Carbonatation and Decarbonatation Kinetics in the La₂O₃-La₂O₂CO₃ System under CO₂ Gas Flows

Abstract

The carbonatation of La₂O₃ oxide and the decarbonatation of lanthanum carbonate phase La₂O₂CO₃ are investigated using thermal and thermogravimetry analyses under CO₂ gas flow. The initial phase La₂O₃ is first elaborated from pyrolysis of a LaOHCO₃ precursor. Then, thermal and thermogravimetry analyses are carried out under CO₂ flow, as temperature increases then decreases. The carbonatation kinetics of La₂O₃ is determined at three fixed temperatures. Electrical impedance spectroscopy is performed to determine the electrical responses associated with ionic mobilities and phase changes, in the temperature range 25 to 900^∘C. The electrical conduction during heating under CO₂ gas flow should be linked to two regimes of ionic conduction of the carbonate ions. From these electrical measurements, the ionic mobility of carbonate ions CO³²⁻ is found to be close to 0.003 ·10⁻⁴ cm² s⁻¹ V⁻¹ at 750^∘C for the monoclinic La₂O₂CO₃ phase.

1. Introduction

The lanthanum-based system La₂O₃-CO₂-H₂O is characterized by successive phases LaOHCO₃, La₂O₂CO₃, and La₂O₃, stable in various temperature ranges [1–9], depending on the partial pressures of CO₂ and H₂O [10–15]. It has been established that the decomposition of the hydroxycarbonate LaOHCO₃ under air generally gives the La₂O₂CO₃ dioxycarbonate phase [6]. However, this last phase exists under three polymorphic structural varieties with tetragonal, hexagonal, and monoclinic crystal lattices [16–19]. The carbonatation of La₂O₃ under pure CO₂ generally gives the main La₂O₂CO₃ phase; however, the obtained system can be complex, with presence of several polymorphic structures depending on the experimental synthesis conditions [10, 18, 20]. In their work concerning the TG analysis of the La₂O₂CO₃ phase under CO₂ gas, the authors of [2] showed that a small amount of La₂O(CO₃)₂ was probably formed as an additional phase. This phase was also studied by authors of [21].

In room conditions, the lanthanum oxide is highly sensitive to environmental water. In a previous study [22], we have established correlations between the thermal decomposition and the electrical responses of compacted pellets of this LaOHCO₃ phase, subjected to pyrolysis under air: we have shown that strong variations in conductances accompanied these phase changes. We have also established that these LaOHCO₃, La₂O₂CO₃, and La₂O₃ phases have the capacity to convert carbon monoxide into CO₂ at relatively low temperature: at 200–300^∘C, the L phase is a good catalyst converting CO into CO₂, while it might be sensitive to CO₂ only above 500^∘C.

In the present study, we focus our attention on phase changes during carbonatation and decarbonatation processes, respectively, of the La₂O₃ phase and of the La₂O₂CO₃ phase. The main objective of this approach should reside in connecting the weight variations due to these phase transformations with electrical responses, in order to appreciate their potential efficiency in gas sensing devices. These correlations between mass losses and electrical responses are not known, and they could deliver interesting information on the electrical sensitivity of such systems.

2. Experimental Details

The LaOHCO₃ hydroxycarbonate was first prepared via a specific route [22, 23] based on a thermal treatment at 80^∘C of three aqueous solutions of La(NO₃)₃⋅6H₂O, urea CO(NH₂)₂, and polyvinyl-pyrrolydine (PVP) polymer. The La₂O₃ oxide was obtained by pyrolysis of this LaOHCO₃ precursor.

The various chemical steps can be summarized as follows:

(i)First initial decomposition processes under air as temperature increases (25–1200^∘C): LaOLHaC₂O₃⟶La₂O₂CO₃[1]+H₂O+CO₂,(1)O₂CO₃[1]⟶La₂O₃[1]+CO₂.((1^′))(ii)Carbonatation and decarbonatation under pure CO₂ as temperature increases (25–1200^∘C): La₂La₂O₃[1]+CO₂⟶La₂O₂CO₃[2],(2)O₂CO₃[2]+CO₂⟶La₂O₃[2]+2CO₂.((2^′))(iii)Recarbonatation under pure CO₂ as temperature decreases (1200 to 25^∘C):

La₂O₃[2]+CO₂⟶La₂O₂CO₃[3].(3)

In the previous equations, in bracket [1 to 3] we have designated phases obtained after a transformation process (decomposition, carbonatation, and decarbonatation). Theses phases have not the same characteristics (various morphologies and specific surfaces).

The polycrystalline samples were systematically analyzed by X-ray diffraction, using a D5000 Siemens-Bruker diffractometer, equipped with a copper X-ray source (wavelength 𝜆=1.5410⁻¹⁰ m), and with a monochromator eliminating K𝛽 radiation. The experiments were carried out using classical 𝜃−2𝜃 configuration.

Thermal and Thermogravimetric analyses (DTA-TG) were carried out using SETARAM DSC 92 equipment, with a thermal rate of 10^∘C/minute, under CO₂ pure gas (rate of flow of 33 cm³⋅s^-1).

Electrical measurements under CO₂ gas flow were performed using a Solartron electrical impedance spectrometer working with a maximal tension of 1 V, in the frequency range 100 to 10⁷ Hz. A reactive homemade cell was used to perform experiments under various gas flows (air, CO₂) at various temperatures ranging between 25 and 900^∘C. The spectrometer delivers Nyquist representations of the resulting impedances recorded at fixed temperatures: the resistance value is classically obtained by extrapolation of the experimental Nyquist circles, and using electrical equivalent circuits (parallel 𝑅-𝐶 circuits) generated by the software. We have selected specific electrical circuits with a resistance (𝑅) parallel to a constant phase element CPE=(𝑗𝐶^∗𝜔)^𝑛 where the exponent n is comprised between 1 and 0, and 𝐶^∗ is a term similar to a capacitance for 𝑛=1 (the unit of 𝐶^∗ depends on 𝑛).

To obtain electrical analyses of sample surfaces reacting with gas flows, the powder samples were first compacted under a pressure of 5 kbar in a cylindrical cell. Then, the obtained cylindrical pellet was cut in form of a rectangular plate, with platinum electrodes fixed on two parallel faces (dimensions 2.3 × 8 mm). The distance between the electrodes is 9 mm. This configuration (adapted to the reactive cell) allows a determination of the electrical properties of a significant material surface exposed to gas action. In a later step, these results might be used to test a hypothetical gas sensor sensitive to CO₂.

3. Results

3.1. Carbonatation-Decarbonatation Processes

3.1.1. Heating Process under CO₂ Flow

The La₂O₃ sample, initially obtained from thermal decomposition of LaOHCO₃, has been subjected to thermal and thermogravimetry analyses under CO₂ gas flow, with temperature increasing from 25 to 1200^∘C. The resulting TG-DTA curves are reported on Figure 1. A strong exothermic DTA peak is observed at 525^∘C: it is related to the carbonatation of La₂O₃ with formation of the La₂O₂CO₃ phase. Then, at 960^∘C, we observe an endothermic feature corresponding with the decomposition of the carbonate phase. Above 980^∘C the La₂O₃ phase stabilizes. A small endothermic feature is observed at 375^∘C: it might be associated with a partial dehydration of the sample due to the high sensitivity to environmental water of La₂O₃. The progressive mass evolution observed in the TG curve of Figure 1, as temperature increases, is directly associated with the classical buoyancy. A similar effect will be observed during the cooling process.

Figure 1 

Weight gain associated with carbonatation of La2O3 (first step): exothermic peak at 520∘C linked with weight gain due to formation of the monoclinic La2O2CO3 phase; endothermic peak due to decarbonatation of La2O2CO3 phase and formation of final La2O3 (second step).

3.1.2. Cooling Experiments under CO₂ Flow

Using cooling experiments, we have analyzed the carbonatation of La₂O₃ from 1200^∘C to 25^∘C. The results are represented on Figure 2. The formation of La₂O₂CO₃ starts from 820^∘C and is maximum at 750^∘C. The exothermic peak associated with the crystallization of La₂O₂CO₃ carbonate is observed at 790^∘C. This temperature of carbonatation is strongly different from the one obtained during the heating process.

Figure 2 

Evolution of La2O3 weight during cooling process, under CO2 flow: formation of La2O2CO3 phase (exothermic peak) then relative stabilization of this phase as temperature decreases.

At each step involving a stabilized phase, we have carried out X-ray diffraction analyses to identify the obtained phases. We have confirmed that, in the case of thermal decomposition under air of LaOHCO₃ phase, two different tetragonal and hexagonal La₂O₂CO₃ structures are simultaneously observed. In the case of carbonatation of the La₂O₃ phase in the temperature range 500 to 700^∘C, we observe the formation of the La₂O₂CO₃ phase. The La₂O(CO₃)₂ phase was not observed in our experiments. This fact was previously reported by other authors [10, 18]. On Figure 3, we have reported the X-ray diffraction pattern characteristic of the monoclinic La₂O₂CO₃ phase heated at 520^∘C under CO₂ flow, during 3 hours. The refined cell parameters are 𝑎=0.4073±0.0003 nm; 𝑏=1.3503±0.0008 nm; 𝑐=0.4079±0.0005 nm; 𝛽=90.89^∘. In the pattern, a weak trace of the hexagonal phase (noted as *) is observed.

Figure 3 

X-ray diffraction pattern of La2O2CO3 (monoclinic) obtained by heating La2O3 at 520∘C under CO2 flow. Trace of hexagonal phase (noted *).

3.2. Kinetics Study of Carbonatation of La₂O₃ at Fixed Temperatures

We have performed a weight analysis of the La₂O₃ powder, obtained from the thermal decomposition of the initial LaOHCO₃ phase, under CO₂ gas flow at three constant temperatures. The CO₂ gas flow rate was 33 cm³⋅s^-1. In the SETARAM equipment, a fast temperature increase is first applied to the sample, and then, the temperatures are successively fixed to 450, 480, and 500^∘C. The three initial masses of La₂O₃ are successively (at 𝑇=450, 480, and 500^∘C) 𝑚₀=74.36 mg, 70.73 mg, and 39.26 mg. The data evolutions have been interpreted in terms of an elemental Avrami’s model [24] (using a single mechanism approach): Δ𝑚=Δ𝑚₀1−exp(−𝑘⋅𝑡^𝑝),(4)(i)𝑡 is the reaction time;(ii)Δ𝑚₀ is the limit mass of CO₂ involved in the carbonate formation La₂O₂CO₃ from a mass m₀ of La₂O₃;(iii)Δ𝑚 is the CO₂ mass having reacted with La₂O₃ at the time 𝑡;(iv)𝑘 is a kinetics parameter depending of temperature;(v)𝑝 is the exponent characteristic of the reaction mechanism (𝑝>2 for complex mechanisms, 𝑝<1, for example, for mechanisms involving diffusion barriers).

To test the degree of validity of this Avrami’s model, we have reported the function 𝑌 versus ln(𝑡) on Figure 4: 𝑌=ln−lnΔ𝑚⁰−Δ𝑚Δ𝑚₀=ln(𝑘)+𝑝ln(𝑡).(5) For a single crystal growth mechanism, the variation of 𝑌 versus ln(𝑡) should have been linear. Presently, the representation of Figure 4 is not linear: this should be mainly due to the existence of at least two different crystal growth mechanisms, with two periods of mass gain corresponding to two behaviors.

Figure 4 

Test of Avrami’s model validity: the representation of 𝑌=ln[−ln(Δ𝑚0−Δ𝑚)/Δ𝑚0] versus ln(𝑡) (𝑡 in mn) shows that two main types of behaviors can be observed as time increases.

In Table 1, we have reported the values of the kinetics parameters 𝑘₁ and 𝑘₂ and exponents 𝑝₁ and 𝑝₂, corresponding with the two different behaviors in which a linear correlation might be observed. The parameters 𝑘₁, 𝑝₁ are relative to the first period depending on temperature, and the parameters 𝑘₂, 𝑝₂ are relative to the second period. The 𝑘₁ and 𝑘₂ are thermally activated with activation energies of, respectively, 7.6 and 2.8 eV. The 𝑝₁ exponent is quasi-constant, while the 𝑝₂ exponent is close to 1 at 450^∘C and becomes very weak at higher temperatures.

Table 1 

Parameters extracted from Avrami’s model: 𝑘₁ and 𝑘₂ kinetics parameters and 𝑝₁ and 𝑝₂ exponents, respectively, associated with the fast and slow regimes (1st and 2nd periods).

The first growth regime should be associated with a fast carbonatation of grain surfaces associated with complex diffusion mechanisms. During this period, a carbonate shell enveloping oxide grains probably should be formed. The second growth regime should be associated with reaction and diffusion in grain cores, with a decrease of the reaction rate due to the carbonate shell: the resulting slow diffusion regime could govern the global reaction speed.

3.3. Electrical Analyses under CO₂ Gas Flows

To correlate the phase modifications to electrical behaviors, we have analyzed compacted powder samples in the electrical cell. In this experiment, a rectangular compacted sample resulting from the total decomposition of the initial LaOHCO₃ sample has been subjected to a progressive heating, under pure CO₂ gas flow. Between 600 and 700^∘C, carbonatation occurs, thus involving a strong increase in conductance mainly due to the ionic mobility of CO₃₂₋ carbonate ions. Then, above 750^∘C decarbonation occurs, involving a decrease of conductance due to CO₃₂₋ carbonate ions elimination and formation of La₂O₃. This oxide should be formed at 950^∘C.

On Figure 5, we have reported the ln(Σ) values versus temperature (total Σ values). We observe a strong decrease of ln(Σ) between 450^∘C and 750^∘C: the carbonatation of La₂O₃ should start from 450^∘C, with a first regime up to 600^∘C and a second regime up to 750^∘C. Two activation energies for the conduction behavior can be determined: 2.5 (first regime) and 1.4 eV (second regime). In this carbonatation domain, the ionic conduction plays a major role with mobile species CO₃₂₋.

Figure 5 

Evolution of ln(Σ) versus 103/T of initial La2O3 sample during its carbonatation and decarbonatation, in the temperature range 300 to 950∘C, under a constant CO2 gas flow: (a) starting step of carbonatation between 450 and 750∘C with two conduction regimes; (b) decarbonatation step above 750∘C.

Above 750^∘C, we observe a strong decrease in the ln(Σ) values: in this temperature range, decarbonatation occurs in a continuous way, with the elimination of CO₃₂₋ ions. As La₂O₃ phase stabilizes, the resistance reaches a stabilized value.

Using the observed value Σ=1.7⋅10⁻⁴ Ω⁻¹ at 750^∘C (on Figure 5: maximum value of Σjust before decomposition), and considering as negligible the conductance of La₂O₃ at the same temperature (close to 10^-8 to 10^-9 Ω⁻¹ at 750^∘C), we have evaluated an ionic conductance due to CO₃₂₋ ions to ΔΣ=1.7⋅10⁻⁴ Ω⁻¹. From this evaluation of ΔΣ, we have determined the order of magnitude of the carbonate ion mobility u(CO₃₂₋) at 750^∘C. Other values could be derived from the data obtained in the temperature range 450 to 700^∘C. The concentration of carbonate ions 𝐶_ion has been calculated from the effective density of the sample 𝜇=5.1 g⋅cm^-3 (for a theoretical crystal density of 6.51 g.cm^-3) and using the sample volume 𝑉=0.1656 cm³. A value of 𝐶_ion=0.0155 mol⋅cm^-3 has been obtained. To determine the mobility, we have used the classical relations: ΔΣ=𝜎𝑆𝐿,𝜎=𝑄⋅𝐶^ion⋅𝑢(ion),(6) where 𝜎 is the conductivity, 𝑆 and 𝐿 are the surface and separation distance of the two electrodes, and where 𝑄=193000 C⋅mol^-1. The relation giving the conductivity assumes an activity coefficient of 1: it only delivers an order of magnitude for the mobility.

We have obtained an order of magnitude of 𝑢(CO₃₂₋) = (0.003±0.001) 10^-4 cm² s^-1 V^-1 for a carbonate ion moving at 750^∘C mainly along grain boundaries (or grain surfaces), and partly in the grain cores. This relatively high mobility can be associated with the activation energy of 1.4 eV (in the temperature range 600 to 750^∘C) as calculated above.

4. Discussion-Conclusions

The carbonatation kinetics of La₂O₃ has been determined at various temperatures. In the case of mass gain analyses, an elemental Avrami’s approach has allowed determining a complex two-step mechanism of growth: (i) a fast surface carbonatation with carbonate shell formation and (ii) a diffusion mechanism in grain cores with slower kinetics. The electrical analyses argue in favor of two different conduction mechanisms: during carbonatation at increasing temperature, the first activation energy (2.5 eV) should be associated with ionic conduction at grain surfaces, and the second activation energy (1.4 eV) should due to an increasing contribution of the conduction in the bulk. Correlatively, it should be remarked that, in thermal analyses, the stability range is observed from 500 to 850^∘C, while in electrical analyses, this stability range is observed from 500 to 750^∘C. This can be explained by the two different heating kinetics conditions used in the two experiments.

Finally, we observe a relatively high ionic mobility mainly due to the CO₃₂₋ ions in La₂O₂CO₃ at 750^∘C. In our evaluation, we have neglected the ionic conduction of oxygen ions.

It should be concluded that these phase modifications associated with high ionic conduction might be used as electrical sensitive material to detect CO₂, provide temperatures that could be fixed close to 400–550^∘C (carbonatation of La₂O₃ phase) and 750^∘C to restore the initial La₂O₃ phase.

Acknowledgments

The authors gratefully acknowledge the Provence-Alpes-Côte d’Azur Regional Council, the General Council of Var, and the agglomeration community of Toulon Provence Mediterranean for their helpful financial supports. This paper was developed in the general framework of ARCUS CERES project (2008–2010).