(86e) Comparative Study of the Photocatalytic Performance of An Immobilized TiO2 Nanocatalyst and TiO2 Nanoparticles in Slurries

Titanium dioxide (TiO₂) is well known for the light induced catalytic ability of oxidizing recalcitrant organic compounds (Matthews, 1992). Due to favorable bandgap energy, TiO₂ on irradiance with light (of less than 380 nm wavelength) produces an electron-hole pair. Electron and hole migrates to TiO₂ surface and oxidizes the organic molecules to carbon dioxide (Lee and Mills, 2004).
Crystal structure of TiO₂ dictates the bandgap energy and specific surface area (SSA) control the number of site for photocatalytic reactions (Gogate and Pandit, 2004; Hurum et al., 2006). Thus SSA and crystal structure are the two most important factors affecting photocatalytic performance of TiO₂.

Anatase (kinetically stable), rutile (thermodynamically stable), brookite and monoclinic-TiO₂ are four crystalline structures of TiO₂. The anatase crystal form has distorted orthogonal structure with highest bandgap energy (E_g) and more photocatalytic activity than other crystal form of TiO₂ (Carp et al., 2004). Increasing innovations in manufacturing have permitted processes to produce particle sizes in nanometer range. Nanometric sized TiO₂ by virtue of smaller particle size have large SSA, more photocatalytic sites and better photocatalytic performance (Bhatkhande et al., 2001).

In many of the reported photocatalytic applications TiO₂ was used as nanoparticles slurry (Bhatkhande et al., 2001). But using nanoparticles in form of slurry is associated with several practical constraints. Higher amount of remnant particle in the process effluent due to slow settling velocity, increased process cost due to additional solid/liquid separation, reduced penetration depth of incident light due to high solution turbidity and health hazard due to fugitive emissions of nanoparticles during slurry preparation, are some of the practical constraints (Houari et al., 2005; Baan et al., 2006).

Another major practical constraint of utilizing nanoparticles is related to particle-particle aggregation.  The large surface areas associated with nanoparticles is a major factor responsible for reducing the surface charge density and promoting particle-particle contact. Also, as the particle size decreases, the pHzpc (point of zero charge) decreases and the decreasing repulsive forces between particles increases the aggregation tendency (Guzman et al., 2006). Particle aggregation is a major mechanism responsible for the loss of photocatalytic activity because light cannot impinge upon areas which are associated with particle-particle contact.

Immobilization of the TiO₂ nanoparticles onto a support medium can eliminate most of the limitation linked to the use of nanoparticles in form of slurry. However, a major disadvantage of supporting catalyst is related to the loss of surface area which is caused by the sintering or aggregation of the nano-catalyst onto the support surface during the thermal stabilization step (Carp et al., 2004). Particle sintering results in formation of film or sheet on the support surface and the resultant supported catalyst system has catalytic surface area smaller than that of discrete nanoparticles by few orders of magnitude. Hence, development of an immobilized TiO₂nanocatalyst system with SSA comparable to that of discrete nanoparticles is a research priority for enhanced catalytic performance.

Recently, a method of producing TiO₂nano-structures with a high aspect ratio (fibers/wires) has been reported by Li and Xia (2003). In this method, the sol-gel conversion is coupled with an electrospinning process to produce TiO₂ fibers with nanometric diameters.  Electrospinning is a process of producing nanofibers by applying a high voltage to a capillary filled with a conductive solution. Using the sol-gel electrospinning method to fabricate immobilized TiO₂nanocatalyst for photocatalytic application has only been reported in a few studies (Ramaseshan et al., 2007). In addition, no reports have successfully developed an immobilized TiO₂nanocatalyst with an SSA comparable to that of nanoparticles (Madhugiri, et al., 2004; Alves et al. 2009).

The objective of the present study is to examine the aggregation levels of different TiO₂ nanoparticles sizes in aqueous slurries and to develop an immobilized TiO₂nanocatalyst with photocatalytic characteristics which are similar to nanoparticles.

 A sol solution of TiO₂ was prepared by dissolving titanium tetraisopropoxide (TTIP) in acetic acid. The viscosity of the sol solution was adjusted between 130-160 cps by adding polyvinylacetate (PVAc) mixture containing 45% PVAc (w/v) in dimethylformamide and tetrahydrofuran (1:1 (v/v)).  The TiO₂-PVAc composite nanofibers are produced by applying a 40 kV positive potential across two electrodes which are separated by a distance of 32.5 cm. The positive terminal was connected to a metallic needle and the negative or ground terminal was connected to a surface treated PVAc coated aluminum foil.  After electrospinning, the TiO₂-PVAc composite nanofibers were calcined at 400^oC for 2 hours to produce fibers with an anatase (confirmed by X-ray) crystal structure. Three sets of TiO₂ nanofibers with different fiber diameters were produced by varying the Ti-content (1.3%, 2.6%, 3.9% (w/w)). Three different sizes (5, 10, 32 nm) of TiO₂ (anatase) nanoparticles were evaluated in the present study.

The images of the TiO₂nano-catalyst (both immobilized nanofibers as well as nanoparticles) were obtained using a field emission-gun scanning electron microscope (SEM) using secondary electron detector under high vacuum mode (for nano-fibers) and extended vacuum mode (for wet nano-particles). The maximum resolution capacity of the microscope was 0.8nm. The diameter of the nanofibers and wet nanoparticle aggregates was measured from SEM images using SCANDIUM image processing software. Dilute suspension (0.05 g×l^-1) of the nanoparticles were prepared by agitating requisite amount of dry particles in ultrapure water under ambient condition. A micro-droplet (25-30ml) of the TiO₂nanoparticle suspension was imaged to determine the extent of wet aggregation of the nanoparticles in the extended vacuum mode of SEM. The diameter of the wet aggregates was measured. The SSA (m²·g^-1) of the TiO₂nano-catalyst (both immobilized nanofibers as well as nanoparticles) was determined using the Brunauer-Emmett-Teller (BET) gas adsorption technique The BET instrument temperature was set at 77.34K and N₂was used as the adsorbate. The SSA of the immobilized TiO₂nanocatalysts was measured in a dry state between a N₂ relative pressure (P/P_o) from 0.05 to 0.3.

Photocatalytic experiments were conducted with TiO₂ nanoparticles and immobilized TiO₂nanocatalyst with comparable SSA. Phenol was chosen as model degradant. Photocatalysis of phenol was conducted in quartz tubing at 37^oC under 300 nm UV-radiation with 9 mW/m² irradiance (Ray, et al., 2009). The initial dissolved oxygen level was maintained at 7.8 mg/L, catalyst loading was 0.5 mg/mL for all photocatalytic experiments. Liquid samples were drawn at regular intervals and analyzed for the residual phenol concentration in solution.

  Data from these studies demonstrated that smaller TiO₂ nanoparticles tend to form larger aggregates in aqueous slurries.  Also, the size variability in the wet aggregates was large for smaller particles.  In general, the average dimension of the wet aggregates was an order of magnitude larger than that for the dry nanoparticles. Larger diameter TiO₂ fibers were produced when the Ti-content was increased in the electrospinning mixture. Results from the BET measurements demonstrated that the SSA increased with decreasing TiO₂nanoparticle diameter.  For the immobilized TiO₂ nanofibers, a higher SSA was observed as the fiber diameter decreased. The SSA for the TiO₂ nanofibers (electrospun from three solutions with three different Ti-content) were comparable to the areas obtained for the three different TiO2 nanoparticles. The apparent first-order rate constant (min-1) was used as a measure to compare the catalytic efficiency for the 5 nm TiO₂ nanoparticles (SSA = 275 ± 15 m²·g^-1) and the immobilized TiO₂ nanofibers which were produced from using 1.3% Ti-content (SSA = 259 ± 23 m²·g^-1).  The photocatalytic rate of the immobilized TiO₂ nanofibers (0.0085 ± 0.0001 min^-1) was greater than the rate for the 5 nm TiO₂ nanoparticles (0.0032 ± 0.0005 min^-1).  A possible explanation for lower photocatalytic rates of the larger surface area 5 nm TiO₂ nanoparticles in comparison to the immobilized TiO₂ nanofibers could be attributed to the loss of photocatalytic surface area during particle-particle aggregation in the slurry.

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