J. Phys. Chem. 1992, 96, 9979-9983
9979
C-H Bond Activation in Cyclohexane: Photochemical Active Site Generation on a Rh/Ai203Heterogeneous Catalyst Todd H. Ballinger and John T. Yates, Jr.* Surface Science Center, Department of Chemistry, University of Pittsburgh, Pittsburgh, Pennsylvania 15260 (Received: June 30, 1992; I n Final Form: September 9, 1992)
The activation of a C-H bond in cyclohexane has been accomplished by the UV photolysis of the Rh1(C0)2species on a Rh A1203catalyst in the presence of cyclohexane. The photodesorption (hv = 325 nm) of a CO moiety from the surface Rh (C0)2 species creates an unstable =nakednmetal atom center which then inserts into the C-H bond by oxidative addition. The Rh(CO)(H)(cyclohexyl) species produced in this manner yields a cyclohexyl surface species which is stable up to 600 K, it may be an anchored cyclohexyl species on the A1203support. A complete set of control experiments was performed to assure that Rh1(CO)2ultraviolet photolysis caused the activation of cyclohexane and that thermal effects were not involved.
/
Introduction The activation of C-H bonds in saturated hydrocarbons has long been a desirable goal due to the large natural abundance of these hydrocarbons and their utility as sources of synthetic fuels and petrochemicals. However, because the C-H bond is one of the strongest single bonds that exist, efforts to activate this bond have proven to be challenging. Despite this, numerous C-H bond activation reactions have been successfully demonstrated. One class of C-H activation reactions, first demonstrated in alkane solvents 10 years ago, involves organometallic compounds of the form Cp*(L)MH21v2or Cp*ML2’ (where Cp* = pentamethylcydopentadienyl (q5-C5Me5),M = Ir, and L = PPh,, PMe3 or CO). When irradiated by UV light, these complexes lose H2 or CO to form a ‘haked” 16-electron metal center which then undergoes oxidative addition with the alkane (R-H) solvent to form Cp*(L)M(R)(H). Alkane solvents such as cyclohexane, neopentane, cyclopentane, and cyclopropane were successfully activated in this manner. Subsequently, using a Rh center, the Cp*(PMe3)Rh(H2)complex was irradiated in pentane4and cyc l o p r o p 2 to form Cp*(PM@)Rh(alkyl)(H),which is only stable at temperatures below 273 K. The Rh complexes, however, showed greater selectivity between different C-H bonds than the Ir complexes. In low-temperature matrices such as methylcyclohexane (93 K)6 and methane (12 K),’,* irradiation of CP*M(CO)~or CpM(CO)* (where Cp = q5-C5H5and M = Ir, Rh, or Co) forms Cp*(CO)(M)(R)(H) or Cp(CO)(M)(R)(H). Reactivity at such low temperatures shows that the kinetic barrier for such reactions at naked metal centers is low. Furthermore, the methane matrix studies dowed IR spectroscopicobservations of the intermediates. For the Rh complex, v(CO) features were assigned to the following species: Cp(CO)Rh(Me)(H), vm = 2024 an-’;Cp(CO)Rh-CH4, VCO = 1992 m-’;and Cp(CO)Rh, vco = 1969 an-’.Spectroscopic observation of the latter species was the first evidence that the 16-electron metal center was an intermediate during UV irradiation. Similar transient species were later detected at higher temperatures by time-resolved IR spectrocopy during UV laser flash photolysis. Weiller et al? irradiated C P * R ~ ( C Oin ) ~liquid Xe at 242 K and observed a vCoband at 1943 cm-I; the intensity decayed with a rate constant of k = 4 X lo4 s-l. This decay was due to the regeneration of the starting material. In liquid Kr the band appeared at 1947 cm-’and the intensity decayed with a rate constant of k = 5 X lo3 s-l. These bands were assigned to Cp*Rh(CO)(Xe) and Cp*Rh(CO)(Kr) species, respectively. When cyclohexane was added to the complex in liquid Kr, the rate of decay of the above intermediate species concentration increased with inaeaSing cyclohexane concentration. Furthermore, the decrease in absorbance of the band due to the intermediate scaled with the growth of a new IR band at 2003 cm-I. This new spectral feature was assigned to vCoin Cp*Rh(CO)(C6H11)(H) which formed from the Cp*Rh(CO)(Kr) intermediate. The 0022-3654/92/2096-9979S03.00/0
frequency increase in vco is consistent with oxidation of the Rh center by insertion into the C-H bond. In parallel studies, Belt et a1.I0 flash irradiated C ~ R h ( c 0 in ) ~liquid cyclohexane at room temperature. A single IR band, observed at 2018 cm-’, was assigned to the cpRh(cO)(c&11)(H) Species, Which Was formed by the insertion of the CpRhCO species into the C-H bond of cyclohexane within 400 ns. The alkyl hydride complex decayed rapidly at room temperature (k = 50 s-’) to form [ c ~ R h ( p - c O ) ] ~ and ultimately Cp2Rh2(CO)3. Very recently, Wasserman et al.” have observed alkane activation in the gas phase using CpRh(CO),. Photolysis of this compound caused the loss of the dicarbonyl features at 2060 and 2003 cm-’, and the growth of a 1985-cm-’ band due to the CpRhCO species. This species decayed at room temperature with a half-life of 1 X lo4 s, and vco bands due to Cp&(CO)3 were observed to form at 1990 and 1858 cm-’. In the presence of alkanes, the naked metal center decayed more rapidly with a half-life of 2.50 X lo-’ s and formed, with neopentane, the CpRh(CO)(neopentyl)(H) complex, exhibiting vm at 2037 an-’. This species was less reactive, with a half-life greater than 1 X 10-3 s. These results prompted us to ask if a similar alkane activation process occurs when the activated metal carbonyl center is s u p ported on a surface. Numerous studies have photolyzed metal carbonyls to form supported metal catalysts, but alkane activation by photolyzing the supported metal carbonyl has not been reported. The only studies which are somewhat related to the experiments described here were carried out by Wrighton and ~ o - w o r k e r s , ~ ~ ~ ~ ~ who functionalized the surface-OH groups on A 1 2 0 3 and Si02 with C%(CO),, to form anchored -Co(CO), groups. When these catalysts were irradiated in the presence of the unsaturated hydrocarbons ethylene or propylene12at temperatures below 223 K, the anchored species -SiC~(CO)~(alkene)was formed. In this work, a reduced Rh/Al2O3 catalyst was exposed to CO to form a surface carbonyl species. Yang and Garland first observed that CO adsorption on a Rh/A1203catalyst caused the formation of the gem-dicarbonyl species, Rh1(CO)2,yielding a characteristic doublet in the IR spectrum at 2107 and 2034 This surface species was later studied in detail by a variety of techniques by many ~ t h e r s . ’It~was ~ postulated that the species was formed on individual Rh atoms which break away from Rh crystallites on the support as a result of CO ~hemisorption.’~-~’ This adsorbate-induced structural modification of the Rh crystallites was clearly demonstrated by EXAFS and XPS ~ t u d i e s ~ ~ * ~ ~ that showed a reduction in the Rh-Rh coordination number and an increase in the oxidation state of Rh following exposure to CO. The surface AI-H groups were postulated to be responsible for this oxidation; removal of these groups at high temperature was found to decrease Rh1(C0)2f ~ r m a t i o n . ~ ~IR - ’ ~spectroscopic studies by Basu et a1.34J5conclusively showed that the Al-OH groups were quantitatively consumed as R~I’(CO)~ was formed, presumably liberating H2 when Rh’(C0)2 was formed from Rho 0 1992 American Chemical Society
9980
The Journal of Physical Chemistry, Vol. 96, No. 24, 1992
Ballinger and Yates
species. Further proof was later given by chemically reacting away the Al-OH g r o ~ p s ~ " and ~ * observing the suppression in the Rh1(C0)2formation. Thus, by forming these individual Rh'(CO)2 sites on A1203, one can make the surface analog to the gas and solution-phase metal carbonyl species used previously for alkane activation by photochemical methods. This paper describes the study of the photolysis of the carbonyl species to form an anchored RhCO species which, in the presence of cyclohexane, activates C-H bonds to form an adsorbed Rh(CO)(H)(cyclohexyl) intermediate species, as shown in the reaction:
hu
Rh(CO)2(a)
Rh(CO)(a)
+ CO(g)
T=297K
Io.f Q)
u c
0
tu, = 0 hr
I
0 u)
n
a
-
tu"=0.42 hr
C-cd12(8)
Rh(CO)(H)(C6Hll)(a) ( l )
In contrast to the previous studies in liquid and gas phases, the bound C6Hllsurface species is stable on the surface at 300 K and is only completely decomposed at 600 K suggesting that it may be held on the A1203support rather than on the Rh center. This study details the previous preliminary report3' and represents, to the best of our knowledge, the first observation of alkane activation on a surface by this method.
Experimental Section The infrared cell used in these studies has been described previously.a The unreduced powder (RhC13/A1203) is spray deposited from solvent on a tungsten grid held on an electrical feedthrough that is then transferred to the center of a stainless steel cube-shaped cell containing KBr windows. The KBr windows are sealed with differentially pumped Viton O-rings. The tungsten support grid could be oooled using an anilintliquid nitrogen slush bath in the reentrant dewar containing the feedthrough and was electrically heated using an electronic feedback controller.4l This controller maintains the grid temperature within i l K, as measured by a thermocouple spot-welded to the top-central region of the grid. The cell is C O Mto a~stainless steel vacuum system (base pressure I 1 X Torr) that is pumped by a 60 L/s turbomolecular pump and a 30 L/s ion pump. The system also contains a Dycor M 100 quadrupole mass spectrometer and a MKS Baratron capacitance manometer (range 10-3-103 Torr). Degussa aluminum oxide C (101 m2/g) and Alfa RhCI3.3Hz0 were used as the starting materials. The appropriate amounts of the materials to form a 2.2% Rh/Alz03 substrate were slurried in a liquid mixture (1 10 mL/g of support) of distilled water and acetone (1:9 volume ratio) and agitated ultrasonically for 45 min. The resulting slurry was uniformly sprayed by a N,-pressurized atomizer onto the entire exposed grid area (5.2 cm2). The grid was electrically heated during spraying to 323-333 K to flash evaporate the liquid phase.@ The net weight of the deposit sprayed onto the grid was 19.0-26.8 mg (3.7-5.2 mg/cm2). The powder deposit was treated in situ by heating under vacuum to 475 K for 12 h. Reduction was achieved at 475 K with four successive exposures of 400 Torr of H2 (99.995%pure, Matheson) for 15-60 min (after each exposure the cell was evacuated for 30 min), followed by evacuation at 475 K for another 12 h. IR spectra were obtained in a purged, double beam PerkinElmer Model 580B grating spectrometer, coupled with a Model 3600 data station for data storage and manipulation. The spectra of the adsorbed species, using a resolution of 5.3 a d , were signal averaged five times. The spectra shown are the resulting difference spectra after background (vacuum) subtraction. In addition, the spectra in the u(C-H) region have been smoothed with a 19-point smoothing function and flattened for display purposes. The UV light source used was a broad band (23&900 nm) 35@W Hg arc lamp (Oriel Corp. Model 66014). A lOCm distilled water filter, fitted on the output of the lamp, filters out infrared radiation, and a 200 mm focal length quartz condensing lens focuses the light onto a front-surface reflecting mirror. The motor-driven movable mirror is contained inside the sample compartment of the IR spectrometer and reflects the UV light onto the catalyst in the cell. Using the movable mirror, the cell was not moved for either IR or UV measurements. The appro-
1
tu,= I 1
I
2200
1
1.92 hr
I
1900
Wavenumber (cm" ) F i g w 1. Infrared spectra in the v ( C 0 ) region showing the photodecomposition of the A1203supported Rh'(C0)2 species in vacuum at 297 K. The IR spectra were obtained (top spectrum) before photolysis, (middle spectrum) after 0.42 h photolysis at 325 nm, and (bottom spectrum) after 1.92 h photolysis which shows a 26% loss in absorbance of the 2103-cm-' band.
priate wavelength range was selected by using a 325-nm band-pass filter (325 i 50 nm, Oriel). The filtered light intensity, 8.6 X 10l6 photons/(cm2.s), was measured using a calibrated photodiode42with an absolute accuracy of flOQ. The carbon monoxide used was 99.9% pure obtained from Matheson Gas Products in a break-seal glass storage bulb. The UV-grade cyclohexane was purchased from F l u b Chemical Gorp. (stated purity 99.5%) and transferred into a glass storage bulb under nitrogen. It was further purified by several freezepumpthaw cycles. GC analysis indicated that the amount of cyclohexene and benzene impurities was less than 10 ppm. RCLSultS
A. Photolysis of RaX(CO),in Vacuum. The Rh'(CO)* was formed by exposing 5 Torr of CO to the reduced supported Rh for IS min and then evacuating the cell for another 5 min. As previously mentioned, such a complex is produced by the disruption of Rh cry~tallites~~,~ on the surface to produce isolated R~I'(CO)~ species using as oxidizing agents the surface hydroxyl groups present on the A1z03.34,35A characteristic doublet14 in the IR spectrum identifies this species. Such a doublet is seen in the top spectrum of Figure 1 after CO adsorption. The 2103- and 2034-cm-' features are due respectively to the symmetric and antisymmetric C-O stretching modes of Rh'(CO)*. Terminally-bound Rh-CO can also be seen to produce the small band between the doublet at -2065 cm-',along with bridgebound CO at -1860 cm-' (not shown). The terminally-bonded and bridgebonded species are present on metallic Rhoxclusters which have not undergone complete disruption by CO adsorption. However, it is clear that the spectroscopicallypredominant species is Rh1(C0)z.17 After the IR spectrum was collected, the UV lamp irradiated the catalyst in vacuo for 0.42 h. The IR spectrum obtained after this exposure is the middle spectrum in Figure 1. There is a 16% decrease in the measured intensity in the 2103-cm-' Rh1(C0)2 band. UV irradiation was initiated again until the total time of UV exposure to the catalyst was 1.92 h. The resulting IR spectrum obtained after photolysis is the bottom one in Figure 1. The intensity of the 21O3cm1band has demased 26% from the initial intensity. The 2034cm-' band has not decreased quite as much, due to the growth of an additional CO band in the same spectral region. At present, we cannot assign this band to a defdte surface species. The rate of Rh1(CO)2photodecomposition is shown in Figure 2, obtained by measuring the peak absorbance of the 2103-cm-'
The Journal of Physical Chemistry, Vol. 96, No. 24, 1992 9981
C-H Bond Activation in Cyclohexane
A. Mass Spectrometer Measurements 0.25
f
-
l
r
I
6 0
-0 Is)
Time (hr)
0.2
N
a
I 0 20
f.0
0
2
4
6
8
Time (hours) Figure 2. Plot of the kinetics of the Rh'(C0)z photodecomposition in vacuum at 297 K. monitored by the intensity of the 2103-cm-' feature. Before photolysis, there is a small amount of thermal desorption in the "dark". When the powdered ~UI'(CO)~ sample is irradiated, the rate of decompositionincreasa dramatically. The crm-hatched bars indicate that time of photolysis. Finally, control experiments with visible light indicate that the daampxition is induced only by UV photons. The inset shows the mass spectrometer signal for CO(g) when the RII'(CO)~ species is photodecomposed in the dynamically-pumped cell. band. Before photolysis, at 297 K, there is a slow thermal desorption rate in the "dark". This rate was monitored for -2.5 h. Then the UV lamp was tumed on for 10 min using the feedback controller to maintain the surface temperature constant at 297 K. During this exposure, the m a s 28 C o b ) signal in the pumped IR cell was monitored with the mass spectrometer, as shown in the inset of Figure 2. As soon as the shutter on the lamp was opened, the CO(g) pressure increased quickly and then fell gradually during the remainder of the UV exposure. When the lamp was turned off, the CO pressure quickly dropped back to the baseline value. The rapid increase in the CO pressure indicates that UV-induced decomposition of the Rh1(CO)2 species has d. The measured decrease in IR intensity of the 2103Cm' band after the UV exposure also reveals the depletion of the surface species by photodecomposition. The cross-hatched bars in Figure 2 indicate the time period of UV exposure, while the points indicate the time at the beginning of each IR scan. Several 10-min UV exposures were carried out, as indicated in the plot. With each successive W exposure, continued CO photodesorption was observed with the mass spectrometer as well as by the IR intensity decrease at 2103 cm-'. A smooth line has been drawn between the points to guide the eye. Control experiments using visible light irradiation, as described below, indicate that no thermal effects due the the UV lamp occur in these experiments. B. coatrd Exphenb: To avoid confusion between possible thermal effects and photo effects in the decomposition of the R~I'(CO)~ species, electrical control of the grid temperature was employed to maintain constant substrate temperature (297 K) before and during UV irradiation. In addition, control experiments in which the UV lamp was replaced with a tungsten-halogen incandescent lamp (Atlas, 24 V, 250 W) were carried out. The visible lamp power was adjusted to deposit the same thermal energy into the substrate as for the UV lamp by using the substrate/grid as a calorimeter, adjusting the visible lamp power to cause essentially the same change in grid temperature as the W lamp (-10 K) when the electrical feedback controller was turned off. Figure 3 shows the results of these control experiments. In Figure 3a, electrical heating is employed to cause an 10 K increase in substrate temperature; a small evolution of co(s)is obsemd in the upper panel. Figure 3b shows equivalent heating by the incandescent lamp, and again only a small evolution of CO(g) is observed. Figure 3c shows the UV lamp heating effect and also shows a large amount of CO(g) evolution caused by the UV photodecompition of the R~II(CO)~ species. Note the time scale differences in part A and B of Figure
-
3.
The results of Figures 2 and 3 unequivocally demonstrate that
UV light photodecompoeea the €UI'(CO)~ species as measured by
@..Thermocouple
I Measurements
-I - [
[Incandescent Lamp
I
- 300
-> -> E
Figure 3. Mass spectrometer (A) and thermocouple (B) responses to various forms of heating the Rh/A1203 catalyst. Electrical heating (a) and incandescent lamp heating (b) of the grid and powder show that, although there is a temperature increase, only small amounts of CO are desorbed. In contrast, UV irradiation (c) of the powder shows a similar temperature change, but a large rise in the CO pressure.
-
T=297K
0
To.003
c
* t +'***
Q)
0 E 0
2 5: n a !btuV=l5hr
J '%tUV= I
3050
.
5hr
2850
Wavenumber (cm") Qure 4. Infrared spectra in the v(C-H) region showing the adsorbed cyclohexyl species on the surface after photolysis of R~I'(CO)~ in the presence of cyclohexane at 297 K after (bottom spectrum) 5 h of photolysis and (top spectrum) 15 h of photolysis. The stability of the surface species is shown in the inset. This inset plot shows the dmmpoeition of the adsorbed alkyl species begins at 350 K and continues to 600 K.
both infrared and mass spectrometric methods. This is in agreement with both liquid- and gas-phase photodecomposition studies of organometallic complexes of similar Rh dicarbonyl speciw.3s7-11 C. Wotolysis Of Rh'(C0)z A ~ ~ ~ m p l aby i eC d y c l O a ~ r n eACtivation. The activation of the C-H bond in cyclohexane was undertaken by photolyzing the Rh'(CO)2 species in the presence of 2.4 Torr of cyclohexane gas. After formation of the Rh*(CO)2, cyclohexane was added to the IR cell. An IR spectrum obtained before photolysis exhibited strong features at 2936 and 2863 cm-' which agree with the known v(C-H) bands at 2935 and 2864 cm-' of gas-phase ~yclohexane!~Then the shutter to the UV lamp was opened, and the catalyst and gas were exposed to the UV irradiation for 10 min with no pumping of the gas phase. After this exposure, no changes were observed in the gas phase u(C-H) features of the spectrum. However, the 2103-cm-' band of the Rh'(CO)2 species decreased by 396, indicating that the photolysis of this species also occurs under 2.4 Torr of cyclohexane. The catalyst and cyclohexane were then exposed to UV irradiation for 5 h in the isolated IR cell. After the lamp was turned off, the cell was evacuated at 297 K for 25 min before IR spectra were collected. The resulting spectra in the v(C-H) region are shown in Figure 4. Two weak bands are seen at 2937 and 2864
Ballinger and Yates
9982 The Journal of Physical Chemistry, Vol. 96, No. 24, 1992
cm-'. These bands have only slightly shifted from the measured bands of gas phase cyclohexane; we assign these bands to a chemisorbed cyclohexyl species. It should be noted that the u(C-H) bands are the most intense bands in the IR spectrum of cyclohexane; thus, because only a small amount of cyclohexyl species is adsorbed on the surface, we are unable to detect any other IR bands of the surface cyclohexyl species. Simultaneous with the changes in the C-H region, the 2103-cm-' band of Rh'(CO), decreased by 10%. This rate of decrease in the 2103-cm-I intensity is less than that observed in vacuum and is due to the use of a closed system for the cyclohexane reaction studies, where the pressure of CO(g) increases during photolysis. A second period of photolysis was then carried out. Cyclohexane (2.4 Torr) was again added to the cell, and a 10-h UV exposure was performed. After evacuation of the cell, the resulting IR spectrum in Figure 4 shows much stronger bands at 2937 and 2864 cm-], indicating that more cyclohexyl had formed on the surface. As before, the 2103-cm-l CO feature due to Rh'(C0)2 continued to decrease in intensity, with a 29% total decrease after 15 total hours of UV irradiation. D. Control Experiments: Cyclohexane Activation. Three control experiments were performed in order to check the results reported above. In the first experiment, cyclohexane gas was exposed to Rh'(CO), for 1 h at 297 K and then evacuated. No chemisorbed alkyl species were observed. In the second experiment, a freshly-reduced Rh/Al2O3 substrate was exposed to the UV light in the presence of 2.4 Torr of cyclohexane, in the absence of CO, for a total of 10 h. At the end of this period, following evacuation of the cell, only a tiny u(C-H) intensity was measured, amounting to 6% of that achieved in a similar experiment involving Rh1(CO)2surface species. Then CO was adsorbed on the Rh catalyst, and photolysis under cyclohexane for 5 h yielded larger coverages of the adsorbed cyclohexyl species. This verifies that the photolysis of R~I'(CO)~ is necessary to produce a site capable of rapidly activating cyclohexane. A third control experiment, designed to study the effect of working in a closed system during photolysis, was performed using 1.84 Torr of He background gas. It was observed that the initial rate of loss of Rh1(C0)2absorbance during photolysis was very small compared to that seen in vacuum (Figure 2) and that a steady-state absorbance of the 2103-cm-' band was quickly achieved during UV irradiation. Similar effects were also seen by Wrighton et upon photolysis of anchored CO(CO)~ species on SiOz in an Ar gas background. In both cases, the decrease in the apparent photodecomposition efficiency is likely due to the buildup of CO(g) pressure in the cell and to CO(g) diffusion limitations in the presence of background gas. This effect will permit readsorption of CO(g) and is responsible for the lower apparent photodecomposition rate in the cyclohexane + RII'(CO)~ experiments where a closed cell condition prevailed; in Figure 2, dynamic pumping of CO(g) produced during photolysis causes relatively rapid loss of 2103-cm-' absorbance.
Discussion Mechsdam of C-H Bond Activation. This reaction is postulated to occur by the oxidative addition of the unstable metal carbonyl into the C-H bond of the alkane. It is further postulated that the transition state consists of a triangular a-bonded species"*45 where the metal can coordinate to both the C and H of the alkane. In this configuration there should be a maximum interaction between (1) the low-lying unfilled orbitals of the metal with the filled u-bonding orbital of the alkane, and (2) the high-lying fiied orbitals of the metal with the unfilled u*-antibonding orbital of the alkane.",45 For the 16-electronspecies, it is likely that the first kind of interaction may be of greater importance. In thermodynamic terms, Halpem& proposed that oxidative addition is possible if the M-C and M-H bonds are strong relative to the C-H bond of the alkane. Such effects are expected for coordinatively unsaturated metal complexes generated photochemically. When oxidative addition of the alkane occurs, the oxidation state of Rh must change formally from Rh' to Rh"' in the Rh-
(CO)(H)(cyclohexyl) species. Since the CO oscillator is sensitive to the surrounding environment, it should be p i b l e to distinguish the oxidation state of Rh by the frequencies of the adsorbed CO features. Previously, Rice et al.23have made the following assignments for Rh monocarbonyls on a Rh/AI2O3 catalyst: Rh"'C0, uco = 2136 cm-I; Rh"C0, uco = 2120 cm-I; Rh'CO, uco z 2100 cm-I. In our experiment the large absorbance of the Rh1(C0)2symmetric stretch band at 2103 an-'prevents observing these species. The similarity between the gas-phase Rh dicarbonyl complexes" and the surface-supported Rh dicarbonyl species reported here is such that both types of species can be irradiated with 300350-nm UV light to cause the desorption of a CO moiety, producing an unstable 16-electron species. Such a species is involved in the activation of a C-H bond in an alkane to form a Rh(CO)(H)(R) species by insertion into the C-H bond of the alkane. The difference between the gas phase and surface Rh(CO)(H)(R) species is that the surface seems to further stabilize the adsorbed cyclohexyl species. In the gas phase at 300 K, Wasserman et al. found the half-life of the Rh(CO)(H)(alkyl) species to be greater than 1 ms." In contrast, the (adsorbed cyclohexyl) species produced in our experiments is stable indefinitely on the surface at 300 K. The surface species is stable at higher temperatures; the inset in Figure 4 shows this stability in a plot of the intensity of the 2937-cm-' band versus temperature. As seen in this plot, the adsorbed cyclohexyl species begins to disappear above -350 K. However, a small amount of this species is present on the surface up to 600 K. The reasons for this alkyl stability on the surface compared to that in the gas phase are currently not well understood. It is possible that the CsHll species produced photolytically may have transferred from the Rh center to the AZO3 support. Such stability may be important in controlling the reactivity of anchored alkyl groups to form more useful products.
Conclusions The following observations can be made about cyclohexane activation over a supported R~I'(CO)~ species, activated photochemically: (1) The Rh*(C0)2surface species can be photodecomposed by 325-nm UV light. This decomposition is caused by UV photons and not by thermal effects of the light on the catalyst. (2) When photodecomposition of the Rh'(CO)z occurs in the presence of cyclohexane, the "naked- metal sites generated are active in breaking the C-H bond to form a Rh(CO)(H)(cyclohexyl) species on the surface. This species yields a stable (to 600 K) adsorbed C6H1 group which may be present on the support. Almost no adsorbed cyclohexyl is formed under the conditions of these experiments on reduced Rh sites before CO adsorption, with or without UV irradiation. Acknowledgment. We would like to acknowledge, with thanks, the support of this work by the Department of Energy, Office of Basic Energy Sciences. References and Notes (1) Janowicz, A. H.; Bergman, R. G. J. Am. Chem. Soc. 1982,104,352. (2) Janowicz, A. H.; Bergman. R. G. J. Am. Chem. Soc. 1983,105,3929. (3) Hoyano, J. K.;Graham, W. A. G. J. Am. Chem. Soc. 1982,101,3723, (4) Jones, W. D.; Feher. F. J. J. Am. Chem. Soc. 1984,106, 1650. (5) Periana, R. A.; Bergman, R. G. J. Am. Chcm. Soc. 1984,106,7272. ( 6 ) Anderson, F. R.; Wrighton, M. S.Inorg. Chem. 1986, 25, 112. (7) Rest, A. J.; Whitwell, I.; Graham, W. A. G.; Hoyano, J. K.;McMasta, A. D.J. Chem. Soc.. Chem. Commun. 1984, 1984,624. (8) Rest, A. J.; Whitwell, I.; Graham, W. A. G.; Hoyano, J. K.;McMastcr, A. D. J. Chem. Soc., Dalton Trans. 1987, 1987, 1181. (9) Wcillcr, B. H.; Wasserman, E. P.; Bergman, R. G.; Moore. C. B.; Rmentel, G. C. J. Am. Chem. Soc. 1989. 111, 8288.
(IO) Belt, S. T.; Grevels, F. W.; Klotzhticher, W. E.;McCamley, A.; Perutz, R. N. J. Am. Chem. Soc. 1989, 111, 8373. (1 1) WaaPerman, E. P.; Moore, C. Bradley; Bergman, R. G. Science 1992, 225, 315. (12) Kinney, J. B.;Stalcy, R. H.; Reichel, C. L.; Wrighton, M. S.J. Am. Chem. Soc. 1981, 103,4273. (13) Reichel, C . L.; Wrighton. M. S . J. Am. Chem. Soc. 1981, 103,7180. (14) Yang, A. C.; Garland, C. W. J. Phys. Chem. 1957, 61, 1504. (15) Primct, M. J. Chem. Soc., Faraday Tram. 1 1978, 74, 2570. (16) Yao, H. C.; Rothachild, W. G. J. Chem. Phys. 1978, 68, 4774.
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A Numerical Study of the Equilibrium and Nonequillbrlum Diffuse Double Layer in Electrochemical Cells W.D.Murphy? J. A. Manzanares,t**S.Maf6,t.s and H.Reiw*** Science Center Division, Rockwell International, Thousand Oaks, California 91 360, and Department of Chemistry. University of California, Los Angeles. California 90024 (Received: July 6, 1992; In Final Form: September 1. 1992)
A numerical solution of the Nernst-Planck and Poisson equations is presented. The equations are discretized in a finite difference scheme using the method of lies on variable spatial and temporal grids. A Gear’s stiffly stable p r e d i c t m e c t o r integration procedure which automatically adjusts the order of the predictor and corrector equations (and the step size) to ensure the accuracy of the results is incorporated. Some advantages of our approach over more classical ones are discussed. The numerical solution is applied to the study of the equilibrium and nonequilibrium diffuse electrical double layer (EDL) at the metal electrode/electrolyte solution interface. The prescription of this layer is similar to that used in the classical Gouy-Chapman theory. Electrode kinetics are described by the Butler-Volmer equation. Concentration, faradaic and displacement electric current densities,and electric potential profdea as functions of time athe cell thickness, and partiahly in the EDL regions at the metal electrode/solution interfaces, are obtained. Two physical problems are studied: (i) the formation of the equilibrium EDL, and (ii) the transient response of the system to an electrical perturbation. Thew examplea illustrate the potential applications of the numerical method.
Numerical techniques for the solution of transport problems in electrochemistry have contributed significantly to the analysis of many complex processes which are difficult to deal with using more conventional approaches. Though other techniques like the boundary element method are experiencing an increasing popularity,’ fmite differences2Jhas been one of the most widely used in electrochemistry since the pioneering work by Feldberg!.s The need for numerical solutions appears in a multitude of problems of practical interest, e.g., transport phenomena described by diffusional type or second-order parabolic PDEs (partial differential equations) subjected to temporal boundary conditions,*J diffusion-migration situations involving the coupled, nonlinear Nemst-Planck and Poisson equations,b10 convective diffusion problems in electrochemical c e l l ~ , ~etc. J Modern numerical solutions to the Nemst-Planck and Poisson equations in electrochemicalcells including electrical double layer (EDL) effects seem to be lacking.’ We present, here, a numerical solution of these equations for a system consisting of three ions To whom correspondence should be addressed. Rockwell International. ‘Permanent add=: Department of Thermodynamics, University of Valencia, 46100 Burjasot, Spain. 8 University of California.
having different charges and undergoing transport in an electrochemical cell. Transport is assumed to occur in one dimension. Special attention is paid to the evolution of the diffuse EDL. This layer is considered to consist of point ions in a continuous dielectric solvent with local potential of mean force taken to be the electrical potential, i.e., the system is prescribed in a manner similar to that employed in the classical Gouy-Chapman theory.’* Electrode kinetia are described by a Butler-Volmer-type equation that takes into account the structure of the nonequilibrium EDL.12 The numerical solution gives concentration, faradaic and displacement electric currents, the electric potential profiles as functions of time across the cell thickness, and, particularly in the EDL regions, at the metal electrode/solution interfaces. The numerical technique used is of the finite difference type, and incorporates the Gear’s stiffly stable predictorcorrector method. The equations are discretized using the method of lines on a variable spatial grid because large gradients are present in the EDL region. The time grid is automatically generated by the procedure in order to satisfy a prescribed error tolerance. These characteristia result in a stable and accurate numerical integration of the Nernst-Planck and Poisson equations. We have considered two problems of well-known complexity: (1) the formation of the equilibrium EDL at the electrode/solution interface, and (2) the electrical relaxation that occurs when an exponentially-shaped external potential perturbation is applied
0022-3654/92/2096-9983$03.00/0Q 1992 American Chemical Society
