Adsorption of Electron Acceptors on Alumina K E N J I R O MEGURO AND KUNIO ESUMI
Department of Chemistry, Science University of Tokyo, Kagurasaka, Shinjuku-ku, Tokyo, Japan Received December 29, 1975; accepted May 22, 1976 The adsorption of electron acceptors with electron affinity from 1.26 to 2.84 eV on the surface of alumina has been studied by measuring the adsorption isotherms, ESR, and electronic spectra. The limiting adsorbed amounts were decreased with decreasing electron affinity of the electron acceptors. The radical ions were formed as a result of electron transfer to the electron acceptors from the surface hydroxyl ions on the oxide. The radical concentrations formed were decreased with decreasing electron affinity of the electron acceptors from 2.84 to 1.77 eV, and in the acceptor with electron affinity of 1.26 eV, the radical ions were not formed. The results suggest that the limit of electron transfer from the alumina surface to the electron acceptor will be ranged between 1.77 and 1.26 eV in the affinity of the electron acceptor. INTRODUCTION
Recently, the adsorption of electron acceptors on metal oxides has been investigated to study and characterize the electron donor properties of metal oxides 0 - 5 ) . Flockhart et el. (6) attempted the adsorption of tetracyanoethylene (TCNE) for the estimation of the electron donor properties of alumina surface. In this respect, they associated the electron donor sites with the unsolvated hydroxyl ions and the defect centers involving oxide ions. Che el al. (7) have carried out a systematic study of the adsorption of T C N E on the surface of titanium dioxide and magnesium oxide, and showed that the electron donor sites were associated with the surface hydroxyl ions for the samples activated at the lower temperatures, but the oxide ions played an important role at higher temperatures. In this paper we will report the adsorption of some electron acceptors having different electron affinity on metal oxide. For this purpose, we employed electron acceptors, such as 7, 7,8,8-tetracyanoquinodimethane (TCNQ), 2,5-dichloro-p-benzoquinone (DCQ), p-dinitrobenzene (PDNB) and m-dinitrobenzene (MDNB), and metal oxide, alumina.
Materials. The alumina used for our experiment is one for chromatography, and has specialty high purity supplied from Kokusan Chemical Works, Ltd. The sample was treated in air for 2 hr at 500°C in an electric furnace, followed by cooling in a desiccator. The specific surface area of the sample measured by BET method was about 168 m~/g. The crystal structure of the sample was determined as "r-alumina by X-ray diffractometry. TCNQ was obtained from Dainippon Ink Chemical, Ltd., and further recrystallized from acetonitrile. DCQ, PDNB, and MDNB were obtained from Tokyo Kasei, Ltd. DCQ was purified by recrystallization from ethanol, PDNB and MDNB from chloroform and carbon tetrachloride. Acetonitrile used as solvent was G.R. one obtained from Kokusan Chemical Works, Ltd. Apparalus and procedure. The sample was placed in an L-shaped test tube which was attached directly to a high vacuum line, and then kept at 10-~ Torr at 100°C for 1 hr, and cooled at 25°C in vacuo prior to the adsorption experiment. Then, each acetonitrile solution, such as TCNQ, DCQ, PDNB, and MDNB, 93
Copyright ~ 1977 by Academic Press, Inc. All rights of reproduction in any form reserved.
Journal of Colloid and Inlerface Science, Vol. 59, No. 1, March IS, 1977 ISSN 0021-9797
MEGUR0 AND ESUMI
acceptor in solution, before and after the adsorption.
0 TCNQ ID PDNB
/ . / . (i t~. t / _
/----~-A4DT~D ',m -
RESULTS AND DISCUSSION
-©l - I D -
Equilibrium Concentration (m moltl)
FIG. 1. Adsorption isotherm of electron acceptor on alumina (at 25°C). was poured into the L-shaped test tube through a stop cock keeping vacuo state and shaken at 25°C. After at least 2 hr, having reached adsorption equilibrium, the sample was collected by centrifuging and dried at room temperature i n vacuo. The dried sample after the adsorption was used for the measurement of the electronic spectrum and the ESR measurement at 10-5 Torr was applied on the dried sample after the adsorption. The electronic spectra of the dried samples were measured by uv spectrophotometer (Hitachi Co. ESP-3T) with a standard reflectance attachment. The ESR spectra were measured by means of a Japan Electron Optics Laboratory JES-3BS-X-type ESR spectrometer operating at cavity resonance frequency of 9400 Hz with 100 kHz modulation. The g-value was estimated by comparison with the value for Mn 2+. Radical ion concentrations were calculated by comparison of the areas obtained by double integration of the first derivative curves for the sample and standard solutions of 1,1diphenyl-2-picrylhydrazyl in benzene. The concentration of electron acceptor in solution was determined with uv spectrophotometer (Hitachi Co. ESP-3T) by measuring the absorbancy at 395, 335, 263, and 237 nm, each of which was due to TCNQ, DCQ, PDNB, and MI)N.B in acetonitrile, respectively. The amount of electron acceptor adsorbed on the alumina was determined from the concentration difference of electron
Figure 1 shows the adsorption isotherms of TCNQ, DCQ, PDNB, and MDNB from acetonitrile solution at 25°C on the alumina surface. All isotherms except MDNB are convex to the concentration axes, and the adsorption isotherms became linear to the Langmuir plots. In the case of MDNB, the adsorption was so negligible that the amount was hardly estimated. The values of the limiting amounts of TCNQ, DCQ, and PDNB adsorbed on the alumina surface estimated from the Langmuir plots, are given in the second column of Table I. The electron affinity values (8, 9) of TCNQ, DCQ, PDNB, and MDNB are given in the first column of Table I. As shown in Table I, it is noticeable that the limiting amounts decrease with decreasing the electron affinity of the acceptors. When TCNQ, DCQ, PDNB, and MDNB were adsorbed from solution in acetonitrile on the surface of alumina, the color of the alumina surface showed remarkable coloration characterized with the kind of the acceptors. That is, blue green for TCNQ, yellow for DCQ, pale orange for PDNB and colorless for MDNB. These colorations will be owing to the interaction between the acceptor adsorbed and the alumina surface. To confirm the nature of the interaction, the electronic spectra of the
spectra from e]eetron aceeptors on alumina: (a) TCNQ; (b)DCQ; (c)PDNB; (d) MDNB. Fzo. 2. Electronic
Journal of Colloid and Interface Science, Vol. 59, No. 1, M a r c h 15, 1977
A D S O R P T I O N OF E L E C T R O N A C C E P T O R S
colored samples were measured. The electronic spectra of the colored samples are illustrated in Fig. 2. In Fig. 2(a), the band observed below about 450 nm may correspond to the physically adsorbed state of neutral TCNQ which has the absorption band at 395 nm in acetonitrile (10). The absorption band near 600 nm observed can be attributed to dimer TCNQ anion radical which absorbs light at 643 nm (11). This feature is supported by the facts that TCNQ has a high electron affinity, and that the TCNQ anion radical derivatives are stable even at room temperature (12-14). The band about 330 nm as shown in Fig. 2(b) may correspond to the physically adsorbed state of neutral DCQ which has the absorption band at 335 nm in acetonitrile. The absorption band about 400 nm may be attributed to DCQ anion radical (15). In Fig. 2(c), the band about 420 nm observed may be attributed to PDNB anion radical. In Fig. 2(d), the band observed below 360 nm may correspond to the chemically adsorbed state of MDNB. If the acceptor anion radicals are formed on the alumina surface after the adsorption, the colored samples will show ESR signal. So, the ESR spectra of the colored samples were measured. The colored samples gave an unresolved ESR spectra with a g-value of 2.003 for TCNQ, 2.005 for DCQ, and 2.004 for PDNB as shown in Fig. 3. But, the sample of M D N B adsorbed showed no ESR signal. The unresolved ESR spectra can be due to a hindered freedom of the adsorbed species, which obscures a hyperfine structure of the spectra. Therefore, the formation of TCNQ, TABLE I S u m m a r y of the Electron Acceptor Adsorption Results Electron acceptor TCNQ DCQ PDNB MDNB
Electron Adsorbed Radical affinity amount concentration (eV) (mole/m2) ) 10-7 (spins/m2) 2.84 2.30 1.77 ~ 1.26
9.9 6.2 0.25 Negligible
1.5 X i0 is 6.3 X 1015 2.0 X 1013 0
a T h e value given b y Briegleb (9) with 1.07 eV added.
Fro. 3. Electron spin resonance spectra of electron acceptors adsorbed on alumina: (a) T C N Q ; (b) D C Q ; (c) P D N B .
DCQ, and PDNB anion radicals as a result of electron transfer from the alumina surface to TCNQ, DCQ, and PDNB was confirmed by the electronic and the ESR spectra. i The nature of the site responsible for the electron transfer process is not well understood. However, it may be suggested that two possible electron sources exist on the alumina surface. One of these has electrons trapped at intrinsic defects, and the other has hydroxyl ions. If TCNQ, DCQ, and PDNB radical ions are formed by interaction with free electrons derived from the intrinsic defects on the surface or in the bulk of the alumina, which is insulator, they would not normally be expected to participate in an electron transfer adsorption because of the lack of a free electron. Flockhart et al. (6) have suggested that the electron donor defect site on the surface of alumina was created at activation temperatures of above 500°C. The other site may be the surface hydroxyl ion. It has been reported that the surface of alumina has hydroxyl groups (16, 17). The ionization potential of hydroxyl ions (18) is comparatively small ('-~2.6 eV in the gas phase) ; therefore, the possibility of its participation in oxidation-reduction processes of the type : OH- + A --+ OH + A where A is an electron acceptor, can be included. Fomin et al. (19) have shown that electron transfer from hydroxyl ions occurs in certain solvent systems provided a suitable
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MEGURO AND ESUMI
10 20 Equilibrium Conc.(m mo[l[)
a: 5 10 Equilibrium Conc.(m moll[)
10 ' 2o Equilibrium Conc.(rnmo[/I)
FIG. 4. Radical concentration on alumina versus equilibrium concentration of electron acceptor (at 25°C). acceptor molecule is present. I t has been reported that the electron donor site on the alumina surface might be associated with the presence of unsolvated hydroxyl ions on the surface (6). As the alumina surface is covered with hydroxyl groups (20), the electron transfer adsorption of TCNQ, DCQ, and P D N B on the alumina m a y result from surface hydroxyl ions. The other product of the electron transfer adsorption is mainly the OH radical although only the TCNQ, DCQ, and P I ) N B anion radicals are observed in the ESR spectra. Figure 4 shows the radical concentrations formed on the alumina surface plotted against the equilibrium concentration of the acceptor. All isotherms showed a Langmuir type. The limiting radical concentration of TCNQ, DCQ, and P D N B on the alumina surface estimated to the Langmuir plots is given in the third column of Table I. I t is found that the limiting radical concentration decreases with decreasing electron affinity of the acceptor and steeply between P D N B and M D N B . The similar behavior has been found for the change of the limiting radical concentration/the limiting adsorbed amount as a function of electron affinity of the acceptor. These suggest that the surface hydroxyl ions on the alumina surface m a y act as electron donors to adsorbed molecules with electron affinity of 1.77 eV or
above but not that of 1.26 eV. T h a t is to say, the limit of electron transfer from the surface hydroxyl ions on the alumina to the acceptor will be ranged between 1.77 and 1.26 eV in the affinity of the acceptor. REFERENCES 1. EsuMI, K. ANDMEOURO,K., Shikizai Kyokaishi 48,
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