Langmuir 1992,8, 74-79
74
Rhenium Subcarbonyls on Magnesium Oxide: Identification of the Surface Oxo and Hydroxo Ligands by Infrared Spectroscopy Christopher J. Papile and Bruce C. Gates' Center for Catalytic Science and Technology, Department of Chemical Engineering, University of Delaware, Newark, Delaware 19716 Received February 27, 1991.I n Final Form: September 12, 1991 Rhenium subcarbonyls on MgO powder were prepared by adsorption and decomposition of [Rez(CO)101 and characterized by infrared spectroscopy. MgO samples were treated at various temperatures to give surfaces ranging from those that were virtually fully dehydroxylated to those that were highly hydroxylated; methoxylated surfaces were also prepared by treatment of MgO with methanol. The rhenium subcarbonyls on the surface have distinctive carbonyl infrared spectra that are very similar to those of molecular analogues;the spectra allow identification of the surface ligands bonded to the rhenium as 02on the highly dehydroxylated surface, OH- on the hydroxylated surface, and CH30- on the methoxylated surface. A family of surface species is identified, including Re(CO)s{OMgja,Re(CO)3{HOMg}3,Re(C0)3{HOMg)z{OMg},and Re(C0)3(CH30MgJ3,where the braces refer to the surface ligands terminating the MgO.
Introduction The metal-metal oxide interface is important in the performance of supported metal catalysts and electronic materials that incorporate conducting metal structures on semiconductor surfaces. The structures of metalsupport interfaces are poorly understood, as the surface species are typically extremely small and nonuniform. Spectra characterizing the surface species may provide information about the metal-support interface, but it is usually obscured by the other contributions. Good opportunities for understanding the metal-support interface are offered by molecular analogues on surfaces, such as metal carbonyls, but even for these, structures of the metalsupport interface are not known. This research is built on a platform of surface organometallic chemistry, whereby structures on surfaces are identified by comparison of their spectra with those of molecular analogues.' The goal was to prepare and characterize nearly uniform surface structures and to identify the groups through which the metal is bonded to the metal oxide surface. The plan was (1) to use organometallic synthesis to prepare stable, structurally simple surface species with strong covalent metal-oxygen bonds, (2) to vary the composition of the support to give systematic changes in the metal-upport bonds, and (3)to use infrared spectroscopy to characterize the structures. The surface species were chosen to be metal carbonyls because the infrared spectra are sensitive to the ligand environment of the metal. The metal was chosen to be rhenium, as there is an extensive literature of rhenium carbonyls, including surface s p e ~ i e s , and ~ - ~ the oxophilic rhenium forms strong bonds with metal oxide surfaces. The strategy was (1)to adsorb a simple rhenium carbonyl precursor, [Re&O)lol, on a metal oxide powder having a relatively well understood structure and surface (1)Lamb, H. H.; Gates, B. C.; Knozinger, H. Angew. Chem.,Znt. Ed. Engl. 1988, 100, 1127. (2)McKenna, W. P.; Higgins, B. E.; Eying, E. M. J.Mol. Catnl.1985, 31, 199.
(3) Kirlin, P. S.; DeThomas, F. A.; Bailey, J. W.; Gold, H. S.; Dybowski, C.; Gates, B. C. J. Phys.Chem.1986,90, 4882. (4) Kirlin, P. S.; van Zon, F. B. M.; Koningsberger, D. C.; Gates, B. C.
J. Phys.Chem.1990,94,8439. ( 5 ) Guczi, L. Proc.-Int.
Congr. Catal., 9th 1988, 5, 114.
chemistry, namely, MgO, for which the (100) face is predominant;6-8 (2) to treat the precursor to form rhenium subcarbonyls (Le., Re complexes with some of the CO ligands replaced by ligands provided by the metal oxide surface); and (3) to characterize the surface with infrared spectroscopy. The literature of rhenium carbonyl compounds incorporating oxygen-containingligands provides a basis for interpreting the spectra of the surface species, which are expected to incorporate oxo (02-) and hydroxo (OH-) ligands.
Experimental Methods All samples were handled in the virtual absence of air on a standard Schlenk vacuum line and in a nitrogen-purged glovebox (VacuumAtmospheresor Braun MB150; the latter typically contained C0.2 ppm of H20 and >1 C, 2022 8 0.36 0.33 >>>1 C3" 2014 0 0.00 0.00 Re(COMHOMghd [Rez(COhol 390 >>le
ref this work 4
this work this work
a Determined by composition of organometallic precursor. Relative to Re(C0)3(HOMg)3. Fraction of noncarbonyl ligands which are OHin the model structure. Sample prepared from commercial MgO. e Estimate from literature based on MgO pretreatment temperature.E
Table 111. Absorption Bands in the C-0 Single Bond Region Characterizing Methanol Adsorbed on MgO with Adsorbed Rhenium Subcarbonyl temp,
"C
CH30H partial pressure in dose, atm
25
1.W
25
0.88
25
0.63
25
o.Oo0 53c
200
O.Oo0 13
peak position, cm-l
peak width, cm-1
absorbance, arbitrary units
1088 1058 1051 1044 1033 1088 1058 1051 1044 1033 1088 1058 1051 1044 1033 1088 1058 1051 1044 1033 1088 1058 1051 1044 1033
b b b b b b b 5 b 5 b 5 5 b 5 25
1.48 b b b >3.5 1.44 2.4 2.8 b 2.2 1.45 1.85 1.85 0.45 1.6 1.5 0 0 0.45 0 1.2 0 0 0 0
26 25
CH30H vapor in helium carrier stream. Band present, but width could not be determined. Sample evacuated for 45 min after dosing with CH30H at 1 atm.
species in the sample are characterized by narrower (about 5 times narrower) peaks than the chemisorbed species (Table 111). The relatively narrow peak at 1058cm-', which has been attributed above to hydrogen-bonded methanol in the first layer, was always similar in intensity to the peak at 1051 cm-l. Therefore, the 1051-cm-' peak is also assigned to physisorbed methanol, presumably also hydrogen-bonded. The data are therefore consistent with two types of firstlayer sites, and the species adsorbed on them are inferred to have been present in approximately equal amounts, as shown by the approximately equal infrared peak intensities. The available information is not sufficient to identify these (presumably hydrogen-bonded) species characterized by the 1051- and 1058-cm-I peaks, but we speculate that they may be assigned to methanol hydrogen bonded to the oxygen of chemisorbed CH30- on the one hand and to the oxygen of OH- groups on the other. Now, on basis of the remaining plausible possibilities, an assignment is suggested for the 1044-cm-' peak. This peak was not observed except when methanol was present with the rhenium carbonyl on the MgO surface. It is therefore plausible to attribute this band to CH30-groups associated with rhenium subcarbonyls. This assignment is supported by a comparison of the spectrum of the surface species with that of a presumed molecular analogue, K[Re2(CO)&H30)3] (the CH30- group is a ligand bonded to
the rhenium), which is characterized by a band at 1040 cm-1.20 Furthermore, the width of the 1044-cm-l peak characterizing the surface species is similar to that of the 1088-cm-' peak, which has also been assigned above to a chemisorbed CH30- species. The evidence, although not unequivocal, therefore supports the assignment of the 1044-cm-' band to the C0 stretching frequency of the methoxide bonded to the rhenium in the subcarbonyl. An important consequence of this assignment is the inference that the rhenium subcarbonyls are covalently bonded to the MgO surface through the methoxy ligands. Taking all these results together, we formulate the rhenium carbonyl on the methoxylated surface as fac-Re(CO)&H30Mg)3; the C3" symmetry is indicated by the carbonyl spectrum. The ideal structure is depicted as follows, with the oxygen of the OCH3-group bonded to Re and also to C: r
1
L
J
"-
In summary, a list of the surface structures that have been inferred is given with some approximate surface composition and spectroscopic results in Table 11. The internal consistency of the results is sufficient to provide a basis for suggestinga more precise structural formulation of the rhenium subcarbonyl formed on the partially hydroxylated MgO and that reported by Kirlin et aL4 The goal now is to examine the data in detail to provide a basis for inferring whether the structures incorporate one hydroxo and two oxo ligands or vice versa. The reasoning is as follows: There is a pattern of decreasing frequency of the high-frequency carbonyl band in the rhenium subcarbonyl with increasing hydroxylation of the MgO surface. Using literature estimates of the surface densities of the hydroxo and oxo groups on MgO, we interpolate to suggest that the structure reported here for the partially hydroxylated MgO that had been pretreated at 400 OC was predominantly Re(C0)3(HOMg]Z{OMg];this is plausible, since the OH-groups on the surface outnumber the 02groups by about 2 to 1 (Table 11). Similarly, we suggest that the structure reported by Kirlin et aL4 was predominantly Re(CO)3{HOMg](OMgJ2, rather than the Re(CO)3{HOMg)z{OMg)that the researchers had formulated (recognizing that they could not distinguish the two). This assignment is also plausible, since the surface of the MgO had a low population of OHgroups, which were greatly outnumbered by 02groups. (The hydrogen from the precursor [H3Re3(C0)12] contributed to the formation of OH- groups on their MgO.) (20) Ciani, G.; Sironi, A. Cazz. Chim. Z t d . 1979, 109, 615.
Rhenium Subcarbonyls on Magnesium Oxide
We recognize that the structural postulates for the partially hydroxylated surfaces are oversimplified, as mixtures of the various species likely existed. Furthermore, all the interpretationsgiven here neglect the intrinsic nonuniformity of the MgO surface. The predominant face on the MgO is the square (100)face, yet the spectra indicate bonding of the Re to three, not four, oxygensof the surface. Perhaps the structures are formed predominantly at defects where groupings of three oxygens predominate over the square arrangement. The present results do not allow conclusions about this issue. Notwithstanding the approximations of the structural models, the results summarized in Table I1 show that there is a strong correlation between the shift of the highfrequency CO stretch in the rhenium subcarbonyl and the
Langmuir, Vol. 8, No. 1, 1992 79
number of OH ligands inferred to be bonded to the Re. The structural models are thus characterized by a satisfying internal consistency that suggests that many other metal subcarbonyls bonded to metal oxide surfaces may yield to rather exact structural determinations. As the results of Kirlin et al.4show, EXAFS spectroscopy will be valuable in determining quantitative structural data.
Acknowledgment. This research was supported by The U.S.Department of Energy, Office of Energy Research, Office of Basic Energy Sciences (Grant FG0287ER13790). Registry No. Rez(CO)lo, 14285-68-8; MgO, 1309-48-4; Re, 7440-15-5.
