Langmuir 1988,4, 533-543
533
Adsorption and Decomposition of Aliphatic Alcohols on TiOa K. S. Kim and M. A. Barteau* Center for Catalytic Science and Technology, Department of Chemical Engineering, University of Delaware, Newark, Delaware 19716
W. E. Farneth Central Research and Development Department, E. I. du Pont de Nemours & Co., Wilmington, Delaware 19898 Received June 25, 1987. I n Final Form: November 12,1987 Methanol, ethanol, 1-propanol,and 2-propanol were adsorbed at ambient temperature on Ti02anatase powders. Temperature-programmeddesorption spectra were obtained by using two different experimental apparatuses: a carrier-gas flow reactor system and a high-vacuum microbalance system. Two adsorption states were observed molecular alcohol and alkoxide. Molecular alcohols desorbed intact upon slight heating (at 350'K) while alkoxides were removed from the surface via two channels: recombination at 390 K and decomposition at higher temperatures. The concentration of alkoxide species was equal for all of the primary alcohols but was less for the secondary alcohol due to steric effects. Temperatures at which the maximum rate of alkoxide decomposition occurred decreased in the sequence methoxide > ethoxide > n-propoxide > isopropoxide. Both unimolecular and bimolecular reaction products were observed in the decomposition of the primary alkoxides; only unimolecular decomposition was observed for the isopropoxide. Dehydrogenation as well as dehydration products were obtained from the decomposition of each of the alcohols studied, with smaller alcohols favoring dehydrogenation. The patterns of sample weight change during TPD experiments also reflected the reaction selectivits those species favoring dehydration produced smaller ultimate sample weight losses than those favoring oxidative dehydrogenation. The selectivity of alkoxide decomposition depended on both intramolecular bond energies and intermolecular interactions, suggesting that the interpretation of the selectivity in terms of surface acid-base properties alone is inadequate.
Introduction The decomposition of alcohols has been widely used as a probe of the acid-base properties of metal oxide catalysta.14 While it is generally assumed that acidic oxides catalyze dehydration and basic oxides catalyze dehydrogenation, recent studies have shown that the selectivity of alcohol decomposition on metal oxides is a function of reactant structure" as well as surface structure? Further, the mechanisms assumed in equating dehydration with acidity and dehydrogenation with basicity may be incorrect. Oxides which act as solid bases to abstract protons from alcohols can also produce net dehydration products via alkoxide d e c o m p o s i t i ~ n . ~ On ~ ~ Jzinc ~ oxide,1° in particular, dehydration and dehydrogenation of ethanol via surface ethoxides occur in parallel on the same crystal plane. Thus the interpretation of alcohol decomposition selectivity in terms of surface acid-base properties alone is likely to be misleading. Besides ita potential as a catalyst for photoassisted reaction~"-~~ and as a support for metal catalysts exhibiting so-called SMSI effects,14J5titanium dioxide is considered to be one of the ideal materials for the study of gas-solid interactions involving surface defects.16J7 The reactivities of the common bulk structures, anatase and rutile, have been extensively studied and reviewed in the literature.lsJg The surfaces of anatase and rutile are composed of cleavage planes with differing degrees of coordinative unsaturation of titanium with respect to oxygen. It has been suggested that the reactivity of these surfaces is closely related to the degree of coordinative unsaturation. Although the stoichiometric surface of titanium dioxide possesses Ti4+ ions, the concentration of Ti3+increases as the surface is reduced. The surface reactivity of this material is usually explained in terms of the Lewis acidity of the titanium cations. Interactions of aliphatic alcohols with titania have been studied in p h o t o a s ~ i s t e d , ~ thermal J ~ J ~ oxidation,20i21 and
* Author to whom correspondence should be addressed.
d e c o m p o ~ i t i o n ~reactions. J ~ ~ ~ ~ -In~ spite ~ ~ ~of~ ~claims ~ for its use as a model of metal oxide surface chemistry, there (1)Tanabe, K.Solid Acids and Bases; Academic: New York, 1970. (2)Krylov, 0.V. Catalysis by Nonmetals; Academic: New York, 1970. (3)Ai, M. Bull. Chem. SOC.Jpn. 1976,49,1328. (4)Winterbottom, J. M. In Catalysis; Royal Society of Chemistry: London, 1981;Vol. 4, p 141. (5)Noller, H.; Ritter, G. J. Chem. SOC., Faraday Trans. 1 1984,80, 275. (6)Bowker, M.; Houghton, H.; Waugh, K. C. J. Chem. SOC., Faraday Trans. 1 1982,78,2573. (7) Bowker, M.; Petta, R. W.; Waugh, K. C. J . Catal. 1986,99, 53. (8) Cunningham, J.; Hodnett, B. K.; Ilyas M.; Tobin, J.; Leahy, E. L.; Fierro, J. L. G. Faraday Discuss. Chem. SOC. 1981,72, 283. (9)Parrott, S. L.; Rogers, J. W., Jr.; White, J. M. Appl. Surf. Sci. 1978, 1, 443. (10)Vohs, J. M.; Barteau, M. A., to be published. (11)Sato, S.;White, J. M. J . Phys. Chem. 1981,85,592. (12)Cunningham, J.; Morrissey, D. J.; Goold, E. L. J. Catal. 1978,53, 68. (13)Harvey, P. R.; Rudham, R.; Ward, S. J . Chem. SOC., Faraday Trans. 1 1983,79, (a) 1381,(b) 2975. (14)Greenlief, C. M.; White, J. M.; KO,C. S.;Gorte, R. J. J. Phys. Chem. 1985,89,5025. (15)Raupp, G. B.; Dumesic, J. A. J. Phys. Chem. 1985, 89, 5240. (16)Lo, W. J.; Chung, Y. W.; Somorjai, G. A. Surf. Sci. 1978,71,199. (17)Gopel, W.; Rocker, G.; Feierabend, R. Phys. Rev. B Condens. Matter 1983,28,3427. (18)Henrich, V. E.Prog. Surf. Sci. 1983,14,175. (19)Parfitt, G. D. In Progress i n Surface and Membrane Science; Academic: New York, 1976;Vol. 11, p 181. (20) Groff, R. P.; Manogue, W. H. J . Catal. 1984,87, 461. (21)Nakajima, T.;Miyata, H.; Kubokawa, Y. Bull. Chem. SOC.Jpn. 1982,55,609. (22)Jackson, P.; Parfitt, G. D. J . Chem. SOC.,Faraday Trans. 1 1972, 68,1443. (23)Gentry, S.J.; Rudham, R.; Wagstaff, K. P. J . Chem. SOC.,Faraday Trans. 1 1975,71,657. (24)Rochester, C. H.; Graham, J.; Rudham, R. J. Chem. SOC.,Faraday Trans. 1 1984,80,2459. (25)Graham, J.; Rudham, R.; Rochester, C. H. J. Chem. SOC., Faraday Trans. 1 1984,80,895. (26)Carrizosa, I.; Munuera, G. J . Catal. 1977,49,(a) 174, (b) 189. (27)Carrizosa, I.; Munuera, G.; Castanar, S. J . Catal. 1977,49,265.
0143-1463 f 88/24Q4-Q533$Q1.5Q/Q0 1988 American Chemical Society
534 Langmuir, Vol. 4, No. 3, 1988
is surprisingly poor agreement among studies of the surface reactivity of titania Further, due to the variety of behavior reported for both anatase and rutile, it is not possible to identify unambiguously the differences in reactivity between these two materials. For example, although much of the decomposition chemistry of alcohols has been ascribed to the formation of surface alkoxides and their subsequent reaction, other postulated intermediates leading to decomposition products include molecularly adsorbed al~ohols,2~J~ alkyl^,^^^^' carboxylate^,^^^^^ and carbonate^.^^.^^ Even on the same Ti02sample different types of intermediates have been claimed for different alcohols; Carrizosa et al.26*27 postulated that methanol decomposes via surface alkoxides, while the higher alcohols adsorb molecularly and decompose via these molecular species. The apparent need for this variety of adsorbates (and of the surface sites on which each is postulated to form) arises from the complexity of the distribution of gaseous products formed by alcohol decomposition on titania. Typical products include those from net dehydration processes (olefins and ethers) and dehydrogenation processes (aldehydes, ketones, and carbon monoxide). Other side reactions, such as production of alkanes via alkyl coupling, have been shown to be due to the presence of impurities such as sulfates.28 Even when the dehydration/dehydrogenation selectivity alone is considered, this ratio varies considerably from study to study, and with the alcohol under consideration. This again suggests that caution is in order when applying the dehydrationldehydrogenation selectivity as a probe of surface acid-base properties. Since some of the products above, e.g., ethers, require bimolecular surface reactions, it is likely that the product distribution will be sensitive to the adsorbate coverage. Here, also, there is a notable lack of consistency among the various studies. Relative saturation coverages of irreversibly adsorbed species have been reported to be in the order 2-propanol = 2-methyl-2-propanol > ethanol on rutile,12 ethanol > 2-propanol > 1-propanol > 2-methyl2-propanol on anatase,26methanol > 2-propanol on anat a ~ eand , ~ methanol ~ = ethanol = l-propanol on rutile.30 As noted previously by Sleight and co-workers31for alcohol decomposition on MOO,, both the surface activity and reaction selectivity will be sensitive to the coverage of alcohols and of contaminants such as water and hydroxyl species. We report here studies, utilizing temperature-programmed desorption (TPD) techniques, of the decomposition reactions of methanol, ethanol, 1-propanol,and 2-propanol adsorbed at ambient temperature on polycrystalline anatase powders. This work involved two different experimental apparatuses: a carrier-gas flow reactor system (hereinafter referred to as the flow-reactor system) and a high-vacuum microbalance system (hereinafter referred to as the high-vacuum system). The adsorption uptakes of alcohols in the high-vacuum system were measured gravimetrically. The surface species formed after adsorption and upon heating were characterized by IR studies, the results of which will be presented in detail else~here.,~These studies suggest that all of the products of alcohol decomposition o n Ti02 can be accounted for by (28) Busca, G.;Forzatti, P.; Lavalley, J. C.; Tronconi, E. In Catalysis
by Acids and Bases; Elsevier: Amsterdam, 1985; p 15. (29) Rossi,P.F.; Busca, G.; Lorenzelli, V.; Saur, 0.; Lavalley, J. C. Langmuir 1987, 3, 52. (30) Suda, Y.;Morimoto, T.; Nagao, M. Langmuir 1987, 3, 99.
(31) Machiels, C. J.; Cheng, W. H.; Chowdhry, U.; Farneth, W. E.; Hong, F.; McCarron, E. M.; Sleight, A. W. Appl. Catal. 1986,25, 249. (32) Boaventura, J. S.; Staley, R. H.; Barteau, M. A., to be published.
Kim et al.
reaction of surface alkoxide species, with quite consistent patterns of reactivity and moderate variations of selectivity among the primary alcohols. These studies also suggest that the saturation coverages of primary alkoxides are not dependent upon alkyl chain length up to C,; steric limitations are apparent only for the secondary alcohol, 2propanol.
Experimental Section Materials. Ti02 anatase powder was obtained from American Instrument Co.; and the BET surface area of 10.3 m2/g was measured by using nitrogen at 77 K (Micromeritics 2100D surface area-pore volume analyzer). Elemental analysis with X-ray fluorescence (Philips APD 3500 automated X-ray spectrometer) and atomic emission33showed that the anatase originated from the chloride process (chlorine content < 0.1%) and that the amounts of other impurities were negligible. X-ray powder diffraction patterns showed that the powder was substantially free of rutile phase initially and no transformation to rutile took place during the pretreatment procedures described below. Helium (99.995%), used as the carrier gas for the flow-reactor system, was further purified by passage through a 5A molecular sieve trap immersed in liquid nitrogen. Oxygen (99.993%) was used without further purification. Alcohols (spectranalyzedgrade) were subjected to three freeze-pump-thaw cycles before being used. Apparatus. 1. Flow-Reactor System. TiOz powder samples of ca. 25-100 mg were placed on the fritted glass disk support (length 1.5 mm, diameter 10 mm) within a quartz reactor and were pretreated by oxidation in an oxygen flow and 30 mL/min at 673 K for 1 h, followed by a purge with helium at the same temperature and flow rate for 1 h. The reactor bed depth was ca. 3 mm for a 1 " g sample and legs than 1mm for a 25-mg sample. The pretreated sample was exposed to alcohol vapor at room temperature by directing the carrier gas (flow rate 100 mL/min) through the vaporizer. The reactor was then purged with helium (flow rate 100 mL/min) in order to remove residual alcohol vapor. After sufficient evacuation, the reactor was enclosed within a furnace and the sample was heated at the controlled rate of 12 K/min for TPD experiments. The carrier gas was split downstream from the reactor by a leak valve, and a portion was directed to the mass spectrometer for analysis. The analysis stream was reduced to lower pressure with a molecular jet-separator that was differentially pumped. The mass spectrometer was maintained at lo4 Torr with a diffusion pump. A UTI lOOC quadrupole mass spectrometer interfaced with a computer allowed five mass fragments to be monitored simultaneously. The saturation coverage was determined by increasing the alcohol exposure until no changes of the desorption peak shapes, peak temperatures, and peak heights were observed. Exposures of ca. 15 min were required for each of the alcohols to reach saturation of a 100-mgTi02sample. AU of the TPD data reported in this study was obtained for saturation coverages. Variation of the sample amount from 100 to 25 mg did not affect the TPD peak shapes, peak temperatures, and product distributions, indicative of the absence of diffusional limitations and of chromatographic effects due to product r e a d ~ o r p t i o n . ~I t~was not possible to reduce the sample mass below 25 mg due to the difficulty in obtaining accurate measurement of the reactor bed temperature. Variation of the carrier gas flow rate from 100 to 300 mL/min was performed for several of the reactants and was found not to affect the peak shapes, peak temperatures, and product distributions, again verifying the absence of significant transport limitations. Used samples were reoxidized in an oxygen flow of 30 mL/min at 673 K for 1h, and TPD data obtained from reoxidized samples reproduced those of fresh samples. 2. High-Vacuum System. A high-vacuum system was also used to study the chemisorption and TPD of alcohols on titania. The system consisted of a stainless steel high-vacuum chamber equipped with a Cahn RG microbalance. The typical background pressures were 1 X lo4 TOR. Alcohol vapor was admitted through an attached manifold, and uptake data were obtained gravime(33) Schwarzkopf Microanalytical Laboratory, Woodside, NY. (34) Falconer, J. L.; Schwarz, J. A. Catal. Reu.-Sci. Eng. 1983,25, 141.
Langmuir, Vol. 4, No. 3, 1988 535
AdsorptionlDecomposition of Alcohols on TiOz Table I. Chemisorption of Alcohols on the Titania (Anatase) Surface total molecular coverage,* chemisorption,' alcohol pg (rmol) molecules/nm2 CHSOH 160 (5.06) 3.18 CDSOD 180 (5.00) 3.14 CZHSOH 240 (5.22) 3.28 n-C3H70H 308 (5.13) 3.22 i-CSH70H 240 (4.00) 2.51
"Sample mass is 100.6 mg. bSurface area is 10.3 m2/g. trically with the microbalance. Details of the system have been described by Fameth and co-workers.3s The sample was prepared by pressing ca. 100 mg of anatase powder into a pellet. A a/8-in. pellet die was used with powder paper on each side of the sample. The pellet was p r d at 5OOO psi for 10 min at room temperature. Initial treatment of the sample included (i) evacuation at room temperature for 1h, (ii) oxidation at 400 OC in 100 torr of oxygen for 2 h, and (i) evacuation overnight with cooling. After the initial treatment, the weight of the sample was measured. The sample treatment between TPD experiments involved annealing at 400 "C in 1x lo4 Torr followed by oxidation at the same temperature in 10 Torr of oxygen for 1 h. Infrared experiments have shown that this procedure yields an essentially hydroxyl-free and carbonate-free surface. Exposure of the pretreated sample to alcohol vapor was carried out a t room temperature. The saturation coverage was determined by measuring the stable weight gain obtained after pumpout to less than lo-' Torr. All TPD experiments were conducted for saturation coverages. TPD mass spectra were obtained by monitoring the effluent gas stream from the microbalance chamber with a UTI 100C quadrupole mass spectrometer. A controlled heating rate of 5 K/min was used for temperature programming, and the maximum temperature was limited to ca. 673 K by the mechanical integrity of the microbalance. The changes of sample weights were monitored during the TPD experiments and the subsequent reoxidation steps.
Results Adsorption of Alcohols. Adsorption of alcohol onto the anatase surface was carried out on the high-vacuum system by admission of alcohol vapor into the microbalance chamber. Exposures were carried out at room temperature for 10 min at an initial pressure of 0.1 Torr. The weight gain of a pelleted sample was monitored gravimetrically, and a decrease in the ambient pressure was observed. After the exposure, the microbalance chamber was pumped out until the weight gain was stabilized. This procedure was repeated for exposures at initial pressures of 1and 10 Torr. The stable weight gain relative to the clean sample was identical after the second and third exposures, indicating that saturation coverage was achieved after the second exposure cycle. Since the period of evacuation of the system at lo-' Torr was sufficient to achieve stable and reproducible weight gains, the contribution from reversibly adsorbed molecular species was considered to be negligible. This assumption was verified by comparison of TPD data obtained from the high-vacuum system with those from the flow-reactor system, which are presented later. The chemisorption data exhibited a dependence on alcohol structure. The primary alcohols, methanol, ethanol, and 1-propanol,exhibited the same molecular coverage per unit surface area to within 2%, ca. 3.2 molecules/nm2. The saturation coverage of the secondary alcohol, 2-propanol, was approximately 25% less, 2.5 molecules/nm2. Completely deuteriated methanol, CD30D, exhibited the same chemisorption behavior as nondeuteriated methanol. Chemisorption data are summarized in Table I. (35) Farneth, W. E.; Ohuchi, F.; Staley, R. H.; Chowdhry, U.; Sleight, A. W. J . Phys. Chem. 1985,89, 2493.
TPD of Adsorbed Alcohols. In order to identify and quantify the desorption products, TPD mass spectra were analyzed in the following manner: (i) TPD spectra were collected for more than 20 mass fragments, (ii) the contribution from the desorption of the parent alcohol was subtracted, (iii) the remaining portion of each spectrum was integrated to obtain peak areas, (iv) peak areas were assigned to the appropriate products in accordance with their fragmentation patterns,3638and (v) the quantitative distribution of molecular products was calculated by using the method of KO et al.39 It was found that the results of alcohol TPD from the two different apparatuses were in quite good agreement with respect to the product distribution as well as desorption peak temperatures. Minor differences were observed for the desorption of HzO with the two systems: water desorption was typically coincident with that of organic species in the flow-reactor system but was shifted upward by 10-15 K in the high-vacuum system. This small lag is likely due to the facile adsorption and desorption of water during passage from the powdered sample. Experiments with the two experimental systems were complementary: (i) Monitoring of the mle 28 spectrum, which was difficult for the flow-reactor system due to the interference of the background gas, was possible for the high-vacuum system. (ii) Detection of the desorption products above 675 K, which was impossible for the high-vacuum system, was possible for the flow-reactor system. TPD spectra of alcohols chemisorbed at room temperature on the polycrystalline titania exhibited two principal groups of desorption peaks. The first group appeared near 390 K and was identified as the parent alcohol accompanied by water. The other group appeared at higher temperatures and was considered to result from the decomposition of chemisorbed alcohol species. Parallel infrared spectroscopy studies revealed that the chemisorbed species responsible for the desorption of the parent alcohols, as well as of the decomposition products, were alkoxides. While the peak temperatures €or parent alcohol desorption were essentially independent of alcohol identities, those for alkoxide decomposition were dependent upon alcohol structure. Peak temperatures for decomposition of the primary alkoxides were near 650 K for methoxide and near 600 K for ethoxide and n-propoxide. The secondary alkoxide formed from 2-propanol decomposed near 550 K. Both dehydration and dehydrogenation products were observed from the decomposition of each of alkoxides studied; however, dehydration/ dehydrogenation selectivities were less easily correlated with alcohol structure. The behavior of sample weight decreases in the course of TPD experiments clearly reflected the chemistry of alcohol decomposition on the anatase. For every alcohol studied, the sample weight loss during temperature programming exceeded the initial weight gain due to alcohol adsorption. The temperatures for the transition from net weight gain to net weight loss, together with the extents of the net weight loss at the end of temperature programming, were directly related to the product distribution and t h e desorption peak temperatures of alkoxide decompo~~
(36) The fragmentation patterns of most compounds were experimentally obtained from the apparatuses used in this study. (37) Cornu, A.; Massot, R. Compilation of Mass Spectral Data; Heyden: London, 1966. (38) Eight Peak Index of Mass Spectra, 2nd ed.; Mass Spectrometry Data Centre: Aldermaston, Reading, England, 1974; Vol. 1. (39) KO,E. I,; Benziger, 3. B.; Madix, R. J. J. Catal. 1980, 62, 264. (40) Farneth, W. E.; Staley, R. H.; Sleight, A. W. J. Am. Chem. SOC. 1986, 108, 2327.
Kim et al.
536 Langmuir, Vol. 4, No. 3, 1988
a
400
600 TEMPERATURE ( K )
so0
IO00
Figure 2. Desorption of formaldehyde and dimethyl ether from the TPD of methanol-d, and methanol-d4obtained with the flow-reactor system.
400
600
800
IO00
TEMPERATURE ( K 1
Figure 1. TF'D spectra of adsorbed methanol (mass spectrometer sensitivity factors are not included): (a) flow-reactor system; (b)
high-vacuum system.
sition. Those net weight losses were restored completely by reoxidation at 400 "C, indicating that they resulted from the removal of oxygen from the TiOz lattice by reaction with the alcohols. 1. Methanol TPD. Adsorbed methanol gave rise to the TPD spectra shown in Figure 1. For the flow-reactor system, low-temperature desorption peaks for the parent alcohol appeared at 350 and 390 K. The peak at 350 K decreased with increasing the period of the reactor purge between the end of the methanol exposure and the start of the TPD experiment. This peak could be eliminated by purging for 4 h before initiation of temperature programming, demonstrating that it was due to reversibly adsorbed molecular methanol species. The desorption peak at 390 K did not disappear even after a 4-h purge, demonstrating that it originated from an irreversibly chemisorbed species. In the high-vacuum system, only the desorption peak at 390 K was observed, indicating that all of the reversibly adsorbed methanol was efficiently removed from the surface by evacuation a t 300 K. The methanol peak at 390 K was accompanied by water. Desorption peaks appearing at higher temperatures (near 650 K) were due to decomposition of surface methoxy species. Major reaction products were formaldehyde, dimethyl ether, methane, carbon monoxide, and water. Minor products were methanol and hydrogen. A small amount of methanol ( m / e 31) was observed to desorb at 645 K for the flow-reactor system but was not detected with the high-vacuum system. Peaks for m / e 45 and 46 centered around 640 K were assigned to dimethyl ether rather than formic acid because of the coincident signal for m / e 15 (expected for dimethyl ether but not for formic acid). The peak for m / e 30 observed at 660 K with the flow-reactor system was assigned to the desorption of formaldehyde. The corresponding peak for m / e 29, the primary fragment of formaldehyde, was shifted downward slightly, to 653 K, due to the m / e 29 contribution from dimethyl ether. No desorption peak for m / e 28, another fragment of formaldehyde, could be resolved because of the high intensity of the m / e 28 signal from the background gas (most likely nitrogen). Comparison of the m / e
30 peak area with the m / e 29 peak area remaining after subtraction of the contribution of dimethyl ether supported the assignment of the m / e 30 peak to formaldehyde. For the high-vacuum system, TPD spectra corresponding to the formation of formaldehyde exhibited the onset of desorption at 500 K with intensity continuing to increase at 675 K, the temperature limit of the instrument. Hence, instead of the peak area analysis, the following procedure was used to identify formaldehyde desorption. The TPD spectrum of m / e 29 free of the methanol contribution was further reduced by the contribution of dimethyl ether; the spectrum thus obtained corresponded to the net contribution of m / e 29 to formaldehyde desorption. The intensity of this spectrum above the base line at 675 K was compared with that of the m / e 30 spectrum from which the corresponding methanol and dimethyl ether contributions had been removed; this intensity ratio was in good agreement with the ratio obtained from the analysis of peak areas from the flow-reactor system. CO desorption was resolved in the high-vacuum data by subtracting the m l e 28 contribution of formaldehyde from the TPD spectrum of m / e 28 remaining after subtraction of methanol contribution. CO exhibited the onset of desorption at ca. 600 K, with a desorption rate continuing to increase at 675 K. The assignment of the formaldehyde and ether products was also supported by data obtained with the flow-reactor system for decomposition of CD30D. Desorption peaks for m / e 50 and 52 from CD30D TPD exhibited intensities, shapes, and temperatures in good agreement with those for m / e 45 and 46 from CH30H. The m / e 50 and 52 peaks may be assigned to fully deuteriated dimethyl ether but not formic acid. Likewise, peaks for products with m l e 30 and 32 from CD30D were in agreement with those for m / e 29 and 30 for the formaldehyde produced from CH,OH. Additional decomposition products included water and methane, with peaks observed at 660 and 675 K, respectively, with the flow-reactor system. Methane exhibited a rather narrow peak shape compared to those of other decomposition products. A small hydrogen peak was also observed at 665 K. With the high-vacuum system, the desorption rates of these three products did not reach maxima below 675 K but simply continued to increase. A small C 0 2 peak was observed above 700 K with the flowreactor system. As no other products such as hydrogen or water accompanied C 0 2 desorption, this product most likely resulted from the oxidation of adsorbed carbon rather than the decomposition of adsorbed organic fragments. Ethane has been reported to be one of the major
AdsorptionfDecomposition of Alcohols on TiOp
Langmuir, Vol. 4, No. 3, 1988 537
Table XI. Distributions and Peak Temperatures of Methanol TPD Products molecule relative yield peak temp, K Low Temperature CH30H 100.0" (100.0)* 390" (390)b HZO CHsOH CHBOCHS HCHO H20 H2 CH,
co co2
28.0 (19.0)
390 (405)
Higher Temperature 3.0 (neg) 645 8.0 (15.1) 15.1 4.1 0.5 5.0
645 660 660 665 675
not resolved 2.4
w t-
(635)
(above 675) (above 675) (above 675) (above 675) (above 675)
710
U
a
z 2
t-
a LT
0
In w
0
"Data from the flow-reactor system. *Data from the high-vacuum system.
products from methanol decomposition on anatase" In the present study, however, ethane was excluded from the desorption products since peaks for m l e 26 and 27 were not detected. In Figure 1, the desorption spectrum of each product is presented as the mass spectrum of the primary fragment (mass exhibiting the largest relative abundance) from which the contributions from other products have been subtracted. However, for water, the raw mass spectrum for m l e 18 was presented without subtracting the m l e 18 contribution from methanol desorption. The TPD spectra for the rest of the alcohols are presented in the same way as above. Table I1 lists the distributions and peak temperatures of desorption products obtained from both systems. The product distributions are expressed in yields relative to the amount of low-temperature alcohol desorption, since the CO desorption could not be quantified with the flow-reactor system, precluding normalization based on total carbon-containingproducts. The product distribution obtained from the high-vacuum system is necessarily incomplete since the desorption of most of the decomposition products was not complete at 675 K, the maximum operating temperature of the instrument. The peak temperatures of major products observed with the flow-reactor system were in the order dimethyl ether < formaldehyde < methane. The order could be only partly reproduced with the high-vacuum system due to the temperature limitations of the instrument. 2. Ethanol TPD. Adsorbed ethanol exhibited the TPD spectra displayed in Figure 3. For the flow-reactor system, ethanol desorption peaks at 350 and 390 K, attributed to condensed ethanol and to ethoxide species, respectively, were observed. Only the 390 K peak was observed in the high-vacuum system. Approximately 60% of the ethoxide species desorbed intact via this low-temperature (390 K) desorption state; the remainder decomposed at higher temperature. The ethanol peak at 390 K was accompanied by water in the same manner as observed for methanol TPD. Desorption peaks resulting from the decomposition of surface ethoxy species appeared at higher temperatures. Major desorption products were acetaldehyde, ethylene, diethyl ether, and water. Minor products were ethanol, butene, and hydrogen. Diethyl ether and ethanol peaks were observed near 590 K for both the flow-reador system and the high-vacuum system. Deconvolution of these peaks was rather difficult since these two molecules have common principal fragments, i.e., m l e 31,29, and 45. The TPD spectrum for m / e 46 and the fragmentation pattern of gas-phase ethanol were employed to determine the contribution of ethanol desorption to the area of the m l e
II 400
600
IO00
000
TEMPERATURE ( K )
Figure 3. TPD spectra of adsorbed ethanol (mass spectrometer sensitivity factors are not included): (a) flow-reactor system; (b) high-vacuum system. Table 1x1. Distributions and Peak Temperatures of Ethanol TPD Products molecule relative yield Deak temp, K Low Temperature C2HSOH 58.8" (63.3)b 390" (390)b HZO
17.6 (5.7)
390 (405)
Higher Temperature C2HSOH (CzH6)zO CH3CHO H2 C2H4 HZO C4H8
co2
3.0 6.5 9.4 0.6 8.8 4.1 3.5 2.2
(0.6) (3.2) (7.6) (0.3) (17.1) (2.5)
590 590 590 605 615 615 620 710
(590) (590) (600) (630) (640) (655) (640)
a Data from the flow-reactor system. * Data from the high-vacuum system.
31 peak at 590 K, and the remainder of the peak area was assigned to diethyl ether. The possible contribution of methanol as a product was excluded due to the absence of a m l e 32 peak. The desorption of diethyl ether at this temperature was confirmed by the observation of a desorption product with m l e 59 and 74 fragments with the high-vacuum system. The area of the m l e 29 peak around 590 K remaining after subtraction of the contributions of ethanol and diethyl ether was assigned to acetaldehyde. Comparison of this area with those for the m l e 15,44, and 43 peaks at the same temperature, from which the ethanol and ether contributions had also been removed, provided good agreement with the fragmentation pattern of acetaldehyde. The desorption peak for acetaldehyde was followed by that for ethylene, and the temperature lag between those two peaks was ca. 30 K. For the flow-reactor system, spectra for m l e 27 and 26 (instead of m l e 28) were used to identify ethylene desorption quantitatively for the reasons noted above for CO detection from methanol TPD. A small hydrogen peak was located between the peaks for acetaldehyde and ethylene desorption. Minor desorption peaks for m l e 41,39, and 56 appeared simultaneously with ethylene. These peaks were assigned to butene, presumably formed by coupling of the product
Kim et al.
538 Langmuir, Vol. 4, No. 3, 1988 ethylene. Water desorption was observed to accompany ethylene desorption. A C02peak was also observed above 700 K with the flow-reactor system. Molecules which were checked for but not detected in significant amounts included methane, ethane, and acetic acid. Table I11 lists the distributions and peak temperatures of ethanol TPD products. The product distributions are expressed in yields relative to the saturation coverage of chemisorbed alcohol. Water desorption at higher temperature could not be quantified for the high-vacuum system, since the peak was located near the operating limit, 675 K. The product distributions obtained with both instruments were in good agreement, although more ethylene was observed in the high-vacuum experiments. 3. 1-Propanol TPD. Adsorbed 1-propanol exhibited TPD behavior similar to that for ethanol. Approximately 50% of the n-propoxide species desorbed as the parent alcohol at 390 K. Water was observed to accompany the 1-propanol peak at 390 K. For the flow-reactor system, a desorption peak due to condensed propanol was also observed at 350 K. Desorption products appearing at higher temperatures (above 575 K) qualitatively resembled those for ethanol TPD; these included the parent alcohol as well as the corresponding ether, aldehyde, and olefin products, plus water and hydrogen. The major products were propylene, di-n-propyl ether, propionaldehyde, and water. Minor products were 1-propanoland hydrogen. Desorption peaks for 1-propanol and di-n-propyl ether appeared first, followed by propionaldehyde and propylene. The m l e 31 peak at 575 K was unambiguously assigned to 1-propanol desorption, since the possible contributions of methanol or ethanol as products could be excluded due to the absence of peaks for m l e 32 and m l e 46, respectively. The mle 43 peak appearing at 580 K with the flow-reactor system was assigned to di-n-propyl ether desorption. This assignment was further supported by the observation of coincident peaks for mle 43,73, and 102 at 590 K with the high-vacuum system. The possibility of a contribution from acetone ( m l e 43, 15, 58) was excluded due to the absence of a m l e 15 peak around 585 K and consideration of the 1-propanol structure. The area of the m l e 29 peak (at 595 K for the flow-reactor system and 620 K for the high-vacuum system) was assigned to propionaldehyde after subtraction of contributions of products such as 1propanol, di-n-propyl ether, etc. The propionaldehyde peak was apparently followed in temperature by the propylene peak, although the separation was not as large as that between the acetaldehyde and ethylene products from ethanol TPD. The prominent peak for m l e 41, centered at 600 K for the flow-reactor system and 625 K for the high-vacuum system, was assigned to propylene; propylene was the dominant product from n-propoxide decomposition. As was the case with ethanol TPD, water desorption accompanied propylene desorption. CO and COPexhibited small desorption peaks around 620 K in the high-vacuum system. With the flow-reactor system, only a small C 0 2 peak was observed above 700 K. Molecules which were checked for but not detected included propane, acetone, and propionic acid. However, for the high-vacuum system, a small peak corresponding to acrolein was detected at 620 K; its relative yield was less than 2% of the saturation coverage of n-propoxide. TPD spectra for 1-propanol are displayed in Figure 4, and the product distributions and peak temperatures of desorption products are listed in Table IV. 4. 2-Propanol TPD. Adsorbed 2-propanol exhibited TPD spectra that were somewhat different from those for
400
600
800
IO00
TEMPERATURE ( K )
Figure 4. TPD spectra of adsorbed 1-propanol (mass spectrometer sensitivity factors are not included): (a) flow-reactor system; (b) high-vacuum system. Table IV. Distributions and Peak Temperatures of 1-Propanol TPD Products molecule relative yield peak temp, K Low Temperature 1-propanol 48.0" (52.4)b 390" (390)* HZO 21.0 (7.3) 390 (405) 1-propanol (C~H,)ZO CzHSCHO C3H6
HZO
HZ
co COZ
Higher Temperature 1.4 (3.7) 9.6 (7.3) 6.2 (5.2) 25.0 (23.6) 9.1 1.0 (neg) not resolved (0.5) 1.8 (1.0)
575 (575) 580 (590) 595 (620) 600 (625) 600 (640) 600 (620) 710 (625)
Data from the flow-reactor system. *Data from the high-vacuum system.
1-propanol. Desorption peaks a t 390 K were assigned to 2-propanol. Approximately 50 % of the isopropoxide species desorbed intact from this low-temperature desorption state, accompanied by water. The flow-reactor system provided evidence for an additional parent alcohol peak at 350 K, as was observed for the other alcohols. The higher temperature desorption products were, in increasing order of peak temperature, 2-propanol, acetone, propylene, and water. The principal products were propylene and water, and minor products included 2-propanol and acetone. Unlike the TPD of the primary alcohols, no desorption of ether species (diisopropyl ether) was observed. The small m l e 45 peak at 500 K was assigned solely to 2-propanol, with no contribution from diisopropyl ether. This assignment was supported by the following observations: (i) The areas of m l e 45 and 29 peaks around this temperature were in good agreement with the fragmentation pattern of gas-phase 2-propanol. (ii) No peak for mle 87, the characteristic fragment of diisopropyl ether, was observed. The m l e 29 peak was chosen for the peak area analysis instead of mle 43,27, or 15 (major fragments of 2-propanol other than 45 and 29) since m l e 29 is not a major fragment of acetone or propylene, which began to
AdsorptionlDecomposition of Alcohols on TiOz
Langmuir, Vol. 4, No. 3, 1988 539
300 250 200 W
l-
a
2 150
a
-
z 0 c a
z
$
a
0
I-
v)
2
S
W
0
100
50
Y 0
- 50 -100
400
600
800
1000
- I 50
300
400
TEMPERATURE ( K )
Figure 5. TPD spectra of adsorbed 2-propanol (mass spectrometer sensitivity factors are not included): (a) flow-reactor system; (b) high-vacuum system. Table V. Distributions and Peak Temperatures of 2-Propanol TPD Products molecule
relative vield
ueak temu, K
Low Temperature 2-propanol H20 2-propanol CH3COCH3 HZ C3H6 HzO
co cos
48.7" (51.5)* 17.1 (7.2) Higher Temperature 1.5 (2.6) 5.3 (5.2) 0.3 (0.3) 44.4 (40.7) 12.1 (3.1) not resolved (0.5) 2.7 (neg)
500
600
700
TEMPERATURE ( K 1
3 W (390)* 390 (405) 500 (490) 540 (540) 540 (550) 550 (560) 550 (570) (560) 710
OData from the flow-reactor system. *Data from the high-vacuum system.
desorb near 450 K. The areas of the m l e 43, 15, and 58 peaks at 540 K corresponded to the fragmentation pattern of acetone, the dehydrogenation product from isopropoxide decomposition. A very small hydrogen peak was observed near 540 K. A prominent mle 41 peak was observed at 550-560 K and was assigned to propylene. The relative amount of propylene desorption was even greater than in the case of 1-propanolTPD. As for ethanol and 1-propanol TPD, water desorption accompanied propylene desorption. A small CO peak was detected a t 560 K for the highvacuum system. As in the case of 1-propanol TPD, a small amount of C02 was detected above 700 K for the flowreactor system. TPD spectra of adsorbed 2-propanol are displayed in Figure 5. Table V lists the distribution and peak temperatures of 2-propanol TPD products. The results for 2-propanol TPD again demonstrated the close agreement obtained between experiments with the two different apparatuses. The peak separation between acetone and propylene in this case resembled those between aldehyde and olefin products in ethanol and 1-propanol TPD. Gravimetric Results. The weight of the sample initially saturated with alkoxide species decreased continuously during temperature programming, due to the desorption of the adsorbed species. Figure 6 displays the
Figure 6. Sample weight decrease during TPD experiments. Table VI. Net Weight Losses Measured at 675 K alcohol methanol ethanol
net w t loss, pg 74
95
alcohol 1-propanol 2-propanol
net wt loss, fig 30 30
changes of sample weight in the course of TPD experiments obtained from the high-vacuum system. For each of the alcohols studied, a transition from net weight gain (relative to the clean sample) to net weight loss was observed to occur in the middle of temperature ramping: at 523 K for methanol, 548 K for ethanol, 623 K for 1propanol, and 573 K for 2-propanol. This order does not coincide with the apparent order of alkoxide thermal stability observed from TPD experiments: Me0 > Et0 > n-Pro > i-Pro. This difference is due to the dependence of the weight loss upon product selectivity. While oxidative dehydrogenation (to aldehyde plus water) results in the net removal of oxygen atoms from the lattice, dehydration (to olefin or ether plus water) does not produce a net change in the sample mass. Thus those alcohols exhibiting a higher selectivity toward dehydrogenation, methanol and ethanol, produced a net weight loss earlier in the temperature ramp than the propanols, although the methoxide and ethoxide were more stable than either of the propoxides. The dependence of the weight loss on the product selectivity is further illustrated by order of the absolute weight loss measured at the end of temperature programming (at 673 K). These losses decreased in the order ethanol > methanol > 1-propanol = 2-propanol as shown in Table VI, again reflecting the greater dehydrogenation selectivity of ethanol and methanol.
Discussion The uptake data obtained from the microbalance experiments clearly demonstrate that irreversible chemisorption of alcohols on titania is dependent upon the structure of the alcohol. All of the primary alcohols studied exhibited greater molecular coverages per unit surface area than the secondary alcohol. This effect is presumably a steric one and suggests that a large fraction of the titania surface participates in alcohol adsorption. Steric effects would be expected to be less important if the active sites for alcohol adsorption were present in low density. Steric
540 Langmuir, Vol. 4, No. 3,1988 Table VII. Surface Ti Densities Calculated for the Cleavage Planes of Anatase surface Ti density, surface T i density, plane T i atoms/nm2 plane Ti atoms/nm2 (001) 7.00 (100) 2.79 (101) 5.18 (111) 1.90 (110) 3.93
effects have also been noted by Farneth and co-workersM for chemisorption of methanol, ethanol, 2-propanol and 2-methyl-2-propanol on Moo3. In the present study, the primary C1 to C3 alcohols exhibited molecular saturation coverages for irreversible chemisorption that were nearly independent of the alcohol identity. These results are in good agreement with those reported for rutile by Suda et aL30but are not in complete agreement with those reported for anatase by Carrizosa and Munueramband for rutile by Cunningham and co-workers.12 Carrizosa and Munuera observed that the increase in alkyl chain length from C2 to C5produced a progressive decrease in alcohol saturation coverages, while the change from C1 to C2 did not affect the saturation coverage. They also observed that secondary and tertiary alcohols were adsorbed in slightly larger amounts than primary alcohols with the same number of carbon atoms, suggesting that branching did not influence the adsorption capacity of the surface. Cunningham and co-workers reported that ethanol exhibited much less irreversible chemisorption than 2propanol or 2-methyl-2-propano1, which implied that branching increased the adsorption capacity. The origin of the conflicts between those studies and the present study is not clear, but these differences may have originated from different adsorption measurement techniques. The inconsistencies between these studies serve to illustrate the need for determination of absolute adsorbate coverages on these materials. These provide an additional indication of differences in the surface properties of samples produced by different preparation techniques. Further, both steric effects and the selectivity toward bimolecular reaction pathways will depend strongly on the adsorbate packing density. The polycrystalline anatase surface is a combination of several crystallographic planes. Assuming that the (001) plane is the most likely cleavage plane for and calculating the number of Ti atoms exposed per unit surface area42as tabulated in Table VII, we find the molecular coverage of 3.14 molecules/nm2 for the irreversible chemisorption of methanol corresponds to 0.45 molecules/surface Ti atom on the basis of the (001) surface plane model. Since the (001) plane is the cleavage plane with the highest surface Ti density for anatase, and since the bulk structure requires that planes with lower atomic densities also be exposed, this value of 0.45 molecules/ surface Ti represents a lower bound on the adsorbate density. For example, if one assumes that the surface is composed of equal areas of (001) and (100) planes, an average atomic density of 4.90 Ti/nm2 is obtained, which leads to the coverage of 0.64 molecules/surface Ti. Thus, it is reasonable to suggest that 50% or more of the surface Ti atoms participates in the irreversible chemisorption of methanol if a one-to-one correspondence is assumed between the primary alkoxides and surface Ti atoms. It has generally been proposed that the dehydroxylated titania surface is mostly composed of the active Ti-0-Ti (41) Primet, M.; Pichat, P.; Mathieu, M. V. J.Phys. Chen. 1971, 75, (a) . . 1216. (b) 1221. (42) Wyckoff, R. W. G. Crystal Structures; John Wiley: New York, 1963; Vol. I.
Kim et al.
b r i d g e ~ , ' ~which , ~ ~ *dissociate ~~ to form Ti-OR and Ti-OH species upon the chemisorption of alcohols or water. According to this scheme, uptake of 0.5 alcohol molecules/ surface Ti would require the participation of all of the titanium atoms at the surface. Thus, steric effects, such as those observed for 2-propanol adsorption, are not surprising. It has been suggested that multiple adsorption states on titania are indicative of different types of surface sites and that the characteristics of those sites are closely related to the coordinative unsaturation of the exposed Ti cati o n ~ The . ~TPD ~ results ~ ~ of~the~present ~ ~ study showed essentially two different adsorbed alcohol states on titania: reversible adsorption responsible for intact desorption at 350 K and irreversible adsorption to form surface alkoxides. The latter species were removed from the surface by two processes: recombination to produce molecular alcohols at 390 K and decomposition to various products at higher temperatures. The decomposition of alkoxides was a complex function of alcohol structure and temperature. We propose below a mechanism to account for the adsorption, desorption, and decomposition of alcohols on the polycrystalline titania surface. Only one type of site for alkoxide formation is required to explain all of the decomposition products observed in this study. adsorption of alcohols at 300 K molecular adsorption: ROH(g) -+ ROH(ad) dissociative chemisorption: ROH(g) + O(1)
-
RO(ad) + OH(ad)
(1)
(2)
desorption at low temperatures desorption of molecularly adsorbed alcohols at 350 K:
-
ROH(ad) ROH(g) (3) desorption of dissociatively adsorbed species at 390 K: RO(ad) + OH(ad)
ROH(g) + O(1)
(4)
where (g) and (ad) denote gas and adsorbed phases, respectively, and O(1) denotes lattice oxygen. The alcohol desorption peak at 350 K observed with the flow-reactor system, but not in the high-vacuum system, clearly demonstrates that the molecularly adsorbed alcohols as well as the dissociatively adsorbed species (alkoxides) are produced from adsorption of alcohola on titania as depicted by pathways 1and 2. The enhanced evacuation efficiency of the high-vacuum system removed this weakly adsorbed species from the surface before the start of the TPD experiment; as a result, only the 390 K alcohol desorption peak was observed. IR spectra obtained following alcohol adsorption at 325 K with a high-vacuum system showed evidence for surface alkoxides but none for molecularly adsorbed alcohols,32suggesting that the alcohol desorption peak at 390 K results from the recombination of dissociatively adsorbed species via pathway 4. The formation of surface alkoxides via pathway 2 is usually described as the deprotonation of gas-phase alcohol molecules by the basic sites (oxide anions) and the coordination of alkoxides to the acidic sites (surface cations). (43) Jones, P.; Hockey, J. A. Trans. Faraday. SOC.1971,67, (a) 2669, (b) 2679. (44) Tanaka, K.; White, J. M. J. Phys. Chem. 1982, 86, 4708.
Langmuir, Vol. 4,No. 3, 1988 541
AdsorptionlDecomposition of Alcohols on TiOa
~~
Table VIII. Dehydration Selectivities of Alkoxide Decomnosition" alkoxide selectivity, % alkoxide selectivity, % 58b n-propoxide 88 (84) methoxide 75 (79)C isopropoxide 89 (89) ethoxide
For the methoxide decomposition, the dehydration selectivity was not compared between the two sets of TPD data since a complete product distribution could not be obtained for the high-vacuum system. For the flow-reactor system, carbon monoxide was not included in the estimate of the dehydration selectivity of methoxide decomposition. *Data from the flow-reactor system. Data from the high-vacuum system.
Scheme I. Decomposition of Alkoxides at Higher Temperatures bimolecular dehydration to form dialkyl ethers: PRO(ad) RORIg) + O ( I )
-
a
- hydrogen elimination to form carbonyl products : A'
a
The peak temperature for alcohol desorption via pathway 4 was essentially independent of the identity of alcohol adsorbed. Water was observed to desorb coincident with each of the alcohols at 398 K. The formation of water at this temperature can be explained by the condensation of hydroxyl groups produced in the course of dissociative adsorption of alcohol molecules. The mechanistic expression is usually depicted as 2OH(ad) H,O(g) + O(1) + V, (5)
-
where V, denotes the oxygen vacancy. If no water were formed at 390 K,there would exist a one-to-one correspondence between surface alkoxide and hydroxyl species throughout the course of alcohol desorption in this temperature range. Thus the adsorption of alcohols would be dissociative but reversible: all of the alkoxides would be removed by reaction 4 at 390 K, and none would remain to decompose at higher temperature. Such dissociative but completely reversible adsorption of alcohols has been observed on nonreducible oxides such as Mg0.45 On TiO, the recombination of alkoxide and hydroxyl species competes with the reaction of hydroxyls to form water. Roughly half of the alkoxides are removed via low-temperature recombination. The remaining half decompose at higher temperature, presumably due to the shortage of hydroxyl species available for recombination. The relative insensitivity of the alkoxide fraction remaining after reaction 4 to alcohol structure suggests that the rate of reaction 4 does not depend strongly on alcohol structure. This may either reflect the intrinsic rate of the alkoxide recombination reaction or control of the rates of alkoxide and hydroxyl removal by the mobility of protons on the surface. Desorption peaks at higher temperatures (above 490 K) were assigned to the products of alkoxide decomposition. Both dehydrogenation and dehydration pathways were observed, with selectivities dependent upon the alcohol structure. Decomposition of the primary alkoxides produced a common pattern of sequential desorption with increasing temperature: ether-aldehyde-olefin. In the case of the methoxide, methane was the product at high temperature, as no olefins can be formed by simple dehydration. The secondary alkoxide exhibited a different pattern: ketone-olefin. Pathways representing the multiple channels for alkoxide decomposition may be depicted as in Scheme I. The formation of dialkyl ethers from primary alkoxides via the bimolecular interaction between the alkoxide species is most likely to be important when the titania surface has a high density of adsorbed alkoxides. Gravimetric data indicate that there is near stoichiometric uptake of alcohol at saturation; thus the surface alkoxide species have the possibility of mutual interaction. Evidence for interactions between surface alkoxides to form
-
I
R-C-H(ad)
I
R'
I
R-C=O(g)
+
H(ad)
Martinez, R.; Barteau, M. A. Langmuir 1986, 1, 684.
(7)
0
8-hydrogen elimination plus C - 0 bond cleavage in form olefins:
7 7'
R-C-C-H(ad)
I 1 H O
+
O(I) H
I
R'
t
ethers may be also found in the methanol decomposition results reported by Matsushima and White& on alumina. They observed that in the presence of gas-phase CD,OD the ether produced by thermal desorption from a surface on which CH30H had been preadsorbed was primarily CH30CH3 An alternative mechanism for ether formation, involving reaction of a surface alkoxide with an adsorbed alcohol molecule by electrophilic attack on the hydroxyl oxygen, has also been pr~posed.~? This mechanism does not appear to be applicable to the present study since (i) molecularly adsorbed alcohols were not present on the surface once the 350 K desorption state was removed and (ii) the kinetics and selectivity for ether formation were independent of the method of gas-phase removal (whether by carrier gas or by evacuation), although alcohols desorbed from the surface a t temperatures up to 600 K. Indeed, the selectivity for ether formation depended most strongly on alkoxide structure; the peak temperatures for dialkyl ether desorption reflected the thermal stabilities of the primary alkoxides as evidenced by their correspondence with other alkoxide decomposition products. Further, no ether species were produced from isopropoxide decomposition. This latter effect is presumably due to steric limitations on the interaction of secondary alkoxides to form ethers. The formation of ethers was followed (in temperature) by that of aldehydes for primary alkoxide decomposition. These reactions are typically accounted for by a-H elimination?*@ as depicted by pathway 7. The secondary alkoxide from 2-propanol also underwent dehydrogenation to produce a small amount of acetone. The desorption peak temperatures for dehydrogenation products were in the order HCHO > CHBCHO> CzH5CH0> CH3COCH3, reflecting the strengths of a-C-H bonds. If we define the dehydrogenation selectivity as the ratio of the amount of alkoxide converted to dehydrogenation produds (aldehyde or ketone) to the total amount of alkoxide decomposed, the dehydrogenation selectivities decreased in the order Me0 > Et0 > n-Pro > i-Pro. Thus, the selectivity for dehydrogenation decreases as the kinetics for this reaction become more favorable, implying that the rate of dehydration also increases with increasing alkoxide chain length as shown in Table VIII. A small peak for hydrogen desorption which followed the peak for the carbonyl product in every case can be explained by the recombination of surface hydrogen species produced via pathway 7 as follows: ~
(45)
(6)
(46) (47)
Matsushima, T.; White, J. M. J. Catal. 1976,44, 183. Knozinger, H.; Buhl, H.; Ress, E. J. Catal. 1968, 12, 121.
542 Langmuir, Vol. 4, No. 3, 1988 H(ad)
+ Mad)
-
Kim et al. H2(g)
(9)
The formation of carbonyl products was followed (in temperature) by that of olefins for the decomposition of alkoxides other than methoxide, as depicted by pathway 8. The dehydration of alkoxides to olefins plus water must involve cleavage of P-C-H and C-0 bonds. Further, while the Brernsted acid catalyzed dehydration of alcohols results in removal of the oxygen from alcohol to form water, such a mechanism would require considerable intramolecular rearrangement for dehydration of a surface alkoxide intermediate. As discussed above, the evolution of H 2 0 at 390 K produces an oxygen-deficient surface. The stoichiometry of the surface can be restored during TPD by C-0 bond cleavage of adsorbed alkoxides, resulting in oxygen deposition on the surface. A similar mechanism has been proposed by Bowker et aL6 to describe the net dehydration of ethanol to ethylene via ethoxide intermediates on ZnO. For methanol, ether formation was the only dehydration pathway due to the absence of a /?-carbon. The dehydration selectivity of alkoxides to olefins was in the order E t 0 < n-Pro < i-Pro. The water desorption peak which followed olefin desorption for every alcohol (except methanol) was associated with alcohol dehydration and could be explained either by the condensation of the surface hydroxyl species produced via pathway 8 or by the reaction between the hydroxyl species and the surface hydrogen species formed via pathway 7. For methanol decomposition, the surface hydroxyl species participating in this reaction are likely to be formed by the interaction of lattice oxygen with the hydrogen species produced from the decomposition of surface methyl species as discussed below. In most cases, a small peak for desorption of the parent alcohol was observed to accompany the peaks for the alkoxide decomposition products. This parent alcohol desorption is considered to result from the recombination of surface alkoxides with the available hydrogen species as follows: RO(ad) + H(ad) ROH(g) (10)
-
In addition to the expected dehydrogenation and dehydration products, several other products were observed. For methanol TPD, methane and carbon monoxide were produced and desorption peak temperatures for these products were higher than those of the major products, formaldehyde and dimethyl ether. One possible mechanism to explain the formation of methane and carbon monoxide involves the reaction of the methoxide with oxygen vacancies: CH,O(ad) + V, CH,(ad) + O(1) (11) CH3(ad) CH,(ad)
-
C(ad) + 3H(ad)
-
+ H(ad)
C(ad) + O(1)
(12)
CH,(g)
(13)
+ V,
(14)
CO(g)
Sat0 and Whitell suggested that the reduced titania surface tends to scavenge the oxygen from the adsorbed species in order to fill the oxygen vacancies. Similar results have also been obtained in our laboratories for the decomposition of aliphatic alcohols on reduced titania single-crystal surfacesaa The CH3 species produced by step 11 would decompose into surface carbon and hydrogen species via pathway 12, with the hydrogens rapidly consumed by step 13 to produce methane. Surface carbon formed through step 12 is likely to be oxidized to carbon (48)
Kim, K. S.; Barteau, M. A., manuscript in preparation.
monoxide by step 14. The evolution of C02observed above 700 K for every alcohol studies can be similarly explained by the oxidation of the deposited carbon. Parallel IR studies demonstrated that no formation of formate species from methoxide occurred in the absence of gas-phase oxygen, supporting the conclusion that the CO desorption observed above 600 K from methanol TPD did not result from the decomposition of formate species. Groff and ManogueMreported that formate species were formed from methanol oxidation at 673 K on titania (rutile) and were decomposed into CO and C02. In order to further investigate the possible formation of the carboxylate species as intermediates derived from alcohols, a separate series of experiments was carried out for carboxylic acid decomposition on anatase." Surface formate species formed following formic acid adsorption at 300 K decomposed mostly into CO a t 525 K. Since neither CO nor C02 desorption was observed below 600 K from the methanol TPD experiments carried out with both apparatuses, the formation of formate species via the oxidation of surface methoxide below 600 K could be unambiguously excluded. Above 600 K, as the titania surface must be highly oxygen depleted due to the desorption of formaldehyde and water, the oxidation of methoxide into formate would be strongly unfavorable in the absence of gas-phase oxygen. The desorption products and their peak temperatures observed from acetic acid and propionic acid TPD were also different from those observed from ethanol, 1-propanol, and 2-propanol TPD. Therefore, it may be concluded that the formation of carboxylate species via oxidation of surface alkoxides is not a major pathway of alcohol decomposition in the absence of oxygen in this study.
Conclusions The adsorption and decomposition of small aliphatic alcohols on polycrystalline titania may be summarized as follows. (1) Two adsorption states of alcohol were observed: molecularly adsorbed alcohol and dissociatively adsorbed species (alkoxides). Molecularly adsorbed alcohols desorbed intact at 350 K, demonstrating the reversibility of the adsorption state. Alkoxides were removed via two channels upon heating: alcohol desorption via recombination at 390-400 K and decomposition at higher temperatures. (2) Coverages of the primary alkoxides were of the order of 0.5fsurface Ti cation. At these high coverages, significant yields of bimolecular reaction products were observed. The saturation coverage of surface isopropoxide species was ca. 75% that of the primary alkoxides, presumably due to steric effects on the adsorption of secondary alcohols. (3) The decomposition kinetics and selectivity of alkoxides were dependent upon the alcohol structure. The decomposition temperatures of surface alkoxides were in the order Me0 > E t 0 > n-Pro > i-Pro. The desorption sequence for the decomposition products from ethanol and 1-propanol TPD exhibited a common pattern: dialkyl ether (bimolecular interaction)-aldehyde (a-H abstraction)-olefin (P-H abstraction and oxygen deposition). For methanol TPD, aldehyde was followed by methane due to the absence of @-H.The absence of dialkyl ether formation from 2-propanol TPD again suggested steric constraints on secondary alcohol adsorption and decomposition. (49)
mukr.
Kim, K. S.; Barteau, M. A., submitted for publication in Lang-
Langmuir 1988,4, 543-546 (4) Dehydrogenation/dehydration selectivities for alcohol decomposition also depended upon alcohol structure. The dehydration selectivity increased in the order Me0 < Et0 < n-Pro < i-Pro. ( 5 ) The characteristics of sample weight change during TPD experiments reflected the thermal stabilities of alkoxides and the selectivities of the decomposition pathways. The reduction of the surface after TPD and subsequent reoxidation supported the conclusion that alcohol decomposition on titania follows the redox chemistry of oxide catalysis.
543
(6) The formation of carboxylates via the oxidation of alkoxides in the absence of gas-phase oxygen was not observed. Acknowledgment. We are grateful for the support of thii work by the National Science Foundation (Grant CBT 8311912). Registry No. Methanol, 67-56-1;ethanol, 64-17-5; 1-propanol, 71-23-8; 2-propanol, 67-63-0; titanium dioxide, 13463-67-7; methoxide, 3315-60-4; ethoxide, 16331-64-9; n-propoxide, 26232-83-7; isopropoxide, 15520-32-8.
Quenching of Excited (4-( 1-Pyreny1)butyl)trimethylammoniumBromide on Synthetic Hectorites Containing Lattice Copper J. Wheeler and J. K. Thomas*l Chemistry Department, University of Notre Dame, Notre Dame, Indiana 46556 Received August 4, 1987 Several hectorite clays have been made with different percentages of copper bound into the clay lattice. The fluorescent probe molecule (441-pyreny1)butyl)trimethyla"onium bromide (PN+)was located on the clay surface by cationic exchange, and ita photophysical properties were used to describe certain features of the clays. Marked quenching of excited PN+ occurred on clays containing Cu,the lattice copper being more efficient than adsorbed Cu2+. Analysis of the data indicates that an electron-tunnelingreaction occurs from excited PN' to Cu. The addition of cetyltrimethylammonium bromide (CTAB) to the system leads to a marked decrease in the Cu quenching efficiency. A model describing all the events is eventually given. The data indicate that small amounts of transition metals bound into the clay lattice can have a marked effect on the chemistry of molecules adsorbed to the clays.
Introduction In earlier work we noted that excited ruthenium tris(bipyridine), Ru(II)*, was partially quenched on a number of natural montmorillonite and hectorite clay^.^*^ The Ru(I1) was located on the clay surfaces by cationic exchange with the natural Na+ of the clay. Several features could account for such an effect. (a) The transition-metal content of the clay lattice could quench the Ru(II)* via e- exchange, as in aqueous solution. (b) Transition metals could exchange onto the clay and act as in a (c) High laser intensity effects could shorten the Ru(II)* life via second-order effects. (d) The clay itself could be reactive. Synthetic clays such as laponite in purified form showed little quenching of Ru(II)*, which suggested that feature d is not correct. Deliberate addition of Cu2+or Fe3+to the clay did produce quenching. This provided sufficient information to show that feature b was not operative in the earlier work. High laser intensities did shorten the lifetime of Ru(II)+ via second-order effect^,^-^ but under the experimental conditions used, i.e., low laser powers, this was not operative. Process a, quenching by lattice transition metals, seemed likely, in particular as synthetic clays, e.g., laponite, were inert. Hence, a series of synthetic clays were made, which were deliberately doped with Cu2+into the lattice structure, and the photophysics of (4-(l-pyrenyl)butyl)trimethylammonium bromide (PN') were studied in these systems. (1) We thank the Army Research Office for support of this work. (2) Dellaguardia, R.; Thomas, J. K. J. Phys. Chem. 1983, 87, 990. (3) Dellaguardia, R.; Thomas, J. K. J. Phys. Chem. 1984, 88, 964. (4) Nakamura, T.; Thomas, J. K. Langmuir 1985, 1, 568.
0743-7463f 88f 24O4-0543$01.50f 0
Experimental Section Clay Synthesis. Granquist and Pollacks demonstrated that
synthetic hectorite could be prepared by hydrothermaltreatment of an aqueous solution of silica gel and freshly precipitated Mg(OH),. A 0% Cu clay was made in the following manner: 24.64 g of Na2Si03.9Hz0was dissolved in 100 mL of distilled deionized water with acid-charged Dowex 50x8 cation-exchangeresin for 20 min. The resin was first converted to the H+ form with 2 N HC1 and was then washed repeatedly with distilled deionized water until a negative test for C1- was obtained at 0.001 M Ag NO3for at least 10 min. The resin was suction filtered to remove excess water prior to the addition of the silicate mixture, and the resin was similarly removed at the end of the ion-exchangeprocess. This produces a silicate solution that is pH 7 and essentiallyfree of sodium.6 The Mg(OH), was prepared by dissolving 13.25 g of MgCl2.6Hz0in 80 mL of water and precipitating the mixture with 10 mL of 15 M NH40H. The Mg(OH), was washed by means of differential centrifugation with at least 4 volumes of water to remove excess ions. Once the pH of the discarded supernatant was neutral by litmus paper test, the moist Mg(OH), was transferred to a Waring blender by using 50 mL of HzOto complete the transfer. LiF, 0.4194 g, was added, as well as the silica solution. These were mixed at high speed for 5 min to form a slurry of about 6% solids that was then refluxed with constant stirring for 2 weeks. To produce the clay designated later as 3% Cu hectorite, the silica gel was produced as previously described, but only 12.85 g of MgC1,-6Hz0 was used to form the Mg(OH),; 0.3334 g of CuC12.2Hz0was precipitated with 4 N NaOH, and the Cu(OH), was again washed by means of differentialcentrifugationto remove (5) Granquist, W. T.; Pollack, S. S.Eighth National Conference on Clays and Clay Minerals; 1959;p 150. ( 6 ) Thomas, J. K,;Wheeler, J. J. Photochem. 1985,28, 285.
1988 American Chemical Society
