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Diffuse Reflectance Infrared Spectroscopy Study of Co2(CO)8 Supported on Alumina Maria Kurhinen and Tapani A. Pakkanen* University of Joensuu, Department of Chemistry, P.O. Box 111, FIN-80101 Joensuu, Finland Received March 24, 1998. In Final Form: September 9, 1998 Surface species formed by the deposition of Co2(CO)8 on partially dehydroxylated aluminas were identified by diffuse reflectance IR spectroscopy (DRIFT). Co2(CO)8 decomposes when it interacts with the surface of alumina, giving rise to smaller surface species, such as Co(CO)4- and Co2+(CO)x in which cobalt is supposed to be in octahedral coordination, as well as to larger Co6 species. The behavior of these species and the formation of new ones, a divalent cobalt carbonyl, in which cobalt is in tetrahedral coordination, and one unidentified, were studied by heating the sample gradually until ν(CO) due to carbonyl species had disappeared. Co2+(CO)x with cobalt in octahedral coordination is the first surface species to disappear. Tetrahedrally coordinated divalent cobalt carbonyl, in turn, was found to be the most stable of the cobalt carbonyl surface species. As side reactions, bicarbonates, carbonates, and formates have been formed. A bicarbonate species has been proposed to be formed simultaneously with the disproportionation of Co2(CO)8. Liberating CO reacts with O- sites, forming monodentate carbonate, which turns to bidentate carbonate species during heating. At higher temperatures, hydrogen releases and reacts with CO forming formate species. We proposed the disproportionation of Co2(CO)8 for the main initial surface reaction on alumina 200. Besides the disproportionation, other initial reactions, such as a formation of Co6 species, exist on aluminas with more Lewis acid/basic sites. Formation of Co6 species and bicarbonates was found to be competitive reactions, most probably they adsorb at similar sites.
Introduction Cobalt is widely used as a catalyst in Fischer-Tropsch synthesis1 and as a promoter in hydrodesulfurization (HDS).2 The traditional precursor for a supported CoMo HDS catalyst is cobalt nitrate.2a-c Recently, however, Okamoto et al.2d have obtained promising results with use of Co2(CO)8 as precursor. Many studies by IR spectroscopy have been made of the interactions between Co2(CO)8 and Co4(CO)12 and oxidic supports.3-13 Silica is the simplest of these supports. Since “Co2(CO)8(ads)” and Co4(CO)12(ads) on silica give similar IR spectra, it has been suggested that dicobalt dimerizes to tetracobalt (1) (a) Beitel, G. A.; de Groot, C. P. M.; Oosterbeek, H.; Wilson, J. H. J. Phys. Chem. B 1997, 101, 4035. (b) Wang, W.-J.; Chen, Y.-W. Appl. Catal. 1991, 223. (c) van de Loosdrecht, J.; van der Haar, M.; van der Kraan, A. M.; van Dillen, A. J.; Geus, J. W. Appl. Catal. A: General 1997, 150, 365. (2) (a) Chianelli, R. R. Catal. Rev.-Sci. Eng. 1984, 26, 361. (b) Prins, R.; De Beer, V. H. J.; Somorjai, G. A. Catal. Rev.-Sci. Eng. 1989, 31, 1. (c) Kora´nyi, T. I.; Paa´l, Z. Appl. Surf. Sci. 1991, 52, 141. (d) Okamoto, Y.; Odawara, M.; Onimatsu, H.; Imanaka, T. Ind. Eng. Chem. Res. 1995, 34, 3703. (3) Schneider, R. L.; Howe, R. F.; Watters, K. L. Inorg. Chem. 1984, 23, 4593. (4) Iwasawa, Y.; Yamada, M.; Sato, Y.; Kuroda, H. J. Mol. Catal. 1984, 23, 95. (5) Nakamura, R.; Oomura, A.; Okada, N.; Echigoya, E. Chem. Lett. 1982, 1463. (6) Nakamura, R.; Okada, N.; Oomura, A.; Echigoya, E. Chem. Lett. 1984, 119. (7) Homs, N.; Choplin, A.; de la Piscina, P. R.; Huang, L.; Garbowski, E.; Sanchez-Delgado, R.; The´olier, A.; Basset, J.-M. Inorg. Chem. 1988, 27, 4030. (8) Rao, K. M.; Spoto, G.; Guglielminotti, E.; Zecchina, A. J. Chem. Soc., Faraday Trans. 1 1988, 84, 2195. (9) Schneider, R. L.; Howe, R. F.; Watters, K. L. Inorg. Chem. 1984, 23, 4600. (10) Alve`s, R.; Ballivet-Tkatchenko, D.; Coudurier, D.; Duc Chau, N.; Santra, M. Bull. Soc. Chim. Fr. 1985, 3, 386. (11) Connaway, M. C.; Hanson, B. E. Inorg. Chem. 1986, 25, 1445. (12) Shen, G.-C.; Shido, T.; Ichikawa, M. J. Phys. Chem. 1996, 100, 16947. (13) Lisitsyn, A. S.; Kuznetsov, V. L.; Yermakov, Yu. I. React. Kinet. Catal. Lett. 1980, 14, 445.
species.3,4 The surface of alumina is much more heterogeneous than silica, and the surface reactions are different; various surface species, including Co2(CO)8, Co4(CO)12, Co6(CO)16-x, and Co(CO)4- and also some unidentified R and β species, have been observed by IR spectroscopy.3-6 Similar surface species have been found on magnesia,7,8 which is more basic than alumina, and also on zeolites, although the species formed on zeolites depend on the type of zeolite and the exchanged cation.9-12 The identification of these surface species is not always unambiguous although clear identifications are essential if we are to understand reaction mechanisms. Diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy is widely used in characterizing supports and catalysts and studying catalysis in situ on the surface.14 The aim of our work was to study by a DRIFT technique the thermal stability of the initial surface species of cobalt carbonyls, further surface reactions, and decarbonylation processes by heating the “Co2(CO)8”/alumina samples gradually under an inert atmosphere. The effect of the dehydroxylation of the support on the reactions and formed surface species was studied. This study may be of help in characterizing the cobalt species on the surface, although the final structure of totally decarbonylated cobalt species remains, unfortunately, undetermined by IR spectroscopy, since alumina has a very strong absorption under ∼1000 cm-1, which region is characteristic for Co-Co vibrations. The results of this study could also be useful in developing new catalysts, for example, for hydrodesulfurization. (14) (a) Boroumand, F.; van der Bergh, H.; Moser, J. E. Anal. Chem. 1994, 66, 2260. (b) Lee, D. H.; Condrate Sr., R. A. Mater. Lett. 1995, 23, 241. (c) Capita´n, M. J.; Centeno, M. A.; Male´t, P.; Carrizosa, I.; Odriozola, J. A.; Ma´rquez, A.; Ferna´ndez Sanz, J. J. Phys. Chem. 1995, 99, 4655. (d) Horr, T. J.; Ralston, J.; Smart, R. St. C. Colloids Surf. 1992, 64, 67. (e) van Every, K. W.; Griffiths, P. R. Appl. Spectrosc. 1991, 45, 347. (f) Tynja¨la¨, P.; Pakkanen, T. T.; Mustama¨ki, S. J. Phys. Chem. B, in press.
10.1021/la9803326 CCC: $15.00 © 1998 American Chemical Society Published on Web 10/31/1998
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Experimental Section Co2(CO)8 was supplied by Fluka Chemika and used without further purification. γ-Alumina (Brockmann I, standard grade 150 mesh, 58 Å, surface area 155 m2/g, Aldrich) provided the support and was calcined (about 70 g) at 200-600 °C for 10 h under vacuum (∼8 × 10-3 Torr). The pretreated alumina is referred to here as alumina 200, 300, 400, 500, and 600 later in the text. The BET surface areas for the pretreated aluminas were 176, 178, 180, 187, and 179 m2/g, respectively. The calcined supports were stored and packed into a fluidized bed reactor in a glovebox (purity of nitrogen 99.999%). The samples were prepared in a fluidized bed reactor (upper diameter 3.7 cm, lower diameter 1.2 cm, length ∼7 cm) by a gas-phase adsorption. Co2(CO)8 was deposited on alumina at 45 °C under carbon monoxide (Technohaus, 99.997%). Typical loading of cobalt was 0.6 wt % on alumina 200 and 0.3 wt % on aluminas 300-600, there were no remarkable changes in the amount of cobalt after deposition and after total decarbonylation. The samples were decarbonylated under nitrogen (AGA, 99.999%) flow gradually for 5 + 5 h at 75, 100, 150, and 200 °C until ν(CO) bands due to carbonyl species had disappeared from the spectra. The carrier gas was circulated via a cold trap at a flow rate of 450-500 mL/min depending on the amount of the sample (5-10 g). Pure gas was added at a flow rate of 30 mL/min, and the normal pressure was retained with a pressure-release mercury bubbler. The samples were characterized after the deposition and decarbonylation steps with a DRIFT spectrometer (Nicolet Impact 400D) connected to a glovebox.15 Spectral resolution was 2 cm-1, and the spectra were presented in a reflectance format. The spectra of the ν(CO) region, 2500-1200 cm-1, reported here are difference spectra from which the pure alumina spectrum has been subtracted (the normalization has not been conducted for subtractions, since alumina has no IR bands at the ν(CO) region); no subtraction was made for the OH region spectrum (39003100 cm-1). The given intensities should be considered as relative, and they are comparable only for bands in the same spectrum: the strongest bands are indicated as very strong (vs) and the intensities of other bands in the same spectrum are compared with these. For comparison, alumina (∼1 g) was also impregnated with Co2(CO)8 and Co4(CO)12 (Co contents under 0.1 wt %) from n-pentane (dried over molecular sieves and purged with N2 before use) and with NaCo(CO)4 from distilled THF (tetrahydrofuran). The solvents were removed with a double-ended needle under slight positive nitrogen pressure and the samples were dried overnight under N2 flow; all handling was done in the absence of air.
Results ν(OH) Region of Aluminas. Calcination most dramatically affects the number of OH groups on the alumina surface: the higher the calcination temperature, the fewer OH groups and the more Al3+ and O- sites are exposed. IR spectra of the ν(OH) region of alumina 200, 400, and 600 are shown in Figure 1. The various OH groups were best resolved in the spectrum of alumina 600 (Figure 1C), in which there are bands at 3784, 3756 (sh), 3731, and 3680 cm-1. The first two are due to OH groups coordinated to tetrahedral and octahedral Al3+ atoms, and the latter two are due to OH groups coordinated to two and three Al3+ atoms, respectively.16 The changes in the intensity of the ν(OH) bands were best seen on alumina 600 (Figure 1C spectra b-c). The OH group giving a stretching band at the highest wavenumber was most affected during deposition and decarbonylation processes. Also the intensity of the other ν(OH) bands diminished compared to the region under 3650 cm-1. After deposition of Co2(15) Kurhinen, M.; Vena¨la¨inen, T.; Pakkanen, T. A. J. Phys. Chem. 1994, 98, 10237. (16) Kno¨zinger, H.; Ratnasamy, P. Catal. Rev.-Sci. Eng. 1978, 17, 31.
Figure 1. OH region of aluminas pretreated at (A) 200, (B) 400, and (C) 600 °C: (a) pure alumina spectra; (b) Co2(CO)8 deposited on alumina; (c) after total decarbonylation (no ν(CO) due to carbonyl species).
(CO)8 on the surface, there was no ν(OH) bands resolved on alumina 200. During heat treatment, however, the bands reappeared and were resolved after total decarbonylation. The ν(OH) band at the lowest wavenumber, 3680 cm-1, was the band that decreased least. The hydrogen bonding between adjacent OH groups is seen particularly well in the spectra of the aluminas calcined at low temperatures, as a broad band at around 3600 cm-1 (Figure 1A spectrum a). The interaction between surface species and OH groups can be seen in the spectra as increased intensity of the broad band at around 3600 cm-1 compared to the same region of the pure alumina spectrum. Co2(CO)8 Deposited on Alumina. The most straightforward spectrum of supported cobalt carbonyls was recorded on alumina 200 (Figure 2a). As the degree of dehydroxylation of the support increases, the IR spectra become more complicated (Figure 2b-d). Relative to alumina 200, the features at 1890-1850 cm-1 are less intense on alumina 300, and the bands at around 17971730 cm-1 are resolved and more intense in comparison with the strongest bands (at around 2034 and 1950 cm-1). The band at 2061 cm-1 is less resolved, and the shoulder at 1992 cm-1 is shifted to 2008 cm-1 and a new one appears at 1971 cm-1. These changes in the shape of the spectrum continue for the supports dehydroxylated at higher temperatures, so that the band at 2035 cm-1 remains the strongest. The bands at 1971-1951 cm-1 diminish in intensity relative to the band 2035 cm-1, as do those at 1890-1850 cm-1, and they all coalesce into the main band. Finally, on alumina 600 (Figure 2e) there are only four resolved bands, the strongest one at 2035 cm-1 and the others at 2132, 1791, and 1724 cm-1. The carbonate region, 1700-1200 cm-1, is discussed below. The weak band at around 2120 cm-1 is present in every spectrum, and it has a shoulder at 2132 cm-1 in the spectra of alumina 300, 400, and 500. On alumina 600 the situation is reversed and there is a shoulder at around 2120 cm-1 on the 2132 cm-1 band. The color of all “Co2(CO)8/alumina” samples was reddish brown regardless of the pretreatment temperature of alumina. Decarbonylation. Alumina 200 seems to be the most reactive of the supports with Co2(CO)8: no ν(CO) due to carbonyl species was seen after decarbonylation for 5 h at 150 °C (Figure 3A spectrum f). More decarbonylation cycles were required (Figure 3) to reach this stage on the other supports. The shape of the spectra of Cox(CO)y/ alumina 200 (Figure 3A) continues to be very clear: the bands at 1947 (vs) and 1886-1850 (vs) cm-1 form the
DRIFT Study of Co2(CO)8 on Alumina
Figure 2. Spectra of Co2(CO)8 supported on alumina pretreated at (a) 200 °C, (b) 300 °C, (c) 400 °C, (d) 500 °C, and (e) 600 °C. The spectra were recorded prior to thermal treatments. The wavenumbers of the bands and shoulders marked with vertical lines are presented in Table 1.
main feature during the decarbonylation. The first band to disappear is the weak one at 2122 cm-1, and the next to diminish and disappear fairly easily is that at 2090 cm-1. Although heating also affects the intensity of the other bands (2058, 2033, and 1985 cm-1), these are still seen after decarbonylation at 100 °C. The bands at 18861850 cm-1 coalesce into the other main band, which has shifted to 1937 cm-1. After 5 h at 150 °C, all ν(CO) due to carbonyl species have disappeared. Alumina 300, 400, and 500 behave in a similar way during the first decarbonylation steps; the band at 2090 cm-1 coalesces into the main feature (2034 cm-1) and new bands are resolved at 2075 and 2061 cm-1 (see Figure 3B for alumina 400). Alumina 300 also shows a new band at 2010 cm-1, but this is seen on alumina 400 and 500 already after deposition. The band at 2075 cm-1 is still seen on alumina 300 after decarbonylation for 5 + 5 h at 75 °C, on alumina 400 after 5 h at 100 °C, and on alumina 500 after 5 + 5 h at 100 °C. During the next decarbonylation step, the band (2075 cm-1) coalesces into the one at 2061 cm-1. The band at 2032 cm-1 begins to diminish during the first 5 h at 75 °C, and during the next steps it coalesces into the band at around 2010 cm-1. Alumina 600 behaves in the opposite way to alumina 200 and also quite simply (Figure 3C). During the first decarbonylation step (5 h at 75 °C), the band at 2091 cm-1 coalesces into the main feature at 2035 cm-1, and a new band appears at 2064 cm-1, with a shoulder at 2071 cm-1 and others at 2015 and 1976 cm-1. During the following steps, the main band at 2035 cm-1 vanishes and coalesces into the band at 2013 cm-1. After 5 h at 100 °C, the 2125 cm-1 band disappears and the three bands at 2064, 2015,
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and 1976 cm-1 become sharper and better resolved. These three bands are the last ones to be seen in the spectra as decarbonylation proceeds; however, they are shifted to 2063, 1999, and 1952 cm-1, respectively. The color of samples turned to light blue at 150 °C in the case of alumina 200 and 300 and on other aluminas at 200 °C. Carbonates. In the region of ν(CO) of carbonate species, ∼1700-1200 cm-1, three main bands appear at around 1650, 1440, and 1230 cm-1 after the deposition of Co2(CO)8 on alumina 200 (Figures 2a and 3A). The carbonate bands are also affected by the degree of dehydroxylation of the surface: the higher the degree, the less the intensity relative to the carbonyls (cf. Figure 2). Also other features, which resolve as bands during decarbonylation, appear in the spectra of alumina 300600 after deposition of Co2(CO)8. Besides the three bands, a shoulder appears at around 1508 cm-1 along with a new broad band with peaks at 1289 and 1273 cm-1 on alumina 300 and 400. On alumina 500, a new band appears at around 1481 cm-1, between the shoulder at 1510 cm-1 and the band at 1450 cm-1, and the two peaks at 1289 and 1273 cm-1 are coalesced into a broad band at 1275 cm-1. On alumina 600, the 1644 cm-1 band is seen as a shoulder on the ν(CO) due to carbonyl species, the 1450 cm-1 band does not resolve under the new band at 1485 cm-1, and the other two bands appear at 1285 (broad) and 1232 cm-1. Relative to the intensities of ν(CO) due to carbonyl species, the intensities of the ν(CO) due to carbonates are strongest on alumina 200 and weakest on alumina 600. The broad band at around 1288-1275 cm-1 and the shoulder at around 1508 cm-1 are the first features to disappear during the decarbonylation at 75 °C for aluminas 200 and 300 and at 100 °C for aluminas 400-600 (Figure 3). After the final decarbonylation step (i.e., no ν(CO) due to carbonyl species are seen), three main bands at 1654 (vs), 1438 (vs), and 1228 (m) cm-1, as at the beginning, remain on alumina 200, and a new one is resolved at 1593 (s) cm-1 with a broad shoulder at around 1529 (s) cm-1 and other shoulders at 1394 (m) and 1373 (m) cm-1. The intensity of the triplet (at around 1650, 1440, and 1230 cm-1) decreases with respect to the other bands in the carbonate region as a function of the degree of dehydroxylation (Figure 3A g spectrum vs Figure 3B,C h spectrum). Impregnations. To assist in the assignment of the observed species, we impregnated NaCo(CO)4 and Co4(CO)12 on aluminas 200-600 and Co2(CO)8 on alumina 200 and 500 (spectra not shown). Since NaCo(CO)4 is highly reactive, some of it reacted further with the surface, while most of it adsorbed on Al3+ sites. The adsorbed species gave bands at 1930, 1898, and 1860 cm-1. The intensity ratios of these three bands were independent of the alumina pretreatment. Although minor changes occurred in the intensities on alumina 600, most probably they were due to interaction with THF adsorbed on the surface. No disturbance due to THF was observed on the other supports. There were further bands at 2102, 2047, and 1979 cm-1, but their relative intensities varied with the support, and since we assumed them to be due to some reacted species of NaCo(CO)4 we do not discuss those bands here. There was also a band at around 2018 cm-1, the wavenumber of which depended on the degree of dehydroxylation of the support, being 2016 cm-1 on alumina 200 and 2022 cm-1 on alumina 600. The origin of this band is adsorbed or reacted cobaltate. Impregnation of Co4(CO)12 on alumina 200 gave a spectrum with one major band at 2027 cm-1. On alumina 300-600, the wavenumber of the band increased a little
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Table 1. Wavenumbers of Co2(CO)8 Adsorbed on Aluminas Pretreated at Different Temperatures (Figure 2)a (a) alumina 200 2122 (vw) 2090 (wm) 2061 (m) 2034 (vs) 1992 (s) 1947 (vs) 1886 (br vs) 1850 (sh vs) 1768 (sh m) 1649 (m) 1593 (sh w) 1440 (m) 1392 (vw) 1372 (vw) 1274 (vw) 1233 (w)
(b) alumina 300
(c) alumina 400
(d) alumina 500
(e) alumina 600
2132 (sh vw) 2122 (vw) 2089 (m)
2133 (sh vw) 2122 (w) 2089 (m)
2133 (sh vw) 2122 (w) 2089 (sh m)
(2063 (sh s)) 2032 (br vs) 2006 (br vs) 1970 (sh vs) 1953 (br vs) 1892 (sh s) 1860 (sh sm)
(2059 (sh vs)) 2034 (br vs) 2010 (vs) 1971 (br vs) 1954 (sh vs) (1893 (sh s)) 1858 (sm)
2134 (w) 2125 (sh w) 2091 (sh s) (2076 (sh vs)) (2062 (sh vs)) 2035 (br vs) (2010 (sh vs))
1797 (sm) 1760 (br sm) 1738 (sh sm) 1650 (m) 1588 (br w) 1508 (sh w)
1794 (sm) 1754 (br sm) 1731 (sm) 1649 (w) 1588 (sh vw) 1505 (br w)
1444 (m)
1447 (w)
1794 (s) 1757 (br sm) 1726 (s) 1651 (w) 1590 (sh w) 1510 (sh w) 1481 (br w) 1452 (w)
1289 (br vw) 1273 (br vw) 1229 (vw)
1288 (br vw) 1275 (vw) 1229 (vw)
1275 (br vw) 1232 (w)
(2063 (sh s)) 2032 (br vs) 2008 (sh vs) 1970 (sh vs) 1951 (vs) 1894 (br s) 1857 (sh s)
1855 (br m) 1828 (br m) 1791 (m) 1753 (br m) 1724 (m) 1644 (sh w) 1486 (br w)
1285 (br vw) 1232 (vw)
a
Intensities of bands: (vw) very weak, (w) weak, (wm) weak medium, (m) medium, (s) strong, (vs) very strong, (sh) shoulder, (br) broad. The values in parentheses are marked with dotted lines in the figure.
Figure 3. Spectra recorded after deposition and decarbonylation cycles of aluminas pretreated at (A) 200 °C, (B) 400 °C, and (C) 600 °C: (a) after deposition and after decarbonylation for (b) 5 h at 75 °C, (c) 5 + 5 h at 75 °C, (d) 5 h at 100 °C, (e) 5 + 5 h at 100 °C, (f) 5 h at 150 °C, (g) 5 + 5 h at 150 °C, (h) 5 h at 200 °C. The wavenumbers of the bands and shoulders marked with vertical lines are presented in Table 2 for alumina 200, Table 3 for alumina 400, and Table 4 for alumina 600.
with pretreatment temperature and was 2038 cm-1 on alumina 600. The other bands, at 2073, 2057, 1990, 1957, 1944, 1889, and 1854 cm-1, were sharpest on alumina 200, and on alumina 600 were coalesced into the main band (2038 cm-1). IR spectra of samples stored in a
glovebox for a few weeks showed increased intensities for the bands at 1949, 1890, and 1854 cm-1 and decreased intensities for the other bands. The changes were most marked on alumina 200 and least on alumina 600. There was also a triplet of bands at around 1650, 1440, and 1230
DRIFT Study of Co2(CO)8 on Alumina
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Table 2. Wavenumbers of Co2(CO)8 on Alumina 200 after Deposition and Decarbonylation Cycles (Figure 3A)a (a) deposition
(b) 5 h at 75 °C
(c) 5 + 5 h at 75 °C
(d) 5 h at 100 °C
(e) 5 + 5 h at 100 °C
2122 (vw) 2090 (wm) 2061 (m) 2034 (vs) 1992 (s) 1947 (vs) 1886 (vs) 1850 (sh vs) (1768 (sh m)) 1649 (m) 1593 (sh w)
2090 (vw) 2059 (m) 2035 (vs) (1995 (sh s)) 1945 (br vs) 1886 (br vs) 1855 (sh vs) (1770 (sh m)) 1651 (m) 1595 (sh w)
2090 (vw) 2058 (s) 2033 (vs) 1990 (sh s) 1944 (vs) 1889 (br vs) 1853 (sh vs) (1768 (sh m)) 1651 (m) (1596 (sh w))
2057 (m) 2033 (s) 1988 (sh s) 1945 (vs) 1891 (vs) 1854 (sh vs) (1760 (sh w)) 1650 (m) 1595 (sh w)
2057 (m) 2027 (br vw) 1985 (m) 1937 (br s) (1900 (sh m)) (1860 (sh w))
1440 (m) 1392 (vw) 1372 (vw) 1274 (vw) 1233 (w)
(f) 5 h at 150 °C
1652 (br vs) 1592 (sh m)
1445 (w) 1394 (vw) 1375 (vw)
1443 (m) 1393 (vw) 1373 (vw)
1437 (m) 1393 (vw) 1375 (vw)
1436 (vs) 1392 (m) 1375 (m)
1654 (vs) 1593 (m) 1529 (sh m) 1438 (vs) 1394 (m) 1375 (m)
1228 (vw)
1229 (w)
1229 (w)
1228 (m)
1228 (m)
a
Intensities of bands: (vw) very weak, (w) weak, (wm) weak medium, (m) medium, (s) strong, (vs) very strong, (sh) shoulder, (br) broad. The values in parentheses are marked with dotted lines in the figure. Table 3. Wavenumbers of Co2(CO)8 on Alumina 400 after Deposition and Decarbonylation Cycles (Figure 3B)a (a) deposition (2133 (sh vw)) 2122 (w) 2089 (m) (2063 (sh s)) 2032 (vs) 2006 (br vs) 1970 (sh vs) 1953 (br vs) 1892 (sh s) 1860 (sh sm) 1794 (sm) 1754 (br sm) 1731 (sm) 1649 (w) (1588 (sh vw)) 1505 (br w)
(b) 5 h at 75 °C (2133 (sh vw)) 2124 (vw) 2074 (s) 2061 (s) 2034 (vs) 2009 (vs) 1970 (sh vs) 1954 (vs) (1894 (sh vs)) 1860 (sh s) 1795 (s) 1753 (sh s) 1732 (s) 1650 (m) 1590 (sh w)
(c) 5 + 5 h at 75 °C
(d) 5 h at 100 °C
(e) 5 + 5 h at 100 °C
2073 (vs) 2062 (vs) 2041 (s) 2009 (vs) 1972 (sh vs) 1954 (vs) (1894 (sh vs)) 1859 (sh s) 1795 (s) (1752 (sh sm)) 1733 (s) 1650 (m) 1588 (sh w)
(2073 (sh s)) 2063 (s) 2039 (sm) 2008 (vs) (1967 (sh vs)) 1953 (br vs) (1894 (br s)) 1858 (sh m) 1797 (m) 1760 (sh m) 1732 (m) 1650 (m) 1590 (sh w)
(f) 5 h at 150 °C
(g) 5 + 5 h at 150 °C
2061 (m) 2029 (sh m) 1998 (s)
2060 (vw) 2029 (sh vw) 1995 (m)
(h) 5 h at 200 °C
2124 (vw) 2074 (s) 2060 (s) 2036 (vs) 2009 (vs) 1970 (sh vs) 1955 (br vs) (1891 (sh vs) 1859 (sh s) 1796 (s) 1752 (sh sm) 1733 (s) 1650 (m) 1587 (sh w)
1447 (w)
1445 (m)
1444 (m)
1288 (vw) 1275 (vw) 1229 (vw)
1291 (vw) 1276 (sh vw) 1232 (w)
1290 (br vw) 1229 (vw)
1950 (br s) (1891 (sh m)) 1858 (w)
1947 (vw)
1769 (br w) 1650 (vs) 1593 (br sh s)
1650 (vs) 1590 (sh s)
1734 (sh vw) 1653 (s) 1594 (vs)
1444 (m) 1394 (w)
1445 (m)
1535 (br m) 1442 (vs) 1396 (sh m)
1540 (br s) 1449 (s) 1391 (m)
1541 (s) 1441 (vs) 1395 (m) 1377 (sh m)
1229 (vw)
1229 (w)
1229 (w)
1229 (w)
1229 (w)
a
Intensities of bands: (vw) very weak, (w) weak, (m) medium, (sm) strong medium, (s) strong, (vs) very strong, (sh) shoulder, (br) broad. The values in parentheses are marked with dotted lines in the figure.
cm-1 on each alumina, and over time the intensity increased with respect to the bands at 2100-1700 cm-1. The IR spectra of Co2(CO)8 impregnated on alumina 200 and 500 were similar to those recorded after gasphase adsorption, except that the bands were better resolved on impregnated alumina 500. Extraction. To find out if species could be extracted from the surface after the deposition, we washed cobalt carbonyl-alumina 200-600 samples with dried pentane. Although the IR spectra of the extract solution showed no other bands than those due to pentane, the spectra of washed cobalt carbonyl-alumina were changed from the parent spectra. Whatever the pretreatment of the support, the bands at around 2120, 2090, and 2030 cm-1 were diminished, those at 1800-1730 cm-1 had disappeared, and those at 2060, 1990, and 1945 cm-1 remained. On alumina 200, the bands at 1890 and 1855 cm-1 also remained, whereas on the other supports they were slightly resolved as shoulders. The spectra of cobalt carbonyl-aluminas pretreated at higher temperatures became sharper after pentane extraction.
Discussion The surface of Al2O3 is heterogeneous: OH groups are bonded to one or more tetrahedrally or octahedrally coordinated aluminum ions. There are also chemisorbed and physisorbed water molecules on the surface. During heating (calcination), physisorbed water desorbs at 150 °C and dehydroxylation begins at about 200 °C as a reaction between two adjacent OH groups.17 The more basic OH group combines with a hydrogen of the more acidic OH group, forming H2O and exposing coordinatively unsaturated Al3+ and O- sites.16 Approximate numbers of OH groups, Lewis acid/base sites, and the Lewis/OH ratio for aluminas pretreated at different temperatures are given in Table 5. Reactions of Co2(CO)8 on γ-Al2O3. Various surface reactions occur and compete when Co2(CO)8 is supported on alumina. Zecchina and Otero Area18 divide the interactions between carbonyl compounds and oxidic (17) Howe, R. F. in Tailored Metal Catalysts; Iwasawa, Y., Ed.; Reidel Publishing Co.: Dorchrecht, The Netherlands, 1986; p 143.
6912 Langmuir, Vol. 14, No. 24, 1998
Kurhinen and Pakkanen
Table 4. Wavenumbers of Co2(CO)8 on Alumina 600 after Deposition and Decarbonylation Cycles (Figure 3C)a (a) deposition 2134 (w) (2125 (sh w)) 2091 (s) (2076 (sh vs)) (2062 (sh vs)) 2035 (br vs) (2010 (sh vs))
(c) 5 + 5 h at 75 °C
(b) 5 h at 75 °C (2136 (sh w)) 2126 (w) (2093 (sh s)) (2073 (sh vs)) 2064 (vs) 2036 (br vs) 2015 (sh vs) 1976 (sh vs) (1754 (sh vs))
2126 (w)
(d) 5 h at 100 °C
(e) 5 + 5 h at 100 °C
(f) 5 h at 150 °C
(g) 5 + 5 h at 150 °C
(h) 5 h at 200 °C
2124 (vw)
(2074 (sh s)) 2066 (s) 2033 (vs) 2013 (vs) 1972 (br vs) 1954 (sh vs)
2069 (vs) (2032 (sh vs)) 2013 (br vs)
2066 (vs)
2066 (s)
2063 (w)
2009 (vs)
2003 (vs)
1999 (w)
1960 (br vs)
1959 (vs)
1959 (s) (1923 (sh s)) 1857 (sh m)
1952 (br w) (1922 (sh w))
1789 (br w)
1767 (br w)
1760 (br vw)
1855 (br m) 1828 (br m) 1791 (sm) 1753 (br m) 1724 (m) 1644 (sh w)
1857 (br m) 1831 (br m) 1793 (m) 1756 (br m) 1723 (m) 1649 (w)
1857 (br m) 1831 (br m) 1793 (m)
1857 (sh m)
1857 (sh m)
1793 (m)
1791 (m)
1722 (m) 1646 (m)
1652 (vs) 1603 (br s) 1537 (br s)
1655 (sh m) 1597 (vs) 1541 (sh s)
1481 (w) 1452 (w)
1481 (wm) 1450 (wm)
1721 (wm) 1654 (sm) 1598 (br m) 1523 (br m) 1481 (m) 1452 (m) 1391 (sh w)
1655 (s) 1598 (br s) 1530 (br s)
1486 (br w)
1721 (m) 1648 (m) 1607 (br m) 1525 (br m) 1484 (br m) 1457 (br m) 1395 (sh w)
1454 (m) 1399 (sh m)
1447 (s) 1394 (sh m)
1454 (s) 1395 (m)
1280 (br vw) 1232 (vw)
1285 (br vw) 1234 (vw)
1236 (vw)
1233 (w)
1233 (w)
1231 (w)
1234 (vw)
1285 (br vw) 1232 (vw) a
Intensities of bands: (vw) very weak, (w) weak, (wm) weak medium, (m) medium, (sm) strong medium, (s) strong, (vs) very strong, (sh) shoulder, (br) broad. The values in parentheses are marked with dotted lines in the figure. Table 5. Approximate Numbers of OH Groups after Calcination at Temperature T (N(T)), Lewis Acid/Base Sites (NL), and the Ratio of NL/OH of Aluminas Calcined at Different Temperatures N(T),a N(T),b T(calcination), OH/nm2 OH/nm2 NL,b °C (this work) (lit.32) nm-2 NL/OHb (OH/NL) 200 300 400 500 600 a
10 8 7 5 3
11 7.9 5.4 3.7 2.3
0.75 2.30 3.55 4.40 5.10
0.07 0.29 0.66 1.19 2.22
(14.3) (3.4) (1.5) (0.8) (0.5)
groups/nm2
The number of OH of aluminas analyzed by hydrogen elemental analysis. b Values in bold are from ref 32, others are taken from plot 3.17 of ref 32 and calculated in the same way as the bold values, by assuming 12.5 OH/nm2 for fully hydroxylated alumina.
surfaces into metal-centered and ligand-centered ones. On partially dehydroxylated alumina, the incoming carbonyl compound interacts via its basic CO ligand with a Bro¨nsted acidic hydroxyl site. This interaction shifts the ν(CO) of the involved CO ligand to lower wavenumbers, in an amount depending on the acidity of the site. The hydrogen bonding to an OH site can also be seen in the OH region as a shift and broadening of the band.18 Formation of a σ-bond between surface Al3+ and oxygen of the CO ligand draws electron density from the carbonyl compound and causes cis-labilization. In IR spectra, the σ-bond to a Lewis acid site can be seen as shifts of 100150 cm-1 and 200-400 cm-1 to lower wavenumbers for the involved terminal and bridging CO stretchings, respectively, and as a simultaneous slight increase of wavenumbers of the CO ligands not involved. The cislabilization leads to decarbonylation and possibly to clustering of migrating subspecies.18 Another way that surface reactions can proceed is through nucleophilic attack by the Lewis base center (O-) or by the Bro¨nsted basic OH- group; as a side reaction, carbonate-like species are formed by the Lewis base center, and bicarbonate by (18) Zecchina, A.; Otero Arean, C. Catal. Rev.-Sci. Eng. 1993, 35, 261.
Table 6. Wavenumbers for ν(CO) Due to Co(CO)4- on Various Supports aluminaa
alumina3,19
1947 1889 1855
1940 (sh) 1900 1870 (sh)
a
zeolite9 zinc NaY NaX magnesia20 oxide21 1945 1938 1941 1899 1901 1891 1876 (sh) 1868 1870
2030 1940 1880 1855
1896 1875
This work.
OH-. Oxygen-bonding to a Lewis site may also cause a transformation of the noninvolved bridging ligand into a terminal position, while the carbonyl dimer decomposes into anionic and cationic carbonyl subspecies.18 Assignment of ν(CO) Bands Due to Carbonyl Species. The easiest surface species to identify was Co(CO)4-, which gave ν(CO) bands at 1889 and 1855 (sh) cm-1 and a third band at around 1947 cm-1. The assignment was made on the following reasoning: In THF, NaCo(CO)4 has three ν(CO) bands, at 2005 (w), 1889 (br, vs), and 1858 (m). If the symmetry of the cobaltate in solution were Td, it would have only one IR active CO stretching (T2). An interaction with a counterion or a solvating molecule, however, most probably would distort the symmetry from Td to C3v, whose 2A1 and E modes are IR active. A corresponding triplet at around 1930, 1889, and 1859 (sh) cm-1 was seen in the IR spectra when NaCo(CO)4 was impregnated from THF onto alumina; the interaction with the surface can be assumed to be stronger but otherwise similar to that in solution. Analogous surface Co(CO)4- species on alumina,3,19 magnesia,7,8,20 zeolites,9-11 and zinc oxide21 are reported in the literature (Table 6). Cobaltate was the major species on alumina 200, as would be expected; the number of OH groups was greater on this than on the other aluminas (see Table 5) and the presence of OH groups favors the formation of anionic (19) Schneider, R. L.; Howe, R. F.; Watters, K. L. J. Catal. 1983, 79, 298. (20) Rao, K. M.; Spoto, G.; Zecchina, A. J. Catal. 1988, 113, 466. (21) Llorca, J.; Homs, N.; Sales, J.; de la Piscina, P. R. J. Mol. Catal. A: Chem. 1995, 96, 49.
DRIFT Study of Co2(CO)8 on Alumina
carbonyl cobaltate.22 The surface cobaltate species was relatively stable; it could be seen in the IR spectra after heating at 100 °C for 5 h. Upon heating for another 5 h at 100 °C the bands at 1890 and 1855 cm-1 coalesced into the main band at 1937 cm-1, which is due to some other surface species (discussed later) than Co(CO)4-. After heating at 150 °C, no ν(CO) due to carbonyl was seen for alumina 200. On aluminas 300-600, the presence of cobaltate was observed as a shoulder on the main band, and other species (discussed later) were dominant. Evidently, the decarbonylation of Co(CO)4- is facilitated by OH groups since on higher dehydroxylated aluminas the shoulder due to cobaltate species was still seen after heating at 150 °C. The cobaltate species is formed by disproportionation of Co2(CO)8 in the presence of a Lewis base22 (B) with simultaneous formation of Co2+, which in the presence of CO may appear in the form Co2+(CO)xBy. The existence of Co2+ species has been indicated by UV spectroscopy on partially dehydroxylated magnesia7 and zinc oxide.21 The Co2+ occurs in an octahedral environment7 and typically is pale red or purple.23 Our results are in agreement with this, for the samples became reddish brown after deposition of Co2(CO)8. Since the bands at around 2120, 2090, and 2030 cm-1 appeared in the first spectrum, they could have been due to the divalent cobalt carbonyl formed during decomposition of Co2(CO)8. This species was not very stable, however, and began to disappear during the first decarbonylation steps. The band at 2120 cm-1 was the first to disappear, but it was also the weakest at the beginning. The extraction experiment likewise showed the three bands to behave in the same way. In reports in the literature, Rao et al.8 sublimed Co2(CO)8 on fully dehydroxylated magnesia, and after exposure of the sample to CO, they observed ν(CO) due to Co2+(CO)x at 2097 and 2040 cm-1.8 Similar bands were observed by Schneider et al.3 at 2098 and 2038 cm-1 and were assigned to R species. The existence of dicobalt octacarbonyl and the formation of tetracobalt dodecacarbonyl have been proposed in previous studies of Co2(CO)8 supported on alumina, by IR spectroscopy and TPDE.3-5 These species were proposed to give ν(CO) bands at 2127 (w), 2070 (s), and 1795 (w) due to Co2(CO)8 and at 2108 (w), 2075 (s), and 1795 (w) due to Co4(CO)12.3 In pentane, Co2(CO)8 gave ν(CO) bands at 2113 (vw), 2070 (vs), 2060 (m), 2044 (vs), 2033 (s), 2024 (vs), 2003 (w), 1993 (w), 1868 (m), and 1868 (m) cm-1, and Co4(CO)12 at 2064 (vs), 2055 (vs), 2038 (w), 2029 (w), and 1868 (m) cm-1. Although in the spectra of Co2(CO)8/Al2O3 adsorbed from the gas phase there are bands at wavenumbers the same as or very similar to those in solution, no bands can be assigned solely to supported Co2(CO)8 or Co4(CO)12. If these species were to interact with the surface via bridging carbonyls, the bands would shift about 200-400 cm-1 to lower wavenumber18 and would appear at around 1600-1400 cm-1. In our studies, bands in that region are assigned to carbonate and bicarbonate species (see below). If, in turn, the interaction were via terminal CO, the shift would be about 100-150 cm-1 to lower wavenumber, and the bands in that region likewise find another explanation. On the other hand, the bands due to noninvolved CO groups should appear at slightly higher wavenumbers. The IR spectra of both gas-phase adsorbed Co2 carbonyl (see, e.g., Figure 2) and impregnated Co2 and Co4 carbonyls (22) Wender, I.; Sternberg, H. W.; Orchin, M. J. Am. Chem. Soc. 1952, 74, 1216. (23) Cotton, F. A.; Wilkinson, G. Advanced Inorganic Chemistry, 5th ed.; John Wiley & Sons: New York, 1988; p 730.
Langmuir, Vol. 14, No. 24, 1998 6913
(not shown) show the surface reactions to have begun: in all cases bands due to bicarbonate are observed. Furthermore, without thermal stabilization the surface species of impregnated “Co4”/alumina samples reacted further, as was observed in IR spectra (not shown) run a few weeks later on the samples stored in the glovebox. The amount of cobaltate species was increased, as well as the amount of bicarbonates, while the bands of other species were weakened. Thus, we suppose that Co4(CO)12 has decomposed to cobaltate and to other smaller clusters that are slightly stabilized by the alumina surface but which decompose further over time. According to the IR spectra, the structure of that unidentified surface species is highly symmetric, giving rise to only one strong band and a few others of weak intensity; this species is not discussed further. Our thermal studies also speak against adsorbed Co2(CO)8 and Co4(CO)12 species: Co2(CO)8 decomposes above 52 °C24a and Co4(CO)12 at 60 °C,24b which means that ν(CO) bands due to those species should not be seen after decarbonylation at 75 °C. The bands assigned by Schneider et al.3 to one of these cobalt species (for example, those at 2127, 2070, and 1795 cm-1) were seen in our studies in IR spectra of samples decarbonylated at 100 °C. On the basis of the above, we suppose that Co2(CO)8 has decomposed and formed cobaltate, Co2+(CO)x, and surface species (discussed later) other than Co4(CO)12. Certainly, existence of Co4 carbonyl as an intermediate structure is possible, and would explain why such species were observed in previous studies.3-5 In our studies, where the interaction time was longer, the Co2 and Co4 species had time to react further. Nakamura et al.6 found Co6 species after deposition of Co2(CO)8 on highly dehydroxylated alumina (pretreated at 950 °C). We also found ν(CO) bands assignable to Co6 species, but on alumina pretreated at lower temperatures (300-600 °C). In solution, Co6(CO)16 gives ν(CO) bands at 2113 (w), 2061(vs), 2057 (sh), 2026 (w), 2020 (w), 2018 (w), 1806 (w), and 1772 (s) cm-1,25 whereas anionic Co6(CO)152- gives them at 2042 (m), 1982 (s), 1959 (sh), 1778 (s), 1737 (s), and 1685 cm-1.26 The terminal CO stretchings are seen at the same wavenumbers as those of other surface species of cobalt carbonyl. However, the hexacobalt species could be assigned with its µ3-CO stretching. We found sharp bands at 1797 and 1730 cm-1 for alumina 300-600, but only a weak shoulder at 1770 cm-1 for alumina 200. This might be due to Co6 species, but it is mostly hidden by Co(CO)4- species. The µ3-CO stretchings were still seen in the spectra after decarbonylation at 100 °C but disappeared after treatment at 150 °C. This behavior lends further support to the assignment since Co6(CO)16 decomposes at 110-120 °C under nitrogen.25 According to the group theory, Co6(CO)16 belongs to the Td point group and has four IR active ν(CO) bands (A1, E, and 2T2) for terminal carbonyls and two (A1 and T2) for bridging carbonyls. Co6(CO)152-, with its C3v symmetry, has 3A1 + 3E IR active modes for terminal carbonyls and 2A1 + 2E for bridging carbonyls. If anionic Co6 interacts with the surface via three terminal CO ligands, the number (24) (a) The MERCK Index, An Encyclopedia of Chemicals and Drugs, 9th ed.; Windholz, M., et al., Eds.; Merck & Co., Inc.: Rahway, NJ, 1976; p 407. (b) Wilkinson, G.; Stone, F. G. A.; Abel, E. W. Comprehensive Organometallic Chemistry, The Synthesis, Reactions and Structures of Organometallic Compounds; Pergamon Press: Oxford, U.K., 1982; Vol. 5, p 2. (25) Chini, P. Inorg. Chem. 1969, 8, 1206. (26) Chini, P.; Albano, V. J. Organomet. Chem. 1968, 15, 433.
6914 Langmuir, Vol. 14, No. 24, 1998
of terminal ν(CO) bands decreases to four (2A1 and 2E), but the number of stretchings due to the bridging carbonyls remains the same. Interaction instead via bridging carbonyls would shift the ν(µ3-CO) to lower wavenumber, but in our spectra they appear at wavenumbers close to those in solution. For our Co6 species we propose the anionic Co6(CO)152-, which has bands due to the two E modes at 1797 and 1730 cm-1. The anionic Co6 species is highly reactive and, after pentane wash, the bands at 1800-1730 disappeared, possibly due to decomposition of Co6 species. The formation of the Co6 species is blocked by the bicarbonate species, or vice versa. On alumina 200, where the amount of bicarbonates is greatest before decarbonylation, the possible Co6 species are seen only as a slight shoulder. The amount of bicarbonate species, in turn, does not increase significantly during decarbonylation until the Co6 species have disappeared. A few more ν(CO) bands remain to be assigned. Band patterns were found that behaved in a similar way in decarbonylation and extraction studies and could thus be assigned to a certain species. The most persistent ν(CO) bands due to carbonyl species were at average wavenumbers 2060, 1995, and 1945 cm-1; the last two bands were a little lower on alumina 200 and a little higher on alumina 600. Apparently, the dehydroxylation of the support affects the interactions. The blue color may imply this surface species to be a divalent cobalt carbonyl in tetrahedral environment (cobalttet).23 Schneider et al.3 found bands at 1995-2005 and 1948-1955 cm-1 and assigned them to an unidentified β species. We suggest that the third band, 2060 cm-1, belongs to this same divalent cobalttet carbonyl species. One interesting feature is observed during heating: There are “counterparts” at around 2070, 2010, and 1970 cm-1 for the triplet bands mentioned above. If only the first triplet is observed, the middle band appears at around 2000-1990 cm-1 (depending on the support pretreatment). If the 2070/2060 and 1970/1950 cm-1 bands are seen, there is one middle band at around 2010 cm-1. This is so even when the 2070 and/or 1970 cm-1 bands appear only as slight shoulders on the 2060 and 1950 cm-1 bands. During heating, when the doublet at higher wavenumbers disappears, the intermediate band shifts to ∼1995 cm-1. Unfortunately, by DRIFT spectroscopy it is not possible to give a suggestion for the surface species giving the ν(CO) bands at 2070, 2010, and 1970 cm-1. The changes on the ν(OH) regions are not very characteristic since both surface cobalt carbonyls and carbonate species interact with the OH groups. However, the very initial interaction between a carbonyl compound and an oxidic surface is a hydrogen bonding. This was seen best in the case of alumina 200 as the ν(OH) were not resolved. The hydrogen bonds break as the cobalt carbonyl reacts with the surface. On the other hand, various carbonate species that involve OH groups are formed (see below), increasing the intensity of the hydrogen bonding region (∼3600 cm-1). The most basic OH groups (i.e., those giving ν(OH) at highest wavenumber)16 are the most reactive: their intensity decreased the most. The band at 3680 cm-1, in turn, was affected the least. It is the most acidic in character and also difficult to attain due to the long distance (it is in the middle of three aluminum ions16). Carbonates. Several carbonate species formed simultaneously when Co2(CO)8 reacted with the surface and during decarbonylation steps. The three bands at around 1650, 1450-1440, and 1230 cm-1 could be assigned as ν(CO)as, ν(CO)s, and δ(OH), respectively, due to bicarbon-
Kurhinen and Pakkanen
ate. The assignment was supported by the ν(OH) of the bicarbonate band at around 3620 cm-1. Analogous bands are reported in the literature after CO2 chemisorption on alumina.27-30 Morterra et al.27 proposed two forms, B1 and B2, for the bicarbonate species. The B1 form would adsorb as a monodentate species, having the same wavenumbers for ν(CO)as, ν(CO)s, and δ(OH) as found in this study, whereas the B2 form would have a bridged structure between a tetrahedral Al3+ and an adjacent “reactive” OH group, giving a band at around 1480 cm-1.27 We found such a band in our studies and the behavior of the bridged bicarbonate species during thermal treatment is in agreement with the findings of Morterra et al.27 This species was best resolved on alumina 500 and alumina 600, where the OH/Al3+ ratio can be supposed to be the most suitable for formation of the B2 bicarbonate. On alumina 300 and 400, the ν(CO) due to B2 might be hidden under the broad shoulder at around 1510 cm-1. That shoulder was also seen on alumina 200 but with very weak intensity. The thermal stability of the B2 form was lower than that of the B1 form. The monodentate adsorbed species could still be seen in IR spectra after decarbonylation at 200 °C, whereas the ν(CO) due to the bridged form decreased in intensity during the decarbonylation and disappeared or was hidden by bands due to ν(CO)s of B1 bicarbonate and ν(CO)as of monodentate carbonate. Liberated CO can also react with the surface of alumina. Fredriksen et al.31 found surface formate and carbonate species after exposing γ-alumina (or Co/γ-alumina) to a CO/H2 gas mixture. In the carbonate region, the bands at 1595 and 1393 cm-1 were assigned to ν(CO)as and ν(CO)s of formate species, and the band at around 1370 cm-1 was assigned to δ(CH).31 In our study, counterparts for the formate bands were found at the same wavenumbers, at around 1595, 1395, and 1375 cm-1, and were assigned to formate species on the surface. Another carbonate species, assigned by Fredriksen et al.31 to bidentate carbonate, has bands at 1622 and 1280 cm-1 due to asymmetric and symmetric CO stretchings, respectively. The band at about 1285 cm-1 in our study was thus assigned to bidentate carbonate species. On alumina 300 and 400, the band is broad and has splitted into two peaks at around 1288 and 1275 cm-1, perhaps due to the interaction of bidentate species with tetrahedral and octahedral Al3+ sites. The ν(CO)as is hidden by the much more intense bicarbonate band at around 1650 cm-1. The interaction between the bidentate species and the surface cannot be very strong since the bands due to that species readily disappeared during heating. On pure metal oxides, the monodentate carbonate gives ν(CO)as and ν(CO)s at 1540-1420 and 1390-1330 cm-1, respectively.29 In our study, a broad ν(CO) band appeared at around 1540 cm-1 during heating and became more intense with an increase in temperature. This band was assigned to ν(CO)as of monodentate carbonate. The region of the symmetric stretching is dominated by the formate bands and the tail of the bicarbonate band (1440 cm-1). (27) Morterra, C.; Zecchina, A.; Coluccia, S.; Chiorino, A. J. Chem. Soc., Faraday Trans. 1 1977, 73, 1544. (28) Parkyns, N. D. J. Phys. Chem. 1971, 75, 526. (29) Turek, A. M.; Wachs, I. E.; DeCanio, E. J. Phys. Chem. 1992, 96, 5000. (30) Manchado, M. C.; Guil, J. M.; Masia´, A. P.; Paniego, A. R.; Menayo, J. M. T. Langmuir 1994, 10, 685. (31) Fredriksen, G. R.; Blekkan, E. A.; Schanke, D.; Holmen, A. Ber. Bunsen-Ges. Phys. Chem. 1993, 97, 308. (32) Morrow, B. A. In Spectroscopic Characterization of Heterogeneous Catalysts Part A: Methods of Surface Analysis; Fierro, J. L. G., Ed.; Elsevier: Amsterdam, 1990; p A161.
DRIFT Study of Co2(CO)8 on Alumina
Most probably, the monodentate species is adsorbed on Al3+ sites, since the ν(CO) band is the weakest on alumina 200, where the population of Al3+ sites is lowest, and the strongest on alumina 600. After the final decarbonylation step, there was a weak shoulder at around 1760 cm-1. The band was present, between the µ3-(CO) bands, right from the beginning, and remained in the spectrum while the µ3-(CO) bands disappeared. Following Turek et al.,29 the band could be assigned to ν(CO)s of the bridged carbonate. Conclusions We carried out a systematic study on the interactions of Co2(CO)8 on alumina pretreated at 200-600 °C after deposition and during a gradual rise in temperature. Pretreatment of the support considerably influenced the initial reaction, but the most stable species formed during the subsequent heat treatments were similar, independent of the degree of dehydroxylation of the support. The presence of OH groups favors decarbonylation: the ν(CO) due to carbonyl species disappeared at lower temperatures on alumina 200 than on the higher dehydroxylated aluminas. Our results agree with those of earlier studies, but also new species were identified. Both Co(CO)4- and Co6 species are reported in the literature. In our studies, ν(CO) at 1889 and 1855 cm-1 and a third band at around 1947 cm-1 were assigned to Co(CO)4-. The counterpart for Co(CO)4-, formed simultaneously by decomposition of dicobalt octacarbonyl, is reddish brown Co2+(CO)x, in which cobalt is proposed to be in an octahedral environment, and gives ν(CO) bands at 2120, 2090, and 2030 cm-1. This was the first species to disappear during heating. Blue Co2+(CO)z, in which cobalt is proposed to be, in turn, in a tetrahedral environment, was the most stable species, giving bands at around 2060, 1995, and 1945 cm-1, the last two being dependent on the degree of dehydroxylation of the support. On alumina 200, they disappeared after 5 h at 150 °C, but on other supports only at 200 °C. The most probable structure of the Co6 species is Co6(CO)152-, whose µ3-(CO) stretchings appeared at around 1797 and 1730 cm-1. There was also a third band at that region, between the two µ3-(CO) bands, but it most probably originated from a bridged carbonate species. In agreement with the literature, the bands due to Co6 species disappeared after treatment at 150 °C. The most remarkable difference compared to the literature is our finding of Co6 species already on partially dehydroxylated aluminas. There remains one unidentified species, giving
Langmuir, Vol. 14, No. 24, 1998 6915
ν(CO) bands at 2070, 2010, and 1970 cm-1. These bands were resolved during heating but disappeared easily during the subsequent decarbonylation steps. Monodentate bicarbonate B1 gave ν(CO)as, ν(CO)s, and δ(OH) bands at around 1650, 1450-1440, and 1230 cm-1, respectively. The ν(OH) of the bicarbonate band was at around 3620 cm-1. The bridged bicarbonate B2 is proposed to give the ν(CO) band at around 1480 cm-1. The formate species showed ν(CO)as and ν(CO)s bands at 1595 and 1393 cm-1 and the δ(CH) band at around 1373 cm-1. Bidentate carbonate had the ν(CO)as band at 1285 cm-1, and in some cases the broad band was split into two peaks at around 1288 and 1275 cm-1, perhaps due to the interaction of bidentate species with tetrahedral and octahedral Al3+ sites. Monodentate species formed during heating gave rise to a ν(CO) band at 1540 cm-1. On the basis of the formation of various carbonic species, we propose the following reaction mechanisms on the alumina surface. The initial reaction involves disproportionation of the parent cobalt carbonyl cluster with the formation of cobaltate and bicarbonate species. This, we propose, is the main reaction on alumina 200. On aluminas dehydroxylated at higher temperatures, where more Lewis acid/base sites are exposed, also other initial reaction mechanisms occur, such as the formation of Co6 species. The formation of Co6 species and bicarbonates are competing processes, most probably they both use the same surface sites. Some CO is liberated right from the beginning, giving rise to bidentate carbonates. As monodentate species only appear during heating, they may be supposed to form from the bidentate species. As the decarbonylation temperature increases, the surface species of cobalt carbonyls decarbonylate and zerovalent cobalt oxidizes with a simultaneous liberation of hydrogen from the surface hydroxyl groups and the formation of formate species. As the preparation times were relatively long (in 5 h periods), unstable species were difficult to observe and characterize. An equilibrium was achieved, however, and as the amounts prepared were about 5-10 g, our results may be more informative of a real catalyst preparation process than the microscale studies. Acknowledgment. Financial support of this research by the Technology Development Center of Finland and the Academy of Finland is gratefully acknowledged. LA9803326
