J. Phys. Chem. 1993,97, 6843-6852
6843
Infrared Spectroscopic Characterization of Molybdenum Carbonyl Species Formed by Ultraviolet Photoreduction of Silica-Supported Mo(V1) in Carbon Monoxide Clark C. Williamst and John G. Ekerdt' Department of Chemical Engineering, University of Texas at Austin, Austin, Texas 7871 2 Received: December 15, 1992; In Final Form: April 13, 1993
The molybdenum carbonyl species formed by ultraviolet photoreduction of Mo6+/Si02 in C O were characterized with Fourier transform infrared spectroscopy and temperature-programmed decomposition (TPDE). Mo6+/ Si02 samples containing 0.07-6.4% Mo were prepared from MoC15, Mo2(q3-C3H5)4,and ( N H ~ ) ~ M o ~ O ~ ~ ~ H Z O . Four molybdenum carbonyl species have been identified: mer-Mo4+(CO)3,cis-Mo4+(CO)2,linear Mo4+(CO), and Mo(CO)~.The stoichiometry of photoreduction, decomposition, and reoxidation supports the +4 oxidation state for the mono-, di-, and tricarbonyls. The CzUsymmetry of mer-Mo(CO)3 resulted in an I R spectrum consisting of a weak AI)^ symmetric trans C-0 stretch at 2181 cm-l, a strong B2 antisymmetric trans C-0 stretch at 2141 cm-l, and a strong AI)^ cis C-0 stretch at 2108 cm-l. The mer-Mo(CO)3 structure successfully predicted the observed frequencies and intensities of partially substituted M o ( ~ ~ C * ~ O ) , ( ~ ~and C~~O)~~, MO(~~C~~O),(~~C O 2). ) ~ , Mo(CO)3 was stable at 300 K with C O partial pressures above 60 Torr. ( x~=* 1, At 193 K, this species was stable under vacuum. Evacuation of mer-Mo(C0)3 at 300 K led to C O ligand loss, resulting in the sequential formation of cis-Mo(CO)Z, linear Mo(CO), and finally, CO-free Mo4+. The tricarbonyl assignment is supported by the TPDE pattern, in which two C O ligands were released near 350 K, followed by desorption of the final C O group near 440 K. The stable species are photoformed in the sequence merMo4+(CO)3, cis-Mo4+(C0)2,and Mo(CO)~. Photoformed Mo(CO), was found to both physisorb on silica and chemisorb on reduced molybdenum cations.
Introduction Oxide-supported molybdenum carbonyls have been prepared by direct adsorption of Mo(CO)~onto oxide supports and by thermal or photochemical reduction of supported molybdenum oxide catalysts. The adsorption of Mo(CO)~on oxides such as silica and alumina has been extensively studied with IR and temperature-programmed decomposition(TPDE) because of the catalytic properties of these systems.14 The molybdenum carbonyl species formed by thermals7 and photochemica1*-l4 reduction have primarily been studied to characterize supported molybdenum oxide catalyst systems. The use of the CO infrared stretching frequency in providing information about the oxidation state of the molybdenum cation adsorption site has recently been r e v i e ~ e d . Interpretation ~ of the IR spectra of molybdenum carbonyl speciesformed by thermal reduction has proved difficult because the spectra often consist of broad overlapping bands characteristicof a range of Mo oxidation states and environments. Photochemical reduction of Mo6+/SiOzin CO, on the other hand, has been found to result in molybdenum carbonyl species that display narrow peaks and are easily differentiated. Significant disagreement remains, however, in the interpretation of these spectra. The infrared frequencies and assignments of the most commonly observed molybdenum carbonyl species formed from molybdena/silica by UV photoreduction in CO are presented in Table I. Guglielminotti and Giamello reported the IR spectra of three photoformed molybdenum carbonyl species, designated by them as X,Y, and Z.8 The "X"bands at 2128 and 2080 cm-1 were assigned to MoZ+(CO)z, which was thought to be formed by reduction of a pair of molybdate tetrahedra. Evacuation of this dicarbonyl led to the formation of the =Y'' band at 2043 cm-l, which was assigned as a bridging CO group. Peaks at 2 140 and 2108 cm-1 were designated as "Z" bands. It was proposed that these bands resulted from a Mo4+dicarbonyl because continued
* Author to whom correspondence should be addressed.
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photoreduction transformed this species into the 2128- and 2080-cm-1 dicarbonyl on MoZ+. Gerasimovg also reported two sets of peaks, at 2142 and 2109 cm-I and at 2130 and 2085 cm-I. In agreement with others,8Jl a monocarbonyl band at 2050 cm-l was formed when CO was removed from the 21 30- and 2085-cm-1 dicarbonylspecies. These two species were proposed to form on isolated Moo4tetrahedra, because they were observed at low Mo loadings, where it was thought that Mo tetrahedra existed.15 The bands at 2142 and 2 109cm-l were observed at higher weight loadings, where surface molybdate chains were thought to exist.I5 Removal of the bands at 2142and2109cm-I byevacuationwasreported to havecaused the growth of a band at 2021 cm-1. It was proposed that these two species were formed on polymolybdate chains and that they were related in the same way as were the 2130- and 2085-cm-1 species ("X")and the 2050-cm-1 species ("Y"). A band at 1983 cm-l was found to increase in intensity with increasing Mo weight loading. Rodrigo et al." assigned the bands at 2135 and 2100 cm-1 to a M O ~ + ( C Ospecies )~ that could be photoreduced to the MoZ+(C0)z species, characterized by the 2125- and 2075-cm-I bands. Comparison with UVDRS results led to the conclusion that photoreduction occurred on dimeric surface molybdates. Results of the present study are also presented in Table I. The band at 2178-2181 cm-1 and the "Z" bands will be referred to as a bands. The bands at 2125-2130 and 2075-2085 cm-l will be referred to as 5, bands, and that at 2035-2050 cm-1 will be designated by 7. In previous studies,8-" the a bands at 21302142 and 2105-2109 cm-1 were assigned to molybdenum dicarbonyls. However, the spectrum of isotopically substituted a species was not reported, as had been done for the 8 species.8 As will be shown below, the IR frequencies resulting from partial substitutionof the a specieswith W16O or W18O are inconsistent with the Mo(C0)z model. Rather, a meridionally-coordinated molybdenum tricarbonyl model was found to best represent the spectroscopicdata. The temperature-programmeddecomposition pattern also supports the tricarbonyl stoichiometry. This paper addresses the IR spectroscopic identification of molybdenum 0 1993 American Chemical Society
6844
Williams and Ekerdt
The Journal of Physical Chemistry, Vol. 97, No. 26, 1993
TABLE I: Frequencies, Structures, and Transformations of Molybdenum Carbonyl Species Formed by UV Photoreduction of Mo(VI)/Si02 , ,. - in CO a species ref 2178-2181 cm-1 "Z" bands fl species, "X"bands y species, "Y" band 1982-1997 cm-I 2021-2030 cm-1 2181 cm-l. 2 140.2 108 cm-I. 2128.2080 cm-I. 2043 cm-I. n.r." n.r. 8 M~~+(CO)~; M&(C0)2; (Mo2+);CO; MO~+CO; formed on Mo(Td) formed on Mo(Td) bridging CO formed by thermal pairs, evacuation of 'Z" pairs by prolonged formed by reduction in H2 results in increase photoreduction of "Z" evacuation of 'X" in "X" 2142,2109 cm-I, 2130,2085 cm-l, 1983 cm-', 2021 cm-1, 2050 cm-I, 9 2178 cm-l, Mo~+(CO)~; MO~+CO; MO'+CO; MO~+(CO)~; MO~+CO; Mo C0; formed on molybdate formed on isolated formed on formed by formed by formed by CO chains Mo(Td) evacuation molybdate evacuation photoreduction aggregates of "Z" of "X" n.r. 1990 cm-l, n.r. 2 140.2 108 cm-I n.r. 10 2180cm-I, formed by CO Mo(C016; formed by CO photoreduction photoreduction n.r. 2135,2100 cm-I, 2125,2075 cm-I, 2035 cm-l, n.r. 11 n.r. Mo2+(C0)2; Mo~+(CO)~; Mo2+C0or formed on Mo pairs, formed on Mo pairs (Mo2+)2C0 from photoreduction removed by prolonged linear or bridging photoreduction,"Z" of "Z" CO, formed by more readily evacuated evacuation than "X" of "X" 2141.2108. 2128.2080 cm-l. 2045 cm-l. 1982-1997 cm-l. 2030 cm-1. this 2181 cm-l. study mer-Mo4+(CO)3; mer-M~i+(CO)~; M~~+(CO)~; ~04+(co); MO(CO)~; Mo(C0)s; (&)I mode B2 and AI)^ modes formed by evacuation formed by adsorbed on adsorbed on or prolonged UV evacuation silica, two other reduced Mo sites exposure of of MO~+(CO)~ modes observed '
+
'
MO~+(CO)~
* Not reported. reached 423 K during a 20-min photoreduction. Samples were photoreduced between 0.5 and 120 min. The gas phase was not monitored during the isotope experiments to determine if isotopic scrambling occurred. After photoreduction, spectra of adsorbed molybdenum carbonyl species were obtained either at room Experimental Section temperature with subtraction of the CO gas phase and silica Infrared Studies. Mo6+/Si02 samples were prepared from background or a t temperatures as low as 127 K with the surface MoC15, Mo*($-C,H5)4, and AHM ( ( N H . & M o ~ O ~ ~ - ~ H as~ O ) molybdenum carbonyl species under helium purge so that only described elsewhere.16 Transmission infrared (IR) spectra were the silica background required subtraction. obtained from 100 scans (except where noted) at a resolution of Flow Experiments. A flow-through apparatus was used to 2 cm-1 with a Digilab FTS-15/90 Fourier transform infrared determine the amount of carbon dioxide generated during (FTIR) spectrophotometer. Three different IR cells were used: photoreduction, the temperature-programmed desorption pattern (1) a stainless steel cryogenic cell capable of cooling to 193 K, of the adsorbed carbon monoxide, and the amount of oxygen which used self-supporting sample wafers; (2) a stainless steel necessary to reoxidize the sample after photoreduction. Two cryogenic cell capable of cooling to 127K, which employed catalyst different quartz U-tubes were used to contain the samples. A 6 pressed into a tantalum mesh; and (3) a stainless steel cell coupled X 4-mm tube was used for samples of 50-75 mg and a 12 X to a quartz furnace, which also employed self-supporting wafers. 10-mm tube was used for samples from 0.5 to 2.0 g. The quartz The cryogenic cells and the IR spectrometer have been described U-tubes were located inside the oven of a Varian 3700 gas previously.17 The meshes and self-supporting wafers contained chromatograph (GC), capable of heating to 773 K and cooling 40 to 160 mg/cm2 of powder and were pressed under 7500 lbs/ to 173 K. The U-tube effluent was analyzed a t intervals of 1, in.2 for 20 s. Samples were calcined at 673 K for 10 min and 1.5,2, or 3 min with a thermal conductivity detector in a Hewlettevacuated while cooling for 1 h in the two cryogenic cells or were Packard 5880A GC. calcined in the quartz/steel cell a t 873 K for 30 min and evacuated Photoillumination was performed with the mercury lamp at 773 K for 15 min. All three cells were capable of flow-through positioned approximately 25 cm from the sample U-tube, just gas treatments and evacuation to 1 X 1WTorr. After evacuation short of the focal length. Unless specified below, the lamp was and cooling, CO was admitted to the cells for photoreduction at operated a t 100 W. The minimum UV wavelength was limited pressures ranging from 20 to 760 Torr. by the quartz U-tube or by Pyrex filter plates. Room temperature photoillumination was performed with a Samples were pretreated by heating for 1 h at 773 K in a 30% Photon Technology International (PTI) high-intensity arc lamp O2/He mixture. The samples were purged in He for 20 min (02-Al010Q) and power supply (02-LPS200) with a 100-W while cooling to room temperature and were then photoreduced mercury lamp. The broad-band optical output was approximately from 3 min to 6 h in pure CO or in CO diluted with helium. 12 W. The minimum UV wavelength was limited by quartz Samples were then cooled to 173 K in pure CO and purged with plates to 180 nm, by KBr optics to 220 nm, or by Pyrex plates He for 15-20 min. TPDE was performed while ramping the to 280 nm. The lamp was placed 50 cm from the IR wafers samples a t rates between 1.5 and 15 K/min under 22-50 cm3/ (approximately twice the focal length) to reduce the intensity. min (STP) of He. The UV beam was passed through a water filter 10 cm long to minimize sample heating by removal of infrared radiation. Even For some samples, the amount of CO2 formed during phowith this water filter, a thermocouple pressed into a sample wafer toreduction was compared with theamount of 0 2 consumed during carbonyls formed by UV photoreduction of Mo6+/Si02 in CO and the transformation of these species with temperature, CO partial pressure, and photoreduction.
Characterization of Molybdenum Carbonyl Species reoxidation. After photoreduction, these samples were either heated in helium to rapidly remove CO or cooled, purged, and heated to measure the amount of adsorbed CO, as described above. Both CO removal procedures resulted in similar patterns of oxygen consumption. After CO removal, the cooled samples were reoxidized with 15.2 cm3/min of a mixture of 0.63% O2in helium. Oxygen uptake at room temperature and subsequent oxygen desorption during the temperature ramp were measured. When the UV lamp was turned off after photoreduction, the C02 signal at the GC dropped to zero within 2 min. The presence of C02 on the photoreduced samples was tested by ramping a sample to 773 K immediately after photoreduction. No C 0 2 desorbed. At attempt was made to photoreduce Moo3 and CaMoOd, crystalline compounds found to occur commonly on Mo/Si02 samples.16 These compounds were not photoreducible; no C 0 2 could be generated under the conditions used in this study, i.e., X > 180 nm. Materials. Carbon monoxide (l2CI60,Matheson UHP 99.9% or Liquid Carbonic research grade 99.99%) was first passed through a bed of molecular sieves at 473 K to decompose iron carbonyls and then passed through water and oxygen traps (Scientific Glass Engineering (SGE)) and finally through a bed of molecular sieve 4A (W. R. Grace) to remove carbon dioxide. (98.6 13C160(99.1 atom % 13C and 14.4 atom % 180) and atom % I3C and 96.6 atom % l80) were used as received from Isotec Inc. Oxygen (Liquid Carbonic 99.995%)and 2.00%oxygen in helium (Lindecertified) were used as received. A0.63% oxygen in helium mixture was prepared from UHP oxygen for the reoxidation results presented here. The oxygen composition was calibrated against the 2.00% Linde certified mixture. Helium (99.99+%, UT Physics Dept.) was passed through an oxygen trap (SGE), and dry air was passed through a water trap (SGE).
The Journal of Physical Chemistry, Vol. 97, No. 26, 1993 6845 2141
11
4 2 2 8
I
2250
2200
I 2150
G/
I 2100
I 2050
I 2000
1950
WAVENUMBERS Figure 1. FTIR spectra of mer-Mo(CO)s. (a) Sample scanned at 198 K under He purge. Photoreduced in 771 Torr of CO for 14 min (7 min each side) through 3 mm of a Pyrex filter. Sample was 5.0% Mo/SiOl prepared from AHM on Cab-0-Si1EH-5. (b) Same sample scanned at 298 K in 486 Torr of CO. CO gas phase subtracted. Photoreduced in 760 Torr CO for 50 min (25 min each side) through 13 mm of a b e x
Results
filter.
Although the j3 and y bands will be discussed in subsequent subsections, their assignments are given here to facilitate interpretation of the a bands. We are in agreement with previous workers in assigning the j3 bands to a molybdenum dicarbonyl species.s+'Jl Our assignment of the y species as a linear molybdenum monocarbonyl is different from that of others.8.1' a Bands. When Mo6+/Si02 samples undergo UV photoreduction in CO, the first bands observed occur at 2181,2141, and 2108 cm-1 (Figure 1). These peaks are most readily observed after relatively short photoreductiontimes, at higher Mo loadings, and at CO partial pressures above 200 Torr. Continued photoreduction8.11 of the a species, or evacuation of the CO gas phases exposed to the a species, causes formation of the j3 bands at 2128 and 2080 cm-l (Figure lb). Further evacuation results in the removal of the j3 bands and in the formation of the y band at 2045 ~m-l.~.9Jl The peaks at 1997 cm-l (Figure la) and 1990 cm-l (Figures l a & b) are due to physisorbed Mo(CO)~.It has been proposed elsewhere6 that the weak peak at 2030 cm-l (Figure la) results from Mo(CO)~chemisorbed on reduced molybdenum sites. The peak at 2064 cm-I (Figure la) is caused by a linear Moe+(CO) species (t unknown).18 This peak was only observed on samples containing more than 2% Mo. It has been proposed*-9Jlthat the peaks at 2141 and 2108 cm-1 belong to the same dicarbonyl species because they always occur with the same intensity ratio, regardless of Mo content or degree of reduction. In this study, an additional IR peak at 2181 cm-1 was found toaccompany thepeaksat 2141 and2108 cm-I (Figure 1). At room temperature, thea species decomposed in theabsence of a CO gas phase. Subtraction (performed by the IR computer) of the CO gas phase from the IR spectrum of the a species causes noise that obscures the peak at 2181 cm-l (Figure lb). The signal to noise ratio of the 2181-cm-1 peak can be improved by
scanning at low temperature (below 220 K), where the a species was stable in the absence of gaseous CO (Figure la). Althoughthepeakat 2181 cm-Ialwaysaccompanied thepeaks at 2141 and2108 cm-1, therewasachangeintheapparentintensity ratios between the 2181- and 2141-cm-l peaks during removal of the a species. For example, when a photoreduced Mo/SiOz sample, whichdisplayed the peaks at 2181,2141, and 2108 cm-l, was heated from 200 to 310 Kin a helium purge, the 2141-cm-1 band underwent a 5-fold decreasein intensity. During this heating process, theintensityratioofthe2181-cm-l peakto the2141-cm-I peak decreased from 0.064 to 0.049, approximately 23%. When a sample containing the 2181-, 2141-, and 2108-cm-l peaks was evacuated at room temperature (as in Figure 5), the intensity ratio of the 2181-cm-1 peak to the 2141-cm-1 peak decreased from 0.064 to 0.055, approximately 14%. This apparent change in the intensity ratio of the 2181- and 2141-cm-l peaks during removal of the a species is caused by an increasing contribution to the 2141-cm-1 peak from the 2128-cm-1 peak of the cis-Mo(C0)z species, which forms from the a species. The assignment of the three a peaks to the same species is supported by the fact that they wereobserved togetherwithessentiallythesameintensity ratios at temperatures from 173 K to 330 K, at CO partial pressures from 35 to 760 Torr, at Mo loadings from 0.4% to 6.4% Mo, and for Mo6+/SiO2 samples made from all three precursors: MoCls, Mo2($-C,Hs)4, and AHM. The four other molybdenum carbonyls occurred in vastly different ratios as the Mo loading increased and as the carbonyls were decomposed by heating or evacuation. Guglielminottiand Giamellos attributed the band at 218 1cm-l to CO adsorbed at a thermally reduced Mo site, which formed during the hydrogen prereduction step at 823 K. Other researchers, in addition to those in the present study, have found
Williams and Ekerdt
6846 The Journal of Physical Chemistry, Vol. 97, No. 26, I993
7r
TABLE II: Observed a Bands and Predicted Infrared Frequencies for cisMo(CO)z, c i s M o ( C 0 ) ~ cisMo(CO)z(CO)', and mer-Mo(CO), Substituted with ' Z C W and 1jC160. predicted ~
cis-
observed 12C160 2181 (w) 2141 (s)
21 6 9
cis-
2141 (s)
2141 (s)
2169 2160
(2141 0.92
;:!,
~
2120
2080
2040
12108 1.00 2064 1.01
2169 ,0.14
:E CL. , : : ,:ll6,7 ~
, ; : :E
2115 0.67
:;I
2141
2141
2131
2130
21 14 2108
I
: :o: 20631 0.96
:oCq
2108
2181 2179b 2169 2167
2141 2134
2141
2142 2141
2131
2131
2116 2108 2107
2115 2108 2106 2100 2099
2072
that this band can be formed by UV photoreduction in CO,9JO without the thermal prereduction of Mo6+/Si02 in H2. It should be noted that a number of bands have been reported near 2180 cm-l for CO adsorbed on reduced molybdenum supported on alumina, titania, and ceria at temperatures as low as 77 K.5 Ghiotti et al.19 have assigned peaks at 2155 and 2133 cm-1 to CO physisorbed on silica at 77 K. In this study, no bands other than the 2181-cm-1 band (and a very weak band, which occurred irreproducibly at 2205 cm-I) were observed on photoreduced samples cooled in CO to 127 K. Figure 2 presents the IR spectrum of partially substituted CY species prepared by photoreduction of Mo6+/Si02 in a mixture of 49% W I 6 0 and 5 1% l3C16O. The spectrum displays a weak, broad band near 2169 cm-I; two sharp intense bands at 2141 and 2063 cm-1; and a cluster of intense bands from 2114 to 2094 cm-1, with discernible peaks at 21 14,2108,2100, and 2094 cm-1 anda shoulder at 2 104cm-I. Spectra of Mo6+/Si02photoreduced with different W60and W 1 6 0 mixtures show the same bands with different intensities, as well as additional bands reported in Table 11. The spectrum of Mo6+/Si02 photoreduced in a mixture of 50.1% K ! I 6 0 and 49.9% I3Cl8O is presented in Figure 3. This spectrum consists of a weak broad band around 2169 cm-I and four sharp peaks at 2141,2108,2043, and 2010 cm-I. The peak at 2043 cm-I has shoulders at 2052 and 2061 cm-l. Spectra of the CY species partially substituted with I3Cl8O, obtained by photoreduction in 90.9%, 78.3%, and 33.4% 13C180,are presented in Figure 4. The 2080-, 2043-, and 2010-cm-I peaks of the CY species substituted completely with 13C180are prominent in the spectrum obtained by photoreduction in 9.1% I2C16Oand 90.9% I3CI*O(Figure 4a). Inaddition, peaksdue to partially-substituted aspeciesappearat2169,2159,and2121 cm-1. Thereisashoulder centered near 2111 cm-I on the 2121-cm-I peak, and several
2108
2104 2100
Figure 2. Top: observed FTIR spectrum of m e r - M 0 ( W ~ O ) ~ ~ ( l ~ C ~ 6 0 ) , obtained by photoreduction in 813 Torr of 49% I2CL6O and 51% 13C160. Spectrum was obtained at 213 K under He purge. Sample was 5.0% Mo/SiO2 prepared from AHM and photoreduced for 10 min (5 min each 2063 side) through a 3-mmPyrex filter. Bottom: predicted IR frequencies I3c160 and intensities. Black dot represents l3CI6O.
:09q
2181 2178 2169 2166 2143
2094
L
2181 (w) 2141 (s)
2162
2094
O
2181 (s) 2141 (s)
2125
I Iio:;
2106, 0.38
mer-
Mo(CO)~ Mo(C0h Mo(C0)dCO)' Mo(C0)p
218i 2200
cis-
~ _ _ _ _ _ _ _ _ _
2131 (w) 2094 (s) 2063 (s) a
2094 2077
2097 2096 2094
2094
2068 2063
2063
2064 2063 2062
2064 2063
2094 (9) 2063 (s)
2094 (s) 2063 (s)
2131 (s) 2094 (s) 2063 (s)
2131 (w) 2094 (s) 2063 (s)
All values in cm-'.
* Band not observed; a low intensity is predicted.
shoulders appear on the high-frequency side of the 2043-cm-1 peak. A very weak band is discernible at 2141 cm-I. An increase in the fraction of 12C'60in the reducing mixture to 21.7% leads to an increase in the intensity of peaks at 2141 and 2108 cm-1 (Figure 4c). In addition, a peak has developed at 2061 cm-1, which becomes more prominent in Figure 4c, for a sample photoreduced in 66.6% l2CI6O. The contribution from the Mot+(I2C16O)peakat 2064 cm-l (Figure la) is minimal in this spectrum (Figure 4c), because there is no corresponding peak at 1967 cm-1 due to M O ~ + ( ' ~ C ~The ~ O )2121-cm-1 . peak now appears only as a shoulder on the 2108-cm-' peak (Figure 4c). Other bands resulting from substitution with l3CI8Oare reported in Table 111. B Bands. In agreement with others,8J1 it has been found in this study that prolonged photoreduction of the CY species leads to the p species, a molybdenum dicarbonyl species characterized by two strong peaks at 2128 and 2080 cm-1 (Figure 1b). The IR spectrum (not shown) of M O ( ~ ~ C ~ ~ O ) ( obtained ~ ~ C ~ ~byO ) , photoreduction in 760 Torr of 52% W 1 6 0 and 48% I3C160, was found to display intense peaks at 21 14 and 2047 cm-I. Mo(C0)z was also formed by submitting Mo(CO)~to evacuation at room temperature (Figure 5). As the CO pressure over Mo(CO)~was gradually reduced (approximate time for complete experiments: 1 h), the Mo(CO)~band at 2141 cm-I (and those at 2108 and 2181 cm-1, not shown) was replaced by the 2080-cm-1 (and 2128-cm-1, not shown) band of Mo(C0)z. Evacuation below 180-60 Torr of CO, depending on the Mo/Si02 sample, led to a rapid drop in the intensity of the 2141-cm-I band. As the CO pressure decreased, the peak height of the 2080-cm-I mode of Mo(C0)2 increased from 50 to 100 Torr, where Mo(C0)z
Characterization of Molybdenum Carbonyl Species
The Journal of Physical Chemistry, Vol. 97, No. 26, 1993 6847
IT
2200
7-
2160
2120
2080
2040
2178 2014 0.96
21 6 6
J 4
2000
;;::1 I:
1,E;
J-
1
2043
0.19
2111 0.93
I
I
2054 0.29
~-
21 6 3 ,0.16
2058 0.31
2080, 0.07
I
20431 0.92
201 2 0.95
:O;tl
J l
A Figure3. Top: observed F T I R s ~ t r u m o f m e r - M 0 ( ~ ~ C ~ ~ O ) ~ - ~ ( ~ ~ C ~ ~ 0 ) ~
obtained by photoreduction in 775 Torr of 50.1% lZcl6O and 49.9% 13C180.Spectrum was obtained at 193 K under He purge. Sample was 5.0% Mo/SiOz prepared from MQ($-C~H~)~ and Cab-O-Sil and was photoreduced for 6 min (3 min each side) through a 3-mm pyrex filter. Bottom: predicted IR frequenciesand intensities. Black dot represents 13~180.
decomposed as described below. After Mo(CO)~was evacuated to Mo(C0)2 at room temperature in the IR cells, or after all three CO groups were removed from MO(CO)~ by TPDE or room temperature evacuation,repopulationof Mo(C0)3 by readmission of CO was observed, when extreme care was taken to avoid O2 contamination of the CO. y Band. Evacuation of Mo(C0)z at 298 K (as illustrated in Figure 6) or decomposition of Mo(CO)3 at 340 K under an inert purge led to the formation of Mo(CO), characterized by a single IR peak at 2045 cm-l. (This peak has also been reported at 2043 cm-l,8 2050 cm-l,9 and 2035 cm-l.ll) Initially (Figure 6), the peak height of the 2080-cm-l band of Mo(C0)2 was nearly 6 times greater than that of the 2045-cm-1 band of Mo(C0). The IR cell was then evacuated for 1 s, and the IR spectrum was recorded (requiring 2 min). This was repeated several times at increasing time intervals. After 2 s of total evacuation time, less than 1 Torr of CO remained in the cell. Evacuation first lead to a rapid decrease in the intensity of the 2080-cm-1 peak (as well as that of the 2128-cm-l peak, not shown) accompanied by an increase in the 2045-cm-I peak. After evacuation for a total of 30 s, the 2045-cm-1 peak also decreased in intensity. Continued room temperature evacuation of Mo(C0) led to the complete removal of the CO ligand for samples in IR cells. Readmission of CO to the reduced Mo/SiOz wafer, after complete or partial removal of CO from Mo(C0)2 or Mo(CO), led to repopulation of the Mo(C0)2 species. Mo(C0)h Prolonged photoreduction,or photoreduction with wavelengthslower than 280 nm, led to the formation of an intense broad band near 1990 cm-1 (Figure lb), which is assigned to physisorbed Mo(CO)~. At lower temperatures the mode of interaction with the surface changes, resulting in a higher frequency band at 1997 cm-1 (Figure la).l8 Room temperature
00
2150
I
I
2100
2050
2000
l!50
WAVENUMBERS
Figure 4. FTIR spectra of mer-M0(~2C~60)r,(~~C~80), obtained by photoreduction in (a) 9.1% W 6 0 and 90.9% W8O, (b) 21.7% W 6 0 and 78.3% l3C180, and (c) 66.6% W160and 33.4% 13C180. Spectra were obtained at 193 K under He purge. Sample was 5.0% Mo/SiOz prepared from MQ(#-C~H~)~ and was photoreduced for 6 min (3 min each side) through a 3-mm Pyrex filter under a total pressure CO of (a) 773, (b) 807, and (c) 828 Torr.
evacuation of the a,8, and y species resulted in the spectrum presented in Figure 7, which consists of peaks at 21 18,2028, and 1990 cm-1. TemperaturePmg”d Decomposition. TheTPDEpatterns of Mo(CO)~and Mo(C0)2 are presented in Figure 8. Experimental conditionswere employed to avoid Mo(CO)~formation.’* A 2.0-g sampleof 1.7% Mo/Si02, prepared from Mo2(q3-C3H5),, was photoreduced in 80 cm)/min of 50% CO (balance He) for 15 min without a UV filter. After cooling to 173 K and He purging, the sample was ramped at 5 K/min under 39 cm3/min of He, resulting in CO peaks at 350 and 440 K (curve a of Figure 8). Deconvolution of these peaks results in an area ratio of 2: 1. For low degreesof UV exposure,a similar 2:l pattern was observed universally, regardless of precursor and Mo weight loading. Only with increasing degrees of photoreduction, favored by the absence of UV filters, by extended photoreduction time, and by low Mo loadings did the TPDE pattern change. With prolonged exposure, the area of the second peak in the TPDE pattern was found to approach that of the first peak. In curve b of Figure 8, the areas of the two CO peaks in the TPDE pattern are nearly equal. This pattern was obtained after 4 h of photoreduction of a 60-mg sample of 1.54% Mo/SiOz prepared from M02(qWjHg)d. Photoreduction was performed in 10 cm3/ min of pure CO with a 5-mm Pyrex filter. The sample was ramped at 15 K/min with 22 cm3/min of He. The CO peaks occurred at 325 and 445 K. This 1.54% Mo/Si02 material produced a TPDE pattern with a 2:l peak ratio similar to that in curve a of Figure 8 when photoreduced for shorter time periods (for a 500mg sample reduced less than 1 h). The change in TPDE pattern, such as that depicted in Figure 8 with increasing extent of photoreduction, is accompanied by a change in the number of CO ligands associated with the reduced molybdenum cations. The degree of photoreduction can be
6848 The Journal of Physical Chemistry, Vol. 97, No. 26, 1993
Williams and Ekerdt
TABLE III: Observed a Bands and Predicted Infrared
W 6 0
2181(~) 2141 (s) 2108 (s)
12C160/13C180
-
-
2141 ( 8 ) 2108 (s)
2141 (s) 2108 (s)
2181
2181 (s) 2141 (s) 2108 (s)
2181 (w) 2141 (8) 2108 (s)
2181 2176
2181 21786
2166 2162
2166 2163
2141
2141
200
0
400
600
BOO
30
Frequencies for cisMo(C0)g cis-Mo(CO), and me~Mo(CO)3Substituted with ~SMO(CO)~(CO)', WW and W W predicted
a)
.
80>
2169 2159 2141
2141
2141 2132
2121 2111 2108 2080
2122 2108
2108
206 1 2052 2043
2043
2043 2027
2112 2108 2080 2062 2057 2050 2043
2122 2111 2108 2080 2067* 2058 2054 2043
2022
2010
2010
2010
2080(~) 2043 (s) 2010 (8)
-
-
2043 (8) 2010 (s)
2043 (8) 2010 (s)
a All
200
0
2017
ISC180
I
n '
2125
2127
2013 201 1 2010
2014 2012 2010
2080 (s) 2043 (s) 2010 (9)
2080 (w) 2043 (s) 2010 (s)
values in cm-'. Band not observed, a low intensity is predicted.
expressed in molecules of C02 formed during photoreduction divided by the number of molybdenum atoms in the sample. For lower degrees of photoreduction (C02/Mo < 0.5), three CO molecules desorb during TPDE for each C02 formed during photoreduction.'* The CO/CO2 ratio was 3.05 and the COz/ Mo ratio was 0.33 for the TPDE spectrum in Figure 8a. As the C02/Mo ratio approaches 1 (Mo average oxidation state +4), the CO/CO2 ratioapproaches 2-16Concomitantly,the area ratios of the first and second TPDE peaks decrease from ca. 2:l and approach ca. 1:1. Reoxidation of Photoreduced Mo*c/SiOz. Reoxidation of photoreduced, CO-free Mo cations occurred in a two-step process.18 At room temperature, oxygen uptake occurred and the green samples became tan. When the samples were then heated, oxygen began to desorb at about 90 OC. The rate of O2 desorption peaked and the samples changed from tan to white at approximately 315 O C . The net oxygen uptake was determined by subtracting the excess oxygen desorbing at elevated temperature from the amount adsorbed at room temperature. The net oxygen uptakes are within 10% of the amount of C02 formed during reduction (Table IV). The reduction of C02, previously found to occur at 90 OC for photoreduced Mo/Si02,14 can be ruled out as the source for this excess oxygen because, as discussed in the Experimental Section, virtually no C02 was present on these samples after photoreduction. A precedent for molybdenum cations adsorbing excess oxygen is the Mo5+/02- system, observed with electron paramagnetic resonance by Che et a1.20 Discussioo Mo(C0)s The a peaks are assigned to a tetravalent molybdenum cation with three CO ligands distributed meridionally,
400
800
800
CO PRESSURE (Torr) Figure 5. Transformation of mr-Mo(CO), (2141 cm-l) into cis-Mo(C0)z (2080 cm-l) by isothermalevacuation. (a) Samplewas 1.7% Mol Si02 prepared from M@(S-C!~HS)~ and was photoreduced in 660 Torr of CO without Pyrex filters for 2 min. (b) Sample was 5.0% Mo/SiOz prepared from Mq(q3-C3H,)d and was photoreduced in 740 Torr of CO for 2 min (1 min each side) with a 2.5-mm Pyrex filter. (c) Sample was 5.0% Mo/SiOz prepared from AHM and was photoreduced in 690 Torr of CO for 50 min (25 min each side) through 13 mm of Pyrex filter. IR spectra were recorded using 50 scans. -18 0
15 X
-12
w
0
z g $ 6 0
3 3 < 0
20 30 40 50 60 70 80 EVACUATION TIME (s) Figure 6. Transformationof cis-Mo(C0)z (2080 cm-')into linear Mo(CO) (2045 cm-1) by isothermal evacuation. Sample was 0.41% Mo/ Si02 prepared from MoCl8 and photoreduced in 40 Torr of CO for 50 min through 5-mm Pyrex filters. IR spectra were recorded using 50 scans. 0
10
mer-M04+(CO),. This assignment is based on (1) the presence of three bands in the unsubstituted IR spectrum; (2)the relative intensities of these bands, one weak and two strong; and (3) the ability of the mer structure to predict the IR frequencies and intensities after partial substitution with isotopes of CO. The Mo oxidation state of mer-MoM(CO)3 was determined by the stoichiometry of its formation and decomposition, which also supports the CO/Mo ratio of 3. To interpret the substitution data, the following molybdenum carbonyl structures have been considered: (1) cis-L4Mo(CO)z (or cis-LzMo(CO)z) of symmetry C%, (2) cis-L3Mo(CO)3 of
Characterization of Molybdenum Carbonyl Species
rgO
T0.125
I \
1 & 2Yl \ I
I
I
2200
I
I
2100
I
I
I
I
1800
1900
2000
WAVENUMBERS
Figure7. IR spectra of photoformed Mo(CO)6adsorbed on silica. Sample was 5% Mo/SiOZ prepared from AHM. Photoreduced for 50 min in 700 Torr of CO without a Pyrex filter. CO, Mo(CO)3, Mo(C0)2, and Mo(CO) were evacuated for 5 min at 298 K. Cell was back-filled with air and re-evacuated for 1 min before scanning at 298 K. 30000 25000
B\
20000
15000
150
200
250
300
350
400
450
500
550
600
TEMPERATURE (K)
Figure 8. Temperature-programmeddecomposition of Mo(C0)p (curve a) and Mo(C0)z (curveb). MO(CO)3was formed by photoreduction in 80 cm3/min of 50% CO in helium for 15 min. The sample was 1.7%
Mo/Si02 prepared from Mo2(q3-C3Hj)4. Ramping rate was 5 K/min,
and carrier was 39 cm3/min of He. No Pyrex UV filters were used. Mo(C0)z was formed by photoreduction in 10 cm3/min of pure CO for 4 h with 5 mmof Pyrex filters. The sample was 1.54% Mo/SiOzprepared from Mo~($-C~HS)~. Heating rate was 15 K/min with 22 cm3/min of
He.
TABLE I V Oxygen Adsorption and Des0 tion during Reoxidation of Photoreduced, CO-Free Mo?atioW
co2
formed during 0 adsorbed 0 desorbed net 0 exccss Mo photoreduction at 298 K by heating consumed O/Mo4+ 86.5 356.8 356.8 356.8 a
15.4 38.9 39.4 34.4
29.7 62.6 63.8 66.0
14.4 25.5 20.5 29.1
15.3 37.1 43.3 36.9
0.94 0.66 0.52 0.85
Values in first five columns are pmol.
symmetry C3”, (3) mer-L3Mo(C0)3 of symmetry CZ, (4) cisL3Mo(C0)2(CO)’ of symmetry C,, and (5) distorted trans-12Mo(CO)~of symmetry C,.18 These compounds were modeled assuming nonmechanical coupling between the CO groups.21 Frequency positions and intensities were predicted using the GF matrix approach.22-26 A summary of the predictions is presented in Tables I1 and I11 for I3C16Oand 13C180substitution, respectively. Only the mer-
The Journal of Physical Chemistry, Vol. 97,No. 26, 1993 6849 L ~ M o ( C O )structure ~ fits the experimental data. A detailed discussionof the calculationsand of the stretching and interaction parameters can be found elsewhere.18 Here we focus on the key features of these calculations. The Mo(C0)2 and cis-Mo(C0)~models failed to predict the observed spectra resulting from partial substitution with 13Cl6O or 13C180. Monosubstitutionof cis-L4Mo(CO)2to M O ( ~ ~ C ~ ~ O ) ( l V 8 O ) should lead to two intense peaks, which are predicted at 2127 and 2022 cm-l. Intense peaks were observed at 2141, 2043, and 2010 cm-’ (Figure 3). Similarly, monosubstitution of cis-LpMo(CO)p with W80would cause peaks at 2132, 2108, and 2017 cm-1, and disubstitution would result in peaks at 2122, 2027, and 2009 cm-l. There is a peak at 2120 cm-1 in the spectra of a! substituted with 90.98 W180(Figure 4a). However, the peaks predicted at 2132,2017, and 2027 cm-’ are clearly absent from the observed spectra (Figures 3 and 4). Another failure for both the Mo(C0)2 and cis-Mo(C0)p models is that they fail to predict any bands at frequenciesgreater than 2141 cm-’, whereas in the observed spectra substitution causes an increase in the number and intensity of bands above 2141 cm-1. The cis-L3Mo(CO)z(CO)’ model represents the case where one of the CO ligands is different from the other two. There are then two stretching parameters (k and k’) and one interaction parameter (i).22 Mathematically, this model would be identical to the mer model if its interaction parameters, k, and k,, were set equal. The mer parameters determined from the data yield a k,/k, ratio of 1.26. The cis-L3Mo(C0)2(CO)’ model predicts frequencies that fit the substitution data better than all of the models considered, except for the mer model. However, the cisL3Mo(C0)2(’CO) model can readily be ruled out, because of its three unsubstituted modes would be expected to occur with similar intensities, contrary to observation. Occasionally,a peak approximately 10 times lower in intensity than the 2181-cm-l peak appeared near 2205 cm-1 with the 01 peaks (see Figure la). A distorted rrans-L2Mo(CO), model was developed,18 by analogy with cis-L2Mo(C0)4,21 which could account for peaks at 2205, 2181, 2141, and 2108 cm-1. This model could not predict the frequencies resultingfrom substitution, and the tetracarbonyl stoichiometry was not supported by the C02/C0/02 uptake and desorption measurements. The observed spectra and predicted peak positions and intensities for m e r - M 0 ( ~ ~ C ~ ~ O ) ~ - ~ and ( ~ ~merC’60), M O ( ~ ~ C ~ ~ O ) ~ are ~ (presented ~ ~ C ~ in ~O Figures ) . 2 and 3 and in Tables I1 and 111. Mer-Mo(CO), can be described with two stretching parameters, trans (k2) and cis (kl), and two interaction parameters, rrans-trans (k,) and cis-trans (kc) (Figure 9). Because there are four vibrational parameters and only three nonsubstituted infrared bands, an additional substituted band is required to determinethe four parameters. Rather than determine the vibrational parameters with just one substituted mode, the parameters were chosen to best fit all of the observed substituted bands. Following the established procedure,22 three of the stretching parameters, k2, k,,and k,, were expressed in terms of the fourth parameter, kl. The cis stretching parameter kl was varied to fit, by least squares, as many as could be observed of the 10 nonredundant bands resulting from the four substituted species (5 of the 10 bands for W60substitution and 6 of the 10bands for W 8 0 substitution were observed). The parameters that best predicted the frequencies of 13C16O- and W18Osubstituted mer-Mo(C0)3 are kl = 1805 N/m, k2 = 1882 N/m, kt = 30.0 N/m, and k, = 23.9 N/m. The error analysis for these parameters is given below. In addition to accurately predictingthe frequenciesof partiallysubstituted a! species, the mer-Mo(C0)~model also predicts the relative intensities of both unsubstituted and substituted species. With the assumption that CO ligands occur on photoreduced molybdenum at nearly regular positions of tetrahedral or octahedral coordination spheres, the only geometry consistent
6850 The Journal of Physical Chemistry, Vol. 97,No.26,1993 Mo(C0) 3 STRETCHING PARAMETERS
0
integrated area ratios and of the stretching constants: kl, k2, and k,. A 3% error in one of the area ratios leads to a maximum 0.6O error in the interdipole area. A 3% error in the sum (k2 k, k l )leads to a 1.5' angle error. It is therefore estimated that the interdipole angle (e) for mer-Mo(CO)s is 91.5' f 2'. The rate of change of molecular dipolemoment with mass-weightednormal coordinates of the cis CO group is 2.16 times greater than that of the trans groups. Mo(C0)z. In agreement with Guglielminotti and Giamello? we have found that the l3CW monosubstitution pattern can be predicted with the dicarbonyl model. The stretching and interaction parameters, k = 1789 N/m and i = 40.8 N/m, vibrational respectively, can be determined from the Mo( 12C160)z frequencies at 2128 and 2080 cm-l. With these parameters, the IR frequencies of M O ( ~ ~ C ~ ~ O )are ( ~predicted ~ C ~ ~atO 21 ) 13.9 and 2047.3 cm-l. These predicted peaks were observed at 21 14 and 2047 cm-l. Mo(C0). It has been suggesteds.1' that the 2045-cm-1 peak is due to a bridging CO group because its frequency was lower than the range observed for the dicarbonyl, 2128-2080 cm-1. This suggestioncan be rejected for two reasons. First, the position of the 2045-cm-1 band is well outside of the range of bridging CO groups on molybdenum, which are generally known to absorb at frequencies lower than 1860cm-l .22.29 Second, the stoichiometry of decomposition (Figure 8) supports a mechanism whereby Mo(C0)z is decomposed by sequential loss of two CO ligands. In the first step, one CO group is desorbed from Mo(CO)2, leading to the formation of Mo(C0). In the second step, the second CO ligand desorbs. This desorption stoichiometry is not consistent with the proposed bridging CO mechanism in which three CO ligands (initially on adjacent dicarbonyls) desorb, leaving one bridging CO on the surface.sJ1 Mo(C0)6. Thepeakfrequenciesat 21 18,2028,and 1990cm-' (Figure 7) correspond well with those found for MO(CO)~ in CC4 at 2116.7, 2018.8, and 1986.1 30 and with MO(CO)~in hydrocarbonat 21 16,5,2018.9,and 1989.3.31A 1989-cm-1 band on thermally reduced samples was attributed to Mo(CO)~by Louis et a1.,6 who reported a band at 2020 cm-l but not that at 2118 cm-1. The AI, symmetric stretching mode at 21 18 cm-1 and the E, stretching mode at 2028 cm-l arevanishingly weakin theinfrared for perfectly octahedral Mo(CO)~but can be observed when Mo(CO)6 is distorted by a solvent or by adsorption on a surface. The TI, antisymmetric stretch produces the strongest peak near 1990 cm-l in both solution and when adsorbed.2 When physisorbed Mo(CO)~is formed in IR experiments, the weak bands at 21 18 and 2028 cm-l can be hidden by the bands of other molybdenum carbonyl species. For example, Ogata et alaLo observed a band at 1990 cm-l for both Mo(CO)~deposited on silica and Mo6+/SiOZ photoreduced in CO. Becausethe 21 18and 2128-cm-1 bands were not observed, however, the 1990-cm-1 photoreduced specieswas not specificallyattributed to Mo(CO)~. Gerasimovgreported peaks at 21 18,2021, and 1988 cm-1, which, in retrospect, were most likely caused by physisorbed Mo(CO)~. The peak at 1988 cm-l, however, was assigned to Mo'+(CO) on molybdate aggregates. Zecchina et aL2have proposed that chemisorption Of Mo(C0)6 on alumina occurs through AP+- -O==C-Mo bonds at surface tetrahedral and octahedral Al3+ sites, resulting in an increase in the frequency of the TI, mode by as much as 50 cm-1. For silicasupported samples, chemisorption of Mo(CO)6 on Man+ sites (n = 3,4, or 5), was proposed6 to increase the frequency of the T1, mode to 2032 cm-1. Mo(CO)~chemisorbed on reduced Mo sites may therefore be responsible for the relatively weak mode at 2030 cm-l, frequently observed in this study (Figure la). This is supported by the fact that the 2030-cm-l band does not always accompany the 1990-cm-l band of Mo(CO)6 (Figure lb), suggesting that, for some cases (such as that in Figure la), the
+
SUBSTITUTION
MODES
B2
I
(A')2
.-L
Williams and Ekerdt
(A')3
t
Figure 9. Vibrational parameters and modes of mer-Mo(CO),.
with the observed intensities (Figure 1) is the mer configuration. The near collinear symmetric stretching mode ((AI)', Figure 9) results in a very weak mode at 21 8 1 cm-l because the individual CO dipole moments perpendicular to the C2 symmetry axis cancel each other out. Only the resultant dipole vector along the C2 symmetry axis contributes to the intensity of this weak mode.27 The other two modes, AI)^ and B2, are intense because there is no similar dipole cancellation. Other tricarbonyls ( ~ i s - M o ( C 0 ) ~ and cis-Mo(CO)z(CO)') would not display one weak and two strong IR modes, like those of the a species, because they do not possess near-collinear CO groups. A detailed analysis's of the effect of isotopic substitution on the intensities of the modes of mer-Mo(CO)S demonstrated that this model successfully predicted the observed intensities. The predicted intensitiesare those displayed in Figures 2 and 3. The relative integrated intensities of the 2181-, 2141-, and 2108-cm-I peaksof mer-Mo(CO)3 are0.07:0.92:1.0. These peaks were deconvoluted to modified Lorentzian shapes before integration.28 The intensity ratios, along with the stretching parameters, have been used to calculate the angle between CO dipolemoments and the relative IR sensitivityof the CO ligands.22 Overlap between the Mo(CO)3 bands, and between these bands and the dicarbonyl bands at 2128 and 2080 cm-I, prohibits a completely accurate integration of the Mo(CO)~bands. The error in the area ratios of these bands is therefore estimated to be up to 3%. The cis stretching constant, kl,varies by only 0.8% to cover the complete range where real solutions to the secular equation exist, subject to the constraint that kt > k,. Over this rangeofk1(1795-1809N/m),kzvariesfrom 1886 to 1879N/m, about 0.4%. Because good fits to the substituted data can be obtained only over a much narrower range of these constants, the actualerrorsin kl and kzareless thsn0.3%andO.l%,respectively. Over the range for which the best fit is obtained, k,can vary from 29.5 to 30.5 N/m, approximately 3%, and kc can vary from 23 to 25 N/m, about 8%. As mentioned above, the best set of constants is kl = 1805 N/m, kz = 1882 N/m, kt = 30.0 N/m, and k, = 23.9 N/m. The interdipole angle is a function of the
- -
Characterization of Molybdenum Carbonyl Species
SCHEME I
The Journal of Physical Chemistry, Vol. 97, No. 26, 1993 6851
is favored by short photoreduction time, high CO pressure, and high Mo loading. The phototransformationof thespecies responsiblefor the 2141M O ~ + ( C O ) ~-L M04+(co), Mo4+(co)Jl_ y4+ and 2108-cm-1 bands into that responsible for the 2128- and 2080-cm-l bands has been noted previously.8J1 Indeed, it was 2 co proposed that the 2141- and 2108-cm-l bands were caused by a transient dicarbonyl species on Mo cations with a higher oxidation SCHEME I1 state than that of the stable 2128/208O-cm-l dicarbonyl. ConMO~+ MO~+(CO)~ 7MO'+(CO), 7MO(CO)~ tinued photoreduction was thought to decrease the Mo oxidation hv state, and the alleged dicarbonyl bands shifted to 2128 and 2080 2030-cm-l mode is caused by something other than the weak cm-1.8J1 Consistent with our IR band assignments, this shift in 2028-cm-' mode of Mo(C0)6 physisorbed on silica. frequency with increasing photoreduction times, however, is actually due to the transformation of molybdenum tricarbonyl Transformation of Molybdenum Carbonyls. The room temperature transformations between M04+(C0)3, M O ~ + ( C O ) ~ , to molybdenum dicarbonyl. It is likely that the oxidation state Mo4+(CO),and Mo4+on photoreducedMo/SiOz are summarized is the same for both of these species, because of the facile in Scheme I. The CO removal steps readily occurred at room interconversions illustrated in Scheme I. The photoformation temperature during IR experiments. Previousauthors have found stoichiometry of Mo(CO)3 supports a +4 oxidation state for this that, for IR experiments, room temperature evacuation of the species. The reoxidation stoichiometry of the reduced Mo gas-phase CO in equilibrium with Mo(C0)z removes one CO remaining after all the CO ligands have been removed from Moligand, resulting in M o ( C O ) . ~ ~Evacuation ~J~ at 373 K was (CO)3 also supports a +4 oxidation state for the tri-, di-, and necessary to remove the CO ligand from Mo(CO).8J4 During monocarbonyls as well as the CO-free Mo cation. IR experiments in this study, the Mo4+(CO) species responsible The TPDE results are also consistent with the transformations for the band at 2045 cm-l could be removed by evacuation at shown in Scheme I and the photoreduction sequence shown in room temperature. During TPDE experiments under a He purge, Scheme 11. Figure 8 demonstrates that two desorption peaks are however, the removal of CO from Mo4+(CO)did not occur until observed during TPDE with the area of the low-temperature 440 K. This apparent higher desorption temperature may have peak being either 2 times greater than or equal to the area of the been caused by CO readsorption in the larger samples used for high-temperature peak. This two-peak pattern was observed TPDE. The previously r e p ~ r t e d ~ *reversibility ~Jl of the Mo(CO)~ under a variety of conditions and heating rates. We propose that to Mo(C0) transformation was also observed in this study. The the low-temperature peak is associated with CO ligand loss from transformation of CO-free Mo4+to Mo(C0) is probably possible, Mo4+(C0)3and M o ~ + ( C Oand ) ~ that the high-temperature peak although attempts to observe it at 298 K were unsuccessful; the is associated with CO ligand loss from Mo4+(CO). (It was not dicarbonyl could be repopulated at pressures less than 1 Torr of possible to resolve the decomposition of Mo4+(CO)3and Mo~+CO, and it was not possible, in our apparatus, to form only Mo4+(C0)z into two different TPDE peaks by lowering the heating (CO) from Mo4+. rate.) The TPDE pattern (Figure 8) occurring after relatively The transformation, in Scheme I, of Mo(CO)~to Mo(C0)z is short periods of photoreduction, in which the first CO peak demonstrated by IR results such as those presented in Figure 5. contains twice the area of the second CO peak, is consistent with Repopulation of Mo(CO)~from CO-free Mo4+was established thedecomposition of a species containing three CO ligands, where by IR and by TPDE. The 2:l TPDE pattern presented in Figure Mo4+(CO)3and its transient product, M O ~ + ( C Odecompose )~, at 8a is due to Mo(CO)~.When CO was reintroduced to CO-free similar temperatures. When samples were photoreduced for long Mo4+,which had been formed by ramping Mo(CO)~to 525-625 periods of time, the areas of the two TPDE peaks became similar K under He, the Mo(CO)~TPDE pattern could be reproduced (with slightly less CO evolved due to poisoning of reduced Mo (Figure 8b). In this case, the 1:l TPDE pattern results from a sites by trace impurities), indicating that Mo(CO)~had been two-step decomposition of Mo(C0)2. reformed from CO-free Mo4+. The oxidation state of +4, for Mo(CO)3, Mo(CO)z, and MoFor the samples in parts b and c of Figures 5, much of the (CO), agrees with the value reported for photoformed molybconversion of Mo(CO)~to Mo(C0)z occurred over a narrow denum carbonyls by Elev et al., on the basis of C02 and partial pressure range. Below 60-80 Torr of CO, Mo(C0)2 was CO balances,12and by Anpo et al.,13for AHM-derived samples, prevalent. This may explain why Guglielminotti and Giamellos again established by redox stoichiometry. In this study, it has and Rodrigo et al.11 primarily observed IR bands associated with been found that the same molybdenum carbonyl species (MoMo(C0)z. Their samples were photoreduced and scanned at (CO)3, Mo(C0)2, and MO(C0)) are formed, with the same +4 CO pressures at which M O ( C O ) ~is not stable:
