Ultrahigh Vacuum Studies of the Reaction Kinetics and Mechanisms of Nitrate Radical with Model Organic Surfaces
Detailed understanding of the kinetics and mechanisms of heterogeneous reactions between gas-phase nitrate radicals, a key nighttime atmospheric oxidant, and organic particles will enable scientists to predict the fate and lifetime of the particles in the atmosphere. In an effort to acquire knowledge of interfacial reactions of nitrate radical with organics, model surfaces are created by the spontaneous adsorption of methyl-/vinyl-/hydroxyl-terminated alkanethiols on to a polycrystalline gold substrate. The self-assembled monolayers provide a well-defined surface with the desired functional group (-CH3, H2C=CH-, or HO-) positioned precisely at the gas-surface interface. The experimental approach employs in situ reflection-absorption infrared spectroscopy (RAIRS) to monitor bond rupture and formation while a well-characterized flux of NO3 impinges on the organic surface. Overall, the reaction kinetics and mechanisms were found to depend on the terminal functional group of the SAM and incident energy of the nitrate radical (NO3). For reactions of the H2C=CH-SAM with NO3, the surface reaction kinetics obtained from RAIRS reveals that the consumption rate of the terminal vinyl groups is nearly identical to the formation rate of a surface-bound nitrate species and implies that the mechanism is one of direct addition to the vinyl group rather than hydrogen abstraction. Upon nitrate radical collisions with the surface, the initial reaction probability for consumption of carbon-carbon double bonds was determined to be (2.3 ± 0.5) -- 10-3. Studies of reactions of HO-SAM with the effusive source of NO3 suggest that the reaction between NO3 and the HO-SAM is initiated by hydrogen abstraction at the terminal - 'CH2OH groups with the initial reaction probability of (6 ± 1)-- 10-3. An Arrhenius plot was obtained to measure the activation energy of the H abstraction from the HO-SAM. Further, for reactions of the HO-SAM with the high incident energy of NO3 molecules created by molecular beam, the reaction probability for H abstraction at the hydroxyl terminus was determined to be ~0.4. The significant increase in the reaction probability was attributed to the promotion in the ability of NO3 abstracting hydrogen atom at the methylene groups along hydrocarbon chains. The reaction rates of NO3 with the model organic surfaces that have been investigated are orders of magnitude greater than the rate of ozone reactions on the same surfaces which suggests that oxidation of surface-bound organics by nighttime nitrate radicals may play an important role in atmospheric chemistry despite their relative low concentration. X-ray photoelectron spectroscopy (XPS) data suggests that oxidation of the model organic surfaces by NO3 leads to the production of organic nitrates, which are stable for a period time. In addition, the effect of background gases on reactions of NO3 with model organic surfaces needs further investigations at atmospheric pressures. The results presented in this thesis should help researchers to predict the fate and environmental impacts of organic particulates with which nitrate radicals interact.