Photochemistry of Iron Pentacarbonyl Adsorbed on Au (111)

reflection absorption spectroscopy (IRAS), thermal desorption spectroscopy ... IRAS and TDS show that Fe(CO)5 is molecularly adsorbed on the surface a...
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J. Phys. Chem. 1996, 100, 14769-14775

14769

Photochemistry of Iron Pentacarbonyl Adsorbed on Au(111) Shinri Sato* and Toshihiro Suzuki Catalysis Research Center, Hokkaido UniVersity, Sapporo 060, Japan ReceiVed: March 13, 1996; In Final Form: May 22, 1996X

The photodecomposition of Fe(CO)5 adsorbed on the Au(111) surface at ∼90 K has been studied using IR reflection absorption spectroscopy (IRAS), thermal desorption spectroscopy (TDS), and X-ray photoelectron spectroscopy (XPS). IRAS and TDS show that Fe(CO)5 is molecularly adsorbed on the surface and desorbed completely around 180 K. A totally symmetric vibrational band appears at 2114 cm-1 in IRAS, indicating that structural deformation of Fe(CO)5 or charge transfer between the substrate and the adsorbate occurs to some extent upon adsorption. Irradiation of the adsorbed Fe(CO)5 leads to decarbonylation with a high quantum yield, especially at around the absorption threshold wavelengths of Fe(CO)5 (300-360 nm). The enhanced photodecomposition at the absorption threshold is found limited to the close vicinity of the surface. At shorter wavelengths than 260 nm, however, the photolysis yield increases with increasing distance from the surface, suggesting significant quenching of photoexcited states near the surface. The average CO/Fe ratio in a photoproduct is close to 4 for monolayer (ML) coverage, suggesting intermediate species such as Fe(CO)4, and decreases to ∼3 as the coverage exceeds 3 ML. Since XPS shows formation of Fe-surface bonding, further photodecarbonylation may be inhibited by fast relaxation through the bonding. The photoproduct exhibits a sharp, single C-O stretching band at 2081 cm-1 in IRAS, indicating formation of the carbonyl ligands with vertical vibrational modes in the same symmetry. Its structure is discussed in detail.

1. Introduction Photodecomposition of Fe(CO)5 adsorbed on solid surfaces has been found to give a variety of products depending upon the nature of surface,1-6 and the yield of the photodecarbonylation is significantly high as compared to other surface photochemical reactions.2,7 For example, Fe3(CO)12 is typically formed on illuminated SiO21 or porous Vycor glass (PVG)2 at room temperature, while Fe2(CO)9 is mainly produced on Al2O3.4 On the other hand, the photolysis of Fe(CO)5 over semiconductor at room temperature leads to the deposition of Fe metal on the surface.5 Darsillo et al.2 have measured the quantum yield of Fe(CO)5 disappearance during the 350 nm photolysis of Fe(CO)5 over PVG using a Kr ion laser. The quantum yield determined at 22 °C was 0.96. Celii et al.7 have measured the cross section of photodecarbonylation at 337 nm for Fe(CO)5 adsorbed on single-crystal Al2O3 and Ag(110) surfaces at temperatures lower than 150 K using a N2 laser, and they found that the cross sections obtained for these surfaces are close to the gas phase absorption cross section for Fe(CO)5 at 337 nm ((1-1.5) × 10-18 cm2). Henderson et al.8 have studied the photodecomposition of Fe(CO)5 adsorbed on the Ag(111) surface at 90 K and determined the cross sections to be ∼9 × 10-18 and (3-6) × 10-20 cm2 at 256 and 365 nm, respectively, using a high-pressure Hg arc. These results imply that the rate of Fe(CO)5 photofragmentation is competitive with energy transfer to the surface, though the quenching of photoexcited states is rapid, especially for metal substrates. Henderson et al.8 also showed that when the Ag(111) surface is covered with n-decane in advance, the cross section increased at 256 nm but decreased at 365 nm. This result indicates that the surface effects in surface photochemistry may be different with the radiation energy. Bottka et al.9 studied the Fe(CO)5 photolysis on GaAs(100) surface at 77 K using a low-pressure Hg lamp and concluded the deposition of Fe film from the reflection change of a HeX

Abstract published in AdVance ACS Abstracts, August 1, 1996.

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Ne laser beam. Temperature-programmed desorption (TPD) measurements carried out by others, however, indicate the presence of CO-containing species on Al2O3,7 Ag(110),7 Ag(111),8 and Si(111)10 surfaces after irradiation. Swanson et al.11,12 tried to analyze the surface species formed by the 337 nm photolysis of Fe(CO)5 on Si(111) at 120 K using multiple internal reflection IR spectroscopy and concluded the absence of any surface stable Fe(CO)x (x < 5) species. Gluck et al.,10 on the other hand, investigated the 257 nm photolysis of Fe(CO)5 on Si(111) at 90 K using TPD and high-resolution electron energy loss spectroscopy (HREELS), but they could not reach any definite conclusions about the structure of surface species. We have studied the photodecomposition of Fe(CO)5 on Ag polycrystal13 and SiO2 film (with a buried Al layer)14 surfaces at temperatures lower than 140 K using TPD, IR reflection absorption spectroscopy (IRAS), and X-ray photoelectron spectroscopy (XPS). The average CO/Fe ratio in the photoproduct was found to be ∼4 for monolayer coverage, and no deposition of Fe metal was observed even after prolonged irradiation at 150 nm.13,14 The IRA spectra of the products on the Ag substrate show two C-O stretching bands, while those on the SiO2 substrate exhibit a broad, featureless band in the C-O stretching region, when the coverage is lower than monolayer. These species was tentatively assigned to an oligomer species, {Fe(CO)4}n (n ) 2, 3),13,14 but a further investigation on a well-defined surface is required to assign in more detail the photoproducts. In the present study, we have used Au(111) single-crystal surface as a substrate for the surface photochemistry of Fe(CO)5 and investigated the adsorption state of Fe(CO)5, the dependence of the photodecarbonylation yield on light wavelength and on Fe(CO)5 coverage, and the structure of the photoproducts. 2. Experimental Section Au(111) single crystal (12 mm in diameter, 1.5 mm thick, 99.999% purity) was purchased from Mateck. The surface of © 1996 American Chemical Society

14770 J. Phys. Chem., Vol. 100, No. 35, 1996 the Au disc had been polished within a roughness of 0.03 µm. Three holes were bored along the edge of the disc; two of them were for Ta heating wires, and another was for a thermocouple. The sample was cleaned by Ar ion sputtering at 700 K and annealed at 800 K. The experimental apparatus used was similar to that described in previous papers.13,14 An ultrahigh-vacuum chamber was pumped by a turbomolecular pump (Leybold, 400 L/s) and an ion pump (Anelva, 270 L/s), and the base pressure was 5 × 10-10 Torr. The sample holder was able to heat the sample to temperatures higher than 1000 K and to cool it to 90 K. The substrate was held on the sample holder using Ta wires (0.3 mm in diameter). The sample was heated by heating the Ta wires electrically. Thermal desorption spectra (TDS) of adsorbed species were obtained using a quadrupole mass spectrometer (Anelva AQA-200) and a programmable temperature regulator (Chino KP-31E), both of which were controlled by a computer. Because CO+ (m/e ) 28) is produced from both Fe(CO)5 and CO in a mass spectrometer, Fe(CO)5 desorption was distinguished from CO desorption by observing Fe+ (m/e ) 56) simultaneously. IRAS measurements were carried out using an FTIR spectrometer (Bio-Rad FTS-155) equipped with an MCT detector. The incident angle of IR beam was ∼85°, and a wire grid polarizer was used to select p-polarized light, since only p-polarized light is effective for IRAS. The IRA spectra were recorded with 4 cm-1 resolution and 200 scans. XPS measurements were carried out at the Institute for Molecular Science. The XPS apparatus was equipped with a VSW CLASS150 electron energy analyzer and a dual-anode X-ray source (Mg and Al). The energy calibration of the XPS spectra was carried out with reference to the Au 4f7/2 and 4f5/2 peaks. The light source was a 300 W Xe lamp (ILC LX300UV), which was monochromatized by a grating monochromator (Ritsu MC20L) and focused by a CaF2 lens onto the sample through an MgF2 window. A water filter (20 cm long) was used to remove heat. Incident photon flux into the chamber was measured using chemical actinometry. In the XPS measurements the light source was synchrotron orbital radiation (SOR), which was filtered by a sapphire filter (∼150 nm cutoff). The rate of CO evolution during photolysis was determined from CO mass signal intensity and the pumping speed of the system for CO. To measure the pumping speed, CO was introduced into the chamber at a constant pressure from the reservoir of known volume, and the CO pressure decrease in the reservoir was monitored with a precision pressure gauge (Baratron µBar). During this procedure the increase in CO mass signal was integrated by a computer, and the relation between the CO mass signal intensity and the CO dosing (evolution) rate was calculated. The CO photoevolution from Fe(CO)5 adsorbed on the sample holder occurs to a small extent due to stray light. This extent was determined from the CO photoevolution after removing Fe(CO)5 from the sample by TPD. Fe(CO)5 was obtained from Aldrich, vacuum-distilled, and stored in the dark. Fe(CO)5 was introduced into the apparatus through a variable-leak valve. The coverage of Fe(CO)5 on the surface was controlled by pressure and exposure time (in langmuirs, 10-6 Torr‚s). 3. Results and Discussion 3.1. Adsorption State of Fe(CO)5. Figure 1 shows TDS from Fe(CO)5 adsorbed on Au(111). Adsorbed Fe(CO)5 is molecularly desorbed at ∼180 K, indicating the associative adsorption. The desorption peak increases with increasing exposure, and a shoulder peak begins to emerge at a lowtemperature side at 8 langmuir exposure, indicating the forma-

Sato and Suzuki

Figure 1. TDS from Fe(CO)5 adsorbed on Au(111) as a function of exposure. The heating rate ws 4 K/s.

Figure 2. IRA spectra of Fe(CO)5 adsorbed on Au(111) at 90 K as a function of coverage.

tion of second layer. Monolayer coverage is, therefore, achieved at an exposure between 6 and 8 langmuirs. IRAS of Fe(CO)5 on Au(111) shows three bands in the C-O stretching region as shown in Figure 2, and no other bands are detectable. The bands at 2061 and 2015 cm-1 are typically observed for Fe(CO)5 physically adsorbed on Pt(uncleaned),15 Si,16 Ag,13 and SiO2/Al.14 The weak band at 2113 cm-1 is only observed at submonolayer coverages and disappears when the surface is covered with monolayer amounts of n-decane before adsorption. This result indicates that the directly adsorbed molecular species exhibit the band. The 2113 cm-1 band has been observed in the IRAS for Fe(CO)5 chemisorbed on the Ag polycrystalline surface cleaned by Ar ion sputtering at room temperature.13,17 The species giving the 2113 cm-1 band is not desorbed from the Ag surface at ∼180 K during TPD and totally decomposes at ∼300 K.13,17 For the present Au(111) surface, however, this species is also desorbed at ∼180 K. Although one may suppose that adsorbed CO is responsible for the 2113 cm-1 band, neither TDS nor IRAS indicates CO adsorption on the present Au(111) surface at 90 K. Since CO adsorbed on metal surfaces is readily exchanged with gas phase CO even at low temperatures, adsorbed Fe(CO)5 was tried to expose to gas phase 13CO, but no shift of the 2113 cm-1 band was observed. The IRA spectra of adsorbed Fe(CO)5 are quite different in

Photochemistry of Fe(CO)5 on Au(111)

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Figure 4. Coverage dependence of the photodecarbonylation yield in 250 and 330 nm photolysis of Fe(CO)5 adsorbed on Au(111) at 90 K.

Figure 3. Wavelength dependence of the yield of the photodecarbonylation from 1 ML of Fe(CO)5 on Au(111) at 90 K. Inset is the absorption spectrum of gas phase Fe(CO)5,9 and the right ordinate indicates an optical cross section. O, for bare Au(111) surface; 0, for the surface covered with 1 ML of n-decane prior to Fe(CO)5 adsorption.

position and shape from its transmission spectra in which two intense C-O stretching bands are observed at around 2024 and 2002 cm-1 and assigned to ν10 (A2′′ mode) and ν6 (E′ mode), respectively.1,2 A big blue shift as well as band distortion is often observed in IRAS for molecules with a high extinction coefficient and has been ascribed to the anomalous dispersion of refractive index in the region of the observed band.18 We have obtained the refractive index of adsorbed Fe(CO)5 from the IR transmission spectra of Fe(CO)5 adsorbed on SiO2 films coated on a sapphire plate using the Kramers-Kronig relation and then calculated IRA spectra of Fe(CO)5 adsorbed on Au using the Fresnel formula.19 The observed IRA spectra are basically reproduced by calculation. The 2061 and 2015 cm-1 bands are, therefore, assigned to ν10 and ν6, respectively.19 The 2113 cm-1 band is little shifted in the IRAS calculation due to its small extinction coefficient.19 This band appears in the Raman spectrum of liquid Fe(CO)5 with medium intensity and was assigned to IR-inactive, totally symmetric vibrational mode, ν1.20 The band also appears when Fe(CO)5 is adsorbed on silica,1 PVG,2 alumina,4 or zeolite21 at room temperature, and its appearance in IR was ascribed to a decrease in molecular symmetry upon adsorption.21 A charge transfer interaction between adsorbate and metal surface has been also proposed to make totally symmetric vibrational modes IR-active.22 Thus, the adsorption state of Fe(CO)5 on the Au(111) surface is the physisorbed state influenced either by structural deformation or by charge transfer. 3.2. Photodecarbonylation. Mass spectrometric analysis during the photolysis of adsorbed Fe(CO)5 shows that only CO is evolved, and no Fe-containing species are desorbed. Upon illumination, CO pressure promptly increased and decayed exponentially with illumination time. The rate of photodecarbonylation was determined from the initial rise-up of CO pressure, and the apparent quantum yield of CO evolution (the number of CO molecules desorbed per incident photon) was then calculated using the incident photon flux at each wavelenth as shown in Figure 3 (open circle). The inset in the figure represents the absorption spectrum of gas phase Fe(CO)5 cited from ref 9, and the right ordinate indicates an optical cross section. A similar absorption spectrum was reported by Nathanson et al.,23 but the cross sections at shorter wavelengths are somewhat greater than those shown in the inset. The

absorption spectrum in the inset shows an intense charge transfer band around 200 nm, a secondary intense band at ∼250 nm assigned to a metal-ligand charge transfer band, and a broad and weak band near 330 nm assigned probably to a ligandfield transition band.24 The cross section for the photodecarbonylation was calculated from the apparent quantum yield and the number of Fe(CO)5 molecules at monolayer coverage, which was assumed to be 2 × 1014 molecules/cm2 based on the density of solid Fe(CO)5. The calculated cross section, σ, is shown at the right ordinate of Figure 3. Although the cross section for photodecarbonylation is much lower than that for the Fe(CO)5 absorption at wavelengths shorter than 300 nm, these two cross sections become the same order of magnitude at longer wavelengths (σ ) 1.0-1.5) × 10-18 cm2), indicating a remarkably high quantum yield for the photodecarbonylation at the absorption threshold wavelengths of Fe(CO)5. We have also measured the apparent quantum yield of photodecarbonylation at 250 and 330 nm as a function of coverage (adsorption layer). The yield at 250 nm increases with increasing coverage up to ∼7 ML, while the yield at 330 nm levels off at 2-3 ML as shown in Figure 4. Although the yield increases with increasing coverage due to increase in total cross section, this kind of increase would be limited in a few monolayers if the absorption cross section is large. Since the optical cross section of Fe(CO)5 is fairly large at 250 nm, a further increase in the yield is due to another reason. Such coverage-dependent increase in photoreaction yield has been often observed in surface photochemistry and ascribed to a decrease in energy relaxation of photoexcited species with increasing absorption layer, i.e., increasing distance from the surface.25 The coverage dependence observed in the present study is, however, different from that obtained by Henderson et al.8 for Fe(CO)5 photolysis on the Ag(111) surface. They calculated the cross section from the Fe(CO)5 peak areas of TDS before and after the photolysis and found that the increase in coverage from 1 to 5 ML results in a little decrease in the cross section at 365 nm, while virtually unchanging the cross section at 256 nm. We also measured the cross section by our method using a polycrystalline Ag surface to examine their result. The result is very similar to the present Au result, but the effect of surface plasmon excitation appears at around 320 nm26 as has been observed for the photolysis of Mo(CO)6 adsorbed on the Ag(111) surface.27 The method of Henderson et al.8 tends to involve a significant error at higher coverages, since the amount of Fe(CO)5 remaining undecomposed is so large that the amount of Fe(CO)5 photodecomposed is not exactly obtained from the TDS data. Although the present result at 250 nm can be explained in terms of the quenching of excited states of the molecules near the surface, the curve obtained at 330 nm indicates that the

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photodecomposition occurs under the influence of the surface. To examine the influence of surface, the metal surface was covered with a chemically and photolytically inert material as a spacer layer. For this purpose n-decane was chosen. After n-decane was preadsorbed at 1 ML, the monolayer amount of Fe(CO)5 was adsorbed and the photodecarbonylation yield was measured as a function of wavelength. The results are plotted in Figure 3 (open square). The yield is significantly suppressed at wavelengths longer than 260 nm, while the yield at 250 nm exceeds the one observed on the bare surface. This result clearly shows that the photodecarbonylation in a longer wavelength region takes place under the influence of the surface. Henderson et al.8 also obtained a similar result: the Ag(111) covered with multilayer n-decane gives a lower cross section at 365 nm, while a higher cross section at 256 nm, than the bare surface. They discussed this enhanced photodecomposition at the threshold wavelengths for possible surface effects: (a) metal to molecule charge transfer, (b) photoelectron capture, (c) red-shifted absorption threshold, and (d) resonant coupling with excited surface plasmon. They excluded (d) since the energy of 365 nm light (3.4 eV) is below the surface plasmon energy of Ag(111). Also, (a) was ruled out for the reason that the anion states of Fe(CO)5 are not known to exist in the gas phase, and (b) was eliminated because the light energy is not enough to produce photoelectrons from Ag(111) surface which has a work function of 4.6 eV. Finally, they ascribed their result to a red shift of the Fe(CO)5 absorption threshold, which could result from an increased exchange-correlation interaction between the photoexcited state species and the substrate electrons and/or from lowering of the excited state energy by interaction with the electrostatic potential of the surface. For the present case, the photoelectron capture could be ruled out since the work function of Au(111) (5.3 eV) is greater than that of Ag(111). Although the surface plasmon energy of Au(111) is ∼2.5 eV, no photodecarbonylation takes place at wavelengths longer than 400 nm. Because the results of IRAS indicate a change in molecular symmetry of Fe(CO)5 or charge transfer from the surface, the enhancement of the photodecarbonylation at the absorption threshold wavelengths will be attributed to one of these factors. 3.3. Photolysis Products. Surface species produced by the photolysis of adsorbed Fe(CO)5 is thermally decomposed and evolves CO remaining Fe on the surface. TDS shows a broad CO desorption peak centered at 300 K as depicted in Figure 5. The peak maximum is positioned at almost the same temperature as observed for the photoproduct on the Ag polycrystalline surface,13 but the peak is very sharp in the latter. TDS feature is independent of the wavelength of irradiation. As described in the previous paper,14 the average CO/Fe ratio, x, in the surface photoproducts of Fe(CO)5 is given as

x ) 5n2/(n1 + n2)

(1)

where n1 and n2 are the numbers of CO molecules desorbed during irradiation and TDS, respectively. The n1/n2 ratio can be obtained from the intergrated CO mass signals during irradiation and TDS. The CO/Fe ratio calculated for 250 nm photolysis is shown in Figure 6 as a function of the initial coverage from 1 to 13 ML. The ratio is close to 4 in the vicinity of the surface, as observed for the Ag13 and SiO2/Al14 substrates, and decreased to ∼3 with increasing adsorption layer. This result strongly suggests the formation of surface Fe(CO)4 intermediate at a primary process, which hardly undergoes further photodecarbonylation. Although the photolysis of matrix-isolated Fe(CO)5 is known to produce the Fe(CO)4 intermediate, a prolonged irradiation leads to further

Figure 5. TDS of the photoproduct of Fe(CO)5 adsorbed on Au(111). The sample was irradiated at 250 nm for 10 min. The heating rate was 2 K/s.

Figure 6. Average CO/Fe ratio in the photoproduct as a function of initial Fe(CO)5 coverage.

decomposition,28-32 similarly to the photolysis of gas phase Fe(CO)5.23 There should be some mechanism which makes the surface intermediate more stable against irradiation. XPS of adsorbed Fe(CO)5 exhibits the peaks of Fe 2p3/2 and 2p1/2 at 709.5 and 722.5 eV, respectively, as shown in Figure 7. These binding energies are in agreement with those observed for condensed Fe(CO)5 on the Ag surface.17 The peaks shfit to higher binding energies upon irradiation by ∼2 eV, indicating a drastic change in the electronic state of center Fe. This shift may be due not only to the decarbonylation but also to the formation of Fe-Fe bonding among intermediates or Fesubstrate bonding. After irradiation the substrate was heated in a stepwise manner to 300 K, and XPS spectra were recorded. The Fe 2p peaks are remained unchanged in position during the elevation of temperature. The surface species should be totally decomposed and deposit Fe on the surface during the XPS measurements at 300 K, since no CO is evolved in TDS after the XPS measurement. The disappearance of CO may be due to a prolonged time expended for the XPS measurements. The binding energies observed for the photoproduct are, therefore, the same as those for Fe deposited on the surface. For the Fe(CO)5/Ag13 case, irradiation leads to the shift of Fe 2p3/2 and 2p1/2 peaks toward lower binding energies, i.e., the inverse direction of the present Au case. Upon heating the Ag sample to 280 K, the peaks slightly shift further to lower binding energies and remains unchanged by further heating to 380 K, at which TDS shows the total decomposition of the surface species.13 This difference in the shifting direction of Fe 2p peaks

Photochemistry of Fe(CO)5 on Au(111)

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Figure 9. Illustrations of the structures of Fe(CO)5, possible intermediates and photoproducts: (a) Fe(CO)5, (b) intermediate formed during low-temperature matrices, (c) and (d) possible surface intermediates, (e) unbridged Fe2(CO)8 formed in low-temperature matrices, and (f) possible surface dimer species. Figure 7. Fe 2p XPS spectra of Fe(CO)5 adsorbed on Au(111) at 140 K. The sample was irradiated with SOR through a sapphire filter (150 nm cutoff) for 40 min and then heated to the indicated temperature.

Figure 8. Time dependence of the IRAS of Fe(CO)5 adsorbed on Au(111) at 90 K during 320 nm photolysis.

during the photolysis indicates the formation of Fe-substrate bonding rather than Fe-Fe bonding, since the former bonding would give rise to the substrate-dependent shift of XPS peaks while the latter would not. It is therefore reasonable to conclude that the photodecarbonylation of adsorbed Fe(CO)5 leads to the formation of surface bound [Fe(CO)4]n species. Such species could be very stable against irradiation, since fast relaxation can occur through the bonding. Figure 8 shows the time dependence of the IRAS of adsorbed Fe(CO)5 during 320 nm photolysis. All of the initial bands belonging to adsorbed Fe(CO)5 disappear, and a sharp, single band appears at 2081 cm-1, the intensity of which is comparable to that of the 2054 cm-1 band. The band position of the product is independent of the wavelength of irradiation but slightly shifts to lower frequencies as the initial coverage of Fe(CO)5 is decreased. The present IRAS of the photoproduct is quite different from those observed for the Ag13 and SiO2/Al14 substrates on which the photoproduct shows broad bands. Since IRAS is sensitive only to vibrational modes perpendicular to the surface (the surface selection rule of IRAS),33 this result indicates that the photoproduct has carbonyl ligand(s) with vertical vibrational modes of the same symmetry, and the other carbonyl ligands are parallel to the surface. One may suppose that the photoproduct is pyramidal Fe(CO)4 with its triangle

base toward the surface (see Figure 9c), since this species has Fe-surface bonding and only one carbonyl ligand perpendicular to the surface. Another possible structure of surface bound Fe(CO)4 is shown in Figure 9d, which can exhibit a single C-O stretching band in IRAS. An intermediate species, Fe(CO)4, formed during Fe(CO)5 photolysis in low-temperature matrices (Figure 9b) exhibits C-O strethcing bands at lower frequencies than Fe(CO)5.28-32 When Fe(CO)4 reacts with a molecule or a ligand larger than CO, Fe(CO)4L (L ) molecule or ligand) shows C-O stretching bands at higher frequencies than Fe(CO)5. For example, Fe(CO)4(C2H4) in methylcyclohexane at 77 K shows the most intense C-O stretching band at 2088 cm-1, while Fe(CO)4(PPh3) in methylcyclohexane at 298 K shows a band at 2052 cm-1.34 Since the photoproduct on the Au surface is thought to have Fe-surface bonding, its C-O stretching band may be shifted to higher frequencies than the bands of adsorbed Fe(CO)5. The extent of such blue-shifting is, however, hardly presumed. The surface moiety formed by the photolysis of Fe(CO)5 adsorbed on the Ag polycrystalline surface shows a narrow band at 2062 cm-1 and a broad band centered at 2038 cm-1, when its initial coverage is lower than 1 ML.13 This species has been tentatively assigned to oligomer species. We have recently calculated the IRAS of Fe2(CO)9 from the IR transmission spectrum of Fe2(CO)9 adsorbed on silica at room temperature, using the Kramers-Kronig and the Fresnel relations as described before.26 The calculation was done under the assumption that the extinction coefficient of Fe2(CO)9 is lower than that of Fe(CO)5, since the observed bands of the photoproduct are much weaker than those of Fe(CO)5. The calculated IRAS shows a narrow band at 2064 cm-1 and a broad band at 2040 cm-1, in good agreement with the observed IRAS. Because the average CO/Fe ratio in the photoproduct is ∼4, the surface species would be Fe2(CO)8, which shows an IR spectrum similar to Fe2(CO)9.35 The IRAS of the photoproduct on the Ag surface is changed with raising temperature and exhibits a sharp, single C-O stretching band at 2058 cm-1 around 280 K.13 This result suggests that the oligomer product decomposes to a monomer species at 280 K. TDS shows a sharp CO peak at ∼320 K, also suggesting that the species formed at 280 K has a simpler structure than the oligomer species, and no other CO peaks present before the 320 K peak.13 We have assumed from these results that the oligomer product is changed to the Fe(CO)4 species which exhibits a single C-O stretching band in IRAS. Because many surface species are mobile at temperatures as low as 90 K even though they are bound to the surface, the

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Figure 10. Changes in the IRAS of the photoproduct upon heating the sample to the indicated temperature. Initial Fe(CO)5 coverage was 0.5 ML.

initial photolysis intermediate may react with each other to produce a dimer or higher oligomer species. In the Fe(CO)5 photolysis in low-temperature matrices, oligomer species such as Fe2(CO)8 are formed, which show C-O stretching bands at higher frequencies than Fe(CO)5.28-32,35 Therefore, there is some possibility that the photoproduct giving the 2081 cm-1 band is the dimer or trimer of Fe(CO)4, similar to the product on the Ag surface. To obtain further information about the photoproduct, the Au substrate was heated in a stepwise manner after the photolysis, and IRAS was observed. As seen in Figure 10, new bands at 2104 and 2059 cm-1 emerge at a similar rate as the 2081 cm-1 band decreases in intensity and then decreases at temperatures above 240 K. At temperatures around 300 K, a single band appears at 2055 cm-1, which is very similar in shape, position, and formation temperature to the 2058 cm-1 band observed on the Ag surface.13 No bands were observed at temperatures above 340 K. During the thermal decomposition process, a small band appears at 2071 cm-1 around 240 K. This band is not observed when the initial Fe(CO)5 coverage is decreased to less than 0.5 ML, but other bands are observed at the same positions. The species giving the 2071 cm-1 band is, therefore, an additional product which appears as the photoproduct coverage is increased. The 2104 and 2059 cm-1 bands may be originated from the same species, probably oligomer species, judging from the positions of the bands. If the species exhibiting the single band at 2055 cm-1 is Fe(CO)4, then the photoproduct exhibiting the 2081 cm-1 band is likely to be an oligomer species. To assign the product to be oligomer species, we must check first whether or not the surface oligomer species of Fe(CO)4 is able to exhibit a single C-O strethcing band in IRAS. The stable oligomers of iron carbonyl, such as Fe2(CO)9 and Fe3(CO)12, are known to contain bridging carbonyl.36 It is, however, unlikely that the bridging carbonylcontaining species exhibits a single C-O stretching band in IRAS, because the formation of the bridging carbonyl inevitably reduces the symmetry of linear carbonyl ligands. The dimer species, Fe2(CO)8, that has no bridging carbonyl has been observed in Fe(CO)5 photolysis of low-temperature matrices,35 and its structure is shown in Figure 8e. The highest frequency in the C-O stretching bands of unbridged Fe2(CO)8 is 2038 cm-1.35 Since the surface photoproduct is taken to have Fesurface bonding, the structure shown in Figure 8f is plausible

Sato and Suzuki for a surface dimer species which exhibits a single C-O stretching band in IRAS. Although the coordination number of Fe in this surface dimer becomes greater than that of Fe(CO)5, Fe atoms in Fe2(CO)9 or Fe3(CO)12 have coordination numbers greater than 5.36 Thus, the surface dimer of Fe(CO)4 can be a strong candidate for the photoproduct, but its C-O stretching frequency in IRAS was not assumed because of unknown effects of the surface bonding on band shifting. Although IRAS is able to give valuable information about orientation of surface species,33 it provides less information for the assignment of chemical species than ordinary IR transmission spectroscopy, since vibrational modes parallel to the surface are not observed. Thus, IR transmission spectra were supplemented for the photoproduct formed on an IR-transparent Au film coated on a sapphire plate, which has a surface-enhanced infrared absorption (SEIRA) effect.36,37 The details of this experiments will be reported elsewhere.19 The spectra were about 10 times enhanced as compared to an ordinary transmission spectrum, and surface species of submonolayer coverages could be distinctly detected. The SEIRA spectrum of the photoproduct formed at 115 K from 1 ML of Fe(CO)5 shows a broad intense C-O stretching band centered at 2025 cm-1 and weak bands at 1867 and 1827 cm-1.19 The latter two bands are assignable to bridging CO and, therefore, indicate the presence of oligomer species. The oligomer Fe2(CO)9 adsorbed on silica at room temperature exhibits two intense linear C-O stretching bands at 2054 and 2014 cm-1, while Fe3(CO)12 2056 and 2034 cm-1.1,2,4 The photoproduct on the Au/sapphire substrate may be a mixture of Fe2(CO)9 and Fe3(CO)12 since two kinds of bridging carbonyls are involved in the product. The theoretical transformation of the SEIRA spectrum of the photoproduct to IRAS, however, cannot reproduce the observed IRAS on the Au(111) surface. The evaporated Au film surface is probably contaminated during evaporation or transportation in the air so that the photolysis product is different from that formed on the clean Au(111) surface. In this connection, the surface photoproduct of Fe(CO)5 is very sensitive to a surface contamination as well as to the nature of substrate surface itself. For example, before the sputter cleaning of the Au(111) surface the IRAS of the photoproduct is different from that observed on the clean surface even though the average CO/Fe ratio in the product is the same. We assume that intermediate Fe(CO)4 is first produced from molecularly adsorbed Fe(CO)5 irrespective of substrate, and the thermal oligomerization of Fe(CO)4 proceeds under a catalytic influence of the surface yielding different photoproducts. In summary, Fe(CO)5 adsorbed on the Au(111) surface at 90 K undergoes photodecarbonylation to produce a surface species in which the average CO/Fe ratio is close to 4 for initial Fe(CO)5 coverages lower than 1 ML. The cross section for photodecarbonylation is so high as to match the absorption cross section of gas phase Fe(CO)5 around the absorption threshold. The high cross section is limited to the surface layer within 3 ML, probably because adsorption-induced structural deformation or charge transfer enhances the photodecomposition. XPS measurements demonstrate the formation of the bonding between the surface and the Fe atom of carbonyl moiety upon irradiation. This bond formation will enhance the quenching of photoexcited states which inhibits further photodecarbonylation of intermediate Fe(CO)4. The photoproduct exhibits a sharp, single C-O strethcing band in IRAS, unlike the products formed on the Ag13 and SiO2/Al14 substrates, which show broad C-O stretching bands. The photoproduct was tentatively assigned to unbridged Fe2(CO)8.

Photochemistry of Fe(CO)5 on Au(111) Acknowledgment. We thank Prof. M. Osawa and Dr. K. Ataka for preparation of the IR-transparent evaporated Au films. This work was partly supported by the Grant-in-Aid on PriorityArea-Research on “Photoreaction Dynamics” from the Ministry of Education, Science, Sports, and Culture of Japan (No. 06239110) and by the Joint Studies Program (1995) of the Institute for Molecular Science. References and Notes (1) Trusheim, M. R.; Jackson, R. L. J. Phys.Chem. 1983, 87, 1910. Jackson, R. L.; Trusheim, M. R. J. Am. Chem. Soc. 1982, 104, 6590. (2) Darsillo, M. S.; Gafney, H. D.; Paquette, M. S. J. Am. Chem. Soc. 1987, 109, 3275. (3) Sato, S.; Ohmori, T. J. Chem. Soc., Chem. Commun. 1990, 1032. (4) Sato, S.; Ohmori, T. J. Phys. Chem. 1991, 95, 7778. (5) Ukisu, Y.; Sato, S.; Ohmori, T. Appl. Organomet. Chem. 1991, 5, 243. (6) Ohmori, T.; Sato, S. J. Photochem. Photobiol. A: Chem. 1992, 64, 201. (7) Celii, F. G.; Whitmore, P. M.; Janda, K. C. Chem. Phys. Lett. 1987, 138, 257; J. Phys. Chem. 1988, 92, 1604. (8) Henderson, M. A.; Ramsier, R. D.; Yates, J. T., Jr. J. Vac. Sci. Technol. A 1991, 9, 1563; Surf. Sci. 1992, 275, 297. (9) Bottka, N.; Walsh, P. J.; Dalbey, R. Z. J. Appl. Phys. 1983, 54, 1104. (10) Gluck, N. S.; Ying, Z.; Bartosch, C. E.; Ho, W. J. Chem. Phys. 1987, 86, 4957. (11) Swanson, J. R.; Friend, C. M.; Chabal, Y. J. J. Chem. Phys. 1987, 87, 5028. (12) Swanson, J. R.; Friend, C. M. J. Vac. Sci. Technol. A 1988, 6, 770. (13) Sato, S.; Ukisu, Y.; Ogawa, Y.; Takasu, Y. Appl. Surf. Sci. 1994, 79/80, 428. (14) Sato, S.; Minoura, S.; Urisu, T.; Takasu, Y. Appl. Surf. Sci. 1995, 90, 29. (15) Sato, S.; Ukisu, Y. Surf. Sci. 1993, 283, 137. (16) Sato, S. In New Functionality Materials; Tsuruta, T., Doyama, T., Seno, M., Eds.; Elsevier: Amsterdam, 1993; Vol. C, p 201.

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