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Al2O3 nanoparticle catalysts
Applied Catalysis A: General 261 (2004) 77–86
Partial oxidation of methanol for hydrogen production using ITO/Al2 O3 nanoparticle catalysts Ames Kulprathipanja, John L. Falconer∗ Department of Chemical Engineering, University of Colorado, Boulder, CO 80309-0424, USA Received in revised form 22 October 2003; accepted 26 October 2003
Abstract An indium tin oxide/alumina (ITO/Al2 O3 ) nanoparticle catalyst was used for the selective oxidation of methanol to form hydrogen and carbon dioxide. At 68% methanol conversion, the hydrogen selectivity was 73%, and carbon monoxide was only 1–2% of the products. Four reactors were used, and the highest hydrogen selectivity was obtained in a tank reactor that contained a thin catalyst film. A packed bed reactor did not effectively remove the heat of reaction, but coating the catalyst as a thin film gave better control. The product selectivity dependence on temperature and the methanol/oxygen feed ratio was measured. © 2003 Elsevier B.V. All rights reserved. Keywords: Methanol oxidation (partial); Indium tin oxide; Hydrogen production; Nanoparticle catalyst; Reactor set-up; Thin films
1. Introduction Hydrogen, which has potential as a clean energy fuel, can be extracted from water, biomass, natural gas, and other carbon sources [1]. One use of hydrogen is as an energy source for fuel cells [2]. Hydrogen fuel cells produce less pollutants (NOx and SOx ) and green house gases than internal combustion engines [1]. Due to safety concerns regarding hydrogen storage and transport, research has focused on on-site hydrogen production from a high-energy liquid fuel such as methanol. Four methods of hydrogen production from methanol have been considered [3–7]. CH3 OH ↔ 2H2 + CO,
H = 92 kJ/mol
CH3 OH + H2 O ↔ 3H2 + CO2 , CH3 OH + 21 O2 ↔ 2H2 + CO2 ,
H = 49 kJ/mol
(1) (2)
H = −192 kJ/mol (3)
4CH3 OH + 3H2 O +
1 O ↔ 11H2 2 2
+ 4CO2 ,
H = −44 kJ/mol
(4) ∗ Corresponding author. Tel.: +1-303-492-8005; fax: +1-303-492-4341. E-mail address: [email protected] (J.L. Falconer).
0926-860X/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2003.10.034
Methanol decomposition (Eq. (1)) produces carbon monoxide. Proton exchange membrane fuel cells require that the hydrogen feed contain <20 ppm carbon monoxide because carbon monoxide poisons the platinum anode [8]. Thus, an additional reformer would be needed to reduce the carbon monoxide concentration. Steam reforming (Eq. (2)) is endothermic and thus requires energy input, which makes transient operation difficult when bursts of energy are needed. Partial oxidation of methanol (POM, Eq. (3)) has a higher reaction rate than steam reforming, but half the hydrogen selectivity [9]. Furthermore, POM is highly exothermic, so temperature control can be difficult. Oxidative steam reforming (Eq. (4)) is a combination of steam reforming and partial oxidation. This process uses the energy produced from partial oxidation to supply the endothermic, steam-reforming reaction, and thus can be run adiabatically [10]. Oxidative steam reforming of methanol has not been extensively studied, but initial results indicate low carbon monoxide and high hydrogen concentration in the products [7]. Partial oxidation and steam reforming reactions have mostly used CuZnAl-oxide based catalysts. Both methanol conversion and hydrogen selectivity >75% have been attained for POM on a 1% Pd/ZnO catalyst [5], but the carbon monoxide concentration in the products was 10%. Alejo et al. [6] used a Cu/ZnO/Al2 O3 catalyst for POM that
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produced <1% carbon monoxide but methanol conversion was <15% due to high methanol/oxygen feed ratios. Reitz et al. [11] reported that a commercial CuO/ZnO catalyst were active for complete oxidation of methanol to water and carbon dioxide, but once the catalyst was reduced to metallic Cu, hydrogen formed due to steam reforming. Oxidative steam reforming has also used CuZnAl-oxide catalysts. Velu et al. reported methanol conversions close to 100% with no carbon monoxide detected in the products [7]. The ratio of the hydrogen production rate versus methanol conversion rate was approximately 2.5. Adding zirconium to the catalyst increased both hydrogen and carbon monoxide selectivities. The indium tin oxide/alumina (ITO/Al2 O3 ) catalyst used in the current study is a semiconductor catalyst that has low carbon monoxide selectivity and high hydrogen selectivity. A previous study showed that the ITO/Al2 O3 catalyst could be heated thermally or electrically [12]. In the current study, the reaction sensitivities to reactor type, methanol/oxygen feed ratio, space time, and temperature were examined. A packed bed reactor did not effectively transfer the heat, but coating the ITO/Al2 O3 catalyst as a thin film in three of the reactors allowed studies at lower temperatures and longer space times. The four reactors provided a better measure of reproducibility, allowed the reaction parameters to be varied, and allowed the selectivity to be measured at high conversions.
2. Experimental methods 2.1. Catalyst The ITO nanoparticle catalyst (Nanomaterials Research Corporation, Longmont, CO) consisted of 90 wt.% In2 O3 mixed with 10 wt.% SnO2 and had a BET surface area of 15.7 m2 /g [12]. The average diameter of the particles was 10 nm. They were prepared by precipitating a mixed chloride from an aqueous solution by adding ammonia. The precipitate was filtered, washed, and calcined at 723 K. The Al2 O3 nanoparticles had a BET surface area of 61.7 m2 /g. A mixture of 75% ITO and 25% Al2 O3 nanoparticles was milled together and deposited on a porous (0.2–0.3 mm pores) alumina monolith (Vesuvius Hi-Tech Ceramics) by dip coating using a 2-propanol slurry. The ITO/Al2 O3 was also deposited from aqueous solution onto the thin-film tubular reactor using a water slurry. The catalyst was then reduced in 20% hydrogen in helium at 575 K for 2 h; the catalyst changed from a light green to a light blue color. 2.2. Kinetic measurements The gas feed was 20–60% oxygen in a helium carrier gas. Helium and oxygen flow rates were controlled between 60 and 140 cm3 /min (STP) with mass flow controllers. The
oxygen–helium mixture was bubbled through a tank containing liquid methanol. The temperature of the tank was changed to control the methanol/oxygen ratio. A bypass line was used to monitor the feed composition. The tubing throughout the system was heated to prevent condensation. The feed and product streams were analyzed by a Hewlett-Packard gas chromatograph (model 6890) equipped with a thermal conductivity detector. A 2 m Porapak-T column was used to separate carbon dioxide, formaldehyde, methanol, water, and methyl formate. A 1 m HayeSep-DB column was used to separate hydrogen, carbon monoxide, oxygen, and methane. Catalyst temperatures were measured using 0.5 mm chromel–alumel thermocouples. A packed bed reactor (PBR) consisted of 35 mg of catalyst in a 7 mm i.d. quartz tube. Five millimeters of quartz wool was above and below the catalyst, and two thermocouples were about 1 mm apart in the center of the catalyst bed. The catalyst bed was heated by a tubular oven. The thin-film tubular reactor (TFTR) had a 1 mm annular spacing and a 2 cm o.d. Its interior surface was coated with 30 mg of catalyst by evaporative deposition of a sonicated slurry of catalyst and distilled water. The approximate thickness of the catalyst film was 200 nm. Its temperature was measured with two thermocouples that contacted the catalyst thin-film. The TFTR was also used for steam reforming, methanol decomposition, and the water gas shift reaction with helium as the carrier gas. The catalyst was heated by a tubular oven. The thin-film recirculating reactor (TFRR) was a constant-volume batch reactor. It had 2.1 mg of catalyst on a tungsten foil (0.13 mm thick, 99.9%) that was used as a resistive heater. The approximate thickness of the catalyst film was 350 nm. A thermocouple was spot welded to the foil, which was dip-coated in a 2-propanol catalyst slurry. Reactants and products were recirculated through the reactor and GC valve using a bellows pump. The tank reactor (TR-volume of 362 cm3 ) contained an alumina monolith support (3 cm × 1.5 cm × 0.5 cm) that was dip coated in a 2-propanol slurry to deposit 320 mg of catalyst. A ceramic heater was clamped to the monolith. Resistive heating of the ITO/Al2 O3 was also studied in a similar TR. Note that the space time (the reactor volume divided by the feed flow rate) was used for the three flow reactors, but residence time (the time in the reactor) was used for the TFRR since it is a batch reactor.
3. Results 3.1. Thin-film tubular reactor (TFTR) Both the methanol conversion and hydrogen selectivity [H2 /(H2 + H2 O + CH2 O)] for POM on the ITO/Al2 O3 catalyst in the TFTR increased with temperature, as shown in Fig. 1. The lines in this and other graphs are trend lines. At 725 K, the methanol conversion was 58%, and the hydrogen
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Fig. 1. Methanol conversion and hydrogen selectivity vs. temperature for methanol oxidation on ITO/Al2 O3 in a thin-film tubular reactor: space time = 20 s; methanol/oxygen feed ratio = 1.9.
selectivity was 53%. The C/H ratio in the products was within 6% of that in the feed, and the C/O ratio was within 11%. As methanol conversion increased, the water selectivity decreased and hydrogen selectivity increased. This trend was seen in all the reactors and at most feed conditions except high oxygen feed concentrations. Methanol was mainly oxidized to hydrogen, carbon dioxide, and water. The product formation rates and feed exit rates are shown in Fig. 2. The methanol:oxygen stoichiometry of the reaction was near 1:1 at low temperatures and 2:1 at higher temperatures for a methanol/oxygen feed ratio of 1.9 (Fig. 3). The reaction
coefficient in Fig. 3 is the ratio of the amount of methanol reacted divided by the amount of oxygen reacted for a given temperature increase. When a methanol/oxygen feed ratio of 3.5 was used, coefficients were >2 at temperatures above 730 K. Note, in Fig. 2, that water and carbon dioxide formation was detectable at 590 K, and at approximately 600 K, H2 formed. Most significantly, carbon monoxide product selectivity [CO/(H2 + CO + CO2 + H2 O + CH2 O + CH3 COOH)] was <2% for all runs except at methanol/oxygen feed ratios >3.5, where it was 5%. Methyl formate was only
Fig. 2. Product formation and feed exit rates vs. temperature for methanol oxidation on ITO/Al2 O3 in a thin-film tubular reactor: space time = 20 s; methanol/oxygen feed ratio = 1.9.
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Fig. 3. Reaction coefficients vs. temperature for methanol oxidation on ITO/Al2 O3 in a thin-film tubular reactor for methanol/oxygen feed ratios = 0.9, 1.9 and 3.5: space time = 12 s.
detected in trace amounts below 20% methanol conversion. At methanol/oxygen feed ratios <1, water formed more readily than hydrogen even at higher temperatures (Fig. 4). When the space time in the TFTR increased from 8.5 to 20 s at 650 K and the methanol/oxygen feed ratio was 1.9, the methanol conversion and hydrogen selectivity both increased by 15% (Fig. 5). More hydrogen formed at longer space times and this increased the hydrogen selectivity. The carbon monoxide and formaldehyde product selectivities were both lower at longer space times.
3.2. Packed bed reactor (PBR) The PBR exhibited a temperature jump due to the exothermic reaction. Below 550 K, the methanol conversion was <5%. When the temperature increased by approximately 15 K, it jumped another 100 K and methanol conversion increased to 66%. Particles near the center of the catalyst bed turned bright orange and these hot spots were only eliminated by turning off the feed. Diluting the catalyst with Al2 O3 did not decrease the temperature jump. Moreover, the Al2 O3 nanoparticles were catalytically active and increased
Fig. 4. Product formation and feed exit rates vs. temperature for methanol oxidation on ITO/Al2 O3 in a thin-film tubular reactor: space time = 12 s; methanol/oxygen feed ratio = 0.9.
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Fig. 5. Methanol conversion and hydrogen and carbon monoxide selectivity vs. space time for methanol oxidation on ITO/Al2 O3 at 650 K in a thin-film tubular reactor: methanol/oxygen feed ratio = 1.9.
carbon monoxide and decreased hydrogen production. Pure Al2 O3 nanoparticles (60 mg) at a methanol/oxygen feed ratio of 1.5 and 650 K produced a methanol conversion and hydrogen selectivity of 83 and 20%, respectively. Decreasing the ITO concentration in Al2 O3 to 15% (total catalyst weight = 60 mg) at a methanol/oxygen feed ratio of 1.5 and 620 K produced a methanol conversion and hydrogen selectivity of 69 and 40%, respectively. A temperature jump was still observed after diluting the ITO concentration to 15%, but not with pure Al2 O3 . Agrell et al. reported temperature differences of 60–120 K between their PBR catalyst bed (i.d. = 6 mm) and the furnace walls for POM at 525–605 K [13]. However, they did not report a temperature jump. 3.3. Thin-film recirculating reactor (TFRR) At 550 K and a methanol/oxygen feed ratio of 1.4, essentially all the oxygen was consumed after 450 min in the TFRR. At that time, the methanol conversion was 65%, and the hydrogen selectivity was 43%. At 565 K and a methanol/oxygen feed ratio of 1.1, all the oxygen was consumed after 90 min, but the methanol continued to react and the hydrogen selectivity also increased, as shown in Fig. 6. At approximately 75 min, the methanol conversion and hydrogen selectivity were 75 and 50%, respectively. After 200 min, 95% of the methanol was converted and the hydrogen selectivity was 67%. Carbon monoxide product selectivity increased from 1% at 75 min to 3% at 200 min. 3.4. Monolith tank reactor (TR) A temperature gradient of 100 K was measured in the monolith of the TR. The maximum temperature, measured
closest to the heater, was used as the TR temperature. Heaters were placed on both sides of the monolith to decrease the temperature gradient, but the catalyst did not attain higher conversion at the same maximum temperature, possibly because the heaters hindered accessibility of the reactants to the catalyst. The alumina monolith alone was not active at 470 K, but at 530 K, methanol conversion was 50%, carbon monoxide product selectivity 11%, formaldehyde product selectivity 5%, and no hydrogen formed. For the ITO/Al2 O3 catalyst on the alumina monolith, both hydrogen selectivity and methanol conversion increased to over 60% at 530 K, and <2% of the product was carbon monoxide. The TR temperatures were approximately 130 K lower than for the tubular reactor temperatures at the same methanol conversion. In addition, the space time, the reactor volume divided by reactant flow rate, was on the order of 180–240 s in the TR compared to 10–20 s in the tubular reactors. At lower temperatures, water and carbon dioxide were the major products formed in the TR (Fig. 7). Above 500 K, the hydrogen and carbon dioxide formation rate increased, but the water formation rate decreased. Similar to the TFTR, the reaction coefficient was close to 1 at low temperatures, but increased more rapidly with temperature to 2 than the TFTR. Formaldehyde only formed at high temperatures and in small amounts. At high methanol/oxygen feed ratios, hydrogen selectivity was >85% and methanol conversion was 35% for a TR at 530 K and 210 s space time (Fig. 8). At low feed ratios, but the same temperature and space time, hydrogen selectivity fell below 10%, though methanol conversion was >70%. The carbon monoxide percentage in the products increased with increasing methanol/oxygen ratio from non-detectable to 5% at a methanol/oxygen feed ratio of 3.5.
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Fig. 6. Methanol conversion and hydrogen selectivity vs. time for methanol oxidation on ITO/Al2 O3 at 565 K in a thin-film recirculating reactor: methanol/oxygen feed ratio = 1.1.
The catalyst deactivated, as shown in Fig. 9, for measurements at a methanol/oxygen feed ratio of 1.7 and a space time of 210 s. Measurements were made between 10 and 48 h, but at different flow conditions or feed ratios. The methanol conversion decreased from 68 to 56% and hydrogen selectivity decreased from 73 to 56%. The carbon monoxide percentage in the products remained at 1–2%. The runs included time to ramp up to the maximum reactor temperature (3 h) and down to ambient (1 h). The catalyst was held at the maximum temperature of 530 K for approx-
imately 2 h. After eight runs, the catalyst turned slightly brown on the ceramic heater side. Reducing or oxidizing the catalyst after the eight runs did not affect the catalyst activity. 3.5. Reactor comparison Hydrogen selectivity, catalyst temperature, space time, and catalyst weight at the same methanol conversion and a methanol/oxygen ratio of 1.5 are shown for the four reactor types in Table 1. The space time was 180–240 s in the
Fig. 7. Product formation and feed exit rates vs. temperature for methanol oxidation on ITO/Al2 O3 in a monolith tank reactor: space time = 210 s; methanol/oxygen feed ratio = 1.6.
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Fig. 8. Effect of methanol/oxygen feed ratio on methanol conversion and hydrogen selectivity for methanol oxidation on ITO/Al2 O3 at 530 K in a monolith tank reactor: space time = 210 s.
Fig. 9. Effect of catalyst deactivation on methanol conversion and hydrogen selectivity after 48 h on-line for methanol oxidation on ITO/Al2 O3 in a monolith tank reactor: space time = 210 s; methanol/oxygen feed ratio = 1.7; catalyst temperature = 530 K.
Table 1 Reactor type comparison of methanol conversion, hydrogen selectivity, catalyst temperature, space time, and catalyst weight for methanol oxidation on ITO/Al2 O3 (methanol/oxygen feed ratio = 1.5) Reactor set-up
Methanol conversion (%)
Packed bed Thin-film tubular Thin-film recirculating Monolith tank
66 64 63 62
± ± ± ±
2 4 3 2
Hydrogen selectivity (%) 45 49 45 64
± ± ± ±
3 4 3 4
Catalyst temperature (%)
Space time (%)
Catalyst weight (%)
660 665 550 430–530
12 12 27000 210
35 30 2 320
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Fig. 10. Hydrogen selectivity vs. methanol conversion for methanol oxidation on ITO/Al2 O3 in the monolith tank, thin film tubular, and thin film recirculating reactors: methanol/oxygen feed ratio = 1.5.
TR but only 10–20 s in the tubular reactors. The TFRR is a batch reactor, so instead of the space time, the reaction time was used. The molecules were recirculated to keep the gas mixture in the reactor uniform. The TR produced the highest hydrogen selectivity (64%) and at the lowest temperature (530 K). The two tubular reactors had the same conversion at temperatures that were almost 130 K higher than the TR and TFRR temperatures, because of their shorter space times. Fig. 10 plots the hydrogen selectivity versus methanol conversion for the monolith tank, thin film tubular, and thin film recirculating reactors. The TFTR and TFRR behaved similarly at all methanol conversions, but the TR had higher hydrogen selectivity at methanol conversions above 40%. 3.6. Steam reforming and oxidative steam reforming A methanol/water feed ratio of 1.4 was used in the TFTR to determine if ITO/Al2 O3 is a steam reforming catalyst. The methanol conversion was 7% and hydrogen, carbon monoxide, carbon dioxide, and formaldehyde formed at 725 K. Oxidative steam reforming using the TFTR at a methanol/oxygen ratio of 0.7 and methanol/water ratio of 1.4 showed 86% methanol conversion and 28% hydrogen selectivity at 725 K. Formaldehyde product selectivity was 2%, and only trace amounts of carbon monoxide were detected. In the absence of water at the same methanol/oxygen ratio and methanol conversion, the hydrogen selectivity was 22%. The low hydrogen selectivity is due to the low methanol/oxygen feed ratio.
3.7. Methanol decomposition and water gas shift reaction Methanol decomposed on the ITO/Al2 O3 catalyst in the TFTR above 650 K in the absence of oxygen. Hydrogen, carbon monoxide, and formaldehyde formed at 6% methanol conversion and 725 K. For water/carbon monoxide and hydrogen/carbon dioxide feed ratios from 0.5 to 1.5, no reaction was detected up to 750 K using the TFTR and PBR at a space time of 20 s; the ITO/Al2 O3 catalyst was not active for the water gas shift or reverse water gas shift reaction under these conditions. 3.8. Discussion The product selectivity indicates that the primary reactions on the ITO/Al2 O3 catalyst are complete oxidation of methanol to carbon dioxide and water at low temperatures and partial oxidation to water, carbon dioxide, and hydrogen at high temperatures. Hydrogen selectivity was higher at high methanol/oxygen feed ratios and longer space times. The hydrogen formation rate increases with temperature; at low temperature the methanol:oxygen stoichiometry was approximately 1:1, and water and carbon dioxide are the main products. At high temperature, the stoichiometry is closer to 2:1 as methanol is partially oxidized to hydrogen. Reaction coefficients were >2 at a methanol/oxygen feed ratio of 3.5 because of side reactions that did not consume oxygen, such as methanol decomposition and steam reforming. Hydrogen selectivity increased and methanol conversion decreased with increasing methanol/oxygen feed ratio. At the same reactor conditions, when the oxygen feed
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concentration was increased and the methanol feed concentration kept constant, the rate of methanol oxidation increased. The additional oxygen reacted with the methanol to form water, so hydrogen selectivity was lower. Some methanol and water produced from POM further reacted to form hydrogen, since the ITO/Al2 O3 catalyst promotes steam reforming under these conditions. When the oxygen feed concentration was decreased and the methanol feed kept constant, less water formed and the hydrogen selectivity was higher. However, as illustrated in the TFRR, after all the oxygen reacted, methanol continued to react and form hydrogen and carbon monoxide. As methanol is converted for a methanol/oxygen feed ratio >1 (Fig. 3), the methanol/oxygen ratio increases in the reactor because the feed ratio is higher than the reaction stoichiometry. For a feed ratio <1, the methanol/oxygen ratio in the reactor decreases and water formation is favored, as seen in Fig. 3, with a methanol/oxygen feed ratio of 0.9. Similarly, for a methanol/oxygen ratio of 1.9, hydrogen selectivity increased at longer space times (Fig. 5) because the methanol/oxygen ratio in the reactor increased. Since steam reforming has a lower reaction rate than POM, a longer space time would allow more methanol to react through steam reforming. Oxidative steam reforming (Eq. (4)) also occurs on the ITO/Al2 O3 catalyst. Adding water to POM increased hydrogen selectivity and decreased the carbon monoxide product selectivity from 2% to undetectable limits. Oxidative steam reforming is also less exothermic, which makes temperature control more manageable. The most significant characteristic of the ITO/Al2 O3 catalyst was that only 1–2% carbon monoxide formed at higher temperatures for most feed conditions. Though this is higher than the 20 ppm level necessary for fuel cell applications, it is a significant improvement over previous studies of POM. Carbon monoxide forms at approximately 650 K during POM on the ITO/Al2 O3 catalyst using the TFTR. Similarly, methanol starts decomposing at the same temperature to produce carbon monoxide, formaldehyde, and hydrogen. Only 6% methanol conversion was obtained at 725 K during methanol decomposition. The TR had higher hydrogen selectivity; the transition to a 2:1 stoichiometry took place at low temperature. The TR operated at 100 K lower temperature than the tubular reactors, but 20 times longer space time. To mimic these conditions, the TFRR was run at 550 K and space times up to 27,000 s, but achieved the same results as the tubular reactors. Another difference between the TR and other reactors is the alumina monolith support. The alumina support was catalytic and formed carbon monoxide, carbon dioxide, formaldehyde, and water at similar temperatures. Burcham and Wachs showed a strong influence of the oxide support on the turnover frequency during methanol oxidation on metal catalysts [14]. The other difference that may account for the higher hydrogen selectivity in the TR (Table 1) is that backmixing
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lowers the methanol and oxygen concentrations in the TR, and the selectivity for hydrogen formation (POM) increases at lower oxygen concentrations. The methanol/oxygen feed ratio in the tubular reactors gradually increase along the length of the reactor (or with time in a batch reactor), and this favors hydrogen production with distance down the reactor. Because of backmixing, the TR is more like a CSTR, which operates at the final methanol/oxygen ratio, and this higher ratio favors hydrogen formation. The adiabatic temperature for POM in 20% oxygen and 80% helium for a stoichiometric feed is 1500 K. Temperatures over 750 K were not measured in any of the reactors due to the loss of heat through the reactor walls and the presence of endothermic side reactions such as methanol decomposition and steam reforming, which would absorb some of the exothermic heat of reaction. Visible hot spots in the PBR indicated large temperature gradients, whereas hot spots were not observed in the other reactors, apparently because of better heat transfer. The TFTR did not have significant temperature gradients, because the thin-film coated on the supports provided sufficient surface area for heat transfer. The TR had a 100 K temperature gradient across the monolith. Models and experiments on heat transfer in non-adiabatic metal monolith reactors show that a 6 cm diameter cylindrical monolith with a wall temperature of 925 K and constant nitrogen gas flow at approximately 500 K can have a centerline temperature of 475 K [15]. Therefore, a 100 K temperature gradient in the alumina monolith in this study is not unexpected. In addition, hot spots and temperature gradients were seen in other studies with exothermic reactions [16]. The ITO/Al2 O3 catalyst deactivated after approximately 50 h on-line. This process was not reversed by reduction or oxidation of the catalyst, which suggests the deactivation was not due to a change in surface oxidation state. Furthermore, when oxidized and reduced catalysts were compared for catalytic activity at the same conditions, no difference in the final conversion and product selectivity was detected. Annealing the ITO catalyst may cause crystallization and growth of grain boundaries [17–23]; these could lead to the changes in the catalytic activity.
4. Conclusions An indium tin oxide/Al2 O3 catalyst was used to produce hydrogen and carbon dioxide by partial oxidation of methanol. The product selectivity depends on temperature and the methanol/oxygen ratio. The transition from mainly water to hydrogen product occurred when the methanol:oxygen reaction stoichiometry changed from a 1:1 ratio at low temperatures to 2:1 at higher temperatures. A tank reactor may have increased hydrogen selectivity by operating at high methanol/oxygen reactant ratios because of mixing. The small amount of carbon monoxide formed at higher temperatures is due to methanol decomposition. Steam reforming also occurred on the ITO/Al2 O3
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catalyst and ITO/Al2 O3 can catalyze oxidative steam reforming. Reactors made using catalyst thin films have better heat exchange than packed bed reactors and can eliminate auto-thermal temperature jumps due to exothermic reactions. Acknowledgements We gratefully acknowledge support from the National Science Foundation, Grant CTS-0072193. We also thank Drs. Bijan Miremadi and Tapesh Yadav of Nanomaterials Research Corporation for providing the catalyst and for valuable discussions. References [1] G.P. Nowell, The Promise of Methanol Fuel Cell Vehicles, American Methanol Institute, Washington, DC, 2000. [2] R. Alpert, Sci. Am. 281 (1999) 72. [3] Y. Usami, K. Kagawa, M. Kawazoe, Y. Matsumura, H. Sakurai, M. Haruta, Appl. Catal. A 171 (1998) 123. [4] B.A. Peppley, J.C. Amphlett, L.M. Kearns, R.F. Mann, Appl. Catal. A 179 (1999) 21. [5] M.L. Cubeiro, J.L.G. Fierro, Appl. Catal. A 168 (1998) 307.
[6] L. Alejo, R. Lago, M.A. Pena, J.L.G. Fierro, Appl. Catal. A 162 (1997) 281. [7] S. Velu, K. Suzuki, T. Osaki, Chem. Commun. (1999) 2341. [8] H.F. Oetjen, V.M. Schmidt, U. Stimming, F. Trila, J. Electrochem. Soc. 143 (1996) 3838. [9] L.F. Brown, Int. J. Hydrogen Energy 26 (2001) 381. [10] S. Ahmed, M. Krumpelt, Int. J. Hydrogen Energy 26 (2001) 291. [11] T.L. Reitz, S. Ahmed, M. Krumpelt, R. Kumar, H.H. Kung, J. Mol. Catal. A 162 (2000) 275. [12] B. Miremadi, T. Yadav, J.Z. Zhang, J.L. Falconer, Chem. Commun. (2000) 1875. [13] J. Agrell, K. Hasselbo, K. Jansson, S.G. Jaras, M. Boutonnet, Appl. Catal. A 211 (2001) 239. [14] L.J. Burcham, I.E. Wachs, Catal. Today 49 (1999) 467. [15] M. Flytzani-Stephanopoulos, G.E. Voecks, T. Charng, Chem. Eng. Sci. 41 (1986) 1203. [16] N. Gupta, V. Balakotaiah, Chem. Eng. Sci. 56 (2001) 4771. [17] W.-F. Wu, B.-S. Chiou, Appl. Surf. Sci. 68 (1993) 497. [18] T. Omata, H. Fujiwara, S. Otsuka-Yao-Matsuo, H. Ikawa, Jpn. J. Appl. Phys. 37 (1998) 879. [19] S. Otsuka-Matsuo-Yao, Z. Kozuka, N. Ono, J. Electrochem. Soc. 144 (1997) 1488. [20] F.O. Adurodija, H. Izumi, T. Ishihara, H. Yoshioka, M. Motoyama, J. Vac. Sci. Technol. A 18 (2000) 814. [21] J.-G. Na, Y.-R. Cho, Y.-H. Kim, T.-D. Lee, J. Am. Ceram. Soc. 72 (1989) 698. [22] R.B.H. Tahar, T. Ban, Y. Ohya, Y. Takahashi, J. Appl. Phys. 85 (1997) 2631. [23] H. Morikawa, M. Fujita, Thin Solid Films 339 (1998) 309.
Partial oxidation of methanol for hydrogen production using ITO/Al2 O3 nanoparticle catalysts Ames Kulprathipanja, John L. Falconer∗ Department of Chemical Engineering, University of Colorado, Boulder, CO 80309-0424, USA Received in revised form 22 October 2003; accepted 26 October 2003
Abstract An indium tin oxide/alumina (ITO/Al2 O3 ) nanoparticle catalyst was used for the selective oxidation of methanol to form hydrogen and carbon dioxide. At 68% methanol conversion, the hydrogen selectivity was 73%, and carbon monoxide was only 1–2% of the products. Four reactors were used, and the highest hydrogen selectivity was obtained in a tank reactor that contained a thin catalyst film. A packed bed reactor did not effectively remove the heat of reaction, but coating the catalyst as a thin film gave better control. The product selectivity dependence on temperature and the methanol/oxygen feed ratio was measured. © 2003 Elsevier B.V. All rights reserved. Keywords: Methanol oxidation (partial); Indium tin oxide; Hydrogen production; Nanoparticle catalyst; Reactor set-up; Thin films
1. Introduction Hydrogen, which has potential as a clean energy fuel, can be extracted from water, biomass, natural gas, and other carbon sources [1]. One use of hydrogen is as an energy source for fuel cells [2]. Hydrogen fuel cells produce less pollutants (NOx and SOx ) and green house gases than internal combustion engines [1]. Due to safety concerns regarding hydrogen storage and transport, research has focused on on-site hydrogen production from a high-energy liquid fuel such as methanol. Four methods of hydrogen production from methanol have been considered [3–7]. CH3 OH ↔ 2H2 + CO,
H = 92 kJ/mol
CH3 OH + H2 O ↔ 3H2 + CO2 , CH3 OH + 21 O2 ↔ 2H2 + CO2 ,
H = 49 kJ/mol
(1) (2)
H = −192 kJ/mol (3)
4CH3 OH + 3H2 O +
1 O ↔ 11H2 2 2
+ 4CO2 ,
H = −44 kJ/mol
(4) ∗ Corresponding author. Tel.: +1-303-492-8005; fax: +1-303-492-4341. E-mail address: [email protected] (J.L. Falconer).
0926-860X/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2003.10.034
Methanol decomposition (Eq. (1)) produces carbon monoxide. Proton exchange membrane fuel cells require that the hydrogen feed contain <20 ppm carbon monoxide because carbon monoxide poisons the platinum anode [8]. Thus, an additional reformer would be needed to reduce the carbon monoxide concentration. Steam reforming (Eq. (2)) is endothermic and thus requires energy input, which makes transient operation difficult when bursts of energy are needed. Partial oxidation of methanol (POM, Eq. (3)) has a higher reaction rate than steam reforming, but half the hydrogen selectivity [9]. Furthermore, POM is highly exothermic, so temperature control can be difficult. Oxidative steam reforming (Eq. (4)) is a combination of steam reforming and partial oxidation. This process uses the energy produced from partial oxidation to supply the endothermic, steam-reforming reaction, and thus can be run adiabatically [10]. Oxidative steam reforming of methanol has not been extensively studied, but initial results indicate low carbon monoxide and high hydrogen concentration in the products [7]. Partial oxidation and steam reforming reactions have mostly used CuZnAl-oxide based catalysts. Both methanol conversion and hydrogen selectivity >75% have been attained for POM on a 1% Pd/ZnO catalyst [5], but the carbon monoxide concentration in the products was 10%. Alejo et al. [6] used a Cu/ZnO/Al2 O3 catalyst for POM that
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produced <1% carbon monoxide but methanol conversion was <15% due to high methanol/oxygen feed ratios. Reitz et al. [11] reported that a commercial CuO/ZnO catalyst were active for complete oxidation of methanol to water and carbon dioxide, but once the catalyst was reduced to metallic Cu, hydrogen formed due to steam reforming. Oxidative steam reforming has also used CuZnAl-oxide catalysts. Velu et al. reported methanol conversions close to 100% with no carbon monoxide detected in the products [7]. The ratio of the hydrogen production rate versus methanol conversion rate was approximately 2.5. Adding zirconium to the catalyst increased both hydrogen and carbon monoxide selectivities. The indium tin oxide/alumina (ITO/Al2 O3 ) catalyst used in the current study is a semiconductor catalyst that has low carbon monoxide selectivity and high hydrogen selectivity. A previous study showed that the ITO/Al2 O3 catalyst could be heated thermally or electrically [12]. In the current study, the reaction sensitivities to reactor type, methanol/oxygen feed ratio, space time, and temperature were examined. A packed bed reactor did not effectively transfer the heat, but coating the ITO/Al2 O3 catalyst as a thin film in three of the reactors allowed studies at lower temperatures and longer space times. The four reactors provided a better measure of reproducibility, allowed the reaction parameters to be varied, and allowed the selectivity to be measured at high conversions.
2. Experimental methods 2.1. Catalyst The ITO nanoparticle catalyst (Nanomaterials Research Corporation, Longmont, CO) consisted of 90 wt.% In2 O3 mixed with 10 wt.% SnO2 and had a BET surface area of 15.7 m2 /g [12]. The average diameter of the particles was 10 nm. They were prepared by precipitating a mixed chloride from an aqueous solution by adding ammonia. The precipitate was filtered, washed, and calcined at 723 K. The Al2 O3 nanoparticles had a BET surface area of 61.7 m2 /g. A mixture of 75% ITO and 25% Al2 O3 nanoparticles was milled together and deposited on a porous (0.2–0.3 mm pores) alumina monolith (Vesuvius Hi-Tech Ceramics) by dip coating using a 2-propanol slurry. The ITO/Al2 O3 was also deposited from aqueous solution onto the thin-film tubular reactor using a water slurry. The catalyst was then reduced in 20% hydrogen in helium at 575 K for 2 h; the catalyst changed from a light green to a light blue color. 2.2. Kinetic measurements The gas feed was 20–60% oxygen in a helium carrier gas. Helium and oxygen flow rates were controlled between 60 and 140 cm3 /min (STP) with mass flow controllers. The
oxygen–helium mixture was bubbled through a tank containing liquid methanol. The temperature of the tank was changed to control the methanol/oxygen ratio. A bypass line was used to monitor the feed composition. The tubing throughout the system was heated to prevent condensation. The feed and product streams were analyzed by a Hewlett-Packard gas chromatograph (model 6890) equipped with a thermal conductivity detector. A 2 m Porapak-T column was used to separate carbon dioxide, formaldehyde, methanol, water, and methyl formate. A 1 m HayeSep-DB column was used to separate hydrogen, carbon monoxide, oxygen, and methane. Catalyst temperatures were measured using 0.5 mm chromel–alumel thermocouples. A packed bed reactor (PBR) consisted of 35 mg of catalyst in a 7 mm i.d. quartz tube. Five millimeters of quartz wool was above and below the catalyst, and two thermocouples were about 1 mm apart in the center of the catalyst bed. The catalyst bed was heated by a tubular oven. The thin-film tubular reactor (TFTR) had a 1 mm annular spacing and a 2 cm o.d. Its interior surface was coated with 30 mg of catalyst by evaporative deposition of a sonicated slurry of catalyst and distilled water. The approximate thickness of the catalyst film was 200 nm. Its temperature was measured with two thermocouples that contacted the catalyst thin-film. The TFTR was also used for steam reforming, methanol decomposition, and the water gas shift reaction with helium as the carrier gas. The catalyst was heated by a tubular oven. The thin-film recirculating reactor (TFRR) was a constant-volume batch reactor. It had 2.1 mg of catalyst on a tungsten foil (0.13 mm thick, 99.9%) that was used as a resistive heater. The approximate thickness of the catalyst film was 350 nm. A thermocouple was spot welded to the foil, which was dip-coated in a 2-propanol catalyst slurry. Reactants and products were recirculated through the reactor and GC valve using a bellows pump. The tank reactor (TR-volume of 362 cm3 ) contained an alumina monolith support (3 cm × 1.5 cm × 0.5 cm) that was dip coated in a 2-propanol slurry to deposit 320 mg of catalyst. A ceramic heater was clamped to the monolith. Resistive heating of the ITO/Al2 O3 was also studied in a similar TR. Note that the space time (the reactor volume divided by the feed flow rate) was used for the three flow reactors, but residence time (the time in the reactor) was used for the TFRR since it is a batch reactor.
3. Results 3.1. Thin-film tubular reactor (TFTR) Both the methanol conversion and hydrogen selectivity [H2 /(H2 + H2 O + CH2 O)] for POM on the ITO/Al2 O3 catalyst in the TFTR increased with temperature, as shown in Fig. 1. The lines in this and other graphs are trend lines. At 725 K, the methanol conversion was 58%, and the hydrogen
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Fig. 1. Methanol conversion and hydrogen selectivity vs. temperature for methanol oxidation on ITO/Al2 O3 in a thin-film tubular reactor: space time = 20 s; methanol/oxygen feed ratio = 1.9.
selectivity was 53%. The C/H ratio in the products was within 6% of that in the feed, and the C/O ratio was within 11%. As methanol conversion increased, the water selectivity decreased and hydrogen selectivity increased. This trend was seen in all the reactors and at most feed conditions except high oxygen feed concentrations. Methanol was mainly oxidized to hydrogen, carbon dioxide, and water. The product formation rates and feed exit rates are shown in Fig. 2. The methanol:oxygen stoichiometry of the reaction was near 1:1 at low temperatures and 2:1 at higher temperatures for a methanol/oxygen feed ratio of 1.9 (Fig. 3). The reaction
coefficient in Fig. 3 is the ratio of the amount of methanol reacted divided by the amount of oxygen reacted for a given temperature increase. When a methanol/oxygen feed ratio of 3.5 was used, coefficients were >2 at temperatures above 730 K. Note, in Fig. 2, that water and carbon dioxide formation was detectable at 590 K, and at approximately 600 K, H2 formed. Most significantly, carbon monoxide product selectivity [CO/(H2 + CO + CO2 + H2 O + CH2 O + CH3 COOH)] was <2% for all runs except at methanol/oxygen feed ratios >3.5, where it was 5%. Methyl formate was only
Fig. 2. Product formation and feed exit rates vs. temperature for methanol oxidation on ITO/Al2 O3 in a thin-film tubular reactor: space time = 20 s; methanol/oxygen feed ratio = 1.9.
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Fig. 3. Reaction coefficients vs. temperature for methanol oxidation on ITO/Al2 O3 in a thin-film tubular reactor for methanol/oxygen feed ratios = 0.9, 1.9 and 3.5: space time = 12 s.
detected in trace amounts below 20% methanol conversion. At methanol/oxygen feed ratios <1, water formed more readily than hydrogen even at higher temperatures (Fig. 4). When the space time in the TFTR increased from 8.5 to 20 s at 650 K and the methanol/oxygen feed ratio was 1.9, the methanol conversion and hydrogen selectivity both increased by 15% (Fig. 5). More hydrogen formed at longer space times and this increased the hydrogen selectivity. The carbon monoxide and formaldehyde product selectivities were both lower at longer space times.
3.2. Packed bed reactor (PBR) The PBR exhibited a temperature jump due to the exothermic reaction. Below 550 K, the methanol conversion was <5%. When the temperature increased by approximately 15 K, it jumped another 100 K and methanol conversion increased to 66%. Particles near the center of the catalyst bed turned bright orange and these hot spots were only eliminated by turning off the feed. Diluting the catalyst with Al2 O3 did not decrease the temperature jump. Moreover, the Al2 O3 nanoparticles were catalytically active and increased
Fig. 4. Product formation and feed exit rates vs. temperature for methanol oxidation on ITO/Al2 O3 in a thin-film tubular reactor: space time = 12 s; methanol/oxygen feed ratio = 0.9.
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Fig. 5. Methanol conversion and hydrogen and carbon monoxide selectivity vs. space time for methanol oxidation on ITO/Al2 O3 at 650 K in a thin-film tubular reactor: methanol/oxygen feed ratio = 1.9.
carbon monoxide and decreased hydrogen production. Pure Al2 O3 nanoparticles (60 mg) at a methanol/oxygen feed ratio of 1.5 and 650 K produced a methanol conversion and hydrogen selectivity of 83 and 20%, respectively. Decreasing the ITO concentration in Al2 O3 to 15% (total catalyst weight = 60 mg) at a methanol/oxygen feed ratio of 1.5 and 620 K produced a methanol conversion and hydrogen selectivity of 69 and 40%, respectively. A temperature jump was still observed after diluting the ITO concentration to 15%, but not with pure Al2 O3 . Agrell et al. reported temperature differences of 60–120 K between their PBR catalyst bed (i.d. = 6 mm) and the furnace walls for POM at 525–605 K [13]. However, they did not report a temperature jump. 3.3. Thin-film recirculating reactor (TFRR) At 550 K and a methanol/oxygen feed ratio of 1.4, essentially all the oxygen was consumed after 450 min in the TFRR. At that time, the methanol conversion was 65%, and the hydrogen selectivity was 43%. At 565 K and a methanol/oxygen feed ratio of 1.1, all the oxygen was consumed after 90 min, but the methanol continued to react and the hydrogen selectivity also increased, as shown in Fig. 6. At approximately 75 min, the methanol conversion and hydrogen selectivity were 75 and 50%, respectively. After 200 min, 95% of the methanol was converted and the hydrogen selectivity was 67%. Carbon monoxide product selectivity increased from 1% at 75 min to 3% at 200 min. 3.4. Monolith tank reactor (TR) A temperature gradient of 100 K was measured in the monolith of the TR. The maximum temperature, measured
closest to the heater, was used as the TR temperature. Heaters were placed on both sides of the monolith to decrease the temperature gradient, but the catalyst did not attain higher conversion at the same maximum temperature, possibly because the heaters hindered accessibility of the reactants to the catalyst. The alumina monolith alone was not active at 470 K, but at 530 K, methanol conversion was 50%, carbon monoxide product selectivity 11%, formaldehyde product selectivity 5%, and no hydrogen formed. For the ITO/Al2 O3 catalyst on the alumina monolith, both hydrogen selectivity and methanol conversion increased to over 60% at 530 K, and <2% of the product was carbon monoxide. The TR temperatures were approximately 130 K lower than for the tubular reactor temperatures at the same methanol conversion. In addition, the space time, the reactor volume divided by reactant flow rate, was on the order of 180–240 s in the TR compared to 10–20 s in the tubular reactors. At lower temperatures, water and carbon dioxide were the major products formed in the TR (Fig. 7). Above 500 K, the hydrogen and carbon dioxide formation rate increased, but the water formation rate decreased. Similar to the TFTR, the reaction coefficient was close to 1 at low temperatures, but increased more rapidly with temperature to 2 than the TFTR. Formaldehyde only formed at high temperatures and in small amounts. At high methanol/oxygen feed ratios, hydrogen selectivity was >85% and methanol conversion was 35% for a TR at 530 K and 210 s space time (Fig. 8). At low feed ratios, but the same temperature and space time, hydrogen selectivity fell below 10%, though methanol conversion was >70%. The carbon monoxide percentage in the products increased with increasing methanol/oxygen ratio from non-detectable to 5% at a methanol/oxygen feed ratio of 3.5.
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Fig. 6. Methanol conversion and hydrogen selectivity vs. time for methanol oxidation on ITO/Al2 O3 at 565 K in a thin-film recirculating reactor: methanol/oxygen feed ratio = 1.1.
The catalyst deactivated, as shown in Fig. 9, for measurements at a methanol/oxygen feed ratio of 1.7 and a space time of 210 s. Measurements were made between 10 and 48 h, but at different flow conditions or feed ratios. The methanol conversion decreased from 68 to 56% and hydrogen selectivity decreased from 73 to 56%. The carbon monoxide percentage in the products remained at 1–2%. The runs included time to ramp up to the maximum reactor temperature (3 h) and down to ambient (1 h). The catalyst was held at the maximum temperature of 530 K for approx-
imately 2 h. After eight runs, the catalyst turned slightly brown on the ceramic heater side. Reducing or oxidizing the catalyst after the eight runs did not affect the catalyst activity. 3.5. Reactor comparison Hydrogen selectivity, catalyst temperature, space time, and catalyst weight at the same methanol conversion and a methanol/oxygen ratio of 1.5 are shown for the four reactor types in Table 1. The space time was 180–240 s in the
Fig. 7. Product formation and feed exit rates vs. temperature for methanol oxidation on ITO/Al2 O3 in a monolith tank reactor: space time = 210 s; methanol/oxygen feed ratio = 1.6.
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Fig. 8. Effect of methanol/oxygen feed ratio on methanol conversion and hydrogen selectivity for methanol oxidation on ITO/Al2 O3 at 530 K in a monolith tank reactor: space time = 210 s.
Fig. 9. Effect of catalyst deactivation on methanol conversion and hydrogen selectivity after 48 h on-line for methanol oxidation on ITO/Al2 O3 in a monolith tank reactor: space time = 210 s; methanol/oxygen feed ratio = 1.7; catalyst temperature = 530 K.
Table 1 Reactor type comparison of methanol conversion, hydrogen selectivity, catalyst temperature, space time, and catalyst weight for methanol oxidation on ITO/Al2 O3 (methanol/oxygen feed ratio = 1.5) Reactor set-up
Methanol conversion (%)
Packed bed Thin-film tubular Thin-film recirculating Monolith tank
66 64 63 62
± ± ± ±
2 4 3 2
Hydrogen selectivity (%) 45 49 45 64
± ± ± ±
3 4 3 4
Catalyst temperature (%)
Space time (%)
Catalyst weight (%)
660 665 550 430–530
12 12 27000 210
35 30 2 320
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Fig. 10. Hydrogen selectivity vs. methanol conversion for methanol oxidation on ITO/Al2 O3 in the monolith tank, thin film tubular, and thin film recirculating reactors: methanol/oxygen feed ratio = 1.5.
TR but only 10–20 s in the tubular reactors. The TFRR is a batch reactor, so instead of the space time, the reaction time was used. The molecules were recirculated to keep the gas mixture in the reactor uniform. The TR produced the highest hydrogen selectivity (64%) and at the lowest temperature (530 K). The two tubular reactors had the same conversion at temperatures that were almost 130 K higher than the TR and TFRR temperatures, because of their shorter space times. Fig. 10 plots the hydrogen selectivity versus methanol conversion for the monolith tank, thin film tubular, and thin film recirculating reactors. The TFTR and TFRR behaved similarly at all methanol conversions, but the TR had higher hydrogen selectivity at methanol conversions above 40%. 3.6. Steam reforming and oxidative steam reforming A methanol/water feed ratio of 1.4 was used in the TFTR to determine if ITO/Al2 O3 is a steam reforming catalyst. The methanol conversion was 7% and hydrogen, carbon monoxide, carbon dioxide, and formaldehyde formed at 725 K. Oxidative steam reforming using the TFTR at a methanol/oxygen ratio of 0.7 and methanol/water ratio of 1.4 showed 86% methanol conversion and 28% hydrogen selectivity at 725 K. Formaldehyde product selectivity was 2%, and only trace amounts of carbon monoxide were detected. In the absence of water at the same methanol/oxygen ratio and methanol conversion, the hydrogen selectivity was 22%. The low hydrogen selectivity is due to the low methanol/oxygen feed ratio.
3.7. Methanol decomposition and water gas shift reaction Methanol decomposed on the ITO/Al2 O3 catalyst in the TFTR above 650 K in the absence of oxygen. Hydrogen, carbon monoxide, and formaldehyde formed at 6% methanol conversion and 725 K. For water/carbon monoxide and hydrogen/carbon dioxide feed ratios from 0.5 to 1.5, no reaction was detected up to 750 K using the TFTR and PBR at a space time of 20 s; the ITO/Al2 O3 catalyst was not active for the water gas shift or reverse water gas shift reaction under these conditions. 3.8. Discussion The product selectivity indicates that the primary reactions on the ITO/Al2 O3 catalyst are complete oxidation of methanol to carbon dioxide and water at low temperatures and partial oxidation to water, carbon dioxide, and hydrogen at high temperatures. Hydrogen selectivity was higher at high methanol/oxygen feed ratios and longer space times. The hydrogen formation rate increases with temperature; at low temperature the methanol:oxygen stoichiometry was approximately 1:1, and water and carbon dioxide are the main products. At high temperature, the stoichiometry is closer to 2:1 as methanol is partially oxidized to hydrogen. Reaction coefficients were >2 at a methanol/oxygen feed ratio of 3.5 because of side reactions that did not consume oxygen, such as methanol decomposition and steam reforming. Hydrogen selectivity increased and methanol conversion decreased with increasing methanol/oxygen feed ratio. At the same reactor conditions, when the oxygen feed
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concentration was increased and the methanol feed concentration kept constant, the rate of methanol oxidation increased. The additional oxygen reacted with the methanol to form water, so hydrogen selectivity was lower. Some methanol and water produced from POM further reacted to form hydrogen, since the ITO/Al2 O3 catalyst promotes steam reforming under these conditions. When the oxygen feed concentration was decreased and the methanol feed kept constant, less water formed and the hydrogen selectivity was higher. However, as illustrated in the TFRR, after all the oxygen reacted, methanol continued to react and form hydrogen and carbon monoxide. As methanol is converted for a methanol/oxygen feed ratio >1 (Fig. 3), the methanol/oxygen ratio increases in the reactor because the feed ratio is higher than the reaction stoichiometry. For a feed ratio <1, the methanol/oxygen ratio in the reactor decreases and water formation is favored, as seen in Fig. 3, with a methanol/oxygen feed ratio of 0.9. Similarly, for a methanol/oxygen ratio of 1.9, hydrogen selectivity increased at longer space times (Fig. 5) because the methanol/oxygen ratio in the reactor increased. Since steam reforming has a lower reaction rate than POM, a longer space time would allow more methanol to react through steam reforming. Oxidative steam reforming (Eq. (4)) also occurs on the ITO/Al2 O3 catalyst. Adding water to POM increased hydrogen selectivity and decreased the carbon monoxide product selectivity from 2% to undetectable limits. Oxidative steam reforming is also less exothermic, which makes temperature control more manageable. The most significant characteristic of the ITO/Al2 O3 catalyst was that only 1–2% carbon monoxide formed at higher temperatures for most feed conditions. Though this is higher than the 20 ppm level necessary for fuel cell applications, it is a significant improvement over previous studies of POM. Carbon monoxide forms at approximately 650 K during POM on the ITO/Al2 O3 catalyst using the TFTR. Similarly, methanol starts decomposing at the same temperature to produce carbon monoxide, formaldehyde, and hydrogen. Only 6% methanol conversion was obtained at 725 K during methanol decomposition. The TR had higher hydrogen selectivity; the transition to a 2:1 stoichiometry took place at low temperature. The TR operated at 100 K lower temperature than the tubular reactors, but 20 times longer space time. To mimic these conditions, the TFRR was run at 550 K and space times up to 27,000 s, but achieved the same results as the tubular reactors. Another difference between the TR and other reactors is the alumina monolith support. The alumina support was catalytic and formed carbon monoxide, carbon dioxide, formaldehyde, and water at similar temperatures. Burcham and Wachs showed a strong influence of the oxide support on the turnover frequency during methanol oxidation on metal catalysts [14]. The other difference that may account for the higher hydrogen selectivity in the TR (Table 1) is that backmixing
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lowers the methanol and oxygen concentrations in the TR, and the selectivity for hydrogen formation (POM) increases at lower oxygen concentrations. The methanol/oxygen feed ratio in the tubular reactors gradually increase along the length of the reactor (or with time in a batch reactor), and this favors hydrogen production with distance down the reactor. Because of backmixing, the TR is more like a CSTR, which operates at the final methanol/oxygen ratio, and this higher ratio favors hydrogen formation. The adiabatic temperature for POM in 20% oxygen and 80% helium for a stoichiometric feed is 1500 K. Temperatures over 750 K were not measured in any of the reactors due to the loss of heat through the reactor walls and the presence of endothermic side reactions such as methanol decomposition and steam reforming, which would absorb some of the exothermic heat of reaction. Visible hot spots in the PBR indicated large temperature gradients, whereas hot spots were not observed in the other reactors, apparently because of better heat transfer. The TFTR did not have significant temperature gradients, because the thin-film coated on the supports provided sufficient surface area for heat transfer. The TR had a 100 K temperature gradient across the monolith. Models and experiments on heat transfer in non-adiabatic metal monolith reactors show that a 6 cm diameter cylindrical monolith with a wall temperature of 925 K and constant nitrogen gas flow at approximately 500 K can have a centerline temperature of 475 K [15]. Therefore, a 100 K temperature gradient in the alumina monolith in this study is not unexpected. In addition, hot spots and temperature gradients were seen in other studies with exothermic reactions [16]. The ITO/Al2 O3 catalyst deactivated after approximately 50 h on-line. This process was not reversed by reduction or oxidation of the catalyst, which suggests the deactivation was not due to a change in surface oxidation state. Furthermore, when oxidized and reduced catalysts were compared for catalytic activity at the same conditions, no difference in the final conversion and product selectivity was detected. Annealing the ITO catalyst may cause crystallization and growth of grain boundaries [17–23]; these could lead to the changes in the catalytic activity.
4. Conclusions An indium tin oxide/Al2 O3 catalyst was used to produce hydrogen and carbon dioxide by partial oxidation of methanol. The product selectivity depends on temperature and the methanol/oxygen ratio. The transition from mainly water to hydrogen product occurred when the methanol:oxygen reaction stoichiometry changed from a 1:1 ratio at low temperatures to 2:1 at higher temperatures. A tank reactor may have increased hydrogen selectivity by operating at high methanol/oxygen reactant ratios because of mixing. The small amount of carbon monoxide formed at higher temperatures is due to methanol decomposition. Steam reforming also occurred on the ITO/Al2 O3
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catalyst and ITO/Al2 O3 can catalyze oxidative steam reforming. Reactors made using catalyst thin films have better heat exchange than packed bed reactors and can eliminate auto-thermal temperature jumps due to exothermic reactions. Acknowledgements We gratefully acknowledge support from the National Science Foundation, Grant CTS-0072193. We also thank Drs. Bijan Miremadi and Tapesh Yadav of Nanomaterials Research Corporation for providing the catalyst and for valuable discussions. References [1] G.P. Nowell, The Promise of Methanol Fuel Cell Vehicles, American Methanol Institute, Washington, DC, 2000. [2] R. Alpert, Sci. Am. 281 (1999) 72. [3] Y. Usami, K. Kagawa, M. Kawazoe, Y. Matsumura, H. Sakurai, M. Haruta, Appl. Catal. A 171 (1998) 123. [4] B.A. Peppley, J.C. Amphlett, L.M. Kearns, R.F. Mann, Appl. Catal. A 179 (1999) 21. [5] M.L. Cubeiro, J.L.G. Fierro, Appl. Catal. A 168 (1998) 307.
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