Heterogeneous Hydrocarbon Oxidation - American Chemical Society

Chapter 8

Oxidative Coupling of Methane by Adsorbed Oxygen Species on SrTi Mg O Catalysts 1-x

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Keiichi Tomishige, Xiao-hong Li, and Kaoru Fujimoto

Downloaded by UNIV LAVAL on October 24, 2015 | http://pubs.acs.org Publication Date: August 13, 1996 | doi: 10.1021/bk-1996-0638.ch008

Department of Applied Chemistry, Graduate School of Engineering, University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113, Japan

It was found that ethane was formed by the stoichiometric reaction between methane and an adsorbed oxygen species on SrTi Mg O with high selectivity (> 80 %) at much lower temperatures (550-750 K) than those typically used for the catalytic reaction. The properties of the adsorbed oxygen species were investigated by means of temperature programmed desorption, and the role of Mg was studied using XRD, BET, and the measurements of the exchange rate between lattice oxide ions and gas phase oxygen. The added Mg ions seem to be located at the surface and bulk Ti site of Sr-Ti mixed oxides, where oxide ion defects are formed because of differences in ion charges. Surface oxide ion defects play an important role in oxygen adsorption, while bulk defects promote the mobility of oxide ions. SrTi Mg O was initially active for oxidative coupling of methane at low temperature (873 K), but combustion of methane predominated under steady state conditions, due to a change in adsorbed oxygen species induced by the adsorption of CO . 1-x

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Oxidative coupling of methane to produce C hydrocarbons has been recognized as a promising route for the direct conversion of methane to C hydrocarbons, and this reaction is very important in terms of the chemical utilization of natural gas. This is because methane is the main component of natural gas, whose reserve is comparable to that of petroleum. Numerous metal oxides have been known to be effective catalysts for the oxidative coupling of methane. Much research on the reaction mechanism and the active site for the oxidative coupling of methane has shown that the activation of methane molecule to the methyl radical plays an key role in this reaction, and that certain kinds of oxide ion species are responsible for the activation of methane to the methyl radical (1, 2, 3). These results indicate that activation of molecular oxygen is as important as activation of methane in the oxidative coupling of methane. On some alkali and alkaline earth metal oxides the surface O species has been reported to be responsible for the activation of methane, and oxide ion vacancies were the active sites for the activation of oxygen molecules to the O " species (4, 5, 6). Superoxide ions (0 ~) were recognized to be active for the coupling reaction from the investigation on N a ^ , Ba0 , and S r 0 (7, 8) 2

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0097-6156/96/0638-0109$15.00/0 © 1996 American Chemical Society In Heterogeneous Hydrocarbon Oxidation; Warren, B., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1996.

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HETEROGENEOUS HYDROCARBON OXIDATION

catalysts, and it has been suggested that on alkaline earth metal oxides, an oxygen molecule interacted with a coordinately unsaturated lattice oxide ion O to form the 0 species (9). Itis well known that in perovskite-type oxide (ABO3) systems, the replacement of A and/or B site cations by other metal cations often brings about the formation of lattice defects (10, 11). Perovskite oxides with lattice oxide ion defects can adsorb and activate oxygen for the oxidative coupling of methane. It was reported that compounds related to perovskite with the formula S r T i ^ M g ^ O ^ have the ability to adsorb oxygen, and that these can be utilized as lean-burning oxygen sensors (72). In addition, we recently found that the adsorbed oxygen species on S r T i ^ M g ^ C ^ had the ability to activate methane at much lower temperatures than the usual conditions for this catalytic reaction, to form ethane with high selectivity (75). In the current study the properties of the oxygen species adsorbed on SrTij. M g 0 catalysts which are active for oxidative coupling were investigated. The formation of and role of the lattice oxide ion defects which occur by replacing Ti * with M g ions were studied by means of temperature programmed desorption of adsorbed oxygen species and the exchange reaction between lattice oxide ions and 0 oxygen in the gas phase. 2 -

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Experimental SrTi0 , S r T i 0 and S r T i ^ M g ^ O ^ (X= 0.1, 0.3, 0.5) catalysts were prepared by calcining the proper stoichiometric mixture of powdered SrC0 (Wako Pure Chemical industries), T i 0 (Aerosil), and MgO (Koso Chemical) at 1473 K in air for 2h. MgO and SrO were prepared by calcining MgO and SrC0 , respectively, under the same conditions. Crystal structures were investigated by X-ray diffraction (Rigaku RINT2400, Cu K ^ . Surface areas were measured by the BET method. Temperature programmed desorption (TPD) of 0 was obtained using a closed circulating system equipped with a quadrupole mass spectrometer. After the sample was evacuated at 1123 K for 0.5 h, 6.7 kPa of oxygen gas (Takachiho Trading, 99.9%) was exposed to the sample ( sample weight: 0.65 g) at a particular temperature, and then the sample was cooled to room temperature under an 0 atmosphere. Under the 6.7 kPa of oxygen used, the amount of 0 adsorption reached a saturation level. TPD spectra were measured by heating the oxides at the heating rate of 10 K/min after evacuation at room temperature. The desorbed 0 was analyzed by the mass signal intensity of m/e=32. The amount of 0 adsorption was measured by a volumetric method using the vacuum line (with a dead volume of 45 cm ). The sample weight was 1.0 g in this experiment. The evacuation treatment was carried out at 1123 K for 0.5 h and oxygen adsorption conditions were the same as those used for the TPD experiment. The exchange reaction rate between the lattice oxide ions and the 0 oxygen (Isotec Inc, 98.5 atom% O ) in the gas phase was measured in a closed circulating system (with a dead volume of 190 cm ) equipped with a quadrupole mass spectrometer. The sample (sample weight: 0.40 g) was evacuated at 1123 K for 0.5 h, cooled down to the reaction temperature, then the 0 gas was introduced into the sample. The isotopic distribution of gas phase oxygen was estimated by the mass signal intensities of m/e=32, 34, and 36, and the exchange reaction rate was calculated from the concentration of 0 (2 0 + 0 0 ) in the gas phase. The reaction pressure was 2.0 kPa, and the reaction temperature was 473 — 673 K . Temperature programmed reaction (TPR) of methane with adsorbed oxygen species on the catalysts was carried out in a fixed bed flow reaction system equipped with an FID gas chromatograph. The 0 adsorption was done at 1123 K and then the sample was cooled to 373 K under flowing air. Next, C H (Takachiho Trading, 99.99%) diluted with A r (Takachiho Trading, 99.995%) was passed through the 3

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In Heterogeneous Hydrocarbon Oxidation; Warren, B., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1996.

8. TOMISHIGE ET AL.

Coupling of Methane by Adsorbed Oxygen Species

reactor with a partial pressure of methane of 20 kPa. The sample weight was 0.50 g. The TPR of methane was measured in the range of 373— 773 K at first, then TPD spectra were recorded by heating in an A r flow after the reactor was purged once with A r in order to analyze the adsorbed product in the range of 773 —1200 K . The amounts of the products were analyzed by using FID gas chromatograph equipped with methanator which converted C O and C 0 into methane. We used the Porapak QS was used as the separating column. Selectivity was calculated using by the TPR result combined with the TPD result. For the series of these experiments the heating rate was 6.3 K/min. Catalytic oxidative coupling of methane was carried out in the same apparatus as that used for TPR measurement. Reaction conditions were a total pressure 0.1 MPa, C H : 0 : N : Ar= 5: 1:4: 20, a total flow rate 30 ml/min, a sample weight 0.5 g, and a reaction temperature of 873 K . 2

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Results and Discussion Crystal Structure and Surface Area. Figure 1 shows the X R D patterns of SrTi0 , SrTi . Mg 0 ^ (X=0.1, 0.3, 0.5), and S r T i 0 . The structure of SrTi0 was found to be the perovskite type, whereas that of S r T i 0 was K j h h ^ type. For these oxides no impurities were observed. In contrast, SrTi _ Mg 0 _ (X=0.1, 0.3, 0.5) oxides seem to consist of more than two phases. For SrTi . Mg 0 ^ (X= 0.3, 0.5), the peak at the diffraction angle 26 =62.3° which was attributed to MgO was especially clearly observed. This indicates that all the M g ions were not incorporated into the Sr-Ti mixed oxides. In the diffraction angle range of 26 = 31.0°-33.0°, Sr-Ti mixed oxides have characteristic strong peaks with diffraction angles for the samples in Figure 1 listed in Table 1. The positions of the diffraction peaks are highly dependent on the amount of the additive M g ions and the atomic ratio of Sr* to T r \ If all of the additive M g ions replaced the B sites of SrTi0 , only the peaks at the same positions as those of SrTi0 would be observed. But fact new peaks not observed in the X R D pattern of SrTi0 were observed in the X R D of SrTi . Mg 0 . (X=0.1, 0.3, 0.5). This is the consistent with the presence of MgO which was not incorporated. According to the JCPDS data, the peaks at 31.39° and 32.41° are from Sr Ti O , and the peaks at 31.59° and 32.53° are correspond to Sr Ti O From the comparison between these standard data and the results in Table 1, SrTi Mg _ 0 . phase probably consists of SrTi0 as the major phase and Sr Ti O , as the minor phase, and S r T i M g O . consists of S r T i 0 as the major phase and S r T i 0 as the minor one. This interpretation can explain the position of the peaks smaller than the characteristic peaks in the diffraction angle range 26 = 31.0°-33.0°. 3

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Table 1. The diffraction angle in the X R D patterns of SrTi0 , S r T i ^ M g P ^ , Sr Ti0 oxides. 3

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Diffraction angle 261 degree* 32.42 31.79 32.39 31.74 32.41 31.38 31.67 32.44 31.28 32.53 31.31


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Figure 1. Powder X-ray diffraction patterns of (a) SrTi0 , (b, c, d) SrTij M g 0 (x= 0.1,0.3,0.5), and (e)Sr Ti0 . 3

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TOMISHIGE ET AL.

113 Coupling of Methane by Adsorbed Oxygen Species

In addition, each peak width observed on X R D patterns of S r T i M g O ^ was much broader than those for SrTiO, and Sr Ti0 . The BET surface area of these three oxides was so small (< 3.3 m g" ) and was not so different from each oxide listed in Table 2. Therefore this peak broadening does not seem to be due to the size of crystallites. We think the partial substitution of M g (with an ionic radius: 0.066 nm) for T i ^ w i t h an ionic radius: 0.068 nm) in Sr-Ti mixed oxides distorts the crystal structure. We do not understand clearly why the relative peak intensity of the first and second maximum peaks on S r T i M g O , and Sr Ti6 is different. Table 2 shows the BET surface area of the oxides. The oxides except for MgO have very low surface areas. The surface area of the S r T i ^ M g P ^ (X=0.1, 0.3,0.5) oxides was a little higher than that of SrTi0 and Sr Ti0 . This phenomenon is probably due to the MgO impurity in the S r T i ^ M g ^ ^ (X=0.1,0.3,0.5) oxides, each major phase seems to have a lower surface area than that listed in Table 2. 05

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Table 2. Surface areas and the amounts of oxygen adsorption. The amount of oxygen adsorption Surface area /m g /^molg SrTiO, not determined 2.0 Sr Ti0 0.7 3.5 SrTi Mg O . 1.6 3.3 SrTi Mg O . 3.1 5.8 SrTi^Mg^ 2.5 8.2 MgO 16.5 not determined SrO 5.6 0.4 a) The surface area was estimated by the BET method. b) The amount of oxygen adsorption was determined by the volumetrical method. The temperature at which 6.7 kPa of oxygen gas was exposed to the 1.0 g sample was 1123 K.

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Oxygen Adsorption and Temperature Programmed Desorption. Table 2 shows the amount of oxygen adsorption for each oxide. The amount of oxygen adsorption on the S r T i M g 0 oxides increased with increased amounts of M g . While the amount of oxygen adsorption on MgO is too small to be determined by the volumetric method even though MgO had rather a high surface area, SrO adsorbed a large amount of oxygen. Oxygen adsorbed on Sr Ti0 , but it did not adsorb on SrTiO,. Figure 2 shows temperature programmed desorption of adsorbed oxygen on SrTi . Mg 0 . (x=0.1,0.3,0.5), SrTiO, (Figure 2a), S r T i 0 , MgO, and SrO (Figure 2b). TPD profiles on SrTij Mg 0 _ (x=0.1, 0.3, 0.5) were very different from those on S r T i 0 and SrO. Oxygen species on S r T i ^ M g P ^ (x=0.1, 0.3, 0.5) desorbed at temperatures of 400- 850 K with a maximum at about 620 K , while oxygen on S r T i 0 desorbed at 550-850 K with a maximum at about 720 K . S r T i M g O . was found to contain the S r T i 0 phase from X R D results, but the oxygen desorption temperature on SrTi Mgj O _ was about 100 K lower than that on S r T i 0 . This indicates that the surface structure of S r T i M g O . was different from that of S r T i 0 though they have similar bulk structures. This difference of the surface structure is caused by the addition of M g . The amount of oxygen adsorption on SrTij Mgfi^ (x=0.1, 0.3, 0.5) is highly dependent on the amount of the additive M g ions, but the oxygen desorption temperature is independent of the amount of M g . These results strongly suggest that similar oxygen adsorption sites were formed on SrTi . Mg 0 ^ (x=0.1, 0.3, 0.5) oxide surfaces, though they have different bulk structures as indicated by their X R D 2+

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Figure 2. Temperature programmed desorption profiles of adsorbed oxygen on samples, (a) the dependence on the amount of M g on SrT^ xMg^C^ x=0.5 ( ), 0.3 (), 0.1( ) and SrTi0 (-). (b) Sr Ti0 ( ), SrO ( ) and MgO ( ).Sample weights: 0.65 g except for SrO and MgO (0.30 g), heating rate: 10 K/min, temperature samples were exposed to oxygen gas : 1123 K , pressure: 6.7 kPa. 2+

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8. TOMISHIGE ET AL.

Coupling of Methane by Adsorbed Oxygen Species 4

patterns. We think that the replacement of surface T i * sites of Sr-Ti mixed oxides of various compositions by M g forms the oxygen adsorption site. In this case, oxide ion defects are formed because of the ion charge difference between Mg * and Ti *, and this oxide ion defects seem to be involved in oxygen adsorption. The total of mass signal intensities for each oxides is almost proportional to the amount of oxygen adsorption for each, as determined by the volumetric method. Figure 3 shows the TPD profiles of adsorbed oxygen on S r T i M g O , and its dependence on the temperature at which oxygen gas is exposed to the sample. The TPD profiles for samples exposed at temperatures 573- 1123 K are almost the same. When the exposure temperature was 523 K or 473 K , the amount of oxygen adsorption was about 65% and 33% of the saturation amount, respectively. From these results it appears that most adsorbed oxygen species are formed in the temperature range of 473 - 573 K , indicating that oxygen adsorption has as activation energy. This activation energy is probably due to electron transfer from the lattice oxide ion (0 ~) to the oxygen molecule in order to form an adsorbed oxygen ion species. According to Figure 2a, the temperature range of the desorption peaks were found to be very wide, indicating that some kinds of oxide species were present on the surfaces on these oxides. The peak can be divided into three parts, the first low temperature peak (peak top: 450 K), the second large peak (peak top: 620 K), and the third part which is at temperatures above 650 K . The existence of the third part seems to make this TPD profile broad. The largest peak is attributed to the oxygen species adsorbed on the oxide ion defect. As the temperature range of the third peak apparently agrees with the TPD profile of adsorbed oxygen of the Sr Ti0 , this is attributed to the oxygen species adsorbed on the Sr Ti0 -like surface of the SrTi Mg O3_ i d e . In addition, this peak temperature range agrees with that of oxygen desorption from SrO. It has been reported that oxygen interacts with coordinatively unsaturated oxide ions of alkaline earth oxide surfaces, or surface basic sites (9). We think that oxygen adsorption occurs on a surface basic site of Sr Ti0 . We do not clearly understand the cause of the lowest temperature peak on S r T ^ J v l g p ^ (x=0.1,0.3,0.5). 2+

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Exchange Reaction between 0 in the Gas Phase and Lattice Oxide Ions. Figure 4 shows the Arrhenius plot of the exchange reaction between 0 i n the gas phase and lattice oxide ions on SrTi Mgj O . , S r T i 0 , and SrTi0 . It was found that the activity of the oxygen exchange reaction on SrTi Mg ^O . is much higher than that on S r T i 0 and SrTi0 . The activation energy of this reaction was 41, 54, and 60 kJ m o l on S r T i ^ M g ^ O ^ , S r T i 0 , and SrTi0 , respectively. The addition of M g decreased the activation energy for the exchange reaction. The amount of surface oxide ion can be estimated from the lattice constant (a ) and the BET surface area (5), assuming that the surface is the (001) face. The calculated amount of the surface oxide ion is 2.7 x 1 0 mol g-caf on S r H ^ M g ^ O ^ (5=2.5 m g \

Heterogeneous Hydrocarbon Oxidation - American Chemical Society

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