J. Phys. Chem. 1984,88, 4041-4044 amount of methane at 100 "C. Hydrogen cyanide had two intense overlapping peaks at about 90 and 125 "C and a weaker peak at -230 "C. A molecule of mass 41 (probably acetonitrile) desorbed at about 50 "C (broad) and dimethylamine (mass 44) desorbed at about 90 "C. Acetyl cyanide also decomposed to give a variety of desorption products. Hydrogen (1 30 and 205 "C), carbon monoxide (130 and 185 "C), hydrogen cyanide (broad desorption from 100 to 390 "C), methane (broad desorption from 0 to 185 "C), and apparently acetonitrile (200 "C) were evolved upon heating. The low-temperature part (100-200 "C) of the mass 27 spectrum may be due to fragmentation of desorbing acetyl cyanide, however. The acetyl cyanide desorption occurred in two overlapping peaks at -35 and 70 "C. Decomposition of hydrogen cyanide from Ni( 1 11)-HCN explicably had only two decomposition products that desorbed from the surface: H2which peaked at 100 "C and Nz which showed two maxima, -400 and 750 "C. The decomposition data for the functionalized nitriles are interesting but do not provide fundamental information about how such nitriles are bound to the surface and how the molecule is oriented on the surface. For some nitriles, the binding may be through both the nitrile group and the second functional group-e.g., benzonitrile may bond to the flat N i ( l l 1 ) surface through both the nitrile and the aromatic ring. Such multiple interactions might facilitate decomposition of the nitrile. Nitriles like acrylonitrile and methacrylonitrile could be oriented initially so as to place the olefinic carbon atoms nearly parallel to the surface plane providing them maximal r-a* interaction with surface metal atoms. This could place the CN group also in a plane parallel to the surface plane. In such a configuration high thermal reactivity-as is observed-should obtain. If the activation energy for nitrile decomposition is very low, then there is a reasonable probability that the ground-state structure of the chemisorbed species is reasonably related to the transition state for the decomposition pathway(s). Thus, the fact that as many functionalized nitriles undergo gross decomposition at low temperatures suggests that the functional group or specific C-H hydrogen atoms in the nitrile may be very close to surface metal atoms: if we consider the specific case of chloroacetonitrile,
4041
the chlorine atom would closely approach the surface only if the cyanide group is nearly parallel to the surface plane (or if perpendicular, the C-C(C1) bond were bent toward the surface). On the basis of an ab initio calculation by Howell et a1.,15 a decrease in the C-C-N angle of acetonitrile bound to a metal atom only through the nitrogen lone pair would decrease its a acceptor capability. This would tend to support a decomposition pathway which involves binding of the C N group in a side-on fashion as observed in Fe3(p3-v2-NCCH2CH2CH3)(CO)9.'6 A similar coordination geometry has been proposed recently for acetonitrile on Pt(l11) based on electron energy loss spectra." To resolve key issues raised in this exploratory work, we plan near-edge X-ray absorption fine structure studies of nitriles on several metal surfaces to establish orientation of the nitriles as a function of metal (d-level filling), surface topography, and the nature of the nitrile (substituent groups). Also, the nitrile orientation on some surfaces may be a function of surface and possible coverage effects will be examined. Acknowledgment. This research was supported by the Director, Office of Energy Research, Office of Basic Energy Sciences, Chemical Sciences Division of the US.Department of Energy, under Contract No. DE-AC03-76SF00098. We are also indebted to the Fannie and John Hertz Foundation for a graduate fellowship (R.M.W.). Registry No. Ni, 7440-02-0; CH3CN, 75-05-8; CF3CN, 353-85-5; CZHSCN, 107-12-0; i-C3H$N, 78-82-0; t-C,H,, 630-18-2; C2F,CN, 422-04-8; CHSOCHICN, 1738-36-9; CHSCOCN, 631-57-2; CH2= C(CH,)CN, 126-98-7; CH,=CHCN, 107-13-1; ClCHZCN, 107-14-2; (CH,)2NCN, 1467-79-4; HCN, 74-90-8. Supplementary Material Available: Subroutine EQN and subroutine CALCY (2 pages). Ordering information is given on any current masthead page. (15) J. A. S.Howell, J.-Y. Saillard, A. Lebeuze, and G . Jaouen, J. Chem. SOC.,Dalton Trans. 2533 (1982). (1 6) M. A. Andrews, C. B. Knobler, and H. D. Kaesz, J . Am. Chem. SOC., 101, 7260 (1979). (17) B. A. Sexton and N. R. Avery, Surf. Sci., 129, 21 (1983).
Adsorption of Water, Ammonia, and Carbon Dioxide on Zinc Oxide at Elevated Temperatures Isao Yasumoto Department of Chemistry, Yonago Technical College, Yonago, 683, Japan (Received: February 28, 1984) The adsorption isotherms of H20, NH,, and C 0 2 were measured at a variety of temperatures on the surface of ZnO which was pretreated at 723 K in vacuo. The adsorption isotherm of HzO at room temperature showed the type of multilayer adsorption, involving two-dimensional condensation of H20, but when the adsorption temperature exceeded 373 K, it changed to the Langmuir type which represents the chemisorptionof H20. The desorption of H 2 0 was so retarded in a lower pressure region that a small number of strongly adsorbed H 2 0 molecules were left on the surface even after their evacuation into a trap cooled by liquid nitrogen, and they were found to decrease with elevating adsorption temperatures. The isosteric heat of adsorption, qst,of H 2 0 in the chemisorption region showed a monotonous decrease with the increase of the surface coverage, while the qst value in the physisorption region showed a maximum before the completion of the monolayer. It was found that there is a wide temperature range, 373-623 K, at which the coverage of about 3 molecules/nm2 is thermally stable for chemisorbed H20, whereas for chemisorbed NH, and C 0 2 it shifts to a lower temperature range, corresponding to the qst values of 110, 80, and 60 kJ/mol for H20, NH3, and C 0 2 ,respectively.
Introduction It has been clarified that metal oxides adsorb water molecules chemically to form surface hydroxyls, on which physisorbed multimolecular films are formed in the atm~sphere.'-~ The
surface hydroxyls thus formed can be desorbed successively by evacuating at high temperatures, and in some cases remain even after heating at 1300 K.*Hitherto, metal.oxides have been used as catalysts frequently at elevated temperatures, and a number
(1) Morimoto, T.; Nagao, M.; Tokuda, F. Bull. Chem. SOC.Jpn. 1968, 42, 1533. (2) Morimoto, T.; Nagao, M.; Tokuda, F. J. Phys. Chem. 1969,73,243. (3) Morimoto, T.;Nagao, M.; Imai, J. Bull. Chem. SOC.Jpn. 1971, 44, 1282.
(4) Kittaka, S.; Kanemoto, S.; Morimoto, T. J . Chem. SOC.,Faraday Trans. 1 1978, 74, 676. ( 5 ) Kittaka, S.;Nishiyama, J.; Morishige, K.; Morimoto, T. Colloids Surf. 1981, 3, 51.
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The Journal of Physical Chemistry, Vol. 88, No. 18, 1984
Yasumoto
Pressure
High
Gauge
Vacuum
i
1i K
I' B
0'
u 1000 2000 3000 N/mP
Chromel-Alumel
Figure 1. Schematic diagram of the adsorption apparatus: (S) sample
cell, (K) buret, (F) furnace, (U) trap, (C) tap, (X and Y) terminals connected to a controller for the furnace, (B) stainless steel cylinder 89 mm wide, 250 mm long, and 3 mm thick.
Figure 2. Adsorption isotherms of H 2 0 on ZnO at 283 and 298 K. Open symbols indicate adsorption, and solid symbols desorption. a=0.324nrn
of adsorption studies6-" have been carried out on metal oxide-gas systems, but few studies have dealt with the adsorption at elevated temperatures. In the present investigation, the adsorption isotherms of H 2 0 , NH,, and COz on ZnO are volumetrically measured over a wide range of temperatures. The adsorption state of each molecule is also discussed on the basis of the isosteric heat of adsorption and the population of adsorbed molecules.
Experimental Section Materials. The zinc oxide used in this study was produced by burning zinc in the air. According to the maker's assay (Sakai Kagaku Kogyo Co.), the purity of the sample is 99.8%, and the main impurities detected are 0.15% moisture, 0.002% Pb, 0.0002% Cd, and 0.02% water-soluble substances. Zinc oxide powder thus produced is known as having an excellent surface crystallinity and well-developed (lOT0)plane of wurtzite structure.12 The gases which were used were supplied by the Takachiho Shoji Co., and their nominal purities in volume percent are 99.99 for C 0 2 and 99.5 for NH,. Surface Area Measurement. The adsorption of nitrogen on the oxide powder was measured at 77 K by a conventional adsorption apparatus,13 and the specific Burface area was calculated to be 4.30 m2/g by the application of the BET equation, by assuming that the cross-sectional area of a nitrogen molecule is 0.162 nmz. Adsorption Isotherms of Water Vapor. The apparatus for the adsorption measurement is illustrated in Figure 1. A pressure gauge of the stainless steel bellows type made by the Wakaida Scientific Instruments Inc., Model WAP-5250, was employed. Water vapor was supplied through the taps C, and C2 by evaporating liquid water in a calibrated microburet K of diameter 1.12 mm, the level of water being read by a cathetometer, and the volume of water evaporated being measured. Before the measurement of adsorption isotherm, the sample in the cell S was heated at 723 K for 8 h under a pressure below (6) (7) (8) (9) 29.
Hart, P. M. G.; Sebba, F. Trans. Faraday SOC.1960, 56, 551. Kobes, R. J.; Glemza, R. J. Phys. Chem. 1965, 69, 17. Levy, 0.; Steinberg, M. J . Catal. 1967, 7, 159. Morimoto, T.; Nagao, M.; Hirata, M. Kolloid Z . Z . Polym. 1968, 225,
(10) Morishige, K.; Kittaka, S.; Morimoto, T. Surf. Sci. 1981, 109, 291. (1 1) Kittaka, S.;Morishige, K.; Nishiyama, J.; Morimoto, T. J . Colloid Inferface Sci. 1983, 91, 117. (12) Morimoto, T.; Nagao, M. Bull. Chem. SOC.Jpn. 1970, 43, 3746. (13) Joy, A. S. Vacuum 1953, 3, 254.
Figure 3. Crystal structures of (1070) plane of ZnO: planar view of (1070) plane (a); side view perpendicular to c axis (b); and side view parallel to c axis (c). A water molecule chemisorbed on a site A is drawn schematically by dotted circles according to the model of the closed
hydrogen-bonding structure. 1.3 X lo-, N/m2. The adsorption isotherm of HzO was measured at 283, 298, 313, and 323 K on the sample kept in a water bath controlled within &O.l K, and at 373, 523, 577, 623, and 673 K in an electric furnace F controlled within *1 K. In order to obtain the desorption isotherm at lower pressures less than lo3N/mZ,the trap U in Figure 1 was used: the desorbed water vapor was once condensed in the trap cooled with cold water for 30 min and measured after reevaporating at room temperature. After the reevaporated vapor was exhausted, the tap C, was opened, and 3 h later the pressure in a stationary state was measured. The same procedure was repeated successively by cooling the trap with ice, cold acetone, dry ice-acetone mixture, and finally liquid nitrogen, as the pressure decreased. The equilibrium pressure to be measured was calibrated against the pressure difference due to the thermal tran~piration'~ of the vapor. Adsorption Isotherms of Carbon Dioxide and Ammonia. The experimental procedure of adsorption measurement for C 0 2 and NH, was almost the same as that for HzO.The adsorption isotherm was measured at 426, 474, 524, 583, 623, and 675 K for COz and at 298, 322, and 370 K for NH3.
Results and Discussion The first adsorption isotherm of H 2 0 measured at 283 or 298 K after treating the sample at 723 K shows the hysteresis as illustrated in Figure 2. However, when the following second adsorption isotherm is measured after the final desorption point, A or B, it goes on and comes back along the first desorption isotherm. In other words, no hysteresis appears on the second (14) Yasumoto, I. J . Phys. Chem. 1980, 84, 589.
Adsorption of H20, NH3,and C02 on ZnO
’1
’
0 313K 0 323K
A
313K Second
6
/
373K
Temperature
P
‘h ‘0
b
The Journal of Physical Chemistry, Vol. 88, No. 18, 1984 4043
0 623K
Figure 5. Dependence of the amount of irreversibly adsorbed gas on adsorption temperature.
673K
500
1000
1500
N/m2
2000
2500
adsorption isotherm. The isotherm obtained here has a remarkable inflection which appears over the relative pressure range from 0.2 to 0.3, and the inflectional region is steeper in the second adsorption isotherm than in the first one. The inflection was first observed by Morimoto et al.’ They explained it as the two-dimensional condensation of H 2 0 on the (1010) plane which occurs on the inert surface of surface hydroxyls made by the closed-hydrogen bonding.15 Figure 3 shows the (lOT0)plane of the oxide surface which is parallel to the c axis, where the centers of zinc and oxygen atoms are coplanar on the plane, each atom coordinates three counterions, and both kinds of atoms are arranged in pairs parallel to the c axis. Figure 3 shows also the dissociative adsorption of a water molecule: hydroxyl is bonded to zinc atom and hydrogen to oxygen atom, when hydration occurs on the oxide surface. This leads to the result that the area, (0.324)(0.520) nm2, has a single active site for the molecule. According to the report of Morimoto et al.,” the inflectional region of the H 2 0 adsorption isotherm becomes steeper when the surface is more homogeneous, especially when the (1010) plane is well developed. As stated above, the first adsorption isotherm shows hysteresis, but the second one does not. At the same time, the vertical part of inflectional region becomes more distinguished in the second isotherm than in the first one. Therefore, it is clear that this phenomenon is quite different from the hysteresis which appears in the adsorption systems with such adsorbents as charcoal,I6 silica gel,I7J8and porous g1a~s.I~The reason that this phenomenon appears can be elucidated as follows: the surface hydroxylation becomes complete and ordered on exposing the surface to water vapor near the saturated vapor pressure, though most of the hydroxylation is completed at a very low pressure near the point A or B in Figure 2. Thus, in the first adsorption isotherm the two-dimensional condensation of water occurs on somewhat disordered hydroxyls on the (1010) plane, but in the second one it occurs on more ordered and homogeneous hydroxyls, which results in the formation of a more vertical inflectional region in the isotherm. Figure 4 represents the water adsorption isotherms on ZnO, which were measured at temperatures from 313 to 673 K. As seen, the isotherms measured at 313 and 323 K are of the multimolecular layer adsorption type, and the isotherm obtained at 3 13 K can be seen slightly inflected, but the inflection cannot be observed in the isotherm obtained at 323 K, because the measured pressures are below the relative pressure range, 0.2-0.3. When the adsorption is measured at temperatures over 373 K, the adsorption isotherm transfers to that of the Langmuir type, which implies that the chemisorption of water only occurs in this temperature range. In the individual isotherms measured, the desorption curve goes backward on the adsorption one, but in the (15) Morimoto, T.; Nagao, M. J . Phys. Chem. 1974, 78, 1116. (16) Emmett, P. H. Chem. Rev. 1948, 43, 69. (17) Pidgeon, L. M. Can. J . Res. 1934, 10, 713. (18) Rao, K. S . J . Phys. Chem. 1941, 45, 513. (19) Emmett, P. H.; Cines, M. J . Phys. Chem. 1947, 51, 1248.
e
100~\100’
Figure 4. Adsorption isotherms of H20on ZnO at elevated temperatures. Open symbols indicate adsorption, and solid symbols desorption.
- 0
L
2.9 3.0 3.1 3.2 3.3 ‘5rnolecules/nrnz
10
-
15
Figure 6. qatof H 2 0 on ZnO at temperatures of 283 and 298 ( O ) , 577 and 623 (A),577 and 673 ( O ) , and 523 and 623 (0)K. EL denotes the heat of liquefaction of H20.
COz
6-
0 426K 0 474K A 524K
$
N
E .c
@ 583K
A
623K 675K
474K Second
qF==L ys-
E L ? O O
::
---T
3 2 2 ~ 370K
-
+
500
NH3 (3 298K
1000
1500
2000
.
6
2500
3000
N/m’ Figure 7. Adsorption isotherms of C02and NH, on ZnO. Open symbols indicate adsorption, and solid symbols desorption.
final region of the pressure the desorption of water is so retarded that the desorption curve deviates to intersect the ordinate at a point over the original point. Points A-I in Figures 2 and 4 indicate the intersections, which represent the amounts remaining on the oxide surface in the last stage of each desorption isotherm. The remaining amount of water thus obtained is plotted against the temperature of measurement in Figure 5. As seen in Figure 5 , the strongly adsorbed water amounts to about 5 H 2 0 molecules/nm2 at room temperature, but it decreases to about 3 H 2 0 molecules/nm2 at 373 K, the latter value being constant approximately over a wide range of temperatures from 373 to 623 K. Water molecules have been found to be dissociativelyadsorbed on the surface to form surface hydroxyls at these higher temperatures.20 Figure 5 shows that the number of surface hydroxyls decreases abruptly in vacuo at temperatures over 623 K. This coincides with the desorbability of surface hydroxyls of ZnO measured by another method.’ By applying the Clausius-Clapeyron equation to the data in Figures 2 and 4, we can obtain the isosteric heat of adsorption of water, qst,in both high- and low-coverage regions as shown in Figure 6, which correspond to physical and chemical adsorption regions, respectively. The qstcurve obtained in the physisorption region is very similar to that obtained by Nagaozl and shows a (20) Morishige, K.; Kittaka, S.; Moriyasu, T.; Morimoto, T. J. Chem. SOC., Faraday Trans. 1 1980, 76, 738. (21) Nagao, M. J . Phys. Chem. 1971, 75, 3822.
J . Phys. Chem. 1984,88, 4044-4049
4044
con 0 Y
%
501
0
2
1
3
molecules/nmz
Figure 8. qstof C 0 2 and NH,.
maximum which reveals the lateral interaction of water molecules over the coverage of the inflectional region. On the other hand, the qsl values in the chemisorption region are comparable to the heat of chemisorption of water measured by the heat-of-immersion meth~d.~ Figure 7 illustrates the adsorption isotherms of C 0 2 and NH3 on ZnO in temperature ranges of 426-675 and 298-370 K, respectively. All the isotherms obtained are found to be of Langmuir type, indicating the chemisorption of the molecules. The desorption of C 0 2and NH, is also retarded at lower pressures as in the case of H20. The desorption curve deviates to intersect the ordinate at a point over the original point. Points J-R indicate the intersections, which are also plotted in Figure 5 against the temperature of measurement, together with those obtained in the case of H20. From the comparison of the results in Figures 4 and 7, similar adsorption isotherms of the Langmuir type can be observed in the temperature ranges of 373-673 K for H20, 298-370 K for NH,, and 426-524 K for C 0 2 ,whereas the amounts remaining on the oxide surface in the last stage of each desorption isotherm are very different as seen in Figure 5. This will depend on the interaction force between each molecule and the ZnO surface as described later. C 0 2 can be chemisorbed on ZnO, according to infrared spectroscopic st~dies:~,~~ to form carbonate and carboxylate ions. In Figure 5 these species are found to be thermally unstable. The second adsorption isotherm of C 0 2 ,which is measured at 474 K (22) Taylor, J. H.; Amberg, C. H. Can. J . Chem. 1961, 39, 535. (23) Matsushita, S.; Nakata, T. J . Chem. Phys. 1962, 36, 665.
after the attainment of the final point of desorption, K in Figure 7, goes on the first desorption isotherm all over the range of pressure measured as in the case of H 2 0 adsorption. For this reason, the isosteric heat of adsorption is calculated on the first desorption curve and is plotted against the adsorbed amount as shown in Figure 8. As seen, the isosteric heat of adsorption is 60-120 kJ/mol for C 0 2 and 60-80 kJ/mol for NH,. Several authors have reported the heat of adsorption of C 0 2 on ZnO to be, eg., 71-926 and 69-105' kJ/mol by isosteric heat measurements, and 87-124 kJ/mol by gas chromatography.* These values agree with the present data. It can be also seen that the qstvalues at a coverage of about 3 molecules/nm2 for H 2 0 , NH,, and C 0 2 are 110, 80, and 60 kJ/mol, respectively. It is interesting to realize from the results in Figures 4, 5, and 7 that there are temperature ranges favorable to the coverage of 3 molecules/nm2 for the adsorbed species. As mentioned already, the ZnO sample used here has well-developed (1070) planes. The chemisorption sites for H 2 0 on this plane have been considered to be A and B in Figure 3, though A is quite equivalent to B, the population of them being calculated to be 5.94 sites/nm2. Therefore, the coverage of 3 molecules/nm2 equals just half of the number of full chemisorption of H20. It is a reasonable consideration that the vibrational motion of the H 2 0 molecules chemisorbed fully on the sites becomes greater when the temperature is raised, and the motion extends to the region of the nearest neighboring sites, which results in the separation of the adsorbed molecules on the surface, thus covering half of the available active sites. The coverage of 3 molecules/nm2 for H 2 0 seems to be very adequate over the wide temperature range to balance the adsorption force with the repulsive force. In the cases of C 0 2 and NH3, this temperature range largely shifts to a lower temperature range, below 373 K, corresponding to lower chemisorption energies. Acknowledgment. I thank Professor Tetsuo Morimoto, Okayama University, for his interest and encouragement in this work. Registry No. ZnO, 1314-13-2;C 0 2 , 124-38-9;NH,, 7664-41-7;H,O, 7732-18-5.
Effect of Environment on Decay Pathways of the Singlet Excited State of N,N,N',N'-Tetramethylbenzldine S. Hashimoto and J. K. Thomas* Department of Chemistry, University of Notre Dame, Notre Dame, Indiana 46556 (Received: November 3, 1983)
Singlet decay pathways of tetramethylbenzidine (TMB) and its protonated forms (monoprotonated and diprotonated TMB, TMBH+, and TMBH22+)were investigated in several media, including sodium lauryl sulfate (NaLS) and cetyltrimethylammonium bromide (CTAB) micellar systems. Quantum yields of fluorescence, triplet formation, and monophotonic ionization were determined. The predominant mode of deactivation of the excited singlet state (SI*)of TMBH' was fluorescence emission, while that of TMB(S,*) decay was triplet formation. TMBHz2' was expected to undergo deprotonation in the SI*state. Unusual features were observed in micellar systems, efficient monophotonic ionization was the principal mode of decay of TMB in NaLS micelles, while deprotonation of the excited state of TMBHf was observed in CTAB micelles.
Introduction Tetramethylbenzidine (TMB) is characterized by its low gas-phase ionization potential (6.1 to 6.8 eV1) or by its strong reductive nature in solution ( E , 2(TMB/TMBc) = 0.36-0.43 V vs. SCE2). Photoionization phenomena and formation of the (1) (a) Fulton, A.; Lyons, L. E. &SI. J . Chem. 1968, 21, 873-82. (b) Nakajima, A.; Akamatsu, H. Bull. Chem. SOC.Jpn. 1969,42, 3030-2. (2) For example: (a) Nocera, D. G.; Gray, H. B. J. Am. Chem. SOC.1981, 103, 7349-50. (b) Kalyanasundaram, K. Coord. Chem. Rev. 1982, 46, 159-244.
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TMB cation radical are well d o ~ u m e n t e d . ~ The dynamic features of photoinitiated reactions of TMB, both in organic solvents, and in micellar solutions, have been demonstrated by Alkaitis and Gratzel? with studies of the photoionization (3) For example: (a) Lewis, G . N.; Lipkin, D. J . Am. Chem. SOC.1942, 64, 2801-8. (b) Krog, U.; Ruppel, H.; Witt, H. T. Ber. Bunsenges. Phys. Chem. 1963,67, 795-6. (c) Hester, R. E.; Williams, K. P. J. J . Chem. Soc., Faraday Trans. 2 1981, 77,541-7. (d) Narayana, P. A,; Li, A. S. W.; Kevan, L. J . Am. Chem. SOC.1981, 103, 3603-34. (e) Narayana, P. A,; Li, A. S. W.; Kevan, L. Ibid. 1982, 104, 6502-5.
0 1984 American Chemical Society
