J . Phys. Chem. 1992, 96, 7714-7718
7714
are intensity related and may be a consequence of waterframework interaction. Thus,we are left with the fact that a subtle structural change, not detectable by the present powder X-ray analysis, is necessary to explain the N M R spectra. This fact prompted our suggestion of the presence of P-OH or p-0 groups in sufficient amount to account for about oneninth of the phosphate groups. A key point demonstrated by the present study is that the difference between H1 and VPI-5 arises from the disorder in the former structure. This disorder probably results from the missing framework phosphate groups and produces a 10-membered ring structure lining the larger cavities a feature not present in VPI-5. Such a ring structure is inherently less stable than the six-membered rings of VPI-5. This feature together with the missing framework phosphate groups is apparently responsible for the ready conversion of this phase to AlP04-8. Finally, it should be stated that the presence of even a fraction of a mole of water starts the process of bonding of water to Al( 1) to increase its coordination number. Thus, the water molecules must prefer this framework site to the emptiness of the channel and use these fixed water molecules as the anchor for formation of the triple helix.I2 We are attempting to carry out a structure study of these 18-ring aluminum phosphates at several intermediate stages of hydration to shed further light on the waterframework interaction.
Acknowledgment We gratefully acknowledge financial support of this study by the Regents of Texas A&M University through the Commitment of Texas Program and the State of Texas through the Advanced Technology Program. Registry No. AlPO,, 7784-30-7.
References a d Notes (1) Davis, M. E.; Saldarriaga, C.; Montes, C.; Garces, J. M.; Crowder, C. Nature 1988, 331, 698. (2) Davis, M. E.; Montes, C.; Hathaway, P. E.; Arhancet, J. P.; Hasha, D. L.; Garces, J. M. J . Am. Chem. Soc. 1989, 1 1 1 , 3919. (3) Wilson, S.;Lok, B.; Flanigan, E. M. U S . Patent 4,310,440, 1982. (4) Perez, J. 0.;McGuire, N . K.; Clearfield, A. Coral. Lett. 1991,8, 145. (5) Clearfield, A.; Perez, J. 0. In Synthesis of Microporow Materials, Molecular Sieues; Occclli, M. L., Robson, T., E&.; Van Nostrand Reinhold New York, 1992; p 266. (6) DYoire, F. Bull. Soc. Chim. Fr. 1961, 1762. (7) Duncan, B.; Szostak, R.; Sorby, K.; Ulan, J. G. Coral. Lett. 1990, I , 367.
(8) Crowder, C. E.; Garces, J. M.; Davis, M. E. Adu. X-ray A M / . 1988, 32, 507. (9) Smith, J. V.; Dytrych, W. J. Nature 1984, 309, 607. (10) Richardson, J. W., Jr.; Smith, J. V.; Ruth, J. J. J. Phys. Chem. 1989, 93, 8212. (11) Rudolf, P. R.; Crowder, C. E. Zeolites 1990, IO, 163. (12) McCusker, L. B.; Baerlocher, Ch.; John, E.; BBlow, M. Zeolites 1991, 1 1 , 308. (1 3) Grobet, P.; Martens, J. A.; Balakrishanan, I.; Martens, M.; Jacobs, P. A. Appl. Coral. 1989, 56. 121. (14) Rudolf, P. R.; Clearfield, A. Inorg. Chem. 1989, 28, 1706. ( I 5) TEXSAN:TEXRAY Structure Analysis Package for Single Crystal Data; Molavlar Structure Corp: The Woodlands, TX, 1986 (revised 1987). (16) PATSEE:Sheldrick, G. M. SHELXTL PLUS Usen Manual; Siemans Analytical X-ray Inst., Inc.: Madison, WI, 1989. (17) Rudolf, P. R.; Clearfield, A. Acta Crystallogr. 1985, B41, 418. (18) GSAS: Generalized Structure Analysis System, Larson, A.; von Dreele, R. B. LANSCE, La Alamos National Laboratory, copyright 1985-88 by the Regents of the University of California. (19) Rudolf, P. R.; Soldarriaga, C.; Clearfield, A. J . Phys. Chem. 1986, 90, 6122. (20) Van Braam Houckgeest, J. P.; Kraushaar-Czarnetzki, B.; Dogterom, R. J.; de Groot, A. J . Chem. Soc., Chem. Commun. 1991, 666. (21) Perez, J. 0.;Chu, P.-J.;Clearfield, A. J. Phys. Chem., 1991,95,9994. (22) Martens, J. A,; Feijen, E.; Lievens, J. L.; Grobet. P. J.; Jacobs. P. A. J . Phys. Chem. 1991, 95, 10025.
FT-IR Spectroscopic Investigation of Methane Adsorption on Cerium Oxide Can Li* and Qin Xin State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, People's Republic of China (Received: February 3, 1992; In Final Form: April 28, 1992)
The methane adsorption on cerium oxide has been studied by Fourier transform infrared (FT-IR) spectroscopy equipped with a specially designed IR cell capable of a wide temperature range of 100-lo00 K. Ce02 was well outgassed at lo00 K prior to the methane adsorption performed at 173-273 K. When the well-outgassed CeOz was exposed to CH, at 173 K four IR bands at 3008, 2990, 2875, and 1308 cm-' were observed, and the four bands shifted to 2249,2235,2076, and 998 cm-', respectively, for the adsorption of CD,. The bands of adsorbed CH4in the C-H vibrational region evidently shifted to lower wavenumbers relative to those of gas-phase CHI. Moreover, an infrared-forbidden vibration at 2917 cm-I of free CHpbecame the infrared-active mode and appeared at 2875 cm-' for the adsorbed CH,. This is accounted for by the severe distortion of Td symmetry of free CH4 caused by the adsorption. These observed IR bands can be attributed primarily to the chemically adsorbed methane. For partially reduced or C02- and H20-covered Ce02 the chemisorbed methane with the characteristic band at 2875 cm-l was significantly prohibited. It is suggested that two types of chemisorbed methane are formed on the well-outgassed CeO,; one interacts with the surface lattice oxygen anions, and the other interacts with the coordinatively unsaturated oxygen species.
Introduction The adsorption and activation of hydrocarbons on metal and oxide surfaces have received much attention in the past several decades'-' since most of the catalytic processes involve hydrocarbons. The adsorption of unsaturated hydrocarbons, mainly the adsorption of olefin, has been extensively studied." However, the adsorption and activation of saturated hydrocarbons on surfaces are not well understood. Recently, the activation of saturated hydrocarbons,8v9particularly that of methane, attracts much interest from both academia and industry. Because of the abundant
resources of natural gas, an extensive worldwide effect is now underway to develope the catalytic oxidative coupling of methane to petrochemicals.I0-l2 Methane is the most chemically inert molecule among hydrocarbons. The activation of the C-H bond of CHI has been considered as one of the toughest subjects in catalysis ~hemistry.2~ But so far, how the C-H bond of CH4is cleaved is still not clear. A promising way to approach the methane activation is by using surface-active oxygen species as that in methane oxidative couplingl"I2 and partial oxidation13on oxide catalysts. In light of
0022-365419212096-77 14503.OO/O 0 1992 American Chemical Society
Methane Adsorption on Cerium Oxide
The Journal of Physical Chemistry, Vol. 96, No. 19, 1992 7715
heterogeneous catalysis, the adsorption of a molecule is the initial step necessary for the activation of the molecule. Unfortunately, studies on methane adsorption on either metals or metal oxides have been far from extensive due in part to the fact that it is difficult for methane to be captured by the surfaces. Recently, we have studied the surface oxygen ~ p e c i e s ' ~and J~ their reactivities toward hydrocarbon~'~J' on CeO,. It has been found that there are various oxygen species on CeO,, and these oxygen species exhibit quite different activities and selectivities in the oxidation of hydrocarbons. In the present contribution, the adsorption of CHI on cerium oxide has been investigated with an intention to understanding the initial interaction of CH, with surface oxygen species on Ce02. IR spectra of chemisorbed methane species are reported, and the adsorption models of CH, are proposed to highlight the chemisorbed methane species in the methane activation and methane oxidation.
Experimental Section Cerium oxide (CeO,, BET area 20 m2/g) was prepared by calcinating cerium hydroxide gel at 773 K as described in a previous paper.I5 The sample was pressed into a self-supporting wafer for IR study. A quartz IR cell available for the wide temperature range 100-1000 K was specially designed for this study. A sample in the IR cell can be treated in situ in various ways. A well-outgassed CeO, sample was obtained in the IR cell by oxidizing in 0,at 873 K and then outgassing at 1000 K so as to remove surface contaminants, such as carbonate species. Most of the surface hydroxyls were also removed by the outgassing. A partially reduced cerium oxide was prepared by treating the well-outgassed CeO, in H2 at 673 K for 4 h. When the welloutgassed CeO, was exposed to air or to C 0 2and H20, the sample was contaminated, and accordingly the strong IR bands due to the adsorbed carbonate and water were detected. CHI and CO with a purity of 99.99% were further purified by passing them through a liquid nitrogen trap before their admission to the IR cell. The isotope CD, was obtained via the following reaction at 673 K on Ni/A120, catalyst: CO + 3D2 CD4 D20
-
+
The reaction was carried out in an internal cycle system, and the byproducts such as DzO, CO,, and higher hydrocarbons were removed by a cooling trap. The prepared CD4 contains a small amount of CO and D, residuals and trace amounts of CD,H, CD2H2,and CDH3 because there is a small amount of H2 impurity in D,. The adsorption of CHI and CO was performed at temperatures 173-273 K. The lowest temperature set for recording IR spectra is 173 K, which is much higher than the boiling point of CH,, 112 K, in order to avoid the heavier IR absorbance of physically adsorbed CH,. The sample wafer outgassed at loo0 K was cooled to 173 K in vacuum, and the background spectra were recorded during the cooling process. The low temperature of the sample was achieved by liquid nitrogen. The IR experiments were performed using a double-beam FT-IR spectrophotometer (Perkin-Elmer 1800) equipped with a liquid nitrogen cooled mercury-calcium-telluride (MCT) detector with a low-frequency cutoff of 700 cm-I. All IR spectra were recorded with 4-cm-' resolution and eight scans. Absorbances are calculated from the spectra ratioed to the background spectra at corresponding temperatures. Because the IR bands observed in this study are very narrow, the peak height in absorbance was simply taken as the band intensity. ReSUltS Figure l a shows the IR spectrum recorded at 173 K after the admission of CH, on a well-outgassed CeO,. Four distinct IR bands at 3008,2990,2875, and 1308 cm-' are observed. These bands are most possibly attributed to the adsorbed CH,. To confirm the bands arising from the CH4 adsorption, CD, adsorption under the same conditions was performed. There are six IR bands at 2249,2235,2177,2157,2076, and 998 c m - I which appear in Figure 1b for the CD, adsorption. There might be bands
3500
3000
2500
2000
1500
Wavenumber/cm-'
Figure 1. IR spectra of adsorbed CH4, CD,, and CO on CeO, at 173 K. (a) CH4, 1 1 Torr;(b) CD4,5 Torr, containing a small amount of CO, (c) CO, 2 Torr. TABLE I: Vibrational Modes of CHI and Observed IR Bands of CH, Adsorbed on CeOl(in cm-')
vibrational mode sym stretch deg deform deg deform deg stretch
gas phase
adsorbed
29 17' 1533' 1306 3019
2875
freu shift 42
b
b
1308 3008 2990
-2 11 29
'Infrared inactive. bNot observed. due to adsorbed CO in Figure 1b because the CD, containsa small amount of CO. Figure IC gives the IR spectrum recorded for the adsorption of CO on Ceo,at 173 K. The bands at 2157 and 2177 cm-' in Figure IC are readily assigned to CO adsorbed on CeTherefore, the remaining four bands at 2249,2235,2076, and 998 cm-'in Figure 1b are due to adsorbed CD, on Ceo2 The isotopic results unambiguously demonstrate that the four bands in Figure l a arise from the adsorbed CH,, and these bands shift correspondingly to the four bands in Figure 1b, namely, 3008 2249,2990 2235,2875 2076, and 1308 998 cm-l. The fine structure of rotation along two sides of the band at 3019 cm-' of free CH, entirely disappears for the adsorbed CH,, indicating that the rotation of CHI is strictly limited for CH4 adsorbed on Ce02. In an early study, Sheppard and Yaksz0have found that some reduction in rotational degree of freedom had occurred for CH, adsorbed on glass. Table I lists the vibrational modes of free CH4and the IR bands observed for the methane adsorption on Ceo,at 173 K. Gas-phase CHI has four vibrational modes: 2917, 1533, 3019, and 1306 cm-'.2' Of these modes, the vibrations at 3019 and 1306 cm-' are infrared active as usually observed and the other two vibrations at 2917 and 1533 cm-'are infrared inactive. However, there are more than two IR bands observed for methane adsorption on Ce02. The bands at 3008 and 2990 cm-l of the adsorbed CH, could be assigned to the vibrations originating from 3019 cm-' whereas the band at 2875 cm-' could not be simply assigned to a band shifted from 3019 cm-I. We believe that this band might come from the vibrational mode at 2917 c m - I of free CH,. The assignment is further substantiated by the isotope ratio listed in Table 11. It is of particular importance that the infrared-forbidden vibration at 2917 cm-l of free CH, not only became infrared active but also shifted to 2875 cm-l. The mode at 2917 cm-' is an 0218319
-
-
-
-
7716 The Journal of Physical Chemistry, Vol. 96, No. 19, 1992
Li and Xin
TABLE 11: Isotopic Shifts (cm-I) Observed for the IR Bands of Adsorbed CHI on CeO,
gas phase" CD4 CH4/CD4b 2109 1.383' 996 1.311 2259 1.336
CH4 2917 1306 3019
CHI 2875 1308 3008 2990
adsorbed CD4 CH4/CDt 2076 1.385 998 1.311 2249 1.338 2235 1.338
O.
I -
0.6-
0.5-
6 e
0.4-
" From ref 21. Frequency ratio. Infrared inactive. 1
I
0
5
10
I
I
I
I
I
I
15
20
25
30
35
40
06 1308 cm''
p-;.j 236 K
258 K 3400
3000
2600
1400
1000
Wavenumber/cm-' Figure 2. IR spectra of CHI adsorbed on CeOzat elevated temperatures from 173 to 273 K in the presence of 11 Torr of CH,. infrared-inactivevibration since the vibration is the symmetric stretch of CH, in the Td point group. It became infrared active mainly owing to the strong chemical interaction of CH4 with the Ce02surface which leads to a violent distortion of spatial structure of CH4. The considerable frequency shifts displayed in Table I imply that the C-H bond is weakened remarkably for the adsorbed CH4. The vibration at 1306 cm-l of CH, is a deformation mode which seems to be insensitive to the adsorption perturbation and only shows a slight shift to higher wavenumbers for the adsorbed CHI. The frequency ratios of isotope, CH4/CD4, for gas-phase and adsorbed methane are summarized in Table 11. The ratios of gas-phase methane are in excellent agreement with those of the adsorbed methane on CeO,. If the band at 2875 cm-I were assigned to the degeneracy stretch, the frequency ratio of CH4/CD4would be close to 1.336 instead of 1.385. Moreover, the frequency shifts of the adsorbed CH, would be 144 cm-l for CHI and 183 cm-I for CD, if 2875 and 2076 cm-l were ascribed to the degeneracy stretch of CHI. This assignment is theoretically unacceptable because the frequency shift of a given vibrational mode of a molecule with heavier isotope atom must be smaller than that of the normal molecule. The band at 2875 cm-I could only be assigned to the symmetric stretch of CH,, and the wavenumber shifts are 42 and 33 cm-' for the adsorbed CH4 and CD,, respectively. CO has been frequently used as a probe to identify the surface sites of metal oxides.22 Adsorbed CO is suggested to coordinate to the cationic site of the oxide surface via a a-donor bond which leads to positive C - O frequency shifts relative to the gas phase (2143 cm-l). The bands at 2157 and 2177 cm-' in Figure lb,c are ascribed to the CO adsorbed on the surface cerium cations of Ce02.18J9The fact that the coadsorbed CO scarcely affects
0.1
-i
I oq
,
,
0
1
2
I
I
I
3 4 5 6 7 Methane pressure/torr
8
9
10
Figure 4. Methane pressure dependences of IR band intensities of ad-
sorbed CHI on Ce02 at 173 K.
the IR spectrum of the adsorbed methane is indicative of the fact that adsorbed methane solely interacts with the surface oxygen anions instead of the cerium cations of Ce02. Figure 2 displays a series of IR spectra obtained at elevated temperatures for the adsorbed CHI on CeOz. The adsorbed methane was formed at 173 K, and then the sample was warmed in a stepwise fashion from 173 to 273 K. On warming, these bands were attenuated; especially the bands at 3008 and 1308 cm-' became much weaker at temperatures higher than 200 K. All these bands disappeared at temperatures close to 273 K,indicating that the adsorbed methane species are quite unstable. The band at 2875 cm-I persisted at little higher temperatures than bands at 3008 and 1308 cm-I. The adsorbed CH4 on CeOz was hardly detected at room temperature or higher temperatures. Figure 3 illustrates the growth of the bands at 3008,2875, and 1308 cm-l of adsorbed CH, on CeO, at 173 K with time under a CHI atmosphere. The bands reached their maxima for more than 60 min. The bands at 3008 and 1308 cm-' developed in parallel while the band at 2875 cm-l grew in a different way. At early stages of the adsorption the band intensity of 2875 cm-' is close to that of the band at 3008 cm-I and is stronger than the band at 1308 cm-I. With prolonged time, the bands at 3008 and 1308 cm-l grew much stronger than the band at 2875 cm-I. At the beginning of the adsorption, the band at 1308 cm-'was weaker than the band at 3008 cm-I but finally became stronger than the 3008-cm-I band. Obviously, the band at 2875 cm-' is not from the same adsorbed species as the band at 3008 cm-'. The band at 2875 cm-I might be due to the adsorbed methane species more strongly interacted with surface. The band at 2990 cm-1 is not shown in Figure 3 since this band is a shoulder of 3008 cm-', and its variation is similar to the band at 2875 cm-'. It is suggested that there are at least two types of chemically adsorbed methane which are denoted as species I and species 11. The bands at 2990, 2875, and 1308 cm-I should be assigned to species I and the bands
The Journal of Physical Chemistry, Vol. 96, No. 19, 1992 7717
Methane Adsorption on Cerium Oxide
H
H
I
H
U aJ
c
0
Species I Species I I Figure 6. Proposed interaction models of CHI with surface oxygen species. Species I (2990,2875,1308cm-l) formed on surface cus oxygen, and species I1 (3008,1308 cm-') formed on surface lattice oxygen anions.
fl v1 0
n Q
3 IO
3000
2600
1400
1000
Waven u mber /c m-'
Figure 5. IR spectra of adsorbed CHI at 173 K on (a) well-outgassed Ce02, (b) partially reduced cerium oxide, and (c) air-contaminated
Ce02. at 3008 and 1308 cm-' to species 11. The adsorption isotherm of methane on Ce02 was measured at 173 K. Figure 4 presents the isotherms of the bands at 3008, 2875, and 1308 cm-' at 173 K. All these band intensities varied with methane pressure in accordance with the Langmuir curve, indicating that the methane mostly chemisorbed on Ce02. The band intensities could be reduced to zero after a prolonged evacuation even at 173 K. This means that the interaction between adsorbed methane and surface is relatively weak although the adsorption behavior is chemisorption. In the low-pressure region the difference of these bands can be clearly observed. The bands at 3008 and 1308 cm-I are attenuated more sharply than the band at 2875 cm-I with the decrease of methane pressure. This further proves that the band at 2875 cm-l is due to a more strongly adsorbed methane species. When methane pressure was increased to more than 11 Torr, the band at 2875 cm-' grew slightly but the bands at 3008 and 1308 cm-I developed significantly. The results corroborate two types of adsorbed methane, species I and species 11, formed on Ce02, and species I is more strongly adsorbed than species 11. To clarify the surface sites of Ce02for the methane adsorption and the nature of the interaction between methane and the surface sites, methane adsorption on differently-pretreated Ce02 was made. Figure 5 is a collection of the IR spectra recorded for the methane adsorption on well-outgassed, partially reduced, and contaminated Ce02at 173 K. The bands in the 3ooO-rm-I region are very sensitive to the pretreatment of Ce02. For the partially reduced Ce02,the bands at 3008,2990, and 2875 cm-' are much weaker than those for well-outgassed Ce02 as shown in Figure 5a. The band at 2875 cm-I was not observed, but a weak band at 2893 cm-' appeared. The bands at 3008 and 2990 cm-' are almost absent, and a weak band centered at 3000 cm-I can be observed. The band at 1308 cm-I is reduced as well. For the contaminated Ce02, the bands at 3008 and 2875 cm-I were not detected, but the bands at 3000 and 1308 cm-' were still observed. For the Ce02 treated in air at high temperatures the bands in Figure 5c became much weaker. The spectra in Figure 5 strongly suggested that the adsorption of methane on C e 0 2 is highly selective to the surface state. The adsorbed methane can only be formed on the surface definite sites where chemical interaction with methane is possible. Therefore, the bands at 3008, 2990, 2875, and 1308 cm-I are arising mostly from the chemisorbed methane.
Discussion There is a general consensus that the activation of methane on oxide is via the interaction of methane with surfaceactivate oxygen species,10J1.23-24 because the most inert property of methane, the first hydrogen abstraction, can only be realized by the surfaceactive oxygen on oxide catalyst. The studies on methane oxidative coupling proposed various models of methane activation depending on the different surface oxygen species involved in the hydrogen abstraction of CH4. For example, Lunsford and c o - w ~ r k e r s ~ ~ - ~ ' have extensively studied the methane oxidative coupling on Li/ MgO catalyst, and they concluded that the CH3 radical was produced from the interaction of CH4 with the 0-of the [LPO-] center. Otsuka and co-workers28asuggested that the active species for methane activation on Sm203oxide is a diatomic oxygen species such as OZ2-.Some authors30have postulated that lattice oxide ions, 02-,involved in the generation of activated surface methyl species in methane oxidative coupling over reducible oxides. Although a variety of active oxygen species were proposed for the methane activation, the initial intermediate of methane to be activated on surface has been poorly known. The chemisorbed methane species derived on C e 0 2 are the intermediates formed by virtue of initial interaction with surface oxygen. C e 0 2 had been found to be a poor cataly~tl'.~' for methane oxidative coupling but an active catalyst for complete oxidation of methane. It was observed that the surface oxidation of methane on Ce02 could even be initiated at 473 K, and the surface formate species were always derived." This surface reaction was limited for C02- and H20-covered Ce02. It was concluded that the surface oxygen species play a vital role in the mild oxidation of methane on Ce02. Although the adsorbed methane at low temperatures may not be directly correlated with the methane oxidation at high temperatures, the chemisorbed methane species observed at low temperatures can be reasonably assumed as one of the important precursors for the methane activation. The hydrogen abstraction during the methane oxidation is perhaps via this precursor whose lifetime is so short that it is difficult to be detected at high temperatures. Taking into account the fact that the coadsorbed CO did not inhibit the formation of chemisorbed methane, the adsorbed methane interacts mainly with surface oxygen anions. There are two types of the surface oxygen anions on well-outgassed Ce02: surface lattice oxygen anions and surface coordinatively unsaturated (cus) oxygen. The latter might be like 0-species which could be formed via oxygen adsorption. The cus oxygen could be also generated at the expense of reduction of surface Ce4+to Ce3+ as proposed by Che and Tench32 for reducible oxide. Methane interacted with the two oxygen sites, and accordingly two types of adsorbed methane could be derived. The cus oxygen has a high electron affinity and interacts with methane more strongly than the surface lattice oxygen anion. It is assumed that species I is formed on the cus oxygen and species I1 is formed on the surface lattice oxygen anions as schematicallyshown in Figure 6. A partial reduction treatment will reduce the surface oxygen and both the cus oxygen and surface lattice oxygen anions; therefore, the band intensities due to chemically adsorbed methane are remarkably decreased for the methane adsorption on the partially reduced C e 0 2 as shown in Figure Sb. H 2 0 and C 0 2
J. Phys. Chem. 1992,96, 7718-7724
7718
will react preferably with surface cus oxygen and produce hydroxyls and carbonate species, and correspondingly species I failed to be detected for the methane adsorption on the contaminated CeOz as shown in Figure 5c. The four C-H bonds of CH4 are equivalent and arranged so that their respective axes define a regular tetrahedron; i.e., methane belongs to the Tdpoint group. The Tdsymmetry of CH, is reduced to lower symmetry, such as C3, when CH4 is strongly adsorbed on surface. Accordingly, the four C-H bonds become unequivalent and one of them may be weakened, ready for cleavage. For species I as depicted in Figure 6 the strong chemical interaction causes a structural distortion of CH4and the Tdsymmetry possibly turns into C, symmetry, and the mode at 2917 cm-'therefore becomes infrared active. Species I1 exhibits a similar spectrum as that of free CHI, indicating that the adsorbed methane still keeps the identity of free CHI owing to the relative weak interaction between
oxygen and CH4. Acknowledgment. We gratefully acknowledge the Natural Science Foundation of China (NSFC) for support of this research. Registry No. Ce02, 1306-38-3; CH4,74-82-8; C02, 124-38-9; H20, 7732-18-5.
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(7) Busca, G.;Marchetti, L.; Xerlia, T.; Girelli,A,; Sorlino, M.; Lorenzelli, V. 8th Int. Congr. Catal., Tokyo 1984, 111-299. (8) Eedohelyi, A.; Solymosi, F. J. Catal. 1990, 123, 31. (9) Centi, G.; Trifiro, F. Chem. Rev. 1988, 88, 55. (IO) Lee, J. S.;Oyama, S.T. Catal. Rev.-Sei. Eng. 1988, 30, 249. (1 1) Hutchings, G.J.; Scurrell, M. S.; Woodhouse, J. R.Chem. Soc. Reu. 1989, 18, 251. (12) Pokier, M.; Breault, R.Appl. Carol. 1991, 71, 103. (13) Parkyns, N. D.; Brown, M. J. Catal. Today 1991,8, 305. (14) Li, C.; Domen, K.; Maruya, K.; Onishi, T. J . Chem. Soc., Chem. Commun. 1988, 1541. (15) Li, C.; Domen, K.; Maruya, K.; Onishi, T. J . Am. Chem. Soc. 1989, 1 1 1 , 7683. (16) Li, C.; Xin, Q.; Guo, X.-X. (a) Chin. J . Mol. Caral. 1991, 5, 193; (b) Catal. Lett. 1992, 12, 297. (17) Li, C.; Xin, Q.; Guo, X.-X.;Onishi, T. Symp. 10th Inr. Congr. Catal.,
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Influence of the Electrolytic Medium Composition on the Structural Evoiutlon of Thin Eiectrochromlc MOO, Films Probed by X-ray Absorption Spectroscopy Daniel Guay,**fGrard Tourillon, LURE, Bdtiment 209 D, Universiti Paris-Sud, Orsay 91 405, France
Guylaine Laperrik, and Daniel Wlanger Dipartement de Chimie, Universiti du Quibec 6 Montrial, C.P. 8888, Succursale A , Montrial, Quibec, Canada H3C 3P8 (Received: February 3, 1992; In Final Form: April 30, 1992) We have performed an X-ray absorption spectroscopy study at the Mo K edge of Moo3thin films to determine the effect of the composition of the electrolyte on the structural modifications occurring in the layer during the electrocoloration process. Both Li+ and H+cations containing solutions were studied. The M a 3 layers were obtained by thermal decomposition of electrodeposited MoS3 and the structure of the resulting material was confirmed by X-ray absorption spectroscopy. Upon incorporation into the M a 3 material, a 5% (2.3%) increase of the shortest Mo-Mo separation distance is observed. Li+ (H+) This structural deformation occurs in a direction perpendicular to the layered structure of the material. In potential sweep experiments, the structural modification induced by the incorporation of the cations within the M a 3 material is not totally reversible, and a buildup of chemical disordering is observed with the number of sweeps. This chemical disorder affects both the first 0 and the second Mo coordination shell and is more important when Li+ rather than H+cations are incorporated into the Moo3 material. The perturbing effect of H+and Li+ is best understood if the cations are located near the bridging 0 atoms within the M a 3 layer rather than between the M a 3 sheets of the layered compound. The cation-specific structural deformation and chemical disorder induced into the Moo3 material are responsible for the specific deterioration of the electrochemical and electrochromic properties of the layer in potential sweep experiments. This effect most probably arises from a variation of the diffusivity of the cation according to the extent of structural deformation and disorder in the material.
Introduction Transition-metal oxides make up a t&nologially important ~ ~ t e lie r i ~ l ~ wo3,and v,oSchange class *To whom correspondence should be sent. 'Permanent address: INRS-Encrgieet Mat&iaux, 1650 MontQ Ste-Julie, C.P. 1020 Varennes, Qu&ec, Canada J3X 1S2.
their light-absorbing properties under an externally applied electric fieldi-' can be in nonemissive d i s p l a ~ . ~Moreover, .~ hey have attracted a lot of attention as positive electrodes for ambient temperature secondary lithium batteries? Molybdenum trioxide has been prepared by several methods: thel'n'lal evaporation? sPutk&? andimtion? spray PYrol@%'o colloidal sol-gel method,l'J2 chemical vapor deposition,13J4 and
0022-36S4/92/2096-7718$03.00/00 1992 American Chemical Society
