J. Phys. Chem. 1994,98, 4336-4342
4336
Infrared Spectroscopic Characterization of Chromium Carbonyl Species Formed by UItraviolet Photoreduction of Silica-Supported Chromium(V1) in Carbon Monoxide Steven D. Kohlert and John G. Ekerdt' Department of Chemical Engineering, University of Texas a t Austin, Austin, Texas 78712 Received: October 20. 1993'
The chromium carbonyl species that form during ultraviolet (UV) photoreduction of Cr6+/Si02 in 12C160 and I3Cl6O were analyzed by Fourier transform infrared spectroscopy (FTIR). Four carbonyl species were identified: linear Cr4+(CO), mer-Cr2+(CO)3,linear Cr2+(CO), and Cr2(C0)2 (band a t 2099 cm-1). The linear Cr4+(CO) band a t 2206 cm-l, formed a t short UV exposures, increased to a maximum and decreased a t long UV exposures. Both the mer-Cr2+(C0)3, with bands a t 2212,2190, and 2178 cm-I, and linear CrZ+(CO), with a band a t 2184 cm-', were present a t all UV exposures. The IR bands arising from these two species were isolated during isothermal evacuation. The oxidation state of the photoformed Cr2+species was assigned by comparing the IR bands following thermal reduction of Cr6+/Si02 in CO to those formed following photoreduction.
Introduction Chromium(V1) supported on silica is one of the most used and studied catalyst systems in the petrochemical industry. The Phillips catalyst, CrO3/SiO2, is utilized extensively for the lowpressure polymerization of ethylene. Factors affecting the polymerization of ethylene have been reviewed by McDaniel.1 One area of disagreement centers on the valence state of the active chromiumcenter. Hexavalent chromium will not catalyze the polymerization of ethylene;2 however, the active oxidation state of Cr/SiOz during industrial production of polyethylene is not known. Researchers have examined supported, reduced chromium in the laboratory to learn about the chemical and catalytic properties of chromium. Five different approaches have been used to produce supported chromium in a reduced oxidation state on silica. These approaches are (1) thermal reduction of Cr6+/Si02in CO or H2,3-13(2) photochemical reduction of Cr6+/ Si02 in COor H2,"I9 (3) deposition of Cr(CO)6,2sz6(4) exchange of silica hydroxyls with organometallic reagents containing reduced chr0mium,9*~~-~~ and (5) ion exchange with aqueous solutions of Cr3+.31-33 Thermal reduction of Cr6+/Si02 is a common method of producing catalytically active reduced chromium centers. The structure and the valence state of the reduced chromium on the surface of the silica are in much dispute. The onset of thermal reduction of Cr6+/Si02 occurs about 373 K.34 Fubini et al.5 report reduction in CO a t 623 K yields 98%of the silica-supported chromium in the 2+ state as determined from oxygen uptake measurements. The remaining 2%of the metal was proposed to be clustered in a-chromia-like particle^.^ There are two general interpretations of the carbonyl modes for thermally reduced Cr/SiOz. One interpretation has assigned room temperature infrared spectroscopy (IR) bands to mononuclear chromium carbonyls. At low temperatures, these bands shift to lower frequencies. The other representation uses the same room temperature IR bands and the additional lowtemperature bands to assign the bands to both mononuclear and dinuclear chromium carbonyls. In the first interpretation, three different types of mononuclear Cr2+ ions have been proposed. The assignments for IR bands formed after reduction at 633 K are shown in Table 1. The CrZ+ ions, labeled A, B,and C, have two, three, and four oxygen ligands ~~
* To whom correspondence should be addressed.
~~
Current address: Sandia National Laboratories, Division 6212, P.O.Box 5800, Albuquerque, NM 87185. a Abstract published in Aduance ACS Abstracts, April 1, 1994.
TABLE 1: Assignments (in cm-l) for CO Adsorbed on Cr/SiOl species terminal CO band bridging CO bands ref 2181 CrA"(C0) none 7 Cr.t,II(CO)s 2184,2178 none 7 2191 Cr#(CO) none 7 2095 7 Crz(C0) none CrlI-Ama 2183 none 13 Cr11-b
CrI1-Bm CrII-Cd
2178 2190 2188 2200 2200
2119,2098,2034
none 2120,2100,2043
none none m = mononuclear chromium, d = dinuclear chromium.
CrIII,
CrlIId
13 13 13 13 13
coordinated to the Cr2+center, respectively (Cr$ not shown). Vacuum thermal treatments converted type A chromium into type C chromium.6 Type B chromium was unaffected. This allowed for the discrimination between the behavior of the chromium centers. CrcII was inactive to CO at room temperature and would not adsorb CO. Cr#(CO) could only coordinate one CO ligand. Thestretching frequency of theCr#CO was assigned to 2191 cm-'. Coordination of CO to the CrAIr was proposed to occur in a two-step process. CrAII(C0) coordinated one COligand a t low CO pressure. This monocarbonyl stretch was assigned to the 2181-cm-l band. The second adsorption step coordinated two additional CO ligands, forming CrAI1(C0)3. The tricarbonyl was proposed to have a trigonal-bipyramidal-likestructure with theoxygen ligands in thevicinal p ~ s i t i o n .Two ~ of the CO ligands were in the trans configuration, and the remaining CO was in the cis position. The symmetric mode of the trans CO was reasoned to be inactive in IR. The two other modes, antisymmetric mode of the trans CO ligand and the stretch of the apical CO, were assigned to a pair of bands a t 21 78 and 2 184 cm-l, respectively. These bands were assigned to the tricarbonyl because they did not show the intensity ratios expected of a dicarbonyl. The assignment of these two bands to the same Cr species has been q u e ~ t i o n e d .An ~ ~ additional carbonyl band a t 2095 cm-l was seen at pressures above 10 Torr.4 This band was assigned to CO ligands bridge-bonded to adjacent Cr ions. Carbon dioxide also adsorbed on the Cr ions; a band at 2350 cm-l was assigned to the antisymmetric stretching mode of linear COz molecules loosely bound to the surface.3 Six different chromium centers, mononuclear and dinuclear, are proposed by a different research group to form from the thermal reduction of Cr6+/Si02 at 623 K in CO.I3 From O2 uptake measurements, the chromium existed in the +2 and +3
0022-365419412098-4336%04.50/0 0 1994 American Chemical Society
IR Spectroscopy of Chromium Carbonyl Species oxidation states. The assignments are also shown in Table 1. Isolated CrI1-Amwas proposed to be the dominant species on thermally reduced Cr/SiOz.lI However, repeated oxidation and reduction cycles converted the mononuclear species to the dinuclear species, Cr11-h. The two chromium centers of the dinuclear species are connected with a bridging oxygen. The type A species differs from the types B anc C species by the coordination of the chromium to the surface. All three species coordinate with two oxygen ions to the surface;12in addition, type B chromium is coordinated to an isolated surface silanol ligand and type C chromium is coordinated to either two surface silanols or a siloxane bridge. It must be noted that the species notation, CrII-B and C r W , is different from the notation used by other researchers,3-8 who did not distinguish between the two species and used the notation C P - B for both species. The deposition of metal carbonyls on high surface area metal oxides is a convenient method of producing well-dispersed metals and metal clusters. Cr(C0)6 was only physically adsorbed on Si02.23Around 423 K, the Cr(C0)6 decomposed by losing CO to form very small metallic agglomerates. The loss of CO ligands was not sequential, and this process was only partially reversed upon reexposure to CO. Under oxidizing conditions, the Cr(CO)6 decomposed to fully oxidized chromium species. Thermal reduction of this Cr/SiO2 catalyst produced coordinately unsaturated Cr2+ species typical of a reduced chromia/silica cataly~t.~3 IR spectroscopyof the physically adsorbed Cr(C0)6/ Si02 revealed a strong band at 1990 cm-l and a weak shoulder at 2015 cm-1.23 On highly siliceouszeolite(Si/Al = 1000), similar bands were assigned to the Tu and to the E, modes of Cr(C0)6, respectively.26 Photoreduced catalysts have been found to be highly active for certain catalytic reactions.17J9 In some cases, the photoreduced catalyst was more active than the conventionallyreduced catalyst. Cr6+/Si02 can be reduced by ultraviolet (UV) light at room temperature in either H2 or C0.l4l8 The optical spectra of the photoreduced catalyst contained a band at 11 800 cm-l, which was assigned to Cr2+ existing in a dichromate structure with Cr6+.15 The coordinative symmetry of these chromium sites was not investigated. From oxygen uptake measurements, the average oxidationstateof the photoreducedcatalyst was 4+. These results suggested a photoreduction scheme in which one Cr6+ of a dinuclear site was reduced to Cr2+,and the oxidation state of the other chromium was unchanged. However, recent Raman spectra have shown that a majority of the Cr6+ is isolated, with very small amounts of oligomeric species.3638 This review illustrates the lack of consensus regarding the structure of reduced chromium supported on silica. Although the average oxidation state of thermally reduced Cr/SiOz has been shown repeatedly to be approximately 2+, there is still uncertainty surrounding the oxidation state of photoreduced Cr/ SiOz. We report a study of Cr/SiO2 photoreduction that was directed toward understanding the photoreduction process given the importance of reduced chromium species in catalysis and because photoreduction can provide a convenient method of producing coordinatively unsaturated chromium sites. Assignments of FTIR bands to Cr*+(CO)xstructures are made, and the dependence of these bands on UV exposure and evacuation time is discussed. In addition to bands commonly seen for thermally reduced Cr/SiOz catalysts, new bands, seen only for photoreduction, are reported.
Experimental Methods The Cr6+/Si02 samples were prepared by a standard wet impregnation method from Cr(NO3)s (Aldrich) and Cab-0-Si1 EH-5 (380 m2/g). Cr(N03)3 was dissolved in just enough distilled, deionized water to wet the silica surface. The samples were dried for 3 h at 410 K and calcined for 16 h at 773 K in hydrocarbon-free air (HFA). The calcined samples were cooled
The Journal of Physical Chemistry, Vol. 98, No. 16, 1994 4331 in ambient air, crushed with a mortar and pestle to facilitate wafer pressing, and stored in sealed glass sample bottles until use. Weight loadings were varied from 0.1% to 2.0% Cr (metal basis). Raman spectra show that, in the dehydrated and fully oxidized state, wet-impregnated Cr6+/Si02 samples form primarily isolated chromates and very small amounts of oligomeric chromate species.3638 There were no Raman bands associated with crystalline Cr2O3 in the samples used herein. The sample colors ranged from yellow for low Cr loading to orange-yellow for high Cr loadings. Transmission IR spectra were measured with a Mattson Research Series 1 Fourier transform IR spectrometer equipped with an air-cooled source and a mercury+admium-telluride detector. The spectra were obtained at a resolution of 0.75 cm-1 after 100-200 scans. Two IR cells were used. The first IR cell consisted of a stainless steel base coupled to a quartz furnace.39 The quartz furnace was capable of thermal treatments up to 1273 K. A magnetically coupled quartz manipulator was within the quartz furnace and 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 second IR cell, capable of subambient temperatures, has been described previously.“ This stainless steel cell with CaF2 optics was capable of thermal treatments to 700 K. The wafers were pressed from 40-160 mg of fresh Cr/SiO;! under 6000 lb/ in.2 for 30 s. Wafers were calcined in HFA for 1 h at 873 K in the first cell or for 15 min at 673 K in the second oell. After calcination, the wafer was evacuated for 30 min at 773 K in the first cell or evacuated for 5 min at 673 K and cooled to room temperature in He in the second cell. Approximately 1 atm of CO was added to either cell for photoreduction. Room temperature photoillumination was performed with a Photon Technology International (PTI) high-intensity arc lamp (02-Al010Q) and power supply (02-LpS200) with a 100-W mercury lamp. The minimum UV wavelength was limited by Pyrex plates to 280 nm. The UY beam was passed through 10 cm of water to minimize IR heating of the sample. The illumination time was varied from 0.1 to 100 min. After photoreduction, the IR spectra of the absorbed metal carbonyl were obtained at room temperature with subtration of both the Si02 background and the CO gas phase or at subambient temperatures with subtraction of just the Si02 background. A flow-throughreactor system was used to measure the amount of C02 produced during photoreduction and to determine the average oxidation state of the reduced catalyst by titration with an Oz/He mixture. Cr/SiO2 sampleswere held in quartz U-tubes. A 6 X 4 mm2 tube was used for sample masses of 4&150 mg, and a 12 X 10 mm2 tube was used for sample masses of 03-13 g. Glass wool was used to position the samples in the UV beam. Products were analyzed using the thermal conductivity detector of a Hewlett Packard 5880A gas chromatograph (GC). The GC was equipped with an automatic sampling valve that injected 1 cm3 of the reactor effluent into the GC. Carbon monoxide and C02 were analyzed during photoreduction every 2 min using a 1.83-m X 6.3-mm Porapak T column. During redoxidation, 02, N2, and CO were analyzed using a 1.83-m X 6.3-mm molecular sieve SA column. The samples were held in the oven of a Varian 3700 GC. The samples in the U-tubes were positioned about 0.25 m from the Hg lamp during photoreduction. Samples were manually shaken thoroughly to expose all of the sample to the light and held in a beaker of water to minimize IR heating of the sample. Thermally reduced samples were ramped at 20 K/min to 623 K in CO and held at that temperature until reduction was complete (usually 20 min). Samples were calcined for 1 h at 773 K in 40 cm3/min HFA. The samples were cooled to room temperature in 40 cm’/min of He and then photoreduced for 0.5-240 min in CO while
4338 The Journal of Physical Chemistry, Vol. 98, No. 16, 1994 2.0
Kohler and Ekerdt
1
A
A
b
b
8 0
8 0
r b a n
r b a
n
C
C
e
e
2225 2200 2175 2150 2125 2100
Wavenumbers Figure 1. IR spectra following photoreduction of 2.0% Cr/SiO2 for (A) 25 s and (B)10 min through a 3-mm Pyrex filter in 775 Torr of CO. The sample spectra were obtained at 220 K in a He purge.
monitoring the C02 production with the GC. The C02 signal was detected after 1-2 min of photoreduction and attenuated within 2 min of turning off the lamp. The C02 production for thermally reduced samples was determined in a similar fashion. Following reduction, the samples were heated in 40 cm3/min of He to 573 K to remove adsorbed CO. After cooling in He to 3 13 K, the samples were reoxidized in 10 cm3/min of a 2% O2/He mixture. Oxygen uptake was measured at 313 K and while ramping the catalyst at 10 K/min to 773 K. Carbon monoxide ( W 1 6 0 , Liquid Carbonic, research grade 99.99%) was passed through a bed of molecular sieves at 473 K to decompose iron carbonyls and further purified by passing through water and oxygen traps (SGE). 13C160(99.3 atom % W a n d 12.3atom% l*O,IsotecInc.) wasusedasreceived.Helium (99.99+% UT Physics Department) was passed through oxygen and water traps (SGE). Hydrocarbon-freeair (Liquid Carbonic) was passed through a water trap. The 2% O2/He mixture (Linde certified) was purified by passing through a water trap.
Results The IR spectrum resulting from short duration photoreduction of Cr6+/Si02 (Figure 1A) has bands in three regions: (1) 20902110, (2) 2170-2195, and (3) 2200-2220 cm-l. The lowfrequency region consists of a single, broad peak at 2099 cm-'. The mid-frequency region contains the well-known chromium carbonyl triplet of peaks at 2178, 2184, and 2190 cm-l. The high-frequency region consists of a single peak at 2206 cm-I and a shoulder at 2212 cm-l. With more extensive photoreduction (Figure lB), the peaks at 2099, 2178, 2184, and 2190 cm-1 increased in intensity. The shoulder at 2212 cm-l in Figure 1A is a full peak in Figure 1B. The intensity of the 2206-cm-' peak decreased in intensity relative to the other peaks. The species responsible for the band at 2206 cm-l is designated as CY. As seen in Figure 2, the peak at 2206 cm-1 was not stable during isothermal evacuation; however, the triplet of 2178,2184, and 2190 cm-1 and the peak at 2212 cm-1 were stable. Though it is not shown in Figure 2, the peak at 2099 cm-1 was not stable during isothermal evacuation. After 20 s of UV exposure (Figure
2225
2200
2175
2150
Wavenumbers Figure 2. IR spectra following photoreduction of 2.0% Cr/SiO2 for (A) 5 min, (e) 1 min, and (C) 20 s through a 3-mm Pyrex filter in 770 Torr of CO. The sample spectra were obtained at 298 K after isothermal evacuation.
2C), the peak at 2178 cm-l was most intense, but after 5 min of photoreduction, the peak at 2184 cm-1 was largest (Figure 2A). Since the rate of intensity increase with UV exposure for these two peaks was different, they must belong to two different carbonyl species. Thisresult is incontrast withGhiotti et a1.,6 whoassigned peaks at 2178 and 2184 cm-l to the same species. The spectrum of Figure 2A is fit to three Lorentzian peaks in Figure 3 of equal width, located at 2178,2184, and 2190 cm-1, The Lorentzian peak representation is most accurate at the peak maxima and is less accurate in modeling the peak tails. The curve-fitting procedure allows for the contributionof each specific peak to the total spectrum to be determined. The integrated intensity ratios of the peaks at 2178,2184,2190, and 2212 cm-1 are shown in Figure 4. The peak area ratios of 2190/2178 cm-1 and 2212/2190 cm-l wereconstant during isothermal evacuation, which resulted in a 5-fold decrease in the integrated intensity of the 2190-cm-l peak. This suggests that these threepeaks, 2178, 2 190, and 22 12 cm-1, all arise from a single chromium carbonyl species, designated as species 8. The 2184 cm-l peak is assigned to a third species, y. Since the band at 2099 cm-1 was not associated with any peaks of the chromium carbonyl triplet or the peak at 2206 cm-l, the 2099-cm-l peak is assigned to species 6. The IR spectrum resulting from partial isotopic substitution of the photoformed chromium carbonyls is presented in Figure 5 . The spectrum was taken after photoreduction of C++/SiO2 in a mixture of 50% 12C160and 50% 13C160and exhibits strong bandsat 2130and2179cm-1. Thereareweak bandsat 2150and 2210 cm-1 and shoulders at 2140 and 2188 cm-1. Figure 6 is the spectrum of Cr6+/SiO2 photoreduced in 69% 12C160and 31% l3C16O. This spectrum exhibits strong bands at 2130 and 2180 cm-1 and weak bands at 2149 and 2210 cm-1. Shoulders of the strong peaks were visible at 2141 and 2188 cm-l. Figure 7 is the spectrum resulting from photoreductionof Clb+/SiO2 in a mixture of 24% 12CW and 76% 13CW. This spectrum displays strong bandsat 2131 and2179cm-1,weakbandsat 2149and2205cm-1, and shoulders at 2141 and 2189 cm-'. An IR band was observed at 2345 cm-1 (not shown) that is assigned to physisorbed C02. The weak interaction between C02
IR Spectroscopy of Chromium Carbonyl Species
The Journal of Physical Chemistry, Vol. 98, No. 16, 1994 4339 I
'0~1
A b
0.8
0
4
1 2130,
1.4-
1.2
A b S 0
S
r b a n
r b a n
C
C
e
e
0.0
2220
2200
2180
2160
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
,
,
,
,
2225 2200 2175 2150 2125
Wavenumbers
Wavenumbers
Figure 3. Fit of three Lorentian peaks (2190,2184, and 2178 cm-l) to the IR spectrum of 2% Cr/SiOz photoreduced for 5 min through a 3-mm Pyrex filter in 770 Torr of CO. The sample spectra were obtained at 298
K after isothermal evacuation.
Figure 5. Observed and predicted IR spectra following photoreduction of 2.0% Cr/Si02 for 13 min through a 3". Pyrex fdter in 780 Torr of 50% l2CL60 and 50% W60. The sample spectrum was obtained at 298 K under evacuation. The predicted spectrum was generated using the
mer-Cr(CO)s model.
AREA RATIO 0.8 1
l
0.6
0
2190/2184
1I
0
I A b
0
S
+
0.5
o
0
!-
0.7
+
0
+
r b a n
2190/2178
+
C 0.05
1
e
1 0
I
j
I 2
4
6
8
10
12
14
0.5
16
EVACUATION TIME -4. IRintegratedintensityratiosofthe bandsat 2190,2184,2178, and 2212 cm-* during isothermal evacuation at 298 K. The 2.0% Cr/
Si02 sample was photoreduced for 10 min through a 3-mm Pyrex filter in 780 Torr of CO. Spectra were obtained at 298 K. and reduced Cr/SiOz has already been ~bserved.~ The adsorbed COz desorbed quickly upon evacuation. Various papers have reported 0 2uptake measurements, which show that the averageoxidation state of thermally reduced catalyst is approximately 2.1.5~~This study also reports this result along with average oxidation state determinations for photoreduced catalyst in Table 2. From mass balances, the average oxidation state of the photoreducedcatalyst was 4.0. The sample color was orange-yellow after calcination and turned a blackish, blue-green upon photoreduction. Figure 8 presents carbonyl modes formed after thermal reductionin770TorrofCOat773K. Thermalreductionfollowed 5 min of photoreduction. The IR band shapes and positions were
0.
Wavenumbers Figure 6. Observed and predicted IR spectra following photoreduction of 2.0% Cr/SiOz for 11 min through a 3-mm Pyrex filter in 770 Torr of 69% l2Cl60and 13% 13C160.The sample spectrum was obtained at 298 K under evacuation. The predicted spectrum was generated using the
mer-Cr(CO), model.
nearly identical for photoreduction and thermal reduction. Thermal reduction led to more intense bands.
Discussion Four different species formed during photoreduction of silicasupported C++. The 2206-cm-1 band assignment (species a) is
4340 The Journal of Physical Chemistry, Vol. 98, No. 16, 1994
A b
Kohler and Ekerdt
A b
S
8
0
0
r b a n
b a n
C
C
e
e
r
Wavenumbers
Wavenumbers
Figure 7. Observed and predicted IR spectra following photoreduction
Figure 8. IR spectra of photoreduced and thermally reduced 2% Cr/ SiOz. Spectra were taken at 298 K under evacuation. The sample was photoreduced for 5 min through a 3-mm Pyrex filter in 780 Torr of CO (Spectrum A). After obtaining spectrum A, the sample was thermally reduced for 15 min at 773 K in 770 Torr of CO (spectrum B).
TABLE 2 Uptake Measurements for Thermal Reduced and Photoreduced Cr/SiOl pmol of COz net pmol of 0 pmol formed during consumed during reduction of Cr reduction reoxidation photo 54.7 53.6 54.1 113.1 116.1 photo 114.8 133.6 134.3 photo 134.2 31.2 30.5 photo 30.4 260.4 261.2 photo 256.6 188.2 185.4 thermal 95.6 299.5 301.2 thermal 152.5 260.1 258.2 thermal 133.8
photoreduction of supported metal oxides in CO has been proposed to occur as a two-electron process.1k1g The response of the 2206cm-l band to UV exposure suggests a series reduction of Cr6+ to Cr4+and ultimately to CrZ+. Since IR peaks associated with the B, y, and 6 species are evident a t the shortest reduction times, the first reduction step to C++ is the most likely rate-limiting step. B Species. B is assigned a mer-Cr2+(CO)3 structure because it generated three bands in the unsubstituted IR spectrum, because of the intensity of these bands, two strong and one weak, and by the ability of the mer model to predict the IR frequencies and intensities of chromium carbonyls photoformed in an isotopic mixture of CO. The oxidation state was established by analogy to thermally reduced Cr2+/Si02. Peaks a t 2178,2190, and 2212 cm-’ are assigned to the merCr2+(CO)s structure of C, symmetry. This assignment is both similar and contrasting to previous assignmentsmade for thermally reduced chromium carbonyls. The trigonal-bipyrimidal-like structure, first proposed by Ghiotti et ale,’ has the same carbonyl structure proposed herein (two trans CO ligands and one cis CO ligand); we disagree in the assignment of particular IR bands to this structure. As seen in Figure 2, the bands a t 2178 and 2184 cm-1, assigned to the tricarbonyl by Ghiotti et al., did not increase a t the same rate with increasing UV exposure. Furthermore, the changes in relative integrated intensities during evacuation (Figure 4) clearly demonstrate the 2184- and 2178-cm-I bands are not associated with the same carbonyl species. The association of the 2178- and 2184-cm-I bands to the same carbonyl species has also been rejected by another research g r o ~ p . 3 ~ The three CO ligands of B are distributed meridionally around the Cr metal center. The (Al)1 mode at 2212 cm-l is very weak because the individual dipole moments of the trans carbonyls nearly cancel each other out. The resultant dipole points along the Cz symmetry axis. The lack of dipole cancellation enables the B2 (2190 cm-1) and (Al)2 (2178 cm-l) modes to be much more intense. The mer-Cr2+(CO)s model, using the 2212-cm-I peak, does the best in the prediction of spectra from isotopic mixtures of
of 2.0% Cr/SiOa for 10 min through a 3-mm Pyrex filter in 750 Torr of 24% l2CI6Oand 76% l3CI6O. The sample spectrum was obtained at 298 K under evacuation. The predicted spectrum was generated using the mer-Cr(C0)3 model.
based on the changes in band intensity to photoreduction time. Assignment of carbonyl structures for the B and y species is based on the number and relative intensities of the bands in the unsubstituted IR spectra and on the ability of various carbonyl models to predict the substituted IR spectra. The 6 species assignment is consistent with past assigments of the band at 2099 cm-1 to a bridging carbonyl. a Species. The 2206-cm-1 band for CY is assigned to the stretch of linear Cr4+(CO). This stretch is prominent in the IR spectrum of Cr6+/Si02 photoreduced for 20 s as shown in Figure 1A. The peak at 2206 cm-l grew in intensity for UV exposures up to 20 s. After 20 s of UV exposure, the peak decreased in intensity but was still present after 10 min of UV exposure (Figure 1B). All other IR peaks were still growing in intensity after 5 min of UV exposure. The oxidation state of a is assigned 4+ by comparison to literature values for other linear Cr carbonyls and by the response of the carbonyl stretching mode to UV exposure. Linear Cr2+ carbonyls have been assigned to IR peaks in the range 21752191 cm-1,7.13while linear Cr3+ carbonyls have been assigned peaks at 21917 and 2200 cm-1.7.13.41+42 Given the agreement for the peak positions of the 8, y, and 6 species with the frequencies quoted in the literature, it is possible to discount the 2+ and 3+ oxidation states for the a species. Odd oxidation states are not expected as a result of photoreduction of Cr6+/SiO2 since
The Journal of Physical Chemistry, Vol. 98, No. 16, 1994 4341
IR Spectroscopy of Chromium Carbonyl Species
TABLE 3: Predicted Fr uenciea and Intensities of ~~~Cr2+('F'LO)ru('fl~)~ ~
predicted predicted case and symmetry mode freq, cm-1 intensity nonsubstituted (C~)(MO(~ZC~~O), (AI)I 2212 0.053 BZ 2190 0.558 2178 221 1 Bz 2190 ( A ~ ) ~ 2130 asymmetricallymonosubstituted (C,) A' 2204 A' 2180 A' 2149 symmetricallydisubstituted (Ca) (AI)\ 2181 Bz 2160 ( A ~ ) ~ 2141 asymmetricallydisubstituted (C,) A' 2203 A' 2150 A' 2130 completely substituted (Cb) (Aih 2163 B2 2141 ( A ~ ) ~ 2130 (A112
symmetricallymonosubstituted (Ck)
(Al)1
1.o 0.016 0.583 1.045 0.139 1.058 0.198 1.034 0.062 0.555 0.099 0.227 1.005 0.053 0.558 1.o
Relative to 2178 cm-l lZCW-13C16O when compared to other models (see below). A
CF matrix approach that assumed nonmechanical coupling between CO ligands was used for modeling the detailed procedures are reported el~ewhere.3~ Table 3 lists the relative intensities for various substituted structures. The values in Table 3 were used to generate the predicted spectra that are presented in Figures 5-7. The predicted spectra exhibit the same shape and location as the measured spectra, two sets of one major peak with two high-frequency shoulders. As explained earlier, the tails of the spectrum could not be fit properly due to the inability of the Lorentzian function to properly cut off the tails of the individual peaks. Expected contributions for the unsubstituted (2184 cm-1) and substituted (21 34 cm-l) y speciesbands were also included in the predicted spectra. The expected y contributions were based on the 2178/2184-cm-l intensity ratio recorded under similar photoreduction experiments using 12C160. A key assumption of this mer model is that the 13C160ligands to not have a preference for either cis or trans substitution. Hence, substitution should yield statistical ratios of the different substituted species. This assumption predicts that certain species should be more prevalent depending upon the degree of isotopic substitution. This result can be seen in Figures 5-7. At equal amounts of isotopic substitution, 50% of the intensity of the predicted spectrum should be from the asymmetrically monosubstituted and the asymmetrically disubstituted species. Each of the remaining species only contributes 12.5% to the total intensity. Hence, the major peaks of the Figure 5 are located near the most intense predicted frequencies of the asymmetric cases at 2130 and 2179 cm-1. Three other possible chromium carbonyl structures were considered: cis-Cr2+(CO)Z of C, symmetry, cis-Cr2+(CO)3of C3, symmetry, and cis-Cr2+(C0)2(CO)' of C, ~ymmetry.~'The Cr2+(C0)2(CO)' model results when one of the CO groups is trans to a unique ligand, such as a double-bonded oxo anion. The cis-Cr2+(CO)Zandcis-Cr2+(C0)3modelscan berejectedbecause they are fit using a two-band unsubstituted carbonyl spectrum. The results reported in Figures 1, 2, and 4 demonstrate the @ species was associated with three carbonyl bands, and neither cis-Cr2+(CO)znor cis-CrZ+(CO)3can account for the three-band spectrum. The cis-Crz+(CO)2(CO)' model can be rejected because it required the unsubstituted spectrum to consist of three bands of similar intensity, which differs significantly from the 0.053:0.588:1.0 2212:2190:2178 cm-1 ratio reported in Figure 4. In the me~-Crz+(CO)~model, there are four stretching parameters and only three nonsubstituted modes.39 Three of the stretching parameters, the trans stretching parameter (kz), the trans-trans interaction parameter (kt),and the cis-trans interac-
tion parameter (kc), were expressed in terms of the fourth parameter, the cis stretching parameter (kl). The parameter k1 was varied to best fit the spectra for the substituted species. The parameters which best predicted the observed spectra are kl = 1918.5 N/m, kz = 1953.8 N/m, kt = 17.0 N/m, and k, = 9.5 N/m. Therelativeintegrated intensitiesofthe 2212,2190,and 2178cm-l peaks for met-Crz+(CO)3 were 0.053:0.558:1.0 (Figure 4, Table 2). These peaks were deconvoluted using a Lorentzian peak shape. The integrated intensities and the stretching parameters have been used to calculate the angle between the CO dipole moments and the relative sensitivityof the CO ligands.6 The estimated interdipole angle is 90.8 f 1.Oo. The relative rate of change of dipole moment with mass-weighted normal coordinates of the cis CO group is 2.7 times greater than that of the trans group. These parameters compared well to those calculated for mer-Mo(CO)s. The interdipole angle was 9 l 0 , and the rate of change of the cis dipolemoment was 1.8 times that of the trans dipole moment for mer-M0(C0)3.'~ y Species. Room temperature photoreduction of C#+/SiO2 in CO at long UV exposures produced both the mer-Crz+(CO)a and y species (2184-cm-1peak). y is assigned to a monocarbonyl Cr2+ species, most likely Cr2+(CO). Both of and y were stable at 298 K in 1 atm of CO and slowly desorbed CO ligands during isothermal evacuation. The 2184-cm-1 peak was shown to be separate from the peaks of the mer-Crz+(C0)3in Figures 2 and 4. Other studies have assigned the 2184-cm-1 peak as one mode of a mer-Crz+(CO)37or to a carbonyl singly bonded to a dinuclear chromium site.13 6 Species. The IR mode at 2099 cm-1 (Figure 1) is assigned to a carbonyl bridging between two chromium centers. This peak was present at low UV exposures, and the intensity increased to a constant value. The bridging carbonyl was not stable without CO in the gas phase. The assignment of the peak at 2099 cm-1 to a bridging carbonyl has been made previ0usly.~J3 As the amount of chromium supported on the silica is increased, the amount of dinuclear chromium centers should also increase. The integrated intensity ratio of the 2099-cm-l peak to the 2190 cm-1 increased with Cr loading. For chromium loadings of 0.5 wt % Cr, 1.0 wt % Cr, and 2.0 wt % Cr, the integrated intensity ratio was 0.17, 0.23, and 0.33, respectively. Oxidation State. This study has shown that a mer-Cr(CO)3 species and a linear Cr(C0) form during the photoreduction of Cr6+/SiOZ in CO at long UV exposures. The CO triplet seen in the IR spectra of photoreduced CrlSiO2 is the same triplet as seen by other research groups for thermally reduced Cr/SiOz. This is demonstrated in Figure 8. Thermal reduction caused an increase in the IR intensity for each mode. Thermal reduction of Cr6+/Si02 in CO has been shown to produce Cr2+.5.9 The results in Figure 8 suggest the oxidation state of the mer-Cr(CO)3,and the linear Cr(C0) speciesis 2+. The results in Table 2 show nearly complete thermal reduction to 2+ whereas the mass balance for photoreduction led to an average oxidation state of 4+. For some reason not all the Cfl+underwent photoreduction. Since the mer-Cr(CO)s species is in the 2+ oxidation state, it seems reasonable to suggest it has two additional oxygen ligands that bridge it to the surface. Each oxygen ligand, L, in such a CrZ+(C0)3L*structure would contribute a negative charge of 1. Any IR modes associated with the C r - o S i would be at a wavelength below the silica cutoff (1300 cm-1); therefore, no direct structural information regarding the nature of the ligands at the non-carbonyl sites of the mer-Cr(CO)s coordinationsphere can be determined.
Conclusions Ultraviolet photoreduction of Cr/SiOz produced several of the same chromium carbonyl species that form during thermal reduction on CO. Photoreductionwas not as efficient in reducing
4342 The Journal of Physical Chemistry, Vol. 98, No. 16, 1994 the supported chromium as was thermal reduction. Short UV exposures produced a linear Cr4+(CO) carbonyl species that cannot be prepared through thermal reduction of C1.6+. Long UV exposures produce three chromium carbonyl species: merCrZ+(C0)3,linear CrZ+(CO), and a carbonyl bridging between two chromium centers. These latter species were identified by the number and intensity of IR bands, the response of these bands to isothermal evacuation, and, for mer-Crz+(C0)3,partial isotopic substitution using W 6 O . COz, which is formed during photoreduction, physisorbed onto CrZ+and produced a band at 2344 cm-1.
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