Daniel M. Blake

Application of the Photocatalytic Chemistry of Titanium Dioxide to Disinfection and the Killing of Cancer Cells

Abstract This article will review the work that has been published on disinfection and the killing of cancer cells using photocatalytic chemistry with titanium dioxide (TiO2). This is an application of photocatalytic chemistry that has been under active investigation since 1985. Because the nature of the research is such that it brings together disparate disciplines, this review provides background on photocatalytic chemistry, fundamental characteristics of target organisms, potential applications, and the toxicology of titanium dioxide. Literature identified in searches done through September 1998 is included.

Assessment of a Molten Salt Heat Transfer Fluid in a Parabolic Trough Solar Field

An evaluation was carried out to investigate the feasibility of utilizing a molten salt as the heat transfer fluid (HTF) and for thermal storage in a parabolic trough solar field to improve system performance and to reduce the levelized electricity cost. The operating SEGS (Solar Electric Generating Systems located in Mojave Desert, California) plants currently use a high temperature synthetic oil consisting of a eutectic mixture of biphenyl/ diphenyl oxide. The scope of this investigation included examination of known critical issues, postulating solutions or possible approaches where potential problems exist, and the quantification of performance and electricity cost using preliminary cost inputs. The two leading candidates were the so-called solar salt (a binary salt consisting of 60% NaNO 3 and 40% KNO 3 ) and a salt sold commercially as HitecXL (a ternary salt consisting of 48% Ca(NO 3 ) 2 , 7% NaNO 3 , and 45% KNO 3 ). Assuming a two-tank storage system and a maximum operation temperature of 450°C, the evaluation showed that the levelized electricity cost can be reduced by 14.2% compared to a state-of-the-art parabolic trough plant such as the SEGS plants. If higher temperatures are possible, the improvement may be as high as 17.6%. Thermocline salt storage systems offer even greater benefits.

Engineering aspects of a molten salt heat transfer fluid in a trough solar field

An evaluation was carried out to investigate the feasibility of utilizing a molten salt as the heat transfer fluid (HTF) and for thermal storage in a parabolic trough solar field to improve system performance and to reduce the levelized electricity cost. The operating large-scale solar parabolic trough plants in the USA currently use a high temperature synthetic oil in the solar field consisting of a eutectic mixture of biphenyl/diphenyl oxide. The scope of the overall investigation included examination of known critical issues, postulating solutions or possible approaches where potential problems existed, and the quantification of performance and electricity cost using preliminary, but reasonable, cost inputs. The two leading candidates were the so-called solar salt (a binary salt consisting of 60% NaNO3 and 40% KNO3) and a salt sold commercially as HitecXL (a ternary salt consisting of 48% Ca(NO3)2, 7% NaNO3, and 45% KNO3). Operation and maintenance (O&M) becomes an important concern with molten salt in the solar field. This paper addresses that concern, focusing on design and O&M issues associated with routine freeze protection, solar field preheat methods, collector loop maintenance and the selection of appropriate materials for piping and fittings.

Heterogeneous photocatalysis for control of volatile organic compounds in indoor air

Research results concerning the photocatalytic activity and selectivity of benzene are discussed. This compound, which represents one of an important class of volatile organic compounds found in indoor air, was oxidized in an annular photocatalytic reactor featuring a thin film of titanium dioxide and illuminated by a fluorescent black light. The gas phase products, carbon dioxide and carbon monoxide, were quantified with a Fourier transform infrared spectrometer (FTIR). Adsorbed intermediates were extracted from the surface with water. The extract was analyzed via high performance liquid chromatography and some of the adsorbed species were provisionally identified by retention time matching. The adsorption of reactants on the catalyst surface was studied explicitly, particularly with respect to the effect of near-UV radiation on adsorption processes. Maximum and steady-state rates of the surface reactions are reported here as functions of the operating conditions. Deactivation of the catalyst surface is characterized and methods of regeneration of catalyst activity are explored. This established research methodology provides the framework for a broader outline of research into enhancement of indoor air quality via photocatalytic oxidation. The results of investigations are discussed that pertain to a variety of classes of compounds representative of indoor air pollutants.

Products, Intermediates, Mass Balances, and Reaction Pathways for the Oxidation of Trichloroethylene in Air via Heterogeneous Photocatalysis.

Studies of the photocatalytic reaction of a solution of trichloroethylene in the air and in contact with UV-irradiated titanium dioxide have produced conflicting reports in regard to the composition of the product mixture. This paper resolves these discrepancies by reporting the results of experiments designed to identify and quantify intermediates, products, and reaction pathways. Mass balances are closed in differential and integral modes to ascertain the effects of factors such as the extent of conversion, feed composition, and photon energy on the composition of the product stream

Advanced Thermal Storage Fluids for Solar Parabolic Trough Systems

It has been established that the development of a storage option and increasing the operating temperature for parabolic trough electric systems can significantly reduce the levelized electricity cost (LEC) compared to the current state of the art. Both improvements require a new heat transfer fluid that must have a very low vapor pressure at the hot operating temperature and combined with a high thermal stability, higher than 450°C. Further, the piping layout of trough plants dictates that the fluid not be allowed to freeze, which dictates the use of extensive insulation and heat tracing unless the fluid has a freezing point near 0°C. At present, it seems likely that this “ideal” fluid will have to be found among organic rather than inorganic salts. We are therefore investigating the chemical and thermal properties of ‘room temperature ionic liquids’ (RTILs) that hold much promise as a new class of heat transfer or storage fluids.© 2002 ASME

Kinetic and mechanistic overview of TiO2-photocatalyzed oxidation reactions in aqueous solution

Abstract Destruction of trace organic compounds in water by photocatalytic oxidation over titanium dioxide is the subject of worldwide interest. In this paper we review chemical mechanisms that have been put forward to account for the observed reactions and identify some band gap and thermodynamic limitations that must be considered in developing a solar-driven process. Recent work will be discussed which tests predictions made by the mechanism concerning dependence of rates of destruction on light flux and on the presence of hydrogencarbonate ion, a known hydroxyl radical scavenger.

Solar photocatalytic processes for the purification of water: State of development and barriers to commercialization

Abstract Semiconductor-based photocatalytic processes for removing hazardous chemicals from contaminated water have been studied for nearly 20 years. One goal of this research is to use the sun as the light source. This article assesses the state of development of solar heterogeneous photocatalytic processes for treating contaminated water and identifies key barriers that must be overcome for the technology to achieve commercial success. Some industry members estimate that the cost of using solar technology for waste treatment will need to be less than half the cost of a “conventional” technology in order to gain market acceptance. The number of applications that are near commercial viability could be expanded with significant progress in the improvement of the photo-efficiency of the photocatalytic process.

Mechanism of Hydrogen Formation in Solar Parabolic Trough Receivers

Solar parabolic trough systems for electricity production are receiving renewed attention, and new solar plants are under construction to help meet the growing demands of the power market in the Western United States. The growing solar trough industry will rely on operating experience it has gained over the last two decades. Recently, researchers found that trough plants that use organic heat transfer fluids (HTF) such as Therminol VP-1 are experiencing significant heat losses in the receiver tubes. The cause has been traced back to the accumulation of excess hydrogen gas in the vacuum annulus that surrounds the steel receiver tube, thus compromising the thermal insulation of the receiver. The hydrogen gas is formed during the thermal decomposition of the organic HTF that circulates inside the receiver loop, and the installation of hydrogen getters inside the annulus has proven to be insufficient for controlling the hydrogen build-up over the lifetime of the receivers. This paper will provide an overview of the chemical literature dealing with the thermal decomposition of diphenyl oxide and biphenyl, the two constituents of Therminol VP-1.

Tetrazete (N4). Can it be prepared and observed

Abstract The nitrogen plasma generated by microwave or electrical discharge in gaseous N 2 was quenched on a cold window (6.2–35 K) and resulting matrix was examined by IR and UV–Vis absorption spectroscopies. In samples prepared with 14 N 2 we observe a weak infrared transition at 936.7 cm −1 . It shifts to 900.0 cm −1 when 15 N 2 is used. Both peaks do not correlate with any other features in the spectra and are best explained as originating from tetrahedral tetrazetes (N 4 ). Their positions are compatible with quantum chemical estimates for 14 N 4 , at 936.0 cm −1 and 15 N 4 , at 904.4 cm −1 .