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2007_6_Nr6_EEMJ

Chirila et al

Chirila et al /Environmental Engineering and Management Journal 6 (2007), 6, 549-553 drying in oven at 80 o C and iii) calcination in muffle oven of the dried gel at three different temperatures: 600, 800 and 1000 o C (5 hours for each sample). 2.2. Characterization The calcined solids were characterized by different methods: X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), gas physisorbtion (N 2 ) and temperature programmed reduction (TPR). The crystallographic data were obtained using a Siemens D5000 diffractometer with CuKα radiation for crystalline phase detection between 5 and 100 o (2θ). XPS surface analysis were performed with a LHS 10 (Leybold AG) spectrometer using MgKα radiation (λ = 1256.6 eV). The specific surface area (BET) was determined by nitrogen adsorption at 300 o C using a Micrometics model ASAP 2000. TPR analysis was carried out under H 2 atmosphere, in a temperature range between 30 and 900 o C. 2.3. Catalytic activity testing The catalytic reduction of nitrogen oxides was carried out in a SCR-HC equipment with a gas mixture consisted by hydrocarbon (C 3 H 6 – 600 ppm and C 3 H 8 -800 ppm respectively), nitrogen oxides (600 ppm) under rich oxygen atmosphere (5%). 3. Results and discussions 3.1. The characterization of LaMnO 3 perovskite samples: XRD, XPS, gas physisorption and TPR In order to assess the perovskite-like structure, XRD analysis was recorder. The diffractograms obtained for LaMnO 3 perovskite samples are shown in Fig.1. The strong line of each XRD pattern showed the presence of single perovskite phase. In accordance with JCPDS-1998 data, all synthesized samples correspond to perovskite structure as is presented in Table 1. 1000 °C 800 °C 600 °C 0 10 20 30 40 50 60 70 80 90 2 Theta Fig.1. X-Ray diffractograms of LaMnO 3 perovskites calcined at 600, 800 and 1000 o C, respectively It was found a full perovskite structure like LaMnO 3 for the synthesized sample at 600 o C while the synthesized sample at 800 o C has shown a lanthanum deficit of perovskite structure. An oxygen excess in the perovskite structure has shown the last sample, synthesized at 1000 o C. Table 1. The samples correspondence with perovkite structure Samples Perovskite JCPDS-1998 Symmetry structure 600 o C LaMnO 3 86-1234, 75-440 Cubic 800 o C La 0.92 MnO 3 82-1152 Rhombohedra 1000 o C LaMnO 3+δ 32-848 Hexagonal These deviations from the perfect stoichimetry result from calcinations process when the perovskite phase is under transformation. Alongside with the temperature increasing, the crystallographic structure is changing due to octahedral distortion. For all perovskite synthesized samples, the La 3d 5/2 signal it was found around 833,6 eV. The Mn 2p 3/2 binding energy is 640 eV for LaMnO 3 structure and 641 eV for the others that showing the presence of Mn(III) in perovskite structure of the samples synthesized at 800 and 1000 o C. O 1s signal was appeared in three peaks typically corresponding to binding oxygen, the oxygen from hydroxyl or carbonate and oxygen from humidity. The values of surface atom composition are presented in Table 2 and showed enrichment in lanthanum of the perovskite surface for all analyzed samples. Table 2. Surface atom composition, BET surface area and reduction degree Atom % T cal. BET Mn La O 1s °C (m 2 /g) I II II 600 15,4 20,4 41 18,6 4,6 24,7 800 15,1 20,2 39 18,7 7 13,1 1000 15 21 39 19,4 6 2,4 BET surface area (m 2 /g) has values which decrease parallel with increasing of calcination temperatures at which the samples were obtained. Thus, sample obtained at 600 o C have higher value while the less value is given by surface of the sample calcined at 1000 o C, Table 2. TPR shape is given by reduction behavior of manganese oxides in analyzed perovskite in the presence of reducing gas, behavior very important which is reflected in catalytic activity of whole structure. In order to characterize resulted MnOx phases during synthesis of perovskite, it has to take into account that Mn 4+ reduces at lower temperature (331- 351 o C) than Mn 3+ (443-526 o C) (Buciuman et. al., 2000; Stephan et. al., 2002). TPR curves of three LaMnO3 perovskite samples presented in Fig. 2 are characterized by two reduction regions. 550

Syntheis, characterization and catalytic reduction of NOx emissions over LaMnO 3 perovskite 60 (c) 50 (b) (a) 0 200 400 600 800 1000 Conversia NO x [%] 40 30 20 10 600 °C 800 °C 1000 °C Temperature (°C) 0 150 200 250 300 350 400 450 500 550 600 650 Fig. 2. TPR curves for three LaMnO 3 perovskite samples calcined at 600, 800 and 1000 o C, respectively 100 Temperatura [°C] The first region corresponds to manganese oxides reduction, already discussed, reduction that gradually takes place as the corresponding temperature peaks show. The first peak corresponds to Mn 4+ la Mn 3+ reduction when reduction performs between 78 331-351 o C, followed by Mn 3+ la Mn 2+ reduction ate temperatures comprised between 421- 471 o C. The high consuming oh hydrogen happens in the second region of reduction curves due to both, Mn 3+ reduction to Mn 2+ in LaMnO 3 perovskite and carbonate species such as La 2 O 2 CO 3 those reduction corresponds to this temperature interval. Considering that these perovskites were prepared by citric method it is very plausible that carbonates traces are in their structure (Hackenberger, 1998; Stephan et. al., 2002). 3.2. Catalitic activity testing SCR-C 3 H 6 The perovskite samples were tested in nitrogen oxides removing by SCR-C 3 H 6 in presence and also in absence of oxygen atmosphere. In oxygen atmosphere (5%), the experimental results indicated of 100% propene oxidation activity between 300 and 450 o C for the samples obtained at 600 and 1000 o C while for sample obtained at 800 o C, the maxim activity was around 78% after which the oxidation activity had kept just below this value. The maxim point of propene conversion corresponded to the temperature interval in which propene could decompose to carbon dioxide and water. The perovskite synthesized sample at 1000 o C achieved maximal activity at 300 o C then it sharply deactivated. Regarding nitrogen oxides reduction as it can be seen in the experimental data processing (Fig. 3) all three LaMnO 3 perovskite samples practically showed negligible conversion values. The tests performed on synthetic gas mixture without oxygen have shown high activity for propene oxidation process but these were moved on high temperature range, over 400 o C. Conversia C 3 H 6 [%] 90 80 70 60 50 40 30 20 10 0 600 °C 800 °C 1000 °C 150 200 250 300 350 400 450 500 550 600 650 Temperatura [°C] Fig. 3. Nitrogen oxides (a) and propene (b) conversin for the LaMnO 3 perovskite samples (propene 600 ppm, NOx 600 ppm, 5% O 2 ) For LaMnO 3 perovskite sample obtained at 600 o C the catalytic activity test was carried out on a large temperatures range between 150 and 600 o C. Therefore, it can be observed for this sample that oxidation activity becomes maxim over 500 o C and it remains constant until 600 o C, experimental limit temperature. The other two perovskites samples, synthesized at 800 and 1000 o C, the values of nitrogen oxides conversion was increasing until ending of experiment, 450 o C. At this temperature, LaMnO3 sample calcined at 800 o C presents the higher value (100%) while sample calcined at 1000 o C has the lower catalytic activity (65.24%) (Fig. 4). 3.3. Catalytic activity testing SCR-C 3 H 8 Data processing obtained in selective catalytic reduction of nitrogen oxides using propane as reduction agent with 5% oxygen in synthetic gas mixture has led to the curves grouped in Fig.5. In this case, oxidation reaction reaches maximal values at high temperatures interval (350-450 o C). As it is shown in Fig. 5 diagram b, the LaMnO 3 perovskite samples achieve maximal hydrocarbon conversion point at 450 o C. 551

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