Formation Of Nitrous Acid During Ammonia Photooxidation On Tio2 Under Atmospherically Relevant Conditions

Ammonia (NH3) from natural and anthropogenic sources is a globally distributed air pollutant, with emission rates expected to double over the next several decades. Selective catalytic oxidation (SCO) on TiO2 photocatalysts represents a potential strategy for NH3 pollution abatement. We show here that irradiation of TiO2 in the presence of NH3 under atmospheric conditions releases gas-phase nitrous acid (HONO), a potentially harmful precursor to atmospheric ozone, before it can be further reduced to N-2. Chemical ionization mass spectrometry and diffuse reflection infrared spectroscopy measurements indicate that HONO is derived from the reduction of NO2 and NO3-, which are products of the water-catalyzed photooxidation of NH3. This represents the first experimental evidence of HONO formation from NH3 photooxidation and has important implications for understanding SCO processes and devising air pollution control strategies to remove NH3 from ambient air.