Matrix Reactions of Bromine Atoms and NO2
The Journal of Physical Chemistry, W . 83, No. 17, 1979 2217
Matrix Reactions of Bromine Atoms and NO, Molecules David E. Tevault Chemistry Division, Naval Research Laboratory, Washington, D.C. 20375 (Received February 1, 1979) Publication costs assisted by the Nwal Research Laboratory
Argon matrix reactions of atomic bromine and NOz produce nitryl bromide, BrN02. Near-ultraviolet photolysis of the matrices effects growth of the oxygen-bonded isomer, bromine nitrite, ONOBr. Structural identifications of the products have been obtained by infrared spectroscopy with the aid of nitrogen-15 and oxygen-18 isotopically enriched NOz reactions with bromine atoms. The photolysis behavior of bromine nitrite contrasts that observed for fluorine and chlorine nitrites.
Introduction Nitryl fluoride and chloride are stable, well-characterized gases at room temperature. The gas phase infrared spectra of these compounds and their nitrogen-15 isotopic counterparts have been reported.' Several attempts to prepare and isolate the analogous nitryl bromide have met with little S U C C ~ S S . ~ -Efforts ~ to generate BrNOz in situ as a possible Friedel-Crafts type nitrating reagent were unsuccessful due to excessive cleavage of the N-Br bond during r e a ~ t i o n .Because ~ of the apparent weakness of the bromine-nitrogen bond of nitryl bromide, the equilibrium between bromine, NO2, and nitryl bromide lies strongly in favor of bromine and NO2 in the gas phase at room temperature. Only two reports of the possible detection of B r N 0 2 have been made. In the first, anomalous pressures in Br2-N02 mixtures were attributed to the presence of BrNOP3 More recently, a transient ultraviolet absorption observed following flash photolysis of Brz-NO mixtures was attributed to reactions of nitryl bromide.6 In previous work from this laboratory, matrix reactions of fluorine7ss or chlorineg atoms with NOz yielded the infrared spectra of both the nitryl halides (FN02 and C1N02) and the halogen nitrites (ONOF and ONOC1). As an extension of that work, matrix reactions of bromine atoms with NO2 have been performed. The importance of stratospheric bromine atom chemistry is less well-known than that of chlorine; however, bromine undergoes the same catalytic ozone depletion reactions as chlorine.1° Additionally, since the natural removal rate for bromine is lower than that of chlorine in the stratosphere, a lower concentration of bromine may be as effective as larger amounts of chlorine.1° The present work has been performed with the purpose of more fully elucidating the chemistry of NO2 molecules and atomic bromine. Experimental Section Bromine (Fisher, Reagent) was stored over P20, and degassed at -196 "C prior to use. Nitrogen dioxide and 99% nitrogen-15-enriched NO2 were prepared by adding excess oxygen to nitric oxide (Matheson, 99%) or 15N0 (Analytical Supplies Development Corp., 99 % 15N), respectively. Oxygen-18-enriched NO2 (Miles Laboratories, Inc., 75% " 0 ) was used without purification. All NOz samples were degassed at -196 "C prior to use. Argon (Matheson, 99.9995%) was added to each reactant to produce the desired matrix-to-reactant ratios by using standard vacuum line techniques on a stainless steelTeflon vacuum line. The matrix isolation apparatus has been described in detail in earlier publication^.^,^ As before, a small quantity of pure argon was initially deposited to preclude any
undesired window reactions. The predeposit was followed by a 4-6-h deposition of atomic bromine, produced by the microwave discharge of Ar/Br,, with undischarged NOz also diluted in argon such that the final argon-to-N02ratio was greater than 700/1. Total sample in most cases was about 20 mmol as measured by the pressure drop in the known volumes during the course of each experiment. In situ photolysis experiments were performed with a high-pressure mercury arc lamp (Phillips HPK, 125 W) with all quartz optics and a 5-cm water bath to remove unwanted near-infrared radiation. In most cases, the radiation was Pyrex filtered to remove light with wavelengths shorter than about 300 nm. Infrared spectra were recorded on a Beckman IR-12 filter-grating infrared spectrophotometer and calibrated with standard techniques. Survey scans were taken in the 200-4000-~m-~ spectral region after sample deposition by using the standard chart expansion and a 40 cm-l/min scan speed. A 20 cm-l/in. chart expansion and 8 cm-'/min scanning speed were used to precisely measure band frequencies.
Results and Discussion When argon-diluted bromine molecules and NO2 samples were co-deposited without discharging, the resulting infrared spectrum was indistinguishable from experiments performed in which only NO2 in argon was deposited. The lone exception was a weak band centered at 311 cm-l, which was also observed in the absence of NOz. Its intensity was dependent on the Br, concentration and is most likely due to aggregated bromine molecules, (Br2)n, perturbed enough to give a weak infrared absorption. When discharged argon (without bromine) was co-condensed with NOp in a blank experiment, only the wellknown spectrum of monomeric and dimeric NO2 in solid argon was observed.ll When Ar/Br2 samples were discharged and condensed with NOz, more complex infrared spectra were produced, as shown in Figure 1,which also shows the spectral changes produced by near-ultraviolet light photolysis. Figure 2 shows the major spectral changes observed after photolysis of a matrix formed by the bromine atom-N02 reaction under higher resolution conditions. The frequencies and intensities of all new infrared absorptions observed in a typical experiment before and after photolysis are listed in Table I. Two distinct reaction products can be readily recognized on the basis of the photolysis results. The bands at 1660, 1289, and 784 cm-' all lie in close proximity to strong absorptions of Nz04. The photolytic behavior of these bands is difficult to determine because of overlap with the strong N204absorptions which grow with photolysis due
This article not subject to U S . Copyright. Published 1979 by the American Chemical Society
2218
The Journal of Physical Chemistry, Vol. 83, No. 17, 1979
David E. Tevault
I -
t
$1
Z
21 u,
1500
2300
1000 \NAVENUMBER (cm-')
7i.c
200
500
( b l 75m photolysis
1'00
" 843
A
78C
MLvEP~UIViGER
"
,
630
\-&La'
56C
123
380
C~T I )
Figure 1. Survey spectrum (200-2000 cm-') observed following cocondensation of microwavedischargedbromine and NO2with excess argon onto a 10 K optical window (top), and the spectrum observed after exposing the same sample to 1 h of Pyrex-filtered mercury arc photolysis (bottom).
Figure 2. High-resolution spectra observed after codeposition of microwave-discharged bromine and NO2 with excess argon at 10 K in regions of special interest (top). Spectra observed after 75 mln of Pyrex-filtered mercury arc photolysis (bottom) showing growth of new Droduct bands.
TABLE I : Frequencies, Photolysis Behavior, and Identification of New Infrared Absorptions due t o Reaction upon Codeposition of Bromine Atoms and NO, Molecules onto a 10 K Cesium Iodide Window
as ONOBr below). Two very weak bands were observed at 1203 and 1197 cm-l. The former decreased in intensity with photolysis, while the latter increased. In order to identify the new reaction products, experiments have been performed with nitrogen-15-enriched NO2, as well as NO2 with 30-7596 oxygen-18 enrichment. The results of these experiments are discussed below and summarized in Table 111. Identification of the Prephotolysis Reaction Product. All prephotolysis reaction product absorptions shifted preceptibly with N-15. The smallest N-15 isotopic shift was found for the 420.7-cm-l band which shifted only 0.9 em-l, which we deem to be larger than our experimental uncertainty. The reaction product bands at 1660, 1289, and 784 cm-' split into apparent triplets, all partly overlapped by nearby N204 absorptions when partially oxygen-18-enriched NOz was codeposited with bromine atoms, while the 574-cm-' absorption had a single lowfrequency isotopic counterpart at 559 cm-' (Figure 3). The 421- and 402-cm-' absorptions were essentially unshifted. The three strong absorptions at 1660,1289,and 784 cm-l are assignable to a molecule with a bromine atom, a nitrogen atom, and two equivalent oxygen atoms on the basis of the isotopic data. The simplest molecule to satisfy these requirements is nitryl bromide, BrN02. We also tentatively assign the 574- and 402-cm-l absorptions to BrNOZ, based on their proximity to frequencies expected for this molecule. Additionally, the observed nitrogen-15 isotopic shifts are in reasonable accord with the analogous shifts of UNO2 for these lower frequency vibrations.'?' The BrNOz frequencies and assignments are shown in Table IV. Vibrational assignments were made on the basis of the similarity to the spectra of nitryl chloride and
intensity
freq, cm-'
initial deposit
after photolysis
1724.7 1659.9 1655.3 1288.9 1202.6 1197.4 836.6 784.0 587.6 574.4 420.7 401.8 391.8
0.07 0.15 0.37 0.39 0.03 0.02 0.06 0.33 0.09 0.35 0.18 0.04 0.02
0.28 0.35 0.32 0.33 0.02 0.Q3 0.22 0.44 0.36 0.20 0.12 0.03 0.10
assign uN= 0 ONOBr v 4 BrNO, u4 BrN0,a v 1 BrNO,
b b VN-QONOBr v 2 BrNO, VB,+O ONOBr v 6 BrNO, b u 5 BrNO, CYONOONOB~
a This band is assigned to a second matrix site of BrNO,. See Table IV for approximate description of BrNO, vibrations. Assignment uncertain (see text).
to warming of the matrix. Bands at 574,421, and 402 cm-l decrease in intensity with photolysis. On the other hand, the weak bands initially found at 1725,837,588,and 392 cm-l grow markedly during photolysis, in some cases increasing up to fourfold in intensity. The relative intensities of these latter four absorptions are constant (within experimental error) over a wide range of reactant concentrations, as shown in Table 11, and on this basis are assigned to a common new species (which will be identified
TABLE 11: Absolute and Relative Intensities of Infrared Absorptions of ONOBr Molecule at Various Concentrations after Mercury Arc Photolysis of Argon Matrices Containing Bromine Atoms and NO, 1300/20/1 1000/14/1 625/12/1 1000/20/1 400/16/1 Ar/Br/NO, ~
~~
cm-l 1725 8 37 588 392
v,
a
~~
la = I ,
0.29 0.19 0.32 0.095
Optical density.
~
~
III, 1.00 1.00 1.00 1.00
I 0.26 0.22 0.36 0.10
III, 0.90 1.16 1.13 1.05
I 0.145 0.09 0.17 Wb
1/10
0.50 0.47 0.53
I 0.15 0.07 0.15 0.035
Denotes band too weak for accurate intensity measurement.
III0 0.52 0.37 0.47 0.37
I 0.13 0.08 0.16 0.03
III, 0.45 0.42 0.50 0.32
Matrix Reactions of Bromine Atoms and NOp
TABLE 111: Frequencies and Isotopic Shifts (in Parentheses) of Reaction Product Infrared Absorptions Observed When Bromine Atoms Were Condensed at 10 K with 99%N-15 Enriched NO, or 75% 0.18 Enriched NO, in Excess Argon Br + 15N0, Br + N'6*'80, assignment __ BraON' 6O 1724.4 (0.3) BrO' 5 N 0 1694.3 (30.4) BraON1aO 1681.4 (43.3) BrN160, 1659.8 (0.1) BrN'60180 1646.3 (13.6) Br15N0, b BrN"0, 1629.4 (30.5) BrNO, 1288.6 ( 0 . 3 ) Br15N0, 1274.7 (14.2) aO BrN' 601 1265.0 (23.9) BrN"0, 1244.2 (44.7) 1197.9 (-0.5) d d 1192.3 (10.3) d 1187.7 ( 9 . 7 ) d 1184.9 ( 1 7 . 7 ) d 1181.6 (21.0) d 1179.0 (18.4) d 1173.0 (24.4) Brl 6 0 N '6O 836.7 (-0.1) Br'60N'80 828.9 (7.7) BrO'5N0 826.6 (10.0) Br' sON16O 818.8 (17.8) Brl80N'8O 808.3 (28.3) BrNO, 783.6 (0.4) Brl 5 N 0 773.3 (10.7) BrNI6O1' 0 770.0 (14) BrN"0, 756.4 (27.6) Br160Na0 587.2 ( 0 . 4 ) BrO'SNO 582.7 (4.9) BrNO, 573.8 (0.6) Br ' 5 N 0, 566.4 ( 8 . 0 ) Br' 80Na0 566.3 (21.3) 559.1 (15.3) BrN' 80,c d 420.7 (0.0) 419.8 ( 0 . 9 ) d BrN ' 6 * 401.6 ( 0 . 2 ) '400.5 ( 1 . 3 ) Br15N0, 389.1 (2.7) BrO' 5 N 0 Br'80N'80 387.2 (4.6) a Denotes either oxygen isotopes; unlabeled atoms denote naturally abundant isotopes. Isotopic assignment of 1187.7-cm-' band is not clear. u4 of Brl5NO, is expected to lie at ca. 1622 cm-' (cf. C115N0,,ref 9), but was not observed due t o interfering absorptions of 14N0, and 15N,04. Tentative assignment since no intermediate band attributable to the BrN160180molecule has been observed. Assignment uncertain (see text).
fluoride which are also shown for comparison in Table IV. The observed oxygen isotopic doublet of the 574-cm-l absorption is not well understood, but may arise because only one of the oxygen atoms is significantly displaced in this vibration. The assignment of these reaction product bands to BrON02 instead of BrN02 cannot be excluded on the basis
The Journal of Physical Chemistry, Vol. 83, No. 17, 1979 2219
I
1740
I
I
I
--
1680 600 WAVENUMBER (cm-')
563
Flgure 3. The 560-600- and 1680-1740-cm-' spectral regions following codeposition of bromine atoms and (a) NOpin natural isotopic abundance (top), (b) NOp 99% enriched with nitrogen-15 (middle), and (c) NOp 75% enriched with oxygen-18 (bottom) in argon onto a 10 K cesium iodide window. All spectra were taken after near-ultravioletirradiation of the matrices. Small diagonal arrows denote absorptions which grow with irradiation (ONOBr).
of the isotopic data. It is conceivable that BrO could be formed if a leak existed upstream of the discharge tube; BrO could then react with NO2 to form BrON02. Present evidence disfavors this hypothesis since strong bands of bromine oxides were observed when equal amounts of bromine and oxygen were deliberately premixed before discharging.12J3 Additionally, the antisymmetric NOz stretching and NO2 scissoring vibrations of BrONOz lie at 1711 and 802 cm-l, respectively, in the gas phase,l0 while the NOz antisymmetric stretching and NO2 scissoring vibrations of BrN02 reported here lie at 1660 and 784 cm-l. Finally, if we were to assume for the moment that ONOBr (vide infra) is produced by photolysis of BrON02, a depletion of the oxygen-18 NOz isotopic enrichment in the 837-cm-l multiplet would have been expected. Since no such depletion was observed, we feel safe in concluding that ONOBr is formed by isomerization of BrN02. Identification of the Photolysis Product. As already noted, the four bands which grow in unison with photolysis (Figure 2) are due to a single new molecule. The nitrogen-15 and oxygen-18 shifts of the 1725-cm-l band are 30.4 and 43.3 cm-l, respectively. These shifts are very close to
TABLE IV: Fundamental Infrared Absorption Frequencies (cm-') of Nitryl Halides, XNO, (X = F, C1, or Br) gasa 1309.6(vs) 822.4(vs) 567.8(s) 1 79 1.5(vs) 559.6(s) 742.0(m)
FNO, N, matrixb 1312.0 (0.19) 813.1 (0.20) 562.9 (0.06) 1802.9 (0.33) 559.7 (0.03) 737.9 (0.05)
ClNO, gasa Ar matrixC 1267.1e(vs)h 1264.3e (0.13) 792.6(vs) 7 8 7 . 0 f (-) 369.6(vs) 365.0 (0.09) 1 684.6(vs ) 1674.8 (0.07) 408.l(vvw) 652.3(m)
BrN 0, Ar matrixd 1289(s) 784(s) g 1660(m) 402(w) 574(s)
assign u,(a,) NO, symmetric stretch u,(a,) NO, scissor u,(a,) N-X stretch v4(bl)NO, asymmetric stretch u,(b,) NO, wag u6(b2)out-of-plane deformation
Reference 1. Reference 8 (optical densities in parentheses). Reference 9. This work, eFrequency perturbed by Fermi resonance interaction with 2 u 6 (see ref 1). f Overlaps N,O, absorption. IF Not observed. v = very; s = strong; m = medium; w = weak.
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The Journal of Physical Chemistry, Vol. 83,No. 17, 1979
David E. Tevault
the isotopic quartet structure observed for partially oxygen-18-enriched experiments, shown in the lower traces of Figure 4, demonstrates the presence of two inequivalent oxygen atoms in this species. The most logical assignment for this vibration is the 0-N=O "symmetric" stretch. It is obvious that this vibration is not a symmetric stretch as vl of NOz, but is more like an N-0 bond stretching with a smaller contribution from the N=O bond stretching. The combined isotopic data for the new photolysis product clearly indicate that its identity is bromine nitrite, ONOBr. The alternative assignment, bromine nitrate, formed by reactions such as (1)and (2) was discounted on
x
(30% 0-18)
8 50
800
WAVEN UMBER (cm-') Flgure 4. High-resolution spectra in the 780-870-cm-' region observed following codeposition of atomic bromine with isotopic NO2 samples in excess argon at 10 K and Pyrex-filtered mercury arc photolysis. The weak band at 804 cm-' in the two upper spectra is due to a trace of the BrBrO molecule (ref 13).
the calculated shifts for a harmonic diatomic N-0 stretching vibration, 30.9 cm-' for nitrogen-15 and 45.3 cm-' for oxygen-18 substitution. The absence of an intermediate absorption in the partially oxygen-18-enriched NOz experiments shows that this vibration is not an antisymmetric NOz stretch. The 588-cm-l photolysis band exhibited a 5-cm-l nitrogen-15 isotopic shift and appeared as a doublet in partially oxygen-18-enriched experiments with the new isotopic component at 566 cm-', a shift of 21 cm-'. These isotopic shift data are indicative of a Br-0 stretching vibration. The calculated bromine 79-81 shift for a diatomic Br-0 stretch at 588 cm-' is only 1.2 cm-l, and therefore it is not surprising that the individual bromine isotopic peaks could not be resolved. The 560-600- and 168@174O-cm-l spectral regions (after photolysis) for these NOz isotopic reactions are shown in Figure 3. The 392-cm-l photolysis product absorption shifted 3 cm-I upon nitrogen-15 substitution and 4.6 cm-l in the 75% oxygen-18-enriched experiment. Unfortunately, the band was too weak in the latter experiment for any detailed isotopic splitting patterns to be observed. Assuming that the oxygen shift is the result of the species completely substituted with oxygen-18, the most likely assignment for this vibration is an O N 0 scissoring mode. The 392-cm-l frequency is higher than expected for a BrON bend which should lie near 265 cm-', the bending frequency of BrN0.14 Spectra in the 800-850-cm-' region following reactions of isotopic NOz molecules with bromine atoms and photolysis are illustrated in Figure 4. The 10-em-' nitrogen15 shift shows the presence of nitrogen. More importantly,
NO^ A, NO + o 0 + Br-N02
-
BrONOZ
(1) (2)
the basis of the isotopic evidence, specifically the oxygen-18 isotopic splitting pattern of the 1725-cm-' absorption. Bromine nitrate has a strong absorption a t 1711 cm-' in the gas phase;'" however, this absorption is an antisymmetric stretching of the terminal NOz group and would be expected to have isotopic frequency shifts and isotopic splitting patterns (i.e., a triplet in partially oxygen-18enriched experiments) characteristic of that vibration (cf. N-15 shift of v1 of CIONOz and FON025), whereas the 1725-cm-I feature seen in our work is clearly a terminal N=O stretching vibration. In addition, experiments were performed by codepositing BrO, generated by discharging Ar/Brz/02 = 200/1/1 mixtures, and either Ar/NO or Ar/NOz. Previously, we had shown that similar reactions of C1+ 0 with NO produced smali yields of C10N0.9 The reaction of Br 0 with NO produced a higher yield of BrONO than did the Br + 0 reaction with NOz, as expected. Other Absorptions. The very weak 1203-cm-l absorption had isotopic components a t 1192 and 1182 cm-' in oxygen-18-enriched experiments and was found to have a weak nitrogen-15 counterpart at 1185 cm-'. Each of these bands was attributed to isotopically substituted species of the 1203-cm-' feature since each decreases in intensity upon near-ultraviolet photolysis. It is very tempting to assign this absorption to a combination band of BrNOz because of the isotopic splitting patterns and the photolysis behavior; however, no combination of observed bands (or of an observed and the unobserved v3 bands) produces a suitable sum and isotopic shifts for such an assigment. The 1197-cm-' absorption was found to have three isotopic counterparts at lower frequencies in oxygen-18-enriched runs and a sizeable nitrogen-15 shift. Its assignment to ONOBr seems to be favored on this basis, but it only grew slightly with photolysis in contrast to ONOBr absorptions, which increased dramatically in intensity. In addition, no combination of observed and/or expected ONOBr fundamental frequencies leads to a suitable assignment of this band to ONOBr. The 421-cm-I feature had no measurable oxygen isotopic shift and a small nitrogen isotopic shift, and thus definitive assignment cannot be made confidently. Identification of these bands must await further work. Stability of Halogen Nitrites. In earlier work, it was shown that ONOF rapidly isomerizes to FNOz in solid nitrogen matrices with UV radiationa8 ONOCl isomerizes more slowly through a metastable intermediate, OClNO, to ClNOD9 On the other hand, the present results clearly show that BrNOz photolyzes with near-U.V. light to produce ONOBr. This suggests that the halogen nitrites increase in stability as the size of the halogen atom increases. This effect may be due in part to the charge
+
Photoelectrolysis of Water with Solar Energy
distribution on NOz and electronegativity of the halogens. The fluorine atom, being the most electronegative, strongly seeks the more positive nitrogen atom during photolysis. The chlorine atom also apparently prefers the nitrogenbonded configuration, but not as much as fluorine. Bromine, which is less electronegative than oxygen or nitrogen, favors the more negative oxygen atom with photolysis. It is also interesting to note the very small amount of ONOBr formed in the initial matrix deposit. In an earlier a c c ~ u n tthe , ~ 1714-cm-l absorption of ONOCl was found to be approximately twice as intense as the 1675-cm-’ absorption of ClN02 under identical conditions as the present study, although no quantitative significance could be inferred at that time. Subsequently, Niki et a1.,16 in an infrared study of the gas phase reaction of chlorine atoms and NO2, estimated that ONOCl was formed initially at more than four times the rate of C1NO2. The agreement between these two reports appears to be excellent. Fluorine at,oms have also been shown to react with the oxygen atom of NOz to form ONOF at low temperatures with no activation energy. The present results, however, show very little initial ONOBr formation during matrix condensation, and it is not clear why a barrier to the direct formation of ONOBr should exist. Recently, Molina and MolinaI7 and Spencer and Rowlandlo have discounted the role of ONOCl and ONOBr, respectively, as stratospheric halogen atom sinks because of the measured high photodissociation cross section of ONOCl in the near-UVi7 and the belieflo that ONOBr is not expected to be significantly more stable than ONOC1. The present results clearly show that ONOBr is the most stable of the halogen nitrites with respect to the corresponding nitryl halides. However, it is not clear from our work that ONQBr is more stable than ONOCl since the photolysis behavior could be easily attributed to the instability of BrN02 as compared to C1N02. On the other hand, the present results certainly do not rule out the possibility that the stability of ONOBr in the presence of near-UV irradiation is greater than the stability of ONOC1.
The Journal of Physical Chemistry, Vol. 83, No. 17, 1979 2221
Conclusions The reaction of bromine atoms and NO2 in argon matrices leads to the isolation of nitryl bromide, BrN02. Near-UV photolysis causes &NOz to photoisomerize to bromine nitrite, ONOBr. This photolysis behavior is in sharp contrast to that of ONOF and ONOCl which photoisomerize to the respective nitryl halides. This work represents the initial unambiguous detection of nitryl bromide as well as bromine nitrite. The latter is a possible metastable stratospheric sink for bromine atoms. Acknowledgment. The author expresses his thanks to the National Research Council for support in the form of an NRL/NRC Research Associateship held during the initial portion of this work and to Dr. R. R. Smardzewski and Dr. R. L. Mowery for several helpful suggestions. References and Notes Bernitt, D. L.; Miller, R. H.; Hisatsune, I. C. Spectrochim. Acta, Part A 1967, 23, 237. Jolles, 2. E.; “Bromine and Its Compounds”; Academic Press: New York, 1966;p 209. Martin H.; Seidel, W.; Cnotka, H. G.; Hellmayr. Z. Anorg.
[email protected]. 1964, 331,333. Kuhn, S. J.; Olah, G. A. J. Am. Chem. SOC. 1961, 83, 4564. Uthman, A. P.; Demlein, P. J.; Allston, T. D.; Withiam, M. C.; McClements, M. J.; Takacs, G. A. J . Phys. Chem. 1978, 82, 2252. Hippler, H.; Luu, S.H.; Teitelbaum, H.; Troe, J. Int. J. Chem. Kinet. 1978, IO, 155. Smardzewski, R. R.;Fox, W. B. J . Chem. Soc., Chem. Commun. 1974, 241. Smardzewski, R. R.; Fox, W. B. J. Chem. Phys. 1974, 60, 2980. Tevault, D. E.;Smardzewski, R. R. J. Chem. Phys. 1977, 67, 3777. Spencer, J. E.; Rowland, F. S. J . Phys. Chem. 1978, 82, 7. Fateley, W. G.;Bent, H.A.; Crawford, Jr., B. J . Chem. Phys. 1959, 31, 204. Tevault, D. E.;Smardzewski, R. R. J. Am. Chem. SOC.1978, 100,
3955. Tevault, D. E.; Walker, N.; Smardzewski, R. R.; Fox, W. B. J . Phys. Chem. 1978, 82, 2733. Laane, J.; Jones, L. H.; Ryan, R. R.; Asprey, L. B. J. Mol. Spectrosc. 1869, 30, 485. Miller, R. H.; Bernitt, D. L.;Hisatsune, I. C. Spectrochim. Acta, Part A 1967. 23. 223. Niki, H.;’Maker, P. D.; Savage, C. M.; Breitenbach, L. P. Chem. Phys. Lett. 1978, 59, 78. Molina, L. T.; Molina, M. J. Geophys. Res. Lett., 1978, 4, 83.
Design and Evaluation of New Oxide Photoanodes for the Photoelectrolysis of Water with Solar Energyt R. D. Rauh,” J. M. Buzby, T. F. Reise, and S. A. Alkaitis EIC Corporation, Newton, Massachusetts 02 158 (Received March 8, 1979)
Ideal photoanodes for the efficient photoelectrolysis of water with solar energy would be low electron affinity, robust oxide semiconductors with band gaps which are less than -2.5 eV. It is probable that only “d-band” oxides would have valence bands high enough in energy to give rise to this combination of properties. In order to produce such materials, highly insulating perovskites and rutiles have been substituted with d” transition metals. Sensitization of photocurrents to the visible portion of the spectrum was noted in several cases, although in this series of compounds, the photocurrent quantum yields were decreased by the substituents.
Introduction Much research has been directed recently toward the problem of photoelectrolysis, particularly for the purposes of converting solar energy to electricity or directly to a ‘This work was supported by the US.Department of Energy under Contract No. EC-77-C-01-5060. 0022-3654/79/2083-2221$01 .OO/O
transportable fuel. In the latter category, the demonstration of semiconducting electrodes for the efficient photoelectrochemical splitting of water has been an elusive goal. In this paper we report our considerations regarding the design of n-type photoanodes for this application, and present preliminary results concerning the properties of some new electrode materials. 0 1979 American Chemical Society
