Research Interests

The research activities of Prof. Hoffmann have been focused on the topical areas of 1) heterogeneous atmospheric chemistry, and 2) advanced oxidation and reduction chemistry and technology, 3) basic aquatic chemistry, 4) semiconductor photocatalysis, and 5) hydrogen production from sunlight via electrochemical water splitting. His research activities over the years have involve both field and laboratory experimentation.

In the subject area of heterogeneous atmospheric chemistry (e.g., clouds, fogs, and haze aerosol), Hoffmann and his group have made significant advances in the determination of the detailed kinetics and mechanisms of the reactions of dissolved sulfur dioxide with hydrogen peroxide, oxygen, and ozone, in advancement of our knowledge of the detailed thermodynamics, kinetics and mechanisms of the equilibrium formation of hydroxyalkylsulfonates from the reaction of dissolved sulfur dioxide with dissolved aldehydes (i.e., aldehyde-bisulfite complexes, in determination of the detailed chemistry of clouds and fogs in coastal and mountainous environments, in determination of the role of aqueous-phase photochemistry in relation to the in situ production of hydrogen peroxide, hydroperoxyl radical, and hydroxyl radical in illuminated clouds, and in the field-based determination of the redox states of dissolved metal ions (e.g., Fe(II) vs. Fe(III) and Cu(I) vs. Cu(II)) in clouds, fogs, and haze aerosol. Recent projects have focused on aerosol reactions at liquid droplet interfaces, on the photolysis of organic molecules in the droplet phase, on the production of humic like organic compounds via photolysis of small molecular organic compounds.

In the subject area of pollution control chemistry and technology, Hoffmann and his group have made significant advances in our understanding of a) the basic chemistry and applications of homogeneous and heterogeneous catalysis involving metal-phthalocyanine complexes, b) in the chemistry and application of hydrogen peroxide for the oxidative elimination of hydrogen sulfide and other reduced sulfur compounds in water and wastewater, c) in the chemistry and application of quantum-sized semiconductor colloids for the oxidative and reductive elimination of chlorinated hydrocarbons, d) in the examination of the underlying chemistry and physics of the application of ultrasonic irradiation in water for the destruction of chemical contaminants, e) for the development and advancement of the pulsed-power plasma process for water treatment, f) for the development and advancement of metal-doped semiconductor electrodes for the electrochemical production of hydroxyl radical from water, g) for the development and characterization of semiconductor-coated fiber optic cable reactors for the heterogeneous photochemical destruction of chemical contaminants in water, and for h) the application of photochemically-activated periodate solutions for chemical compound oxidation.

In the subject area of aquatic chemistry, Hoffmann and his research group focused their research efforts on metal-ligand chemistry relevant to fresh waters, on the chemistry of aquatic humic substances and their interactions with first-row transition metal ions, on the photochemistry of iron oxides and oxyhydroxides in the presence of naturally-occurring organic compounds, on the kinetics and mechanisms of redox reactions involving oxidants such as oxygen, hydrogen peroxide, peroxydisulfate, peroxydiphosphate, ozone, periodate, and hydroxyl radical, and on the kinetics and mechanisms of selected microbial processes involving the oxidation of reduced sulfur compounds, the oxidation of ferrous iron, and the reduction of ferric iron.

Current Research Topics

The Hoffmann group’s current research projects are focused on 1) the photodynamics and photophysics of hybrid semiconductor suspensions, 2) on the kinetics and mechanisms of reactions in haze aerosol with a particular focus on ozone reactions at droplet interfaces, 3) on the photochemical transformation of chemical compounds in ice, 4) on the fundamentals and applications of ultrasonic irradiation for water and wastewater treatment, 5) on photoelectrochemical systems for the production of hydrogen from water, 6) on the photochemical fixation of CO2 using heterogeneous catalysts and semiconductor electrode systems, 7) on the destruction and elimination of perfluorocompounds from water, 8) on the use of photovoltaic panels to drive electrochemical water treatment coupled with CO2 and water reduction, 9) and on the detailed photochemical transformations of small molecular weight organic compounds to produce higher molecular weight oligomers and polymers that mimic the behavior of colored humic-like substances (HULIS) and secondary organic aerosols.

Overview of Past Research Accomplishments

The research accomplishments of Michael Hoffmann are in the topical areas of 1) heterogeneous atmospheric chemistry, and 2) pollution control chemistry and technology, and 3) aquatic chemistry. His research activities involve both field and laboratory experimentation.

In the subject area of heterogeneous atmospheric chemistry (e.g., clouds, fogs, and haze aerosol), Hoffmann and his group have made significant advances in the determination of the detailed kinetics and mechanisms of the reactions of dissolved sulfur dioxide with hydrogen peroxide, oxygen, and ozone, in advancement of our knowledge of the detailed thermodynamics, kinetics and mechanisms of the equilibrium formation of hydroxyalkylsulfonates from the reaction of dissolved sulfur dioxide with dissolved aldehydes (i.e., aldehyde-bisulfite complexes, in determination of the detailed chemistry of clouds and fogs in coastal and mountainous environments, in determination of the role of aqueous-phase photochemistry in relation to the in situ production of hydrogen peroxide, hydroperoxyl radical, and hydroxyl radical in illuminated clouds, and in the field-based determination of the redox states of dissolved metal ions (e.g., Fe(II) vs. Fe(III) and Cu(I) vs. Cu(II)) in clouds, fogs, and haze aerosol.

In the subject area of pollution control chemistry and technology, Hoffmann and his group have made significant advances in our understanding of a) the basic chemistry and applications of homogeneous and heterogeneous catalysis involving metal-phthalocyanine complexes, b) in the chemistry and application of hydrogen peroxide for the oxidative elimination of hydrogen sulfide and other reduced sulfur compounds in water and wastewater, c) in the chemistry and application of quantum-sized semiconductor colloids for the oxidative and reductive elimination of chlorinated hydrocarbons and other contaminants from hazardous waste streams, d) in the examination of the underlying chemistry and physics of the application of ultrasonic irradiation in water for the destruction of chemical contaminants, e) for the development and advancement of the pulsed-power plasma process for the direct in situ destruction of hazardous wastes dissolved in water, f) for the development and advancement of metal-doped semiconductor electrodes for the electrochemical production of hydroxyl radical from water, g) for the development and characterization of semiconductor-coated fiber optic cable reactors for the heterogeneous photochemical destruction of chemical contaminants in water, and for h) the application of photochemically-activated periodate solutions for chemical compound oxidation.

In the subject area of aquatic chemistry, Hoffmann and his research group focused their efforts on metal-ligand chemistry relevant to fresh waters, on the chemistry of aquatic humic substances and their interactions with first-row transition metal ions, on the photochemistry of iron oxides and oxyhydroxides in the presence of naturally-occurring organic compounds, on the kinetics and mechanisms of redox reactions involving oxidants such as oxygen, hydrogen peroxide, peroxydisulfate, peroxydiphosphate, ozone, periodate, and hydroxyl radical, and on the kinetics and mechanisms of selected microbial processes involving the oxidation of reduced sulfur compounds, the oxidation of ferrous iron, and the reduction of ferric iron.

Some of the noteworthy contributions in environmental science and technology originating from the Hoffmann group will be highlighted in the several paragraphs emphasizing contributions in heterogeneous atmospheric chemistry and in pollution control chemistry and technology.

Hoffmann’s detailed investigations of the kinetics and mechanism of the oxidation of dissolved sulfur dioxide in the form of bisulfite by hydrogen peroxide in acidic aqueous solution (Hoffmann and Edwards, 1975; McArdle and Hoffmann, 1983) established the state-of-the-art for our understanding of important pathways for the oxidation of sulfur dioxide to sulfuric acid in the atmosphere. This fundamental reaction is now recognized as the single most important pathway for the conversion of sulfur dioxide to sulfuric acid on a global basis. As much as 80% of the total sulfur dioxide on a global basis is oxidized via the cloud-processing pathway originally proposed by Hoffmann and Edwards (1975). The reaction mechanism involves the nucleophilic addition of hydrogen peroxide to the bisulfite ion to form a peroxymonosulfite intermediate, which, in turn, undergoes a proton-catalyzed rearrangement to give bisulfate as the final product. This mechanism has been formally named the “Hoffmann and Edwards’ Mechanism. In a subsequent study, McArdle and Hoffmann (1983) extended the pH range of the initial study to very low pH values and confirmed an important prediction of the Hoffmann and Edwards’ mechanism that the principal reactive species was bisulfite over the pH range of 0 to 8. Many other investigators over the next 10 years reexamined the kinetics of this reaction only to confirm the validity of the Hoffmann and Edwards’ mechanism over a broad range of conditions relevant to atmospheric conditions.
The next major advancement by Hoffmann and his group involved a detailed field-oriented study of the chemistry of clouds and fogs. The primary focus of these studies was to obtain solid field-based observations of dynamic chemical changes within liquid water droplets and to very predictions of chemical kinetic models (e.g., Jacob and Hoffmann, 1983) for cloudwater acidification. In this regard, Hoffmann and co-workers developed several patented (US Patent Numbers 4,697,462 and 4,732,037) devices for the automatic time-series collection of cloud and fog water from ground-based sampling stations. In 1982, they published a seminal paper (Waldman et al., 1982) in Science entitled “The Chemical Composition of Acid Fogs,” in which they determined the detailed chemical composition of hyperacidic clouds and fogs. Hoffmann and his students measured pH values below 1.7 in coastal marine clouds and fogs. The occurrence of such highly acidic clouds and fogs was not predicted. In later reports, Hoffmann and his group extended these early studies to a variety of different locations in California (Munger et al., 1983; Jacob et al., 1985; Jacob et al., 1986) and around the country.

During the course of their field studies, Hoffmann and his students discovered that many cloud systems in near urban environments were enriched in dissolved sulfur dioxide (i.e., S(IV)) and aldehydes such as formaldehyde, methyl glyoxal, glyoxal, and hydroxyacetaldehyde. Based on their initial observations they (Boyce and Hoffmann, 1984; Munger et al., 1984; Munger et al., 1986; Olson and Hoffmann, 1989) proposed that bisulfite and the aldehydes react in situ within cloud droplets via a classical reaction to form reversibly aldehyde-bisulfite adducts or hydroxyalkylsulfonates as reservoir species for S(IV). A short time later, Hoffmann and his group a new analytical methods that allowed for the direct chromatographic determination of the hydroxyalkylsulfonates in ambient samples. They reported their discovery of the actual occurrence and quantification of these compounds in Science (Munger et al., 1986). In the laboratory, they studied the detailed thermodynamics (Betterton and Hoffmann, 1988), kinetics, and mechanisms of formation of a wide-range of hydroxyalkylsulfonates (e.g., Olson and Hoffmann, 1988ab; Olson et al., 1988)).
Recent work in the area of cloud and fog chemistry has been focused on the detailed chemical speciation and photochemistry of iron-containing solids (e.g., Fe2O3, FeOOH, Fe(OH)3) present in aquated aerosol systems. Hoffmann and his co-workers were able to show that a substantial fraction of the total iron present in aerosols and clouds is found in the reduced ferrous state and that iron species mediate the in situ production of hydrogen peroxide and hydroxyl radical with the concomitant oxidation of organic compounds and sulfur dioxide (e.g., Faust et al., 1989; Erel et al., 1993; Pehkonen et al., 1993).

In the field of pollution control chemistry, Hoffmann (1977) investigated the detailed kinetics and mechanism of the oxidation of hydrogen sulfide by hydrogen peroxide over a broad pH range and proposed that this reaction could be used to conveniently control H2S in water and wastewater systems. Today this simple chemical system is widely used to control the odor, corrosion, and toxicity problems posed by H2S in engineered systems. In the next major advance, Hoffmann and his students (Hoffmann and Lim, 1979; Boyce et al., 1983; Hong and Hoffmann, 1987ab, Leung and Hoffmann, 1989ab) synthesized a series of metal phthalocyanine complexes that were highly effective homogeneous catalysts of the oxidation of hydrogen sulfide in water by molecular oxygen, which is a slow reaction in the absence of any catalytic influences. These catalytic systems were then extended to active (Hong et al., 1987ab) and passive (Hong et al., 1989) porous support systems that could be used in industrial-scale catalytic reactors.

Over the same period of time, Hoffmann and his students (Kormann et al, 1988; Bahnemann, et al., 1987; Hong et al., 1987ab; Choi et al., 1994; Martin et al., 1994abc) carried out a detailed series of investigations in the photochemistry and photophysics of colloidal metal oxide semiconductor systems that could be used for the effective elimination of chemical contaminants from water or for the in situ production of hydrogen peroxide. For example, Hoffmann and colleagues were the first to synthesize nano-sized colloidal ZnO in the quantum-size domain (i.e., the photochemical characteristic such as the band-gap energy increase with decreasing particle size) and show that as the particle sizes decrease toward 2 nm in diameter the quantum yield for the photoreduction of dioxygen to form hydrogen peroxide approaches unity (Hoffman et al., 1994). This research also led to the development of hybrid photocatalytic systems involving the chemical coupling of Co(II)tetrasulfophthalocyanine catalysts to the surface of TiO2 to produce an unusually high reactivity photocatalyst (Hong et al., 1987ab). In the case of the photooxidation of SO2 by oxygen, the measured quantum yield in the presence of the hybrid photocatalyst was greater than 1000, which indicated that the reaction proceeded via the photolytic induction of a free-radical chain reaction on the surface of the functionalized titanium dioxide (Hong et al., 1987b).

Research in the area of metal-doping of the quantum-size semiconductor photocatalysts led directly to the successful development of niobium (V)-doped TiO2-coated titanium anodes for the direct electrochemical production of hydroxyl radical from the oxidation of hydroxide ion and water with current efficiencies approaching 98%. This novel electrochemical system (Kesselman et al., 1997) led to the granting of 3 patents (Weres and Hoffmann, U.S. Patents 5,364,508, 5,419,824; and 5,439,577) and to direct commercialization of the electrochemical reactor system for water and wastewater treatment applications. Several Fortune 500 companies such as the Eaton Corporation are using these commercially-available treatment systems in large-scale applications.

Further advances in the application of semiconductor photocatalysis by the Hoffmann group; include the development of TiO2-coated fiber optic cable reactors for heterogeneous photocatalysis (Peill and Hoffmann, 1995; 1996; 1997; 1998). These novel photoreactors employ quartz fiber-optic cables to deliver focused UV or solar irradiation over long-distances to an active reaction zone in which the polymeric coating of the fiber optic cables has been replaced by TiO2 or metal-doped TiO2 coating. Once the propagated light reaches the semiconductor coating the UV light is refracted out of the fiber and into the TiO2 coating, which then leads directly to photoactivation of the catalyst. This invention has also led to the formal approval of another US Patent (Serial Number 08/654,093)). These reactors can be used, for example, for the in situ photocatalytic treatment of contaminated groundwater or for the photocatalytic treatment of contaminated air streams from industrial sites. At present, this technology is being explored by the Northrop-Grumman Corporation for chemical and biowarfare agent control.

Hoffmann and his group have been exploring the application of several forms of electrohydraulic cavitation for the elimination of hazardous chemical compounds from water. They have advanced our understanding of the chemistry and physics of ultrasonic irradiation in water and advocated its application of the destructive elimination of chemical contaminants such as carbon tetrachloride, pentachlorophenol, TNT, parathion, triethanolamine, and methyl tertiary-butyl ether from groundwater. Hoffmann and his students have demonstrated that electrohydraulic cavitation induced by ultrasonic irradiation over the frequency range of 16 to 1,100 kHz results in the complete oxidative and pyrolytic degradation of a wide range of organic and inorganic compounds in water (Kotronarou et al., 1991; Kotronarou and Hoffmann, 1992; Hua and Hoffmann, 1996; Hua and Hoffmann, 1997; Kang and Hoffmann, 1998). They have shown that the chemical pathways resulting from the violent collapse of cavitation bubbles include direct pyrolytic decomposition within the vapor-phase of the collapsing bubbles, oxidation by hydroxyl radical produced from the pyrolytic decomposition of water, and by transient supercritical water reactions resulting from the extremely high temperatures and pressures generated within a bubble during sonolytic cavitation. This technology also has many commercial applications.
In a related application of electrohydraulic phenomena, Hoffmann and his group (Wilberg et al., 1996; Lang et al., 1998) have developed a reactor system known as the pulsed-power plasma discharge reactor. In this unique reactor system, a large capacitor bank is charged up to a level of several thousand volts (i.e., 10,000 volts) and then over a period of nanoseconds this stored energy (up to 10 kJ) is discharged across two tantalum electrodes with the resulting conversion of water directly into a 50,000 K plasma state. The H2O+ and electron plasma produces a blackbody emission centered at about 100 nm. At these wavelengths, water is photodecomposed to produce hydroxyl radical and hydrogen atom. Within the plasma volume (4 mL), all chemical compounds are instantly degraded during the net lifetime of the plasma (e.g., 40 ?s). At the plasma water interface, the H2O+ ions from the gas-phase react with water to produce hydroxyl radical, which in turn is used to oxidize organic compounds and their degradation intermediates. Recent work (Lang et al., 1998) has shown that 200 ?M TNT can be totally eliminated from 4 liters of water within a power utilization time of only 0.2 ms. This technology is also being developed for commercial applications by several small technology companies.

References Cited

D. W. Bahnemann, C. Kormann, and M. R. Hoffmann (1987) "Preparation and Characterization of Quantum Size Zinc Oxide: Fluoresence and Non Linear Optical Effects," J. Phys. Chem. 91, 3789-3798.

E. A. Betterton and M. R. Hoffmann (1988) Henry's Law Determinations for Some Environmentally Important Aldehydes," Environ. Sci. Technol., 22, 1415-1418.

S. D. Boyce, M. R. Hoffmann, P. A. Hong and L. M. Moberly (1983) "Catalysis of the Autoxidation of Aquated Sulfur Dioxide by Homogeneous Metal-phthalocyanine Complexes." Environ. Sci. Tech. 17, 602-611.

S. D. Boyce and M. R. Hoffmann, (1984)) "Kinetics and Mechanism of the Formation of Hydroxymethanesulfonic Acid at Low pH." J. Phys. Chem. 88, 4740-4746.

W. Choi, A. Termin and M. R. Hoffmann (1994) "The Role of Metal-Ion Dopants in Quantum-Sized TiO2: Correlation between Photoreactivity and Charge-Carrier Recombination Dynamics," J. Phys. Chem. , 98, 13669-13679.

B. C. Daube, Jr., R. C. Flagan, and M. R. Hoffmann (1987) "Active Cloudwater Collector," United States Patent, Patent Number: 4,697,462; Date: Oct. 6, 1987.

B. C. Daube, Jr., R. C. Flagan, and M. R. Hoffmann (1988) "Automated Rainwater Collector," United States Patent, Patent Number: 4,732,037; Date: Mar. 22, 1988.

Y. Erel, S. O. Pehkonen and M. R. Hoffmann (1993) "Redox Chemistry of Fe in Coastal Fog and Stratus Clouds, J. Geophys. Res., 98, 18423-18434.

B. C. Faust, Bahnemann, and M. R. Hoffmann (1989) "Kinetics and Mechanism of the Photoassisted Oxidation of Sulfur Dioxide on Hematite (?-Fe2O3) J. Phys. Chem., 93, 6371-6381.

I. Hua and M. R. Hoffmann (1996) "The Kinetics and Mechanism of the Sonolytic Degradation of CCl4 in Aqueous Solution: Reaction intermediates and By-products," Environ. Sci. Technol., 30, 864-871.

I. Hua and M. R. Hoffmann (1997) "Optimization of Ultrasonic Irradiation as an Advanced Oxidation Technology," Environ. Sci. Technol., 31, 2237-2243.

A. J. Hoffman, E. R. Carraway and M. R. Hoffmann (1994) “Photocatalytic Production of Hydrogen Peroxide and Organic Peroxides on Quantum-sized Semiconductor Colloids,” Environ. Sci. Technol., 28, 776-785.

M. R. Hoffmann and J. O. Edwards (1975) "Kinetics and Mechanism of the Oxidation of Sulfur Dioxide by Hydrogen Peroxide in Acidic Solution." J. Phys. Chem. 79, 2096-2098.

M. R. Hoffmann (1977) "Kinetics and Mechanism of the Oxidation of Hydrogen Sulfide by Hydrogen Peroxide in Acidic Solution." Environ. Sci. Tech. 11, 61-66.

M. R. Hoffmann and B. C. H. Lim (1979) "Kinetics and Mechanism of the Oxidation of Sulfide by Oxygen: Catalysis by Homogeneous Metal Phthalocyanine Complexes." Environ. Sci. Tech. 13, 1406-1414.

M. R. Hoffmann and N. Peill (1999) "TiO2-Coated Fiber Optic Cable Reactor," United States Patent Approved, Ser. No. 08/654,093; Issue Date: March, 1999.

A. P. Hong, D. W. Bahnemann, and M. R. Hoffmann (1987a) "Co(II) tetrasulfophthalocyanine on Titanium Dioxide: A New Efficient Relay for the Photocatalytic Formation and Depletion of Hydrogen Peroxide in Aqueous Suspensions," J. Phys. Chem. 91, 2109-2116.

A. P. Hong, D. W. Bahnemann, and M. R. Hoffmann (1987b) "Co(II) tetrasulfophthalocyanine on Titanium Dioxide II. Photocatalytic Oxidation of Aqueous Sulfur Dioxide," J. Phys. Chem. 91, 6245-6251.

A. P. Hong, S. D. Boyce, M. R. Hoffmann (1989) "Catalytic Autoxidation of Chemical Contaminants by Hybrid Complexes of Co(II) Phthalocyanine," Environ. Sci. Technol. 23, 533-540.

D. J. Jacob and M. R. Hoffmann (1983) "A Dynamic Model for the Production of H+, NO3- and SO42- in Urban Fog." J. Geophys. Res. 88, 6611-6621.

D. J. Jacob, J. M. Waldman, J. W. Munger, and M. R. Hoffmann (1985) "Chemical Composition of Fogwater Collected along the California Coast." Environ. Sci. Tech., 19, 730-735.

D. J. Jacob, J. M. Waldman, J. W. Munger, and M. R. Hoffmann (1986) "The H2SO4-HNO3-NH3 System at High Humidities and in Fogs: I. Spatial and Temporal Patterns in the San Joaquin Valley of California." J. Geophys. Res. 91D, 1073-1088.

J. W. Kang and M. R. Hoffmann (1998) “The Kinetics and Mechanism of the Sonolytic Destruction of Methyl Tertiary Butyl Ether (MTBE) by Ultrasonic Irradiation in the Presence of Ozone, Environ. Sci. Technol., 32, 3194-3199.

J. M. Kesselman, O. Weres, N. S. Lewis and M. R. Hoffmann (1997) "Electrochemical Production of Hydroxyl Radical at Polycrystalline Nb-doped TiO2 Electrodes and Estimation of the Partitioning between Hydroxyl Radical and Direct Hole Oxidation Pathways," J. Phys. Chem., 101, 2637-2643.

A. Kotronarou, G. Mills and M. R. Hoffmann (1991) "Ultrasonic Irradiation of p-nitrophenol in Aqueous Solutions," J. Phys. Chem., 95, 3630-3638.

A. Kotronarou and M. R. Hoffmann (1992) "Ultrasonic Irradiation of Hydrogen Sulfide in Aqueous Solutions," Environ. Sci. Technol., 26, 2420-2428.

C. Kormann, D. W. Bahnemann, and M. R. Hoffmann (1988) "Preparation and Characterization of Quantum-Size Titanium Dioxide," J. Phys. Chem., 92, 5196-5201.

P. S. Lang, W. K. Ching, D. M. Willberg and M. R. Hoffmann, “Oxidative Degradation of 2,4,6-trinitrotoluene by Ozone in an Electrohydraulic Discharge Reactor,” Environ. Sci. Technol., 32, 3142-3148 (1998).

K. Leung and M. R. Hoffmann (1989a) "Kinetics and Mechanism of Reduction of Co(II)-4,4',4'',4'''-tetrasulfophthalocyanine by 2-mercaptoethanol under Anoxic Conditions,", J. Phys. Chem., 93, 431-433.

K. Leung and M. R. Hoffmann (1989b) "Kinetics and Mechanism of Autoxidation of 2-aminoethanethiol and Ethanethiol Catalyzed by Co(II)-4,4',4'',4'''- tetrasulfophthalocyanine in Aqueous Solution", J. Phys. Chem., 93, 434-441.

J. V. McArdle and M. R. Hoffmann (1983) "Kinetics and Mechanism of the Oxidation of Aquated Sulfur Dioxide by Hydrogen Peroxide at Low pH." J. Phys. Chem. 87, 5425-5429.

S. T. Martin, H. Herrmann, W. Choi and M. R. Hoffmann (1994a) "Time-Resolved Microwave Conductivity (TRMC) 1. TiO2 Photoactivity and Size Quantization," J. Chem. Soc. Faraday Trans., 90, 3315-3322.

S. T. Martin, H. Herrmann, and M. R. Hoffmann (1994b) "Time-Resolved Microwave Conductivity (TRMC) 1. Quantum-sized TiO2 and Effects of Adsorbates and Light Intensity on the Charge Carrier Dynamics," J. Chem. Soc. Faraday Trans., 90, 3323-3330.

S. T., Martin, C. L. Morrison and M. R. Hoffmann (1994c) "Photochemical Mechanism of Size-Quantized Vanadium-Doped TiO2," J. Phys. Chem., 98, 13695-13704 (1994).

J. W. Munger, J. M. Waldman, D. J. Jacob and M. R. Hoffmann (1983) "Fogwater Chemistry in an Urban Atmosphere." J. Geophys. Res. 88, 5109-5123.

J. W. Munger, D. J. Jacob, and M. R. Hoffmann (1984) "The Occurence of Bisulfite-Aldehyde Addition Products in Fog and Cloudwater." J. Atmos. Chem. 1, 335-350.

J. W. Munger, C. Tiller, and M. R. Hoffmann (1986) "Identification and Quantification of Hydroxymethanesulfonic Acid in Atmospheric Water Droplets," Science, 231, 247-249.

T. M. Olson and M. R. Hoffmann (1988a) "The Kinetics, Mechanism, and Thermodynamics of Glyoxal-S(IV) Adduct Formation," J. Phys. Chem., 92, 533-540.

T. M. Olson and M. R. Hoffmann (1988b) "The Kinetics, Mechanism, and Thermodynamics of Glyoxylic Acid-S(IV) Adduct Formation," J. Phys. Chem., 92, 4246-4253.

T. M. Olson, L. A. Torry, and M. R. Hoffmann (1988) "Kinetics of the Formation of Hydroxyacetaldehyde-S(IV) Adducts at Low pH," Environ. Sci. Technol., 22, 1284-1289.

T. M. Olson and M. R. Hoffmann (1989) "Hydroxyalkylsulfonate Formation: Its Role as a S(IV) Reservoir in Atmospheric Water Droplets," Atmospheric Environment, 23, 985-997.

S. O. Pehkonen, R. L. Siefert, S. Webb, Y. Erel and M. R. Hoffmann (1993) "Photo-reduction of Iron Oxyhydroxides In The Presence of Important Atmospheric Organic-Compounds," Environ. Sci. Technol., 27, 2056-2062.

N. J. Peill and M. R. Hoffmann (1995) "Development and Optimization of a TiO2-Coated Fiber Optic Cable Reactor: Photocatalytic Degradation of 4-Chlorophenol," Environ. Sci. Technol. 29, 2974-2981.

N. J. Peill and M. R. Hoffmann (1996) "Chemical and Physical Characterization of a TiO2-Coated Fiber Optic Cable Reactor," Environ. Sci. Technol., 30, 2806-2812.

N. J. Peill and M. R. Hoffmann (1997) "Solar-Powered Photocatalytic Fiber Optic Cable Reactor for Waste Stream Remediation," J. Solar Energy Eng., 119, 229-236.

N. J. Peill and M. R. Hoffmann (1998) "Mathematical Model of Photocatalytic Fiber-Optic Cable Reactor for Heterogeneous Photocatalysis," Environ. Sci. Technol. 32, 398-404.

J. M. Waldman, J. W. Munger, D. J. Jacob, R. C. Flagan, J. J. Morgan and M. R. Hoffmann (1982) "The Chemical Composition of Acid Fog." Science 218, 677-680.

O. Weres and M. R. Hoffmann (1994) "Electrochemical Process and Device for Generating Hydroxyl Free Radicals and Oxidizing Chemical Substances Dissolved in Water," United States Patent, Patent Number: 5,364,508; Date: Nov. 15, 1994.

O. Weres and M. R. Hoffmann (1994) "Electrode Manufacturing Process and Electrochemical Cell", United States Patent, Patent Number: 5,419,824; Date: May 30, 1995.

O. Weres and M. R. Hoffmann (1994) "Electrochemical Device for Generating Hydroxyl Free Radicals and Oxidizing Chemical Substances Dissolved in Water", United States Patent, Patent Number: 5,439,577; Date: Aug. 8, 1995.

D. M. Willberg, P. S. Lang, R. H. Höchemer, A. Kratel and M. R. Hoffmann (1996) "Degradation of 4-chlorophenol and 2,4-dichloroaniline in an Electrohydraulic Discharge Reactor," Environ. Sci. Technol. 30, 2526-2534.