Surface-Enhanced Nitrate Photolysis on Ice

Heterogeneous nitrates photolysis is the trigger for many chemical processes occurring in the polar boundary layer and is widely believed to occur in a quasi-liquid layer (QLL) at the surface of ice. The dipole forbidden character of the electronic transition relevant to boundary layer atmospheric chemistry and the small photolysis/photoproducts quantum yields in ice (and in water) may confer a significant enhancement and interfacial specificity to this important photochemical reaction at the surface of ice. Using amorphous solid water films at cryogenic temperatures as models for the disordered interstitial air/ice interface within the snowpack suppresses the diffusive uptake kinetics thereby prolonging the residence time of nitrate anions at the surface of ice. This approach allows their slow heterogeneous photolysis kinetics to be studied providing the first direct evidence that nitrates adsorbed onto the first molecular layer at the surface of ice are photolyzed more effectively than those dissolved within the bulk. Vibrational spectroscopy allows the ~3-fold enhancement in photolysis rates to be correlated with the nitrates’ distorted intramolecular geometry thereby hinting at the role played by the greater chemical heterogeneity in their solvation environment at the surface of ice than in the bulk. A simple 1D kinetic model suggests 1-that a 3(6)-fold enhancement in photolysis rate for nitrates adsorbed onto the ice surface could increase the photochemical NO[subscript 2] emissions from a 5(8) nm thick photochemically active interfacial layer by 30%(60)%, and 2-that 25%(40%) of the NO[subscript 2] photochemical emissions to the snowpack interstitial air are released from the top-most molecularly thin surface layer on ice. These findings may provide a new paradigm for heterogeneous (photo)chemistry at temperatures below those required for a QLL to form at the ice surface

Chlorine activation indoors and outdoors via surface-mediated reactions of nitrogen oxides with hydrogen chloride

Gaseous HCl generated from a variety of sources is ubiquitous in both outdoor and indoor air. Oxides of nitrogen (NOy) are also globally distributed, because NO formed in combustion processes is oxidized to NO2, HNO3, N2O5 and a variety of other nitrogen oxides during transport. Deposition of HCl and NOy onto surfaces is commonly regarded as providing permanent removal mechanisms. However, we show here a new surface-mediated coupling of nitrogen oxide and halogen activation cycles in which uptake of gaseous NO2 or N2O5 on solid substrates generates adsorbed intermediates that react with HCl to generate gaseous nitrosyl chloride (ClNO) and nitryl chloride (ClNO2), respectively. These are potentially harmful gases that photolyze to form highly reactive chlorine atoms. The reactions are shown both experimentally and theoretically to be enhanced by water, a surprising result given the availability of competing hydrolysis reaction pathways. Airshed modeling incorporating HCl generated from sea salt shows that in coastal urban regions, this heterogeneous chemistry increases surface-level ozone, a criteria air pollutant, greenhouse gas and source of atmospheric oxidants. In addition, it may contribute to recently measured high levels of ClNO2 in the polluted coastal marine boundary layer. This work also suggests the potential for chlorine atom chemistry to occur indoors where significant concentrations of oxides of nitrogen and HCl coexist

Discovering Inexpensive, Effective Catalysts for Solar Energy Conversion: An Authentic Research Laboratory Experience

Electrochemical water oxidation is a major focus of solar energy conversion efforts. A new laboratory experiment has been developed that utilizes real-time, hands-on research to discover catalysts for solar energy conversion. The HARPOON, or Heterogeneous Anodes Rapidly Perused for Oxygen Overpotential Neutralization, experiment allows an array of mixed-metal oxide compositions to be analyzed in parallel to test their activity as water oxidation catalysts. Students create unique combinations of mixed-metal oxide materials, which are then analyzed utilizing a simple, inexpensive system that detects the amount of oxygen evolved during electrolysis. This experiment has the flexibility to be implemented at a variety of educational levels with the depth and breadth of the material covered accordingly. Concepts such as stoichiometry, materials, solutions, and fluorescence can be emphasized, while the research-like experience strengthens students’ independence, critical-thinking skills, and excitement for science. An online questionnaire was developed to measure various effects of the experiment on students, including learning gains, attitudes toward chemistry, and motivation to pursue a career in scientific research. The assessment results indicate positive gains for students in their understanding of the social nature of scientific work, scientific literacy, and interest in pursuing additional research opportunities

Discovering Inexpensive, Effective Catalysts for Solar Energy Conversion: An Authentic Research Laboratory Experience

Electrochemical water oxidation is a major focus of solar energy conversion efforts. A new laboratory experiment has been developed that utilizes real-time, hands-on research to discover catalysts for solar energy conversion. The HARPOON, or Heterogeneous Anodes Rapidly Perused for Oxygen Overpotential Neutralization, experiment allows an array of mixed-metal oxide compositions to be analyzed in parallel to test their activity as water oxidation catalysts. Students create unique combinations of mixed-metal oxide materials, which are then analyzed utilizing a simple, inexpensive system that detects the amount of oxygen evolved during electrolysis. This experiment has the flexibility to be implemented at a variety of educational levels with the depth and breadth of the material covered accordingly. Concepts such as stoichiometry, materials, solutions, and fluorescence can be emphasized, while the research-like experience strengthens students’ independence, critical-thinking skills, and excitement for science. An online questionnaire was developed to measure various effects of the experiment on students, including learning gains, attitudes toward chemistry, and motivation to pursue a career in scientific research. The assessment results indicate positive gains for students in their understanding of the social nature of scientific work, scientific literacy, and interest in pursuing additional research opportunities