4059
J. Phys. Chem. 1991,95,4059-4063
Photocatalytic Activity of AikaiEMetai Titanates Combined wlth of Water
Ru in the Decomposition
Yasunobu Inoue,* Toyoyuki Kubokawa, and Kazunori Sat0 Analysis Center, Nagaoka University of Technology, Nagaoka, Niigata 940- 21, Japan (Received: July 20, 1990)
Alkali-metal titanates having a chemical formula of M2Ti,0W1 (M = Na, K, Rb and n = 2, 3, 4, 6) were employed as semiconductor oxide supports for the photoassisted decomposition of water under Xe light. In their association with oxidized Ru, the photocatalytic activity increased in the order Na > K > Rb (n = 6), with a tendency for higher activity with increasing n (M = K). Na2Ti6013associated with oxidized Ru was found to have not only the highest activity but also a capability for the stoichiometric production of Hz and 02.In this catalyst, Ru ions were intercalated through ion exchange with Na ions and were favorably activated by oxidation rather than by reduction treatment. X-ray photoelectron spectra showed that the oxidized states of Ru analogous to Ru02are the active species. It is suggested that the higher activity of the photocatalyst with Na2Ti6013,compared to other alkali-metal titanates, is associated with the framework of Ti06 octahedra forming the tunnel structures. The effects of alkali-metal ions in M2Ti6013on the exchangeability of alkali-metal ion with Ru ion and on the wavelength of light absorption were explained in terms of the distortion of the host framework caused by increasing size of the alkali-metal ions.
Introduction In heterogeneous photocatalysis, the development of new semiconducting photocatalysts is of particular importance. We have reported that ferroelectric semiconductors of PbZrl-xTix031-3and host-guest type oxides of Pb,,K,Nb2Os4 have interesting photocatalytic and photovoltaic properties. In a recent work, we have shown that sodium hexatitanate, Na2Ti6013,combined with the active phase such as Ru, has the capability of invoking a photocatalytic decomposition of water to produce Hzand O2 at approximately the stoichiometric ratio.5 This mixed oxide is one of the alkali-metal titanates belonging to a chemical formula of MzTin02,,+l ( M = alkali-metal atom). A considerable amount of the research has been performed on their crystal structures, and it has been shown that the structures of the titanates in this series are constructed from a sheet characterized by edge- and/or corner-sharing of Ti06octahedra, thus forming zigzag ribbon or tunnel structures. As photocatalysts, these structures are suitable for the accommodation of the catalytic active phases such as noble metal atoms which work to raise the efficiency of the separation of photoexcited charges and also for their transfer to adsorbed reactants a t the surface. Thus, it would be expected that these titanates become promising photocatalytic oxides. Domen et al. reported that K4Nb6ol7oxide, having layer structure, became an excellent photocatalyst when this mixed y i d e was combined with Ni0.6 In the abovementioned titanates, it is of particular interest to compare the photocatalytic activity between zigzag ribbon-type and tunnel-type structures, but there has been so far little application of these titanates as photocatalysts. In the present work, alkali-metal titanates, M2Tin02,,+l,associated with Ru, with different alkali-metal atoms (M= K, Na, or Rb) and with different number of n ( n = 2, 3, 4, or 6) were employed as photocatalysts. A detailed study on the photocatalytic properties was done on sodium hexatitanate, Na2Ti6013,which (1) Inoue, Y.; Sato, K.; Sato, K.; Miyama, H. J . Phys. Chem. 1986, 90,
2809.
(2) Inoue, Y.; Hayashi, 0.;Sato. K.; Kubokawa, T. Proceedings ojthc 9th International Congress on Catalysis; Phillips, M. J., Ternan. M., Eds.; The Chemical Institute of Canada: Ottawa, 1988; Vol. IV, p 1497. (3) Inoue, Y.; Sato, K.; Sato, K. J . Chem. Soc.. Faraday Trans. 1. 1989. 85. 1765. (4) Inoue,Y.; Hayashi, 0.; Sato, K. J . Chem. Soc., Faraday Trans. I 1990, 86, 2217. ( 5 ) Inoue, Y.; Kubokawa, T.; Sato, K. J . Chem. SOC.,Chem. Commun. 1990. 1298. ( 6 ) Domen, K.; Kudo, A.; Shibata, M.; Tanaka, A,; Maruya, K.; Onishi, 7. J . Chem. Sm., Chem. Commun. 1986, 1706. Domen, K.; Kudo, A.; Shinozaki, A.; Tanaka, A.; Maruya, K.; Onishi, T. J . Chem. Soc., Chem. Commun. 1986, 356.
gave rise to the highest activity among the employed oxides.
Experimental Section In the preparation of polycrystalline samples of M2TinOb+,, a molar ratio of extra-pure-grade reagent powders of Ti02 (Junsei Chemicals Co.) and the corresponding carbonates (Nakarai Chemicals Co.) were mixed thoroughly in an agate mortar, heated in an alumina crucible between 1173 and 1273 K over a period of 18 h, and then cooled to room temperature. The formation of M2TinOWloxides was confirmed by X-ray diffraction patterns. In the preparation of Nb-doped Na2T&OI3oxides, a small amount of Nb205 was added to the starting materials and then calcined under almost the same conditions as described above. Photocatalysts were prepared by impregnating MZTinOWl oxides with RuCI, aqueous solution at 333 K and were then subjected to heat treatment such as reduction in a H2 atmosphere or oxidation in air at various temperatures. In the immersion of a Na2Ti6Ol3oxide in a RuCI, aqueous solution, it was found that Na ions were liberated from the Na2Ti6013oxide, while Ru ions were taken up by the oxide. A relationship between the liberated Na and deposited Ru was obtained by analyzing the concentrations of each component in the solution. The former were analyzed by atomic absorption spectrometry while in the latter, the remaining Ru ions in the solution were converted by the reaction with thiourea to thiourea complexes (Ru[SC(NH)NH2I2+,Ru[SC(NH)NH213),and their quantity was obtained from the peak intensity at 620 nm in the visible absorption spectra. The amounts of the deposited Ru were calculated from differences before and after the exchange. In the following description, the amount of Ru taken up by the titanates is given as weight percent of the metallic state. A closed gas-circulation system connected to a high-vacuum line was used for the photocatalytic reaction. About 250 mg of powder photocatalysts was placed in a quartz reaction cell which was filled with ca. 20 mL of distilled and deionized pure water. Dissolved gas residues in water were flushed off by a stream of Ar gas through water. The reaction system was evacuated once and then filled with ca. 27 kPa of Ar gas. The decomposition of water was carried out under irradiation with light from a 500-W Xe lamp operated at 400 W through a water filter. During the reaction, Argas was bubbled through the water, and a suspension of the photocatalyst powder was created by the stirring of the solution by the Ar gas. The evolved gases were analyzed by a gas chromatograph connected directly to the reaction system, which is described el~ewhere.~ High-resolution electron microscopic images of Na2Ti6013 oxides were obtained at 400 kV with JEOL GMJOOFX. X-ray
0022-365419 1 12095-4059%02.50/0 0 1991 American Chemical Society
Inoue et al.
4060 The Journal of Physical Chemistry, Vol. 95, No. 10, 1991
Figure 1. A high-resolution electron microscopic image of Na2Ti6Ol3 oxide. TABLE I: Various Heat Treatments and Photocatalytic Activity of Ru-Free and Ru-Impregnated Na2Ti6013 activity/ pmol h-' active phase treatmenta (temp/K) H2 O2 0.1 0 RD (873) 0.4 0 Ru (0.23 wt '36) dry (373) 0.8 0.15 1.5 0 RD (973) 1.7 0.7 RD (973)-0X (573) 6.1 3.1 RD (973)-0X (773) 3.3 1.6 ox (573) ox (773) 7.3 3.5
TABLE II: Binding Energy of Ru 3d5l2
OThe symbols RD and OX refer to the reduction with hydrogen and the oxidation in air, respectively. The numbers in parentheses are the temperature of the treatment.
K,the photocatalysts exhibited not only a remarkable increase in the activity but also the nearly stoichiometric production of H2and 02.The direct oxidation, without reduction, of the Ruimpregnated oxides at 573 K, resulted in 2 times higher activity with a better stoichiometric ratio of the products than did the oxidation of the reduced catalysts at the same temperature. The direct oxidation at 773 K produced the highest activity among the various heat treatments for the decomposition of H20.From these results, it is evident that oxidation is more favorable than reduction for the activation of the Ru-impregnated Na2Ti6Ol3 photocatalytic system. Oxidation at 773 K, which was simple and gave the highest activity, was employed, unless otherwise specified, and the photocatalyst thus prepared was described as Ru0,(773)/M2T@,, where the number in parentheses is the oxidation temperature (K) and M represents one of the specified alkali-metal atoms. Table I1 shows the results of X-ray photoelectron spectroscopic measurements for Ru-impregnated and heat-treated Na2Ti6013 oxides, together with metallic Ru and Ru02 powder. The binding energy of Ru 3d5,2 for Ru metal reduced with H2 was 280.1 eV. This value was about the same as reported previously for Ar ion-sputtered Ru metal powder.' Its oxidation at 823 K led to a higher value for the binding energy, which was similar to that of commercially available Ru02 oxide. These results show that
photoelectron spectra were recorded on a JEOL JPS lOOX with a source of Mg Ka. UV diffuse reflectance spectra were obtained from JASCO UNIDEC 660. Results Figure 1 shows a high-resolution electron microscopic image of a Na2Ti6Ol3oxide with the incident beam parallel to the [OlO] direction. The regular lattice image can be seen, in which the dark spots are related to the sites of Ti atoms. This image demonstrates that the prepared oxide preserves a well-defined layerlike structure up to the topmost surface. Table 1 shows the effect of heat treatment on the photocatalytic activities of pure and Ru-impregnated Na2Ti6013oxide. In the absence of Ru, very little activity was seen for the titanate, and the reduction at 873 K brought about a slight increase in the activity of hydrogen production. For Ru-impregnated oxides, drying at 373 K led to both the evolution of hydrogen and oxygen, although their activity still remained at a low level. By the reduction in a hydrogen atmosphere at 973 K,the activity for H2 evolution increased by a factor of 2, whereas that for oxygen disappeared completely. The subsequent oxidation in air at 573 K gave rise to a considerable recovery of the activity for O2 production. When the temperature of oxidation was raised to 773
samdes Ru powder Ru powder Ru02 powder Ru impregnated Na2Ti6013
treatment RD (473) OX (823)" dry (373) RD (973) RD (973)-0X (573) RD (973)-0X (773) ox (773)
binding energy of Ru 3 d d e V 280.1 280.6 280.7 28 1.7 280.2 280.6 280.7 280.6
"In 13.3 kPa of OF See Table I for the symbols RD and OX.
~~~
~
~~
(7) Kim, K. S.; Winograd, N. J. Cutal. 1974,35,66.
The Journal of Physical Chemistry, Vol. 95, No. 10, 1991 4061
Photocatalytic Activity of Alkali-Metal Titanates
1 oc
TABLE 111: Compsrison of the Photocatalytic Activity of R~0,(773)/M~Ti.O~~~' activity/pmol h-'
M Na K
Rb
n
H2
3 6 2 4 6 6
0.8
02 0 3.5
7.3
0
I-
s
0 0
0.4 1.2
5C
0.4 0
0.9
'0.23 wt % Ru content.
1 0
I
I
300
400
500
Wavelength Inm Figure 3. UV diffuse reflectance spectra of M2TinOWltitanates: (----) M = Rb, n = 6; (---) M = K, n = 6; (-) M = Na, n = 6; M = K, n = 4; (---) M = Na, n = 3. (..e)
0.2
0.4
0.6
0.8
3.8
4.0
Ru contentlwt% Figure 2. Photocatalytic activity of Ru0,(773)/Na2Ti6OI3for H2O decomposition as a function of Ru content: 0, H2; 0 , 02.
the chemical shift between Ru(0) and Ru(1V) is in the range 0.5-0.6 eV. The impregnation of RuC13 and drying at 373 K produced a considerably higher binding energy, which was probably due to the remaining CI ligand. The reduction with hydrogen at high temperature caused a large chemical shift toward lower binding energy up to the level of metallic Ru. The subsequent oxidation at 573 or 773 K gave almost the same binding energy, 280.6-280.7 eV, as observed for the oxidized Ru metal and R u 0 2 powder. The direct oxidation at 773 K without prereduction also resulted in the similar value, 280.6 eV. Table 111 compares the photocatalytic activities of Ru0,(773)/M2Tin02,,+, (M = Na, K, R b and n = 2 , 3 , 4 , 6 ) with the same Ru content. The activity for H2 production increased in the order Na > K > Rb for the same number of n ( n = 6) and also with increasing number of n, as seen in the case of M = K. There was a similar trend in the activity for O2evolution. Among the titanates examined, Na2Ti6OI3oxide permitted us to obtain the photocatalyst having the highest activity for the stoichiometric production of H2and 02. In the immersion of Na2Ti6013oxides in RuC13 solutions at 333 K for 2 h, the amounts of the liberated Na and the Ru take up were respectively 38 and 18 pmol at a solution concentration of 30 pmol dm-3 and 50 and 26 pmol at 630 pmol dm-3. It is to be noted that the ratio of Na to Ru maintains nearly a constant value of ca. 2 in different concentrations. The extent of alkalimetal ions exchanged with Ru ion was compared among M2Ti6013 (M = Na, K, or Rb) oxides. The relative ratio of the exchanged alkali-metal ions for M = Na, K, and R b was respectively 1 .OO, 0.82, and 0.69 (when the value for M = N a was taken as reference), whereas the ratio of the Ru take-up was respectively 1 .oO, 0.83, and 0 . 7 3 . I t is to be noted that there is a close similarity between the liberated alkali-metal ions and the Ru take-up for these three oxides. These results evidently indicate that the ion exchangeability is enhanced in the order N a > K > Rb, which is in agreement with the order observed for the photocatalytic activity of Ru0,(773)/M2Ti6Ol3. Figure 2 shows the photocatalytic activity of Ru0,(773)/ Na2Ti6Ol3as a function of Ru content. With an increase in the content, both the activity of H2 and O2production increased almost proportionally to Ru content until they reached a maximum at
0O -
400 Wavelength I nm Figure 4. Photocatalytic activity as a function of wavelength and an absorption spectrum: (0)H2,( 0 )02,(---) UV diffuse reflectance spectrum. Photocatalyst: Ru0,(773)/Na2Ti601,,0.23 wt W Ru content. 300
ca. 0.23 wt %. The activity then decreased considerably in the region 0.44.7 wt %, followed by a very slow decrease with further Ru loading. The activity for H2 and O2 production was reproducible to about *7% on 0.23 wt % Ru-containing photocatalysts prepared in different batches. It should be noted that the stoichiometric production of H2 and O2is preserved nearly over the entire range of the Ru content. In our preliminary experiments using a band-pass filter (wavelength of passing light is between 330 and 410 nm), the quantum yield for the highest activity was estimated to be 1.5%. Figure 3 shows the UV diffuse reflectance spectra of M2Ti6Ol3 (M = K, Na, and Rb) oxides, together. with those of Na?Ti307 and K2Ti409oxides. The threshold wavelength of absorption for M2Ti6Ol3oxides was around 410 nm and there was a tendency for the main absorption region to shift slightly (-6 nm) toward longer wavelength when M was varied in the order of N a K Rb. For Na2Ti307and K2Ti409oxides, the main absorption occurred at a wavelength shorter by 21-26 nm than that of M2Ti6Ol3. Figure 4 shows the wavelength dependence of the photocatalytic activity of R ~ 0 , ( 7 7 3 ) / N a ~ T i ~together 0 ~ ~ , with absorption characteristics of the titanate. The activities for both the hydrogen and oxygen evolution became significant at ca. 350 nm, which was in agreement with the main absorption region, and continued to increase with decreasing wavelength. When Na2Ti6013titanate was doped with a small amount of N b ions, as shown in Figure 5, bulges appeared between 370 and 400 nm in the UV absorption spectra, which grew with increasing amount of N b ions. Table 1V shows the photocatalytic activity b ~a function T i ~ ~ ~of)the ~ ~amount ~~ of R ~ 0 , ( 7 7 3 ) / N a ~ ~ ~ ( N as of y . The addition of even a small amount of a N b component
-
-
Inoue et al.
4062 The Journal of Physical Chemistry, Vol. 95, No. 10, 1991 10
t
s
5(
c
I
300
400
WavelengthI nm
, 500
Figure 5. Changes in UV diffuse reflectance spectra of NazTi6013with N b doping. In Naz4y(Nb,,Til,)6013,(1) y = 0, (2) y = 0.01, (3) y = 0.03,(4) y = 0.1. Figure 6. Schematic representation of (a) zigzag structure of NazTi307 and (b) tunnel structure of NaZTi60,,oxide: (0)Na, ( 0 )Ti0&
TABLE I V Changes in Photocatalytic Activity of Ru0,(773)/Na~,(Nb,Til,),01, with y
Y
0 0.01 0.03 0.1
photocatalytic activity/pmol h-I HZ 02 7.3 1.4 1 .o
3.5 0.6 0.2
0
0
"0.23 wt % Ru content.
resulted in a drastic decrease in photocatalytic activities, e.g., ca. one-fifth the original value with I mol '75 Nb. The activity for oxygen production was influenced more drastically.
Discussion The previous studys on either Pt or Rh deposited on Na2Ti6Ol3 titanate showed that reduction treatment with hydrogen gave rise to not only a low activity for H2 production but also to a negligibly small production of 02.It is demonstrated here that the photocatalytic activity of Ru-impregnated Na2Ti6Ol3was lower when subjected to reduction treatment, but increased considerably for the production of both H2 and O2 by oxidation treatment. Furthermore, an interesting feature of the present catalyst is that catalyst activation can be achieved without any reduction treatment through its preparation. This contrasts with the characteristics of the conventional photocatalysts using Ti02and SrTiO, oxides, in which reduction treatment at higher temperatures is required for activation before the deposition of the active phases. In a typical photocatalytic run on R ~ 0 , ( 7 7 3 ) / N a ~ T i ~ 0the ~,, total number of hydrogen atoms produced for 20-h irradiation was calculated to be 28 times larger than that of the surface Ti atoms, provided that the (010) plane of the oxide is exposed. No significant deterioration can been seen over a prolonged irradiation up to 60 h. It appears that such durable activity is related to the feasibility of activation without reduction treatment, since the incorporation of the produced oxygens into the lattice of oxides, which is frequently associated with the deterioration of the photocatalytic activity, occurs to a lesser extent in the present case, unlike conventional semiconductor oxides. The X-ray photoelectron spectra of Ru deposited on Na2Ti6013 showed that the binding energy of Ru 3d512level for the photocatalysts treated by reduction 973 K was similar to that of metallic Ru, whereas the oxidation led to a binding energy level close to that observed for R u 0 2 powder and oxidized Ru metal. This indicates that Ru(1V) is an active species, evidently existing in a chemical form analogous to Ru02. The appearance of high activity through this active phase is associated with the fact that R u 0 2 electrodes have a high capability to decrease overvoltage
for oxidation reactions. The effective role of R u 0 2 for O2production from water has been so far reported.*-I0 In immersion of Na2Ti6ol3oxides in RuC13 sohtions, a relationship is found between the liberated Na ions and the deposited Ru ions; the ratio of the former to the latter was close to ca. 2. This ratio suggests that an exchange reaction proceeds according to the equation 2Na + RuCI, >RuCl + 2NaCl
-
The amount of Na ions replaced was calculated to correspond to at least nine layers of the oxide at 0.23 wt '75 Ru content, which is indicative of the intercalation of Ru ions deep in the oxide and not on its surface. As shown in Figure 2, the photocatalytic activity increased almost proportionally to the content of Ru species up to a maximum at 0.23 wt '75. It is assumed here that Ru also deposited inside the titanate and not only on the surface layer which is available for the reaction. This may give rise to another explanation based on the membrane effect of the titanate which allows not only separation of charges but also separation of reaction products, O2and H2, which are not recombined. The decrease in the activity with increasing Ru content over a maximum indicates that the photocatalytic role of Ru species is different from that of the intercalated one. In TiOl photocatalysts, it was demonstrated that the deposition of a large amount of Ru02 leads to the production of a recombination center for the photoexcited carriers and hence lowers the photocatalytic activity." Thus, it is likely that excess loading of Ru produces the agglomeration of Ru at the surface, a part of which works as the recombination center. As shown in Table 111, the photocatalytic activity of Ru0,(773)/M2Ti6Ol3(M = Na, K, Rb) increased in the order Na > K > Rb. With the same alkali-metal atom, there was a trend that larger number of n produced higher activity. Thus,the highest activity was obtained for R ~ 0 , ( 7 7 3 ) / N a ~ T i(M ~ 0= ~ ~Na and n = 6). In addition, for the catalysts other than Ru0,(773)/ Na2Ti6Ol3,oxygen production was partially or completely depressed. From a structural point of view, the most interesting feature is that Na2Ti6013has a framework enclosing tunnel structures formed by edge- and corner-shared Ti06 octahedra, whereas other titanates are represented by zigzag ribbon structures consisting of edge-shared Ti06 octahedra. The structures of triLehn, J. M.;Sauvage, J. p.; Zieasel, R. N o w . J . Chim. 1979,3,423. (9) Kiwi, J.; Gratzel, M.Chimica 1979, 33, 289. (IO) Blondeel, G.;Harrlman, A.; Porter, G.;Urwin, D.;Kiwi, J. J . Phys. Chem. 1983,87, 2629. ( I I ) Sakata, T.; Hashimoto, K.; Kawai, T. J . Phys. Chem. 1984,88.5214. (8)
J . Phys. Chem. 1991, 95,4063-4069 and hexatitanates are schematically given in Figure 6. The present results suggest that such structural differences are responsible for the different photocatalytic properties; the framework of the tunnel structure is evidently suitable for accommodating Ru species to induce a strong interaction between the active species and semiconductor oxide. This situation appears to enhance the efficiency for the transfer of the photoexcited charges to the adsorbed species. From the X-ray diffraction data, Wadsley et al.12 showed that the lattice parameters of alkali-metal hexatitanates varied considerably depending on the kinds of alkali-metal ions involved. With increasing atomic number of alkali-metal atoms, i.e., in the order Na K Rb, both the a and 6 axes expand, whereas the c axial length is decreased; for example, a comparison between N a and Rb ions showed that the increase in a and 6 axes was respectively 76 and 7.5 pm, whereas the compression of c axis was 4.9 pm. This means that a larger alkali-metal ion causes such a distortion of the tunnel framework that the ion-exchange reaction between the alkali-metal ion and Ru ion is depressed. Therefore, the oxide involving Na ion is suitable for the intercalation, whereas Rb ion is unfavorable for the ion exchange with Ru. The differences in the amount of the intercalated Ru among the tunnel-structural alkali-metal hexatitanates are in line with the photocatalytic activity sequence, Le., higher activity in the order K Rb. In addition to this, we cannot exclude the Na possibility that the extent of distortion of the tunnel framework negatively influences the efficiency of photoexcitation. The shift of the light absorption region toward longer waveK R b (Figure 3) is also probably length in the order N a
--
+
+
--
(12) Andersson, S.; Wadsley, A. D. Acta. Crystallogr. 1962, I S , 194.
4063
associated with the effect of alkali-metal ion size which causes the distortion of Ti06 octahedra due to the expansion and compression of the framework. For example, the distance of 0-0 which forms one of the edges is reduced from 0.248 nm in Na2Ti6Ol3to 0.229 nm in RbzTi6013,12and, provided that a distortion from Ohto DM symmetry occurs,the tD level splits into eg + alglevels. The splitting of degeneracy leads to the lowering of the conduction level and hence to a decrease in the band gap. The addition of N b ions caused the shift of absorption toward visible light region but unfortunately a marked decrease in the photocatalytic activity as well. This indicates that the addition of N b ions increases the concentration of recombination centers for the photoexcited carriers, similar to the case of Cr3+-doped SrTiO3.I3*l4 In conclusion, a combination of Na2Ti6013and Ru species produces a photocatalyst effective for the decomposition of water into H2 and 02.Its characteristic is that it should be activated in an oxidative atmosphere, and it is suggested that the framework of the tunnel structures is responsible for the generation of the high activity. Acknowledgment. This work was supported by a Grant-in-Aid for Scientific Research on Priority Area for the Ministry of Education, Science and Culture of Japan. We thank the EM application laboratory of JEOL Ltd. for observations of high-resolution electron microscopic images. (13) Campet, G.; Dare-Edwards, M. P.; Hamnett, A.; Goodenough, J. B. Nouu. J . Chim. 1%0, 4, 501. (14) Mackor, A.; Blasse, G. Chem. Phys. Lerr. 1981, 77, 6.
Adsorption of Pyridlne on Silica Gels L.Nikiel and T.W.Zelda* Physics Department, Texas Christian University, P.O. Box 32915, Fort Worth, Texas 76129 (Received: August 27, 1990)
Raman spectra of pyridine adsorbed at silica surface are reported as a function of surface coverage, ranging from a fraction of a monolayer to completely filled pores. It is observed that pyridine is preferentially adsorbed by silica, and it is suggested that the process results in a bilayer structure of the interface. The increased concentration of pyridine in silica pores in comparison with the concentration of pyridine in binary mixtures in the reservoir outside of the sample is reported for mixtures with CC14, CH3N02,and, to a lesser extent, (CH3)$0 and CHCI,. For mixtures of pyridine with CH3CN,silica preferentially adsorbs acetonitrile, and the final concentration of pyridine is less than that in the reservoir in contact with the sample. This effect is also attributed to the bilayer structure of the interface. The diffusion rate is determined, and it is shown that pyridine diffusion is slower in mixtures with CS2, an inert solvent, and faster in mixtures with acetonitrile, a polar solvent.
Introduction Molecular motion, energy transfer, and chemical reactions in porous materials have been recently investigated by NMR,' Rayleigh scattering? fluore~cence,~ induced birefringence? and IR ~pectroscopy.~Optical techniques require the porous host material to be transparent, and it is important to collect the signal from molecules trapped within small pores and not from molecules inside macropores or macroscopic cracks. It is also important to analyze samples of narrow pore size distribution and to be able to control the surface groups throughout the sample. Most of the optical studies of molecules confined within small volumes have (1) Bernstein, T.; Kitaev, L.; Michel, D.; Pfeifer, H. J . Chem. Soc., Faraday Trans. i 1982, 78,76 1, (2) Dozier, W. D.; Drake, J. M.; Klafter, J. Phys. Rev. Lett. 1986, 56, 197. (3) Fujii. T.; Ishii, A.; Suzuki, S.; Anpo, M. Chem. Express 1989,4, 471. (4) Awschalom, D. D.;Warnock. J. In Molecular Dynamics in Restricted Geometries; Klafter, J., Drake. J . M., Eds.; J. Wiley: New York, 1989. ( 5 ) Bebe, T.P.; Crowell, J. E.; Yates, J. T. J . Chem. Phys. 1990,92,5119; J . Phys. Chem. 1988, 92, 1296.
been performed for Vycor g l a ~ swhose ~ , ~ physical and chemical properties are well-known. Optical studies of adsorption on silica powders, although difficult, are also possible.' Recent advances in the silica sol-gel process enable to extend those measurements to other transparent porous materials obtained via the sol-gel process. This process enables to control the pore sizes and pore size distribution: as well as to control the functional groups on the s ~ r f a c e . Moreover, ~ the manufacturing of silica monoliths of diameters exceeding 2 cm is relatively simple. All those properties make the silica sample an excellent host for investigating various molecular processes within the pores. R e ~ e n t l y we , ~ discussed molecular reorientational and vibrational relaxation of simple molecules inside silica gels of various pore diameters and of different surface groups. We showed that (6) Dierker, S. 8.;Dennis. B. S.; Wiltzius, P. J . Chem. Phys. 1990, 92, 1320. (7) Simpson, S. F.; Harris, J. M. J . Phys. Chem. 1990, 94, 4649. ( 8 ) Hench, L. L.; West, J . Chem. Rev. 1990, 90,73. (9) Nikiel, L.;Hopkins, B.; Zerda, T. W. J . Phys. Chem. 1990, 94, 7458.
0022-365419112095-4063%02.50/0 0 1991 American Chemical Society
