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Journal ArticleDOI: 10.1016/J.JHAZMAT.2020.124223

Adjustment of operation temperature window of Mn-Ce oxide catalyst for the selective catalytic reduction of NOx with NH3.

05 Mar 2021-Journal of Hazardous Materials (Elsevier)-Vol. 405, pp 124223-124223
Abstract: In order to enhance the catalytic activity of Mn-Ce oxide catalyst for the selective catalytic reduction of NOx with NH3 (NH3-SCR), W was introduced as a promoter. With the doping of W, the NOx conversion over Mn3CeOx catalyst above 150 °C was increased, and the N2O production was significantly decreased. Even in the present of water vapour, Mn3CeW0.3Ox still showed a good SCR activity. H2-TPR and XPS results suggested that the doping of tungsten could inhibit the charge imbalance and reducibility, which would inhibit NO oxidation to NO2 over Mn3CeOx. As a result, the NOx conversion below 150 °C over Mn3CeW0.3Ox was slightly lower than that over Mn3CeOx. Since the NOx production and the NH3 conversion during the NH3 oxidation of Mn3CeOx were inhibited after the doping of W, the NOx conversion above 150 °C over Mn3CeW0.3Ox was higher than that over Mn3CeOx. The transient reaction demonstrated that the doped W species on Mn3CeW0.3Ox could inhibit the N2O produced by the Langmuir-Hinshelwood mechanism. Kinetic study proved that νSCR over Mn3CeW0.3Ox was obviously higher that over Mn3CeOx, νNSCR and νC-O over Mn3CeOx were much higher than those of Mn3CeW0.3Ox, which were consistent with the SCR activity.

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Topics: NOx (58%), Selective catalytic reduction (52%), Catalysis (50%)

5 results found

Journal ArticleDOI: 10.1016/J.JRE.2021.05.001
Lanyi Wang1, Xuehua Yu2, Yuechang Wei1, Jian Liu1  +2 moreInstitutions (2)
Abstract: Nowadays, air pollution has become a prominent environmental problem and has attracted much attention. With the increase of vehicle retention quantity, the exhaust emissions have become the main sources of air pollution. To reduce pollution and hazards, vehicle exhaust emission regulations are becoming stricter and stricter, which puts forward higher requirements for purification of vehicle exhausts. At present, rare earths have been widely applied in vehicle exhaust purification because of their good catalytic performance, which is attributed to their unique 4f electron layer structure occupied without full electrons, excellent oxygen storage/release capacity and redox ability. In this paper, the current status of rare earth catalysts and application of rare earth in different fuel vehicle exhaust catalysts, including three-way catalysts (TWCs) for gasoline vehicles, diesel exhaust catalysts for different pollutants (particulate matter (PM), NOx, CO and HC) and catalysts for new energy vehicles with different fuels, are summarized in detail. Meanwhile, the corresponding mechanisms and the role of rare earth in vehicle exhaust catalysts are also simultaneously described. Furthermore, the challenges and development directions of rare earth catalysts for the purification of vehicle exhausts are also proposed.

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Topics: Diesel exhaust (53%)

2 Citations

Journal ArticleDOI: 10.1016/J.JES.2021.06.029
Gui-huan Yao1, Yuliang Wei2, Keting Gui2, Xiang Ling1Institutions (2)
Abstract: Iron-based catalysts have been explored for selective catalytic reduction (SCR) of NO due to environmentally benign characters and good SCR activity. Mn-W-Sb modified siderite catalysts were prepared by impregnation method based on siderite ore, and SCR performance of the catalysts was investigated. The catalysts were analyzed by X-ray diffraction, H2-temperature-programmed reduction, Brunauer-Emmett-Teller, Thermogravimetry-derivative thermogravimetry and in-situ diffused reflectance infrared Fourier transform spectroscopy (DRIFTS). The modified siderite catalysts calcined at 450°C mainly consist of Fe2O3, and added Mn, W and Sb species are amorphous. 3Mn-5W-1.5Sb-siderite catalyst has a wide temperature window of 180-360°C and good N2 selectivity at low temperatures. In-situ DRIFTS results show NH4+, coordinated NH3, NH2, NO3− species (bidentate), NO2− species (nitro, nitro-nitrito, monodentate), and adsorbed NO2 can be discovered on the surface of Mn-W-Sb modified siderite catalysts, and doping of Mn will enhance adsorbed NO2 formation by synergistic catalysis with Fe3+. In addition, the addition of Sb can inhibit sulfates formation on the surface of the catalyst in the presence of SO2 and H2O. Time-dependent in-situ DRIFTS studies also indicate that both of Lewis and Bronsted acid sites play a role in SCR of NO by ammonia at low temperatures. The mechanism of NO removal on the 3Mn-5W-1.5Sb-siderite catalyst can be discovered as a combination of Eley-Rideal and Langmuir-Hinshelwood mechanisms with three reaction pathways. The mechanism of NO, oxidized by synergistic catalysis of Fe3+ and Mn4+/3+ to form NO2 among three pathways, reveals the reason of high NOx conversion of the catalyst at medium and low temperatures. Download : Download high-res image (64KB) Download : Download full-size image

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Journal ArticleDOI: 10.1016/J.APSUSC.2021.151047
Xiaofeng Wang1, Zhe Zhao1, Yang Xu1, Qingbo Li1Institutions (1)
Abstract: A series of 3DOM-MnxCeyTiz catalysts with three-dimensionally ordered macroporous (3DOM) structure were synthesized by a soft template method. The high SCR activity of the 3DOM-Mn3Ce1 catalyst at low temperature was owing to the excellent redox ability and rich Lewis and Bronsted acid sites of the catalysts, which were beneficial to improve the catalytic activity. However, the strong oxidizability of MnOx resulted in the nonselective oxidation of NH3 and generation of N2O, decreasing the N2 selectivity. After addition of Ti, the redox capacity of the 3DOM-Mn3Ce1Ti1 catalyst decreased, and the NH3 were strongly adsorbed on the acid sites, which enhanced the N2 selectivity with a broadened temperature window of 240–440 °C and a high GHSV of 120000 h−1. Furthermore, both the 3DOM-Mn3Ce1 and 3DOM-Mn3Ce1Ti1 catalysts showed remarkable tolerance of H2O. Finally, through in situ DRIFT analysis, the Langmuir-Hinshelwood (L-H) mechanism was confirmed as the dominant reaction pathway over the 3DOM-Mn3Ce1Ti1 catalyst.

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Topics: Catalysis (55%), Selectivity (50%)

Open accessJournal ArticleDOI: 10.3390/NANO11040988
12 Apr 2021-Nanomaterials
Abstract: The Mn-Ce oxide catalysts active in the oxidation of CO were studied by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), temperature-programmed reduction (TPR), transition electron microscopy (TEM), energy dispersive X-Ray (EDX), and a differential dissolution technique. The Mn-Ce catalysts were prepared by thermal decomposition of oxalates by varying the Mn:Ce ratio. The nanocrystalline oxides with a fluorite structure and particle sizes of 4–6 nm were formed. The introduction of manganese led to a reduction of the oxide particle size, a decrease in the surface area, and the formation of a MnyCe1−yO2−δ solid solution. An increase in the manganese content resulted in the formation of manganese oxides such as Mn2O3, Mn3O4, and Mn5O8. The catalytic activity as a function of the manganese content had a volcano-like shape. The best catalytic performance was exhibited by the catalyst containing ca. 50 at.% Mn due to the high specific surface area, the formation of the solid solution, and the maximum content of the solid solution.

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Topics: Manganese (61%), Oxide (56%), Specific surface area (55%) ... show more

Journal ArticleDOI: 10.1039/D1CY01424B
Shuqi Zhao1, Jian-Wen Shi1, Cihang Niu1, Baorui Wang1  +6 moreInstitutions (1)
Abstract: Iron vanadate (FeVO4) nanorods are used as a carrier to support manganese (Mn) and cerium (Ce) oxides for the selective catalytic reduction (SCR) of nitrogen oxides (NOx) with NH3 for the first time. Among these developed Ce–Mn/FeVO4 catalysts with different molar ratios of Ce/Mn, the Ce0.2Mn0.2/FeVO4 catalyst exhibits the best de-NOx performance and N2 selectivity, which are higher than 90% in a wide temperature window of 90–420 °C. Through a series of characterization techniques, it is found that the synergistic effect between Ce and Mn enhances the reduction ability and the number of acid sites on the catalyst, which facilitates the adsorption and conversion of flue gas. The introduction of an appropriate ratio of Ce/Mn increases the concentration of Mn4+ and chemisorbed oxygen (OS) on the catalyst, leading to a “fast SCR reaction” with oxidizing NO to NO2, which significantly improves the low-temperature de-NOx efficiency. In addition, the interaction between the active components (Ce/Mn) and the support (FeVO4) increases the de-NOx performance of Ce0.2Mn0.2/FeVO4 at high temperatures. In the meantime, the Mn4+ + Ce3+ ↔ Mn3+ + Ce4+ reduction electron pair formed between Ce and Mn promotes the transport of electrons, which is also beneficial for the SCR reaction at low temperature. The in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) reveals that the SCR reaction over Ce0.2Mn0.2/FeVO4 catalyst follows both Eley–Rideal (E–R) and Langmuir–Hinshelwood (L–H) reaction mechanisms.

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Topics: Selective catalytic reduction (54%), Catalysis (53%), NOx (52%) ... show more

54 results found

Journal ArticleDOI: 10.1016/S0926-3373(98)00040-X
Abstract: The open literature concerning chemical and mechanistic aspects of the selective catalytic reduction of NO by ammonia (SCR process) on metal oxide catalysts is reviewed. Catalytic systems based on supported V2O5 (including the industrial TiO2-supported V2O5–WO3 and/or V2O5–MoO3 catalysts) and catalysts containing Fe2O3, CuO, MnOx and CrOx are considered. The results of spectroscopic studies of the adsorbed surface species, adsorption–desorption measurements, flow reactor and kinetic experiments are analyzed. The proposed reaction mechanisms are described and critically discussed. Points of convergence and of disagreement are underlined.

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1,758 Citations

Journal ArticleDOI: 10.1016/0021-9517(84)90371-3
H.C. Yao1, Y.F. Yu Yao1Institutions (1)
Abstract: Oxygen storage capacities of CeO 2 , CeO 2 Al 2 O 3 , and PM/CeO 2 /Al 2 O 3 , measured by a pulse injection method, are affected by pretreatment temperature, pulsing temperature, partial pressure of CO, presence of precious metals (PM), and the concentration of CeO 2 on alumina They are lowered by higher pretreatment temperature but increase with the pulsing temperature in the range of application At a pulsing temperature ≤500 °C, the capacities are not affected by oxygen pressure but increase with partial pressure of CO The presence of PM lowers the reduction temperature and increases the oxygen storage capacity of CeO 2 TPR was used to measure the oxygen removal at various temperatures At 900 °C, the maximum amount of oxygen removed from unsupported or alumina-supported ceria is about 25% The TPR traces of the unsupported ceria show two peaks at 500 and 750 °C which are associated with the reduction of surface capping oxygen and bulk oxygen anions, respectively For alumina-supported ceria, the TPR traces show a third peak at 850 °C which is associated with the reduction of the shared oxygen anions at the interface The presence of PM lowers only the reduction temperature of the capping oxygen anions but not of the other two oxygen species Both oxygen chemisorption and TPR were used to measure the oxygen anion restoration at various temperatures following the reduction at 500 and 900 °C, respectively Chemisorption data show that the oxygen uptake per CeO 2 is highest at the lowest CeO 2 concentration The TPR traces show that a new oxygen species, probably a molecular oxygen anion, is formed at 25 °C which converts slowly at 500 °C to the capping oxygen anion Complete restoration of all three types of oxygen anions is accomplished at 850 °C in air

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Topics: Limiting oxygen concentration (63%), Oxygen storage (61%), Oxygen (59%) ... show more

1,647 Citations

Journal ArticleDOI: 10.1039/A908800H
Abstract: The results of a systematic XPS study, under high controlled conditions, of different basic oxides of transition metals, alkali and alkaline-earth metals are presented; the XPS data of some hydroxides and peroxides are also reported. Variations of the O 1s binding energies are analysed and one point of interest is the large binding energy scale obtained for O 1s peaks all associated with a ‘‘2− ’’ formal charge. Through extended Huckel theory-tight binding (EHT-TB) calculations, attempts are made to rationalize the observed variations. The results illustrate the significant differences between real charges on oxygen atoms in transition metal and alkaline-earth oxides.

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Topics: X-ray photoelectron spectroscopy (51%), Binding energy (51%), Transition metal (51%) ... show more

1,328 Citations

Journal ArticleDOI: 10.1021/JA808433D
Xiangwen Liu1, Kebin Zhou1, Lei Wang1, Baoyi Wang1  +1 moreInstitutions (1)
Abstract: CeO2 is a catalytic material of exceptional technological importance, and the precise role of oxygen vacancies is crucial to the greater understanding of these oxide materials. In this work, two ceria nanorod samples with different types and distributions of oxygen vacancies were synthesized. A direct relationship between the concentration of the larger size oxygen vacancy clusters and the reducibility/reactivity of nanosized ceria was revealed. These results may be an important step in understanding and designing active sites at the surface of metal oxide catalytic materials.

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Topics: Oxide (52%), Nanorod (51%)

838 Citations

Journal ArticleDOI: 10.1039/DT9760001686
Abstract: A study of the core-electron X-ray photoelectron (X-p.e.) spectra of the f0 compounds La2O3, LaMO3(M = Fe and Co), CeO2, and BaCeO3 is described. Results on the chelate species [La(tmhd)3] and [Ce(tmhd)4](tmhd = 2,2,6,6-tetramethylheptane-3,5-dionato) are included for comparison. Special precautions have been taken to ensure an optimal degree of surface purity of the samples. Satellite structure has been observed for the 4p, in addition to the 3d and 4d, signals in both the lanthanum(III) and cerium(IV) compounds. These satellites arc discussed in terms of coexcitations of the charge-transfer type, principally O 2p→ metal 4f transitions. In the cerium(IV) oxides the satellites are apparently due to energy-gain (representing ‘ shake-down ’) rather than energy-loss (shake-up) processes.

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Topics: Cerium (57%), Lanthanum (55%)

762 Citations