Synthesis, characterization and activity of titanium dioxide based (Ca, Ce, W)-TiO2 photocatalysts for degradation of dye and pesticide
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Date
2011
Authors
Uduak, Akpan (George)
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Abstract
Global industrialization is not without its attendant challenges. The release of
unwanted by-products and pollutants which are carcinogenic and toxic into the
environment by textiles, chemicals and processing industries is directly proportional
to industrial growth. The environment must be kept safe. Therefore, photocatalysis
leading to complete mineralization of pollutant(s) was adopted to treat wastewaters
containing dyes (acid red 1, reactive orange 16, reactive blue 19, direct blue 71 and
acid blue 25) and a pesticide (2,4-dichlorophenoxyacetic acid; 2,4-D). A composite
photocatalyst, electronically stable of the type Ti(l-x-y)Ca(3x-y)Ce(2x-y)W(yt6)02(1-2(y-x))
(at y<2x and x+y<1) with an enhanced photocatalytic activity was developed by
doping Ti02 with Ca, Ce and W. The photocatalyst was prepared by sol-gel method,
hydrothermally treated and employed in the degradation of the above mentioned
pollutants. The effectiveness of the composite photocatalyst was verified by
comparing its activity under the same experimental conditions with two commercial
photocatalysts; Degussa P25 and Ti~-Sigma product CAS No. 1317-70-0. The
developed photocatalyst was better than Ti02-Sigma product in solar photocatalytic
degradation of ARl. The reusability test of the developed photocatalyst makes it
superior to Degussa P25 (which could not settle out of solution seven months after
photocatalytic degradation of 2,4-D), hence rendering it non-reusable. On the other
hand, the composite photocatalyst settled out of solution in less than 1 h after
irradiation and proved to be as efficient at the fourth cycle as in the first, as it
accomplished a complete degradation at the same irradiation time. This advantage
of the composite photocatalyst over Degussa P25 limits the number of comparison
made between them. Other catalysts were also developed and tested as described in
the body of the Thesis. The developed photocatalysts were characterized by X-ray
photoelectron spectroscopy (XPS) for the chemical states of the elements in the
developed photocatalysts; X-ray diffraction (XRD) and Fourier Transformed Infra
Red (FTIR) for structural and functional groups analysis respectively; surface
scanning electron microscopy (SEM) for microstructure and morphology; Nitrogenphysisorption
for surface area and pore size distributions; and UV-Vis diffused
reflectance for band gap evaluations. The doping resulted in a reduction in the band
gap of Ti02 from 3.2 eV to 2.94 eV, and hence the photocatalytic reaction was
pushed into the visible region. The high resolution XPS analysis revealed that the
photocatalysts is stable as its chemical constituents were found to exist in the
proposed oxidation states. The photocatalysts have high surface areas available for
photocatalysis and are of N2 adsorption.:.desorption isotherms of type IV with type
H2 hysteresis loops. V arlo us operational parameters such as initial pollutants
concentration, initial pH, calcination/hydrotreatment temperatures and dopant
contents were investigated. While pH greatly influenced the photocatalytic process
in all cases, initial concentration does not seem to influence the process, except for
2,4-D degradation. The kinetic study revealed that reaction order that best describes
the whole process is first order, except for the visible light degradation of ARl
where the rate is independent of initial concentration.
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Keywords
Titanium dioxide based