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Combination of carbon dioxide reforming and partial oxidation of methane over supported platinum catalysts
Applied Catalysis A: General 255 (2003) 83–92
Combination of carbon dioxide reforming and partial oxidation of methane over supported platinum catalysts Mariana M.V.M. Souza a , Martin Schmal a,b,∗ a
NUCAT/PEQ/COPPE, Universidade Federal do Rio de Janeiro, C.P. 68502, 21945-970 Rio de Janeiro, Brazil Escola de Qu´ımica, Universidade Federal do Rio de Janeiro, C.P. 68542, 21940-900 Rio de Janeiro, Brazil
b
Received 29 October 2002; received in revised form 21 January 2003; accepted 21 January 2003
Abstract CO2 reforming and partial oxidation of methane were carried out using Pt/Al2 O3 , Pt/ZrO2 , and Pt/10%ZrO2 /Al2 O3 catalysts, in the temperature range 450–900 ◦ C. The Pt/10%ZrO2 /Al2 O3 was found to be the most active and stable catalyst for combined CO2 reforming and partial oxidation of methane. The addition of O2 to the feed increases CH4 conversion and the catalyst stability. The deactivation is primarily due to coke formation, which was characterized by thermogravimetric analysis and transmission electron microscopy. The reduction of carbon deposition over zirconia-alumina catalyst is related to Pt-Zrn + interactions and the high oxygen mobility on zirconia. © 2003 Elsevier B.V. All rights reserved. Keywords: CO2 reforming; Methane; Supported platinum catalysts; Partial oxidation
1. Introduction Carbon dioxide reforming has attracted substantial interest over the past 10 years for both economic and environmental reasons. The CH4 /CO2 reforming produces a mixture of hydrogen and carbon monoxide (syngas) which may be used industrially for the synthesis of chemical products with high added values, such as hydrocarbons, oxygenated compounds, and polycarbonates [1,2]. Traditionally, synthesis gas has been produced by the steam reforming of methane, despite its high operation cost due to heat demand [3]. Reforming with CO2 , rather than steam, is attractive for certain applications because it yields syngas with lower H2 /CO ratios, which is a preferable feedstock for Fischer–Tropsch synthesis and production of oxy∗ Corresponding author. Fax: +55-21-2562-8300. E-mail address: [email protected] (M. Schmal).
genated compounds, possible usage in energy storage or transmission systems and it provides a means of disposing CO2 , which is considered as harmful greenhouse gas [2,4,5]. The primary difficulty associated with the applicability of the CO2 reforming is the high thermodynamic potential to form coke under the high operating temperatures required to obtain significant conversions. The higher carbon content in the feedstream compared with partial oxidation or steam reforming is thought to be responsible for the higher level of carbon deposition on CO2 reforming catalysts [6,7]. Addition of oxygen to the carbon dioxide reforming can reduce the carbon deposition on the catalytic surface and increase methane conversion, although this can also cause the reduction of process selectivity [8]. Additionally, as CO2 reforming is a highly endothermic process and thus requires a large amount of energy to proceed, its coupling with the exothermic partial
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oxidation would also have economic advantages. The combination of these reactions can improve the reactor temperature control and reduce the formation of hot spots, besides allowing the production of syngas with a wider range of H2 /CO ratios (H2 /CO can be varied between 1 and 2 by manipulating the relative concentrations of O2 and CO2 in the feed). Combined partial oxidation and CO2 reforming of methane was first studied by Vernon et al. [9], which reported that high yields of synthesis gas can be obtained using a 1%Ir/Al2 O3 catalyst at 1050 K, without carbon deposition. They also manipulated the CH4 :CO2 :O2 ratio in order to achieve a thermoneutral reaction, potentially advantageous from an engineering point of view. Choudhary and coworkers [10,11] have reported simultaneous catalytic reactions of methane with CO2 and O2 over NiO/CaO and NiO/MgO catalysts, obtaining conversions above 95% without deactivation during 20 h on stream at 800 ◦ C. Ruckenstein and Hu [12] also found that NiO/MgO catalyst has high activity and selectivity in the combined reaction and the formation of a solid solution inhibited the carbon deposition. Our own research has shown that Pt/ZrO2 /Al2 O3 catalysts are active for the CO2 reforming of methane and the catalyst with 10 wt.% of ZrO2 remained stable up to 60 h of reaction at 800 ◦ C [13,14]. In this paper, we investigate the coupling between the catalytic partial oxidation and the CO2 reforming of methane over the Pt/10%ZrO2 /Al2 O3 catalyst and, for comparison purposes, Pt/Al2 O3 and Pt/ZrO2 were also investigated. 2. Experimental 2.1. Catalyst preparation The ␥-Al2 O3 support (Harshaw, Al 3996) was calcined in flowing air at 550 ◦ C for 2 h. The ZrO2 support was prepared by calcination of zirconium hydroxide (MEL Chemicals) using the same conditions. The 10%ZrO2 /Al2 O3 was obtained by impregnation of alumina with a nitric acid solution of zirconium hydroxide, as described elsewhere [13]. The supports were impregnated with an aqueous solution of chloroplatinic acid (H2 PtCl6 , Aldrich) by incipient wetness
technique. The catalysts were subsequently dried overnight at 120 ◦ C and calcined in air at 550 ◦ C for 2 h. The platinum content was around 1 wt.%. 2.2. Catalyst testing Before each catalytic test, the catalysts were dried in situ with flowing nitrogen at 150 ◦ C for 30 min, following the reduction with 10%H2 /N2 at a rate of 10 ◦ C/min to 500 ◦ C and remaining at this temperature for 1 h. After reduction, the catalyst was purged with N2 for 30 min at the same temperature. The reaction was carried out in a fixed-bed flow-type quartz reactor loaded with 20 mg of catalyst, under atmospheric pressure. A thermocouple was placed on top of the catalyst bed to measure catalyst temperature. The total feed flow rate was held constant at 200 cm3 /min (WHSV = 600,000 cm3 /h g cat), with flowing He. The gas compositions are listed in Table 1. The activity tests were performed at different temperatures, ranging from 450 to 900 ◦ C in steps of 50 ◦ C that were kept for 30 min at each temperature. The loss in catalyst activity at 800 ◦ C was monitored up to 60 h on stream. The reaction products were analyzed by on-line gas chromatograph (CHROMPACK CP9001), equipped with a Hayesep D column and a thermal conductivity detector. 2.3. Carbon deposition measurements The amount of coke formed over the catalysts after deactivation tests at 800 ◦ C was examined by thermogravimetric analysis (TGA), using a RIGAKU thermoanalyzer (model TAS 100). The samples were pretreated at 150 ◦ C under flowing nitrogen and then heated at a rate of 10 ◦ C/min to 800 ◦ C in a flow of 15%O2 /N2 (50 cm3 /min). TEM measurements were performed in a JEOL JEM-2000FX apparatus. The samples were not Table 1 Flow rates for activity and deactivation tests (cm3 /min) Test no.
CH4
CO2
O2
He
Total
1 2 3 4
10 20 20 10
10 10 10 0
0 5 10 5
180 165 160 185
200 200 200 200
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pretreated. Particle diameters were measured using the dark field enlargements micrographs and counting over 200 particles.
Table 3 CH4 conversions during partial oxidation of methane from 450 to 800 ◦ C Temperature (◦ C)
CH4 conversion (%) Pt/Al2 O3
Pt/ZrO2
Pt/10%ZrO2 /Al2 O3
450 500 550 600 650 700 750 800
1.8 34.0 62.5 72.5 80.3 84.5 90.1 100
39.9 43.8 50.9 57.4 61.4 77.9 90.4 100
3.4 56.5 66.4 73.9 74.2 85.3 93.0 100
3. Results and discussion 3.1. CO2 reforming and partial oxidation of methane The activities for both CO2 reforming and partial oxidation of methane on the PtAl, PtZr, and Pt10ZrAl catalysts were investigated using diluted feeds; the deactivation behavior of the catalysts at 800 ◦ C were also examined. The results for CO2 reforming and partial oxidation are presented in Tables 2 and 3, respectively. The Pt10ZrAl catalyst emerged as the most active for CO2 reforming over the whole temperature range investigated, as described in our previous paper [13]. This catalyst was also the most stable at 800 ◦ C: the deactivation rate was only 0.1%/h during 60 h on stream. The PtAl catalyst exhibited a high deactivation rate of 4%/h, during 20 h on stream at 800 ◦ C, due to the fast deposition of inactive carbon [13]. The rapid deactivation of Pt/Al2 O3 catalysts during CO2 reforming was also reported in the literature by Van Keulen et al. [3] and Bitter et al. [15], which was attributed to carbon deposition on metallic sites. For the case of partial oxidation, all three catalysts presented similar activities, mainly at high temperatures. The Pt10ZrAl catalyst presented the higher Table 2 CH4 conversions during CO2 reforming of methane between 450 and 900 ◦ C [13] for reaction conditions of test 1, as presented in Table 1 Temperature (◦ C)
450 500 550 600 650 700 750 800 850 900 Deactivation rate at 800 ◦ C
CH4 conversion (%) Pt/ZrO2
Pt/10%ZrO2 /Al2 O3
4.5 9.1 18.0 30.3 43.7 56.5 68.6 76.8 81.5 83.6
4.3 8.5 15.4 21.5 29.6 47.8 71.2 86.9 90.1 90.1
5.5 10.7 19.9 32.5 46.9 61.6 74.8 84.6 90.9 93.5
4.0%/h
0.3%/h
Deactivation rate at 800 ◦ C
0.21%/h
0.1%/h
0.18%/h
0.03%/h
Conditions: test 4, Table 1 (conversion of O2 = 100%, except for Pt/Al2 O3 and Pt/10%ZrO2 /Al2 O3 at 450 ◦ C).
stability again and the light deactivation of PtAl and PtZr at 800 ◦ C is most likely due to coking. The activity of PtAl was similar to that reported by Vernon et al. [9], which closely attained the thermodynamic equilibrium values. For all catalysts, it was observed the formation of CO2 and H2 O with temperatures up to 650 ◦ C, which clearly indicates the total combustion of CH4 . CH4 +2O2 ⇔ CO2 +2H2 O H298 K = −802 kJ/mol (I) However, with increasing temperature, the selectivity of syngas increased, due to the reforming of methane with steam and carbon dioxide produced in reaction (I). This was also observed in the literature for different catalysts [8,9,16]. CH4 + H2 O ⇔ CO + 3H2
Pt/Al2 O3
85
H298 K = 206 kJ/mol (II)
CH4 + CO2 ⇔ 2CO + 2H2
H298 K = 247 kJ/mol (III)
3.2. Combined CO2 reforming and partial oxidation of methane The comparison of catalyst activities for combined CO2 reforming and partial oxidation of methane is displayed in Figs. 1 and 2, in terms of CH4 conversion
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CH4 conversion (%)
80
CO2/CH4= 0.5 O2/CH 4= 0.25
60
40
PtAl PtZr Pt10ZrAl
20
0
300
400
500
600
700
800
900
o
Temperature ( C) Fig. 1. CH4 conversion of Pt catalysts for combined CO2 reforming and partial oxidation of methane as a function of temperature (each point was taken after 30 min on stream). Condition: test 2, Table 1.
1,5
PtAl PtZr Pt10ZrAl
1,4
H2/CO
1,3
1,2
1,1
CO 2/CH 4= 0.5 O 2/CH4= 0.25
1,0
0,9 400
500
600
700
800
900
o
Temperature ( C) Fig. 2. H2 /CO product ratio for combined CO2 reforming and partial oxidation of methane as a function of temperature. Condition: test 2, Table 1.
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O2/CH4= 0.25 CO2/CH 4=0.5
composition (%)
60
H2 O2 CO
87
CH4 CO2 H 2O
40
20
0
300
400
500
600
700
800
900
o
Temperature ( C) Fig. 3. Composition profile for combined CO2 reforming and partial oxidation of methane as a function of temperature over the Pt/10%ZrO2 /Al2 O3 catalyst. Condition: test 2, Table 1.
and H2 /CO product ratio, respectively. All three catalysts exhibited similar activities, with the PtAl being the most active at lower temperatures and the PtZr the most active one at temperatures higher than 700 ◦ C. The activity was almost constant in temperature range between 450 and 600 ◦ C, which can be related to the combustion of methane to CO2 and H2 O, a very exothermic reaction. With increasing temperature, the conversion of CH4 and CO2 increased, while the H2 /CO ratio decreased. At low temperatures, the H2 /CO ratio is most influenced by the partial oxidation of methane and the water-gas shift reaction: CH4 + 21 O2 ⇔ CO + 2H2 H298 K = −36 kJ/mol (IV) CO2 + H2 ⇔ CO + H2 O
H298 K = 41 kJ/mol (V)
At high temperatures, the H2 /CO ratio is practically only due to CO2 reforming of methane (reaction (III)). Using the same feed ratio, Ruckenstein and Hu [12] obtained a H2 /CO ratio of 1.3 over nickel catalysts, at 790 ◦ C. These catalysts probably have higher ac-
tivity to CH4 oxidation than the Pt catalysts studied here. The composition profiles for combined CO2 reforming and partial oxidation of methane, as a function of temperature over Pt10ZrAl catalyst, are shown in Fig. 3. The O2 conversion is 100% starting from 450 ◦ C. The production of steam starts at 450 ◦ C and begin to decrease at 600 ◦ C, disappearing almost completely at 800 ◦ C. This profile also confirms that methane combustion occurred to a greater extent at lower temperatures, which explains the H2 O production and decreased CO2 conversion. The effect of oxygen addition on CH4 conversion at various temperatures between 350 and 900 ◦ C for Pt10ZrAl catalyst is shown in Fig. 4. The addition of O2 to the feed increased methane conversion, mainly at low temperatures, when the combustion of methane is enhanced. The same trend as that predicted by thermodynamic calculations [17,18] was observed here; the effect of increasing CH4 conversion is more pronounced at lower temperatures and the H2 selectivity decreased with increased amounts of O2 in the feed stream. However, the occurrence of methane combustion at lower temperatures does not fully account for the high CH4 conversion, because the high
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100
CH4 conversion (%)
80
60
40
CO2 reforming 2.5 vol% O2 5 vol% O 2 partial oxidation
20
0 300
400
500
600
700
800
900
o
Temperature ( C) Fig. 4. CH4 conversion vs. temperature for combined CO2 reforming and partial oxidation of methane using different gas compositions over the Pt/10%ZrO2 /Al2 O3 catalyst. Condition: Table 1.
selectivity to syngas. As the methane combustion is highly exothermic, it should increase the catalyst bed temperature and probably causes an increase in CO2 and steam reforming activity, leading to higher CH4 conversion and selectivity to syngas. 3.3. Deactivation studies The comparison of catalyst stabilities for combined CO2 reforming and partial oxidation of methane at 800 ◦ C is displayed in Fig. 5. Although all catalysts exhibit similar initial activities, the stability behavior differs completely with time on stream. The PtAl and PtZr deactivated very fast during 30 h on stream, with a deactivation rate of about 0.9 and 1.1%/h, respectively, while the Pt10ZrAl remained stable up to 54 h of reaction, with a deactivation rate of only 0.15%/h. The amount of coking on these catalysts after the deactivation test at 800 ◦ C was quantified by TGA measurements, carried out in an oxygen-containing atmosphere (Fig. 6). The stability of the Pt10ZrAl catalyst is really associated with the observation of little coke formation during the reaction. On the other hand, TGA experiment showed a weight loss of about 8 and 10% on treating the PtAl and PtZr catalysts in oxygen, indicating a significant amount of car-
bon deposition during 30 h on stream (approximately 6.7 mg coke/g cat h). Indeed, TEM measurements after the deactivation tests on PtZr and Pt10ZrAl are clearly distinguishable (Fig. 7). Firstly, the catalysts themselves are different. The PtZr displays large and heterogeneous ZrO2 particles and a large amount of carbon deposition in distinguished zones (with an average size of about 10 nm). On the other hand, Pt10ZrAl shows very homogeneous distribution of small ZrO2 particles and, in addition, small amount of carbon deposited in very small particles over the zirconia surface, in good agreement with the TGA measurements. The effect of oxygen addition on CH4 conversion with time on stream at 800 ◦ C of the Pt10ZrAl catalyst is shown in Fig. 8. For CO2 reforming, the CH4 conversion decreased by 5% over 60 h on stream, while for partial oxidation it decreased by only 2% over the same period of time. When the feed contains CH4 , CO2 and O2 , the CH4 conversion decay was dependent on the oxygen content and it decreased to a greater extent when less oxygen is present: CH4 conversion decreased by 6 and 3% on addition of 2.5 and 5 vol.% O2 , respectively. Thus, the addition of O2 greatly prevents the deactivation of the catalyst, which can be related to less carbon formation over the catalyst when
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89
95
CO 2/CH4= 0.5 O 2/CH4= 0.25
CH4 conversion (%)
90 85 80 75 70
PtAl PtZr Pt10ZrAl
65 60 0
5
10
15
20
25
30
35
40
45
50
55
60
time on stream (h) Fig. 5. CH4 conversion as a function of time on stream at 800 ◦ C for combined CO2 reforming and partial oxidation of methane. Condition: test 2, Table 1.
O2 is present in the feed above 5 vol.% of O2 . The effect of oxygen in the methane reforming was also observed previously in reference [8], and indeed, in our case, 2.5% of oxygen in the feed was not sufficient
to prevent coke formation. TEM pictures display coke formation on these samples, which confirm the TGA and catalytic deactivation test, that even with partial oxidation coke cannot be eliminated at all, after more
100
Pt10ZrAl (54 h on stream)
weight loss (%)
98
96
94
PtAl (30h on stream) 92
PtZr (30 h on stream)
90
0
200
400
600
800
1000
o
Temperature ( C) Fig. 6. TGA of PtAl, PtZr, and Pt10ZrAl catalysts after combined CO2 reforming and partial oxidation of methane at 800 ◦ C. Conditions: 15%O2 /N2 , 10 ◦ C/min, and feed flow rate = 50 cm3 /min.
90
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Fig. 7. Transmission electron micrographs of (a) PtZr and (b) Pt10ZrAl catalysts after combined CO2 reforming and partial oxidation of methane at 800 ◦ C.
than 50 h on stream. The main reason is that the concentration of oxygen needed to burn coke formation is not sufficient. However, due to the mobility of oxygen in the Pt10ZrAl catalyst, coke formation can be inhibited. The higher stability of the catalyst with 10% of ZrO2 is closely related to its coking resistivity, which has been attributed to Pt-Zrn + interactions at metal–support interface [13]. The stability of this catalyst is comparable with that related for nickel catalysts [12] or Pt/CoAl2 O4 /Al2 O3 [19] under similar conditions of temperature and molar feed ratio. As reported in our previous paper [13], the interfacial sites on Pt-ZrOx are active for CO adsorption and CO2 dissociation, providing active species of oxygen that may react with carbon formed by CH4 decomposition on the metal particle, suppressing carbon accumulation. Moreover, zirconia is a well-known oxygen supplier, and its oxygen mobility is fast, which helps to keep the metal surface free of carbon. Therefore, the presence of molecular oxygen in the feed promotes the oxygen supply, enhancing the combustion of deposited carbon and preventing the carbon deposition. When zirconia is dispersed over alumina, Pt surface is not extensively recovered by ZrOx [13]. Thus, the Pt-ZrOx interface in ZrO2 /Al2 O3 systems appears to be more active and stable. In fact, we attribute mainly to the formation of Pt-Zrδ+ interface the stability of the catalyst in the reforming of methane. The proof of these interfaces was obtained by FTIR of CO, as reported in our paper [13]. In addition, the particle size of Pt on Pt10ZrAl was much smaller than on PtZr catalyst. These complementary results agreed with Bitter et al. [20], enhancing the stability of the catalyst for CO2 reforming of methane and partial oxidation of methane. Moreover, the number of interfacial sites is higher on the Pt10ZrAl catalyst compared to PtZr itself, because, as shown in the micrographs (TEM), the particle sizes of ZrO2 on Al2 O3 are very small compared to the ZrO2 particles. Therefore, the Pt particles are better dispersed on highly dispersed ZrO2 particles over Als O3 . As reported, we determined first the ZrO2 particle sizes using ISS and XPS measurements in situ [21] and observed formation of nanoparticles on Pt10ZrAl catalysts and secondly, the particle sizes of Pt by chemisorption measurements [13], revealing that on this system, the dispersion of Pt is very high. These
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91
100
CH4 conversion (%)
95
90
CO2 reforming 2.5 vol% O2 5 vol% O 2 partial oxidation
85
80
0
5
10
15
20
25
30
35
40
45
50
55
60
65
time on stream (h) Fig. 8. CH4 conversion as a function of time on stream at 800 ◦ C for combined CO2 reforming and partial oxidation of methane using different gas compositions over the Pt/10%ZrO2 /Al2 O3 catalyst. Condition: Table 1.
results are in good agreement with above statements supporting markedly the enhancing stability.
4. Conclusions The Pt/10%ZrO2 /Al2 O3 is the most active and stable catalyst for carbon dioxide reforming, partial oxidation of methane and for these two reactions combined, when compared to Pt/Al2 O3 and Pt/ZrO2 . The composition profile for combined CO2 reforming and partial oxidation of methane showed that methane combustion is the preferential reaction at lower temperatures, followed by reforming of the remainder of the methane by CO2 and the resultant steam. The addition of O2 to the feed increases methane conversion and decreases the H2 selectivity. The loss of catalyst activity with time on stream decreases markedly with the amount of oxygen added to the feed, preventing or reducing coke formation over the catalyst surface. The nature of coke formed during combined reaction is influenced by the support, with small and highly dispersed particles over zirconia-alumina catalyst. The higher stability of the Pt/10%ZrO2 /Al2 O3 catalyst is closely related to its coking resistivity, which is attributed to the Pt-Zrn + interactions at metal–support interface.
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Combination of carbon dioxide reforming and partial oxidation of methane over supported platinum catalysts Mariana M.V.M. Souza a , Martin Schmal a,b,∗ a
NUCAT/PEQ/COPPE, Universidade Federal do Rio de Janeiro, C.P. 68502, 21945-970 Rio de Janeiro, Brazil Escola de Qu´ımica, Universidade Federal do Rio de Janeiro, C.P. 68542, 21940-900 Rio de Janeiro, Brazil
b
Received 29 October 2002; received in revised form 21 January 2003; accepted 21 January 2003
Abstract CO2 reforming and partial oxidation of methane were carried out using Pt/Al2 O3 , Pt/ZrO2 , and Pt/10%ZrO2 /Al2 O3 catalysts, in the temperature range 450–900 ◦ C. The Pt/10%ZrO2 /Al2 O3 was found to be the most active and stable catalyst for combined CO2 reforming and partial oxidation of methane. The addition of O2 to the feed increases CH4 conversion and the catalyst stability. The deactivation is primarily due to coke formation, which was characterized by thermogravimetric analysis and transmission electron microscopy. The reduction of carbon deposition over zirconia-alumina catalyst is related to Pt-Zrn + interactions and the high oxygen mobility on zirconia. © 2003 Elsevier B.V. All rights reserved. Keywords: CO2 reforming; Methane; Supported platinum catalysts; Partial oxidation
1. Introduction Carbon dioxide reforming has attracted substantial interest over the past 10 years for both economic and environmental reasons. The CH4 /CO2 reforming produces a mixture of hydrogen and carbon monoxide (syngas) which may be used industrially for the synthesis of chemical products with high added values, such as hydrocarbons, oxygenated compounds, and polycarbonates [1,2]. Traditionally, synthesis gas has been produced by the steam reforming of methane, despite its high operation cost due to heat demand [3]. Reforming with CO2 , rather than steam, is attractive for certain applications because it yields syngas with lower H2 /CO ratios, which is a preferable feedstock for Fischer–Tropsch synthesis and production of oxy∗ Corresponding author. Fax: +55-21-2562-8300. E-mail address: [email protected] (M. Schmal).
genated compounds, possible usage in energy storage or transmission systems and it provides a means of disposing CO2 , which is considered as harmful greenhouse gas [2,4,5]. The primary difficulty associated with the applicability of the CO2 reforming is the high thermodynamic potential to form coke under the high operating temperatures required to obtain significant conversions. The higher carbon content in the feedstream compared with partial oxidation or steam reforming is thought to be responsible for the higher level of carbon deposition on CO2 reforming catalysts [6,7]. Addition of oxygen to the carbon dioxide reforming can reduce the carbon deposition on the catalytic surface and increase methane conversion, although this can also cause the reduction of process selectivity [8]. Additionally, as CO2 reforming is a highly endothermic process and thus requires a large amount of energy to proceed, its coupling with the exothermic partial
0926-860X/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0926-860X(03)00646-X
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oxidation would also have economic advantages. The combination of these reactions can improve the reactor temperature control and reduce the formation of hot spots, besides allowing the production of syngas with a wider range of H2 /CO ratios (H2 /CO can be varied between 1 and 2 by manipulating the relative concentrations of O2 and CO2 in the feed). Combined partial oxidation and CO2 reforming of methane was first studied by Vernon et al. [9], which reported that high yields of synthesis gas can be obtained using a 1%Ir/Al2 O3 catalyst at 1050 K, without carbon deposition. They also manipulated the CH4 :CO2 :O2 ratio in order to achieve a thermoneutral reaction, potentially advantageous from an engineering point of view. Choudhary and coworkers [10,11] have reported simultaneous catalytic reactions of methane with CO2 and O2 over NiO/CaO and NiO/MgO catalysts, obtaining conversions above 95% without deactivation during 20 h on stream at 800 ◦ C. Ruckenstein and Hu [12] also found that NiO/MgO catalyst has high activity and selectivity in the combined reaction and the formation of a solid solution inhibited the carbon deposition. Our own research has shown that Pt/ZrO2 /Al2 O3 catalysts are active for the CO2 reforming of methane and the catalyst with 10 wt.% of ZrO2 remained stable up to 60 h of reaction at 800 ◦ C [13,14]. In this paper, we investigate the coupling between the catalytic partial oxidation and the CO2 reforming of methane over the Pt/10%ZrO2 /Al2 O3 catalyst and, for comparison purposes, Pt/Al2 O3 and Pt/ZrO2 were also investigated. 2. Experimental 2.1. Catalyst preparation The ␥-Al2 O3 support (Harshaw, Al 3996) was calcined in flowing air at 550 ◦ C for 2 h. The ZrO2 support was prepared by calcination of zirconium hydroxide (MEL Chemicals) using the same conditions. The 10%ZrO2 /Al2 O3 was obtained by impregnation of alumina with a nitric acid solution of zirconium hydroxide, as described elsewhere [13]. The supports were impregnated with an aqueous solution of chloroplatinic acid (H2 PtCl6 , Aldrich) by incipient wetness
technique. The catalysts were subsequently dried overnight at 120 ◦ C and calcined in air at 550 ◦ C for 2 h. The platinum content was around 1 wt.%. 2.2. Catalyst testing Before each catalytic test, the catalysts were dried in situ with flowing nitrogen at 150 ◦ C for 30 min, following the reduction with 10%H2 /N2 at a rate of 10 ◦ C/min to 500 ◦ C and remaining at this temperature for 1 h. After reduction, the catalyst was purged with N2 for 30 min at the same temperature. The reaction was carried out in a fixed-bed flow-type quartz reactor loaded with 20 mg of catalyst, under atmospheric pressure. A thermocouple was placed on top of the catalyst bed to measure catalyst temperature. The total feed flow rate was held constant at 200 cm3 /min (WHSV = 600,000 cm3 /h g cat), with flowing He. The gas compositions are listed in Table 1. The activity tests were performed at different temperatures, ranging from 450 to 900 ◦ C in steps of 50 ◦ C that were kept for 30 min at each temperature. The loss in catalyst activity at 800 ◦ C was monitored up to 60 h on stream. The reaction products were analyzed by on-line gas chromatograph (CHROMPACK CP9001), equipped with a Hayesep D column and a thermal conductivity detector. 2.3. Carbon deposition measurements The amount of coke formed over the catalysts after deactivation tests at 800 ◦ C was examined by thermogravimetric analysis (TGA), using a RIGAKU thermoanalyzer (model TAS 100). The samples were pretreated at 150 ◦ C under flowing nitrogen and then heated at a rate of 10 ◦ C/min to 800 ◦ C in a flow of 15%O2 /N2 (50 cm3 /min). TEM measurements were performed in a JEOL JEM-2000FX apparatus. The samples were not Table 1 Flow rates for activity and deactivation tests (cm3 /min) Test no.
CH4
CO2
O2
He
Total
1 2 3 4
10 20 20 10
10 10 10 0
0 5 10 5
180 165 160 185
200 200 200 200
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pretreated. Particle diameters were measured using the dark field enlargements micrographs and counting over 200 particles.
Table 3 CH4 conversions during partial oxidation of methane from 450 to 800 ◦ C Temperature (◦ C)
CH4 conversion (%) Pt/Al2 O3
Pt/ZrO2
Pt/10%ZrO2 /Al2 O3
450 500 550 600 650 700 750 800
1.8 34.0 62.5 72.5 80.3 84.5 90.1 100
39.9 43.8 50.9 57.4 61.4 77.9 90.4 100
3.4 56.5 66.4 73.9 74.2 85.3 93.0 100
3. Results and discussion 3.1. CO2 reforming and partial oxidation of methane The activities for both CO2 reforming and partial oxidation of methane on the PtAl, PtZr, and Pt10ZrAl catalysts were investigated using diluted feeds; the deactivation behavior of the catalysts at 800 ◦ C were also examined. The results for CO2 reforming and partial oxidation are presented in Tables 2 and 3, respectively. The Pt10ZrAl catalyst emerged as the most active for CO2 reforming over the whole temperature range investigated, as described in our previous paper [13]. This catalyst was also the most stable at 800 ◦ C: the deactivation rate was only 0.1%/h during 60 h on stream. The PtAl catalyst exhibited a high deactivation rate of 4%/h, during 20 h on stream at 800 ◦ C, due to the fast deposition of inactive carbon [13]. The rapid deactivation of Pt/Al2 O3 catalysts during CO2 reforming was also reported in the literature by Van Keulen et al. [3] and Bitter et al. [15], which was attributed to carbon deposition on metallic sites. For the case of partial oxidation, all three catalysts presented similar activities, mainly at high temperatures. The Pt10ZrAl catalyst presented the higher Table 2 CH4 conversions during CO2 reforming of methane between 450 and 900 ◦ C [13] for reaction conditions of test 1, as presented in Table 1 Temperature (◦ C)
450 500 550 600 650 700 750 800 850 900 Deactivation rate at 800 ◦ C
CH4 conversion (%) Pt/ZrO2
Pt/10%ZrO2 /Al2 O3
4.5 9.1 18.0 30.3 43.7 56.5 68.6 76.8 81.5 83.6
4.3 8.5 15.4 21.5 29.6 47.8 71.2 86.9 90.1 90.1
5.5 10.7 19.9 32.5 46.9 61.6 74.8 84.6 90.9 93.5
4.0%/h
0.3%/h
Deactivation rate at 800 ◦ C
0.21%/h
0.1%/h
0.18%/h
0.03%/h
Conditions: test 4, Table 1 (conversion of O2 = 100%, except for Pt/Al2 O3 and Pt/10%ZrO2 /Al2 O3 at 450 ◦ C).
stability again and the light deactivation of PtAl and PtZr at 800 ◦ C is most likely due to coking. The activity of PtAl was similar to that reported by Vernon et al. [9], which closely attained the thermodynamic equilibrium values. For all catalysts, it was observed the formation of CO2 and H2 O with temperatures up to 650 ◦ C, which clearly indicates the total combustion of CH4 . CH4 +2O2 ⇔ CO2 +2H2 O H298 K = −802 kJ/mol (I) However, with increasing temperature, the selectivity of syngas increased, due to the reforming of methane with steam and carbon dioxide produced in reaction (I). This was also observed in the literature for different catalysts [8,9,16]. CH4 + H2 O ⇔ CO + 3H2
Pt/Al2 O3
85
H298 K = 206 kJ/mol (II)
CH4 + CO2 ⇔ 2CO + 2H2
H298 K = 247 kJ/mol (III)
3.2. Combined CO2 reforming and partial oxidation of methane The comparison of catalyst activities for combined CO2 reforming and partial oxidation of methane is displayed in Figs. 1 and 2, in terms of CH4 conversion
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CH4 conversion (%)
80
CO2/CH4= 0.5 O2/CH 4= 0.25
60
40
PtAl PtZr Pt10ZrAl
20
0
300
400
500
600
700
800
900
o
Temperature ( C) Fig. 1. CH4 conversion of Pt catalysts for combined CO2 reforming and partial oxidation of methane as a function of temperature (each point was taken after 30 min on stream). Condition: test 2, Table 1.
1,5
PtAl PtZr Pt10ZrAl
1,4
H2/CO
1,3
1,2
1,1
CO 2/CH 4= 0.5 O 2/CH4= 0.25
1,0
0,9 400
500
600
700
800
900
o
Temperature ( C) Fig. 2. H2 /CO product ratio for combined CO2 reforming and partial oxidation of methane as a function of temperature. Condition: test 2, Table 1.
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O2/CH4= 0.25 CO2/CH 4=0.5
composition (%)
60
H2 O2 CO
87
CH4 CO2 H 2O
40
20
0
300
400
500
600
700
800
900
o
Temperature ( C) Fig. 3. Composition profile for combined CO2 reforming and partial oxidation of methane as a function of temperature over the Pt/10%ZrO2 /Al2 O3 catalyst. Condition: test 2, Table 1.
and H2 /CO product ratio, respectively. All three catalysts exhibited similar activities, with the PtAl being the most active at lower temperatures and the PtZr the most active one at temperatures higher than 700 ◦ C. The activity was almost constant in temperature range between 450 and 600 ◦ C, which can be related to the combustion of methane to CO2 and H2 O, a very exothermic reaction. With increasing temperature, the conversion of CH4 and CO2 increased, while the H2 /CO ratio decreased. At low temperatures, the H2 /CO ratio is most influenced by the partial oxidation of methane and the water-gas shift reaction: CH4 + 21 O2 ⇔ CO + 2H2 H298 K = −36 kJ/mol (IV) CO2 + H2 ⇔ CO + H2 O
H298 K = 41 kJ/mol (V)
At high temperatures, the H2 /CO ratio is practically only due to CO2 reforming of methane (reaction (III)). Using the same feed ratio, Ruckenstein and Hu [12] obtained a H2 /CO ratio of 1.3 over nickel catalysts, at 790 ◦ C. These catalysts probably have higher ac-
tivity to CH4 oxidation than the Pt catalysts studied here. The composition profiles for combined CO2 reforming and partial oxidation of methane, as a function of temperature over Pt10ZrAl catalyst, are shown in Fig. 3. The O2 conversion is 100% starting from 450 ◦ C. The production of steam starts at 450 ◦ C and begin to decrease at 600 ◦ C, disappearing almost completely at 800 ◦ C. This profile also confirms that methane combustion occurred to a greater extent at lower temperatures, which explains the H2 O production and decreased CO2 conversion. The effect of oxygen addition on CH4 conversion at various temperatures between 350 and 900 ◦ C for Pt10ZrAl catalyst is shown in Fig. 4. The addition of O2 to the feed increased methane conversion, mainly at low temperatures, when the combustion of methane is enhanced. The same trend as that predicted by thermodynamic calculations [17,18] was observed here; the effect of increasing CH4 conversion is more pronounced at lower temperatures and the H2 selectivity decreased with increased amounts of O2 in the feed stream. However, the occurrence of methane combustion at lower temperatures does not fully account for the high CH4 conversion, because the high
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100
CH4 conversion (%)
80
60
40
CO2 reforming 2.5 vol% O2 5 vol% O 2 partial oxidation
20
0 300
400
500
600
700
800
900
o
Temperature ( C) Fig. 4. CH4 conversion vs. temperature for combined CO2 reforming and partial oxidation of methane using different gas compositions over the Pt/10%ZrO2 /Al2 O3 catalyst. Condition: Table 1.
selectivity to syngas. As the methane combustion is highly exothermic, it should increase the catalyst bed temperature and probably causes an increase in CO2 and steam reforming activity, leading to higher CH4 conversion and selectivity to syngas. 3.3. Deactivation studies The comparison of catalyst stabilities for combined CO2 reforming and partial oxidation of methane at 800 ◦ C is displayed in Fig. 5. Although all catalysts exhibit similar initial activities, the stability behavior differs completely with time on stream. The PtAl and PtZr deactivated very fast during 30 h on stream, with a deactivation rate of about 0.9 and 1.1%/h, respectively, while the Pt10ZrAl remained stable up to 54 h of reaction, with a deactivation rate of only 0.15%/h. The amount of coking on these catalysts after the deactivation test at 800 ◦ C was quantified by TGA measurements, carried out in an oxygen-containing atmosphere (Fig. 6). The stability of the Pt10ZrAl catalyst is really associated with the observation of little coke formation during the reaction. On the other hand, TGA experiment showed a weight loss of about 8 and 10% on treating the PtAl and PtZr catalysts in oxygen, indicating a significant amount of car-
bon deposition during 30 h on stream (approximately 6.7 mg coke/g cat h). Indeed, TEM measurements after the deactivation tests on PtZr and Pt10ZrAl are clearly distinguishable (Fig. 7). Firstly, the catalysts themselves are different. The PtZr displays large and heterogeneous ZrO2 particles and a large amount of carbon deposition in distinguished zones (with an average size of about 10 nm). On the other hand, Pt10ZrAl shows very homogeneous distribution of small ZrO2 particles and, in addition, small amount of carbon deposited in very small particles over the zirconia surface, in good agreement with the TGA measurements. The effect of oxygen addition on CH4 conversion with time on stream at 800 ◦ C of the Pt10ZrAl catalyst is shown in Fig. 8. For CO2 reforming, the CH4 conversion decreased by 5% over 60 h on stream, while for partial oxidation it decreased by only 2% over the same period of time. When the feed contains CH4 , CO2 and O2 , the CH4 conversion decay was dependent on the oxygen content and it decreased to a greater extent when less oxygen is present: CH4 conversion decreased by 6 and 3% on addition of 2.5 and 5 vol.% O2 , respectively. Thus, the addition of O2 greatly prevents the deactivation of the catalyst, which can be related to less carbon formation over the catalyst when
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89
95
CO 2/CH4= 0.5 O 2/CH4= 0.25
CH4 conversion (%)
90 85 80 75 70
PtAl PtZr Pt10ZrAl
65 60 0
5
10
15
20
25
30
35
40
45
50
55
60
time on stream (h) Fig. 5. CH4 conversion as a function of time on stream at 800 ◦ C for combined CO2 reforming and partial oxidation of methane. Condition: test 2, Table 1.
O2 is present in the feed above 5 vol.% of O2 . The effect of oxygen in the methane reforming was also observed previously in reference [8], and indeed, in our case, 2.5% of oxygen in the feed was not sufficient
to prevent coke formation. TEM pictures display coke formation on these samples, which confirm the TGA and catalytic deactivation test, that even with partial oxidation coke cannot be eliminated at all, after more
100
Pt10ZrAl (54 h on stream)
weight loss (%)
98
96
94
PtAl (30h on stream) 92
PtZr (30 h on stream)
90
0
200
400
600
800
1000
o
Temperature ( C) Fig. 6. TGA of PtAl, PtZr, and Pt10ZrAl catalysts after combined CO2 reforming and partial oxidation of methane at 800 ◦ C. Conditions: 15%O2 /N2 , 10 ◦ C/min, and feed flow rate = 50 cm3 /min.
90
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Fig. 7. Transmission electron micrographs of (a) PtZr and (b) Pt10ZrAl catalysts after combined CO2 reforming and partial oxidation of methane at 800 ◦ C.
than 50 h on stream. The main reason is that the concentration of oxygen needed to burn coke formation is not sufficient. However, due to the mobility of oxygen in the Pt10ZrAl catalyst, coke formation can be inhibited. The higher stability of the catalyst with 10% of ZrO2 is closely related to its coking resistivity, which has been attributed to Pt-Zrn + interactions at metal–support interface [13]. The stability of this catalyst is comparable with that related for nickel catalysts [12] or Pt/CoAl2 O4 /Al2 O3 [19] under similar conditions of temperature and molar feed ratio. As reported in our previous paper [13], the interfacial sites on Pt-ZrOx are active for CO adsorption and CO2 dissociation, providing active species of oxygen that may react with carbon formed by CH4 decomposition on the metal particle, suppressing carbon accumulation. Moreover, zirconia is a well-known oxygen supplier, and its oxygen mobility is fast, which helps to keep the metal surface free of carbon. Therefore, the presence of molecular oxygen in the feed promotes the oxygen supply, enhancing the combustion of deposited carbon and preventing the carbon deposition. When zirconia is dispersed over alumina, Pt surface is not extensively recovered by ZrOx [13]. Thus, the Pt-ZrOx interface in ZrO2 /Al2 O3 systems appears to be more active and stable. In fact, we attribute mainly to the formation of Pt-Zrδ+ interface the stability of the catalyst in the reforming of methane. The proof of these interfaces was obtained by FTIR of CO, as reported in our paper [13]. In addition, the particle size of Pt on Pt10ZrAl was much smaller than on PtZr catalyst. These complementary results agreed with Bitter et al. [20], enhancing the stability of the catalyst for CO2 reforming of methane and partial oxidation of methane. Moreover, the number of interfacial sites is higher on the Pt10ZrAl catalyst compared to PtZr itself, because, as shown in the micrographs (TEM), the particle sizes of ZrO2 on Al2 O3 are very small compared to the ZrO2 particles. Therefore, the Pt particles are better dispersed on highly dispersed ZrO2 particles over Als O3 . As reported, we determined first the ZrO2 particle sizes using ISS and XPS measurements in situ [21] and observed formation of nanoparticles on Pt10ZrAl catalysts and secondly, the particle sizes of Pt by chemisorption measurements [13], revealing that on this system, the dispersion of Pt is very high. These
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91
100
CH4 conversion (%)
95
90
CO2 reforming 2.5 vol% O2 5 vol% O 2 partial oxidation
85
80
0
5
10
15
20
25
30
35
40
45
50
55
60
65
time on stream (h) Fig. 8. CH4 conversion as a function of time on stream at 800 ◦ C for combined CO2 reforming and partial oxidation of methane using different gas compositions over the Pt/10%ZrO2 /Al2 O3 catalyst. Condition: Table 1.
results are in good agreement with above statements supporting markedly the enhancing stability.
4. Conclusions The Pt/10%ZrO2 /Al2 O3 is the most active and stable catalyst for carbon dioxide reforming, partial oxidation of methane and for these two reactions combined, when compared to Pt/Al2 O3 and Pt/ZrO2 . The composition profile for combined CO2 reforming and partial oxidation of methane showed that methane combustion is the preferential reaction at lower temperatures, followed by reforming of the remainder of the methane by CO2 and the resultant steam. The addition of O2 to the feed increases methane conversion and decreases the H2 selectivity. The loss of catalyst activity with time on stream decreases markedly with the amount of oxygen added to the feed, preventing or reducing coke formation over the catalyst surface. The nature of coke formed during combined reaction is influenced by the support, with small and highly dispersed particles over zirconia-alumina catalyst. The higher stability of the Pt/10%ZrO2 /Al2 O3 catalyst is closely related to its coking resistivity, which is attributed to the Pt-Zrn + interactions at metal–support interface.
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