NO Oxidation Performance Research Articles

Impact of Preparation Method and Hydrothermal Aging on Particle Size Distribution of Pt/γ-Al2O3 and Its Performance in CO and NO Oxidation

The influence of the preparation method and the corresponding particle size distribution on the hydrothermal deactivation behavior at 600–800 °C and performance during CO/NO oxidation was systemati...

Performance and mechanism comparison of manganese oxides at different valence states for catalytic oxidation of NO

Manganese oxides with different valences (MnO2, Mn2O3 and Mn3O4) synthesized by a hydrothermal synthesis method were investigated to evaluate their catalytic performances for NO oxidation. MnO2 with the Mn4+ ion exhibited the best catalytic activity with a maximum NO conversion of 91.4% at 250 °C under the GHSV of 48 000 mL g−1 h−1, and the low-temperature catalytic activity for NO oxidation decreased in the order of MnO2 > Mn2O3 > Mn3O4. Compared with Mn2O3 and Mn3O4, the catalytic performance of MnO2 was less affected by NO concentrations, O2 contents, GHSV, H2O and SO2, though all the three catalysts show stable catalytic properties. To investigate the factors influencing the catalytic activity, their properties were characterized by XRD, SEM, N2 adsorption, H2-TPR, O2-TPD and XPS, and these results indicated that the redox property and active oxygen species of MnO2 were proposed to be the main factors that contributed to the excellent performance in NO oxidation. For the three catalysts, in-situ DRIFTS studies further indicated that both the lattice oxygen and the chemically adsorbed oxygen participated in the NO oxidation process, but the adsorbed species were more easily desorbed from MnO2, probably benefiting for the catalytic performance of MnO2.

NO oxidation in dry and humid conditions using hyper-cross-linked polymers: Impact of surface chemistry on catalytic conversion efficiency

Microporous and hydrophobic polymers were synthesized and tested as NO oxidation catalysts to overcome water co-adsorption in combustion flue gas streams. The self-cross-linked 4,4′-bis(chloromethyl)-1,1′-biphenyl polymer (PBCMBP), with micropore volume of 0.38 cm3/g and surface area of 1430 m2/g, provided 62% NO oxidation efficiency at 25 °C in dry conditions but its performance dropped to 50% in the presence of 1.6 vol% moisture (wet conditions). To decrease performance loss, PBCMBP was modified with benzene (PBCMBP-BZ) to remove pendant chloromethyl groups, improving resistance to surface reactions with NO2 that add hydrophilicity. In wet conditions, PBCMBP-BZ has 59% NO oxidation efficiency, an 18% increase compared to PBCMBP at 25 °C. PBCMBP was also functionalized with dimethylamine (PBCMBP-DMA) to increase surface basicity, increasing NO oxidation by 11% in dry conditions, but decreasing NO oxidation in wet conditions by 30% due to increased proclivity to react with NO2. The combined impact of temperature and humidity was measured up to 100 °C, showing that moisture’s impact decreases with increasing temperature and that PBCMBP-DMA outperforms other catalysts at higher temperatures. Findings suggest that catalyst surface chemistry can be manipulated to further improve NO oxidation performance, especially in the presence of moisture.

In-Situ DRIFTS for Reaction Mechanism and SO2 Poisoning Mechanism of NO Oxidation Over γ-MnO2 with Good Low-Temperature Activity

In this study, the γ-MnO2 catalyst modified with PEG exhibits outstanding low-temperature performances for NO oxidation, and in-situ DRIFTS experiments were used to systematically investigate the low-temperature NO oxidation mechanisms over γ-MnO2. These results demonstrated that NO was first adsorbed on the surface of γ-MnO2 to form the nitrosyls, which could be further oxidized to nitrates under the action of the chemisorbed oxygen or lattice oxygen, and afterwards the formed nitrates were decomposed into nitrogen dioxide. Moreover, the inhibitory mechanism of SO2 on γ-MnO2 was also studied, and SO2 severely inhibit the NO oxidation performance of γ-MnO2 through forming stable sulfates that could easily consume the active sites of the catalyst to hinder the formation of nitrates, resulting in the termination of oxidation of NO to NO2. Clarifying the mechanisms of NO oxidation and SO2 poison is very essential for developing better NO oxidation catalysts.

High-efficient catalysts of core-shell structured Pt@transition metal oxides (TMOs) supported on 3DOM-Al2O3 for soot oxidation: The effect of strong Pt-TMO interaction

Diesel soot particles are important components of atmospheric fine particles, and the key factor in controlling soot emissions is the performance of catalysts in after-treatment systems. Herein, a series of high-efficient nanocatalysts of Pt@transition metal (Mn, Fe, Co, Ni, Cu) oxides (TMOs) core-shell nanoparticles (NPs) supported on 3D ordered macroporous (3DOM) Al2O3 (Pt@TMO/3DOM-Al2O3) were designed and synthesized. The strong Pt-TMO interaction at the optimized interface of Pt@TMO core-shell NPs induces the formation of coordination unsaturated active sites for activated reactants (O2 and NO). Pt@TMO/3DOM-Al2O3 catalysts exhibit high catalytic activity dependence on the strong Pt-TMO interaction for soot oxidation, and Pt@CoOx/3DOM-Al2O3 catalyst shows the highest catalytic activity (T50 = 357 °C, TOF = 2.76 S−1 ×10−3) and the lowest apparent activation energy (52 kJ mol−1) in the presence of O2 (5%), NO (0.2%) and H2O (5%). In addition, Pt@TMO/3DOM-Al2O3 catalysts exhibit excellent H2O and SO2 resistances during catalytic soot oxidation. Based on the results of characterizations and density functional theory calculations, the coordination unsaturated Co-based active sites have efficient catalytic performance for NO oxidation to NO2 intermediate, which is an important reaction pathway for catalytic soot oxidation. Insight into the activity dependence on the strong Pt-TMO interaction is not only meaningful for development of advanced catalysts, but also supports the development of Pt@TMO nanoparticles for practical applications of emission reduction of diesel soot.

High Performance of NO Oxidation Over Co/Zr0.2Ce0.8O2 Catalysts Prepared by One-pot Method

Cobalt supported on Zr0.2Ce0.8O2 with different cobalt loading for catalytic oxidation of NO to NO2 were prepared by the one-pot method using citric acid as complexing agent. The samples were characterized by XRD, H2-TPR, O2-TPD and NO-TPD and the catalytic performance was investigated. The results showed that the Co/Zr0.2Ce0.8O2 catalyst containing 10 wt.% of Co3O4 exhibited excellent activity at a wide temperature window ranged from about 240 °C to 380 °C with NO conversion above 50%. The high catalytic performance of Co/Zr0.2Ce0.8O2 samples should be attributed to the active component related with cobalt species.

Influence of Ce-doping on MnOx-ZSM-5 catalysts for the selective catalytic reduction of NO/NO2 with NH3

A series of MnOx-CeO2/ZSM-5 catalysts with different Ce loadings were prepared through the impregnation method and characterized by BET, XRD, SEM, H2-TPR, and XPS. The performances of selective catalytic reduction (SCR) of NO/NO2 with NH3 and NO oxidation over MnOx-CeO2/ZSM-5 catalysts were investigated. The result showed that the MnOx dispersions, redox abilities and proportions of Mn4+ of catalysts were enhanced by the doping of CeO2. However, the excessive Ce loadings led to the accumulation of active components on the catalyst surface. The sample with Ce/Mn molar ratio of 0.39 exhibited the best SCR performance, the NOx conversion achieved 95.4% at 150 °C, and maintained higher than 95% in a wide operating temperature range (150–350 °C). MnOx-CeO2/ZSM-5 also exhibited considerable catalytic performance of NO oxidation, the maximum conversion of NO achieved 56.5% at 350 °C.

Shape-controlled synthesis of MnOx–CeO2 oxides and their catalytic performance in NO oxidation

MnOx–CeO2 nanocubes and nanorods with the pure fluorite-type ceria crystalline structure have been synthesized, in order to investigate the effect of catalysts shape in NO oxidation. The results indicated that both of the nanomaterials growing from Ce(OH)3 nanorods nuclei, and higher hydrothermal temperature and alkali concentration facilitated the conversion of nanorods to nanocubes. During the synthesis process, manganese ions could incorporate into the ceria lattice without affecting its crystalline structure and growth mechanism. The morphology of this binary oxide determines NO oxidation rate, while the specific reaction rate on the reactive {100} planes was two times higher than that on the {111} planes, due to co-effect of the valence state of manganese ions, surface energy and the ratio of surface lattice oxygen species. The results could provide guidance for understanding of the importance of catalyst design in NO oxidation.

Origin of the superior activity of surface doped SmMn2O5 mullites for NO oxidation: A first-principles based microkinetic study

Using first-principles based microkinetic analysis, we conduct a comprehensive investigation on the NO oxidation process on SmMn2O5 mullite’s active surface under experimentally relevant conditions. The influencing factor for NO oxidation activity is identified and Mars-van Krevelen (MvK) and Eley-Rideal (ER) mechanisms are found to contribute to the high activity of pristine SmMn2O5 mullites in high and low temperature regions, respectively. We further study the activity of surface (Ba/Sr/La) doped SmMn2O5. It is found that surface doping of Ba primarily destabilizes the nitrite (NO∗) species to promote NO oxidation performance via MvK mechanism. Due to the stronger ability of O2 (O2∗) dissociation along the ER route, Sr and La doped mullites are predicted to have greatly enhanced reaction activity in a wide temperature region. Our study gives insight into the NO oxidation ability of pristine and surface doped SmMn2O5 that are beneficial for further optimization of mullite based catalyst performance.

A Study on the NO Treatment using Dry Oxidant Produced by H2O2

In this study, the NO oxidation using dry oxidant produced by catalytic H2O2 conversion was conducted. It was shown that Mn-based Fe2O3 support catalyst has the best performance in the catalytic H2O2 conversion and it’s combined-NO oxidation. The reaction characteristics of NO oxidation was investigated by the various operation conditions such as H2O2 amount, oxidation temperature and space velocity. As a results, the oxidation efficiency of NO greatly depends on the oxidation reaction temperature, H2O2 amount and space velocity. The performance of NO oxidation was increased with increasing the oxidation temperature and H2O2 amount. Also performance of NO oxidation was decreased with increasing the space velocity.

The formation of 3D spherical Cr-Ce mixed oxides with roughness surface and their enhanced low-temperature NO oxidation

3D spherical Cr-Ce mixed oxides were successfully synthesized via the hydrothermal method in different solvents, and their efficiencies of catalytic oxidation NO were evaluated. Among all catalysts, CrCeEt (ethanol adopted as the solvent) exhibited the best low-temperature catalytic NO oxidation performance, which was over 40% at 200 °C and reached 50% at 230 °C. The high activity at low temperature could be ascribed to the particle size and roughness on the surface, which could make the surface with active site more exposed, thus enhancing the catalytic performance at low temperature. Further, the exposed surface could generate more Cr6+ and Oβ ratios, which played the key role in NO oxidation. Moreover, CrCeEt catalyst exhibited the stable catalytic performance and the strong H2O(g) resistance, promoting the industrial application of new SCO technique, which the denitration system was placed after the desulfurization system.

The inhibition effect of oxygen in the calcination atmosphere on the catalytic performance of MnOx–CeO2 catalysts for NO oxidation

In this study, the widely studied MnCe (MnOx–CeO2) catalyst prepared by the co-precipitation method was used for investigating the effects of oxygen in the calcination atmosphere (referred as Oca) on its catalytic performance for NO oxidation. The evaluation of catalytic performance showed that MnCe-O (calcined in O2/Ar) exhibited a much lower NO conversion in comparison with MnCe-A (calcined in Ar) catalyst. The characterization results revealed that Oca could cause the sintering of MnCe catalysts and then reduce the surface area, which decreased the active sites for catalytic reaction. Simultaneously, Oca inhibited the incorporation of Mn ions into CeO2 lattice, which could weaken the interaction between MnOx and CeO2 supports. This weak interaction decreased the amounts of active oxygen species and weakened the redox properties of MnCe catalyst. Furthermore, the nitrate species adsorbed on MnCe-O was more stable than that on MnCe-A. Therefore, the inferior catalytic performance of MnCe-O could be attributed to the collective effect of low concentration active sites and active oxygen species, poor redox properties and stable adsorbed nitrate species.

The influence of xMnyCe/γ-Al2O3 on NOx catalysts on the properties of NOx storage and reduction over Pt-Ce-Ba/γ-Al2O3 catalysts

The aim of the study was to investigate the influence of Diesel Oxidation Catalysts (DOC) xMnyCe/γ-Al2O3catalysts on the properties of NOx storage and reduction (NSR) under the treatment of Pt-Ce-Ba/γ-Al2O3 catalysts. The physicochemical properties of DOC and NSR were determined with XRD, TEM and XPS. The results showed that Pt-CeO2 interaction had a great impact on the dispersion of the NSR catalyst. Furthermore, the performance of NO oxidation by DOC and NOx desorption by DOC/NSR combined system were evaluated based on the simulation experiments. Moreover, optimal operation of the single NSR catalyst and the DOC/NSR combined system were attained at 400°C, while the durations of the lean and rich periods were tL=240s and tR=30s/45s/60s, respectively. The single NSR catalyst showed that the highest NOx conversion and maximum NOx adsorption conversion were 58.0% and 74.8%. The DOC/NSR combined system allowed Pt10Ce15Ba/γ-Al2O3 catalyst storing the NO2 oxidized on the 6Mn10Ce/γ-Al2O3 catalyst, favoring the NOx storage process during the lean phase. And it also allowed the stored NOx species of NSR catalysts desorbing due to H2 oxidation by the 6Mn10Ce/γ-Al2O3 catalyst, favoring the NOx desorption process during the rich phase.

Key factors in Sr-doped LaBO3 (B = Co or Mn) perovskites for NO oxidation in efficient diesel exhaust purification

Perovskites have attracted attention in recent years as an economic alternative to noble metals in oxidation processes. Synthesis conditions of LaCoO3 and LaMnO3 perovskites have been studied varying citrate to nitrate molar ratio in the starting solution, pH and calcination protocol, with the aim of obtaining high purity perovskites, absence of impurities, and with enhanced textural properties. Once synthesis conditions were established, strontium was incorporated in the perovskite lattice as a textural and structural promoter, by substituting lanthanum with different doping levels, i.e. La0.9Sr0.1BO3, La0.8Sr0.2BO3, La0.7Sr0.3BO3, La0.6Sr0.4BO3 and La0.5Sr0.5BO3 with B = Co or Mn. The prepared solids were characterized in terms of crystalline phase identification (XRD), specific surface area (N2 adsorption–desorption at −196 °C), reducibility and oxidation state of transition metal ions (H2-TPR), quantification of adsorbed oxygen species (O2-TPD) and surface elemental composition (XPS). Charge imbalance associated to strontium (Sr2+) incorporation in the perovskite lattice in substitution of lanthanum (La3+) was preferentially balanced by Mn4+ promotion in La1−xSrxMnO3 perovskites, whereas formation of oxygen vacancies seems to be the mechanism for charge compensation in La1−xSrxCoO3 perovskites, where Co ions remained as Co3+ ions. Strontium doped perovskites further improved NO conversion compared to the non-substituted formulations. The best NO oxidation performance was obtained with La0.7Sr0.3CoO3 and La0.9Sr0.1MnO3 samples, achieving maximum NO conversion of 83 and 65% at 300 and 325 °C, respectively. Higher oxidation capacity of La0.7Sr0.3CoO3 sample was associated to the higher oxygen mobility and exchange capacity between oxygen in the lattice and gas phase oxygen. It is worth noting that prepared perovskites presented far higher NO oxidation capacity than platinum-based NSR model catalysts, confirming perovskites as an economic alternative to catalyze NO oxidation reactions in automotive catalysis.

Investigation of the hydrothermal aging of an Mn-based mullite SmMn2O5 catalyst of NO oxidation

The decrease in the catalytic performance for NO oxidation of the Mn-based mullite SmMn2O5 after being subjected to hydrothermal aging was mainly caused by an ∼3-fold decrease in its specific surface area (BET).

The NO oxidation performance over Cu/Ce0.8Zr0.2O2 catalyst

The Cu-doped Ce0.8Zr0.2O2 (CZ) catalyst was investigated in terms of its NO oxidation activity, structure and surface characteristics. The Cu/CZ catalyst with 6 wt% Cu loading and 550°C calcination temperature of CZ support showed the best catalytic performance, and over 50% NO conversion was obtained in a wide temperature range (250–370°C). Moreover, this catalyst showed good resistance to H2O and SO2. The dispersion of CuO species on the smaller crystallite size CZ support was favorable for NO adsorption. The Cu–Ce–Zr solid solution provided the sites of oxygen vacancies, prompted the formation of active oxygen, which in sum improved the oxidation activity of NO.

Electronic properties and native point defects of high efficient NO oxidation catalysts SmMn2O5

Mn-based oxide SmMn2O5 exhibits great catalytic performance in NO oxidation [Wang et al., Science 337, 832 (2012)]. Nevertheless, the fundamental understanding of SmMn2O5 properties is so far not fully accessible. Here, the SmMn2O5 nanoparticles are synthesized through hydrothermal methods, and the pure phase of triclinic SmMn2O5 is characterized by high-resolution tunneling electron microscope and X-ray diffraction. Furthermore, the X-ray photoelectron spectroscopy, absorption, photoluminescence spectra (PL), and density functional theory based first-principles calculations are employed to explore the fundamental electronic structures of pristine and defective SmMn2O5. Combined with band structure calculations, light absorption, and PL spectra, we first show that SmMn2O5 presents an insulating behavior with an indirect band gap of ∼1.0 eV. Between the two types of crystal fields, i.e., octahedral and tetrahedral, the later one contributes to the dz2 of the valence band edge, resulting in superior catalytic performance of NO oxidation. Furthermore, the native point defects in SmMn2O5 are first reported. Among the various native point defects, we demonstrate that oxygen vacancy (VO) shows the lowest formation energy in oxygen poor conditions, while the oxygen interstitial (Oi) and Mn vacancies are energetically favorable in oxygen rich situations. In other words, SmMn2O5 could be potentially utilized as an oxygen storage material.

Identifying the descriptor governing NO oxidation on mullite Sm(Y, Tb, Gd, Lu)Mn2O5for diesel exhaust cleaning

Mn-based oxides Sm(Y, Tb, Gd, Lu)Mn2O5are synthesized to identify a general descriptor that governs the catalytic performance in NO oxidation.

Promotional role of La addition in the NO oxidation performance of a SmMn2O5 mullite catalyst

A series of LaxSm1−xMn2Oδ (x = 0, 0.1, 0.3, 0.5) catalysts were synthesized through a co-precipitation method. The catalytic activity for NO oxidation was enhanced with La substitution, and the maximum activity was achieved at x = 0.3.

Nonprecious mixed oxide catalysts Co3AlO and Co2NiAlO derived from nanoflowerlike cobalt-based hydrotalcites for highly efficient oxidation of nitric oxide

Nanoflowerlike Co3AlO-500 and Co2NiAlO-500 exhibit excellent NO oxidation performance upon abundant active sites Co3+/Ni3+–Oads through formation of bridged bidentate nitrate.