Yuichi Ichihashi

Photocatalytic reduction of CO2 with H2O on various titanium oxide catalysts

Abstract UV irradiation of highly dispersed, anchored titanium oxide catalyst and finely powdered TiO 2 catalysts in the presence of a mixture of CO 2 and H 2 O led to the formation of (CH 4 + CH 3 OH + CO) and CH 4 , respectively, into the gas phase at 275 K. The efficiency of these photoreactions depended strongly on the kind of catalyst, the ratio of CO 2 to H 2 O, and the reaction temperature. In the case of highly dispersed titanium oxide catalysts, the charge transfer excited state, i.e. (Ti 3+ O − ) 3* of the tetrahedral coordinated titanium oxide species was found to play an important role in the appearance of a highly efficient reactivity. The anatase-type TiO 2 catalyst with large band gap and numerous surface OH groups showed high efficiency for the formation of CH 4 . Detection of Ti 3+ , H, C and CH 3 radicals by electron spin resonance at 77 K suggests that the photoreaction of CO 2 with H 2 O proceeds via the formation of CO and C radicals from CO 2 . The Cu-loading on the small powdered TiO 2 catalyst brought about new formation of CH 3 OH, and the Cu + species on TiO 2 were found to play a significant role in this photocatalytic reaction. In the case of single crystals of rutile TiO 2 , the surface of TiO 2 (100) exhibited a higher efficiency for the formation of CH 4 and CH 3 OH than TiO 2 (110). Measurements of the intermediate species formed on the TiO 2 (100) single crystal were carried out using high resolution electron energy loss spectroscopy.

Characterization of metal ion-implanted titanium oxide photocatalysts operating under visible light irradiation.

The metal ion-implantation of titanium oxide with metal ions (Cr + or V +) at high energy acceleration and the subsequent calcination of these metal ion-implanted TiO 2 in oxygen at around 725 K resulted in a large shift in the absorption spectra toward visible light regions. These catalysts exhibited effective and remarkable photocatalytic reactivity for various reactions such as the decomposition of NO into N2, 02 and N20 at 275 K not only under UV but also under visible light irradiation. XAFS, ESR and UV-Vis studies have revealed that the implanted metal ions (Cr + or V +) are located at the lattice positions of Ti 4+ in TiO2 and stabilized as Cr 3+ or (V 3+ or V 4+) species after the calcination of the sample in 0 2 at around 775 K. Thus, these spectroscopic studies show that the presence of these substitutional metal ion species are, in fact, responsible for the large shift in the absorption spectra of these catalysts toward visible light regions.

Application of ion beam techniques for preparation of metal ion-implanted TiO2 thin film photocatalyst available under visible light irradiation: metal ion-implantation and ionized cluster beam method.

Transparent TiO2 thin film photocatalysts have been prepared on silica glass plate by an Ionized Cluster Beam (ICB) method. In order to improve the electronic properties of these photocatalysts, transition metal ions (V+, Cr+, Mn+, Fe+) were implanted into the TiO2 thin films at high energy acceleration using an advanced metal ion-implantation technique. The combination of these ion beam techniques can allow us to prepare the TiO2 thin film photocatalysts which can work effectively under visible light (lambda>450 nm) and/or solar light irradiation. The investigation using XAFS and ab initio molecular calculation suggests that the substitution of octahedrally coordinated Ti ions in TiO2 lattice with implanted metal ions is important to modify TiO2 to be able to adsorb visible light and operate under visible light irradiation.

Photocatalytic synthesis of CH4 and CH3OH from CO2 and H2O on highly dispersed active titanium oxide catalysts

Abstract UV-irradiation of the highly dispersed titanium oxide catalysts in the presence of CO2 and H2O at 275 K led to the formation of CH4, CH3OH, C2-compounds, CO and O2. The yields of the photocatalytic reaction strongly depended on the types of catalysts, the ratio of CO2/H2O and the reaction temperature. The photocatalytic reduction of CO2 with H2O is linked to the much higher reactivity of the charge transfer excited state, i.e., (Ti3+-O−)∗ of the tetrahedral coordinated titanium oxide species formed on the surfaces. Based on the detection of the reaction intermediate species such as Ti3+, H atoms, and C radicals, a molecular scale reaction mechanism has been proposed.

Application of Metal Ion Implantation for the Design of Visible Light-Responsive Titanium Oxide Photocatalysts

An ion beam technology using accelerated metal ions, a metal ion implantation and an ionized cluster beam (ICB) method have been applied to design unique photocatalysts as new concept for the design of effective photocatalysts. The metal ion implantation of TiO2 and Ti-containing zeolites with highly accelerated metal ions (V+, Cr+, etc.) is useful in designing photocatalysts which can operate efficiently under visible light irradiation. On the other hand, an ICB method with a low acceleration energy is useful in preparing transparent TiO2 thin films on various types of substrates for the efficient photocatalytic degradation of pollutants diluted in water and air. The combination of the ICB method and metal ion implantation can develop the TiO2 thin films which are able to operate not only under UV light but also visible light irradiation.

Direct photo-oxidation of benzene to phenol over Ti/Si binary oxide prepared by sol-gel method

Titanium-silicon (Ti/Si) binary oxides having a varying Ti content were prepared using the sol-gel method and used as photocatalysts. The photo-oxidation of benzene to phenol was carried out using Ti/Si binary oxide catalysts in the presence of benzene, water and gaseous oxygen. The amounts of benzene used hardly affected the phenol yield. On the other hand, the addition of sulfuric acid into reaction solution led to the improvement of phenol formation. The H2O2 formation rate seemed to be related to the phenol yeilds. The photocatalytic reactivity of Ti/Si binary oxide having different TiO2 contents was investigated and it was found to be dramatically enhanced in the range of lower TiO2 contents. XANES, ESR and XRD spectroscopic investigations of these Ti/Si binary oxide catalysts indicated that Ti species were highly dispersed in SiO2 matrices and existed in a tetrahedral coordination. The photo-irradiation of catalysts having tetrahedral Ti species seemed to effectively lead to the formation of H2O2 as an intermediate.

Effect of H2O on the partial photo-oxidation of ethylene over Cu/SiO2 photocatalyst

Ethylene can be catalytically photo-oxidised with water as an oxidant over silica-supported copper oxide evacuated at 973 K. Presence of Cu+ cations in the catalyst is evidenced by X-ray Absorption Near-Edge Structure (XANES). Ethylene oxide is formed at 278 K with a selectivity higher than 80% by the irradiation with UV-filtered light longer than 280 nm.

Cyclohexane photooxidation under visible light irradiation by WO 3 –TiO 2 mixed catalysts

Cyclohexane photocatalytic oxidation was performed over WO3 photocatalysts under visible light irradiation with molecular oxygen. Cyclohexanone and cyclohexanol were obtained with high selectivity. The yield of cyclohexanol and cyclohexanone was increased by using a Pt-loaded WO3 catalyst (Pt/WO3). It was demonstrated by isotope-labeled experiments that cyclohexanone formed through the reaction with lattice oxygen in WO3. Electron spin resonance measurements showed that cyclohexyl radicals form as a reaction intermediate before reacting with the WO3 lattice oxygen. The binary catalyst obtained by physical mixing of TiO2 with Pt/WO3 exhibited a higher photocatalytic activity than that of the Pt/WO3 catalyst. The photocatalytic activity further increased with increasing BET surface area of TiO2 in the binary catalyst. The formation of cyclohexyl radicals was observed to be accelerated over the binary catalyst with respect to Pt/WO3. It is therefore speculated that the surface of TiO2 contributed to the formation of cyclohexyl radicals and/or their stabilization.

Study of Cyclohexane Photooxidation over Pt-WO3 Catalysts Mixed with TiO2 under Visible Light Irradiation

The photooxidation of cyclohexane to cyclohexanone, cyclohexanol and CO2 was carried out by visible light irradiation to tungsten oxide photocatalysts. The physical mixing of tungsten oxide and titanium oxide (WO3+TiO2) led to the high photocatalytic activity. It was speculated that the physical mixing of WO3 and TiO2 took place the charge transfer between WO3 and TiO2, and this inhibited the recombination between electrons and holes on the WO3 surface. The platinum loading on WO3 further developed the photocatalytic activity of WO3+TiO2 photocatalyst. Hence, the degree of the recombination between electrons and holes may dominate the photocatalytic activity of cyclohexane oxidation.

Photocatalytic Decomposition of NH3 Over Fe-Doped TiO2 Prepared by Solid-State Impregnation

Pt/Fe-TiO2 photocatalysts which deposited Pt on Fe-doped TiO2 catalyzed the photodecomposition of NH3 aqueous solution to H2 and N2 under UV irradiation at room temperature. In addition, it was indicated that hydrogen formation by the photodecomposition of NH3 aqueous solution was caused under even visible light irradiation to Pt/Fe-TiO2 photocatalyst. Electron spin resonance spectroscopy showed that the dopant Fe species substituted a portion of the Ti4+ sites in the TiO2 crystal without changing TiO2 structure. From UV–visible diffuse reflectance measurements of Fe-doped TiO2, it was also indicated that the absorption edge of TiO2 was shifted from the ultraviolet to the visible light region by substitution with Fe. Hence, Fe-TiO2 photocatalyst seemed to allow for the effective utilization of irradiation light owing to the presence of a Fe impurity band, thereby leading to its higher activity.