N2O N2 Research Articles

Substrate Competition of Diazotrophic Nitrous Oxide Assimilation Over Dinitrogen Fixation

AbstractNitrous oxide (N2O) is a potent greenhouse gas and is depleting the stratospheric ozone layer. Diazotrophic N2O assimilation to biomass represents a novel biological N2O consumption pathway in addition to canonical denitrification. Thermodynamically, N2O assimilation is more favorable than dinitrogen (N2) fixation in natural environments, especially under higher N2O concentration and cooler conditions. Via isotopic tracing experiments, N2O assimilation was detected on cultured diazotrophs Crocosphaera and Trichodesmium with specific rates from 1.27 ± 0.16 × 10−4 to 2.00 ± 0.25 × 10−4 hr−1 under elevated [N2O]/[N2] conditions (0.0005–0.01) within 24‐hr incubation. The rates of N2O assimilation during the light and dark periods were statistically insignificant compared with N2 fixation activity. In a eutrophic estuary, N2O assimilation was not detected in the absence of diazotrophic activity. A competitive substrate kinetic model with experimentally calibrated parameters successfully quantified rate ratios of N2O assimilation and N2 fixation in varying substrate concentrations. The low [N2O]/[N2] ratio in natural conditions leads to N2O assimilation rate being <0.1% of N2 fixation rate, rendering negligible impact of N2O assimilation. The model was also used to predict the time required for experimental detection of N2O assimilation in isotopic tracing experiments under varying [N2O]/[N2] ratios. This study enhances the mechanistic understanding of N2O assimilation by diazotrophs, broadening the microbial nitrogen cycle by a potential N2O sink and nitrogen source for production.

Effects of Straw Amendment in Combination with Synthetic N Fertilizer Addition on N2O, N2, and Their Stoichiometric Ratios in Three Different Agro-Ecosystems

Nitrogen (N) fertilizer and crop residue amendments are important agricultural practices that could increase soil health, fertility, and crop yield. Such practices may also change soil denitrification processes where contradictory observations have been reported on soil N2O emissions with fewer studies on N2 emissions due to its large atmospheric background concentrations limiting its soil-borne measurement. This study aims to investigate N2O production and reduction of N2 emissions under a conducive denitrifying environment (like anaerobic microsites, 80% WFPS, available N and C) after rice straw amendment and KNO3 application to three different soil types (fluvo-aquic, black, and paddy soils). In this regard, three treatments for three different soil types were set consisting of (a) a non-amended treatment (control), (b) a KNO3 treatment (KNO3, 20 mM KNO3), and (c) a straw plus KNO3 treatment (2.5 g rice straw kg−1 dry soil and 20 mM KNO3), which were incubated under 80% WFPS. Moreover, direct N2O and N2 fluxes were measured over 17 days in the current incubation experiment with a robotized incubation system using a helium atmosphere. Results showed that rice straw amendment combined with N fertilizer increased both N2O and N2 fluxes compared with control or KNO3 treatments in all three soil types. Overall, compared with the black and paddy soils, the N2O and N2 fluxes were higher in the fluvo-aquic soil, with a maximum of 234.2 ± 6.3 and 590.1 ± 27.3 g N ha−1 from F_SK treatment, respectively, during the incubation period. The general trends in three soil types of both N2O and N2 emissions were control < KNO3 < rice straw plus KNO3 treatments. Straw amendment in combination with KNO3 can stimulate a high denitrification rate (less N2O and higher N2), whereas their effect on stoichiometric ratios of N2O/(N2O + N2) highly depends on soil nitrate concentration, oxygen level, soil moisture content, and labile C. The current study underscores that the rice straw amendment in combination with N fertilizer can trigger denitrification with less increment on soil N2O but higher N2 emissions under conditions favoring denitrification.

A new ab initio potential energy surface and rovibrational spectra for the N2–N2O complex

We propose a new potential energy surface for the N2–N2O complex, which encompasses the Q3 normal mode for the ν3 antisymmetric stretching coordinate of N2O. The interaction potential energies were obtained at the explicitly correlated coupled cluster with single, double, and perturbative triple excitations [CCSD(T)-F12a] level with aug-cc-pVTZ basis set plus mid-bond functions. Four-dimensional vibrationally averaged potentials with N2O in the ground and ν3 excited states were generated through integration over the Q3 coordinate. Both potentials exhibit two equivalent skew T-shaped global minima. The rovibrational energies were calculated by employing the radial discrete variable representation/angular finite basis representation approach. The determined fundamental band origin shift of the infrared spectra in the N2O ν3 region, transition frequencies and rotational constants are all consistent with the experimental values.

N2O consumption, uptake, and microbial reduction processes in flooded sandy loamy paddy soils

AbstractSandy loamy soils are widely distributed in fluvial floodplains and experience flooding events frequently, resulting in a large amount of nitrous oxide (N2O) emissions. This case is more serious when the soil use is changed to paddies. It is of great significance to figure out the N2O consumption and its influencing factors in sandy loamy paddy soils to mitigate N2O emissions. In this study, three sandy loamy paddy soils (0–5 cm) originated from lake deposits were selected (S1, S2, and S3) as objectives. A certain concentration of exogenous N2O was added at the bottom of the flooded soil column to monitor the dynamics of N2O and nitrogen (N2) on the soil surface. Total N2O consumption, N2O uptake, and N2 production were quantified, and the abundance of nitrous oxide reductase genes (nosZI, and nosZII) and other soil properties (ammonium‐nitrogen, nitrate‐nitrogen, and dissolved organic carbon [DOC] content) were analyzed. The results showed that the sandy loamy paddy soil column with a depth of 0–5 cm could intercept more than 95% of the exogenous N2O under the flooded anaerobic condition, indicating that the three sandy loamy paddy soils all had extremely strong N2O consumption capacities. And the increment of N2 accounted for 68.73%–76.09% of the total N2O consumption, which had a stronger relationship with the increase of nosZI gene abundance than nosZII gene. In addition, the total N2O consumption and N2 increment of S1 and S3 soils were significantly higher than those of S2 soil. This difference was mainly related to soil organic matter content, total nitrogen content, DOC consumption, and the increase of nosZI gene abundance (p < 0.05). The strong N2O consumption potential of sandy loamy soils can provide feasible solutions for regulating N2O emissions in a wide range of similar environments in fluvial floodplains.

Near-ambient pressure velocity map imaging.

We present a new velocity map imaging instrument for studying molecular beam surface scattering in a near-ambient pressure (NAP-VMI) environment. The instrument offers the possibility to study chemical reaction dynamics and kinetics where higher pressures are either desired or unavoidable, adding a new tool to help close the "pressure gap" between surface science and applied catalysis. NAP-VMI conditions are created by two sets of ion optics that guide ions through an aperture and map their velocities. The aperture separates the high pressure ionization region and maintains the necessary vacuum in the detector region. The performance of the NAP-VMI is demonstrated with results from N2O photodissociation and N2 scattering from a Pd(110) surface, which are compared under vacuum and at near-ambient pressure (1 × 10-3 mbar). NAP-VMI has the potential to be applied to, and useful for, a broader range of experiments, including photoelectron spectroscopy and scattering with liquid microjets.

Stronger link of nosZI than nosZII to the higher total N2O consumption in anoxic paddy surface soils

Rice paddy surface soils emit substantial amounts of atmospheric nitrous oxide (N2O). However, the soil can also act as sites for the abiotic and biological consumption of some of the N2O produced in the soil profile, thus, reducing net N2O emissions. Determining the proportion of N2O consumed in the soil, as well as the amount of N2O microbially reduced to nitrogen (N2) is critical for developing N2O mitigating strategies in paddy soils. However, few studies have focused on these processes. Furthermore, the association of microorganisms containing nosZI and nosZII genes with variations in biological N2O consumption potential remains largely unexplored. Here, moisture-controlled (60% gravimetric water content, GWC) and completely anoxic (helium [He] system) microcosm experiments were conducted on paddy surface soils (0–5 cm deep) treated with N2O. During a 96-h incubation period, the N2O cumulative emissions were monitored, the total N2O consumption and N2 production were quantified, and the population sizes of nosZI- and nosZII-containing microorganisms were analyzed. The results showed that 97.87%–99.99% of the N2O that accumulated in the 5 cm soil profile were consumed before emission, and the N2 increase in these soils accounted for 64.50%–82.44% of the total N2O consumption, indicating that the total N2O consumption potential and biological N2O sink capacity of paddy surface soils were considerable under experimental conditions. Furthermore, the production of N2 from N2O seemed to be positively related to soil pH, with less N2 being produced in more acidic soil. In addition, nosZI gene abundance had a strong positive correlation with N2 production, while nosZII had a much weaker correlation. Considering the positive correlation between N2 production and soil dissolved organic carbon (DOC) consumption and available potassium content (AK), it is speculated that complete denitrification by nosZI-N2O reducers dominate the N2O biological sink capacity of the paddy surface soils investigated in this study.

Nitric Oxide Generation in N2-Diluted H2–N2O Flames: A Computational Study

A computational study was conducted on the addition of nitrogen (0–60 vol%) to stoichiometric hydrogen–nitrous oxide flames at various initial pressures (1–10 bar) and temperatures (300–500 K), with a special emphasis on NO, which is a major pollutant that is generated by these flames. The detailed kinetic modeling of H2–N2O–N2 flames, which was performed using the GRI 3.0 mechanism, produced the structure of the H2–N2O–N2 flames for the temperature and mass fraction profiles of 53 species, which participated in 325 elementary reactions. The NO profiles that were computed by the detailed kinetic modeling followed the trend of experimental NO profiles that have been reported in the literature for laminar premixed flames at sub-atmospheric pressures. For the examined H2–N2O–N2 flames, an increase in initial pressure resulted in a significant reduction in the NO mass fraction of the burned gas. The increase in initial temperature was also accompanied by an increase in the mass fractions of the generated NO, as well as the flame temperature.

Numerical Study of Laminar Flame Propagation in CH4–N2O–N2 at Moderate Pressures and Temperatures

Numerical Study of Laminar Flame Propagation in CH4–N2O–N2 at Moderate Pressures and Temperatures

N2 and N2O mitigation potential of replacing maize with the perennial biomass crop Silphium perfoliatum—An incubation study

AbstractSustainability of biogas production is strongly dependent on soil‐borne greenhouse gas (GHG) emissions during feedstock cultivation. Maize (Zea mays) is the most common feedstock for biogas production in Europe. Since it is an annual crop requiring high fertilizer input, maize cropping can cause high GHG emissions on sites that, due to their hydrology, have high N2O emission potential. On such sites, cultivation of cup plant (Silphium perfoliatum) as a perennial crop could be a more environmentally friendly alternative offering versatile ecosystem services. To evaluate the possible benefits of perennial cup plant cropping on GHG emissions and nitrogen losses, an incubation study was conducted with intact soil cores from a maize field and a cup plant field. The 15N gas flux method was used to quantify N source‐specific N2 and N2O fluxes. Cumulated N2O emissions and N2+N2O emissions did not differ significantly between maize and cup plant soils, but tended to be higher in maize soil. Soils from both systems exhibited relatively high and similar N2O/(N2+N2O) ratios (N2Oi). N2O emissions originating from sources other than the 15N‐labelled NO3 pool were low, but were the only fluxes exhibiting a significant difference between the maize and cup plant soils. Missing differences in fluxes derived from the 15N pool indicate that under the experimental conditions with high moisture and level, and without plants, the cropping system had little effect on N fluxes related to denitrification. Lower soil pH and higher bulk density in the cup plant soil are likely to have reduced the mitigation potential of perennial biomass cropping.

Photocatalytic deoxygenation of N-O bonds with rhenium complexes: from the reduction of nitrous oxide to pyridine N-oxides.

The accumulation of nitrogen oxides in the environment calls for new pathways to interconvert the various oxidation states of nitrogen, and especially their reduction. However, the large spectrum of reduction potentials covered by nitrogen oxides makes it difficult to find general systems capable of efficiently reducing various N-oxides. Here, photocatalysis unlocks high energy species able both to circumvent the inherent low reactivity of the greenhouse gas and oxidant N2O (E0(N2O/N2) = +1.77 V vs. SHE), and to reduce pyridine N-oxides (E1/2(pyridine N-oxide/pyridine) = −1.04 V vs. SHE). The rhenium complex [Re(4,4′-tBu-bpy)(CO)3Cl] proved to be efficient in performing both reactions under ambient conditions, enabling the deoxygenation of N2O as well as synthetically relevant and functionalized pyridine N-oxides.

Impact of N2O admixture on the characteristics of pulsed dielectric barrier discharges in N2 at atmospheric pressure

The impact of N2O admixtures on the inception and development of pulsed dielectric barrier discharges (DBDs) in N2 at atmospheric pressure is studied using optical and electrical diagnostics supported by 0D modelling of the reaction kinetics. The N2O concentration in the N2–N2O gas mixture was varied between virtually pure N2 and 10 vol% N2O. A significant impact of N2O on the discharge characteristics was found for concentrations above 1000 ppm. In particular, a decrease of the transferred charge, and discharge energy and an increase of breakdown voltage, maximal discharge current, discharge emission diameter and velocity of the cathode-directed ionisation front (positive streamer) were observed. Additionally, a strong increase of breakdown voltage and discharge inception jitter accompanied by a decrease of the emission duration was found for N2O concentrations above 1 vol%. The modelling results indicate that for N2O concentrations in this range a transition from an electropositive to an electronegative regime takes place.

Theoretical studies of infrared spectra for the N2-N2O complex: The tunneling effects of fundamental and combination bands.

This study is a continuation of our previously published research for the N2-N2O complex [Journal of Chemical Physics 143 (2015) 154304], and focuses on predicting the quantum tunneling effects of infrared fundamental and combination bands. A new four-dimensional intermolecular potential energy surface (PES) was constructed for the vibrational excited state upon the N2O ν1 excitation at the same calculated level with the previous PES of ground state. Compared with the ground state, two equivalent T-shaped global minima are found to be slightly different in both structural parameters and binding energies. Based on the PESs of ground and vibrational excited states, the vibrational shift of infrared spectrum for the fundamental band in the N2O ν1 region is determined to be a blue shift of 1.29 cm-1 for 14N2-N2O, which is in qualitative agreement with the experimentally observed value of 2.233 cm-1 [Journal of Chemical Physics 140 (2014) 044332]. Furthermore, for the fundamental and disrotation bands, the calculated tunneling splitting is almost the same for the ground and vibrational excited states, so the tunneling effects cannot be observed for these bands using the infrared spectroscopic technique. Nevertheless for the infrared combination bands, our calculated results suggest that the tunneling effects for the torsion and twice disrotation bands are significantly larger, and the predicted infrared spectra display obvious differences for the different sub-states. If the sensitivity of infrared spectrometer is high enough, it is interesting to investigate the quantum tunneling effects experimentally.

Straw amendment with nitrate-N decreased N2O/(N2O+N2) ratio but increased soil N2O emission: A case study of direct soil-born N2 measurements

Straw application in combination with synthetic N fertilizer could increase crop yield and improve soil fertility, however, contradictory observations have been reported on the effects of straw addition on soil N2O emission. Straw application can affect both denitrification rate and its product stoichiometry (N2O/(N2O + N2) ratio), whereas the latter remains rather unclear since the ratio is strongly regulated by other soil parameters, e.g. nitrate and oxygen concentrations at denitrifying micro-sites. In this context, we conducted an incubation experiment with a robotized continuous flow incubation system using a He/O2 atmosphere and measured N2O and direct N2 fluxes over 22 days. Soil amended with and without rice straw (2.5 g kg−1 soil) in conjunction with nitrate fertilizer (10 mM KNO3) and non-amended control soil were incubated under 85% water-filled pore space. To simulate a short soil anoxic period, three different O2 partial pressures phases were set (20%, 5% and 10%). Additionally, N2O site preference signatures of soil-emitted N2O were analyzed to identify the processes contributing to N2O fluxes. Addition of nitrate increased cumulative N2O fluxes and decreased cumulative N2 fluxes compared with non-fertilized control, while rice straw amendment increased both N2O and N2 emissions drastically compared with the nitrate only treatment. The N2O SP values ranged from 0.4 to 2.7‰ among all treatments, indicating denitrification/nitrifier denitrification was the dominating source. The results suggest that straw amendment can trigger high denitrification rate, whereas the effect of straw amendment on the amount of emitted N2O and the N2O/(N2O + N2) product ratio strongly depends on soil NO3− concentration. As a conclusion, the present study suggests that straw amendment in conjunction with nitrate-N can increase soil N2O emissions under conditions favoring denitrification, even though it may decrease the overall N2O/(N2O + N2) product ratio.

Influence of Deposition Temperature and Source Gas in PE-CVD for SiO2 Passivation on Performance and Reliability of In–Ga–Zn–O Thin-Film Transistors

In this paper, we investigated the influence of both source gas and deposition temperature in plasma-enhanced chemical vapor deposition (PE-CVD) for a SiO2 passivation layer on the electrical properties and reliability of a bottom-gate In–Ga–Zn–O thin-film transistor (IGZO TFT). Two gas chemistries consisting of SiH4–N2O–N2 and tetraethoxysilane (TEOS)–O2 were utilized as the source gases for the PE-CVD SiO2 deposition, and the deposition temperature ( ${T}_{D}$ ) was adjusted from 180 °C to 380 °C. The TFT properties were basically identical for both gas chemistries at ${T}_{D}$ of 180 °C. When ${T}_{D}$ increased to 300 °C or higher, the TFTs with the SiO2 passivation deposited by SiH4–N2O–N2 gas chemistry (SiH4-SiO2) drastically changed from the transistor to the conductor. In contrast, the TFT with TEOS-SiO2 passivation maintained its TFT characteristics even at ${T}_{D}$ of 380 °C, despite the degradation of subthreshold characteristics and a negative shift of turn-on voltage were observed due to an electron injection barrier lowering as ${T}_{D}$ increased to 310 °C or higher. We also revealed that a stacked SiO2 passivation that is deposited at a different ${T}_{D}$ is an effective technique to improve the performance and reliability of the IGZO TFT.

Methane-unconventional oxidant flames. Laminar burning velocities of nitrogen-diluted methane–N 2 O mixtures

Abstract The flame propagation in nitrogen-diluted CH4–N2O mixtures was monitored by pressure measurements during explosions in a spherical vessel with central ignition. The burning velocities were obtained from experimental measurements of pressure variation during closed vessel explosions and from the detailed modelling of free laminar premixed flames. Lean- and stoichiometric methane–nitrous oxide mixtures (equivalence ratios: 0.8 and 1.0) diluted by various amounts of nitrogen between 40 and 60 vol% were studied at various initial pressures between 0.3 and 1.8 bar and ambient initial temperature. Nitrogen addition to each CH4–N2O mixture results in the decrease of laminar burning velocity and flame temperature, along with the increase of flame width. Examination of burning velocity variation against average flame temperature in experiments at constant initial pressure and various inert concentrations allowed the determination of the overall activation energy; examination of burning velocity dependence on pressure, at constant inert concentration, allowed the determination of the overall reaction orders. For all CH4–N2O–N2 mixtures, the temperature, volumetric rate of heat release and reactive species concentration profiles across the flame front were examined versus similar data characteristic to stoichiometric methane–air mixtures. The most important elementary reactions in CH4–N2O–N2 and CH4–air mixtures were identified by means of sensitivity analysis.

Hydrogenation of aqueous nitrate and nitrite with ruthenium catalysts

Historically, development of catalysts for treatment of nitrate-contaminated water has focused on supported Pd-based catalysts, but high costs of the Pd present a barrier to commercialization. As part of an effort to develop lower cost hydrogenation catalysts for water treatment applications, we investigated catalysts incorporating Ru with lower cost. Pseudo-first-order rate constants and turnover frequencies were determined for carbon- and alumina-supported Ru and demonstrated Ru’s high activity for hydrogenation of nitrate at ambient temperature and H2 pressure. Ex situ gas pretreatment of the catalysts was found to enhance nitrate reduction activity by removing catalyst surface contaminants and exposing highly reducible surface Ru oxides. Ru reduces nitrate selectively to ammonium, and no aqueous nitrite intermediate is observed during reactions. In contrast, reactions initiated with nitrite yield a mixture of two endproducts, with selectivity shifting from ammonium towards N2 at increasing initial aqueous nitrite concentrations. Experimental observation and Density Functional Theory calculations together support a reaction mechanism wherein sequential hydrogenation of nitrate to nitrite and NO is followed by parallel pathways involving the adsorbed NO: (1) sequential hydrogenation to ammonium, and (2) N–N coupling with aqueous nitrite followed by hydrogenation to the detected N2O intermediate and N2 endproduct. These findings open the door to development of alternative catalysts for purifying and recovering nutrients from nitrate-contaminated water sources, and insights into the controlling surface reaction mechanisms can guide rational design efforts aimed at increasing activity and tuning endproduct selectivity.

Theoretical studies for the N2-N2O van der Waals complex: The potential energy surface, intermolecular vibrations, and rotational transition frequencies.

Theoretical studies of the potential energy surface (PES) and bound states are performed for the N2-N2O van der Waals (vdW) complex. A four-dimensional intermolecular PES is constructed at the level of single and double excitation coupled-cluster method with a non-iterative perturbation treatment of triple excitations [CCSD(T)] with aug-cc-pVTZ basis set supplemented with bond functions. Two equivalent T-shaped global minima are located, in which the O atom of N2O monomer is near the N2 monomer. The intermolecular fundamental vibrational states are assigned by inspecting the orientation of the nodal surface of the wavefunctions. The calculated frequency for intermolecular disrotation mode is 23.086 cm(-1), which is in good agreement with the available experimental data of 22.334 cm(-1). A negligible tunneling splitting with the value of 4.2 MHz is determined for the ground vibrational state and the tunneling splitting increases as the increment of the vibrational frequencies. Rotational levels and transition frequencies are calculated for both isotopomers (14)N2-N2O and (15)N2-N2O. The accuracy of the PES is validated by the good agreement between theoretical and experimental results for the transition frequencies and spectroscopic parameters.

Nitrous oxide emission from agricultural soils

Anthropogenic nitrogen loading, particularly fertiliser usage in agricultural soils is thought to be a potentially important source of nitrous oxide (N2O) emission, which can be controlled by properly managed fertiliser usage. In this research, laboratory scale experiments were conducted to evaluate N2O production and emission from agricultural soils under variable oxygen availability conditions. N2O was observed to be released from the reactors in the absence of oxygen as well as at low concentrations of oxygen. With the increase of oxygen concentration, N2O production decreased. In addition, N2O emission was found to coincide with corresponding nitrate (NO3−) depletion, indicating that denitrification was the dominating process that was responsible for N2O production. The depletion of NO3− was described by the sum of two exponential functions, i.e., NO3− reduction for N2O and NO3− reduction for N2. The simulation of the experimental results using above model demonstrated that N2 production dominated over N2O production in the absence of oxygen. Both N2O production rate and N2 production rate decreased with the increase of oxygen concentration. However, the decrease of N2 production rate was more pronounced than that of N2O production rate.

Direct N2O decomposition over La2NiO4-based perovskite-type oxides

Direct decomposition of N2O by perovskite-structure catalysts including La2NiO4, LaSrNiO4, and La0.7Ce0.3SrNiO4 was investigated. The catalysts were prepared by the Pechini method and characterized by x-ray diffraction (XRD), BET, scanning electron microscopy (SEM), and O2-TPD. Experimental results indicate that the properties of La2NiO4 are significantly improved by partially substituting La with Sr and Ce. N2O decomposition efficiencies achieved with LaSrNiO4 and La0.7Ce0.3SrNiO4 are 44 and 36%, respectively, at 400ºC. As the temperature was increased to 600ºC, N2O decomposition efficiency achieved with LaSrNiO4 and La0.7Ce0.3SrNiO4 reached 100% at an inlet N2O concentration of 1,000 ppm, while the space velocity was fixed at 8,000 hr−1. In addition, effects of various parameters including oxygen, water vapor, and space velocity were also explored. The results indicate that N2O decomposition efficiencies achieved with LaSrNiO4 and La0.7Ce0.3SrNiO4 are not significantly affected as space velocity is increased from 8,000 to 20,000 hr−1, while La0.7Ce0.3SrNiO4 shows better tolerance for O2 and H2O(g). On the other hand, N2 yield with LaSrNiO4 as catalyst can be significantly improved by doping Ce. At a gas hour space velocity of 8,000 hr−1, and a temperature of 600ºC, high N2O decomposition efficiency and N2 yield were maintained throughout the durability test of 60 hr, indicating the long-term stability of La0.7Ce0.3SrNiO4 for N2O decomposition.Implications:Nitrous oxide (N2O) not only has a high global warming potential (GWP100 = 310), but also potentially destroys ozone in the stratosphere. Pervoskite-type catalysts including La2NiO4, LaSrNiO4, and La0.7Ce0.3SrNiO4 are applied for direct N2O decomposition. The results show that N2O decomposition can be enhanced as Sr and Ce are doped into La2NiO4. At 600ºC, N2O decomposition efficiencies achieved with LaSrNiO4 and La0.7Ce0.3SrNiO4 reach 100%, demonstrating high activity and good potential for direct N2O decomposition. Effects of O2 and H2O(g) contents on catalytic activities are also evaluated and discussed.

Selective oxidation of ammonia on mixed and dual-layer Fe-ZSM-5 + Pt/Al2O3 monolithic catalysts

The development of improved Ammonia Slip Catalysts (ASCs) was pursued by changing the composition and architecture of the critical functions, Selective Catalytic Reduction (SCR) and oxidation, needed in the catalyst for high NH3 oxidation activity and selectivity to N2. Selected space velocities and reactant compositions were used to probe chemical and physical processes that limit the performance of ASCs. On a single component Pt/Al2O3 catalyst, at the lower space velocity (66kh−1) light-off was characterized by a sharp increase in NH3 conversion to >98%, while at high space velocity (265kh−1) the transition in conversion was more gradual and the limiting conversion was below 98%, indicative of transverse transport limitations. The light-off temperature of ammonia oxidation on Pt/Al2O3 catalyst decreased with Pt loading from 0.7 to 10g Pt/ft3 monolith and, in general, the selectivity to N2 decreased while that of N2O and NOx increased. A dual-layer ASC comprising a top layer of Fe-ZSM-5 and a bottom layer of Pt/Al2O3 resulted in higher selectivity and yield to N2, due to SCR reactions between the counter diffusing NOx formed in the Pt/Al2O3 and NH3 reactant in the Fe-zeolite layer. However, the diffusion resistance provided by the Fe-ZSM-5 layer inhibited the overall ammonia conversion at high space velocity. When Fe-zeolite and Pt/Al2O3 particles were mixed and washcoated as a single layer, this led to an increase in NH3 conversion at high space velocity due to a decrease in the diffusion barrier that was observed with the dual-layer structure. When SCR and oxidation catalyst particles were contiguous in the washcoat structure as in single-layer mixed catalysts, the N2 yield was lower due to Pt-catalyzed NH3 oxidation, compared to the dual-layer ASC, which was especially apparent at low space velocity. The dual-layer catalyst was superior to the mixed layer catalyst at high temperatures, exhibiting lower NOx and higher N2 yields whereas the mixed catalyst out-performed the dual-layer catalyst at low temperature by exhibiting lower N2O and higher N2 yield.