A cascade solar powered photocatalytic reactor for wastewater treatment
Sponsor: Hong Kong Research Grants Council Earmarked Grant (HKUST6026/98P)
PM and PI: Dr. Chak K. Chan; CI: Dr. John Porter (HKUST) and Dr. King Lung Yeung (HKUST)
Status: Completed

In this research, electrophoretic deposition and subsequent thermal treatment in the range of 473K to 873K have been applied to produce stable TiO₂ (Degussa P-25) coatings on 316 stainless steel plates. The effects of the thermal treatment on the photoactivities of the supported catalysts have also been investigated. Compared to the unheated coating, the photoactivity to degrade benzoic acid was found to decrease by 52% when the coating was heated at 873K. Possible reasons accounting for the drop in the photoactivity exhibited by the coating heated at 873K included the decrease in catalyst surface area and the presence of Fe³⁺ ions, which were considered by other researchers to act as electron hole recombination centers, at the catalyst surface. A lower heating temperature, such as 473K as examined was suggested to ensure the catalyst photoactivity and mechanical stability. A bench-top three-plate photoreactor and a pilot scale nine-plate photoreactor have been constructed for the treatment of benzoic acid solutions. Artificial UV-A lamps and solar UV photons have been employed as the UV source of the bench-top and the pilot scale reactors respectively. The TiO₂-coated plates have been arranged in a unique cascade configuration. The “waterfall” introduced when the solution flows from one plate to another can reduce mass transfer limitations and enhance the transfer of oxygen into the solution. For the bench-top reactor, the degree of TOC removal of benzoic acid was positively affected by UV light intensity, but was independent of solution flowrate from 2l/min to 5l/min. A Langmuir-Hinshelwood form of rate equation was found to be suitable for modeling the degradation of benzoic acid solutions at TOC_o from 15.4ppm-68.9ppm. For the pilot scale reactor, the proposed rate equations involving the Langmuir type and power law dependence on I_mean provided good fits to 90 data points from 17 experiments carried out at 18^oC. Experiments also showed that the percentage removal of TOC slightly reduced at elevated temperatures, but significantly increased with the addition of hydrogen peroxide solutions in general.

Publications:

 
Student Trained:

Alex Chan, MPhil in Chemical Engineering, graduated 2001.

Controlled Synthesis of Submicron Titanium Dioxide Particles for Wastewater Treatment
Sponsor: Hong Kong Research Grants Council Earmarked Grant (HKUST582/94E)
PM and PI: Dr. Chak K. Chan; CI: Dr. John Porter (HKUST)
Status: Completed

Ultrafine titanium dioxide powders were produced in an aerosol reactor using vapor hydrolysis of titanium tetraisopropoxide (TTIP) at 260 °C and higher temperatures (600, 700, 800, and 900 °C). The effect of calcination on the microstructure characteristics and the photoactivity was studied. The powders were characterized using BET surface area, XRD, and TEM analyses.  The photocatalytic activity of the powders was also studied using degradation of phenol in water as a test reaction. Powders produced at 260 °C were calcined at 500 to 900  °C  while those produced at higher temperatures were calcined at 600 °C for 3 hours. Raw powders produced at 260  °C are amorphous but become crystalline after calcination.  As the calcination temperature increases, the surface area decreases but the rutile to anatase ratio and the anatase and rutile crystallite sizes increase.  The photoactivity increases when calcination temperature increases until 900 °C when the powders become densified and the surface area drops significantly because of sintering. Powders produced at higher temperatures are predominantly anatase and are generally more photoactive.  Calcination of the powders enhances the photoactivity.  Among the factors examined, large surface area and good dispersion of the powders in the reaction mixture are favorable to photoactivity. Conversely,  prolonged calcination at high temperatures is detrimental to photoactivity.  However, surface area, crystallite size, anatase to rutile ratio, and dispersity of the powders alone cannot account for the observed trend of photoactivity. Similar observations have been found for a commercial powder (Degussa P-25). The role of crystallinity needs to be further investigated.
 

In addition to BET, XRD and TEM analysis, we have carried out a detailed Micro-Raman analysis to study the structure of TiO2 powders produced by the aerosol reactor.  Spatial inhomogeneity was discovered after the amorphous TiO2 powders produced at low temperature were calcined at 700, 800, and  900 °C for 3 hours.  The TiO2 powders produced at high temperatures (from 600 to 900 °C) were found to be spatially homogeneous and predominately anatase in structure.  Small amounts of rutile and brookite are found for powders produced at 700, 800, and 900 °C after calcination at 600 °C for 3 hours.  The rutile and brookite impurities are believed to be concentrated on the surface of anatase based on a comparison of results of Raman and x-ray diffraction studies.  The more surface sensitive Micro-Raman analysis may be better suited to characterize the structural parameters of photocatalytic powders than bulk x-ray techniques.

Publications:

Zhang, Y. H., Chan, C. K., Porter, J. F., and Guo, W. (1998) “Micro-Raman spectroscopic characterization of nano-sized TiO2 powders prepared by vapor hydrolysis”, J. Materials Research, 13(9), 2602-2609.

Chan, C. K., Porter, J. F., Li, Y. G., Guo, W., and Chan, C-M. (1999) “The effect of calcination on the microstructures and photocatalytic properties of nano-sized TiO2 powders prepared by vapor hydrolysis”, J. Am. Ceram. Soc., 82(3), 566-572.

Porter, J. F., Li, Y. G., and Chan, C. K. (1999) “The effect of calcination on the microstructural characteristics and photoreactivity of Degussa P-25 TiO2”, J. Materials Science, 34(7), 1523-1532.

Guo, W., Porter, J. F., Chan, C. M., and Chan, C. K. (1997) “Characterization of ultrafine titanium dioxide powders produced by vapor phase hydrolysis of titanium tetraisopropoxide”, European Aerosol Conference, 15th-19th Sept., Hamburg, Germany, J. Aerosol Sci., 28, Suppl. 1, S485-S486.

Guo W., Porter, J. F., Chan, C. M. , and Chan, C. K (1996) "The synthesis of TiO2 photocatalysts by the vapor phase hydrolysis of titanium tetraisopropoxide", Proceeding of the 1996 Asian Pacific Chemical Reaction Engineering Forum, Beijing, China, June 26-28, 549-554.