3934
Ind. Eng. Chem. Res. 1996, 35, 3934-3939
Simultaneous Carbon Dioxide and Steam Reforming of Methane to Syngas over NiO-CaO Catalyst Vasant R. Choudhary* and Amarjeet M. Rajput Chemical Engineering Division, National Chemical Laboratory, Pune 411008, India
Steam reforming, CO2 reforming, and simultaneous steam and CO2 reforming of methane to CO and H2 over NiO-CaO catalyst (without any prereduction treatment) at different temperatures (700-850 °C) and space velocities (5000-70 000 cm3‚g-1‚h-1) are investigated. The catalyst is characterized by XRD, XPS, and temperature-programmed reduction (TPR). The catalyst showed high activity/selectivity in both the steam and CO2 reforming reactions and the simultaneous steam and CO2 reforming. In the CO2 reforming, the coke deposition on the catalyst is found to be very fast. However, when the CO2 reforming is carried out simultaneously with the steam reforming, the coke deposition on the catalyst is drastically reduced. By the simultaneous CO2 and steam reforming (at g800 °C and space velocity of about 20 000-30 000 cm3‚g-1‚h-1), methane can be converted almost completely to syngas with 100% selectivity for both CO and H2. The H2/CO ratio in products can be varied between 1.5 and 2.5 quite conveniently by manipulating the relative concentration of steam and CO2 in the feed. Introduction
Experimental Section
Release of large quantities of carbon dioxide in the atmosphere causes a global warming/greenhouse effect. Hence, research activities on the activation of carbon dioxide for its conversion to useful products are gaining more and more importance. In the last few years, CO2 activation, particularly by its reaction with methane to CO and H2 (commonly known as CO2 reforming of methane), has gained a lot of momentum. Several investigations have been reported on the CO2 reforming of methane to syngas using Pt group metals supported on different metal oxides (Richardson and Paripatyadar, 1990; Solymosi et al., 1991; Perera et al., 1991; Ashcroft et al., 1991; Rostrup-Nielson and Bak Hansen, 1993; Erdohelyi et al., 1993; Nakamura et al., 1994), Ni/MgO or MgAl2O4 (Rostrup-Nielson and Bak Hansen, 1993), Ni/Al2O3 (Chen and Ren, 1994), and Ni/MgO-CaO (Yamazaki et al., 1992) catalysts. A rapid coke deposition on catalyst is, however, a serious problem in the CO2 reforming of methane, particularly when nickel catalyst is used (Rostrup-Nielson and Bak Hansen, 1993). A catalytic process for producing Oxo feed with a H2/CO ratio of about 1.0 by reforming hydrocarbons with a mixture of steam and CO2 has also been reported earlier (Teuner, 1987). In our earlier studies (Choudhary et al., 1994), NiOCaO catalyst showed high activity, selectivity, and productivity in the oxidative conversion of methane to syngas. By carrying out the oxidative conversion of methane simultaneously with the steam and CO2 reforming (Choudhary et al., 1994) or with the CO2 reforming alone (Choudhary et al., 1995) over NiO-CaO catalyst, methane can be converted to CO and H2 with high conversion (e95%) and selectivity for CO (100%) and H2 (95-100%) and also with high CO productivity without catalyst deactivation due to coking for a long period under the most energy-efficient and safe manner, requiring little or no external energy. This investigation was undertaken to study the performance of NiO-CaO catalyst in the steam reforming, CO2 reforming, and simultaneous steam and CO2 reforming of methane to syngas.
NiO-CaO catalyst (Ni/Ca mole ratio ) 0.2, 1.0, 3.0, and 10.0) was prepared by mixing thoroughly finely ground nickel (Loba Chemi, GR) and calcium hydroxide (Qualigens SQ) with the required Ni/Ca ratio in the presence of enough distilled water to form a thick paste, drying at 120 °C for 4 h, decomposing the mass at 600 °C for 4 h, powdering, pressing, and crushing to 22-30 mesh size particles, and then calcining in air at 930 °C for 4 h. The surface area of the catalyst was measured by the single-point BET method using a monosorb surface area analyzer (Quantachrome Corp., USA). Crystalline phases in the catalyst were detected by X-ray powder diffraction using a Holland Phillips PW/1730 X-ray generator with Cu KR radiation. Surface chemical analysis of the catalyst was done by X-ray photoelectron spectroscopy (XPS) using a VG-scientific ESCA-3 MK II electron spectrometer (C 1s with binding energy ) 285 eV was used as an internal standard). Before these measurements, the catalyst was preteated at 900 °C for 1 h in a flow of moisture-free N2 and it was used immediately with its minimum exposure to atmospheric moisture and CO2. The basicity of the catalyst was measured in terms of the chemisorption of carbon dioxide at 50 and 500 °C by the method based on stepwise thermal desorption of CO2 (Choudhary and Rane, 1990). Temperature-programmed reduction (TPR) of the NiO-CaO catalysts was carried out in a quartz reactor (o.d. 6 mm and i.d. 4.5 mm packed with 0.05-0.1 g of catalyst) in a flow (60 cm3‚min-1) of a H2-Ar (2 mol % H2) mixture, from 50 to 900 °C at a heating rate of 10 °C‚min-1 and measuring a change in the H2 concentration (due to the reduction of NiO in the catalyst) by a thermal conductivity detector. Before measuring the basicity or carrying out the TPR, the catalyst was pretreated in situ at 900 °C for 1 h in a flow of moisture-free nitrogen (50 cm3‚min-1). The catalytic steam reforming, CO2 reforming, and simultaneous CO2 and steam reforming reactions over the catalyst were carried out at atmospheric pressure in a continuous-flow quartz reactor (i.d. 1.0 cm) packed with 0.3-0.5 g of catalyst particles (22-30 mesh) and provided with a chromel-alumel thermocouple in the center (axially) of the catalyst bed. The reactor was kept
* To whom correspondence should be addressed.
S0888-5885(96)00002-4 CCC: $12.00
© 1996 American Chemical Society
Ind. Eng. Chem. Res., Vol. 35, No. 11, 1996 3935 Table 1. Surface Area and Chemisorption of CO2 on NiO-CaO Catalyst with Different Ni/Ca Mole Ratios Ni/Ca ratio
surface area (m2‚g-1)
0.2 1.0 3.0 10.0
1.5 2.1 2.5 1.6
CO2 chemisorption (µmol‚g-1) 50 °C
500 °C
191 156 114 85
64 58 55 51
in a tubular furnace, and the reaction temperature was controlled by the thermocouple in the catalyst bed. The temperature gradient in the catalyst bed was 5-10 °C. The feed was a mixture of pure methane (99.95%), CO2 (99.995%), and/or steam. Water was added to the feed using a SAGE syringe pump and a specially designed evaporator. Before carrying out the reaction, the catalyst was heated in situ at 900 °C in a flow (50 cm3‚min-1) of moisture-free nitrogen for 1 h. The catalytic reactions were carried out at different temperatures, gas hourly space velocities (GHSV measured at 0 °C and 1 atm), and relative concentrations of methane, CO2, and steam. The product gases, after condensation of water at 0 °C, were analyzed by on-line gas chromatography with a thermal conductivity detector, using a Spherocarb column. The amount of coke deposited on the catalyst was measured in terms of carbon (wt %) determined by the microanalysis. The pulse reaction of methane (in the absence of free oxygen) over NiO-CaO (Ni/Ca ) 3.0) catalyst at 700 °C was carried out by passing a pulse of pure methane (pulse size ) 1.0 mL) over the catalyst (0.02 g) packed in a quartz microreactor (i.d. ) 0.5 mm), which is directly connected to a gas chromatograph provided with a thermal conductivity detector and Spherocarb column. The pulse reaction was carried out as a function of the pulse number. The products of each pulse experiment were analyzed by the gas chromatograph before injecting the next pulse. The catalytic activity is expressed in terms of the conversion of methane.
conversion of CH4 (%) ) [CH4 (in feed) CH4 (in products)/CH4 (in feed)] × 100 The selectivity for CO or H2 is based on the methane conversion alone.
selectivity for CO or H2 (%) ) (conversion of CH4 to CO or H2/total conversion of CH4) × 100 yield of CO or H2 (%) ) conversion of methane to CO or H2 Results and Discussion Catalyst Characterization. Results on the surface area and basicity [measured in terms of the CO2 chemisorbed at 50 °C (total basicity) and 500 °C (strong basicity)] of the NiO-CaO catalyst with different Ni/ Ca ratios are presented in Table 1. The surface area passes through a maximum (at Ni/Ca ) 3.0) with increasing Ni/Ca ratios, whereas both the total and strong basicity are decreased quite significantly with increasing Ni/Ca ratio. The XRD analysis shows the presence of two distinct crystalline phases, CaO and NiO, in all the catalysts before their use in the reforming reactions. A representative XRD spectrum of the NiO-CaO catalyst
Figure 1. XRD spectra of NiO-CaO (Ni/Ca ) 3.0): (A) fresh catalyst; (B) catalyst used in the steam reforming (for 10 h); (C) catalyst used in the simultaneous steam and CO2 reforming (for 10 h). [a ) CaO; b ) NiO; c ) Ni; d ) Ca(OH)2.]
Figure 2. C 1s (A, B) and Ni 2p3/2 (C, D) XPS spectra of NiOCaO (Ni/Ca ) 3.0) catalyst before use (A, C) and after use (B, D) in the simultaneous steam and CO2 reforming reaction (at 850 °C for 10 h).
(before use) is shown in Figure 1A. However, after using the catalyst in the time-on-stream activity runs for the steam reforming and simultaneous steam and CO2 reforming reactions (at 850 °C for 10 h), a metallic Ni (i.e., Ni0) phase, instead of the NiO phase, is observed (Figure 1B,C). The observed Ca(OH)2 phase instead of the CaO phase for the catalyst used in the steam reforming reaction is due to the conversion of CaO by its reaction with water vapors at lower temperatures when the reaction is stopped. The presence of Ni0 and the absence of NiO in the used catalyst indicate that the NiO in the catalyst is reduced completely during the reforming reactions and the catalyst under operating conditions is essentially Ni0 supported on CaO. XPS spectra for C 1s and Ni 2p3/2 of the fresh and used (after the time-on-stream activity run for the simultaneous steam and CO2 reforming at 850 °C for 10 h) catalyst (with Ni/Ca ) 3.0) are presented in Figure 2, and the binding energy data for the different elements and the surface Ni/Ca ratio are given in Table 2. The C 1s peak at the higher binding energy (≈289.6 eV) (Figure 2A,B) indicates the presence of surface carbonate (CO32-) species in both the fresh and used catalysts. The surface carbonate species are expected to be formed by the interaction of CO2 from atmosphere (for the fresh catalyst) or from the feed/reaction products with the catalyst. However, the C 1s peak at the lower binding energy (281.5 eV), observed for the used catalyst (Figure 2B), may be due to partially hydrogenated carbonaceous
3936 Ind. Eng. Chem. Res., Vol. 35, No. 11, 1996 Table 2. XPS Data for NiO-CaO (Ni/Ca ) 3.0) Catalyst before and after the Simultaneous Steam and CO2 Reforming Reactions binding energy (eV) catalyst
O 1s
C 1s
Ni 2p3/2
Ca 2p3/2
surface Ni/Ca ratio
before reaction
531.4
1.40
531.5
854.9 861.5 852.2 (shoulder) 855.1 861.7 (satellite)
347.3
after reaction
285.0 289.6 285.0 289.7 281.5 (shoulder)
347.2
0.44
Figure 3. TPR profiles for NiO-CaO catalyst with different Ni/ Ca ratios.
species. The XPS spectra for Ni 2p3/2 (Figure 2C,D) show the presence of Ni2+ (BE ) 855 eV) on the surface of both the fresh and used catalysts. However, a smaller concentration of surface Ni0 (BE ) 852.2 eV) is observed for the used catalyst. The observed high surface concentration of Ni2+ for the used catalyst is expected due to the oxidation of its surface Ni0 by atmospheric oxygen. It is also interesting to note that the surface Ni/Ca ratio of the catalyst is decreased very appreciably after the reaction (Table 2). However, the binding energies for O 1s and Ca 2p3/2 are not changed significantly due to the reaction. The above results reveal that NiO in the catalyst is reduced during the reforming reactions. Hence, it is interesting to study the catalyst reduction. Results of TPR of the NiO-CaO (Ni/Ca ) 0.2-10.0) catalysts by hydrogen (2 mol % H2 in Ar) are presented in Figure 3. The TPR curves for all the catalysts show a single peak with a maximum between 400 and 450 °C. It may be noted that the TPR peak maximum temperature for all the catalysts is quite close to that (418 °C) observed earlier (Brown et al., 1982) for the reduction of bulk NiO at the same heating rate. This shows an absence of chemical interactions between NiO and CaO. The observed small difference in the peak maximum temperature for the catalysts may be due to physical interactions between NiO and CaO, causing a change in the particle size or dispersion of NiO with a change in the Ni/Ca ratio.
Figure 4. Time-on-stream activity/selectivity and pressure drop (∆p) across a catalyst bed in (a) CO2 reforming, (b) steam reforming, and (c) simultaneous CO2 and steam reforming of methane over NiO-CaO (Ni/Ca ) 3.0) at 850 °C. [(a) GHSV ) 11 500 cm3‚g-1‚h-1 and CO2/CH4 ) 1.0; (b) GHSV ) 13 900 cm3‚g-1‚h-1 and H2O/CH4 ) 1.1; (c) GHSV ) 32 250 cm3‚g-1‚h-1, CO2/CH4 ) 0.55, and H2O/CH4 ) 0.55.]
Time-on-Stream Activity/Selectivity. Figure 4 shows the time-on-stream activity and selectivity of the NiO-CaO (Ni/Ca ) 3.0) catalyst in the CO2 reforming, steam reforming, and simultaneous CO2 and steam reforming of methane at 850 °C. In the case of the CO2 reforming, although the selectivity to syngas is not affected significantly, the methane conversion activity is decreased appreciably and also the pressure drop across the catalyst bed is increased very sharply in the short initial period. This is due to a rapid coke deposition on the catalyst and also between the catalyst particles, causing a resistance to a gas flow through them. However, in the case of the steam reforming and simultaneous CO2 and steam reforming reactions, no significant decrease in the methane conversion activity and selectivity of the catalyst and also no significant increase in the pressure drop across the catalyst bed are observed. The data on the carbon deposition on the catalyst in the three reactions are presented in Table 3. The rate of carbon deposition in the CO2 reforming is very rapid, whereas it is comparable for the steam reforming and the simultaneous CO2 and steam reforming. This shows that the carbon deposition in the CO2 reforming can be drastically reduced by carrying it out simultaneously
Ind. Eng. Chem. Res., Vol. 35, No. 11, 1996 3937 Table 3. Carbon Deposition in the Methane-to-Syngas Conversion Reactions at 850 °C reaction
time-on-stream (h)
carbon deposition (wt %)
rate of carbon deposition (×103 g(c)‚g-1(cat)‚h-1)
steam reforminga CO2 reformingb simultaneous steam and CO2 reformingc
10 2 10
0.53 25.96 1.08
0.5 130 1.1
a GHSV ) 13 900 cm3‚g-1‚h-1 and H O/CH ) 1.1. b GHSV ) 11 500 cm3‚g-1‚h-1 and CO /CH ) 1.0. c GHSV ) 32 250 cm3‚g-1‚h-1, 2 4 2 4 CO2/CH4 ) 0.55, and H2O/CH4 ) 0.55 (CO2/H2O ) 1.0).
Table 4. Results of Pulse Reaction of Methane over NiO-CaO (Ni/Ca ) 3.0) Catalyst at 700 °C (Pulse, 1 mL of Pure Methane; Carrier Gas, He at Flow Rate of 30 cm3‚min-1; Amount of Catalyst, 20 mg) pulse number CH4 conversion (%)
1 0.5
2 1.8
4 4.1
6 6.8
10 6.4
15 1.4
20 0.9
with the steam reforming. For the feed of composition CH4:CO2:H2O ) 1.0:0.55:0.55, the coke formation on the catalyst at 850 °C is expected to be thermodynamically much less favored than that for the feed of composition CH4:CO2 ) 1:1 (Rostrup-Nielsen, 1984). Hence, the coke formation in the simultaneous steam and CO2 reforming is much lower than that in the CO2 reforming alone. In the case of steam reforming, the feed composition (CH4:H2O ) 1.0:1.1) at the reaction conditions lies outside the carbon-forming regime, but for the CO2 reforming (CH4:CO2 ) 1:1) case, there is a possibility of carbon formation from an equilibrium gas mixture (Rostrup-Nielsen, 1984). There is a small but significant increase in the methane conversion activity in the steam reforming (Figure 4b) as well as in the simultaneous CO2 and steam reforming (Figure 4c) in the short initial period. This is mostly due to reduction of the catalyst (NiO f Ni0) followed by some changes in the surface properties of the catalyst during the short initial period. The catalyst is expected to be reduced in the reforming reactions during the short initial period first by methane by its reaction with the lattice oxygen from NiO
NiO f Ni0 + carbon oxides and water
(1)
and then by the H2 produced in the reforming reactions over the partially reduced catalyst. The initiation of the catalyst reduction by reaction 1 has been confirmed by the pulse reaction of pure methane over the catalyst at 700 °C (Table 4). The methane conversion passes through a maximum with increasing pulse number. The increase in the methane conversion for the first few pulses suggests that the catalyst reduction process is an autocatalytic one. The decrease in the methane conversion after the sixth pulse is due to depletion of the lattice oxygen from the NiO of the catalyst. CO2 Reforming of Methane Results showing the influence of temperature and GHSV on the conversion of methane and CO2, H2 selectivity, and H2/CO product mole ratio in the CO2 reforming of methane are presented in Figure 5. Because of the rapid coke deposition on the catalyst, a fresh catalyst is used for each run. The following important observations are made from these results: (a) The influence of the reaction temperature and space velocity is quite strong. (b) The H2 selectivity and H2/CO product ratio increased with increasing temperature but decreased with increasing space velocity. (c) The catalyst shows high activity and selectivity in the CO2 reforming of methane.
Figure 5. Influence of (a) temperature and (b) space velocity on the CO2 reforming of methane to syngas over NiO-CaO (Ni/Ca ) 3.0). [(a) GHSV ) 27 690 cm3‚g-1‚h-1 and CO2/CH4 ) 1.1; (b) T ) 800 °C (O,4) and 850 °C (b,2); CO2/CH4 ) 1.1.]
Reactions involved in the CO2 reforming of methane are as follows:
CH4 + CO2 f 2CO + 2H2
(2)
CO2 + H2 f CO + H2O
(3)
The selectivity for CO (based on CH4 conversion) in this process is always 100%, and the H2/CO product ratio is less than 1 due to the reverse shift reaction (reaction 3). The observed increase in the H2 selectivity with increasing temperature or with decreasing space velocity indicates that the CO2 hydrogenation (reaction 3), though thermodynamically favored at high temperatures, becomes less important than the CO2 reforming (reaction 2) at higher temperatures and/or at higher contact times (i.e., at lower space velocities). Steam Reforming of Methane Results of the steam reforming of methane over the NiO-CaO catalyst (Ni/Ca ) 3.0) at different temperatures and space velocities (H2O/CH4 mole ratio in feed ) 1.1) are presented in Figure 6. The conversion of methane and water is increased markedly with increasing reaction temperature and/or contact time. The influence of the contact time at higher temperature (850 °C) is, however, less pronounced. In this case, the H2 selectivity (based on methane conversion) is always
3938 Ind. Eng. Chem. Res., Vol. 35, No. 11, 1996
Figure 6. Influence of (a) temperature and (b) space velocity on the steam reforming of methane to syngas over NiO-CaO (Ni/Ca ) 3.0). [(a) GHSV ) 27 600 cm3‚g-1‚h-1 and H2O/CH4 ) 1.1; (b) T ) 800 °C (O,4) and 850 °C (b,2); H2O/CH4 ) 1.]
100%. However, the CO selectivity is increased and consequently the H2/CO product mole ratio is decreased, with increasing the temperature and/or contact time. The H2/CO ratio is always greater than 3 because of the water gas shift reaction (which is thermodynamically favored at lower temperatures)
CO + H2O f CO2 + H2
(4)
occurring simultaneously with the steam reforming of methane.
CH4 + H2O f CO + 3H2
(5)
The catalyst shows high activity (high methane conversion at low contact time) and high CO selectivity, particularly at the higher temperatures (g800 °C), in the steam reforming process. Simultaneous CO2 and Steam Reforming The simultaneous CO2 and steam reforming of methane was carried out by passing a mixture of methane, CO2, and steam with different concentrations and space velocities over the NiO-CaO catalyst (Ni/Ca ) 3.0) at different temperatures. The results on the simultaneous reforming reactions at different process conditions are presented in Figure 7 and Table 5. The influence of the Ni/Ca ratio of the catalyst on its activity and on the H2/CO product ratio selectivity in the simultaneous reforming reactions at different temperatures (600-850 °C) is also investigated. The conversion of the three reactants is increased and the H2/CO ratio is decreased with increasing temperature (Figure 7a,b). However, when the contact time is increased (i.e., space velocity is decreased), both the conversion of all the reactants and the H2/CO product ratio are increased (Figure 7b). It may be noted that when the CO2 reforming and steam reforming reactions occur simultaneously, the selectivity for both H2 and CO (based on methane conversion) is always 100%. This is because of the fact
Figure 7. Influence of (a) temperature and (b) space velocity on the simultaneous CO2 and steam reforming of methane to syngas over NiO-CaO (Ni/Ca ) 3.0). [(a) GHSV ) 26 900 cm3‚g-1‚h-1, CO2/CH4 ) 0.55, and H2O/CH4 ) 0.55; (b) T ) 800 °C (O) and 850 °C (b); CO2/CH4 ) 0.55 and H2O/CH4 ) 0.55.] Table 5. Influence of (CO2 + H2O)/CH4 and CO2/H2O Ratios in Feed on the Conversion and H2/CO Ratio in the Simultaneous Steam and CO2 Reforming of Methane over NiO-CaO (Ni/Ca ) 3.0) Catalyst (GHSV ) 19 700 cm3‚g-1‚h-1) feed ratio CH4 H2/CO ratio temp (°C) (CO2 + H2O)/CH4 CO2/H2O conversion (%) in products 800 800 850 850 850 850 850
1.1 1.7 1.7 1.1 1.1 1.1 1.1
0.90 0.45 0.45 0.05 0.30 1.00 1.82
96.8 98.8 99.8 98.5 99.0 98.5 99.3
1.65 2.07 2.13 2.80 2.20 1.62 1.40
that there is no net formation of CO2 or H2O in the simultaneous reactions. The observed variation in the H2/CO ratio is, therefore, expected to be due to the relative variation in the conversion of H2O and CO2 with the changes in the process parameters. The result in Table 5 shows a strong influence of the relative concentration of CO2 and steam (i.e., CO2/H2O mole ratio) in the feed, particularly on the H2/CO product ratio. The product ratio is decreased from 2.8 to 1.4 with increasing the CO2/H2O ratio in the feed from 0.05 to 1.82. Thus, by carrying out the reforming reactions simultaneously, it is possible to obtain syngas with a H2/CO ratio between 1.0 and 3.0 (more conveniently between 1.5 and 2.5) by manipulating the relative concentration of CO2 and steam in the feed (i.e., by controlling the extent of methane reforming by CO2 and steam, according to reactions 2 and 5, respectively). Moreover, very high conversion of methane (almost close to 100%) can be obtained (Table 5). By carrying out the CO2 reforming simultaneously with the steam reforming over the NiO-CaO catalyst, the following three major advantages of great practical importance can be achieved:
Ind. Eng. Chem. Res., Vol. 35, No. 11, 1996 3939
the NiO-CaO catalyst, (a) the carbon deposition on the catalyst (which is rapid in the case of the CO2 reforming alone) is drastically reduced, (b) there is no net formation of CO2 and H2O and hence the selectivity for both H2 and CO is 100%, and (c) a desirable H2/CO ratio (about 2.0 or between 1.5 and 2.5) can be obtained conveniently by manipulating the CO2/H2O ratio in the feed. Moreover, methane can be converted to syngas with almost complete conversion (at g800 °C) at low contact time (GHSV ≈ 20 000 cm3‚g-1‚h-1). Literature Cited
Figure 8. Influence of Ni/Ca mole ratio in NiO-CaO catalyst on its performance in the simultaneous CO2 and steam reforming of methane at different temperatures. [GHSV ) 31 500 cm3‚g-1‚h-1, CO2/CH4 ) 0.55, and H2O/CH4 ) 0.55.]
1. The carbon deposition, which is very rapid in the CO2 reforming, is drastically reduced. 2. Since there is no net formation of CO2 and H2O (as these are reactants), the selectivity (based on the methane conversion) for both H2 and CO is always 100%. 3. Syngas with the desirable H2/CO ratio (about 2.0 or between 1.5 and 2.5) required for the methanol and Fischer-Tropsch synthesis processes can be obtained by manipulating the CO2/H2O ratio in the feed. The results in Figure 8 show the influence of the Ni/ Ca ratio of the catalyst on the conversion of methane, CO2, and H2O and on the H2/CO product ratio in the simultaneous reforming reaction. The influence on the catalytic activity at different temperatures is quite complex, and it is relatively less pronounced at the higher temperature. Conclusions This investigation leads to the following important conclusions: 1. NiO-CaO (containing two distinct phases, NiO and CaO) shows high catalytic activity and selectivity in both the CO2 and steam reforming reactions at g800 °C for the conversion of methane to syngas. However, the carbon deposition in the CO2 reforming is quite fast. 2. The nickel oxide from the catalyst is reduced to metallic nickel during the short initial period of the reforming reactions; the catalyst in its operating state is Ni0 dispersed on CaO. 3. When the CO2 reforming reaction is carried out simultaneously with the steam reforming reaction over
Ashcroft, A. T.; Cheetham, A. K.; Green, M. L. H.; Vernon, P. D. F. Partial Oxidation of Methane to Synthesis Gas Using Carbon Dioxide. Nature 1991, 352, 225-226. Brown, R.; Cooper, M. E.; Whan, D. A. Temperature Programmed Reduction of Alumina-Supported Iron, Cobalt and Nickel Bimetallic Catalysts. Appl. Catal. 1982, 3, 177. Chen, Y.; Ren, J. Conversion of Methane and Carbon Dioxide into Synthesis Gas over Alumina-Supported Nickel Catalysts: Effect of Ni-Al2O3 Interaction. Catal. Lett. 1994, 29, 39-48. Choudhary, V. R.; Rane, V. H. A Novel Method for Measuring Base Strength Distribution on Solid Catalysts Under Operating Conditions. Catal. Lett. 1990, 4, 101-106. Choudhary, V. R.; Rajput, A. M.; Prabhakar, B. Low Temperature Oxidative Conversion of Methane to Syngas over NiO-CaO catalysts. Catal. Lett. 1992, 15, 363-370. Choudhary, V. R.; Rajput, A. M.; Prabhakar, B. NiO/CaOCatalyzed Formation of Syngas by Coupled Exothermic Oxidative Conversion and Endothermic CO2 and Steam Reforming of Methane. Angew. Chem., Int. Ed. Engl. 1994, 33, 2104-2106. Choudhary, V. R.; Rajput, A. M.; Prabhakar, B. Energy Efficient Methane-to-Syngas Conversion with Low H2/CO Ratio by Simultaneous Catalytic Reaction of Methane with Carbon Dioxide and Oxygen. Catal. Lett. 1995, 32, 391-396. Erdohelyi, A.; Cserenyi, J.; Solymosi, F. J. Activation of CH4 and its Reaction with CO2 over Supported Rh Catalysts. J. Catal. 1993, 141, 287-299. Nakamura, J.; Aikawa, K.; Sato, K.; Uchijima, T. Role of Support in Reforming of CH4 with CO2 over Rh Catalysts. Catal. Lett. 1994, 25, 265-270. Perera, J. S. H. Q.; Couves, J. W.; Sankar, G.; Thomas, J. M. The Catalytic Activity of Ru and Ir Supported on Eu2O3 for the Reaction, CO2 + CH4 S 2H2 + 2CO; a Viable Solar-thermal Energy System. Catal. Lett. 1991, 11, 219-226. Richardson, J. T.; Paripatyadar, S. A. Carbon Dioxide Reforming of Methane with Supported Rhodium. Appl. Catal. 1990, 61, 293-309. Rostrup-Nielsen, J. R. Catalytic Stream Reforming. In Catalysis, Science and Technology; Anderson, J. R., Boudart, M., Eds.; Springer: Berlin, 1984; Vol. 5, p 1. Rostrup-Nielsen, J. R.; Bak Hansen, J. H. CO2-Reforming of Methane over Transition Metals. J. Catal. 1993, 144, 38-49. Solymosi, F.; Kutsan, G.; Erdohelyi, A. Catalytic Reaction of CH4 with CO2 over Alumina-supported Pt Metals. Catal. Lett. 1991, 11, 149-156. Teuner, S. A New Process to Make Oxo-feed. Hydrocarbon Process. 1987, 52. Yamazaki, O.; Nozaki, T.; Omata, K.; Fujimoto, K. Reduction of Carbon Dioxide by Methane with Ni-on-MgO-CaO Containing Catalysts. Chem. Lett. 1992, 1953-1954.
Received for review January 2, 1996 Revised manuscript received May 13, 1996 Accepted August 15, 1996X IE960002L
X Abstract published in Advance ACS Abstracts, October 1, 1996.
