Theoretical Study The Role Of The Urea In The Hydrolysis Reaction Of NO₂

The nitrous acid?HONO?in atmospheric pollution chemistry and nitrogen-containing particulate matter are both typical secondary pollutants.Ammonia,methylamine,dimethylamine,etc.can promote the hydrolysis of NO₂ and release HONO.In recent years,urea as a double amino organic amine has received attention,because it is detected in aerosols and rainwater.However,the formation mechanism of urea-related aerosols in the atmosphere has not been reported theoretically.In this paper,the formation and structural changes of the molecular cluster2NO₂-mH₂O-CON₂H₄?m=1-3?was calculated by quantum chemistry theory.The effect of urea on the gas phase hydrolysis of NO₂ was studied,the catalytic effect is compared with ammonia and methylamine.The total reaction process is:2NO₂+mH₂O+NH₂CONH₂?NH₂CONH₃⁺NO₃^-/NH₂CONH₂·HNO₃+HONO?m=1-3?The results show that urea as a catalyst during the hydrolysis reaction,the catalytic effect is better than ammonia and methylamine.The energy barrier of urea participation in the reaction is-2.67 kcal/mol,which is 14.37 kcal/mol lower than that of the hydrolysis reaction of NO₂.Under the urea-promoted reaction,the hydrolysis reaction becomes a no energy barrier process,eventually releasing HONO.Because urea has amino and carbonyl functional groups,two urea nitrate isomers formed in the subsequent reaction,its hydrates may form aerosol molecules.When amino act as a proton acceptor,the formation of the zwitterions NH₂CONH₃⁺NO₃^-?UN-1?,this process needs to overcome an energy barrier;When carbonyl as a proton acceptor,the formation of acid-base complex NH₂CONH₂·HNO₃?UN-2?,this process approaches barrierless,and it is more favorable than zwitterions UN-1 in kinetic and thermodynamic.Zwitterions formation and hydrolysis reaction are affected by humidity.The multi-water promoted hydrolysis reaction and urea nitrate hydrate are shown more better thermodynamic properties,when the humidity is increasing.The extra water molecules act as solvents to reduce the energy barrier.Ultimate,use the molecular electrostatic potential surface?MEPS?charge profile to detect the active site of the complexes,it was found that both the urea nitrate UN-1 and its hydrate tended to form zwitterions.The donor-acceptor interactions of the urea nitrate and its hydrate were analyzed by the natural bond orbital?NBO?method in this paper,the result indicates that the interaction between carbonyl and nitric in acid-base complex is the strongest,also explains the formation process of RNH₃⁺and transformation form hydrogen bond to the ion interactions.Besides,thermodynamic data shows that all the hydrolysis and subsequent reactions are spontaneous exothermic processes.