COMMUNICATIONS TO THE EDITOR
3172 h
I
c
I
1
I
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1
I
1
t -> a
e
0
500 U
0
f
I
I
I
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220 240 WAVELENGTH ( n m )
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260
Figure 1. Absorption spectrum of the hydrated electron a t 25’ from pulse radiolysis of: 0, 2 X M NaOH 0.027 M Hz (vertical bars give standard deviation and the curve Y NaOH 0.027 M HS (after gives best fit); A, radiation clean-up); . , from ref 1.
+
+
only H, e,,-, Hz, H202, and OH- could still be present a few microseconds after the electron pulse. Under our conditions OH reacts with H2 to form H (half-time 0.5 psec) and Hs0+ reacts with OH- (half-time 0.3 psec). The pulse-radiolytic absorbance transients were extrapolated back to the end of the electron pulse (disregarding essentially the first 2 psec after the electron pulse) and corrected for radical recombination during the pulse. The absorbances obtained were corrected for those due to H , 2 J H z O ~and , ~ OH- using G(H) = 3.2, G(H202) = 0.7, and G(-OH-) = 2.6. The calculated molar decadic absorptivity, of en,- using G(e,,-) = 2.6 is given by the curve and the points 0 in Figure 1. The values of determined similarly by M NaOH 0.027 M Hz (not pulse radiolysis of purified) after radiation clean-up at low dose rates were in good agreement with the points 0 in Figure 1 except at 200 nm where values were found to be consistently higher than those in Figure 1. The values of ee given by the curve in Figure 1 agree fairly well with those of Fielden and Hart above 220 nm (points in Figure 1) and with obtained by repeating their experiments with nonpurified Hz and radiation clean-up at pH 12 (points A in Figure 1). It has previously been showna that only very weak pulse-radiolytic absorbance transients at 200-250 nm are present in the pressure cell windows used. If new transients produced from soluble impurities were the cause of the increased absorptivity at 200 nm shown in Figure 1, they would have to be present in concentrations of a t least 5 X 10-5 M in order to have reacted substantially with H, OH, and ea,- in a few microseconds after the pulse. Such high concentrations of impurities are unlikely in view of the triply distilled water, analytical grade NaOH (diluted from a concentrated, stored solution), and purified H2 (passage through 2 m of molecular sieve at -196”) used. Fur-
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The Journal of Physical Chemistry
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m
200
thermore, after 15 identical electron pulses were given to the same solution it was found that the transient absorbance following the 15th pulse was identical within experimental error with the absorbance following the first pulse. The assignment of the increased absorptivity at 200 nm in Figure 1 to ea,- is consistent with H30+ --f the observation that the reaction ea,H HzO, when monitored at 200 nm in dilute HC1O4, is accompanied by only a relatively small change in absorbance. BH at 200 nm was previously measured to be 900 f 30 M-’ ~ m - l . ~ JWe therefore assign the new absorption band with maximum below 200 nm in Figure 1 to ea,- rather than to some unknown species not listed on the right-hand side of reaction scheme 1. The proposed new uv absorption band of the hydrated electron is similar in appearance around 200 nm to an absorption band associated with H atoms in aqueous solution2J and a similar interpretation of it may be offered. We attribute this new absorption band of e,,- to a red shift of the liquid water absorption band beginning at -200 nm caused by the presence of ea,-, The perturbation may involve a partial electron transfer from the first excited singlet of water to a “neighboring” ea,-. The uv absorption band of ea,- here proposed is analogous to the p bands associated with F centers in alkali A similar band may exist in metal-ammonia solutions.8 We wish to thank Professor J. Jortner for calling these p bands to our attention. (5) Landolt-Bornstein Tables, 6th ed, Vol. I, “Atomic and Molecular Physics,” Part 3 “Molecules 11,” Springer-Verlag, Berlin, 1961, p 231. (6) C. J. Delbecq, P. Pringsheim, and P. Yuster, J . Chem. Phys., 19,574 (1951); 20,746 (1952). (7) J. J. Markham, Solid State Phys., Suppl., 8, 106 (1966). (8) J. J. Lagowski and P. Rusch, private communication.
DANISH ATOMICE N E R G Y COMMISSION RESEARCH ESTABLISHMENT Rise, DENMARK
S. 0. NIELSEN P. PAQSBERG
ARGONNENATIONAL LABORATORY ARGONNE,ILLINOIS
EDWINJ. HART
AXTIEBOLAGET ATOMENERGI SWEDEN STUDSVIK, NYKOPING,
H. CHRISTENSEN G. NILSSON
JUNE12, 1969 RECEIVED
Paramagnetic Resonance and Phosphorescence of Chromia-Silica Catalyst with
Chemisorbed Oxygen’
Xir: Electron paramagnetic resonance (epr) techniques have revealed the presence of several distinct states of chromia adsorbed on high surface area alu(1) Based on work performed under the auspices of the U.S. Atomic Energv Commission.
3173
COMMUNICATIONS TO THE EDITOR
VI
A 6 -PHASE 0 y-PHASE (STABLE)
16
+ > 2; t-
z W
I
0
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200 400
I
1
600
600
1
,
,
I
,
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/
0 '
1000 1200 1400 1600 I600 2000 2200 2400
E
PlOzLp
Figure 3. Calibrated intensity of the y-phase (stable) resonance and relative intensity of the &phase peak a t 1500 G OS. P(0n).
I, !
v
Figure 1. Epr first derivative spectra a t 9.129 GHz of ( a ) 6 phase (b) y phase, P(Oa)= 7.7 Torr, (e) y phase, P ( 0 z ) = 1.0 Torr. Field markings are in G. Resonances on the right of or 0 (b) and (e) are tentatively ascribed to 02on the surface.
i
/\
SbC1
\,/
13400
pni
'iI h
$200
I
\ Figure 2. Epr first derivative of stable y phase a t P ( 0 2 )= 1 3 2 ~ .
mina.2-4 At low concentrations of chromia, magnetically isolated Cra+ions are predominant (6 phase) ; a t higher concentrations clusters of Cra+ ions occur (p phase) which are antiferromagnetic with a distribution2 of Curie temperatures. Upon oxidation with oxygen the valence state Cr5+ is produced (y phase) which has a relatively sharp epr absorption line and partially resolved g-factor anisotropy. I n the present work the paramagnetic resonance of chromia (0.53 wt % Cr) dispersed on a high surface area silica aerogel (BET area = 600 m2 g-') was studied as a function
of oxygen pressure over the system which had been reduced in hydrogen a t 500". Epr spectra of the catalyst were generally taken a t -180" to suppress the resonance of the ,&phase chromia and allow precise measurements on the y-phase signal, Spectra similar to those given in Figure 1 and 2 were measured a t -180" after exposure to various pressures of oxygen a t 25". The sample was exposed to oxygen at 25" and the time following the exposure was recorded. The peak in the derivative at 1500 G is due to the &phase chromium. The intensity of this peak decreases with increase in oxygen pressure in the range of 20-2400 p of oxygen as shown in Figure 3. This behavior is i n sharp contrast to the 6 phase of chromia-alumina which remains essentially unaltered in intensity even after exposure to oxygen a t 500°.3 In the vicinity of 3300 G, spectra similar to those of Figure l b and IC were observed. The y-phase resonance in Figure l b consisted of two partially resolved components: y phase (stable) and y phase (unstable). The y phase (stable) remained constant in intensity and has a powder pattern indicating axial symmetry with gL = 1.970 and gll = 1.92. The y-phase (unstable) resonance intensity decayed exponentially with time for initial oxygen pressure (P(O2)) less than about 250 p and appears to have axial symmetry with gL = 1.952 and gll = 1.92. The half-life of the y-phase (unstable) resonance increased with increase in P(02) (
