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Decomposition of N2O Over Ru/Fe/Al2o3 Catalyst Nanoparticles Gaston O

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Decomposition of N2O Over Ru/Fe/Al2o3 Catalyst Nanoparticles Gaston O Decomposition of N2O over Ru/Fe/Al2O3 catalyst nanoparticles Gaston O. Larrazabal Particle Technology Laboratory, Department of Mechanical and Process Engineering, ETH Zurich, 8092 Zurich, Switzerland Report handed in on December 23, 2011 Supervisor: Dr. Robert Büchel Abstract. As the third-largest greenhouse gas and the most important ozone-depleting substance, nitrous oxide (N2O) represents a significant environmental issue. In this work, the catalytic activity towards N2O decomposition of six different Ru/Fe/Al2O3 nanoparticle powders synthesized by flame spray pyrolysis (FSP) was investigated. The powders were characterized by X-ray diffraction and N2 adsorption with the BET method. The catalytic tests were conducted with a feed of N2O dissolved in helium in a tubular fixed bed reactor. The prepared powders were found to have a BET specific surface area of approximately 170 g/m2. The prepared powders were catalytically active in the direct decomposition of N2O; 50% conversion of N2O was achieved at approximately 380 °C with the best-performing catalyst. The activity of the catalysts improved as the content of ruthenium increased until 2 wt%, after this point no improvement of the activity with higher Ru content was observed. The addition of iron when Ru was already present did not result in any improvement of the catalyst performance under the studied conditions compared to a Ru-only sample, however, a Fe/Al2O3 powder was found to shift the decomposition of N2O towards lower temperatures compared to the spontaneous thermal decomposition. 1. INTRODUCTION Nitrous oxide (N2O) is very harmful to the environment. It is the third most important greenhouse gas after carbon dioxide and methane: in 2004, nitrous oxide accounted for 7.9% of total greenhouse gas 1 (GHG) emissions in the world on a CO2-equivalent basis . Furthermore, N2O has a global warming potential 1 which is 310 times higher than that of CO2 over a 100-year timeframe as well as a very long lifetime . 2 Additionally, N2O has replaced chlorofluorocarbons (CFC) as the largest ozone-depleting substance (ODS) . However, nitrous oxide emissions are not regulated by the 1987 Montreal Protocol2. Sources of nitrous oxide emissions are both natural and anthropogenic. The main anthropogenic sources are agriculture, mostly due to the use of fertilizers and the decomposition of livestock manure, the mobile and stationary combustion of fossil fuels and some industrial production processes, of which nitric acid and adipic acid production are particularly relevant12. For example, in the United States agricultural soil management represented two-thirds of N2O emissions in 2008, while combustion of fossil fuels and production of nitric and adipic acids accounted for 13% and 7%, respectively12. In the European Union, the share of N2O emissions by the chemical industry is much larger (29%), while agriculture and combustions account for 57% and 10% respectively7. Despite their larger share of emissions coming from agriculture, it has been proposed that the N2O emissions that can be more effectively reduced in the short-term are associated with industrial processes (e.g. production of nitric and adipic acid) and stationary and mobile combustions3. It is relevant to note that some N2O is produced by the incomplete reduction of NOx in three-way catalytic converters and that N2O particularly as the catalyst ages12. Hence, it has been suggested that a new catalytic converter for road vehicles might consist of a traditional three-way catalyst followed by a N2O decomposition catalyst. In light of increased awareness of the environmental impact of nitrous oxide, regulations are expected to come into force in the near future. For example, N2O emissions from nitric, adipic and glyocalic acid production plants are scheduled to enter the European Union Emission Trading Scheme (EU ETS) in 5 2013 . Because of the high global warming potential of N2O, a small reduction of N2O emissions results into large savings in CO2-equivalent terms under the ETS. Therefore, there is an increased interest in the development of catalysts capable of reducing the content of N2O in the off-gases of such processes. 1 However, N2O abatement in nitric acid plants and mobile and stationary combustions remains very challenging. The main limitations are the low content of N2O in the off-gases (up to ca. 1 vol%, compared to 30-50 vol% in adipic acid production)2, which requires catalysts with high activity capable of operating at moderate temperatures (e.g. 300-500 °C), and the presence in the feed of common inhibitors such as H2O, 3 O2, SO2 and NOx . Research on N2O abatement has been mostly concentrated on two reactions: selective catalytic reduction (SCR) and direct decomposition. However, the cost and complexity of SCR is considered to be higher, in part because a reducing agent (e.g. hydrocarbons) must be added to the gas stream4. Due to their high intrinsic activity, early efforts focused on noble metal catalysts. In 1997, Oi et al.6 studied the decomposition of N2O (feed composition of 950 ppm N2O and 5% O2) over rhodium catalysts supported on different media (i.e. ZnO, CeO2, ZSM-5 and hydrotalcite). Of these, the Rh/ZnO catalyst (metal content of 0.5 wt%) showed the highest activity, achieving a conversion of 100% at approximately 300°C. However, the addition of 1% H2O and 900 ppm NO2 to the feed strongly inhibited the catalyst’s activity. In the presence of these inhibitors, 100% conversion of N2O was reached at approximately 500°C. This strong inhibition by H2O and NOx on the activity of noble metal catalysts has been widely reported in the 3 literature and is the largest limitation for their use in N2O abatement under realistic conditions. Iron zeolite-based catalysts have been widely investigated for N2O decomposition. These have shown higher stability in the presence of inhibitors. Pérez-Ramírez et al.7 prepared ex-framework Fe-ZSM-5 catalysts which, despite having a lower activity than noble metal catalysts in the presence of N 2O alone, showed remarkable stability in the simulated off-gas feeds. In fact, the activity of these Fe-ZSM-5 catalysts is inhibited by H2O but promoted by NO and SO2. It has been proposed that NO and SO3 promote N2O conversion by accelerating the desorption rate of adsorbed O* species while not forming nitrites/nitrates or sulfates with the metal catalyst, as it occurs in the case of noble metals7. 4 More recently, Komvokis et al. studied the use of γ-Al2O3-supported ruthenium nanoparticles for the decomposition of N2O. The nanoparticles were prepared by reduction with ethylene glycol; achieving a small particle size (‹3 nm) and high dispersion. Despite having a low metal content (0.38 wt%), the catalyst with the smallest particle size (1-2 nm) and highest dispersion (71%) showed the highest activity. In presence of H2O, SO2 and NO in the feed, the activity was similar to that of FeZSM-5 catalysts and the inhibition was found to be reversible4. This suggests that a small particle size and high dispersion might help mitigate the effect of inhibition and improve the applicability of noble metal catalysts in N2O abatement. In this regard, it has been shown that flame spray pyrolysis (FSP) is effective for the synthesis of metal catalysts with well-dispersed nanometer-sized particles8. Interestingly, Pieterse et al.9 reported that a bimetallic Fe-Ru catalyst supported on a ferrierite (FER) zeolite, in the presence of NO, showed better performance in the decomposition of N2O than either Fe-FER and Ru-FER individually. Pirngruber et al.10 identified two reasons for this effect: first, the catalytic cooperation between iron and ruthenium, which occurs through different mechanisms depending on the presence or absence of O2, and second, the preferential adsorption of NO on Fe in catalysts with high iron loading (2.2 wt% Fe, 0.4 wt% Ru), which helps protect Ru from inhibition. The goal of this project was to obtain an initial insight into the behavior of FSP-synthesized nanoparticle powders, containing ruthenium and iron supported on Al2O3, in the direct catalytic decomposition of nitrous oxide. For this, the prepared powders were used as catalysts in a tubular fixed bed reactor. The feed consisted of N2O dissolved in helium, that is, neither inhibitors nor external reducing agents were fed into the reactor. 2. EXPERIMENTAL SECTION 2.1. Synthesis and characterization The first part of the synthesis consisted in the preparation of the precursors for each powder containing the desired amounts of aluminum, iron and ruthenium. A 0.5 mol/l solution of Al(III) was prepared by dissolving 36.94 g of aluminum tri-sec-butoxide (Aldrich, >98%) in 300 ml of a previously- prepared solvent mixture consisting of 2 parts by volume of diethylene glycol monobutyl ether (Sigma- 2 Aldrich, >99%) and 1 part by volume of acetic anhydride (Sigma-Aldrich, >99%). Each precursor was then prepared separately by dissolving the required amounts of ruthenium (III) 2-4-pentanedionate (Aldrich, >99.9%) and iron (III) acetylacetonate (Aldrich, >99.9%) in 50 ml of the Al(III) mother solution. The weighed amounts of each compound for each one of the precursor solutions are shown in Table 1; these were calculated based on the expected content of metallic iron and ruthenium of the Fe/Ru/Al2O3 powders after reduction with hydrogen. Table 1. Metal content of powders and amount of Fe(III) and Ru(III) compounds in precursor solutions Metal content of powder Amounts in precursor solution (wt%) Powder 0.5 mol/l solution name Ru(III) Fe(III) Ruthenium Iron 2,4.pentanedionate acetylacetonate of Al(III) (mg) (mg) (ml) 0 Ru 2.2 Fe 0 2.2 0 181.7 50 0.5 Ru 2.2 Fe 0.5 2.2 26.4 182.3 50 1 Ru 2.2 Fe 1 2.2 52.2 182.9 50 2 Ru 2.2 Fe 2 2.2 104.9 182.9 50 4 Ru 2.2 Fe 3 2.2 214.8 188.9 50 0.5 Ru 0 Fe 4 0 25.8 0 50 In the flame spray pyrolysis setup, each precursor (50 ml) was fed in the center of a methane/oxygen flame by a syringe pump (Lambda VIT-FIT).
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