lOs0
J. Phvs. Chem. 1993,97, 1050-1054
Induced IR Spectra of Nz and 0 2 Adsorbed on Evaporated Films of Ionic Crystals A. Lubezky Haifa University, Oranim, Kiryat Tivon 36000. Israel
Y. Kozirovski and M.Folman' Department of Chemistry, Technion-Israel Institute of Technology, Haifa 32000, Israel Received: July 20, 1992; In Final Form: November 9, 1992
Induced I R absorptions of adsorbed nitrogen and oxygen were investigated. Evaporated films of alkali halides and CaF2 served as adsorbents. In the case of Nz adsorbed on LiF and CaF2 two absorption bands were found. Only one band was found in all the other cases. Adsorption potentials and spectral shifts were calculated. These were in good agreement with experimental results.
Introduction In our laboratory a considerable amount of work has been done over the years on IR spectra of small polar molecules (CO, N20,NO, etc.) adsorbed on ionic ~urfaces.l-~ Ionic crystals are known to exert strong electric fields on the surfaces. Molecules adsorbed on these surfaces are stronglybound and their IR spectra are influenced by the electric fields. Compared to the gas phase, absorption bands of the adsorbed molecules show changes in intensities and shifts in frequencies of the allowed absorption bands. Moreover, appearance of forbidden transitions and splitting of the degeneracy were also found (C02, H2).5 It is well-known that the field of the adsorbent is responsible for the changes in the charge distribution of the adsorbed molecule. In the case of homonucleardiatomic molecules, a momentary dipole moment is formed which in turn causes the relaxation of the selection rules. It was therefore expected that induced absorption spectra of N2 and 02 would be found on adsorption of these molecules on ionic surfaces. Some of our preliminary results were reported recently.6 IR spectra of N2 and 02 adsorbed on various zeolites are k n o ~ n . ~IR J spectra of N215adsorbed on NaCl were reported by Richardson and M ~ D o n a l dwhere ,~ three absorption bands were found and attributed to N2 adsorbed on smooth surface faces and steps.
Experimental Section The apparatus and experimentalprocedurehave been described earlier.'-5 The adsorbent films were prepared in a low-temperature IR adsorption cell, which was specially built to serve simultaneously for spectroscopic and adsorption measurements. Films of LiF, NaF, KF, CaF2, NaCl, and KCl were prepared in situ, by evaporating the respective salts, from a tantalum wire basket, in a helium atmosphere of 0.15 Torr. The vapors were condensedon a central window cooled to 77 K. In previous studies it was found that this procedure gave rise to films having high surface areas of the order of 200 m2/g. In some cases the films were annealed at about 200 Korat room temperature for several hours and then cooled again. The annealing increased the homogeneity of the surfaces but reduced the area of films. Nitrogen and oxygen were adsorbed at 77 K at different equilibrium pressures, on freshly prepared films as well as on annealed ones. The spectra of the adsorbed molecules were recorded by means of a 580B Perkin-Elmer spectrophotometer with a dedicated computer. Materials (gases and salts) of spectroscopicor analytical grade were used. 0022-3654/58/2097-1050$04.00/0
oL----f 2400
1
,
'
'
'
2300 '2400 2300' FREQUENCY (cm')
L-2400
2300
Figure 1. IR spectra of nitrogen adsorbed on LiF film. Adsorbed on a freshly deposited LiF film (nine scans). Equilibrium pressures of N2: (a) 1.5 Torr, (b) 0.065 Torr. (c) Adsorption after the LiF film was annealed at room temperature (25 scans). N2 at 5 Torr of equilibrium pressure.
Results and Discussion Spectra of N2 adsorbed on LiF and CaF2are shown in Figures 1 and 2. The spectra of 0 2 adsorbed on CaF2 are shown in Figure 3. The frequencies of the absorption bands are summarized in
Table I. The spectra of N2 adsorbed on freshly deposited films of LiF and CaF2 consisted of one sharp band with a shoulder on the low-frequency side of the band. On annealed films, a closely spaced doublet at 2341 and 2332 cm-I, originating probably from molecules adsorbed on two different sites, was observed. In the case of CaF2 when the film was annealed at room temperature there was a change in the relative intensitiesof the two components and the splitting was much more pronounced (Figure 2). Nitrogen adsorbed on NaF, KF, NaCl and KCl gave rise to only one band (on each substrate) in the range 2329-2335 cm-I (Table I). The spectra of adsorbed 0 2 consisted of one band at frequencies of 1550-1 560 cm-I, depending on the substrate. A shoulder was evident on the high frequency side of the band in the case of 0 2 adsorbed on NaF and KF (Table I). The shoulder could not be resolved from the main band even on annealed films. The intensities of the absorption bands of N Zwere generally larger compared with those of 0 2adsorbed on the same substrates at about the same equilibrium pressure: e.g., the absorbance was equal to 0.1 1 4 . 1 3 for N2 and 0.07 for 0 2 , for the same NaCl adsorbent. Moreover, from the ease of desorption, as judged
63 1993 American Chemical Society
The Journal of Physicd Chemistry, Vol. 97, No. 5, 1993 loSl
Spectra of N2 and O2
the modification of Kiselev and co-workersll were used in the calculations. Details are given in the Appendix. The other contributions to the attraction potentials are the electrostatic interactions of the adsorbed molecules with the electrostatic field of the ionic crystal and the gradient of this field. Analytical expressions for the electrostatic potential outside the respective equilibrium surfaces were wed in the present calculation. The expression for the (1 11) plane of CaF2 has been reportedina previous work.l2 InalltheothercasesofN2adsorbed on (100) plane of the fcc structure, the well-known expression given by Lennard-Jonesand Dent13for the electrostatic field and its derivatives, was used. The interaction potentials due to the induced dipole (@I)and the quadrupole ( @ Q ) were calculated from
2341
234
0
I ' " . . 2400
2300
FREQUENCY (cm-9
Figure 2. IR spectra of nitrogen adsorbed on CaF2 film. (a) Adsorbed
= ' / 2 a [ a 0 ( x , y , z ) / a t ]= 2 '/*a(Ft)2
(2)
onafreshlydepositedCaF2film(fivescans). N2at 2.5Tonofequilibrium pressure. (b) Adsorbed after the CaF2 film was annealed at 200 K (nine scans). N2 at 48 Torr of equilibrium pressure. (c) Adsorbed after the CaF2 film was annealed at room temperature (25 scans). N2 at 27 Torr of equilibrium pressure.
1556
Y
E! y
1556
60-
)I
z
2 f
40-
a
+ K
20-
1556
j
FREQUENCY (cm-' 1
Figure 3. IR spectra of oxygen adsorbed on CaF2 film (nine scans). Equilibrium pressures of 02:(a) 0.24 Torr,(b) 3 Torr, (c) 33 Torr.
TABLE I: IR Absorption Fn~wncies(cm-I) of NZand 0 Adsorbed on the Evaporated Films' film LiF (F) LiF (A) N a F (F) KF (F) NaCl (F) KCI (F) CaF2 CaF2 (A)
N2 2340; 2334, sh 2341; 2332 2333 2329 2335 2332 2338; 2332, sh 2341; 2332
2
0 2
1558 1560; 1566, sh 1554; 1561, sh 1552 1555 1556
0 F, freshly deposited film. A, annealed film. Vibration frequencies in the gas phase (Raman): N2, 2330.1 cm-l.lsa 02,1556.2 cm-l.Isb
from the change in the band intensities, it is inferred that the adsorption energy of N2 is higher than that of 0 2 . Clllculatioas of the adsorption potentials were carried out for N2 adsorbed on CaF2, LiF, NaF, KF, NaCl, and KCl. The total adsorption potential, @, has been calculated as a sum of dispersion, induction, quadrupole, and repulsion potentials:
+
0 = 0, + aQ+ 0 R (1) where each individual potential is given as a sum of pair interactions between the adsorbed molecule and each ion of the adsorbent. Such a procedure has been applied in the past for molecules adsorbed on films of alkali metal halides.1J.kv5b The two major contributions to the dispersion potential are the induced dipole-induced diple and the induced dipole-induced quadrupoleinteractions. The Kirkwood-Muller expressionloand
where a is the polarizability and Q is the quadrupole of the adsorbed molecule. F, is the gradient of the potential @(xy,z) in the direction t of the adsorbed molecule, where tis taken along the axis of symmetry of the quadrupole Q. The repulsion potential @R was of the form B exp(-cr). The constants B and c were taken as geometrical and arithmetical means, respectively, of the corresponding constants for repulsion between the adsorbed nitrogen molecules and the adsorbent ions. Calculations were carried out for N2 adsorbed on the q u i librium planes of the various salts. CaFz has a face-centered cubic structure. The (1 11) face, which contains only negatively charged F- ions, is the equilibrium surface. This is also the dominant face in the films, although the (1 10) plane has also beenfound.14 Theothersalts (LiF,etc.) havealsoanfccstructure but the dominant equilibrium surface is the (100) ~ 1 a n e . I ~ In previous works calculations were done for molecules adsorbed on different sites and 0rientations.I.3.~~It was shown that the most energetic site was thecation, where the molecule wasoriented normal to the surface. Another site, where the molecule was adsorbed parallel to the surface was also found to be energetic. The adsorption potentials for these two sites were considered also in the present work. The CaF2 crystal has a lattice constant of (I = 5.463 A. In the (1 11) face the anions form equilateral triangles, the F-ions being ca. 4 A ( a / d 2 ) apart. The Ca2+ions are positioned in the centers of these triangles in a plane which is ca. 0.8 A (a/4d3) below the plane of the anions. Due to the relatively small size of the F- ions (ionic radius r = 1.36 A) the Ca*+ions are only partially screened. These Ca2+ions are the main sites for theN2 adsorption. The second site considered in the case of CaF2 is the one where N2 is adsorbed in a parallel orientation, above the midpoint between two anions." As mentioned above, the cation M+was the main site considered for adsorption on the (100) plane of the various MX substrates. The second site was found to be the center of the lattice square where N2 is adsorbed parallel to the surface. The different contributions to the adsorption potential at the minimum are summarized in Table 11. For the two cases, CaF2 and NaCl, the adsorptionpotentials on the two sites, as a function of the distance from the surface, are given in Figures 4 and 5. The physical constants used in the calculations are collected in the Appendix. As expected,the energy of adsorption on the cation has a larger value compared to the one obtained for the second site. The difference between the two values depends on the substrates. The calculated adsorptionpotential for N2 on the Ne+of NaCl surface is 3.56 kcal/mol and is consistent with the experimental result obtained earlier.9.16 A lower calculated result, 3.10 kcal/ mol, was obtained for N2adsorbed on KCl. A similar value, of
Lubezky et al.
1052 The Journal of Physical Chemistry, Vol. 97, No. 5, 1993
TABLE II: Contributions (cal/mol) of tbe Different Potentials in tbe Adsorption Potential at tbe Respective Minim
film LiF
site and orientation' A C
dist from the surface at the minimum
1.69 1.89 1.81 1.40 1.47 1.26 1.20 1.06 1.20 1.02 0.96 0.67 0.73
D A
NaF
D A
KF
D A
NaCl
D A
KCI
D A
CaF2
rrA 3.395 3.797 3.636 3.230 3.392 3.358 3.198 2.982 3.376 3.203 3.014 3.660 3.988
p units
D'
*DO
@w
1515 855 1085 1661 1297 1897 1940 3218 2102 2783 3023 3575 2634
127 43 65 155 88 183 146 332 161 290 265 419 200
*Q 392 81 1 I5 939 335 1135 741 2346 630 2016 1316 2104 905
@R
@I
5
amin
838 307 439 1203 682 1338 1222 2648 882 2282 2005 3129 1644
34 66 315 29 1
183 16
1200 672 826 1586 1038 1943 1605 3563 201 1 3098 2599 3152 2111
0 (A) Cation, normal to the surface. (C) Anion, parallel to the surface in x or y direction. (D) Midpoint of unit-cell, parallel xy direction. (D') Midpoint between two anions, parallel.
versus 2.5 X esu cm). The contributions of the other electrostatic interactions are also smaller, since Nz has no dipole moment and its polarizability is somewhat smaller than that of
co.
-41
' ' ' 0.8 1.0 1.2
'
1.4
'
1.6
'
'
1
1.8 2.0
P Figure 4. Potential energy of N l adsorbed on (100) plane of NaCI: (A) perpendicular orientation above Na+; (D) parallel orientation above the center of a lattice square (xy direction).
The calculated adsorption potential for N2 adsorbed on KF, NaF, and LiF are lower than those mentioned above. These results are not in complete agreement with the experimental fact that the adsorption of N2on these fluorides, (as judged from the slow desorption), seems to be quite strong. The spectral shift of Nz adsorbed on NaCl was calculated following a procedure described earlier.'*" In calculating the spectral shift, it has been assumed that the adsorbed molecule behaves as a one dimensional anharmonic oscillator perturbed by the adsorbent. This perturbation difference causes the appearanceof the forbidden stretchingvibration of the adsorbed N2 and the shift in the vibration frequency as known from the Raman spectrum. The spectral shift was calculated by the perturbation method, the perturbing potential being the interaction energy of the adsorbed molecule with the surface of the adsorbent. The Hamiltonianofthemoleculein thegasphasewascalculated according to the method of Hulburt and Hirshfelder" with the molecular parameters taken from Herzberg:l*
4 7 H = Ho + H,' = (1/2)klx2 + (1/6)k,x3 + (1/24)k,x3 + ... 231
I
(4)
= (1/2)2.2946
X
+
106x2- (1/6)1.696 X 10i5x3 (1/24)9.98 X lOZ3x4
The potential energy of interaction of N2 adsorbed in a perpendicular orientation on Na+ ion around the minimum was made to fit a power series in the vibration coordinate x. The terms contributing to the total Hamiltonian:
H = Ho+H,' + H,'
(5)
where
H,'= (1 /2)fix2 + (1 /6)f2x'
+ (1/24)j3x4 = (1 /2) 1.039 X 104x2 (1/6)4.79 X 10'zx3 (1/24)1.77 X 1022x4 The1;. were added to the corresponding k, coefficients. The perturbed energy levels and the shift in frequency were calculated in the usual way.Ig The value of Av = +5.6 cm-1 obtained from the calculation is in good agreement with the experimental value of +5 cm-1. Therefore, the 2335-cm-1 absorptionis attributed to N2 adsorbed above the Na+ ion perpendicular to the surface. A similar calculation was carried out for N2 adsorbed perpendicular on a CaZ+ ion of the CaF2 surface.4~ The
+
P Figure 5. Potential energy of N2 adsorbed on (1 1 1) plane of CaF2: (A) perpendicular orientation above Ca2+; (D') parallel orientation above midpoint between two anions.
3.15 kcal/mol, was found for N2 adsorbed on Ca2+on the CaF2 surface. This value is lower compared to the one obtained for CO adsorbed on the same substrate.& This is expected since the quadrupole moment of N2 is smaller than that of CO (1.52 X
+
Spectra of N2 and O2
The Journal of Physical Chemistry, Vol. 97, No. 5, 1993 1053
TABLE IIk P h y S i d Const.nb of N2
TABLE I V Physical Constants of the Various IOM ion 102Saav,23a cm3 IO6x23b cm3/mol 1066,*4erg’/2 ~~
av polarizability a polarizability along the bond a polarizability perpendicular to the bond XM diamagnetic susceptibility Q quadrupole moment rm dist at the minimum of the potential c / k potential well depth B steepness of the potential o, vibrational frequency o.x, anharmonic constant B, rotational constant r. bond length at quilibrium aau
17.5 X 23.8 X 14.5 X
cm3 (ref 20a) cm3 (ref 20a) cm3 (ref 20a)
12.0 X 10” cm3/mol,(ref 21a) 1.52 X esu cm (ref 22) 4.01 1 A (ref 2Ob) 101.2 (K) (ref 20b) 17 (ref 20b) 2358.57 cm-I (ref 18) 14.324cm-I (ref 18) 1.998 24 cm-’ (ref 18) 1.097 68 A (ref 18)
F-
Li+
Na+ K+ FCa2+
X
+
+
S”ry The results presented here once again prove that strong surface fields above ioniccrystalsgive rise to stronginduced IR absorptions in adsorbed molecules. Also, the knowledge of exact surface potentials makes it possible to evaluate quantitatively with a reasonable accuracy the adsorption potentials and the spectral shifts. Appendix The formulas used for the calculations of the dispersion potentials are for pair interaction:
*DQ=-
45h2 -a 1 a ([2( 32r2mr8
’
*)
+ I]-’+
a2/X2
[2(
*“2) , / X I
+ 11-l)
(A2)
where m is the mass of the electron, c the velocity of light, aithe polarizability of the ions or adsorbate, xi their magnetic susceptibility, r the distance between the two species, and h Planck’s constant. The other contributions to the potential are based on the expressions for the electrostatic and the repulsion potentials as mentioned in the text. The constant c used in the calculationsof the repulsion potential was 2.898 A-l for all the ions.24 In the case of CaF2 the value of 3.00 A-1 and somewhat different values for b were used.25 The lattice constants used were uL~F= 4.01 1 A, aNaF = 4.615 A, aKF 5.33 A, U N ~ C I= 5.627 A, aKcI 6.28 A , ” b and aCaFl = 5.463 A.26 On the basis of the relevant data (Tables I11 and IV), the following expressions were obtained for the calculations of the
24.96 71.9 3.962 12.63 3 1.02 22.249 (ref 25) 45 (ref 25)
(a) NaCl:
103x2 (1/6)4.21 X lOI2x3
(1/24)1.50 X 102’x4 A value of Av = +5 cm-1was obtained from the calculation. The experimental Av value was +8 cm-1 for the main band. Here, the agreement between the calculated and experimental shift is less satisfactory than that of the previous case. However, in the case of N2 adsorbed on CaF2 surface, it was also concluded that the main adsorption sites are the Ca2+ ions where the molecules are adsorbed perpendicular to the surface. A similar conclusion, namely, that the adsorbed N2 molecules are localized on the cations of the zeolites in a perpendicular orientation, was reached by de Lara et al.’ and by Yamazaki et a1.8 The second absorption band at 2332 cm-I obtained for N2 on CaF2 is probably due to molecules adsorbed on a different site or on a different equilibrium plane, such as the (1 10) plane.14
9.4 24.2 0.7 6.1 14.6
various contributions to the adsorption potential of N2adsorbed on site A (in kcal/mol) of (a) NaCI and (b) CaF2.
Hamiltonian (Zfl) in this case is given by
H,’= (1/2)9.267
8.7 30.0 0.29 2.2 9.7
c1-
-@DD
= 0.5292+(/~)“+ 5.2462-(~)”
-@DQ
= 0 . 1 0 8 2 + ( ~ ) - ~1 . 2 2 7 2 - ( ~ ) - ~
+
-9q = 260.4Fz7,,
-01 = 3882.4(FJ2
aR = 77985 exp(-10.04p) + 4 X 443950 exp[-10.04( 1
+
(b) CaF2: 4 D D
-@Dq
+ = 0 . 0 4 0 2 + ( ~ ) -+~ 0 . 0 2 9 2 - ( ~ ) - ~ = 0.3302+(~)” 0.2522-(~)”
-9Q= 41 .5Fz, = 93.3(FJ2
aR = 2.78 X lo5 exp[-13.98(p + (24)-’/2)] + 3 X 1.37 X lo5 exp[-13.98(p2
+ ‘/3)’/2]
is the distance r from the adsorbed molecule to the respective ions, expressed as a function of the lattice constant a. In the case of NaCl p = r(2/a) and for CaF2 p = r(d2/a). Analogous expressions were found for the different sites and salts. p
References and Notes (1) (a) Gevirzman, R.; Kozirovski, Y.; Folman, M. Trans. Faraday Soc. 1969, 65, 2206. (b) Gevirzman, R.; Kozirovski, Y. Trans. Faraday Soc. 1971, 67, 2686. (2) (a) Kozirovski, Y.; Folman, M. Trans. Faraday Soc. 1%9,65,244. (b) Kozirovski, Y.; Folman, M. Isr. J . Chem. 1969, 7, 595. (3) Lubezky, A.; Folman, M. Trans. Faraday Soc. 1971,67, 586. (4) (a) Schmidt, J.;Marcovitch, 0.;Lubezky, A.; Kozirovski. Y.; Folman, M. J . Colloid Interface Sci. 1980, 75, 85. (b) Lubezky, A.; Kozirovski, Y.; Folman, M. J . Colloid Interface Sci. 1983, 92, 525. (c) Schmidt, J.; Marcovitch, 0.;Lubezky, A.; Kozirovski, Y. J . Chem. Soc., Faraday Trans, I 1984, 80, 1. ( 5 ) (a) Kozirovski, Y.; Folman, M. Trans.Faraday SOC.1966,62,1431. (b) Folman, M.; Kozirovski, Y . J . Colloid Interface Sci. 1972, 38, 51. (6) (a) Lubezky, A.; Kozirovski, Y.; Folman, M. Israel Chem. Soc., 58th Annual Meeting, 1989, Rehovot, Israel. (b) Lubezky, A.; Kozirovski, Y.; Folman, M. XXVI Colloquium Spectroscopicum Internationale. 1989, Sofia, Bulgaria. (7) (a) Banachin, B.; Cohen de Lara, E. J . Chem. SOC.,Faraday Trans. 2 1986, 82, 1953. (b) Soussen-Jacob, J.; Tsakiris, J.; Cohen de b r a , E . J . Chem. Phys. 1989,91,2649. (c) Cohen de Lara, E.Mol. Phys. 1989,66,479. (8) (a) Yamazaki, T.; Watamiki, J.; Ozawa, S.;Ogino, Y. Bull. Chem. SOC.Jpn. 1988, 61, 1039; (b) Mol. Phys. 1991, 73, 649. (9) Richardson, H. H.; McDonald, T. L. J . Electron Spectrosc. Relat. Phenom. 1990, 54/55, 1003. (10) Young, D. M.; Corwell, A. D. Physical Adsorprion of Gases; Butterworth: London, 1962; Chapter 2. (1 1) Avgul, N. N.; Isirikyan, A. A.; Kiselev, A. V.;Lygina, J. A.; hshkus, D. P.Bull. Akad. Sci. USSR, Diu. Chem. Sci. 1957, 1314. (12) Marcovitch, 0.;Kozirovski, Y. J . Chem. SOC.,Faraday Trans, 2 1975, 71, 1302. (1 3) Lennard-Jones, J. E.; Dent, B. M. Trans. Faraday Soc. 1928,2492. (14) (a) Bannon, J.; Curnov, C. E. Nature (London) 1948,161, 136. (b) Bannon, J.; Coogan, C. E. Nature (London) 1948, 163, 62. (15) Schulz, L. G. J . Chem. Phys. 1949, 17, 1153.
1M4 The Journal of Physical Chemistry, Vol. 97, No. 5, 1993 (16) Jackson, D. J.; Davis, E.W. J . Colloid Interface Sci. 1974.47,499. (17) Hulbert, H. M.;Hirschfelder, J. 0. J . Chem. fhys. 1941,9, 61. (18) Huber, K. D.; Herzberg. G. Molecular Spectra and Molecular Srrucrure IY: Comranrs of Diaromic Molecules; Van-Nostrand Reinhold: New York, 1979; (a) p 420,(b) p 489. (19) Wu, T. Y. Vibrarional Spectra and Srrucrure of folyaromic Molecules; Edwards: Michigan, 1946. (20) Hirschfelder, J. 0.; Curtis, C. F.;Bird, R. B.TheMolecular Theory of Gases and Liquids; Wiley: New York, 1954; (a) p 950,(b) p 181.
Lubezky et al. (21) HandbookofChemisrryondfhysics,67thcd.;CRCPrcu Inc.: Boa Raton, FL. 1986-1987;(a) p E-122,(b) p E-190. (22) Stogryn, D.E.; Stogryn, A. D. Mol. Phys. 1966, 11, 371. (23) Dekker, A. J. Solid Srare Physics; Prcntice-Hall: N e w York, 19S9; (a) p 136, (b) p 453. (24) Huggins, M . L.;Mayer, J. E. 1. Chem. fhys. 1933, I , 643. (25) Harris, H. J.; Morris, D. F. C. Acro Crysrallogr. 1959, 12, 657. (26) Wyckoff, R. W. G . Crysral Srrucrure, 2nd ed.; Interacicnce Pub lishers: New York, 1963;Vol. 1, Chapter 4.
