Showing papers by "Michael R. Hoffmann published in 1995"

Environmental Applications of Semiconductor Photocatalysis

Abstract: The civilian, commercial, and defense sectors of most advanced industrialized nations are faced with a tremendous set of environmental problems related to the remediation of hazardous wastes, contaminated groundwaters, and the control of toxic air contaminants. For example, the slow pace of hazardous waste remediation at military installations around the world is causing a serious delay in conversion of many of these facilities to civilian uses. Over the last 10 years problems related to hazardous waste remediation have emerged as a high national and international priority.

Development and Optimization of a TiO2-Coated Fiber-Optic Cable Reactor: Photocatalytic Degradation of 4-Chlorophenol.

Abstract: We have developed, characterized, and utilized a photochemical reactor system that employs an optical fiber cable as a means of light transmission to solid supported TiO_2. Light energy is transmitted to TiO_2 particles, which are chemically anchored onto quartz fiber cores, via radial refraction of light out of the fiber. Operational factors that influence the efficiency of the bundled-array optical fiber reactor are as follows: the uniformity and extent of light propagation down the fiber, the degree of light absorption by the TiO_2 coating of the refracted light, and the ability of the chemical substrates to diffuse into the TiO_2 coating. A TiO_2 coating layer that minimizes the interfacial surface area of the quartz core and TiO_2 particles and operation with incident irradiation angles near 90" enhance light propagation down the fibers. A maximum quantum efficiency of Φ = 0.011 for the oxidation of 4-chlorophenol was achieved. This can be compared to a maximum quantum efficiency of Φ = 0.0065 for 4-chlorophenol oxidation in a TiO_2 slurry reactor operated under similar conditions.

Chemical mechanism of inorganic oxidants in the TiO2/UV process: increased rates of degradation of chlorinated hydrocarbons.

Abstract: Particulate suspensions of TiO_2 irradiated with UV light at wavelengths shorter than 385 nm catalyze the autooxidation of chlorinated hydrocarbons such as 4-chlorophenol (4-CP). The addition of oxyanion oxidants such as CIO_2^-, CIO_3^-, IO_4^-, S_2O_8^(2-), and BrO_3^- increases the rate of photodegradation of 4-CP in the following order: CIO_2^- > IO_4^- > BrO_3^- > CIO_3^-. In the absence of TiO_2, CIO_3^- shows no photoreactivity toward 4-CP, while HSO_5^- and MnO_4^- exhibit rapid thermal reactivity with 4-CP. BrO_3^- appears to increase photoreactivity by scavenging conduction-band electrons and reducing charge-carrier recombination. With CIO_3^- as an oxidant, the degradation of 4-CP appears to follow three concurrent pathways. Kinetic equations for the rate of degradation of 4-CP as a function of [4-CP], [CIO_3^-], and [O_2] and of the light intensity are derived from a proposed mechanism.

Photoreductive Mechanism of CCl4 Degradation on TiO2 Particles and Effects of Electron Donors.

Abstract: The photoreductive degradation of CCl_4 in TiO_2 particulate suspensions in the presence of a variety of organic electron donors (alcohols, carboxylic acids, and benzene derivatives) has been examined. The rate of CCl_4 dechlorination can be enhanced significantly when alcohols and organic acids are used as electron donors. Alcohols with α-hydrogens show complex behavior due to the formation of intermediate α-hydroxyalkyl radicals, which directly reduce CCl_4. Kinetic isotope effects and structurere-activity relationships show that hydrogen-abstraction by hydroxyl radicals plays an important role in the hole-scavenging mechanism. The pH of the TiO_2 suspension influences the rate of CCl_4 reduction either by altering the electrostatic interactions of electron donors on the TiO_2 surface or by changing the reduction potential of the conduction band electron in a Nernstian fashion. It is demonstrated that CCl_4 can be fully degraded under both oxic and anoxic conditions. CHCl_3, C_2Cl_4, and C_2Cl_6 are detected as intermediates during photolysis at pH 2.8 while no intermediates are formed at pH 12.4. A photodegradation mechanism of CCl_4 that includes both one-electron and two-electron transfer processes is proposed. Dichlorocarbene, which is formed through a two-electron reduction of CCl_4, is directly trapped during the photolysis.

Sonolytic hydrolysis of p-nitrophenyl acetate: the role of supercritical water

Abstract: Ultrasonic irradiation is shown to accelerate the rate of hydrolysis of p-nitrophenyl acetate (PNPA) in aqueous solution by 2 orders of magnitude over the pH range of 3-8. In the presence of ultrasound, the observed first-order rate constant for the hydrolysis of PNPA is found to be independent of pH and ionic strength with k_(obs) = 7.5 x 10^(-4) s^(-1) with Kr as the cavitating gas, and k_(obs) = 4.6 x 10^(-4) s^(-1) with He as the cavitating gas. The apparent activation parameters for sonolytic catalysis are ΔH^≠ (sonified) = 211 Kj/mol, ΔS^≠(sonified) = -47/Jl(mol K), and ΔG^≠ (sonified) = 248 kJ/mol. Under ambient conditions and in the absence of ultrasound, k_(obs) is a strong function of pH where k_(obs) = k_(H_2O)[H_2O] + k_(OH)[OH-] with k_(H_2O) = 6.0 x 10^(-7) s^(-1) and k_(OH)- = 11.8 M^(-1) s^(-1) at 25 °C. The corresponding activation parameters are ΔH^≠ = 71.5 kJ/mol, ΔS^≠ = -107 J/(mol K), and ΔG^≠ = 155 kJ/mol. During cavitational bubble collapse, high temperatures and pressures exceeding the critical values of water (T > T_c = 647 K and P > P_c = 221 bar) occur in the vapor phase of the cavitating bubbles and at the interfaces between the hot vapors and the cooler bulk aqueous phase. The formation of transient supercritical water (SCW) appears to be an important factor in the acceleration of chemical reactions in the presence of ultrasound. The apparent activation entropy, ΔS^≠, is decreased substantially during the sonolytic catalysis of PNPA hydrolysis, while ΔG^≠ and ΔH^≠ are increased. The decrease ΔS^≠ is attributed to differential solvation effects due to the existence of supercritical water (e.g., lower ρ and є) while the increases in ΔG^≠ and ΔH^≠ are attributed to changes in the heat capacity of the water due to the formation of a transient supercritical state. A dynamic heat-transfer model for the formation, lifetime, and spatial extent of transient supercritical water at cavitating bubble interfaces is presented.

Sonochemical degradation of p-nitrophenol in a parallel-plate near-field acoustical processor.

Abstract: The sonochemical degradation of p-nitrophenol (p-NP) in a near-field acoustical processor (NAP) is investigated. The pseudo-first-order rate constant, k, for p N P degradation increases proportionally from 1.00 x to 7.94 x 10^(-4) s^(-l) with increasing power to volume ratio (i.e., power density) over the range of 0.98-7.27 W/cm^3. An increase in the power-to-area ratio (i.e., sound intensity) results in an increase in k up to a maximum value of 8.60 x 10^(-4) s^(-1) a sound intensity of 1.2 W/cm^2. A mathematical model for a continuous-flow loop reactor configuration is required in order to extract k from the experimentally observed rate constant, k_(obs), which is a function of the relative volumes of reactor and reservoir. The nature of the cavitating gas (Ar, O_2) is found to influence the overall degradation rate and the resulting product distribution. The rate constant for p-NP degradation in the presence of pure O_2, k_(O_2), = 5.19 x 10^(-14) s^(-1), is lower than in the presence of pure Ar, k_(Ar) = 7.94 x 10^(-4) s^(-1). A 4:l (v/v) Ar/O_2 mixture yields the highest degradation rate, k_(Ar/O_2) = 1.20 x 10^(-3) s^(-1). Results of these experiments demonstrate the potential application of large-scale, high-power ultrasound to the remediation of hazardous compounds present at low concentrations. The NAP is a parallel-plate reactor that allows for a lower sound intensity but a higher acoustical power per unit volume than conventional probe-type reactors.

Photoreduction of Iron Oxyhydroxides and the Photooxidation of Halogenated Acetic Acids

Abstract: The photolytic reduction of ferrihydrite (am-Fe_2O_3*3H_2O), lepidocrocite (γ-FeOOH), goethite (a-FeOOH), hematite (α-Fe_2O_3), maghemite (γ-Fe_2O_3) and iron-containing aerosol particles (Fe_(aerosol)) in the presence of a series of halogenated acetic acids has been investigated. The fastest rates of photoreduction of Fe(lll) to Fe(ll) were achieved with ferrihydrite as an electron acceptor and fluoroacetic acid as an electron donor. The relative rates of photooxidation of the monohalogenated acetic acids with ferrihydrite in order of decreasing reactivity were as follows: FCH_2CO_2H > CICH_2CO_2H > BrCH_2CO_2H > ICH_2CO_2H; for multiple substituents the relative order of reactivity was as follows: FCH_2CO_2H > F_2CHCO_2H > F_3CCO_2H. With respect to the iron oxide electron acceptors, the relative order of reactivity toward monohaloacetate oxidation was am-Fe_2O_3-3H_2O > γ-Fe_2O_3 > γ-FeOOH ≥ α-Fe_2O_3 ≥ Fe_(aerosol) > α-FeOOH. Strong kinetic isotope effects observed for the photooxidation of CICD_2CO_2H suggest that the oxidation of the mono- and disubstituted haloacetic acids proceeds via hydrogen-atom abstraction by surface-bound hydroxyl radicals to produce haloacetate radicals, which in turn yield the corresponding halide and glycolic acid. Fully halogenated haloacetic acids appear to be oxidized via a photo-Kolbe mechanism to yield the corresponding halo acids and CO_2.

Time-Resolved Radio Frequency Conductivity (TRRFC) Studies of Charge-Carrier Dynamics in Aqueous Semiconductor Suspensions

Abstract: The time-resolved radio-frequency conductivity (TRRFC) method provides a useful tool for in situ measurements of charge carrier dynamics in aqueous suspensions of semiconductor particles. In this report, the effects of pH on surface states of ZnO and the effects of hole scavengers (2-propanol) are examined. The experimental results are interpreted in terms of surface mediated recombination processes, in which holes are trapped in a fast process by surficial sites on the ZnO. Recombination rates appear to be governed by the reaction rate of electrons with surface-trap sites. At higher pH (i.e., pH 12), electrostatic repulsion due to a negatively charged ZnO surface leads to slower surface recombination rates compared to lower pH (Le., pH 7) conditions. Addition of a hole scavenger significantly decreases the absolute charge-carrier concentration as detected by TRRFC. This decrease is attributed to the loss of trapped surface holes or surface-bound OH due to oxidation of the hole scavenger. When 2-propanol is used as a solvent, the holes react in a fast step with the solvent at the semiconductor interface within the time resolution of the experiment. The observed TRRFC signal is then due to electrons which are thought to be predominantly transferred to dissolved oxygen (O_2) leading to the formation of hydroperoxyl radicals (HO_2) and subsequently hydrogen peroxide (H_2O_2).

Photochemical destruction of chemical contaminants on quantum-sized semiconductor particles

Abstract: When the crystallite dimension of a semiconductor particle falls below a critical radius of approximately 10 nm, the charge carriers appear to behave quantum-mechanically as a simple particle in a box. As a result of this confinement, the bandgap increases and the band edges shift to yield larger redox potentials. The solvent reorganization free energy for charge transfer to a substrate, however, remains unchanged. The increased driving force and the unchanged solvent reorganization free energy in size-quantized systems are expected to increase the rate constant of charge transfer in the normal Marcus region. Thus, the use of size-quantized semiconductor TiO{sub 2} particles may result in increased photoefficiencies for systems in which the rate-limiting step is charge transfer. One such system is the oxidation of many common organic pollutants in the presence of TiO{sub 2} irradiated with bandgap illumination. In the present study, the photodegradation of several chlorinated compounds in the presence of Q-TiO{sub 2} are used as control reactions to study the size-quantization effect on photoreactivity. The stoichiometry of the reactions is given.