Langmuir 1989, 5, 1051-1056 parently exhibit a positive departure from the additive ideality of the area over the whole composition range. The concept "condensing effect" is based on the experimental fact that the mean area of a mixed spread monolayer deviates negatively from the straight line joining those of the pure monolayers. However, monolayer studies suffer some disadvantages. Because of the strong lateral interaction of cholesterol molecules, the measurements can be made only for the condensed state of the cholesterol film. Since biological membranes are usually in a liquidcrystal state, the condensed-state film is inadequate to a model membrane to verify the condensing effect. In the condensed film, moreover, the cholesterol molecules have a limited miscibility with hydrocarbon chains! This means that we have no method to obtain the straight line used commonly as a reference curve of the ideal mixing. In addition, the compressing process of the spread monolayer usually accompanies the hysteresis. Comparing the adsorbed film with the spread monolayer shows that the present study is free from the difficulties mentioned here. The above experimental results do not conform with those of the condensing effect of cholesterol. If the variation of the mean area with the composition is assumed to be unaffected by carbon tetrachloride and water molecules in the film, the partial molar areas of the components are obtained from the intercepts of the tangent line on the coordinates. As can be seen from Figure 6, the partial molar area of octadecanol almost remains constant up to the composition of about 0.5 and then increases strongly with increasing composition. On the other hand, the partial molar area of cholesterol has a larger value than the molar area of the pure one and decreases to that with increasing composition. Such a variation in the partial molar area is attributable to the stronger cohesive force
1051
between cholesterol molecules than that between octadecanol and cholesterol molecules. Thus, we can conclude that the cholesterol molecule does not act as the condensing agent in a hydrocarbon chain region of membrane. This conclusion is supported by the fact that the condensed film is composed almost completely of cholesterol (see Figures 2 and 3). It is also supported by the experimental work of Motomura et a1.: in which fatty acids do not mix with cholesterol in the condensed state and mix in a limited manner in the expanded state. The partial miscibility of cholesterol in the condensed film corresponds to the phase separation of the phospholipid vesicle suspensions into cholesterol-rich and cholesterol-poor regions below the gel/liquid-crystalline phase transition temperature.16-18 Although cholesterol molecules are located so that their hydroxyl groups can be in the immediate vicinity of the phospholipid ester carbonyl group^,^^^^^ the role of cholesterol molecules in biological membranes involves primarily the lateral interaction with fatty acyl chains. The effect of cholesterol on these acyl chains should be the same for any model membrane system. Thus, our conclusion conflicts with the current knowledge that cholesterol decreases the mean area per molecule occupied by saturated and monosaturated phospholipids in monolayers a t the air/ water i n t e r f a ~ e . ~Taking -~ account of the disadvantages of spread monolayers, accordingly, the data on the spread monolayers must be analyzed with great caution. (16)Blume, A.; Griffin, R. G. Biochemistry 1982,21,6230. (17)Presti, F.T.;Chan, S. I. Biochemistry 1982,21,3821. (18)Recktenwald, D.J.; McConnell, H. M. Biochemistry 1981,20, 4505. (19)Franks, N.P. J. Mol. Biol. 1976,100,345. (20)Wroester, D.L.;Franks, N. P. J. Mol. Biol. 1976,100,359.
Adsorption of Formaldehyde on Model MgO Surfaces: Evidence for the Cannizzaro Reaction X. D. Peng and M. A. Barteau* Center for Catalytic Science and Technology, Department of Chemical Engineering, University of Delaware, Newark, Delaware 19716 Received January 3, 1989. In Final Form: April 17, 1989 Simultaneous formation of formate and methoxide species from formaldehyde on model MgO surfaces was observed with XPS and UPS. This reaction can be explained by a mechanistic route similar to the liquid-phase Cannizzaro reaction. The surface sites involved in this reaction are the base sites which are also responsible for the dissociation of methanol and water, as demonstrated by blocking experiments with these reagents. The difference between this reaction and the reaction of formaldehyde on other oxides to produce formates exclusively suggests that the operative pathways for consumption of hydrogen atoms eliminated from the carbonyl group may determine the reaction selectivity for formaldehyde conversion on oxide surfaces. Introduction Surface formates and methoxides are two of the most ubiquitous reaction intermediates encountered in C1 chemistry on solid surfaces. Formation of surface methoxides from methanol and of surface formates from formic acid, methanol, CO/H,, and CO/H20 on metal and metal oxide surfaces has been reported in numerous adsorption
* Author t o whom t h e correspondence should be addressed.
studies.'-12 Interconversion of these surface species also represents an important reaction sequence in certain (1)Bowker, M.; Madix, R. J. Surf. Sci. 1981,102,542. (2)Bowker, M.; Madix, R. J. Surf. Sci. 1980,95,190. Egawa, C.; Aruga, T.; Iwasawa, Y. Surf. Sci. 1987,191, (3)Onishi, H.;
479. (4)Vohs, J. M.; Barteau, M. A. Surf. Sci. 1986,176,91. (5)Vohs, J. M.;Barteau, M. A. Surf. Sci. 1988,197,109. (6)Kim, K. S.; Barteau, M. A.; Farneth, W. E. Langmuir 1988,4 , 533-543.
0743-7463/89/2405-lO51$01.50/0 0 1989 American Chemical Society
1052 Langmuir, Vol. 5, No. 4, 1989
Peng and Barteau
catalytic processes. For instance, surface formate species provided by radiation from a tungsten filament and conduction to a liquid nitrogen reservoir, respectively. The sample temare believed to be one of the precursors of surface methperature was monitored with a chromel-alumel thermocouple. oxides, which are then converted into methanol, in the The thermocouple was wedged between the crystal and the synthesis of methanol from CO and HPl3J4 Conversely, tantalum clip in the case of the metal sample; it was attached to surface methoxides have been proposed to be responsible the edge of the oxide crystal with high-temperature cement. for the formation of formaldehyde via methanol oxidation The procedures for preparation of the clean Mg(0001)surface over metal and metal oxide ~ a t a 1 y s t s . l ~ In order to unby argon ion sputtering the annealing have been described prederstand C1 chemistry on solid surfaces, the adsorption viously.26 MgO films were formed on this surface by a ca. 100properties of formaldehyde on various materials have been langmuir exposure to O2 in the vacuum chamber, followed by studied by surface science techniques, as have those of annealing at 470 K. The detailed characterization of these films is reported elsewhere;n however, angle-resolved XPS experiments methanol and formic acid. In general, formaldehyde demonstrated that these stoichiometric films were of the order molecules are oxidized to formate species at room temof 20 A thick. Thin films were used in order to circumvent perature or below on metal oxide and oxygen-dosed tranelectrostatic charging problems associated with insulatingsamples sition-metal s ~ r f a c e s . ' J " ~ ~A different reaction scheme, as bulk MgO. In fact, it was the charge-free feature of the however, has been proposed by Busca and c o - w ~ r k e r s . ~ ~such ~ ~ ~ thin-fii sample that made the characterizationof surface species Both methoxy and formate species were detected by IR by UPS possible. XPS spectra were easily obtained for both spectroscopy in their studies of formaldehyde-dosed Ti02, thin-film and bulk oxide samples. Charging of the bulk oxide Tho2,and V-Ti oxide surfaces. T h e observation of both was observed in the form of a shift in the apparent kinetic energy hydrogenation and oxidation of formaldehyde molecules of the photoelectrons;however, this effect appeared to be uniform in that the line widths were comparable to those observed for the was attributed to a Cannizzaro-type disproportionation thin-film samples. No charge dissipation measures, e.g., use of reaction taking place on surface base sites. an electron flood gun, were therefore required. The binding energy In order t o understand the routes for surface formate references for XPS and UPS spectra from these model MgO and methoxide formation, the adsorption of formaldehyde surfaces are discussed in detail elsewhere.2s For the thin-film on MgO, known for its strong base character,24 was insample, XPS spectra were referred to the Mg(2p) peak at 49.5 vestigated by XPS, UPS, and TPD. The MgO(100) sineV arising from metallic magnesium beneath the oxide layers, and gle-crystal plane and magnesium oxide thin films formed UPS spectra were aligned relative to the low binding energy edge on the Mg(0001) surface were used as model MgO surfaces. of the O(2p) emission at 6.1 eV. In the case of the MgO(100) These surfaces have been shown to be models for the surface, XPS spectra were referenced to the Mg(2p) peak of acid-base properties of MgO powders.25 Formation of magnesium oxide at 50.8 eV. Formaldehyde vapor was introduced from a sample tube both methoxy and formate species was observed by XPS containing solid paraformaldehyde through a manifold into the and UPS on formaldehyde-dosed MgO surfaces, characUHV chamber. The paraformaldehyde was heated to ca. 350 K teristic of the reaction of formaldehyde on surface base to provide a vapor pressure of a few hundred millitorr of HCHO. sites. Since methyl formate has been reported to be a common contaminant in f~rmaldehyde?~ the purity of the formaldehyde Experimental Section sample was checked with a mass spectrometer. The possibility A VG Scientific ESCALAB system, equipped with a mass of contamination by methanol was also checked, because the spectrometer, twin-anode X-ray source, ultraviolet lamp, and presence of such an impurity would introduce uncertainty in the hemispherical energy analyzer, was used for this study. The data analysis. All fragments with intensities larger than 2% of magnesium anode, operated at 600 W, was chosen for XPS exthe principal cracking fragment could be well resolved in the periments to minimize interference from Auger transitions. The cracking patterns. Three major peaks at mle 28, 29, and 30 were base pressure in this instrument was 1 X Torr, maintained observed from the formaldehyde sample, in agreement with the by diffusion and sublimation pumping. This system was also result reported by Yates and co-w~rkers.~~ For methyl formate equipped with two stainless steel needles attached to variable leak and methanol, the leading peaks are at m / e 31,15, and 29, the valves for the introduction of reactants onto the solid samples. first two of which did not appear in the formaldehyde cracking Both magnesium and magnesium oxide single crystals were pattern. The deuterated formaldehydesample exhibited principal utilized in this investigation. The magnesium crystal (99.999%, fragments with mle 28, 30, and 32. If one assumes that any Metallschmeltz Gesellschaft) was oriented to expose the (0001) impurities in the deuterated formaldehyde were also deuterated, face, cut, polished, and mounted on the specimen manipulator then the corresponding major peaks from methanol and methyl with a tantalum wire clip. The MgO(100) sample (Atomergic) formate would be expected to be mle 34,30, and 18. Again, the was mounted in a similar fashion. Heating and cooling were two distinctive peaks at mle 34 and 18 were not observed in the deuterated formaldehyde cracking pattern. Thus the possibility (7)Isa, S. A.; Joyner, R. W.; Matloob, M. H.; Roberta, M. W. Appl. of contamination of the formaldehyde sample by methanol and Surf. Sci. 1980,5, 345. methyl formate can be eliminated. Since the cracking pattern (8) Steinbach, F.; Shutte, J. Surf. Sci. 1984, 146,537. was taken under the same conditions under which the adsorption (9)Tindall, I. F.;Vickerman, J. C. Surf. Sci. 1985,149,577. experiments were performed, contamination of the surface by (10)Wang, G.-W.; Hattori, H. J . Chem. Soc., Faraday Trans. 1 1984, methanol and methyl formate from the chamber background can 80,1039. (11)Rethwisch, D. G.; Dumesic, J. A. Appl. Catal. 1986,21,97. also be excluded. (12)Gopal, P. G.;Schneider, R. L.; Watters, K. L. J . Catal. 1987,105, 366. (13)Kung, H. H. Catal. Reu.-Sci. Eng. 1980,22,235. (14)Egawa, C.; Doi, 1.;Naito, S.;Tamaru, K. Surf. Sci. 1986,176,491. (15)Wachs, I. E.; Madix, R. J. Surf. Sci. 1979,84, 375. (16)Sexton, B. A.; Hughes, A. E. Surf. Sci. 1982,146,L561. (17)Sexton, B. A.; Hughes, A. E.; Avery, N. R. Surf. Sci. 1985,155, 366. (18)Barteau, M. A.; Bowker, M.; Madix, R. J. Surf. Sci. 1980,94,303. (19)Busca, G.; Lorenzelli, V. J . Catal. 1980,66,155. (20)Yates, J. T., Jr.; Cavanaugh, R. R.J. Catal. 1982,74,97. (21)Groff, R. P.; Manogue, W. H. J . Catal. 1983,79,462. (22)Lavalley, J. C.; Lamotte, J.; Busca, G.; Lorenzelli, V. J . Chem. Soc., Chem. Commun. 1985,1006. (23)Busca, G.;Elmi, A. S.; Forzatti, P. J. Phys. Chem. 1987,91,5263. (24)Tanabe, K.Solid Acids and Eases; Academic Press: New York, 1970. (25)Peng, X. D. Ph.D. Dissertation, University of Delaware, 1988.
Results Formaldehyde was adsorbed on the MgO model surfaces at low temperature (around 170 K), and saturation exposures were used in all experiments. T h e nature of the adsorbates as a function of temperature was monitored by XPS and UPS. In the MgO thin-film case, the sample temperature was restricted to 550 K and below by the low (26)Barteau, M.A,; Peng, X. D. Mater. Chem. Phys. 1988,18, 425. (27)Peng, X.D.; Barteau, M. A., in preparation. (28)Peng, X. D.; Barteau, M. A. Surf. Sci., submitted. (29)Yates, J. T., Jr.; Madey, T. E.; Dresser, M. J. J. Catal. 1973,30, 260.
Langmuir, Vol. 5, No. 4, 1989 1053
Adsorption of Formaldehyde on MgO C(lS)XPS
HCHO/HCHO c.)
=!
0
v
W
ta ........ ......... ....:.:..:. ...... .... . ....... ...... . .. . . .... . . . . ...... . . . .... .... .... ... ....... (b) .... ... .......... ................ :. ..::‘ ( c......... )
. . . ’
.
1300 CPS.: :.
285
..
287 289 291 BINDING ENERGY(eV1
Figure 1. C(1s) spectra of the formaldehyde-dosedMgO thin-film surface a t (a) 170, (b) 250, (c) 300, (d) 350, (e) 450, and (f) 550
K. melting point and high volatility of magnesium metal. Figure 1 presents a set of C(ls) spectra from the formaldehyde-dosed MgO thin-film surface a t different temperatures. A broad (fwhm = 2.3 eV) peak with a maximum at 289.4 eV was observed at 170 K. This peak was slightly asymmetric, with a tail on the low binding energy side. As the sample temperature was increased, the peak area decreased and two shoulders on both sides of the original peak developed gradually and finally emerged by 350 K as two peaks at 287.4 and 290.3 eV. The spectrum at 170 K cannot envelop the spectra at higher temperatures, indicative of the development of the peaks at 287.4 and 290.3 eV with increasing temperature. The difference between the spectra at 170 and 350 K yields a narrower (fwhm = 1.9 eV) C(1s) peak a t 289.4 eV with negative intensity below ca. 287.5 eV (Figure la-d). This peak corresponds roughly to the species lost from the surface between 170 and 350 K. T P D experiments detected formaldehyde as the only desorption product in this temperature region (Figure 2a). Similar T P D results have been obtained in this laboratory previously.w Thus the C(1s) peak at 289.4 eV can be assigned to molecularly adsorbed formaldehyde. The two new peaks at 287.4 and 290.3 eV, which emerged a t elevated temperature, can be assigned to surface methoxides and formates, respectively, as they agree well in peak position with the well-characterized surface methoxy and formate species observed on the methanol, formic acid, and methyl formate dosed MgO surfaces.28 Since no contamination from methanol and methyl formate, both of which can result in the formation of surface methoxides,28 was detected, and since this methoxy intermediate was produced by increasing the sample temperature without additional dosing, the methoxy species on the formaldehyde-dosed surface must have been derived from formaldehyde. Therefore, it can be concluded that three different species were present on the form(30)Martinez, R.; Barteau, M. A. Langmuir 1985, I, 684.
z P ! i a
%W 0
I ’
200
I
I
1
I
350
500
650
800
TEMPERATURE ( K )
Figure 2. Temperature-programmed desorption spectra for formaldehyde desorption from (a) MgO thin-film and (b) MgO(100) surfaces.
aldehyde-dosed surface, characterized by different C(1s) peak positions: molecularly adsorbed formaldehyde at 289.4 eV, methoxides at 287.4 eV, and formates a t 290.3 eV. The molecularly adsorbed formaldehyde desorbed from the surface below 350 K, while the surface methoxides and formates increased in population over the temperature region from 170 to 350 K. The presence of a small amount of methoxy species a t the adsorption temperature can be identified by the shoulder on the lower binding energy side of the major formaldehyde peak shown in Figure la. The presence of the peak at 290.3 eV (surface formate) a t the adsorption temperature could not be resolved, as it would be too close to the major formaldehyde peak at 289.4 eV. The two peaks a t 287.4 and 290.3 eV were roughly equal in area and reached their maximum intensities a t 350 K (Figure Id). At higher temperatures, both peaks decreased gradually with the diminution of the formate peak somewhat greater than that of the methoxide peak by 500 K. Note that both peaks remained finite a t 550 K, although the TPD peaks for formaldehyde appeared only below 400 K. Examination of desorption products above 550 K could not be carried out in this case, because the thin-film sample could not be heated to higher temperature. Analogous but less intense XPS spectra were obtained following formaldehyde adsorption on the annealed (770 K, 1 h) MgO(100) surface and are shown in Figure 3. It can be seen that a large and broad peak at 289.4 eV was present a t 180 K on the formaldehyde-dosed MgO(100) surface; this peak evolved into two peaks at 287.0 and 290.3 eV at elevated temperatures. This behavior is essentially the same as that on the thin-film surface. (The peak at 287.0 eV appears lower in binding energy owing to the overlap of the C(ls) signal from residual carbon on the “clean”surface (Figure 3) on its lower binding energy side.) The two peaks at 287.0 and 290.3 eV disappeared below 770 K, and the surface a t this temperature exhibited a small peak a t low binding energy, corresponding to deposited carbon. TPD experiments were unable to resolve products other than desorption of the parent molecule, most probably owing to the large separation between the sample and the mass spectrometer ionizer in this apparatus. Figure 4 illustrates the original and difference UPS spectra from the formaldehyde-dosed MgO thin-film surface at different temperatures. Difference spectra were obtained by normalizing the intensity of the O(2p) peak a t 6 eV prior to subtraction. Spectrum e’ shows the difference between the spectra from the formaldehyde-dosed
1054 Langmuir, Vol. 5, No. 4, 1989
Peng and Barteau
c (Is) XPS 7MK..
C(lS)XPS
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287 289 291 BINDING ENERGY (eV)
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Figure 3. C(1s) spectra of the formaldehyde-dosed MgO(100) surface at different temperatures. The clean surface was produced by argon ion bombardment, followed by annealing at 770 K. Adsorption was carried out at 180 K, and the sample was heated sequentially to the temperatures noted. ORIGINAL SPECTRA
He II
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e e’=e-a
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BINDING ENERGY (eV) Figure 5. Adsorption of formaldehyde on the MgO thin-film predosed with H20: (a) 170 and (b) 250 K.
......
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. . l8OK . : ............. ...... ... .. . .. . . . . .. .
285
. :. .
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Figure 4. UPS spectra of the formaldehyde-dosed MgO thin-film surface at different temperatures. (a) Clean surface of the thin oxide film. (b-e) HCHO-dosed surface at 150,250,350,and 480 K, respectively.
surface at 480 K and the surface prior to adsorption. This spectrum exhibits four peaks, located at 7.1, 10.5,12.0, and 15.5 eV, similar to spectra of surface formate species derived from HCOOH and HCOOCH, on MgO thin-film surfaces.28 This suggests that surface formates were formed by adsorption of formaldehyde on the MgO thinfilm surface, in agreement with the XPS results. As reported in another publication regarding the methyl formate dosed surface,28the characteristic peaks of surface methoxides cannot be distinguished in the UPS spectrum when a comparable population of formate species is present. The difference spectrum following adsorption of formaldehyde a t 180 K (Figure 4b’) did not contain all of the features of the surface formate species (Figure 4e’). The emission from the surface formate species at 15.5 eV, for example, appeared only above 250 K (Figure 4b’-e’). This is indicative of the evolution of the surface formate species with increasing temperature, in agreement with the XPS results discussed above.
In order to ensure that the surface methoxides on the formaldehyde-dosed surface did not originate from contaminants, and to probe the mechanism of the formation of surface methoxides and formates, formaldehyde adsorption was also conducted on the MgO thin-film surfaces predosed with either water or methanol to saturation. In the former set of experiments, the sample at 300 K was dosed with water directly from the dosing needle. The exposure based on the background pressure rise was 1 langmuir; typical in-beam fluxes are up to 2 orders of magnitude greater. On the water-predosed surface, a single (fwhm = 1.9 eV), symmetric C(1s) peak at 289.4 eV was observed following formaldehyde adsorption at 170 K (Figure 5a), corresponding to molecularly adsorbed formaldehyde. In contrast to the results for adsorption of formaldehyde on the clean oxide thin-film surface, no shoulder on the lower binding energy side was observed in this case. This suggests that there were no methoxides formed on the surface at the adsorption temperature and that there was no molecularly adsorbed methanol or methyl formate from background contamination, since these species would exhibit a peak around 287.6 eV (methoxide and molecularly adsorbed methyl formate) or 288.3 eV (molecularlyadsorbed methanol).28 Upon heating of the sample of 250 K, the C(ls) peak disappeared, and the clean surface was essentially regenerated (Figure 5b), suggesting that the only adsorbate on the H,O-preadsorbed surface was molecularly adsorbed formaldehyde at low temperature. In other words, preadsorption of water completely blocked the reaction of formaldehyde to surface methoxide and formate species. The corresponding results from a methanol-predosed surface are depicted in Figure 6. Spectrum a in Figure 6 is characteristic of a surface dosed with methanol at 170 K and then heated to 300 K. This surface exhibited a single methoxide peak at 287.6 eV. After the sample was cooled to 165 K and exposed to formaldehyde, a large amount of formaldehyde was adsorbed on the surface (Figure 6b). Upon heating of the sample back to 300 K, formaldehyde disappeared completely, and the original spectrum was restored (Figure 6c). This experiment demontrates that neither methoxides nor formate species were formed from formaldehyde on the methanol-predosed surface; i.e., methanol adsorption blocked the reactions of formaldehyde, as did water.
Discussion Formation of formates and methoxides from formaldehyde on MgO can be represented as follows: 2HCHO(ad) + O(1attice) CH,O(ad) + HCOO(ad) which involves essentially a disproportionation reaction
-
Langmuir, Vol. 5, No. 4, 1989 1055
Adsorption of Formaldehyde on MgO
c (ISIXPS :. ... ,:
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drogen with its electron pair. The eliminated hydride attacks another molecule of formaldehyde. This concerted hydride transfer results in the formation of a formic acid salt and a methoxide ion.35 Similar to this solution reaction, the carbonyl carbon of an adsorbed formaldehyde molecule was most likely attached to a surface base site (02-ion) on MgO, forming a surface dioxymethylene intermediate. The transfer of electrons from the surface oxygen to the carbonyl carbon would weaken the bond between the hydrogen and the carbonyl carbon. Meanwhile, an adsorbed formaldehyde molecule in the vicinity could accommodate this hydride, forming a surface methoxide group bound on a weakly acidic site (Mg2+ion). The reaction can be represented by
.......... . ._.....:........ ..,.* 1
1
I
I
I
287 289 291 BINDING ENERGY (eV)
Figure 6. Adsorption of formaldehyde on the MgO thin-film surface predosed with CH30H. (a) Clean MgO thin-film surface exposed to CH30H at 170 K and then heated to 300 K. (b) Surface in a exposed to HCHO at 165 K. (c) Surface in b heated to 300 K.
with addition of lattice oxygen from the surface. This reaction is similar to the Cannizzaro reaction taking place in basic aqueous solution and has been used to explain the simultaneous formation of methoxides and formates from formaldehyde on Ti02, Tho2, and V-Ti oxide catalysts by Busca and c o - w ~ r k e r s . Similar ~ ? ~ ~ reactions have also been observed on MgO p o ~ d e r s . ~Tanabe ~ ? ~ ~ and Saito31 reported that, in the reaction of benzaldehyde on alkaline earth metal oxides to form benzoic acid and benzyl alcohol, both surface benzoate and benzylate species were observed by IR spectroscopy as possible reaction intermediates. A Cannizzaro-type disproportionation reaction was proposed to explain the formation of these two surface species from adsorbed benzaldehyde. The same scheme was also employed to explain the formation of surface H2C=CHCH2COO- and H2C=CHCH2CH20- species from H2C=CHCH2CH0 on MgO powder surfaces;32these three surface species were detected by IR spectroscopy. Surface base sites have been suggested to be responsible for these disproportionation reactions in these studies, consistent with the results of the blocking experiments in this study. Concepts drawn from solution-phase acid-base chemistry have been applied successfully to explain certain reactions on metal oxide and oxygen-dosed metal surf a c e ~ .The ~ ~relative ~ ~ ~ acidity scale of several Brclnsted acids such as carboxylic acids and alcohols on MgO and ZnO surfaces was reported to agree with the ranking of their aqueous dissociation constant^.^^^^^ This agreement was attributed to a surface “solvation” effect, which stabilizes conjugate base anions produced by heterolytic dissociation of these molecules on the surface. The reaction reported here appears to provide another example of the similarity between the surface- and solution-phase acid-base chemistry. In the Cannizzaro reaction taking place in basic aqueous solutions, a hydroxyl anion first adds to the carbonyl carbon of a formaldehyde molecule. The strong electron-donating character of 0- in this intermediate facilitates the departure of the aldehydic hy(31) Tanabe, K.; Saito, K. J. Catal. 1974, 35, 247. (32) Garrone, E.; Stone, F. T. In Proc. 8 t h Intern. Congr. Catal., Berlin, 1984, Verlag Chemie: Berlin, 1984; p 111-441. (33) Stair, P. C. J. A m . Chem. SOC.1982, 104, 4044. (34) Spitz, R. N.; Barton, J. E.; Barteau, M. A.; Staley, R. H.; Sleight, A. W. J. Phys. Chem. 1986, 90,4067.
2HCHO
+
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-
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II
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H
\/-
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-Mg-O-Mg-
I
-
H37
0
I
7
7 7 7
-Mg-O-Mg-
The base-catalyzed character of this disproportionation reaction can be illustrated by the water and methanol blocking experiments shown above. It has been demonthat these two molecules dissociate on MgO to form hydroxyls, methoxides, and protons. The methoxide and hydroxyl coverages produced on the thin-film surface by adsorption of methanol and water, respectively, have been estimated to be ca. 0.5 monolayer from angle-resolved XPS experiment^.^^ The protons adsorbed on surface 02sites may block the disproportionation reaction of formaldehyde by blocking the sites for dioxymethylene formation. The results above also show that the coverage of surface formates from formaldehyde on MgO is similar to that of surface methoxides from formaldehyde, which in turn is similar to the coverage of surface methoxides derived from methanol on the oxide film (Figures l and 6). This suggests that the same sites responsible for the methanol dissociation are also responsible for the disproportionation of formaldehyde. These sites have been shown to be coordinatively unsaturated MgO pairs of fairly strong basicity.25 Two important steps in the above mechanism are the adsorption of a formaldehyde molecule on the nucleophilic through the carbonyl carbon, Le., the formacenter (02-) tion of a surface dioxymethylene intermediate, and the elimination of a hydrogen from the carbon atom. These two initial steps appear to be general for the react.> IS of formaldehyde on metal oxide and oxygen-dosed nsition-metal In fact, dioxymethylerl lave been observed on Ti-V oxide surfaces by IR spectrc- . ~ y , ~ and have been inferred on oxygen-dosed Ag(1l and Cu(ll0) surfaces from T P D experiment^.^^,'^ In spite of these two common steps, only formates were observed on other metal oxide surfaces such as a-Fe203,19A1203?0 Ti02,2*and Zn04 and on oxygen-dosed transition-metal s u r f a ~ e s . ’ J ~ - ~This ~ difference may result from the availability of different routes for the consumption of the hydrogen eliminated from the carbonyl carbon. On nonbasic metal oxides, surface acid sites may be responsible for accommodation of the eliminated hydrogen. Although little evidence for this phenomenon has been reported for formaldehyde-dosed oxide surfaces, metal ions on acidic oxide surfaces have been proposed to be responsible for hydride abstraction from surface m e t h o x i d e ~ . ~Magne~ sium ions on MgO surfaces are known as weak acids and 7
(35) March, J. Aduanced Organic Chemistry, 3rd ed., Wiley: New York, 1985. (36) Parrott, S. L.; Rogers, J. W., Jr.; White, J. M. Appl. Surf. Sci. 1978, I, 443.
Langmuir 1989, 5 , 1056-1061
1056
therefore may not be able to abstract and accommodate the hydride from the carbonyl carbon. In this case, the eliminated hydrides from the carbonyl carbon atoms may attack adsorbed formaldehyde molecules to form surface methoxides.
nizzaro reaction. This reaction may be suppressed by preadsorption of Bransted acids such as methanol and water. The observation of the Cannizzaro reaction on clean oxide surfaces, including both thin oxide films on Mg(0001) and the MgO(100) surface, suggests that this reaction can occur on basic MgO sites of high coordination number.
Conclusions Formaldehyde reacts on basic sites on MgO surfaces to form both CH,O(ad) and HCOO(ad) species via the Can-
Acknowledgment. We gratefully acknowledge the support of the National Science Foundation (Grant CBT8451055).
Photoactivity of Zeolite-Supported Cadmium Sulfide: Hydrogen Evolution in the Presence of Sacrificial Donors Marye Anne Fox* and Thomas L. Pettit' Department of Chemistry, University of Texas, Austin, Texas 78712 Received January 4, 1989. I n Final Form: April 18, 1989 Photocatalytic hydrogen evolution can be observed on zeolite-supportedCdS particles modified by surface modification with an appropriate hydrogen evolution catalyst (Pt or ZnS). Particles formed within the zeolite cavities aggregate upon exposure to water, and small clusters (Q-particles)are isolated within individual cavities only in nonaqueous solvents at low (ca. 3%) CdS loading levels. Sustained hydrogen evolution requires the presence of an added sacrificial donor. Platinum deposited on the photoactive CdS surface is exclusively found inside the cavity, where it is inaccessible to large anionic reagents.
Introduction Photocatalysis on semiconductor surfaces has been widely investigated as a potential route for solar energy conversion. Practical applications of such materials demand that an effective method for their dispersal be devised, so that better selectivity for a desired photoinduced redox reaction (from among several possible routes) can be attained. It is important therefore to study the photophysical properties of semiconductors as a function of supporting material as well as a function of the particle size itself. Zeolites are attractive candidates for supports for these photoactive species. Their aluminum silicate frameworks render them highly stable under ambient conditions, and their varied structural porosity allows for controlled variance in the introduction or growth of the semiconductor particle. We report herein our investigations of the photoactivity of CdS particles grown inside zeolitic cavities.2 Our studies have aimed to answer the following questions: (a) Can zeolites be used as supports for photocatalytically active semiconductors particles? (b) If so, where will the particles be located: in the cavities or on the external surface? (c) Will the cavities of the zeolites limit the growth of the semiconductor particles to very small sizes? (d) Do such encapsulated semiconductor particles retain the photoactivity observed when unsupported? Experimental Section Preparation of Semiconductor-Loaded Zeolites. The
general procedure for the preparation of CdS-loaded zeolites involves sequential ion exchange with Cd2+and sulfidation with H2S. Co-catalysts for hydrogen evolution were deposited either by platinization or by treatment with Znf2and final resulfidation. (1) Current address: SRI International, Menlo Park, CA 94025. (2) For a more complete discussion and more experimental detail, see: Pettit, T. L., Ph.D. Dissertation, University of Texas, Austin, 1986.
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Table I. Deposition of CdS Particles i n Several Zeolites loading level, zeolite Cd2+loading exchange mg of CdS/g time," h of catalyst* exchanged level, wt %" 21 LZ-Y52 2.2 24 48 41 LZ-Y52 3.2 24 78 LZ-Y52 6.1 20 140 LZ-Y52 11' 20' 1l e 130 LZ-Y52 24 28 LZ-Y52 2.2 24 140 LZ-Y52 10 24 130 10 13X 20 260 13X 20 20 130 13X 10 20 150 4A 10 20 280 4A 23 Loading levels established by digestion, followed by atomic absorption analysis, f20%. CdS produced, assuming quantitative sulfidation of Cd2+in the exchanged zeolites, &20%. 'After sulfidation and exchange with Zn2+ (28.6 mg of Zn2+/mg of catalyst).
This sequence produced CdS particles within the zeolite pores, in which surface complexation with Pt or ZnS3 had occurred. Zeolites A and LZY52 were 120-meshpowders (Union Carbide), while the 13X sample was a 600-mesh powder (Alfa). Fumed amorphous silica (Cab-o-sil)was used as supplied. Cation exchange (Cd2+for 2Nat) of the zeolites, used as received, was carried out at room temperature by stirring slurries of the powders (ca. 5 g) for periods ranging from 20 to 48 h with solutions of Cd2+ (ca. 25 mL) prepared from reagent-grade cadmium chloride hemipentahydrate (Aldrich). The concentration of the exchange solutions was varied from 0.025 to 0.5 M Cd2+. After exchange, the ion-loaded zeolites were washed with water until the filtrate gave a negative Ag+ test. After filtration,the powders were dried at 110 O C for 22 h. Immersion of the Cd2+-exchangedzeolites into 0.1 M NazS in H 2 0 for 30 min, followed by filtration,yielded clear filtrates and colored powders. Alternatively, 200-mg samples of Cd2+-loaded (3) Sobczynski, A,; Bard, A. J.; Campion, A.; Fox, M. A.; Mallouk, T.; Webber, S. E.; White, J. M. J . Phys. Chen. 1987, 91, 3316.
'C 1989 American Chemical Society
