ISSN 1063455X, Journal of Water Chemistry and Technology, 2012, Vol. 34, No. 1, pp. 17–23. © Allerton Press, Inc., 2012. Original Russian Text © E.E. Levitskaya, R.V. Prikhod’ko, V.V. Goncharuk, 2012, published in Khimiya i Tekhnologiya Vody, 2012, Vol. 34, No.1, pp. 28–39.

PYSICAL CHEMISTRY OF WATER TREATMENT PROCESSES

Synthesis and Investigation of Photocatalytic Activity of Lanthanum–Titanium Oxides Promoted by Metal–Oxide Systems Based of Copper and Silver E. E. Levitskaya, R. V. Prikhod’ko, and V. V. Goncharuk Dumanskii Institute of Colloid and Water Chemistry, National Academy of Sciences of Ukraine, Kiev, Ukraine Received September 2, 2011

Abstract—We have investigated factors affecting the formation of the structure and properties of active centers of catalysts of hydrogen synthesis of organic compounds on the basis of hybrid lanthanum–tita nium oxide systems based on copper and silver. It has been shown that the conditions for the preparation of synthesized materials substantially affect their physicochemical and catalytic properties. We have revealed main regularities for the formation of catalysts investigated under nonhydrothermal conditions. It has been determined that this process is affected by such factors as the concentration of organic com pounds, temperature, the pH of the reactive medium, the amount of the catalyst and the prompting phase. DOI: 10.3103/S1063455X12010031 Keywords: heterogeneous photocatalysis, synthetic titanium dioxide.

INTRODUCTION The concern with the pollution of the environment, restrictions of minerals and oil reserves compel the researchers to search for alternative sources for the production of energy as well as effective methods for the treatment of water of pollutants of various origin, in particular organic compounds. A promising are of con temporary science is combination of two different independent technologies: water purification of organic pollutants and hydrogen generation under the effect of light with the use of effective catalytic systems. In recent years the modern power system offers attracting possibilities of using hydrogen as fuel for transport and generation of electric power from fuel elements. Most of hydrogen is produced from natural gas and oil, which so far are the most spread and economically costeffective. Since the opening in 1972 of the reaction of photocatalytic destruction of water [1] throughout the world the production of hydrogen from it is not a subject of intensive research. Since that time the efforts of scientists have been aimed at making catalysts of this reaction converting light energy into a chemical reaction. How ever, the given process remains far from effective use since existing catalysts do not ensure its sufficient effi ciency. It causes the necessity of developing new catalytic systems. The other source of hydrogen is biomass and watersoluble products of vital activity of microorganisms [2, 3]. The objective of the present paper is the investigation of photoreforming of watersoluble organic alcohols and acids with the use of an ultraviolet radiation on hybrid catalysts of the system of the blended oxides of lan thanum and titanium with nanoparticles of copper and silver applied to their surface and also to determine the impact of some parameters affecting the rate of hydrogen formation, namely: he amount of metal, a photo catalyst in the reaction medium, the concentration of organic compounds, and the pH of the medium and temperature. EXPERIMENTAL Samples of titanium dioxide modified with lanthanum oxides were synthesized by the method close to one described in [4, 5]. Titanium tetraisopropylate (Ti(iPrO)4) in a blend with absolute isopropanol was used as a source of TiO2. This solution was topped up by drops, in intensive stirring, the reactor, having a water jacket, was additionally filled with a solution of sodium lanthanum in an amount necessary for obtaining a 0.5% solu tion in a water–isopropylene blend (1 : 20). After a complete blend of the components the temperature in the reactor was raised to 358 constantly adding 15 cm3 of water every 15 K. Upon achieving the set temperature the solution of NH4OH was added to the reactor to pH 9. Then the blend was stirred for 60 min and after that 17

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filtered and washed with distilled water until the pH neural value of washing waters. Further on synthesized samples were dried for 18 h in the air at 373 K. TiO2 synthesized in the given research as well as TiO2 P25 of the grade Degussa was modified with oxides of copper and silver according to the technique given in [6]. According to this technique driedout TiO2 was dispersed in 100 cm3 of ethylene alcohol with triblock copolymer (EO20PO70EO20, EO—ethylene oxide, PO—propylene oxide (BASF, Pluronic P123). The resultant blend was stirred for 15–20 min. Later on it was added (with intensive agitation) (O–2% of metal) with a solution of ammonium complexes of copper and sil ver. The process of photoreduction took place for one hour using incandescent lamp of 100 W. A solid phase that formed was filtered, washed thrice with ethanol and dried for 18 h in the air at 373 K. The Xray phase analysis (XRP) was conducted by means of Rigaku RINT2000 and DRON 3 M diffrac tometers (CuKα and CoKαradiation) in the range of the Brag angles (2θ) 5–80. Infrared research in the field of oscillations of the crystal lattice of synthesized materials (1500–200 cm–1) was conducted on an infra red spectrometer Specord M80 (Carl Zeiss, Jena, Germany). Electronic spectroscopy of diffusion reflec tion—on a Shimadzu 2401 spectrophotometer (Japan), BaSO4 standard. Adsorption isotherms–nitrogen desorption were obtained at 77 K by the use of an ASAP 2010 Micromer itics vacuum device (USA). Before doing the experiments the samples were calcined in vacuum for 5 h at 473 K, 10 Pa. Electronic microscopic pictures were obtained by means of an JEOLL 840 scanning electron microscopy integrated with a Tracor Northen Xray spectrometer (Japan). The reaction of photocatalytic hydrogen obtaining was conducted in a quarts reactor by means of a DRT240 highpressure mercury ultraviolet lamp by using the H2O blend—methanol at the concentration) to 1 mol/dm3 (the reactor volume: 1 dm3, the weighted sample of the catalyst: 0.1–3 g. The analysis of the gas blend of obtained products was carried out by means of a Tsvet500M chromatographer with a detector on heat conduction and a twometer column with Paropac Q. The catalytic process lasted for 50 h and after that the stirring of the reaction blend was terminated. RESULTS AND DISCUSSION The Xray diffractograms of mixed oxide La–TiO2 (Fig. 1a) contain basal reflexes (101), (110), (211), and (200) in the range of the Bragg angles (2θ) 20–70 deg., which are characteristic of titanium dioxide in the ana tas isomorphous form. Temperature treatment of obtained samples at 573 K and higher results in the formation of new isomor phous phase of rutile, which can be seen from the emergence of basal reflections (110), (111), and (221) (see

R(221)

R(101) Cu2O(100) R(111)

2

R(110)

A(101)

A(200) A(211)

Ag(200)

Ag(110) A(004)

I

A(101)

I

A(200)

1

A(004)

5 3

4

40 60 2θ, deg 20 30 40 50 2θ, deg (a) (b) Fig. 1. Impact of modification on crystalline structure of La–TiO2: a—Ag/La–TiO2 calcined at 573 K (1(1), 2 wt % (2)); b— Cu/La–TiO2, calcined at 873 K (1/2 wt % (3–5)). I—intensity, arb. units; 2θ—Bragg’s angle (deg.). 20

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Fig. 1b). In addition, to the Xray diffractograms shown, sharply defined reflexes formed by oxides of silver and copper are superimposed, which is an evidence of the presence on the surface of a catalyst. In infrared spectra synthesized there are absorption bands at 3300; 2400; 1680; 1200, and 780 cm–1 corre sponding to fluctuations of links of TiO2 structural fragments [7] and also to fluctuations of ν(CH)links of the structureforming agent at 2800–3000 cm–1. When the said material is calcined in the air current fluctuations disappear, which is caused by complete destruction of the organic phase. In addition, one observes a slight shift of the remaining bands, while their intensity effectively does not change (see Fig. 1). The obtained data are an evidence of the absence of a disruption in the ordered structure of synthesized La–TiO2 as a result of intro ducing of a modifying additive in the form of lanthanum oxide. The adsorption isotherm of N2 on calcined La–TiO2 ought to be referred to type IV according to the IUO PAC classification, which characterizes its texture as mesoporous. However, a part of the porous structure of this material is formed by micropores. The results obtained agree with the literature data [5, 7] and make it possible to come to a conclusion that the synthesized material presents a well ordered mesostructured La–TiO2. The introduction of the phase of oxides of silver and copper does not cause a noticeable change of reflexes in the range of small angles. It means that mesoporous structure dies not undergo a substantial deformation in synthesis of catalysts and confirms the correctness of the choice of the said material as a carrier of the active phase. In the range of the Bragg angles (2θ) 30–50 deg. for calcined catalysts one can observe the reflexes of the introduced oxide phase. Since these reflexes are intensive and narrow one may state that the oxide phase formed from relatively large crystals located mainly on the external surface of the carrier particles. Synthesized catalysts contain aggregates formed by flat particles with the size equal to 5–10 nm (Fig. 2). Ag/LaTiO2

Cu/LaTiO2

Ag

Cu

50 nm

50 nm

(a) (b) Fig. 2. Microphotographs of La–TiO2 samples obtained by means of scanning electron microscopy: a—Ag/La–TiO2, b— Cu/La–TiO2.

The oxide phase of the promoting oxide is localized on the surface of these aggregates, which agrees with the data of the Xray phase analysis of the composition of the samples Ag/La–TiO2 and Cu/La–TiO2. As can be seen from the table the introduction of copper oxide into titanium dioxide results in a substantial change of the textural parameters of the later. As a result of this process the specific surface and volume of the pores of the carrier decrease, which may be determined by blocking mesopores with the oxide phase located on the external surface of crystallites. The oxide phase of the promoting oxide is localized on the surface of these aggregates, which agrees with the data of the Xray phase analysis of the composition the samples Ag/LaTiO2 and Cu/La–TiO2. As can be seen from the table, the introduction of copper oxide to titanium dioxide results in a substantial change of the textural parameters of the latter. As a result of this process the specific surface and the volume of the pores of the carrier decrease, which can be determined by the blocking of the mesopores by the oxide phase located on the external surface of crystallites. JOURNAL OF WATER CHEMISTRY AND TECHNOLOGY

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Parameters of the porous structure of synthesized samples Vpr

SBET, m2/g

Sample

cm3/g

259 346 150

LaTiO2 Ag/LaTiO2 Cu/LaTiO2,

Vmicro

0.3 0.3 0.3

0.003 0.004 –

According to the data obtained by means of the Xray phase analysis synthesized catalysts have a developed specific surface and are formed from the phase of anatase (~ 75%). Dispersion (D) of the applied phase of the samples with the content 0.5; 1, and 1.5 weight % of silver is within the range 70–75%, which was calculated by the expansion of basal reflexes and corresponded to the average size of crystallite equaling 1.4–1.6 nm. Cat alysts with a higher amount of the metal phase on the carrier surface are characterized by a lower dispersion and larger size of crystallites, i.e., D = 52% (dAg = 2.0 nm) for the 2% Ag/La–TiO2. As is known the yield of the electron–hole photogenerated pair depends, first of all, on intensity of pho tons; their energy should be equal or larger than the energy of the prohibited zone TiO2. The effect of the impact of the introduced silver and copper on the energy of the prohibited zone of TiO2 was investigated by means of spectroscopy of diffusion reflection. The comparison of the spectra and reflection coefficient of pure La–TO2 and Ag/La–TiO2 points to the fact that the energy of the La–TiO2 band and for the samples contain ing silver and copper did not change, while the prohibited zone of the particles constituted 3.1 eV. From these data one may come to a conclusion that the photocatalytic activity of the particles of silver and copper is not an outcome of the change of the energy of the band of the prohibited zone of TiO2. Spectra of diffusion reflec tion of the samples Ag/La–TiO2 and Cu/La–TiO2 contain bands at 360; 480, and 600 nm, which refer to octahedrally coordinated cations of silver and copper due to electron junctions. In the spectra of samples dur ing the change of concentration one can observe a clear change in the intensity of the reflection band at 360 nm and the shoulder at 480 nm, which refer to junctions typical of silver and copper (Fig. 3). R, %

R, %

4 5 1

6

2

3

300 450 600 750 λ 600 750 λ (a) (b) Fig. 3. Electronic spectra of diffusion reflection (R): a—Ag/La–TiO2 with silver concentration 0.5 (1), 1.0 (2), and 2.0 wt % (3); b—Cu/La–TiO2 with copper concentration 0.5 (5), 1.2 wt % (6); 4—pure La–TiO2. λ—wavelength (nm). 300

450

For integral understanding of the processes taking place on the surface of the catalysts one proposed a sche matic representation of a hypothetical reaction mechanism of transformation of water–methanol blends into hydrogen and carbon dioxide (Fig. 4). For clearing up the reality of this schematic, in the first place, investigations on the impact of the amount of methanol of photocatalytic activity were carried out. The investigation results of the hydrogen formation rate over time in absence and in the presence of methanol (0.738 mmol) in the solution demonstrated that in JOURNAL OF WATER CHEMISTRY AND TECHNOLOGY

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the last case the hydrogen formation rate goes through the maximum 0.35 μmol/min for 40 min, which is more than by an order of magnitude compared with obtained data for pure water.

Fig. 4. Schematic representation of the processes taking place on the catalyst surface.

Prolonged radiation is conducive to a decrease of the hydrogen formation rate, which, in the long run, results in values comparable with such obtained in the absence of methanol. The production of H2 is accom panied b the formation of CO2, whose rate also passes the maximum approximately 75 min after and then gradually decreases over time. The obtained results may be explained by the fact that methanol acts as a “sac rificial” agent rapidly removing a hole being photogenerated and oxidants (e.g., OH and oxygen) obtained from water [8–15]. This leads to the suppression of the electron–hole recombination and a decrease of the O2 + H2 reverse reaction rate, i.e., processes, which reduce the efficiency of converting light into hydrogen [13– 15]. When the “sacrificial” agent is oxidized completely into CO2, the hydrogen formation rate decreases to the values comparable with ones obtained in the absence of methanol. Qualitatively selected results were obtained by the use of different concentration of methanol in the range 0.2–1000 mmol. In all the cases the amount of H2 and CO2 is in good correlation with the theoretical yield calculated by stoichiometry of the reac tion (the molar ratio H2 : CO2 = 7 : 3). A number of catalysts was investigated with the content of the introduced metal in the range 0–2.0 wt %. One can note that under existing experimental conditions fabrication of H2 and CO2 insufficiently changes compared with pure L–TiO2. This is an indication of the fact that the presence of nanocrystallites of silver and copper is a prerequisite for the photoconversion reaction. The H2 formation rate lasts 0.1 μmol/min at the maximum for 40 min and then gradually decreases. An increase of the amount of introduced metal up to 1.0 wt % results in a growth of the rate maximum up to 0.2 μmol/min. A further increase of the amount of silver and copper to 2 wt % does not result in an increase of the hydrogen formation rate. Optimal photocata lytic activity for the reaction of obtaining hydrogen from the water–methanol blend is possible on samples with the content of active phase 1.0–1.5 wt 5%. An increase of photocatalytic activity when introducing a metal phase is related to the ability of metals to retain the electron–hole recombination [7, 16]. In addition, silver and copper are effective catalysts of oxidation and participate in the reaction of oxidizing methanol into intermediate products of photogenerated oxygen. The relationship of the reaction rate being observed occurs due to the change in the number of active centers located on the metal/substrate surface. The effect of the concentration of a photocatalyst was determined on aqueous solutions containing meth anol in the amount 0.368 mmol/dm3 and metal on the La–TiO2 surface—1.0 wt %. The amount of the cata lyst was varied within the interval 0–3 g/dm3. In the absence of a photocatalyst in the gas phase H2 and CO2 were not detected. The introduction of 1 g/dm3 of the photocatalyst led to a substantial increase of the yield of H2 and CO2. Further increase of the content of the photocatalyst up to 23 g/dm3 was conducive to a growth of the initial rate of the formation of H2 and CO2. JOURNAL OF WATER CHEMISTRY AND TECHNOLOGY

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At an increased concentration of methanol in the range 0–1000 mmol/dm3 there happens a monotonic increase of the formation rate of H2. For relatively low concentrations of methanol (< 1 mmol/dm3) the rate in the long run decrease to the values comparable with ones obtained in pure water, which points to the com plete transformation of methanol into H2 and CO2 and is completely corroborated by the theoretical calcu lated. An increase of the pH from 3 to 8 results in a substantial increase of the initial rate of hydrogen formation from 0.2 to 0.4 μmol/min–1 and an increase of the rate of the photoconversion reaction. Further increase of the pH of the solution to 10 does not cause a substantial increase of the rate compared with pH 8. This rela tionship may be explained by several reasons [10, 11, 17]: the impact of the pH on the position of the valence zone and the level of the semiconductor conduction band with respect to oxidation–reduction pairs in the solution; by the charge on the semiconductor surface; chemical transformations of the matter in the solution and by the change in the size of conglomerates of the photocatalyst particles that have taken shape. The impact of the solution temperature on photocatalytic activity was investigated in the range 310–360 K. Heating from 310 to 333 K results in an increase of the maximum of the hydrogen formation rate. A further increase of the temperature to 553 K leads to a small growth of the reaction rate. Thus, such an increase of the reaction rate at T = 333–353 K (compared with T = 313 K) can be explained by the effect of temperature on the “shadow” stages of the reaction. They include adsorption–desorption equilibrium of the reagents and reaction products, stabilization of intermediate products, the exits off the surface of an adsorbed particle, etc. [10]. It should be noted that temperature affects heat catalytic ways of the reaction taking place on the surface of the semiconductor and on the surface of crystallites of the metal such as oxidation of methanol and inter mediate reaction products. As can be seen from Fig. 5 the energy of activation in a small degree differs from the data in [18]. It does no depend on such factors as the intensity of UVradiation, the pH of the medium, the amount of the cata lysts, however, in great degree correlates with TiO2 photoconduction. 333 K

323 K

2.5

313 K

303 K

1.5 –1

–1

–14.10

1.0

ln, k/mol s

Hydrogen yield, mmol

2.0

0.5

EA = 5.73 kJ mol

–14.15 –14.20 –14.25 –14.30 0.0030 0.0031 0.0032 0.0033 –1 1/T, K

0

20

40 Reaction time, min

60

Fig. 5. Relationship between the hydrogen yield at different temperature of the reaction medium and the activation energy of this process on the Ag/La–TiO2 catalyst.

For clearing up the efficiency of the operation of the catalyst of Ag/La–TiO2 in systems involving partici pation of watersoluble organic compounds additional experiments were carried out and the following regu larities were obtained: maximum amount of hydrogen is observed in the methanol–water system, then meth anol > ethanol > isopropanol > ethylene glycol > butanol–glycerin > acetaldehyde > vinegar.

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CONCLUSIONS Thus, based on the above it was shown that modification of titanium dioxide by lanthanum ions makes it possible to obtain active catalysts of the photolysis reaction of aqueous–alcohol systems. The activity of these catalysts depends on the content of silver, copper, and the values of the specific surface. The presence of meth anol is conducive to the growth of the reaction rate and an increase of hydrogen yield. The catalysts based on La2O3–TiO2 obtained by way of coprecipitation under conditions of the controlled hydrolysis with the con tent of 1% of silver are the most active catalysts for obtaining hydrogen from the water–organic compounds system. Effective photocatalytic systems have been developed for water treatment and decomposition pro cesses in simultaneous generation of hydrogen under the effect of light. We have investigated factors affecting the formation of the structure and properties of the active centers of catalysts of Ag/La–TiO2, Cu/La–TiO2. The relationship between conditions of preparing synthesized materials and their physicochemical and cata lytic properties, has been revealed. The hydrogen yield depends on the initial concentration of organic compounds and increases roughly by two orders of magnitude under conditions of increasing the concentration of organic compounds from 0 to 1 mol. Activity of these catalysts depends on the content of metal, the pH, and the temperature of the reaction medium. The reaction rate increases at neutral and alkaline values of the pH of the solutions and an increase of the temperature from 40 to 80°C. REFERENCES 1. Fujishima, I. and Honda, K., Nature, 1972, vol. 238, pp. 37–38. 2. Mohan, D. and Pittmann, C.U., Energy and Fuel, 2006, vol. 20, pp. 848–889. 3. Fundamntal’ni problemmy vodneboyi energetyky (Fundamental Issues of Hydrogen Power Engineering), Pokhodenko, V.D., Skorokhod, Yu.M., and Solonin, Yu.M., (Eds.), Kiev: KIM, 2010. 4. Zalas, M. and Laniecki, M. , Solar Energy Materials and Solar Cells, 2005, vol. 89, pp. 287–296. 5. Gnanaskar, K.I., Subrmanian, V., and Robinson, J., J. Material Res., 2002, vol. 17, pp. 1507–1512. 6. Zhan, F., Guan, N., Zhang, Y., Li, X., Chen, J., and Zeng, H., Langmuir, 2003, vol. 19, pp. 8230–8234. 7. Luo, N., Fu, X., Cao, F., Xiao, Edwards, P.P., Fuel, 2008, vol. 87, pp. 3483–3489. 8. Choi, H.J. and Kang, M., Int. J. Hydrogen Energy, 2007, vol. 32, pp. 3841–3848. 9. Nada, A.A., Barakat, M.H., Hamed, H.A., Mohamed, N.R., and Veziroglu, T.N., ibid., 2005,vol. 30, pp. 687–691. 10. Ni, M., Leung, M..K.H., Leung, D.Y.C., and Sumathy, K., Renewable and Sustainable Energy Rev., 2007, vol. 11, pp. 401–425. 11. Park, M.S. and Kang, M., Materials Lett., 2008, vol. 62, pp. 183–187. 12. FuentesPerujo, D., SantamariaGonzalez, J., MeridaRobbles, J., et al., J. Solid State Chem, 2006, vol. 179, pp. 2182–2189. 13. Garcia, R., Besson, M., and Gallezot, Appl. Catal., A., 1995, vol. 127, pp. 165–176. 14. Sasiharan, M. and Kumar, R., J. Mol. Catal., A., 2000, vol. 210, pp. 93–98. 15. Garg, S., Soni, K., Rumaran, G., et al., Catal. Today., 2008, vol. 12, p. 275. 16. Yamada, T., Zhou, HS., Hiroshi, D., Tomita, M., Ueno, Yu., Asi, K., and Honma, I., Adv. Materials, 2003, vol. 15, no. 6, pp. 511–513. 17. Fernandez, J.M., Barriga, C., Ulibarri, M.A., Labajos FM., Rives, V., Chem. Materials, 1997, vol. 9, pp. 312–318. 18. Alejander, A., Medina, F., Rodriguez, X., Salagre, P., and Sueiras, J.E., J. Catal., 1999, vol. 188, pp. 311–324.

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