The effect of the preparation method on the properties of LaMnO 3 and La 0.8Sr 0.2MnO 3 perovskite was studied. Materials were prepared by four methods: sol-gel, chemical combustion, solvothermal and spray pyrolysis and characterized. The effect of the synthesis method on the texture, acid-base character of the surface, reducibility with hydrogen, oxygen desorption, surface composition and catalytic activity for combustion of lean methane was studied. It was found that synthesis method affects physicochemical properties of obtained materials-solvothermally produced materials exhibit well-developed surface area, presence of reactive oxygen species on surface and high catalytic activity for CH 4 combustion. Generally, LaMnO 3 and La 0.8Sr 0.2MnO 3 perovskites show catalytic activity for lean CH 4 combustion comparable or higher than the activity of 0.5 wt.% Pt/Al 2O 3 but lower than 1 wt.% Pd/Al 2O 3.
Huge amounts of methane are emitted into atmosphere every year from numerous sources like coal mines, animal waste or landfill. Concentration of methane in the atmosphere has been increasing strongly since beginning of XX century. Methane is a greenhouse gas showing 23 times higher global-warming potential than CO2 [
Crystal structure of ABO3 mixed metal oxides with the perovskite like structure is very flexible towards substitutions and thus allows modifying their physicochemical properties including catalytic activity. Perovskites have been the subject of many studies regarding use their as high-temperature oxidation catalysts [
The most important advantages of the solvothermal method of perovskites synthesis are high quality and purity of the products, short reaction time, low dispersion of grain size and no need to final high temperature calcination of product. The main disadvantage is a relatively high cost. F. Teng et al. [
H. Najjar et al. [
A number of other methods of perovskites synthesis has been elaborated, these ones mentioned above belongs to the most commonly used. Ch. Zhang et al. [
This paper reports results of comparative investigation of the physicochemical properties of the LaMnO3 and La0.8Sr0.2MnO3 perovskites prepared by different synthesis routes (spray pyrolysis, sol-gel with citric acid, solvothermal and chemical combustion). The goal of the work was to determine effect of the synthesis procedure on the properties and catalytic performances for lean methane oxidation of the LaMnO3 and La0.8Sr0.2MnO3 perovskites.
Mixed oxides with general formula LaMnO3 (LM) and La0.8Sr0.2MnO3 (LSM) were prepared using following methods of synthesis: sol-gel (SG), chemical combustion (CC), microwave-assisted solvothermal (MS) and spray-pyrolysis (SP) method. The details of preparation are as follows:
Sol-gel method: Nitrates of corresponding metals (molar ratio 1:1 for LM and 0.8:0.2:1 for LSM) were dissolved in demineralised water acidified by HNO3. To the homogeneous solution citric acid was added (molar ratio metal/citric acid = 1:2.5). The solution was then evaporated at 80˚C to produce viscous syrup, which was dried for 24 h at 110˚C and then grinded and calcined by using 3 steps during calcination: 4 h/180˚C, 1 h/300˚C and 4 h/750˚C.
Chemical combustion method: This method is based on the highly exothermic reaction between the metal salts and organic fuel―here glucose was used. It is self-sufficient energy synthesis because the heat required to the synthesis are provided by the reaction itself. Nitrates of corresponding metals (in molar ratio 1:1 for LM and 0.8:0.2:1 for LSM) were dissolved in a small amount of demineralized water. Glucose was then added to the obtained solution, a mass of glucose was equal to the mass of all salts. The resulting solution was concentrated in a rotary evaporator at 80˚C to produce viscous syrup, which was dried for 24 h at 110˚C and then calcined for 6 h at 750˚C.
Microwave-assisted solvothermal method: nitrates of corresponding metals were dissolved in ethylene glycol and reacted in a teflon melting pot in an autoclave (90 min, 200˚C, 40 bar) under microwave heating. The resulting suspensions were centrifuged, washed with acetone, dried (24 h/110˚C) and then calcined (4 h/750˚C, 10˚C/min).
Spray-pyrolysis method: The hydrated nitrates of corresponding metal were dissolved in distilled water (0.45 mol/dm3). The resulting solution was filtered and then sequentially pumped into the chamber system by a peristaltic pump. Powder synthesis reaction was carried out under the following conditions: solution flow rate 30 ml/min with performance of 0.1137 dm3/min, pressure 13.8 bar, the temperature 500˚C. The resulting powder was filtered through a cotton filter, dried 24 h/110˚C and then calcined 4 h at 750˚C (10˚C/min).
The prepared mixed oxides were characterized by means of: X-ray diffraction (XRD), specific surface area determination, scanning electron microscopy (SEM), temperature programmed reduction with hydrogen (TPR H2), decomposition of cyclohexanol (CHOL), temperature programmed desorption of CO2 (TPD CO2), temperature programmed desorption of oxygen (TPD O2), X-ray photoelectron spectroscopy (XPS) and tests of catalytic activity for lean methane combustion.
By using an X'Pert Powder Diffractometer (PANalytical), working in the reflection geometry and using CuKα radiation (λ = 1.54056 Ǻ) X-ray diffraction (XRD) patterns were recorded. The data were collected in the 2Θ range 10˚ - 80˚ (step of 0.026˚). The fitting of the diffraction peaks was done by WinPLOTR programme [
The specific surface area was determined by the BET method (Brunauer-Emmett-Teller) using the Quantachrome Autosorb-1C. The BET surface was calculated from the N2 adsorption isotherm at −196˚C (77 K), in the range of relative pressures p/po from 0.05 to 0.35. Prior to the measurements, samples were degassed at 200˚C for 2 h to remove adsorbed contaminants such as water.
The morphology of the samples was determined using a scanning electron microscope SEM/Hitachi S-3400N while elemental composition of selected sites on their surface were determined using energy-dispersive X-ray spectroscopy (EDS).
The susceptibility of the prepared materials to reduction was determined by temperature programmed reduction with hydrogen (TPR H2). TPR H2 measurements were made using Pulse ChemiSorb 2705 apparatus. The 50 mg of sample was placed in a U-tube quartz reactor and then heated to 900˚C (heating rate of 10˚C/min). Through the catalyst bed was passed the mixture of 5% vol. of H2 in Ar (30 ml/min). Hydrogen consumption was measured by thermal conductivity detector (TCD).
The share of acid-base centers on the surface of tested materials were determined by decomposition of cyclohexanol (CHOL), determining the selectivity of the conversion of this compound to dehydration and dehydrogenation products. The dehydration of CHOL occurs on acid sites and leads to cyclohexene (CHEN), while the dehydrogenation of CHOL, leading to cyclohexanone (CHON), requires the interaction both acid and basic centers. It is accepted, that selectivity of CHOL decomposition to CHEN (SCHEN) is a direct measure of acidic character of material surface, while the CHON/CHEN selectivities ratio (SCHON/SCHEN) gives information about basic nature of surface [
Measurements of temperature programmed desorption of oxygen (TPD O2) was performed by using a Micromeritics AutoChem II 2920 analyzer equipped with a TCD detector. The 100 mg of samples were placed in a quartz reactor, then moved into the furnace chamber and heated in a stream of O2 from 25˚C to 500˚C for 1 h. The reactor was cooled to 40˚C and a stream of inert gas was passed through the catalyst bed (He, 30 ml/min) for 0.5h. Next the samples were heated, in the stream of He, from 40˚C to 950˚C (10˚C/min.). The content of O2 in the effluent gas was determined by a mass spectrometer (OmniStar QMS 200, Pfeiffer Vacuum) while monitoring the m/e signal 32 (O2).
The surface composition of the prepared materials was determined by X-ray photoelectron spectroscopy (XPS), using a SPECS XPS/UHV system equipped with a PHOIBOS 100 spectrometer. The X-ray source was an Mg anode operating at 100 W (survey scan) and 250 W (high resolution spectra), Ar(+) sputtering (90'', 7 μA/cm2, 3 keV). The analyzer mode was set at constant analyzer energy mode. Sample charging was compensated using an electron flood at 0.5 mA current and 0.1 eV energy. The C1s peak of the contamination carbon, at 284.6 eV, was taken as reference. The detection angle was normal to the surface. The spectra were collected and processed using SPECLAB software. Non-linear least-squares fitting algorithm was performed using peaks with a mix of Gaussian-Lorenzian shape and a Shirley baseline.
Catalytic activity in lean CH4 combustion was tested in a continuous-flow fixed bed quartz reactor, placed in a tube furnace with a single heating zone. The temperature was measured with a thermocouple positioned a few millimeters below the top of the catalyst bed. Temperature of reactor was raised (5˚C/min) from 25˚C to 550˚C. The 200 mg of catalyst diluted in 200 mg of SiC was loaded into a tubular quartz reactor on a thin layer of quartz wool. The reagents mixture (0.6 vol% of CH4 and 21 vol% of O2 in argon) was fed to the reactor with flow rate corresponding to GHSV = 40,000 h−1. The gas mixture flow rate was 8 dm3/h and was adjusted by mass flow controllers. The reaction products were analyzed by GC (PERKIN ELMER Autosystem XL with capillary column (30 m × 0.25 mm × 0.25 µm), equipped with FID detector. The reaction rate per m2 ( r S S A ) of the catalyst was calculated using formula:
r S S A = F CH 4 S S A × α × 10 6
where F CH 4 is the flow rate of methane ( mol ⋅ s − 1 ), a is methane conversion and SSA is specific surface area.
Regardless of the method of synthesis used, the X-ray diffraction patterns of all materials are similar to each other and characteristic for perovskite-like structure (PDF card No. 00-032-0484 for LM and PDF card No. 00-040-1100 for LSM), as can be seen from
Method of synthesis | Sample | Crystal. System | Space group | a*, **, Å | c, Å | γ ***, ° | Average size of crystal., nm | |
---|---|---|---|---|---|---|---|---|
from XRD | from SSA | |||||||
SG | LM | Trig. | R-3c | 5.506(3) | 13.335(7) | 120 | 37 | 83 |
LSM | Reg. | Pm-3m | 3.878(2) | - | 90 | 25 | 34 | |
CC | LM | Trig. | R-3c | 5.504(4) | 13.332(13) | 120 | 32 | 61 |
LSM | Trig. | R-3c | 5.503(2) | 13.360(5) | 120 | 36 | 51 | |
MS | LM | Trig. | R-3c | 5.503(3) | 13.321(8) | 120 | 25 | 57 |
LSM | Reg. | Pm-3m | 3.881(2) | - | 90 | 22 | 48 | |
SP | LM | Trig. | R-3c | 5.506(2) | 13.321(5) | 120 | 44 | 152 |
LSM | Reg. | Pm-3m | 3.855(2) | - | 90 | 16 | 83 |
* - For trigonal system a = b ≠ c ** - For regular system a = b = c *** - The angle between the axes.
67˚, PDF card No. 00-005-0602) (
All studied LM and LSM mixed oxides show relatively low specific surface area (
Method of synthesis | Sample | SBET, m2/g | Decomposition of cyclohexanol | |||
---|---|---|---|---|---|---|
Conversion of CHOL, % | Selectivity to CHEN, % | Selectivity to CHON, % | Y | |||
SG | LM | 11 | 9 | 71 | 29 | 0.4 |
LSM | 27 | 9 | 51 | 49 | 1.0 | |
CC | LM | 15 | 15 | 63 | 37 | 0.6 |
LSM | 18 | 12 | 80 | 20 | 0.3 | |
MS | LM | 16 | 10 | 63 | 37 | 0.6 |
LSM | 19 | 16 | 71 | 29 | 0.4 | |
SP | LM | 6 | 8 | 84 | 16 | 0.2 |
LSM | 11 | 21 | 88 | 12 | 0.1 |
among the LSM materials, this one obtained by the sol-gel method. Both samples prepared by spray pyrolysis are characterized by the smallest value of SSA. The partial substitution of lanthanum by strontium, for all of used methods of synthesis, leads to a material with a higher specific surface area.
Acid-base properties of the studied materials were determined by the method of cyclohexanol (CHOL) decomposition. On the surface of the mixed metal oxides with perovskite like structure nucleophilic oxide ions O2− are present (basic sites of a Lewis type) [
The morphology of the studied perovskites was characterized by scanning electron microscopy (SEM) technique.
Typical SEM pictures of LM surface are presented in
The TPR H2 profiles of studied materials are shown in
SG | CC | MS | SP | |||||
---|---|---|---|---|---|---|---|---|
LM | LSM | LM | LSM | LM | LSM | LM | LSM | |
TI, ˚C | 352 | 314 | 357 | 364 | 343 | 361 | 397 | 464 |
TII, ˚C | 835 | 716 | 830 | 794 | 792 | 794 | 849 | 726 |
The TPR H2 profiles of Mn-containing materials show two intensive peaks of hydrogen consumption (
Mn 3 + + O 2 → O 2 − / O − + Mn 4 +
The shapes of TPR H2 profiles of all LM and LSM materials are similar, regardless of the method of synthesis. Nevertheless, the influence of the synthesis method on the temperature of maximum of H2 consumption is observed. The temperatures of reduction TI and TII of LM perovskites lower in the following order of synthesis method: SP > SG > CC > MS. LM-SP exhibits the highest value of TI (397˚C) and TII (849˚C) (
The TPD O2 profiles of studied materials are shown in
Depending on the desorption temperature, three forms of oxygen are distinguished on the perovskite surface [
sample | Amount of oxygen desorbed, µmol O2/g (µmol O2/m2) | |||
---|---|---|---|---|
α1 | α2 | β | Total | |
LM-SG | 7 (0.6) | 427 (39) | - | 434 (39) |
LSM-SG | 14 (0.5) | 130 (5) | 62 (2) | 206 (8) |
LM-CC | 14 (1) | 511 (34) | - | 525 (35) |
LSM-CC | 5 (0.3) | 97 (5) | 63 (4) | 165 (9) |
LM-MS | 28 (2) | 197 (12) | 341 (21) | 566 (35) |
LSM-MS | 35 (2) | 346 (18) | 146 (8) | 527 (28) |
LM-SP | 6 (1) | 105 (18) | 61 (10) | 172 (29) |
LSM-SP | 5 (0.5) | 48 (4) | - | 53 (5) |
400˚C is associated with the surface oxygens, described as α1-oxygen, which are commonly attributed to weakly chemisorbed oxygen molecules upon surface-oxygen vacancies. The oxygen described as α2 is a near-surface oxygen associated with lattice defects such as dislocations and grains frontiers. This form of oxygen desorbs at temperatures from the range 400˚C - 700˚C and is completely associated with the oxidation of diluted methane, together with α1-oxygen [
All studied LM and LSM perovskites desorb α1, α2 and β oxygen (
The onset temperature of α1-oxygen desorption from LM perovskites increased with the order MS < CC < SG < SP while in the case of materials substituted by strontium the order is following: MS < SG < CC < SP (
Oxides of lanthanum and strontium are very reactive and in contact with the atmospheric air form surface carbonates, hence on the surface of materials large amount of carbon are present. The surface of all the studied materials is enriched with La (
Method of synthesis | SG | CC | MS | SP | |||||
---|---|---|---|---|---|---|---|---|---|
Sample | LM | LSM | LM | LSM | LM | LSM | LM | LSM | |
La, % at. | 20.8 | 12.7 | 18.9 | 16.4 | 19.3 | 17.4 | 18.5 | 15.8 | |
Mn, % at. | 10.0 | 12.8 | 11.3 | 11.9 | 10.2 | 11.8 | 12.0 | 13.4 | |
share of Mn4+/Mn, % | 25.4 | 25.8 | 23.9 | 24.1 | 14.2 | 41.4 | 17.7 | 21.7 | |
Sr, % at. | - | 2.8 | - | 2.6 | - | 2.3 | - | 4.3 | |
O, % at. | 56.6 | 59.0 | 59.2 | 57.2 | 58.7 | 56.5 | 56.4 | 58.9 | |
C, % at. | 12.6 | 12.7 | 10.6 | 11.9 | 11.8 | 12.0 | 13.1 | 7.6 | |
La:Mn | T | 1:1 | 0.8:1 | 1:1 | 0.8:1 | 1:1 | 0.8:1 | 1:1 | 0.8:1 |
E | 2.1:1 | 1:1 | 1.7:1 | 1.4:1 | 1.9:1 | 1.5:1 | 1.5:1 | 1.2:1 | |
La:Sr | T | - | 4:1 | - | 4:1 | - | 4:1 | - | 4:1 |
E | - | 4.5:1 | - | 6.3:1 | - | 7.6:1 | - | 3.7:1 |
T―theoretical ratio; E―experimental ratio.
termination. The surface characterized by such termination adsorbs oxygen atoms in the place of bridge La-La, which results in the more energetically favorable configuration of the material [
S. Ponce et al. [
Form of oxygen | SG | CC | MS | SP | ||||
---|---|---|---|---|---|---|---|---|
LM | LSM | LM | LSM | LM | LSM | LM | LSM | |
O2−, % at. | 64.9 | 57.1 | 66.2 | 61.7 | 43.6 | 67.0 | 64.6 | 66.7 |
OH- and CO 3 2 − , % at. | 35.1 | 42.9 | 33.8 | 38.3 | 47.1 | 33.0 | 26.4 | 29.9 |
H2O, % at. | - | - | - | - | 9.3 | - | 9.0 | 3.4 |
Olat/Oads | 1.85 | 1.33 | 1.96 | 1.61 | 0.93 | 2.03 | 2.45 | 2.23 |
In addition, as it was shown above, the morphological and textural properties of studied perovskites are strongly affected by the method of synthesis. S. Royer et al. have shown that amounts of desorbed different α-oxygens is mainly dependent on the specific surface area of the material [
Three forms of oxygen are present (
However, both the method of synthesis and substitution by Sr exert a significant influence on the share of oxygen forms on the surface of these materials (
The results of lean methane combustion on studied LM and LSM perovskites are reported in
All prepared perovskites exhibit catalytic activity in lean methane combustion. Depending on the method of synthesis, the temperature of 10%, 50% and 90% conversion of CH4 differs significantly. The highest activity exhibits LM-MS, it enables the total methane conversion at 451˚C. Slightly less active is LSM-CC (T100% = 455˚C). The catalytic activity of 0.5% Pt/Al2O3 is low and comparable with the activity of both samples prepared by spray pyrolysis method while 1% Pd/Al2O3 is significantly more active than all other studied materials. The partial
SG | CC | MS | SP | 0.5% Pt/Al2O3 | 1% Pd/Al2O3 | |||||
---|---|---|---|---|---|---|---|---|---|---|
LM | LSM | LM | LSM | LM | LSM | LM | LSM | |||
T10%, | 313 | 270 | 237 | 266 | 269 | 280 | 330 | 359 | 278 | 205 |
T50%, | 379 | 333 | 351 | 329 | 331 | 363 | 410 | 452 | 446 | 252 |
T90%, | 443 | 397 | 375 | 389 | 392 | 439 | 496 | 537 | 499 | 296 |
substitution of 20% at. of La by Sr in the structure of LM perovskite decreases its activity for combustion of lean methane―only LSM-SG has higher activity than LM-SG.
It seems, that the high catalytic activity of LM samples prepared by MS and CC methods may be explained by their ability to release large amounts of α oxygen at relatively low temperatures (
Method | Sample | Temperature, ˚C | ||
---|---|---|---|---|
300 | 350 | 400 | ||
SG | LM | 0.4 | 2.0 | 5.4 |
LSM | 0.7 | 2.0 | 3.1 | |
CC | LM | 0.9 | 3.3 | 5.5 |
LSM | 1.2 | 3.4 | 4.7 | |
MS | LM | 1.2 | 3.7 | 5.3 |
LSM | 0.7 | 1.8 | 3.5 | |
SP | LM | 0.5 | 2.1 | 6.3 |
LSM | 0.2 | 0.6 | 1.8 |
form of oxygen species are responsible for the oxidation of methane [
Another factor contributing to higher catalytic activity of LM-MS and LM-CC, is relatively better developed specific surface area of this material, composed of fine crystal particles. In addition, LM-MS reaction with hydrogen starts at the lowest temperature, which suggests the presence of the most reactive oxygen species on the surface.
For the samples containing Sr there are no simple correlations between their properties and the catalytic activity. The highest rate of methane combustion shows material prepared by CC method while the lowest this one prepared by SP method (
TPR-H2, possess close temperatures TI and TII and textures. In addition, the LSM-MS desorbs substantially more oxygen (α and β) counterpart prepared by CC. However, the latter perovskite shows a significantly higher catalytic activity than the previous one. The only one which differs these two materials is strontium content―its insufficiency in respect to the stoichiometric in the LSM-MS. Presently we can not explain why the abundance and availability of α-oxygen on the LSM-MS surface is not sufficiently reflected in its activity. Probably β-oxygen present in large amounts on the surface LSM-MS, but desorbing at relatively higher temperatures than in case of other LSM play more important role in methane combustion on this material. In this way one can suppose that unusual catalytic behavior of LSM-MS might be associated with lower than stoichiometric content of Sr resulting in a high share of Mn4+, whereby results of neither TPD-O2 nor TPR-H2 can be simply related to the rate of methane oxidation. Relatively low activity of LSM-SP should be attributed to its low SSA and a small amount of adsorbed oxygen species on its surface. High catalytic activity of materials prepared by CC method can be explained on the basis of H. Najjar, H. Batis conclusions who stated that this method of synthesis resulted in a decrease in superficial La/Mn atomic ratio and an increase of relative content of the surface oxygen species what leading to high catalytic activity of corresponding material in the combustion reactions [
LaMnO3 and La0.8Sr0.2MnO3 perovskites were synthesized by four different preparation routes. It appeared that synthesis method affects physicochemical properties of obtained materials including catalytic activity in lean methane combustion. There is no simple answer to the question on the physicochemical reason for differences of catalytic behavior of studied perovskites. The use of solvothermal method allows one to obtain materials characterized by relatively well-developed surface area, the presence of reactive oxygen species on surface as well as high catalytic activity for lean methane combustion. Properties of the materials prepared by chemical combustion are very similar to that of solvothermally prepared. Spray pyrolysis leads to materials possessing the lowest specific surface area and porosity, characterized by the acidic character of the surface. LM and LSM perovskites prepared by the spray pyrolysis method show low susceptibility to reduction with H2 and the highest share of lattice oxygen on the surface which can be correlated with the lowest activity in lean methane combustion. LM and LSM prepared by sol-gel method exhibit an intermediate properties between that ones prepared by solvothermal (or chemical combustion) and spray pyrolysis. LSM perovskites prepared by solvothermal and spray pyrolysis have a less than stoichiometric content of Sr.
All prepared LM and LSM materials show catalytic activity for lean CH4, comparable to (or higher) than the activity of 0.5 wt.% Pt/Al2O3 but lower than 1 wt.% Pd/Al2O3. Qualitatively catalytic activity of lean methane combustion on studied perovskites can be correlated with the presence of α and β oxygen forms on their surface as well as availability of these species and the degree of surface development. The rate of methane combustion on LM perovskites quite well correlates with temperature of first maximum of hydrogen consumption. In case of LSM perovskites neither results of TPR-O2 nor TPR-H2 could be simply correlated with rates of methane combustion. Catalytic properties of LSM prepared by MS are affected by lower than assumed content of strontium leading to high content of β-oxygen on its surface. These species desorb from the LSM-MS surface at relatively higher temperatures than in the case of other LSM materials what may results in lower rate of methane combustion at low temperatures range.
The work was financed by a statutory activity subsidy from the Polish Ministry of Science and Higher Education for the Faculty of Chemistry of Wrocław University of Technology. We would like to thank dr W. Miśta for the TPD O2 measurements.
Miniajluk, N., Trawczyński, J., Zawadzki, M. and Tylus, W. (2018) LaMnO3 (La0.8Sr0.2MnO3) Perovskites for Lean Methane Combustion: Effect of Synthesis Method. Advances in Materials Physics and Chemistry, 8, 193-215. https://doi.org/10.4236/ampc.2018.84013