J. C. MCMANUSAND M. J. D. Low
2378 droxylated at increasingly higher temperatures, and they suggested that the polymerization was a basecatalyzed reaction with oxygen atoms acting most probably as adsorption sites, the HCN molecule being adsorbed through the hydrogen. The second HCN for dimerization would then come from the gas phase, adsorption of the second reacting HCN on OH groups being unlikely because the polymerization was enhanced by dehydroxylation.1 However, the present results point to another mechanism. The bonding of HCN to aluminum ions and the poisoning effect described earlier suggest that aluminum ions acted not only as adsorption sites but also as centers for the polymerization. After HCN had adsorbed on an aluminum site, a second HCN could come from a neigh-
boring oxygen atom. The polymer thus formed could block the center and lead to the observed poisoning. The polymerization would be enhanced because increased dehydroxylation would lead to the formation of a greater number of oxygen atoms at which HCN could adsorb. Alternatively, or in addition to this, the progressive degassing may lead to the formation of additional centers through the migration of aluminum to the glass surface, in analogy to the enrichment of boron on the surface via migration from the bulk of the glass. a
Acknowledgment. Support by a grant from the Communicable Disease Center of the National Institutes of Health and National Science Foundation Grant GP1434 is gratefully acknowledged.
Interactions of Propionic and Acetic Acids with Germania by J. C. McManus and M. J. D. Low Department of Chemistry, New York University, New Y o ~ kNew , York 10469
(Received February 16, 1968)
Infrared spectra were recorded from 4000 to 1000 cm-1 of acetic and propionic acid sorbed on partially hydrated, dehydroxylated, and deuterated germania surfaces, and also after degassing at temperatures up to 600". In general, the same results were obtained with both acids. Some HPr was physically adsorbed at high surface coverage, hydrogen bonded to surface Ge-OH groups, and could be desorbed at 100". Dissociative chemisorption resulted in the formation of surface esters. At least some of the acids reacted with Ge-0-Ge bridges to yield the ester and Ge-OH structures, but the extent of a direct esterification reaction of acids with preexisting Ge-OH groups is uncertain. A small amount of Ge-H and propionate ion may be formed. The surface esters decomposed above 300". Various data suggest that the surface ester decomposed to form surface carbonate and ketone within the pores of the gel. The ketone then formed a ketene, which became chemisorbed.
There have been several infrared studies on the interactions of carboxylic acids with oxide surfaces.1 It was shown with formic acid adsorbed on N O , ZnO, A1203,and MgO that ionic formate species were formed, but the results with Si02 were not so clear. Hirota, et a1.,2 claimed that formic acid was not dissociated and was present in monomeric form on SiOz, while Eischens3 suggested that carboxylic acids were sometimes dissociatively adsorbed on SiOz with the formation of a covalently bonded carboxylate radical. These interpretations were not unequivocal, because the opacity of the SiOz samples prevented examining the spectral region where C-0 stretching and OH deformation would occur. Germania is, however, reasonably transmitting to 1000 cm-I. We have consequently recorded infrared spectra over the 4000-1000cm-l region of the interactions of propionic and acetic acids with germania gel. The Journal of Physical Chemistry
Experimental Section Germania gel was prepared in a manner similar to that described by Laubengayer and Brandt,4 except that double the amount of water recommended by them was used. The gel was air dried at 110" for 3 hr, was ground, and was compressed at approximately 25 tons/in.Z to form self-supporting disks. Sample disks (approximately 25 mg/cm2) were degassed in a cell6 by pumping for 12-14 hr, at temperatures from 450 to 600°, depending upon the degree of dehydroxylation (1) L. H. Little, "Infrared Spectra of Adsorbed Species," Academic Press Inc., New Yorlr, N. Y.,1966. (2) K. Hirota, K. Kuwata, T. Otaki, and S. Aaai, Proc. Intern. Congr. Catalysis, Bnd, Paris, 1960 809 (1961). (3) R. P. Eischens, Science, 146, 486 (1964). (4) A. W. Laubengayer and P. L. Brandt, J. Amer. Chern. SOC.,54, 549 (1932). ( 6 ) J. B. Peri and R. B. Hannan, J . Phys. Chem., 64, 1521 (1960).
2379
INTERACTIONS OF PROPIONIC AND ACETICACIDSWITH GERMANIA
Figure 1. Propionic acid on hydroxylated germania: (A) background spectrum of gel after degassing; (B) after exposure to 10 torr of H P r for 2 min and degassing for 30 rnin a t 25'; (C) after degassing for 30 rnin a t 122" and 30 rnin a t 153".
required. To remove carbonaceous material, the samples were heated in 10 torr of oxygen for 2 hr, a t the temperature used for degassing. Propionic acid (HPr) was degassed by alternate freezing and thawing in uacuo until no pressure burst was detected by a Pirani gauge. Acetic acid (HAC)was dried by CuSOe and vacuum distillation and was degassed by means of freezing-thawing cycles. Each acid was always degassed before use. Acids were sorbed a t 25". Spectra were recorded with the samples a t room temperature using a PerkinElmer Model 621 spectrophotometer. A second cell was used in the reference beam of the instrument.
Results and Discussion Various sorption experiments were made with HPr and HACa t pressures up to 10 torr on partially hydrated, dehydroxylated, and deuterated surfaces. Spectra were also recorded after various heat treatments up to 600" subsequent to adsorption. The results obtained with the two acids were similar, and several reactions were observed. However, the data clearly show that two temperature regions were involved, in that the nature of the surface species changed drastically above 300". Consequently, each of the temperature regions is considered separately. The Nature of the Adsorbed Layer (25-SOO0). Spectra of specimens which had been degassed and oxygen treated below 550" showed sharp bands a t 3672 em-' and a broad band near 3500 cm-l, e.g., Figure 1, spectrum A. The sharp 3672-cm-' band, hereafter termed the Ge-OH band, is attributed to the 0-H stretching fundamental of isolated Ge-OH surface s t r u ~ t u r e s ,while ~ J the broader absorption is ascribed to hydrogen-bonded hydroxyls.' The hydroxyls could be removed by outgassing a t 600" (Figure 2, spectrum A) or could be converted to OD groups by exchange with DzO (Figure 3, spectrum A). With some specimens a band was observed a t 2330 cm-l, which could not be removed by further degassing or oxygen treatment; it
L,
,
,
35w'
"
' JpO ' O"'~
, 2oM) ' '
'
' 1500&
'
'
Figure 2. Propionic acid on dehydroxylated germania: (A) germania gel dehydroxylated a t 620'; (B) after exposure to 0.6 torr of H P r for 20 min and degassing for 5 min a t 25" (the trace is slightly displaced to avoid overlapping).
was probably caused by COz trapped in closed pore^.^^^ Strong absorptions below 1550 cm-' were caused by the adsorbent. Changes occurred over the entire spectral range when acids were sorbed. Some effects produced by HPr are shown in Figure 1, spectrum B. On exposing a partially hydroxylated surface to 10 torr of HPr, the Ge-OH band disappeared and the transmittance decreased from 3600 to 1800 cm-'. The broad absorption peaked near 3400 cm-l and showed satellite bands in the 2800-2400-~m-~ region. Bands formed a t 2986, 2950, and 2890 cm-' in the C-H region. A pronounced, somewhat asymmetric, band formed near 1700 cm-l (approximately 1730-1696 cm-l). Other bands occurred near 1462, 1420,1380, 1360, 1320, 1276, 1230, and 1180 cm-'. Degassing under relatively mild conditions substantially reduced some of the absorptions. As shown by the example (Figure 1, spectrum C), a short degassing removed most of the 3600-1800-~m-~absorption. A broad band remained near 3500 cm-l and the Ge-OH band reappeared. The (6) M. J. D. Low and P. Ramamurthy, Chem. Commun., 609 (1967). (7) M. J. D. Low and P. Ramamurthy, to be submitted for publica-
tion. (8) The air-dried gel contains some adsorbed ethanol which partly desorbs and decomposes on heating the solid. Volume 72, Number 7 J u l y 1968
2380
Figure 3. Acetic acid on deuterated germania: (A) gel subsequent to degassing a t 500°, after deuteration and degassing a t 420', (B) after exposure to 5 torr of HACfor 21 min; (C) after exposure to 10 torr of HAC for 35 min and degassing for 1 hr a t 25"; (D) after degassing for 30 min a t 210'. (The ordinates are displaced to avoid overlapping of traces.)
bands in the C-H region became more distinct. The intense band near 1700 cm-l lost some of its asymmetry on its low wave number side and appeared near 1720 cm-l. Other bands occurred near 1462, 1360, 1273, and 1180 em-'. An additional weak band was found a t 1080 cm-l in some spectra. The various "residual" bands, i e . , those remaining after an initial degassing a t 100-150", decreased continuously and in concert on progressive degassing up to approximately 300". Residual bands a t the same positions and exhibiting the same behavior on degassing were observed on exposing the adsorbent to lower HPr pressures, e.g., Figure 2. The intense 3600-1800 cm-l absorption and 1418-, 1380-, and 1230-em-' bands were not observed a t low surface coverage. Also, on first exposing a fresh, hydroxylated specimen to HPr at low pressure, e.g., a fraction of 1 torr, the Ge-OH band increased slightly but then decreased with increasing HPr pressure and surface coverage. Essentially the same behavior was observed when a hydroxylated sample was exposed to HAC and was then degassed, except that the broad absorption was not so extensive and the satellite bands were not found, Distinct bands indicating that physical adsorption had occurred were not observed. The bands subsequent to degassing a t 100-150" were a t 3028, 2940, 1715, 1370, and 1250 cm-l. The spectrum of HAC adsorbed on a deuterated surface is shown in Figure 3. The band in the 2700-2400-cm-' region is attributed to perturbed surface deuteroxyls (Figure 3, spectrum B). With HPr sorption a t 10 torr, the band positions observed were like those found with liquid HPr, with the exception of the 1360- and 1180-em-' bands. Liquid HPr also exhibits a broad absorption in the 3700-2000-~m-~region. These absorptions could easily be decreased, and in the case of those at 1418, 1380, and 1230 cm-I, can be removed entirely, by pumping a t The Journal of Physical Chemistry
J. C. MCMANUSAND M. J. D. Low relatively low temperature. Also, the apparently reversible behavior of the Ge-OH band was typical of that exhibited by surface hydroxyls in the presence of a weakly bound adsorbate. In view of such behavior, some of the effects observed a t high surface coverage are ascribed to physically adsorbed HPr. The satellite bands in the 2800-2400-~m-~region, found in the spectra of dimeric fatty acids,$ indicate that a t least a part of the physically adsorbed HPr was dimeric, The bands produced by sorbing acid at low pressure, where no significant amounts of physically adsorbed material were detected, and the identical bands observed on degassing subsequent to exposure to acids a t high pressure, indicate the existence of more tightly bound species. However, the possibility of HPr or HAC adsorption in undissociated, monomeric form must be , ~ suggested this to be considered, as Hirota, et ~ l . have the case for formic acid adsorbed on SiOz. There has been some controversy about the assignments and positions of bands of monomeric fatty acids. Wilmshurst claimed that v(C-0) occurred a t 1279 cm-1 for HAC and a t 1200 cm-' for formic acid, while the corresponding bands for 6(OH) occurred at 1192 and 1105 cm-l, respectively.lo These assignments were criticized,"J2 and now it appears that the situation is more complex. For example, it is thought13 that the v(C-0), 6(OH), and 6(CH3) vibrations of HAC interact but that a band a t 1182 cm-I can be assigned primarily to v(C-0), while another a t 1264 cm-l resulted mainly from 6(OH). Hadzi and Pintar12 found two characteristic bands in the spectra of monomeric acids in the regions 1380-1280 cm-l and 1190-1075 cm-' and concluded that while there is much coupling, the first band is due mainly to s(OH) and second band is due to v(C-0). For adsorbed HAC,the intense 1250-cm-' band might be attributed to v(C-0). That absorption occurs a t 1190 cm-l with the free monomer, but an upward shift would be expected if the monomer were held to the surface by hydrogen bonding. However, there was no band ascribable to 6(OH). The latter is of medium intensity under conditions of m e d i ~ m ~ hydrogen ,'~ bonding and is absent12 when hydrogen bonding is strong, and this might account for the absence of such a band. For adsorbed HAC, however, hydrogen bonding was weak, as evidenced by the relatively small perturbation of surface hydroxyls. Also, if the 1728-cm-' band of adsorbed HAC is taken as v(C=O), its shift from 1788 cm-l for gaseous12 HAC would be 2-3 times the shift usually found14for that band under conditions of weak hydrogen bonding. The situation is more difficult for adsorbed HPr, (9) D. Hadzi and N. Sheppard, Proc. Roy. Soc., A216, 247 (1953). (10) J. K. Wilmshurst, J. Chem. Phys., 25, 478, 1171 (1956). (11) R. C. Millikan and K. S. Pitzer, ibid., 27, 1305 (1957). (12) D. Hadzi and M. Pintar, Spectrochim. Acta, 12, 162 (1958). (13) M. Haurie and A. Novak, J . Chim. Phys., 62, 137 (1965).
INTERACTIONS OF PROPIONIC AND ACETICACIDSWITH GERMANIA because monomeric HPr does not appear to have been examined. However, from results for HACand butyric acid,la v(C-0) would be expected near 1190-1150 cm-1 and 6(OH) would be expected near 1380-1320 cm-l. For adsorbed HPr it seems most likely that the 1180cm-1 band is due to v(C-0), because it is intense. However, for monomeric HPr held to the surface by hydrogen bonding, v(C-0) would be expected a t frequencies higher than the observed 1180 cm-l. Also, 6(OH) would be shifted to higher wave numbers by hydrogen bonding; the observed 1280-cm-' band is lower than would be expected. The spectral evidence thus outlined does not favor the concept of HACor HPr adsorption in undissociated, monomeric forms, and this is supported by chemical evidence. I n view of the relatively small perturbation of surface hydroxyls, shown by the shift of only approximately 200 cm-l, more extensive desorption would be expected if the adsorbed layer consisted of monomeric acid molecules. As the adsorbed layer was retained a t 300", more tightly bound species are indicated. Comparison of what were earlier termed the residual bands with spectra of organic ester^'^^^^ and silyl esters1' shows remarkably good agreement in band positions in the 1800-1000-cm-1 region. Also, the relative band intensities are of the same order of magnitude as those for organic esters.16 Unfortunately, there appear to be no published spectra of germy1 esters, but such compounds exisP and have similar properties to their silicon and carbon counterparts; also, germanium tetraacetate exists.19 I n the absence of preferable alternatives, the present results can be best explained in terms of analogous surface structures. The residual bands of the tightly bound species are consequently attributed to surface esters. The prominent bands a t 1715 and 1720 cm-1 are attributed to v(C=O) modes, and those a t 1260 and 1180 cm-l are attributed predominantly to v(C-0) modes of acetate and propionate surface esters, respectively. Assignments for other bands follow those outlined by Katritzky, et d , l e who point out that there is a great deal of coupling. Thus the 1278-cm-1 band of propionate ester may have some contribution from the v(C-0) mode. Ester formation could occur in two ways R-C=O
/I\ H
R-C=O
H
2381
I n an attempt to distinguish between these mechanisms, other sorption experiments were made. On addition of acid to deuterated surfaces (Figure 3), the Ge-OH band was observed along with bands of hydrogenbonded OH groups, free Ge-OD groups a t 2707 cm-1, and hydrogen-bonded OD groups and those of the adsorbate. However, those results are not unequivocal, because exchange reactions7 may have occurred. Conversely, acid addition to completely dehydroxylated germania (Figure 2) clearly showed that free Ge-OH groups and hydrogen-bonded ones were formed. Thus reaction 1 is definitely operative. The extent of the esterification reaction (eq 2) remains uncertain. Occasionally small bands at 2190 and 1633 cm-l were observed when small quantities of HPr were added to either hydroxylated or dehydroxylated surfaces. The bands appeared together and declined and vanished on degassing a t 200" and were removed by exposure to HzO; they were not formed with HAC. The bands are shown in spectrum B of Figure 2, but these are more intense than those normally observed. Other work has shown that a band can form in the region 21702190 cm-' when Hz, HzO, or NHB are adsorbed on germania under certain conditions, and by comparison with the spectra of various germanes, the band was attributed to surface Ge-H species.'JO The 2190-cm-l band of spectraof adsorbed HPr can be similarly assigned, but the cause of its comparison band is uncertain. Carboxylate species would give rise to bands near 16501550 cm-l and 1440-1360 cm-l, the latter being the less intense.21 The present spectra showed no additional absorption near 1400 cm-l, but the germania itself was highly absorbing in that region, so that detection of a weak band would be difficult. However, the general behavior of the 2190- and 1633-cm-' bands leads to the speculation that propionate ions were responsible for the 1633-cm-1 band. A reaction of acid leading to carboxylate ions and Ge-H structures might occur a t a nonstoichiometric portion of the germania surface. The ions may be precursors of the ester species. As the compound germanium tetraacetate exists, the formation of bulk esters must be considered. Taking the surface area7 as 80 m2/g, it can be deduced that the surface coverage by adsorbed species did not exceed one monolayer at acid pressures of 1 torr (the amount adsorbed could not be measured, (14) 8. Bratoz, D. Hadzi, and N. Sheppard, Spectrochim. Acta, 8, 249 (1956). (15) H. W. Thompson and P. Torkington, J . Chem. Soc., 640 (1945). (16) A. R. Katritzky, J. M. Lagowski, and J. A. T. Beard, Spectrochim. Acta, 16, 964 (1960). (17) A. G. E. Robiette and J. C. Thompson, ibid., 21, 2023 (1965). (18) H. H. Anderson, J . Amer. Chem. Soc., 72, 2089 (1950). (19) H. Schmidt, C. Blohm, and G. Jandes, Angew. Chem., A59, 233 (1947). (20) M. J. D. Low and K. Matsushita, to be aubmitted for publioation. (21) N. B. Colthup, J . Opt. Soc. Amer., 40, 397 (1950). Volume 72,Number 7 J u l y 1068
2382
Figure 4. Propionic acid on hydroxylated surface. Continuation of the sequence of spectra of Figure 1. The spectra were recorded after degassing a t the following temperatures and times (in min): (A) 398", 30; (B) 433", 30; (C) 490°, 45; (D) 527", 105 plus 569", 60. The D ordinate is displaced.
and the values of extinction coefficients for the surface species are unknown and must be estimated). However, the skeleton of the spectrum of bulk germania was retained when acids were added, and considerable amounts of unreacted acid remained on the surface after acid addition a t high pressure. Thus if bulk ester is formed a t these high pressures, it must be limited to only a few monolayers. Behavior of the Adsorbed Layer above 300". When hydroxylated samples were treated with acid and subsequently degassed, the bands ascribed to the surface ester decreased and new bands appeared. Bands formed a t 2131 and 1767-1769 cm-' a t 300-320" with HPr-treated samples. The bands were not related, e.g., the intensity of the 2131-cm-1 band reached a maximum on heating to 420°, while the other band had begun to decline. The 2131-cm-' band could be detected after all ester bands and the second band had disappeared. Some spectra of a degassing sequence are shown in Figure 4. On degassing from 300 to 600", the Ge-OH band a t first increased but then decreased continuously with increasing temperature; the broad band due to hydrogen-bonded hydroxyls decreased continuously. The C-H bands also declined and, a t the highest degassing temperature, changed slightly in position and relative intensities. HAc-treated hydroxylated samples exhibited very similar behavior, except that bands formed at 2150 cm-l and at 1769-1776 cm-l. In general, the intensities of the new bands depended on the original outgassing temperature of the adsorbent and on the history of the sample. The bands were intense with a fresh sample outgassed a t relatively low temperature but, as the sample was subjected to sorption-degassing cycles, became less prominent. Also, the relative intensities changed, the bands near 1770 cm-1 decreasing to a lesser extent than the other bands. The new bands were not observed at all with samples which had been completely dehydroxylated by degassing at high temperature. However, when a completely dehydroxylated sample was rehydroxylated by reaction The Journal of Physical Chemistru
with HzO, the new bands were also not observed. Such effects would indicate that it is the texture of the adsorbent rather than the concentration of surface hydroxyls which governs the concentration of decomposition products. Assignments for the two pairs of bands are made difficult by the lack of other bands. Some spectra tended to show quite weak absorptions near 1180 cm-I for adsorbed HAC and near 1120 cm-1 for adsorbed HPr during the desorption process, but such results are of little value because the sloping background in that region obviates reliable measurements of very weak bands. However, the similarities of the two pairs of intense bands suggest that the products of the decomposition of each ester must have very similar structures; this assumption is implicit in the interpretation adopted. The single, fairly intense band at 2150 or 2130 cm-1 caused by one of the products might be attributed to either the Ge-CO or Ge-H structures. The latter is rejected because the band position is a function of the nature of the acid and the band intensities and positions are much different from those observed for the Ge-H bands. It is also unlikely that the bands were caused by adsorbed CO. The observation of the bands both above and below the frequency of v(C-0) for gaseous CO would indicate peculiar behavior of the C-0 bond of a hypothetical Ge-CO structure. There are no published data for CO adsorption on germania, but Bennett and TompkinsZ2have shown that CO is weakly adsorbed on oxidized germanium. It is unlikely that CO would remain adsorbed on germania above 100". In view of these objections, it seems that the 2150- and 2131-cm-l bands are best interpreted in terms of ketenic structures. It is suggested that the decomposition of the acetate and propionate esters produces adsorbed ketene and methyl ketene, respectively. Some indirect support (22) M. J. Bennett and F. C. Tompkins, Trans. Faraday Soc., 58, 816 (1962).
2383
INTERACTIONS OF PROPIONIC AND ACETICACIDSWITH GERMANIA for this suggestion comes from reports that methyl ketene is producedz3by the pyrolysis of HPr, and that ketene is formed when HACis passedz4over ZrOz-AlzOa. There is strong absorption in the ketene spectrumz5 near 2150 cm-l, owing to v(C=C=O); with methyl ketene,23v(C=C=O) absorbs at 2150 cm-l, although the data are meager. The v(C=C=O)' absorption of ketene is also approximately one order of magnitude more intense than other ketene absorptionslZK and this can readily explain the absence of other bands in the present spectra. I n view of the similarity of the band positions, the surface structures
I , , ,
'
3800"
HsC\
H
c=c=o
\C=C=O
I
/I\
I
Ge
\I/
Ge
are suggested. The hydrogen abstracted on adsorption of ketenes would form surface hydroxyls. Assignments of the bands near 1770 cm-1 are again made difficult because of the absence of other bands. However, the similarities of position and behavior of the bands derived from two different acids suggest that one surface structure is responsible for one 1770-cm-l band. Organic carbonates absorb near that frequency,z6and it is suggested that a surface carbonate
Ge
Ge
I
/I\
I
0
+
I
0 -
0
/I c
' 1500 ' onit ' '
'
''
I + H&=C=O I
(4)
/7\
0
0 R-C=O
'
or from a ketone, e.g.
is formed, giving rise to the 1770-cm-1 band. Indirect support for this suggestion comes from the observation that ketones formedz7from fatty acids over germania at Detailed kinetic studies were not made, but 360-450'. mixed ketones occurred when acid mixtures were used, Ketones were not indicating a bimolecular observed in the present study, but these would not be expected to bond strongly to the surface. A similar bimolecular process is suggested for the present thermal decompositions R-C=O
'
+0
A
I
ZOW
H
HsC-C=O
II
0
"' '
surface carbonate. The latter would decompose to Ge-0-Ge bridges and COZa t high temperatures. The formation of ketenes, and the subsequently adsorbed ketenes, could occur through the decomposition of the surface ester, e.9.
I 0 I
0
'
Figure 5. Propionic ester decomposition. After the gel had been degassed a t 530' (spectrum A), it was exposed to 1 torr of H P r for 5 min and then degassed for 1.5 hr (spectrum B). The sample was then heated in the isolated cell for 30 min at the following temperatures: (C), 325", 365'; (D) 410°,480'. Spectra were recorded with the sample a t 25". The ordinates are displaced.
0
\I/
3 'm
It /"\
+o
0
(3) resulting in the formation of ketone and the postulated
I/
CzHs-C-CzHs
GeOz
-+H&HC=C=O
+ C2HO
(5) I n view of the earlier generalizations relating the intensities of the 1770- and 2150-2130-~m-~bands to the sample history and pretreatments, it is suggested that most of the ketenes were derived from ketones produced, held, and allowed to react within the pores of the gel, This would readily explain why ketenes were not observed with samples which had been subjected to very severe degassing and consequently were sintered. Some experiments were carried out in which an acidtreated sample was heated with the cell isolated from the pumping system. Spectra were recorded after the (23) P. G. Blake and K. Hole, J . Phys. Chem., 70, 1464 (1966). (24) V. I. Yakerson, L. I. Lafer, A. L. Klyachko-Gurvioh, and A. M. Rubinshtein, Izu. Akad. Nauk SSSR, Ser. Khim., 1, 83 (1966). (25) F. Halverson and V. A. Williams, J . Chem. Phys., 15, 552 (1947). (26) B. M. Gatehouse, 9. E. Livingstone, and R. S. Nyholm, J . Chem. SOC., 3137 (1958). (27) P. Mastagli, P. Lambert, C . Hirigoyen, and J. Pourriere, Compt. Rend., 250, 1507 (1960).
Volume 78, Number 7 July 1968
H. E. SPENCER AND J. 0. DARLAK
2384 sample had cooled. Some results are shown in Figure 5 . On progressive heating, the following occurred. (a) The ester concentration decreased, but more slowly than when the cell was evacuated. This suggests that some ester decomposed by the reverse of reaction I; the ester would re-form when the adsorbent cooled. (b) Free and hydrogen-bonded hydroxyls increased and bands in the C-H region changed. The band at 2990 cm-I shifted to 2976 cm-l, while the relative intensity of the 2890-cm-l band increased, indicating the formation of a new surface structure. It is known that acetone is decomposed to 2-methylpropene7
COz,and HzOover the SiOz-A1zOa catalyst.28 A similar reaction is suggested, ketone being formed from surface ester and decomposing to yield chemisorbed water and olefin. (c) The 2130-~m-~band was very small and the 1770-cm-l band was absent. The decrease in ketene bands could result from reaction of the ketene with a neighboring OH to form the surface ester or with the acid to yield the ketone and COz. The surface carbonate could be destroyed by reaction with the acid produced by the reversal of eq 1. (28) M. Demorest, D. Mooberry, and J. D. Danforth, I n d . Eng. Chem., 43, 2569 (1961).
Lead Bromide Photochemistry: Reduction of Lead Ion and Oxidation of Leucocrystal Violet by H. E. Spencer and Jennifer 0. Darlak Research Laboratories, Eastman Kodak Company, Rochester, N e w York 14660
(Received October 26, 1967)
Ultraviolet irradiation of microcrystals of lead bromide at 22" produces metallic lead and induces the oxidation of adsorbed leucocrystal violet to crystal violet. In terms of incident quanta, the sensitivity for production of either lead or crystal violet is higher at 313 and 335 mp than at 254 or 366 mp. The quantum yield for crystal violet production is 0.15 molecule/absorbed quantum at 254 mp and 0.31 molecule/absorbed quantum at 313, 335, and 366 mp. Adsorbed 3,3'-diethylthiacyanine ethyl sulfate and 3-carboxymethyl-5-(3ethyl-2-benzothiazoliny1idene)rhodaninespectrally sensitize crystal violet production, but not lead production, to about 500 mp. The rates of production of lead and of crystal violet are proportional to the first power of the intensity of irradiation. Increasing the temperature to 100" appreciably changes the sensitivity for lead production only at the longer wavelengths at which absorption of radiation is increased. Possible mechanisms are discussed in terms of the band structure of PbBrz.
Introduction
crystalline lead bromide dispersions were, therefore, used in this work. The electron-hole pair created in a solid by the absorption of radiation can react further; electrons can cause reduction, whereas holes can cause oxidation. We shall report here an electron reaction, the reduction of lead ion to lead, and a hole reaction, the oxidation of
Among the recent papers dealing with the photodecomposition of inorganic solids are those concerned with lead halides, namely: PbF2,' PbC12,2-4 PbBrz,4 and PbIzU4,5In this paper we shall discuss the photochemistry of lead bromide. Wells6first observed that lead bromide blackens when exposed to sunlight. Norris' showed that lead was (1) D. A. Jones, Proc. Phys. Soc., B68, 165 (1955). formed and that bromine was lost during irradiation. (2) W. C . Tennant, J . Phys. Chem., 70, 3523 (1966). Sanyal and Dhar8 also reported a loss of bromine. (3) A. Kaldor and G. A. Somorji, ibid., 70, 3538 (1966). ~ ~(a) J. F. Verwey, J . Phys. Chem. Solids, 27, 468 (1966); (b) Using an optical method for measurement, V e r ~ e y ~ a (4) calculated the quantum efficiency of lead production in J. F. Verwey and J. Schoonman, Physica, 35, 386 (1967); (c) J. Arends and J. F. Verwey, P h y s . Status Solidi, 23, 137 (1967); (d) irradiated single crystals. No measurements have J. F. Verwey, Ph.D. Thesis, University of Utrecht, 1967. been made of the quantum yield (@)for the production ( 5 ) (a) R. I. Dawood and A. J. Forty, Phil. Mag., 7 , 1633 (1982); (b) R. I. Dawood and A. J. Forty, ibid., 8, 1003 (1963); (c) M. R. of bromine or for the production of either bromine or Tubbs and A. J. Forty, Brit. J . A p p l . Phys., 15, 1553 (1964). lead when microcrystals of lead bromide are used. (6) H. L. Wells, Amer. J . Sci., 45, 121 (1893). Because microcrystals have a large surface-to-volume (7) R.S . Norris, A m e r . Chem. J., 17, 189 (1895). ratio, they are particularly suited to the study of solid(8) A. K. Sanyal and N. R. Dhar, 2. Anorg. AZZgem. Chem., 128, 212 (1923). state reactions which occur a t surfaces. hficroT h e Journal of Physical Chemistry
