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Hydrogen Production from Steam Reforming of Hydrocarbons over Alkaline-Earth Metal-Modified Fe- or Ni-Based Catalysts Kazuhisa Murata,* Linsheng Wang, Masahiro Saito, Megumu Inaba, Isao Takahara, and Naoki Mimura National Institute of Advanced Industrial Science and Technology (AIST), AIST Tsukuba Central 5, 1-1-1, Higashi, Tsukuba, Ibaraki 305-8565, Japan Received June 23, 2003. Revised Manuscript Received October 21, 2003
The steam reforming of isooctane and methylcyclohexane (MCH) was investigated over Feand Ni-based catalysts. FeMg/Al2O3 catalyst, effective for CH4 decomposition in the presence of O2/CO2, was active for steam reforming of isooctane, being a little easier to cause C-C bond cleavage than MCH, but the rate of hydrogen production was insufficient, although the rate was only slightly improved by Rh modification, due to raised formation of CH4 and C2. FeMg/Al2O3 catalyst was also less active for the steam reforming of MCH than Ni systems. Modification of FeMg/Al2O3 catalyst with rhodium resulted in the increased formation of aromatic byproducts. The rate of H2 production with Ni/ZrO2 catalyst was found to be 7 times higher than that with FeMg/Al2O3. Furthermore, the stability of the Ni/ZrO2 catalyst was found to be improved by the addition of alkaline-earth metals into the catalyst (M/Ni ) 1:2 wt ratio). In particular, the addition of Sr was the most effective and the activity of NiSr/ZrO2 catalyst for the reaction at 973 K remained stable after 100 hours, although below 10% decrease in the rate of H2 formation was observed. On the contrary, above 50% decrease in the rate was observed over Ni/ZrO2 catalyst, after 100 hours. This Sr effect, possibly, could be associated with the formation of mixed oxides consisting of Sr and Zr (or Ni). The NiSr/ZrO2 catalyst was also effective for the steam reforming of model gasoline, which contained organic mixtures with different reforming reactivities such as naphthenes, aromatics, and n- and iso-paraffins.
Introduction Much attention has been paid to hydrogen production from reforming of hydrocarbons, such as methanol, natural gas, gasoline, and diesel. Reforming has been intensively developed for not only on-board (vehicle), but also off-board (stationary and residential) applications.1 Partial oxidation (PO), autothermal reforming (ATR), and steam reforming (SR) are the primary methods used in reforming hydrocarbons to produce hydrogen for use in polymer electrolyte membrane (PEM) fuel cells. Much effort in the development of fuel processors has been made on the partial oxidation and autothermal reforming processes (Pettersson1). Partial oxidation and autothermal reforming offer faster startup time and better transient response, but result in poor quality of feed to fuel cells. Compared these two methods, catalytic steam reforming offers higher hydrogen concentrations (7080% for steam reforming versus 40-50% for partial oxidation and autothermal reforming on a dry basis) in the crude reformate gas. Our final goal is to develop a miniature reformer that can process liquid hydrocarbon fuels such as gasoline, diesel, and kerosene. For many years, nickel has been the most suitable metal for steam * Corresponding author. E-mail:
[email protected]. (1) Brown, L. F. Int. J. Hydrogen Energy 2000, 26, 381-382; Pettersson, L.; Westerholm, R. Int. J. Hydrogen Energy 2000, 26, 243; Avci, A. K.; Onsan, Z. I.; Trimm, D. L. Appl. Catal., A: General 2001, 216, 243.
reforming of hydrocarbons. The current steam reforming catalysts are mainly nickel supported on alumina.2 However, nickel catalysts with improved coke resistance for steam reforming of readily available hydrocarbons such as gasoline, and diesel are required. Borowiecki et al. have reported the effect of potassium on coke resistance for steam reforming of hydrocarbons catalyzed by Ni-Mo/Al2O3.3 The Ni0.03Mg0.84Ca0.13O and CaO-promoted Ni/γ-Al2O3 catalysts have been reported to be effective for the CO2 reforming of methane by preventing the carbon deposition.4 Usually, gasoline consists of olefine, paraffins, naphthene, and aromatics, although their contents are dependent on types of gasolines. Large C-C bond energies would render steam reforming of naphthenes and aromatics more difficult than that of olefins and paraffins. So, to examine catalyst performance for steam reforming, isooctane and methylcyclohexane (MCH) were chosen as typical model compounds. And a mixture of hexane, isooctane, MCH, and benzene was used as model gasoline. We have already reported that Fe/Mg/ Al2O3 catalyst was active for methane decomposition in the presence of O2/CO2 to form hydrogen and carbon.5 (2) Trimm, D. L. Catal. Today 1997, 37 (3), 233-238. (3) Borowiecki, T.; Giecko, G.; Panczyk, M. Appl. Catal., A: General 2002, 230 (1-2), 85-97. (4) Zhang, Z. L.; Verykios, X. E. Catal. Today 1994, 21 (2-3), 589595; Yamazaki, O.; Nozaki, T.; Omata, K.; Fujimoto, K. Chem. Lett. 1992, 1953-1954.
10.1021/ef0301295 CCC: $27.50 © 2004 American Chemical Society Published on Web 12/11/2003
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So, Fe/Mg/Al2O3 catalysts were first tested for steam reforming of isooctane, and the performance of these Febased catalysts was found to be insufficient even to cause the reforming of isooctane. Therefore, Ni-based catalysts were explored for the steam reforming of MCH, and we found that Sr-modified Ni/ZrO2 catalyst was more effective for the steam reforming of MCH than Ni/ ZrO2 catalyst, because of reduction of coke formation. The test results of model gasoline are also reported in this paper. Experimental Section Catalyst Materials. Fe and Ni catalysts supported on Al2O3 were prepared by impregnation of iron and nickel nitrate into Al2O3, followed by drying at 373 K and calcination at 973 K for 3 h. Fe and Ni/M/Al2O3 catalysts were prepared by impregnation of iron (or Ni) and metal nitrates into Al2O3, followed by the same calcination procedure. The iron, nickel, and additive metal concentration were 10 wt %, respectively. Precious metal-based catalysts were prepared by impregnation of precious salts such as rhodium nitrate, tetraammine platinum nitrate, dinitrodiamine palladium, and ammonium perrhenate into supports. The calcination procedures were the same as those for the iron- or Ni-based catalysts. Reaction Procedure. These catalysts were tested using a fixed-bed reactor, with an alumina tube (inside diameter 10 mm). The catalyst (1 g) was diluted with quartz sand (5 g) for steam reforming reactions. The catalysts were activated by reduction with H2 at 873 K for 3 h. Then, liquid hydrocarbon and water were introduced and vaporized in a preheater, and then fed into the catalyst bed at atmospheric pressure. Pure nitrogen was used as carrier gas and internal standard (40 mL min-1). The temperature of the catalyst bed was set and maintained using a temperature controller. isooctane (>99%) and methylcyclohexane (>99%) were purchased from Wako Pure Chemical and used in this investigation without further treatment. Gas Analysis. The raw reformate was cooled to condense the water. The gaseous products were analyzed by on-line gas chromatography (GC) equipped with a Porapack Q and MS 5A columns for the TCD detector and another Porapack Q column for the FID detector. Conversion, selectivity, and formation rates of products were calculated by an internal standard analyzing method. Catalyst Characterization. The fresh and used catalysts were analyzed by powder X-ray diffraction (XRD), which was performed with a Philips-1800 type diffractometer. The carbonaceous material on the used catalysts was analyzed in air in the temperature-programmed mode at a rate of temperature increase equal to 10 °C/min by thermo-gravimetric (TG)/ differential-thermal analysis (DTA) methods, which was measured with a MAC Science TG/DTA-2000 analyzer. The TG/ DTA analysis used 10 mg of the catalyst.
Results and Discussion Preliminary Experiments on Steam Reforming of Isooctane at 823 K. To investigate a performance of Fe-based catalyst, the steam reforming of isooctane was carried out at 823 K. The Fe/Mg/Al2O3 catalyst, which was active for CO2 reforming of methane,5 produced hydrogen, CH4, and CO as detectable gaseous products, while C2 hydrocarbons were not formed, as shown in Figure 1. The addition of Rh to Fe/Al2O3 catalyst resulted in an increase in CH4 and C2 forma(5) Murata, K.; Inaba, M.; Saito, M.; Takahara, I.; Mimura, N. J. Jpn. Pet. Inst. 2003, 46 (3), 196-202.
Figure 1. Product compositions in steam reforming at 823 K for 5 h. Conditions were catalyst: 1 g; isooctane/H2O/N2 ) 5.58: 63.71:30.71 (vol %, S/C ) 1.43); N2 flow: 40 mL min-1; total pressure: 0.1 Mpa.
Figure 2. Correlation between isooctane conversion and the rate of H2 formation in steam reforming at 823 K for 5 h. Conditions: see Figure 1.
tions. In fact, Rh-, Pd-, and Pt-based catalysts not containing Fe produced pronounced amounts of CH4 and C2. Correlations between the conversion of isooctane and the rate of hydrogen formation in the steam reforming using various catalysts are shown in Figure 2.6 The rate of hydrogen formation for Fe/Mg/Al2O3 catalyst was 0.0121 mol h-1 and the conversion of isooctane was ca. 93%. The rate for Rh-modified Fe/ Al2O3 was 0.014 mol h-1, and the conversion was 92.4%. The precious and rare-earth metals-based catalysts have been reported to be more effective catalysts for steam reforming by preventing the carbon deposition than Febased catalysts.7 However, metals such as Rh, Pt, Pd, and Ce were a little less active for the steam reforming of isooctane than Fe/Mg/Al2O3 catalyst, because of CH4 and C2 formation (Figure 1). Thus, not only precious metals alone, but also Febased catalysts, even by Rh-modification, seem to be (6) In Figures 1 and 2, there was observed a little difference in the time dependence on the activity between catalyst systems employed. However, to compare these catalyst performances, the data after reaction for 5 h were taken for convenience. (7) Aupretre, F.; Descorme, C.; Duprez, D. Ann. Chim. 2001, 26 (4), 93-106; Ribeiro, F. H.; Bonivardi, A. L.; Somorjai, G. A. Catal. Lett. 1994, 27 (1-2), 1-10.
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Figure 4. Effect of Sr additive on the rate of H2 production and gaseous product composition in steam reforming of MCH at 973 K. Reaction conditions are the same as in Figure 3. Inset: product compositions (%).
Figure 3. Correlation between MCH conversion and the rate of H2 formation in steam reforming at 973 K for 5 h. Conditions were catalyst: 1 g; MCH/H2O/N2 ) 6.86:81.18:11.96 (vol %, S/C ) 1.67); N2 flow: 40 mL min-1; total pressure: 0.1 Mpa; H2 reduction at 873 K for 2 h.
insufficient to produce hydrogen from the steam reforming of even isooctane. Therefore, Ni-based catalysts, as well as Fe, were explored for the steam reforming of methylcyclohexane (MCH), being more difficult to cause the reaction than isooctane. Steam Reforming of Methylcyclohexane (MCH). The steam reforming of methylcyclohexane (MCH) over Fe/Mg/Al2O3 catalyst was first tested at 973 K. As shown in Figure 3, the MCH conversion was ca. 30% and the rate of hydrogen formation was 0.011 mol h-1, a little lower than that from the steam reforming of isooctane at 823 K. The gaseous products were H2, CH4, CO2, and CO and a small amount of organic compounds such as benzene was also detected by GC analyses. Rhmodified Fe/Al2O3 catalyst improved the activity and stability for steam reforming of MCH, and the activity of Rh/Fe/Mg/Al2O3 catalyst remained unchanged after 5 h. The MCH conversion of the catalyst was 67%, higher than that of Fe/Mg/Al2O3 catalyst, but the rate of hydrogen formation was 0.013 mol h-1, being slightly higher than 0.011 mol h-1 for Fe/Mg/Al2O3. This is, likely, due to organic byproducts such as benzene and lower aliphatic hydrocarbons. The hydrogen formation rate of Rh/Al2O3 catalyst was 0.0037 mol h-1, one-third that of Rh/Fe/Mg/Al2O3. The rate of hydrogen formation of RhFe/ZrO2 was close to that of Fe/Mg/Al2O3. On the contrary, the rates of hydrogen formation of Ni-based catalysts such as Rh/Ni/Al2O3 (ca. 0.02 mol h-1) and Ni/ Al2O3 (ca. 0.045 mol h-1) were much higher than that of Fe/Mg/Al2O3. This indicates that Fe could be a major species in hydrogen formation catalyzed by Rh/Fe/Mg/ Al2O3 and the addition of Rh resulted in a slight increase in hydrogen formation, but in a pronounced increase in formation of organic byproducts. Investigation of all these data on Fe-based catalysts from Figures 1-3 reveals that the Fe-based catalyst systems would not be suitable for the steam reforming of MCH, as well as isooctane. Thus, Ni-based catalysts were further tested for steam reforming of MCH. The rate of hydrogen formation of Ni/ZrO2 catalyst was higher than that of Ni/Al2O3. To improve the catalyst performance, the additive effects of alkali- and
Figure 5. Time dependences of product composition in steam reforming of methylcyclohexane (MCH) at 973 K. Reaction conditions are the same as in Figure 3.
alkaline-earth metals on the Ni/ZrO2 were examined for the steam reforming of MCH, as summarized in Figure 3. The Na and K additives increased the activity, while the Li-modified catalyst decreased the activity. However, the additions of these alkali metals resulted in the increase in the formation of organic byproducts such as benzene. On the contrary, the alkaline-earth metals increased the activity in the order Sr > Ba > Ca > Mg. Gaseous products were hydrogen, methane, CO, and CO2 as shown in Figure 4, inset. The formation of organic byproducts for these catalysts was much less than those of alkali metals. As a result, the rate of hydrogen formation of Sr-modified Ni/ZrO2 was 0.07 mol h-1. The activity increased with the increase in the Sr content up to 5 wt %, and further addition decreased the activity. The performance of the Mg-modified catalyst was close to that of Ni/Sr/ZrO2. Gaseous product compositions on the Ni/Sr/ZrO2 catalyst at 973 K remained constant after 300 min, as shown in Figure 5A. The compositions on the Ni/ZrO2 were analogous to those of Ni/Sr/ZrO2. Figure 6 shows typical XRD patterns for alkaline-earth metal-modified Ni/ZrO2 and Ni/ZrO2 catalysts, freshly prepared and used for 5 h at 973 K in the steam reforming of MCH. The XRD data of the Ni/Sr/ZrO2 catalyst freshly prepared shows the absorptions characteristic of mixed oxide species such as SrZrO2 (cubic and tetragonal) and SrNiO2 (orthorhombic) absorptions. Similarly, the XRD
H2 Production from Steam Reforming of Hydrocarbons
Energy & Fuels, Vol. 18, No. 1, 2004 125 Table 1. Carbon Content after 100-h Reaction at 973 Ka carbon content (wt %)
a
catalyst
elemental analysis
TGb
Ni/ZrO2 Ni/Sr/ZrO2 RUA
46.6 15.3 0.497
48 10.9 1.08
Experimental conditions: See Figure 7. b Thermo-gravimetry.
Figure 6. XRD patterns of fresh and used Ni/M/ZrO2 catalysts. Fresh catalyst: after calcination at 973 K for 3 h. Used catalyst: after reaction at 973 K for 5 h. F and U denote fresh and used catalysts.
Figure 8. (A) Dependences of benzene content on the rate of H2 formation in steam reforming of MCH at 973 K. Reaction conditions are the same as in Figure 3. (B) Product composition in the case of benzene (5 vol %).
Figure 7. Time dependences of rate of H2 formation and product composition in steam reforming of MCH at 973 K. Reaction conditions are the same as in Figure 3.
data of the Ni/Ba/ZrO2 and Ni/Mg/ZrO2 exhibit the absorptions characteristic of BaNiO3 (hexagonal), BaZrO3 (cubic), ZrMgO (cubic), and MgNiO2 (cubic). These absorptions remained intact after reaction, although structural transformation of ZrO2 between monoclinic and tetragonal could occur after reaction, as shown in Figure 6 (used catalysts). The formations of these mixed oxides could be consistent with the formation of MgFe2O4 in the CH4 decomposition/reforming with CO2/O2.5 Therefore, the mixed oxide species are, possibly important in preventing carbon accumulation and sintering of Ni particles during steam reforming. The stable performance of the Ni/Sr/ZrO2 catalyst during a 300 min run have led us to conduct a longerterm test of the durability of the catalyst during steam reforming of MCH. Figure 7A shows the product composition over a 100 h test. The hydrogen concentration was sustained at above 70% throughout the test, while CH4 concentration was very low. The catalyst maintained its activity and selectivity for the entire 100 h, although below 10% decrease in the rate of H2 formation was observed. In contrast, above 50% decrease in the rate of H2 formation was observed for Ni/ZrO2 catalyst
at 973 K after 100 h. Product compositions on the Ni/ Sr/ZrO2 catalyst remain unchanged after 100 h, as illustrated in Figure 7B. For comparison, RUA catalyst, purchased from Sud Chemie Catalysis Inc., showed above 20% decrease in the hydrogen formation rate, as shown in Figure 7A. Table 1 shows carbon contents, obtained from elemental analyses and TG weight losses of used catalysts after 100 h reaction at 973 K. The carbon content of Ni/ZrO2 was approximately 3 times or 4 times higher than that of Ni/Sr/ZrO2, estimated by elemental or thermo-gravimetric (TG) analysis.8 These findings could be consistent with the results of catalyst stability, as illustrated in Figure 7. Thus, the Ni/Sr/ZrO2 catalyst, possibly, is regarded as a candidate of catalyst effective for steam reforming of liquid hydrocarbons. Steam Reforming of MCH Containing Benzene. Gasoline normally consists of paraffins and isoparaffins, aromatics, and naphthenes in concentrations ranging from 5 to 30 vol %, respectively. So, MCH containing 1 or 5 vol % of benzene were first tested for steam reforming at 973 K over the Ni/Sr/ZrO2 catalyst. As shown in Figure 8, during the entire testing period, the catalyst maintained its high selectivity and activity, irrespective of the presence of benzene. The product hydrogen concentration was above 70% (dry basis). The modest CO concentration for the test was a result of a steam/C ratio ()1.67), lower than stoichiometric value of 2. (8) Ni/Sr/ZrO2 catalysts supported on SiO2, Al2O3, and CeO2 were prepared by impregnation method, followed by calcination at 973 K. Thus prepared catalysts were found to be less active and stable than Ni/Sr/ZrO2 alone, due to a lot of coke formation.
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which contained organic mixtures with different reforming reactivities such as naphthenes, aromatics, and nand iso-paraffins. Further work is currently underway to test a longer reaction time and to try the use of retail gasoline. Conclusions
Figure 9. Dependences of time on stream on rate of H2 formation (A) and product distribution (B) in steam reforming of MCH at 973 K. Model gasoline: hexane/iso-octane/MCH/ benzene ) 30:30:35:5 (vol %). Other conditions are the same as in Figure 3.
Thus, the presence of below 5vol % of benzene did not affect the catalyst performances. Steam Reforming of Model Gasoline. The model gasoline contains hexane (30vol %), isooctane (30vol %), MCH (35vol %), and benzene (5vol %). The model gasoline was then tested for steam reforming at 973K over the Ni/Sr/ZrO2 catalyst. The test was conducted intermittently for 380 min without any noticeable deactivation of the catalyst even at a modest H2O/C ratio of 1.67, although the catalyst performances slightly fluctuated at initial stage up to ca. 120 min, as shown in Figure 9. Not only the rate of hydrogen formation but also the hydrogen concentration was close to that of the reforming of MCH alone. The methane concentration was very low and other organic byproducts were not detected during reaction. Thus, the Ni/Sr/ZrO2 catalyst was found to be effective for model gasoline,
The steam reforming of isooctane and methylcyclohexane (MCH) was investigated over Fe- and Ni-based catalysts. FeMg/Al2O3 catalyst, effective for CH4 decomposition in the presence of O2/CO2, was active at 823 K for steam reforming of isooctane, being easier to cause C-C bond cleavage than MCH, but the rate of hydrogen production was insufficient, although the rate was improved by Rh modification. FeMg/Al2O3 catalyst was less active for the steam reforming of MCH than Ni systems. Modification of FeMg/Al2O3 catalyst with rhodium resulted in the increased formation of aromatic byproducts. The rate of H2 production of Ni/ZrO2 catalyst was found to be 7 times higher than that of FeMg/Al2O3. Very small amounts of aromatics were formed with gaseous products (CO, CO2, and CH4). Furthermore, the stability of the Ni/ZrO2 catalyst was found to be improved by the addition of alkaline-earth metals into the catalyst (M/Ni ) 1:2 wt ratio). In particular, the addition of Sr was the most effective and the activity of NiSr/ZrO2 catalyst for the reaction at 973 K remained stable after 100 h, although below 10% decrease in the rate of H2 formation was observed. On the contrary, above 50% decrease in the rate was observed over Ni/ZrO2 catalyst, after 100 h. This Sr effect, possibly, could be associated with the formation of mixed oxides consisting of Sr and Zr (or Ni). The NiSr/ZrO2 catalyst was also effective for the steam reforming of model gasoline, which contained organic mixtures with different reforming reactivities such as naphthenes, aromatics, and n- and iso-paraffins. EF0301295
