The hypothesis to be tested in this project is that atmospheric heterogeneous photocatalysis of isoprene may play an important role in secondary organic aerosol (SOA) formation. The atmospheric relevance of transition metal-based particulate semiconductors in the photochemical oxidation of isoprene to low volatility products will be investigated. Photocatalysts will include atmospherically-relevant transition metal oxide particles, desert sand, and volcanic ash in the 0.1 to 10 micrometer size range. Liquid-solid photocatalytic reactions will be studied in terms of the organic substrate (isoprene and the primary products of isoprene gas-phase photochemical oxidation, methacrolein and methyl vinyl ketone), the presence of a sacrificial electron donor (e.g., acetate), oxygen concentration, and pH. The potential for photochemical Diels-Alder reactions to occur between isoprene (a diene) and a range of relevant dienophiles will be investigated, followed by analysis for oligomeric products. The research will be overseen by senior investigators from CalTech and the Oak Crest Institute of Science.

These studies will potentially advance the field of aerosol research by guiding future smog chamber studies; identifying chemical markers characteristic of heterogeneous photooxidation pathways; and investigating the polymerization products, which may account for a significant portion of the SOA mass. The research will be carried out by a post-doctoral scholar, and a graduate student at Caltech, as well as community college students and high school teachers working in teams with high school students via Oak Crest. These efforts will help to broaden the participation of underrepresented groups in scientific research.

Project Report

The collaborative research among three institutions, the California Institute of Technology, St. Andrews University (Scotland), and Robert Gordon University (Scotland) resulted in the synthesis and characterization of composite catalytic systems composed of earth-abundant metal oxide semiconductors, mixed metal oxide semiconductors, coupled with carbon nitride for the reduction of carbon dioxide into hydrogen carbons injunction with the reduction of protons and water to produce hydrogen. The various composite catalysts were able to split water photochemically into hydrogen and oxide, to reduced carbon dioxide into methane, ethane, propane, butane, ethene, propene, butene, formate, oxalate, and methanol. A composite catalyst consisting of multi-walled sodium titanate nanotubes, nano-particle elemental copper embedded in the walls of the nanotube base support, and nano-particle cadmium sulfide as a light-absorbing semiconductor was shown to be able to simply use CO2, water, and light under-illumination with visible light to produce methane, ethane, and propane along with the corresponding alkenes. Higher reactivity was obtained by increasing the non-stoichiometric fraction of sodium in the multi-walled titanate nanotubes. Results of this collaborative project illustrate approaches to achieve artificial photosynthesis using earth-abundant inorganic materials to produced useful hydrocarbons for chemical synthesis, for energy production, and for carbon dioxide capture and recycling into useful materials. With increased photo-efficiencies the use of composite inorganic materials for CO2 capture and transformation in to useful byproducts may have a significant impact on reducing worldwide carbon footprints, less the impact of increasing CO2 concentrations in the atmosphere and lead to greater energy and environmental sustainability.

National Science Foundation (NSF)
Division of Atmospheric and Geospace Sciences (AGS)
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Sylvia A. Edgerton
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California Institute of Technology
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