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Al2O3 catalyst
Applied Catalysis B: Environmental 38 (2002) 259–269
Fuel cell grade hydrogen from methanol on a commercial Cu/ZnO/Al2 O3 catalyst Yongtaek Choi, Harvey G. Stenger∗ Department of Chemical Engineering, Lehigh University, Bethlehem, PA 18015, USA Received 20 October 2001; received in revised form 4 March 2002; accepted 7 March 2002
Abstract This paper presents the results of experiments of the methanol decomposition reaction catalyzed by a commercial Cu/ZnO/ Al2 O3 in the absence and presence of water. Methanol decomposition of 100% in the absence of water was obtained at 290 ◦ C and a space velocity of 2 cm3 /h g cat. At these conditions, the hydrogen yield was 1.9–2.0. Water addition to the feed increased the yield of hydrogen and reduced the formation of: dimethyl ether; methyl formate and methane. The variation of the catalyst’s activity and selectivity with time, temperature and feed composition was consistent with previous studies of methanol–steam reforming and water–gas shift reaction, however, this appears to be the first study over the same catalyst of methanol decomposition and methanol–steam reforming. XPS analysis of used catalyst samples and time on-stream data showed that the Cu2+ oxidation state of copper favors methanol decomposition, and we propose that the deactivation of the catalyst is mainly caused by the change in the oxidation state of copper. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Methanol decomposition; Copper catalyst; Methanol–steam reforming; Reformer; Fuel cell; Hydrogen
1. Introduction Combustion of hydrocarbon fuels is the world’s largest source of the air pollution, responsible for atmospheric presence of CO, CO2 , SO2 , NOx , VOCs and indirectly O3 . Using hydrogen in fuel cells produces useful energy at high efficiencies and generates only heat and water as emissions. Therefore, the use of fuel cells for transportation and electric power could reduce toxic air pollutants and reduce green house gas emissions. Today’s rapid development of fuel cell technology have been motivated by its environmental aspects. The advantages of high energy density, easy availability and safe handling/storage are now making ∗ Corresponding author. E-mail address: [email protected] (H.G. Stenger).
methanol one of the most promising sources of hydrogen for fuel cell systems. While extensive work has been done for decades on how to synthesize methanol, recent research is now concentrating on how to produce hydrogen efficiently from methanol. The goal of our work is to maximize hydrogen yield while minimizing the size of the reforming unit. Therefore, to design a compact and efficient methanol reformer, reaction data and kinetics are needed for methanol decomposition (1), methanol–steam reforming; (2) the water–gas shift reaction (3). CH3 OH = CO + 2H2
(1)
CH3 OH + H2 O = CO2 + 3H2
(2)
CO + H2 O = CO2 + H2
(3)
The reversibility of these reactions makes it plausible that the results of methanol synthesis studies of the
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past decades can be used to interpret them. However, even though knowledge concerning the active site, the reaction mechanism, and the kinetics of methanol synthesis can be related to methanol decomposition, one cannot use the results and the catalysts directly. For example, Cheng pointed out that good methanol synthesis catalysts are not always good methanol decomposition catalysts due to the rapid deactivation in the decomposition environment [1,2]. The proposed causes of deactivation are Cu sintering, carbon deposition, and change of catalyst structure [1]. For Cu/ZnO/Al2 O3 methanol synthesis catalysts, it has been shown that the reduction of ZnO and formation of Cu–Zn alloys cause an initial and rapid decrease of activity for methanol decomposition [2]. There are few published studies addressing the mechanism and kinetics of methanol decomposition on Cu/ZnO/Al2 O3 catalysts, however, the accepted mechanism of methanol synthesis on copper–zinc oxide is shown in Fig. 1. In step 1 carbon dioxide, which is oxidized from feed CO, interacts with adsorbed hydrogen to produce a formate species CO2 H [3]. This CO2 H species is hydrogenated to form CO2 H2 through step 2. Methanol is finally formed by further hydrogenation of these formate species via methoxy groups as shown in step 3 and 4. Although several schemes are possible to make CO2 H from CO2 or CO, it has been generally accepted that the rate determining step of this reaction path is the hydrogenation of the CO2 H species [4]. Similarly the mechanism of methanol decomposition on Cu/SiO2 reported by Fisher and Bell [5], proposes that CO2 and H2 are liberated from decomposing formate species on Cu while CO and H2 are formed from the decomposition of adsorbed methyl formate. Several groups have studied the mechanism and rate-expression for methanol–steam reforming [6–9]. An early hypothesis believed that H2 /CO formation occurred first followed by the water–gas shift
reaction. This was later modified by Peppley et al. [6,7] who showed that the rates for all three reactions (methanol–steam reforming, water–gas shift reaction and methanol decomposition) must be included in the kinetic analysis [6]. In their work, it is proposed that hydrogen adsorption does not compete with oxygen and that there exists separate sites for each species. In their mechanism the rate determining step is either the dehydrogenation of methoxy groups or methyl formate hydrolysis [7–9]. In spite of the extensive literature on Cu/Zn/Al2 O3 catalysts, much remains in dispute concerning the role of the oxidation states of Cu. In methanol synthesis, Klier et al. propose and show [10] that oxidized Cu is active and reduced Cu is not active. In their work, copper was thought to be incorporated into the Zn lattice as Cu+ and was thought to be the active site for reactant adsorption [10,11]. In the case of methanol decomposition, both Cu+ and Cu0 seem to be active. However, because of the reducing environment of methanol decomposition, Cu0 was thought to be the active site for the Cu/ZnO/Al2 O3 catalyst [2]. In other work Fisher and Bell [5] suggested that metallic Cu serves only as a sink for atomic hydrogen during methanol decomposition in Cu/ZrO2 /SiO2 catalyst. Although most studies use various X-ray, IR, and TPR/TPD techniques, etc. it is hard to find a study which relates the product distribution to the catalyst state. Furthermore, there are few papers which address the effects of water addition on methanol decomposition. The purpose of our study is to examine the reaction mechanism, by product formation and kinetics of methanol decomposition in the absence and presence of water. Cu oxidation state and its relation to deactivation is also determined through XPS analysis of the catalyst and time on-stream tests. Determining the exact rate-expression to fit the experimental data is in progress and will follow in a subsequent publication.
Fig. 1. The schematic mechanism of methanol synthesis.
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2. Experimental The Cu/Zn/Al2 O3 catalyst used in this study is a commercial catalyst manufactured by Sud-Chemie (catalyst #EX-2248). The molar Cu:Zn ratio was determined to be 2:1 by XPS analysis and verified by Galbraith Lab (Knoxville, TN). The catalyst was ground and sieved for particle diameter of the catalyst was chosen to eliminate internal diffusion resistance. An effectiveness factor of η > 0.95 required the size of the catalyst to be below 2 mm. Our studies used a particle size of 200–250 m. A stainless steel tubular reactor, 1/2 in. in diameter and 12 in. long was used for all reaction tests. To ensure isothermal conditions along the bed length, a split tubular furnace was used and the temperature of catalyst bed was measured directly by a 1/16 in. J-type thermocouple. The reaction tests were performed at temperatures between 110 and 360 ◦ C. Two grades of methanol, laboratory grade (Fisher Scientific, water content ∼1 wt.%) and HPLC grade (Aldrich, water content ∼0.03 wt.%), were used. The methanol feed rate was controlled precisely by a syringe pump, 74,900 Series (Cole Palmer), from 0.5 to 8 ml/h, giving a volume of vaporized methanol at STP flow of 277–4427 cm3 /h. The catalyst load was between 0.25 and 1.0 g and the GHSV at reaction temperature was controlled between 1000 and 10,000 h−1 . No catalyst pre-treatments were used. The catalyst was started up as it is.
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The effluent of the reactor was maintained at 120 ◦ C with heating tapes to avoid liquid condensation and connected directly to a CARLE Series S gas chromatograph using a Hydrogen Transfer System (Pd membrane) for hydrogen analysis. Two columns: Alltech Chemisorb 107 (80–100 mesh, 6 ft × 1/8 in.) and Supelco Carboxen 1000 (60–80 mesh, 15 ft × 1/8 in.) were connected in series to analyze the condensible and light gas components. Eight components: water, methanol, dimethyl ether, methyl formate, H2 , CO, CH4 , CO2 were measured during each test run. Material balances on carbon were calculated to verify measurement accuracy. The fresh and used catalyst were analyzed using a Scienta X-ray Photoelectron Spectrometer.
3. Results 3.1. Methanol decomposition in the absence of water Fig. 2 shows a plot of methanol conversion in the absence of water versus reaction temperature at various space velocities. For the lowest space velocity of 280 h−1 (0.5 cm3 of liquid/h g cat), the conversion reached 100% at approximately 260 ◦ C. A temperature greater than 330 ◦ C was needed for 100% conversion at the highest space velocity of 4400 h−1 (8.0 cm3 of liquid/h g cat). Careful viewing of the conversion
Fig. 2. Conversion of methanol with various space velocities in the absence of water as a function of reaction temperature (1.0 g of Cu/ZnO/Al2 O3 catalyst; methanol feed, laboratory grade; GHSV, 280–4400 h−1 ).
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Fig. 3. By-product distribution for methanol decomposition in the absence of water as a function of reaction temperature (DME, dimethyl ether; MF, methyl formate; catalyst, 1.0 g; GHSV, 1100 h−1 ).
versus temperature profiles shows two distinct temperature regions, which cannot be explained by typical “S” curve temperature dependence. In the lower temperature region, below 230 ◦ C, the rate of decomposition increases slowly with temperature, while at temperatures greater than 230 ◦ C, the decomposition rate increases more rapidly with temperature. The causes of these two regions will be discussed later.
If the reaction pathways of methanol decomposition and steam reforming are similar to the reverse pathways of methanol synthesis, by-products such as methane, dimethyl ether, methyl formate and formaldehyde can be expected. Although formaldehyde is difficult to detect using gas chromatography, we are fairly certain that formaldehyde was not formed under any conditions in our experiments. If it
Fig. 4. Methanol conversion in methanol–steam reaction as a function of reaction temperature (water content in feed, 1.8, 8.6, 24, 43, 64 mol%; catalyst loading, 1.0 g; GHSV, 1100 h−1 ).
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was, it would be less than the error in our carbon balances which was typically less than 1%. Fig. 3 shows a concentration profile of the observed by-products with temperature. The largest by-product was methyl formate (HCO2 CH3 ), which increased with temperature, reaching a maximum at approximately 250 ◦ C, and then declining to zero as methanol conversion reached 100%. Dimethyl ether formation also increased with temperature, reaching a maximum at 275 ◦ C and then decreased with further increasing temperature; following a similar pattern to methyl formate. Methane, as shown in Fig. 3 was formed to a smaller extent (below 0.5 mol%) and only at high space velocities and high temperatures. In the case of carbon dioxide, formation was essentially constant throughout the temperature range. 3.2. Methanol–steam reaction Fig. 4 shows the conversion of methanol versus reaction temperature when water is added to the methanol feed. When water is added to the feed, methanol decomposes more rapidly at lower temperatures. When water addition is more than 30 wt.% of the feed (43 mol%), complete methanol conversion occurs near 250 ◦ C. To evaluate the effect of water the temperatures required to achieve 50% con-
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version of methanol were measured. At equal residence times, this “50% conversion temperature” was 45–60 ◦ C lower for a feed with 30 wt.% versus pure methanol. Also as the amount of water is increased, the temperature-conversion curves have higher slopes in the low temperature region. This is in contrast to the methanol-only results where the conversion versus temperature slopes were much lower at low temperatures. The most notable effect of water addition is the reduction of all the observed by-products: dimethyl ether; methyl formate; methane (Fig. 5). Methyl formate, the main by-product for methanol-only decomposition was reduced significantly as water was added. No methyl formate was detected when the feed contains 30% water by weight (43 mol%) or greater (Fig. 6). Also there was no DME formation when the feed had more than 15 wt.% (24 mol%) water (Fig. 7) and no methane was detected after a feed of 5 wt.% (8.6 mol%) of water. 3.3. Hydrogen and CO yield The yield of hydrogen from methanol decomposition and methanol–steam reforming can be defined as the molar ratio of hydrogen produced to methanol reacted (4). Similarly the yield of carbon monoxide
Fig. 5. Dimethyl ether and methyl formate formation in the methanol–steam reaction with stoichiometric feed of water to methanol (DME, dimethyl ether; MF, methyl formate; reaction temperature, 200, 250 and 280 ◦ C; catalyst loading, 1.0 g; GHSV, 1100 h−1 , methanol conversion, 95–100%).
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Fig. 6. Methyl formate formation in the methanol–steam reaction as a function of reaction temperature (water content in mixed feed, 1.8, 8.6, 24, 43 mol%; catalyst loading, 1.0 g; GHSV, 1100 h−1 ; methanol conversion, 95–100%).
can be defined as the molar ratio of CO produced to methanol converted (5). mole H2 FH2 YH2 = (4) FMo − FM mole of methanol mole CO FCO YCO = (5) FMo − FM mole of methanol
where, FH2 is the molar flow rate of hydrogen, reactor outlet (mol/h); FMo the molar flow rate of methanol, reactor inlet (mol/h); FM the molar flow rate of methanol, reactor outlet (mol/h); FCO the molar flow rate of carbon monoxide, reactor outlet (mol/h). Fig. 8 shows hydrogen yield versus the stoichiometric ratio of water to methanol in the feed. With no water in the feed, the hydrogen yield varies between
Fig. 7. Dimethyl ether formation in the methanol–steam reaction as a function of reaction temperature (water content in mixed feed, 1.8, 8.6, 24 mol%; catalyst loading, 1.0 g; GHSV, 1100 h−1 ; methanol conversion, 95–100%).
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Fig. 8. Hydrogen yield for methanol decomposition and steam reaction versus feed ratio of water to methanol when methanol conversion is 50 and 100% (catalyst loading, 1.0 g; GHSV, 1100–4400 h−1 ).
1.8 and 2.0 at high methanol conversion, but only 1.3–1.7 at lower conversion. As water is added the yield increases steadily with increasing ratio of water to methanol approaching a value around 2.5–2.6 for all methanol conversion levels. The CO yield for methanol-only feed was approximately 0.9 and decreased significantly with the addition of water. This is due primarily to the water–gas shift reaction, which causes the CO yield to decrease
continuously even after the stoichiometric ratio is over 1.0 (Fig. 9). 3.4. Deactivation It is well known that the activity of Cu/ZnO/Al2 O3 catalyst for methanol decomposition reaction as well as methanol synthesis decreases rapidly during the initial period of operation. The activity of this catalyst
Fig. 9. Carbon monoxide yield in the methanol decomposition and steam reaction versus feed ratio of water to methanol (catalyst loading, 1.0 g; GHSV, 280–4400 h−1 ; methanol conversion, 95–100%).
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Fig. 10. Methanol conversion versus time on-stream with various feed water contents (reaction temperature, 300 or 250 ◦ C; catalyst loading, 1.0 g; GHSV, 1100 h−1 ).
was evaluated for 100 h using various methanol feeds and constant reaction condition (GHSV, 1100 h−1 ; temperature, 300 ◦ C). It was observed that the deactivation pattern varied with the water content in the methanol feed. In absence of water, methanol conversion decreases rapidly during the initial period and reaches steady state after 20 h. When water is present the initial deactivation is less evident; however some slow deactivation occurs over the course of the run (Fig. 10). 4. Discussion
in Fig. 2, each profile was seen to have two temperature regions, inferring that two different rate determining steps may exist. In the low temperature region, we observed no dimethyl ether while methyl formate increased significantly with temperature (Fig. 3). This infers that methyl formate decomposition to form CO, CO2 and H2 is most likely the rate determining step in the low temperature region (reaction 5 and 6 in Fig. 11). In the high temperature region, the formation of dimethyl ether increased with temperature and methyl formate decreased rapidly. This indicates that formation of formaldehyde from methoxy is now the slow step at higher temperatures (reaction 2).
4.1. Reaction mechanism and rate determining steps of methanol decomposition
4.2. Copper oxidation state
It is difficult to select just one reaction path for methanol decomposition to hydrogen, CO and CO2 because of the various intermediates formed. However, after reviewing several hypotheses detailed in the literature, we believe they can be simplified to eight major steps shown in Fig. 11. The intermediate compound formed to the largest extent was methyl formate. Therefore, we can infer that the rate determining step is decomposition of methyl formate to form H2 , CO or CO2 . For the methanol conversion versus temperature data shown
The state of Cu in the active catalyst can be either Cu2+ (Cu–O), Cu+ (Cu–O–Cu) or Cu0 (metallic copper) and most likely all three states exist simultaneously. To form methoxy from adsorbed methanol or gas phase methanol requires an oxidized Cu site (Cu–O) (6) to remove hydrogen. If CO2 is formed from formaldehyde (7) or methyl formate (8), an oxidized Cu site is again needed. Cu–O + 2CH3 OH → Cu–CH3 O + H2 O
(6)
Cu–O + CH2 O → Cu–CH2 O2 → CO2 + H2
(7)
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Fig. 11. Schematic diagram of the mechanism of methanol decomposition.
Cu–OH + HCOOCH3 → HCOOH + CH3 O → CO2 + H2
(8)
It is generally accepted that the source of oxygen for the Cu–O site is water in the feed and that Cu can be repeatedly oxidized through a redox mechanism (9–11). H2 O → 2H(a) + O–Cu(a)
(9)
CO + O–Cu(a) → CO2
(10)
2H(a) → H2
(11)
When a fresh catalyst is calcined and is largely in the form of oxidized copper, it should have a higher methanol decomposition rate. If little or no water is fed, the oxidized copper sites rapidly reduce to metallic copper, explaining the rapid initial decrease of activity shown in Fig. 10. 4.3. Methanol decomposition versus steam reforming Methanol conversion for this catalyst at moderate conditions of 250 ◦ C, 1 atm and GHSV = 1100 (or
W/F = 2 cm3 /h g cat) is less than 30% when the feed contains little or no water (<1%). This is consistent with Cheng’s results [1,2], where he reported that a Cu/ZnO catalyst is less active than a Cu/Cr/Mn catalyst, which did not contain ZnO. However, in our work, when water in the feed is more than 43 mol%, methanol conversion reached almost 100%. This is explained by the number of Cu–O active sites formed by water’s dissociative adsorption of water. Thus, the Cu/ZnO/Al2 O3 catalyst seems more suited for methanol–steam reforming than methanol decomposition. Earlier a work on methanol–steam reforming proposed a two steps reaction scheme consisting of methanol decomposition to CO and hydrogen and subsequent CO + H2 O shift reaction [12]. This hypothesis is consistent with the knowledge that methanol is synthesized directly from CO. However, later work discovered that CO2 has an important role in methanol synthesis [4,10,11] and the formation of CO2 by direct reaction of methanol and steam was proposed [9]. In our study, we have confirmed that methanol decomposition to CO and the steam reforming reaction occur simultaneously (in contrast to the
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Fig. 12. XPS spectrum of copper in the fresh and used catalyst for methanol-only feed and methanol-water feed.
work of Jiang et al. [9]) by observation of the H2 and CO yields, CO2 formation characteristics and time on-stream data. As shown in Fig. 8, at 50% methanol conversion (which means relatively low temperatures) the hydrogen yield increases significantly with the feed ratio of H2 O to methanol, while the hydrogen yield for 100% conversion has a relatively steady value of 2.1–2.2 at the low H2 O to methanol molar feed ratio. This is consistent with the rate determining step in the low temperature region being methyl formate hydrolysis because CO and CO2 formation from methyl formate is slow and the direct methanol–steam reaction is relatively fast. Therefore, a faster methanol–steam reaction relative to methanol decomposition would explain the higher yield for hydrogen. This data cannot be explained equally as well if methanol decomposition and water–gas shift are the only reaction occurring.
During the time-on-stream experiments, we found that our Cu/ZnO/Al2 O3 catalyst had a significant weight loss (16–18%, excluding moisture) caused most likely by the loss of O from CuO in both cases of with and without water. This was confirmed by XPS analysis. As shown in Fig. 12 the fresh catalyst has copper in the state of CuO (Cu2+ ) while the spent catalyst after 100 h run is mostly metallic copper. This change in oxidation state correlates with the loss of activity shown in Fig. 10. We conclude that the main cause of the initial deactivation is the change of this oxidation state. However, there is still a slow activity decrease for all runs including those with water in the feed. Thus, a second deactivation mechanism exists such as chemically induced sintering or carbon fouling. Also from these results we conclude that reduced copper is not inactive and contributes significantly to methanol decomposition.
4.4. Cu oxidation state and deactivation There has been an extended debate about the active site for copper-zinc catalysts in methanol synthesis (Cu0 versus Cu+ ) during the last two decades. While the case of methanol synthesis is summarized well through several review papers [4,11,13,14], there are only a few papers which deal simultaneously with the cases of methanol decomposition and methanol–steam reforming.
5. Conclusions 1. The activity and yield of the methanol decomposition reaction on a commercial Cu/ZnO/Al2 O3 catalyst was characterized. Water addition to methanol significantly increased the activity of catalyst and decreased by-product formation. H2 yield increased with increasing ratio of water to
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methanol and CO yield decreased continuously with water content. 2. A simplified eight-step reaction mechanism is inferred from our data and two rate determining steps are proposed: decomposition of methyl formate at temperatures below 230 ◦ C and the formation of formaldehyde at temperatures greater than 230 ◦ C. 3. The active site for methanol decomposition and steam reforming is Cu2+ . The loss of Cu–O is the main cause of initial catalyst deactivation. Deactivation is worsened when water is absent from the feed. References [1] W.H. Cheng, Acc. Chem. Res. 32 (1999) 685–691. [2] W.H. Cheng, Appl. Catal. A 130 (1995) 13–30.
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[3] J.M. Thomas, W.J. Thomas, Principle and Practice of Heterogeneous Catalysis, VCH, New York, 1997, pp. 515–524. [4] K.C. Waugh, Catal. Today 15 (1992) 51–75. [5] I.A. Fisher, A.T. Bell, J. Catal. 184 (1999) 357–376. [6] B.A. Peppley, J.C. Amphlett, L.M. Kearns, R.F. Mann, Appl. Catal. A 179 (1999) 21–29. [7] B.A. Peppley, J.C. Amphlett, L.M. Kearns, R.F. Mann, Appl. Catal. A 179 (1999) 31–49. [8] R.O. Idem, N.N. Bajhshi, Chem. Eng. Sci. 51 (14) (1996) 3697–3708. [9] C.J. Jiang, D.L. Trimm, M.S. Wainwright, Appl. Catal. A 93 (1993) 245–255. [10] K. Klier, V. Chatikavanij, R.G. Herman, G.W. Simmons, J. Catal. 74 (1982) 343–360. [11] K. Klier, Advances in Catalysis, Vol. 31, p. 243. [12] J.C. Amphlett, M.J. Evans, R.A. Jones, R.F. Mann, R.D. Weir, Can. J. Chem. Eng. 59 (1981) 720–727. [13] G.C. Chinchen, P.J. Denny, J.R. Jennings, M.S. Spencer, K.C. Waugh, Appl. Catal. 36 (1988) 1–65. [14] J.C. Bart, R.P. Sneeden, Catal. Today 2 (1987) 1–124.
Fuel cell grade hydrogen from methanol on a commercial Cu/ZnO/Al2 O3 catalyst Yongtaek Choi, Harvey G. Stenger∗ Department of Chemical Engineering, Lehigh University, Bethlehem, PA 18015, USA Received 20 October 2001; received in revised form 4 March 2002; accepted 7 March 2002
Abstract This paper presents the results of experiments of the methanol decomposition reaction catalyzed by a commercial Cu/ZnO/ Al2 O3 in the absence and presence of water. Methanol decomposition of 100% in the absence of water was obtained at 290 ◦ C and a space velocity of 2 cm3 /h g cat. At these conditions, the hydrogen yield was 1.9–2.0. Water addition to the feed increased the yield of hydrogen and reduced the formation of: dimethyl ether; methyl formate and methane. The variation of the catalyst’s activity and selectivity with time, temperature and feed composition was consistent with previous studies of methanol–steam reforming and water–gas shift reaction, however, this appears to be the first study over the same catalyst of methanol decomposition and methanol–steam reforming. XPS analysis of used catalyst samples and time on-stream data showed that the Cu2+ oxidation state of copper favors methanol decomposition, and we propose that the deactivation of the catalyst is mainly caused by the change in the oxidation state of copper. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Methanol decomposition; Copper catalyst; Methanol–steam reforming; Reformer; Fuel cell; Hydrogen
1. Introduction Combustion of hydrocarbon fuels is the world’s largest source of the air pollution, responsible for atmospheric presence of CO, CO2 , SO2 , NOx , VOCs and indirectly O3 . Using hydrogen in fuel cells produces useful energy at high efficiencies and generates only heat and water as emissions. Therefore, the use of fuel cells for transportation and electric power could reduce toxic air pollutants and reduce green house gas emissions. Today’s rapid development of fuel cell technology have been motivated by its environmental aspects. The advantages of high energy density, easy availability and safe handling/storage are now making ∗ Corresponding author. E-mail address: [email protected] (H.G. Stenger).
methanol one of the most promising sources of hydrogen for fuel cell systems. While extensive work has been done for decades on how to synthesize methanol, recent research is now concentrating on how to produce hydrogen efficiently from methanol. The goal of our work is to maximize hydrogen yield while minimizing the size of the reforming unit. Therefore, to design a compact and efficient methanol reformer, reaction data and kinetics are needed for methanol decomposition (1), methanol–steam reforming; (2) the water–gas shift reaction (3). CH3 OH = CO + 2H2
(1)
CH3 OH + H2 O = CO2 + 3H2
(2)
CO + H2 O = CO2 + H2
(3)
The reversibility of these reactions makes it plausible that the results of methanol synthesis studies of the
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past decades can be used to interpret them. However, even though knowledge concerning the active site, the reaction mechanism, and the kinetics of methanol synthesis can be related to methanol decomposition, one cannot use the results and the catalysts directly. For example, Cheng pointed out that good methanol synthesis catalysts are not always good methanol decomposition catalysts due to the rapid deactivation in the decomposition environment [1,2]. The proposed causes of deactivation are Cu sintering, carbon deposition, and change of catalyst structure [1]. For Cu/ZnO/Al2 O3 methanol synthesis catalysts, it has been shown that the reduction of ZnO and formation of Cu–Zn alloys cause an initial and rapid decrease of activity for methanol decomposition [2]. There are few published studies addressing the mechanism and kinetics of methanol decomposition on Cu/ZnO/Al2 O3 catalysts, however, the accepted mechanism of methanol synthesis on copper–zinc oxide is shown in Fig. 1. In step 1 carbon dioxide, which is oxidized from feed CO, interacts with adsorbed hydrogen to produce a formate species CO2 H [3]. This CO2 H species is hydrogenated to form CO2 H2 through step 2. Methanol is finally formed by further hydrogenation of these formate species via methoxy groups as shown in step 3 and 4. Although several schemes are possible to make CO2 H from CO2 or CO, it has been generally accepted that the rate determining step of this reaction path is the hydrogenation of the CO2 H species [4]. Similarly the mechanism of methanol decomposition on Cu/SiO2 reported by Fisher and Bell [5], proposes that CO2 and H2 are liberated from decomposing formate species on Cu while CO and H2 are formed from the decomposition of adsorbed methyl formate. Several groups have studied the mechanism and rate-expression for methanol–steam reforming [6–9]. An early hypothesis believed that H2 /CO formation occurred first followed by the water–gas shift
reaction. This was later modified by Peppley et al. [6,7] who showed that the rates for all three reactions (methanol–steam reforming, water–gas shift reaction and methanol decomposition) must be included in the kinetic analysis [6]. In their work, it is proposed that hydrogen adsorption does not compete with oxygen and that there exists separate sites for each species. In their mechanism the rate determining step is either the dehydrogenation of methoxy groups or methyl formate hydrolysis [7–9]. In spite of the extensive literature on Cu/Zn/Al2 O3 catalysts, much remains in dispute concerning the role of the oxidation states of Cu. In methanol synthesis, Klier et al. propose and show [10] that oxidized Cu is active and reduced Cu is not active. In their work, copper was thought to be incorporated into the Zn lattice as Cu+ and was thought to be the active site for reactant adsorption [10,11]. In the case of methanol decomposition, both Cu+ and Cu0 seem to be active. However, because of the reducing environment of methanol decomposition, Cu0 was thought to be the active site for the Cu/ZnO/Al2 O3 catalyst [2]. In other work Fisher and Bell [5] suggested that metallic Cu serves only as a sink for atomic hydrogen during methanol decomposition in Cu/ZrO2 /SiO2 catalyst. Although most studies use various X-ray, IR, and TPR/TPD techniques, etc. it is hard to find a study which relates the product distribution to the catalyst state. Furthermore, there are few papers which address the effects of water addition on methanol decomposition. The purpose of our study is to examine the reaction mechanism, by product formation and kinetics of methanol decomposition in the absence and presence of water. Cu oxidation state and its relation to deactivation is also determined through XPS analysis of the catalyst and time on-stream tests. Determining the exact rate-expression to fit the experimental data is in progress and will follow in a subsequent publication.
Fig. 1. The schematic mechanism of methanol synthesis.
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2. Experimental The Cu/Zn/Al2 O3 catalyst used in this study is a commercial catalyst manufactured by Sud-Chemie (catalyst #EX-2248). The molar Cu:Zn ratio was determined to be 2:1 by XPS analysis and verified by Galbraith Lab (Knoxville, TN). The catalyst was ground and sieved for particle diameter of the catalyst was chosen to eliminate internal diffusion resistance. An effectiveness factor of η > 0.95 required the size of the catalyst to be below 2 mm. Our studies used a particle size of 200–250 m. A stainless steel tubular reactor, 1/2 in. in diameter and 12 in. long was used for all reaction tests. To ensure isothermal conditions along the bed length, a split tubular furnace was used and the temperature of catalyst bed was measured directly by a 1/16 in. J-type thermocouple. The reaction tests were performed at temperatures between 110 and 360 ◦ C. Two grades of methanol, laboratory grade (Fisher Scientific, water content ∼1 wt.%) and HPLC grade (Aldrich, water content ∼0.03 wt.%), were used. The methanol feed rate was controlled precisely by a syringe pump, 74,900 Series (Cole Palmer), from 0.5 to 8 ml/h, giving a volume of vaporized methanol at STP flow of 277–4427 cm3 /h. The catalyst load was between 0.25 and 1.0 g and the GHSV at reaction temperature was controlled between 1000 and 10,000 h−1 . No catalyst pre-treatments were used. The catalyst was started up as it is.
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The effluent of the reactor was maintained at 120 ◦ C with heating tapes to avoid liquid condensation and connected directly to a CARLE Series S gas chromatograph using a Hydrogen Transfer System (Pd membrane) for hydrogen analysis. Two columns: Alltech Chemisorb 107 (80–100 mesh, 6 ft × 1/8 in.) and Supelco Carboxen 1000 (60–80 mesh, 15 ft × 1/8 in.) were connected in series to analyze the condensible and light gas components. Eight components: water, methanol, dimethyl ether, methyl formate, H2 , CO, CH4 , CO2 were measured during each test run. Material balances on carbon were calculated to verify measurement accuracy. The fresh and used catalyst were analyzed using a Scienta X-ray Photoelectron Spectrometer.
3. Results 3.1. Methanol decomposition in the absence of water Fig. 2 shows a plot of methanol conversion in the absence of water versus reaction temperature at various space velocities. For the lowest space velocity of 280 h−1 (0.5 cm3 of liquid/h g cat), the conversion reached 100% at approximately 260 ◦ C. A temperature greater than 330 ◦ C was needed for 100% conversion at the highest space velocity of 4400 h−1 (8.0 cm3 of liquid/h g cat). Careful viewing of the conversion
Fig. 2. Conversion of methanol with various space velocities in the absence of water as a function of reaction temperature (1.0 g of Cu/ZnO/Al2 O3 catalyst; methanol feed, laboratory grade; GHSV, 280–4400 h−1 ).
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Fig. 3. By-product distribution for methanol decomposition in the absence of water as a function of reaction temperature (DME, dimethyl ether; MF, methyl formate; catalyst, 1.0 g; GHSV, 1100 h−1 ).
versus temperature profiles shows two distinct temperature regions, which cannot be explained by typical “S” curve temperature dependence. In the lower temperature region, below 230 ◦ C, the rate of decomposition increases slowly with temperature, while at temperatures greater than 230 ◦ C, the decomposition rate increases more rapidly with temperature. The causes of these two regions will be discussed later.
If the reaction pathways of methanol decomposition and steam reforming are similar to the reverse pathways of methanol synthesis, by-products such as methane, dimethyl ether, methyl formate and formaldehyde can be expected. Although formaldehyde is difficult to detect using gas chromatography, we are fairly certain that formaldehyde was not formed under any conditions in our experiments. If it
Fig. 4. Methanol conversion in methanol–steam reaction as a function of reaction temperature (water content in feed, 1.8, 8.6, 24, 43, 64 mol%; catalyst loading, 1.0 g; GHSV, 1100 h−1 ).
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was, it would be less than the error in our carbon balances which was typically less than 1%. Fig. 3 shows a concentration profile of the observed by-products with temperature. The largest by-product was methyl formate (HCO2 CH3 ), which increased with temperature, reaching a maximum at approximately 250 ◦ C, and then declining to zero as methanol conversion reached 100%. Dimethyl ether formation also increased with temperature, reaching a maximum at 275 ◦ C and then decreased with further increasing temperature; following a similar pattern to methyl formate. Methane, as shown in Fig. 3 was formed to a smaller extent (below 0.5 mol%) and only at high space velocities and high temperatures. In the case of carbon dioxide, formation was essentially constant throughout the temperature range. 3.2. Methanol–steam reaction Fig. 4 shows the conversion of methanol versus reaction temperature when water is added to the methanol feed. When water is added to the feed, methanol decomposes more rapidly at lower temperatures. When water addition is more than 30 wt.% of the feed (43 mol%), complete methanol conversion occurs near 250 ◦ C. To evaluate the effect of water the temperatures required to achieve 50% con-
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version of methanol were measured. At equal residence times, this “50% conversion temperature” was 45–60 ◦ C lower for a feed with 30 wt.% versus pure methanol. Also as the amount of water is increased, the temperature-conversion curves have higher slopes in the low temperature region. This is in contrast to the methanol-only results where the conversion versus temperature slopes were much lower at low temperatures. The most notable effect of water addition is the reduction of all the observed by-products: dimethyl ether; methyl formate; methane (Fig. 5). Methyl formate, the main by-product for methanol-only decomposition was reduced significantly as water was added. No methyl formate was detected when the feed contains 30% water by weight (43 mol%) or greater (Fig. 6). Also there was no DME formation when the feed had more than 15 wt.% (24 mol%) water (Fig. 7) and no methane was detected after a feed of 5 wt.% (8.6 mol%) of water. 3.3. Hydrogen and CO yield The yield of hydrogen from methanol decomposition and methanol–steam reforming can be defined as the molar ratio of hydrogen produced to methanol reacted (4). Similarly the yield of carbon monoxide
Fig. 5. Dimethyl ether and methyl formate formation in the methanol–steam reaction with stoichiometric feed of water to methanol (DME, dimethyl ether; MF, methyl formate; reaction temperature, 200, 250 and 280 ◦ C; catalyst loading, 1.0 g; GHSV, 1100 h−1 , methanol conversion, 95–100%).
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Fig. 6. Methyl formate formation in the methanol–steam reaction as a function of reaction temperature (water content in mixed feed, 1.8, 8.6, 24, 43 mol%; catalyst loading, 1.0 g; GHSV, 1100 h−1 ; methanol conversion, 95–100%).
can be defined as the molar ratio of CO produced to methanol converted (5). mole H2 FH2 YH2 = (4) FMo − FM mole of methanol mole CO FCO YCO = (5) FMo − FM mole of methanol
where, FH2 is the molar flow rate of hydrogen, reactor outlet (mol/h); FMo the molar flow rate of methanol, reactor inlet (mol/h); FM the molar flow rate of methanol, reactor outlet (mol/h); FCO the molar flow rate of carbon monoxide, reactor outlet (mol/h). Fig. 8 shows hydrogen yield versus the stoichiometric ratio of water to methanol in the feed. With no water in the feed, the hydrogen yield varies between
Fig. 7. Dimethyl ether formation in the methanol–steam reaction as a function of reaction temperature (water content in mixed feed, 1.8, 8.6, 24 mol%; catalyst loading, 1.0 g; GHSV, 1100 h−1 ; methanol conversion, 95–100%).
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Fig. 8. Hydrogen yield for methanol decomposition and steam reaction versus feed ratio of water to methanol when methanol conversion is 50 and 100% (catalyst loading, 1.0 g; GHSV, 1100–4400 h−1 ).
1.8 and 2.0 at high methanol conversion, but only 1.3–1.7 at lower conversion. As water is added the yield increases steadily with increasing ratio of water to methanol approaching a value around 2.5–2.6 for all methanol conversion levels. The CO yield for methanol-only feed was approximately 0.9 and decreased significantly with the addition of water. This is due primarily to the water–gas shift reaction, which causes the CO yield to decrease
continuously even after the stoichiometric ratio is over 1.0 (Fig. 9). 3.4. Deactivation It is well known that the activity of Cu/ZnO/Al2 O3 catalyst for methanol decomposition reaction as well as methanol synthesis decreases rapidly during the initial period of operation. The activity of this catalyst
Fig. 9. Carbon monoxide yield in the methanol decomposition and steam reaction versus feed ratio of water to methanol (catalyst loading, 1.0 g; GHSV, 280–4400 h−1 ; methanol conversion, 95–100%).
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Fig. 10. Methanol conversion versus time on-stream with various feed water contents (reaction temperature, 300 or 250 ◦ C; catalyst loading, 1.0 g; GHSV, 1100 h−1 ).
was evaluated for 100 h using various methanol feeds and constant reaction condition (GHSV, 1100 h−1 ; temperature, 300 ◦ C). It was observed that the deactivation pattern varied with the water content in the methanol feed. In absence of water, methanol conversion decreases rapidly during the initial period and reaches steady state after 20 h. When water is present the initial deactivation is less evident; however some slow deactivation occurs over the course of the run (Fig. 10). 4. Discussion
in Fig. 2, each profile was seen to have two temperature regions, inferring that two different rate determining steps may exist. In the low temperature region, we observed no dimethyl ether while methyl formate increased significantly with temperature (Fig. 3). This infers that methyl formate decomposition to form CO, CO2 and H2 is most likely the rate determining step in the low temperature region (reaction 5 and 6 in Fig. 11). In the high temperature region, the formation of dimethyl ether increased with temperature and methyl formate decreased rapidly. This indicates that formation of formaldehyde from methoxy is now the slow step at higher temperatures (reaction 2).
4.1. Reaction mechanism and rate determining steps of methanol decomposition
4.2. Copper oxidation state
It is difficult to select just one reaction path for methanol decomposition to hydrogen, CO and CO2 because of the various intermediates formed. However, after reviewing several hypotheses detailed in the literature, we believe they can be simplified to eight major steps shown in Fig. 11. The intermediate compound formed to the largest extent was methyl formate. Therefore, we can infer that the rate determining step is decomposition of methyl formate to form H2 , CO or CO2 . For the methanol conversion versus temperature data shown
The state of Cu in the active catalyst can be either Cu2+ (Cu–O), Cu+ (Cu–O–Cu) or Cu0 (metallic copper) and most likely all three states exist simultaneously. To form methoxy from adsorbed methanol or gas phase methanol requires an oxidized Cu site (Cu–O) (6) to remove hydrogen. If CO2 is formed from formaldehyde (7) or methyl formate (8), an oxidized Cu site is again needed. Cu–O + 2CH3 OH → Cu–CH3 O + H2 O
(6)
Cu–O + CH2 O → Cu–CH2 O2 → CO2 + H2
(7)
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Fig. 11. Schematic diagram of the mechanism of methanol decomposition.
Cu–OH + HCOOCH3 → HCOOH + CH3 O → CO2 + H2
(8)
It is generally accepted that the source of oxygen for the Cu–O site is water in the feed and that Cu can be repeatedly oxidized through a redox mechanism (9–11). H2 O → 2H(a) + O–Cu(a)
(9)
CO + O–Cu(a) → CO2
(10)
2H(a) → H2
(11)
When a fresh catalyst is calcined and is largely in the form of oxidized copper, it should have a higher methanol decomposition rate. If little or no water is fed, the oxidized copper sites rapidly reduce to metallic copper, explaining the rapid initial decrease of activity shown in Fig. 10. 4.3. Methanol decomposition versus steam reforming Methanol conversion for this catalyst at moderate conditions of 250 ◦ C, 1 atm and GHSV = 1100 (or
W/F = 2 cm3 /h g cat) is less than 30% when the feed contains little or no water (<1%). This is consistent with Cheng’s results [1,2], where he reported that a Cu/ZnO catalyst is less active than a Cu/Cr/Mn catalyst, which did not contain ZnO. However, in our work, when water in the feed is more than 43 mol%, methanol conversion reached almost 100%. This is explained by the number of Cu–O active sites formed by water’s dissociative adsorption of water. Thus, the Cu/ZnO/Al2 O3 catalyst seems more suited for methanol–steam reforming than methanol decomposition. Earlier a work on methanol–steam reforming proposed a two steps reaction scheme consisting of methanol decomposition to CO and hydrogen and subsequent CO + H2 O shift reaction [12]. This hypothesis is consistent with the knowledge that methanol is synthesized directly from CO. However, later work discovered that CO2 has an important role in methanol synthesis [4,10,11] and the formation of CO2 by direct reaction of methanol and steam was proposed [9]. In our study, we have confirmed that methanol decomposition to CO and the steam reforming reaction occur simultaneously (in contrast to the
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Fig. 12. XPS spectrum of copper in the fresh and used catalyst for methanol-only feed and methanol-water feed.
work of Jiang et al. [9]) by observation of the H2 and CO yields, CO2 formation characteristics and time on-stream data. As shown in Fig. 8, at 50% methanol conversion (which means relatively low temperatures) the hydrogen yield increases significantly with the feed ratio of H2 O to methanol, while the hydrogen yield for 100% conversion has a relatively steady value of 2.1–2.2 at the low H2 O to methanol molar feed ratio. This is consistent with the rate determining step in the low temperature region being methyl formate hydrolysis because CO and CO2 formation from methyl formate is slow and the direct methanol–steam reaction is relatively fast. Therefore, a faster methanol–steam reaction relative to methanol decomposition would explain the higher yield for hydrogen. This data cannot be explained equally as well if methanol decomposition and water–gas shift are the only reaction occurring.
During the time-on-stream experiments, we found that our Cu/ZnO/Al2 O3 catalyst had a significant weight loss (16–18%, excluding moisture) caused most likely by the loss of O from CuO in both cases of with and without water. This was confirmed by XPS analysis. As shown in Fig. 12 the fresh catalyst has copper in the state of CuO (Cu2+ ) while the spent catalyst after 100 h run is mostly metallic copper. This change in oxidation state correlates with the loss of activity shown in Fig. 10. We conclude that the main cause of the initial deactivation is the change of this oxidation state. However, there is still a slow activity decrease for all runs including those with water in the feed. Thus, a second deactivation mechanism exists such as chemically induced sintering or carbon fouling. Also from these results we conclude that reduced copper is not inactive and contributes significantly to methanol decomposition.
4.4. Cu oxidation state and deactivation There has been an extended debate about the active site for copper-zinc catalysts in methanol synthesis (Cu0 versus Cu+ ) during the last two decades. While the case of methanol synthesis is summarized well through several review papers [4,11,13,14], there are only a few papers which deal simultaneously with the cases of methanol decomposition and methanol–steam reforming.
5. Conclusions 1. The activity and yield of the methanol decomposition reaction on a commercial Cu/ZnO/Al2 O3 catalyst was characterized. Water addition to methanol significantly increased the activity of catalyst and decreased by-product formation. H2 yield increased with increasing ratio of water to
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methanol and CO yield decreased continuously with water content. 2. A simplified eight-step reaction mechanism is inferred from our data and two rate determining steps are proposed: decomposition of methyl formate at temperatures below 230 ◦ C and the formation of formaldehyde at temperatures greater than 230 ◦ C. 3. The active site for methanol decomposition and steam reforming is Cu2+ . The loss of Cu–O is the main cause of initial catalyst deactivation. Deactivation is worsened when water is absent from the feed. References [1] W.H. Cheng, Acc. Chem. Res. 32 (1999) 685–691. [2] W.H. Cheng, Appl. Catal. A 130 (1995) 13–30.
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