Langmuir 1994,10, 4077-4082
4077
A Family of Rhenium Subcarbonyls on MgO: Structural Characterization by Extended X-ray Absorption Fine Structure Spectroscopy N. D. Triantafillou,? S. K. Purnell,tl* C. J. Papile,? J.-R.Chang,? and B. C. Gates*>?,* Center for Catalytic Science and Technology, Department of Chemical Engineering, University of Delaware, Newark, Delaware 1971 6, and Department of Chemical Engineering and Materials Science, University of California, Davis, California 95616 Received November 1, 1993. In Final Form: August 22, 1994@ Stable rhenium tricarbonyls bonded to the surface of MgO powder, prepared from a variety of precursors including [Rez(CO)lo], [DRe(C0)61, and [Re4(C0)12(OH)41,have been characterized by extended X-ray absorption fine structure (EXAFS)spectroscopy. The conversion of the initially adsorbed species into the rhenium subcarbonyl has been carried out by a variety of routes at relatively mild temperatures, including treatment in H2, He, 0 2 , or under vacuum. The EXAFS and infrared data indicate a C3usymmetry of the surface-bound rhenium subcarbonyls. Each Re atom is bonded to three surface oxygen atoms, and the complex is formulated as [Re(CO)3{OMg},{HOMg}~-,I (where the braces denote groups terminating the MgO bulk). The surface species are precise analogues of molecular rhenium carbonyls, as indicated by both the coordination numbers and bonding distances for all the members of the family. The indications of this symmetry demonstrate that the rhenium subcarbonyls on the surface of MgO powder are bound to faces other than the (100) face and/or at surface defects. These rhenium subcarbonyls are among the best characterized examples of adsorbates on metal oxide powders.
Introduction Metal subcarbonyls on metal oxide surfaces have been investigated with a variety of spectroscopic methods to provide quantitative characterizations of the bonding of metals to the metal oxide surfaces and, by inference, information about metal-support interactions in supported metal catalysts. Rhenium subcarbonyls have been more thoroughly investigated than others, because they offer the advantages of being easily prepared, stable, and catalytically active for several reactions. Here we bring together new and published data characterizing rhenium subcarbonyls on MgO made from a variety of rhenium carbonyl and rhenium salt precursors. A compilation of the available structural data for of all these materials shows that rhenium subcarbonyls on MgO are among the simplest and best characterized species adsorbed on metal oxides.
Experimental Section Sample Handling. Preparation and handling of the airsensitive materials were carried out on a Schlenk vacuum line and in a Vacuum Atmospheres or a Braun (MB-15OM)glovebox filled with Nz that was recirculated through traps to remove 0 2 and water. Typical 0 2 and water concentrations were < 1ppm. Preparation of Rhenium Subcarbonyls from [DRe(CO)d/ MgO. The MgO support (EM Science)was prepared by treatment under vacuum for 15 h, followed by 1h in flowing dry 0 2 , and finally 1 h under vacuum. The MgO was then rehydroxylated by exposing it to Nz saturated with water at room temperature and calcining at 250 "C in vacuum for 10 h.' [DRe(CO)sl was prepared from [Re(CO)sBr] by a literature method2 and allowed to react with the MgO surface as reported previ~usly.~,~ [DRe(C0)6]was slurried with MgO and stirred for 12 h at room temperature. The solvent was then removed by
University of Delaware. University of California. * AbstradpublishedinAdvanceACSAbstracts,October 15,1994. (1) Triantafillou,N.D. Ph.D. Dissertation, University of Delaware, t
*
1993.
(2) Urbancic, M. A.Znorg. Synth. 1989,26,77. (3)Kirlin, P. S.; van Zon, F. B. M.; Koningsberger, D. C.; Gates, B. C. J . Phys. Chem. 1990,94,8439.
evacuation to yield a sample containing 2 wt % rhenium.6 The sample prepared by adsorption of [DRe(C0)5]was treated in flowing HZat 80 "C for 4 h t o form the surface-bound rhenium s u b c a r b ~ n y lA . ~fraction ~~ of this sample was methoxylated by exposure to CHaOH-saturated He at 150 "C for 30 min.l Preparationof Rhenium Subcarbonylsfrom [€&4(C0)12(OH)41/MgO. The MgO powder was prepared from MgC03 [Re4(C0)12(0H)41 calcined at 390 "C as reported el~ewhere.~,~ was prepared by a combination of two literature methods,819 as in distilled water was placed follows: A suspension of [Re~(CO)lol in an autoclave and degassed with nitrogen and purged with He. The autoclave was heated to 200 "C with the final pressure being 17.3 atm. It was held under these conditions for 7 h and then at 130 "C for 10 h. The product was purified by diethyl ether extraction, passage through a column packed with silica gel, and repeated crystallization. The purity was verified by infrared spectroscopy (2026 (s), 1922 (vs);literature8j92025 (s), 1918 (vs) cm-l). [Re4(C0)12(0H)4]was slurried withMgO in diethyl ether, stirred, and dried. The sample was subsequently treated in vacuo at 225 "C for 4 h. Preparationof RheniumSubcarbonylsfrom [Rea(CO)lol/ MgO. The sample preparation is described elsewhere.6 Characterization by EXAJ?S Spectroscopy. The experiments were performed on beamline X-11A of the National Synchrotron Light Source at Brookhaven National Laboratory, Upton, Long Island, NY. The storage ring operated with an electron energy of 2.5 GeV the beam current was typically between 45 and 180 mA. Wafers for the transmission EXAFS experiments were prepared in a glovebag under Nz,as follows: Each powder sample was placed in a holder in the glovebag. The holder was placed in a pressing die and the sample pressed into a self-supporting wafer. The wafer was then loaded into an EXAFS celllOJ1 in the glovebag, and the cell was sealed. (4)Chang, J.-R.; Gron, L.; Honji, A.; Sanchez,K. M.; Gates, B. C. J. Phys. Chem. 1991,95,9944. ( 5 ) Honji, A.; Gron, L. U.; Chang, J. R.; Gates, B. C. Langmuir 1992, R -. 0716 -, --.
(6)Papile, C.J.; Gates, B. C. Langmuir 1992,8, 74. (7)Papile, C. J. Ph.D. Dissertation, University of Delaware, 1991. (8)Herberhold, M.;Suss, G.; Ellerman, J. Chem. Ber. 1978,111, 2931. (9)Gard, D. R.; Brown, T. L. J. Am. Chem. SOC.1982,104,6340.
0743-7463/94/2410-4077$04.50/00 1994 American Chemical Society
4078 Langmuir, Vol. 10, No. 21, 1994
Triantafillou et al.
Table 1. Ranges Used in the Isolation of the Major Contributions in the EXAFS Data supporthreatment k range, A-1 r rahge, A FT weighting kn MgO calcined at 700 "C 2.8-11.5 -0-4 3 [H3RedC0)1~1 MgO calcined at 700 "C 3.5-10.2 0.2-4.2 3 [HRe(CO)sl precursor
MgO calcined a t 400 "C MgO calcined a t 250 "C MgO calcined at 250 Wtreated with CHaOH-saturated He at 150 "C MgO calcined at 390 "C [Rez(CO)lol [ R ~ ~ ( C O ) ~ Z ( O H ) ~MgO ] calcined a t 390 "C [HRe(CO)sl [DRe(CO)d EDRe(C0)d
The rhenium subcarbonyls were prepared in the sealed E M S cells as gases flowed through them or as they were evacuated. The samples prepared from [DRe(C0)51 were treated in HZand scanned under an atmosphere of Hz for the E M S measurements. The samples prepared from [Re4(C0)12(0H)4]and from [Rez(CO)lo] were treated under vacuum and scanned in an atmosphere of He. Each sample was scanned twice with the cell at approximately liquid nitrogen temperature. Higher harmonics in the X-ray beam were minimized by detuning the Si(ll1) double-crystal monochromator by 30%; the energy resolution AEIE was 2.0 x The samples were scanned at energies near the Re LIII edge (10535 eV). Characterization by Infrared Spectroscopy. Infrared spectra were measured with a Nicolet 510M or a Nicolet 7199 FTIR spectrometer. Solid samples were pressed into semitransparent wafers in the drybox and loaded into air-tight cells with NaCl windows. Details are given e l ~ e w h e r e . ~
3.5-10.3 3.6-10.4 3.6-10.5
0.6-3.7 0.9-4.1 0.9-4.3
3 2 2
ref 5 9 3 this work this work
3.5-11.0 3.6-13.3
0.5-3.4 0.5-3.4
3 3
this work this work
Table 2. EXAFS Results Characterizing the Rhenium Subcarbonvls PreDared from [DRe(CO)d on M f l a a o " shell Re-C Re-0, Re-O*
N 2.8 2.5 3.0
R,A 1.88 2.16 3.09
OC"
EXAFS N
The EXAFS data were analyzed by essentially the same procedure that has been r e p ~ r t e d . ~[Os3(CO)121 -~ (which has only terminal CO ligands) and Reo3 were used as reference materials in the analysis. The EXAFS parameters characteristic ofthese reference materials and details of the preparation of the reference files are reported e l ~ e w h e r e . The ~ averaged data for each sample were analyzed by the difference file technique of Koningsberger et al.12J3 The data were Fourier-filtered over the useful range. The k ranges (k is the wave vector) used for the forward Fourier transformations and the r ranges ( r is the distance from the absorbing Re atom) used for the inverse transformations are shown in Table 1. The k and T- ranges used are nearly the same for all the samples. The kl- and k3-weightedFourier transforms ofthe EXAFS data were used to optimize the fits.
Re-C Re-0, Re-O*
2.8 3.1 2.8
a
R.A 1.85 2.13 3.07
(10)Lorntson,J.M.Ph.D. Dissertation,UniversityofDelaware, 1980. (11)Kampers, F.W.H. Ph.D. Dissertation, Eindhoven Universityof Technoloev. The Netherlands., 1988. _", (12)Duivenvoorden,F.B. M.; Koningsberger,D. C.; Uh,Y.S.;Gates, B. C. J.A m . Chem. SOC. 108. -.. 1986. ~ . , -. _ - , 6264. -_ -- (13)van Zon, J. B. A. D.; Koningsberger, D. C.; van't Blik, H. F. J.; Sayers, D. E. J. Chem. Phys. 1986,82,5742. ~~~~
Auz.Az
AEn. eV
0.0011 0.0029 -0.0020
10.4 3.1 -0.8
reference
os-c Re-0
os-o*
Notation as in Table 2.
Table 4. EXAFS Results Characterizing the Rhenium Subcarbonyls Prepared from [Rea(CO)iol on M&SSO= shell Re-C Re-0, Re-O* Re-Ozb a
N 2.8 3.1 2.9 4.9
R,A 1.88 2.18 3.08 3.68
Auz,A2 0.0035 0.0034 0.0001 0.0028
A E o , eV 8.0 -0.2 3.9 0.4
EXAFS reference
os-c Re-0
os-o*
Re-0
Notation as in Table 2. Higher-shell Re-0 contribution.
Table 5. EXAFS Results Characterizing the Rhenium Subcarbonyls Prepared from [Re4(CO)ls(OH)4]on
Results
MgO-supported rhenium subcarbonyls have been prepared from a variety of precursors, including [HRe-
os-o*
Table 3. EXAFS Results Characterizing the Rhenium Subcarbonyls Prepared from [DRe(CO)5l on MgOmo After the Sample Was Exposed to CHsOH-SaturatedHe at 150 shell
Discussion
E G S reference os-c Re-0
Notation: N , coordination number for absorber-backscatterer pair; R,radial distance; A$, Debye-Waller factor, difference with respect to reference compound; -0, inner potential correction (correction on the edge position). The subscript on MgO refers to the calcination temperature in "C.
Analysis of EXAFS Spectra
The coordination parameters obtained from the detailed analysis of the E M S data characterizing the preparations reported here are shownin Tables 2-5. The EXAFS parameters characterizing the complete family of rhenium subcarbonyls are summarized in Table 6. The infrared spectra for the complete family of rhenium subcarbonyls on MgO are summarized in Table 7. The raw EXAFS data characterizing the samples prepared from [DRe(C0)61, [ReACO)d, and [Re4(CO),z(OH),] on MgO are shown in Figure 1A-C, respectively. The fits, both in K space and in r space, are shown in Figures 2-4, respectively. The fits are very good.
0.0033 0.0150 -0.0018
AEo,eV 8.8 -2.5 -1.6
MgOseo" shell Re-C Re-0, Re-O* Re-02
N 3.2 3.1 3.3 5.6
R,A 1.90 2.17 3.07 3.63
Aaz,A2
0.0027 0.0020 0.0000 0.0028
A E o , eV 5.3 0.2 4.0 5.9
EXAFS reference
os-c
Re-0
os-o*
Re-0
Notation as in Table 4.
(CO)51,3-5 [Re&O)l01,~ [ H ~ R ~ ~ ( C O ) and I ~ I ,K~~JR~e c 1 6 . ~ ~ The very weakly acidic [HRe(C0I51has been reported to bond to the basic MgO surface through a n acid-base interaction. Kirlin et al.3reported evidence consistent with the partial deprotonation of [HRe(C0I5]on the MgO surface to form ion pairs, represented as [Re(C0)61-.* .H+(OMg}, where the braces refer to groups terminating the MgO surface. Essentially the same acid-base chemistry has been observed for other weakly acidic metal carbonyl hydrides on MgO, e.g., [HzO~(C0)41.'~Treatment of the sample containing the ion pairs, [Re(C0)51-- .H+{OMg),
~
(14)Kirlin, P. S.; DeThomas, F. A.; Bailey, J. W.; Gold, H. S.; Dybowski, C.; Gates, B. C. J. Phys. Chem. 1986,90,4882. (15)Purnell, S. K.;Gates, B. C. J. Mol. Cutal., in press. (16)Lamb, H. H.; Gates, B. C. J.Am. Chem. SOC. 1983,105, 1179.
Langmuir, Vol. 10,No. 11, 1994 4079
Rhenium Subcarbonyls on MgO
Table 6. S u m m a r y of EXAFS Results Characterizing a Family of Rhenium Subcarbonyls on MgOa Re-C precursor [H3RedCO)izI [HRe(C0)51 [HRe(CO)sl [DRe(C0)51 [DRe(C0)53 [ReZ(CO)lOl [Re4(C0)1~(0H)41 a
supportkreatment MgO calcined at 700 "C MgO calcined at 700 "C MgO calcined at 400 "C MgO calcined at 250 "C MgO calcined at 250 Wtreated in CHaOH-saturated He at 150 "C MgO calcined at 390 "C MgO calcined at 390 "C
Re-O*
N 3 3 3.3 2.8 3.1
R,A
N
R,A
1.88 1.91 1.87 1.88 1.85
3 3.2 3.1 3.0 2.8
3.09 3.12 3.11 3.09 3.07
Re-Osupport N R,A 2.7 2.15 2.8 2.13 2.8 2.13 2.5 2.16 3.1 2.13
3.3 3.2
1.89 1.90
2.9 3.3
3.08 3.07
3.3 3.1
2.17 2.17
ref 5 3 3 this work this work this work this work
Notation as in Table 2.
Table 7. CO Stretching Frequencies in Rhenium Carbonyls Drecursor
solvent or suuport/treatment uncalcined MgO, treated with CH3OH-saturatedCO at 350 "C MgO calcined at 390 "C, dosed with CH30H at 200 "C MgO calcined at 400 "C MgO calcined at 250 "C, then treated with CH30Hsaturated He at 150 "C MgO calcined at 390 "C, dosed with water at 200 "C MgO calcined at 700 "C MgO calcined at 250 "C MgO calcined at 390 "C, spectrum at 395 "Cin 1 atm of equimolar CO H2 MgO calcined at 700 "C MgO calcined at 700 "C, spectrum at 200 "C,in vacuo MgO(ll1) ultrathin film
vco bands, cm-l 2006 (s), 1880 (vs), 1862 (sh)
15
ref
2008 (s), 1885 (vs)
6
2011 (vs), 1895 (vs), 1862 (sh) 2013 (s), 1894 (SI, 1885 (sh)
5 this work
2014 (91, 1894 (vs)
6
2017 (vs), 1908 (vs), 1867 (sh) 2020 (s), 1894 (s), 1885 (sh) 1905 (9) 2022 (4,
5 this work 6
+
3, 14 6
a t 80 "C in Hz, in He, or under vacuum resulted in the partial decarbonylation of the rhenium carbonyl and the formation of three Re-0 bonds anchoring the rhenium subcarbonyl to the surface through oxygen atoms or hydroxyl group^.^ These rhenium subcarbonyls were formulated as [Re(C0)3{OMg},{ HOMg)3-,1 (0 < x .e 3).14 [H3Re3(C0)1~l also reacts with the MgO surface through a n acid-base i n t e r a ~ t i o n . ~Although J~ the very weakly acidic [HRe(C0)51is only partially deprotonated on the MgO surface, the more strongly acidic [H3Re3(C0)121 is quantitatively deprotonated by the basic MgO to form [HzRe3(C0)12]- ions, which could be extracted cleanly from the ~ u r f a c e . ~Treatment J~ of these samples in air a t 25 "C for 18 h, or in Hz or in He a t 225 "C for 4 h, resulted in the formation of rhenium subcarbonyls by oxidative fragmentati~n.'~The formation of numerous other metal subcarbonyls also takes place by oxidative fragmentation.17 Papile7 reported t h a t [Rez(CO)lol reacts rapidly with the MgO surface to form [Re2(C0)&. Upon treatment at 125 "C in vacuo or in He or in air, the surface-bound anions undergo oxidative fragmentation to form mononuclear rhenium subcarbonyls like those formed from [HRe(C0)51.6 Rhenium subcarbonyls have also been prepared directly from adsorbed KzReCl6 on MgO powder by treatment in flowing CH3OH-saturated CO a t 350 "C for 1.5 h.15 Evidently, reductive carbonylation occurred to reduce the rhenium from the +4 oxidation state to that reported for the rhenium subcarbonyls, namely, + l . 1 4 The results reported here confirm the expected preparation of rhenium subcarbonyls from [DRe(C0)51on MgO. The infrared bands (Table 7)agree well with those reported for rhenium subcarbonyls, [Re(C0)3{OMg},{HOMg}3-,1, prepared by other methods. The EXAFS coordination
parameters (Tables 2-4) are also consistent with the presence of this species on the MgO surface. The EXAFS data characterizing the sample prepared from [Rez(CO)lolon MgO (Table 4) likewise confirm the presence of [Re(C0I3{OMg},(HOMg}3-,1. The infrared spectra6 (Table 7) are consistent with the conclusion. The EXAFS data characterizing the sample prepared from [Re4(C0)12(0H)41onMgO (Table 5) are also consistent with the presence of [Re(C0)3{HOMg},{ OMg}3-,1. However, these results are not sufficient to demonstrate the presence of this surface-bound rhenium subcarbonyl because the EXAFS results that would be expected for [Re4(CO)12(OH)Jitself (which in the crystalline state has the cubane structure18) would be indistinguishable from the observed E M S results unless evidence of a Re-Re contribution would be available. Since (1)the data quality was very high, even giving evidence ofa higher-shell Re-0 contribution (with a distance of 3.63 that was detected in only one of the other data sets, namely, that for the sample prepared from [Rez(CO)lol, and since (2) no evidence of a Re-Re contribution was found (which would be expected at a distance of 3.5 Afor the cubane structure) for the supported rhenium species, we infer that the cubane structure was indeed broken up and that rhenium subcarbonyls formed. The summary of the EXAFS data (Table 6) shows that a whole family of rhenium subcarbonyls have been prepared, each with nearly the same structure, namely, [Re(C0)3{OMg},{HOMg}3-,l. The EXAFS results show a remarkable internal consistency. The Re-C coordination number for each member of the family is represented as 3.0 f 0.3, and the Re-C coordination distance is represented as 1.88 f 0.03 These results show that the coordination numbers and distances characterizing
(17) Lamb, H. H.; Gates, B. C.; Knozinger,H. Angeru. Chem.,Int. E d . Engl. 1988,100, 1127.
(18) Nuber, B.; Oberdorfer, F.; Ziegler, M. L. Acta Crystallogr., Sect. B 1981, 37, 2062.
A)
A.
~~
Triantafillou et al.
4080 Langmuir, Vol. 10, No. 11, 1994
= u
. I
0.000
-
-
0.020
$
. I
1
1
1
-0.020 .0.040 3 4.080
6
4
~
k
0'080 0.040 0.040
(i.L)
IO
ia
I4
-0.040
8 i9
'
'
0
'
'
1
'
'
'
'
4
3
2
Vt
,
5
"
'
I
"
, , ,
,
7
6
k 0.12;
,
,
,
'i
,
,
,
,
,
,:
9 10111213
8
(A") I
"
0.08
0.04 L *0.040
I
1
Id
-0.04
-1.1-1
a
4
0.00
6
k
(2')
is
io
14
.0.08
,
r
i
.0.12
'
'
0
1
2
0.040
.0.020
I
I -
l
'
'
~
'
'
"
'
'
l
m
'
5
(A)
r 4
4
3
'
'
'
I
"
'
~
~
'
, ' " ' I
2
i
1 -0.060
2
4
6
8
IO
12
14
16
k (A,') Figure 1. Raw EXAFS data measured near the Re LIII absorption edge for the MgO-supported rhenium subcarbonyls prepared from (A) [RedCO)lol,(B)[Re4(C0)12(0H)41,and (C) [DRe(C0)5]. Treatment conditions are stated in the text.
the Re-C contribution for the entire family of samples agree within 10 and 2%, respectively, and these error bounds are within the expected experimental errors of the EXAFS data analysis. The error in the Re-C coordination number is typically expected to be f 2 0 % ; the results presented here suggest that this is a conservative statement. The Re-O* coordination number (where O* is the oxygen of the carbonyl ligand) for the family is represented as 3.0 f 0.2, and the Re-O* coordination distance is represented as 3.09 f 0.03 A. These results show that the coordination numbers and distances characterizing the Re-0" contribution for the entire family of samples agree within 7 and 1%, respectively. Again the agreement for the Re-O* coordination number is better than what
- 4 ~ ' " " " ' 0 1
~
"
"
'
"
2
3
r
"
'
4
'
'
5
(A)
Figure 2. Results of E M S analysis obtained with best calculated coordination parameters characterizing the MgOsupported rhenium subcarbonyls prepared from [DRe(CO)d (A) Experimental EXAFS (solid line) and the sum of the calculated Re-0, Re-C Re-O* contributions; (B)imaginary part and magnitude of the Fourier transform (kl-weighted, Ak = 3.6-10.4A-')of experimental EXAFS (solid line) and the Re-O* contributions sum of the calculated Re-0, + Re-C (dashed line);(C) imaginary part and magnitude of the Fourier transform (k3-weighted,Ak = 3.6-10.4A-l) of experimental EXAFS (solid line) and the sum of the calculated Re-0, + Re-C + Re-O* contributions (dashed line).
+
+
+
is usually expected on the basis of the EXAFS data analysis. The Re-0, coordination number (where 0, is oxygen of the MgO support) for the family of rhenium subcarbonyls is represented as 2.8 f 0.3,and the Re-0, coordination distance can be represented as 2.15 f 0.03 A. These results show that the coordination numbers and distances
Langmuir, Vol. 10,No.11, 1994 4081
Rhenium Subcarbonyls on MgO
I
L
0.02
-
0.00
:
.om
-
. I
-O.0Ct. 3
"
"
"
"
.a
n
v
v
"
J
'
"
9
u
''
-0.04
I
11
;kl)
k
(k')
k
e c2
e c2
0.08 0.04
P e c.
f
b
0.00
L
.%
4.04
g
.0.01
L 3
'
.0.12
.-.
'
'
"
'
'
"
'
'
0
1
2
3
4
5
0
1
2
3
4
5
L 0)
-'v / \I IY
L
a
" ,Y Y, u
k
,
-4
0
a
1
,
,
,
3
r
,
,
(
,
,
,
,
j 5
4
(A>
r
(A,
Figure 3. Results of EXAFS analysis obtained with best calculated coordination parameters characterizing the MgOsupported rhenium subcarbonyls prepared from [Re2(CO)lol. (A) Experimental EXAFS (solid line) and the sum of the Re-C Re-O* Re-02 contributions calculated Re-0. (dashed line);(B)imaginary part and magnitude of the Fourier transform (kl-weighted,Ak = 3.7-10.5 A-l) of experimental EXAFS (solid line) and the sum of the calculated Re-0, + Re-C Re-O* Re-02 contributions (dashed line); (C) imaginary part and magnitude of the Fourier transform (K3weighted, Ak = 3.7-10.5 Awl)of experimental EXAFS (solid Re-C Re-O* line) and the sum of the calculated Re-0, Re-02 contributions (dashed line).
Figure 4. Results of EXAFS analysis obtained with best calculated coordination parameters characterizing the MgOsupported rhenium subcarbonyls prepared from [Re4(C0)12(0H)dI. (A) Experimental EXAFS (solid line) and the sum of the calculated Re-0, Re-C Re-O* Re-02 contributions (dashed line);(B) imaginary part and magnitude of the Fourier transform (kl-weighted, Ak = 3.7-12.3 A-1) of experimental EXAFS (solid line) and the sum of the calculated Re-0, Re-C Re-O* Re-02 contributions (dashed line); (C) imaginary part and magnitude of the Fourier transform (k3weighted, Ak = 3.7-12.3 A-l) of experimental EXAFS (solid Re-C Re-O* line) and the sum of the calculated Re-0, Re-02 contributions (dashed line).
characterizing the Re-0, contribution for the entire family of samples agree within 11and 2%, respectively. Again the agreement for the Re-0, coordination number is better than what is normally expected. The Re-0, distance is a bonding distance, matching that observed for numerous metal subcarbonyls and analogous compounds with metal-oxygen bonds4 and also matching the Pt-0 distance in supported platinum crystallites following treatment in Hz a t high temperatures or in the absence ofH2.I9 The distance has been associated with metal atoms bearing a small positive charge and oxygen a t the metalsupport interface.*
In summary, we conclude that rhenium subcarbonyls with nearly uniform structures can be prepared by a variety of routes using several different precursors. More importantly, the agreement in the EXAFS data is better than expected. Typically, coordination numbers obtained from EXAFS are reported to be within a n experimental error of f20-30%. The EXAFS results compiled here show t h a t for these simple precise structures, the error may be only about 10%. The coordination distances for
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
~~~~
+
+
~
(19) Vaarkamp, M.; Modica, F. S.; Miller, J. T.; Koningsberger, D. C. J . Catal. 1993,144, 611.
4082 Langmuir, Vol. 10, No. 11, 1994 the family of rhenium subcarbonyls are in agreement within 1-2%, as is typically reported for EXAFS results. Papile and Gates6 reported that the positions of the vco bands characterizing rhenium subcarbonyls are sensitive to the degree of hydroxylation of the support. Rhenium subcarbonyls prepared on highly dehydroxylated MgO exhibited bands at higher energies than those observed for the same species on moderately dehydroxylated MgO. Rhenium subcarbonyl samples subsequently treated in He saturated with water or CH30H were characterized by bands at the lowest energies.6 Table 2 shows that this trend broadly holds for all the samples. Careful attention must be given to the support treatment when vco bands are compared. Consistent with the conclusions of Deutsch et a1.,20who investigated osmium subcarbonyls on y-AlzO3, the results summarized here are consistent with the coordinative saturation of the metal oxide-supported metal subcarbonyls, which are inferred from the electron counts.21These show that the rhenium in the surface species was present in the +1oxidation state. l4 The EXAFS and infrared data indicate a Cav symmetry of the surface-bound rhenium subcarbonyls. The indications of this symmetry have led to the suggestion that the rhenium subcarbonyls on the surface of MgO powder were bonded not to the predominant square (100) faces, but rather were bonded to other faces andor a t defect sites.6 Recent infrared results obtained for rhenium subcarbonyls bonded to the hex(20) Deutsch, S. E.; Chang, J.-R.; Gates, B. C. Langmuir 1993,9, 1284. (21)Collman, J. P.; Hegedus, L. S.; Norton, J. R.; Finke, R. G. Principles and Applications of Organotransition Metal Chemistry; University Science Books: Mill Valley, CA, 1987; Chapter 2.
Triantafillou et al. agonal (111)face of MgO thin films confirm the symmetry and reinforce the inference that the rhenium subcarbonyls bonded to MgO powder are present on faces other than the (100)face or a t surface defects.22 Thus, these surface species are precise analogues of molecular rhenium carbonyls. The analogy is supported by both the coordination numbers and coordination distances for all the members of the family.
Conclusions This compilation of infrared and EXAFS data shows that rhenium subcarbonyls can be prepared on MgO from a variety of rhenium precursors, including rhenium carbonyls and a rhenium salt. The decomposition of the initially chemisorbed species t o give the rhenium subcarbonyl can be accomplished by a variety of routes at relatively mild temperatures, including treatment in H2, He, 0 2 , or under vacuum. The ease of preparation of the rhenium subcarbonyls reflects the stability of these species. EXAFS results show that it is possible to obtain precise information about the structure of the rhenium subcarbonyls, including the structure of the metalsupport interface. These rhenium subcarbonyls are among the best characterized examples of adsorbates on metal oxide powders.
Acknowledgment. This work was supported by the
U.S.Department of Energy, Office of Energy Research, Office of Basic Energy Sciences (Contract No. F W 2 87ER13790). (22) Purnell, S. K.; Xu, X.; Goodman, D. W.; Gates, B. C. J . Phys. Chem. 1994,98,4076.
