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ZrO2
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 0 ( 2 0 1 5 ) 8 7 0 6 e8 7 1 2
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Elimination of acetaldehyde from hydrogen rich streams employing Ni/ZrO2 Daniela C.D. da Silva a,b, Sonia Letichevsky a, Luiz E.P. Borges b, Lucia G. Appel a,* lise, Instituto Nacional de Tecnologia, Av. Venezuela 82/518, Rio de Janeiro, RJ 21081-312, Laborat orio de Cata Brazil b Instituto Militar de Engenharia, Rio de Janeiro, RJ 22290-270, Brazil a
article info
abstract
Article history:
The elimination of acetaldehyde from H2 streams was studied employing the TPSR of
Received 18 February 2015
acetaldehyde decomposition, thermodesorption of acetaldehyde followed by FTIR and
Received in revised form
catalytic tests employing Ni/ZrO2 catalyst and also Ni/SiO2 catalyst, being the latter a
16 April 2015
reference. The results show that Ni/ZrO2 is a very promising catalytic system for the CO
Accepted 1 May 2015
and acetaldehyde simultaneous elimination from H2 rich streams. Indeed, the outstanding
Available online 28 May 2015
performance of Ni/ZrO2 when compared with Ni/SiO2 might be related to two factors: its high activity toward acetaldehyde and CO elimination and its large number of active sites
Keywords:
located on the metal and mainly on the support.
Hydrogen
Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Purification Acetaldehyde Carbon monoxide
Introduction Ethanol steam reforming has been proposed for the generation of renewable H2, which can be employed as fuel in the Proton Exchange Membrane Fuel Cells (PEMFC). In general, H2 is produced simultaneously with CO2 and small amounts of CO, CH4, ethylene, acetaldehyde, acetone and coke [1e4]. The PEMFC electrocatalysts are based on Pt. As well known, CO is a poison for them. In spite of the recent developments of CO resistant electrocatalysts, this compound must be yet removed from the reformed streams (less than 100 ppm) [5]. Starz et al. [6] verified that acetaldehyde adsorbs strongly on Pt inhibiting almost completely the H2 adsorption. Moreover, De Lima et al. [7] employing Pt/SiO2, Pt/USY and Pt/C (eletrocatalyst) verified that this aldehyde is very reactive and
readily decomposes on Pt. This precious metal promotes the CeC bond scission of acetaldehyde at a wide range of temperatures producing CH4 and CO. At low temperatures (<200 C), the CeO bond breaking also occurs, generating CO, H2, oxygen and carbon residues. Heinen et al. [8] employing Pt thin film electrode under continuous electrolyte flow verified that, first acetaldehyde is adsorbed and loses one hydrogen atom (acetyl formation). After that, the dissociation (CeC bond breaking) of these adsorbed species occurs. These authors suggested that the rate limiting step (RLS) for the dissociative acetaldehyde adsorption at low potentials is the CeC bond scission of the adsorbed acetyl species. At higher potentials, the rate for acetyl decomposition increases which results in a fast poisoning of the Pt surface due to the formation of COad and CHx,ad species.
* Corresponding author. E-mail address: [email protected] (L.G. Appel). http://dx.doi.org/10.1016/j.ijhydene.2015.05.005 0360-3199/Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
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Acetaldehyde also decomposes on oxides and other compounds. Thus, even when electrocatalysts without precious metal are to be available, acetaldehyde should be removed from the H2 stream in order to avoid equipment damage. However, despite the relevance of this subject there are very few pieces of information related to the elimination of acetaldehyde from hydrogen rich streams. Lima et al. [2], in order to eliminate CO in the presence of acetaldehyde, employed the preferential CO oxidation reaction (PROX) using Pt/CeO2 as catalyst. Due to this aldehyde decomposition, CH4 and more CO were observed. They inferred that the PROX reaction shifts the equilibrium of the acetaldehyde decarbonylation. However, this oxidation reaction was not able to decrease the CO concentration to acceptable levels. Colman et al. [9] suggested that in the presence of acetaldehyde, the hydrogen purification can be carried out employing two catalysts arranged in a double bed reactor. In the first layer, Ni based catalyst decomposes acetaldehyde generating mainly CO and CH4. In the second one, CO is almost totally converted to CH4 employing a Ru based catalyst. As acetaldehyde is a poison for the Ru catalyst, it is crucial that it must be completely eliminated before the methanation reaction. The methanation of CO is very well described in the literature [10,11]. Ruthenium is considered to be the most active metal for this reaction. Nickel, however, not only has high level of activity for the methanation reaction, but also can be acquired at low price rendering it the most employed metal. The catalytic performance (activity and selectivity) of the Ni catalysts changes according to: composition of the support, metal-support interactions, Ni metal particles sizes and the amount of Ni in the catalyst [10]. Andersson et al. [12] suggested that under methanation conditions, the CO dissociation proceeds most favorably over undercoordinated sites (step/edge), which are the primary sites for the carbon monoxide dissociation on Ni (111) surfaces. This step is the RLS (rate limiting step) of the carbon monoxide methanation. Recently, Silva et al. [13] showed that Ni/ZrO2 catalyst is very active for the carbon monoxide methanation reaction. Ni/ZrO2 shows at least two active sites for this reaction. One on Ni and the other on ZrO2, being the latter much more active than the metal one. These authors proposed that the high activity of Ni/ZrO2 is associated with the hydrogen spillover from Ni to the support, and mainly, the ability of ZrO2 to dissociate carbon monoxide. Taking into account that Ni/ZrO2 catalyst is very active for the CO methanation [13] and that Ni is able to promote the acetaldehyde decomposition [9], it is relevant to evaluate the performance of Ni/ZrO2 in the simultaneous elimination of acetaldehyde and CO from a H2 rich stream. Thus, this work describes the performance of Ni/ZrO2 in the acetaldehyde and CO elimination from H2 streams using Ni/SiO2 as a reference.
Experimental Synthesis of the catalysts The Ni/ZrO2 and Ni/SiO2 catalysts were synthesized employing the wet impregnation method using Ni(NO3)2.6H2O. The Ni
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concentration for both catalysts was 10 wt.%. The ZrO2 (monoclinic) and SiO2 were supplied by Saint-Gobain NorPro. After the impregnation, the solids were dried at 100 C and calcined at 500 C for 5 h in air (20 mL min1).
Catalytic test The catalytic tests were carried out using a fixed-bed reactor. The reactants and products were monitored by an Agilent 6890 GC gas chromatograph equipped with flame ionization (with a methanator) and thermal conductivity detectors. The samples were pretreated at 500 C (5oCmin1) under N2 flow (30 mL min1) for 30 min and then reduced at the same temperature under H2 flow (30 mL min-1) for 30 min. The reaction was conducted from 150 up to 400 C at 1 atm. The flow rate, the catalyst and SiC mass were: 70 mL min-1, 300 mg and 300 mg, respectively. A mixture comprised of 1% of acetaldehyde, 50% of H2 and 49% of N2 was employed. These experiments were designed in order to analyze the CO elimination. 1% v/v of acetaldehyde was chosen for this study considering the work of Lima et al. [2] which observed this same concentration in the exit stream of the ethanol reforming.
Characterization Temperature programmed surface reaction (TPSR) The TPSR of the acetaldehyde decomposition was carried out employing a micro reactor coupled to a Balzers quadrupole QMS200 mass spectrometer. The catalysts (Ni/SiO2 and Ni/ZrO2) were pretreated at 500 C (5oCmin1) under N2 flow (30 mL min-1) for 30 min and then reduced at the same temperature under H2 flow (30 mL min-1) for 30 min, as well. After that, the solids were cooled down and exposed to acetaldehyde (4% acetaldehyde/N2, 20 mL min-1) for 1 h at room temperature. The TPSR of acetaldehyde measurements were carried out heating the samples (10oCmin1) up to 500 C under H2 flow (80 mL min-1). The samples were kept at this temperature for 30 min. The fragments m/z ¼ 44, 31, 29, 28, 16 and 2 related to CO2, ethanol, acetaldehyde, CO, CH4 e H2, respectively, were continuously monitored. The intensities of these fragments were mathematically treated in order to eliminate contributions of more than one species.
The adsorption of acetaldehyde on Ni/ZrO2 and Ni/SiO2 analyzed by FTIR The spectra were recorded employing a Nicolet Magna-IR 560 spectrometer using thin self-supporting wafers (~20 mg). The catalysts samples (Ni/ZrO2 and Ni/SiO2) were pretreated according to the procedure described above. Afterwards, they were exposed to high vacuum for 30 min at room temperature. Next, acetaldehyde was admitted in the cell (933 Pa) for 30 min and then the system was exposed to vacuum again. Using the transmission mode the spectra were collected after desorption for 30 min at 25, 100, 200, 300, 400 and 500 C under high vacuum.
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Results and discussion The catalysts The catalysts selected (Ni/ZrO2 and Ni/SiO2) for the acetaldehyde elimination are the same ones described and used by Silva et al. [13] in the CO methanation reaction. The Ni concentration for both catalysts is 10 wt %. The ratio between the methanation rates of Ni/ZrO2 and Ni/SiO2 is almost 20 at 150 C. Silva et al. [13] suggested that the CO methanation occurs not only on the metallic sites but also on the support sites, being the latter the most active. This proposal was based on data generated by the TPSR of the methanation reaction, TPD of CO and other techniques. These authors also evaluated the Nio metallic surface areas employing the cyclohexane dehydrogenation rate (ratCH), which is directly proportional to the number of metallic atoms on the surface per catalyst mass unit (NiS). According to Mahata et al. [14], the metal particle size (dNi) is inversely proportional to the dispersion of the metal (D), which is defined as the ratio between the number of Ni atoms on the surface per catalyst mass unit (Nis) and the total number of Ni atoms (NiT) of the catalyst per catalyst mass unit (D ¼ NiS/NiT). As these catalysts show the same amount of Ni per mass unit (NiT), their particle size ratio (dNi/ZrO2/dNi/SiO2) is inversely proportional to the ratio of the number of Ni atoms on the surface of these catalysts (NiS) dNi/ZrO2/dNi/SiO2¼1/(NiS-ZrO2/ NiS-SiO2) or to the cyclohexane dehydrogenation rate of the same catalysts dNi/ZrO2/dNi/SiO2¼1/(ratCH-ZrO2/ratCH-SiO2). Silva et al. [13] showed that the cyclohexane dehydrogenation rate of Ni/ZrO2 is 60% higher than the one of Ni/SiO2 (ratCH-ZrO2/ ratCH-SiO2 ¼ 1.6). Therefore, the particle size ratio of these two catalysts (dNi/ZrO2/dNi/SiO2) is equal to 0.62. Thus, the Ni particle size of Ni/SiO2 is 60% larger than the one of Ni/ZrO2. According to Andersson et al. [12], the smaller the Ni particle size, the higher the activity in the methanation reaction of the catalyst. Thus, the Ni/ZrO2 activity can also be associated with its particle size. Accordingly, two main factors (support activity and particle size) might contribute to the outstanding performance of Ni/ZrO2 in the methanation reaction.
The acetaldehyde elimination Figs. 1 and 2a depict the selectivities to CO, CH4 and ethanol versus the temperature for Ni/SiO2 and Ni/ZrO2 catalysts, respectively. Fig. 1 and 2b show the CO concentrations (ppm) in the gaseous mixture at the output of the reactor at different temperatures. The acetaldehyde conversion is around 100% for both catalysts from 150 to 300 C, except for Ni/ZrO2, which shows a lower conversion (70%) at 150 C. When Ni/SiO2 catalyst is employed at low temperatures (T < 250 C), ethanol generated by acetaldehyde hydrogenation is the main compound observed (Fig. 1a). At these temperatures, small amounts of diethoxyethane (acetal) and ethyl acetate are also synthesized by condensation reactions between acetaldehyde and ethanol [15,16]. At high temperatures, the acetaldehyde decarbonylation is prevalent. This reaction produces equimolecular amounts of CH4 and CO. However, above 200 C, due to the CO methanation, the
Fig. 1 e a. Selectivities of Ni/SiO2 catalyst to carbon monoxide (*), methane (+) and ethanol ( ) versus temperature. b. Concentration of CO (ppm) in the outlet stream of the catalytic test versus temperature, Ni/SiO2 catalyst.
selectivity to CH4 is higher than the one to CO. Fig.1b depicts that the CO concentration increases from 150 C up to 250 C and it is only eliminated above 300 C.
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Employing Ni/ZrO2 catalyst (Fig. 2a), the same compounds cited for Ni/SiO2 are produced including the ones obtained at very low amounts by condensation reactions, which are also generated at low temperatures (not shown). Fig. 1 and 2a depict the competition between the acetaldehyde decarbonylation and the hydrogenation of this aldehyde to ethanol. At high temperature, the former is the most relevant reaction and at low temperature, the latter. At low temperature, Ni/ ZrO2 is less active for the acetaldehyde hydrogenation than Ni/SiO2 (lower conversion). It can also be verified that the CO generated by the decarbonylation of acetaldehyde on the ZrO2 catalyst begins to be hydrogenated at much lower temperature (200 C) than on the SiO2 catalyst. Moreover, from 150 to 300 C the Ni/ZrO2 catalyst shows much lower concentrations of CO in the hydrogen stream when compared with Ni/SiO2 (Fig. 1 and 2b). Indeed, Ni/ ZrO2 reaches CO concentrations lower than 20 ppm at 250 C while Ni/SiO2 needs higher temperature to eliminate CO.
The adsorption of acetaldehyde over Ni/SiO2 and Ni/ZrO2 analyzed by FTIR Firstly, acetaldehyde was adsorbed at 25 C. After that, the catalysts were heated up to 500 C. Fig. 3 shows the FTIR spectra collected in the range of 1800e1300 cm1 at 25, 100, 200, 300, 400 and 500 C. As it can be observed, the Ni/SiO2 catalyst spectra (Fig. 3a) depict three bands at 1718, 1574 and 1420 cm-1. The first one can be associated with nas(CO) of acetaldehyde adsorbed on Ni. The other two vibrations can be assigned to nas(COOH) and na(COOH) of the bidentate acetate species adsorbed on this metal, respectively [17]. The intensity of the vibrations associated with acetaldehyde and acetates decrease as the temperature increases. However, above 200 C only acetate species are observed. The Ni/ZrO2 catalyst spectra (Fig. 3b) show two broad bands at low temperatures: 1575 and 1428 cm-1, which can be assigned to nas(COOH) and ns(COOH) of the bidentate acetate species, respectively. These species might be adsorbed on both Ni and ZrO2 surface [18]. A shoulder at 1329 cm-1 can also be observed at these low temperatures. As the temperature increases, the intensity of this shoulder increases as well, whereas, the vibration at 1575 cm-1 shifts to 1611 cm-1. These bands, not observed in the case of Ni/SiO2, can be assigned to monodentate acetate species adsorbed on ZrO2 [18] which show higher thermal stability than the ones on Nio. These results agree with the findings of Rodrigues et al. [19], who proposed that as ZrO2 shows superficial O mobility, it is able to oxidize acetaldehyde to acetate species.
Fig. 2 e a. Selectivities of Ni/ZrO2 catalyst to carbon monoxide (*), methane (+) and ethanol ( ) versus temperature. b. Concentration of CO (ppm) in the outlet stream of the catalytic test versus temperature, Ni/ZrO2 catalyst.
TPSR of acetaldehyde over Ni/ZrO2 and Ni/SiO2 Fig. 4a and b shows the results of the TPSR of acetaldehyde over Ni/SiO2 and Ni/ZrO2, respectively. At low temperatures (t < 180 C), Fig. 4a depicts the ethanol desorption, showing that acetaldehyde is hydrogenated to ethanol. This result is in line with the ones of the catalytic tests. At these low temperatures, the TPSR of acetaldehyde on Ni/SiO2 also shows the generation of CO.
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Fig. 3 e a. FTIR spectra of acetaldehyde adsorbed on Ni/SiO2 at 25 C (a), 100 C (b), 200 C (c), 300 C (d), 400 C (e) and 500 C (f). b. FTIR spectra of acetaldehyde adsorbed on Ni/ZrO2 at 25 C(a), 100 C (b), 200 C (c), 300 C (d), 400 C (e) and 500 C (f).
Mc Cabe et al. [20] proposed the following mechanism for the acetaldehyde decomposition on Pt: the first step is related to the adsorption of acetaldehyde; after that, this aldehyde loses one H generating acetyl species, which undergo the CeC bond scission producing CH3 (adsorbed) and CO (decarbonylation reaction). The CH3 species are hydrogenated to CH4 by the H of the acetyl synthesis. They also proposed that C]O bond cleavage of the acetyl species produces C2H3 adsorbed (ethylidine) and O as well. The C2H3 species are decomposed generating C (residues) and H2. Finally, the reaction between two molecules of acetyl species produces
hydrocarbons residues, CO and H2. This last step is related not only to the C]O bond scission but also to the CeC cleavage. Therefore, the CO desorption (Fig. 4a), observed at low temperature, can be associated with the acetaldehyde decomposition on Ni (reaction between two molecules of acetyl species). It can also be proposed that the O from the C]O bond cleavage of the acetyl species [20] might oxidize the remaining acetaldehyde generating the acetate species observed by the FTIR experiments.
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Fig. 4 e a. TPSR spectra of acetaldehyde over Ni/SiO2. b. TPSR spectra of acetaldehyde over Ni/ZrO2.
At higher temperatures (CH4 maximum at 250 C), the CO and CH4 desorption (Fig. 4a) might be related to the decomposition of the acetaldehyde and mainly of acetate species [20e22]. Moreover, considering the CH4 high intensity spectra and the catalytic results (Fig. 1a), it can be inferred that the methanation reaction should occur after the decomposition of these species consuming CO. The TPSR spectra intensities of the Ni/ZrO2 catalyst are much higher than the ones of SiO2 (Fig. 4a and b). This occurs
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due to the adsorption of acetaldehyde on the ZrO2 sites, as already observed by Rodrigues et al. [19] and Zonetti et al. [15]. However, the Ni/ZrO2 FTIR spectra show slightly higher intensity when compared with the ones of Ni/SiO2. It can be suggested that acetaldehyde adsorbed on the Ni/ZrO2 support is oxidized or desorbed when the wafer is exposed to vacuum at low temperature. According to the catalytic tests results, the TPSR spectra of acetaldehyde over Ni/ZrO2 (Fig. 4b) show the ethanol desorption at low temperature (85 C). This figure also depicts the simultaneous desorption of acetaldehyde, CO and CH4 at two temperatures: 194 C and 298 C. As well known, Nio dissociates H2. Then, H can migrate toward ZrO2 [13] and might react with the acetate species adsorbed on the support, generating acetaldehyde (intermediate), CO and CH4. These two temperatures (194 C and 298 C (Fig. 4b)) might be associated with the decomposition of the two different acetate species observed by the FTIR. Moreover, considering the catalytic behavior described above (Fig. 2a), it can be inferred that the methanation reaction occurs at both temperatures, whereas, at 298 C, it generates more CH4. Fig. 4a and b exhibit that Ni/ZrO2 desorbs CH4 at much lower temperature (first peak) than Ni/SiO2, showing that the former is more active for the acetate elimination than Ni/SiO2. This result is in line with the ones of the catalytic tests. According to the TPSR spectra of Ni/ZrO2 catalyst, the decomposition of acetate species might be described as follows: first, the acetaldehyde generation on the support surface occurs (reduction of the acetate species), followed by the decarbonylation of this aldehyde (CO and CH4 generation), and, finally, by the CO methanation. It can be proposed that the high activity toward methanation of the Ni/ZrO2 catalyst [13] might promote (shift) the reduction of the acetates species and also the decarbonylation reaction. Taking the results described above into account, it can be suggested that Ni and ZrO2 adsorb acetaldehyde during the reaction. In fact, Ni/ZrO2 adsorbs larger amounts of acetaldehyde than Ni/SiO2. Moreover, H2 is adsorbed and dissociated by Nio. Hydrogen promotes the reduction of acetate species, the decarbonylation of acetaldehyde and the methanation reaction. Furthermore, in the case of Ni/ZrO2, the oxygenate species are mainly adsorbed on ZrO2. For this reason, hydrogen species might spillover from the metal to the support during the reaction. The outstanding performance of Ni/ZrO2 when compared with Ni/SiO2 might be related to two factors: its high activity toward acetate, acetaldehyde and CO elimination and its large number of active sites located on the metal and mainly on the support.
Conclusions The results show that the Ni/ZrO2 catalyst is a very promising system for the simultaneous elimination of CO and acetaldehyde from H2 rich streams generated by ethanol reforming.
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Acknowledgments via Almeida The authors acknowledge Renata Santos and Fla (INT) for the TPSR analyses.
references
[1] Palma V, Castaldo F, Ciambelli P, Laquaniello G. CeO2supported Pt-Ni catalyst for the renewable and clean H2 production via ethanol steam reforming. Appl Catal B 2014;145:73e84. [2] Lima SM, Colman RC, Jacobs G, Davis BH, Souza KR, Lima AFF, et al. Hydrogen production from ethanol for PEM fuel cells. An integrated processor comprising ethanol steam reforming and preferential oxidation of CO. Catal Today 2009;146:110e23. [3] Llorca J, Homs N, Sales J, Fierro JLG, Piscina PR. Effect of sodium addition on the performance of Co-ZnO-based catalysts for hydrogen production from bioethanol. J Catal 2004;222:470e80. [4] Segal SR, Carrado KA, Marshall CL, Anderson KB. Catalytic decomposition of alcohols, including ethanol, for in situ H2 generation in a fuel stream using a layered double hydroxide-derived catalyst. Appl Catal A 2003;248:33e45. € mer M, Sto € we K, Duisberg M, Mu¨ller F, Reiser M, Sticher S, [5] Kra et al. The impact of dopants on the activity and selectivity of a Ni-based methanation catalyst. Appl Catal A 2009;369:42e52. [6] Starz KA, Auer E, Lehmann T, Zuber R. Characteristics of platinum-based electrocatalysts for mobile PEMFC applications. J Power Sources 1999;84:167e72. [7] De Lima AFF, Colman RC, Zotin MFZ, Appel LG. Acetaldehyde behavior over platinum based catalyst in hydrogen stream generated by ethanol reforming. Int J Hydrogen Energy 2010;35:13200e5. [8] Heinen M, Jusys Z, Behm RJ. Ethanol, acetaldehyde and acetic acid adsorption/electrooxidation on a Pt thin film electrode under continuous electrolyte flow: an in situ ATR-FTIRS flow cell study. J Phys Chem C 2010;114:9850e64. [9] Colman RC, Torres LA, Lima AFF, Appel LG. Removing CO and acetaldehyde from hydrogen streams generated by ethanol reforming. Int J Hydrogen Energy 2009;34:9832e7.
[10] Gao J, Liu Q, Gu F, Liu B, Zhong Z, Su F. Recent Advances in methanation catalyst for the production of synthetic natural gas. RSC Adv 2015;5:22759e76. [11] Park ED, Lee D, Lee HC. Recent progress in selective CO removal in a H2-rich stream. Catal Today 2009;139:280e90. [12] Andersson MP, Abild-Pedersen F, Remediakis IN, Bligaard T, Jones G, Engbæk J, et al. Structure sensitivity of the methanation reaction:H2-induced CO dissociation on nickel surfaces. J Catal 2008;255:6e19. [13] Silva DCD, Letichevsky S, Borges LEP, Appel LG. The Ni/ZrO2 catalyst and the methanation of CO and CO2. Int J Hydrogen Energy 2012;37:8923e8. [14] Mahata N, Vishwanathan V. Influence of palladium precursors on structural properties and phenol hydrogenation characteristics of supported palladium catalyst. J Catal 2000;196:262e70. [15] Zonetti PC, Celnik J, Letichevsky S, Gaspar AB, Lucia GA. Chemicals from ethanol e the dehydrogenative route of the ethyl acetate one-pot synthesis. J Mol Catal A Chem 2011;334:29e34. [16] Capaletti MR, Balzano L, de La Puente G, Laborde M, Sedran U. Synthesis of acetal (1,1-diethoxyethane) from ethanol and acetaldehyde over acidic catalysts. Appl Catal A 2000;198:1e2. [17] Madhavaram H, Idriss H. Acetaldehyde reactions over the uranium oxide system. J Catal 2004;224:358e69. [18] Pan Q, Peng J, Sun T, Wang S, Wang S. Insight into the reaction route of CO2 methanation: promotion effect of medium basic sites. Catal Commun 2014;45:74e8. [19] Rodrigues CP, Zonetti PC, Silva CG, Gaspar AB, Appel LG. Chemicals from ethanol e the acetone one-pot synthesis. Appl Catal A General 2013;458:111e8. [20] McCabe RW, DiMaggio CL, Madix RJ. Adsorption and reactions of acetaldehyde on platinum (S)-[6(111).Times. (100)]. J Phys Chem 1985;89:854e61. [21] Silva AM, Costa LOO, Barandas APMG, Borges LEP, Mattos LV, Noronha FB. Effect of the metal nature on the reaction mechanism of the partial oxidation of ethanol over CeO2supported Pt and Rh catalysts. Catal Today 2008;133e135:755e61. [22] Lima SM, Cruz IO, Jacobs G, Davis BH, Mattos LV, Noronha FB. Steam reforming partial oxidation, and oxidative steam reforming of ethanol over Pt/CeZrO2 catalyst. J Catal 2008;257:356e68.
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/he
Elimination of acetaldehyde from hydrogen rich streams employing Ni/ZrO2 Daniela C.D. da Silva a,b, Sonia Letichevsky a, Luiz E.P. Borges b, Lucia G. Appel a,* lise, Instituto Nacional de Tecnologia, Av. Venezuela 82/518, Rio de Janeiro, RJ 21081-312, Laborat orio de Cata Brazil b Instituto Militar de Engenharia, Rio de Janeiro, RJ 22290-270, Brazil a
article info
abstract
Article history:
The elimination of acetaldehyde from H2 streams was studied employing the TPSR of
Received 18 February 2015
acetaldehyde decomposition, thermodesorption of acetaldehyde followed by FTIR and
Received in revised form
catalytic tests employing Ni/ZrO2 catalyst and also Ni/SiO2 catalyst, being the latter a
16 April 2015
reference. The results show that Ni/ZrO2 is a very promising catalytic system for the CO
Accepted 1 May 2015
and acetaldehyde simultaneous elimination from H2 rich streams. Indeed, the outstanding
Available online 28 May 2015
performance of Ni/ZrO2 when compared with Ni/SiO2 might be related to two factors: its high activity toward acetaldehyde and CO elimination and its large number of active sites
Keywords:
located on the metal and mainly on the support.
Hydrogen
Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Purification Acetaldehyde Carbon monoxide
Introduction Ethanol steam reforming has been proposed for the generation of renewable H2, which can be employed as fuel in the Proton Exchange Membrane Fuel Cells (PEMFC). In general, H2 is produced simultaneously with CO2 and small amounts of CO, CH4, ethylene, acetaldehyde, acetone and coke [1e4]. The PEMFC electrocatalysts are based on Pt. As well known, CO is a poison for them. In spite of the recent developments of CO resistant electrocatalysts, this compound must be yet removed from the reformed streams (less than 100 ppm) [5]. Starz et al. [6] verified that acetaldehyde adsorbs strongly on Pt inhibiting almost completely the H2 adsorption. Moreover, De Lima et al. [7] employing Pt/SiO2, Pt/USY and Pt/C (eletrocatalyst) verified that this aldehyde is very reactive and
readily decomposes on Pt. This precious metal promotes the CeC bond scission of acetaldehyde at a wide range of temperatures producing CH4 and CO. At low temperatures (<200 C), the CeO bond breaking also occurs, generating CO, H2, oxygen and carbon residues. Heinen et al. [8] employing Pt thin film electrode under continuous electrolyte flow verified that, first acetaldehyde is adsorbed and loses one hydrogen atom (acetyl formation). After that, the dissociation (CeC bond breaking) of these adsorbed species occurs. These authors suggested that the rate limiting step (RLS) for the dissociative acetaldehyde adsorption at low potentials is the CeC bond scission of the adsorbed acetyl species. At higher potentials, the rate for acetyl decomposition increases which results in a fast poisoning of the Pt surface due to the formation of COad and CHx,ad species.
* Corresponding author. E-mail address: [email protected] (L.G. Appel). http://dx.doi.org/10.1016/j.ijhydene.2015.05.005 0360-3199/Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
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Acetaldehyde also decomposes on oxides and other compounds. Thus, even when electrocatalysts without precious metal are to be available, acetaldehyde should be removed from the H2 stream in order to avoid equipment damage. However, despite the relevance of this subject there are very few pieces of information related to the elimination of acetaldehyde from hydrogen rich streams. Lima et al. [2], in order to eliminate CO in the presence of acetaldehyde, employed the preferential CO oxidation reaction (PROX) using Pt/CeO2 as catalyst. Due to this aldehyde decomposition, CH4 and more CO were observed. They inferred that the PROX reaction shifts the equilibrium of the acetaldehyde decarbonylation. However, this oxidation reaction was not able to decrease the CO concentration to acceptable levels. Colman et al. [9] suggested that in the presence of acetaldehyde, the hydrogen purification can be carried out employing two catalysts arranged in a double bed reactor. In the first layer, Ni based catalyst decomposes acetaldehyde generating mainly CO and CH4. In the second one, CO is almost totally converted to CH4 employing a Ru based catalyst. As acetaldehyde is a poison for the Ru catalyst, it is crucial that it must be completely eliminated before the methanation reaction. The methanation of CO is very well described in the literature [10,11]. Ruthenium is considered to be the most active metal for this reaction. Nickel, however, not only has high level of activity for the methanation reaction, but also can be acquired at low price rendering it the most employed metal. The catalytic performance (activity and selectivity) of the Ni catalysts changes according to: composition of the support, metal-support interactions, Ni metal particles sizes and the amount of Ni in the catalyst [10]. Andersson et al. [12] suggested that under methanation conditions, the CO dissociation proceeds most favorably over undercoordinated sites (step/edge), which are the primary sites for the carbon monoxide dissociation on Ni (111) surfaces. This step is the RLS (rate limiting step) of the carbon monoxide methanation. Recently, Silva et al. [13] showed that Ni/ZrO2 catalyst is very active for the carbon monoxide methanation reaction. Ni/ZrO2 shows at least two active sites for this reaction. One on Ni and the other on ZrO2, being the latter much more active than the metal one. These authors proposed that the high activity of Ni/ZrO2 is associated with the hydrogen spillover from Ni to the support, and mainly, the ability of ZrO2 to dissociate carbon monoxide. Taking into account that Ni/ZrO2 catalyst is very active for the CO methanation [13] and that Ni is able to promote the acetaldehyde decomposition [9], it is relevant to evaluate the performance of Ni/ZrO2 in the simultaneous elimination of acetaldehyde and CO from a H2 rich stream. Thus, this work describes the performance of Ni/ZrO2 in the acetaldehyde and CO elimination from H2 streams using Ni/SiO2 as a reference.
Experimental Synthesis of the catalysts The Ni/ZrO2 and Ni/SiO2 catalysts were synthesized employing the wet impregnation method using Ni(NO3)2.6H2O. The Ni
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concentration for both catalysts was 10 wt.%. The ZrO2 (monoclinic) and SiO2 were supplied by Saint-Gobain NorPro. After the impregnation, the solids were dried at 100 C and calcined at 500 C for 5 h in air (20 mL min1).
Catalytic test The catalytic tests were carried out using a fixed-bed reactor. The reactants and products were monitored by an Agilent 6890 GC gas chromatograph equipped with flame ionization (with a methanator) and thermal conductivity detectors. The samples were pretreated at 500 C (5oCmin1) under N2 flow (30 mL min1) for 30 min and then reduced at the same temperature under H2 flow (30 mL min-1) for 30 min. The reaction was conducted from 150 up to 400 C at 1 atm. The flow rate, the catalyst and SiC mass were: 70 mL min-1, 300 mg and 300 mg, respectively. A mixture comprised of 1% of acetaldehyde, 50% of H2 and 49% of N2 was employed. These experiments were designed in order to analyze the CO elimination. 1% v/v of acetaldehyde was chosen for this study considering the work of Lima et al. [2] which observed this same concentration in the exit stream of the ethanol reforming.
Characterization Temperature programmed surface reaction (TPSR) The TPSR of the acetaldehyde decomposition was carried out employing a micro reactor coupled to a Balzers quadrupole QMS200 mass spectrometer. The catalysts (Ni/SiO2 and Ni/ZrO2) were pretreated at 500 C (5oCmin1) under N2 flow (30 mL min-1) for 30 min and then reduced at the same temperature under H2 flow (30 mL min-1) for 30 min, as well. After that, the solids were cooled down and exposed to acetaldehyde (4% acetaldehyde/N2, 20 mL min-1) for 1 h at room temperature. The TPSR of acetaldehyde measurements were carried out heating the samples (10oCmin1) up to 500 C under H2 flow (80 mL min-1). The samples were kept at this temperature for 30 min. The fragments m/z ¼ 44, 31, 29, 28, 16 and 2 related to CO2, ethanol, acetaldehyde, CO, CH4 e H2, respectively, were continuously monitored. The intensities of these fragments were mathematically treated in order to eliminate contributions of more than one species.
The adsorption of acetaldehyde on Ni/ZrO2 and Ni/SiO2 analyzed by FTIR The spectra were recorded employing a Nicolet Magna-IR 560 spectrometer using thin self-supporting wafers (~20 mg). The catalysts samples (Ni/ZrO2 and Ni/SiO2) were pretreated according to the procedure described above. Afterwards, they were exposed to high vacuum for 30 min at room temperature. Next, acetaldehyde was admitted in the cell (933 Pa) for 30 min and then the system was exposed to vacuum again. Using the transmission mode the spectra were collected after desorption for 30 min at 25, 100, 200, 300, 400 and 500 C under high vacuum.
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Results and discussion The catalysts The catalysts selected (Ni/ZrO2 and Ni/SiO2) for the acetaldehyde elimination are the same ones described and used by Silva et al. [13] in the CO methanation reaction. The Ni concentration for both catalysts is 10 wt %. The ratio between the methanation rates of Ni/ZrO2 and Ni/SiO2 is almost 20 at 150 C. Silva et al. [13] suggested that the CO methanation occurs not only on the metallic sites but also on the support sites, being the latter the most active. This proposal was based on data generated by the TPSR of the methanation reaction, TPD of CO and other techniques. These authors also evaluated the Nio metallic surface areas employing the cyclohexane dehydrogenation rate (ratCH), which is directly proportional to the number of metallic atoms on the surface per catalyst mass unit (NiS). According to Mahata et al. [14], the metal particle size (dNi) is inversely proportional to the dispersion of the metal (D), which is defined as the ratio between the number of Ni atoms on the surface per catalyst mass unit (Nis) and the total number of Ni atoms (NiT) of the catalyst per catalyst mass unit (D ¼ NiS/NiT). As these catalysts show the same amount of Ni per mass unit (NiT), their particle size ratio (dNi/ZrO2/dNi/SiO2) is inversely proportional to the ratio of the number of Ni atoms on the surface of these catalysts (NiS) dNi/ZrO2/dNi/SiO2¼1/(NiS-ZrO2/ NiS-SiO2) or to the cyclohexane dehydrogenation rate of the same catalysts dNi/ZrO2/dNi/SiO2¼1/(ratCH-ZrO2/ratCH-SiO2). Silva et al. [13] showed that the cyclohexane dehydrogenation rate of Ni/ZrO2 is 60% higher than the one of Ni/SiO2 (ratCH-ZrO2/ ratCH-SiO2 ¼ 1.6). Therefore, the particle size ratio of these two catalysts (dNi/ZrO2/dNi/SiO2) is equal to 0.62. Thus, the Ni particle size of Ni/SiO2 is 60% larger than the one of Ni/ZrO2. According to Andersson et al. [12], the smaller the Ni particle size, the higher the activity in the methanation reaction of the catalyst. Thus, the Ni/ZrO2 activity can also be associated with its particle size. Accordingly, two main factors (support activity and particle size) might contribute to the outstanding performance of Ni/ZrO2 in the methanation reaction.
The acetaldehyde elimination Figs. 1 and 2a depict the selectivities to CO, CH4 and ethanol versus the temperature for Ni/SiO2 and Ni/ZrO2 catalysts, respectively. Fig. 1 and 2b show the CO concentrations (ppm) in the gaseous mixture at the output of the reactor at different temperatures. The acetaldehyde conversion is around 100% for both catalysts from 150 to 300 C, except for Ni/ZrO2, which shows a lower conversion (70%) at 150 C. When Ni/SiO2 catalyst is employed at low temperatures (T < 250 C), ethanol generated by acetaldehyde hydrogenation is the main compound observed (Fig. 1a). At these temperatures, small amounts of diethoxyethane (acetal) and ethyl acetate are also synthesized by condensation reactions between acetaldehyde and ethanol [15,16]. At high temperatures, the acetaldehyde decarbonylation is prevalent. This reaction produces equimolecular amounts of CH4 and CO. However, above 200 C, due to the CO methanation, the
Fig. 1 e a. Selectivities of Ni/SiO2 catalyst to carbon monoxide (*), methane (+) and ethanol ( ) versus temperature. b. Concentration of CO (ppm) in the outlet stream of the catalytic test versus temperature, Ni/SiO2 catalyst.
selectivity to CH4 is higher than the one to CO. Fig.1b depicts that the CO concentration increases from 150 C up to 250 C and it is only eliminated above 300 C.
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Employing Ni/ZrO2 catalyst (Fig. 2a), the same compounds cited for Ni/SiO2 are produced including the ones obtained at very low amounts by condensation reactions, which are also generated at low temperatures (not shown). Fig. 1 and 2a depict the competition between the acetaldehyde decarbonylation and the hydrogenation of this aldehyde to ethanol. At high temperature, the former is the most relevant reaction and at low temperature, the latter. At low temperature, Ni/ ZrO2 is less active for the acetaldehyde hydrogenation than Ni/SiO2 (lower conversion). It can also be verified that the CO generated by the decarbonylation of acetaldehyde on the ZrO2 catalyst begins to be hydrogenated at much lower temperature (200 C) than on the SiO2 catalyst. Moreover, from 150 to 300 C the Ni/ZrO2 catalyst shows much lower concentrations of CO in the hydrogen stream when compared with Ni/SiO2 (Fig. 1 and 2b). Indeed, Ni/ ZrO2 reaches CO concentrations lower than 20 ppm at 250 C while Ni/SiO2 needs higher temperature to eliminate CO.
The adsorption of acetaldehyde over Ni/SiO2 and Ni/ZrO2 analyzed by FTIR Firstly, acetaldehyde was adsorbed at 25 C. After that, the catalysts were heated up to 500 C. Fig. 3 shows the FTIR spectra collected in the range of 1800e1300 cm1 at 25, 100, 200, 300, 400 and 500 C. As it can be observed, the Ni/SiO2 catalyst spectra (Fig. 3a) depict three bands at 1718, 1574 and 1420 cm-1. The first one can be associated with nas(CO) of acetaldehyde adsorbed on Ni. The other two vibrations can be assigned to nas(COOH) and na(COOH) of the bidentate acetate species adsorbed on this metal, respectively [17]. The intensity of the vibrations associated with acetaldehyde and acetates decrease as the temperature increases. However, above 200 C only acetate species are observed. The Ni/ZrO2 catalyst spectra (Fig. 3b) show two broad bands at low temperatures: 1575 and 1428 cm-1, which can be assigned to nas(COOH) and ns(COOH) of the bidentate acetate species, respectively. These species might be adsorbed on both Ni and ZrO2 surface [18]. A shoulder at 1329 cm-1 can also be observed at these low temperatures. As the temperature increases, the intensity of this shoulder increases as well, whereas, the vibration at 1575 cm-1 shifts to 1611 cm-1. These bands, not observed in the case of Ni/SiO2, can be assigned to monodentate acetate species adsorbed on ZrO2 [18] which show higher thermal stability than the ones on Nio. These results agree with the findings of Rodrigues et al. [19], who proposed that as ZrO2 shows superficial O mobility, it is able to oxidize acetaldehyde to acetate species.
Fig. 2 e a. Selectivities of Ni/ZrO2 catalyst to carbon monoxide (*), methane (+) and ethanol ( ) versus temperature. b. Concentration of CO (ppm) in the outlet stream of the catalytic test versus temperature, Ni/ZrO2 catalyst.
TPSR of acetaldehyde over Ni/ZrO2 and Ni/SiO2 Fig. 4a and b shows the results of the TPSR of acetaldehyde over Ni/SiO2 and Ni/ZrO2, respectively. At low temperatures (t < 180 C), Fig. 4a depicts the ethanol desorption, showing that acetaldehyde is hydrogenated to ethanol. This result is in line with the ones of the catalytic tests. At these low temperatures, the TPSR of acetaldehyde on Ni/SiO2 also shows the generation of CO.
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Fig. 3 e a. FTIR spectra of acetaldehyde adsorbed on Ni/SiO2 at 25 C (a), 100 C (b), 200 C (c), 300 C (d), 400 C (e) and 500 C (f). b. FTIR spectra of acetaldehyde adsorbed on Ni/ZrO2 at 25 C(a), 100 C (b), 200 C (c), 300 C (d), 400 C (e) and 500 C (f).
Mc Cabe et al. [20] proposed the following mechanism for the acetaldehyde decomposition on Pt: the first step is related to the adsorption of acetaldehyde; after that, this aldehyde loses one H generating acetyl species, which undergo the CeC bond scission producing CH3 (adsorbed) and CO (decarbonylation reaction). The CH3 species are hydrogenated to CH4 by the H of the acetyl synthesis. They also proposed that C]O bond cleavage of the acetyl species produces C2H3 adsorbed (ethylidine) and O as well. The C2H3 species are decomposed generating C (residues) and H2. Finally, the reaction between two molecules of acetyl species produces
hydrocarbons residues, CO and H2. This last step is related not only to the C]O bond scission but also to the CeC cleavage. Therefore, the CO desorption (Fig. 4a), observed at low temperature, can be associated with the acetaldehyde decomposition on Ni (reaction between two molecules of acetyl species). It can also be proposed that the O from the C]O bond cleavage of the acetyl species [20] might oxidize the remaining acetaldehyde generating the acetate species observed by the FTIR experiments.
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Fig. 4 e a. TPSR spectra of acetaldehyde over Ni/SiO2. b. TPSR spectra of acetaldehyde over Ni/ZrO2.
At higher temperatures (CH4 maximum at 250 C), the CO and CH4 desorption (Fig. 4a) might be related to the decomposition of the acetaldehyde and mainly of acetate species [20e22]. Moreover, considering the CH4 high intensity spectra and the catalytic results (Fig. 1a), it can be inferred that the methanation reaction should occur after the decomposition of these species consuming CO. The TPSR spectra intensities of the Ni/ZrO2 catalyst are much higher than the ones of SiO2 (Fig. 4a and b). This occurs
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due to the adsorption of acetaldehyde on the ZrO2 sites, as already observed by Rodrigues et al. [19] and Zonetti et al. [15]. However, the Ni/ZrO2 FTIR spectra show slightly higher intensity when compared with the ones of Ni/SiO2. It can be suggested that acetaldehyde adsorbed on the Ni/ZrO2 support is oxidized or desorbed when the wafer is exposed to vacuum at low temperature. According to the catalytic tests results, the TPSR spectra of acetaldehyde over Ni/ZrO2 (Fig. 4b) show the ethanol desorption at low temperature (85 C). This figure also depicts the simultaneous desorption of acetaldehyde, CO and CH4 at two temperatures: 194 C and 298 C. As well known, Nio dissociates H2. Then, H can migrate toward ZrO2 [13] and might react with the acetate species adsorbed on the support, generating acetaldehyde (intermediate), CO and CH4. These two temperatures (194 C and 298 C (Fig. 4b)) might be associated with the decomposition of the two different acetate species observed by the FTIR. Moreover, considering the catalytic behavior described above (Fig. 2a), it can be inferred that the methanation reaction occurs at both temperatures, whereas, at 298 C, it generates more CH4. Fig. 4a and b exhibit that Ni/ZrO2 desorbs CH4 at much lower temperature (first peak) than Ni/SiO2, showing that the former is more active for the acetate elimination than Ni/SiO2. This result is in line with the ones of the catalytic tests. According to the TPSR spectra of Ni/ZrO2 catalyst, the decomposition of acetate species might be described as follows: first, the acetaldehyde generation on the support surface occurs (reduction of the acetate species), followed by the decarbonylation of this aldehyde (CO and CH4 generation), and, finally, by the CO methanation. It can be proposed that the high activity toward methanation of the Ni/ZrO2 catalyst [13] might promote (shift) the reduction of the acetates species and also the decarbonylation reaction. Taking the results described above into account, it can be suggested that Ni and ZrO2 adsorb acetaldehyde during the reaction. In fact, Ni/ZrO2 adsorbs larger amounts of acetaldehyde than Ni/SiO2. Moreover, H2 is adsorbed and dissociated by Nio. Hydrogen promotes the reduction of acetate species, the decarbonylation of acetaldehyde and the methanation reaction. Furthermore, in the case of Ni/ZrO2, the oxygenate species are mainly adsorbed on ZrO2. For this reason, hydrogen species might spillover from the metal to the support during the reaction. The outstanding performance of Ni/ZrO2 when compared with Ni/SiO2 might be related to two factors: its high activity toward acetate, acetaldehyde and CO elimination and its large number of active sites located on the metal and mainly on the support.
Conclusions The results show that the Ni/ZrO2 catalyst is a very promising system for the simultaneous elimination of CO and acetaldehyde from H2 rich streams generated by ethanol reforming.
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Acknowledgments via Almeida The authors acknowledge Renata Santos and Fla (INT) for the TPSR analyses.
references
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