Solar Light-Responsive Pt/CdS/TiO2 Photocatalysts for Hydrogen Production and Simultaneous Degradation of Inorganic or Organic Sacrificial Agents in Wastewater
VASILEIA M. DASKALAKI,† MARIA ANTONIADOU,‡ GIANLUCA LI PUMA,§ DIMITRIS I. KONDARIDES,† AND P A N A G I O T I S L I A N O S * ,‡ Department of Chemical Engineering, University of Patras,GR-26504 Patras, Greece, Engineering Science Department, University of Patras, 26500 Patras, Greece, and Photocatalysis & Photoreaction Engineering, Department of Chemical and Environmental Engineering, The University of Nottingham, University Park, Nottingham NG7 2RD, United Kingdom
The reaction is initiated by excitation of a semiconductor photocatalyst, which results in the promotion of anelectron (e-) from the valence band (VB) to the conduction band (CB) of the semiconductor and the creation of a hole (h+) in the VB 8 SC 9 e- + h+
hν g Ebg
In the absence of suitable traps or scavengers of photogenerated carriers, electrons and holes recombine releasing heat e- + h+ f heat (3)
Received December 22, 2009. Revised manuscript received March 30, 2010. Accepted April 1, 2010.Recombination is mainly responsible for the generally low quantum efﬁciency of semiconductor-mediated photocatalytic processes (2, 3). Under favorable conditions, photogenerated carriers can migrate to the photocatalyst surface where electrons may reduce protons to produce hydrogen, whereas holes may oxidize water to produce oxygen 2e- + 2H+ f H2 (4) (5)
Photocatalytic degradation of wastematerial in aqueous solutions and simultaneous production of hydrogen was studied with the double purpose of environmental remediation and renewable energy production. Both powdered and immobilized Pt/CdS/TiO2 photocatalysts were used to oxidize model inorganic (S2-/SO32-) and organic (ethanol) sacriﬁcial agents/ pollutants in water. Powdered Pt/CdS/TiO2 photocatalysts of variable CdS content(0-100%) were synthesized by precipitation of CdS nanoparticles on TiO2 (Degussa P25) followed by deposition of Pt (0.5 wt %) and were characterized with BET, XRD, and DRS. Immobilized photocatalysts were deposited either on plain glass slides or on transparent conductive ﬂuorinedoped SnO2 electrodes. The results show that it is possible to produce hydrogen efﬁciently (20% quantum efﬁciency at 470 nm) byusing simulated solar light and by photocatalytically consuming either inorganic or organic substances. CdS-rich photocatalysts are more efﬁcient for the photodegradation of inorganics, while TiO2-rich materials are more effective for the photodegradation of organic substances.
2h+ + H2O f
1 O + 2H+ 2 2
The production of hydrogen by solar photocatalytic reforming oforganic/inorganic substances dissolved in water and wastewater is a green, carbon neutral and an environmentally signiﬁcant process for simultaneous removal of pollutants and photochemical conversion and storage of solar energy (1-3). Splitting of water over irradiated semiconductors can be described by the reaction 1 H2O 9 H2 + O2 ∆Go ) 237 kJ mol-1 8 2 semiconductor
hν g Ebg
The ability ofphotogenerated charge carriers to split water depends on the relative positions of the energy levels of the conduction band (ECB) and the valence band (EVB) of the semiconductor with respect to the hydrogen and oxygen evolution potentials of water, respectively. In particular, water cleavage may occur in a cyclic manner only when ECB < E(H+/ H2) and EVB > E(O2/H2O). Among the semiconductor materials whichfulﬁll these requirements (e.g., TiO2, ZnO, SiC, CdS, CdSe), titanium dioxide (TiO2) has attracted the most attention because of its chemical and photochemical stability, biological inertness, and low cost. The major disadvantage of TiO2 is that, due to its relatively large energy band gap (3.2 eV for anatase), it can be activated only by photons in the near UV-region. Theoretically, the lowest...