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Catalytic performances of perovskite oxides for CO oxidation under microwave irradiation
Chemical Engineering Journal 283 (2016) 97–104
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Chemical Engineering Journal journal homepage: www.elsevier.com/locate/cej
Catalytic performances of perovskite oxides for CO oxidation under microwave irradiation Hisahiro Einaga a,⇑, Yusaku Nasu a, Manabu Oda a, Hikaru Saito b a b
Department of Energy and Material Sciences, Faculty of Engineering Sciences, Kyushu University, 6-1, Kasugakoen, Kasuga, Fukuoka 816-8580, Japan Center of Advanced Instrumental Analysis, Kyushu University, 6-1, Kasugakoen, Kasuga, Fukuoka 816-8580, Japan
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Microwave-assisted catalytic CO
oxidation was carried out with perovskite oxides. Heating properties depended on physical state of perovskite oxide particles. Catalytic properties were controlled by changing the chemical compositions.
a r t i c l e
i n f o
Article history: Received 28 March 2015 Received in revised form 13 July 2015 Accepted 15 July 2015 Available online 21 July 2015 Keywords: Perovskite Microwave heating CO oxidation Transition metals
a b s t r a c t Microwave-assisted heterogeneous catalytic oxidation of CO over ABO3 type perovskite oxides (A: La; B: Mn, Fe, Co) was carried out and the factors controlling the catalytic properties were investigated. The heating behavior of perovskite oxides depended on the physical state of catalyst particles and catalyst compositions. Catalytic activity of the perovskite oxide decreased in the order of LaCoO3 > LaMnO3 > LaFeO3 under conventional heating, although LaMnO3 exhibited similar activity to LaCoO3 under microwave heating. The partial substitution of A site cation La by Sr enhanced the heating and catalytic properties under microwave heating conditions. Catalytic properties of the perovskite oxides under microwave heating were compared with those under conventional external heating. Ó 2015 Elsevier B.V. All rights reserved.
1. Introduction Microwave-assisted chemical reactions have attracted a lot of attention because microwave can rapidly and selectively increase the temperature of target compounds that can convert microwave energy to heat [1]. Microwave heating processes have been therefore used for various types of chemical processes including organic synthesis [2–3], materials productions [4–5], and catalytic reactions [6–8]. Microwave heating shortened reaction times and improved high product yields in these reactions. As for ⇑ Corresponding author. Tel.: +81 92 583 7525; fax: +81 92 583 8853. E-mail address: [email protected] (H. Einaga). http://dx.doi.org/10.1016/j.cej.2015.07.051 1385-8947/Ó 2015 Elsevier B.V. All rights reserved.
microwave-assisted gas–solid heterogeneous catalytic reactions, solid catalysts such as zeolites [9], metals [10], supported metals [11], and metal oxides [12] have been used. One of the potential applications of microwave-assisted catalytic oxidation processes is control of polluted gases emitted from stationary sources and mobile sources. Although so many kinds of gas purification methods have been developed, microwaveassisted catalytic oxidation process has the advantages over conventional combustion processes in that we can construct compact and highly operable systems for gas purification. Perovskite-type mixed metal oxides, represented by general formula ABO3 are potential substitutes of noble metal catalysts [13]. In the perovskite oxides, B metal ions are coordinated with
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six oxide ions, and the BO6 units are connected by corner-sharing manner, making networks in which A-site metal ions are located in the dodecahedral site of framework. Perovskite oxides containing transition metals such as Mn and Co at B sites have high oxidation ability toward CO [14], methane [15], volatile organic compounds (VOCs) [16,17] and diesel soot [18,19]. The catalytic properties of perovskite oxides can be controlled by substitution of A site cations by other kinds of metals [20]. In the case of LaMnO3 and LaCoO3 perovskite oxides, for instance, the substitution of La3+ ions by Sr2+ ions gives rise to the increase in the oxidation state of B site cations, such as Co3+ to Co4+ and Mn3+ to Mn4+, which improves the oxidizing ability of the oxides. Perovskite type mixed oxides have been also used for the catalytic oxidation processes under microwave heating. Catalytic oxidation of CO [21], methane [22,23], propane [21,24], benzene [25,26], and soot [27] has been so far reported. One of the advantages of perovskite oxides over other metal oxides in catalytic reactions under microwave heating is that the electromagnetic properties of perovskite oxides that contribute to microwave heating can be also controlled by changing the constituent elements. As the oxidation catalysts, so many kinds of perovskite oxides have been used under microwave heating [21–27] and the perovskite oxides containing La in the A site and Mn, Fe, and Co in the B site were effective for the oxidation reactions. To clarify the relationship between the composition, the textural properties, the heating behavior under micro-wave irradiation and the CO oxidation activity of the catalysts, the effect of the B site cation in LaBO3 (B = Mn, Fe, Co) type perovskite oxides and the partial substitution of La by Sr were investigated.
2. Materials and methods 2.1. Catalyst preparation In this study, we prepared perovskite oxide catalysts using two kinds of methods: LaBO3 (M = Mn, Co, Fe) type perovskite oxides were prepared by hydrolysis-coprecipitation of metal nitrates with TMAH in aqueous solution because impurity phases such as single metal oxides can be easily suppressed by this method. On the other hand, the Sr-containing perovskite oxides were prepared by evaporation-to-dryness method because Sr ions were not completely precipitated in the hydrolysis-coprecipitation method and therefore this method cannot be applied to the preparation of Sr-containing samples. LaBO3 (B = Mn, Fe, and Co) type perovskite oxides were prepared by hydrolysis of metal nitrates in aqueous solutions according to the previously reported method [28]. In a typical preparation method for LaMnO3, appropriate amounts of La(NO3)36H2O (Kishida Chemical Co. Ltd., Japan, >99%) and Mn(NO3)24H2O (Wako Pure Chemical, Japan, >99.9%) were dissolved into water, and the aqueous solution was added dropwise to an aqueous solution containing tetramethylammonium hydroxide (TMAH, Kishida Chemical Co., Ltd.). The resultant colloidal particles were filtered off, washed several times with water, dried at 100 °C for 24 h and calcined at 950 °C for 5 h. For the preparation of LaFeO3 and LaCoO3, the same procedure was adopted using Fe(NO3)39H2O and Co(NO3)26H2O instead of using Mn(NO3)24H2O. Sr-substituted perovskite oxides La0.8Sr0.2BO3 (B = Fe, and Co) and La1xSrxBMnO3 (x = 0.2, 0.5, 0.6) were prepared by evaporation-to-dryness method using malic acid as the complexing reagent [29]. In a typical preparation method for La0.8Sr0.2MnO3, appropriate amounts of La(NO3)36H2O, Sr(CH3COO)20.5H2O (Wako Pure Chemicals, Japan), Mn(NO3)26H2O, and malic acid were dissolved into water with the 2-to-3 M ratio of metal ions to malic acid, and the aqueous
solution was heated and evaporated to dryness under stirring. The residual powder was heated at 950 °C for 5 h. The catalysts were pressed into disks at a pressure of 30 MPa for 3 min, pulverized using mortar and pestle, and then sieved to the sizes of 250–710 lm before use for catalytic reactions. Although the catalyst temperatures increased with decreasing the meshed particle sizes, mass transfer in the reactor was more homogeneous when the 250–710 lm-sized particles were used as the catalysts. 2.2. Catalyst characterization X-ray diffraction (XRD) patterns were taken using a RIGAKU RINT2200 diffractometer using Cu Ka radiation (k = 1.5418 Å) at 40 kV and 30 mA. The measurements were carried out with a step size of 0.02° and a scan speed of 2.0°/min. Catalyst samples were ground with mortar and pestle before measurements. Catalyst surface area was determined by BET method from N2 adsorption isotherm at 196 °C with Belsorp mini (BEL Japan Co. Ltd.). TEM and STEM observations were carried out using an FEI Titan Cubed equipped with a Schottky electron source and an aberration corrector of the probe-forming lens. The microscope was operated at 300 kV in TEM and scanning probe modes. The convergence semiangle was 21.4 mrad for STEM imaging. The inner and outer semiangles of the HAADF detector were 38 and 184 mrad, respectively. 2.3. Measurement of heating behavior of perovskite oxides under microwave heating Fig. 1 shows the schematics of the reaction system used in this study. A triple tube type reactor made of quartz glass which had the inlet and outlet pipes was used in this study (Fig. 1(a)). The microwave irradiation system was a modified system of Green Motif II (IDX Co. Ltd., Japan) and composed of a rectangularshaped waveguide (27 mm 96 mm) and a power source and reflecting plate, which produced single mode microwaves (Fig. 1(b)). The strength distribution of electric field and magnetic field in the wave guide is demonstrated in Fig. 1(c). In the microwave irradiation system, microwaves were reflected by the reflecting plate and interfered with incident microwaves, giving rise to the formation of standing wave. Consequently, the strength distribution for electric field was shifted by one-fourth of the microwave length to that of magnetic field. The catalyst (0.20 g) was placed in the triple tube type reactor. The reactor was placed in the waveguide as the catalyst sample was located in the middle of the waveguide (Place C), at which the strength of the electric field was the highest. Microwave was irradiated for the catalyst with the power range of 30–500 W. The catalyst temperature was pursued with a fiber optic thermometer (Anritsu Keiki, Co. Ltd.; AMOTH FL-2000, FS400), which was contacted with the catalyst across the inner glass tube. 2.4. Catalytic oxidation under conventional heating and microwave heating Catalytic oxidation of CO under microwave heating was carried out with a fixed bed flow reactor. Catalyst particle (0.10 g) was placed in the triple-tube type reactor, which was located in the waveguide. As a pretreatment, the catalyst was heated in O2–He flow by microwave irradiation at 300 W until the catalyst temperature was raised to 300 °C for the catalyst samples except for LaFeO3. In the case of LaFeO3, the catalyst temperature did not reach the temperature of 300 °C (see below). Therefore, the pretreatment was carried out at 150 °C with the power of 300 W for several minutes. After the heat treatment under microwave heating, the catalyst samples were cooled down to room temperature. The reaction gases for CO oxidation were prepared by mixing CO
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Fig. 1. Schematic of microwave heating system. (a) triple tube type reactor, (b) waveguide, (c) distribution of electric and magnetic field strengths.
(1%, He balance) and O2 (10%, He balance; He balance) in cylinders. The reaction gas composition was as follows: CO concentration 0.5%, O2 5%; gas flow rate 100 ml min1; WHSV = 60,000 mL g1 h1. Gas samples were analyzed on a GC-TCD (GL Science GC 390). Catalytic oxidation of CO under conventional heating was carried out with the triple-tube type reactor. Prior to catalytic reaction, the catalyst was heated at 300 °C in a flow of pure O2 for 1 h, and then thermostated at a reaction temperature with a heating apparatus. The reaction gas composition and gas flow rate was the same as those in the catalytic reaction under microwave heating. Gas samples were analyzed on a GC-TCD (GL Science GC 390).
3. Results and discussion 3.1. Textural properties of perovskite oxides Fig. 2 shows the XRD patterns of LaBO3 (M = Mn, Co, Fe) type perovskite oxides prepared by hydrolysis-precipitation method and Sr-substituted perovskite oxides, La0.8Sr0.2MO3 (M = Mn, Co, Fe) prepared by evaporation to dryness method, followed by calcination at 950 °C. For both systems, characteristic X-ray diffraction patterns for perovskite phases were observed. XRD spectra showed the formation of rhombohedral crystalline phases for of LaMnO3 and LaCoO3 catalysts (JCPDS ID 032-0484, 25-1060), whereas LaFeO3
has an orthorhombic perovskite structure (JCPDS ID 015–0148). For the Sr-substituted perobskite oxides, La0.8Sr0.2MnO3, La0.8Sr0.2CoO3, and La0.8Sr0.2FeO3, the crystal systems were the same as the corresponding Sr-free perovskited oxides. The surface areas of perovskite oxides are listed in Table 1. The surface area of the perovskites prepared in this study was in the range of 1–4 m2/g. LaMnO3 have larger surface area than LaCoO3 and LaFeO3. The partial substitution of La by Sr led to the increase in surface area of LaCoO3 and LaFeO3, indicating that surface area of the perovskite oxides depended on their compositions. The improvement of surface area by Sr substitution for B site metal cation has been also reported [17,30,31]. Fig. 3 shows TEM and STEM images of LaMnO3 catalyst prepared by hydrolysis-coprecipitation method and calcined at 950 °C. Catalyst particles with the ranges of 100–500 nm were observed (Fig. 3(a)). The HAADF-STEM and HRTEM images (Fig. 3(b) and (c)) and selected area electron diffraction (SAED) (Fig. 3(d)) confirmed that the highly ordered crystalline structure for the LaMnO3 catalyst.
3.2. Heating behavior of perovskite oxides under microwave irradiation Fig. 4 shows the heating behavior of perovskite oxides LaMnO3 under microwave irradiation. Here, the catalyst sample was
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LaMnO3
LaFeO3
LaCoO3
La0.8Sr0.2MnO3
La0.8Sr0.2FeO3
La0.8Sr0.2CoO3
Fig. 2. XRD patterns of perovskite oxides.
located inside the waveguide at the point where the electric field strength was the maximum (Fig. 1c, Place C). When microwave irradiation for the fresh samples started, the temperature increases with time on stream and reached a steady-state value. The steady state temperature increased with the increase in microwave power in the range of 50–250 W. When the microwave power was raised to 300 W, the catalyst temperature drastically increased after the temperature exceeded 180 °C (Fig. 4(a)). After the catalyst sample was cooled down to room temperature and then heated again with microwave irradiation. The steady state temperature for the catalyst for this time was higher than that under the microwave heating for the first time (Fig. 4(b)). When the microwave on–off cycles were repeated, the steady state temperature was comparable to that under microwave irradiation at second time irradiation. Thus, the microwave irradiation improved the heating properties of perovskite oxides. The ‘‘pre-irradiation effect’’ was observed for LaCoO3 and LaFeO3. Hereafter, heating properties of the perovskite oxides were investigated after the microwave heating was carried out for the oxides. The mechanism for microwave heating of materials can be categorized into dielectric heating and magnetic heating [30]. In the dielectric heating process, dipolar polarization formed in dielectric materials under microwave irradiation is rotated and deformed by the oscillating electric field, leading to heat generation [5,32]. In
Table 1 BET surface area of perovskite oxides. Catalyst
Surface area/m2 g1
LaCoO3 LaFeO3 LaMnO3 La0.8Sr0.2CoO3 La0.8Sr0.2FeO3 La0.8Sr0.2MnO3
1.4 1.5 3.2 3.7 3.1 3.8
the magnetic heating for solid materials, one of the mechanisms for heating is eddy current loss: magnetic field generates eddy current in the solids, which gives rise to resistance heating. To reveal the factors controlling the heating behavior of perovskite oxides, we subsequently investigated the effect of the location of catalyst samples in the wave guide on their heating behaviors. When the catalyst positions in the waveguide were changed from Place C to B and A, where the strength of the electric field decreased and that of magnetic field increased, the steady-state temperature of the catalysts greatly decreased (Supporting information Fig. S2), indicating that the physical properties that contributed to the heating behavior of perovskite oxides were the dielectric loss rather than the magnetic eddy current loss. The heating properties of the perovskite oxides also depended on the physical state of the catalyst particles. When the meshed catalysts were heated by microwave irradiation, the ‘‘pre-irradiation effect’’ was observed, as described above. After the catalyst samples were pulverized to fine powders and sieved to the particle sizes of 250–710 lm and then heated again by microwave irradiation, the catalyst temperature profile was similar to that of the fresh sample. The heating behavior was also improved by calcination of the meshed particles: the catalyst particles which had been sieved to the particles sizes of 250–710 lm were heated at 800°C for 5 h and microwave irradiation was carried out for the samples, the temperature of LaMnO3 catalyst rapidly increased and heating behavior was similar to that of microwave-preirradiated samples. These findings suggested that the ‘‘pre-irradiation effects’’ were ascribed to the changes in the physical state and interaction between crystallites and primary particles contained in the meshed particles. On the other hand, the catalysts samples that were pretreated by microwave heating were taken out from the reactor and shuffled and located again into the reactor. When the microwave was irradiated again for the catalyst, ‘‘pre-irradiation effects’’ was also observed. This indicates that the ‘‘pre-irradiation effects’’ was not ascribed to the changes in interaction between the meshed catalyst particles. The ‘‘pre-irradiation effects’’ were observed for LaCoO3, confirming that the heating behavior of the perovskite oxides depend not only on the B site cation but also on the state of primary and secondary particles. The heating behavior depended on the B site cations of LaBO3 perovskite oxides. The steady state temperature also increased with the increase in microwave power for LaCoO3 and LaFeO3 (Supporting information Fig. S2). When the catalyst temperature was compared, the value decreased in the order of LaMnO3 > LaCoO3 > LaFeO3. The temperature for LaMnO3 catalyst under microwave heating was much higher than that for manganese oxide (Mn2O3) prepared by the AMP method and calcined at 650 °C at any given power, indicating that Mn-containing perovskite oxides are effective for microwave heating compared with Mn single oxides. 3.3. Catalytic properties of LaBO3 type perovskite oxides under conventional heating Fig. 5 shows the CO oxidation over perovskite oxides containing Mn, Fe, and Co in the temperature range of 150–400 °C. Here, the conversions were evaluated at steady state. CO oxidation activity increased monotonically with the increase in catalyst temperature for all the catalyst. The order of CO oxidation activity was LaCoO3 > LaMnO3 > LaFeO3 at any reaction temperature. The difference in the catalytic activity was not merely ascribed to the difference in catalyst surface area because the surface areas of the perovskite oxides were similar (1.4–3.7 m2/g). It should be note that when the LaMnO3 perovskite oxides were irradiated with microwave at 300 W and used for CO oxidation under external
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(a)
101
(b)
La Mn
(c)
(d)
Fig. 3. TEM and STEM images of LaMnO3 catalyst. (a) and (b) HAADF-STEM images, (c) HRTEM image, (d) selected area electron diffraction (SAED).
Temperature / °C
(a) 350
300 W
300 250
250 W 200 W 150 W 100 W 50 W
200 150 100 50 0
0
200
400
600
800
1000
Time / sec
Temperature / °C
(b) 350
200 W
300 250
300 W 250 W
200
150 W
100 W Fig. 5. Catalytic oxidation of CO over perovskite oxides under conventional external heating. (s) LaMnO3, (d) La0.8Sr0.2MnO3, (h) LaCoO3, (j) La0.8Sr0.2CoO3, (4) LaFeO3, (N) La0.8Sr0.2FeO3. Catalyst weight 0.10 g, CO concentration 0.5%, O2 5%; gas flow rate 100 ml min1; WHSV = 60,000 mL g1 h1.
150 50 W
100 50 0
0
200
400 600 Time / sec
800
1000
Fig. 4. Heating behaviors of LaMnO3 under microwave irradiation in air. Catalyst weight 0.20 g. (a) fresh sample, (b) the sample pre-irradiated by microwave .
heating, the activity was comparable to that for the LaMnO3 catalyst pretreated by conventional heating. This indicates that the CO oxidation activity of LaMnO3 catalyst under conventional heating was not affected by the pre-irradiation of microwave.
It has been reported that CO oxidation on perovskite oxide proceeds by the adsorption of CO on the perovskite oxides and the adsorbed CO reacts with surface oxygen species formed by dissociative adsorption of molecular O2 (Eqs. 1–4) [33].
O2ðgÞ ! O2 ðadÞ ! 2O2 ðadÞ
ð1Þ
COðgÞ ! COðadÞ
ð2Þ
COðadÞ þ 2OðadÞ ! CO2 3 ðadÞ
ð3Þ
2 2 CO2 3 ðadÞ ! CO2ðadÞ þ OðadÞ ! CO2ðgÞ þ OðadÞ
ð4Þ
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Therefore, the CO oxidation activity is dominated by the reactivity of oxygen species with CO (Eq. (3)) and desorption of CO2 from the perovskite oxide surfaces (Eq. (4)). The activity of LaCoO3 for CO oxidation was higher than the activities of LaMnO3 and LaFeO3 catalysts. The activity order was the same as that reported earlier for catalytic oxidation of propane over perovskite oxides, LaMO3 (M = Mn, Fe, and Co) [34]. The incorporation of Co into the B site of perovskite oxides gives rise to the formation of Co3+ in the B site of perovskite oxides, which are highly reducible and therefore have oxidizing ability toward CO [15]. The substitution of A site cation La3+ with Sr2+ ions greatly improved the oxidation ability of Co, Mn, an Fe-containing perovskite oxides because the substitution changed the valence of B-site cations, such as Co3+ to Co4+, which further improved the oxidizability of the B site cations [15]. The catalysts prepared in this study also showed the similar phenomena: the partial substitution of 20% La by Sr greatly increased the CO oxidation activity of the Mn-, Fe-, and Co-containing perovskite oxides, as shown in Fig. 5. The order of the CO oxidation activity for the Sr-substituted perovskite oxides La0.8Sr0.2BO3 was Co > Mn > Fe, which was also consistent with that of LaBO3 perovskite oxide series.
require for obtaining the steady-state temperature depended on the catalyst composition. Here, the catalyst temperatures were measured after they reached steady-state values. The steady-state temperature during the CO oxidation increased with the increase in microwave power (Fig. 7(a)). The catalyst temperature decreased in the order of Mn > Co > Fe. Thus, the LaMnO3 catalyst exhibited superior heating behavior than other catalysts under the reaction conditions. The substitution of La by Sr strongly influenced the heating behavior and catalytic properties of perovskite oxides. The heating behaviors of Sr-substituted perovskite oxides LaSrMO3 (M = Mn, Fe, Co) under microwave irradiation were also shown in Fig. 7(a). The steady-state temperature was also obtained for the catalysts and the temperature strongly depended on the microwave power. Thus, the catalyst temperature could be controlled by changing the microwave power, as in the case of LaBO3 type perovskites. The heating behavior of the LaSrMO3 type perovskites was almost independent of the B site cations. The temperature for the LaSrMO3
3.4. Catalytic properties of LaBO3 type perovskite oxides under microwave heating One of the important features of perovskite oxides was high stability during catalytic oxidation under microwave heating. Fig. 6 shows a typical time course for CO oxidation with LaMnO3 catalyst under microwave heating. Here, the catalyst samples were located at Place C. The temperature for catalyst bed increased as the power was switched on and the temperature reached steady state after 10 min. Moreover, good reproducibility was observed when the MW power was switched on and off (data not shown). Thus, the temperature of catalyst bed could be also controlled under the reaction conditions with microwave heating. CO oxidation behavior was also plotted in Fig. 6. No significant changes in CO conversions were observed. Thus, the steady-state activity for CO oxidation could be measured under the microwave heating. Fig. 7 shows the effect of microwave power on the catalyst temperature and CO oxidation activity of LaMO3 (M = Mn, Fe, Co) perovskite oxides under microwave irradiation. It needs 10–20 min before the catalyst reached steady-state temperatures and the time
Fig. 6. Time course for catalyst temperature and CO conversion under microwave heating. Catalyst weight 0.10 g, CO concentration 0.5%, O2 5%; gas flow rate 100 ml min1; WHSV = 60,000 mL g1 h1; microwave power 200 W.
Fig. 7. Catalytic oxidation of CO over perovskite oxides under microwave heating. (a) Heating behaviors of perovskite oxides. (b) CO conversions. (s) LaMnO3, (d) La0.8Sr0.2MnO3, (h) LaCoO3, (j) La0.8Sr0.2CoO3, (4) LaFeO3, (N) La0.8Sr0.2FeO3. Catalyst weight 0.10 g, CO concentration 0.5%, O2 5%; gas flow rate 100 ml min1; WHSV = 60,000 mL g1 h1.
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type perovskite was much higher than that for LaMO3 type perovskite when the same B site cations were used. Particularly, the substitution of La by Sr greatly improved the heating behavior of the Fe-containing perovskite oxides. The CO oxidation activity of LaMO3 (M = Mn, Fe, Co) perovskite oxides under microwave irradiation depended on the B-site cations. Fig. 7(b) shows the CO conversion against the microwave power for the perovskite oxides. The CO conversion increased with the increase in the microwave power. The activity of LaMnO3 was comparable with that of LaCoO3 in contrast with the CO oxidation under conventional heating. LaFeO3 exhibited almost no activity even the microwave power was raised to 280 W. The order of the activity can be explained in terms of heating properties of perovskite oxides: the LaMnO3 catalyst exhibited CO oxidation activity comparable with the LaCoO3 catalyst because the temperatures for LaMnO3 catalyst under microwave heating were higher than that for LaCoO3 catalyst. The temperatures for LaFeO3 catalyst under microwave heating were much lower than the light-off temperature for CO oxidation. Fig. 7(b) also compares the CO oxidation with the Sr-substituted perovskite oxides under microwave heating. The La0.8Sr0.2MO3 (M = Mn, Fe, Co) type perovskite oxides exhibited the CO oxidation activities and the CO conversion monotonically increased with the increase in microwave power. The La0.8Sr0.2MO3 catalysts exhibited much higher activity for CO oxidation than the corresponding LaBO3 type oxides having the same B site cations. Therefore, the substitution of La by Sr raised the catalyst temperatures and CO oxidation activities under microwave heating. The improved activity by Sr substitution was ascribed to the improved heating properties and inherent CO oxidation properties of the perovskite oxides. The effect of partial substitution of La by Sr was prominent for the LaFeO3 catalyst. Fig. 8 compares the CO oxidation activity as the function of catalyst temperature for perovskite oxides. When the CO oxidation activity of LaBO3 type perovskite oxides under the microwave heating with that under the conventional heating, the activity decreased in the order of LaCoO3 > LaMnO3 > LaFeO3 under the microwave heating and the order was consistent with that under the conventional heating (Fig. 5). The activities under microwave heating were much higher than those under conventional heating and the reaction temperature required for CO oxidation was much lowered (50–100 °C). Similar results were obtained for microwave-assisted heterogeneous catalytic reactions [21,22,35]. It has been suggested that the catalytic active sites are selectively heated by microwave irradiation and the temperatures of catalyst active sites are much higher than those in the surroundings [36]. The adsorbed reactants on such hot active sites can be activated by energy transfer from the active sites, facilitating the surface chemical reaction (‘‘thermal effect’’). The formation of these hot spot sites is possible for perovskite oxides because dipole strengths in the perovskite oxides are different depending on the A and B sites [37]. Another possible explanation is ‘‘non-thermal’’ effects, in which the electromagnetic field of the microwave interacts with the reactants, promoting the surface chemical reaction [32]. The higher activity of the perovskite oxides for CO oxidation under microwave heating than that under conventional heating was ascribed to either the ‘‘thermal effect’’ or ‘‘non-thermal effect’’. However, we cannot differentiate the effect affecting the microwave-assisted catalytic oxidation process at present. For the LaSrMO3 (M = Mn, Fe, Co) perovskite oxides, the activity decreased in the order of Co > Mn > Fe, which was consistent with the order of LaMO3 (M = Mn, Fe, Co) catalysts. As described above, heating property of perovskite oxides is one of the important factors affecting their catalytic properties under microwave heating, and the heating behaviors of Mn-containing perovskites LaMnO3 and La0.8Sr0.2MnO3 were much higher than
103
Fig. 8. Comparison of CO oxidation activity of perovskite oxides under microwave heating and conventional heating. Conditions are the same as those in Figs. 4 and 6. (a) (s,d) LaMnO3, (h,j) LaCoO3, (4,N) LaFeO3. (b) (s,d) La0.8Sr0.2MnO3, (h,j) La0.8Sr0.2CoO3, (4,N) La0.8Sr0.2FeO3. Open symbols: reaction under microwave heating; closed symbols: reaction under conventional heating.
those of Co- and Fe-containing perovskite oxides. These findings urged us to investigate the effect of Sr/La molar ratio on the catalytic properties of La–Sr–Mn type perovskite oxides under microwave heating because the CO oxidation activity of the catalysts under conventional heating can be greatly improved by increasing the Sr/La ratio [20]. For the perovskite oxides of La1xSrxMnO3 (x = 0.5, 0.6) prepared by evaporation-to-dryness methods, perovskite oxides were mainly formed with almost no impurity phases in the XRD patterns (Fig. S4). With the increase in Sr/La ratio, the crystalline phase was changed to cubic perovskite structure. As shown in Fig. 9, the heating behaviors of the La–Sr–Mn type perovskite oxides were almost unchanged when the Sr/La ratio was changed. On the other hand, the CO oxidation activity greatly improved when the Sr/La ratio increased to 1/1. The improved activity was probably ascribed to the improved activity of Mn sites due to the changes in their oxidation state. This finding further suggested that the substitution of A site cation was also effective for improving the catalytic properties of ABO3 type perovskite oxides under microwave heating.
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LaMnO3 exhibited much lower activity than LaCoO3 under conventional heating. The substitution of La by Sr improved the heating behavior and CO oxidation ability of the perovskite oxides under microwave heating. The highest activity was obtained at the La/Sr ratio of 1/1 for La–Sr–Mn–O system. Acknowledgement This work was financially supported by Environmental Technology Development Fund from the Ministry of the Environment, Government of Japan. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.cej.2015.07.051. References [1] [2] [3] [4] [5]
[6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16]
Fig. 9. Catalytic oxidation of CO over La1x-SrxMnO3 type perovskite oxides under microwave heating. (a) Heating behaviors of perovskite oxides. (b) CO conversions. (d) x = 0, (s) 0.2, (h) 0.5, (j) 0.6. Catalyst weight 0.10 g, CO concentration 0.5%, O2 5%; gas flow rate 100 ml min1; WHSV = 60,000 mL g1 h1.
4. Conclusions In this study, we prepared perovskite oxides containing transition metals (Mn, Fe, Co) by precipitation method and evaporation-to-dryness methods, followed by calcination at 950 °C. XRD studies confirmed the formation of perovskite oxide phases for all the catalysts. The heating behavior of the perovskite oxides was improved by the pre-irradiation of microwave, and the ‘‘pre-irradiation effects’’ were ascribed to the changes in the intra-particle interaction. The physical state of perovskite oxide particles affected their heating behaviors. The temperature of perovskite oxide particles reached steady-state value under microwave irradiation conditions and monotonically increased with the microwave power. Heating and catalytic properties of perovskite oxides strongly depended on the catalyst composition. LaMnO3 was more easily heated by microwave irradiation than LaCoO3 and LaFeO3. The CO oxidation activity of LaMnO3 was comparable to that LaCoO3 under microwave irradiation, although
[17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34] [35] [36] [37]
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Contents lists available at ScienceDirect
Chemical Engineering Journal journal homepage: www.elsevier.com/locate/cej
Catalytic performances of perovskite oxides for CO oxidation under microwave irradiation Hisahiro Einaga a,⇑, Yusaku Nasu a, Manabu Oda a, Hikaru Saito b a b
Department of Energy and Material Sciences, Faculty of Engineering Sciences, Kyushu University, 6-1, Kasugakoen, Kasuga, Fukuoka 816-8580, Japan Center of Advanced Instrumental Analysis, Kyushu University, 6-1, Kasugakoen, Kasuga, Fukuoka 816-8580, Japan
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Microwave-assisted catalytic CO
oxidation was carried out with perovskite oxides. Heating properties depended on physical state of perovskite oxide particles. Catalytic properties were controlled by changing the chemical compositions.
a r t i c l e
i n f o
Article history: Received 28 March 2015 Received in revised form 13 July 2015 Accepted 15 July 2015 Available online 21 July 2015 Keywords: Perovskite Microwave heating CO oxidation Transition metals
a b s t r a c t Microwave-assisted heterogeneous catalytic oxidation of CO over ABO3 type perovskite oxides (A: La; B: Mn, Fe, Co) was carried out and the factors controlling the catalytic properties were investigated. The heating behavior of perovskite oxides depended on the physical state of catalyst particles and catalyst compositions. Catalytic activity of the perovskite oxide decreased in the order of LaCoO3 > LaMnO3 > LaFeO3 under conventional heating, although LaMnO3 exhibited similar activity to LaCoO3 under microwave heating. The partial substitution of A site cation La by Sr enhanced the heating and catalytic properties under microwave heating conditions. Catalytic properties of the perovskite oxides under microwave heating were compared with those under conventional external heating. Ó 2015 Elsevier B.V. All rights reserved.
1. Introduction Microwave-assisted chemical reactions have attracted a lot of attention because microwave can rapidly and selectively increase the temperature of target compounds that can convert microwave energy to heat [1]. Microwave heating processes have been therefore used for various types of chemical processes including organic synthesis [2–3], materials productions [4–5], and catalytic reactions [6–8]. Microwave heating shortened reaction times and improved high product yields in these reactions. As for ⇑ Corresponding author. Tel.: +81 92 583 7525; fax: +81 92 583 8853. E-mail address: [email protected] (H. Einaga). http://dx.doi.org/10.1016/j.cej.2015.07.051 1385-8947/Ó 2015 Elsevier B.V. All rights reserved.
microwave-assisted gas–solid heterogeneous catalytic reactions, solid catalysts such as zeolites [9], metals [10], supported metals [11], and metal oxides [12] have been used. One of the potential applications of microwave-assisted catalytic oxidation processes is control of polluted gases emitted from stationary sources and mobile sources. Although so many kinds of gas purification methods have been developed, microwaveassisted catalytic oxidation process has the advantages over conventional combustion processes in that we can construct compact and highly operable systems for gas purification. Perovskite-type mixed metal oxides, represented by general formula ABO3 are potential substitutes of noble metal catalysts [13]. In the perovskite oxides, B metal ions are coordinated with
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six oxide ions, and the BO6 units are connected by corner-sharing manner, making networks in which A-site metal ions are located in the dodecahedral site of framework. Perovskite oxides containing transition metals such as Mn and Co at B sites have high oxidation ability toward CO [14], methane [15], volatile organic compounds (VOCs) [16,17] and diesel soot [18,19]. The catalytic properties of perovskite oxides can be controlled by substitution of A site cations by other kinds of metals [20]. In the case of LaMnO3 and LaCoO3 perovskite oxides, for instance, the substitution of La3+ ions by Sr2+ ions gives rise to the increase in the oxidation state of B site cations, such as Co3+ to Co4+ and Mn3+ to Mn4+, which improves the oxidizing ability of the oxides. Perovskite type mixed oxides have been also used for the catalytic oxidation processes under microwave heating. Catalytic oxidation of CO [21], methane [22,23], propane [21,24], benzene [25,26], and soot [27] has been so far reported. One of the advantages of perovskite oxides over other metal oxides in catalytic reactions under microwave heating is that the electromagnetic properties of perovskite oxides that contribute to microwave heating can be also controlled by changing the constituent elements. As the oxidation catalysts, so many kinds of perovskite oxides have been used under microwave heating [21–27] and the perovskite oxides containing La in the A site and Mn, Fe, and Co in the B site were effective for the oxidation reactions. To clarify the relationship between the composition, the textural properties, the heating behavior under micro-wave irradiation and the CO oxidation activity of the catalysts, the effect of the B site cation in LaBO3 (B = Mn, Fe, Co) type perovskite oxides and the partial substitution of La by Sr were investigated.
2. Materials and methods 2.1. Catalyst preparation In this study, we prepared perovskite oxide catalysts using two kinds of methods: LaBO3 (M = Mn, Co, Fe) type perovskite oxides were prepared by hydrolysis-coprecipitation of metal nitrates with TMAH in aqueous solution because impurity phases such as single metal oxides can be easily suppressed by this method. On the other hand, the Sr-containing perovskite oxides were prepared by evaporation-to-dryness method because Sr ions were not completely precipitated in the hydrolysis-coprecipitation method and therefore this method cannot be applied to the preparation of Sr-containing samples. LaBO3 (B = Mn, Fe, and Co) type perovskite oxides were prepared by hydrolysis of metal nitrates in aqueous solutions according to the previously reported method [28]. In a typical preparation method for LaMnO3, appropriate amounts of La(NO3)36H2O (Kishida Chemical Co. Ltd., Japan, >99%) and Mn(NO3)24H2O (Wako Pure Chemical, Japan, >99.9%) were dissolved into water, and the aqueous solution was added dropwise to an aqueous solution containing tetramethylammonium hydroxide (TMAH, Kishida Chemical Co., Ltd.). The resultant colloidal particles were filtered off, washed several times with water, dried at 100 °C for 24 h and calcined at 950 °C for 5 h. For the preparation of LaFeO3 and LaCoO3, the same procedure was adopted using Fe(NO3)39H2O and Co(NO3)26H2O instead of using Mn(NO3)24H2O. Sr-substituted perovskite oxides La0.8Sr0.2BO3 (B = Fe, and Co) and La1xSrxBMnO3 (x = 0.2, 0.5, 0.6) were prepared by evaporation-to-dryness method using malic acid as the complexing reagent [29]. In a typical preparation method for La0.8Sr0.2MnO3, appropriate amounts of La(NO3)36H2O, Sr(CH3COO)20.5H2O (Wako Pure Chemicals, Japan), Mn(NO3)26H2O, and malic acid were dissolved into water with the 2-to-3 M ratio of metal ions to malic acid, and the aqueous
solution was heated and evaporated to dryness under stirring. The residual powder was heated at 950 °C for 5 h. The catalysts were pressed into disks at a pressure of 30 MPa for 3 min, pulverized using mortar and pestle, and then sieved to the sizes of 250–710 lm before use for catalytic reactions. Although the catalyst temperatures increased with decreasing the meshed particle sizes, mass transfer in the reactor was more homogeneous when the 250–710 lm-sized particles were used as the catalysts. 2.2. Catalyst characterization X-ray diffraction (XRD) patterns were taken using a RIGAKU RINT2200 diffractometer using Cu Ka radiation (k = 1.5418 Å) at 40 kV and 30 mA. The measurements were carried out with a step size of 0.02° and a scan speed of 2.0°/min. Catalyst samples were ground with mortar and pestle before measurements. Catalyst surface area was determined by BET method from N2 adsorption isotherm at 196 °C with Belsorp mini (BEL Japan Co. Ltd.). TEM and STEM observations were carried out using an FEI Titan Cubed equipped with a Schottky electron source and an aberration corrector of the probe-forming lens. The microscope was operated at 300 kV in TEM and scanning probe modes. The convergence semiangle was 21.4 mrad for STEM imaging. The inner and outer semiangles of the HAADF detector were 38 and 184 mrad, respectively. 2.3. Measurement of heating behavior of perovskite oxides under microwave heating Fig. 1 shows the schematics of the reaction system used in this study. A triple tube type reactor made of quartz glass which had the inlet and outlet pipes was used in this study (Fig. 1(a)). The microwave irradiation system was a modified system of Green Motif II (IDX Co. Ltd., Japan) and composed of a rectangularshaped waveguide (27 mm 96 mm) and a power source and reflecting plate, which produced single mode microwaves (Fig. 1(b)). The strength distribution of electric field and magnetic field in the wave guide is demonstrated in Fig. 1(c). In the microwave irradiation system, microwaves were reflected by the reflecting plate and interfered with incident microwaves, giving rise to the formation of standing wave. Consequently, the strength distribution for electric field was shifted by one-fourth of the microwave length to that of magnetic field. The catalyst (0.20 g) was placed in the triple tube type reactor. The reactor was placed in the waveguide as the catalyst sample was located in the middle of the waveguide (Place C), at which the strength of the electric field was the highest. Microwave was irradiated for the catalyst with the power range of 30–500 W. The catalyst temperature was pursued with a fiber optic thermometer (Anritsu Keiki, Co. Ltd.; AMOTH FL-2000, FS400), which was contacted with the catalyst across the inner glass tube. 2.4. Catalytic oxidation under conventional heating and microwave heating Catalytic oxidation of CO under microwave heating was carried out with a fixed bed flow reactor. Catalyst particle (0.10 g) was placed in the triple-tube type reactor, which was located in the waveguide. As a pretreatment, the catalyst was heated in O2–He flow by microwave irradiation at 300 W until the catalyst temperature was raised to 300 °C for the catalyst samples except for LaFeO3. In the case of LaFeO3, the catalyst temperature did not reach the temperature of 300 °C (see below). Therefore, the pretreatment was carried out at 150 °C with the power of 300 W for several minutes. After the heat treatment under microwave heating, the catalyst samples were cooled down to room temperature. The reaction gases for CO oxidation were prepared by mixing CO
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Fig. 1. Schematic of microwave heating system. (a) triple tube type reactor, (b) waveguide, (c) distribution of electric and magnetic field strengths.
(1%, He balance) and O2 (10%, He balance; He balance) in cylinders. The reaction gas composition was as follows: CO concentration 0.5%, O2 5%; gas flow rate 100 ml min1; WHSV = 60,000 mL g1 h1. Gas samples were analyzed on a GC-TCD (GL Science GC 390). Catalytic oxidation of CO under conventional heating was carried out with the triple-tube type reactor. Prior to catalytic reaction, the catalyst was heated at 300 °C in a flow of pure O2 for 1 h, and then thermostated at a reaction temperature with a heating apparatus. The reaction gas composition and gas flow rate was the same as those in the catalytic reaction under microwave heating. Gas samples were analyzed on a GC-TCD (GL Science GC 390).
3. Results and discussion 3.1. Textural properties of perovskite oxides Fig. 2 shows the XRD patterns of LaBO3 (M = Mn, Co, Fe) type perovskite oxides prepared by hydrolysis-precipitation method and Sr-substituted perovskite oxides, La0.8Sr0.2MO3 (M = Mn, Co, Fe) prepared by evaporation to dryness method, followed by calcination at 950 °C. For both systems, characteristic X-ray diffraction patterns for perovskite phases were observed. XRD spectra showed the formation of rhombohedral crystalline phases for of LaMnO3 and LaCoO3 catalysts (JCPDS ID 032-0484, 25-1060), whereas LaFeO3
has an orthorhombic perovskite structure (JCPDS ID 015–0148). For the Sr-substituted perobskite oxides, La0.8Sr0.2MnO3, La0.8Sr0.2CoO3, and La0.8Sr0.2FeO3, the crystal systems were the same as the corresponding Sr-free perovskited oxides. The surface areas of perovskite oxides are listed in Table 1. The surface area of the perovskites prepared in this study was in the range of 1–4 m2/g. LaMnO3 have larger surface area than LaCoO3 and LaFeO3. The partial substitution of La by Sr led to the increase in surface area of LaCoO3 and LaFeO3, indicating that surface area of the perovskite oxides depended on their compositions. The improvement of surface area by Sr substitution for B site metal cation has been also reported [17,30,31]. Fig. 3 shows TEM and STEM images of LaMnO3 catalyst prepared by hydrolysis-coprecipitation method and calcined at 950 °C. Catalyst particles with the ranges of 100–500 nm were observed (Fig. 3(a)). The HAADF-STEM and HRTEM images (Fig. 3(b) and (c)) and selected area electron diffraction (SAED) (Fig. 3(d)) confirmed that the highly ordered crystalline structure for the LaMnO3 catalyst.
3.2. Heating behavior of perovskite oxides under microwave irradiation Fig. 4 shows the heating behavior of perovskite oxides LaMnO3 under microwave irradiation. Here, the catalyst sample was
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LaMnO3
LaFeO3
LaCoO3
La0.8Sr0.2MnO3
La0.8Sr0.2FeO3
La0.8Sr0.2CoO3
Fig. 2. XRD patterns of perovskite oxides.
located inside the waveguide at the point where the electric field strength was the maximum (Fig. 1c, Place C). When microwave irradiation for the fresh samples started, the temperature increases with time on stream and reached a steady-state value. The steady state temperature increased with the increase in microwave power in the range of 50–250 W. When the microwave power was raised to 300 W, the catalyst temperature drastically increased after the temperature exceeded 180 °C (Fig. 4(a)). After the catalyst sample was cooled down to room temperature and then heated again with microwave irradiation. The steady state temperature for the catalyst for this time was higher than that under the microwave heating for the first time (Fig. 4(b)). When the microwave on–off cycles were repeated, the steady state temperature was comparable to that under microwave irradiation at second time irradiation. Thus, the microwave irradiation improved the heating properties of perovskite oxides. The ‘‘pre-irradiation effect’’ was observed for LaCoO3 and LaFeO3. Hereafter, heating properties of the perovskite oxides were investigated after the microwave heating was carried out for the oxides. The mechanism for microwave heating of materials can be categorized into dielectric heating and magnetic heating [30]. In the dielectric heating process, dipolar polarization formed in dielectric materials under microwave irradiation is rotated and deformed by the oscillating electric field, leading to heat generation [5,32]. In
Table 1 BET surface area of perovskite oxides. Catalyst
Surface area/m2 g1
LaCoO3 LaFeO3 LaMnO3 La0.8Sr0.2CoO3 La0.8Sr0.2FeO3 La0.8Sr0.2MnO3
1.4 1.5 3.2 3.7 3.1 3.8
the magnetic heating for solid materials, one of the mechanisms for heating is eddy current loss: magnetic field generates eddy current in the solids, which gives rise to resistance heating. To reveal the factors controlling the heating behavior of perovskite oxides, we subsequently investigated the effect of the location of catalyst samples in the wave guide on their heating behaviors. When the catalyst positions in the waveguide were changed from Place C to B and A, where the strength of the electric field decreased and that of magnetic field increased, the steady-state temperature of the catalysts greatly decreased (Supporting information Fig. S2), indicating that the physical properties that contributed to the heating behavior of perovskite oxides were the dielectric loss rather than the magnetic eddy current loss. The heating properties of the perovskite oxides also depended on the physical state of the catalyst particles. When the meshed catalysts were heated by microwave irradiation, the ‘‘pre-irradiation effect’’ was observed, as described above. After the catalyst samples were pulverized to fine powders and sieved to the particle sizes of 250–710 lm and then heated again by microwave irradiation, the catalyst temperature profile was similar to that of the fresh sample. The heating behavior was also improved by calcination of the meshed particles: the catalyst particles which had been sieved to the particles sizes of 250–710 lm were heated at 800°C for 5 h and microwave irradiation was carried out for the samples, the temperature of LaMnO3 catalyst rapidly increased and heating behavior was similar to that of microwave-preirradiated samples. These findings suggested that the ‘‘pre-irradiation effects’’ were ascribed to the changes in the physical state and interaction between crystallites and primary particles contained in the meshed particles. On the other hand, the catalysts samples that were pretreated by microwave heating were taken out from the reactor and shuffled and located again into the reactor. When the microwave was irradiated again for the catalyst, ‘‘pre-irradiation effects’’ was also observed. This indicates that the ‘‘pre-irradiation effects’’ was not ascribed to the changes in interaction between the meshed catalyst particles. The ‘‘pre-irradiation effects’’ were observed for LaCoO3, confirming that the heating behavior of the perovskite oxides depend not only on the B site cation but also on the state of primary and secondary particles. The heating behavior depended on the B site cations of LaBO3 perovskite oxides. The steady state temperature also increased with the increase in microwave power for LaCoO3 and LaFeO3 (Supporting information Fig. S2). When the catalyst temperature was compared, the value decreased in the order of LaMnO3 > LaCoO3 > LaFeO3. The temperature for LaMnO3 catalyst under microwave heating was much higher than that for manganese oxide (Mn2O3) prepared by the AMP method and calcined at 650 °C at any given power, indicating that Mn-containing perovskite oxides are effective for microwave heating compared with Mn single oxides. 3.3. Catalytic properties of LaBO3 type perovskite oxides under conventional heating Fig. 5 shows the CO oxidation over perovskite oxides containing Mn, Fe, and Co in the temperature range of 150–400 °C. Here, the conversions were evaluated at steady state. CO oxidation activity increased monotonically with the increase in catalyst temperature for all the catalyst. The order of CO oxidation activity was LaCoO3 > LaMnO3 > LaFeO3 at any reaction temperature. The difference in the catalytic activity was not merely ascribed to the difference in catalyst surface area because the surface areas of the perovskite oxides were similar (1.4–3.7 m2/g). It should be note that when the LaMnO3 perovskite oxides were irradiated with microwave at 300 W and used for CO oxidation under external
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(a)
101
(b)
La Mn
(c)
(d)
Fig. 3. TEM and STEM images of LaMnO3 catalyst. (a) and (b) HAADF-STEM images, (c) HRTEM image, (d) selected area electron diffraction (SAED).
Temperature / °C
(a) 350
300 W
300 250
250 W 200 W 150 W 100 W 50 W
200 150 100 50 0
0
200
400
600
800
1000
Time / sec
Temperature / °C
(b) 350
200 W
300 250
300 W 250 W
200
150 W
100 W Fig. 5. Catalytic oxidation of CO over perovskite oxides under conventional external heating. (s) LaMnO3, (d) La0.8Sr0.2MnO3, (h) LaCoO3, (j) La0.8Sr0.2CoO3, (4) LaFeO3, (N) La0.8Sr0.2FeO3. Catalyst weight 0.10 g, CO concentration 0.5%, O2 5%; gas flow rate 100 ml min1; WHSV = 60,000 mL g1 h1.
150 50 W
100 50 0
0
200
400 600 Time / sec
800
1000
Fig. 4. Heating behaviors of LaMnO3 under microwave irradiation in air. Catalyst weight 0.20 g. (a) fresh sample, (b) the sample pre-irradiated by microwave .
heating, the activity was comparable to that for the LaMnO3 catalyst pretreated by conventional heating. This indicates that the CO oxidation activity of LaMnO3 catalyst under conventional heating was not affected by the pre-irradiation of microwave.
It has been reported that CO oxidation on perovskite oxide proceeds by the adsorption of CO on the perovskite oxides and the adsorbed CO reacts with surface oxygen species formed by dissociative adsorption of molecular O2 (Eqs. 1–4) [33].
O2ðgÞ ! O2 ðadÞ ! 2O2 ðadÞ
ð1Þ
COðgÞ ! COðadÞ
ð2Þ
COðadÞ þ 2OðadÞ ! CO2 3 ðadÞ
ð3Þ
2 2 CO2 3 ðadÞ ! CO2ðadÞ þ OðadÞ ! CO2ðgÞ þ OðadÞ
ð4Þ
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Therefore, the CO oxidation activity is dominated by the reactivity of oxygen species with CO (Eq. (3)) and desorption of CO2 from the perovskite oxide surfaces (Eq. (4)). The activity of LaCoO3 for CO oxidation was higher than the activities of LaMnO3 and LaFeO3 catalysts. The activity order was the same as that reported earlier for catalytic oxidation of propane over perovskite oxides, LaMO3 (M = Mn, Fe, and Co) [34]. The incorporation of Co into the B site of perovskite oxides gives rise to the formation of Co3+ in the B site of perovskite oxides, which are highly reducible and therefore have oxidizing ability toward CO [15]. The substitution of A site cation La3+ with Sr2+ ions greatly improved the oxidation ability of Co, Mn, an Fe-containing perovskite oxides because the substitution changed the valence of B-site cations, such as Co3+ to Co4+, which further improved the oxidizability of the B site cations [15]. The catalysts prepared in this study also showed the similar phenomena: the partial substitution of 20% La by Sr greatly increased the CO oxidation activity of the Mn-, Fe-, and Co-containing perovskite oxides, as shown in Fig. 5. The order of the CO oxidation activity for the Sr-substituted perovskite oxides La0.8Sr0.2BO3 was Co > Mn > Fe, which was also consistent with that of LaBO3 perovskite oxide series.
require for obtaining the steady-state temperature depended on the catalyst composition. Here, the catalyst temperatures were measured after they reached steady-state values. The steady-state temperature during the CO oxidation increased with the increase in microwave power (Fig. 7(a)). The catalyst temperature decreased in the order of Mn > Co > Fe. Thus, the LaMnO3 catalyst exhibited superior heating behavior than other catalysts under the reaction conditions. The substitution of La by Sr strongly influenced the heating behavior and catalytic properties of perovskite oxides. The heating behaviors of Sr-substituted perovskite oxides LaSrMO3 (M = Mn, Fe, Co) under microwave irradiation were also shown in Fig. 7(a). The steady-state temperature was also obtained for the catalysts and the temperature strongly depended on the microwave power. Thus, the catalyst temperature could be controlled by changing the microwave power, as in the case of LaBO3 type perovskites. The heating behavior of the LaSrMO3 type perovskites was almost independent of the B site cations. The temperature for the LaSrMO3
3.4. Catalytic properties of LaBO3 type perovskite oxides under microwave heating One of the important features of perovskite oxides was high stability during catalytic oxidation under microwave heating. Fig. 6 shows a typical time course for CO oxidation with LaMnO3 catalyst under microwave heating. Here, the catalyst samples were located at Place C. The temperature for catalyst bed increased as the power was switched on and the temperature reached steady state after 10 min. Moreover, good reproducibility was observed when the MW power was switched on and off (data not shown). Thus, the temperature of catalyst bed could be also controlled under the reaction conditions with microwave heating. CO oxidation behavior was also plotted in Fig. 6. No significant changes in CO conversions were observed. Thus, the steady-state activity for CO oxidation could be measured under the microwave heating. Fig. 7 shows the effect of microwave power on the catalyst temperature and CO oxidation activity of LaMO3 (M = Mn, Fe, Co) perovskite oxides under microwave irradiation. It needs 10–20 min before the catalyst reached steady-state temperatures and the time
Fig. 6. Time course for catalyst temperature and CO conversion under microwave heating. Catalyst weight 0.10 g, CO concentration 0.5%, O2 5%; gas flow rate 100 ml min1; WHSV = 60,000 mL g1 h1; microwave power 200 W.
Fig. 7. Catalytic oxidation of CO over perovskite oxides under microwave heating. (a) Heating behaviors of perovskite oxides. (b) CO conversions. (s) LaMnO3, (d) La0.8Sr0.2MnO3, (h) LaCoO3, (j) La0.8Sr0.2CoO3, (4) LaFeO3, (N) La0.8Sr0.2FeO3. Catalyst weight 0.10 g, CO concentration 0.5%, O2 5%; gas flow rate 100 ml min1; WHSV = 60,000 mL g1 h1.
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type perovskite was much higher than that for LaMO3 type perovskite when the same B site cations were used. Particularly, the substitution of La by Sr greatly improved the heating behavior of the Fe-containing perovskite oxides. The CO oxidation activity of LaMO3 (M = Mn, Fe, Co) perovskite oxides under microwave irradiation depended on the B-site cations. Fig. 7(b) shows the CO conversion against the microwave power for the perovskite oxides. The CO conversion increased with the increase in the microwave power. The activity of LaMnO3 was comparable with that of LaCoO3 in contrast with the CO oxidation under conventional heating. LaFeO3 exhibited almost no activity even the microwave power was raised to 280 W. The order of the activity can be explained in terms of heating properties of perovskite oxides: the LaMnO3 catalyst exhibited CO oxidation activity comparable with the LaCoO3 catalyst because the temperatures for LaMnO3 catalyst under microwave heating were higher than that for LaCoO3 catalyst. The temperatures for LaFeO3 catalyst under microwave heating were much lower than the light-off temperature for CO oxidation. Fig. 7(b) also compares the CO oxidation with the Sr-substituted perovskite oxides under microwave heating. The La0.8Sr0.2MO3 (M = Mn, Fe, Co) type perovskite oxides exhibited the CO oxidation activities and the CO conversion monotonically increased with the increase in microwave power. The La0.8Sr0.2MO3 catalysts exhibited much higher activity for CO oxidation than the corresponding LaBO3 type oxides having the same B site cations. Therefore, the substitution of La by Sr raised the catalyst temperatures and CO oxidation activities under microwave heating. The improved activity by Sr substitution was ascribed to the improved heating properties and inherent CO oxidation properties of the perovskite oxides. The effect of partial substitution of La by Sr was prominent for the LaFeO3 catalyst. Fig. 8 compares the CO oxidation activity as the function of catalyst temperature for perovskite oxides. When the CO oxidation activity of LaBO3 type perovskite oxides under the microwave heating with that under the conventional heating, the activity decreased in the order of LaCoO3 > LaMnO3 > LaFeO3 under the microwave heating and the order was consistent with that under the conventional heating (Fig. 5). The activities under microwave heating were much higher than those under conventional heating and the reaction temperature required for CO oxidation was much lowered (50–100 °C). Similar results were obtained for microwave-assisted heterogeneous catalytic reactions [21,22,35]. It has been suggested that the catalytic active sites are selectively heated by microwave irradiation and the temperatures of catalyst active sites are much higher than those in the surroundings [36]. The adsorbed reactants on such hot active sites can be activated by energy transfer from the active sites, facilitating the surface chemical reaction (‘‘thermal effect’’). The formation of these hot spot sites is possible for perovskite oxides because dipole strengths in the perovskite oxides are different depending on the A and B sites [37]. Another possible explanation is ‘‘non-thermal’’ effects, in which the electromagnetic field of the microwave interacts with the reactants, promoting the surface chemical reaction [32]. The higher activity of the perovskite oxides for CO oxidation under microwave heating than that under conventional heating was ascribed to either the ‘‘thermal effect’’ or ‘‘non-thermal effect’’. However, we cannot differentiate the effect affecting the microwave-assisted catalytic oxidation process at present. For the LaSrMO3 (M = Mn, Fe, Co) perovskite oxides, the activity decreased in the order of Co > Mn > Fe, which was consistent with the order of LaMO3 (M = Mn, Fe, Co) catalysts. As described above, heating property of perovskite oxides is one of the important factors affecting their catalytic properties under microwave heating, and the heating behaviors of Mn-containing perovskites LaMnO3 and La0.8Sr0.2MnO3 were much higher than
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Fig. 8. Comparison of CO oxidation activity of perovskite oxides under microwave heating and conventional heating. Conditions are the same as those in Figs. 4 and 6. (a) (s,d) LaMnO3, (h,j) LaCoO3, (4,N) LaFeO3. (b) (s,d) La0.8Sr0.2MnO3, (h,j) La0.8Sr0.2CoO3, (4,N) La0.8Sr0.2FeO3. Open symbols: reaction under microwave heating; closed symbols: reaction under conventional heating.
those of Co- and Fe-containing perovskite oxides. These findings urged us to investigate the effect of Sr/La molar ratio on the catalytic properties of La–Sr–Mn type perovskite oxides under microwave heating because the CO oxidation activity of the catalysts under conventional heating can be greatly improved by increasing the Sr/La ratio [20]. For the perovskite oxides of La1xSrxMnO3 (x = 0.5, 0.6) prepared by evaporation-to-dryness methods, perovskite oxides were mainly formed with almost no impurity phases in the XRD patterns (Fig. S4). With the increase in Sr/La ratio, the crystalline phase was changed to cubic perovskite structure. As shown in Fig. 9, the heating behaviors of the La–Sr–Mn type perovskite oxides were almost unchanged when the Sr/La ratio was changed. On the other hand, the CO oxidation activity greatly improved when the Sr/La ratio increased to 1/1. The improved activity was probably ascribed to the improved activity of Mn sites due to the changes in their oxidation state. This finding further suggested that the substitution of A site cation was also effective for improving the catalytic properties of ABO3 type perovskite oxides under microwave heating.
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LaMnO3 exhibited much lower activity than LaCoO3 under conventional heating. The substitution of La by Sr improved the heating behavior and CO oxidation ability of the perovskite oxides under microwave heating. The highest activity was obtained at the La/Sr ratio of 1/1 for La–Sr–Mn–O system. Acknowledgement This work was financially supported by Environmental Technology Development Fund from the Ministry of the Environment, Government of Japan. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.cej.2015.07.051. References [1] [2] [3] [4] [5]
[6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16]
Fig. 9. Catalytic oxidation of CO over La1x-SrxMnO3 type perovskite oxides under microwave heating. (a) Heating behaviors of perovskite oxides. (b) CO conversions. (d) x = 0, (s) 0.2, (h) 0.5, (j) 0.6. Catalyst weight 0.10 g, CO concentration 0.5%, O2 5%; gas flow rate 100 ml min1; WHSV = 60,000 mL g1 h1.
4. Conclusions In this study, we prepared perovskite oxides containing transition metals (Mn, Fe, Co) by precipitation method and evaporation-to-dryness methods, followed by calcination at 950 °C. XRD studies confirmed the formation of perovskite oxide phases for all the catalysts. The heating behavior of the perovskite oxides was improved by the pre-irradiation of microwave, and the ‘‘pre-irradiation effects’’ were ascribed to the changes in the intra-particle interaction. The physical state of perovskite oxide particles affected their heating behaviors. The temperature of perovskite oxide particles reached steady-state value under microwave irradiation conditions and monotonically increased with the microwave power. Heating and catalytic properties of perovskite oxides strongly depended on the catalyst composition. LaMnO3 was more easily heated by microwave irradiation than LaCoO3 and LaFeO3. The CO oxidation activity of LaMnO3 was comparable to that LaCoO3 under microwave irradiation, although
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