La–Mn perovskite-type oxide prepared by combustion method: Catalytic activity in ethanol oxidation

Applied Catalysis A: General 383 (2010) 192–201

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La–Mn perovskite-type oxide prepared by combustion method: Catalytic activity in ethanol oxidation H. Najjar, H. Batis ∗ Unité de Recherche d’Elaboration de Nanomatériaux et leurs Applications, University of El Manar, Faculty of Sciences, Chemistry Department, 2092 El Manar II, Tunis, Tunisia

a r t i c l e

i n f o

Article history: Received 8 February 2010 Received in revised form 26 May 2010 Accepted 28 May 2010 Available online 4 June 2010 Keywords: Perovskite Combustion synthesis Ethanol oxidation Basic and redox sites

a b s t r a c t The present work deals with the use of the combustion method to synthesize a series of lanthanum manganite (LM) and to study their physico-chemical and catalytic properties in ethanol deep oxidation. Some synthesis parameters such as chemical nature of the fuel molecules (glycine and citric acid) and additional thermal treatment were used and their effects on the powder characteristics were studied. XRD patterns showed a single phase perovskite-type oxide for all studied samples. Additional lanthanum oxocarbonate was observed on powder patterns of samples obtained with citric acid. The presence of chemisorbed carbonate species was studied by IR spectra analysis in the 2000–500 cm−1 , XRD and XPS analyses of the C1s and O1s levels. Their concentration was higher on surface samples prepared with citric acid than on those prepared with glycine. From XPS results, the use of glycine as gelling agent rather than citric acid and additional thermal treatment resulted in a decrease in superficial La/Mn atomic ratio and carbonate content and in an increase in relative content of the surface O2 2− /O− species. These preparation parameters determined the best catalytic activity of the studied LM materials. Our results indicated that the catalytic activity in deep oxidation of ethanol over lanthanum manganite may be described as a synergetic effect of lanthanum and manganese. Ethanol activation was favoured by the strong basicity of lanthanum oxide at the surface of perovskite-structure and deep oxidation to CO2 was enhanced by the activation of oxygen species on manganese site through a redox cycle. © 2010 Elsevier B.V. All rights reserved.

1. Introduction Perovskite-type oxides (ABO3 ) containing a transition metal ion (B) are of high technological importance because of their interesting electric and magnetic properties [1,2]. Moreover, they have received remarkable attention over the last years in view of their activity and significant thermal resistance in the catalytic complete oxidation of volatile organic compounds (VOC) [3–7]. A special consideration is devoted to ethanol since it has been suggested as an alternative fuel for vehicle activity in order to reduce gasoline and diesel consumption [8]. Efficient decontamination of exhaust pollutants produced by alcohol-fueled automotive takes part in the major preoccupation of the air purification. Catalytic total combustion of ethanol represents a promising way for limiting the formation of unburned ethanol and undesirable partially oxidized products such as acetaldehyde. Manganese containing perovskites and particularly lanthanum manganite oxide of formula LaMnO3 have received great attention due to their high catalytic activity in complete oxidation of ethanol [3,4]. Many efforts are devoted to the development of perovskite-

∗ Corresponding author. Tel.: +216 71872600; fax: +216 71871666. E-mail address: [email protected] (H. Batis). 0926-860X/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2010.05.048

type catalysts in order to better understand the contribution of the active sites in the total oxidation of ethanol. Investigating the La0.8 Sr0.2 MnO3 , Blasin-Aubé et al. [3] reported a high activity of these catalysts in ethanol total combustion. Whereas, Shimizu [5] showed that LaMO3 (M = Mn, Fe, Co, Ni) is active in partial and deep ethanol oxidation at higher temperature and partial pressure. The success of these materials is related to the high availability of adsorbed oxygen which determine the surface basicity and to the presence of a mixed valence of transition metal which determine the redox property of the catalyst [9,10]. In order to achieve these characteristics, many researches have been devoted to develop alternative chemical processes. Among them, combustion synthesis is particularly versatile to synthesize an impressive variety of fine particles of metallic oxides including perovskite-type oxides as related by Civera et al. [11]. The basis of the combustion synthesis technique comes from the thermochemical concepts used in the field of propellant and explosives [12–17]. Its extrapolation to the synthesis of ultrafine powders of ceramic oxides and thermodynamic interpretation was extensively discussed elsewhere [11,18–20]. The success of this process was due to an intimate blending among the constituents using a suitable fuel or complexing agent (citric acid and glycine) in an aqueous medium and an exothermic redox reaction between the fuel and an oxidant (nitrate). Different organic fuels can be used for the

H. Najjar, H. Batis / Applied Catalysis A: General 383 (2010) 192–201

combustion reaction such as urea, formaldehyde, glycine, and citric acid. They differ in the amount of gases generated by one mole of perovskite. These amounts range from 21.4 mol/mol of perovskite for citric acid to 15.8 mol/mol of perovskite for formaldehyde. This obviously affects the characteristics of the reaction product. The reaction is not isothermal and large amounts of gases dissipate more heat, thereby preventing the powder sintering. The use of glycine and citric acid gave satisfactory results, i.e., high specific surface area values and formation of the desired perovskite phase. The mechanism of the combustion reaction is quite complex because many parameters influence the reaction including the chemical nature of the fuel molecule, the fuel-to-oxidant ratio, the use of excess oxidant, the ignition temperature and the water content of the precursor mixture. The use of this preparation route in the present work allows the achievement of many valuable physico-chemical properties of lanthanum manganite oxide, especially the specific surface area which determines significantly the catalytic activity of the studied material. In this work, the combustion process with synthesis parameters like the chemical nature of the fuel molecule (glycine and citric acid) and additional thermal treatment was investigated with the goal to study their effect on physico-chemical properties, surface composition and catalytic activity in ethanol total combustion. 2. Experimental 2.1. Samples preparation A series of lanthanum manganite oxides were prepared by aqueous combustion synthesis. An aqueous solution of lanthanum nitrate was prepared by dissolving La metallic powder (Prolabo) in a suitable amount of HNO3 (Merck, 65%). The appropriate volume of Mn(NO3 )2 solution (Fisher Scientific Company) was added under a continuous stirring in such a way that the atomic ratio La/Mn was fixed to 1. This less straightforward route for obtaining nitrate precursors was chosen because the commercially available manganese and lanthanum salts do not have a well-defined hydration degree. Syntheses were carried out maintaining the ratio between fuel organic molecules and nitrates constant and equal to 0.25. This fuel/oxidizer ratio corresponds to the case where lanthanum manganite composition can be formed directly from the reaction between fuel and oxidizer, with no additional ambient oxygen required (Eqs. (1) and (2)). Additionally, as shown below, enthalpy calculation clearly shows that this under-stoichiometric composition was sufficient to generate the heat necessary for the synthesis of lanthanum manganite powder. After a few minutes stirring on a heating plate, the so-prepared solution was transferred in a ceramic dish, which was placed into an oven preheated and kept at 700 ◦ C. After a dehydration step, the mixture frothed and swelled, until a fast and explosive reaction took off, and large amounts of gases evolved. The whole process was achieved after almost few minutes. Four samples were prepared and used in this study. Two samples designated LMG0 and LMA0 obtained with glycine (G) and citric acid (A), respectively, and without additional calcination treatment. Two samples, designated LMG24 and LMA24, were obtained with additional calcination treatment at 700 ◦ C for 24 h under a synthetic flowing air (N2 /O2 = 4/1). 2.2. Samples characterization Powder X-ray diffraction (XRD) patterns, recorded on a diffractometer with Co K␣ radiation, were performed on all samples to assess the presence and purity of the expected phases and to gather information about their degree of crystallisation.

193

Specific surface areas (SSA) were determined by N2 adsorption at 77 K in a volumetric all glass apparatus. Prior to each measurement, the samples were degassed 4 h at 250 ◦ C under vacuum (7 × 10−4 Pa). Investigations of the particle microstructure were performed on a Philips XL 30 FEG Scanning Electron Microscopy (SEM). The composition of every sample was determined by X-ray fluorescence on a PW 1400 Philips instrument. The calibration was made using calibration samples with known amounts. Concentrations of Mn3+ and Mn4+ ions were determined by redox titration [21,22]. The same procedure of redox titration was also used to determine the oxygen excess, y, in a perovskite lanthanum manganite oxide of formulae LaMnO3+y . Infrared spectra were recorded on a BRUKER IFS 66 V/S in the range of 4000–400 cm−1 using the KBr pellet method. The sample is placed in a controlled environment transmission cell with CaF2 windows. Spectra were recorded for LMA0 and LMG0 catalysts pretreated under vacuum at beam temperature and for LMA24 and LMG24 pre-treated under a flow of synthetic air (20% O2 /N2 ) at 700 ◦ C and evacuated at beam temperature. XPS analysis was performed with an ESCALAB 200 R electron spectrometer using monochromatised focused Al K␣ X-ray source (1486.6 eV). The base pressure in the analysis chamber was less than 5 × 10−8 Pa. All binding energy (BE) measurements were corrected for charging effects with reference to the C1s peak at 284.6 eV (adventitious carbon). This reference gave BE values with an accuracy of ±0.2 eV. Collected data were analysed with a Shirley background subtraction and a least-squares fit routine using the simplex algorithm. For quantitative analysis, the signal intensities were measured using integrated area under the detected peak. To evaluate the surface atomic ratios, we used sensitivity factors provided by the VG software (inelastic mean free path and transmission function) and the cross-section for X-ray excitation as calculated by Scofield [23]. Programmed thermodesorption analysis (TPD) was carried out on the fresh solid LMA0. The catalyst (100 mg) was submitted to the following conditions: 30 mL/min He, temperature from 25 to 900 ◦ C with a ramp of 10 ◦ C/min. The outlet gases, i.e., CO2 (m/z = 44) and O2 (m/z = 32) were identified using a mass spectrometer. Catalytic activity in the oxidation of ethanol was determined using a catalyst charge of about 0.1 g (granulometry = 0.06–0.12 mm) loaded in a U-shaped quartz microreactor. The reactor was operated in a down-flow mode at atmospheric pressure. The reactor was placed in a tubular furnace equipped with a temperature programmer. The reaction temperature was controlled with a K-type thermocouple placed alongside the quartz tube and its tip inserted in the catalytic bed. The feed gas at 100 mL/min is composed of a stoichiometric mixture of ethanol vapour (molar ratio O2 /C2 H6 O = 3) diluted in a mixture of oxygen and helium in the proportion (vol.% C2 H6 O/O2 /He = 9.3/27.9/62.8). In order to adjust the ethanol vapour concentration, two streams of helium are employed: one is introduced into a regulated bubble saturator at a fixed temperature (25 ◦ C) to carry out a constant flow rate of organic vapour and the other is used to balance the overall concentration. In order to control reactor isothermicity, catalytic tests were carried, under the same conditions, with pure and diluted catalyst with quartz powder of the same dimension. No changes were detected in the catalytic activity of all samples. The analysis is conducted isothermally at 150 ◦ C. Under the present experimental conditions, the only reaction products were acetaldehyde (ACA), CO2 and H2 O. The data obtained (given by the ethanol conversion Xethanol (%) = ((Ethanol)in − (Ethanol)out )/(Ethanol)in × 100) at each temperature were the average of three steady state measurements. The absence of gas phase mass transfer limitations has been verified by reacting, at constant contact time, differ-

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Fig. 1. XRD patterns of samples (K␣Co = 1.789 Å), *: La2 O2 (CO3 ) or La2 O(CO3 )2 .

ent volumes (v; 2v; 3v, etc.) of a given catalyst with a moving gas stream at various gas velocities (F; 2F; 3F, etc.). It was verified that ethanol conversion is constant at a flow rate higher than 70 mL/min. The conversion was investigated in a reaction temperature ranging from 120 to 210 ◦ C. Each temperature was maintained for 90 min to attain steady state conversion. No changes were detected in the catalytic activity of all samples when the reaction was carried out in two cycles of increasing and decreasing temperatures. Carbon balance was close to within ±4% in all catalytic tests. 3. Results and discussion

It is known that formation of bulk carbonate species seems to occur when CO2 produced by the combustion reaction (Eqs. (1) and (2)) interacts with a very basic surface oxide may be lanthanum oxide [26–28]. Some physico-chemical characteristics of the studied samples are summarized in Table 1. The effective La/Mn ratio in the final compounds is close to that of the precursor solution. Chemical analysis results show that all synthesised samples exhibit nonstoichiometry on one or more sub-lattices (Table 1). The excess oxygen content y in LaMnO3+y varies in the range 0.15–0.16 (% Mn4+ = 30–32) in agreement with XRD results. The SSA values seem to be sensitive to the chemical nature of the gelling agent. Indeed, SSA of the fresh LaMnO3 prepared with citric acid is higher than that of the perovskite prepared with glycine. Moreover, a thermal treatment at 700 ◦ C for 24 h results in a decrease of SSA values indicating a grain agglomeration effect which seems to be kinetically slow at this temperature. To help understand the evolution of SSA values and the formation of carbonaceous residue with the change in gelling agent, a brief coming back to the preparation route is useful. According to the propellant chemistry [13] and following the works of many other researchers [18], the global redox reactions between lanthanum and manganese nitrates to form stoichiometric LaMnO3 in the presence of citric acid and glycine can be written as: La(NO3 )3 · 6H2 O(c) + Mn(NO3 )2 (c) + (5/4)C6 H8 O7 (c) → LaMnO3 (c) + (5/2)N2 (g) + (15/2)CO2 (g) + (3/8)O2 (g) +11H2 O(g)

(1)

La(NO3 )3 · 6H2 O(c) + Mn(NO3 )2 (c) + (5/4)C2 H5 NO2 (c)

3.1. General characterisations

→ LaMnO3 (c) + (25/8)N2 (g) + (5/2)CO2 (g) + 73/8H2 O(g)

The diffraction patterns, given in Fig. 1 are closely related to the patterns that would be expected of simple perovskite-structure and show the absence of simple oxides such as La2 O3 , Mn2 O3 and MnO2 . Differences between ideal and observed patterns are the split reflections, implying distortion of the lattice from cubic symmetry. XRD analysis reveals that the samples consist of a rhombohedral perovskite-structured oxide as the major phase (space group R-3c). The comparison with the PDF card 50-0298 suggests the formula LaMnO3.15 which presents an oxygen excess. This excess of oxygen in the LaMnO3+y structure was often observed after calcination under air. Such a feature was already noticed in the literature, and a number of studies agreed for a y value between 0.15 and 0.17 [24,25]. The presence of manganese (IV) is the direct consequence of this over stoichiometry in oxygen. Moreover, the powder patterns of samples prepared with citric acid show a small and wide peak at 2 ≈ 35.2–35.4◦ (d ≈ 2.94–2.96 Å) which could be assigned to the formation of lanthanum dioxocarbonate La2 O2 (CO3 ) (PDF card 84-1963) or oxocarbonate La2 O(CO3 )2 (PDF card 41-0672).

+(51/16)O2 (g)

(2)

CO2 , H2 O and N2 were considered the most stable products of the combustion synthesis with respect to other theoretical acceptable combinations that might be considered, including the formation of nitrogen oxides, CO and so forth. It can be seen from the above reactions that, if the quantity of oxygen in the combustible mixture was in excess of that required for complete combustion of the fuel, then a portion of the oxygen did not react and appeared in the exhaust. Note that, with citric acid, the amount of evolved carbon dioxide was three times as much as that obtained when glycine was used as gelling agent. Available thermodynamic data in the literature [29,30] for various reactants and products are presented in Table 2. It is well known that the enthalpy of combustion can be expressed as: r H ◦ = (



n f H ◦ )



products

−(

n f H ◦ )

reactants

Table 1 Physico-chemical characteristics of studied samples. Sample

Formulae

Mn4+ (%)a

SSA (m2 g−1 )

Lab (wt.%)

Mnb (wt.%)

La/Mnb

LMG0 LMG24 LMA0 LMA24

LaMnO3.16 LaMnO3.16 LaMnO3.15 LaMnO3.15

32 32 30 30

24 16 32 19

57.1 (56.8) 57.0 (56.8) 56.5 (56.9) 56.7 (56.9)

21.7 (22.5) 21.9 (22.5) 21.9 (22.5) 21.6 (22.5)

1.04 1.03 1.02 1.04

a b

For LaMnO3+y , Mn4+ (%) = 2y. Element contents determined by X-ray fluorescence spectroscopy (XRF), nominal values are in parentheses.

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195

Table 2 Thermodynamic characteristics of combustion reactions. Compounda

f H◦ (kJ mol−1 )

Cp (J mol−1 K−1 )

Eq. no.

r H◦ (kJ mol−1 )

Tf (◦ C)

Gases (mol/mol perovskite)

CO2 (mol/mol perovskite)

La(NO3 )3 ·6H2 O(c) Mn(NO3 )2 (c) C2 H5 NO2 (c) C6 H8 O7 (c) LaMnO3 (c) O2 (g) N2 (g) CO2 (g) H2 O(g)

−3060.6 −573.5 −333.2 −1644.4 −1425.8 0 0 −393.1 −241.6

– – – – 104.1 24.7 + 0.01534Tb 27.2 + 0.00418Tb 43.2 + 0.01145Tb 30.1 + 0.01505Tb

(1) (2)

−1342.1 −562.6

1692.0 814.7

21.4 17.9

7.5 2.5

a b

c, crystalline; g, gas. Absolute temperature.

and



Q = −r H ◦ =

Tf



(

n.Cp)products dT,

298

Tf =



−r H ◦ nCp (products)

+ 298

where Q is the heat absorbed by products under adiabatic condition, Cp is the heat capacity of the products at constant pressure and Tf is the adiabatic flame temperature. The powder characteristics like SSA and structure are primarily governed by the heat generated during combustion, which itself depend on fuel chemical nature and fuel-to-oxidant ratio [11]. In our case, syntheses were carried out maintaining the ratio between fuel organic molecules and nitrates constant and equal to 0.25. Then, differences in thermodynamic characteristics reported in Table 2, could be considered as due to the nature of the used fuel molecule. It can be seen that the adiabatic flame tempera-

ture and the volume of escaped gaseous products are higher for citric acid than for glycine. According to the theoretical calculations of r H◦ , the energy released from the exothermic reaction between fuel and nitrates, could rapidly heat the system to a high temperature. This effect seems to favour an easier achievement of a perovskite crystal structure, but a decrease of SSA as a result of possible particles agglomeration. On the other hand, a large volume of gaseous products can be released during the combustion reaction. As a consequence, as the amount of evolved gas increases, agglomerates are more likely to break up which may produce the powder with more porosity and high SSA. Then, there could be competition between the effects of these two factors in influencing the SSA of the powder product. Our results tend to indicate that the amount of evolved gas plays a more dominant role. Thus, higher number of mole of gases evolved during the combustion seems to have a positive effect on SSA as shown in Table 1. It is thought that this increase in SSA when citric acid is used as gelling agent, is due to a faster reaction rate, which pro-

Fig. 2. SEM micrographs of (a) LMG0, (b) LMG24, (c) LMA0 and (d) LMA24.

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motes the nucleation of perovskite grains with respect to their growth.

species (Eq. (4)). This may be expressed by the following equations:

In order to observe the microstructure of the synthesized perovskites, SEM analysis was performed. Fig. 2a and c shows SEM pictures of perovskite prepared without additional thermal treatment, whereas Fig. 2b and d refers to those obtained after additional thermal treatment at 700 ◦ C for 24 h. All samples exhibit foamy agglomerated particles with a wide distribution and presence of large voids in their structure. Moreover, in Fig. 2a and c the spongelike structure is more evident than in Fig. 2b and d. Formation of these features would be attributed to the escaping of gas during the combustion of organic species used in the preparation process. The primary particle size increases after thermal treatment, in qualitative agreement with the decrease in specific surface area (Table 1). Moreover, a comparison between samples obtained with citric acid and glycine shows that the formers exhibit particles with larger voids in their structure.

Characteristic bands of bidentate carbonates still observable in the spectra even after additional thermal treatment indicating that these species are strongly chemisorbed to the surface. These results are in agreement with the presence of lanthanum oxocarbonate as observed on the diffraction patterns (Fig. 1). Their formation could be explained by the reaction of the evolved CO2 gas and the high local temperature under the combustion reaction with superficial LaO and/or MnO sites.

3.2. FTIR study To get information about adsorbed species on powder surface, IR spectra were recorded (Fig. 3). Bands were observed at 1615, 1470, 1370, 1350 (shoulder), 1060 and 840 cm−1 . Those at 1470 and 1370 cm−1 due to symmetric and asymmetric stretching vibration modes of carbonate species are produced by the splitting of the doubly degenerate band at 1440 cm−1 (3 , asymmetric CO stretching) of the free CO3 2− ion [31]. This splitting of only 100 cm−1 indicates the formation of monodentate carbonate species formed by interaction of CO2 molecules with basic oxide ions at the surface. The relative breadth of the 1370 and 1470 cm−1 pair reflects the extent of heterogeneity of the surface basic oxide ions due to the presence of La–O and Mn–O sites. Indeed, it was shown in a recent study [32] using experimental and theoretical results that CO2 molecule can be adsorbed on La–O and Mn–O sites leading to carbonate formation. Moreover, the O from the LaO-terminated surface is stronger basic site than MnO one. Bands at 1615 and 1350 cm−1 (3 = 265 cm−1 ) can be attributed to bidentate or bridged carbonate structure. Bands at 1060 and 840 cm−1 are due to a stretching vibration of the carbonate and to vibration of the carbon atom out of the plane of the group, respectively. As expected, no evidence was observed for bicarbonate (HCO3 − ) species, due to the lack of surface hydroxyl groups in 3500–4000 cm−1 region. The absence of bands in the region 1700–1900 cm−1 indicates that carboxylate (CO2 − ) formation does not occur in the studied perovskite systems. The presence of carbonate species is supported by the similarity of the observed bands with those reported for La2 (CO3 )3 ·8H2 O [33] and in the system CO2 /La2 O3 [34]. Referred to the relative intensity of these bands, it seems that the surface of samples prepared with citric acid is enriched with carbonate species comparatively to those obtained with glycine. Additional thermal treatment at 700 ◦ C under air caused a narrowing of the 1470–1370 cm−1 doublet and a gradual decrease in their intensity. This indicates a decrease of monodentate carbonate surface population as CO2 desorbed (Eq. (3)) and/or a rearrangement of these species to bidentate carbonate

3.3. Surface analysis The XPS levels for La3d, Mn2p, O1s and C1s were recorded for all studied samples. Typical XPS peaks for LMA24 are shown in Fig. 4. A distinct variation in the intensity was observed with the fresh and calcined samples. Using the appropriate signal intensities of La, Mn, C and O elements, it was possible to check the chemical composition of all samples. Note that, a careful examination of the O1s and C1s core levels (Figs. 4 and 5) indicated more complex situation at the surface concerning the number of chemical species. Similar situation was usually observed with basic catalysts such as pure and doped alkaline earth oxides [35]. Table 3 lists the corresponding binding energy (BE) of La3d, Mn2p, O1s and C1s. The C1s level (Fig. 4) shows peaks at 284.6 eV (adventitious carbon) and 288.9–289.3 eV due to the presence of surface carbonates [36]. Besides, another peak appears at 286.0–286.4 eV, which compares well with the 286.0 eV value reported for La2 (CO3 )3 [37]. It can be seen from the estimated surface concentrations (Table 3, column 7) that a higher carbonate concentration is systematically obtained on the surface of samples prepared with citric acid compared to those obtained with glycine. This result is consistent with that obtained by FTIR and XRD.

Fig. 3. Infrared spectra of the studied samples.

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197

Fig. 4. Typical C1s, O1s, La3d and Mn2p levels for LMA24.

Fig. 5. Ethanol conversion and selectivities to ACA and CO2 as a function of reaction temperature over LMA0 catalyst.

The O1s region (Fig. 4) also shows a consistent behaviour with these observations. The O1s spectra reveal the existence of oxygen species in mixed valence states: lattice O2− (1s) at 528.9–529.2 eV, adsorbed O2 2− /O− (1s) at 530.5–531.0 eV and the O1s of oxygencontaining contaminants CO3 2− at 532.4–532.8 eV [38–41]. The relative content of the three kinds of oxygen species in the total

surface oxygen was estimated from the relative area of these subpeaks. Note that, the relative content of the adsorbed O2 2− /O− species in the total surface oxygen increased when the fresh samples were additionally calcined at 700 ◦ C (LMG24 and LMA24). The desorption and/or the rearrangement of monodentate carbonate species as given above by Eqs. (3) and (4) may explain the increase of basic oxygen species. This result is consistent with the decrease of the O1s signal corresponding to oxygen-containing contaminants (CO3 2− ) in LMG24 and LMA24 samples. The La3d features are located at 851.0–851.6 and 855.0–855.7 eV (Fig. 6) for La3d3/2 and at 834.2–834.8 and 838.0–838.9 eV for La3d5/2 (Fig. 4 and Table 3). The spin–orbit splitting of La3d level (16.8 eV) is almost identical in all the compounds. The BE values recorded for pure lanthanum oxide at 837.8 and 834.4 eV [42] are close to those observed for perovskite samples. These data indicate that lanthanum ions are present in the trivalent form for all the samples. The Mn2p BE values are located at 641.9–642.2 eV (Fig. 4). The multiplet splitting of Mn2p3/2 and Mn2p1/2 is 11.6–11.8 eV. The peak positions of Mn2p3/2 level of MnO, ␣Mn2 O3 and ␤MnO2 are 640.6, 641.9 and 642.2 eV, respectively [43]. The splitting between Mn2p3/2 and Mn2p1/2 levels of these oxides is the same, 11.6 eV. As reported in other studies [44,45], the mean oxidation state of Mn ions at the surface layers is extremely difficult to detect by XPS. However, the literature data suggest that the BE differ-

Table 3 Binding energy (BE) and surface concentration ratio (values in parentheses) of the different components of LM samples. Sample

La3d5/2 (eV)

Mn2p3/2 (eV)

O1s (eV) 2−

O LMG0 LMG24 LMA0 LMA24

834.2 838.0 834.4 838.1 834.8 838.9 834.3 838.5

641.9 642.2 642.0 641.9

(lattice)

528.9 (58%) 529.2 (55%) 529.0 (56%) 529.1 (56%)

C1s (eV) O2

2−

−

/O

530.5 (20%) 531.0 (28%) 530.7 (18%) 530.9 (24%)

CO3

2−

532.5 (22%) 532.8 (17%) 532.4 (26%) 532.6 (20%)

286.4 (20%)289.0 (22%) 286.2 (17%)288.9 (19%) 286.3 (26%)289.1 (27%) 286.0 (23%)289.3 (22%)

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Fig. 6. Conversion per SSA unit at different temperatures as a function of catalyst weights.

ence between Mn2p3/2 and O1s emissions increases with about 0.6–0.7 eV for the change of the oxidation state between Mn3+ and Mn4+ [46]. As reported in Table 3, this BE difference, in the range 112.8–113.0 eV, was within the uncertainty in the BE measurements (±0.2 eV) and seems unlikely that this BE difference results in a change of Mn4+ /Mn3+ ratio. The surface atomic compositions are summarized in Table 4. The surface layers of perovskite powders display different atomic concentrations with respect to the bulk. The theoretical values are determined from sample’s formula (Table 1). Results indicated a pronounced enrichment of lanthanum on the surface of all samples. This might correspond to excess LaOx species on the surface when not incorporated in the solid solution. These species are probably carbonated in part as indicated by the analysis of the C1s spectra and IR analysis. The sample prepared with glycine and additionally calcined shows a minimum La/Mn atomic ratio. The superficial La/Mn ratios determined for LM samples obtained by combustion route is lower (<2) than those observed with other perovskite systems (>2) prepared by conventional methods [5]. This may indicate that, higher the La/Mn ratio, less the Mn cation can be exposed near the surface. 3.4. Catalytic activity in ethanol oxidation Catalytic activities were measured for all studied samples in ethanol oxidation reaction. In the blank experiment using empty reactor, it was stated that at the highest reaction temperature (210 ◦ C), only 2% conversion of ethanol was reached. Typical conversion and selectivity evolutions as a function of reaction temperature are given for LMA0 catalyst (Fig. 5). The curves for the other catalysts have the same shape that for LMA0. The only products detected were water, carbon dioxide and acetaldehyde (ACA). ACA production was found to pass through a maximum with increasing reaction temperature over all four catalysts. The product distribution indicates that ACA yield reaches a maximum value between 180 and 190 ◦ C (Table 4). For all catalysts, complete oxidation of near 100% ethanol to form CO2 and H2 O (with the content of ACA in the exit-gas under the GC-detectable limits) can be realized

Fig. 7. Reaction rate per surface area unit as function of superficial CO3 2− and ACA yield as function of superficial La/Mn ratio.

at 210 ◦ C. The catalytic activity of LM samples obtained by combustion route is higher than that observed with other perovskite systems prepared by conventional methods (solid state, citrate, etc.), such as LaMnO3 [5], LaFeO3 [6] and LaCoO3 [7] for which 100% conversion occurs at a temperature higher than 250 ◦ C. Comparison between the four catalysts shows that activities follow roughly the same trend as SSA values (Table 1) although not proportionally. Indeed, when the intrinsic conversion (defined as ethanol conversion per surface area units) is considered, the activity difference is still observable and it can be claimed that the most active catalyst is LMG24, i.e., the unit SSA of LMG24 catalyst (with the lowest SSA value) works better than the corresponding SSA of the LMA0 catalyst (with the highest SSA value). In Fig. 6 are drawn the curves of catalytic activities per surface area unit at different temperatures as a function of catalysts weights. It can be claimed, from an engineering point of view, that the catalyst LMA0 would allow to obtain the same catalytic conversion with the lower mass of catalytic material, so in a reactor of lower volume and cost. This reveals that, in addition to the surface area, the chemical composition of the surface which is related to the preparation method, may have a pronounced effect on the performance of the catalysts. So, to provide the basis of an objective assessment of the effect of SSA, the specific (per g; rw ) and areal (per m2 ; rs ) activities were calculated at 140 ◦ C at which ethanol conversion is lower than 10% (Table 4). The deduced values indicate that the ranking of areal activities is somewhat different from that of specific activities. To identify which is the phenomenon governing the difference in activity between the four catalysts, we intent to find a correlation between the catalytic behaviour and the perovskite characterisation results. Referring to the above XPS results and the measured catalytic activities, some correlations are suggested in Fig. 7. The catalytic activity seems to be closely related to the concentration of carbonates. Fig. 7 clearly shows that the catalytic activity decreases as the surface carbonate concentration increases. Their formation may cause the active oxidation sites to become inaccessible. Indeed, in the whole range of the temperature at which the oxidation of ethanol occurs, the carbonates remain adsorbed on the

Table 4 Surface atomic concentrations (in %) of the different components and catalytic activities of the LM catalysts for complete oxidation of ethanol. Sample

La

Mn

La/Mn

T50 a (◦ C)

T90 a (◦ C)

YACA maxb (%) [T (◦ C)]

rs c (mmol m−2 h−1 )

rw c (mmol g−1 h−1 )

LMG0 LMG24 LMA0 LMA24

23.0(19.3) 22.0(19.4) 25.0(19.3) 25.0(19.4)

15.0(19.3) 16.0(19.4) 14.0(19.3) 16.0(19.4)

1.53 1.38 1.78 1.56

185 192 180 190

204 207 202 206

37 [190] 30 [180] 43 [190] 34 [180]

1.47 1.77 1.26 1.53

35.4 28.3 40.5 29.1

Numbers in parentheses concern the bulk concentrations, determined from sample formula. a T50 and T90 are temperatures needed for 50 and 90% ethanol conversion, respectively. b Maximum ACA yield. c Reaction rates at a temperature = 140 ◦ C.

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Fig. 9. TPD signal of fresh LMA0. MS signal of CO2 (m/z = 44) and O2 (m/z = 32). Fig. 8. Effect of pre-treatment on the ACA (solid line) and CO2 yield (dotted line) over LMA0 and LMA24 catalysts.

surface of the perovskite since according to IR results, the desorption of these carbonates requires higher temperature. Moreover, another possible explanation of the observed results can be given considering the evolution of superficial Mn concentration (Table 4). In fact, the ratio between the rates per unit surface area (rs ) (Table 4) and the Mn surface atomic fraction gives practically the same value, i.e., 0.10 ± 0.01 for the four samples, indicating that surface concentration in manganese is another possible parameter in controling its activity. Further ethanol oxidation measurements were carried out after pre-treatment of LMA0 catalyst which present a slow oxidation rate. Fig. 8 shows ACA and CO2 evolution during ethanol oxidation after He and O2 pre-treatment at 700 ◦ C for 1 h. The curve corresponding to LMA24 is added for comparison. As can be seen, the effect of pre-treatment on the activity of LMA0 catalyst in both ACA and CO2 formation is benefit especially when the surface of these catalysts was pre-treated under helium. When catalyst surface is pre-treated under O2 atmosphere, the slight enhancement of ACA formation may be attributed to the increase of basic sites concentration (O− and O2 2− ) as a consequence of carbonate species decomposition (Eq. (5)). Whereas, pre-treatment of surface catalyst under helium at 700 ◦ C improves

significantly the ACA and CO2 formation (Fig. 8). This pre-treatment allows not only the adsorbed carbonates decomposition (Eq. (5)), but also increases the amounts of the surface anionic vacancies as a consequence of oxygen species desorption (Eq. (6)). Indeed, the MS signal of CO2 (m/z = 44) and O2 (m/z = 32) recorded during thermodesorption experiments under a flow of He showed the desorption of both CO2 and O2 in the temperature range of 350–700 ◦ C (Fig. 9). Two signals of CO2 were detected at 330 and 670 ◦ C. The first one is ascribed to the desorption of the monodentate carbonates. The second signal is due to the desorption of the bidentate carbonates which are more stable than the monodentate carbonates and require higher temperature to desorb. Comparison with other perovskite systems prepared with conventional methods [7] shows the absence of the low temperature signal indicating the presence of only strong basic sites which are not active in ethanol

Fig. 10. Ethanol conversion as a function of reaction temperature over LMA0, La2 O3 , Mn2 O3 and a mechanical mixture La2 O3 + Mn2 O3 solids.

oxidation. Concerning the O2 , the patterns obtained showed two peaks of oxygen desorption, one at 425 ◦ C and one at 670 ◦ C. The O2 desorption signal at low temperature may be assigned to oxygen species adsorbed at the surface oxygen vacancies [47]. These oxygen species are very reactive and they can improve notably the catalytic activity considering the reaction temperature for ethanol oxidation.

It can be assumed that ethanol oxidation is enhanced by the availability at the catalyst surface of both basic and reductible sites. These sites are related to the presence of lanthanum and manganese. Then, with the objective to obtain more information about the probable synergy of the lanthanum manganite system, the catalytic activities of related simple oxides La2 O3 and Mn2 O3 were investigated. Moreover, in order to understand the contribution of lanthanum and manganese in the catalytic mechanism of ethanol oxidation, a mechanical mixture of pure oxides (atomic ratio La/Mn = 1) was prepared. The catalytic tests were carried out under the same conditions as over LM samples. Results are given in Fig. 10 and Table 5. As it can be seen, the higher catalytic activity of LM catalyst in comparison with pure oxides seems related to the existence of mixed oxide. Indeed, the measured activities indicate that the onset

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temperature (i.e., the temperature at which CO2 begins to form) is about 30–40 ◦ C higher on simple oxides and mechanical mixture than on mixed oxide. The highest ACA maximum yield obtained over La2 O3 compared to that obtained over Mn2 O3 is explained by the stronger basicity of lanthana surface [48,49]. One could suggest that the strong basic site (La–O) is favourable to activate ethanol molecule and hence to form acetaldehyde via ethoxide intermediates. When considering CO2 formation, it could be noted from Table 5 that ACA degradation to CO2 is more favourable on Mn2 O3 than on La2 O3 . Two possible explanations can be proposed for this fact. The first is related to the activation of oxygen species on manganese cations and likely oxidized the ACA into CO2 . The co-presence of two oxidation states of manganese (+III and +IV) in the perovskite-structure (Table 1) favours the activation of oxygen species and hence oxidation of ACA in order to produce CO2 . The second is related to the higher basicity of ACA (pKa = 13.3) [50] compared to that of ethanol (pKa = 16.0) [51]. The lower the surface acidity and the higher the basicity, the weaker the adsorption of ACA should be. It should be suggested that the stronger basicity rendered difficult the adsorption of the produced ACA on the catalyst surface thus avoiding their further oxidation to carbon dioxide in secondary reactions. A multiple step mechanism can be suggested for the ethanol oxidation (Eqs. (7) and (8)). As stated in the literature, alcohol adsorption is known to occur to form ethoxide species [52,53]. Thus the adsorption of ethanol requires anionic vacancy Mn(n−1)+ and a basic site LaO− . These two species are produced after desorption of carbonate species as indicated by Eqs. (5) and (6). Then ethoxide species may react with hydroxyl groups to produce water and anionic vacancy. After reoxydation of the manganese site with gaseous oxygen, two ways are suggested depending on the reaction temperature. At low temperature, ACA can be desorbed and the surface would be regenerated and ready for the continuation of the catalytic process. At higher temperature, the degradation of the organic fragment into CO2 probably occurs through hypothetical “multi-oxidized” intermediates.

On the basis of the obtained results and the above suggested reactions scheme, it could be noted that the use of glycine as gelling agent is suitable to have enough basic surface with low concentration of adsorbed carbonate species and appropriate superficial La/Mn ratio and O2 2− /O− concentration. These optimal values are favourable for high catalytic activity implying jointly basic and redox sites with the minimum production of undesirable ACA by product. Table 5 Results of the activity assays of the LMA0 catalyst and relative simple oxides for complete oxidation of ethanol. Sample a

◦

T20 ( C) rw b (mmol g−1 h−1 ) TCO2,onset c (◦ C) YACA,max d (%) [T (◦ C)] a b c d

LMA0

La2 O3

Mn2 O3

No catalyst

158 40.5 140 43 [190]

188 25.3 180 55 [250]

191 20.2 170 42 [250]

285 0 280 26 [330]

T20 is temperature needed for 20% ethanol conversion. Specific rate of ethanol conversion at 140◦ C. Temperature at which CO2 formation starts (c.a. 1% of CO2 reached). Maximum ACA yield.

4. Conclusion Lanthanum manganite (LM) perovskite-type oxides have been synthesized by means of combustion route and by controlling some operating conditions, i.e., the chemical nature of the fuel molecules (glycine or citric acid) and additional thermal treatment at 700 ◦ C for 24 h. Adequate specific surface areas, 16–32 m2 /g, for oxide catalysts to be used in ethanol total combustion are measured. The catalysts are single perovskite phases with additional lanthanum oxocarbonate phase when citric acid is used as fuel molecule. From IR and XPS results, carbonaceous residues are detected and their concentration is higher for catalysts obtained with citric acid than those obtained with glycine. The use of combustion method is a suitable preparation procedure to obtain perovskite materials with controlled superficial La/Mn ratio which seems to have a large impact on the catalytic performance in ethanol total oxidation. The activity was found to be directly linked to the basic character of La–O− sites and to the redox properties of manganese. Since during ethanol oxidation under studied conditions, consecutive reactions occur with the undesirable acetaldehyde formation as an intermediate, the optimum LaMnO3 catalyst efficiency (total combustion activity) can be achieved by precise control of acid–base properties of this material. Promising catalytic results have been obtained on LMG24 (prepared with glycine as fuel molecule and calcined at 700 ◦ C), a catalyst whose high activity is associated with the lowest La/Mn ratio and carbonate concentration. Acknowledgment The authors thank Prof. J. Vedrine for the useful discussion and help.

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