Combined plasma-catalytic processing of nitrous oxide

The gliding arc discharge, combined with a catalytic bed, has been applied for nitrous oxide processing in oxygen containing gases. It has been found that under conditions of the gliding arc, nitrous oxide in mixtures with oxygen or air not only decomposes to oxygen and nitrogen, but is also oxidised to nitric oxide. The overall conversion of nitrous oxide, as well as the degree of N₂O oxidation to NO were studied as a function of its initial concentration, flow rate, and discharge power. The overall N₂O conversion and degree of oxidation to NO decreased with increasing flow rate and initial N₂O concentration, and increased with increasing discharge power. The degree of N₂O oxidation to NO varied within 20–37%. The overall conversion and degree of N₂O oxidation increased when granular dielectric materials (TiO₂, SiO₂ (quartz glass), and γ-Al₂O₃) were introduced into the reaction zone. The energy efficiency and the overall conversion of N₂O were still further increased due to catalytic effects of a number of metal oxides (CuO, NiO, MnO₂, Fe₂O₃, Co₃O₄, ZrO₂) deposited on γ-Al₂O₃. The activity of the oxide catalysts within the active power range of 300–360 W decreased in the order: CuO>Fe₂O₃>NiO>MnO₂>Co₃O₄>ZrO₂. It has been concluded that the combined plasma-catalytic processing may be an efficient way for the reduction of N₂O emissions.     

Effects of methane and oxygen on decomposition of nitrous oxide over metal oxide catalysts

Catalytic activities of various metal oxides for decomposition of nitrous oxide were compared in the presence and absence of methane and oxygen, and the general rule in the effects of the coexisting gases was discussed. The reaction rates of nitrous oxide were well correlated to the heat of formation of metal oxide, i.e., a V-shaped relationship with a minimum at −ΔH_f⁰ around 450 kJ (O mol)⁻¹ was observed in N₂O decomposition in an inert gas. In the case of metal oxides having the heat of formation lower than 450 kJ (O mol)⁻¹, CuO, Co₃O₄, NiO, Fe₂O₃, SnO₂, In₂O₃, Cr₂O₃, the activities were strongly affected by the presence of methane and oxygen. On the other hand, the activities of TiO₂, Al₂O₃, La₂O₃, MgO and CaO were almost independent. The reaction rate of nitrous oxide was significantly enhanced by methane. The promotion effect of methane was attributed to the reduction of nitrous oxide with methane: 4N₂O+CH₄→2N₂+CO₂+2H₂O. The activity was suppressed in the presence of oxygen on the metal oxides having lower heat of formation. On the basis of Langmuir–Hinshelwood mechanism, the effect of oxygen on nitrous oxide decomposition was rationalized with the strength of metal–oxygen bond.     

Surface-nitrogen removal in NO and N2O reduction on Pd(1 1 0): Angular distribution studies of desorbing products

This paper is a report of angle-resolved product desorption measurements in the course of catalyzed NO and N₂O reduction on Pd(1 1 0). Surface-nitrogen removal processes show different angular distributions, i.e. normally directed N₂ desorption takes place in process (i) 2N(a) → N₂(g). Highly inclined N₂ desorption towards the [0 0 1] direction is induced in process (ii) N₂O(a) → N₂(g) + O(a). N₂O or NH₃ desorption follows the cosine distribution characterizing the desorption after the thermalization in process (iii) N₂O(a) → N₂O(g) or (iv) N(a) + 3H(a) → NH₃(a) → NH₃(g). Thus, a combination of the angular and velocity distributions provides the analysis of most of surface-nitrogen removal processes in the course of catalyzed NO reduction.

At temperatures below 600 K, processes (ii) and (iii) dominate and process (iv) is enhanced at H₂ pressures higher than NO. Process (i) contributes significantly above 600 K. Only three processes except for NH₃ formation are operative when CO is used. Only process (ii) was observed in a steady-state N₂O + CO (or H₂) reaction.     

Catalytic reduction of N2O over steam-activated FeZSM-5 zeolite: Comparison of CH4, CO, and their mixtures as reductants with or without excess O2

The catalytic reduction of N₂O by CH₄, CO, and their mixtures has been comparatively investigated over steam-activated FeZSM-5 zeolite. The influence of the molar feed ratio between N₂O and the reducing agents, the gas-hourly space velocity, and the presence of O₂ on the catalytic performance were studied in the temperature range of 475–850 K. The CH₄ is more efficient than CO for N₂O reduction, achieving the same degree of conversion at significantly lower temperatures. The apparent activation energy for N₂O reduction by CH₄ was very similar to that of direct N₂O decomposition (140 kJ mol⁻¹), being much lower for the N₂O reduction by CO (60 kJ mol⁻¹). This suggests that the reactions have a markedly different mechanism. Addition of CO using equimolar mixtures in the ternary N₂O + CH₄ + CO system did not affect the N₂O conversion with respect to the binary N₂O + CH₄ system, indicating that CO does not interfere in the low-temperature reduction of N₂O by CH₄. In the ternary system, CO contributed to N₂O reduction when methane was the limiting reactant. The conversion and selectivity of the reactions of N₂O with CH₄, CO, and their mixtures were not altered upon adding excess O₂ in the feed.     

Nitrous oxide formation in low temperature selective catalytic reduction of nitrogen oxides with V2O5/TiO2 catalysts

Nitric oxide and nitric dioxide compounds (NO_x) present in stack gases from nitric acid plants are usually eliminated by selective catalytic reduction (SCR) with ammonia. In this process, small quantities of nitrous oxide (N₂O) are produced. This undesirable molecule has a high greenhouse gas potential and a long lifetime in the atmosphere, where it can contribute to stratospheric ozone depletion. The influence of catalyst composition and some operating variables were evaluated in terms of N₂O formation, using V₂O₅/TiO₂ catalysts. High vanadia catalyst loading, nitric oxide inlet concentration and reaction temperature increase the generation of this undesirable compound. The results suggest that adsorbed ammonia not only reacts with NO via SCR, but also with small quantities of oxygen activated by the presence of NO. The mechanism proposed for N₂O generation at low temperature is based on the formation of surface V–ON species which may be produced by the partial oxidation of dissociatively adsorbed ammonia species with NO + O₂ (eventually NO₂). When these active sites are in close proximity they can interact to form an N₂O molecule. This mechanism seems to be affected by changes in the active site density produced by increasing the catalyst vanadia loading.     

Reduction of nitric oxide to nitrous oxide by cobalt porphyrins and corrins

The reduction of nitric oxide (NO) to nitrous oxide (N₂O) by dithiothreitol in the presence of cobalt-centered corrin and porphyrin are presented. Reactions were monitored directly using Fourier transform infrared (FTIR) spectroscopy of vapor phase spectra. Reaction rates were two-fold faster for cobalt corrin than cobalt-centered porphyrins. The stoichiometry showed loss of two NO molecules per N₂O generation.     

Development of a new Rh/TiO2–sepiolite monolithic catalyst for N2O decomposition

Monolithic catalysts based on Rh/TiO₂–sepiolite were developed and tested in the decomposition of N₂O traces. Several effects such as the presence of NO, O₂ and NO + O₂ in the gas mixture, the catalysts pre-treatment and the metal loading were evaluated. The system was extremely sensitive to the amount of rhodium, passing through a maximum in the catalytic activity at a Rh content of 0.2 wt.%. It has been demonstrated that both NO and O₂ compete for the same adsorption sites as N₂O; however, this effect was not as severe as for other previously reported Rh systems. For NO + O₂ gas mixtures the inhibition effect was stronger than when only NO or O₂ was present. Analysis of the pre-reduced sample by XPS showed Rh mainly in the metal state, even after treatment with N₂O + O₂ mixtures, suggesting that the oxygen consumption observed in the Temperature Programmed Reaction experiments was related to the oxygen uptake by vacancies in the support. The presence of sepiolite in the support preparation and its role as a matrix over which TiO₂ particles were distributed, seems to play an important effect in the migration process of oxygen species through the support vacancies. The Rh/TiO₂ monolithic system is an attractive alternative for the elimination of N₂O traces from stationary sources due to the combination of high catalytic activity with a low pressure drop and optimum textural/mechanical properties.     

N2O catalytic decomposition over various spinel-type oxides

Various spinel-type catalysts AB₂O₄ (where A = Mg, Ca, Mn, Co, Ni, Cu, Cr, Fe, Zn and B = Cr, Fe, Co) were prepared and characterized by XRD, BET, TEM and FESEM-EDS. The performance of these catalysts towards the decomposition of N₂O to N₂ and O₂ was evaluated in a temperature programmed reaction (TPR) apparatus in the absence and the presence of oxygen. Spinel-type oxides containing Co at the B site were found to provide the best activity. The half conversion temperature of nitrous oxide over the MgCo₂O₄ catalyst was 440 °C and 470 °C in the absence and presence of oxygen, respectively (GHSV = 80,000 h⁻¹).

On the grounds of temperature programmed oxygen desorption (TPD) analyses as well as of reactive runs, the prevalent activity of the MgCo₂O₄ catalyst could be explained by its higher concentration of suprafacial, weakly chemisorbed oxygen species, whose related vacancies contribute actively to nitrous oxide catalytic decomposition. This indicates the way for the development of new, more active catalysts, possibly capable of delivering at low temperatures amounts of these oxygen species even higher than those characteristic of MgCo₂O₄.     

Catalytic decomposition of N2O over CeO2 promoted Co3O4 spinel catalyst

A series of CeO₂ promoted cobalt spinel catalysts were prepared by the co-precipitation method and tested for the decomposition of nitrous oxide (N₂O). Addition of CeO₂ to Co₃O₄ led to an improvement in the catalytic activity for N₂O decomposition. The catalyst was most active when the molar ratio of Ce/Co was around 0.05. Complete N₂O conversion could be attained over the CoCe0.05 catalyst below 400 °C even in the presence of O₂, H₂O or NO. Methods of XRD, FE-SEM, BET, XPS, H₂-TPR and O₂-TPD were used to characterize these catalysts. The analytical results indicated that the addition of CeO₂ could increase the surface area of Co₃O₄, and then improve the reduction of Co³⁺ to Co²⁺ by facilitating the desorption of adsorbed oxygen species, which is the rate-determining step of the N₂O decomposition over cobalt spinel catalyst. We conclude that these effects, caused by the addition of CeO₂, are responsible for the enhancement of catalytic activity of Co₃O₄.     

Role of active oxygen transients in selective catalytic reduction of N2O with CH4 over Fe-zeolite catalysts

Sharp NO and O₂ desorption peaks, which were caused by the decomposition of nitro and nitrate species over Fe species, were observed in the range of 520–673 K in temperature-programmed desorption (TPD) from Fe-MFI after H₂ treatment at 773 K or high-temperature (HT) treatment at 1073 K followed by N₂O treatment. The amounts of O₂ and NO desorption were dependent on the pretreatment pressure of N₂O in the H₂ and N₂O treatment. The adsorbed species could be regenerated by the H₂ and N₂O treatment after TPD, and might be considered to be active oxygen species in selective catalytic reduction (SCR) of N₂O with CH₄. However, the reaction rate of CH₄ activation by the adsorbed species formed after the H₂ and N₂O or the HT and N₂O treatment was not so high as that of the CH₄ + N₂O reaction over the catalyst after O₂ treatment. The simultaneous presence of CH₄ and N₂O is essential for the high activity of the reaction, which suggests that nascent oxygen species formed by N₂O dissociation can activate CH₄ in the SCR of N₂O with CH₄.     

The importance of Fe loading on the N2O reduction with NH3 over Fe-MFI: Effect of acid site formation on Fe species

Effect of the loading amount of Fe over ion-exchanged Fe-MFI catalysts on the catalytic performance of N₂O reduction with NH₃ was investigated, and the results indicated that the turnover frequency (TOF) was almost constant in the Fe/Al range between 0.05 and 0.40. The activity of N₂O + NH₃ reaction was much lower than that of N₂O + CH₄ reaction over Fe-MFI (Fe/Al = 0.40), and the preadsorption of NH₃ decreased drastically the activity of N₂O + CH₄ reaction. The temperature-programmed desorption (TPD) of NH₃ showed the formation of stronger acid sites on Fe-MFI (Fe/Al = 0.24 and 0.40), and the amount of the acid sites agrees well with the desorption amount O₂ in O₂-TPD in the low temperature range. The acid sites gave a 3610 cm⁻¹ peak (Brønsted acid) in FTIR observation. These results suggest that the acid sites were formed on the bridge oxide ions in binuclear Fe species. Adsorbed NH₃ on the strong acid sites inhibited N₂O dissociation, which can be related to the low activity of N₂O + NH₃ reaction over Fe-MFI with high Fe loading.     

Mechanistic insights into the formation of N2O and N2 in NO reduction by NH3 over a polycrystalline platinum catalyst

The reaction pathways of N₂ and N₂O formation in the direct decomposition and reduction of NO by NH₃ were investigated over a polycrystalline Pt catalyst between 323 and 973 K by transient experiments using the temporal analysis of products (TAP-2) reactor. The interaction between nitric oxide and ammonia was studied in the sequential pulse mode applying ¹⁵NO. Differently labelled nitrogen and nitrous oxide molecules were detected. In both, direct NO decomposition and NH₃–NO interaction, N₂O formation was most marked between 573 and 673 K, whereas N₂ formation dominated at higher temperatures. An unusual interruption of nitrogen formation in the ¹⁵NO pulse at 473 K was caused by an inhibiting effect of adsorbed NO species. The detailed analysis of the product distribution at this temperature clearly indicates different reaction pathways leading to the product formation. Nitrogen formation occurs via recombination of nitrogen atoms formed by dissociation of nitric oxide or/and complete dehydrogenation of ammonia. N₂O is formed via recombination of adsorbed NO molecules. Additionally, both products are formed via interactions between adsorbed ammonia fragments and nitric oxide.     

污水微生物脱氮过程中N2O产生机理及影响因素研究进展

产生于生物脱氮过程的N₂O是一种强效的温室气体并会导致臭氧层破坏。本文综述了污水脱氮过程中N₂O的产生机理及影响因素。羟胺氧化和AOB反硝化是硝化过程产生N₂O两种主要路径,诸如溶解氧、氨氮和亚硝酸盐等因素主要通过影响微生物的活动或酶的活性而间接影响硝化过程中N₂O的产生。反硝化过程是N₂O的另一重要产生来源,其N₂O生成量的多少与N₂O酶有直接关系,而溶解氧、有机碳源和亚硝酸盐等因素会影响反硝化过程中N₂O酶的活性。目前新型脱氮工艺也成为N₂O的潜在来源,但其N₂O产生机理还有待深入研究。尽管N₂O释放与周围环境变化密切相关,但本质原因还是由于微生物的作用及酶活性受到影响所致。文章最后指出污水生物脱氮过程中N₂O产量控制与减量化策略是今后研究的主要方向,并给出了几点建议。     

Effect of operating conditions on the reduction of nitrous oxide by propane over a Fe-zeolite monolith

Fluidised bed combustion is an important source of nitrous oxide emissions. The influence of different operating parameters, such as catalyst volume, temperature, gas hourly space velocity, and hydrocarbon addition, on the activity, selectivity, and poisoning tolerance of a Fe-ZSM-5 monolith for the nitrous oxide selective catalytic reduction, has been investigated under realistic conditions, at bench scale.

Both in the absence or in the presence of poisons, such as H₂O, NO, and SO₂, the optimisation of operating conditions gives rise to a broadening of the temperature window for N₂O reduction, making it more compatible with real application conditions, with a simultaneous reduction in hydrocarbon fugitive emission, resulting in an environmental friendly process.

Excessively high reaction temperatures seem to be needed to obtain an acceptable level of N₂O decomposition. On the contrary, high N₂O reduction conversions are obtained, even in the presence of poisons and at relatively low temperatures, which is the preferred situation in the processes of pollutants removal from stationary combustion sources.

The optimum value of C₃H₈/N₂O ratio to be used for reducing N₂O over the catalyst system seems to be about the unity, since higher N₂O and C₃H₈ conversions and lower hydrocarbon unwanted emissions are attained, with a low consume of propane as selective reductant.     

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.     

Low temperature decomposition of nitrous oxide over Fe/ZSM-5: Modelling of the dynamic behaviour

The kinetics of N₂O decomposition to gaseous nitrogen and oxygen over HZSM-5 catalysts with low content of iron (<400 ppm) under transient and steady-state conditions was investigated in the temperature range of 250–380 °C. The catalysts were prepared from the HZSM-5 with Fe in the framework upon steaming at 550 °C followed by thermal activation in He at 1050 °C. The N₂O decomposition began at 280 °C. The reaction kinetics was first order towards N₂O during the transient period, and of zero order under steady-state conditions. The increase of the reaction rate with time (autocatalytic behaviour) was observed up to the steady state. This increase was assigned to the catalysis by adsorbed NO formed slowly on the zeolite surface from N₂O. The formation of NO was confirmed by temperature-programmed desorption at temperatures >360 °C. The amount of surface NO during the transient increases with the reaction temperature, the reaction time, and the N₂O concentration in the gas phase up to a maximum value. The maximum amount of surface NO was found to be independent on the temperature and N₂O concentration in the gas phase. This leads to a first-order N₂O decomposition during the transient period, and to a zero-order under steady state. A kinetic model is proposed for the autocatalytic reaction. The simulated concentration–time profiles were consistent with the experimental data under transient as well as under steady-state conditions giving a proof for the kinetic model suggested in this study.     

Improvement in selective catalytic reduction of nitrogen oxides by using dielectric barrier discharge

The behavior of the selective catalytic reduction of nitrogen oxides (NO_x) assisted by a dielectric barrier discharge was investigated. The principal function of the dielectric barrier discharge in the present system is to generate ozone, which is continuously fed to a chamber where the ozone and NO-rich exhaust gas (NO forms the large majority of NO_x) are mixed. In the ozonization chamber, a part of NO contained in the exhaust gas is oxidized to NO₂, and then the mixture of NO and NO₂ enters the catalytic reactor. The ozonization method proposed in this study was found to be more energy-efficient for the oxidation of NO to NO₂ than the typical nonthermal plasma process. The degree of NO oxidation was approximately equal to the amount of ozone added to the exhaust gas, implying that the decomposition of ozone into molecular oxygen was relatively slow, compared to its reaction with NO. When the exhaust gas was first treated by ozone to produce a mixture of NO and NO₂, a remarkable enhancement in the catalytic reduction of nitrogen oxides was observed. Neither NO₃ nor N₂O₅ was formed in the present system, but small amounts of ozone and N₂O (less than 5 ppm) were detected in the outlet gas.     

Catalytic decomposition of nitrous oxide over calcined cobalt aluminum hydrotalcites

Catalytic decomposition of nitrous oxide has been carried out over calcined cobalt aluminum hydrotalcites of general formula [Co_1−xAl_x(OH)₂[CO₃]_x/2 H₂O where x = 0.25–0.33 at 50 Torr (1 Torr = 133 Pa) initial pressure of N₂O in a static glass recirculatory reactor (130 cc) in the temperature range 150–280°C. All catalysts showed a first order dependence in N₂O without significant oxygen inhibition. The activity increased with an increase in cobalt concentration present in the sample. The catalyst precursor synthesized under low supersaturation (LS) exhibited a higher activity than the precursor synthesized by sequential precipitation (SP) method. The observed trend in the activity is explained based on the surface concentration of cobalt, determined by XPS and matrix effects. Prior to catalytic studies, the fresh and calcined samples were characterized by various physicochemical techniques such as XRD, FT–IR, TG–DSC, TEM (with EDAX) and BET surface area measurements.     

Development of carbon supported metal catalysts for the simultaneous reduction of NO and N2O

The development of an activated carbon supported bimetallic catalyst for the simultaneous reduction of NO and N₂O is described. Base metal catalysts were found to deactivate due to the oxidation of the metallic phase, suggesting the use of a noble metal. Platinum catalysts were very active for NO reduction, but they lacked selectivity towards N₂, since N₂O was produced. The association of Pt and K, a good N₂O reduction catalyst, was capable of achieving the complete conversion of both gases at 350 °C, the catalyst being stable over extended periods of time (up to 17 h). A synergistic effect between Pt and K was observed, similar to the effect previously reported for Ni and K.     

Determination of the surface area of dispersed ruthenium by reactive nitrous oxide chemisorption

The interaction of nitrous oxide with ruthenium surfaces was studied in order to develop a fast and reliable method for the determination of the metallic surface area and dispersity of ruthenium catalysts as alternative to the established chemisorption methods using hydrogen, carbon monoxide or oxygen as probe molecule. Studies of the temperature dependence of the reactive or dissociative chemisorption of nitrous oxide using pulse technique showed that an oxygen monolayer of almost constant stoichiometry is formed in a narrow temperature range around 403 K. However, so-called ‘rest nitrogen peaks' were observed after saturation of an O-monolayer revealing a small additional nitrous oxide conversion which could be attributed to formation of subsurface oxygen by means of back-titration of the oxygen totally uptaken. This additional nitrous oxide consumption can be minimised by an adjustment of both the N₂O-sampling loop volume and the catalyst sample weight in order to obtain an oxygen monolayer saturation by few pulses, and moreover, it can be considered as a correction for the calculation of the oxygen monolayer. A specific nitrous oxide consumption of 13.5 μmol/m²_Ru has been determined on ruthenium black by means of alternating measurements of the nitrogen physisorption and N₂O-pulse chemisorption. The reliability of the new method for characterisation of the dispersity or surface area of supported ruthenium was proved with ruthenium highly dispersed on γ-alumina.