Hydrogen production by ethanol reforming over NiZnAl catalysts

Applied Catalysis A: General 304 (2006) 116–123 www.elsevier.com/locate/apcata

Hydrogen production by ethanol reforming over NiZnAl catalysts M. Noelia Barroso, Manuel F. Gomez, Luis A. Arru´a, M. Cristina Abello * Instituto de Investigaciones en Tecnologı´a Quı´mica (UNSL-CONICET), Chacabuco y Pedernera, San Luis 5700, Argentina Received 23 September 2005; received in revised form 2 February 2006; accepted 12 February 2006 Available online 22 March 2006

Abstract NiZnAl catalysts with different Ni loading (1–25 wt.%) and a Zn:Al atomic ratio nearly constant (Zn:Al ffi 0.6) were prepared by the citrate sol gel method and characterized by different techniques such as TG, BET, TPR, XRD and FT-IR. They were active in the ethanol steam reforming at atmospheric pressure in the temperature range 500–600 8C. However, there were significant differences in their performance. On catalysts with high Ni loading (18–25 wt.%) high hydrogen selectivities around 85% were obtained. CO, CO2 and small amount of CH4 were the only byproducts at 600 8C. # 2006 Elsevier B.V. All rights reserved. Keywords: Ethanol; Steam reforming; Hydrogen production; NiZnAl catalysts

1. Introduction Hydrogen is considered the future fuel and it will become the major energy source as fossil resources become insufficient to satisfy the global energy demand. Different hydrogen production methods can be used: water electrolysis, gasification, partial oxidation reactions of heavy oil and steam reforming reactions. Hitherto, the hydrocarbon steam reforming, specially of methane, is the most widespread and economic way to produce hydrogen [1]. An attractive alternative to hydrogen production is the ethanol steam reforming. Ethanol has several advantages compared to other raw materials but the most important is its renewable origin and the consequent reduction in CO2 emission. It has a relatively high hydrogen content and its reaction in the presence of water is able to produce six moles of hydrogen per mol of fed ethanol: C2 H5 OH þ 3H2 O ! 2CO2 þ 6H2

(1)

From a thermodynamic point of view, the reaction (1) is highly favored, DG500  C = 125.7 kJ mol1, but several reactions are involved and in general, the reactor effluent contains a wide range of liquid and gaseous products. The best catalyst for the reaction (1) requires a surface capable of breaking the * Corresponding author. Tel.: +54 2652 426711; fax: +54 2652 426711. E-mail address: [email protected] (M.C. Abello). 0926-860X/$ – see front matter # 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2006.02.033

ethanol C–C bond and of oxidizing both carbon atoms to CO2. Besides, it should be unable to promote the oxidation of H2 and other C–C bond formation reactions. Different catalytic systems have been studied in this reaction using different metals (Rh, Pt, Pd, Ru, Ni, Cu, Zn, Fe, Co) and various supports (Al2O3, ZrO2, MgO, etc.) [2–12]. Aupreˆte et al. [12] have studied the influence of metal nature on Al2O3, ZrO2, CeO2, CeO2–Al2O3 and CeO2–ZrO2. They found that g-Al2O3 supported 1% Rh or 9.7% Ni catalysts were the most active and selective catalysts for ethanol steam reforming, with H2 yield of 2.3 and 3:1 gh1 g1 cat ; respectively at 700 8C. A very good activity was reported on Cu–Ni–K/g-Al2O3 at a temperature as low as 300 8C [13] with H2 selectivities around 1.3 mol per each ethanol converted. Ni supported on MgO also showed an excellent performance with 100% of ethanol conversion at 650 8C. On this catalyst, H2 yield was very high (5.2 mol H2/mole C2H5OH) with a modest coke formation [9]. The addition of alkali on Ni/MgO catalysts was also investigated [14]. Recently, Ni/Al2O3 catalysts were also examined for reforming crude ethanol (fermentation broth) [15]. One additional limiting factor influencing the steam reforming is the coke formation, mainly caused by hydrocarbon decomposition [9]. The support of catalysts influences the product distribution and also the deactivation rate by coking. The search of a new carrier able to work at high temperature under steam with a low coke formation is an important challenge.

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In this work, the preparation, characterization and catalytic activity of NiZnAl catalysts for ethanol steam reforming are presented. The influence of Ni loading is discussed as well.

Pt cell and heated from room temperature to 1000 8C at a heating rate of 10 8C min1 with a gas feed (air or N2) of 50 ml min1.

2. Experimental

2.2.5. Temperature programmed reduction (TPR) Studies were performed in a conventional TPR equipment. This apparatus consists of a gas handling system with mass flow controllers (Matheson), a tubular reactor, a linear temperature programmer (Omega, model CN 2010), a PC for data retrieval, a furnace and various cold traps. Before each run, the samples were oxidized in a 50 ml min1 flow of 20 vol.% O2 in He at 300 8C for 30 min. After that, helium was admitted to remove oxygen and then the system cooled at 25 8C. The samples were subsequently contacted with a 50 ml min1 flow of 5 vol.% H2 in N2 and heated, at a rate of 5 8C min1, from 25 8C to a final temperature of 700 8C and held at 700 8C for 1 h. Hydrogen consumption was monitored by a thermal conductivity detector after removing the water formed. The characteristic P number proposed by Malet and Caballero [16] defined as bS0/V*C0, where S0 is the initial amount of reducible species in the sample (mmol), V* is the total flow rate (ml min1), C0 the initial hydrogen concentration in the feed (mmol ml1) and b the heating rate (8C min1) was 10 8C in order to obtain an unperturbed reduction profile.

2.1. Catalyst preparation NiZnAl catalysts were prepared by the citric method. Citric acid was added to an aqueous solution that contained all the required ions as metal nitrates (Ni(NO3)26H2O; Zn(NO3)26H2O; Al(NO3)39H2O). An equivalent of acid per total equivalent of metals was used. The solution was stirred for 10 min and held at boiling temperature for 30 min. Then, it was evaporated in a revolving flask under a pressure of a few mmHg at 75 8C until a viscous liquid was obtained. Finally, dehydration was completed by drying the sample in a vacuum oven at 100 8C for 16 h. Samples were calcined under the following program: at 450 8C in N2 flow for 2 h, a heating from 450 to 500 8C in O2 (10%)/N2 flow, then at 500 8C for 5 h and finally the temperature was raised to 700 8C for 2 h. The samples were denoted as NZAx where ‘‘x’’ indicates the nickel nominal weight%. Binary catalysts with composition leading to ZnAl2O4 and NiAl2O4 spinel formations were also prepared under identical conditions and they were named ZA and NA, respectively. 2.2. Catalyst characterization All samples were characterized using different physicochemical methods. 2.2.1. BET surface area BET surface areas were measured by using a Micromeritics Accusorb 2100E instrument by adsorption of nitrogen at 196 8C on 200 mg of sample previously degassed at 200 8C for 2 h under high vacuum atmosphere. 2.2.2. X-ray diffraction (XRD) XR diffraction patterns were obtained with a RIGAKU diffractometer operated at 30 kV and 20 mA by using Nifiltered Cu Ka radiation (l = 0.15418 nm) at a rate of 38 min1 from 2u = 108 to 908. The powdered samples were analyzed without previous treatment after deposition on a quartz sample holder. The identification of crystalline phases was made by matching with the JCPDS files. 2.2.3. Infrared spectroscopy (IR) IR spectra were recorded by a Nicolet Protege´ 460 Infrared spectrometer, in the region 4000–200 cm1 with a resolution of 4 cm1. Compressed KBr pellets containing 1 wt.% of a sample were employed. Each spectrum was collected by co-adding of minimum 32 scans. 2.2.4. Thermal gravimetry (TG) The analyses were recorded by using TGA 51 Shimadzu equipment. The samples, ca. 15 mg, have been placed in a

2.3. Catalytic test The ethanol steam reforming reaction was carried out in a fixed-bed quartz tubular reactor operated at atmospheric pressure. The reactor is encased in a furnace, which is controlled by a programmable temperature controller. The reaction temperature was measured with a coaxial thermocouple. The feed was a gas mixture of ethanol, water and helium (free of oxygen). Ethanol and water were fed through independent saturators before mixing. The flow rates of gas stream were controlled by mass flowmeters. The flow rate was 70 ml/min at room temperature with an ethanol molar composition of 3%. The H2O:C2H5OH molar ratio was 3.6– 3.8 in all experiments. The catalyst weight was 300 mg (0.3– 0.4 mm particle size range selected after preliminary mass transport experiments to minimize diffusional resistances). The catalyst was heated to the reaction temperature under He flow, then the mixture with C2H5OH + H2O was allowed to enter into the reactor to carry out the catalytic test. In all the cases fresh samples were used. The reactants and reaction products were analyzed on-line by gas chromatography. H2, CH4, CO2 and H2O were separated by a 1.8 m Carbosphere (80–100 mesh) column and analyzed by TC detector. Besides, CO was analyzed by a flame ionization detector after passing through a methanizer. Higher hydrocarbons and oxygenated products (C2H4O, C2H4, C3H6O, C2H5OH, etc.) were separated in Rt–U PLOT capillary column and analyzed with FID using N2 as carrier gas. The activity was measured at 500 and 600 8C, for four hours at each temperature. The homogeneous contribution was tested with the empty reactor. These runs showed no activity at 500 8C whereas the ethanol conversion was 3% at 600 8C being acetaldehyde the only product.

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3. Results and discussion 3.1. Structural features Catalysts with different Ni/Zn/Al composition as it is shown in Table 1 were prepared by citrate sol-gel method. Ni loading varied between 1 and 25 wt.% and the Zn/Al molar ratio was kept nearly constant. In all the cases, homogeneous precursors were obtained and their decomposition was studied by thermogravimetry. The TG curves of dried precursor for catalysts with 0 and 25 wt.% Ni under a N2 flow are shown in Fig. 1. The derivatives of TG curves are also included to facilitate interpretation. In both cases the most important weight loss is between 370 and 500 8C with a maximum around 410–420 8C. The total weight loss amounts to 73.1 and 69.7% of the original weight, respectively. No significant weight loss is detected after 500 8C and only an increase in phase crystallinity is observed (see further XRD). Taking into account these results, the first step of calcination procedure was carried out under nitrogen flow as it was described under Section 2. Specific surface areas also shown in Table 1 vary between 58 and 27 m2 g1. They are low compatible with the relatively high calcination temperature. The calcination temperature was chosen to ensure that there will be no structural changes in the catalyst within the temperature range used for ethanol steam reforming [17]. The X-ray patterns of catalysts calcined at 700 8C are shown in Fig. 2. In all the samples, the diffraction lines of the ZnAl2O4 spinel phase (2u = 36.98, 31.38, 59.48, 65.38, JCPDS-5-669) are intense and symmetric, indicating a high crystallinity. For ZA sample the intensity ratio I3 1 1/I2 2 0 shown in Table 1 is strongly indicating that ZnAl2O4 is a normal spinel [18]. This ratio increases with Ni loading which can suggest an extend of inversion or a contribution to reflection lines due to the formation of NiAl2O4 (2u = 378, 458, 31.48, 59.68, 65.58, JCPDS-78-0552). Besides, weak bands corresponding to ZnO (2u = 36.28, 31.88, 34.48, JCPDS-36-1451) are also observed. On samples with Ni loading 8 wt.% NiO is also detected (2u = 43.38, 37.38, 62.98, JCPDS-4-835). Reflections of Al2O3 are absent although their presence could not be ruled out. Al2O3 usually shows poor crystallinity and its amount should be small due to its consumption in the spinel formation. Fig. 3 reports the IR spectra of ZA, NA and NZAx samples. The spectrum of ZA reveals the characteristic of spinel structure. Three peaks centered at 660, 550 and 495 cm1 due to the stretching mode of Al–O in octahedral coordination state Table 1 Characteristics of NiZnAl catalysts prepared by citric method Sample

Ni (wt.%)

Zn/Al

SBET (m2 g1)

I3 1 1/I2 2 0

ZA NZA1 NZA8 NZA18 NZA25

0 1 8 18 25

0.48 0.62 0.63 0.60 0.62

58 37.2 28.3 31.5 27.0

1.4 1.6 1.8 1.9 2.1

I3 1 1 and I2 2 0 reflection line intensities of basal planes 3 1 1 and 2 2 0.

Fig. 1. TG curves of thermal decomposition of (a) ZA and (b) NZA25 precursors.

(AlO6 octahedral units) [19] are observed in agreement with XRD results. Residual organic and hydroxy groups are absent. IR spectra of catalysts are dominated by a strong and broad absorption band in the region of 900–400 cm1 related to the inorganic network corresponding to Al–O stretching frequencies (AlO6 and AlO4). IR spectra are suggesting the presence of Al in tetrahedral coordination and then the formation of Ni spinel or Zn spinel with an extend of inversion could be inferred. In general, ZnAl2O4 is a normal spinel [20] in which the divalent cations (i.e. Zn2+) occupy tetrahedral holes and trivalent cations (i.e. Al3+) are in octahedral holes but it is reported that ZnAl2O4 can be partially inverse below 900 8C [19]. In the case of NiAl2O4 the parameter h (defined as the fraction of divalent metal ions in octahedral coordination) is 0.76 which is indicating that Ni tends to form a spinel with some inversion extent [20]. After calcination at 700 8C the intensities of bands between 1600 and 1400 cm1 corresponding to the antisymmetric and symmetric vibrations of the COO groups [21] have not completely disappeared. Likewise, those between 3500 and 3000 cm1 assigned to the overlapping of bands due to surface adsorbed water and chemically bonded hydroxy groups are still being observed. For NA sample a similar spectrum is obtained. The reducibility of NZA catalysts is examined by temperature programmed reduction, Fig. 4. TPR profile of NiO as reference compound illustrated in Fig. 4(a) shows one peak centered at 364 8C. Almost flat TPR profiles were obtained for binary oxide ZA and pure ZnO under the same reduction conditions. The results for catalysts depend on nickel

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Fig. 2. Diffraction patterns of fresh NiZnAl catalysts calcined at 700 8C: (a) ZA; (b) NZA1; (c) NZA8; (d) NZA18; (e) NZA25; (f) NA. () NiO, (&) ZnO, ( ) ZnAl2O4, (^) NiAl2O4.

Fig. 3. IR spectra of NZA catalysts: (a) ZA; (b) NZA1; (c) NZA8; (d) NZA18; (e) NZA25; (f) NA.

composition. On NZA1 catalyst, Fig. 4(b), TPR profile shows a peak around 364 8C similar to that found in pure oxide, then NiO should be forming weakly interacted crystallites with the spinel matrix. For NZA8, a weak band at 364 8C should be assigned again to the reduction of NiO whereas the band centered at 538 8C suggests a strong interaction of Ni2+ species with the spinel phase (mainly NiO in agreement with XRD results and/or NiAl2O4). The second peak can be attributed to the reduction of Ni2+ to Ni0 and also a contribution of Zn2+ to Zn0. On Cu1xNixZnAl-mixed oxide catalysts, Velu et al. [22] have reported that Ni induces the ZnO reduction to Zn8 around 500 8C. A similar behavior could be observed on other samples with a slight shift in the maximum at high temperature, 550 8C. On sample NZA25 the band at low temperature is not observed. The high reduction temperature on catalysts with Ni loading 8% indicates difficulty in reduction whereas the wide band indicates a strong degree of interaction between Ni2+ species and spinel phase. These species could be completely reduced at temperatures higher than 700 8C. It is important to note that the reducibility lightly decreases with the Ni loading and it is similar to that reported in literature for catalysts prepared by coprecipitation and sol gel methods [23,24]. XRD

of reduced samples shown in Fig. 5 reveal that peaks assigned to ZnAl2O4 are not practically altered and broad peaks corresponding to Ni0 are observed. ZnO reflection lines have also disappeared or their intensities have been strongly decreased. However, the peaks corresponding to Zn0 are not clearly observed. The highest intensity peak of Ni0 should be at 2u = 44.58 (JCPDS 4-0850). However, a shift of Ni0 reflection is detected in reduced samples, ca. 0.58. These shifts could suggest a partial dissolution of Zn0 in the Ni0 crystallites. Matulewicz et al. [25] have reported a shift of Cu0 reflections in X-ray diffraction caused by a partial dissolution of Zn0 in the Cu0 crystallites. Ni0 and Cu0 have the same spatial group, Fm3m, then the partial dissolution could be possible. For comparing the Ni–Al binary sample, NA, was reduced under the same reduction conditions. The XRD of this sample, Fig. 2(f), shows broad peaks which can be attributed to NiAl2O4 whereas the H2 consumption in TPR becomes significant around 600 8C, Fig. 4(f). Guo et al. [26] have studied nickel catalysts supported on magnesium aluminate spinel in dry reforming of methane. They found two reduction peaks but the second peak temperature decreased from 800 to 705 8C when the Ni loading increased from 1 to 15 wt.%

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Fig. 4. TPR profiles of NZA catalysts: (a) NiO; (b) NZA1; (c) NZA8; (d) NZA18; (e) NZA25; (f) NA.

Fig. 5. Diffraction patterns of reduced NiZnAl catalysts calcined at 700 8C: (a) ZA; (b) NZA1; (c) NZA8; (d) NZA18; (e) NZA25; (f) NA. (&): ZnO, ( ) ZnAl2O4, (!) Ni8, (^) NiAl2O4.

3.2. Catalytic steam reforming results

At 600 8C, a significant amount of CO is produced whereas CH4 formation strongly decreases and CH formation is not observed. This behavior induces to think that steam reforming or decomposition reactions of methane could be promoted by these catalysts. It can also be observed that CO2 selectivity decreases from 59–56% at 500 8C to 40–38.7% at 600 8C, which could be an indication of WGS inverse reaction occurrence. Recently, nickel/zinc aluminate catalysts were used for hydrogen production by methane decomposition at 580 8C [27].

The activity of NiZnAl catalysts is tested in steam reforming of ethanol. The ethanol conversion is 100% in all catalysts in the studied temperature range. As expected Ni loading has an important effect on the product distribution. Experimental results are shown in Tables 2 and 3. The catalysts can be divided in two groups: NZA catalysts with a low Ni loading (8 wt.%) and those with a high Ni loading (18 wt.%). NZA catalysts with low Ni loading produce an important fraction of undesirable compounds, HC (C3H6O, C3H6, C2H4, etc.) at 500 8C and after ca. 70 min of time on stream. The H2 yield changes from 2.2 to 3 mol H2/mol C2H5OH for catalysts with 1% and 8% of Ni, respectively. At 600 8C, CH4, CO and CO2 increase by decreasing CH products. Other products (C2H4, C3, etc.) are detected only on the NZA1 catalyst. The hydrogen yield also increases to 2.7 and 4 mol H2/mol C2H5OH. On the other hand, NZA catalysts with high Ni loading show a better performance to hydrogen. At 500 8C a strong decrease in CH products is observed, being the most important products CO2, CO and CH4. Hydrogen yields also increase to 4.9–4.7 mol H2/mol C2H5OH. These results clearly reveal that the presence of Ni favors the C–C bond rupture [22].

Table 2 Catalytic results in ethanol steam reforming at 500 8C on NZA catalysts

NZA1 NZA8 NZA18 NZA25

Time min

SCH4 (%)

SCO (%)

SCO2 (%)

SCH (%)

mol H2/mol C2H5OH

63 75 61 76

18.0 16.3 12.8 15.5

1.8 8.2 25.1 23.7

29.3 34.2 58.9 56.5

50.9 41.2 3.2 4.3

2.2 3.0 4.9 4.7

Ethanol conversion = 100%; selectivity in C-products is defined as in out Si = Fiout =nðFEtOH  FEtOH Þ; F: molar flow; n: stoichiometry factor; i = CH4, CO, CO2 and CH.

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Table 3 Catalytic results in ethanol steam reforming at 600 8C on NZA catalysts

NZA1 NZA8 NZA18 NZA25

Time (min)

SCH4 (%)

SCO (%)

SCO2 (%)

SCH (%)

mol H2/mol C2H5OH

67 71 80 63

38.7 24.2 3.9 3.0

11.9 31.2 57.4 57.1

38.4 44.6 38.7 39.9

10.9 0 0 0

2.7 4.0 5.3 5.1

Ethanol conversion = 100%.

If the reforming reaction (1) is taken into account, the complete conversion of ethanol and water could generate 6 H2 moles per C2H5OH mol which represents 100% selectivity to hydrogen. From the experimental results a significant increase in H2 selectivity is observed with increasing Ni loading. Thus, H2 selectivity increases from 45% for NZA1 to 85% for NZA18 at 600 8C. After ca. 230 min of time on stream, all catalysts show good stability when they are running at 600 8C under reforming conditions, Table 4. However, when the reaction temperature is 500 8C, differences in the catalyst behavior are observed, Table 5. For catalysts with low Ni loading, a decrease in CH products is observed and concomitantly an increase of H2 production. In catalysts with high Ni loading, the product distribution changes in the opposite way, with an important increase in CH products and a decrease in hydrogen yield. The experimental tests are carried out without previous reduction of catalysts. XRD patterns of spent catalysts shown in Figs. 6 and 7 could lead to a plausible explanation. They reveal that Ni2+ species have been reduced to Ni0 under the reforming environment (500–600 8C), as expected. As it was mentioned the reduction of Ni species under H2-TPR occurs at the same temperature range. Then, it is reasonable that Ni2+ was reduced to the metallic state during the reaction. The peak of Ni0 at 2u = 44.58 is clearly observed and is more intense when the reaction is carried out at 600 8C. Besides, XRD results reveal

Fig. 6. Diffraction patterns of NiZnAl catalysts after being used at 500 8C: (a) NZA1; (b) NZA8; (c) NZA18; (d) NZA25. (&) ZnO, ( ) ZnAl2O4, (!) Ni0, (*) Ni3ZnC0.7.

Table 4 Catalytic results in ethanol steam reforming at 600 8C on NZA catalysts

NZA1 NZA8 NZA18 NZA25

Time (min)

SCH4 (%)

SCO (%)

SCO2 (%)

SCH (%)

mol H2/mol C2H5OH

240 240 230 230

36.2 22.8 4.9 3.1

11.3 31.9 57.9 56.2

38.8 45.2 37.1 40.7

13.6 0 0 0

2.4 4.2 5.2 5.0

Ethanol conversion = 100%. Table 5 Catalytic results in ethanol steam reforming at 500 8C on NZA catalysts

NZA1 NZA8 NZA18 NZA25

Time (min)

SCH4 (%)

SCO (%)

SCO2 (%)

SCH (%)

mol H2/mol C2H5OH

240 250 250 220

24.6 14.1 10.4 11.8

2.6 13.0 17.2 18.0

41.3 43.0 52.4 48.8

31.5 30.0 20.0 21.4

2.9 3.9 4.3 4.2

Ethanol conversion = 100%.

Fig. 7. Diffraction patterns of NiZnAl catalysts after being used at 600 8C: (a) NZA1; (b) NZA8; (c) NZA18; (d) NZA25. (&) ZnO, ( ) ZnAl2O4, (!) Ni0, (*) Ni3ZnC0.7.

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the formation of Ni3ZnC0.7 (2u = 42.78, 49.88, 73.18, JCPDS28-0713). The presence of this carbide in the spent catalyst evidences a partial dissolution of Zn0 and C in the Ni0 crystallites. Similar results about carbides formation were reported in literature [27]. A clue of this dissolution was partially revealed by XRD patterns of reduced samples in hydrogen, Fig. 4. The carbide formation in NZA catalysts with a Ni loading 18 wt.% is higher at 500 8C (higher intensities in reflection lines assigned to the carbide). Then, the changes observed in the product distribution on these samples are likely related to carbide formation. In Fig. 8, TG curves of spent NZA25 catalyst are shown. Both samples present the same behavior up to 390 8C: a small weight loss (0.4%) up to 150 8C due to dehydratation, a constant weight from 150 to 263 8C and an increase up to 390 8C due to reoxidation of catalyst. From this point the sample used at 500 8C under reforming conditions shows a weight loss of 7.8%. However, the sample used at 600 8C continues gaining weight up to 497 8C and then remains constant. This behavior is in agreement with catalytic results and with the carbide formation. The presence of Ni3ZnC0.7 deserves a deep study to determine its influence in the steam reforming reaction and in the deactivation rate. This investigation is being carried out in our laboratory.

Fig. 8. TG curves of thermal decomposition of spent NZA25 at reaction temperature (a) 500 8C and (b) 600 8C.

4. Conclusions NiZnAl catalysts have been prepared by the citrate sol gel method. Zn and Al species are mainly incorporated into the spinel matrix with ZnO segregations. The amount of nickel was varied from 0 to 25 wt.% and predominantly forms NiO and likely NiAl2O4. The catalysts were very active in the steam reforming of ethanol. A complete ethanol conversion was obtained at 500 and 600 8C. The product distribution depends on nickel loading the catalysts with Ni amounts between 18 and 25 wt.% being the ones showing the best performance. At 600 8C, these catalysts are stable with high selectivities to hydrogen (around 85%) and to carbon monoxide. The present results show that NiZnAl catalysts are a good choice to be used in ethanol steam reforming. Acknowledgment Financial supports are acknowledged to CONICET, ANPCyT and Universidad Nacional de San Luis. References [1] J.N. Armor, Appl. Catal. A: Gen. 176 (1999) 159–176. [2] C. Diagne, H. Idriss, A. Kiennemann, Catal. Commun. 3 (2002) 565– 571. [3] S. Cavallaro, V. Chiodo, S. Freni, N. Mondello, F. Frusteri, Appl. Catal. A: Gen. 249 (2003) 119–128. [4] D. Liguras, D. Kondarides, X. Verykios, Appl. Catal. B: Environ. 43 (2003) 345–354. [5] F. Frusteri, S. Freni, L. Spadaro, V. Chiodo, G. Bonura, S. Donato, S. Cavallaro, Catal. Commun. 5 (2004) 611–615. [6] V. Klouz, V. Fierro, P. Denton, H. Katz, J. Lisse, S. Bouvot-Mauduit, C. Mirodatos, J. Power Sources 105 (2002) 26–34. [7] A. Kaddouri, C. Mazzocchia, Catal. Commun. 5 (2004) 339–345. [8] F. Marin˜o, M. Boveri, G. Baronetti, M. Laborde, Int. J. Hydrogen Energy 29 (2004) 67–71. [9] S. Freni, S. Cavallaro, N. Mondello, L. Spadaro, F. Frusteri, Catal. Commun. 4 (2003) 259–268. [10] J. Llorca, N. Homs, J. Sales, J.L.G. Fierro, P. Ramirez de la Piscina, J. Catal. 222 (2004) 470–480. [11] J. Sun, X. Qiu, F. Wu, W. Zhu, W. Wang, S. Hao, Int. J. Hydrogen Energy 29 (2004) 1075–1081. [12] F. Aupreˆte, C. Descorme, D. Duprez, Catal. Commun. 3 (2002) 263– 267. [13] F. Marin˜o, G. Baronetti, M. Jobbagy, M. Laborde, Appl. Catal. A: Gen. 238 (2003) 41–54. [14] F. Frusteri, S. Freni, V. Chiodo, L. Spadaro, O. Di Blasi, G. Bonura, S. Cavallaro, Appl. Catal. A: Gen. 270 (2004) 1–7. [15] A.J. Akande, R.O. Idem, A.K. Dalai, Appl. Catal. A: Gen. 287 (2005) 159–175. [16] P. Malet, A. Caballero, J. Chem. Soc., Faraday Trans. 84 (1988) 2369– 2375. [17] M.N. Barroso, M.F. Gomez, L.A. Arrua, M.C. Abello, Chem. Eng. Trans. 4 (2004) 265–270. [18] M.N. Barroso, M.F. Gomez, J. Andrade Gamboa, L.A. Arrua, M.C. Abello, J. Phys. Chem. Solids, in press. [19] S. Mathur, M. Veith, M. Haas, H. Shen, N. Lecerf, V. Huch, J. Am. Ceram. Soc. 84 (2001) 1921–1928. [20] A. Bielanzki, J. Haber, Oxygen in Catalysis, Marcel Dekker, New York, 1991, Chapter I, p. 29. [21] J.I. Di Cosimo, C.R. Apesteguı´a, J. Catal. 116 (1989) 71–81.

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