J. Phys. Chem. 1994, 98, 10237-10242
10237
Subcarbonyl Species of Molybdenum Hexacarbonyl Supported on Silica. A DRIFT Study Maria Kurhinen, Tapani Venalkiinen, and Tapani A. Pakkanen* Department of Chemistry, University of Joensuu, P.O. Box I I I , FIN-80101 Joensuu, Finland Received: May 11, 1994; In Final Form: July 25, 1994@
Subspecies of partially decarbonylated molybdenum hexacarbonyl supported on silica were studied by diffuse reflectance IR spectroscopy. Mo(CO)dSi02 was prepared in a fluidized bed reactor by vapor-phase adsorption of molybdenum hexacarbonyl under nitrogen flow. Decarbonylation begins when Mo(CO)~has adsorbed onto the silica. Dehydroxylation of the support during calcination facilitates the formation of subspecies of Mo(CO)~. The activation energy needed for bond formation between a transition metal and silica and for decarbonylation is lower than the desorption energy of physisorbed Mo(CO)~,and this was seen in the IR spectra as a disappearance of bands due to subspecies. When the supported Mo(CO)~was reheated the physisorption bands were the last to disappear from the IR spectra.
Introduction
possible effects of solvents. Samples were characterized by diffuse reflectance IR spectroscopy under nitrogen atmosphere.
Supported molybdenum catalysts are used in metathesis's2 and disprop~rtionation~,~ of olefins and in Fischer-Tropsch synExperimental Section thesis5 Promoted molybdenum catalysts, both supported and Mo(CO)~was supplied by Aldrich Chemical Co. Ltd. and unsupported, have been studied intensively because of their used without further purification. Nitrogen (99.999%) and CO importance as hydrodesulfurization (HDS) catalysts.6-' A (99%) were used as carrier gases. Silica gel 60 (particle size common way to prepare molybdenum catalysts, especially ones 0.063-0.200 mm, surface area 540 m2/g, impurities Fe < for HDS catalysis, is impregnation with ammonium molyb0.02%, C1 < 0.02%, supplied and reported by E. Merck) date.2,6-8,10Carbonyl-based supported catalysts are more active provided the support. Silica was sieved, and the fraction left in hydrogenation and methanation and more selective in on the sieve 0.200 mm was used. Calcination of the support Fischer-Tropsch synthesis than are traditional catalysts made was carried out at 100 and 600 "C. When calcinations at 100 by impregnation with molybdenum salt.12 In addition, metal and 600 "C were carried out for 0.5-6 h in air, the hot support dispersion is higher than in traditional catalysts.13J4 The (100 "C) was packed into the reactor in air. When the support oxidation state of molybdenum can be determined by EPR,15-18 was calcined at 600 "C for 10 h under vacuum, it was stored in XPS,15.19 and UV-vis'' spectroscopy. IR spectroscopy has been a glovebox. Packing into the fluidized bed reactor was also used in the investigation of carbonyl precursors and in the done in the glovebox, so the support was never exposed to air. determination of CO adsorption onto catalysts.4~10~17~18~20-23 MoThe samples were prepared in a fluidized bed reactor by vapor (CO)6 supported on alumina decomposes to subcarbonyl species, which have been observed by IR s p e c t r o s ~ o p y .Another ~ ~ ~ ~ ~ ~ ~ phase adsorption. Molybdenum in a small boat and the support in a fluidized bed reactor were maintained at the same method of studying the decomposition of supported carbonyl temperature during the sublimation: Temperatures were from compound is TPDE.'J2 53 to 100 "C. Carrier gas (N2 or CO) flowed over the MoNo subspecies have been found by IR spectroscopy in studies of molybdenum hexacarbonyl supported on s i l i ~ a The . ~ ~ ~ ~(CO)6 ~ ~ boat, ~ transferring sublimed carbonyl compound through the fluidized bed reactor, where Mo(CO)~adsorbed onto the only IR bands found for Mo(C0)dSiOz were attributed to silica. Carrier gas flows varied from 30 to 160 " i n . Excess physisorbed Even during heating under vacuum, Mo(CO)~was collected in a cold trap after the reactor. when the intensities of the bands decreased due to desorption In some experiments the sample was reheated at 75 or 100 or decomposition, no subspecies were d e t e ~ t e d . ~In, ~contrast ~ "C after sublimation. During the reheating total decarbonylation to this, when Mo(CO)~was adsorbed onto Mo/Si0221 or Cr/ took place. After the sublimation and reheating, samples were Si02,22 carbonyl stretchings due to M-OC-Mo(C0)5 (M = transferred to the glovebox by Schlenk technique. Mo or Cr) were detected by IR. Infrared spectra were recorded at a resolution of 2 cm-' using Brenner et al. have reported indirect evidence of subcarbonyl a Galaxy 6020 Fourier transform infrared spectrometer with an species of Mo(CO)~supported on silica. After activation of MCT (mercury-cadmium-tellurium) detector. Spectra reMo(C0)dSiOz at 65 "C, physisorbed Mo(CO)6 sublimed from ported here are difference spectra, from which the pure silica silica at 45 "C under He flow. When the gas was changed to spectra have been subtracted. IR spectra were run in a glovebox carbon monoxide, however, a discontinuity was observed in the under nitrogen atmosphere by a diffuse reflectance (DR) amount of Mo(CO)~recovered and in explanation of this they technique. In a specially designed system, a metal box with suggested the formation of subcarbonyl species of molybdenum NaCl windows, containing the DR apparatus, was placed under hexacarbonyl on silica.' the glovebox (Figure 1). The IR spectrometer could be lifted The aim of this work was to support molybdenum on silica up so that the IR beam traveled through the NaCl window to using Mo(CO)~as a precursor. The samples were prepared by the DR apparatus and further through the second NaCl window vapor-phase adsorption in a fluidized bed reactor to avoid the to the IR detector. Molybdenum contents were determined with an energydispersive X-ray fluorescence (ACAX 300 EDXRF), but since Abstract published in Advance ACS Abstracts, September 1, 1994.
'
'
@
0022-3654/94/2098- 10237$04.50/0
0 1994 American Chemical Society
10238 J. Phys. Chem., Vol. 98, No. 40, 1994
Kurhinen et al.
Figure 1. View of the measuring setup: a = glove box. b = infrared spectrometer, c = metal box with diffuse reflectance apparatus. d =
NaCl windows, e = MCT detector.
gives hands at 2120 (AI,), 2024 (G),and 2003 (TI,) cm-1?4 Thus these three hands at 1994, 2020, and 2119 cm-I are suggested to he due to TI., E,. and A,, modes of physisorbed Mo(CO)s, respectively. Even with careful handling, some decarbonylation takes place, as can he seen in the weak band at 2388 cm-l and a slight broadening of the hand at 1994 cm-l. New hands due to formation of subcarbonyl species during chemisorption appeared in the spectrum at 2205, 2112, 2088, and 2004 cm-l (Figure 2b) when the amount of molybdenum was increased and the sample was ground. The TI. mode of physisorbed Mo(CO)~at 1994 cm-I can be seen as a part of a shoulder at lower wavenumber side of the hand at 2004 cm-l. The two other modes (AI, and Eg)of physisorhed Mo(CO)6 can also be seen (at 2118 and 2027 cm-I), hut the intensities are too strong compared with the intensity of TI,mode (at 1994 cm-I) to he entirely due to physisorhed species. After grinding, the intensity of the hand at 2388 cm-l increased due to further decarbonylation and a new hand appeared also at 2483 cm-I. In TF'DE studies Brenner et a1.l have observed evolution of CO, Hz, C € b ,and COz during decarbonylation of Mo(CO)~on si1ica.l For the formation of COz, the following scheme is presented in the literature:25
co + 0'-
2400
2200
2000
1800
1600
Wavenumber (em-11
Figure 2. Dry mix of Mo(C0)s and silica (pretmted at 600 "C) (a) after small addition of Mo(C0)s and very careful mixing and (b) after addition of a greater amount of Mo(C0)s and grinding. The wavenumben of the bands marked with vertical lines are presented in Table 1.
measurements were done with powder samples, the results should be considered only approximate. For a few samples with no or a very small amount of physisorbed Mo(CO)~(content of chemisorbed molybdenum 0.09-0.18 wt %) the molybdenum content was also determined by AAS for reference purposes. These AAS results together with the XRF results of the same samples were used to calculate the molybdenum content measured by XRF in other samples.
Results The first interaction between a carbonyl compound and a surface is physisorption. A dry mix of Mo(CO)~and silica, previously calcined for 10 h under vacuum at 600 "C, was very carefully mixed to find out where the v(C0) hands of physisorbed Mo(CO)6 appear in an IR spectrum measured by the DR technique. The spectrum is shown in Figure 2a. The strongest band is at 1994 cm-l with a shoulder at 2023 cm-I and a third and weaker band at 21 19 cm-I. Gaseous Mo(CO)~
-CO, +
2e-
Physisorhed COz gives a hand at 2382 cm-l on aluminaz6and at 2346 cm-l on silica?' The hand at 2388 cm-l may therefore be due to physisorhed COz. The hand at 2483 cm-l is unidentified hut may he due to COz adsorbed on molybdenum on the surface. The bands at 2362 and 2341 cm-I marked with an asterisk are due to gaseous COz in the atmosphere. After sublimations of Mo(CO)~on the silica (at 53-100 "C), IR spectra showed more bands than the three due to the physisorbed species, and these other bands were assigned to partially decarbonylated subspecies of Mo(CO)~formed during chemisorption. Although Mo(CO)6 interacts with the hydroxyl groups of the surface, no remarkable changes in v(OH) bands, except a slight broadening at the lower wavenumber side of the band at 3745 cm-I, were seen because the number of OH groups interacting with Mo(CO)6 was low as compared to the total number of surface OH and had thus no great effect on the intensity of the v(OH) band. Effects of Support Pretreatment. Figure 3 presents the IR spectra of MO(CO)~sublimed at 53 'C with gas flow 160 mU min for 30 minutes onto silica pretreated at 100 OC in air (spectrum a) and at 600 OC under vacuum (spectrum h). There is a monolayer of hydroxyl groups and physically adsorbed water on the surface of silica whenever it is exposed to a i z 8 The physisorhed water can be removed by heating at 150 OC in vacuum.28 After Mo(CO)6 sublimation onto silica pretreated at 100 "C, strong IR bands appear at 2029 (s) and 1996 (vs) cm-I. A very sharp hand appears at 2119 (m) cm-I and a hand which is broad and weak at 2085 cm-l (Figure 3a). The shape of the spectrum and the positions of the bands are similar to those of physisorhed MO(CO)~(Figure Za), hut decarbonylation is observed as increased intensity of the bands at 2029 and 2118 cm-l compared with the intensity of 1996 cm-I, and there is a broadening of the hand at 1996 cm-I. In addition to decarhonylation, the oxidation of molybdenum occurred, observable as a color change from white to light yellowish, when molybdenum chemisorbs on the silica. Based only on the IR results, it is not possible to determine reliable the oxidation state(s) of molybdenum on silica. If the sample came in contact with air, the yellowish color disappeared.
J. Phys. Chem., Vol. 98, No. 40, 1994 10239
Molybdenum Hexacarbonyl Supported on Silica
I
2400
2200
ZOO0 1800 Wavenumber [em-11
1600
Figure 3. IR spectra of Mo(CO)6 sublimed at 53 "C for 30 min with 160 d m i n N2 flow onto silica pretreated (a) at 100 "C in air and (b) at 600 "C for 10 h under vacuum. The wavenumbers of the bands marked with vertical lines are presented in Table 1. TABLE 1: Wavenumbers of Spectra in Figures 2 and J Figure 2a Figure 2b Figure 3a Figure 3b 2483(m) 2483(m) 2390(w) 2389(m) 2390(vs) 2390(vs) 2362(*) 2362(*) 2341(*) 234 1(*) 234 1(*) 234 1(*) 2205(w) 2205(w) 2118(vs) 2118(m) 2119(w) 21 19(s) 2112(s) 21 12(m) 2085(w) 2089(s) 2088(m) 2027(vs) 2029(s) 2023(m) 2026(vs) 2088(s) 1996(vs) 1994(vs) 2004(vs) 1980(sh) 1973(sh) 1970(sh) ~
I
2400
I
2200
I 2000
I
I
1800
1600
~~
Intensities of bands: (w) = weak, (m) = medium, (s) = strong, (vs) = very strong, (sh) = shoulder. Bands marked with asterisks are due to gaseous COz in atmosphere. During the IR runs, the IR beam created light brown spots on samples that had lost their color in air. More bands are seen in the b than the a spectrum of Figure 3. The shift of the strongest intensity from 1994 to 2008 cm-' might be explained as a formation of bands due to decarbonylated species, and the TI,mode of physisorbed Mo(CO)~is not resolved at all. The two other modes (A', and E,) of physisorbed Mo(CO)~can be seen at 2118 and 2027 cm-', but the intensities are too strong compared with the intensity of TI, mode (at 1994 cm-') to be entirely due to physisorbed species. In view of this we would propose that there are bands due to partially decarbonylated species very near to or even at the same wavenumber (21 18 and 2027 cm-') as the physisorption bands. After calcination of the support at 600 "C a new sharp band of Mo(CO)6-dSiO2 appears in the spectra at 21 12 cm-' (Figure 3b). In spectra of samples calcined at 100 "C (Figure 3a) it is seen only as a shoulder of the band at 2119 cm-'. This new band could be assigned to a subcarbonyl bound to siloxane (-Si -0-- Si-). Dehydroxylation occurs during calcination at 600 "C, and the literaturez9reports an average concentration of 1.5 nmW2 for OH groups. When the gas flow was reduced from 160 to 30 " i n , more CO bands appeared in the spectrum (not shown) than in
Wavenumber (cm-11
Figure 4. Effects of reheating: (a) Mo(CO)~sublimed onto silica The pretreated at 600 "C (T= 75 "C, t = 60 min, f = 160 " i n ) . sample was reheted at 75 "C for (b) 15 min, (c) 30 min, (d) 45 min, (e) 60 min, (0, 75 min, and (g) 90 min. The wavenumbers of the bands marked with vertical lines are presented in Table 2. the spectrum 3a after sublimation of Mo(CO)~onto silica pretreated at 100 "C. This might be explained as follows: water adsorbed onto silica prevents or slows down the reaction between Mo(CO)~and the surface, but under slow flow, Mo(CO)6 has more time to react with the surface. While the shape of the spectrum is similar to that in Figure 3b, there are the following differences: The intensities of bands at 2120-2080 cm-' are weaker than those in Figure 3b, when comparison is made with the bands at 2030-1970 cm-', and the most intense band is at 2030 cm-' (2029 cm-' in Figure 3b). Because the IR runs were made with the diffuse reflectance technique, the intensities can be compared only on a relative basis. The band at 1996 cm-' is not resolved under the band at 2000 cm-' due to the stretching modes of subspecies. At about 1975 cm-' there is a shoulder just like that in Figure 3b. With long sublimation times (60-75 min) at 53,75, and 100 "C, IR spectra of Mo(CO)6 supported onto silica pretreated at 600 "C showed the most intense bands at wavenumbers 21202075 cm-'. IR spectra produced at higher sublimation temperatures over a short time were similar to those produced at lower temperatures over a longer time. Mo(CO)~reacts first with surface OH groups, and since supports pretreated in milder conditions have more OH groups than supports pretreated at higher temperatures, they can be assumed to adsorb greater amounts of Mo(CO)~. Effects of Reheating. Samples prepared at 75 "C were reheated for various lengths of time at 75 and 100 "C. IR spectra of the samples reheated at 75 "C are shown in Figure 4. Immediately after sublimation (Figure 4a), several broad bands appeared in the spectrum. During reheating (Figure 4b-g),
10240 J. Phys. Chem., Vol. 98, No. 40, I994
Kurhinen et al.
TABLE 2: Wavenumbers of Spectra in Figure C b
a
C
2483(w) 2482(w) 2483(vw) 2390(vs) 2390(s) 2389(m) 2462(*) 2462(*) 2341(*) 2341(*) 2341(*) 2204(w) 2205(w) 2205(vw) 2119(vs) 2119(m) 2119(m) 2112(s) 2112(m) 2112(sh) 2090(s) 2089(m) 2083(w) 2028(s) 2027(s) 2025(m) 2008(sh) 2001(s) 1995(s) 1976(sh)
d
f
e
2388(vw) 2391(vw) 2462(*) 2462(*) 2462(*) 2341(*) 2341(*) 2341(*)
g
-
2462(*) 2341(*)
2119(vw) 2119(vw) 2119(vw) 2025(w) 1995(m)
2026(w) 1993(m)
2027(vw) 2028(vw) 1991(w) 1991(w)
Intensities of bands: (w) = weak, (m) = medium, (s) = strong, (vs) = very strong, (sh) = shoulder. Bands marked with asterisks are due to gaseous CO2 in atmosphere. a
0.6 O.'
K
0 re-heating at 75 O C o re-heating at 100 O C
a
o*21 0.1
- .
0
20
40 60 Re-heating time [min]
80
100
Figure 5. Amount of Mo(C0)6 as weight percent of molybdenum after sublimation and reheating, measured by XRF.
intensities of all bands decreased, but the bands assigned to subcarbonyl species disappeared at a faster rate. When samples were reheated at 100 "C (spectra not shown), the v(C0) bands due to physisorption were the only v(C0) bands to be seen after 15 min, and all the carbonyl bands had disappeared after 45 min. Samples were dark yellow to light brown after sublimation and darkened during the reheating, especially when reheated at 100 "C. Figure 5 shows the amounts of molybdenum determined by XRF. After sublimation there was 0.65 wt % molybdenum (equivalent to about 1.8 wt % Mo(CO)6) on silica. After reheating, the physisorbed Mo(CO)~had desorbed and only chemisorbed molybdenum remained. The amount of molybdenum remaining on silica, 0.1 wt % Mo (corresponding to 0.3 wt % Mo(CO)~),was almost 3 times as great as the amount given in the literature, 0.1% Mo(CO)~.~ Evidently the fluidized bed method is more effective than earlier methods' as a means of getting molybdenum as a hexacarbonyl compound onto the support. Carrier Gas. Nitrogen was used as a carrier gas except in a few experiments in which CO was used. Samples prepared under nitrogen and carbon monoxide flow gave similar IR spectra (not shown) like the spectrum 4a, immediately after Mo(CO)6 sublimation. Differences appeared after reheating, however: the bands of the sample prepared under CO flow were narrower than those of the sample prepared under nitrogen. As reheating was continued, the intensity of the carbonyl bands decreased and after 60 min reheating under CO at 100 "C only a weak band at 1991 cm-' could be seen. The failure at the same time to find any molybdenum by XRF may be explained by the better sensitivity of IR than XRF. This experiment showed that CO can re-form Mo(CO)~from partially decarbonylated species, probably from Mo(CO)3-5/
Si02. At 100 "C Mo(CO)~desorbs from the surface under CO flow. Samples remained white under CO but turned yellowish (or darker to brownish depending on the temperature) during heating under nitrogen. Likewise, samples prepared under CO slowly changed color from white to yellowish during a few weeks storage in the glovebox. On the basis of these results the nitrogen atmosphere seems to favor oxidation of molybdenum on silica.
Discussion Gaseous Mo(CO)~has an octahedral (Oh)structure. Molecules with o h symmetry have only one IR active v(C0) band (TI,)and two Raman active bands (AI, and Eg). In solution and in solid state, the symmetry of the molecule may become lower and the Raman active and other inactive bands may become IR active. The weak interaction between Mo(CO)~and a surface perturbs the symmetry of the molybdenum compound, and the IR spectrum of physisorbed Mo(CO)~can be assumed to be similar to the spectrum of Mo(CO)~in solution. The first interaction between a support and a carbonyl compound is physisorption.20 If adsorption site energetics are favorable, the interaction may then change to chemisorption, in which a bond is formed between the metal and the surface, and the carbonyl compound loses at least one carbonyl ligand. The missing ligand is replaced by an OH or 0- group of the support. On hydroxylated or mildly dehydroxylated supports, oxidation of the metal may occur when an OH group loses hydrogen during heating.15 A more detailed discussion of interactions between carbonyl compounds and oxidic surfaces is reviewed by Zecchina and Otero The carbonyl ligand has the strongest known trans effect (together with CN- and C2H4 ligands) and favors a substitution, in a trans position to itself, of some other ligand with weaker x-bonding ability. In addition, because of the strong z-acceptor ability of CO, further substitution of CO ligand by most other ligands occurs in the cis position to the first replaced ligand, due to the cis labilization e f f e ~ t . ~In' a hexacarbonyl compound with oh structure, three of the carbonyls have trans ligands. Thus three of the carbonyls in Mo(CO)~should be replaced quite easily and the most probable positions of the new ligands are facial. Substituted carbonyl compounds can be used as model compounds to help in the identification of supported subcarbonyl species. In this case the positions of the CO bands are less important than the number of bands and the shape of the IR spectrum. Provided the substituting ligands are heavy enough, the stretching of the remaining carbonyl ligands are not significantly perturbed. Turner et al.32found that the subcarbony1 species of hexacarbonyl have the highest possible symmetry.32 Applying the above knowledge we used model compounds to study subspecies Mo(CO),, x = 3-5. For Mo(CO)& and Mo(C0)3L3 to serve as model compounds, the replacing ligands have to be in a cis position to each other and in a fac position, respectively. cis-Mo(CO)4L2 has Czv symmetry when the ligands L are similar and C, when bonding properties of the ligands are different. The symmetry coordinates for C2" and C, molecules are presented in Figure 6a. As can be seen, the number of IR active coordinates does not change, so the only effects to be expected in the IR spectrum are some shifts in wavenumber. The wavenumbers are influenced by the different ligands and their bonding properties. The C3" symmetry of fuc-Mo(CO)3L3 with similar ligands L changes to C, symmetry when there are differences in the ligands
Molybdenum Hexacarbonyl Supported on Silica
A'
c,
t
A'
t
A'
A"
t
>Me
y G
>MX
A'
A'
A"
1
J. Phys. Chem., Vol. 98, No. 40, 1994 10241
1
Figure 6. Vibrational symmetry coordinates for (a) symmetries of CzV and C, and (b) symmetries of C3"and C,.
L. The E mode is degenerated in C3, symmetry, but when the symmetry changes to C, the degeneration splits into A' and A" modes. Due to this splitting the position of bands changes as well. As can be seen in Figure 6b, the symmetry coordinates of A1 in C3, and A' in C, are identical. The bands will be approximately at the same wavenumbers in the IR spectrum. However, some shift may occur depending on how much the ligands L differ from each other in the C, molecule. Mo(C0)sL. Marx et al.33 have isolated and characterized M(C0)5(en) complexes, where M = Cr, Mo, or W and en = ethylenediamine. Shapes of the IR spectra of the three complexes are similar, the only difference being in the position of the CO stretchings. For the molybdenum complex, the IR spectrum shows four bands (All, B1,E, and A12) at 2078 (w), 1992 (w), 1946 (s), and 1904 ( ~ h ) . ~ ~ After adsorbing Mo(CO)~onto Mo/SiOz, Louis et al.21found IR bands at 2132 (w), 2088 (w), 2032 (s), and 1775 (m) cm-'. The first three they attributed to terminal carbonyls in Man+OC-Mo(C0)5 (n < 6), and the band at lowest wavenumber (1775 cm-l) to the bridging CO. Mo(CO)~adsorbed onto pure silica, in contrast, gave only two bands in the IR spectrum (2020 and 1990 cm-'), due to physisorbed M o ( C O ) ~ . ~ ~ The shape of the spectra is similar for Mo(CO)s(en) and for Mo(CO)~supported on Mo/SiO2. There is one strong band which is assigned to the E mode of C4,, symmetry, and at higher wavenumbers there are two weak bands (AI' and B1). The A? mode is not reported by Louis et al.21and in the work of Marx et al.33it is observed as a shoulder. In the present study the strong band at about 2028 cm-l (Figures 2b, 3a,b, and 4a-c) may be due to E mode of Mo(CO)5. There is also a band, at 2090-2085 cm-l, which with reference to the work of Louis et al.21 suggests should be assigned to Mo(CO)5. Probably there are other stretchings in the same region as well, because the band at 2090 cm-' is quite broad. However, in our experiments there are no bands in the bridging carbonyl region, indicating a direct bond formation between molybdenum and surface oxygen. Molybdenum oxidizes during bond formation to the surface and changes color. The brownish color of the samples supports this interpretation.
cis-Mo(CO)Lz and fuc-Mo(CO)sLs. When the replacing ligands are the same or similar in their bonding properties or the two leaving CO ligands are replaced by a bidentate ligand, the symmetry of the carbonyl compound is C2,. Such a molecule has four IR-active CO bands (2A1, B1, and B2). The model compound cis-Mo(CO)4[P(OPh)3]2 has four bands in its IR spectrum. The very sharp and intense band at 2020 cm-' is assigned to A1 mode. Two other bands lie at 19651945 cm-' (A1 and Bl), and these are broader than the 2020 cm-l band. The B2 mode is resolved as a shoulder at 1941 cm-1.34 When two CO ligands are replaced by a bidentate ligand such as dithiahexane (DTH)34 or bis(dipheny1phosphine)methane ( d ~ p m )the , ~ IR ~ spectra continue to contain sharp and intense bands at about 2028-2020 cm-'. The B2 modes are now resolved at 1866-1860 cm-l but they are much broader than the All modes (2020 cm-'). The A12 and B1 modes are also very broad and only poorly resolved from each other. The bands appear at wavenumbers about 1925-1900 cm-1.34,35 Can0 et al.35and Asali et al.36have recorded the IR spectra of molybdenum carbonyl complexes in which three COS are replaced by a bidentate phosphine ligand and a nitrogen or phosphorus-bound ligand. In both studies the symmetry of the molecules is C,. The most intense band is now A' at higher wavenumber. The other A' and A" modes are also strong in intensity. Comparing the shape of our IR spectra with the spectra of model compounds suggested the assignment of the two sharp bands at 2119 and 2112 cm-' to A1 or A' mode. Exact assignment cannot be made since the 21 19 cm-l band may be due to A1 mode of either Mo(CO)4 or Mo(CO)3. When there are lots of OH groups on the surface of silica, as when the support is calcined under mild conditions, the intensities of A1 (or A') modes are not very strong. Decreasing the amount of silanol groups and thus increasing the amount of siloxane groups increase the intensities of bands at 2119-2112 and at about 2090 cm-'. After sublimation of Mo(CO)~at 75-100 "C with a gas flow of 30-160 " i n onto a support pretreated at 600 "C, the bands at 2119-2112 cm-l due to the A1 (or A') modes are the strongest bands in the IR spectrum. Likewise at 53 "C and slow (75 " i n ) gas flow, the bands at 2119-2112 cm-' are the strongest. This would indicate that at the higher temperatures, enough energy is available for the formation of more of those subspecies that give bands at 2119-2112 cm-l and also that the amount of sublimed Mo(CO)~is great enough to react with the support. Another explanation is that the E mode splits due to changes in symmetry into A' and A" giving bands at different wavenumbers. Hydrogen-bonded OC-Cr(C0)5 adsorbed onto a Bronsted acid site H+ gives a v(C0) band at about 50 cm-l lower than the other free carbonyl ligand.37 The probable effect of hydrogen bonding can also be seen for Mo(CO)&i02: the IR spectra show a shoulder at 1970-1965 cm-l. The interaction between carbonyl of Mo(CO)~and hydrogen of the surface silanol group is not very strong. During reheating, the shoulder at 1970-1965 cm-l disappeared due to the breaking of the hydrogen bond. Mo(CO)~may desorb from the surface or chemisorb and decarbonylate losing one or more CO ligands. In summary, we propose that, after sublimation of Mo(CO)6 onto silica, there are not only physisorbed species but partially decarbonylated species and perhaps Mo(CO)~which has hydrogen bonded to silanol. During chemisorption, oxidation visible as a color change occurs, and in that case at least one carbonyl ligand is replaced by oxygen of siloxane (-Si-0--
10242 J. Phys. Chem., Vol. 98, No. 40, 1994 Si-). Although the different species of partially decarbonylated Mo(CO)~cannot be characterized definitely, some proposals can be made with the help of model compounds. The sharp bands at 2119 and 2112 cm-' are due to A1 or A' mode of Mo(CO)4 or Mo(CO)3. Of these, the more probable subspecies is Mo(CO)3 because of the trans effect (see above). The band at 2090-2085 cm-' can be assigned to B1 mode of Mo(C0)s. There may also be other bands in that area, as suggested by the broadness of the band, but no suggestions can be made. The strong band at about 2035-2025 cm-' would be assigned to the E mode of Mo(C0)s. And the strong shoulder at 19751960 cm-' may be due to hydrogen-bonded Mo(CO)~. Physisorbed Mo(CO)6 gives bands at about 2118, 2023, and 1994 cm-', and these increase the intensities of bands due to subspecies, at the same wavenumbers. The differences in IR spectra of earlier studies, i.e., bands due to only physisorption, and in this study, i.e., bands due to physisorption and partially decarbonylated species of Mo(CO)~, are most probably due to the sublimation conditions. In this study the sublimations were made at 53-100 "C under inert gas flow. According to these results it seems to be possible to influence on the formation of subspecies with the inert gas pressure and sublimation temperature. The partially decarbonylated subspecies of Mo(CO)~on silica may provide new reactive centres in reactions and catalysis.
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