Preparation And Performance Optimization Of Nano-TiO₂/Fly Ash Beads Composite Photocatalyst

Nano-TiO₂has good chemical stabiltiy and excellent photocatalyticactivity under UV irradiation, so it has been widely used in organic pollutansttreatment, wastewater treatment, air purification, self-cleaning andbacteriostasis as the most promissing photocatalytic material. But there aresome disadvantages of nano-TiO₂in industrial applications. For example,nano-TiO₂is easy to aggregate and difficult to recycle nano powder afterdegradation in the liquid system. In addition, nano-TiO₂can only response toUV light which wavelength is less than387nm because it’s a wide bandgapsemiconductor. While this part of UV-light accounts for only3%～5%of thesolar energy, which seriously restrict the application of nano-TiO₂. Thisresearch work is focused on the preparation of nano-TiO₂photocatalyst withgood dispersion and easy recycling. Hydrothermal growth and N, S co-dopingof nano-TiO₂were conducted to improve its photocatalytic properties.Using TiCl4as Ti precursor, fly ash beads as the carrier, urea （NH2）2CO asrelease agent, nano-TiO₂were deposited on the surface of fly ash bead viahydrolysis-precipitation method. The influence of catalyst amount andintensity of light source on the photocatalytic properties was discussed. Thenthiourea （NH2）2SC and urea （NH2）2CO were used as the source of N and Srespectively, and the powders mixed with nano-TiO₂/fly ash beads compositematerial according to a certain proportion, N, S co-doped TiO₂/fly ash beadscomposite photocatalyst was prepared after calcining at the500℃for2h.Then chemical state of N, S and the doping mechanism were studied. FinallyKOH was used as the mineralizer, hydrothermal method was used to make thefurther growth of nano TiO₂/fly ash beads composite materials. The influenceof the KOH concentration and hydrothermal time on its morphology wereanalysised. XRD, SEM, EDS, XPS, UV-vis and TEM methods were used tocharacterize the samples.The results show that: the thickness of the TiO₂deposition film on thesurface of the fly ash bead via the hydrolysis-precipitation is about1²Î¼m.The TiO₂crystal phase contains the anatase and rutile phase. As thecalcination temperature increased to900℃, anatase phase transformed intorutile phase. The samples after calcination at500℃for2h has the best photocatalytic activity, for which the degradation rate of methyl orange canreach99%under300W UV irradiation for45mim. XPS analysis showed thatthe main doping mechanism of nitrogen was that nitrogen atoms substituteoxygen atoms to form O-Ti-N bond in TiO₂lattices. S elements are in twoconditions: S2-substitutes O₂-and S6+substitute Ti4+. The visible absorptionedge had been expanded from390nm to500nm by the doping of N and S. Thedegradation rate of MO reached the maximum value of82.48%afterirradiation under250W halogen lamp for1h. On the same condition, thedegradation rate of undoped samples is less than10%. In the process ofhydrothermal method, the morphology of nano-TiO₂was influenced by KOHconcentration, washing method and hydrothermal time. When KOHconcentration was7mol/L, the hydrothermal time was72h,0.1mol/L HClwas used for washing, the prepared TiO₂/fly ash beads composite showed thebest photocatalytic properties. TEM observation revealed the multilayertubular structure of the material. The mechanism of hydrothermal process isthat the TiO₂changed into lamellar K2TiO3, then curled up by ion exchangeand surface effect, after calcining, formed one-dimensional nano TiO₂/fly ashbead composite photocatalyst. Photocatalytic results showed that thedegradation rate to methyl orange was36%under300W mercury lampirradiation for1h. The reaction of KOH and beads can be redused in this wayof using piranha solution for pretreatment on the beads.The degradation rate ofthe prepared nano-TiO₂/fly ash beads can reach99%in40min under the samecondition.