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Suppression of carbonaceous depositions on nickel catalyst for the carbon dioxide reforming of methane
Applied Catalysis A: General 177 (1999) 15±23
Suppression of carbonaceous depositions on nickel catalyst for the carbon dioxide reforming of methane Mitsunobu Ito, Tomohiko Tagawa, Shigeo Goto* Department of Chemical Engineering, Nagoya University, Chikusa, Nagoya 464-8603, Japan Received 12 February 1998; received in revised form 10 July 1998; accepted 10 July 1998
Abstract A new pretreatment of catalyst, deposition±removal (D±R) treatment, is proposed to suppress carbonaceous depositions on Ni/Al2O3 catalyst for carbon dioxide reforming of methane. This treatment is based on the hypothesis that active cores forming carbon whiskers are different from surface active sites for the main reaction. In the deposition step, active cores of nickel are detached from the bulk nickel surface as growing cores in carbon whiskers. In the removal step, the carbon whiskers are removed from the nickel surface. These two steps are repeated by ¯owing pure methane gas and then pure carbon dioxide gas. The thermogravimetric (TG) measurements showed that the repeated D±R treatments reduced the carbonaceous deposition. Carbon monoxide adsorption measurements showed that the nickel surface area was decreased by the treatments. The reaction rate and the turnover frequency were increased by the treatments. The increase in the activity was explained by the development of active sites on newly exposed nickel layers which strongly interacted with the support. # 1999 Elsevier Science B.V. All rights reserved. Keywords: Carbon dioxide reforming of methane; Nickel catalyst; Carbonaceous deposition; Carbon whisker
1. Introduction Methane and carbon dioxide are both major green house gases. The low-grade natural gas resources mainly consist of methane and carbon dioxide. To utilize more such gases as chemical raw materials has been strongly desired [1]. The carbon dioxide reforming of methane (Eq. (1)) is a particularly ef®cient process for producing synthesis gas from these two substrates. Though steam reforming of methane (Eq. (2)) is widely industrialized as a *Corresponding author. Tel.: +81-52-789-3261; fax: +81-52789-3261; e-mail: [email protected]
method of producing synthesis gas, the carbon dioxide reforming of methane has advantages in producing synthesis gas with low H2/CO ratio which is ef®cient for producing oxygenates such as alcohol or aldehyde [2]: CH4 CO2 2H2 2CO;
(1)
CH4 H2 O 3H2 CO:
(2)
Several supported transition metal catalysts (Ni, Ru, Rh, Pd, etc.) were used for the carbon dioxide reforming of methane [3]. Nickel catalysts are mainly used in industry due to their low costs. We reported the performance of several industrial nickel catalysts for this reaction [4]. Among the tested supports,
0926-860X/99/$ ± see front matter # 1999 Elsevier Science B.V. All rights reserved. PII: S0926-860X(98)00251-8
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alumina has been concluded to be essential to the reaction. During the reaction, a serious pressure drop in the reactor due to carbonaceous deposition was frequently observed [5]. To avoid carbonaceous deposition, several methods were proposed. For example, alkaline or alkaline earth metal was added on catalysts [6], or active sites for carbonaceous deposition were covered with sulfur [7]. In a previous paper [8], we reported that the effect of carbonaceous deposition could be suppressed using a macroporous ceramic foam catalyst. In the present paper, a new pretreatment of catalyst, deposition±removal (D±R) treatment is proposed to suppress carbonaceous deposition on Ni/Al2O3 catalyst for the carbon dioxide reforming of methane. The in¯uences of this treatment on carbonaceous deposition and on the activity of carbon dioxide reforming of methane are reported.
2. Deposition±removal (D±R) treatment The D±R treatment is designed according to the proposed reaction mechanism of this reaction. Fig. 1 shows the summary of reaction mechanism proposed in the previous paper [8]. The mechanism has been constructed on the basis of steam reforming mechanism [9]. Two types of active sites were proposed on the catalyst, the one interacted with the alumina support, the other at the surface of bulk nickel. The former mainly participated in the reforming reaction while the latter was involved with carbonaceous deposition. Carbon deposited on the surface of bulk nickel is well known to form carbon whiskers having nickel in the growing top [10]. If the nickel in the growing core can only be removed, carbonaceous deposition will be suppressed without any deactivation of reforming activity because the nickel interacted with support
Fig. 1. Model of the reaction and carbonaceous deposition.
M. Ito et al. / Applied Catalysis A: General 177 (1999) 15±23
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Fig. 2. Model of the D±R treatment.
which mainly participates in the reforming reaction is not detached. Based on the above model of carbonaceous deposition, the D±R treatment of Ni/Al2O3 catalyst is proposed for the removal of nickel in the growing core. Fig. 2 shows the model of the D±R treatment. This treatment consists of two steps. Deposition step. Pure methane gas is introduced and decomposed. CH4 C 2H2 :
(3)
The produced carbon is deposited on the catalyst as carbon whiskers. Active cores of nickel are detached from the surface of bulk nickel as growing cores in carbon whiskers. Removal step. Pure carbon dioxide gas is introduced and forced to react with carbon whiskers to produce
carbon monoxide. CO2 C 2CO:
(4)
Carbon whiskers having nickel cores can thus be removed from the surface of the catalyst. Repetition. These two steps are repeated several times. To con®rm the ef®ciency of this treatment, three kinds of measurement were conducted; thermogravimetry, nickel surface area and reaction rate of carbon dioxide reforming of methane. Additional measurements were conducted to observe the state of nickel which was removed by the D±R treatment using scanning electron microscope (SEM) and energy dispersive X-ray analyzer (EDX). Inactive nickel in the lower part of Fig. 2 can be presumed from these measurements.
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3. Catalyst preparation Catalyst was prepared by impregnating alumina (JRC-ALO4; reference catalyst of Syokubai Gakkai) with an aqueous solution of nickel nitrate. The precipitates were separated by ®ltration and washed with hot water. Then, each catalyst was dried overnight at 383 K, and calcined at 603 K for 5 h. The catalyst was crushed into 20±32 mesh size. Loading of nickel was adjusted to 40 wt% which was commonly used in the industrial steam reforming catalysts. 4. Thermogravimetric measurements 4.1. Apparatus
Fig. 3. Changes in catalyst weight in methane flow (carbon deposition step). T1000 K FCH4 1.510ÿ5 mol/s.
A 20 mg of the catalyst was placed on a plate of thermogravimetric analyzer (TGA-50, SHIMADZU). Changes in catalyst weight were measured in methane or carbon dioxide ¯ow. 4.2. Procedure Pre-step. The catalyst is reduced in hydrogen ¯ow of 5.010ÿ6 mol/s for 1 h at 1073 K. Deposition step. The change in catalyst weight is measured at 1000 K in methane ¯ow of 1.5 10ÿ5 mol/s. Temperature decrease step. The temperature is decreased to 700 K in helium ¯ow. Removal step. The change in catalyst weight is measured with increasing temperature at 5 K/min from 700 to 1000 K in carbon dioxide ¯ow of 3.010ÿ5 mol/s. Repetition. One cycle from the deposition step to the removal step (D±R treatment) is repeated several times. 4.3. Results Fig. 3 shows the effect of the cycle number n on the changes in catalyst weight, Wn, during the deposition step. The catalyst weights are increased by carbonaceous deposition. The Y-axis is normalized by the initial weight of the catalyst in the reduced state at each cycle, Wr,n. The weight of the fresh catalyst extremely increased as indicated in cycle 1.
Fig. 4. Changes in catalyst weight in carbon dioxide flow (carbon removal step) with an increase in temperature (5 K/min). FCO2 3.010ÿ5 mol/s.
As the D±R treatment is repeated, the amount of deposited carbon decreases. This suggests that this treatment makes the catalyst inactive for the carbonaceous deposition. Fig. 4 shows the effect of the cycle number on the changes in catalyst weight during the removal step. The catalyst weight decreases rapidly above 800 K. Carbon monoxide was detected in the outlet gas. Therefore, the decrease in catalyst weight was due to the reaction between carbon and carbon dioxide.
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areas of catalysts (SNi,n) were calculated by Eq. (5). SNi;n m2 =kg CNi-s;n mol=kg=Ni-s mol=m2 ; (5) where CNi-s,n (mol/kg) is the amount of carbon monoxide molecule adsorbed on the surface nickel based on the catalyst weight and Ni-s (mol/m2) is the density of surface nickel atom2.5610ÿ5 mol/m2. 5.2. Procedure
Fig. 5. Rates of the decrease in catalyst weight in carbon dioxide flow with an increase in temperature (5 K/min). FCO2 3.0 10ÿ5 mol/s.
Fig. 5 shows time differentiations of catalyst weights calculated from Fig. 4 at the removal step. Every cycle has two peaks, smaller peak at 800 K and larger one at 950 K. This suggests that two types of carbon species exist on the catalyst. The heights of smaller peaks are almost unchanged in every cycle. On the other hand, heights of larger peaks at 950 K extremely decrease as the cycle number increases. The existence of two types of carbon species coincides to the assumption that there existed two types of active sites such as whisker carbon deposition and reforming reaction. The changes in the large peak and the amount of carbonaceous deposition in Fig. 3 showed a similar tendency. Thus, the carbon species with a larger peak at 950 K might be related with carbon whiskers which were accumulated during the deposition step. The sites of former peak (800 K) might relate to the reforming active site which has stable nature with repeated the D±R treatment. 5. Nickel surface area measurements 5.1. Apparatus A pulse CO adsorption apparatus was used to determine the nickel surface area [11]. Carbon monoxide adsorbed was assumed to be the linear-type (Ni:CO1:1) equimolar adsorption. Nickel surface
A 20 mg of catalyst was packed in a quartz tube of 4.0 mm inner diameter. Pre-step. The catalyst was reduced in hydrogen ¯ow of 5.010ÿ6 mol/s for 1 h at 1073 K. Deposition step. Carbon is deposited on the catalyst by the contact with methane at 1000 K for 1 h in methane ¯ow of 1.510ÿ5 mol/s. Removal step. Deposited carbon is removed by the contact with carbon dioxide at 1000 K for 1 h in carbon dioxide ¯ow of 3.010ÿ5 mol/s. Temperature decrease step. The temperature is decreased to 293 K in helium ¯ow. CO-adsorption step before reduction. Nickel surface area is measured. Temperature increase step. The temperature is increased to 1000 K in helium ¯ow. Reduction step. The catalyst is reduced in hydrogen ¯ow of 5.010ÿ6 mol/s at 1000 K for 30 min. Temperature decrease step. The temperature is decreased to 293 K in helium ¯ow. CO-adsorption step after reduction. Nickel surface area is measured again. Repetition. One cycle from deposition step to the CO-adsorption step after reduction is repeated several times. 5.3. Results Fig. 6 shows the changes in the nickel surface area measured by the method of carbon monoxide adsorption after conducting the D±R treatment several times. The value at cycle 0 indicates the nickel surface area just after the pre-step. Once the D±R treatment is conducted, the nickel surface area decreases extremely and becomes almost zero after several cycles. However, 16% of original value were recovered by the hydrogen reduction step. This suggests that a part of
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Fig. 6. Change in the nickel surface area caused by the D±R treatment cycles: (*) before hydrogen reduction; (*) after hydrogen reduction.
Removal step. Deposited carbon is removed by a carbon dioxide ¯ow of 3.010ÿ5 mol/s at 1000 K for 1 h. Reaction step before reduction. Reaction rate is measured under standard conditions. Reduction step. The catalyst is reduced in a hydrogen ¯ow of 5.010ÿ6 mol/s at 1000 K for 30 min. Reaction step after the reduction. Reaction rate is measured again under standard conditions. Repetition. One cycle from the deposition step to the reaction step after the reduction is repeated several times. 6.3. Results
nickel remains on the support against the D±R treatment while others were removed from the catalyst. The fact that nickel surface area was observed after hydrogen reduction suggests that a part of the surface nickel might be oxidized by carbon dioxide during the removal step of the D±R treatment. 6. Reaction rate measurements 6.1. Apparatus A 10 mg of catalyst and 5 g of quartz powders of the same size were packed in a continuous ¯ow ®xed bed reactor of a quartz tube of 8.0 mm inner diameter. The reaction temperature was measured with a thermocouple inserted in the catalyst bed. The catalysts were reduced in hydrogen ¯ow of 5.010ÿ6 mol/s at 1073 K for 1 h just before the reaction was started. The reaction rates were calculated on the basis of methane. The standard reaction conditions were as follows: Reaction temperature T1000 K, methane ¯ow rate FCH4 1.510ÿ4 mol/s, carbon dioxide ¯ow rate FCO2 3.010ÿ4 mol/s, and feed ratio FCO2 =FCH4 2.0.
Fig. 7 shows the changes in the reaction rate of carbon dioxide reforming of methane measured after the several D±R treatment cycles and the hydrogen reduction. The value at cycle 0 indicates the reaction rate just after the pre-step. The D±R treatment accelerates the reaction rate of carbon dioxide reforming. The reason will be discussed later. Each catalyst has good stability. In the previous paper [5], we reported that during the carbon dioxide reforming of methane over nickel catalyst, a rapid pressure drop occurred in the plug ¯ow reactor for nickel catalyst by carbonaceous deposition, causing
6.2. Procedure Pre-step. The catalyst is reduced in a hydrogen ¯ow of 5.010ÿ6 mol/s at 1073 K for 1 h. Deposition step. Carbon is deposited on the catalyst in a methane ¯ow of 1.510ÿ5 mol/s at 1000 K for 1 h.
Fig. 7. Changes in the reforming reaction rate caused by the D±R treatment. Reaction conditions: T1000 K, FCH4 1.510ÿ4 mol/s, FCO2 3.010ÿ4 mol/s and W10 mg.
M. Ito et al. / Applied Catalysis A: General 177 (1999) 15±23
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Fig. 9. Change in TOFn with the D±R treatment cycles.
Fig. 8. Changes in the catalyst weight (a) and the reaction rate (b) with the D±R treatment cycles: (&) catalyst weight; (*) reaction rate measured before hydrogen reduction; (*) reaction rate measured after hydrogen reduction; T1000 K; FCH4 1.5 10ÿ5 mol/s (a); FCH4 1.510ÿ4 mol/s (b); FCO2 3.010ÿ4 mol/s (b); W20 mg (a); W10 mg (b).
the plugging of the reactor while the catalytic activity was stable for a long time. These ®ndings suggested that active cores for forming carbon whiskers were different from surface active sites for the main reaction. Fig. 8(a) and (b) show the changes in catalyst weights and changes in reforming reaction rates with the number of cycles of the D±R treatment. The changes in catalyst weights were represented by the values at 60 min after introducing methane in Fig. 3. These were normalized by data at cycle 1. After cycle 5, the carbonaceous deposition decreased to one-tenth of that of fresh catalyst. The reaction rates at 24 min were plotted against the cycle number both before and after the hydrogen reduction step. The arrows indicate the order of treatments. The rates were normalized by data at cycle 0 (2.4 mol/(s kg)) in Fig. 7. Before the hydrogen reduction step, the reaction rate was decreased by conducting the D±R treatment. After the reduction step, the reaction rates are higher by about 25% than that of fresh catalyst (cycle 0) and are almost independent of the cycle number. This
agrees with the tendency of nickel surface area. As shown in Fig. 6, surface nickel increases after the reduction. From the result of nickel surface area, when carbon was removed by carbon dioxide, surface nickel atoms became inactive for the reforming reaction. Therefore, the hydrogen reduction regenerates the active site for the reforming reaction. The differences in reaction rate between before and after the D±R treatment indicate that new active sites which participate in the reforming can be developed on the surface. These facts agree with our hypothesis that active sites for carbonaceous deposition and those for the reforming reaction are different and that new active sites will be developed by removing bulk nickel which is growing core of carbon whiskers. Fig. 9 shows the turnover frequency, TOFn (sÿ1). It can be determined by Eq. (6) from the reaction rate and the amount of surface nickel atoms after the reduction. TOFn sÿ1 ÿrCH4 ;n mol= s kg=CNiÿs;n mol=kg: (6) The values after the D±R treatment increase gradually with the number of treatment cycles and become a constant value. This can be explained as follows: conducting the D±R treatment increases the ratio of active nickel to the total surface nickel; new active sites are developed by removing inactive bulk nickel. The density of active sites becomes almost constant after cycle 2 of the D±R treatment. These results agree with the hypothesis that nickel atoms near the alumina support mainly participate to the
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reforming reaction and are not removed by the D±R treatment because they interact strongly with the support. 7. SEM±EDX measurements From the model of suppressing carbonaceous deposition by the D±R treatment, there must be a lot of nickel which turned inactive after conducting the D±R treatment. After the experiments of the D±R treatment and carbon dioxide reforming reaction, black powders appeared on the wall of catalyst bed. It was thought that powders were separated from the catalyst during the D±R treatment or carbon dioxide reforming reaction and derived from separated nickel cores. 7.1. Apparatus SEM±EDX (HITACHI S-570 and HORIBA EMAX 5770) measurements were conducted to determine the composition of powders on the wall of catalyst bed. 7.2. Results Fig. 10 shows SEM photograph of powders sampled from the wall of the catalyst bed after ninth D±R treatment cycle and carbon dioxide reforming reaction. The powders consisted of many particles with the size of ca. 1 mm diameter. It was found from the EDX measurement that these particles consisted of only nickel. No carbon species were found on the nickel particles. This suggests that the nickel particles separated from catalyst are inactive for the carbon deposition. The same type of nickel particles were also found on the surface of the catalyst. From the above observations, the behaviors of nickel in the D±R treatment may be explained as follows; the nickel cores (ca. 0.1 mm size) detached from nickel bulk by the D±R treatment coalesce to be particles with the size of ca. 1 mm. Some of the coalesced nickel particles are on the catalyst surface and others are on the wall or the exit of reactor. They are inactive for carbonaceous deposition once separated from catalyst surface. On the same catalyst surface, a considerable amount of nickel other than the nickel particles was observed
Fig. 10. SEM photograph of nickel particles (after ninth D±R treatment and carbon dioxide reforming reaction).
from the EDX measurement. This means that the nickel which strongly interact with support still remains on the surface. This nickel is active for the reforming reaction. In Fig. 2, we have assumed two kinds of active sites; one is bulk nickel which is active for carbonaceous deposition and is easily removed from catalyst with the growth of carbon whiskers, the other is nickel site strongly interacted with alumina support which may remain on the catalyst surface during the reaction. These assumptions can explain the two kinds of nickel observed by SEM±EDX experiments. 8. Conclusion We have proposed the new pretreatment technique, the D±R treatment for suppressing carbonaceous deposition which is the most serious problem in the carbon dioxide reforming of methane on the Ni/Al2O3 catalyst. There are two types of nickel on the catalyst; the one mainly participates in the reaction, while the other
M. Ito et al. / Applied Catalysis A: General 177 (1999) 15±23
causes a carbonaceous deposition. In the early stages of the deposition step, deposited carbon whiskers could detach small nickel cores from bulk nickel on the support. Then, the detached nickel cores which act as the growing cores of whisker carbon accelerated the carbonaceous deposition. When the deposited carbon whiskers were removed by the reaction with carbon dioxide, the growing nickel cores cohered and were deactivated. A part of the nickel was removed from catalyst. The D±R treatment decreased the bulk nickel, which became the growing cores of carbon whiskers, and made catalyst inactive for carbonaceous deposition. Moreover, this treatment accelerated the reforming activity, because the nickel located near the support was not in¯uenced by the D±R treatment and the new active sites for the reforming reaction were exposed by the removal of the bulk nickel. 9. Nomenclature FCH4 (mol/s) FCO2 (mol/s) T (K) ÿrCH4 ;n (mol/(s kg)) SNi,n (m2/kg) CNi-s,n (mol/kg) Ni-s (mol/m2) Wr,n (mg)
flow rate of methane flow rate of carbon dioxide reaction temperature reaction rate of methane at cycle n nickel surface area at cycle n amount of surface nickel atom at cycle n density of surface nickel atom catalyst weight in the reduced state at cycle n
Wn (mg) TOFn (sÿ1)
23
change in catalyst weight at cycle n turnover frequency based on surface nickel atom at cycle n
Acknowledgements The authors wish to thank Mr. T. Osaki of Nagoya Government Industrial Research Institute for the use of TGA system, Dr. A. Takano for his kind and suitable supports and encouragements and Mr. H. Ito for his skilled technical assistance. References [1] H. Koide, M. Iijima, Kagaku Kogaku 57 (1993) 614. [2] S. Michel, Hydrocarbon Process. April (1989) 37. [3] J.R. Rostrup-Nielsen, J.-H. Bak Hansen, J. Catal. 144 (1993) 38. [4] A. Takano, T. Tagawa, S. Goto, J. Chem. Eng. Jpn. 27 (1994) 727. [5] A. Takano, T. Tagawa, S. Goto, Sekiyu Gakkaishi 39 (1996) 144. [6] T. Horiuchi, K. Sakuma, T. Fukui, Y. Kubo, T. Osaki, T. Mori, Appl. Catal. A 144 (1996) 111. [7] T. Osaki, T. Horiuchi, K. Suzuki, T. Mori, Catal. Lett. 35 (1995) 39. [8] A. Takano, T. Tagawa, S. Goto, Kagaku Kogaku Ronbunshu 21 (1995) 1154. [9] J.R. Rostrup-Nielsen, Catal. Sci. Technol. 5 (1984) 50. [10] E. Tracz, R. Scholz, T. Borowiecki, Appl. Catal. 66 (1990) 133. [11] G.S. Brooks, V.J. Kehrer, Anal. Chem. 41 (1969) 103.
Suppression of carbonaceous depositions on nickel catalyst for the carbon dioxide reforming of methane Mitsunobu Ito, Tomohiko Tagawa, Shigeo Goto* Department of Chemical Engineering, Nagoya University, Chikusa, Nagoya 464-8603, Japan Received 12 February 1998; received in revised form 10 July 1998; accepted 10 July 1998
Abstract A new pretreatment of catalyst, deposition±removal (D±R) treatment, is proposed to suppress carbonaceous depositions on Ni/Al2O3 catalyst for carbon dioxide reforming of methane. This treatment is based on the hypothesis that active cores forming carbon whiskers are different from surface active sites for the main reaction. In the deposition step, active cores of nickel are detached from the bulk nickel surface as growing cores in carbon whiskers. In the removal step, the carbon whiskers are removed from the nickel surface. These two steps are repeated by ¯owing pure methane gas and then pure carbon dioxide gas. The thermogravimetric (TG) measurements showed that the repeated D±R treatments reduced the carbonaceous deposition. Carbon monoxide adsorption measurements showed that the nickel surface area was decreased by the treatments. The reaction rate and the turnover frequency were increased by the treatments. The increase in the activity was explained by the development of active sites on newly exposed nickel layers which strongly interacted with the support. # 1999 Elsevier Science B.V. All rights reserved. Keywords: Carbon dioxide reforming of methane; Nickel catalyst; Carbonaceous deposition; Carbon whisker
1. Introduction Methane and carbon dioxide are both major green house gases. The low-grade natural gas resources mainly consist of methane and carbon dioxide. To utilize more such gases as chemical raw materials has been strongly desired [1]. The carbon dioxide reforming of methane (Eq. (1)) is a particularly ef®cient process for producing synthesis gas from these two substrates. Though steam reforming of methane (Eq. (2)) is widely industrialized as a *Corresponding author. Tel.: +81-52-789-3261; fax: +81-52789-3261; e-mail: [email protected]
method of producing synthesis gas, the carbon dioxide reforming of methane has advantages in producing synthesis gas with low H2/CO ratio which is ef®cient for producing oxygenates such as alcohol or aldehyde [2]: CH4 CO2 2H2 2CO;
(1)
CH4 H2 O 3H2 CO:
(2)
Several supported transition metal catalysts (Ni, Ru, Rh, Pd, etc.) were used for the carbon dioxide reforming of methane [3]. Nickel catalysts are mainly used in industry due to their low costs. We reported the performance of several industrial nickel catalysts for this reaction [4]. Among the tested supports,
0926-860X/99/$ ± see front matter # 1999 Elsevier Science B.V. All rights reserved. PII: S0926-860X(98)00251-8
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M. Ito et al. / Applied Catalysis A: General 177 (1999) 15±23
alumina has been concluded to be essential to the reaction. During the reaction, a serious pressure drop in the reactor due to carbonaceous deposition was frequently observed [5]. To avoid carbonaceous deposition, several methods were proposed. For example, alkaline or alkaline earth metal was added on catalysts [6], or active sites for carbonaceous deposition were covered with sulfur [7]. In a previous paper [8], we reported that the effect of carbonaceous deposition could be suppressed using a macroporous ceramic foam catalyst. In the present paper, a new pretreatment of catalyst, deposition±removal (D±R) treatment is proposed to suppress carbonaceous deposition on Ni/Al2O3 catalyst for the carbon dioxide reforming of methane. The in¯uences of this treatment on carbonaceous deposition and on the activity of carbon dioxide reforming of methane are reported.
2. Deposition±removal (D±R) treatment The D±R treatment is designed according to the proposed reaction mechanism of this reaction. Fig. 1 shows the summary of reaction mechanism proposed in the previous paper [8]. The mechanism has been constructed on the basis of steam reforming mechanism [9]. Two types of active sites were proposed on the catalyst, the one interacted with the alumina support, the other at the surface of bulk nickel. The former mainly participated in the reforming reaction while the latter was involved with carbonaceous deposition. Carbon deposited on the surface of bulk nickel is well known to form carbon whiskers having nickel in the growing top [10]. If the nickel in the growing core can only be removed, carbonaceous deposition will be suppressed without any deactivation of reforming activity because the nickel interacted with support
Fig. 1. Model of the reaction and carbonaceous deposition.
M. Ito et al. / Applied Catalysis A: General 177 (1999) 15±23
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Fig. 2. Model of the D±R treatment.
which mainly participates in the reforming reaction is not detached. Based on the above model of carbonaceous deposition, the D±R treatment of Ni/Al2O3 catalyst is proposed for the removal of nickel in the growing core. Fig. 2 shows the model of the D±R treatment. This treatment consists of two steps. Deposition step. Pure methane gas is introduced and decomposed. CH4 C 2H2 :
(3)
The produced carbon is deposited on the catalyst as carbon whiskers. Active cores of nickel are detached from the surface of bulk nickel as growing cores in carbon whiskers. Removal step. Pure carbon dioxide gas is introduced and forced to react with carbon whiskers to produce
carbon monoxide. CO2 C 2CO:
(4)
Carbon whiskers having nickel cores can thus be removed from the surface of the catalyst. Repetition. These two steps are repeated several times. To con®rm the ef®ciency of this treatment, three kinds of measurement were conducted; thermogravimetry, nickel surface area and reaction rate of carbon dioxide reforming of methane. Additional measurements were conducted to observe the state of nickel which was removed by the D±R treatment using scanning electron microscope (SEM) and energy dispersive X-ray analyzer (EDX). Inactive nickel in the lower part of Fig. 2 can be presumed from these measurements.
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3. Catalyst preparation Catalyst was prepared by impregnating alumina (JRC-ALO4; reference catalyst of Syokubai Gakkai) with an aqueous solution of nickel nitrate. The precipitates were separated by ®ltration and washed with hot water. Then, each catalyst was dried overnight at 383 K, and calcined at 603 K for 5 h. The catalyst was crushed into 20±32 mesh size. Loading of nickel was adjusted to 40 wt% which was commonly used in the industrial steam reforming catalysts. 4. Thermogravimetric measurements 4.1. Apparatus
Fig. 3. Changes in catalyst weight in methane flow (carbon deposition step). T1000 K FCH4 1.510ÿ5 mol/s.
A 20 mg of the catalyst was placed on a plate of thermogravimetric analyzer (TGA-50, SHIMADZU). Changes in catalyst weight were measured in methane or carbon dioxide ¯ow. 4.2. Procedure Pre-step. The catalyst is reduced in hydrogen ¯ow of 5.010ÿ6 mol/s for 1 h at 1073 K. Deposition step. The change in catalyst weight is measured at 1000 K in methane ¯ow of 1.5 10ÿ5 mol/s. Temperature decrease step. The temperature is decreased to 700 K in helium ¯ow. Removal step. The change in catalyst weight is measured with increasing temperature at 5 K/min from 700 to 1000 K in carbon dioxide ¯ow of 3.010ÿ5 mol/s. Repetition. One cycle from the deposition step to the removal step (D±R treatment) is repeated several times. 4.3. Results Fig. 3 shows the effect of the cycle number n on the changes in catalyst weight, Wn, during the deposition step. The catalyst weights are increased by carbonaceous deposition. The Y-axis is normalized by the initial weight of the catalyst in the reduced state at each cycle, Wr,n. The weight of the fresh catalyst extremely increased as indicated in cycle 1.
Fig. 4. Changes in catalyst weight in carbon dioxide flow (carbon removal step) with an increase in temperature (5 K/min). FCO2 3.010ÿ5 mol/s.
As the D±R treatment is repeated, the amount of deposited carbon decreases. This suggests that this treatment makes the catalyst inactive for the carbonaceous deposition. Fig. 4 shows the effect of the cycle number on the changes in catalyst weight during the removal step. The catalyst weight decreases rapidly above 800 K. Carbon monoxide was detected in the outlet gas. Therefore, the decrease in catalyst weight was due to the reaction between carbon and carbon dioxide.
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areas of catalysts (SNi,n) were calculated by Eq. (5). SNi;n m2 =kg CNi-s;n mol=kg=Ni-s mol=m2 ; (5) where CNi-s,n (mol/kg) is the amount of carbon monoxide molecule adsorbed on the surface nickel based on the catalyst weight and Ni-s (mol/m2) is the density of surface nickel atom2.5610ÿ5 mol/m2. 5.2. Procedure
Fig. 5. Rates of the decrease in catalyst weight in carbon dioxide flow with an increase in temperature (5 K/min). FCO2 3.0 10ÿ5 mol/s.
Fig. 5 shows time differentiations of catalyst weights calculated from Fig. 4 at the removal step. Every cycle has two peaks, smaller peak at 800 K and larger one at 950 K. This suggests that two types of carbon species exist on the catalyst. The heights of smaller peaks are almost unchanged in every cycle. On the other hand, heights of larger peaks at 950 K extremely decrease as the cycle number increases. The existence of two types of carbon species coincides to the assumption that there existed two types of active sites such as whisker carbon deposition and reforming reaction. The changes in the large peak and the amount of carbonaceous deposition in Fig. 3 showed a similar tendency. Thus, the carbon species with a larger peak at 950 K might be related with carbon whiskers which were accumulated during the deposition step. The sites of former peak (800 K) might relate to the reforming active site which has stable nature with repeated the D±R treatment. 5. Nickel surface area measurements 5.1. Apparatus A pulse CO adsorption apparatus was used to determine the nickel surface area [11]. Carbon monoxide adsorbed was assumed to be the linear-type (Ni:CO1:1) equimolar adsorption. Nickel surface
A 20 mg of catalyst was packed in a quartz tube of 4.0 mm inner diameter. Pre-step. The catalyst was reduced in hydrogen ¯ow of 5.010ÿ6 mol/s for 1 h at 1073 K. Deposition step. Carbon is deposited on the catalyst by the contact with methane at 1000 K for 1 h in methane ¯ow of 1.510ÿ5 mol/s. Removal step. Deposited carbon is removed by the contact with carbon dioxide at 1000 K for 1 h in carbon dioxide ¯ow of 3.010ÿ5 mol/s. Temperature decrease step. The temperature is decreased to 293 K in helium ¯ow. CO-adsorption step before reduction. Nickel surface area is measured. Temperature increase step. The temperature is increased to 1000 K in helium ¯ow. Reduction step. The catalyst is reduced in hydrogen ¯ow of 5.010ÿ6 mol/s at 1000 K for 30 min. Temperature decrease step. The temperature is decreased to 293 K in helium ¯ow. CO-adsorption step after reduction. Nickel surface area is measured again. Repetition. One cycle from deposition step to the CO-adsorption step after reduction is repeated several times. 5.3. Results Fig. 6 shows the changes in the nickel surface area measured by the method of carbon monoxide adsorption after conducting the D±R treatment several times. The value at cycle 0 indicates the nickel surface area just after the pre-step. Once the D±R treatment is conducted, the nickel surface area decreases extremely and becomes almost zero after several cycles. However, 16% of original value were recovered by the hydrogen reduction step. This suggests that a part of
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Fig. 6. Change in the nickel surface area caused by the D±R treatment cycles: (*) before hydrogen reduction; (*) after hydrogen reduction.
Removal step. Deposited carbon is removed by a carbon dioxide ¯ow of 3.010ÿ5 mol/s at 1000 K for 1 h. Reaction step before reduction. Reaction rate is measured under standard conditions. Reduction step. The catalyst is reduced in a hydrogen ¯ow of 5.010ÿ6 mol/s at 1000 K for 30 min. Reaction step after the reduction. Reaction rate is measured again under standard conditions. Repetition. One cycle from the deposition step to the reaction step after the reduction is repeated several times. 6.3. Results
nickel remains on the support against the D±R treatment while others were removed from the catalyst. The fact that nickel surface area was observed after hydrogen reduction suggests that a part of the surface nickel might be oxidized by carbon dioxide during the removal step of the D±R treatment. 6. Reaction rate measurements 6.1. Apparatus A 10 mg of catalyst and 5 g of quartz powders of the same size were packed in a continuous ¯ow ®xed bed reactor of a quartz tube of 8.0 mm inner diameter. The reaction temperature was measured with a thermocouple inserted in the catalyst bed. The catalysts were reduced in hydrogen ¯ow of 5.010ÿ6 mol/s at 1073 K for 1 h just before the reaction was started. The reaction rates were calculated on the basis of methane. The standard reaction conditions were as follows: Reaction temperature T1000 K, methane ¯ow rate FCH4 1.510ÿ4 mol/s, carbon dioxide ¯ow rate FCO2 3.010ÿ4 mol/s, and feed ratio FCO2 =FCH4 2.0.
Fig. 7 shows the changes in the reaction rate of carbon dioxide reforming of methane measured after the several D±R treatment cycles and the hydrogen reduction. The value at cycle 0 indicates the reaction rate just after the pre-step. The D±R treatment accelerates the reaction rate of carbon dioxide reforming. The reason will be discussed later. Each catalyst has good stability. In the previous paper [5], we reported that during the carbon dioxide reforming of methane over nickel catalyst, a rapid pressure drop occurred in the plug ¯ow reactor for nickel catalyst by carbonaceous deposition, causing
6.2. Procedure Pre-step. The catalyst is reduced in a hydrogen ¯ow of 5.010ÿ6 mol/s at 1073 K for 1 h. Deposition step. Carbon is deposited on the catalyst in a methane ¯ow of 1.510ÿ5 mol/s at 1000 K for 1 h.
Fig. 7. Changes in the reforming reaction rate caused by the D±R treatment. Reaction conditions: T1000 K, FCH4 1.510ÿ4 mol/s, FCO2 3.010ÿ4 mol/s and W10 mg.
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Fig. 9. Change in TOFn with the D±R treatment cycles.
Fig. 8. Changes in the catalyst weight (a) and the reaction rate (b) with the D±R treatment cycles: (&) catalyst weight; (*) reaction rate measured before hydrogen reduction; (*) reaction rate measured after hydrogen reduction; T1000 K; FCH4 1.5 10ÿ5 mol/s (a); FCH4 1.510ÿ4 mol/s (b); FCO2 3.010ÿ4 mol/s (b); W20 mg (a); W10 mg (b).
the plugging of the reactor while the catalytic activity was stable for a long time. These ®ndings suggested that active cores for forming carbon whiskers were different from surface active sites for the main reaction. Fig. 8(a) and (b) show the changes in catalyst weights and changes in reforming reaction rates with the number of cycles of the D±R treatment. The changes in catalyst weights were represented by the values at 60 min after introducing methane in Fig. 3. These were normalized by data at cycle 1. After cycle 5, the carbonaceous deposition decreased to one-tenth of that of fresh catalyst. The reaction rates at 24 min were plotted against the cycle number both before and after the hydrogen reduction step. The arrows indicate the order of treatments. The rates were normalized by data at cycle 0 (2.4 mol/(s kg)) in Fig. 7. Before the hydrogen reduction step, the reaction rate was decreased by conducting the D±R treatment. After the reduction step, the reaction rates are higher by about 25% than that of fresh catalyst (cycle 0) and are almost independent of the cycle number. This
agrees with the tendency of nickel surface area. As shown in Fig. 6, surface nickel increases after the reduction. From the result of nickel surface area, when carbon was removed by carbon dioxide, surface nickel atoms became inactive for the reforming reaction. Therefore, the hydrogen reduction regenerates the active site for the reforming reaction. The differences in reaction rate between before and after the D±R treatment indicate that new active sites which participate in the reforming can be developed on the surface. These facts agree with our hypothesis that active sites for carbonaceous deposition and those for the reforming reaction are different and that new active sites will be developed by removing bulk nickel which is growing core of carbon whiskers. Fig. 9 shows the turnover frequency, TOFn (sÿ1). It can be determined by Eq. (6) from the reaction rate and the amount of surface nickel atoms after the reduction. TOFn sÿ1 ÿrCH4 ;n mol= s kg=CNiÿs;n mol=kg: (6) The values after the D±R treatment increase gradually with the number of treatment cycles and become a constant value. This can be explained as follows: conducting the D±R treatment increases the ratio of active nickel to the total surface nickel; new active sites are developed by removing inactive bulk nickel. The density of active sites becomes almost constant after cycle 2 of the D±R treatment. These results agree with the hypothesis that nickel atoms near the alumina support mainly participate to the
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reforming reaction and are not removed by the D±R treatment because they interact strongly with the support. 7. SEM±EDX measurements From the model of suppressing carbonaceous deposition by the D±R treatment, there must be a lot of nickel which turned inactive after conducting the D±R treatment. After the experiments of the D±R treatment and carbon dioxide reforming reaction, black powders appeared on the wall of catalyst bed. It was thought that powders were separated from the catalyst during the D±R treatment or carbon dioxide reforming reaction and derived from separated nickel cores. 7.1. Apparatus SEM±EDX (HITACHI S-570 and HORIBA EMAX 5770) measurements were conducted to determine the composition of powders on the wall of catalyst bed. 7.2. Results Fig. 10 shows SEM photograph of powders sampled from the wall of the catalyst bed after ninth D±R treatment cycle and carbon dioxide reforming reaction. The powders consisted of many particles with the size of ca. 1 mm diameter. It was found from the EDX measurement that these particles consisted of only nickel. No carbon species were found on the nickel particles. This suggests that the nickel particles separated from catalyst are inactive for the carbon deposition. The same type of nickel particles were also found on the surface of the catalyst. From the above observations, the behaviors of nickel in the D±R treatment may be explained as follows; the nickel cores (ca. 0.1 mm size) detached from nickel bulk by the D±R treatment coalesce to be particles with the size of ca. 1 mm. Some of the coalesced nickel particles are on the catalyst surface and others are on the wall or the exit of reactor. They are inactive for carbonaceous deposition once separated from catalyst surface. On the same catalyst surface, a considerable amount of nickel other than the nickel particles was observed
Fig. 10. SEM photograph of nickel particles (after ninth D±R treatment and carbon dioxide reforming reaction).
from the EDX measurement. This means that the nickel which strongly interact with support still remains on the surface. This nickel is active for the reforming reaction. In Fig. 2, we have assumed two kinds of active sites; one is bulk nickel which is active for carbonaceous deposition and is easily removed from catalyst with the growth of carbon whiskers, the other is nickel site strongly interacted with alumina support which may remain on the catalyst surface during the reaction. These assumptions can explain the two kinds of nickel observed by SEM±EDX experiments. 8. Conclusion We have proposed the new pretreatment technique, the D±R treatment for suppressing carbonaceous deposition which is the most serious problem in the carbon dioxide reforming of methane on the Ni/Al2O3 catalyst. There are two types of nickel on the catalyst; the one mainly participates in the reaction, while the other
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causes a carbonaceous deposition. In the early stages of the deposition step, deposited carbon whiskers could detach small nickel cores from bulk nickel on the support. Then, the detached nickel cores which act as the growing cores of whisker carbon accelerated the carbonaceous deposition. When the deposited carbon whiskers were removed by the reaction with carbon dioxide, the growing nickel cores cohered and were deactivated. A part of the nickel was removed from catalyst. The D±R treatment decreased the bulk nickel, which became the growing cores of carbon whiskers, and made catalyst inactive for carbonaceous deposition. Moreover, this treatment accelerated the reforming activity, because the nickel located near the support was not in¯uenced by the D±R treatment and the new active sites for the reforming reaction were exposed by the removal of the bulk nickel. 9. Nomenclature FCH4 (mol/s) FCO2 (mol/s) T (K) ÿrCH4 ;n (mol/(s kg)) SNi,n (m2/kg) CNi-s,n (mol/kg) Ni-s (mol/m2) Wr,n (mg)
flow rate of methane flow rate of carbon dioxide reaction temperature reaction rate of methane at cycle n nickel surface area at cycle n amount of surface nickel atom at cycle n density of surface nickel atom catalyst weight in the reduced state at cycle n
Wn (mg) TOFn (sÿ1)
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change in catalyst weight at cycle n turnover frequency based on surface nickel atom at cycle n
Acknowledgements The authors wish to thank Mr. T. Osaki of Nagoya Government Industrial Research Institute for the use of TGA system, Dr. A. Takano for his kind and suitable supports and encouragements and Mr. H. Ito for his skilled technical assistance. References [1] H. Koide, M. Iijima, Kagaku Kogaku 57 (1993) 614. [2] S. Michel, Hydrocarbon Process. April (1989) 37. [3] J.R. Rostrup-Nielsen, J.-H. Bak Hansen, J. Catal. 144 (1993) 38. [4] A. Takano, T. Tagawa, S. Goto, J. Chem. Eng. Jpn. 27 (1994) 727. [5] A. Takano, T. Tagawa, S. Goto, Sekiyu Gakkaishi 39 (1996) 144. [6] T. Horiuchi, K. Sakuma, T. Fukui, Y. Kubo, T. Osaki, T. Mori, Appl. Catal. A 144 (1996) 111. [7] T. Osaki, T. Horiuchi, K. Suzuki, T. Mori, Catal. Lett. 35 (1995) 39. [8] A. Takano, T. Tagawa, S. Goto, Kagaku Kogaku Ronbunshu 21 (1995) 1154. [9] J.R. Rostrup-Nielsen, Catal. Sci. Technol. 5 (1984) 50. [10] E. Tracz, R. Scholz, T. Borowiecki, Appl. Catal. 66 (1990) 133. [11] G.S. Brooks, V.J. Kehrer, Anal. Chem. 41 (1969) 103.