|Title of Invention||
MINERALIZATION OF A MODEL POLLUTANT OVER SILVER MODIFIED TIO2 IMMOBILIZED ON A ROTATING CYLINDRICAL REACTOR
|Abstract||Ti02 a stable metal oxide semiconductor is a better choice because it is inexpensive, non-toxic and does not undergo photo corrosion although it is UV sensitive. Since the advent of Fujishima and Honda's water electrolysis cell utilizing to decompose water into hydrogen and oxygen, reports on the decomposition of organic contaminants have also been published. But, efficient models are yet to be identified for commercialization. Efficiency of a system is related to various parameters like (i) development/preparation of photocatalysts, (ii) quantifying the absorption capacity of photocatalysts, (iii) design of the photo reactors and (iv) the use of the photcatalysts in the form of powders/slurries/films, A model is proposed which utilizes UV radiation as the light source and silver doped Ti02 thin film photocatalyst immobilized on a cylindrical rotating reactor as the photon absorber to decompose a typical pollutant, methyl orange.|
Studies on photocatalytic properties of oxide semiconductors have been advanced since the photodecomposition of water using the photoelectrode of titanium dioxide (Ti02) was reported, because such materials are expected to be very effective for solar energy conversion such as photodecomposition of water and photosynthesis of organic molecules. In recent years, Ti02 photocatalysis as an attractive technique is applied for the complete destruction of undesirable contaminants in both liquid and gaseous phase by using solar or artificial light illumination. The principle of any such degradation lies in initiating an oxidation or reduction process at the semiconductor surface with light as the only source. TiOa, when used in the form of fine particles, powders or colloids have various advantages such as the increment of surface area and facility in diffusion of excited electrons and holes towards a reaction surface before recombination apart from low operation temperature, low cost and significantly low energy consumption. Most interesting aspect of titanium dioxide photocatalysis is its stability towards photocorrosion, acidic and alkaline environment even though they absorb radiations in the near visible or far ultraviolet region.
Most investigations relating to photocatalytic transformation and mineralization of model pollutants has been studied on aqueous slurries, on powders and on immobilized particulate film of TiOa. A severe handicap noted with use of slurries, powders and/or colloids etc., is loss of photocatalyst materials or a recovery process deems essential. The immobilized particulate thin films of Ti02 on the other hand, used in electrochemically assisted photocatalysis for the degradation of several organic contaminants posed with the problem of agglomeration of Ti02 particles. In a particular investigation, the photoactive Ti02 film for a two compartment photo electrochemical cell was prepared by applying slurry of unmodified Ti02 powder (particle size less than 30 nm) on a conducting glass substrate which overcame the problem of material loss.
Hence, to overcome the existing difficulty prevailing in the aqueous slurry, powder and/or colloidal systems, this invention provides an improved process for immobilization of Ti02 film modified with silver deposits onto a glass substrate, since, most recent investigations dealing with photocatalytic degradation employing Ti02 semiconductors are oriented towards photocatalyst immobilization in the form of a thin film.
Moreover, it is well established that the efficiency of immobilized systems is significantly lower than that of corresponding slurries. An efficiency increase places the photocatalyst immobilization technique even more appealing for wide range applications for which metal loading is beneficial. Hence, a proficient way to enhance the photocatalytic reaction rate is doping of transition metals to semiconductor matrix, whereby, creation of localized energy levels is facilitated for efficient removal of electron-hole pair formed due to band gap irradiation.
So far, noble metal doping has been performed on TiOi matrix in order to hinder the photo generated electron-hole pair recombination and accelerate the photo excitation and formation of oxidizing species. In this context, deposition of silver on titanium dioxide has been of considerable interest for both mechanistic and applicability reasons.
The ultimate objective of this invention is to photomineralize a model pollutant, namely, methyl orange on a rotating cylindrical reactor immobilized with silver modified titanium dioxide and in turn, commercializes the same to treat effluent of any kind. Further, this immobilization technique still in its primitive stage enables industrial applications benefits eliminating the majority of problem encountered with slurries: such as
(a) The need for separation and filtration step.
(b) The problematic use in continuous flow system and
(c) The particles aggregation, especially at high concentrations.
Methodology and Operation
Upon excitation with light radiation equal to the band gap energy of the immobilized silver doped titanium dioxide photocatalyst film, charge carriers such as electrons and holes (e" & h"^) are produced possessing strong reducing and oxidizing power according to the equation :
Since silver metal is doped on the Ti02 matrix, recombination of charge carriers is inhibited necessitating the charge carriers to individually react with reactants at semiconductor surface.
Titanium dioxide is associated with loosely bound surface hydroxyl groups when in contact with aqueous / gaseous phase. The adsorbed OH groups from solution are attacked by holes converting them to OH radicals. Since most photocatalytic degradations are performed in air
saturated conditions, the electrons reduce oxygen to oxygen radicals, O2". The so formed OH and O2" radicals being extremely reactive will oxidize methyl orange into CO2 and H2O with release of mineral acids according to the following reactions.
Preparative procedure & Operation
In the first step, an aqueous solution of 30% water glass is sprayed on a glass pipe with dimensions of 5 cm diameter and 17 cm in length to be used as rotating cylindrical reactor. It is then dried at 80^ C. In the second step, silver loaded titanium dioxide is prepared by addition of 0.5 gms silver nitrate to a dispersion containing Ti02 powders (20 gms) and 0.25 ml methanol in a 1 liter double distilled water. The dispersion is illuminated for 4 hours by a 450 W high pressure mercury lamp. During the course of illumination, the dispersion is purged with nitrogen gas and agitated by a magnetic stirrer. The silver doped titanium dioxide powders are later subjected to filtration, further washed with water and dried at 105^ C. The loading ratio of silver is 11 mg Ag- per gm Ti02.
The thin film of immobilized photocatalyst was obtained by initially dispersing 25 wt% of silver doped Ti02 powders in water and spraying the dispersion onto the outer surface of the continuously rotating glass pipe so as to obtain a uniform coat. This thin film of silver doped Ti02 immobilized on the glass pipe is dried at 500^C for 1 hour. The amount of silver doped Ti02 from a cut section of the film is found to be 12.5 mg Ag-doped Ti02 / m .
The apparatus designed to perform the operation is illustrated in figure-1. The apparatus consists of a silver doped Ti02 immobilized thin film rotating cylindrical reactor (A) made of glass pipe sealed at both ends, one end resting on the container and the other end sliding over the container connected to the motor shaft (B). The rotating speed of the immobilized cylindrical reactor was fixed at 25 rpm. The UV light source (C) is a F 15 W / T 8 black light blue light tube with light concentrating system held attached to source stand (D). The model pollutant, methyl orange filled in the container (E) is degraded completely upon irradiation with light source by the action of exposed immobilized layers of Ti02. Here is an illustrative example of the process: The methyl orange contaminated water of 0.02 mM (mille Molar) concentration is subjected to photo degradation. The
reaction liquid volume of 50 ml is taken in the container (E). Upon continuous rotation fresh layers of the immobilized film is exposed to UV radiation, where the electrons and holes generated due to illumination react with adsorbed pollutant and thus decomposing it into non-toxic substances according to the above equation (eqn 4). The operation of degradation process was performed batch wise.
The degradation of methyl orange contaminated water is studied both in presence and absence of ultraviolet radiation. Without ultraviolet irradiation, silver doped Ti02 photocatalyst did not bleach the dye contaminated water, instead bleaching occurred only when irradiation commenced. Due to continuous rotation, the pollutant in the liquid phase is drawn upward due to viscous sheer and the thin film of the pollutant is now exposed to ultraviolet radiation from UV lamp. The degradation of methyl orange contaminated water is assessed by drawing aliquots of the solution (about 2 ml) at equal intervals of time and analyzing its absorption characteristics at wavelength of 525nm in a UV-VIS Systolic spectrophotometer 119, (^^max of methyl orange is 525 nm). At regular intervals of time (every 10 minutes), the absorption characteristics of the sample solution represented the degradation kinetics and a maximum of 90 % degradation was realized after 1 hour of illumination. Further, complete pollutant depolarization was achieved after 2 hours of illumination. Hence, this photocatalyst immobilization technique works well in achieving complete mineralization of organic pollutants.
1. An apparatus and process for degradation of a model pollutant methyl orange, comprising a photocatalyst namely silver modified Ti02 immobilized on a rotating cylinder made of glass substrate.
2. The apparatus and process as claimed in claim - 1 wherein the liquid phase pollutant is in communication with photocatalyst surface immobilized on glass substrate.
3. The apparatus and process as claimed in claims, 1 and 2 wherein formation of the immobilized photocatalyst is an oxide semiconductor, namely, titanium dioxide, Ti02.
4. The apparatus and process as claimed in claims, 1 to 3 wherein selection of platinum group metal, especially silver as dopant is dispersed to sensitize titanium dioxide immobilized over glass substrate.
5. The apparatus and process as claimed in claims, 1 to 4 wherein the unique immobilization of photocatalyst over water glass in the immobilization process adhering well with the glass substrate.
6. The apparatus and process as claimed in claims, 1 to 5 wherein occurrence of agglomeration of silver doped titanium dioxide particles due to excellent binding with water glass applied on the glass substrate.
7. The apparatus and process as claimed in claims, 1 to 6 wherein the adherence of the silver doped titanium dioxide on the water glass and subsequently the water glass over the glass substrate.
8. The apparatus and process as claimed in claims, 1 to 7 wherein unique and sturdy
existence of both the immobilized film of silver doped T1O2 and water glass
throughout the cycle of operation.
9. The apparatus and process as claimed in claims, 1 to 8 wherein an ultraviolet source
to excite the silver doped titanium dioxide immobilized on glass substrate.
10. The apparatus and process as claimed in claims, 1 to 9 wherein an ultraviolet lamp as
a source of excitation for the immobilized catalyst attached with concentrator.
11. The apparatus and process as claimed in claims, 1 to 10 wherein emitting of radiation
by an ultraviolet lamp source in the wavelength range 200-400 nm.
|Indian Patent Application Number||944/CHE/2004|
|PG Journal Number||38/2007|
|Date of Filing||21-Sep-2004|
|Name of Patentee||M/S. KONGU ENGINEERING COLLEGE|
|Applicant Address||PERUNDURAI,ERODE, TAMIL NADU-638 052|
|PCT International Classification Number||B 01 J 32/00|
|PCT International Application Number||N/A|
|PCT International Filing date|