La2O3 catalyst prepared from LaNiO3 perovskite-type oxides for the production of hydrogen through steam reforming and oxidative steam reforming of ethanol

Applied Catalysis A: General 377 (2010) 181–190

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Evaluation of the performance of Ni/La2O3 catalyst prepared from LaNiO3 perovskite-type oxides for the production of hydrogen through steam reforming and oxidative steam reforming of ethanol Sania M. de Lima a,1, Adriana M. da Silva a, Lı´dia O.O. da Costa a, Jose´ M. Assaf b, Gary Jacobs c, Burtron H. Davis c, Lisiane V. Mattos a,2, Fa´bio B. Noronha a,* a

Instituto Nacional de Tecnologia - INT, Av. Venezuela 82, CEP 20081-312, Rio de Janeiro, Brazil Universidade Federal de Sa˜o Carlos - UFSCar, Laborato´rio de Cata´lise, Via Washington Luiz, Km 235, CEP 13565-905, Sa˜o Carlos, Brazil c Center for Applied Energy Research, The University of Kentucky, 2540 Research Park Drive, Lexington, KY 40511, USA b

A R T I C L E I N F O

A B S T R A C T

Article history: Received 8 October 2009 Received in revised form 15 January 2010 Accepted 26 January 2010 Available online 2 February 2010

This paper studies the performance of LaNiO3 perovskite-type oxide precursor as a catalyst for both steam reforming and oxidative steam reforming of ethanol. According to results of temperatureprogrammed desorption of adsorbed ethanol and by carrying out diffuse reflectance infrared Fourier transform spectroscopy analyses of ethanol steam reforming, ethanol decomposes to dehydrogenated species like acetaldehyde and acetyl, which at moderate temperatures, convert to acetate by the addition of hydroxyl groups. Demethanation of acetate occurs at higher temperatures, leading to a steady state coverage of carbonate. Catalyst deactivation occurs from the deposition of carbon on the surface of the catalyst. Both thermogravimetric and scanning electron microscopy analyses of postreaction samples indicate that lower reaction temperatures and lower H2O/EtOH ratios favor the deposition of filamentous carbon. However, less carbon formation occurs when the H2O/EtOH ratio is increased. Increasing reaction temperature or including O2 in the feed suppresses filamentous carbon formation. ß 2010 Elsevier B.V. All rights reserved.

Keywords: Perovskite-type oxides Hydrogen production Ethanol steam reforming Ethanol oxidative steam reforming Deactivation mechanism Nickel catalyst

1. Introduction The development of new processes for the production of chemicals and fuels from biomass requires new catalysts tailored to convert these feedstocks with high conversion rates, selectivities, and stabilities. Hydrogen emerges as an energy carrier with high potential and if produced from renewable sources, may contribute to the sustainable production of energy, as it can be directly converted electrochemically in PEM fuel cells to produce electricity for use in transportation applications and portable power devices. Hydrogen can be produced through the steam reforming of biomass-derived liquids such as bioethanol, a water and ethanol mixture that may be obtained by biomass fermentation [1–3]. However, there are currently no viable commercial catalysts for bio-ethanol steam reforming. Different catalysts, including metal oxides [4–7], mixed metal oxides [8–10], supported base metals (Ni, Co, Cu) [11–20] and

* Corresponding author. Tel.: +55 21 2123 1177; fax: +55 21 2123 1051. E-mail address: [email protected] (F.B. Noronha). 1 Present Address: Universidade Estadual do Oeste do Parana´ - Unioeste, Campus de Toledo, Rua da Faculdade, 645, Jd. La Salle, CEP 85903-000, Toledo - Brazil. 2 Present Address: Universidade Federal Fluminense, Rua Passo da Pa´tria, 156, Nitero´i, RJ CEP 24210-240, Brazil. 0926-860X/$ – see front matter ß 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2010.01.036

supported noble metals (Pd, Pt, Rh, Ru, Ir) [21–30], have been extensively studied for the steam reforming (SR), partial oxidation (POX) and oxidative steam reforming (OSR) of ethanol. In spite of their lower activity relative to supported metal catalysts, metal oxides are capable of producing hydrogen free of CO as well as carbon deposits, depending on the reaction conditions used [19,26]. However, a wide range of undesirable by-products (e.g., ethene, acetaldehyde and acetone) is formed during steam reforming of ethanol over metal oxides in comparison with supported metal catalysts, depending on the metal oxide properties. Supported metal catalysts are more active and selective to hydrogen than metal oxides but they undergo significant losses in activity with time on stream (TOS) [6,29]. Catalyst deactivation during ethanol conversion reactions may be associated with:  metal particle sintering;  metal oxidation (mainly for Co- and Ni-based catalysts);  carbon deposition, including both filamentous carbon and amorphous carbon covering the metallic particle and the support. The type of carbon formed and the mechanism of catalyst deactivation depends on the nature of the metal employed.

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Both types of carbon are formed on Co- and Ni-based catalysts [8,10,13,31]. Galeti et al. [31] studied the effect of reaction temperature on the performance of CuCoZnAl catalyst during SR. The nature of the carbon deposits formed was evaluated by Raman spectroscopy and temperature-programmed oxidation (TPO) experiments. CuCoZnAl strongly deactivated during SR at 673 and 773 K, whereas the activity remained unchanged at 873 K. Regardless of the reaction temperature used, Raman spectra revealed the presence of two types of carbon: ordered (graphitic) and disordered (amorphous or filamentous) carbon. Scanning electron microscopy (SEM) analysis confirmed the formation of carbon filaments after reaction at 773 K, while amorphous carbon was present at 873 K. They proposed that coke was removed from the surface of the catalyst by the reverse of Boudouard reaction, improving catalyst stability at this high temperature. For supported noble metal catalysts, filamentous carbon is not formed. In this case, the deactivation mechanism is still a matter of debate. According to Erdo˜helyi et al. [21], the catalytic deactivation during steam reforming of ethanol was due to an inhibiting effect caused by surface acetate species. Platon et al. [28] studied the deactivation of Rh/Ce0.8Zr0.2O2 catalysts during low temperature ethanol steam reforming. The results did not reveal any significant metal particle sintering or carbon deposits. However, an important build-up of carbonaceous intermediates was observed, which may be a contributing factor in the deactivation of the catalyst. These carbonaceous intermediates were less stable at higher reaction temperatures. Among different reaction intermediates, acetone and ethene were the main ones suggested to be responsible for catalyst deactivation. Recently, Lima et al. [25,26] have studied the performance of Pt/ CeZrO2 catalyst during steam reforming of ethanol at 773 K, which significantly deactivated during the reaction. An accumulation of acetate species on the Pt/CeZrO2 catalyst surface was also observed but this seemed to be symptomatic and not the root cause for the deactivation of the Pt/CeZrO2 catalyst. They proposed that the presence of the metal promotes the decomposition of acetate species to CO and CHx, which may be further dehydrogenated to H and C. The CHx species formed may lead to the blockage of the Ptsupport interface, deleteriously impacting the acetate turnover rate and thereby leading to a steadily increasing inventory of its steady state coverage, as well as catalyst deactivation. Therefore, a proper balance between the rate of acetate decomposition and the rate of desorption of CHx species is necessary in order to avoid catalyst deactivation. Suppressing carbon formation during ethanol conversion reactions is therefore a main issue in catalyst development. As deactivation due to carbon deposition has often been generally observed, efforts have been undertaken to develop new catalysts that are more resistant to coke formation. An interesting class of material is the perovskite-type oxide (ABO3); these mixed oxides are able to produce very small metal particles upon reduction [32,33]. Taking into account the catalyst deactivation mechanism proposed [25,26], carbon formation could be decreased or inhibited on these highly dispersed metal particles [34,35]. In addition, perovskites may exhibit structural defects due to deficiencies of cations at the A or B sites or of oxygen anions [36]. The oxygen vacancies are more common and affect directly the catalytic properties of the material. The literature reports that the higher reducibility and oxygen storage/release capacity of perovskite-type oxides catalysts may promote the mechanism of continuous removal of carbonaceous deposits from the active sites in reactions like partial oxidation and the CO2 reforming (i.e., dry reforming) of methane [32,33,36]. LaNiO3, a more easily reducible perovskite-type oxide, displayed high activity and good stability during CO2 reforming of methane [37]. However, there are only a few studies about

perovskites employed as catalysts [38,39] for the steam reforming of ethanol. Urasaki et al. [38] studied the performance of Co and Ni catalyst supported on perovskite-type oxides for the steam reforming of ethanol. The perovskite-type oxides tested (LaAlO3, SrTiO3 and BaTiO3) exhibited low activity and tended to produce C2H4 as the main product. SR was investigated on a series of La0.6Sr0.4Co1xFexO3 perovskite-type oxides [39]. The activation treatment of the sample significantly affected the activity and selectivity obtained. For the unreduced samples, acetone, ethene and acetaldehyde were the main products observed until 800 K, whereas ethanol was completely converted to syngas only above 900 K. The reduction at 873 K favored ethanol conversion to syngas and acetone production at low temperature (600–900 K). Longterm stability tests were not conducted in this study. The aim of this work is to study the performance of LaNiO3 perovskite-type oxide precursor, following activation, as a catalyst for both SR and OSR of ethanol reactions. The LaNiO3 perovskitetype oxide catalyst precursor has never been used in the SR or OSR of ethanol for hydrogen production. Therefore, a study of the reaction mechanism was carried out in order to shed light on the mechanistic pathways of ethanol reactions using a combination of reaction testing, temperature programmed desorption (TPD), and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) measurements taken under suitable reaction conditions. The second goal of this work was to gain insight into the deactivation mechanism by analyzing the nature of carbon formed by scanning electron microscopy (SEM) and the amount of carbon formed by thermogravimetric analysis. The holdup of adsorbed species during reaction was assessed by DRIFTS experiments. 2. Experimental 2.1. Catalyst preparation The precursor oxide was prepared by a precipitation method [40]. Aqueous solutions of lanthanum and nickel nitrates (Aldrich) in appropriate quantities to give perovskites with the desired formula were added rapidly to an aqueous solution of sodium carbonate, with vigorous stirring. The resultant mixed precipitate was washed and filtered under vacuum, until free of contaminating ions. The washed sample was dried in air at 333 K for 20 h, crushed and then calcined in two stages, first at 823 K for 3 h and then at 1173 K for 10 h, to obtain the final perovskite structure. A La2O3 supported Ni catalyst was also prepared by wet impregnation of the support with an aqueous solution containing the appropriate amount of nickel nitrate to have the same Ni content of the perovskite-type oxide precursor (23 wt.%). After the impregnation, the sample was dried in a rotary evaporator followed by calcination at 773 K. 2.2. BET surface area The BET surface areas of the samples were carried out using a Quantachrome NOVA 1200 instrument by nitrogen adsorption at the boiling temperature of liquid nitrogen. 2.3. X-ray Diffraction (XRD) The X-ray powder diffraction pattern of the calcined and reduced/passivated samples were obtained with nickel-filtered CuKa radiation (l = 1,5418 A˚) using a Siemens D5005 diffractometer. The XRD data were collected between 2u = 5 and 808 (in steps of 28/min). The sample was reduced with H2 (30 mL/min) at 973 K/ 1 h, cooled to room temperature, and passivated with 5.6% O2/He mixture.

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2.4. Temperature-programmed reduction (TPR)

2.8. SEM

TPR experiments were carried out on a Micromeritics TPD/TPR 2900 apparatus interfaced with a microcomputer. The sample (30 mg) was pretreated at 773 K for 1 h under a flow of air (50 mL/ min) prior to the TPR experiment in order to remove traces of water. Reduction profiles were then recorded by passing a stream of 10% H2/Ar at a flow rate of 50 mL/min, while heating the sample at a rate of 10 K/min from ambient temperature to 1173 K. A coldtrap was placed just before the thermal conductivity detector (TCD) of the instrument to remove the water from the exit stream.

SEM analyses of the fresh and spent catalysts were carried out using a JEOL JSM-6460 LV scanning electron microscope equipped with a secondary electron analyzer. The microscope was also equipped with a Thermo/Noran (System Six 200) energy dispersive spectrometer (EDS).

2.5. Temperature-programmed desorption (TPD) of ethanol TPD experiments of adsorbed ethanol were carried out using a micro-reactor coupled to a quadrupole mass spectrometer (Omnistar, Balzers). Prior to TPD analyses, the samples were reduced in flowing H2 (30 mL/min) by ramping to 973 K (10 K/min) and holding at this temperature for 1 h. After reduction, the system was purged with helium at 973 K for 30 min and cooled to room temperature. The adsorption of ethanol was carried out at room temperature using an ethanol/He mixture, which was obtained by flowing He through a saturator containing ethanol at 298 K. After adsorption, the catalyst was heated at 20 K/min to 773 K under flowing helium (60 mL/min). The products were monitored using a quadrupole mass spectrometer. 2.6. Diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) DRIFTS spectra were recorded using a Nicolet Nexus 870 spectrometer equipped with a DTGS-TEC detector. A Thermo Spectra-Tech cell capable of high pressure/high temperature operation and fitted with ZnSe windows served as the reaction chamber for in-situ adsorption and reaction measurements. Scans were taken at a resolution of 4 to give a data spacing of 1.928 cm1. The number of scans taken was 512. The amount of catalyst was  40 mg. Samples were first prereduced in an external fixed bed reactor by ramping in 200 mL/min H2:He (1:1) at 10 K/min to 973 K, and holding at this temperature for 1 h. After cooling to room temperature and purging in He, the gas flow was switched to 1% O2/He (25 mL/min) for 4 h for passivation. Samples were re-reduced in-situ by ramping in 200 mL/min H2:He (1:1) at 10 K/min and holding at 773 K for 1 h. The catalyst was purged in flowing He at 773 K, prior to cooling in flowing He to 313 K. For ethanol steam reforming reaction tests, He was bubbled at 15 mL/min through a saturator filled with ethanol and held at 273 K. A second helium stream (15 mL/min) was bubbled through a saturator filled with water and held at 298 K. The two streams were joined at a tee-junction, prior to which 1 psig check valves were placed on the lines to prevent a back-flow condition. The saturator gas flows and temperatures were set to provide a H2O: CH3CH2OH stoichiometric ratio of 2:1. Adsorption/reaction measurements were started at 323 K, and then the temperature was increased at 10 K/min; measurements were recorded at 373, 473, 573, 673 and 773 K. 2.7. Thermogravimetric analysis (TGA) TPO experiments were performed using a TA Instruments TGA analyzer (SDT Q 600) in order to quantify the amount of carbon formed over the catalyst. Approximately 10 mg of spent catalyst was heated under air flow from room temperature to 1173 K at a heating rate of 20 K/min and the weight change was measured.

2.9. Reaction conditions SR and OSR of ethanol were performed in a fixed-bed reactor at atmospheric pressure. Prior to reaction, catalysts were reduced under pure H2 (30 mL/min) at 973 K for 1 h and then purged under N2 at the same temperature for 30 min. All reactions were carried out at 773 K except for SR, which was also performed at 1073 K. For SR, H2O/ethanol molar ratios of 2.0, 3.0 and 10.0 were utilized. OSR was performed employing a H2O/ ethanol molar ratio of 3.0 and an O2/ethanol molar ratio of 0.5. The reactant mixtures were obtained using two saturators containing water and ethanol, which were maintained at the temperature required to obtain the desired H2O/ethanol and O2/ ethanol molar ratios. In the case of SR, the reactant mixture was obtained by flowing two N2 streams (30 mL/min) through each saturator containing ethanol and water separately. For OSR, a flow of 5.6 mole % O2 in N2 (28 mL/min) and a flow of N2 (32 mL/ min) were passed through the saturators containing ethanol and water, respectively. The partial pressure of ethanol was maintained constant for all experiments. The variation of partial pressure of water was compensated by a decrease in the partial pressure of N2. In order to observe the deactivation of the catalyst within a short period of time, a small amount of catalyst was used (20 mg). The samples were diluted with inert SiC (SiC mass/catalyst mass = 3.0). The reaction products were analyzed by gas chromatography (Micro GC Agilent 3000 A containing two channels for dual thermal conductivity detectors (TCD) and two columns: a molecular sieve and a plot U column). The ethanol conversion and selectivity to products were determined from: X ethanol ¼

Sx ¼

ðnethanol Þfed  ðnethanol Þexit  100 ðnethanol Þfed

ðnx Þproduced ðntotal Þproduced

 100

(1)

(2)

where (nx)produced = moles of x produced (x = hydrogen, CO, CO2, methane, acetaldehyde or ethene) and (ntotal)produced = moles of H2 + moles of CO + moles of CO2 + moles of methane + moles of acetaldehyde + moles of ethene (the moles of water produced are not included). 3. Results and discussion 3.1. Catalyst characterization The BET surface area of the perovskite-type oxide was very low (3.8 m2/g), which is typical of these materials prepared after calcination at high temperature. The XRD pattern of the calcined sample is displayed in Fig. 1a. The diffraction lines at 2u = 23.2, 32.8, 40.5, 41.2, 47.0, 53.0, 53.6, 58.0, 68.7, 69.78 are characteristic of the LaNiO3 rhombohedral phase (JCPDF 330711) [41], indicating that a perovskite structure was the main phase obtained after calcination. The TPR profile of LaNiO3 is shown in Fig. 1b. The reduction profile of LaNiO3 perovskite-type oxide exhibits three peaks at 614, 640 and 770 K. The reduction peak at 614 K is due to the

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Fig. 1. (a) XRD pattern of calcined LaNiO3 catalyst; (b) TPR profile of LaNiO3 catalyst; (c) XRD pattern of LaNiO3 catalyst after reduction at 973 K and passivation; (d) XRD pattern of Ni/La2O3 catalyst after reduction at 973 K and passivation.

reduction of Ni3+ to Ni2+, and the peak at 770 K corresponds to the reduction of Ni2+ to Ni0, both coming from the perovskite structure [37]. This result indicates that, after reduction at 973 K, the perovskite structure was destroyed and Ni0 particles are deposited over lanthanum oxide. The intermediate peak at around 640 K is associated with the reduction of Ni2+ to Ni0 of a nickel oxide phase [33]. In addition to the LaNiO3 perovskite-type oxide phase, it is likely that well-dispersed crystallites of NiO were also present on the calcined sample but remained undetected because only crystals larger than 5 nm are detectable by the X-ray diffractometer. Fig. 1c shows the diffratogram of the LaNiO3 perovskite-type oxide after reduction at 973 K and passivation. This diffratogram reveals the presence of lines typical of La2O3 (2u = 26.3, 30.4, 40.0, 46.6, 52.7, 56.0, 62.58), La(OH)3 (2u = 15.8, 28.1, 49.1, 70.58) and Ni0 (2u = 44.9, 52.7, 76.0), confirming that the perovskite structure was destroyed during reduction. For comparison, the diffratogram of the Ni/La2O3 sample after reduction at 973 K and passivation is displayed in Fig. 1d. Basically, this diffratogram shows the same diffraction lines observed for the reduced LaNiO3 perovskite-type oxide. From the diffraction line of Ni0 at 2u = 44.9o, the nickel crystallite size was calculated through the Scherrer equation for LaNiO3 perovskite-type oxide (dp = 8.8 nm) and La2O3 supported Ni catalyst (dp = 21.5 nm). This result demonstrates that the perovskite structure is able to produce and stabilize smaller nickel particles during the reduction treatment relative to those obtained from the catalyst prepared by standard impregnation.

3.2. Reaction testing The TPR and XRD experiments revealed that the active sites for the steam reforming of ethanol (metallic nickel) were produced during the reduction step. However, the LaNiO3 perovskite-type oxide phase was no longer detected after reduction and the working catalyst is nickel dispersed over lanthana. Therefore, the nomenclature ‘‘LaNiO3 catalyst’’ is retained during the discussion of the catalytic results to emphasize that the LaNiO3 perovskitetype oxide phase was the precursor to the catalyst. 3.2.1. SR and OSR at 773 K Fig. 2a–d shows the performance of LaNiO3 catalyst for the SR reaction under H2O/ethanol ratio of 2.0, 3.0, 10.0 and OSR reaction at 773 K. The initial ethanol conversion for SR under H2O/ethanol ratio of 2.0 was around 100% but significantly decreased after 6 h, achieving the ethanol conversion characteristic of the homogeneous reaction (around 22%). The product distributions mainly exhibited the formation of hydrogen, CO2, CO and CH4, which were constant during the reaction. In addition, small amounts of acetaldehyde were also detected while ethylene was not formed. The effect of the H2O/ethanol molar ratio during SR at 773 K was also evaluated. Ethanol conversion and product distributions as a function of time on stream (TOS) obtained for LaNiO3 catalyst during SR with H2O/ethanol molar ratio of 3.0 and 10.0 are presented in Fig. 2b and c. The catalyst remained stable during 24 h under a H2O/ethanol molar ratio of 3.0 but after this period, it deactivated very fast reaching a final ethanol conversion of 75%.

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Fig. 2. Ethanol conversion (Xethanol) and product distributions versus time on stream over LaNiO3 catalyst obtained during (a) SR under H2O/ethanol molar ratio = 2.0 at 773 K; (b) SR under H2O/ethanol molar ratio = 3.0 at 773 K; (c) SR under H2O/ethanol molar ratio = 10.0 at 773 K; (d) OSR under H2O/ethanol molar ratio = 3.0 and O2/ethanol molar ratio = 0.5 at 773 K; (e) SR under H2O/ethanol molar ratio = 3.0 at 1073 K. (Mass of catalyst = 20 mg and residence time = 0.02 g.s/mL).

Actually, the curve indicates that ethanol conversion was not stable and it should further deactivate if the reaction continues. On the other hand, the deactivation begins after 6 h for the SR under a H2O/ethanol molar ratio of 10.0. However, the catalyst is quite stable after 48 h TOS. Furthermore, the final ethanol conversion was higher in this experiment (82%). A comparison between the results presented in Fig. 2a–c shows that increasing the H2O/ethanol molar ratio from 2.0 to 10.0 slightly promoted long-term catalyst stability. Nevertheless, the rate of deactivation observed was quite high. Fig. 2d shows the ethanol conversion and product distributions as a function of TOS for LaNiO3 catalyst during OSR at

773 K. The simultaneous addition of O2 and H2O to the feed significantly improved catalyst stability. Fierro et al. [15] also observed that the introduction of a higher amount of oxygen increased the stability of Ni-Cu/SiO2 catalysts during OSR. Frusteri et al. [42] studied the performance of Ni/CeO2 and Ni/ MgO for SR and OSR. TEM revealed that both catalysts deactivated during SR due to carbon formation. In the presence of oxygen, carbon formation was significantly decreased on both catalysts, which resulted in enhanced stability mainly on the ceria supported catalyst. The high oxygen storage capacity of the ceria favored the reaction of gasification of carbonaceous residues, thereby promoting catalyst stability.

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Compared with SR, the selectivity to hydrogen decreased whereas the formation of CO2 increased during OSR. In addition, only trace amounts of acetaldehyde were detected during OSR. Kugai et al. [14] reported that the addition of water and oxygen promoted C-C bond cleaving, explaining the decrease in acetaldehyde formation during OSR in comparison with SR. 3.2.2. Effect of the reaction temperature on SR SR was also carried out at high temperature (1073 K) and the results are presented in Fig. 2e. The catalyst exhibited the same initial ethanol conversion level at both temperatures (Fig. 2b: 773 K and Fig. 2e: 1073 K). However, increasing the reaction temperature from 773 to 1073 K greatly improved the stability of the catalyst, such that it remained quite stable at the higher temperature. The lower stability at low temperature has been attributed to carbon deposition [10,13]. Fatsikostas and Verykios [13] reported that the rate of carbon deposition over Ni/Al2O3 and Ni/La2O3 catalysts during SR was strongly dependent on the reaction temperature, decreasing with increasing temperature. They proposed that the reactions of carbon with steam or CO2, which are responsible for carbon removal, are facilitated by higher temperatures. Galetti et al. [10] studied the performance of NiZnAl catalysts modified by Ce for SR at different temperatures (673, 773, and 973 K). They also observed that carbon formation was a strong function of reaction temperature. TGA, XRD, Raman and SEM analyses revealed that with increasing reaction temperature, carbon formation decreased significantly. At 973 K, the amount of carbon formed was very low and carbon filaments were not detected by SEM. They proposed that the reverse of Boudouard reaction (Eq. (3)) and the gasification of coke reaction (Eq. (4)) are favored at high temperature. The occurrence of such reactions of carbon removal improved the resistance to carbon deposition and as a result, the rate of deactivation decreases. 2 CO ! C þ CO2

(3)

H2 O þ C ! CO þ H2

(4)

Fig. 3. Ethanol conversion (Xethanol) and product distributions versus time onstream over Ni/La2O3 catalyst obtained during OSR under H2O/ethanol molar ratio = 3.0 and O2/ethanol molar ratio = 0.5 at 773 K; (Mass of catalyst = 20 mg and residence time = 0.02 g.s/mL).

of the LaNiO3 catalyst after exposure to different reaction conditions are shown in Fig. 4. Essentially, the TPO profile for SR reaction under H2O/ethanol molar ratios of 3.0 and 10.0 exhibited two peaks, the first being around 796–813 K and the second in the

H2, CO, CO2 and CH4 were the only products detected at high temperature whereas acetaldehyde was also observed during the run at 773 K. When reaction temperature was increased, the selectivity to CH4 and CO2 decreased while the selectivity to CO increased. Since the stability of LaNiO3 catalyst was improved during the OSR at 773 K, we compared the performance of Ni metal catalyst derived from LaNiO3 perovskite-type oxide precursor and La2O3 supported Ni metal catalyst that was prepared by a conventional catalyst preparation method (e.g., impregnation method). Ethanol conversion (Xethanol) and product distributions versus time on stream over Ni/La2O3 catalyst obtained during OSR at 773 K are shown in Fig. 3. The Ni/La2O3 catalyst was also quite stable during OSR similar to the LaNiO3 catalyst. However, lower hydrogen selectivity and higher methane formation were observed on Ni/ La2O3 catalyst. In addition, significant formation of acetaldehyde was observed on the Ni/La2O3 catalyst, which continuously increased with TOS. 3.2.3. Catalyst deactivation Catalyst stability is one of the main issues for the commercialization of ethanol conversion reactions for hydrogen production. The main cause of catalyst deactivation during SR is the formation of carbon deposits, comprising both filamentous carbon and amorphous carbon covering the metallic particle and the support. The nature of carbon formed depends on both the reaction conditions and choice of catalyst. We studied the effect of reaction conditions on the nature of the carbon deposits formed by TGA and SEM analyses. The TPO profiles

Fig. 4. TPO profile of LaNiO3 catalyst obtained following: (a) SR under H2O/ethanol molar ratio = 2.0 at 773K; (b) SR under H2O/ethanol molar ratio = 3.0 at 773K; (c) SR under H2O/ethanol molar ratio = 10.0 at 773K; (d) OSR under H2O/ethanol molar ratio = 3.0 and O2/ethanol molar ratio = 0.5 at 773K; (e) TPO profile of Ni/La2O3 catalyst obtained following OSR under H2O/ethanol molar ratio = 3.0 and O2/ ethanol molar ratio = 0.5 at 773K; (f) TPO profile of reference sample containing multiwalled carbon nanotubes (MWNT) and graphitic carbon physically mixed with the perovskite material.

S.M. de Lima et al. / Applied Catalysis A: General 377 (2010) 181–190 Table 1 Amount of carbon deposited on LaNiO3 and Ni/La2O3 catalyst after different reaction conditions, as determined by TGA, using 5% O2:He and a heating rate of 10 K/min. Catalyst

Reaction conditions

Mg carbon/g catalyst

LaNiO3 LaNiO3 LaNiO3 LaNiO3

SR; H2O/Ethanol = 2.0; 773 K SR; H2O/Ethanol = 3.0; 773 K SR; H2O/Ethanol = 10.0; 773 K OSR; H2O/Ethanol = 3.0; O2/Ethanol = 0.5; 773 K OSR; H2O/Ethanol = 3.0; O2/Ethanol = 0.5; 773 K SR; H2O/Ethanol = 3.0; 1073 K

44.1 12.0 9.3 1.9

23%Ni/La2O3 LaNiO3

11.8 0.0

range of 851–864 K. In the case of SR under a H2O/ethanol molar ratio of 2.0, there is a shoulder at around 790 K and a peak at 833 K. For OSR, only one small peak at 830 K was observed whereas no CO2 formation was detected during TPO analysis after SR at 1073 K. The TPO profile of Ni/La2O3 catalyst after OSR exhibits two peaks at 790 and 843 K. Fig. 4 also showed the TPO profile of a reference sample containing multi-walled carbon nanotubes (MWCNT) and graphitic carbon physically mixed with the perovskite material. The preparation method of the MWCNT is described in reference [43]. The oxidation of this reference sample exhibited two peaks at 833 and 873 K, corresponding to the MWCNT and graphitic carbon, respectively. Table 1 lists the amounts of carbon deposited on the LaNiO3 catalyst after different reaction conditions. Increasing the H2O/ ethanol molar ratio from 2.0 to 10.0 significantly decreased the amount of carbon deposited on the catalyst following SR at 773 K. Carbon deposition significantly decreased when oxygen was added to the feed whereas it was no longer observed during SR at high temperature. However, the amount of carbon deposited during OSR at 773 K over Ni/La2O3 catalysts prepared by a conventional method was significantly higher (six fold) than the one observed for Ni/La2O3 catalysts derived from LaNiO3 perovskite-type oxide. These results are in agreement with the higher stability of perovskite-type oxides catalysts on reactions like partial oxidation and the CO2 reforming of methane [32,33,36,37]. In these cases, the higher Ni dispersion obtained

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from perovskite-type oxide catalysts is responsible for the decrease of carbon formation. There are very few studies in the literature reporting on the use of TPO after SR of ethanol to study the nature of the carbonaceous deposits formed and to quantify the amount of carbon deposited over supported Ni catalysts [10,13,20]. The two CO2 peaks observed during TPO may be attributed to the presence of two different types of carbon on the surface of the spent catalyst. Fatsikostas and Verykios [13] carried out TPO analysis after SR of ethanol at 873 K over Ni/Al2O3 and Ni/La2O3/Al2O3 catalysts. The two peaks of CO2 evolved were assigned to polymeric carbon stemming from ethylene polymerization and to CHx species produced. Galetti et al. [10] observed a peak at around 859 K and a broad peak between 673–873 K during TPO of different Ni catalysts supported on ZnAl2O4, with spinel structure and having been modified by CeO2, after SR of ethanol at 773 K. The peak at low temperature was assigned to the oxidation of amorphous carbon overlaying the nickel surface whereas the peak at high temperature was due to oxidation of filamentous carbon. Sanchez-Sanchez [20] carried out TPO-TG analysis of Ni/MxOy/Al2O3 (M = Ce, La, Zr, Mg) catalysts after steam reforming of ethanol. The TPO profiles identified the presence of two shoulders at 723 and 853 K and a peak at 773 K. The oxidation peaks at low temperatures were attributed to carbon deposited over the nickel surface and carbon filaments, respectively. The peak above 823 K was due to oxidation of coke deposits having different degrees of graphitization. The amount of each type of carbon depended on the nature of the metal oxide. These assignments were also proposed in TPO studies of carbon nanotubes prepared using Co-based catalysts [44,45]. The oxidation peak located below 673 K was ascribed to amorphous carbon whereas the peak above 773 K corresponds to carbon nanotubes. The oxidation of graphite occurred at high temperature (at around 873 K). Therefore, our TPO profiles indicate the presence of filamentous carbon (SWNT and MWNT) and graphitic carbon over LaNiO3 and Ni/La2O3 catalysts after SR and OSR of ethanol. Electron microscopy techniques have been used to characterize the nature of the carbonaceous species formed over Ni-based catalysts after SR of ethanol. In our work, SEM images were analyzed to study the effect of reaction conditions on the nature of

Fig. 5. SEM images of the LaNiO3 catalyst after: (a) SR under H2O/ethanol molar ratio = 3.0 at 773K ( 10,000); (b) SR under H2O/ethanol molar ratio = 10.0 at 773K ( 30,000); (c) OSR under H2O/ethanol molar ratio = 3.0 and O2/ethanol molar ratio = 0.5 at 773K ( 20,000); (d) SR under H2O/ethanol molar ratio = 3.0 at 1073K ( 30,000).

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carbonaceous deposits formed. Fig. 5 shows the SEM micrographs of the LaNiO3 catalyst after exposure to different reaction conditions. The morphology changed significantly depending on the reaction conditions used. A large amount of carbon, in the form of carbon filaments, is clearly observed after running SR under a H2O/ethanol ratio of 3.0 (Fig. 5a). However, increasing the water content of the feed decreased the amount of carbon filaments formed, and they were not as well defined (Fig. 5b). No filaments were detected after OSR at 773 K (Fig. 5c) and SR at 1073 K (Fig. 5d), which agrees very well with the TPO and catalytic activity results. 3.3. Reaction mechanism 3.3.1. TPD of ethanol The TPD profiles of adsorbed ethanol for the LaNiO3 catalyst are shown in Fig. 6. No ethanol desorption peak was observed, while a small amount of acetaldehyde was formed at 398 K. At low temperatures, there are also peaks corresponding to H2 production (416 and 476 K) and CH4 formation (at 407 and 475 K). The literature reports the appearance of peaks for CO, CH4, and H2 in the low-temperature region during TPD analysis of adsorbed ethanol, which was attributed to the decomposition of dehydrogenated species [25,26,46,47]. Above 500 K, hydrogen (578 K), methane (580 K), CO (596 and 753 K) and CO2 (605 and 750 K) were detected. The formation of these products at high temperature has been attributed to the decomposition of acetate and carbonate species [25,26,46,47]. 3.3.2. DRIFTS analysis under reaction conditions The mechanism of the ethanol steam reforming reaction has been extensively discussed in the literature [1–3]. However, only little evidence has been obtained from in situ experiments [25,26] and there are few, if any, reports of DRIFTS experiments under reaction conditions over Ni-based catalysts to be found in the open literature. Therefore, DRIFTS tests under steam reforming of ethanol were carried out to provide further insight into the mechanistic pathways involved in ethanol steam reforming. DRIFTS spectra of the LaNiO3 catalyst under a reaction mixture containing ethanol and water are shown in Fig. 7. Table 2 lists vibrational wavenumbers and mode assignments for the different adsorbed species observed during the DRIFTS experiments at

Fig. 6. TPD profiles of ethanol desorption obtained for LaNiO3 catalyst.

different temperatures. At room temperature, only two weak bands are observed in the range between 1000–2000 cm1. The band at 1245 cm1 has been attributed to ethanol molecularly adsorbed on the Lewis acid sites of the oxide [48], while the band at 1636 cm1 is likely associated with the n(CO) vibrational mode of acetyl species. It has been suggested that a fraction of the ethoxy species is dehydrogenated to acetaldehyde, which may be further dehydrogenated to acetyl species [48]. The bands positioned between 2900 and 3000 cm1 are also associated with the different vibrational modes (e.g., n(CH) stretching mode) of dehydrogenated species (acetyl species in our work) [25,26]. However, the bands characteristic of ethoxy and acetate species were not detected in the spectrum at room temperature. When the temperature was increased to 373 K, the intensity of the bands assigned to both molecularly adsorbed ethanol and acetyl species only slightly decreased, whereas a new band at 1557 cm1 appears, indicating

Fig. 7. DRIFTS spectra obtained on LaNiO3 catalyst at different temperatures and under the reaction mixture containing ethanol and water (water/ethanol ratio = 2.0).

S.M. de Lima et al. / Applied Catalysis A: General 377 (2010) 181–190 Table 2 I.R. vibrational wavenumbers and mode assignments for the different adsorbed species observed during the DRIFTS experiments at different temperatures. Vibrational mode

Wavenumber (cm1)

Adsorbed species

Reference

d (OH) n(CO) ns(CH3) nas(CH3) ds(CH3) ns(OCO) nas(OCO) n(CO) ns(OCO) nas(OCO) n(C=O) nas(OCO)

1245 1636 2901 2978 1356 1445 1547 1063 1318 1547 1763 2362

Molecular ethanol Acetyl Acetyl Acetyl Acetate Acetate Acetate Carbonate Carbonate Carbonate Acetaldehyde Gas phase CO2

[48] [48] [48] [48] [48,49] [48,49] [48,49] [49,52] [49,52] [49,52] [50,51] [26]

the formation of acetate species. The appearance of the bands assigned to acetate species indicates that dehydrogenated species were oxidized to acetate species. According to the literature [25,26], ethoxy species are dehydrogenated to acetaldehyde, which may in turn be further dehydrogenated to acetyl species. The acetaldehyde reaction with the surface OH groups and/or the reaction between the acetyl species and the oxygen from the support can form acetate species [25,26]. At 473 K, the spectrum exhibits bands at 1356, 1445 and 1547 cm1, which are assigned to ds(CH3), ns(OCO), na(OCO) vibrational modes of acetate species, respectively [48,49]. There is also a poorly resolved band at 1764 cm1, which is likely due to acetaldehyde [50,51]. Increasing the temperature from 573 to 773 K, the intensities of the bands at 1547 and 1343 cm1 increased while the intensities of n(CH) stretching bands between 2900 and 3000 cm1 continued to decrease. The disappearance of these bands at high temperature suggests that carbonate species were formed through acetate decomposition [25,26] and dominate the spectra at high temperatures. The appearance of a band at 1063 cm1 in the spectra at 673 and 773 K also confirms the presence of carbonate species [52]. However, the presence of the acetate species may not be ruled out, since it is difficult to distinguish between carbonate species and acetate species by the vibrational modes in the (OCO) stretching region [24]. In addition, gas phase CO2 bands at 2326

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and 2362 cm1 were clearly observed, arising from carbonate decomposition [25,26]. The intensity of the band related to acetaldehyde species also increased and it is clearly present at 773 K. Recently, we have proposed a reaction mechanism for the steam reforming of ethanol over ceria and ceria-zirconia supported Pt catalysts [25,26,53]. The reaction mechanism proceeded by :  dissociative adsorption of ethanol to ethoxy species followed by;  decomposition and production of CO, CH4 and H2 or dehydrogenation of ethoxy species to acetaldehyde or acetyl species;  oxidation of acetaldehyde species by OH groups to acetate;  acetate demethanation to CH4 and carbonate species;  carbonate decomposition to CO2;  CH4 decomposition steps. The reaction pathways observed over the LaNiO3 catalyst were, in general, similar to the ones proposed for supported Pt catalyst. However, the results revealed significant differences in the relative intensity of certain intermediates. Actually, the TPR and XRD result showed that the reduction of LaNiO3 perovskitetype oxide led to the formation of Ni0 particles over a La2O3 support. Therefore, the differences observed are likely due to the nature of the metal (Pt or Ni) and possibly the support type (La versus Ce). Ethoxy and acetate species are preferentially formed over ceria and ceria-zirconia supported Pt catalysts whereas acetyl and carbonate are the main species for lanthana supported Ni catalyst. In addition, Pt based catalysts exhibit higher activity at high temperature because with increasing reaction temperature, the intensities of the bands related to acetate and/or carbonate species were very low (i.e., low steady state coverage suggest high turnover rates of the intermediates). On the other hand, the band intensities attributed to carbonate species remained approximately constant as temperature was increased for the LaNiO3 catalyst. In order to follow the evolution of intermediate species with TOS, DRIFTS spectra were recorded during 6 h under the reaction mixture ethanol + water at 773 K (Fig. 8). The first spectrum was very similar to the one obtained at 773 K from Fig. 7, revealing the presence of mainly carbonate species and a contribution from acetaldehyde, as previously noted. After an increase in the intensity of carbonate bands during the first hour, no significant changes were detected in the DRIFTS spectra of LaNiO3 with TOS. Thus, the coverage of carbonate species on the catalyst surface remained constant and could not be responsible for the deactivation observed. According to the reaction mechanism proposed, the decomposition of the dehydrogenated species (e.g., acetyl, acetaldehyde) and the acetate species produces hydrogen, CO and CHx species, which may in turn result in carbon formation depending on the rate of hydrogen recombination. This carbon may react with water or remain on the catalyst surface, resulting in :  encapsulation of the nickel metal particle, leading to catalyst deactivation or;  diffusion through the metal particle and precipitation behind it, resulting in the formation of carbon filaments.

Fig. 8. DRIFTS spectra obtained on LaNiO3 catalyst at 773 K and under the reaction mixture containing ethanol and water (water/ethanol ratio = 2.0) during 6 hours TOS.

The formation of filamentous carbon lifts the nickel particle from the support surface but may not necessarily result in loss in activity, since the metal surface may itself remain clean. It is clear that when O2 is present along with H2O, the catalyst is more stable (i.e., OSR versus SR). When SR is carried out at high temperature (1073 K), carbon does not accumulate, and this may be due to:

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 the steam and CO2 reforming of methane reactions;  the reverse of the disproportion of CO reaction (Eq. (3));  the carbon gasification reaction favored at high temperatures. 4. Conclusions Activation of LaNiO3 perovskite-type oxide led to the formation of La2O3-supported Ni0 particles. In TPD of adsorbed ethanol and by carrying out DRIFTS of ethanol steam reforming as a function of temperature, ethanol was suggested to decompose to dehydrogenated species like acetaldehyde and acetyl in the low temperature range, and can further decompose to gas phase byproducts. At moderate temperatures, acetate was suggested to form by the addition of–OH to the dehydrogenated species. At higher temperatures, acetate was suggested to demethanate, resulting in a steady state coverage of carbonate, which is the precursor to CO2 formation. Catalyst deactivation was determined to be due to the deposition of carbon; both the nature and amounts of carbon were characterized extensively by TG, and SEM. Clearly, lower temperatures and lower H2O/EtOH ratios favored the deposition of filamentous carbon (e.g., nanotubes), whereas less carbon was observed when the H2O/EtOH ratio was increased. Raising the temperature of SR or including O2 suppressed the formation of carbonaceous overlayers. Conflict of interest statement There is no conflict of interest. Acknowledgements This work received financial support of CTENERG/FINEP01.04.0525.00. CAER acknowledges the Commonwealth of Kentucky for financial support. The authors thank Victor Teixeira da Silva for providing the multi-walled carbon nanotubes. We would like to thank the Material Laboratory from Instituto Militar de Engenharia for the SEM images. References [1] [2] [3] [4] [5] [6] [7] [8] [9]

P.R. de la Piscina, N. Homs, Chem. Soc. Rev. 37 (2008) 2459–2467. P.D. Vaidya, A.E. Rodrigues, Ind. Eng. Chem. Res. 45 (2006) 6614–6618. A. Haryanto, S. Fernando, N. Murali, S. Adhikari, Energy. Fuels. 19 (2005) 2098–2106. N. Laosiripojana, S. Assabumrungrat, Appl. Catal. B. 66 (2006) 29–39. T. Nishiguchi, T. Matsumoto, H. Kanai, K. Utani, Y. Matsumurab, W.J. Shen, et al. Appl. Catal. A. 279 (2005) 273–277. J. Llorca, N. Homs, P.R. de la Piscina, J. Catal. 227 (2004) 556–560. J. Llorca, P.R. de la Piscina, J. Sales, N. Homs, Chem. Commun. (2001) 641–642. J.C. Vargas, S. Libs, A.C. Roger, A. Kiennemann, Catal. Today. 107 (2005) 417–425. S. Velu, K. Suzuki, M. Vijayaraj, S. Barman, C.S. Gopinath, Appl. Catal. B. 55 (2005) 287–299.

[10] A.E. Galetti, M.F. Gomez, L.A. Arru´a, M.C. Abello, Appl. Catal. A. 348 (2008) 94–102. [11] H. Song, L. Zhang, R.B. Watson, D. Braden, U.S. Ozkan, Catal. Today. 129 (2007) 346–354. [12] M. Vero´nica, B. Graciela, A. Norma, L. Miguel, Chem. Eng. J. 138 (2008) 602–607. [13] A.N. Fatsikostas, X.E. Verykios, J. Catal. 225 (2004) 439–452. [14] J. Kugai, S. Velu, C. Song, Catal. Lett. 101 (2005) 255–264. [15] V. Fierro, O. Akidim, C. Mirodatos, Green. Chem. 5 (2003) 20–24. [16] E.B. Pereira, N. Homs, S. Martı´, J.L.G. Fierro, P.R. de la Piscina, J. Catal. 257 (2008) 206–214. [17] J. Llorca, N. Homs, J. Sales, P.R. de la Piscina, J. Catal. 209 (2002) 306–317. [18] H. Song, U.S. Ozkan, J. Catal. 261 (2009) 66–74. [19] J.M. Guil, N. Homs, J. Llorca, P.R. de la Piscina, J. Phys. Chem. B. 109 (2005) 10813– 10819. [20] M.C. Sa´nchez-Sa´nchez, R.M. Navarro, J.L.G. Fierro, Int. J. Hydrogen. Energy. 32 (2007) 1462–1471. [21] A. Erdo˜helyi, J. Rasko´, T. Kecske´s, M. To´th, M. Do¨mo¨k, K. Baa´n, Catal. Today. 116 (2006) 367–376. [22] L.V. Mattos, F.B. Noronha, J. Power. Sources. 145 (2005) 10–15. [23] L.V. Mattos, F.B. Noronha, J. Power. Sources. 152 (2005) 50–59. [24] L.V. Mattos, F.B. Noronha, J. Catal. 233 (2005) 453–463. [25] S.M. de Lima, I.O. da Cruz, G. Jacobs, B.H. Davis, L.V. Mattos, F.B. Noronha, J. Catal. 257 (2008) 356–368. [26] S.M. de Lima, A.M. Silva, U.M. Graham, G. Jacobs, B.H. Davis, L.V. Mattos, et al. Appl. Catal. A. 352 (2009) 95–113. [27] W. Cai, F. Wanga, E. Zhan, A.C. Van Veen, C. Mirodatos, W. Shen, J. Catal. 257 (2008) 96–107. [28] A. Platon, H.S. Roh, D.L. King, Y. Wang, Top. Catal. 46 (2007) 374–379. [29] D.K. Liguras, D.I. Kondarides, X.E. Verykios, Appl. Catal. B. 43 (2003) 345–354. [30] F. Aupreˆtre, C. Descorme, D. Duprez, Catal. Commun. 3 (2002) 263–267. [31] A.E. Galetti, M.F. Gomez, L.A. Arrua, A.J. Marchi, M.C. Abello, Catal. Commun. 9 (2008) 1201–1208. [32] G. Valderrama, M.R. Goldwasser, C.U. de Navarro, J.M. Tatiboue¨, J. Barrault, C. Batiot-Dupeyrat, et al. Catal. Today. 107 (2005) 785–791. [33] S.M. Lima, J.M. Assaf, M.A. Pen˜a, J.L.G. Fierro, Appl. Catal. A. 311 (2006) 94–104. [34] D.L. Trimm, Catal. Today. 37 (1997) 233. [35] J.R. Rostrup-Nielsen, J. Catal. 85 (1984) 31. [36] M.A. Pen˜a, J.L.G. Fierro, Chem. Rev. 101 (2001) 1981–2018. [37] S.M. de Lima, M.A. Pen˜a, J.L.G. Fierro, J.M. Assaf, Catal. Lett. 124 (2008) 195–203. [38] K. Urasaki, K. Tokunaga, Y. Sekine, M. Matsukata, E. Kikuchi, Catal. Commun. 9 (2008) 600–604. [39] M.M. Natile, F. Poletto, A. Galenda, A. Glisenti, T. Montini, L. De Rogatis, et al. Chem. Mater. 20 (2008) 2314–2327. [40] S.M. de Lima, J.M. Assaf, Mater. Res. 05 (2002) 329–335. [41] H. Provendier, C. Petit, C. Estourne`s, S. Libs, A. Kienemann, Appl. Catal. A. 180 (1999) 163–173. [42] F. Frusteri, S. Freni, V. Chiodo, S. Donato, G. Bonura, S. Cavallaro, Int. J. Hydrogen. Energy. 31 (2006) 2193–2199. [43] B. Oliveira, O. Antunes, V. Silva, in: book of abstracts of 15th Brazilian Congress on Catalysis and 5th Mercosur Congress on Catalysis, Armac¸a˜o de Bu´zios–Brazil, September 13–17, 2009, pp. 587. [44] B. Kitiyanan, W.E. Alvarez, J.H. Harwell, D.E. Resasco, Chem. Phys. Lett. 317 (2000) 497–503. [45] J.E. Herrera, D.E. Resasco, Chem. Phys. Lett. 376 (2003) 302–309. [46] E.M. Cordi, J.L. Falconer, J. Catal. 162 (1996) 104–116. [47] L.F. de Mello, F.B. Noronha, M. Schmal, J. Catal. 220 (2003) 358–371. [48] J. Rasko´, A. Hancz, A. Erdo˜helyi, Appl. Catal. A. 269 (2004) 13–25. [49] A. Yee, S.J. Morrison, H. Idriss, J. Catal. 186 (1999) 279. [50] H. Idriss, C. Diagne, J.P. Hindermann, A. Kiennemann, M.A. Barteau, J. Catal. 155 (1995) 219–237. [51] A.M. Silva, L.O.O. Costa, A.P.M.G. Barandas, L.E.P. Borges, L.V. Mattos, F.B. Noronha, Catal. Today. 133 (2008) 755. [52] B. Klingenberg, M.A. Vannice, Chem. Mater. 8 (1996) 2755. [53] S.M. de Lima, R.C. Colman, G. Jacobs, B.H. Davis, K.R. Souza, A.F.F. de Lima, et al. Catal. Today. 146 (2009) 110–123.