Carbonatation and Decarbonatation Kinetics in the La 2 O 3-La 2 O 2 CO 3 System under CO 2 Gas Flows

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 CO 2− 3 is found to be close to 0.003 ·10−4 cm2 s−1 V−1 at 750◦C for the monoclinic La2O2CO3 phase.

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 LaOHCO 3 phase, subjected to pyrolysis under air: we have shown that strong variations in conductances accompanied these phase changes.We have also established that these LaOHCO 3 , La 2 O 2 CO 3 , and La 2 O 3 phases have the capacity to convert carbon monoxide into CO 2 at relatively low temperature: at 200-300 • C, the L phase is a good catalyst converting CO into CO 2 , while it might be sensitive to CO 2 only above 500 • C.
In the present study, we focus our attention on phase changes during carbonatation and decarbonatation processes, respectively, of the La 2 O 3 phase and of the La 2 O 2 CO 3 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.

Experimental Details
The LaOHCO 3 hydroxycarbonate was first prepared via a specific route [22,23] based on a thermal treatment at 80 • C of three aqueous solutions of La(NO 3 ) 3 •6H 2 O, urea CO(NH 2 ) 2 , and polyvinyl-pyrrolydine (PVP) polymer.The La 2 O 3 oxide was obtained by pyrolysis of this LaOHCO 3 precursor.
The various chemical steps can be summarized as follows: (i) First initial decomposition processes under air as temperature increases (25-1200 • C): (ii) Carbonatation and decarbonatation under pure CO 2 as temperature increases (25-1200 • C): (iii) Recarbonatation under pure CO 2 as temperature decreases (1200 to 25 • C): 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 = 1.54 10 −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 CO 2 pure gas (rate of flow of 33 cm 3 •s −1 ).
Electrical measurements under CO 2 gas flow were performed using a Solartron electrical impedance spectrometer working with a maximal tension of 1 V, in the frequency range 100 to 10 7 Hz.A reactive homemade cell was used to perform experiments under various gas flows (air, CO 2 ) 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 CO 2 .

Carbonatation-Decarbonatation Processes
3.1.1.Heating Process under CO 2 Flow.The La 2 O 3 sample, initially obtained from thermal decomposition of LaOHCO 3 , has been subjected to thermal and thermogravimetry analyses under CO 2 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 La 2 O 3 with formation of the La 2 O 2 CO 3 phase.Then, at 960 • C, we observe an endothermic feature corresponding with the decomposition of the carbonate phase.Above 980 • C the La 2 O 3 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 La 2 O 3 .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.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 LaOHCO 3 phase, two different tetragonal and hexagonal La 2 O 2 CO 3 structures are simultaneously observed.In the case of carbonatation of the La 2 O 3 phase in the temperature range 500 to 700 • C, we observe the formation of the La 2 O 2 CO 3 phase.The La 2 O(CO 3 ) 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 La 2 O 2 CO 3 phase heated at 520 • C under CO 2 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.[24] (using a single mechanism approach):

Kinetics Study of Carbonatation of La
(i) t is the reaction time; (ii) Δm 0 is the limit mass of CO 2 involved in the carbonate formation La 2 O 2 CO 3 from a mass m 0 of La 2 O 3 ; (iii) Δm is the CO 2 mass having reacted with La 2 O 3 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: 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.
In Table 1, we have reported the values of the kinetics parameters k 1 and k 2 and exponents p 1 and p 2 , corresponding with the two different behaviors in which a linear correlation might be observed.The parameters k 1 , p 1 are relative to the first period depending on temperature, and the parameters k 2 , p 2 are relative to the second period.The k 1 and k 2 are thermally activated with activation energies of, respectively, 7.6 and 2.8 eV.The p 1 exponent is quasiconstant, while the p 2 exponent is close to 1 at 450 • C and becomes very weak at higher temperatures.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.

Electrical Analyses under CO 2
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 LaOHCO 3 sample has been subjected to a progressive heating, under pure CO 2 gas flow.Between 600 and 700 • C, carbonatation occurs, thus involving a strong increase in conductance mainly due to the ionic mobility of CO 3 2− carbonate ions.Then, above 750 • C decarbonation occurs, involving a decrease of conductance due to CO 3 2− carbonate ions elimination and formation of La 2 O 3 .This oxide should be formed at 950 • C.
where σ is the conductivity, S and L are the surface and separation distance of the two electrodes, and where 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(CO 3 2− ) = (0.003 ± 0.001) 10 −4 cm 2 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.

Discussion-Conclusions
The carbonatation kinetics of La 2 O 3 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 CO 3 2− ions in La 2 O 2 CO 3 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 CO 2 , provide temperatures that could be fixed close to 400-550 • C (carbonatation of La 2 O 3 phase) and 750 • C to restore the initial La 2 O 3 phase.

2
Flow.Using cooling experiments, we have analyzed the carbonatation of La 2 O 3 from 1200 • C to 25 • C. The results are represented on Figure 2. The formation of La 2 O 2 CO 3 starts from 820 • C and is maximum at 750 • C. The exothermic peak associated with the crystallization of La 2 O 2 CO 3 carbonate is observed at 790 • C.This temperature of carbonatation is strongly different from the one obtained during the heating process.

Figure 1 :
Figure 1: Weight gain associated with carbonatation of La 2 O 3 (first step): exothermic peak at 520 • C linked with weight gain due to formation of the monoclinic La 2 O 2 CO 3 phase; endothermic peak due to decarbonatation of La 2 O 2 CO 3 phase and formation of final La 2 O 3 (second step).

Figure 2 :
Figure 2: Evolution of La 2 O 3 weight during cooling process, under CO 2 flow: formation of La 2 O 2 CO 3 phase (exothermic peak) then relative stabilization of this phase as temperature decreases.

Figure 4 :
Figure 4: Test of Avrami's model validity: the representation of Y = ln[− ln(Δm 0 − Δm)/Δm 0 ] versus ln(t) (t in mn) shows that two main types of behaviors can be observed as time increases.

Figure 5 :
Figure 5: Evolution of ln(Σ) versus 10 3 /T of initial La 2 O 3 sample during its carbonatation and decarbonatation, in the temperature range 300 to 950 • C, under a constant CO 2 gas flow: (a) starting step of carbonatation between 450 and 750 • C with two conduction regimes; (b) decarbonatation step above 750 • C.

Table 1 :
Parameters extracted from Avrami's model: k 1 and k 2 kinetics parameters and p 1 and p 2 exponents, respectively, associated with the fast and slow regimes (1st and 2nd periods).
, obtained from the thermal decomposition of the initial LaOHCO 3 phase, under CO 2 gas flow at three constant temperatures.The CO 2 gas flow rate was 33 cm 3 •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 La 2 O 3 are successively (at T = 450, 480, and 500 • C) m 0 = 74.36mg, 70.73 mg, and 39.26 mg.The data evolutions have been interpreted in terms of an elemental Avrami's model 2 O 3 at Fixed Temperatures.We have performed a weight analysis of the La 2 O 3 powder