Increased silver activity for direct propylene epoxidation via subnanometer size effects.

Production of the industrial chemical propylene oxide is energy-intensive and environmentally unfriendly. Catalysts based on bulk silver surfaces with direct propylene epoxidation by molecular oxygen have not resolved these problems because of substantial formation of carbon dioxide. We found that unpromoted, size-selected Ag3 clusters and approximately 3.5-nanometer Ag nanoparticles on alumina supports can catalyze this reaction with only a negligible amount of carbon dioxide formation and with high activity at low temperatures. Density functional calculations show that, relative to extended silver surfaces, oxidized silver trimers are more active and selective for epoxidation because of the open-shell nature of their electronic structure. The results suggest that new architectures based on ultrasmall silver particles may provide highly efficient catalysts for propylene epoxidation.

Experimental shock chemistry of aqueous amino acid solutions and the cometary delivery of prebiotic compounds.

A series of shock experiments were conducted to assess the feasibility of the delivery of organic compounds to the Earth via cometary impacts. Aqueous solutions containing near-saturation levels of amino acids (lysine, norvaline, aminobutyric acid, proline, and phenylalanine) were sealed inside stainless steel capsules and shocked by ballistic impact with a steel projectile plate accelerated along a 12-m-long gun barrel to velocities of 0.5-1.9 km sec-1. Pressure-temperature-time histories of the shocked fluids were calculated using 1D hydrodynamical simulations. Maximum conditions experienced by the solutions lasted 0.85-2.7 microseconds and ranged from 5.1-21 GPa and 412-870 K. Recovered sample capsules were milled open and liquid was extracted. Samples were analyzed using high performance liquid chromatography (HPLC) and mass spectrometry (MS). In all experiments, a large fraction of the amino acids survived. We observed differences in kinetic behavior and the degree of survivability among the amino acids. Aminobutyric acid appeared to be the least reactive, and phenylalanine appeared to be the most reactive of the amino acids. The impact process resulted in the formation of peptide bonds; new compounds included amino acid dimers and cyclic diketopiperazines. In our experiments, and in certain naturally occurring impacts, pressure has a greater influence than temperature in determining reaction pathways. Our results support the hypothesis that significant concentrations of organic material could survive a natural impact process.

Small-angle X-ray studies of soot inception and growth.

The high spectral intensity of X-rays produced by the undulator at the Basic Energy Sciences Synchrotron Radiation Center of Argonne's Advanced Photon Source has allowed us to perform small-angle X-ray scattering (SAXS) studies of the initial distribution of soot particles formed by various fuels. SAXS provides an in situ probe of the morphology of soot in the region between 1 and 100 nm and complements the ex situ technique of electron microscopy. The basic aspects of SAXS and its potential are illustrated with measurement on a laminar flame of acetylene in air. The more complex fuel toluene has been studied in a flat-flame burner that supports a CH4/H2/air or CO/H2/air diffusion flame stabilized by N2 co-flow. This burner produces a nearly constant temperature region above the flame where the pyrolysis and combustion of the heavier fuels occurs. Kinetic information is obtained by performing measurements of the scattered intensity profile as a function of the height above the burner. These profiles have been reduced to give the mean radius and dispersion of a distribution of spherical particles. Mean radii between 0.8 and 18 nm have been observed. The smallest of these is a factor of ten smaller than previously detected with Lorentz-Mie scattering. Near 1550 K, the soot distribution found in toluene shows a distinct step behavior that is consistent with model calculations.

Fullerenes in the 1.85-billion-year-old Sudbury impact structure.

Fullerenes (C60 and C70) have been identified by laser desorption, laser desorption post-ionization, and high-resolution electron-impact mass spectrometry in shock-produced breccias (Onaping Formation) of the Sudbury impact structure in Ontario, Canada. The C60 isotope is present at a level of a few parts per million. The fullerenes were likely synthesized within the impact plume from the carbon contained in the bolide. The oxidation of the fullerenes during the 1.85 billion years of exposure was apparently prevented by the presence of sulfur in the form of sulfide-silicate complexes associated with the fullerenes.

Phenolic ethers in the organic polymer of the murchison meteorite.

Seven phenolic acids and many nonphenolic organic acids, including large amounts of meta-hydroxy (3-hydroxy) benzoic acid and 3-hydroxy-1,5-benzene-dicarboxylic acid, were obtained from the organic polymer of the Murchison C2 chondrite upon oxidation with alkaline cupric oxide. The phenolic acids apparently were derived from phenolic ethers in the polymer, which in turn probably were formed from carbon monoxide and hydrogen by catalytic Fischer-Tropsch type reactions in the solar nebula. In contrast, terrestrial polymers such as lignin, humic acid, and coal yield mainly para-hydroxy (4-hydroxy) benzene derivatives by the same oxidation procedure.