Photooxidation of ammonia on TiO₂ as a source of NO and NO ₂ under atmospheric conditions

Ammonia is the most abundant reduced nitrogen species in the atmosphere and an important precursor in the industrial-scale production of nitric acid. A coated-wall flow tube coupled to a chemiluminescence NO_x analyzer was used to study the kinetics of NH₃ uptake and NO_x formation from photochemistry initiated on irradiated (λ > 290 nm) TiO₂ surfaces under atmospherically relevant conditions. The speciation of NH₃ on TiO₂ surfaces in the presence of surface-adsorbed water was determined using diffuse reflection infrared Fourier transform spectroscopy. The uptake kinetics exhibit an inverse dependence on NH₃ concentration as expected for reactions proceeding via a Langmuir-Hinshelwood mechanism. The mechanism of NO_x formation is shown to be humidity dependent: Water-catalyzed reactions promote NO_x formation up to a relative humidity of 50%. Less NO_x is formed above 50%, where increasing amounts of adsorbed water may hinder access to reactive sites, promote formation of unreactive NH₄⁺, and reduce oxidant levels due to higher OH radical recombination rates. A theoretical study of the reaction between the NH₂ photoproduct and O₂ in the presence of H₂O supports the experimental conclusion that NO _x formation is catalyzed by water. Calculations at the MP2 and CCSD(T) level on the bare NH₂ + O₂ reaction and the reaction of NH₂ + O₂ in small water clusters were carried out. Solvation of NH₂OO and NHOOH intermediates likely facilitates isomerization via proton transfer along water wires, such that the steps leading ultimately to NO are exothermic. These results show that photooxidation of low levels of NH₃ on TiO₂ surfaces represents a source of atmospheric NO_x, which is a precursor to ozone. The proposed mechanism may be broadly applicable to dissociative chemisorption of NH ₃ on other metal oxide surfaces encountered in rural and urban environments and employed in pollution control applications (selective catalytic oxidation/reduction) and during some industrial processes.