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(14) W. N. Lipscomb and F. E. Wang, Acta Crystallog., 14, 1100 (1961). (15) L. Pauling, "The Nature of the Chemical Bond", Cornell University Press, Ithaca, N.Y., 1960, p 260. (16) D. M. Young and A. D. Crowell, "The Physical Adsorption of Gases", Butterworths, London, 1962. (17) E, is further divided into two terms, E,,, and E,,,, which relate to the dispersion force and the interaction of the induced dipoleelectrostatic field, respectively. cm3 for NO, (18) The values of 01 are 1.72, 1.95, 1.73, and 1.60 X CO, N, and 0, (ref 18a and 18c). The values of Qare 0.28, 0.34, and 0.27 X cm2 for NO, CO, and N, (ref 18b and 18c). The values of p are 0.150, 0.112, and 0 D for NO, CO, and N, (ref Ma). (a) "Kagaku Binran", The Chemical Society of Japan, Maruzen, 1966, p 1226; (b) R. M. Hill and W. V. Smith, Phys. Rev., 82, 451 (1951);
Nagao et al.
(c) W. V. Smith and R. Howard, ibid., 79, 132 (1950). (19) A. L. Smith and H. L. Johnston, J. Am. Chem. SOC.,74, 4696 (1952). (20) T. Takaishi and M. Mohri, J . Chem. SOC., Faraday Trans. 1 , 68, 1921 (1972). (2 1) G. J. Janz, "Thermodynamic Properties of Organic Compounds", Academic Press, New York, N.Y., 1967, p 20. (22) The observed foundamental frequencies are 1865, 1760, 262, 196, and 167 cm-', respectively (A. L. Smith, W. E. Keller, and H. L. Johnston, J. Chem. phys., 19, 189 (1951)). One missing foundamentai frequency is assumed to be 100-1,000 cm-' in the present study. (23) I f the average value of the three vextvlb is assumed to be 10-100 cm-', the soextvlb amounts to 9.4 - 2.6 cal/mol deg for (N0),/2 at 121 K, and 19 - 5 cal/mol deg for the other diatomic molecules around their boiling points.
Differential Heat of Chemisorption. 1. Chemisorption of Water on Zinc Oxide and Titanium Dioxide Mahiko Nagao," Koji Yunoki, Haruto Muraishi, and Tetsuo Morimoto Department of Chemistry, Faculty of Science, Okayama University, Okayama 700, Japan (Received Ju/y 2 1, 1977; Revised Manuscript Received December 19, 1977)
The heat of immersion in water was measured at 28 "C on samples of ZnO and Ti02 (rutile) having different amounts of surface hydroxyls, from which the integral heat of water chemisorption was calculated. The differential heat of water chemisorption, Le., that of formation of hydroxyl groups, was obtained by differentiating the curve of the integral heat of chemisorption vs. the amount of prechemisorbed water. The differential heat on Ti02decreases with increasing amount of chemisorbed water, suggesting an ordinary type of surface heterogeneity. However, ZnO gives an unusual differential heat curve which exhibits a maximum in the vicinity of monolayer coverage. This phenomenon is best understood in terms of the additional contribution of mutual hydrogen bondings between surface hydroxyls formed on the well-developed (1010) plane of ZnO, on the basis of the model postulated previously for the elucidation of an adsorption anomaly in the syst,em ZnO-H20.
Introduction In a series of investigations on water adsorption on ZnO surfaces, we have found an adsorption anomaly, that is, the appearance of discontinuity (or jump) in the physisorption isotherm of water on fully hydroxylated ZnO s ~ r f a c e s . l -This ~ phenomenon was interpreted by introducing a new concept, namely, a peculiar configuration of hydroxyl groups on the well-developed (1010) plane of Zn0.4 Therefore, it may be reasonable to expect that the surface hydroxyl groups formed on this plane should also have a peculiar effect on the heat of chemisorption of water on ZnO. A criterion for whether a solid surface is homogeneous or heterogeneous with respect to energy distribution is given by the variation of differential heat of adsorption with surface coverage. There are essentially three methods prevailing for this purpose: direct calorimetry for gas adsorption, application of the Clausius-Clapeyron equation to adsorption data, and use of immersional wetting calorimetry. These methods have been extensively employed to determine a heat of physisorption in a variety of systems, making the most of their own features. Among the three methods, the heat-of-immersion technique is believed to provide an appropriate means for the heat-of-adsorption measurement on systems involving such a chemisorption process as hydroxylation on a metal oxide. In the present work, the effect of surface crystallinity of ZnO on the differential heat of chemisorption of water was investigated by measuring the heat of immersion of samples having different amounts of surface hydroxyls, and the results were compared with those for Ti02which was considered to have a typically heterogeneous surface. 0022-3654/78/2082-1032$01 .OO/O
Experimental Section Materials. The ZnO sample used in this study was obtained by burning zinc metal in air (Sakai-KagakuKogyo Co.). It is known that the ZnO sample stored in the atmosphere adsorbs simultaneously a considerable amount of water and carbon dioxide to produce an amorphous surface product with a composition similar to basic zinc ~ a r b o n a t e . Surface ~ cleaning of the sample, therefore, was carried out by heating the sample a t 600 "C for 4 h under a vacuum of 1.33 X N m-2,which resulted in the removal of almost all the adsorbed carbon dioxide and water. Next, the sample was exposed to water vapor of 2.7 X lo3 N m-2 at room temperature for 2 h to ensure full rehydroxylation of the surface. By evacuating this sample at various temperatures between 25 and 600 "C (Zn0-600), we obtained samples with different amounts of surface hydroxyls. Furthermore, the original ZnO sample was calcined at 700 or 900 "C (Zn0-700 and Zn0-900) for 4 h, and then degassed at 600 "C for 4 h, followed by the same treatment as that for Zn0-600. Ti02 (rutile) used as a reference sample was the same as that in the previous work,6 and was pretreated in the same manner as Zn0-600. Specific Surface Area. The specific surface area of the sample was determined by applying the BET method to the nitrogen-adsorption data obtained a t liquid-nitrogen temperature, assuming the cross-sectional area of a nitrogen molecule to be 16.2 A2. For Zn0-600, Zn0-700, Zn0-900, and Ti02 samples, pretreated a t 600 "C, the specific surface areas were 3.20, 2.73, 1.73, and 9.45 m2 g-', respectively. Determination of Surface Hydroxyl Groups. The 0 1978 American
Chemical Society
The Journal of Physical Chemistry, Vol. 82, No. 9, 1978
Differential Heat of Chemisorption
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$ 5.0 8 0
E
2 4-0
htiri
Y
E 3.0 2
s8
2 Figure 1. Thermodynamic cycle containing immersion processes of solid surfaces: S,sample solid; L, liquid water; V, water vapor.
2.0
2
*
0
1.0
c
amount of surface hydroxyls remaining on the sample was determined by the successive-ignition-loss method as described previously.' The sample which had experienced the 600 "C treatment and succeeding exposure to water vapor was first evacuated at 25 "C for 2 h under a pressure of 1.33 X N m-2. Next, the vacuum system was closed, the sample was heated to 100 "C, and kept for 2 h at this temperature, during which the vapor liberated was condensed into a trap kept a t liquid-nitrogen temperature. The' condensed gas was reevaporated in a measuring system at room temperature and determined volumetrically. The same operation was repeated by raising the temperature stepwise from 100 to 1000 "C. No gases other than water vapor were detected between 25 and 600 "C, but a trace of carbon dioxide was evolved above 600 "C, which was separated and determined by means of the two-step trap m e t h ~ d . For ~ ? ~the calculation of the amount of surface hydroxyls, the content on the 1000 "C treated sample was assumed to be nil. Measurement of Heat of Immersion. The measurement of heat of immersion was carried out at 28 f 0.1 "C by using an adiabatic calorimeter. A quartz thermometer was used as a temperature-sensing element and its sensitivity "C. A glass ampoule containing a sample of was 1 X ca. 5 m2 was sealed off in vacuo after the final treatment. The heat of breaking of the ampoule was found to be 0.504 f 0.035 J and it was used to correct the measured values of the immersion heats. The data obtained were expressed as the average value of at least six determinations for each point, and the reproducibility was always found to be better than 3 % . Slow heat evolution was not observed during heat measurements.
Results and Discussion On immersing metal oxide samples covered partially with surface hydroxyls into water, both chemisorption and physisorption of water take place, and hence the heat evolved is associated with the formation of surface hydroxyls as well as with physical immersional wetting. The heat due to the former process should vary with the prechemisorbed amount of water which can be regulated by the pretreatment temperature. A thermodynamic cycle containing the immersional processes of such surfaces in water is represented in Figure 1. Here, hI and hI(r, are the heats of immersion per unit area of the bare surface and the unit surface covered with chemisorbed water of r molecules, Lo is the heat of liquefaction of water, and Qa(r) is the integral heat of chemisorption of r molecules of water. The relationship among these heats is given by the following equation: = h~ - h
r ) + LO
(1)
The integral heat of chemisorption Qa(r)can be calculated by introducing the experimental results for immersional heats into this equation. A precaution should be taken against the preparation
5
c
5
0
100
300 400 Treatment temperature , "C
200
500
600
Figure 2. Amount of surface hydroxyls on ZnO and TiOP treated at Zn0-900; 0 ,Ti02 various temgeratures: O,Zn0-600; @, Zn0-700; 0, (rutile).
1000-
E
v
2 800'0
.s600-
f
'-
400-
r c
? 2000
100
300 400 Treatment temperature ,O C
200
500
600
Ftgure 3. Heat of immersion in water for ZnO and TiO, treated at various temperatures. Symbols are the same as those in Figure 2.
of surfaces with controlled amount of prechemisorbed species. As has been pointed out by Zettlemoyer and Narayan,* the first increment of vapor introduced to the sample would not tend to distribute throughout the sample tube, but all the vapor would be adsorbed by the sample near the entrance port and distribution throughout on the most active sites would not occur. If such circumstances progress through the bed of adsorbent for every increment of adsorbate, the heat-of-adsorption data on this sample will give an apparent homogeneity of the surface. Therefore, such a progressive adsorption technique for preadsorption will probably lead to an erroneous evaluation about the surface homogeneity of the solid. On the contrary, the desorption method for regulating the amount of chemisorbed species is believed to be adequate for leaving molecules chemisorbed on stronger sites by treatment at higher temperatures. This is the reason why the desorption method was employed in the present experiment. A similar method has been applied to such systems as n-butylamine-attap~lgite,~ -silica-alumina and -kaolin.lo Figure 2 shows the amount of surface hydroxyls on the samples treated at various temperatures, expressed as the number of water molecules per 100 A2 of surface. The results indicate that the desorbability of surface hydroxyls depends greatly upon the nature of the metal oxide, as has been observed previous1y.l The amount of surface hydroxyls on Ti02decreases monotonously with increase of treatment temperature, while ZnO gives a characteristic curve in the sense that the amount decreases slowly up to 200 "C and then sharply between 200 and 400 "C. The heat of immersion of ZnO and T i 0 2 in water is plotted against the pretreatment temperature as illustrated
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The Journal of Physical Chemistry, Vol. 82, No. 9, 1978
Nagao et al.
L 2
40t
n~
0
Amount of chemlsorbed water
ir), mo1ecules/iooA2
Figure 4. Integral heat of chemisorption of water on ZnO and TiO,. Symbols are the same as those in Figure 2.
in Figure 3. An excellent correspondence between the decrease in surface hydroxyl amount (Figure 2) and the increase in the heat-of-immersion value (Figure 3) can be observed as the evacuation temperature is raised. In other words, the heat of immersion of TiOz increases monotonously with an increase of evacuation temperature, while all three samples of ZnO provide sigmoid curves showing a sharp increase in immersional heats between 200 and 400 "C. For the calculation of the integral heat of chemisorption according to eq 1, an evaluation of the heat of immersion of the bare surface is necessary. It has been known that the surface hydroxyls on metal oxides can be removed through a condensation dehydration by heat treatment in vacuo to leave a "true" oxide surface, which is reversibly converted to a hydroxylated surface on immersing into water, and that the treatment of a sample at elevated temperatures gives rise to a stabilization of the oxide structure, which makes it difficult to recover the hydroxylated surface in the immersion process into water.11-16 The present results indicate that the heat of immersion of the 600 "C treated sample is almost equal to that of the sample treated again at 600 "C after rehydroxylation of the former sample. This close agreement suggests that the rehydroxylation of the 600 "C treated sample is almost perfect on immersion into water, and the number of hydroxylating sites on the surface remains unchanged by the second treatment at 600 "C. Thus, we may regard the surface of the 600 "C treated sample as a bare surface which is able to fully chemisorb water molecules during the immersion process, except for a small part covered tenaciously with remaining surface hydroxyls. The monolayer capacity of chemisorbed water was obtained by subtracting the amount of hydroxyls on the 600 "C treated sample from that on the 25 "C treated sample. can be calcuThe integral heat of chemisorption lated on the basis of these assumptions and by introducing the value of Lo (43.9 k J mol-l), and is plotted against the amount of chemisorbed water l? in Figure 4. The shape of Qa(r,curves for ZnO is somewhat different from that for TiO,; the slope of the curves on ZnO increases with moderate coverage of chemisorbed water, but on TiOz it shows only a gradual decrease. These conclusions are demonstrated more clearly by graphical differentiation of these curves. The result gives the differential heat of chemisorption of water Q d or that of formation of surface hydroxyls, as shown in Figure 5. The differential heat of chemisorption of water on TiOz decreases with increasing amount of chemisorbed water, indicating that the surface of TiOz has a usual type of heterogeneity. Hollabaugh and Chessickl' have also obtained the differential heat of adsorption of water on TiOz
, 1.0
1
2.0
3.0 4.0 Amount of chemisorbed water lr), rnolecules/iooA'
5.0
Figure 5. Differential heat of chemisorption of water on ZnO and TiO,: Zn0-600; - - -, Zn0-700; -, Zn0-900; --, TiO, (rutile).
-,
---
--
(13324A
Figure 6. Hydroxylated (1070) plane of ZnO. Shaded circles represent underlying atoms and dotted lines indicate hydrogen bondings.
(rutile) by differentiating the heat of immersion curve against preadsorbed amount of water, and they found the heat of adsorption to be nearly constant (ca. 120 kJ m o l 3 over the whole chemisorption range. A similar surface homogeneity for the chemisorption of water has been observed by Holmes et al. on the system Th0z-Hz0.18J9 However, in these cases, the authors employed the progressive adsorption technique instead of the desorption method for the preadsorption of water, which accordingly might have the possibility of leading to an erroneous distribution curve representing surface homogeneity as described above. In addition, they did not take into account the heat of liquefaction of water in the calculation of differential heat of adsorption, which should result in a constant depression of adsorption heat, though the shape of differential heat curve would remain unchanged. The differential heat of chemisorption for the ZnO sample gives a peculiar curve that has never been observed in systems of metal oxide-water; it increases with increasing surface coverage and reaches a maximum in the vicinity of monolayer coverage. The shape of these curves apparently resembles that of adsorption-heat curve measured calorimetrically by Beebe and Youngz0for argon adsorption on graphitized Spheron with a homogeneous surface. it has been found that the same In our previous ZnO sample as used here had an excellent surface crystallinity, as well as well-developed (1070) plane of wurtzite structure, and gives an adsorption anomaly for water physisorption. This phenomenon, representing a discontinuity in the adsorption isotherm of water, has been elucidated from the crystallographic point of view, and interpreted as follows. When a water molecule is chemisorbed on the (1010) plane of ZnO, two hydroxyl groups are formed to complete the tetrahedral coordination with respect to both surface zinc and oxygen atoms (Figure 6). Furthermore, each hydrogen atom newly bonded to oxygen
Differential Heat of Chemisorption
in the original coplanar Zn-0 surface is in a suitable position to undergo hydrogen bonding with the oxygen in the hydroxyl newly bonded group. At the same time, hydrogen in the newly formed hydroxyl groups will be favorably oriented to the oxygen in the neighboring hydroxyl groups on the same row. Eventually, all the hydrogens on the surface hydroxyl groups take part in the formation of hydrogen bonding or of the closed hydrogen-bonding structure. As a result, adsorption force emanating from such surface structure for approaching water molecules is so weakened that the amount of physisorbed water is depressed at a pressure lower than that at which two-dimensional condensation of water starts. Taking into consideration such a peculiar picture on the configuration of surface hydroxyls on ZnO, the variation of the differential heat of chemisorption in Figure 5 seems to be best understood. An additional energy due to the formation of mutual hydrogen bondings between surface hydroxyls, as well as the energy of formation of hydroxyl groups, will contribute cooperatively to the heat of chemisorption of water on ZnO. According to the model postulated for the formation of a closed hydrogen-bonding structure as mentioned above, the contribution of hydrogen bonding should be small a t low coverages because of the small density of surface hydroxyls, but it becomes increasingly prominent with increasing surface coverage and reaches a maximum near the completion of a monolayer. The heat of chemisorption at low coverage, therefore, is associated only with the heat of chemisorption of water, i.e., the heat of formation of hydroxyl groups. This can be read from Figure 5 to be 100-120 kJ mol-l, which may be a reasonable value for the heat of a chemical reaction. On the basis of this level, the additional heat due to mutual hydrogen bonding can be estimated from the curves, being 70-80 kJ mol-l at the top of the curves. A comparison of these data with the well-known hydrogen-bonding energy value for the OH- - -0 bond, 21-25 kJ mo1-1,21,22leads to the striking result that the former is three times greater than the latter. This manifests that the hydrogen-bonding effect on the heat of chemisorption of water increases from nil to three bonds per water molecule as the amount of chemisorbed water increases, in agreement with the model described above. Crystallographically, the present model gives 5.93 HzO molecules/100 w2 as the amount of chemisorbed water on the (1070) plane. On the other hand, the total amounts of chemisorbed water on Zn0-600,Zn0-700, and Zn0-900 samples are 4.89,4.54, and 4.20, respectively, as shown in Figure 2. These experimental values are smaller than the calculated one, probably because of the coexistence of other planes such as (OOOl), (OOOT), ( l O T l ) , and (1051). In addition, it is well known that the treatment of metal oxides at higher temperatures reduces the number of sites which can be rehydroxylated.l1-l6 In view of these facts, the observed monolayer capacities denoted by arrows in Figure 5 seem to be reasonable values. As stated above, the particles of the present sample are
The Journal of Physical Chemistry, Vol. 82, No. 9, 1978
1035
well-developed prisms of the wurtzite type, on which besides the (lOT0) plane there should be (0001) and (0001) planes and others. The latter planes should have more or less contributions to the heat-of-chemisorption data, though their extent is uncertain. If these planes are present only in small portion on the real surfaces, the importance of the above discussion will become greater. Finally, it is to be noted that the monolayer capacity of chemisorbed water decreases with the increase of calcination temperature of the sample. In the previous paper reporting the adsorption anomaly of water on Zn0,394we have described that the evacuation or calcination of the ZnO sample at elevated temperatures gives rise to a decrease in the height of the discontinuity in the adsorption isotherms. Since this fact is directly indicative of the decrease in the number of sites themselves for water physisorption, especially of the sites which contribute to the occurrence of the discontinuity, this further states the decrease in the extent of the presence of well-developed (1010) planes on ZnO surfaces, probably because of the partial stabilization of the surface oxide structure. If the heat values in Figure 5 are replotted against the surface coverage as an abscissa, the resulting curves give the peaks at the same coverage of 0.82. The shift of peaks of the curves, which is caused by the progressive treatment of the samples, may also be due to the same reason as that for the decrease in the height of the discontinuity in the adsorption isotherm.
References and Notes T. Morimoto, M. Nagao, and F. Tokuda, Bull. Chem. SOC.Jpn., 41, 1533 (1968). T. Morimoto and M. Nagao, Bull. Chem. SOC.Jpn., 43, 3746 (1970). M. Nagao, J . Phys. Chem., 75, 3822 (1971). T. Morimoto and M. Nagao, J. Phys. Chem., 78, 1116 (1974). M. Nagao, K. Morishige, T. Takeshita, and T. Morimoto, Bull. Chem. SOC. Jpn., 47, 2107 (1974). T. Morimoto, M. Nagao, and F. Tokuda, J. Phys. Chem., 73, 243 (1969). T. Morimoto, K. Shiomi, and H. Tanaka, Bull. Chem. SOC.Jpn., 37, 392 (1964). A. C. Zettlemoyer and K. S. Narayan, "The Solid-Gas Interface", Vol. 1, E. A. Flood, Ed., Marcel Dekker, New York, N.Y., 1967, Chapter 6. J. J. Chessick and A. C. Zettlemoyer, J. Phys. Chem., 62, 1217 (1958). A. C. Zettlemoyer and J. J. Chessick, J . Phys. Chem., 64, 1131 (1960). G. J. Young and T. P. Bursh, J . Colloid Sci., 15, 361 (1958). J. W. Whaien, Adv. Chem. Ser., No. 33, 281 (1961). W. H. Wade and N. Hackerman, J. Phys. Chem., 65, 1681 (1961). T. Morimoto, M. Nagao, and T. Omori, Bull. Chem. SOC. Jpn., 42, 943 (1969). T. Morimoto, N. Katayama, H. Naono, and M. Nagao, Bull. Chem. SOC. Jpn., 42, 1490 (1969). T. Omori, J. Imai, M. Nagao, and T. Morimoto, Bull. Chem. SOC. Jpn., 42, 2198 (1969). C. M. Hollabaugh and J. J. Chessick, J. Phys. Chem., 65, 109 (1961). H. F. Holmes, E. L. Fuller, Jr., and C. H. Secoy, J . Phys. Chem., 70, 436 (1966). H. F. Holmes, E. L. Fuller, Jr., and C. H. Secoy, J . Phys. Chem., 72, 2095 (1968). R. A. Beebe and D. M. Young, J. Phys. Chem., 58, 93 (1954). L. Pauling, "The Nature of the Chemical Bond", Cornell University Press, Ithaca, N.Y., 1960, p 468. K. B. Harvey and G. B. Porter, "Introduction to Inorganic Physical Chemistry", Addison-Wesley, London, 1963, p 257.
