Atmospheric photodissociation rate coefficients and photodissociation lifetimes for nitromethane, methyl nitrite, and methyl nitrate were calculated as a function of altitude from their measured visible and near ultraviolet photoabsorption cross sections at 298 K. The lifetime of methyl nitrite is nearly independent of altitude and is approximately 2 min. From 0 to 50 km the lifetime of nitromethane varies from 10 to 0.5 hr, while that of methyl nitrate changes from 5.3 to 0.09 days, respectively.

Nanoceria was produced by an aqueous precipitation technique in the presence of an organic stabilizer. The stable suspensions were diafiltered to remove reaction byproducts. Particles were characterized by transmission electron microscopy with images used to size the particles, and selected-area electron diffraction was used to determine the lattice structure and the lattice constant. For all particles studied, the electron diffraction data were consistent with that of CeO2 and not Ce2O3, as predicted by some researchers for very small particles sizes. At particle diameters of ∼1 nm, the lattice expansion approached 7%. In agreement with earlier researchers, we interpret this effect as due to the formation of substantial amounts of Ce3+ with corresponding oxygen vacancies, but within the fluorite lattice structure of CeO2. Even at a particle size of 1 nm, there was a measurable oxygen storage capacity, consistent with a still-reducible CeO2 structure, rather than the fully oxidized Ce2O3.

reaction enthalpy of AHr0 = -13.3, -10.1, and -2.3 kcal mol-', respectively, is calculated for the dissociation into the fragments R and C1-. Dissociation reactions yielding the fragments F-, C12-, Fz-, and/or CIF-are all endothermic. On the other hand, in aliphatic chlorocarbons a very small abundance of Clz-was detected in a low-energy (0-10 eV) electron impact study with a time-of-flight mass s p e~t r o m e t e r .~~ It is interesting to note that changes of the relative attachment rates as a function of electron energy can be related to the thermodynamics of the capture reaction. For the most exothermic reaction, i.e., capture by CC&F, only a decrease in the relative ECR line intensity is observed on increasing the electron energy. The less exothermic reaction between free electrons and CClZFz, for which an activation energy of 4.5 kcal mol-l was obtained from kinetic measurements, shows a flat maximum around 0.05 eV. Only an increase in relative line intensity with electron energy is observed for the inefficient scavenger CCIFB ( Figure 1). The calculated AHro yielded -2.3 kcal mol-'. The determined activation energy was 5.7 kcal mol-'. Unfortunately, the change in relative cross section could not be followed to higher electron energies to obtain the position of the maximum.Acknowledgment. The technical assistance of Mr. C. Lygris is gratefully acknowledged.

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Global interest in both renewable energies and reduction in emission levels has placed increasing attention on hydrogen-based fuel cells that avoid harm to the environment by releasing only water as a byproduct. Therefore, there is a critical need for education and workforce development in clean energy technologies. A new undergraduate laboratory course for students has been developed, entitled, Clean Energy: Hydrogen/Fuel Cells. If hydrogen is to be used on a large-scale basis, the storage of hydrogen becomes a crucial issue for mobility and transport applications. Fuel cell technology would be practical if hydrogen could be stored in a safe, efficient, compact, and economic manner. The experiments reported here train students to measure the hydrogen storage capacity of aqueous hydrochloric acid solution and solid magnesium hydride by measuring the consumption of hydrogen with a hydrogen fuel cell and evaluating its performance. In addition, students also gain fundamental experience handling gases and using the gas law equations.