J. PhyS. Chem. 1881, 85, 537-541
III but in annealing experiments where the chloride density decreased from 4.6 Cl/nm2, and in other experiments where coverage decreased between 400 and 800 "C. For the samples treated with Sic& at 400 "C or higher, this
537
could imply a higher degree of pairing than actually exists, because for straight chlorination Cl/A(OH) = 1. Samples chlorinated with CCll did not exhibit this thermal instability.
Surface Halides of Silica. 2. Bromide and Iodide M. P. McDanlel Phillips Research Center, Bartlesville, Oklahoma 74004 (Received: Aprll30, 1980; In Finel Form: October 24, 1980)
The surface bromide and iodide of silica were prepared in a simple flow system and briefly examined. In the presence of carbon monoxide, both silanol and siloxane groups reacted with the free halogen above about 500 "C. The maximum coverages found were 3.5 Br/nm2 and 2.0 I/nm2 at 800-900 "C. Each halide could be removed yielding a hydrophobic silane surface, or to 02, yielding siloxane. In order of reactivity, by exposure to Hz, =Si-I> =Si-Br > =Si-Cl. The iodide could not be completely removed by O2 even at 800 "C, but rather formed a heat-stable, yellow-orange species having a strong ESR signal. This species was a powerful oxidant and could even be regenerated by 0,after reduction in CO. However, reduction in H2 or hydrocarbons was permanent. Moisture also destroyed the species, but left most of the oxidizing capacity. These fads and others suggest an unusual oxyanion of iodine(V1) stabilized by the silica surface.
Introduction Although there are a few references in the literature to the surface chloride of silica, practically nothing has been devoted to the lower halides of silica. Perhaps this is because they are more difficult to prepare, owing to the weaker oxidizing strength of bromine or iodine. In this paper the bromide and iodide of silica were prepared, the surface coverages measured, and their reactivities briefly examined. Experimental section Sample Preparation. The gas purification and flow system used here has already been described in part 1of this series. Liquids, such as Br, or SOBr,, were injected into the gas stream ahead of the fluidizing silica. Iodine, when used, was packed (25 g) on a quartz wool plug in the gas stream below the silica bed and just outside of the furnace. At the desired time, it was slowly raised into the furnace over 1h so that it evaporated and passed through the silica. All portions of the sample tube were eventually heat treated to prevent any contamination by unreacted halogen. Halide Analysis. The analysis of bromide and iodide on silica was identical with that used for chloride in part 1. Controls were made, again in 1 N NaOH, to contain iodide, iodate, and various combinations of the two species, which can react with each other in neutral or acidic solution. Neutralization of the NaOH occurred only in the presence of mecuric thiocyanate. The method was found to be an accurate measure of iodide concentration without interference from iodate, to which it was not sensitive. Iodometric titrations were performed under N,. Silica samples were added to acidic 1 N KI solution, protected from air by Schlenkware. 1, was liberated immediately and then stopped. This solution was then titrated against 0.04 N sodium thiosulfate to the starch endpoint. Instrumentation. X-ray photoelectron spectra were taken on a Varian IEE spectrometer with Mg Ka source (1253.6 eV) and retarding grid mode at 100-eV pass energy. Electron spin resonance samples were sealed by flame 0022-365418112085-0537$01.25/0
under N2 inside quartz tubes. Spectra were obtained on a Varian V-4502 double-cavity spectrometer modulated at 100-kHz and 400 Hz for the reference side. Results Bromination of Silica. Figure 1 plots the bromide concentration on the surface of Davison 952 wide-pore silica after being treated with SOBrz. In Figure 1A each sample was calcined and then treated with SOBr2at the same temperature. Bromide adsorption peaked at 500 "C and then declined at higher temperatures as the OH population (Figure 1C) also declined. This suggests less reactivity with siloxane than was found in part 1 when SOCl, was used. In Figure lB, all samples were first dried at 800 "C, leaving 0.9 OH/nm2 on the surface. Bromination with SOBr2then followed a t the temperature indicated. The low bromide density, even at 800 "C, again indicates little reactivity with siloxane. The bromination of siloxane was accomplished more completely in the presence of a strong reducing agent like carbon monoxide and a large excess of bromine. Si-0-Si CO Br, 2Si-Br + COz Figure 2 indicates the surface bromide density after each sample was dried at the temperature shown and then treated with a large excess of Br2vapor in carbon monoxide at the same temperature. At 750 "C, all hydroxyls were gone and at 900 "C a maximum bromide density of 3.5 Br/nm2 was found, which was also the highest chloride level found by any direct treatment in part 1. No carbon deposits were found on any of these CO-treated samples, nor was surface area or porosity affected. Exposing silica to bromine vapor at 750 "C in a Nzatmosphere, instead of CO, left only 0.9 Br/nm2 on the surface. When dry air was used as the carrier, little or no bromide was found on the sample, but the hydroxyl population decreased 10%. Iodination of Silica. Figure 2B shows the iodide density found when silicas were treated with Iz in carbon monoxide. Since iodine is a weaker iodizing agent than bromine or chlorine, and makes a larger anion, it is perhaps
+
+
0 1981 American Chemical Society
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538
The Journal of Physical Chemistry, Vol. 85, No.
5, 1981
McDanlel Br OR I/nm 2.0
2.0 '
t
* BROMIDE
Br OR OH/nm
1.0
.
---1.o a@
IODIDE
I Flgure 1. BromMe concentration on silica (A) calcined and treated with SOBr, at the temperature shown, and (B) calcined at 800 "C and then treated with SO&, at the temperature shown. (C) Hydroxyl concentration on virgin silica dried at the temperature shown.
I
300 400 500 600
700 800C
Flgure 3. HalMe concentration on halogenatedslllca after treatment In H2 at the temperature shown for 15 mln. Br OR I/nmz
X/nm 2
3.0
3.0 2.0
2.0
1.0
1.o
300 400 500 600 700 800 9OOC
I
1
1
1
1
1
1
1
300
400
500
600
700
800
9001
Flgure 2. Halide concentration on silica calcined in CO and (A) Br, or (B) I, at the temperature shown.
Flgure 4. Hallde concentratlon on halogenated silica after exposure to dry air at temperature shown for 10 mln.
not surprising that lower coverages were obtained. Even with a large excess of I2the maximum iodide concentration found was only 2.0 I/nm2, or less than 60% that of chloride or bromide. These results tended to be variable. For example, the 800 "C experiment was repeated nine times. The measured iodide content ranged from 0.9 to 2.0 I/nm2, with the mean at 1.4 I/nm2. Two other types of silica were also iodinated at 800 "C with about the same result. Cabosil S-17, a nonporous, very pure silica prepared by flame hydrolysis of Sic&, adsorbed 1.2 I/nm2. Silica A from part 1, having many small pores, was prepared from purified ethyl silicate. It adsorbed 0.8 I/nm2. This indicates that highly pure silica is equally reactive, and that porosity is not a controlling factor in these experiments. Iodination was accomplished by another reaction at as low as 300 "C. The samples in Table 1 were dried at the temperature shown and then treated with dry HI in N2at temperatures ranging from 200 to 800 "C. The maximum coverage, about 0.5-0.6 I/nm2, was obtained at 500-600 "C. By measuring the change in hydroxyl population after treatment with HI, it should be possible to determine whether the reaction occurs with siloxane or silanol. In the former case, the silanol level should increase, whereas in the later case it should decrease. Si-0-Si + HI Si-I + Si-OH Si-OH + HI Si-I + H20
loxane, predrying at 800 "C and HI at 500 "C, the A(OH)/I ratio was -0.5 which corresponds to 75% reaction with silanols. Reactivity with Hydrogen. The surface halide could be partially displaced by hydrogen above 600 "C. This is shown in Figure 3 where silica bromide and iodide were exposed to a 50% H2 atmosphere for 15 min at various temperatures. By 800 "C, the bromide concentration had dropped rearly 40% and the iodide level by 90%. A hydrophobic surface was produced in both cases, with zero hydroxyl population. Exposure to dry air at 400 "C or even 600 "C failed to produce any silanol groups, but at 800 "C, both samples lost their hydrophobic character and an almost normal hydroxyl population of 0.8 OH/nm2 reappeared. This suggests that =Si-Br or =Si-I was replaced by =Si-H, which was then oxidized to =Si-OH. If a silane was produced, its resistance to oxidation is surprising. Reactivity with Oxygen. Like the chloride, surface bromide and iodide of silica could also be removed by exposure to oxygen at elevated temperature. Figure 4 plots the halide concentration on two samples raised in N2 to various temperatures and then exposed to dry air. Immediately the red-brown or violet color of Br2 or I2 could be seen in the exit stream. The higher the temperature, more halogen was lost. Usually Br2 stopped coming off within 1 min or so, whereas I2was slower. Even switching back to N2did not always immediately stop the evolution of 12. Some traces of violet could still be seen in the exit long afterward. Remember that the halide analyses reported in Figure 4 measured only the halide anion (-1).
--
The A(OH)/I values listed in Table I are all negative, indicating that the reaction is primarily with silanols. Even under the most favorable conditions for reaction with si-
n., TA
The Journal of Physical Chemistty, Vol. 85, No. 5, 7981 539
Surface Halides of Silica
(I
A0
= 2.0083
g
2.0020
g 3 2.0057
g = 2.0048
0 -
I
I
I
400
I
I
I
500
800
I
I
I
700 nm
Figure 5. Visible reflectance spectrum (transmittance) of oxidized silica iodide.
g = 2.0035
Flgure 7. Electron spin resonance of silica iodide: (A) exposed to dry air for 5 mln at 600 "C; (B) exposed to air for 30 min at 800 "C. e/nm2 1.5
1.0
0.5
W
t
(I :2.0028
-,In 0:
Figure 6. Electron spin resonance of silica iodide: (A) raised in 0, to 450 "C and cooled in N2; (B) raised in 0,to 650 "C and held 20 min; (C) held 1 h in O2at 800 "C;N, flush.
Higher oxidation states would not have been detected. Like the untreated silica, all the halides of silica were white, as well as the oxidized silica bromide and chloride. However, the oxidized silica iodide was yellow or even orange sometimes. Figure 5 shows the visible reflectance spectrum of this species. At first it was suspected that this might be free I2adsorbed by the silica during cooling, but long periods of flushing with N2 at high temperature (even 800 "C)did not lessen the color. A pretreater column of quartz wool at 800 "C was added to remove traces of iron carbonyl from the CO stream during the initial iodination treatment, but again the yellow species was obtained upon oxidation. Electron Spin Resonance. This colored silica also had a strong electron spin resonance. Two types of silica were tested by ESR-Davison 952 and the ultrapure silica A whose preparation from tetraethyl orthosilicate has already been described in part 1. In both cases only the colored, oxidized silica iodide produced a signal; its unoxidized parent silica iodide did not. Neither did the chloride or bromide even after displacing some halide by oxygen, nor did silica controls treated with CO and O2 as above but without halogen. The ESR at -196 "C of three silica iodides are shown in Figure 6. The parent silica iodide was white, had no ESR signal, and contained 0.6 I/nm2. This material was heated in oxygen over about 30 min to 450 O C . The first traces if I2 could be seen in the exit at about 200 "C. After the temperature reached 450 "C, the sample was flushed with N2 and cooled. It had a pale yellow tint, contained 0.6 I/nm2, and produced the spectrum shown in Figure 6A. Actuallly two signals seem to be present, a broad and
Flgure 8. Oxidizing capacity of silica iodide exposed to air for 5 min at the temperature shown.
narrow one, both centered in the g = 2 vicinity. Spectrum B was obtained by heating the above sample in O2at 650 "C for 20 min. This sample had a dark orange color and contained 0.4 I/nm2. Sample C was held in O2at 800 "C until I2 stopped coming off (1h). The orange color remained but the iodide level had decreased to less than 0.1 I/nm2. In this series, the broad signal seemed to increase slightly and then decrease as the oxidation treatment became more severe, whereas the narrow signal became more pronounced. The narrow signal is better defined in Figure 7 where the parent silica iodide held 1.1I/nm2 before oxidation. The spectrum shown in Figure 7A developed when a sample was exposed to dry air at 600 "C for 5 min. Again it released I2and acquired a yellow-orangecolor; the iodide level fell to 0.9 I/nm2. The signal in Figure 7A was ten times stronger than that in Figure 6. The broad signal, however, was not evident. In Figure 7B the same silica iodide was exposed to dry air at 800 "C for 0.5 h until no more I was evolved. The iodide concentration on the surface decreased to 0.7 I/nm2 but the ESR signal increased fourfold and lost its well-defined assymetry. The yellow color remained. In another experiment, a silica iodide was exposed at 800 "C to oxygen which had been enriched to 20% I7O. The ESR spectrum obtained was almost identical with that in Figure 7B; no hyperfine splitting was observed. In these and other runs, no correlation was noticed between the ESR signal intensity and iodide content (or loss). Even the most powerful ESR signals observed at -196 "C
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The Journal of Physical Chemistry, Vol. 85, No. 5, 1981
almost disappeared when run at 25 "C. Reactivity of Oxidized d i - I and =Si-Br. The yellow oxidized form of silica iodide was found to be a strong oxidant. Figure 8 plots the oxidizing capacity as determined by aqueous iodometry of the same series of samples shown in Figure 4. The parent silica iodide had no oxidizing capacity but became oxidizing after exposure to dry air at 400 "C or higher. A t 600 "C, the oxidizing capacity peaked at over 1.5 e (atom equivalents)/nm2. This was 2.2 times the measured iodide concentration at 600 "C; at 700 "C the ratio was 1.9 and at 800 "C it dropped to 1.3. Holding the silica iodide in dry air at 800 "C for 2 h did not further decrease the iodide concentration or oxidizing capacity. The oxidizing capacity could be removed reversibly by exposure to carbon monoxide. For example, a sample containing 0.8 I/nm2 and an equal oxidizing capacity of 0.8 e/nm2, was treated with carbon monoxide for 5 min at 350 "C; no change in color was noticed. By 500 "C in CO, however, the sample turned white and lost its ESR signal and oxidizing capacity. The measured iodide concentration had increased to 0.9 I/nm2. Exposure again to dry air at 600 "C for 5 rnin restored the yellow color and 65% of the oxidizing capacity, while the iodide concentration again measured 0.8 I/nm2. Another sample, containing 0.8 I/nm2 and 1.4 e/nm2, was treated with dry hydrogen at 300 "C for 10 min. The orange-yellowcolor lightened, and both iodide content and oxidizing capacity decreased to -0.6 I/nm2 and 0.5 e/nm2. Further treatment with Hz removed all color, iodide, and oxidizing capacity by 500 "C. Reoxidation in dry air at 500 "C or even at 800 "C did not restore the oxidizing capacity or color. When the oxidized silica iodide was submerged in a dry hydrocarbon like heptane, the liquid turned dark violet, even at 20 "C, indicating liberation of 1% Since these samples were quite stable at 600 "C, they could not have contained free I2 and therefore the evolution of Iz in heptane must indicate a reaction of the sample with heptane. Even heptane vapor at 150 "C decreased or removed the oxidizing capacity. Ethylene at 200 "C had the same effect, removing both the oxidizing capacity and the iodide. However, when dry CC1, liquid was used in place of heptane, no Iz was generated. Brz vapor at 300 "C also decreased the oxidizing capacity but did not change the halide content. Treating the oxidized silica iodide with water vapor removed the color and ESR signal but left most of the oxidizing capacity. For example, a sample containing 1.0 I/nm2 and 1.2 e/nmz was exposed to wet Nz(23 mmH20) at 200 "C for 30 min. The violet color of Iz could be seen in the off gas and afterward it held only 0.1 I/nm2 whereas the oxidizing capacity diminished just 30% to 0.9 e/nmz. At 600 "C the sample ceased to be an oxidant. The oxidized silica bromide samples in Figure 3 had no oxidizing capacity as measured by this technique. Exposing the silica bromide to dry air at 600 "C in the presence of I2vapor did not produce the yellow colored product, nor did this treatment produce any oxidizing potential. Neither could the yellow species be made by exposing the parent iodide to Izvapor at 375 "C; the I2was not adsorbed, ruling out =Si-13- as the oxidized species. Iodate on Silica. An effort was also made to generate the yellow oxidized species from a higher valent precursor. Two batches of silica were impregnated aqueously to 5% iodate, one with HI03 and the other with HI04. After being dried on a hot plate with stirring, samples of these two batches were raised in dry air to temperatures ranging
McDaniel
from 300 to 900 "C and held there 15 min. In both cases, I2vapor could be seen in the exit stream at as low as 200 "C and it's evolution stopped somewhere between 400 and 600 "C. None of the samples had any detectable iodide concentration, nor was the yellow color found. However, a pink tint, probably adsorbed Iz, was noticed on some of the samples treated at the lower temperatures. Oxidizing capacity decreased quickly with increasing temperature, to about 0.2 e/nmz at 500 "C and finally to zero a t 600 "C. Another series of samples impregnated with HIOBwas heated in carbon monoxide to temperatures between 400 and 800 "C. Again I2evolved near 200 "C and stopped after 1 h at 400 "C. All were white and none contained any iodide. X P S of =Si-X. X-ray photoelectron spectroscopy (XPS) of brominated silica (1.8 Br/nm2) indicated only one type of bromine species present which was identified as bromide. The 3d,,, and 3d5/2 peak, an unresolved doublet, was found at 70.5 eV. As the sample was treated with oxygen at 300,600, and then 800 "C, the intensity of this peak declined exactly like the bromide concentration in Figure 3, but no shifts or new peaks were observed. The separation between bromide and say bromate, which is nearly 6 eV, would have been noticeable. Comparison of the intensities of bromide to oxide and silicon, using the method of Dreiling,l agreed well with the chemical analysis of bromide, inidicating that the bromide was bound to the surface only. The XPS spectrum of silica iodide was more confusing. It revealed two surface species with the 3d5,, peaks occurring at 622.1 eV, comprising about 73% of the signal, and the remainder at 619.8 eV. (The 3d3lZemissions were found at 633.5 and 631.2 eV.) The minor component corresponded well to iodide, but the larger one is unknown. Iodate occurs at about 624.0 eV. The oxidized (600 "C) silica iodide degenerated very quickly under X rays, making the data of questionable reliability. Even the first scan, obtained after only 15 min under X rays, had less than a tenth the signal intensity of the parent silica iodide. Only a single species at 620.8 eV (3d5l2)was found. Discussion The differences in reactivity between C12, Br2, and Iz became visible in the preparation of the corresponding halide of silica. Chlorine reacted directly with the surface, even in the presence of air, whereas bromine reacted only slightly in N2 and not at all in air. For maximum coverage, both bromine and iodine had to be accompanied by a reducing agent like carbon monoxide. Under these conditions, both silanol and siloxane surface groups were attacked. The maximum bromide concentration found was about the same as chloride, 3.5 Br/nm2. Coverage by iodide was considerably lower a t 2.0 I/nm2. All three surface halides could be partially displaced by hydrogen, yielding a hydrophobic surface, probably a silane. The iodide was most reactive, going almost to completion in 15 min, whereas the chloride was least reactive. Simply heating the unhalogenated silica in Hz did not produce silane, or greatly lower the OH population. Once formed, this surface silane was surprisingly reluctant to be oxidized by air to silanol at 600 "C or below. All three surface halides could also be displaced by oxygen, and again the iodide was most reactive. However, unlike chloride or bromide, oxidation of the iodide produced a new species, paramagnetic and having an orange-yellow color. This species was a strong oxidant, with (1) M. J. Dreiling, Surf. Sci., 71, 231 (1978).
J. Phys. Chem. 1981, 85, 541-547
I (+7)
H 2 52 0 C
11
WHITE
1
f )I*
200C
H20
pq -F I 300 C
/I\
WHITE
CO 300 C
ORANGE
Flgure 9.
maximum capacity found near 1.5 e/nmz. Using oxygen enriched with 1 7 0 did not split the ESR signal, as might be expected if the spin density had been concentrated on the oxygen. Furthermore, X-ray photoelectron spectroscopy indicated increased sensitivity to X rays, as might be expected from a higher valent species. No oxide of iodine has been reported which is stable at 800 "C, paramagnetic, and highly colored. Nor could this or 1207 onto species be reproduced by impregnating 1205 the silica, both of which completely decomposed well below 600 "Cleaving neither iodide nor oxidizing potential. And even the intermediate oxides such as 1204 and 1409 are thought to be combinations of I5and 13. However, white or yellow alkaline earth salts of paramagnetic iodine(V1) oxy anions have been Formed by decompo~
~
~~~~~~
(2)M.Dratovsky and L. Pacesova, Russ. Chern. R e a , 37,243(1968).
541
sition of certain periodates at 200 "C, they are stable in air to near 400 "C, where they also decompose. In water, they hydrolyze to I(VI1) and I(V) anions. No other stable paramagnetic iodine species have been reported. Therefore, the most likely assignment for the yelloworange species seems to be I(VI) attached to and stabilized by the surface. Ita one unpaired electron would fit the sharp ESR signal obtained, and its high oxidation state would account for the large oxidizing capacity when only a small amount of iodine seemed to be involved. (Remember that the iodide analysis increased only slightly when the yellow-orange iodate product was reduced in carbon monoxide to iodide.) The high thermal stability is admittedly unusual, but transition metal oxides are also known to be stabilized by isolation on silica. Figure 9 summarizes our current understanding of the iodineailica system. Notice that moisture probably caused a disproportionation from I(V1) to I2(which was observed in the off gas) and I(VI1) (or perhaps I(V1) and 02). Thus the paramagnetic species was destroyed but the oxidizing capacity remained intact. Further heating merely decomposed I(VI1) to I2 and 02. This sensitivity to moisture would also explain why reduction in Hz (which produces HzO) was permanent, whereas reduction in CO was reversible. The highest I(V1) concentration found, based on the measured oxidizing capacity, would then be about 0.25 I(VI)/nm2. Acknowledgment. Help from the following people is gratefully acknowledged George D. Parks and Charles W. Krueger who obtained X-ray photoelectron spectra, and Morgan A. Waldrop who obtained ESR spectra. (3)G.S. Sanyal and K. Nag, J. Znorg. Nucl. Chem., 39,1127(1977). (4)K.H.Stern, J. Phys. Chern. Ref. Data, 3,481 (1974).
Laser Photodetachment from Aromatic Anions In Nonpolar Solvents Ulrlch Sowada and Richard A. Holroyd" Department of Chmistty, Brookhaven National Laboratoty, Upton, New York 11973 (Received: April 29, 1980; In Final Form: October 29, 1980)
A combined X-ray visible-dye-laserdouble-pulse technique is used to determine the photodetachment (action) spectra of the anions of biphenyl, trans-stilbene, pyrene, perylene, benzperylene, coronene, and nitrobenzene in nonpolar solvents. Quantum yields of electrons (4.J are reported for the visible transitions and photodetachment is shown to be a major process for visible light. For pyrene and trans-stilbene anions the quantum yields are 0.47 and 0.31 independent of solvent. For perylene and benzperylene anions 4, is solvent dependent. The results are interpreted in terms of autoionizing excited states which can also internally convert to ground state anions. The mechanism of autoionization and the lifetime of the excited states in solution are discussed. Introduction The anions of aromatic molecules have been experimentally investigated by condensed-phase absorption spectroscopy1p2as well as by gas-phase electron transmission spectroscopy (ETS).3 The absorption spectra are characterized by several sharp bands extending from the infrared to the ultraviolet and a correspondence of the (1)Shida, T.;Iwata, S. J. Am. Chem. Soc. 1973,95,3473. (2) Balk, P.; Hoijtink, G. J.; Schreurs, J. W. H. Recueil 1957,76,813. Hoijtink, G.J.; Velthorst, N. H.; Zandstra, P. J. Mol. Phys. 1960,3,533. (3) Jordan, K.D.; Burrow, P. D. Acc. Chem. Res. 1978,1I,341. Chern. Phys. 1980,45, 171. 0022-3654/81/2085-0541$01.25/0
energies found in absorption and ETS has been noted.3 These transitions lead to states whose energies are, in general, above the minimum detachment energy and an important physical problem is to understand the dynamical behavior of such discrete states within the continuum. Tsubomura and Sunakawa4 examined the problem of excited states of aromatic anions which are above the detachment threshold. Their model showed that the existence of discrete states in the continuum can be accounted for by the limited range of the potential. Using (4)Tsubomura, H.;Sunakawa, S. Bull. Chem. Soc. Jpn. 1967,40,2468.
0 1981 American Chemical Society
