Al2O3 catalysts: The influence of the variation of support synthesis conditions

Applied Surface Science 317 (2014) 350–359

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Oxidative-reforming of model biogas over NiO/Al2 O3 catalysts: The influence of the variation of support synthesis conditions Yvan J.O. Asencios a , Kariny F.M. Elias b , Elisabete M. Assaf b,∗ a b

Departamento de Ciências do Mar, Universidade Federal de São Paulo, Av. Alm. Saldanha da Gama, 89, Ponta da Praia, CEP: 11030-400, Santos-SP, Brazil Instituto de Química de São Carlos, Universidade de São Paulo, Av. Trabalhador Sãocarlense, 400, 13560-970, São Carlos-SP, Brazil

a r t i c l e

i n f o

Article history: Received 14 March 2014 Received in revised form 1 August 2014 Accepted 12 August 2014 Available online 20 August 2014 Keywords: Alumina Synthetic-conditions Bayerite Nickel catalysts Biogas Oxidative-reforming Syngas

a b s t r a c t In this study, nickel catalysts (20 wt%) supported on ␥-Al2 O3 were prepared by the impregnation method. The ␥-Al2 O3 , was synthesized by precipitation of bayerite gel obtained from aluminum scrap. The synthetic conditions of the bayerite gel varied as follows: precipitation pH ranging from 6 to 7; ageing temperature ranging from 25 to 80 ◦ C, the calcination temperature for all samples was 500 ◦ C. The catalysts and the supports were analyzed by temperature programmed reduction (H2 -TPR), X-ray diffraction (XRD), physisorption of N2 (BET), X-ray absorption near-edge structure (XANES) and scanning electron microscopy (SEM). Isopropanol decomposition reactions over the catalysts were carried out to evaluate their acidity. SEM images of the spent catalysts showed that the morphology of the carbon formed during the reaction is of the filamentous type. The TPR analysis of the catalysts showed the presence of NiO species weakly interacted with the support as well as stoichiometric and non-stoichiometric nickel aluminate, the reduction of these species was also observed by XANES analysis. XRD analysis of the fresh catalyst showed peaks assigned to NiO, NiAl2 O4 and ␥-Al2 O3 . The best catalysts (samples NiAl7-25 and NiAl7-80) synthesized in this report showed high stability and high conversion values (CH4 (70%) and CO2 (78%)). These catalysts showed better performance than the catalyst supported on commercial ␥-Al2 O3 , which showed a high coke formation which affected the course of the reaction. The ␥-Al2 O3 synthesized from bayerite obtained at neutral pH conditions was the best support for nickel catalysts in the oxidative-reforming of model biogas. © 2014 Published by Elsevier B.V.

1. Introduction Biogas is produced by anaerobic fermentation of biomass and is composed primarily of methane and carbon dioxide (molar ratio CH4 :CO2 = 1.5), and other gaseous by-products such as H2 S, NH3 , H2 . Its composition can vary with the origin of the biomass feedstock and the operational conditions during fermentation [1,2]. Most of the hydrogen and syngas for industrial use is produced through steam reforming of methane. The biogas can be used as a substitute for natural gas in reforming processes, which is advantageous since biogas is considered a renewable source. Thus, due to its principal components, biogas can be transformed into synthesis gas (syngas = H2 /CO) through the dry reforming of methane (DRM, reaction (1)) in a CH4 :CO2 molar ratio of 1, and the excess methane can be partially oxidized by the

∗ Corresponding author. Tel.: +55 1633739918. E-mail addresses: [email protected] (Y.J.O. Asencios), [email protected] (E.M. Assaf). http://dx.doi.org/10.1016/j.apsusc.2014.08.058 0169-4332/© 2014 Published by Elsevier B.V.

POM reaction (reaction (2)). These coupled processes result in the oxidative-reforming of biogas [3,4]: DRM :

1.5CH4 + 1CO2 → 2CO + 2H2 + 0.5 CH4 H ◦ = 260.5 kJ mol−1

POM :

(1)

0.5CH4 + 0.25O2 → 0.5CO + 1H2 H ◦ = −22.6 kJ mol−1

(2)

It is known that the POM reaction over nickel catalysts follows the combustion-reforming pathway [5,6], where the total combustion of methane (TCM), dry reforming of methane (DRM) and steam reforming of methane (SRM) occur in parallel: TCM : CH4 + 2O2 → CO2 + 2H2 O H ◦ = −890 kJ mol−1 (3) DRM : SRM :

CH4 + CO2 → 2CO + 2H2 CH4 + H2 O → CO + 3H2

H ◦ = 260.5 kJ mol−1 ◦

H = 225.4 kJ mol

−1

(4) (5)

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All these reactions (1)–(5), occur in parallel during the oxidativereforming of biogas. The water–gas shift reaction (WGSR) is a reversible and exothermic reaction, and its reverse direction (RWGSR, reaction (6)) is favored at high temperatures where the reforming of methane takes place; so the occurrence of the RWGSR is very likely to occur during the reforming of biogas: RWGSR :

H ◦ = 34.3 kJ mol−1

CO2 + H2 ↔ CO + H2 O

(6)

The syngas (H2 /CO) produced by the reforming of the biogas can be used in the production of dimethyl ether (DME), a promising substituent for commercial diesel, through the syngas to DME process (known as STD process). Syngas can also be used as a feedstock in the Fischer-Tropsch process. The oxidative-reforming of biogas (reactions (1)–(5)) produces syngas with a H2 /CO ratio of 1.2, which allows its direct use in the STD process [7,8]. Several metal catalysts have been proposed for methane decomposition: (i) the non-noble group VIII: Ni, Co, Fe; (ii) the noble group VIII: Ru, Rh, Pd, Pt, Ir; and (iii) transition metal carbide catalysts. Within non-noble group VIII, nickel based catalysts are the most promising as they are inexpensive, of high availability and show catalytic activity comparable to that of noble metals [9]. Nevertheless, nickel based catalysts are affected by carbon incrustation in the metallic nickel particles which leads to deactivation, additionally it was found that the carbon is more soluble in nickel than in noble metals (e.g. Rh, Ru, Ir, Pt) [10]. The methane reforming industry uses the commercial catalyst, nickel supported on alumina (Ni/Al2 O3 ); however, it presents problems due to carbon deposits that lead to deactivation. This problem is related to coking (deposition of coke) caused by nickel incrustation, and can also be promoted by the acidity of the alumina that also favors the carbon deposition. Some pathways have been proposed to explain the carbon deposition during the reforming of methane, among them, are the cracking reaction of methane (reaction (7)) and the Boudouard reaction (8): CH4 → C + 2H2 2CO → C + CO2

H ◦ = 74.8 kJ mol−1 ◦

H = −172.8 kJ mol

(7) −1

(8)

Coke formation is inevitable under the reforming process operating conditions (<500 ◦ C), hence various alternatives have been proposed to minimize the carbon deposits including: the use of solid solutions as catalytic supports [3,6,11,12], the reduction of the metallic nickel crystallite size [13,14] and the addition of promoters [4,15,16], among others. Another proposal to improve the catalytic performance, thereby reducing the coke deposits, is to modify the acidic–basic properties of the catalyst by the addition of alkalineearth cations (e.g. Ca2+ , Mg2+ , Ba2+ ) or rare earth metal oxides (e.g. La2 O3 , Y2 O3 ) [17]. The ␥-alumina is a solid-acid which is extensively used as catalytic support due to its high surface area, acidic properties (due to Bronsted acid centers: Al–OH), thermal stability, low toxicity and low cost [18–20]. The properties of alumina depend on the synthetic method which can affect the catalytic properties of the resulting catalyst for the methane reforming reaction, and can lead to different conversion rates and different types of carbon deposits [21]. Currently, there are not many studies focused on the variation of the synthesis conditions of ␥-alumina obtained by the sol–gel method, and its influence on the catalysis of the reforming of biogas. Aluminum is an abundant raw material on earth and is mainly extracted from bauxite, much of this aluminum is recycled. In our previous work [18], various high-surface-area aluminas (the highest values recorded 371 m2 g−1 ) were obtained from aluminum scrap (aluminum cans of >99% purity) by the sol–gel method under moderate synthesis conditions using inexpensive starting materials. In that report, the influence of the variation of the synthesis conditions (precipitation pH and ageing temperature) in the

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physicochemical properties of the resulting ␥-aluminas was studied. The present paper is a continuation of that previous report [18]; hence, the present study focuses on the influence of this variation, on the catalytic properties of the resulting nickel catalysts. The physicochemical properties of the resulting Ni/␥-Al2 O3 catalysts were correlated with their catalytic behavior in the oxidativereforming of model biogas and with their coke deposition rates; these results were compared with a nickel catalyst supported on a commercial ␥-alumina. 2. Experimental 2.1. Preparation of catalysts The aluminas were prepared by the sol–gel method from aluminum scrap (>99.9% Al). The methodology was detailed in our previous report [18]. In this methodology, the sodium aluminate was obtained from the stoichiometric reaction between a beverage can (>99% Al purity) and NaOH solution (2N). The resulting sodium aluminate solution was filtered and then separated in different aliquots to be precipitated with an H2 SO4 solution (2N). The precipitation pH was varied between 6 and 7, once obtaining the different precipitates (sol), some of them were further aged at temperatures of 25 ◦ C (48 h) and the others were aged at 80 ◦ C (4 h) in order to obtain the gel; the diffraction patterns (obtained by X-ray diffraction analysis: XRD) of all these samples revealed the bayerite structure as the main phase. These bayerites were subsequently calcined at 500 ◦ C (5 ◦ C min−1 ) for 1 h under synthetic air flow (30 mL min−1 ) in order to obtain ␥-Al2 O3 . The previous XRD analysis of the calcinated samples indicated the formation of ␥-Al2 O3 nano-crystals with a size near 5 nm [18]. The supports were named as: Al6-25, Al6-80, Al7-25 and Al7-80 alluding to the synthetic condition used. For example the name Al7-80 indicates that this alumina was precipitated at pH 7 and aged at 80 ◦ C. After obtaining the aluminas, they were impregnated with a determined amount of nickel nitrate salt (Ni(NO3 )2 ·6H2 O, 99.99%) (Alfa-Aesar) to reach 20% of nickel in total weight, in a rotaevaporator. The mixture, composed of the catalytic support and the aqueous solution of nickel precursor, was maintained at 80 ◦ C under reduced pressure to completely remove the solvent. After that, this mixture was dried at 80 ◦ C (24 h) and then calcined at 750◦ C (5 ◦ C min−1 ) for 3 h, under synthetic air flow (30 mL min−1 ) to obtain the oxide phases. The catalysts were named NiAl6-25, NiAl680, NiAl7-25 and NiAl7-80, according to the catalytic supports used. For comparison purpose, a commercial gamma alumina (crystallite size = 6.96 nm; 220 m2 g−1 ) was impregnated with nickel nitrate (Ni(NO3 )2 ·6H2 O, 99.99%) with a charge of 20 wt% Ni, and was then treated under the same thermal conditions as the other catalysts, this sample was named as NiAl commercial. 2.2. Characterization of catalysts The crystal phases were identified with a Rigaku Multiflex Xray diffractometer (40 kV, 30 mA) scanning in the range 2 = 5–80◦ ˚ Temperatureat 2◦ min−1 , using Cu K␣ radiation ( = 1.5406 A). programmed reduction (TPR) was performed in a quartz tube reactor, in which H2 consumption was measured with a thermal conductivity detector (TCD). 100 mg of sample was placed in the reactor and reduced with a 1.96% H2 /He (v/v) gas mixture flowing at 30 mL min−1 . The temperature was increased to 1000 ◦ C at a heating rate of 5 ◦ C min−1 . The specific surface area was estimated by the BET method from adsorption–desorption of N2 at liquid nitrogen temperature, using a Quantachrome Nova 1200 instrument. Scanning electron microscopy (SEM) of the catalysts (before and after

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Table 1 Textural analyses of aluminas, mass composition obtained by EDX and average crystallite size of NiO (XRD).

*

Catalyst

Mass composition Ni (%, EDX)

Surface area of support* (m2 g−1 )

Average pore diameter of support* (Dp : nm)

Surface area of catalysts (m2 g−1 )

Average pore diameter of catalysts (Dp : nm)

Average crystallite size of NiO (D: nm)

NiAl7-80 NiAl6-80 NiAl7-25 NiAl6-25 NiAl commercial

21.8 22.1 20.6 23.4 21.6

273 276 280 371 220

5.70 4.97 4.33 2.96 6.94

87 72 73 81 91

7.47 8.55 6.95 7.17 8.58

25 20 32 28 –

Data reported in reference [18].

reaction) was carried out in a LEO 440 microscope with an Oxford detector, operating with a 20 kV electron beam. The composition of the catalysts was determined by energy-dispersive X-ray spectroscopy (EDX), the measurements were made in five regions of the image using a LEO 440 scanning electron microscope with a tungsten filament coupled to an energy-dispersive X-ray detector. X-ray absorption near edge structure (XANES) analysis was carried out by dispersive-geometry X-ray absorption spectroscopy at the D06A-DXAS beamline of the Brazilian Synchrotron Light Laboratory (LNLS) in Campinas, Brazil. A Si(1 11) monochromator was used to select the energy, and the beam was focused at the sample. The catalyst was pelletized and then fixed in a stainless steel holder placed in the center of the quartz tube, and oriented relative to the beam by fine adjustment of the table position. Prior to each measurement, the reactor was purged with He flowing at 30 mL min−1 for 30 min In the TPR-XANES measurement, the reducing gas (5%H2 /N2 ) was flowed through the sample (inside the reactor) at 30 mL min−1 , while a temperature ramp of 10 ◦ C min−1 was applied by the furnace (from room temperature to 800 ◦ C) and XANES spectra were collected at each temperature step. Next, the sample was brought to the reaction temperature (750 ◦ C) under a flow of pure He (30 mL min−1 ). Subsequently, the XANES spectra were acquired in situ under reaction conditions (750 ◦ C) in the presence of CH4 (20% CH4 /He), CO2 (20% CO2 /He) and O2 (21% O2 /N2 ) streams, in a molar ratio: 1.5CH4 :1CO2 :0.25O2 , for 3 h. A Ni foil spectrum was recorded to calibrate the photon energy; all spectra were collected in the range from 8200 to 8550 eV. 2.3. Oxidative-reforming of a model biogas The catalytic reactions were carried out in a fixed-bed downflow quartz reactor (i.d. = 10 mm) connected in-line to a gas chromatograph fitted with thermal conductivity detectors. Prior to reactions, the catalysts were reduced with H2 (30 mL h−1 ) at 750 ◦ C for 1 h. The reactions were carried out on 100 mg of catalyst at 750 ◦ C, with a feed containing a model biogas and oxygen, in a molar ratio of 1.5CH4 :1CO2 :0.25O2 flowing at 107.5 mL min−1 . The oxygen was added in the form of synthetic air (79% N2 , 21% O2 ). The reaction temperature was measured and controlled by a thermocouple inserted directly into the top of the catalyst bed. All the reaction products were analyzed in-line with a gas chromatograph (Varian, Model 3800) with an automatic injection valve. The products at the reactor outlet were divided into two streams which were analyzed separately, in order to obtain a complete analysis of the reaction products. In one stream, hydrogen and methane were separated in a 13× molecular sieve packed column, with nitrogen as carrier gas. In the other stream, helium was used as the carrier gas and CO2 , CH4 and CO separation were separated in 13× molecular sieve and Porapak-N packed columns. Both stream outlets were equipped with thermal conductivity detectors. Carbon deposition was determined as the apparent gain in mass of the catalyst during the reaction.

The CH4 and CO2 conversion was calculated as: XR (%) =

(Mols Rin − Mols Rout ) (Mols Rin )

where R = CH4 or CO2 . 2.4. Decomposition isopropanol reactions To evaluate the acidity of the catalysts, decomposition reactions of isopropanol were carried out. The tests were conducted in the same reaction facilities used in the catalytic tests. 150 mg of catalyst was used for each test, pure isopropanol was fed with a pump with a flow rate of 2.5 mL h−1 and diluted in N2 (flow rate of 75 mL min−1 ) in the inlet stream. The reaction was carried out during 90 min, at temperatures of 325, 350 and 375 ◦ C. Before the reaction, the catalysts were activated by reduction with H2 at 800 ◦ C (10◦ C min−1 ) for 1 h under an H2 flow at 30 mL min−1 . The gaseous and liquid products were also analyzed by chromatography. 3. Results and discussion 3.1. Characterization The scanning electron micrograph (SEM) images of the fresh catalysts are shown in Fig. 1a–e. The SEM images of samples NiAl625 (Fig. 1c), NiAl7-25 (Fig. 1d) and NiAl6-80 (Fig. 1e) are similar to each other, in these images it is possible to observe the formation of agglomerates in the form of round distorted platelets, the platelets in NiAl6-25 and NiAl7-25 are larger and have similar sizes. The SEM image of NiAl commercial (Fig. 1a) shows an amorphous morphology. NiAl7-80 (Fig. 1b) formed plates of parallelepiped form that correspond to Al2 O3 (according to EDX analysis made over this region), the neighboring agglomerates are principally nickel. The overall results by SEM analysis of the fresh catalysts, indicates that despite the aluminas being obtained by the same sol–gel method starting from the same precursor materials, the variation of the ageing temperature and precipitation pH of aluminas influences the morphology of the resultant catalysts. The textural properties of the catalysts and the respective supports, as well as the mass composition for the catalysts, are given in Table 1. Briefly, in our previous work [18], various ␥-Al2 O3 were obtained by the sol–gel method. The aluminas named as: Al6-25, Al6-80, Al7-25 and Al7-80 (alluding to the synthetic condition used, as explained in the methods) reported different physical–chemical properties, meaning that the variation of precipitation pH and ageing temperature strongly affected the characteristics of the aluminas. At the same ageing temperature (25 or 80 ◦ C in each case), the aluminas prepared at pH 6 (Al6-25 and Al6-80) reported higher surface areas and smaller pores than their counterparts obtained at pH 7 (Al7-25 and Al7-80). Additionally, the ageing temperature of 80 ◦ C led to aluminas (Al7-80 and Al6-80) with larger pores and lower surface area than the aluminas prepared at 25 ◦ C (Al7-25 and Al6-25). The effects of the variation of these parameters during

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Fig. 1. SEM images of the fresh catalysts: (a) NiAl commercial, (b) NiAl7-80, (c) NiAl6-25, (d) NiAl7-25 and (e) NiAl6-80.

the sol–gel synthesis of the ␥-Al2 O3 were widely discussed in our previous report [18]. In Table 1, it can be observed that, despite the aluminas (the catalytic supports) presenting high surface area, most part of this area was lost when nickel was impregnated on the surface (see surface areas for catalysts). This loss can be related to the deposition of nickel particles on the alumina pores during the impregnation method (thus covering the pores of alumina), this effect is more noticeable in this case due to the relative high concentration of nickel (20 wt%). The impregnation of nickel led to the formation of larger pores on the catalysts. The mass composition of Ni in the catalysts (%, EDX) is very close to the stoichiometric (20 wt% nickel). Fig. 2 shows the XRD patterns of the fresh catalysts. All samples show peaks that correspond to the structure of ␥-Al2 O3 (JCPDS 79-1558), these peaks are in the same positions as those

for nickel aluminate, NiAl2 O4 (JCPDS 10-339). This superposition occurs because both the ␥-Al2 O3 and NiAl2 O4 have the spinel structure [22], implying a set of crystallographic planes which have the same Bragg angles (2). Additionally, peaks located at 2 = 43.3◦ , 62.8◦ , 75.5◦ which correspond to the cubic structure of NiO (JCPDS 47-1049), were observed. Only in the XRD pattern of NiAl commercial the peaks related to the cubic structure of NiO were not observed, suggesting that in this sample, NiO is segregated or that the NiO species do not form large crystalline structures (undetectable by XRD); this sample is composed principally of nickel aluminate, these findings were subsequently confirmed by the TPR analysis (that will be discussed in the following paragraphs). The NiO crystallite size of each catalyst was calculated by the Scherrer equation (Table 1). These values indicate that the catalysts

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Fig. 2. XRD patterns of catalysts (o: ␥-Al2 O3 , #: NiAl2 O4 , +: NiO).

supported on Al6-80, Al7-80 and on commercial alumina formed the smallest NiO crystallites. According to the TPR analysis shown in Fig. 3, samples NiAl625 and NiAl7-25 show similar reduction profiles. This may be due to these catalysts being supported on aluminas with very similar pore diameters (see the textural properties of the aluminas Al625 and Al7-25, Table 1). This suggests that the NiO particles were deposited on similar chemical environments in the pores of these aluminas. Note that all catalysts present three main peaks named: ␣, ␤ and ␦. The ␣ peak corresponds to free NiO species located on the support surface (present in all samples), which due to their weak interaction with the supports, are reduced in the relatively low temperature range of 400–550 ◦ C [23]. The ␤ peak, present in all catalysts, corresponds to the reduction of stoichiometric nickel aluminate with spinel structure (NiAl2 O4 ) [24,25]. The nickel aluminate, due to its high stability, reduces above 700 ◦ C [24–26].

Fig. 3. TPR profiles of catalysts.

The peak ␦ presents in all samples, corresponds to the reduction of NiO bonded to Al2 O3 [23], which in some cases is described as non-stoichiometric nickel aluminate (NiO-Al2 O3 ) [27]. This non-stoichiometric nickel aluminate, due to its low stability, decomposes at relatively low temperatures (550–700 ◦ C). This peak is more noticeable in NiAl6-25 and NiAl7-25, however, this peak is less noticeable in NiAl6-80 and NiAl7-80. In the NiAl 6-80 and NiAl7-80 profiles, two types of nickel oxide species can be clearly observed: ␣ and ␤, peak ␦ is barely noticeable, probably peak ␣ is overlapping peak ␦. In the NiAl6-80 sample profile, peak ␣ was split in two parts. The profile of NiAl commercial shows peaks ␣ and ␤ only, peak ␦ is absent. The assignment of such species was based on the XRD analysis of the catalysts and the adsorption–desorption of nitrogen analyses of the supports (Figs. 2 and 3 and Table 1). At this point, the supports Al6-25 and Al7-25, which have the smallest pore sizes: 2.96 and 4.33 nm, respectively (see Table 1), were suitable for the formation of non-stoichiometric nickel aluminate (more appearance of peak ␦) [23–24,28]; and not all NiO species defunded into the alumina pores for the formation of nickel aluminate. Consequently, the remaining NiO species may have led to the formation of nonstoichiometric nickel aluminate and therefore to NiO crystallites of larger size; this is consistent with the NiO crystallite sizes calculated for samples NiAl6-25 and NiAl7-25 (the largest size reported among all samples, Table 1). On the other hand, the TPR profiles of the catalysts NiAl680 (average pore size of the support Al6-80 = 4.97 nm), NiAl 7-80 (average pore size of the support Al7-80 = 5.70 nm) and NiAl commercial (average pore size of the commercial support ␥Al2 O3 = 6.96 nm) show reduction peaks assigned to free NiO species and to stoichiometric nickel aluminate (peaks ␣ and ␤, respectively), principally. This finding suggests that the pores formed on these aluminas (the largest pore size reported in Table 1) were suitable for the diffusion of NiO species into the ␥-Al2 O3 structure, leading to the conformation of the spinel structure (NiAl2 O4 ) [23–24,28]; that pores were unsuitable for the conformation of non-stoichiometric nickel aluminate (peak ␦ is barely noticeable). Consequently, this led to the formation of small NiO crystallites; this observation is consistent with the smaller NiO crystallite size found in NiAl6-80 (20 nm), NiAl7-80 (25 nm) and NiAl Commercial (NiO crystallite size not detectable by XRD). The verification of the formation of the free NiO, nonstoichiometric nickel aluminate and nickel aluminate is in agreement with the XRD results. The isopropanol decomposition reaction has been considered as a typical reaction to explore the acidic/basic properties of the catalysts [29]. The dehydration of isopropanol is catalyzed by acid sites while the dehydrogenation of isopropanol is catalyzed by both acidic and basic sites. As a result, it was established that the isopropanol dehydration rate measures the acidity of the catalyst, while the ratio between dehydrogenation and dehydration rates measures the basicity of the catalyst. Fig. 4 shows the selectivity of the catalysts to propylene and acetone, given in mol produced/mol of isopropanol converted. Taking into account that a high selectivity to propylene and a low selectivity to acetone means that the catalyst has a strong acidity, the catalyst supported on commercial ␥-alumina reported the highest selectivity to propylene, thus demonstrating its strong acidic character compared with the other samples. The catalysts supported on Al6-80 and Al6-25, which were precipitated under acidic pH conditions, have high selectivity to propylene, indicating their strong acidity. Contrarily, the catalysts supported on Al7-25 and on Al7-80, which were precipitated at pH 7, have higher selectivity for acetone, indicating their relative weak acidity.

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Fig. 4. Selectivity in function of reaction temperature: (a) propylene and (b) acetone.

Fig. 5. Conversion values for CH4 and CO2 during the oxidative-reforming of biogas.

3.2. Catalytic test

but its high coke deposition rate increased the pressure inside the reactor which did not allow completing the 6 h of reaction. The values of reactant conversion rates (Fig. 5) and the percentages of CO2 that contributes to RWGSR indicate that the best performance in the catalysis occurs over the NiAl7-25 and NiAl7-80 catalysts (there was a lower incidence of RWGSR as well as the maximum conversion of reactants). The trend found for the coke deposition rate was: NiAl commercial > NiAl7-80 > NiAl725 > NiAl6-80 > NiAl6-25, which is almost the same decreasing order of conversion rates. The catalyst supported on commercial ␥-alumina reached, by far, the highest carbon deposit rate among all catalysts in this study. Samples NiAl7-80 and NiAl7-25, which showed good conversion rates, reported low coke deposition rates in comparison to the Sample NiAl commercial. Furthermore, in the isopropanol decomposition analysis (Fig. 4) we observed that the catalysts show the following decreasing order of acidity: NiAl commercial > NiAl6-80 > NiAl6-25 > NiAl725 > NiAl7-80. In this analysis, the catalysts NiAl commercial, NiAl6-80 and NiAl6-25 were more acidic than the other catalysts. The most active catalysts, NiAl7-25 and NiAl7-80, demonstrated weak acidity which may be related to their low coke deposition rates and high catalytic performance. Contrarily the catalysts NiAl680 and NiAl6-25 demonstrated stronger acidic properties which may have caused their high carbon deposition rates (these values were high considering their poor catalytic performance), subsequently leading to deactivation.

Fig. 5 shows the conversion values for CH4 and CO2 obtained during 6 h of reaction of the oxidative-reforming of biogas. According to previous studies reported in the literature [30–33], the mixture based on Ni/Al2 O3 result in a catalyst with notable performance in the reforming of biogas. Contrary to expected; in the present paper the reactant conversion rates found decreased in the following order: NiAl commercial > NiAl7-80 > NiAl7-25 > NiAl625 > NiAl6-80; meaning that the variation of support synthesis conditions strongly affects the catalytic performance. The best performances in the catalysis were reported for samples NiAl7-80 and NiAl7-25. Samples NiAl6-80 and NiAl6-25 deactivated during the reaction time, NiAl6-80 reported the worst catalytic performance. After 6 h of reaction, traces of water were collected; this water may come from the RWGSR (reaction (6)) as well as from the TCM (reaction (3)). The conversion of CO2 is slightly higher than the conversion of CH4 suggesting that the collected water comes principally from RWGSR (CO2 being consumed to produce CO and H2 O, reaction (6)). The percentages of CO2 of the inlet-stream that contributes to RWGSR (calculated from the collected water) were 1%, 10%, 9%, 30% and 16% and the coke deposition rates were 1.42, 0.12, 0.11, 0.08 and 0.06 mmol h−1 for NiAl commercial, NiAl7-80, NiAl7-25, NiAl6-80 and NiAl6-25 catalysts, respectively. The Sample NiAl commercial reported the highest initial conversion rate,

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It is known that the addition of alkaline oxides in the catalysts supported on alumina/or silica, can activate CO2 molecules (during the DRM) more readily due to the decreasing of the acidic sites of the support [9,17]. In this context, taking into account that CO2 is an acid compound, the CO2 molecules can react readily with basic centers formed on alumina, an acid-base interaction favors this reaction [3,31]. This fact could explain the low conversion values reported by the catalysts NiAl6-25 and NiAl6-80, probably, the high number of acidic centers of these catalysts did not favor the interactions between the catalyst surface and the CO2 molecules, thus leading to low conversion rates. The high initial conversion rate of NiAl commercial may be due to the large amount of NiAl2 O4 present in its composition (as was found in the characterizations by XRD and TPR). The reduction of part of the NiAl2 O4 at activation temperature (750 ◦ C) provides a good conformation of Ni◦ active sites for the C-H bond breakdown of CH4 . The reduction of NiAl2 O4 produces a fine dispersion of Ni◦ particles in the catalytic support [24,25]. Moreover, according to the TPR analysis, NiAl commercial has less free-NiO species on the surface (the ␣ reduction peak is of very low intensity; Fig. 2), thus the majority of the active sites come from the reduction of NiAl2 O4 . On the other hand, it is likely that the strong acidic properties of NiAl commercial (as found in the decomposition of isopropanol analysis) was not very favorable for the activation of CO2 molecules; the activation/decomposition of CO2 is essential to the removal of surface carbon species; this may explain the high carbon deposition rate reported by this sample. Although the conversion rate of the NiAl commercial was high, the catalytic test was carried out only for 3 h due to the large amount of carbon deposited (1.42 mmol h−1 ) which raised the reactor pressure. The high conversion of CO2 and the high carbon deposition rate of NiAl commercial suggests that carbon formation is assisted by the Boudouard reaction (2CO → C + CO2 ) [34]. The result of the catalytic tests indicates that the catalysts supported on aluminas synthesized under a precipitation pH of 7, were optimal for the catalysis of oxidative-reforming of biogas. The ageing temperature of the alumina seems not to significantly influence in the catalysis of the reaction since NiAl7-25 and NiAl7-80 recorded similar catalytic activity. The catalysts were investigated by XANES analysis; the Ni Kedge XANES spectra for NiAl7-80, NiAl6-80 and NiAl6-25 samples (which correspond to the catalysts with the best and the worst catalytic performance) were monitored during the reduction and the reaction process; these results are shown in Figs. 6 and 7. The spectra for the fresh catalysts and for pure NiO and Ni◦ are shown in Fig. 6. As can be observed, the spectra for the fresh catalysts and for pure NiO are very similar. The Ni◦ spectra shows a pre-edge feature that is due to the transition from the Ni 1s orbitals to Ni 3d orbitals which is dipole forbidden and quadrupole allowed. The high white-light intensity at 8350 eV (present in all catalysts analyzed) is caused by (1s → 4p) and is typical of Ni–O interaction of nickel oxide [35,36]. The spectra for catalysts and pure NiO at the pre-edge and at the edge regions are very similar, however the spectra of the NiAl780, NiAl6-25 and NiAl6-80 at the post-edge are a little different from that of pure NiO (see Fig. 6). At this region, the spectras are almost equal to those of pure NiAl2 O4 found by Meng et al. [35] and by Jordão et al. [37], who studied mixtures of NiO and NiAl2 O4 . These results suggest the existence of nickel in the form of NiO, NiOAl2 O3 and NiAl2 O4 in the catalysts, in agreement with the findings in the TPR and XRD analysis of the present report. These characteristics were used for monitoring the reduction of the NiO species to Ni◦ . As the temperature was raised under reduction conditions with H2 over the fresh catalysts, the white line intensity at 8350 eV fell and the spectrum assumes a profile that resembles that of Ni◦ ,

Fig. 6. Ni K-edge XANES spectra of the fresh NiAl6-25, NiAl6-80, NiAl7-80, pure NiO and Ni foil.

showing that most NiO was reduced to Ni◦ (see Fig. 7a–d). Fig. 7a and d shows that the reduction of NiAl6-25 catalyst occurred in three steps, the first starting at 457 ◦ C, where there is a slight decrease in the white line intensity that correspond to the reduction of NiO surface species. The second step began at 523 ◦ C, where the decreasing of white-line intensity is more noticeable, and corresponds to the reduction of non-stoichiometric nickel aluminate (NiO-Al2 O3 ). The third fall is observed at 645 ◦ C, and corresponds to the reduction of the stoichiometric nickel-aluminate, which due to its high stability, requires a high temperature in order to be reduced. A similar reduction profile was observed for NiAl6-80 (Fig. 7b and d), with a first fall at 440 ◦ C (reduction of surface NiO), the second fall located at 520 ◦ C (reduction of non-stoichiometric nickel aluminate) and third located at 650 ◦ C (reduction of stoichiometric nickel aluminate). Sample NiAl7-80 (Fig. 7c and d) shows only two noticeable falls of the white line intensity located at 420 and at 700 ◦ C which correspond to the reduction of surface NiO and NiAl2 O4 , respectively. The reduction temperatures denoted by the XANES analysis are in good agreement with those temperatures found in TPR analysis (Fig. 3). The XANES analysis under reaction conditions (at 750 ◦ C in stream of reactants: CH4 , CO2 and O2 , and reaction products formed: H2 , CO, and traces of H2 O) reveals that the deactivation of NiAl6-80 and NiAl6-25 is due to the oxidation of Ni◦ active sites. As can be observed in Fig. 7a and b, the XANES profiles of these two samples under reaction conditions progressively return to NiO. Samples NiAl6-80 and NiAl6-25 needed only 12 and 20 min, respectively, to be deactivated. On the other hand, the metallic nickel phase of NiAl7-80 was not affected during 2 h of reaction, thus demonstrating its good performance in the catalysis of the oxidative-reforming of biogas. The above observations are in good agreement with the catalytic performance profile observed in Fig. 5. The images obtained by SEM analysis (in a region rich in carbon) of the catalysts after the oxidative-reforming of biogas are shown in Fig. 8a–c. The images correspond to the best (NiAl7-80), the worst (NiAl6-80) and the commercial catalysts (NiAl commercial) according to the catalytic test, respectively. These images show the presence of filamentous carbon, on the surface of the three catalysts, no great difference was found among them.

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Fig. 7. XANES profile of catalysts under reduction (800 ◦ C) and reaction conditions (750 ◦ C): (a) NiAl6-25, (b) NiAl6-80, (c) NiAl7-80 and (d) profiles of white-line intensity of the catalysts at 8350 eV vs. reduction temperature (◦ C). For more details see Section 2.

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The catalysts supported on alumina precipitated at pH7 (NiAl780 and NiAl7-25), favored high conversion rates. The highest conversion values was recorded by the sample supported on ␥alumina obtained at pH 7 and ageing temperature of 80 ◦ C (NiAl 7-80); additionally this catalyst produced a relatively low carbon deposition rate. The NiAl7-80 and NiAl7-25 catalysts are promissory for the catalysis of the oxidative-reforming of model biogas, they are more efficient in the catalysis than the commercial sample. The commercial catalyst recorded the highest carbon deposition rate (1.42 mmol h−1 ) that caused the pressure inside the reactor to rise rapidly. The catalysts supported on aluminas obtained at acidic pH 6 (NiAl6-25 and NiAl6-80) were not good for the catalysis, they deactivated readily. Their low performance can be related to the high acidity found through the isopropanol decomposition reaction. Acknowledgements The authors thank the Brazilian National Council for Scientific Development (CNPq) for the fellowship, the FAPESP agency for the financial support, the Brazilian Synchrotron Light Laboratory (LNLS) in Campinas, Brazil, for the XANES analysis, and the Department of Chemical Engineering of the Universidade Federal de São Carlos for the N2 adsorption–desorption analysis. References

Fig. 8. SEM images of the catalysts after reaction: (a) NiAl commercial, (b) NiAl7-80 and (c) NiAl6-80 after reaction.

4. Conclusions The variation of the parameters of synthesis of sol–gel alumina (precipitation pH and ageing temperature) influenced the catalytic performance of the resultant catalysts. The sol–gel aluminas precipitated at pH 7 and aged at 25 or 80 ◦ C were optimal catalytic supports for nickel based catalysts applied in the reforming of the model biogas.

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