12640
J. Phys. Chem. 1995,99, 12640-12646
Heterogeneous Photochemical Activation of Methane on a Rhodium Catalyst. Formation of an Acetyl Moiety on the Surface Jason C. S. Wong and John T. Yates, Jr.* Surface Science Center, Department of Chemistry, University of Pittsburgh, Pittsburgh, Pennsylvania 15260 Received: May 8, 1995; In Final Form: June 13, 1999
The photochemical activation of methane over a substrate containing Rh'(C0)z species has been studied using transmission infrared spectroscopy. At 233 K, methane activation is achieved by the oxidative addition of the C-H bond to a postulated 16-electron surface species, Rh'(CO), which is produced by UV (325 nm) photodecomposition of the Rh1(CO)2on the surface. The oxidative addition product, Rh(CO)(H)(CH3), further undergoes a migratory-insertion reaction in the presence of CO to form a proposed Rh(CO)(H)(COCH3) surface species. Isotopic experiments using CI8O have been performed to confirm the product assignment. An IR spectroscopic study of acetaldehyde and acetone adsorption following the photoreaction suggests that other CH3CO-containing species may also be formed on the surface as a result of C-H bond activation followed by CH3 insertion into the Rh-CO bond region.
Introduction Methane is the most stable and abundant hydrocarbon. Conversion of methane to other useful petrochemicals has long been a desirable goal, which has proven to be challenging. Generally, methane can be activated via two different approaches which are (i) the oxidative addition of the C-H bond to metal centers and (ii) the generation of the methyl radical.'-3 Extensive studies of these two approaches have been reported in the literature!-24 The oxidative addition of the 16-electron Cp*ML (Cp* = pentamethylcyclopentadiene, v5-C5(Me)5; M = Ir or Rh; L = PR3 or CO) intermediates to the C-H bonds of methane and other higher alkanes is the best known example of the first approach.I0 The oxidative coupling of methane over alkali-promoted alkaline earth oxide catalysts to form higher hydrocarbons (mainly ethane and ethylene) has been shown to occur via methyl radical formation.8 When Cp*MLz complexes are irradiated with UV light, these complexes lose a ligand, L, to form unstable, 16-electron Cp*ML intermediates. In the presence of an alkane (R-H), the Cp*ML intermediates undergo oxidative addition to form alkyl hydrido complexes, Cp*ML(R)(H).'O," At room temperature, the alkyl hydrido complexes decay rapidly to form RH and Cp*2MzL3 c0mp1exes.l~ The mechanism and the kinetics of the Cp*ML2 photoreaction have been well studied experimentally in different environments, e.g., in alkane solvent^,'^-'^,'^,^^ in low-temperature mat rice^,'^^'^^^' in liquid Xe and Kr,16323and also in the gas phase.20 Recent theoretical calculation^^^^^^ of the reaction barrier and enthalpy of the oxidative addition of Cp*ML to methane support recent experimental results in the homogeneous phase.20 Recently, we r e p ~ r t e dthat ~ ~this , ~ ~oxidative addition reaction can also be performed heterogeneously on an A 1 2 0 3 surface containing the Rh'(C0)z species. The W photolysis of the Rh1(C0)2 adsorbate in the presence of gaseous cyclohexane produces an active 16-electron Rh'cO surface intermediate. This intermediate activates the C-H bonds of cyclohexane, producing a cyclohexyl surface species which is stable up to 600 K. This paper presents the experimental results of the photoactivation of methane by the Rh1(C0)2 surface species using the same principles. In addition to the formation of the methyl hydrido @
Abstract published in Advance ACS Abstracts, August 1, 1995.
0022-3654/95/2099- 12640$09.00/0
adsorbate Rh(CO)(H)(CH3), a CO insertion reaction is observed to occur immediately after the oxidative addition reaction, producing an acetyl moiety on the surface as observed by highsensitivity transmission infrared spectroscopy. Only a few studies regarding the heterogeneous photoactivation of methane have been reported in the literature. Kaliaguine and co-workersZ7first showed that exposing various supported metal oxides to y or W radiation produced 0- anion radical species; 0- species reacted with methane to form a small amount of formaldehyde and a large amount of C02(g). Ward et aLZ8reported that methanol was produced from the partial oxidation of methane in the presence of oxygen at 373 K by W-irradiated copper molybdate (CuMo04). Wada et aLZ9 demonstrated that UV irradiation of several n-type solid metal oxide semiconductors in the presence of both oxygen and methane was able to convert methane to formaldehyde and COZ at 493 K. All of the above studies suggested that the methyl radical was the key intermediate in the photoactivation of methane. Rhodium(1) gem-dicarbonyl, Rh1(CO)2, is a surface species analogous to the Cp*MLz organometallic complexes. This surface species has been well investigated in this laboratory and elsewhere by trqnsmission infrared s p e c t r o s c ~ p y ~and ~ - ~in~ other research groups by X-ray photoelectron spectroscopy (XPS),4'-43 extended X-ray absorption fine structure (EXAFS),4',42 and mass spectroscopy-temperature-programmed reaction (MS-TPR).43 The Rh'(CO)* surface species can be formed readily at room temperature when oxide-supported Rh containing surface hydroxyl groups is exposed to C0(g).36.37It is now generally accepted that the Rh1(C0)2exists as an isolated surface species on oxide surfaces3' having been fully dispersed by oxidative degradation of small metallic Rh particles under
co.
Experimental Section Transmission infrared spectroscopy was the major technique used to monitor the photoreaction of methane on the Rh1(CO)2/ A1203 substrate. Figure 1 shows the high-temperature IR cell used in these studies. The construction and the use of this IR cell have been discussed previously.44 It is a bakeable stainless steel ultrahigh-vacuum cell containing KBr windows. The KBr windows are sealed with differentially pumped Viton O-rings. 0 1995 American Chemical Society
J. Phys. Chem., Vol. 99, No. 33, 1995 12641
Photochemical Activation of Methane on a Rhodium Catalyst Thermocouple and power leads to temperature controller
IR cell
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Figure 1. Schematic diagram showing the high-temperature IR cell.
The Rh/A1203 samples were supported on a 0.0025 cm thick tungsten mesh. The sample temperature was measured by a chromel/alumel thermocouple which was spot welded to the topcentral region of the tungsten mesh. Using an electronic temperature controller, a constant sample temperature (from 150 to 1400 K, within f l K) can be maintained by passing electrical current and using liquid nitrogen coolant.45 The IR cell is connected to a stainless steel vacuum system (base pressure -= 1 x IO-* Torr) which is pumped by a 60 L s-l turbomolecular pump and a 30 L s-I ion pump. The system also contains a differentially pumped Dycor M 1OOM quadrupole mass spectrometer and an MKS 116A Baratron capacitance manometer (range: to lo3 Torr). A Hewlett-Packard (5890 Series 11) gas chromatograph, equipped with a flame ionization detector, is also connected to the vacuum system for gas analysis. Degussa aluminum oxide C (101 m2 g-l) and Alfa RhC13*3H20 (99.9% pure) were starting materials used to prepare the 0.5% Rh/A1203 samples. The appropriate amounts of the materials were slurried with 10 mL of distilled water in an ultrasonic bath for 45 min. Acetone (Mallinckrodt, analytical reagent grade, 90 mL) was then added. The resulting mixture was uniformly sprayed by a nitrogen-pressurized atomizer onto the entire exposed area (5.2 cm2) of the tungsten mesh which was heated electrically at about 330 K to flash evaporate the solvent.a The net weight of the samples sprayed on the tungsten mesh was from 41.9 to 58.0 mg (from 8.06 to 11.2 mg cm-*). After the sprayed powder deposit (RhC13/A1203)was mounted into the IR cell, the powder was outgassed in situ at 475 K for at least 12 h under vacuum before reduction. Reduction was carried out at 475 K with four exposures (15, 30, 45, and 60 min) of 200 TOITof hydrogen gas (Matheson 99.9995% pure); the cell was evacuated for 15 min after each exposure. Then the fully-reduced sample (Rh/A1203) was allowed to outgas under vacuum at 475 K for another 12 h. Before the photo-
Figure 2. Schematic diagram showing the optical design of the IR cell for simultaneous photochemistry and IR spectroscopy on high surface area substrates.
reaction studies were carried out, the metallic Rh deposit was converted to Rh'(C0)2 by CO adsorption. Infrared spectra were recorded with a nitrogen gas-purged Mattson Fourier transform infrared spectrometer (Research Series 1) equipped with a liquid nitrogen-cooled wide-band HgCdTe (MCT) detector. The spectra shown here were taken by averaging from lo00 to 4000 scans, depending on the relative signal-to-noise ratio, with a resolution of 4 cm-'. Difference spectra (Rh/Al203 background subtracted) are shown except where otherwise specified. The UV light source is a 350 W high-pressure mercury arc lamp. The lamp is mounted inside a lamp housing equipped with an F/1 two-element UV fused silica condensing lens, a 10 cm water filter, an iris diaphragm, and a shutter. The 325 nm UV light was selected by using a 325 f 50 nm band-pass filter. The photoflux of the filtered UV light as measured by a calibrated EG&G (HUV-4000B) p h ~ t o d i o d ewas ~ ~ 1.1 x 1OI7 photon cm-2 s-l, with an absolute accuracy of &lo%. Figure 2 is a schematic diagram showing the optical alignment of the UV lamp with the Rh/A1203 sample in the IR cell. This alignment allows the simultaneous photochemistry and IR spectroscopic measurements without moving either the IR cell or the UV lamp. A 2.40 cm diameter UV grade sapphire viewport was installed onto the IR cell; this provides an optical window for the UV light used in the photochemistry. The Rh/ A1203 substrate supported on the tungsten mesh is positioned in such a way that both the IR beam from the FI'IR spectrometer and UV light from the mercury arc lamp are focused at an angle of incidence of about 45" to the normal of the mesh. Using this orthogonal arrangement, the interference of the UV light on the IR spectra collected is negligible. CO (Matheson, 99.9% pure) and ClXO(MSD Isotope, 97.4 atom % lXO,99.9% pure) were obtained in break-seal glass storage bulbs. Research grade methane (99.999% pure) was purchased from Matheson. Acetone (Mallinckrodt, 99.8% pure) and acetaldehyde (Fisher Scientific, 99.5% pure) were transferred under a nitrogen atmosphere into glass storage bulbs; they were further purified using freeze-pump-thaw cycles.
Results and Discussion A. Activation of Methane by UV Photolysis of Rh'(C0)Z. The Rh*(CO):!was formed by introducing 10.6 TOITof CO into the IR cell containing the 0.5% Rh/A1203 substrate at 310 K for 20 min, followed by evacuation to < 1 x Torr for 15 min. The IR spectrum was recorded after cooling the sample to 198 K, and it is shown in Figure 3, curve A. The RhL(C0)2 is identified by two sharp IR bands at 2099 and 2028 cm-I. These two bands were assigned respectively to the coupled symmetric and the antisymmetric C-0 stretching modes of the R h l ( C 0 ) ~ .The ~ ~ RhL(C0)2surface species are produced by the
12642 J. Phys. Chem., Vol. 99, No. 33, 1995
Wong and Yates
TABLE 1: Comparison of the Carbonyl Stretching Frequencies of Acetyl Ligands of Different Fth(II1) Organometallic ComJexes Y(CH~C=O), Rh(II1) organometallic complexes cm-' reference [RhI(COCH3)(PPh3)(mnt)]1690 55 mnt = malenonitriledithiolate R~C~Z(COCH~)(CO)(PM~ZP~)~1680, 1652 56 (2 isomers) RhBr2(COCH3)(CO)(PMezPh)z 1673, 1653 (2 isomers) RhCII(CO)(CH3CO)[P(n-C4H9)3]2 1670 57 RhI2(CO)(CH3CO)[P(n-C4H9)312 1665 [AsP~~~][R~(COCH~)(CO)(NCSH~)I~] 1709 58 [AsPltlI[Rh(COCH3)(C0)2I31 1703 [AsP~~~][R~(COCH~)(CO)I~(PM~~P~)] 1671 59 Rh(COCH3)12(Ph2PCHzPPhz) 1709 R~(COCH~)IZ(P~~PCHZCH~PP~~) 1713 CSH&~I(COCH~)PP~~ 1666, 1643(sh) 60 Rh(CO)(H)(COCH3)surface species 1693 this work
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Figure 3. Top panel showing the infrared spectra of Rh1(C0)2/A1203 (A) before and (B) after UV photolysis in the presence of methane. Bottom panel showing the difference spectra (B - A). Consumption of Rh1(C0)2is indicated by two negative bands at 2099 and 2028 cm-I. Formation of Rh(CO)(H)(COCH3)species is shown by the developing bands in the v(C-H) region and in the v(C-0) region at 2060 and 1693 cm-I. Rh2(C0)3 dinuclear species was also formed and identified by two developing bands at 1980 and 1833 cm-', in agreement with previous disruption of metallic Rh crystallites, Rhxo,on the surface during the chemisorption of C0(g).31336337No IR feature was observed in the v(C-H) region of the IR spectrum before the addition of methane, as shown in Figure 3, curve A. After the formation of the Rh1(C0)2, 9.17 Torr of methane was added to the IR cell, and the sample was slowly warmed to 216 K. An IR spectrum recorded before photolysis displayed two strong vibration-rotation bands centered at 3016 and 1304 cm-I, which agree well with the reported v(C-H) and 6(CH) bands at 3018 and 1306 cm-' of gaseous methane."* IR features derived from adsorbed methane on oxide surface^^^.^^ were not observed in the IR spectrum at 216 K. Then the shutter of the W lamp was opened, and the UV photolysis of Rh'(CO);? in the presence of C&(g) was carried out for 153 min. During the W photolysis, the temperature of the sample never exceeded 233 K. After the shutter of the W lamp was closed, the IR cell was evacuated to (1 x Torr for 20 min and the sample was slowly cooled down to 198 K before an IR spectrum was collected. Figure 3, curve B, shows the C-H and C-0 stretching regions of the resulting spectrum. In the v(C-H) region, three IR features at 3022,2984, and 2910 cm-' are seen. The frequencies of these IR bands are too high to be assigned to the stretching modes of methylene (CH;!) groups (vas(CH2) = 2936-2916 cm-' and vS(CH2) = 2863-2843 cm-') of aliphatic hydrocarbon^.^' Here, we assign these IR bands to a methyl (CH3) moiety. It is highly likely that the methyl moiety binds to a carbonyl group to form an acetyl (CH3CO) surface species. This is supported by the following facts: (i) a carbonyl stretching IR
band at 1693 cm-' was found to develop during the UV light exposure; this band is shown in the v(C-0) region of the IR spectrum in Figure 3, curve B; (ii) the methyl stretching frequencies of acetyl groups are generally in the range of 30202920 the stretching frequencies of the methyl IR bands shown in Figure 3, curve B also fall within this range. The possibility of assigning these IR bands to the methoxy (CH3O) species formed on the alumina surface can be eliminated. The methoxy surface species on the alumina support has been previously studied in detail in this 1aborato1-y.~~ The methoxy is identified by the asymmetric and the symmetric C-H stretching IR bands respectively at 2960 and 2849 ~ m - ' ? ~ which are lower than the stretching frequencies of the methyl groups produced in this study. Further, the characteristic C - 0 stretching band of CH3O species on alumina at 1055 cm-' 53 was not found in the IR spectrum of this work (not shown). In the v(C-0) region of Figure 3, curve B, the consumption during photolysis of the Rh'(CO);! was observed by the decrease in absorbance of the 2099 and 2028 cm-' bands. Using the absorbance of the 2099 cm-' band, it was estimated that 29% of the Rh'(CO);! was consumed assuming that Beer's law applies approximately in this case. An IR feature between the Rh1(C0)2 doublet, at -2060 cm-', was observed to increase in intensity during photolysis. The change in intensity of all of the carbonyl bands could be seen more easily in the difference spectrum, Figure 3(B - A), which was obtained by subtracting spectrum A from spectrum B. Figure 3(B - A) shows clearly that the Rh'(CO)2 was depleted, producing two negative IR difference bands at 2099 and 2028 cm-I. Four positive difference bands at 2060, 1980, 1833, and 1693 cm-' were also found. The 1980 and 1833 cm-' bands were assigned respectively to the terminal and bridging C - 0 stretching modes of the Rh2(CO)3 surface dinuclear species.54 The Rh2(C0)3 surface dinuclear species was produced by the coupling reaction of the photogenerated surface intermediate Rh'(C0) with unphotolyzed Rh1(C0)2 species and was first reported by us.54 We assign the 2060 cm-' band to the C - 0 stretching mode of a newly formed Rh-CO surface functionality. Since this IR band is located between the doublet of the Rh1(C0)2, the frequency of this band may not be exactly at 2060 cm-'. As previously discussed, the 1693 cm-I band is assigned to the C-0 stretching mode of the acetyl surface species. This assignment is c o n f i i e d by comparing the stretching frequency of the v(C-0) band of the acetyl surface species at 1693 cm-' with some R h I n acetyl-containingorganometallic c o m p o ~ n d s ~ ~ - ~ as shown in Table 1; the 1693 cm-I band falls within the
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Photochemical Activation of Methane on a Rhodium Catalyst
J. Phys. Chem., Vol. 99, No. 33, 1995 12643
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