J . Phys. Chem. 1994, 98, 1276-1281
1276
Infrared Spectroscopic Characterization of Tungsten Carbonyl Species Formed by Ultraviolet Photoreduction of Silica-Supported W (VI) in Carbon Monoxide Steven D. Kohlert and John G. Ekerdt' Department of Chemical Engineering, University of Texas a t Austin, Austin, Texas 78712 Received: September 30, 1993"
The tungsten carbonyl species that form during ultraviolet photoreduction of W6+/Si02 in CO were analyzed by Fourier transform infrared spectroscopy (FTIR). Two carbonyl species were identified, mer-W(CO)3 and cis-W(CO)2, by the number and intensities of the respective I R peaks and through isotopic substitution. The C2" symmetry of mer-W(C0)3 was associated with three I R bands: a very weak AI)^ symmetric trans mode a t 2145 cm-I, a strong B2 symmetric mode a t 21 12 cm-', and a strong AI)^ antisymmetric mode a t 2179 cm-l. mer-W(CO)3 was stable a t 298 K i n 25 Torr of CO. Isothermal evacuation of the mer-W(C0)3 species resulted in the loss of a single C O ligand, forming cis-W(CO)2. The W ( C 0 ) 2 species formed during both isothermal evacuation of the mer-W(C0)3 species and during the initial photoreduction process. c i s - W ( C 0 ) ~was identified by its I R spectrum consisting of a strong symmetric mode a t 21 12 cm-l and a strong antisymmetric mode a t 2040 cm-'.
Introduction Supported tungsten oxide catalysts have wide use in industry to catalyze such reactions as metathesis, hydrodesulfurization, and hydrodenitrogenation.l,2Despite the wide use of supported tungsten oxide catalysts, very little is known about the coordination and oxidation state of supported tungsten oxide catalysts. This paper analyzes the reduction of W6+/Si02 by ultraviolet (UV) light and carbon monoxide because photoreduction can provide a convenient method of producing coordinatively unsaturated tungsten sites in a reduced oxidation state that may subsequently be used in catalytic reaction studies. Carbon monoxide is a commonly used probe molecule to examine reduced cation sites. The number and intensity of CO vibrational modes are indicative of the number C O ligands a t the site and of the coordination of the reduced metal site. A recent review by Kung and Kung has examined the use of CO as a probe molecule on a variety of supported, reduced metal sites.3 Much information exists for C O adsorption on supported chromium and molybdenum, but there is very little concerning supported tungsten. This lack of information for CO adsorption on supported tungsten has been attributed to the difficulty in reducing tungsten as compared to chromium and m o l y b d e n ~ m . ~ . ~ Infrared spectroscopy has been used to investigate CO adsorption by W/A1203 that was thermally reduced in H2$ three different surface tungsten sites, each adsorbing a single carbonyl ligand, were identified by this study: Ws+,W4+(I),and W4+(II). The oxidation state of the reduced tungsten sites was assigned by analogy to thermally reduced MO/A&. IR bands at 2198, 2176, and 2154 cm-I were assigned to the W5+, W4+(I), and W4+(II)sites, respectively. These assignments were based upon similar spectra for CO adsorption on thermally reduced Cr/ Si02.' The three overlapping bands at 2191, 2186, and 2181 cm-' were assigned to Cr3+and two types of Cr2+sites, respectively. However, recent literature has assigned the 21 86- and the 2 181cm-I bands to the same Cr2+ species.8 The most common procedure to produce supported tungsten in a reduced oxidation state involves deposition of W(CO)6 on a high-area oxide. Only physically adsorbed W(CO)6 was formed t Current address: Sandia National Laboratories, Division 6212, P.O.Box 5800, Albuquerque, NM 87185.
* Address correspondence to John G. Ekerdt, Department of Chemical Engineering, University of Texas at Austin, Austin, TX 7 8 7 1 2 .
[email protected]. @
Abstract published in Aduance ACS Abstracts. January
1, 1994.
by the deposition of W(CO)6 onto fully hydroxylated Si02 and A1203.9*10IR bands at 1980 (s), 2015 (sh), and 2120 cm-I were observed for the physically adsorbed species? During the removal of the CO ligands, the support partially oxidized the Wo to W4+.lo The C O ligands could not be repopulated. Additional adsorption studies have been conducted with W(CO)6 on alumina dehydroxylated at 623 K" and 1073 K.I2 Monomeric W(CO)3 and dimeric W2(CO)611as well as W(CO)6 coordinated to different alumina sites12 were reported. With the lack of information about oxidation state and coordination of reduced tungsten supported on a high-area oxide, photoreduced Mo/SiOz could be a useful reference because of the isostructural nature of Mo and W oxides. Carbon monoxide adsorbed on photoreduced molybdenum catalysts has been characterized in our laboratories using FTIR spectroscopy, temperature-programmed desorption (TPD), and 0 2 titration.13 Four primary molybdenum carbonyl species formed under photoreduction. These include a mer-Mo4+(CO)3, a cis-Mo4+(C0)2, a linear Mo4+(CO), and MoO(CO)~. Experimental Methods The W6+/Si02samples were prepared from (C5H&W*(C0)4 and Cab-0-Si1 EH-5 (380 m2/g). Raman spectra show that in the dehydrated and fully oxidized state W6+/Si02samples made from (CsHs)2Wz(C0)4 formed isolated tungstates on the silica surface.14 The Raman band at 984 cm-' that has been associated with isolated tungstates was observed, and there were no bands associated with crystalline W 0 3 . Silica was dried a t 673 K under vacuum or an Ar purge for 20 min to remove both adsorbed water and excessvicinal hydroxyl groups. A benzene solution of ( C ~ H ~ ) Z W ~ ( was C O transferred )~ to the dehydroxylated silica at 298 K under Ar.I5 The samples were evacuated at 373 K until dry of the benzene solvent. The samples were then heated at 5 K/min to 623 K in He. After cooling to room temperature, the samples were calcined in hydrocarbon-free air at 823 K for 1 h, then cooled, exposed to ambient air, and crushed with a mortar and pestle to facilitate wafer pressing. Samples were kept in sealed glass sample bottles. All results reported herein were measured using a 4.5 wt % W (metal basis) sample. Transmission infrared spectra were obtained with a Mattson Research Series 1 FTIR spectrometer equipped with an air-cooled source and a mercury-cadmium-teluride detector. The spectra were obtained at a resolution of 4 cm-I after 100-200 scans. The
0022-3654/94/2098-1276%04.50/0 0 1994 American Chemical Society
The Journal of Physical Chemistry, Vol. 98, No. 4, 1994 1277
IR of Tungsten Carbonyl Species
0.30 20401'
0.25-
A
0.15-
A
b
b
2040
I l i l
0
r b a n
S
0.20-
0
r
0.10-
C
e
b a n
0.
C
0.
e
I 0.00-
V
\
0.
9 2150
2100
2050
2000
Wavenumbers Figure 1. FTIR spectrum following photoreduction of 4.5% W/Cab0-Si1 for 5 min through a IO-mmwater filter in 770 Torr of CO. The sample spectrum was obtained at 289 K in 770 Torr CO. IR cell consisted of a stainless steel base coupled to a quartz furnace.16 The quartz furnace was capable of thermal pretreatments up to 1273 K. A magnetically coupled quartz manipulator was within the quartz furnace. The manipulator allowed the sample, a self-supporting wafer, to be lifted into the quartz furnace, or lowered into the IR beam passing through the stainless steel base and KBr optics. The wafers were pressed from 40-160 mg of fresh W6+/Si02 under 6000 1 b / h 2 for 30 s. The wafer was calcined in hydrocarbon-free air for 1 h at 873 K and evacuated (using a mechanical pump) for 30 min at 773 K. After evacuation and cooling, approximately 1 atm of C O was added to the cell for photoreduction. Room temperature photoillumination was performed with a Photon Technology International (PTI) high-intensity arc lamp (02-A1010Q) and power supply (02-LPS200) with a 100-W mercury lamp. The minimum UV wavelength was limited by KBr optics to 220 nm. The UV beam was passed through 10 cm of water to minimize I R heating of the sample. The lamp was placed 10 in. from the wafer. The illumination time was varied from 0.1 to 1000 min. After photoreduction, the IR spectra of the adsorbed metal carbonyl were obtained at room temperature with subtraction of both the Si02 background and the CO gas phase. Carbon monoxide (I2Cl6O, Liquid Carbonic research grade 99.99%) was first passed through a bed of molecular sieves at 473 K to decompose iron carbonyls. The CO was further purified by passing through water and oxygen traps (SGE). I3Cl6O(99.3 , Inc.) was used as received. atom % 13Cand 12.3 atom % I80Isotec Helium (99.99%+ UT Physics Dept.) was passed through both oxygen and water traps (SGE). Hydrocarbon-free air (Liquid Carbonic) was passed through a water trap (SGE). Hydrogen (99.999% Liquid Carbonic) and argon (99.999% Liquid Carbonic) were purified by passing through oxygen and water traps (SGE).
Results The IR spectrum resulting from the photoreduction of W6+/ Si02 consisted of three major peaks a t 21 12,2079, and 2040 cm-I and a very weak peak at 2145 cm-I (Figure 1). It will be shown that the peak at 21 12 cm-I is the superposition of peaks from two
0.
2150
2100
2050
2000
Wavenumbers Figure 2. FTIR spectrum following photoreduction of 4.5% W/Cab0-Si1 for 5 min through a 10-mm water filter in 770 Torr of CO. The sample spectrum was obtained at 289 K after isothermal evacuation. W carbonyl species. Isothermal evacuation at room temperature removed the peakat 2079 cm-I, and the 2040-cm-l peakincreased in intensity. Only two peaks are seen in the IR spectrum (Figure 2) of photoreduced W/SiO2 after isothermal evacuation. The ratio of the 2212- and 2040-cm-I peaks in Figure 2 is obviously different from the ratio in Figure 1. Changes in the integrated intensity areas of the 21 12-, 2079-, and 2040-cm-l peaks reveal the presence of two carbonyl species. The 2079-cm-l peak was completely attenuated after removal of all C O from the gas phase. After removal of the species responsible for the 2079-cm-l peak, a single species remained on the surface that gaveriseto twopeaksat2112and2040cm-',whoseintegrated intensity ratio remained constant during further evacuation (Figure 3). The average value of the integrated intensity ratio of the 21 12-cm-' peak to the 2040-cm-' peak was determined to be 0.60, during which time the area of 2040 cm-l underwent a 16-fold decrease. Theintegratedintensityofthepeakat21 12cm-'didnot change much during isothermal evacuation when it was present along with the 2079-cm-I peak. This peak either is separate from both the 2079- and the 2040-cm-' peaks or is the superposition of the IR peaks of two species, called species A and species B. Species A was less resistant to evacuation than species B and is responsible for the band at 2079 cm-I. Species B was more resistant to evacuation and produced the band at 2040 cm-I. The respective contributions of the two species to the peak at 21 12 cm-I can be determined by isothermal evacuation. If the contribution to the peakat 21 12 cm-I by species B (0.6 times theintegrated intensity of the 2040-cm-' peak) is subtracted from the total integrated area of the 21 12-cm-' peak, the remaining portion should be the contribution of species A. The ratio of the integrated intensity of the peak at 2079 cm-' to the contribution that species A makes to the 2112-cm-1 peak is plotted in Figure 4 with respect to the C O pressure. This ratio is constant over more than one order of magnitude change in C O pressure and a 7-fold decrease in the 2079-cm-' peak area. Therefore, species A gives rise to an IR band at 2079 cm-I and contributes to the peak at 21 12 cm-l. In all cases, the integrated intensity ratio of the 2079-cm-1 band and the 21 12-cm-' band, when corrected for overlap by
Kohler and Ekerdt
1278 The Journal of Physical Chemistry, Vol. 98, No. 4, 1994
AREA RATIO
AREA RATIO
0.07
0.8
0.06
-
0.7 -
I I
0.05I
AVG = 0.045
AVG, = O h 0 I
I
0.04
I
0.6.
0.03-
I
0.50.022112/2040
2145/2079
0.4
0.0 1
10
5
0
15
25
20
0
20
EVACUATION TIME (MIN)
60
40
80
100
120
140
160
UV EXPOSURE (MIN)
Figure3. IR integratedintensityratioof2112-and2040-~m-~absorbances after removal of species A during isothermal evacuation at 298 K. The 4.5% W/Cab-0-Si1 sample was photoreduced for 50 min through a 10mm water filter under a total CO pressure of 780 Torr. Spectra were obtained at 298 K.
Figure 5. IR integrated intensity ratio of 2145- and 2079-cm-I bands after varying times of photoreduction. The 4.5% W/Cab-0-Si1 sample was photoreduced through a 10-mmwater filter under a total CO pressure of 775 Torr. Spectra were obtained at 298 K in 775 Torr of CO.
4
AREA RATIO 0.8
2036 2095
a . 0.75
I
-
I
.I
I
I I
AVG. * 0.748
I
2065
0.58
3.005
I,
1
-
0.38 0.27
0.08
I
-L
!
I 0.7
0.13
-
0.34 0 . 2 1
A 0.42
0.23
0.07
I
7
1.35 o.03
0.65 -
2150
0.6
200
400
'Ic A 2050
2000
Wavenumbers
I 2079/(2112-0.6'2040)
0
2100
1
600
800
CO PRESSURE Figure 4. IR integrated intensity ratio of 2079-cm-I absorbance and the speciesAcontribution to the 2112-cm-I band during isothermal evacuation at 298 K. The 4.5 wt % W/Cab-0-Si1 sample was photoreduced for 50 min through a IO-" water filter under a total CO pressure of 780 Torr. Spectra were obtained at 298 K. species B, was constant above 50 Torr of CO. A very weak band a t 2145 cm-I was also associated with these two peaks. The attenuation of the 2145-cm-1 band could not be monitored with decreasing C O pressure due to the weak intensity of the band. The ratio of the integrated intensity of the 2145-cm-I band to that of the 2079-cm-I band is plotted versus increasing UV
Figure 6. Top: FTIR spectrum immediately following photoreduction of 4.5% W/Cab-0-Si1 for 12 min through a IO-" water filter in 740 Torr of 73% I2CI6Oand 27% I3CI6O. The spectrum was obtained at 298 K. Bottom: Predicted IR frequencies and intensities of mer-W(12C160)rx(13C160)~. The black dots represent substitution by I3CL6O. exposure in Figure 5 . The 2145-cm-1 band showed a reasonable correlation with the 2079-cm-l band of species A. Furthermore, the 2145-cm-I band attenuated with evacuation. Figure 6 is the spectrum resulting from photoreduction of W6+/ Si02 in a mixture of 73% l2Cl6O-27% 13C'60. The spectrum exhibits bands at 2141,2112,2082,2065,2036, and 2011 cm-l and a shoulder at 2095 cm-I. The band at 201 1 cm-I is due to I3CIsO(12.8% of 13C0from cylinder consisted of 13ClsO). The frequency of the peaks shifted slightly in the spectra obtained for W6+/Si02 photoreduced in different 12C160-13C160 mixtures.
IR of Tungsten Carbonyl Species
The Journal of Physical Chemistry, Vol. 98, No. 4, 1994 1279 W ( C O ) ~Stretching
Parameters
0
y
1.0
0 74
w L
t
I o,13
0 . 3 4 0.21
el.
A 0.42
0.23
0.07
1 J - o
I 1.35
MQdea
I
LL
.___.__________________________________ Asymmetric
2150
2100
I
I
2000
2050
Wavenumbers
t
Figure 7. Top: FTIR spectrum immediately following photoreduction of 4.5% W/Cab-0-Si1 for 13 min through a IO-" water filter in 740 Torr of 21% I2CL6O and 79% l3CI6O. The spectrum was obtained at 298 K. Bottom: Predicted IR frequencies and intensities of mer-W(12CL60)3-x(13CL60)~. The black dots represent substitution by 13C160.
(A') 3
I
t
Figure 9. Vibrational parameters and modes of mer-W(C0)3. The black dots represent substitution by 13C160.
0.034
w 2( ' J c
' 60)z
A' AI
evacuation to remove species A. The spectrum consists of six and 1995 major peaks located at 2112,2095,2064,2041,2010, cm-l. Silica-supported W6+ photoreduced more slowly than silicasupported Mob+ and Cr6+, with Cr6+ being the easiest to photoreduce.I7 During Cr6+ photoreduction, 1.01 mol of COz was produced per mole of Cr metal in 30 min. Mo6+ and W6+ were photoreduced for longer times than was necessary for Cr6+. Assuming a constant photoreduction rate, 0.10 mol of COZwas produced per mole of Mo metal and 0.02mol of COz was produced per mole of W in 30 min. Cr6+ reached an average oxidation state of 4+ in 30 min, whereas W6+ required 960 min to reach an average oxidation state of 5.4+.**
Discussion Two different species formed during photoreduction of silicasupported W6+. I R bands for species A attenuated first during isothermal evacuation. The unsubstituted IR bands for A were .. found at 2145, 2112, and 2079 cm-l, with an average relative 2150 2100 2050 2000 intensity of 0.05:1:0.75.The unsubstituted bands for B were found at 21 12and 2040 cm-I, with an average intensity of 0.60: 1. Wavenumbers The assignment of carbonyl structures is based on the number Figure 8. FTIR spectrum following photoreduction of 4.5% W/Caband relativeintensities of these bands in the unsubstituted spectra water filter in 740 Torr of 21% l2CL60 0-Si1 for 13 min through a IO-" and on the correspondence between observed and predicted spectra and 79% 13CL60.The spectrum was obtained at 298 K under evacuation. following isotopic substitution. The frequencies and intensities were predicted using the GF matrix approachl9-2I and assumed This shift is due to changing amounts of 13C'60substitution in nonmechanical coupling between the C O groups.z2 A similar the different W carbonyl species and varying degrees of species procedure was followed in a recent study of Mo6+/Si02 phoAandB. Thespe~trumofW~+/Si02photoreducedin21%~~C~~O toreduction.I3 and 79% 13C160is presented in Figure 7. The main peaks of this The I R bands for species A (2145,21 12,and 2079 cm-I) are and 2010 spectrum are evident a t 2140,2112,2078,2064,2033, assigned to a tungsten cation, having Cz, symmetry, with three cm-l. CO ligands distributed meridionally, mer-W(CO)g. The merAfter isothermal evacuation at 298 K, new sets of bands for w(co)3structure is shown in Figure 9. The peak at 2145 cm-', species B were observed. Figure 8 is the IR spectrum of W6+/ assigned to the symmetric (AI)] mode of mer-W(CO)s, is very weak because the individual dipole moments nearly cancel each Si02 photoreduced in 21% 12CW-79% I3Cl6Oafter isothermal ~
~
Kohler and Ekerdt
1280 The Journal of Physical Chemistry, Vol. 98, No. 4, 1994
TABLE 1: Species A Bands and Predicted Infrared Frequencies (in cm-I) for cis-W(CO)z, cis-W(CO):, cis-W(CO)z(CO)‘, and mer-W(C0)3 Substituted with 1zC160 and I3Cl6O predicted observed
cis-
cis-
cis-
mer-
W(C0)2
w(co)3
W(CO)2(CO)’
w(co)3
-
-
2112(s) 2079 (s)
2112(s) 2079 (s)
2145 (s) 2112(s) 2079 (s)
2145 (vw) 2112(s) 2079 (s)
12cl60
2145 (vw) 2112(s) 2079 (s) 12Cl 6 0 / 13CI 6 0
2145 2144
2144 2143 2141 2137 2114 2112
2134 2133 21 12
21 12
21 12 2111
2112
2106 2103 2097 2079 2076 2069 2066 2064
2079
2078
2079 2069 2065
2084 2079 2076 2070 2065
TABLE 2: Observed and Predicted Infrared Frequencies and Predicted Intensities for mer-W(12C160)3~x(~JC~60)x predicted intensity case and frequency (cm-’) (relative to symmetry mode obsd predicted 2112 cm-l) 2145 (AI), 2145 0.03 nonsubstituted 2112 1.o Bz 2112 (C2J (A1)2 2079 2079 0.74 w(12c160)3 2144 (AI)] a 0.005 symmetrically 2112 2112 0.583 monosubstituted B2 0.425 (CZ”) (A1)2 2035’ 2034 asymmetrically A’ 2135’ 2134 0.08 monosubstituted A‘ 2083 2084 0.38 0.27 A’ 2068 2070 (CA 2104 2104 AI)^ 0.13 symmetrically 2073 B2 2075 0.34 disubstituted 0.21 2065’ 2065 AI)^ (C2J A‘ 2136’ 2133 asymmetrically 0.07 disubstituted A‘ 2079 2076 0.23 A‘ 2033’ 2033 0.42 (CJ 2097 AI)^ 2091 0.03 completely B2 2065 2065 1.35 substituted 2033 1.o (A1)2 2032 (C2J Band not observed. Band could not be isolated.
’
model nor the cis-W(CO)3model predicts any bands above 21 12 cm-I. The cis-W(CO)2model predicts intense peaks at 2103 and 2043 cm-1 for the monosubstituted dicarbonyl, and these peaks 2049 are missing in the spectra. Furthermore, no peaks are visible at 2043 2106 and 2049 cm-I as predicted by the cis-W(CO)3model. These 2039 2038 observations suggest species A is not cis-W(CO)2or cis-W(CO)3 2036 2034 2036 2033 2033 2032 2033 2032 and support assigning the bands at 2145, 21 12, and 2079 cm-I to the same species. 13CI60 The cis-W(CO)z(CO)’and mer-W(C0)3models fit a three2091 (vw) 2100 2091 (vw) band unsubstituted carbonyl pattern. Both models fit the partial 2067 (s) 2065 2065 (s) 2065 (s) 2067 (s) 2033 2033 (s) 2032 (s) 2032 (s) 2032 (s) substitution data better than the models that use only two unsubstituted bands. However, the cis-W(CO)1(CO)’model can be eliminated because the unsubstituted spectrum should consist other out. The resultant dipole moment of the AI)^ points along of three bands of similar intensity, which is contrary to what we the C2 symmetry axis. Peaks at 21 12 and 2079 cm-I are assigned observed. totheBZand(Al)2 modes, respectively. Thelackofdipolemoment cancellation results in these two modes being much more intense The predicted frequencies and intensities of the mer-W(C0)j modes are shown in Table 2 for various 1 2 C W - 1 3 C 1 6combi0 than the (Al)1 mode. Three other W carbonyl models were considered: cis-W(C0)j nations. (See ref 16 for the procedure used.) The mer-W(CO)3 with C3” symmetry, cis-W(CO)2 with C2, symmetry, and cisstructure predicts the IR spectrum obtained from photoreduction W(CO)2(CO)’with C, symmetry. A summary of all predictions of W 6 +/Si02 in an isotopic CO mixture. Six of the frequencies predicted by the mer-W(C0)3model are coincident or very close is contained in Table 1 . A complete discussion of the various cases can be found e1sewhere.l’ In theunsubstituted cis-W(CO)3 to another predicted frequency of a substituted mode of the mermodel, all of the carbonyl ligands are equivalent and the spectra W ( C O ) 3species. The largest difference discrepancy between the observed and predicted frequencies of the mer-W(CO)3model consists of two modes: a symmetric stretch, (AI),and a degenerate antisymmetric stretch, E. The unsubstituted c i s - W ( C 0 )spec~ is those frequencies of the AI)^ mode of the symmetric species. trum of the two equivalent carbonyl ligands consists of two This discrepancy is due to the low IR intensities of the AI)^ modes: symmetric A1 and antisymmetric B1. In the cis-W(CO)2modes. (CO)’ model, one of the CO ligands is different than the other Comparison between the predicted and observed frequencies two carbonyl ligands. The physical interpretation of this model can be made in Figures 6 and 7. Although the band at 2145 cm-1 suggests that one of the CO groups could be trans to a unique is very weak in the unsubstituted spectrum, the band at 2141 ligand or to a coordinately unsaturated position. The unique cm-I in Figure 6 and the peak at 2140 cm-I in Figure 7 are ligand might possibly be a double-bonded oxo anion or an oxygen pronounced. The mer-W(CO)3predicts the absorbanceintensity bridging a tungsten and silicon cation. Mathematically, this model should increase for bands above 21 12 cm-I upon substitution. would be equivalent to the mermodel if the interaction parameters The peak at 2082 cm-’ in Figure 6 decreased in frequency to 2078 of the mer model, k, and kt, were equal; however, in the mer cm-I in Figure 7. This decrease in frequency is explained by an model we find, k,/kt = 0.71. increase in the amount of W ( ’ 2 C ’ 6 0 ) ( 1 3 C 1 6and 0 ) 2W ( 1 3 C 1 6 0 ) z The cis-W(C0)z and cis-W(CO)3 models fit a two-band species and a corresponding decrease in the amount of W(1zC160)2unsubstituted carbonyl pattern. Since the 2145-cm-l peak in (I3Cl6O)species. This explanation also describes the decrease Figure 1 is weak and the peak area ratio in Figure 5 shows some in frequency from 2036 to 2033 cm-I. scatter about the average value of 0.045, it seems prudent to The idealized modes and the four vibrational parameters of determine if the two remaining bands, 21 12 and 2079 cm-1, are the mer-W(C0)3 species are presented in Figure 9. The four the only bands associated with species A. Figures 6 and 7 parameters are the cis stretching parameters (kl),the trans demonstrate an increase in absorbances above 21 35 cm-I following stretching parameter (k2),the cis-trans interaction parameter partial substitution. Table 1 reveals that neither the ~ i s - W ( C 0 ) ~ (k,), and the trans-trans interaction parameter (k,). Mathe2067
2067
IR of Tungsten Carbonyl Species matically, the values of three of the parameters, k2, k,, and k,, are dependent upon the value of the fourth parameter, kl. This parameter was varied to best fit the observed spectra reuslting from photoreduction of W6+/Si02in a isotopic mixture of CO. The parameters that best predicted the frequencies of I3Cl6Osubstituted mer-W(C0)3 were kl = 1752.3 N/m, k2 = 1827.3 N/m, k, = ls.ON/m,and kt = 25.5N/m. Therelativeintegrated intensities of the 2145-, 2112-, and 2179-cm-I peaks of merW(CO)3 have beencalculated to be0.03:1.0:0.74. Thevibrational parameters and the integrated intensities were used to calculate the angle between the dipole moments and the relative IR sensitivity of the C O ligands.I9 The estimated interdipole angle was determined to be 90.5' f 2.0°.17 The IR bands for species B (21 12 and 2040 cm-l) are assigned to the cis-W(CO)2 structure. Species B was present at all UV exposures. Isothermal evacuation a t room temperature transformed mer-W(CO)3 into cis-W(CO)2. The stretching and interaction parameters, k = 1741.9 N / m and i = 59.9 N/m, were calculated using the frequencies of the l2CI6O bands and the dicarbonyl model. These parameters were used to predict IR frequencies for W(~2CI60)(~3Cl60) at 2095 and 201 1 cm-l and for W(13C160)2 a t 2065 and 1995 cm-I. As can be seen in the spectrum (Figure 8) obtained by photoreduction in 740 Torr of 21% I2Cl60and 79% 13C160,the frequencies predicted by the cis-W(CO)2 model correlate well with the actual peaks. Ten sets of 21 12- and 2040-cm-1 peaks were used to determine that the integrated area ratio of the antisymmetric (2040 cm-l) mode to the symmetric (21 12 cm-I) mode was 1.67 f 0.01. The interdipole angle calculated using this ratio is 104.5' f 0.7'. This interdipole angle suggests that the W sites are oriented in a tetrahedral coordination. Although FTIR cannot provide direct information about the ligands occupying the other sites of the tetrahedral coordination sphere, two oxygen anions probably occupy the other two sites. In agreement with thermal reduction results,@ supported tungsten oxide was much more difficult to reduce via photoreduction than either chromium oxide or molybdenum oxide. The lowest averageoxidation state of a photoreduced W/SiO2 catalyst determined by mass balance was 4.8+. Photoreduction in C O is a two-electron process. IR spectra did not indicate the formation of intermediate carbonyl structures at short photoreduction times or additional structures at very long photoreduction times; only mer-W(C0)3 and cis-W(CO)2 were observed. By analogy to photoreduced Mo/Si02,13 we suggest W is photoreduced to the 4+ oxidation state. Since the reduction is slow, most of the W is present in the 6+ oxidation state for photoreduction times less than 6 h." Conclusions (1) Ultraviolet photoreduction of W/SiO2 produced two tungsten carbonyl species that have not been prepared through
The Journal of Physical Chemistry, Vol. 98, No. 4, 1994 1281 thermal reduction of W6+/Si02 or by attachment of tungsten carbonyl compounds, such as W(CO)6, directly to the silica support. (2) The two carbonyl species, A and B, were discriminated by their response to isothermal evacuation. (3) Species A was assigned to a mer-W(CO)s structure through the intensity and frequency of its three IR bands at 2145,2 112, and 2079 cm-l and through the changes to these IR bands following partial substitution with I3Cl6O. The interdipole angle for mer-W(CO)3 is 90.5' f 2'. The W(CO)3 stretching parameters are kl = 1752.3 N/m, k2 = 1827.3 N/m, kt = 25.5 N/m, and k, = 18.0 N/m. (4) Species B with bands at 21 12 and 2040 cm-l is formed during the initial photoreduction of W6+/Si02 in CO and by loss of a CO ligand from mer-W(CO), during isothermal evacuation. (5) Species B was assigned to a cis-W(CO)2 structure. Acknowledgment. This work was supported by the U S . Department of Energy, Office of Basic Energy Sciences. Ryan Roark prepared the W6+ samples. References and Notes (1) Katzer, J. R.; Sivasubramanian, R. Catal. Rev. 1979, 20, 155. (2) Andreini, A.; Mol, J. C. J . Chem. SOC.,Faraday Trans. I 1985,81, 1705. (3) Kung, M. C.; Kung, H. H. Catal. Rev. 1985, 27, 425. (4) Thomas,R.;VanOers,E.M.;deBeer,V.H. J.;Medema, J.;Moulijn, J. A. J . Catal. 1982, 76, 241. (5) Kazusaka, A,; Howe, R. F. J . Catal. 1980, 63, 447. (6) Yan, Y.; Xin, Q.;Jiang, S.; Guo, X . J . Catal. 1991, 131, 234. (7) Zecchina, A,; Garrone, E.; Coluccia, S. J . Phys. Chem. 1975, 79, 972. (8) Ghiotti, G.; Garrone, E.; Zecchina, A. J . Mol. Carol. 1988, 46, 61. (9) Howe, R. F. Inorg. Chem. 1976, 15, 486. (IO) Brenner, A,; Hucul, D. A.; Hardwick, S.J. Inorg. Chem. 1979,18, 1478. (11) Kazusaka, A,; Howe, R. F. J . Mol. Catal. 1980, 9, 199. (12) Zecchina, A.; Platero, E. E.; Arean, C. 0 .Inorg. Chem. 1988, 27, 102. (13) Williams, C. C.; Ekerdt, J. G. J . Phys. Chem. 1993, 97, 6843. (14) Roark, R. D.; Wachs, I. E.; Ekerdt, J. G., unpublished results. (15) Roark, R. D.; Narayanan, C. R.; Sullivan, D. L.; Ekerdt, J. G. Formation of Molybdenum (IV) and Tungsten (IV) Cation Pairs on Silica from CpzMM(C), (M=Mo, W;n=4, 6), submitted to Chemistry Materials. (16) Williams, C. C. Ph.D. Dissertation, University of Texas at Austin, 1989. (17) Kohler, S. D. Ph.D. Dissertation, Universityof Texas at Austin, 1993. (18) The average oxidation state was determined using [&(moles of COz formed)/(mole of metal present)]. (19) Braterman, P. S. Metal Carbonyl Spectra; Academic Press: New York, 1975. (20) Wilson, E. B., Jr.; Decius, J. C.; Cross, P. R. Molecular Vibrations; McGraw-Hill: New York, 1955. (21) Nakamoto, K. Infrared and Raman Spectra of Inorganic and Coordination Compounds, 3rd ed.; Wiley: New York, 1978. (22) Cotton, F. A,; Kraihanzel, C. S. J . Am. Chem. SOC.1962, 84, 4432.
