1634
Ind. Eng. Chem. Res. 1991,30, 1634-1638
Brindley, B. D. The Effect of Grain or Particle Size on X-Ray Reflections from Mixed P o d e r a and Alloys: considered in to the Quantitative Determination of Crystalline Substances by X-Ray Methods. Philos. Mag. 1945,6,347-369. Colombo, U.; Gazzarrini, F.; Lanzavecchia, G. Mechanism of Iron Oxide Reduction at Temperatures below 400 OC. Mater. Sci. Eng. 1967,2,125-135. Cullity, B. D. Elements of X-Ray Diffraction; Addison-Wesley: Reading, MA, 1978; pp 132-135. Levenspiel, 0. Chemical Reaction Engineering; Wiley: New York, 1972;pp 357-377. McKewan, W. M.Kinetics of Iron Oxide Reduction. Trans. AZME 1960,218,1-6. Nix, R. M.; Rayment, T.; Lambert, R. M.; Jennings, J. R.; Owen, G. An in Situ X-Ray Diffraction Study of the Activation and Performance of Methanol Synthesis Catalysts Derived from Rare
Earth-Copper Alloys. J. Catal. 1987,106,216-234. Taylor, A.; Sinclair, H. The Influence of Absorption on the Shape and Positions of Lines in Debye-Scherrer Powder Photographs. h o c . Phys. SOC.1945,576,108-125. Themelis, N. J.; Gauvin, W. H. A G e n e r a l i i Rate Equation for the Reduction of Iron Oxides. Trans. AZME 1963,227,290-300. Thomson, W. J. Dynamic X-Ray Diffraction: A Technique for High Temperature Ceramic Processing. Ceram. Trans. 1989,5,131. Walker, A. P.; Rayment, T.; Lambert, R. M. A Controlled Atmosphere in Situ X-Ray Diffraction Study of the Activation and Performance of Ammonia Synthesis Catalysts Derived from CeRu,, CeCo,, and CeFe,. J. Catal. 1989,117,102-120. Received for review July 31, 1990 Revised manuscript received January 22, 1991 Accepted February 4, 1991
Hydrogen Evolution over a Powdered Semiconductor Photocatalyst Toshiro Maruyama* and Tadashi Nishimoto Department of Chemical Engineering, Faculty of Engineering, Kyoto University, Kyoto 606,J a p a n
Hydrogen evolution from a liquid water-methanol mixture that dispersed with semiconductor photocatalyst Pt/Ti02 (anatase) powders was studied experimentally. The hydrogen evolution rates under various conditions are compared, and the mechanism involved in the rapid degradation of the activity of the catalyst is suggested on the basis of some factors that affect the activity of the catalyst. The rates of hydrogen evolution over catalysta of various shapes, i.e., colloid, porous particle, and thin film, are compared with respect to the morphology of the catalysts. A recommendation is made about the use of the porous catalyst, which has a surface area larger than ita light-absorbing area and maintains a high catalyst activity over a long period of time. Hydrogen evolved in the solar collector in response to solar power with about a 15-min time lag.
Introduction Recently, photocatalysis using a semiconductor powder has attracted attention from the viewpoint of solar energy storage in the form of chemical bonding potential energy via a positive change in free energy. It is also attractive in ita application to the synthesis of valuable organic substances even if the change in potential energy is negative (Kawai and Sakata (1980)). In this study, photochemical hydrogen production from a water-methanol mixture dispersed with a semiconductor photocatalyst, platinized titania (anatase) Pt/Ti02, will be described. On the basis of the experimental hydrogen evolution rates under various conditions and morphologies of catalysts, some factors affecting the activity of the catalyst and the morphology of the catalyst, which maintains a high rate of the hydrogen evolution over a long period of time, will be discussed. Experiments Undoped n-type titania powder (Merk anatase) was used as a catalyst in the main part of the experiments. It was partially covered with platinum ( 5 wt % ) using the photoreduction method (Kraeutler and Bard (1978)). The platinized catalyst was crushed in a mortar (the median particle diameter was 3.2 pm and specific surface area was 9.27 m2/g). A known amount of catalyst taken from the same batch was used in a series of experiments. Figure 1shows the schematics of the experimental apparatus. A Pyrex photolysis cell of about 700 cm3 in volume was internally irradiated with an unfiltered 500-Whigh-pressure mercury lamp, which emits intense UV light (wavelength X .( 300 nm; cut by Pyrex glass). To maintain a constant reaction temperature, cooling water was circulated from a temperature-controlling bath to jackets situated in the outer surfaces of the mercury lamp and reactor. The re0888-5885/ 91/ 2630-1634$02.50/0
action temperature was measured with a thermometer or a thermocouple inserted into the cell. For the dispersion of the catalyst particle and the mixing of the reacting fluid during the experiment, the slurry was thoroughly stirred by a magnetic stirrer at the bottom of the reactor and recirculated with an external pump to cause an upflow in the reactor. At the beginning of a run, the cell was filled with a methanol-water mixture (500-650 cm9)and catalysts. Argon gas was bubbled through the suspension for 20 min to eliminate dissolved oxygen. The gas that evolved was bubbled through an aqueous solution of Ba(OH)2to remove C02,and then, the gas was bubbled through water to collect Hp A glass electrode was situated at the external circulating pipe to measure the pH of the reacting liquid. A quantitative analysis of the reaction products was done by using a gas chromatograph and a total organic carbon meter. The particle diameter, the reduced surface area, and the pore distribution of the catalyst are measured by centrifugally depositing particle-diameter distribution measuring equipment, an automatic surface-areaanalyzer, and a mercury porosimeter.
Results Reaction Products. The gaseous products were H2 v d COP. H20, CH,OH, and H2 COSwere detected in liquid phase. The amounts of the other byproducts of oxidation of CH30H, i.e., HCHO and HCOOH, were below the detection limits. Hydrogen Evolution Rate. Figure 2 shows typidal profiles for H2 evolution. The H2 evolution rate rapidly turns into a lower value (t > 400 min) preceded by maintaining a high rate for a certain time from the start of light irradiation. After that (t > 800 min), the lower rate of H2 evolution remained nearly constant over a long period. The time it took for the change to occur was dominated Q 1991 American
Chemical Society
Ind. Eng. Chem. Res., Vol. 30, No.7,1991 1636
1100-
High -pressure Hg l a m p
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Figure 3. Temporal Variation of H&03 concentration and hydrogen evolution rate.
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Figure 2. Amount of hydrogen evolved and hydrogen evolution rate.
by the total amount of illumination time, but it was not affected by introducing either interruptions in irradiation or renewals of the solution. The H2evolution rate was obtained under the following conditions: a catalyst particle concentration of 4 g/L; a methanol concentration C of 2.5-22.3 M; a reaction temperature T of 25.4-53.4 OC; and d[H2]/dt = kCMCHnexp(-E/RT) mol/min (1) Before the change in the rate k = 2.1 X lom3, E = 19.6 kJ/mol, n = 1 (2) After the change in the rate k = 3.45, E = 37.6 kJ/mol,
-0.0
0 1000
n = 0.5 (3) The rate of C02 production was 0.6-5.070 of that of H2 production. Figure 3 shows an example of the temporal variation in the concentrationof H&03 in comparison with the H2evolution rate. The concentration of HZCO3 shows a higher (1.5-2 times) level after rapid change (t > 400 min) when the concentration of C02 increased in the solution and the pH of the solution decreased slightly. The following are the results for a methanol concentration of 12.4 mol/L (equivolume mixture of methanol and water), which showed the highest H2 evolution rate. All the experimenta were carried out at room temperature. Effects of a n Additive. The initial rate of the H2 evolution depends on the pH of the solution as shown in Figure 4. The yield of H2decreases at both higher (NaOH addition) and lower (H2S0, addition) pH values. When a trace quantity of NaOH was added to the solution during UV irradiation, the Hz evolution rate decreased. Furthermore, the neutralization of this Solution by adding HCl enabled the return of the evolution rate to near that of the
1 3 5 7 9 1 1 1 3 PH [-I
Figure 4. Hydrogen evolution rate as a function of the pH of the solution. Q2
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Figure 6. Temporal variation of the hydrogen evolution rate and the pH of the solution.
original solution. Since titania is an amphoteric oxide, the reaction solution changes from neutral to acidic (pH 3.3-4.8) when the catalyst was added to the solution, and the value of the pH decreases slightly as the reaction proceeds. As shown in Figure 5, however, the temporal variation in pH during a run is too small to affect the H2 evolution rate. Figure 6 shows the effects of reaction products on the H2evolution rate. The addition of HCHO or C02does not change the H2evolution rate and time to an abrupt change in the H2 evolution rate. The addition of HCOOH, however, decreases the time of an abrupt change, as shown in Figure 6. Effect of Treatment of the Catalyst. The activities of a powdered catalyst were measured after the following three kinds of pretreatment: (i) crushing in a mortar (the median particle diameter was 3.2 pm, and the specific surface area was 9.27 m2/g); (ii) irradiation by ultrasonic waves for 10 min in solution (the median particle diameter
1636 Ind. Eng. Chem. Res., Vol. 30,No. 7, 1991 I
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i
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Figure 8. Hydrogen evolution rate after varioue treatments for reactivation. Fresh catalyst (- - -), deactivated catalyst before treatment (0)and after treatments: washing with water (4, HCOOH ( 0 )and methanol (A); crushing in mortar (V); heating under low , dipping in water after heating under low preasure pressure ( e )and (0).
3wt
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Figure 7. Amount of hydrogen evolved with and without irradiation of an ultrasonic wave.
was 0.48 pm); (iii) dipping in water for a long time (300 h). It was observed that these kinds of pretreatment increased the fraction of small particle diameters. In H2 evolution, the treated catalyst maintained initial high activity over a longer period without resulting in any real improvement in the H2 evolution rate. The effects of irradiation by ultrasonic waves are shown in Figure 7. The degraded catalysts were treated in the following ways to recover the initial activity: (i) irradiation by ultrasonic waves in the reaction solution; (ii) washing with distilled and deionized water, methanol, and HCOOH; (iii) heating at 250 "C under low pressure. Figure 8 shows the results. Washing with water or HCOOH is effective in returning activity to its original level. On the other hand, washing with methanol and irradiation of the catalyst immersed in reaction solution by ultraeonic waves are not effective. Heating at 250 "C under low preaaure lowered the catalyst activity, but it can be recovered by dipping the catalyst in water. Effects of Concentration and Dispersion of the Catalyst. Figure 9 shows the H2evolution rate for various concentrations of catalyst particles. The initial H2 evolution rate does not show a dependence on concentration, because it is photon-flux limiting under the experimental conditions in this study. However, the period of duration depends on the concentration, and it corresponds to the time when the amount of evolved Hzper unit mass of catalyst takes a constant value, as shown in Figure 10. The degradation of the catalyst for a higher concentration of particle (>7g/L) begins at a shorter radiation time. Probably, the degradation is accelerated by the possible nonhomogeneous dispersion of the catalyst in the photo-
0
100 200 300 LOO 500 600 700 800 900 1000 t
lminl
Figure 9. Amount of hydrogen evolved for varioue concentrations of catalyrrt particles.
0
100 200 300
a0 t
500 600 700 800 900 1000 [minl
Figum 10. Amount of hydrogen evolved per unit of concentration of catalyst.
lysis cell at the higher concentration of particles. Effect of the Morphology of the Catalyst, Besides titania powder, a colloid, porous particles, and a thin film were prepared to study the morphology of the catalyst that is effective in maintaining a high catalyst activity over a long period. The colloid catalyst was made from titanium alkoxide (Dounghong et al. (198111, and the porous catalyst was prepared from titanium chloride by using the inorganic method (Kruczynski et al. (1981)). In the preparation of the thin-film catalyst, a thin film of titanium dioxide was deposited on a glass bead substrate of 1-mm particle di-
Ind. Eng. Chem. Res., Vol. 30, No. 7, 1991 1637 300 c
A
d
E
Sol-Gel Mcthod(HzPtCl6) Sol-Gel MethodIPt black) Inorganic Method(H2PtCle)
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Porous
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500
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Figure 11. Amount of hydrogen evolved by using a colloid catalyst.
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.,
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Figure 13. Amount of hydrogen evolved by using a thin film catalyst coated on glass beads.
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500
1000 t
1500
2000
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Pt K C i Solution Clark Type Hydrogen Electrode
Figure 12. Amount of hydrogen evolved by using a porous catalyst.
Figure 14. Solar collector and a Clark-typehydrogen electrode.
meter by using either the sol-gel method or the inorganic method (Mukherjeeand Loedermilk (1982)). Platinization occurred when H2PtC& or Pt black was added to the coating solution or by reducing H2PtCbon the coated film. Although there are some difficulties in preparation of the colloid catalyst, deposition of platinum and recovery from the solution, it has the advantages of resulting in a large surface area and maintaining suspension without adding any mechanical energy. As shown in Figure 11,the colloid catalyst showed a H2evolution rate lower than the powdered catalyst, and in the experiment, the colloid suspending solution, which was colored platinum black before irradiation, turned blue. The medium diameter and the surface area of the porous catalyst were 26 pm and 78.2 m2/g, respectively, which were 1order of magnitude larger than the powdered catalyst. As shown in Figure 12, H2 evolution rate using the porous catalyst showed the catalyst-particle concentration limiting process (C, < 4 g/L) in addition to the photonflux limiting process (C, 1 4 g/L), because the porous catalyst used in this study has a larger particle diameter and consequently has a smaller light-absorbing area at a fiied particle concentration. In the photon-flux limiting process, the H2 evolution rate is about 20% higher than that of the powdered catalyst. Since the porous catalyst has a larger surface area compared to the light-absorbing area at fixed particle concentration, it maintains a high catalyst activity over a very long irradiation period, which is longer than the powdered catalyst. In addition, the decrease in the H2 evolution rate after the rapid change in catalyst activity is relatively small. Figure 13 shows temporal variations in the amount of H2 produced by using thin-film catalysts prepared by various methods. Among catalysts of the various morphologies used in this study, the thin-film catalyst showed the highest H2 evolution rate per unit mass of titanium oxide, because the catalyst is effectively localized at the
surface of the supporting particle. However, the H2 evolution rate per unit reactor volume was very low, because the particles coated by thin-film catalysts were packed in the reactor to form the packed bed. The H2evolution rate showed no decay in catalyst activity under the experimental condition of this study. Solar Energy Conversion. Figure 14 shows the schematics of the solar reactor and the hydrogen electrode used in the experiments for solar energy conversion. A parallel-plate light collector of 0.135-cm2surface area and 1-cm depth was made from UV transparent acrylic resin, and it was then exposed to sunlight. The surface of the collector was directed south at slope of 36O. An equivolume methanol-water mixture (about 2 L) dispersed with the powdered catalyst (4 g/L) flowed uniformly upward and was circulated by an external circulating pump. The evolved H2 gas was entrained by an inert carrier gas (argon), and the temporal variation of the H2concentration was measured by using the Clark-type hydrogen electrode (Griiniger et al. (1978)). The variation with time of the effective solar power was monitored by measuring the light intensity in the wavenumber range 310-400 nm with a UV power meter. As shown in Figure 15, the H2evolution rate (output of Clark-type hydrogen electrode, VH,)fluctuated in response to the output of the UV meter (I) and consequently to the effective power of the solar energy. It is noted that there appears to be a prominent time lag of about 15 min. between the variations of solar energy and the H2 evolution rate. Discussion The degradation in the rate of H2 evolution by heating the catalyst or by changing the pH of the solution is attributable to the change in the quantity of OH radical adsorbed on the surface of the catalyst. The incipient H2 evolution rate is not affected by the particle diameter, the
1638 Ind. Eng. Chem. Res., Vol. 30,No. 7, 1991 in Kyoto 1985 Fchl8
L
"t
0
Time
Figure 16. Example of temporal variations in radiation energy and output of hydrogen electrode.
concentration, or the dispersion of the catalyst when the reaction proceeds under a photon-flux limiting condition. However, the duration of this process is largely affected by these facton. Decreasing the catalyst particle diameter and consequentlyincreasing the surface area lengthens the duration of initial high evolution rate, and the rate rapidly drops when the total volume of the evolved H2 per unit mass of catalyst amounts to a constant value. These facts suggest that a mechanism of the rapid degradation of the activity of the catalyst is as follows. Some chemical species are generated during the oxidation process of HCOOH, and they are adsorbed on the surface of catalyst. When the quantity of the adsorbed species becomes large enough so that an electric charge at the surface of the suspending particle is counterbalanced, the particle agglomerates to make the particle diameter larger within a short time. After that, the evolution rate depends on particle concentration, and the reaction mechanism changes. With respect to the pretreatments of a fresh catalyst which can extend the period of incipient high activity of the catalyst, ultrasonic radiation and dipping in water are effective due to an increase in catalyst surface area by increasing the smaller particle diameter by a fraction. Ultrasonic radiation is not effective in treatments for catalyst reactivation because it cannot remove the adsorbed species so that the dispersed particles agglomerate again when radiation is ceased. On the other hand, the effectivenessof washing the deactivated catalyst with water or HCOOH is attributable to possible desorption of the adsorbed species. The experimental results on the dispersion of the catalyst suggest that the rate of agglomeration is much faster in localized dispersion than in homogeneous dispersion. On the basis of the above considerations, the porous catalyst is effective in offering a large catalyst surface area with a relatively low particle concentration. However, no conclusions about the colloid catalyst can be made from the experimental results of this study because coloration with light irradiation indicated that the colloid catalyst that included hydroxytitanium used in the study was not anatase. It was inferred that the thin-film catalyst coated on small supporting particles was effective in retarding the agglomeration of catalyst particles in addition to its effectiveness in the utilization of catalyst mass. However, the supporting particle used in this study was 80 large that it formed a packed bed of catalyst that immobilized the irradiated surfaces and consequently resulted in a low H2 evolution rate. As a result, a recommendation is made to
use a porous catalyst that has a larger surface area than the light-absorbing area. The large time lag obtained in solar radiation experiments is attributable to the time that elapsed from the evolution of the H2 bubble to the detachment from the catalyst surface. In a particle suspending system, the H2 bubbles attached to the catalyst particle accelerate the rotational motions of the particle and enhance the renewal of the irradiated surface of the catalyst. In the fmed bed, however, the attached H2 bubble prevents the contact of the reactant solution with the catalyst surface and consequently lowers the H2evolution rate. This clearly shows the other advantage of the dispersion of the catalyst particle, in addition to its large surface areas advantage. Conclusion The incipient hydrogen evolution reaction to a powdered catalyst proceeds under a photon-flux limiting condition, but the duration of this process is largely affected by the particle diameter, the concentration, and the dispersion of the catalyst. Decreasing the catalyst particle diameter and consequently increasing the surface area lengthens the duration of the initial high H2 evolution rate. The rate rapidly drops when the total volume of hydrogen evolved per unit mass of catalyst is a constant value. On the basis of the suggested mechanism of the abrupt degradation of the activity of the catalyst, a recommendation was made on the use of the porous catalyst. The porous catalyst is able to maintain high catalyst activity for a long period of time because it has a larger surface area compared to the light-absorbingarea at a fixed particle concentration. In a solar collector, the hydrogen evolution rate changes in response to the effective power of solar energy. There is a prominent time lag between variations in the solar radiation power and the hydrogen evolution rate. Nomenclature Cat= concentration of catalyst, g/L CH = concentration of water, g/L CM = concentration of methanol, g/L E = activation energy, kJ/mol I = light intensity, mW/cm2 kI = rate constant of reaction (initial),L/mol.min kL = rate constant of reaction (later), L/mol.min T = temperature, O C t = time, min VH, = output potential of hydrogen electrode, rV Registry No. HO,1333-74-0; Pt, 7440-06-4;TiOz,13463-67-7; CH30H, 67-56-1;H20, 7732-18-5;NaOH, 1310-73-2;H2S04, 7664-93-9;HCHO, 50-00-0; COZ, 124-38-9;HCOZH, 64-18-6.
Literature Cited Duonghong, D.; Borgarello, E.; Gratzel, M. Dynamics of Light-Induced Water Cleavage in Colloidal System. J . Am. Chem. SOC. 1981,103,4685-4690. Griiniger, H.-R.; Sulzberger, B.; Calzaferri, G. Clark-Like Hydrogen Detector. Helv. Chim. Acta 1978,61, 23752380. Kawai, T.; Sakata, T. Photocatalytic Hydrogen Production from Liquid Methanol and Water. J. Chem. SOC.Chem. Commun. 1980,24,694-695. Kraeutler, B.; Bard, A. J. Heterogeneous Photocatalytic Synthesis of Methane from Acetic Acid-New Kolbe Rsaction Pathway. J. Am. Chem. SOC.1978,100,2239-2240. Kruczynski, L.; Gesser, H.D.; Turner, C. W.; Speers, E. A. Porous Titania Glass as a Photocatalyst for Hydrogen Production from Water. Nature 1981,291,399-401. Mukherjee, S.P.; Loedermilk, W. H. Gel-Derived Single Layer Antireflection Films. J. Non-Cryst. Solids 1982,48, 177-184. Received for review Auguet 27, 1990 Revised manuscript received January 22, 1991 Accepted February 13, 1991
