August, 1958
PHOTOCHEMICAL PROPERTIES OF ANTIMONY TRIOXIDE
known practical f a c P J 9 that salts which are neither detergents nor strongly adsorbable materials may also promote detergency. Surface Coverage.-By analogy with the behavior of condensed films on water it might seem reasonable that changes in the adsorption isotherm could arise from changes in packing or orientation of the adsorbed film. However, a t the c.m.c. the of adsorption of 18.6 mg. of SDS per 22.4 surface corresponds to an area per adsorbed molecule of 57.4 if they are uniformly distributed over the surface. Even a t maximum adsorption the area is still 44.8 A.2 per molecule. Since the amount adsorbed is insufficient t o form a closepacked film over the whole surface it therefore seems (28) P. W. 0.Wijga, Proc. First World Conoress on StbTfaCe Active Agents, 1, 19 (1944), Chatnbre Syndicate Tramopras, Paris, France. (29) W . W . Niven, “Fundamentals of Detergency,” Reinhold Publ. Corp.. New York, N. Y.,1960.
989
unlikely that changes of state such as can occur in such films play any part in determining the shape of the isotherm. Effect of Electrolytes.-If the assumption of surface micellisation to account for the maximum in the adsorption of SDS by carbon is correct its position should he shifted to lower concentrations by the addition of electrolyte. This is in fact the case as already has been shown by Phansalkar.21 Quantitative treatment of these data will be communicated later. Acknowledgment.-The authors wish to thank Dr. M. J. Vold for assistance in quantitative formulation of the present hypothesis and Professor K. R. Krishnaswami for his encouragement throughout the course of the work. They also wish to express their appreciation to Dr. S. R. Sivaraja Iyer for spectroscopic analysis of the ash.
PHOTOCHEiMICAL PROPERTIES OF ANTIMONY TRIOXIDE BY M.
CLARE
MARKHAM, M. CONSILIA HANNAN, LILY LIN, CLAIRE COFFEY AND BARBARA JONES Department of Chemistry, Saint Joseph College, West Hartford, Connecticut Received April 89, 1968
Photochemical properties of antimony trioxide depend on the crystal structure. The rismatic form of SbzOa darkens on exposure to light, presumably undergoing oxidation-reduction. This darkening takes pkce even in the presence of oxygen in aqueous suspensions and is enhanced by organic substances. Rhombic antimony trioxide does not darken under these conditions. It acts in part like a photocatalyst for the oxidation of organic substances in a manner similar t o zinc oxide. I n both cases hydrogen peroxide is one of the products. I n the absence of oxygen, but in presence of water and organic materials-or in absence of water, but in presence of oxygen and organic materials-rhombic antimony trioxide also darkens. Similarities to and differences from the zinc oxide reaction are discussed and interpreted. The glycerol was Matheson, Coleman and Bell, reagent grade. Antimony pentoxide was prepared by oxidizing Sb203with aqua regia and heat,ing a t 300” for several hours. The apparatus.consisted of a quartz jacket through which water a t constant temperature was circulated. Stirring was effected by bubbling- air (or nitrogen) through the suspensions. The suspensions were irradiated bv light from a Hnnovia high pressure quartz mercury arc. -Antimony trioxidr absorbs strongly below 3250 A . in the ultraviolet. The reflectance curves of zinc oxide and of the two forms of antsimony trioxide are compared in Fig. 1. The radiation at 3650 A. which produces photochemical reactions on zinc oxide is only very slightly effectipe for antimony trioxide. The radiation at 3130 and 3025 A . initiates reactiona with both oxides. Measurements of quantum yields were made with a Bausch and Lomb monochromator and a mercury arc lamp, a t 3130, 3025 and at 2537 A. Uranyl oxalate actinometer solution was used to determine radiation Materials and Arrangement of Apparatus.-Antimony intensit8yaccording to the method of Leighton and Forbes.2 trioxide obtained in reagent grade from Coleman and Bell, Identification of Products .-The irradiated suspensions Mallinckrodt, Baker, and Hopkins and Williams, Ltd., were were filtered and the filtrates analyzed for (1) substances all tested and failed to darken in aqueous suspension with capable of oxidizing iodide ion (hydrogen peroxide, organic organic substances in presence of air. Antimony trioxide pcroxides, pentavaleiit antimony); (2) peroxides capahlc of Coleman and Bell urns used for most of the tests described of forming a yellow color wit,h titanium sulfak; (3) electroI d o w ; X-ray analysis showed it to be almost purr rhombic rcducible siibstenccs with the polarograph; (4)prescnce of form and it will be designated sulmquently as Sbz03(rh). aldehydes in the ahsorption spect,re; (5) changes i n PIT. Prismatic antimony trioxide was prepared, as described hy (J3l;tnks wrre run on Sb#& wit,h water and with glyc-erol Cohn and Goodeve,’ by precipitation from solutions of solutions of varying concentration in the dark and with antimony trichloride on addition of ammonia. The pre- glycerol solutions in absence of SbzOa in the light.) cipitated antimony trioxide was then washed until it gave no Measurements of oxygen uptake hv the irradiated suspentest for chloride ion. X-Ray analysis showed that this sions were made manometrically in Warburg flaslrs. antimony trioxide was approximately a 50-50 mixture of Results gismatic and rhombic forms, and it will be designated as bzOdpr ) . Photochemical Reactions of Prismatic Antimony
Introduction The darkening of antimony trioxide on exvosure t o ultraviolet light was studied by Cohn and Goodeve.’ They found that certain naturally occurring and synthetic forms showed more intense darkening than others and that the addition of organic materials and ammonia increased the darkening. Glycerol was among the most effective additives. As part of a general program of study of photochemical reactions on the surface of photoconducting solids, it was decided t o investigate the products of these photochemical changes on antimony trioxide and compare them with photochemical reactions catalyzed by zinc oxide and light. Experimental
(1) Q. Cohn and C. F . Goodeve, Trans. Faraday Sac.. 36, 433-440 (1940).
(2) W.G. Leighton and G. 8. Forbes, J . Am. Chem. Sac., 62, 3139 (1930).
M. C. MARKHAM, M. C. HANNAN, L. LIN, C. COFFEY AND B. JONES
990
Vol. 62
1
The darkening is reversible. After the light is turned off the darkened powder slowly returns to its original white color on exposure t o moist air. Photochemical Reactions of Rhombic Antimony Trioxide.-SbO&h) shows no darkening when irradiated in suspensions of water or glycerol solutions of varying concentration in presence of oxygen. There is a marked drop in pH indicating the accumulation of hydrogen ion. Tests with Ti(SO& reagent show amounts of peroxide varying with the concentration of glycerol as indicated in Table I. This table also shows the amounts of substances 310 330 350 370 390 410 430 capable of oxidizing iodide ion as measured by titraWave length, mp. tion of the liberated iodine with standard thiosulFig. 1.-Reflectance curves for: (A) ZnO; (B) Sbz03(pr.); fate solution. Note that the two analyses do not (C) Sbz03(rh.). give equal results; therefore it seems likely that in the titration with thiosulfate one is also measuring pentavalent antimony. Specially prepared antimony pentoxide was suspended in water and the d, 100 filtrate was found t o oxidize iodide ion readily under the conditions of these titrations in approxid! mately N sulfuric acid. Typical polarograms 80 shown in Fig. 2 indicate only hydrogen peroxide 3 j 60 (0.8 to 1.2 v.) and hydrogen ion (1.4 to 1.6 v.) when performed on the irradiated filtrates which have a 3 $ 40 pH of about 3. On neutralizing the filtrates with u lithium hydroxide, the current rise a t 1.4 to 1.6 v. is 20 suppressed. A new rise in current appears a t 1.55 to 1.8 v., characteristic of glyceraldehyde or dihydroxyacetone. Absorption spectra, Fig. 3, 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 also indicate the presence of glyceraldehyde. AlVolts us. S.C.E. dehydes in general are hydrated in acid solution Fig. 2.-Polarograms: (A) 1.0 M glycerol, SbzOs(rh.), and are not reduced quantitatively a t dropping (B) 1.0 M glycerol, mercury electrodes until the solution is neutralized. stirred with air one hour in dark, ;":1 100 90
t
/-
SbtOs(rh.), irradiated one hour, 15 ; pH 3.1; (C) 1.0 M glycerol, Sbz03(rh.),as in (B); filtrate adjusted to pH 7.1 with 0.1 M LiOH. 1.0 0.8
1
A
TABLE I IDENTIFICATION OF PRODUCTS FROM IRRADIATED SUSPENSIONS OF SbzOs(rh) 25 ml., 0.2 g. of SbzOa(rh), 15' Total Substances capable of oxidizing iodide ion and t.itrated with Siosx 103
o.2k 3
,-,
,
~
, ,
230 240 250 260 270 280 290 300 310 Fig. 3.-Absorption spectra: (A) 1.0 M glycerol; (B) 10-3 M glyceraldehyde; (C) 0.5 M glycerol irradiated 30 minutes with Sb203(rh.),303-313 mp, 20'.
Trioxide.-The precipitated antimony trioxide, SbzOa(pr),darkened rapidly on exposure to ultraviolet light, even on bubbling oxygen through the suspensions, and using the concentration of glycerol and the temperature found to be optimum for peroxide formation with the rhombic crystal form. Some hydrogen peroxide and aldehyde or ketone was found in the filtrate, but since X-ray analysis showed that approximately 50% was the rhombic form, it seems likely that this form was responsible for the peroxide formation. I n fact initial yields of hydrogen peroxide with this precipitated antimony trioxide were only about half those of Sbz03 (rh).
Peroxides measured colorimetrically with Ti(S03z x 108
"P
3.9 39.6 11.2 Water 15.0 3.4 58.0 0.10 M glycerol 3.3 89.4 20.0 .50 hl glycerol 3.2 30.0 102.0 .75 M glycerol 3.1 32.2 96.0 1.00 M glycerol 3.1 37.5 95.0 2.00 M glycerol 3.1 36.2 69.0 3.00 M glycerol 3.1 27.4 39.6 5.00 M glycerol .. 0.0 0.0 Pure glycerol .OM a The pH of suspensions of SbsOs(rh) in water or glycerol in the dark in presence or absence of air, is approximately 5.5.
Optimum concentrations of glycerol for peroxide and glyceraldehyde formation range from about 0.5 t o 1.5 M . Highest yields of these oxidation products are obtained between 15 and 25'. Measurements of oxygen uptake indicate a ratio of 2:1, for moles of O2 t o moles of peroxide. The differences between the number of milliequivalents resulting from the oxidation of iodide ion and the amount of peroxide found on analysis with Ti(SO&, as well as the ratio of oxygen to peroxide found in measurements of oxygen uptake, seem t o indicate formation of pentavalent antimony. Sbz05
August, 1958
PHOTOCHEMICAL PROPERTIES OF ANTIMONY TRIOXIDE
forms a definitely acid solution in water, pH 2-3. Furthermore, as the concentration of organic substance is increased, the ratio of milliequivalents in columns 2 and 3 of Table I decreases, indicating competing reactions. As more of the organic substance is oxidized, less pentavalent antimony seems t o be present in proportion t o the amount of peroxides. Since the pH remains about 3 it is likely that glyceraldehyde is also being oxidized t o glyceric acid. When the suspensions of Sbz03(rh)in 1.0 M glycerol are irradiated in the absence of air, using nitrogen for stirring, only insignificant amounts of peroxide and glyceraldehyde are formed. The Sbz03 shows visible darkening. Whether these minute quantities are significant, or merely the result of adsorbed oxygen, needs further investigation. In the case of ZnO, irradiated in aqueous suspension under nitrogen, no peroxide is detectable. Furthermore when suspensions of Sb203(rh)in pure glycerol are irradiated in the presence of air, no peroxides are detectable in the reaction mixture, either by test with iodide ion or titanium sulfate. I n this case also SbzOa(rh)shows noticeable darkening. Comparison of the Photochemical Oxidation of Glycerol on Sb203(rh)and Zn0.-Figure 4 demonstrates clearly that the optimum concentration of glycerol for peroxide formation is the same for both Sb203(rh)and ZnO. Both reactions have a similar dependence on temperature, with an optimum temperature for formation of peroxide a t about 1525", as shown in Table 11.
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1.0 1.5 2.0 2.5 3.0 3.5 Concn. glycerol (M). Fig. 4.-Concentrations of peroxide formed in 30 minutes in solutions of glycerol of varying concentration irradiated: (A) in suspensions of SbzOs(rh.); (B) in suspensions of ZnO; 303-313 mp, 15". 0.5
radiation in the ultraviolet absorption band causes electron transfer within the crystal, and that the excited electron can be transferred in aqueous suspensions to oxygen adsorbed on the surface, forming a reduced oxygen molecule having properties of a peroxide radical ion .OZ:-. I n the absence of organic additives the peroxide accumulates to a small but easily detectable extent. The reaction is reversible. Excess added hydrogen peroxide decomposes on irradiation at 3650 A. in suspensions of ZnO. Oxidizable organic additives act either to return electrons t o the photo-positive crystal and prevent polarization, or t o furnish hydrogen atoms to the peroxide ion. The result is a large increase in concentration of peroxide and eventual oxidation TABLE I1 of the organic additive to carbon dioxide and water. COMPARISON OF AMOUNTS OF PEROXIDE FORMED IN SusThe net result is the reduction of oxygen from the PENSIONS OF SbtOa(rh) AND ZnO IN 1.0 M GLYCEROL AT air to peroxide and the oxidation of the organic subDIFFERENT TEMPERATURES, IRRADIATED 0.5 HR. stance. ZnO is a true photo-catalyst. ZnO A remarkable feature of the irradiation of Sb203Temp., meq. SzOs-)25 ml. Sb2Oa(rh) (rh) in pure water, in absence of organic matter, 10 170 35 but in presence of oxygen, is the accumulation of 15 48 larger amounts of peroxide than are found in the 20 219 ZnO systems. Another important feature of the 25 54 reaction with Sbz03(rh) is the presence of an oxidiz30 162 ing substance which liberates iodine but does not, 35 42.5 give an equivalent peroxide test with Ti(SO&. AnZinc oxide however has much higher quantum timony (plus 3) can go to a stable higher valence yields for peroxide formation. That zinc oxide state, whereas Zn (plus 2) cannot. Probable reacshould be more effective than Sb203as a photo-oxi- tions in absence of organic matter are dation catalyst in the light of the full mercury arc is hu not surprising, since the high pressure mercury arc Sb103(rh) + 2Sb (reduced) + SbzOs emits considerable radiation at 3650 A., where zinc dark oxide absorbs strongly, but which affects only a O2 JH20 small fraction of the surface of Sbz03(rh). HOTever, with the monochromator at 3130 and 3030 A. Sb203 + .OnH + (HzSb04)ZnO has 4-5 times the activity of Sbz03(rh). The quantum yield of SbaOo(rh)is somewhat higher at 2537 A. HOz. The p H of irradiated ZnO suspensions changes 2H02. +H202 + 02 very little from 6.5 to 5.8 with 1.0 M glycerol a t At any rate, both O2 and HzOseem to be necessary 15", whereas the p H of the irradiated Sb203(rh) for the reoxidation of the photo-reduced antimony. changes from about 5.6 to 3.1. If either one is missing the Sbz03(rh) darkens and Interpretation and Conclusions peroxide formation is inhibited. The principal difNumerous studies on Zn03+ have shown that ir- ference between the Sb203(pr)and Sb203(rh)would OC.
.1
'
4"
(3) M . C. Markham and K . J. Laidler, THISJOURNAL, 67, 363 (1953). (4) T. R. Rubin, J. G. Cslvert, G. T. Rankin and W. MacNevin, J . Am. Chem. SOC.,75, 2850 (1053).
(5) J. G. Calvert, K. Theuer, G. T. Rankin and W. MacNevin, ibid., 76, 2575 (1954). (6) G. V. Elmore and H. A . Tanner, THISJOURNAL, 60, 1328 (1956).
C.H. HOLM,C. R. ADAMSAND JAMESA. IBERS
992
thus seem to be the ease of reoxidation of antimony in the two crystal structures. Znl is probably more easily reoxidized than antimony; however, zinc has no stable higher oxidation state, therefore the accumulation of peroxide in suspensions of ZnO without organic additives reaches a low steady-state concentration. I n the case of Sb20s(pr)there must be greater difficulty of reoxidation and the crystal structure favors photolysis rather than photo-catalytic oxidation properties. In the case of SbzOa(rh) the crystal structure permits rapid reoxidation with the net result that 0 2 from the air is reduced in part to peroxide and both antimony and organic additives are oxidized. The hydrolysis of Sb20swould produce hydrogen ion. It is not certain that all the peroxide measured is hydrogen peroxide. I n fact there is some indication that organic peroddes are present in intermediate stages. Ammonium molybdate increases the rate of reaction of H202with iodide ion. There is no visible additional release of iodine on adding molybdate to the filtrates from suspensions of these oxides in glycerol solutions, whereas the filtrates of irradiated suspensions of ZnO with phenolic substances show increased iodine formation. In addition there is often a repeated appearance of starchblue after apparently completing the titration of the iodine originally released. Sometimes the titration must be continued over a period of an hour or more, and it is necessary to run blanks with every titration. A probable series of reactions in presence of glycerol might be
eo2-(on surface of irradiated oxides) + CH2-CH-CHz bH
+
(1)
bH dH
HOz-
+ .CH-CH-CHz
VOl. 62
(or CHFCH-CH~) 0 OH OH
OH AH AH * .CH-CH-CH2 0 2 +*02-CH-CH-CHz
+
dH dH dH *Oz--CH-CH-CHz
dH dH dH
+ CH2-CH-CH*+
dw
AH dH CH-CH-CHz
AH AH
(2)
I
+
OH
dH dH dH HOz-CH-CH-CH2
I
(3)
1
OH OH d H
The latter substance is an a-hydroxyalkyl hydroperoxide and would be equivalent t o an addition product of H202and glyceraldehyde. This addition product probably would dissociate in dilute solution to give HzOzand glyceraldehyde, thus explaining the increased concentration of peroxide and the presence of glyceraldehyde (or dihydroxy acetone) in the reaction products. The temperature dependence in the presence of organic material indicates the necessity of an activation energy for some step, possibly step 3. The falling off of peroxide formation at higher temperatures probably is due to the instability of some of the peroxides at higher temperatures. It is significant to note that the amount of glyceraldehyde also decreases at higher temperatures. Acknowledgments.-The authors are indebted to Dr. Ray Pepinsky of the X-Ray and Crystal Analysis Laboratory of Pennsylvania State University for performing the X-ray analyses of the antimony trioxide samples. This work was made possible in part by grants from Research Corporation and Sigma Xi-RESA Research Fund.
THE HYDItOGEN BOND I N BOEHMITE BY C. H. HOLM,C. R. ADAMSAND JAMES A. IBERS Shell Development Company, Emeryville, Calijornia Received April 10,1068
Although the crystal structure of boehmite a-AlOOH, has been the subject of two X-ray investigations, the symmetry of the 0-H-0 bond in boehmite has remained in doubt. The determination by a nuclear magnetic resonance technique of the symmetry of this bond is reported upon here. The 0-H-0 bond in boehmite is found t o be asymmetric.
Introduction The crystal structure of boehmite, (r-AlOOH,has been the subject of two X-ray Typical of X-ray investigations, neither provides any direct information on the nature of the hydrogen bond in boehmite. Moreover, the two investigations are in disagreement with regard to the length of that oxygen-oxygen interaction which is hydrogen bonded: Reichertz and Yost find the while , Milligan and McAtee distance to be 2.47 i% find it to be 2.69 A. The general question of the symmetry of hydrogen bonds is an important one in structure chemistry, and it has been discussed by (1) P. P. Reichertr and W. J. Yost, J . Chcm. P h w . , 14, 495 (1946). (2) W. 0. Milligan and J. L. McAtee, THISJOURNAL, 60, 273 (1956).
many workers. Donohues in his summary states that it is safe to conclude that all 0-H-0 bonds with the 0-0 distance greater than 2.54 A. are asynimetric.4 On this basis, primarily because of the disagreement between the two X-ray investigations on the length of the 0-0 distance, the symmetry of the hydrogen bond in boehmite remains in doubt. Here we report on the application of a nuclear magnetic resonance (n.m.r.) method to the determination of the symmetry of this hydrogen bond. (3) J. Donohue, {bid., 66, 502 (1952). (4) This statement is perhaps not s d e , for R. M. Douglass (Acta Cruet., 10, 423 (1957)) in his study of the crystal structure of HCrOp finds an 0-0distance of 2.55 A. and concludes, largely on the basis of indirect evidence, s.g., the absence of a piezoelectric effect, that the 0-H-0 bond is symmetric.
