Effect of surface area, pore volume and particle size of P25 titania on the phase transformation of anatase to rutile

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1 Indian Journal of Chemistry Vol. 48A, October 2009, pp Notes Effect of surface area, pore volume and particle size of P25 titania on the phase transformation of anatase to rutile K Joseph Antony Raj & B Viswanathan* National Centre for Catalysis Research, Indian Institute of Technology Madras, Chennai , India bvnathan@iitm.ac.in Received 9 June 2009; revised and accepted 11 September 2009 The effect of calcination on P25 titania at different temperatures has been studied by XRD, nitrogen adsorptiondesorption isotherm, BET-surface area, pore volume, pore size, particle size and crystallite size in order to understand the conversion of anatase phase to rutile phase. The fraction of anatase and rutile has been determined using XRD. Although the crystallinity remains the same, the phase change of anatase to rutile is inevitable with temperature. The isotherms show the nonporous characteristics of the P25 samples. In addition, nitrogen adsoption by the samples decreased appreciably with crystallinity. The pore volume and BET surface area show a large decrease at 900 o C. The particle size, crystallite size, and pore size show an increasing trend with increase in temperature. Analysis of the physical characteristics indicates that P25 titania samples can be used for catalytic applications at less than 500 o C. Keywords: Catalysts, Phase transformations, Titania, Pore volume, Crystallinity, Particle size, Surface area IPC Code: Int. Cl. 9 B01J21/00 Titania is an efficient catalyst for photo-oxidation of organic pollutants 1. The photocatalytic properties are influenced by physicochemical variables such as particle size, pore volume, surface area, and crystallinity. Generally, researchers use Langmuir- Hinshelwood kinetic expressions to model photocatalytic reaction data 2,3, showing the importance of substrate adsorption and thus surface area. Studies do not conclusively support this interpretation of dependence of surface area and photocatalytic activity by TiO 2 4,5. Influence of particle size of titania on the photocatalytic activity has been demonstrated. Rivera et al. 6 reported increasing rate of photocatalytic degradation with anatase crystallite size, while Anpo et al. 7 have reported that for anatase particles in the range of 4 50 nm, photocatalytic activity increased with decreasing size. A fundamental understanding of the characteristics affecting photocatalytic activity is necessary to make efficient use of existing catalysts and to provide design criteria for the preparation of new catalysts. Commercial Degussa P25 titania is commonly used as a basis for comparison with titania photocatalysts prepared by other procedures. Hence, a study of those microstructural characteristics relevant to the photoreactivity of P25 titania and its effect on calcination should prove useful in developing a good understanding of the effect of these characteristics on photocatalytic reactivity. The present study is primarily concerned with the adsorption and desorption of nitrogen by a series of calcined P25 titania samples. The principal objective of this investigation is to determine the effect of pore volume, pore size, particle size, BET surface area and crystallinity with temperature. This study is expected to improve the understanding of the catalyst structure and adsorption phenomenon. Experimental Degussa P25 titania was used as supplied. The samples were calcined in a muffle furnace at 500, 700 and 900 o C for two hours. One sample was calcined at 900 o C for four hours to examine the effect of calcination time. The calcined P25 titania and untreated Degussa P25 samples were characterized for evaluating the changes in properties. The calcined P25 samples were analyzed for XRD by powder diffraction technique using a Rigaku Miniflex II with CuKα irradiation. After each thermal treatment, the percentage of anatase and rutile was calculated from XRD data. The mass fraction of rutile in each sample was determined from the relative XRD diffraction intensities corresponding to anatase [1 0 1] and rutile [1 1 0] reflections. The mass fraction of rutile and anatase was determined using Eqs (1) and (2) 8,9, Anatase (%) = [0.79I A / (I R I A )] (1) Rutile (%) = {1 / [(I R I A ) / I R ] } (2) where I A and I R are the peak intensities of [1 0 1] and [1 1 0] reflections for anatase and rutile respectively. The average size of crystallites was determined based

2 NOTES 1379 on XRD peak broadening using the Scherrer equation (Eq. (3)) 10. t = 0.9 λ / (B cos θ) (3) In Eq. (3), t is the average crystallite size, B is the broadening of the diffraction line measured at half maximum intensity, λ is the wavelength of the X-ray radiation and θ is the Bragg angle. The average particle size was estimated 11 by assuming all the particles to have same spherical shape and size. The particle size, D, is given by Eq. (4), D = 6000 / (S BET ρ) (4) where S BET is the BET-specific surface area and ρ is the true density (ρ for titania is 4.2 g /ml). The nitrogen adsorption and desorption isotherms at -195 o C were measured using Micromeritics ASAP surface area and porosity analyzer after the samples were degassed in vacuum at 300 o C for 3 h. BET-surface area was calculated from the BET plot. Pore-size distributions were calculated using the BJH model. The pore volume was measured at the single point of p/p o = Helium gas used for dead-space measurements and nitrogen employed as an adsorbate gas were obtained from Inox Chemical Co. Results and discussion The XRD patterns obtained for the P25 samples calcined at various temperatures are shown in Fig. 1. The rutile content was found to increase with temperature at the cost of anatase. The XRD patterns of 100% crystalline anatase and rutile are given in the Fig. 1 for comparing the peak patterns of the P25 samples. Figure 1 shows significant lowering of intensities for anatase peaks with temperature. The effect of rutilation is not significant at less than 500 o C. However, the treatment at 700 o C and 900 o C showed significant rutilation. Figure 2 shows the nitrogen adsorption-desoption isotherm for the P25 titania samples calcined at various temperatures. The isotherms obtained for the samples demonstrate that it belongs to the Type II isotherm. Although the samples calcined at various temperatures showed the same type of isotherm, the quantity of adsorbed nitrogen was found to be different for different samples. The untreated P25 sample with high surface area showed higher nitrogen adsorption than the heat treated samples. The nitrogen Fig. 1 XRD patterns of P25 titania samples calcined at various temperatures.

3 1380 INDIAN J CHEM, SEC A, OCTOBER 2009 adsorption property of the samples showed a decreasing trend with temperature. For the P25 titania treated at 900 o C, rutile and anatase samples showed virtually the same amount of adsorbed nitrogen. In addition, the adsorption and desorption were found to fall on the same line for these samples and demonstrate non-utility for catalytic applications. Although the type of isotherm obtained for P25 showed it to be non-porous, the extensive photocatalytic application of this material is attributed to the nano sized particles (26 nm), high surface area (56 m 2 /g) and optimum pore volume (0.25 cm 3 /g). This furthermore indicates that ceramic materials may be used for catalytic applications provided the particle size is lowered to nano range. It is noted from the isotherms that higher surface area and optimum anatase content is essential for adsorption. The data on BET-surface area, pore volume and pore size of the P25 titania samples are presented in Table 1. The surface area and pore volume show a decreasing trend with temperature. The surface area showed a decrease of 7% up to 500 o C, while the pore volume showed a decrease of 28%. Heat treatment at 700 o C for 2 hours showed a surface area decrease of Fig. 2 Nitrogen adsorption-desorption isotherm of the P25 titania samples calcined at various temperatures.

4 NOTES 1381 Table 1 BET-Surface area, pore volume and pore size of the P25 titania samples Sample BET surface area (m 2 /g) Pore vol. (cm 3 /g) Pore size (nm) P P o C (2 h) P o C (2 h) P o C (2 h) P o C (4 h) Anatase Rutile %. Although the calcination performed at 900 o C for 2 hours showed a decrease in surface area of 17 m 2 /g, the calcination performed at 900 o C for 4 hours showed a similar surface area with the commercially available 100% rutile. The value of pore volume for the sample calcined at 900 o C for 4 hours was the same as that of the rutile sample. The surface area and pore volume data obtained for the P25 samples calcined at various temperatures reveal that the sample can be used for catalytic applications below 500 o C. In contrast to the effect of surface area and pore volume, the pore size showed an increasing trend with the calcination temperature. Although the P25 samples showed a mesopore range of nm, the surface area, adsorption-desorption isotherm and XRD patterns did not match with the well-known mesostructured MCM and SBA materials. Table 2 shows the characteristics of particle size, crystallite size, % rutile and % anatase for the P25 titania samples calcined at various temperatures. The % of anatase, % of rutile, crystallite size and particle size were calculated using Eqs (1) (4) respectively. The particle size showed an increasing trend with temperature. At 700 o C, it showed a 55% increase in particle size compared to that of the untreated P25. A three-fold increase in particle size was observed for the sample treated at 900 o C for 2 hours. The sample calcined at 900 o C for 4 hours showed a drastic increase in particle size (180 nm) and this is essentially attributed to the particle aggregation followed by crystal growth. This fact is additionally supported by the crystallinity data calculated using the XRD data. The crystallinity and particle size did not show a significant variation after the calcination at 500 o C. Nevertheless, calcination beyond 500 o C showed significant change in particle size and rutile content. The rutile form is the most stable polymorph of titania, which is 57% and 75% for the samples Table 2 Particle size, crystallite size, % rutile and % anatase of the P25 titania samples Sample Particle size a (nm) Crystallite size b (nm) Rutile (%) Anatase (%) P P o C (2 h) P o C (2 h) P o C (2 h) P o C (4 h) Anatase Rutile a S BET method; b XRD-method. calcined for 2 hours at 700 o C and 900 o C respectively. The temperature treatment showed a decreasing trend of anatase content for the samples. The P25 sample calcined at 900 o C for 4 hours showed the complete conversion of anatase to rutile. The crystallite size calculated using the Scherrer equation (Eq. 3) showed a trend similar to that of the particle size obtained by S BET method. The crystallite size data were nearly consistent with the particle size up to the calcination temperature of 500 o C and thereafter were different. In the present study, the nitrogen adsorptiondesorption isotherm, crystallinity, pore volume, pore size, surface area, particle size, and crystallite size were analysed for the P25 samples calcined at various temperatures. The isotherms essentially showed the non-porous characteristics and appreciable lowering of nitrogen adsorption with crystallinity. The pore volume of the samples showed a decreasing trend with rutilation. The heat treatment beyond 700 o C showed significant conversion of anatase into rutile and also lowered the surface area and pore volume. The pore size, particle size and crystallite size increased with temperature. The sample calcined at 900 o C for 4 hours showed complete conversion of anatase into rutile. The studies conclusively indicate that P25 titania samples can be used for catalytic applications at less than 500 o C. Acknowledgement The authors acknowledge the Department of Science and Technology, Government of India for funding the National Centre for Catalysis Research (NCCR) at IIT-Madras. Thanks are also due to M/s. Shell India (P) Limited for a fellowship to one of us (KJAR).

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