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.     

Toward Improved Coefficients for Predicting Direct N<Subscript>2</Subscript>O Emissions from Soil in Canadian Agroecosystems

Agricultural soils emit nitrous oxide (N₂O), a potent greenhouse gas. Predicting and mitigating N₂O emissions is not easy. To derive national coefficients for N₂O emissions from soil, we collated over 400 treatment evaluations (measurements) of N₂O fluxes from farming systems in various ecoregions across Canada. A simple linear coefficient for fertilizer-induced emission of N₂O in non-manured soils (1.18% of N applied) was comparable to that used by the Intergovernmental Panel on Climate Change (IPCC) (1.25% of N applied). Emissions were correlated to soil and crop management practices (manure addition, N fertilizer addition and inclusion of legumes in the rotation) as well as to annual precipitation. The effect of tillage on emissions was inconsistent, varying among experiments and even within experiments from year to year. In humid regions (e.g., Eastern Canada) no-tillage tended to enhance N₂O emissions; in arid regions (e.g., Western Prairies) no-tillage sometimes reduced emissions. The variability of N₂O fluxes shows that we cannot yet always distinguish between potential mitigation practices with small (e.g., <10%) differences in emission. Our analysis also emphasizes the need for developing consistent experimental approaches (e.g., ‘control’ treatments) and methodologies (i.e. measurement period lengths) for estimating N₂O emissions.     

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.     

Nickel-impregnated zirconium-doped mesoporous molecular sieves as catalysts for the hydrogenation and ring-opening of tetralin

Catalysts based on nickel supported on zirconium-doped mesoporous silica, with nickel loadings of 15 and 30 wt.%, have been prepared, characterized and tested in the hydrogenation of tetralin at high hydrogen pressures (6.0 MPa). The zirconium-doped mesoporous silica, used as support, is revealed as appropriate for attaining a high loading of metallic nickel with a suitable degree of dispersion, although the large metallic particles are located on the external surface of the support. These catalysts are very active in the hydrogenation of tetralin at 350 °C, specially that with the highest nickel content, with no deactivation by coke deposition. Concomitantly, the mild acidic nature of these catalysts favor the ring-opening of the tetralin molecules, up to 28% of cracking products at moderate temperatures are detected and the amount of volatile compounds is almost negligible under these experimental conditions.     

Direct conversion of syngas to ethanol over Rh/Mn-supported on modified SBA-15 molecular sieves: Effect of supports

SBA-15 molecular sieves with a tunable pore size (SBA-15-HCl, SBA-15-HPMo, SBA-15-HPW, and SBA-15-HSiW) were synthesized through adding heteropoly acids to a template and extending crystallization time. Loaded with RhCl₃ and Mn(NO₃)₂, SBA-15-HPMo catalyzed well the direct conversion of syngas to ethanol. In the first 2 h the CO conversion over the Rh-Mn/SBA-15-HPMo catalyst was up to 100%. Average CO conversion over the SBA-15-HPMo-supported catalyst increased significantly from 4.2 to 21.9% and selectivity to ethanol from 9.3 to 12.8% compared with the amorphous silica-supported one. The enhanced redox property of the encapsulated Rh and Mn oxides within the mesoporous channels, and multi-dispersed centers formed by confined metal particles contributed to improved catalytic performances.     

Influence of preparation methods of LaCoO3 on the catalytic performances in the decomposition of N2O

This study reports the potential interest of LaCoO₃ in the catalytic decomposition of N₂O from nitric acid plants. Typically, the exhaust gas contains NO, water and O₂ which usually induce strong inhibiting effects depending on the surface properties of the solids particularly the surface mobility of oxygen from LaCoO₃. Different preparation methods have been implemented, involving citrate route, reactive grinding and the use of templates, which lead to different structural and textural properties examined by X-ray diffraction, transmission electron microscopy and N₂ physisorption. EDX analysis and XPS measurements also revealed that different surface composition may alter subsequent interactions between the surface and the reactants and related catalytic performances. LaCoO₃ prepared by reactive grinding was found to be the most active catalyst due to a high specific surface area but the presence of Fe and Zn impurities inherent to the preparation method were suggested to interfere on the catalytic performances.     

Temperature-programmed desorption vs. N2 desorption in determining pore-size distribution of mesoporous silica molecular sieves

Two mesoporous silica molecular sieves, one synthesized by cationic template method and another by neutral template route, were characterized for their pore-size distribution by a novel Temperature-Programmed Desorption (TPD) method and the widely used N₂ desorption method. The pore-size distributions determined by the two methods agree quite well and are within experimental errors. For example, TPD method gave a pore size distribution (radius) centered at 14 Å while N₂ desorption method showed a peak centered at 13.3 Å. for the mesoporous silica prepared by cationic template route. The new TPD method based on thermogravimetric analysis is a viable option for mesopore characterization of silica-based materials.     

Surface characterization of sulfated zirconia and its catalytic activity for epoxidation reaction of castor oil

The solid acid catalysts SO₄^2?/ZrO₂ were prepared by impregnation technique at different calcination temperatures. The surface characterizations were carried out by using scanning electron microscope (SEM), Fourier transform infrared spectrometer (FTIR), X-ray diffraction (XRD), temperature programed desorption of NH₃ (NH₃-TPD), and N₂-BET. The SEM results showed that the size of the SO₄^2?/ZrO₂ was not uniform and varied from about 1 to 20?µm. The characteristic peaks in FTIR spectra were essentially the same within the calcination temperature range of 400–700?°C. The XRD results indicated that the transition temperature from amorphous to tetragonal phase was up to 500?°C. The strong acid and superacid sites of the samples could be observed by the NH₃-TPD results. The largest BET surface area was 140 m²/g, when the calcination temperature was at 500?°C, and all the pore size distributions belong to mesoporous range. The solid acid SO₄^2?/ZrO₂ was used for the epoxidation of castor oil. When the calcination temperature of SO₄^2?/ZrO₂ was 600?°C, reaction temperature 45?°C, and reaction time 8?h, the reaction effect was better with an iodine value of 33.0?±?1.6?g/100?g and an epoxy value of 2.45?±?0.11?mol/100?g.     

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.     

Mixed oxides as catalysts for the selective reduction of nitrobenzene

The catalytic behaviour of the PbO-Mn₃O₄ and the Bi₂O₃-MoO₃ systems was investigated in the selective reduction of nitrobenzene to nitrosobenzene. Lead compounds appeared to be good catalysts, and co-catalysts when used with Mn₃O₄. Different from oxidations by di-oxygen, Bi₃O₃ alone is a good catalyst and formation of mixed Bi-Mo-O compounds does not enhance the catalytic activity. It is suggested that the difference between these catalysts in the mentioned reaction is related to the way in which the oxygen vacancy is represented by the oxygen donor.     

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.     

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.     

Effect of sulfur dioxide on nitric oxide adsorption and decomposition on Cu-containing micro- and mesoporous molecular sieves

NO decomposition was studied at different temperatures on copper-exchanged ZSM-5, AlMCM-41 and NbMCM-41 molecular sieves. Cu-ZSM-5 zeolites presented the highest activity. SO₂ poisoning was also performed and Cu–NbMCM-41 was found to be more resistant. IR results of NO and SO₂ coadsorption either at room temperature or at 573 K show evidence that sulfate formation occurred at 573 K and partially prevented NO adsorption on Cu²⁺ in square planar structure in Cu-ZSM-5. Sulfation of Cu–NbMCM-41 was quite low due to niobium incorporated into the lattice. By contrast, niobium present in the extra-lattice position in CuNb-ZSM-5 and CuNb–AlMCM-41 did not protect the catalyst from sulfation. H₂-TPR results suggested that sulfates were formed on copper sites. IR spectra after treatment under SO₂ + O₂ at 673 K indicated that sulfated species were covalently bonded, their structure varying according to the nature of the support. This revised version was published online in August 2006 with corrections to the Cover Date.     

Nitrous oxide emissions for 6 years from a gray lowland soil cultivated with onions in Hokkaido, Japan

We studied nitrous oxide (N₂O) emissions every growing season (April to October) for 6 years (19952000), in a Gray Lowland soil cultivated with onions in central Hokkaido, Japan. Emission of N₂O from the onion field ranged from 0.00 to 1.86 mgN m^–2 h^–1. The seasonal pattern of N₂O emission was the same for 6 years. The largest N₂O emissions appeared near harvesting in August to October, and not, as might be expected, just after fertilization in May. The seasonal patterns of soil nitrate (NO₃ ^–) and, ammonium (NH₄ ⁺) levels and the ratio of N₂O to NO emission indicated that the main process of N₂O production after fertilization was nitrification, and the main process of N₂O production around harvest time was denitrification. N₂O emission was strongly influenced by the drying–wetting process of the soil, as well as by the high soil water content. The annual N₂O emission during the growing season ranged from 3.5 to 15.6 kgN ha^–1. The annual nitrogen loss by N₂O emission as a percentage of fertilizer-N ranged from 1.1 to 6.4%. About 70% of the annual N₂O emission occurred near harvesting in August to October, and less than 20% occurred just after fertilization in May to July. High N₂O fluxes around the harvesting stage and a high proportion of N₂O emission to total fertilizer-N appeared to be probably a characteristic of the study area located in central Hokkaido, Japan.     

Comparing a process-based agro-ecosystem model to the IPCC methodology for developing a national inventory of N2O emissions from arable lands in China

Nations are now obligated to assess their greenhouse gas emissions under the protocols of Article 4 of the United Nations Framework Convention on Climate Change. The IPCC has developed `spreadsheet-format' methodologies for countries to estimate national greenhouse gas emissions by economic sector. Each activity has a magnitude and emission rate and their product is summed over all included activities to generate a national total (IPCC, 1997). For N₂O emissions from cropland soils, field studies have shown that there are important factors that influence N₂O emissions at specific field sites that are not considered in the IPCC methodology. We used DNDC, a process-oriented agroecosystem model, to develop an unofficial national inventory of direct N₂O emissions from cropland in China. We assembled county-scale data on soil properties, daily weather, crop areas, N-fertilizer use, livestock populations (for manure inputs to cropland), and agricultural management for the 2500 counties in mainland China. Total 1990 cropland area was 0.95 million km². Total N-fertilizer use in China in 1990 was 16.6 Tg N. The average fertilization rate was 175 kg N ha⁻¹ cropland. One-year simulations with DNDC were run for each crop type in each county to generate estimates of direct N₂O emissions from soils. National totals were the sum of results for all crop simulations across all counties. Baseline simulations estimated that total N₂O emission from arable land in China in 1990 was 0.31 Tg N₂O-N yr⁻¹. We also ran simulations with zero N-fertilizer input; the difference between the zero-fertilizer and the baseline run is an estimate of fertilizer-induced N₂O emissions. The fertilizer-induced emission was 0.13 Tg N₂O-N yr⁻¹, about 0.8% of total N-fertilizer use (lower than the mean but within the IPCC range of 1.25±1.0%). We compared these results to our estimates of county-scale IPCC methodology emissions. Total emissions were similar but geographical patterns were quite different. This revised version was published online in August 2006 with corrections to the Cover Date.     

Effect of pretreatment and regeneration conditions of Ru/γ-Al2O3 catalysts for N2O decomposition and/or reduction in O2-rich atmospheres and in the presence of NOX, SO2 and H2O

The effect of the pretreatment (inert, oxidative, and reducing) of Ru/γ-Al₂O₃ catalyst on its activity and stability in the decomposition of N₂O in the absence or presence of O₂, SO₂, H₂O and NO_X was studied in the present work. Decomposition of pure N₂O was slightly enhanced by the H₂-pretreated catalyst (metallic Ru) compared to the O₂- or He-pretreated ones, owing to a cyclic oxidation–reduction pathway of metallic Ru. The observed decrease of activity by O₂ or H₂O addition was reversible compared to SO₂ which caused a strong, irreversible deactivation of the catalyst, irrespective of the type of pretreatment. This was attributed to the formation of stable sulphates, mainly those on RuO₂ surface, which could only be removed by regeneration under reducing (H₂ in He) atmosphere at temperatures of ca. 500 °C. Oxidative or inert regeneration required very high temperatures (i.e. >700 °C) in order to decompose these sulphates. A method of retaining N₂O conversion activity very high (≥98%) for long reaction times is suggested and is based on frequent and short-time (ca. 10 min) regenerations of the catalyst under reducing atmosphere (ca. 5% H₂ in He). The effect of co-feeding various reducing agents, such as CO or C₃H₆, on the N₂O conversion activity in the presence of O₂, SO₂, H₂O and NO_X is negligible, mainly because they are oxidized at relatively low temperatures in the O₂-rich feeds used in this study.     

Remarks on the passivation of reduced Cu-, Ni-, Fe-, Co-based catalysts

Catalysts containing metals such as Cu, Ni, Fe, Co in their reduced state are often subjected to passivation procedures prior to characterization. Passivation with N₂O or O₂ to create a protective oxide layer also results in a certain degree of sub-surface oxidation. The heat released during oxidation is a critical parameter. The extent of bulk oxidation depends on the type of oxidant as well as on the size of the metal particles, as shown for copper catalysts. The final, meta-stable passivation layer requires a certain thickness to sustain exposure to ambient atmosphere. The encapsulation of metal particles in carbon is an efficient method for preserving the metallic state, as demonstrated for metallic nickel and iron with carbon nanofibers. The use of passivated samples for characterization of the active, i.e., reduced, catalyst has limited value.     

Preparation of carbon molecular sieves from carbon micro and nanofibers for sequestration of CO2

In this study, a hierarchal web of carbon micro and nanofibers was used as a precursor for the synthesis of a carbon molecular sieve (CMS). CMSs were prepared by thermal treatment of carbon fibers using a microwave heating device. The heating power and treatment time were optimized for the maximum performance of the prepared CMS for the separation of CO₂ at low concentrations from the gaseous mixture of CO₂ and air under dynamic (flow) conditions. Based on the experimental data, microwave power input of 240 W and treatment time of 15 min were found to be suitable for the maximum uptake of CO₂ by CMS. Adsorption breakthrough curves were obtained at different gas flow rates and CO₂ concentrations. CMSs prepared from the hierarchal web of carbon micro and nanofibers were found to be superior to those prepared from ACF. The CO₂ uptake was determined to be approximately 0.88 mg/g and 10 mg/g at concentrations of 500 ppm and 5000 ppm, respectively, in air.