A, J. WOODWARD AND NEVILLE JONATHAN
2930
Infrared Spectra of Nitric Oxide Adsorbed on Evaporated Alkali Halide Films by A. J. Woodward* and Neville Jonathan Chemistry Department, The University, Southampton, SO0 6 N H England
(Received January 1 2 , 1972)
Publication costs borne completely by The Journal of Physical Chemistry
Infrared spectra are reported for nitric oxide adsorbed at different surface coverages on high surface area films of evaporated SaCl, XaBr, NaI, KCl, and CsC1. The data are interpreted in terms of monomeric and dimeric species adsorbed on the surface planes and at the edges of the alkali halide crystallites. Possible adsorption sites and orientations of the adsorbate are discussed. The spectra provide support for the surface structures previously proposed for alkali halide films.
Introduction Kozirovski and Folman have pioneered the production of alkali halide films in a form suitable for infrared spectroscopic studies of adsorption.' I n the present work an apparatus of similar design has been used to study the adsorption of nitric oxide on films of Tc'aC1, KaBr, NaI, KC1, and CsCl. The interest in these systems arises because of the possibility of determining the nature of the adsorption, particularly as there is some doubt as to whether or not nitric oxide exists in a dimerized form on these surfaces. Adsorption isotherms have been reported for systems of this type and the data used to determine the extent of interaction of the NO quadrupole with the alkali halide surface and also to find whether or not dimerization accompanies the adsorption.2 It was argued that in the absence of quadrupole interaction the heats of adsorption of argon and NO should be in a similar ratio to that of their equilibrium separation energies obtained from 9 : 6 intermolecular potential function^.^ The agreement between heats of adsorption estimated on this basis and the experimental low-coverage heats for KO adsorption was good except for a LiCl surface. It was therefore concluded that the quadrupole moment of KO is not of sufficient magnitude to have any significant effect on the heats of adsorption of KO on NaCl, KCl, and CsCl but it does seem to have an appreciable effect where the much smaller cation of LiCl is concerned. No evidence was found for dimerization. These conclusions are now considered open to question since the parameters used for the 9:6 potential of KO are known t o relate to the double m ~ l e c u l e . ~The agreement between experimental and calculated heats of adsorption is therefore reinterpreted as being indicative of a significant contribution to the heat of adsorption from dimerization and/or quadrupole interaction. Experimental Section The adsorbent films were prepared in situ by evaporation from a small tantalum furnace onto a window cooled with liquid nitrogen at approximately -196". The Journal of Physical Chemistry, Vol. 76,No. 19,1971
The films were then annealed a t -80" for 10-24 hr prior to adsorption studies. This treatment increases the homogeneity and stability of the films' and after it they are believed to have surface areas of ca. 200 m2/g. Ron and Folman5 have previously discussed the adsorptive and thermodynamic properties of alkali halide films prepared under similar conditions. A conventional high-vacuum system with a mercury diffusion pump and liquid nitrogen trap was used for evacuating the cell and admitting gas to the film. Adsorption studies were made in the temperature range -145 to -150", the spectra being recorded with a GrubbParsons GSBA grating spectrometer. The salts used were of analytical grade. Nitric oxide supplied by Air Products Ltd. was purified by repeated fractional sublimations from liquid argon to liquid nitrogen.
Results Figures 1-6 show spectra obtained from the various KO-alkali halide systems for different surface coverages, although the actual extent of the coverage could not be determined. I n all cases the variations in band intensities on the desorption cycle were the exact reverse of the adsorption cycle data, The observed frequencies and their shifts with coverage are summarized in Table I. Changes in band shapes, widths, and frequencies with temperature were small and attempts to make systematic studies of these factors proved to be inconclusive. All spectra were accompanied by a very weak absorption in the region 2.9-3.0 pm. This problem which has been referred to previously' arises from traces of water which desorb slowly from the walls of the metal cell and become adsorbed on the alkali halide film. However, in view of the low (1) Y . Korirovski and M . Folman, Trans. Faraday Soc., 62, 808 (1966). (2) A. Granville and P. G.Hall, ibid., 63, 701 (1967).
(3) E. A. Moelwyn-Hughes, "Physical Chemistry," Pergamon Press, London, 1961, p 335. (4) E. A. Moelwyn-Hughes, private communication. (5) I. Ron and M. Folman, I s r . J . Chem., 3, 18 (1965).
INFRARED SPECTRAOF NITRIC OXIDE
2931
-
60 w
.-0'
g o8
P
m 20
I
I
I
'
B
L
1700
1800 frequency cm-I
1900
Figure 1. Spectra of NO adsorbed at different coverages on NaCl.
1700
1700
I
I
I
1900
Figure 3. Spectra of NO adsorbed at different coverages on a NaI film annealed a t -80" for 10 hr.
20 I1
1800 frequency cm-I
-
I
1800 1900 frequency cm-1
Figure 2. Spectra of NO adsorbed at different coverages on NaBr.
intensity of the band the usual assumption is made that only a small fraction of the surface is affected.
Discussion The existence of the nitric oxide dimer in the lowtemperature gas and in the condensed phase is now firmly established. A bent ONNO structure for the molecule was first suggested by Raman and infrared studies of NO in the solid and liquid states and in matrix i s ~ l a t i o n . ~The , ~ infrared data for the low-temperature gas is consistent with an N-IS bond distance of about 1.75 8 and an N-N-0 angle of around 90°s while for the solid dimer the dimysions N - . . N = 2.16 A, 0 . . -0 = 2.62 8, N-0 = 1.12A have been determined by X-ray crystallography. The heat of dissociation of the dimer in the gas phase has been found to be 2.7 kcal/mol.lO
1700
1800 frequency cm"
1900
Figure 4. Spectra of NO adsorbed a t different coverages on a NaI film annealed a t -80" for 18 hr.
I n addition to the normal cis dimer, monomeric and trans dimeric NO species have been isolated in a COz matrix.' Table I1 gives relevant data from these studies. A recent communication has reported an infrared spectroscopic study of NO physisorbed on lithium halide filrns.l1 On LiF, LiC1, and LiBr four absorp(0) A. L. Smith, W. H. Keeler, and H. L. Johnston, J . Chem. Phys., 19, 189 (1951). (7) W.G. Fateley, H. A. Bent, and B. Crawford, J r . , ibid., 31, 204
(1959). (8) C.E. Dinerman and G. E. Ewing, ibid,, 53, 626 (1970). (9) W.J. Dulmage, et al., Acta Crystallogr., 14, 1100 (1961). (10) C. E. Dinerman and G. E. Ewing, J . Chem. Phys., 54, 3669 (1971). (11) A. Lubezky and M. Folman, Proc. Israel Chem. Soc., Suppl., 8, 99 (1970). The Journal o/ Physical Chemistry, Vol. 76, N o . 19,1971
A. J. WOODWARD AND NEVILLEJONATHAN
2932
Table I: Frequencies Observed for the NO-Alkali Halide Systems, cm-'
60
x 540
.-
. I ,
E
t: P m
20
NaI
L t
1700
.
I
'
I
1800
frequency cm-'
1900
1889-1886
~1876
-
'
1815 1817 -1785 1764 + 1757 ~ 1 7 3 7
I
Figure 5. Spectra of NO adsorbed at different coverages on KC1.
I
I
Table I1 : Infrared Spectral Data for NO, Cm-1, Taken from Ref 6-8 --Matrix
InNO
NO (monomer)
I
1700
I
I
1800
* 1900
sym cis-(NO)z asym t r a ~ ( N 0asym )~
1862 1768
isolated'-
InCOz
In argon
1883
1875
1862 1768 1740
1866 1776
Gas8
at
Liquid6
1872 (in krypton) 1865 1770
123'K
1876
1860 1788
frequency cm-I Figure 6. Spectra of NO adsorbed at different coverages on CsC1.
tion bands were observed, two in the region 1900-1830 cm-1, the other two in the 1800-1760-cm-1 region. It was concluded that two different adsorption sites exist a t these surfaces and that the nitric oxide is adsorbed as a dimer at both types of site. On LiI only two absorptions were obtained. Adsorption on Sodium Salts. For the NO-NaC1 system, four ir absorption bands were observed at each of the surface coverages studied. With increasing coverage the absorptions at 1893 and 1809 cm-I showed markedly decreasing rates of growth compared with those of the 1860- and 1751-cm-l absorptions. A similar effect has been noted previously in a study of NzO adsorption on these surfaces,12although here some bands were actually found to cease growing from a certain coverage onward. Such bands were assigned to adsorbate at sites of limited number which were thought The Journal of Physical Chemistry, Vol. 76, No. 19, 1971
to be located at the edges of the alkali halide crystallites. It seems reasonable to assume a similar assignment for the present spectra; ie., the bands at 1893 and 1809 cm-I are attributed to adsorption a t sites which are limited in number and possibly located a t the crystallite edges, while the 1860- and 1757-cm-' bands are attributed to adsorption on the surface planes of the crystallites. A close correlation was observed between the optical density data for the 1860- and 1757-cm-I bands. I n view of this and of the close similarity of the frequencies to those previously reported for the stretching modes of the cis (NO)zdimer (Table 11),these two bands are assigned to a cis dimeric species on the NaCl surface planes. The asymmetric stretching mode at 1751 em-l is shifted downward by 37 cm-I with respect to the gas-phase frequency indicating that the interaction of (12) Y. Kosirovski and M. Folman, Trans. Faraday Soc., 65, 244 (1969).
INFRARED SPECTRA OF NITRICOXIDE the dimer with the NaCl surface is greater than that occurring between dimer molecules in the solid for which the corresponding shift is 20 cm-'. A dimer molecule could be adsorbed at the surface plane by interaction of either one or both of the NO structures with the surface. If the separation of adjacent adsorption sites is suitable, adsorption through both structures will be favored. Calculations of the heats of adsorption for N21a and CO14 on the (100) surface plane of NaCl have indicated that as a result of a large quadrupole interaction with the surface an adsorption perpendicular to the surface and above a cation is the most favorable. The same site is anticipated for NO adsorption on this plane since the quadrupole moments of NO and Nz have been found to be similar.l69l6 This is supported by the recent calculations on the NO-LiC1 system which found a normal orientation above the Li+ ion to be most favorable. The separation of adjoacent cations on the NaCl surface plane is about 4.0 A and is suitable for the adsorption of a dimer by the interaction of each NO structure with a surface cation (taking into account the ionic radius for the oxygen of NO). The structure of the adsorbed molecule would be similar to that of the solid-state dimer and each NO molecule would be slightly displaced from the surface normal. This is believed to be the predominant mode of adsorption on the surface planes of the crystallites a t the lower surface coverages. At high coverages there is probably a contribution from dimers adsorbed by the interaction of one NO structure with a surface cation. This could explain the shift in dimer frequencies with increasing coverage (see Table I), although this could also be due to energetic heterogeneity of the surface sites and adsorbate-adsorbate interactions. The lack of correlation in the optical density data for the 1893- and 1809-cm-' absorptions would seem to preclude a simple assignment of these bands to a second dimeric species, although in the study of NO adsorption on LiF, LiC1, and LiBrl' bands a t 1890 and 1800 cm-' were assigned in this manner. The two alternative assignments are either (a) there is an overlap of absorptions due to monomeric and dimeric adsorbate molecules or (b) each band arises from a monomeric species a t the NaCl surface. An assignment of the bands in part to a dimer is considered unsatisfactory since it requires that the symmetric and asymmetric stretching modes of the dimer are shifted upward by 33 and 21 cm-l, respectively, from the corresponding gas-phase frequencies. A downward frequency shift would be anticipated as observed for the 1860- and 1757-cm-l bands. An assignment of the 1893-cm-' absorption to a monomeric species is suggested by its similarity to the frequency reported for the NO monomer isolated in a COZ matrix.' Since the NO+ ion absorbs a t -2210 cm-l the upward shift of the band by 17 cm-I from
2933 the gas-phase frequency represents a slight increase in NO bond order on adsorption. Such a change can only occur as a result of close association with surface cations. Adsorption above a cation located a t a crystallite edge would be even more favorable than above an in-plane cation due to the somewhat greater surface field a t this point. A perpendicular orientation is indicated by the lack of any significant frequency shift with a change in anion from C1- to I-, and although the dipole moment of NO is small (0.16 D18), an adsorption through oxygen would give a contribution to the interaction potential from dipole interaction with the surface field. Adsorption as a dimer at these sites in the manner proposed for the in-plane adsorptions is prevented byJhe large separation of cations in a crystallite edge (5.6 A). Adsorption as a dimer by the interaction of one NO structure with an edge-located cation will be less favorable than adsorption as two monomers provided the heat of adsorption is significantly greater than the heat of dimerization. The 1 8 0 9 - ~ m -frequency ~ is shifted 67 cm-1 downward from the gas-phase monomer frequency indicating an increase in electron density in the ir system of the NO and hence association with the anion. The shift is considerably larger than that which one normally associates with physisorption. However, such a shift only involves a small change in the NO bond order since the NO- ion absorbs in the region 1100-1000 cm-l l9 (gas-phase frequency NO monomer 1876 cm-l). The change is of similar magnitude to that accompanying adsorption above a cation. A significant shift in the low-coverage frequency of this band with a change in cation indicates that the adsorption is not perpendicular to the crystallite edge. It therefore seems probable that the adsorption is such that the nitrogen atom is almost above the edge-located anion and that the oxygen is directed toward the cation. Such an adsorption would be expected to be accompanied by a far lower heat of adsorption than that due to monomeric adsorption above an edge-located cation. This would explain the somewhat lower intensity of the 1809-cm-l band (relative to the 1893-cm-1 band intensity) at the lowest coverage studied. Furthermore, since the inplane sites are far more numerous than edge sites, the fact that all four absorptions appear for the lowest coverage studied suggests that the edge sites are the more energetic. It should be noted that both argu(13) T. Hayakawa, Bull. Chem. SOC. Jup., 30, 236 (1957). (14) R. Gevirzman, Y. Kozirovski, and 111. Folman, Trans. Farad& SOC., 65, 2206 (1969). (15) B. T. Berendts and A. Dymanus, J. Chem. Phys., 49, 2632 (1968). (16) J. S. Murphy and J. E. Boggs, ibid., 49, 3338 (1968). (17) D. W. Turner and D. P. May, ibid., 45, 471 (1966). (18) H. E. Watson, G. G. Rao, and K . L. Ramanaswamy, Proc. R o y . SOC.,Ser. A, 143, 558 (1934). (19) W. P. Griffith, J. Lewis, and G. Wilkinson, J. Inorg. Nucl. Chem., 7, 38 (1958).
The Journal of Physical Chemistry, Vol. 76,N o . 10,1071
2934 ments make the reasonable assumption of a similarity in the extinction coefficients of the various absorptions. The spectral data for NO-NaBr (Figure 2) were almost identical with the above and bands are assigned in a similar manner; i.e., the low-coverage frequencies of 1893 and 1807 cm-' are attributed to monomeric NO adsorbed at edge sites, while those at 1867 and 1746 cm-l are attributed to dimeric adsorption on the surface planes. I n a previous study of h'zO-.NaBr,12 absorptions were detected for adsorption at Bites in a second surface plane. It is possible that the slight asymmetry in the 1746-cm-l band at low coverage might be due to dimers adsorbed on such a plane, but it would appear that the films were either of slightly lower surface area or annealed to a somewhat greater extent than those of the previous study. The data for the NO-NaI system were complicated by the fact that the spectra obtained were strongly dependent on the time for which the film was annealed a t -80". Figures 3 and 4 show examples of the spectra obtained from films annealed for 10 and 18 hr, respectively. Eight absorptions were distinguished for NO adsorbed on the 10-hr film and from their intensity variation with surface coverage these are readily divided into two sets of four, each set behaving in an analagous manner to the bands described for NO-NaCl. The bands are labeled A and B in Figure 3. The approximate low-coverage frequencies are: for set A, 1876, 1785 cm-l and 1852, 1737 cm-l; for set B, 1889,1815 cm-l and 1860,1764 cm-l. Only the bands of set A and the highest frequency of set B were observed at the lowest coverage studied. At the higher coverages the bands of set B increased in intensity at a much greater rate than did those of set A. The spectra from the 18-hr film showed a greater correspondence with those for NO-NaC1. Four main absorptions were observed v i t h frequencies similar to those of set B described above, i.e., 1889, 1815 cm-l and 1861, 1757 cm-l. Additional absorptions were present as shoulders on the low-frequency sides of the 1861- and 1757-cm-I absorptions, their frequencies being similar to those of the strong absorptions of set A, namely, -1739 and -1853 cm-l. Each set of bands is assigned to monomeric edge and dimeric in-plane adsorptions on a particular type of crystallite. Set A arises from adsorption on a high-energy form which is somewhat unstable a t - 80". The number of these crystallites is dependent on the annealing time and in the 18-hr film is not sufficient to allow detection of edge adsorptions. Bands of set B arise from adsorption on a more stable crystallite type and in view of the similarity of frequencies with those recorded for the NO-NaC1-NaBr systems the crystallite type is probably the same as that existing in the other sodium halides. The difference in frequencies is attributed to the different arrangement of ions in the surface and underlying planes of the two crystallites. This different arrangement could also mean that for the The Journal of Physical Chemistry, Vol. 76, N o . 19, 1971
A. J. WOODWARD AND NEVILLEJONATHAN less stable form the sites and orientations of the adsorptions are somewhat different from those of the more stable form. The latter are assumed to be similar to those discussed for NaCl, although it is probable that in view of the larger cation separation in NaI the mode of dimer adsorption may be somewhat different, viz,, adsorption by interaction of only one ATOmolecule of the dimer with a surface cation. The above analysis supports the interpretation given by Rodrovski and Folman for the N20-ISaI system.12 This work indicated that two crystallite types were present in NaI films annealed for 24 hr at -80". Absorptions were observed for edge and in-plane adsorptions on one crystallite, whereas only in-plane adsorptions could be detected for the other. The slower rate of annealing for the S a 1 film is attributed to the size of the anion. Studies of the dielectric properties of alkali halide films formed in this manner have shown pronounced aging eff ectsa20 These occur as the result of high vacancy concentrations, the excess of which is steadily annealed out. I n order to maintain electrical neutrality the cation vacancies can only condense as the less mobile anion vacancies condense. The annealing rate is therefore determined by the anion mobility which is highest for a small anion and/or a large cation and vice versa. It must be pointed out that in view of the above data for NaI, care was taken to ensure that films of all the other alkali halides were fully annealed. Adsorption on Potassium Chloride. Three of the four bands observed for NO-KC1, viz., the high-frequency monomer absorption (1889 cm-l) and the dimer absorptions (1860, 1757 ern-'), followed a similar pattern to that described for NO-NaX. However, for all coverages the intensity of the monomer absorption relative to those of the dimer bands was weaker than the corresponding cases for the KaX surfaces. This effect is attributed to the somewhat larger cation which leads to a more extensive annealing in the KC1 films than in the NaX films and hence larger crystallites with a higher ratio of in-plane to edge sites. For all except the lowest coverage studied a fourth absorption was observed at 1790 crn-l, its increase in intensity with coverage being more rapid than that of the other three absorptions. The band is again assigned to monomeric NO molecules associated with surface anions, but in view of the intensity variations of the band, these anions are considered to be located a t both edge and in-plane sites. The calculations for N221have indicated that the difference in energy between anion and cation sites in a (100) surface plane decreases for a change from KaC1 to KCl which fits in with this explanation. Adsorption on Cesium Chloride. Two regions of (20) C. Weaver and S. Lorenzoni, J . Vac. Sci. Technol, 6,597 (1969) (21) T. Hayakawa, Bull. Chem. SOC.Jap., 30, 332 (1957).
INFRARED SPECTRA OF NITRIC OXIDE absorption were observed for this system, viz., 18501880 cm-I and 1740-1780 cm-'. The high-frequency region appeared to arise from two overlapping absorptions of similar intensity and approximate frequencies 1877 and 1858 cm-l. Similarly the low-frequency region appeared to be due to two or perhaps three overlapping bands (-1778, -1753 cm-I and -1760 cm-'(?)). An assignment to monomeric species at edge and inplane sites and dimeric species a t in-plane sites is considered unsatisfactory. No frequency showed an upward shift from the gas-phase monomer frequency as would be expected for monomeric adsorption over an edge-located cation and the apparent similarity in intensity of the overlapping bands would not be expected for edge and in-plane adsorptions. Kozirovski and Folman in previous studies of CsCl have considered the films to be a mixture of bcc crystallites exposing (110) faces and fcc crystallites exposing (100) faces.1*22$2a Spectra of adsorbed HCN, COe, and NzO were then interpreted in terms of adsorption on these two planes. The lack of adsorptions due to edge adsorptions (assuming the above assignment to be correct) was not discussed. Using such a structure for the present surfaces, our interpretation of the data for the NO-NaX and NO-KC1 systems may now be extended to give a satisfactory explanation of the NOCsCl data. It is reasonable that due to the large cation, the annealing in the CsCl films is even more extensive than for the KC1 films. As a result the crystallites are relatively large and prevent detection of edge adsorptions. Absorption bands are observed for
2935
NO dimers adsorbed on the surface planes of the two crystallite types. I n view of the similarity of the frequencies at -1858 and -1753 cm-' to those recorded for the other surfaces these absorptions are assigned to dimeric adsorption on the fcc structure. The bands a t -1877 and -1760 cm-l are attributed to dimeric adsorption on the bcc structure. The spectra indicate that a third absorption overlaps in the low-frequency region (-1753 cm-l) and this is probably due to adsorption above anions located in the surface planes of the two crystallite types. I n this paper we have considered the various possible assignments of the spectral data in relation to our present knowledge of the structures of the alkali halide films. We believe that our final assignments are the most coherent and logical since they satisfactorily explain the changes in spectra which accompany a change in the alkali halide surface. Finally we note that the consistent detection of a dimer on all of these surfaces strongly supports the conclusions reached by reinterpretation of the heats of adsorption for low surface coverages of NO on alkali halide films.
Acknowledgments. We wish to thank the Science Research Council for the award of a studentship to A. J. W. (22) Y. Kozirovski and M. Folman, Trans. Faraday Soc., 62, 1431 (1966). (23) Y. Kozirovski and M . Folman, Isr. J . Chem., 7, 596 (1969)
The Journal of Physical Chemistry, Vol. 76, No. 10, 1071
