Langmuir 1986,2, 528-532
528
to infer that the pair of HCO, and OH- ions exist a t the reaction interface through the following reaction. 02-(s)
-
+ H2C03(ad)
OH-(ad)
+ HC03-(ad)
(2)
This is the dissociative adsorption of H2C03(ad),which represents the fixation of C02. The next stage of the reaction will be the formation of CO?-(s) ions in the solid phase, as manifested by the IR spectra. OH-(ad)
+ HC03-(ad)
-
C032-(s) + H20(ad) (3)
This corresponds to the second ionization stage of H2C03(ad).Thus, the adsorbed H 2 0 acts as a catalyst for
the absorption reaction of C 0 2 by Ag20. As shown in Figure 3, surface hydroxyls operate alone as a catalyst for the C 0 2 absorption reaction, though the effect is not so marked. This may be due to the fact that surface hydroxyls on Ag20 react with C02to form surface carbonate ions like Ag-C03H, which will carry the C02 molecule to the inner phase to form CO:-(s) ions. Acknowledgment. We thank Dr. Yasushige Kuroda of Okayama University for his kind cooperation in the experiments. This work was partially supported by a Grant-in-Aid from the Japanese Ministry of Education. Registry NO.AgZO, 20667-12-3;COP, 124-38-9;HZO, 7732-18-5.
Reactive Adsorption of a,a-Diphenyl-0-picrylhydrazylon Alumina As Studied by Inelastic Electron Tunneling, FT-IR, and EPR Spectroscopy K. W. Hipps,* E. A. Dunkle, and Ursula Mazur Department of Chemistry and Chemical Physics Program, Washington State University, Pullman, Washington 99164-4630 Received February 11, 1986. In Final Form: March 20, 1986 The adsorption from benzene solution of cY,cY-diphenyl-8-picrylhydrazyl(DPPH) on thin-film and high surface area alumina was studied by vibrational and EPR spectroscopy. Thin-layer chromatography (TLC) analysis of material in solution and eluted from the oxide surface was also performed. On thin-film native oxides, and on basic and acid y-alumina, DPPH is converted to the hydrazine, a,cY-diphenyl-@-picrylhydrazine (HDPPH), with small amounts of other reaction products. Of these minor products, we have identified diphenylamine and trinitroaniline. On basic (natural) alumina the surface reaction seems to take place in at least two steps. There is a fast step, T = 10 min, in which much of the DPPH is reacted, and a slower step, T = 5 h. For solutions containing sufficient DPPH to yield ultimate surface coverages ranging from 0.058 to 0.38, the adsorption is irreversible and HDPPH is the primary surface product. Introduction During the past 20 years, the study of adsorbate-oxide redox processes has been a subject of continuing interest. The great majority of the work in thisarea utilized electron paramagnetic resonance (EPR) as the primary investigative tool. A small number of papers have directly probed the surface species through the use of vibrational spectroscopy. The use of both types of spectroscopy is particularly desirable, since they are both of only limited utility for studies of oxide-supported materials. EPR is exceedingly sensitive to radical species but offers no information about nonradical components. IR spectroscopy can provide structural information about all of the surface species, but only those species having more than about 10% abundance (relative t o the principal adsorbate component) can be identified. If too many high-yield products occur, analysis is also quite difficult. Further, the characteristic oxide absorption makes much of the IR spectral region inaccessible (below 1000 cm-’ in the case of alumina). Raman spectroscopy generally yields narrower spectral lines and is much less sensitive to interference from support scattering. However, many of the interesting redox systems are highly colored and easily photodegraded and have small amounts of side produds which are fluorescent. These are all anathema to the Raman spectroscopist.
* Author t o whom correspondence should be addressed
Inelastic electron tunneling spectroscopy (IETS)provides many of the benefits of IR spectroscopy, has low sensitivity to the oxide bands (spectral range extends from 150 to greater than 4000 cm-l), and requires the least amount of sample for a given SIN ratio of any of the three techniques. On the negative side of the ledger are the requirements that the support be a thin film (15 nm) and that the species studied survive in a high-vacuum environment. Further, the statements made earlier concerning detection of side products also apply. Thus, where applicable, IETS is an especially useful technique for studying adsorbateoxide systems. Tunneling spectroscopy has been extensively reviewed.l” It has been applied to surface chemistry and vibrational spectroscopy problems ranging the full spectrum of biochemical, organic, inorganic, and chemisorption problems. Its application to redox processes, however, has been relatively limited. In a pioneering paper, Korman and Coleman6 observed the reduction of TCNQ to the radical anion TCNQ-’ on alumina by tunneling spectroscopy. (1)Hansma, P.K. Tunneling Spectroscopy; Plenum Press: New Ork, 1982. (2) Wolf, E.L. Principles of Electron Tunneling Spectroscopy; Oxford University Press: New York, 1985. (3) Walmsley, D.G.; Tomlin,J. L. Compilation of Inelastic Electron Tunneling Spectra of Molecules Chemisorbed on Metal Oxides; Pergamon Press: New York, 1985. (4) Weinberg, W. H. Annu. Reu. Phys. Chem. 1978,29,115. (5)Hansma, P.K. Phys. Rep. 1977,30C,145. (6)Korman, C. S.; Coleman, R. V. Phys. Rev. B 1977,15, 1877.
0743-7463/86/2402-0528$01.50/00 1986 American Chemical Society
Reactive Adsorption of DPPH on Alumina
Hipps and M a w 7used tunneling spectroscopyto observe the reduction of ferricyanide (paramagneticion) to ferrocyanide (diamagnetic ion) on alumina. Mazur and H i p p ~ & ~ also studied the adsorption of tetracyanoethylene(TCNE) and its radical anion on alumina by tunneling spectroscopy. Due to the extreme reactivity of the species involved, this required the use of a special controlled atmosphere tunnel junction fabrication system? Most recently, Monjushiro'O et al. have utilized IETS to observe the reduction of some nitrobenzene derivatives on alumina, a process that they believe has a short-lived radical species as an intermediate. Although the above work does demonstrate the utility of IETS for some surface redox studies, it is not a sufficient basis for assuring that processes observed on the thin films used in IETS are identical with processes on bulk high surface area oxides. Thus, any tunneling study of such reactions should be done in concert with more conventional studies on bulk materials. The present work combines several techniques to study the reaction of the stable radical DPPH with thin-film and bulk alumina. The reaction of DPPH with alumina was chosen for several reasons. First, it is a stable organic radical which forms a stable organic nonradical on alumina under both air and inert atmosphere adsorption conditions. No such system has been previously studied by tunneling spectroscopy. Second, the reaction of DPPH with oxides and has been extensively studied by EPR and elution techniques, and the reaction chemistry seems to vary with the type of surface studied. Third, only one study of DPPH on alumina" has appeared in the literature, and that work indicates that the adsorption is weak and reversible. This latter result is inconsistentwith much of the work on other oxides, and we felt that it was important to question the accuracy of this result. Experimental Section Materials: The bulk aluminas used were purchased from Baker and their surface areas were measured by nitrogen adsorption, using the BET method. The acid alumina (no. 0538) had a surface area of 140 m2/g and the basic alumina (no. 0539) had a surface area of 163 m2/g. For some of the experiments, the aluminas were 'activated" by heating at 145O C under vacuum for 18h followed by cooling to room temperature under Ar. These activated samples were sealed under Ar in Kontea Micro-flex flasks with rubber septa. Solutions were introduced and removed by hypodermic needles and syringes. DPPH and 1,l-diphenyl-2picrylhydrazine (HDPPH) were purchased from Kodak (a picture of DPPH can be found in the Aldrich chemical catalog). The DPPH was recrystallized twice from benzene/ligroin before use and stored under vacuum. Benzene was Baker reagent grade with less than 0.004% water content. Methylene chloride was HPLC (7) Hipps, K. W.; Mazur, U. J.Phys. Chem. 1980,84, 3162. (8)Mazur, U.; Hipps, K. W. J. Phys. Chem. 1984,88, 1555. (9) Hipps, K. W.; Mazur, U. Rev. Sci. Instrum. 1984,55, 1120. (10) Monjushiro, H.; Murata, K.; Ikeda, S. Chem. SOC.Jpn. 1985,58, 957. (11) Golubev, V. B.; Evdokimov, V. B.; Kireenko, G. M. Rws. J.Phys. Chem. 1965.39. 195. (12) Aston, J. G.; Misra, D. N.; Cresswell, K. M. Nature (London) 1965,208,181. (13) Aston, J. G.; Misra, D. N. J.Phys. Chem. 1965, 69, 3219. (14) Misra, D. N. J.Phys. Chem. 1967, 71, 1552. (15) Imai, H.; Ono, Y.; Keii, T. J.Phys. Chem. 1968, 72, 46. (16) Bernard, H. W.; Aston, J. G. J. Colloid Interface Sci. 1968,26, 276. (17) Misra, D. N. Tetrahedron 1970,26, 5447. (18)Misra, D. N. J. Catal. 1970, 19, 204. (19) Slinkin, A. A.; Loktev, M. I.; Rubinshtein, A. M. Dokl. Akad. Nauk. SSSR 1970, 191,625. (20) Solomon, D. H.; Hawthorne,D. G. J. Macromol. Sci., Chem. 1971, A5, 575. (21) Misra, D. N. J. Colloid Interface Sci. 1975,50, 108. (22) Hasegawa, S.; Miyazawa, N.; Kitano, S.; Kawaguchi, T. Nippon Kagaku Kaishi 1976,2, 246.
Langmuir, Vol. 2, No. 4, 1986 529 grade, and the anhydrous diethyl ether was reagent grade. TLC was performed by using EM Reagents plastic-backed silica gel sheets (no. 5735-7). Aluminum and lead metals were >99.995% purity. Tunneling Data. Tunnel junctions were prepared both by standard methods' and in a unique system designed to allow fabrication in an inert atm~sphere.~ In all cases the adsorption was accomplished by exposing the thin-film alumina to a 0.1 mg/mL solution of DPPH or HDPPH in benzene, followed by spin drying the substrate. The spectrometer used is a modified version of the one described in ref 7. The spectra reported are the sum of 10-15 scans taken with a 0.3-5 time constant at 1.7-mV rms modulation. All data were taken with the sample at 4 K. IR Data. Infrared spectra presented in this paper were obtained on an IBM Instruments Fourier Transform IR Model 98 vacuum instrument using a liquid nitrogen cooled MCT detector. Spectra were collected at 4-cm-' resolution in transmission mode and are plotted in absorbance units. Surface samples were mixed 10% by weight with KBr, ground in an agate capsule, and pressed into pellets. In the case of all oxide-supported samples, a reference spectrum of the oxide treated in the same manner as the sample but with benzene only adsorption was subtracted from the data presented. Typically 1200scans (each) of sample, reference, and background were coadded for analysis. To ensure that the pellet pressing operation did not modify the observed results, a few of the samples were run in reflectance mode without KBr dilution or pressing. The only apparent difference in these latter spectra and those taken by transmission was increased noise in the reflectance data. Nonsupporkd samples (authentic materials and TLC isolated fractions) were also mixed with KBr and pressed into pellets. Reference KBr spectra were subtracted from the data presented. The number of scans varied from 300 to 800 depending on the amount of material available for analysis. EPR Data. EPR spectra were obtained on a Varian Instruments E-3 spectrometer operating near 9.5 GHz and 3389 G. Liquid samples were made up as benzene solutions in 5-mmdiameter sample tubes and filled with sufficient liquid to extend well beyond the height of the cavity. The spectrometer signal response was calibrated over the desired range by using known concentrations of DPPH. Amplitudes were determined by double integration of the spectra. Typically, the scan width was 100 G and the modulation amplitude was 1.25 G. All spectra were obtained at room temperature. TLC Data. TLC was performed with methylene chloride or ether as the developing solvent. For qualitative analysis, plates were first observed visually and then under 254-nm excitation and finally developed in I, vapor. Several fractions were isolated by using larger sheets (3 in. X 3 in.) and scraping off the bands of interest. The silica-compound mixture was then washed with the appropriate solvent and filtered. The resulting solution was evaporated to dryness and the solid material was again subjected to TLC to ensure its purity. The isolated compounds were analyzed by FT-IR, NMR, or both. We frequently ran authentic DPPH and HDPPH spots adjacent to unknowns to ensure that we were not reporting reactions that might occur on the TLC plate. Bulk Adsorption Experiments. Both as-supplied and activated aluminas were used for the bulk adsorption experiments. The adsorption process was carried out in air with the as supplied aluminas and under Ar with the activated samples. Amounts of DPPH (in benzene solution) ranging from 0.1 to 30 mg/g of Al,O, were used. Adsorption times ranging from 15 min to 96 h were used. Typically, at the end of a selected adsorption time a sample of the liquid phase was subjected to TLC analysis and the EPR spectrum was measured to determine the number of radicals in solution. The solid fraction was then separated by filtration. In some cases, this solid was immediately studied by FT-IR or EPR. In other cases, the solid fraction was then washed with a small amount (=lo mL) of benzene and vacuum-dried at 50 " C for several hours before spectral analysis. In order to characterizethe reaction products, several samples were subjected to the following treatment. After adsorption the sample was washed with benzene until the wash solution was colorless. The color of the oxide at this stage was near black. The liquid fraction was reduced in volume and TLC performed on it. The solid was then washed with methylene chloride, again until
530 Langmuir, Vol. 2, No. 4 , 1986
Hipps et al.
w HDPPH
FT-IR
0
2000
4000
ENERGY (cm-')
Figure 1. Tunneling and infrared spectra of an authentic sample of HDPPH. The band marked A1-0 is due to the alumina sub-
strate used in tunneling spectroscopy. the wash was colorless. This solution was also reduced in volume and TLC performed. Finally, the solid was washed with diethyl ether and TLC performed on the liquid phase. The multiply washed solid was still somewhat colored being beige to khaki depending on the amount of material originally adsorbed. In order to obtain an estimate of the time required for the surface reaction, duplicate 3-g samples of activated basic alumina were stirred with 80 mg of DPPH in 20 mL of benzene. At several different times 1 mL of solution was withdrawn and the EPR spectrum measured. These spectra were double-integrated to obtain the number of radicals removed from solution as a function of time and these values were corrected for the amounts withdrawn. We also allowed 12.5 mg of HDPPH in benzene to interact with 1g of both basic and acid activated alumina for 24 h. The solid material was sequentially washed in benzene, methylene chloride, and ether. Each wash was analyzed by TLC.
Results Figure 1 presents the IETS and FT-IR spectra of authentic HDPPH. The small broad band near 3000 cm-' in the IETS is due to benzene coadsorbed on alumina. The most prominent alumina band in the tunneling spectrum (near 950 cm-') appears as a broad shoulder in the figure. The weak broad feature near 3600 cm-' is due to surface OH. The FT-IR spectrum is of pure HDPPH in KBr, not of adsorbed material. Considering the lower resolution of the tunneling data and the difference in selection the similarity of the two spectra is quite good. Figure 2 displays the tunneling and IR spectra obtained from DPPH adsorbed on alumina. Note first that the tunneling spectra shown in Figures 1 and 2 are essentially identical. Spectra taken from junctions fabricated in air or under inert conditions were also the same. The FT-IR spectrum was obtained from a 0.5-g sample of basic alumina after a 48-h period of reaction with 5 X mol of DPPH in 14 mL of benzene. The solid and matching reference were dried by vacuum filtration, only. The large negative feature near 3000 cm-' (benzene CH bands) is probably due to slightly less efficient drying of the reference than the sample. As mentioned earlier, the strong oxide absorption below about 1000 cm-' makes the lowfrequency region inaccessible in the IR. Spectra taken at much shorter exposure time intervals (15 min to 2 h) were essentially the same but the signal strength increased with exposure time. Please note the similarity of the tunneling and IR spectra. Figure 3 contrasts the IR spectra obtained from an authentic sample of DPPH in KBr (Figure 3B) with that obtained from DPPH on basic alumina after vacuum
2000
4000
ENERGY (crn-'1
Figure 2. Tunneling and infrared spectra of DPPH adsorbed on alumina from benzene solution.
drying at 50 "C. The alumina-supported material was the result of a 24-h exposure of 3 g of activated basic alumina to 80 mg of DPPH in 20 mL of benzene under inert conditions. Recall that all surface IR spectra reported are difference spectra; the matching benzene only reference oxide spectra have been subtracted. Thus the long negative excursion near 3000 cm-' is due to a loss of acidic surface hydroxyls. Figure 4 is a plot of the disappearance of DPPH from solution with time under conditions described in the Experimental Section. The smooth curve is a nonlinear least-squares fit of the function M ( t ) = al (1 - e x p ( - t / ~ ~ )+) a,(l - exp(-t/r2)) where M ( t ) is the number of moles of DPPH lost from solution with time. The values obtained for the parameters are a, = 1.32 X a2 = 0.594 X 7 1 = 0.216 h, and 7 , = 4.53 h. While we did not analyze the oxide during the course of the experiment, there was no DPPH present on the surface at 24 h. Thus, we believe that Figure 4 is an approximation of the loss of DPPH from the entire sample as a function of time. This idea is strengthened by the TLC results obtained from the solution phase samples represented by the points in Figure 4. In the TLC (run in methylene chloride) of the solution removed a t 0.5 h, one observes a strong purple spot at Rf 0.5 and a yellow halo just above it. These were subsequently identified as DPPH and HDPPH (vide infra). At 1h the TLC is quite similar to the 0.5-h result. In the TLC of the solution after 6 h has elapsed, however, the HDPPH spot has become much more prominent and a fluorescence quenching compound near Rf0.7 is seen. After 21 h, no DPPH spot is observed and the yellow HDPPH spot is the most prominent. In addition to the Rr 0.7 component, there is a weaker component a t R, 0.76 (fluorescence quenching). After 24 h had elapsed, the reaction mixture was filtered and the solid washed with about 20 mL of benzene. The total40 mL of benzene solution was reduced in volume and TLC (in methylene chloride) was performed. The results were quite colorful. Although no DPPH was observed, a number of other spota appeared. In addition to the three compounds produced after 21 h, a grey band at Rf0.55, a yellow spot a t R, 0.29, a blue spot at R, 0.27, a magenta spot very near Rf0, and several extremely faint bands in the region below R, 0.27 were observed. The solid was then eluted with methylene chloride and TLC (in methylene chloride) performed. The methylene chloride wash yielded a small yellow spot near R, 0.5, a yellow spot near R, 0.29,
Reactive Adsorption of DPPH on Alumina
2000
1000
Langmuir, Vol. 2, No. 4, 1986 531
3000
WAVENUMBERS CM-1
Figure 3. Infrared spectra. Curve A is the IR spectrum of alumina withdrawn from an initially 4 mg/mL solution of DPPH in benzene after a 24-h reaction time. The spectrum of alumina in contact with pure benzene for the same period of time has been subtracted. Curve B is the spectrum of an authentic sample of DPPH pressed in a KBr wafer. ( v ,
1
!r
Discussion
X
"
T
i
m
w
B El
6A is the spectrum of the species isolated a t Rf 0.7 and is identical with the IR of diphenylamine. NMR spectra of this sample also indicate that it is diphenylamine. Figure 6B is the IR spectrum of the material with Rf 0.29and is identical with the spectrum of trinitroaniline. The bands marked with * are due to hydrocarbon contaminants picked up from the air by the TLC plates. While the experiments performed on the acid alumina samples were not so extensive, several differences were observed. The acid alumina samples were not as active in the decomposition of DPPH, and the magenta compound seen as a very minor component on basic alumina was much more prominent. So much so, in fact, that we were able to isolate sufficient material to obtain the IR spectrum given in Figure 6C. Although the signal was quite weak, we were able to obtain a partial NMR spectrum of the magenta component. A well-defined singlet occurred a t 8.64 ppm (relative to Me,Si). A complex multiplet was also present near 7.47 ppm. Other signals were present but were so weak that they could not be associated with the magenta species with certainty. Please note, however, that we cannot be sure that the material obtained is not itself a mixture of components. HDPPH on both acid and basic alumina adsorbed reversibly for the most part, with a small amount of diphenylamine being produced after several hours of reaction time. Trinitroaniline and the unidentified material a t Rf 0.76 were also observed in the TLC of the ether extract from the basic alumina surface. At the solution concentration used (see Experimental Section) part of the HDPPH was always in solution.
6
1 2
T I M E
C h o u r r >
1 8
24
Figure 4. A plot of the number of moles of DPPH lost from solution with time. Initially there was 80 mg of DPPH in 20 mL of benzene in contact with 3 g of activated basic alumina.
and a blue spot near Rf 0.27 as the principal components but with other very weak colored regions. The primary product of the diethyl ether wash was a magenta material that resolved into several bands when TLC was performed in ether. Amid this confusion of colorful products, it is important to note that the masses of the solids recovered from the benzene, methylene chloride, and ether washes were in the ratio of about 100:3:0.2. We were able to separate sufficient amounts of the compounds a t Rf 0.7, 0.51, and 0.29 to obtain NMR and IR spectra. The major product is the yellow material with Rf near 0.51, and its IR spectrum is shown in Figure 5B. Also shown (Figure 5A) is the spectrum of an authentic sample of HDPPH. Parts A and B, Figure 5, are clearly IR spectra of the same species. Thus the major overall product of the reaction of DPPH with basic alumina is HDPPH. Figure
Consideration of Figures 1and 2 clearly shows that the primary species present on the oxide surface after DPPH adsorption is the nonradical hydrazine HDPPH. While the tunneling spectrum provides poorer resolution of spectral features than does FT-IR, significantly more low-frequency information is available in the tunneling spectrum. Further, no spectral subtractions were performed on the tunneling data. The main features observed are those of the adsorbate rather than of the oxide as in FT-IR. Thus, the data is not prejudiced by the subjective judgment of how much reference to subtract from the data. Since the thin-film oxides used in IETS are free of pores, the data obtained are certainly that of an adsorbed surface species. In bulk oxide IR studies of this type, one must always be concerned whether a surface species, or a crystallite formed during drying of the solution, is observed. This is not the case in tunneling, where no signal is observed from surface layers more than about 3 nm The similarity of tunneling data taken under both inert and air atmospheric conditions shows that the production of HDPPH on the surface is not significantly affected by oxygen or water on the time scale of the tunnel junction preparation (=l h). Bulk oxide IR spectra taken a t exposure times ranging from 10 min to a few hours also are qualitatively insensitive to whether the adsorption is performed in air or under Ar. The intensity of the bands does increase with solution exposure time and activation of the oxide. During the first few hours of the reaction, therefore, oxygen and atmospheric water do not affect the qualitative course of the reduction of DPPH to HDPPH on alumina. These spectral results are completely consistent with the loss of DPPH from solution shown in Figure 4. About 70% of the DPPH has been consumed within the first hour of exposure under the conditions appropriate to Figure 4. We stress here that we are not attempting to understand the
Hipps et al.
532 Langmuir, Vol. 2, No. 4, 1986
WAVENUMBERS (cm-')
Figure 5. Infrared spectra of authentic HDPPH (trace A) and of the primary product of the reaction of DPPH with alumina (Rf 0.51) (trace B).
1000
2000
3000
WAVENUMBERS (cm-' )
Figure 6. Infrared spectra of other products of the reaction of DPPH with basic alumina and isolated by TLC. (A) diphenylamine, (B) trinitroaniline, and (C) magenta compound.
kinetics of the processes. The form of Figure 4 will certainly change with the concentration of DPPH or the amount of alumina used. The purpose of the figure is solely to provide an indication of the time scale associated with the chemical and spectroscopic studies reported. With this in mind, it is interesting to determine the maximum possible surface coverage associated with the two exponential steps. In order to do so, we take the area/(DPPH or HDPPH molecule) to be 160.2 A2/molecule.14 The first step saturates a maximum of 26% of the basic aluminum surface while the sum of the steps accounts for only 38% of the available surface area. Clearly these are maximum values since much of the product HDPPH returns to solution after it is formed on the surface. None of the spectroscopic techniques used were able to discern any of the many side products of the surface reaction. This is not too surprising since we found no radical products and the amount of any one side product was less than 10% of the HDPPH formed. We could see weak IR bands due to DPPH in spectra obtained from acid alumina removed from strong DPPH solutions a t short times. Of the many side products, only diphenylamine, trinitroaniline, and the unidentified material with R f 0.76 were
produced in significant amounts on adsorption of HDPPH on alumina. These species, therefore, may be produced in the DPPH adsorption experiments by a subsequent reaction of HDPPH on alumina. The other products may require the presence of DPPH to form. The magenta material which forms more readily on acid alumina is quite similar to a compound reported by Solomon and Hawthorne.20 They found that a magenta compound having similar R and color and a 1600-cm-l vibrational band was producedin significant amounts on the surface of minerals during the chemisorption of DPPH. They proposed that it was 4,4'-oxybis[N-[4-(picrylimino)cyclohexa-2,5-dienylidenelaniline]. It is also instructive to compare our results with those of Monjushiro et al.1° They report that several nitrobenzene derivatives are reduced on alumina to form amino compounds. Further, they suggest that this type of reduction of nitro groups is common on alumina. In our present studies, the nitro groups of DPPH and HDPPH are relatively unaffected by adsorption. The azide linkage is apparently much more reducible than the nitro groups in DPPH. Further, the nitro groups of HDPPH are apparently much more protected from reduction than those studied by Monjushiro. The production of HDPPH during the reaction of DPPH with various minerals and semiconductors is well-known.12-22There is general agreement that surface OH plays a role in the chemisorption process, but the number of mechanisms proposed is almost as great as the number of materials studied. The primary difficulty seems to be that the species formed, other than HDPPH, vary with the oxide under study. On carbon black, for example, Aston and Misra13 observed the production of one molecule of benzoquinone for every four molecules of DPPH added. On aluminum silicates, however, Solomon and HawthorneZ0find no benzoquinone produced. From our HDPPH adsorption studies, it would appear that a t least part of the confusion is due to secondary reactions of the product HDPPH with the oxide surface. In the case of the alumina-DPPH system studied here, at least three of the products may be due to secondary reactions. We intend to report on the simpler HDPPH-alumina system in a future publication.
Conclusion We have demonstrated that tunneling spectroscopy is an effective tool for studying the primary product of the reduction of DPPH on alumina, HDPPH. While the resolution achieved is inferior to that of IR spectroscopy, the increased spectral range and sensitivity of IETS are compensatory factors. We find that the initial reaction of DPPH (in benzene) with alumina to form HDPPH is quite fast initially but that final equilibrium is not achieved for several hours. A large number of very complex species are found to reside on the oxide surface at equilibrium, some of which may be eluted with polar solvents. At least three of these may be due to secondary reactions of the product HDPPH with the oxide surface. An earlier paper on the adsorption of DPPH on alumina reported that the adsorption was weak and reversible.'l Our results are not consistent with theirs. We find the adsorption to be very irreversible and far from weak. Acknowledgment. We thank the National Science Foundation for supporting this work under Grant DMR8414566 and for partial support under equipment Grant DMR-8320556. We thank Prof. Manning P. Cooke for introducing us to the TLC technique. K.W.H. also thanks the late James Lopez. Registry No. DPPH, 1898-66-4; A1203, 1344-28-1.
