Hydrogen Production by Methane Steam Reforming ...

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The good reactivity of ruthenium impregnated catalyst was attributed to the .... reactivity ranking for the different calcined solids: 1%Ru/Co6Al2 ... [6] J.H. Jeong, J.W. Lee, D.J. Seo, Y. Seo, W.L. Yoon, D.K. Lee and D.H. Kim: Appl. Catal. A. Vol.
Advanced Materials Research Vol. 324 (2011) pp 453-456 © (2011) Trans Tech Publications, Switzerland doi:10.4028/www.scientific.net/AMR.324.453

Hydrogen Production by Methane Steam Reforming Over Ru and Cu Supported on Hydrotalcite Precursors Doris Homsi1,2,3,a, Samer Aouad3,b,*, Cedric Gennequin1,2,c, Antoine Aboukaïs1,2,d and Edmond Abi-Aad1,2,e 1

Univ Lille Nord de France, F-59000 Lille, France 2

ULCO, UCEIV, F-59000 Dunkerque, France

3

Department of Chemistry, University of Balamand, P.O. Box 100, Tripoli, Lebanon

a

[email protected], [email protected], [email protected], d [email protected], [email protected]

Keywords: copper, hydrotalcites, methane steam reforming, ruthenium, TPR, XRD

Abstract. Co6Al2 oxide was prepared using the hydrotalcite route. The obtained solid was thermally stabilized at 500°C and then impregnated with 5 wt.% copper or 1 wt.% ruthenium nitrate solution followed by calcination at 500°C under an air flow. X-ray diffraction results showed that the calcination of the impregnated solids led to the formation of various oxides (CuO, RuO2, Co3O4, CoAl2O4, CoAl2O4). The different impregnated and non impregnated solids were tested in the methane steam reforming reaction (MSR). Methane conversion did not exceed 5% at 800°C in the case of the non impregnated solid, whereas the impregnation strongly enhanced the reactivity: ~89% and ~92% conversions were reached at 600°C for Cu and Ru respectively. The good reactivity of ruthenium impregnated catalyst was attributed to the formation of easily reducible ruthenium and cobalt oxide species at the surface of the support. The addition of ruthenium made the reduction of surface and bulk cobalt oxides possible at lower temperatures. Introduction In recent years, hydrogen is increasingly regarded as the clean energy source of the future [1,2]. Hydrogen is expected to be an important alternative energy resource in the near future. In order to produce hydrogen, catalytic reforming reactions have always been applied. From these reactions, methane steam reforming (MSR) is one of the most economical ways to produce hydrogen. It consists of two reactions: the first one is the decomposition of methane (Eq. 1) and the second is the water gas shift reaction WGS (Eq. 2) to produce additional hydrogen: CH4 + H2O ↔ CO + 3H2 CO + H2O ↔ CO2 + H2

∆H = 206.2 kJ.mol-1 ∆H = - 41.2 kJ.mol-1

Eq. 1 Eq. 2

This process suffers from coke formation, at low steam/methane ratios in the feed, leading to catalyst deactivation. A higher ratio in the feed favors high conversions and minimizes carbon accumulation. However, unnecessary generation of more steam than the reaction stoichiometry represents an extra energy consumption, which means a higher cost for the process [3]. In addition, studies have shown that catalysts obtained via hydrotalcite-type precursors have shown resistance to carbon deposition and could be applied to the MSR process, with a potential to minimize catalyst deactivation [4]. These compounds have received a great attention since the 1970’s due to their high specific surface area, basic character and memory effect. They gain increasing importance as catalyst precursors in steam reforming of methane [5]. Jeong et al. [6] investigated the effect of using ruthenium as a promoter for Ni catalysts supported on different carriers. They reported that the presence of Ru in a highly dispersed state appears to facilitate the reduction of Ni oxides and to decrease the coke deposition over the catalyst during the steam reforming reaction. Huang and Jhao [7] showed that water gas shift activity in MSR can be improved by the presence of copper. In this paper, Co6Al2 was used as the hydrotalcite precursor on which 5 wt.% Cu or 1 wt.% Ru were impregnated, characterized using XRD and TPR techniques and tested in the MSR reaction. All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of TTP, www.ttp.net. (ID: 193.227.175.28-22/08/11,10:06:23)

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Experimental Co6Al2 hydrotalcite was prepared by precipitating appropriate quantities of Co(NO3)2.6H2O (SIGMA-ALDRICH, 98%) and Al(NO3)3.9H2O (FLUKA, 98%) into 1M sodium carbonate Na2CO3 (HIMEDIA, 98%) solution at 60°C. The pH of the solution was maintained constant (pH~10). The resulting slurry was heated at 60°C for 2 hours and then placed in a drying oven for 24 h. The precipitate was filtered, washed with hot deionized water until its pH reached 6-7 and dried for 48 h at 60°C, then grinded to obtain fine powders. The calcination treatment was performed under an air flow (33 mL.min-1) at 500°C in order to stabilize the mixed oxide. 5 wt.% Cu/Co6Al2 and 1 wt.% Ru/Co6Al2 were prepared by adding an adequate volume of Cu(NO3)2.3H2O or Ru(NO)(NO3)3 to calcined Co6Al2. The obtained mixture was then stirred during 2 hours and dried overnight in a drying oven at 60°C. Catalysts were then thermally stabilized by calcination at 500°C (1°C.min-1) under air flow (33 mL.min-1) during 4 hours. X-ray diffraction (XRD) experiments were performed at ambient temperature on a BRUKER D8 Advance diffractometer using CuKα radiation (1.5405 Å). The diffraction patterns were indexed by comparison with the JCPDS files. Temperature programmed reduction (TPR) was carried out on a ZETON ALTAMIRA apparatus. Hydrogen (30 mL.min-1/5 vol.% in Ar) was passed through a U-shaped reactor containing the catalyst under atmospheric pressure. The tube was heated with an electric furnace (5°C.min-1), and the amount of H2 consumed was monitored with a thermal conductivity detector (TCD). MSR test was carried out under atmospheric pressure in a catalytic reactor coupled to a micro GC (Varian CP-4900) equipped with a TCD. Two hundred milligrams of the catalyst were used and the catalytic reactivity was studied in the range of 400°C to 800°C. The reactant gas flow consisted of a steam to methane ratio of 3 to 1 and the total flow was 50 mL.min-1. Results and Discussions Fig. 1 (a) represents XRD patterns of Co6Al2, 5%Cu/Co6Al2 and 1%Ru/Co6Al2 calcined at 500°C. Non calcined hydrotalcites show diffraction peaks at 2θ = 23.23°, 34.91°, 39.35°, 46.76°, 60.77°, 62.05° and 66.01° (result not presented). It is clearly shown that these latter are absent in Fig. 1 (a). Diffraction peaks marked with “s” are attributed to Co3O4 (JCPDS N° 42-1467), CoAl2O4 (JCPDS N° 44-0160), Co2AlO4 (JCPDS N° 38-0814), CoxMgyO4 (JCPDS N° 81-0667). They show close 2θ values and similar intensities in the three samples. The diffraction peak marked with “t” was only observed for 5%Cu/Co6Al2. It is attributed to CuO in the “tenorite” type phase (JCPDS N°45-0937) and indicates the presence of some agglomerated copper oxide species in this catalyst. On the other hand, the XRD pattern for 1%Ru/Co6Al2 did not show any diffraction peak corresponding to the RuO2 phase. Fig. 1 (b) shows TPR profiles of the different calcined Co6Al2, 5%Cu/Co6Al2 and 1%Ru/Co6Al2 solids. Two reduction peaks at 343°C and 702°C were obtained for the Co6Al2 support. The first peak can be attributed to Co3O4 reduction into metallic Co while the second peak corresponds to the reduction of mixed Co2+-Al3+ or Co3+-Al3+ oxide species [8]. For 5%Cu/Co6Al2 catalyst, one peak attributed to the reduction of Cu2+ into Cu0 is observed at 173°C. The large peak at 455°C followed by a small shoulder at around 634°C are attributed to the reduction of mixed Co3+-Al3+ or Co2+Al3+ oxide species respectively. The 1%Ru/Co6Al2 catalyst presents two reduction peaks at 177°C and 479°C. These latter are attributed to the reduction of ruthenium oxide and cobalt oxide species. According to the literature [9], the small peak at 80°C can be attributed to the reduction of small well dispersed ruthenium oxide species which are in interaction with the surface of catalyst. Table 1 shows H2 consumptions of the two catalysts. It is clear that the experimental H2 consumptions at low temperatures (1937.5 and 3301.9 µmol H2.g-1 catalyst) are greater than the theoretical values (197.8 and 786.8 µmol H2.g-1 catalyst) calculated for RuO2 and CuO reductions respectively.

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s s: Co3O4/CoAl2O4/Co2AlO4 Spinel t: CuO: Tenorite

Intensity (a.u.)

s

s s

s

s

s Co6Al2

s

t

s

s

s

s

s

5%Cu/Co6Al2

s

s

s

s

s

702

(b) Hydrogen Consumption (a.u.)

(a)

455

343 173

Co6Al2 455 634

177

479

80

1%Ru/Co6Al2 20

30

5%Cu/Co6Al2

1%Ru/Co6Al2

40

50 60 70 80 50 250 450 650 850 2θ (°) Temperature (°C) Fig. 1 (a) XRD patterns and (b) H2 consumption profiles for Co6Al2, 5%Cu/Co6Al2 and 1%Ru/Co6Al2 calcined at 500ºC.

This result suggests that the cobalt oxide species that were reduced at 343°C in the support became more easily reducible following the establishment of an interaction between the impregnated metal and the mixed oxides support upon impregnation and calcination at 500°C. Table 1: Experimental and theoretical H2 consumptions of Co6Al2, 1%Ru/Co6Al2 and 5%Cu/Co6Al2 H2 consumption [µmol H2.g-1 catalyst] Experimental Co6Al2

2568.5 (343ºC)

9132 (702ºC)

Theoretcial -

1%Ru/Co6Al2

1937.5 (80ºC and 177ºC)

8344.1 (479ºC)

197.8 (RuO2 → Ru)

3301.9 (173ºC) 8377.9 (455ºC and 634ºC) 786.8 (CuO → Cu) 5%Cu/Co6Al2 Fig. 2 (a) displays methane conversion in the steam reforming reaction over the three calcined solids. It is observed that once 5 wt.% of copper or 1 wt.% of ruthenium were impregnated on calcined Co6Al2, methane conversion reached 100% at 700ºC for 5%Cu/Co6Al2 and 1%Ru/Co6Al2 whereas it did not exceed 3% for Co6Al2 at the same temperature. For 1%Ru/Co6Al2, methane conversion reached 95% at 600°C. At the same temperature, the conversion was 88% in the presence of 5%Cu/Co6Al2. This result shows that the ruthenium based catalyst is the most reactive at lower temperatures. Fig. 2 (b) shows H2/CO ratio for 1%Ru/Co6Al2 and 5%Cu/Co6Al2. It is noticed that the ratio decreased with increasing temperature (> 600°C) due to increased CO yield. In fact, increasing the temperature favors endothermic reactions. Therefore, additional CO quantities were produced according to the reversed WGS reaction (Eq. 2). For the highest temperatures (700 to 800°C), 1%Ru/Co6Al2 gives a higher H2/CO ratio than 5%Cu/Co6Al2 indicating better selective H2 formation in the presence of the ruthenium based catalyst. Thus, the results obtained for the methane conversion and H2/CO ratio lead to the following reactivity ranking for the different calcined solids: 1%Ru/Co6Al2 > 5%Cu/Co6Al2 > Co6Al2. The best reactivity for 1%Ru/Co6Al2 may be due to well dispersed ruthenium oxide species obtained following calcination as revealed in the XRD study. Moreover, these oxides are easily reducible at the surface of the support as revealed in the TPR study.

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100

15 (b)

5%Cu/Co6Al2 1%Ru/Co6Al2

80 10 60

H2/CO

CH4 Conversion (%)

(a)

40 Co6Al2 5%Cu/Co6Al2 1%Ru/Co6Al2

20 0

5

0 400

500 600 700 Temperature (ºC)

800

600

650 700 750 Temperature (ºC)

800

Fig. 2 (a) Methane conversion and (b) H2/CO ratio for Co6Al2, 5%Cu/Co6Al2 and 1%Ru/Co6Al2 tested in the MSR reaction. Conclusion In this study, the catalytic behavior of Co6Al2, 5%Cu/Co6Al2 and 1%Ru/Co6Al2 catalysts calcined at 500ºC was studied in the MSR reaction. It was demonstrated that the highest catalytic reactivity was obtained with the 1%Ru/Co6Al2 catalyst due to the easily reducible ruthenium species and their good dispersion at the surface of Co6Al2 calcined support. Acknowledgments The authors thank the “09 Sci F 7/L 22 - CEDRE” 2009 program, the AUF-CNRS-L and the BRG 8/2009 for financial support. References [1] M.A. Pena, J.P. Gomez and J.L.G. Fierro: Appl. Catal. A Vol. 144 (1996), p. 7 [2] J.N. Armor: Appl. Catal. A Vol. 176 (1999), p. 159 [3] A.F. Lucrédio, G.T. Filho and E.M. Assaf: Appl. Surf. Sci. Vol. 255 (2009), p. 5851 [4] A.F. Lucrédio and E.M. Assaf: J. Power Sources Vol. 159 (2006), p. 667 [5] C.E. Daza, J. Gallego, F. Mondragon, S. Moreno and R. Molina: Fuel Vol. 89 (2010), p. 592 [6] J.H. Jeong, J.W. Lee, D.J. Seo, Y. Seo, W.L. Yoon, D.K. Lee and D.H. Kim: Appl. Catal. A Vol. 302 (2006), p. 156 [7] T-J. Huang, S-Y. Jhao: Appl. Catal. A Vol. 302 (2006) p. 325 [8] C. Gennequin, S. Kouassi, L. Tidahy, R. Cousin, J-F. Lamonier, G. Garcon, P. Shirali, F. Cazier, A. Aboukaïs and S. Siffert: C. R. Chim. Vol. 13 (2010), p. 494 [9] D. Homsi, S. Aouad, J. El Nakat, B. El Khoury, P. Obeid, E. Abi-Aad and A. Aboukaïs, Catal. Comm. Vol. 12 (2011), p. 776.