Temperature-Programmed Decomposition, Oxidation, and Reduction

programmed processes (temperature-programmed decomposition, TPDE; temperature-programmed oxida- tion, TPO; and temperature-programmed reduction, ...
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Langmuir 2000, 16, 2658-2664

Temperature-Programmed Decomposition, Oxidation, and Reduction Studies of Co2(CO)8 Supported on Alumina Maria Kurhinen and Tapani A. Pakkanen* Department of Chemistry, University of Joensuu, P.O. Box 111, FIN-80101 Joensuu, Finland Received May 11, 1999. In Final Form: October 18, 1999 Co2(CO)8 supported on alumina pretreated at different temperatures was studied by temperatureprogrammed processes (temperature-programmed decomposition, TPDE; temperature-programmed oxidation, TPO; and temperature-programmed reduction, TPR). The gaseous decomposition and reaction products were analyzed by mass spectrometry and gas-phase IR spectroscopy. Oxygen and hydrogen consumptions by the surface species were followed with a TCD. The pretreatment of the support was found to affect the TPDE profile of cobalt carbonyls. CO ligands were removed more straightforwardly in the presence of OH groups producing a narrow TPDE profile. A broader TCD profile was found for the support with predominantly Lewis acid/base sites. The presence of OH groups also favored decomposition of surface cobalt carbonyl species, giving the lowest Tmax. Oxidation and reduction of samples were observed to depend on the preceding treatment. Decarbonylation of the sample under inert gas intensified the interactions between cobalt and the surface, and the further reactivity of cobalt decreased. The cobalt species remained more active when decarbonylation was carried out under reactive gas. A Fischer-Tropsch reaction occurred when aluminasupported cobalt carbonyls were treated with hydrogen. The samples were active even at room temperature, producing methane, ethene, and methanol. The lack of CO ligands at higher temperatures prevented the formation of higher hydrocarbons.

Introduction Temperature-programmed decomposition (TPDE) is a useful method for studying the decomposition of carbonyl compounds on oxidic supports. Samples are heated with a linear heating rate, and decomposition products can be analyzed by mass spectrometry or gas chromatography. The first TPDE studies of alumina-supported Co2(CO)8 were published in the 1980s.1-6 In studies concerned with the oxidation of cobalt during TPDE3 and with the hydrogenation of CO under various carrier gases during heating,1 it was assumed that the supported carbonyls were preserved as parent compounds.1-3 Other surface species were identified when Co2(CO)8 lost CO ligands during interaction with a dehydroxylated surface and formed, for example, Co4(CO)12(ads),4-6 Co2(CO)6L2,4,5 or Co6(CO)16-nLn5 on the surface of the support (L denotes a surface site). Our preparation methods differ from those reported earlier. Supporting Co2(CO)8 on alumina by vapor-phase adsorption in a fluidized bed reactor7 resulted in various surface species of cobalt carbonyls, from Co(CO)4-(ads) to Co6(CO)x(ads), depending on the pretreatment temperature of the support. In our earlier work7 we studied the surface reactions of alumina-supported Co2(CO)8 by DRIFT spectroscopy. In the new study now described we expand our study of cobalt carbonyl supported on alumina pretreated at different temperatures. The aim of this work was to find out effects of the support pretreatment as well as oxidation and reduction of supported cobalt studied by temperature* Corresponding author. Fax: +358 (0)13 251 3390. E-mail: [email protected]. (1) Brenner, A.; Hucul, D. A. J. Am. Chem. Soc. 1980, 102, 2484. (2) Hucul, D. A.; Brenner, A. J. Am. Chem. Soc. 1981, 103, 217. (3) Hucul, D. A.; Brenner, A. J. Phys. Chem. 1981, 85, 496. (4) Nakamura, R.; Oomura, A.; Echigoya, E. Chem. Lett. 1982, 1463. (5) Nakamura, R.; Okada, N.; Oomura, A.; Echigoya, E. Chem. Lett. 1984, 119. (6) Iwasawa, Y.; Yamada, M.; Sato, Y.; Kuroda, H. J. Mol. Catal. 1984, 23, 65. (7) Kurhinen, M.; Pakkanen, T. A. Langmuir, 1998, 14, 6907.

programmed processes. Samples in carbonyl form were heated under an inert (He) or a reactive (H2) gas and the gases evolved were analyzed by mass spectrometry and gas-phase infrared spectroscopy. Temperature programmed oxidations (TPOs) and reductions (TPRs) were carried out on decarbonylated samples, and the reactivity of the cobalt species was interpreted in terms of the results. Experimental Section Co2(CO)8 (Fluka Chemika) was used without further purification. γ-Alumina (Brockmann I, standard grade, ∼150 mesh, 58 Å, surface area 155 m2/g, Aldrich) provided the support, and amounts of about 70 g were pretreated at 200, 400, and 600 °C for 10 h under vacuum (∼8 × 10-3 Torr). The pretreated alumina is subsequently referred to as alumina 200, 400, and 600. The BET (Brunauer-Emmet-Teller) surface areas for the pretreated alumina were 176, 180, and 179 m2/g, respectively, determined by N2 adsorption. Co2(CO)8 was deposited on the alumina at 45 °C under carbon monoxide (Technohaus, 99.997%) flow in a fluidized bed reactor, by gas-phase adsorption.7 Cobalt loadings were in the range 0.3-0.7 wt %. The results were normalized to correspond to 1 mg of Co to allow comparison of the individual samples. The temperature-programmed treatments (TPDE, TPR, TPO, and pulse chemisorption) were carried out using a Micromeritics AutoChem 2910 analyzer equipped with a thermal conductive detector (TCD). Samples (0.5-0.8 g) were packed into a U-shaped quartz tube in a glovebox (N2 atmosphere), and the sealed tube was attached to the analyzer by using a glovebag with an inert gas flush. The samples were heated in a clamshell-type furnace with a linear heating rate. All samples were handled in the absence of air. TPDE experiments were carried out under helium (purity 99.9999%) flow (10 mL/min) with heating rates of 5 and 10 K/min. For TPR (5% H2/He, 40 mL/min or 10% H2/Ar, 10 mL/min) the samples were heated at 10 K/min. Helium was used as a carrier gas when mass spectra were recorded; otherwise, argon was the carrier gas. Oxidation was carried out by pulse chemisorption (PC) of O2 (purity 99.9999%) at room temperature, followed by TPO using 5% O2/He with a flow rate of 40 mL/min and a heating rate of 10 K/min. Co/alumina 200 samples were heated only to 200 °C to avoid possible dehydroxylation reactions of the support. Except for a

10.1021/la990569h CCC: $19.00 © 2000 American Chemical Society Published on Web 02/23/2000

Co2(CO)8 Supported on Alumina

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Table 1. Run Sequences and Maxima of TCD Signals (°C) during TPDE, TPO, and TPR Co alumina 200

sequence i ii iii

TPDE TPO TPR PC(O2)e TPO TPR TPR TPO

alumina 400

alumina 600

150a/168b 175a/185b 187a/188b 200c 300,d 400c 160,d246, 345 50,d 180 105, 348 177, 293, 395 98, 200c 180 85, 200c 200c

117, 350c 200, 330 160 300c

118f 170, 320, 343, 380, 463 163f 150,g 300c

a Heating rate 5 K/min. b Heating rate 10 K/min. c No return to baseline. d Negative signal. e At rt. f Very broad. g Shoulder.

few samples of Co/alumina 600 that were heated to 500 °C, the maximum temperature for Co/aluminas 400 and 600 was 350400 °C to avoid Co penetration into the bulk of the alumina. The following sequences were used to determine the effects of various treatments (Table 1): TPDE, TPO, and TPR; PC(O2) at room temperature, TPO, and TPR; and TPR and TPO. Quadrupole mass spectrometry (Hewlett-Packard 5971A) and gasphase IR spectroscopy (Galaxy 6020 Fourier transform infrared spectrometer with MCT detector) were used to detect the gaseous decomposition and reaction products. In mass spectra, a total ion flow was collected (range of m/z ) 10-550), from which the different m/z ratios were picked up. Background spectra were subtracted from both the mass and the IR spectra. The amounts of CO or CO2 were determined as integrated surface areas of CO and CO2 bands. The effluent flowed through a cold trap (2-propanol-liquid nitrogen, ∼ -40 °C) before reaching the detectors when samples in carbonyl form were studied. After the temperature-programmed treatments, X-ray diffraction analyses were carried out on some of the samples using a Bruker D8 Advance powder diffractometer with Cu KR radiation (wavelength 154.2 pm). No peaks due to crystalline phases of cobalt oxides were observed.

Results and Discussion Temperature-Programmed Decomposition. The cobalt carbonyl on alumina samples were heated at two different linear rates to study the gases released due to decomposition of surface cobalt carbonyl species. As can be seen in Table 1, increasing the heating rate from 5 to 10 K/min increased the Tmax of the TPDE profile, but the higher the pretreatment temperature of the support the smaller the increase. Furthermore, the higher the pretreatment temperature of the support the higher the Tmax. Thus, Tmax values were 150 and 168 °C for Co/alumina 200 and 187 and 188 °C for Co/alumina 600. The width of the TPDE profile was narrowest for Co2(CO)8 on alumina 200 (Figure 1a), and the heating rate had no significant effect on the width. (The TPDE profiles in Figure 1a were run at the heating rate 5 K/min.) Decomposition of surface cobalt carbonyls on aluminas 400 and 600 began in the same way (Figure 1a), but after 100 °C, the amounts of gases released increased noticeably on Co/alumina 400 and the profile had a similar maximum to that of the Co/alumina 200 sample. None of the TPDE profiles returned to the baseline, probably because decomposition of surface carbonate species released CO2 (see discussion of the IR results below). The Co/alumina 200 sample was heated only to 200 °C, and evidently this was too low a temperature for the decomposition of the carbonate species. Some CO2 was liberated even at 200 °C, however. TPDE was combined with IR spectroscopy to determine the percentage amounts of CO and CO2 in the effluent flow (Figure 1b-d). Because the shapes of the sum spectra

Figure 1. Decomposition of cobalt carbonyls on alumina pretreated at different temperatures: (a) TPDE profile (a.u.); (b-d) percentage of CO (dotted lines) and CO2 (broken lines) in the effluent and their sum (solid line), normalized to correspond to 1 mg of Co, on (b) alumina 200, (c) alumina 400, and (d) alumina 600.

(Figure 1b-d) and the TPDE signal (Figure 1a) were alike, the analysis of CO and CO2 released during heating could also be used to interpret the TPDE profiles. Carbon dioxide began to form on Co/alumina 200 in the initial stages of the heating and reached a maximum at about 100 °C, after which the liberation of CO increased sharply (Figure 1b). The CO2 was responsible for the broad shoulder of the TPDE profile. The dehydroxylation of the support increased the desorption temperature of CO2: on Co/alumina 400, an increase in the amount of CO2 can be seen above 100 °C, and on Co/alumina 600 a smaller increase can be seen above 130 °C (Figure 1c,d). Thus, the higher the pretreatment temperature of the support the higher the temperature needed to release the CO2 but also the smaller the amount of CO2 released. Because of the extreme sensitivity of IR to CO2, even after background subtraction, the atmospheric CO2 might interfere with determination of quantitative amounts of liberated CO2, especially when those concentrations are low (under 0.05%).

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The band in the CO2 curve of Co/alumina 400 at about 250 °C (Figure 1c, broken line), where the decomposition of carbonyl species is decreasing, suggests that at this point the decomposition of surface carbonate species begins. Calculated as the surface area of the CO2 curves, the amount of CO2 released, whether from decomposition of surface cobalt carbonyls or from decomposition of surface carbonate species, is greatest on alumina 200salmost 3-fold the value on alumina 600 (Figure 1b-d). Evidently, since physisorbed H2O has been removed and chemisorbed H2O can be considered as dissociated to OH groups after heating at 200 °C under vacuum, OH groups play an important role in the evolution of CO2 during decomposition of Co2(CO)8 on pretreated alumina. For all samples, the liberation of CO began above 50 °C, reaching sharp maxima at 150 °C on Co/alumina 200 and 175 °C on Co/alumina 400 and a broad maximum at about 180 °C on Co/alumina 600. All CO ligands were desorbed by 200 °C on Co/alumina 200 and by about 260 °C on Co/alumina 400. The desorption profile of CO was much broader for Co/alumina 600 than for the other samples. This would imply interaction of liberated CO with Al3+ sites before desorption. Although this interaction could not have been strong, it could have slowed the removal of CO to the effluent. Another explanation for the broad CO profile may lie in the various surface cobalt species, which decomposed at different rates. These results are in agreement with our previous studies.7 In the DRIFT spectra of cobalt carbonyl on alumina 200, the ν(CO) bands due to carbonyl ligands disappeared after 5 h of heating at 150 °C. On the other supports, heating at 200 °C was needed to remove the ν(CO) bands due to the carbonyl ligands. The longer heating time (5 h) in the DRIFT study7 allowed the process to reach equilibrium at each temperature level. With the faster heating rate in the present study, some liberation of CO was observed at higher temperatures than those in the DRIFT study. There were no marked differences in the amounts of CO released (calculated as surface area of the CO curves) from the samples. As with CO2, the greatest amount was from the Co/alumina 200 sample. The Co/alumina 400 and 600 samples released almost equal amounts of CO, only slightly less than that from the Co/alumina 200 sample. Determination of the amounts of CO or CO2 liberated per cobalt atom was not possible, since formation of surface carbonaceous species affected the composition of the effluent. Moreover, preparation of the samples in the fluidized bed reactor under CO flow prevented measurement of the amounts of liberated CO or CO2 during the formation of surface cobalt carbonyl species. The effect of the support pretreatment temperature on the decomposition process was clearly seen in our results. The OH groups of the support favored the decomposition, and the Tmax of the TPDE profile was lowest for cobalt carbonyls on alumina 200. The CO profile was narrowest on Co/alumina 200, which would indicate either a similarity of the surface cobalt species or different species decomposing by similar processes. The amount of CO2 released was least for the Co/alumina 600 sample. There were more Lewis acid sites (Al3+) on alumina 600 than on alumina 200 or 400, and liberated CO2 may have reacted with those sites, forming surface carbonaceous species, rather than being removed to the effluent. The surface properties of alumina 400 lie between those of alumina 200 and 600: whereas OH groups dominate on alumina 200 and Lewis acid/base sites on alumina 600,

Kurhinen and Pakkanen

neither dominates on alumina 400.7 This was seen in our present work: the OH groups were responsible for the sharp maximum in the CO profile whereas the interaction with Lewis acid sites broadened the TPDE profile of Co/ alumina 400. Temperature-Programmed Oxidations. To determine the effect of the preceding treatment on the oxidizability of the cobalt, we carried out oxidations on decarbonylated samples (sequence i), samples whose CO ligands were removed by oxygen pulse chemisorption (sequence ii), and samples that were reduced in carbonyl form (sequence iii). Sequence i. Samples decarbonylated during TPDE were oxidized by heating under a 5% O2/He flow. For Co/alumina 200, the TCD signal had no distinct maximum assignable to oxidation of cobalt. Some consumption of oxygen was observed from the beginning, but the consumption increased markedly only near the calcination temperature of the support; the total O2 consumption was 0.7 calculated as the molar ratio of O2/Co. The TPO profiles of Co/aluminas 400 and 600 were different from the profile of Co/alumina 200 right from the beginning: A negative TCD signal was observed at about 300 and 160 °C for Co/alumina 400 and 600, respectively, indicating the release of gaseous products rather than the consumption of O2. This may be explained as follows: During TPDE (and preparation of the samples), more surface carbonaceous species formed on Co/alumina 400 and 600 than on Co/alumina 200. Some of the carbonate species, possibly bicarbonates, began to decompose during heating (TPDE), but others may need oxygen to decompose. CO and CO2 have thermal conductivities close to that of O2 with respect to the carrier gas He,8 and when liberation of CO2, CO, and/or O2 is greater than the consumption of O2, a negative signal can be expected. Some increases in the O2 consumptions were observed after the maxima of the negative signals, but it was unclear whether the increase in the TCD signal was due to a consumption of oxygen or a decrease in the liberated CO and CO2. However, for Co/alumina 400 the consumption of oxygen increased, and the signal did not return to the baseline during heating. In the case of Co/ alumina 600, maxima were observed at 246 and 345 °C, the latter one with greater intensity. Sequence ii. Carbonyl ligands react readily with oxygen, and this may cause such marked changes in the TCD signal as to mask the oxidation of cobalt species. To avoid this, in sequence ii we removed CO ligands before temperature-programmed oxidation by feeding oxygen in pulses over the samples at room temperature. Of the gases released, only CO2 was detected by IR during pulse chemisorption. After the removal of the CO ligands, the samples were heated under a 5% O2 flow. Maxima in O2 consumptions were observed at 98, 117, and 118 °C for Co/alumina 200, 400, and 600, respectively (Figure 2). The TPO profile was narrowest for Co/alumina 200, indicating that the cobalt species were more homogeneous there than on the other supports. The results are in agreement with the TPDE results regarding surface cobalt species. Total oxygen consumptions per Co (mole/mole) were 3.0, 3.3, and 3.4 for Co/alumina 200, 400, and 600, respectively. Note that some oxidation of cobalt may have occurred during O2 pulse chemisorption, and the figures should therefore be considered approximate. (8) Thermal conductivity (relative to air): CO2 ) 0.62, CO ) 0.97, O2 ) 1.02, He ) 5.84, H2 ) 7.07, from Micromeritics AutoChem 2910 Operator’s manual V4; Micromeritics Instrument Corporation: Norcross, GA, 1998; Appendix C.

Co2(CO)8 Supported on Alumina

Figure 2. TPO profiles (a.u.) for temperature-programmed oxidation after pulse chemisorption of oxygen at room temperature (sequence ii): (a) Co/alumina 200; (b) Co/alumina 400; (c) Co/alumina 600.

The particle size and dispersion of cobalt could not be estimated from the PC(O2) results. The oxygen consumption is calculated as the surface area of the O2 pulses, and since the thermal conductivities of O2 and CO2 with respect to He are similar,8 consumption of oxygen may be compensated by release of CO2. Therefore, the changes in the surface area of O2 pulses are not totally accurate. In the case of Co/alumina 200, for example, the first pulse showed a release of gases even though oxygen was supposed to be consumed. Sequence iii. In the third sequence, samples in carbonyl form were heated under H2 flow (discussed below as temperature-programmed surface reaction) to remove CO ligands and to reduce surface cobalt species prior to TPO. Oxidation of the reduced Co/alumina 200 sample gave a TPO profile similar to that of sequence i. However, in this run the total consumption of O2 was 2.0, calculated as the molar ratio of O2/Co, and it was more than twice the consumption of sequence i. Evidently, more active (oxidizable) cobalt species are produced in decarbonylation under reducing conditions than in decarbonylation under inert gas. The TPO profiles of Co/alumina 400 and 600 also showed consumption of O2 from the beginning, but there were no distinct maxima. Because the samples were reduced prior to TPO, some cobalt may have become oxidized immediately after oxygen was passed over the sample, that is, before the baseline had stabilized and recording of the TCD profile began. Oxidation of the carbonaceous species might also affect the consumption of oxygen. Discussion. To our knowledge, no TPO studies of alumina-supported Co2(CO)8 have been reported in the literature. Some related studies9,10 on alumina-supported cobalt nitrate prepared by impregnation can be used to assist our interpretation of results, though the oxidation level and surface species are different. Using unsupported (9) Sewell, G. S.; van Steen, E.; O’Connor, C. T. Catal. Lett. 1996, 37, 255. (10) Arnoldy, P.; Moulijn, J. A. J. Catal. 1985, 93, 38.

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metallic cobalt (Co0) as a model to analyze their TPO profile, Sewell et al.9 assigned a maximum at 298 °C to the oxidation of metallic Co to CoO. The maximum we found at 300 °C (Table 1, Co/alumina 400 and 600) might be due to oxidation of surface cobalt species to CoO(ads). It is unclear, however, whether metallic cobalt species were actually present on the surface after the preceding heating under hydrogen flow or only surface cobalt species with low oxidation states. Iwasava et al.6 supported Co2(CO)8 on alumina by dry mixing, and during aging at 40 °C, under vacuum, they observed evolution of 0.95 CO/Co. This they attributed to the formation of Co4(CO)12(ads). The sample was treated further with oxygen at room temperature to obtain (CoO)n surface species. Heating of (CoO)n under O2 at 500 °C led to Co3O4 on the surface, whereas heating under vacuum to 240 °C led to a distorted tetrahedrally coordinated Co2+ species.6 Unfortunately, Iwasawa et al.6 did not report a TCD profile for their oxygen treatments, which would have helped us in the interpretation of our data. Earlier,7 however, the blue color of the sample led us to propose the existence of tetrahedrally coordinated Co2+. In the present study, too, the sample was blue after oxygen treatment, so that in accordance with the results of Iwasawa et al.6 the cobalt might well be tetrahedrally coordinated with an oxidation state of 2+. Schneider et al.11 identified Co(CO)4- species on alumina after subliming (CO)9Co3CCH3. The cobaltate species were highly sensitive to O2, and exposure to oxygen resulted in their decarbonylation and the appearance of carbonate bands in the IR spectra.11 This sensitivity could explain the greater consumption of oxygen on Co/alumina 200, on which we proposed there to be more cobaltate species.7 Temperature-Programmed Reduction. Reductions of the samples were carried out by heating under H2 (with argon or helium as carrier gas) flow after oxidation (sequences i and ii) but also without pretreatments, so that CO ligands of the surface cobalt species were still in place (sequence iii, discussed below as temperatureprogrammed surface reactions). Sequence i. In this run, the samples were decarbonylated during TPDE and oxidized during TPO before the temperature-programmed reduction. In the case of Co/alumina 200, after an initial drop in the TCD signal, the consumption of hydrogen increased, reaching a maximum at about 180 °C (Figure 3a, Table 1). The differences in the thermal conductivities of hydrogen and helium are much smaller than the differences between CO or CO2 and helium.8 Thus, even minimal liberation of CO or CO2 may disturb the TPR profile by compensating the consumption of hydrogen or even masking it. We propose the initial drop in the TCD signal to be a result of decomposition of the surface carbonaceous species and liberation of CO or CO2. For the Co/alumina 400 and 600 samples, hydrogen consumption increased from the beginning. Maxima for these samples appeared at different temperatures: at 105 and 348 °C for Co/alumina 400 and at 177, 293, and 395 °C for Co/ alumina 600 (Figure 3c,e). Sequence ii. In sequence ii, samples in carbonyl form were oxidized before temperature-programmed reduction. With the initial drop in the case of Co/alumina 200 disregarded, the shape of the TPR profiles for Co/alumina 200 and 400 (Figure 3b,d) differed only slightly from the shape of the profiles in sequence i. The basic form of the TPR profile for Co/alumina 600 was similar to that of sequence i (Figure 3e,f). However, the maxima appeared at different temperatures (Table 1). (11) Schneider, R. L.; Howe, R. F.; Watters, K. L. J. Catal. 1983, 79, 298.

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Kurhinen and Pakkanen Table 2. Consumption of H2 in TPRa

sequence

sample

(H2/Co)35-200 (mol/mol)

(H2/Co)200-400 (mol/mol)

(H2/Co)35-400 (mol/mol)

(H2/Co)400-500 (mol/mol)

(H2/Co)35-500 (mol/mol)

i ii i ii i ii

Co/alu 200 Co/alu 200 Co/alu 400 Co/alu 400 Co/alu 600 Co/alu 600

0.02 0.08 0.02 0.10 0.07 0.06

0.05 0.19 0.27 0.21

0.07 0.29 0.34 0.27

0.12 0.08

0.46 0.35

a

Subscripts represent temperature range (°C); values in bold represent total hydrogen consumption.

Figure 3. TPR profiles (a.u.) for temperature-programmed reduction (sequences i and ii) of Co/alumina 200 (a and b), Co/ alumina 400 (c and d), and Co/alumina 600 (e and f).

Discussion. The amounts of hydrogen consumed in relation to cobalt (mole/mole) are listed in Table 2. Because the separation of maxima from one another was difficult due to tailing, the temperature ranges were chosen according to the calcination temperature of the support. Detailed calculations of cobalt reduction were not practical because cobalt species on the surface represented various oxidation states and hydrogen was also consumed by the carbonaceous species, especially at higher temperatures. For Co/alumina 200 and 400, hydrogen consumption was greater for sequence ii than for sequence i. This was in good accord with the above TPO results: the first temperature-programmed treatment had a great effect on the further reactivity of the surface cobalt species. Decomposition of the cobalt carbonyl species by heating under inert gas (sequence i) resulted in more stable interaction with the surface than decomposition under oxygen did (sequence ii). In the case of Co/alumina 600, in contrast, more hydrogen was consumed during TPR of sequence i than TPR of sequence ii. The existence of the Co6 species on the surface may be one reason for this. However, the difference in hydrogen consumption of the two sequences was only minor. In a study of the reduction of alumina-supported cobalt carried out by temperature-programmed reduction (TPR), X-ray diffraction (XRD), and diffuse reflectance spectroscopy (DRS), Arnoldy and Moulijn10 found four separate cobalt phases. As noted above, the initial conditions were different for their alumina-supported cobalt prepared by impregnation of a nitrate precursor and ours prepared by gas-phase adsorption of a carbonyl precursor. The closest maximum to our results was the one at about 330 °C (600 K) that was assigned to the oxidation of Co3O4. We did not find any peaks due to crystalline Co3O4 by powder XRD, but our metal contents were small, so that observation of

peaks would in any case have been difficult. Wang and Chen12 found Co3O4 only on high metal loadings. Temperature-Programmed Surface Reaction. Sequence iii. Heating of alumina-supported cobalt carbonyls under a 5% H2/He flow is better described as a temperature-programmed surface reaction (TPSR) than as a temperature-programmed reduction. The carbonyl ligands react readily with hydrogen, forming various products. Cobalt is well-known as a Fischer-Tropsch catalyst, that is, a catalyst for hydrogenation of carbon monoxide, and the presence of H2, CO ligands, and surface hydroxyls provides suitable conditions for Fischer-Tropsch reaction. Brenner and Hucul1 were the first to demonstrate that supported carbonyl compounds could function as FischerTropsch catalysts. The reaction products of our study are proposed, according to mass numbers, as follows: m/z ) 12 is carbon (formed by decomposition of CO), m/z ) 16 is oxygen (formed by decomposition of CO) or CH4, m/z ) 28 is C2H4, m/z ) 32 is CH3OH, and m/z ) 44 is CO2, C3H8, or CH2CHCO. It should be noted that CO decomposed totally to carbon and oxygen (1:1) and did not give a m/z ) 28 signal. However, formation of methane (m/z ) 16) increased the intensity of the m/z ) 16 signal. In the total ion flow, the relative amount of the m/z ) 16 signal due to oxygen can be determined by comparing the intensity to that of the signal for m/z ) 12. The presence of CO and CO2 was confirmed by gas-phase IR spectroscopy. Figures 4, 5, and 6 present the TPSR results for Co/alumina 200, 400, and 600, respectively. CO ligands, and perhaps also surface carbonaceous species, reacted at room temperature immediately upon the exposure to hydrogen. Carbon monoxide was seen in the IR spectra of the effluent only at the very beginning, quickly disappearing through the formation of CH4 and other reaction products. According to the gas-phase IR results, carbon dioxide formed on all supports, but the amounts formed at different stages of the reactions depended on the pretreatment temperature of the support. During heating, however, the m/z ) 44 profile could mainly be attribute to C3H8 or CH2CHCO. Methane began to form under mild conditions (rt), but with increasing temperature other reaction mechanisms became dominant, giving rise to heavier reaction products. Dehydroxylation of the support seemed to affect the formation of methane and ethane: the broadest initial peak due to methane was regarded to be for Co/alumina 600 produced and ethene formed during stabilization (before the temperature ramp began). The OH groups on the surface played an important role in the formation of methanol. For cobalt carbonyls on aluminas 200 and 400, methanol was observed in the effluent immediately after CH4 had reached its maximum. Methanol was also observed on Co/alumina 600, but in smaller amount than that on Co/alumina 200 or 400. (12) Wang, W.-J.; Chen, Y.-W. Appl. Catal. 1991, 77, 223.

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Figure 6. Temperature-programmed surface reactions of Co/ alumina 600 induced by a 5% H2/He flow: (a) components in the total ion flow; (b) percentages of liberated CO and CO2 in the effluent, determined by gas-phase IR, and the temperature profile.

Figure 4. Temperature-programmed surface reactions of Co/ alumina 200 induced by a 5% H2/He flow: (a) components in the total ion flow; (b) percentages of liberated CO and CO2 in the effluent, determined by gas-phase IR, and the temperature profile.

over supported cobalt catalysts prepared from carbonyl precursors. In Brenner and Hucul’s study, methane appeared at 150 °C during heating under H2 flow, and small amounts of C2H4, C2H6, and CO2 formed.1 Lisitsyn et al.13 decomposed Co2(CO)8 on the support at 260 °C under vacuum. The hydrocarbons were lighter, and the activity of CO hydrogenation was lower with Co/alumina catalysts than with the other supports studied, namely SiO2, TiO2, and ZrO2.13 The lower activity for Co/alumina may have been due to the decomposition conditions: 260 °C under vacuum may have been be too violent, so that the advantage of the low oxidation state of cobalt was lost. The different methods of catalyst preparation in our work and that of Brenner and Hucul1 may explain why methane formed at a much lower temperature for us. Even though the cobalt content in our study was low (