx‐CuO/TiO2 catalysts

AbstractNi modified MnOx‐CuO/TiO2 catalysts were synthesized by the sol‐gel method and investigated for NO‐CO reaction. The results showed that 0.05Ni‐Mn‐Cu/TiO2 (the Ni/Ti molar ratio was 0.05) had the highest NO conversion efficiency with a maximum value of 85.4 % at 350 °C, which was 10 % higher than that of Mn‐Cu/TiO2. XPS analysis showed that more chemisorbed oxygen formed on the Ni‐modified catalyst surface, which could act as active sites for CO adsorption and oxidation. CO‐TPD results confirmed that the amount of adsorbed CO on the catalyst surface increased after Ni modification. H2‐TPR results suggested that the Ni‐modified catalyst had better redox properties, which favoured the reduction of NO to N2O and N2O to N2. Furthermore, in situ FTIR showed that CO2 from the production of the NO‐CO reaction could be easily formed on the surface of 0.05Ni‐Mn‐Cu/TiO2, and it indicated that the NO‐CO reaction could easily occur after Ni doping. All these factors were responsible for the better catalytic activity of Ni‐modified catalysts.