STRUCTURESOF C,H,O COMPOUNDS ADSORBED ON Fe
that some process of equilibrium with the atmosphere is taking place.
Acknowledgment. The author thanks B. J. Corbitt for his assistance in making the measurements and R. &I.Youmans and P. V. Vittorio for making avail-
893
able the furnace. He also is indebted to Dr. M. N. Plooster for valuable discussions on the properties of A1203 melts and to the Crystal Products Department of the Linde Division for permission to publish these results.
Structures of Some C,H,O Compounds Adsorbed on Iron'
by G. Blyholder and L. D. Neff Department of Chemistry, University of Arkansas, Fayetteville, Arkansas
(Receined October 22, 1966)
The infrared spectra over the range 4000 to 300 cm-' of methyl, ethyl, n-propyl, n-butyl, isopropyl, isobutyl, and t-butyl alcohols, acetaldehyde, ethylene oxide, acetone, methyl ethyl ketone, diethyl ether, methyl vinyl ether, and tetrahydrofuran adsorbed on Fe have been obtained. All except diethyl ether and tetrahydrofuran, which gave no infrared evidence of adsorption, adsorb at 25" to give an alkoxide structure as the main stable surface species. The alkoxide structure is shown to be in accord with the main stream of thought on the stability of organometallic compounds, thus linking surface chemistry and organometallic chemistry firmly on a structural basis. An alkoxide structure is proposed for intermediates in the Fischer-Tropsch synthesis reaction.
Introduction Carbon-, hydrogen-, and oxygen-containing molecules have been found to undergo many interesting and occasionally useful reactions on metal surfaces. As well as hydrogenation and hydrogenolysis of many kinds of compounds, dehydration of many compounds including alcohols, which yield aldehydes and esters, have been observed. CO and Hzinteract on metal surfaces under appropriate conditions to produce hydrocarbons and alcohols. Although there is a voluminous literature on reactions occurring on metal surfaces, there is relatively little direct experimental evidence about the structure of species adsorbed on metal surfaces. Infrared spectroscopy has proved to be one of the most effective ways of obtaining structural information about adsorbed species. Most of the spectral work has concerned CO and hydrocarbons with carbon-, hydrogen-, and oxygen-containing molecules being somewhat neglected. The interaction of
methyl and ethyl alcohols2 and a variety of alcohols and other oxygen-containing molecules3 adsorbed on Ni have been studied by infrared spectroscopy in this laboratory. On Ni these studies indicated that at room temperature chemisorbed CO and acyl structures are the most stable species. When the carbon atom to which the oxygen is bonded is bonded to only one other carbon atom, that C-C bond is readily broken to produce chemisorbed CO. This decomposition to CO and acyl structures indicates that hydrogen atoms are fairly readily removed by the surface interaction. In this paper the infrared spectra of a variety of C,H,O compounds adsorbed on Fe are examined. Since Fe is generally found to be not so good a dehy(1) This paper is taken in part from the Ph.D. dissertation of L. D. Neff, University of Arkansas, 1964. (2) G. Blyholder and L. D. Neff, J. Catalysis, 2 , 138 (1963). (3) G. Blyholder and L. D. Neff, in preparation.
Volume YO, Sumber 3 March 1066
G. BLYHOLDER AND L. D. NEFF
Table I: Spectra and Assignments of Adsorbed Species on Fe
Adsorbate
Methyl alcohol 1.5 cm for 1.5 hr
Ethyl alcohol 1.5 cm for 4.5 hr
Frequencies,
Model compound frequencies,
cm-1
om-'
825 m
Methyl alcohols 1112 (liq), CHI rocking 1057 (gas) 1030 s 1030 s CO str 1800 vw Added CO Ethyl alcohol8 Chemisorbed
1920 vw
888 s
co
1273 m 1149 vw 1089 s 1050 s 880 s 802 w 657 vs 433 m
476 w 1800 w 571 w
n-Butyl alcohol 0.7 cm for 8 hr
Adsorbate
1058 s
1148 vw 1095 s 1050 s
n-Propyl alcohol 1.6 cm for 1 hr
Assignments
Frequencies, om -1
915 m 895 sh 820 w 560 w 1875
1158m
1130 sh
1118s 950 s 935 sh
The Journal of Physical Chemistry
1025 m 985 m
Added CO
Isopropyl alcoholli 1250 s 1162 s
1113sh 950 s 933 sh
1160 w
M-0 str Added CO
n-Butyl alcohol10 1114m 1070s 1043s 1021 w 997 m 960m 954m 890 w 904 w 840 vw 853 m 540 w XI-0 st r 1800 w Added CO
Isopropyl alcohol 2.5 cm for 2.5 hr
Isobutyl alcohol 1.4 cm for 6 hr
1120 sh 1100 m
1112vw 1060 s 1040s
OH bend Skeletal str and bend Skeletal str CH3 rocking CHa rocking Skeletal plus CH3
cm-1
818 m 485 m 420 m
530 w 1850 m
CH2 twist CH, rocking CH1 rocking Skeletal str Skeletal str CH2 OH bend Skeletal bend
%-Propyl alcohol# 1140 w 1095 sh 1100m 1075 sh 1055 s 1060s 1015 m 1020s 990 w 970 sh 972 s 918 w 908 w 890 w 890 m 860 w 540 w M-0 str 1800 w Added CO
Model compound frequencies, Assignments
Skeletal str Skeletal bend Skeletal bend M-0 str Added CO
Isobutyl alcohol'* 1150 m 1135 sh 1125 sh 1112 s 1085 w 1048 w 1030 s 990 s 968 m 942 vw 912 s 895 sh 820 m
11-0 str Added CO t-Butyl
&Butyl alcohol
1165 m 920 m 875 w 765 w
1242 m 1201 s 1185 s, sh 1023 m 915 s 880 sh 750 s 465 42 4
1900 m 580 w Acetaldehyde 2.6 cm for 3 hr
Ethylene oxide 2.7 cm for 3 hr
Acetone 2 cm for 5 hr
1150 w 1090 m 1045 m 885 w 1850 w
4dsorbed ethyl alcohol above 1148 vw 1095 s 1050 s 888 s
Adsorbed ethyl alcohol above 1095 s 1090 w 1050 5 1035 w 888 s 885 vw 610 w, vb
1140w 1115 w 950 w
Skeletal str Skeletal str Skeletal str CH, rocking CHJ rocking Skeletal str Skeletal bend Skeletal bend Added CO Added CO
CH, rocking CH3 rocking Skeletal str Skeletal str Added CO
CH3 rocking Skeletal str Skeletal str Chemisorbed oxygen
-4dsorbed isopropyl alcohol above 1158 m Skeletal str and bend 1118 CH, rocking 950 s CHI rocking
STRUCTURES OF C,H,O COMPOUNDS ADSORBED ON Fe
Table I (Continued)
Adsorbate
Frequencies,
Model compound frequencies,
cm-1
cm-1
Assignments
930 sh
935 sh
820 w
825 m 530 w
Skeletal plus CHa Skeletal str
1925 m
Methyl ethyl ketone 2.4 cm for 4.5 hr
1160 vw 1100 w 1030 w 990 w 91Ovw 820vw 560 vw 1900 m
Diethyl ether 4 cm for 5 hr
Tetrahydrofuran 3 cm for 3 hr
Adsorbed isobutyl alcohol above 1160 w 1120 sh 1095 m 1025 m 985 m 915m 895 sh 820 w 560 w Added CO
Xo bands
1950 b
Methyl vinyl ether 2 cm for 2.5 hr
Added CO
1048 1015m 1920 m
Added CO Adsorbed methyl alcohol above 105s s CHa rocking 1030s CO str Added CO
X o bands
1950 s
Added CO
drogenation catalyst as Xi, the hydrogen-stripping reactions observed with Xi are expected to be less prominent. The interactions of alcohols with Fe are of interest in Fischer-Tropsch mechanism studies since Fe is a good catalyst for the reaction, and alcoholictype intermediates have been p r ~ p o s e d . ~It has also been found that alcohols fed into the reactant gas stream are incorporated into the product^.^ We have previously studied the interaction of CO and Hz on silica-supported Fee6 Because of the silica support, the usable infrared range is limited to from about 4000 to 1350 cm-l. This limited range makes identification of surface species difficult in many cases. I n this study a wide spectral range technique developed in this laboratory is used to examine the structure of stable surface species at 25”. By determining the structure of stable surface species it is hoped that the relationship between surface chemistry and the rest of organometallic chemistry can be made more explicit.
895
Experimental Section The wide spectral range experimental technique, which has been described in detail elsewhere,’ consists of evaporating Fe from an electrically heated tungsten filament in the presence of a small pressure of helium. The metal particles formed in the gas phase deposit in an oil film on the salt windows of an infrared cell. The gas to be studied is then admitted to the cell, and the spectrum of the chemisorbed species is obtained. Spectra are recorded before and after admission of the gas to the cell. Five minutes of pumping has been found sufficient to remove all spectra due to gas phase molecules. For three- and four-carbon atom molecules 0.5 hr may be required to pump out molecules dissolved in the oil film. The spectra were obtained using Perkin-Elmer Model 21 and 337 spectrophotometers. The Model 21 is equipped with CsBr optics which permit scanning from 715 to 250 cm-’. The 337, which is a grating instrument, is used to scan the region from 4000 to 400 cm-l. The adsorbates were obtained as reagent grade chemicals from commercial sources. They were degassed by repeated freeze-thaw cycles with pumping and distilled into storage vessels on the vacuum system. The CO was passed through an activated-charcoal trap cooled with liquid air. Results The experimental results are given in Table I n column 1 are listed the adsorbates and the length of their exposure to the adsorbent. In the second column are listed the observed bands for the adsorbed species. These spectra were recorded after the adsorbate was evacuated from the gas phase and the oil film. I n the third column are listed the bands for appropriate comparison compounds while the last column lists assignments for the comparison compounds that are assumed also to apply to bands for adsorbed species on the same line in the table. (4) H. H. Storch, N. Golumbic, and R. B. Anderson, “The FischerTropsch and Related Synthesis,” John Wiley and Sons, Inc., New York, N. Y., 1951. (5) R. B. Anderson, Catalysis, 4, 257 (1965). (6) G. Blyholder and L. D. Neff, J . Phys. Chem., 66, 1664 (1962). (7) G. Blyholder, J . Chem. Phys., 36, 2036 (1962). (8) C. Tanaka, Nippon Kagaku Zasshi, 83, 792 (1962). (9) American Petroleum Institute, Project 44, Spectrum No. 427. (10) American Petroleum Institute, Project 44, Spectrum No. 429. (11) C. Tanaka, -VITippon Kagaku Zasshi, 83, 521, 657 (1962). (12) American Petroleum Institute, Project 44, Spectrum N o . 431. (13) C. Tanaka, Nippon Kagaku Zasshi, 83, (1962). (14) American Petroleum Institute, Project 44, Spectrum No. 432.
Volume 70, Number S March 1966
896
All adsorptions were done at about 25”, which is the average room temperature. After the spectra of the adsorbed species were recorded, the samples were exposed to about 10 mm of CO, and the spectra were again recorded. The bands resulting from this treatment are also listed in column 2 of Table I and are labeled “added CO.” Several spectra are shown in the figures so that an idea of the band shapes observed for the adsorbed species may be gained. Adsorption bands ascribed to C-H stretching and deformation vibrational modes are obscured by corresponding bands of the hydrocarbon oil into which the Fe is evaporated. These bands are located in the general region of 2900, 1460, and 1370 cm-l. Adsorption of a gas sample results in an increase in the band intensities in these regions. Since these changes are difficult to interpret with any sense of assurance, they are not discussed.
Discussion Surprisingly uniform behavior is found for the interaction of the alcohols, aldehydes, ketones, and ethers tested. I n all cases the assignments in Table I lead to principal stable surface species which have an alkoxide structure. The assignments have been made by comparison with the spectra of alcohols. I n some cases detailed assignments were not available in the literature, but even here the spectrum of the adsorbed species is in good agreement with that of the alcohol. In fact, the agreement in all cases is so good that one may wonder why substituting a metal atom for an H atom does not have a larger effect on skeletal vibrations. This agreement appears to be due to both the H and the metal having little effect on the skeletal vibrations. The H atoms have little effect because, while the 0-H force constant is large, the hydrogens are too light to greatly affect skeletal vibrations. I n the case of metal atoms the metal-oxygen bond is relatively weak so that the metal-oxygen frequencies are expected to be around 400 to 500 cm-l. It is a well-known principle that, where there is a large separation in force constants or vibrational frequencies, the modes of motion are fairly independent.l6 This agreement between alkoxide spectra and free alcohol spectra has also been found in the few infrared spectra that have been reported for AI and Ti alko~ides.’~-’~ The possibilities of some other structures were considered and discarded for a variety of reasons. Struttures containing hydrogen atoms attached to unsaturated carbon atoms were eliminated because of the absence of C-H stretching vibrations above 3000 Out the saturated C-H the Oil stretching region, bands for unsaturated C-H groups The Journa2 of Physical Chemistry
G. BLYHOLDER AND L. D. REFF
which usually occur near 3100 cm-1 *O should be clearly visible. Structures containing 0-H groups were eliminated because of the absence of 0-H stretching and bending vibrations which are clearly evident when gas phase alcohol is in the cell. The intensity of OH bands for free alcohols is near that of the other bands so that, if the surface structures contain OH groups, their bands should have been strong. Likewise, structures containing carbon-oxygen double bonds were eliminated because of the absence of a band around 1700 cm-l. Even s-complexed double bonds are only shifted within about 100 cm-’ so these too are presumed absent. Two-point (or more) attachment to the surface, in which both a carbonmetal and an oxygen-metal bond are present, is eliminated in the case of methyl alcohol adsorption because of the presence of the CH3 rocking band at 1060 cm-I and in the other cases because multiple attachment is expected to perturb the skeletal vibrations more than is observed. When it is stated that these structures are eliminated, it is not meant that they cannot exist in small concentrations on the surface but only that they do not exist in sufficient concentration to be observed in our spectra. It may be noted in Table I that the exposure times vary considerably. It was found that the spectra were unchanged for exposure times greater than 1 hr so the actual times used were dictated by convenience.
# 30
20 2000
Alcohol
b ---Chemisorbed c -.-Effect of
I
I I800
I
I 1600
I
I
I
I I200
1400
I
Chemisorbed
I I000
I
I 800
CO I
I 600
cm-l
Figure 1. Spectrum of methyl alcohol adsorbed on Fe at 25’.
(15) E. B. Wilson, Jr., J. C. Decius, and P. C. Cross, “Molecular Vibrations,” McGraw-Hill Book Co., Inc., New York. N. Y.,1955. (16) J. V. Bell, J. Heisler, H. Tannenbaum, and J. Goldenson, Anal. Chem.9 25, 1720 (1953). H. Bauer, and J. Goldenson. (17) D. L.Guertin, 8. E. Wiberley, J . Phys. Chem., 60, 1018 (1956). (18) R.C.Wilhoit, J. R. Burton, F. Kuo, S. Huang, and A. Viguesnel, J . Inorg. Nucl. Chem., 24, 851 (1962). (19) V. H. Kriegamann and K. Licht, Z . Elektrochem., 6 2 , 1163 (1958). (20) K. Nakanishi, “Infrared Absorption Spectroscopy,” HoldenDay, Inc., San Francisco, Calif., 1962.
w.
897
STRUCTURES OF C,H,O COMPOUNDS ADSORBED ON Fe
'""I
I
I I
8 2 0 L
3800
GO
I
l
3600
i
l
3400
l
I
3200
I
I
3000
z40
1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1
2800
2600--,2400 2200
2000
I800
1600
1400
1200
1000
800
-
+30-
$20IO
F
800
Figure 2.
700
600
cm''
500
400
300
Spectrum of ethyl alcohol adsorbed on Fe at 25'.
The spectrum of the surface species produced by methanol is shown in Figure 1. The band produced by the addition of CO is seen to be very weak at 1800 cm-1. Since CO is known to adsorb strongly with an intense band at about 1970 cm-l, the fact that only a very weak band is produced is interpreted as indicating that the alkoxide structure is strongly adsorbed and covers most of the adsorption sites. The spectrum of ethanol adsorbed on Fe is shown in Figure 2. The very weak band at 1920 em-l is attributed to Chemisorbed CO from decomposition of ethyl alcohol on the surface. While alcohol decomposition has been found to be quite extensive on Nil2 here the amount is small. Ethyl alcohol is the only compound in the series dealt with here that gave observable decomposition products. One new feature in this spectrum is the appearance of a weak band at 476 cm-'. This is more than 40 cm-' from the nearest alcohol skeletal vibration while the other alkoxide vibrations have been found within 10 cm-l of the corresponding free alcohol vibrations. One new feature which may be expected in the spectrum is a metaloxygen stretching vibration. For aluminum alkoxides, bands in this region have been assigned to the metaloxygen stretching ~ i b r a t i 0 n . l ~While this and similar bands for the other adsorbates have been assigned in Table I to the metal-oxygen stretch, this assignment is very tentative. The CO added after ethyl alcohol
produced weak bands at 1800 and 570 cm-l, again indicating the ethyl alcohol adsorption covered most of the surface. The remaining primary alcohols, n-propyl and nbutyl, follow the same pattern. The adsorption seems to be a little less strong since the addition of 10 mm of CO for 1 hr to the cell as usual produces a little larger adsorbed CO band in these cases than for ethyl alcohol, and a small amount of the surface species desorbs. The bands around 1050 em-' are also shifted a few wavenumbers to lower frequencies. Since the CO interacted strongly enough to result in some alkoxide desorption, it is not surprising that the bands near 1050 cm-l which involve the C-0 stretching vibration are affected some. Bands in other regions are not noticeably shifted by the CO treatment. The secondary alcohols, isopropyl and isobutyl, do not seem to be so tightly held or to occupy the surface sites so extensively as the primary alcohols. These conclusions are based on the facts that the CO treatment appears to cause the desorption of a few per cent of the adsorbed species and that the band from added CO reaches medium intensity. The lower coverage of surface sites may be due to the branched carbon chain blocking the approach to adjacent surface sites of other bulky molecules. The smaller CO molecules might still be able to adsorb on these sites. Volume 70.Xumber S March 1066
898
The changes on going from the primary to secondary alcohols are carried further upon going to the tertiary alcohol, t-butyl. Here the bands for the adsorbed species are less intense than for the other alcohols, and the added CO band is of medium to strong intensity. The bands at 116.5, 920, and 875 cm-1 were all shifted by a few cm-l to lower frequencies by the CO adsorption but were not noticeably decreased in intensity. One possible cause of the shift is steric interaction with CO adsorbed on adjacent sites. The first of the nonalcohols to be considered is acetaldehyde. Acetaldehyde might have been expected in a straightforward associative or dissociative manner to produce chemisorbed CO upon dissociation or an acyl structure by losing hydrogen or to bond coordinatively through the oxygen while maintaining a C-0 double bond. Comparison of the observed bands in Table I to known spectra leads to the conclusion that an adsorbed two-carbon alkoxide structure is produced. The band positions all agree well with those produced by adsorbed ethyl alcohol. This structure requires the addition of a hydrogen atom to acetaldehyde. Two possible sources for this hydrogen are cracking of the hydrocarbon oil matrix and decomposition of some of the acetaldehyde. A certain amount of hydrogen from the first source would not be unexpected since the Fe particles are expected still to be fairly hot when they enter the oil during the evaporation process. Beeck2’ has reported that hydrogen layers on Fe are mobile at room temperature so hydrogen produced anywhere on the surface is available everywhere. The bands for the alkoxide structure produced from acetaldehyde are somewhat less intense than those from ethyl alcohol so the surface may well be partially covered with decomposition products vhich do not have any bands other than C-H bands, which would be masked by the oil, strong enough to be observed. The interaction of acetaldehyde would seem to have to be classed as adsorption plus reaction. The production of the alkoxide structure from acetaldehyde is taken as an indication of the stability of the structure on Fe. Ethylene oxide is a structural isomer of acetaldehyde, but the spectrum listed in Table I indicates that one of the stable structures formed is an alkoxide structure similar to that formed by ethyl alcohol and acetaldehyde. However, the bands are weak and a new feature enters the spectrum in that a weak, broad band at 610 cm-l is observed. This band has been previously observed upon oxygen treatment of an Fe surface and so is ascribed to an oxide structure. Since CO did not adsorb after the ethylene oxide treatment and the intensities of the alkoxide bands do not indiThe Journal of Physical Chemistry
G. BLYHOLDER AWD L. D. NEFF
cate nearly enough of that structure to account for this behavior, the surface is presumed to be largely covered by decomposition products. One of these is apparently a surface oxide. Although no other adsorbed species were detected, if ethylene, produced by removing an oxygen atom from ethylene oxide, is associatively adsorbed, only saturated C-H stretching vibrations would be present, and these would be masked by the oil. If gas phase ethylene or ethane had been produced even in relatively small quantities, they should have been detected by characteristic bands at 949 and 821 cm-l, respectively. No bands were observed at these places before the gas phase was evacuated from the cell. The two ketones, acetone and methyl ethyl ketone, followed the path of the aldehydes to produce the corresponding alkoxide structures. I n both cases mediumintensity chemisorbed CO bands resulted from subsequen t CO exposure, indicating only partial surface coverage. Fractional surface coverage by the ketones is also indicated by the weakness of the alkoxide bands produced. One ordinary and one cyclic ether, diethyl ether and tetrahydrofuran, were exposed to the surface with the result that no bands for adsorbed species were observed in either case. Subsequent CO exposure produced a strong band at 1950 cm-’ in each case indicating that the surface had not accidently become deactivated. While ordinary ethers do not appear to produce stable surface species at 25” on our samples, methyl vinyl ether was observed to give them. The bands listed in Table I are most consistent with a methoxide surface species. Since ordinary ethers did not permanently interact with the surface, the strong interaction here is presumed to be due to the presence of the vinyl group. Ethylene is known to chemisorb readily on Fe.22 Presumably, once the vinyl group has secured the molecule to the surface, the methoxy group migrates from carbon attachment to metal attachment. Looked at from the standpoint of organometallic chemistry, which has recently received a great deal of attention, the alkoxide structure is not a surprise. Alkoxides of most transition metals are well known.23 What is perhaps a little unexpected is the uniformity with which the compounds investigated produced the alkoxide structure with very few side reactions. This is a function of the specificity of the Fe surface since this same series of compounds when exposed to a Ki (21) 0.Beeck, Advan. Catalvsis, 2 , 151 (1950). (22) D.0.Hayward and B . M. W. Trapnell, “Chemisorption,” 2nd ed, Butterworth Inc., Washington, D.C., 1964. (23) D. C. Bradley, Progr. Inorg. Chem., 2, 303 (1960).
STRUCTUREB OF C,H,O COMPOUNDS ADSORBED ON Fe
surface either decomposes to give chemisorbed CO or produces an acyl surface ~ t r u c t u r e . ~Acetaldehyde could have readily produced an acyl structure on Fe by adsorption with dissociation of the hydrogen attached to the carbonyl carbon, but instead it hydrogenated to give the alkoxide structure in spite of the limited hydrogen supply. For the alcohols to form an alkoxide structure, they need only lose the hydroxyl hydrogen. The lability of this hydrogen on metal surfaces has been demonstrated by deuterium-exchange experiments. 24 The synthesis of several Fe(II1) alkoxides has been reported.26 They have been found to be polymeric with presumably alkoxide oxygen bridges between Fe atoms. This raises the question as to whether the alkoxide oxygen of the surface species is bonded to one or more Fe surface atoms. The data so far obtained for the spectra in the metal-oxygen stretching region are not sufficient to answer this question. Considerations from organometallic and coordination chemistry on bond stabilities are expected to apply to these surface species. Aquo ligands are among the less stable ligands for Fe(I1) and Fe(II1) ions in solution. If the charged ions show little tendency to accept a pair of electrons to form a coordination bond, the neutral atoms on the surface may be expected to show even less tendency to accept electrons in a coordination bond. This is in accord with the fact that the ethers, which without extensive decomposition can only adsorb by coordination of the oxygen lone pair electrons, do not adsorb. Apparently pure donation of charge from the ligand to an iron surface atom does not produce a stable structure. Where a pure u bond is not stable, the additional formationbf some T bonding sometimes leads to stability. However, the oxygen p orbitals which might participate in a ?r bond with the metal d orbitals are filled so that this interaction can lead only to more charge transfer to the metal, which would apparently already be too much in a pure u-coordination bond. I n the case of the alkoxide structure, the oxygen ligand is donating only one electron into the u bond. This apparently leads to stability. Now the appropriately oriented vacant d orbitals of the Fe atom can accept some charge from the filled ligand p orbitals. The full utilization of metal d orbitals is apparently desirable to achieve maximum ~tability.~'It may be noted that, in the of x i , the d orbitals are nearly filled SO the interaction with filled ligand orbitals is not expected to lead to stability, and indeed the alkoxide structure is not found for adsorption on There is an uncertainty in illfOrmatiOn gained about stable surface Species to pz6
899
reaction mechanism considerations since the reaction may proceed through a small number of active sites which contain species different from those adsorbed on the majority of sites. However, on the assumption that reaction intermediates based on stable structures are at least as worthwhile considering as those formulated in the absence of such information, some comments on the Fischer-Tropsch synthesis will be made. One further reservation is that iron synthesis catalysts are not pure iron but are promoted with varying percentages, usually less than lo%, of various oxides such as 5502, Al2O3,Thoz, and KzO. Storch, Golumbic, and Anderson4 proposed oxygenated intermediates containing mainly OH groups. Emmett and co-workers in a series of tracer experiments on the incorporation of alcohols and other compounds in the synthesis products have found evidence supporting the idea of oxygenated intermed i a t e ~ . ~ * In - ~ ~these papers the attachment of the complex to the surface has been presumed to be through a carbon-metal bond. It has been suggested that attachment to the surface may be through both carbon-metal and oxygen-metal bonds.33 The mechanism presented below is based on the finding of this paper that the alkoxide structure seems to enjoy a special stability on Fe. It is assumed that what is a stable structure at room temperature will be a moderately reactive intermediate at a synthesis temperature of about 200". The Fischer-Tropsch synthesis does require an intermediate that is stable enough on the surface to undergo repeated chainaddition steps. The chain-propagation step could be
R 1
0
I Fe
+ CO(g)
I
+
/
H
0 0
I/
---f
/
Fe
R
R
\
\
C I
0
OH2
\ / Fe
C& H
+
I
(1)
0
I Fe
(24) J. R. Anderson and C. Kemball, Trans. Faraday SoC., 51, 966 (1955). (25) c. Keml.mIl and Stoddart, P T O C . ROY. SOC. (London), A241, 208 (1957). (26) D.C.Bradley, R. K. Multani, and W. Wardlaw, J. Chem. Soc., 4153 (1958). (27) J. W. Richardson, “Organa Metallic Chemistry," H. Zeiss, Ed., Reinhold Publishing Corp., New York, N. Y., 1960.
Volume 70, Xiamber S March 1966
900
G. BLYHOLDER AND L. D. NEFF
There are a numerous ways in which the chain-lengthening process could occur using an alkoxide structure. The second step in the above scheme, rather than being hydrogenation, could be a rearrangement to an acid structure.
R
o+o
0
\
/’
Fe
R
O+
\/ Fe
7+
Fe
H20 (2)
Concerning the possible occurrence of an acid intermediate, in an infrared study of the interaction of CO and H2 at 180” on silica-supported Fe, bands for a surface species were found at 1440 and 1560 cm-1.6 KO assignments were made for these bands in that study, but it may be noted that a bidentate acetate ligand has 6-0 stretching vibrations at about these f r e q u e n ~ i e s . ~These ~ bands could not be produced by the interaction of CO and 0 2 alone in the infrared study. Information on the incorporation of acids in Fischer-Tropsch products might shed some light on the possibility of an acid intermediate. For a chain-branching step the addition of a second alkyl group from an adjacent alkoxide structure, rather than a second hydrogen atom in the last step of eq 1, may be proposed. For a chain-initiating step the hydrogenation of a temporarily oxygen-attached CO molecule may be proposed to produce a methoxy structure. The great power of alcohols to act as chain initiators is seen as the result of alcohol adsorption to produce an alkoxide intermediate being more probable than CO hydrogenation to methoxide. The finding that propi~naldehyde~~ is also a good chain initiator is in accord with the alkoxide intermediate mechanism because aldehydes were found here to adsorb to produce an alkoxide structure. I n eq 1 and 2 the presence of oxygen-attached CO on the surface is not suggested as a stable form since infrared study of CO on Fe35 indicates carbon attachment as the stable form at 25”. However, in the infrared study of the interaction of CO and H2 on silica-
T h e Joitmal of Physical Chemistry
supported Fe, it was shown that no reaction occurred until a high enough temperature mas reached that the gas phase CO was in dynamic equilibrium with the adsorbed CO. Under reaction conditions CO molecules will be continually colliding with the surface, and oxygen-end first collisions will be as likely as carbon-end first collisions. The oxygen-attached CO molecules in eq 1 and 2 are regarded as molecules from the gas phase colliding with the surface. The kinetics of the Fischer-Tropsch reaction have received a thorough treatment by Hall, Kokes, and Emmett.32 Changing the structures of the intermediates they wrote down to ones like those proposed here does not change their kinetic equations so these intermediates are in accord with the observed kinetics. In summary, our infrared study indicates that the stable structure produced by the adsorption of a nuniber of C,H,O compounds on evaporated-into-oil Fe at 25” is an alkoxide structure. This structure is seen to be in accord with the main stream of thinking on the stability of organometallic compounds. Thus, a relationship between surface chemistry and organometallic chemistry is firmly established on a structural basis. The intermediates in the Fischer-Tropsch reaction are proposed to have an alkoxide structure. These intermediates have at least the virtue, if no other, of being in accord with the latest findings on the structure of surface species. Acknowledgment. Acknowledgment is made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, for partial support of this research. (28) J. T. Kummer, H. H. Podgurski, W. B. Spencer, and P. H. Emmett, J. Am. Chem. SOC.,73, 5641 (1951). (29) J. T. Kummer and P. H. Emmett, ibid., 75, 5177 (1953). (30) W. K. Hall, R. J. Kokes, and P. H. Emmett, ibid., 79, 2983 (1957). (31) R. J. Kokes, W. K. Hall, and P. H. Emmett, ibid., 79, 2989 (195i). (32) W. K. Hall, R. 3. Kokes, and P. H. Emmett, ibid., 8 2 , 1027 (1960). (33) G. Blyholder and P. H. Emmett, J . P h y s . Chem., 63, 962 (1959). (34) K. Nakamoto, “Infrared Spectra of Inorganic and Coordination Compounds,” John Wiley and Sons, Inc., New York, N. Y., 1963. (35) G. Blyholder, in preparation.
