Catalytic reduction of nitric monoxide by ethene over Ag/TiO2 in the presence of excess oxygen

Selective reduction of nitric oxide (NO) by ethene in the presence of excess oxygen was investigated using a silver supported on TiO₂ (Ag/TiO₂) catalyst. Ag/TiO₂ showed high catalytic activity for the reduction of NO to N₂ and N₂O. The activity for the reduction of NO to N₂ and N₂O was enhanced with an increase up to 3 wt.% Ag loading level. On increasing the concentration of ethene, the catalytic activity for the reduction of NO to N₂ and N₂O was enhanced. The reduction of NO over Ag/TiO₂ catalyst never proceeds without coexistent oxygen.     

In situ FT-IR studies of NO decomposition on Pt/TiO2 catalyst under UV irradiation

Photodecomposition of NO on the well-dispersed Pt/TiO₂ catalyst under UV irradiation was studied by in situ DRIFT (Diffuse-Reflectance Infrared Fourier-Transform) spectroscopy. 2 wt% Pt/TiO₂ catalyst was prepared by photochemical deposition method. The photocatalytic activity of Pt/TiO₂ is highly dependent on its pretreatment. Although the catalyst exhibited a highly adsorption capability to NO after hydrogen reduction or thermal evacuation at 500°C, no evidence upon NO decomposition was observed under UV irradiation. While reducing the catalyst at 300°C in the hydrogen flow, it not only exhibited an intense NO adsorption but also conducted a direct decomposition of NO to N₂ and O₂ under UV irradiation. The hydrogen reduction at 200°C led to a weaker NO adsorption. During UV irradiation, the IR peaks of NO fully disappeared and N₂O was formed. It is concluded that the photochemical prepared Pt/TiO₂ catalyst after activating at mild reduction conditions is highly active for NO photodecomposition. The effective oxidation states of the active components, the surface structure and the reaction mechanisms will be discussed.     

On the kinetics of catalytic reduction of nitrogen oxide by alkenes in oxidizing atmosphere

A kinetic model is presented which considers the catalytic reduction of NO to N₂ by hydrocarbons in oxidizing atmosphere (NO−HC−O₂) as the sum of two simultaneous competitive reactions: the red-ox reaction between NO and hydrocarbon (NO−HC) and the hydrocarbon oxidation (O₂−HC). The model is developed for alkenes employed as reductants and noble metal catalysts. The ratio of the kinetic constants of the two reactions can be considered as a selectivity index in evaluating the ability of a catalyst to favor the desired NO reduction with respect to the undesired O₂−HC oxidation.     

Catalytic reduction of nitrogen monoxide with propene over Nb/TiO2 mixed mechanically with Mn2O3

Selective catalytic reduction of nitrogen monoxide (NO) over a catalyst of mechanically mixed Nb/TiO₂ and Mn₂O₃ (Mn₂O₃+Nb/TiO₂) in an oxidizing atmosphere with propene (C₃H₆) was studied. The Mn₂O₃+Nb/TiO₂ catalyst showed high activity for the reduction of NO to N₂. The maximum conversion of NO to N₂ was observed at 200∼300°C, with about 80% reduction of NO to N₂. Mn₂O₃ enhanced the formation of NO₂ from NO and the activation of propene to react with NO₂ for reduction to N₂.     

Oxide Nanoparticles within a host microporous matrix: Polynuclear copper species in Cu-ZM5 and their role in the reduction of NO

The role of copper-oxide nanoparticles inside a host ZSM5 zeolite, identified by UV-Visible-NIR diffuse reflectance spectroscopy on the reactivity in NO reduction to N₂ in the presence of propane/O₂ or ammonia/O₂ is discussed. These species lead to an enhancement in the rate of alkane depletion in the reduction of NO in the presence of propane/O₂, whereas they cooperate with isolated ions at exchangeable sites in determining the overall catalytic behaviour in NO reduction in the presence of ammonia/O₂. Also the stability of these species depends on the type of reductant.     

Modelling reduction of nitric oxide by hydrogen over supported catalysts

A mean field model for NO oxidation with H₂ over supported catalysts is proposed and solved numerically. The model is composed of a system of PDEs subject to nonclassical conjugate conditions at the catalyst–support interface and includes the bulk diffusion of reactants and reaction products and surface diffusion of all intermediate products. The influence of the particle jumping rate constants via the catalyst–support interface and reaction rate constants on the evolution of the reactivity of the catalyst surface is investigated. It is shown that the conversion rates (turnover frequencies) of NO and H₂ into products, N₂, H₂O, NH₃, and N₂O, are nonmonotonous functions of time. The conversion rates of NO and H₂ into N₂ and N₂O can have one or two local maxima, while their conversion rates into H₂O and NH₃ can possess one, two, or three local maxima. The mechanism and conditions for arising of the second maximum are discussed and reaction steps that essentially increase the surface reactivity are indicated.

     

Ni-Ti-O混合氧化物的制备、表征及其富氧选择性催化还原NO

采用均匀共沉淀法制备了不同Ni/Ti摩尔比的Ni-Ti-O混合氧化物,考察了它们在富氧条件下丙烯选择性催化还原NO反应中的催化性能,并运用X射线衍射,N₂吸附-脱附、吡啶吸附、程序升温脱附和原位红外光谱对催化剂进行了表征.结果表明,Ni/Ti摩尔比为1的催化剂表现出最佳催化活性,430℃时NO_x转化率达68%.该催化剂具有锐钛矿结构,比表面积较高(149m²/g),有利于提高催化活性;其表面Lewis酸性位有利于硝酸盐物种的吸附,而硝酸盐物种是该反应的重要中间体.     

Fe/Al-PILC催化C_3H_6选择性还原NO的实验研究

采用浸渍法制备了负载于铝柱撑黏土的铁基催化剂(Fe/Al-PILC),在固定床反应器上测试其催化C3H6选择性还原NO的性能。通过N2吸附-脱附、X射线衍射(XRD)、H2的程序升温还原(H2-TPR)、紫外可见光谱(Uv-vis)、吡啶吸附红外光谱(Py-FTIR)等手段对催化剂的物理化学性质进行表征。结果表明,9Fe/Al-PILC在400-550℃能够还原98%以上的NO,而且SO2和水蒸气对其催化性能的影响很小。XRD、N2吸附-脱附表征结果表明,Fe/Al-PILC催化剂中铁氧化物高度分散在载体表面,催化剂有较大的比表面积和孔容。H2-TPR结果表明,催化剂的活性主要由Fe_2O_3物相的还原性能决定。Uv-vis结果表明,催化剂的活性与铁氧低聚物种FexOy呈正相关性。Py-FTIR结果表明,催化剂表面同时存在Lewis酸和Brnsted酸,L酸性位是NO和C3H6反应的主要催化活性中心。     

Catalytic reduction of U(VI) to U(IV) using hydrogen with platinum loaded on alumina and silica

During the reprocessing of spent nuclear fuel, uranium (U) and plutonium (Pu) are together extracted by employing tri-n-butyl phosphate (TBP)/dodecane mixture and their partitioning is achieved by adding uranous nitrate. The partitioning agent, uranous is conventionally produced by the electrolytic reduction of uranyl nitrate. An alternate route for the reduction of U from (VI) to (IV) using hydrogen (H₂) as reductant was developed using platinum (Pt) based catalyst. Improvements in the development of the catalyst have been carried out in order to reduce the requirement of Pt without affecting the reduction performance. Experiments using 2 wt% Pt loaded on alumina beads and alumina powder have been performed and results are discussed. As the catalyst supported on alumina was found to be unstable in acidic environment, Pt loaded on silica powder has also been developed. Pt loaded on alumina and silica substrates have been tried to envisage the reduction behaviour using H₂ as reductant in presence of hydrazine nitrate which acts as U(IV) stabiliser as well as reductant. Parametric studies have been carried out to optimise the process parameters namely pressure, temperature, U concentration, free acidity, hydrazine concentration and catalyst to U (C/U) ratio. 2 wt% Pt loaded on silica has been selected for further scale up studies for making uranous.     

Surface Engineering of g-C3N4 by Stacked BiOBr Sheets Rich in Oxygen Vacancies for Boosting Photocatalytic Performance

BiOBr containing surface oxygen vacancies (OVs) was prepared by a simple solvothermal method and combined with graphitic carbon nitride (g-C₃N₄) to construct a heterojunction for photocatalytic oxidation of nitric oxide (NO) and reduction of carbon dioxide (CO₂). The formation of the heterojunction enhanced the transfer and separation efficiency of photogenerated carriers. Furthermore, the surface OVs sufficiently exposed catalytically active sites, and enabled capture of photoexcited electrons at the surface of the catalyst. Internal recombination of photogenerated charges was also limited, which contributed to generation of more active oxygen for NO oxidation. Heterojunction and OVs worked together to form a spatial conductive network framework, which achieved 63 % NO removal, 96 % selectivity for carbonaceous products (that is, CO and CH₄). The stability of the catalyst was confirmed by cycling experiments and X-ray diffraction and transmission electron microscopy after NO removal.     

Microkinetic model for NO–CO reaction: Model reduction

The objective of this work is to elucidate controlling mechanisms in NO_x reduction, develop reduced‐order reaction models, and analyze the reactor performance using the reduced‐order reaction model for the NO–CO reaction. We start with the microkinetic model on platinum, which describes the mechanism of catalytic reduction of NO by CO. The formation of the main product N₂O and the competitive formation of the side product N₂ are accounted for in the microkinetic model. Sensitivity and reaction path analysis have been carried out to determine the rate‐limiting steps as well as the most abundant reactive intermediates in the system. Owing to the differences between system performance at high and low temperatures, the model has been analyzed in detail in these temperature regimes. Two closed‐form expressions, corresponding to the two global reactions involved, have been derived. The characteristic features of the microkinetic model such as the sharp increase in NO conversion and the selectivity to N₂O are captured well by the reduced model. The reduced‐order model has been extended to the rhodium catalyst as well. © 2012 Wiley Periodicals, Inc. Int J Chem Kinet 44: 577–585, 2012     

Surface Engineering of g‐C3N4 by Stacked BiOBr Sheets Rich in Oxygen Vacancies for Boosting Photocatalytic Performance

BiOBr containing surface oxygen vacancies (OVs) was prepared by a simple solvothermal method and combined with graphitic carbon nitride (g‐C₃N₄) to construct a heterojunction for photocatalytic oxidation of nitric oxide (NO) and reduction of carbon dioxide (CO₂). The formation of the heterojunction enhanced the transfer and separation efficiency of photogenerated carriers. Furthermore, the surface OVs sufficiently exposed catalytically active sites, and enabled capture of photoexcited electrons at the surface of the catalyst. Internal recombination of photogenerated charges was also limited, which contributed to generation of more active oxygen for NO oxidation. Heterojunction and OVs worked together to form a spatial conductive network framework, which achieved 63 % NO removal, 96 % selectivity for carbonaceous products (that is, CO and CH₄). The stability of the catalyst was confirmed by cycling experiments and X‐ray diffraction and transmission electron microscopy after NO removal.     

Enzymatic and catalytic reduction of dinitrogen to ammonia: Density functional theory characterization of alternative molybdenum active sites

We used density functional calculations to model dinitrogen reduction by a FeMo cofactor containing a central nitrogen atom and by a Mo‐based catalyst. Plausible intermediates, reaction pathways, and relative energetics in the enzymatic and catalytic reduction of N₂ to ammonia at a single Mo center are explored. Calculations indicate that the binding of N₂ to the Mo atom and the subsequent multiple proton–electron transfer to dinitrogen and its protonated species involved in the conversion of N₂ are feasible energetically. In the reduction of N₂ the Mo atom experiences a cycled oxidation state from Mo(IV) to Mo(VI) by nitrogenase and from Mo(III) to Mo(VI) by the molybdenum catalyst, respectively, tuning the gradual reduction of N₂. Such a wide range of oxidation states exhibited by the Mo center is crucial for the gradual reduction process via successive proton–electron transfer. Present results suggest that the Mo atom in the N‐centered FeMo cofactor is a likely alternative active site for dinitrogen binding and reduction under mild conditions once there is an empty site available at the Mo site. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2005     

Effect of the composition and method of preparation of iron-containing and cobalt-containing catalysts on the combined reduction of NO and N<Subscript>2</Subscript>O by hydrocarbons

Binary iron-containing and cobalt-containing catalysts derived from the hydrogen form of Pentasil, zirconium dioxide, and binary supports (ZrO₂ + zeolite) display significant activity in the combined reduction of nitrous oxide and nitric oxide by C₁ and C₃-C₄ alkanes, including conditions for the selective catalytic reduction of NO + N₂O. The activity of the catalyst in the conversion of NO and N₂O depends on the method of preparation of the binary composites and the amount of the cobalt and iron components.     

Fe- and Cu-oxides supported on γ-Al2O3 as catalysts for the selective catalytic reduction of NO with ethanol. Part I: catalyst preparation,characterization, and activity

Fe- and Cu-oxides supported on γ-alumina (γ-Al₂O₃; metal loading of 3 mass %) were investigated as alternative catalysts to the conventional Ag-based system in the selective catalytic reduction of NO with ethanol (EtOH-SCR). The catalysts were characterized by elemental analysis, N₂ sorption, X-ray diffraction, temperature-prgrammed desorption of NH₃, temperature-programmed reduction with H2, diffuse reflectance UV-VIS (DR-UV-VIS) spectroscopy, and compared with 3 mass % Ag/γ-Al₂O₃ as a reference catalyst. Catalytic experiments were carried out between 423 K and 773 K in the steady state and by temperature-programmed surface reaction (TPSR) experiments. For all catalysts, the highest NO conversion (900 ppm (ppm = parts of the mixture component per million parts of all mixture components) NO, 900 ppm EtOH, 0.5 vol. % H₂O, 4 vol. % O₂ in He) was found at 573 K. While 84 % of NO were converted over the Ag-based catalysts, only 20–60 % NO conversion was observed for the Fe- and Cu-containing catalysts. Total oxidation of ethanol as an unwanted side reaction occurs over 3 mass % Cu on γ-Al₂O₃ already at 573 K, whereas the highest activity of 3 mass % Fe on γ-Al₂O₃ for this conversion was reached at 743 K. For lower temperatures, partial oxidation of ethanol leads to organic by-products which can act as active intermediates in EtOH-SCR. TPSR experiments show that ethanol reacts over both the Fe- and the Cu-based catalysts to organic by-products, such as ethene or acetaldehyde, which affect the EtOH-SCR reaction.     

Biomimetic and microbial reduction of nitric oxide

The biomimetic reduction of nitric oxide (NO) to nitrous oxide (N₂O) by dithiothreitol in the presence of cyanocobalamin and cobaltcentered porphyrins has been investigated. Reactions were monitored directly using Fourier Transform Infrared (FTIR) Spectroscopy vapor-phase spectra. Reaction rates were twofold faster for the corrin than for the cobalt-centered porphyrins. The stoichiometry showed the loss of two molecules of NO per molecule of N₂O produced. We have also demonstrated that the facultative anaerobe and chemoautotroph,Thiobacillus denitrificans, can be cultured anoxically in batch reactors using NO as a terminal electron acceptor with reduction to elemental nitrogen (N₂). We have proposed that the concentrated stream of NO_x, as obtained from certain regenerable processes for the gas desulfurization and NO_x removal, could be converted to N₂ for disposal by contact with a culture ofT. denitrificans. Four heterotrophic bacteria have also been identified that may be grown in batch cultures with succinate, yeast extract, or heat and alkali pretreated sewage sludge as carbon and energy sources and NO as a terminal electron acceptor. These areParacoccus dentrificans, Pseudomonas denitrificans, Alcaligens denitrificans, andThiophaera pantotropha.     

富氧条件下乙炔选择催化还原NOx

Acetylene as a reducing agent of metal exchanged HY catalysts, for selective catalytic reduction of NO in the reaction system of 0.16% NO, 0 （C2H2-SCR） was investigated over a series 08% C2H2, and 9.95% O2 （volume percent） in He. 75% of NO conversion to N2 with hydrocarbon efficiency about 1.5 was achieved over a Ce-HY catalyst around 300 ℃. The NO removal level was comparable with that of selective catalytic reduction of NOx by C3H6 reported in literatures, although only one third of the reducing agent in carbon moles was used in the C2H2-SCR of NO. The protons in zeolite were crucial to the C2H2-SCR of NO, and the performance of HY in the reaction was significantly promoted by cerium incorporation into the zeolite. NO2 was proposed to be the intermediate of NO reduction to N2, and the oxidation of NO to NO2 was rate-determining step of the C2H2-SCR of NO over Ce-HY. The suggestion was well supported by the results of the NO oxidation with O2, and the C2H2 consumption under the conditions in the presence or absence of NO.     

Effect of Fe-zeolite on formation of N<Subscript>2</Subscript>O in selective reduction of NO by NH<Subscript>3</Subscript> over V<Subscript>2</Subscript>O<Subscript>5</Subscript>–WO<Subscript>3</Subscript>/TiO<Subscript>2</Subscript> catalyst

An approach for significantly suppressing N₂O formation in reduction of NO by NH₃ over V₂O₅–WO₃/TiO₂ (VWT) catalyst has been studied by coating different amounts of a Fe-exchanged zeolite (FeZ) onto the catalyst. FeZ-promoted VWT samples were characterized using N₂ sorption, X-ray diffraction (XRD) analysis, and NH₃ adsorption/desorption techniques to understand the primary role of FeZ in lowering N₂O production levels. At high temperatures (≥450 °C), VWT gave N₂O production with high concentrations, while N₂O formation was noticeably reduced when using FeZ-promoted catalysts, which also showed somewhat lower NO removal activities (<5 %) at all temperatures. N₂ sorption and XRD measurements revealed no perceptible physical or chemical alterations of each constituent, even in VWT catalysts after FeZ coating following high-temperature calcination. Adsorption of NH₃ on unpromoted and FeZ-promoted catalysts and subsequent desorption yielded very complicated spectra for N₂O that might primarily come from NH₃ oxidation, and the interaction between V–NO species at temperatures >580 °C. NO on neighboring sites seems to be produced via decomposition of N₂O generated at lower temperatures. The FeZ in the promoted VWT catalysts could be responsible for N₂O decomposition and N₂O reduction with unreacted NH₃ at temperatures >400 °C, thereby significantly lowering N₂O emission levels. This promotional effect bodes well for use in many industrial deNO _x applications.     

Temperature programmed reduction of unsupported molybdena and vanadia catalysts by ammonia and hydrogen

Temperature-programmed reduction (TPR) is a valuable tool for the characterisation of catalysts. The reductant mostly used is hydrogen. Hydrogen reduces the catalyst by producing water. The amount of hydrogen reacted is measured by menas of a thermal conductivity detector. In this paper the temperature-programmed reduction of vanadia and molybdena catalysts by ammonia is emphasized. During the temperature-programmed reduction ammonia is consumed. The compounds formed in this process are detected by means of a mass spectrometer. It was found that almost exclusively N₂, H₂O and H₂ were formed. Hydrogen formed by the decomposition of ammonia over vanadia and molybdena above 600° produces hydroxyl groups, which are a source for the formation of water at temperatures above 600°.NH₃-TPR gives more relevant information than H₂-TPR for processes in which ammonia is used as a reactant.     

Experimental and kinetic modeling of the reduction of NO by isobutane in a Jsr at 1 atm

A kinetic study of the reduction of nitric oxide (NO) by isobutane in simulated conditions of the reburning zone was carried out in a fused silica jet‐stirred reactor operating at 1 atm, at temperatures ranging from 1100 to 1450 K. In this new series of experiments, the initial mole fraction of NO was 1000 ppm, that of isobutane was 2200 ppm, and the equivalence ratio was varied from 0.75 to 2. It was demonstrated that for a given temperature, the reduction of NO is favored when the temperature is increased and a maximum NO reduction occurs slightly above stoichiometric conditions. The present results generally follow those reported in previous studies of the reduction of NO by C₁ to C₃ hydrocarbons or natural gas as reburn fuel. A detailed chemical kinetic modeling of the present experiments was performed using an updated and improved kinetic scheme (979 reversible reactions and 130 species). An overall reasonable agreement between the present data and the modeling was obtained. Furthermore, the proposed kinetic mechanism can be successfully used to model the reduction of NO by ethylene, ethane, acetylene, a natural gas blend (methane‐ethane 10:1), propene, and HCN. According to this study, the main route to NO reduction by isobutane involves ketenyl radical. The model indicates that the reduction of NO proceeds through the reaction path: iC₄H₁₀ → C₃H₆ → C₂H₄ → C₂H₃ → C₂H₂ → HCCO; HCCO + NO → HCNO + CO and HCN + CO₂; HCNO + H → HCN → NCO → NH; NH + NO → N₂ and NH + H → followed by N + NO → N₂; NH + NO → N₂O followed by N₂O + H → N₂. © 2000 John Wiley & Sons, Inc. Int J Chem Kinet 32: 365–377, 2000