4128
J . Phys. Chem. 1989, 93, 4128-4132
the validity of assuming that the O2concentration is equal to that in the gas phase. The 0, concentration surrounding the excited pyrene in zeolite X or Y might be higher than that in the gas phase if the microcondensation of the 0, did occur inside the cage. In fact, there is evidence for 0, condensed in the pore of the silica ge1.l) The deviation from the first-order decay at 0, pressure less than 7 Torr could be caused by impurities such as Fe3+ in the framework. ESR studies show a weak Fe3+ signal (g = 4.3), which indicates a trace amount of tetrahedral Fe3+ present in the framework. ( B ) High pyrene loading sample, where both monomer and dimer emission were observed. Comparing Figure 5B (500-11sfull time scale) and Figure 8 C (200-11sfull time scale) shows that both exhibit monomer emission under vacuum. The one with high pyrene loading decays much faster than the one with low loading. This difference is probably caused by self-quenching a t this high pyrene concentration. The data in the 0, quenching studies (see Table 11) reveal that the 0, movement is limited under the high loading condition and suggests that some blocking of the tunnel inside the zeolite cage by the pyrene molecule occurs and that this blocking slows down the 0, quenching-a diffusion-controlled process. The formation of the pyrene dimer is static. This was confirmed by the following observations: ( 1) Ground-state dimer absorption was observed from the steady-state excitation spectrum (Figure 3). (2) The formation of the dimer occurred directly with the
laser pulse (Figure 8A). The time-resolved 0, quenching experiments indicated a decreased 0, quenching efficiency for the dimer compared with monomer (Figure 10 and Table II), which suggests that space was limited in the vicinity of the dimer. The 0, quenching of excited pyrene requires close contact of reactants; the space limitation in the dimer samples is unfavorable for reactant contact. It should be also mentioned that other mechanisms could cause the slow equilibrium, for example, a special geometry between the reactants. Cu2+Quenching. Cu2+ quenching of excited pyrene monomer follows Perrin-electron tunneling mechanisms. The electron tunneling reaction between Cu2+ and excited pyrene takes place spontaneously within a radius of 13.6 f 0.2 A in zeolite X. This implies that if an active site is present in the next neighboring supercage (such as Cu2+in the quenching reaction), the reaction between the reactant and the active site can occur, even if they are not in the same supercage. These results are consistent with the recent investigation by Dutta and Incavo19 that electron transfer can take place between R ~ ( b p y ) ~in ~one + supercage and MV2+ in the other.
Acknowledgment. We thank R . Krasnansky for his helpful discussion. Registry No. 02,7782-44-7; Cu2+, 15158-1 1-9; pyrene, 129-00-0: pyrene dimer, I7441 - 16-6. (19) Dutta, P. K.; Jncavo. J. A. J . Phys. Chem. 1987, 91, 4443-4446.
Methane Oxidation at Room Temperature and Atmospheric Pressure Activated by Light via Polytungstate Dispersed on Titania M. Gratzel, K. R. Thampi, and J. Kiwi* Institut de chimie physique, Ecole Polytechnique FZdPrale, CH- 101.5 Lausanne, Switzerland (Received: February 10, 1988: In Final Form: September 21, 1988)
This paper presents a new type of oxidation catalyst which in a photoexcited state activates methane. Tungstosilicate (SiWl,0a)4--loaded titania is active at ambient temperature and atmospheric pressure in the photoinduced oxidation of methane. Reaction products are CO, C02,and H 2 0 . Conversion increases with reaction temperature. In the absence of tungstosilicate total catalytic oxidation of methane to C 0 2and H 2 0 occurs. The results of the present study reveal a profound modification of the photocatalytic effect of the Ti02 surface due to the polytungstate.
Introduction The activation of C H bonds in methane for its selective oxidation into oxygen-containing chemicals is a subject of current interest.' The stability of the C H bond in methane (104 kcal/mol)2 makes it very difficult to activate this material into more valuable oxygenated or higher hydrocarbons. In practice, methane is converted to HCHO, CHIOH, CO, CO,, and higher hydrocarbons at high temperatures in exoergic reactions. These consecutive oxidative reactions are difficult to control and total oxidation to CO, occurs frequently. Catalysts containing transition-metal ions and heteropolymetalates have been widely used for dehydrogenation or oxidation of alcohols and other organic substance^.^ Photoassisted catalysis
was also reported for such reactions involving heteropolymetalate~.~ The present study shows that heteropolyacids, e.g., H4SiW12040, deposited on a suitable support material act as heterogeneous catalysts in the partial oxidation of methane. Kinetically, the challenge was to inhibit the undesirable formation of carbon dioxide instead of C O which is useful as synthesis gasS
( I ) (a) Pitchai, R.; Klier, K. Catal. Rea. Sci. Eng. 1986, 28, 13. (b) Mimoun, H. New J . Chem. 1987, / I , 5 1 3 . (c) Jones, C.; Leonard, J.; Sofranko, J . Energy Fuels 1987, l , 12. (2) Benson, S . W . Thermochemical Kinetics, 2nd ed., Wiley: New York,
(4) (a) Papaconstantinou, E.; Dimotikal, D.; Politov, A. Inorg. Chim. Acta 1980,46, 155. (b) Ward, M. D., Brazdil, J. F., Grasselli. R. K. J . Phys. Chem. 1984, 88, 4210. (c) Baba, T.; Watanabe, H.; Ono, Y. J . Phys. Chem. 1983, 87,2406. (d) Darwent, J. R. J . Chem. Soc., Chem. Commun. 1982,798. (e) Hill, C. L.; Bouchard, D. A. J . Am. Chem. SOC.1985, 107. 5148. (f) Hiskia, A.; Papaconstantinou, E. Polyhedron 1988, 7, 477. (5) Hahn, G . The Petrochemical Industry; McGraw-Hill: New York. 1970; p 20. (6) Kasztelan. S . ; Moffat, J . B. J . Catal. 1987. 106, 512.
1976; p 309. ( 3 ) (a) Pope, M. Heteropoly and Isopolyoxometalates; Springer Verlag: Berlin, 1983. Inorg. Chem. Concepts, 8 . (b) Weakley, T.Struct. Bonding 1983, 18, 131. ( c ) Akamoto, M.; Ikeda, H.; Okabe. H.: Echigoya, E. J . Catal. 1984, 89, 196.
0022-3654/89/2093-4l28$01.50/0
CH4
+ 202
-
C02 + 2H20
(1)
Recent work6 has shown that heteropolyoxometalates a t temperatures >700 K are effective in oxidizing methane to CO, C 0 2 , HCHO, and C H 3 0 H . We use here for the first time light-activated processes in methane oxidation on (SiWl,0w)4-/Ti02 and (SiW 12040)4-/A1203 powders. The objective was to determine
C 1989 American Chemical Society
Methane Oxidation Activated by Light whether or not it was possible to photoactivate methane a t low temperature and pressure, and at the same time inhibit complete oxidation. This paper presents first results concerning some of the factors which control the activity and selectivity in the photoinduced oxidation of CH, on polyacid-supported oxide catalysts. Some time ago Kazanskii7 showed that 0- reacts with methane over various supported oxides. The 0- centers were induced by y and UV radiation. More recently, Lunsford* has shown that CH, undergoes oxidative dimerization on Li20-doped MgO in which the active catalytic site is an 0- radical on the catalyst surface. A stable form of adsorbed C 0 2 was induced on M g O only a t temperatures above 673 K.8a SomorjaiQ has recently reported the kinetics of partial oxidation of methane with N20. Sofrankolo has shown that lattice oxygen from the catalyst was necessary to perform oxidative coupling of C H 4 via oxides and alkali-promoted oxides. Otsuka" has carried out efficient oxidative coupling of methane into ethane and ethylene using S m 2 0 3and Li-promoted SmzO3. Using a different approach, activation of methane with metal complexes has recently been reported by Bergman.I2 Various recent papers' have reported methane oxidation via MoO3I3in the temperature range from 750 to 850 K using O2 and N 2 0 to induce 0- and 02,the active oxidative species. Several theoretical and experimental papers have appeared in the past few years reporting photon-assisted C H bond activation over heteropolymetallates, oxyanions, and oxides.I4
Experimental Section Catalysts were prepared by impregnating Ti02 (Degussa P-25) with aqueous solutions containing different concentrations of H4(SiW12040)Merck p.a. The Ti02 support had a surface area of ca. 50m2/g. The impregnated pastes (ca. 0.5 mL of solution per gram of T i 0 2 ) were dried in air a t 448 K overnight, calcined at 723 K for 24 h, and then ground to a fine powder. Irradiations were performed with a solar simulator (Hanau GmbH) with 100 mW/cm2. Catalyst compositions reported in Figures 1 and 2 refer :o weight percentage loading of H4(SiWI2Oa) on 200 mg of Ti02 used in each run based on concentration used during the impregnation step. Prior to photolysis the samples were heated at 393 K in the reaction vessel with a stream of Ar to eliminate physisorbed impurities and moisture. Detection of C O was done on a Gow Mac T C detector equipped with a molecular sieve 5A column with He carrier gas a t 70 OC. Absence of methanol formation was confirmed by using a Porapak QS column. Determination of C 0 2 and CHI was performed in a Carlo Erba 5300 Mega gas chromatograph equipped with Carbosieve column and T C detector at 200 OC with H e as carrier gas. Our experiments did not show any C2H2,C2H4, C2Hs, or H C H O as a result of irradiations on CH4 and O2with the catalyst. Electron microscopy (TEM) was done with a Phillips-300s instrument. The resolution for such an instrument is 3.0 A.
Results and Discussion Figure l a (trace 1) presents the amount of C 0 2 generated on
Ti02support material without H,SiWl20, when it was illuminated (7) (a) Kaliaguine, S.;Shelimov, B.; Kazanskii, V. J . Catal. 1978.55, 384. (b) Lipatkina, N.; Shvets, V.; Kazanskii, V. Kinet. Katal. 1987, 19, 979. (8) (a) Aika, J.; Lunsford, J. J . Phys. Chem. 1977, 14, 1393. (b) Liv, R.; Lunsford, J. J . Chem. SOC.,Chem. Commun. 1982, 78. (c) Ito, T.; Wang, Ji.; Lin, Ch.; Lunsford, J. J . Am. Chem. SOC.1985, 107, 5062. (d) Lin, Ch.; Ito, T.; Wang, Ji.; Lunsford, J. J . Am. Chem. SOC.1987, 109, 4808. (9) (a) Khan, M.; Somorjai, G. A. J . Catal. 1985, 9, 263. (b) Zhen, K.; Khou, M.; Mak, C.; Lewis, B.; Somorjai, G. A. J . Caral. 1985, 94, 501. (10) (a) Sofranko, J.; Leonard, J.; Jones, C. J . Caral. 1987,103, 302. (b) Jones, C.; Leonard, J.; Sofranko, J. U S . Patent 4 499 322, 1985. (1 1) (a) Otsuka, K.; Komatsu, T. Chem. Lett. 1987. (b) Otsuka, K.; Liv, Q.; Morikawa, A. J . Chem. SOC.,Chem. Commun. 1986, 586. (12) Bergman, R. Science 1984, 223, 902. (13) Machiels, C.; Steight, A. Proceedings of the 4th International Conference on Chemistry and Uses of Molybdenum; Barry, H., Mitchell, P., Eds.; Climax Molybdenum: Ann Arbor, MI, 1982; p 411. (14) (a) Anderson, B.; Ray, K. J . Am. Chem. SOC.1985, 107, 253. (b) McCarron, M.; Harlow, R. J . Am. Chem. SOC.1983, 105, 6179. (c) Mehandru, P.; Anderson, B.; Brazdil, F.; Grasselli, K. J . Phys. Chem. 1987, 91, 2930. (d) Ward, M.; Brazdil, J.; Mehandru, S.;Anderson, A. J . Phys. Chem. 1987, 91, 6515.
The Journal of Physical Chemistry, Vol. 93, No. 10, 1989 4129
I-
0
12
6
18
---
TIME lh)
3
300
1
I
Lo w
4
U I Ln
200
LII 6
z
ri
N
0 U
100
a 0 U
0
I
I
,
6
12
18
'600
T I M E (h)
Figure 1. ( a ) (top) G a s consumed or produced when 2 m L of CH, and 1 m L of O2 were reacted on 200 m g of T i 0 2 (1) under illumination a t 47 OC, (2) in d a r k a t 47 OC,and (3) under illumination a t 47 O C , but with only 1 m L of O2present in the cell. Cell volume = 20 m L with Ar as diluent gas. (b) G a s consumed or produced when 2 m L of CH, and 1 m L of O2 were reacted on 200 mg of 37.5% (SiW,2040)4-/Ti02 (1) under illumination a t 47 OC, (2) in d a r k a t 4 7 "C, and (3) under illumination a t 47 OC, but with only 1 m L of O2present in the cell. O t h e r conditions a r e similar to Figure l a .
with 2 mL of CH4 and 1 mL of 02. C O was not observed. Concomitantly, a marked decrease in the initial CH4 used is noted. Dark experiments on the same system (trace 2) indicate similar values for CH4 adsorption but almost no C 0 2 evolution takes place as compared to irradiated samples (1). Control experiments to assess the amount of C impurity on the Ti02 surface are shown in trace 3 where the catalyst was irradiated in the presence of 1 mL of 02.Only 12 pL of C 0 2 evolved as compared to 300 p L in the presence of CH,. The impurity C of the Ti02surface was found to be 0.003% in agreement with the manufacturer's speci f i c a t i o n ~ . ~ ~ Figure l b shows results from a 37.5% (SiW12040)4-/Ti02catalyst irradiated under the same experimental conditions as in Figure l a , using 2 mL of CH, and 1 mL O2in each run. As seen from Figure 1b, 300 pL of CO is produced within 20 h. Concomitantly, 350 pL of C 0 2 was evolved, making the conversion of initial methane into carbon oxides of the order of 32.5%, with a 46% selectivity in CO, without considering the adsorbed products on the sample surface. In the presence of excess 02,C O initially grows during the first 12-15 h and then starts to decrease, due to further oxidation. One can suggest that heteropolyblue species3are involved in the catalysis because under illumination the catalyst changes color from light yellow to grey-blue. Exposure to air or O2 at 250-300 " C reverts the grey-blue color to yellow. Since CH, adsorption already takes place in the dark, it is not possible to judge the course of the reactions
-
CHI
+ Y2O2
CH4
+ 202
+
CO
+ 2H20
(2)
COZ
+ 2H20
(3)
(15) Degussa Bulletin 56, 1980, Frankfurt 1, B.R.D
4130
The Journal of Physical Chemistry, Vol. 93, No. 10, 196!9
from the C H 4 present in the gas phase. Since 300 pL of C O and 350 pL of C 0 2 was formed and no other products, other than H,O, were detected, reactions 2 and 3 would require I . 15 mL of 0,. This amount is somewhat higher than the oxygen initially added. It is possible tnat some TiO, lattice oxygen will intervene in the reaction. Similar observations have been reported for oxidation and photooxidation of C H 4 by Kazanskii,' Lunsford,* Sofranko.Io Seiyama,I6 and Baerns." Illumination of H4(SiW12040) alone in the presence of 2 mL CH4 and 1 mL 0, produces only 9 pL of CO, over a period of 20 h. Several polyoxometallates based on W, Mo, V, Nb, and Ta have been shown to catalyze the photodehydrogenation of alcohols in liquid phase.4 The present results show that it does not exhibit significant activity in the photooxidation of methane when used without TiO, or A1203supports. A catalyst containing 3 7 . 5 4 (SiW12040)4supported onto AI203 irradiated under the present experimental condition as shown for (SiWI2O,)"/TiO2 in Figure l b evolved 100 p L of CO and 200 p L of CO, whereas (SiW12040)4-/Ti02evolved 300 pL of CO and 350 p L of CO, over a span of 24 h adsorbing about 1200 WL of the CH, initially added. The results reported in Figure l a and I b show that the active sites on Ti02- and (SiW120,)4--loaded TiO, are kinetically d i ~ t i n g u i s h a b l e ~and * * ~that ~ the two types of sites have different specific photoefficiencies. Photooxidation of methane was also studied as a function of (SiW12040)4-loading on TiO, up to 75% by weight loading of heteropolyacid. A catalyst with 37.5% (SiW12040)4loaded on TiO, performed best in this reaction. At 37.5% (SiW12040)4loading on TiO, we have slightly more than an equivalent monolayer coverage. Assuming hexagonal packing and an equivalent radius of 5.6 8, for the (SiW12040)4-unit,20one equivalent sphere of this material will occupy an area of 108.6 8,'. A monolayer coverage on 5.5 m2 of TiOz (P25-Degussa) will contain 6.06 X I O 1 * spheres. Taking 2800 as the M W of the polytungstate, a monolayer is attained with 30.1 mg loading on 100 mg of TO,. By electron microscopy, the size of the heteropolyacid deposit was assessed to be below 20 A. The high dispersion attained indicates that the tungstate anions bind very tightly and uniformly to Ti02 until an equivalent of a monolayer is reached. However, since yields of CO and CO, are similar at coverages above 30% polyacid i t can be concluded that overall selectivity is not significantly influenced by coverage during reaction following the primary oxidation step. Experiments were also carried out in which the amount of oxygen added was varied between 0.5 and 6 mL, the methane being kept at I mL on a 37.5% (SiW12040)4-/Ti02 catalyst. The yields of CO, went up with the amount of 0, added, but the amount of C O observed never exceeded 300 pL (as reported for a 2 mL of C H 4 + I mL of O2 run in Figure Ib). It appears that, while 0, is required for the regeneration of the active oxygen atoms of the catalyst and for continuation of the oxidation cycle, an excess of this oxidant increases the conversion to CO, while decreasing the conversion and selectivity toward CO. (SiWl,040)4~/A1203 showed about one-half the activity of a with the same heteropolyacid loading as re(Si W 12040)4-/Ti02 ported earlier. Electron microscopy revealed heteropolyacid deposits ranging from 100 to 150 A. Additional experiments have been carried out using 37.5% (SiW,,0,0)4-/Si02 under illumination. This was selected as a stable photoinsensitive support. No C O was formed and very little CO,. We have also irradiated 2 mL of CH, + 1 mL of 0, on 37.5% (SiW,2040)4-/Zn0using ZnO as photosensitive support.
Gratzel et al.
No C O or C 0 2 was observed. Our initial experiments under illumination showed that ZnO, S O z , and A1203could produce small quantities of CO, from C H 4 + O2 reaction. They fall in the order, ZnO > SiO, = A1203. The activity of various supports in this reaction follow the order Ti02 > A1203 > Si02 ZnO when they are loaded with (SiW12040)4-. We have seen that TiO, and ZnO as such are photocatalysts and are capable of oxidizing C H 4 to CO, but not to CO selectively. Ti02 and ZnO are photosensitive materials where Si02is not. For A1203surface state mediated photoprocesses such as the dehydrogenation of alcohols have been reported.*I ZnO, being an amphoteric oxide, could react with H,SiW120,0, an acid, while loading to form a Zn2+ salt layer masking ZnO surface and that could explain its photoinactivity. Therefore, there are three possibilities: (a) (SiW,20,0)4- itself is photocatalytically active and the support only modifies it: (b) oxide support is photoactive and (SiWl,040)4- modifies it; and (c) both are photoactive and there is a synergy between the oxide and polytungstate. Experiments show that unsupported (SiWl,0,0)4- is not at all photocatalytically active in the present reaction. Also it was found to be inactive after getting dispersed over SiO,. Therefore, a dispersion of the polytungstate by itself cannot be involved in the primary photoexcitation process. Al2O3, though an insulator. behaved differently compared to SiO, and this will be discussed i n a later paragraph. I f the oxide support, TiO,. is taking part in the primary photoprocess, then TiO,
hv
TiO, (ecb-, hvb+)
(4)
the eCb-would migrate to (SiW120,)4- sites on T i 0 2 reducing the heteropolyacid in a one or two electron process.
-
ecb- + (SiW12040)4- (SiW12040)s-
+
(SiW12040)5- ecb-
(SiWl,040)6-
(5) (6)
Heteropolyblue species where tungsten has valence states IV and V have been widely r e p ~ r t e d .This ~ ~ ~explains ~ ~ our observations that the catalyst changed its original color from yellow to greyblue. At the end of the reaction, the catalyst was inefficient and the original color and activity was regained only by heating the catalyst in air or 0,. This shows that W6+ is not regenerated efficiently during the reactions. The holes generated under illumination would react with TiO, surface hydroxyde groups:
-
hVb++ (SiW12040)5-Ti02(OH-) H + + (SiWi2040)5-Ti02(O-)( 7 ) Equation 7 suggests that the oxide support used serves to provide Bronsted acid or base sites that mediate these reactions. A n alternate mechanism is possible where the primary photoprocess is the excitation of (SiWI2O4,J4- to form W5+and 0- sites, with charge trapping of the Ws+ by the oxide conduction band. This should regenerate W6+ and 0- in close proximity preventing recombination and resulting in longer life times of the 0- species. Such a mechanism has been invoked to explain the photoactivity for copper molybdate in ref 14d. However, an efficient regeneration of W6+ ions was not found as discussed before, suggesting that such a photoprocess of (SiW12040)4-is unlikely on this catalyst. The formation of 0- would then allow 0'-(ads)
+ CH,
Ti0,
OH-(ads)
+ CH,'
(8)
( 1 6 ) Arai. H.: Yaniada. T.; Eguchi, K . : Seiyama, T. A p p l . Caral. 1986,
26, 265.
( 1 7 ) Hinsen, W . ; Bytyn, W . ; Baerns, M . Proceedings. 8th International Congress on Cafaly,sis:Verlag Chemie: Weiheim, B. R . D , 1984: Vol. 3. p 586. ( 1 8 ) K i w i , J.. Gritzel. M. J . Phys. Chem. 1987, 9 / . 6673. ( 19) Morrison. S. R . The Chemical Physics of Surfacec.; Plenum Press: h e w York. 1980. ( 2 0 ) L e \ ) , H.. Agron, P.; Danford, M. J . C'heni. Phys. 1959, 30. 1486.
(21) ( a ) Mansour, A.; Balard, H . ; Papirer, E. J . C h i m Phys. 1987, 84, 569. (b) Mansour, A.; Balard. H.: Papirer, E. Bull. Soc. Chim. Fr. 1981. / , 236. (c) Balard. H.: Vansour, A.; Papirer, E.: Pichat, P. J . Chim. Phj's. 1985. 82. 1051. ( 2 2 ) Suarer. W . : Dumesic. 4 . ; Hili, G . J. Catal. 1985, 94, 408. (23) ( a ) Papaconstantinou, E. Homogeneous and Heierogeneoirc. Cara/y,sis NATO AS1 Series 1986, 7 4 , 415. ( b ) Kasansky, L.:L a u n a q . .I J . Cherri Phj.s. h i i . 1977. i i . 242
The Journal of Physical Chemistry, Vol. 93, No. 10, 1989 4131
Methane Oxidation Activated by Light CH3'
+ O-(ads)
s (CH,O-)(ads)
(9)
The mechanism suggested above would necessitate that CH3' remains on the surface. This is in agreement with the experimental evidence found in the activation of CHI on V/Si0224*25 and W03/Si0,26 systems. Kaliaguine et al?a have reported that, when CH4 was contacted at room temperature with 0- centers created on vanadium-supported silica catalysts by y radiation, they could observe reactions 8 and 9. Since a UV-irradiated (SiW12040)4-/Si02 catalyst did not produce any product, we can conclude that reactions leading to the formation or consumption of 0- did not occur on its surface. But photoexcitation of (SiW12040)4-is possible, generating Ws+ and 0-. Since its recombination rate was higher over Si02 due to lack of charge injection onto the support, the catalyst might be less active but could not be fully inactive. However, its total inactivity suggests that the photoprocess happening on polytungstate was not very significant in this class of catalysts. The observation that A1203 is an active support material for the heteropoly tungstate mediated photooxidation of C H 4 may be rationalized in terms of the availability of intra-band-gap states on the surface of this oxide. Earlier studies by Mansour et aL2] have shown that the dehydrogenation of alcohols on A1203 is enhanced by near-UV light. Since the band gap of A1203exceeds 6 eV, such processes must involve intra-band-gap surface states. A similar mechanism is likely to be operative in the photoxidation of methane observed with this support. The reaction is initiated by the light-induced electron transfer from a basic surface hydroxide group to the (SiW12040)4-:
I/
-AI-OH-
/I
+
(SiW12040)~-
- /I I/
+ H* + ( S I W , ~ O ~ O ) ~ -
-AI-O'-
hv
(10)
The 0- species subsequently abstracts hydrogen from CH4, as shown by eq 8. The conduction band electrons are also removed from the catalyst by reaction with O2resulting in the formation of adsorbed superoxide anion r a d i c a l ~ . ~ * ~ J ~ (TiO2)eCb-+ O2
-
02-(Ti02)
(11)
CH3' can as well react with 02CH3'
+ 02-
-+
CH302*-
(12)
H+ formed in reaction 7 and OH- from reaction 8 can form water, facilitating a series of reactions such as (13) to (15). In fact, H 2 0 has been detected as one of the products. 02-(ads)
+ H20
CH30- + H 2 0 CH3'
+ HO2'
-+
-
H02'
+ OH-
+ OHH2CO + H 2 0
CH30H
CH300H
-+
(13) (14) (1 5)
At low temperatures these products remain adsorbed to the catalyst (hence cannot be detected) and are accessible to further oxidation by photogenerated h+ resulting in the formation of C O and C 0 2 .
-
+ H2C0 4h+ + C H 3 0 H C O + H2O + 2h+ 2hf
+ CO C O + 4H+ C 0 2 + 2H+
2H+
-*
(16) (17) (18)
The overall reaction corresponds to the partial or complete oxidation of CHI as stated in reactions 2 and 3. Figure 2a shows runs at 473 K in 60-mL cells, 6 mL of CHI and 3 mL of 0, being reacted over 200 mg of T i 0 2 in the dark (24) Gerasimov, F. Reacr. Kinet. Carol. Lett. 1986, 32, 57. (25) Gerasimov, S.;Filimonov, V. Kiner. Karal. 1981, 22, 469. (26) Liv, H.; Liv, R.;Liew, K.; Johnson, J.; Lunsford, H. J . Am. Chem. SOC.1984, 106, 41 17.
-
-1
1 .
5000 m U
a r ( 6 I]
4000 H
u I
0
2
4
6
T I M E (hl
0
2
4
TIME
6
(h)
Figure 2. (a) (top) Gas consumed or produced when 6 mL of CH4 and 3 mL of O2were reacted at 473 K on 200 mg of Ti02 ( I ) under illumination, (2) in dark, and (3) under illumination, but with only 3 mL of O2present in the cell. Cell volume = 60 mL with Ar as diluent gas. ( b ) Gas consumed or produced when 6 mL of CH4 and 3 mL of O2were reacted at 473 K on 200 mg of 37.5% (SiW,,0.,,J4-/Ti02 ( 1 ) under illumination, (2) in dark, and (3) under illumination, but with only 3 mL of O2 present in the cell. Other conditions are similar to Figure 2a.
and under light. The ratio of methane to oxygen is 2 as in Figure la. When light activation is applied, C O (280 pL in 6 h) and CO, (420 pL in 6 h) are evolved. This is shown in trace 1. Therefore, an increase in reaction temperature affords C O which is not formed under the same experimental conditions a t 320 K (Figure la). Trace 3 shows that when 3 mL of O2is added under illumination only C 0 2 is formed as reaction product (120 r L in 6 h). When 6 mL of CH4 and 3 mL of 0, were reacted on T i 0 2 in the dark, only a very small quantity of C 0 2 was observed (trace 2). Figure 2b shows that an illuminated sample at 473 K evolves up to 500 pL of C O in 6 h (trace 1) over a 37.5% (SiW12040)4-/Ti02catalyst in the presence of 6 mL of C H 4 and 3 mL of 02.The yields of C 0 2 observed in a dark reaction (trace 2) and under illumination at 473 K (trace 3) are higher than the respective values found in Figure 2a. Values for C O photoproduction on a 37.5% (SiW120,)"/Ti02 catalyst at 300, 393, and 573 K were 45, 250, and 900 pL, respectively. This shows that the product of interest, CO, is favored by both photo and thermal activation. Our results confirm the choice of polytungstates as catalytic species capable of intervening favorably in C H 4 activation. Recently, Moffat6 has reported thermal reactions leading to C O and C 0 2 evolution accompanied by very small amounts of H,CCHO and C H 3 0 H via polytungstates at 873-973 K. The present study shows that partial oxidation of methane on a photoactivated catalyst proceeds at room temperature as compared to the latter study. Conclusions
It has been demonstrated that a properly prepared (SiW1204q)4-/Ti02combines a good level of selectivity and photoactivlty to evolve C O during methane photooxidation. This confirms the initial goal of our study, namely, photoassisted ex-
J . Phys. Chem. 1989, 93, 4132-4138
4132
citation of the catalyst at low temperature and pressure to inhibit the total oxidation reaction. It remains to be seen whether or not a more selective and active catalyst can be found in this direction. Our work has provided information on the nature of the participation of the catalyst in the oxidation process. This study has shown that the valence state of the transition-metal surface ions and their reducing properties are responsible for electron transfer from the active catalyst adsorption center to the adsorbed mol-
ecule(s). 0-and O2lattice species and surface anion radicals seem to be involved in the oxidative dehydrogenation of hydrocarbons on titanium polytungstate due to their reducing properties. These observations are in agreement with thermally activated processes reported for CH4 oxidation.'S6-* Registry No. TiOz, 7440-32-6; H4(SiWI2Oa), 12027-38-2; CH4, 7482-8; CO, 630-08-0; C 0 2 , 124-38-9; HzO,7732-18-5.
Surface Tension of Simple Mixtures: Comparison between Theory and Experiment B. S. Almeida* Departamento de Engenharia Qurmica do IST e Centro de Qdmica Estrutural, Av. Rovisco Pais, 1096 Lisboa Codex, Portugal
and M. M. Telo da Gama Departamento de F k c a da FCUL e Centro de F h c a da MatPria Condensada, Au. ProJ Gama Pinto, 2, 1699 Lisboa Codex, Portugal (Received: February 22, 1988; In Final Form: August 22, 1988)
We report a study of the surface tension of both pure and mixed fluids of simple molecules using a microscopic mean-field theory (MFT). The pure components are modeled by Lennard-Jones potentials with two different sets of intermolecular parameters reported in the literature. One of these yields results that are in much better agreement with the experimental data. We have also studied the dependence of the interfacial properties of binary mixtures on the cross-interaction parameters and it was found that (for mixtures of krypton, ethene, and ethane) reasonable agreement with experiment can only be obtained when the Lorentz-Berthelot combining rule is relaxed.
1. Introduction
Although in the past few years considerable effort has been made by theoreticians to understand the surface behavior of pure fluids and mixtures, in most cases the calculations were carried out for model systems that bear little resemblance to real fluids. I n general's2 the choice of potential parameters was made for theoretical reasons, preventing a direct comparison with real mixtures. As a matter of fact, the number of experimental results for surface properties of relatively simple binary mixtures has not been a b ~ n d a n t ~and . ~ the first measurements for a series of mixtures have only appeared r e ~ e n t l y . ~Consequently .~ our study is the first systematic comparison between the results of a microscopic theory and experiment. In this paper we apply mean-field theory (MFT) to predict the surface tension of krypton, ethane, and ethene and their binary mixtures. The experimental results7~* for these systems cover the whole range of composition and a fairly wide temperature range ( 1 16-1 35 K) allowing comparisons with theoretical predictions over an unprecedented range of intermolecular and thermodynamic parameters. A previous comparison of the results of this theory (1) Telo da Gama, M . M.; Evans, R. Mol. Phys. 1983.48, 229. Telo da Gama, M. M.; Evans, R. Mol. Phys. 1983, 48, 251. (2) Lee, D.J.: Telo da Gama, M. M.; Gubbins, K. E. J . Phys. Chem. 1985, 89, 1514. ( 3 ) Sprow, F. B.; Prausnitz, J. M . Trans. Faraday SOC.1966, 62, 1097. (4) Fuks, S.; Bellemans, A. Physica 1966, 32, 594. ( 5 ) Almeida, B. S. Ph.D. Thesis, Technical University of Lisbon, 1986. (6) Nadler, K. C.; Zollweg, J. A.; Street, W. B.; McLure, I. A . J . Colloid Interface Sci., in press. (7) Soares, V . A. M.; Almeida, B.S.; McLure. I. A,; Higgins, R. A. Fluid Phase Equilib. 1986, 32, 9 . (8) Almeida, B. S.; Soares, V . A. M.; McLure, I. A,; Calado, J. C. G . J . Chem. Soc., Faraday Trans. I , submitted for publication.
0022-3654/89/2093-4132$01.50/0
and experiment was restricted to mixtures of Ar + Kr9 and Ar + CH4.9 (We will comment on the results of this study in section 5.) On the other hand Lee et a1.I0 carried out a detailed comparison between the results of M F T and molecular dynamics for model mixtures of argon and krypton. The agreement was fair in both cases. In a different study Lee et aL2 have also applied M F T to calculate the surface tension, adsorption, and density profiles of binary mixtures of Lennard-Jones fluids in which one of the components is a supercritical vapor. The agreement with the results of molecular dynamics for these systems was also found to be satisfactory. Earlier studies by Falls et al.," Carey et a1.,I2and Telo da Gama and EvansI3 used the square gradient approximation to calculate the interfacial properties of several binary mixtures but these results have been superseded by the results of MFT. The latter takes into account the long-range nature of the intermolecular interactions, which is important, for example, in determining the equilibrium thickness of wetting 1 a ~ e r s . l In ~ addition, M F T is easier to implement, which makes its practical application quite feasible. Other theories (the so-called classical thermodynamic theories) have been used in the study of interfaces, namely corresponding states, regular solution, and lattice theory. However, an accurate prediction of the'experimental results for the surface tension of (9) Thurtell, J. H.; Chapman, W. G.; Nadler, K. C., preprint. (10) Lee, D.J.; Telo da Gama, M . M.; Gubbins, K. E. Mol. Phys. 1984, 53, 1 1 13. ( 1 1) Falls, A. H.; Scriven, L. E.; David, H.
T.J . Chem. Phys. 1983, 78,
7300. (12) Carey, B. S.; Scriven, L. E.; David, H. T . AIChE J . 1980, 26, 705. (13) Telo da Gama, M. M.; Evans, R. Mol. Phys. 1980, 41, 1091. (14) Telo da Gama, M . M.; Sullivan, D.In Fluid Interfacial Phenomena; Croxton, C. A,, Ed.; Wiley: London 1986, Chapter 2, p 45.
0 1 9 8 9 American Chemical Society
