Catalytic - Photocatalytic

Laboratory of Catalytic Photocatalytic Processes (Solar Energy – Environment)

Research Director

Dr. Elias Papaconstantinou, Rank A

Permanent Scientific Personnel

Dr. Anastasia Hiskia, Research Rank B.

Graduate Students and Research Associates

Aris Troupis

Stamatia Antonaraki

Eleni Gkika

Pigi Kormali

Katerina Tsimili

Research Facilities

Variety of photolysis apparatus, UV-Visible-near IR spectrophotometers, Ultra Sound apparatus, Cyclic Voltametry, GC-MS, GC with various detectors (ECD, FID, TCD), HPLC with various detectors (UV, FL, Cond.), Solid Phase Extraction apparatus, Solid Phase Micro-Extraction apparatus, Ion Chromatography, Atomic Absorption, Pilot unit for decontamination of aquatic media.

Research interests

Catalytic photocatalytic reactions for light (solar) energy utilization, environmental detoxification (Advanced Oxidation Processes) and environmentally friendly methods.

Metal oxides, mainly TiO₂, and polyoxometallates (POM) mainly of W, have been used in thermal and mainly photochemical reactions according to the following photocatalytic cycle.

Our pioneer contribution in this photocatalytic cycle has been with the use of POM whose representative structures and spectra are shown below:

The structures of several categories of POM. PW₁₂O₄₀^3-, Keggin structure; P₂W₁₈O₆₂^6-, Dawson-Wells structure. The stucture of isopoly W₁₀O₃₂^4-. Mixed Dawson-Wells structure, for instance P₂W₁₅Mo₃O₆₂^6- in which three W atoms have been replaced by Mo atoms. Transition metal substituted polyoxometalate (TMSP) with Keggin structure, for instance, [PW₁₁O₃₉Mn(H₂O)]^6-. Oxidized and reduced (by one and two electrons) spectra of PW₁₂O₄₀³, showing the O® M CT band, d-d transitions and the intervalence electron transfer band (M—M CT).

POM upon illumination at the O® M CT band (i.e., in UV and near-visible area) become powerful oxidizing reagents able to oxidize various organic compounds. In the process POM undergo stepwise reduction accumulating electrons that can be subsequently delivered, via thermal reactions to a variety of oxidants. This way a great variety of organic compounds are oxidized and indeed mineralize to CO₂, H₂O and inorganic anions and several organic and inorganic compounds can be reduced via a photocatalytic process in which POM serve as electron relays. Thus, in principle, POM can serve as decontamination photocatalysts of aquatic media removing both organic pollutants and metal ions. Under special conditions controlled photocatalytic reactions have been reported. In addition the reduction precipitation of metal ions mediated by POM may lead to formation of metal nanoparticles in which POM serve as reducing reagents and stabilizers.

Our main contribution has been with the use of POM in the following areas, (representative references are given in parentheses):

(a) Water splitting (Hydrogen production) [J. Chem. Soc., Chem. Commun.., 12 (1982); , Inorg. Chem. 1985, 24, 439]

(b) Photoelectrochemical production of electricity [, Inorg. Chim. Acta, 1983, 75, 235]

(c) Modification of electrodes (photoelectrochemical reactions) [J. Electroanal. Chem.. 1997, 435, 17]

(d) Selective oxidation-synthesis of organic chemicals [Chem. Soc. Rev.,., 1989, 16, 1-31; Phys. Chem. Chem. Phys., 1999, 1, 437-440
(e) Non-selective oxidation (photodegradation) of organic pollutants to CO₂, H₂O and inorganic anions.

A great variety of toxic compounds such as incecticides, herbicides, etc., undergo complete photodecomposition (mineralization) upon illumination in the presence of POM. [Chem. Soc. Rev., 2001, 30, 62-69]

Photodecomposition of ortho-chlorophenol (2.0 mM), formation of CO₂ and Cl^- and (insert) formation and decay of some intermediates, upon photolysis of substrate in the presence of W₁₀O₃₂^4- (0.7 mM); λ > 320 nm, pH 2.5 (HClO₄), T 20 ⁰C.

(f) Reduction-removal of metallic ions.

Recently, a novel photocatalytic method for the selective reduction and recovery of several metal ions from their aqueous solutions based on POM has been reported By suitable choice of POM and organic substrate, several metal ions (Ag⁺, Cu²⁺, Pd²⁺, Au³⁺, Hg²⁺ etc.), can be reduced to a lower or zero oxidation states. This way, photocatalytic decontamination of aqueous solutions, from organic pollutants and metal ions can take place, upon illumination in the presence of POM [New Journal of Chemistry, 2001, 25, 361; Environ. Sci. Technol., 2002, 36, 5355-5362]

(g) Synthesis of metal nanoparticles.

The diversified redox chemistry of POM and the fact that the reactions

POM + S POM(e^-) + S_ox

POM(e^-) + Mⁿ⁺ ® POM + M⁰_coll

(S = organic substrate, Mⁿ⁺ = metal ion)

can be separated in time and space or take place in one pot system, makes these compounds capable of functioning as new multifunctional mediators showing both reducing and stabilizing ability in the synthesis of metal nanoparticles [Angewandte Chemie, Intern. Ed., 2002, 41, 1911 – 1914].

Absorption spectra of metal nanoparticles (Ag, Au, Pd, Pt) in solution, obtained after thermal reaction of photogenerated 1-equivalent reduced POM, SiW₁₂O₄₀^5-, ca 0.35 mM (spectra also shown), with the corresponding metal ions [Ag⁺, Pd²⁺, PtCl₆^2-(0.1mM) or AuCl₄^-(1mM)]; pH 5.

Other research interests:

(i) Immobilization of photocatalysts in optically active and/or inert substrates

(ii) Sensitisation of photocatalysts towards the visible light,

(iii) Use of high frequency sonar system for remediation of aquatic media [Environ. Sci. Technol., 2001, 35, 2358 – 2364].

(iii) Development of new methods of analysis for trace organic pollutants and interservices [Analyst, 1999, 124, 473; Chemosphere, 2003, 51, 69-75]

Research Grants

Research programs and collaborations.

· Ministry for Development, General Secretariat of Research and Technology "ΠΕΝΕΔ', « Polyoxotungstates in : (A) photocatalytic decomposition of pollutants and (B) sensitization of surface areas of conducting and semiconducting materials » 1996 – 98

· Ministry for Development, General Secretariat of Research and Technology. Joint Research and Technology Programs. University of Torino, Italy. « Degradation of Pollutants through homogeneous and heterogeneous photocatalysis» 1997 – 98

· Ministry for Development, General Secretariat of Research and Technology. Joint Research and Technology Programs. University of Leipzig, Germany. « Photocatalysis in Waste Decontamination »), 1997 - 99.

· Ministry for Development, General Secretariat of Research and Technology. Joint Research and Technology Programs. Ecole Central de Lyon, France, « Comparison of mechanism of photocatalytic degradation of organic pollutants in aqueous solutions in the presence of polyoxometalates or TiO₂", 1998 – 99 (one year). (Project leader on the Greek side Dr. A. Hiskia of this Lab.).

· Ministry for Development, General Secretariat of Research and Technology. Joint Research and Technology Programs. University of Leipzig, Germany. «Sonochemical and photocatalytic destruction of triazine herbicides » ,1999 - 01.

· Ministry for Development, General Secretariat of Research and Technology, joint project with University of Ioannina and Greek State Chemical Laboratories. «Development of decontamination processes. Polyoxides in photodegradation of herbicides » (Project leader Dr. A. Hiskia)

· Ministry for Development, General Secretariat of Research and Technology. « Industrial Waste treatment project », jointly with the University of Patras and the Company CHEMO, Greece. 1999 (for 18 months)

· Ministry for Development, General Secretariat of Research and Technology. «ΠΕΝΕΔ» jointly with the Inst. of Material Science, Demokritos and the University of Kriti, "Natural and Technical Photosynthesis" 1999-00 (18 months) (Project leader Dr. V. Petrouleas, Inst. of Material Science.)

· Ministry for Development, General Secretariat of Research and Technology, joint research project with Fudam University, Shanghai, China. "Immobilized polyoxometallates on inert and active supports employed as heterogeneous photocatalysts for decontamination of aqueous media" 2003 – 05.

Selected Publications