J. Phys. Chem. 1983, 8 7 , 1906-1910
1906
Temperature Effect on the Removal of Hydroxyl Radicals by a Polycrystalline Platinum Surface G. T. Fu/lmoto,+0. S. Selwyn,t J. 1. Kelser, and M. C. Lln' Chemistry Division, Code 6 105, Naval Research Laboratory, Washington, D.C. 20375 (Received: October 4, 1982; In Final Form: December 9, 1982)
The catalytic removal of OH radicals by a polycrystalline platinum surface has been studied in a low-pressure discharge-flow system. OH radicals, generated in a microwave discharge, were passed over the Pt surface and detected downstream by using the laser-induced fluorescence (LIF) technique. The activation energy for the removal of OH by Pt over the temperature range of 300-900 K was found to be 3.6 f 1.3 kcal/mol. Above 950 K, OH started to desorb from Pt as was previoulsy demonstrated. A reinvestigation of the thermal desorption of OH over a broad range of temperature and the O/H ratio revealed that the activation energy for OH desorption depends strongly on the O/Hratio, varying from 27 kcal/mol at O/H = 100 to -51 at O/H= 0.037. Both the removal and desorption processes are believed to occur primarily on weaker surface sites because of the high coverages under the present flow conditions. Introduction We have recently applied the discharge-flow/laser-induced fluorescence method to study the catalytic removal of NH and NH2 radicals by polycrystalline Pt and Fe surfaces at different temperatures.l Because of the specificity and intensity of the laser, these radical species can be readily differentiated and detected to a level of 107/cm3(or torr) by means of the laser-induced fluorescence (LIF) technique. Due to the sensitivity of this technique in the detection of appropriate species, reactions involving a very low concentration of reactive species can now be investigated routinely.2 We were the first to apply the LIF technique to studies of heterogeneous catalytic processes. These studies include the activation energy measurement for desorption of OH3+ and NH7 radicals from catalyst surfaces and the determination of the internal energy of the OH radical desorbed from a polycrystalline Pt surface.8 In our previous study of the catalytic removal of NH and NH2 radical by Pt and Fe,' it was found that the NH2 radical was considerably less stable on both surfaces than the NH radical. By varying the temperature of both catalysts, one can determine the apparent activation energy for the removal of these species by a particular catalyst surface. In this work, we have employed the same method to study the stability of OH radicals on Pt surfaces. Additionally, we have reinvestigated the thermal desorption of OH from the surface over a broad range of temperatures and oxygen/hydrogen (O/H) ratios. The results are reported herein.
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Experimental Section A complete description of the experimental setup has been presented e1sewhere.l Briefly, 0.5-1.0-torr mixtures of HzO (0.5-1.3%) in argon were passed through a microwave discharge to produce a mixture containing OH radicals. Generated OH radicals were then flowed over a temperature-controlled polycrystalline Pt surface. The platinum surface consisted of a 99.99% pure platinum wire (50 cm long, 0.2 mm in diameter) which was coiled into a flat spiral and oriented in a plane perpendicular to the
gas flow. Flow rates in the range of -300 cm/s were employed. The concentration of OH radicals was monitored 2.8 cm downstream of the platinum surface by using the LIF techniques. The Pt wire was resistively heated. Temperature over the range of 300-1400 K were obtainable. The temperature of the catalyst was monitored by the change in the resistance of the wire in accordance with standard tables and occasionally checked with an optical pyrometer, particularly at higher temperatures. The Pt surface was conditioned before each experiment at 1000-1300 K in a manner similar to previous studies.*8 Oxygen (-3 torr) was flowed over the Pt wire for -30 min followed by flowing 1torr of hydrogen over the Pt wire at similar temperatures for an additional 15 min. The OH radical concentration was monitored by LIF using a flash-lamp-pumped dye laser. The laser was operated with a Rhodamine 590 water/methanol solution. OH radicals were excited in the A28 X2n transition. Laser light in the region of 308 nm was generated by intracavity doubling of the dye laser. The OH fluorescence was focused through a band-pass interference filter centered at 309 nm and imaged onto an 1P28 photomultiplier tube. The output of the photomultiplier tube was amplified (Tektronix 1A7A) and averaged with a Nicolet 1174 signal averager. The averaged signal was ratioed against the excitation energy of the laser pulse to obtain a measurement of the relative OH radical concentration. Gas mixtures of 02,Hz, and H 2 0 were made with Matheson UHP argon (99.999%). The H 2 0 used was degassed by a freeze-pump-thaw procedure using deionized distilled water. Experiments were also run using H2/02/Ar and H20/02/Ar mixtures to produce the OH radicals. Results obtained from these mixtures did not differ significantly from those obtained by using the H20/Ar mixtures.
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(1) Selwyn, G. S.; Fujimoto, G. T.; Lin, M. C. J. Phys. Chem. 1982,86, 760. (2) Lin, M. C.; McDonald, R. J. In "Reactive Intermediates in the Gas Phase: Generation and Monitoring"; Setser, D. W., Ed.; Academic Press: New York, 1979; p 233. (3) Talley, L. D.; Tevault, D. E.; Lin, M. C. Chem. Phys. Lett. 1979, 66, 584.
(4) Umstead, M. E.; Talley, L. D.; Tevault, D. E.; Lin, M. C. Opt. Eng.
1980, 10, 94.
(5) Talley, L. D.; Lin, M. C. AIP Conf. Proc. 1980, 61, 297.
NRC/NRL Postdoctoral Associate (1979-1981); present address: NBS, Washington DC 20234. NRC/NRL Postdoctoral Associate (1980-1982); present address: IBM Research Laboratory, San Jose, CA 95193.
*
(6) Tevault, D. E.; Talley, L. D.; Lin, M. C. J. Chem. Phys. 1980, 72, 3314. (7) Selwyn, G. S.; Lin, M. C. Chem. Phys. 1982, 62, 213. (8) Talley, L. D.; Sanders, W. A.; Bogan, D. J.; Lin, M. C. Chem. Phys. Lett. 1981, 78, 500; J . Chem. Phys. 1981, 75, 3107.
This article not subject to US. Copyright. Published 1983 by the American Chemical Society
The Journal of Physical Chemisfty, Vol. 87, No. 11, 1983 1907
Removal of OH Radicals by a Pt Surface TemDerature (K) 1000 7 0 0
500
400
)
300
Temperature (K)
2
1000 7 0 0
400
300
t
-2/
0.5
1.0
1.5 2.0 2.5 3.0 1IT (1000IK)
l
-3 0.5
3.5
1.0 1.5
,
,
2.0
2.5
,yj 3.0
3.5
l / T (looO/K)
Flgure 1. Relative OH concentration as measured by LIF in OH/Ar mixtures after passing them over a Pt wire. Data were taken as a function of Pt wire temperature. The different symbols represent different mixtures. The solM line Is a nonlinear least-squares fit of eq I to the data. The increase in OH concentration observed above 900 K was caused by OH desorption as described in the text and ref 3-8.
Flgure 2. OH concentration as a function of the Pt wire temperature. Data are plotted in the In (-In [OH]/[OH],) form to be compatible with eq 11. The solid line is a least-squares fit to eq 11.
1o01300 1
Results Catalytic Removal of OH Radicals. Figure 1 shows a typical set of data obtained for the effect of the Pt catalyst temperature on the OH radical concentration. One-torr mixtures of 1% H20 in argon were used in this case. The concentration of OH radicals was found to decrease rapidly as the temperature of the catalyst was increased to about 900 K. Above 950 K, the OH LIF intensity increased abruptly, apparently due to the desorption of the OH from the surface. This has been discussed in detail in our earlier works3* and will also be discussed later. The model used to analyze the data is based on the assumed first-order kinetics. As OH radicals flow down the tube, the radical species is in contact with the Pt wire for a finite period of time during which adsorption onto the Pt surface may occur. I t was assumed that an Arrhenius type rate constant was applicable to this adsorption process. The fraction of OH radicals that impinge the surface without being adsorbed or destroyed can be estimated by the following expression: [OHl/[OHlo = exp(-7[A exp(-E,/RT)lI
,
,
'
Temperature (K) 1200 1100 1000 1
'
I
I
J
L \
-am --
K
-
c
-
.-- 0
EL
fn
0"' or I 0
-
a , -
.-
-ma
c)..
K
I
I
I
I
(1)
where E, is the activation energy for the removal process, 7 is the average duration time that the OH radicals are in contact with the Pt wire as they flow by it, A is the Arrhenius preexponential factor, [OH], is the total initial concentration of OH radicals that come in contact with the Pt wire, and [OH] is the concentration of OH radicals that come in contact with the Pt wire but are not removed by surface reactions. Both [OH] and [OH], come from experimental fluorescence measurements that are made downstream from the wire. The fluorescence measurements are corrected for the amount of OH radicals that flow past the wire without impinging on the wire. This procedure was discussed previous1y.l Figure 1 shows a nonlinear least-squares fit of eq I to the data. The fit yields a value of 3.6 f 1.3 kcal/mol for the activation energy of the adsorption process. This value was independent of the concentration and pressure of the mixture in which the OH radicals were generated. Eq I can also be rearranged to In [-ln ([OH]/[OH],)] 0: -E,/RT (11) A plot of In [-ln ([OH]/[OH],)] vs. 1/T is presented in
(9) Talley, L.
D.;Lin, M. C. Chem. Phys. 1981, 61, 249.
1908
The Journal of Physical Chemistry, Vol. 87, No. 11, 1983
Fujimoto et al.
TABLE I : Apparent Activation Energies for OH 6 o Desorption Determined at Different O/H Ratios L
0,i
I
\
I
O/H
I
I
10
100
Figure 4. Activation energy of the OH desorption process measured by passing O,/H,/Ar mixtures over a Pt wire to produce adsorbed OH. The activation energy was measured as a function of varying O/H ratios.
with O/H ratios varying in the range of 0.5-10. A further study carried out recently, however, revealed that the values of activation energy for OH desorption depend rather strongly on the O/H ratio. The result of this study is presented in the following section. Further Study of OH Desorption. Our recent study of OH desorption from the Pt-catalyzed oxidation of NH, in this laboratory has shown that the apparent activation energy for OH desorption depends strongly on the 02/NH3 ratio.'O Previously we had reported a reaction mechanism for the production of OH from the decomposition of 02/H2 mixtures on Pt. The OH desorption was found to be the rate-limiting step and an apparent activation energy of 31 1 kcal/mol was determined.6 However, this activation energy was not examined over large variations in the 02/H2 ratio. Accordingly, we carried out an additional study to determine the variation in OH desorption energies using 02/H2ratios varied from 0.032:l to 1OO:l. The experimental procedure was essentially the same as described earlier in this paper, with the exception of the absence of microwave discharge. The partial pressures of 0, and H2were increased to compensate for the lower OH concentration from desorption, especially under highly hydrogen-rich conditions. O2 and H, were prepared separately as 8% mixtures in Ar or were used as pure gases and then premixed before flowing into the catalytic reador. To increase pumping speed and also to minimize the flow effects of differing partial pressures of the 0, and Hz mixtures in each run, 1.00 torr of pure Ar was added to the flow in each run. Two sets of data were obtained, one using the highest temperature limited to 1389 K with the lowest temperature determined by the detection limit of the particular run or by a drop in [OH] by a factor of lo2 as the temperature was decreased. The other set used a higher temperature limit of 1660 K and the lower temperature corresponding approximately to the upper limit of the previous set. Within the large uncertainty under small O/H conditions (f3 kcal/mol), no significant deviation between the two data sets was observed. We also tested for the possibility of surface saturation by increasing the absolute pressures of O2 and Hz, keeping the same mixture ratios. The signal was found to increase significantly, indicating that surface saturation was not a problem at these high temperatures. Whenever possible, the pressures of H2and O2mixtures were each kept below
+
-
(10)Selwyn, G. S.; Lin, M. C., to be submitted for publication.
Ei
mtorr
HJ mtorr
Ari torr
160'" 161'" 7 6'" 39'" 4.64 2.48 4.00 2.40 2.16 1.44 7.12 3.84 2.64 2.40 2.72 2.72 2.56 2.08
1.60 1.68 1.68 4.08 1.28 2.40 4.00 3.76 5.12 3.92 22'" 42'" 39'" 40'" 66'" 71'" 69'" 66'"
1.00 1.01 1.01 1.01
a
1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.01 1.00 1.00 0.99
1.00 1.01
(kcali
TIK
O/H
879-1375 899-1384 842-1376 905-1366 913-1389 939-1377 941-1372 955-1388 1210-1628 1133-1440 971-1372 1083-1302 1203-1387 955-1386 1248-1526 1320-1662 1217-1384 1312-1609
100 96 45 9.6 3.6 1.0
1.0 0.64 0.42 0.37 0.32 0.091 0.068 0.060 0.040 0.038 0.037 0.032
mol)
27.4 27.4 29.1 30.3 32.3 35.8 35.1 38.2 36.1 40.2 37.9 38.4 42.9 48.1 51.0 56.2 47.9 50.4
Pure gas instead of 8% Ar mixtures was used.
100 mtorr so that the predominant gas species was always the large excess of Ar added. Table I shows the results obtained: these data are also summarized in Figure 4. A clear trend is established. The activation energy for OH desorption increases from as low as 27 f 1 kcal/mol at O/H = 100 to as high as 51 f 3 kcal/mol at O/H = 0.037 f 0.02. This trend differs strongly from the results obtained for NH3+ O2,loin which a downturn was observed to occur for Oz/NH3 C 1.
Discussion The apparent activation energy for the removal of OH by the polycrystalline Pt catalyst in this flow study was measured to be -3.6 kcal/mol. This value was found to be independent of the concentration and pressure of the mixtures in which the OH radicals were generated. Under these conditions the surface is expected to be well covered, particularly for catalysts near room temperature. Accordingly, OH removal is probably the result of adsorption onto weaker surface sites which are not already occupied by H, 0, or OH. This could acount for the observed high activation energy, 3.6 f 1.3 kcal/mol, which is higher than the values of 2.4 and 0.9 kcal/mol previously determined for the removal of NH and NH,, respectively, by the same catalyst under similar experimental conditions. The removal of OH radicals by Pt in the present study could result from the following surface reactions: OH(g) + Pt OH(ads) (1)
- + + + - +
OH(ads)
O(ads)
H(ads)
OH(ads) OH(ads)
H(ads) H,O(g) O(ads) 02(g) H(ads)
(2)
(3)
(4) The rate of removal is probably controlled mainly by the initial adsorption (reaction 1) rather than by subsequent surface reactions (reactions 2-4). There is now strong and direct evidence that the Pt-OH species does exist'lJ2 and is sufficiently stable to desorb from the Pt surface as shown here and by earlier studies.3+ Reaction 3 was found to affect strongly the desorption of OH radicals at high temperatures (>950 K). The concentration of the OH detected in the gas phase during the (11) Fisher, G. B.; Sexton, B. A. Phys. Rev. Lett. 1980,44, 683. (12)Ibach, H.; Lehwald, S.Surf. Sci. 1980, 91, 187.
Removal of OH Radicals by
a Pt Surface
The Journal of Physical Chemistry, Vol. 87, No. 11, 1983 1000
by the energy diagram shown in Figure 5, which was constructed by using the energetics for O(ads)15J7and H(ads)17obtained from UHV single-crystal studies and the values for OH(ads) on Pt polycrystalline surfaces from our flow studies,3* assuming Edes= 30 kcal/mol. The heat of formation of the OH(ads) species thus obtained is some 22 kcal/mol higher than those of H(ads) O(ads). This cannot be reconciled with the fact that OH(ads) can be formed at 5120 K from the reaction of H2 O2 on Pt (111) surfaces.15 The association reaction, H(ads) + O(ads) OH(ads), apparently can occur with little or no activation energy, in contradiction to our previous results. Additionally, our low activation energy for OH desorption cannot account for the observation of the reaction O(ads) H20(ads) 20H(ads) on Pt (111)at 2150 K.l’ The reaction would be endothermic by as much as 60 kcal/mol if Edes = 30 kcal/mol is adopted. The observation of the continued increase in the activation energy for OH desorption from 27 f 1 kcal/mol at O/H = 100 to 51 f 3 at O/H = 0.037 f 0.002 could help resolve the difficulty. Since a high concentration of O(ads) promotes the formation of OH(ads) by reaction -2, the reverse of reaction 2, the desorption of OH(ads) under the O/H >> 1 conditions probably occurs predominantly at weaker and more labile surface sites as reflected by a much lower desorption energy, -30 kcal/mol. Under the H/O >> 1conditions, however, reaction 3 is expected to dominate the catalytic oxidation process, removing OH(ads) predominantly from those labile sites on which species move around more rapidly. Accordingly, under these conditions, the desorbed OH would come mainly from more populated stronger and less labile sites with a considerably higher activation energy. Interestingly, the highest averaged value observed in the present work, Edes = 51 kcal/mol for O/H = 0.037, lowers the energy for the association reaction H(ads) + O(ads) = OH(ads) to nearly heat neutral. The use of the extrapolated value at O/H = 0, Ed&, = 60 f 3 kcal/mol, makes the above reaction exothermic by about 10 kcal/mol. Thus, it is believed that, at high temperatures (>950 K), excess H(ads) effectively lowers the steady-state coverage of O(ads) and OH(ads) by converting them into HzO which does not remain on the surface at these temperatures. Therefore, at O/H 0, we expect to have an extremely low OH coverage and the activation energy measured for its desorption corresponds to the dissociation energy for only the strongest Pt-OH bond. In evaluation of the thermochemistry of H(ads), O(ads), and OH(ads) on Pt surfaces, perhaps, only this high value for D(Pt-OH) should be used in conjunction with other quantities obtained from UHV low-coverage experiments.
+ +
I
I
-50
1
[
I
(
Pt;yH
Pt-0
p”
LOW O/H
Figure 5. Energy level diagram for the OH on platinum system. Data for this figure came from the following: OH(g), ref 18; 0-Pt, ref 15 and 17; and H-R, ref 15 and 17. The HIGH O/H and LOW O/H OH-Pt levels refer to the range of OH-Pt sites that are Important in the OH desorption process. The HIGH O/H level refers to the site that plays the major role In the desorption process under the situation of a high O/H ratio. Conversely, the LOW O/H level refers to a low O/H ratio.
catalytic oxidation of H2by 02 and N20,5as well as in the catalytic decomposition of Hz0,9 was found to be influenced by the presence of HP Addition of an excess amount of H2 (Le., H/O >> 1)effectively eliminates the desorption process OH(ads) OH(g)
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presumably by reaction 3, producting H20.6 On the other hand, reaction 4 was found to be unimportant at high temperatures. Continued addition of an oxidant such as N205or Oz6typically resulted in a linear increase in OH desorption until a constant plateau was reached after a large excess of the oxidant had been introduced. This suggests that reaction 4 is ineffective in reducing the concentration of OH(ads) even in the presence of a large amount of ~ x i d a n t s . ~ ~ ~ The removal of OH(g) could also result from a RidealEley type process. O W ) + Wads)
-
H Z W
This is conceivable in view of the relatively weak Pt-H bond,13 in comparison with that of the C-H bonds in alkanes, for example. The gas-phase reaction of OH with alkanes (RH) OH + RH H2O R +
+
is known to take place with a typical activation energy of 2-5 kcal/mol.14 The dissociation energy of a secondary C-H bond is about 94 kcal/mol, which is amost 40 kcal/mol stronger than the Pt-H bond. The observed dependence of OH desorption activation energy on the O/H ratio is most interesting. It is also important because this observation could resolve a puzzling difficulty brought about by our previous report of a relatively low activation energy, 30 kcal/mol, for the thermal desorption This difficulty can be demonstrated (13) Ertl, G. In “The Nature of the Surface Chemical Bond”;Rhodin, T. N., Ertl, G. Eds.; North-Holland Publishing Co.: Amsterdam, 1979; p 313. (14) Kerr, J. A.; Moss, S. J. ‘Handbook of Bimolecular and Thermolecular Gas Reactions”;CRC Press: Boca Raton, FL, 1981; Vol. 1, p 380.
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+
-
-
Conclusion The catalytic removal of OH radicals by a highly covered polycrystalline Pt surface has been studied in a discharge-flow system by using the laser-induced fluorescence technique. The activation energy for the removal process was found to be about 3.6 kcal/mol. Above 950 K, the OH (15) Fisher, G. B.; Gland, J. L.; Schmieg, S. J., submitted to Surf. Sci. (16) “JANAF Thermochemical Tables”, 2nd ed.; National Bureau of Standards: Washington DC, Natl. Stand. Ref. Data Ser. (US., Natl. Bur. Stand.) 1971, No. 37. (17) Fisher, G. B.; Sexton, B. A.; Gland, J. L. J. Vac. Sci. Technol. 1980,17, 144. Gland, J. L.;Sexton,B. A.; Fisher, G. B. Surf. Sci. 1980, 95, 587.
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J. Phys. Chem. 1983, 87, 1910-1916
was found to desorb from the Pt surface with or without microwave discharge. A reinvestigation of the thermal desorption of the OH radical indicates that the desorption energy depends strongly on the O/H ratio of reactant mixtures, varying from 27 kcal/mol at O/H = 100 to 51 kcal/mol at O/H 0.037, with an extrapolated value of Edes= 60 f 3 kcal/mol at O/H = 0. On the basis of the
=
established mechanism for the H2 + O2 reaction on Pt, it is concluded that, under excess H(ads) conditions, OH(ads) coverage becomes extremely low and the value, 60 kcal/ mol, corresponds to the dissociation energy of the strongest Pt-OH bond. Registry No. OH, 3352-57-6; Pt, 7440-06-4.
Observation of Photochemical Intermediates in the Low-Temperature Photolysis of Silica-Adsorbed Fe(CO), Mark R. Trushelm and Robert L. Jackson’ Corporate Research Center, Allied Corporation, Morristo wn, New Jersey 07960 (Received: October 4, 7982; In Final Form: December 28, 1982)
The photochemistry of silica-adsorbed Fe(C0)5has been examined at reduced temperatures with a frequency-tripled Nd:YAG laser (355 nm) as the light source. The only significant photoproduct observed by IR spectroscopy upon photolysis at temperatures between 200 and 300 K is Fe,(CO),,; no Fe2(C0)9is detected. Photolysis at 100 to 150 K also yields Fe3(CO)12,but another major product is formed as well. On the basis of IR spectra obtained in the carbonyl stretching region, this product is assigned as Fe(CO)4(Si02), which denotes the species formed upon addition of a silica surface hydroxyl group or siloxane bridging oxo group to the primary photoproduct, Fe(CO)+ This species is a key intermediate in the photoinitiated conversion of Fe(CO), to Fe3(C0)12 in this system. Photolysis experiments were also carried out at temperatures below 100 K, but here Fe(CO)S apparently aggregates on the silica surface, complicating interpretation of the photochemistry.
Introduction Heterogeneous photochemistry, involving light-initiated processes occurring on solid surfaces, is a growing field which promises to yield a number of important applications.’v2 Our work in this area3 has concentrated on the photochemistry of metal complexes adsorbed onto a solid surface, with emphasis on possible applications to heterogeneous c a t a l y ~ i s . In ~ ~a~previous paper we discussed ways in which photoinitiated processes could provide an important alternative to present methods for the preparation of supported transition-metal catalyst^.^ One attractive approach involves the single-step preparation of supported transition-metal cluster complexes via irradiation of s m d e r polynuclear or mononuclear carbonyl complexes adsorbed onto a support surface. This idea was illustrated by the photolysis of silica-adsorbed Fe(C0)5, which leads almost exclusively to the formation of Fe3(CO)12.This was not the expected cluster product, however, since photolysis of Fe(C0)5in the gas or liquid phase or in solution yields mainly Fe2(C0)9.6 The formation of Fe,(C0)12was interpreted as evidence that silica plays an active role in photoinitiated reactions of adsorbed Fe(CO)5. Specifically, we proposed that silica surface hydroxyl (1) Wrighton, M. S., Ed. “Interfacial Photoprocesses: Energy Conversion and Synthesis”; American Chemical Society: Washington, DC, 1980;Adv. Chem. SOC.,No. 184. (2) Ehrlich, D. J.; Osaood, R. M. Chem. Phys. Lett. 1981, 79, 381. (3) Jackson, R. L.; h h e i m , M. R. J.Am. Chem. Soc. 1982,104,6590. (4) See also: Nagy, N. B.; Eenoo, M. V.; Derouane, E. G. J. Catal.
1977, 58, 230. (5) See also: Kinney, J. B.; Staley, R. H.; Reichel, C. L.; Wrighton, M. S. J. Am. Chem. SOC.1981, 103,4273. (6) (a) Chisolm, M. H.; Massey, A. G.; Thompson, N. R. Nature (London) 1966,211,67. (b) Dewar, J.; Jones, H. 0. Proc. R. SOC.London, Ser. A 1905, 76, 558. (c) Braye, E. H.; Huebel, W. Znorg. Synth. 1966, 8, 178. 0022-3654/83/2087-1910$01.50/0
groups or siloxane bridging oxo groups participate as weak ligands, thus altering the outcome of secondary thermal processes involving unsaturated iron carbonyl intermediates. In an effort to understand these secondary thermal processes, we have examined the photolysis of silica-adsorbed Fe(C0)6at reduced temperatures. In the present study we have used spectroscopic methods to follow both photochemical and subsequent thermal processes in this system between 10 and 300 K. Our results indicate that silica surface groups indeed act as ligands, trapping the primary photoproduct Fe(C0)4on the surface as a saturated but thermally labile species, Fe(CO)4(Si02).This species is shown to be a likely intermediate in the silica surface-assisted photochemical conversion of Fe(CO)5to FedCO)12. Experimental Section Fe(C0)5was obtained from Strem and was purified by vacuum transfer to a glass bulb. The bulb was stored in liquid nitrogen except when in use and was thoroughly degassed before each use. The silica used in most experiments was specially prepared in high purity by Dr. John N. Armor and Mr. Emery J. Carlson of Allied Corporate Research Laboratory and is the same as that used in our previous experiment^.^ The BET surface area of this material was 470 m2 g-l. The silica particles were 20-60 mesh (0.25-0.85-mm diameter). The samples were pretreated in air for 2 2 h at 560 “C. Transmittance of the samples was -25% over a path length of 2.5 mm at the photolysis wavelength. As b e f ~ r e a, ~commercial silica (Davison PA-400) was also used in a few experiments, with qualitatively identical results. AU gas manipulations were carried out on a glass vacuum line fitted with greaseless stopcocks and capable of lo4
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0 1983 American Chemical Society
