Key factors in Sr-doped LaBO3 (B = Co or Mn) perovskites for NO oxidation in efficient diesel exhaust purification

1 Jon A. Onrubia, B. Pereda-Ayo, U. De-La-Torre, Juan R. González-Velasco* Departamento de Ingeniería Química, Facultad de Ciencia y Tecnología, Universidad del País Vasco, UPV/EHU, Campus de Leioa, P. O. Box 644, ES-48080 Bilbao, Bizkaia, Spain KEYWORDS: perovskite, LaCoO3, LaMnO3, Sr-doping, surface oxygen vacancies, NO oxidation, Pt-free catalyst *Corresponding author: [email protected] This is the accepted manuscript of the article that appeared in final form in Applied Catalysis B: Environmental 213 :198–210 (2017), which has been published in final form at https://doi.org/10.1016/j.apcatb.2017.04.068. © 2017 Elsevier under CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/) Key factors in Sr-doped LaBO3 (B = Co or Mn) perovskites for NO oxidation in efficient diesel exhaust purification 2 ABSTRACT Perovskites have attracted attention in recent years as an economic alternative to noble metals in oxidation processes. Synthesis conditions of LaCoO3 and LaMnO3 perovskites have been studied varying citrate to nitrate molar ratio in the starting solution, pH and calcination protocol, with the aim of obtaining high purity perovskites, absence of impurities, and with enhanced textural properties. Once synthesis conditions were established, strontium was incorporated in the perovskite lattice by substituting lanthanum with different doping levels, i.e. La0.9Sr0.1BO3, La0.8Sr0.2BO3, La0.7Sr0.3BO3, La0.6Sr0.4BO3 and La0.5Sr0.5BO3 with B=Co or Mn. The prepared solids were characterized in terms of crystalline phase identification (XRD), specific surface area (N2 adsorption-desorption at -196 ˚C), reducibility and oxidation state of transition metal ions (H2-TPR), quantification of adsorbed oxygen species (O2-TPD) and surface elemental composition (XPS). Charge imbalance associated to strontium (Sr2+) incorporation in the perovskite lattice in substitution of lanthanum (La3+) was preferentially balanced by Mn4+ promotion in La1-xSrxMnO3 perovskites, whereas formation of oxygen vacancies seems to be the mechanism for charge compensation in La1-xSrxCoO3 perovskites, where Co ions remained as Co3+ ions. Strontium doped perovskites further improved NO conversion compared to the non-substituted formulations. The best NO oxidation performance was obtained with La0.7Sr0.3CoO3 and La0.9Sr0.1MnO3 samples, achieving maximum NO conversion of 83 and 63% at 300 and 325 C, respectively. Higher oxidation capacity of La0.7Sr0.3CoO3 sample was associated to the higher oxygen mobility and exchange capacity between oxygen in the lattice and gas phase oxygen. It is worth noting that prepared perovskites presented far higher NO oxidation capacity than platinum-based NSR model catalysts, confirming perovskites as an economic alternative to catalyze NO oxidation reactions in automotive catalysis. 3 1. INTRODUCTION Diesel engines present higher fuel efficiency than stoichiometric gasoline engines and thereby emit less CO2 to the atmosphere. In contrast, the clean-up of diesel exhaust gases is even more challenging than gasoline engines exhaust gases, due to the nature of the emission, including soot, and also due to the net oxidizing environment that limits NOx reduction [1] In order to meet stringent emission standards, diesel engines implemented catalytic processes such as Diesel Oxidation Catalyst (DOC) [2] or Diesel Particulate Filter (DPF) [3] In the DOC, unburned hydrocarbons as well as carbon monoxide are completely oxidized to CO2 and H2O while NO is converted into NO2. Afterwards, soot is trapped in DPF and continuously regenerated by oxidation with NO2 produced in the DOC. However, this strategy fails in meeting EURO VI standards regarding NOx emissions, and consequently additional catalytic strategies are mandatory to be implemented, such as NOx Storage and Reduction (NSR) [4] or Selective Catalytic Reduction (SCR) [5] In the NSR technology, NOto-NO2 oxidation is considered a primary step to promote NOx storage via nitrites or nitrates [6, 7] formed over an alkali or alkali-earth material, because NO2 adsorption occurs much faster than NO [4] In the SCR technology NO-to-NO2 oxidation is also of critical importance [8] being well known that the fast SCR reaction activates when NO/NO2=1/1 (molar ratio), achieving high NOx conversion even at lower temperature. Thus, the significance of NO-toNO2 oxidation reaction in automotive exhaust purification is relevant and merits further research on novel catalyst formulations. Noble metal based catalysts have been commonly employed in order to promote NO oxidation in automotive catalysis. However, the use of noble metal (especially Pt) results in high cost and poor thermal stability under highly oxidative conditions [9] . In that sense, perovskites have attracted attention in recent years as potential, low-cost alternative to noble metal in oxidation processes [10] In the perovskite lattice (with the general formula ABO3), 4 the B cation coordinates with oxygen in octahedral structure, and A cation locates in the center of the dodecahedral structure. Perovskites activity for oxidation reactions seems to be related with some specific structural properties, such as change of oxidation state of B cation, active oxygen mobility and ion vacancy defect [11] During oxidation reactions, lattice oxygen is thought to be catalytically active and its consumption and regeneration is relative to cycling the oxidation state of neighboring transition metal ions (B+3  B+4) and/or (B+2  B+3) [12] In that sense, it has been reported that the catalytic activity of perovskites is far influenced by the type of B cation and its oxidation state cycling feasibility [13] LaMnO3 and LaCoO3 perovskites have been proposed as active perovskites for NO oxidation reactions [11, 14] . In particular, LaMnO3 perovskite in which Mn3+/Mn4+ mixed oxidation state is usually observed even for nominal stoichiometry LaMnO3+ (with Mn4+ content of 2 per formula unit), opposite to most other perovskites. However, it has been proposed that electroneutrality of the lattice is accomplished by generating cation vacancies, instead of oxygen over-stoichiometry, as perovskite lattice cannot accommodate interstitial oxygen ions [15, 16] . On the other hand, stoichiometric LaCoO3 perovskite usually presents some oxygen deficiency in the lattice, represented by LaCoO3-, and the charge balance is accomplished by the presence of some cobalt as Co2+. This Mn3+/Mn4+ and Co2+/Co3+ reversible oxidation state is thought to be a key factor for oxidation reactions [17-20] The oxidation state of B cation can be modulated by varying preparation steps during the perovskite synthesis, such as calcination temperature [21] , non-stoichiometry of cations A or B [12, 14] or substituting La+3 by lower oxidation state cations, such as Ca2+, Ba2+ or Sr2+, or even higher oxidation state cation such as Ce4+. Strontium-doped LaMnO3 and LaCoO3 perovskites have been used for a wide range of applications [10, 22-24] . The introduction of lower oxidation state Sr2+ in substitution of La3+ in LaMnO3 and LaCoO3 lattice generates a net charge imbalance that may be compensated by alteration of the oxidation state of a 5 fraction of transition metal, e.g. Mn4+ or Co4+. Alternatively, the oxidation state of transition metal could be maintained unaltered (Mn3+ or Co3+), but instead oxygen vacancies could be generated in the lattice to attain the charge balance. Even a mixed situation showing altered oxidation state of transition metal along with oxygen vacancies in the lattice could be expected. Oxide-based catalysts have typically shown much lower activity than Pt-based catalysts under the kinetics-controlled temperature regime. However, Kim et al [25] prepared La0.9Sr0.1CoO3 and La0.9Sr0.1MnO3 perovskites and reported NO oxidation activity similar to or higher than those of Pt-based catalysts under realistic automotive conditions. Li el al. [26] prepared a series of La1-xSr0.xCoO3 (x=0.1, 0.2, 0.3, 0.4 and 0.5), and they found, after the NOx storage tests, the La0.7Sr0.3CoO3 perovskite had the best NO-to-NO2 performance and the largest NOx storage capacity (NSC) at 300 ºC. These authors also suggested possible NOx storage routes on La1-xSrxCoO3. To our knowledge, a similar study of strontium-doped Mnbased perovskites has not been reported and results compared with those of Co-based perovskites. More recently, Dong et al. [22] prepared a series of La0.7Sr0.3MnO3 perovskite-type catalysts by the sol-gel method using citric acid as the complexant. They concluded that variation of synthesis conditions, namely the calefactive velocity, the calcination temperature and the pH of the precursor solutions, greatly affect the morphology of the perovskite catalyst and, consequently, the NO-to-NO2 activity and NOx storage capacity of La0.7Sr0.3MnO3 perovskite-type catalysts. In this work, we look deeply into the identification of the main changes in physicochemical properties induced by the modification of different parameters (i.e. synthesis conditions or La partial substitution by strontium). Consequently, relevant information on the key factors for the NO-to-NO2 oxidation is extracted, along with the main differences derived 6 from the chemical nature of B cation (Co or Mn) in the based-perovskite catalyst, to gain understanding on key factors to design noble-metal-free perovskite-based catalysts competitive to substiture material for the conventional Pt-containing NSR catalysts. We prepare, characterize and test NO oxidation activity of stoichiometric LaMnO3 and LaCoO3 as well as strontium doped La1-xSrxMnO3 and La1-xSrxCoO3 perovskites (x=0.1, 0.2, 0.3, 0.4 and 0.5). After optimizing conditions during the sol-gel synthesis, including citrate to nitrate ratio, pH of the gel and and calcination protocol to get pure perovskites with no phase segregation and enhanced textural properties, special attention is focused on the correlation of NO-to-NO2 activity with the physico-chemical properties of the prepared strontium-doped perovskite-based catalysts. 2. EXPERIMENTAL 2.1. Perovskite catalyst preparation. All perovskites were prepared the by citric acid solgel method [27] . Appropriate amounts of La(NO3)3·6H2O (Fluka), Co(NO3)2·6H2O (Sigma Aldrich), Mn(NO3)2·4H2O (Merck) were dissolved in distillated water under vigorous stirring. Then citric acid (C6H8O7·H2O, CA) was added as a complexing agent and organic loading for the combustion process with a citrate to nitrate (CA/N) molar ratios of 0.7, 1.1 or 1.5. The pH value was adjusted to 3, 4.5, 6, 7, 8 or 9 by ammonia (25% as NH3, Panreac). After solvent evaporation at 80 °C, the gel was further dried at 120 °C overnight and then calcined in 5% O2/He (60 ml min-1) or static air at desired temperature (600 °C, 700 °C, 800 °C or 900 °C) for 4h. Table 1 resumes the nomenclature and synthesis conditions of all catalyst. The general nomenclature set for each sample was as follows: LaCoO3 (LCO) or LaMnO3 (LMO) with the specific synthesis conditions in brackets (citrate to nitrate molar ratio/pH/calcination temperature), as for example LCO (1.1/7/700). In order to determine if the synthesis method was repetitive this sample was prepared twice (with * in Table 1). 7 In order to synthesize strontium doped La1-xSrxCoO3 and LaxSr1-xMnO3 perovskites, adequate amount of Sr(NO3)2 (Sigma Aldrich) was added to the initial solution following the same procedure as stated before. The following samples with increasing La substitution degree by Sr were synthetized: La0.9Sr0.1BO3, La0.8Sr0.2BO3, La0.7Sr0.3BO3, La0.6Sr0.4BO3, La0.5Sr0.5BO3, with B=Mn or Co. 2.2. Catalyst characterization. X-ray diffraction (XRD) patterns were obtained on a Philips PW1710 difractometer. The samples were finely ground and were subjected to Cu Kα radiation in a continuous scan mode from 5° to 70° 2θ with 0.02° per second sampling interval. PANalytical X`pert HighScore specific software was used for data treatment. Textural properties of the samples were determined by N2 adsorption-desorption at - 196 °C using Micromeritics TriStar equipment. The thermogravimetric (TG) analysis was conducted with Setaram Setsys Evolution TG instrument coupled to the mass spectrometer Pffeifer Vacuum DUO 2.5, using 10 mg of sample. The experiment was conducted in 5% O2/He mixture or air (100 mL min-1) from room temperature to 950 °C at a heating rate of 2 °C min-1. The reducibility of the samples was investigated by temperature programmed reduction (H2-TPR) using Micromeritics AutoChem II equipment. The quartz tube reactor was loaded with 0.1 g of sample and pretreated with 30 mL min-1 of 5% O2/He mixture at 600 C for 30 min, and then cooled down to 50 C. Afterwards, samples were heated from room temperature to 900 C with 10 C min-1 heating rate in a 5% H2/Ar gas mixture with 30 mL min-1 flowrate. Water generated during sample reduction was removed using a cold trap before gas analysis by TCD. The strength and concentration of different oxygen species was investigated by Oxygen temperature programmed desorption (O2-TPD) using Micromeritics AutoChem II equipment. The sample (0.1 g) was introduced in a quartz tube reactor and pretreated with 50 mL min-1 8 of 5% O2/He mixture at 600 °C for 5 min and then cooled down to 40 C. Afterwards, samples were heated from room temperature to 900 ˚C with a heating rate of 10 C min-1 in a 50 mL min-1 of Helium gas flow. X-ray photoelectronic spectroscopy (XPS) characterization was carried out in a SPECS electron spectrometer with a Phoibos 150 1D-DLD energy analyzer using Al-Kα (1486.7 eV) radiation source. To obtain the XPS spectra, the pressure of the analysis chamber was maintained at 5 × 10−10 mbar. The binding energy (BE) scale was adjusted by setting the C1s transition at 284.6 eV. 2.4. NO-to-NO2 oxidation experiments. Oxidation tests were carried out in a vertical stainless steel reactor filled with 0.5 grams of 0.3-0.5 mm pelletized catalyst, and placed inside a 3-zone tube furnace. The feed composition was 650 ppm of NO, 6% O2 and Ar as balance gas. Gases were feed via mass flow controllers and the total flow rate was set at 634 mL min-1, which corresponded to a space velocity (GHSV) of 123,500 h-1. Temperature was measured by a thermocouple inside the catalyst bed. The outlet gas composition was continuously measured using a MKS MultiGas 2030 FT-IR analyzer for quantitative analysis of NO and NO2 (no additional nitrogen compounds are detected). The conversion of NO-to-NO2 was calculated after steady state at even temperature was reached, which usually needed around 20 minutes after temperature stabilization, according to Equation (1): 2 in out NO NO NO-to-NO in NO (%) 100 FF XF   (1) 3. RESULTS ANS DISCUSSION 9 3.1. LaCoO3 perovskites. There are several parameters affecting perovskites physicochemical properties during the synthesis, such as citrate to nitrate molar ratio in the starting solution, pH value of the gel and calcination protocol [22, 28, 29] 3.1.1. Influence of citrate to nitrate ratio. The influence of citrate to nitrate ratio (CA/N) was studied by preparing LaCoO3 perovskite with a citrate to nitrate molar ratio of 0.7, 1.1 and 1.5 (first section in Table 1), i.e. defect, near stoichiometric and excess of citrate for the stoichiometric reaction with nitrates, respectively. The thermal decomposition of the prepared samples was characterized by thermogravimetric analysis, as shown in Figure 1. Prior to the analysis, the obtained gels were dried overnight in an oven at 120 C and thus, only a weak weight loss (<5%) was observed below 150 C, which was ascribed to removal of residual adsorbed and hydrated H2O. From 150 C and onwards, two main weight losses were identified centered around 200 C and 300-350 C (DTGA curve), ascribed to the reaction between citrates and nitrates. FIGURE 1 In order to gain understanding on the decomposition reactions leading to the final formation of the perovskite, TGA-MS experiments in 5% O2/He were carried out with bare precursors, i.e. cobalt nitrate, lanthanum nitrate and citric acid (not shown). Excluding the weight loss attributed to dehydration processes, it was observed that citric acid started to decompose at low temperature with a maximum weight loss detected at 170 C and a progressive decomposition until 400 C. CO2 was the main gas product along with traces of higher molecular weight intermediates. On the other hand, cobalt nitrate decomposition started at 180 C and was completed at 280 C, whereas lanthanum nitrate was more stable and its decomposition only started after temperature was raised above 400 C, concluding the 16 In order to determine the influence of strontium doping, La1-xSrxBO3 perovskites (B=Mn or Co) were synthesized with La substitutions x = 0, 0.1, 0.2, 0.3, 0.4 and 0.5, using Sr(NO3)2 as precursor, and the optimum synthesis procedure determined in the previous section. 3.3.1. X-ray diffraction (XRD). Figure 6 shows the XRD patterns of the prepared La1-xSrxCoO3 (Figure 6a) and La1-xSrxMnO3 (Figure 6b) perovskites. Pure perovskite diffraction patterns were obtained for low Sr substitution degrees, in the absence of impurities. However, phase segregation in the form of Co3O4, La(OH)3 and SrCO3 start to be detectable by XRD for La0.6Sr0.4CoO3 perovskite, and become even more significant for La0.5Sr0.5CoO3. On the other hand, in the case of Mn-based perovskites, the presence of impurities in the form of SrCO3 was only evident for the highest Sr substituted sample, i.e. La0.5Mn0.5CoO3. Thus, it can be concluded that LaMnO3 perovskites allow higher Sr accommodation in the lattice without phase segregation in comparison to LaCoO3 perovskites. FIGURE 6 Irrespective of B cation, Mn or Co, increasing Sr substitution degree leads to a widening of the diffraction peaks (Figure S3) due to smaller crystal size of the solids, which has been quantified by Scherrer´s equation (Table 3). In fact, the largest crystal size, 32 and 27 nm, is detected for the non-substituted LaCoO3 and LaMnO3 samples, respectively, and the smallest, 15 and 14 nm, for the highest substitution degree La0.5Sr0.5CoO3 and La0.5Sr0.5MnO3, respectively. Furthermore, the intensity decrease and of the diffraction peaks with the displacement towards higher diffraction angles by Sr doping, suggests that La+3 cations were successfully substituted by Co+3 or by Mn+3/Mn+4 in the perovskite lattice structure. In addition, gradual changes in the characteristic pattern are observed from rhombohedral LaCoO3 (PDF number: 17 048-0123) and La0.88MnO2.91 (PDF number: 089-0679) to rhombohedral distorted La0.5Sr0.5CoO2.91 (PDF number: 048-0122) and rombohedral La0.5Sr0.5MnO3 (PDF number: 048-0122), respectively [10, 33-35] 3.3.2. Textural properties. The influence of Sr content on textural properties was studied by N2 adsorption-desorption at low temperature (Table 3). The evolution of the specific surface area (SSA) with respect to the substitution degree reveals a maximum for La0.8Sr0.2CoO3 and La0.6Sr0.4MnO3, samples. The SSA is enhanced by 32% for La0.8Sr0.2CoO3 (21.4 m2 g-1) with respect to the non-substituted sample (16.2 m2 g-1) whereas the SSA enhancement was notably higher (70%) for La0.6Sr0.4MnO3 (47.9 m2 g-1) in comparison to the non-substituted sample (28.2 m2 g-1). The SSA development for Sr substituted samples can be directly linked to the lower crystal size of the samples, as it was revealed by XRD (Table 3). Although crystal size was further decreased for higher Sr substitution, SSA was penalized, which was attributed to the presence of phase segregations that may block the access to the pores [26] Note that this SSA penalization resulted evident for Sr substitutions higher than 0.3 and 0.4 for LaCoO3 and LaMnO3 perovskites, respectively, which again is a clear evidence of the higher ability of LaMnO3 to accommodate Sr in the lattice with no phase segregation, and thus allowing higher promotion effect on textural properties due to Sr incorporation. TABLE 3 3.3.3. Reducibility (H2-TPR). It has been found that the catalytic activities of lanthanum perovskites were mainly determined by the B site element properties [20] . In this sense, the redox properties of Mn and Co-based perovskites were investigated by H2-TPR experiments. Figure 7 illustrates the H2-TPR for La1-xSrxCoO3 samples, where two main H2 consumption regions can be distinguished, i.e. below and above 500 C. Hydrogen consumption below 500 C can be deconvoluted into three different contributions centered around 200, 325 and 375 18 C, which are assigned to the reduction of quimisorbed oxygen in the catalyst surface, superficial reduction of Co3+ to Co2+ and bulk reduction of Co3+ to Co2+, respectively, maintaining the perovskite structure, 1-x x 3 2 1-x x 2.5 2 La Sr CoO +1 2H La Sr CoO +1 2H O (2) On the other hand, hydrogen consumption above 500 C can be assigned to the final reduction of Co2+ to Co0 which is again accomplished in two different consumption peaks attributed to superficial and bulk reduction, resulting in the destruction of the perovskite structure, 0 1-x x 2.5 2 2 3 2 (1-x) La Sr CoO + H La O + xSrO + Co H O 2  (3) The aforementioned reduction peaks can be clearly observed for perovskite samples with low Sr substitution up to La0.7Sr0.3CoO3. In contrast, reduction peak assignation is not straightforward for higher Sr substitution degree samples. In those cases, an additional reduction peak can be observed at higher temperature (800 C) which is not related with hydrogen consumption but instead is ascribed to surface decomposition of residual carbonates in the form of CO2 which alters the TCD signal, as detected by MS [26, 36] . As a general trend, it can be observed that increasing Sr content results in the displacement of reduction peaks to lower temperature, related with increasing BET surface area (SSA) which promotes sample reducibility. FIGURE 7 Focusing on low Sr substituted samples, the total hydrogen consumption related to the total amount of cobalt in the sample (Table 4) can be used to estimate the mean oxidation state of cobalt. Perovskite reduction stoichiometry (equations 2 and 3) reveals that 1.5 mol of H2 is needed to reduce 1 mol of Co3+ to Co0. Calculated H2/Co ratios are in all cases close to 1.5, 19 this confirming an oxidation state of Co3+ in the initial perovskite and discard the presence of Co4+. Consequently, it can be deduced that generation of oxygen vacancies is the only mechanism available to accomplish charge compensation in LaCoO3 perovskites when strontium is doped into the structure. In fact, increasing H2 consumption due to reduction of chemisorbed oxygen in surface vacancies was observed with increasing Sr doping (Table 4), reaching this contribution about 4 times for La0.5Sr0.5CoO3 with respect to non-substituted LaCoO3 perovskite. Furthermore, it was experimentally verified that the amount of hydrogen consumed in the reduction of Co3+Co2+ was half of that consumed for the reduction of Co2+Co0, which validates the peak deconvolution assignments. Co3+ relative content with respect to total cobalt in the sample was calculated from deconvoluted peaks areas relation, i.e. dividing the H2 consumption related to Co3+Co2+ reduction by half the H2 consumption related to Co2+Co0 reduction. It can be observed that cobalt exists only in the 3+ oxidation state in the perovskite lattice for low Sr substituted samples. On the contrary, for high Sr substituted samples lower Co3+/Co ratios were observed. This fact can be explained by the presence of a significant amount of cobalt as Co3O4, as observed by XRD, with a mixed +2/+3 oxidation state, which contributes to reduce H2 consumption and decreases the observed Co3+/Co ratio. TABLE 4 Figure 8 shows TPR profiles of Mn perovskites, all curves exhibiting similar shapes with two main reduction peaks, below and above 550 C. H2 consumption below 550 C was deconvoluted into three contributions located at 150, 300 and 400 C, which are ascribed to the reduction of nonstoichiometric excess oxygen accommodated within the lattice, reduction of Mn4+ to Mn3+ and reduction of Mn+3 to Mn+2 located in a coordination unsaturated microenvironment [22, 37, 38] , respectively. Theoretically, same contents of Sr2+ and Mn4+ 20 are expected in the perovskite structure in order to accomplish charge balance. Furthermore, it has been reported that even stoichiometric LaMnO3 perovskites can show mixed Mn3+/Mn4+ oxidation state balanced by excess oxygen in the lattice. Thus, reduction sequence of La1-xSrxMnO3 perovskites can be written as: 3+ 4+ 3+ 4+ 1-x x 1-x x+δ/2 3 2 1-x x 1-x x 3 2 La Sr Mn Mn O + δH La Sr Mn Mn O + δH O (4) 3+ 4+ 3 1-x x 1-x x 3 2 1-x x 32 2 11 La Sr Mn Mn O + xH La Sr Mn O + xH O 22 x   (5)   3 1-x x 32 2 2 3 2 1 1 1 La Sr Mn O + H 1-x La O +MnO+xSrO+ H O 2 2 2 x  (6) where x represents the Sr molar fraction and  the excess of oxygen accommodated within the lattice. On the other hand, H2 consumption above 550 C was attributed to bulk reduction of Mn3+ to Mn2+ and destruction of the perovskite. As previously observed for LaCoO3 perovskites, an additional peak was observed at higher temperature (800 C) for high Sr substituted samples due to SrCO3 decomposition. FIGURE 8 Total H2 consumption related to Mn content in the sample (H2/Mn) is indicative of an average Mn oxidation state. 1 mol of H2 is needed to reduce 1 mol of Mn4+ to Mn2+ whereas 0.5 moles of H2 is needed to reduce 1 mol of Mn3+ to Mn2+. Calculated H2/Mn ratios resulted in between 0.5 and 1 for all samples, revealing a hybrid Mn4+/Mn3+ oxidation state (Table 5). Observing numerical values of deconvoluted and integrated signals, it can be observed that H2 consumption related to reduction of oxygen excess is maintained fairly constant with Sr doping. The observed increasing amount of H2 consumption is attributed to reduction of Mn4+ to Mn3+, revealing that Sr doping promotes the presence of Mn4+ in the perovskite structure to compensate the charge imbalance produced by Sr. A similar trend observed for H2 consumption is assigned to superficial reduction of Mn3+ to Mn2+, which correlates 21 adequately with increasing surface area of the samples. Reduction of bulk Mn3+ to Mn2+ is complementary to surface Mn3+ to Mn2+ reduction. Thus, H2 consumption related to bulk reduction of Mn3+ to Mn2+ decrease with Sr doping. Mn4+ content determined from integrated H2 consumption of deconvoluted signals show, as a general trend, an increasing content of Mn4+ with increasing Sr doping, obtaining almost 50% of manganese as Mn4+ for high Sr doped perovskites. TABLE 5 As a general trend it can be observed that increasing Sr content results in a displacement of reduction peaks to lower temperature, related with an increasing BET surface area (SSA) which promotes sample reducibility, as it was observed for LaCoO3 perovskites. 3.3.3. Concentration and strength of the oxygen species (O2-TPD). Partial substitution of La+3 by Sr+2 can modify Co or Mn oxidation state as previously observed by H2-TPR, but oxygen vacancies can be also promoted in order to accomplished charge compensation. In this sense, temperature programmed desorption of oxygen (O2-TPD) is a reliable technique to analyze the amount and the strength of adsorbed oxygen species. Figures 9a and 10a show the evolution of TCD signal obtained during O2-TPD experiments for La1-xSrxCoO3 and La1-xSrxMnO3 samples, respectively. The profiles clearly show two distinct regions where oxygen is desorbed, related to different O2 species: the so-called α-oxygen (< 550 ˚C ) and βoxygen (> 550 ⁰C) [39] . α-Oxygen comprises weakly adsorbed species at the surface and lattice oxygen generated from the dislocations or grain frontiers [27] . Only the oxygen released from vacancies located very near to or on the surface is likely to desorb at this temperature. On the other hand, β-oxygen accounts for oxygen being released from inner layers of the lattice, which could cause the reduction of B site cation [27] 22 Figure 9a shows O2-TPD profiles of La1-xSrxCoO3 samples and Figure 10a the evolution of different oxygen species (Oα, Oβ and OTOTAL) with substitution degree. As already observed by H2-TPR strontium doping does not induce any change in the cobalt oxidation state, and thus, the positive charge defect due to the substitution of La+3 by Sr+2 should be balanced by the formation of oxygen vacancies [33, 40-44] . As can be observed, substitution degree up to 30%, i.e. La0.7Sr0.3CoO3, nearly triples the amount of Oα species with respect to the nonsubstituted sample, which could be also related to the promotion of specific surface area [45] On the contrary, substitution degrees higher than 0.4 penalize the amount of Oα species due to formation of surface impurities which reduce surface area. With respect to Oβ, La substitution by Sr enhanced the formation of these species in the whole range. However, we verified by MS that TCD signal variations for temperatures higher than 600 C were not only due to O2 desorption but also due to CO2 formation by surface carbonates decomposition. This explains the remarkable increase in the TCD signal at temperatures higher than 600 C for perovskite samples with high substitution degree, as those samples presented high amount of surface impurities, as revealed by XRD. FIGURE 9 La1-xSrxMnO3 samples (Figure 9b) show the same desorption regions identified for Mn based samples attributed to Oα and Oβ. However, strontium doping does not result in a significant change of Oα species in La1-xSrxMnO3 samples, as opposite to La1-xSrxCoO3 (Figure10b). Thus, charge compensation in La1-xSrxMnO3 samples seems to be accomplished preferably by modification of Mn oxidation state (as observed by H2-TPR) rather than formation of oxygen vacancies. In the high temperature region, the oxygen desorption peak situated around 600-700 C is assigned to the release of lattice oxygen which results in the reduction of Mn4+ ions to Mn3+. This peak shifts to lower temperature and tends to increase as 23 Sr substitution degree is increased, which suggests a higher facility of oxygen to diffuse along the lattice and a higher Mn4+ content [33] . It is worth to note that an additional peak appears around 800 C for high Sr substituted samples which is assigned to the decomposition of surface impurities in the form of carbonates, mainly SrCO3. Higher content of impurities changes the O2 desorption profile for La0.5Sr0.5MnO3. FIGURE 10 3.3.5. Surface elemental composition and oxidation states (XPS). X-Ray photoelectron spectroscopy (XPS) is a surface-sensitive technique and provides information of the surface elemental composition and the oxidation state of each compound, which could be modified by Sr doping [44, 46-48] . Surface composition and binding energies of La 3d5/2, Co 2p3/2, Sr 3d5/2 and O 1s core levels were recorded by XPS for LaCoO3 perovskites (Figure S4). La 3d5/2 transition was deconvoluted in two different contributions located at 832.9 and 834.6 eV with their corresponding shake up satellites. These contributions were assigned to La accommodated within the lattice and segregated lanthanum as an oxide (La2O3), respectively. Similar behavior was observed for Sr 3d5/2 transition, fitting the observed spectra with two contributions located at 131.6 and 133.5 eV with their corresponding shake up satellites, and again assigned to contributions due to lattice and segregated strontium phases, mainly as SrCO3, as observed by XRD. O 1s transition shows a broad emission line between 526 eV and 536 eV, which can be deconvoluted into three distinct peaks around 528.8, 531.2 and 534.0 eV. Lower BE value signal corresponds to lattice oxygen bonded to cations in the crystal structure (O2-latt). Intermediate binding energy contribution (Oads) is ascribed to O-C or O=C bonds, as for example in carbonates, oxygen bonded to cations as segregations (La2O3 or Co3O4) or weakly bonded O2 (associated with surfaces defects, i.e. surface vacancies). The signal at highest binding energy values is due to water and hydroxyl groups on the surface 24 [33, 49-52] . Co 2p3/2 transition shows a strong asymmetric peak around 779.5 eV with a weak satellite peak at 790 eV. The former signal could be resolved into two components attributable to Co+3 (779.5 eV) and Co+2 (780.7) in a hybrid oxidation state environment, respectively. Besides, an additional contribution at 782.1 eV (with a shake-up satellite at 784.5) is needed in order to fit experimental data, which is assigned to Co2+ in a CoO segregated phase [37] . Table 6 presents Sr/Co and La/Co atomic surface ratios along with ratios of lattice oxygen and segregated strontium with respect to total oxygen and strontium content, respectively. As can be observed there is a surface enrichment of Sr and La irrespective of strontium doping degree, i.e. surface atomic ratio of strontium and La with respect to cobalt is notably higher than the theoretical value shown in brackets, which is in line with lanthanum and strontium segregates observed by XRD. On the other hand, it can be observed that segregated strontium fraction increases with increasing strontium doping degree, revealing a limited accommodation capacity of Sr in the La1-xSrxCoO3 lattice. With respect to O2, decreasing contribution of lattice oxygen is detected in the O 1s transition as Sr doping increases, balanced with an increase contribution of oxygen adspecies or oxygen bonded to cations as segregations. TABLE 6 Similar procedure was followed with Sr doped LaMnO3 perovskites, recording La 3d5/2, Mn 2p3/2, Sr 3d5/2 and O 1s transitions by XPS (Figure S5). For these samples, it was possible to deconvolute La 3d5/2 transition in a unique component situated at 834.0 eV along with the shake-up satellite, assigned to lanthanum being part of the perovskite lattice. In contrast, as already observed for LaCoO3 perovskites, two different components were needed to fit experimental Sr 3d5/2 transition, assigned to lattice and segregated strontium. Asymmetrical 25 signal at 641.6 eV was recorded for Mn 2p3/2 transition with a weak satellite. Quantification of Mn oxidation state is rather difficult by XPS, due to small differences in the binding energy of Mn+4 and Mn+3 ions. Binding energy of Mn 2p3/2 transition appears near to the theoretical value attributed to Mn+4 and Mn+3, which is indicative of a mix oxidation state [10, 33, 49, 53, 54] . O 1s transition for La1-xSrxMnO3 samples can be deconvoluted following the same procedure as that followed for LaCoO3. Table 6 presents Sr/Mn and La/Mn atomic surface ratios along with ratios of lattice oxygen and segregated strontium with respect to total oxygen and strontium content, respectively. Surface enrichment of Sr and La can also be observed for La1-xSrxMnO3. In contrast, segregated strontium versus total strontium content shows a constant value irrespective of strontium doping degree, which reveals a larger accommodation capacity of Sr in the lattice of La1-xSrxMnO3 samples with respect to La1-xSrxCoO3. Note also that for the same Sr doping degree segregated strontium is much higher in La1-xSrxCoO3 than La1-xSrxMnO3. 3.3.6. NO-to-NO2 oxidation of Sr-doped perovskites. NO-to-NO2 oxidation capacity of Co and Mn perovskites with different Sr contents are plotted in Figure 11, including the evolution of the equilibrium conversion with temperature for the chosen reaction conditions (broken lines). The oxidation capacity of a platinum model catalyst (1.5% Pt5% CeO2/Al2O3, is also included in both cases for comparison purpose. Strontium doped LaCoO3 samples further improve NO conversion in comparison to non-substituted sample, especially at intermediate temperatures. Among prepared samples, La0.7Sr0.3CoO3 perovskite presented the best NO-to-NO2 performance (Figure 11a), with a maximum NO conversion of 83% at 300 C (71% achieved with LCO and 30%, and only 30% at 420 ºC with Pt-based conventional catalyst) . We suggest that optimum activity of La0.7Sr0.3CoO3 perovskite is the result of best balance between α oxygen species (O2-TPD) and BET surface area. It has been observed that strontium doping promotes the formation of α oxygen species but does not induce any change in the oxidation state of Co, remaining as Co3+ irrespective of Sr doping. 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NO-to-NO2 oxidation activity of different perovskite formulations reported in the literature, under different reaction conditions, in comparison to La1-xSrxMnO3 and La1xSrxCoO3 prepared in this work. Figure 1. TGA and DTGA curves for LaCoO3 gel precursor prepared with different citric acid to nitrate molar ratios (CA/N): 0.7, 1.1 and 1.5. Figure 2. XRD diffractograms of LaCoO3 perovskites synthesized with different citric acid to nitrate molar ratios (CA/N): 0.7, 1.1 and 1.5. 37 Figure 3. NO-to-NO2 oxidation capacity of LaCoO3 perovskites prepared with different synthesis conditions: a) citric acid to nitrate molar ratios (CA/N = 0.7, 1.1 and 1.5), b) pH values of the gel (pH = 3, 6, 7, 8 and 9) and c) calcination temperature (Temp. = 600, 700, 800, 800 °C). Figure 4. NO-to-NO2 oxidation capacity of LaCoO3 and LaMnO3 perovskites prepared by optimal synthesis conditions. Figure 5. Linearization of Arrhenius equation in order to obtain activation energies of NO-toNO2 reaction over LaCoO3 and LaMnO3 perovskites. Figure 6. XRD diffractograms of a) La1-x SrxCoO3 and b) La1-x SrxMnO3 with x ranging from 0 to 0.5 (▲ SrCO3, ■ La(OH)3 and ○ Co3O4). Figure 7. H2-TPR profiles of La1-x SrxCoO3 samples with x ranging from 0 to 0.5. Figure 8. H2-TPR profiles of La1-x SrxMnO3 samples with x ranging from 0 to 0.5. Figure 9. O2-TPD profiles of a) La1-x SrxCoO3 and b) La1-x SrxMnO3, with x ranging from 0 to 0.5 perovskites. Figure 10. Evolution of ,  and total desorbed oxygen species with substitution degree for: a) La1-x SrxCoO3 and b) La1-x SrxMnO3 perovskites. Figure 11. NO-to-NO2 oxidation capacity of a) La1-xSrxCoO3 and b) La1-xSrxMnO3 perovskites with x ranging from 0 to 0.5, together with model Pt based catalyst. 38 TABLE 1 Nomenclature, synthesis conditions and textural properties of the prepared LaCoO3 and LaMnO3 perovskites. Perovskite Nomenclature CA/N pH T. Calc., C SSA, m2 g-1 dc, nm LaCoO3 LCO (0.7/7/700) 0.7 7 700 5.1 51 LCO (1.1/7/700) 1.1 7 700 10.6 40 LCO (1.5/7/700) 1.5 7 700 10.1 67 LCO (1.1/3/700) 1.1 3 700 11.3 57 LCO (1.1/4.5/700) 1.1 4.5 700 9.7 45 LCO (1.1/6/700) 1.1 6 700 9.7 41 LCO (1.1/8/700) 1.1 8 700 12.6 38 LCO (1.1/9/700) 1.1 9 700 13.2 (15.2*) 46 (34*) LCO (1.1/7/600) 1.1 7 600 14.6 24 LCO (1.1/7/700)(+) 1.1 7 700 11.0 (11.6*) 41 (39*) LCO (1.1/7/800) 1.1 7 800 7.0 164. LCO (1.1/7/900) 1.1 7 900 4.3 212 LaMnO3 LMO (1.1/4.5/700) 1.1 4.5 700 21.6 42 LMO (1.1/6/600) 1.1 6 600 31.3 31 LMO (1.1/6/700) 1.1 6 700 26.0 34 LMO (1.1/7/700) 1.1 7 700 23.0 45 LMO (1.1/8/700) 1.1 8 700 13.9 51 (*) Calcination in 5% O2/He flow. 39 TABLE 2 NO-to-NO2 oxidation conversions and reaction rates of LaMnO3 and LaCoO3 perovskites in a differential reactor. LaCoO3 (LCO 1.1/8/700*) LaMnO3 (LMO 1.1/6/600*) Temperature, °C XNO-to-NO2, % (-rA), µmol min-1 m-2 Temperature, °C XNO-to-NO2, % (-rA), µmol min-1 m-2 140 2.64 0.260 140 5.74 0.281 164 3.72 0.367 163 5.85 0.286 188 7.56 0.746 189 8.10 0.396 211 14.57 1.437 218 12.71 0.621 236 25.65 2.531 238 19.01 0.929 40 TABLE 3 Textural properties of La1-x SrxCoO3 and La1-x SrxMnO3 perovskites, with x from 0 to 0.5. Sample Surface area, m2 g-1 Vp, cm3 g-1 dc, nm LaCoO3(*) 16.2 0.09 32 La0.9Sr0.1CoO3(*) 17.2 0.09 27 La0.8Sr0.2CoO3(*) 21.4 0.11 18 La0.7Sr0.3CoO3(*) 20.3 0.12 18 La0.6Sr0.4CoO3(*) 18.8 0.11 16 La0.5Sr0.5CoO3(*) 15.7 0.09 15 LaMnO3(**) 28.2 0.14 27 La0.9Sr0.1MnO3(**) 33.1 0.17 21 La0.8Sr0.2MnO3(**) 35.8 0.18 20 La0.7Sr0.3MnO3(**) 39.1 0.17 15 La0.6Sr0.4MnO3(**) 47.9 0.19 15 La0.5Sr0.5MnO3(**) 39.6 0.16 14 (*) All samples synthesized under same conditions that LCO (1.1/8/700) (**) All samples synthesized under same conditions that LMO (1.1/6/600) 41 TABLE 4 Deconvoluted hydrogen consumption related to different reduction steps for La1-xSrxCoO3 perovskites, with x from 0 to 0.5. Sample H2/Co O2 chem (a), µmol H2 g-1 Co+3Co+2 (b), µmol H2 g-1 Co+2Co0 (c), µmol H2 g-1 Co+3/Co (d) LaCoO3 1.53 145 2028 3990 0.98 La0.9Sr0.1CoO3 1.55 222 2144 4130 0.96 La0.8Sr0.2CoO3 1.53 282 2139 4357 1.02 La0.7Sr0.3CoO3 1.49 315 2158 4237 0.98 La0.6Sr0.4CoO3 1.38 542 2137 3245 0.76 La0.5Sr0.5CoO3 1.34 590 1796 2086 0.58 (a) Integration of peak centered around 225 C. (b) Sum of integrated peaks centered around 325 and 375 C. (c) Sum of integrated peaks centered around 500, 575 and 650 C. (d) Estimation of relative Co3+ content based on deconvoluted peak areas relation, Co3+/Co= (Co+2Co0)/(Co+3Co+2/2). Figure 2. Figure 3. Figure 4. Figure 5. Figure 6. Figure 7. Figure 8. Figure 9. Figure 10. Figure 11.