SiO2 Catalyst

Journal of Natural Gas Chemistry 16(2007)316–321

Article

In-Situ FT-IR Investigation of Partial Oxidation of Methane to Syngas over Rh/SiO2 Catalyst Tinghua Wu1∗ , Dongmin Lin1 , Ying Wu1 , Xiaoping Zhou1 , Qiangu Yan2,3 , Weizheng Weng1,2 , Huilin Wan1,2 1. Zhejiang Key Laboratory for Reactive Chemistry on Solid Surfaces, Institute of Physical Chemistry, Zhejiang Normal University, Jinhua 321004, Zhejiang, China; 2. State Key Laboratory for Physical Chemistry of Solid Surfaces, Xiamen University, Xiamen 361005, Fujian, China; 3. Center for Advanced Vehicular Systems, Box 5405, Mississippi State University, Starkville, MS39762, USA [ Manuscript received February 1, 2007; revised May 6, 2007 ]

Abstract: Partial oxidation of methane to syngas (POM) over Rh/SiO2 catalyst was investigated using in-situ FT-IR. When methane interacted with 1.0wt%Rh/SiO2 catalyst, it was dissociated to adsorbed hydrogen and CHx species. The adsorbed hydrogen atoms were transferred to SiO2 surface by “spill-over” and reacted with lattice oxygen to form surface −OH species. POM mechanism was investigated over Rh/SiO2 catalyst using in-situ FT-IR. It was found that CO2 was formed before CO could be detected when CH4 and O2 were introduced over the preoxidized Rh/SiO2 catalyst, whereas CO was detected before CO2 was formed over the prereduced Rh/SiO2 catalyst. Key words: partial oxidation; syngas; Rh/SiO2 catalyst; in-situ FT-IR

1. Introduction Syngas (H2 and CO) is an important feedstock for the synthesis of methanol and dimethyl ether. Traditionally, syngas can be produced from hydrocarbons and natural gas as well as coal steam reforming. However, these processes need high temperatures, for example for hydrocarbon compounds reforming, a temperature higher than 800 ℃ is needed, while for coal reforming, higher than 1200 ℃ is necessary. To heat up the steam to such a high temperature, more than one fourth of the hydrocarbon compounds or coal must be burned. As an alternative energy saving process to produce synthesis gas, catalytic partial oxidation of methane (POM) was extensively investigated in recent years. The catalytic reaction of POM to hydrogen and carbon monoxide, which was first studied by Prettre et al. [1], has been intensively studied in the past decade [2−15]. Different results can be ∗

obtained over different catalysts or under different reaction conditions. Several reaction mechanisms have been suggested for POM. However, there are still disputes on its mechanisms. Some POM reaction mechanism proposed by Prettre et al. [1] was in dominant. It involves the total oxidation of a portion of CH4 followed by reforming the unconverted CH4 with CO2 and H2 O to produce CO and H2 . The other scheme proposed by Schmidt et al. [4] postulates the direct partial oxidation of CH4 to CO and H2 without the production of CO2 and H2 O in the reaction. The reaction steps involved in the oxidation of CH4 to CO and H2 over Rh (1.0 wt%)/γ-Al2 O3 catalyst were studied using in-situ DRIFTS at 700 ℃ by Buyevskaya et al. [16,17]. The product distribution and the absorption band intensities of the adsorbed species were strongly influenced by oxygen coverage and carbon deposits on the surface. These authors assumed that surface carbon contributed to CO formation through

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Journal of Natural Gas Chemistry Vol. 16 No. 3 2007

reaction with CO2 . FT-IR has been used to study the POM mechanism over different metal supported catalysts [18−22]. Elmasides and Verykios [18] investigated the partial oxidation of methane to synthesis gas over Ru/TiO2 employing nonsteady state and steady state isotopic transient experiments combined with in-situ DRIFT spectroscopy. Gas-phase CH4 interacts with the catalyst surface, producing CHx surface species. Elmasides and Verykios determined that CO is the primary product, resulting from the surface reaction between carbon and adsorbed atomic oxygen on metallic Ru sites, and CO2 is formed during the oxidation of CO on oxidized Ru sites [18]. Chen et al. [19] studied the chemisorption of methane over Ni/Al2 O3 catalysts by in-situ FT-IR. They concluded that the dissociation of chemisorbed methane in the participation of chemisorbed O is the key step in the partial oxidation of methane. Tsipouriari et al. [20] investigated the carbonate species formed over the Ni/La2 O3 catalyst under POM reaction conditions using FT-IR and found that surface carbon species formed over the supported catalyst do not decompose under He and O2 treatment at 600 ℃. Weng et al. [21,22] employed in-situ TR-FT-IR to investigate the partial oxidation of methane to syngas over supported Rh and Ru catalyst at 500 ℃. When supported Rh catalysts were used instead of supported Ru catalysts, the TR-FT-IR spectra indicated a significant difference in POM mechanisms. CO was the primary POM product over the hydrogen reduced and working state Rh catalysts according to these TR-FT-IR observations. In contrast, CO2 was the primary POM product over the supported Ru catalysts. Therefore, the direct oxidation of CH4 was proposed as the main pathway over Rh/SiO2 , whereas the reforming of unreacted CH4 to syngas dominated over Ru/SiO2 and was accompanied by the complete oxidation of a portion of CH4 to CO2 and H2 O. To gather more information on the catalytic reaction over Rh/SiO2 , this investigation concerns the partial oxidation of methane to syngas over Rh/SiO2 catalyst. Interaction of CH4 /Ar, CD4 /Ar or CH4 /O2 /Ar with Rh/SiO2 catalysts was examined by in-situ FT-IR.

110 ℃ overnight and calcined at 500 ℃ for 6 h. The catalyst was then pressed to a self-supporting disk (10−20 mg, 1 cm diameter) for IR study. The in-situ FT-IR experiments were performed on a Perkin Elmer Spectrum 2000 FT-IR spectrometer using a homebuilt high temperature in-situ IR cell with quartz lining and CaF2 windows. Feed gases such as CH4 , CD4 , or a gas mixture of CH4 /O2 /He (2/1/45, molar ratio) were used in the TR-FT-IR study. The spectra were recorded at a resolution of 16 cm−1 with an average of 24 scans. The time resolution of the spectra was chosen between 0.28 and 0.60 s depending on the reaction rate. The spectrum was referenced to that of the catalyst before the introduction of feed gases. 3. Results and discussion 3.1 Activation of methane (CH4 /CD4 ) over Rh/SiO2 catalyst Figure 1 shows the FT-IR spectrum of gas phase methane. There are four bands for gaseous CH4 , i.e. 3017, 1342, 1304 cm−1 , and one weak band at 2828 cm−1 . Only one band at 2253 cm−1 was observed for CD4 . The FT-IR spectrum of CD4 over pure SiO2 is shown in Figure 2. CD4 was introduced into the sample cell after SiO2 was reduced by pure hydrogen at 500 ℃ for 1 h and then heated to 700 ℃ and evacuated for 10 min at 700 ℃. The spectra are the same for CD4 in Figure 2 and Figure 1(2). There is one band at 2253 cm−1 . These results indicated that there was no reaction between methane and pure SiO2 . The IR spectra of methane interacted with the prereduced Rh/SiO2 catalyst were obtained after the

2. Experimental Rh/SiO2 catalyst was prepared by incipient wetness impregnation method. The catalyst was dried at

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Figure 1. FT-IR spectrum of CH4 (1) and CD4 (2)

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Figure 2. FT-IR spectrum of CD4 over pure SiO2 at 700 ℃ (the catalyst was prereduced by hydrogen at 500 ℃ for 1 h and then evacuated at 700 ℃ for 10 min)

catalyst was reduced at 500 ℃ by hydrogen for 1 h, then the sample was heated to 700 ℃ and evacuated for 10 min before methane was introduced into the sample cell at 700 ℃. Spectra were recorded at different time and are shown in Figure 3.

Figure 3. TR-FT-IR spectra of reaction of CH4 over 1.0%Rh/SiO2 catalyst at 700 ℃ (catalyst was prereduced by H2 for 1 h at 500 ℃ and then evacuated for 10 min at 700 ℃ )

A new band was observed at 3741 cm−1 , which was assigned to surface SiO−H bond vibration [23]. The intensity of the peak increases with the reaction time. These results indicated that there is surface SiO−H bond formation. The hydrogen atom on the bond is assumed to come from the dissociation of methane. The process could be assumed as follows:

methane is first dissociated on Rh surface to form adsorbed atomic hydrogen and CHx species, while atomic hydrogen is spilled-over to SiO2 surface and combined with the lattice oxygen to form SiO−H. The strong bands observed at 3112 and 3009 cm−1 were assigned to the adsorbed CH2 species due to the asymmetric and symmetric stretching vibrations respectively. The band at 1399 cm−1 represented the scission vibration of CH2 [24]. Similarly, the weak bands appeared at 1489 and 1358 cm−1 represented the asymmetric deformation vibration and the symmetric deformation vibration of CH3 species, respectively. The weak band at 1442 cm−1 was attributed to the scission vibration of CH3 . These results indicated that methane was dissociated over Rh surface to form adsorbed atomic hydrogen and CHx fragments. The atomic hydrogen was highly mobile, which could migrate from the Rh active sites to SiO2 surface and combine with the lattice oxygen of SiO2 to form SiO−H surface species. Similar to the previous case, the spectra of CD4 activation over Rh/SiO2 catalyst are shown in Figure 4.

Figure 4. FT-IR spectra of CD4 over 1.0% Rh/SiO2 at 700 ℃ (before reacting with methane, the catalyst was prereduced by hydrogen at 500 ℃ for 1 h and then evacuated at 700 ℃ for 10 min)

In this case, strong peaks at 2751 and 2253 cm−1 were observed. A weak peak at 2990 cm−1 and a negative peak at 3733 cm−1 were also found in this reaction. The peak at 2253 cm−1 is the absorbing band of gas phase CD4 (Figure 1(2)). The one at

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2751 cm−1 is assigned to the SiO–D band vibration. The intensity of the band increased with the reaction time. The weak band at 2990 cm−1 is assigned to the impurity C−H vibrations. The negative band at 3733 cm−1 was also observed because of the interaction of pure CD4 with pure SiO2 according to Figure 2. The bands at 2990 and 2253 cm−1 disappeared upon the evacuation of the gas phase (shown in Figure 5). This confirmed that the bands at 2990 and 2253 cm−1 were the bands of the gas phase methane, whereas the band at 2751 cm−1 was generated from the surface SiO–D. This isotopic study helps us to identify SiO−H.

Figure 5. FT-IR spectra of CD4 over 1.0%Rh/SiO2 catalyst after their contacting 40 min at 700 ℃ and then evacuated for a period of time

3.2 Reaction of CH4 /O2 /He gas mixture over Rh/SiO2 catalyst According to the results of in-situ microprobe Raman experiments, before passing CH4 /O2 /He (2/1/45) gas mixture into the IR cell, the catalyst was preoxidized in oxygen at 700 ℃ for 30 min, and then the gas phase oxygen was pumped out at 500 ℃ . A clear base line was obtained as shown in Figure 6. Within the primary 0.3 s contacting time, gas phase CH4 band at 3017 cm−1 and gas phase CO2 bands at 2350 and 2305 cm−1 [25] were observed. The CO2 concentration increased with the reaction time. There was almost no CO visible within the first 10 s. It is clear that at the beginning of methane contacting with the preoxidized catalyst surface, CO2 was generated before CO. The most possible case can be explained as follows: methane that was adsorbed onto the catalyst surface interacts with RhOx to generate CO2 and reduce RhOx into metal Rh, because it has been proved that the methane does not react with

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pure SiO2 at 500 ℃ , as shown in Figure 2. The band which belongs to CO at 2015 cm−1 appeared after reaction for about 10 s. It seems that CO was formed over a reduced Rh/SiO2 catalyst.

Figure 6. FT-IR spectra of CH4 /O2 /He gas mixture (2/1/45) over 1.0%Rh/SiO2 catalyst at 500 ℃ (before recording spectra, catalyst was preoxidized by O2 at 700 ℃ for 30 min and evacuated for 3 min at 500 ℃)

To explore the reaction, further experiments were performed over the prereduced catalyst. Before feeding in CH4 /O2 /He (2/1/45) gas, the catalyst was prereduced by hydrogen at 700 ℃ for 30 min and evacuated at 500 ℃ for 3 min. The resulted catalyst contacted with CH4 /O2 /He (2/1/45) mixture at 500 ℃ . The spectra are shown in Figure 7. In this case, bands belong to gas phase methane (3017 cm−1 ), CO2 (2308, 2348 cm−1 ), and CO (2017 cm−1 ) were observed. At the beginning, CO was formed before CO2 . This is different from that over preoxidized catalyst in Figure 6. It seems that the primary oxidation product over prereduced catalyst is CO. One may wonder at this point what happens over a stable working catalyst. Is the catalyst in a reduced state or oxidized state or in between? To answer this question, we carried out the reaction described below.

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Figure 7. FT-IR spectra of CH4 /O2 /He (2/1/45) over 1.0%Rh/SiO2 catalyst at 500 ℃ (before recording spectra, catalyst was prereduced by H2 at 700 ℃ for 30 min and evacuated for 3 min at 500 ℃)

The catalyst was pretreated with the reaction mixture CH4 /O2 /He (2/1/45) at 500 ℃, and then the gas phase was pumped out. The reactant mixture CH4 /O2 /He (2/1/45) was passed into the sample cell and spectra were recorded. The spectra are shown in Figure 8. From the spectra, it was found that the reaction process was very similar to that over hydrogen prereduced catalyst. It means that in the CH4 /O2 /He (2/1/45) reactant stream at 500 ℃, Rh was most likely in its reduced state. Over this Rh, CO was formed before CO2 . It is most likely that under the real reaction conditions, Rh is in the reduced state.

Figure 8. FT-IR spectra of CH4 /O2 /He (2/1/45) over 1.0%Rh/SiO2 catalyst at 500 ℃ (before recording spectra, catalyst was pretreated with CH4 /O2 /He (2/1/45) (4 ml each) at 700 ℃ and evacuated)

when methane was activated over prereduced Rh/SiO2 . The atomic hydrogen may migrate to SiO2 surface by “spillover” process and combine with the lattice oxygen of SiO2 to form SiO−H species. Pure SiO2 cannot break the C–H bond in methane. When methane and oxygen reacted over the reduced Rh/SiO2 catalyst, methane was first dissociated to atomic hydrogen and CHx species. And then atomic hydrogen can combine to form molecular hydrogen over Rh surface or react with oxygen to form water, while the adsorbed CHx species might react with the adsorbed oxygen species to form CO. CO2 was generated before CO when CH4 and O2 were introduced over the preoxidized Rh/SiO2 catalyst. However, CO was detected before CO2 over the prereduced Rh/SiO2 catalyst.

4. Conclusions Acknowledgements

On the basis of this study, the following conclusions are made: methane dissociated to adsorbed atomic hydrogen and CHx species over Rh surface

This study was supported by the grant of 2004C31053 from the Ministry of Science and Technology of Zhejiang Province, China, and the grant of Y404305 from the Natural Science Foundation of Zhejiang Province, China, and

Journal of Natural Gas Chemistry Vol. 16 No. 3 2007

the grant of 20673101, 20673102 from National Natural Science Foundation of China.

[12]

References

[13]

[1] Prettre M, Eichner C, Perrin M. Trans Faraday Soc, 1946, 42: 335b [2] Ashcroft A T, Cheetham A K, Foord J S, Green M L H, Grey C P, Murrell A J, Veron P D F. Nature, 1990, 344(6264): 319 [3] Tsipouriari V A, Zhang Z, Verykios X E. J Catal, 1998, 179(1): 283 [4] Hickman D A, Schmidt L D. Science, 1993, 259(5093): 343 [5] Horn R, Williams K A, Degenstein N J, Schmidt L D. J Catal, 2006, 242(1): 92 [6] Hickman D A, Schmidt L D. J Catal, 1992, 138(1): 267 [7] Choudhary V R, Mondal K C, Choudhary T V. Appl Catal A: Gen, 2006, 306: 45 [8] Yan Q G, Wu T H, Weng W Z, Toghiani H, Toghiani R K, Wan H L, Pittman C U Jr. J Catal, 2004, 226(2): 247 [9] Au C T, Wang H Y. J Catal, 1997, 167(2): 337 [10] Dissanayake D, Rosynek M P, Kharas K C C, Lunsford J H. J Catal, 1991, 132(1): 117 [11] Drago R S, Jurczyk K, Kob N, Bhattacharyya A,

[14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25]

321

Masin J. Catal Lett, 1998, 51(3,4): 177 Nakagawa K, Ikenaga N, Teng Y, Kobayashi T, Suzuki T. J Catal, 1999, 186(2): 405 Gesser H D, Hunter N R. Catal Today, 1998, 42(3), 183 Mirodatos C. Stud Surf Sci Catal, 1998, 119: 99 Yiannis B, Zhang Z, Verykios X E. Catal Lett, 1996, 40(3-4): 189 Walter K, Buyevskaya O V, Wolf D, Baerns M. Catal Lett, 1994, 29(1-2): 261 Buyevskaya O V, Walter K, Wolf D, Baerns M. Catal Lett, 1996, 38(1,2): 81 Elmasides C, Verykios X E. J Catal, 2001, 203(2): 477 Chen Y, Hu C, Gong M, Chen Y, Tian A. Stud Surf Sci Catal, 2000, 130D: 3543 Tsipouriari V A, Verykios X E. J Catal, 1998, 179(1): 292 Weng W Z, Chen M S, Yan Q G, Wu T H, Chao Z S, Liao Y Y, Wan H L. Catal Today, 2000, 63(2-4): 317 Weng W Z, Yan Q G, Lou C R, Liao Y Y, Chen M S, Wan H L. Stud Surf Sci Catal, 2001, 136: 233 Uansant E F, Voort P V D, Vrancken K C. Stud Surf Sci Catal, 1995, 93: 59 He H, Okawa Y, Tanaka K. Surf Sci, 1997, 376(1-3): 310 Weng W, Chen M, Yan Q, Wu T, Chao Z, Liao Y, Wan H. Chin Sci Bull, 2000, 45(24): 2236