M. CODELL,H. GISSER,J. WEISBERG,AND R. D. IYENGAR
2460 rangement must occur in the solid during the course of an experiment, it never seems to attain a highly crystalline state under the given treatment. It was observed that the lattice vibration of the LiF-HX solids occurs a t considerably higher frequencies than for pure LiF. Pronounced environmental shifts of the FHF- vibration frequencies were observed, especially for va. There was no significant evidence in the spectra for the presence of XHX- (of course, assuming the primary band assignments to FHF- are correct), and, consequently, no statements concerning the linearity of those ions can be made.. There is no evidence to suggest nonlinearity of FHF- in the given systems. (The integrity of the FHF- species in simple solids is indicated by the virtually identical F-F bond distance of 2.26 or 2.27 A in LiHF2,20NaHF2,21KHF2,22and "4-
HF22*). There is no reliable spectral evidence for the presence of higher bifluoride ions, although bands occur which could possibly be assigned to these species. The presence of moisture results in a large number of sharp absorption lines. While it is hoped that other bihalide ions may be formed in analogous solid mixtures, the present experience suggests that the solids may be amorphous and that the absorption bands may be too broad to yield accurate frequency values.
Acknowledgment. Mr. K. Ranjan Guha and Mr. Jack K. Gregersen helped with the early experiments. (20) L.K.Frevel and H. W. Rinn, Acta Crystallogr., 15, 286 (1962) (21) B.L. McGaw and J. A. Ibers, J . Chem. Phys., 39, 2677 (1963). (22) J. A. Ibers, ibid., 40,402 (1964);S. W.Peterson and H. A. Levy, ibid., 20, 704 (1951). (23)T. R. R.McDonald, Acta Crystallogr., 13, 113 (1960).
Electron Spin Resonance Study of Hydroperoxide on Zinc Oxide by M. Codell, H. Gisser, J. Weisberg, Pitman-Dunn Research Laboratories, Frankford Arsenal, Philadelphia, Pennsylvania
and R. D. Iyengar Center for Surface and Coatings Research, Lehigh University, Bethlehem, Pennsylvania
(Received December 18, 1967)
The esr signal observed on zinc oxide (Q 1.961) following oxygen treatment a t 500' decreases in intensity on exposing the degassed sample to t-butyl hydroperoxide (TBHP) vapors. However, with samples vacuum outgassed at 500°,the adsorption of TBHP leads to the formation of two signals (g 1.965 and 1.961) in place of the original signal. No spectra of free radicals resulting from decomposition of TBHP have been observed. When vacuum-outgassed ZnO is treated with oxygen a t 700 torr, reoutgassed at 25", and left under vacuum for more than 24 hr, two signals are again observed which correspond to the two signals observed earlier on the TBHP treatment. The formation of the two signals and the variation in their intensities have been discussed in terms of oxygen ion vacancies and interstitial Zn+ ions.
Introduction Hydroperoxide association with the catalysis of oxidation of organic lubricants has long been known, and while the oxidation of these lubricants has been studied extensively, details of the mechanism of hydroperoxide participation are still not clear. Since metal surfaces exposed to lubricants are oxide coated, it was considered desirable to explore the interaction of hydroperoxides with metal oxides. Since the surface chemistry of zinc oxide has been extensively investigated, zinc oxide was considered an appropriate experimental component. Of the various techniques utilized in attempts to elucidate the nature of the surface species and defects on solids, the application of electron spin resonance has The Journal of Physical Chemistry
been of considerable interest recently since it permits an analysis of different forms of chemisorbed paramagnetic species. The esr spectrum of ZnO (vacuum outgassed as well as oxygen treated) has been studied by numerous investigators,2-* and signals corresponding to g values of (1) For a summary of earlier work see: 8. R. Morrison, Advan. CataE., 7 , 213 (1955); G. Heiland, E. Mollwo, and F. Stockman, Solid State Phys., 8, 191 (1959). (2) J. H. Lunsford and J. P. Jayne, J . Chem. Phys., 44, 1487 (1966). (3) Y.Fujita and J. Turkevich, Discussions Faraday SOC.,41, 407 (1966). (4) K. M. Sancier, J . Catal., 5 , 314 (1966). ( 5 ) M.Sedaka and T. Kwan, Bull. Chem. SOC.Jap., 38, 1414 (1965). (6) P.H.Kasai, Phys. Rev., 130, 989 (1963). (7) R.J. Kokes, J . Phys. Chem., 66, 99 (1962).
2461
ESRSTUDYOF HYDROPEROXIDE ON ZINC OXIDE 1.96, 2.003, 2.008, 2.01, 2.04, and 2.049 have been reported. The triplet with g values 2.003, 2.008, and 2.049 has been attributed to an 0%-specie^.^,^ However, according to Sancier,4 adsorbed 02- does not give any observable resopance, and the signal at 2.01 should be assigned to an 0- species. Kwan5 attributes the signal at 2.003 to 0- ions which transform to 02-ions above 370". I n support of this view, it has been observed9 that the isobar for oxygen on ZnO reveals two peaks, corresponding to 170 and 370". The signal which appears in the vicinity of g 1.96 has been variously assigned to Zn+ ions3 possibly in shallow donor bands,8 to oxygen vacancies,6 and to conduction electrons.' Though the early work on ZnO6-l0 has revealed a complexity in the signal appearing a t g 1.96, the emphasis in subsequent investigations has been clearly on adsorbed oxygen. I n the present study the adsorption of t-butyl hydroperoxide was found to affect primarily the signal at g 1.96 and to modify it. This modification not only establishes clearly the complexity of the signal but also renders further investigations on zinc oxide possible. An attempt, therefore, has been made to elucidate the nature of the interaction between the hydroperoxide and zinc oxide by the observed effects of adsorption on the various resonance signals initially present on differently pretreated samples.
Experimental Section Materials. Two zinc oxide samples, both prepared by the vapor-phase oxidation of zinc and furnished by the Sew Jersey Zinc Co. (Palmerton, Pa.), were used in this work. The first sample, designated Kadox-15, had a surface area of approximately 10 m2/g. The second sample, designated SP 500, was of exceptionally high purity and had a surface area of -5 m2/g. The chloride content of this sample as analyzed by the turbidimetric method was less than 1 ppm. Most of our experiments were done with SP 500. Essentially similar results were obtained in our studies with Kadox-15. &Butyl hydroperoxide, TBHP (Lucidol Division, Wallace and Tiernan Inc., Buffalo, N. Y.), was distilled at 18 mm and the fraction boiling between 36 and 37.5" was collected. The material was degassed by multiple freeze and thaw cycles prior to adsorption. Apparatus. The vacuum treatment of ZnO and subsequent adsorption of oxygen and t-butyl hydroperoxide were carried out in a gas-handling apparatus attached to a mercury-diffusion pump backed by a mechanical pump (Welch Scientific Co.) and an ion gauge. A vacuum of torr could be attained in this system. During outgassing, the ZnO samples were protected from mercury vapors by the use of a Iiquidnitrogen trap. The sample tubes were made of annealed Pyrex glass, 4-mm id., and were attached to the system through a vacuum stopcock and a detachable joint. Thus suc-
cessive treatments could be carried out on a single sample. Apiezon-N grease was used to lubricate all stopcocks and joints. Approximately 25 mg of the sample was taken in the tube for each experiment. An esr spectrometer (Varian &!Iode1V-4500) with a TE-104 mode dual cavity at a frequency of approximately 9.3 khlcps was employed. The magnetic field was controlled by a Fieldial regulator V-FR-2100. 1,l-Diphenylpicrylhydrazyl (DPPH) was used as a standard for g measurements. All measurements were made at liquid-nitrogen temperature. Procedure. Pretreated zinc oxide samples were exposed for 5 min to t-butyl hydroperoxide maintained at 25" (the vapor pressure of TBHP at 25" is 11 mm); the sample was then removed and the esr spectrum was recorded with and without subsequent degassing. Essentially the same spectrum was obtained. Extending the exposure time up to 30 min produced no changes in the spectrum.
Results Oxygen-Treated Samples. Heating zinc oxide in oxygen at 1 atm for 2 hr at different temperatures in the range 100-500" does not completely eliminate the signal at g -1.96 (Figure la). The intensity of these signals increases somewhat on outgassing at room temperature for 30 min (Figure lb). Oxygen treatment is also observed to produce a signal at g 2.012. The addition of TBHP to the sample at room temperature causes a decrease in the intensity of the signal at g -1.96; however, the peak to peak width of the signal remains constant (Figure IC).
-'
Figure 1. Esr (x-band) spectra of zinc oxide: (a) heated in oxygen for 2 hr at 500'; (b) outgassed a t room temperature for 30 min; (c) exposed to t-butyl hydroperoxide (TBHP). DPPH (upper spectra) is used as a standard for the g value. The field increases from left to right. (8) K. A . Muller and J. Schneider, Phys. Lett., 4 , 2288 (1963). (9) T. Kwan, personal communication.
(10) J. Sohneider and A . Raober, Z . Naturforsch., A16, 712 (1961).
Volume 72,Number 7 Julg 1968
M. CODELL,H. GISSER,J. WEISBERG,AND R. D. IYENGAR
2462
av
b Y
Figure 3. Esr spectra of zinc oxide outgassed for 2 hr a t (a) 375', (b) 400°, (c) 450', (d) 525", and (a', b', c', and d') after the addition of TBHP. Figure 2. Esr spectra of ZnO: (a) outgassed a t 500'; (b) TBHP added; (c) outgassed a t 25' for 30 min; (d) Oxygen added (700 torr); (e) reoutgassed a t 25' for 30 min. All spectra were recorded at the same signal level.
Vacuum-Outgassed Samples. On subjecting zinc oxide to vacuum treatment a t 500" for 2 hr, a broad signal (7.6 G) is observed at g -1.96 (Figure 2a). On adding TBHP, the sample surprisingly showed the presence of two signals a t g values of 1.965 and 1.961, respectively (Figure 2b). A series of experiments with zinc oxide outgassed at different temperatures in the range 100-525" revealed that the formation of two signals at g -1.96 is observed only with samples outgassed a t temperatures above 350". The combined width of the two signals, however, appeared to be identical with the width of-thesignal prior to the addition of TBHP. The variation of the intensity ratio of the two signals (g 1.965 and 1.961) on subsequent oxygen treatment (Figure 2d) and vacuum treatment (Figure 2e) showed that the two signals were independent of each other and could not result from a single species. The esr spectra (signal a t g -1.96) of zinc oxide, outgassed a t 375, 400, 450, and 525", and changes in the spectra as a result of TBHP adsorption are shown in Figure 3. Apparently the intensity of the signal shows a decrease a t higher outgassing temperatures. The spectrum of the sample outgassed a t 525" in addition A indicates the presence of two signals a t g -1.96. decrease in the intensity of the signal at g -1.96 owing to the added TBHP is obvious for samples outgassed below 475" but is not so apparent at the higher temperatures, 500 and 525" (Figures 2 and 3). However, in all cases following TBHP adsorption, two signals are clearly observable. Zinc oxide vacuum outgassed for 2 hr at 500" and subsequently treated with oxygen at room temperature and reoutgassed shows the typical spectrum of 0 2 (Figure 4a). (This spectrum is not observed on samples heated in oxygen at 500".) The signal a t g -1.96 shows a decrease in intensity following the addition of oxygen, and two signals are not observed immediately. The Journal of Physical Chemistry
Figure 4. Esr spectra of zinc oxide vacuum outgassed for 2 hr a t 500': (a) 100 mm of oxygen added and reoutgassed 10 min a t 25'; (b) after standing for 24 hr following treatment a; (c) after standing for 70 hr following treatment a.
However on standing for 24 hr at room temperature, the sample shows the presence of two signals at g -1.96. After 70 hr, the intensity of these signals increased with a simultaneous decrease in the signal for 02-. It appears that 02- is undergoing a surface transformation to a different species. Samples which showed the presence of two signals at g -1.96 as a result of treatment with TBHP or oxygen exhibited a restoration of the broad signal at g -1.96 on being reoutgassed at 500" for 2 hr. The Combined Effect of Oxygen and Hydrocarbons. While the addition of oxygen at 25" to einc oxide vacuum outgassed at 500" does not immediately reveal the presence of two signals a t g -1.96, the presence of hydrocarbons in addition to oxygen leads to the immediate formation of two signals. In fact, it is not possible to differentiate between the spectra at g -1.96 of samples treated with hydrocarbon oxygen mixtures from those treated with TBHP. The results were
+
2463
ESRSTUDYOF HYDROPEROXIDE ON ZINC OXIDE Table I : g Values and Signal Widths of Zinc Oxide Subjected to Various Treatments Q
values Std
-
1.2 0.9 0.5 1.0 0.6 0.9 1.3
11 6 6 15 16 12 13
14 16
5.3 3.1
1.0 0.6
13 15
0.0005 0.0006
3 5
6.1 3.2
0.4 0.8
3 4
0.0006
12
3.7
0.8
12
Outgassed 2 hr at 500' Outgassed 2 hr a t 500'; 0 2 added; reoutgassed a t 25' Outgassed 2 hr a t 500"; exposed to TBHP vapors Outgassed 2 hr a t 500'; exposed to TBHP vapors; outgassed 0.5 hr a t room temperature Outgassed 2 hr a t 500'; exposed to TBHP vapors; outgassed 0.5 hr a t room temperature; 02 admitted Heated 2 hr in 0 2 a t 500' Heated 2 hr in 02 at 500'; exposed to TBHP vapors Heated 2 hr in 02 a t 500'; outgassed a t room temperature
1.9657 1,9660 1.9620 1.9651 1,9609 1.9654 1.9614
0.0007 0.0005 0.0007 0.0009 0.0007 0.0007 0.0007
16 6 6 16 16 16 16
1.9655 1.9617
0.0005 0.0007
1.9630 1,9606 1.9607
Discussion Schneider and RauberlO in their early experiments on vacuum-outgassed zinc oxide have reported a variation not only in the g values but also in the width of the principal signal. The observed g values varied from 1.956 to 1.962 and the peak to peak width of the signal ranged from 4.6 to 10.5 G a t 300°K and from 2.7 to 6.1 G at 77°K. The g values reported in Table I show fluctuations of a similar magnitude to those of Schneider and Rauber in the range 1.961-1.966. The signal widths in the present investigation ranged from 3 G, for samples vacuum outgassed at 500" and exposed to TBHP vapors, to 7.6 G, for samples vacuum outgassed at 500" for 2 hr. These variations in signal width are also similar to those reported by 8chneider and Raiiber. Kasai6 has attributed the scattering of g values at g -1.96 to the existence of two types of defects:
7
7.6 5.9 3.0 4.4 3.0 4.6 3.4
Treatment
No. of samples
essentially the same for C1to C4 hydrocarbons. Figure 5 shows the spectrum of zinc oxide treated with methane containing 2% oxygen.
to peak widtha-
No. of samples
dev
Figure 5. Esr spectrum of zinc oxide outgassed a t 500' and treated with methane containing 2y0 oxygen.
-Peak Width, G
Std dev
0
oxygen ion vacancies and halogen ions in substitutional positions (donors). The signal which he attributes to chloride ions in substitutional positions saturates at a power level of 16 mW. I n the present investigation, no saturation was observed for either of the signals at g -1.96 in the range 2.2-22 mW. It seems, therefore, that none of these signals can be due to chloride ions in substitutional positions, and the fluctuations observed in the present study do not result from halogen contamination. A comparison of the two signals at g -1.96 on vacuum-outgassed samples treated with oxygen (and reoutgassed at room temperature) with those obtained with the TBHP treatment shows that the g values compare within the limits of experimental error (Table I). The absence of any signal, with or without hyperfine structure, close to g -2 on samples exposed to TBHP vapors indicates that no free radicals are formed from TBHP or any of its decomposition products. These observations suggest that the signals produced by the addition of TBHP arise from the interaction of either oxygen (02)or oxygen ions derived from TBHP. Previous investigators have attributed the signal at g -1.96 to ionic vacancies or interstitial Zn+ ions. According to Thomas" most of the interstitial zinc in zinc oxide should be removed by oxygen treatment of the sample at 500" for a few minutes. Our observations that heating the sample in oxygen for 2 hr a t 500' does not remove the signal at g -1.96 clearly shows the presence of a defect other than Zn+. The experiments of Kasai6 suggest that the signal which survives the oxygen treatment can be attributed to electrons in oxygen vacancies. It appears that TBHP interacts
(11) D.G.Thomas, J . Phys. Chem. Solids, 3, 229 (1957). Volume 78, Number 7 July 1068
M. CODELL, H. GISSER,J. WEISBERG, AND R. D. IYENGAR
2464 with the oxygen vacancies to restore stoichiometry as in (CH3)3COOH + (CHs),O* 0-
+ e-
+ 0- + H+
(oxygen vacancy) +02-
(1) (2)
The H + ions can interact with lattice oxygen as in 02-
+ H+
OH-
(3)
The formation of 0- at the surface, for example, resulting from the TBHP decomposition illustrated above, would explain the observed decrease in the intensity of the signals in samples heated in oxygen in the range 100-500". The portion of the signal remaining after the addition of TBHP ( g 1.961) to the sample heated in oxygen probably arises from anion vacancies in the bulk not readily accessible to the 0-. A straightforward explanation of the other signal at g 1.965 is more difficult. It is possible that this signal is caused by zinc ions in interstitial positions. The absence of a hyperfine structure would then be due to the loosely bound electron in Zn+.lo An explanation can now be offered for the formation of two signals at g -1.965 and 1.961 either by oxygen treatment followed by outgassing or by TBHP treatment. The samples used were prepared a t approximately 1000" by the oxidation of zinc vapors and contain anionic vacancies which are not removed by the oxygen treatment at temperatures below 900°6 except at the surface. Vacuum outgassing a t temperatures above 350" can be expected to produce additional anionic vacancies and interstitial zinc, the majority of which are at the surface. Adsorption of TBHP to the vacuum-outgassed sample leads to removal of Zn+ as in Zn+
+ 0- (from TBHP) +Zn2+ + 02-
This reaction leads to a decrease in the signal at g 1.965 and takes place in preference to the competing reaction
0-
+ e- +02-
which leads to a decrease in the signal at g 1.961. Thus the decrease at g 1.965 is larger than the decrease at g 1.961 and the change in the relative intensity of the two signals leads to their resolution, making observation of the two signals possible. Further, according to the above, a decrease in the intensity of the signal at g -1.96 would normally be expected on the adsorption of TBHP. However, thic.,
The Journal of Physical Chemistry
is not obvious for samples outgassed at or above 500°, as seen in Figure 3. The reason for this behavior appears to be as follows: outgassing at higher temperatures produces a larger concentration of defects and the signal observed at g -1.96 shows a decrease in amplitude and is comparatively broad because of the ensuing dipolar interactions. This can be seen in Figure 3b-d as the outgassing temperature of the sample is increased from 400 to 525". If the subsequent interaction with TBHP reduces the concentration of the paramagnetic species just enough t o eliminate the dipolar interactions, the increase observed in the intensity of the resolved signals can be explained. The formation of two signals at g 1.96 on vacuumoutgassed samples subsequently exposed to oxygen (700 torr) and outgassed at room temperature is not immediate but occurs after a period of time. Simultaneously with the observation of two signals at g -1.96, a decrease in the intensity of the 02- signal (g -2) is also observed. It appears here that the 02ions are desorbed as oxygen molecules leaving behind electrons. Thus 02- (adsorbed) .--+ O2
+ e-
(to the surface)
The electrons are localized a t either the anionic vacancies or the Zn2+ions, thereby contributing to an increase in the amplitude of the signals at g 1.961 and 1.965, respectively, as shown in Figure 4. The resulting increase in the concentration of Zn+ and oxygen ion vacancies accompanied by some broadening of the signals appears to be responsible for the poor resolution observed. Zinc oxide vacuum outgassed and treated with a hydrocarbon containing oxygen (total pressure 700 torr) not only produced immediately the two signals corresponding to treatment with TBHP but also led to the formation of an 0 2 - species. Hence it appears that both an 0- and an 02- species are formed. A possible mechanism involves the formation of a hydroperoxide through the activation of the hydrocarbon by an active site on zinc oxide. A mechanism for the formation of hydroperoxide from hydrocarbons has been suggested and discussed by Ingold.12
Acknowledgment. One of the authors, R. D. I., wishes to acknowledge support by the Advanced Research Projects Agency of the Department of Defense, monitored by the Naval Research Laboratory under Contract No. NONR 610(09). (12) K. U . Ingold, Chem. Rev., 61, 563 (1961).
