Propene versus propane steam reforming for hydrogen production over Pd-based and Ni-based catalysts

Size: px

Start display at page:

Download "Propene versus propane steam reforming for hydrogen production over Pd-based and Ni-based catalysts"

Camron Leonard
5 years ago
Views:

1 Catalysis Communications 6 (25) Propene versus propane steam reforming for hydrogen production over Pd-based and Ni-based catalysts Carlo Resini a,b,c, Maria Concepción Herrera Delgado d, Laura Arrighi c,e, Luis J. Alemany d, Rinaldo Marazza c,e, Guido Busca a,b,c, * a Dipartimento di Ingegneria Chimica e di Processo, Università di Genova, P.le J.F. Kennedy, 1, Genova, Italy b Centro Interuniversitario di Ricerca di Monitoraggio Ambientale (CIMA), Via Cadorna 7; 171 Savona c Consorzio INSTM, Via Benedetto Varchi n. 59, 5132 Firenze d Departamento de Ingenieria Quimica, Universidad de Malaga, 2971 Malaga, Spain e Dipartimento di Chimica e Chimica Industriale, Università di Genova, Via Dodecaneso 31, Genova, Italy Received 25 January 25; revised 25 March 25; accepted 27 March 25 Available online 2 June 25 Abstract Experiments of propane and propene steam reforming have been performed over a Pd Cu/c-Al 2 O 3 catalyst and over a Ni/NiAl 2 O 4 catalyst. Over the palladium-based catalyst, the steam reforming of propene is faster and more selective than steam reforming of propane. Over the Pd catalyst, the steam reforming of propane is likely inhibited by site poisoning. In contrast, over Ni-based catalyst the reforming of propane and propene have similar reaction rates and product selectivity. This suggests a different mechanisms for the two catalysts. The well-known ability of Pd centers to adsorb olefins and catalyze their transformation suggests that over this catalyst p-bonding or di-r-bonding of the olefin may be a key feature allowing its transformation. Ó 25 Elsevier B.V. All rights reserved. 1. Introduction The methane or natural gas steam reforming represents since decades the most efficient way for the largescale production of hydrogen [1], like in the cases of ammonia and methanol synthesis processes [2], steel and iron recovering processes [3] and hydrogenation of organics, e.g., for the production of amines [4] and also to supply additional hydrogen to refineries for hydrotreatings [5]. The typical catalyst is based on Ni Al 2 O 3 [6] and may be prepared either by impregnation or by co-precipitation [7]. Noble metal catalysts may display even higher activity than Ni-based ones, but are usually not applied for economic reasons. Alternative hydrogen production can be carried out by steam reforming and/ * Corresponding author. Tel.: ; fax: address: guido.busca@unige.it (G. Busca). or partial oxidation of higher hydrocarbons like, e.g., heavy naphta [8]. The successful development of hydrogen fuel cells and their forthcoming wide application make necessary the development of small-scale hydrogen production systems. On the other hand, direct fuel cell [9,1] are also under development where the fuel is different from hydrogen and is directly fed to the anode that also catalyzes the hydrogen forming reaction. For these smallscale applications, the use of noble metals as anode-catalysts materials can be more convenient. For on-board fuel cell applications (ships, cars, trucks), gaseous fuels (like hydrogen, methane, natural gas) may be conveniently substituted by liquid fuels (like gasoline or alcohols) or by easily condensable fuels like propane. On the other hand, the presence of propane in natural gas is relevant because it favors coking of the catalysts: to limit it propane pre-reforming may be an option [1]. Noblemetal-based catalysts including Pd-based systems seem /$ - see front matter Ó 25 Elsevier B.V. All rights reserved. doi:1.116/j.catcom

2 442 C. Resini et al. / Catalysis Communications 6 (25) to offer good performances for hydrogen production from propane [11 13]. However, propene has been found among the products in this case. On the other hand, no data are available in the literature on propene reforming. Pd Cu alloys show good hydrogen permeance [14], while supported Pd Cu catalysts are also active in steam reforming reactions [15]. In the present investigation, we focused on the steam reforming of propane and propene over two catalysts which may also be used as anodes for directly fuelled FCs, i.e., a Pd Cu-alumina and a Nialumina catalyst. 2. Experimental 2.1. Catalyst preparation The catalyst Pd Cu/c-Al 2 O 3 catalyst. has been prepared by co-precipitation of the two metal hydroxides over c-al 2 O 3 and subsequently dried at 8 C for 48 h. The catalyst composition Pd Cu (1:5 atomic ratio, 3% metal w/w) supported on c-al 2 O 3 has been confirmed by microanalysis (EPMA). XRD of the fresh catalyst shows the typical broad pattern of c-al 2 O 3 together with very weak peaks of Pd and Cu oxides while reduction in mild conditions gives rise to metal particles. The Ni/NiAl 2 O 4 catalyst has been prepared by coprecipitation via an hydrotalcite-like precursor as reported previously [16], similar to those reported in the literature [6,7]. Its composition is Ni.5 Al 2 O 3.5. The catalyst has been reduced in hydrogen flow for 1 h at 723 K before catalytic experiments Catalytic tests The catalytic experiments were carried out in a fixed-bed tubular quartz reactor containing 15 mg of catalyst mixed with 35 mg of quartz particles. Product analysis was performed with an on-line gas-chromatographs (GC) in order to detect both carbon species and. A Agilent 489 GC equipped with a Varian capillary column Select Permanent Gases Molsieve 5A/Porabond Q Tandem and TCD and FID detectors in series. Between them a Nickel Catalyst Tube has been employed to reduce CO to. A six-port valve with a.5 cm 3 loop was used for the gas sampling of the outlet gases. The catalytic tests were performed by feeding stoichiometric water-to-organic ratios sufficient to obtain the complete steam reforming and WGS reactions this means a ratio O/organic = 6/1. Results are reported based on reactant conversion and product selectivities. Selectivity to has been defined as the fraction of hydrogen produced with respect to the theoretical full conversion of the organic reactant to hydrogen obtainable from its complete reforming. Selectivity to carbon compounds has been defined as: moles of C-compound produced/moles of C-reactant reacted/stoichiometric coefficient. 3. Results Fig. 1 shows the activity of the catalyst in the propane steam reforming reaction over the Pd Cu/Al 2 O 3 catalyst. Propane conversion starts above 8 K. Propylene (7% selectivity) and (3%) are the only products detectable at low temperature and very low conversion, whose selectivity decreases by increasing temperature, in favor of CO selectivity which grows first and keeps a constant selectivity of 22% in the range 9 11 K. In the same range of temperature, and C 2 H 4 are also observed, their selectivity rising as well. Hydrogen selectivity shows a maximum of 45% at low propane conversion (around 5%) and decreases by increasing temperature down to a minimum of 22% at 11 K. At the same time, O conversion slightly increases to 12% at 11 K. These data indicate that under SR conditions, propane dehydrogenation actually occurs already near 8 K, H 8 ¼ H 6 þ The outlet amounts of, CO and hydrogen indicate, according to our thermodynamic calculations, that steam reforming of propane, H 8 þ 3 O ¼ 3CO þ 7 is likely followed by water gas shift reaction CO þ O ¼ þ H 8 O C 2 H 4 H 6 CO Fig. 1. Conversion/selectivities vs. temperature of propane SR over Pd Cu/c-Al 2 O 3 catalyst.

3 C. Resini et al. / Catalysis Communications 6 (25) whose equilibrium is actually not reached even at 11 K, although the equilibrium tends to shift towards CO and water by increasing temperature. The conversion of propane is strongly enhanced at higher temperatures, when methane and ethylene are observed. The far higher selectivity to ethylene with respect to methane suggests that, besides the likely occurrence of the cracking reaction H 8 ¼ C 2 H 4 þ also another reaction producing ethylene occurs, possibly the following one: H 8 þ O ¼ C 2 H 4 þ CO þ 3 According to this picture, the conversion of water is much lower than the conversion of propane, which are fed with the correct stoichiometry for the SR reaction. In spite of the hydrogen selectivity decrease, the hydrogen yield actually increases by increasing temperature and reaches 16% at 11 K. This shows that Pd Cu/ Al 2 O 3 is not a selective catalyst for propane steam reforming under these conditions, with quite large productions of propene, ethylene and methane. Actually, at the highest temperature ethylene is the main byproduct. The curves relative to steam reforming of propene performed in the same conditions over Pd Cu/Al 2 O 3 are shown in Fig. 2. Conversion of both propene and water grows steadily above 55 K giving rise to CO and and hydrogen as the largely predominant products. Only above 85 K, methane is also formed in small amounts. Hydrogen selectivity is always very high approaching 1%, but decreases at the highest temperature in concomitance with the production of methane. According with these data in the range 55 7 K, the propene steam reforming can be considered the main reaction. Propene and water conversions in fact have the same trend and the selectivity to reaches 1% meaning that SR is the largely predominant reaction. These data show that Pd Cu/Al 2 O 3 catalyst is an excellent catalyst for propene steam reforming, although CO/ ratio is quite high showing that at low temperature the water gas shift reaction occurs to a limited extent. Propene SR is over this catalyst faster and much more selective to CO x and hydrogen than propane SR. The catalytic activity of Ni/NiAl 2 O 4 catalyst in H 8 steam reforming is shown in Fig. 3. Conversion of propane and O start to be detectable above 65 K. At conversion values around 35% for both propane and O, the main products detected are and. Propane conversion increases by increasing temperature, whereas at the same time selectivity decreases from 9% at 75 75% at 1 K. Selectivity to dramatically drops to zero giving place to CO which rises reaching its maximum of 9% at 1 K. Water conversion does not exceed 5% at 1 K. A small amount of methane has been detected in the range 8 1 K, but with a selectivity not higher than 5%. Our data suggest that the steam reforming of propane coupled with WGS are the main reactions occurring in the range 65 8 K. This finds a confirmation in the decrease of selectivity by increasing temperature, in agreement with the WGS equilibrium. The low amount of detected at 8 1 K can be ascribed to the hydrogenation of by, according with the behavior of the Ni-based materials as a methanation catalysts, and this causes the decrease of hydrogen selectivity. þ 4 ¼ þ 2 O Conversion/selectivities vs. temperature of the catalytic test of Ni-based catalyst in the propene steam reforming are shown in Fig. 4. A detectable conversion of the reactants starts above 7 K and in the case of propylene reaches 95% at 1 K whereas for Ois 5% at the same temperature. At low temperature H 6 O CO H 8 O C 2 H 4 CO Fig. 2. Conversion/selectivities vs. temperature of propene SR over Pd Cu/c-Al 2 O 3 catalyst Fig. 3. Conversion/selectivities vs. temperature of propane SR over Ni/NiAl 2 O 4 catalyst.

4 444 C. Resini et al. / Catalysis Communications 6 (25) (75 K), the main products are (6% of selectivity) and more than 95% of selectivity. Taking into consideration the all range of temperature selectivity varies from 95% to 75% at 95 K. decreases from 6% at 75 K to 7% at 95 K. In the same range of temperature, CO rises reaching 95% of selectivity at 1 K. The small production of, which selectivity does not exceed 5%, can be related to methanation of in the range 75 1 K. Even in this case, like for propane over Ni-based catalyst, the reaction occurring at low temperature is the steam reforming of the olefin followed by the WGS, whereas by increasing temperature since WGS is less favored, the main reaction is steam reforming. A part of the hydrogen produced in the range 85 1 K participates in methanation of as suggested by the small amount of detected. þ 4 ¼ þ 2 O 4. Discussion H 6 O C 2 H 4 CO Fig. 4. Conversion/selectivities vs. temperature of propene SR over Ni/ NiAl 2 O 4 catalyst. The data reported here allow to compare the steam reforming reactions of propane and propene over a Pd-based and a Ni-based catalyst. The behavior of our Ni NiAl 2 O 4 catalyst is quite comparable with that reported in the literature for similar materials [11]. Over this catalyst, the conversion of propane and propene and the selectivities to the products, CO, and are strictly similar, in similar conditions. The activation energy for propane steam reforming found in this study (see Table 1), as calculated taking low conversion experiments in the approximation of a differential reactor, is a little higher than that reported in the literature for similar systems [17,18]. A definitely higher activation energy is found here for propene conversion over the same catalyst. The product distribution is such that the water gas shift reaction approaches equilibrium here. Table 1 Apparent activation energy of steam reforming of propane and propene over Ni.5 Al 2 O 3.5 and Pd Cu/c-Al 2 O 3 Catalyst H 8 SR (kj/mol) H 6 SR (kj/mol) Ni.5 Al 2 O Pd Cu/c-Al 2 O The propane steam reforming over the Pd catalyst is definitely worst than over the Ni catalyst. Propene formation, likely produced by propane dehydrogenation, limits steam reforming at low temperature, while the formation of ethylene and methane limit it at high temperature. Also, the /CO ratio is far from equilibrium, with an excess of formation. Well in contrast with this, propene steam reforming is definitely faster and more selective than propane steam reforming over our Pd-based catalyst. This behavior, according to the fast conversion of propene but also with its production from propane in the parallel conditions, provides evidence of some kind of Pd-based catalyst poisoning by propane, unlike propene. Coking of Pd by propane is a wellknown phenomenon and may not involve propene as an intermediate [19]. Propene steam reforming occurs over our Pd Cu catalyst about 1 K below propene steam reforming on Nichel, although over Ni, likely due to the faster water gas shift, hydrogen yield is better at the same propene conversion level. The higher activity of Pd Cu catalyst may be related to the well-known ability of Pd metal to adsorb olefins forming di-r-bonded species [2] and to act as hydrogenation/dehydrogenation catalyst. Also the ability of Cu + centers to adsorb olefins through a p- bonding [21] may be relevant here. These interactions may activate propene towards steam reforming. References [1] J.R. Rostrup Nielsen, J. Sehested, J.K. Nørskov, Adv. Catal. 47 (22) 66. [2] Petrochemical Processes 23, Hydrocarbon Process., 3 (23) 7. [3] N. Eliaz, D. Eliezer, D.L. Olson, Mat. Sci. Eng. A 289 (2) 41. [4] J. Barrault, Y. Pouilloux, Catal. Today 37 (1997) 137. [5] J.H. Gary, G.E. Handwerk, Petroleum Refining, Technology and Economics, fourth ed., Marcel Dekker, New York, 21. [6] D.E Ridler, M.V. Twigg, in: M.V. Twigg (Ed.), Catalyst Handbook, Wolfe Publication, 1989, p [7] J.R.H. Ross (Ed.), Catalysis, vol. 7, The Royal Society of Chemistry, London, 1985, p. 1. [8] H. Gunardson, Industrial Gases in Petrochemical Processing, Marcel Dekker, New York, [9] S. Katikaneni, C. Yuh, S. Abens, M. Farooque, Catal. Today 77 (22) 99. [1] K. Föger, K. Ahmed, J. Phys. Chem. B (in press, published in the web). [11] T. Maillet, J. Barbier Jr., D. Duprez, Appl. Catal. B 9 (1996) 251. [12] S. Ayabe, H. Omoto, T. Utaka, R. Kikuchi, K. Sasaki, Y. Teraoka, K. Eguchi, Appl. Catal. A 241 (23) 261. [13] G. Kolb, R. Zapf, V. Hessel, H. Löwe, Appl. Catal. A 277 (24) 155.

5 C. Resini et al. / Catalysis Communications 6 (25) [14] B.H. Howard, R.P. Killmeyer, K.S. Rothenberger, A.V. Cugini, B.D. Morreale, R.M. Enick, F. Bustamante, J. Membrane Sci. 241 (24) 27. [15] K. Takeishi, H. Suzuki, Appl. Catal. A 26 (24) 111. [16] G. Busca, V. Lorenzelli, V. Sanchez Escribano, Chem. Mater. 4 (1992) 595. [17] A.K. Avc, D.L. Trimm, A.E. Aksoylu, Z.Í. Önsan, Appl. Catal. A 258 (24) 235. [18] J.R. Rostrup-Nielsen, in: J.R. Anderson, M Boudart (Eds.), Catalysis, Science and Technology, vol. 5, Springer, Berlin, 1984, p. 3. [19] P. Quicker, V. Höllein, R. Dittmeyer, Catal. Today 56 (2) 21. [2] A.M. Doyle, Sh.K. Shaikhutdinov, H.-J. Freund, J. Catal. 223 (24) 444. [21] G. Busca, V. Lorenzelli, G. Ramis, V. Sanchez Escribano, Mater. Chem. Phys. 29 (1991) 175.

Hydrogen Production by Non Thermal Plasma Steam Reforming of alkanes and ethanol

Hydrogen Production by Non Thermal Plasma Steam Reforming of alkanes and ethanol A. Khacef, F. Ouni, E. El Ahmar, O. Aubry, and J. M. Cormier GREMI-Polytech'Orléans, 14 rue d'issoudun, BP 6744, 4567 Orléans