The Journal of Physical Chemistry, Vol. 82, No. 8, 1978 969
Communications to the Editor
-
following reaction scheme. R-C
//O \
OCHXY
-
0-
R-C
(10)
0-
R-C
\
A t B
f
vis h v
.f
(7) (8) (9)
OCHXY
Department of Chemistry University of Tennessee Knoxville, Tennessee 379 16
t CHXY
\\0
which correspondto the most intense components of the ethyl radical spectrum, S. Noda, K. Fueki, and 2 . Kuri, J . Chem. Phys., 49, 3287 (1968). J. E. Bennett and B. Mile, Trans. Faraday Soc., 87, 1587 (1971). A. Faucitano and F. Faucitano Martinotti, J. Chem. Soc., Perkin Trans. 2, 1563 (1973). V. I.Mal'tsev, B, N., Shelimov, and A. A. Petrov, Zhur. Org. Khim., 4, 545 (1968).
A
Reggle L. Hudson" Ffrancon Williams"
Received January 11, 1978
0 /I
R-C
/p
/'
2 8 0 < X < 420nm
>R-C
\
BO
C
X
Y
L
C
+X
:I
/ \
Y
(2)
R* + CO t XCOY X a n d Y = H and/or alkyl groups
Although the overall reaction represented by (2) has been previously reported, the intermediacy of the acyl radical was not recognized owing to the unselective nature of the photobleaching experiments.lt2 In fact, it has been suggested that the photobleaching of the hydrogen-loss radical from methyl acetate to give the methyl radical proceeds by a carbene mechanism.1° The mechanism proposed here also resolves the apparent contradictions in the previous work. Thus, methyl radical production from methyl pivalate by photobleaching immediately after y-irradiation1 is attributable to reaction 1, whereas the generation of tert-butyl radicals on photobleaching the y-irradiated sample after it had been allowed to stand at 77 K2 can be explained by reaction 2. In the latter case it seems likely that the radical anion had decayed and hydrogen-loss radicals were mainly present before photobleaching. Also, the nearly exclusive production of R'. radicals by UV photolysis of N,N,N',N'tetramethyl-p-phenylenediamine (TMPD)-ester mixtures2 is readily understandable in terms of TMPD photoionization and radical anion formation followed by reaction 1. In summary, we feel that consideration of the two pscission reactions 1 and 2 allows a correct and unified interpretation of the photobleaching reactions of y-irradiated carboxylic esters in the solid phase. We also note that the reactions are independent of whether the trapped radicals are produced in the crystalline or glassy phase. Acknowledgment. This work was supported by the Division of Basic Energy Sciences, US.Department of Energy (Document No. 0R0-2968-112).
References and Notes Y. Nakajima, S. Sato, and S. Shida, Bull. Chem. SOC.Jpn., 42, 2132 (1969). P. B. Ayscough and J. P. Oversby, J . Chem. SOC.,Faraday Trans. 1 , 68, 1153 (1972). J. Kroh, A. Plonka, and K. Wyszywacz, Bull. Acad. Pol. Sci., Ser. Sci. Chim., 25, 295 (1977). F. J. Adrian, E. L. Cochran, and V. A. Bowers, J . Chem. Phys., 38, 1661 (1962). On standing at 77 K, the signal from the methyl radical decays and this is accompanied by a growth in the intensityof the CH,CHC(0)OCH3 spectrum. Such hydrogenatom abstraction reactionscommonty occur in irradiated organic solids even at 77 K and below; see R. L. Hudson, M. Shiotani, and F. Williams, Chem. Phys. Lett., 48, 193 (1977), and references cited. In ref 3 the spectra of y-irradiated ethyl acetate taken after photobleaching with visible light show two lines separated by ca. 27 G
0022-365417812082-0969$01,0010
Formation of 0- from O2 on Molybdena-Alumina Catalysts Publication costs assisted by the National Science Foundation
Sir: The 0- ion may function as an intermediate in oxidation and exchange reactions over metal oxide catalysts1 and several EPR studies have been made of this ion stabilized on oxide surfaces. The conventional method for producing 0- involves dissociative adsorption of NzO on the reduced or irradiated oxide surface. Experiments with I7O labeled N 2 0 have confirmed that the observed spectrum is due to a monatomic form of oxygen on Mg02 and on silica supported MOO^.^ If 0- does indeed function as an intermediate in oxidation and exchange reactions, as postulated, this species must necessarily be formed from molecular 02.A spectrum obtained from O2 on supported V205 has been assigned to 0-,4but the presence of overlapping vanadium hyperfine lines prevented complete identification in this case.5 Evidence is presented herein demonstrating the formation of 0- when molecular O2 is adsorbed on a reduced molybdena-alumina catalyst. These observations have considerable significance in view of the importance of supported molybdena catalysts for oxidation reactions. Samples of the same 8% molybdena-alumina catalyst were prepared and reduced in hydrogen as described previously.6 Conventional controlled high vacuum procedures were used. Exposure of the reduced catalysts to O2 at 93 K produced a signal at both X- and &-band frequencies, identical with that previously assigned7 to the 0, ion stabilized on Mo6+centers. Exposure to O2at room temperature gave a X-band spectrum identical with that previously assigned7>*to a combination of signals from 02on Mo6+ and on exposed A13+ ions. When the latter spectrum was recorded at &-band, however (Figure la), a third low-field maximum appeared which was not evident in the previous (X-band) work. The signal responsible for this new feature could be separated from the Oz- signals by evacuating the gas phase oxygen at room temperature and heating the sample very slowly over a period of 6 h to a final temperature of 423 K. As shown in Figure l b , this treatment almost completely eliminated both of the 02-species, leaving an axially symmetric signal with parameters given in Table I. A t 93 K, a six-line hyperfine pattern with a splitting of 3.4 G centered on gllcould be resolved; this is seen most clearly in the second derivative spectrum (Figure IC). Comparison of the parameters in Table I with the values reported for 0- formed by decomposition of NzO on the surfaces of molybdena-silica catalyst^^,^^^ and with the predicted values for 0- in an environment of tetragonal symmetry1 has led us to conclude that this species must be responsible for the axially symmetric signal of Figure 0 1978 American Chemical Society
970
The Journal of Physical Chemistry, Vol. 82,
No. 8, 1978
say at present whether or not 02-is a necessary intermediate in the formation of 0-,although we were unable to form the latter by dissociative adsorption of NzO. The 0- species observed here does not show the high reactivity found for 0- formed from NzO on molybdena-silica. For example, the signal was not affected by exposure to CO or H2at room temperature. The '0- is located at the surface, however, since the signal was reversibly broadened by exposure to a high pressure of oxygen. Attempts to confirm the identification of 0- by use of "0 labeled oxygen were unsuccessful. Adsorption of 59 % 170enriched oxygen at room temperature gave hyperfine lines associated with I7O1'O- and 170160-, but after heating species, only a I 6 0 - signal remained. to remove the 02Partial exchange between 170-and lattice oxide ions has been reported for molybdena-~ilica,~and complete exchange for zinc oxide.1° For the molybdena-alumina system, we envisage initial formation of 0- at an oxide vacancy adjacent to a molybdenum ion, followed by migration of the resulting hole to an oxide ion adjacent to an aluminum ion:
2.024
2.037
i'f"
m
a.
6 I
50G
2.008
Communications to the Editor
N
IR
1 7 0 -
+
1602-
~
1701-
+ 160-
I
Figure 1. 35-GHz EPR spectra of oxygen radical anions formed on the reduced surface of molybdena-y-alumina (8 % Mo). (a) composite 0,-/Mo6+, and 0-observed after the room spectrum of O;/A13+, temperature adsorption of oxygen. (b) 0-spectrum observed after slowly heating the sample to 423 K in vacuo. (c) Expanded second derivative recording of the high field region of spectrum b. Spectra a and b were recorded at room temperature and spectrum c at 9 3 K.
TABLE I: EPR Parameters of Oxygen Species on an 8% Molybdena-Alumina Catalyst Species
g,
g2
0,-/A13+
2.037 i 0.002 2.017 2.008 (gli)
2.010
0,-/Mo6+
O-/A13+
2.008
g3 2.002 2.006 2.024
The failure of earlier attempts to stabilize 0- on alumina supported transition metal oxides was attributed to the lack of tetrahedrally coordinated ionsa4This is inconsistent with the observations reported here.
Acknowledgment. The EPR spectrometer was purchased through NSF Departmental Grant No. MPS7506216. This work was supported by grants from the National Science Foundation (No. CHE-74-11539 A02) which we gratefully acknowledge. References and Notes
(a)
lb. The principle components of its g tensor were not identical with those reported for 0- on molybdena-silica, however, and the observed six-line hyperfine pattern centered on gI1did not have the intensity distribution expected for interaction of the unpaired electron with the non-zero-spin nuclei of 98Mo ( I = 5/2, 25% natural abundance). On the other hand, the observed pattern was consistent with interaction with a 27Alnucleus ( I = 5/2, 100% natural abundance). Thus, we infer that the 0- ion is stabilized on exposed A13+ions of the alumina support. Therefore, the interaction of O2with our catalyst involved three distinct paramagnetic species. At 93 K, electron transfer from Mo5+ sites to O2 produced the 0
