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Langmuir 1988,4, 430-432
Interaction of Water with the Surface of SrF2. 2. Physisorption of Water on Water Strongly Adsorbed onto SrF2 Yasushige Kuroda and Tetsuo Morimoto* Department of Chemistry, Faculty of Science, Okayama University, Tsushima, Okayama 700, Japan Received July 27, 1987. I n Final Form: September 25, 1987 When physisorption of HzO occurs on the strongly adsorbed HzO on SrFz, the IR peaks at 3684,2561, 1947,1656,and lo00 cm-l, which originate from the strongly adsorbed H20 being hydrogen-bonded to the surface F- ion in the form of OH-F, are widely varied: the consolidation of the peak at 3684 cm-' into a broad band centered at 3400 cm-', the shift of the peak at 2561 cm-' to 2980 cm-', the disappearance of the peak at 1947 cm-l, the appearance of the peak at 2280 cm-', an increase in the intensity of the peak at 1656 cm-l, and a gradual shift of the peak at 1000 cm-' to 880 cm-'. Most of these phenomena were interpreted in terms of the physisorption of H20 on top of the surface F ion, which results in a weakening of the hydrogen bonding of the preadsorbed HzO. An additional effect comes from the physisorbed H 2 0 itself.
Introduction There exists molecularly and strongly adsorbed HzO on SrFz,which can be desorbed by evacuation at 200 "C. The IR spectra of the strongly adsorbed HzO exhibit characteristic absorption peaks at 2561, 1947, and 1000 cm-', which have not been observed on the surface of metal oxides.'" It has been found that this kind of H 2 0 is located a t the F ion-deficient site on the well-developed (100)surface of SrF,, being hydrogen-bonded in the form of Furthermore, it is known that the two-dimensional condensation of H 2 0 occurs on the strongly adsorbed HzO on SrFz.6 In the present work, a new experimental fact is reported that when HzO is physisorbed on SrF2which has strongly adsorbed H20 the characteristic absorption peaks stated above become extinct and the different peaks appear. Experimental Section The SrF2sample used in this study is the same as that used in the previous paper^,^^^ being prepared through precipitation by mixing two aqueous solutions of Sr(NO& and NH4F. Prior to the measurements of the HzO adsorption isotherms and IR spectra, the sample was evacuated at 500 O C in a vacuum of Torr for 4 h, rehydrated by exposure to saturatedH20vapor for 12 h, and again evacuated at room temperature to remove physbrbed HzO. First, the adsorption isotherm of H20was measured at 25 "C, which involves only physisorption of H20. For the measurement of IR spectra, a self-supporting disk 20 mm in diameter was prepared by compressing 200 mg of sample under a pressure of 400 kg/cm2and evacuating the disk for 2 h. Then the disk was equilibrated with H20vapor at various pressures, and the IR spectrum was measured at every stage of the equilibration by using a spectrometer,Type 810, manufactured by the Nippon Bunko Co. Results and Discussion Figure 1shows the adsorption isotherm of H20 at 25 OC on SrFz which has strongly adsorbed H20. In this iso(1)McDonald, R. S.J. Phys. Chem. 1958, 62, 1168. (2) Lewis, K. E.; Parfitt, G. D. Trans. Faraday SOC.1966, 62, 204. (3)Morimoto, T.; Nagao, M.; Tokuda, F. BULL Chen. SOC.Jpn. 1968, 41, 1533. (4) Deane, A. M.; Grifiths, D. L.; Lewis, I. A.; Winter, J. A.; Tench, A. J. J. Chem. Soc., Faraday Trans. 1 1975, 71,1005. ( 5 ) Kuroda, Y.; Morimoto, T. Langmuir, in press. (6) Kuroda, Y.; Kittaka, S.; Miura, K.; Morimoto, T. Langmuir, in press.
Table I. Relationship between Pressure (P)and Coverage (e) of H20for the IR Measurement P (25 "C)/ P (25 " C ) / spectrum Torr (PIP,)" a 0.0 (0) b 0.2 (0.01) C 0.5 (0.02) d 1.3 (0.05) e 1.9 (0.08)
"Po= 23.756 Torr
0
0 0.16 0.50 0.79 0.86
spectrum Torr (PIP$ f 2.6 (0.11) g 4.9 i0.21j h 9.7 (0.41) 1 18.8 (0.79)
at 25 "C.
therm, a step appears near the relative pressure of 0.025, which manifests the occurrence of two-dimensional condensation of H20. The monolayer capacity of H20 can be estimated by the B-point method to be 0.380 cm3 m-2 = 10.22 H20 molecules nm-2 (STP). Figure 2 illustrates the IR spectra measured at various stages of physisorption of H20,the relationship between the vapor pressure and the coverage 6 at every stage of measurements being listed in Table I. The IR spectrum measured at 8 = 0 gives the absorption peaks at 3684,3214,2561,1947,1656,1000, and ca. 750 cm-' as reported previ~usly.~ Among them, the peaks at 3214 and 750 cm-' were found to become almost extinct after the evacuation at 100 "C and to originate in a weakly adsorbed HzO, while the other peaks a t 3684, 2561,1947, 1656, and 1000 cm-' were found to disappear a t last after outgassing a t 200 O C and to be due to a strongly adsorbed H20 in a fixed state.5 Furthermore, the latter group of peaks was elucidated and found to originate in the HzO molecules, located at the F- ion-deficient sites on the (100) surface of SrF2 and hydrogen bonded in the form of OH-F-. An interesting feature appears when 6 increases. The intensity of the peaks near 3210,3400, and 700-800 cm-l, which are due to weakly adsorbed HzO, increases with increasing 6, while that of the peaks at 2561,1947, and lo00 cm-l, which come from strongly adsorbed H20, decreases at the same time. Especially, the peaks at 2561 and 1947 cm-' entirely vanish at e = 0.86, and instead the peaks at 2950 and 2280 cm-l come out and continue to keep an almost constant intensity at 0 > 1. On the other hand, the intensity of the peak at 1000 cm-' becomes weak, accompanied by a decrease in absorption frequency down to 880 cm-' at 8 = 0.86; further increase in 8 does not alter the peak intensity. The peak a t 3684 cm-' decreases in intensity when 8 increases,and it is consolidated into a broad
0743-7463/88/2404-0430501.50/0 , 0 1988 American Chemical Society I
,
0 0.91 0.99 1.07 1.58
Langmuir, Vol. 4 , No. 2, 1988 431
Interaction of Water with the Surface of SrF, 0.7
d
0.6 f i
N
'E h
a.
g
F
0.1 r\
U
0
20
15
10
5
Pressure /Torr
Figure 1. Adsorption isotherm of HzO on SrFz at 25 "C. Dotted line indicates the monolayer capacity of HzO. Letters represent the pressures at which the IR spectra were measured.
40
30
20 15 Wavenumber 1 O2 cnil
/
10
5
Figure 2. IR spectra of SrFzwith different amounts of physisorbed HzO. Relationship between pressure and coverage of HzO is listed in Table I. Spectrum j was measured when NH3 of 70 Torr was introduced. band centered at 3400 cm-'. Thus, all the absorption peaks concerned with the strongly adsorbed H20,which belongs to the B site on SrFZ,Schange simultaneously in various ways, as B increases, and drastically near B = 1. This can be naturally considered to be caused by a remarkable change in the adsorption mode of the strongly adsorbed H20. In the previous paper,S it has been demonstrated that the electroneutrality of the well-developed (100) surface of SrF2is satisfied by removing half the number of F ions on the surface with fully packed F- ions and that all the
absorption peaks concerned with the strongly adsorbed H 2 0 come from a special adsorption mode of H20, fixed a t the F- ion-deficient site of the (100) surface and hydrogen bonded to the neighboring F ion in the form OHF- (Figure 3-1). It is reasoned that the OH stretching vibration of the preadsorbed H 2 0 forming the OH-Fhydrogen bond should be weakened by the physisorption of the second H 2 0 molecule on the same surface F- ion, as depicted in Figure 3-111, which will result in a decrease in intensity of the peak a t 2561 cm-' as well as a shift of the frequency, e.g., from 2561 to 2950 cm-'.' Such an
Kuroda and Morimoto
432 Langmuir, Vol. 4, No. 2, 1988
P
580 A
:F-
:Sr*+ 0 : H
i,, 'u
Figure 3. Adsorption model of H 2 0 on the (100) plane of SrF, (side view). I indicates strongly adsorbed H20 at the B site, and I1 and I11 are two kinds of physisorbed H20.
adsorption of H 2 0 molecule on the F ion will simultaneously affect the out-of-plane bending vibration of the OH-F bond a t 1000 cm-'? which results in a decrease in the frequency, e.g., from 1000 to 880 cm-'. As 0 increases, the adsorption will also occur on the free OH of the strongly adsorbed H,O, which will bring about a decrease in intensity of the peak a t 3684 cm-' and a t the same time an increase in intensity of a broad band centered a t 3400 cm-', being assigned to hydrogen-bonded OH stretching vibrations. P r e v i o ~ s l ythe , ~ peak a t 1947 cm-' has been considered to be the overtone 2yOHof the librational vibration a t 1000 cm-'. A band a t 2280 cm-', which newly appears a t 6 > 1, cannot be assigned to the shift of the overtone of loo0 cm-', because the latter is a bending-type vibration. Therefore, it is reasonable to infer that the peak at 2280 cm-' is a combination band of the bending vibration v2 (1656 cm-l) and a kind of librational vibration near 600 cm-' of newly physisorbed H20, as a band of similar frequency (2240 cm-l) has been reported on i ~ e . ~ In J~ addition, the intensity of the peak a t 1656 cm-' due to the bending vibration v2 of H 2 0 continues to increase when 0 increases, since the peak originates in the strongly adsorbed H 2 0 as well as the newly physisorbed H20. If we assume that the actual surface of SrF, is composed only of the (100) surface, each density of the F- ion and (7) Takenaka, T., private communication. (8) Novak, A. Struct. Bonding (Berlin) 1974, 18, 177. (9) Bertie, J. E.;Whalley, E.J. Chem. Phys. 1964, 40, 1637. (10) Walrafen, G . E. J. Chem. Phys. 1964,40, 3249.
the strongly adsorbed H20 is equal to 5.95 molecules/nm2, and the total sum is 11.89 molecules/nm2." As stated above, the monolayer capacity of H 2 0 on the present SrF, sample is 10.22 molecules/nm2. Taking into account a possibility that the actual exposed surface of SrF, may contain crystal planes other than the (100) plane which exhibits the highest F ion density, the ratio of the number of physisorbed H 2 0to the total number of the underlying F ions and strongly adsorbed H 2 0will approximate unity at 0 = 1. On the other hand, three peaks a t 2561, 1947, and lo00 cm-l, which are characteristic of the OH-F bond on the strongly adsorbed H20,disappear or shift when 6 approaches unity. Therefore it follows that at the initial stage of adsorption H 2 0 is not physisorbed preferentially on top of the F- ion (Figure 3-111) but on both kinds of sites, the F- ion and the strongly adsorbed H20,through bridging the neighboring sites (Figure 3-11); a t the final stage of the monolayer physisorption the adsorption of one H 2 0molecule on top of every F ion is established (Figure 3-111). In Figure 2, a spectrum is added, which was measured when NH, of 70 Torr was introduced to a cell a t room temperature, instead of the physisorption of H20. The spectrum j in Figure 2 manifests that the adsorption of NH, causes a decrease in intensity of the peaks a t 3684, 2561, and 1947 cm-' in a degree corresponding to a moderate 0 of H20. In other words, the adsorption of NH, has the same effect as that of H,O; that is, it makes the original OH-F- bond weak. Previously,5 it was reported that the strongly adsorbed H 2 0 can be exchanged by methanol, which also forms the same OH-F- bond. In the present work, therefore, the physisorption of CH,OH was examined on the CH,OH-exchanged sample of SrF,. As the result, it was found that the physisorption of CH30H does not affect the intensity or the shift of peaks a t 2575 and 1942 cm-l, unlike the physisorption of H20 or NH, on the H20-preadsorbed sample.
Acknowledgment. This work was partly supported by a Grant-in-Aidfor Scientific Research, No. 62740258, from the Ministry of Education, Science, and Culture of Japanese Government. Registry No. H20, 7732-18-5; SrF2, 7783-48-4. (11) Wells, A. F. Structural Inorganic Chemistry; Clarendon: Oxford,
1975.
