분무건조법을 이용한 TiO2 나노분말의 팩키징

Theories and Applications of Chem. Eng., 2002, Vol. 8, No. 1

김진수, Sotiris E. Pratsinis*

경희대학교 화학공학과, 스위스연방공대(ETH-Zurich)*

화학공학의 이론과 응용 제8권 제1호 2002년

Packaging of TiO2 Nanoparticles by Spray Drying

Jinsoo Kim and Sotiris E. Pratsinis*

Dept. of Chemical Engineering, Kyung Hee University

Institute of Process Engineering, Swiss Federal Institute of Technology*

INTRODUCTION

Nanoparticles are receiving a lot of attention as they exhibit different electrical, magnetic, optical, and mechanical properties than bulk materials [1]. As applications of these particles become more and more apparent, there is an increasing need to bring them to a size that industry is accustomed to dealing with (particles larger than 1 □m) maintaining desired nanostructures and properties [2-4]. Here we present on packaging and comparison of sol-gel made titania nanoparticles and flame-made P25 titania nanoparticles (Degussa) that were spray dried into micron size granules. Since all these nanoparticles have different characteristics attributed to different preparation techniques, the product granules show also different pore structure and phase composition.

EXPERIMENTAL

Four types of feed solution were prepared for spray drying: sol-gel titania precipitates, hydrothermally processed titania slurry, peptized titania sol and flame-made Degussa P25 suspension. For sol-gel made solutions, sol-gel titania precipitates were synthesized first by hydrolysis and condensation of titanium tetraisopropoxide (TTIP) [2,4,5]. The TTIP was dissolved in anhydrous ethyl alcohol at a molar ratio of C2H5OH/TTIP=5. Hydrolysis and condensation were carried out at room temperature by adding the mixed solution slowly into the distilled water until a molar ratio of H2O/TTIP=200. On adding the water into the mixed solution, white precipitates were formed and the reaction time was fixed at 1hr [5]. For hydrothermal processing, the white precipitates were transferred into Teflon beaker in the metal jacket (autoclave). This autoclave was aged in the preheated oven at 135℃ for 1 day. After cooling down, this slurry was used as hydrothermally processed slurry. For peptized titania sol, the above white precipitates were peptized with 1 M HNO3 to make a molar ratio of [H+]/[Ti]=0.5. Then, the solution was refluxed at 80□C for 12hr [4]. The resulting well-dispersed sol was used as peptized titania sol. The P25 suspension was prepared by adding deionized water into the as-desired powder (2 wt%).

The above described feed solution was spray dried. The feed solution was injected into a two phase nozzle by a peristaltic pump at the constant speed of 600ml/h, and it was mixed with pressurized air (2 bar) to give droplets. The sprayed droplets were dried in the co-current hot air which was maintained at the temperature of 100-200□C. The prepared granules were collected by a cyclone, and then they were dried at room temperature for 24hr.

RESULTS AND DISCUSSION

The granules from sol-gel precipitates were amorphous after spray drying, but were crystallized to anatase by heating at 450□C and 600□C for 2hr. In contrast, the as-spray dried granules from peptized titania sol and hydrothermal processed titania slurry showed anatase phase and the granules from P25 suspension showed a combination of anatase and rutile even after spray drying. With further heat treatment, the granules from peptized sol started showing rutile peaks at 600□C, while the granules from hydrothermally processed sol-gel titania exhibited only anatase phase even after calcined at 600□C for 2hr. Generally, the granules from hydrothermal processing or peptization had smaller XRD crystallites than those from conventional sol-gel precipitates.

The shape and microstructure of the spray dried granules depend on feed solution characteristics, especially the extent of flocculation [6]. Figures 1a-d show the SEM images of spray dried granules prepared from sol-gel precipitates, peptized titania sol, hydrothermally processed titania slurry and P25 suspension, respectively. Most of granules obtained by spray drying had diameters ranging from 1 to 10 □m. The granules from sol-gel precipitates are more or less spherical in shape, looking like a cotton ball. Also, it appears that the granules consist of relatively hard agglomerates which are 50-100 nm in size and the voids between the hard aggregates give a lot of inter-particle pores (Figure 1a). In contrast, the granules from peptized sol show deformed structrure (doughnut shape or sphere with dimples) or spherical, depending on spray drying conditions [4]. However, the microstructure of these granules shows very well packed smooth surface without any agglomerates (Figure 1b). The granules from hydrothermally processed titania slurry are always spherical consisting of agglomerate-free nanocrystalline primary particles (less than 10 nm in size, Figure 1c). The granules from P25 also show the similar appearance with those from sol-gel precipitates. However, they are dense particles of about 25nm resulting in only inter-particle pores.

The pore structures of the prepared granules were studied by N2 adsorption/desorption at 77K in a Micrometrics ASAP 2010. All granules show a decrease of the specific surface area (SSA) with increasing calcination temperature due to the phase transformation and/or crystallite size growth. The SSA of P25 granules was 53-59 m2/g. The calculated porosity slightly decreased from 49% to 47% after calcined at 600℃. In contrast, the other sol-gel made granules showed a drastic decrease of SSA with increasing calcination temperature. The granules from sol-gel precipitates showed a big decrease in SSA at 450℃ that is attributed to the phase transformation from amorphous to anatase [4], resulting in a porosity decrease from 48% to 31%. For the granules from hydrothermal processing, the SSA decreased relatively slowly, maintaining the anatase phase. This decrease is attributed to the crystallite growth only. In contrast, the granules from peptized sol showed a rather large decrease of SSA at 600℃, indicating the collapse of pore structure by the phase transformation from anatase to rutile [4].

Figure 1. Microstructures of granules made from sol-gel titania precipitates (a), peptized titania sol (b), hydrothermally processed titania slurry (c) and Degussa P25 suspension (d).

Figure 2 shows the pore size distribution (PSD) of these granules after calcined at 450℃ for 2hr. All granules have different pore modes depending on their primary particle and agglomerate sizes. The granules from P25 nanoparticles have 5-40 nm pore diameters. These pores are coming from voids among dense primary particles. Since these particles were prepared at much higher temperature (> 1000℃) than the calcination temperature, they maintained similar PSD with further heat treatment up to 600℃. The granules made by hydrothermal processing show monomodal PSD in the range from 2 to 15 nm. This is reasonable, considering its particle size of 12 nm. With further heat treatment, the PSD shifts to the larger pore size, indicating crystallite growth. The granules from peptized sol are well packed with small primary particles of less than 10 nm, resulting in pore sizes from 2 to 6 nm. The granules from sol-gel precipitates consist of hard agglomerates in the size range from 50 to 100 nm (Figure 1a). Therefore, the product granules have bimodal PSDs consisting of intra-particle pores (2-5 nm) among nanoparticles inside agglomerates and inter-particle pores (10-120 nm) that are the voids among the hard agglomerates in granules [4].

Figure 2. Pore size distributions (PSD) of granules made from sol-gel titania precipitates, peptized titania sol, hydrothermally processed titania slurry and Degussa P25 suspension.

REFERENCES

화학공학의 이론과 응용 제8권 제1호 2002년

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