J. Phys. Chem. 1986, 90, 637-640 TABLE II: Heats of Crystallization vs. R for the Water/AOT/Isooctane System 8 7 6 n n14 5 6.3 15.6 20.7 22.5 L, cal/g water 0
9
10
29.0
30.6
1
L (cal/g water)
40i -1 20
0
I
5
IO
I
15
b
n
Figure 9. Variations of the heat of crystallizationL (cal/g of water) vs. n: water/AOT/isooctane system. P, = 0.1. -, theoretical curve (eq 1) with n, = 4.5.
is so strongly bound to polar heads that crystallization does not occur. In such conditions, the measured crystallization heat can be written as
L = L,(1
- n,/n)
where L,, represents the water crystallization heat, Le., 56 cal/g at about -40 O C I 6 and n, represents the number of bound water
637
moles per AOT mole. Numerical analysis indicates that, at given n, = 6.5, the theoretical curve giving L according to n agrees perfectly with experimental results as demonstrated in Figure 8. For the water/AOT/isooctane system we made a similar analysis (Table 11). At n, = 4.5 the theoretical curve giving L according to n (eq 1) agrees with experimental results (Figure 9).
Conclusion This study has proved that, in the case of a ternary system, differential scaning calorimetry enables us to determine clearly and directly the water state within microemulsions. We have determined the required minimum number of water molecules per AOT molecules (n,) to obtain water with "bulk" properties within the micelles. The data show an effect of the oil nature, which results certainly from the penetration of the oil into the interfacial layer. In a recent paper Angel et al." point out that oil penetration decreases for higher chain length. Perhaps n, increases when the oil is higher because the penetration of oil is smaller. Because of the absence of alcohol, we are able to identify the peaks, unlike the case of quaternary system." Besides our thermograms do not exhibit a reversible behavior, unlike previous observations,' according to the supercooling breakdown which is an irreversible phenomenon. Acknowledgment. We are indebted to the Soci&t&Nationale ELF-Aquitaine (P) for the financial support of this work. Registry No. AOT, 577-11-7; water, 7732-18-5; dcdecane, 112-40-3; isooctane, 540-84-1. (16) Clause, D.; Dumas, J. P.; Broto, F. C. R. Acad. Sci. Paris 1974,
2798,415-418,
(17) Angel, L. R.;Evans, D. F.; Ninham, B. W. J . Phys. Chem. 1983.87, 538-540.
Heterogeneous Photocatalysis: Enhanced H, Production in TiO, Dispersions under Irradiation. The Effect of Mg Promoter at the Semiconductor Interface J. Kiwi* and M. Gratzel Institut de Chimie Physique, Ecole Polytechnique F?d&rale, CH- 101 5 Lausanne, Switzerland (Received: May 29, 1985; In Final Form: August 30, 1985)
Promoter action in semiconductorswas studied by using Mg2+doping of Ti02 powders as a model system. This paper describes a novel series of Pt-Mg2+-Ti02 catalysts. The catalytic properties of these novel catalysts were determined in relation to their efficiency in water-splitting processes under UV irradiation. The most efficient catalysts range up to 4 times in activity over the catalysts without Mg2+. A proportionality is observed for the activity of the catalyst with magnesium loading reaching a plateau at 1-276.
Introduction One of the effects of the projected shortage of oil and natural gas has been the growing interest in recent years in the study of alternative energy sources. Research from different laboratories reflects this interest.' More recently, work in our laboratory has been performed with the goal of optimizing the deposition of noble metals and oxides on T i 0 2 supports.2 We have attempted to (1) (a) Kiwi, J.; Kalyanasundaram, K.; Gritzel, M. Struct. Bonding (Berlin) 1982, 49, 3. (b) Harriman, A., West, G. Eds. "Photogeneration of Hydrogen"; Academic Press: London, 1983. (c) Gritzel, M., Ed. "Sources of Energy through Photochemistry and Catalysis"; Academic Press: New York, 1983. (2) (a) Yesodharan, E.; Gritzel, M. Helu. Chim. Acta 1983,66,2145. (b) Kiwi, J.; Gritzel, M. J . phys. them. 1984,88, 1302. (c) ~ i ~J.;i soss, , J,; Szapiro, S. Chem. Phys. Lett. 1984, 206, 135.
0022-3654/86/2090-0637$01.50/0
TABLE I mixed atom % Mg
0.5 1 2 3
5 7
10 12.5
mixing temp, OC 500 500
500 500 500 500 500 500
mixing time, h 2 2 2 2 2 2 2 2
atom % Mg found 0.48 0.91 1.8
2.7 5.0 6.9 9.3
10.2
improve the efficiency Of the photoc'eavage process through semiconductor loaded materials. In the course of this work it has been found that alkaline ion dopants in T i 0 2 such as Li3 are 0 1986 American Chemical Society
638
The Journal of Physical Chemistry, Vol, 90, No. 4, 1986
Kiwi and Gratzel
H ia) b,
2
4
6
8
10 12 % M g in T i 0 2 -
Figure 1. (a) Media rate per hour of hydrogen induced by UV in 25 cm3 of 1 N NaOH solutions using 40 mg of impregnated 0.5% Pt-Mg2+-Ti02 catalyst. All irradiations were carried out with a Xe lamp 90-100 mW/cmz. (b) Media rate per hour of hydrogen under UV irradiation when 40 mg of exchanged 0.05% Pt-Mg2+-Ti02 was irradiated in 1 N NaOH. Other conditions as in (a).
instrumental in accelerating H2 generation under UV light. Although much is known regarding the overall H2 generation process: little information exists about details related to the charge transfer taking place at the TiO, i n t e r f a ~ e . ~It, ~is our aim in this work to assess the catalytic efficiency due to ion-doping and to understand the observed variations in terms of a reaction mechanism.
Experimental Section The Mg2+-Ti02 doped samples were prepared by Dr. Panek (Bayer AG, Krefeld, Uerdingen, West Germany) using the slurry technique as outlined by Teichner in ref 5b. In order to produce bulk magnesium-doping within a reasonable time, a dried slurry of each Mg2+-Ti02 sample was heated for 2 h at 500 O C . The actual magnesium content of the catalyst was analyzed through atomic absorption by Dr. Panek and the results are shown in Table I. X-ray diffraction analysis was performed on all samples up to 12.5% Mg2+ content. No evidence for titanate formation (MgTi03 or Mg2Ti04)was found. This experimental observation indicates that lattice substitution by Mg2+has taken place in the doping process without formation of a new phase. About 1000 O C has been shown necessary5ato form MgTi03 under the present experimental conditions. Impregnated samples of Pt-Mg2+-Ti02 were obtained by using H,PtCl, dissolved in a minimum amount of alcohol as reported in ref 2b. Ion-exchanged Pt-Mg2+-Ti02 samples were prepared in the same way as previously reported.2b,38,3bThe Pt content of the samples was analyzed in the following way:' A 6-times excess of N a 2 0 2was added to the sample in a Zr crucible. Fusion at 500 OC was performed and after cooling the material was dissolved in HCl to a convenient volume. Continuous photolysis experiments (3) (a) Kiwi, J.; Morrison, C. J. Phys. Chem. 1984,88,6146. (b) Kiwi, J.; Gratzel, M. 'Proceedings of the 8th International Congress on Catalysis"; Verlag Chemie: West Berlin, 1984; Vol. 3, p 395. (4) (a) Wagner, F.; Somorjai, G. J. Am. Chem. Soc. 1980, 102, 5494. (b) Ferrer, S.; Somorjai, G. J . Phys. Chem. 1981, 85, 1464. (5) (a) Barksdale "Titanium"; Ronald Press: New York, 1966. (b) Teichner, S. Adu. Carol. 1%9, 20, 107. (c) Kruczynski, L.; Gesser, D. Inorg. Chim.Acta 1983, 72, 161. (6) (a) Anderson, J. 'The Structure of Metallic Catalysts"; Academic Press: New York, 1975. (b) Pajonk, G.; Teichner, S.;Germain, J., Eds. 'Spillover of Adsorbed Species"; Elsevier: Amsterdam, 1983. (7) Bock, R. "Aufschlufsmethoden der Anorganische u. Organische Chemie"; Verlag Chemie: West Berlin, 1972; p 59.
0
3
6
9 A t % Mg a d d e d
-
12
Figure 2. Variation in the amount of magnesium employed in the preparation of the Mg2+-Ti02samples and the amount of magnesium found by atomic absorption in the actual samples.
were performed with a Rofin 150-W Xe lamp. The lamp provided a total flux of 95 mW/cm* on the external surface of the irradiated flask (4.5cm2). All irradiations were carried out in a 25-cm3flask. H2was analyzed as previously reported.28*bPeroxotitanates were M at pH determined by titration in the dark with K M n 0 4 0.2.33
Results and Discussion Photocatalytic H2 Evolution through Pt-M2+-Ti02 Catalysts. Figure 1 presents the average rate per hour for H2 evolution measured during the first 4 h of photolysis. Trace a shows 0.5% Pt-TiO, impregnated catalysts of Mg2+-Ti02 Bayer samples containing different amounts of Mg2+. The platinization process was performed by impregnation of the Mg2+-Ti0, samples as described in the Experimental Section. From the experiments shown in trace a it is readily seen that Mg2+-doping affects the rate of H2 generation. The samples containing 1% and 2% represent the most favorable conditions for H2 evolution. Mg2+-dopingwas chosen due to its favorable charge to size ratio for this ion (+2/0.78 %.)when compared to Li ion (+1/0.68 A).9*'0 An enhancement factor of 2 in relation to similar Li-doped samples was observed.3a A decrease in H2 generation activity was observed at higher Mg doping. Trace b presents the average rate of H2 evolution per hour for 0.05% Mg2+-Ti02 Bayer catalysts as a function of Mg-doping. Higher yields were observed for promoted catalysts as compared to the runs carried out on the impregnated samples. From Figure 1 it is seen that the highest yields for H2 evolution under irradiation took place at similar loadings of magnesium in both cases. Figure 2 presents the results for the atomic percentage of Mg2+ found vs. the amount of Mg2+added during the preparation. A 1:l linear correlation is formed and we ascribe this to be the substitutional doping of Ti4+ with Mg2+. At high Mg2+dopant levels not all the Mg2+ initially added is incorporated in the TiO, solid. The point defect model is used as a simple qualitative model throughout this study only as a basis for the discussion of the (8) Yesodharan, E.; Yesodharan, S.; Gratzel, M. Sol. Energy Mater. 1984, 10, 287. (9) Shanon, R. Acta Crystallogr., Sect. A 1976, 32, 751. ( ! O ) (a) Kofstadt, P. "on-Stoichiometry, Diffusion and Electrical Conductivity in Binary Metal Oxides"; Wiley: New York, 1972. (b) Smith, R. "Semiconductors"; Cambridge University Press: Cambridge, England 1976. (c) Hannay, N. Semiconductors"; Reinhold: New York, 1959. (d) Shobaky, G.; Gravelle, P.; Teichner, S. Bull. SOC.Chim.Fr. 1967, 3246.
The Journal of Physical Chemistry, Vol. 90, No. 4, 1986 639
Heterogeneous Photocatalysis
800b
t l i l
I
J
I
I
J
0
2
4
6 hours-
hours
Figure 4. O2photouptake (left ordinate) and photoproduced H2 (right ordinate) when solutions containing 750 pL of O2were irradiated in 25 cm3 of 1 N N a O H containing 40 mg of the following catalysts: (a) 0.05% Pt-Ti02, (b) 0.05% Pt-1% Mg2+-Ti02.
-c
Figure 3. UV irradiation of 0.05% Pt-1% Mg2+-Ti02 catalyst, 40 mg in 25 cm3 of 1 N NaOH, as a function of time of irradiation. Recyclings
-16
are carried out over 4-h periods. Other experimental conditions as cited in Figure 1.
electronic character of the solids. As such, it should provide an indicator of the reactivities as a function of the increasing conT i 0 2 is known to be an centration of added n-type metal excess semiconductor.Ioa Compensation of n-type character is apparently achieved by magnesium-doping. If we use the KrOger-Vinkl3 notations, the defects introduced into Ti02 may be expressed in the following way: Oo = Y202 V0” 2e’ (1)
+
or 2 T i ~4, 00=
f/z02
+
+ vo” + 2TiT{
K 1= [ Vo”][e’]2p021/2
(2) (3)
where Oo = oxygen ions on normal lattice sites; VO“ = oxygen anion vacancy doubly positive charged; e’ = electrons in lattice; TiTi= Ti on a normal lattice position; TiT/ = Ti on a normal lattice position with a negative charge; pol = partial pressure of the oxygen atmosphere. In addition to eq 1-3, lattice incorporation of MgO could be stated by eq 4-7, using Hauffe’s notation,’&12 h+ being holes in the valence band
+ v, MgO + ‘/,0, = MgTp + TiO, + 2h+ 00 =
‘/02
Adding eq 4 and 5: MgO oo = MgTl”
+
Since Vo
+ Vo + Ti02 + 2h+
(4)
(5) (6)
+ 2h+ = Vo“ MgO + oo = MgT,” + VO” + TiOz
(7) Equations 1-7 then indicate that the Mg2+added increases the concentration of ionized vacancies. This would in turn compensate n-type conductance. Since a decrease in the observed H2evolution rate seems to take place above 2% Mg2+-doping, it is possible that the added Mg2+ ion would neutralize the strongest acidic surface OH g r o ~ p ’ ~ of - ~TiOz * above this level. (1 1) Drauglis, E.; Jaffe, R. “The Physical Basis for Heterogeneous Catalysis”; Plenum Press: New York, 1975. (12) Dowden, D. A. “Surface Science 2”; International Atomic Energy Agency: Vienna, 1979; p 215. ( 1 3) Kroger, F.; Vink, H. In ‘Solid State Physics”; Scitz, F., Turnbell, D., Eds.; Academic Press: New York, 1956; Vol. 3, 307. (14) Hauffe, K.; Schlosser, S . DECHEMA-Monogr. 1956, 26, 222. (1 5) (a) Parfitt, G. Prog. Surf. Membr. Sci. 1976, I I, 18 1. (b) Parfitt, G . Pure Appl. Chem. 1976,48,415. (c) Boehm, H. Discuss. Faraday Soc. 1971, 52, 264.
h o u r s -c
Figure 5. Amount of peroxotitanate as a function of irradiation time for 1 N NaOH solution of 40 mg of 0.05% Pt-7% Mg2+-Ti02 catalyst. H2 production as a function of time is shown on the right-hand side.
Figure 3 presents repetitive cycles for H, formation when 0.05% Pt-1.0% Mg2+-Ti02 Bayer was irradiated during 12 runs (ca. 48 h). Irradiations were carried out for 4 h, bubbling afterwards .~ for ~ 2 min 0 with Ar to eliminate the H2 p r ~ d u c e d . ~During 48 h of irradiation, 9 mL of Hz were produced, corresponding to 8.0 X IO4 mol of reduction equivalents. This exceeds the amount of TiO, (40 mg = 6.3 X IO4 mol) present in the irradiated system. Therefore, repetitive irradiations produce more H, than there are Ti ions present in solution. The mechanism taking place during H2 evolution has been previously The Effect of M p on Photouptake of Oxygen. Figure 4 shows the doping of 0.05%Pt-TiO, Bayer with 1% Mg2+ions enhances the rate of photouptake of oxygen. Mgz+ion doping will increase therefore the amount of oxygen which will be chemisorbed. As shown r e ~ e n t l y , the ~ ~ photouptake ~J~ of O2leads to formation of peroxide adsorbed on solid TiO,. This capacity is of the order of 20 nmol/cm2. The effect of 1% Mg2+ doping is to increase 0, uptake by a factor of 2. Oxygen photoadsorption is favored in the 1 % Mg2+-Ti02 catalyst under study since each vacancy (16) Grange, P.; Jacobs, P.; Poncelet, G. “Scientific Basis for the Preparation of Heterogeneous Catalysts”; Elsevier: Amsterdam, 1980. (17) (a) Tanabe, K. “Solid Acids and Bases”; Academic Press: Tokyo, 1970. (b) Yashima, T.; Sakaguchi, Y.; Namba, S. “New Horizons in Catalysis”; Elsevier: Amsterdam, 1981; Vol. 7, p 739. (!8) Anderson, J.; Boudart, M., Eds. “Catalysis”; Springer-Verlag: West Berlin, 1983; Vol. 3.
640
Kiwi and Gratzel
The Journal of Physical Chemistry, Vol. 90, No. 4, 1986
I
f
Figure 6. Adsorption isotherm of N2 on Ti02. n represents the number of adsorbed molecules of N2,P the pressure of gas adsorption, and P, the saturation pressure of liquid N2.
is capable of accommodating one-half extra oxygen during 0, photouptake. Increased photoadsorption seems to occur on these vacancies up to 1-2% dopant level. From eq 4-7 it can be seen that the Mg-doping process increases the number of vacant cation lattice positions and oxygen ion vacancies. Finally, in the right-hand side of Figure 4,hydrogen evolution is only possible when more than 90% of the O2 added initially has been photoadsorbed. It has been observed (Figure 1) that the formation of H2 during photolysis is not accompanied by the appearance of 0, in the gas phase. Instead, peroxotitanium complexes are formed which were analyzed by K M n 0 4 titration as previously r e p ~ r t e d . ~ It , ] is ~ readily seen from Figure 5 that peroxotitanate detected by this method corresponds to about 40% of the possible stoichiometric amount in relation to the H2 evolved during the reaction. When recyclings were repeated many times, as shown in Figure 3, a faint yellow color was observed. Electron Microscopy and Surface Area Studies on PtM F - T i O , Catalysts. Electron microscopy studies were carried out on 0.5% Pt-1% Mg-Ti0 samples. Sixty percent of the Pt islands counted were 18-20 in size. More defined structures were found in the 0.5% Pt-Ti02 samples than in the Mgz+-doped catalyst. The differences in activity for H2 evolution reported in trace 2, Figure 1 (0.5% Pt and 0.05% Pt-MgZ+-Ti02 catalysts), were probably due to the difference in the particle size of the metal. Actually, for a 0.05% Pt-1% Mg2+-Ti02 catalyst about 60% of the particles counted had a size of 8-10 A. This is a valid consideration since approximately the same dispersion of Pt is attained in the different catalysts reported in trace b in this same figure. Lower concentrations of Pt in the dispersed phase seem to allow stronger Pt-TiO, interactions with the support. Figure 7 shows the diffuse reflectance spectra (DRS) for (a) TiO,, (b) 0.05% Pt-TiO,, (c) 0.05% Pt-0.5% Mg2+-Ti02, (d) 0.05% Pt-1% Mg2+-Ti02, (e) 0.05% Pt-2% Mg2+-Ti02, and (f) 0.05% Pt-3% Mg2+-Ti02. The absorption increases with Mg2+ ion doping up to 3%. Traces c-f show that the absorption increases due to Mg-loading of the catalyst up to a limiting value of 2-3% Mg2+. The inflection in the curves around 400 nm is due to the band gap of TiO, (3.2 eV). The absorbance up to 800 nm due to Pt is shown in trace b. The absorbance of Mg2+-dopedcatalysts (traces c-f) is slightly stronger as compared to the unpromoted catalyst. Diffuse reflectance spectroscopy shows an increase in absorbance in the visible region indicating that lower energy transitions than that of the band gap are possible. Therefore, in the highly dispersed state, the Pt particles are in different environments when Mg2+dopant is present up to the described limit.
8.
(19) (a) Muhlebach, J.; Muller, K.; Schwarzenbach, G. Inorg. Chem. 1970, 9, 2381. (b) Mori, M.; Skibata, M.; Kyuno, E.; Ito, S. Bull. Chem. SOC. Jpn. 1956, 29, 904. (c) Paul Patai, Ed. "The Chemistry of Peroxides"; Wiley: New York, 1983. (20) Munuera, G.; Navio, J. Span. Symp. Adsorpt., Sevilla 1979, 25.
I
0
nm-
Figure 7. Diffuse reflectance spectra (DRS) of (a) Ti02, (b) 0.05% Pt-TiO,, (c) 0.05% F't-0.5% Mg2+-Ti02,(d) 0.05% Pt-1% Mg2+-Ti02, (e) 0.05% Pt-2% Mg2+-Ti02,and (0 0.05%-3% Mg2+-Ti02.
A word of caution should be added here since it may be very well possible that the Mg2+-doped TiOz changes the distribution and size of the Pto clusters. This effect will then suffice to confer to the surface a different capacity for H2 generation as shown for the different Mgz+-doped samples shown in Figure 1. Two explanations are possible for these DRS spectra obtained: (a) SMSI condition involving very short Pt-Ti interaction distances would be considerably modified by dopants such as the one used in the present case; (b) a p o x 0 bridge structure is formed when Pt and Ti02interact at 425 0C.6,18 In the promoted catalyst, these latter bands may contribute much more to the observed intensity in the DRS bands than in the case of unpromoted catalysts. Surface area was measured in a multipoint N2BET installation. For undoped catalyst 180 f 7 mZ/g surface area was found. For 0.05% Pt-Mg-loaded catalysts, areas of 33-40 m2/g were found. Specific activities for H2production reported in Figures 1 (traces a and b) are valid since this comparison is based on similar available surface area. BET results therefore show sintering upon Mgz+and FY loading. Figure 6 shows the results of the multipoint BET experiment where P is the absorbing pressure of N,, n the number of Nz molecules adsorbed on TiOz, and P,the saturation pressure of liquid N2 (731 torr). The adsorption heat (AH) liberated by the adsorption of N2 on the TiO, surface can be calculated by the expression: AH=AHc+RTInc (8) AHc is the enthalpy of condensation (1286 cal/mol) and c is the ratio of the intercept and tangent in Figure 6. The positive heat effect when N2is captured on the TiO, surface in the adsorption process is 2350 cal/mol. Conclusion
A detailed study has been presented on the effect of Mg2+doping on the behavior of TiOz under photolytic conditions. Mg2+-doped Ti0, has been described by a defect model that involves Vo", e', Oo, and Mg2+ acceptor. Mg-doping has been shown to enhance the water cleavage process and 0, photouptake. The most efficient catalyst was found to be 0.05% Pt-1% Mg2+-Ti02,showing a 0.26% efficiency in light energy conversion.2 The overall efficiency observed is low since both oxidized and reduced species are produced simultaneously at the Ti02-water interface in the course of the photolysis.
Acknowledgment. This study was supported by the Swiss National Science Foundation. We thank Dr. Panek of Bayer AG for the preparation of the catalyst samples and M. Buffat for his assistance with the electron microscopy work. Helpful discussions with Professor N. Eror of the Oregon Graduate Center, Beaverton, OR, are appreciated.
