support surfaces are heterogeneous. ... models of supported metal clusters on metal oxide surfaces; ..... inferred from the infrared spectra to have pseudo CsU.
Langmuir 1994,10, 3057-3062
3057
Adsorption and Reaction of [HRe(C0)5]on Ultrathin MgO( 111) and Amorphous Si02 Films Grown on a Mo( 110) Surface: Characterization by Infrared Reflection-Absorption Spectroscopy and Temperature-Programmed Desorption S. K. Purrtell,? X. Xu,*D. W. Goodman,*>* and B. C. Gates*>+ Department of Chemical Engineering and Materials Science, University of California, Davis, California 9561 6,and Department of Chemistry, Texas A&M University, College Station, Texas 77843 Received December 6,1993. In Final Form: June 10, 1994@ The adsorption and reaction of [HRe(C0)5]on ultrathin MgO(ll1) and on amorphous Si02 films grown on a Mo(ll0)substrate were investigated with infrared reflection-absorption spectroscopyand temperatureprogrammed desorption. Similar adsorption and reaction chemistry were observed for samples prepared on both MgO(ll1) and Si02 surfaces. [HRe(C0)5]was physisorbed in multilayers at 100 K. A fraction of the [HRe(C0)5]present in multilayers desorbed at 200 K, and as the temperature was raised, more strongly bound species were formed, which desorbed at 340 K. Heating of the sample in the range 100-300 K produced rhenium pentacarbonyls such as [Re(CO)5{OM}l(M = Mg, Si), as indicated by the infrared spectra. Species suggested to be [Re(C0)4{0M)]2 (M = Mg, Si) were formed in the temperature range 100-350 K, as indicated by the infrared spectra and a companson of the spectra with those observed for samplesprepared similarlyfrom [Re2(CO)lo].Heatingto 400-500 Kresulted in significant decarbonylation. The major species formed on both surfaces during this decarbonylation were surface-bound rhenium tricarbonyls, identified by infrared spectroscopy as [Re(C0)3{OM}31 (M = Mg, Si). These results demonstrate strong parallels between the chemistry of rhenium carbonyls adsorbed on metal oxide thin films and those adsorbed on metal oxide powders.
Introduction Notwithstanding the importance of supported metals as industrial catalysts, their structures are not well understood because the metal clusters (crystallites) in them are highly nonuniform in size and shape, and the support surfaces are heterogeneous. In particular, much remains to be learned about the structure of the metalsupport interfaces in these materials. Thus experiments have been done with mononuclear (single-metal-atom) complexes bonded to the surfaces of metal oxide powders, since the metal-support bonding in these relatively simple materials may provide some insight into the metalsupport interface in supported metal catalysts. The results of a variety oftechniques, including extended X-ray absorption fine structure (EXAFS) spectroscopy, have shown that the metal-oxygen bonding in metal oxidesupported mononuclear metal complexesis characterized by metal-oxygen distances (2.1-2.2 A)that are also found for almost all the metal oxide-supported metal clusters that have been characterized by EXAFS spectroscopy.l Thus, to a degree, the mononuclear metal complexes are models of supported metal clusters on metal oxide surfaces; however, the models are limited in their validity, because the metal-support interface in metal oxide-supported metal catalysts is typically characterized by both this short metal-oxygen distance and a longer distance of about 2.7
was chosen to be the precursor of the adsorbed species because it has been shown to lead to well-defined structures on MgO powder surface^^-^ and has a vapor pressure high enough to allow vapor deposition in ultrahigh vacuum apparatus. The surfaces were chosen to be those of MgO and Si02 thin films because they can be easily prepared and characterized with an array of surface science technique^.^-^ The samples were characterized by temperature-programmed desorptioddecomposition (TPD) and infrared reflection-absorption (IRAS) spectroscopy because the results allow a direct comparison with results characterizing the powder samples.
A.1
(2)Kirlin, P.S.;van Zon, F. B. M.; Koningsberger, D. C.; Gates, B. C. J . Phys. Chem. 1990,94,8439. (3)Honji, A.;Gron, L. U.; Chang, J.-R.; Gates, B. C. Langmuir 1992,
The goals of this work were to understand the nature of the metal-metal oxide bonding better by investigating relatively well-defined and stable adsorbed species (rhenium subcarbonyls)on relatively well-defined metal oxide surfaces under ultrahigh vacuum conditions. [HRe(CO)sI ~~
~
+ University of California.
* Texas A&M University.
Abstract publishedinAduanceACSAbstracts, August 1,1994. (1)Koningsberger, D.C.; Gates, B. C. Catal. Lett. 1992,14,271and references cited therein. @
Experimental Methods The TPD and IFUS experiments were performed in an ultrahigh vacuum chamber described previously.1° This chamber is equipped with a UTI mass spectrometerand a Mattson Cygnus 100 FTIR spectrometer; it was also possible to characterize the samples by Auger electron spectroscopy. The characterization of the MgO and Si02 films was done in a separate chamber equipped with X-ray photoelectron (XPS), Auger electron (AES), and ion scattering (ISS) spectroscopies as well as low-energy electron diffraction (LEED). The procedures used to prepare the MgO films in the two chambers were identical;likewise, the
8,2715. (4)Chang, J.-R.; Gron, L. U.; Honji,A.; Sanchez, K. M.; Gates, B. C. J . Phys. Chem. 1991,95,9944. (5)Xu, X.; Goodman, D. W. Su$. Sci. 1993,284,103. (6)Xu, X.; Goodman, D. W. J . Phys. Chem. 1993,97,683. (7)Xu, X.; Goodman, D. W. Appl. Phys. Lett. 1992,61, 774. (8) He, J.-W.; Xu, X.; Corneille, J. S.; Goodman, D. W. Surf. Sci. 1992,279,119. (9)Xu,X.; Goodman, D.W. Surf. Sci. 1993,282,323. (10)Leung, L.-W.; He, J.-W.; Goodman, D. W. J . Chem. Phys. 1990, 93,8328.
0743-746319412410-3057$04.50/0 0 1994 American Chemical Society
Purnell et al.
3058 Langmuir, Vol. 10,No. 9,1994 procedures used to make the Si02 films in the two chambers were identical. The base pressure in each chamber was Torr. Preparationand Characterization of ultrathin MgO(ll1) Filmson a Mo(110)Substrate. Thin MgO films were prepared by evaporating pure metallic magnesium in 5 x lo-' Torr of oxygen onto a clean Mo(110)substrate at room temperature; the samples were subsequently annealed to 800 K. This method is similar to that used t o prepare MgO(100)on M0(100).~lMo(ll0) was cleaned by annealing in oxygen at 1200 K and flashing to 2000 K in uucuo. The MgO films were characterized by LEED and He+ISS. LEED exhibited a good (1x 1)hexagonal pattern, demonstrating (111)orientationofthe MgO films. XPS and AES showedthe film to have the stoichiometryofMgO and the absence ofthe M$ state. ISS showed the surface to be composedofboth oxygen and magnesium. The stability of the MgO films was investigated with temperature-programmeddesorption;the films were found to be stable at temperatures -= 1400K, and at higher temperatures the MgO was reduced by the Mo substrate,forming volatile MOOS and Mg vapor.l2 The MgO films were approximately 50 A thick. Preparation and Characterization of Ultrathin Si02 Films on a Mo(l10)Substrate. The ultrathin Si02 films were Torr of oxygen by a prepared by evaporating silicon in reported method.'-g After being annealed to 1400 K, the
stoichiometric Si02 films exhibited the electronic and bonding structures of vitreous silica. The Si02 films are stable at temperatures > 1500 K. They were approximately 50 A thick. Materials. [HRe(C0)5]was prepared at the University of California by a literature method.l3 [Re(CO)sBr](Strem),Zn powder (Fisher), and H3P04 (Fisher)were used as received for this preparation. The synthesis of the air-sensitive [HRe(CO)sl was done with the exclusion of air on a double-manifold Schlenk vacuum line, either under vacuum or under high-purity Nz (Liquid Carbonic, 99.997%). The [HRe(C0)51was transported to Texas A&M University in an air-tight flask. It was then transferred to the dosing system under a blanket of Nz. Before being admitted to the vacuum chamber, the [HRe(C0)5]was subjected to several freeze-pump-thaw cycles to remove any residualcontaminants. The purity of the [HRe(C0)5]was verified by mass spectrometry. Dosing of [HRe(CO)a]onto the Surfaces. [HRe(CO)sl,a liquid at room temperature, was introduced into the UHV chamber at ambient temperature. The flux was monitored by a mass spectrometer mounted in line with the dosing line. The surfaces were held at approximately 100K during the adsorption. Temperature-ProgrammedDesorptioflecomposition.
The temperature of the sample was ramped at a rate of 5 Ws in the range 100-1000 K, and the products were monitored with the mass spectrometer. The temperature was measured with a W-5%Re/W-26%Rethermocouple that was spot-welded to the back of the Mo(ll0) crystal. The upper limit of detectabilityof the mass spectrometer was mlq = 300. Fourier Transform Infrared Reflection-Absorption Spectroscopy. The crystal was lowered into the infrared beam
path and aligned. The incident angle of the infrared beam with respectt o the surfacenormal was 85". Spectra with a resolution of 4 cm-I were recorded after each cycle in which the sample was heated at a rate of about 5 Ws t o a particular temperature and immediately cooled to about 110-120 K. The temperatures to which the sample was heated ranged from 150 t o 1000 K.
Results Temperature-Programmed Desorption of Samples Prepared by Adsorption of [HRe(co)~lon Si02 Films. Figure 1shows typical TPD spectra characterizing a sample prepared from [HRe(C0)51 on a SiOz thin film. Because of the limitation of our mass spectrometer, which has a range of 0-300 amu, the parent ion of [HRe(C0)51 could not be monitored. Five rhenium-containing fragments were monitored, with mlq = 186, 214, 242, 270, (11)Wu, M. C.; Corneille, J. S.; Estrada, C. A.; He, J. W.; Goodman, D. W. Chem. Phys. Lett. 1991,182, 472. (12) Xu, X.; Goodman, D. W. Manuscript in preparation. (13) Urbancic, M. A. Inorg. Synth. 1989,26,77.
TPD.
~
HRe(CO)5/Si0,/Mo(l10) 1
=!
m
Y
l x /,(( 1b Re(C0): m/e-298
Re(C0); m/e-270
~x10
AJ
(II
100
300
500
700
900
Temperature (K) Figure 1. Temperature-programmed desorption spectra for samples prepared by the adsorption of [HRe(C0)51on SiOd Mo(ll0). [HRe(c0)~] was adsorbed at about 100 K to give a coverage of -1 monolayer. One monolayer is defined as the coverage at which the peak observed at 200 K and attributed to the sublimation of [HRe(C0)51appeared. and 298, corresponding to Re+, Re(CO)+, Re(C0)2+, Re(C0)3+,and Re(C0I4+,respectively. Each of the TPD spectra (Figure 1) characteristic of rhenium-containing fragments includes two distinct peaks centered at 200 and 340 K. When the exposure of the films to [HRe(C0)5] was increased, the intensity of the peak centered at 200 K increased without saturation. Therefore, the peak at 200 K is attributed to the sublimation of [HRe(C0)51. I n contrast,the peak at 340 Kreached saturation;Le., beyond a certain level of exposure to [HRe(CO)5]the peak intensity did not increase. The peak at 340 K, with a tail extending to 400 K,is also attributed to a mononuclear(sing1e-metalatom) rhenium pentacarbonyl species because the relative intensities of the five rhenium-containing fragments associated with the peak at 340 K (namely, 100:50:140: 100:45) are nearly identical to those observed when gasphase [HRe(C0)51 was admitted to the same vacuum chamber (namely, 100:50:130:100:45). The major CO evolution was observed in a narrow peak at 430 K (Figure 1). Small CO desorption features were also observed at 200-400 K. The evolution of CO during the TPD of the sample prepared from [HRe(C0)51 was monitored at mlq = 12 (C+). C+ (mlq = 12) was chosen instead of CO+ (mlq = 28) to allow simultaneous monitoring of CO at mlq = 12 (C+) and the rhenium-containing fragments with the same mass spectrometer sensitivity scale. The contribution of the fragmentation of desorbed rhenium carbonyls to the C+ signal was negligible. Temperature-ProgrammedDesorption of Samples Prepared by Adsorption of [HRe(CO)al onMgO(ll1) Films. The TPD spectra of the samples formed from [HRe(CO)5]on MgO(111)thin films were observed to be identical to those characterizing the samples formed from [HRe(CO)5]on Si02 films. Rhenium carbonyls desorbed at 200 and 340 K. The principal CO evolution peak was observed at 430 K. Infrared Reflection-Absorption Spectroscopy of Samples Prepared by Adsorption of [HRe(C0)51on MgO(ll1) Films. Figure 2 shows IRAS spectra characterizing samples prepared by adsorption of [HRe(C05)1 on MgO films; the samples had been heated to the indicated temperatures prior to the measurement of the spectra. The initial coverage of the film by [HRe(C0)51 was
Adsorption and Reaction of [HRe(CO)J
I@
Langmuir, Vol. 10,No.9,1994 3059
IRAS. HRe(CO), IMgOIMo(ll0) 1
IRAS HRe(CO)S/SiO,/Mo(l 10)
,
I
~
j069
]0002
p 2200
2100
2000
1900
1800
Frequency (cm-l)
2200
5 - ,,/ 2100
'\ ->-
._ 2!q 500 K ,
-1
2000
1900 (cm -l)
1800
Frequency Figure 3. Fourier transform infrared reflection-absorption spectra for samples prepared by the adsorption of 1monolayer of [HRe(c0)~] on SiO&lo(llO). The spectra were recorded after the surface had been briefly heated to the indicated temperatures. [HRe(C0)5]was adsorbed at about 100 K.
1 'y 2027
i
2104
r-
t2200
Ji --' 2100
I
\
/-< -L--43 . I -
,-500 K I
2000
1900
1800
Frequency (cm -') Figure 2. Fourier transform infrared reflection-absorption spectra for samples prepared by the adsorption of 4monolayers
of[HRe(CO)5]onMgO(lll)/Mo(llO).The spectrawere recorded after the surface had been briefly heated to the indicated temperatures. [HRe(C0)51was adsorbed at about 100 K. estimated to be 4 monolayers, where one monolayer is taken to be the coverage a t which the 200 K desorption peak appeared in the TPD spectrum. After adsorption of [HRe(C0)51on the MgO films a t 100 K, a very strong, broad band was observed in the vco region ofthe spectrum a t 2068 cm-', and weak bands were evident a t 2138 and 1950 cm-' (Figure 2A). After the sample had been heated to 250 K, the intensity of the 2068 cm-' band decreased, the weak band a t 2138 cm-' disappeared, and two lowintensity bands appeared, a t 2155 and 2102 cm-'. After the sample had been heated to 300 K, resolved peaks a t 2158,2104,2066, and 2027 cm-l, as well as an unresolved shoulder at about 2000 cm-l, were observed (Figure 2B). Heating to 350-400 K resulted in the disappearance of the band at 2158 cm-l and a decrease in intensity of the band a t 2066 cm-l. After the sample had been heated further to 450 K, only two bands were observed, a t about 2066 and 1960 cm-'. No vco bands were observed after the sample had been heated to temperatures 2600 K. Infrared Reflection-Absorption Spectroscopy of Samples Prepared by Adsorption of [HRe(C0)51on Si02 Films. The IRAS spectrum recorded after the initial
adsorption of the [HRe(C0I51on Si02 films was identical to that observed for samples prepared on MgO(ll1) films. The spectra characterizing the species formed from [HRe(CO),] on Si02 films, recorded after the samples had been heated to temperatures > l o 0 K, were observed to be slightly different from those characterizing samples prepared from [HRe(C0I51on MgO films. Figure 3 shows I U S spectra for a sample prepared by adsorption of -1 monolayer of [HRe(CO)51on a SiOz thin film which had been heated to the indicated temperatures. At 100 K, a principal band a t 2069 cm-l was observed in addition to several weak shoulders, a t -2135,2100,2020, and 1950 cm-'. When the sample was heated to 200 K, two bands became resolved, a t 2153 and 2029 cm-l. After the sample had been heated to 350 K, the band a t 2153 cm-l disappeared and YCO bands were observed a t 2101,2070, 2033, and 1990cm-'. After the sample was further heated to 400-500 K, only two broad bands were observed, a t about 2050 and 1990 cm-l. No vco bands were observed after the sample had been heated to temperatures '600
K. Discussion Multilayers of [HRe(CO)slon MgO(ll1) and SiOz Films. The TPD data are consistent with the inference that exposure ofboth the MgO and the Si02 films to [HRe(C0)51 a t 100 K led to the formation of [HRe(CO)5I multilayers. The TPD spectra in Figure 1 show a peak a t 200 K associated with rhenium-containing fragments that increased without saturation with increasing exposure of the surface to [HRe(C0)51. This peak is attributed to the sublimation of [HRe(CO)sImultilayers. The infrared spectra (Figures 2A and 3) are consistent with the formation of [HRe(C0)5] multilayers after adsorption on the MgO and Si02 films a t 100 K. A very strong, broad band was observed in the YCO region of the spectrum a t 2068 cm-l, and weak bands were evident a t 2138 and 1950 cm-l (Figure 2A). Solvated [HRe(C0)51 (which has C, symmetry) exhibits three principal infraredactive vco bands, al(e:equatorial),e, and al(a:axial),with frequencies of 2014 (vs), 2007 (s), and 1982 cm-', respect i ~ e 1 y . IThe ~ infrared bands characteristic of molecules in solution are different from those of the compound in
3060 Langmuir, Vol. 10, No. 9, 1994
Purnell et al.
Table 1. CO Stretching Frequencies in Rhenium Carbonyls precursor
solvent or support'treatment
YCO bands,
[HRe(C0)51 [Re(CO)sBrl [Re(CO)sBrl
cyclohexane solution cyclohexane solution none (single crystal)
[Re(C0)4C112 [Re(C0)4IIz [Rez(CO)lol [HRe(C0)53 [Rez(CO)lol [HRe(C0)51 [Rez(CO)lol
C c 4 solution CC4 solution MgO calcined a t 663 Wdosed with methanol a t 473 K MgO calcined a t 673 K MgO calcined a t 663 Wdosed with water at 473 K MgO calcined a t 973 K MgO calcined a t 663 K, spectrum at 668 K i n 1atm of equimolar CO H2 MgO calcined at 973 K MgO calcined at 973 K, spectrum a t 373 K, in vacuo
EH3RedC0)1~1 [Rez(CO)lol
+
the crystalline state, which are usually split into the longitudinal- (LO) and transverse-optical (TO)branches. The selection rules of infrared reflection-absorption spectroscopy allow only bands of the LO branch to be detected. No infrared data have been reported for solid [HRe(C0)5]; however, single-crystal infrared data for a similar compound, [Re(CO)5Brl, have been reported.15 Three principal vco bands have been reported for solvated [Re(CO)5Brl, at 2151 (w), 2044 (s), and 1985 (m) cm-', which correspond to the al(e), e, and al(a) bands, respectively.16 The LO branches of the al(e), e, and al(a) bands for [Re(CO)5Br]single crystals exhibit frequencies of2153, 2075, and 2002/1970 cm-l, re~pective1y.l~ By comparison, we assign the band a t 2068 cm-l (Figure 2A) to the LO branch of the e vibration, the weak band at 2138 cm-' to al(e)-LO, and the shoulder at 1950-2000 cm-l to al(a)LO. The locations of these infrared bands for [HRe(C0)51 multilayers a t lower energies than those reported for [Re(C0)5Br]single crystals can be explained by the different ligands on the rhenium pentacarbonyl moiety. The hydride ligand would be expected to withdraw less electron density from the rhenium atom than would the Br ligand. Thus the greater electron density on the rhenium atom in [HRe(C0)51 would result in more efficient n-backbonding, which in turn would result in the appearance of the YCO bands at lower energies, consistent with the observations. Reaction and Decarbonylation of [HRe(CO)alon MgO(ll1) Films. The infrared reflection-absorption spectra clearly demonstrate that [HRe(CO)bI underwent a series of reactions upon heating to temperatures > 100 K (Figure 2). After the sample had been heated to 300 K, two species were identified. The bands a t 2158 and 2066 cm-l are close to those reported for single-crystal [Re(C0)5Br1.l5 However, because the TPD spectra indicated the desorption of [HRe(C0)5]multilayers a t 200 K, these bands cannot be attributed to [HRe(CO)51multilayers. McKenna et al." reported a band a t 2150 cm-' to be characteristic of a rhenium pentacarbonyl bound to the surface of Si02 powder. They formulated this surface-bound rhenium pentacarbonyl as [Re(C0)5{OSi}], where the braces represent oxygen atoms terminating the metal oxide surface. One might expect that this surface-bound rhenium pentacarbonyl would retain nearly CqUsymmetry and exhibit bands close to those reported for [Re(CO)sBr]. Consistent with this expectation, we attribute the bands at 2158 and 2066 cm-' to a MgO-supported rhenium (14) Braterman, P. S.;Harrill, R. W.; Kaesz, H. D. J . Am. Chem.SOC. 1967,89,2851. (15)Adams, D. M.; Taylor, I. D. J . Chem. SOC.,Faraday Trans.2 1982,78,1065. (16)Kaesz, H. D.; Bau, R.; Hendrickson, D.; Smith, J. M. J . Am. Chem. SOC.1967. 89. 2844. (17) McKenna; W.' P.; Higgins, B. E.; Eyring, E. E. J . Mol. Catal. 1986,31, 199.
cm-l
ref
2014 (vs), 2007 (91, 1982 (w) 2151 (w), 2044 (s), 1985 (m) LO: 2153,2076,2074,2002,1970 TO: 2153,2061,2034,1974,1962 2114 (w), 2031 (s), 2000 (m), 1959 (m) 2106 (w), 2029 (s), 2001 (m), 1965 (m) 2008 (s), 1885 (vs) 2011 (vs), 1895 (vs), 1862 (sh) 2014 (s), 1894 (vs) 2017 (vs), 1908 (vs), 1867 (sh) 2022 (s), 1905 (9)
14 16 15
2028 (s), 1905 (s) 2036 (s), 1921 (5)
2 19
18 18 19
3 19
3 19
pentacarbonyl species, [Re(CO)5{OMg}]. The spectrum recorded after the initial adsorption of [HRe(C0)51on the MgO films includes a very weak shoulder a t 2153-2155 cm-l (Figure 2A), which suggests that some of the [HRe(CO)5]reacted with the metal oxide surface to form the surface-bound rhenium pentacarbonyl even a t 100 K. The 2104-cm-l peak (Figure 2B) is suggestive of species with structures related to those of the rhenium tetracarbonyl dimers [Re(CO)312 (X= C1, Br, I).18 The other two bands, a t 2027 and 2000 cm-l, are also similar to those observed for these rhenium tetracarbonyl dimers. For example, [Re(C0)&1]2 is characterized by four YCO bands, a t 2114, 2032, 2000, and 1959 cm-l (Table 1).l8 McKenna et al." reported a band a t 2101 cm-l to be characteristic of the surface-bound rhenium tetracarbonyl dimer with a Re-Re bond, formed from [Rez(CO)l~l, i.e., [Re2(C0)4{OSi}l2. Thus the bands a t 2110,2027, and 2000 cm-', observed after the sample had been heated to 300 K (Figure 2B), are suggested to be an indication of a surface-bound rhenium tetracarbonyl dimer, [Re(C0)4{ OMg}]2. Alternatively,the spectrum might be attributed to a n analogous mononuclear rhenium subcarbonyl, [Re(C0)4{0Mg)2],but there is a lack of molecular analogues for a comparison of spectra. As the sample was heated from 300 to 400 K, the peaks in the YCO region generally decreased in intensity. The peak a t 340 K, with a tail extending to 400 K, observed in the TPD data, has been attributed above to a mononuclear rhenium pentacarbonyl species. The TPD spectra (Figure 1)show the desorption of rhenium pentacarbonyl was accompanied by desorption of only very little CO. The near absence of desorbed CO is consistent with the inference that [Re(CO),j{OMg)l and [Re(CO)r{OMg}12(or related species) had been formed on the MgO surface a t these temperatures, because formation of [Re(C0)5{0Mg}l from [HRe(C0I51on MgO requires no loss of CO, and formation of [Re(C0I4{0 M g ) l ~from [HRe(C0)5]on MgO involves loss of only one CO ligand per rhenium atom. The TPD data show that significant decarbonylation occurred in the range 400-500 K. Since no rheniumcontaining fragments were observed to desorb in this temperature range, the decarbonylation is attributed to surface reactions leading to CO evolution without the desorption of rhenium. After the sample had been heated to 450 K, two vco bands were observed, a strong band a t 2066 cm-l and a broad, weak band at about 1960 cm-'. These bands are attributed to rhenium subcarbonyls because the bands resemble those reported for rhenium tricarbonyls on MgO p o ~ d e r . (Presumably, ~ ~ ~ J ~ ~ metallic ~ ~ ~~~
(18)Hileman,J. C.; Huggins, D. K.; Kaesz, H. D. I n o g . Chem. 1962, 1 , 933.
(19) Papile, C. J.; Gates, B. C. Langmuir 1992,8, 74. (20) Kirlin, P. S.; DeThomas, F. A.; Bailey, J. W.; Gold, H. S.; Dybowski, C.; Gates, B. C. J . Phys. Chem. 1986,90,4882.
Langmuir, Vol. 10, No. 9, 1994 3061 Table 2. Summary of Infrared Band Assignments surface species 100-300 K
-CO
>600K
(OMg)
Figure 4. Reaction scheme proposed for [HRe(C0)5] on MgO( 11l)/Mo( 110).
rhenium was formed simultaneously.1 These rhenium subcarbonyls have been prepared from a variety of rhenium carbonyl precursors, including [HRe(C0)51.2p3J9 These surface-bound rhenium subcarbonyls have been formulated on the basis of infrared, TPD, extended X-ray absorption fine structure, and other data as [Re(CO)3{OMg}x{HOMg}3-,1 (0 5 x 5 3).2,3J9~20 The bands were not attributed to CO chemisorbed on rhenium metal clusters, because CO chemisorbed on rhenium crystallites exhibits a single band a t 2035 cm-1.21 The vco frequencies characterizing MgO powder-supported rhenium tricarbonyls have been reported to increase with the degree of dehydroxylation of the s u p ~ o r t . ~The J ~ bands characterizing the MgO filmsupported rhenium tricarbonyls were observed a t higher frequencies than those characterizing the powder-supported samples, consistent with the negligible degree of hydroxylation of the MgO(111) surface as indicated by the lack of evidence of surface hydroxyl groups in the infrared spectrum.22 Thus we infer that the species supported on the MgO film was [Re(CO)3{OMg}31. The MgO powder-supported species [Re(C0)3{OMg)31has been inferred from the infrared spectra to have pseudo CsU symmetry.lg Therefore we assign the bands a t 2066 and 1960 cm-' characterizing the MgO film-supported sample to the symmetric and antisymmetric carbonyl stretching vibrations in [Re(CO)3{OMg}3],respectively. No vco bands were observed after the sample had been heated to temperatures >600 K. We infer that the treatment a t high temperature resulted in the formation of metallic rhenium clusters. Metallic rhenium clusters were observed to form a t similar temperatures for samples prepared by the adsorption of [Re2(CO)l~lon MgO(ll1) films.22 In summary, Figure 4 illustrates a proposed scheme that accounts for the species observed to result from the adsorption and reaction of [HRe(CO)sI on MgO(ll1) ultrathin films grown on a Mo(ll0) surface. Reaction and Decarbonylation of [HFte(CO)51on Si02Films. The TPD spectra recorded for [HRe(CO)51 on Si02 films were identical to those on MgO films (data not shown), suggesting that the reaction and/or decomposition of [HRe(C0)51on Si02 were similar to those occurring on the MgO surface. By comparison with the results observed for the rhenium carbonyls on MgO, the following structures are inferred for the rhenium carbonyls on SiOz: [Re(CO)5{OSi}], [Re(C0)4{0 S i ) l ~(or related species),and [Re(CO)3{OSi}3]. After the sample had been heated to temperatures in the range 200-250 K, a band was observed in the vco region of the infrared spectrum (21)Bolivar, C.; Charcosset, H.; Frety, R.; Primet, M.; Toumayan, L.; Betizeau, C.; Leclercq, G.; Maurel, R. J . Cutul. 1976,45, 163. (22) Purnell, S. K.; Xu, X.; Goodman, D. W.; Gates, B. C. J. Phys. Chem. 1994,98,4076.
treatment temp, K
VCO,
cm-1
100
2138 (w), 2068 (vs),1950 (w)
300 300
2158 (w), 2066 (vs) 2104 2066 (91, (m),1960 2027(w,br) (s), 2000 (sh)
200-250 450 350
2153 (w), 2070 (S) 2101 (w), 2070 (91, 2033 (m), 1990 (w)
400-450
2050 (s), 1990 (m,br)
a t 2153 cm-'. As discussed above, this band is characteristic of the [Re(C0I51moiety.15J7 McKenna et al.17 observed a band a t 2150 cm-l for a sample prepared by adsorption of [Re2(CO)l~lon Si02 powder, followed by photolysis in tetrahydrofuran. They attributed this band to surface-bound rhenium pentacarbonyls, formulated as [Re(CO)5{OSi}l. Thus we suggest that the band observed a t 2153 cm-' is characteristic of the same SiO2-bound rhenium pentacarbonyl formed by reaction of [HRe(C0)51 with the Si02 surface. After the SiO2-supported sample had been heated to 350 K, four vco bands were observed, a t 2101,2070,2033, and 1990 cm-' (Figure 3). McKenna et al.17attributed a band a t 2101 cm-l to a surface-bound rhenium tetracarbony1 dimer, [Re(CO)4{OSi)lz. An identical band was observed for our Si02 film-supported sample. The bands a t 2033 and 1990 cm-' can also be attributed to [Re(C0)4{OSi}12. Hileman et al.'* reported similar bands for [Re(C0)4C112a t 2032 and 2000 cm-l (Table 1).As for the MgO film-supported sample, we again recognize that the spectrum might be attributed to a n analogous mononuclear rhenium subcarbonyl, [Re(C0I4{OSi}J We attribute the vco band a t 2070 cm-' observed for samples heated in the temperature range 100-350 K to physisorbed [HRe(CO)sI or to [Re(CO)s(OSi}l. The overall infrared absorbance decreased significantly as the temperature of treatment was increased from 100 to 400 K, and no significant CO evolution was observed. By reasoning analogous to that stated above for the MgOsupported samples, we infer that these results are consistent with the formation of [Re(CO)5{OSi}l and [Re(C0)4{OSi)]2 (or related mononuclear species). When the sample had been heated to temperatures in the range 400-500 K, in which significant CO evolution occurred as evidenced by the TPD data, the sample was characterized by two broad vco bands, a t approximately 2050 and 1990 cm-'. By reasoning analogous to that stated above for samples prepared on MgO films, we assign these two bands to the surface-bound rhenium tricarbonyl [Re(C0)3{OSi}3]. McKenna et al.17reported bands a t 2032 and 1935 cm-l for this rhenium tricarbonyl bound to Si02 powder. The appearance of the thin-film bands a t higher frequencies than those observed by McKenna et al.17for rhenium subcarbonyls on Si02 powders is consistent with the presence of hydroxyl groups on the Si02 powder surfaces,but not on the Si02 films. The relative intensities of the bands a t 2050 and 1990 cm-' are not the same as those observed for the rhenium tricarbonyls on MgO(ll1) films. Other species in addition to the rhenium tricarbonyl may have been present. Comparison of the Chemistry of [HRe(CO)sl on MgO( 111) and on Si02 Films. On both surfaces, [HRe(CO)5]reacted to form [Re(CO)s{OM}] and [Re(C0)4{OM}l2 (or related species) (M = Mg, Si). [Re(C0)5{OM)l (M = Mg, Si) was observed to be stable a t temperatures (300 K on both surfaces. The lack of exact agreement between the vco spectra of the MgO- and SiO2-supported species in the temperature range of 350-400 K (Table 2) suggests
3062 Langmuir, Vol. 10, No. 9, 1994 the presence of a variety of structures on the Si02 surface, which, in contrast to the MgO(111)surface, lacks rigorous threefold symmetry. When the sample temperature was increased to 350 K, a fraction of these surface-bound rhenium pentacarbonyls desorbed as volatile rhenium pentacarbonyls(asevidenced by the TPD data), and the remainder reacted to form other surface-boundrhenium carbonyls. The inferred formation of molecularly adsorbed rhenium pentacarbonyl, [Re(CO)5{OM}] (M = Mg, Si), is consistent with the chemistry of rhenium carbonyls on metal oxide powder surfaces. [HRe(C0)J is a weak proton donor and has been reported to react with the basic surface of MgO, leading to partial deprotonation by 02-groups to form ion pairs such as {MgO}H+-[Re(CO)51-.2 Such a species, upon dissociation of the proton, would give a chemisorbed species analogous to [Re(CO)5{OMg}l. Furthermore, the formation of [Re(CO)5{OSi}lhas been reported to result from the photolysis of [Re2(CO)l~ladsorbed on Si02 p0wder.l' The suggested formation of dinuclear rhenium subcarbonyls {[Re(C0)4{OM}l2 (M = Mg, Si)} from the mononulear precursor is consistent with the solution chemistry of rhenium carbonyls. Rhenium pentacarbonyl halides {e.g., [Re(CO)&] (X = C1, Br, I)} are readily converted into rhenium tetracarbonyl halide dimers in solution at 100-120 0C.23 We suggest that analogous, presumably bimolecular, reactions occurred on the MgO(ll1) and Si02 surfaces, with [HRe(CO)5I or [Re(C0)5{OM}l (M = Mg, Si) reacting to form the surface-bound rhenium tetracarbonyl dimer, [Re(C0)4{0M}12(M = Mg, Si). The principal species formed on each surface in the range 400-500 K were rhenium tricarbonyls, inferred from the infrared reflection-absorption spectra to be [Re(C0)3{OMg}J on MgO(ll1) films (Figure 2B) and [Re(C0)3{OSi}31 on Si02 films (Figure 3). Other surface species may also have been present. Comparison of the Chemistry of [HRe(CO)aland [Rez(CO)lolon MgO(ll1) Films. The adsorption and reaction of [Re2(CO)lolon MgO(l11) films has already been reported.22 The two rhenium carbonyl precursors show very similar chemistry on the MgO surface. Both precursors have been suggested to react to form rhenium tetracarbonyl dimers, [Re(C0)4{OMg}l2, and the stable surface-boundrhenium tricarbonyl, [Re(CO)3{OMg}J. The observations that (1)the dimeric precursor [Re2(CO)101 reacted on the MgO(l11) surface to give dimeric rhenium (23)Abel, E. W.; Hargreaves, G. B.; Wilkinson, G. J . Chem. SOC. 1958, 3149.
Purnell et al. tetracarbonyls and that (2) the infrared spectrum attributed to the latter is nearly the same as that suggested here to be indicative of the dimeric rhenium tetracarbonyl formed from the mononuclear precursor [HRe(CO)5Ilend support to the suggestion that the mononuclear precursor was converted into a dimeric surface species. No infrared evidence of surface-bound rhenium pentacarbonyls such as [Re(CO)5{OMg}I was observed for the reaction of [Re2(CO)lolonMg0.22 However, the TPD data suggested the presence of rhenium pentacarbonyls on the MgO(ll1) surface.22 Thus the chemistry of rhenium carbonyls on MgO(ll1) is almost the same, irrespective of whether the precursor is [HRe(CO)51 or [RedCO)loI. Nature of the Bonding of the Rhenium Tricarbonyls to the Surface. As discussed separately, the results are consistent with the bonding of Re(1) in the rhenium subcarbonyl to three oxygen atoms on the hexagonal MgO(ll1) surface.22 The data presented here indicate similar bonding on the surface of Si02. We infer that the amorphous Si02 offered the appropriate surface bonding sites.
Conclusions Similar adsorption and reaction chemistry were observed for samples prepared on both MgO(ll1) and Si02 surfaces. [HRe(C0)51was physisorbed in multilayers a t 100 K. Afradion ofthe [HRe(C0)5]present in multilayers desorbed a t 200 K, and. as the temperature was raised, more strongly bound species were formed, which desorbed a t 340 K. Heating of the sample in the range 100-300 K produced rhenium pentacarbonyls such as [Re(CO)E{OM}] (M = Mg, Si), as indicated by the infrared spectra. Species suggested to be [Re(C0)4{0M}l2 (M = Mg, Si) were formed in the temperature range 100-350 K, as indicated by the infrared spectra and a comparison of the spectra with those observedfor samples prepared similarly from [Re2(CO)lo]. Heating to 400-500 K resulted in significant decarbonylation. The major species formed on both surfaces during this decarbonylation were surfacebound rhenium tricarbonyls, identified by infrared spectroscopy as [Re(C0)3{0M}31 (M = Mg, Si). The infrared assignments are summarized in Table 2. Acknowledgment. This work was supported by the Department of Energy, Office of Energy Research, Office of Basic Energy Sciences, Division of Chemical Sciences, Contract No. FG05-88ER13954at Texas A&M University and Contract No. FG02-87ER13790 at the University of California a t Davis.
