CO2 Conversion and Utilization - American Chemical Society

Chapter 12

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on September 8, 2015 | http://pubs.acs.org Publication Date: January 24, 2002 | doi: 10.1021/bk-2002-0809.ch012

Methane Dry Reforming over Carbide, Nickel-Based, and Noble Metal Catalysts Abolghasem Shamsi National Energy Technology Laboratory, U.S. Department of Energy, 3610 Collins Ferry Road, Morgantown,WV26507

Carbide catalysts of molybdenum and tungsten were prepared and tested for reaction of methane with CO at atmospheric pressure. At this pressure the catalysts were not stable and the tungsten carbide irreversibly deactivated after 35 hours on stream. The carbide catalysts produced lower H /CO ratios at lower temperatures of 650 and 750°C compared to noble metal or nickel-based catalysts. Tungsten carbide was partially oxidized to tungsten oxide during the reactions, resulting in lower catalytic activity. We also tested 1 wt% rhodium supported on alumina and two commercial Ni-based catalysts, R-67 and G-56B. Significant amounts of carbon formed on the commercial catalysts that plugged the reactor after 5 hours on stream. 2

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U.S. government work. Published 2002 American Chemical Society In CO2 Conversion and Utilization; Song, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2002.

183 Introduction Recently many researchers have concentrated their efforts toward catalytic reforming of methane with carbon dioxide. This process can be very useful for converting thermal energy into chemical energy. For example, energy losses in combustion/gasification systems and in advanced gas turbines can be captured by reacting natural gas with the by-product of combustion (C0 ) over a catalyst for producing syngas which can be converted into liquid fuels and chemicals, ultimately increasing the system efficiency. Although this concept has many environmental and economic incentives, unfortunately, there are no commercial processes for reforming of methane with C 0 (1). The main problem is that there are several carbon-forming reactions associated with this concept that deactivate the conventional steam reforming, nickel-based, catalysts. Nickel catalyzes carbon formation via hydrocarbon decomposition and CO disproportionate reactions, which greatly contributes to catalyst deactivation. Three phenomena are known to be responsible for the deactivation of nickel catalyst (2). 1) Carbon deposition, 2) metal sintering, and 3) phase transformation such as N i A l 0 * Ni/y-Al 0 . Choudhary et al. (3) reported that the pressure drop across a fixed bed reactor, containing NiO supported on CaO, increased rapidly due primarily to rapid coke formation on the catalyst surfaces. However, when they added steam to the reaction the amount of carbon was significantly reduced. Chang et.al. (4) found a coke formation rate of 7.0 wt% per hour for pentasil zeolite-supported nickel catalyst at 700°C. Furthermore, they indicated that addition of promoters such as potassium and calcium plus altering the preparation method reduced the coke formation rate to less than 0.1 wt% per hour. Addition of CaO to Ni/y-Al 0 catalyst is reported to improve the catalyst stability and also increases the reaction rate (5).


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Operating conditions and catalyst support (6) also play an important role in catalyst particle size distribution, kinetics, and the reactivity of carbon deposited on the catalyst surfaces. Gadalla and Bower (7) recommended that a C0 /CH4 ratio of greater than one should be used for reducing carbon deposition. Furthermore, they have calculated the optimum range of temperatures for each feed composition and pressure in which carbon deposition and carbide formation are minimized. It is reported (8) that the activity and stabilty of methane reforming catalysts are significantly affected by the support and by the active metals. Reducing the concentration of Lewis acid sites on the support and reducing nickle particle size will significantly lower carbon formation. Basini and Sanfilippo (9) studied the molecular aspects of syngas production and proposed that the formation of highly reactive oxidic species, formed from breaking one 2

In CO2 Conversion and Utilization; Song, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2002.

184 of the two C 0 bonds, is responsible for inhibiting carbon formation on the surfaces of the dry reforming catalysts such as Rh, Ru, and Ir. The effect of the support on ruthenium catalyst, under dry reforming conditions, has also been studied (10) and the catalytic activity decreases in the following order: Rh/Al 0 > Rh/Ti0 > Rh/Si0 . Several studies (11-13) have shown that the high-surface area tungsten and molybdenum carbide materials are active for methane dry reforming reactions. These catalysts appeared to be stable at higher pressures without forming significant amounts of carbon on the catalyst surfaces. However, these catalysts are very sensitive to oxidation by oxygen or H 0 , forming oxides that are not active for dry reforming. Review of the current literature indicates that developing an inexpensive catalyst, which exhibits a high selectivity toward hydrogen and carbon monoxide without forming carbon remains a challenge, specifically at higher pressure (14). In this paper, we report the preparation and testing of carbide, Ni-based and noble metal catalysts and compare catalytic activity, selectivity, and stability of these catalysts for reaction of methane with carbon dioxide. 2


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Experimental Section The tungsten carbide catalyst was prepared as described (15) in U.S. patent 5321,161. The molybdenum carbide catalyst was prepared by temperatureprogrammed reduction (TPR) of molybdenum oxide in a flow of 11.6% ethane or methane in hydrogen at a flow rate of 55 ml/min. Molybdenum carbide supported on T i 0 was prepared by mixing M0O3 with T i 0 in ethyl alcohol slurry. The dried mixture was reacted with 11.6 vol.% ethane in hydrogen to form carbide. More information on carbide preparation can be found in a paper published by Lee et.al. (16). The nickel-based catalysts were prepared from water soluble nitrate solutions with proper metal ratios. A 0.3-m long quartz reactor tube (6.35-mm o.d., 4.0-mm i.d.) with a quartz thermocouple well was used as afixed-bedreactor with 0.5 grams of catalyst (-28/+48 mesh) held in place by quartz wool. Electronic mass flow controllers fed methane and carbon dioxide into the reactor, GHSV=5040 cm ^" .^ . The reactor was electrically heated to reaction temperatures. Products were analyzed by on-line gas chromatography (GC). Samples were analyzed for hydrogen, carbon monoxide, methane and carbon dioxide. A thermal conductivity detector was used with a 1-m by 3.2-mm-o.d. stainless steel molecular sieve 5A column and a 3.66-m by 3.2-mm-o.d. stainless steel HayeSep C (80/100 mesh) column at isothermal oven temperatures of 115 and 52 °C, respectively. Argon was used as carrier gas at 20 ml/min. 2

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185 Results and Discussion A series of carbide, Ni-based, and noble metal catalysts were prepared and tested for methane dry reforming at a temperature range of 650-950°C and at atmospheric pressure. The results at 750 and 850°C are shown in Table I and the results at other temperatures are discussed in the text. Pure SiC was tested at 950°C and no significant methane or C 0 conversion was observed. We also tested pure tungsten carbide and tungsten carbide supported on silica. Methane conversion of less than 41% was obtained for the supported catalyst at 950°C. The activity of the unsupported tungsten carbide was tested at 650, 750, and 850°C. At lower temperatures of 650 and 750°C, methane and C 0 conversions were less than 15% with a H /CO ratio of 0.2. However, higher methane and C 0 conversions with a H /CO ratio of 1.1 were obtained when the temperature was raised to 850°C, at which the reaction was continued for about 45 hours as shown in Figure 1. The catalyst irreversibly deactivated after 35 hours on stream and we were not able to regenerate it with flowing hydrogen or a mixture of 11.6% ethane in hydrogen over the catalyst. Identical results were obtained when a fresh catalyst, from the same batch, was tested at a similar reaction condition (not shown). Since the freshly prepared catalyst was very sensitive to air oxidation the reactor was loaded under an argon atmosphere in a glove box. Although we were not able to determine the exact cause of the sharp decrease in catalytic activity, characterization of the sample


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Table I. Methane dry reforming over carbide and Ni-based catalysts

Temperature, °C Catalysts: Mo C (me) Mo C (et) Mo C/Ti0 Tungsten carbide R-67, Topsoe* G-56B, UCI* 2

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%CO > Conversion

750

850

750

850

750

850

750

850

18.3 36.9 69.9 5.4 94.2 96.1

92.1 89.5 70.1 90.7 98.9 99.1

37.3 59.1 87.1 12.0 91.1 90.3

99.8 99.9 88.7 99.7 95.2 97.1

37.3 51.6 76.2 13.4 95.3 93.2

96 89.4 77.8 86.6 99.1 99.8

0.3 0.7 0.9 0.2 1.0 1.0

1.0 1.1 0.9 1.1 1.0 1.0

:

(me)- TPR of Mo0 using 11.6% methane in H (et) -TPR of Mo0 using 11.6% ethane in H * Plugged the reactor after 5 hours 3

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H /CO ratio

%CO Yield

%CH< Conversion

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186 by X-ray diffraction after the reaction indicated that the tungsten carbide was partially oxidized to tungsten oxide, suggesting that some of the active sites had been destroyed. Molybdenum carbide catalysts were prepared by temperature-programmed reduction of molybdenum oxide with a mixture of 11.6 vol.% methane or ethane in hydrogen. The catalyst prepared with methane mixture was tested at 750 and 850°C as shown in Table I. Methane and C 0 conversions of less than 18% with a H /CO ratio of 0.3 were obtained at 650°C. At 850°C methane and C 0 conversions were higher than 90% with a H /CO ratio of 1.0. Higher catalytic activity was observed for the catalyst prepared with ethane mixture compared to that prepared with methane mixture as shown in Table I. The carbide catalysts produced low H /CO ratios and deactivated rapidly at temperatures lower than 750°C. Molybdenum carbide prepared with ethane mixture was tested for 16 hours and the results are shown in Figure 2. After 10 hours the catalytic activity continuously decreased until the reaction was terminated. This could result from oxidation of carbide active sites into the oxide, which is inactive for dry reforming reactions as reported by Claridge et. al. (13). Molybdenum carbide mixed with T i 0 was tested at 650, 750, 850, and 900°C. Methane and C 0 conversions of 21 and 42% with a H /CO ratio of 0.5 were obtained at 650°C. However, after 30 minutes on stream these numbers decreased to 9.5%, 21.8%, and 0.3, respectively. The results for the initial activity at 750 and 850°C are shown in Table I. The mixed catalyst was also unstable at temperatures of less than 850°C. Less deactivation was observed at higher temperatures and this could be due to higher stability of carbide catalyst at higher temperatures. Thermodynamically, the reduction of oxide into carbide is more favorable at higher temperatures while forming C 0 as shown in Table II. The table shows the temperatures beyond which the equilibrium constants are greater than one (K>1). At 900°C a methane conversion of 95% and C 0 conversion of 100% with a H /CO ratio of 1.0 were measured and the test was continued for 2.5 hours without a significant drop in catalytic activity or selectivity as shown in Figure 3. At temperatures of less than 850°C higher methane and C 0 conversions with a higher H /CO ratio were obtained for molybdenum carbide mixed with T i 0 than the sample without it. This shows that T i 0 has a promoting effect on the catalytic activity of the carbide catalysts. 2


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The tungsten carbide catalyst was characterized before and after reaction using X-ray diffraction, Scanning Electron Microscopy (SEM), and X-ray Photoelectron Spectroscopy (XPS). The X-ray diffraction patterns show that the carbide catalysts were partially oxidized to W 0 during the reaction as shown in Figures 4. This is in good agreement with a study done by Clarige and his coworker (13), reporting that molybdenum and tungsten carbides were 2


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Figure 1. Dry reforming of methane over tungsten carbide at 850 "C, CH4/CO2 = 1.1

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- · — C02 conversion -at— % CO yield ••— CH4 conversion - H2/CO ratio

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Time, h Figure 3. Dry reforming of methane over molybdenum carbide mixed with Ti0 , at 900 °C, CH/C0 = 1.12 2

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50 60 2-Theta (deg)

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Figure 4. X-ray diffraction pattern of tungsten carbide catalyst: (a) before reaction and φ) after reaction at 850 °C, CH4/C0 =1.15 2


190 Table Π. Thermodynamic calculations for Molybdenum and Tungsten Carbide Species Found in the HSC Database*

Temperature

Reaction

Equilibrium Constant (K)


°c 364 379 581 385 589 604 549 567 604 644

1.087 2Mo0 + 4CH4 (g) = 3C0 (g) + Mo C + 8H (g) 1.006 Mo0 + 2.5CIL, (g) = 1.5C0 (g) +MoC + 5H (g) Mo0 + 2CIL, (g) = C 0 (g) + MoC + 4 H (g) 1.050 3Mo0 + 6.5 C H 4 (g) = 4.5C0 (g) +M03C2 + 13 H (g) 1.011 1.067 2Mo0 + 3CH, (g) = 2C0 (g) + M02C + 6H (g) 1.024 3Mo0 + 5CH4 (g) = 3C0 (g) + Mo G + 10H (g) 1.032 3W0 + 2.5CH4 (g) = 1.5C0 (g) + WC + 5H (g) W 0 + 2CH4 (g) = C 0 (g) + WC + 4H (g) 1.043 1.060 2W0 + 4CH4 (g) = 3C0 (g) + W C + 8H (g) 1.16 2W0 + 3CH4 (g) = 2C0 (g) + W C + 6H (g) 3

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Calculated using HSC Chemistry, Outokumpu Research Oy in Finland

converted to oxides during methane dry reforming. Further oxidation of the deactivated sample occurred when it was exposed to air. Tungsten carbides of W C and WC were detected in both samples. The SEM photographs of tungsten carbide catalyst, before and after reaction, clearly show differences between the two samples as shown in Figure 5. The particles in the sample before the reaction have sharper edges than the particles in the sample after the reaction, indicating that some of the active sites have been destroyed or covered with carbon during the reactions. The XPS analysis of tungsten carbide catalyst showed higher concentrations of carbon and oxygen on the surfaces of the sample after reaction compared to that before reaction. The ratio of carbon (C Is) to tungsten (W Is) increased from 7 to 50 while the ratio of oxygen (O Is) to tungsten (W Is) increased from 2.8 to 12.6 during 35 hours of reactions. Furthermore, no significant changes were observed for carbon (C Is) or the oxygen (O Is) peaks on these samples. The tungsten (W Is) for both samples before and after the reactions is shown in Figure 6. Two commercial nickel-based catalysts (R-67 and G-56B) were tested at 750 and 850°C as shown in Table I. Although these catalysts showed methane and C 0 conversions of more than 90% at 750°C they produced significant amounts of carbon in the catalyst bed which eventually plugged the reactor and stopped the flow as shown in Figure 7. 2

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In CO2 Conversion and Utilization; Song, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2002. 2

Figure 5. SEM photographs of tungsten carbide catalyst: (a) before reaction and (b) after reaction at 850 °C, CH/C0 = 1.15


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-100

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Figure 6. Photoelectron spectra of tungsten (W Is) carbide catalyst: (a) before reaction and (b) after reaction at 850 °C, CHyC0 =7.15 2


193

A noble metal catalyst of 1% rhodium supported on alumina was tested at 650, 750, and 850°C and the results are listed in Table HI. Methane and C 0 conversions of more than 59% with a H /CO ratio of 0.9 were obtained at 650 °C. At this temperature the catalyst showed higher activity, selectivity, and stability than carbide and Ni-based catalysts. The test was continued for more than 25 hours at 850°C, as shown in Figure 8, with no sign of deactivation or formation of significant amounts of carbon on the catalyst surface. When the space velocity (GHSV) was raised from 5040 to 7800 and 10200 cm^g^.h" no significant changes in catalytic activity or selectivity were observed. 2

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Table ΙΠ. Methane dry reforming over 1% Rh/alumina catalyst Temperature, °C 1

GHSV.cnyV.h

H /CO ratio % CO yield % CH4 conversion % C 0 conversion 2

2

650 5040 0.9 65.6 59.2 65.2

750 5040 1.0 88 86.9 88.1

850 5040 1.0 95.7 97.2 97.4

850 7800 1.0 96.1 97.2 93.0

850 10200 1.0 94.7 95.9 92.3

The initial catalytic activity of carbide, Ni-based, and noble metal catalysts were similar at temperatures higher than 800°C. However, carbide catalysts were oxidized to oxide during the reaction and lost their catalytic activity after several days on stream. The commercial (Ni-based) catalysts formed significant amounts of carbon that plugged the reactor. Furthermore, at 650°C the initial activity of Ni-based and 1% Rh/alumina catalysts were significantly higher than the carbide catalysts. Although rhodium catalyst is much more expensive than nickel-based and carbide catalysts, it is more stable and produces little or no carbon during the reaction. Therefore, it is a good candidate for dry reforming reaction, producing syngas from methane and C 0 . The high cost of rhodium metal could be tolerated considering higher activity, low metal loading, and reduced carbon deposition. The existing nickel-based commercial catalysts are not suitable for dry reforming because significant amounts of carbon formed on the catalyst surfaces, plugging the reactor. We found that the carbide catalysts are also not reliable and deactivate irreversibly during the reaction at atmospheric pressure. However, it has been shown that these catalysts are much more stable at higher pressure (13). 2



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195 Conclusion Carbide catalysts of molybdenum and tungsten were prepared and tested for reaction of methane with C 0 . The catalysts were not stable at atmospheric pressure and irreversibly deactivated after several days on stream. The carbide catalysts produced a lower H /CO ratio than nickel-based and noble metal catalyst at lower temperatures of 650 and 750 °C. The rhodium supported catalyst was active and more stable than the carbide and Ni-based catalysts at lower temperatures of 650 and 750 °C. Significant amounts of carbon formed on the commercial catalysts, plugging the reactor after 5 hours on stream. Tungsten carbide catalyst was analyzed before and after methane dry reforming reactions using XRD, SEM, and XPS. The XRD and the XPS results showed that the tungsten carbide was partially oxidized to tungsten oxide during the reactions and may have resulted in the loss of some of the active sites. 2


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Acknowledgment Author acknowledges the Natural Gas Processing and Utilization team of National Energy Technology Laboratory for funding this work. He also thanks Dr. Ranjani V. Siriwardane, James A. Poston and Elizabeth A. Frommell for carrying out the XPS, SEM and XRD measurements.

References 1.

2.

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Goula, Μ. Α.; Lemonidou, Α. Α.; Efstathiou, Α. Μ. Characterization of Carbonaceous Species Formed During Reforming of CH4 with C O over Ni/CaO-Al O Catalysts Studied by Various Transient Techniques. J. Catal. 1996, 161, 626-640. Gadalla, A. M.; Bower, B. The Role of Catalyst Support on the Activity of Nickel for Reforming Methane with CO . Chem. Eng. Sci. 1988, 43 (11), 3049-3062. Lercher, J. Α.; Bitter, J. H.; Hally, W.; Niessen, W.; Seshan, K. Design of Stable Catalysts for Methane-Carbon Dioxide Reforming. Studies in Surface Science and Catalysis. 1996, 101, 463-472. Nakamura, J.; Aikawa, K.; Sato, K.; Uchijima, T. Role of Support in Reforming of CH4 with CO over Rh Catalysts. Catalysis Letters. 1994, 25, 265-270. Basini, L.; Sanfilippo, D. Molecular Aspects in Syn-Gas Production: The CO -Reforming Reaction Case. J. Catal. 1995, 157, 162-178. York, P. E.; Claridge, J.B.; Marquez-Alvarez C.; Brungs, A. J.; Green, M.L.H. Group (V) and (VI) Transition Metal Carbides as New Catalysts for the Reforming of Methane to Synthesis Gas. Prepr. Am. Chem. Soc., Div. Fuel Chem. 1997, 42(2), 606-610. York, A. P. E.; Claridge, J. B.; Brungs, A. J.; Tsang, S.C.; Green, M.L.H. Molybdenum and Tungsten Carbides as Catalysts for the Conversion of Methane to Synthesis Gas Using Stoichiometric Feedstock. Chem. Commun. 1997, 39-40. Claridge, J. B.; York, A.P.E.; Brungs, A. J.; Marquez-Alvarez, C.; Sloan, J.; Tsang, S.C.; Green, M.L.H. New Catalysts for the Conversion of Methane to Synthesis Gas: Molybdenum and Tungsten Carbides. J. Catal. 1998, 180, 85-100. Song, C.; Srimat, S. T.; Sun, L.; Armor, J. N. Comparison of High -Pressure and Atmospheric-Pressure Reactions for C O Reforming of CH4 over Ni/Na-Y and Ni/Al O Catalysts Prepr. Am. Chem. Soc., Div. Petrol. Chem. 2000, 45 (1), 143-148. Vreugdenhil, W.; Sherif, F.G.; Burk, J.H.; Gadberry, J.F. U.S. Patent 5,321,161, 1994. Lee, J.S.; Oyama, S.T.; Boudart, M . Molybdenum Carbide Catalysts I. Synthesis of Unsupported Powders J. Catal. 1987, 106, 125-133. 2

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CO2 Conversion and Utilization - American Chemical Society

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