NO Oxidation Activity Research Articles

Au monolayer on WC(0001) with enhanced activity towards NO oxidation: A theoretical study

Catalysts with fast NO oxidation kinetics and moderate adsorption towards NO2 are urgently required for fast selective catalytic reduction of NO. Based on density functional calculations, we report in this work that Au monolayer on WC(0001) (Au/WC) has both high NO oxidation activity and strong adsorption to NO2. The WC substrate shifts the d-band center of Au monolayer to the vicinity of the Fermi level and the binding strength of reactants on Au/WC is consequently enhanced. The rate constants of gas desorption obtained in this work are significantly lower than those of forward reactions, in accordance with previous molecule dynamics simulations. We anticipate that the present results will facilitate the development of high-performance catalysts for NO oxidation.

Positive effects of K+ in hybrid CoMn-K and Pd/Ba/Al2O3 catalysts for NOx storage and reduction

CoMn-K was mechanically mixed with a Pd/Ba/Al2O3 catalyst to enhance its NOx storage and reduction properties. The Pd/Ba/Al2O3 and CoMn-K mixture showed a synergetic effect with respect to NOx storage capacity and improved NOx reduction ability. The addition of K+ ions promoted the catalytic performance by increasing NO oxidation activity and therefore NOx storage via the creation of active surface oxygen species on the catalysts. The stored NOx reduction by H2 was also improved, being ascribed to the fact that Pd was mainly present as Pd0 due to the electronic interactions between Pd and K+ ions. NOx conversions and N2 selectivity at low temperatures (80–200 °C) could be further enhanced by assistance of non-thermal plasma in rich phase.

Synergistic Effect of Cu/CeO2 and Pt-BaO/CeO2 Catalysts for a Low-Temperature Lean NO x Trap.

A lean NO x trap (LNT) catalyst has been widely used for removing NO x exhaust from lean-burn engines. However, the operation range of LNT has been limited because of the poor activity of LNT catalysts at low temperatures (≤300 °C), especially in urban driving conditions. To increase NO x removal efficiency during lean-rich cycle operation, a Cu/CeO2 (CC) catalyst was added to a Pt-BaO/CeO2 (PBC) catalyst. In comparison to PBC- or CC-only catalysts, the physical mixture of PBC and CC catalysts (PBC + CC) exhibited a significant synergy for both NO x storage and reduction efficiencies. In particular, low-temperature activity below 200 °C was greatly enhanced. A Pt-BaO-Cu/CeO2 (PBCC) catalyst, which was synthesized by depositing Pt and Cu together on a ceria support, showed poorer NO x removal efficiency. The origin of the synergistic effect over PBC + CC was investigated. Under lean conditions, the CC showed much better activity for NO oxidation, allowing for faster NO x storage on PBC. Under rich conditions, H2 was generated in situ on the CC by a water-gas shift reaction then accelerated the reduction of NO x, which had been stored on PBC, with a higher selectivity to N2. This simple modification in the catalyst can provide an important clue to enhance low-temperature activity of the commercial LNT system.

Russian operator commissions methanol-ammonia plant

The effects of Mn precursors on structure defects and NO catalytic mechanism over Ce0·6Mn0.4Ox catalysts were fully investigated. The Ce0·6Mn0.4Ox-Ac catalyst, synthesized by using MnAc2 as a Mn precursor, showed the best catalytic activity for NO conversion (86.9%) at 250 °C under high space velocity (40,000 mL g−1 h−1). Detailed structure-activity relationship reveals that the abundant oxygen vacancies and the highly migratory oxygen species formed on Ce0·6Mn0.4Ox are the crucial factors that leading to the better NO oxidation activity than that of the other Ce0·6Mn0.4Ox-Y (YNO3, SO4, Cl) catalysts. In situ DRIFTS technique confirms that the differences in formation mode and desorption ability of N-based (nitrates, nitrites, and dimer nitroso) intermediate species are the vital factors for NO high-efficiency catalytic oxidation. The highly reactive surface intermediate species, like monodentate nitrates, were observed particularly on Ce0·6Mn0.4Ox-Ac catalyst, so that the NO oxidation performance on Ce0·6Mn0.4Ox-Ac catalyst was more active comparing with other Ce0·6Mn0.4Ox-Y catalysts. This study can broaden the horizons for understanding NO catalytic oxidation mechanism on serial Ce0·6Mn0.4Ox catalysts and serve as a reference guide in design of structure defects for functional materials by modulating precursor species.

High-efficiency catalytic oxidation of nitric oxide over spherical Mn[sbnd]Co spinel catalyst at low temperature

The spherical MnCo spinel catalyst were prepared by (NH4)2CO3-assisted co-precipitation method and were used for the catalytic oxidation of NO at low temperature. The experimental results showed that the optimized Mn0.52Co2.48O4 spinel catalyst, preparing under conditions of Co/Mn molar ratio as 5 and calcination temperature at 450° for 5 h, has the best catalytic activity with about 65% NO oxidation at 150 °C, 80% at 200 °C and 90% at 250 °C, respectively. The main reasons for good NO oxidation activity over Mn0.52Co2.48O4 spinel were attributed to the high surface area of 182.8 m2/g, low apparent activation energy of 30.8 kJ/mol, high content of high surface concentrations of chemisorbed oxygen, Mn4+/Mn3+ and Co2+/Co3+, and the efficient synergetic effect between Co and Mn ions (Co2+ + Mn4+ ↔ Mn3+ + Co3+; Co3+ + Mn2+ ↔ Mn3+ + Co2+). The monodentate nitrite and bridged nitrate species were the important intermediates in the oxidation process of NO to NO2, and the adsorption-reaction pathways were further proposed over MnCo spinel catalyst.

Enhancement of NO oxidation activity and SO2 resistance over LaMnO3+δ perovskites catalysts with metal substitution and acid treatment

NOx control by catalytic oxidation combined with wet absorption has been a promising technology for the flue gas treatment of industrial boiler and furnace. LaMnO3+δ perovskites were investigated as catalysts for NO oxidation. The comparison between metal substitution of La and Mn cations was researched. The activity results indicated that La site substitution was more favorable for NO oxidation than Mn site substitution. LaCoMnO exhibited the highest activity in NO oxidation with maximum value of 83.74% at 250 °C. That value further increased to 89.96% after acid etching (LaCoMnO-A), and closely, such high value of 89.78% was achieved at 220 °C. The surface area was significantly improved by acid etching, i.e., from 47.0 m2/g to 212.0 m2/g, Tremendous changes in crystalline structure and morphology were taken place, i.e., perovskites phase disappeared and MnO2 was detected instead; compact amorphous particle morphology was replaced by multiple layer structure. Even more, Mn4+ ions and oxygen vacancies accumulated on the surface were improved. Deservedly, the redox property was enhanced enormously, especially at low temperature. Subsequently, SO2 poisoning tests were conducted at 250 °C under 20 ppm SO2. Interestingly, no decrease was observed for LaCoMnO-A within 480-min SO2 poisoning test, which was totally contrasted with the nearly complete deactivation results of LaCoMnO. In-situ DRIFTS measurements validated nitrate disappeared and replaced by sulfate formation on the catalyst surface. Merely, under low-concentration SO2 atmosphere, the monodentate nitrate and bidentate nitrate were destroyed, while the free nitrates exhibited higher resistance. Overall, the acid etching improved the catalyst resistance to SO2, which may have the potential to apply in the flue gas treatment of low-sulfur and sulfur-free fuel fired boiler and furnace.

Electrospun YMn2O5 nanofibers: A highly catalytic activity for NO oxidation

Nanofiber catalysts are potentially promising to be equipped in the lean-burned vehicle exhaust emission system considering its facile synthesis, low-cost, long durability in contrast to the traditionally complicated powder based catalyst. Here, via electrospinning method, we synthesized YMn2O5 nanofibers (average diameter: ˜93 nm) including nanowire stacked with particles, and hollow tubes with or without inside nanowires. The morphologies remained even after hydrothermal aging at 800 ℃ for 10 h with 10% H2O stream. X-ray photoelectron spectrum revealed Mn4+/Mn3+ ratio 0.589 on the surface of YMn2O5 catalyst (YMO), indicating the existence of oxygen vacancies. The highest conversion of electrospun mullite fibers at 310 ℃ is ˜18% higher than powder-based YMO at 352 ℃ in the presence of 5% H2O at WHSV = 240,000 ml g−1 h−1. The extracted activation energy from NO-to-NO2 conversion curves of YMO nanofiber (61.68 kJ/mol) is slightly lower than that of powder-based mullite (91.45 kJ/mol), consistent with calculated 68.16 kJ/mol on the fiber YMO surface (121) with a rate-limiting step of the NO2 desorption. The catalytic activity can be attributed to the unit occupy of the eg (dz2 and dx2-y2) orbitals of Mn-dimer atoms around fermi level based on the combination of the theoretical calculations and DRIFTS spectra analysis. Importantly, after the hash hydrothermal-aging, the highest NO oxidation conversion of powder-based mullite is less than 50%, while the fiber maintains a conversion of 62%. Moreover, the fresh YMO fiber maintained superior oxidation stability for 12 h at its maximum conversion of 66%. This work provides the possibility for mullite oxide fibers to replace powder-based precious metal catalyst coating on monolith converter.

Monolayer Epitaxial Heterostructures for Selective Visible‐Light‐Driven Photocatalytic NO Oxidation

AbstractConstruction of vertical heterostructures by stacking two‐dimensional (2D) layered materials via chemical bonds can be an effective strategy to explore advanced solar‐energy‐conversion systems. However, it remains a great challenge to fabricate such heterostructures based on conversional oxide‐based compounds, as they either do not possess a 2D layered structure or are not suitable for epitaxial growth due to large lattice mismatch. Here, a vertical heterostructure of bismuth oxyhalide semiconductors fabricated through a heteroepitaxial anion exchange method is reported. Monolayer Bi2WO6 is epitaxially grown on the exposed surface of BiOI to inhibit photocorrosion and introduce active sites. Theoretical and experimental results reveal that electrons generated under visible‐light irradiation can directly transfer to surface coordinatively unsaturated (CUS) Bi atoms, which contribute to the adsorption and activation of reactant molecules. As a result, the Bi2WO6/BiOI vertical heterostructures exhibit significantly enhanced visible‐light‐driven NO oxidation activity compared with BiOI and Bi2WO6.

NOx Adsorption Over Ce/Zr-Based Catalysts Doped with Cu and Ba

In this study, Ce/Zr-based materials without noble metals in their formulation have been prepared, characterized and tested in NOx adsorption at different temperatures. In particular, the effect of the presence of Ba and Cu in the catalytic formulation has been investigated. The catalyst characterization shows that the redox properties of the catalysts are enhanced by Cu and inhibited by Ba. Upon interaction with NO/O2, it is found that NOx are stored at low temperatures (150 °C) in the form of nitrites, without any relevant NO oxidation activity. This suggests a direct NO uptake. At higher temperatures (300 °C) a significant NO oxidation activity is observed, that is enhanced by Cu and depressed by Ba. In this case, NOx are stored in the form of nitrates; the samples containing both Ba and Cu exhibits the better performances in terms of NOx adsorption capacity.

Non-thermal Plasma Assisted Preparation of MnCeOx, MnOx and CeO2 Catalysts for Enhancement of Surface Active Oxygen and NO Oxidation Activity

ABSTRACT Non-thermal plasma was used to enhance the synthesis of nanostructured manganese oxide, cerium oxides and MnCeOx composite oxide catalysts via the decomposition of organics and nitrates in an integrated network gel. Pure oxides prepared with plasma formed larger crystallites and exhibited more crystallization. The active oxygen species in the plasma were adsorbed through interaction with ions on the catalyst surface; compared to calcined catalysts, the percentage of active oxygen was 33%, 7% and 20% higher for the plasma-treated MnOx, CeO2 and MnCeOx, respectively. The percentage of unsaturated Ce3+ was also 10% higher on the plasma-treated MnCeOx. Furthermore, the NO oxidation efficiency at 275°C for plasma-treated MnOx and MnCeOx was 28% and 21% higher, respectively, than for their calcined counterparts. Due to the lack of significant thermal effects during preparation, the plasma-treated catalysts better retained the structures of their precursors, the even mixtures of manganese oxides and ceria in gel. Additionally, the plasma-treated MnOx and MnCeOx displayed higher formation and decomposition rates for nitrates. The continuous and rapid transformation of NO into nitrates and of nitrates into NO2 contributes to the excellent oxidation efficiency of these catalysts.

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.

Synthesis of Bi2WO6 with gradient oxygen vacancies for highly photocatalytic NO oxidation and mechanism study

We successfully synthesized the Bi2WO6 (BWO) with gradient concentration of oxygen vacancies and exhibited the excellent photocatalytic NO oxidation activity. And the oxygen vacancy introduced in photocatalyst is a popular method to enhance the photocatalytic performance. The formation of oxygen vacancy defects in BWO is responsible for the tuning band structure and modifying surface chemical state to improve carriers separation efficiency, enlarge visible light absorption range and facilitate reactant activation, which is unambiguously verified by the UV–vis DRS, XPS, EPR and DFT calculations. Moreover, the role of surface oxygen vacancy defects and reaction mechanism of photocatalytic NO oxidation were discussed in detail by combing the in situ DRIFT and theoretical calculations. This work could be broadly used to the design and synthesize high efficient photocatalysts and pave a way to understand the reaction mechanism of NO oxidization.

Anion intercalated layered-double-hydroxide structure for efficient photocatalytic NO remove

Due to the easily controllable interlayer anions, metal cation composition proportion and thickness, which is beneficial to modify surface chemical state and tune bandgap, layered double hydroxides (LDHs) have great promising potential for photocatalytic applications. In this study, we have successfully synthesized the ZnAl–LDH intercalated the single anion between ZnAl cationic interlayer without anionic impurities by using a facile calcining and reconstructing routes. The electron structure and surface chemical state of the prepared products have been investigated by combining the DFT calculation and experimental characterization methods. UV–vis DRS was used to certify the light absorption of the prepared products, and we performed the DFT calculation to demonstrate the density of state and activation of reactant. These results suggested that the ZnAl–LDH–CO3 possessed the more proper band structure and superior ability to activate NO and O2 for accelerating the photocatalytic NO oxidation activity. Moreover, the in situ DRIFTS with dynamically monitoring intermediates and products over the ZnAl–LDH–CO3 was adopted to declare the photocatalytic NO oxidized process during the photocatalytic reaction process. This work illustrated the influence of different interlayer anions to the electron structure and surface chemical state of ZnAl–LDH structure through the experimental verification combined DFT calculation and the photocatalytic NO oxidized process via in situ DRIFTS analyzing, which would provide a novel way to design and fabricate the efficient photocatalysis, and understand the reaction process.

Differentiation between O2 and NO2 impact on PtOx formation in a diesel oxidation catalyst

NO oxidation activity of platinum-based catalysts undergoes reversible changes during operation in oxidative atmosphere typical for Diesel engine exhaust gas due to platinum oxides (PtOx) formation and reduction. In recent years, these changes were studied mostly with the mixtures containing both O2 and NO2 so that it was difficult to quantify individual impacts of the oxidants on PtOx formation. In this paper, Pt/Al2O3 catalyst deactivation and PtOx formation with O2 and NO2 was examined separately by the means of transient isothermal experiments at 150, 175 and 200 °C with alternating deactivation and probe periods. The largest extent of deactivation was detected at 175 °C where the NO conversion in the isothermal experiment dropped from 85% (reduced catalyst) down to 10% (steady-state PtOx level) over 4 h. The experimental data were then utilized for parameter evaluation in global kinetic model of NO oxidation on a diesel oxidation catalyst extended by the reactions describing PtOx formation and reduction. Though the rate coefficient for PtOx formation with NO2 is higher than with O2, the overall PtOx formation rate and the steady-state PtOx level are higher with O2 at typical component concentrations (8% O2 or 250 ppm NO2). Similar activation energies were observed for both PtOx formation reactions. Validity of the model was verified also at lower O2 and NO2 concentrations (4% O2 or 125 ppm NO2). The developed model is thus capable of sufficiently precise prediction of catalyst deactivation at various O2 and NO2 concentrations relevant to diesel exhaust gas.

Influence of B-site transition metal on NO oxidation over LaBO3 (B=Mn, Fe and Co) perovskite catalysts

A comprehensive understanding of NO catalytic oxidation on different La-based perovskites LaBO3 (B=Mn, Fe, Co) enables to ultimately utilize the catalyst in the lean-burn NOx after treatment system. Here, we report a comparative study of the NO oxidation on LaBO3 (B = Mn, Fe and Co) surfaces by first-principles calculations though density functional theory (DFT). Based on the adsorption of NOx (x=1, 2 and 3) on the LaO and BO2 terminations of (001) surface, we find that the NOx adsorbates are bound stronger on the LaO terminations than BO2 ones. Infrared vibrational spectra and the NO oxidation reactions calculations suggest that BO2 surfaces are more active compared to LaO ones. The primary step for NO oxidation is the desorption of NO2* from the BO2 surfaces with a sequence of barrier 1.43eV, 1.60eV, 1.68 eV for CoO2, MnO2, and FeO2 terminations, respectively. Fundamentally, least charge transfer from CoO2 surface to NO2 ensures its smallest activation energy in contrast to the other two BO2 terminations. These findings provide insights into the influence of B-site transition metal and different terminations on NO oxidation activity of La-based perovskites which might be extended to design of other NO oxidation catalysts.

Comparison of O2 and NO2 impact on PtOx and PdOx formation in diesel oxidation catalysts and their reduction by CO and C3H6 pulses

NO oxidation on diesel oxidation catalyst (DOC) increases NO2/NO ratio in the exhaust gas, which is beneficial for both NOx reduction and soot oxidation processes in the catalysts and filters located downstream, such as SCR, LNT and DPF. The impact of O2 and NO2 on the PtOx and PdOx formation and subsequent changes in NO oxidation activity was studied on Pt/γ-Αl2O3 and Pd/γ-Αl2O3 catalysts. Repeated heat-up and cool-down temperature ramps in the range of 80–450 °C and isothermal deactivation/reactivation experiments at 150, 175 and 200 °C with NO oxidation as a probe reaction were performed. Inverse hysteresis of NO2 yield during temperature ramps was observed with both catalysts. Though NO2 is stronger oxidizing agent than O2, PtOx and PdOx formation was induced to a similar extent by both O2 and NO2 when present at their typical concentration levels (8% O2, 250 ppm NO2). The NO oxidation activity of both Pt and Pd sites was restored by CO or C3H6 pulses while keeping overall lean conditions (excess of oxygen). The maximum efficiency of CO pulses was achieved at 200 °C for PtOx reduction, and at 150 °C for PdOx reduction.

Effect of surface morphology on catalytic activity for NO oxidation of SmMn2O5 nanocrystals

SmMn2O5 has been reported to be a promising alternative to substitute the current commercial Pt-based catalysts for NO oxidation. In this work, single-crystalline orthorhombic SmMn2O5 nanorods and nanoparticles are successfully synthesized through a tunable hydrothermal route. The pH value of the precursor solution is a decisive factor for morphology control of SmMn2O5 nanocrystals, with decreasing pH value the morphology of SmMn2O5 nanocrystals converts from nanoparticles to nanorods. The nanoparticles exhibit a better catalytic activity for NO oxidation than the nanorods, because they can efficiently convert NO at lower temperature. By the analysis of X-ray photoelectron spectrum and high-resolution transmission electron microscope, the oxidation activity is found to be dependent on the high specific surface area and surface crystal planes of SmMn2O5 nanocrystals, which widens and deepens the oxygen chemistry and NO oxidation mechanism on the surfaces of SmMn2O5. This work could not only provide new insights into the morphology control of SmMn2O5 nanocrystals, but also pave a new way for the crystal planes modification of SmMn2O5 nanocrystals as NO oxidation catalyst to achieve an enhanced performance for environmental applications.

Simulation of diesel exhaust aftertreatment system DOC—pipe—SCR: The effects of Pt loading, PtOx formation and pipe configuration on the deNOx performance

A combined exhaust aftertreatment system consisting of a diesel oxidation catalyst (DOC), pipe and selective catalytic reduction of nitrogen oxides (SCR) is studied in this paper by the means of mathematical modeling and simulations. Pt/γ-Al2O3 DOC and V2O5-WO3/TiO2 SCR catalysts for heavy-duty Diesel engines are examined. First, spatially 1D models of the DOC, SCR and pipe are introduced and calibrated using the measured engine test data. The models are then employed in a simulation study of the effects of Pt loading, PtOx formation and pipe configuration on the NO2 yield in DOC and the resulting deNOx performance of the SCR in the driving cycles ETC and WHTC. It is shown that there exists optimum washcoat loading in the DOC with respect to NOx conversion in SCR and that the minimization of heat losses in the connecting pipe can further improve the NOx conversion. Finally, the decrease of NO oxidation activity in DOC due to PtOx formation and its impact on NOx conversion in SCR is quantified over the repeated driving cycles, showing 2–10% difference between the deNOx performance with the pre-reduced DOC and after few hours of operation under oxidizing conditions. Based on the simulation results, an improved configuration of the system is proposed that achieves up to 6% higher overall NOx conversion.

Bimetal MOF derived mesocrystal ZnCo2O4 on rGO with High performance in visible-light photocatalytic NO oxidization

A mesocrystal ZnCo2O4 on reduced graphene oxide (rGO) nano-composite was successfully synthesized by a low-temperature annealing of bimetal organic frameworks on rGO. In this nano-composite, rGO nanosheets worked as the two-dimensional support to significantly improve the dispersity and conductivity of nano-composites. The mesoporous ZnCo2O4 crystal derived from ZnCo-ZIF worked as the main active sites with the optimized Zn/Co ratio to ensure an ideal valence band structure as well as enhanced visible-light harvesting properties. The as-prepared rGO@ZnCo2O4 nano-composites showed a remarkable activity in visible-light photocatalytic NO oxidation with the conversion of 83.8% under visible light and 92.6% under simulated solar light. It also exhibited a higher stability than that of N-doped TiO2, thereby making this nano-composite as a promising and stable photocatalyst for environmental remediation.

Rational design, synthesis and evaluation of ZnO nanorod array supported Pt:La0.8Sr0.2MnO3 lean NOx traps

In this study, a new type of lean NOx traps (LNT) based on Pt:La0.8Sr0.2MnO3 (LSMO) nanoparticles decorated on ZnO nanorod array integrated monoliths have been successfully designed and demonstrated. Compared to the commercial catalyst, the nanorod array based catalyst demonstrates competitive NO oxidation activity, and exceptional hydrothermal stability and recoverability after sulfur poisoning. With uniform distribution of perovskite and Pt nanoparticles, the ZnO nanorod array retains its structure after long-time exposure to the reducing gaseous atmosphere. The H2 treatment induces the surface Mn4+ reduction to Mn3+ and subsequently to Mn2+, affecting oxygen mobility, and decreasing the amount of surface lattice oxygen on the LSMO perovskite, thus leading to the catalyst performance decay according to a Mars-Van Krevelen reaction mechanism. Such surface state change was found to be reversible, with the catalytic performance fully recovered after O2 re-treatment, making it suitable for NOx storage and reduction (NSR). The complete lean NOx traps are formulated with rationally-designed sequential BaCO3 and Pt loading, improving NOx storage capacity. The ZnO nanorod array supported LNT shows good water compatibility under simulated exhaust. The nanorod array catalysts may be potentially suitable for vehicular NOx emission control.