N. D. PARKYNS
526
The Influenee of Thermal Pretreatment on the Infrared Spectrum ioxide Adsorbed on Alumina AI. D. Parkyns Gas Council,London Research Station, Michael Road, London, S.W.6., Great Britain
(Received October SO, 1970)
PuSlioation costs assisted by the Gas Council (London, England)
The idfeet of heating different aluminas from 400 to 1000" on the infrared spectrum of adsorbed carbon dioxide has been investigated and shown to be the major cause of discrepancies of data previously published on this system. The aluminas used were almost entirely dehydroxylated by heating at 800". As a result, the bicarbonate bands, which appear when carbon dioxide is added after lower temperature treatments, do not appear, but are replaced by uncoordinated, uni- and bidelitate carbonate ion bands. Other bands, attributed to "organic-type" carbonate species, are affected to a far lesser extent,
Introduction Recently, four papers dealing with the infrared spectra of carbon dioxide adsorbed on alumina have appeared. 1--4 When these are considered together with earlier certain discrepancies and mutually contradictory features are observed. Table I shows the main spectra features which appear a t pressures of about 1-2 Torr, the degassing conditions used, and the phase of the alumina under study. Table I gives only an outline of the observed spectra. When the conditions of adsorption (COz pressure a few Torr, adsorpticrn temperature 20-25") were departed from, new bards appeared in several instances. In addition, banda at 25150-2370 cm-' were also observed in all eases: there is general agreement among the authors that these are due to some form of physically adsorbed carbon dinxide. Without making detailed comparisons between the spectra, we can separate them into two or three main classes. (a) Rea and Lindquist,' Perij6and Parkyns,3 all agree in findtng a key band at 3610 f 5 cm-1 (2667 emiL for deuterated tsurfaces) which is ascribed t o the hydroxyl stretching mode of a surface bicarbonate group. Parkyns further showed that the sharp and intense bands at 16410, 1480, and 1232 em-1 were all different modes of the bicarbonate species. (b) ASa rule, two or three hands of variable intensity are found betwem 8740 and 1870 em-' but assignments which have been made to particular species are much less certain than for the bicarbonate bands. Neither amsay' nor Uakerson, et aLj2 observed bands in this region. (c) Gregg and Ramsay observed no bicarbonate bands at all but described their spectra in terms of uncoordinated and bidentate carbonate ions. The purpose of the present paper is to show how the nature of the spectra of adsorbed carbon dioxide on alumina is influenced by increasing the degassing temThe Journal of Physical Chemistry, V o l . 76, NO.4, 1971
perature prior to adsorption and to see whether these results can be used to reconcile the observed differences in the spectra hitherto published.
Experimental Section The spectroscopic and other apparatus were as previously d e ~ c r i b e d . Alumina ~,~ samples used were: (A) A very pure alumina, made by P. Spence and Sons, Widnes, England, described as "Gibbsite-rich. " This was similar to the material used by Gregg and Ramsay.' After caIcination in air at 600" before use (to remove organic matter) it was in the x phase. The RET surface area as determined by the manufacturers ~ 7 a s 420 mz/g after heating at 400". (B) Alumina, as above, which had been heated for 2 hr at 1000" in vacuo and then allowed to rehydrate by exposure to the air for a few weeks. After the conclusion of experiments, which entailed a final further heating to IQQQ", the surface area was 122 m2/g and a mixture of 8-, x-,8-, and K-alumina was found by X-ray diffraction. ( C ) Alumina, type "C" from Degussa (Germany) was as described previously.8 The surface area ,after degassing at room temperature, was 96 m2/g, but after 2 hr evacuation a t 900" rose to 103 mz/g. It was found to be in the y phase. The three types of sample were activated by stepwise heating in vacuo for 2 br, starting at 400". The carbon dioxide was added to a pressure (1) S. J. Gregg and J. D. F. Ramsay, J . Phys. Chem., 73, 1243 (1969). (2) V. I. Yakerson, L. I. Lafer, V. Ya. Danyushevskii, and A. M. Rubenstein, I z v . Akad. Nauk S S S R Ser. Khim., 19, (1969). (3) N. D. Parkyns, J . Chem. SOC.A , 410 (1969). (4) P. Fink, 2. Chem., 7, 324 (1967). (5) L. H , Little and C. H. Amberg, Can. J. C h e m ~40, , 1997 (1962). (6) J. B. Peri, J . Phys. Chem., 70, 3168 (1966). (7) D. G . Rea and R. H. Lindquist, paper presented to the 136th National Meeting of the American Chemical Society, Atlantic City, N. J., Sept 1959. (8) N. D. Parkyns, J . Chem. SOC.A , 1910 (1967).
IR SPECTRA 017 CB2 ADSORBED ON ALUMINA Table 1
Ref
527
Pnf rarcd Spectra of Carbon Dioxide on Alumina Degassing temp end phnse of AlzQa, 06
Adsorption bands and intensities
c -
2650 (m) 3605 (m) 2666 3618 (m) 3610 3610 (m)
1820 and 1780 (m) 1820 and 1780 1790 (m) 1750 1870-1800 1825 and 1790 (m) 1856 (m)
1646 (m), 1600 1640 ( s ) 1615 1652 (s) 1645 (s) 1635 1650 (s) 1639 ( 6 ) 1652 (m)
(-7)
1440 (s), 1370 (w) 1230 (m) 1480 (s) 1232 (s) 1475
