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ISRN Materials Science
Volume 2012 (2012), Article ID 804763, 9 pages
Time-Dependent Oxidative Capacities of , , , and Materials Interacting with Air-CO or Air- Flows
1Institut Matériaux Microélectronique et Nanosciences de Provence (IM2NP), UMR CNRS 6242, Université du Sud Toulon-Var, BP 20132, 83957 La Garde Cedex, France
2Laboratoire Matériaux et Environnement (LME), Faculté des Sciences, Université Ibn Zohr, BP 32/S, Cité Dakhla, Agadir 8106, Morocco
Received 22 August 2012; Accepted 29 September 2012
Academic Editors: I. Imae, G. C. Mather, and R. A. Varin
Copyright © 2012 Bahcine Bakiz et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
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 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.
In the general framework of catalytic gas sensing or depolluting microsystems development, we study the time-dependent oxidation capacities of polycrystalline materials interacting with air-CO or air-CH4 gas flows. The main objective is to convert toxic CO or undesired CH4 gases into CO2 gas. To follow the reaction processes, we make use of Fourier transform infrared spectroscopy and determine the time- and temperature- dependent intensities of CO2 vibrational bands resulting from the oxidation processes of CH4 or CO in air-CH4 or air-CO gas flows. In this study, we determine the catalytic efficiencies of La2O3, Lu2O3, CeO2, and Bi2O3 polycrystalline oxides as a function of reaction time, at various temperatures: the air-CH4 or air-CO flows cross through porous polycrystalline walls of these rare earth-based oxides and a system based on the Bi2O3 phase .
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 [2–5] or semiconducting material [6–8]. The catalytic properties of ceria have been ascribed to the capacity of cerium to change its valence from Ce4+ to Ce3+ and thus to oxidize or reduce gases. In the case of lanthanum oxide (lantana), catalytic behaviours have been studied in the past . This oxide is used in the conversion of CH4 into syngas (H2 + CO) . 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 [11–14]. 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 . 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 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 Bi2O3-CeO2 , 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-CH4 gas flows.
In these experimental studies , the evolution versus time of oxidation capacities (CH4 or CO being converted into CO2) presents a common behavior, characterized by at least three periods: initiating, intermediate, and stabilization periods. Some samples present abnormal maxima of 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 time-dependent evolutions of the FTIR intensities using elemental Avrami models [16–18]. The modeling approach is applied to La2O3, CeO2, Lu2O3, and Bi2O3 phases.
2. Experimental Section
The various samples were synthesized via specific routes previously described by us [1, 19–22] and based on wet chemistry processes involving nitrate-based solutions. The ceria phase was elaborated in nanocrystalline form at 200°C. The La2O3 and Lu2O3 were obtained after final thermal treatment at 750°C. The Bi2O3 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)  analyses were carried out to determine the sample-specific surface areas noted as in m2/g. This method delivers the effective surface exposed to gas adsorption. In the case of absence of agglomerations, the values can be related to individual grain dimensions. The values were found to be 11.5 m2/g for La2O3, 82.0 m2/g for CeO2, 5.0 m2/g for Lu2O3, and 2.0 m2/g for Bi2O3.
2.2. FTIR Spectroscopy and Reactivity Analyses
The rare earth or bismuth-based materials were exposed to air-CH4 and air-CO gas flows in a homemade cylindrical reaction cell (see Figure 1). The transformation of CH4 or CO into CO2 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 was used in each experiment.
All experimental details have been extensively described in previous works. The conversion reactions are analyzed from the infrared absorption band intensities of CO2 (doublet at 2340–2360 cm−1), resulting from one of the overall reactions: The conversion intensity was determined from the measurements of CO2 absorption bands, at a certain time t of the gas/solid interaction. Each vibrational spectrum was recorded over a period of with intervals of 30 s between two spectra. The total exposure time was two hours. For a given total time of reaction and a given temperature , the intensity was assumed to be proportional to the amount of CO2 molecules formed during the time of FTIR record. All experiments were characterized by a first initiating regime in which the CO2 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 CH4 and CO after solid gas interactions. The CO2 FTIR band intensity increases as the CH4 or CO band intensities decrease.
To compare the various reactivities, we have normalized the curves using the specific surfaces by calculating the values . 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-CH4 flows in order to avoid direct oxidation of CO or CH4 by oxygen (this direct oxidation was tested in our experimental device in the absence of active sample).
3. Modeling Approaches
In our previous work , we proposed a semiempirical model based on typical catalytic steps summarized as follows:(i)surface adsorption of gas and O2 (air) on the surface of the solid; (ii)surface reaction CH4 + 2O2 CO2 + 2H2O or CO + O2 CO2, with reduction of catalyst surface, followed by CO2 desorption; (iii)regeneration of the solid (with oxidation of surfaces) due to adsorbed oxygen from the air; (iv)desorption of gases.
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 (CO2) 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 of one active site , delivering one reduced site “”: or (i)Formation of mobile oxygen species from adsorbed dioxygen from air.(ii)Regeneration (oxidation) of reduced sites “”: or
The general expression delivering the CO2 vibrational band intensity is assumed to result from sites delivering oxygen and sites regenerated from oxygen from air : In this expression, the symbol [CO2] designates the total amount of CO2 molecules formed after a reaction time 0074. The derivative is proportional to the instantaneous amount recorded at time , during the period . The term describes the adsorption of gases on the solid surfaces, describes the reaction between oxidative active sites and reducing gas molecules, and describes the site regeneration due to oxygen from air adsorbed at the solid surface. Each term was previously expressed using three distinct Avrami’s approaches. The final general expression adapted to the normalized data was as follows: In this expression, the parameter is proportional to the composition of the air-gas flow. This composition (2500 ppm in air) was fixed for all experiments. As the normalized values characterize the instantaneous amounts of molecules formed per surface unit of material, the parameter can act as a scaling factor. The kinetics factors result from first-order classical adsorption law. The parameters and are linked to the initial active sites (oxygen donors) and to the final re-activated sites, respectively, due to oxygen (air) action. The and parameters and , 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 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 species. This interaction is complex and could be divided into three main steps: (i)reduction step with formation of one oxygen vacancy and one free oxygen species, (ii) migration out of the solid,(iii)reaction between gas (CH4 or CO) and giving rise to CO2.
In our calculations, we tried some adjustments of this parameter. Finally, the value was found to be satisfactory.
Surface regeneration is associated with a first-order reaction between surface molecules (or oxygen O2 from air) and the solid. To regenerate active sites, a lot of surface species (from air) are available on the solid surface: no substantial migration is required to interact with one active site. This is the reason why an exponent might be expected for such a mechanism. We have fixed this value in our calculations.
As is a scale factor, five pertinent parameters result from our hypotheses. The constant is the adsorption parameter, the constant is the kinetics parameter associated with adsorption and reaction with one active site, and the constant is the corresponding kinetics parameter of oxygen regenerating one deactivated site. The and 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 and should be equal. Therefore, the conversion rate can be expressed as follows: Apart from the parameter (scaling factor), each parameter could be dependent on the temperature. The parameters 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 observed for long reaction times can directly deliver the term, with being fixed to an arbitrary value.
The first step of calculation consists in assimilating the values (constant values reached after a long time of reaction) to . Then, the various parameters have been adapted to obtain a good fit to the various experimental curves.
Figures 5(a) and 5(b) compare the parameters as a function of temperatures in the case of air-CO and air-CH4 gases. It should be noted that the calculated curves (Figures 3 and 4) fit well the experimental data in the initiating periods of catalytic process.
The three , , and parameters vary with temperature (Tables 1 and 2): they are associated with the adsorption, degradation, and regeneration mechanisms. The parameter 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 parameter is strongly coupled to the one. It plays a very important role in the short time reactions.
In the case of air-CO flow interacting with the present oxides, the (and ) 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 , 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 regeneration parameters (Tables 1 and 2) are observed for Bi2O3 and Lu2O3.
The 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 La2O3, they strongly decrease; in the case of Lu2O3, they slowly decrease. Finally, in the case of Bi2O3, the values irregularly vary.
The parameters are correlated with the parameters, and they play an important role in the initiating and intermediate periods, before the 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 parameters are relatively weak in the case of CeO2 and Bi2O3. They have large values in the case of La2O3 and Lu2O3. This suggests different degradation processes associated with reduction of the solid surfaces. The 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 CeO2 and Bi2O3, and secondly in the case of Lu2O3 and Bi2O3.
In the case of air-CH4 flow interacting with the present oxides, all calculated curves fit well the experimental data. The and parameters increase with temperature. The parameters all increase with temperature. The highest values are observed for Lu2O3 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 CH4 giving rise to two molecules CO2 and H2O.
To better interpret the abnormal evolutions observed in the case of Bi2O3 oxide (Figure 3(d)), with a maximum of 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 ), 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.
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 parameter, (ii) the intermediate period is governed by both and parameters, and (iii) the stabilization period is governed by the 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 and parameters represent successively the adsorption kinetics of CH4 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 parameter can be used to interpret the experimental undulations of the intensity , just before this intensity reaches a constant value. Both and parameters might be directly linked to the presence or not of adsorbed species on the surfaces of polycrystalline solids. The parameter plays a moderate role in fitting calculated curves to experimental data.
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 work was developed in the general framework of ARCUS CERES project (2008–2011).
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