TiO2

NATURAL GAS CONVERSION V Studies in Surface Science and Catalysis, Vol. 119 A. Parmaliana et al. (Editors) o 1998 Elsevier Science B.V. All rights reserved.

203

DISPERSION AND REDUCIBILITY OF Co/SiO 2 AND Co/TiO 2 Roberto Rivaa, Hans Miessner "*, Gastone Del PierC, Bernadette Rebours b, Magalie Royb aEniricerche, via Maritano 26, 1-20097 San Donato Milanese (MI), Italy blnstitute Francais du Petrole, B.P.311, 92852 Rueil-Malmaison Cedex, France *present address: Gesellschaft zur F6rderung Forschung in Berlin-Adlershof, Germany

der

naturwissenschaftlich-technischen

INTRODUCTION The interaction of cobalt with various supports has been widely studied, as cobalt has important catalytic properties both in hydrodesulfurization reactions and in the Fischer Tropsch synthesis (1-8). Much effort has been devoted to understanding the relationship between the dispersion of cobalt and the activity of the catalyst in the Fischer Tropsch synthesis (9,10). The formation of surface compounds between cobalt and the support has been reported to decrease the activity of the catalyst (2,5,11). Moreover, strong metal-support interaction has been found to affect the dispersion of supported metals (12). According to the literature, the interaction of cobalt with titania is much stronger than with silica. The present study deals with the interaction between cobalt and the support, either silica or titania. It aims to understand how the interaction with the support affects both the reducibility and the dispersion of cobalt. The response of cobalt to reduction is studied with TPR experiments, in which the temperature is raised at a steady rate, and with XPS after reduction treatments at constant temperature. The dispersion of cobalt is studied with XPS. EXPERIMENTAL Preparation of the samples Johnson&Matthey Co304 was used as a reference compound for XPS spectra. The quality of the sample was checked by X-ray diffraction (XRD) before XPS analysis. Silica supported samples with various degrees of cobalt loading (from 2wt% to 27wt%) were prepared following the incipient wetness impregnation method. After impregnation the samples were calcined at 400~ in air for 4 hours. The surface area of the Merck silica was 430 m2/g, its particle size being in the range 15-45 mm with an average pore radius of 35A0 Titania supported samples containing 12wt% Co were prepared with the same procedure, using Degussa P25 titanium dioxide. After this treatment the surface area of the support was found to be ca. 40 m2/g with an average particle size around 0.1 mm. X-Ray Diffraction (XRD) The XRD data were collected at ambient conditions using a Philips diffractometer with monochromatic Cu Ka radiation (1=1.5418A). Qualitative phase analysis was carried out using the Siemens Diffrac AT package run on a IBM PC330 P-75. For titania supported samples, the quantitative phase analysis was carried out by using the Rietveld profile fitting method (13) with the procedure proposed by Hill and Howard (14). Structural data were taken from Wyckoff (15). For silica supported samples, the conventional method reported by Klug and Alexander was used (16). Crystal size was calculated from line broadening applying the Scherrer equation (16).

204

Temperature Programmed Reduction (TPR) TPR experiments were performed in a U-shaped tubular quartz reactor. After loading the sample, the reactor was flushed with He at 150~ for 1 hour, then cooled down to 50~ in flowing He. The gas flow (2%Hz-He) was adjusted for each sample in such a way as to maintain a roughly constant ratio between the amount of cobalt contained in the sample and the H 2 available. The temperature was then raised at the constant rate of 10~C/min from 50~ to the desired temperature (700-900~ The content of H z and HEOin the outflowing gas was monitored with a VG-Fisons quadrupole mass spectrometer.

X-Ray Photoelectron Spectroscopy (XPS) The XPS spectra were collected with a VG Escalab MKII spectrometer. A non-monochromatic A1 X-ray source was used. The binding energy values given in the literature for the following peaks were used as a reference: Si 2p 103.3 eV for silica supported samples, Ti 2p 458.7 eV for titania supported samples, O ls 530 eV for unsupported Co304 (17,18). A reaction chamber connected to the vacuum system of the spectrometer allowed the samples to be transferred into the measurement chamber without exposure to air after reducing and oxidizing treatments. The reducing treatments were carried out in 3%HE-Ar at various temperatures and for various lengths of time. The oxidizing treatments were done in synthetic air at 400~ for 5 hours at least. The Co 2p and the Si 2s or Ti 2p peaks were used for the quantitative analysis, by assuming the composition of the sample to be uniform throughout the volume probed by XPS (18,19,20). The dispersion of cobalt over the two supports was studied by analysis of Co/Si and Co/Ti atomic ratios respectively. RESULTS AND DISCUSSION

Unsupported cobalt I

,

,

r

,

,

'

r,,

3% H2 300C 32 hours

]

[

-

. . . . . . . .

770

t

780

....

._.L..____

l

J.

790 800 810 Binding Energy / ~V

[

1

820

Figure 1: XPS Co 2p peak of unsupported cobalt.

Figure 1 shows how the Co 2p photoelectron peak is affected by reducing treatments. Co304 is stable up to 200~ and is reduced completely at 300 ~C. The reduction occurs in two steps: first Co304 is reduced to CoO (third curve from the bottom), then CoO is reduced to metallic Co (curve at the top). Metallic cobalt is easily distinguished from oxidized cobalt because of the large difference in binding energy. The difference in lineshape make it possible to distinguish between Co304 and CoO. In fact a satellite peak appears on the high binding energy side of both Co 2p3/2 and Co 2pl/2, due to multiplet splitting. These assignments are in agreement with literature data (1,21). Reportedly,

205 the lineshape of CoO applies to Co 2§ in general, even when cobalt forms silicate or titanate through reaction with the support.

Silica supported cobalt Only cubic Co304 is detected by XRD in silica supported calcined samples. The amount of this phase increases with the increase in cobalt loading, the quantity being always close to that calculated from chemical analysis data. The size of the C0304 crystallites, evaluated by XRD, tends to increase with increasing cobalt loading (from 120 to about 160 A), even though the values are rather scattered. The same measurements have been made on reduced and passivated l T "l' (l%O/-N z at room temperature Co 2p s for 2 hours before exposure to air) samples. At high temperature (900"C) cobalt crystallizes as cubic metal, while at lower temperature (400 ~C) a fraction of -g Co crystallizes also in the hexagonal form and some residual CoO is present, probably due to the passivation process. Crystal size tends to increase with cobalt loading, as found for calcined samples, with a strong dependence on the reduction temperature. Reducing treatments of the 18wt%Co-SiO 2 sample, 770 780 790 800 810 820 studied with XPS, show that the Binding Energy / eV surface cobalt oxide is Figure 2: XPS Co 2p peak of Co/SiO 2. completely reduced at 300~ just like unsupported cobalt (fig. 2). Compared to unsupported cobalt, shorter treatments were sufficient to achieve the complete reduction. Treatments at higher temperatures do not affect the Co 2p peak any more. Reduction experiments on the sample containing 9.7wt% cobalt confirm that cobalt is completely reduced at 300~ in 2 hours and give no indication of the presence of unreducible cobalt. The TPR profiles contain two major peaks at 340~ and 430 ~C, and a broad peak at higher temperatures (not shown). The two major peaks are similar to those obtained with pure Co304 . The ratio between the H 2 consumed at 430~ and that consumed at 340~ is 3:1. A similar behaviour was observed by other authors (8,22,24). It is generally agreed that this represents the reduction of Co304 particles to metallic cobalt through the CoO step, as already pointed out for unsupported C%O 4. The response of the dispersion of cobalt to reducing and reoxidizing treatments has also been studied. The samples were exposed to air after 700~ reduced again for 2 hours at 400~ in the reaction chamber connected to the XPS spectrometer. The samples were then reoxidized and their spectra were collected again. The measured Co/Si ratios are listed in table 1 and deserve some comment. The Co/Si ratios of the samples containing 5.1 and 9.7 Co wt% are not significantly affected by reduction and reoxidation. On the other .

~

I

'

J

----r

206 hand the Co/Si ratios of the samples containing 18.4 and 22.8 Co wt% decrease appreciably after reduction and reoxidation. This leads to the conclusion that sintering of the cobalt particles occurred in the two samples with the highest content of cobalt. The tendency of the supported particles towards sintering proves that the interaction between cobalt and silica is not strong, since sintering causes the area of the interface between the two phases to decrease. Table 1 also shows that the Co/Si ratio increases strongly when the content of cobalt changes from 9.7% to 18.4%. Then the Co/Si ratio levels off at a constant value. This behaviour is attributed to the progressive development of a Co rich outer shell on the surface of the SiO 2 particles, followed by the onset of the growth of the cobalt particles. SEM, TEM and XPS data, not shown in this paper, support this conclusion. TABLE 1: Silica supported samples: XPS Co/Si atomic ratios after different treatments. Co w t % calcined reduced & reoxidized 5.1

0.15

0.14

9.7

0.20

0.19

18.4

0.72

0.56

22.8

0.70

0.52

Titania supported cobalt The XRD spectra of several titania supported samples (with ca 12% Co and Rutile/Anatase ratio ranging from 76/24 to 85/15) indicate that all the cobalt contained in the calcined samples is in the form of crystalline Co304 . After reduction and passivation most of the cobalt is amorphous and only a small fraction crystallizes as cubic Co (table 2), whereas neither the rutile to anatase ratio nor the morphology of the support changes. Therefore, the reduction treatment affects the phase composition of cobalt quite strongly, turning the oxidized crystalline phase into a mainly amorphous phase after reduction~ TABLE 2: Titania supported samples (12wt%Co-TiOz): cobalt phases composition (XRD). sample C0304 (%) cubic C0(%) amorphousCo(%) crystal size(A) 85% rutile- calc.

100%

-

0%

300(CosO4)

red. & passivated

-

17%

83%

220(Co cubic)

76% futile - calc.

100%

-

0%

red. & passivated

-

17%

83%

400(Co304) 190(Co cubic)

The response of cobalt to reducing treatments has been studied with XPS. The results are shown in figure 3. Co304 is readily reduced to Co 2+ with a 2 hour treatment at 300*C, but the complete reduction of Co 2~ to metallic cobalt is not accomplished even after 66 hours at 300 ~ In fact, a high binding energy shoulder indicates that a fraction of the cobalt is not reduced and is probably in the Co 2~ oxidation state. This behaviour is markedly different from that of unsupported Co304 and is probably due to the partial formation of cobalt titanate, which is less reducible than CosO4 , according to the literature. Treatments at higher temperatures increase the degree of reduction. This behaviour is confirmed by tests on samples that were prepared in different batches and can be regarded as XPS evidence of the well known metal-support interaction.

207 Table 3 gives the atomic ratio and the binding energy values obtained after two consecutive reducing and oxidizing treatments. The increase in the dispersion of cobalt is very strong after the first reduction-reoxidation step, since the Co/Ti ratio increases from 0.53 to 0.94. The Co/Ti ratio does not vary appreciably after the second redox treatement (final Co/Ti ratio 0.92), which means that the second redox step does not affect the dispersion of cobalt any more. It must be remarked that the reduction step is necessary in order to obtain an increase in the dispersion of cobalt. In fact, treating the calcined samples in air at 400"C for 10 hours does not affect the Co/Ti atomic ratio. This behaviour is consistent with the model proposed by Horseley, which depicts the metal-support interaction as an electron exchange between a partially reduced support and the metal (23). TABLE 3: Response of 12wt%Co-TiO z to various treatments. treatment atomic Co/Ti calcined

0.53

reduced & reoxidized

0.94

twice reduced & reoxidized

0.92

The TPR profiles of titania supported samples are quite different from those of silica supported samples: only two peaks are detected and their maxima occur at higher temperatures, 380-400* C and 500-600 ~C respectively, the latter being very broad. The conclusion that a reaction occurs between the cobalt particles and titania during the reduction treatment is supported by the following arguments: XRD data indicate that cobalt is prevailingly amorphous in the reduced and passivated samples, while it had completely crystallized 770 780 790 800 810 820 as C%O 4 in the calcined samples. - Contrary to both unsupported Co304 and silica supported Co304, Figure 3: XPS Co 2p peak of Co/TiO 2. XPS reduction tests show that titania supported cobalt is not completely reducible at 300*C in 3%H z . Moreover, the dispersion of cobalt (Co/Ti atomic ratio) increases appreciably after reduction and reoxidation, compared to the starting calcined samples. - The TPR peaks fall at higher temperatures for titania supported samples than for silica supported samples.

I Co2p. . . . . .

e-.c~ e--

-

I

BindingEnergy/ eV

208 CONCLUSIONS

This study has addressed the interaction of cobalt with two different kinds of support: silica and titania. The formation of a surface compound between cobalt and titania that is more resistant to reduction than Co304 shows that the interaction is much stronger in the case of titania. On the contrary, the behaviour of silica supported samples is very similar to that of unsupported C%O4 under reducing treatments. The different reactivity of cobalt with silica and titania explains why reducing and reoxidizing treatments have opposite effects on the dispersion of cobalt depending on whether it is supported on SiOz or TiO 2 . The low reactivity of cobalt with silica favours sintering effects after reduction and reoxidation treatments. In contrast, the level of dispersion of titania supported cobalt tends to increase after the same treatments owing to the high reactivity of cobalt with titania. REFERENCES

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