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NiBa catalysts for CO2-reforming of methane
Catalysis Communications 11 (2010) 1133–1136
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Catalysis Communications j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / c a t c o m
NiBa catalysts for CO2-reforming of methane M. García-Diéguez, M.C. Herrera, I.S. Pieta, M.A. Larrubia, L.J. Alemany ⁎ Departamento de Ingeniería Química, Facultad de Ciencias, Campus de Teatinos, Universidad de Málaga, Málaga, E-29071, Spain
a r t i c l e
i n f o
Article history: Received 3 February 2010 Received in revised form 8 June 2010 Accepted 9 June 2010 Available online 18 June 2010 Keywords: Dry reforming Methane Nickel Barium
a b s t r a c t Ni–Ba catalysts supported on γ-Al2O3 for the dry reforming of methane were prepared, characterized and studied under reaction conditions. Ba incorporation inhibits the formation of Ni spinel. All the Ni–Ba catalysts studied are highly active for the CO2-reforming of methane. However, the Ni–Ba catalyst with high Ba and Ni content was the most active and stable catalyst, due to the presence of accessible Ni particles stabilized by the formation of BaAl2O4. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Recently, there has been an increasing interest in light hydrocarbons' reforming, due to the use of synthesis gas as feedstock for several chemical processes [1–8]. Furthermore, dry reforming could be an alternative route to convert natural gas components into valuable products. The most commonly used catalysts for dry reforming are Ni-based catalysts, due to their low cost and high availability. Unfortunately, under reforming conditions Ni catalysts are quickly deactivated because of coke deposition and Ni sintering [3,5,9–12]. Alumina is widely used as support for Ni catalysts; however, during catalyst pretreatments the Ni spinel phase (NiAl2O4) is generally formed. There is not an agreement about the Ni spinel effect on the CO2reforming performance. N. Sahli et al. [9] reported that in CH4 steam reforming NiAl2O4 favours the coke deposition. K. Y. Koo et al. [5] found that Ni spinel is inactive for the dry reforming of methane (DRM) and the promotion of MgO on Ni/Al2O3 catalysts prevents its formation and suppresses the carbon formation. On the other hand, some authors have shown that carbon deposition is markedly suppressed if NiAl2O4 is formed during the pretreatment procedure [2,13] and its presence is crucial for the high stability of the Ni/Al2O3 catalysts [14,15]. It is well known that the incorporation of basic metals into Ni-based catalysts improves their activity and stability. Several studies have been done with Mg [4–6,12,16], Ca [6,17], La [18] and K [18,19] incorporated in Ni supported catalysts. However, little investigations have been done with Ba, because Ni-based catalysts containing Ba tend
⁎ Corresponding author. Tel./fax: +34 952 13 1919. E-mail address: [email protected] (L.J. Alemany). 1566-7367/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.catcom.2010.06.008
to form Ni aggregates and barium carbonates [20]. L. Xiancai et al. [10] found that BaTiO3 is an excellent support for Ni-based catalysts. Moreover, Narayanan et al. [21] observed that the incorporation of Ba to Ni/Al2O3 catalysts improves the reducibility of Ni. The aim of the present work is to develop effective NiBa catalysts with good performance under DRM conditions. 2. Experimental 2.1. Catalysts preparation A series of three catalysts was prepared; two of the catalysts were synthesized by incipient wetness impregnation of the support (gamma alumina PURALOX TH 100/150, ABET = 144 m2 g−1, VP = 0.97 cm3 g−1) with a solution of Barium Acetate and/or Nickel Nitrate. A Ni monometallic catalyst (4 at/nm2 of Ni, denoted as Ni/Al2O3(IM) and a bimetallic catalyst (Ni/Ba/Al2O3 = 1/1/10), denoted as NiBa/Al2O3(IM)) were prepared. After impregnation the catalysts were dried overnight at 100 °C and calcinated in two steps, at 300 °C for 1 h (2 °C min−1) and at 800 °C for 2 h (5 °C min−1). The third catalyst (denoted as NiBa/Al2O3 (M)) was synthesized by mixing, under continuous stirring, Barium Acetate, Nickel Nitrate and Alumina to get a Ni/Ba/Al2O3 molar ratio of 1/ 1/2.5. The gel obtained was dried overnight at 100 °C and calcinated at 800 °C for 2 h. 2.2. Characterization X-ray Powder Diffraction data have been recorded with an X'Pert MPD PRO diffractometer (PANalytical) using CuKα1 radiation (λ = 1.54059 Å) and a Ge(111) primary monochromator. The X-ray tube worked at 45 kV and 35 mA. The measurements were done from 10° to 70° (2θ). X-ray Photoelectron Spectra were recorded on a
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M. García-Diéguez et al. / Catalysis Communications 11 (2010) 1133–1136 Table 1 Average particle size of Ni0 and carbon content in the catalysts after reaction. Catalyst
DPNi0 (nm)a
C (%)b
Ni/Al2O3(IM) NiBa/Al2O3(IM) NiBa/Al2O3(M)
16 20 21
24 23 31
a b
Calculated by the Scherrer equation. Elemental Analysis.
the reactor bed. The reactor effluent was analyzed by GC (Agilent 4890D). The stability of the catalysts was tested for several hours at 700 °C and at the same reaction conditions as in the activity tests. Mass balances were closed with deviations lower than 5%. 3. Results and discussion
Fig. 1. XRD patterns of the catalysts before (BR) and after (AR) reaction. a. Ni/Al2O3 (IM); b. NiBa/Al2O3(IM); c. NiBa/Al2O3(M). (○) γ-Al2O3; (●) BaCO3; ( ) NiO; ( ) Ni0; (■) BaAl2O4; (♦) NiAl2O4; (G) graphitic carbon; (*) holder.
Physical Electronic 5701 equipped with a PHI 10–360 analyzer using the MgKα X-ray source. Binding Energy (BE) values were referred to the C1s peak (284.8 eV) from the adventitious carbon for the catalysts before reaction and to the Al2p peak for the catalysts after reaction. All deconvolutions of experimental curves were done with Gaussian and Lorenzian line fitting, minimizing the (χ2) chi-square values. TEM images were taken with a Philips CM 200 of 200 kV. The carbon content of the catalysts after reaction was obtained via Elemental Analysis using an Elemental Analyzer Perkin-Elmer 2400 CHN.
2.3. Reactivity Steady state experiments were carried out in a MicroactivityReference reaction system from PID-Eng&Tech (Spain) at atmospheric pressure in a temperature range between 400 and 700 °C. A tubular fixed bed stainless steel reactor (i.d. 9 mm) with 100 mg of catalyst (250–420 μm) was employed. The total gas flow rate was kept constant at 50 N cm3/min with stoichiometric composition diluted in He (CH4/CO2/He = 20/20/60). The space velocity and the contact time were 6000 h−1 and 0.8 g hmol−1, respectively; operating under plug flow conditions. Preliminary reactivity tests with different catalyst particle sizes and dilutions, measuring the radial and axial temperatures at different points, were performed to confirm the non-existence of heat or mass transfer limitations. Before reaction, catalysts were activated in situ with H2 (3% in He, 30 N cm3/min) at 700 °C for 2 h. The reaction temperature was measured with a thermocouple placed in
The XRD patterns of the catalysts before and after reaction are shown in Fig. 1. It can be observed that diffraction lines associated with gamma alumina (JCPDS 75–0921) were detected in the XRD patterns of the Ni/Al2O3(IM) and NiBa/Al2O3(IM) catalysts before reaction. Besides, for the Ni/Al2O3(IM) catalyst a slight shift in the alumina signal was detected, suggesting the presence of Ni spinel (JCPDS 10– 0339). On the contrary, for the NiBa/Al2O3(IM) there was no evidence of NiAl2O4 formation. However, an additional phase was detected, which is related to BaCO3 (JCPDS 71–2394). In addition, for the NiBa/ Al2O3(M) catalyst before reaction the presence of BaAl2O4 (JCPDS 72– 0387), NiO (JCPDS 78–0643) and BaCO3 (JCPDS 71–2394) were principally registered. XRD results before reaction indicate that Ba incorporation in Ni/Al2O3 catalysts inhibits the diffusion of Ni into the alumina structure to form NiAl2O4, in agreement with data reported in the literature [5] for Ni/Al2O3 catalysts promoted by other basic metals. After reaction, two additional phases to those registered before reaction were observed for all of the catalysts. These are related to metallic Ni (JCPDS 70–1849) and graphitic carbon (JCPDS 89–8489). The average particle size of metallic Ni after reaction, calculated by the Scherrer equation, is presented in Table 1, as well as the carbon content, obtained by Elemental Analysis. It can be noticed that apparently the Ba in the IM catalysts does not considerably affect the net carbon formation; this trend was also observed by TEM (Fig. 2). Moreover, it is interesting to observed that the NiBa/Al2O3(M) catalyst presents a global carbon formation similar to that registered for the IM catalysts, despite that this catalyst has a higher content of Ni. Additionally, it should be pointed out that the reoxidation of nickel during catalysts handling cannot be excluded, thus the Ni0 obtained after reaction could not be so precise, if we consider that the small Ni0 particles could have been reoxidized. However, taking into account the results here presented and data reported previously for Ni-K catalysts [19], where the potassium addition did not modify the size or structure
Fig. 2. TEM images for the catalysts after reaction. a. Ni/Al2O3(IM); b. NiBa/Al2O3(IM); c. NiBa/Al2O3(M).
M. García-Diéguez et al. / Catalysis Communications 11 (2010) 1133–1136
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Table 2 BE and surface atomic ratios of catalysts, before (BR) and after (AR) reaction. Catalyst
Ni2p3/2a
Ba3d5/2a BR
Ni/Al2O3(IM)
NiBa/Al2O3(IM)
NiBa/Al2O3(M)
a b
0
Ni NiO NiAl2O4 Ni/Alb Ni0 NiO Ni/Alb Ni0 NiO NiAl2O4 Ni/Alb
855.4 856.3 0.05 855.7 0.08 855.4 856.8 0.1
AR (36) (64)
(100)
(95) (5)
852.8 855.2 856.8 0.02 852.4 855.8 0.02 852.7 855.9 856.9 0.08
(33) (17) (50) (43) (57) (41) (55) (4)
BR
AR
-
-
-
BaCO3 Ba/Alb BaAl2O4 BaCO3 Ba/Alb
780.2 (100) 0.06 779.8 (63) 780.4 (37) 0.1
780.3 (100) 0.04 779.6 (72) 780.2 (28) 0.06
Binding Energy (± 0.2 eV), in brackets () relative contribution of the species in %. Atomic superficial ratio.
of the Ni particles, it is presumed that the Ni0 particle size is not significantly affected by Ba incorporation. The Binding Energies and the surface atomic ratios of the Ni2p3/2 and Ba3d5/2 core electrons for the catalysts before and after reaction are summarized in Table 2. For all of the catalysts the presence of Ni2+ as NiO [3,20] was registered. The absence of XRD lines of NiO for the IM catalysts can be due to the presence of non-crystalline NiO, as has also been reported by Scheffer et al. [22]. For the Ni/Al2O3(IM) and NiBa/ Al2O3(M) catalysts an additional signal related to Ni spinel [3,20] was also detected, being its relative contribution almost insignificant for the NiBa/Al2O3(M) catalyst, in agreement with XRD results. Additionally, in the Ba3d5/2 core electron spectrum, for the NiBa/ Al2O3(IM) catalyst before reaction a unique signal was registered, which is associated with the presence of barium carbonate [23]. For the NiBa/Al2O3(M) catalysts two different Ba contributions were detected;
one related to BaCO3 (37%) and another one associated with BaAl2O4 (63%) [24], confirming that for this catalyst formation of BaCO3 was partially inhibited. After reaction, in the Ni2p3/2 spectral region, an additional signal was detected, which corresponds to Ni0 [3]. The relative contribution of this species was slightly higher for the NiBa/Al2O3(IM) and NiBa/ Al2O3(M) catalysts than for the Ni/Al2O3(IM). This latter observation together with the higher proportion of NiO for the catalysts containing Ba before reaction suggests that Ba incorporation could promote Ni reducibility. Juan-Juan et al. [19] studied the effect of K content on Ni/ Al2O3 catalysts for the DRM. They reported that K incorporation increases the Ni reducibility, because K modifies the interaction of nickel oxide with alumina. However, the differences in reducibility after reaction that we observed could be affected by the carbon formation and Ni reoxidation prior to the XPS analysis, as was pointed
Fig. 3. Activity and stability results. a. CH4 conversion vs. temperature; b. CO2 conversion vs. temperature; c. H2/CO ratio vs. temperature; d. CO2 conversion vs. time on stream (700 °C).
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M. García-Diéguez et al. / Catalysis Communications 11 (2010) 1133–1136
out above. All the samples were kept under inert atmosphere after reaction, but the exposition to the air during catalyst handling cannot be excluded. On the other hand, for the Ba containing catalysts after reaction the same contributions observed before reaction were detected, with a decrease in the relative contribution of BaCO3 for the NiBa/Al2O3(M) catalyst. Moreover, for the NiBa/Al2O3(M) catalyst the Ni/Al superficial atomic ratio does not considerably change after reaction, indicating that the Ni superficial population is not modified, keeping its dispersion. Activity results are summarized in Fig. 3 as well as the equilibrium conversions values, calculated considering simultaneous CO2-CH4 reforming and reverse water gas shift reaction (RWGS) at the reactivity test conditions (atmospheric pressure and CH4/CO2/He = 20/20/60). Equilibrium calculations were done by the minimization of Gibbs Energy Method taking into account CH4, CO2, H2, CO, H2O and He as species in the system, solid carbon was not considered. The computer program for calculation of complex chemical equilibrium composition and applications was used [25]. With respect to catalysts' activity, it can be observed (Fig. 3a,b and c) that in the temperature range 300–700 °C there are not significant differences among the catalysts studied. All the variations were in the error range of the analysis technique. Furthermore, it is interesting to note that the H2/CO ratio values obtained are lower than the thermodynamic ones in the whole temperature range, indicating the occurrence of side reactions; which were not considered for the equilibrium calculations. Despite the apparent higher reducibility of the Ba containing catalysts, registered by XPS, there is no a direct effect on activity. This suggests the presence of different Ni2+ species, as NiO-amorphous species, with diverse stability, affecting the population of Ni0 sites; whose proportion is apparently the same for the catalysts studied at the onset of the reaction. To clarify the effect of Ba on the reactivity of Ni/Al2O3 catalysts for the DRM, stability runs were performed at 700 °C (Fig. 3d). It is worth noticing that, even when the activity and Elemental Analysis results did not show important differences between the catalysts, the catalysts' stability for the DRM is quite different. For the Ni/Al2O3 (IM), the NiBa/Al2O3(IM) and the NiBa/Al2O3(M) catalysts values of CO2 conversion loss of 1.9% h−1, 1.7% h−1 and 0% h−1 were registered. The low stability of the impregnated catalysts can be attributed to the encapsulation of Ni particles observed by TEM (see Fig. 2a,b), decreasing the available Ni sites as was observed by XPS. Apparently, the kind of carbon formed with the IM catalysts differs from that observed for the NiBa/Al2O3(M) catalyst. Together characterization and reactivity results indicate that all the NiBa catalysts were highly active for the DRM and apparently the carbon formed affects the catalysts' stability. The addition, by impregnation, of low quantities of Ba in the Ni/Al2O3 catalyst does not considerably affect the catalyst activity under the DRM conditions. However, depending on the amount of Ba added and the preparation procedure the stability of these catalysts can be modified. The catalyst with higher NiBa content and prepared by mixture (NiBa/Al2O3(M)) presented high stability, which can be attributed to the in situ formation of a stable BaAl2O4 phase. The Ba spinel acts as the real support for the NiBa/Al2O3(M) catalysts stabilizing the Ni particles over the catalyst surface. It has been already reported that Mg spinels are high stable supports for reforming processes [26,27].
It is important to point out that in a previous study (data not shown) a Ni catalyst supported on a BaAl2O4 spinel, synthesized ex situ, was prepared; this catalyst presented very low activity for the DRM. In addition, a NiBa unsupported catalyst was also prepared and tested under reaction conditions. It was not active for the DRM, due to the high and non-reversible carbonation of the catalyst. Therefore, simultaneous incorporation of Ni, Ba and alumina facilitates the attainment of stable and effective NiBa catalysts for the DRM. 4. Conclusion The addition of 4 at/nm2 of Ba to Ni-based catalysts supported on alumina does not considerably affect the overall activity and stability for the DRM. However, it inhibits the formation of Ni spinel. The NiBa catalyst with high content of Ni–Ba and prepared by mixture, allows a system with a low carbonation level, characterized by the formation of a highly stable Ba spinel. This catalyst reaches good activity levels and is rather stable for the DRM. Acknowledgement MGD acknowledges the Spanish Minister of Education and Science for an FPI grant and for the financial support to the projects ENE200406176 and ENE2007-67926-C02-02/ALT. References [1] J.R. Rostrup-Nielsen, Catal. Today 18 (1993) 305–324. [2] M.C.J. Bradford, M.A. Vannice, Catal. Rev.-Sci. Eng. 41 (1999) 1–42. [3] B. Pawelec, S. Damyanova, K. Arishtirova, J.L. Fierro, L. Petrov, Appl. Catal. A 323 (2007) 188–201. [4] C.E. Daza, J. Gallego, J.A. Moreno, F. Mondragón, S. Moreno, R. Molina, Catal. Today 133–135 (2008) 357–366. [5] K.Y. Koo, H.-S. Roh, Y.T. Seo, D.J. Seo, W.L. Yoon, S.B. Park, Appl. Catal. A 340 (2008) 183–190. [6] E. Ruckenstein, Y.H. Hu, Appl. Catal. A 133 (1995) 149–161. [7] M. García-Diéguez, I.S. Pieta, M.C. Herrera, M.A. Larrubia, L.J. Alemany, J. Catal. 270 (2010) 136–145. [8] M. García-Diéguez, I.S. Pieta, M.C. Herrera, M.A. Larrubia, L.J. Alemany, Appl. Catal. A 377 (2010) 191–199. [9] N. Sahli, C. Petit, A.C. Roger, A. Kiennemann, S. Libs, M.M. Bettah, Catal. Today 113 (2006) 187–193. [10] L. Xiancai, W. Min, L. Zhihua, H. Fei, Appl. Catal. A 290 (2005) 81–86. [11] F. Pompeo, N.N. Nichio, M.M.V.M. Souza, D.V. Cesar, O.A. Ferretti, M. Schmal, Appl. Catal. A 316 (2007) 175–183. [12] J. Zhang, H. Wang, A.K. Dalai, J. Catal. 249 (2007) 300–310. [13] Y.-G. Chen, J. Ren, Catal. Lett. 29 (1994) 39–48. [14] R. Lamber, G. Schulz-Ekloff, Surf. Sci. 258 (1991) 107–118. [15] R. Lamber, G. Schulz-Ekloff, J. Catal. 146 (1994) 601–607. [16] S. Corthals, J. Van Nederkassel, J. Geboers, H. De Winne, J. Van Noyen, B. Moens, B. Sels, P. Jacobs, Catal. Today 138 (2008) 28–32. [17] Z.L. Zhang, X.E. Verykios, Catal. Today 21 (1994) 589–595. [18] M. Rezaei, S.M. Alavi, S. Sahebdelfar, Peng Bai, Xinmei Liu, Zi-Feng Yan, Appl. Catal. B 77 (2008) 346–354. [19] J. Juan-Juan, M.C. Román-Martínez, M.J. Illán-Gómez, Appl. Catal. A 301 (2006) 9–15. [20] M. García-Diéguez, I. Pieta, F. Sánchez-García, B. Guerrero-Klein, M.C. Herrera, M. A. Larrubia, L.J. Alemany, Proceedings del XXI Simposio Iberoamericano de Catálisis (SICAT). ISBN 978 84 691 4234 9 (Málaga 2008), 2008, p. 1594. [21] S. Narayanan, K. Uma, J. Chem. Soc. Faraday Trans. I 84 (1988) 521–527. [22] B. Scheffer, P. Molhoek, J.A. Moulijn, Appl. Catal. 46 (1989) 11–30. [23] A.B. Christie, J. Lee, I. Sutherland, J.M. Walls, Appl. Surf. Sci. 15 (1983) 224–237. [24] C. Zhang, L. Wang, L. Cui, Y. Zhu, J. Cryst. Growth 255 (2003) 317–323. [25] B.J. Mc Bride, S. Gordon, NASA Reference Publication, 1311, 1996. [26] S. Corthals, J. Van Nederkassel, J. Geboers, H. De Winne, J. Van Noyen, B. Moens, B. Sels, P. Jacobs, Catal. Today 138 (2008) 28–32. [27] J. Sehested, J.A.P. Gelten, S. Helveg, Appl. Catal. A 309 (2006) 237–246.
Contents lists available at ScienceDirect
Catalysis Communications j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / c a t c o m
NiBa catalysts for CO2-reforming of methane M. García-Diéguez, M.C. Herrera, I.S. Pieta, M.A. Larrubia, L.J. Alemany ⁎ Departamento de Ingeniería Química, Facultad de Ciencias, Campus de Teatinos, Universidad de Málaga, Málaga, E-29071, Spain
a r t i c l e
i n f o
Article history: Received 3 February 2010 Received in revised form 8 June 2010 Accepted 9 June 2010 Available online 18 June 2010 Keywords: Dry reforming Methane Nickel Barium
a b s t r a c t Ni–Ba catalysts supported on γ-Al2O3 for the dry reforming of methane were prepared, characterized and studied under reaction conditions. Ba incorporation inhibits the formation of Ni spinel. All the Ni–Ba catalysts studied are highly active for the CO2-reforming of methane. However, the Ni–Ba catalyst with high Ba and Ni content was the most active and stable catalyst, due to the presence of accessible Ni particles stabilized by the formation of BaAl2O4. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Recently, there has been an increasing interest in light hydrocarbons' reforming, due to the use of synthesis gas as feedstock for several chemical processes [1–8]. Furthermore, dry reforming could be an alternative route to convert natural gas components into valuable products. The most commonly used catalysts for dry reforming are Ni-based catalysts, due to their low cost and high availability. Unfortunately, under reforming conditions Ni catalysts are quickly deactivated because of coke deposition and Ni sintering [3,5,9–12]. Alumina is widely used as support for Ni catalysts; however, during catalyst pretreatments the Ni spinel phase (NiAl2O4) is generally formed. There is not an agreement about the Ni spinel effect on the CO2reforming performance. N. Sahli et al. [9] reported that in CH4 steam reforming NiAl2O4 favours the coke deposition. K. Y. Koo et al. [5] found that Ni spinel is inactive for the dry reforming of methane (DRM) and the promotion of MgO on Ni/Al2O3 catalysts prevents its formation and suppresses the carbon formation. On the other hand, some authors have shown that carbon deposition is markedly suppressed if NiAl2O4 is formed during the pretreatment procedure [2,13] and its presence is crucial for the high stability of the Ni/Al2O3 catalysts [14,15]. It is well known that the incorporation of basic metals into Ni-based catalysts improves their activity and stability. Several studies have been done with Mg [4–6,12,16], Ca [6,17], La [18] and K [18,19] incorporated in Ni supported catalysts. However, little investigations have been done with Ba, because Ni-based catalysts containing Ba tend
⁎ Corresponding author. Tel./fax: +34 952 13 1919. E-mail address: [email protected] (L.J. Alemany). 1566-7367/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.catcom.2010.06.008
to form Ni aggregates and barium carbonates [20]. L. Xiancai et al. [10] found that BaTiO3 is an excellent support for Ni-based catalysts. Moreover, Narayanan et al. [21] observed that the incorporation of Ba to Ni/Al2O3 catalysts improves the reducibility of Ni. The aim of the present work is to develop effective NiBa catalysts with good performance under DRM conditions. 2. Experimental 2.1. Catalysts preparation A series of three catalysts was prepared; two of the catalysts were synthesized by incipient wetness impregnation of the support (gamma alumina PURALOX TH 100/150, ABET = 144 m2 g−1, VP = 0.97 cm3 g−1) with a solution of Barium Acetate and/or Nickel Nitrate. A Ni monometallic catalyst (4 at/nm2 of Ni, denoted as Ni/Al2O3(IM) and a bimetallic catalyst (Ni/Ba/Al2O3 = 1/1/10), denoted as NiBa/Al2O3(IM)) were prepared. After impregnation the catalysts were dried overnight at 100 °C and calcinated in two steps, at 300 °C for 1 h (2 °C min−1) and at 800 °C for 2 h (5 °C min−1). The third catalyst (denoted as NiBa/Al2O3 (M)) was synthesized by mixing, under continuous stirring, Barium Acetate, Nickel Nitrate and Alumina to get a Ni/Ba/Al2O3 molar ratio of 1/ 1/2.5. The gel obtained was dried overnight at 100 °C and calcinated at 800 °C for 2 h. 2.2. Characterization X-ray Powder Diffraction data have been recorded with an X'Pert MPD PRO diffractometer (PANalytical) using CuKα1 radiation (λ = 1.54059 Å) and a Ge(111) primary monochromator. The X-ray tube worked at 45 kV and 35 mA. The measurements were done from 10° to 70° (2θ). X-ray Photoelectron Spectra were recorded on a
1134
M. García-Diéguez et al. / Catalysis Communications 11 (2010) 1133–1136 Table 1 Average particle size of Ni0 and carbon content in the catalysts after reaction. Catalyst
DPNi0 (nm)a
C (%)b
Ni/Al2O3(IM) NiBa/Al2O3(IM) NiBa/Al2O3(M)
16 20 21
24 23 31
a b
Calculated by the Scherrer equation. Elemental Analysis.
the reactor bed. The reactor effluent was analyzed by GC (Agilent 4890D). The stability of the catalysts was tested for several hours at 700 °C and at the same reaction conditions as in the activity tests. Mass balances were closed with deviations lower than 5%. 3. Results and discussion
Fig. 1. XRD patterns of the catalysts before (BR) and after (AR) reaction. a. Ni/Al2O3 (IM); b. NiBa/Al2O3(IM); c. NiBa/Al2O3(M). (○) γ-Al2O3; (●) BaCO3; ( ) NiO; ( ) Ni0; (■) BaAl2O4; (♦) NiAl2O4; (G) graphitic carbon; (*) holder.
Physical Electronic 5701 equipped with a PHI 10–360 analyzer using the MgKα X-ray source. Binding Energy (BE) values were referred to the C1s peak (284.8 eV) from the adventitious carbon for the catalysts before reaction and to the Al2p peak for the catalysts after reaction. All deconvolutions of experimental curves were done with Gaussian and Lorenzian line fitting, minimizing the (χ2) chi-square values. TEM images were taken with a Philips CM 200 of 200 kV. The carbon content of the catalysts after reaction was obtained via Elemental Analysis using an Elemental Analyzer Perkin-Elmer 2400 CHN.
2.3. Reactivity Steady state experiments were carried out in a MicroactivityReference reaction system from PID-Eng&Tech (Spain) at atmospheric pressure in a temperature range between 400 and 700 °C. A tubular fixed bed stainless steel reactor (i.d. 9 mm) with 100 mg of catalyst (250–420 μm) was employed. The total gas flow rate was kept constant at 50 N cm3/min with stoichiometric composition diluted in He (CH4/CO2/He = 20/20/60). The space velocity and the contact time were 6000 h−1 and 0.8 g hmol−1, respectively; operating under plug flow conditions. Preliminary reactivity tests with different catalyst particle sizes and dilutions, measuring the radial and axial temperatures at different points, were performed to confirm the non-existence of heat or mass transfer limitations. Before reaction, catalysts were activated in situ with H2 (3% in He, 30 N cm3/min) at 700 °C for 2 h. The reaction temperature was measured with a thermocouple placed in
The XRD patterns of the catalysts before and after reaction are shown in Fig. 1. It can be observed that diffraction lines associated with gamma alumina (JCPDS 75–0921) were detected in the XRD patterns of the Ni/Al2O3(IM) and NiBa/Al2O3(IM) catalysts before reaction. Besides, for the Ni/Al2O3(IM) catalyst a slight shift in the alumina signal was detected, suggesting the presence of Ni spinel (JCPDS 10– 0339). On the contrary, for the NiBa/Al2O3(IM) there was no evidence of NiAl2O4 formation. However, an additional phase was detected, which is related to BaCO3 (JCPDS 71–2394). In addition, for the NiBa/ Al2O3(M) catalyst before reaction the presence of BaAl2O4 (JCPDS 72– 0387), NiO (JCPDS 78–0643) and BaCO3 (JCPDS 71–2394) were principally registered. XRD results before reaction indicate that Ba incorporation in Ni/Al2O3 catalysts inhibits the diffusion of Ni into the alumina structure to form NiAl2O4, in agreement with data reported in the literature [5] for Ni/Al2O3 catalysts promoted by other basic metals. After reaction, two additional phases to those registered before reaction were observed for all of the catalysts. These are related to metallic Ni (JCPDS 70–1849) and graphitic carbon (JCPDS 89–8489). The average particle size of metallic Ni after reaction, calculated by the Scherrer equation, is presented in Table 1, as well as the carbon content, obtained by Elemental Analysis. It can be noticed that apparently the Ba in the IM catalysts does not considerably affect the net carbon formation; this trend was also observed by TEM (Fig. 2). Moreover, it is interesting to observed that the NiBa/Al2O3(M) catalyst presents a global carbon formation similar to that registered for the IM catalysts, despite that this catalyst has a higher content of Ni. Additionally, it should be pointed out that the reoxidation of nickel during catalysts handling cannot be excluded, thus the Ni0 obtained after reaction could not be so precise, if we consider that the small Ni0 particles could have been reoxidized. However, taking into account the results here presented and data reported previously for Ni-K catalysts [19], where the potassium addition did not modify the size or structure
Fig. 2. TEM images for the catalysts after reaction. a. Ni/Al2O3(IM); b. NiBa/Al2O3(IM); c. NiBa/Al2O3(M).
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Table 2 BE and surface atomic ratios of catalysts, before (BR) and after (AR) reaction. Catalyst
Ni2p3/2a
Ba3d5/2a BR
Ni/Al2O3(IM)
NiBa/Al2O3(IM)
NiBa/Al2O3(M)
a b
0
Ni NiO NiAl2O4 Ni/Alb Ni0 NiO Ni/Alb Ni0 NiO NiAl2O4 Ni/Alb
855.4 856.3 0.05 855.7 0.08 855.4 856.8 0.1
AR (36) (64)
(100)
(95) (5)
852.8 855.2 856.8 0.02 852.4 855.8 0.02 852.7 855.9 856.9 0.08
(33) (17) (50) (43) (57) (41) (55) (4)
BR
AR
-
-
-
BaCO3 Ba/Alb BaAl2O4 BaCO3 Ba/Alb
780.2 (100) 0.06 779.8 (63) 780.4 (37) 0.1
780.3 (100) 0.04 779.6 (72) 780.2 (28) 0.06
Binding Energy (± 0.2 eV), in brackets () relative contribution of the species in %. Atomic superficial ratio.
of the Ni particles, it is presumed that the Ni0 particle size is not significantly affected by Ba incorporation. The Binding Energies and the surface atomic ratios of the Ni2p3/2 and Ba3d5/2 core electrons for the catalysts before and after reaction are summarized in Table 2. For all of the catalysts the presence of Ni2+ as NiO [3,20] was registered. The absence of XRD lines of NiO for the IM catalysts can be due to the presence of non-crystalline NiO, as has also been reported by Scheffer et al. [22]. For the Ni/Al2O3(IM) and NiBa/ Al2O3(M) catalysts an additional signal related to Ni spinel [3,20] was also detected, being its relative contribution almost insignificant for the NiBa/Al2O3(M) catalyst, in agreement with XRD results. Additionally, in the Ba3d5/2 core electron spectrum, for the NiBa/ Al2O3(IM) catalyst before reaction a unique signal was registered, which is associated with the presence of barium carbonate [23]. For the NiBa/Al2O3(M) catalysts two different Ba contributions were detected;
one related to BaCO3 (37%) and another one associated with BaAl2O4 (63%) [24], confirming that for this catalyst formation of BaCO3 was partially inhibited. After reaction, in the Ni2p3/2 spectral region, an additional signal was detected, which corresponds to Ni0 [3]. The relative contribution of this species was slightly higher for the NiBa/Al2O3(IM) and NiBa/ Al2O3(M) catalysts than for the Ni/Al2O3(IM). This latter observation together with the higher proportion of NiO for the catalysts containing Ba before reaction suggests that Ba incorporation could promote Ni reducibility. Juan-Juan et al. [19] studied the effect of K content on Ni/ Al2O3 catalysts for the DRM. They reported that K incorporation increases the Ni reducibility, because K modifies the interaction of nickel oxide with alumina. However, the differences in reducibility after reaction that we observed could be affected by the carbon formation and Ni reoxidation prior to the XPS analysis, as was pointed
Fig. 3. Activity and stability results. a. CH4 conversion vs. temperature; b. CO2 conversion vs. temperature; c. H2/CO ratio vs. temperature; d. CO2 conversion vs. time on stream (700 °C).
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out above. All the samples were kept under inert atmosphere after reaction, but the exposition to the air during catalyst handling cannot be excluded. On the other hand, for the Ba containing catalysts after reaction the same contributions observed before reaction were detected, with a decrease in the relative contribution of BaCO3 for the NiBa/Al2O3(M) catalyst. Moreover, for the NiBa/Al2O3(M) catalyst the Ni/Al superficial atomic ratio does not considerably change after reaction, indicating that the Ni superficial population is not modified, keeping its dispersion. Activity results are summarized in Fig. 3 as well as the equilibrium conversions values, calculated considering simultaneous CO2-CH4 reforming and reverse water gas shift reaction (RWGS) at the reactivity test conditions (atmospheric pressure and CH4/CO2/He = 20/20/60). Equilibrium calculations were done by the minimization of Gibbs Energy Method taking into account CH4, CO2, H2, CO, H2O and He as species in the system, solid carbon was not considered. The computer program for calculation of complex chemical equilibrium composition and applications was used [25]. With respect to catalysts' activity, it can be observed (Fig. 3a,b and c) that in the temperature range 300–700 °C there are not significant differences among the catalysts studied. All the variations were in the error range of the analysis technique. Furthermore, it is interesting to note that the H2/CO ratio values obtained are lower than the thermodynamic ones in the whole temperature range, indicating the occurrence of side reactions; which were not considered for the equilibrium calculations. Despite the apparent higher reducibility of the Ba containing catalysts, registered by XPS, there is no a direct effect on activity. This suggests the presence of different Ni2+ species, as NiO-amorphous species, with diverse stability, affecting the population of Ni0 sites; whose proportion is apparently the same for the catalysts studied at the onset of the reaction. To clarify the effect of Ba on the reactivity of Ni/Al2O3 catalysts for the DRM, stability runs were performed at 700 °C (Fig. 3d). It is worth noticing that, even when the activity and Elemental Analysis results did not show important differences between the catalysts, the catalysts' stability for the DRM is quite different. For the Ni/Al2O3 (IM), the NiBa/Al2O3(IM) and the NiBa/Al2O3(M) catalysts values of CO2 conversion loss of 1.9% h−1, 1.7% h−1 and 0% h−1 were registered. The low stability of the impregnated catalysts can be attributed to the encapsulation of Ni particles observed by TEM (see Fig. 2a,b), decreasing the available Ni sites as was observed by XPS. Apparently, the kind of carbon formed with the IM catalysts differs from that observed for the NiBa/Al2O3(M) catalyst. Together characterization and reactivity results indicate that all the NiBa catalysts were highly active for the DRM and apparently the carbon formed affects the catalysts' stability. The addition, by impregnation, of low quantities of Ba in the Ni/Al2O3 catalyst does not considerably affect the catalyst activity under the DRM conditions. However, depending on the amount of Ba added and the preparation procedure the stability of these catalysts can be modified. The catalyst with higher NiBa content and prepared by mixture (NiBa/Al2O3(M)) presented high stability, which can be attributed to the in situ formation of a stable BaAl2O4 phase. The Ba spinel acts as the real support for the NiBa/Al2O3(M) catalysts stabilizing the Ni particles over the catalyst surface. It has been already reported that Mg spinels are high stable supports for reforming processes [26,27].
It is important to point out that in a previous study (data not shown) a Ni catalyst supported on a BaAl2O4 spinel, synthesized ex situ, was prepared; this catalyst presented very low activity for the DRM. In addition, a NiBa unsupported catalyst was also prepared and tested under reaction conditions. It was not active for the DRM, due to the high and non-reversible carbonation of the catalyst. Therefore, simultaneous incorporation of Ni, Ba and alumina facilitates the attainment of stable and effective NiBa catalysts for the DRM. 4. Conclusion The addition of 4 at/nm2 of Ba to Ni-based catalysts supported on alumina does not considerably affect the overall activity and stability for the DRM. However, it inhibits the formation of Ni spinel. The NiBa catalyst with high content of Ni–Ba and prepared by mixture, allows a system with a low carbonation level, characterized by the formation of a highly stable Ba spinel. This catalyst reaches good activity levels and is rather stable for the DRM. Acknowledgement MGD acknowledges the Spanish Minister of Education and Science for an FPI grant and for the financial support to the projects ENE200406176 and ENE2007-67926-C02-02/ALT. References [1] J.R. Rostrup-Nielsen, Catal. Today 18 (1993) 305–324. [2] M.C.J. Bradford, M.A. Vannice, Catal. Rev.-Sci. Eng. 41 (1999) 1–42. [3] B. Pawelec, S. Damyanova, K. Arishtirova, J.L. Fierro, L. Petrov, Appl. Catal. A 323 (2007) 188–201. [4] C.E. Daza, J. Gallego, J.A. Moreno, F. Mondragón, S. Moreno, R. Molina, Catal. Today 133–135 (2008) 357–366. [5] K.Y. Koo, H.-S. Roh, Y.T. Seo, D.J. Seo, W.L. Yoon, S.B. Park, Appl. Catal. A 340 (2008) 183–190. [6] E. Ruckenstein, Y.H. Hu, Appl. Catal. A 133 (1995) 149–161. [7] M. García-Diéguez, I.S. Pieta, M.C. Herrera, M.A. Larrubia, L.J. Alemany, J. Catal. 270 (2010) 136–145. [8] M. García-Diéguez, I.S. Pieta, M.C. Herrera, M.A. Larrubia, L.J. Alemany, Appl. Catal. A 377 (2010) 191–199. [9] N. Sahli, C. Petit, A.C. Roger, A. Kiennemann, S. Libs, M.M. Bettah, Catal. Today 113 (2006) 187–193. [10] L. Xiancai, W. Min, L. Zhihua, H. Fei, Appl. Catal. A 290 (2005) 81–86. [11] F. Pompeo, N.N. Nichio, M.M.V.M. Souza, D.V. Cesar, O.A. Ferretti, M. Schmal, Appl. Catal. A 316 (2007) 175–183. [12] J. Zhang, H. Wang, A.K. Dalai, J. Catal. 249 (2007) 300–310. [13] Y.-G. Chen, J. Ren, Catal. Lett. 29 (1994) 39–48. [14] R. Lamber, G. Schulz-Ekloff, Surf. Sci. 258 (1991) 107–118. [15] R. Lamber, G. Schulz-Ekloff, J. Catal. 146 (1994) 601–607. [16] S. Corthals, J. Van Nederkassel, J. Geboers, H. De Winne, J. Van Noyen, B. Moens, B. Sels, P. Jacobs, Catal. Today 138 (2008) 28–32. [17] Z.L. Zhang, X.E. Verykios, Catal. Today 21 (1994) 589–595. [18] M. Rezaei, S.M. Alavi, S. Sahebdelfar, Peng Bai, Xinmei Liu, Zi-Feng Yan, Appl. Catal. B 77 (2008) 346–354. [19] J. Juan-Juan, M.C. Román-Martínez, M.J. Illán-Gómez, Appl. Catal. A 301 (2006) 9–15. [20] M. García-Diéguez, I. Pieta, F. Sánchez-García, B. Guerrero-Klein, M.C. Herrera, M. A. Larrubia, L.J. Alemany, Proceedings del XXI Simposio Iberoamericano de Catálisis (SICAT). ISBN 978 84 691 4234 9 (Málaga 2008), 2008, p. 1594. [21] S. Narayanan, K. Uma, J. Chem. Soc. Faraday Trans. I 84 (1988) 521–527. [22] B. Scheffer, P. Molhoek, J.A. Moulijn, Appl. Catal. 46 (1989) 11–30. [23] A.B. Christie, J. Lee, I. Sutherland, J.M. Walls, Appl. Surf. Sci. 15 (1983) 224–237. [24] C. Zhang, L. Wang, L. Cui, Y. Zhu, J. Cryst. Growth 255 (2003) 317–323. [25] B.J. Mc Bride, S. Gordon, NASA Reference Publication, 1311, 1996. [26] S. Corthals, J. Van Nederkassel, J. Geboers, H. De Winne, J. Van Noyen, B. Moens, B. Sels, P. Jacobs, Catal. Today 138 (2008) 28–32. [27] J. Sehested, J.A.P. Gelten, S. Helveg, Appl. Catal. A 309 (2006) 237–246.