React. Kinet. Catal. Lett., Vol. 21, No. 4,517-520 (1982)

S T U D Y O F T H E A D S O R P T I O N O F C 1 8 0 O N I R O N C A T A L Y S T S

V. Perrichon

Institut de Recherches sur la Catalyse du C. N. R. S. 2, avenue Albert Einstein, 69626 Villeurbanne. France

Received May 31, 1982 Accepted October 5, 1982

The adsorption of C 1 s O on Fe/A12 O 3 and Fe/SiO 2 at room temperature was studied by TPD-MS. Two desorption peaks were obtained, which corresponded to the evolution of C 1 s O at low temperature and that of C 16 O at higher temperature. These results are in favor of a completeexchange of oxygen through the dissociation of the CO molecule.

AJICOp6ttH~ C 18 O Ha Fe/A12 03 H Fe/SiO 2 npa KOMHaTHoit reMnepaType 6una riccne- ~OBaI-Ia MffrO~OM TII~-MC. Bsmo nonyqeHo RBa ~tecop6tmomIslx naKa, COOT~eXerBy~O- H.~X BbI~OHeHHIO C 1 sO IlpH HO3KOffl H C 160 IIpH IIOBI~II.IIeHI-IOI~ TeMIIepaType. 3TH pe-

3yJI~TaT/~I yKa3bIBaIOT Ha HOHH/aI~ O6MeH KHCHOpo~a qepe3 ~Hccotr~at~IO MOYleKy- n].I CO.

INTRODUCTION

We had previously identified /1, 2/, by means o f infrared spectroscopy and tempera-

ture-programmed desorption associated with mass spectrometry (TPD-MS), two modes of

adsorption of carbon monoxide at 294 K on iron-alumina catalysts. The a-form, desorbing

at low temperature corresponds to a molecular adsorption, whilst the B-form desorbing

between 573 and 773 K has been associated with a dissociative adsorption. The latter

~ fo rm has also been observed by others for different iron contact masses including

single crystals o f iron / 3 - 7 / and its dissociated nature has been deduced for single

crystals of Fe, by electron spectroscopy /5, 7/. However, it is still not dear, particu-

larly in the case o f our alumina supported catalyst, whether the oxygen required for

the formation of the CO which desorbs at ~ 700 K comes from the original CO

(immobile FeC and FeO type entities) or whether it comes from other sources (mo-

bile surface entities). For this reason, we have studied the adsorption and desorption of labelled CO (C 1 SO) on Fe/A1203 and Fe/SiO2 contact masses.

517

PERRICHON: ADSORPTION OF C 1 sO

EXPERIMENTAL

The Fe/A1203 contact mass (10 wt. % iron) was prepared by precipitation of iron chlorides on Degussa alumina (oxyd C) /8/. It was reduced (80% metallic iron) by heating at 873 K for 18 h under hydrogen.

The Fe/SiO2 catalyst (10 wt. % iron) was prepared by impregnation ((Fe(NO3)3) of silica "Aerosil" Degussa. In order to avoid excessive sintering of the iron, it was reduced at 653 K according to Blyholder /9/. The degree of reduction calculated from magnetic measurements was 75%.

99.8% isotopically pure ClsO was obtained from Ventron. The TPD-MS experi- ments were carried out in a microbalance connected to an ion pump allowing a background pressure of 10 -8 Torr/10/ . Analysis of thermodesorbed gases was ef- fected by means of a quadrupole mass spectrometer (MQ 63 Riber). Each contact mass was evacuated at its reduction temperature and cooled to room temperature under H2 (400 T o r r - 99.9995%). ClsO was introduced at a pressure ~ 3 Torr, and the chemisorbed part was thermodesorbed under vacuum with a 0.13 K s -~ heating rate.

RESULTS AND DISCUSSION

1. Fe/Al203 catalyst

The number of c l S o molecules chemisorbed at 294 K (1.4 X 1020 per gram of iron) is in good agreement with that obtained previously on the same solid (1.7 X 102~ /1/. Figure 1 shows the desorption spectra obtained between 294 and 873 K for the masses 28 and 30. The a and [3 peaks are confirmed. However, the analysis of the respective contributions of C 160 (mass 28) and C 1 sO (mass 30) reveals that the a-form is almost entirely composed of c l S o , whilst the /3-form corresponds mainly to C160. The reference curve (Fig. la) corresponding to the thermodesorption of the catalyst after cooling under hydrogen without added C 1 sO or C 160, indicates that surface pollution phenomena, by the CO of residual atmosphere, are negligible under our experimental conditions.

The peak positions at 360 and 730 K are similar to those measured previously /1/, though the/3-peak is slightly shifted towards higher temperatures (730 instead of 670 K).

No appreciable amount of CO2 is detected, and at the end of the TPD, the total weight loss (0.82 mg) is nearly equal to the sum of the weights of the ini- tially adsorbed CO (0.68 mg) and of the desorbed hydrogen (0.21 rag) indicating a reasonable CO mass balance,

518

PERRICHON: ADSORPTION OF C ~ sO

15 t_ t - O

I - - q-

0

10

:3 O) t/)

I -

5 '6 t - O CL

0 C.3

,c, Z~ C 18

o o C 16

Q

i

300 500 700 9 0 0

Temperature (K)

Fig. 1. Carbon monoxide thermodesorption (masses 28 and 30) after adsorption of C t s O at 294 K on Fe/AI~O a . (a) is the background pressure curve

,- 15

I - -

0

~0 3 LO

L r~

"6 5

17)

,~ n CleO

c o C~60

300 SO0 700

Temperoture (K)

I

900

Fig. 2. Carbon monoxide thermodesorption (masses 28 and 30) after adsorption of C ~ 80 at 294 K on Fe/SiO 2

2. Fe/Si02 catalyst

The TPD curves shown in Fig. 2 indicate that similar results are obtained with

this catalyst, namely: a C 1 So a-peak at 345 K and a C 160 /~.peak at 645 K. The

lower temperature o f the/~-peak observed with Fe/SiO2 reflects the differences in

the metal support interaction for Fe/A1203 and Fe/SiO2 contact masses.

519

PERRICHON: ADSORPTION OF C 1 sO

CONCLUSIONS

TPD studies of ClSO adsorbed on reduced Fe/A1203 or Fe/SiO2 contact masses

confirm that the CO a-peak corresponds to molecularly adsorbed CO, which is de- sorbed (as C 180) without any exchange or prior transformation.

The fact that the/~-peak is desorbed as C 160 and not as C 1 So indicates a com-

plete breaking of the carbon-oxygen bond in the initial CO molecule. It requires a flat adsorption of CO in which both carbon and oxygen are bonded to the ca- talyst, in agreement with our previous results /1 -2 / .

One may ask whether the exchange takes place after the dissociation of CO, as

supported by electron spectroscopy, or whether it occurs via the formation of 1 6 0 - C - 1 8 0 intermediates without dissociation, the oxygen coming from oxidized

iron species present on the surface (reduction rate lower than 100%) or possibly ~

from the support. No clear-cut evidence is available from these results. However,

the correlation between the catalytic activity and the /~-peak found for a series of

alumina supported catalysts /2/gives support to the dissociation hypothesis asso- ciated with a great mobility of the surface species.

In conclusion, these results are in favor of the dissociation of the CO molecule

on iron catalysts and put in evidence the important role of the support in the ~ -

sorption properties of the contact masses.

Acknowledgements. I am pleased to thank Dr. R.P.A. Sneeden for his stimulat- ing interest in this work and the C.N.R.S. (GRECO, CO) for financial support.

REFERENCES

1. V. Petriehon, M. Primet, N. Nahon, P. Turlier: C. R. Acad. Sci., 289 C, 149 (1979). 2. V. Perrichon, P. Turlier, J. Barrault, C. Forquy, J. C. Menezo: Applied Catalysis, 1, 169

(1981). 3. G. Wedler, K. G. Colb, G. McElhiney, W. Heinrich: Appl. Surface Sci., 2, 30 (1978). 4. K. Yoshida: J. Res. Inst. Catal. Hokkaido Univ., 28, 105 (1980). 5. J. Benziger, R. J. Madix: Surface Sei., 94, 119 (1980). 6. H. Matsumoto: Chem. Lett., 1041 (1981). 7. G. Broden, G. Gafner, H. P. Bonzel: Surface Sci., 84, 295 (1979). 8. N. Nahon, V. Perrichon, P. Turlier, P. Bussi~re: React. Kinet. Catal. Lett., 11,281 (1979). 9. G. Blyholder, L. D. Neff: J. Phys. Chem., 66, 1464 (1962).

10. V. Perrichon, J. P. Candy, P. Fouilloux: "Progress in Vacuum Microbalance Techniques", Vol. 3, p. 18. Heyden and Son, London 1975.

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