Showing papers by "Wonyong Choi published in 2003"

Highly enhanced photoreductive degradation of perchlorinated compounds on dye-sensitized metal/TiO2 under visible light.

Abstract: This study reports an example of visible-light photocatalyst based on TiO2 modified by ruthenium-complex sensitizers and noble metal deposits. The photodegradation of trichloroacetate (TCA) and carbon tetrachloride was used as a probe reaction for evaluating the visible light activity of the photocatalyst under the illumination of λ > 420 nm. Photodeposition of platinum nanoparticles on dye-sensitized TiO2 (Pt/TiO2/RuIIL3) drastically enhanced the degradation rate of TCA and CCl4. The visible light reactivity of Pt/TiO2/RuIIL3 was optimal with [RuIIL3] = 10 μM, [TiO2] = 0.5 g/L, and Pt loading of about 0.2 wt %. Although no electron donors to regenerate the oxidized Ru-sensitizers were added in the aqueous suspension, the photoreductive dechlorination of perchlorinated compounds proceeded far beyond the stoichiometric limit of the initial sensitizer concentration. Water acted as an electron donor to regenerate the sensitizer with a concurrent production of dioxygen. On the other hand, Pt/TiO2/RuIIL3 was c...

Visible light and Fe(III)-mediated degradation of Acid Orange 7 in the absence of H2O2

Abstract: The photodegradation of Acid Orange 7 (AO7) was successfully achieved in the presence of Fe(III) ions only under visible light (λ≥420 nm). Upon adding Fe(III) to AO7 solution, ferric ions formed complexes with AO7 mainly through the azo chromophoric group. This AO7–Fe(III) complex formation was highly pH-sensitive and maximized around pH 3.7. The visible light-induced degradation of AO7 was effective only when the complex formation was favored. The AO7 photodegradation accompanied the production of ferrous (Fe2+) ions and was not inhibited in the presence of excess OH radical scavenger (2-propanol), which indicated that OH radicals were not responsible for the dye degradation. The proposed mechanism of the dye degradation is the visible light-induced electron transfer from the azo chromophoric group to the iron center in the complex. Therefore, when the formation of AO7–Fe(III) complex was inhibited in the presence of excess interfering anions such as sulfites and sulfates, the photodegradation of the dye was also prevented. The photodegradation of AO7 under visible light produced o-phthalate and 4-hydroxybenzenesulfonate (4-HBS) as major products but did not reduce the total organic carbon (TOC) concentration. Since this process does not require the addition of hydrogen peroxide, it might be developed into an economically viable method to pretreat or decolorize azo-dye wastewaters using sunlight.

Photoelectrochemical Investigation on Electron Transfer Mediating Behaviors of Polyoxometalate in UV-Illuminated Suspensions of TiO2 and Pt/TiO2

Abstract: The role of electron acceptors in semiconductor photocatalysis is critical in determining the overall photoefficiencies, kinetics, and mechanisms. Polyoxometalates (POMs) are efficient electron acceptors and carriers in TiO2 photocatalysis. The electron transfer mediating behaviors of a POM (PW12O403-/PW12O404-) couple were investigated photoelectrochemically in the UV-illuminated TiO2 or Pt/TiO2 suspensions and compared with those of an Fe3+/Fe2+ couple. POMs successfully transfer CB (conduction band) electrons on TiO2 particles to an inert collector electrode with generating photocurrents under UV illumination. The magnitude of photocurrent is directly proportional to the reduced POM concentration ([POM-]) and markedly decreases in the presence of dissolved O2 due to the rapid reoxidation of the reduced POMs by O2. Electron donors are essential in inducing photocurrents, which are negligibly small in the absence of them. The photocurrent generation also depends on the kind of electron donors. Whereas fo...

Effects of surface fluorination of TiO2 on the photocatalytic degradation of tetramethylammonium

Abstract: Photocatalytic degradation (PCD) of tetramethylammonium (TMA, (CH3)4N+) ions in water was studied using both naked-TiO2 and fluorinated-TiO2 (F–TiO2) in order to investigate how the modification in TiO2 surface functional groups affects the PCD reaction. A comparison between the naked-TiO2 and F–TiO2 systems shows that their relative photoreactivities strongly depend on pH. At pH 3, the addition of fluoride decreases the PCD rate whereas higher degradation rates are obtained at pH 5 and 7 with F–TiO2. Little fluoride effect on the TMA degradation rate is observed at pH 9. The addition of fluoride affects not only the PCD rate but also the mechanistic pathways of TMA degradation and subsequently the intermediates and product distribution. The modeling result of TiO2 surface speciation shows that the fluoride addition at pH 3 shifts the dominant surface species from Ti–OH2+ and Ti–OH to Ti–F (to near completion). This reduces the surface positive charge of TiO2 (at pH 3) upon adding fluoride and consequently lowers the electrostatic repulsion between the TMA cations and TiO2 surface. Accordingly, ATR-FTIR spectroscopic measurements show that the TMA concentration at the water/TiO2 interface is higher on F–TiO2 than naked-TiO2 film at pH 3. However, the PCD of TMA on F–TiO2 at pH 3 is reduced on the contrary, which is ascribed to the depletion of surface OH groups that are the site of surface OH radical formation. At pH 5 and 7, the surface OH sites are not completely diminished even in the presence of fluoride and the presence of surface Ti–F species in fact increases the TMA degradation rate. The fluoride-induced enhancement of PCD is yet to be understood although some speculative arguments are presented.

Highly enhanced photocatalytic oxidation of CO on titania deposited with Pt nanoparticles: kinetics and mechanism

Abstract: The effects of nanosized Pt deposits on TiO 2 particles on the photocatalytic oxidation of gaseous CO were investigated. Platinum was photodeposited directly on a TiO 2 -coated glass plate with a typical Pt loading of ∼3 wt.%. Platinum deposition on TiO 2 greatly enhances the photooxidation rate of CO in the concentration range 30–500 ppmv, whereas naked TiO 2 samples exhibit much slower oxidation rates. The degree of Pt-induced enhancement strongly depends on the kind of TiO 2 substrate. Three commercial TiO 2 samples (Degussa P25, ISK STS-01, Hombikat UV-100) were platinized and compared for their photoactivity of CO oxidation. Platinized Hombikat TiO 2 shows the best activity among them. The photoconversion of CO to CO 2 is quantitative in the presence of O 2 . The oxidation states of photodeposited Pt include both Pt 0 and Pt II O with the former species dominant. The photocatalytic CO oxidation rates and the amounts of Pt deposited on TiO 2 surface show a close correlation. The CO photooxidation is strongly inhibited in the absence of O 2 or in the presence of alternative oxidants such as N 2 O and H 2 O. The addition of water vapor little affects the CO photooxidation kinetics in the presence of O 2 . Hydroxyl radicals do not seem to play a significant role in CO photooxidation on Pt/TiO 2 . The main oxidants in CO photocatalytic oxidation seem to be produced from adsorbed O 2 . The active oxygen species generated on illuminated Pt/TiO 2 surface are stable enough to survive for several minutes even in a dark period, and take part in a successive photooxidation reaction. The photocatalytic oxidation kinetics was investigated in detail as a function of Pt loading, coated TiO 2 mass, CO concentration, O 2 concentration, light intensity, additives, and humidity.

DFT calculation on the thermodynamic properties of polychlorinated dibenzo-p-dioxins: Intramolecular Cl-Cl repulsion effects and their thermochemical implications

Abstract: Polychlorinated dibenzo-p-dioxins (PCDDs) are one of the most intensively studied chemical pollutants. However, the absence of reliable thermodynamic data on PCDDs seriously limits quantitative understanding of their formation and distribution at combustion sources. By carrying out a DFT calculation (B3LYP/6-31G**) for thermodynamic properties of 75 PCDD congeners, we find that previous thermodynamic estimations on the gas-phase enthalpies of formation (ΔHf) and Gibbs free energy of PCDDs were seriously misestimated, particularly for highly chlorinated congeners, largely as a result of underestimating the intramolecular Cl−Cl repulsion energy. The disagreement between the DFT calculation and other methods becomes progressively larger with increasing chlorine substitution. Octachlorodibenzo-p-dioxin (OCDD) that has been considered as the most thermodynamically stable congener in previous calculations turns out to be much less stable. The differences in calculated ΔHf values between OCDD and the most toxic ...

DFT Calculation on the Electron Affinity of Polychlorinated Dibenzo-p-dioxins

Abstract: Polychlorinated dibenzo-p-dioxins (PCDDs) are extremely toxic and persistent environmental pollutants. Their chemical reactivities and other physicochemical/biological properties show a strong dependence on the chlorination pattern. With increasing the number of chlorines, dioxin congeners become more electronegative and gain higher electron affinities. The vertical electron affinities (VEA) are related with the LUMO energies of neutral molecules. LUMO energies of all PCDD congeners were calculated at the B3LYP/6-31G** level and those of some selected congeners at the level of B3LYP/6-311G**//B3LYP/6-31G** and B3LYP/cc-pvtz/ /B3LYP/6-31G**. The total energies of neutral and anionic species for dibenzo-p-dioxins (DD), 1469-TCDD, 2378-TCDD, and OCDD were calculated at the level of B3LYP/6,31G**, B3LYP/aug-cc-pvdz, and B3LYP/ aug-cc-pvtz//B3LYP/6-31G**. By using the four congeners with D 2 h symmetry as reference molecules, we could estimate VEA (B3LYP/aug-cc-pvdz) of 75 PCDD congeners based on the linear correlations between LUMO energy and VEA (B3LYP/6-31G**) and between VEA (B3LYP/6-31G**) and VEA (B3LYP/aug-cc-pvtz//B3LYP/6-31G**). Results show that all PCDDs with the number of Cl ≥ 3 have positive electron affinities. The PCDD electron affinity values provided in this work can be a useful data set in understanding the congener-specific reactivities of dioxins in various environmental media.

Electronic compass using two-axis micro fluxgate sensing element

Abstract: This paper presents a micro fluxgate sensor fabricated by Si-based Micro Electro Mechanical Systems (MEMS). To measure X- and Y-axis magnetic fields, two fluxgate sensing elements were perpendicularly aligned. The fluxgate sensor was composed of rectangular-ring shaped magnetic core, solenoid excitation and pick-up coils. The 2 /spl mu/m thick Ni/sub 0.8/Fe/sub 0.2/ (permalloy) magnetic core layer was electroplated with photoresist frame using sputtered Ni/sub 0.8/Fe/sub 0.2/ seed layer. The fabricated fluxgate chip size was 4.5/spl times/2.7 mm/sup 2/. Excellent linear response over the range of -100 /spl mu/T to +100 /spl mu/T was obtained with 210 V/T sensitivity at excitation square wave of 2.8 V/sub p-p/ and 1.2 MHz. When two-axis fluxgate sensing element was rotated in terrestrial field, induced second harmonic voltages from X-and Y-axis were sine and cosine wave without distortion, respectively. It is very useful to commercialize the portable navigation system including north-up and map matching, military research, medical research, and space research.

TiO2 광촉매 반응 연구

Abstract: 반도체 광촉매는 태양에너지 전환, 환경오염물질 분해, 초친수성 및 자정기능 소재 등으로 최근 활발히 연구되고 있는 분야이다. 광촉매 연구는 기초 및 응용과학 분야에 걸쳐 매우 다양한 관점에서 진행되고 있으며 관련 연구자들의 학문적 배경도 매우 다양한 학제적 성격을 띠고 있다. 본 총설에서는 광촉매로 가장 널리 사용되고 있는 TiO₂의 특성에 대하여 개관하고, 이를 이용하여 본 연구실에서 수행한 여러 가지 난분해성 오염물질들의 광촉매 분해반응 연구 사례들을 소개한다. 기술된 광촉매 분해반응은 액체/TiO₂, 기체/TiO₂, 고체/TiO₂ 계면 시스템을 모두 포함하여 대상 물질들은 다이옥신, 유기염소화합물, 암모니아, 비소이온, 일산화탄소, 검댕, 고분자 필름 등 이다. 광촉매 상에서의 오염물질 분해반응은 대부분의 경우 표면에서 생성된 OH 라디칼의 강력한 산화력에 기인하나, 전체적인 메커니즘은 OH 라디칼 뿐만 아니라 공유대 정공, 전도대 전자, O₂, superoxides (O₂^(-), HO₂) 등이 관련되는 일련의 산화환원 표면반응이 복잡하게 연계되어 일어난다. 광촉매 분해반응 메커니즘은 대상물질에 따라 매우 다양한 양상을 보이며 일반화시키기 매우 어렵다. 또한, TiO₂ 광촉매에 가시광 활성을 부여하기 위하여 제조된 ruthenium complex-sensitized TiO₂의 특성과 가시광 반응 활성, 표면에서의 광여기 전자전이 반응을 증대시키기 위해 제조된 surface platinized TiO₂ (Pt/TiO₂)의 특성과 몇 가지 광촉매 반응 시스템들에서의 메카니즘의 관련성 등을 논한다.