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Deactivation of supported nickel catalysts during the reforming of methane by carbon dioxide
ELSEVIER
catalysis today Catalysis Today 21 ( 1994) 571-578
Deactivation of supported nickel catalysts during the reforming of methane by carbon dioxide H.M. Swaan, V.C.H. Kroll, G.A. Martin, C. Mirodatos * Institut de Recherches sur la Catalyst-, 2 avenue Albert Einstern. 69626 Villeurbanne Cedex, France
Abstract Several supported nickel catalysts were tested for the methane reforming reaction at 700°C. The initial activity is found to depend essentially on the state of the nickel phase (reduction and dispersion) and little on its environment (support, additive). The product distribution is controlled by the WGS equilibrium. The zero order aging process is mostly due to carbon deposition, though slight Ni sintering also occurs. Among the deposited carbon, only a stable form, possibly arising from the CO disproportionation, would poison the Ni particles. Another form, less stable and arising from methane activation, is rapidly accumulated on the catalyst, but at a low level and limited extent.
1. Introduction The reforming of methane by carbon dioxide has recently received a renewed interest being able to generate syngas with a CO/H2 ratio around 1 / 1, adapted to specific syntheses such as the production of alcohols. Gadalla et al. [ 1 ] have studied nickel based catalysts for this reaction at 900°C using a C02/CH4 ratio equal to 2/ 1. It was reported that only nickel supported on alpha alumina was stable, while other supports showed decomposition or reaction with nickel. Sintering of Ni particles was also observed, as expected when working above the Tamman temperature (600°C). Operating at lower temperatures enhances however the coke formation. Recently, Rostrup-Nielsen et al. [ 21 have shown that Ni/MgO is coke resistant only after being passivated by sulphur. Precious metals have also successfully been employed for this reaction [ 3-51; however, their market price renders their industrial use questionable. The present work is aimed at investigating the deactivation process on nickel based catalysts in more detail. Several catalysts using various supports and pro* Corresponding author. 0920-5861/94/$07.00 0 1994 Elsevier Science B.V. All rights reserved ssDI0920-5861(94)00124-3
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H.M. Swaan et al. /Catalysis
Today 21 (1994) 571-578
moters known to inhibit coke deposition were prepared and tested and the possible causes of deactivation (coke deposition or nickel sintering) were successively analysed.
2. Experimental All catalysts, presented in Table 1, were prepared by impregnating various supports with [ Ni( NH3)J NO, solutions, including promoter addition [ 61. After evaporation, the precursors were dried and calcined at 750°C for 8 h. A quartz tubular reactor was filled with 0.10 g of catalyst (particle size 0.2-0.3 mm) and consequently reduced in situ at 750°C for 10 h under flowing HZ/He ( 10/90) or pure H, (flow rate: 1.2 l/h, temperature rise: 2.5”C/min). The catalysts were tested between 400 and 750°C under a standard feed consisting of CO,/CH,/ Ar= 15/7.5/77.5 with a flow rate of 6 l/h. Consequently the stability of the catalysts was tested at 700°C using the feed CO,/CH,/Ar= 15/15/70. Temperature programmed oxidation (TPO) of carbon deposits was performed by flowing over the aged catalysts a mixture Oz/He = 30/70 (4.2 l/h) from 25 to 800°C with a temperature rise of 20”C/min. In order to determine the origin of the carbon deposits, a catalytic test was carried out under a mixture ‘3CI&/COJ He = 919182 for 10 min followed by a flash TPO (by introducing the reactor into the furnace, preheated at 700°C). Gases were analysed by gas chromatography and on-line mass spectrometry. The degree of Ni reduction and the average metal particle size were determined using the Weiss extraction magnetic method [6] in a special cell allowing both catalysts testing and magnetic measurements. Table I Characterisation and activity of nickel based catalysts for methane reforming ’ Catalyst
N1/SiOZ NJLazOl NJMgO NLIZIQ Ni/TiO, NI/A120j-SiO? Ni-K/SiO, N+CulSiO,
Ni wt.-%
Promoter wt.-%
4.0 4.4 3 41
_ _
2.6 6.2 40 34
0.12 3.6
Percentage of reduction
42 n.d. 31 100 100 100 40 50
NI particle size/nm Before reaction
After reaction
5.4 n.d. 8.3 13.5 13.4 11.0 5.2 5.3
5.9 n.d. n.d. n.d. n.d. n d. 6.1 n d.
Reaction temp. (“C) required for X(CH,)=50%
Percentage of deactivation (%)/h
550 550 730 530 > 750 625 740 575
0.7 1.3 1.3 1.1 n.d. 3.7 0.8 b 3.1
a n.d. = not determined. “In order to compare the stability at a slmdar degree of conversIon, the stabihty of the Ni-KISiO, using a flow of 3 I/h and 100 mg of catalyst.
catalyst was tested
H.M. Swaan et al. /Catalysis
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3. Results 3.1. Catalyst characterisation
before reaction
As reported in Table 1, a large range of values was observed for nickel reduction and particle size after catalyst calcination and reduction. It is observed that the catalysts with a larger degree of reduction show a smaller nickel dispersion. This is in agreement with data already obtained at this laboratory though after slightly different activation conditions [ 61. This correlation probably arises from several factors such as the size of the precursor particle, the support basicity and the chemical interaction which may develop between nickel ions and the support. The reduction of NilMgO is probably hindered while the Ni2+ ions are inserted in the MgO matrix during the calcination. The rather low degree of reduction of reduced Ni/SiOz can be caused by the formation of either nickel silicate with the support or a Si-Ni alloy during the reduction, which is not fe~oma~etic 171. 3.2. Catalyst activity and aging As can be seen in Table 1, the reactor temperature needed for achieving 50% CH4 conversion varies in a wide range as a function of support: Ni/SiO;?, Ni/Z& Ni--Cu/SiOl and Ni/Laz03 presented a similar activity while a conversion of only 5% was achieved at 550°C for NilMgO. NifA1,03-Si0, also showed relatively low activity. For Ni/TiO,, some conversion was observed at 400°C however the activity was lost within 1 h on stream. A similar and moderate deactivation (Table 1) was observed for Ni/ZrO,, Nil La203, NilSiO:! and Ni-K/SiOz: after 12 h on stream the activity was about 80% of the initial activity; furthermore the activity decreased linearly indicating a zero order process. In contrast, the rate of deactivation of Ni/A1203-Si02 and Ni-Cu/ SiO, was found to be much higher. From a morphological point of view, only slight sintering of the nickel phase occurred during the reforming reaction (Table l), while the degree of reduction was found to increase significantly (from 42 to 64% in the case of the Ni/SiOz catalyst). Actually, this implies that the active surface has increased during time on stream. Accordingly, the catalyst aging has to be related to another factor such as coke formation, rather than to a sintering effect. 3.3. Product distribution The product distribution was studied in more detail during the aging test at 7OO”‘C.Fig. 1 plots the experiments concentration of hydrogen as a function of the concentration of carbon monoxide, obtained for several catalysts at different conversions, along with the theoretical relationship determined from the water-gas shift (WGS) equilibrium. It can be seen that the Hz/CO ratio varies from almost
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20
* XX
0 0
15
5
20
[C&Ol%
Fig. 1.H, vs. CO concentration during C& reforming at 700°C for various catalysts. ( A, Ni/ZrO,; 0, Ni/La203; 7, Ni-K/SiO?, X , Ni-Cu/SiOz; X, Ni/SiO,-A&O3 and the corresponding curve calculated from the WCS equilibrium, feed CO>/CH,/Ar= 15/15/70).
1 / 1 at complete
methane conversion to l/2 at low conversion, following strictly the WGS equilibrium. This feature strongly suggests that, due to the WGS reaction, a certain amount of CO2 and H, is converted to CO and H,O, or the reverse. At low CH4 conversion, this amount of converted Hz is relatively large, giving a low HZ/CO ratio. At high CH4 conversion, the amount of H2 converted by WGS is relatively low, giving a HZ/CO ratio of almost 1 to 1. These results indicate that the CH4 reforming selectivity is thermodynamically determined in the case of all the tested nickel catalysts, irrespective of the supports and promoters. 3.4. Aging process of the NiLGO, sample In order to determine the exact nature of the aging process, a combined study of (i) the changes in morphology and state of the nickel phase during the reaction and (ii) the nature of the coke deposits was carried out on one of the most stable and active catalysts, Ni/SiO,?. The catalyst was initially reduced for 10 h, used for reaction during 2 h and afterwards oxidised by TPO. Consequently, the same material was re-reduced for 10 h, used for the reaction during 6 h and reoxidised again by TPO (Table 2). A similar catalytic cycle including an aging step of 20 h was also carried out. A marked effect of the first reaction step (2 h on stream) is to increase the degree of nickel reduction from 47 to 78%, along with a slight particle sintering. After the TPO procedure the nickel was converted to nickel oxide (disappearance of ferromagnetic signal). The subsequent reduction of the catalyst material gave an enhanced reduction (61%) and a particle size almost the same as before the TPO experiment. The increase of nickel reduction during the reforming reaction was unexpected since the reaction occurred at a lower temperature than during the reduction step (700°C vs. 750°C) and in an atmosphere with a lower reduction potential (10 vs. 100 vol. % H,) . A plausible explanation is that the water formed during the reaction
H.M. Swam et al. /Catalysts Today 21(1994) 571-578
515
Tabte 2 Changes in nickel morphology and carbon deposition observed on Ni/SiO? during methane reforming a Procedure steps
Degree of reduction/
Average particle size/
%
nm
10 h reduction at 750°C
47
2 h reaction at 700°C 10 h reduction at 750°C 6 h reaction at 700°C 20 h reaction at 700°C
78 61 69 n.d.
Dispersion/ %
Atomic C/Ni’ from TPO
6.0
18
_
6.6 6.6 6.9 n.d.
17 1-l 16 n.d.
0.11 0.25 2.43
a n.d. = not determined
enhances the decomposition of the nickel silicate or Ni-Si alloy formed during the reduction [ 6,7]. This decomposition tends to be essentially irreversible since after the second reduction step, a higher degree of reduction was achieved, followed by a smaller effect of the reforming reaction (Table 2). Fig. 2 shows the CO, signal as a function of temperature during the TPO experiments performed after 2,6 and 20 h on stream. No CO was formed during the TF’O experiments while only traces of Hz0 were detected. This indicates that the carbon deposits oxidised into CO, still contained some hydrogen. The atomic ratio of the total amount of carbon (calculated from the integration of the CO2 signal) to the amount of reduced nickel (from magnetic measurements after the reaction) is reported in Table 2. All TPO spectra show essentially two peaks which markedly increase with time on stream: a first low temperature peak of weak intensity (at around 35O’C) and a second one of high intensity at higher temperature (between 600 and 750°C). Note that the shift towards Iower temperatures observed for the latter peak as the amount of carbon increases is most likely related to a in-away phenomenon caused by the oxidation exothermicity. Note also that the increase of the amount of carbon, especially the most stable one oxidised at high temperature, is nearly proportional to the time on reforming stream, which is in line with the linear deactivation observed previously. Both forms of carbon do not affect signif-
J 0
100
zoo
300 400 500 TemperatureI “C
600
700
800
Fig. 2. TPO profiles after different times on stream of CH, reforming at 700°C on Ni/SiO? 20 h.
(a), 2 h; (b), 6 h; (c),
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H.M. Swaan et al. /Catalysis Today 21 (1994) 571-578
Fig. 3. Flash TPO profile after 10 min on “CH.,/“CO,/He
stream at 700°C on Ni/Si02.
icantly the ferromagnetic signal, which discards any hypothesis of bulk nickel carbide or interstitial carbon formation (indeed the two latter species are not ferromagnetic)
.
Fig. 3 reports a flash TPO experiment carried out after an aging period of 10 min under ‘3CH4/‘2C02/He stream. Two peaks are observed for both 12C02 (amu 44) and 13C02 signals (amu 45) : a sharp one at around 500°C and a broad one at around 650°C. It can be deduced from the previous TPO experiments carried out with a much slower temperature rise, that the two peaks of the flash TPO correspond to the two peaks previously detected by the normal TPO. It can thus be noted that the low temperature peak is predominantly formed of 13C (76%)) i.e., arising from 13CH4 in the gas feed, while the broad high temperature peak is predominantly formed from 12C02 in the gas feed.
4. Discussion Close catalytic activities were found for a number of catalysts, except for Nil MgO, Ni/Ti02 and Ni-K/Si02 which are less active. For Ni/MgO, a possible stabilisation of Ni2+ ions into the MgO matrix would limit the reducibility of nickel, as observed experimentally, and therefore the active phase for the reforming reaction. For Ni/Ti02, a decoration of the active phase by titania, which has often been reported after high temperature reduction (SMSI effect, [ 81) would also limit the nickel accessibility, in a way similar to Ni-K/Si02 for which a decoration effect of the active phase by K+ ions has been demonstrated elsewhere [9]. Therefore, except for the case where an additive or a support may hinder the formation of nickel particles, the catalytic activity of nickel-based catalysts appears to depend essentially on the nickel phase and not on the support nature. In line with this, it is also noted that a catalyst presenting a low dispersion of nickel but a high degree of reduction (Ni/ZrO,) is initially as active as a partially reduced catalyst with a high nickel dispersion ( Ni/Si02). This feature tends to indicate that the active Ni phase is actually the metal and not the oxide. Note however that the less dispersed catalyst is more easily deactivated than the highly dispersed one (Table 1) .
H.M. Swaan et al. /Catalysis Today 21 (1994) 571-578
577
The selectivity appears to be controlled only by the WGS equilibrium, confirming the negligible influence of the nickel environment (support, additive, aging) on the catalytic process. Due to a minor sintering effect, the main origin of the catalyst aging has clearly been assigned to carbon deposition. From TPO experiments, at least two types of carbon deposits are identified on Ni/SiO,: The first type of carbon, oxidised at low temperature, would preferentially be formed within the early period of the catalytic test. Tracer experiments indicate that it mostly originates from the methane molecules. The accumulation of this unstable carbon remains limited whatever the aging time, and there is no indication of a large chemical interaction of that carbon with the nickel phase (leading, for instance, to the formation of a bulk carbide). However the formation of a surface nickel carbide arising from the dissociative adsorption of methane could be considered. As a matter of fact, N&C surface carbide species are generally observed in CO/H2 chemistry [ 10,111. Unfortunately, the low dispersion and the low content of nickel prevented any clear detection of such a carbide phase by magnetic measurements. The second type of carbon identified by TPO is much more stable towards oxidation, and its accumulation closely follows the deactivation rate. Not interacting chemically with nickel, this noxious form of carbon could progressively encapsulate the nickel particles, hindering the access of the reacting gases to the active surface, in agreement with early studies of carbon deposits in steam reforming and methanation units [ 10,111. This progressive poisoning of the active surface agrees with the zero order observed in the deactivation process. For the case of porous supports such as Ni/A1,03-SiO,, the deactivation rate could be accelerated by effects of pore blocking. From the tracer TPO experiment, it is observed that this toxic form of carbon is formed both from methane and carbon dioxide reacting molecules. Recent isotopic transient experiments [ 121 have revealed that the gaseous CO formed during the reforming reaction originates both from CH4 and CO*, in close proportion to the one observed for the toxic carbon (around 40160). It is therefore tentatively suggested that this type of carbon arises from the Boudouard reaction 2 CO + C + CO,, which would act as a slow side-reaction in the overall reforming process.
Acknowledgements This work has been supported by the EEC Joule II Programme. Dr S. Lacombe is acknowledged for isotopic experiments and fruitful discussions.
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References [I] [2] [3] [4] [5]
A.M Gadalla and B. Bower, Chem. Eng. Sci., 43 ( 1988) 3049. J.R. Rostrup-Nielsen and J.-H. Bak Hansen, J. Catal., 144 (1993) 38. J.T. Richardson and S.A. Paripatyadar, Appl. Catal., 61 (1990) 293. A.T. Ashcroft. A.K. Cheetman, M.L.M. Green and P.D.F. Vernon, Nature, 352 (1991) 225. K. Seshan, H.W. ten Barge, W. Hally, A.N.J. van Keulen and J.R.H. Ross, Proc. Natural Gas Conversion Symposium, Sydney 1993. [6] P. Turlier, H. Praliaud, P. Moral, G.A. Martin and J.A. Dalmon, Appl. Catal., 19 ( 1985) 287. [7] H. Praliaud and G.A. Martin, J. Catal., 72 (1981) 394. [8] R. Burch and A.R. Flambart, Stud. Surf Sci. Catal., 11 (1982) 193. [9J H. Praliaud, M. Primet and G.A. Martin, Applic. Surf. SC., 17 (1983) 107. [ IO] C.H. Bartholomew. Catal. Rev.-Sci. Eng., 24 (1982) 67. [ 111J.R. Rostrup-Nielsen and P.E. Hojlund Nielsen, in J. Oudar and H. Wise (Eds.), Deactivation and Poisoning of Catalysts, Marcel Dekker Inc., New York, 1985, p. 259. [ 121 V.C.H. Kroll, S. Lacombe, H.M. Swaan and C. Mirodatos, to be published.
catalysis today Catalysis Today 21 ( 1994) 571-578
Deactivation of supported nickel catalysts during the reforming of methane by carbon dioxide H.M. Swaan, V.C.H. Kroll, G.A. Martin, C. Mirodatos * Institut de Recherches sur la Catalyst-, 2 avenue Albert Einstern. 69626 Villeurbanne Cedex, France
Abstract Several supported nickel catalysts were tested for the methane reforming reaction at 700°C. The initial activity is found to depend essentially on the state of the nickel phase (reduction and dispersion) and little on its environment (support, additive). The product distribution is controlled by the WGS equilibrium. The zero order aging process is mostly due to carbon deposition, though slight Ni sintering also occurs. Among the deposited carbon, only a stable form, possibly arising from the CO disproportionation, would poison the Ni particles. Another form, less stable and arising from methane activation, is rapidly accumulated on the catalyst, but at a low level and limited extent.
1. Introduction The reforming of methane by carbon dioxide has recently received a renewed interest being able to generate syngas with a CO/H2 ratio around 1 / 1, adapted to specific syntheses such as the production of alcohols. Gadalla et al. [ 1 ] have studied nickel based catalysts for this reaction at 900°C using a C02/CH4 ratio equal to 2/ 1. It was reported that only nickel supported on alpha alumina was stable, while other supports showed decomposition or reaction with nickel. Sintering of Ni particles was also observed, as expected when working above the Tamman temperature (600°C). Operating at lower temperatures enhances however the coke formation. Recently, Rostrup-Nielsen et al. [ 21 have shown that Ni/MgO is coke resistant only after being passivated by sulphur. Precious metals have also successfully been employed for this reaction [ 3-51; however, their market price renders their industrial use questionable. The present work is aimed at investigating the deactivation process on nickel based catalysts in more detail. Several catalysts using various supports and pro* Corresponding author. 0920-5861/94/$07.00 0 1994 Elsevier Science B.V. All rights reserved ssDI0920-5861(94)00124-3
572
H.M. Swaan et al. /Catalysis
Today 21 (1994) 571-578
moters known to inhibit coke deposition were prepared and tested and the possible causes of deactivation (coke deposition or nickel sintering) were successively analysed.
2. Experimental All catalysts, presented in Table 1, were prepared by impregnating various supports with [ Ni( NH3)J NO, solutions, including promoter addition [ 61. After evaporation, the precursors were dried and calcined at 750°C for 8 h. A quartz tubular reactor was filled with 0.10 g of catalyst (particle size 0.2-0.3 mm) and consequently reduced in situ at 750°C for 10 h under flowing HZ/He ( 10/90) or pure H, (flow rate: 1.2 l/h, temperature rise: 2.5”C/min). The catalysts were tested between 400 and 750°C under a standard feed consisting of CO,/CH,/ Ar= 15/7.5/77.5 with a flow rate of 6 l/h. Consequently the stability of the catalysts was tested at 700°C using the feed CO,/CH,/Ar= 15/15/70. Temperature programmed oxidation (TPO) of carbon deposits was performed by flowing over the aged catalysts a mixture Oz/He = 30/70 (4.2 l/h) from 25 to 800°C with a temperature rise of 20”C/min. In order to determine the origin of the carbon deposits, a catalytic test was carried out under a mixture ‘3CI&/COJ He = 919182 for 10 min followed by a flash TPO (by introducing the reactor into the furnace, preheated at 700°C). Gases were analysed by gas chromatography and on-line mass spectrometry. The degree of Ni reduction and the average metal particle size were determined using the Weiss extraction magnetic method [6] in a special cell allowing both catalysts testing and magnetic measurements. Table I Characterisation and activity of nickel based catalysts for methane reforming ’ Catalyst
N1/SiOZ NJLazOl NJMgO NLIZIQ Ni/TiO, NI/A120j-SiO? Ni-K/SiO, N+CulSiO,
Ni wt.-%
Promoter wt.-%
4.0 4.4 3 41
_ _
2.6 6.2 40 34
0.12 3.6
Percentage of reduction
42 n.d. 31 100 100 100 40 50
NI particle size/nm Before reaction
After reaction
5.4 n.d. 8.3 13.5 13.4 11.0 5.2 5.3
5.9 n.d. n.d. n.d. n.d. n d. 6.1 n d.
Reaction temp. (“C) required for X(CH,)=50%
Percentage of deactivation (%)/h
550 550 730 530 > 750 625 740 575
0.7 1.3 1.3 1.1 n.d. 3.7 0.8 b 3.1
a n.d. = not determined. “In order to compare the stability at a slmdar degree of conversIon, the stabihty of the Ni-KISiO, using a flow of 3 I/h and 100 mg of catalyst.
catalyst was tested
H.M. Swaan et al. /Catalysis
Today 21 (I994) 571-578
513
3. Results 3.1. Catalyst characterisation
before reaction
As reported in Table 1, a large range of values was observed for nickel reduction and particle size after catalyst calcination and reduction. It is observed that the catalysts with a larger degree of reduction show a smaller nickel dispersion. This is in agreement with data already obtained at this laboratory though after slightly different activation conditions [ 61. This correlation probably arises from several factors such as the size of the precursor particle, the support basicity and the chemical interaction which may develop between nickel ions and the support. The reduction of NilMgO is probably hindered while the Ni2+ ions are inserted in the MgO matrix during the calcination. The rather low degree of reduction of reduced Ni/SiOz can be caused by the formation of either nickel silicate with the support or a Si-Ni alloy during the reduction, which is not fe~oma~etic 171. 3.2. Catalyst activity and aging As can be seen in Table 1, the reactor temperature needed for achieving 50% CH4 conversion varies in a wide range as a function of support: Ni/SiO;?, Ni/Z& Ni--Cu/SiOl and Ni/Laz03 presented a similar activity while a conversion of only 5% was achieved at 550°C for NilMgO. NifA1,03-Si0, also showed relatively low activity. For Ni/TiO,, some conversion was observed at 400°C however the activity was lost within 1 h on stream. A similar and moderate deactivation (Table 1) was observed for Ni/ZrO,, Nil La203, NilSiO:! and Ni-K/SiOz: after 12 h on stream the activity was about 80% of the initial activity; furthermore the activity decreased linearly indicating a zero order process. In contrast, the rate of deactivation of Ni/A1203-Si02 and Ni-Cu/ SiO, was found to be much higher. From a morphological point of view, only slight sintering of the nickel phase occurred during the reforming reaction (Table l), while the degree of reduction was found to increase significantly (from 42 to 64% in the case of the Ni/SiOz catalyst). Actually, this implies that the active surface has increased during time on stream. Accordingly, the catalyst aging has to be related to another factor such as coke formation, rather than to a sintering effect. 3.3. Product distribution The product distribution was studied in more detail during the aging test at 7OO”‘C.Fig. 1 plots the experiments concentration of hydrogen as a function of the concentration of carbon monoxide, obtained for several catalysts at different conversions, along with the theoretical relationship determined from the water-gas shift (WGS) equilibrium. It can be seen that the Hz/CO ratio varies from almost
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H.M. Swaan et al. /Catalysts
Today 21 (1994) 571-578
20
* XX
0 0
15
5
20
[C&Ol%
Fig. 1.H, vs. CO concentration during C& reforming at 700°C for various catalysts. ( A, Ni/ZrO,; 0, Ni/La203; 7, Ni-K/SiO?, X , Ni-Cu/SiOz; X, Ni/SiO,-A&O3 and the corresponding curve calculated from the WCS equilibrium, feed CO>/CH,/Ar= 15/15/70).
1 / 1 at complete
methane conversion to l/2 at low conversion, following strictly the WGS equilibrium. This feature strongly suggests that, due to the WGS reaction, a certain amount of CO2 and H, is converted to CO and H,O, or the reverse. At low CH4 conversion, this amount of converted Hz is relatively large, giving a low HZ/CO ratio. At high CH4 conversion, the amount of H2 converted by WGS is relatively low, giving a HZ/CO ratio of almost 1 to 1. These results indicate that the CH4 reforming selectivity is thermodynamically determined in the case of all the tested nickel catalysts, irrespective of the supports and promoters. 3.4. Aging process of the NiLGO, sample In order to determine the exact nature of the aging process, a combined study of (i) the changes in morphology and state of the nickel phase during the reaction and (ii) the nature of the coke deposits was carried out on one of the most stable and active catalysts, Ni/SiO,?. The catalyst was initially reduced for 10 h, used for reaction during 2 h and afterwards oxidised by TPO. Consequently, the same material was re-reduced for 10 h, used for the reaction during 6 h and reoxidised again by TPO (Table 2). A similar catalytic cycle including an aging step of 20 h was also carried out. A marked effect of the first reaction step (2 h on stream) is to increase the degree of nickel reduction from 47 to 78%, along with a slight particle sintering. After the TPO procedure the nickel was converted to nickel oxide (disappearance of ferromagnetic signal). The subsequent reduction of the catalyst material gave an enhanced reduction (61%) and a particle size almost the same as before the TPO experiment. The increase of nickel reduction during the reforming reaction was unexpected since the reaction occurred at a lower temperature than during the reduction step (700°C vs. 750°C) and in an atmosphere with a lower reduction potential (10 vs. 100 vol. % H,) . A plausible explanation is that the water formed during the reaction
H.M. Swam et al. /Catalysts Today 21(1994) 571-578
515
Tabte 2 Changes in nickel morphology and carbon deposition observed on Ni/SiO? during methane reforming a Procedure steps
Degree of reduction/
Average particle size/
%
nm
10 h reduction at 750°C
47
2 h reaction at 700°C 10 h reduction at 750°C 6 h reaction at 700°C 20 h reaction at 700°C
78 61 69 n.d.
Dispersion/ %
Atomic C/Ni’ from TPO
6.0
18
_
6.6 6.6 6.9 n.d.
17 1-l 16 n.d.
0.11 0.25 2.43
a n.d. = not determined
enhances the decomposition of the nickel silicate or Ni-Si alloy formed during the reduction [ 6,7]. This decomposition tends to be essentially irreversible since after the second reduction step, a higher degree of reduction was achieved, followed by a smaller effect of the reforming reaction (Table 2). Fig. 2 shows the CO, signal as a function of temperature during the TPO experiments performed after 2,6 and 20 h on stream. No CO was formed during the TF’O experiments while only traces of Hz0 were detected. This indicates that the carbon deposits oxidised into CO, still contained some hydrogen. The atomic ratio of the total amount of carbon (calculated from the integration of the CO2 signal) to the amount of reduced nickel (from magnetic measurements after the reaction) is reported in Table 2. All TPO spectra show essentially two peaks which markedly increase with time on stream: a first low temperature peak of weak intensity (at around 35O’C) and a second one of high intensity at higher temperature (between 600 and 750°C). Note that the shift towards Iower temperatures observed for the latter peak as the amount of carbon increases is most likely related to a in-away phenomenon caused by the oxidation exothermicity. Note also that the increase of the amount of carbon, especially the most stable one oxidised at high temperature, is nearly proportional to the time on reforming stream, which is in line with the linear deactivation observed previously. Both forms of carbon do not affect signif-
J 0
100
zoo
300 400 500 TemperatureI “C
600
700
800
Fig. 2. TPO profiles after different times on stream of CH, reforming at 700°C on Ni/SiO? 20 h.
(a), 2 h; (b), 6 h; (c),
516
H.M. Swaan et al. /Catalysis Today 21 (1994) 571-578
Fig. 3. Flash TPO profile after 10 min on “CH.,/“CO,/He
stream at 700°C on Ni/Si02.
icantly the ferromagnetic signal, which discards any hypothesis of bulk nickel carbide or interstitial carbon formation (indeed the two latter species are not ferromagnetic)
.
Fig. 3 reports a flash TPO experiment carried out after an aging period of 10 min under ‘3CH4/‘2C02/He stream. Two peaks are observed for both 12C02 (amu 44) and 13C02 signals (amu 45) : a sharp one at around 500°C and a broad one at around 650°C. It can be deduced from the previous TPO experiments carried out with a much slower temperature rise, that the two peaks of the flash TPO correspond to the two peaks previously detected by the normal TPO. It can thus be noted that the low temperature peak is predominantly formed of 13C (76%)) i.e., arising from 13CH4 in the gas feed, while the broad high temperature peak is predominantly formed from 12C02 in the gas feed.
4. Discussion Close catalytic activities were found for a number of catalysts, except for Nil MgO, Ni/Ti02 and Ni-K/Si02 which are less active. For Ni/MgO, a possible stabilisation of Ni2+ ions into the MgO matrix would limit the reducibility of nickel, as observed experimentally, and therefore the active phase for the reforming reaction. For Ni/Ti02, a decoration of the active phase by titania, which has often been reported after high temperature reduction (SMSI effect, [ 81) would also limit the nickel accessibility, in a way similar to Ni-K/Si02 for which a decoration effect of the active phase by K+ ions has been demonstrated elsewhere [9]. Therefore, except for the case where an additive or a support may hinder the formation of nickel particles, the catalytic activity of nickel-based catalysts appears to depend essentially on the nickel phase and not on the support nature. In line with this, it is also noted that a catalyst presenting a low dispersion of nickel but a high degree of reduction (Ni/ZrO,) is initially as active as a partially reduced catalyst with a high nickel dispersion ( Ni/Si02). This feature tends to indicate that the active Ni phase is actually the metal and not the oxide. Note however that the less dispersed catalyst is more easily deactivated than the highly dispersed one (Table 1) .
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The selectivity appears to be controlled only by the WGS equilibrium, confirming the negligible influence of the nickel environment (support, additive, aging) on the catalytic process. Due to a minor sintering effect, the main origin of the catalyst aging has clearly been assigned to carbon deposition. From TPO experiments, at least two types of carbon deposits are identified on Ni/SiO,: The first type of carbon, oxidised at low temperature, would preferentially be formed within the early period of the catalytic test. Tracer experiments indicate that it mostly originates from the methane molecules. The accumulation of this unstable carbon remains limited whatever the aging time, and there is no indication of a large chemical interaction of that carbon with the nickel phase (leading, for instance, to the formation of a bulk carbide). However the formation of a surface nickel carbide arising from the dissociative adsorption of methane could be considered. As a matter of fact, N&C surface carbide species are generally observed in CO/H2 chemistry [ 10,111. Unfortunately, the low dispersion and the low content of nickel prevented any clear detection of such a carbide phase by magnetic measurements. The second type of carbon identified by TPO is much more stable towards oxidation, and its accumulation closely follows the deactivation rate. Not interacting chemically with nickel, this noxious form of carbon could progressively encapsulate the nickel particles, hindering the access of the reacting gases to the active surface, in agreement with early studies of carbon deposits in steam reforming and methanation units [ 10,111. This progressive poisoning of the active surface agrees with the zero order observed in the deactivation process. For the case of porous supports such as Ni/A1,03-SiO,, the deactivation rate could be accelerated by effects of pore blocking. From the tracer TPO experiment, it is observed that this toxic form of carbon is formed both from methane and carbon dioxide reacting molecules. Recent isotopic transient experiments [ 121 have revealed that the gaseous CO formed during the reforming reaction originates both from CH4 and CO*, in close proportion to the one observed for the toxic carbon (around 40160). It is therefore tentatively suggested that this type of carbon arises from the Boudouard reaction 2 CO + C + CO,, which would act as a slow side-reaction in the overall reforming process.
Acknowledgements This work has been supported by the EEC Joule II Programme. Dr S. Lacombe is acknowledged for isotopic experiments and fruitful discussions.
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