La2O3, Lu2O3, CeO2, and Bi2O3 Materials Interacting with Air-CO or Air-CH4 Flows

Using Fourier transform infrared (FTIR) spectroscopy analyses, we have studied the oxidation processes of methane or carbon monoxide in air-CH4 or air-CO flows interacting with polycrystalline catalytic oxides, as a function of temperature (T) and time (t). The gas flows crossed through La2O3, Lu2O3, CeO2, or Bi2O3 porous walls with constant composition and rate. The oxidation capacities of materials were determined from the intensities I(t,T) of CO2 vibrational bands resulting from the total oxidation of CH4 or CO into CO2. To interpret the time-dependent variations of these intensities, we have applied a model derived from Johnson-Mehl-Avrami approaches. This simple approach delivers pertinent parameters describing time-dependent oxidation rates.


Introduction
In the general framework of catalytic gas sensing or depolluting microsystems development, we study the timedependent oxidation capacities of polycrystalline materials interacting with air-CO or air-CH 4 gas flows.The main objective is to convert toxic CO or undesired CH 4 gases into CO 2 gas.To follow the reaction processes, we make use of Fourier transform infrared spectroscopy and determine the time-(t) and temperature-(T) dependent intensities I(t, T) of CO 2 vibrational bands resulting from the oxidation processes of CH 4 or CO in air-CH 4 or air-CO gas flows.In this study, we determine the catalytic efficiencies of La 2 O 3 , Lu 2 O 3 , CeO 2 , and Bi 2 O 3 polycrystalline oxides as a function of reaction time, at various temperatures: the air-CH 4 or air-CO flows cross through porous polycrystalline walls of these rare earth-based oxides and a system based on the Bi 2 O 3 phase [1].
Rare earth oxides are well known for the similarity of their chemical and physical properties, in relation with the electronic structures (4f-5d-6s) of rare earth cations.Despite this similarity, these oxides generally present diversified physical and chemical properties allowing their use in many applications.The cerium oxide (ceria) has been extensively studied as a catalyst [2][3][4][5] or semiconducting material [6][7][8].The catalytic properties of ceria have been ascribed to the capacity of cerium to change its valence from Ce 4+ to Ce 3+ and thus to oxidize or reduce gases.In the case of lanthanum oxide (lantana), catalytic behaviours have been studied in the past [9].This oxide is used in the conversion of CH 4 into syngas (H 2 + CO) [10].However, very few studies on the reactivity of lutetium oxide (lutetia) in presence of gases are available in the literature: this oxide is mainly involved in optical, luminescence applications [11][12][13][14].In 1976, the catalytic oxidation of butane was studied over a series of lanthanide oxides in order to investigate the effect of the electronic configuration on the catalytic activity [15].The authors showed that lanthanum oxide was one of the least active catalysts; cerium oxide was the most active one, and the activity decreased with an increase in the atomic ISRN Materials Science number from cerium to gadolinium.They also observed that the activity of terbium oxide was the second highest, and again the activity decreased from terbium to lutetium.In other terms, for the authors, lutetia should have the weakest activity in the case of oxidation of butane.
In the case of our previous studies on the system Bi 2 O 3 -CeO 2 [1], we clearly observed a high catalytic activity of the bismuth oxide interacting with air-CO gas flows and a poor activity of this oxide in presence of air-CH 4 gas flows.
In these experimental studies [1], the evolution versus time of oxidation capacities I(t, T) (CH 4 or CO being converted into CO 2 ) presents a common behavior, characterized by at least three periods: initiating, intermediate, and stabilization periods.Some samples present abnormal maxima of I(t, T) in the intermediate period of reaction.Such behaviors might be conditioned by the nature of grain surfaces in the materials.
In the present study, we try to interpret the timedependent evolutions of the FTIR intensities I(t, T) using elemental Avrami models [16][17][18].The modeling approach is applied to La 2 O 3 , CeO 2 , Lu 2 O 3 , and Bi 2 O 3 phases.

Experimental Section
2.1.Samples.The various samples were synthesized via specific routes previously described by us [1,[19][20][21][22] and based on wet chemistry processes involving nitrate-based solutions.The ceria phase was elaborated in nanocrystalline form at 200 • C. The La 2 O 3 and Lu 2 O 3 were obtained after final thermal treatment at 750 • C. The Bi 2 O 3 phase was obtained after thermal treatment at 600 • C during 4 hours.Each polycrystalline phase was clearly identified using classical X-ray diffraction from D5000 Bruker equipment in θ-2θ configuration.Brunauer-Emmett-Teller (BET) [23] analyses were carried out to determine the sample-specific surface areas noted as A BET in m 2 /g.This method delivers the effective surface exposed to gas adsorption.In the case of absence of agglomerations, the A BET values can be related to individual grain dimensions.The A BET values were found to be 11.5 m 2 /g for La 2 O 3 , 82.0 m 2 /g for CeO 2 , 5.0 m 2 /g for Lu 2 O 3 , and 2.0 m 2 /g for Bi 2 O 3 .

FTIR Spectroscopy and Reactivity
Analyses.The rare earth or bismuth-based materials were exposed to air-CH 4 and air-CO gas flows in a homemade cylindrical reaction cell (see Figure 1).The transformation of CH 4 or CO into CO 2 was analyzed by Fourier transform infrared (FTIR) spectroscopy, using a FTIR Unicam-Mattson-Bruker spectrometer.The gases pass through a polycrystalline porous walls constituted of the various phases.The polycrystalline walls are fixed between two porous separators.A constant mass of m 0 = 0.1 g was used in each experiment.
All experimental details have been extensively described in previous works.(doublet at 2340-2360 cm −1 ), resulting from one of the overall reactions: The conversion intensity I(t, T) was determined from the measurements of CO 2 absorption bands, at a certain time t of the gas/solid interaction.Each vibrational spectrum was recorded over a period of Δt = 10 s with intervals of 30 s between two spectra.The total exposure time was two hours.For a given total time t of reaction and a given temperature T react , the intensity I(t, T) was assumed to be proportional to the amount of CO 2 molecules formed during the time Δt of FTIR record.All experiments were characterized by a first initiating regime in which the CO 2 intensities increase upto a maximal value after a time of about 10-15 minutes.Figures 2(a) and 2(b) represent typical FTIR spectra associated with conversions of CH 4 and CO after solid gas interactions.The CO 2 FTIR band intensity increases as the CH 4 or CO band intensities decrease.
To compare the various reactivities, we have normalized the curves I(t, T) using the specific surfaces A BET by calculating the values I * (t, T) = I(t, T)/A BET .It should be noted that, in our experiments, the temperature ranges were limited to 275 • C for air-CO flows and 525 • C for air-CH 4 flows in order to avoid direct oxidation of CO or CH 4 by oxygen (this direct oxidation was tested in our experimental device in the absence of active sample).

Modeling Approaches
In our previous work [24], we proposed a semiempirical model based on typical catalytic steps summarized as follows: (i) surface adsorption of gas and O 2 (air) on the surface of the solid; (ii) surface reaction CH 4 + 2O  It should be important to note that, in the present experiments, the gas flows cross through polycrystalline porous walls of materials, with a fixed speed and a fixed gas composition: in these circumstances, the gas-solid interactions result from a complex equilibrium involving four distinct kinetics of adsorption, desorption, degradation, and regeneration of active sites.In our model, we have assumed that the desorption steps of final gases (CO 2 ) might be described by surface reaction laws.
The intermediate surface reactions between adsorbed molecules and active sites might be described as follows.
(i) Oxidation step of adsorbed molecules from mobile surface oxygen O * of one active site S red -O * , delivering one reduced site "S red ": (i) Formation of mobile oxygen species O * from adsorbed dioxygen from air: (O 2 ) ads → 2O * .
(ii) Regeneration (oxidation) of reduced sites "S red ": or The general expression delivering the CO 2 vibrational band intensity is assumed to result from sites delivering oxygen S red -O * and sites regenerated from oxygen from air S red : In this expression, the symbol [CO 2 ] designates the total amount of CO 2 molecules formed after a reaction time 0074.The derivative d[CO 2 ]/dt = Δ[CO 2 ]/Δt is proportional to the instantaneous amount Δ[CO 2 ] recorded at time t, during the period Δt.The term A(t, T) describes the adsorption of gases on the solid surfaces, D(t, T) describes the reaction between oxidative active sites and reducing gas molecules, and R(t, T) describes the site regeneration due to oxygen from air adsorbed at the solid surface.Each A, D, R term was previously expressed using three distinct Avrami's approaches.The final general expression adapted to the normalized data I * = I/A BET was as follows: In this expression, the parameter X 0 is proportional to the composition of the air-gas flow.This composition (2500 ppm in air) was fixed for all experiments.As the normalized I * values characterize the instantaneous amounts of CO 2 molecules formed per surface unit of material, the X 0 parameter can act as a scaling factor.The K 0 kinetics factors result from first-order classical adsorption law.The parameters S 1 and S 2 are linked to the initial active sites (oxygen donors) and to the final re-activated sites, respectively, due to oxygen (air) action.The K 1 and K 2 parameters and p, q exponents result from specific Avrami models and are conditioned by the chemical processes of degradation or reconstructive growth.Surface degradation is conditioned by the presence of initial O * species reacting with adsorbed "gas" molecules in increasing number (because of the constant gas flow).The degradation mechanism requires interaction between one adsorbed molecule "gas" and one active site delivering a free O * species.This interaction is complex and could be divided into three main steps: (i) reduction step with formation of one oxygen vacancy and one O * free oxygen species, (ii) O * migration out of the solid, (iii) reaction between gas (CH 4 or CO) and O * giving rise to CO 2 .
In our calculations, we tried some adjustments of this parameter.Finally, the value p = 2 was found to be satisfactory.
Surface regeneration is associated with a first-order reaction between surface molecules [O * ] air surf (or oxygen O 2 from air) and the solid.To regenerate active sites, a lot of O * surface species (from air) are available on the solid surface: no substantial O * migration is required to interact with one active site.This is the reason why an exponent q = 1 might be expected for such a mechanism.We have fixed this value in our calculations.
As X 0 is a scale factor, five pertinent parameters result from our hypotheses.The K 0 constant is the adsorption parameter, the K 1 constant is the kinetics parameter associated with adsorption and reaction with one active site, and the K 2 constant is the corresponding kinetics parameter of oxygen regenerating one deactivated site.The S 1 and S 2 constants are proportional to the initial surface area of active sites and final surface area of reactivated sites, respectively.
Regarding the surface reactions, the active sites subjected to degradation (oxygen loss) can be considered as being regenerated by oxygen from air (oxidation), so the numbers of active and regenerated sites are assumed to be equal.Thus, as a first simplified hypothesis, we could assume that S 1 and S 2 should be equal.Therefore, the conversion rate can be expressed as follows: Apart from the X 0 parameter (scaling factor), each parameter could be dependent on the temperature.The parameters S 1 = S 2 depend on the nature of the surface and on the active site densities at a fixed temperature.For a short experimental time, the expression can be reduced to For long experimental time, the expression can be reduced to: The maximal values of I * observed for long reaction times can directly deliver the S 2 term, with X 0 being fixed to an arbitrary value.

Results
The first step of calculation consists in assimilating the I * max values (constant values reached after a long time of reaction) to X 0 • S 2 .Then, the various parameters have been adapted to obtain a good fit to the various experimental curves.
In Tables 1 and 2, we have reported the optimized parameters in the cases of air-CO and air-CH 4 gas flows interacting with La 2 O 3 , CeO 2 , Lu 2 O 3 , and Bi 2 O 3 .
Figures 3 and 4 give the experimental data and the simulated values of the catalytic efficiencies I(CO 2 ) of the (1 − x)CeO 2 -x/2Bi 2 O 3 (0 ≤ x ≤ 1) phases, respectively, interacting with air-CO and air-CH 4 flows (2500 ppm CO or CH 4 in air).
Figures 5(a  gases.It should be noted that the calculated curves (Figures 3  and 4) I * (t, T) fit well the experimental I * exp (t, T) data in the initiating periods of catalytic process.
The three K 0 , K 1 , and K 2 parameters vary with temperature (Tables 1 and 2): they are associated with the adsorption, degradation, and regeneration mechanisms.The S 2 parameter (S 1 = S 2 ) increases with temperature: it is characteristic of the surface of each solid, at specific temperatures and under partial pressures of oxygen from air.
In the case of abnormal behaviors observed during the initiating period, it has been possible to simulate such undulations or maxima.The K 1 parameter is strongly coupled to the K 0 one.It plays a very important role in the short time reactions.

Discussion
In the case of air-CO flow interacting with the present oxides, the S 2 (and S 1 = S 2 ) parameters are characteristic of long time behaviors (after initiating period) for all samples and each temperature.Their values directly depend on the choice of unique scale factor X 0 , identical in each experiment.They systematically increase with temperature, and their activation energies can be representative of thermal equilibrium at the solid surfaces.The highest values of S 2 regeneration parameters (Tables 1 and 2) are observed for Bi 2 O 3 and Lu 2 O 3 .
The K 0 parameters (Tables 1 and 2) characterize the progressive adsorption of gases and behave diversely as temperature increases.In the case of ceria interacting with air-CO gas, we observe small increasing values; in the case of La 2 O 3 , they strongly decrease; in the case of Lu 2 O 3 , they slowly decrease.Finally, in the case of Bi 2 O 3 , the K 0 values irregularly vary.
The K 1 parameters are correlated with the K 0 parameters, and they play an important role in the initiating and intermediate periods, before the I max values have been reached.They allow good fitting of undulations observed on several curves (for intermediate time) even if this observation is not systematic.These K 1 parameters are relatively weak in the case of CeO In the case of air-CH 4 flow interacting with the present oxides, all calculated I * curves fit well the experimental I * data.The K 0 and K 1 parameters increase with temperature.The S 2 = S 1 parameters all increase with temperature.The highest values are observed for Lu 2 O 3 sample.These evolutions of parameters are probably conditioned by the specific high stability of methane in presence of oxygen and by a complex decomposition of CH 4 giving rise to two molecules CO 2 and H 2 O.

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To better interpret the abnormal evolutions observed in the case of Bi 2 O 3 oxide (Figure 3(d)), with a maximum of I(t, T) during the intermediate period, at 200 and 225 • C, it is possible to invoke the presence of oxygen, carbonate, or hydroxide species, initially adsorbed on grain surfaces of solids, and favor additional oxidation of CO.Such adsorbed species are generally eliminated between 150 and 250 • C.After thermal elimination of these surface species (presently for T > 175 • C), the oxidation process is governed firstly by the oxygen molecules present in gas flows and then by oxygen species delivered by the solids.In the case of methane, as oxidation occurs above 400 • C and desorption of species occurs below 400 • C, the elimination of such surface species cannot be observed.

Conclusions
The Johnson-Avrami Mehl models allowed us to determine pertinent parameters characteristic of the porous media and of the two kinds of gas-solid interactions involving methane or carbon monoxide gases.The different time-dependent oxidation processes characterized by at least three steps have been well described from these parameters: (i) the initiating period is governed by the K 0 parameter, (ii) the intermediate period is governed by both K 0 and K 1 parameters, and (iii) the stabilization period is governed by the S 2 parameter.This last parameter represents the number of active sites at the internal solid surfaces and conditions the catalytic efficiencies for long reaction time: it is directly related to regeneration of solid surfaces via oxygen from air.The K 0 and K 1 parameters represent successively the adsorption kinetics of CH 4 or CO molecules and the reduction kinetics of active sites at the solid surfaces.They condition the initiating period of oxidation reaction (ranging between 10 and 20 minutes in the present experiments).It should be interesting to note that the K 1 parameter can be used to interpret the experimental undulations of the intensity I * , just before this intensity reaches a constant value.Both K 0 and K 1 parameters might be directly linked to the presence or not of adsorbed species on the surfaces of polycrystalline solids.The K 2 parameter plays a moderate role in fitting calculated curves to experimental data.

Figure 2 :
Figure 2: FTIR spectroscopy of emitted gases.Transmittances without any catalytic sample and in presence of a catalyst Lu 2 O 3 : (a) CH 4 converted into CO 2 and H 2 O and (b) CO converted into CO 2 .

Figure 3 :
Figure 3: Simulation of CO oxidation efficiencies in the case of air-CO flows (2500 ppm CO) interacting with: (a) CeO 2 ; (b) La 2 O 3 ; (c) Lu 2 O 3 ; (d) Bi 2 O 3 as a function of time and for various temperatures.Experimental data: dotted line; model data: continuous line.

Figure 4 :
Figure 4: Simulation of CH 4 oxidation efficiencies in the case of air-CH 4 flows (2500 ppm CH 4 ) interacting with: (a) CeO 2 ; (b) La 2 O 3 ; (c) Lu 2 O 3 ; (d) Bi 2 O 3 as a function of time and for various temperatures.Experimental data: dotted line; model data: continuous line.

Figure 5 :
Figure 5: (a) Evolution of S 1 = S 2 parameter as a function of temperature in the case of air-CO flows interacting with La 2 O 3 , CeO 2 , Lu 2 O 3 , and Bi 2 O 3 .(b) Evolution of S 1 = S 2 parameter as a function of temperature in the case of air-CH 4 flows interacting with La 2 O 3 , CeO 2 , Lu 2 O 3 , and Bi 2 O 3 .
2 and Bi 2 O 3 .They have large values in the case of La 2 O 3 and Lu 2 O 3 .This suggests different degradation processes associated with reduction of the solid surfaces.The K 2 parameters characterize the oxidation capacity of the solid surfaces due to oxygen from air.They present two types of similar values, firstly in the case of CeO 2 and Bi 2 O 3 , and secondly in the case of Lu 2 O 3 and Bi 2 O 3 .
The conversion reactions are analyzed from the infrared absorption band intensities of CO 2

Table 1 :
Simulation parameters for solids interacting with air-CO gas flows at various temperatures.

Table 2 :
Simulation parameters for solids interacting with air-CH 4 gas flows at various temperatures.La 2 O 3 , CeO 2 , Lu 2 O 3 , and Bi 2 O 3 interacting with air-CH 4 gas flows La 2 O 3