1078
NOTES
metal treatment as were the methane and argon. The product yields given in Table I1 for 100% liquid ethane are in qualitative agreement with the results of Gillisll for the'radiolysis i f liquid ethane at higher temperatures.
Table I1 : Radiolysis of Liquid Ethane-Argon at
%
CZHE converted? %
100 55. 8d 10.0 10.0 10.0 10.0 0.89 0.10 0.048
0.1 0.2 0.03 0.06 0.12 0.18 1.4 5.6# 1.4
CZHf, electron
- 182'
CHI
CzHi
CsHs
n-C~Hlo
0.33 0.49
0.89 1.03 0.15 0.12 0.09 0.10 0.00 0.00 0.00
0.24 0.30 0.29 0.29 0.28 0.34 0.14 0.12 0.12
1.81 1.57 1.00 0.99 0.86 1.10 0.59 0.40 0.39
... ... ... ... ...
0.046
...
Inorganic Photosensitizer
by W. C. Tennant Chemistry Division, Department of Scientific and Industrial Research, Wellington,New Zealand (Received October 19, 1967)
Q value^^^^-----
I
The Action of Lead Monoxide as an
These values are merely estimates made in order to indicate the approximate amount of ethane decomposed in each experiment. They are based on a hypothetical value of 10 for G(-&He) (see footnote e ) . * G values are based on the total energy absorbed by the solutions. ' Acetylene was detected in several experiments. I n all cases C(C2H,) was less than 0.1. A product, probably butenes, was produced with a G value of 0.08. This conversion was experimentally determined. It corresponds to a value of 3.3 for G(-C2HB). This suggests that some high molecular weight products may have been formed.
The radiolyses of argon solutions containing less than 1% of methane, kthane, or ethylene provide interesting data. The initial absorption of energy in these solutions is almost entirely by argon. Yet it is obvious that, in each case, a large fraction of the absorbed energy is ultimately utilized to destroy the hydrocarbon solute. For all solutes, the G value of each product was constant over the range 1-0.0570 solute. Thus, it has been shown that the constancy of G values at low solute concentrations is not peculiar to the ethylene solutions studied previously.3 It seems that, at low hydrocarbon concentrations, energy transfer occurs through one or more long-lived species which can react completely even with 0.05% hydrocarbon. The constant G values seem to preclude argon ions as the energy-transfer species, since geminate recombination of argon ions and electrons should compete with charge transfer to hydrocarbon solute in the low-concentration range. For this reason, it was stated previouslya that electronically excited argon is the most likely energy-transfer species at low solute concentrations.
I n a recent paper,' a mechanism was proposed for the zinc oxide photosensitized photolysis of PbC12 and certain other inorganic salts in aqueous mixtures. Photolysis was shown to result from electron transfer from ZnO to the decomposing molecule following the absorption of radiation by the sensitizer. Aqueous interaction of the two components is a necessary prerequisite to the electron transfer. Lead monoxide also undergoes aqueous interaction with many heavy-metal salts,2 it has a favorable absorption spectrum3 and possesses n-type photoconducting proper tie^.^ Hence it might be expected that PbO could parallel ZnO in its action as an inorganic photosensitizer, although no instances have apparently been reported in the literature. This note discusses the action of lead monoxide in photosensitizing the photolyses of PbClz, PbBr?, and HgClz in aqueous mixtures. The photosensitizing action is shown to be a function of the ye1lobT form of lead monoxide only, and possible reasons for this are discussed.
Experimental Section A Norelco X-ray diffractometer was used to study the products of aqueous interaction of PbO with PbCI2, PbBr2, and HgCl2. The energy dependences of the photolyses studied were determined with a Philips 120-W medium-pressure quartz mercury lamp in conjunction with the appropriate color filters. Absorption spectra of the red and yellow forms of PbO and of the three salts studied were measured with a Beckmann DU spectrophotometer, using the standard reflectance attachment and using magnesium oxide as the reference. The spectra are shown in Figure 1. The extent of decomposition of irradiated mixtures was taken as proportional to the blackening produced and was measuredo as the percentage decrease in reflected light at 7000 A, where the salts and both forms of PbO are completely reflecting.
I
(1) W. C. Tennant, J. Phys. Chem., 70, 3523 (1900). (2) H. R. Oswald and W. Feitkneckf, Helv. Chim. Acta, 44, 847 (1961). (3) C . F.
(11) H. A.
Gillis, J. Phys. Chem., 67, 1399 (1903).
The Journal of Phyaical Chemistry
Goodeve, Trans. Faraday Soc., 33, 340 (1937). (4) L. Neijne, Philips Res. Rept. Suppl., No. 4 (1961).
1079
NOTES
1C0
-
z
u
w
V
60
V
w
-1
a
50
20
10
-
0
4000
3000
6000
SO00 WAVELENOTH
7000
(i)
Figure 1. Absorjhion spectra of solid powders.
reacts with PbClz and PbBrz to form Pb(0H)Cl and Pb(OH)Br, respectively. A stoichiometric equation of the form
5 o r v1
W
PbO
r?
9a
+ PbXz + HzO + 2Pb(OH)X
(X = C1, Br)
(1)
30-
a
RED PbO (per cent)
0
20
40
60
80
YELLOW PbO (per c e n t )
Figure 2. Relation between PbO-PbBr, blackening and the crystal form of PbO.
Results and Discussion X-Ray examination of wetted powder mixtures showed that the yellow orthorhombic form of PbO
can be proposed, although the reactions are never complete. X-Ray powder patterns of wetted yellow PbO-HgClz mixtures showed almost complete disappearance of PbO and HgClz and the formation of a new compound (or compounds) which, however, could not be identified from the ASTRiI powder file. However, a reaction similar to eq 1, with the formation of a basic chloride of mercury, appears almost certain to occur. The red tetragonal form of PbO also reacts with the three salts in aqueous mixtures, but the reaction products were in each case amorphous and could not be identified by X-rays. Wetted mixtures of yellow PbO with PbC12, PbBrz, or HgClz blackened markedly within 10 min when exposed to light of wavelength shorter than about 4300 A. Each of the three salts alone is decomposed by radiations on the short-wavelength side of its absorption edge Volume 7!2, Number 8 March 1068
1080
(see Figure 1). The photosensitized reactions only took place in mixtures which had been wetted, and only the yellow form of PbO was effective as the photosensitizer. This is shown clearly in Figure 2, where the photosensitized decomposition (blackening) of PbBr2 in admixture with various proportions of red and yellow PbO is shown to be nearly proportional to the amount of yellow oxide present. From the description of the photochemical reactions given above, it is clear that the photosensitizing action of the yellow form of PbO closely parallels that of Zn0.l In the case of ZnO photosensitization, the decomposition of lead chloride arises from electron transfer from the ZnO to PbCl2 and this only takes place following aqueous interaction of the two. It was proposed' that this aqueous interaction was necessary to lock the two components rigidly together in a solid mass of basic chlorides, thus providing an efficient contact for electron transfer to occur. An entirely similar interpretation is obviously applicable
The Journal of Physical Chemistry
NOTES to the PbO-photosensitized photolyses of PbC12, PbBr2,and HgC12. The ineffectiveness of the red form of PbO as a photosensitizer is, a t first sight, somewhat surprising, since both forms of PbO are strongly absorbing in the photoactive region (see Figure 1) and both are known to exhibit n-type photoconductivity, at least to some extente4v6 However, in the case of the tetragonal PbO, the photoconductivity has its maximum value at which is outside the photosensitizing about 6000 range, and hole conduction is predominant.6 A further possibility, as suggested by the X-ray evidence, is that aqueous interaction of the red form with the salts studied does not result in efficient contact between the sensitizer and decomposing crystal, so that electron transfer does not readily occur. ( 5 ) V. A. Izvozchikov and G. A. Bordovskii, Rokl. Akad. Nauk SSSR, 145, 1253 (1962). (6) 0. I. Kolesova, Chem. Abstr., 58, 6302e (1963).
