Role of surface structures on the selective reduction of nitrogen oxides with hydrocarbons on oxide catalysts

The infrared spectrum in situ was used to study the mechanism of selective reduction of nitrogen oxides on a series of oxide catalysts. It is shown that under the reaction conditions, nitrites, nitrates, and nitroorganic and acetate complexes are formed. Qualitative comparison of the rate of decomposition of the complexes and the rate of formation of the process products was performed using the spectral kinetic technique. Based on this comparison, the role of surface complexes in the process was determined. A mechanism showing the main stages of the reduction of nitrogen oxides is proposed.     

Snx.Ti1‐xO2 solid solution catalysts for nitrogen oxide selective catalytic reduction by propene in presence of oxygen

A series of SnO₂‐TiO₂ binary oxide catalysts prepared by co‐current precipitation method was found to be a novel and good system for the selective catalytic reduction of NO by propene in the presence of oxygen with high activity and good selectivity to N₂. The NO conversion to N₂ over SnO₂‐TiO₂ oxide catalysts varied with SnO₂ content and attainted a maximum at 65% over the catalyst with SnO₂ content at 40wt% for a feed with 1186 ppm NO, 948 ppm propene and 2.23% O₂, in He and a space velocity of 15000 h^?1 at 350°C. The SnO₂TiO₂ catalysts could sustain moderate activity in the presence of 10% steam. Because of the identical valence and the similar radius of Sn⁴⁺ and Ti⁴⁺, SnO₂‐TiO₂ binary oxides can form solid solution in three different phases as proved by XRD, electron diffraction and TPR. Sn⁴⁺ is the main active species in the SnO₂‐TiO₂ catalysts, and it is enriched on the solid solution surface as tested by XPS analysis. H₂‐TPR, NH₃‐TPD and BET tests show that SnO₂‐TiO₂ solid solution can dilute SnO₂ and suppress the activity of propene complete oxidation over SnO₂. This may be beneficial to the reactivity enhancement of NO conversion.     

Metal oxide promoted TiO2 catalysts for photo-assisted selective catalytic reduction of NO with NH3

The photo-assisted selective catalytic reduction (SCR) of NO with NH₃ (Photo-SCR) was performed over TiO₂ modified by supporting 1 wt% of various transition metal (V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Y, Zr, Nb, Mo, Ta or W) oxides aiming at the improvement of the photo-SCR activity. The addition of Nb, Mo or W oxide to TiO₂ was found to enhance the photo-SCR activity. We have reported that the amount of acid sites on TiO₂ is one of the key factors to the photo-SCR activity. The increase in the activity depends on the enhancement of acidity of catalyst by the addition of Nb, Mo or W oxide. In contrast, the addition of V, Cr, Mn, Fe, Co, Ni or Cu oxide to TiO₂ lowered the photo-SCR activity, although addition of metal cations also changed the acidity of TiO₂. We guess that the reduction of the activity was caused by two reasons; the first is that the sites newly formed on these transition metal oxides is not photoactive and the second is that TiO₂ supporting V, Cr, Mn, Fe, Co, Ni or Cu oxides had low stabilities under the reaction conditions, i.e., the chemical state of the cations changed during the reaction. Therefore, we concluded that the increase in the acid sites that are active sites for photo-SCR and the stability of the catalysts are important for the photo-SCR.     

The structure and catalytic activity of anatase and rutile titania supported manganese oxide catalysts for selective catalytic reduction of NO by NH3

The structure and catalytic properties of anatase and rutile supported manganese oxide catalysts prepared by impregnation method have been studied by using X-ray diffraction (XRD), laser Raman spectroscopy (LRS), X-ray photoelectron spectroscopy (XPS), H(2) temperature-programmed reduction (H(2)-TPR) and BET surface area measurements combined with activity testing of selective catalytic reduction (SCR) of NO by NH(3). It has been shown that the manganese oxide loadings on the two TiO(2) supports exert great influences on the SCR activity. For the rutile supported manganese oxide catalysts, increasing manganese oxide loading leads to the increase of reducibility of dispersed manganese oxide species and the rate constant k, which reaches a maximum around 9.6 × 10(-6) mol g(Mn)(-1) s(-1) at 0.5 mmol Mn per 100 m(2) TiO(2). When the manganese oxide loading is beyond this value, the existence of amorphous MnO(x) multiple layers will certainly reduce the ratio of manganese oxide species exposed on the surface and the reducibility of dispersed manganese oxide species, resulting in the rapid decrease of rate constant k. The LRS and XPS results have revealed that for the anatase supported manganese oxide catalysts manganese oxide species exist in Mn(+4) as a major species with Mn(+3) species and partially undecomposed Mn-nitrate as the minor species. Under the SCR reaction conditions, Mn(+3) species on anatase are oxidized to Mn(+4) species, inserting in the surface of anatase and promoting the anatase-to-rutile transformation in the surface layers of the anatase support. Since these Mn(4+) cations are actually dispersed on the support with a rutile shell-anatase core structure and its concentration is very near to that of MnO(x)/TiO(2) (R) catalyst, the relation between the rate constant k and the MnO(x) loading on the anatase support is similar to that on the rutile support, and that the rate constant k values for anatase and rutile supported manganese oxide catalysts are very close at the same MnO(x) loading.     

环境友好的选择性催化还原氮氧化物催化剂

富氧条件下的氮氧化物（NO。）选择性催化还原（SCR）是目前环境催化领域的研究热点,其核心问题是研发环境友好、高效稳定的SCR催化剂．目前,以NH3为还原剂的NH3-SCR已经大规模应用于固定源烟气脱硝和柴油车尾气净化,以碳氢化合物（Hc）为还原剂的HC-SCR也有望实际应用．针对NH3-SCR,本文以作者研究的铁钛复合氧化物催化剂、铈基氧化物催化剂以及国际上新兴的小孔分子筛催化剂为例,从催化剂结构、SCR反应机理、催化剂低温活性改进以及抗中毒性能等诸多方面对该领域的研究进展做了较为全面的论述．针对HC-SCR催化剂,本文在综述长链HC及柴油选择性还原NQ研究现状的基础上,结合作者在HC-SCR反应机理方面的研究成果,展望了实现柴油-SCR的发展方向．     

Kinetics of ethylene polymerization reactions with chromium oxide catalysts

Principal kinetic data are presented for ethylene homopolymerization and ethylene/1‐hexene copolymerization reactions with two types of chromium oxide catalyst. The reaction rate of the homopolymerization reaction is first order with respect to ethylene concentration (both for gas‐phase and slurry reactions); its effective activation energy is 10.2 kcal/mol (42.8 kJ/mol). The r₁ value for ethylene/1‐hexene copolymerization reactions with the catalysts is ～30, which places these catalysts in terms of efficiency of α‐olefin copolymerization with ethylene between metallocene catalysts (r₁ ～ 20) and Ti‐based Ziegler‐Natta catalysts (r₁ in the 80–120 range). GPC, DSC, and Crystaf data for ethylene/1‐hexene copolymers of different compositions produced with the catalysts show that the reaction products have broad molecular weight and compositional distributions. A combination of kinetic data and structural data for the copolymers provided detailed information about the frequency of chain transfer reactions for several types of active centers present in the catalysts, their copolymerization efficiency, and stability. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 5315–5329, 2008     

Nitrogen doped tin oxide nanostructured catalysts for selective electrochemical reduction of carbon dioxide to formate

Tin/tin oxide materials are key electrocatalysts for selective conversion of CO_2 to formate/formic acid.Herein we report a tin oxide material with nitrogen doping by using ammonia treatment at elevated temperature. The N doped material demonstrated enhanced electrocatalytic CO_2 reduction activity, showing high Faradaic efficiency(90%) for formate at -0.65 V vs. RHE with partial current density of 4 mA/cm~2.The catalysis was contributed to increased electron negativity of N atom compared to O atom. Additionally, the N-doped catalyst demonstrates sulfur tolerance with retained formate selectivity. The analysis after electrolysis shows that the catalyst structure partially converts to metallic Sn, and thus the combined Sn/N-SnO_2 is the key for the active CO_2 catalysis.     

Catalytic properties of chromites with a spinel structure in the oxidation of CO and hydrocarbons and reduction of nitrogen oxides

The catalytic activity of supported chromites MCr₂O₄/-Al₂O₃ (M = Cu, Co, Mn, Zn, Mg) in the oxidation of CO, C₃H₆, and o-xylene and NO_x reduction was studied. The catalytic activity depends on the calcination temperature and cation nature. The features of the formation of the catalysts were studied by the UV-Vis diffuse reflectance and IR spectroscopies.     

Ag(x)/ZSM-5催化剂的CH4-SCR脱硝性能

采用浸渍法制备Ag(x)/ZSM-5(x=3、6、9)催化剂。采用XRD、SEM、NH_3-TPD、Py-FTIR、XPS和NO-TPD等手段对催化剂的理化性质进行表征,在常压固定床微型反应器中评价催化剂甲烷选择性催化还原(CH_4-SCR) NO催化性能,考察Ag负载量对Ag(x)/ZSM-5催化剂CH_4-SCR脱硝性能的影响。结果表明,ZSM-5分子筛负载Ag,催化剂的酸性和酸量发生变化,改善了催化剂对NO的吸附脱附性能。随着Ag负载量增加,形成较大的Ag晶粒,有利于甲烷活化,Ag(x)/ZSM-5催化剂CH_4-SCR脱硝活性提高。Ag(9)/ZSM-5催化剂CH_4-SCR脱硝性能较好,在350℃时NO转化率为41.87%。     

The determination of ammonia in flue gas from the selective catalytic reduction of nitric oxide with ammonia

The emission of NO_x from coal-fired boilers can be limited by means of the selective catalytic reduction of NO_x with ammonia. The amounts of unreacted ammonia downstream should be low to avoid processing and environmental problems. Continuous measurement of the ammonia in the flue gas is needed. The determination of ammonia and flue gas sampling techniques are discussed. Measurements of ammonia in exhausts of a laboratory reactor and of a pilot plant for the selective catalytic reduction of NO_x with ammonia are presented. Ammonia was determined by mass spectrometry and chemiluminescence in the gas phase, and by spectrophotometric (Nessler and Berthollet reactions) or potentiometry in aqueous solution, in low (<5 μl l^?1) and high (<1000 μl l^?1) concentration ranges.     

Kinetic study of ethylene polymerization with chromium oxide catalysts by a radiotracer technique

The quenching of polymerization with a chromium oxide catalyst by radioactive methanol ¹⁴CH₃OH enables one to determine the concentration of propagation centers and then to calculate the rate constant of the propagation. The dependence of the concentration of propagation centers and the polymerization rate on reaction time, ethylene concentration, and temperature was investigated. The change of the concentration of propagation centers with the duration of polymerization was found to be responsible for the time dependence of the overall polymerization rate. The propagation reaction is of first order on ethylene concentration in the pressure range 2–25 kg/cm². For catalysts of different composition, the temperature dependence of the overall polymerization rate and the propagation rate constant were determined, and the overall activation energy E_ov and activation energy of the propagation state E_p were calculated. The difference between E_ov and E_p is due to the change of the number of propagation centers with temperature. The variation of catalyst composition and preliminary reduction of the catalyst influence the shape of the temperature dependence of the propagation center concentration and change E_ov.     

Cooperation of catalytic species for the selective reduction of nitrogen monoxide with hydrocarbons

Studies on the cooperation effect of catalytic species for the selective reduction of NO with hydrocarbons under oxidizing con-ditions have been reviewed. Based on the catalyst form, the effect can be classified into multiple-stage catalysts, mechanical or physi-cal mixture catalysts, and uniform catalysts with two or more catalytic components. Broadening of the temperature window for NO reduction by using two or more catalysts with different windows in series is an example for the first case. For the other two cases, bifunctionality of catalytic species is expected since the reaction is considered to proceed via a series of sequential and parallel steps. Actually, the promotion of NO reduction activity has been reported by the addition of catalysts or catalytic species having NO oxi-dation activity, where the NO oxidation step was slow. Another example is the combination of acidity and metal species to create NO reduction activity.     

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.

     

Zn-La复合氧化物催化剂用于甲醇与碳酸乙烯酯酯交换反应的研究

ZnO、La₂O₃和Zn-La复合氧化物催化剂用于甲醇与碳酸乙烯酯反应制备碳酸二甲酯和乙二醇。催化剂采用共沉淀法进行制备,并用BET、XRD、TG-DSC、CO₂-TPD和Hammett滴定等对催化剂进行表征。考察了Zn-La物质的量比、焙烧温度,反应条件（反应温度、反应时间、催化剂用量等）对催化剂活性的影响。结果表明,ZnLa复合氧化物物质的量比为2：1,焙烧温度为500℃时,催化剂表现了较好的催化效果。催化剂的活性与催化剂表面的碱性强度和碱量有关,碱量越多催化剂的活性越好。     

Interaction of nitrogen monoxide with alumina-iron oxide catalysts in chemisorption-regeneration regime

IR spectroscopy and thermodesorption have been used to study the surface reactions taking place in chemisorption of NO and NO +02 mixtures on alumina-iron oxide catalysts and palladized alumina-iron oxide catalysts, as well as the reactions that take place upon subsequent heating of these catalysts in helium or in the presence of hydrogen. The role of catalyst promotion by palladium in these reactions is examined critically.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 32, No. 6, pp. 362–366, November-December, 1996.     

Mechanism of reduction of nitrogen oxides with propene in excess oxygen catalyzed by bifunctional catalysts

HC-SCR (selective reduction of nitrogen oxide by hydrocarbon) and our view on its reaction mechanism are briefly described, and then our attempts in the last few years to design catalysts based on the concept of bifunctional catalysis for HC-SCR are reviewed. The example chosen is the NO-C₃H₆-O₂-H₂O reaction catalyzed by mechanical mixtures of transition metal oxides (e.g., Mn₂O₃) and metal-loaded ZSM-5 zeolites. The synergistic effect of two components was explained based on a reaction mechanism comprising NO oxidation, oxidative decomposition of the products of NO₂-C₃H₆ reactions and polymerization of (or carbon deposition from) C₃H₆.     

Catalytic reduction of nitrogen oxides via nanoscopic oxide catalysts within activated carbons at room temperature

Cabin air filters consisting of activated carbon infiltrated with nanoscopic metal oxide particles as catalysts have been investigated for the reduction of nitrogen oxides within motor-car cabins. In that concept, nitrogen dioxide is adsorbed on the activated carbon during operation conditions of the car and then reduced by the catalysts within the pores. The conversion has to take place at ambient temperature during the relatively long standstill periods of motor-cars. In this article we are going to discuss the manufacturing of the adsorbents by “liquid phase infiltration” and their characterization by techniques, such as nitrogen sorption analysis, X-ray diffraction, thermogravimetry, energy dispersive X-ray spectroscopy, and electron microscopy. The new adsorbents were evaluated in repeated breakthrough tests using NO₂ (4 ppm_V as feed concentration) in humid air as the adsorptive. In the intermittent rest periods of varying duration the volume flow through the fixed bed of adsorbent was stopped. The measured breakthrough curves indicate a catalytic conversion of the nitrogen dioxide in the filter beds.     

Molybdenum complexes as catalysts for the reduction of molecular nitrogen in protic media

N₂-fixing systems in protic media consisting of different reducing agents and molybdenum complexes (initially Mo(V)) as N₂ activators form the same Mo(III) carbonyl upon replacing N₂ by carbon monoxide. When CO is removed from the carbonyl, the Mo(III) complex formed becomes again capable of catalyzing the reduction of N₂ to hydrazine by Ti(III) or Cr(II) hydroxide in aqueous alcohol. Sodium amalgam forms a particularly active system, reducting N₂ catalytically in the presence of a Mo(III) complex already at room temperature.

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