2054
J. Phys. Chem. 1980, 84, 2054-2058
Adsorption of Alcohols on Zinc Oxide Surfaces Mahlko Nagao" Research Laboratory for Surface Science, Faculty of Science, Okayama University, Tsushima, Okayama 700, Japan
and Tetsuo Morlmoto Department of Chemistry, Faculty of Science, Okayama University, Tsushima, Okayama 700, Japan (Received: November 26, 1979)
Adsorption isotherms of normal aliphatic alcohols (MeOH, EtOH, and 1-PrOH) were measured at 25 "C for ZnO samples having differentnumbers of surface hydroxyls, from which the amounts of irreversibly and reversibly adsorbed alcohols were calculated. An excellent linear relationshipwas found between the amount of adsorbed alcohol and the surface hydroxyl content of the sample, independent of the carbon number of the alcohols; with decreasing surface hydroxyl content of the sample, the irreversible adsorption of alcohol increases, while the reversible one decreases. Infrared absorption spectra of adsorbed species suggested that the alcohols used here can be chemisorbed dissociatively on the dehydroxylated ZnO surface to form hydroxyl groups as well as alkoxyl groups. A small amount of irreversible adsorption was also observed on the fully hydroxylated surface of ZnO, being greater for alcohols with longer carbon chains, which was interpreted in terms of the strong hydrogen-bonding interaction between alcohol molecules and surfacehydroxyls. It was also revealed that when the ZnO sample with prechemisorbed alcohol is contact with water vapor, the substitution reaction occurs to form surface hydroxyls instead of surface alkoxyl groups, as in the cases of chemisorbed COz and NH3 reported previously.
Introduction The adsorption of organic molecules on metal oxide surfaces is a basic phenomenon in the catalytic reaction occurring on the surface, and hence, much research has been done in this area. A study on the adsorption of alcohols on metal oxide surfaces is of special interest in connection with catalytic decomposition of alcohol such as dehydration and dehydrogenation1+ and with surface modification of solids for practical purpose^.^-^ Since the surface of metal oxide is usually covered with chemisorbed water, i.e., surface hydroxyls, its adsorptive behavior and catalytic action should be affected by the presence of surface hydroxyls.6J0J1The knowledge of the numbers of surface hydroxyls, therefore, is necessary for the critical discussion of the interaction between metal oxide surfaces and adsorbate molecules. Taking into consideration the presence of surface hydroxyls, Borello e t a1.12 have investigated the methanol adsorption on Aerosil by means of infrared spectroscopy and have found that the number of chemisorbed methoxyls can be correlated with the number of surface hydroxyls. The present study was devoted to a detailed investigation of adsorption of low-molecular-weight, normal aliphatic alcohols, i.e., methanol (MeOH), ethanol (EtOH), and 1-propanol (1-PrOH), on zinc oxide samples which have various numbers of surface hydroxyls. The quantitative relationship between the amount of adsorbed alcohol and the surface hydroxyl content of the sample was studied by measuring the adsorption isotherms, surface hydroxyl contents, and infrared spectra. The possibility of a substitution reaction of water for chemisorbed alcohol was also investigated, similar to the cases of chemisorbed C 0 2 and NH, on zinc oxide surfaces reported p r e v i ~ u s l y . l ~ - ~ ~ Experimental Section Materials and Pretreatment. The original ZnO sample used in this study was Kadox 15 produced by New Jersey Zinc Co., which had been prepared by burning zinc metal in air. Since the ZnO sample stored in the atmosphere is known to have surface compounds like basic zinc carbonate,17Je the sample was first degassed a t 600 "C for 4 h 0022-3654/80/2084-2054$01.0010
in vacuo at 1 X N m-2 in order to remove such surface contaminations. This sample was then exposed to saturated water vapor at room temperature for 15 h to ensure a complete surface hydroxylation. Next, the sample was evacuated for 4 h in a vacuum of 1 X N m-2a t various temperatures between 25 and 600 "C to leave different numbers of hydroxyls on the surface. Spectroscopic grade MeOH and EtOH and guaranteed grade 1-PrOH were passed through a trapping tube containing a molecular sieve 3A previously degassed a t 350 "C. These liquids were allowed to evaporate into gas reservoirs and subjected to several freeze-pump-thaw cycles before use. Determinations of Surface Hydroxyl Content and Surface Area. The surface hydroxyl content, which is the number of hydroxyl groups remaining on the surface after evacuating the fully hydroxylated sample a t a given temperature, was determined by the successive-ignition-loss methodlg and is plotted against the evacuation temperature in Figure 1. It is seen from Figure 1that the completely hydroxylated surface of ZnO evacuated at 25 "C has 5.5 water molecules or 11.10 hydroxyl groups per nm2 and that on the 600 "C evacuated sample 0.53 water molecules or 1.06 hydroxyl groups still remain on the unit area. Besides the 25 "C and 600 "C treated samples being regarded as the fully hydroxylated and the mostly dehydroxylated surfaces, respectively, the other three samples having intermediate hydroxyl contents were prepared by evacuating the fully hydroxylated samples a t each temperature of 200, 300, and 350 O C . The specific surface area of the sample degassed at 600 "C, which was determined by the BET method based on the N2 adsorption, was found to be 9.11 m2 8-l. Measurements of Adsorption Isotherms of Alcohol and Water. The adsorption isotherm of alcohol vapor was measured a t 25 f 0.01 "C by using a conventional volumetric adsorption apparatus equipped with greaseless stopcocks. An equilibrium pressure was read to a precision of 0.01 mmHg by using a cathetometer. Adsorption equilibrium was attained in a short time (within 20 min). The adsorption isotherm of water vapor was also deter@ 1980 American Chemical Society
Adsorption of Alcohols
on Zinc Oxide Surfaces
The Journal of Physical Chemistty, Vol. 84, No. 16, 1980 2055
1 1.0
Relative pressure IP/P.)
Evacuation tempera t urc! ,"C
Figure 1. Surface hydroxyl content of ZnO sample evacuated at various temperatures.
Figure 3. Adsorption isotherms of EtOH on the ZnO sample evacuated at various temperatures after complete hydroxylation. Symbols are the same as those in Figure 2.
0.3 -
E
A (
-
L
-
v)
*)
6 0.2-
0~ - 7 + - - - - - % - + 0 06 Relative pressure (P/W
Figure 2. Adsorption isotherms of MeOH on the ZnO sample evacuated at various temperatures after complete hydroxylation: (A) 25; (V) 200; (0) 300;(0)350;(0) 600 OC. Open and filled marks represent the first and second adsorptions, respectively.
mined for the sample with prechemisorbed alcohol. Measurement of Infrared Spectra. Infrared spectra of adsorbed species were measured for both surfaces of mostly dehydroxylated and of fully hydroxylated samples by means of a Nippon Bunko IR-G infrared spectrophotometer. A self-supportimg sample disk of 2 cm in diameter (ca. 200 mg) was placed in an in-situ cell with fluorite windows. After the sample disk was degassed a t 600 "C, O2 treatment was carried out in a similar manner as described previously in order to recover the original transmittance.la Otherwise high-temperature treatment of the ZnO disk might change its color to gray, resulting in a decrease in IR transmittance.20
Results and Discussion Adsorption Isotherms of Alcohols. Figures 2-4 illustrate the first and second adsorption isotherms of MeOH, EtOH, and 1-PrOH,respectively, on the ZnO samples with various hydroxyl contents. 'The first adsorption refers to the surface pretreated at a given temperature, which involves both irreversible and reversible adsorptions of alcohol. After the measuremelnt of the first adsorption isotherm, the sample was furtheir exposed to saturated alcohol vapor at 25 "C for 15 h, followed by evacuation at 25 O C for 4 h; and then the second adsorption isotherm was determined at the same temiperature as before, which permitted only reversible physisorption. All of the isotherms obtained have highly developed knees in the range of lower relative pressures (PIP, 5 0.05)
Relatlve pressure (P/P.)
Figure 4. Adsorption isotherms of I-PrOH on the ZnO sample evacuated at various temperatures after complete hydroxylation. Symbols are the same as those in Flgure 2.
and almost flat regions in the intermediate pressure range, followed by a succeeding increase at the highest pressure, showing an onset of multilayer formation. This multilayer character decreases with increasing adsorbate chain length, as would be expected from the gas-phase autophobicity developed by Barto et al.?l in which multilayer adsorption of alcohol is restricted and the bulk liquid forms a nonzero contact angle. It is noted in Figures 2-4 that at a given relative pressure the adsorbed amount for the samples treated at the same temperature, and hence covered with the same number of hydroxyls, decreases in the order MeOH > EtOH > 1PrOH in both cases of the first and second adsorptions, Moreover, for a given adsorbate, with increasing hydroxyl content of the sample, the adsorbed amount of the first adsorption decreases, while that of the second one increases. The shapes of all of the isotherms belong to the Langmuir type rather than to the BET type except for the final parts of the isotherms. The extent of the linearity of the Langmuir plots increases in going from MeOH to 1-PrOH. A similar tendency has been reported by Barto et a1.21for the adsorption of normal aliphatic alcohols on A1203.
A parallelism between the first and second adsorption isotherms is very excellent for every adsorbate and for all of the samples containing different numbers of surface hydroxyls. The difference in the adsorbed amounts between the first and second adsorptions appears in a gap
The Journal of Physical Chemistry, Vol. 84, No. 16, 1980
2056
Nagao and Morimoto
L
' i
E
:
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Figure 5. Infrared spectra of adsorbed species on the dehydroxylated ZnO surface: (a) background spectrum of ZnO dehydroxylated at 600 O C ; (b) adsorption of MeOH; (c) adsorption of EtOH; (d) adsorption of
3600
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2
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3000I
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Figure 6. Infrared spectra of adsorbed species on the hydroxylated ZnO surface: (a)background spectrum of hydroxylated ZnO; (b) adsorption of MeOH; (c) adsorption of EtOH; (d) adsorption of 1-PrOH.
1-PrOH.
at zero pressure. This fact gives strong evidence that the first adsorption includes both irreversible and reversible adsorptions of alcohols and that the second adsorption MeOH does only reversible adsorption. It is also found that the difference between the amounts of the first and second MeOH adsorptions, that is, the amount of chemisorbed alcohol, is greater for the sample having lower hydroxyl content. EtOH Infrared Spectroscopic Observation. Infrared spectra I-PrOH of adsorbed species on the mostly dehydroxylated surface EtOH of ZnO are shown in Figure 5. In the case of MeOH adsorption, the most significant bands due to the adsorbed I-PrOH species are observed at 2930 and 2813 cm-l. In addition, the absorption bands characteristic of surface hydroxyls on ZnO appear over the wavenumber region of 3670-3440 cm-', as reported previously.16 It is known that a free 0 2 4 6 8 10 12 Hydroxyl content , OH groups/nm* methyl group in an alcohol molecule gives two distinct bands at 2962 and 2872 cm-l, which are due to asymmetric Figure 7. Relationship between the monolayer capacity of adsorbed and symmetric CH stretching vibrations, r e ~ p e c t i v e l y . ~ ~ t ~alcohol ~ and the surface hydroxyl content of the ZnO sample. Open The absorption bands observed in the present study should and filled marks represent V,, and Vm2,respectively. be obviously assigned to the stretching mode of the CH3 droxylated ZnO surface, which suggests the interaction of group, though they shift largely to lower wavenumber surface hydroxyls with alcohol molecules through hydrogen compared with the values cited above or reported in the bondings. literature for MeOH adsorbed on a silica surface (about The present spectroscopic observation did not provide 2960 and 2850 cm-1). 12,24,25 any evidence of the formation of formate or carboxylate Four distinct bands at 2955,2928, 2875, and 2844 cm-l, ions by catalytic decomposition of chemisorbed alcohols. as well as characteristic bands due to surface hydroxyls, It might be due to the fact that the present sample having are observed in the spectrum for EtOH adsorbed on the chemisorbed alcohol has never been treated at a higher dehydroxylated ZnO surface. In addition to the vibration temperature than room temperature, in contrast with the of the methyl group, the vibration due to the methylene condition reported in the l i t e r a t ~ r e . ~ ~ ~ ~ - ~ ~ group should appear as the carbon chain of alcohol beRelation between the Surface Hydroxyl Content and comes longer. Absorption bands at 2955 and 2875 cm-l the Amount of Adsorbed Alcohol. Figure 7 shows the can be assigned to asymmetric and symmetric CH relationship between the surface hydroxyl content of the stretching vibrations in the methyl group, respectively, and ZnO sample and the adsorbed amount of alcohol. Vml and the 2928 and 2844 cm-' bands are due to asymmetric and V,, are monolayer capacities of adsorbed alcohol obtained symmetric vibrations of the methylene group, respectiveby applying the Langmuir equation to the first and second ly.22J3 In the case of 1-PrOH adsorption, the spectrum adsorption isotherms in Figures 2-4, respectively. I t is pattern is almost the same as that for EtOH, as can be seen obvious from Figure 7 that an excellent linearity exists in Figure 5. On the basis of these facts, it can be concluded between the monolayer capacity of adsorbed alcohol and that alcohols used in the present study can be chemisorbed the surface hydroxyl content of the sample; with increasing dissociatively to form surface alkoxy1 and hydroxyl groups hydroxyl content, the V,, value increases, while the V,, on the dehydroxylated ZnO surface. value decreases. For the samples having the same hydroxyl Figure 6 represents the infrared spectra of strongly adcontents, both amounts of first and second adsorptions sorbed alcohols on the fully hydroxylated ZnO surface. decrease in the order MeOH > EtOH > 1-PrOH. This Absorption bands due to methyl and methylene groups are tendency agrees qualitatively with that obtained by Barto almost the same as those on the dehydroxylated surface et a1.21 for the adsorption of normal aliphatic alcohols (Figure 5). However, absorption bands in the OH (Ci-C4) on A1203,though they have not measured the stretching region become broader and shift to lower hydroxyl content of the sample. The fact that Vm, inwavenumber when alcohols are adsorbed on the hy-
The Journal of Physical Chemistry, Vol. 84, No. 16, 1980 2057
Adsorption of Alcohols on Zinc Oxide Surfaces
--
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Flgure 9. Adsorption isotherms of water on the ZnO sample with prechemisorbed MeOH at 25 "C. Broken line indicates physisorption isotherm of water on the fully hydroxylated ZnO surface.
can be seen from Figure 8: MeOH, 0.67; EtOH, 1.02; 1creases with increasing hydroxyl content of the sample PrOH, 1.34 molecules nm-2. On the basis of the above suggests that physisorption of alcohol occurs on the surface consideration and the spectroscopic observation, it seems hydroxyls, coinciding with the spectroscopic results dereasonable to assume that when the surface is completely scribed above. covered with hydroxyls, alcohol molecules can be physiThe amount of chemisorbed alcohol (V,) can be obtained sorbed on the surface hydroxyls by forming strong hyby subtracting Vm2from Vml, as stated above, which is drogen bondings. plotted against the hydroxyl content of the sample in An alkyl group in organic molecules has an inductive Figure 8. As can be seen from Figure 8, the amount of effect, which is likely to thrust its electrons to the opposite chemisorbed alcohol decreases linearly with an increase side of the molecules.35 This effect is greater in molecules in the surface hydroxyl content of the sample. Furtherwith a larger number of carbon atoms: MeOH C EtOH more, it is noteworthy that this relationship gives almost C 1-PrOH. The greater the 6- character of oxygen atoms the same straight line for every alcohol with different in alcohol molecules due to the inductive effect, the more carbon-chainlength, which indicates that alcohol molecules strongly they attract electron-accepting atoms (in the are adsorbed with their chains perpendicular to the surface. present system hydrogen atoms in surface hydroxyls), Here, if we assume that normal aliphatic alcohols with resulting in the formation of strong hydrogen bondings. different carbon-chainlengths occupy the same area of 0.25 Substitutional Reaction of Water for Chemisorbed nm2 on the oxide four alcohol molecules should Alcohol. Figure 9 represents the adsorption isotherms of be present on the surface of 1 nm2. The observed values water vapor at 25 "C on a ZnO surface covered with preof V, on the hydroxylated ZnO surface are 3.96,3.90, and chemisorbed MeOH. The lowest curve shows the first 3.58 molecules nm-2 for MeOH, EtOH, and 1-PrOH, rewater adsorption isotherm on the sample which had been spectively, being comparable to the calculated value of 4. fully covered with chemisorbed alcohol. After this meaThis agreement implies that the dehydroxylated ZnO surement, the system was evacuated at 25 "C for 4 h, and surface can be covered almost completely by chemisorbed then the second measurement of the adsorption isotherm alcohols. The inverse linearity of the amount of chemisorbed of water vapor was carried out at 25 "C. These procedures were repeated several times on the same sample. It is alcohol against the surface hydroxyl content of the sample apparent from Figure 9 that the discontinuity of the isomanifests that the adsorption sites available for alcohol chemisorption are the same as those for the formation of therm, which is a characteristic phenomenon of water surface hydroxyls, in contrast to the case of physisorption. physisorption on the completely hydroxylated ZnO surThis gives strong support for the suggestion from specface,36t37becomes progressively greater as the water adtroscopic observation that alcohol can be chemisorbed sorption is carried out repeatedly. In this case, three-times dissociatively on the dehydroxylated sites. repetition of water adsorption was sufficient to give an Two different interpretations have been offered on the isotherm similar to that for the hydroxylated ZnO surface. adsorption of MeOH on a silica surface: (1) surface meThus, we can conclude that MeOH prechemisorbed on the thylation takes place through the esterification reaction ZnO surface had been replaced by hydroxyl groups. A between surface hydroxyls and methanol molecule^,^^^^^ similar substitution reaction has been observed when the and (2) MeOH is chemisorbed dissociatively on the siloxZnO sample having prechemisorbed C 0 2 or NH3 is exposed anes to form methoxyl and hydroxyl g r o ~ p s . ~ In~the p ~ ~ ~to~water ~ vapor at room temperature;13-16in these cases C 0 2 former cases, the number of methoxyls formed should be or NH3 molecules are released into the gaseous phase, resulting in the formation of surface hydroxyls. equal to the number of hydroxyls consumed, and the reaction should be favorable for the sample with higher The present substitution reaction was also confirmed hydroxyl content, while the latter reaction should be by infrared spectroscopic observation, as can be seen in preferable for the sample with lower hydroxyl content. Figure 10. The characteristic bands of surface hydroxyls The present results evidently substantiate the dissociative on ZnO appear at 3672,3644,3618,3550, and 3442 cm-l, chemisorption of alcohols on the dehydroxylated ZnO accompanied by the disappearance of the bands at 2930 surface. and 2813 cm-l assigned to methyl groups as water adOn the other hand, the fully hydroxylated ZnO surface sorption proceeds. Similar results were obtained for the has the hydroxyl content of 11.10 OH groups nm-2 (Figure sample with prechemisorbed EtOH or 1-PrOH; a more l ) , but even on this surface V , gives a nonzero value, as drastic condition, e.g., frequent repetition of water ad-
2058
J. Phys. Chem. 1980, 84, 2058-2061
I
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1 1 1 3800 3600
4
8
3400
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I
I
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3200 3000 2900 Wovenumber, cm-'
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2600
A J
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Flgure 10. Infrared spectra of adsorbed species when water is adsorbed on the ZnO surface with prechemisorbed MeOH: (a)background spectrum of dehydroxylated ZnO; (b) chemisorption of MeOH; (c)water adsorption on the ZnO surface with prechemisorbed MeOH.
sorption, was needed to complete the substitution reaction as the carbon chain in alcohol molecules became longer.
References and Notes (1) Blyholder, G.; Richardson, E. A.
J . Phys. Chem. 1962, 66, 2597; 1964, 68, 3882. (2) Hunter, G. L.; Brogden, Jr. J . Org. Chem. 1963, 28, 1679. (3) Trifiro, F.; Pasquon, I. J . Catal. 1966, 72,412; 1971, 22,324. (4) Ueno, A.; Onishi, T.; Tamaru, K. Bull. Chem. SOC.Jpn. 1970, 43, 2652. (5) Nondek, L.; Kraus, M. J . Cafal. 1975, 40, 45. (6) Seiyama, T. "Kinzoku-Sankabutsuto Sono Shokubal-Sayo"; Kodansha: Tokyo, 1978; Chapter 4. (7) Boehm, H. P. Adv. Cafal. 1966, 16, 193. (8) Chahal, R. S.; Pierre, L. E. St. Macromolecules 1968, 1 , 152.
(9) Hodgkin, J. H.; Solomon,D. H. J. Macromol. Sci., Chem. 1974, 8 , 621. (10) Little, L. H. "InfraredSpectra of Adsorbed Species";Academic Press: London, 1966. (1 1) Kiselev, A. V.; Lygin, V. I. "Infrared Spectra of Surface Compounds"; Keter Publlshina House: Jerusalem. 1975. (12) Borello, E.; Zeichina, A.;Morterra,C. J.Phys. Chem. 1967, 77, 2938; 1967, 77,2945. (13) Morirnoto, T.; Morishbe, K. J. Phvs. Chem. 1975. 79.1573. (14) Nagao, M.; Morimoto,-T. Bull. Chem. SOC.Jpn. 1976, 49,2977. (15) Nagao, M.; Kiriki, M.; Mwaishi, H.; Morimto,T. J. phys. Chem. 1978, 82,2561. (16) Morimoto, T.; Yanai, H.; Nagao, M. J. Phys. Chem. 1976, 80, 471. (17) Taylor, J. H.; Amberg, C. H. Can. J. Chem. 1961, 39,535. (18) Nagao, M.; Morlshige, K.; Takeshita, T.; Morimoto, T. Bull. Chem. SOC.Jpn. 1974, 47,2107. (19) Morlmoto, T.; Shiomi, K.; Tanaka, H. Bull. Chem. SOC.Jpn. 1964, 37,392. (20) Atherton, A.; Newbdd, G.; Hockey, J. A. D&cuss. Fara&ysoC. 1971, 52,33. (21) Barto, J.; m m , J. L.; Baston, V. F.; Wade, W. H. J. Colbk3Interface Sci. 1966, 22,491. (22) Bellamy, L. J. "The Infrared Spectra of Complex Molecules", 2nd ed.; Wlley: New York, 1958; Chapter 2. (23) Silverstein,R. M.; Bassler, G. C. "SpectroscopicIdentlflcation of Organic Compounds",2nd ed.;Wlley: New York, 1967; Chapter 3. (24) Sidorov, A. N. Zh. Fiz. Khlm. 1956, 30,995. (25) Folman, M.; Yates, D. J. C. Trans. Faraday SOC.1956, 54,1684. (26) Kagel, R. 0. J. Phys. Chem. 1967, 71, 844. (27) Deo, A. V.; Dalla Lama, I. 0 . J . Phys. Chem. 1969, 73,716. (28) Thornton, E. W.; Harrison, P. G. J . Chem. SOC.,Faraday Trans. 7 1975, 77,2468. (29) Knozlnger, H.; Stubner, B. J . Phys. Chem. 1978, 82,1526. (30) Jura, G. Phys. Methods Chem. Anal. 1951, 2. (31) Dzisko, V. A.; Krausnopol'skova, V. N. Zh. Flz. Khim. 1952, 26, 1841. (32) McClellan, A. L.; Harnsberger, H. F. J. Collold Interface Sci. 1967, 23,577. (33) McDonald, R. S. J. Phys. Chem. 1958, 62,1175. (34) Beliakova, L. D.; Kiselev, A. V. Zh. Flz. Khlm. 1959, 33, 1534. (35) E.g., Fieser, L. F.; Fieser, M. "Textbook of Organic Chemistry": Maruzen: Tokyo, 1952. (36) Nagao, M. J. Phys. Chem. 1971, 75,3822. (37) Morimoto, T.; Nagao, M. J. Phys. Chem. 1974, 78,1116.
Differential Heat of Chemisorption. 4. Chemisorption of Methanol and I-Propanol on Zinc Oxide Tetsuo Morimoto," Masafumi Klrlki, Department of Chemistry, Faculty of Science, Okayama University, Tsushima, Okayama 700, Japan
and Mahiko Nagao Research Laboratory for Surface Sclence, Faculty of Science, Okayama University, Tsushima, Okayama 700, Japan (Received November 26, 1979)
The heats of chemisorption of alcohols (MeOH and 1-PrOH) on ZnO surfaces were obtained by measuring the heat of immersion in water for the samples having different amounts of prechemisorbed alcohols which could be replaced by water. The differential heat of chemisorption of alcohol was given by graphical differentiation of the integral chemisorption heat, which showed a maximum at low coverage and then decreased with increasing coverage of chemisorbed alcohol. The higher heat value at low coverage was interpreted in terms of the additional heat effect due to an enhanced probability of hydrogen bonding of surface hydroxyls formed by dissociative chemisorption of alcohol with the surrounding surface hydroxyls remaining on the surface, in addition to the genuine heat of chemisorption of alcohol. It was also found that the differentialheat of chemisorption for 1-PrOH is larger than that for MeOH. This was explained on the basis of the inductive effect of alkyl groups.
Introduction If chemisorbed species on metal oxide surfaces are replaced by water to form surface hydroxyls, we can measure the heat of chemisorption of the species by means of the immersion calorimetry. This method has been applied to
the systems ZnO-C02 and -NH3 to give the heat of chemisorption of COz and "3 on the ZnO surface.' The results obtained supported the idea that the hydroxylation of the ZnO surface results in the formation of a peculiar structure of surface hydroxyls strongly bonded with each
0022-3654/80/2084-2058$01.00/00 1980 American Chemical Society
