J . Phys. Chem. 1993,97, 13810-13813
13810
In Situ Raman Spectroscopy of Peroxide Ions on Ba/MgO Catalysts Jack H. Lunsford,' Xueming Yang, Kristjan Hailer,' and Jaan Laane Department of Chemistry, Texas A&M University, College Station, Texas 77843
Gerbard Mestl and Helmut Kniizinger Institut fur Physikalische Chemie. Universitiit Miinchen, Sophienstrasse 1 1 , 80333 Miinchen, Germany Received: August 30, 1993"
In situ Raman spectroscopy has been used to show that peroxide ions are present on Ba/MgO catalysts at temperatures up to 800 OC. The surface 0z2ions - are believed to be responsible for the activation of CH4 during the oxidative coupling reaction which occurs at these elevated temperatures. At 100 OC the peroxide ion spectrum is characterized by a major line at 842 cm-l, with minor peaks at 829 and 821 cm-l. As the temperature was increased, all of the peaks shifted to lower wavenumbers and broadened, until at 800 OC only a broad asymmetric band remained. Introduction of l 8 0 z also caused the peaks to shift to lower wavenumbers, consistent with the assignment of the spectra to peroxide ions. In the presence of COz and H20 the 0 z 2 - ions reacted to form carbonate ions at T 1 500 OC, which is in agreement with the poisoning effect of C02 in the catalytic experiments. At Ba loadings 5 2 mol ?6 a two-dimensional layer of BaOz is present on the surface of MgO, but at larger loadings crystallites of BaOz contribute significantly to the Raman spectra. Introduction The partial oxidation of methane to ethane and ethylene (CZ products) has been extensively studied, as the process offers the potential for converting an abundant hydrocarbon resource to more useful chemicals and fuels. The catalysts that are effective for this reaction and the mechanistic details have recently been reviewed.'" The active and selective catalysts generally are strongly basic oxides themselves (e.g., LazO,), or they contain group IA or group IIA promoters, which largely cover the surface and result in a basic oxide surface (e.g.,Na/Ce02). It is believed that reactive forms of surface oxygen in the ionic state are responsible for the activation of CHI to produce CH3' radicals, which emanate into the gas phase where they couple as CzH6. The oxygen ions 0-,02-, 03-, 022-, and 02-at low coordination sites have been proposed as active centers, and indeed there is ex situ spectroscopic evidence for each of these species.u A growing body of evidence suggests that peroxide ions are either directly or indirectly responsible for the activation of CH4. For example, Na202 and BaO2 react with CH4, even at 400 OC, to give C2 products, although not in a catalyti~cycle.~.~ Yamashita et demonstrated by X-ray photoelectron spectroscopy (XPS) and thermal decomposition experiments that 0 2 2 - ions were present on the surface of Ba/La203catalysts, but they found that thecatalysts containing a larger amount of barium (e.g.,50 atom % Ba) were relatively inactive for the conversion of CHI. They concluded that the dispersion of 0z2-ions was poor at the large concentrations of Ba. Moreover, it was suggested that 0- ions were the actual active centers and that these ions resulted from the decomposition of 0z2-ions. The formation of 0- ions was related to the BaO2 dispersion. More recently, Dissanayake et aL9 examined the XPS and catalytic properties of a series of Ba/MgO catalysts with Ba loadings between 0.2 and 25 mol %. With a 2 mol 76 Ba/MgO catalyst, a C2+selectivityof 80% was achieved at a CH4 conversion of 17%. There was a good correlation between the intrinsic catalytic activity and the near-surface concentration of 0 z 2 - ions over the entire range of compositions. Both the activity and the near-surface concentration of 0 z 2 - ions reached a maximum at t Institute of Physics, Estonian Acad. Sci., 142 Riia Street, EE2400 Tartu,
Estonia. e Abstract published in Aduance ACS Absrracrs. November 15, 1993.
about 3 mol % Ba for the used catalysts. It was also found that at thegreater loadings thecatalysts were moresubject to poisoning by COz (i.e., C0,2-); thus, the maxima were attributed to the conversion of peroxide ions into inactive carbonates. The same explanation would account for the maximum in activity over Ba/ La2O3, observed by Yamashita et ~ l . ~ To determine whether peroxide ions were indeed present on the Ba/MgO catalysts in the temperature range of the oxidative coupling reaction (750-850 "C), in situ Raman studies were carried out. The catalysts were studied both in the presence of pure oxygen and in mixtures of CH4, COZ,and HzO.Raman spectroscopy is particularly suitable for this type of investigation because the 0-0 stretching mode in peroxide ions is Ramanactive, and the technique is well-suited for observing samples at elevated temperatures.
Experimental Section The Ba/MgO catalysts used in this study were selected from the larger set prepared by Dissanayake et aL9 A solution of Ba(N03)~was added to a slurry of MgO in water, and after evaporating to dryness the material was calcined in air for 1 h at 800 OC. The fused-quartzRaman cell used in the experimentsis depicted in Figure 1. Catalysts in the form of a powder were pressed into the sampleholder a. Thelight entered window b and was scattered out window c. The external surface of the catalyst was at 45O with respect to both windows, a geometry that gave the highest sensitivity. The cell was resistively heated to a maximum temperature of 850 'C. The temperature was measured and controlled using a thermocouple placed just below the sample holder. Most of the Raman spectra were recorded using a Jobin Yvon Model U- 1000double monochromator with holographicgratings (1800 grooves/mm), together with an Innova 20 argon ion laser. The excitation line was at 514.5 nm. For a given spectrum, the excitation power at the sample was constant to an accuracy of &5%,but for different experiments it was varied between 10 and 500 mW. To determine the limits of linear Raman response with respect to excitation density, between lo2and 3 X los W/cmZ, the laser power and focusing conditions of the laser beam. were varied for different samples. The excitation power was kept within the linear response region for all experiments described here.
0022-3654/93/2097- 13810%04.00/0 0 1993 American Chemical Society
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Peroxide Ions on Ba/MgO Catalysts
The Journal of Physical Chemistry, Vol. 97, No. 51, 1993 13811
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Figure 1. Schematic of the Raman sample cell: (a) sample holder, (b) laser light, (c) scattered light, (d) gas inlet, (e) gas outlet, (f)thermocouple.
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Wavenumber (cm") Figure 3. Raman spectra of 15 mol 5% Ba/MgO at 700 OC: (a) after
treatment to flowing 0 2 at 700 "C, (b) after exposure to 1 8 0 2 at 700 O C . '+ ---I
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Wavenumber (cm'') Figure 2. Raman spectra of 0.5 mol 5% Ba/MgO with the sample at the indicated temperatures. The sample had been cooled from 800 O flowing 0 2 .
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Because the crystallite size was small relative to the diameter of the focused laser beam, the fluctuation of the Raman signal was
