Superior performance of Ir-substituted hexaaluminate catalysts for N2O decomposition

Novel Ir-substituted hexaaluminate catalysts were developed for the first time and used for catalytic decomposition of high concentration of N₂O. The catalysts were prepared by one-pot precipitation and characterized by X-ray diffraction (XRD), N₂-adsorption, scanning electronic microscopy (SEM) and temperature-programmed reduction (H₂-TPR). The XRD results showed that only a limited amount of iridium was incorporated into the hexaaluminate lattice by substituting Al³⁺ to form BaIr_xFe_1−xAl₁₁O₁₉ after being calcined at 1200 °C, while the other part of iridium existed as IrO₂ phase. The activity tests for high concentration (30%, v/v) of N₂O decomposition demonstrated that the BaIr_xFe_1−xAl₁₁O₁₉ hexaaluminates exhibited much higher activities and stabilities than the Ir/Al₂O₃-1200, and the pre-reduction with H₂ was essential for activating the catalysts. By comparing BaIr_xFe_1−xAl₁₁O₁₉ with BaIr_xAl_12−xO₁₉ (x = 0–0.8), it was found that iridium was the active component in the N₂O decomposition and the framework iridium was more active than the large IrO₂ particles. On the other hand, Fe facilitated the formation of hexaaluminate as well as the incorporation of iridium into the framework.     

The influence of silver on the properties of cryptomelane type manganese oxides in N2O decomposition reaction

The cryptomelane type oxides were prepared by the redox precipitation technique using Mn(CH₃COO)₂ and KMnO₄ precursors. Nitrogen sorption, XRD, TEM and TPD of oxygen studies showed changes of the surface, structural and redox properties of the samples upon silver introduction. Samples were investigated in the N₂O decomposition reaction. Direct introduction of silver to the synthesis mixture caused partial distortion of regular channel-like structure of the oxides, leading to the decrease of Mn–O bonds strength. As results samples showed slightly better catalytic activity at low temperatures, but were less stable at high temperatures. An introduction of silver by the impregnation method caused the decrease of the surface area of the samples, increase of surface oxygen mobility, leading to the small changes of activity.     

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.     

The influence of precursors on Rh/SBA-15 catalysts for N2O decomposition

Series of Rh/SBA-15 catalysts were prepared by impregnation and grafting method applying different Rh precursors. The catalytic behaviors of N₂O decomposition over these catalysts were tested in an automated eight flow reactor system. The catalysts were characterized by X-ray fluorescence spectroscopy (XRF), X-ray diffraction (XRD), N₂ adsorption/desorption, transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS) techniques. The results showed that the dispersion of Rh species on the catalysts is closely related to the molecular size and the hydrophobic property of the precursors comparing to the hydrophilic support, better dispersion results were found in catalysts by impregnation of smaller precursors, while by grafting better dispersion resulted from big precursor. On the other hand, the activities of the catalysts match well with the Rh dispersion status. Rh/SBA-15-CDCR starting from [(CO)₂RhCl]₂ showed good dispersion and gave the best N₂O decomposition activity.     

N2O decomposition over the circulating ashes from coal-fired CFB boilers

The catalytic decomposition of N₂O over the circulating ashes from coal-fired circulating fluidized bed (CFB) boilers was investigated with a fixed bed reactor. The associated kinetics was mimicked by four surrogate metal oxides of SiO₂, Al₂O₃, CaO and Fe₃O₄, which were found as the main components of the circulating ashes. The activation energies and collision coefficients for N₂O thermal decomposition over the circulating ashes and surrogates were individually measured. Experimental results showed that different metal oxides play different roles in the catalytic decomposition of N₂O. Among the components, CaO and Fe₃O₄ are very active, while Al₂O₃ and SiO₂ contribute much less to N₂O destruction. A model based on the specific surface-area-weighted kinetic data of individual surrogates was developed to predict the catalytic decomposition of N₂O over circulating ashes. The predictions agreed with the experimental data with a minor discrepancy acceptable in an engineering view. Some discussions on the discrepancy were given. The O₂ effect on the N₂O decomposition over the circulating ashes was experimentally assessed. It was found that the presence of O₂, even with a small amount, would deteriorate the catalytic decomposition of N₂O.     

N2O decomposition on Fe- and Co-ZSM-5: A density functional study

Density functional theory (DFT) calculations are employed to study N₂O decomposition on relaxed [(SiH₃)₄AlO₄M] (where M = Fe, Co) cluster models representing Fe- and Co-ZSM-5 surfaces and Fe-ZSM-5 channel cluster. The catalytic cycle steps are completed both for Fe- and Co-ZSM-5 clusters. It is found that the general trend of the results obtained is in agreement with experimental and theoretical literature: Co-ZSM-5 has a lower activation energy barrier than Fe-ZSM-5 and O₂ desorption step is the rate-limiting step for both clusters. The activation barrier for the decomposition of the first N₂O molecule inside a Fe-ZSM-5 channel cluster increases in comparison with that of the cluster model indicating a channel effect on the activation barrier. The activation barrier reported for the channel cluster is 12.63 kcal/mol. This is also in good agreement with experimental literature.     

Highly active and stable bimetallic Ir/Fe-USY catalysts for direct and NO-assisted N2O decomposition

The current research investigated N₂O decompositions over the catalysts Ir/Fe-USY, Fe-USY and Ir-USY under various conditions, and found that a trace amount of iridium (0.1 wt%) incorporated into Fe-USY significantly enhanced N₂O decomposition activity. The decomposition of N₂O over this catalyst (Ir/Fe-USY-0.1%) was also partly assisted by NO present in the gas mixture, in contrast to the negative effect of NO over noble metal catalysts. Moreover, Ir/Fe-USY-0.1% can decompose more than 90% at 400 °C (i.e. the normal exhaust temperature) under simulated conditions of a typical nitric acid plant, e.g. 5000 ppm N₂O, 5% O₂, 700 ppm NO and 2% H₂O in balance He, and such an activity can be kept for over 110 h under these strict conditions. The excellent properties of bimetallic Ir/Fe-USY-0.1% catalyst are presumably related to the good dispersion of Fe and Ir on the zeolite framework, the formation of framework Al–O–Fe species and the electronic synergy between the Ir and Fe sites. The reaction mechanism for N₂O decomposition has been further discussed on the temperature-programmed desorption profiles of O₂, N₂ and NO₂.     

DRIFTS study of the catalytic N2O reduction by SO2 on FeZSM-5

SO₂ has been recognized as an effective reducing agent for N₂O over iron-containing zeolite catalysts, lowering the operation temperature up to 100 K with respect to the direct N₂O decomposition. This unique behavior contrasts with the common poisoning effect of SO₂ over other active de-N₂O metals (e.g. Co, Cu, Rh, and Ru). The formation of surface sulfates has been generally posed as the main cause for catalyst deactivation by SO₂. Through the use of in situ infrared spectroscopy (DRIFTS), we show that steam-activated FeZSM-5 indeed builds up stable sulfate species during the N₂O + SO₂ reaction. Significant amounts of sulfur were detected in the used catalyst by elemental analysis and X-ray photoelectron spectroscopy. However, the enhanced N₂O conversion is remarkably stable, indicating that the reducing action by SO₂ and the sulfation of the surface are decoupled. The resulting sulfate species are thus spectators in the catalytic process and do not block or alter the structure of the active sites for N₂O reduction and decomposition.     

Catalytic decomposition of N2O on supported Pd catalysts: Support and thermal ageing effects on the catalytic performances

This study is devoted to the catalytic decomposition of N₂O over noble metal-based catalysts under lean conditions in the presence of O₂, NO and water. A particular attention has been paid toward the influence of the support and the thermal ageing-induced effects on the catalytic properties of palladium species. In those operating conditions, the deposition of palladium on reducible supports, such as LaCoO₃, leads to higher activity in comparison with conventional supports such as alumina. Surface reconstructions take place during thermal ageing under reactive conditions on pre-reduced perovskite-based catalysts which lead to a significant rate enhancement in the decomposition of N₂O. On the other hand, it was found that oxygen and water strongly inhibit the surface reconstructions associated with changes in the selectivity towards the production of NO₂.     

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 reduction of N2O by ethylbenzene over novel hydrotalcite-derived Mg–Cr–Fe–O as an alternative route for simultaneous N2O abatement and styrene production

New hydrotalcite-like materials containing magnesium, chromium, and/or iron were synthesized by the coprecipitation method and then thermally transformed into mixed metal oxides. The obtained catalysts were characterized with respect to chemical composition (XRF), structural (XRD, Mössbauer spectroscopy) and textural (BET) properties. The catalytic performance of the hydrotalcite-derived oxides was tested in the N₂O decomposition and the N₂O reduction by ethylbenzene. An influence of N₂O/ethylbenzene molar ratio on the process selectivity was studied. The relationship between catalytic performance and structure of catalysts was discussed.     

The destruction of N2O in a pulsed corona discharge reactor

The removal of N₂O by a pulsed corona reactor (PCR) was investigated. Gas mixtures containing N₂O were allowed to flow in the reactor at various levels of energy input, and for different background gases, flow rates, and initial pollutant concentrations. The reactor effluent gas stream was analyzed for N₂O, NO, NO₂, by means of an FTIR spectrometer. It was found that destruction of N₂O was facilitated with argon as the background gas; the conversion dropped and power requirements increased when nitrogen was used as the background gas.Reaction mechanisms are proposed for the destruction of N₂O in dry argon and nitrogen. Application of the pseudo-steady state hypothesis permits development of expressions for the overall reaction rate in these systems. These reaction rates are integrated into a simple reactor model for the pulsed corona discharge reactor. The reactor model brings forth the coupling between reaction rates, electrical discharge parameters, and fluid flow within the reactor. Comparison with experiment is encouraging, though the needs for additional research are clearly identified.     

Real time intracellular pH dynamics in Listeria innocua under CO2 and N2O pressure

This article investigates the inactivation mechanism of high-pressure food treatment, considered as alternative to conventional biocidal processes. We aimed to determine intracellular pH decrease under CO₂ and N₂O pressure, so far postulated as one of the main causes of inactivation. Working with a lab-scale bioreactor in mild conditions – 25 °C and pressures up to 8 MPa – we monitored – for the first time during pressurization – cytoplasmic pH variations of Listeria innocua labeled with pH-sensitive fluorophores based on fluorescein.We show that carbonic acid, due to solubilization of CO₂ into the aqueous phase, causes a rapid pH drop in the cytosol, reaching pH 4.8 at 1 MPa and falls below the detection limit of the indicator fluorophore of pH 4.0. This correlates with a reduced viability (below 90%) in all the pressure ranges investigated. Contrarily, treatment under N₂O pressure reduces cell viability without significant pH-drop neither of intra- nor extra-cellular liquid at any pressure investigated. The pH value remains between 7 and 6 while an inactivation of more than 80% is achieved at 8 MPa.Our data clearly demonstrate that, as a critical pressure is achieved, microbial inactivation is mainly due to pressure-induced membrane permeation – stimulated by non-acidifying fluids as well, rather then cytoplasmic acidification, as widely argued so far. A definitive understanding of the microbial inactivation mechanism due to CO₂/N₂O under pressure has been advanced significantly.     

Modelling of NO and N2O emissions from biomass-fired circulating fluidized bed combustors

A model for NO and N₂O emissions from biomass-fired circulating fluidized bed (CFB) combustors has been developed and evaluated in this study. All the model parameters were chosen for a typical woody biomass-pinewood chips. Both drying and devolatilization of biomass particles were modelled with limited rates, which were selected from the literature based on woody biomass fuels. The partition of fuel-nitrogen between volatiles and char was also specifically chosen for pinewood based on available experimental data from the literature. Volatile nitrogen was assumed to consist of NH₃, HCN and N₂ with the distribution between three species as input parameters to the model. Twenty-five homogenous and heterogeneous global chemical reactions were included in the model, of which 20 reactions represents the global fuel-nitrogen reactions. Both gaseous and solid phase were assumed to be in plug flow. The model has been applied to the modelling of a 12 MW_th CFB boiler. The predicted N₂O emissions were always less than 5 ppmv for pinewood combustion, which was consistent with the experimental results. The predicted NO emissions increased with the total excess air of the riser and the fuel-N content while the predicted percentage conversion of fuel-N to NO decreased with increasing fuel-N content. The NO emissions were also predicted to decrease with increasing primary zone stoichiometry. These predictions agree with the experimental results. The predicted NO emissions decreased slightly with increasing bed temperature, whereas experiments showed that NO emissions slightly increased with bed temperature for birch chips combustion and did not change with bed temperature for fir chips combustion. Sensitivity analyses reveal that the reaction between NO and char is the key reaction to determine the NO emissions.     

Mechanism of N2O decomposition over a Rh black catalyst studied by a tracer method: The reaction of N2O with O(a)

N₂O decomposition on an unsupported Rh catalyst has been studied using tracer technique in order to reveal the reaction mechanism. N₂¹⁶O was pulsed onto ¹⁸O/oxidized Rh catalyst at 220°C and desorbed O₂ molecules (m/e=32,34,36) were monitored by means of mass spectrometer. The ¹⁸O fraction in the desorbed dioxygen was the same value as that on the surface oxygen. The result shows that the O₂ molecules desorb via Langmuir–Hinshelwood mechanism, i.e., the desorption of dioxygen through the recombination of adsorbed oxygen. On the other hand, TPD measurements in He showed that desorption of oxygen from the Rh black catalyst occurred at the higher temperatures. Therefore, reaction-assisted desorption of oxygen during N₂O decomposition reaction at the low temperature was proposed.     

N2O decomposition over wet- and solid-exchanged Fe-ZSM-5 catalysts

The N₂O decomposition activity of Fe-ZSM-5 strongly depends on the iron content and the preparation methods, including wet (WIE) and solid state ion exchanges (SSIE). The state of Fe species formed on the surface of a series of Fe-ZSM-5 catalysts containing a variety of Fe contents with respect to the preparation method and their role for N₂O decomposition activity have been systematically examined. The general trend for the decomposition activity of Fe-ZSM-5-SSIE is higher than that of Fe-ZSM-5-WIE, indicating the formation of a distinctive local structure of Fe on the catalyst surface during the course of the ion-exchange procedure. Based upon the Fourier transformed Fe K-edge EXAFS spectra for the series of Fe-ZSM-5-SSIE and -WIE catalysts, most of the Fe species on the surface of Fe-ZSM-5-SSIE with high Fe loading are well dispersed in the form of oxygen-bridged binuclear Fe species. The turnover frequency (TOF) for N₂O decomposition under dry and wet conditions has been confirmed assuming that Fe-ZSM-5-SSIE samples with Fe/Al = 0.20 and Fe/Al = 0.65 only contain mononuclear and binuclear Fe species, respectively, as active reaction species on their surface. The high performance of Fe-ZSM-5-SSIE may be mainly due to the formation of the binuclear Fe species onto its surface during the preparation of the catalyst.     

Potassium-doped Co3O4 catalyst for direct decomposition of N2O

Direct decomposition of nitrous oxide (N₂O) on K-doped Co₃O₄ catalysts was examined. The K-doped Co₃O₄ catalyst showed a high activity even in the presence of water. In the durability test of the K-doped Co₃O₄ catalyst, the activity was maintained at least for 12 h. It was found that the activity of the K-doped Co₃O₄ catalyst strongly depended on the amount of K in the catalyst. In order to reveal the role of the K component on the catalytic activity, the catalyst was characterized by XRD, XPS, TPR and TPD. The results suggested that regeneration of the Co²⁺ species from the Co³⁺ species formed by oxidation of Co²⁺ with the oxygen atoms formed by N₂O decomposition was promoted by the addition of K to the Co₃O₄ catalyst.     

Decomposition of N2O over the surface of cobalt spinel: A DFT account of reactivity experiments

The DFT molecular modeling of N₂O decomposition over cobalt spinel (1 0 0) plane was performed using a cluster approach, and applied to rationalize the experimental reactivity data. The energetics of the postulated elementary steps such as N₂O adsorption, N₂O activation through dissociative electron or oxygen atom transfer, surface diffusion of resultant oxygen intermediates, and their recombination into O₂, was evaluated and discussed. The geometry and electronic structure of the implicated active sites and intermediates were determined. Three different transition states were found for the activation of nitrous oxide molecule. In the preferred electron transfer mechanism, involving a monodentate transition state, the N₂O activation and the formation of dioxygen are energetically the most demanding steps, whereas the barrier for the oxygen surface diffusion was found to be distinctly smaller. For the oxygen atom transfer the reaction is energetically constraint by the NO bond-breaking step. The inhibiting effect of co-adsorbed water and oxygen on the particular reaction steps was briefly addressed.     

Calcined hydrotalcites for the catalytic decomposition of N2O in simulated process streams

Various hydrotalcite based catalysts were prepared for testing for the catalytic decomposition of N₂O. CoAl, NiAl, Co/PdAl, Co/RhAl, and Co/MgAL substituted hydrotalcites and CoLaAl hydroxides offer very good activity at modest temperatures. Precalcination of these materials at ca. 450–500°C, which destroys the hydrotalcite phase, is necessary for optimum activity and life. For Co substituted hydrotalcites, the optimal ratio of Co/Al is 3.0. The temperature for 50% conversion of N₂O of these calcined cobalt hydrotalcites is ca. 75°C lower than for the previous highly active Co-ZSM-5. These calcined cobalt hydrotalcite materials display sustained life at temperatures in excess of 670°C in an O₂ rich, wet stream with high levels of N₂O [10%]. Excess O₂ does not seriously impact N₂O decomposition, but the combination of both water vapor and O₂ does reduce activity by ca. 50%.     

Novel Mo/Ga/H-ZSM-5 catalyst in environmentally important reductive transformation of N2O in presence of CH4

Direct decomposition of N₂O and the reduction of N₂O with CH₄ over Ga/H-ZSM-5 and Mo/Ga/H-ZSM-5 (Si/Al = 40) catalysts in a plug flow reactor under steady-state conditions as well as by temperature programmed surface reaction (TPSR) have been investigated. Ga ions were ion-exchanged from liquid phase while Mo was deposited onto the Ga/H-ZSM-5 sample using incipient wetness technique. The catalysts were characterized by means of XRF, XPS, TPR, CO chemisorption, TEM and EDS. The N₂O forms redox centers in the Mo/Ga/H-ZSM-5 catalysts at elevated temperatures, which are extremely active in the reaction with CH₄ already at around 373 K. Addition of Mo to the Ga/H-ZSM-5 decreased the T₅₀ temperature in the N₂O decomposition and reduction of N₂O with CH₄ from 819 to 787 K and from 755 to 646 K, respectively. The oxidation/reduction of the Mo/Ga/H-ZSM-5 sample is more favoured in the interaction with N₂O/CH₄ as compared to that using O₂/H₂ and the mechanism of the redox reactions might also be different. The reduction of N₂O with CH₄ cannot be described with the Mars–van Krevelen redox mechanism, but by the participation of CH₄ via MoGa–OCH₃ species in a complex oxygen transfer mechanism is proposed at which N₂O does not directly reoxidise the reduced active centers.