Lanthanum based high surface area perovskite-type oxide and application in CO and propane combustion

The perovskite-type oxides using transition metals present a promising potential as catalysts in total oxidation reaction. The present work investigates the effect of synthesis by oxidant co-precipitation on the catalytic activity of perovskite-type oxides LaBO3 (B= Co, Ni, Mn) in total oxidation of propane and CO. The perovskite-type oxides were characterized by means of X-ray diffraction, nitrogen adsorption (BET method), thermo gravimetric and differential thermal analysis (ATG-DTA) and X-ray photoelectron spectroscopy (XPS). Through a method involving the oxidant co-precipitation it’s possible to obtain catalysts with different BET surface areas, of 33-44 m2/g, according the salts of metal used. The characterization results proved that catalysts have a perovskite phase as well as lanthanum oxide, except LaMnO3, that presents a cationic vacancies and generation for known oxygen excess. The results of catalytic test showed that all oxides have a specific catalytic activity for total oxidation of CO and propane even though the temperatures for total conversion change for each transition metal and substance to be oxidized.


Introduction
The removal of CO, unburned hydrocarbons (HC), and NO from automotive exhaust requires catalytic devices in which these pollutants are eliminated.Catalytic combustion offers one of the most efficient means for controlling atmospheric pollution (1).Although noble metals, such as palladium, platinum, and Rhodium are well known with higher activity (per site than metal-oxide catalysts, they present several disadvantages, such as higher volatility, high cost, and poor availability.Compared with noble metals, base metal catalysts present a lower but still sufficient activity as oxidation catalysts, and have the advantages of lower costs and the potential market in energy generation systems in domestic and small scale industrial applications.For this reason perovskite type compounds have received wide attention, which have been incorporated into the design of the novel combustors (2,3).
A perovskite-type oxide has an ABO 3 type crystal structure wherein cations with a large ionic radius have twelve coordination to oxygen atoms and occupy A-sites, and occupy B sites.A and O form a cubic closest packing, and B is contained in the octahedral voids in the packing.If the ionic radii are r A , r B and r O , to form a perovskite crystal structure, the tolerance factor (t) = (r A + r O )/ (r B + r O ) must lie within the range 0.8 < t < 1.0, and r A > 0.090 nm, r B > 0.051 nm.A great many elements can form ideal or modified perovskites depending on the tolerance factor.The ideal perovskites structure appears only in a few cases for tolerance factors very close to 1 and at right temperatures In others conditions different distortions of the perovskites structure will appear.Deviations from the ideal structure with orthorhombic, rhombohedral, tetragonal, monoclinic and triclinic symmetry are known, although the latter three are scarce and poorly characterized [4].Nonstoichiometry in perovskites can arise from cation deficiency (in the A or B site), anion deficiency or anion excess.Much attention has particularly been paid to lanthanum -transition metal based perovskites oxides, which were introduced into catalysis some 30 years ago (5,6).but they are available for perovskites in which the metal in position A and/or B has been partly substituted with another metal, thus establishing a large group of active oxides described by the general formula A 1-x A´xB 1-y B´yO 3±d , where d stands for excess oxygen resulting from non-stoichiometry of these species [7].
Lanthanum-based perovskites containing transition metal in B-site, (LaBO 3 , B = Co, Fe, Ni or Mn), show catalytic activity close to the noble metal, presenting low cost and high thermal stability [8].The perovskites type oxides are of interest for catalytic oxidation and reduction reactions associated with automotive exhaust emission control [9].They have been tested in oxidation of CO and hydrocarbons, NOx reduction and hydrogenation and hydrogenolysis reactions [7].
In this work, the effect of oxidant -coprecipitation synthesis on the catalytic activity of perovskite-type oxides LaBO 3 (B= Co, Ni, Mn) is investigated, using as a model the total oxidation of propane and CO.

Experimental
A number of methods have been used in the synthesis of perovskites; the choice of a particular one depends mostly on the expected use for these oxides.The LaCoO 3 , LaMnO 3 and LaNiO 3 perovskites oxides were prepared by oxidant -coprecipitation.The catalysts were pre-pared using 20 mmol of aqueous solution (100 mL) of metal's salts (Co(NO 3 ) 2 .6H 2 O to LaCoO 3 , MnCl 2 .4H 2 O to LaMnO 3 and Ni(NO 3 ) 2 .6H 2 O to LaNiO 3 ) mixed with 100 mL of aqueous solution of lanthanum nitrate (20 mmol).The mixture was added drop wise to a rapidly stirred (nitrogen stream) solution of sodium hydroxide (7.2g 0.18 mol) in sodium hypochlorite (3.5 M. 30ml).Use of inert atmosphere minimizes carbonate contamination in the final product.The resultant gel was washed repeatedly with deionized water (3x200 ml), then washed with acetone (3x 50ml) and dried under vacuum.
The powder was dried at 60 ºC for 12h, after that it was calcinated at 600ºC for 2 hours.
The catalysts were characterized by X-ray powder diffraction (XRD) on a 65-Rich Seifert & Co diffractometer by using CuKa radiation for determination of the final structure.Patterns were recorded between a 2q of 20º and 65º with step size of 0.05º with 30kV and 40mA.
The specific surface areas were obtained by nitrogen adsorption at 77 K, evaluated using the BET equation on an automatic Quantachrome Aparatus, Autosorb -1 C, in the 0, 05-0,095 relative pressure range.The powders were degassed at 300ºC for 1h before the measurements.
The thermal decomposition with the temperature of the powder before heating at 60°C for 12h and passed by a sieve of 200 mesh was performed in N 2 by using a TGA-DTA of Schimadzu and TA Instruments, under N 2 flow (50 m l min -1 ), and 10 0 C min -1 from room temperature to 600ºC.
The X-ray photoelectron spectra (XPS) were obtained with a VGESCALAB MKII, employing Mg Ka radiation (1253.6 eV) under a high vacuum of 3x10 -8 Torr.The binding energies were calibrated by using a C1s peak (284.6 eV) as a background.
Temperature programmed reduction (TPR) experiments were performed in a system with a thermal conductivity detector.The reducing gas was a mixture of 3% H 2 /N 2 (1.8 l/h) and heating 10°C/min was employed.
The catalysts were tested in combustion of propane and CO.The catalytic measurements were realized in a fixed-bed reactor, working in atmospheric pressure.For characterization of the catalytic activity, it was used 10mg of powder.
Before the reaction, the powder is pretreated for 1.5h at 400ºC with synthetic air: O 2 (20±0.5)%and N 2 (80±0.5)%,1.8 l/h.The feed gas composition was CO (0.97%), C 3 H 8 (0.507%), O 2 (2.02%) and N 2 (balance), 1.8 l/h, on-line continuous analysis of reactants and products was performed using a gas chromatograph CG 9001 Finnigan, provided with a thermal conductivity detector.For CO and CO 2 analyses were utilized an independent ND-IR detector.

Results and discussion
The method employed was oxidant-coprecipitacion, developed by Barnard et al [10] to LaCoO 3 catalyst synthesis.
The catalyst preparation in basic conditions, due to the presence the sodium hydroxide and sodium hypochlorite as oxidant agent, yields probably LaBOH (B= Co, Mn, Ni).This was reported by Vydiasagar et al [11] as the hydroxigel to be dehydrated.
The advantage in this method is that the dehydratation of the gel occurs at low temperatures (60°C), due to the initial dehydratation in the presence of acetone.
Controlled dehydratation of hydroxi-gel, using water and a miscible solvent, presents positive results in obtain solids with high specific surface areas.The displacement of water by acetone allows a reduction of superficial and leads a smallest capillary force; producing weak agglomeration and keeping opened the particle structure.The result is a lowest collapse during calcination [12].
Figure 1a, 1b and 1c display the TGA and DTA curves of respective catalysts.It can be observed that the samples present endothermic peaks at low temperatures, corresponding to the fisissorved water elimination.
The strong mass loss region from 300 and 500°C was attributed to the decarbonization and dehydratation, process accompanying the perovskite structure formation.DTA curves of these samples exhibit essentially only endothermic peak the range, centered at 318 and 440 0 C for LaCoO 3 , 340 and 420 0 C for LaMnO 3 , 318 and 470 0 C for LaNiO 3 .This different behavior probably due to the different decomposition mechanism of hydroxide gels.In LaNiO 3 catalyst, the carbonate quantity is bigger, when compared to another catalyst, due to the catalyst has been stayed more time during the filtration process (72 h), once fine grain was formed, difficulting the filtration process.The oxides at low temperatures present typical structure of perovskite (Figure 2), what can be confirmed comparing with standard archives JCPDS and literature [13][14][15].The catalyst LaNiO 3 and LaCoO 3 presented rhombohedrical structure and the LaMnO 3 presented orthorhombic structure.In the catalysts LaCoO 3 and LaNiO 3 there's a peak represented by (*), referring to lanthanum oxide (La 2 O 3 ), indicating a cationic excess.In the LaNiO 3 exists a peak (¨), which confirms the presence of NiO, and was reported by others authors [16].The catalyst LaMnO 3 doesn't present a La 2 O 3 peak; it is possible that this oxide presents cationic vacancies coming with an excess of negative species of oxygen.This fact can originate a spontaneous transition of 15-30% of Mn 3+ to Mn 4+ to reestablish the eletroneutrality of system [17].
data indicate that lanthanum is probably present in trivalent form.
The sign in 779.9 and 795.5 eV corresponds to the Co 2p 3/2 and Co 2p 1/2 respectively, which is typical when Co 3+ is present.On the other hand, the absence of peaks in 785 -788 eV proves the inexistence of Co 2+ , and then the theory about the existence of cations in trivalent form.
The XPS spectra of LaCoO 3 , present two O 1s photolines that correspond to two different oxygen species.The lower binding energy, 529.8 eV, can be associated with lattice, while the higher energy, 532.5 eV, can be associated with the species of absorbed oxygen, although this latter fraction should also contain adsorbed oxygen.The amount of oxygen absorbed-adsorbed can be represented by the species O -and OH - [16].
In the literature [20], the binding energy corresponding to the oxygen species appear in 528.3 and 531.9 eV.For the species OH -, these values are 529.7 and 532.2 eV, respectively.The similarity between OH -binding energies results in that the values presented in this work indicate a higher possibility of this species to exist.
In the catalyst LaMnO 3 the absence of peak in high energy value indicates the presence only of the lattice oxygen.Alterations in the crystalline structure and also in the electronic structure can be responsible by displacement of the peak for higher energy values.The surface characterization was realized for XPS analysis using C1s as reference.Table 1 contains binding energies values: O 1s, La 3d, Co 2p, Mn 2p and Ni 2p.Several works demonstrate that the majority of cations are present in trivalent form [18,19].
The analysis date for LaCoO 3 and references to the La 3d 5/2 are registrated in 835.3 e 838.8 eV.These values are similar to that presented in the literature [20] for the pure lanthanum oxide (La 2 O 3 ) in 834.4 e 837.8 eV.These Table 1.XPS results to perovskite type-oxides.The Mn 2p spectra presents three signals which corresponds to Mn 3+ (647.40 eV), Mn 4+ (658.7 eV) and the presence of a peak, indicating the absence of Mn 2+ .
The information obtained by XPS is important to compare the stoichiometry of compounds (Table 1).The atomic relation La/Co on surface is 1.8.This suggests a lanthanum excess or Co-deficiency on LaCoO 3 surface.Some authors have observed the presence of lanthanum excess in the perovskite structure (La/Co >1).Tejuca et al. [19] reported the relation La/Co is 1.2 in the catalyst LaCoO 3 , obtained by acetate synthesis.The more pronounciated case was observed by O'Connell et al. [20], which prepared the catalyst LaCoO 3 through precipitation of nitrate solutions of salts, with thetraethilamonium hydroxide.In this case the relation La/Co was 4.26, showing the lanthanum excess on surface.This excess on surface catalyst is attributed to two phases formation; the first corresponding to the pure perovskite and the second for La/perovskite.In the first, the cationic vacancies are compensated by high valences of oxygen and cobalt.In the second, the cationic vacancies are compensated by anionic vacancies, being more interesting for catalysis.
It has been reported by others authors that the LaCoO 3 is considered as a stoichiometric system.On the other hand, this system was showed as non-stoichiometric, as reported by Spinicci et al [21].In the non-stoichiometric system, generally there is a deficiency of net lanthanum, which can be compensated, by increasing the metal valence or by oxygen or by creation of anionic vacancies.
In this work, it can be observed that the lanthanum ions quantity is almost twice of cobalt ions.It is possible to suppose the formation of stoichiometric LaCoO 3 with other lanthanumbased compounds, like lanthanum oxide, proved by X-ray diffraction.
In LaMnO 3 catalyst, the relation La/Mn is 0.79, suggesting the non-stoichiometric lanthanum manganite phase with lanthanum deficiency.To keep the charge equilibrium, the conversion can be considered: Mn 3+ ® Mn 4+ .This transition comes with vacancies formation and presence of oxygen excess.Taguchi et al. [22] found the relation La/Mn changing from 1.5 to 2, depending on method used to obtain LaMnO 3 .However, part of lanthanum isn't incorporated in perovskite, but presented as oxide, and part of this oxide is carbonated.
The peaks of lanthanum and nickel are located in the same energy range (835-855 eV).There is a overlapping for La3d 3/2 and Ni2p 3/2 .The peak near 870 eV is characteristic for Ni 2+ in NiO, that was reported by Guo et al [23].
The atomic relation La/Ni on surface is near 1.6, which suggests the excess of lanthanum or deficiency of nickel on oxide surface of LaNiO 3 .In this case, the lanthanum excess is the more probable factor, due to X-ray diffraction, that shows both the nickel oxide and the perovskite phases.
Due to this quantity of positive ions in the structure, there is an oxygen ion excess, and part of this oxygen is absorbed in the structure.
The relation between oxygen and total metallic ion [O/(La + B)] in the catalyst LaCoO 3 is near to 3.4, according to the result obtained by O'Connell et al [20].In the catalyst LaMnO 3 this relation is near 2.5, both higher than that would be expected if the stoichiometry exists.This proved the materials can present oxygen excess in case of LaCoO 3; for LaMnO 3 there are cationic vacancies.By the way, there is a catalyst LaCoO 3 with cobalt deficiency and LaMnO 3 with manganese deficiency.However, the majority of non-stoichiometric oxygen in LaCoO 3 catalyst, when compared to LaMnO 3 , can indicate the major cationic vacancies in first case.In the LaMnO 3 , vacancies are minimized by transition of Mn 3+ ® Mn 4+ .This relation for LaNiO 3 is 2.5, which confirms the oxygen excess.
To obtain information about reducibility of studied catalyst and to identify the reduction temperature, it was realized a temperature-programmed reduction.The profile is showed on figure 3.
In the figure 3a, which refers to LaCoO 3 , according to Guo et al. [23], it is observed the catalyst reduction in two steps: in the first, the Co 3+ to Co 2+ reduction; in the second, Co 2+ to Co 0 .
In the case of LaMnO 3 catalyst, figure 3b, it is observed a peak due to the d-oxygen, non-stoichiometric reduction and, consequently, reduction of Mn +3 , as reported by Tejuca et al. [24].In figure 3c, which refers to LaNiO 3 , it is observed a small shoulder near 451 °C, corresponding to La 2 Ni 2 O 5 formation, which results of Ni 3+ to Ni 2+ reduction, and the second peak (600 °C), to Ni 2+ to Ni 0 reduction.This transformation, reported by Lima & Assaf [11], is according reaction bellow: This shoulder indicates both the NiO presence and the perovskite phase.Through an X-ray diffraction it was possible to identify the NiO phase.Furthermore, the first peak corresponds simultaneously to the Ni 3+ to Ni 2+ reduction in the perovskite structure, and Ni 2+ to Ni 0 reduction of the nickel oxide contained in the sample.The second one corresponds to the Ni 2+ to Ni 0 reduction of La 2 Ni 2 O 5 , according to equation 5.
The conversion results were determined according to the reactions: The CO oxidation (Figure 4) occurs at lower temperatures, when compared to propane.According values to CO conversion it was observed that the more efficient catalyst for this reaction was LaCoO 3 , that started the conversion at 130ºC and presented a maximum under 200ºC, while LaMnO 3 presented this maximum near 300ºC.LaNiO 3 started the conversion near 300ºC that is a high temperature when compared to the others catalysts.The good conversion is mentioned to the LaCoO 3 by Shu & Kaliaguine [13].
The results to propane oxidation are presented in figure 5.It shows that conversion grows up according to the temperature for all catalysts.The perovskites LaCoO 3 and LaMnO 3 presented the better conversion than LaNiO 3 .
There are many discussions about perovskites activity.The more commons are: Oxidation states of transition metal ion, the amount of non-stoichiometric oxygen, the deviation of the lattice, it is observed in literature that the non-stoichiometric oxygen can be a considerable factor for interpretation of catalytic activity in a perovskite series of transition metals [19].Therefore, the net chemical structure in the perovskite is very complex, due hexagonal, rhombohedral or tetragonal distortions.These deviations can result in a cationic or anionic non stoichiometry.
For lanthanum manganite, the transition Mn 3+ à Mn 4+ is accompanied by cationic vacancies generations and for known oxygen excess related to necessary stoichiometry.

Conclusions
We have examined co-precipitation of a stoichiometric Co/La, Mn/La and Ni/La mixed  hydroxide, in which simultaneous oxidation of Co (II), Mn (III) and Ni (II) was induced by co -addition of an oxidizing agent.High-area samples can be obtained by this method only if the hydroxide gel is dehydrated in a controlled manner by washing with a water-miscible nonaqueos solvent.Subsequent calcinations afforded a crystalline LaCoO 3 , LaMnO 3 and LaNiO 3 powder with perovskite structure and a surface area determined by the calcination temperature.
It was verified that the LaCoO 3 catalyst presented more efficiency to oxidation reactions when compared to LaMnO 3 and LaNiO 3. The LaCoO 3 presented the highest specific surface.

Figure 4 .
Figure 4. Conversion of perovskite type oxide for CO oxidation.

Figure 5 .
Figure 5. Conversion of perovskite type oxide for propane oxidation.