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

Home , Catalytic converter

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.

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). A sketch of the FSP nozzle by Strobel et al.11 is shown in Figure 1; the same configuration was used in this work.

11 Figure 1. Sketch of the flame pyrolysis unit by Strobel et al. for the synthesis of Pd/Al2O3 catalysts. The liquid precursor is dispersed by a gas stream and ignited by a methane/oxygen flame. The feed of methane (PanGas, 99.95%) and oxygen (PanGas, 99.95%) to the flame was 1 l/min and 2 l/min, respectively. Both the flow of the liquid precursor and of the dispersion gas (oxygen) was 5 ml/min. The pressure drop across the nozzle was 1.6 bar. The produced particles were collected on a glass fiber filter (Whatman GF/D, diameter 25.7 cm), which were then removed from the filter by scratching. To remove leftover fibers from the filter, the powders were sieved prior to the characterization and the catalytic tests. The specific surface area (SSA) of the produced powders was determined by nitrogen adsorption at 77 K with the BET method, employing a Micromeritics TriStar unit. The samples were previously degassed for 1 hour at 150 °C in nitrogen. The x-ray diffraction (XRD) spectra of the powders, for a range of angles (2) from 10 to 70 degrees, were obtained with a Bruker AXS B8 Advance diffractometer operating with Cu K radiation with a step size of 0.01972° and a scan speed of 12 °/min. 3

2.2. Catalytic tests The catalytic tests were performed using a glass tubular fixed bed reactor with an inner diameter of 4 mm. A diagram of the unit used for these tests is shown in Figure 2. For each test, 40 mg of powder were loaded in the glass tube and secured with quartz wool (Carl Roth KG). In all cases, the length of the catalyst bed before the start of the experiment was approximately 35 mm.

FIC Exhaust FIC

Hydrogen Exhaust FIC

Helium Mass spectrometer FIC TIC

Nitrous oxide

Figure 2. Scheme of the experimental setup for the catalytic tests. The catalysts were first treated with H2 for 30 minutes at 350 °C. Then the decomposition test was carried out with N2O dissolved in He (N2O concentration of 1 vol%). Temperature in the reactor was set by controlled heating elements. The outlet gas from the reactor was sampled with a mass spectrometer. The powders were reduced in situ with hydrogen (PanGas, 99.9999%) during 30 minutes at 350 °C. After the reduction, the hydrogen was purged from the system with pure helium during 30 minutes with no heating. The catalytic tests were then carried out with a a feed of 1 vol% of nitrous oxide (PanGas, 99.5%) dissolved in helium (PanGas, 99.999%); no external reducing agent was added to the feed. The volume flow was 40 ml/min. The experimental conditions for the N2O decomposition are summarized in Table 2.

Table 2. Experimental conditions for tests on the catalytic decomposition of N2O Catalyst weight 40 mg Feed flow rate 40 ml/min

Concentration of N2O in feed 1 vol% W/F ratio 0.06 g.s/ml Gas hourly space velocity (GSHV) ~ 5500 h-1 Reactor temperature 130-700 °C

The outlet of the reactor was sampled by a quadrupole mass spectrometer (Pfeiffer Vacuum

ThermoStar GSD). The decomposition of N2O was studied by varying the temperature of the reactor from 130 to 700 °C with a heating ramp of 10 °C/min.

3. RESULTS AND DISCUSSION This results and discussion section is structured in two parts: first the characterization is presented and then in the second part the catalytic tests are discussed. In Figure 3 a picture of the synthesized powders is shown.

Figure 3. Picture of synthesized powders of Fe/Ru/Al2O3. The powders appeared darker with the increase in the ruthenium content, ranging from white (0 wt% Ru, 2.2 wt% Fe) to black (4 wt% Ru, 2.2 wt% Fe). 3.1. Characterization The BET surface specific area (SSA) of the FSP-synthesized powders is shown in Table 3. For comparison with the present work, Table 3 also shows the SSA of the the Ru/Al2O3 nanoparticle catalysts reported by Komkovis et al.4 prepared by impregnation-calcination (imp-calc) as well as by in situ reduction with ethylene glycol (EG). Table 3. Surface specific area (m3/g) of synthesized powders and comparison with literature SSA (m2/g) BET-N Catalyst sample Source 2 adsorption 0 Ru 2.2 Fe This work 172.74 0.5 Ru 2.2 Fe This work 170.81 1 Ru 2.2 Fe This work 171.84 2 Ru 2.2 Fe This work 173.55 4 Ru 2.2 Fe This work 167.32 0.5 Ru 0 Fe This work 171.58 4 Ru/γ-Al2O3-imp-calc (Ru 0.95 wt%) Komkovis et al. 194 4 Ru/γ-Al2O3-imp-calc (Ru 1.94 wt%) Komkovis et al. 169 4 Ru/γ-Al2O3-EG (Ru 0.38 wt%) Komkovis et al. 198 4 Ru/γ-Al2O3-EG (Ru 0.98 wt%) Komkovis et al. 199

The results shown in Table 3 indicate that there are no major differences among the SSA of the FSP- synthesized catalysts, as most of the values are close to 170 m2/g. However, except in one case they have a lower specific surface area compared to the catalysts prepared by Komkovis et al.4, which have an SSA of close to 200 m2/g. The samples were also studied by XRD; the obtained spectra are shown in Figure 4. It can be observed that the spectra of the synthesized powders are all very similar, as they are characteristic of an

Al2O3 sample. Nonetheless, the diffraction peaks at 28° and 35° 2θ in the Ru 4 wt% powder are characteristic of the RuO2 phase. It is likely that no iron or ruthenium oxides are observed in the other samples because the concentration of Ru and Fe is too low compared to Al2O3 and not enough crystals are formed in order to produce distinct diffraction peaks.

Figure 4. XRD spectra of the synthesized powders. In the 4 Ru, 2.2 Fe sample, the diffraction peak identified with ▼ corresponds to the (1 1 0) plane and ▽ corresponds to the (0 1 1) plane of ruthenium (IV) oxide.

3.2. Catalytic activity of Ru/Fe/Al2O3 catalysts towards N2O decomposition

The activity towards N2O decomposition of the prepared catalysts was evaluated for a feed of 1 vol% of nitrous oxide in helium using a fixed bed reactor setup, as described in Section 2.2. The outlet stream was sampled with a mass spectrometer, which produced ion current data for each relevant species (e.g. N 2O, N2, O2). The MS signal was assumed to be linearly related to the concentration of the species in the stream. Additionally, it was assumed that at low temperature the concentration of each component is unchanged between the inlet and the outlet of the reactor, i.e. the decomposition of N 2O is negligible. This assumption is supported by the fact that the variations of the N2O, N2 and O2 signals at low temperature (150-200°C) are indeed small compared to the changes of these signals as the temperature increased (> 300°C). This means that, for each species, the ratio of the MS signals at any given temperature and at the start of the experiment (the relative intensity) corresponds to the ratio of the outlet and feed concentrations. The results of the catalytic tests with the ruthenium-containing powders as well as an empty tube are shown in Figure 5a, and the MS signals of N2O, N2 and O2 for the 1 Ru 2.2 Fe sample are shown in Figure 5b. The results with the empty tube, in which no powder was present, show that the thermal decomposition of N2O occurs at very high temperatures. For instance, it was observed that a temperature of approximately 580°C was required in order to reduce the content of N 2O of the feed by 20%. On the other hand, Figure 5a also shows that in presence of the prepared powders, the decomposition of nitrous oxide occurs at much lower temperatures, confirming that the prepared powders are indeed catalytically active for the decomposition of N2O. Figure 5b shows that the decrease in the concentration of nitrous oxide is accompanied by an increase in the concentration of nitrogen and oxygen, which are products of the decomposition reaction. This further confirms the occurrence of the catalytic decomposition of N 2O without the addition of any external reducing agent. It can also be seen from Figure 5a that increasing the ruthenium content of the powder until 2 wt% improves the catalytic activity, but that the performance difference between the sample with 2 wt% and 4 wt% of ruthenium is negligible. Because increasing the ruthenium content increases the amount of active sites available for the N2O decomposition, this explains the improvement of the catalyst activity up to a ruthenium content of 2 wt%. However, it is possible that at a Ru content of 2 wt% and under the applied experimental conditions the catalyst activity is high enough so that rate-determining step is not in the reaction itself but rather in the mass transfer. Hence, after this transition an increase of the amount of active sites brings little benefit to the performance of the system. This would explain the similarity in the behavior of the 2 Ru 2.2 Fe and 4 Ru 2.2 Fe powders observed in Figure 5a.

1 1 0.9 0.9 (b) 0.8 (a) 0.8

0.7 0.7

O MS signal at at signal MS O

2 0.6 0.6 1 Ru, 2.2 Fe 0.5 0.5 Empty tube [a.u] 0.4 0.4 outlet [a.u] outlet 0.5 Ru, 2.2 Fe 0.3 0.3 N2ON2O 1 Ru, 2.2 Fe 0.2 0.2 Series2 2 Ru, 2.2 Fe N2

0.1 0.1 O2O2

Relative intensity of N of intensity Relative 4 Ru, 2.2 Fe Relative intensity of MS signal at outlet at signal MS of intensity Relative 0 0 100 300 500 700 100 300 500 700 Temperature [°C] Temperature [°C]

Figure 5. (a) Relative intensity of the N2O MS signal versus temperature for an empty tube and the ruthenium- containing catalysts. The shift of the curves towards lower temperatures indicates an increase of catalytic activity. However, no improvement of the catalytic performance was observed when the Ru content was changed from 2 wt% to 4 wt%. (b) Relative intensity of the N2O, N2 and O2 MS signals versus temperature for the 1 Ru 2.2 Fe catalyst. The decomposition of N2O is evidenced by the decrease of the N2O signal accompanied with the increase of the N2 and O2 signals. One way in which different catalysts can be compared is by observing the temperature required to achieve a conversion of 50%, that is, to reduce the amount of N 2O to half of the feed concentration. This is called the light-off temperature (T50 ). Table 4 shows the T50 of the powders synthesized in this work with 4 the Ru/Al2O3 nanoparticle catalysts reported by Komkovis et al.

Table 4. Light-off temperatures of nanoparticle catalysts in an N2O/He feed Ru content Light-off temperature Sample Source (wt%) T50 (°C) 1 Ru 2.2 Fe 1 This work ~ 417 4 Ru/γ-Al2O3-imp-calc (a) 0.95 Komkovis et al. ~ 461 4 Ru/γ-Al2O3-EG 0.98 Komkovis et al. ~ 395 2 Ru 2.2 Fe 2 This work ~ 380 4 Ru/γ-Al2O3-imp-calc (b) 1.94 Komkovis et al. ~ 415

It can be observed that for comparable concentrations of ruthenium, the FSP-synthesized catalysts showed a higher activity than those prepared by impregnation-calcination reported by Komkovis et al.4 However, the catalyst prepared by in situ reduction with ethylene glycol and a Ru content of 0.98 wt% had a higher activity than the corresponding FSP-prepared powder. Komkovis et al.4 list the small particle size and high metal dispersion (2-3 nm and 35%, respectively) of the catalysts prepared by reduction with ethylene glycol as contributing factors to their high activity. It is possible that the FSP-prepared powders have a larger particle size or lower metal dispersion which would contribute to the lower catalytic activity. However, it must be noted that FSP, as a whole, offers several practical advantages over traditional wet - phase catalyst preparation, and that it might be possible to change the parameters of the spraying itself (e.g. precursor flow, dispersion flow, etc.) in order to adjust the characteristics of the obtained product and improve its catalytic activity. While most of the results presented so far focus on the influence of the ruthenium content on activity, it is also important to discuss the effect of iron. The results of the relevant catalytic tests are shown 7 in Figure 6. Figure 6a shows the decomposition of N2O on two powders with the same ruthenium content (0.5 wt%) of which one contains Fe (2.2 wt%) while the other does not contain any iron. It can be observed that the presence of iron does not increase the activity compared to the Ru-only catalyst, and might in fact slightly worsen the performance. This suggests that the iron in the FSP-synthesized Fe/Ru/Al2O3 nanoparticles is not catalytically active for N2O decomposition at the temperatures at which the decomposition on Ru takes place. Figure 6a shows no synergistic effect between Ru and Fe in the 10 decomposition of N2O, however, Pieterse et al. only reported such an effect in the presence of NO, which was not part of the experiments carried out in this work. The fact that the activity of the 0.5 Ru 2.2 Fe catalyst was slightly inferior to the 0.5 Ru 0 Fe powder might have been caused by the “blocking” of a fraction of the Ru surface by iron, but further characterization of the samples would be needed in order to confirm this.

1 1 0.9 0.9 (a) (b) 0.8 0.8

0.7 0.7

O MS signal at at signal MS O at signal MS O 2 2 0.6 0.6 0.5 0.5

0.4 0.4

outlet [a.u] outlet [a.u] outlet 0.3 0.3 0.2 0.2 0.5 Ru, 0 Fe 0 Ru, 2.2 Fe

0.1 0.1 Relative intensity of N of intensity Relative Relative intensity of N of intensity Relative 0.5 Ru, 2.2 Fe Empty tube 0 0 100 200 300 400 500 600 700 100 200 300 400 500 600 700 Temperature [°C] Temperature [°C]

Figure 6. (a) Relative intensity of the N2O signal at the outlet versus temperature for two catalysts with the same amount of Ru (0.5 wt%), but one contains Fe (2.2 wt%) while the other does not. The presence of iron did not improve the activity in comparison to the Ru-only catalyst. (b) Result of the catalytic tests of the empty tube and the 0 Ru 2.2 Fe powder.

On the other hand, Figure 6b shows the decomposition of N2O in the empty tube and on the 0 Ru 2.2 Fe powder, which contains only iron (2.2 wt%). It can be observed that on 0 Ru 2.2 Fe the decomposition occurs at lower temperatures than in the empty tube but at higher temperatures than on the least active Ru-containing powders shown in Figure 6a and Figure 5. Because no other Fe-only powders were prepared, it is difficult to discern whether this occurred due to the iron becoming active at higher temperatures than the ruthenium or due of the decomposition occurring over the alumina surface. However, it is inte resting 10 to note that Pieterse et al. reported that, for a pure N2O feed (i.e. no inhibitors), the activity of FER zeolite catalysts varied with the metal as follows: the ruthenium-based catalyst showed the highest activity, then the Ru-Fe hybrid and last the iron-based catalyst. The results obtained in this work coincide with this observation.

4. CONCLUSIONS

Six different Fe/Ru/Al2O3 nanoparticle powders were synthesized by flame spray pyrolysis. The BET specific surface area of the prepared powders was approximately 170 m2/g. The XRD spectra of the samples are characteristic of an Al2O3 sample, but in the case of the powder with a ruthenium content of 4 wt%, diffraction peaks corresponding to a RuO2 phase were observed. The powders were found to be catalytically active in the decomposition of nitrous oxide without the addition of an external reducing agent. Increasing the content of ruthenium up to 2 wt% resulted in an 8 improvement of the catalyst’s performance; however, the activity did not change significantly when the Ru content was increased from 2 wt% to 4 wt%. The lowest light-off temperature among the prepared catalysts, corresponding to the 2 Ru 2.2 Fe and 4 Ru 2.2 Fe powders, was approximately 380°C. Iron was not found to increase the activity of a 0.5 wt% ruthenium-containing catalyst, hence, no synergistic effect between Fe and Ru was observed for a pure N2O feed. However, it was observed that a 2.2 wt% Fe/Al2O3 (Ru

0 wt%) powder did decrease the temperature at which the decomposition of N 2O takes place. Nonetheless, even the least-active Ru-containing powder was more effective for reducing the content of nitrous oxide in the stream.

5. OUTLOOK  A more detailed characterization of the synthesized powders, incorporating techniques such as transmission electron microscopy, would provide valuable information to interpret the behavior of

the Fe/Ru/Al2O3 catalysts.  To better explain the role of Fe in the prepared catalysts, it would be helpful to perform a series of

tests of Fe/Al 2O3 powders with varying iron content.  Further efforts could be oriented towards fine-tuning the FSP synthesis to minimize the particle size and maximize the metal dispersion of the prepared powders.  As the next step in the study of these catalysts, it would be necessary to conduct tests including

common inhibitors (e.g. NO, SO2 and H2O) in the feed and study the effect on activity of catalyst aging, for example.

6. REFERENCES 1. Forster P, Ramaswamy V. Changes in Atmospheric Constituents and in Radiative Forcing. In: Solomon S, Qin D, Manning M, Chen Z, Marquis M, Averyt KB, Tignor M and HL Miller, eds. Contribution of the Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. Cambridge, UK and New York, USA: Cambridge University Press; 2007; 212.

2. Ravishankara AR, Daniel JS, Portmann RW. Nitrous oxide (N2O): The dominant ozone-depleting substance emitted in the 21st Century. Science. 2009; 326: 123-125.

3. Centi G, Perathoner S, Vazzana F, Marella M, Tomaselli M, Mantegazza M. Novel catalysts and catalytic technologies for N2O removal from industrial emissions containing O2, H2O and SO2. Adv. Environ. Res. 2000; 4: 325-338.

4. Komkovis VG, Marti M, Delimitis A, Vasalos IA, Triantafyllidis KS. Catalytic decomposition of N2O over highly active supported Ru nanoparticles (<3 nm) prepared by chemical reduction with ethylene glycol. Appl. Catal. B. 2011; 103: 62-71. 5. European Union. Directive 2009/29/EC of the European Parliament and of the Council of 23 April 2009 amending Directive 2003/87/EC so as to improve and extend the greenhouse gas emission allowance trading scheme of the Community. Official Journal of the European Union. 2009; OJ L 140: 63-85. 6. Oi J, Obuchi A, Bamwenda GR, Ogata A, Yagita H, Kushiyama S, Mizuno K. Decomposition of nitrous oxide over supported rhodium catalysts and dependency on feed gas composition. Appl. Catal. B. 1997; 12: 277-286.

7. Pérez-Ramírez J, Kapteijn F, Mul G, Xu X, Moulijn JA. Ex-framework FeZSM-5 for control of N2O in tail-gases. Catal. Today. 2002; 76(1): 55-74. 8. Strobel R, Baiker A, Pratsinis SE. Aerosol flame synthesis of catalysts. Adv. Powder Technol. 2006; 17(5): 457-480.

9. Pieterse JAZ, Booneveld S, Mul G, Van den Brink RW. A synergistic effect in Iron-Ruthenium-FER catalyst for N2O decomposition in the presence of NO. Stud. Surf. Sci. Catal. 2005; 158: 1915-1920.

10. Pirngruber GD, Frunz L, Pieterse JAZ. The synergy between Fe and Ru in N2O decomposition over FeRu-FER catalysts: a mechanistic explanation. J. Catal. 2006; 243: 340-349.

11. Strobel R, Krumeich F, Stark WJ, Pratsinis SE, Baiker A. Flame spray synthesis of Pd/Al2O3 catalysts and their behavior in enantioselective hydrogenation. J. Catal. 2004; 222(2): 307-314. 12. United States Environmental Protection Agency. Inventory of US Greenhouse Gas Emissions and Sinks 1990-2009. Washington DC: US EPA; 2011