PHOTOCHEMICAL PROCESSES IN

HETEROGENEOUS SYSTEMS  

TiO2 PHOTOCATALYSIS

TiO2 is a powerful oxidizer. Light energy from ultraviolet radiation in the form of photons excites the electrons on the surface of titanium atoms, moving them from "the valence band to the conductance band". The result of this energy change is the formation of holes in the surface of the titanium atom, and free electrons which are now available to form hydroxide, superoxide or other radicals, which can oxidize organic chemicals, or reduce metal species. TiO2 powder is added to the solution of a given chemical and exposed to ultraviolet light, either from natural sunlight or commercial lamps. Photocatalytic detoxification process consists on the utilization of the wavelength shorter than 400 nm. to promote a strong oxidation reaction. This reaction takes place when such UV radiation photoexcites a semiconductor catalyst in the presence of oxygen. In this circumstances hydroxyl radicals which attack oxidizable contaminants are generated, producing a progressive breaking of molecules yielding only to carbon dioxide, water and dilute mineral acids. Initially, the process is able to oxidize almost any chemical substance owing to its very high positive oxidation potential. In our research laboratory an intensive program of research is being performed to, firstly, demonstrate the feasibility of the process with the most used organic contaminants, and secondly, to outline the main physical and chemical characteristics such as mechanistic and kinetics of this type photocatalytic reactions.

Photochemical processes in heterogeneous systems have gained wide popularity in recent years because of their practical applications in xerography, photography, chemical synthesis, waste treatment and convertion and storage of solar light energy. In this respect the photochemical processes on inorganic oxide surfaces are particularly important. The surface and the intrinsic properties of the support materials are actually control the photochemical behavior of an adsorbed substrate. Reactive inorganic oxide surfaces can participate in photochemical reactions by absorbing the incident photon and transferring charge to an adsorbed molecule or by quenching the excited state of the adsorbed molecule. Recently, the utilization of semiconductor particulate systems as a reactive heterogeneous medium has reached to a tremendous growth in carrying out photochemical transformations of organic and inorganic compounds. One of the particular area of applied research for the application of the semiconductors are the deposition of metals on semiconductor powders. Attractive practical applications include, particularly in terms of photocatalytic reduction of metals, the recovery of metals from industrial wastes or dilute solutions, and selective recovery of one specific metal over several others in solution.

In general, solids with bandgap energies less than 3 eV are considered to be semiconductors. In a semiconductor the highest occupied and the lowest empty energy bands are particularly important. These two bands are separated by an energy gap called the bandgap.The highest occupied energy band is called the valence band and the lowest empty energy band is the conduction band. Semiconductors can be made conductive either by putting extra electrons into the conduction band or by removing electrons from the valence band. Removal of an electron from the valence band creates a positively charged vacancy called a hole. The hole can be regarded as the mobile entity because annihilation of a hole by a nearby electron effectively moves the hole over in space. So electrical current can be carried by either electrons in the conduction band or holes in the valence band, or by both type of charge carriers. Mobile charge carriers can be generated by three different mechanisms. Thermal excitation, photoexcitation and doping. If the bandgap energy is sufficiently small (less than half an electron volt) thermal excitation can promote an electron from the valence band to the conduction band. In a similar manner, an electron can be promoted from the valence band to the conduction band upon the absorption of a photon of light, photoexcitation, provided that hv>Ebg. The third mechanism of generating mobile charge carriers is doping.

Doping is the process of introducing new energy levels into the bandgap. Two types of doping can be distinguished. For n-type doping, occupied donor levels are created very near the conduction band edge. In this case current is carried mainly by negative charge carriers. Likewise, p-type doping corresponds to the formation of empty acceptor levels near the valence band, creating positive charge carriers. In this particular case current is carried mainly by positive charge carriers.

A single particle of semiconductor may provide both the oxidizing and reducing sides for the reaction. With an n-type powder of TiO2 photooxidation occurs and the reduction is the dark side reaction. The reverse is true with a p-type powder. The efficiency of the dark side reactions is not high enough. Higher efficiencies may be obtained by modifying a portion of the powder surface by deposition of metal. Metal doping of the semiconductor can easily be carried out by a photocatalytic process. A particle of metal doped semiconductor can be thougth of as a short-circuated photoelectrochemical cell where the desired overall reaction occurs by electron and hole transfer at two sides on the particle.When light of energy greater than the band gap energy is absorbed by the semiconductor electron-hole pairs are generated. The bands in the bulk of the semiconductor will move with changes in Fermi energy level and the potential drop will appear in a region near the surface of the semiconductor.

This region of potential drop is called the space charge region. Electron-hole pairs formed in the space charge region are separated by the electric field. The holes, in the valence band , move to the surface and the electrons, in the conduction band, move into the bulk semiconductor. Electron-hole pairs generated beyond the space charge region will suffer recombination, except for those that diffuse back into space charge region before recombination occurs. Semiconductors are usually covered with hydroxyl groups on their surfaces as well as physisorbed water molecules. We know that the surface of TiO2 is promtly hydroxylated when the TiO2 is contact with water. On the other hand when water dissociates on a pure TiO2 surface two distinctive hydroxyl groups are formed. Assuming that anatase particles consists of a mixture of these surface planes, complete surface coverege by OH- should be around 5-15 OH-/nm2 on room temperature.

 

First-principles molecular dynamics calculations of water absorption indicates that dissociative adsorption occurs at the five fold-coordinated Ti-site. The vibrational spectra calculated from this hydroxylated surface shows that a clear streching frequency is present for only one of these groups, with vibrations from the other hydroxyl broadened due to hydrogen bonding between the two hydroxyl groups. For oxidation of OH- or H2O to occur the oxidation potential for corresponding reactions must lie above the position of the TiO2 valence band.The band potentials are functions of pH. The oxidation potentials for the reactions are thermodynamically favorable when they remain above valence band throughout the entire pH range. It is therefore under both acidic and basic conditions the oxidation of surface bound OH- and H2O by TiO2 valence band holes to form OH. is thermodynamical possible and expected. The oxidation potentials for many organic compounds are above the valence band of the TiO2. So what we can expect at least thermodynamically they should be able to interact directly with holes at the TiO2 surface. Recent experiments run in water free aerated organic solvents have displayed only partial oxidation of organics.These results indicates that the presence of water or hydroxyl groups appears to be essential for complete oxidative destruction of organic reactants. Also the intermediates detected during the photocatalytic dergradation of organic reactants are typically hydroxylated structures, further suggesting that OH- is the primary attacking species. CH3OH (a hole scavenger) are examined for the photocatalytic degradation of 1,3-dihydroxybenzene (1,3-DHB), by following the carbon dioxide (CO2) formation in a gas recycling reactor. Addition of H2O2 as an electron trap enhances the CO2 formation rate for 1,3-DHB, but there is a limit for the concentration of H2O2 above which hydroxyl radicals (*OH) are consumed in other reactions rather than taking part in oxidative routes. CH3OH being an hole scavenger decreases the degradation rate by reacting with photogenerated holes as well as *OH radicals.

 

 

ARTICLES and PAPERS PRESENTED AT MEETINGS

RELATED TO TiO2 HETEROGENEOUS PHOTOCATALYTIC SYSTEMS

 

 

A. NEREN ÖKTE and Y. İNEL

''Kinetic and Mechanistic Studies in TiO2 Photocatalyzed Degradation of 

1,3-Dihydroxybenzoic acid'': The First European Conference on Oxidation and Reduction Technologies for Ex-Situ and In-Situ Treatment of Water, air and Soil, Sartorius Collge, Gottingen, Germany (2004)

 

A. NEREN ÖKTE and Y. İNEL

''Photocatalytic Degradation of 3,5-dihydroxybenzoic acid: Influence of Metal Deposition on TiO2 (P-25)'': The Ninth International Conference on Advanced Oxidation Technologies for Waret nad Air Remediation, Sheraton Centre Montreal, Montreal, Quebec, Canada (2003)

 

  1. A.NEREN ÖKTE, M. SOWA REŞAT and Y. İNEL 

"Photocatalytic Degradation of 1,3-Dihydroxy-5-Methoxybenzene in Aqueous Suspensions of TiO2: An Initial Kinetic Study", Journal of Catalysis 198, 172 (2001)

 

ABSTRACT

Photocatalytic degradation of 1,3 -dihydroxy-5-methoxybenzene (1,3-DHMB) is investigated in the presence of TiO2. Effects of initial concentration of 1,3-DHMB, pH of the medium, irradiation time, and temperature are examined. Degradation of 1,3-DHMB obeys zero-order kinetics. Following the formation of CO2, the reaction rate constant, 

k, is found as 217 mu mol m(-3) s(-1) and the adsorption constant, K, is found as 4.22 m(3) mol(-1). The highest degradation of 1,3-DHMB and the highest concentration of CO2 are both obtained at pH 9.0. The apparent activation energy is calculated as 17.1 kJ/mol. For the route of degradation, a mechanism is postulated.

 

A. NEREN ÖKTE, M. SOWA REŞAT and Y. İNEL 
"Influence of Hydrogen Peroxide and Methanol on the Photocatalytic Degradation of 1,3-dihydroxybenzene": Toxicol Environ Chem., 79, 171 (2001) 

 

ABSTRACT

Effects of scavengers; hydrogen peroxide, H2O2 (an electron scavenger), and methanol, CH3OH (a hole scavenger) are examined for the photocatalytic degradation of 1,3-dihydroxybenzene (1,3-DHB), by following the carbon dioxide (CO2) formation in a gas recycling reactor. Addition of H2O2 as an electron trap enhances the CO2 formation rate for 1,3-DHB, but there is a limit for the concentration of H2O2 above which hydroxyl radicals (*OH) are consumed in other reactions rather than taking part in oxidative routes. CH3OH being an hole scavenger decreases the degradation rate by reacting with photogenerated holes as well as *OH radicals.

 

A. NEREN ÖKTE, M. SOWA REŞAT and Y. İNEL

"TiO2 Photocatalytic Degradation of 1,3-Dihydroxytoluene'': First International Conference on Semiconductor Photochemistry, University of Strathclyde, Glaskow United Kingdom (2001)

 

Y. İNEL and A. UZUNOĞLU

''Photocatalytic Degradation of Oxalic acid in a Water Suspension of TiO2: An Initial Kinetic Study of CO2Photogeneration'': The Second European Workshop on Water, Air  and Soil Treatment by Advanced Oxidation Technologies: Innovative and Commercial Applications, Ecole Superieure d'Ingenieurs de Poitiers (ESIP), Universite de Poitiers, France (2001) 

 

A. NEREN ÖKTE, M. SOWA REŞAT and Y. İNEL 
"Quantum Yields and Relative Photonic Efficiencies of Substitude 1,3-dihydroxybenzenes” 
J.Photochem. Photobiol. A: Chem.134, 59 (2000)

 

ABSTRACT

1,3-dihyroxybenzene (1,3-DHB) is selected as the standard probe and Degussa P25 TiO2 as the standard photocatalyst for the determination of quantum yields of 3,5-dihydroxytoluene (3,5-DHT), 1,3-dihydroxy-5-methoxybenzene (1,3-DHMB), and 3,5-dihydroxybenzoic acid (3,5-DHBA) based on the relative photonic efficiency concept. A detailed analysis about the incident photon flu and rate of formation of CO2 for the photocatalytic degradation of 1,3-DHB is done. Fraction of Light absorption is measured as a function of incident photon flux and TiO2 loading. When a TiO2 concentration of 1 g/l and an incident photon flux of 10.8 x 10(-6) Einstein/min are used, the quantum yields for the photocatalytic degradation of I,3-DHB, based on disappearance of I,3-DHB, and formation of CO2 are found as 0.06 and 0.34, respectively. The difference between two quantum yields indicate the formation of intermediates during the photodegradation process. The effect of reactant concentration, pH of the medium and temperature of the system are investigated to determine the relative efficiencies of the reactant molecules. Quantum yields of the reactants are found from phi=zeta(r)phi(1,3-DHB) at their natural pH values as 0.22, 0.35, and 0.38 for 3,5-DHT, 1,3-DHMB and 3,5-DHBA, respectively.

 

Y. İNEL, A. NEREN ÖKTE, and M. SOWA REŞAT

''Substituted Recorcinols in Metal Doped TiO2 Aqueous Suspension'': The Fourth International Conference on TiO2Photocatalytic Purification and Treatment of Water and Air, Sheraton Old Town Hotel,Albuquerque, New Mexico, USA (1999)

 

Y. İNEL and A. NEREN ÖKTE 
"TiO2 Sensitized Photomineralization Kinetics of Pthalic Anhydride": Chemosphere 36, 2969 (1998)

 

ABSTRACT

The photomineralization of phthalic anhydride (PA) in aqueous solutions over TiO2 particles has been investigated under UV-illumination, and then kinetics of CO2 generation have been followed. Effects of the aqueous suspension pH, PA concentration, temperature, irradiation time, and light intensity are examined. A correction term, alpha(T), the decrease in the solubility of CO2 with increasing temperature, is taken into account. The activation energy is found to be 3.19 kJ/mol. A quantum yield of 0.22 for CO2 formation is also calculated.

 

Y. İNEL and A. NEREN ÖKTE 
"Photodegradation Kinetics of Adipic Acid in the presence of TiO2": Toxicol Environ Chem., 65, 123 (1998) 

 

ABSTRACT

The kinetics of CO2 photogeneration from adipic acid (AA) sensitized by TiO2 were studied in a gas recycling reactor. The rate of formation of CO2 was examined as a function of [AA], [TiO2], pH of the aqueous suspension, temperature, irradiation time, flow rate, and light intensity. It is found that the rate of CO2 generation conforms to a Langmuir-Hinshelwood type of kinetic scheme. Including a correction term a(T) which accounts for the decrease in the solubility of CO2 with increasing temperature, the activation energy is found to be 14.38 kJ/mole. We propose a possible mechanism for the kinetics of CO2 photogeneration from adipic acid. Based on this mechanism a quantum yield of 0.27 for the CO2 formation is calculated.

 

Y. İNEL, A.NEREN ÖKTE, and M. SOWA REŞAT

"Photocatalytic Oxidation of Recorcinol in a UV-İlluminated Aqueous TiO2 Suspensions"

The 1998 European Workshop on Water and Air Treatment by Oxidation Technologies: Innovative and Commercial Applications, EPLF, Lausanne, Switzerland (1998)

 

Y. İNEL, A.NEREN ÖKTE, and M. SOWA REŞAT

''Heterogeneous Photocatalysis: TiO2 Sensitizad Degradation of Recorcinol; Effect of Methanol, Hydrogen Peroxide, and Metal Ion Dopants'': Chemical Physics III,Boğaziçi Universiy, Bebek, İstanbul, Turkey (1998)

 

I. AKMEHMET BALCIOĞLU and Y. İNEL

“Photocatalytic Degradation of Organic Contaminants in Semiconductor Suspensions with Added H2O2”: J.Environ.Sci.Health, A31,123 (1996)

 

ABSTRACT

The photocatalytic oxidation of aqueous solutions of phenol, p-nitrophenol, 

4-chlorophenol, aniline, hydroquinone and pyridine have been studied in gas recycling reactor utilizing TiO2 as photocatalyst in the presence and in the absence of hydrogen peroxide. The rate of oxidation of organics to CO2 was measured by using natural and artificial near u.v. light. The disappearance of each solute obeys pseudo first order kinetics with respect to organic whereas the rate of mineralization is zeroth order. While the addition of small amounts of hydrogen peroxide significantly enhanced the photomineralization rate of all organic compounds, high concentration of it inhibited the reaction. The optimum concentration of H2O2 ranges from 1x10(-2) to 0.5x10(-2) M for 10(-4) M organic solutions. The complete mineralization for 10(-4) M of each organic resulted in approximately 50-110 minutes whereas this period was shortened to 6-28 minutes by the addition of an optimum amount of H2O2.

 

A. NEREN ÖKTE and Y. İNEL 
"Photocatalytic Degradation of Malonic Acid in Aqueous Suspensions of Titanium Dioxide: An Initial Kinetic Investigation of CO2 Photogeneration": J.Photochem. Photobiol. A: Chem.96, 175 (1996)

 

ABSTRACT

The photocatalytic oxidation of malonic acid in aqueous suspensions of TiO2 was investigated by following the formation of CO2. The rate of formation of CO2 is affected by the pH of the aqueous suspension, TiO2 loading, malonic acid concentration, temperature, irradiation time, flow rate and light intensity. The disappearance of malonic acid fits a Langmuir-Hinshelwood kinetic model. Rate constants were calculated by including a correction term alpha(T) to fake into account the decrease in the solubility of CO2 with increasing temperature. The activation energy is 9.99 kJ mol(-1). A quantum yield of 0.26 for CO2 formation was estimated from the proposed mechanism. 

 

A. NEREN ÖKTE and Y. İNEL 
"Photocatalytic Degradation of Succinic Acid in Aqueous Suspensions of Titanium Dioxide: An Initial Kinetic Investigation of CO2 Photogeneration": Toxicol Environ Chem., 55, 115 (1996) 

 

ABSTRACT

Photocatalytic oxidation of succinic acid in aqueous suspensions of TiO2 has been investigated in a gas recycling reactor, following the CO2 formation. The rate of formation of CO2 was examined as a function of pH of the aqueous suspension, TiO2 loading, the concentration of succinic acid, temperature, irradiation time, flow rate, and light intensity. It is found that the rate of CO2 generation conforms a Langmuir-Hinshelwood kinetic model. Rate constants were calculated by including a correction term  α(T) which stands in order to take into account the decrease in the solubility of CO2 with increasing temperature. Activation energy is found to be 15.59 kJ/mole. A quantum yield of 0.27 for the CO2 formation has been estimated from the proposed mechanisms.

 

Y. İNEL and D. ERTEK (HACIU) 
“Photocatalytic Deposition of Bismuth(III) Ions onto TiO2 Powder”: J.Chem. Soc. Faraday Trans., 89, 129 (1993)

 

ABSTRACT

Photochemical reduction of Bi(III) on illuminated TiO2 suspensions has been investigated. The photoreduction leads to the formation of metallic Bi on TiO2. Adsorption of Bi(III) species in the dark was examined. The adsorption parameters were determined using the classical Langmuir adsorption isotherm. The photocatalytic deposition rate was determined under various experimental conditions, such as pH, dissolved oxygen and Bi(III) initial concentration. The effect of alcohols as hole-scavengers was also investigated. It was found that addition of hole-scavengers increases the efficiency of the photocatalytic deposition process, by impeding recombination of holes and electrons on the catalyst's surface.

 

I. AKMEHMET BALCIOĞLU and Y. İNEL 
“Photocatalytic Degradation of 4-chlorophenol in Aqueous TiO2 Suspensions: the Influence of H2O2 on the Photocatalytic Mineralization Rate”: Doğa-Tr.J. of Chemistry, 17, 125 (1993)

 

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