Activity during reduction of NO by CO over bimetallic palladium-rhodium/silica catalysts

A study of the activity of bimetallic Pd-Rh catalysts supported on silica in the reduction of NO by CO is presented. The catalysts were prepared by three different methods: (1) Pd and Rh were coimpregnated on the support, (2) Rh was impregnated first and, after calcining, the sample was impregnated with Pd, (3) the monometallic Pd and Rh catalysts were physically mixed. The results showed that the activity of the catalysts prepared by coimpregnation was much lower than that of the other two catalysts.     

Oxidation of carbon monoxide over barium cuprate catalysts

Catalytic activities of BaCuO₂, Ba₂Cu₃O₅ and CuO for CO oxidation were investigated. At 250 ° C, BaCuO₂ was found to be about 7 times more active than CuO, while Ba₂Cu₃O₅ was found to be only slightly more active than CuO. This result also demonstrates that expensive rare earth elements such as La and Y are not necessary for a cuprate to have good activity for CO oxidation. After sintering at 940 ° C in air, the conversion substantially decreased for CuO. At steady state, both barium cuprates exhibited higher activity than in the fresh state. Based on the absence of significant changes in the XRD spectra, the change in catalytic activity is attributed to changes at the surface and possibly slight reduction of Cu²⁺. Reaction orders of CO were found to be 1.2 and 0.3, and reaction orders of O₂ were found to be 0 and 0.3 for BaCuO₂ and Ba₂Cu₃O₅, respectively.     

Decomposition of nitric oxide over perovskite oxide catalysts: effect of CO2, H2O and CH4

Direct nitric oxide decomposition over perovskites is fairly slow and complex, its mechanism changing dramatically with temperature. Previous kinetic study for three representative compositions (La_0.87Sr_0.13Mn_0.2Ni_0.8O_3−δ, La_0.66Sr_0.34Ni_0.3Co_0.7O_3−δ and La_0.8Sr_0.2Cu_0.15Fe_0.85O_3−δ) has shown that depending on the temperature range, the inhibition effect of oxygen either increases or decreases with temperature. This paper deals with the effect of CO₂, H₂O and CH₄ on the nitric oxide decomposition over the same perovskites studied at a steady-state in a plug-flow reactor with 1 g catalyst and total flowrates of 50 or 100 ml/min of 2 or 5% NO. The effect of carbon dioxide (0.5–10%) was evaluated between 873 and 923 K, whereas that of H₂O vapor (1.6 or 2.5%) from 723 to 923 K. Both CO₂ and H₂O inhibit the NO decomposition, but inhibition by CO₂ is considerably stronger. For all three catalysts, these effects increase with temperature. Kinetic parameters for the inhibiting effects of CO₂ and H₂O over the three perovskites were determined. Addition of methane to the feed (NO/CH₄=4) increases conversion of NO to N₂ about two to four times, depending on the initial NO concentration and on temperature. This, however, is still much too low for practical applications. Furthermore, the rates of methane oxidation by nitric oxide over perovskites are substantially slower than those of methane oxidation by oxygen. Thus, perovskites do not seem to be suitable for catalytic selective NO reduction with methane.     

Selective catalytic reduction of nitric oxide by ethylene in the presence of oxygen over Cu ion-exchanged pillared clays

Cu²⁺ ion-exchanged pillared clays are substantially more active than Cu²⁺-ZSM-5 for selective catalytic reduction (SCR) of NO by hydrocarbons. More importantly, H₂O (or SO₂) has only mild effects on their activities. First results on Cu²⁺-exchanged TiO₂-pillared montmorillonite were reported by this laboratory (Yang and Li, Ref. [1]), that showed overall activities two to four times higher than Cu²⁺-ZSM-5.

A delaminated pillared clay was subjected to Cu²⁺ ion-exchange and studied for SCR by C₂H₄ in this work. The Cu²⁺ ion-exchanged delaminated Al₂O₃-pillared clay yielded substantially higher SCR rates than both Cu²⁺-exchanged TiO₂-pillared clay and Cu²⁺-ZSM-5 at temperatures above 400°C. The peak NO conversion was 90% at 550°C and at a space velocity of 15,000 h⁻¹ (with O₂ = 2%). The peak temperature decreased as the concentration of O₂ was increased. The macroporosity in the delaminated pillared clay was partially responsible for its higher peak temperatures (than that for laminated pillared clays). At 1000 ppm each for NO and C₂H₄, the NO conversion peaked at 2% O₂ for all temperatures. H₂O and SO₂ caused only mild deactivation, likely due to competitive adsorption (of SO₂ on Cu²⁺ sites and H₂O on acid sites). The high activity of Cu²⁺-exchanged Al₂O₃-pillared clay was due to a unique combination of the redox property of the Cu²⁺ sites and the strong Lewis acidity of the pillared clay. The suggested mechanism involved NO chemisorption (in the presence of O₂) on Cu²⁺OAl³⁺-on the pillars, and C₂H₄ activation on the Lewis acid sites to form an oxygenated species.     

Competition between NO reduction and NO decomposition over reduced V–W–O catalysts

The SCR of NO and NO decomposition were investigated over a V–W–O/Ti(Sn)O₂ catalyst on a Cr–Al steel monolith. The conversions of NO and NH₃ over the reduced and oxidised catalysts were determined. The higher conversion of NO than of NH₃ was observed in SCR over the reduced catalyst and very close conversions of both substrates were found over the oxidised one. The increase of the pre-reduction temperature was found to cause an increase in catalyst activity and its stability in direct NO decomposition. The surface tungsten cations substituted for vanadium ones in vanadia-like active species are considered to be responsible for the direct NO decomposition. The results of DFT calculations for the 10-pyramidal clusters: V₁₀O₃₁H₁₂ (V–V) and V₉WO₃₁H₁₂ (V–W) modelling (0 0 1) surfaces of vanadia and WO₃–V₂O₅ solid solution (s.s.) active species, respectively, show that preferable conditions for NO adsorption exist on W sites of s.s. species and that reduction causes an increase in their ability for electron back donation to the adsorbed molecule. Electron back donation is believed to be responsible for the electron structure reorganisation in the adsorbed NO molecule resulting in its decomposition. The high selectivity of NO decomposition to dinitrogen was considered to be connected with the formation of the tungsten nitrosyl complexes solely via the W–N bond.     

Differences in the structure of copper active sites for decomposition and selective reduction of nitric oxide with hydrocarbons and ammonia

Cu ion co-ordination-location in zeolites of MFI, erionite, mordenite matrices has been determined and the activity of the individual Cu sites compared for NO decomposition and its selective reduction by hydrocarbons or ammonia. It appears that Cu ions in the vicinity of one framework Al (site II), able to form stable Cu⁺-dinitrosyl complexes, and abundant in MFI structure, are responsible for high activity in NO decomposition. The Cu ions neighbouring two framework Al atoms (site I), and forming mostly mononitrosyl complexes, which dominate in erionite structure, provide a high activity in selective reduction of NO.     

Transient and resonant behavior for no reduction by CO over a Pt/Al2O3 catalyst during forced composition cycling

The reduction of NO by CO over a Pt/Al₂O₃ catalyst has been investigated using the technique of forced concentration cycling in an isothermal recycle reactor at 485 K. Time-average conversions exhibit resonant behavior with increasing frequency. Maximum time-average NO conversion of 78%, compared with the steady-state conversion of 3.8%, was attained during out-of-phase feed concentration cycling. The effect of the phase angle between the NO and CO feed cycles has been examined. Higher conversions are obtained by decreasing the NO phase lead below 180°. The convergence to cycle-invariance was slow for high frequency cycling.     

金属氧化物催化CO还原NO的研究进展

一氧化碳（CO）广泛存在于烧结/球团/焦化烟气或汽车尾气中,应用CO-选择性催化还原（SCR）技术同时脱除烟气中CO和NO是烟气治理的理想方案之一。目前,在NO-CO反应研究中较多的是贵金属催化剂,但由于其价格昂贵、高温失活、易中毒等问题难以在工业中实现应用。本文将近几年来金属氧化物催化CO还原NO的研究成果进行了系统的梳理与总结,重点介绍Fe基、Ce基、Co基、Cu基这4种金属氧化物催化剂的研究进展,分析催化剂的制备方法、掺杂助剂种类和比例、NO-CO反应条件等因素与催化活性之间的关系,总结催化剂抗水抗硫性能及可能的CO-SCR反应机理,并探讨O₂存在的条件下对催化剂活性的影响,为提高金属氧化物催化剂抗氧性研究提供理论参考。     

Reduction of nitric oxide with ammonia on silica-supported vanadium oxide catalysts

Highly active catalysts for the reduction of nitric oxide with ammonia can be obtained by supporting vanadium oxide, more than 15% by weight, on a silica gel with micropores of a mean diameter larger than 10 nm, followed by calcining it in the temperature range from 250 to 350°C. Both pre-impregnation with a small amount of titania and addition of ammonium bromide increased the activity markedly. The catalyst gave an NO conversion level of 100% at 150°C. A series of life tests of the catalysts, which was performed at 230°C using a simulated flue gas containing 700 parts/10⁶ of sulphur dioxide, demonstrated that their activities were stable for more than 300 h. Little irreversible change in the catalyst properties was observed after the test.     

The effect of particle size on nitric oxide decomposition and reaction with carbon monoxide on palladium catalysts

The effect of palladium particle size on its catalytic activity was investigated by the decomposition of chemisorbed nitric oxide and the reaction of nitric oxide with carbon monoxide in flow conditions. Palladium particles (30–500 Å) were prepared on silica thin films (100 Å) which were supported on a Mo(110) surface. The reactivity of the supported palladium varied with the metal particle size. On large palladium particles, nitric oxide (NO) reacts to form nitrous oxide (N₂O), dinitrogen (N₂) and atomic oxygen during temperature-programmed reaction, whereas on small particles (< 50 Å), nitrous oxide is not formed. Similarly, reactions of NO with CO on large particles, in flow conditions produce N₂O, N₂ and CO₂, whereas N₂O is not produced on small particles. In addition, more extensive NO decomposition is observed on the smaller particles.     

Catalytic reduction of NO by CO over NiO/CeO2 catalyst in stoichiometric NO/CO and NO/CO/O2 reaction

A new catalyst composed of nickel oxide and cerium oxide was studied with respect to its activity for NO reduction by CO under stoichiometric conditions in the absence as well as the presence of oxygen. Activity measurements of the NO/CO reaction were also conducted over NiO/γ-Al₂O₃, NiO/TiO₂, and NiO/CeO₂ catalysts for comparison purposes. The results showed that the conversion of NO and CO are dependent on the nature of supports, and the catalysts decreased in activity in the order of NiO/CeO₂ > NiO/γ-Al₂O₃ > NiO/TiO₂. Three kinds of CeO₂ were prepared and used as support for NiO. They are the CeO₂ prepared by (i) homogeneous precipitation (HP), (ii) precipitation (PC), and (iii) direct decomposition (DP) method. We found that the NiO/CeO₂(HP) catalyst was the most active, and complete conversion of NO and CO occurred at 210 °C at a space velocity of 120,000 h⁻¹. Based on the results of surface analysis, a reaction model for NO/CO interaction over NiO/CeO₂ has been proposed: (i) CO reduces surface oxygen to create vacant sites; (ii) on the vacant sites, NO dissociates to produce N₂; and (iii) the oxygen originated from NO dissociation is removed by CO.     

On the mechanism of nitric oxide decomposition over Cu-ZSM-5

Cu-ZSM-5, a copper-containing zeolite, catalytically decomposes NO at temperatures below those of other catalysts. A mechanism is proposed which is based on active sites consisting of coordinatively unsaturated cupric (Cu²⁺) ions in a square planar configuration. These sites are posited to chemisorb NO molecules in the gem-dinitrosyl form. The pair of adsorbed NO molecules desorbs as N₂ and O₂. This mechanism accounts for the experimental behavior in chemisorption and decomposition without invoking a cyclical oxyreduction of the surface sites.     

Reactivity of V2O5-WO3/TiO2 catalysts in the selective catalytic reduction of nitric oxide by ammonia

The physico-chemical characteristics and the reactivity of sub-monolayer V₂O₅-WO₃/TiO₂ deNO_x catalysts is investigated in this work by EPR, FT-IR and reactivity tests under transient conditions. EPR indicates that tetravalent vanadium ions both in magnetically isolated form and in clustered, magnetically interacting form are present over the TiO₂ surface. The presence of tungsten oxide stabilizes the surface V^IV and modifies the redox properties of V₂O₅/TiO₂ samples. Ammonia adsorbs on the catalysts surface in the form of molecularly coordinated species and of ammonium ions. Upon heating, activation of ammonia via an amide species is apparent. V₂O₅-WO₃/TiO₂ catalysts exhibits higher activity than the binary V₂O₅/TiO₂ and WO₃/TiO₂ reference sample. This is related to both higher redox properties and higher surface acidity of the ternary catalysts. Results suggest that the catalyst redox properties control the reactivity of the samples at low temperatures whereas the surface acidity plays an important role in the adsorption and activation of ammonia at high temperatures.     

Catalytic reduction of nitric oxide by methane over CaO catalyst

The selective catalytic reduction of nitric oxide by methane was studied over CaO catalyst in a bubbling fluidized bed in the temperature range of 800–900 °C, in which NO cannot be reduced by CH₄ without CaO catalyst. The nitric oxide conversion was found to depend on oxygen and CH₄ feed concentration, and also on temperature. In addition, the presence of water vapors in the flue gas enhanced the NO reduction admirably well in the absence of O₂. But water vapor has an inhibiting effect on the reaction while O₂ is present in the flue gas. The addition of CO₂ poisoned the CaO catalyst and exhibited a detrimental effect on NO conversion at the working temperature range, 800–900 °C. However, with a temperature rise to 900 °C the CO₂ poisoning effect on NO reduction was weakened. The mechanism was studied and discussed according to the references in the paper. This work was presented at the 6 ^th Korea-China Workshop on Clean Energy Technology held at Busan, Korea, July 4–7, 2006.     

Hydrogenation of CO and CO2 on carbon black-supported Ru catalysts

The activity and selectivity of both unsupported Ru and carbon black-supported Ru catalysts toward the hydrogenation of CO and CO₂ have been investigated in order to learn about the effect of metal particle size on both reactions. The specific activity for both reactions decreases with metal particle size and the product distribution obtained in the hydrogenation of CO (the hydrogenation of CO₂ only yields methane) is also a function of metal dispersion; thus, the proportion of methane produced increases and the olefin/paraffin decreases with decreasing Ru particle size. This behaviour is attributed to changes in the electronic properties of the Ru crystallites in close contact with the graphitised carbon black used as support.     

NO reduction by CO over automotive exhaust gas catalysts in the presence of O2

The reduction of NO by CO in absence and presence of O₂ has been investigated by transient experiments at automotive cold-start conditions over Pt/Rh/CeO₂/-Al₂O₃, and derived model catalysts. A high-resolution magnetic sector mass spectrometer was used for distinguishing CO/N₂ and CO₂/N₂O. Mechanistic comparisons are made between the catalyst formulations. A kinetic scheme of elementary reaction steps is proposed, which highlights the various contributions of the catalyst constituents.     

Vanadium loaded carbon-based catalysts for the reduction of nitric oxide

Carbon-based SCR catalysts for the reduction of NO with NH₃ at low temperatures have been prepared using activated carbons obtained from a local Spanish coal, doped with several vanadium compounds. Among them, the ashes of a petroleum coke (PCA) were also employed. Both the catalysts and the carbon supports have been characterized by means of N₂ and CO₂ physisorption, NH₃ and O₂ chemisorption and temperature programmed desorption (TPD). The activity of the catalysts has been tested in a laboratory-scale unit, measuring significant conversions of NO (above 50%) with almost 100% selectivity toward N₂ at 150 °C. The feasibility of using the petroleum coke ashes as the active phase was confirmed comparing the activity of the catalysts doped with these residues, with the one measured for the catalysts prepared using model vanadium compounds. The physical–chemical features of the carbon support resulted of key importance for achieving a considerable catalytic activity. The values of apparent energy of activation calculated for the catalysts presented in this paper were very similar to other carbon-based catalysts and smaller than the ones corresponding to TiO₂-supported systems. The gas residence time on the catalytic bed influences the catalytic activity to a great extent, thus being a determinant parameter for designing the SCR de-NO_x unit. To avoid ammonia slip, inlet concentrations of NH₃ has to be little under the stoichometric NH₃/NO ratio (0.7). The catalysts stability was tested in terms of carbon support gasification followed by termogravimetric analysis and gas chromatography. The activity of the catalysts was maintained at least over 24 h of reaction.     

Mesoporous Fe-containing ZSM-5 zeolite single crystal catalysts for selective catalytic reduction of nitric oxide by ammonia

Mesoporous and conventional Fe-containing ZSM-5 catalysts (0.5–8 wt% Fe) were prepared using a simple impregnation method and tested in NO selective catalytic reduction (SCR) with NH₃. It was found that mesoporous Fe-ZSM-5 catalysts exhibit higher SCR activities than comparable conventional catalysts. Furthermore, conventional Fe-ZSM-5 catalysts have maximum activity at ~2.5 wt% Fe while for the mesoporous system, optimal NO conversion is obtained for the catalysts with ~6 wt % Fe.     

Highly dispersed MgO-supported model Pd-Mo catalysts prepared from bimetallic clusters: Chemisorption and selective catalytic reduction of NO

Model supported palladium and palladium-molybdenum catalysts prepared from organometallic precursors and previously characterized by a variety of chemical and physical methods were examined by IR spectroscopy of NO chemisorption and tested for their activities as catalysts in the competitive NO + CO + O₂ reaction. The IR results reveal distinctive behavior of the catalyst made from a bimetallic precursor, and the activity results show that this catalyst is more selective for NO reduction than the other catalysts, but its stability is vulnerable to the reaction conditions. The high selectivity is attributed to Pd-Mo interactions, which are inferred to be stronger in the catalyst prepared from a bimetallic precursor than in catalysts prepared from monometallic precursors.