Indian Journal of Chemistry Vol. 51A, July 2012, pp. 931-936

Effect of glycine concentration on the properties of LaCoO3 perovskite prepared

by the glycine-nitrate process

Rajender R Kondakindi College of Science, Technology, Engineering and Mathematics,

Youngstown State University, One University Plaza, Youngstown, OH 44555, USA

Email: rajender.kondakindi@gmail.com

Received 12 July 2011; revised and accepted 20 June 2012

LaCoO3 nanocrystals have been prepared by the glycine-nitrate process. The effect of glycine content in the synthesis solution on various properties of LaCoO3 has been studied in the glycine-to-nitrate molar ratio between 0.75 and 2.0. The samples are characterized by X-ray diffraction for phase purity and particle size, by transmission electron microscopy for particle size, by BET for surface area and by temperature-programmed reduction for reducibility of Con+. XRD reflections show the presence of impurities in the as-prepared samples while only rhombohedral phase could be identified in the samples after calcination at 700 °C. For all the glycine-to-nitrate ratios tested, the particle size estimated from XRD data ranges from 58-69 nm, which are systematically lower than those determined by BET which ranged from 97-118 nm. TPR profiles of the samples show two major peaks; one due to the reduction of Co3+ to Co2+ and the other due to the reduction of Co2+ to Co0. The samples prepared under fuel-deficient and fuel-rich conditions show higher reducibility as compared to that of the sample prepared under stoichiometric conditions.

Keywords: Glycine-nitrate process, Perovskites, Mixed oxides, Lanthanum cobaltate, Surface area, Particle size

Perovskites are mixed oxides of the general formula ABO3, where both A and B metal ions can be partially substituted to introduce structural and electronic defects resulting in mixed valence states and enhanced mobility of oxygen in the lattice. Perovskite-type oxides have application as catalysts in solid oxide fuel cells1,2, in DeNOx process3 and in catalytic combustion of volatile organic compounds (VOC)4,5. One such material is LaCoO3 and this perovskite has applications in automotive pollution abatement6-8, electrochemical reduction of oxygen9, chemical sensors9 and diesel reformation10,11. Commonly reported methods for the synthesis of LaCoO3 include solid state synthesis12, sol-gel method13, freeze drying method14,15, citrate method16

and coprecipitation method17. In the conventional solid state method, the oxides are ground and calcined at high temperatures to obtain the desired phase. Due to the high temperature calcination, impurities are formed and the specific surface area is low. The wet chemical methods result in fine particles with homogeneous size, but are complex in process and require a long preparation time. Glycine-nitrate process (GNP) could be an alternative method for perovskite synthesis because of its simplicity. Although, LaCoO3 has been prepared by GNP18,19, there is no systematic study on the effect of glycine concentration in the precursor solution on various properties of LaCoO3.

In the present study, LaCoO3 perovskites have been prepared by GNP with varying glycine-to-nitrate ratios, covering the fuel-deficient, stoichiometric and fuel-rich conditions. The influence of synthesis mixture composition on the surface area, particle size, microstructure and the reducibility of prepared samples has been examined. Experimental

LaCoO3 perovskites were prepared by the combustion method using glycine as the fuel. Lanthanum nitrate (La(NO3)3.6H2O, 99.9%), cobalt nitrate (Co(NO3)2.6H2O, 99.98%) and glycine (NH2CH2COOH, 99.5%) were obtained from Alfa Aesar, USA. Glycine-to-nitrate ratios (GNRs) were calculated based on the oxidizing and reducing valencies of nitrate and glycine, respectively. The oxidizing valency of lanthanum nitrate is -15 [La(NO3)3 = 3+3(0+(-6))] and the reducing valency of glycine is +9 [H2NCH2COOH = 2(1)+0+4+2+4-2-2+1]. Hence, 1 mol of lanthanum nitrate requires 1.66 moles of glycine. A GNR of 1.66 is considered stoichiometric such that higher and lower GNRs are defined as fuel-rich and fuel-deficient, respectively. In a typical GNP, lanthanum nitrate and cobalt nitrate were dissolved in 50 mL of deionized water in a beaker (2 g batch). Glycine was added to this nitrate solution according to the desired GNR. The solution, while stirred, was heated on a hot plate to vaporize most of the water. Once the solution became sufficiently thick for the combustion, the beaker was placed in a furnace that was maintained at 300 °C to

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facilitate combustion. The combustion reaction occurred rapidly under fuel-deficient and stoichiometric conditions. The reaction was slower under fuel-rich conditions but was still completed within a few seconds. The powder obtained after the combustion (as-prepared sample) was ground and a portion of the sample was calcined at 700 °C for 2 h to study the effect of calcination on physical properties of LaCoO3.

X-ray diffraction (XRD) patterns were recorded on a Scintag X-ray diffractometer with Cu-Kα radiation in the 2θ range 20-80°. The particle size was calculated by Scherrer equation after subtracting the instrument broadening. The BET specific surface areas were measured on an Autosorb-I instrument at liquid nitrogen temperature. The samples were degassed at 200 °C for 2 h before the analysis. The scanning electron micrographs (SEM) of perovskite samples were recorded on a Philips XL30 instrument after gold coating the samples. For transmission electron microscopy (TEM) analysis, the samples were dispersed in ethanol and sprayed onto formvar coated Cu grids using an air gun. The TEM micrographs were recorded on a Philips CM20 instrument at 200 kV. Temperature-programmed reduction (TPR) experiments were carried out on a Zeton Altamira AMI-200 instrument. Approximately, 0.03 g of the catalyst was loaded in a quartz tube and held between two quartz wool plugs. The temperature of the catalyst was measured using a thermocouple

placed in contact with the sample. The sample was pre-treated at 300 °C for 30 min in a flow of Ar (50 mL/min). After the pre-treatment, the reactor was cooled to room temperature and the gas flow changed to 10 % H2-Ar mixture (50 mL/min). The sample temperature was then ramped to 900 °C at a heating rate of 10 °C/min. Changes in the exit gas stream were monitored by continuous measurement of the thermal conductivity of the gas. Results and discussion

XRD patterns of the as-prepared and the calcined samples are shown in Figs 1 and 2, respectively. Although the as-prepared samples prepared under fuel-deficient, stoichiometric and fuel-rich conditions exhibit different material compositions, the XRD patterns of calcined samples exhibited essentially the same characteristics, i.e., corresponding to LaCoO3. The sample prepared at a GNR of 1.0 exhibited the diffraction lines corresponding to La2CoO4, indicating that the formation of LaCoO3 proceeds through La2CoO4 phase during the calcination. The samples prepared under fuel-deficient and fuel-rich conditions exhibited the reflections corresponding to La2O3 and LaCoO3 perovskite phases. This indicates that under these conditions, the formation of LaCoO3 proceeds through the reaction between La2O3 and another material phase. From elemental constraints, the other phase can be deduced to be the Co-phase though no Co-phase was observed in XRD, which may be due to smaller crystallite size of the phase. It has been reported that material prepared by freeze-dried and Pechini methods20 is a mixture of La2O2CO3, Co3O4

20 30 40 50 60 70 80

Inte

nsity

(a.u

)

Glycine:nitrate = 0.75

1.0

1.5** * *=

♦♦

♦♦

♦ ♦♦

♦•

2θ°

20 30 40 50 60 70 80

Inte

nsity

(a.u

)

Glycine:nitrate = 0.75

1.0

1.5** * *=

♦♦

♦♦

♦ ♦♦

♦•

2θ°

Fig. 1—XRD patterns of as-prepared LaCoO3 samples. [*Peaks due to La2O3; • peak due to LaCoO3 perovskite; ♦ peaks due to La2CoO4].

20 30 40 50 60 70 80

Inte

nsity

(a.u

)

Glycine:nitrate = 0.75

1.0

1.5

2.0012

110 104

202 006

024 214 018

2θ°

20 30 40 50 60 70 80

Inte

nsity

(a.u

)

Glycine:nitrate = 0.75

1.0

1.5

2.0012

110 104

202 006

024 214 018

2θ°

Fig. 2—XRD patterns of calcined LaCoO3 samples.

2θ (deg.)

2θ (deg.)

NOTES

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and LaCoO3. In the present study, we did not observe the presence of La2O2CO3 and Co3O4 phases in the XRD analysis, but all the samples exhibited LaCoO3 phase after calcination at 700 °C for 2 h (Fig. 2).

The lattice parameters of the sample prepared at a GNR of 1.0 (calcined at 700 °C for 2 h) are a = b = 5.4439 Å and c = 13.1265 Å, which correspond to rhombohedral LaCoO3 phase20. The crystallite size measurements were carried out by Scherer equation, D = kλ/βcosθ, where D is the crystallite size, k is a constant (= 0.9 assuming that the particles are spherical), λ is the wavelength of the X-ray radiation, β is the line width and θ is the angle of diffraction. The crystallite sizes of the calcined samples are in the range of 58-69 nm (Table 1). It has been reported previously that the temperature during the combustion process exceeds 1000 °C in the case of Fe-based samples prepared under fuel-deficient and stoichiometric conditions21. This high temperature causes sintering of the particles and hence leads to the formation of larger particles at GNR less than 1.0. As the glycine concentration increases, the maximum temperature decreases to ~500 °C (GNR ≥ 2.0) (ref. 21). The lower combustion temperature leads to the formation of smaller sized particles of LaCoO3 under fuel-rich conditions. This is assuming that the LaCoO3 system behaves similar to LaFeO3 system in terms of the combustion temperature. The amount of gaseous products produced also increases drastically at a GNR exceeding 1.0. The large volume of the gas products generated under these conditions inhibits the sintering of the particles and hence, helps preserve smaller particle size.

The specific surface areas of calcined samples are presented in Table 1. The BET specific surface areas of the samples show small variations from each other. Among the various samples, the sample prepared at a GNR of 1.0 showed the lowest specific surface area (Table 1). The grain equivalent diameter (DBET) has

been calculated using BET specific surface area according to the following equation assuming that all the particles are spherical, DBET = 6/ρS, where ρ is the theoretical density and S is the specific surface area of LaCoO3. The particle sizes obtained from the BET are higher than those obtained by Scherer equation (Table 1). This is due to the loss of a fraction of surface by contact between the crystal domains22.

The SEM micrographs of the calcined samples are shown in Fig. 3. The samples prepared at lower GNRs (≤1.0) did not show any agglomeration but well-connected porosity can be observed. The samples prepared at higher glycine concentration showed agglomeration. The energy dispersive X-ray spectroscopy (EDS) analysis (results are not shown here) of these samples did not show any La-rich or Co-rich regions, which indicates the homogeneous distribution of La and Co in the samples. The TEM micrographs of various calcined LaCoO3 perovskites, shown in Fig. 4, indicate that the particle size is in the range of 60-120 nm. The perovskite prepared at a GNR of 0.75 contains particles with a size of ~100-120 nm whereas the particle size of the samples prepared under fuel-rich conditions decreased with the increase of glycine concentration in the precursors. In a previous study, Jadhav et al.23 have reported a particle size of ~80 nm for LaCoO3 prepared by coprecipitation method and calcined at 450 °C. In the present work, similar particle sizes were obtained for LaCoO3 samples (GNR≥1.5) after calcination at 700 °C, which is 250 °C higher than that used by Jadhav et al.23 The particles synthesized by Jadhav et al.23 were rectangular polygon whereas those synthesized in the current work are spherical.

It is evident from the TPR profiles of calcined LaCoO3 samples are shown in Fig. 5 that the samples are reduced in two major steps. However, it is observed that the first reduction process is complex with multiple peaks (Fig. 5). The low-temperature peak (330-420 °C) can be assigned to the reduction of Co3+ to Co2+ and the high-temperature peak (550-560 °C) is due to the reduction of Co2+ to Co0 (ref. 24). The two-step process for LaCoO3 reduction is shown in the following equations24:

LaCo3+O3 + 21

H2 → 21

La2Co +22 O5 +

21

H2O ... (1)

21

La2Co +22 O5 + H2 →

21

La2O3 + Co0 + H2O ... (2)

Table 1—BET areas and particle sizes of various LaCoO3 calcined samples

Particle size (nm) Glycine to nitrate molar ratio

BET area (m2/g) BETa XRDb

0.75 8.8 104 62 1.0 7.7 118 69 1.5 9.1 100 58 2.0 9.4 97 69 aCalculated from BET specific surface area. bCalculated by Scherrer’s equation.

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Fig. 3—SEM micrographs of calcined LaCoO3 samples at varying glycine:nitrate ratio. [a, 0.75; b, 1.00; c, 1.50; d, 2.0].

Fig. 4—TEM micrographs of calcined LaCoO3 samples at varying glycine:nitrate ratio. [a, 0.75; b, 1.00; c, 1.50; d, 2.0].

NOTES

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In the first step, there is a release of oxygen from LaCoO3 leading to the formation of a phase from Brownmillerite series. In the second step, the reduction of Brownmillerite phase takes place resulting in the formation of Co metal and La2O3. In the present study, the first reduction peak is broad and contains two reduction maxima, which may be attributed to the reduction of Co3+ and to the formation of Co metal as reported elsewhere22,25. The formation of Co metal can be explained by considering the instability of Brownmillerite phase in inert atmosphere. La2Co2O5 phase decomposes to CoO and Ruddlesden-Popper (Lax+1CoxO3x+1) phases in inert atmosphere, but CoO reduces to Co metal under the TPR gas environment20. The appearance of multiple peaks in the low temperature region may also be attributed to the distortion of perovskite lattice and oxygen defects, which could create distinct Co3+ sites and reducibilities26. The peak maxima shifted to higher temperature in the sample prepared at a GNR of 1.0, and in the other samples, it did not shift much. The high temperature peak maximum also shifted to higher temperature in the sample prepared at a GNR of 1.0. The shifting of the peaks to higher temperature is attributed to the diffusional resistance of H2 to the core of the LaCoO3 lattice (low porosity and lower surface area)27.

GNP is a rapid and self-sustaining method for the preparation of oxides. It is also a very simple method that can be an alternative to wet chemical methods to produce homogeneous nanopowders. In the present study, LaCoO3 was prepared by GNP with varying glycine concentration to examine the effect of glycine

on various properties of LaCoO3. XRD results showed the presence of rhombohedral perovskite phase after calcination at 700 °C for 2 h. The BET specific surface area of the sample prepared at a glycine-to-nitrate ratio (GNR) of 1.0 had a slightly lower surface area in comparesion with those of the samples prepared under fuel-deficient and fuel-rich conditions. The particle size of perovskites decreased with an increase in glycine concentration in the precursors due to the liberation of higher amount of gaseous products, which inhibited the sintering of particles. The particle size of the samples was found to be ~100 nm as determined by TEM. The perovskites were reduced in two major steps, i.e., first, via the reduction of Co3+ to Co2+ and, second, by the reduction of Co2+ to Co0. The reducibility of the sample prepared at a GNR of 1.0 was the lowest compared to the samples prepared under fuel-deficient and fuel-rich conditions due to diffusional constraints. Acknowledgement

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Glycine:nitrate = 0.75

1.0

1.5

2.0

328 390 550

366 420560

335 389547

557395330

Temperature (°C)

H2

cons

umpt

ion

(a.u

)

50 150 250 350 450 550 650 750 850

Glycine:nitrate = 0.75

1.0

1.5

2.0

328 390 550

366 420560

335 389547

557395330

Temperature (°C)

H2

cons

umpt

ion

(a.u

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Fig. 5—TPR profiles of various LaCoO3 samples.

Temp. (°C)

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