A study of nitrous oxide decomposition over calcium oxide in combustion condition

A study of N₂O decomposition reaction over a bed of CaO particles in a fixed bed reactor has been conducted. Effects of parameters such as concentration of inlet N₂O, reaction temperature and effects of CO, CO₂, O₂, and NO gas presented in the combustion gas environment have been investigated. The experiment showed that the N₂O decomposition reaction was accelerated by the increase of reaction temperature, and the existence of CO, while the reaction was hindered by the existence of CO₂, NO. O₂ also affected the N₂O decomposition. Heterogeneous gas-solid reaction kinetics were proposed for the reaction conditions and compared with homogeneous reaction kinetics.     

Direct decomposition of nitrous oxide in the presence of oxygen over iridium catalyst supported on alumina

Direct decomposition of nitrous oxide (N₂O) on noble metal catalysts supported on alumina was examined in the presence of oxygen. The iridium catalysts supported on alumina showed higher activities than the other noble metal catalysts. Although the catalyst activity was affected by oxygen formed by N₂O decomposition at lower temperatures, desorption of oxygen proceeded promptly at the temperature , and the catalytic activity was recovered by increasing the reaction temperature from 350 to . Therefore, the Ir/Al₂O₃ catalyst can be used for N₂O decomposition in the presence of oxygen at relatively higher temperatures.     

Effect of noble metals in the decomposition of nitrous oxide over Fe-ferrierites

The decomposition of nitrous oxide was studied over Fe-ferrierite, Me-ferrierites and Fe/Me-ferrierites (Me: Pt, Rh and Ru). Flow as well as batch experiments were carried out and showed a synergy between Fe and Me ions. Ions of noble metals in Fe-ferrierite increased the catalytic activity in the sequence Pt < Rh ≅ Ru. Addition of NO substantially decreased the decomposition of N₂O over Rh/ferrierite and Ru/ferrierite, but not over bimetallic ferrierites. NO _x species created during the decomposition of nitrous oxide alone as well as with addition of NO, and employment of nitrous oxide labeled with ¹⁸O allowed us to assume a changing decomposition mechanism in the presence of Me ions in Fe-ferrierites.     

Isotopic study of nitrous oxide decomposition on an oxidized Rh catalyst: mechanism of oxygen desorption

N₂O decomposition on an oxidized Rh catalyst (unsupported) has been studied using a tracer technique in order to reveal the reaction mechanism. N₂ ¹⁶O was pulsed onto an ¹⁸O/oxidized Rh catalyst at 493 K and desorbed O₂ molecules were monitored. The ¹⁸O fraction in the desorbed oxygen had the same value as that on the surface oxygen. The result shows that the oxygen molecules do not desorb via the Eley–Rideal mechanism, but via the Langmuir–Hinshelwood mechanism. On the other hand, desorption of oxygen from Rh surfaces (in vacuum or in He) occurs at higher temperatures, which suggests reaction-assisted desorption of oxygen during the N₂O decomposition reaction at low temperature. This revised version was published online in July 2006 with corrections to the Cover Date.     

Sources of nitrous oxide in soils

Research to identify sources of nitrous oxide (N₂O) in soils has indicated that most, if not all, of the N₂O evolved from soils is produced by biological processes and that little, if any, is produced by chemical processes such as chemodenitrification. Early workers assumed that denitrification was the only biological process responsible for N₂O production in soils and that essentially all of the N₂O evolved from soils was produced through reduction of nitrate by denitrifying microorganisms under anaerobic conditions. It is now well established, however, that nitrifying microorganisms contribute significantly to emissions of N₂O from soils and that most of the N₂O evolved from aerobic soils treated with ammonium or ammonium-yielding fertilizers such as urea is produced during oxidation of ammonium to nitrate by these microorganisms. Support for the conclusion that chemoautotrophic nitrifiers such as Nitrosomonas europaea contribute significantly to production of N₂O in soils treated with N fertilizers has been provided by studies showing that N₂O emissions from such soils can be greatly reduced through addition of nitrification inhibitors such as nitrapyrin, which retard oxidation of ammonium by chemoautotrophic nitrifiers but do not retard reduction of nitrate by denitrifying microorganisms. This revised version was published online in August 2006 with corrections to the Cover Date.     

Emission of nitrous oxide from soils used for agriculture

Nitrous oxide is emitted into the atmosphere as a result of biomass burning, and biological processes in soils. Biomass burning is not only an instantaneous source of nitrous oxide, but it results in a longer term enhancement of the biogenic production of this gas. Measurements of nitrous oxide emissions from soils before and after a controlled burn showed that significantly more nitrous oxide was exhaled after the burn. The current belief is that 90% of the emissions come from soils. Nitrous oxide is formed in soils during the microbiological processes nitrification and denitrification. Because nitrous oxide is a gas it can escape from soil during these transformations. Nitrous oxide production is controlled by temperature, pH, water holding capacity of the soil, irrigation practices, fertilizer rate, tillage practice, soil type, oxygen concentration, availability of carbon, vegetation, land use practices and use of chemicals. Nitrous oxide emissions from agricultural soils are increased by the addition of fertilizer nitrogen and by the growth of legumes to fix atmospheric nitrogen. A recent analysis suggests that emissions of nitrous oxide from fertilized soils are not related to the type of fertilizer nitrogen applied and emissions can be calculated from the amount of nitrogen applied. Legumes also contribute to nitrous oxide emission in a number of ways, viz. atmospheric nitrogen fixed by legumes can be nitrified and denitrified in the same way as fertilizer nitrogen, thus providing a source of nitrous oxide, and symbiotically living Rhizobia in root nodules are able to denitrify and produce nitrous oxide. Conversion of tropical forests to crop production and pasture has a significant effect on the emission of nitrous oxide. Emissions of nitrous oxide increased by about a factor of two when a forest in central Brazil was clear cut, and pasture soils in the same area produced three times as much nitrous oxide as adjacent forest soils. Studies on temperate and tropical rice fields show that less than 0.1% of the applied nitrogen is emitted as nitrous oxide if the soils are flooded for a number of days before fertilizer application. However, if mineral nitrogen is present in the soil before flooding it will serve as a source of nitrous oxide during wetting and drying cycles before permanent flooding. Thus dry seeded rice can be a source of considerable nitrous oxide. There are also indirect contributions to nitrous oxide emission through volatilization of ammonia and emission of nitric oxides into the atmosphere, and their redistribution over the landscape through wet and dry deposition. In general nitrous oxide emissions can be decreased by management practices which optimize the crop's natural ability to compete with processes whereby plant available nitrogen is lost from the soil-plant system. If these options were implemented they would also result in increased productivity and reduced inputs. This revised version was published online in August 2006 with corrections to the Cover Date.     

Ammonia and nitrous oxide emissions from grass and alfalfa mulches

Ammonia (NH₃) and nitrous oxide (N_-2O) emissions were measured in the field for three months from three different herbage mulches and from bare soil, used as a control. The mulches were grass with a low N-content (1.15% N in DM), grass with a high N-content (2.12% N in DM) and alfalfa with a high N-content (4.33% N in DM). NH₃ volatilization was measured using a micrometeorological technique. N_-2O emissions were measured using closed chambers. NH₃ and N_-2O emissions were found to be much higher from the N-rich mulches than from the low-N grass and bare soil, which did not differ significantly. Volatilization losses of NH₃ and N_-2O occurred mainly during the first month after applying the herbage and were highest from wet material shortly after a rain. The extent of NH₃-N losses was difficult to estimate, due to the low frequency of measurements and some problems with the denuder technique, used on the first occasions of measurements. Nevertheless, the results indicate that NH₃-N losses from herbage mulch rich in N can be substantial. Estimated losses of NH₃-N ranged from the equivalent of 17% of the applied N for alfalfa to 39% for high-N grass. These losses not only represent a reduction in the fertilizer value of the mulch, but also contribute appreciably to atmospheric pollution. The estimated loss of N_-2O-N during the measurement period amounted to 1% of the applied N in the N-rich materials, which is equivalent to at least 13 kg N_-2O-N ha^-1 lost from alfalfa and 6 kg ha^-1 lost from high-N grass. These emission values greatly exceed the 0.2 kg N_-2O-N ha^-1 released from bare soil, and thus contribute to greenhouse gas emissions.     

Electroreduction of nitrous oxide on platinum and palladium: Toward selective catalysts for methanol–nitrous oxide mixed-reactant fuel cells

The kinetics of N₂O electroreduction in the absence and presence of methanol was studied between 295 and 333 K on polycrystalline Pt and Pd electrodes in 0.1 M NaOH. In the absence of methanol the reduction of N₂O on Pd is more facile than on Pt as shown by the approximately four times lower apparent activation energy and lower Tafel slope (Pt: 0.111 ± 0.019 V dec⁻¹, Pd: 0.084 ± 0.007 V dec⁻¹ at 295 K). Two different electroreduction mechanisms are proposed for Pt and Pd with and without participation of underpotential deposited hydrogen, respectively. The selectivity of Pt and Pd electrodes toward both N₂O electroreduction and methanol (0.5 and 1 M) oxidation at 295 K was also investigated. Pt based electrocatalysts are promising candidates for the anode of a mixed reactant CH₃OH–N₂O fuel cell due to inhibition of N₂O reduction by chemisorbed methanol. Pd on the other hand is a selective cathode electrocatalyst since N₂O reduction takes place fairly actively in the presence of 1 M methanol, while methanol oxidation is inhibited.     

Heterogeneous catalytic decomposition of nitrous oxide

An overview is given on the ongoing activities in the area of the decomposition of nitrous oxide, N₂O, over solid catalysts. These catalysts include metals, pure and mixed oxides, supported as well as unsupported, and zeolitic systems. The review covers aspects of the reaction mechanism and kinetics, focusing on the role of surface oxygen, the inhibition by molecular oxygen, water and other species, poisoning phenomena and practical developments.     

Self oscillatory behaviour in N2O decomposition over Co-ZSM-5 catalysts

Isothermal oscillations developed during N₂O decomposition over Co-ZSM-5 catalysts with different Si/Al ratios have been investigated. Spontaneous oscillations were observed between 350 and 450 °C. The maximum amplitude has been obtained for the catalysts having Si/Al of 40 and 50. The activation energies of the obtained oscillations were calculated in respect to cobalt concentration. The results showed that the E_a values increase linearly with an increasing Si/Al ratio of the zeolite. For Co-ZSM-5 catalyst (Si/Al = 25), increasing cobalt content in the catalyst led to a decrease in the frequency as well as the amplitude of the oscillations. Meanwhile, the increase in the E_a values was observed. The calculated reaction rate was found to be first order with respect to nitrous oxide concentration. Moreover, the developed oscillations were found to be sensitive to inlet N₂O concentration, catalyst weight and milling time duration. Decreasing the N₂O inlet concentration as well as the catalyst weight and increasing the milling time would lead to a quenching of the developed oscillations.     

Possible role of nitrite/nitrate redox cycles in N2O decomposition and light-off over Fe-ZSM-5

Surface nitrite/nitrate redox cycles were proposed to explain light-off behavior that was observed during the decomposition of N₂O over Fe-ZSM-5. Further study has demonstrated that while the nitrite/nitrate model can explain the original observations as an isothermal, mechanistic phenomenon, the light-off behavior is thermal, and not a mechanistic effect. Nonetheless, a pathway involving nitrite/nitrate redox cycles appears to be more consistent with experimental observation than the simple two-step pathway involving cation redox cycles. In particular, the nitrite/nitrate pathway can explain the effect of added NO upon the reaction kinetics and the reported isotopic product composition when unlabeled N₂O reacts over an oxygen-labeled catalyst. Further, a nitrite/nitrate pathway is consistent with the steady-state kinetics as well as published thermal desorption and infrared spectroscopic results.     

Kinetics of nitrous oxide decomposition over Cu-ZSM-5

The kinetics of nitrous oxide decomposition on an overexchanged Cu-ZSM-5 catalyst were measured using a gradientless reactor. Isothermal oscillations of nitrous oxide and oxygen concentrations can be observed in a broad range of experimental conditions. A transition of the catalytic activity during oscillations is accompanied by a change in the oxygen content of the catalyst and by the formation of traces of nitric oxide. The presence of excess oxygen does not significantly alter the behaviour of the catalyst whereas NO concentrations as low as 10 ppm quench the oscillations in the whole temperature range studied (375 to 450°C), maintaining steady-state operation at maximum catalytic activity. Reaction rates in this ‘ignited’ state are first order with respect to nitrous oxide concentration and not affected by either oxygen or nitric oxide. At temperatures above 400°C, the observed reaction rates are influenced by pore diffusion effects. In the region of intrinsic kinetics, the temperature dependence of the first order rate constant can be described by an activation energy of ca. 100 kJ/mol.     

Fluxes of methane and nitrous oxide in water-saving rice production in north China

Lowland rice production is currently facing serious water shortages in numerous Asian countries. In the North China Plain water limitations are severe. Water-saving rice production techniques are therefore increasingly searched for. Here we present the first study of trace gas emissions from a water-saving rice production system where soils are mulched and are kept close to field capacity in order to compare their contribution to global warming with traditional paddy rice. In a two-year field experiment close to Beijing, CH₄ and N₂O fluxes were monitored in two forms of the Ground Cover Rice Production System (GCRPS) and in traditional paddy fields using closed chambers. With paddy rice the observed CH₄ emissions were very low, about 0.3 g CH₄ m⁻² a⁻¹ in 2001 and about 1 g CH₄ m⁻² a⁻¹ in 2002. In GCRPS, the CH₄ emissions were negligible. N₂O fluxes in GCRPS were similar, 0.5 to 0.6 g N₂O m⁻² a⁻¹ in 2001 and 2002, and emission peaks mainly followed fertilizer applications. In paddy rice, N₂O fluxes were unexpectedly low throughout the year 2001 (0.03 g N₂O m⁻² a⁻¹), and in 2002 larger emissions occurred during the drainage period. So with 0.4 g N₂O m⁻² a⁻¹ the cumulative flux was similar to emissions in GCRPS. Total CO₂ equivalent fluxes calculated according to IPCC methodology were tenfold higher in GCRPS compared to paddy in 2001. In 2002, fluxes from both systems were similar with 175 and 141 g CO₂ equivalents m⁻² a⁻¹ from GCRPS and paddy. Burkhard Sattelmacher deceased.     

The photocatalytic reduction of nitrous oxide with propane on lead(II) ion-exchanged ZSM-5 catalysts

UV irradiation of the Pb²⁺/ZSM-5 catalyst prepared by an ion-exchange method in the presence of N₂O leads to the decomposition of N₂O into N₂. This reaction is found to be dramatically enhanced by the addition of propane to produce N₂ and oxygen-containing compounds such as ethanol or acetone. UV light effective for the reaction lies in wavelength regions shorter than 250 nm where the absorption band of the Pb²⁺ ion ([Xe] 4f¹⁴5d¹⁰6s² [Xe] 4f¹⁴5d¹⁰6s¹6p¹) exists, indicating that the excited state of the isolated Pb²⁺ ions plays a significant role in this decomposition of N₂O both in the absence and the presence of propane, and the role of propane is found to be a capture of oxygen atoms formed by the decomposition of N₂O.     

Measurement of nitrous oxide emissions from two rice-based cropping systems in China

A field experiment was conducted to investigate the effects of winter management and N fertilization on N₂O emission from a double rice-based cropping system. A rice field was either cropped with milk vetch (plot V) or left fallow (plot F) during the winter between rice crops. The milk vetch was incorporated in situ when the plot was prepared for rice transplanting. Then the plots V and F were divided into two sub-plots, which were then fertilized with 276 kg urea-N ha^–1 (referred to as plot VN and plot FN) or not fertilized (referred to as plot VU and plot FU). N₂O emission was measured periodically during the winter season and double rice growing seasons. The average N₂O flux was 11.0 and 18.1 g N m^–2 h^–1 for plot V and plot F, respectively, during winter season. During the early rice growing period, N₂O emission from plot VN averaged 167 g N m^–2 h^–1, which was eight- to fifteen-fold higher than that from the other three treatments (17.8, 21.0 and 10.8 g N m^–2 h^–1 for plots VU, FN, and FU, respectively). During the late rice growing period, the mean N₂O flux was 14.5, 11.1, 12.1 and 9.9 g N m^–2 h^–1 for plots VN, VU, FN and FU, respectively. The annual N₂O emission rates from green manure-double rice and fallow-double rice cropping systems were 3.6 kg N ha^–1 and 1.3 kg N ha^–1, respectively, with synthetic N fertilizer, and were 0.99 kg N ha^–1 and 1.12 kg N ha^–1, respectively, without synthetic N fertilizer. Generally, both green manure N and synthetic fertilizer N contribute to N₂O emission during double rice season.     

N2O decomposition over K-promoted Co-Al catalysts prepared from hydrotalcite-like precursors

N₂O decomposition was investigated over a series of K-promoted Co-Al catalysts. The activity tests showed that doping with K greatly enhanced the catalytic activity of the Co-Al catalyst, and the enhancement was critically dependent on the amount of K and the calcination temperature. When the catalyst had a K/Co atomic ratio of 0.04 and was calcined at 700–800 °C, a full N₂O conversion could be reached at a reaction temperature of 300 °C. Moreover, even under the simultaneous presence of 4% O₂ and 2.6% water vapor, such high-temperature treated K/Co-Al catalyst exhibited high reactivity and stability, with the N₂O conversion remaining at a constant value of 92% over 40 h run at 360 °C. In contrast, non-doped Co-Al catalyst showed a severe activity loss under such reaction conditions. A combination of characterization techniques was employed to reveal the promoting role of K and the effect of calcination temperature. The results suggest that doping with K increases the electron density of Co and weakens the Co–O bond, thus promoting the activation of N₂O on the Co sites and facilitating the desorption of oxygen from the catalyst surface. High-temperature calcinations made the desorption of O₂ proceed more readily.     

N2O decomposition over [Fe]-ZSM-5 and Fe-HZSM-5 zeolites

The N₂O decomposition over an [Fe]-ZSM-5 and an Fe-HZSM-5 zeolite was studied. We found that framework incorporated iron species were much more active than Fe(III) introduced as framework charge countercations by ion exchange (TOF at 0.1 vol% N₂O:1.47 × 10^–4 at 280°C for [Fe]-ZSM-5 vs. 2.58 × 10^–4 at 468°C for Fe-HZSM-5). The higher activity of [Fe]-ZSM-5 was attributed to the uniqueness of framework iron species. Both [Fe]-ZSM-5 and Fe-HZSM-5 zeolites showed enhanced activity in the presence of excess oxygen. This is in sharp contrast to ruthenium exchanged zeolites which showed strong oxygen inhibiting effect on the rate of N₂O decomposition.     

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.     

Nitrous oxide formation potential of various humid tropic soils of Malaysia: A laboratory study

Nitrous oxide (N₂O) is formed mainly during nitrification and denitrification. Inherent soil properties strongly influence the magnitude of N₂O formation and vary with soil types. A laboratory study was carried out using eight humid tropic soils of Malaysia to monitor NH₄ ⁺ and NO₃ ^– dynamics and N₂O production. The soils were treated with NH₄NO₃ (100 mg N kg^–1 soil) and incubated for 40 days at 60% water-filled pore space. The NH₄ ⁺ accumulation was predominant in the acid soils studied and NO₃ ^– accumulation/disappearance was either small or stable. However, the Munchong soil depicted the highest peak (238 g N₂O-N kg^–1 soil d^–1) at the beginning of the incubation, probably through a physical release. While the Tavy soil showed some NO₃ ^– accumulation at the end of the study with a maximum N₂O flux of 206 g N₂O-N kg^–1 soil d^–1, both belong to Oxisols. The other six soils, viz. Rengam, Selangor, Briah, Bungor, Serdang and Malacca series, formed smaller but maximum peaks in an decreasing order of 116 to 36 g N₂O-N kg^–1 soil d^–1. Liming the Oxisols and Ultisols raised the soil pH, resulting in NO₃ ^– accumulation and N₂O production to some extent. As such the highest N₂O flux of 130.2 and 77.4 g N₂O-N kg^–1 soil d^–1 was detected from the Bungor and Malacca soils, respectively. The Selangor soil, belonging to Inceptisol, did not respond to lime treatment. The respective total N₂O formations were 3.63, 1.92 and 1.69 mg N₂O-N kg^–1 soil from the Bungor, Malacca and Selangor soils, showing an increase by 49 and 99% over the former two non-limed soils. Under non-limed conditions, the indigenous soil properties, viz. Ca⁺⁺ content, %clay, %sand and pH of the soils collectively could have influenced the total N₂O formation.     

A kinetic study of NOx reduction over Pt/SiO2 model catalysts with hydrogen as the reducing agent

Flow reactor experiments and kinetic modeling have been performed in order to study the mechanism and kinetics of NO_x reduction over Pt/SiO₂ catalysts with hydrogen as the reducing agent. The experimental results from NO oxidation and reduction cycles showed that N₂O and NH₃ are formed when NO_x is reduced with H₂. The NH₃ formation depends on the H₂ concentration and the selectivity to NH₃ and N₂O is temperature dependent. A previous model has been used to simulate NO oxidation and a mechanism for NO_x reduction is proposed, which describes the formation/consumption of N₂, H₂O, NO, NO₂, N₂O, NH₃, O₂ and H₂. A good agreement was found between the performed experiments and the model.