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Mechanism of partial oxidation of methane over a nickel-calcium hydroxyapatite catalyst
Applied Catalysis A: General 312 (2006) 27–34 www.elsevier.com/locate/apcata
Mechanism of partial oxidation of methane over a nickel-calcium hydroxyapatite catalyst Jin Hyuk Jun a,1, Tae Hoon Lim b, Suk-Woo Nam b, Seong-Ahn Hong b, Ki June Yoon a,* b
a Department of Chemical Engineering, Sungkyunkwan University, Suwon 440-746, South Korea Battery and Fuel Cell Research Center, Korea Institute of Science and Technology (KIST), Seoul 136-791, South Korea
Received 16 March 2006; received in revised form 13 June 2006; accepted 14 June 2006 Available online 24 July 2006
Abstract The mechanism of partial oxidation of methane to synthesis gas over a nickel-calcium hydroxyapatite catalyst was studied by employing pulse experiments for the powder catalyst and by measuring temperature profiles of the activated, washcoated monolith catalyst. The pulse study showed that after the catalyst was partially reduced, carbon deposition occurred to a great extent and CO was predominantly produced over CO2. Temperature profiles of the monolith catalyst showed that the highest temperature difference between the furnace and the monolith became smaller as the furnace temperature increased. We propose that the reaction occurs primarily via the pyrolysis mechanism or direct dissociation of methane. Adsorbed CO (COs) is a common intermediate and it is rapidly desorbed to produce CO(g), especially at high temperature, or converted to CO2(g), especially at low temperature. The observation that the fully reduced catalyst exhibited lower activity suggests that both metallic Ni and partially oxidized nickel are required in order to exhibit high activity and selectivity. # 2006 Elsevier B.V. All rights reserved. Keywords: Calcium hydroxyapatite; Mechanism; Methane partial oxidation; Nickel; Pulse experiment
1. Introduction Catalytic partial oxidation of methane (POM) for synthesis gas and hydrogen production has been an active research subject in recent years. This process arouses interest when compared with the well-established steam reforming of methane, since POM is mildly exothermic and does not require high operating pressures and hence is more energy efficient [1–14]. Although many studies on the mechanism for POM have been reported, the mechanism is still debated. In the literature, two alternative mechanisms for POM have been proposed: (i) the two-step mechanism or the indirect scheme [6,15–17], according to which the initial complete oxidation of methane is followed by reforming of the residual methane with CO2 and * Corresponding author. Tel.: +82 31 290 7244; fax: +82 31 290 7272. E-mail addresses: [email protected] (J.H. Jun), [email protected] (K.J. Yoon). 1 Present address: Doosan Technical Center, Electro-Materials BG, Doosan Corp., 39-3 Sungbok-dong, Yongin-si 449-795, South Korea. 0926-860X/$ – see front matter # 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2006.06.020
H2O that are produced primarily; (ii) the pyrolysis mechanism or the direct scheme [1–3,14,18–22], according to which methane first dissociates to generate hydrogen and carbon, then H2 desorbs and carbon is oxidized to carbon monoxide by surface oxygen species. The two-step mechanism is characterized by a hot spot near the entrance of the catalyst bed and a temperature drop in the rear part of the bed. On the other hand, Schmidt and coworkers [10,18] proposed the pyrolysis mechanism and several researchers have supported this mechanism. Au and coworkers [1,19] have reported from pulse studies that the reaction over reduced Ni/SiO2 and Rh/ SiO2 catalysts occurs via the pyrolysis mechanism, while CO2 is produced by subsequent oxidation of CO. They also reported that methane dissociation and adsorption was accelerated by adsorbed oxygen on the surface of the catalyst, and this was later theoretically verified [20]. Liu et al. [14] have reached similar conclusions from a pulse study over a reduced Ni/CeZrO2/u-Al2O3 catalyst. Some investigators have reported that the reaction pathway depends strongly on the metal as well as the support. Nakagawa et al. [23] reported that the synthesis gas was formed via a
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pyrolysis mechanism over Rh/Al2O3 and Rh/TiO2, while it was produced via a two-step mechanism over Rh/SiO2 and Ir/TiO2. Weng et al. [24] reported that it was via the pyrolysis mechanism over reduced Rh/SiO2 while it was via the two-step mechanism over Ru/SiO2 and Ru/Al2O3. On the other hand, Verykios and coworkers [25,26] suggested direct formation of CO and H2 over Ru/TiO2 catalysts. Recent investigations in this laboratory have demonstrated that nickel-calcium hydroxyapatite catalysts exhibit excellent performance for POM [27–29]. Although an essential active component is metallic Ni, these catalysts contain no oxide support unlike most of the supported Group VIII metal catalysts. Another salient feature for these catalysts is that the catalyst activation can be done with the reactant gases (CH4 + O2) at a relatively low temperature (at or below 923 K). The main components in the catalyst with the optimum composition are metallic Ni, NiO and calcium hydroxyapatite (Ca10(OH)2(PO4)6) but the calcium phosphate phase is minor. In this work, the reaction mechanism for POM over a nickel-calcium hydroxyapatite catalyst was studied by employing pulse experiments and by measuring temperature profiles of the monolith catalyst bed. The reaction results from CH4–O2–CH4 and CH4/O2 pulses over reduced and unreduced catalysts were compared. The active constituents of the catalyst are also discussed. 2. Experimental 2.1. Catalyst preparation A Ni-calcium hydroxyapatite powder catalyst having a composition of Ca9.5Ni2.5(PO4)6 was prepared from nearly saturated aqueous solutions of calcium nitrate, nickel nitrate and dibasic ammonium phosphate as described in the previous works [27,28]. The pH of each of the aqueous solutions was adjusted to 10–11 by adding ammonia water, and predetermined amounts of the solutions were mixed with vigorous stirring. The mixture was dried at 383 K overnight and finally heat-treated in air at 1073 K for 2 h to obtain the catalyst. The solid catalyst was crushed and sieved, and particles of 40- to 80mesh size were used for the pulsed reaction. This catalyst has the composition near the optimum, as reported in the previous works [27,28]. A honeycomb support was employed to prepare the monolith catalyst. The honeycomb is made from cordierite (2Al2O32MgO5SiO2) with channels of square cross-section, and the cell density is 400 cells per square inch (cpsi). The honeycomb was cut so that the monolith had 17 cells, the length of 28 mm, and the diameter of ca. 8 mm. Four walls of the cells at the center of the monolith were removed in order that a 1/ 16 in. (1.6 mm) thermocouple could be inserted. The weight of the monolith before the washcoating was about 0.42 g. In order to use as the washcoat, the catalyst powder was crushed into fine particles (below a few tens of micrometers). The fine powder was suspended in water, and the washcoated monolith catalyst was prepared by repeated dipping into the suspension. After each dipping, it was dried at 423 K for 1 h. This step was repeated until the net weight of the coated powder catalyst
became 0.1 g. The monolith catalyst was employed for reaction after calcinations at 923 K in air. 2.2. Pulse experiments Pulse experiments using CH4, O2 and/or CH4/O2 with a molar ratio of 2 were carried out in a quartz micro-reactor, as described in an earlier work [14]. Prior to each reaction, 30 mg of the powder catalyst was loaded in the reactor and treated at the desired temperature in He (30 ml/min) for 90 min to remove any residual gases in the system, and then the powder was exposed to pulses of CH4, O2 and/or CH4/O2. The amount of each pulse was 0.25 cm3 (STP) and the time interval between pulses was about 10 min. After each pulse, the exit gas was analyzed on-line with a gas chromatograph equipped with a TCD (Carbosphere column, 353 K, helium as carrier gas). The conversion and selectivity were calculated on the basis of carbon contents in the products and 100% carbon and oxygen balances. 2.3. Measurement of temperature profiles For the measurement of the temperature profiles in the reactor, the reaction was carried out by a conventional method using an 8 mm i.d. quartz-tube flow reactor with down-flow of the gas. The middle of the tube was narrowed somewhat so that the monolith could be located there. The reaction temperature was controlled with an electric furnace. The partial pressures of methane and oxygen were 16.2 and 8.1 kPa (0.16 and 0.08 atm), respectively. Ar was used as the diluent gas and the total flow rate was 100 cm3 (STP)/min. The catalyst activation was first carried out at the furnace temperature of 1023 K for 30 min with the reactant gas (without employing the hydrogen pretreatment), since this temperature had been found to be sufficiently high for the activation of the catalyst [28], and then the temperature was set to the desired reaction temperature. During the reaction, the temperatures in the reactor, including the inside of the monolith, were measured at several positions by using the inserted thermocouple. The thermocouple could be moved up and down with negligible leaking by slightly loosening the nut with a Teflon ferrule. One thing to note here is that the measured temperature does not indicate the true temperature of the catalyst powder or the active component inside due to the limitation of the measuring device but is a measure of relative difference. The product gas was analyzed using two gas chromatographs with Carboxen 1004 columns (Supelco); one used Ar as the carrier gas and the other used He. The latter was needed especially when the concentration of produced CO2 was too low to detect by using the Ar carrier gas. 3. Results 3.1. Pulse study The catalyst employed in this work had been characterized by several methods in a previous work [28]. In order to better understand the characteristics of this catalyst, the main results
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of the previous work are briefly introduced here. From XRD analysis for the fresh, unreduced catalyst, it was observed that calcium hydroxyapatite and NiO were the major crystalline phases, but that calcium phosphate was present in a very small amount and no metallic nickel was present. For the used catalyst, which had been reduced at 1023 K with a reactant gas (CH4 + O2 + Ar) and then experienced a set of POM experiments at temperatures from 1073 to 673 K, the amounts of calcium hydroxyapatite and calcium phosphate were observed to change little during the reaction, but most of NiO was reduced to metallic Ni. It was confirmed that some of the nickel in the fresh catalyst was present in the hydroxyapatite and phosphate structures by substituting the calcium ions but this nickel in those structures came out as fine, reduced nickel particles during the POM reaction. From the TEM results for the used catalyst, many particles of a few nanometers in size were observed, although large particles, their size being several tens of nanometers, were also present in a considerably small number. When the fresh catalyst was treated with a reactant gas, only CO2 and H2O were produced below 873 K, but large amounts of CO and H2 were produced at and above 923 K. This shows that the Ni2+ in the catalyst can be reduced by the reactant gas at and above 923 K. To obtain some mechanistic information for methane partial oxidation over nickel-calcium hydroxyapatite, pulse experiments with CH4, O2 and/or CH4/O2 with a molar ratio of 2 were carried out. Fig. 1 shows the results for the reaction with sequential pulses of CH4–O2–CH4 over the fresh, unreduced catalyst at 1023 K. The methane conversion for the first and second CH4 pulses was very high but it decreased rapidly from the third pulse. The CO2 selectivity was high for the first and second pulses. But the CO2 selectivity drastically decreased at the third pulse and was virtually 0% from the fourth pulse, while the CO selectivity increased sharply and reached the
Fig. 1. Results for sequential pulses of CH4, O2 and CH4 over unreduced Ca9.5Ni2.5(PO4)6 at 1023 K.
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maximum of about 60% at the third pulse and then rapidly decreased. The above results indicate that, in the first place, the nickel in the catalyst is reduced mainly with the formation of CO2 by the reaction of CH4 with the lattice oxygen and then with the formation of CO. The increase of the CO selectivity is due to the small amount of the lattice oxygen left, which is not sufficient for deep oxidation of the carbon. From the amount of oxygen in the CO2 and CO products for the first three pulses, it was estimated that more than 70% of the total nickel in the catalyst was reduced. As the nickel was reduced to the metallic state with the exhaustion of the oxygen, carbon deposition occurred to a great extent and the methane conversion rapidly decreased. The selectivity to the deposited carbon, Cs, rapidly increased from the third pulse and it reached about 90% at the fifth pulse. These results are similar to those reported in an earlier work on Ni/SiO2 [19]. For the subsequent oxygen pulses, oxygen was all consumed up to the fifth pulse but negligible amounts of CO and CO2 were produced. This indicates that the deposited carbon in this catalyst is very difficult to oxidize. Moreover, the total amount of oxygen injected by the five pulses is well above the amount needed to re-oxidize the reduced nickel. Therefore, we consider that there are some species other than Ni or deposited carbon that could react with the injected oxygen. This is not well cleared up at the moment, and that is why we used the term ‘sorbed oxygen’. Those species might be pyrophosphate compounds, such as Ca2P2O7 that can be oxidized to the orthophosphate Ca3(PO4)2 (note: the presence of the pyrophosphate is hard to identify by XRD and other methods). The results for the oxygen pulses are peculiar for the catalyst in this work, and are markedly different from those for the Ni/CeZrO2/u-Al2O3 and Ni/SiO2 catalysts [14,19]. For O2 pulses after CH4 pulses, quite large amounts of CO and CO2 were produced by the Ni/Ce-ZrO2/u-Al2O3 and Ni/SiO2 catalysts, which means that Cs in these catalysts could easily be oxidized. For the subsequent second-series CH4 pulses, the results were similar to those for the first-series CH4 pulses. However, production of CO2 was smaller for the first pulse compared with that for the first series. For the second pulse, the CO2 selectivity was very small, while the CO selectivity was high. This indicates that the amount of reactive oxygen after the oxygen pulses is smaller compared with the amount for the original fresh sample. One thing to note here is that not all of the sorbed oxygen during the oxygen pulsing was used for the CH4 oxidation. That is, this means that a portion of the sorbed oxygen is present in a form that is difficult to use for the CH4 oxidation. In other words, a large part of the sorbed oxygen may be present in the inner of nickel particles or bonded with other elements such as P. Fig. 2 shows the results of CH4/O2 (2:1) pulse reactions over the fresh, unreduced catalyst at 1023 K. As with the results in Fig. 1, the CO2 selectivity was high for the first and second pulses but then decreased sharply. This indicates that the nickel was reduced with consumption of the lattice oxygen by CH4. From the fourth pulse, the CO selectivity was very high and remained nearly 100%. This suggests that the POM follows the pyrolysis mechanism on the reduced Ni atoms. Although from
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the analysis the Cs selectivity was calculated to be negligible, the activity of the catalyst decreased gradually from the fourth pulse, and this is considered most probably due to the deposited carbon. Fig. 3 shows the results at 923 K for sequential pulses of CH4– O2–CH4 over the fresh, unreduced catalyst. For the first-series CH4 pulses, the amount of the consumed oxygen for the production of CO and CO2 were significantly lower compared with that at 1023 K due to the lower temperature. However, since a considerable portion of the nickel was reduced, the Cs
selectivity increased sharply while the CH4 conversion decreased rapidly. This trend is similar to the case at 1023 K. For the subsequent O2 pulses, the results were again similar to those for the case at 1023 K. Most of the O2 was sorbed, but the amounts of CO and CO2 were somewhat larger than those at 1023 K. For the second-series CH4 pulses, the CO2 selectivity decreased gradually while the CO selectivity increased gradually. The CH4 conversion was rather low (less than 20%) but a rapid decrease was not observed since the Cs selectivity was not high. This can be explained by the low reaction rate owing to the low temperature and low active surface area. During the first-series CH4 pulses, the active nickel surface was covered with the deposited carbon but only a minor part of it was restored during the O2 pulses. Thus, during the second-series CH4 pulses, the oxygen in the catalyst was gradually consumed and the Cs selectivity gradually increased. Nevertheless, these results also support the pyrolysis mechanism on the reduced nickel. For the pulses of CH4/O2 (2:1) mixture over the unreduced catalyst at 923 K, CH4 conversions were quite low (about 10%) but remained almost constant up to 10 pulses, and the product was exclusively CO2 (Fig. 4). Since gaseous oxygen was cofed, the catalyst would not have the opportunity to be reduced because the conversion was considerably lower than 25%, which is the theoretical value needed to provide a reducing environment. This indicates that the complete combustion dominates over the oxidized nickel and that carbon deposition is negligible. Fig. 5 shows the results of CH4–O2–CH4 pulse reactions at 1023 K over the catalyst that was pre-reduced by H2 at 1023 K for 3 h. For the first-series CH4 pulses, the methane conversion was only 24% for the first pulse and then decreased to 5% for the fifth pulse. Cs was the main product and only a small amount of CO was produced, but no CO2 was detected. This is certainly due
Fig. 3. Results for sequential pulses of CH4, O2 and CH4 over unreduced Ca9.5Ni2.5(PO4)6 at 923 K.
Fig. 4. Results for CH4/O2 (2/1) pulses over unreduced Ca9.5Ni2.5(PO4)6 at 923 K.
Fig. 2. Results for CH4/O2 (2/1) pulses over unreduced Ca9.5Ni2.5(PO4)6 at 1023 K.
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Fig. 5. Results for sequential pulses of CH4, O2 and CH4 over reduced Ca9.5Ni2.5(PO4)6 at 1023 K.
to the deficiency of oxygen source to react with methane owing to the pre-reduction of the catalyst. During the subsequent oxygen pulses, injected oxygen was almost all sorbed in the catalyst and was not used to produce the CO and/or CO2. This is similar to the results of Figs. 1 and 3. For the next-series CH4 pulses, the methane conversion was low, but the CO selectivity was significantly higher and the Cs selectivity was lower compared with the case for the first-series CH4 pulses. These results show that carbon deposition occurs to a great extent on the reduced nickel and that oxygen in the catalyst is beneficial for the prevention of carbon deposition. Fig. 6 shows the results of CH4/O2 (2:1) pulse reactions at 1023 K over the same pre-reduced catalyst. The CH4 conversion was very low, but it increased slowly from about 1% at the first pulse to about 3% at the tenth pulse. The CO selectivity was virtually 100%. The reduced catalyst would have been expected to exhibit high activity since most of the nickel in the catalyst is present as the metallic nickel. But the above results show that fully reduced Ni is not so active for POM.
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Fig. 6. Results for CH4/O2 (2/1) pulses over reduced Ca9.5Ni2.5(PO4)6 at 1023 K.
between the measured temperature in the reactor during the reaction (the reactor or bed temperature) and the temperature in the reactor without the reaction (the furnace temperature). Fig. 7 shows that the temperature rises in the monolith. The positive DT before the monolith may be due to heat transfer or
3.2. Temperature profiles of a monolith reactor The temperatures measured in the reactor during the reaction over the activated monolith catalyst were compared with those with Ar-only flow. The temperatures measured with only Ar flow, or the furnace temperatures, are shown by the dotted lines in Fig. 7. The actual furnace temperatures were not uniform due to the buoyancy and the temperature difference between the ends of the monolith was about 15 K. For convenience sake, the mentioned temperature hereafter denotes the representative (set) furnace temperature at the 15 mm position unless specified otherwise. DT is the temperature difference at the same position
Fig. 7. Temperature profiles of the monolith reactor in POM for different furnace temperatures ((*) 770 K; (&) 860 K; (~) 1023 K; ( ) temperature profile at each furnace temperature without reaction; PCH4 = 0.16 atm; CH4/ O2 = 2; total flow rate = 100 cm3/min).
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the back mixing of the gas. The maximum DT became smaller as the reaction temperature increased: the maximum DTs were +70, +60 and +40 K at 770, 860 and 1023 K, respectively. As the reaction temperature increased, the position of the highest temperature moved toward the inlet: 11 mm at 770 K, 5 mm at 875 K and 3 mm at 1023 K. This is surely because the reaction rate becomes faster at the higher temperature. The temperature decrease afterward was partly due to the heat loss through the reactor sidewall [30] and partly due to the endothermic reactions. After the highest temperature, however, the bed temperature decreased faster as the reaction temperature increased even though DT was smaller. At 770 K, the decreasing temperature curve appeared convex and the temperature at the monolith outlet was significantly higher than the furnace temperature. This indicates that the extent of endothermic reactions in the rear part of the bed is small and/or that a mildly exothermic reaction such as partial oxidation may be occurring to some extent even though the heat loss is taken into account. At 860 K, the decreasing temperature curve was concave and the outlet temperature became nearly the same as the furnace temperature. At 1023 K, the temperature decreased more rapidly and at about 15-mm position the bed temperature became the same as the furnace temperature; at the outlet the bed temperature became slightly lower than the furnace temperature. The above results indicate that endothermic reactions are occurring faster in the rear part of the bed as the temperature increases but that the extent of the endothermic reactions was not large; otherwise, more rapid decrease of the temperature and a large negative DT would have been observed. Fig. 8 shows the product analysis results. These results are nearly the same as those obtained over 0.2 g of the powder catalyst reported in the previous work [28]. At 770 K, the CH4 conversion was somewhat lower than the equilibrium, but the CO selectivity was considerably lower than the equilibrium.
Fig. 8. CH4 conversion, CO selectivity and H2 yield over the monolith catalyst (PCH4 = 0.16 atm; CH4/O2 = 2; total flow rate = 100 cm3/min).
This indicates that the CO2 formation is dominant. However, H2 yield was much closer to the equilibrium, indicating that H2O formation is small. At 860 K, the CH4 conversion and CO selectivity were slightly lower than the values at equilibrium but the H2 yield was a little bit higher than the value at equilibrium. At 1023 K, the CH4 conversion and CO selectivity were slightly lower than the values at equilibrium and the H2 yield was nearly the same as the value at equilibrium. These results strongly support the claim that the direct scheme is dominant. 4. Discussion 4.1. POM mechanism As shown in the CH4 pulse study on the oxidized nickel, the carbon deposition was small and the CO2 formation was dominant. This has been reported by several authors [5,11,14,15] and it is attributed to the fact that the metal oxide does not easily dissociate methane to surface carbon species. On the nickel reduced by the CO2 formation, however, the carbon deposition occurred to a great extent and the CO formation was predominant over CO2 formation. During the second-series CH4 pulses after O2 pulses, the CO selectivity became considerably higher compared with that during the first-series CH4 pulses (Figs. 1 and 3). This is because the catalyst can be more easily reduced since it has once been reduced during the first-series pulses, as reported in the previous work [28]. The CH4/O2 (2:1) pulse study at 1023 K on the fresh, unreduced catalyst also showed that the catalyst was reduced by forming CO2 and then the CO selectivity became near 100% (Fig. 2). These results are in agreement with the earlier works on Ni/Ce-ZrO2/u-Al2O3 and Ni/SiO2 [14,19] and strongly support the claim that decomposition of CH4 primarily occurs on the reduced nickel. The CH4/O2 (2:1) pulse study at 923 K on the fresh, unreduced catalyst showed that only CO2 was formed (Fig. 4). In this case, the reason is that the nickel could not be reduced because oxygen was cofed and a reducing environment was not provided. On the contrary, when only methane was introduced, the catalyst was reduced and carbon deposition occurred to a great extent, as shown in Fig. 3. The measured temperature profiles of the monolith also support the pyrolysis mechanism. If the highly exothermic complete oxidation takes place first, a larger and sharper temperature increase would have been observed as the temperature increases, since the reaction rate becomes faster. However, the observation was the opposite (Fig. 7). As the temperature increases, it is apparent that the mildly exothermic partial oxidation occurs predominantly. The H2 yield at 770 K was closer to the equilibrium than the CO selectivity, and the H2 yield at 860 K was slightly higher than the equilibrium (Fig. 8). This means that the H2O formation is significantly lower than the value at equilibrium, and this could not occur if the complete oxidation takes place first. The surface hydrogen atom may be combined and desorbed as H2(g) or oxidized and desorbed as H2O(g). The activation energy for combination of surface hydrogen atoms so as to desorb as H2 on nickel clusters
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(124 kJ/mol) has been reported to be nearly the same as that for combination of surface carbon and oxygen so as to desorb as CO (125 kJ/mol) [20]. Moreover, formation of H2O on Ni is much less favorable than that on Pt or Pd due to relative instability of OH species on Ni [10,20]. This suggests that H2(g) can be more readily produced on Ni than CO(g) can. The higher CO2 formation at low temperatures may also be explained by the pyrolysis mechanism. As suggested by Au et al. [19], this may be caused by the difference of the activation energies of desorption and oxidation of adsorbed CO (COs). The surface C species reacts with the surface oxygen species to give adsorbed COs. The activation energy of COs desorption on Ni(1 1 1) surface (113 kJ/mol) is about two times higher than that of COs oxidation (64 kJ/mol). Moreover, the surface oxygen species may be more abundant than Cs at lower temperature. Therefore, the lower temperature would favor COs oxidation rather than COs desorption, leading to higher CO2 selectivity. This suggestion has also been supported by other investigators [1,4,14]. In summary, based on the experimental results and the above discussion, the following mechanism is proposed as shown below in Scheme 1. On metallic nickel, methane is primarily dissociated into surface carbon and hydrogen. The surface carbon is then oxidized to COs with surface oxygen, and then COs may be desorbed as CO(g) or oxidized to CO2s and desorbed as CO2(g). At low temperature, COs oxidation is faster, resulting in high CO2 selectivity. However, the CH4 conversion is limited due to limited supply of gaseous oxygen, and the subsequent CO2 reforming will proceed slowly due to low temperature. At high temperature, COs desorption is faster and the temperature increase will be smaller. The rate of CO2 reforming may be high due to high temperature, but the extent of reaction is low due to a small amount of CO2, resulting in a small temperature decrease. The same explanation could be given to the surface hydrogen. Desorption as H2 and oxidation to H2O compete, and the H2 selectivity may be primarily determined by how fast the desorption of hydrogen occurs. The subsequent steam reforming occurs slowly at low temperature
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or the extent of this reaction is small due to the small amount of produced H2O at high temperature. 4.2. Active species While completely oxidized products are formed over NiO, methane decomposition occurs primarily on the metallic nickel and/or partially reduced nickel. It is certain that metallic nickel is an essential active component in POM. However, fully reduced nickel alone, treated in H2 at 1023 K for 3 h, did not appear to be so active (Figs. 5 and 6), while the catalyst activated by CH4/O2 (2/1) gas, which is a weaker reducing agent, showed much higher activity (Fig. 2). A similar suggestion has been reported from a study on POM over Ni/ Ce-ZrO2/u-Al2O3, where it is concluded that the reactive oxygen in partially oxidized nickel species (NiOx) produces CO and H2 more rapidly, although POM still calls for the presence of metallic nickel [14]. It is suggested that the reactive oxygen species in NiOx is formed from gas phase oxygen by the metallic nickel and then reacts with surface carbon species to produce CO. Meanwhile, Au et al. [1,20] showed that oxygen co-adsorbed at metal on-top sites promoted methane dissociation. Therefore, the results in this work also suggest that partially reduced nickel species is needed to exhibit high activity and selectivity. The carbon deposited on our catalyst appeared to be rapidly converted to an unreactive form, since very small amounts of CO and CO2 were produced during oxygen pulsing after methane pulses. This is different from the results on other Ni catalysts supported on oxides [14,19]. This is a peculiar characteristic of our catalyst, and might be due to the absence of the oxide support. It has been shown that carbon deposited on Ni can form very unreactive graphitic layers and that the coking can be prevented more effectively as the oxygen mobility of the support becomes higher [31]. Since the support of our catalyst is an apatite, the oxygen mobility may be relatively low. However, when gaseous oxygen is cofed, the deposited carbon can be continuously eliminated before it is transformed to an unreactive form. Actually, our catalyst showed stable behavior in POM over 80 h at 1023 K [27]. 5. Conclusions
Scheme 1. Proposed POM reaction routes on Ni0 and NiOx.
Based on the results of the pulse study and temperature profiles of the monolith catalyst bed of a nickel-calcium hydroxyapatite catalyst, the following conclusions were drawn. On NiO, the complete oxidation occurs exclusively. However, on metallic nickel and partially reduced nickel, direct dissociation of methane, or the pyrolysis mechanism, primarily occurs. This is supported by the observation that carbon deposition occurred to a great extent on reduced nickel. Although the partial oxidation and the complete oxidation occur competitively, COs is a common intermediate and it is rapidly desorbed to produce CO(g) especially at high temperature or converted to CO2(g) especially at low temperature. Consequently, the temperature increase in the monolith becomes
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smaller as the furnace temperature increased. The subsequent steam and CO2 reforming reactions take place slowly at low temperature or the extents of the reforming reactions are small at high temperature, and the resulting temperature decrease is not large. The observation that the fully reduced catalyst exhibited lower activity suggests that both metallic Ni and partially oxidized nickel are required in order to exhibit high activity and selectivity.
[10] [11] [12] [13]
Acknowledgement
[17]
This work was financially supported by the Korea Institute of Science and Technology.
[18] [19] [20] [21]
[14] [15] [16]
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Mechanism of partial oxidation of methane over a nickel-calcium hydroxyapatite catalyst Jin Hyuk Jun a,1, Tae Hoon Lim b, Suk-Woo Nam b, Seong-Ahn Hong b, Ki June Yoon a,* b
a Department of Chemical Engineering, Sungkyunkwan University, Suwon 440-746, South Korea Battery and Fuel Cell Research Center, Korea Institute of Science and Technology (KIST), Seoul 136-791, South Korea
Received 16 March 2006; received in revised form 13 June 2006; accepted 14 June 2006 Available online 24 July 2006
Abstract The mechanism of partial oxidation of methane to synthesis gas over a nickel-calcium hydroxyapatite catalyst was studied by employing pulse experiments for the powder catalyst and by measuring temperature profiles of the activated, washcoated monolith catalyst. The pulse study showed that after the catalyst was partially reduced, carbon deposition occurred to a great extent and CO was predominantly produced over CO2. Temperature profiles of the monolith catalyst showed that the highest temperature difference between the furnace and the monolith became smaller as the furnace temperature increased. We propose that the reaction occurs primarily via the pyrolysis mechanism or direct dissociation of methane. Adsorbed CO (COs) is a common intermediate and it is rapidly desorbed to produce CO(g), especially at high temperature, or converted to CO2(g), especially at low temperature. The observation that the fully reduced catalyst exhibited lower activity suggests that both metallic Ni and partially oxidized nickel are required in order to exhibit high activity and selectivity. # 2006 Elsevier B.V. All rights reserved. Keywords: Calcium hydroxyapatite; Mechanism; Methane partial oxidation; Nickel; Pulse experiment
1. Introduction Catalytic partial oxidation of methane (POM) for synthesis gas and hydrogen production has been an active research subject in recent years. This process arouses interest when compared with the well-established steam reforming of methane, since POM is mildly exothermic and does not require high operating pressures and hence is more energy efficient [1–14]. Although many studies on the mechanism for POM have been reported, the mechanism is still debated. In the literature, two alternative mechanisms for POM have been proposed: (i) the two-step mechanism or the indirect scheme [6,15–17], according to which the initial complete oxidation of methane is followed by reforming of the residual methane with CO2 and * Corresponding author. Tel.: +82 31 290 7244; fax: +82 31 290 7272. E-mail addresses: [email protected] (J.H. Jun), [email protected] (K.J. Yoon). 1 Present address: Doosan Technical Center, Electro-Materials BG, Doosan Corp., 39-3 Sungbok-dong, Yongin-si 449-795, South Korea. 0926-860X/$ – see front matter # 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2006.06.020
H2O that are produced primarily; (ii) the pyrolysis mechanism or the direct scheme [1–3,14,18–22], according to which methane first dissociates to generate hydrogen and carbon, then H2 desorbs and carbon is oxidized to carbon monoxide by surface oxygen species. The two-step mechanism is characterized by a hot spot near the entrance of the catalyst bed and a temperature drop in the rear part of the bed. On the other hand, Schmidt and coworkers [10,18] proposed the pyrolysis mechanism and several researchers have supported this mechanism. Au and coworkers [1,19] have reported from pulse studies that the reaction over reduced Ni/SiO2 and Rh/ SiO2 catalysts occurs via the pyrolysis mechanism, while CO2 is produced by subsequent oxidation of CO. They also reported that methane dissociation and adsorption was accelerated by adsorbed oxygen on the surface of the catalyst, and this was later theoretically verified [20]. Liu et al. [14] have reached similar conclusions from a pulse study over a reduced Ni/CeZrO2/u-Al2O3 catalyst. Some investigators have reported that the reaction pathway depends strongly on the metal as well as the support. Nakagawa et al. [23] reported that the synthesis gas was formed via a
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pyrolysis mechanism over Rh/Al2O3 and Rh/TiO2, while it was produced via a two-step mechanism over Rh/SiO2 and Ir/TiO2. Weng et al. [24] reported that it was via the pyrolysis mechanism over reduced Rh/SiO2 while it was via the two-step mechanism over Ru/SiO2 and Ru/Al2O3. On the other hand, Verykios and coworkers [25,26] suggested direct formation of CO and H2 over Ru/TiO2 catalysts. Recent investigations in this laboratory have demonstrated that nickel-calcium hydroxyapatite catalysts exhibit excellent performance for POM [27–29]. Although an essential active component is metallic Ni, these catalysts contain no oxide support unlike most of the supported Group VIII metal catalysts. Another salient feature for these catalysts is that the catalyst activation can be done with the reactant gases (CH4 + O2) at a relatively low temperature (at or below 923 K). The main components in the catalyst with the optimum composition are metallic Ni, NiO and calcium hydroxyapatite (Ca10(OH)2(PO4)6) but the calcium phosphate phase is minor. In this work, the reaction mechanism for POM over a nickel-calcium hydroxyapatite catalyst was studied by employing pulse experiments and by measuring temperature profiles of the monolith catalyst bed. The reaction results from CH4–O2–CH4 and CH4/O2 pulses over reduced and unreduced catalysts were compared. The active constituents of the catalyst are also discussed. 2. Experimental 2.1. Catalyst preparation A Ni-calcium hydroxyapatite powder catalyst having a composition of Ca9.5Ni2.5(PO4)6 was prepared from nearly saturated aqueous solutions of calcium nitrate, nickel nitrate and dibasic ammonium phosphate as described in the previous works [27,28]. The pH of each of the aqueous solutions was adjusted to 10–11 by adding ammonia water, and predetermined amounts of the solutions were mixed with vigorous stirring. The mixture was dried at 383 K overnight and finally heat-treated in air at 1073 K for 2 h to obtain the catalyst. The solid catalyst was crushed and sieved, and particles of 40- to 80mesh size were used for the pulsed reaction. This catalyst has the composition near the optimum, as reported in the previous works [27,28]. A honeycomb support was employed to prepare the monolith catalyst. The honeycomb is made from cordierite (2Al2O32MgO5SiO2) with channels of square cross-section, and the cell density is 400 cells per square inch (cpsi). The honeycomb was cut so that the monolith had 17 cells, the length of 28 mm, and the diameter of ca. 8 mm. Four walls of the cells at the center of the monolith were removed in order that a 1/ 16 in. (1.6 mm) thermocouple could be inserted. The weight of the monolith before the washcoating was about 0.42 g. In order to use as the washcoat, the catalyst powder was crushed into fine particles (below a few tens of micrometers). The fine powder was suspended in water, and the washcoated monolith catalyst was prepared by repeated dipping into the suspension. After each dipping, it was dried at 423 K for 1 h. This step was repeated until the net weight of the coated powder catalyst
became 0.1 g. The monolith catalyst was employed for reaction after calcinations at 923 K in air. 2.2. Pulse experiments Pulse experiments using CH4, O2 and/or CH4/O2 with a molar ratio of 2 were carried out in a quartz micro-reactor, as described in an earlier work [14]. Prior to each reaction, 30 mg of the powder catalyst was loaded in the reactor and treated at the desired temperature in He (30 ml/min) for 90 min to remove any residual gases in the system, and then the powder was exposed to pulses of CH4, O2 and/or CH4/O2. The amount of each pulse was 0.25 cm3 (STP) and the time interval between pulses was about 10 min. After each pulse, the exit gas was analyzed on-line with a gas chromatograph equipped with a TCD (Carbosphere column, 353 K, helium as carrier gas). The conversion and selectivity were calculated on the basis of carbon contents in the products and 100% carbon and oxygen balances. 2.3. Measurement of temperature profiles For the measurement of the temperature profiles in the reactor, the reaction was carried out by a conventional method using an 8 mm i.d. quartz-tube flow reactor with down-flow of the gas. The middle of the tube was narrowed somewhat so that the monolith could be located there. The reaction temperature was controlled with an electric furnace. The partial pressures of methane and oxygen were 16.2 and 8.1 kPa (0.16 and 0.08 atm), respectively. Ar was used as the diluent gas and the total flow rate was 100 cm3 (STP)/min. The catalyst activation was first carried out at the furnace temperature of 1023 K for 30 min with the reactant gas (without employing the hydrogen pretreatment), since this temperature had been found to be sufficiently high for the activation of the catalyst [28], and then the temperature was set to the desired reaction temperature. During the reaction, the temperatures in the reactor, including the inside of the monolith, were measured at several positions by using the inserted thermocouple. The thermocouple could be moved up and down with negligible leaking by slightly loosening the nut with a Teflon ferrule. One thing to note here is that the measured temperature does not indicate the true temperature of the catalyst powder or the active component inside due to the limitation of the measuring device but is a measure of relative difference. The product gas was analyzed using two gas chromatographs with Carboxen 1004 columns (Supelco); one used Ar as the carrier gas and the other used He. The latter was needed especially when the concentration of produced CO2 was too low to detect by using the Ar carrier gas. 3. Results 3.1. Pulse study The catalyst employed in this work had been characterized by several methods in a previous work [28]. In order to better understand the characteristics of this catalyst, the main results
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of the previous work are briefly introduced here. From XRD analysis for the fresh, unreduced catalyst, it was observed that calcium hydroxyapatite and NiO were the major crystalline phases, but that calcium phosphate was present in a very small amount and no metallic nickel was present. For the used catalyst, which had been reduced at 1023 K with a reactant gas (CH4 + O2 + Ar) and then experienced a set of POM experiments at temperatures from 1073 to 673 K, the amounts of calcium hydroxyapatite and calcium phosphate were observed to change little during the reaction, but most of NiO was reduced to metallic Ni. It was confirmed that some of the nickel in the fresh catalyst was present in the hydroxyapatite and phosphate structures by substituting the calcium ions but this nickel in those structures came out as fine, reduced nickel particles during the POM reaction. From the TEM results for the used catalyst, many particles of a few nanometers in size were observed, although large particles, their size being several tens of nanometers, were also present in a considerably small number. When the fresh catalyst was treated with a reactant gas, only CO2 and H2O were produced below 873 K, but large amounts of CO and H2 were produced at and above 923 K. This shows that the Ni2+ in the catalyst can be reduced by the reactant gas at and above 923 K. To obtain some mechanistic information for methane partial oxidation over nickel-calcium hydroxyapatite, pulse experiments with CH4, O2 and/or CH4/O2 with a molar ratio of 2 were carried out. Fig. 1 shows the results for the reaction with sequential pulses of CH4–O2–CH4 over the fresh, unreduced catalyst at 1023 K. The methane conversion for the first and second CH4 pulses was very high but it decreased rapidly from the third pulse. The CO2 selectivity was high for the first and second pulses. But the CO2 selectivity drastically decreased at the third pulse and was virtually 0% from the fourth pulse, while the CO selectivity increased sharply and reached the
Fig. 1. Results for sequential pulses of CH4, O2 and CH4 over unreduced Ca9.5Ni2.5(PO4)6 at 1023 K.
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maximum of about 60% at the third pulse and then rapidly decreased. The above results indicate that, in the first place, the nickel in the catalyst is reduced mainly with the formation of CO2 by the reaction of CH4 with the lattice oxygen and then with the formation of CO. The increase of the CO selectivity is due to the small amount of the lattice oxygen left, which is not sufficient for deep oxidation of the carbon. From the amount of oxygen in the CO2 and CO products for the first three pulses, it was estimated that more than 70% of the total nickel in the catalyst was reduced. As the nickel was reduced to the metallic state with the exhaustion of the oxygen, carbon deposition occurred to a great extent and the methane conversion rapidly decreased. The selectivity to the deposited carbon, Cs, rapidly increased from the third pulse and it reached about 90% at the fifth pulse. These results are similar to those reported in an earlier work on Ni/SiO2 [19]. For the subsequent oxygen pulses, oxygen was all consumed up to the fifth pulse but negligible amounts of CO and CO2 were produced. This indicates that the deposited carbon in this catalyst is very difficult to oxidize. Moreover, the total amount of oxygen injected by the five pulses is well above the amount needed to re-oxidize the reduced nickel. Therefore, we consider that there are some species other than Ni or deposited carbon that could react with the injected oxygen. This is not well cleared up at the moment, and that is why we used the term ‘sorbed oxygen’. Those species might be pyrophosphate compounds, such as Ca2P2O7 that can be oxidized to the orthophosphate Ca3(PO4)2 (note: the presence of the pyrophosphate is hard to identify by XRD and other methods). The results for the oxygen pulses are peculiar for the catalyst in this work, and are markedly different from those for the Ni/CeZrO2/u-Al2O3 and Ni/SiO2 catalysts [14,19]. For O2 pulses after CH4 pulses, quite large amounts of CO and CO2 were produced by the Ni/Ce-ZrO2/u-Al2O3 and Ni/SiO2 catalysts, which means that Cs in these catalysts could easily be oxidized. For the subsequent second-series CH4 pulses, the results were similar to those for the first-series CH4 pulses. However, production of CO2 was smaller for the first pulse compared with that for the first series. For the second pulse, the CO2 selectivity was very small, while the CO selectivity was high. This indicates that the amount of reactive oxygen after the oxygen pulses is smaller compared with the amount for the original fresh sample. One thing to note here is that not all of the sorbed oxygen during the oxygen pulsing was used for the CH4 oxidation. That is, this means that a portion of the sorbed oxygen is present in a form that is difficult to use for the CH4 oxidation. In other words, a large part of the sorbed oxygen may be present in the inner of nickel particles or bonded with other elements such as P. Fig. 2 shows the results of CH4/O2 (2:1) pulse reactions over the fresh, unreduced catalyst at 1023 K. As with the results in Fig. 1, the CO2 selectivity was high for the first and second pulses but then decreased sharply. This indicates that the nickel was reduced with consumption of the lattice oxygen by CH4. From the fourth pulse, the CO selectivity was very high and remained nearly 100%. This suggests that the POM follows the pyrolysis mechanism on the reduced Ni atoms. Although from
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the analysis the Cs selectivity was calculated to be negligible, the activity of the catalyst decreased gradually from the fourth pulse, and this is considered most probably due to the deposited carbon. Fig. 3 shows the results at 923 K for sequential pulses of CH4– O2–CH4 over the fresh, unreduced catalyst. For the first-series CH4 pulses, the amount of the consumed oxygen for the production of CO and CO2 were significantly lower compared with that at 1023 K due to the lower temperature. However, since a considerable portion of the nickel was reduced, the Cs
selectivity increased sharply while the CH4 conversion decreased rapidly. This trend is similar to the case at 1023 K. For the subsequent O2 pulses, the results were again similar to those for the case at 1023 K. Most of the O2 was sorbed, but the amounts of CO and CO2 were somewhat larger than those at 1023 K. For the second-series CH4 pulses, the CO2 selectivity decreased gradually while the CO selectivity increased gradually. The CH4 conversion was rather low (less than 20%) but a rapid decrease was not observed since the Cs selectivity was not high. This can be explained by the low reaction rate owing to the low temperature and low active surface area. During the first-series CH4 pulses, the active nickel surface was covered with the deposited carbon but only a minor part of it was restored during the O2 pulses. Thus, during the second-series CH4 pulses, the oxygen in the catalyst was gradually consumed and the Cs selectivity gradually increased. Nevertheless, these results also support the pyrolysis mechanism on the reduced nickel. For the pulses of CH4/O2 (2:1) mixture over the unreduced catalyst at 923 K, CH4 conversions were quite low (about 10%) but remained almost constant up to 10 pulses, and the product was exclusively CO2 (Fig. 4). Since gaseous oxygen was cofed, the catalyst would not have the opportunity to be reduced because the conversion was considerably lower than 25%, which is the theoretical value needed to provide a reducing environment. This indicates that the complete combustion dominates over the oxidized nickel and that carbon deposition is negligible. Fig. 5 shows the results of CH4–O2–CH4 pulse reactions at 1023 K over the catalyst that was pre-reduced by H2 at 1023 K for 3 h. For the first-series CH4 pulses, the methane conversion was only 24% for the first pulse and then decreased to 5% for the fifth pulse. Cs was the main product and only a small amount of CO was produced, but no CO2 was detected. This is certainly due
Fig. 3. Results for sequential pulses of CH4, O2 and CH4 over unreduced Ca9.5Ni2.5(PO4)6 at 923 K.
Fig. 4. Results for CH4/O2 (2/1) pulses over unreduced Ca9.5Ni2.5(PO4)6 at 923 K.
Fig. 2. Results for CH4/O2 (2/1) pulses over unreduced Ca9.5Ni2.5(PO4)6 at 1023 K.
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Fig. 5. Results for sequential pulses of CH4, O2 and CH4 over reduced Ca9.5Ni2.5(PO4)6 at 1023 K.
to the deficiency of oxygen source to react with methane owing to the pre-reduction of the catalyst. During the subsequent oxygen pulses, injected oxygen was almost all sorbed in the catalyst and was not used to produce the CO and/or CO2. This is similar to the results of Figs. 1 and 3. For the next-series CH4 pulses, the methane conversion was low, but the CO selectivity was significantly higher and the Cs selectivity was lower compared with the case for the first-series CH4 pulses. These results show that carbon deposition occurs to a great extent on the reduced nickel and that oxygen in the catalyst is beneficial for the prevention of carbon deposition. Fig. 6 shows the results of CH4/O2 (2:1) pulse reactions at 1023 K over the same pre-reduced catalyst. The CH4 conversion was very low, but it increased slowly from about 1% at the first pulse to about 3% at the tenth pulse. The CO selectivity was virtually 100%. The reduced catalyst would have been expected to exhibit high activity since most of the nickel in the catalyst is present as the metallic nickel. But the above results show that fully reduced Ni is not so active for POM.
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Fig. 6. Results for CH4/O2 (2/1) pulses over reduced Ca9.5Ni2.5(PO4)6 at 1023 K.
between the measured temperature in the reactor during the reaction (the reactor or bed temperature) and the temperature in the reactor without the reaction (the furnace temperature). Fig. 7 shows that the temperature rises in the monolith. The positive DT before the monolith may be due to heat transfer or
3.2. Temperature profiles of a monolith reactor The temperatures measured in the reactor during the reaction over the activated monolith catalyst were compared with those with Ar-only flow. The temperatures measured with only Ar flow, or the furnace temperatures, are shown by the dotted lines in Fig. 7. The actual furnace temperatures were not uniform due to the buoyancy and the temperature difference between the ends of the monolith was about 15 K. For convenience sake, the mentioned temperature hereafter denotes the representative (set) furnace temperature at the 15 mm position unless specified otherwise. DT is the temperature difference at the same position
Fig. 7. Temperature profiles of the monolith reactor in POM for different furnace temperatures ((*) 770 K; (&) 860 K; (~) 1023 K; ( ) temperature profile at each furnace temperature without reaction; PCH4 = 0.16 atm; CH4/ O2 = 2; total flow rate = 100 cm3/min).
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the back mixing of the gas. The maximum DT became smaller as the reaction temperature increased: the maximum DTs were +70, +60 and +40 K at 770, 860 and 1023 K, respectively. As the reaction temperature increased, the position of the highest temperature moved toward the inlet: 11 mm at 770 K, 5 mm at 875 K and 3 mm at 1023 K. This is surely because the reaction rate becomes faster at the higher temperature. The temperature decrease afterward was partly due to the heat loss through the reactor sidewall [30] and partly due to the endothermic reactions. After the highest temperature, however, the bed temperature decreased faster as the reaction temperature increased even though DT was smaller. At 770 K, the decreasing temperature curve appeared convex and the temperature at the monolith outlet was significantly higher than the furnace temperature. This indicates that the extent of endothermic reactions in the rear part of the bed is small and/or that a mildly exothermic reaction such as partial oxidation may be occurring to some extent even though the heat loss is taken into account. At 860 K, the decreasing temperature curve was concave and the outlet temperature became nearly the same as the furnace temperature. At 1023 K, the temperature decreased more rapidly and at about 15-mm position the bed temperature became the same as the furnace temperature; at the outlet the bed temperature became slightly lower than the furnace temperature. The above results indicate that endothermic reactions are occurring faster in the rear part of the bed as the temperature increases but that the extent of the endothermic reactions was not large; otherwise, more rapid decrease of the temperature and a large negative DT would have been observed. Fig. 8 shows the product analysis results. These results are nearly the same as those obtained over 0.2 g of the powder catalyst reported in the previous work [28]. At 770 K, the CH4 conversion was somewhat lower than the equilibrium, but the CO selectivity was considerably lower than the equilibrium.
Fig. 8. CH4 conversion, CO selectivity and H2 yield over the monolith catalyst (PCH4 = 0.16 atm; CH4/O2 = 2; total flow rate = 100 cm3/min).
This indicates that the CO2 formation is dominant. However, H2 yield was much closer to the equilibrium, indicating that H2O formation is small. At 860 K, the CH4 conversion and CO selectivity were slightly lower than the values at equilibrium but the H2 yield was a little bit higher than the value at equilibrium. At 1023 K, the CH4 conversion and CO selectivity were slightly lower than the values at equilibrium and the H2 yield was nearly the same as the value at equilibrium. These results strongly support the claim that the direct scheme is dominant. 4. Discussion 4.1. POM mechanism As shown in the CH4 pulse study on the oxidized nickel, the carbon deposition was small and the CO2 formation was dominant. This has been reported by several authors [5,11,14,15] and it is attributed to the fact that the metal oxide does not easily dissociate methane to surface carbon species. On the nickel reduced by the CO2 formation, however, the carbon deposition occurred to a great extent and the CO formation was predominant over CO2 formation. During the second-series CH4 pulses after O2 pulses, the CO selectivity became considerably higher compared with that during the first-series CH4 pulses (Figs. 1 and 3). This is because the catalyst can be more easily reduced since it has once been reduced during the first-series pulses, as reported in the previous work [28]. The CH4/O2 (2:1) pulse study at 1023 K on the fresh, unreduced catalyst also showed that the catalyst was reduced by forming CO2 and then the CO selectivity became near 100% (Fig. 2). These results are in agreement with the earlier works on Ni/Ce-ZrO2/u-Al2O3 and Ni/SiO2 [14,19] and strongly support the claim that decomposition of CH4 primarily occurs on the reduced nickel. The CH4/O2 (2:1) pulse study at 923 K on the fresh, unreduced catalyst showed that only CO2 was formed (Fig. 4). In this case, the reason is that the nickel could not be reduced because oxygen was cofed and a reducing environment was not provided. On the contrary, when only methane was introduced, the catalyst was reduced and carbon deposition occurred to a great extent, as shown in Fig. 3. The measured temperature profiles of the monolith also support the pyrolysis mechanism. If the highly exothermic complete oxidation takes place first, a larger and sharper temperature increase would have been observed as the temperature increases, since the reaction rate becomes faster. However, the observation was the opposite (Fig. 7). As the temperature increases, it is apparent that the mildly exothermic partial oxidation occurs predominantly. The H2 yield at 770 K was closer to the equilibrium than the CO selectivity, and the H2 yield at 860 K was slightly higher than the equilibrium (Fig. 8). This means that the H2O formation is significantly lower than the value at equilibrium, and this could not occur if the complete oxidation takes place first. The surface hydrogen atom may be combined and desorbed as H2(g) or oxidized and desorbed as H2O(g). The activation energy for combination of surface hydrogen atoms so as to desorb as H2 on nickel clusters
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(124 kJ/mol) has been reported to be nearly the same as that for combination of surface carbon and oxygen so as to desorb as CO (125 kJ/mol) [20]. Moreover, formation of H2O on Ni is much less favorable than that on Pt or Pd due to relative instability of OH species on Ni [10,20]. This suggests that H2(g) can be more readily produced on Ni than CO(g) can. The higher CO2 formation at low temperatures may also be explained by the pyrolysis mechanism. As suggested by Au et al. [19], this may be caused by the difference of the activation energies of desorption and oxidation of adsorbed CO (COs). The surface C species reacts with the surface oxygen species to give adsorbed COs. The activation energy of COs desorption on Ni(1 1 1) surface (113 kJ/mol) is about two times higher than that of COs oxidation (64 kJ/mol). Moreover, the surface oxygen species may be more abundant than Cs at lower temperature. Therefore, the lower temperature would favor COs oxidation rather than COs desorption, leading to higher CO2 selectivity. This suggestion has also been supported by other investigators [1,4,14]. In summary, based on the experimental results and the above discussion, the following mechanism is proposed as shown below in Scheme 1. On metallic nickel, methane is primarily dissociated into surface carbon and hydrogen. The surface carbon is then oxidized to COs with surface oxygen, and then COs may be desorbed as CO(g) or oxidized to CO2s and desorbed as CO2(g). At low temperature, COs oxidation is faster, resulting in high CO2 selectivity. However, the CH4 conversion is limited due to limited supply of gaseous oxygen, and the subsequent CO2 reforming will proceed slowly due to low temperature. At high temperature, COs desorption is faster and the temperature increase will be smaller. The rate of CO2 reforming may be high due to high temperature, but the extent of reaction is low due to a small amount of CO2, resulting in a small temperature decrease. The same explanation could be given to the surface hydrogen. Desorption as H2 and oxidation to H2O compete, and the H2 selectivity may be primarily determined by how fast the desorption of hydrogen occurs. The subsequent steam reforming occurs slowly at low temperature
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or the extent of this reaction is small due to the small amount of produced H2O at high temperature. 4.2. Active species While completely oxidized products are formed over NiO, methane decomposition occurs primarily on the metallic nickel and/or partially reduced nickel. It is certain that metallic nickel is an essential active component in POM. However, fully reduced nickel alone, treated in H2 at 1023 K for 3 h, did not appear to be so active (Figs. 5 and 6), while the catalyst activated by CH4/O2 (2/1) gas, which is a weaker reducing agent, showed much higher activity (Fig. 2). A similar suggestion has been reported from a study on POM over Ni/ Ce-ZrO2/u-Al2O3, where it is concluded that the reactive oxygen in partially oxidized nickel species (NiOx) produces CO and H2 more rapidly, although POM still calls for the presence of metallic nickel [14]. It is suggested that the reactive oxygen species in NiOx is formed from gas phase oxygen by the metallic nickel and then reacts with surface carbon species to produce CO. Meanwhile, Au et al. [1,20] showed that oxygen co-adsorbed at metal on-top sites promoted methane dissociation. Therefore, the results in this work also suggest that partially reduced nickel species is needed to exhibit high activity and selectivity. The carbon deposited on our catalyst appeared to be rapidly converted to an unreactive form, since very small amounts of CO and CO2 were produced during oxygen pulsing after methane pulses. This is different from the results on other Ni catalysts supported on oxides [14,19]. This is a peculiar characteristic of our catalyst, and might be due to the absence of the oxide support. It has been shown that carbon deposited on Ni can form very unreactive graphitic layers and that the coking can be prevented more effectively as the oxygen mobility of the support becomes higher [31]. Since the support of our catalyst is an apatite, the oxygen mobility may be relatively low. However, when gaseous oxygen is cofed, the deposited carbon can be continuously eliminated before it is transformed to an unreactive form. Actually, our catalyst showed stable behavior in POM over 80 h at 1023 K [27]. 5. Conclusions
Scheme 1. Proposed POM reaction routes on Ni0 and NiOx.
Based on the results of the pulse study and temperature profiles of the monolith catalyst bed of a nickel-calcium hydroxyapatite catalyst, the following conclusions were drawn. On NiO, the complete oxidation occurs exclusively. However, on metallic nickel and partially reduced nickel, direct dissociation of methane, or the pyrolysis mechanism, primarily occurs. This is supported by the observation that carbon deposition occurred to a great extent on reduced nickel. Although the partial oxidation and the complete oxidation occur competitively, COs is a common intermediate and it is rapidly desorbed to produce CO(g) especially at high temperature or converted to CO2(g) especially at low temperature. Consequently, the temperature increase in the monolith becomes
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smaller as the furnace temperature increased. The subsequent steam and CO2 reforming reactions take place slowly at low temperature or the extents of the reforming reactions are small at high temperature, and the resulting temperature decrease is not large. The observation that the fully reduced catalyst exhibited lower activity suggests that both metallic Ni and partially oxidized nickel are required in order to exhibit high activity and selectivity.
[10] [11] [12] [13]
Acknowledgement
[17]
This work was financially supported by the Korea Institute of Science and Technology.
[18] [19] [20] [21]
[14] [15] [16]
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