MICROSTRUCTURE OF MESOPOROUS TiO2-Al2O3 MIXED OXIDE AS A PERSPECTIVE PHOTOCATALYSTS

I. Sizeneva, I. Lebedeva, D. Kiselkov, V. Valtsifer

Institute of Technical Chemistry, UB RAS, ak. Koroleva, 3, Perm, Russia

Abstract

Two series of mesoporous TiO2-Al2O3 mixed oxides with various Ti:Al atomic ratios and pure TiO2 and Al2O3 mesoporous oxides were prepared by means of co-precipitation method with followed calcination. Evolution of phase composition, morphology, microstructure, and texture properties was investigated with use of X-ray diffraction, scanning electron microscopy and low-temperature N2 physisorption. The effect of precipitator type on microstructure, textural properties and on photocatalyst activity of TiO2-Al2O3 mesoporous mixed oxides was observable.

Introduction

Titanium dioxide (TiO2) is a photocatalyst widely used due to it redox properties, and high resistance to photo-corrosion. Moreover, TiO2 is inexpensive, non-toxic, chemically stable compound available in large amounts. However, its photocatalytic efficiency is limited due to recombination of photo generated electrons and holes [1, 2]. To overcome this problem, many attempts were made with use of doping, metal coating, surface sensitization, combined operations, increased surface area, porosity, or crystallinity. Recent findings on this subject are summarized in comprehensive reviews [3-5].

A strategy we investigate in with use of TiO2 photocatalysts is preparation of composites with metal oxide adsorbents. This method provides pre-concentration of material near photoactive sites; in addition, it also enables a possibility to adsorb material in darkness, whereupon material undergoes irradiaton to decompose it and to restore the original photocatalyst. Moreover, this strategy allows production of matrix-isolated quantum particles of TiO2 with a different band gap, as opposed to bulk material [6].

Photocatalytic activity of mesopous TiO2 was demonstrated to be closely related to its surface area, pore structure, crystalline phase, crystallinity, and particle size [7]. Thus, it is important to develop methods controlling textural properties and crystal structure.

In the presented article, we report synthesis of mesoporous TiO2-Al2O3 mixed oxides with various Ti:Al ratios by means of co-precipitation of TiOSO4 and Al2(SO4)3 with aqueous ammonia or urea. Porous textures and crystalline structures of mesoporous TiO2-Al2O3 mixed oxides were controlled by means of variable Ti and Al atomic ratios and precipitation conditions. Effect of the precipitator type and of Ti:Al atomic ratio on microstructure, phase composition, textural properties of mesoporous TiO2-Al2O3 mixed oxides was investigated.

Experimentalpart

All chemicals were analytic-grade reagents used without subsequent purification. Mesoporous TiO2-Al2O3 composites with various Ti:Al atomic ratios were prepared by means of co-precipitation of a mixed solution of TiOSO4 and Al2(SO4)3 with use of aqueous ammonia or urea as precipitation agent according to procedure described in [8]. Unlike the mentioned method, TiOSO4 solution was used instead of Ti(SO4)2 solution in the presented work. All composites were calcined at 500 ˚C in air for 3h. The resulting mesoporous TiO2-Al2O3 mixed oxides are denoted as A-n-500 and M-n-500 for synthesis with aqueous ammonia and urea, respectively. The n value varies from 1 to 5 for Ti:Al atomic ratios varying from 0:1 to 1:0; 500 is calcination temperature, ˚C.

Crystalline phase of TiO2-Al2O3 samples was investigated with use of X-ray diffraction (XRD) method (XRD-7000 diffractometer, Shimadzu, Japan) using the CuKa-radiations (λ = 1.5406 Å) in the 2Θ interval of 10-80. Average dimensions of crystallites were determined with use of the Scherrer equation, D = 0.89λ/βcosθ, using (101) reflection for anatase phase and (110) for TiO2-B phase. Here, D is crystallite size, λ is the wavelength of X-ray radiation (1.5406 Å), 2θ is the scattering angle and β is the full width at half maximum of the Bragg reflection, after correction of instrumental broadening [1].

The shape and microstructure of TiO2-Al2O3 samples were observed with use of scanning electron microscope (model XR-3000, Evex, USA).

Isotherms of low-temperature nitrogen adsorption were registered at -196 C by means of the ASAP 2020 device (Micrometrics, USA), after degassing the material under vacuum at 350 C for 3 h. Specific surface area (SBET) of samples was calculated as per the BET method in 0.05-0.25 p/p0 interval of relative pressure values. Total pore volume (Vtot) was calculated from the quantity of nitrogen adsorbed at relative pressure p/p00.99. Pore size distribution and average pore diameter values were determined from desorption isotherms using the BJH method.

Photocatalytic activity of TiO2-Al2O3 mesoporous composites was appraised from degradation of salicylic acid (SA) under UV radiation (luminescent lamp, 78 W, base line of spectrum 365 nm). Magnetic stirring in darkness was exposed for 60 min. Degradation (X) of salicylic acid was determined by means of absorbance measured at 298 nm with UV-vis spectrofotometer (PortLab 511).

Results and discussions

XRD patterns in Figure 1 illustrate crystalline phase of mesoporous TiO2-Al2O3 mixed oxides with various Al:Ti atomic ratios. As shown in Figure 1, the A-5-500, and M-5-500 samples (Ti:Al=1:0) are pure anatase (ICDD No. 00-021-1272), whereas the A-3-500, M-3-500 (Ti:Al = 1:1) and A-4-500, M-4-500 (Ti:Al = 9:1) samples consist of both anatase and amorphous alumina. A peak at around 2θ = 25.0400 ˚ corresponding to TiO2-B crystallite (ICDD No. 00-035-0088) [9] was observed in A-4-500 sample. As is apparent from Figure 1, only sharp diffraction peaks are observed for pure TiO2 samples (A-5-500 and M-5-500). These peaks indicate precipitation with aqueous ammonia and urea with subsequent calcination to completely crystallize the samples, no matter which precipitator is used. The absence of peaks in XRD patterns of the A-1-500, A-2-500, M-1-500, and M-2-500 sample confirms amorphous structure of these samples.

Apparently, the type of precipitator and Al:Ti atomic ratio influence the crystalline structure of TiO2 in TiO2-Al2O3 composites. Relatively weak diffraction peaks of the A-3-500 (Al:Ti = 1:1) and A-4-500 (Al:Ti = 1:9) samples indicate their low crystallinity. The well crystallized anatase structure of samples M-3-500 and M-4-500 signifies precipitation in homogenous conditions (with urea) to result in better crystallinity, as compared with inhomogeneous conditions (using ammonia).

Fig. 1. XRD patterns of mesoporous TiO2-Al2O3 mixed oxides. A: anatase; B: TiO2-B crystalline phases of TiO2.

Average dimensions of crystallites of anatase and of TiO2-B calculated in accord with the Sherrer equation are presented in Table 1. As shown in Table 1, A-n-500 samples have larger anatase crystallites as compared with M-n-500 samples. This is explained by the fact that hydrolysis of urea proceeds slowly, thus limiting hydrolysis of titanium and aluminum precursors. At this point, groups of unhydrolyzed anions remain in precursors and prevent aggregation of particles by means of electrostatic repulsion and growth of the TiO2 crystallites.

SEM images (Figure 2) show micro-scale morphologies of mesoporous TiO2-Al2O3 mixed oxides (A-n-500 and M-n-500) markedly differing from one another. As is apparent, A-n-500 samples have fibrous structure of anatase, only some irregular aggregates presumably composed of amorphous alumina can be observed on surface of particles. Microphotograph of M-n-500 samples show porous agglomerates, irregularly sized and shaped.

Nitrogen physisorption analysis provides detailed information on pore texture. Specific surface area and total pore volume values for all samples investigated are summarized in Table 1. As is apparent, surface area value of A-n-500 samples increases, as Ti:Al atomic ratio increases, being the largest for M-n-500 samples at Ti:Al= 1:1 atomic ratio. The presence of TiO2 (anatase) in all samples leads to enhanced surface area.

Table 1. Crystalline structures and textural properties of mesoporous TiO2-Al2O3 mixed oxides

Sample / Al:Ti / SBET, m2g-1 / Vtot, cm3g-1 / DBJH,
nm / Phase composition / Cryst size, nm / X SA, %
A-1-500 / 1:0 / 97 / 0.153 / 3.4 / amorph / - / 10
A-2-500 / 9:1 / 96 / 0.130 / 3.5 / anatase / - / 6
A-3-500 / 1:1 / 199 / 0.406 / 3.8 / anatase / 9.8 / 26
A-4-500 / 1:9 / 269 / 0.287 / ≤2.5, 3.9 / anatase,
TiO2-B / 15.5 (A),
36.6 (TiO2-B) / 50
A-5-500 / 0:1 / 486 / 0.562 / ≤1.7, 3.7 / anatase / 12.4 / 15
M-1-500 / 1:0 / 9 / 0.015 / 2.4 / amorph / - / -
M-2-500 / 9:1 / 11 / 0.019 / 3.4 / amorph / - / 18
M-3-500 / 1:1 / 284 / 0.556 / 7.6 / anatase / 6.9 / 48
M-4-500 / 1:9 / 251 / 0.486 / 5.9 / anatase / 6.1 / 76
M-5-500 / 0:1 / 136 / 0.207 / 5.0 / anatase / 8.9 / -

Fig. 2. SEM images of mesoporous TiO2-Al2O3 mixed oxides, 5.0x103 magnifications

Isotherms of nitrogen adsorption-desorption, and pore size distribution patterns of mesoporous TiO2-Al2O3 mixed oxides are presented in Figure 3. All isotherms are of IV type (IUPAC classification) and indicate availability of mesoporous in samples. However, as shown in Figure 3, isotherms of pure Al2O3 and of pure TiO2 as well as of TiO2-Al2O3 composites prepared with use of a different precipitation method differ markedly and evince different textural properties in these samples. Moreover, isotherms of A-n-500 samples show H2-type hysteresis loop for pure Al2O3 samples and for samples with small TiO2 percentage (A-1-500, A-2-500); a combination of H2- and H3-types for samples with larger TiO2 percentage (A-3-500, A-4-500), and H3-type hysteresis loop for pure TiO2 samples (anatase), thus indicating differences in pore structure of these oxides.

Fig. 3. Isotherms of N2 adsorption-desorption and pore size distribution patterns of mesoporous TiO2-Al2O3 mixed oxides.

A triangularly-shaped hysteresis loop with a steep desorption branch belonging to H2-type hysteresis loop indicates availability of pores with narrow mouth (ink-bottle pores). H3- type hysteresis loop is attributed to aggregates of plate-like particles forming slit-like pores [1]. This phenomenon is confirmed by corresponding pore size distribution calculated with use of the BJH method (Figure 3).

For M-3-500, M-4-500, M-5-500 samples, isotherms show H2-type hysteresis loop in relative pressure interval exceeding 0.6. As is apparent from pore size distribution patterns of these samples, pure TiO2 sample is characterized availability of narrow pore size distribution with a peak centered at 5.0 nm, whereas TiO2-Al2O3 mixed oxides (M-4-500 and M-3-500) have a wider distribution centered at 5.9 and 7.6 nm, respectively.

While comparing TiO2-Al2O3 samples prepared with use of different precipitators, we had ascertained variations in synthesis conditions to be reflected in phase composition and in textural properties of samples and, therefore, in their catalytic activity. The measured grades of photocatalytic decomposition of SA are shown in Table 1. The samples with atomic ratio Ti:Al = 9:1 are more efficient photocatalysts. This can be attributed to large surface area and pore volume in these samples.

Conclusions

Mesoporous TiO2-Al2O3 mixed oxides were prepared by means of co-precepitation method under homogenous and inhomogeneous conditions. Their porous and crystalline structures were tailored with use of variable type of precipitator and Ti:Al atomic ratios. Results show evinidence that mesoporous TiO2-Al2O3 mixed oxides prepared under homogenous conditions have fibrous structures of anatase with some irregularly-shaped aggregates presumably composed of amorphous Al2O3 on surface of TiO2 particles, whereas mesoporous TiO2-Al2O3 mixed oxides prepared under inhomogeneous conditions are porous irregularly-shaped agglomerates. It has been ascertained that variations in synthesis conditions are reflected in phase composition and in textural properties of samples and, therefore, in its catalytic activity. Mesoporous TiO2-Al2O3 mixed oxides with atomic ratio Ti:Al = 9:1 are more efficient photocatalysts. This can be attributed to large surface area and pore volume in these samples.

Acknowledgement

The reported study was partially supported by RFBR, research project No. 13-03-96111 r_ural_a.

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