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Low temperature steam reforming of methanol over layered double hydroxide-derived catalysts
Applied Catalysis A: General 231 (2002) 215–226
Low temperature steam reforming of methanol over layered double hydroxide-derived catalysts夽 Scott R. Segal a , Ken B. Anderson a , Kathleen A. Carrado a , Christopher L. Marshall b,∗ a
b
Chemistry Division, Argonne National Laboratory, 9700 S. Cass Avenue, Argonne, IL 60439, USA Chemical Technology Division, Argonne National Laboratory, 9700 S. Cass Avenue, Argonne, IL 60439, USA Received 22 August 2001; received in revised form 12 January 2002
Abstract The catalytic production of hydrogen by steam reforming of methanol (SRM) has been carried out over Mg/Al, Cu/Al, Co/Al, and Ni/Al layered double hydroxides (LDHs). The catalytic reactions were performed at temperatures of 150–400 ◦ C and atmospheric pressure. The most efficient catalyst was the Cu/Al LDH, which became active at ∼230 ◦ C, with concomitant H2 production. The Ni/Al and Co/Al LDHs were also active in SRM, however, the activation temperature was significantly higher (315–320 ◦ C). No catalytic activity was observed for the Mg/Al LDH. Significant LDH decomposition occurred during the catalytic reactions. The reducibility of the divalent cations present in the LDH was a crucial parameter in determining the steam reforming activity of the catalysts. Pre-activation of the Cu/Al LDH by calcination in air (400 ◦ C), followed by reduction in dilute H2 , did not significantly change the catalytic activity. The onset of H2 production was slightly lower for the pre-activated versus as-prepared Cu/Al LDH (∼218 ◦ C), also the CH3 OH conversion was 5–10% lower. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Methanol; Copper; Cobalt; Nickel; Layered double hydroxide; Reforming; Hydrogen
1. Introduction Catalytic steam reforming of methanol (SRM) is a well-established process used for the production of hydrogen. CH3 OH + H2 O 3H2 + CO2 ; 0 = 49.4 kJ/mol H298 夽
(1)
The submitted manuscript has been created by the University of Chicago as Operator of Argonne National Laboratory (Argonne) under Contract no. W-31-109-ENG-38 with the US Department of Energy. The US Government retains for itself, and others acting on its behalf, a paid-up, non-exclusive, irrevocable worldwide license in said article to reproduce, prepare derivative works, distribute copies to the public, and perform publicly and display publicly, by or on behalf of the Government. ∗ Corresponding author. E-mail address: [email protected] (C.L. Marshall).
This process is especially important for proton exchange membrane fuel-cells, which generate electrical power by electrochemical oxidation of hydrogen with atmospheric oxygen [1]. The use of methanol as an on-board hydrogen source is attractive for fuel-cell engines in transportation applications because of its safe handling, low cost and ease of synthesis from a variety of feedstocks (biomass, coal and natural gas) [2]. Copper-containing catalysts, especially CuZn or CuZnAl mixed oxides, have been the most well-studied catalysts in the SRM, due to their high selectivity and activity [3–12]. It is believed that the active sites in Cu-containing catalysts are metallic Cu species [11]. For example, recent in situ X-ray diffraction (XRD) and absorption spectroscopy studies showed that the active Cu phase for SRM in Cu/ZnO catalysts were completely reduced Cu species [13]. To
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produce a large number of active sites, the catalysts are generally activated prior to the SRM reaction. Activation is usually accomplished by reducing the catalysts in a dilute hydrogen stream at temperatures of 200–500 ◦ C. Layered double hydroxides (LDHs) belong to a class of anionic clays having a hydrotalcite-like structure. The general molecular formula can be represented as: [M1−x 2+ Mx 3+ (OH)2 ][An− ]x/n · mH2 O, where M2+ and M3+ are metal cations which are cross-linked through hydroxide groups to form a dimetal hydroxide sheet, similar to that of brucite [Mg(OH)2 ]. Some of the divalent metal cations are substituted by trivalent metal cations to form positively charged sheets [14,15]. The excess positive charge of the layered network is compensated by anions (An − ), which occupy the interlayer space along with water molecules [16]. Numerous LDHs having a wide variety of M2+ /M3+ cation pairs with different anions in the interlayer have been reported [17]. They have many applications, including catalysts, catalyst supports, ion exchangers, adsorbents, antiacids, flame retardants, and corrosion inhibitors [17,18]. Many reports have concentrated on the use of LDHs as precursors to mixed oxide catalysts formed during thermal decomposition. Some have reported the use of non-calcined LDHs in catalytic applications, mainly in liquid-phase oxidations of organic substrates such as Baeyer–Villiger oxidations [19,20], olefin epoxidation [21], and phenol hydroxylation [22]. The mixed metal oxides formed during thermal decomposition of LDHs are excellent catalysts due to their large surface areas, basic properties, high metal dispersions and stability against sintering [23]. Such metal oxides are known to promote base-catalyzed reactions such as polymerization [24], condensation [25], hydrogenation [26], and total oxidation of volatile organic compounds [27]. A few reports have shown that mixed metal oxides derived from LDHs are active catalysts in the SRM, including those with the metal combinations Cu/Zn/Al [8], Ni/Al, and Co/Al [3]. However, in these studies, no catalytic data were presented using as-prepared LDHs that had not been pre-activated. In this work, we prepared a series of LDH catalysts containing different metal cation combinations and examined the catalysts in the SRM. In particular, we carried out steam reforming reactions using as-prepared LDHs and characterized their performance by
elemental analyses, XRD, temperature-programmed reduction (TPR), and thermal gravimetric analyses (TGA). We also compared these results to those obtained using pre-activated LDHs (calcination in air, followed by reduction in hydrogen).
2. Experimental section 2.1. Catalyst preparation The LDHs were prepared using a standard co-precipitation technique similar to that reported by Valente et al. [28]. The syntheses were performed at room temperature in air. All chemicals were purchased from Aldrich (Milwaukee, WI). The LDHs were synthesized by drop-wise addition of an aqueous solution containing 2 M NaOH and 0.5 M Na2 CO3 to an aqueous M2+ /M3+ nitrate salt solution and vigorous stirring. The M2+ /M3+ ratio is 3 (M2+ = 0.75 M, M3+ = 0.25 M). The NaOH/Na2 CO3 solution was added using a pH stat instrument (718 Stat Titrino, Metrohm), to maintain constant pH. All syntheses were done at pH = 9.00 ± 0.02, except for the Cu/Al LDH (pH = 8.00 ± 0.02). The M2+ /M3+ solution was added at a constant rate (0.4 ml/min) using a 776 Dosimat (Metrohm) instrument. After addition of all reagents, the solutions were heated overnight at 65 ◦ C with stirring. The samples were then filtered, washed several times with deionized water and dried in air at 80 ◦ C for 18 h. 2.2. Characterization techniques XRD was conducted with a Rigaku Miniflex + instrument (30 kV, 15 mA) using Cu K␣ radiation, a step size of 0.05◦ , and a scan rate of 2◦ 2θ /min from 5 to 65◦ 2θ . TPR measurements were made with an Altamira AMI-1 equipped with a thermal conductivity detector (TCD). The sample (0.100 g) was reduced with 4% H2 /Ar from 50 to 800 ◦ C at a heating rate of 5 ◦ C/min and a flow rate of 30 ml/min. TGA were performed on a HAAKE instrument EXSTAR 6000. Samples of ∼5 mg were heated from 25 to 800 ◦ C at 5 ◦ C/min in flowing N2 (100 ml/min). The samples were measured against an alumina reference. Metals analysis was performed on a Perkin-Elmer Optima 3300 DV inductively coupled plasma-atomic
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emission spectrometer instrument. Microanalysis (% C, H, N) data were collected using a LECO CHN-900 analyzer. 2.3. Catalytic activity Activity tests were performed in a flow reactor system at atmospheric pressure. A 1/1 molar mixture of CH3 OH/H2 O was introduced into a 2 cm long stainless steel reactor (0.5 cm diameter) that contained 0.150 g of the catalyst. The reactants were delivered into the catalytic reactor using a Hewlett-Packard 1050 HPLC pump at a total flow rate of 0.25 ml/min. The reactor was placed in a temperature-controlled oven and experiments were performed by increasing the temperature from 150 to 400 ◦ C at 1 ◦ C/min. Prior to all experiments, the catalyst was purged with He at 150 ◦ C for 30 min. After flowing through the reactor, the product feed was passed through a condenser (chilled to −16 ◦ C) to trap any unreacted CH3 OH and H2 O. The gaseous products were then injected into a Hewlett-Packard 5890 Series II gas chromatograph (J&W GasPro GSC column with TCD) to measure products such as H2 , CO2 , CH4 , and CO. The GC was operated using He as a carrier gas with temperature programming from −40 (held for 1 min) to 185 ◦ C, using a ramp rate of 20 ◦ C/min. To measure reactant conversion (CH3 OH and H2 O), a separate experiment was performed in which the product stream was passed directly into a Stanford Research System RGA 200 mass spectrometer.
3. Results and discussion 3.1. Catalyst characterization Chemical formulas for the various LDH catalysts are given in Table 1. A M2+ /M3+ ratio of ∼3 for
217
Cu/Al, Ni/Al, and Co/Al LDHs is consistent with the M2+ /M3+ ratio used in the starting solutions. The Mg/Al ratio of 2.5, however, suggests the incomplete precipitation of Mg2+ ions. The CHN analyses indicated that the LDH samples contain both carbonate and nitrate anions within the interlayer, however, carbonate anions are the dominant interlayer anion. The XRD patterns for the LDHs are shown in Fig. 1 and are typical of hydrotalcite-like materials with intercalated carbonate anions. The most crystalline LDH is Mg/Al, which has well-defined peaks at 7.73 and 3.87 Å, corresponding to the 0 0 3 and 0 0 6 reflections, respectively. The positions of these peaks are consistent with ordering of the layers in the c-axis direction [29]. The peaks at 2.60, 2.33, 1.97, 1.54, and 1.51 Å can be assigned to the 0 1 2, 0 1 5, 0 1 8, 1 1 0, and 1 1 3 reflections of hydrotalcite (JCPDS file no. 70-2151). The XRD patterns of the Ni/Al, Co/Al, and Cu/Al LDHs (Fig. 1B–D) also indicate the presence of a hydrotalcite-like phase, however, the XRD pattern for the Cu/Al LDH contains several extra peaks, suggesting the presence of another phase. For the Cu/Al LDH, the peaks at 6.09, 5.12, 2.88, 2.75, and 2.44 Å can be assigned to malachite, a copper hydroxide carbonate material (JCPDS file no. 76-0660). The malachite phase is known to form in Cu/Al LDHs, especially at high Cu/Al ratios [30]. The lower stability of the copper hydrotalcite phase, and the subsequent formation of malachite, can be explained by the Jahn–Teller effect in the Cu2+ ion [17,23]. Fig. 2 displays TGA data for the LDH samples. The curves show two principal regions where weight loss occurs in the LDHs. The first weight loss occurs below 200 ◦ C and corresponds to removal of interlayer water molecules. The second weight loss occurs at temperatures greater than 200 ◦ C and is attributed to removal of hydroxyl groups from the brucite layers as water molecules. This second weight loss is also due to loss of interlayer carbonate anions as CO2 [31].
Table 1 Chemical formulas of layered double hydroxides LDH sample
M2+ /M3+
Composition of solids
Mg/Al Ni/Al Co/Al Cu/Al
2.51 2.98 3.00 3.07
[Mg0.80 Al0.32 (OH)2 ](CO3 2− )0.24 (NO3 − )0.05 ·0.86H2 O [Ni0.79 Al0.26 (OH)2 ](CO3 2− )0.18 (NO3 − )0.05 ·0.65H2 O [Co0.80 Al0.27 (OH)2 ](CO3 2− )0.18 (NO3 − )0.04 ·0.64H2 O [Cu0.86 Al0.28 (OH)2 ](CO3 2− )0.25 (NO3 − )0.10 ·0.54H2 O
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Fig. 1. XRD patterns for (A) Mg/Al; (B) Ni/Al; (C) Co/Al and (D) Cu/Al LDH catalysts. (䊏) malachite.
The TGA data indicate well-defined decomposition ranges for the Mg/Al, Ni/Al, and Co/Al LDHs but not for the Cu/Al LDH. The well-defined decomposition ranges observed with Mg/Al, Ni/Al, and Co/Al LDHs may be correlated with their higher crystallinity and phase purity compared to the Cu/Al LDH, in which malachite is known to be present [32]. Temperature-programmed reduction was used to examine the redox properties (i.e. ease of reduction) of the LDH catalysts. Fig. 3 gives TPR data for the LDHs. For the Mg/Al LDH, a small reduction peak appears at 440–550 ◦ C. This peak is probably due to reduction of the interlayer NO3 − into NO. No other reduction peaks occur in the TPR curve of the Mg/Al LDH, as is expected considering the highly unfavorable reduction potentials of Mg2+ /Mg0 and Al3+ /Al0 (E 0 = −2.375 and −1.706 V versus SHE, respectively) [33]. Reduction potentials for all cations used in the LDHs are listed in Table 2. The TPR curve for the Ni/Al LDH shows a small, broad peak at 300–400 ◦ C, which is due to reduction of NO3 − into NO [34]. A much larger and broader peak is observed at ∼425–800 ◦ C. The large area of this peak suggests the reduction of Ni2+ to Ni0 . The Co/Al LDH shows reduction trends similar to the Ni/Al LDH. The small peak at around 470 ◦ C probably correlates
to NO3 − reduction. A large, broad peak then begins at 480 ◦ C and extends beyond 800 ◦ C. This peak indicates reduction of Co2+ to Co0 , however, the reduction is incomplete since the baseline never returns to its original value. The Cu/Al LDH exhibits the lowest reduction temperature of all the LDHs. The TPR curve has a large shoulder at ∼275 ◦ C and a large and broad peak that extends up until 370 ◦ C. These peaks are clearly due to Cu2+ reduction, which occurs at a much lower temperature than observed with other LDHs and can be explained by the highly favorable reduction potential of Cu2+ /Cu0 (E 0 = 0.337 V versus SHE) [33]. When the TPR experiment for the Cu/Al LDH is performed at a lower heating rate (2 ◦ C/min instead of Table 2 Reduction potentials of cations used in LDH catalysts Redox couple
Reduction potential (V vs. SHE)a
Cu2+ /Cu0 Ni2+ /Ni0 Co2+ /Co0 Mg2+ /Mg0 Al3+ /Al0
0.337 −0.23 −0.28 −2.375 −1.706
a
Taken from [33].
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Fig. 2. TGA data for (A) Mg/Al; (B) Ni/Al; (C) Co/Al and (D) Cu/Al LDHs.
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Fig. 3. TPR data for (A) Mg/Al; (B) Ni/Al; (C) Co/Al and (D) Cu/Al LDHs.
5 ◦ C/min), the peaks are better resolved and also occur at lower temperatures. The TPR data for the Cu/Al LDH at the slower heating rate are plotted in Fig. 4. The two peaks are now completely resolved and occur at lower temperatures (230 and 280 ◦ C). We reason that the peak occurring at 230 ◦ C is due to reduction of Cu2+ in the malachite phase. The TPR curve for a malachite standard (Cu2 CO3 (OH)2 , Aldrich) is also shown in Fig. 4. The Cu2+ reduction in malachite occurs at much lower temperatures (160–220 ◦ C), closer to the temperature of the first reduction peak in the Cu/Al LDH. All these data indicate that the reduction behavior of Cu2+ varies depending on the environment, structure, support, etc. The TPR data provide further evidence that the Cu/Al LDH is actually a mixture of a hydrotalcite-like and malachite phases. 3.2. Catalytic activity of LDHs In the absence of catalyst, no CH3 OH conversion was observed at <400 ◦ C. Similarly for Mg/Al LDH, besides the production of trace amounts (<1%) of CO2 , no catalytic activity was observed. The most
active catalyst was the Cu/Al LDH. Catalytic data for SRM over the Cu/Al LDH are plotted in Fig. 5. The results show that activation of the catalyst occurs at ∼230 ◦ C. Above this temperature, CH3 OH conversion increases, and both H2 and CO2 are produced in stoichiometric amounts (∼H2 /CO2 = 3). With increasing temperature and CH3 OH conversion, the H2 /CO2 ratio remains the same. Carbon dioxide is by far the major C-containing product, however, small amounts of CO are produced as the temperature increases (0.8 mol% at 400 ◦ C). Most likely, the CO is formed due to the reverse water–gas shift reaction (Eq. (2)), which is commonly observed with Cu-containing SRM catalysts [35]. We have calculated the equilibrium concentration of CO at 400 ◦ C to be 0.6 mol%, which is close to the CO concentration we observed (0.8 mol%). This calculation suggests that the reverse water–gas shift reaction is at equilibrium over the Cu/Al LDH. CO2 + H2 CO + H2 O
(2)
Catalytic data for the Co/Al LDH are plotted in Fig. 6. The behavior of the Co/Al LDH is similar to that observed for the Cu/Al LDH, except that the
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Fig. 4. TPR data for Cu/Al LDH and malachite at lower heating rates (2 ◦ C/min).
Fig. 5. Steam reforming activity as a function of temperature for Cu/Al LDH.
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Fig. 6. Steam reforming activity as a function of temperature for Co/Al LDH.
activation temperature is considerably higher (315 ◦ C) and CH3 OH conversion is significantly less. As with the Cu/Al LDH, trace amounts of CO are produced as temperature increases (1.0 mol% at 400 ◦ C). Fig. 7 displays the catalytic data for the Ni/Al LDH. Note that there is some drift in the mass spectrometer measurements, especially at low methanol conversion (T < 300 ◦ C). The results for the Ni/Al LDH differ from those of the Cu/Al and Co/Al LDHs. As with the Co/Al LDH, the activation temperature for the Ni/Al LDH is ∼320 ◦ C. The CH3 OH conversion is intermediate between that of the Cu/Al and Co/Al LDHs. The major product formed over the Ni/Al LDH is H2 and considerably less CO2 is formed (8.0 mol% at 400 ◦ C). In addition, a significant amount of CO is produced (17 mol% at 400 ◦ C). This result suggests that the major reaction occurring over the Ni/Al LDH is CH3 OH decomposition (Eq. (3)). CH3 OH CO + 2H2
(3)
This reaction is consistent with literature reports, which show that Ni-containing catalysts promote methanol decomposition [2]. The XRD studies indicate that significant structural changes occur in the LDH materials during the
catalytic reactions. Diffraction patterns taken immediately following the catalytic reactions show that the Cu/Al LDH structure has decomposed to a highly crystalline metallic Cu phase (Fig. 8). The result is also consistent with literature reports, which indicate that the active sites in Cu-containing SRM catalysts are reduced Cu species [11]. While some literature reports have suggested that Cu+ species participate in SRM reactions [6], we did not observe Cu+ phases, such as Cu2 O by XRD. Future in situ catalyst characterization by XRD or extended X-ray absorption fine structure spectroscopy (EXAFS) will be done to probe for any Cu2 O formation. The Ni/Al and Co/Al LDHs also undergo significant structural changes during the catalytic reaction. For the Ni/Al LDH, the only crystalline phase detected after reaction is NiO. The Co/Al LDH forms an amorphous structure. As mentioned earlier, the reducibility of the cations in the LDHs appears to be an important factor in determining the catalysts’ activity. Based on methanol conversion, the order of catalytic activity is Cu/Al Ni/Al > Co/Al Mg/Al. The catalytic activity correlates to the redox behavior of the divalent cations in the LDHs. The LDHs containing divalent cations that are easily reduced (i.e. Cu2+ ) exhibit
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Fig. 7. Steam reforming activity as a function of temperature for Ni/Al LDH.
Fig. 8. XRD pattern for the Cu/Al LDH after steam reforming of methanol (400 ◦ C).
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Fig. 9. Steam reforming activity as a function of temperature for pre-treated Cu/Al LDH.
the highest catalytic activity. The LDHs containing divalent cations that are not easily reduced (i.e. Mg2+ ) show no catalytic activity. The Ni/Al and Co/Al LDHs have intermediate reduction potentials and thus exhibit intermediate catalytic activity with respect to the Cu/Al and Mg/Al LDHs. 3.3. Catalytic activity of pre-activated Cu/Al LDH It is clear from these data that the LDH materials undergo significant decomposition during SRM reactions. Under a methanol/water atmosphere (absence of O2 ), the redox-active divalent cations in the LDHs are reduced. This may explain, why in many studies dealing with Cu-containing SRM catalysts, the catalysts are pre-reduced prior to SRM reactions. Therefore, we have compared an as-prepared LDH with one that has been pre-activated by calcining in air, followed by reduction in H2 . Since the Cu/Al LDH showed the highest activity (and is completely reduced below 400 ◦ C), we chose this catalyst for the comparison study. We followed a standard procedure found in the literature for pre-treating Cu-containing catalysts prior to SRM reactions [36]. The Cu/Al LDH was first calcined in
flowing air at 400 ◦ C for 4 h. The XRD data of the calcined Cu/Al LDH indicated that the only crystalline phase present is CuO. The calcined Cu/Al LDH was loaded into the catalytic reactor and reduced in situ by passing a 4.2% H2 /He gas mixture over the catalyst at 400 ◦ C for 1 h. The pre-activated catalyst was cooled to 150 ◦ C and purged with He briefly. Then the SRM reaction was carried out as in previous experiments. If we compare data from the pre-activated Cu/Al LDH (Fig. 9) to that of the as-prepared Cu/Al LDH (Fig. 5), several differences are obvious. Overall, product compositions appear similar, however, the pre-activated Cu/Al LDH becomes catalytically active at a slightly lower temperature. Hydrogen formation is first observed at 218 ◦ C, whereas the as-prepared Cu/Al LDH does not begin H2 production until ∼230 ◦ C. The most surprising finding is that the increase in reaction rate with increasing temperatures occurs more slowly with the pre-treated catalyst than with the non-activated catalyst. The CH3 OH conversion for the as-prepared versus the pre-treated Cu/Al LDH is shown in Fig. 10. The curves indicate that CH3 OH conversion is 5–10% lower for the pre-treated compared with the as-prepared Cu/Al LDH. The differences in catalytic activity are probably due to
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Fig. 10. CH3 OH conversion for pre-treated Cu/Al LDH vs. as-prepared Cu/Al LDH.
differences in surface area, Cu dispersion and the oxidation states of the Cu species in the catalysts. Further catalyst characterization studies are needed to help in explaining the catalytic trends. In situ EXAFS experiments are planned to learn more about the state of the metal in these catalysts during the SRM reaction. The results above suggest that pre-activation of Cu/Al LDHs is not necessary to achieve high conversion and selectivity for H2 production in SRM reactions. For the Cu/Al LDH, sufficient catalyst activation (Cu2+ reduction) occurs in the presence of the CH3 OH/H2 O feed (absence of O2 ) at ∼230 ◦ C. Further studies are being carried out to compare pre-treated versus non-treated Co/Al and Ni/Al LDHs.
4. Conclusions We have prepared and characterized several LDH catalysts that are active for SRM. The most favorable activity is observed with Cu/Al LDHs. Our Cu/Al LDH is actually a mixture of hydrotalcite-like and malachite phases. While Co/Al and Ni/Al LDHs also show catalytic activity, their activation temperature is higher and methanol conversions are lower. For
the Ni/Al LDH, methanol decomposition is the major reaction pathway. No activity was observed for the Mg/Al LDH. Results from TGA, TPR, and XRD experiments indicate that the LDH structure decomposes at temperatures used during SRM reactions (>150 ◦ C). A direct correlation was found relating the reducibility of the divalent metal in the LDH to the catalytic activity. The Cu/Al LDH containing easily reducible Cu2+ had the highest catalytic activity, whereas the Mg/Al LDH showed no catalytic activity due to the unfavorable reduction potential of the Mg2+ /Mg0 redox couple. Pre-activation of the Cu/Al LDH by calcination in air (400 ◦ C), followed by reduction in dilute H2 does not significantly change the catalytic activity of the catalyst. The onset of H2 production is slightly lower for the pre-activated compared with the as-prepared Cu/Al LDH, also the CH3 OH conversion is generally 5–10% lower.
Acknowledgements The work was performed under the auspices of the US Department of Energy, under contract number
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Low temperature steam reforming of methanol over layered double hydroxide-derived catalysts夽 Scott R. Segal a , Ken B. Anderson a , Kathleen A. Carrado a , Christopher L. Marshall b,∗ a
b
Chemistry Division, Argonne National Laboratory, 9700 S. Cass Avenue, Argonne, IL 60439, USA Chemical Technology Division, Argonne National Laboratory, 9700 S. Cass Avenue, Argonne, IL 60439, USA Received 22 August 2001; received in revised form 12 January 2002
Abstract The catalytic production of hydrogen by steam reforming of methanol (SRM) has been carried out over Mg/Al, Cu/Al, Co/Al, and Ni/Al layered double hydroxides (LDHs). The catalytic reactions were performed at temperatures of 150–400 ◦ C and atmospheric pressure. The most efficient catalyst was the Cu/Al LDH, which became active at ∼230 ◦ C, with concomitant H2 production. The Ni/Al and Co/Al LDHs were also active in SRM, however, the activation temperature was significantly higher (315–320 ◦ C). No catalytic activity was observed for the Mg/Al LDH. Significant LDH decomposition occurred during the catalytic reactions. The reducibility of the divalent cations present in the LDH was a crucial parameter in determining the steam reforming activity of the catalysts. Pre-activation of the Cu/Al LDH by calcination in air (400 ◦ C), followed by reduction in dilute H2 , did not significantly change the catalytic activity. The onset of H2 production was slightly lower for the pre-activated versus as-prepared Cu/Al LDH (∼218 ◦ C), also the CH3 OH conversion was 5–10% lower. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Methanol; Copper; Cobalt; Nickel; Layered double hydroxide; Reforming; Hydrogen
1. Introduction Catalytic steam reforming of methanol (SRM) is a well-established process used for the production of hydrogen. CH3 OH + H2 O 3H2 + CO2 ; 0 = 49.4 kJ/mol H298 夽
(1)
The submitted manuscript has been created by the University of Chicago as Operator of Argonne National Laboratory (Argonne) under Contract no. W-31-109-ENG-38 with the US Department of Energy. The US Government retains for itself, and others acting on its behalf, a paid-up, non-exclusive, irrevocable worldwide license in said article to reproduce, prepare derivative works, distribute copies to the public, and perform publicly and display publicly, by or on behalf of the Government. ∗ Corresponding author. E-mail address: [email protected] (C.L. Marshall).
This process is especially important for proton exchange membrane fuel-cells, which generate electrical power by electrochemical oxidation of hydrogen with atmospheric oxygen [1]. The use of methanol as an on-board hydrogen source is attractive for fuel-cell engines in transportation applications because of its safe handling, low cost and ease of synthesis from a variety of feedstocks (biomass, coal and natural gas) [2]. Copper-containing catalysts, especially CuZn or CuZnAl mixed oxides, have been the most well-studied catalysts in the SRM, due to their high selectivity and activity [3–12]. It is believed that the active sites in Cu-containing catalysts are metallic Cu species [11]. For example, recent in situ X-ray diffraction (XRD) and absorption spectroscopy studies showed that the active Cu phase for SRM in Cu/ZnO catalysts were completely reduced Cu species [13]. To
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produce a large number of active sites, the catalysts are generally activated prior to the SRM reaction. Activation is usually accomplished by reducing the catalysts in a dilute hydrogen stream at temperatures of 200–500 ◦ C. Layered double hydroxides (LDHs) belong to a class of anionic clays having a hydrotalcite-like structure. The general molecular formula can be represented as: [M1−x 2+ Mx 3+ (OH)2 ][An− ]x/n · mH2 O, where M2+ and M3+ are metal cations which are cross-linked through hydroxide groups to form a dimetal hydroxide sheet, similar to that of brucite [Mg(OH)2 ]. Some of the divalent metal cations are substituted by trivalent metal cations to form positively charged sheets [14,15]. The excess positive charge of the layered network is compensated by anions (An − ), which occupy the interlayer space along with water molecules [16]. Numerous LDHs having a wide variety of M2+ /M3+ cation pairs with different anions in the interlayer have been reported [17]. They have many applications, including catalysts, catalyst supports, ion exchangers, adsorbents, antiacids, flame retardants, and corrosion inhibitors [17,18]. Many reports have concentrated on the use of LDHs as precursors to mixed oxide catalysts formed during thermal decomposition. Some have reported the use of non-calcined LDHs in catalytic applications, mainly in liquid-phase oxidations of organic substrates such as Baeyer–Villiger oxidations [19,20], olefin epoxidation [21], and phenol hydroxylation [22]. The mixed metal oxides formed during thermal decomposition of LDHs are excellent catalysts due to their large surface areas, basic properties, high metal dispersions and stability against sintering [23]. Such metal oxides are known to promote base-catalyzed reactions such as polymerization [24], condensation [25], hydrogenation [26], and total oxidation of volatile organic compounds [27]. A few reports have shown that mixed metal oxides derived from LDHs are active catalysts in the SRM, including those with the metal combinations Cu/Zn/Al [8], Ni/Al, and Co/Al [3]. However, in these studies, no catalytic data were presented using as-prepared LDHs that had not been pre-activated. In this work, we prepared a series of LDH catalysts containing different metal cation combinations and examined the catalysts in the SRM. In particular, we carried out steam reforming reactions using as-prepared LDHs and characterized their performance by
elemental analyses, XRD, temperature-programmed reduction (TPR), and thermal gravimetric analyses (TGA). We also compared these results to those obtained using pre-activated LDHs (calcination in air, followed by reduction in hydrogen).
2. Experimental section 2.1. Catalyst preparation The LDHs were prepared using a standard co-precipitation technique similar to that reported by Valente et al. [28]. The syntheses were performed at room temperature in air. All chemicals were purchased from Aldrich (Milwaukee, WI). The LDHs were synthesized by drop-wise addition of an aqueous solution containing 2 M NaOH and 0.5 M Na2 CO3 to an aqueous M2+ /M3+ nitrate salt solution and vigorous stirring. The M2+ /M3+ ratio is 3 (M2+ = 0.75 M, M3+ = 0.25 M). The NaOH/Na2 CO3 solution was added using a pH stat instrument (718 Stat Titrino, Metrohm), to maintain constant pH. All syntheses were done at pH = 9.00 ± 0.02, except for the Cu/Al LDH (pH = 8.00 ± 0.02). The M2+ /M3+ solution was added at a constant rate (0.4 ml/min) using a 776 Dosimat (Metrohm) instrument. After addition of all reagents, the solutions were heated overnight at 65 ◦ C with stirring. The samples were then filtered, washed several times with deionized water and dried in air at 80 ◦ C for 18 h. 2.2. Characterization techniques XRD was conducted with a Rigaku Miniflex + instrument (30 kV, 15 mA) using Cu K␣ radiation, a step size of 0.05◦ , and a scan rate of 2◦ 2θ /min from 5 to 65◦ 2θ . TPR measurements were made with an Altamira AMI-1 equipped with a thermal conductivity detector (TCD). The sample (0.100 g) was reduced with 4% H2 /Ar from 50 to 800 ◦ C at a heating rate of 5 ◦ C/min and a flow rate of 30 ml/min. TGA were performed on a HAAKE instrument EXSTAR 6000. Samples of ∼5 mg were heated from 25 to 800 ◦ C at 5 ◦ C/min in flowing N2 (100 ml/min). The samples were measured against an alumina reference. Metals analysis was performed on a Perkin-Elmer Optima 3300 DV inductively coupled plasma-atomic
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emission spectrometer instrument. Microanalysis (% C, H, N) data were collected using a LECO CHN-900 analyzer. 2.3. Catalytic activity Activity tests were performed in a flow reactor system at atmospheric pressure. A 1/1 molar mixture of CH3 OH/H2 O was introduced into a 2 cm long stainless steel reactor (0.5 cm diameter) that contained 0.150 g of the catalyst. The reactants were delivered into the catalytic reactor using a Hewlett-Packard 1050 HPLC pump at a total flow rate of 0.25 ml/min. The reactor was placed in a temperature-controlled oven and experiments were performed by increasing the temperature from 150 to 400 ◦ C at 1 ◦ C/min. Prior to all experiments, the catalyst was purged with He at 150 ◦ C for 30 min. After flowing through the reactor, the product feed was passed through a condenser (chilled to −16 ◦ C) to trap any unreacted CH3 OH and H2 O. The gaseous products were then injected into a Hewlett-Packard 5890 Series II gas chromatograph (J&W GasPro GSC column with TCD) to measure products such as H2 , CO2 , CH4 , and CO. The GC was operated using He as a carrier gas with temperature programming from −40 (held for 1 min) to 185 ◦ C, using a ramp rate of 20 ◦ C/min. To measure reactant conversion (CH3 OH and H2 O), a separate experiment was performed in which the product stream was passed directly into a Stanford Research System RGA 200 mass spectrometer.
3. Results and discussion 3.1. Catalyst characterization Chemical formulas for the various LDH catalysts are given in Table 1. A M2+ /M3+ ratio of ∼3 for
217
Cu/Al, Ni/Al, and Co/Al LDHs is consistent with the M2+ /M3+ ratio used in the starting solutions. The Mg/Al ratio of 2.5, however, suggests the incomplete precipitation of Mg2+ ions. The CHN analyses indicated that the LDH samples contain both carbonate and nitrate anions within the interlayer, however, carbonate anions are the dominant interlayer anion. The XRD patterns for the LDHs are shown in Fig. 1 and are typical of hydrotalcite-like materials with intercalated carbonate anions. The most crystalline LDH is Mg/Al, which has well-defined peaks at 7.73 and 3.87 Å, corresponding to the 0 0 3 and 0 0 6 reflections, respectively. The positions of these peaks are consistent with ordering of the layers in the c-axis direction [29]. The peaks at 2.60, 2.33, 1.97, 1.54, and 1.51 Å can be assigned to the 0 1 2, 0 1 5, 0 1 8, 1 1 0, and 1 1 3 reflections of hydrotalcite (JCPDS file no. 70-2151). The XRD patterns of the Ni/Al, Co/Al, and Cu/Al LDHs (Fig. 1B–D) also indicate the presence of a hydrotalcite-like phase, however, the XRD pattern for the Cu/Al LDH contains several extra peaks, suggesting the presence of another phase. For the Cu/Al LDH, the peaks at 6.09, 5.12, 2.88, 2.75, and 2.44 Å can be assigned to malachite, a copper hydroxide carbonate material (JCPDS file no. 76-0660). The malachite phase is known to form in Cu/Al LDHs, especially at high Cu/Al ratios [30]. The lower stability of the copper hydrotalcite phase, and the subsequent formation of malachite, can be explained by the Jahn–Teller effect in the Cu2+ ion [17,23]. Fig. 2 displays TGA data for the LDH samples. The curves show two principal regions where weight loss occurs in the LDHs. The first weight loss occurs below 200 ◦ C and corresponds to removal of interlayer water molecules. The second weight loss occurs at temperatures greater than 200 ◦ C and is attributed to removal of hydroxyl groups from the brucite layers as water molecules. This second weight loss is also due to loss of interlayer carbonate anions as CO2 [31].
Table 1 Chemical formulas of layered double hydroxides LDH sample
M2+ /M3+
Composition of solids
Mg/Al Ni/Al Co/Al Cu/Al
2.51 2.98 3.00 3.07
[Mg0.80 Al0.32 (OH)2 ](CO3 2− )0.24 (NO3 − )0.05 ·0.86H2 O [Ni0.79 Al0.26 (OH)2 ](CO3 2− )0.18 (NO3 − )0.05 ·0.65H2 O [Co0.80 Al0.27 (OH)2 ](CO3 2− )0.18 (NO3 − )0.04 ·0.64H2 O [Cu0.86 Al0.28 (OH)2 ](CO3 2− )0.25 (NO3 − )0.10 ·0.54H2 O
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Fig. 1. XRD patterns for (A) Mg/Al; (B) Ni/Al; (C) Co/Al and (D) Cu/Al LDH catalysts. (䊏) malachite.
The TGA data indicate well-defined decomposition ranges for the Mg/Al, Ni/Al, and Co/Al LDHs but not for the Cu/Al LDH. The well-defined decomposition ranges observed with Mg/Al, Ni/Al, and Co/Al LDHs may be correlated with their higher crystallinity and phase purity compared to the Cu/Al LDH, in which malachite is known to be present [32]. Temperature-programmed reduction was used to examine the redox properties (i.e. ease of reduction) of the LDH catalysts. Fig. 3 gives TPR data for the LDHs. For the Mg/Al LDH, a small reduction peak appears at 440–550 ◦ C. This peak is probably due to reduction of the interlayer NO3 − into NO. No other reduction peaks occur in the TPR curve of the Mg/Al LDH, as is expected considering the highly unfavorable reduction potentials of Mg2+ /Mg0 and Al3+ /Al0 (E 0 = −2.375 and −1.706 V versus SHE, respectively) [33]. Reduction potentials for all cations used in the LDHs are listed in Table 2. The TPR curve for the Ni/Al LDH shows a small, broad peak at 300–400 ◦ C, which is due to reduction of NO3 − into NO [34]. A much larger and broader peak is observed at ∼425–800 ◦ C. The large area of this peak suggests the reduction of Ni2+ to Ni0 . The Co/Al LDH shows reduction trends similar to the Ni/Al LDH. The small peak at around 470 ◦ C probably correlates
to NO3 − reduction. A large, broad peak then begins at 480 ◦ C and extends beyond 800 ◦ C. This peak indicates reduction of Co2+ to Co0 , however, the reduction is incomplete since the baseline never returns to its original value. The Cu/Al LDH exhibits the lowest reduction temperature of all the LDHs. The TPR curve has a large shoulder at ∼275 ◦ C and a large and broad peak that extends up until 370 ◦ C. These peaks are clearly due to Cu2+ reduction, which occurs at a much lower temperature than observed with other LDHs and can be explained by the highly favorable reduction potential of Cu2+ /Cu0 (E 0 = 0.337 V versus SHE) [33]. When the TPR experiment for the Cu/Al LDH is performed at a lower heating rate (2 ◦ C/min instead of Table 2 Reduction potentials of cations used in LDH catalysts Redox couple
Reduction potential (V vs. SHE)a
Cu2+ /Cu0 Ni2+ /Ni0 Co2+ /Co0 Mg2+ /Mg0 Al3+ /Al0
0.337 −0.23 −0.28 −2.375 −1.706
a
Taken from [33].
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Fig. 2. TGA data for (A) Mg/Al; (B) Ni/Al; (C) Co/Al and (D) Cu/Al LDHs.
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Fig. 3. TPR data for (A) Mg/Al; (B) Ni/Al; (C) Co/Al and (D) Cu/Al LDHs.
5 ◦ C/min), the peaks are better resolved and also occur at lower temperatures. The TPR data for the Cu/Al LDH at the slower heating rate are plotted in Fig. 4. The two peaks are now completely resolved and occur at lower temperatures (230 and 280 ◦ C). We reason that the peak occurring at 230 ◦ C is due to reduction of Cu2+ in the malachite phase. The TPR curve for a malachite standard (Cu2 CO3 (OH)2 , Aldrich) is also shown in Fig. 4. The Cu2+ reduction in malachite occurs at much lower temperatures (160–220 ◦ C), closer to the temperature of the first reduction peak in the Cu/Al LDH. All these data indicate that the reduction behavior of Cu2+ varies depending on the environment, structure, support, etc. The TPR data provide further evidence that the Cu/Al LDH is actually a mixture of a hydrotalcite-like and malachite phases. 3.2. Catalytic activity of LDHs In the absence of catalyst, no CH3 OH conversion was observed at <400 ◦ C. Similarly for Mg/Al LDH, besides the production of trace amounts (<1%) of CO2 , no catalytic activity was observed. The most
active catalyst was the Cu/Al LDH. Catalytic data for SRM over the Cu/Al LDH are plotted in Fig. 5. The results show that activation of the catalyst occurs at ∼230 ◦ C. Above this temperature, CH3 OH conversion increases, and both H2 and CO2 are produced in stoichiometric amounts (∼H2 /CO2 = 3). With increasing temperature and CH3 OH conversion, the H2 /CO2 ratio remains the same. Carbon dioxide is by far the major C-containing product, however, small amounts of CO are produced as the temperature increases (0.8 mol% at 400 ◦ C). Most likely, the CO is formed due to the reverse water–gas shift reaction (Eq. (2)), which is commonly observed with Cu-containing SRM catalysts [35]. We have calculated the equilibrium concentration of CO at 400 ◦ C to be 0.6 mol%, which is close to the CO concentration we observed (0.8 mol%). This calculation suggests that the reverse water–gas shift reaction is at equilibrium over the Cu/Al LDH. CO2 + H2 CO + H2 O
(2)
Catalytic data for the Co/Al LDH are plotted in Fig. 6. The behavior of the Co/Al LDH is similar to that observed for the Cu/Al LDH, except that the
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Fig. 4. TPR data for Cu/Al LDH and malachite at lower heating rates (2 ◦ C/min).
Fig. 5. Steam reforming activity as a function of temperature for Cu/Al LDH.
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Fig. 6. Steam reforming activity as a function of temperature for Co/Al LDH.
activation temperature is considerably higher (315 ◦ C) and CH3 OH conversion is significantly less. As with the Cu/Al LDH, trace amounts of CO are produced as temperature increases (1.0 mol% at 400 ◦ C). Fig. 7 displays the catalytic data for the Ni/Al LDH. Note that there is some drift in the mass spectrometer measurements, especially at low methanol conversion (T < 300 ◦ C). The results for the Ni/Al LDH differ from those of the Cu/Al and Co/Al LDHs. As with the Co/Al LDH, the activation temperature for the Ni/Al LDH is ∼320 ◦ C. The CH3 OH conversion is intermediate between that of the Cu/Al and Co/Al LDHs. The major product formed over the Ni/Al LDH is H2 and considerably less CO2 is formed (8.0 mol% at 400 ◦ C). In addition, a significant amount of CO is produced (17 mol% at 400 ◦ C). This result suggests that the major reaction occurring over the Ni/Al LDH is CH3 OH decomposition (Eq. (3)). CH3 OH CO + 2H2
(3)
This reaction is consistent with literature reports, which show that Ni-containing catalysts promote methanol decomposition [2]. The XRD studies indicate that significant structural changes occur in the LDH materials during the
catalytic reactions. Diffraction patterns taken immediately following the catalytic reactions show that the Cu/Al LDH structure has decomposed to a highly crystalline metallic Cu phase (Fig. 8). The result is also consistent with literature reports, which indicate that the active sites in Cu-containing SRM catalysts are reduced Cu species [11]. While some literature reports have suggested that Cu+ species participate in SRM reactions [6], we did not observe Cu+ phases, such as Cu2 O by XRD. Future in situ catalyst characterization by XRD or extended X-ray absorption fine structure spectroscopy (EXAFS) will be done to probe for any Cu2 O formation. The Ni/Al and Co/Al LDHs also undergo significant structural changes during the catalytic reaction. For the Ni/Al LDH, the only crystalline phase detected after reaction is NiO. The Co/Al LDH forms an amorphous structure. As mentioned earlier, the reducibility of the cations in the LDHs appears to be an important factor in determining the catalysts’ activity. Based on methanol conversion, the order of catalytic activity is Cu/Al Ni/Al > Co/Al Mg/Al. The catalytic activity correlates to the redox behavior of the divalent cations in the LDHs. The LDHs containing divalent cations that are easily reduced (i.e. Cu2+ ) exhibit
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Fig. 7. Steam reforming activity as a function of temperature for Ni/Al LDH.
Fig. 8. XRD pattern for the Cu/Al LDH after steam reforming of methanol (400 ◦ C).
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Fig. 9. Steam reforming activity as a function of temperature for pre-treated Cu/Al LDH.
the highest catalytic activity. The LDHs containing divalent cations that are not easily reduced (i.e. Mg2+ ) show no catalytic activity. The Ni/Al and Co/Al LDHs have intermediate reduction potentials and thus exhibit intermediate catalytic activity with respect to the Cu/Al and Mg/Al LDHs. 3.3. Catalytic activity of pre-activated Cu/Al LDH It is clear from these data that the LDH materials undergo significant decomposition during SRM reactions. Under a methanol/water atmosphere (absence of O2 ), the redox-active divalent cations in the LDHs are reduced. This may explain, why in many studies dealing with Cu-containing SRM catalysts, the catalysts are pre-reduced prior to SRM reactions. Therefore, we have compared an as-prepared LDH with one that has been pre-activated by calcining in air, followed by reduction in H2 . Since the Cu/Al LDH showed the highest activity (and is completely reduced below 400 ◦ C), we chose this catalyst for the comparison study. We followed a standard procedure found in the literature for pre-treating Cu-containing catalysts prior to SRM reactions [36]. The Cu/Al LDH was first calcined in
flowing air at 400 ◦ C for 4 h. The XRD data of the calcined Cu/Al LDH indicated that the only crystalline phase present is CuO. The calcined Cu/Al LDH was loaded into the catalytic reactor and reduced in situ by passing a 4.2% H2 /He gas mixture over the catalyst at 400 ◦ C for 1 h. The pre-activated catalyst was cooled to 150 ◦ C and purged with He briefly. Then the SRM reaction was carried out as in previous experiments. If we compare data from the pre-activated Cu/Al LDH (Fig. 9) to that of the as-prepared Cu/Al LDH (Fig. 5), several differences are obvious. Overall, product compositions appear similar, however, the pre-activated Cu/Al LDH becomes catalytically active at a slightly lower temperature. Hydrogen formation is first observed at 218 ◦ C, whereas the as-prepared Cu/Al LDH does not begin H2 production until ∼230 ◦ C. The most surprising finding is that the increase in reaction rate with increasing temperatures occurs more slowly with the pre-treated catalyst than with the non-activated catalyst. The CH3 OH conversion for the as-prepared versus the pre-treated Cu/Al LDH is shown in Fig. 10. The curves indicate that CH3 OH conversion is 5–10% lower for the pre-treated compared with the as-prepared Cu/Al LDH. The differences in catalytic activity are probably due to
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Fig. 10. CH3 OH conversion for pre-treated Cu/Al LDH vs. as-prepared Cu/Al LDH.
differences in surface area, Cu dispersion and the oxidation states of the Cu species in the catalysts. Further catalyst characterization studies are needed to help in explaining the catalytic trends. In situ EXAFS experiments are planned to learn more about the state of the metal in these catalysts during the SRM reaction. The results above suggest that pre-activation of Cu/Al LDHs is not necessary to achieve high conversion and selectivity for H2 production in SRM reactions. For the Cu/Al LDH, sufficient catalyst activation (Cu2+ reduction) occurs in the presence of the CH3 OH/H2 O feed (absence of O2 ) at ∼230 ◦ C. Further studies are being carried out to compare pre-treated versus non-treated Co/Al and Ni/Al LDHs.
4. Conclusions We have prepared and characterized several LDH catalysts that are active for SRM. The most favorable activity is observed with Cu/Al LDHs. Our Cu/Al LDH is actually a mixture of hydrotalcite-like and malachite phases. While Co/Al and Ni/Al LDHs also show catalytic activity, their activation temperature is higher and methanol conversions are lower. For
the Ni/Al LDH, methanol decomposition is the major reaction pathway. No activity was observed for the Mg/Al LDH. Results from TGA, TPR, and XRD experiments indicate that the LDH structure decomposes at temperatures used during SRM reactions (>150 ◦ C). A direct correlation was found relating the reducibility of the divalent metal in the LDH to the catalytic activity. The Cu/Al LDH containing easily reducible Cu2+ had the highest catalytic activity, whereas the Mg/Al LDH showed no catalytic activity due to the unfavorable reduction potential of the Mg2+ /Mg0 redox couple. Pre-activation of the Cu/Al LDH by calcination in air (400 ◦ C), followed by reduction in dilute H2 does not significantly change the catalytic activity of the catalyst. The onset of H2 production is slightly lower for the pre-activated compared with the as-prepared Cu/Al LDH, also the CH3 OH conversion is generally 5–10% lower.
Acknowledgements The work was performed under the auspices of the US Department of Energy, under contract number
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