2058
J. Phys. Chem. 1980, 84, 2058-2061
I
\Ir"
1 1 1 3800 3600
4
8
3400
I
I
I
I
I
1
3200 3000 2900 Wovenumber, cm-'
I
4
I
2600
A J
2400
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
Heat of Chemisorption of MeOH and 1-PrOH on ZnO
The Journal of Physical Chemistty, Vol. 84, No. 16, 1980 2059
2E N
8ooj
E
i3./ B
,
210
,
g 2.0 r
c
s
a8
IO
0
40 i ' 1 6 10 L - 8 7.0 & Hydroxyl content , OH graups/nm*
Y
d
0
100
200
300 400 Treatment temperature ,"C
500
600
Figure 1. Relationship between the amounts of chemisorbed alcohol and the surface hydroxyl content of the ZnQ sample.
Figure 2. Heat of
other through hydrogen bondingsS2J Our recent study of the alcohol adsorption on ZnO surfaces revealed that the bare surface of ZnO can dissociatively chemisorb normal aliphatic alcohols of small carbon numbers (C1,--C3)at 25 "C and that, when the sample with prechennisorbed alcohol is exposed to water vapor, the prechemiisorbed alcohol can be replaced by water, resulting in thle formation of surface hydroxyls instead of surface alkoxyls.4 This fact suggests the possibility of measuring the heal; of chemisorption of alcohols on ZnO surfaces by the same method as described above. In the present work, the heats of chemisorption of methanol (MeOH) and 1-propanol (1-PrOH) on ZnO surfaces were measured by applying the heat-of-immersion method to the exchange reaction of prechemisorbed alcohol for water. The results obtained were discussed in comparison with those for water chemisorption on the same ZnO surface.
Practically, we employed the following control method for preparing ZnO samples with different amounts of prechemisorbed alcohol, The fully hydroxylated ZnO sample was evacuated at various temperatures between 25 and 600 "C for 2 h, which left different numbers of hydroxyls on the surface. Next, the sample was exposed to MeOH or 1-PrOH vapor at 25 "C for 2 h, followed by evacuation at 25 "C for 2 h; this treatment resulted in ZnO surfaces with chemisorbed water on one part and with chemisorbed alcohol on the remaining part, the ratio of which varied. Here, the evacuation temperature of the fully hydroxylated sample determines the portion of the dehydroxylated surface, by which the amount of chemisorbed alcohol can be controlled. Measurement of Heat of Immersion in Water. The heat of immersion in water for ZnO samples with prechemisorbed alcohol and surface hydroxyls was measured at 28 f 0.1 "C by using an adiabatic calorimeter equipped with a quartz thermometer (sensitivity: 1 X lo4 "C)as a temperature-sensing element. The procedure of the heat-ofimmersion measurement has been described e1sewhere.l~~ The data obtained were expressed as the average value of several determinations for each sample; the experimental error was within 3%.
Experimental Section Materials and Pretreatment. The original ZnO sample used in this study was the same as that in the preceding work: which was Katlox 15 produced by New Jersey Zinc Co. The sample waEi first degassed at 600 "C for 4 h in N m-2 in order to remove surface imvacuo at 1 X purities such as chemisorbed water and carbon dioxide, followed by exposure to saturated water vapor at 25 "C for 15 h, to fully hydroxylate the sample surface. The specific surface area of the sample degassed at 600 "C was found to be 9.11 m2g-l on the basis of N2-adsorptiondata. Control of the Amount of Prechemisorbed Alcohol on ZnO. The primary consideration in this study was to prepare the ZnO samlples with different amounts of prechemisorbed species and without hydroxyls, as in the cases of the measurements of chemisorption heats for C02and NH3 on Zn0.l However, it has been known that the heat treatment of ZnO samples with adsorbed alcohol causes a catalytic decompoeiition of alcohol to leave undesired species such as formate or carboxylate ions on the surfa~e.~-~ Thus, we changed the initial plan to the second-bestone, in which ZnO samples with different amounts of prechemisorbed alcohol and water (hydroxyls) both were prepared, through the environmental situation of prechemisorbed alcohol qwas different from that of the cases of prechemisorbed C 0 2 and NH,. Fortunately, the quantitative relationship bietween surface hydroxyls and chemisorbed alcohol has been obtained in the preceding work: being reproduced in Figure 1.
immersion in water for ZnO samples covered with (a) hydroxyls, (b) chemlsorbed MeOH, and (c) chemisorbed 1-PrOH.
Results and Discussion The heat-of-immersion values for ZnO samples are plotted against the evacuation temperature of fully hydroxylated sample in Figure 2. Among the three curves in Figure 2, the heat-of-immersion curve for the sample covered with surface hydroxyls only is essentially the same as that obtained previously on another ZnO sample3 produced by the same method as the present sample but by a different manufacturer. This curve is characterized by a large increase with rising evacuation temperature from 200 to 400 "C, as stated previ~usly.~ On the other hand, the heat values for ZnO samples having prechemisorbed MeOH or 1-PrOH are significantly smaller than those for the samples containing hydroxyls only. When the ZnO samples covered with chemisorbed alcohol are immersed in water, besides physical wetting of the surface, three processes will proceed concurrently: (1) desorption of chemisorbed alcohol from the surface, (2) dissolution of the desorbed alcohol in water, and (3) formation of surface hydroxyls on the surface. Since the heat effect due to each process depends on the amount of prechemisorbed alcohol on the surface, small heat-of-immersion values for alcohol-covered ZnO samples shown in Figure 2 suggest a large endothermic effect due to the
The Journal of Physical Chemistry, Vol. 84, No. 16, 1980
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Morimoto et al.
1200
E
4 E 1000 1
0
.-+0
800
P
;.
600
c 0) L
r
::
I; 0
&?
400
zoo
c
.+.c
c 0
2.0 3.0 4.0 5.0 Amount of chemisorbed ROH or H,o , moIecules/nm'
1.0
0
Flgure 3. Integral heat of chemisorption of MeOH, 1-PrOH, and H,O on the ZnO surface. Broken lines represent the corrected curves for MeOH and 1-PrOH.
desorption of chemisorbed alcohol. The integral heat of chemisorption of alcohol per unit can be calculated from eq 1, area of the sample, Qa(r)
= h~- h r ) + I'Hml
(1)
similar to that used in the cases of C 0 2 and NH, chemisorption,' where hI is the heat of immersion per unit surface covered with hydroxyls only, and hI(r)is the heat of immersion for the surface covered with both r molecules of prechemisorbed alcohol and the hydroxyls in the same amount as in the case of hI. is the heat of dissolution of I' molecules of alcohol in water. The value of I' can be evaluated at the specified hydroxyl content according to the relationship shown in Figure 1. Figure 3 represents the integral heat of chemisorption of alcohol on a ZnO surface plotted against the chemisorbed amount r, which was calculated by introducing each numerical value into eq 1. Here, the heats of dissolution of alcohols in water at 28 "C are cited from the literature,1° being 43.4 and 57.2 kJ mol-l for MeOH and 1-PrOH, respectively. In Figure 3, the integral heat of chemisorption of water is also added, being calculated in the same manner as described in the earlier paper? Figure 3 shows that the Qa(r)values at low coverage of alcohols are lacking, which is concerned with the fact that a small amount of alcohol can be adsorbed irreversibly even on the fully hydroxylated surface (Figure 1). As is shown in Figure 1, the amount of chemisorbed alcohol decreases linearly with increasing surface hydroxyl content of the sample, which strongly suggests the idea of dissociative chemisorption of alcohols, as discussed in the preceding papera4 Therefore, we may reasonably consider the irreversible adsorption on the fully hydroxylated surface to be caused neither by dissociative chemisorption nor by esterification of alcohol^,^^-^^ but to come from an interaction of another type, namely, a strong physisorption of alcohol molecules on the surface hydroxyls through the formation of hydrogen bondings. For this reason, the heat of chemisorption at lower coverage does not seem to be the genuine one, and hence we should not venture to extrapolate the heat curve to the origin in Figure 3. As is seen from Figure 1, the ZnO sample has 11.10 hydroxyl groups per nm2 after evacuating the fully hydroxylated surface at 25 "C, while the hydroxyl content is reduced to 1.06 hydroxyl groups per nm2 after the evacuation at 600 "C; furthermore, this evacuated sample can chemisorb 3.96 MeOH molecules per nm2. Since MeOH molecules are chemisorbed dissociatively on the
I .o 20 30 40 50 Amount of chemisorbed R O H or H 2 0 , molecule5/nrn2
Flgure 4. Differential heat of chemisorption of MeOH, 1-PrOH, and H,O on the ZnO surface.
bare surface to produce an equal number of hydroxyls and m e t h ~ x y l s7.92 , ~ sites nm-2 are consumed on the chemisorption of MeOH. Therefore, 2.12 sites nm-2 should remain unchemisorbed on the surface as isolated sites or obstructed sites under methoxyls. A similar calculation for 1-PrOH chemisorption gives 2.88 nm-2 as the number of such sites. The above consideration suggests that the calibration should be carried out on the heat of chemisorption for the real number of alcohol molecules which reacted. On the surface having a maximum amount of prechemisorbed MeOH, there exist three kinds of sites: 5.02 (1.06 3.96) sites nm-2 occupied by hydroxyls, 3.96 sites nm-2 occupied by methoxyls, and 2.12 sites nmW2 as vacant ones. When the surface is immersed in water, the substitution reaction of water for chemisorbed alcohol takes place on the second group of sites, and the chemisorption of water occurs on the last group of sites. In order to obtain the true integral heat of chemisorption of MeOH at the maximum coverage, one should rule out the contribution due to the formation of hydroxyls on the vacant sites from the measured heats, the value of which can be estimated by multiplying the heat value at the chemisorbed amount of 3.04 H20 molecules nm-2 (6.08 hydroxyl groups nm-2) in the integral heat curve for water by the ratio 2.12/(3.96 + 2.12). The number of vacant sites on the fully hydroxylated surface is reasonably zero. In addition, the linear relationship between the adsorbed amount of alcohol and the hydroxyl content of the sample as shown in Figure 1 also suggests the linearity between the number of vacant sites and the hydroxyl content. Thus, by applying the same calculation method as adopted at the maximum coverage of MeOH to all of the range of coverage and also to the case of 1-PrOH chemisorption, we can obtain the corrected curves for alcohol chemisorption as shown in Figure 3. Figure 4 represents the differential heat of chemisorption of alcohol, Qd,obtained by graphical differentiation of the corrected integral heat curves in Figure 3. The differential heat of chemisorption of water is also added in Figure 4. This curve is essentially the same as that obtained previously on a different ZnO ample.^ The differential heat of chemisorption of water increases with increasing coverage of hydroxyls and reveals a maximum near the monolayer coverage; this result was interpreted in terms of the formation of mutual hydrogen bondings between surface hydroxyls formed on the well-developed (1010) plane of Zn0.3 In the preceding work,l the differential heat-of-chemisorptioncurve of C 0 2 on ZnO gave a horizontal part over a wide range of coverage, indicating a surface homogeneity of the ZnO sample used, while that
+
J. Phys. Chem. 1980, 84, 2061-2067
of NH3 revealed a large maximum similar to the case of H,O; the latter was explained by the additional contribution of mutual hydrogen bondings between chemisorbed species of NH3, as in the case of water chemisorption on the same ZnO sample. Here, the differeintial heat of chemisorption of alcohol exhibits a maximum at low coverage and then decreases with increasing coverage of chemisorbed alcohol. Moreover, it can be seein that the heat of chemisorption of 1PrOH is larger than that of MeOH. It should be noted that the environment for alcohol adsorption, which was prepared on the ZinO surface in the present study, is quite different from that for C02 and NH3 ads0rptions.l Since the chemisorptionsites for alcohol were prepared by partial desorption of hydroxyls, the environment for alcohol chemisorption is suirrounded by hydroxyls which decrease with increasing amount of alcohol to be chemisorbed, in contrast to the fact ithat the environment for COzand NH3 is composed of the bare surface. Individual areas of bare patches fbrmed by the desorption of hydroxyls must increase on the average with increasing numbers of desorbed hydroxyls. When the area of the bare surface is small, in other words, when a larger portion of the surface is covered with hydroxyls, the surface hydroxyls formed b,y dissociative chemisorption of alcohol will interact with each other and with the neighboring hydroxyls through hydrogen bonding, which results in the enhancement of differential heat of chemisorption. However, when the area becomes larger and hence the amount of chemisorbed alcohol increases, the probability of forming hydrogen bondings between surface hydroxyls themselves is expected to decrease. We can thus explain the decrease in the differential heat of chemisorption with increasing coverage of chemisorbed alcohol. 1-PrOH gives a larger differential heat of chemisorption compared with that of MeOH. The inductive effect of the alkyl group, that iti, t,he tendency of thrusting its electrons to the neighboring atoms in the molecule,15 is greater for alcohols with more carbon atoms. Consequently, the oxygen atom in the propoxyl group becomes more negative than that in the methoxyl group, which results in the enhancement of the bonding force of the propoxyl group
2061
to surface zinc atoms, in comparison with that of the methoxyl group. In addition to this effect, hydrogen bonding of the type CH-016 and mutual van der Waals interaction between hydrocarbon chains themselves may contribute to the heat of chemisorption; these effects are also greater for 1-PrOH than for MeOH. As can be seen in Figure 4, the Qdvalue of water at low coverage. where the mutual hydrogen bondings among surface hydroxyls cannot be formed, gives the genuine heat of chemisorption of water, 100 kJ mol-’. In the case of alcohol chemisorption, the genuine heat of chemisorption, ca. 80 kJ mol-l, is evaluated at high coverage of chemisorbed alcohol, where the interaction of chemisorbed alcohol with the surrounding hydroxyls does not occur. Consequently, the genuine heat of chemisorption of water is larger than that of alcohol, and hence the difference between them (ca 20 kJ mol-l) is consideredto be the heat of exchange of alcohol for water. This exothermic exchange reaction is consistent with the fact that the chemisorbed alcohol can be replaced by water, though entropy effects should also be taken into consideration to discuss thermodynamically the exchange reaction. References and Notes Nagao, M.; Kirlki, M.; Muraishi, H.; Morlmoto, T. J. phys. Chem. 1978, 8 2 , 2561. Morimoto, T.; Nagao, M. J. Phys. Chem. 1974, 78, 1116. Nagao, M.; Yunoki, K.; Muraishi, H.; Morimoto, T. J. Phys. Chem. 1978, 8 2 , 1032. Nagao, M.; Morimoto, T., preceding paper In this Issue. Kagel, R. 0. J. Phys. Chem. 1967, 71, 844. Deo, A. V.; Daila Lama, I. G. J. Phys. Chem. 1989, 73, 716. Ueno, A.; Onishl, T.; Tamaru, K. Bull. Chem. SOC.Jpn. 1970, 43, 2652. Thornton, E. W.; Harrison, P. 0.J. Chem. Soc., Faraday Trans. 1 1975, 71, 2468. Knozlnger, H.; Stubner, B. J. Phys. Chem. 1978, 8 2 , 1526. Nippon Kagaku-kai, “Kagaku Blnran”; Maruzen: Tokyo, 1975. Sidorov, A. N. Zh. Fiz. Khim. 1956, 30,995. McDonald, R. S. J . Phys. Chem. 1058, 62, 1175. Beilakova, L. D.; Kiseiev, A. V. Zh. Fir. Khim. 1959, 33, 1534. Boreiio, E.; Zecchina, A.; Morterra, C. J . Phys. Chem. 1987, 71, 2938. E.g., Fleser, L. F.; Fieser, M. “Textbook of Organlc Chemistry”; Maruzen: Tokyo, 1952. Pimentel, 0.C.; McCielian, A. L. “The Hydrogen Bond”; Freeman: San Franclsco, 1960; Chapter 6.
Charge-Transfer Excited States of Osmium(I1) Complexes. 1. Assignment of the Vlsible Absorption Bandsia B. J. Pankuch,lb D. E. Lacky, and G. A. Crosby*lG Department of Chemistry and Chemical Physics Program, Washington State Unlversity, Pullman, Washington 99 784 (Rectsived: January 9, 1980) Publication costs assisted by the Air Force Office of Scientific Research
The visible and near-UV absorption and photoluminescence spectra of five tris and five bis complexes of osmium(I1)containing a-conjugated ligands have been assigned to charge-transfer-to-ligandexcited configurations. For the trigonal molecules both the emission bands and the weak absorption bands lying contiguous to them have been assigned symmetry labels based on an ion-parent coupling model previously developed for the analogous states of ruthenium(I1) complexes. Substantial d-orbital penetration from osmium(I1) onto the ligand T system is deduced from the optical measurements. Similarities and differences between analogous sets of osmium(I1) and ruthenium(I1) species are discussed. Previously an elwtron-ion coupling model for d-a* charge-transfer-to-ligand (CTTL) excited states was induced from optical data on ruthenium(I1) complexes 0022-3654/80/2084-2061$01 .OO/O
containing r-conjugated ligands.2 The model was developed mathematically3 and used to determine exchange integrals for a group of related trigonal (OJ complexes of 0 1980 American Chemical Society
