INFRARED STUDIESOF ZEOLITESAND ADSORBEDMOLECULES
1413
Infrared Spectroscopic Investigations of Zeolites and Adsorbed Molecules. 11. Adsorbed Carbon Monoxide'
by C. L. Angell and Paul C. Schaffer Union Carbide Research Institute, Tarrytown, New York 10698
(Received September 9, 1965)
On adsorbing carbon monoxide on X- and Y-type zeolites containing a variety of cations, two types of adsorption can be observed by infrared spectroscopy. The first, occurring on bivalent cation-containing zeolites, gives rise to a higher than gas-phase frequency infrared band. The actual frequency depends on the nature of the cation and the zeolite and can be correlated with the strength of the electric field near the cation. The origin of the highfrequency band is explained to be the polarization of the carbon monoxide molecule in the electric field. The second type of adsorptim occurs on all zeolites examined and seems to be associated with two separate adsorption sites. Examples of the use of the high-frequency band for locating the cations in the zeolite structure are given.
Introduction When carbon monoxide is adsorbed on transition metal oxides or on some transition metals in the presence of oxygen a band appear^^-^ at about 2200 cm-'. Its appearance at a higher than gas phase frequency is rather surprising. Several explanations have been offered and were reviewed by Eischens6 recently; the high-frequency band was ascribed to (I) a structure of the type
shift are proportional to the strength of the electrostatic field at the adsorption site. The great advantage of zeolites over other ionic crystals in the elucidation of this phenomenon is that one is able to leave the general structure unchanged while varying the nature of the cation.
Experimental Section The zeolite materials, the preparation of the pellets, and the spectrometers used have been described in part I.s The samples were flash activated; they were evacuated at room temperature very briefly (pressure torr), heated to 500" in less than 10 min, kept at this temperature under vacuum for about 3 hr (final pressure -5 X 10-6 torr), and then allowed to cool under vacuum. The carbon monoxide, of CP grade from Mathieson, was analyzed and found to contain a small amount, 0.17%, of carbon dioxide, which was not
-
(11) a dipolar molecule, -CEO+, attracted to the surface by the oxygen ions in ZnO; or (111) the (CO) + ion. We have found that CO on bivalent cation containing X- and Y-type zeolites also gives a band at about 2200 cm-'. Its actual position depends on which bivalent cation is present and to a smaller extent on the type (X or Y) of zeolite. Univalent-cation zeolites do not show this band. In addition, there are bands at 2170 and 2120 cm-' regardless of the cation composition, even including the case of decationized Y-zeolite. The precise frequency of the 2200-cm-' band can be correlated with the radius of the bivalent cation. We infer that the frequency shift is primarily due to a distortion of the carbon monoxide molecule in the electrostatic field of the cation and that the distortion and frequency
(1) The major portion of this work was presented (Abstract B7) a t the Symposium on Molecular Structure and Spectroscopy, June 15-19, 1964, Columbus, Ohio. (2) C. E. O'Neill and D. J. C. Yates, Spectrochim. Acta, 17, 953 (1961). (3) (a) J. H. Taylor and C. H. Amberg, Can. J . Chem., 39, 535 (1961); (b) L. H. Little and C. H. Amberg, ibid., 40, 1997 (1962). (4) R. P. Eischens and W. A. Pliskin, Advan. Catalysis, 9, 662 (1957). ( 5 ) R. P. Eischens, Science, 146, 486 (1964). (6) C. L. Angell and P. C. Schaffer, J . Phys. Chem., 69, 3463 (1965).
Volume 70,Number 6 M a y 1966
1414
C. L. ANGELLAND PAULC. SCHAFFER
removed. Extra short path length (1 mm) cells, described previouslyIBwere used because of the very strong gas phase spectrum of CO. Also, a new 2-cm cell was constructed in which measurements could be made on samples at liquid-nitrogen temperature; there was no provision for activating the sample in situ in this cell, however, and the samples, preactivated, had to be loaded into it in a vacuum drybox. I n the region 2200-2000 cm-l, slit widths of 0.040 to 0.100 mm were used, corresponding to spectral resolutions of 0.5 to 1.2 cm-'. Band positions could be determined t.o f l cm-l, while band intensities were reproducible to f 2% of transmittance.
Results None of the zeolite samples chemisorbed CO, and a pressure of about 200 torr was used to observe the CO bands. With minor exceptions, all the zeolites tested, including X- and Y-zeolites that contained uni-, bi- or tervalent cations' and Y-zeolites that were partly or almost fully decationized, showed the two bands a t 2170 and 2120 cm-l unchanged in frequency, intensity, and shape. The exceptions were NaX (2164-2121 cm-l), BaY (2105 cm-l), and SrY (2098 cm-l). In several cases the 2170-cm-l band could not be resolved from the high-frequency band (see Table I and Figure 1). At room temperature the 2170-cm-l band is about 1.5 times as strong as the 2120-cm-l band (Figure 2). On cooling to liquid-nitrogen temperature, both bands show increases owing to increased CO adsorption (for example, MnY zeolite at room temperature and at 300 torr adsorbs 7 cc of CO/g of zeolite, while at -75" and the same pressure the adsorption is 52 cc/g), but the
80
50
40
Decationized Na Li + Mgz + Ca2 +
+
SrZ+ Baa
+
MnZ+ Fez+ C02f Nil+ Zn2+ Cd2+ a
Frequencies" on -X-seoiite, cm-*-
A
,Ot
20 I 2500
C
I
I
I
I
I
2000
2000
1 I
2000
cm-1
Figure 1. Spectra of carbon monoxide adsorbed on zeolites in the G O stretching region: A, N a y ; b, C a y ; and C, ZnY.
1-
~
MnY
2000
2000
2000
2000
crn-1
Table I : Carbon Monoxide Adsorbed on Zeolites
Frequencies5 on --Y-zeolite, em-'---.
!I-'
60
2500
Cation
t
Ionic radius. A (Goldschmidt)
Figure 2. Spectra of carbon monoxide adsorbed on MnY zeolite. Variation of the two cation-independent bands with temperature and pressure; ratio of band heights 1.55:2.64:3.28:4.35.
2170 2172 2122 2172 2213 2170 2197 2172 2186 2098 2178 2105 2208 2173 2119 2198 2172 2208 2172 2119 2217 2172 2120 2214 2170 2118 2209 2170 2120
2164 2121 2205 2173 2192 2104 2172 2203 2204 2170 2211
Frequencies are accurate to f l ern-'.
The Journal of Physical Chemistry
2112
0.98 0.70 0.78 1.06 1.27 1.43 0.91 0.83 0.82 0.78 0.83 1.03
2170-cm-' band grows considerably faster until it is approximately four times the size of the 2120-cm-' band. In addition to the above ubiquitous bands, the zeolites containing bivalent cations showed a band of higher frequency (Figure 1). This is also due to weak adsorption, since the band disappears on pumping at room temperature, although not as readily as the other (7) Zeolite Y [described in U. S.Patent 3,130,007(1964) ] has sodium ions aa the structural cations. The notation B a y , CdY, etc., is used throughout this paper to denote zeolites prepared from zeolite Y by ion-exchange substitution of Baa+, Cd*+,etc., cations for the Na+ cations of zeolite Y.
INFRARED STUDIES OF ZEOLITESAND ADSORBED MOLECULES
two bands. On cooling the samples in the presence of excess CO, the 2200-cm-l band from COY and NiY (as typical examples) did not increase while the other two bands increased manyfold. The 2200-cm-l band also did not decrease when the gas pressure was decreased a t low temperature, but the other two bands did decrease. Clearly, the CO molecules responsible for the cation-dependent band and for the other two bands are distinct, and infrared methods provide a means of distinguishing between them. When a sample of a 22% exchanged COY (6.3 bivalent cations per unit cell) and a sample of a 20% exchanged NiY (5.7 bivalent cations per unit cell) were exposed to CO, the high-frequency bands did appear although much smaller than on the respective more highly exchanged zeolite samples. On the other hand, a 35% exchanged CaX sample (15 bivalent cations per unit cell) did not show any trace of the corresponding high-frequency band. It was also noticed that small amounts of water present (
