429
Communications to the Editor
Acknowledgment. We appreciate advice on this study by Alfred Buchler and David J. Meschi. This work was supported by the U.S. Energy Research and Development Administration. References and Notes (1) See, for example, “Chemistry of the Solid State”, W. E. Garner, Ed., Butterworths, London, 1955. (2)R. D. Shannon, Trans. Faraday SOC.,60, 1902 (1964). (3)D. A. Young, “Decomposition of Solids”, Pergamon Press, Oxford, IQdR
(4)K.H.‘ Stern and E. L. Weise, Natl. Stand. Ref. Data Ser., NaN. Bur. Stand., No. 30 (1969). (5) D. Beruto and A. W. Searcy, J. Chem. Soc., Faraday Trans. 1, 70, 2145 (1974).
(6) P. Mohazzabi and A. W. Searcy, J. Chem. Soc., Faraday Trans. 1, 72,
290 (1976). (7)L. S.Darken, :[an& AIM€, 160, 430 (1949). (8)A. W. Searcy, Proceedings of the Second International High Temperature Chemistry Symposium”, Asilomar, Calif., Oct 1959, Wiley, New York, N.Y., 1960,pi57. (9)A. W. Searcy in “Chemical and Mechanical Behavior of Inorganic Materials", A. W. Searcy, D. V. Ragone, and U. Colombo, Ed., Wiley-lnterscience, New York, N.Y., 1970,p 2. (IO)See, for example, K. S. Pitzer and L. Brewer, “Thermodynamics”, McGraw-Hill, New York, N.Y., 1961 Chapters 14, 17,and 20. (11)A general solutlon of the rate equation and an alphabltical list of symbols are available in “Appendices to The Kinetics of Endothermic Decomposition Reaction: l Steady State Chemical Steps” by A. w. Searcy and D. Beruto, LBL-3137Review Supplement. (12)A. W. Searcy, A. Buchler, and D. Beruto. HlQhTemp. Sci., 8,64 (1974). (13)A. W. Searcy in ref 8, Chapter 6. (14)A. W. Searcy and D. Beruto. J. Phys. Chem., 78, 1298 (1974). (15)T. K. Basu and A. W. Searcy, submitted for publication.
COMMUNICATIONS TO THE EDITOR
Electron Paramagnetic Resonance Study of the Diphenylketyl Radlcal at Low Temperatures
the second-order decay of the ketyl radical disappearance was observed at temperatures higher than 140 K, which was expected from the combination reaction of two ketyl Sir: The diphenylketyl radical is known to be an important radicals to form pinacol.2 In this method, no solvent radical intermediate in the photochemistry of ben~ophenone.l-~ was observed. (2) A t 77 K, diphenylketyl radicals and solWe have recently reported the formation mechanisms of vent radicals were obtained by high intensity light irradiaaromatic ketyl radicals in the ultraviolet photolysis of tion (A >300 nm). This process is thought to be a biphotonbenzaldehyde, acetophenone, and benzophenone at low ic process through the higher excited triplet state of benzot e m p e r a t ~ r e s In . ~ this paper, intensive EPR studies of the phenone. By raising the sample temperature to -130 K, diphenylketyl radical have been carried out using benzothe solvent radicals disappeared, and the EPR spectrum of phen~ne-carbonyl-~~C at temperatures from 77 to 140 K. the ketyl radical remained. Both these reaction paths 1 and The reversible temperature dependence of the EPR spec2 were previously discussed in detail.4 trum was observed. The isotropic and anisotropic 13C hyWhen benzophen~ne-carbonyl-~~C in methanol was phoperfine coupling parameters have been estimated from tolyzed, the EPR spectrum of the diphenylketyl radical analysis of the EPR spectra. with carbon-13 hyperfine splitting was obtained in both The EPR spectrometer used was a conventional X-band methods 1and 2. The spectrum was transformed reversibly type (JEOL JES-3BS-X) operated with 100-kHz modulaby changing the temperature between 77 and 138 K, which tion. Methanol was mainly used as solvent, and ethanol, 2is shown in Figure 1. However in the case of benzophenonepropanol, and EPA were also used. Benzophenone (natural carbony1-l2C, only a broad singlet spectrum was obtained abundance of isotopes) and benzophen~ne-carbonyl-~~C and a significant spectrum transformation by temperature (91.9 atom % 13C) were used as solute. The concentration of was not observed. Similar results were also obtained in the benzophenone was M. The solutions were decases of ethanol, 2-propanol, and EPA solvents. The triplet gassed with a high vacuum system. The irradiation source spectrum at 77 K shown in Figure 1 is understood by aswas a high-intensity high-pressure mercury lamp and a suming the anisotropy of Ph213COH radicals in rigid solHalio glass filter which transmitted light of wavelength vent. From this spectrum, ail(13C) was estimated at about longer than 300 nm was used. 50 G and a1(13C) was thought to be below 15 G . There exDiphenylketyl radicals were formed in the following ists an interesting phenomenon. In spite of the averaging of ways. (1) A t -130 K, the radicals were produced by light irthe anisotropic spectrum by raising the sample temperaradiation of wavelength longer than 300 nm. Under this ture, the interval between both side peaks of the first derivcondition, diphenylketyl radicals were easily formed by the ative of the EPR absorption spectra did not apparently hydrogen abstraction reaction through the lowest triplet show a remarkable change. The ketyl radical does not show state ( 3 n , ~ * of ) benzophenone. Stopping the irradiation, the spectrum of complete free motion under this condition,
-
The Journal of Physlcal Chemistty, Vol. 80, NO. 4, 1976
430
Communications to the Editor 3 0 gauss
47-
Mechanism of Catalytic Reactlon between NO and NH3 on V2O5 in the Presence of Oxygen Publication costs assisted by the Department of Chemistry, The Universityof Tokyo
iT"J77K
A
w'28K
V Figure 1. EPR spectra of ketyl radical (Ph213COH)in methanol solution at various temperatures.
since this type of radical is too large and complicated to move freely in methanol solution in this temperature range. The value of aiS0(l3C)of the diphenylketyl radical was estimated at 17-27 G from the values of a ~ l ( ' ~ Cand ) a1(13C). The EPR spectrum at 138 K is explicable using this value. In the cases of H&OH,5 H(OH)CCOOH, and (H0)2CCOOH,6 u ~ , , ( ~ ~was C ) reported to be 47.4, 33, and 29 G, respectively. In comparison with those results, the value of ai,, estimated above is reasonable. The spin densities on 13C are ps 0.02 and 0.4 < pp < 0.5 assuming literature values for atomic orbital coupling^.^ This result shows that the contribution of an unpaired electron to the s orbital is very small; that is, the bonds of the a carbon of the diphenylketyl radical in rigid media are almost sp2 hybridized.
-
Acknowledgments. We are very grateful to Professor I. Tanaka for support and encouragement. We also acknowledge with pleasure the useful discussion with Dr. K. Shimokoshi in particular.
References and Notes (1) G. Porter and F. Wilkinson, Trans. Faraday Soc., 57, 1686 (1961). (2) A. Beckett and G. Porter, Trans. Faraday SOC.,59, 2038 (1963). (3) R. WiIson,'J. Chem. SOC.B, 84, 1581 (1968). (4) H. Murai and K. Obi, J. Phys. Chem., 79, 2446 (1975). (5) A. J. Dobbs, B. C. Gilbert, and R. 0. C. Norman, J. Chem. SOC.A, 124 (1971). (6) L. Bonazzola, C. Hesse-Bezot, and J. Roncln, Chem. Pbys. Left, 20, 479 (1973). (7) P. 8. Ayscough, "Electron Spin Resonance in Chemistry", Methuen, London, 1967, p 438. Department of Chemistry Tokyo Institute of Technology Ohokayama, Meguro-ku Tokyo, Japan Received October 6, 1975 The Journal of Physical Chemistry, Vol. 80. No. 4, 1976
Hlsao Mural Mamoru Jlngujl Klnlchl Obl.
Sir: The catalytic reduction of NO to form nitrogen offers a significant issue for environmental sciences and many varieties of reaction systems have already been studied. Otto and Shelef proposed the mechanism of the reduction of NO by NH3 over Pt and CuO, in which ammonia is dissociatively adsorbed on the catalyst as NH2(ad) and H(ad) reacting with adsorbed NO(ad) via the LangmuirHinshelwood mechanism.lI2 It is generally accepted that the reaction between nitric oxide and ammonia is markedly accelerated by the addition of oxygen. To explain this fact, Markvart and Pour conjectured that the effect may be due to the acceleration of the dissociative adsorption of ammonia by ~ x y g e n The . ~ mechanism of this reaction, however, has not been established. In this communication we will propose a new mechanism of the reduction of nitric oxide by ammonia on V2O5 in the presence of oxygen. V2O5 has a very high activity as well as selectivity to form N2 molecule and is not easily poisoned by gases such as SO2 which are frequently contained in the reacting gas in practical use. For elucidating the reaction mechanism, we separately studied the elementary steps of the reaction, by using volumetric, infrared, x-ray photoelectron spectroscopy, and mass spectrometry techniques. A commercial V2O5 (Nakarai Chemical, special grade) and alumina supported V205 (V205/A1203) were used as catalysts. The V205/A1203 was prepared by impregnating alumina with a V2O5 saturated solution of oxalic acid, heated under vacuum at 400 "C for 2 h and then oxidized in 100 Torr of oxygen at 400 "C for 1 h before use. The x-ray photoelectron spectra of these catalysts after pretreatment showed only V and 0 for V205 and Al, V, and 0 for V2O5/ A1203 (V5+, 02-).No contaminants that would influence the reaction were observed on the catalyst surface except for a small amount of carbon. The infrared measurements of the adsorbed species were carried out by means of a circulating system equipped with an ir cell as described previo~sly.~ The ir spectra after adsorption of ammonia on V205, A1203,and supported V2O5 (v205/&03) are shown in Figure la-c. The adsorbed ammonia on the V2O5 surface exhibited a strong absorption band at 1413 cm-l. This band was assigned to adsorbed NHd+(ad) on VzO5, since the infrared spectrum of meta ammonium vanadate (NH4V03) also showed a very strong band at 1410 cm-l due to NH4+ and the binding energy of nitrogen 1s electron in the adsorbed ammonia on the V2O5 surface obtained by x-ray photoelectron spectroscopy was exactly the same as that of NH4V03. The other two bands at 1610 and 1275 cm-l in the ir spectrum are due to adsorbed "dad) on ~ - & 0 3 , as Eischens et al. have already r e p ~ r t e d . ~ When oxygen was introduced onto the ammonia adsorbed V&,/A1203 surface, the absorption band of NHs(ad) on y-A1203 decreased in intensity, while NHA+(ad) on V2O5 increased. This indicates that "dad) on y-A1203was expelled from A1203and was adsorbed on the V2O5 surface as NHd+(ad) by the introduction of oxygen. No adsorption of NO was observed on the v205 catalyst surface, even when the surface was oxidized by oxygen or
