Nitrous oxide processing by a combination of gliding and microwave discharges

Non-thermal plasma of microwave discharge coupled with gliding discharge was applied to convert nitrous oxide. The experiments were carried out using air or oxygen as carrier gases for N₂O (5%). The overall rates of nitrous oxide conversion determined for the N₂O + air mixture were slightly higher than those for N₂O + oxygen. No significant effect of the carrier gas (air or oxygen) on the rate of N₂O → NO conversion was observed. The effect of the power of the microwave discharge and gas flow rate (air) on the overall rate of nitrous oxide conversion and rate of N₂O conversion to NO was studied. The increase of the gas flow rate from 200 to 400 N l/h resulted in an increase of the N₂O conversion rates both overall (r) and to NO (r_NO). For 400 N l/h, both rates were higher by about 80–100% than those determined in the experiments performed with 200 N l/h.     

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.     

Temperature programmed desorption study of adsorbed species formed by the decomposition of N2O on ion-exchanged copper zeolite catalysts

The nature of the adsorbed species on Cu-ZSM-5 (Cu-Z), Cu-Mordenite (Cu-M), and Cu-Y-zeolite (Cu-Y) was investigated by means of temperature programmed desorption (TPD). When dinitrogen monoxide (N₂O) came into contact with Cu-zeolites above 573 K, the decomposition of N₂O occurred accompanied by the formation of adsorbed oxygen species and adsorbed nitrogen oxide species. In the TPD runs, three O₂ desorption peaks appeared at temperatures of 623, 673, and 753 K and were named -, β-, and γ-peaks, respectively. The O₂ desorption at the - and γ-peaks became quickly saturated after contacting N₂O at 598 K, while the amount of O₂ desorbed at the β-peak increased with time, not reaching a constant level until 120 min of exposure. The activity for the decomposition of N₂O decreased with the accumulation of β-oxygen over the catalyst. The rate of N₂O decomposition depended upon the nature and amount of the copper zeolite catalysts available, as determined by the formation of - and/or β-oxygen.     

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.     

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.     

Investigation of the oxygen storage process on ceria- and ceria–zirconia-supported catalysts

A total of 10 noble metal (Rh, Pt, Pd, Ru and Ir) catalysts, either supported on CeO₂ or Ce_0.63Zr_0.37O₂, were prepared. Catalysts were fully characterized using XRD, N₂ adsorption at −196 °C, TEM and H₂ chemisorption. Oxygen storage processes were carefully investigated. The influence of temperature was checked and a key role of oxygen diffusion was further demonstrated. A review of the reactions involved in the CO transient oxidation reaction is finally proposed.     

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.     

Nitrous oxide as an oxidant for ethane oxydehydrogenation

Waste nitrous oxide was used as an oxidant for ethane oxydehydrogenation performed at the range of temperature from 350 to 450 °C over iron modified zeolite catalysts. Different zeolite matrices (zeolite ZSM-5 of different Si/Al ratio, H-Y, mordenite) modified with iron cations introduced into zeolite by means of ionic exchange were applied as catalysts for the reaction under study. Additionally, amorphous silica and alumina silica as well as silicalite of MFI structure were also used as a matrix for iron ions accommodation and they were tested for oxydehydrogenation reaction. It was found that only iron modified zeolites showed activity for reaction under study. Amorphous oxide supports and crystalline neutral silicalite modified with iron cations by means of impregnation were completely inactive for oxydehydrogenation reaction. The best catalytic performance was found on iron modified zeolites of MFI structure. The Si/Al ratio of the ZSM-5 matrix influenced the activity for ethane oxydehydrogenation reaction insignificantly. N₂O oxidant was partly utilized for ethane oxidation (towards ethene or carbon oxides), while some part of the oxidant was decomposed to nitrogen and oxygen. Performing the reaction at 450 °C resulted in a high ethene yield and complete N₂O removal.     

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 oxygen concentration on the NOx reduction with ammonia over V2O5–WO3/TiO2 catalyst

The catalytic reduction of NO_x in the typical operation temperatures and oxygen concentrations of diesel engines has been studied in the presence of V3W9Ti in a tubular flow reactor. The results have shown that the selective catalytic reduction is strongly affected by the oxygen concentration in low temperature range (150–275 °C). At higher temperatures, the reaction becomes independent of the O₂ concentration. The rate of the selective catalytic reduction of NO with ammonia may be considerably enhanced by converting part of the NO into NO₂. DRIFT measurements have shown that NH₃ and NO₂ are adsorbed on the catalyst surface on the contrary of NO. The experiments have shown that the decrease in N₂ selectivity of the SCR reaction is mainly due to the SCO of ammonia and to the formation of nitrous oxide.     

Modification of the surface reactivity of Cu-MFI during chemisorption and transformation of the reagents in the selective reduction of NO with propane and O2

The steady-and unsteady-state catalytic behaviour of Cu-MFI in the conversion of propane and NO in the presence of O₂ is reported, showing how the chemisorption and transformation of reactants may influence the surface reactivity. Various effects were observed: (i) a change in the surface reactivity and kinetics in going from low to high concentrations of NO or propane, (ii) the transformation of NO to N₂ and N₂O promoted at low temperature (250°C) by oxygen in the absence of hydrocarbon, (iii) the influence of NO over the surface reactivity of the catalyst in the conversion of propane and (iv) the influence of surface precoverage with oxidized nitrogen oxides (N_xO_y) or carboxylate species on the catalyst transient reactivity in the reduction of NO to N₂. In particular, Cu-MFI is initially more active when oxidized nitrogen oxides are present, suggesting that the active intermediate in the reduction of NO with propane is a complex formed by the reaction of nitrate with activated hydrocarbon. It is shown, however, that strongly bound oxidized nitrogen oxides may have also additional effects on the surface reactivity: (i) can promote the conversion of NO to N₂ and N₂O in transient conditions and (ii) can give a partial inhibition of the surface reactivity blocking copper ions due to their strong chemisorption. Furthermore, it is shown that NO reacts faster with oxygen than hydrocarbon forming N_xO_y species which are then the oxidizing agent for the hydrocarbon. It is thus suggested that the surface reactivity of Cu-MFI in the reduction of NO with propane/oxygen depends on the surface population of nitrogen oxide adspecies which influence not only the surface reactivity, but also the pathway of hydrocarbon oxidation.     

Characterization and performance of Pt-USY in the SCR of NOx with hydrocarbons under lean-burn conditions

Pt-USY was used for the selective catalytic reduction of NO_x with hydrocarbons in the presence of excess oxygen. The catalyst was prepared by an ion-exchange method and characterized by XRD, TEM, CO chemisorption, and Ar adsorption at 87 K. The platinum particle size distribution was found to be broad (2–20 nm), with no apparent sintering of the active phase during the HC-SCR process after 25 h time-on-stream. Generally, large metal clusters (>15 nm) are situated at the external surface of the zeolite, while the smaller ones are located in the pores of the support. Pt-USY shows an excellent activity in the deNO_x reaction (molar NO_x conversion 90% at 475 K) with propene as the reductant in 5 kPa O₂, as well as stable operation during time-on-stream. Propane only yields a low NO_x conversion compared to propene. The presence of high oxygen contents (5–10 kPa O₂) slightly inhibits the reaction. No significant decrease in deNO_x activity was observed at high space velocities (up to 100,000 h⁻¹). The presence of SO₂ and H₂O in the feed stream did not significantly affect the deNO_x activity. Pt-USY performs better under lean-burn conditions than other Pt-catalysts supported on e.g. ZSM-5, Al₂O₃, or SiO₂. The selectivity to N₂ was similar to the other Pt-based catalysts (30%), the other major product being N₂O.     

Influence of annealing atmosphere on the structure, resistivity and transmittance of InZnO thin films

Dependence of the electrical and optical properties of In₂O₃–10 wt% ZnO (IZO) thin films deposited on glass substrates by RF magnetron sputtering on the annealing atmosphere was investigated. The electrical resistivities of indium zinc oxide (IZO) thin films deposited on glass substrate can be effectively decreased by annealing in an N₂ + 10% H₂ atmosphere. Higher temperature (200 °C) annealing is more effective in decreasing the electrical resistivity than lower temperature (100 °C) annealing. The lowest resistivity of 6.2 × 10⁻⁴ Ω cm was obtained by annealing at 200 °C in an N₂ + 10% H₂ atmosphere. In contrast, the resistivity was increased by annealing in an oxygen atmosphere. The transmittance of IZO films is improved by annealing regardless of the annealing temperature.     

Comparative study of Pt-based catalysts on different supports in the low-temperature de-NOx-SCR with propene

Pt-based catalysts have been prepared using supports of different nature (γ-Al₂O₃, ZSM-5, USY, and activated carbon (ROXN)) for the C₃H₆-SCR of NO_x in the presence of excess oxygen. Nitrogen adsorption at 77 K, pH measurements, temperature-programmed desorption of propene, and H₂ chemisorption were used for the characterization of the different supports and catalysts. The performance of these catalysts has been compared in terms of de-NO_x activity, hydrocarbon adsorption and combustion at low temperature, and selectivity to N₂. Maximum NO_x conversions for all the catalysts were achieved in the temperature range of 200–250°C. The order of activity was, Pt-USY>Pt/ROXNPt-ZSM-5Pt/Al₂O₃. At temperatures above 300°C only Pt/ROXN maintains a high activity caused by the consumption of the support, while the other catalysts present a strong deactivation. Propene combustion starts at the same temperature for all the catalytic systems (160°C). Complete hydrocarbon combustion is directly related to the acidity of the support, thus determining the temperature of the maximum NO_x reduction. The support play an important role in the reaction mechanism through the hydrocarbon activation. N₂O formation was observed for all the catalysts. N₂ selectivity ranges from 15 to 30% with the order, Pt/ROXN>Pt-USYPt/Al₂O₃>Pt-ZSM-5. The catalytic systems exhibit a stable operation under isothermal conditions during time-on-stream experiments.     

Dynamics of oxygen storage and release on commercial aged Pd-Rh three-way catalysts and their characterization by transient experiments

The present work has provided new fundamental information on the effects of aging (mileage, km) of a commercial Pd-Rh (9:1, w/w) three-way catalyst (TWC) on (a) the H₂ chemisorption, (b) the redox properties of washcoat material, and (c) the dynamic oxygen storage and release properties of TWC. Hydrogen chemisorption in the 25–400 °C range was found to decrease with increasing aging of TWC. On the other hand, H₂ chemisorption performed at 200 and 400 °C resulted in a significantly larger amount of chemisorbed hydrogen (exceeding the monolayer value based on the noble metals) compared to that obtained after chemisorption at 25 °C. This is due to the onset of a hydrogen spill over process in the 200–400 °C range. The features of H₂-TPD spectra were found to strongly depend on the aging of TWC. Hydrogen chemisorption at 25 °C followed by TPD after a given pre-treatment of the catalyst surface might be considered as a procedure for a good estimate of the metal dispersion of a commercial Pd-Rh TWC. The redox properties of the oxygen storage components of the washcoat of a commercial Pd-Rh TWC were found to drastically change with increasing aging of TWC. It was found that the oxygen storage capacity (OSC) of the TWC investigated decreases significantly with catalyst aging in the 0–56,000 km range and in the temperature range of 500–750 °C. In the case of fresh TWC, the presence of 20 ppm SO₂ or 10% CO₂ in a 1.5% O₂/He gas mixture used for oxygen storage (oxidizing gas) was found to result in a large decrease in the amount of dynamic OSC measured by alternating switches between oxidizing and reducing feed gas compositions. The shift in time of the peak maximum of the transient response of CO₂ obtained during the switch O₂/He → He → CO/He (t) can be ascribed to the alteration of the kinetics of the oxygen back-spillover process and not to the kinetics of CO oxidation on the metal surface.     

Characterization and catalytic properties of perovskites with nominal compositions La1-xSrxAl1-2yCuyRuyO3

Nine different metal oxide catalysts were prepared by impregnating alumina washcoats with water solutions containing La³⁺, Sr²⁺, Cu²⁺ and Ru³⁺ ions and calcining them at 900°C. The produced samples were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM) studies combined with energy-dispersive spectroscopy (EDS) analysis, X-ray powder diffraction and specific surface area measurements. A perovskite phase of the nominal composition La_1-xSr_xAl_1-2yCu_yRu_yO₃ was found in all samples, in increasing amount in the samples with increasing contents of strontium and ruthenium. The catalysts were evaluated with respect to light-off temperatures and redox characteristics using two gas mixtures, one containing NO/CO/C₃H₆/O₂/N₂ and the other NO/CO/N₂. The light-off temperatures for nitric oxide reduction decreased from 534 to 333°C for the catalysts without and with strontium and ruthenium, respectively. In the presence of oxygen the conversion of nitric oxide declined rapidly under oxidative conditions whereas in absence of oxygen this decline was less pronounced and found to be linear over the entire redox interval studied. These studies suggest that the perovskite phase takes an active part in the conversion of nitric oxide and carbon monoxide to nitrogen and carbon dioxide.     

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.     

Alumina supported manganese oxides for the low-temperature selective catalytic reduction of nitric oxide with ammonia

Alumina supported manganese oxides exhibit a high and selective activity for the catalytic reduction of nitric oxide with ammonia (SCR) between 385 and 575 K. Samples with 3–15 wt.-% manganese were studied at space velocities between 22 000–116 000 h⁻¹ and at standard conditions of 500 ppm NO, 550 ppm NH₃ and 2% O₂. Manganese acetate results in a better dispersion of the manganese oxide on the support and a higher specific catalyst activity than manganese nitrate as precursor, for which crystalline structures could be detected. Temperature-programmed reduction revealed that acetate yields Mn₂O₃ and nitrate mainly MnO₂ on the γ-alumina support. The nitric oxide conversion per amount of manganese is fairly independent of the loading for the catalysts prepared from each precursor. The use of ¹⁵NH₃ reveals that it reacts in a 1:1 molar ratio with nitric oxide towards ¹⁵NN and/or ¹⁵NNO. The SCR activity (to nitrogen) is strongly dependent on the oxygen partial pressure, whereas water inhibits reversibly. Lattice oxygen of the catalyst is not able to maintain the SCR reaction in the absence of oxygen. The nitrous oxide formation is independent of the oxygen partial pressure, but increases with increasing manganese loading and with temperature, resulting in lower selectivities for nitrogen formation. The nitrogen and nitrous oxide formation probably occur at different sites. Above 525 K ¹⁵NH₃ oxidation occurs, yielding mainly ¹⁵N₂O and ¹⁵NO, depending on the temperature. The nitrous oxide is not further reduced by ammonia over this type of catalyst. The addition of tungsten to the catalyst increases the selectivity for nitrogen considerably. The stability of the ex-acetate catalyst is good, for at least 600 h the activity remained constant. The catalysts are sensitive towards sulphur dioxide, the ex-acetate catalysts the least, due to the strong interaction with the alumina support, as is revealed by TPR.     

Catalytic decomposition of N2O over supported Rh catalysts: effects of supports and Rh dispersion

The study of catalytic decomposition of nitrous oxide to nitrogen and oxygen over Rh catalysts supported on various supports (USY, NaY, Al₂O₃, ZrO₂, FSM-16, CeO₂, La₂O₃) showed that the activities of Rh/Al₂O₃ and Rh/USY (ultrastable Y zeolite) catalysts were comparable to or higher than the other catalysts reported in the literatures. The catalytic activity of N₂O decomposition was sensitive not only to the Rh dispersion but also to the preparation variables such as the Rh precursors and the supports used. A pulsed N₂O experiment over a Rh/USY catalyst suggested that the catalytic N₂O decomposition occurs on oxygen-covered surface and that O₂ may be freed on collision of N₂O molecules with the adsorbed oxygen atoms.