Influence of co-fed gases (O2, CO2, CH4, and H2O) on the N2O decomposition over (Co, Fe)-ZSM-5 and (Co, Fe)-BETA catalysts
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The influence of co-fed gases (O2, CO2, CH4, and H2O) on the N2O decomposition over (Co or Fe)-BETA and (Co, Fe)-ZSM-5 catalysts prepared by ion exchange method was investigated. Co2+ ions and oxo dinuclear Co species were identified in Co-ZSM-5 and Co-BETA catalysts. Isolated and oligomeric Fe3+ species in cationic sites and Fe2O3 particles were found on surface of the Fe-ZSM-5 and Fe-BETA catalysts. Cobalt catalysts were more actives than iron catalysts for the direct decomposition of N2O. Conversion of N2O over Fe-BETA and Fe-ZSM-5 was remained stable when co-fed O2, CO2, and CH4, but decreases with water vapor. However, Co-BETA and Co-ZSM-5 showed much larger reaction rate for N2O decomposition and were very stable when co-fed O2, CO2, CH4, and especially H2O. The results showed that the higher CH4 consumption during N2O reaction over Co-BETA and Co-ZSM-5 was due to CH4 combustion.
KeywordsN2O decomposition Co-fed gases Iron species Cobalt species ZSM-5 zeolite BETA zeolite
Nitrous oxide (N2O) is a potent greenhouse-effect gas with global warming potential (GWP) per molecule of about 300 times and 21 times higher than that of carbon dioxide (CO2) and methane (CH4), respectively. Nitrous oxide is not only a greenhouse gas, but also contributes to stratospheric ozone depletion [1, 2, 3]. The increase in anthropogenic N2O emissions (combustion, production of nitric and adipic acids, etc.) shows that the development of effective methods to reduce these emissions is therefore urgent. Consequently, extensive efforts are focused on catalysts for the decomposition of N2O into harmless N2 and O2 due to their efficiency, simplicity and low preparation costs [4, 5].
N2O decomposition has been evaluated on several catalysts, such as supported noble metals [6, 7, 8], transition metal oxides [8, 9, 10, 11, 12], and metal exchanged zeolites [13, 14, 15]. Noble metal based catalysts as Pt and Rh are active at low temperatures. However, the higher cost of these catalysts and the conversion reduction when co-fed with O2 and/or H2O make their industrial application unviable . Additionally, metal oxide based catalysts have been proposed for high temperature operation conditions. However, Liu et al.  as well as Yu et al.  showed that the N2O conversion decreases in presence of H2O, CO2, and O2, which are commonly found in industrial exhausts. Recent progress in the N2O decomposition reaction has been studied with the focus on transition-metal (Cu, Fe, Co)-modified zeolite catalysts [13, 14, 15, 18, 19]. Efforts dedicated to Cu-ZSM-5 have shown that decomposition of N2O is inhibited by O2 or H2O co-fed . On the other hand, Fe-zeolite catalysts have been outstanding due to their high activity and resistance in co-fed of CH4, CO, O2 and SO2 [20, 21]. Fe-ZSM-5 has been most studied experimentally and theoretically for the decomposition of N2O [22, 23]. It is believed that isolated Fe3+ and oligonuclear Fe x 3+ Oy clusters on the exchanged sites affects the catalytic performance [13, 22]. Furthermore, it is also notable that Co-sites were more active than Fe-sites due to lower activation energy barrier for the direct decomposition of N2O [19, 24]. According to some authors [13, 20], the activity of (Fe, Co)-zeolite catalysts also depends on the intrinsic properties of each zeolitic structure.
Some studies with BETA zeolite have shown attractive and advantageous characteristics of this material for catalysis, such as: wide pore opening, three-dimensional channel system, large specific area, high thermal and hydrothermal stability and shape selectivity [13, 14]. In terms of the zeolite topology, Fe-BETA was the most effective material between various commercial zeolites (MFI, FER, MOR, FAU) with similar Si/Al ratios for the decomposition of N2O  exhibiting superior activity in comparison with Fe-ZSM-5 . Additionally, Liu et al.  reported that the activity of Co-BETA was higher than that of Fe- or Cu-BETA comparing by the turnover frequency (TOF).
Although some studies for N2O decomposition by using beta-type zeolites has been done as previously reported [19, 25, 26], these works were not done with co-fed gases commonly found in industrial emissions. In addition, the performance of cobalt-containing zeolites for N2O decomposition is so far not well established. Therefore, the objective of this work was to study the influence of co-fed O2, CO2, H2O and CH4 on the direct decomposition of N2O over (Co, Fe)-BETA and (Co, Fe)-ZSM-5 catalysts. These catalysts were prepared by ion exchange method and its characterization using XRD, H2-TPR and UV–VIS spectroscopy was also discussed.
Materials and methods
The (Co, Fe)-ZSM-5 and (Co, Fe)-BETA catalysts were prepared by ion-exchange methods using commercial Na-ZSM-5 (ALSI-PENTA Zeolithe Gmbh) and NH4-BETA (TRICAT) zeolites with similar Si/Al molar ratios. The parent zeolite (3 g) was added to 1 mol/L aqueous solutions (150 mL) of iron nitrate (Fe(NO3)3·9H2O) and cobalt nitrate (Co(NO3)2·6H2O) at 50 °C. The mixture was vigorously stirred for 12 h. The sample was exchanged three times under the same conditions. Between each ion-exchange, the mixture was filtered; the solid was washed with distilled water and dried at 110 °C. Finally, the (Co, Fe)-ZSM-5 and (Co, Fe)-BETA catalysts were obtained after further calcination in muffle oven at 650 °C for 2 h under static air.
The catalysts were characterized by X-ray diffraction (XRD), temperature programmed reduction with H2 (H2-TPR) and UV–Visible Diffuse Reflectance Spectroscopy (UV–VIS). XRD analyzes were performed by the powder method using a Rigaku diffractometer (Miniflex 600) with Cu tube, Ni-filtered, operating at 40 kV, 15 mA and Cu Kα radiation. The speed of the goniometer used was 2° (2θ)/min, the angle ranging between 5 and 80° (2θ). The crystal phases were determined by correlating the diffraction patterns with those in the X’Pert HighScore reference.
H2-TPR analyses were performed on SAMP3 apparatus (Termolab Equipment, Brazil) equipped with a thermal conductivity detector (TCD). A trap was used to remove the water stemming from the reduction before the gas of the reactor outlet was sent to the TCD. TPR started with a ramp of 10 °C/min from 100 °C until 1000 °C. A flow of 30 mL/min from a high purity mixture of 2 V % H2 in Ar was used.
UV–VIS analyses were carried out in an Ocean Optics USB2000 spectrometer using a quartz cell. Before the measurement, the samples were dried at 120 °C for 2 h to remove the water molecule or OH groups. Samples were scanned in the range of 200–800 nm. The reflectance data were converted to the Schuster–Kubella–Munk function, F(R) = (1 − R)2/2R, where R is the diffuse reflectance obtained directly from the spectrometer.
Catalytic tests for N2O decomposition over (Co, Fe)-ZSM-5 and (Co, Fe)-BETA catalysts were carried out in U-shaped quartz reactor with an inner diameter of 9 mm. An electric furnace equipped with PID temperature control was used to heat the reactor. About 40 mg of catalyst were placed on fixed bed of quartz-wool inside the reactor. For all the experiments, 60 mL/min from a mixture of 10 V % N2O in He was flowed continuously into the reactor, corresponding to a GHSV of 30,000 h−1. Catalytic performance started with a ramp of 10 °C/min from 25 to 600 °C. The effect of co-fed gases on catalytic performance of catalysts was studied by adding 10 V % O2, 10 V % CO2, 10 V % CH4 and 10 V % H2O vapor (by saturator) into the reaction mixture at 600 °C whereas the concentration of N2O was still 10 V %, and the total flow of the gas was still 60 mL/min. The gaseous mixture flows were adjusted by electronic controllers (Brooks Instrument 0254).
Results and discussion
As for the UV–VIS spectra of Fe-BETA (Fig. 3c) and Fe-ZSM-5 (Fig. 3d), the two bands observed at 200–260 nm and at 260–360 nm corresponding to the typical ligand to metal charge transfer (LMCT) bands of isolated Fe3+ species in the cationic sites [19, 34] and isolated or oligomeric extraframework Fe species in zeolite channels , respectively. The bands at 360–460 nm (iron oxide clusters) and > 450 nm (large surface oxide species) can be associated to Fe2O3 particles on zeolite surface . Note that the spectrum of the Fe-ZSM-5 catalyst was characterized by a low intensity absorption edge at 550 nm reflecting low concentration of Fe oxide nanoparticles, which was not detected by XRD (< 4 nm), however observed by TPR. On the other hand, Fe-BETA showed a significant increase of the band above 500 nm confirming the present of large particles of Fe2O3 on zeolite surface, as already commented by XDR and TPR.
As noted, the nature of the host zeolite has a great influence on the reactivity of Fe and Co species with respect to N2O. Co-BETA was the most active material at low temperatures. This can be understood to both the large amount of active sites and to the widely open porosity of the BEA structure. Co-BETA and Co-ZSM-5 catalysts show similar conversions at higher temperatures due to the mass transfer regime. However, it is known that the decomposition of N2O over catalysts is inhibited mainly by O2 or H2O co-fed. The inhibiting effect of O2 and H2O vapor has been attributed to the competition adsorption between N2O and O2 or H2O [16, 19] and hydroxylation of the active sites by H2O [23, 44].
Our observations suggest that the reactions of N2O decomposition and reduction of N2O by CH4 proceed via different reaction mechanism and might occur over cobalt sites of the zeolite. Nobukawa et al.  measured the oxidation rates of methane with N2O and O2, and found that the intermediates of methoxy were oxidized with N2O more rapidly than O2. However, our results showed that the co-fed with complete gas mixture (N2O + CH4 + O2 + CO2 + H2O) did not change the N2O conversion over Co-BETA and Co-ZSM-5 catalysts.
(Co, Fe)-BETA and (Co, Fe)-ZSM-5 catalysts prepared by ion-exchange method have Co2+ ions, and oxo dinuclear Co species or isolated and oligomeric Fe3+ species compensating the negative charge of the zeolite framework, as shown by UV–VIS spectroscopy. In addition, Fe2O3 particles were found on surface of the Fe-ZSM-5 and Fe-BETA catalysts. Cobalt-catalysts were more actives than iron-catalysts for the direct decomposition of N2O. Conversion of N2O over Fe-BETA and Fe-ZSM-5 practically remained stable in the presence of O2, CO2, and CH4, but decreases slightly in the presence of water vapor. On the other hand, Co-BETA and Co-ZSM-5 were highly actives for the N2O decomposition, showing reaction rate clearly much larger and, interestingly, were very stable in the presence of O2, CO2, CH4, and especially H2O, with a slight decay of conversion of 0.06%/h. The higher CH4 consumption during N2O reaction on Co-BETA (70%) and Co-ZSM-5 (92%) was due to CH4 combustion.
The authors would like to acknowledge FAPEMIG (TEC-APQ-03361-15) for financial support of this research and Nayara Fernandes Biturini is grateful to the CAPES (Brazil) for a scholarship.
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