Topics in Catalysis Vol. 22, Nos. 1/2, January 2003 (# 2003)
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Catalysis of methane coupling with carbon dioxide over binary oxides Ye Wanga,* and Yasuo Ohtsukab a
State Key Laboratory for Physical Chemistry of Solid Surfaces, Institute of Physical Chemistry, Department of Chemistry, Xiamen University, Xiamen 361005, China E-mail:
[email protected] b Research Center for Sustainable Materials Engineering, Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Katahira, Aoba-ku, Sendai 980-8577, Japan
Binary oxides of Ca–Ce, Ca–Cr, and Ca–Mn exhibit good performance and similar kinetic behavior in the conversion of CH4 to C2 hydrocarbons with CO2, whereas the corresponding Sr- and Ba-containing catalysts show lower activity in C2 formation except for Sr–Mn and Ba–Mn oxides. The Sr–Mn oxide provides even higher C2 yield than the Ca-containing catalysts. Characterization reveals that solid solution and composite oxides comprising Ca2þ species and the redox component (Ce, Cr, or Mn) exist at a steady state of reaction and probably account for the synergy in C2 formation over the Ca-containing catalysts. On the other hand, Sr and Ba carbonates are formed along with Ce, Cr, or Mn oxides during the reactions over Sr- and Ba-containing oxides. The carbonates, however, can react with MnO to form SrMnO2.5 and BaMnO2.5, the probable active species for CH4 activation over the Sr–Mn and Ba–Mn catalysts. KEY WORDS: methane coupling; carbon dioxide; C2 hydrocarbons; binary oxide catalysts
1. Introduction Simultaneous utilization of CH4 and CO2 has attracted much attention from an environmental point of view, since natural gas frequently contains a high concentration of CO2 in addition to CH4. The use of CO2 as an oxidant for the selective oxidation of lighter alkanes including CH4 may also become important, because it is expected that the replacement of O2 with CO2 inhibits the gas-phase non-selective oxidation induced by O2 and thus increases the selectivity to a partial oxidation product. Our research group has focused on CH4 coupling to C2 hydrocarbons using CO2 instead of O2. Thermodynamic calculations show that equilibrium conversions of CH4 with CO2 (CO2/ CH4 ¼ 2Þ to C2H6 and C2H4 exceed 15 and 25% at 800 C, respectively. Although several catalysts [1–3] were effective for CH4 coupling with O2 and a large number of unsupported single oxides [4,5] were tested for C2 formation from CH4 and CO2, their performances were not satisfactory and, in some cases, the catalysts were not stable. The present authors have shown that several binary oxides such as Ca–Ce [6,7], Ca–Cr [8], and Ca–Zn [9] are effective for this reaction. It has been proposed that the redox component in each catalyst activates CO2, providing active oxygen species for CH4 coupling, and the basic Ca2þ enhances the selectivity to C2 hydrocarbons [7]. Recently, we have also found that Sr–Mn and Ba–Mn oxides exhibit some unique catalytic properties for C2 formation [10]. This paper highlights the elucidation of the differences in *To whom correspondence should be addressed.
catalysis by these systems and Ca-containing binary oxides.
2. Experimental All binary catalysts were prepared by the impregnation method using a reducible powder oxide, such as CeO2 or MnO2, and an aqueous solution of Ca, Sr, or Ba nitrate, followed by air calcination at 850 C [7]. The atomic ratio of the two components in each catalyst was 1.0, unless otherwise stated. Catalytic reactions were performed using a conventional fixed-bed quartz reactor. Before reaction, each catalyst charged into the reactor was pretreated at 850 C with air for 1 h and then purged with He (>99.9999%) for 1 h. The reaction started with the introduction of feed gas of CH4 (>99.999%) and CO2 (>99.995%) to the reactor. The following conditions were mainly used: T ¼ 850 C, PðCH4 Þ ¼ 30:3 kPa, PðCO2 Þ ¼ 70:7 kPa, W ¼ 2 g, FðtotalÞ ¼ 0:268 mol (STP) h1. Products with H2O removed were analyzed by an online high-speed micro gas chromatograph. The details of data processing have been reported elsewhere [7]. The carbon balance was calculated to be better than 95%, and no carbon deposition was observed over the binary oxide catalysts. Catalyst characterization was carried out by X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). For this purpose, the catalyst after reaction was quenched from 850 C to room temperature in a flow of high-purity He and instantly transferred to the XRD or XPS detection chamber. 1022-5528/03/0100-0071/0 # 2003 Plenum Publishing Corporation
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Ye Wang, Y. Ohtsuka/Methane coupling with carbon dioxide
3. Results and discussion 3.1. Catalytic performance and kinetic behavior Table 1 shows the catalytic performance of a series of binary oxides at steady state. These results were obtained after ca. 15 and 30 h of reaction for the Sr–Mn and Ba–Mn binary oxides, and ca. 10 h for the other catalysts. The amounts of oxygen species needed for such conversions were much larger than those of reactive lattice oxygen atoms existing in the catalysts. Thus, it is clear that the formation of C2 hydrocarbons at steady state is not caused by the stoichiometric reaction between CH4 and the reactive lattice oxygen but by the catalytic reaction with CO2 as an oxidant. Each single component of these binary oxides was almost inactive for C2 formation, C2 yield being 0.3% with Mn oxide and <0.1% with other single oxides. On the other hand, all of the Ca-containing binary catalysts (Ca–Ce, Ca–Cr, and Ca–Mn) enhanced C2 formation drastically and thus C2 yields over these oxides reached 4%, independent of the type of catalyst. Thus, there existed an obvious synergistic effect in C2 formation between Ca and the other component (Ce, Cr, or Mn). When Sr or Ba oxide was used in place of the Ca component, the resulting binary oxides showed much lower C2 yields except for Sr–Mn and Ba–Mn oxides. Among these Mnbased catalysts, the Sr–Mn oxide exhibited higher C2 selectivity and C2 yield than all the Ca-containing binary catalysts. Figure 1 shows the effect of catalyst composition on CH4 conversion, C2 selectivity, and C2 yield over the Sr– Mn binary oxides. The tendencies of the changes in CH4 conversion and C2 selectivity with catalyst composition were similar to those for the Ca-based catalysts reported previously [11], confirming that the synergy in C2 formation also existed for the Sr–Mn binary oxides. The
Figure 1. Effect of catalyst composition on catalytic performance over the Sr–Mn binary oxides: *, CH4 conversion; &, C2 selectivity; *, C2 yield.
composition with Sr/Mn ratio of 1.0 was the most appropriate for C2 formation. Figure 2 compares the dependence of CH4 conversion and C2 selectivity on P(CO2) over the Ca-containing catalysts and the Sr–Mn oxide. In the absence of CO2,
Table 1 Steady-state catalytic performance of binary oxidesa. Catalystb
Ca–Ce (0.5) Ca–Cr (1) Ca–Mn (1) Sr–Ce (0.5) Sr–Cr (1) Sr–Mn (1) Ba–Ce (0.5) Ba–Cr (1) Ba–Mn (0.5) Ba–Mn (1)
CH4 conversion (%)
5.4 6.3 6.6 2.5 3.9 5.5 0.8 1.1 4.9 3.4
Selectivity (%) ——————— C2 CO 74 64 62 54 31 82 49 38 65 78
26 36 38 46 69 18 51 62 33 23
C2 yield (%)
4.0 4.0 4.1 1.4 1.2 4.5 0.4 0.4 3.2 2.7
T ¼ 850 C, PðCH4 Þ ¼ 30 kPa, PðCO2 Þ ¼ 70 kPa, W ¼ 2 g, F ¼ 0:268 mol (STP) h71. a The results were obtained after ca. 15 and 30 h of reaction for Sr–Mn and Ba–Mn, and ca. 10 h for other catalysts. b The numbers in parentheses denote atomic ratio.
Figure 2. Effect of partial pressure of CO2 on (A) CH4 conversion and (B) C2 selectivity over binary oxide catalysts: &, Ca–Ce; ~, Ca–Cr; *, Ca–Mn; *, Sr–Mn. Reaction conditions: T ¼ 850 C, PðCH4 Þ ¼ 30:3 kPa, W ¼ 2 g, FðtotalÞ ¼ 0:268 mol (STP) h71.
Ye Wang, Y. Ohtsuka/Methane coupling with carbon dioxide
CH4 was mainly converted to CO and H2 by the reaction with lattice oxygen atoms, but the conversion was quickly stopped due to the consumption of the reactive lattice oxygen. In other words, the presence of CO2 was essential for achieving steady formation of C2 hydrocarbons, irrespective of the kind of catalyst [7]. As shown in figure 2, over the three Ca-containing binary oxides, CH4 conversion depended strongly on P(CO2) with a typical Langmuir-type curve; it increased remarkably with P(CO2) in the lower region but almost leveled off beyond 10 kPa. C2 selectivity over these catalysts showed a different dependence on P(CO2) from that of CH4 conversion and increased almost linearly up to the higher region of P(CO2). Over the Sr–Mn oxide, on the other hand, both CH4 conversion and C2 selectivity depended slightly on P(CO2) (figure 2). A similar tendency was observed over the Ba–Mn oxide. To obtain an insight into the influence of P(CO2) on C2 formation over the Ca-containing catalysts, the behavior of CO2 adsorption was examined by measuring temperature-programmed desorption (TPD) profiles of the catalysts after reaction [7,11]. As shown in figure 3, the amount of CO2 chemisorbed increased with increasing P(CO2). These profiles resembled the dependency of C2 selectivity observed in figure 2(B). It is therefore likely that the chemisorbed CO2 accounts for selective conversion of CH4 to C2 hydrocarbons. The increase in P(CO2) may inhibit uncatalyzed reactions involving the lattice oxygen that is responsible for formation of H2 and CO and thus increase C2 selectivity. The apparent activation energies for CH4 conversion with CO2 over Ca–Ce, Ca–Cr, and Ca–Mn catalysts were 190–220 kJ mol1 (700–900 C for Ca–Ce and Ca– Cr, and 840–900 C for Ca–Mn) [7,8], whereas those over Sr–Mn and Ba–Mn oxides were 130–140 kJ mol1 [10]. The lower activation energies and the different
P(CO2) dependency of C2 selectivity observed for the Sr–Mn and Ba–Mn catalysts indicate that the reaction mechanism over them is different from that over the Cacontaining binary oxides. 3.2. Characterization and reaction mechanisms The results from catalyst characterization are summarized in table 2. The XRD measurements revealed that a solid solution (Cax Ce1x O2y ) and composite oxides (CaCrO4 and CaMnO3) existed as the main phases of the Ca-containing catalysts before reaction. After reaction, for the Ca–Ce oxide, the bulk phase was retained, but a new Ce3þ species appeared on the surface. With the Ca–Cr and Ca–Mn catalysts, CaCrO4 and CaMnO3 as the initial phases, respectively, changed to Ca(CrO2)2 and Ca0.48Mn0.52O (a solid solution of CaO and MnO) after reaction. Such transformation corresponded well to the changes in surface species (table 2). Ca(CrO2)2 and Ca0.48Mn0.52O as well as CaxCe1x O2y were stable at steady state of reaction and would thus be the active species for C2 formation. The following mechanism over Ca–Ce, Ca–Cr, and Ca–Mn catalysts may be proposed on the basis of the observations mentioned above. The chemisorption of CO2 on basic Ca2þ sites first takes place (equation (1)), and then neighboring species (Ce3þ , Cr3þ , or Mn2þ ) reductively activate the CO2 chemisorbed by electron transfer to provide CO and O , probably through CO 2 (equation (2)). The oxygen species can convert CH4 to CH3 radical, the precursor of C2 hydrocarbons. The formation of CaxCe1x O2y , Ca0.48Mn0.52O, and Ca(CrO2)2 comprising Ca2þ species and the redox component may play crucial roles for CO2 adsorption and subsequent activation and thus lead to synergistic effect in C2 formation. Ca2þ –O2
CO2 ! CO2 ðaÞ Ce3þ ;Cr3þ ; or Mn2þ
Figure 3. CO2 adsorbed on Ca-containing binary oxide catalysts at 850 C under different partial pressures of CO2. The symbols are the same with those in figure 2.
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ð1Þ
! CO CO2 ðaÞ 2 ! CO þ O
ð2Þ
CH4 þ O !CH3 þ OH
ð3Þ
It should be noted in table 2 that, after CH4 coupling with CO2 at 850 C, no bulk carbonates, e.g. CaCO3, are detectable for any Ca-containing catalysts, but contrarily alkaline earth metal carbonates are the dominant phases over all of the Sr- and Ba-containing binary oxides investigated. For example, with Sr–Ce and Ba– Ce oxides, the SrCeO3 and BaCeO3 observed before reaction were transformed to SrCO3 and BaCO3, respectively, with the formation of CeO2. The segregation of the composite oxide phase comprising alkaline earth metal and Ce4þ species may be responsible for the absence of a synergy in C2 formation over these catalysts; in other words, lower CH4 conversion and lower C2 selectivity (table 1). The poor performance of Sr–Cr
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Ye Wang, Y. Ohtsuka/Methane coupling with carbon dioxide Table 2 XRD and XPS results for alkaline earth metal-containing binary oxide catalysts before and after reaction. Catalysta
Before reaction —————————————— XRD XPS
After reactionb —————————————————— XRD XPS
Ca–Ce (0.5) Ca–Cr (1) Ca–Mn (1) Sr–Mn (1) Ba–Mn (1)
Ce1x CaxO2y , CaOc CaCrO4 CaMnO3 SrMnO3 BaMnO3
Ce1x CaxO2z, CaOc Ca(CrO2)2, CaO Ca0.48Mn0.52O SrCO3, MnO, SrMnO2.5c BaCO3, MnO
Ce4þ Cr6þ Mn4þ Mn4þ Mn4þ
Ce4þ , Ce3þ Cr3þ Mn2þ Mn2þ Mn2þ
a
The numbers in parentheses denote atomic ratio. The catalysts after reaction at 850 C for ca. 10, 15, and 30 h (all at steady state) were quenched in He and used for characterization for the Ca-containing oxides, Sr–Mn, and Ba–Mn oxides, respectively. c Weak XRD peaks. b
and Ba–Cr catalysts may be explained by the same reason as stated above. However, Sr–Mn and Ba–Mn oxides were exceptional cases. Compared with the Ca-containing binary catalysts, the Sr–Mn exhibited higher C2 selectivity and higher C2 yield (table 1). Table 2 shows that, for the Sr– Mn and the Ba–Mn catalysts, SrMnO3 and BaMnO3 with perovskite structures were transformed to the corresponding carbonates and MnO after reaction (as mentioned above with other Sr- and Ba-containing binary oxides). Interestingly, however, weak XRD peaks of SrMnO2.5 were also detectable for the Sr–Mn catalyst. In order to clarify the formation process of this species, transient desorption experiments were carried out with the Sr–Mn catalyst by switching the feed gas to inert He at a steady state of reaction. As shown in figure 4, the desorption of not only CO2 but also CO from the
catalyst took place. Furthermore, the XRD and XPS measurements after the desorption experiment showed the existence of SrMnO2.5 as the main phase and an increased intensity of Mn3þ species, respectively. These results strongly suggest that the following reaction proceeds in He: 2SrCO3 þ 2MnO!2SrMnO2:5 þ CO þ CO2
ð4Þ
Since SrMnO2.5 was detectable after a steady-state reaction, it is reasonable to consider that equation (4) also occurs during CH4 coupling reactions with CO2. The ready formation of the stable composite oxide (SrMnO2.5) may be the driving force for this reaction. It is suggested that this oxide with the valence of Mn3þ accounts for the activation of CH4 to CH3 radical through the following overall reaction: 2SrMnO2:5 þ CH4 þ 2CO2 !2SrCO3 þ 2MnO þ CH3 þ OHðaÞ
ð5Þ
Although BaMnO2.5 was not observed after CH4 coupling with CO2 over the Ba–Mn catalyst (table 2), the transient desorption run and subsequent XRD measurement showed the formation of this oxide together with desorption of CO and CO2, which means the occurrence of a reaction similar to equation (4). It is thus likely that the formation of CH3 radical over the Ba–Mn catalyst proceeds via the same mechanism (equation (5)) as with the Sr–Mn oxide. On the other hand, the reaction of equation (4) did not take place over other Sr- and Ba-containing catalysts, which is consistent with the poor performance as reported in table 1. 4. Conclusions Figure 4. Desorption of CO and CO2 upon switching feed gas (PðCH4 Þ ¼ 30:3 kPa, PðCO2 Þ ¼ 70:7 kPa, FðtotalÞ ¼ 100 mL min71) to He (F ¼ 0:268 mol (STP) h1 ) over the Sr–Mn oxide at 850 C: *, CO2; *, CO.
Ca-containing Ce, Cr, and Mn binary oxides all showed good performance in C2 formation with an obvious synergy between the two components. The formation of solid solution or composite oxide com-
Ye Wang, Y. Ohtsuka/Methane coupling with carbon dioxide
pound between Ca2þ and the redox component at a reaction steady state was responsible for the synergy in C2 formation. It is proposed that Ca2þ accounts for the chemisorption of CO2 and the redox site nearby at a low valence state probably reductively activates the adsorbed CO2 to CO and O , the active oxygen for CH4 coupling. On the other hand, the corresponding Sr- and Ba-containing binary oxides showed a remarkably lower C2 yield except for Sr–Mn and Ba–Mn oxides. The formation of Sr or Ba carbonate at steady state led to the segregation of the composite oxide phase and was probably responsible for the lower C2 formation rate. However, SrMnO2.5 and BaMnO2.5 were formed by the reactions of the carbonates and MnO over the Sr–Mn and Ba–Mn catalysts, respectively, and were probably the active species for the conversion of CH4 to C2 hydrocarbons. Acknowledgement This work was supported in part by a Grant-in-Aid for Scientific Research (B) (No. 14350422) from the
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Ministry of Education, Science, Sports and Culture, Japan.
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