Photo-catalysis is a promising method for the removal of organic pollutants, harmful chemicals and dyestuff from water and air. The activity of a photo-catalyst greatly depends on its ability to create electron-hole pairs, which generates free radicals that undergoes further reactions to rich the effluent degradation. An ideal material should combine: high efficiency in solar energy conversion, high activity, biological and chemical non-toxicity, stability, availability and low cost. TiO2 is the most widely used photo-catalyst since it presents most of these properties. However, the rapid recombination of its photo-generated electron-hole pairs as well as its large band gap energy diminishes its effectiveness. Several strategies can be used to improve its catalytic efficiency, among them doping/loading with metal or nonmetal ions. However, controversial results depending on the catalyst preparation method, metal loading, precursor, kind of pollutant to be degraded, etc. have been reported.
Introduction:
Climate change and the consumption of non-renewable resources are considered as one of the greatest problems facing humanity. In this sense, preservation and treatment of water had become a priority over the world. The interest in the reuse and recycling of wastewater effluent is rapidly growing and turning into a necessity for water utilities. Nevertheless, a major fraction of pollutants resulting from chemical industries is presented as wastewaters streams. According to ANDESCO, in Colombia only 30% of wastewaters are treated.
Among water pollutants, phenol and its derivatives are well known for their bio-recalcitrant and acute toxicity, being introduced into the aquatic environment through various anthropogenic inputs: production of resins, herbicides, pharmaceutical products, colorants, paints, etc. According to Central of Pollution Control Board, the discharge limit of phenols in inland water is 1 mg/L and for the Colombian standard for water quality (NTC-813-2010) the maximum permissible value for phenol in drinking water is 0,001 ppm. Conventional wastewater purification systems such as activated carbon adsorption, membrane filtration, chemical coagulation, etc., also generate wastes during the treatment of contaminated water, which requires additional steps and cost.
In a typical photocatalytic system, photo reaction or photoinducedmolecular transformation occurs on the photocatalystsurface. The basic mechanism of photocatalytic reaction isgoverned by the generation of electron’hole pair into thesemiconductor and its transportation to destination (i.e., reactionwith organic pollutants).
TiO2 is the most widely used photo-catalyst since it presents most of these properties. The principal problem of titania is its high band gap. In this way, TiO2 only uses a low fraction of solar spectra: UV light. However, sunlight consists of about 46% visible light, 47% infrared radiation and only 5-7% ultraviolet light. Several strategies have been used to improve photo-catalytic efficiency of TiO2 among them doping/loading with metal or nonmetal ions. However, controversial results were obtained depending on the preparation method, metal loading, metal precursor, etc. Band gap modification by doping with transition metal is commonly used with the purpose of extending light absorption to the visible region creating energy states within the band gap. Doping with transition metals results especially interesting due to their similitude with titania properties such as atomic radius and electronegativity.
Aim of the work:
Synthesis of the modified Tio2 composite for visible light photocatalytic degradation of dyes in aqueous solutions.
Methodology:
The synthesis procedure used for the preparation of Metal/TiO2 systems is presented. Physical, thermal, structural, chemical and photo-catalytic properties of TiO2 and metal/TiO2 systems were studied in order to understand their performance in phenol photo-degradation. Different techniques available in the Universidad Nacional de Colombia (Manizales), the Universidad EAFIT (Medell”n) and the Technical University of Lodz (Poland) were used. Consequently, device specificities of the applied materials and characterization methods are presented below.
1. Chemical reagents
Table 2-1 present the list of the chemical reagents used during the development of this thesis.
Chemical reagent Chemical formula Purity Provider
1 Iron (III) nitrate nonahydrate Fe(NO3)3. 9 H2O ‘ 99% Merck
2 Cobalt (II) nitrate 6-hydrate Co(NO3)2 . 6 H2O ‘ 98% PanReac
3 Copper (II) nitrate 3-hydrate Cu(NO3)2 . 3 H2O ‘ 98% PanReac
4 Ammonium molybdate 4-hydrate (NH4)6 Mo7O24*4 H2O ‘ 99% PanReac
5 Titanium dioxide TiO2 85% Anatase
15% Rutile Degussa P25
6 Phenol C6H5OH ‘ 99% Merck
7 Deionized water H2O 0,06 Micro siems/cm
18 M” cm Thermoscientific
8 Sodium hydroxide NaOH Mayor igual 97 Carlo Erba
reagents
9 Sulfuric Acid H2SO4 95-97% Merk
2. Synthesis of Metal/TiO2 catalysts
Transition metal ion supported TiO2 (M/TiO2) catalysts were prepared by incipient wet impregnation method.
3. Characterization methods:
‘ Thermal Analysis: In order to enable the thermal decomposition of the synthesized samples and eliminate the traces of the unreacted starting materials (if any) and/or products of their decomposition, the as-prepared powders require thermal treatment.
‘ Atomic Absorption Spectroscopy: is based on the fact that metal atoms absorb strongly at characteristic wave lengths, which coincide with the emission spectra lines of the particular metal.
‘ Nitrogen adsorption/desorption isotherms: Gas adsorption measurements are widely used for the characterization of porous solids.
‘ Zero Point Charge: The pH of an aqueous suspension of an oxide depends on the amount of powder in a given volume of water.
‘ Diffused Reflectance Spectroscopy: is a technique based on the reflection of UV-visible radiation by finely divided materials.
‘ Scanning Electron Microscopy – Energy Dispersive Spectroscopy: let to observe and characterize the heterogeneous organic and inorganic materials on a nanometer (nm) to micrometer (”m) scale.
‘ Photo-catalytic Tests: were carried out in a pyrex, batch, cylindrical reactor containing ca. 120 mL of aqueous phenol solution (10-50 ppm) and TiO2 or M/TiO2 catalyst (0.1-0.7 g/L). The pH (1-8) was adjusted with diluted solutions of H2SO4 or NaOH according to acid or basic conditions. Air was continuously bubbling to the agitated solution (10 mL/s).
Conclusions:
Degradation rate decreased due to chemical species adsorbed on the catalyst surface. When used catalyst was employed for the reaction at optimum conditions, a decrease in photo-activity was observed. It was attributed to the organic species absorbed on the surface of catalysts after reaction which cover the active sites, as detected in FTIR and Raman spectroscopy.
References:
Ahmed, S., Rasul, M.G., Brown, R. &Hashib, M.A. (2011a). Influence of parameters on the heterogeneous photocatalytic degradation of pesticides and phenolic contaminants in wastewater: A short review. Journal of Environmental Management. 92. 311’330.
Ahmed, S., Rasul, M.G., Martens, W.N., Brown, R. &Hashib, M.A. (2011b). Advances in heterogeneous photocatalytic degradation of phenols and dyes in wastewater: A review. Water, Air, & Soil Pollution. 215 (1-4). 3’29.
Ahmed, S., Rasul, M.G.G., Martens, W.N., Brown, R. &Hashib, M.A. (2010). Heterogeneous photocatalytic degradation of phenols in wastewater: A review on current status and developments. Desalination. 261 (1-2). 3’18.
Akbal, F. &Onar, A.N. (2003).Photocatalytic degradation of phenol.Environmental Monitoring and Assessment. 83. 295’302.
Akpan, U.G. &Hameed, B.H. (2009). Parameters affecting the photocatalytic degradation of dyes using TiO2-based photocatalysts: A review. Journal of Hazardous Materials. 170 (2-3). 520’529.
Baia, L., Diamandescu, L., Barbu-Tudoran, L., Peter, A., Melinte, G., Danciu, V. &Baia, M. (2011). Efficient dual functionality of highly porous nanocomposites based on TiO2 and noble metal particles. Journal of Alloys and Compounds.509 (6). 2672’2678.
Barrett, E.P., Joyner, L.G. &Halenda, P.P. (1951).The determination of pore volume and area distributions in porous substances. I. Computations from nitrogen isotherms. Journal of American Chemical Society.1896 (1948).
Beaty, R.D. &Kerber, J.D. (1993).Concepts, instrumentation and techniques in atomic absorption spectrophotometry.2nd Ed.The Perkin-Elmer Corporation.
Bellardita, M., Addamo, M., DiPaola, A. &Palmisano, L. (2007).Photocatalyticbehaviour of metal-loaded TiO2 aqueous dispersions and films. Chemical Physics. 339 (1-3). 94’103.
Bottani, E.J. &Tasc”n, J.M.D. (2008). Adsorption by carbons: novel carbon adsorbents. Elsevier Ltd.
Brown, M.E. (2001). Introduction to thermal analysis: Techniques and applications. Kluwer Academic Publishers.
Brundle, R.C., Evans, C.A. & Wilson, S. (1992). Encyclopedia of materials characterization: Surfaces, interfaces, thin films. Butterworth-Heinemann (ed.). Reed Publishing.
Centeno, M.A., Hidalgo, M.C., Dominguez, M.I., Nav”o, J.A. &Odriozola, J.A. (2008).Titania-supported gold catalysts: Comparison between the photochemical phenol oxidation and gaseous CO oxidation performances. Catalysis Letters. 123 (3-4). 198’206.
Choi, W., Termin, A. & Hoffmann, M.R. (1994). The role of metal ion dopants in quantum-sized TiO2: Correlation between photoreactivity and charge carrier recombination dynamics. The Journal of Physical Chemistry.98 (51).13669’13679.
Choudhury, B. &Choudhury, A. (2012). Luminescence characteristics of cobalt doped TiO2 nanoparticles. Journal of Luminescence.132 (1).178’184.
Chun, H., Yuchao, T. &Hongxiao, T. (2004). Characterization and photocatalytic activity of transition-metal-supported surface bond-conjugated TiO2/SiO2. Catalysis Today. 90 (3-4). 325’330.
Colina-M”rquez, J., Zuluaga, L. &MachucaMart”nez, F. (2009).Evaluation of the titanium dioxide photocatalysis for the degradation of a commercial pesticides mixture.Ingenier”a y desarrollo. 26. 156’167.
Delaigle, R., Eloy, P. &Gaigneaux, E.M. (2011). Influence of the impregnation order on the synergy between Ag and V2O5/TiO2 catalysts in the total oxidation of Cl-aromatic VOC. Catalysis Today.
Dieterle, M. (2001). In situ resonance Raman studies of molybdenum oxide based selective oxidation catalysts. Technical University of Berlin.
DiPaola, A., Marc”, G., Garc”a-L”pez, E., Mart”n, C., Palmisano, L., Rives, V. &Venezia, A.M. (2004). Surface characterisation of metal ions loaded TiO2 photocatalysts: structure’activity relationship. Applied Catalysis B: Environmental. 48 (3).223’233.